Metal halide perovskites of the general formular ABX₃ where A is a monovalent cation such as caesium, methylammonium or formamidinium, B stands for divalent lead, tin or germanium and X is a halide anion, have shown great potential as light harvesters for thin film photovoltaics. Amongst a large number of compositions investigated, the cubic α-phase of formamidinium lead triiodide (FAPbI₃) has emerged as the most promising semiconductor for highly-efficient and stable perovskite solar cells (PSCs). Maximizing the performance of α-FAPbI₃ has therefore become of vital importance for perovskite research. Using formate ions to mitigate lattice defects and to augment film crystallinity, we attain a power conversion efficiency of 25.6 % (certified 25.2%). We further enhanced light capture and largely suppress non-radiative recombination at the ETL-perovskite interface, enabling PSCs with a PCE of 25.7% (certified 25.4%) and excellent operational stability as well as intense electroluminescence with external quantum efficiencies of 12.5%. Our findings provide a facile access to solution processable films with unprecedented opto-electronic performance.

Halide perovskite semiconductors can merge the highly efficient operational principles of conventional inorganic semiconductors with the low‑temperature solution processability of emerging organic and hybrid materials, offering a promising route towards cheaply generating electricity as well as light. Following a surge of interest in this class of materials, research on halide perovskite nanocrystals (NCs) as well has gathered momentum in the last years. While most of the emphasis has been put on CsPbX₃ perovskite NCs, more recently the so-called double perovskite NCs, having chemical formula A⁺₂B⁺B³⁺X₆, have been identified as possible alternative materials, together with various other metal halides structures and compositions, often doped with various other elements. This talk will also discuss the research efforts of our group on these materials. I will highlight how for example halide double perovskite NCs are much less surface tolerant than the corresponding Pb-based perovskite NCs and that alternative surface passivation strategies will need be devised in order to further optimize their optical performance.

Perovskite solar cells (PSCs) have reached power conversion efficiency (PCE) of 25.6% [1], for active areas smaller than 1 cm². Even if their operational stability is also improving [2-3], this still represent a key aspect to be tacked to for their industrial development.

Lately, we have demonstrated highly efficient, large area, planar PSCs where the MAPbI₃ perovskite layer has been deposited by thermal co-evaporation of PbI₂ and MAI. The high-quality co-evaporated perovskite thin films are uniform over large areas showing low surface roughness, and a long carrier lifetime. The high-quality perovskite thin films together with vacuum processed charge transport layers PSCs with PCE above 20% in both n.i.p [4, 5] and p.i.n [6] configurations and mini-modules achieved record PCEs of 18.13% and 18.4% for active areas of 20 cm² [4] and 6.4 cm² [7].

Thermal stability is a critical criterion for assessing the long-term stability of PSCs. We have shown that un-encapsulated co-evaporated MAPbI₃ PSCs [8] demonstrate remarkable thermal stability even in an n-i-p structure that employs Spiro-OMeTAD as hole transport material (HTM). MAPbI₃ PSCs maintain over ≈95% and ≈80% of their initial PCE after 1000 and 3600 h respectively under continuous thermal aging at 85 °C. Co-evaporated MAPbI₃ PSCs demonstrate remarkable structural robustness, absence of pinholes, or significant variation in grain sizes, and intact interfaces with the HTM, upon prolonged thermal aging. Here, the main factors driving the  co-evaporated MAPbI₃ stability are assessed. It is demonstrated that the excellent co-evaporated MAPbI₃ thermal stability is related to the perovskite growth process leading to a compact and almost strain-stress-free film. On the other hand, un-encapsulated PSCs with the same architecture, but incorporating solution-processed MAPbI₃ or Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃ as active layers, show a complete PCE degradation after 500 h under the same thermal aging condition. These results highlight that the control of the perovskite growth process can substantially enhance the PSCs thermal stability, besides the chemical composition.

The co-evaporated MAPbI₃ impressive long-term thermal stability features the potential for field-operating conditions.

