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Halide ions are known to be mobile both in dark as well as under bandgap excitation.  The halide ion mobility is often dictated by the defects and halide ion composition. The excited state characterization using emission and transient absorption spectroscopy has allowed us to probe the halide ion migration and segregation in methylammonium lead iodide/bromide (CH3NH3PbBrxI3-x (x=0 to 3)). In addition to composition dependent absorption and emission properties they also undergo phase segregation to create Iodine-rich and Bromide- rich regions when subjected to visible irradiation. This intriguing aspect of halide ion movement in these mixed halide films can be tracked from the changes in the photoluminescence and absorption spectra. The photovoltaic performance decreases with continuous illumination even without external load. Upon storing these perovskite solar cells in dark, the photovoltaic performance recover indicating remixing of the halide ions. This recovery in photovoltaic performance is in line with the absorption recovery of the mixed halide band. The results that show the dependence of the photocurrent and photovoltage recovery on the iodide treatment will be discussed.

The presentation will review recent theoretical, spectroscopy and diffraction results on monocrystals or thin-films of halide perovskites, especially 2D multilayered phases. Optoelectronic devices based on 3D halide perovskites and their alloys have demonstrated attractive photovoltaic and electroluminescence performances, but a number of technological issues have to be solved, related to photo-stability or degradation under ambient conditions. 2D multilayered phases recently demonstrated conversion efficiency in the 10-15% range, but also exhibit solar cell excellent photostability under standard illumination as well as humidity resistance. In 2D multilayered phases, intrinsic quantum and dielectric carrier confinements lead to a stable Wannier exciton at room temperature, which has been investigated in details for a few compounds. Efficient solar cell operation is related to internal exciton dissociation through low energy states, as shown from the investigation of both thin films and small exfoliated single crystals. Chemical engineering offers extensive potential developments for these layered compounds, leading recently to a growing number of multilayered halide perovskites.

The relatively weak bond of metal-halide perovskites (MHPs) gives rise to an inherently soft crystal lattice which is naturally prone to disorder, [1] associated to formation of defects. Defects introducing levels in the material’s band-gap may act as traps and recombination centers for photogenerated charge carriers, limiting the device performance and possibly impacting the device temporal stability. Defects may also introduce ionic mobility channels in MHPs. Ion migration is boosted by the presence of vacancies and interstitial defects, acting as shuttles for ion hopping.[2] If the migrating defects are also charge traps, as it occurs for iodine defects in MAPbI3, one has migrating traps which can respond to the action of an electric field [3] and to the presence of photogenerated carriers.[4, 5] Some of the traps may also undergo photochemical reactions, such as the reported release of molecular iodine under light irradiation[6, 7]. Defects may also lay behind the reported material transformation under light exposure, followed by very slow relaxation to initial conditions.[8]

While bulk defects have been extensively investigated in MHPs,[9] there is increasing awareness that surfaces and grain boundaries may actually represent preferential sites for defect formation. Surfaces are also involved in defining the material work function, thus the interfacial energy level alignment with selective contacts.

We present ab initio modeling results of surfaces, defects and surface defects modeling in MHPs with selected examples of applications related to the effect of electric fields and charge carriers on the structural and electronic properties of perovskites relevant to stability and solar cell operation. A model to account for the surface-related ion migration is also presented.

References:

[1] Conings, B. et al. Adv. Energy Mater. 2015, 5, 1500477.

[2] Mosconi, E.; De Angelis, F. ACS Energy Lett. 2016, 1, 182-188.

[3] Chen, B. et al. Nat. Mater., 2018, in press.

[4] Birkhold, S.T. et al. ACS Energy Lett. 2018, 3, 1279−1286

[5] Meggiolaro, D. et al. Energy Environ. Sci. 2018, 11, 702-713.

[6] Meggiolaro, D. et al. ACS Energy Lett., 2018, 3, 447–451.

[7] Kim, G.Y. et al. Nat Mater 2018, 17, 445-449.

[8] Gottesman, R. et al. J. Phys. Chem. Lett. 2014, 5, 2662-2669.

[9] Meggiolaro, D.; De Angelis, F. ACS Energy Lett. 2018, 2206-2222.

Halide (hybrid) perovskites (HaP) have emerged as a new class of semiconductors that truly encompass all the desired physical properties for building optoelectronic and quantum devices such as large tunable band-gaps, large absorption coefficients, long diffusion lengths, low effective mass, good mobility and long radiative lifetimes. As a result, proof-of-concept high efficiency optoelectronic devices such as photovoltaics and LEDs have been fabricated using both 3D and 2D perovskites. In this talk, I will describe two new results that are open questions and demand an investigation.

First, I will describe our results on demonstration of field effect transistors (FET) operational at room temperature, which despite the excellent electronic transport properties in halide perovskites has remained elusive. Briefly, we fabricated hybrid perovskite-based FETs that operate at room temperature with negligible hysteresis. Extensive current-voltage and quantitative device modeling reveals that the use of high-k dielectrics enables a strong modulation of the channel conductance with gate voltage exhibiting p-type transport characteristics. After gate bias poling, we succeed in achieving ambipolar FETs an on/off ratio >104 with no degradation in transport characteristics for >100 cycles. Our results elucidate the key principles for achieving gate modulated carrier transport in hybrid perovskite thin films.

Second, I will describe the design principles for tailoring achieving phase-purity, crystallinity and orientation in layered 2D perovskites and its implications on optoelectronic application. We will demonstrate proof-of-concept optoelectronic devices that validate our scientific findings.

Perovskite-based solar cells (PSCs) have attracted worldwide interest with a certified 23.3% efficiency in a few years. Currently, stability issues are the key for its future development. Aiming at this point, some distinctive works have been attempted in our lab [1-20]. On one hand, material processing and interface engineering have been deployed to improve the intrinsic stabilities of perovskite absorbers. On the other hand, optimized device structures have also been developed, such as developing new-type charge transporting materials and carbon counter electrodes [8-15]. Furthermore, stability-related theoretical studies have been carried out. A modulated transient photocurrent/photovoltage system (M-TPV/TPC) has been developed and used to investigate stability issues of PSCs. Intrinsic hysteresis behaviour, photocharge accumulation and recombination in PSCs have been systematically investigated [16-20].

Figure 1. (a) Schematic diagrams of a modulated transient photoelectrical system and related results; (b)- (e) Material and interfacial engineering toward PSCs.

References:

[1] Appl. Phys. Lett. 2014, 104, 063901.

[2] ChemPhyschem 2015, 16, 842.

Nano Energy 2015, 15, 540.

[4] ChemPhyschem 2016, 17, 112.

[5] Adv. Funct. Mater. 2018, 28, 1705220.

[6] Nano Energy 2018, 43, 383.

Sunlight concentration allows accelerated stability studies and separating the effect of light on material and cell stability from other factors. Our experimental methodology allows independent control of sunlight intensity, the sample temperature and environment during the exposure. Stress testing of perovskite solar cells (PSCs) showed that faster degradation was found for cells held at short circuit (SC) under concentrated sunlight and on the initial stage of outdoor exposure. However, cells kept at SC showed better long-term stability compared to cells kept at open circuit (OC) upon real operational conditions. We also found that (sun)light intensity was more important than illumination dose for cells degradation at SC conditions, while dose was the determining factor at OC. This indicates that different degradation mechanisms are dominant at different degradation stages and under different bias conditions, and that nano-scale understanding of degradation mechanisms is required to suggest ways to increase the device life-time.[1] Such bias-dependent degradation has been previously observed also in other types of solar cell technologies. Yet, the stability of these solar cells was greatly improved by rationale design. Bias-dependent atomic-scale degradation mechanisms in different photovoltaic technologies are compared to those of PSCs, towards identifying lessons that can be learned to improve PSC stability.[2]

[1] Manuscript in preparation

[2] Mark V. Khenkin, Anoop K.M., Eugene A. Katz, Iris Visoly-Fisher, Bias-Dependent Degradation of Various Solar Cells: Lessons for Stability of Perovskite Photovoltaics, Submitted

[1] Manuscript in preparation

[2] Mark V. Khenkin, Anoop K.M., Eugene A. Katz, Iris Visoly-Fisher, Bias-Dependent Degradation of Various Solar Cells: Lessons for Stability of Perovskite Photovoltaics, Submitted

Two-dimensional (2D) perovskite possess interesting optical and physical properties in addition to the enhanced stability of solar cells which contain 2D perovskite.  In our work¹ we used bromide-based perovskite and varied their dimensionality in the range between a pure 2D structure of (PEA)₂(MA)_n–1Pb_nBr_3n+1 to three dimensional (3D) MAPbBr₃ structure. Optical and physical characterizations were used to characterize the perovskite film, and full perovskite solar cells. Quasi 2D perovskite (having 40-50 layers of perovskite) enabled the increment in the photovoltaic performance (PV). In addition , we studied different barrier molecules demonstrating the significant effect of the barrier molecules on the 2D and quasi 2D perovskites.²

The introduction of different barrier molecules in the quasi 2D perovskite affected the photovoltaic properties of the solar cells, which results with high open circuit voltage (V_oc) of more than 1.4 V with PCE of 9.5% for full bromide-based PSCs. Further manipulation of the barrier molecule enabled us to fabricate low dimensional highly efficient iodide-based PSCs.

