Robust carrier formation process in low-band gap organic photovoltaics

Abstract

By means of femto-second time-resolved spectroscopy, we investigated the carrier formation process against film morphology and temperature (T) in highly-efficient organic photovoltaic, 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]] (PTB7)/[6,6]-phenyl C 71-butyric acid methyl ester (PC 70 BM) solar cells. We found that the carrier formation efficiency (φ CF) from an absorbed photon is nearly independent of the film morphology, indicating that the internal quantum efficiency (φ IQ) is mainly governed by the carrier transfer efficiency (φ CT) to the electrodes. The activation energy (E a = 0.5 - 0.8 meV) of φ CF is significantly low, which suggests an extended charge-transfer state around the PTB7/PC70BM interface. © 2013 AIP Publishing LLC.