Enhanced performance of perovskite solar cells with P3HT hole-transporting materials via molecular p-type doping

The conducting polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) has been widely used as a polymeric hole-transporting material (HTM) in inorganic-organic perovskite solar cells (PSCs). However, pristine P3HT-based PSC devices typically exhibit mediocre overall performance, mainly due to its relatively low conductivity. Herein, we successfully introduced tetrafluoro-tetracyano-quinodimethane (F4TCNQ) as an efficient p-type dopant for P3HT as a HTM in mesoscopic PSCs. The overall performance was significantly enhanced after the introduction of F4TCNQ into P3HT. Under an optimal doping condition (1.0%, w/w), an impressive power conversion efficiency (PCE) of 14.4% was achieved, which was considerably higher than the pristine P3HT based devices (10.3%). The dramatic improvement of the PCE originated from the increase of the photocurrent density and fill factor, strongly correlated to the significant increase of the bulk conductivity of F4TCNQ doped P3HT. After doping with 1.0% F4TCNQ, the conductivity of the P3HT film was significantly increased by more than 50 times. UV-Vis and Fourier transform infrared spectroscopy (FTIR) measurements indicated that p-doping occurs via the electron transfer from the highest occupied molecular orbital (HOMO) level of P3HT to the lowest unoccupied molecular orbital (LUMO) level of the F4TCNQ, which led to a substantial increase of the bulk conductivity. Furthermore, PSCs based on the P3HT: F4TCNQ composite as a HTM also exhibited superior long-term stability under ambient conditions with a humidity of 40%. F4TCNQ was thus demonstrated to be an effective p-dopant for P3HT to improve the electrical properties and thereby the overall performance for highly efficient and stable PSCs.