Effect of 1,8-Diiodooctance additive on the charge carriers behavior in the PCPDTBT:PC71BM BHJ films investigated by using electric-field-induced optical second-harmonic generation measurement

Abstract

Electric-field-induced optical second harmonic generation (EFISHG) measurement was conducted to investigate the effect of 1,8-diiodooctane (DIO) additive, which is relevant to improve the efficiency of bulk heterojunction (BHJ) solar cells, on carrier behaviors in the BHJ layer, where the BHJ layer is a blend of [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) and regioregular poly [2,6-(4,4-bis-(2-Ethylhexyl)-4H-cyclopentane [2,1-b;3,4-b′] dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT). Using the laser wavelength of 1000 nm, we could selectively observe the electric field in the PCPDTBT phase of BHJ layer, which allowed us to determine the difference in charge accumulation at the interface with and without DIO. With adding DIO, a decrease in interfacial stored charge was observed, and this plays a role in improving the efficiency by about 25%. The DIO modified the internal field distribution so that the electric field promotes the electrons toward the ITO electrode and holes toward the PEDOT:PSS/Au electrode.