Combined in Situ Photoluminescence and X-ray Scattering Reveals Defect Formation in Lead-Halide Perovskite Films

Collaborative Fabrication of High-Quality Perovskite Films for Efficient Solar Modules through Solvent Engineering and Vacuum Flash System

The vacuum flash solution method has gained widespread recognition in the preparation of perovskite thin films, laying the foundation for the industrialization of perovskite solar cells. However, the low volatility of dimethyl sulfoxide and its weak interaction with formamidine-based perovskites significantly hinder the preparation of cell modules and the further improvement of photovoltaic performance. In this study, we describe an efficient and reproducible method for preparing large-scale, highly uniform formamidinium lead triiodide (FAPbI3) perovskite films. This is achieved by accelerating the vacuum flash rate and leveraging the complex synergism. Specifically, we designed a dual pump system to accelerate the depressurization rate of the vacuum system and compared the quality of perovskite film formed at different depressurization rates. Further, to overcome the limitations posed by DMSO, we substituted N-methylpyrrolidone as the ligand solvent, creating a stable intermediate complex phase. After annealing, it can be transformed into a uniform and pinhole-free FAPbI3 film. Due to the superior quality of these films, the large area perovskite solar module achieved a power conversion efficiency of 22.7% with an active area of 21.4 cm2. Additionally, it obtained an official certified efficiency of 22.1% with an aperture area of 22 cm2, and it demonstrated long-term stability.

Thermal Stability of Chalcogenide Perovskites

Chalcogenide perovskites (CPs) have recently attracted interest as a class of materials with practical potential in optoelectronics and have been suggested as a more thermally stable alternative to intensely studied halide perovskites (HPs). Here we report a comparative study of the thermal stability of representative HPs, MAPbI3 (MA = CH3NH3+, methylammonium) and CsPbI3, and a series of CPs with compositions BaZrS3, β-SrZrS3, BaHfS3, SrHfS3. Changes in the crystal structure, chemical composition, and optical properties upon heating in air up to 800 °C were studied using thermogravimetric analysis, temperature-dependent X-ray diffraction, energy-dispersive X-ray spectroscopy, and diffuse reflectance spectroscopy. While HPs undergo phase transitions and thermally decompose at temperatures below 300 °C, the CPs show no changes in crystal phase or composition when heated up to at least 450 °C. At 500 °C CPs oxidize on time scales of several hours, forming oxides and sulfates. The structural origins of the higher thermal and phase stability of the CPs are discussed.

Evolution of Defects, Morphology, and Strain during FAMAPbI3 Perovskite Vacuum Deposition: Insights from In Situ Photoluminescence and X-ray Scattering

At present, the power conversion efficiency of single-junction perovskite-based solar cells reaches over 26%. The further efficiency increase of perovskite-based optoelectronic devices is limited mainly by defects, causing the nonradiative recombination of charge carriers. To improve efficiency and ensure reproducible fabrication of high-quality layers, it is crucial to understand the perovskite nucleation and growth mechanism along with associated process control to reduce the defect density. In this study, we investigate the growth kinetics of a promising narrow bandgap perovskite, formamidinium methylammonium lead iodide (FAMAPbI3), for high-performance single-junction solar cells. The temporal evolution of structural and optoelectronic properties during FAMAPbI3 vacuum codeposition was inspected in real time by grazing-incidence wide-angle X-ray scattering and photoluminescence. Such a combination of analytical techniques unravels the evolution of intrinsic defect density and layer morphology correlated with lattice strain from the early stages of the perovskite deposition.

Modular slot-die coater for in situ grazing-incidence x-ray scattering experiments on thin films

Multimodal in situ experiments during slot-die coating of thin films pioneer the way to kinetic studies on thin-film formation. They establish a powerful tool to understand and optimize the formation and properties of thin-film devices, e.g., solar cells, sensors, or LED films. Thin-film research benefits from time-resolved grazing-incidence wide- and small-angle x-ray scattering (GIWAXS/GISAXS) with a sub-second resolution to reveal the evolution of crystal structure, texture, and morphology during the deposition process. Simultaneously investigating optical properties by in situ photoluminescence measurements complements in-depth kinetic studies focusing on a comprehensive understanding of the triangular interdependency of processing, structure, and function for a roll-to-roll compatible, scalable thin-film deposition process. Here, we introduce a modular slot-die coater specially designed for in situ GIWAXS/GISAXS measurements and applicable to various ink systems. With a design for quick assembly, the slot-die coater permits the reproducible and comparable fabrication of thin films in the lab and at the synchrotron using the very same hardware components, as demonstrated in this work by experiments performed at Deutsches Elektronen-Synchrotron (DESY). Simultaneous to GIWAXS/GISAXS, photoluminescence measurements probe optoelectronic properties in situ during thin-film formation. An environmental chamber allows to control the atmosphere inside the coater. Modular construction and lightweight design make the coater mobile, easy to transport, quickly extendable, and adaptable to new beamline environments.

