Fluorination of an N,N,N′,N′-Tetraphenylbenzidine Derivative as a Dopant-Free Hole-Transporting Material for Moisture-Resistant Perovskite Solar Cells

Perovskite solar cells (PSCs) have drawn extensive attention as promising next-generation photovoltaics due to their exceptional power conversion efficiency (PCE) and low fabrication cost over the past 10 years. However, their poor stability remains the biggest barrier for commercialization. Among the many causes for device degradation, the inherent moisture instability of the perovskite structure is one of the main reasons for the poor stability of the PSCs. In addition, chemical dopants used to improve PCE further induce the moisture sensitivity of PSCs. Here, in this study, we report on the stability improvement of the PSCs by incorporating a hydrophobic element, fluorine, into the N,N,N′,N′-tetraphenylbenzidine structure as a dopant-free hole-transporting material (HTM). The optoelectronic, electrochemical, theoretical, and charge transport properties of the synthesized HTM were characterized by UV–vis absorption spectroscopy, cyclic voltammetry, density functional theory calculation, and space charge-limited current method. The hydrophobicity and surface roughness of the HTM film were investigated by the water contact angle analysis and atomic force microscopy, respectively. The PSCs were fabricated with the HTM, a nonfluorinated control molecule, and the commonly used spiro-OMeTAD in an ambient environment without encapsulation. Scanning electron microscopy was used to obtain topographic information of the perovskite layer and cross-sectional images of the devices. The devices with the fluorinated HTM exhibited the best PCE of 13.96%. The stability study under a controlled humid environment showed the improved moisture stability of the fluorinated devices compared to the control devices, indicating that the hydrophobic HTM successfully inhibits the moisture-induced damage to the perovskite structure.