References:

1. NREL. Best Research-Cell Efficiency Chart; U.S. Department of Energy; https://www.nrel.gov/pv/cell-efficiency.htm.

2. S. Yang, S. Chen, E. Mosconi, Y. Fang, X. Xiao, C. Wang, Y. Zhou, Z. Yu, J. Zhao, Y. Gao, Science 2019, 365, 473.

3. Y. Wang, T. Wu, J. Barbaud, W. Kong, D. Cui, H. Chen, X. Yang, L. Han, Science 2019 365, 687.

4. J. Li, H. Wang, X. Y. Chin, H. A. Dewi, K. Vergeer, T. W. Goh, J. W. M. Lim, J. H. Lew, K. P. Loh, C. Soci, T. C. Sum, H. J. Bolink, N. Mathews, S. Mhaisalkar, A. Bruno, Joule 2020, 4, 1035 

5. J Li, HA Dewi, W Hao, N Mathews, S Mhaisalkar, A Bruno, Coatings, 2020 10(12), 1163

6. J Li, HA Dewi, W Hao, J Zhao, N Tiwari, N Yantara, T Malinauskas, V Getautis, T J. Savenije, N Mathews, S. Mhaisalkar, A Bruno, Adv. Funct. Mater. 2021.

7. L Li, HA Dewi, W Hao, L Jia Haur, N Mathews, S Mhaisalkar, A Bruno, Solar RRL, 2020, 4, 2000473

8. HA Dewi, L Li, W Hao, N Mathews, S Mhaisalkar, A Bruno, Adv. Funct. Mater. 2021 2100557,

Recently, there has been a lot of interest in stabilizing organic-inorganic hybrid perovskite solar cells. Sputtering ITO on top of devices has shown to be an effective barrier layer for increasing device stability of planar PSCs.[1], [2] Sputtered ITO stabilizes the devices by suppressing the ingression of moisture, egression of methylamine (MA) cation, metal migration, and corrosion of metal electrodes by interaction with halides in the perovskite. However, sputtering on top of the perovskite solar cell is often reported as a process that can damage the beneath layers.[3] Sputtering damage is generally attributed to the hard bombardment of cathode-generated particles onto the substrate causing the damage. Moreover, the UV/plasma generated during the sputtering process is also linked to "sputter damage". To tackle this damage metal oxide buffer layer is generally used. However, the metal oxide is still prone to damage depending on the sputtering parameters.[4] Here, we made an attempt to understand and decouple the two damages. Further, an in-depth investigation on the role of thickness, target power density, and working pressure was optimized to demonstrate highly efficient stable perovskite solar cells. To demonstrate the efficacy of this optimization, cells and modules were realized achieving improved efficiencies and stabilities. Further, we show that employing such holistic optimization has wide implications in single-junction, semi-transparent, and tandem applications where ITO sputtering is used as the top electrode on PSC.

Despite tremendous interest in inverted architecture perovskite solar cells the PCE of such devices lags behind, with maximum values rarely approaching 23%. The generally lower performance of inverted architecture perovskite solar cells is mainly associated with reduced current extraction and nonradiative recombination losses which limit the device photovoltage and fill factor. The development of strategies to overcome such limitations has been subject to intense research, including interface and bulk passivation of traps, such as crystallographic defects, point defects or higher dimensional defects (at grain boundaries). Predominantly, these approaches led to an improvement in device performance due to increased open-circuit voltage (VOC), with fill factors (FF) remaining in the range of 75-80%. This highlights the need to develop new methods that would simultaneously improve the VOC and the FF of inverted architecture devices. Herein, we demonstrate an innovative strategy for the dual modification of both the HTL/perovskite and perovskite/ETL interfaces by introducing a series of large organic cations: 2-phenylethylammonium iodide (PEAI), 4-chloro-phenylethylammonium iodide (Cl-PEAI) and 4-fluoro phenylethylammonium iodide (F-PEAI) in both these interfaces. Despite this class of cations being commonly utilized for the formation of low dimensional perovskites, in our work, we adopt a different strategy and use a very low concentration to modify both interfaces of inverted perovskite solar cells. We find that this approach does not change the bulk perovskite crystal structure or its of dimensionality, but rather improves the interfaces by facilitating high-quality film formation on top of the HTL and inducing efficient defect passivation at the perovskite/ETL interface. We show that the modification of the buried bottom interface leads a more homogenous film formation and the elimination of nano-voids at the perovskite/HTL interface. These improvements result in a significant increase in the fill factor accompanied by an increase in the short-circuit current (JSC). The modification of the top perovskite surface, on the other hand, leads to its efficient passivation, resulting in a substantial increase in the VOC. Importantly, the implementation of both modifications at the same time results in a simultaneous increase in all of the photovoltaic parameters leading to a superior device performance. As a result, we achieve a high PCE of 23.7%, with a net improvement of device VOC up to 1.184 V and very high FF of 85%.

