Perovskite Quantum Dot Solar Cells with 15.6% Efficiency and Improved Stability Enabled by an α-CsPbI3/FAPbI3 Bilayer Structure

We developed lead halide perovskite quantum dot (QD) solar cells with a combinational absorbing layer based on stacked α-CsPbI3 and FAPbI3. α-CsPbI3 QDs, with a relatively wide bandgap of 1.75 eV, are not ideal for single-junction solar cells. We show that the absorption can be broadened by the introduction of another QD layer with a narrower bandgap like FAPbI3. The α-CsPbI3/FAPbI3 structure together with thermal annealing can improve the electrical coupling in the FAPbI3 layer and induce A-site cation exchange to develop a graded heterojunction for more efficient charge extraction. A highest power conversion efficiency of 15.6% and improved ambient stability have been achieved for the bilayer structured solar cells. More interestingly, the perovskite QDs provided a facile way to fabricate multiple layers and quantum junctions for optoelectronic applications via layer-by-layer deposition, which cannot be realized in solution-processed perovskite thin-film devices.