Halide perovskites are generating enormous excitement for their use in high-performance yet inexpensive optoelectronic applications. Nevertheless, a number of fundamental questions about these materials still remain and need to be answered to push devices to their theoretical performance limits. For example, we still know very little about the specific nature of the defects leading to trap states, and the associated impact on carrier recombination and carrier diffusion.

I will present results on new techniques we are developing to address these open questions in halide perovskite semiconductors. These techniques focus on understanding charge carrier behavior, including recombination, trapping and diffusion, and how these properties link to chemical and material properties. I will present high-resolution luminescence microscopy techniques employing two-photon excitation to allow us to visualize and time-resolve carrier diffusion in three-dimensions, revealing anisotropic and depth-dependent carrier diffusion properties. Furthermore, we link the local luminescence properties to high-resolution crystallographic and chemical properties using synchrotron nano-probe X-Ray beamlines and low-dose scanning electron diffraction measurements. Through these measurements, we reveal the nature of the defects associated with local non-radiative power losses and heterogeneous diffusion. Furthermore, we provide guidelines about how we can ultimately eliminate these unwanted loss pathways and homogenize carrier diffusion.

Organic-inorganic metal halide perovskites have emerged as attractive materials for solar cells with power-conversion efficiencies now exceeding 23%. As these devices are approaching the Shockley-Queisser limit, bimolecular (band-to-band) recombination will dominate the charge-carrier losses, with trap-mediated charge recombination becoming less prominent.

We show that in methylammonium lead triiodide perovskite, bimolecular recombination can be fully explained as the inverse of absorption,^[1] and exhibits a dynamic that is heavily influenced by photon reabsorption inside the material.^[2,3] Such photon recycling is shown to slow charge losses from thin hybrid perovskite films, depending on light out-coupling.^[2] Interestingly, for thin films comprising a quasi-two-dimensional (2D) perovskite region interfaced with a 3D MAPbI₃ perovskite layer the blue-shifted emission originating from quasi-2D regions overlaps significantly with the absorption spectrum of the 3D perovskite, allowing for highly effective “heterogeneous photon recycling”. We show that this combination fully compensates for the adverse effects of electronic confinement, yielding quasi-2D perovskites with highly efficient charge transporting properties.^[3]

In addition, we investigate optoelectronic properties of mixed tin-lead iodide and mixed iodide-bromide lead perovskites. We show how band-gap bowing in tin-lead perovskites is compatible with a mechanism arising from bond bending to accommodate the random placement of unevenly sized lead and tin ions.^[4] While tin-rich compositions exhibit fast, mono-exponential recombination that is almost temperature-independent, in accordance with high levels of electrical doping,^[4,5] lead-rich compositions show slower, stretched-exponential charge-carrier recombination that is strongly temperature-dependent, in accordance with a multiphonon assisted process. Finally, in the context of silicon-perovskite tandem cells, we discuss the mechanisms underlying detrimental halide segregation in mixed iodide-bromide lead perovskites with desirable electronic band gaps near 1.75eV.^[6]

Colloidal organic/inorganic lead halide perovskite nanocrystals (NCs) are considered promising blue and green narrow-band emitters for the next-generation light-emitting diodes.  High photoluminescence efficiencies are attained in these materials without epitaxial overcoating of the NC surfaces for electronic passivation of the surface states [1]. The major practical bottleneck of these materials relates to their labile surface chemistry. In particular, typically used ling chain capping ligands are problematic due to their dynamic and loose binding as well as their highly insulating nature. We have recently rationalized the typical observation of a degraded luminescence upon aging or the luminescence recovery upon post-synthesis surface treatments using a simple surface-structure model, supported by DFT calculations [2]. Healing of the surface trap states requires restoration of all damaged PbX6 octahedra and establishing a stable outer ligand shell. Restoration of such a structure, seen as an increase in the luminescence quantum efficiency to 90-100% and improvement in the overall robustness of CsPbBr3 NCs, was attained using a facile post-synthetic treatment with a PbBr2+DDAB (didodecyldimethylammonium bromimde) mixture. In our most recent work [3], we have used DDAB as a sole ligand directly in the synthesis of perovskite NCs. We then used such NCs in LEDs and demonstrate high external quantum efficiencies of up to 3.6% in blue region (460 nm) and 10% in the green region (520 nm).

1.    M. V. Kovalenko, L Protesescu, M. I. Bodnarchuk. Science 2017, 358, 745-750
2.    M. I. Bodnarchuk,S. C. Boehme, S. ten Brinck, C. Bernasconi, Y. Shynkarenko, F. Krieg, R. Widmer, B. Aechlimann, D. Günther, M. V. Kovalenko, I. Infante. ACS Energy Letters 2019, 4, 63–74
3.    Y. Shynkarenko, M. Bodnarchuk et al. ACS Energy Lett, 2019, 4, 11, 2703-2711
 

Quantitatively predicting the degradation of perovskite materials and photovoltaic devices under different environmental stresses is vital to developing accelerated aging test and developing long-term stable photovoltaic devices. Previously, we have developed methods to quantitatively measure photoluminescence from thin film perovskite materials to determine the quasi-Fermi level splitting (QFLS), effective temperature, and distribution of sub-bandgap states [1,2]. We have shown that the material QFLS accurately predicts device Voc [2] for a wide-range of perovskites (bandgaps from 1.2 to 1.9 eV) when significant voltage losses from HTLs, ETLs, and contacts are not present, and we have used the method to develop several record Voc devices [3,4], particularly for high bandgap guanidinium-based perovskites [3] and 2D/3D perovskites [4]. We have also shown that an estimate of the diffusion length can be obtained from photoconductivity measurements [2]. Here, we present new results from high-throughput measurement of thousands of perovskite material compositions (and a smaller subset of full photovoltaic devices) over a broad range of environmental stresses including temperature, humidity and oxygen. In addition to spatially averaged quantities (like the QFLS and diffusion length), high spatial resolution and time resolution quantitative photoluminescence videos reveal the role of spatial heterogeneity, photo-brightening, and blinking. Using machine learning methods and this large training data set, we are able to quantitatively predict the time it takes for selected material or device properties to degrade to 75% of their initial value for selected important perovskite compositions to an accuracy better than 10% [submitted for publication]. The presentation will summarize the methods we have develop and show how they may be used to design long-term stable perovskite materials and devices.

Halide perovskite semiconductors exhibit a unique combination of very-low electronic defect density, self-healing properties, and low exciton binding. This set of properties is surprising for a solution-processed material and results in an excellent photovoltaic activity.

 I will show that the fundamental property that sets the halide perovskites apart from conventional semiconductors like silicon and GaAs is strongly anharmonic lattice dynamics.  Large amplitude, local polar fluctuations that, result from lattice anharmonicity, localize the electronic states and enhance the screening of electric charges within the material. In other words, in some aspects, halide perovskites behave more like a liquid than a crystalline solid.

First,  I will show that anharmonic lattice dynamics in methylammonium lead bromide lead to strong changes in its dielectric response with temeparture. Then, I will discuss the benefits of  Raman polarization-orientation measurements to investigate temperature-induced symmetry breaking. Finally, I will discuss in detail the mechanisms that lead to strong anharmonicity in methylammonium lead iodide. 

Metastability of defect states is a phenomenon behind many peculiar properties of metal-halide (MH) perovskite semiconductors. For example, irradiation by light can lead to photoluminescence (PL) enhancement, the material self-heals after degradation, solar cells recover their performance after staying in dark. These effects are assigned to instability of concentration of the non-radiative recombination centres. The usual picture is that concentration of the defects and even their type depend on the history of the sample, illumination conditions, environment and so on. In other words, these concentrations are not fixed and can be influenced by external factors. Creation and annihilation of defects and their chemical transformation occur on very different time scales. Due to very large number of defects in the typical sample volume probed by traditional methods individual contributions of each non-radiative recombination centre is averaged over time and space. However, the “top of the iceberg” of this plethora of processes can be nicely seen and studied at the individual defect level due to PL intensity fluctuations of individual nano crystals as well as grains in films.¹

The origin of PL blinking is fluctuations of the non-radiative decay rate. Literature is filled with drastically diverse estimations of the defect concentrations in MH perovskites ranging from 10¹⁰ to 10¹⁷ cm^-3. These concentrations correspond to one defect per cube with sizes from 5000 to 20 nm. A typical grain in a perovskites film of 400 x 400 x 400 nm³ would contain 64 defects if their concentration is 10¹⁵ cm^-3, while a crystal 100 x 100 x 100 nm³ would contain one defect only. Note that it is not necessary that all these defects are effective non-radiative recombination centres, that is why the actual concentration (number N) of the effective non-radiative centres per crystal/grain can be even smaller. Small averaged number of defects per crystal leads to a large statistical fluctuation of this number resulting in different PL brightness of individual grains as well as PL intensity fluctuations when N is changing over time.

This simple consideration is well supported by many reports where PL fluctuations of perovskite crystals of very different sizes (up to micrometres) have been observed. PL micro-spectroscopy providing spatial resolution of 500 nm and, in particular, techniques inspired by single molecule spectroscopy suit very well to study defect metastability. One can see an analogy between a TV screen (representing a whole solar cell) and a one pixel of the TV active matrix (one grain of the film). By investigating the behaviour of small grains individually, we are able to observe and rationalize fundamental properties behind solar cells and other devices operation, life and failure.

I will talk about of PL micro-spectroscopy of individual crystals, discuss the defect dynamics as a function of temperature,² light irradiation and external electric field³ and implications of the very small number of strong non-radiative centres per crystal (N is an integer number, it is “digital”, not a continuous variable) on optical and electrical properties of metal halide perovskites and interpretation of experimental results.

(1)         Merdasa, A.; et al, ACS Nano 2017, 11 (6), 5391–5404.

(2)         Gerhard, M.; et al, Nat. Commun. 2019, 10 (1), 1698.