Perovskite solar cells (PSC) with printable carbon-graphite back electrodes are promising candidates for commercialization of perovskite devices thanks to their low processing costs and extraordinary stability. However, due to the lack of hole selective layers, this device architecture still suffers from severe performance losses at the perovskite/carbon electrode interface. ^[1]

Following recent advances in interface passivation by 2D perovskites,^[2] we report on a novel approach to employ 2D perovskites as an electron blocking hole conducting layer for an efficient surface passivation of the perovskite absorber successfully enhacing the quality of  the corresponding interface with the carbon back electrode. This was achieved through incorporating an octylammonium iodide (OAI) salt on a FAPbI₃ 3D perovskite absorber. Through X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), ultraviolet photoelectron spectroscopy (UPS) and microscopically resolved photoluminescence (PL) microscopy measurements we confirm the formation of a high band gap 2D perovskite layer at the interface between the perovskite and the carbon electrode which is found to behave as an electron blocking layer. We find that the 2D passivation layer does not only increase the steady state photovoltage by 65 mV, but also the fill factor (FF) by 10% absolute. This leads to a champion device yielding a power conversion efficiency (PCE) of 18.5%, which is to the best of our knowledge, the highest reported for HSL-free perovskite solar cells using low temperature carbon back electrodes. Comparative illumination dependent J_SC-V_OC analysis and electrochemical impedance spectroscopy (EIS) measurements were carried out to gain more insights on the reduced interfacial recombination losses. We demonstrate minimized interfacial resistance and non-radiative recombination limitations. Maximum power point tracking (MPPT) measurements demonstrated that the 2D perovskite electron blocking layer also led to an improved device stability, with the hole selective layer-free carbon electrode base PSC still performing at over 82% of its initial normalized efficiency after 500 hours of continuous 1-sun illumination.

We thus highlight the tremendous potential of 2D perovskites in enhancing the photovoltaic performance and stability of hole selective layer free perovskite solar cells. We  demonstrate its electron blocking characteristic which we believe would help pave the way for the future practical development of fully printable carbon based perovskite devices.  

Metal halide perovskite solar cells have reached the recent efficiency breakthrough of 25.6%, higher than silicon polycrystalline photovoltaics. Such fantastic result was only possible due to a precise control and engineering of the morphology, interfaces and the use of multiple cations in perovskite A-site, as Rb, Cs, MA (methylamonnium) and FA (formamidinium). For tandem perovskite solar cells, a mixture of different anions, as Br and I is also desired to adjust the band gap. Such cocktail of different cations and anions influences the formation of intermediates, new phases, favours halide homogenization, etc; so that at the end, the efficiency of the device is closely related to not only the optical quality of the film (e.g. crystallinity), but morphology and composition.

In this presentation, we will summarize important results using in situ experiments to probe perovskite formation (2D and 2D+3D), stability and composition. We employed time-resolved grazing incidence wide angle x-ray scattering (GIWAXS), small angle x-ray scattering (SAXS), high-resolution XRD, and photoluminescence (PL) techniques taken at the Brazilian Synchrotron National Laboratory, SSRL-Stanford and Molecular Foundry.

We used standard organic cations as butylammonium (BA), phenylethylammonium (PEA) and 2-thiophenemethylammonium iodide (2-TMAI) to synthesise pure 2D and 2D+3D halide perovskites. For PEA and 2-TMAI, the threshold from the passivation effect to 2D formation was different for the cations, also reflecting in a different number of slabs (n) of octahedra layers. Data about stability of the 2D/3D structure under external stressors such as temperature, humidity, and O2 exposure will also be presented.

