Spray pyrolysis of CuBi2O4 photocathodes: improved solution chemistry for highly homogeneous thin films

Abstract

Dense, homogeneous CuBi₂O₄ thin films are prepared, for the first time, by spray pyrolysis. Major challenges related to the chemical stability of the precursor solution and spreading behavior of the sprayed droplets are revealed and addressed. Triethyl orthoformate (TEOF) is added as a water scavenger to avoid fast hydrolysis and polycondensation of bismuth ions in the precursor solution, thereby reducing powder formation during the spray deposition process. Polyethylene glycol (PEG) is used to improve the spreading behavior of sprayed droplets over the entire CuBi₂O₄ film surface, which prevents powder formation completely and allows for the deposition of dense, homogeneous films with thicknesses over 420 nm. These highly uniform CuBi₂O₄ thin films are well-suited for fundamental studies on the optical and photoelectrochemical properties. Additionally, they produce record photocurrent densities for CuBi₂O₄ up to 2.0 mA cm⁻² under AM1.5 simulated sunlight along with incident photon-to-current efficiency (IPCE) and absorbed photon-to-current efficiency (APCE) values up to 14% and 23%, respectively (for 550 nm light at 0.6 V_RHE with H₂O₂ as an electron scavenger).