Enhanced optoelectronic quality of perovskite films with excess CH3NH3I for high-efficiency solar cells in ambient air

Colloidal FAPbBr3 perovskite nanocrystals for light emission: what's going on?

Semiconducting nanomaterials have been widely explored in diverse optoelectronic applications. Colloidal lead halide perovskite nanocrystals (NCs) have recently been an excellent addition to the field of nanomaterials, promising an enticing building block in the field of light emission. In addition to the notable optoelectronic properties of perovskites, the colloidal NCs exhibit unique size-dependent optical properties due to the quantum size effect, which makes them highly attractive for light-emitting diodes (LEDs). In the past few years, perovskite-based LEDs (PeLEDs) have demonstrated a meteoritic rise in their external quantum efficiency (EQE) values, reaching over 20% so far. Among various halide perovskite compositions, FAPbBr3 and its variants remain one of the most interesting and sought-after compounds for green light emission. This review focuses on recent progress in the design and synthesis protocols of colloidal FAPbBr3 NCs and the emerging concepts in tailoring their surface chemistry. The structural and physicochemical features of lead halide perovskites along with a comprehensive discussion on their defect-tolerant properties are briefly outlined. Later, the prevalent synthesis, ligand, and compositional engineering strategies to boost the stability and photoluminescence quantum yield (PLQY) of FAPbBr3 NCs are extensively discussed. Finally, the fundamental concepts and recent progress on FAPbBr3-based LEDs, followed by a discussion of the challenges and prospects that are on the table for this enticing class of perovskites, are reviewed.

One-step electrodeposition of CuSCN/CuI nanocomposite and its hole transport-ability in inverted planar perovskite solar cells

The synthesis of CuSCN/CuI nanocomposite by single-step electrodeposition is developed. The surface morphology and film thickness are controlled by changing the electrochemical potential and deposition time. The mixed-phase formation of CuSCN/CuI is confirmed through x-ray diffraction and Raman spectral analysis. Nanopetal (NP) like morphology of CuSCN/CuI is observed in FESEM micrographs. Interestingly, the NPs density and thickness are increased with increasing the deposition potential and time. The device fabricated using CuSCN/CuI nanocomposite as a hole transport layer (HTL) which is grown for 2 min delivers the best photovoltaic performance. The maximum power conversion efficiency of 18.82% is observed for CuSCN/CuI NP with a density of 1153μm^-2and thickness of 142 nm. The charge transfer ability of the CuSCN/CuI NP HTL is analyzed by electrochemical impedance spectroscopy. Based on the observation, moderate charge transport resistance and optimum film thickness are required for achieving maximum photovoltaic performance in perovskite solar cells (PVSCs). Thus, the developed CuSCN/CuI NP HTL is a potential candidate for PVSCs.

Antisolvent engineering on low-temperature processed CsPbI3 inorganic perovskites for improved performances of solar cells

CsPbI₃ inorganic perovskites with ideal bandgap and much enhanced thermal stability compared with organic-inorganic hybrid perovskites, have attracted much interest in the field of solar cells. The performances of solar cells highly depend on the quality of perovskite films, yet the research on fabrication methods of inorganic perovskites is far below that of organic-inorganic hybrid counterparts. Antisolvent engineering is a widely used method in controlling the morphology and crystallinity of organic-inorganic hybrid perovskites. Its effect varies with parameters such as the physicochemical properties of antisolvents and the compositions of perovskite precursors. Specially, there lacks a comprehensive study comparing different antisolvents used in low-temperature processed CsPbI₃ from dimethylammonium-based precursors. In this work, we used three different antisolvents to control the growth of CsPbI₃ films in a low-temperature (<200 °C) processed procedure and systematically compared the properties of resultant films. The green antisolvent ethyl acetate (EA) engineered CsPbI₃ films exhibit improved morphology and crystallinity as well as reduced defects, compared with the counterparts processed without antisolvent or those with widely employed toxic antisolvents toluene and chlorobenzene. The EA antisolvent engineering results in efficient CsPbI₃ perovskite solar cells with a champion power conversion efficiency of 8.8%. Our work thus provides a green and viable way to prepare high quality CsPbI₃ perovskite films for optoelectronic applications.

