Cu-mediated broadening of the absorption band of quaternary Cu–Ag–In–S/CdSe type-II core/shell quantum dot-sensitized solar cells with an efficiency of 12.51% under 0.25 sun

Abstract

We report the effects of Cu substitution on the photovoltaic performance of (a) single-layered Cu–Ag–In–S and (b) core/shell Cu–Ag–In–S/CdSe quantum-dot sensitized solar cells (QDSSCs). Recently, ternary Ag–In–S/CdSe core/shell QDSSCs have been reported with a power conversion efficiency (PCE) of 8.26% under 1 sun. However, the bandgap (E_g) of AgInS₂ (1.9 eV) is higher than the optimal Shockley–Queisser gap (1.4 eV), suggesting room for improvement. Herein, the bandgap engineering strategy was employed to tune the bandgap of AgInS₂ by partially replacing Ag with Cu, forming quaternary Cu–Ag–In–S QDs through the successive ionic layer adsorption and reaction (SILAR) method. Optical measurements revealed a decreased E_g from 1.90 eV (650 nm) in AgInS₂ to 1.76 eV (700 nm) in Cu–Ag–In–S, resulting in an expanded absorption band. Energy dispersive spectroscopy showed that Cu replaced 32% of Ag, leading to the chemical formula of Cu_0.32Ag_0.68InS₂. A CdSe shell layer was subsequently grown on top of the Cu–Ag–In–S QD using SILAR, forming the type-II core/shell Cu–Ag–In–S/CdSe QD structure, as revealed by cyclic voltammetry. The core/shell QDs exhibited an E_g (1.50 eV) lower than 1.76 eV of the Cu–Ag–In–S core and 1.70 eV of the CdSe shell, respectively. Liquid-junction Cu–Ag–In–S/CdSe QDSSCs were fabricated using polysulfide electrolytes and CuS counter electrodes. We compare the photovoltaic performance of QDSSCs with and without Cu substitution. The single-layered Cu-incorporated Cu–Ag–In–S QDSSC achieved an average PCE of 5.71% (J_sc = 12.27 mA cm⁻²) under 1 sun, significantly higher than AgInS₂ QDSSCs (3.10%). Moreover, the J_sc increased from 7.44 (AgInS₂) to 12.27 mA cm⁻² (Cu–Ag–In–S), which is attributable to the broadened absorption band. The Cu–Ag–In–S/CdSe core/shell QDSSCs exhibited a PCE of 9.23% (J_sc = 25.41 mA cm⁻², V_oc = 0.70 V, FF = 51.7%) under 1 sun, also highly improved over 8.39% of the ternary core/shell AgInS₂/CdSe QDSSCs. Under a reduced light intensity of 0.25 sun, the PCE of Cu–Ag–In–S/CdSe QDSSCs further increased to 12.51%. The PCE of 9.23% (1 sun) achieved here is the highest PCE among all type-II core/shell QDSSCs reported to date.