Optimization Of Nickel-Oxide Based Hole Transport Layers For Perovskite Solar Cells

In the past ten years,perovskite solar cells has undergone rapid development and the power conversion efficiency increased from 3.8%to 25.2%,which becomes a rising star in the field of new energy.Hole transport materials with satisfying properties are of crucial importance for the photovoltaic performance and stability of perovskite solar cells.Nickel oxide(NiOx)material becomes one of the most frequently-used hole transport materials for its low cost,relatively suitable band alignment and superior stability.Most of the methods for fabricating NiOx films require high temperature annealing process,which can not be applied to flexible devices in industry.Therefore,spin coating pre-synthesized NiOx nanocrystals was chosen to prepare NiOx films in this article.However,the NiOx films prepared by this method still have some problems such as mismatched band position between NiOx and perovskite and poor conductivity,so graphene quantum dot(GQD)material was chosen as dopant for its excellent conductivity and stability.Herein,we used amino-functionalized GQD,hydroxy-functionalized GQD and imidazole-functionalized GQD to dope NiOx films.It is found that the functional groups on the surface of graphene quantum dots can have significant influence on the dispersivity of NiOx nanocrystals in the corresponding inks.Hydroxy-functionalized GQD and imidazole-functionalized GQD can both result in aggregation of NiOx nanocrystals,so amino-functionalized GQD(AGQD)was chosen as dopant in the end.AGQD doping can improve the performance of perovskite solar cells impressively,and the champion cell based on rigid substrate achieved a power conversion efficiency of 19.55%.AGQD doping can enhance the hole extraction capability of NiOx films and reduce the interfacial recombination of charge carriers.Improved hole extraction can be attributed to two reasons:(1)the nitrogen atoms on AGQD help to increase the grain size of perovskite by a Lewis base-acid interaction,thus reduce the defects in grain boundaries;(2)AGQD doping changes the band structure of NiOx films and improves the energy level match between NiOx and perovskite films.Best flexible device based on AGQ-doped NiOx films obtained a PCE of 18.1%,and our flexible device also showed excellent mechanical stability.The unencapsulated flexible device was kept under ambient conditions to test its stability and the result suggests that AGQD doping can improve the stability of device.In this article,we firstly proposed a strategy to optimize NiOx hole transport layer through AGQD doping,and the mechanisms behind boosted performance of devices was revealed.Flexible perovskite solar cells with high efficiency were also fabricated.This article might throw light on the industrialization of inverted perovskite solar cells.