The authors demonstrate an aqueous solution-processed Cs₂CO₃ thin film with an adjustable work function via MoO₃ and/or Na₂WO₄ doping. The doped Cs₂CO₃ as a cathode interfacial layer is successfully used in poly(3-hexyl-thiophene) (P3HT)/indene-C₆₀ bisadduct (IC₆₀BA) heterojunction based solar cells with improved open-circuit voltage and unaffected short-circuit current density. X-ray photoelectron spectroscopy (XPS) evaluation was conducted to verify the formation of the new composites of W–O–Cs and Mo–O–Cs after doping of MoO₃ and/or Na₂WO₄ into Cs₂CO₃. The change of the work function of MoO₃- and/or Na₂WO₄-doped Cs₂CO₃ was further confirmed by ultraviolet photoelectron spectroscopy (UPS) measurements.