A Facile Surface Passivation Enables Thermally Stable and Efficient Planar Perovskite Solar Cells Using a Novel IDTT‐Based Small Molecule Additive

The degradation of the perovskite solar cell structure was expected recently to be reversible, which opened a new gate to the enhancement of the device lifetime by reversing the process. However, the kinetic details of the structural collapse and recovery are still missing, without which the perovskite reversibility cannot be further explored. Due to the experimental difficulty, a purposeful numerical model was conducted in this report, to simulate the water diffusion process in the perovskite structure in both directions. It was found that the moisture diffusion needs to be initiated by a certain level of structural imperfection and is non-Fickian, as assisted by the collapse of the perovskite into the 1D chains. The reversibility was verified by the back diffusion, but accompanied by hysteresis, stagnancy, and even surprising instability, which initiated the water flow under initial equilibrium, due possibly to the imbalance during the reconstruction of the perovskite lattice. These observations offer new insights to form strategies of improvement, for example, via the possible self-healing perovskite devices.

Hysteresis and Instability Predicted in Moisture Degradation of Perovskite Solar Cells.

A comprehensive study regarding the effect of different solvent vapours on organolead halide perovskite properties is lacking. In the present work, the impact of exposing CH3NH3PbI3 films to the vapours of commonly available solvents has been studied. The interaction with perovskite has been correlated to solvent properties like dielectric constant, molecular dipole moment, Gutmann donor number and boiling point. Changes in the crystallinity, phase, optical absorption, morphologies at both nanometer and micrometer scale, functional groups and structures were studied using X-ray diffraction, UV-visible absorption, FE-SEM, FTIR and Raman spectroscopies. Among the aprotic solvents DMSO and DMF vapours deteriorate the crystallinity, phase, and optical, morphological and structural properties of the perovskite films in a very short time, but due to the difference in solvent property values acetone affects the perovskite properties differently. Polar protic 2-propanol and water vapours moderately affect the perovskite properties. However 2-propanol can solvate the organic cation CH3NH3+ more efficiently as compared to water and a considerable difference was found in the film properties especially the morphology at the nanoscale. Nonpolar chlorobenzene vapour minutely affects the perovskite morphology but toluene was found to enhance perovskite crystallinity. Solvent properties can be effectively used to interpret the coordination ability of a solvent. The present study can be immensely useful in understanding the effects of different solvent vapours and also their use for post-deposition processing (like solvent vapour annealing) to improve their properties.

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Investigating the effect of solvent vapours on crystallinity, phase, and optical, morphological and structural properties of organolead halide perovskite films

2D Ruddlesden‐Popper (RP) perovskites (R‐NH3)2An−1BnX3n+1 with tailorable photovoltaic features of low toxicity, reduced dimensionality, and defect tolerance hold promise to overcome the main constraints of perovskite solar cells (PSCs). A systematic investigation of the structural and electronic properties of pure and mixed (R‐NH3)2Pb1−xGexI4 systems was performed with and without spin‐orbit coupling (SOC), by using density functional theory (DFT). The organic spacer cations (R‐NH3+)2 of methylammonium (MA‐CH3NH3+) and Phenylethyl ammonium (PEA‐C6H5[CH2]2NH3+) with x = 0.0, 0.25, 0.50, 0.75, 1.0, resulted in the compositionally modified 32‐permutations by systematically substituting the inorganic cations. Short‐range ordering has categorized all studied compositions of (R‐NH3)2Pb0.50Ge0.50I4 into column and battenberg schemes depending on the Ge defect site. The competing effects of octahedral tilting and lattice distortion with bandgap bowing and SOC, across Pb‐Ge compositional systems, provide a systematic strategy to tune bandgaps. By controlling the cations' composition, these mixed 2D‐perovskites outlined the strategy to achieve the desired bandgap and band positions. A rational design approach of low dimensional perovskites was adopted to tune the electronic properties through the spacer and inorganic cation engineering. Our findings expose the scope of monolayer‐based systems as an ideal choice of absorber for the bottom sub‐cell in all‐perovskites 2D‐3D tandem solar cell, harvesting light from visible (2.4 eV) up to infrared (1.2 eV) in the solar spectrum with features of defect tolerance, quantum and dielectric confinement, low bandgap, low effective masses, and high mobility.

