Characterizing Sub-Bandgap Traps in Cu(In, Ga)Se2 Solar Cells with Back Contact Barriers
Engineering Physics | Physics
Recent solar cell technology, in striving for lowest cost, results in materials with many imperfections. Capacitance measurements are a common, nondestructive method to identify a type of imperfection, called a trap state, which reduces the solar cell efficiency. Researchers often measure capacitance using positive voltages, known as forward bias, to try to differentiate between a contribution to capacitance caused by a trap state, and a contribution caused by one of the contacts, the ‘back contact barrier’. These data are normally interpreted using a simple device model. However, in forward bias, large diffusion currents flow, making the model inaccurate. Among other things, this can result in modulated conductivity. Although this has been reported in some types of devices, it is neglected in most discussions of solar cell measurements in the literature.
In recent research by Heath, Cox, and Davis, we believed that we saw evidence of a back contact barrier, corroborating other published results.1 However, more careful measurements by Ogle and Cox2,3 in summer 2015 indicated that this initial interpretation was too simple, and the effect of the diffusion current, including modulated conductivity, must be considered. To check our interpretation, we applied two complementary measurement techniques, impedance spectroscopy and current-voltage spectroscopy. We conclude that the assumptions commonly used to interpret the capacitance measurement are not appropriate in this measurement regime, and modulated conductivity can be incorrectly identified as, or conflated with, the signature of a back contact barrier. These results have significant implications for the interpretation of a standard measurement technique, including some published reports. These results also lead to a deeper understanding of the physics underlying solar cell performance.
1 J. Davis, T. Cox, J. T. Heath, “Capacitance measurements of defects in solar cells: checking the model assumptions,” (and winner of an undergraduate presentation award), American Physical Society, San Antonio, TX, March 2015 (poster).
2 T. Cox, A. Ogle, J. T. Heath, "Capacitance of thin film solar cells: violating the depletion approximation," American Physical Society, Baltimore, MD, March 2016 (talk); also presented at Linfield College Student Symposium, May 2016 (talk).
3 A. Ogle, T. Cox, J. T. Heath, “Frequency and voltage dependence of series resistance in a solar cell,” American Physical Society, Baltimore, MD, March 2016 (poster); also presented at Linfield College Student Symposium, May 2016 (poster).
Heath, Jennifer, "Characterizing Sub-Bandgap Traps in Cu(In, Ga)Se2 Solar Cells with Back Contact Barriers" (2017). Post-Grant Reports. Report. Submission 107.