Low Boiling Point Solvent Additives for Improved Photooxidative Stability in Organic Photovoltaics

Abstract

AbstractThe efficiency of organic photovoltaic (OPV) cells is rapidly increasing. However, many of the highest performance OPV devices are also the most unstable, including the two widely used benzodithiophene‐co‐thieno[3,4‐b]thiophene polymers, PTB7 and PTB7‐Th. Here it is shown that both the choice of pendant side chain, and solvents used for film deposition, have an important effect on active layer stability. Specifically, polymers with alkylthienyl side chains are found to be less stable than polymers with alkoxy side chains. Secondly, high boiling point solvent additives such as 1,8‐diiodooctane are found to dramatically accelerate photooxidation unless they are removed fully during device processing, which may be attributed to increased oxygen diffusion in films containing residual solvent. Replacing these higher boiling point solvent additives with lower boiling additives can greatly enhance the polymer stability without requiring high vacuum treatment, making these systems more scalable and compatible with roll‐to‐roll printing. Based on this finding, the non‐halogenated solvent system o‐xylene with 2% N‐methylpyrrolidone is explored as a route to achieving higher active layer stability, while maintaining the favorable, intermixed morphology achieved with higher boiling point, halogenated solvent additives. This result offers a promising way forward in the development of scalable, stable, and high‐efficiency organic solar cells.