Improvement in Power Conversion Efficiency and Performance of P3HT/PCBM Solar Cells Using Dithiafulvalene-Based π-Conjugated Oligomers

The improvement in the power conversion efficiency (PCE) and performance of poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C(61)butyric acid methyl ester (PCBM) solar cells are reported by using dithiafulvalene (DTF)-based conjugated oligomers ((poly[2-(9H-fluoren-9-ylidene)-4,5-bis(hexylthio)-1,3-dithiole-ran-(2,1,3-benzothiadiazole)] (PTBT) and poly[2,7-(9,9-dihexylfluorene)-ran-(2-(9H-fluoren-9-ylidene)-4,5-bis(hexylthio)-1,3dithiole]-ran-(2,1,3-benzothiadiazole)] (PFTBT)) that contain a DTF unit, which serves as an electron-rich donor, and a benzothiadiazole group, which serves as an electron-deficient acceptor in the main chain. A P3HT/PCBM device with 5 wt% PTBT exhibits an efficiency of up to 1.78%, which is higher than that of a device composed only of a P3HT/PCBM blend (1.01%). Introducing 2 wt% PTBT into the P3HT/PCBM blend substantially increases the charge-carrier mobility from 2.01 x 10(-4) to 4.59 x 10(-4) cm(2) V(-1)s(-1). The improvement of the PCE is attributed to improved charge transport in the device and an increased open-circuit voltage, suggesting that blending PTBT increases the intermolecular interaction of the molecules. In addition, doping the oligomer in ambient atmospheric conditions enhances the stability of these devices.