These low dimensional PSCs having n=10, yielded power conversion efficiency of 15.6%, with a current density of 21.5mA/cm². The high PV performance and current density of the low dimensional perovskite were achieved solely by changing the barrier molecule, without any modified deposition methods or additives. Furthermore, photo-electronic characterizations showed different mechanism of the low dimensional PSCs. This work demonstrates the ability to tune the perovskite properties using chemical modifications. The simplicity of the chemical modifications opens the way to simple delicate adjustments, thanks to the wide range of available materials, through combining perovskites of several dimensionalities.

In this contribution, we address some general considerations to succeed in inkjet printing perovskite solar cells: starting from substrate properties to ink development and technical requirements when utilizing inkjet printing as a technique.

We will point out the importance of substrate modification routines to ensure a good wettability and highly reproducible printing results. Determining the substrate surface energy by means of contact angle measurements enables us to define criteria for the ink development. Intrinsic solvent properties such as surface tension, viscosity, boiling point and solubility but also toxicity define a set of parameters to be considered when developing a printable ink formulation. By controlling the ink formulation, as well as the post-processing parameters, we are able to control the film roughness and thickness, thus changing the crystallization dynamics [1], [2]. Preliminary work on multi-cation perovskite solar cells in combination with a vacuum annealing post-processing step leads to power conversion efficiencies of 15% for an n-i-p solar cell on a sub-cm² cell area size [3]. We now focus on in-situ characterization of perovskite film formation kinetics as a function of precursors composition and process conditions to unveil the role of solution chemistry and intermediate states in determining sample morphology and quality.

The presented insights of this study paves the way for large area, low-cost and high-throughput inkjet-printed perovskites for efficient thin film solar cells.

Perovskite based solar cells (PSCs) demonstrated a remarkable increase in terms of power conversion efficiency (PCE) in few number of years exceeding 23% efficiency. Today’s state of the are perovskite solar cells containing two or more cations (MA⁺, FA⁺, Cs⁺ or Rb⁺) together with a mixture of halides (Br^- and I^-) showing long term stability along with high PCE values. In order to further improve the perovskites crystals quality and photovoltaic (PV) performance post deposition treatments are made. In this work we studied the effect of formammidinium iodide (FAI) and formammidinium bromide (FABr) post treatment on Cs_0.2FA_0.8Pb(I_0.75Br_0.25)₃ perovskite. The FAI/FABr post treatment shows that halide exchange can occur in the solid state phase following the fabrication of the thin film perovskite.  Several FAI/FABr ratios (100/0, 75/25, 50/50, 25/75, and 0/100) were studied on films complete solar cells. We found that as the FABr ratio increase the distortion in the perovskite lattice become more significant as a result of large volume differences between already exist to incorporated halides. The influence of halide exchange on a complete cells was studied through charge extraction and voltage decay measurements, which shows that the structural changes due to halide incorporation strongly affect the lifetime of the charge carriers in the perovskite layer eventually reducing the PV performance of the post treated cells compare to standard non-treated mixed cation mixed halide cells. This work demonstrates the halide exchange in the solid state phase of mixed cation mixed halide perovskite.

2D hybrid perovskites are a promising class of materials for optoelectronic applications such as light emitting diodes, photodetectors and solar cells. Compared to the more researched 3D hybrids they have several advantages, including an improved stability in ambient conditions and the possibility of introducing functionality in the organic layer. So far, the organic layer is only used to tune their band gap, dielectric environment and dimensionality, while the transport of charge carriers remains restricted to the inorganic metal-halide octahedrals. In this work we aim at introducing functionality in the organic layer for instance to improve charge separation or tune the sensitivity of photodetectors.  In general, there is a lack of understanding on how the organic cations affect the charge and excited state dynamics of 2D hybrid perovskites and whether the transport can be extended to the organic layer. In this work we use unique time-resolved microwave conductivity techniques, combined with time-resolved fluorescence and femtosecond transient absorption to study the charge and excited state dynamics of a large variety of 2D organic-inorganic perovskites. First, we have found that the thickness of inorganic layers highly affects properties such as mobility, yield of charge dissociation, exciton binding energy. Second, pure 2D perovskites (n=1) with different organic groups (BA, BZA, HA) exhibit new electronic transitions at low temperature that may be associated with different nature of the excitonic states (free exciton, biexcitons, trions). Finally, we have explored the possibility to adapt the organic cations, so that they start playing a role in defining the opto-electronic properties. It is, for example, possible to improve the charge transport to the organic layer through the formation of charge-transfer complexes between a charge-donating and charge-accepting molecule. We synthesized 2D hybrid perovskites containing a donor molecule (Pyr-C₄-NH₃⁺), which serves as a reference material, but with improved transport between the inorganic layers.  And subsequently, we intercalated strong electron acceptors (TCNQ and TCNB), leading to the formation of charge-transfer complexes with the Pyr-C₄-NH₃⁺ donor molecules. We will present several examples where the organic cation affects the charge transport properties, the dissociation of exciton and the optical properties.

Multiple-cation lead mixed-halide perovskites have attracted significant attention in a wide range of applications such as light-emitting devices, lasers and thin-film photovoltaics.^1-3 Solar cells based on mixed perovskites have recently demonstrated high power-conversion efficiency exceeding 22%.⁴ The wide-range tunability by intermixing of different ions, especially halides, facilitates the integration in tandem devices.⁵ However, the compositional stability could be compromised.⁶

In this contribution, we present investigations on reversible changes in the electronic structure of multiple-cation mixed-halide perovskites under illumination and applied electric fields to simulate the solar cell’s operation conditions.

We used electromodulation spectroscopy as a highly sensitive measurement tool for the determination of critical points in the band structure, e.g., the bandgap⁷. The basic principle of this technique relies on the change of the dielectric function under the influence of an applied external bias and allows for the non-invasive investigation of complete solar cells. By periodically applying an external electric field (AC bias), the dielectric function of the absorber and, in turn, the optical properties such as the reflectance R are modulated. The relative change ΔR / R is independent of experimental properties (spectral response of the detector, spectrum of the light source, etc.) and exhibits characteristic line shapes. The analysis of these leads to the precise determination of the optical resonance energy.

We found instabilities in Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃ solar cell absorbers under 1 sun illumination and applied bias leading to a decrease of the bandgap up to 70 meV.  However, interestingly these instabilities are observed to be reversible. They are attributed to segregation of the halides and are confirmed by in-situ X-ray diffraction measurements under the same conditions.

Additionally, we studied the influence of the surrounding atmosphere by changing from air to nitrogen or pure oxygen as well as applying different relative humidity. The resulting bandgap reduction strongly increases by the combined influence of high humidity and oxygen content. Thus, the effect can be minimized by an effective encapsulation. However, since the observed changes are mainly activated by a combination of illumination and electric fields, they are intrinsic to the solar cell’s operation conditions and cannot be prevented completely. Due to the fact, that multiple-cation perovskites have been considered to be stable against halide segregation up to now, a careful stability analysis for mixed perovskites seems to be necessary.⁸

In conclusion, we presented a detailed study of reversible bandgap instabilities in the mixed perovskite Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃. The observed instabilities occur under operation-relevant conditions – 1 sun illumination and applied electric fields – and cannot be avoided completely by encapsulation. This supports the necessity for further stability investigations of mixed perovskites.

A unique aspect of hybrid halide perovskite materials is the presence of an organic cation that occupies the cages formed by lead (Pb) and Iodide (I). In the basic perovskite CH3NH3PbI3, the methylamonium (MA) cation can rotate relatively freely, while slightly large cations can be restricted in their motion. Interestingly, the MA cation is dipolar and the organization of the dipoles in the material depends on the mutual interaction of the dipoles. It has been suggested that the formation of ordered domains can lead to electronic localized states that are different for electrons and holes, and therefore the dipolar disorder should influence the dynamics of charges formed on photo-excitation.

In this work we have performed a combined computational and experiemental study to unravel the relation between dipolar disorder and charge dynamics. Using a combination of Monte Carlo simulations and classical molecular dynamics we have studied the dynamics of the MA dipole at different temperature and show that large ordered domains are formed at low temperature, while above a certain temperature the domains are very small and the dipole are rotation relatively freely. The temperature at which this transition occurs suggests that the phase transitions in the perovskite are induced by dipole alignment.

Subsequently, we have studied the effect of dipole orientation on the delocalization of electronic states in the material. It is shown that for organic cations with a relatively high dipole moment, such as MA, localized states are formed that are different for electrons and holes. For a low-dipole moment cation, formamidinium (FA) such localized states are not observed.