(3)         Chen, R.; Li, J.; et al, Adv. Opt. Mater. 2019, 1901642

Hybrid lead halide perovskites exhibit an atypical temperature dependence of the fundamental gap for the phases stable at ambient conditions: it decreases in energy with decreasing temperature. Reports ascribe such a behavior to a strong electron-phonon renormalization of the gap, neglecting contributions from thermal expansion. However, high pressure experiments performed on the archetypal perovskite MAPbI₃ (MA stands for methylammonium) yield a negative pressure coefficient for the gap of the tetragonal room-temperature phase [1], which speaks against the assumption of negligible thermal expansion effects. Here we show that for MAPbI₃ the temperature-induced gap renormalization due to electron-phonon interaction can only account for about 40% of the total energy shift, thus implying thermal expansion to be more if not as important as electron-phonon coupling [2]. Furthermore, this result possesses general validity, holding also for the tetragonal or cubic phase, stable at ambient conditions, of most halide perovskite counterparts. As an example, recent results obtained for a series of FA_xMA_1-xPbI₃ solid solutions, where FA stands for formamidinium [3], will be also presented. A striking result concerns the temperature dependence of the gap of a presumably tetragonal but disordered phase which is stable in a wide range of intermediate compositions and temperatures lower than ca. 250 K. This phase is found to exhibit a quadratic dependence of the band gap with temperature, which is again interpreted in terms of the combined effects of thermal expansion and electron-phonon interaction.

During the last years, Metal Halide Perovskites (MHPs) have attracted special attention as an efficient conversion films for photovoltaics, or excellent gain media to construct optical sources. Particularly, most of the works have been focused on CH3NH3PbI3polycrystalline thin films, where stimulated emission was observed under pulsed excitation and power density thresholds as low as 1 μJ/cm2at room temperature [1,2]. More recently, laser emission under continuous wave excitation have been also demonstrated [3] and even an integrated optical amplifier-photodetector on a flexible substrate was recently reported [4].MHPs can be also synthesized as colloidal nanocrystals. In particular, CsPbX3 nanocrystals (NCs) revealed extraordinary properties for optoelectronics. In our recently published work [5], thin films of CsPbX3NCs were properly optimized to enhance the generation of photoluminescence, and with it, the optical gain. In particular, Amplified Spontaneous Emission (ASE) is demonstrated with three different compositions (X3=Br3, X3=Br1.5I1.5, X3=I3). Indeed, these films can demonstrate ASE thresholds lower than 5 μJ/cm2at cryogenic temperatures under nanosecond excitation. Finally, the physical origin of ASE is discussed and demonstrated its single exciton origin in contrast to biexcitonic, as claimed in literature. These results pave the road towards the development of an active photonics technology based on CsPbX3NCs. 

Since a couple of years, the halide perovskites CH₃NH₃PbX₃, with X a halogen (I, Br, Cl), have emerged in the framework of photovoltaics and of light-emitting devices such as electroluminescent diodes and lasers. These materials can be solution-processed at low temperature and their emission wavelength can be tuned over the entire visible spectrum via chemistry substitutions. In particular, the halide perovskites could address the "green gap" problem in laser sources, i.e. the drop in efficiency of solid-state LEDs and laser diodes emitting green light.

We consider here a one-dimensional planar microcavity containing a large-surface (1 cm²) spin-coated polycrystalline thin film of the green-emitting CH₃NH₃PbBr₃ perovskite, in which the strong coupling regime at room temperature between the photon mode of the Fabry-Perot cavity and the perovskite excitonic mode was previously demonstrated [1]. The exciton-polaritons, which are a linear and coherent superposition of the exciton and photon states, arise from the strong coupling regime.

Random lasing occurs in highly disordered gain media in which cavity feedback is replaced by multiple scattering. The multi-directionality and low coherence of random lasers can satisfy various applications such as speckle-free imaging. However, for typical laser applications, the directionality of the lasing devices is desired.

We demonstrate here a random lasing emission in the green occurring in the microcavity which is directionally filtered by the lower polariton dispersion curve. The angle of emission can be controlled by changing the microcavity detuning. Angles of emission as large as 22° have been experimentally obtained. This result is interesting from a fundamental point of view because it combines two intriguing physical phenomena: the cavity exciton-polariton and the random lasing. Moreover, the control of the random lasing emission direction is a crucial point for optoelectronic applications, such as LIDAR technology.

In this talk, we will present a new synthetic method for Ruddlesden-Popper Organic Lead Halides (RPOLHs). This new protocol allows a low-cost, room-temperature preparation of polycrystalline materials with general composition L₂[FAPbI₃]_n-1PbI₄, where L is a primary ammonium cation, FA is formamidinium [HC(NH₂)₂]⁺, and n is the number of inorganic octahedron slabs. The butylammonium-based materials presented phase purity above 99% and, in the benzylammonium-based one, some impurities are present in the final product. These polycrystalline materials are easily processed and can be used for thin film fabrication, which is appealing for optoelectronic devices.

In addition to the new synthesis, we also studied the reaction dynamics of one of the materials in situ. We used small angle X-ray scattering (SAXS) to probe the initial and final stages of the formation reaction of BA₂[FAPbI₃]PbI₄. Our results suggest that the formation of the individual slabs is quite fast (within the first 10 s) and, then, these slabs self-assemble into bulk crystallites during the next 40 minutes. By analyzing the variation of the reciprocal space with time and relating these changes with the Scherrer equation, we could calculate the rate and the average velocity of this self-assemble of the slabs. This work offers the material community a new avenue for the synthesis and investigation of RPOLHs, as well as the possibility of their utilization for optoelectronic devices.

Photophysical changes taking place in mixed halide perovskites under illumination are amongst the most striking evidences of the instability of this material which constitutes the main bottleneck for its future commercialization. [1] A vast amount of (seldom conflicting) experimental evidence has appeared over the past few years pointing towards the defect-assisted formation of an iodide-rich phase through which carrier recombination is channeled. Such phase separation constitutes a severe limitation for devices, leading to a decrease in open circuit voltage in tandem solar cells [2] and modifying the spectral ouput of LEDs, the two main application envisaged for mixed halide perovskites. In spite of this evidence, the ultimate mechanism responsible for the migration of halide ions under illumination is still under debate.

In this work we have carried out a spectrally-resolved micro photoluminescence study of CH₃NH₃PbBr_xI_3-x thin films under different external conditions in order to shed more light on the origin of the so-called Hoke effect. This has allowed us to explore the correlation between the dynamics of photophysical changes and the presence of lattice defects. Modifying the surrounding atmosphere, which has been demonstrated to influence these changes, [3,4] and the synthesis conditions to add vacancies or interstitials we discuss on the role of different types of defects on the photophysics of these materials.

Lattice compression of the halide perovskites due to mild mechanical pressure results in decreased bandgap, enhanced photoluminescence (PL), longer carrier lifetime, and reduced trap-states.[1] However, these effects are lost upon decompression. Therefore, developing irreversible lattice compression is needed in order to preserve the enhanced optoelectronic properties. Here, we grow high-quality methylammonium lead bromide (MAPbBr₃) single crystals by developing an antisolvent-assisted solvent acidolysis crystallization technique. The crystals show intense emission at all four edges under UV lamp. Using micro-X-ray diffraction, we examine the structural differences at the edges and central regions; concluding that the enhanced edges emission is due to lattice compression.The structural changes between the edges and the central areas are also confirmed by macro- and micro-PL, and Raman spectroscopy studies. As a consequence of the lattice compression, shallower trap states and/or reduced trap state densities at the edges are expected. In fact, time-resolved PL measurements show 20 times longer photogenerated carriers lifetimes at the edges compared to the central areas. Furthermore, light detectivity is five times enhanced at the compressed edges with respect to the unstrained central regions. Our findings indicate that developments toward realizing the theoretical limit of radiative recombination in the perovskite-based optoelectronic devices could be achieved through a controlled crystallization process where both structural defects and non-radiative pathways can be reduced.

Metal halide perovskite materials have shown a relatively fast evolution in the power conversion efficiency (PCE) reaching the level exceeding those of CIGS and CdTe and approaching those of crystalline Si solar cells. However, low luminescence efficiency of metal halide perovskite in a complete device and non-radiative losses originating from sub gap charge carrier trap states on the grain surfaces (e.g. halide vacancies) are the main barriers against reaching the efficiency limit in solar cells. In addition, the long-term stability of perovskite solar cells (PSCs) remains a pressing challenge that hinders their commercialisation.

Here, we will detail several new and promising passivation approaches through compositional modification and interface engineering aimed at eliminating the sources of instability and loss processes in metal halide perovskites. We demonstrate substantial mitigation of both non-radiative losses and photoinduced ion migration in perovskite films and interfaces by decorating the surfaces and grain boundaries with passivating potassium halide layers. We find significant enhancement in both micro-photoluminescence and photoluminescence quantum efficiency (e.g. internal yields exceeding 95%) while maintaining high mobilities over 42 cm²V^-1s^-1, giving the elusive combination of both high luminescence and excellent charge transport translating into over 21% PCE of the PSCs with entire elimination of hysteresis and an order of magnitude enhancement in the external quantum efficiency and stability of perovskite based light emitting diodes.

One source of instability in PSCs is interfacial defects, in particular, those that exist between the perovskite and the hole transport layer (HTL). Here, we demonstrate that thermally evaporated dopant-free tetracene on top of the perovskite layer, capped with a doped Spiro-OMeTAD layer and top gold electrode offers an excellent hole-extracting stack with minimal interfacial defect levels.  However, we and others find that dopant-free organic semiconductor HTLs introduce undesirable injection barriers to the metal electrode. By capping 120 nm of tetracene with 200 nm solution-processed lithium TFSI - doped Spiro-OMeTAD, we demonstrate a graded hole injection interface to the top gold layer with enhanced ohmic extraction. For a perovskite layer interfaced between this graded HTLs structure and a mesoporous TiO₂ electron-extracting layer its external photoluminescence yield (PLQE) reaches 15%, compared to 5% for the perovskite layer interfaced between TiO₂ and Spiro-OMeTAD alone. For complete solar cell devices containing tetracene/Spiro-OMeTAD as the HTL with graded doping profile, we demonstrate PCEs of up to 21.5% and extended power output over 550 hours continuous illumination at AM1.5 retaining more than 90% of the initial performance, validating our approach. Our findings represent a breakthrough in the construction of stable PSCs with minimized non-radiative losses.

We resolve a controversy surrounding the luminescence properties of low-dimensional halide perovskites and clarify that an often observed broad luminescence arises from defect states instead of commonly invoked self-trapped excitons.

