Side-chain substitution of poly(3-hexylthiophene) (P3HT) by PCBM via postpolymerization: an intramolecular hybrid of donor and acceptor

A novel double cable donor–acceptor poly(3-hexylthiophene) (P3HT) derivative (PCBM-Ph-P3HT) was successfully synthesized via a facile postpolymerization approach followed by a Steglich esterification reaction, which was doped in a P3HT:PCBM (PCBM = [6,6]-phenyl-C61-butyric acid methyl ester) blend solution so as to study its effect on the device performance of P3HT:PCBM bulk heterojunction polymer solar cells (BHJ-PSCs). The successful attachment of PCBM as the side chain onto the P3HT backbone via a phenyl group is confirmed by both 1H NMR and UV-Vis spectroscopic studies. The fluorescence intensity of PCBM-Ph-P3HT in chlorobenzene shows ∼63% quenching of the fluorescence of P3HT, indicating a strong intramolecular photoinduced electron transfer between P3HT and PCBM moieties. PCBM-Ph-P3HT was doped into P3HT:PCBM blend solution with different doping ratios (1, 2 and 5 wt%) and the surface morphology of PCBM-Ph-P3HT-doped film was studied by AFM, indicating that PCBM-Ph-P3HT promotes the bicontinuous interpenetrating networks of the P3HT and PCBM components due to its amphiphilicity toward P3HT and PCBM like a “surfactant”. At the optimum PCBM-Ph-P3HT doping ratio of 1 wt%, the P3HT:PCBM BHJ-PSC devices doped by PCBM-Ph-P3HT show an enhanced power conversion efficiency (PCE), with the maximum PCE being 3.40% which has ca. 12% enhancement compared to that of the reference P3HT:PCBM device. The enhancement of PCE is evidently attributed to the increase of short-circuit current (Jsc), and proposed enhancement mechanism is that the improvement on the bicontinuous interpenetrating networks of the P3HT and PCBM components upon PCBM-Ph-P3HT doping leads to the increase of the charge carrier mobility.