Determination of back contact barrier height in Cu(In,Ga)(Se,S)2 and CdTe solar cells

A relatively straightforward technique has been developed to quantify the energy barrier for holes between a Cu(In,Ga)(Se,S)2 (CIGSeS) or CdTe absorber and the back-contact metallization. The input data is the current-voltage (J-V) curves for the solar cell measured over a range of temperatures. The key parameter is the “turning current” Jt, which is the current at the transition from the positive J-V curvature of a diode to the negative curvature associated with current limitation at a contact barrier. The analytical strategy is to calculate a series of Jt vs. T curves for different values of barrier height and then overlay the experimental values of Jt. Generally the experimental data follow a single barrier-height curve over a wide temperature range. The presentation will describe the turning point technique and apply it to specific solar-cell examples. The range of Jt that can be practically identified extends from approximately 0.1 to 80 mA/cm². Assuming that temperatures between 220 and 340 K are available, the range of barriers that can be determined is between 0.30 and 0.55 eV. This is also the practical range, since lower barriers do not have a measurable effect on the power quadrant and higher ones effectively kill the performance of the cell. Many CIGSeS and CdTe cells, however, do have a back-contact barrier in the 0.30 to 0.55 eV range, and the ability to determine it can assist both cell analysis and process optimization.