Finally, in order to gain insight in the effect of dipole motion on charge dynamics, we have performed time-resolved microwave conductivity measurements combined with generation of charges by irradiation with a short, high-energy electron pulse. We observe substantial changes in mobility and lifetime of charge carriers in CH3NH3PbI3 after the low temperature tetragonal (β) to orthorhombic (γ) phase transition. We observed that the mobility and lifetime of charge carriers increase as the temperature decreases and a sudden increment is seen after the β/γ phase transition. For CH3NH3PbI3 the mobility and the half-lifetime increase by a factor of three to six compared with the values before the β/γ phase transition. We attribute the considerable change in the dynamics at low temperature to the decrease of the inherent dynamic disorder of the organic cation (CH3NH3+) inside the perovskite crystal structure. When replacing the dipolar MA cation by FA such sudden changes in the charge dynamics are not observed. This indicates that the dipolar motion of the organic cation does influence the dynamics of charges, particularly at lower temperature.
 

1. Perovskite/silicon two-terminal tandem photovoltaic modules

The use of perovskite solar cells in tandem photovoltaic (PV) modules based on silicon PV is an attractive application for perovskites, as perovskite-on-silicon tandem solar cells offer a route in exceeding the power conversion efficiency (PCE) of the market-dominant silicon single-junction photovoltaics. In order to improve the PCE of silicon single-junction solar cells, a key strategy in light management is the texturing of the crystalline silicon surface for enhancing light incoupling as well as light trapping[1][2]. Since the micron-scale texture facilitates light incoupling for the complete optical spectrum, the adoption of textures is also of high relevance in perovskite/silicon tandem PV modules[3].

Under realistic conditions, the spectrum and intensity of irradiance is changing continuously with time, and is not comparable to the illumination under conventional standard test conditions (STC) for deriving the solar cell’s PCE. The calculation of energy yield (EY) under realistic irradiation conditions offers the possibility to carefully optimize the structure of perovskite/silicon tandem PV modules. For this, our recently developed EY platform accounts for the variations in the share of specular and diffuse irradiance with time as well as angular dependency of incident light. The model uses the transfer-matrix method combined with statistical ray tracing for the optical modelling of the device and the one-diode model for the electrics. With input of irradiance data of a typical meteorological year (TMY3) from NREL[4] for many locations with various climate zones, we calculate the EY of the solar cell, and predict the optimal structure for different device architectures as well as for different locations. Since the bandgap of perovskites is tunable[5], we further evaluate the EY for different bandgaps finding the optimal bandgap for all investigated architectures and locations.

2. Energy yield modelling of different architectures with planar and textured interfaces

In this work, we focus on the two-terminal (2T) configuration of perovskite/silicon tandem PV modules with monolithic interconnection of perovskite top cell and silicon bottom cell. There is also work on the EY modelling of perovskite/CIGS tandem PV modules, with the comparison of 2T and four-terminal (4T) configuration[6]. For perovskite/silicon 2T tandem PV modules the EY is calculated for three different architectures, a planar with flat front and rear side, a double-side texture with texture at the front and rear side, and rear texture only[7]. The EY of all perovskite/silicon 2T tandem PV modules with a perovskite bandgap of 1.72 eV is compared to the EY of a reference silicon single-junction PV module with a double-side textured architecture, and the relative enhancement in EY is evaluated. These calculations are performed for three locations with various climate conditions with an optimized module tilt angle and an optimal perovskite absorber layer thickness.

We optimize the perovskite absorber layer thickness in order to minimize losses due to mismatch of generated currents in both subcells. Since current matching depends mainly on the spectrum and intensity of irradiance, the maximal EY is found for different perovskite absorber layer thicknesses than under STC. This fact has to be considered in optimizing the layer stack of perovskite/silicon 2T tandem PV modules. The EY increases for all three architectures and the highest relative enhancement in EY with 26–28% is shown for the perovskite/silicon 2T tandem PV module with double-side texture, whereas it is 12–14% for the planar and 19–22% for the rear texture. In order to find the optimal perovskite bandgap for a 2T tandem, we further evaluate various perovskite bandgaps in the range of 1.55–1.88 eV. With this work we demonstrate the importance of wide bandgap perovskites to the application in perovskite/silicon 2T tandem PV modules.

Metal halide perovskites have rapidly become important semiconductor materials for photovoltaic devices. Despite solar cell efficiencies soaring over 20% [1], there are still aspects in regard to understanding the fundamental operational properties that are lacking. One active debate surrounds the influence of lead iodide (PbI2) on device performance as well as fundamental properties of the perovskite absorber [2].  Both beneficial and detrimental traits have been reported, where the ambiguity may not only be related to the amount of residual PbI2, but also to the distribution within the metal halide perovskite absorber layer and whether it is introduced during preparation or created by degradation [3]. Herein, we present a study on the impact of lead iodide on the charge-carrier recombination kinetics. We compare both the case of a vapor-deposited PbI2 layer on top of a methylammonium lead triiodide (MAPbI3) thin film as well as PbI2 generated as a result of photo-induced degradation during several hours of light-soaking under solar illumination conditions. By simultaneously acquiring spectrally-resolved photoluminescence quantum yield and time-resolved photoluminescence lifetime at excitation wavelengths ranging from 450 nm to 780 nm, we identify a unique radiative recombination mechanism occurring at the PbI2/MAPbI3 interface (disordered MAPbI3 region) when charge carriers are generated in PbI2. While PbI2 reportedly acts as a UV-filter,[4] we here demonstrate that charge carriers may in fact funnel via PbI2 and recombine in the perovskite. We thereby provide important insight into the long-debated question of whether excess PbI2 is beneficial or detrimental for charge carrier dynamics in perovskite solar absorber materials.

Methylammonium lead triiodide perovskite (MAPbI₃) semiconductor displays outstanding photovoltaic and light emitting properties. Recently a new property was uncovered, revealing striking features of the transient photoluminescence in perovskite layers [1]. The application of bias voltage can eliminate the luminescence in lateral interdigitated electrode devices under light soaking. Using a wide-field PL imaging microscope it was observed that the PL is progressively suppressed showing a sharp advancing front that moves advances from the positive electrode at a slow velocity of order of 10 mm s^-1. We present a model of ion-induced doping modification governed by local saturation effect `[Li, C.; Guerrero, A.; Hüttner, S.; Bisquert, J. Nat. Commun. 2018.] We have shown that the dominant electrical current flowing across the symmetrical electrodes, is an electronic current. The applied voltage induces an ion movement and influences the majority carrier density turning the material more intrinsic-like and thus the total current changes with time (decreases for p-doped material). The excess iodine vacancies lead to redox reactions as for example in which interstitial Pb2+ is reduced to Pb(0), leading to severe nonradiative recombination that suppresses PL. Significantly, we show that the electron and hole concentration alters with the drift of vacancies under application of an electric field. This insight leads to a direct determination of the diffusion coefficient of iodine vacancies from the measured current and provides detailed information and control on the effect of ionic conduction over the electrooptical properties of hybrid perovskite materials. We also show the identification of the impedance of ionic diffusion by measuring single crystal samples [2].

Organic-inorganic perovskites have attracted tremendous interest due to their exceptional optical and electrical properties (high carrier mobility, long carrier diffusion length, and large absorption coefficient). These lead to high power conversion efficiencies of organic-inorganic perovskites ranging slightly above 22% by using simple spin-coating processing.

This paper addresses the influence of the out-diffusion of the solvent, here dimethyl sulfoxide (DMSO) as often used for such processing techniques, on the device quality. The temperature dependence of the out-diffusion of DMSO from CH₃NH₃PbI₃ precursor layers was investigated by applying the flash-anneal technique and analyzing the S=O vibrational mode in the layers using infrared spectroscopic ellipsometry. The diffusion of DMSO can be well described by diffusion in a homogeneous layer. The diffusion coefficient of DMSO in CH₃NH₃PbI₃ amounts to about 10^-11 cm²/s at 100 °C. Furthermore, the diffusion constant exhibits an activated behavior with activation energies of E = 0.6 and 1.8 eV, respectively, indicating the presence of two different migration processes. The obtained activation energies will be discussed in terms of decomposition and incorporation of DMSO related complexes in the perovskite lattice. Finally the influence of the out-diffusion of DMSO on the formation of the CH₃NH₃PbI₃ lattice will be debated.

Zinc oxide (ZnO) is a well-known electron transport layer (ETL) for efficient Organometal halide perovskite (CH₃NH₃PbI₃) based solar cell with efficiency as high as 17.5% [1]. Over other ETL like TiO₂, it has various advantages: higher electron mobility, easy to etch and ability to be doped. However, perovskite material deposited over ZnO suffers from chemical reaction at interface and fast degradation of perovskite material [2], which leads to less stable device. Interestingly, doping ZnO with aluminum i.e, aluminum doped zinc oxide (AZO) is found to be more suitable for perovskite solar cell, as the latter has more stable interface with perovskite [3], [4]. Also, AZO is highly-conductive (as low as 9 ohm/sq.) and transparent (> 80%), so it doubles up as the transparent conducting oxide (TCO) thus replacing hard-to-etch FTO and expensive ITO.

