Inhibited aggregation of lithium salt in spiro-OMeTAD toward highly efficient perovskite solar cells

Organic small molecule material, spiro-OMeTAD, is widely used in perovskite solar cells as the hole transport material due to its desirable properties, such as suitable highest occupied molecular orbital energy levels, high solubility and good film forming ability. However, the aggregation and hydrolysis of the common additive, LiTFSI, in the spiro-OMeTAD, lead to the existence of voids/pinholes in the hole transport layer, which limits further improvement of perovskite solar cell efficiency. Here, we develop a low-cost and easy-fabrication technique to substantially reduce the undesired voids in hole transport layer by modifying it with commercially available PbI2. The strong interaction between the PbI2 and the TBP retards the evaporation of TBP, which hinders the agglomeration of Li-TFSI and generation of voids in the HTL. Consequently, the HTL shows a dramatically improved mobility and efficiency of charge carrier extraction, leading to a better response to steadystate condition with less than 1s for the corresponding photovoltaic device (where the control device shows more than 26s). A champion power conversion efficiency of over 20% is obtained from PbI2 added devices by optimizing its adding amount. This work provides a facile and effective strategy to improve the performance of the hole transport materials and perovskite solar cells.