Evolution of Electronic Properties of Cu(In, Ga)Se 2 (CIGS)-Based Solar Cells During a 3-stage Growth Process
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Evolution of Electronic Properties of Cu(In,Ga)Se2 (CIGS)-Based Solar Cells During a 3-stage Growth Process Jehad A. AbuShama*, S. Johnston, R. Ahrenkiel, R. Crandall, D. Young, and R. Noufi, National Renewable Energy Laboratory, Golden, CO 80401 *Physics Department, Colorado School of Mines, Golden, CO 80401
ABSTRACT We investigated the electronic properties of ZnO/CdS/CIGS /Mo/SLG polycrystalline thinfilm solar cells with compositions ranging from Cu-rich to In(Ga)-rich by deep-level transient spectroscopy (DLTS) and capacitance-voltage (C-V) measurements. This compositional change represents the evolution of the film during growth by the 3-stage process. Two sets (four samples each) of CIGS thin films were prepared with Ga/(In+Ga) ratios of ~0.3 (low Ga) and ~0.6 (high Ga). The Cu/(In+Ga) ratio ranges from 1.24 (Cu-rich) to 0.88 (In(Ga)-rich). The films were treated with NaCN to remove the Cu2-xSe phase where needed. Key results include: (1) For lowGa devices, DLTS data show that acceptor-like traps dominate in samples where CIGS grains do not go through the Cu-rich to In(Ga)-rich transition, whereas donor-like traps dominate in In(Ga)-rich samples. Therefore, we see a clear transformation of defects from acceptor-like to donor-like traps. The activation energies of these traps range from 0.12 to 0.63 eV. We also observed that NaCN treatment eliminates a deep minority trap in the In(Ga)-rich devices, (2) For high-Ga devices, only majority-carrier traps were detected. These traps again range from shallow to deep, (3) The carrier concentration around the junction and the density of traps decrease as the CIGS becomes more In(Ga)-rich.
INTRODUCTION Cu(In,Ga)Se2 (CIGS)-based polycrystalline thin-film solar cells are promising for photovoltaic applications. The highest conversion efficiency for small laboratory devices is 19.2% [1]. The four elements of this multinary polycrystalline film can be alloyed to form different CIGS phases as dictated by the pseudo-binary CIS phase diagram [2]. Even though this multiplicity makes the material complicated, CIGS nevertheless tolerates defects and impurities by self-adjusting its chemistry and microstructure. In our laboratory, we are investigating the thin-film growth mechanisms using our so-called “3-stage process” as influenced by the specific dynamics of this process. The electronic properties of thin-film CuxIn1-yGaySe2 devices made from films as they transition from Cu-rich to In(Ga)-rich, and for different Ga contents (i.e., different y values), are the subject of this work. Devices made from these films were analyzed using deep-level transient spectroscopy (DLTS) and capacitance-voltage measurements (C-V). The phase and microstructure evolution of Cu(In,Ga)Se2 from Cu-rich to In( Ga)-rich with the addition of Cu has been discussed in our previous publications (e.g. ref. [3]). For comparison of our results with previous studies, see references [4,5].
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