Nano-scale current voltage characteristics of thin film solar cell with light irradiations

Nano-scale current voltage (I–V) characteristics of hydrogenated amorphous silicon (a-Si:H) solar cells were studied with and without light irradiations by using conductive Atomic Force Microscope (conductive-AFM). To obtain proper I–V characteristics, electrical contacts between the n-layer of the a-Si solar cells and the cantilever of the AFM were modified by depositing ZnO films with the thickness of 20nm on the tip using DC sputterer. The I–V characteristics were changed from Schottky like to ohmic like. Surface defects and grain boundaries of the ZnO thin film possibly formed an ohmic-like current path between n-layer and the tip of the cantilever. We measured photo-current map at the bias of −2V with a resolution of nano-meter scales. High photo-current paths appeared at nano-crystallites formed in the nc-Si:H film of the solar cell while small current area was observed at the boundaries of the small and large grains. This suggests that the boundaries have high impedance created by hydrogen passivation or act as a leakage current path for the photo-current recombination. These results imply that our proposed measurement technique using conductive-AFM combined with covering of ZnO film on the cantilever is a useful tool for the evaluation of the short path of the photo-current. In addition, these techniques contribute to increase the conversion efficiency of thin film solar cells.