Halide Segregated Crystallization of Mixed-Halide Perovskites Revealed by In Situ GIWAXS

Mixed-halide perovskites of the composition MAPb(BrxI1-x)3, which seem to exhibit a random and uniform distribution of halide ions in the absence of light, segregate into bromide- and iodide-rich phases under illumination. This phenomenon of halide segregation has been widely investigated in the photovoltaics context since it is detrimental for the material properties and ultimately the device performance of these otherwise very attractive materials. A full understanding of the mechanisms and driving forces has remained elusive. In this work, a study of the crystallization pathways and the mixing behavior during deposition of MAPb(BrxI1-x)3 thin films with varying halide ratios is presented. In situ grazing incidence wide-angle scattering (GIWAXS) reveals the distinct crystallization behavior of mixed-halide perovskite compositions during two different fabrication routes: nitrogen gas-quenching and the lead acetate route. The perovskite phase formation of mixed-halide thin films hints toward a segregation tendency since separate crystallization pathways are observed for iodide- and bromide-rich phases within the mixed compositions. Crystallization of the bromide perovskite phase (MAPbBr3) is already observed during spin coating, while the iodide-based fraction of the composition forms solvent complexes as an intermediate phase, only converting into the perovskite phase upon thermal annealing. These parallel crystallization pathways result in mixed-halide perovskites forming from initially halide-segregated phases only under the influence of heating.

Perovskite Crystallization Regulation via Antimonene Quantum Sheets for Highly Efficient and Stable Solar Cells

The two-step deposition method offers significant advantages in the production of high-performance planar perovskite solar cells (PSCs). Nevertheless, there are still numerous challenges in regulating perovskite crystallization during the two-step process. In this work, two-dimensional (2D) material antimonene quantum sheets (AMQSs) as an additive are introduced to regulate the crystallization process of perovskite. As a result, perovskite films with high crystalline quality and vertical growth orientation are obtained by AMQSs providing heterogeneous nucleation sites with the penetration of a mixture solution of AMQSs and FAI into the PbI2 layer. Also, the influence mechanism of AMQSs on the crystallization of perovskite film is analyzed in details. At the same time, due to the chemical interaction between antimonene and the uncoordinated Pb2+, the defects in the perovskite are efficiently passivated. In addition, the energy level at the perovskite/SnO2 interface becomes more matched, leading to improved charge transport and extraction with the incorporation of AMQSs. Benefiting from the versatile AMQSs, the power conversion efficiency (PCE) of PSCs made by PbI2 + FAI:AMQSs is improved from 20.65 to 22.31% with the vastly enhanced Jsc and Voc. The ambient and operational stability of the unencapsulated PSCs fabricated using the PbI2 + FAI:AMQSs method were significantly improved, retaining 80% of the original PCE after being stored in a dark environment at a relative humidity of 30-40% for 18 days and 83% of the original PCE following continuous AM 1.5G illumination for 200 h.

Evolution of Structure and Optoelectronic Properties During Halide Perovskite Vapor Deposition

The efficiency of perovskite-based solar cells has increased dramatically over the past decade to as high as 25%, making them very attractive for commercial use. Vapor deposition is a promising technique that potentially enables fabrication of perovskite solar cells on large areas. However, to implement a large-scale deposition method, understanding and controlling the specific growth mechanisms are essential for the reproducible fabrication of high-quality layers. Here, we study the structural and optoelectronic kinetics of MAPbI₃, employing in-situ photoluminescence (PL) spectroscopy and grazing-incidence small/wide-angle X-ray scattering (GI-SAXS/WAXS) simultaneously during perovskite vapor deposition. Such a unique combination of techniques reveals MAPbI₃ formation from the early stages and uncovers the morphology, crystallographic structure, and defect density evolution. Furthermore, we show that the nonmonotonous character of PL intensity contrasts with the increasing volume of the perovskite phase during the growth, although bringing valuable information about the presence of defect states.

Life on the Urbach Edge

The Urbach energy is an expression of the static and dynamic disorder in a semiconductor and is directly accessible via optical characterization techniques. The strength of this metric is that it elegantly captures the optoelectronic performance potential of a semiconductor in a single number. For solar cells, the Urbach energy is found to be predictive of a material's minimal open-circuit-voltage deficit. Performance calculations considering the Urbach energy give more realistic power conversion efficiency limits than from classical Shockley-Queisser considerations. The Urbach energy is often also found to correlate well with the Stokes shift and (inversely) with the carrier mobility of a semiconductor. Here, we discuss key features, underlying physics, measurement techniques, and implications for device fabrication, underlining the utility of this metric.

A synergistic effect of the ion beam sputtered NiOxhole transport layer and MXene doping on inverted perovskite solar cells

The synergistic effect of high-quality NiO_xhole transport layers (HTLs) deposited by ion beam sputtering on ITO substrates and the Ti₃C₂T_xMXene doping of CH₃NH₃PbI₃(MAPI) perovskite layers is investigated in order to improve the power conversion efficiency (PCE) of p-i-n perovskite solar cells (PSCs). The 18 nm thick NiO_xlayers are pinhole-free and exhibit large-scale homogeneous surface morphology as revealed by the atomic force microscopy (AFM). The grazing-incidence x-ray diffraction showed a 0.75% expansion of the face-centered cubic lattice, suggesting an excess of oxygen as is typical for non-stoichiometric NiO_x. The HTLs were used to fabricate the PSCs with MXene-doped MAPI layers. A PSC with undoped MAPI layer served as a control. The size of MAPI polycrystalline grains increased from 430 ± 80 nm to 620 ± 190 nm on the doping, as revealed by AFM. The 0.15 wt% MXene doping showed a 14.3% enhancement in PCE as compared to the PSC with undoped MAPI. The energy-resolved electrochemical impedance spectroscopy revealed one order of magnitude higher density of defect states in the band gap of MXene-doped MAPI layer, which eliminated beneficial effect of reduced total area of larger MAPI grain boundaries, decreasing short-circuit current. The PCE improvement is attributed to a decrease of the work function from -5.26 eV to -5.32 eV on the MXene doping, which increased open-circuit voltage and fill factor.