High-resolution mapping of the energy conversion efficiency of solar cells and silicon photodiodes in photovoltaic mode

Abstract

We demonstrate an optical technique to derive the two-dimensional energy conversion efficiency ( η CE), fill factor (FF) and external quantum efficiency ( η QE) distributions across the surface of photovoltaic devices. A compact, inexpensive optical-feedback laser diode microscope is constructed to acquire the confocal reflectance and efficiency maps enabling the observation of the local parametric behavior in silicon photodiodes in photovoltaic mode and single-junction solar cells. The η CE and η QE distributions are greatly influenced by local parasitic resistances that depend on laser irradiance. These parasitic resistances decrease the η CE and η QE values with distance from the contact electrode at high laser irradiance. The optical technique enables microscopic comparison of η CE and η QE within the pn-overlay region of the photodiode sample, revealing its optimization for photodetection rather than power generation. The technique also elucidates the decreasing local η CE of the solar cell under intense irradiation.