Effect of Location of Sodium Precursor on the Morphological and Device Properties of CIGS Solar Cells
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Effect of Location of Sodium Precursor on the Morphological and Device Properties of CIGS Solar Cells Neelkanth G. Dhere1, Ashwani Kaul1 and Helio Moutinho2 1
Florida Solar Energy Center, 1679 Clearlake Road, Cocoa, FL 32922, USA.
2
National Renewable Energy Laboratory, Golden CO, USA.
ABSTRACT Sodium plays an important role in the development of device quality CIGS (Cu-In-GaSe) and CIGSeS (Cu-In-Ga-Se-S) chalcopyrite thin film solar cells. In this study the effect of location of sodium precursor on the device properties of CIGS solar cells was studied. Reduction in the surface roughness and improvement in the crystallinity and morphology of the absorber films was observed with increase in sodium quantity from 0 Å to 40 Å and to 80 Å NaF. It was found that absorber films with 40 Å and 80 Å NaF in the front of the metallic precursors formed better devices compared to those with sodium at the back. Higher open circuit voltages and short circuit current values were achieved for devices made with these absorber films as well. INTRODUCTION Sodium plays a useful role in the growth and doping of CIGS thin film solar cells. Several beneficial effects of sodium in copper chalcopyrite thin-film solar cells are reported in the literature. It has been shown that the presence of sodium plays a very critical role during the growth of CIGS absorber layer and is beneficial for the device performance. Several sodium precursors have been explored to determine their effect on the growth of CIGS solar cells. Among those studied, sodium fluoride, NaF has been found to be the best choice. This is because NaF is non-hygroscopic, stable in air and evaporates stoichiometrically [1]. It has been shown that selenization of the film containing sodium results in the formation of NaSex compounds that delay the growth kinetics of the CIGS phase for better incorporation of selenium in the film [2]. Sodium has a tendency to reduce detrimental point defects. It reduces compensating donors by substituting selenium vacancies Vse and, therefore, increases the p-type doping. Sodium also replaces InCu anti-site defects further reducing the compensating donors [3]. Apart from reducing the compensating donor, sodium also replaces copper vacancies thereby minimizing the IRUPDWLRQRIRUGHUHGGHIHFWFRPSRXQGV 2'& DQGWKXVIDYRUVZLGHQLQJWKHĮ-phase region. Sodium also promotes increase in grain size and preferred (112) orientation of CIGS films [4-6]. Sodium has also been shown to passivate the surface and grain boundaries of CIGS films by promoting oxygen incorporation [2]. The overall effect of sodium on the device performance is noted by an increase in efficiency by improvement in fill factor and open circuit voltage.
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Addition of sodium helps in fabricating Cu-poor films with higher device efficiencies without a KCN treatment. It has been shown that the optimum amount of sodium for better device performance is determined by the process used to prepare the absorbers films [6]. Therefore, films prepared by rapid thermal annealing (RTA) will need
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