Phase Diagram of Conjugated Polymer Blend P3HT/PF12TBT and the Morphology-Dependent Photovoltaic Performance

Abstract

In this study, we systematically investigated the morphological evolution of poly(3-hexylthiophene) (P3HT) and poly{2,7-(9,9-didodecyl-fluorene)-alt-5,5-[4′,7′-bis(2-thienyl)-2′,1′,3′-benzothiadiazole]} (PF12TBT) blend thin films cast from o-dichlorobenzene (oDCB) solution and its effect on photovoltaic performance. Blend ratio and film drying time both played a crucial role in determining polymer blend film morphologies, including phase-separated structure and domain size. An apparent morphological transformation from uniform morphology to droplet and bicontinuous structure could be observed with the P3HT/PF12TBT blend ratio ranging from 10/90 to 90/10. Uniform morphology resulted from one component becoming dominated, and thus no obvious phase separation could be observed. The formation of bicontinuous morphology via a spinodal decomposition mechanism which emerged at nearly equal blend ratio could be attributed to the similar and favorable solubility for both polymers in processing solvent, while the asymmetric composition led to the formation of droplet morphology via a spinodal decomposition mechanism. In addition, the increased domain size which resulted from coarsening of adjacent domains could be observed with the extending film drying time. Further expanding drying time, the thin films exhibited unsharp morphology with large domains which could be attributed to the mixing of polymer blend with the formation of vast P3HT crystallites and the aggregation of both donor and acceptor via molecular diffusion. On the basis of the varying morphologies, an approximate phase diagram of P3HT/PF12TBT blend was depicted. In order to demonstrate the effect of various morphologies on photovoltaic performance, devices based on films with three kinds of blend ratios (40/60, 50/50, and 60/40) while undergoing different drying time (20, 30, and 45 s) were prepared. As the drying time extended, a gradual decreased power conversion efficiency (PCE) could be observed for all blend ratio devices, which resulted from the dramatic morphological transformation from small domains to intermediate domains and then upsharp morphology with large domains ultimately.