摘要
注入金属离子替换有机-无机钙钛矿中参与成键的Pb~(2+)可以有效调控其结晶动力学、薄膜形貌和光电特性,因此,通过优化Pb~(2+)替换比例提高钙钛矿太阳能电池性能是当前的研究热点之一.但是, Pb~(2+)替换对器件异常迟滞现象的影响却缺乏深入的研究.本文采用Cd~(2+)替换的MAPbI_3为模型体系,研究了不同Cd~(2+)浓度下MAPbI_3材料性质及其平面异质结光伏器件(结构为ITO/NiO_x/Cd-MAPbI_3/PCBM/Ag)性能的变化趋势.研究结果表明,优化比例(0.5%)的Cd~(2+)可以有效增强材料结晶性、改善薄膜形貌,降低非辐射复合,提高光生载流子寿命,从而大幅提高器件性能.而Cd~(2+)替换比例过高(>2.5%)时,钙钛矿薄膜中不仅会出现相分离阻碍电荷传输,而且其非辐射复合加剧,光生载流子寿命降低,最终导致器件性能下降.与此同时,过量的Cd~(2+)注入还会引起严重的迟滞现象,利用扫描开尔文探针显微镜(SKPM)证实这一现象与钙钛矿薄膜中显著的离子迁移有关.
Due to long carrier diffusion length,high defect tolerance and adjustable absorption spectra,organic-inorganic perovskite with chemical formula of ABX_3(A=CH_3NH_3(MA) or HC(NH_2)_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 Pb~(2+),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 Pb~(2+),including isovalent substitution(Sn~(2+),Sr~(2+),Cd~(2+),Ca~(2+),Mn~(2+),Fe~(2+),Co~(2+),Ni~(2+),Cu~(2+),Ba~(2+),Zn~(2+),Se~(2+) and Eu~(2+),etc.)and anisovalent substitution(Cu~+,Na~+,Ag~+,In~(3+),Sb~(3+),Al~(3+) and Bi~(3+),etc.),which improved perovskite properties and device performance significantly.However,the effects of Pb~(2+) substitution on the anomalous hysteresis of perovskite solar cells have not been fully explored yet.In this manuscript,isovalent Cd~(2+) substituted MAPbI_3 was employed as the model system,which excluded the interference of halogen ion defect formation if Pb~(2+) was substituted by isovalent metal ions.It is found that the crystallinity,morphology and photogenerated charge carrier dynamics of MAPbI_3 were tuned with different concentrations of Cd~(2+).When the concentration of Cd~(2+) 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/NiO_x/Cd-MAPbI_3/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 Cd~(2+)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 Cd~(2+)(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 Cd~(2+),two-pass scanning Kelvin probe microscopy(SKPM) with additional DC bias(V_(bias)) 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 V_(bias) under Cd~(2+) concentration of 0% and 0.5%.Interestingly,the average SP of the perovskite changed significantly with V_(bias) under Cd~(2+) 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.
引文
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