Hybrid organic-inorganic perovskites remain one of the most promising semiconductor materials in photovoltaics, yet their instability under operating conditions obstructs their application.^[1]  To address this challenge, we rely on supramolecular tools to purposefully tailor noncovalent interactions with organic components that template hybrid perovskite frameworks, such as through π-based interactions and host-guest complexation, which has been uniquely assessed by solid-state NMR spectroscopy and crystallography.^[2–4] In addition, we employ these strategies to form low-dimensional perovskite architectures with enhanced functionalities that further alter the stabilities of materials as well as their photovoltaic performances.^[2,5] As a result, we have achieved perovskite solar cells with superior operational stabilities without compromising their performances,^[2,3] which provides fundamental insights for a versatile strategy in advancing perovskite photovoltaics.

Reference:

[1] M. H. Futscher, J. V. Milić, Front. Energy Res. 2021, 9, 629074.

[2] M. A. Hope et al. J. Am. Chem. Soc. 2021, 143, 1529.

[3] H. Zhang et al. Nat. Commun. 2021, 12, 3383.

[4] P. Ferdowsi et al. J. Phys: Mater. 2021, 4, 042011.

[5] J. Milić, J. Mater. Chem. C 2021, 9, 11428.

The emergence of metal halide perovskites has revolutionized the field of emerging photovoltaics. Typically, the active layers of perovskite solar cells are deposited either from solution, or alternatively by thermal evaporation. In this talk, I will describe how the two methods can be combined to fabricate highly efficient all-inorganic CsPbI₃ perovskite solar cells. Specifically, half of the active layer is deposited by solution processing, following by thermally evaporating the second half. While devices fabricated by each method separately show a reasonable performance of 13-15%, the combination of the two methods leads to perovskite solar cells with improved open-circuit voltage, short-circuit current and fill factor, leading to a maximum photovoltaic performance >20%. Moreover, devices fabricated by this hybrid approach exhibit a significantly reduced hysteresis. We show that the improvement in performance and decrease in hysteresis are associated with a reduced density of defects in the active layers fabricated by the hybrid approach, thus demonstrating its high potential for the fabrication of efficient perovskite solar cells.

Perovskite based diodes using both vacuum and solution processed materials will be presented. Our group has developed several perovskite based solar cells, using vacuum based perovskite preparation methods reaching power conversion efficiencies as high as 20 % in a planar single junction device and similar performance in tandem devices.  Driving these devices under forward bias (in LED modus) allows to obtain important device characteristics. The endeavors related to highly luminescent perovskites and their integration into light-emitting diodes will also be commented upon. The integration of perovskite diodes between semitransparent metal electrodes leads to cavity resonance behavior  which can be tuned by the thickness of the perovskite layer. Using photoluminescence as a detection tool we study the effect of interface materials on the light emission properties of different perovskite thin films prepared using vacuum sublimation techniques. Using combination of organic charge transport layers and perovskite emitters the photoluminescence quantum yield is increased.

Metal halide perovskites are among the most promising materials for next generation photovoltaics, combining a convenient solution processability at mild temperature with excellent power conversion efficiencies, as over 25% in lab-scale devices.[1] Nonetheless, finding a truly scalable manufacturing process and improving the durability of perovskite materials under operational conditions are the next technological challenges to be faced before perovskite solar cells can enter the market.

The inherent and main limitations interfering with perovskite solar cell large-scale production are related to a critical material deposition/reproducibility, as the film formation occurs throughout a complex self-assembly process driven by weak interactions. Herein, we act on the perovskite material by using polymers as cooperative assembling component.[2-4] The use of polymers with pendant hydroxyl/hydroxyethyl groups, which can interact with the perovskite precursors already in solution, allowed to massively interfere with nucleation probabilities and growth rates of perovskite crystals, leading to the formation of compact and uniform film via a single straightforward coating step.[2,3] We found that the organic polymeric nature and the non-covalent interactions between adjacent chains confers superior flexibility and moisture stability to the perovskite-polymer films, enabling the composite material to accommodate a strain, whilst maintaining transport properties suitable for devices,[3] thus very attractive for flexible photovoltaics. A judicious polymer selection can in addition allow to modulate perovskite films transparency and improve its tolerance to thermal stress.[4]

