Self-Organized Hole Transport Layers Based on Polythiophene Diblock Copolymers for Inverted Organic Solar Cells with High Efficiency

Novel fluoroalkyl side-chain diblock copolymers, poly(3-hexylthiophene)-block-poly[3-(4-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyloxy)phenyl)-decyloxy)thiophene] (P3HT-b-P3FAT), were successfully synthesized by Grignard metathesis (GRIM) polymerization. Driven by the low surface energy of fluoroalkyl side chains, the fluorinated polymers can spontaneously segregate on the surface of poly-(3-hexylthiophene) (P3HT) during spin-coating processes. As the P3HT block increases in the copolymer, higher concentrations of fluoropolymers are required to form the self-assembled monolayer on the surface. The fluorinated part forms an interfacial dipole that shifts the work function of the anode metal, while the P3HT block can interact with the P3HT donor for hole transport. With this self-assembly hole transport layer to align the energy levels, P3HT:PCBM photovoltaic devices are easily fabricated to achieve improved performance. Overall, devices prepared with 1.5 mg mL(-1) copolymer PFT-3HT with a 3:1 ratio of P3HT to P3FAT block in the active layer solution displayed PCE values of up to 4.6% (50% PCE increase over a PEDOT:PSS control device) and showed a significant long-term stability in excess of 300 h in air.