Dopant-Free Hole Transporting Molecules for Highly Efficient Perovskite Photovoltaic with Strong Interfacial Interaction

One of the attractive ways to develop efficient and cost-effective inverted perovskite solar cells (PVSCs) is through the use of dopant-free hole transporting materials (HTMs) with facile synthesis and a lower price tag. Herein, two organic small molecules with a fluorene core are presented as dopant-free HTMs in inverted PVSCs, namely, FB-OMeTPA and FT-OMeTPA. The two molecules are designed in such a way they differ by replacing one of the benzene rings (FB-OMeTPA) with thiophene (FT-OMeTPA), which leads to a significantly improved coplanarity as manifested in the redshift of the absorbance and a smaller bandgap energy. Density functional theory calculations show that FT-OMeTPA has a strong Pb2+–S interaction at the FT-OMeTPA/perovskite interface, allowing surface passivation and facilitating charge transfer across interfaces. As a result, the PVSCs based on FT-OMeTPA exhibit a much higher hole mobility, power conversion efficiency, operational stability, and less hysteresis as compared with devices based on FB-OMeTPA.