Efficient and Stable Low-Bandgap Perovskite Solar Cells Enabled by a CsPbBr-Cluster Assisted Bottom-up Crystallization Approach.

Recently, low-bandgap formamidinium lead iodide FAPbI-based perovskites are of particular interest for high-performance perovskite solar cells (PSCs) due to their broad spectral response and high photocurrent output. However, to inhibit the spontaneous α-to-δ phase transition, 15-17% (molar ratio) of bromide and cesium or methylammonium incorporated into the FAPbI are indispensable to achieve efficient PSCs. In return, the high bromide content will increase bandgap and narrow the spectral response region. If simply reducing the bromide content, the corresponding PSCs exhibit inferior operational stability due to α-to-δ phase transition, interface degradation, and halide migration. Herein, we report a CsPbBr-cluster assisted vertically bottom-up crystallization approach to fabricate low-bromide (1% ∼ 6%), α-phase pure, and MA-free FAPbI-based PSCs. The clusters, in the size of several nanometers, could act as nuclei to facilitate vertical growth of high quality α-FAPbI perovskite crystals. Moreover, these clusters can show further intake by perovskite after thermal annealing, which improves the phase homogeneity of the as-prepared perovskite films. As a result, the corresponding mesoporous PSCs deliver a champion efficiency of 21.78% with photoresponse extended to 830 nm. Moreover, these devices show remarkably improved operational stability, retaining ∼82% of the initial efficiency after 1,000 h of maximum power point tracking under 1 sun condition.