Manipulating Bulk Diffusion and Doping of Phosphomolybdic Acid Enables Organic Solar Cells with Improved Homogeneity

The application of organic solar cells (OSCs) significantly contributes to the advancement of sustainable development and the utilization of eco-friendly energy sources. The solution-based phosphomolybdic acid (PMA) doping method has been demonstrated to be effective in simplifying the fabrication of the hole transport layer (HTL) by modulating the optoelectronic properties of the photoactive layer (PAL). Here, the PMA doping method was applied to the state-of-the-art PM6:L8-BO-based OSCs. By laser beam-induced current (LBIC) and time-of-flight-secondary ion mass spectrometry (ToF-SIMS) measurements, we demonstrated that the diffusion of PMA is inhomogeneous with the PM6:L8-BO layer, which yielded poor device performance and broad distribution of power conversion efficiency (PCE) of the cells. To address this issue, a two-step spin-rinsing technique was developed that improved the homogeneity of PMA bulk diffusion over the PAL, leading to significantly improved device performance and narrowed PCE distribution. Most interestingly, this two-step procedure enables manipulation of the vertical diffusion of PMA. This is particularly important in thicker PAL solar cells. The optimized PM6:L8-BO cells with a PAL thickness of 280 nm exhibited an average PCE of 12.96%, which is 0.77% higher than that of the cell with thermal evaporated MoO₃ HTL.