Photostable and Uniform CH3NH3PbI3 Perovskite Film Prepared via Stoichiometric Modification and Solvent Engineering

Solution-processed organometal halide perovskites (OMHPs) have been widely used in optoelectronic devices, and have exhibited brilliant performance. One of their generally recognized advantages is their easy fabrication procedure. However, such a procedure also brings uncertainty about the opto-electric properties of the final samples and devices, including morphology, stability, coverage ratio, and defect concentration. Normally, one needs to find a balanced condition, because there is a competitive relation between these parameters. In this work, we fabricated CH₃NH₃PbI₃ films by carefully changing the ratio of the PbI₂ to CH₃NH₃I, and found that the stoichiometric and solvent engineering not only determined the photoluminescence efficiency and defects in the materials, but also affected the photostability, morphology, and coverage ratio. Combining solvent engineering and the substitution of PbI₂ by Pb(Ac)₂, we obtained an optimized fabrication condition, providing uniform CH₃NH₃PbI₃ films with both high photoluminescence efficiency and high photostability under either I-rich or Pb-rich conditions. These results provide an optimized fabrication procedure for CH₃NH₃PbI₃ and other OMHP films, which is crucial for the performance of perovskite-based solar cells and light emitting devices.

MAPbI3 Quasi-Single-Crystal Films Composed of Large-Sized Grains with Deep Boundary Fusion for Sensitive Vis-NIR Photodetectors

Perovskite single-crystal (SC) or quasi-single-crystal (QSC) films are promising candidates for excellent performance of photoelectric devices. However, it is still a great challenge to fabricate large-area continuous SC or QSC films with proper thickness. Herein, we propose a pressure-assisted high-temperature solvent-engineer (PTS) strategy to grow large-area continuous MAPbI₃ QSC films with uniformly thin thickness and orientation. Dramatic grain growth (∼100 μm in the lateral dimension) and adequate boundary fusion are realized in them, vastly eliminating the grain boundaries. Thus, remarkable diminution of the trap density (n_trap: 7.43 × 10¹¹ cm^-3) determines a long carrier lifetime (τ₂: 1.7 μs) and superior photoelectric performance of MAPbI₃-based lateral photodetectors; for instance, an ultrahigh on/off ratio (>2.4 × 10⁶, 2 V), great stability, fast response (283/306 μs), and high detectivity (1.41 × 10¹³) are achieved. The combination properties and performance of the QSC films surpass most of the reported MAPbI₃. This effective approach in growing perovskite QSC films points out a novel way for perovskite-based optoelectronic devices with superior performance.

Detrimental Effect of Unreacted PbI2 on the Long-Term Stability of Perovskite Solar Cells

Excess/unreacted lead iodide (PbI₂ ) has been commonly used in perovskite films for the state-of-the-art solar cell applications. However, an understanding of intrinsic degradation mechanisms of perovskite solar cells (PSCs) containing unreacted PbI₂ has been still insufficient and, therefore, needs to be clarified for better operational durability. Here, it is shown that degradation of PSCs is hastened by unreacted PbI₂ crystals under continuous light illumination. Unreacted PbI₂ undergoes photodecomposition under illumination, resulting in the formation of lead and iodine in films. Thus, this photodecomposition of PbI₂ is one of the main reasons for accelerated device degradation. Therefore, this work reveals that carefully controlling the formation of unreacted PbI₂ crystals in perovskite films is very important to improve device operational stability for diverse opto-electronic applications in the future.

Off-Stoichiometric Methylammonium Iodide Passivated Large-Grain Perovskite Film in Ambient Air for Efficient Inverted Solar Cells

Hot-casting is a promising technique of depositing high-quality organic-inorganic hybrid perovskite thin films with large crystal grain size. Here, we reported the that the crystallinity and grain size of perovskite films could be systematically tailored by modulating the stoichiometry of the precursor solution in the hot-casting process under ambient condition with a relative humidity of 40%. It was found that a slight excess of methylammonium iodide (MAI) in the precursor solution could effectively compensate the MAI loss due to the high substrate temperature. A significant increase in the grain size and crystallinity of the perovskite film was observed together with a decrease in defect density and a carrier concentration enhancement in MAI-rich samples. The corresponding devices exhibited a notable increase in fill factor (up to 80.70%) and short-circuit current density. In addition, in MAI-deficient samples, an enrichment of PbI₂ at the grain boundaries was directly observed by optical microscopy and laser confocal microscopy. Time-resolved photoluminescence spectroscopy revealed an increase in the charge carrier lifetime in the MAI-deficient samples, which was in line with the previous results with a small amount of excess PbI₂ in the perovskite film. This work highlights a new strategy to prepare high-quality perovskite thin films with excellent crystal quality under ambient condition.