Finally, we demonstrate that though the use of polymeric rheological modifier the viscosity of perovskite-polymer inks could be easily modulated and adapted to the requirements different up-scalable printing techniques. Overall, the superior film forming properties of polymeric materials guarantee the deposition of perovskites on large area flexible substrates without the use of the antisolvent-bath, thus significantly simplifying the large-scale processing.[5,6]

One of the main issues hindering the commercialization of perovskite photovoltaics (PV) is device stability. A myriad of various strategies to stabilize the perovskite layer against degradation under continuous illumination has been demonstrated in perovskite community, such as perovskite passivation with 2D perovskites, organic hydrophobic layers, ionic liquids, as well as the deployment of robust inorganic charge transport layers. However, for commercially established PV technologies, the most detrimental degradation occurs in fact under reverse bias. A cell in a module can be placed under reverse bias due to significant current mismatch between series-interconnected cells, which typically happens under operational conditions when shading takes place. Such situation causes the non-shaded solar cells to act as a reverse-bias voltage source on the shaded cell, causing it to operate at negative voltages, whereas the energy is dissipated by Joule heating. This phenomenon is commonly known as “hot-spot” degradation and is considered to be one of the most severe degradation mechanisms even in crystalline silicon PV. Concomitantly, only few studies on the reverse bias degradation in perovskite PV devices have been published, all of them revealing that this is a severe source of degradation. Thus, reverse bias degradation presents one of the most fundamental challenges for current research and future commercializing not for only single-junction perovskite PV modules, but also tandem architectures.

In this work, we demonstrate that perovskite PV devices with mesoscopic scaffold and carbon-based electrode have outstanding resilience against reverse bias degradation and are able to withstand negative voltages up to -9V. The presence of chemically inert carbon electrode, utilization of single-halide mixed-dimensional 2D/3D perovskite and robust inorganic charge transport layers help to avoid commonly-occurring issues in state-of-the-art cells, like localized melting of metal electrodes, ion interdiffusion and halide segregation. Looking more in-depth at the nature of reverse bias degradation in PSCs we demonstrate that the issue of iodine loss still prevails even in such stable devices. Low activation energy of I^- vacancies causes an accumulation of charges at the interfaces, resulting in significant bend bending at the interfaces between perovskite and charge-transport layer. Under reverse bias, the bending increases until the so-called “breakdown voltage” is reached beyond which holes are able to tunnel into the valence band of perovskite. This hole-tunneling oxidizes incorporated iodine to create iodine vacancies and thermodynamically favorable iodine compounds, which decompose the perovskite structure. We present direct evidence that iodine loss and associated device degradation if it is exposed to reverse-bias for long time durations. Moreover, for reverse bias voltages exceeding -9V, we overserved via thermographic imaging in-operando formation of hotspots and thermal degradation of perovskite into PbI₂.

To demonstrate that modules with carbon electrodes are be able to withstand hot-spot conditions, we manufactured modules with carbon back electrodes of 10x10cm² size and 11.1% power conversion efficiency and subjected them to requirements of IEC61215 hot-spot test at an accredited laboratory Fraunhofer ISE PV Modules Testlab, which has never been done before according to our knowledge. Finally, the modules were able to pass the conditions of the IEC tests for c-Si and thin-film technologies confirming that mesoscopic scaffold and carbon electrodes provide effective stabilization strategy for perovskite PV devices against reverse-bias degradation. [1] This work for the first time demonstrates that perovskite-based modules can withstand severe reverse bias, which is an essential milestone for this emerging technology on its path towards commercialization.

