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

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 AgInS 2 (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 AgInS 2 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 AgInS 2 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 0.32 Ag 0.68 InS 2 . 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 AgInS 2 QDSSCs (3.10%). Moreover, the J sc increased from 7.44 (AgInS 2 ) 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 AgInS 2 /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. 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).