Acidic Additives Enable Tunable Low Temperature Side Chain Cleavage and Performance Enhancement in Organic Photovoltaics

Thermal elimination of alkyl side chains from solution-processed organic semiconductors is a proven technique to maximize desirable properties such as conductivity, optical density, and thermal stability. However, conventional thermolysis (>200 °C, 10-30 min) of tertiary butyloxycarbonyl (Boc)-based side chains has consistently resulted in poor performance metrics across various types of devices (such as organic photovoltaics), which is often attributed to uncontrolled/undesired morphology from high annealing temperatures. To circumvent high temperatures while retaining benefits associated with removing alkyl chains, we present a new acid-catalyzed cleavage (ACC) method where acidic additives coprocessed with organic semiconductors enable alkyl-chain removal at mild annealing temperatures (90-140 °C within 30 min). Based on our mechanistically derived kinetic model, we demonstrate that both acid strength and concentration can effectively tune the temperature required for rapid thermolysis. Furthermore, we find the ACC method to be highly adaptable to numerous Boc-based functionalities. Finally, bulk heterojunction organic solar cells were chosen to showcase the benefits of lowering the cleavage temperature via ACC. With only 5 wt % diphenyl phosphate additive, the required cleavage temperature was lowered by 80 °C to a more amenable 140 °C for the cleavable side chain-based conjugated polymers POET-T2 and PffBT-T4-TCS60. The reduced annealing temperature improves the achievable power conversion efficiency of side chain-cleaved polymer-based organic photovoltaics with appreciable stability.