Insights into Charge Separation and Transport in Ternary Polymer Solar Cells

Although ternary polymer solar cells have more potential in realizing a high power conversion efficiency than the binary counterparts, the mechanism of exciton separation and charge transport in such complicated ternary systems is far from being understood. Herein, we focus on this issue and give a clear view on the detailed roles of the ternary components contributing to the device performance, through utilizing the technique of pump-probe photoconductivity spectroscopy combined with transient photoluminescence spectroscopy, for the first time for ternary polymer solar cells. The ternary photovoltaic devices are based on PBDB-T:ITIC:PC₇₁BM and present a dramatic improvement in efficiency in comparison to that of the binary counterparts. Systematic investigation reveals that the excitons generated in ITIC could be separated at the interface of PBDB-T:ITIC rather than ITIC:PC₇₁BM with holes injecting to PBDB-T. These holes together with those generated in PBDB-T contribute to the photocurrent of the devices. The aggregation of holes in PBDB-T would also weaken the exciton generation herein, and the electron injection to PC₇₁BM and ITIC would also be influenced. The key role of PC₇₁BM in the ternary devices is accepting the electrons from PBDB-T and transporting them to the cathode with a higher rate than that of ITIC. Thus, this article is of importance in constructing high-efficiency ternary polymer solar cells.