Understanding the Role of Additives in Improving the Performance of Bulk Heterojunction Organic Solar Cells

Abstract

Organic solar cells based on the bulk heterojunction (BHJ) represent one of the most promising technologies for next-generation solar energy conversion due to their low-cost and scalability. In the last ten years, the highest power conversion efficiency (PCE) obtained from organic solar cells has risen from 2.5 to 12%. Controlling BHJ morphology is essential for achieving high PCE. Processing BHJ layers with solvent additives has proven to be an effective strategy toward achieving BHJ morphology correlated with high efficiency. As examples, the incorporation of a small percentage of solvent additives, such as 3v% and 0.25 v% of 1,8-diiodooctane (DIO) in PTB7:PC71BM and DTS(PTTh2)2:PC71BM, respectively, results in a nearly doubling of device efficiency. However, the mechanism underlying their role in improving power conversion efficiency (PCE) has remained elusive. The general picture of a correlation between solvent constituents and the morphology within the deposited active layer film is clear, but the nature of this correlation is unknown—and it must be deciphered in order to adopt a more rational approach to improving device function. Using a combination of characterization techniques including x-ray and neutron scattering as well as advanced electron microscopy, we probe the influence of additives on active layer morphology in several state-ofthe-art BHJ blend systems. Through the integrated analysis of realand reciprocal-space data, we uncover new insights into the role of additives that can be generalized across multiple materials.