Theoretical investigation of octahedral tilting and bandgap non‐linearity in monolayer Ruddlesden‐Popper A2Pb1‐xGexI4 perovskites

Flexible and printed perovskite solar cells (PSCs) fabricated on lightweight plastic substrates have many excellent potential applications in emerging new technologies including wearable and portable electronics, the internet of things, smart buildings, etc. To fabricate flexible and printed PSCs, all of the functional layers of devices should be processed at low temperatures. Tin oxide is one of the best metal oxide materials to employ as the electron transport layer (ETL) in PSCs. Herein, the synthesis and application of SnO2 quantum dots (QDs) to prepare the ETL of flexible and printed PSCs are demonstrated. SnO2 QDs are synthesized via a solvothermal method and processed to obtain aqueous and printable ETL ink solutions with different QD concentrations. PSCs are fabricated using a slot-die coating method on flexible plastic substrates. The solar cell performance and spectral response of the obtained devices are characterized using a solar simulator and an external quantum efficiency measurement system. The ETLs prepared using 2 wt% SnO2 QD inks are found to produce devices with a high average power conversion efficiency (PCE) along with a 10% PCE for a champion device. The results obtained in this work provide the research community with a method to prepare fully solution-processed SnO2 QD-based inks that are suitable for the deposition of SnO2 ETLs for flexible and printed PSCs.

https://www.mdpi.com/2079-4991/12/15/2615/pdf?version=1659348337

Solution-Processed SnO2 Quantum Dots for the Electron Transport Layer of Flexible and Printed Perovskite Solar Cells

Single crystalline perovskite solar cells (PSC) are promising for their inherent stability due to the absence of grain boundaries. While the development of single crystals of perovskite with enhanced optoelectronic properties is known, studies on the growth, device performance and understanding of the intrinsic stability of single crystalline perovskite thin film solar cell devices fabricated on electron selective contacts are scarcely explored. In this work, we examine the impact of mesoporous TiO2 (m-TiO2) and planar TiO2 (p-TiO2) on the growth of single crystalline-methyl ammonium lead iodide (SC-MAPbI3) film, PSC device performance and film stability under harsh weather conditions (T ∼ 85 °C and RH ∼ 85%). Self-grown SC-MAPbI3 films are developed on m-TiO2 and p-TiO2 by inverse temperature crystallization under ambient conditions without the need for sophisticated glove-box processing. The best device with m-TiO2 as an electron transport layer showed a promising power conversion efficiency of 3.2% on an active area of 0.3 cm2 in hole transport material free configuration, whereas, only 0.7% was achieved for the films developed on p-TiO2. Complete conversion of precursor to perovskite phase and better surface coverage of the film leading to enhanced absorption and reduced defects of single crystalline perovskite on m-TiO2 compared to its p-TiO2 leads to this large difference in efficiency. Mesoporous device retained more than 70% of its initial performance when stored at 30 °C under dark for more than 5000 h at 50% RH; while the planar device degraded after 1500 h. Thermal and moisture endurance of SC-MAPbI3 films are investigated by subjecting them to temperatures ranging from 35 °C to 85 °C at a constant relative humidity (RH) of 85%. X-ray diffraction studies show that the SC-MAPbI3 films are stable even at 85 °C and 85% RH, with only slight detection (30–35%) of PbI2 at these conditions. This study highlights the superior stability of SC-MAPbI3 films which paves way for further studies on improving the stability and performance of the ambient processed PSCs.