In the search for stable perovskite photovoltaic technology, carbon-based perovskite solar cells (C-PSCs) represent a valid, stable solution for near-future commercialization. However, a complete understanding of the operational device stability calls for assessing the device robustness under thermal stress. Herein, the device response is monitored upon a prolonged thermal cycle aging (heating the device for 1 month up to 80 °C) on state-of-the-art C-PSCs, often neglected, mimicking outdoor conditions. Device characterization is combined with in-house-developed advanced modeling of the current–voltage characteristics of the C-PSCs using an iterative fitting method based on the single-diode equation to extrapolate series (R_S) and shunt (R_SH) resistances. Two temperature regimes are identified: Below 50 °C C-PSCs are stable, and switching to 80 °C a slow device degradation takes place. This is associated with a net decrease of the device R_SH, whereas the R_S is unaltered, pointing to interface deterioration. Indeed, structural and optical analyses, by means of X-ray diffraction and photoluminescence studies, reveal no degradation of the perovskite bulk, providing clear evidence that perovskite/contact interfaces are the bottlenecks for thermal-induced degradation in C-PSCs.

Ruddlesden-Popper low dimensional halide perovskites (2D) have recently attracted researchers’ attention thanks to their unique excitonic properties and superior environmental stability, triggering different exploitation in LED, photodetector, and photovoltaic devices. These materials differ from their 3D counterpart due to the presence of a bulky organic cation that separates the perovskite octahedra in n-dimensional layers connected each other by weak interactions, offering an ideal playground for easily modulating optical and structural properties by compositional and chemical modifications.

In this work, 2D perovskite with different halides composition (I^-, Br^-, Cl^- mixtures) have been synthesized by mean of two different bulky organic cations (thiophene methylammonium -TMA, and thiophene ethylammonium -TEA) demonstrating the easy tunability of the optical properties of the materials as well as modifications in their crystalline structures. Indeed, halide substitution not only progressively modulates the bandgap, but it also proved to be a powerful tool to control phase segregation by rationally adjusting the halide composition, and therefore the spatial distribution of recombination at the nanoscale.

This turned out in the engineering of thin films of chloride-rich 2D perovskites, which appear transparent to the human eye, with tunable and intense emission in the green. This achievement is due to the substitution of Cl^- with the bulkier I^- in the halide site, that lead to structural distortion and spontaneous segregation resulting into a spatial distribution of phases where the minor component is responsible for the tunable emission, as identified by combined hyperspectral photoluminescence imaging and elemental mapping. This work could pave the way for the next generation of highly tunable transparent emissive materials which can be used as light emitting pixels in advanced and low-cost optoelectronics.

The perovskite solar cells have been extensively developed since 2009 but still the ultrafast processes occurring at the interfaces in that system are not fully understood. The main tool providing wide view of a charge transport behavior is transient absorption spectroscopy. We focused on the investigation of electron transport paths: after light absorption, an electron is promoted from the valence to the conduction band, and then several processes can occur. First few hundreds of femtosecond after light absorption are governed by cooling of the hot carriers. When the process is finished, sharp bleach due to the band filling phenomena occurs at absorption edge in the transient absorption measurements. Its decay correlates with photoluminescence kinetics and represents the excited carrier lifetime. That decay proceeds by several paths such as the recombination (first-, second- and third-order) and the charge injection to an electron transporting material (ETM) or a hole transporting material (HTM).       

Despite the fact that scientific world has been better understanding processes within perovskite material, we are still quite far from description of a full cell (with gold electrodes) behavior at working conditions i.e. with applied voltage and under light illumination. Therefore, I will present our recent spectroscopic findings, mainly, the femtosecond transient absorption measured through different thickness gold electrodes in full PSC. The measurements were taken with only femtosecond laser illumination as well as with applied additional potential and with 1Sun illumination during experiments. I will also describe differences in carrier transport behavior between perovskite/ETM and perovskite/HTM interfaces.

