Design, fabrication, and characterization of multi-junction micro-photovoltaic devices

In this paper, the electro-optical characteristics of the smallest-to-date GaAs-based multi-junction micro-photovoltaic devices (PVs) that can be used for ~850nm monochromatic light absorption will be presented. PV devices with circular single-aperture and aperture size of 30-150μm have been fabricated on semi-insulating GaAs substrate. The total number of light-absorbing junctions varies between one to three, and the layer structures are optimized to efficiently absorb a monochromatic light source at the wavelength of ~850nm. The light current-voltage characterization of these devices shows a linear scaling of the open-circuit voltage by increasing the number of active junctions, from 1.2V for single junction devices to ~3.6V for triple junction PVs. It has been observed that changes in the incident light density or the aperture size of the PV do not notably affect the photo-generated voltage. A maximum power conversion efficiency of 55% and fill factor of 84% is achieved for triple junction devices with aperture size of 150Νm, accordingly. However, it is shown that the peak efficiency drops to below 10% and fill factor reduces to 73% for devices with aperture size of 30μm. This is likely due to existence of non-radiative recombination centers near the mesa sidewall area that create shunt conduction paths and can adversely affect the photovoltaic performance of the device. This becomes worse for the smaller devices, as the device’s perimeter to area ratio increases proportionally. Overall, these results demonstrate the feasibility of fabrication of multi-junction micro-PVs for low-power sensing and energy storage application.