Effect of Incorporation of Squaraine Dye on the Photovoltaic Response of Bulk Heterojunction Solar Cells Based on P3HT:PC₇₀BM Blend

We report the improvement of the light-harvesting property in the bulk heterojunction organic polymer solar cell based on poly(3-hexylthiophene) (P3HT) and PC 70 BM, with the incorporation of a near-infrared absorbing squaraine (SQ) dye, bis[4-(2,6-di- tert -butyl)vinyl-pyrylium] squaraine (TBU-SQ). With the incorporation of TBU-SQ dye (2.5% by wt) in a P3HT:PC 70 BM (1:1 wt ratio) blend, the power conversion efficiency (PCE) has been enhanced up to 4.55% as compared to 3.47% for the device based on the P3HT:PC 70 BM binary blend. The improvement in the photovoltaic performance with the incorporation of TBU-SQ attributed to the improvement in the light-harvesting efficiency in the near-infrared region of solar spectrum and increased exciton dissociation into free charge carriers in the ternary blended film. The PCE has been further enhanced to 5.15% when the thermally annealed P3HT:TBU-SQ:PC 70 BM blend was used as the photoactive layer. It was observed that the absorption profile of the active layer was broadened upon thermal treatment as a result of the red shift as well as widening of the P3HT absorption band and the slight red shift of the TBU-SQ absorption peak in the blended film. The improved light-harvesting property of the thermally annealed film and balanced charge transport in the device were attributed to the improvement in the PCE. These results show that TBU-SQ is a promising molecular sensitizer for increasing the PCE of P3HT:PC 70 BM-based polymer solar cells.