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

Abstract

Thin film solar cells have achieved efficiencies up to 20%. Despite these excellent results, the understanding of the underlying mechanisms and the influence of defects on their performance is still incomplete. In thin film solar cells often defect level distributions are present rather than discrete defects. These distributions can be calculated from admittance measurements, however several assumptions are needed which hinder an exact defect density determination. By performing the measurements under different bias voltage conditions the accuracy of the method can be improved and assessed. This is illustrated with measurements on a flexible thin film Cu(In,Ga)Se2-based (CIGS) solar cell.

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

“…Fitting these parameters rather than estimating them improves the accuracy of the final result. 17 Since the fitting requires a spatially uniform value of N t it should be performed at an energy, E x , where this can be assumed. This energy might be different from the energy at which the derivative of the capacitance reaches a maximum.…”

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.…”