The performance improvement of using hole transport layer with lithium and cobalt for inverted planar perovskite solar cell

With the continuous development of solar cells, the perovskite solar cells (PSCs), whose hole transport layer plays a vital part in collection of photogenerated carriers, have been studied by many researchers. Interface transport layers are important for efficiency and stability enhancement. In this paper, we demonstrated that lithium (Li) and cobalt (Co) codoped in the novel inorganic hole transport layer named NiO_x, which were deposited onto ITO substrates via solution methods at room temperature, can greatly enhance performance based on inverted structures of planar heterojunction PSCs. Compared to the pristine NiOx films, doping a certain amount of Li and Co can increase optical transparency, work function, electrical conductivity and hole mobility of NiO_x film. Furthermore, experimental results certified that coating CH₃NH₃PbI_xCl_3-x perovskite films on Li and Co-NiO_x electrode interlayer film can improve chemical stability and absorbing ability of sunlight than the pristine NiO_x. Consequently, the power conversion efficiency (PCE) of PSCs has a great improvement from 14.1% to 18.7% when codoped with 10% Li and 5% Co in NiO_x. Moreover, the short-circuit current density (Jsc) was increased from 20.09 mA/cm² to 21.7 mA/cm² and the fill factor (FF) was enhanced from 0.70 to 0.75 for the PSCs. The experiment results demonstrated that the Li and Co codoped NiO_x can be a effective dopant to improve the performance of the PSCs.