Enhancing Phase Separation and Photovoltaic Performance of All-Conjugated Donor-Acceptor Block Copolymers with Semifluorinated Alkyl Side Chains

Phase separation of all-conjugated donor-acceptor block copolymers is more difficult to achieve compared to classical coil-coil systems owing the intrinsic similarity of the two blocks having both rigid conjugated backbones and alkyl side chains and their generally low degrees of polymerization. Here we demonstrate that side chain fluorination of a poly(carbazole-alt-dithienylbenzothiadiazole) segment (SF-PCDTBT), to be used as electron acceptor block in combination with poly(3-hexylthiophene) P3HT as donor block in all-conjugated donor-acceptor block copolymers of type SF-PCDTBT-b-P3HT, strongly increases dissimilarity between P3HT and SF-PCDTBT leading to phase separation for already moderate molar masses Key to the successful synthesis of a new TBT-monomer with semifluorinated side chains is a direct arylation step that elegantly bypasses classical cross-coupling reactions in which the semifluorinated side chain causes low yields. Suzuki polycondensation of the semifluorinated TBT monomer with a suitable carbazole comonomer and in situ termination by P3HT-Br is optimized extensively with respect to the yield of the end-capping efficiency and molar mass control of the PCDTBT segment. When the fluorinated side chains are replaced by hydrogen (H-PCDTBT) or by n-hexyl chains (hex-PCDTBT), the tendency for phase separation with covalently connected P3HT is much reduced as shown by differential scanning calorimetry and grazing incidence small-angle scattering measurements on thin films. Favorably, of all the block copolymers made only SF-PCDTBT-b-P3HT is inicrophase separated, exhibits face-on orientation of P3HT domains, and additionally displays surface segregation of the SF-PCDTBT segment at the polymer/air interface. All of these properties are beneficial for single layer single component solar cells. SF-PCDTBT-b-P3HT exhibits the best solar cells performance with a high open-circuit voltage of 1.1 V and a power conversion efficiency of similar to 1% which largely outperforms devices based on the analogous H-PCDTBT-b-P3HT and hex-PCDTBT-b-P3HT.