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 (Eg) 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 Eg 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 Cu0.32Ag0.68InS2. 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 Eg (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% (Jsc = 12.27 mA cm−2) under 1 sun, significantly higher than AgInS2 QDSSCs (3.10%). Moreover, the Jsc 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% (Jsc = 25.41 mA cm−2, Voc = 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.