Instability mechanism of phosphomolybdic acid solution and its effect on organic solar cells as a hole-transporting layer

Phosphomolybdic acid (H₃PMo₁₂O₄₀, PMA) containing twelve MoO₃ units exhibits an efficient hole-transporting property in organic electronics. Low-cost, easy synthesis and high thermal-stability PMA is promising solution-processed hole-transporting material for high efficiency organic solar cells (OSCs). We have studied the degradation mechanism of phosphomolybdic acid in methanol solution that was stored for days, and further investigated its effect on the performance of OSC devices. We found that PMA had been slowly reduced to phosphomolybdic heteropoly blue during the solution storage process. Besides the morphology of the PMA film, its energy level and molybdenum valence which changed with solution storage time were also investigated. The instability of PMA in methanol solution resulted in a significant negative effect on the device performance. Comparing to the PM6:Y6 device based on PMA from pristine solution, the power conversion efficiencies of OSCs with PMA hole transporting layer prepared from solution stored for 8 days and 16 days decreased from 14.69% to 3.01% and 0.58%, respectively. Fortunately, using other stable processing solvent systems instead of methanol for the fabrication of the PMA hole transporting layer has been found as an efficient method to avoid the degration of the device performance.