Humidifying, heating and trap-density effects on triple-cation perovskite solar cells

The effect of moisture and heat are important challenges in perovskite solar cells (PSCs). Herein we studied the performance of triple-cation PSCs in different operating environmental conditions. Humidified cells exhibited a hopeful character by increasing the open-circuit voltage (VOC) and short-circuit current density (JSC) to 940 mV and 22.85 mA cm-2 with a power conversion efficiency (PCE) of 14.34%. In addition, further analyses showed that hysteresis index and charge transfer resistance decrease down to 0.4% and 1.67 kΩ. The origin of superior stability is ion segregation to the interface, which removes the antisite defect states. Finally, the effect of operating temperature and trap density on structure and performance was also studied systematically.

Enhanced Photovoltaic Performance of Inverted Perovskite Solar Cells through Surface Modification of a NiOx-Based Hole-Transporting Layer with Quaternary Ammonium Halide-Containing Cellulose Derivatives

In this study, we positioned three quaternary ammonium halide-containing cellulose derivatives (PQF, PQCl, PQBr) as interfacial modification layers between the nickel oxide (NiO_x) and methylammonium lead iodide (MAPbI₃) layers of inverted perovskite solar cells (PVSCs). Inserting PQCl between the NiO_x and MAPbI₃ layers improved the interfacial contact, promoted the crystal growth, and passivated the interface and crystal defects, thereby resulting in MAPbI₃ layers having larger crystal grains, better crystal quality, and lower surface roughness. Accordingly, the photovoltaic (PV) properties of PVSCs fabricated with PQCl-modified NiO_x layers were improved when compared with those of the pristine sample. Furthermore, the PV properties of the PQCl-based PVSCs were much better than those of their PQF- and PQBr-based counterparts. A PVSC fabricated with PQCl-modified NiO_x (fluorine-doped tin oxide/NiO_x/PQCl-0.05/MAPbI₃/PC₆₁BM/bathocuproine/Ag) exhibited the best PV performance, with a photoconversion efficiency (PCE) of 14.40%, an open-circuit voltage of 1.06 V, a short-circuit current density of 18.35 mA/cm³, and a fill factor of 74.0%. Moreover, the PV parameters of the PVSC incorporating the PQCl-modified NiO_x were further enhanced when blending MAPbI₃ with PQCl. We obtained a PCE of 16.53% for this MAPbI₃:PQCl-based PVSC. This PQCl-based PVSC retained 80% of its initial PCE after 900 h of storage under ambient conditions (30 °C; 60% relative humidity).

Roles of Inorganic Oxide Based HTMs towards Highly Efficient and Long-Term Stable PSC-A Review

In just a few years, the efficiency of perovskite-based solar cells (PSCs) has risen to 25.8%, making them competitive with current commercial technology. Due to the inherent advantage of perovskite thin films that can be fabricated using simple solution techniques at low temperatures, PSCs are regarded as one of the most important low-cost and mass-production prospects. The lack of stability, on the other hand, is one of the major barriers to PSC commercialization. The goal of this review is to highlight the most important aspects of recent improvements in PSCs, such as structural modification and fabrication procedures, which have resulted in increased device stability. The role of different types of hole transport layers (HTL) and the evolution of inorganic HTL including their fabrication techniques have been reviewed in detail in this review. We eloquently emphasized the variables that are critical for the successful commercialization of perovskite devices in the final section. To enhance perovskite solar cell commercialization, we also aimed to obtain insight into the operational stability of PSCs, as well as practical information on how to increase their stability through rational materials and device fabrication.

