Influence of water-soluble polythiophene as an interfacial layer on the P3HT/PCBM bulk heterojunction organic photovoltaics

Novel HT-poly[3-(6'-N,N,N-trimethylammonium)-hexyl thiophene] (P3HTN) is developed and the use of the water-soluble thiophene as an interfacial layer for low-cost poly(3-hexylthiophene):phenyl-C61 butyric acid methyl ester (P3HT:PCBM) organic photovoltaic cells with high stability in air is investigated. When P3HTN is simply inserted between the active layer and the cathode as an interfacial dipole layer by spin-coating, the open-circuit voltage (V-oc) and short-circuit current density (J(sc)) of photovoltaic cells with high work function Al metal cathodes dramatically increase. Resulting from a reduction of the metal work function and improved electron extraction efficiency, the power conversion efficiency (PCE) of the devices annealed in air is enhanced from 1.8% to 3.28%. In particular, the analogue of the active layer as a buffer layer could improve interchain interactions between the P3HT and the P3HTN to modify the interfacial contact, consequently obtaining an unattainable enhancement J(sc), with respect to the interlayer polymer replaced with an unanalogous conjugated polymer. The results would supply useful information to understand the contribution of an interfacial layer on the photovoltaic performance.