Aqueous solution-processed off-stoichiometric Cu–In–S QDs and their application in quantum dot-sensitized solar cells

Abstract

Aqueous solution-processed methods offer several advantages such as simplicity, low-cost, environmental friendliness, and easy scale up. Herein, we present an ultrafast and convenient aqueous route to prepare off-stoichiometric Cu–In–S quantum dots (QDs) including CuInS₂, a mixture of In₂S₃ and CuInS₂ (In₂S₃/CIS), and Mn-doped In₂S₃/CIS (Mn:Cu–In–S), obtained by simply adjusting the feed ratio of Cu and In precursors in an identical procedure. Such off-stoichiometric QDs were then exploited as photon-harvesters in quantum dot-sensitized solar cells (QDSSCs), which showed superior photovoltaic performance that is dependent on the [Cu]/[In] feed ratio. The QDSSCs sensitized with In₂S₃/CIS QDs exhibited a cell efficiency as high as 7.53%, a more promising performance than that of the device sensitized with CuInS₂ QDs (1.0%). Such an improvement of the photovoltaic performance can be attributed to the favorable energy level alignment of the adjoining materials (the TiO₂ semiconductor and In₂S₃/CIS QDs). Upon incorporation of Mn²⁺ into In₂S₃/CIS under identical conditions, the power conversion efficiency of the Mn:Cu–In–S-based QDSSC reached 8.0% due to a synergistic effect between the In₂S₃ QDs and Mn²⁺ dopant. Analyses of the Bader charge and projected density of states based on density functional theory (DFT) calculations revealed that the coexistence of In₂S₃ and Mn²⁺ enhances the electronic coupling at the TiO₂/QD interface, leading to a larger number of photoinduced electrons being injected into TiO₂. Electrochemical impedance spectroscopy and intensity-modulated photocurrent/photovoltage spectroscopy measurements confirmed the slower charge recombination and prolonged electron lifetimes that result in the excellent efficiency of the QDSSC device sensitized with Mn:Cu–In–S QDs.