n-type molybdenum-diselenide-based liquid-junction solar cells: A nonaqueous electrolyte system employing the chlorine/chloride couple

Single-crystal, n-type MoSe₂ (E_g=1.4 eV) is shown to serve as a stable photoanode in an electrochemical cell employing a nonaqueous (CH₃CN) solution of Cl₂/Cl^- as the redox active material. At 90 mW/cm² input, 632.8-nm light can be converted to electricity with an efficiency of 5.9-7.5%. The photoanode reaction is 2Cl^-→Cl₂ and the cathode reaction is Cl₂→2Cl^-. The MoSe₂ is qualitatively better than MoS₂ (∼0.5% efficiency) which has a larger band gap (1.7 eV), but both materials are rugged in the nonaqueous solution, while both photocorrode in aqueous Cl^- solutions. In H₂O, the I₃^-/I^- couple is excellent but in CH₃CN it yields lower efficiency than the Cl₂/Cl^- couple. The stable Cl₂/Cl^- system provides evidence that a transparent, reversible, non-O₂-sensitive redox couple can be useful in n-type semiconductor-based liquid-junction cells employing a direct band gap material having optimum solar response.