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.

Measurement of strong photon recycling in ultra‐thin GaAs n/p junctions monolithically integrated in high‐photovoltage vertical epitaxial heterostructure architectures with conversion efficiencies exceeding 60%

“…In this study, we demonstrate experimentally that strong photon coupling and recycling effects are making important contributions to the responsivity of the VEHSA devices, particularly when the optical input is detuned from the peak of the spectral response. The device details have been published previously but here we show that this effect is especially beneficial for the VEHSA structures given that all the subcells have the same relative absorption characteristics. At high input intensities, such photon coupling effects allow to preserve a high responsivity over a broader wavelength range.…”

“…= 61.3%) and 12 (PT12 with P out = 3.4 W, Eff. = 61.8%) thin n/p junctions . The heterostructures are near current‐matched for the indicated wavelengths.…”

“…We can also clearly observe that the responsivity values of the PT6 and the PT12 devices are twice that of the planar PV device. Indeed, it has been shown for the VEHSA structures that EQE values of up to 94% can be readily achieved at the peak of the spectral response . Since the responsivity is directly proportional to the value of the incident wavelength (via the photon flux) and to external quantum efficiency (EQE), it means that the optimum efficiency will be obtained at longer wavelengths near the bandgap and when the EQE is approaching unity.…”

“…Furthermore, high photovoltages and conversion efficiencies have been reported with GaAs side by side planar or vertical arrangements, in particular at high optical intensities with light management or in other material systems . Moreover, record‐high photovoltages and unprecedented monochromatic conversion efficiencies have now been demonstrated for phototransducer applications with the vertical epitaxial heterostructure architecture (VEHSA) design which is engineered with multiple thin, partially absorbing, subcells of the same material . The monolithic epitaxial design of the VEHSA devices naturally allows to construct n/p junction devices with exceptionally low shunting and series resistance issues.…”

Challenges and strategies for implementing the vertical epitaxial heterostructure architechture (VEHSA) design for concentrated photovoltaic applications

“…High conversion efficiencies for GaAs LPCs have been reported in the literature . However, the Vertical Epitaxial HeteroStructure Architechture (VEHSA) technology [30][31][32][33][34][35][36][37][38] has presented a major breakthrough in the field, with reported conversion efficiencies just under 70% (c.f. a previous reporting of 53.4% in Ref.…”