Cs2SnI6 nanocrystals enhancing hole extraction for efficient carbon-based CsPbI2Br perovskite solar cells

Carbon electrode-based perovskite solar cells (C-PSCs) promise long-term stability due to the chemically inertness and moisture resistance of carbon electrodes. However, the larger energy level mismatch between the perovskite and carbon electrode in C-PSCs results in lower hole extraction efficiency and poorer photovoltaic performance compared with the conventional metal electrode-based ones. In this work, a novel hole transport material (HTM) of Cs2SnI6 nanocrystals (CSI NCs) is synthesized and employed in CsPbI2Br C-PSCs. The suitable valence band maximum position of CSI (-5.55 eV) constitutes a gradient energy level alignment in CsPbI2Br/ CSI/carbon, which effectively accelerates the hole extraction from perovskite to carbon electrode and reduces nonradiative recombination loss at the interface. With the introduction of CSI NC HTM between CsPbI2Br perovskite film and carbon electrode, the power conversion efficiency (PCE) of resulting cell devices are increased from 13.16% (Jsc of 14.17 mA cm-2, Voc of 1.221 V, FF of 76.05%) to 14.67% (Jsc of 14.51 mA cm-2, Voc of 1.267 V, FF of 79.81%). To the our best knowledge, this is the first time that CSI was used as a HTM in PSCs and it has shown attractive prospects in improving hole extraction efficiency and photovoltaic performance in resulting PSCs.