Low dimensional halide perovskite are a class of semiconductors with intriguing opto-electronic properties showing already impressive performance metrics in devices. Whilst initial research in this field was predominantly driven by photovoltaic research and more recent efforts focused on the narrow emission linewidth in many of these compounds, the latest hotly examined observation is the presence of broad emission bands. This broad emission has the potential for direct white light generation and significant research is currently conducted to find and optimise compounds for this purpose. Crucially, these efforts commonly base on the assumption that the origin of this luminescence is a so-called self-trapped exciton. Whilst this concept is elegant and theoretical calculations have offered some support, experimental evidence for this interpretation is so far scarce.

We therefore studied single-crystals of two-dimensional lead iodide perovskites through a variety of spectroscopic techniques and prove that the broad emission is in fact due to defect states in the bulk of the material. We study two different compounds to underline the universality of our findings and meticulously exclude all other origins of broad emission bands that have hitherto been proposed.

Surface defect states have been repeatedly identified as a limiting factor for luminescence and photovoltaic device efficiencies in metal halide perovskite materials. [1-3]

Using a state-of-the-art photoemission electron microscopy (PEEM) [4,5] setup, we map locally the distribution of surface defect states on triple cation, mixed halide perovskite films ((CsFAMA)Pb(I_0.83Br_0.17)₃) with 30 nm spatial resolution. Significant photoemission arises from sub-bandgap states at discrete locations which act as hole traps. The formation of these defects is linked to grain-to-grain variation in composition and structure. Confocal photoluminescence maps show a significant quenching of photoluminescence intensity at the locations of surface defects located with PEEM.

Previously, we found that light and atmospheric treatments [6] can improve luminescence yields of perovskite films by the passivation of defect sites, thereby reducing trap densities. Here, we utilise light treatments in a variety of atmospheric conditions as a lever to control surface trap distribution. With PEEM, we observe the creation of nanoscale defect states during in-situ illumination of the perovskite, in ultra-high vacuum conditions. Conversely, illumination in an oxygen-rich environment leads to a tuneable suppression of the photoemission from defect-rich sites. Crucially, we determine that the local change in defect density is non-uniform, and in fact depends strongly on the initial presence of defects. We show that the photoluminescence heterogeneity previously reported for perovskite films is inherently linked to the distribution of these nanoscale defects and can be similarly controlled.

In-situ nanoscopic x-ray diffraction (nXRD) and ex-situ scanning electron diffraction (SED) measurements reveal that complex structural reorganization occurs on the nanoscale following illumination with light. This study unveils crucial details about the mechanisms which govern defect creation and annihilation in the presence of light.

Ruddlesden-Popper 2D Hybrid Organic perovskites (HOP) have remarkable excitonic properties that could be exploited in new generation devices such LEDs or polaritonic lasers or photovoltaic devices. Yet, the interplay between their structural and the optoelectronic properties are not fully understood. In particular, the ability of excitons to diffuse within this ultrathin quantum well structure is not fully understood [1].

In this work, we study the diffusion of carriers in 2D perovskites single crystals. We employed a time resolved photoluminescence (TR-PL) microscopy technique [2] that allows to monitor local variations of the excitonic diffusion.  We highlight the influence of the local structure (including the thickness of the quantum well) and the impact of traps on the diffusion coefficient and the diffusion lengths. If the obtained diffusion coefficients are smaller than in 3D perovskites, they are still relatively large compared to other excitonic materials. To understand the influence of traps in details, we study the changes in diffusion behaviour upon different excitation densities or operating conditions (gas,temperature…). This work provides a better understanding about the chemical and physical factors that still limit the diffusion of carriers in these 2D HOP materials. This study will provide insight to further improve HOP materials and devices performances.

References:

1) G. Delport et al. JPCL, 2019, 10 (17), 5153-5159

2) C. Stravrakas. G Delport et al. , submitted, arXiv:1909.13110 

The extraordinary efficiency values reached with MAPbI₃ (MA: CH3-NH3) based photovoltaic cells has recently boosted the efforts in the search of new hybrid perovskite alternatives for photovoltaic energy production. These perovskites have in common a high sensitivity to water and oxygen as well as to visible irradiation, there issues strongly handicap their stability. Obtaining more stable compounds as well as understanding the complex behavior of the band-to band emission upon illumination and time are main concerns to be tackled. The photoluminescence behavior is strongly sensitive to different parameters, especially to the presence of defects and traps whose evolution with time is related to ion migration and perovskite transformations [1], [2].

Our approach is to introduce BiI₃ in the synthesis of MAPbI3 films to stabilize the compound. The impact of incorporating a non-isovalent cation in the structure and morphology of the films is studied. XRD analysis allows confirming the incorporation of Bi³⁺ into the perovskite lattice up to around 7 at% at the Pb²⁺ site. The grain aspect ratio is reduced and the films are densified. Bi³⁺ incorporation leads to a slight increment of the optical gap due to the reduction of lattice parameters. The presence of empty Bi gap states quenches the 770 nm red interband emission and results in a near-infrared emission at 1100 nm. However, high enough visible irradiation density induces a progressive segregation of Bi³⁺ out of the perovskite lattice and promotes the re-emergence of the red emission. This emission is blue shifted and its intensity increases strongly with time until it reaches a saturation value which remains stable in the transformed films for extremely high power densities, around 1000 times higher than for undoped samples. We propose that the underlying processes include the formation of BiI₃ and BiOI, probably at the surface of the crystals, hampering the usual decomposition pathways into PbI₂ and PbO_x for undoped MAPbI₃ [3]. These results provide a new path for obtaining highly stable materials which would allow an additional boost of hybrid perovskite-based optoelectronics.

Metal-halide perovskites have emerged over the past years as a versatile class of semiconductors for high-performance optoelectronic devices.[1] Despite their unique properties, this family of perovskites has however been observed to display strong instability, especially when electrostimulated, limiting their definitive integration in real-world light-emitting devices.[2,3]

In this talk, we will present a detailed photophysical characterisation of blue-, green- and red-emitting metal halide perovskite films in which different passivation strategies are employed to improve both their optical and electrical properties. We will introduce a new powerful technique with which we can extract the absorptance and photoluminescence quantum efficiency (PLQE) values in the materials at the nanoscale by hyperspectral wide-field imaging. We will then show how our methodology can be extended to characterise operando LEDs and obtain maps at the diffraction limit scale for their External Quantum Efficiency, luminance and luminous efficacy. Our observations allow us to identify the degradation paths of these emerging class of LEDs, revealing microscale heterogeneities that are remarkably suppressed in the passivated samples. Our results open avenues to diagnose the optoelectronic quality of novel semiconductors at the microscale and identify strategies to integrate them in LEDs with spatially maximised performance.

Along with the recent success in photovoltaics, perovskite light-emitting diodes (PeLEDs) are currently receiving great attention. While several groups are reporting high external quantum efficiencies (EQEs) over 20%, such high efficiencies deviate from the predictions using classical models used in the field of organic LEDs. Here, we present the role of photon recycling (PR), as a key to explain the discrepancy between the previous models and present efficiencies. The existence of a PR effect is verified by spatially-resolved photoluminescence (PL) and electroluminescence (EL). In addition, we quantify the practical contribution of PR to device efficiency through the investigation of optical modelling methods for re-absorbing emitters. It is shown that PR can contribute to >70% of light emission in the currently reported PeLEDs, and cause a non-linear relationship between external and internal quantum efficiencies. We also present a few photonic designs to further boost the efficiency with PR in the future.

The presence of lead in novel hybrid perovskite-based solar cells remains a significant issue regarding commercial applications due to toxicity and instability issues. Therefore, perovskite-inspired new lead-free compounds are sought-after as new candidates for photovoltaic applications. However, most of the suggested substitutions are either oxidation unsteady or suffer from indirect and wide bandgaps. Here, we demonstrate the first thin-film synthesis of a stable MA₃Sb₂I₉ in the perovskite-like 2D polymorph with narrow and direct bandgaps. So far, the reported MA₃Sb₂I₉ absorber layers only crystallized in the zero-dimensional dimer structure with wide indirect bandgap properties but already reached photovoltaic efficiencies above 2.8 %. Thus, after being successful with lead acetate for optimal film formation,^[1],[2] we introduce a crystallization process based on antimony acetate precursor and sustainable solvents like MeOH and EtOH for the application of the promising 2D polymorph in solar cells. To confirm the improved absorption properties in the layered structure, we investigated the electronic band structure and experimentally verified the presence of a semi-direct bandgap at around 2.1 eV. Using in situ XRD methods, we validate the stability of the layered phase towards high temperature and moisture. With the incorporation of two-valent metal cations, we show how the bandgap in the layered structure narrows further, revealing a new class for promising lead-free absorber layers. Our work shows that careful control of nucleation via processing conditions can provide access to promising perovskite-like phases for photovoltaic applications.

Nanophotonics and meta-optics based on optically resonant all-dielectric structures is a rapidly developing research area driven by its potential applications for low-loss efficient metadevices. Recently, the study of halide perovskites has attracted enormous attention due to their exceptional optical and electrical properties. As a result, this family of materials can provide a prospective platform for modern nanophotonics and meta-optics, allowing us to overcome many obstacles associated with the use of conventional semiconductor materials. Namely, the perovskites provide simple and cheap wet-chemistry methods of nanofabrication, high quantum yield and pronounced excitonic properties at room temperature, broadband and reversible spectral tunability, high defect tolerance, high enough refractive index for light confinement at subwavelength scale, as well as flexibility regarding integration with various nanophotonics designs. Here, we review the recent progress in our research of halide perovskite nanophotonics starting from single-particle light-emitting nanoantennas [1,2,3] and nano/micro-lasers [4,5] to the large-scale designs working for surface coloration, anti-reflection, optical information encoding, and enhanced solar energy harvesting [6].

