Trends in the open-circuit voltage of semiconductor/liquid interfaces: Studies of n-Al_xGa_1-xAs/CH₃CN-Ferrocene^+/0 and n-Al_xGa_1-xAs/KOH-Se^-/2-(aq) junctions

Trends in open-circuit voltage (V_oc), short-circuit current density (J_sc), and energy conversion efficiency have been determined for the n-type Al_xGa_1-xAs series of photoelectrodes (x = 0.0, 0.09, 0.16, 0.24, 0.31) in contact with CH₃CN-ferrocene^+/0 and KOH-Se^-/2-(aq) electrolytes. V_oc increased linearly with increases in bandgap energy (E_g) of the n-Al_xGa_1-xAs alloy electrodes, with ΔV_oc/ΔE_g = 0.45 ± 0.04 V eV^-1 in CH₃CN and 0.41 ± 0.09 V eV^-1 in KOH-Se^-/2-(aq) at a light intensity sufficient to provide J_sc = 1.0 mA cm^-2. J_sc values under solar-simulated illumination decreased monotonically with increasing bandgap energy. The relatively low value of ΔV_oc/ΔE_g implies decreases in optimal energy conversion efficiency as the mole fraction of Al in the Al_xGa_1-xAs alloy is increased. This is in contrast to the behavior of the n-GaAs_xP_1-x alloy series in the same electrolytes. The lower value of ΔK_oc/ΔE_g for n-Al_xGa_1-xAs also indicates that predictions of the "common anion rule" in solid-state barriers do not apply to this family of III-V semiconductor/liquid junctions.