CuInS₂ quantum dots synthesized by a solvothermal route and their application as effective electron acceptors for hybrid solar cells

This paper describes a solvothermal approach to synthesize CuInS ₂ quantum dots (QDs) and demonstrates their application as a potential electron accepting material for polymer-based hybrid solar cells, for the first time. The CuInS₂ QDs with a size of 2-4 nm are synthesized by the solvothermal method with 4-bromothiophenol (HSPh) as both reduction and capping agents, and characterized by XRD, XPS, TEM, FT-IR, cyclic voltammetry (CV), and absorption and photoluminescence spectra. Results reveal that the CuInS₂ QDs result from the solvothermal decomposition of a soluble organic sodium salt as an intermediate precursor formed by simple reactions among CuCl₂, InCl₃, HSPh and Na₂S at room temperature; they have an ionization potential (IP) of -5.8 eV and an electron affinity (EA) of -4.0 eV and can quench effectively the luminescence of poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV). Due to the favorable IP and EA positions with respect to MEH-PPV, the CuInS₂ QDs act as an effective electron acceptor for the hybrid solar cells based on MEH-PPV/CuInS₂-QDs blends with a wide spectral response extending from 300 to 900 nm, by allowing the efficient charge separation for neutral excited states produced either on the polymer or on the QDs. The MEH-PPV/CuInS₂-QDs solar cells exhibit a promising open circuit voltage (V_oc) of 0.62 V under the monochromic illumination of 15.85 mW cm^-2 at 470 nm. The charge transfer processes in the solar cells are also described. © 2010 The Royal Society of Chemistry.