Enhanced Device Performance and Stability of Organic Photovoltaics Incorporating a Star-Shaped Multifunctional Additive

In this study, we developed a star-shaped diketopyrrolopyrrole (DPP)-based additive as an efficient morphology fixing agent for organic photovoltaics (OPVs). This conjugated small molecule, DPPTPTA, has four arms, with two terphenyl units and four alkyl azide groups. We tested the behavior of DPPTPTA after incorporating it into an active layer comprising poly[4,8-bis(5-(2-ethylhexyl)thien-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene)-2-carboxylate-2,6-diyl)] (PTB7-Th) and the PC61BM fullerene. Atomic force microscopy, UV–vis spectroscopy, optical microscopy, photoluminescence (PL) spectroscopy, and an X-ray photoelectron spectroscopy (XPS) depth profile revealed the effects of the resulting morphological change on the device performance and thermal stability. Compared with the PTB7-Th:PC61BM device prepared without DPPTPTA, the device incorporating this additive exhibited an increase in the power conversion efficiency (from 6.7 to 8.2%) and improved thermal stability. DPPTPTA served as a multifunctional additive, providing ladderlike energy levels for efficient charge separation, altering the morphology of the blend film for improved performance and suppressing the large-scale crystallization of PCBM (only a few fullerene crystals appeared in the active layer after holding the blend film at 150 °C for 18 h) by constructing local borders, ensuring long-term thermal stability. In contrast, the pristine device did not function after accelerated heating. Furthermore, the DPPTPTA-derived blend film displayed excellent solvent resistance and specific selective reactivity, as observed using FTIR and UV–vis spectroscopy.