Self-Induced Type-I Band Alignment at Surface Grain Boundaries for Highly Efficient and Stable Perovskite Solar Cells

The open-circuit voltage (V_OC ) of perovskite solar cells (PSCs) is reported to be significantly weakened by carrier loss at the film surface. Here, the moisture condensation at only the upper surface of perovskite films is controlled by constructing an ultralow surface temperature. Then, type-I band alignment can be formed at the surface grain boundaries due to the etching effect of trace amounts of condensed moisture. The beneficially constructed surface type-I band alignment can effectively repel carriers and return them to the inside of the grain, significantly avoiding the carrier loss at films surface. As a result, a superior carrier lifetime exceeding 2.5 µs is obtained and the V_OC of PSC is remarkably boosted from 1.07 to 1.17 V. The minimum V_OC deficit of only 0.39 V enables a substantial gain in power conversion efficiency (PCE) from 20.2% to 22.4% in one-step spin-coating methods. Moreover, this innovation is versatile and a champion PCE of 23.2% is also achieved in two-step spin-coating methods.