Carrier generation and transport in bulk heterojunction films processed with 1,8-octanedithiol as a processing additive

Abstract

Improved performance of polymer-based solar cells based on poly[2,6-(4,4-bis-(2-ethylhexyl)-$4H$ -cyclopenta[2,1-b;3,4-$b^{\prime }$ ]dithiophene)-alt-4,7-(2,1,3-benzo-thiadiazole)] PCPDTBT has been obtained by using 1,8-octanedithiol (ODT) as a processing additive in the polymer solution used to spin cast the bulk heterojunction films. Although ultrafast spectroscopy studies indicate that the carrier losses are reduced in the films processed with ODT [similar to the reduced carrier losses after thermal annealing of bulk heterojunction (BHJ) materials made from P3HT:PCBM], the magnitude of the reduction is not sufficient to explain the observed factor of 2 increase in the power conversion efficiency. From the analysis of carrier transport in field effect transistors, we find increased electron mobility in the $\text{PCPDTBT}:{\text{PC}}_{70}\text{BM}$ composites when fabricated with ODT, which is indicative of enhanced connectivity of ${\text{PC}}_{70}\text{BM}$ networks. The improved electron mobility appears to be the primary origin of the improved power conversion efficiency in BHJ films.