Carrier density effect on recombination in PTB7-based solar cell.

Organic solar cells (OSCs) are promising alternatives to the conventional inorganic solar cells due to their low-cost processing and compatibility with flexible substrates. The development of low band-gap polymer, e.g., 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]thiophenediyl]] (PTB7), increases the power conversion efficiency (PCE) in the last decade. Here, we investigated the interrelation between the instantaneous carrier density (n) per donor (D)/acceptor (A) interface area and the carrier density (ncollected) collected as photocurrent in PTB7/C70 heterojunction (HJ) device. By means of the time-resolved spectroscopy, we confirmed that the exciton-to-carrier conversion process takes place within ~1 ps at the D/A interface of the PTB7/C70 HJ device. We further determined the absolute magnitude of n by combination of the time-resolved and electrochemical spectroscopies. We found that the carrier recombination becomes dominant if n exceeds a critical concentration (nc = 0.003 carriers/nm(-2)). We confirmed that a similar behaviors is observed in the PTB7/[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) bulk heterojunction (BHJ) device. Our quantitative investigation based on the HJ device demonstrates that the fast carrier escape from the D/A interface region is indispensable for high PCE, because the carrier accumulation nonlinearly accelerates the carrier recombination process.