CH3NH3PbBr3-xIx Quantum Dots Enhance Bulk Crystallization and Interface Charge Transfer for Efficient and Stable Perovskite Solar Cells

Obtaining a perovskite light-absorbing layer with good crystallization, low defect concentration, good stability, and well-matched energy levels is critical to obtaining high-efficiency perovskite solar cells (PSCs). Here, a hybrid PSC with a graded band gap is explored using MAPbBr₃ (MA = CH₃NH₃) and MAPbBr_0.9I_2.1 quantum dots (QDs) as component cells. We have creatively designed a solar cell device with a double-QD structure [indium tin oxide (ITO)/SnO₂/perovskite:MAPbBr₃ QDs/MAPbBr_0.9I_2.1 QDs/Spiro-OMeTAD/Au]. A better crystal film of the perovskite absorption layer can be obtained because the MAPbBr₃ QDs are doped in an antisolvent, which induces nucleation and growth in the polycrystalline perovskite. In addition, we expect that digestive ripening occurred in the crystallization, and the oleic acid ligands on the surface of the QDs disintegrate during the doping process and transfer to the surface of the perovskite absorption layer finally; it follows that the hydrophobicity and stability of the perovskite film are greatly enhanced. Moreover, a thin film of MAPbBr_0.9I_2.1 QDs is introduced between the perovskite absorption layer and the hole layer, acting as an energy-level ladder, which leads to well-matched energy levels, an increase in fill factor (FF), and an enhanced hole transport capability. In particular, the mechanism of the crystallization process involving the effect of oleic acid ligands on the interior and surface of the perovskite film is fully discussed here. The final research results from the PSCs show that both high efficiency and long-term stability are achieved successfully by this design strategy.

Modification of NiO x hole transport layer for acceleration of charge extraction in inverted perovskite solar cells

The modification of the inorganic hole transport layer has been an efficient method for optimizing the performance of inverted perovskite solar cells. In this work, we propose a facile modification of a compact NiO x film with NiO x nanoparticles and explore the effects on the charge carrier dynamic behaviors and photovoltaic performance of inverted perovskite devices. The modification of the NiO x hole transport layer can not only enlarge the surface area and infiltration ability, but also adjust the valence band maximum to well match that of perovskite. The photoluminescence results confirm the acceleration of the charge separation and transport at the NiO x /perovskite interface. The corresponding device possesses better photovoltaic parameters than the device based on control NiO x films. Moreover, the charge carrier transport/recombination dynamics are further systematically investigated by the measurements of time-resolved photoluminescence, transient photovoltage and transient photocurrent. Consequently, the results demonstrate that proper modification of NiO x can significantly enlarge interface area and improve the hole extraction capacity, thus efficiently promoting charge separation and inhibiting charge recombination, which leads to the enhancement of the device performances.

Perovskite solar cells: must lead be replaced - and can it be done?

Perovskite solar cells have recently drawn significant attention for photovoltaic applications with a certified power conversion efficiency of more than 22%. Unfortunately, the toxicity of the dissolvable lead content in these materials presents a critical concern for future commercial development. This review outlines some criteria for the possible replacement of lead by less toxic elements, and highlights current research progress in the application of low-lead halide perovskites as optically active materials in solar cells. These criteria are discussed with the aim of developing a better understanding of the physio-chemical properties of perovskites and of realizing similar photovoltaic performance in perovskite materials either with or without lead. Some open questions and future development prospects are outlined for further advancing perovskite solar cells toward both low toxicity and high efficiency.

Synthesis of SnO2 nanofibers and nanobelts electron transporting layer for efficient perovskite solar cells

The implementation of positive alternative electron transporting layers (ETLs) with excellent electronic properties is a most promising method to up-scale low-cost highly efficient perovskite solar cell (PSC) technology. The present work demonstrates the preparation of tin oxide (SnO2) nanofibers (NF) and nanobelts (NB) as an electron transporting layer (ETL) for PSCs. The smooth and uniform nanofibers and nanobelts have been prepared using an electrospinning technique followed by calcination at 600 °C. Thermogravimetric analysis (TGA) analysis performed on the as-spun polyvinylpyrrolidone-tin oxide (PVP-SnO2) composite suggests that a calcination temperature of 600 °C is required to obtain pure SnO2 and to ensure complete removal of PVP along with other organic solvents. The structural analysis confirmed the presence of the pure tetragonal rutile phase of SnO2 nanofibers and nanobelts. The prepared nanofibers and nanobelts were further used as ETLs for PSCs. Our optimized experimental parameters yielded a JSC of 22.46 mA cm-2, a VOC of 1.081 V and FF of 66%, leading to >16% power conversion efficiency (PCE) for SnO2 nanobelts using an (FAPbI3)0.85(MAPbI3)0.15 perovskite absorber layer with good stability. The obtained PCE is much higher than that of the SnO2 NF (12.893%) morphology. Nevertheless, the synthesis of SnO2 NF/NB ETLs provides a simple, low-cost and large-scale method for PSCs.