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Stability of MAPbI3 perovskite grown on planar and mesoporous electron-selective contact by inverse temperature crystallization

Perovskite solar cells (PSCs) made their mark in the photovoltaic research community due to their accelerating efficiencies, bandgap tunability, ease of fabrication, choice of substrates, etc. The electron transport layer (ETL) plays a decisive role in determining the performance and scalability of PSCs. SnO2 is a potential candidate for ETL in PSCs due to its high transmittance, low sintering temperatures, and suitable deep conduction and valence band positions that allows efficient electron extraction at the interfaces. In this work, we developed a simple, solution-processable low-temperature deposition of SnO2 on fluorine-doped tin oxide (FTO) glass substrates employing dip-coating technique for the first time. The number of dipping cycles are optimized to attain uniform coverage of the SnO2 layer on FTO. Formamidinium lead tri-iodide (FAPbI3) perovskite is deposited onto the dip-coated SnO2 electrode by vapor assisted solution process. XRD reveals the successful SnO2 phase formation by a low-temperature dip-coating method and FAPbI3 perovskite film formation by vapor assisted solution process with an unremarkable presence of the photo inactive δ phase. The surface morphology of the FAPbI3 films is smooth with an average grain size of 220 nm. A preliminary study on the planar device's photovoltaic performance using CuSCN as hole transport material (HTM) and Au as back contact exhibited a power conversion efficiency of 3.2% using four deposition cycles of SnO2 by dip-coating as the electron transport layer. This study demonstrates the utilization of dip-coating as a cost-effective method to deposit ETL that paves the way for PSCs' scale-up.

Dip coated SnO2 film as electron transport layer for low temperature processed planar perovskite solar cells

Temperature dependence of MAPbI3 films by quasi-vapor deposition technique and impact on photovoltaic performance and stability of perovskite solar cells

Perovskite solar cells (PSCs) are hailed as the game-changer technology in solar photovoltaics owing to their excellent optoelectronic properties, ease of fabrication, reduced material consumption, low cost, and flexibility. The architecture of a PSC consists of a transparent conducting substrate, perovskite absorber material sandwiched between electron and hole charge transport layers and metal contact. Oxide materials for charge transport layers are typically employed in high-performance PSCs which require high sintering temperatures. This limits their applications in light weight flexible solar cells, roll-to-roll processing of PSCs, etc. hampering applications to their full potential. On the other hand, nonoxide charge transport layers offer the advantage of low-temperature processing of PSCs which simplifies the fabrication process, cost and choice of substrates for flexible solar cells, and large-scale processing. In this chapter, we discuss in detail the different oxide-free electron and hole transport materials that can potentially replace the oxide materials. Transparent conducting oxide-free and hole transport material-free PSCs are also discussed. The technical aspects covering the performance and stability of the literature are summarized along with the loop-holes that require research to advance the field. 

Oxide free materials for perovskite solar cells

A new platform for research and development of inexpensive and efficient solar cells has evolved based on hybrid perovskite absorber material. The power conversion efficiency of polycrystalline perovskite solar cells shot from 3.8% in 2009 to 25.7% in 2022. Even though perovskite solar cells are close to commercialization, their poor operational stability in real time is a matter of concern. The major roadblock is moisture-induced material degradation at polycrystalline absorber material grain boundaries. Single crystalline perovskite materials are explored to overcome this problem. The superior stability of single-crystalline perovskite over polycrystalline counterpart is the significantly reduced grain boundary regions. Other advantages, including high absorption coefficient, low trap densities, long diffusion lengths, large carrier lifetimes, and increased mobility, have made these single-crystalline perovskites more relevant for solar cell application. This review emphasizes the importance of single-crystalline perovskite over polycrystalline perovskite. Further, it provides a comprehensive account of various methods for the growth of single-crystalline perovskite material, their structural, optical, and electrical properties, emphasizing device applications. The methods employed for developing single crystals by different routes have been elaborately discussed. The challenges encountered in integrating the single-crystal monoliths for device applications, the methods evolved to develop single-crystalline films, performance and stability of single-crystalline devices are presented. The future scope of research including uniform thickness controllable films with proper interface engineering, inorganic perovskites for solar cell applications, and potential optoelectronic applications of single-crystal perovskites are discussed. 

Ramya Krishna Battula

Papers

Stability of MAPbI3 perovskite grown on planar and mesoporous electron-selective contact by inverse temperature crystallization

Dip coated SnO2 film as electron transport layer for low temperature processed planar perovskite solar cells

Temperature dependence of MAPbI3 films by quasi-vapor deposition technique and impact on photovoltaic performance and stability of perovskite solar cells

Oxide free materials for perovskite solar cells

Single-Crystal Hybrid Lead Halide Perovskites: Growth, Properties, and Device Integration for Solar Cell Application