Effect of thermal annealing and incorporating TiO2 layer on the photovoltaic performance of single- and bi-layer bulk heterojunction devices based on phenylenevinylene copolymer and small molecule

We report the fabrication and characterization of organic photovoltaic devices using a conjugated alternating phenylenevinylene copolymer P in bulk heterojunction (BHJ) composites with PCBM. The device with copolymer P:PCBM (1:3) deposited at substrate temperature of 65 °C exhibits power conversion efficiency (PCE) of approximately 1.14% which is further improved up to 1.46%, when thermally annealed composite is used as active layer in the device. We have also studied the bi-layer BHJ device composed of copolymer P:PCBM and BTD-TNP:PPI BHJ layers and found that this device exhibits PCE of approximately 2.68%, which is further enhanced up to 2.98% upon thermal annealing. In both the single-layer and bi-layer BHJ devices the PCE has been enhanced i.e. 1.28% for single P:PCBM BHJ and 2.91% for bi-layer P:PCBM/BTD-TNP:PPI BHJ, when a TiO2 layer is inserted between the active layer and the top Al electrode. This enhancement has been attributed to (i) the increased photon absorption due to optical interference phenomena between the incident light and the reflected light from the top electrode and (ii) the improved electron collection, as the TiO2 layer acts as hole blocking layer. The enhancement in PCE (3.5%) upon post-annealing of the devices was attributed to the improved hole transport in the active layer due to the improved crystallization in the organic layer.