Formamidinium Lead Iodide‐Based Inverted Perovskite Solar Cells with Efficiency over 25 % Enabled by An Amphiphilic Molecular Hole‐Transporter

Abstract

AbstractFormamidinium lead iodide (FAPbI3) represents an optimal absorber material in perovskite solar cells (PSCs), while the application of FAPbI3 in inverted‐structured PSCs has yet to be successful, mainly owing to its inferior film‐forming on hydrophobic or defective hole‐transporting substrates. Herein, we report a substantial improvement of FAPbI3‐based inverted PSCs, which is realized by a multifunctional amphiphilic molecular hole‐transporter, (2‐(4‐(10H‐phenothiazin‐10‐yl)phenyl)‐1‐cyanovinyl)phosphonic acid (PTZ−CPA). The phenothiazine (PTZ) based PTZ−CPA, carrying a cyanovinyl phosphonic acid (CPA) group, forms a superwetting hole‐selective underlayer that enables facile deposition of high‐quality FAPbI3 thin films. Compared to a previously established carbazole‐based hole‐selective material (2‐(3,6‐dimethoxy‐9H‐carbazol‐9‐yl)ethyl)phosphonic acid (MeO−2PACz), the crystallinity of FAPbI3 is enhanced and the electronic defects are passivated by the PTZ−CPA more effectively, resulting in remarkable increases in photoluminescence quantum yield (four‐fold) and Shockley‐Read‐Hall lifetime (eight‐fold). Moreover, the PTZ−CPA shows a larger molecular dipole moment and improved energy level alignment with FAPbI3, benefiting the interfacial hole‐collection. Consequently, FAPbI3‐based inverted PSCs achieve an unprecedented efficiency of 25.35 % under simulated air mass 1.5 (AM1.5) sunlight. The PTZ−CPA based device shows commendable long‐term stability, maintaining over 90 % of its initial efficiency after continuous operation at 40 °C for 2000 hours.