Effects of annealing and degradation on regioregular polythiophene-based bulk heterojunction organic photovoltaic devices

This work investigates the effect of annealing and aging on the performance of devices made from blends of poly(3-hexylthiophene) (P3HT) and 6,6-phenyl C61-butyric acid methyl ester (PCBM) with different regioregularity (RR) for P3HT (98% and 94%). We compare different parameters of fabricated photovoltaic devices, including power conversion efficiency, extracted charge mobilites, external quantum efficiency and optical density as well as current–voltage characteristics, before and after annealing. We find that devices based on higher RR P3HT initially have a better performance but are more prone to degradation due to tendency for micor-phase segregation of donor and acceptor domains, as confirmed by electron and optical microscopy. The lower RR devices, on the other hand, demonstrate a minor increase in hole mobility and PCE in post annealed conditions, since annealing facilitates better π –π stacking when there is a lower tendency for segregation. It was found that the hole mobility is almost unchanged by RR and annealing time, and its minor changes cannot explain the main cause of the performance degradation. The electron mobility, however, decreases with the annealing time for both RR values. The mobility drop for the 98% RR devices was greater than that of the 94% RR ones. This is attributed to extensive phase segregation in blends of 98% RR P3HT and PCBM resulting in discontinuity of the electron transporting phase, i.e, PCBM. As such, the lower RR devices are found to be more stable during a four month degradation experiment, while the efficiency of higher RR devices decreases in the same period of time. These results show that electron transport plays a critical role in the degradation of effective charge mobility of P3HT:PCBM solar cells, and that the selection of RR of the polymer is critical in determining not only the initial performance and efficiency of an organic photovoltaic device but also its durability.