In this work, we present fabrication and device physics of AZO/Perovskite/HTL/Au stack, in which AZO has dual role: as TCO and ETL. AZO replacing the conventional FTO/c-TiO₂/meso-TiO₂ stack, simplifying the fabrication process and reduction in thermal budget. Combining results from Ultraviolet photoelectron spectroscopy (UPS) and UV-Visible spectroscopy (UV-Vis), suggests AZO will act as an effective ETL for perovskite thin-film, with a large valence-band offset and a small conduction-band offset. Constructed electronic band diagram provide the details of possible path for carrier recombination at the interface. Here, we have addressed this problem by ozone treating the surface of AZO (AZO:O₃). Ozone-treated AZO interface yields planar CH₃NH₃PbI₃ solar cells with significantly reduced hysteresis and open-circuit voltage (V_OC) up to 1.05 V.

We show that AZO:O₃ has better charge extraction as compared to AZO and hence a better ETL for perovskite solar cell. By exposing AZO to ozone-gas reduces the oxygen vacancies film (claim supported by O1s spectra of X-ray photoelectron spectroscopy) and hence doping in the 6-10 nm of the AZO thin-film. This gradient in n-type doping at the AZO surface creates electric field at the AZO surface and enhance charge extraction. Steady state photoluminescence (SSPL) was used to experimentally prove the conclusion.

As charge carrier recombination and extraction at the interfaces play a crucial role for device performance enhancement, we believe this controllable ozone treatment has a broader scope and can be a potential tool for various other oxides.   

The metal halide perovskite solar cells (PSCs) have been among the most popular research topics in recent years; the power conversion efficiency rose to 23.3% in a few years making this potentially low cost device a serious alternative for the current solar technologies [1]. However, the number of publications increased dramatically and chaotically in a few years; hence machine learning approaches could be beneficial to extract knowledge from such a large and complex accumulation in the literature. Association rule mining method is one of the most common data mining method to find out the relations, frequent patterns and associations in data which cannot be determined by naked eye.

In this work, the literature on perovskite solar cells was systematically reviewed, and a database containing 1921 data points extracted from 800 publications was created using the research articles published between 2013 and 2018 (until May 31 of 2018). This database was assumed to represent the literature fairly well considering that it covered about 10% of the Web of Science articles. Then, this database was analyzed using association rule mining to capture the major trends and frequent patterns in the collection of works.

The data analyzed using simple descriptive statistics first; the performance evolution with time as well as the average and distribution of PCE values of different materials and deposition methods were investigated. For example, the average efficiencies obtained with MAPbI₃ and MAPbI_3-xCl_x were found to be nearly same while FA based and mixed cation cells gave higher efficiency. Additionally, the potential of some rarely used materials (like Cs containing triple cation perovskites), inorganic HTLs and PTAA as HTL alternative for both regular and inverted cells were also emerged in this simple analysis.

Then the association rule mining analysis was employed to detect the most frequent items appeared in dataset for high performance. The factors such as mixed cation perovskites, DMF+DMSO as solvent, chlorobenzene as anti-solvent and two or three times spinning as the one-step coating technique emerged as the effective ways of obtaining cells with PCE higher than 18.0%. Similarly, relatively less frequently used factors like LiTFSI+TBP+FK209 as HTL additive and SnO₂ as ETL layer were also detected as the alternatives for high performance.

Abstract to NIPHO9

It was reported that the use of inorganic cations affects the long-term stability of perovskite solar cells (PSCs), which remains a major impediment for its industrial application¹. As a result, there have been several attempts to replace the organic cations with inorganic elements to improve the cells’ stability ²_’³ .CsPbI₃ is considered one of the most promising candidates to achieve a stable perovskite structure.

The perovskite’s dimensionality is determined by using a barrier molecule between the inorganic perovskite framework. The barrier molecule is too big to fit into the cage formed by the perovskite octahedrons and therefore, it creates confined inorganic perovskite layers. This low-dimensional perovskite creates a quantum-well structure, where the inorganic framework and the long organic molecules act as wells and barriers, respectively.

Unlike the three dimensional (3D) perovskite, the low-dimensional perovskite has shown promising stability, but it has a much lower power conversion efficiency. The relatively low efficiency of two-dimensional (2D) perovskite solar cells (PSCs) is mainly due to the barrier molecules, which could inhibit charge transport through the film. However, it is likely that combining the high efficiency of 3D perovskite with the enhanced stability of 2D perovskite could be an elegant way to achieve specific requirements from a photovoltaic solar cell⁴.

In this work we demonstrated how the black phase of CsPbI₃ can be stabilized when its dimensionality is reduced. Low-dimensional CsPbI₃ perovskites were fabricated using two different barrier molecules: linear and aromatic barrier molecules. XRD and SEM show the degradation process of these films, where the low-dimensional perovskite, using an aromatic barrier molecule, displays better stability than does the low-dimensional perovskite, using a linear barrier molecule. Both barrier molecules display enhanced photostability under continuous 1 sun illumination compared with 3D CsPbI₃ film, which degrades rapidly. However, the aromatic barrier molecule displays superior photostability compared with the linear barrier. Theoretical calculations explain this observation by the point and extended defects, which increase the time for degradation. Here, we have shown the potential of stabilizing the black phase of CsPbI₃, which provides the possibility to use it efficiently and stably in PV solar cells.

A combination of high-resolution mapping techniques was used to probe defects and homogeneity in perovskite solar cells with different architectures. First, the localized effect of excess PbI₂ on the photophysical and photoelectrical properties of perovskite solar cells with inverted structure was investigated by photoluminescence (PL), photocurrent and Raman mapping with micrometre resolution. On the contrary to other works in which excess PbI₂ is obtained by varying the composition of perovskite precursors solution or by annealing the already formed perovskite film, we use laser irradiation to generate localized PbI₂ in a full device, which amount can be controlled by tuning the laser intensity or irradiation time. We show that whereas a thick PbI₂ film at the perovskite/hole transport layer interface has a detrimental effect on the local photocurrent, a thin PbI₂ film (<20 nm) leads to a significant photocurrent increase, which is ascribed to the passivation of non-radiative defects and reduced charge recombination at the interface¹.

Then, the infiltration mechanisms and homogeneity of mesoscopic perovskite solar cells with structure mesoporous TiO₂/mesoporous ZrO₂/mesoporous carbon were investigated by a combination of photoluminescence, photocurrent, Raman and electroluminescence mapping performed on large and small sample areas. Three different types of cells prepared using a one-step infiltration process with MAPbI₃ or AVAI-MAPbI₃ solution or two-step process with MAPbI₃ were investigated. It is found that the one-step MAPbI₃ cell has very limited infiltration which results in poor device performance. On the contrary, high loading of the mesopores of the TiO₂ and ZrO₂ scaffold is observed when using AVAI-MAPbI₃ solution, but some micrometre-sized areas are not efficiently infiltrated due to the presence of dense carbon flakes hindering perovskite infiltration. Quite differently, the two-step cell has a complex morphology with several types of defects having beneficial or detrimental effects on the local photocurrent.

This work shows how complementary mapping techniques can be used to correlate materials and devices properties on micrometre length scales. PL intensity analysis can be difficult to interpret when measuring a full solar device because of competing photoelectronic effects (quenching, recombination…). Hence, the correlation with photocurrent, electroluminescence and Raman mapping provides a better understanding of the interplay between perovskite infiltration/crystallisation, defects, local PbI₂ excess and charges extraction for perovskite solar cells with various architectures.

Perovskite solar cells  demonstrated impressive power conversion efficiencies of >22%, while their practical application is hampered by poor stability of the active materials. Functional layers of perovskite solar cells have to sustain electric fields generated by built-in and light-induced potentials. Otherwise, the electrochemical degradation of active or charge transport layer materials would ruin the solar cell performance.

In this work, we report a systematic comparative study of the electrochemical stability of a series of hybrid and all-inorganic lead halide based perovskite materials - MAPbI₃, MAPbBr₃, FAPbI₃, FAPbBr₃, CsPbBr₃, CsPbI₂Br. Thin films of these materials were exposed to the potentiostatic polarization under anoxic conditions using the lateral and vertical two-electrode device architectures. We have shown that polarization leads to the appearance of the field-induced gradients in the chemical composition of the films as revealed by PL and Kelvin probe microscopy and ToF-SIMS. The effects of the potentiostatic polarization of the solar cells under forward and reversed bias on their photovoltaic performance will be also discussed and interpreted using results of the light beam induced current (LBIC) mapping. Analysis of the obtained data allowed us to correlate the chemical composition of the perovskite materials with their electrochemical stability, which might guide further design of stable light absorbers for future generation photovoltaics.

