Ion Conducting and Hole Transporting Ta₂O₅ Coupled with MoO₃ and Photoactive BiOI Microflowers in a Durable Photoelectrochromic Device

A self-switching photoelectrochromic (PECD) device capable of transitioning between gray and deep blue hues is presented. The device has a photovoltaic TiO2/N719/BiOI electrode paired with an electrochromic counter electrode of Ta2O5/MoO3. The hole conductivity of Ta2O5 (50 μS cm–1) coupled with its intrinsic proton conductivity of 5.3 μS cm–1 enables it to function as an effective conduit for both electrons and Li+ ions. The PCE of the TiO2/N719/BiOI/I2,LiI gel/Ta2O5/MoO3 PECD is 6.6%, greater by 125 and 317% relative to analogous cells without BiOI and without Ta2O5. The PECD delivers an integrated visible light modulation of 27%, with a photocoloration efficiency of 28.8 cm2 min–1 W–1 under 0.5 sun irradiance in the unbiased mode and switching times of 7 and 4 s under bias. The PECD shows a 1.2-fold drop in efficiency after prolonged sun exposure and a 1.3-fold modulation drop after 1000 photocoloration/dark-bleaching cycles, highlighting the role of Ta2O5 as a barrier layer that inhibits the dissolution of MoO3 in the electrolyte. This work opens opportunities to harness the potential of mixed conductors such as Ta2O5 for developing long-lasting, high-contrast PECDs.