In this work, we introduce a synthesis of ligand-free ABX3 perovskite nanocrystals (nc-ABX3) based on the use of porous metal oxide (MOx) thin films with a controlled nanopore size as a template. The liquid precursors infiltrate these structures helped by spin coater method, then a smooth thermal treatment leads to nc-ABX3s in the voids of the porous network of TiO₂. We demonstrated that the reaction volume imposed by the nanoporous scaffold leads to the strict control of the nanocrystal size, which allows us to observe well-defined quantum confinement effects on the photo-emission. In addition, our methodology provides fine spectral tuning of the luminescence with maximum precision by only changing the concentration of the precursors. Nc-ABX3 /MOx film presents high photoluminescence quantum yield (>60% in many cases), high optical quality and mechanical stability. It also permits subsequent elastomer infiltration to achieve a self-standing flexible film, which keeps the photo-emission efficiency of the nc-AMX3 unaltered and prevents the degradation from the external environment (water, humidity). Applications as adaptable color-converting layers for light-emitting devices are envisaged and demonstrated. Finally, we demonstrate that the embeded nanocrystals support charge transport in a solar cell device configuration achieving 8% photoconversion when we use SiO₂ nanoparticles as porous scaffold.

In this work, lead perovskites quantum dots with a complex cation in A-site and mixed halides, Cs_0.1FA_0.1MA_0.8Pb(I_3-xBr_x), where x= 0, 1, 2 and 3, have been synthesized in toluene and characterized by different spectroscopic techniques. The influence of the complex A-site cation on the fluorescent emission has been studied in comparison with MAPI perovskite. As a result, a red shift in the emission spectrum has been found for the triple cation lead iodide perovskite against the MAPI, due to the composition of the A-site cation and not to a change in the crystalline structure as XRD patterns revealed. In the case of mixed halides perovskites quantum dots, a blue shift from the all iodide perovskite to the all bromide perovskite has been found, as expected, with a displacement in the emission maximum in comparison with the methylammonium lead perovskites quantum dots, demonstrating the influence of both, the mixed halides and A-site cation compositions, in the light emission of the perovskites quantum dots. 

Understanding how grain structure and grain boundaries affect non-radiative recombination is a key challenge facing the use of halide perovskites, indeed any semiconductor, for photovoltaic applications. We use electron backscatter diffraction (EBSD) images to map the local crystal orientations in thin films of CH₃NH₃PbI₃ (MAPI), the archetypal halide perovskite for photovoltaics.  These EBSD images allows the direct identification of grains and grain boundaries in MAPI films. Although this grain structure is broadly consistent with the structures visible in conventional scanning electron microscopy (SEM) and optical microscopy data, the inverse pole figure (IPF) maps taken with EBSD reveal subtle internal crystal orientation variations of the grain structure. This local crystal misorientation leads to orientation spread within grains indicating the presence of local strain which varies from one grain to the next. Furthermore, we use crystallographic identification to demonstrate the presence of sub-grain boundaries and their location within grains. In solar cells, non-radiative recombination is a key figure of merit, which ultimately controls the power conversion efficiency of a given material. To quantify the impact of local grain structure on non-radiative recombination, and hence photovoltaic performance, we also acquire co-aligned confocal optical photoluminescence (PL) microscopy images on the same MAPI samples used for EBSD. By correlating the optical and EBSD data, we find that the PL is anticorrelated with the local grain orientation spread taken near the film surface, suggesting that grains with higher degrees of crystalline orientational heterogeneity exhibit more non-radiative recombination. These results provide critical insight into the interplay between local crystal orientation heterogeneity and local non-radiative recombination in halide perovskite thin films and may explain why the expected correlation between grain size and photovoltaic performance has been difficult to observe in halide perovskite solar cells.

The precise determination of the wavelength dependent complex refractive index of metal-halide perovskite nanocrystals is key for the design a new generation of optoelectronic devices. Herein, we perform an in-depth optical and structural characterization of methyammonium lead iodide and methyammonium lead bromide nanocrystals embedded into silicon dioxide nanoparticle thin films in order to extract, for the first time, the optical constants of metal-halide perovskite nanocrystals. Using a Kramers-Kronig consistent dispersion model along with an effective medium approximation it is possible to derive the real and imaginary parts of the complex refractive index of neat nanocrystals by fitting the spectral dependence of light transmitted and reflected on nanocrystal-based films at different angles and polarizations of the incident beam. Our results yield a strong dependence of the optical constants with the nanocrystal size, featuring small nanocrystals remarkably large values of the extinction coefficient. Based on actual optical characterization data, our analysis opens the door to the rigorous modelling of solar cells and light-emitting diodes with active layers based on perovskite nanocrystals.

Hybrid perovskites have the AMX3 stoichiometry (Figure 1) and are composed of a monovalent cation A (e.g. Cs+, methylammonium (MA) CH3NH3+ or formamidinium (FA) CH(NH2)2+), a metal M (Pb2+,  Sn2+ or Ge2+), and a halide X (Cl–, Br–, or I–). Pb-based perovskites with mixed MA/FA cations and Br/I halides show the most remarkable optoelectronic properties.They can be improved further upon addition of inorganic cations, such as Rb+ or K+. While this approach allowed reaching the present perovskite solar cell (PSC) solar-to-electric power conversion efficiency (PCE) record exceeding 25%, the instability of perovskites towards sunlight, oxygen, and moisture, as well as the environmental impact of lead content, continue to hamper industrial applications. Unlike three-dimensional (3D) perovskites, their layered two-dimensional (2D) analogues have demonstrated promising stabilities against environmental factors. Layered 2D hybrid perovskites are defined by a general formula S2An–1MnX3n+1 (S is a monovalent organic spacer cations,), which represents a layered structure of 3D perovskite slabs separated by the organic spacer layers. In addition to increasing environmental stabilities, 2D layered perovskites act as versatile platforms for realizing lead-free perovskite solar cell architectures that could reduce the detrimental environmental impact of lead. This inspired us to develop a new generation of 2D/3D structures featuring judiciously engineered amphiphilic spacer molecules that exhibit outstanding optoelectronic properties by suppressing non-radiative recombination and achieving very high open-circuit voltages, while retaining excellent operation stability. Several layered 2D perovskite solar cell architectures will be presented which have surpassed the performance of the state-of-the-art. These findings exemplify that molecular engineering based on noncovalent interactions can lead to improved performances, durability, and scalability of perovskite solar cells. Furthermore, the studies on molecular modulation and layered 2D PSCs induced us to monitor atomic-level interactions within hybrid perovskite materials using solid-state 2D-NMR as a powerful tool.

The dynamic response of metal halide perovskite devices shows a variety of physical responses that need to be understood and classified for enhancing the performance and stability and for identifying new physical behaviours that may lead to developing new applications. These responses are the outcome of complex interactions of electronic and ionic carriers in the bulk and at interfaces. Based on a systematic application of frequency modulated techniques and time transient techniques to the analysis of kinetic phenomena, we present a picture of the dominant effects governing the kinetic behaviour of halide perovskite devices. First with impedance spectroscopy we provide an interpretation of capacitances as a function of frequency both in dark and under light, and we discuss the meaning of resistances and how they are primarily related to the operation of contacts in many cases. The capacitance reveals a very large charge accumulation at the electron contact, which has a great impact in the cell measurements, both in photovoltage decays, recombination, and hysteresis. We also show the identification of the impedance of ionic diffusion by measuring single crystal samples. Working in samples with lateral contacts, we can identify the effect of ionic drift on changes of photoluminescence, by the creation of recombination centers in defects of the structure.¹ We also address new methods of characterization of the optical response by means of light modulated spectroscopy. The IMPS is able to provide important influence on the measured photocurrent.² We describe important insights to the measurement of EQE in frequency modulated conditions, which shows that the quantum efficiency can be variable at very low frequencies. The combination of IMPS and Impedance Spectroscopy is able to provide a detailed picture that explains low frequency characteristics, influencing the fill factor of the solar cell. As a summary we suggest an interpretation of the effects of charge accumulation, transport, and recombination. on current-voltage characteristics and time transient properties, and we suggest a classification of the time scales for ionic/electronic phenomena in the perovskite solar cells.

Perovskites have emerged as low-cost, high efficiency photovoltaics with certified efficiencies of 24.0% approaching already established technologies. The perovskites used for solar cells have an ABX₃ structure where the cation A is methylammonium (MA), formamidinium (FA), or cesium (Cs); the metal B is Pb or Sn; and the halide X is Cl, Br or I. Unfortunately, single-cation perovskites often suffer from phase, temperature or humidity instabilities. This is particularly noteworthy for CsPbX₃ and FAPbX₃ which are stable at room temperature as a photoinactive “yellow phase” instead of the more desired photoactive “black phase” that is only stable at higher temperatures. Moreover, apart from phase stability, operating perovskite solar cells (PSCs) at elevated temperatures (of 85 °C) is required for passing industrial norms.
Recently, double-cation perovskites (using MA, FA or Cs, FA) were shown to have a stable “black phase” at room temperature.(1,2) These perovskites also exhibit unexpected, novel properties. For example, Cs/FA mixtures supress halide segregation enabling band gaps for perovskite/silicon or perovskite/perovskite tandems.(3) In general, adding more components increases entropy that can stabilize unstable materials (such as the “yellow phase” of FAPbI₃ that can be avoided using the also unstable CsPbI₃). Here, we take the mixing approach further to investigate triple cation (with Cs, MA, FA) perovskites resulting in significantly improved reproducibility and stability.(4) We then use multiple cation engineering as a strategy to integrate the seemingly too small rubidium (Rb) (that never shows a black phase as a single-cation perovskite) to study novel multication perovskites.(5)
One composition containing Rb, Cs, MA and FA resulted in a stabilized efficiency of 21.6% and an electroluminescence of 3.8%. The V_oc of 1.24 V at a band gap of 1.63 eV leads to a very small loss-in-potential of 0.39 V, one of the lowest measured on any PV material indicating the almost recombination-free nature of the novel compound. Polymer-coated cells maintained 95% of their initial performance at 85°C for 500 hours under full illumination and maximum power point tracking. This is a crucial step towards industrialisation of perovskite solar cells.

