Junjie Ma

Reducing Hysteresis and Enhancing Performance of Perovskite Solar Cells Using Low-Temperature Processed Y-Doped SnO2 Nanosheets as Electron Selective Layers

Despite the rapid increase of efficiency, perovskite solar cells (PSCs) still face some challenges, one of which is the current-voltage hysteresis. Herein, it is reported that yttrium-doped tin dioxide (Y-SnO2 ) electron selective layer (ESL) synthesized by an in situ hydrothermal growth process at 95 °C can significantly reduce the hysteresis and improve the performance of PSCs. Comparison studies reveal two main effects of Y doping of SnO2 ESLs: (1) it promotes the formation of well-aligned and more homogeneous distribution of SnO2 nanosheet arrays (NSAs), which allows better perovskite infiltration, better contacts of perovskite with SnO2 nanosheets, and improves electron transfer from perovskite to ESL; (2) it enlarges the band gap and upshifts the band energy levels, resulting in better energy level alignment with perovskite and reduced charge recombination at NSA/perovskite interfaces. As a result, PSCs using Y-SnO2 NSA ESLs exhibit much less hysteresis and better performance compared with the cells using pristine SnO2 NSA ESLs. The champion cell using Y-SnO2 NSA ESL achieves a photovoltaic conversion efficiency of 17.29% (16.97%) when measured under reverse (forward) voltage scanning and a steady-state efficiency of 16.25%. The results suggest that low-temperature hydrothermal-synthesized Y-SnO2 NSA is a promising ESL for fabricating efficient and hysteresis-less PSC.

Perovskite Solar Cells Based on Low-Temperature Processed Indium Oxide Electron Selective Layers

Indium oxide (In2O3) as a promising n-type semiconductor material has been widely employed in optoelectronic applications. In this work, we applied low-temperature solution-processed In2O3 nanocrystalline film as an electron selective layer (ESL) in perovskite solar cells (PSCs) for the first time. By taking advantages of good optical and electrical properties of In2O3 such as high mobility, wide band gap, and high transmittance, we obtained In2O3-based PSCs with a good efficiency exceeding 13% after optimizing the concentration of the precursor solution and the annealing temperature. Furthermore, to enhance the performance of the In2O3-based PSCs, a phenyl-C61-butyric acid methyl ester (PCBM) layer was introduced to modify the surface of the In2O3 film. The PCBM film could fill up the pinholes or cracks along In2O3 grain boundaries to passivate the defects and make the ESL extremely compact and uniform, which is conducive to suppressing the charge recombination. As a result, the efficiency of the In2O3-based PSC was improved to 14.83% accompanied with V(OC), J(SC), and FF being 1.08 V, 20.06 mA cm(-2), and 0.685, respectively.

MgO Nanoparticle Modified Anode for Highly Efficient SnO2-Based Planar Perovskite Solar Cells

Reducing the energy loss and retarding the carrier recombination at the interface are crucial to improve the performance of the perovskite solar cell (PSCs). However, little is known about the recombination mechanism at the interface of anode and SnO2 electron transfer layer (ETL). In this work, an ultrathin wide bandgap dielectric MgO nanolayer is incorporated between SnO2:F (FTO) electrode and SnO2 ETL of planar PSCs, realizing enhanced electron transporting and hole blocking properties. With the use of this electrode modifier, a power conversion efficiency of 18.23% is demonstrated, an 11% increment compared with that without MgO modifier. These improvements are attributed to the better properties of MgO‐modified FTO/SnO2 as compared to FTO/SnO2, such as smoother surface, less FTO surface defects due to MgO passivation, and suppressed electron–hole recombinations. Also, MgO nanolayer with lower valance band minimum level played a better role in hole blocking. When FTO is replaced with Sn‐doped In2O3 (ITO), a higher power conversion efficiency of 18.82% is demonstrated. As a result, the device with the MgO hole‐blocking layer exhibits a remarkable improvement of all J–V parameters. This work presents a new direction to improve the performance of the PSCs based on SnO2 ETL by transparent conductive electrode surface modification.

