Fabrication and characterization of thin film solar cell using earth abundant elements

Abstract

Current thin-film solar cell technologies based on CuInxGa1-xSe2 (CIGS) and CdTe photo-absorber materials use rare and expensive elements, such as In, Te, Ga, and toxic Cd which severely limit the mass production and deployment of these solar cells. Thus, a major research effort is focused toward the development of new photovoltaic (PV) absorber materials comprising of earth-abundant, low-cost, and environmentally benign constituent elements that can support terawatt (TW)-scale PV generation in the near future and be economically sustainable. Cu-based I2-II-IV-VI4 quaternary kesterite compound Cu2ZnSn(SxSe1-x)4 (CZTSSe) have recently emerged as a potential photo-absorber material for thin-film solar cells. All constituent elements in CZTSSe are abundant in earth’s crust, are much cheaper and possess no acute toxicity. CZTSSe is an intrinsically p-type material with a large optical absorption coefficient (α>104 cm-1) and exhibits a tunable direct optical bandgap in the range of 1.0 eV ≤ Eg ≤ 1.5 eV corresponding to chalcogen ratios of 0 ≤ x ≤ 1. The theoretical Shockley-Queisser efficiency limit for a single junction CZTSSe solar cell is estimated to be ~32% – similar to that of CIGS solar cells. All these merits make CZTSSe an ideal photo-absorber material for thin-film solar cells. In this thesis, a comprehensive investigation is undertaken on the growth and characterization of Cu2ZnSn(SxSe1-x)4 photovoltaic absorber layer, CdS buffer layer, i:ZnO and Al:ZnO window layers using non-vacuum technique to facilitate a way to make low cost solar cells. CZTS and CZTSSe thin film were fabricated by spin coating followed by sulfurization and selenization process respectively. CdS layer was deposited by low-cost chemical bath deposition (CBD) technique. i-ZnO and Al-ZnO window layers were deposited using spin coating technique. The structural, morphological, compositional, optical and electrical properties of all these layers of CZT(S,Se) solar cell were investigated using SEM, EDX, XRD, UV spectroscopy, Hall Effect Measurement system, and Stylus Profilometer to find the optimized fabrication conditions for each layer. Various thin films samples of different layers were fabricated with variation in fabrication conditions and characterization of these films confirm the best one suitable for solar application. SEM micrographs confirm good crystal formation in CZTS and CZTSSe films and smooth surface morphology for non-annealed CdS, i-ZnO and Al-ZnO films. EDX analysis confirms Zn rich, Cu poor composition in the best CZTS film corresponding to elemental ratios of Cu/(Zn+Sn)= 0.98 and Zn/Sn=1.38. But for CZTSSe film, Zn poor, Cu rich composition with Cu/(Zn+Sn)=0.46, Zn/Sn=0.38 and S/Se=0.54 was found. Stylus Profilometer Analyzing measure film thickness of 1.86, 2.66 µm for CZTS and CZTSSe and 151, 68, 492 nm for CdS, i-ZnO, and Al-ZnO. UV spectroscopy, optical band gap of most suitable films were found to have 1.65, 1.5, 2.62, 3.3, and 3.44 eV for CZTS, CZTSSe, CdS, i-ZnO, and Al-ZnO thin film respectively. CdS, i-ZnO and Al-ZnO thin film showed high transmittance and low absorption in visible range. CZTS and CZTSSe thin film confirm absorption coefficient in order of 104 cm-1. XRD analysis confirm Kesterite phased tetragonal structure of CZTS with grain size of 13.7 nm, Wurtzite phased tetragonal structure of CZTSSe with grain size 16.54 nm, hexagonal wurtzite structure of CdS and Al-ZnO. Hall Effect measurement confirms p type nature of CZTS and CZTSSe, and n type nature of CdS, i-ZnO and Al-ZnO. It also measured the carrier concentration of 1.5×1014, 2.74×1014, 2×1012, 8×1013 and 4×1014 cm-3 for CZTS, CZTSSe, CdS, i-ZnO, and Al-ZnO respectively. It also measured Carrier Mobility of 72.40, 46.8, 48.71, 92.4, and 729 cm3/Vs for CZTS, CZTSSe, CdS, i-ZnO, and Al-ZnO respectively. Resistivity of 3.39×103, 4.86×104, 2.55×105, 113.3, 223 Ω cm for CZTS, CZTSSe, CdS, i-ZnO, and Al-ZnO respectively were also measured by Hall effect measurement system. Finally, numerical analysis was carried to find the merit of the fabricated layers in solar cell by simulating Al-ZnO/i-ZnO/n-CdS/p-CZTS and Al-ZnO/i-ZnO/n-CdS/p-CZTSSe cell structure. CZTS based solar cell shows 4.11% efficiency with Voc of 1.62 V and Jsc of 20.074 mA and FF of 12.44%. CZTSSe based solar cell shows 2.96% efficiency with Voc of 1.4 V and Jsc of 5.76 mA and FF of 36.57%. This study will surely work as guideline to fabricate thin film solar cell consist of different layers deposited by nonvacuum techniques using only earth abundant elements for giving low cost solar energy solution.