Direct Observation of CdCl2 Treatment Induced Grain Boundary Carrier Depletion in CdTe Solar Cells Using Scanning Probe Microwave Reflectivity Based Capacitance Measurements

Device grade polycrystalline CdTe films, grown by rf-sputtering (yielding up to 13% efficient devices) and close spaced sublimation (CSS) (yielding up to 16% efficient devices), have been analyzed using scanning microwave impedance microscopy (sMIM). In order to study the effect of Cl on the electronic properties of grain boundaries in CdTe, sMIM based capacitance measurements were performed on the surfaces of as deposited and CdCl2 annealed CdTe films. The measurements were performed with an sMIM instrument operating at similar to 3 GHz probe signal frequency in conjunction with a combination of ac and dc voltage biases applied to the tip. It was found that CdCl2 annealed CdTe samples deposited using both CSS and sputtering exhibit carrier depletion along the grain boundaries as compared to the grain bulk. The grain boundary carrier depletion was not observed in samples that have not been CdCl2 annealed and hence have no incorporated Cl.