Power conversion efficiency (PCE) of lead halide perovskite solar cell (over 23%) has surpassed those of CIGS and CdTe, approaching the top value of crystalline Si cell. However, high PCE of single-cell enabled by lead halide-based perovskite absorbers is now being saturated, taking the Shockley Queisser (SQ) limit of open-circuit voltage (V_OC) (ca.1.32V) into account. A next challenge is to create a single cell which has high PCE comparable with that of GaAs (>28%) by reducing bandgap energy to <1.4 eV without accompaniment of increase in V_OC loss. This possibility will be found in a family of metal halide perovskites or other hybrid materials without depending on use of lead because lead-based semiconductors will limit bandgap energy higher than 1.5 eV. Concerning efficiency enhancement, high performance of organo lead halide materials is not compatible with robust high stability required for practical use. Ensuring the intrinsic thermal stability (desirably >200^oC) of the perovskites is a key issue before industrialization. In addition, toxicity of lead-based perovskites are going to become the most formidable challenges for real use (commercialization), in particular, for applications to IoT society, which is one of the most promising field of perovskite photovoltaic device in terms of high voltage output even under weak illumination. These thoughts urge us to concentrate our next research of perovskite photovoltaics (PV) more on development of non-lead high efficiency absorbers. Sn perovskite is still a strong candidate because Sn(II) has been found to be stabilized against ambient air by metal doping method (such as Ge). Regarding Bi-based perovskites, we found AgBi₂I₇ as a promising all-inorganic absorber having high thermal and moisture stability. Stability also highly depends on the property of charge transport materials (CTMs), especially, the kind of hole transporter. Spiro-OMeTAD does not work at high temperature while P3HT, for example, is thermally stable. In our collaboration with JAXA, P3HT-based perovskite devices showed robust stability by exposure to high (100^oC) and low (-80^oC) temperatures and also to high energy particle radiations (iScience, 2018, 2, 148). Selection of CTMs is another important key in combination with non-lead perovskite materials. In conclusion, next direction of perovskite PV should be to enhance PV performance of non-lead and all-inorganic semiconductor materials by extended compositional engineering, in parallel with developing thermally stable CTMs. Our on-going studies on all-inorganic perovskite absorbers and new CTMs for photovoltaic applications will be introduced in my talk.

Integrating inorganic–organic perovskite top cells with crystalline silicon or CIGS bottom cells into monolithic tandem devices has recently attracted increased attention due to the high efficiency potential of these cell architectures. To further increase the tandem device performance to a level well above the best single junctions, optical and electrical optimizations as well as a detailed device understanding of this advanced tandem architecture need to be developed. Here we present our recent results on monolithic tandem combinations of perovskite with crystalline silicon and CIGS, as well as tandem relevant aspects of perovskite single junction solar cells. By selecting a front contact layer stack with less parasitic absorption and utilizing the p-i-n perovskite top cell polarity, a certified conversion efficiency of 25.0% for a monolithic perovskite/silicon tandem solar cells was enabled. Further fine-tuning of the stack optics as well as contact layers improved the efficiency to 26.0% and we present how highly efficient tandem solar cells behave under current-mismatch conditions. Additionally, the introduction of light trapping foils with textured surfaces is presented together with the influence on texture position on lab performance and outdoor energy yield.^[1] The monolithic combination of perovskite and CIGS was highly challenging up to now as the CIGS surface is rather rough. By implementing a conformal hole transport layer, an efficient monolithic perovskite/CIGS tandem was realised. Absolute photoluminescence of the perovskite and CIGS sub-cells gives insights into the contributions to the tandem open-circuit voltage (Voc). To further improve the tandem efficiency, the Voc of perovskite top cells needs to be enhanced via reduction of non-radiative recombination at the interface between perovskite and the charge selective layers. This can either be done via proper interlayers^[2] or via fine-tuned charge selective contacts. Recently we have shown that self-assembled monolayers (SAM) could be implemented as appropriate hole selective contacts.^[3] The implementation of new SAM molecules enabled further reduction of non-radiative recombination losses with Voc’s up to 1.19 V and efficiency of 21.2% for perovskite single junctions with band gaps of 1.63 eV and 1.55 eV, respectively.  

The low defect density in halide perovskites (HaPs) is a welcome present for most device applications, one that has certainly increased the popularity of these materials. Still, we should pause a minute and ask what are the fundamental reasons that allow a material that can be made in “quick and dirty” (albeit not always reproducible) fashion at low temperatures (around RT), at times in ambient, as films or single crystals to have such low defect density (down to 10¹⁴ cm^-3 for solution-grown and 10¹³ cm^-3 for vacuum-deposited films and 10¹⁰ cm^-3 or less for single crystals). I note that these defect densities are all deduced from the common measurements for charged or neutral imperfections, which are indirect ones. Still also more direct, qualitative measurements, such as surface photovoltage, differential external quantum efficiency, scanning tunneling and photothermal deflection spectroscopies, all indicate GaAs-like or lower defect densities.

I hope to show in 3.5 months from writing these lines that this behaviour reflects a fundamental property of these materials, with a rather simple basis. The talk will combine experimental results from several sources, including our own, for thermodynamic, optical, and electrical data. It is plausible that our conclusions can be generalized to help look for other ultra-low defect density materials.

Since the first report on the high efficiency, stable solid-state perovskite solar cell (PSC) in 2012 by our group, following two seed works on perovskite-sensitized liquid junction solar cells in 2009 and 2011, perovskite photovoltaics have been triggered. AS a result, PSC demonstrated its power conversion efficiency (PCE) of 23.3% in 2018. According to Web of Science, publications on PSC increase exponentially since 2012 and total number of publications reaches already over 10,000 as of October 2018, which is indicative of a paradigm shift in photovoltaics. Although small area cell exhibited superb efficiency surpassing the performance of thin film technologies, scale-up technology is required toward commercialization. In addition, further higher efficiency toward Shockley–Queisser limit is required in parallel. In this talk, Large-area D-bar coating technology is introduced using perovskite cluster embedded coating solution, followed by brief introduction on history of perovskite solar cell. Bi-facial stamping method was developed for not only scale-up technique but also interface modification and low-temperature phase stabilization. For higher efficiency in large-area PSC, managing recombination is critical. Current-voltage hysteresis is also discussed because hysteresis is related to the stability of perovskite solar cell. Methodologies reducing recombination were developed via interface and bulk engineering.

Perovskites have emerged as low-cost, high efficiency photovoltaics with certified efficiencies of 22.1% 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 reproducibality 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 multicatio to multicomponent engineering providing a series of new compounds that are highly relevant candidates for the coming years.(6)

(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)

Hybrid organic-inorganic perovskites have become one of the most promising materials in the photovoltaic field. The best performance was found in compounds with general chemical formula, ABX3, when A is either organic or inorganic cation, like methylammonium (MA+), formamidinium (FA+) or Cs+, B is a bivalent metal cation, such as Pb2+ and X is a halide, Cl−, Br−, or I−. The amazing performance of ABX3 perovskites is attributed to their direct band gap, high absorption coefficient, long carrier diffusion length, hot carrier bottleneck, an ambipolar carrier transport property and low production costs. Perovskites have also been demonstrated as suitable materials for detecting visible light, x-ray, or γ-ray. While remarkable observations were reported in recent years, there is a little knowledge about their spin properties and their on the optical and magneto-optical of perovskite materials.

The present work describes a research that explored the band-edge properties of CsPbBr3 and MAPbBr3 perovskites, to elucidate the electronic origin for some of the unique phenomena. This compound was selected for the study due to its relative chemical and photochemical stability. The study focused on the investigation of single colloidal nanocrystals (NCs) as well as on bulk structures. The band-edge properties were examined by recording the linearly and circularly polarized micro-photoluminescence spectra in the presence of an external magnetic field up to 9 Tesla. The high resolution gained in the measure of a single NC enabled resolving fine split in the exciton emission at zero magnetic field, which grew nonlinearly with the increase of the magnetic field, a fact that indicated a deviation from a linear and from second order corrected Zeeman effects. Theoretical simulations, revealed the existence of a Rashba effect predominantly at field strength < 4 Tesla, explaining the non-linear behavior. Further on, the circular polarized measurement showed an asymmetry between the σ± components, suggesting a partial mixing of one of them with higher electronic states. The Rashba effect emanates in cases experiencing lack or breakage of inversion of symmetry and existence of spin-orbit coupling, both conditions presumably existing in the Perovskites materials, where the small cation A liability induces a crystal distortion and consequent inversion symmetry breaking.

Current observations on bulk CsPbBr3 and MAPbBr3 single crystal reflected similar observations for those viewed in analogous NCs, particularly when examined along unique crystallographic direction. In addition, the observations found in the bulk samples indicate a plausible contribution of cubic Rashba effect, which may indicate a mixing of high lying states to the band-edge properties and may support also the asymmetry in the σ± emission band patterns as was found in a single NC. This open question will be further investigated in the coming months. In any event, the Rashba effect split the band edge extrema to a k¹0 a Brillouin point with momentum forbidden transitions, thus, extending the carriers lifetime at the excited state with a large benefit in photovoltaic devices and in x-ray, or γ-ray detectors.

