Development of Highly Efficient and Stable Lead-Free Perovskite Solar Cells Through Composition and Interface Engineering

Abstract

Solar energy has the highest capacity to power our energy needs sustainably without emitting greenhouse gases. Hybrid organic-inorganic lead halide perovskite solar cells (PVSCs) have emerged in the past decade as a promising low-cost, low energy intensity, thin film solar cell with lab efficiencies reaching 25.2%. The toxicity of Pb perovskites and performance and stability issues with Sn lead-free alternatives remains a major roadblock to commercialization. In this work, novel perovskite compositions are designed and used to study the transition from Pb to Sn perovskites, cations are tuning to implement stabilizing components for pure Sn perovskites, and Sn perovskites performance and stability issues are tackled. The triple cation methylammonium (MA), formamidinium (FA) and Cs, with double halide composition Csx(MA0.17FA0.83)1-xPb1-ySny(I0.83Br0.17)3 films with x = 0.05, 0.10, and 0.20 and y = 0, 0.25, 0.50, 0.75, and 1.0 was used to create a library of new perovskites and study novel band gap trends near the maximum Schockley Quiesser limit. The simple inverted device structure of indium tin oxide (ITO)/poly (3,4, -ethylenedioxythiphene): polystyrene sulfonate (PEDOT:PSS)/Perovskite/[6,6]-phenyl-C60-butyric acid methyl ester (PC60BM)/fullerene (C60)/2,9-dimethyl-7,7-diphenyl-1,10-phenanthroline (BCP)/Ag eliminated dopant instabilities. Due to a high-quality film morphology and optimal bandgap, 3 3 Cs0.05(MA0.17FA0.83)0.95Pb0.25Sn0.75(I0.83Br0.17)3 (band gap = 1.30 eV), achieved a record maximum efficiency of 11.05% for any 75% Sn composition. Moreover, the 75% Sn PVSCs retained 80% of initial PCE after 30 days storage in inert conditions and 100 hours in ambient conditions. After optimizing antisolvent choice, solvent annealing, and hot casting conditions for pure Sn perovskite films, the novel composition with Cs, FA, and guanidinium (GA), (CsGA)xFA100 2xSnI3 was implemented. This cation mixture combines the benefits of a guanidinium cation, such as increased hydrogen bonding and no dipole moment, with Cs to fill point defects and relax the crystal lattice to better integrate a large stabilizing agent, ethylenediammonium diiodide (EDAI2). The EDAI2 additive not only yielded pinhole-free cubic phase (CsGA)xFA100-2xSnI3 perovskite films but also decreased both shallow and deep trap states in the perovskite films. The devices with (CsGA)15FA70SnI3 and 0-2% EDAI2 all achieved a maximum PCE higher than 5% with the highest of 5.72% for a fresh device with (CsGA)15FA70SnI3 and 1% EDAI2. After storage, the maximum PCE was increased from 5.69% to 6.39% for the (CsGA)15FA70SnI3 and 1.5% EDAI2 devices. Finally, to tackle energy loss issues that have plagued pure Sn perovskites (loss = 0.6-0.9 V), the misalignment between the PEDOT:PSS hole transport layer and the Sn perovskite valence band was studied due to the energy misalignment between the hole transport layer and pure Sn perovskite valence band. Cosolvent methods, solvent wash methods, and solvent immersion methods with DMSO, EG, and CH3OH were implemented to alter the PSS content and work function of the HTL to improve alignment. Utilizing the (CsGA)15FA70SnI3+1.0% EDAI2 perovskite film we demonstrated a higher performance of 6.29% and 6.16% with a 5% DMSO cosolvent and methanol solvent wash, respectively. This work is a comprehensive push to improve the performance and stability of non-toxic Sn perovskite devices, bringing this technology one step closer to commercialization.