Small-signal capacitance-frequency modelling of the back contact barrier in Cu(In,Ga)Se2 solar cells

Abstract

Accurate capacitance measurements in Cu(In,Ga)Se₂ solar cells as well as any other material system are important for extracting accurate doping profiles, trap energies, and trap concentrations, but non-Ohmic back contacts can impact the measured capacitance. In this case it is demonstrated that a five-element small-signal model accurately fits the capacitance and conductance frequency dependence where two elements represent and are correlated to the back contact, two others represent the semiconductor junction, and the fifth is a series resistance. A temperature-dependent contact conductance (G_C) is found to cause the inflection in the capacitance-frequency (C-f) measurements, which is shown to be responsible for the signature commonly observed in admittance spectroscopy. Good agreement is observed between measured data and simulations using the five-element model across a wide range of temperatures. This analysis shows the importance of performing a C-f measurement before subsequent capacitance measurements to choose an appropriate measurement frequency.