Defect distributions in thin film solar cells deduced from admittance measurements under different bias voltages

Abstract

The voltage dependence of the derivative of the capacitance to (the logarithm of) the measurement frequency is investigated. Relations describing this dependence are derived for the influence of carrier freeze out, of a defect distribution, and of a back contact barrier. The validity of these relations is investigated with numerical simulations. Considering the extraction of the defect density from capacitance-frequency measurements, the extension of existing formulas to different bias voltages leads to an improved accuracy and the possibility to investigate spatial non-uniformities while preserving a direct link between the defect level energy and the apparent defect density. This is illustrated with voltage dependent admittance measurements of thin film Cu(In,Ga)Se 2-based solar cell devices. V

Accuracy of defect distributions measured by bias dependent admittance spectroscopy on thin film solar cells

“…The presence of a back contact barrier however has a similar influence on the admittance of a solar cell. For example the voltage dependence of the derivative of the capacitance [2] is similar as in (2) and (4): proportional with (Vbi -V)…”

“…This leads to a step in the capacitancefrequency (C-f) characteristic and to an extremum in its derivative. As shown in [1] and [2], the defect energy level which corresponds to such an inflection frequency can be calculated as (1).…”

“…Walter et al [1] developed a straightforward technique to determine defect density distributions from admittance measurements under zero bias conditions. This technique has recently been extended to non-zero bias conditions [2] which improves its applicability, enables to extract additional information from the measurements [3] and helps to improve and assess the accuracy of the results.…”

Grading and metastable effects in admittance spectroscopy of CIGS-based solar cells

“…For CIGS-based solar cells it is adequate to assume parabolic band bending [2,3]. In this case, the position of the defect energy level with respect to the valence band is given by Eq.…”

“…This technique is based on calculating the derivative of the capacitance with respect to the measurement frequency and results in a graph of the defect density as a function of the defect level energy. This technique has recently been extended to non-zero bias conditions [2] which enhances its applicability, helps to improve and assess the accuracy of the results [3] and enables to extract additional information from the measurements.…”