Improvement of bulk heterojunction organic solar cells based on PTB7:PC61BM with small amounts of P3HT

Abstract

The effect of a small amount of poly(3-hexylthiophene) (P3HT) additive on the performance of bulk heterojunction (BHJ) organic solar cells based on poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]:[6,6]-phenyl-C61-butyric acid methyl ester (PTB7:PC61BM) was investigated. The optimized solar cell with 5% P3HT weight fraction showed a marked improvement, with an open-circuit voltage of 0.76 V, a short-circuit current density of 17 mA/cm2, a fill factor of 0.40, and a power conversion efficiency of 5.11%. A notable reduction in the domain sizes of the active layers with 5% P3HT weight fraction was observed by atomic force microscopy. This suggests that the interface area between the PTB7 and PC61BM domains increased, resulting in an enhanced exciton dissociation. This also suggests that the small amount of P3HT in the PTB7:PC61BM active layer acted as a mediator to enhance the hole current from PTB7 domains to the poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) anode buffer layer. Possible mechanisms for these phenomena are discussed on the basis of the results of surface energy analysis.