High performance as-cast P3HT:PCBM devices: understanding the role of molecular weight in high regioregularity P3HT

The performance of bulk heterojunction (BHJ) organic solar cells is well-known to be influenced by the properties of the donor polymer employed such as its molecular weight (MW) and regioregularity. In this study, four different molecular weight batches of high regioregularity (100%) poly(3-hexylthiophene) (DF-P3HT) are investigated. Unlike other studies, here the RR of the P3HT is fixed (to 100%) and the MW of the polymer is varied to understand the influence of MW on P3HT physical properties such as its electrical, optical and thermal properties and microstructure using X-ray synchrotron techniques. Significantly, it is found that annealing has less of an influence the properties of pristine films of P3HT when the RR of the P3HT is very high. A similar approach is used to examine the physical properties and microstructure of P3HT:PCBM blend films. The properties of the blend films for different MW are correlated with the performance of the BHJ solar cells fabricated using P3HT:PCBM blends. A record high average efficiency of 3.8% for as-cast devices (no annealing or solvent additive) with best devices reaching performance over 4% is obtained for DF-P3HT:PCBM with MW of 44 kDa. Though there is a marginal variation in the performance of the devices with change in MW, no systematic variation in device performance as a function of MW is observed in contrast to other MW studies employed P3HT of lower RR. These findings are attractive in the light of the mass production of polymer solar cells such as via roll-to-roll printing where thermal annealing is not possible or desired.