A New Method for Fitting Current-Voltage Curves of Planar Heterojunction Perovskite Solar Cells

Herein we propose a new equivalent circuit including double heterojunctions in series to simulate the current-voltage characteristic of P-I-N planar structure perovskite solar cells. This new method can theoretically solve the dilemma of the parameter diode ideal factor being larger than 2 from an ideal single heterojunction equivalent circuit, which usually is in the range from 1 to 2. The diode ideal factor reflects PN junction quality, which influences the recombination at electron transport layer/perovskite and perovskite/hole transport layer interface. Based on the double PN junction equivalent circuit, we can also simulate the dark current-voltage curve for analyzing recombination current (Shockley-Read-Hall recombination) and diffusion current (including direct recombination), and thus carrier recombination and transportation characteristics. This new model offers an efficacious and simple method to investigate interfaces condition, film quality of perovskite absorbing layer and performance of transport layer, helping us further improve the device efficiency and analyze the working mechanism.

High Efficiency Inverted Planar Perovskite Solar Cells with Solution-Processed NiOx Hole Contact

NiO_x is a promising hole-transporting material for perovskite solar cells due to its high hole mobility, good stability, and easy processability. In this work, we employed a simple solution-processed NiO_x film as the hole-transporting layer in perovskite solar cells. When the thickness of the perovskite layer increased from 270 to 380 nm, the light absorption and photogenerated carrier density were enhanced and the transporting distance of electron and hole would also increase at the same time, resulting in a large charge transfer resistance and a long hole-extracted process in the device, characterized by the UV-vis, photoluminescence, and electrochemical impedance spectroscopy spectra. Combining both of these factors, an optimal thickness of 334.2 nm was prepared with the perovskite precursor concentration of 1.35 M. Moreover, the optimal device fabrication conditions were further achieved by optimizing the thickness of NiO_x hole-transporting layer and PCBM electron selective layer. As a result, the best power conversion efficiency of 15.71% was obtained with a J_sc of 20.51 mA·cm^-2, a V_oc of 988 mV, and a FF of 77.51% with almost no hysteresis. A stable efficiency of 15.10% was caught at the maximum power point. This work provides a promising route to achieve higher efficiency perovskite solar cells based on NiO or other inorganic hole-transporting materials.

Crystal Engineering for Low Defect Density and High Efficiency Hybrid Chemical Vapor Deposition Grown Perovskite Solar Cells

Synthesis of high quality perovskite absorber is a key factor in determining the performance of the solar cells. We demonstrate that hybrid chemical vapor deposition (HCVD) growth technique can provide high level of versatility and repeatability to ensure the optimal conditions for the growth of the perovskite films as well as potential for batch processing. It is found that the growth ambient and degree of crystallization of CH₃NH₃PbI₃ (MAPI) have strong impact on the defect density of MAPI. We demonstrate that HCVD process with slow postdeposition cooling rate can significantly reduce the density of shallow and deep traps in the MAPI due to enhanced material crystallization, while a mixed O₂/N₂ carrier gas is effective in passivating both shallow and deep traps. By careful control of the perovskite growth process, a champion device with power conversion efficiency of 17.6% is achieved. Our work complements the existing theoretical studies on different types of trap states in MAPI and fills the gap on the theoretical analysis of the interaction between deep levels and oxygen. The experimental results are consistent with the theoretical predictions.