Junction Quality of SnO2-Based Perovskite Solar Cells Investigated by Nanometer-Scale Electrical Potential Profiling

Moderate temperature deposition of RF magnetron sputtered SnO2-based electron transporting layer for triple cation perovskite solar cells

The perovskite solar cells (PSCs) are still facing the two main challenges of stability and scalability to meet the requirements for their potential commercialization. Therefore, developing a uniform, efficient, high quality and cost-effective electron transport layer (ETL) thin film to achieve a stable PSC is one of the key factors to address these main issues. Magnetron sputtering deposition has been widely used for its high quality thin film deposition as well as its ability to deposit films uniformly on large area at the industrial scale. In this work, we report on the composition, structural, chemical state, and electronic properties of moderate temperature radio frequency (RF) sputtered SnO₂. Ar and O₂ are employed as plasma-sputtering and reactive gases, respectively. We demonstrate the possibility to grow a high quality and stable SnO₂ thin films with high transport properties by reactive RF magnetron sputtering. Our findings show that PSC devices based on the sputtered SnO₂ ETL have reached a power conversion efficiency up to 17.10% and an average operational lifetime over 200 h. These uniform sputtered SnO₂ thin films with improved characteristics are promising for large photovoltaic modules and advanced optoelectronic devices.

Operando Characterizations of Light-Induced Junction Evolution in Perovskite Solar Cells

Light-induced performance changes in metal halide perovskite solar cells (PSCs) have been studied intensively over the last decade, but little is known about the variation in microscopic optoelectronic properties of the perovskite heterojunctions in a completed device during operation. Here, we combine Kelvin probe force microscopy and transient reflection spectroscopy techniques to spatially resolve the evolution of junction properties during the operation of metal-halide PSCs and study the light-soaking effect. Our analysis showed a rise of an electric field at the hole-transport layer side, convoluted with a more reduced interfacial recombination rate at the electron-transport layer side in the PSCs with an n-i-p structure. The junction evolution is attributed to the effects of ion migration and self-poling by built-in voltage. Device performances are correlated with the changes of electrostatic potential distribution and interfacial carrier dynamics. Our results demonstrate a new route for studying the complex operation mechanism in PSCs.

Pure 2D Perovskite Formation by Interfacial Engineering Yields a High Open-Circuit Voltage beyond 1.28 V for 1.77-eV Wide-Bandgap Perovskite Solar Cells

Surface post-treatment using ammonium halides effectively reduces large open-circuit voltage (VOC ) losses in bromine-rich wide-bandgap (WBG) perovskite solar cells (PSCs). However, the underlying mechanism still remains unclear and the device efficiency lags largely behind. Here, a facile strategy of precisely tailoring the phase purity of 2D perovskites on top of 3D WBG perovskite and realizing high device efficiency is reported. The transient absorption spectra, cross-sectional confocal photoluminescence mapping, and cross-sectional Kelvin probe force microscopy are combined to demonstrate optimal defect passivation effect and surface electric-field of pure n = 1 2D perovskites formed atop 3D WBG perovskites via low-temperature annealing. As a result, the inverted champion device with 1.77-eV perovskite absorber achieves a high VOC of 1.284 V and a power conversion efficiency (PCE) of 17.72%, delivering the smallest VOC deficit of 0.486 V among WBG PSCs with a bandgap higher than 1.75 eV. This enables one to achieve a four-terminal all-perovskite tandem solar cell with a PCE exceeding 25% by combining with a 1.25-eV low-bandgap PSC.

Metastable Dion-Jacobson 2D structure enables efficient and stable perovskite solar cells

The performance of three-dimensional (3D) organic-inorganic halide perovskite solar cells (PSCs) can be enhanced through surface treatment with 2D layered perovskites that have efficient charge transport. We maximized hole transport across the layers of a metastable Dion-Jacobson (DJ) 2D perovskite that tuned the orientational arrangements of asymmetric bulky organic molecules. The reduced energy barrier for hole transport increased out-of-plane transport rates by a factor of 4 to 5, and the power conversion efficiency (PCE) for the 2D PSC was 4.9%. With the metastable DJ 2D surface layer, the PCE of three common 3D PSCs was enhanced by approximately 12 to 16% and could reach approximately 24.7%. For a triple-cation–mixed-halide PSC, 90% of the initial PCE was retained after 1000 hours of 1-sun operation at ~40°C in nitrogen.

Metal oxide charge transporting layers for stable high-performance perovskite solar cells

This review summarizes the recent progress in metal oxide charge transporting layers to achieve stable high-performance perovskite solar cells.

