Amorphous Donor Polymers for Thermally Stable Organic Photovoltaics with Dilute Donor Content

A completely amorphous donor polymer PIDT-T8BT recently showed enhanced photovoltaic performance when paired with Y6 using unconventional low donor:acceptor (D:A) ratios, while its analogue PIDT-T12BT achieved high PCEs at conventional 1:1.2 ratios. Two new polymers, PIDT-T6BT and PIDT-T10BT, were synthesized with varying side-chain lengths. All exhibited amorphous phase behavior, with shorter side-chains yielding higher glass transition temperatures. Photovoltaic devices based on PIDT-TxBT (x=6, 8, 10, 12):Y6 were investigated across broad D:A ratios, revealing different optimal ratios for each donor. Thermal stability testing at 85 °C showed low donor content PIDT-T6BT:Y6 (1:10) devices had significantly enhanced stability. Notably, PIDT-T8BT:Y6 (1:10) devices demonstrated superior thermal stability compared to high-performing PM6:Y6 and D18:Y6 systems at both conventional and low D:A ratios. Dynamic mechanical thermal analysis revealed that blends with less polymer retained storage modulus at higher temperatures, indicating enhanced morphological stability, particularly for 1:10 blends. Charge carrier dynamics studies showed that stable 1:10 device performance results from enhanced carrier mobility offsetting decreased carrier lifetime after thermal aging, minimizing charge collection changes. Conversely, PM6:Y6, D18:Y6, and 1:1.2 PIDT-T6BT:Y6 devices exhibited additional performance degradation from poor charge generation due to undesirable phase-separated morphologies.