Influence of Intermixed Donor and Acceptor Domains on the Ultrafast Charge Generation in Bulk Heterojunction Materials

Abstract

Understanding how intermixed domains and pure domains in bulk heterojunction (BHJ) thin films affect charge generation and recombination in polymer solar cells (PSCs) is important to further improve PSC device performance. We control the proportion of intermixed domains and pure domains in an amorphous polymer and fullerene BHJ thin films by varying the fullerene concentration. By studying these thin films with ultrafast spectroscopy, we found that the coherent ultrafast charge generation process is nonexistent in this BHJ system due to the lack of pure polymer domains. We also found that the BHJ thin film with 100% intermixed domains begins charge recombination immediately after the initial charge generation process, while the BHJ thin film with pure fullerene domains showed an additional slow excitonic charge generation process before a slower charge recombination takes place. These results clearly demonstrate that pure domains are important for optimal BHJ solar cell performance.