Organic inorganic hybrid perovskites (OIHP) is the most promising material to achieved high power conversion efficiency (PCE) at low cost. The high-quality optoelectronic properties in combination with solution-based preparation methods are responsible for the currently certified PCE record of 23.3%, which is close to the PCE of single crystal silicon solar cells (26.1%). OIHP is generally labelled as an ABX3compound, where A is a monovalent cation such methylammonium (MA), formamidinium (FA), or cesium (Cs), B is a divalent metal, such lead (Pb) or tin (Sn), and X is a halide anion, bromide (Br), iodide (I-). The properties of the perovskite film are direct related to film morphology, composition and crystalline structure, thus a clear understanding of how and when the intermediate and the perovskite phases are forming, as well the distribution of these multiple phases in the bulk and grains boundaries are important questions to be addressed in order to improve perovskite film properties and consequently the PCE of the devices. In this presentation, we will summarize our most recent results using in situtime-resolved grazing incidence wide angle x-ray scattering (GIWAXS) and synchrotron infrared nanospectroscopy (nano-FTIR). GIWAXS experiments allowed us to understand the influence of the relative humidity, type of solvent and time to drop the antisolvent during the preparation of mixed cation perovskite films. We also identified intermediates formed before and during the spin coating process. Nano-FTIR technique was applied for the first time on OIHP. Our results revealed a spatial heterogeneity of the vibrational signal, which are associated to different chemical composition. The nano-FTIR permitted to access grain-to-grain chemistry of OIHP and the identification of PbI2and hexagonal phases which are distributed randomly in a background formed by cubic (black phase) perovskite.

Preventing hysteresis and enhancing stability remain key challenges that could be resolved with the aid of judicious device design. We report numerical study of a solar cell model system that is based on a mixed electron-ion conducting perovskite active layer having various device configurations. In the full picture we allow for both mobile ions and the polarizability due to the easy-rotational methylammonium (MA). We then compare with cells where the MA rotation is frozen and/or the ions are non-existing.

We’ll first show that theses simulations can reproduce experimental results of a set of solar cells that was designed to enhance the open circuit voltage through enhancement of the built in potential. Next, several insights, resulting from such detailed simulations, will be presented.

For example: Even when there is no indication of hysteresis and the device’s characteristics can be modelled using ionic free model, the actual electron and hole distributions may be vastly different to the predictions by ionic free model. The low effective DOS promotes higher Voc but makes it more difficult to overcome energy level mismatch. These are related to the fact that the ionic motion is not only causing the hysteresis, it also allows for large deviations between electron and hole densities. Also, when a large energy mismatch exists between the BL and the perovskite the charge density distribution self-adapt to create an effective dipole at the interface. Such self-induced dipole can compensate for 0.4eV mismatch and thus prevent any loss in Voc. In this context formamidinium is preferred to MA.

Many of the physical and engineering aspects that govern the behavior of perovskite photovoltaics occur at interfaces. Especially, the metal oxide-perovskite interface is of great importance for inverted perovskite PV operation, and control of the interfacial materials properties is critical for high performance photovoltaics [1]. As an example, we have recently reported the synthesis and characterization of a low-temperature solution-processable monodispersed nickel cobaltite (NiCo₂O₄) as a hole transporting layer for inverted perovskite PVs. NiCo₂O₄ is a p-type semiconductor consisting of environmentally friendly, abundant elements and higher conductivity compared to NiO. We have shown that interfaces influence the perovskite grain boundaries formation and that control of interfacial energetics are very beneficial for inducing a desired functionality [2]. Hence, the development of intimate interfaces through their fundamental understanding and manipulation is expected to be crucial to the continued progress of perovskite PVs [3]. Up to now the main efforts of the perovskite research community have been focused on improving the power conversion efficiency (PCE). There are fewer reports on the degradation of perovskite PVs. We have recently reported long thermal stability of inverted perovskite photovoltaics [4] by incorporation of fullerene-based diffusion blocking layer (please see figure below) but the underlying degradation mechanisms of the anode-cathode metal oxide-based interfaces remain unclear. The presentation aims in covering a range of interfacial engineering concepts for high performance perovskite photovoltaics. A systematic understanding of the relationship between recently developed metal oxide interface materials [1,2], processing/ electrodes [2,3] and device performance [3,4] will be presented.

Perovskite solar cells have emerged as a new semi-transparent PV technology for urban infrastructures that demands an explicit trade-off between power conversion efficiency (PCE) and average visible transparency (AVT) which can be adjusted by various modifications in the absorber layer. Here, we introduce a scalable and facile “one and a half” step deposition route for mixed cation perovskite patterned in a sub-micron sized grid structure for semi-transparent solar cells. The initial perovskite phase is formed in one step using a grid pattern, while the additional step involves dipping of the pre-deposited perovskite grid in a hot solution of Formamidinium Iodide (FAI) in Isopropanol (IPA). Detailed analysis suggests that the additional step increases pore filling, crystal quality, grain size and lowers the content of residual PbI₂ along with improved photophysical properties. An average PCE~10%with AVT of 28% is attained with gold contact for the champion semi-transparent solar cell. The proposed deposition route can be generalized for all other types of perovskite based devices to yield better efficiency.

In experiments with concentrated sunlight, we previously demonstrated a strong effect of sample temperature on the photochemical decomposition of MAPbI3 thin films [1]. The latter was accompanied with degradation of the perovskite light absorption and growth of PbI2 peaks in the UV-Vis light absorption spectra and XRD patterns. Here, we report a systematic study of the initial stages of photodegradation of MAPbI3 thin films with independent control of the sample temperature and light intensity (from 50 to 700 suns). We demonstrated that photostability of the MAPbI3 film is extremely sensitive to the sample temperature. Under the combined action of light and heat (either by concentrated sunlight or by external heating), a strong reduction of the film photoluminescence (PL) without changes in the perovskite light absorption was observed during the initial stages of degradation. On the contrary, illumination of perovskite films (with intensity up to 500 suns) without heating (using chopped concentrated sunlight) induces considerable PL enhancement while the optical absorption spectrum remains unchanged. Underlying mechanisms for the observed effects are discussed on the basis of micrometer-scale Raman and PL mapping of the samples treated under various experimental conditions.

For some years organic-inorganic perovskites have attracted great interest due to their outstanding electrical and optical properties. Because of their large absorption coefficient, high carrier mobility, and long carrier diffusion length this class of materials is very attractive for opto-electronic applications such as light emitting devices and solar cells. Perovskite solar cells experienced a remarkable development in recent years. Their power conversion efficiency increased from single digit values to more than 22 %. Despite of the high-power conversion efficiencies organic-inorganic perovskites suffer from a number of instability mechanisms. To improve the stability of perovskite solar cells containing these absorbers a fundamental understanding of the governing mechanisms is advantageous.

In this paper we investigate the stability of halide perovskite absorber layers containing methylammonium (CH₃NH₃⁺ - MA) formamidinium (HC(NH₂)₂⁺ - FA), and cesium on the lattice sites of the organic cations. The specimens investigated contain one, two, or all three cations. The samples are prepared by state-of-the-art spin coating processes in nitrogen atmosphere. The thermal stability of the absorber layers is measured by gas evolution measurements. Here, the specimens are mounted in a vacuum tube and then the temperature is increased with a rate of 20 K/min, while gaseous species such as C-H_x and N-H_x are measured with a quadrupole mass-spectrometer. Perovskites containing only MA exhibit a thermal degradation threshold of about 70 °C. This threshold increases to larger temperatures for FAPbI₃ (T = 90 °C) and triple cation perovskites containing MA, FA, and Cs indicating that either the larger molecules for FA or changes in the lattice constants due to the presence of different cations results in a significant improvement of the thermal stability.

Further insight into the improved stability of the perovskites were gained from Fourier-transform infrared absorption (FT-IR) that were performed in vacuum and nitrogen atmosphere prior to illumination with visible and UV light and after the illumination experiments. Interestingly, FT-IR absorption shows the decomposition of the organic cations due to illumination at room temperature. In case of MA the N-H bonds dissociate, while for FA containing perovskites the N – C – N backbone breaks upon illumination. From a thermal point of view this is unexpected. The implication of these results for devices and routes to obtain more stable perovskites will be discussed

We discuss the discovery and recent developments of colloidal lead halide perovskite nanocrystals (LHP NCs_, NCs, A=Cs⁺, FA⁺, FA=formamidinium; X=Cl, Br, I) [1,2,3]. We survey the synthesis methods, optical properties and prospects of these NCs for optoelectronic applications [4,5].

  LHP NCs exhibit spectrally narrow (<100 meV, 12-45 nm from blue-to-near-infrared) sponaneous and stimulated emission, originating form bright triplet excitons [6], and tunable over the entire visible spectral region of 400-800 nm [1-4]. Post-synthestic chemical transformations of colloidal NCs, such as ion-exchange reactions, provide an avenue to compositional fine tuning or to otherwise inaccessible materials and morphologies [7]. Cs- and FA-based perovskite NCs are highly promising for backlighting of LCD displays, for light-emitting diodes and as precursors/inks for perovskite solar cells. In particular, high purity colloids are ideal for further engineering as needed for photochemical/photocatalytic applications. Towards these applications, a unique feature is that perovskite NCs appear to be trap-free without any electronic surface passivaiton [8], making photogenerated electrons and holes readily availably for surface chemical reactions.

  The processing and optoelectronic applications of perovskite NCs are, however, hampered by the loss of colloidal stability and structural integrity due to the facile desorption of surface capping molecules during isolation and purification. To address this issue, we have developed a new ligand capping strategy utilizing common and inexpensive long-chain zwitterionic molecules, resulting in much improved chemical durability [9].

