Enhanced performance in perovskite solar cells via bromide ion substitution and ethanol treatment

Highlights: • Mixed solution of PbI₂ and PbBr₂ with different molar ratio were used as lead precursors to obtain Pb(I_1-xBrx)₂ layer. • Ethanol treatment was performed to modify the morphologies of Pb(I_1-xBr_x)₂ films. • The champion device based on MAPbI_2.6Br_0.4 film was achieved with a PCE of 15.53% and long term stability. Mixed lead halide (Pb(I_1-xBr_x)₂) as the seed layer was employed to prepare mixed lead-halide perovskite (MAPbI_3-2xBr_2x, MA = CH₃NH₃) films through two-step sequential deposition method. Ethanol treatment process was also introduced for the control of morphology and microstructure of Pb(I_1-xBr_x)₂ films. The ethanol treatment accelerates the crystallization of Pb(I_1-xBr_x)₂ and the resulted Pb(I_1-xBr_x)₂ films exhibit a porous structure which facilitates more complete conversion of PbI₂ at the same time. As a result, high purity and highly crystallized MAPbI_3-2xBr_2x films are obtained. The photovoltaic performance of assembled perovskite solar cells based on MAPbI_3-2xBr_2x films are improved upon ethanol treatment and a champion power conversion efficiency (PCE) of 15.53% is obtained with x = 0.2. After exposed in air condition for 14 days, a 86% of initial PCE remains in the champion device.