Lastly, to explore the theme of multicomponent perovskites further, molecular cations were revaluated using a globularity factor. With this, we calculated that ethylammonium (EA) has been misclassified as too large. Using the multication strategy, we studied an EA-containing compound that yielded an open-circuit voltage of 1.59 V, one of the highest to date. Moreover, using EA, we demonstrate a continuous fine-tuning for perovskites in the "green gap" which is highly relevant for lasers and display technology.
The last part elaborates on a roadmap on how to extend the multication to multicomponent engineering providing a series of new compounds that are highly relevant candidates for the coming years.(6,7)
(1) Jeon et al. Nature (2015)
(2) Lee et al. Advanced Energy Materials (2015)
(3) McMeekin et al. Science (2016)
(4) Saliba et al., Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency. Energy & Environmental Science (2016)
(5) Saliba et al., Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance. Science (2016).
(6) Turren-Cruz et al. Methylammonium-free, high-performance and stable perovskite solar cells on a planar architecture. Science (2018)
(7) Saliba. Polyelemental, Multicomponent Perovskite Semiconductor Libraries through Combinatorial Screening. Advanced Energy Materials (2019)

Highly efficient halide perovskite solar cells (PSCs) can only be cost-competitive if their operational stability is ascertained. Defect control and passivation in the halide perovskite absorber is crucial for stability improvement. One recipe to achieve exceptional PV stabilities resides in the engineering and passivation of defects found in any material of the device. The reduction of defect density mitigates recombination and prolongs charge carrier lifetimes leading to efficient and stable PSCs. In the case of perovskite absorbers, low defect concentration has been found for single crystals. Their superior properties in comparison to polycrystalline thin films are ascribed to the presence of 2 – 4 orders of magnitude lower trap densities. Current reports focus on the growth of large perovskite grains and the passivation of defects at grain boundaries and interfaces through additive / interface engineering. Here, we demonstrate the application of an organic additive for the fabrication of high quality, low defect density polycrystalline perovskite thin films. This enables high efficient devices (21.1%) that can retain near 100 % of their original performance after 1,000 h of continuous operation at maximum power point under 1 sun illumination. Understanding the mechanism of defect passivation by organic molecules can facilitate the development of highly stable PSCs.

Solar energy can lead a “paradigm shift” in the energy sector with a new low-cost, efficient, and stable technology. Nowadays, three-dimensional (3D) methylammonium lead iodide perovskite solar cells are undoubtedly leading the photovoltaic scene with their power conversion efficiency (PCE) >25%, holding the promise to be the near future solution to harness solar energy [1]. Tuning the material composition, i.e. by cations and anions substitution, and functionalization of the device interfaces have been the successful routes for a real breakthrough in the device performances [2]. However, poor device stability and still lack of knowledge on device physics substantially hamper their take-off. Here, I will show a new concept by using a different class of perovskites, arranging into a two-dimensional (2D) structure, i.e. resembling natural quantum wells. 2D perovskites have demonstrated high stability, far above their 3D counterparts [3]. However, their narrow band gap limits their light-harvesting ability, compromising their photovoltaic action. Combining 2D and 3D into a new hybrid 2D/3D heterostructure will be here presented as a new way to boost device efficiency and stability, together. The 2D/3D composite self-assembles into an exceptional gradually organized interface with tunable structure and physics. To exploit new synergistic function, interface physics, which ultimately dictate the device performances, is explored, with a special focus on charge transfer dynamics, as well as long term processing happening during aging. As shown in Fig.1, when 2D perovskite is used on top of the 3D, an improved stability is demonstrated. 2D perovskite acts as a sheath to physically protect the 3D underneath. In concomitance, we discovered that the stable 2D perovskite can block ion movement, improving the interface stability on a slow time scale. The joint effect leads to PCE=20% which is kept stable for 1000 h [3,4]. Incorporating the hybrid interfaces into working solar cells is here demonstrated as an interesting route to advance in the solar cell technology bringing a new fundamental understanding of the interface physics at multi-dimensional perovskite junction. The knowledge derived is essential for a deeper understanding of the material properties and for guiding a rational device design, even beyond photovoltaics.

The primary obstacle towards perovskite photovoltaics reliable power generation conversion is the lack of material/device stability. We investigate perovskite’s dynamics from the macro- to the nanoscale by combining game-changing characterization methods that enables us to quantify device performance and material degradation. We emulate realistic solar cells operation conditions while submitting the perovskites to controlled cycles where we vary the device exposure to water, oxygen, bias, temperature, and illumination. We propose the use of machine learning to identify the contribution of each parameter on device performance and recovery, both individually and when acting in combination.

This talk will introduce the technology behind supercontinuum generation and provide application examples of how it is used to advance research within optical material investigations of materials such as perovskites and perovskite devices though hyperspectral analysis and exciton dynamics.

The supercontinuum lasers behind the highest resolution optical microscopes have been around for less than 20 years and allow both full spectroscopic investigations between 260nm to 2200nm, and simultaneous dynamic investigations of effects on the picosecond to nanosecond time scale. These pulsed broadband laser light sources are enabling discoveries in biology, material and chemical sciences not achievable with any other technology.  

Supercontinuum light sources use nonlinear effects in special engineered photonic crystal fibers to create pico second broadband pulses at high repetition rates. The intensity of a focused supercontinuum correspond to many suns allowing fast point-by point interrogation with micro meter resolution and the pulsed nature of the technology allows for further interrogation of the charge carrier dynamics such as fluorescence lifetime imaging (FLIM) and exciton diffusion on the nano second scale. 

Metal halide perovskite materials exhibit exceptional performance characteristics for low-cost optoelectronic applications. Though widely considered defect tolerant materials, perovskites still exhibit a sizeable density of deep sub-gap non-radiative trap states, which create local variations in photoluminescence [1] that fundamentally limit device performance. These trap states have also been associated with light-induced halide segregation in mixed halide perovskite compositions [2] and local strain [3], both of which can detrimentally impact device stability [4]. The origin and distribution of these trap states remains unknown as the optical diffraction-limit does not allow the nature of the traps to be probed on the length scales required. Understanding the nature of these traps will be critical to ultimately eliminate losses and yield devices operating at their theoretical performance limits with optimal stability.

In this talk we outline the distribution and compositional and structural origins of non-radiative recombination sites in (Cs_0.05FA_0.78MA_0.17)Pb(I_0.83Br_0.17)₃ thin films. By combining scanning electron and synchrotron X-Ray microscopy techniques with photoemission electron microscopy (PEEM) measurements we reveal that nanoscale trap clusters are distributed non-homogenously across the surface of high performing perovskite films and that there are distinct structural and compositional fingerprints associated with the generation of these detrimental sites. In addition, our scanning electron diffraction measurements achieve a spatial resolution of 4nm with an accumulated electron dose of only ~6 e/Ų (over an order of magnitude lower than established tolerable dose limits for metal halide perovskites). We will also explore how this combination of high-resolution and low accumulated dose provides new insights into the pristine crystallography of these materials on the nanoscale; thus helping to answer ongoing open questions such as ‘what truly defines a grain?’ and ‘are grain boundaries beneficial or detrimental to performance’?

Compositional engineering of perovskites enables the precise control of key material properties such as the bandgap [1]. This possibility makes perovskites a promising material for multijunction, “tandem” solar cells, where the combination of two different bandgaps allows to easily break the Shockley-Queisser limit and thus improve efficiency [2]. The remaining challenge is to find structures with the target bandgap which are both stable in the environment as well as non-toxic (i.e. lead-free). To this end, computer simulations allow rapid screening of a large array of compositions for a given structure and subsequent data modeling. However typical hightroughput calculation fall short in capturing the effect of different geometries in modeling thus severely constraining the applicability of the result in predicting “new” structures. This becomes especially problematic for larger systems, such as the mixed, lead-free double perovskites, where different compositions might have varying relaxed geometries and sampling the whole feature space becomes infeasible. Incorporating geometrical information into the data-modeling step is typically achieved by variations on the radial-distribution function (RDF) [3], resulting in a large feature vector fed into a machine learning (ML) pipeline. Recently, we published a new RDF based descriptor, merging geometrical description of the structure with compositional information in the “property density distribution function” (PDDF) [4]. Therein we used a LASSO-based scheme to shrink the feature vector to prevent overfitting on a limited set of data. Building upon an improved database of lead-free perovskites, we are currently exploring neural-network based Autoencoders [5] to improve on the current feature-selection process, allowing us to create a good model with a limited amount of datapoints, and eventually extend to the complete compositional space.

Despite tremendous efforts in recent years to study the kinetics of charge recombination in metal-halide perovskite films, the detailed molecular mechanism of these processes is still not fully understood. As an example, photoluminescence (PL) intensity of the perovskite thin films shows greatly varying dependencies on the excitation fluence when measured by different laboratories at different experimental conditions [1-4]. We performed a systematic study of a photoluminescence intensity of the CH₃NH₃PbI₃ thin film at a wide range of the excitation pulse energies with the repetition rate changed from 80MHz to 1 Hz. It was found that for all pulse energies, the dependence of the photoluminescence intensity on the averaged excitation power density lies on one curve corresponding to the high repetition frequency case (quasi-CW regime). With a decrease in the repetition rate, the PL intensity deviates from the quasi-CW dependence and ultimately becomes independent of the frequency (single-pulse regime). We have shown that the frequency of the transition from the quasi-CW regime to the single-pulse regime strongly depends on the pulse energy. In other words, lower charge carrier concentration requires much lower repetition rate to reach the single-pulse regime. It means that by varying the pulse energy at a given repetition rate one can move from the CW-regime to the single-pulse regime. This explains variations of the PL intensity dependences on the pulse energy and the general difficulty to explain them quantitatively. Numerical simulations were carried out to fit the experimental dependencies by several theoretical models.

1. S. D. Stranks, V. M. Burlakov, T. Leijtens, J. M. Ball, A. Goriely, and H. J. Snaith,
 Phys. Rev. Applied  2, 034007 (2014)

2. S. Draguta, S. Thakur, Yu. V. Morozov, Y. Wang, J. S. Manser, P. V. Kamat, and M. Kuno, J. Phys. Chem. Lett. 7, 715−721 (2016)

3. J. M. Richter,  M. Abdi-Jalebi,  A. Sadhanala,  M. Tabachnyk,  J. P.H. Rivett,  L. M. Pazos-Outón, K. C. Gödel, M. Price, F. Deschler, and R. H. Friend, Nature Communications 7, 13941 (2016)

4. S. Feldmann et al. Nature Photonics 14, pages123–128 (2020)

Metal-halide perovskites are one of the most promising active materials for photovoltaic and light emitting technologies, due to their excellent optoelectronics properties and thin films fabrication versatility.  Perovskite solar cells (PSCs), despite their ever improving power conversion efficiency (PCE)[1] and stability [2,3] are still challenging to reach consistency over large area which remains one of the major challenge  to be tackled before their introduction in the photovoltaic market.

