High efficient planar-heterojunction solar cells achieved by using a smooth CH3NH3PbI3 film via a new approach of forming the PbI2 nanostructure together with strategically high CH3NH3I concentration

Abstract

In a typical two-step sequential deposition of perovskites such as MAPbI 3 (MA= CH 3 NH 3 +, PbI 2 is first deposited on the substrate (mesoporous or planar scaffold) by spin-coating or vacuum evaporation, subsequently transformed into the perovskite (MAPbI 3 ) by exposing it to an anhydrous isopropanol (IPA) solution of MAI. For the two-step sequential deposition, the conversion and film morphology of the final perovskite film strongly depend on the initial PbI 2 film during the first step of the process. Conventionally, PbI 2 from dimethyl formamide (DMF) solution tends to form a layered and dense crystalline film on a flat substrate However, the complete conversion of PbI 2 to perovskite on exposure to the MAI solution usually requires several hours. However, this long reaction time in MAI solution could lead to the dissolution of perovskites. These drawbacks make it difficult to fabricate planar-structured PSCs by sequential deposition method. To our knowledge, a simple and effective method for fast conversion of PbI 2 film into perovskite on flat substrate via two-step sequential deposition process at room temperature has not been reported. Although many groups have demonstrated the PbI 2 residue in perovskite film has a positive effect on the efficiency of PSCs, the effect of PbI 2 residue on the long-term stability of PSCs is unclear. In this work, we demonstrate a new approach for forming the PbI 2 nanostructure and the use of high CH 3 NH 3 I concentration which are adopted to form high quality (large crystal size and smooth) perovskite film with better photovoltaic performances. On one hand, self-assembled porous PbI 2 is formed by incorporating small amount of rationally chosen additives into the PbI 2 precursor solutions, which significantly facilitate the conversion of perovskite without any PbI 2 residue. On the other hand, through employing a relatively high CH 3 NH 3 I concentration, a firmly crystallized and uniform CH 3 NH 3 PbI 3 film is formed. As a result, a promising power conversion efficiency (PCE) of 16.21% is achieved in planar-heterojunction PSCs. Furthermore, we experimentally demonstrate that the PbI 2 residue in perovskite film has a negative effect on the long-term stability of devices.