Effect of polymer nanoparticle formation on the efficiency of polythiophene based "bulk-heterojunction" solar cells

Polythiophenes are being intensively studied for use in polymer/fullerene bulk-heterojunction photovoltaic (PV) devices because they absorb light throughout the entire visible spectrum and show high hole mobility, which are the two most important parameters for a successful donor in these devices. A large body of anecdotal information exists about what morphological features lead to a high efficiency bulk-heterojunction PV device, but few design parameters for improved polymers exist. We compare two polythiophene isomers, poly(3-hexylthiophene) (P3HT) and poly(3,3"'-didodecylquaterthiophene) (PQT-12) to determine which parameters make these two polymers more and less suitable for use in bulk-heterojunction solar cells. We find that although they have nearly identical absorption spectra and hole mobilities, the PV devices made using P3HT are far superior to those made using PQT-12 because the PQT-12 forms crystalline nanoparticles in solution that do not form electrical connections in the thin films. The use of heat treatment for P3HT/fullerene films improves the layer morphology for PV applications but the same treatment for PQT-12/fullerene layers forces material separation on the micrometer length scale and further degradation of the electrical properties of the device.