Correlating hysteresis phenomena with interfacial charge accumulation in perovskite solar cells

A generalized charge exchange model is introduced into drift–diffusion equations for modeling the charge extraction in perovskite solar cells.

Room-temperature synthesized SnO2 electron transport layers for efficient perovskite solar cells

Tin oxide (SnO2) is widely used as electron transport layer (ETL) material in perovskite solar cells (PSCs). Numerous synthesis methods for SnO2 have been reported, but they all require a proper thermal treatment for the SnO2 ETLs. Herein we present a simple method to synthesize SnO2 nanoparticles (NPs) at room temperature. By using butyl acetate as a precipitator and a proper UV-Ozone treatment to remove Cl residuals, excellent SnO2 ETLs were obtained without any thermal annealing. The highest power conversion efficiency (PCE) of the prepared PSCs was 19.22% for reverse scan (RS) and 18.79% for forward scan (FS). Furthermore, flexible PSCs were fabricated with high PCEs of 15.27%/14.74% (RS/FS). The low energy consuming SnO2 ETLs therefore show great promise for the flexible PSCs' commercialization.

Point defect-reduced colloidal SnO2 electron transport layers for stable and almost hysteresis-free perovskite solar cells

The commercialization of perovskite solar cells has been investigated, but the instability of their light-absorbing layers remains a problem. We demonstrate that the use of colloidal SnO2 nanoparticles prevents perovskite light absorber decomposition, reduces the hysteresis index to 0.1%, and increases the power conversion efficiency to 19.12%.

Functional Scanning Force Microscopy for Energy Nanodevices

Energy nanodevices, including energy conversion and energy storage devices, have become a major cross-disciplinary field in recent years. These devices feature long-range electron and ion transport coupled with chemical transformation, which call for novel characterization tools to understand device operation mechanisms. In this context, recent developments in functional scanning force microscopy techniques and their application in thin-film photovoltaic devices and lithium batteries are reviewed. The advantages of scanning force microscopy, such as high spatial resolution, multimodal imaging, and the possibility of in situ and in operando imaging, are emphasized. The survey indicates that functional scanning force microscopy is making significant contributions in understanding materials and interfaces in energy nanodevices.

Simultaneous Ligand Exchange Fabrication of Flexible Perovskite Solar Cells using Newly Synthesized Uniform Tin Oxide Quantum Dots

Halide perovskite solar cells (HPSCs) have a significant potential for future photovoltaic systems because of a high power conversion efficiency (PCE) exceeding 23% using solution processing methods. A low-temperature processed oxide layer is a challenging issue for large-scale manufacture of flexible and low-cost HPSCs. Here, we propose a simple reverse micelle-water injection method for highly dispersed ligand-capped ultrafine SnO₂ quantum dots (QD). Interestingly, we observed that the ligands, which help in the formation of a uniform SnO₂ QD thin film, spontaneously exchange for halide through a perovskite solution, and finally we form a suitable SnO₂ QD-halide junction for high-performance HPSCs. The flexible HPSC with the SnO₂ QD-halide junction formed via the ligand exchange exhibits a high PCE of 17.7% using a flexible substrate. It also shows an excellent flexibility, where the initial PCE is maintained within 92% after 1000 bending cycles with a bending radius of 18 mm.

Solution-Processed Cu9S5 as a Hole Transport Layer for Efficient and Stable Perovskite Solar Cells

Organic-inorganic perovskite solar cells have seen tremendous developments in recent years. As a hole transport material, 2,2',7,7'-tetrakis( N, N-di- p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD) is widely used in n-i-p perovskite solar cells. However, it may lead to the perovskite film degradation due to the dopant lithium bis((trifluoromethyl)sulfonyl)amide (Li-TFSI), which has strong hydrophilicity. Cu₉S₅ is considered as a superior p-type transport material, which also has a favorable energy level matching with the highest occupied molecular orbital of Spiro-OMeTAD. Herein, a solution-processed organic-inorganic-integrated hole transport layer was reported, which is composed of the undoped Spiro-OMeTAD and Cu₉S₅ layer. Since there is no Li-TFSI doping, it is extremely conductive to the long-term stability of the solar cells. In the meantime, we proposed a method to adjust the lowest unoccupied molecular orbital (LUMO) of SnO₂ via nitrogen implantation (N:SnO₂). The LUMO of SnO₂ can be tuned from -4.33 to -3.91 eV, which matches well with the LUMO of CH₃NH₃PbI₃ (-3.90 eV), and thus helps to reduce hysteresis. The modified hole and electron transport layers were applied in n-i-p perovskite solar cells, which achieve a maximum power conversion efficiency (PCE) of 17.10 and 96% retention of PCE after 1200 h in air atmosphere without any encapsulation.