V_OC Over 1.4 V for Amorphous Tin-Oxide-Based Dopant-Free CsPbI₂Br Perovskite Solar Cells

CsPbI₂Br perovskite solar cells have attracted much attention because of the rapid development in their efficiency and their great potential as a top cell of tandem solar cells. However, the V_OC outputs observed so far in most cases are far from that desired for a top cell. Up to now, with various kinds of treatments, the reported champion V_OC is only 1.32 V, with a V_OC deficit of 0.60 V. In this work, we found that aging of the SnCl₂ precursor solution for the electron-transporting layer can promote the V_OC of CsPbI₂Br solar cells by employing a dopant-free-polymer hole transport material (HTM) over 1.40 V and efficiency over 15.5% with high reproducibility. With the champion V_OC of 1.43 V, the V_OC deficit was reduced to <0.50 V, which is achieved for the first time. This simple technique of SnCl₂ solution aging forms a uniform and smooth amorphous SnO film with pure Sn⁴⁺, elevates the conduction band of SnO, and reduces the interfacial gaps and the trap state density of the device, resulting in enhancement in average V_OC from ∼1.2 V in the nonaged case to ∼1.4 V in the aged case. Furthermore, the device using an aged SnCl₂ solution also exhibits a much better long-term stability than that made of the fresh solution. These achievements in dopant/additive-free CsPbI₂Br solar cells can be useful for future research on CsPbI₂Br and tandem solar cells.