Hybrid AMX₃ perovskites (A=Cs, CH₃NH₃; M=Sn, Pb; X=halide) have in the last years revolutionized the scenario of emerging photovoltaic technologies. Despite the extremely fast progress, the materials electronic properties which are key to the photovoltaic performance are relatively little understood. Density Functional Theory electronic structure methods have so far delivered an unbalanced description of Pb- and Sn-based perovskites. We developed an effective GW method incorporating spin-orbit coupling[1] which allows us to accurately model the electronic, optical and transport properties of halide perovskites, opening the way to new materials design. In particular, the different CH₃NH₃SnI₃ and CH₃NH₃PbI₃ electronic properties are discussed in light of their exploitation for solar cells and found to be dominantly due to relativistic effects. By applying our computational approach, we moved to investigate the effect of the chlorine doping for the mixed halide perovskites (MAPbI_3-xCl_x)[2] and the role of the different A cation.[3] In parallel, a series of computational simulation carried out using Car-Parrinello molecular dynamics have been performed investigating the nature of the perovskites/TiO₂ interface,[4] the role of moisture in the perovskite degradation[5] process and the effect of the defect on the device working mechanism.[6] Finally, a series of different strategies will be reported to increase the device stability and efficiency.[7] The overall picture of our theoretical investigations underlines a crucial role of computational investigation, casting the possibility of performing predictive modeling simulations, in which the properties of a given system are simulated even before the materials laboratory synthesis and characterization. At the same time, computer simulations are shown to offer the required atomistic insight into hitherto inaccessible experimental observables.

Like any other semiconductors, the defects in halide perovskites determine the performance and long-term stability of the resulting devices. Understanding their electronic properties and their impact on chemical stability and the interplay of the two are paramount. In this talk, I will present our recent findings on these aspects using atomistic simulations from DFT and reactive molecular dynamic simulations. By determining the electronic levels and dynamical properties of several types of defect, we identify harmful defects that lead to recombination losses and chemical degradations. We show several strategies (composition engineering, adding additives, optimizing crystallization kinetics, interface engineering, etc.) in mitigating and passivating these defects.

Energetic disorder in crystalline and amorphous solids is composed of static (structural) and dynamic (vibrational) disorder. The latter depends on phonon occupation and hence scales with the temperature. The temperature dependence of energetic disorder can be described within the framework of Einstein solid model. Temperature-dependent inverse slope of the logarithm of absorption spectrum (Urbach energy) is required to determine these components and to obtain the static disorder. With recent advances in the sensitivity of photocurrent spectroscopy methods providing signal to noise ratios exceeding 90 dB, we determined temperature dependent Urbach energy in lead halide perovskites containing different number of cation components. Urbach energies are obtained with a precision of approximately 1 meV dictated by inevitable optical interference. Using a single mode vibrational solid model we obtain static energetic disorder of 7.1±0.6 meV and 7.3±0.3  meV for single and triple cation perovskites respectively and 5.0±1.7 meV for the double cation system. These value are close to but not smaller than that in inorganic compound semiconductors such as III-V compounds. We also reveal the contribution of mid-gap trap states in the sub-gap quantum efficiency spectra with spectral line shapes heavily affected by the optical interference and hence providing no information about the energy or distribution of the trap states in contrast to previous reports.

The capability of matter to store light energy under the form of electronic excitations constitutes the most fundamental of a series of phenomena classified as “light-matter interactions”. Among them the quantum framework in which an atom or a molecule interacts with a resonant cavity is a very unique system to study the light-matter interaction. A wisely-tailored resonant cavity can enhance the spontaneous emission rate of a fluorophore by Purcell effect [1,2] and in such a scenario, the exciton lifetime is significantly reduced; usually, a resonant cavity is prepared sandwiching a dielectric medium between two metal layers with a metal/dielectric/metal (MIM). On the other side, there is a plethora of applications in which longer exciton lifetimes are required, as for the photovoltaics. Provided such assumptions, it is easy to understand the reason why such two concurrent light-matter interaction regimes are rarely found together in the same technological design. Despite this, we will describe a particular system in which photovoltaic and Purcell effect coexist and their competition can be engineered together to obtain unique properties.

Indeed, we prepared a “Metavoltaic” cell from the fusion of MIM and photovoltaic devices: we used a MaPbI₃ perovskite based photovoltaic cell as dielectric and the two metals forming the cavity also worked as electrodes of the embedded photovoltaic cell. In this system, two regimes coexist and compete so that it is possible to probe the light-matter interaction via a modification of the photovoltaic effect. In particular we studied how the external quantum efficiency (EQE) of the photovoltaic cell measured at different incidence angles of the light can be affected by the cavity resonance modes due to the cavity-atom interaction. Our fundamental investigations provide new insight on the ever-growing interesting field of light-matter interactions, at the same time opening to the possibility to endow photovoltaic systems with an angular dispersion, a property that can help overcoming light-harvesting limitations.

