Cu 2 SnS 3 Inorganic-Organic Hybrid Structures for Photovoltaic Applications
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Cu2SnS3 Inorganic-Organic Hybrid Structures for Photovoltaic Applications Sandra Dias and S. B. Krupanidhi Materials Research Centre, Indian Institute of Science, Bangalore-560012, India ABSTRACT We report the synthesis of Cu2SnS3 (CTS) nanostructures and its incorporation into an inorganic-organic hybrid device to enhance the photoresponse under AM 1.5 G solar illumination. The nanostructures were structurally and optically characterized. From X-ray diffraction (XRD) and Transmission electron microscopy (TEM) the CTS nanocrystals were found to be tetragonal. Flower like structures of CTS were obtained as seen from Scanning electron microscopy (SEM). A band gap of 1.4 eV was obtained from absorption studies. Two devices have been studied, P3HT: PCBM = 1: 1 and CTS: P3HT: PCBM = 8:1:1. The photocurrent increased from a value of 2.33 mA at dark to 2.5 mA for the P3HT-PCBM blend to 3.36 mA for CTS: P3HT: PCBM = 8:1:1 device. The responsivity, sensitivity, external quantum efficiency and specific detectivity increased from 18.81 mA/W, 1.07, 4.25% and 6.88 × 108 Jones respectively for P3HT:PCBM sample to 189.97 mA/W, 1.44, 42.9% and 6.95 × 109 Jones for CTS: P3HT: PCBM = 8:1:1 sample at 1V bias and 1 Sun illumination intensity. The time dependent photoresponse was stable over different ON-OFF cycles. From the fit to the rise and decay curves, the rise and decay time constants were obtained. INTRODUCTION I-IV-VI group materials like Cu2SnS3, Cu3SnS4, Cu2Sn3S7, CuBiS2, CuSbS2 and Cu2GeS3 have gained popularity as photovoltaic materials due to their high absorption coefficient, optimum band gap, earth abundance and non-toxicity of the constituent elements [1, 2, 3]. Cu2SnS3 (CTS) has a high absorption coefficient of 104-105 cm-1 and a direct optimum band gap of 0.93-1.5 eV [4, 5]. Hence it can replace the solar absorber layers like CdTe and Cu(InGa)Se2 which are made up of rare, expensive and toxic elements. The highest reported CTS solar cell efficiency as of now is 2.92% which can be further improved [1]. Inorganic-organic hybrid devices combine the advantages of the high carrier mobility and high absorption coefficient of inorganic materials and the flexibility, low temperature and large area processability of polymers. Bulk heterojunction provides large p-n junction interfacial area, ease of charge separation and collection [6]. In this work we report the incorporation of CTS nanostructures in a P3HT: PCBM polymer matrix and its effect on the solar photoresponse. EXPERIMENTAL CTS synthesis Cationic solution of 1.6 mmol CuCl2.2H2O (99.99%) and 0.8 mmol SnCl2.2H2O (99.99%) in 16 ml ethylene glycol and anionic solution of 2.4 mmol thiourea (99.0%) in 16 ml ethylene glycol were prepared separately. Then thiourea was added to the metal precursor solution dropwise to give a milky white solution. This solution was transferred to a 40 ml Teflon lined stainless steel autoclave upto 80% of the volume and kept at a temperature of 180°C for 8
hours. After cooling to room temperature, the resulting black CTS precipitate was centri
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