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

Abstract

Cu(In,Ga)Se2-based (CIGS) 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. The determination of the energetic position of the defects and of their density of states is important. Admittance spectroscopy is an adequate technique for this.By varying the external voltage during the measurement, the spatial position where the defect distribution is sensed can be varied. However, the application of external biases can lead to metastable effects in the absorber and therefore to defect relaxation and changes in the doping distribution. Hence, it is important to separate between the effects caused by metastable changes and the change in sensing position of the admittance spectroscopy measurement. This can be achieved by varying the applied voltage during the creation of the metastable state and the measurement itself independently or simultaneously.Admittance spectroscopy under different bias voltage conditions performed on a flexible CIGS-based solar cell are presented and assessed.

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

“…This kind of behavior can be observed when the observed defect is one of the configurations of a metastable defect. 18 Since the activation energy remains relatively constant, is unlikely that this increase is due to the influence of a back contact barrier. Otherwise, one would expect a large increase in the activation energy according to Eq.…”

“…(26)-(31), an estimate of the spatial variation of the defect density belonging to a specific defect energy level, E x , can be made. 18 However, care should be taken with the assumptions made.…”