Preparation of Cu(In,Ga)S₂Absorber Layers for Thin Film Solar Cell by Annealing of Electrodeposited Cu-Ga-S Precursor Layers

Abstract

Cu(In,Ga)S 2 (CIGS) thin film was synthesized on ITO glass substrate via electrodeposition of Cu-Ga-S precursor layer followed by thermal annealing treatment. Our results show that annealing temperature played an important role on the formation of CIGS crystallites. The pure quaternary chalcopyrite CIGS crystal phase in good crystallization with an uniform and compact surface morphology was reproducibly achieved after sintering at 400 • C. The metallic In atom diffused from the ITO substrate was found to incorporate to the Cu-Ga-S precursor film and allow the conversion of the quaternary chalcopyrite structure. Several characterization methods including X-ray diffraction (XRD), scanning electron microscope (SEM), energy diffraction spectrum (EDS) and highresolution transmission electron microscopy (HRTEM) certified the incorporation of In. A possible growth mechanism for explaining the formation of CIGS thin films is proposed and briefly discussed. Completed CIGS solar cell device achieved a 5.75% total area power conversion efficiency under a simulated AM 1.5 illumination.The development of low-cost efficient solar cells is one of the ultimate solutions for the renewable and clean energy challenge. Chalcopyrite based solar cells are promising candidates for next generation photovoltaic modules due to their high power conversion efficiency, direct bandgap, high absorption coefficient, low toxicity and excellent stability. 1 Up to now, CuInGaSe 2 absorber layer based solar cells have obtained the highest power conversion efficiency of 20.1% in laboratory among the thin film technology. 2 However, it is approaching the theoretical limit. 3 For a further promotion on conversion efficiency, tandem solar cell structures are currently subject of intensive research. 4 For a double solar cell two-terminal tandem system, it has been estimated that the theoretical efficiency exceeds 30%, which is higher than that for single-junction cells. The optimum bandgap for the top cell is in the range of 1.6 to 2.0 eV. 5 Cu(In,Ga)S 2 solid solutions possesses a bandgap from 1.5 eV to 2.43 eV covers the above mentioned range of the optimum bandgap for the top cell. 6 The incorporation of gallium in CIS leading to a quaternary Cu(In,Ga)S2 (CIGS) structure also bears perspectives of a increasing efficiency due to the enlargement of the bandgap energy (E g ) and correspondingly the open-circuit voltage (V OC ) 7-9 comparing to CIS. Though CIS-based solar cell have a strong potential of achieving high efficiency due to its almost ideal bandgap energy of 1.5 eV, 10,11 debates have been going on regarding the limited efficiency and open circuit voltages of the cells remained below the predicted theoretical values. Significant improvement could not be achieved and open circuit voltage values did not typically exceed 730 mV. A further increase in the efficiency is expected from doping or alloying of the CIS absorber material with isovalent and/or non-isovalent substitutional elements. Promising results have been obtained using silver as dopa...

Electrodeposition of Copper, Selenium, Indium, and Gallium on Molybdenum/Surface Oxides: Unary, Binary, Ternary and Quaternary Compositions

“…Recent reports demonstrated the potential of electrodeposited ternary and quaternary chalcopyrite absorber layers for various solar energy conversion devices. [23][24][25][26] In spite of a good number of investigations on the electrodeposition of Cu, Se, In and Ga and the various compositions on different electrodes, [1][2][3][4] several concepts and explanations on the origin of different voltammetric peaks are still under debates. Reported works that methodically investigated the development of voltammetric peaks in unary, binary, ternary and quaternary compositions on Mo electrode (consolidated and compared in one work) are rare.…”