   Perovskite NCs also readily form long-range ordered asssemblies known as superlattices. These assemblies exhibit accelerated coherent emission (superfluorescence) [10], not observed before in semiconductor nanocrystal superlattices.

References:

L. Protesescu et al. Nano Letters 2015, 15, 3692–3696

L. Protesescu et al. J. Am. Chem. Soc., 2016, 138, 14202–14205

L. Protesescu et al. ACS Nano 2017, 11, 3119–3134

M. V. Kovalenko et al. Science 2017, 358, 745-750

Q.A. Akkerman et al. Nature Materials, 2018, 17, 394–405

M. A. Becker et al, Nature, 2018, 553, 189-193

G. Nedelcu et al. Nano Letters 2015, 15, 5635–5640

M. I. Bodnarchuk et al. ACS Energy Lett., 2018, in press

F. Krieg et al. ACS Energy Letter., 2018, 3, 641–646

G. Raino et al. Nature 2018, in press

Metal halide perovskites are generating enormous interest for their use in optoelectronic devices including photovoltaics and light-emitting diodes. One of their most remarkable properties is their apparent defect tolerance – films can be produced using relatively crude processing methods yet they still exhibit very good device performance. Calculations have suggested that this is at least partly because many defects cause only shallow trap states which may not be catastrophic for device performance (unlike deeper trap states). Nevertheless, there is still substantial non-radiative losses suggesting defects are not entirely benign and they still must be understood and addressed before devices can approaches their performance limits.

Here, I will cover our ongoing work focusing on defects and their impact on non-radiative losses, as well as their mitigation through passivation treatments. I will present recent results in which we use multimodal approaches to determine relationships between local chemistry, structural and luminescence properties in perovskite thin films using synchrotron nano X-Ray Diffraction (n-XRD) and nano X-Ray fluorescence (n-XRF) measurements, as well as confocal and wide-field luminescence imaging. We reveal an intimate connection between strain and non-radiative decay, revealing these strain related defects as a primary origin of non-radiative losses. I will also outline the action of passivation treatments, such as chemical and light-induced treatments, on relieving these strain patterns.

The work provides a platform for designing new and more effective passivation post-treatments or film fabrication methods, which will push devices ever closer to their efficiency limits.

A description of the photoemission properties of perovskite thin films, microcrystals and nanocrystals, and their dependence with the chemical and optical environment, will be provided. The exposure of thin films and microcrystals to different atmospheres has a strong effect on their photoemission, which reveals relevant information about the mechanism behind ion migration, one of the most intriguing observations reported for these semiconductors.[1,2] A detailed analysis of the processes triggered by photoexcitation that lead to activation and latter deactivation of luminescence will be presented. Also, it will be shown that precise control of the spectral features of the luminescence, along with enhanced stability, can be achieved from perovskite nanocrystals synthesized in different types of nanoporous matrices.[3,4] Interestingly, under these conditions, increase of the photoluminescence quantum yield results from the improved optical environment they are exposed to.[5] All this opens the door to the development of novel color converting coatings of application in LED technology, as it will be demonstrated in this talk.

Halide perovskite photovoltaics are receiving a huge attention in the recent few years. Undoubtedly this attention is mainly due to the outstanding power conversion efficiencies, surpassing 23%, reported for photovoltaic devices, fabricated with polycrystalline films from low cost techniques. In addition, the combination of halide perovskite with Si solar cell in a tandem configuration has allowed to prepare devices with efficiency higher than 27%. The great success of halide perovskites boosted also the interest on the nanoparticles (NPs) of these materials. Perovskite NPs are also generating a huge interest as relative easy preparation methods yield a simple core structure, without need for passivating shells, reach photoluminescence quantum yield (PLQY) higher than 90%.This remarkable PLQY points to low non-radiative recombination and consequently shows excellent rationale for the development of solar cells. In this talk I show the interest of perovskite NPs in the development of photovoltaic devices highlighting the advantages and also the current limitations in order to produce high performance devices. The similarities and differences with standard bulk perovskite thin films are analyzed. Interestingly the use of NPs can help to overcome some limitations of bulk perovskites stabilizing new interesting crystalline phases or avoiding mixed halide ion migration.

In this talk we will describe a series of recent ultrafast spectroscopic studies of LHP materials. In the first we have used extreme time resolution pump-probe spectroscopy to characterize the earliest stages of free carrier generation in a thin film of organic lead iodide perovskite. We find an initial short lived state (~20 fs) which reflects localized hot carrier pairs, which evolve rapidly into delocalized free carriers inducing the well known absorption bleaching due to state filling.

In a second study, pump probe data in large cesium lead iodide nanocrustals is compared with that in a thin film of the same material. the results show essentially identical transient transmission spectra in both materials, proving the the gradual buildup of reduced transmission to the blue of the exciton band, assigned by others to light induced changes in reflectivity, are in fact due to induced absorption. The correct assignment of these features is crucial for the correct interpretation of the observed spectral changes.

Halide perovskites have attracted widespread attention for their reported defect tolerance, in stark contrast to traditional semiconductors. In this contribution, I will discuss nanoprobe X-ray microscopy investigations of the relationship between the defects that do appear in thin film hybrid perovskites and optoelectronic performance and stability. In these beam sensitive materials, the weaker X-ray matter interaction relative to E-beam microscopy enables X-ray microscopy to provide a close look at the nanoscale elemental and structural complexities of perovskites. Using a series of model thin film materials, we reveal a wide-ranging heterogeneity in local chemistry and structure and link these nanoscopic variations to impacts on charge collection. We provide direct species-specific evidence of halide migration and its close correlation with photoluminescence and use nano-diffraction to understand the effects of varying crystallinity in perovskite crystals at the nanoscale. Finally, I will share insights from our nanoprobe study of non-stoichiometry and second-phase formation in triple-and quaternary-cation perovskite solar cells to provide feedback for improving performance. By understanding and mitigating defects in the bulk and at interfaces, we aim to systematically accelerate the development of these optoelectronic materials.

Continuous wave (CW) operation of perovskite lasers is of critical importance for many applications. However, most reports on perovskite lasers solely investigate the lasing behavior under short pulsed excitation, since it was found that stimulated emission in methylammonium lead triiodide (MAPI) typically ceases after a few tens to hundreds of nanoseconds for unknown reasons [1, 2]. To date, sustained photon lasing in perovskite thin films was only clearly proved in a pump-induced tetragonal-orthorhombic mixed crystal phase in MAPI, which can only exist at a very special operation temperature [3].

We will present a study on triple cation perovskites (TCP), which shows that CW amplified spontaneous emission (ASE) can be achieved in these kinds of perovskites at any temperatures between 80 K to 140 K. Temperature dependent photoluminescence (PL) from 290 K to 80 K indicates that the phase of the TCP remains the same from room to cryogenic temperatures. A comparison between CW and pulsed excitation over a wide range of pump energies unambiguously demonstrates the ability of TCP to support CW ASE in a single crystalline phase. As the emission of our TCPs occurs from a single phase, the same crystalline phase that they possess at room temperature, this opens a route towards perovskite CW lasers operating at room temperature.

The investigations are performed on TCP, whose emission properties have been optimized by small stoichiometric deviations [4] and imprint lithography [5]. For nanosecond pulsed excitation, the ASE threshold of these TCP is found to decrease exponentially from 95 µJ/cm² at 290 K to 2.2 µJ/cm² at 80 K. The relations between the temperature dependent thresholds, the carrier lifetimes and the recombination mechanisms will be discussed. To support the conclusions, time and temperature dependent PL measurements will be presented. The ASE threshold for CW excitation at 80 K will then be shown to be 387 W/cm². The plausibility of such a CW threshold is discussed in the context of the Bernard-Duraffourg condition [6] and based on the results from the time dependent PL measurement. Furthermore, investigations on the ASE stability under CW excitation will be shown, which reveal that the CW ASE stability is similar to the previously reported ASE stabilities under pulsed excitation. Temperature dependent measurements of the ASE spectra show that ASE can be maintained up to 140 K, where the material degradation threshold drops below the CW ASE threshold. Possibilities for a further increase of the operation temperature will be discussed.

In this work, we consider the Br vacancy (V_Br) in cesium lead bromide (CsPbBr₃) as a prototype for the impact of structural dynamics on defect energetics in halide perovskites (HaPs). Using first principles molecular dynamics based on density functional theory, we find that the static picture of defect energetics breaks down; the energy of the V_Br level is found to be intrinsically dynamic, oscillating by as much as 1 eV on the ps time scale at room temperature. These significant energy fluctuations are found to be correlated with the distance between the neighboring Pb atoms across the vacancy and with the electrostatic potential at their atomic sites. The unusually strong coupling of structural dynamics and defect energetics bears important implications for both experimental and theoretical analysis of defect characteristics in HaPs. Furthermore, it may hold significant ramifications for carrier transport and defect tolerance in this important class of photovoltaic materials.

