Study of Vertical and Lateral Charge Transport Properties of DPP-Based Polymer/PC61BM Films Using Space Charge Limited Current (SCLC) and Field Effect Transistor Methods and their Effects on Photovoltaic Characteristics

We studied the vertical and lateral charge transport characteristics of a diketopyrrolopyrrole polymer donor (D)–PC61BM acceptor (A) system by measuring the space charge limited current (SCLC) mobility and field-effect mobility respectively. It was found that with an increase in annealing temperature, the SCLC hole mobility decreased for the pure polymer (PDBFBT) but increased for the PDBFBT:PC61BM blends, which could be explained by changes in the crystallinity and crystal orientation (edge-on versus face-on). The pure PDBFBT and most blend films showed the maximum field-effect hole mobility (µh) when annealed at 100°C, which then declined as the annealing temperature was further increased. Surprisingly, the D/A = 1/1 blend films annealed at high temperatures exhibited an abrupt increase in the field-effect µh. This unusual phenomenon was interpreted by the antiplasticization effect of PC61BM, which promoted the molecular organization of the polymer. The effect of annealing on the carrier mobility was further correlated with the performance of inverted organic solar cell devices with the PDBFBT:PC61BM blend (D/A = 1/3). Thermal annealing at high temperatures (>100°C) was found to obstruct electron transport and cause the device performance to significantly deteriorate.