Metal-substituted organic-inorganic perovskite photovoltaic device performance and hysteresis behavior

Abstract

Due to long carrier diffusion length, high defect tolerance and adjustable absorption spectra, organic-inorganic perovskite with chemical formula of ABX3 (A=CH3NH3 (MA) or HC(NH2)2 (FA); M=Pb; X=Cl, Br, I) is one of the most promising candidates for low-cost solar energy harvesting. The power conversion efficiency (PCE) of the perovskite solar cells has experienced skyrocket increasing from 3.8% to over 23% in just 10 years after they showed up, which has already outperformed than that of multicrystalline Si. Based on ab initio calculation, it demonstrates that Pb2+, taking part in bonding, is one of the most important factors that determines the crystallization, energy level, charge carrier dynamics as well as film morphology. Thus, a large number of metal ions have been reported to partial substitute Pb2+, including isovalent substitution (Sn2+, Sr2+, Cd2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Ba2+, Zn2+, Se2+ and Eu2+, etc.) and anisovalent substitution (Cu+, Na+, Ag+, In3+, Sb3+, Al3+ and Bi3+, etc.), which improved perovskite properties and device performance significantly. However, the effects of Pb2+ substitution on the anomalous hysteresis of perovskite solar cells have not been fully explored yet. In this manuscript, isovalent Cd2+ substituted MAPbI3 was employed as the model system, which excluded the interference of halogen ion defect formation if Pb2+ was substituted by isovalent metal ions. It is found that the crystallinity, morphology and photogenerated charge carrier dynamics of MAPbI3 were tuned with different concentrations of Cd2+. When the concentration of Cd2+ increased from 0% to 0.5%, the crystallinity and the morphology of perovskite films improved significantly, resulting suppressed non-radiative recombination and increased photogenerated charge carrier lifetime. The PCE of planar heterojunction photovoltaic devices with structure of ITO/NiOx/Cd-MAPbI3/PCBM/Ag enhanced from 15.4% to 17.0%. Importantly, there is negligible hysteresis between forward and reverse current density-voltage (J-V) scan at different scan rate of 0.01, 0.1 and 1.0 V/s. By further increasing concentration of Cd2+ from 1.0% to 5.0%, the crystallinity of perovskite films deteriorated gradually, leading to higher non-radiative recombination and lower photogenerated charge carrier lifetime. The morphology of perovskite films also became worse, i.e., generated insulate phases and pin-holes, which prohibited charge transport. Even worse, incorporating high concentration of Cd2+ (2.5%-5.0%) not only deteriorated the device performance, but also led to serious hysteresis between forward and reverse J-V scan. In order to further understand the hysteresis behavior under high concentration of Cd2+, two-pass scanning Kelvin probe microscopy (SKPM) with additional DC bias (Vbias) applied in the first-pass topography scan have been employed. It is found that the average surface potential (SP) of the perovskite did not change with Vbias under Cd2+ concentration of 0% and 0.5%. Interestingly, the average SP of the perovskite changed significantly with Vbias under Cd2+ concentration of 5.0%, indicating significant ion migration and accumulation. Our work provides a promising way to understand the device performance and hysteresis behavior in metal-substituted perovskite, which benefits the development of perovskite photovoltaic devices, light-emitting diodes, photodetectors etc. with metal ions engineering in future.