Is it possible to get into the bulk of a single crystal and replace a bromide atom with chlorine while keeping the rest of the crystal intact? Generally the answer is negative; nonetheless, herein we have discovered that some lead halide perovskite single crystals can do just this. Methylammonium lead bromide macroscale perovskite single crystals which were reacted in salt solutions of other perovskite suitable ions, namely, Cl^-, I^-, Cs⁺ and Sn²⁺ in proper solvents, exchanged the equivalent ions to a certain detectable degree. While the exchange of Pb²⁺ by Sn²⁺ was minute, chlorides could replace most bromides under ambient conditions. These crystals maintained their crystalline solid structure and initial morphologies throughout the reactions. The optical properties of the crystals changed with the composition and probing the cleaved faces of the reacted single crystals clarified which ions are present in the bulk. Further experiments in the solid phase revealed halide exchange among bromide and iodide based crystals which were ground together. Even though swift ion exchange reactions were identified among various perovskite nanocrystals before, clear differences between nanocrystals ion exchange and this research findings are evident. The experiments, characterizations, and theory behind the results will be discussed.

The outstanding efficiencies of organic-inorganic perovskite thin film based solar cells led to interest in increasing the range of perovskite materials. One such family of materials is the all inorganic CsPbX3 colloidal nanocrystals (NCs). Despite the recent surge of synthetic protocols producing different shapes and crystal structures, there are still significant gaps in the understanding of their formation mechanism. Here we try to address the growth mechanism and reveal the importance of the ligand shell on determining the size, habit and phase.

We have identified that the formation of CsPbX3 NCs follows through two separate stages. First, seed mediated nucleation through the formation of metal Pb NCs. Second, further growth is attained through oriented attachment. We show the impact of delicate changes in the ligand environment on the stoichiometry and crystal structure of cesium lead halide perovskites NCs. We show that small changes in the oleate:ammonium ratio generated by the addition of a Lewis base determines the size, shape and crystallographic structure of CsPbX3 NCs. Using this understanding we could synthesize materials such as CsPbBr3 nanowires, CsPbCl3 bulk-like crystals and CsPbI3 orthorhombic nanowires of length ranging from 200 nm to several microns. Moreover, we show how a robust reversible transformation that involves also a stoichiometric change, from cubic CsPbX3 to rhombohedral Cs4PbX6 which can be achieved via control of the OA to OLAm Brønsted acid−base type equilibrium. We present surface analysis revealing the differences in the ligand shell of cubic CsPbBr3 versus Cs4PbBr6. In addition, we show evidence that the transition mechanism involves an exfoliation and recrystallization processes. This mechanism is supported by crystallographic data showing a Cs4PbBr6, 0D layered rhombohedral phase. The formation of a layered Cs4PbBr6 habit indicates a direct path for transformation that is controlled by thermodynamic surface stabilization provided by the ligand shell.

Finally, we discuss the use of these materials in hybrid semiconductor systems.

All-inorganic perovskite lead halide semiconductors in the form of colloidal nanocrystals have recently caused a stir as an excellent class of materials for optoelectronic applications [1]. Their advantages range from extremely high photoluminescence efficiencies up to 90%, narrow and tunable emission spectra, facile solution deposition on arbitrary substrates, to the presence of surface-capping ligands for further electronic and optical adjustments. An additional feature of this material family stimulated developments in the field of multi-photon optics: Nanocrystals based on all-inorganic cesium lead bromide (CsPbBr₃) perovskite colloidal quantum dots exhibit a giant two-photon absorption cross section in the order of 2·10⁵ GM [2], inspiring applications on low-threshold multi-photon pumped stimulated emission and lasing. Nevertheless, high irradiance levels are generally required for such multi-photon processes. One strategy to enhance the multi-photon absorption is taking advantage of high local light intensities using photonic nanostructures.

In this study, we investigate the two-photon excited photoluminescence of CsPbBr₃ perovskite quantum dots with 9.4 nm size interacting with the leaky modes of a silicon photonic crystal slab with a hexagonal nanohole geometry. [3]. By systematic excitation of optical resonances using a pulsed near-infrared laser beam (fwhm = 40 nm), we observe an enhancement of two-photon-excited photoluminescence by more than one order of magnitude when comparing to using a bulk silicon film. Experimental and numerical analyses allow relating these findings to near-field enhancement effects on the nanostructured silicon surface. We show that the electric field distributions close to the photonic crystal surface can be numerically classified and optimized by means of machine learning techniques [4]. The results reveal a promising approach for significantly decreasing the required irradiance levels for multi-photon processes being of advantage in applications like biomedical imaging, lighting and solar energy.

Electronically coupled systems constructed out of recently developed Lead halide based Perovskite nanocrystals and Quantum dots could be prospective smart hybrid materials for next generation photovoltaic and optoelectronic devices due to their mutually synergistic interactions. In our present work, we have studied the nature of electronic coupling and photoinduced charge transfer dynamics occurring in CsPbBr3-CdSe nanoplatelet hybrid system being cross-linked with various short chain aliphatic (Glycine) and aromatic (p-Aminobenzoic acid) molecular linkers. Combining time-resolved photoluminescence spectroscopy and transient absorption measurements in correlation with in-situ photoresponse X-ray photoelectron spectroscopy, we provide comprehensive information on the effect of charge distribution modifications, realignment of band edge potentials and subsequent change in exciton binding energy in our hybrid films. These hybrid systems generated an effective p-n heterojunction due to the charge distribution with a built in electric field as controlled by the molecular linkers across the interface between these two disparate materials. Our results demonstrate the pivotal role taken by organic linkers in such hybrid assemblies, and shed new light on past results on perovskite-QD hybrid systems. As such, it establishes new insight on hybrid heterojunctions with potential applications in the design of hybrid devices.

Solution-processed organic−inorganic and all-inorganic lead halide perovskite semiconductors have rapidly progressed as versatile and promising materials for a number of optoelectronic applications. Facile solution processing allows for various nano- and microstructures to be fabricated out of perovskite materials of different compositions. The optimization of the performance of potential devices would critically depend on the assessment and understanding of fundamental photophysical properties and processes in perovskites and their dependence on the composition. The intrinsic quantum yield of the photon emission and the character of the emission processes: e.g., free electron-hole recombination vs. excitonic recombination, are among such fundamental properties. Related to them is the nature of the long-range spatial propagation of photoexcitations, which might occur via diffusion of charge carriers and via reabsorption of photons emitted during recombination (photon recycling).

In this work, we use time- and spectrally-resolved double-objective photoluminescence (PL) spectroscopy to directly demonstrate photon reabsorption taking place in cesium-based lead halide perovskite microwires of different compositions, including CsPbBr₃ [1]. We vary the spatial separation between the excitation and collection objectives focused on a single microwire and observe the appearance of time-delayed PL emission at separations exceeding 100 microns from the excitation spot. In independent measurements, we find that the mobility of charge carriers in the studied materials is low enough for the charge carrier diffusion to be irrelevant for observed spatiotemporal evolution of PL. As the separation increases, the data shows a clear pattern of the rise times in PL dynamics thus signifying its origin in the photon reabsorption assisted by the emitted light trapping in the microwire waveguides. We also observe the PL spectrum developing red shifts increasing with the separation. The details of these signatures are found to depend on the composition of the wire material. We observe that the composition can dramatically alter the excitation power dependence of the PL: from the power dependence corresponding to free electron-hole recombination to the one corresponding to the excitonic (or geminate) recombination.

We compare experimental data to the results of a quantitative kinetic model developed to account for contributions of mono- and bimolecular recombination in the presence of waveguide-assisted emission-reabsorption events. This comparison enables us to assess the intrinsic quantum yield of elementary radiative recombination events.

Lead halide perovskites nanocrystals (LHP NCs) exhibit extraordinary light emission properties rendering the materials highly promising for photonic applications. Extensive work on fully inorganic CsPbX₃ NCs with X being Cl, Br, I has already produced impressive results in amplified spontaneous emission and lasing across the visible with thresholds that stand on par with record values reported in traditional colloidal nanocrystals systems.

Almost all such studies, implemented ultrashort photon pulses in the femtosecond regime to initiate the stimulated emission process. Yet the realization of practical lasing applications is crucially dependent on the materials ability to sustain optical amplification at significantly longer timescales, at which major losses associated with spontaneous emission and non-radiative recombination occur. Herein we discuss experiments in which efficient amplified spontaneous emission (ASE) from formamidinium lead iodide perovskite (FAPbI₃) NC films under excitation in the nanosecond regime, is demonstrated¹. Systematic optimization of the processing and thermal treatment of the perovskite NCs, yields solids that exhibit high ASE net modal gain and low ASE thresholds with relatively weak dependence on sample temperature. Furthermore we find evidence that the stimulated emission build-up is readily influenced by the slowdown of the interband relaxation of hot carriers in FAPbI₃ NCs, probed in earlier studies from our group². Our experiments indicate that the retarded carrier relaxation dynamically compete with hot carrier recombination processes such as hot carrier luminescence with the complex interplay influenced by a number of material and experimental parameters such as temperature, material purity, film quality and excitation wavelength.