In this presentation we demonstrate high efficient, large area, planar PSCs with uniform MAPbI₃ perovskite active layer deposited by thermal co-evaporation of PbI₂ and MAI. The high quality co-evaporated perovskite thin films are pinhole-free and uniform over several centimetres, showing large grain sizes, low surface roughness and the long carrier lifetime. The high quality perovskite thin films translates to small area PSCs (0.16 cm²) with PCE above 20% and high reproducibility and also PSCs with area up to 4 cm² did not show a significant drop in PCE. Furthermore, the first mini-modules with active area larger than 20 cm² achieved a PCE well above 18%. Similarly, semitransparent PSCs reached  PCE ~17.0% and transparency above 75% in near infrared using sputtered indium tin oxide (ITO) as semi-transparent electrode. 

Our work represent an important step towards the development of high quality and reproducible large area perovskite solar cells and mini-modules, the main requirements the commercialization of the technology.

Metal-halide perovskites are one of the most promising active materials for photovoltaic and light emitting technologies, due to their excellent optoelectronics properties and thin films fabrication versatility.  Perovskite solar cells (PSCs), despite their ever improving power conversion efficiency (PCE) [1] and stability [2,3] are still challenging to reach consistency over large area which remains one of the major challenge  to be tackled before their introduction in the photovoltaic market.

In this presentation we demonstrate high efficient, large area, planar PSCs with uniform MAPbI₃ perovskite active layer deposited by thermal co-evaporation of PbI₂ and MAI. The high quality co-evaporated perovskite thin films are pinhole-free and uniform over several centimetres, showing large grain sizes, low surface roughness and the long carrier lifetime. The high quality perovskite thin films translates to small area PSCs (0.16 cm²) with PCE above 20% and high reproducibility and also PSCs with area up to 4 cm² did not show a significant drop in PCE. Furthermore, the first mini-modules with active area larger than 20 cm² achieved a PCE well above 18%. Similarly, semitransparent PSCs reached  PCE ~17.0% and transparency above 75% in near infrared using sputtered indium tin oxide (ITO) as semi-transparent electrode. 

Our work represent an important step towards the development of high quality and reproducible large area perovskite solar cells and mini-modules, the main requirements the commercialization of the technology.

[1] https://www.nrel.gov/pv/.

[2] G. Grancini et al., Nat. Communications, 8, 15684 (2017);

[3] L. Meng et al., Nat. Communications, 9, 5265 (2018) A

[4] E.H. Jung et al., Nature, 567, 511 (2019)

The interest in halide perovskites is rising at a rapid pace due to their tremendous success as solution processable, high quality semiconductor for optoelectronic applications. One intriguing property of this material class is the wide range of the possible band-gaps, which can be tuned by changing the perovskite composition.

While such changes in band gap are regularly reported, it is unclear how the respective conduction and valence band positions change and little is understood about the origins of these changes. Knowing the positions of valence and conduction band is however crucial for device design.

To tackle this issue, we use a combination of photoelectron spectroscopy and density functional theory to reliably extract the relevant energy level positions. Furthermore, employing a tight binding model, we are able to explain the origin of these changes based on changes in hybridization strength, atomic level positions, and lattice distortion.

This approach allows us to present the complete set of energy levels for the 3D lead and tin based halide perovskite. [1] In this talk, the focus will be on “pure” systems, but we also show some more recent results on mixed cation or mixed anion materials.

To date there is no entirely systematic theoretical approach with which to analyse the vast quantities of experimental data generated by studies of perovskite solar cell behaviour. This talk aims to go some way in addressing this gap in the literature by formulating [2] and analysing drift-diffusion (DD) models for planar perovskite cells, that incorporate the effects of both ion vacancy and charge carrier motion. These models are analysed using a combination of numerical [1] and approximate asymptotic (e.g. [3]) methods.  This approach is applied to both Kelvin-Probe Force Microscopy measurements and impedance spectroscopy data. In the former case measurements indicate novel physical phenomena that are missing from the standard DD models. In the latter case results, in the form of simple analytic expressions for key experimental quantities, are derived from a systematic approximation of the DD model. These are then used to extract information about the cell's physics from experimental data. 

Perovskite solar cells have gained prominence for their high efficiency and ease of fabrication, yet are hampered by instability. While ion mixing improves device efficiency and stability in general, recent work is demonstrating that incorporation of ions of particularly small or large size (via applying additives in the fabrication process) enhances both further. However there is a lack of understanding of the mysterious mechanisms through which these ions modify material properties (either in the bulk or grain boundaries) or alter the crystallization pathways. In this talk, I will outline recent advances in material characterization (from both experiments and atomistic simulations using DFT) of the impact of several additives (organic salts (PEAI, BAI, GAI) or inorganic salts (alkali halide)) and investigate how they modify the microstructure of the perovskites. I will also discuss the proposed mechanisms and their prospects in the evolution of efficiency and long term stability perovskite solar cells.

Reports of slow charge-carrier cooling in hybrid metal halide perovskites have prompted hopes of achieving higher photovoltaic cell voltages through hot-carrier extraction. However, observations of long-lived hot charge carriers even at low photoexcitation densities and an orders-of-magnitude spread in reported cooling times have been challenging to explain.

In this work we present ultrafast time-resolved photoluminescence measurements on formamidinium tin triiodide, showing fast initial cooling over tens of picoseconds and demonstrating that a perceived secondary regime of slower cooling instead derives from electronic relaxation, state-filling, and recombination in the presence of energetic disorder. We identify limitations of some widely used approaches to determine charge-carrier temperature and make use of an improved model which accounts for the full photoluminescence line shape. The adoption of this model offers a path to more accurate and readily comparable determination of charge-carrier temperatures across perovskite compositions, to assess the true potential for hot-carrier solar cells. Further, we do not find any persistent polarization anisotropy in FASnI₃ within 270 fs after excitation, indicating that excited carriers rapidly lose both polarization memory and excess energy through interactions with the perovskite lattice.

Three-dimensional (3D) hybrid halide perovskites have emerged as a breakthrough in the field of photovoltaic and optoelectronic devices although their low stability against the environmental agents as heat, humidity and oxygen needs to be addressed if commercial devices are considered to be placed on the market. The incorporation of large organic cations in between “perovskite slabs” to form two-dimensional (2D) hybrid perovskites has been reported to mitigate the degradation against the environmental agents of the three-dimensional (3D) perovskites. The 2D hybrid perovskites, in particular the Ruddlesden-Popper (RP) phase, exhibit excellent optoelectronic properties with a wide flexibility in the type of large organic cations that can be employed. However, the small organic cations inserted in the octahedral voids have been limited so far to those three fulfilling the Goldschmidt tolerance factor (t) despite the relaxed structure of the 2D RP perovskites that may open the way to the insertion of other cations.

Three-dimensional (3D) hybrid halide perovskites have emerged as a breakthrough in the field of photovoltaic and optoelectronic devices although their low stability against the environmental agents as heat, humidity and oxygen needs to be addressed if commercial devices are considered to be placed on the market. The incorporation of large organic cations in between “perovskite slabs” to form two-dimensional (2D) hybrid perovskites has been reported to mitigate the degradation against the environmental agents of the three-dimensional (3D) perovskites. The 2D hybrid perovskites, in particular the Ruddlesden-Popper (RP) phase, exhibit excellent optoelectronic properties with a wide flexibility in the type of large organic cations that can be employed. However, the small organic cations inserted in the octahedral voids have been limited so far to those three fulfilling the Goldschmidt tolerance factor (t) despite the relaxed structure of the 2D RP perovskites that may open the way to the insertion of other cations. Here, we present the incorporation of the Gua cation into the octahedral sites of the “perovskite slabs” in 2D RP perovskites. Thus, the methylammonium (MA) cation in the PEA2MA2Pb3I10 perovskite (PEA = phenylethylammonium) has been gradually substituted by the Gua cation to synthesize thin films of the mixed cation PEA2(MA1-xGuax)2Pb3I10 perovskite. X-ray Diffraction (XRD) measurement has revealed a regular expansion of the unit cell when increasing the Gua content up to 90% proving the sequential insertion into the lattice of the Gua. Importantly, the combined analysis of the absorption and photoluminescence (PL) spectra have revealed a change in the distribution of the n-members of the 2D RP perovskites towards phases with low n values upon increasing the Gua content. In particular, a sudden change is observed at 30% Gua content which is related to the impossibility of the phases with high n values to incorporate more than 25% Gua in their structure. Thus, the addition of a large organic cation that substitutes the small MA cation plays a key role to control the distribution of n-members in the 2D RP perovskite films.

Organic-inorganic perovskites are one of the most promising material classes for photovoltaic applications, since the achieved efficiency increased during the last 10 years from 3.8% to over 24% [1,2].While most of the research groups are working on the device efficiency and stability enhancement, there is still a lack of understanding and experimental validation of the influence of trap states on the perovskite charge carrier transport and especially the recombination properties. Besides the radiative and Auger recombinations, which relate to intrinsic recombination processes, the trap-assisted Shockley-Reed-Hall (SRH) and interface recombination play a critical role for the efficiency of perovskite solar cells. Therefore, here we experimentally investigate the energetic distribution of perovskite bulk and metal interface states on the example of methylammonium lead iodide (MAPI) thin-films.

For this purpose we use the thermally stimulated current (TSC) method. To avoid the well-known disadvantages of classical TSC, such as the recombination of complementary charge carriers and high leakage currents, we have recently introduced a modification of this method, by using a metal-insulator-semiconductor (MIS) sample structure for the TSC experiment (MIS-TSC)[3]. The use of the MIS structure allows for a true unipolar high-resolution trap state evaluation,  low leakage currents (jleakage< 1 fA/cm²), and avoids charge carrier recombination due to unipolar trap filling.

To distinguish between defect states in the perovskite bulk and metal-induced interface trap states, we have investigated devices using different metals (Ag, Au and Ni) as top contacts. We detect characteristic trap states independent of the contact metallization, which are probably related to the defect states in the perovskite bulk and are in line with the current literature. We demonstrate further, that the type of top metallizations strongly influences trap states at the MAPI/metal interface. While the formation of hole and electron trap states with low activation energy was observed for Ag contacts, the formation of deeper trap states in the range of 400 meV for electrons and 1400 meV for holes was detected for MAPI/Au and MAPI/Ni interfaces.

