Bismuth doping on CuGaS 2 thin films: structural and optical properties
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Research Letter
Bismuth doping on CuGaS2 thin films: structural and optical properties Marcos A. S. Andrade Jr. and Lucia H. Mascaro, Department of Chemistry, Federal University of São Carlos, Rod. Washington Luiz, Km 235, CEP 13565-905, São Carlos-SP, Brazil Address all correspondence to Marcos A. S. Andrade at [email protected] (Received 21 February 2018; accepted 2 April 2018)
Abstract In this work, we present a solvothermal method to prepare bismuth (Bi)-doped CuGaS2 chalcopyrite nanocrystals ink and apply it to an allsolution-processed approach for the preparation of films with a thickness of approximately 730 nm and with enhanced optical properties and lower band gap energy than the undoped semiconductor films. The low-cost deposition method is comprised by spray deposition of the chalcogenide nanocrystals ink onto the molybdenum substrates, producing microcrystalline films with grains larger than 400 nm originated from coalescence of Bi-doped nanocrystals. Bi-doped CuGaS2 microcrystalline films are a good candidate to be applied as an absorber layer in thinfilm solar cells.
Introduction Thin films solar cells based on chalcopyrite Cu(In,Ga)Se2 (CIGS, η∼21%) are great competitors to achieve the highenergy conversion efficiency of silicon devices (η∼26% for a crystalline cell).[1, 2] These copper-based ternary chalcogenides are among the promising materials to be applied as absorber layers in solar cells due to their low band gap,[3] high absorptivity coefficient into the visible and next infrared (IR) ranges,[4] high tolerance to defects and impurities and have been extensively studied during the last three decades.[5,6] However, efforts have been made about to seek new absorber materials to allow economical and scalable commercialization of chalcopyrite-based devices. CuGaS2 is a wide band gap (2.40 eV) ternary chalcogenide,[7] isostructural to the CIGS and has relatively low toxicity.[8] The CuGaS2 is composed by elements more abundant than that chalcogenides based on In, Cd, Se, and Te: for example, (160 ppb) e Ga (16.9 ppm) e Se (0.05 ppm).[9] However, the optimal band gap energy of a single band gap device was calculated to be approximately 1.4 eV according to the Shockley–Queisser limit. Although CuGaS2 has not suitable band gap to act as a light absorption layer in single junction solar cells, it is possible to modulate its band gap by intermediate band formation or lowering the band gap by metal doping. Experimental and theoretical studies have reported an increased optical absorption intensity of CuGaS2 doped with transition metals (Ti,[10] Cr,[11] Ce,[12] and Fe[13]), post-transition (Sn[14]), and 15 group elements (N, P, As, Sb),[7] in comparison with the undoped semiconductor. The preparation of CuGaS2 thin films have been reported by several physical methods, for example, EBPVD (electron beam physical vapor deposition)[15] and MVPD (metalorganic vapor
phase deposition),[16] and spray pyrolysis.[17] These methods are relatively expensive due to the use of vacuum equipment and may present throughput
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