Effective Carrier‐Concentration Tuning of SnO2 Quantum Dot Electron‐Selective Layers for High‐Performance Planar Perovskite Solar Cells

The carrier concentration of the electron-selective layer (ESL) and hole-selective layer can significantly affect the performance of organic-inorganic lead halide perovskite solar cells (PSCs). Herein, a facile yet effective two-step method, i.e., room-temperature colloidal synthesis and low-temperature removal of additive (thiourea), to control the carrier concentration of SnO2 quantum dot (QD) ESLs to achieve high-performance PSCs is developed. By optimizing the electron density of SnO2 QD ESLs, a champion stabilized power output of 20.32% for the planar PSCs using triple cation perovskite absorber and 19.73% for those using CH3 NH3 PbI3 absorber is achieved. The superior uniformity of low-temperature processed SnO2 QD ESLs also enables the fabrication of ≈19% efficiency PSCs with an aperture area of 1.0 cm2 and 16.97% efficiency flexible device. The results demonstrate the promise of carrier-concentration-controlled SnO2 QD ESLs for fabricating stable, efficient, reproducible, large-scale, and flexible planar PSCs.

Enhanced performance of perovskite solar cells via anti-solvent nonfullerene Lewis base IT-4F induced trap-passivation

The defects at the surfaces and grain boundaries of organic–inorganic halide perovskite films are detrimental to both the efficiency and stability of perovskite solar cells. Here, we introduce an in situ method with a new nonfullerene small molecule (IT-4F) that can effectively passivate ionic defects of hybrid perovskites with their positively charged components, under-coordinated Pb2+, during the anti-solvent process of perovskite film formation. This efficient defect passivation reduces the charge trap density and increases the carrier recombination lifetime. Furthermore, it reduces the open-circuit-voltage deficit of the p–i–n-structured device, and boosts the efficiency to a value of 18.3%. Moreover, the defect healing also significantly enhances the stability of films under ambient conditions. Our findings provide an avenue for defect passivation to further improve both the efficiency and stability of perovskite solar cells.

Surface treatment via Li-bis-(trifluoromethanesulfonyl) imide to eliminate the hysteresis and enhance the efficiency of inverted perovskite solar cells

Recently, inverted planar perovskite solar cells (PSCs) with a p–i–n configuration have drawn tremendous interest due to their high efficiency and reduced hysteresis. It has been demonstrated that p–i–n planar PSCs employing organic hole transport materials can obtain excellent photovoltaic performance, but their high cost and poor stability have restrained their further development. Herein, we employ Li-bis-(trifluoromethanesulfonyl) imide treated NiOx (Li-treated NiOx) as an excellent and robust hole transport layer (HTL) for fabricating highly efficient and stable PSCs. Compared to the pristine NiOx, the Li-treated NiOx exhibits enhanced electrical conductivity and better band alignment, enabling fast and efficient hole transport as well as exciton separation. The PSC with a Li-treated NiOx HTL shows a significantly improved fill factor and efficiency (from 0.75, 14.46% to 0.79, 17.09%) as well as less hysteresis with satisfactory long-term stability. Furthermore, we obtain a champion efficiency of 18.03% via graded heterojunction engineering. Therefore, our results suggest Li-treated NiOx film is an effective and promising hole transport material for efficient PSCs.

Methylammonium, formamidinium and ethylenediamine mixed triple-cation perovskite solar cells with high efficiency and remarkable stability

Solar cells with organic–inorganic perovskites as light absorbers are now delivering more than 22% power conversion efficiencies and are comparable to their inorganic counterparts on a laboratory scale. However, their intrinsic instability hinders the commercialization of this promising photovoltaic technique. Herein we introduced the bivalent alkyldiammonium cation ethylenediamine (EDA2+) into methylammonium (MA)/formamidinium (FA) lead iodide (MA0.7FA0.3PbI3) to improve its stability, forming a triple cation perovskite (MA0.7FA0.3)1−2xEDAxPbI3 as a light absorber layer for planar perovskite solar cells. After delicately tuning the stoichiometric ratio of EDA2+ in the cation, PSCs based on the (MA0.7FA0.3)0.97EDA0.015PbI3 (x = 1.5%) perovskite yielded a highest power conversion efficiency of 20.01% with negligible hysteresis. More importantly, the devices showed greatly enhanced stability compared to the EDA-free ones. The unencapsulated perovskite solar cells with EDA incorporation retained 96% of their initial efficiencies after 25 days of storage in an ambient atmosphere.