We have employed spin- and angle-resolved photoemission spectroscopy of cesium lead tribromide (CsPbBr₃) single crystal to verify the presence of Rashba spin-orbit coupling effect in the highest lying occupied state. We uncover the entire three-dimensional Brillouin zone momentum-space, and the dispersion of the topmost bulk valence band at high symmetry R-point. We use density functional theory (DFT) calculations to compare the ground state electronic structure of the particular perovskite compound with corresponding dispersive bands from photoemission experiments. In particular, using photoemission measurements with circularly polarized light, we elucidate direct evidence of circular dichroism from the uppermost valence band of CsPbBr₃. The spin-resolved ARPES under the same conditions, however, does not show any spin splitting, thus we assign the dichroism effect to the final states with different characters of orbital angular momentum. By direct experimental evidence from the results of spin-resolved band structure experiments, we exclude a large Rashba effect in the global valence band maximum at R-point of bulk Brillouin zone.

An extensive use of renewable and clean energies is required to reduce the strong dependence on fossil fuels and its effect in the climate change. At the same time, new and more advanced systems have to be developed in order to allow a real energy savings. Solar energy, the most abundant renewable energy source, becomes a key actor. Through the photovoltaic effect, this energy can be converted directly into electrical power easy to transport or ready to be consumed in place. In this context, Halide Perovskite (HP) have been emerged as extremely appealing materials with record photoconversion efficiencies higher than 25%. In addition, the goodness of these materials to produce high efficiency solar cells makes them also suitable for the preparation of efficient LED, a key technology to increase energy savings as illumination is the responsible of the 20% of the total electric power consumption. Despite this impressive achievements, ABX3halide materials suffer for some constrains as not always can crystallize with perovskite structure, where B cation is 6-fold coordinated to X anions (a corner sharing [BX6] octahedra) and the A cation occupying the 12-fold cuboctahedral coordination site. In some cases, depending on the Goldschmid tolerance factor and the octahedral factor (determined by A, B and X radii), the octahedra cannot share the corners. This fact influences dramatically the material properties, increasing significantly the bandgap and affecting deleteriously the transport properties. Thus, the interest of the materials with this non-perovskite phase decreases for the fabrication of optoelectronic devices. In this presentation we analyze different ways in which the constrains limiting the phase stability can be surpassed.

Mixed cation and mixed halide perovskites have led to the most stable solar cells reported, so far mostly prepared by solution-processing. This might be due to the technical difficulties associated with the vacuum deposition from multiple thermal sources, requiring a high level of control over the deposition rate of each precursor during the film formation. Here we present multi-component materials obtained by using multiple sources (up to 4) thermal vacuum deposition. Different stoichiometries are studied, varying the A-site cations (methylammonium, formamidinium and cesium), as well as mixed halide systems. Mixed iodide-bromide perovskites with bandgap up to 1.8 eV were prepared, as they are ideal as front cell in perovskite-perovskite tandem devices. Also, we explored the preparation of narrow bandgap FAPbI₃, showing the importance of phase purity for the fabrication of efficient solar cells. We highlight the importance of the control over the film morphology and composition, which differs substantially when these compounds are vacuum processed. Avenues to improve the stability and to maximize the open circuit voltage using additives and/or novel charge transport layers will be discussed.

Stability beside efficiency is rapidly becoming the most important topic in perovskite solar cells research. Spiro-OMeTAD, Poly(3-Hexylthiophene) (P3HT) and poly(triaryl amine) (PTAA) are the most common small molecule and organic semiconducting polymers which are used as  hole transport material (HTM) in perovskite solar cell [1,2]. Regarding the thermal stability issue, Spiro-OMeTAD starts crystallizing at 85 °C due to its low glass transition temperature inducing a significant deterioration of the hole mobility and a consequent thermal instability of PSCs. Thermal stability can significantly improve by replacing the Spiro-OMeTAD with PTAA and P3HT polymers attributed to instinct characteristics of polymer as oxygen impermeability and hydrophobicity [3,4,5].In this work, we fabricated high efficiencyperovskite solar modules using PTAA and Spiro-OMeTAD, which had achieved a hysteresis-freephotoconversion efficiencies above 16.5% with active area of 43 cm²which is among the highest reported in the field of perovskite solar modules. Improving Voc was observed in the case of PTAA as HTM. The photovoltaic (PV) performance, thermal stability and light stability, UV-Vis absorbance, Photoluminscence (PL), Electrochemical Impedance Spectroscopy (EIS), Cyclic Voltammetry (CV), Transient Photovoltage and Photocurrent Fall/Rise analysis have been investigated and compared for Spiro-OMeTAD and PTAA based PSCs. PTAA-based encapsulated devices show more than 1000 h thermal stability at 85 °C under atmospheric condition and it shows better thermal stability respect to the Spiro-OMetad in which is serves as a suitable candidate for scale-up the PSCs.However, in the case of light stability,we did not observedhuge differences between polymeric and small molecules HTM. Non-encapsulated devices show promising shelf life stability around 1800 hunder N2 filled environment equalfor both Spiro-OMeTAD and PTAA HTMs.

Mixed halide perovskites (MHP) have been highlighted as promissory materials in optoelectronics, due to their improved light harvesting, photocarrier generation, and the ease for tuning their optical properties, specially their band gap.[1] These features have opened the door to analogous solar driven process as photocatalysis for carrying out the photodegradation of recalcitrant organic compounds more efficiently.[2] Nonetheless, the photocatalytic (PC) activity of MHP mainly depends on the surface chemical environment formed during their synthesis. This correlation has not been studied yet. In this work, we deduced the nature and the role of surface chemical states of MHP nanocrystals (NC) synthesized by hot-injection (H-I) and anion-exchange (A-E) methods, on their PC performance for the oxidation of β‑naphthol as a model system. We identified iodide vacancies as the main surface chemical states that promote the formation of highly reactive superoxide ions. These species define the PC activity of A‑E-MHP. Conversely, the PC performance of H-I-MHP is dictated by an adequate balance between band gap and highly oxidizing valence band. In this context, MHP can be considered as good photocatalysts for efficient environmental remediation.

CsPbI3 nanocrystals have been proposed as building blocks for photovoltaics and optoelectronics because they show a low bandgap and very high photoluminescent quantum yield (PLQY) near to the unit due to the extreme tolerance to surface defects. However, CsPbI3 shows an important drawback in terms of stability: α-phase can easily experience a phase transition to a non-radiative orthorhombic δ-phase in an ambient environment with moisture. One of the most successful approaches proposed to overcome this problem is to synthesize mixed halide CsPbIXBr3-X perovskites to improve the stability of the α-phase perovskite structure. This can be performed by a post-synthetic anion exchange reaction of halides in CsPbI3 nanocrystals because of the very high ion mobility of halides. Due to the labile nature of the α-phase CsPbI3 nanocrystals, a common strategy is to carry out the ligand exchange from CsPbBr3 as starting material because they show a higher phase stability and a high QY near to 90% in solution.

Although CsPbIXBr3-X nanocrystals can be successfully synthesized in solution with outstanding optical properties, the formation of high-quality thin films of perovskite with high QY is challenging owing to the degradation of their optical properties after deposition on a substrate by the effect of the moisture. Several approaches have been proposed to achieve solid-state anion exchange over CsPbBr3 thin films with discouraging results in terms of low PLQY, poor chemical stability and low film homogeneity. All these drawbacks show the necessity to develop new methods to obtain perovskite thin films with tunable optical properties and high QY.

In this work, we explore spray coating as a new route to carried out the solid-state anion exchange in thin films of CsPbBr3 nanocrystals at room conditions. Basically, spray coating consists in the formation of an aerosol from a solution precursor and its transport by a carrier gas to the CsPbBr3 thin film previously prepared. To the best of our knowledge, this is the first time that a solid-state anion exchange is carried out by spray-coating. As a I- precursor, best results are obtained with a solution of HI in methyl acetate, an antisolvent for CsPbBr3 that keeps unaltered the well-ordered microstructure of the NC thin films. We show how spraying provides a more intense anion exchange than immersion since fresh HI solution is added repeatedly. The light emission of thin films after our anion exchange procedure can be accurately tuned by spraying different HI solution volumes, obtaining a full gamut of emission wavelengths between 520 and 670 nm. However, the most impacting result is that QY improves after anion exchange, from QY=61% in the case of the original CsPbBr3 thin films emitting at 520 nm, to QY=80% in the case of the alloyed film CsPbBr3-xIx emitting at 640 nm after anion exchange. Morphological (TEM), structural (DXR) and optical characterization (UV-Vis spectroscopy, PL and TRPL measurements) were carried out. TEM images and DXR spectra show that the α-phase, cubic shape and the nanocrystal size is conserved after anion exchange. Finally, as a proof of concept, we demonstrated the production of stimulated emission and even random lasing in thin films after applying the anion exchange procedure. This observation is a further proof of the film quality (high emission QY) conserved after anion exchange.

Because of the excellent optoelectronic properties of hybrid organic–inorganic metal halide perovskites, [1-3] the photovoltaic field has undergone a rapid progress in the last decade. Perovskites were first introduced in the field as sensitizers in dye-sensitized solar cells with promising results but very poor stability because of the dissolution of perovskites in liquid electrolytes.[4] Stability and efficiency were dramatically improved by introducing a solid-state hole conductor and a mesoporous TiO₂/perovskite layer.[5] Currently, the certified power conversion efficiency of perovskite solar cells (25.2%)[6] is comparable to the photovoltaic performance of other thin-film photovoltaic technologies (Si, CdTe, and GaAs). Nevertheless, in spite of this progress, the poor-term stability of photovoltaic perovskites complicates their commercialization. Degradation processes in these devices are not only related to the intrinsic properties that determine the thermal and/or electrical stability (device configuration and perovskite composition)[7] but also strongly affected by environmental factors (moisture, light, oxygen, and temperature).[8-9] Specifically, under moisture exposure, as a consequence of, mainly, the reaction between water molecules and methylammonium cations (CH₃NH₃⁺), which acts as a Brønsted/Lewis base, the perovskites tend to be hydrolyzed back to the precursors giving rise to morphological and crystal structural changes, optical absorption decay, and the deterioration of the electronic properties that determine the photovoltaic performance of perovskite solar cells. [8-11]

Different strategies have been employed to prevent degradation and improve the device stability because of the sensitivity of perovskite materials toward ambient moisture. Many of them imply the modification of the perovskite composition by the insertion of ions to achieve a more stable crystal structure,[7] the employment of buffer layers between perovskite films and electron- or hole-selective layers,[12] or even the substitution of the spiro-OMeTAD layer by other more hydrophobic hole-selective materials.[13] On the other hand, different materials have also been employed to encapsulate complete perovskite solar devices and avoid moisture exposure, such as hydrophobic polymers,[14] atomic layer-deposited Al₂O₃ films,[15] or even using sealing glass as a barrier layer.[16] Although successful, many of these approaches involve expensive and complex deposition processes and even restrict the photovoltaic performance of perovskite solar devices.

