Tetrahydrofuran as Solvent for P3HT/F4-TCNQ Hole-Transporting Layer to Increase the Efficiency and Stability of FAPbI3-Based Perovskite Solar Cell

The effect of organic solvents on the crystalline structure of the hole-transporting poly(3-hexylthiophene) (P3HT) layer of perovskite solar cells (PSCs) was investigated in this study. UV–visible absorption spectroscopy revealed a higher degree of aggregation of P3HT in polar solvents, such as chloroform and tetrahydrofuran (THF), resulting in a high degree of crystallinity. The electrical conductance of F4-TCNQ-doped P3HT hole-transport layers was approximately twice as high in THF, compared to other conventional solvents such as 1,2-dichlorobenzene (DCB). The effective reduction of the residual PbI2 on the perovskite surface was observed using X-ray diffraction analysis. The use of THF as the solvent improved the efficiency of the P3HT/F4-TCNQ hole-transporting layer to 13.54% from that of 9.94% with DCB. In addition, the stability was increased when using THF, and the efficiency was maintained at over 80% of the initial value after 1080 h at 85 °C and relative humidity of 40% without encapsulation, whereas 50% of the initial efficiency was lost when DCB was used. The use of THF as the solvent not only controls the crystalline structure of P3HT but also reduces the residual PbI2 on the surface of the perovskite, thereby increasing the efficiency and stability of the PSC.