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 TiO 2 /N719/BiOI electrode paired with an electrochromic counter electrode of Ta 2 O 5 /MoO 3 . The hole conductivity of Ta 2 O 5 (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 TiO 2 /N719/BiOI/I 2,LiI gel/Ta 2 O 5 /MoO 3 PECD is 6.6%, greater by 125 and 317% relative to analogous cells without BiOI and without Ta 2 O 5 . The PECD delivers an integrated visible light modulation of 27%, with a photocoloration efficiency of 28.8 cm 2 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 Ta 2 O 5 as a barrier layer that inhibits the dissolution of MoO 3 in the electrolyte. This work opens opportunities to harness the potential of mixed conductors such as Ta 2 O 5 for developing long-lasting, high-contrast PECDs.