Here,[17] we investigate a simple solvent-free polymer encapsulation method for perovskite solar cells using a conformal plasma polymer thin film. This organic layer is formed by the remote plasma-assisted vacuum deposition of the solid precursor adamantane (ADA). The synthesis is carried out at room temperature and under low-power plasma activation to avoid energetic species or UV radiation of the plasma to reach the substrate surface. This methodology has been successfully applied in recent articles for the fabrication of photonic films based on organic dyes and small functional molecules working as optical sensors, optical filters, tunable photoluminescence emitters, and lasing gain media.[18-20] This deposition process is compatible with opto and microelectronic components and can be scaled to large deposition areas and to wafer-scale fabrication.[21-22] To our best knowledge, this is the first report where the deposition of an organic plasma nanocomposite thin film is employed to encapsulate perovskite solar cells. The ADA precursor molecules (C₁₀H₁₆) consist of a single C–C bond with four connected cyclohexane rings arranged in the armchair configuration. This material is very effectively plasma-polymerized under remote conditions allowing the deposition of homogenous and compact ADA thin films characterized for being insoluble in water and thermally stable up to 250 °C. Additionally, these deposited films show a high transmittance (≈90%) in the low-energy region of the visible spectrum (λ > 300 nm).

After reaching 20% efficiency, research in perovskite photovoltaics has shifted from a race for efficiency to a race for stability. For efficiency, the standard test conditions (STC) set the rules for the race. However, the term stability is used very broadly and assessed in various ways, meaning different groups are running different races. [1] For the application, however, only energy yields that can be achieved under real-world, long-term operation matter. [2]

In this work [3], we characterize and analyse the performance of 20% efficient perovskite solar cells under simulated ambient conditions based on real temperature and irradiance data of 27 selected days during a year in Sion, Switzerland. Working in a controlled lab environment, a much more reliable and systematic collection of data can be carried out, avoiding parasitic failure mechanisms, and weather conditions of an arbitrary location and time of year can be emulated without physical presence outdoors.

We find that the perovskite solar cell shows a low decrease of efficiency with elevated temperature and low light intensity, maintaining almost optimum values for dominant ambient conditions. Therefore, the resulting year-averaged efficiency (energy produced/total illumination energy) is close to the STC value. The overall energy yield is influenced by reversible degradation (<2% a day), delivering the highest performance in the morning, and ~10% permanent degradation, observable during the year.

Finally, we compare the perovskite with 22% efficient silicon heterojunction devices. Whereas the maximum-power-point voltage of these is differently affected by the weather conditions, the current scales linearly with light intensity for both, which is particularly important when considering 2-terminal perovskite-silicon tandem cells.

Organometal halide perovskites have emerged as one of the most promising photoabsorbent materials for efficient and cheap solar cells and have recently reached certified Power Conversion Efficiencies (PCE) of 25.2%.[1–4] The research in Perovskite Solar Cells (PSC) underwent an inflexion point when the group of Miyasaka first[5] and Park later[6] replaced the liquid electrolyte by solid‐state hole conductors (SSHCs), which increased significantly the PCE and stability of the devices. From this moment on, most of the researches on PSCs implement SSHC in the devices. The first and still most used SSHC is the organic molecule Spiro‐OMeTAD [2,2′,7,7′‐tetrakis (N,N‐di‐p‐methoxyphenyl‐amine) 9,9′‐spirobifluorene].

Whereas the use of Spiro‐OMeTAD is widely spread, the undoped form of this material is reported to have very poor intrinsic hole mobility and conductivity, leading to high series resistance in the devices.[7,8] The poor charge transport properties of Spiro‐OMeTAD are solved in the literature by means of dopants. The most common additive for Spiro‐OMeTAD is lithium bis(trifluoromethanesulfonyl) imide (LiTFSI), which dopes the system with Li⁺ cations, improving significantly the hole mobility.[9]

It is clear from the literature that a common strategy to enhance the charge transport properties of some SSHC is the addition of dopants in different relative concentrations. Unfortunately, the main handicap still hindering the eventual exploitation of PSCs is their poor stability under prolonged illumination, ambient conditions, and increased temperatures, which is partially hindered by the use of dopants or additives that can contribute to the degradation of the perovskite film.[7,9] For instance, Li⁺ cations are highly hygroscopic and induce moisture absorption.[10]

However and regardless of the significant number of examples that can be found in literature about Spiro‐OMeTAD layers as SSHC, negligible attention has been paid to the effect of its crystalline structure. This can be understood due to the highly disordered nature of solution‐processed films (the most widely spread) and to its amorphous‐growing tendency related to the spiro-center.[11,12] Despite the low intrinsic hole mobility reported for dopant‐free Spiro‐OMeTAD layers fabricated by wet approaches, the group of Bakr has recently reported significantly improved hole‐transporting properties of pristine Spiro‐OMeTAD single crystals.[12] Their results are supported by theoretical calculations performed by Brédas and Houk,[13,14] who reported a very high intrinsic hole conduction of crystalline Spiro‐OMeTAD layers along the π–π stacking direction of the crystal. Their findings demonstrated that the crystalline order of this SSHC is crucial to promote new charge transport pathways. Even though these results show the potential of crystalline Spiro‐OMeTAD layers to enhance the photovoltaic properties of PSCs, their implementation is not straightforward due to the antisolvent experimental strategy used to grow this organic molecule in its crystalline form.[12]

In this work,[15] we report the unprecedented sublimation under vacuum conditions of the most widely used SSHC in perovskite solar cells, the Spiro‐OMeTAD, in the form of dopant‐free crystalline layers. In addition, we demonstrate the enhanced stability of these layers acting as SSHC in PSCs in comparison with the solution‐processed counterpart. Our results reveal on one hand that the substrate temperature is a critical parameter controlling the microstructure and crystallinity of the layers. On the other hand, the implementation of these vacuum sublimated Spiro‐OMeTAD layers on PSCs have demonstrated two key aspects: i) a considerably increased PCE in comparison to the dopant‐free Spiro‐OMeTAD layers fabricated by a standard wet approach and ii) a significant enhancement of the stability of the cells, which have been tested under continuous illumination during 40 h and after annealing in air up to 200 °C.[15]

Although perovskite solar cells have demonstrated impressive efficiencies in research labs (above 25%), there is a need of experimental procedures to allow their fabrication at ambient conditions, which would decrease substantially manufacturing costs. However, under ambient conditions, a delicate monitoring of moisture level in the atmosphere has to be enforced to achieve efficient and highly stable devices. In this work, we show that it is the absolute content of water measured in the form of partial water vapour pressure (WVP) the only lab parameter that needs to be considered during preparation. Following this perspective, MAPbI₃ perovskite films were deposited under different WVP as derived by climate-determined parameters, i.e., relative humidity (RH) and lab temperature. We found that efficient and reproducible devices can be obtained at given values of WVP. Furthermore, it is demonstrated that small temperature changes, at the same RH value, result in huge changes in performance, due to the non-linear dependence of the WVP on temperature. We have extended the procedure to accomplish high-efficient FA_0.83MA_0.17PbI₃ devices at ambient conditions by adjusting DMSO proportion in the precursor solution as a function of WVP only. As an example of the relevance of this parameter, a WVP value of around of 1.6 kPa appears to be an upper limit for safe fabrication of high efficiency devices at ambient conditions, regardless the specific values of RH and temperature present in the lab.

For successful commercialization of perovskite solar cells, straightforward solutions in terms of environmental impact and economic feasibility are still required. Flash Infrared Annealing (FIRA) is a rapid method to fabricate perovskite solar cells with efficiencies > 20 % on regular architectures which allows a film synthesis in less than 2 seconds^{1, 2}. By varying the FIRA parameters, it is possible to create high quality films of both fully inorganic and hybrid perovskites. This demonstrates the versatility of the FIRA protocol and establishes it as a fast synthesis process, which provides detailed control over the perovskite morphology and crystallinity.To make FIRA effective is necessary to understand the physicochemical phenomena that take place during the crystallization of the synthesized film. Is important, for example, to know the pathways of the nucleation and crystal growth through intermediate phases. In FIRA the crystallization occurs by it intermediate phases until the final crystal phase as it happens on the antisolvent method and posterior thermal low temperature annealing.

Perovskite solar cells (PSCs) are one of the most promising new-generation photovoltaic (PV) technologies combining simple, solution process, fabrication techniques and efficiencies comparable to other well established PV. In this work, we use the Ti3C2TX MXene with various termination groups (TX) to tune the work function of the perovskite absorber and the electron transport layer (ETL), to engineer the perovskite/ETL interface and to improve cell efficiency. Ultraviolet photoemission spectroscopy measurements and Density Functional Theory calculations show that the addition of Ti3C2TX to halide perovskite and ETL permits to tune the materials’ WFs, without affecting other electronic properties. The non linear relation between terminal group mix and the WF is carried out for both MXenes and MXenes/perovskite [1]. In addition, we show that the dipole induced by the Ti3C2TX at the perovskite/ETL interface can be used to change the band alignment between these layers. The combined action of WF tuning and interface engineering can lead to substantial performance improvements in MXene-modified PSCs in direct configuration, as shown by the 26% increase of power conversion efficiency and hysteresis reduction with respect to reference cells without MXene.[2] Similar results have been obtained also for inverted configuration where a NiO/perovskite+MXenes/PCBM stack is used.[3]