Structural, optical, and hole transport properties of earth-abundant chalcopyrite (CuFeS 2 ) nanocrystals
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Research Letter
Structural, optical, and hole transport properties of earth-abundant chalcopyrite (CuFeS2) nanocrystals Ebin Bastola, Khagendra P. Bhandari, Indra Subedi, Nikolas J. Podraza, and Randy J. Ellingson, Department of Physics and Astronomy, The Wright Center for Photovoltaics Innovation and Commercialization (PVIC), University of Toledo, Toledo, Ohio 43606, USA Address all correspondence to Randy J. Ellingson at [email protected] (Received 18 April 2018; accepted 19 June 2018)
Abstract Here, we report thiol-free thermal-injection synthesis of chalcopyrite (CuFeS2) nanocrystals (NCs) using iron (II) bromide (FeBr2), copper (II) acetaylacetonate (Cu(acac)2), and elemental sulfur (S). Controlled reaction temperature and growth time yield stable and phase-pure ternary CuFeS2 NCs exhibiting tetragonal crystal structure. With increasing growth time from 1 to 30 min, absorption peak slightly red shifts from 465 to 490 nm. Based on spectroscopic ellipsometry analysis, three electronic transitions at 0.652, 1.54, and 2.29 eV were found for CuFeS2 NC film. Also, CuFeS2 NC thin films are incorporated as hole transport layers in cadmium telluride solar cells reaching an efficiency of ∼12%.
Introduction Chalcopyrite materials have attracted special attention in the field of opto-electronic applications due to their abundance, and low toxicity composition compared with other semiconducting materials. Semiconducting bulk chalcopyrite, CuFeS2, exhibits an indirect band gap of ∼0.5–0.6 eV and tetragonal crystal structure,[1] in which Cu and Fe ions bond tetrahedrally with sulfur (S).[2] Recent publications indicate that this chalcopyrite is promising for thermoelectric applications due primarily to low resistivity and thermal conductivity.[3,4] Additionally, these narrow gap semiconductors are suitable for the development of counter electrodes,[5] infrared (IR) detectors, and light-to-heat converters.[6] Colloidal nanocrystals (NCs) of the CuFeS2 chalcopyrite materials are being investigated to exploit their unique properties such as quantum confinement, solutionprocessability, and facile synthesis methods. However, development and application of ternary and quaternary colloidal NCs have been challenging due to the required control over phase purity and the stability of the synthesized NC materials.[7] To make the material more device applicable, research has demonstrated various techniques for the fabrication of CuFeS2 chalcopyrite. The common solution-based syntheses employed to prepare CuFeS2 chalcopyrite nanostructures are solvothermal[8] and hydrothermal[9] methods. In addition, a mechanical alloying preparation method has also been reported for CuFeS2 materials.[4,10] Wang et al. have synthesized shape-controlled CuFeS2 NCs with spherical and pyramidal morphology using a solution-based method, and have reported blue shifting features within the absorption spectrum for quantum-confined spherical CuFeS2 NCs as compared with their bulk counterpart.[11]
Kumar et al. have reported solution-based single step synth
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