Experimental coupling process efficiency and benefits of back surface reflectors in photovoltaic multi‐junction photonic power converters

Abstract

Current matching is crucial to maximize the efficiency of two‐terminal multi‐junction photovoltaic devices. However, even in perfectly designed devices, deviation from the target operating temperature and consequent changes in the subcell absorptances causes current mismatch between the subcell currents even at constant spectral conditions. Fortunately, luminescence coupling from current‐overproducing subcells to current limiting subcells mitigates this effect. In this work, the coupling process efficiency in three‐junction photonic power converters based on GaAs/AlGaAs rear hetero‐junction subcells is experimentally quantified. A coupling process efficiency of 32% ± 9% from top and middle subcells to the limiting bottom subcell is found. Under constant monochromatic illumination, the observed coupling reduces the current mismatch, induced by raising the temperature from current matched conditions at 25°C to 70°C, from 4.4% to 1.6%. Furthermore, in this work, three‐junction photonic power converters with back surface reflectors are implemented. Those reflectors improve the device response at elevated temperatures by increasing the optical path length in the limiting subcell. It is shown experimentally how a back reflector effectively redirects photons that are emitted by the bottom subcell towards the upper subcells to reinforce luminescence coupling.

Power and Spectral Range Characteristics for Optical Power Converters

“…Figure 6 therefore confirms, using a DC fixed load method, that for medium-power PT6 and PT12 at 808 nm for 4 W < P in < 6 W, efficiencies greater than 50% are obtained for T < 90 • C. These results illustrate that, for typical operating conditions, the photon recycling effect, inherent in VEHSA based devices, contributes significantly to overall favorable spectral and temperature behaviors [20][21][22][23]. Figure 6 therefore confirms, using a DC fixed load method, that for medium-power PT6 and PT12 at 808 nm for 4 W < Pin < 6 W, efficiencies greater than 50% are obtained for T < 90 °C.…”

“…Because of the vertical stacking design, these OPC devices are highly tolerant to beam non-uniformity, partial illumination, and to beam misalignment [19]. In addition, because of significant photon coupling and photon recycling effects [20][21][22][23], the vertical multijunction OPCs are also tolerant to spectral change variations, and provide the best available conversion efficiencies over large temperature ranges, covering the requirements for the majority of optical wireless power transmission applications.…”

Pushing the Boundaries of Photovoltaic Light to Electricity Conversion: A GaAs Based Photonic Power Converter with 68.9% Efficiency

“…for devices with lower absorber material quality and neither for multi-junction cells. For the latter, the device current often strongly depends on the generation profile due to the series connection of subcells; and, in addition, luminescence coupling between the junctions often causes a non-linear response [5,6].…”

High-Efficiency and High-Power Multijunction InGaAs/InP Photovoltaic Laser Power Converters for 1470 nm

“…The external quantum efficiency (EQE) is shown in Figure 4c. At a voltage bias of 4 V, an EQE of 92% is obtained for input powers up to ~5 W. As the input power is increased, the EQE at 4 V increases slightly, up to 93.5% at 14.5 W. The increased EQE value at higher optical intensities could be caused by a higher photon recycling, giving a better current-matching at higher input powers [79][80][81][82]. The output current is shown in Figure 4d.…”

“…For the InP material system, an interesting design variation consists of reducing the absorber thicknesses of the subcells and using light reflected from the back side. The InP substrate is transparent to the input light and to the InGaAs luminescence, therefore allowing the light reflected [8] from the back side of the device to be recycled [79][80][81][82]. Nevertheless, as described previously [41,58], to realize the required photocurrent matching condition, the structure usually has increasing subcell thicknesses from the top subcell (thinnest) toward the bottom subcell (thickest).…”