Improved Hole Extraction in Carbon-Based CsPbIBr2 Perovskite Solar Cells through Interface Crystalline Reconstruction for High Open-Circuit Voltages over 1.37 V

The interfacial charge recombination at the interface between the carbon electrode and the perovskite layer is a key factor limiting the device performance in the carbon-based hole-free system. The mainstream of the existing strategies is to improve the film quality through the addition of ligands or passivators. As the deposition process of carbon electrodes requires heat treatment, the use of conventional hole transport layer materials (such as Spiro-OMeTAD) is limited, leaving the hole extraction at the carbon–perovskite interface inefficient. In this work, we propose an in situ reannealing strategy that leverages the thermal stabilization to improve the efficiency of interfacial hole extraction in all-inorganic CsPbIBr2 solar cells. The reannealing process changes the δ-phase perovskite layer eroded by moisture back to the α-phase, and this phase transition creates a closer chemical contact between the perovskite and the carbon layer. It was also found that the graphite component in the carbon electrode was involved in the repair of interfacial defects during the reannealing, which achieved the “self-passivation” of the perovskite–carbon interface. With the reannealing process, the PEC of the cell was improved from 9.21% to 11.92% with a high VOC of 1.37 V, and the moisture stability was significantly improved.