Deciphering Haze Formation in Black Electrochromic Polymer Films

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Abstract
Haze is a key parameter that characterizes the optical clarity of transmissive materials. In real‐world applications of organic electrochromic devices, however, the origin of haze formation and its connection to film morphology remain poorly understood, despite its significant impact on user safety and visual comfort. This study elucidates the origin of haze formation in electrochromic polymer (ECP) films by analyzing the morphology of a propylenedioxythiophene (ProDOT)‐based ECP‐black, a broadband, neutral density, absorbing ECP that is one of the most important in electrochromic field. ECP solutions were prepared with a controlled amount of antisolvent, followed by thermal annealing to tune the polymer morphology. Based on Mie scattering theory, surface roughness and polymer aggregate size are identified as the primary factors responsible for haze formation. In addition, haze is shown to vary with film thickness and aggregation‐induced microvoids. This work provides fundamental insight into haze formation in organic electrochromic films and establishes strategies for controlling haze that can impair device performance.

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