The effect of processing additives for charge generation, recombination, and extraction in bulk heterojunction layers of all-polymer photovoltaics

Abstract

Bulk heterojunction all-polymer solar cells, fabricated with poly{[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]-alt-[2-(2-ethyl-hexanoyl)-thieno[3,4-b′]thiophen-4,6-diyl]} (PBDTTT-CT) as a donor polymer, and a acceptor polymer, poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (P(NDI2OD-T2)), have been demonstrated and have achieved a power conversion efficiency exceeding 3.7% by using 1,8-diiodooctane (DIO) as a processing additive. Based on the analysis of charge carrier dynamics (charge generation, separation, and extraction), we found that the appropriate ratio of processing solvent additive (5 vol. % DIO) leads to enhanced device performance and favorable morphological characteristics. This research, therefore, indicates that the incorporation of a DIO additive in all-polymer blends is an effective way to form a morphologically ideal heterojunction network and thereby improve charge carrier kinetics for efficient photovoltaic devices.