Solution processed bulk heterojunction polymer solar cells with low band gap DPP-CN small molecule sensitizer

The improvement of near infrared wavelength sensitivity in the bulk heterojunction organic polymer solar cell based on poly (3-hexylthiophene) (P3HT) and PC70BM, by the addition of soluble DPP-CN small molecule is reported. By incorporating DPP-CN, the photosensitivity in the longer wavelength region was improved and the power conversion efficiency (PCE) has been reached to 4.37% as compared to 3.23% for the device based on P3HT:PC70BM blend. The increase in the PCE is attributed to the increase in light harvesting property of the blend and efficient dissociation of excitons into free charge carriers due to the increased number of D-A sites. The PCE has been further enhanced to 4.70%, when mixed solvent cast P3HT:DPP-CN:PC70BM blend is used as photoactive layer. The optical absorption spectra of the blend showed that the blend film cast from mixed solvent broadened the absorption wavelength range. This occurred as result of a large red shift of P3HT absorption peak and same time a widening and small red shift of DPP-CN absorption peak in the blend film. The improved light harvesting property of thermally annealed film is considered to the factor responsible for the improvement in the PCE. (C) 2012 Elsevier B.V. All rights reserved.