The orthorhombic gamma-phase of CsPbI₃ is the photoactive Perovskite (PSK) with the simplest stoichiometry to be used in tandem with Silicon Solar Cells. It has an energy gap of ~1.75-1.78 eV, well complementing the absorption range of a Silicon.  In addition, it is a fully inorganic Perovskite that does not suffer from degradation through the formation of volatile species, and indeed the risk of mass loss during operation or during ageing is intrinsically prevented.

Nonetheless, bottlenecks on the use of gamma-CsPbI₃ are the formation temperature that requires fast quenching from 320 °C and the intrinsic instability towards its non-photoactive yellow delta-polymorphism, both of which deserve special attention.

Herein, the convenient interplay between Eu incorporation and morphology to form the gamma-phase of CsPbI₃ at 80 C (LT) is unveiled. In contrast, pure CsPbI₃ without Eu is a mixture of gamma-phase and non-PSK delta-phase at 65 C or it is a fully delta-phase at 80 C.

Based on experimental and theoretical findings, we argued about a double beneficial role of Eu. On one hand, it assists in the formation of the gamma-phase either by substituting Pb or by occupying interstitial positions in the CsPbI₃ lattice. On the other hand, it indirectly promotes the formation of a fine-grained layer at LT wherein the high surface-to-volume ratio makes the establishment of the delta-phase unfavourable. Strain accommodation in the fine-grained matrix and the formation of a gluing intergrain-self-material during the kinetics of reaction (snowplow effect) cooperate in extending the lifetime of the LT gamma-phase to ~40 h at 65 C compared to only ~10–15 min in the sample without Eu for the complete phase transformation.

The disclosed phenomena draw a method for the stabilization of the gamma-CsPbI₃ phase that can be further exploited or improved,

Hybrid organic/inorganic perovskites are very versatile materials in terms of composition. Variations within the Goldschmidt’s tolerance factor generally lead to fully miscible structure, which in some cases allow for a continuous composition tuning. In this work we show that two fully miscible phases still exhibit different surface adsorption enthalpies for different substrates, which can lead to microscopic phase separation.

The study focused on the interaction between mixed perovskites and Si/SiO₂ wafer surface partially covered by graphene. We considered two possible variations of the composition: exchange of the organic cation (Piperonylammonium, PipA⁺ and methylammonium, MA⁺) or the anion (iodide and bromide). The obtained films were characterized by photoluminescence spectra. Next, a periodically modulated surface was created using photolithography to prepare a patterned graphene surface on which perovskite mixtures were crystallized. The composition variation of the perovskite assembly was investigated by correlating the photoluminescence spectra with the pattern size. The experiments with cation-varied perovskite mixtures revealed that the structure rich in MA⁺ formed preferentially on SiO₂ while the structure rich in PipA⁺, due to aromatic nature, outcompeted MA+ on graphene. In case of anion varied perovskite mixtures, it is known that the ratio between iodine and bromine in MAPbBr_3-xI_x can be changed continuously (0≤x≤3) and the phases are fully miscible. When we deposited a mixture having 1:1 molar ratio between iodide and bromide perovskite precursors on large-area graphene on SiO₂, the photoluminescence spectra corresponded to the iodine rich phase on both surfaces with only a small amount of MAPbBr₃ observable exclusively on graphene. However, on graphene on SiO₂ sample patterned into narrow stripes, significantly different intensity of the bromide- vs. iodide-rich phase was observed on the two substrates. We have thoroughly investigated the pattern size influence and found out that the feature size of 50 μm is the threshold above which no phase separation can be observed. We speculate that this is the mean diffusion path limit of precursor species during the perovskite crystallization. As a result, phase-separation is observed at shorter distances while uniform mixed phase dominates at longer ones. The study discovered several observations that can spark new directions in microstructured systems involving mixed perovskites and modulated surfaces.