High-photovoltage GaAs vertical epitaxial monolithic heterostructures with 20 thin p/n junctions and a conversion efficiency of 60%

Abstract

Photovoltaic power converting III-V semiconductor devices based on the Vertical Epitaxial HeteroStructure Architecture (VEHSA) design have been achieved with up to 20 thin p/n junctions (PT20). Open circuit photovoltages in excess of 23 V are measured for a continuous wave monochromatic optical input power of ∼1 W tuned in the 750 nm-875 nm wavelength range. Conversion efficiencies greater than 60% are demonstrated when the PT20 devices are measured near the peak of their spectral response. Noticeably, the PT20 structure is implemented with its narrowest ultrathin base having a thickness of only 24 nm. In the present study, the spectral response of the PT20 peaks at external quantum efficiency (EQE) of 89%/20 for an input wavelength of 841 nm. We also performed a detailed analysis of the EQE dependence with temperature and for VEHSA structures realised with a varied number of p/n junctions. The systematic study reveals the correlations between the measured conversion efficiencies, the EQE behavior, and the small deviations in the implementation of the optimal designs. Furthermore, we modeled the photovoltage performance of devices designed with thinner bases. For example, we derive that the narrowest subcell of a PT60 structure would have a base as thin as 8 nm, it is expected to still generate an individual subcell photovoltage of 1.14 V, and it will begin to feature 2-dimensional quantum well effects.