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Pentafluorophenyl substituted fulleropyrrolidine: a molecule enabling the most efficient flexible electrochromic device with fast switching

A fullerene derivative, namely pentafluorophenyl-substituted fulleropyrrolidine (PSF), has been synthesized and reported for application in electrochromic devices as a counter ion electrode material. A polythiophene-based solid-state electrochromic device thus prepared, using the PSF counter ion, breaks the millisecond electrochromic switching limit (300 ms) and presents the most efficient thiophene-based electrochromic device with an efficiency of more than 400 cm2 C−1. Due to its solution processibility and polymer compatibility, a flexible device was also fabricated that makes it possible to realize the most efficient all-organic flexible solid-state electrochromic device. A color contrast of more than 60% between the magenta and transparent states can be achieved with a small bias of ∼±1 V, as observed from the investigation using UV-vis spectroscopy. The PSF counter ion excellently supports the bias-induced absorption change of poly(3-hexylthiophene-2,5-diyl) (P3HT), which has been investigated using in situ Raman spectroscopy. Additionally, electron transport mechanism in the PSF-P3HT device is improved due to addition of PSF. This makes an ultrafast, durable, and power efficient device with a switching speed of the order of milliseconds possible while maintaining an impressive durability in switching cycles for longer than 500 switching pulses. The performance of the device in its flexible form also remains unaltered even after more than 100 bending cycles.