Determining Surface Recombination Velocity and Band Bending at the Back Interface of CdTe Devices Using Back Illuminated Quantum Efficiency

Abstract

In CdTe-based devices the measured open circuit voltage is significantly below the thermodynamic limit. This suggests that significant recombination occurs at one or more of the front interface, back interface, or bulk and that improving device performance will require reduction of this recombination. It is difficult to determine where the device limiting recombination occurs, so attempts to improve the device typically focus on modifying one interface. Unfortunately, if this interface is not limiting the device, improvements will not be observed. A better method would be to measure the key parameters of an interface then use those parameters to determine if that interface is limiting. With this in mind, we developed a method to measure the back surface recombination velocity (SB) and the materials dependent band bending (BB0) using back illuminated quantum efficiency measurements. To determine both parameters, we must fabricate two devices with varying absorber thickness. For a thin device, the back interface is approximately the depletion width from the front interface, so the amount of band bending at the back interface is near zero. As a result, the internal quantum efficiency (IQE) of this device provides a direct measurement of SB. In the thicker device the band bending equals BB0, so the IQE from the thick device and SB determined from the thin device gives BB0. Acquiring these values is the first step in determining the location of limiting recombination.