For solar cells operating under the broad‐band solar spectrum, the photovoltaic conversion efficiency is fundamentally limited by transmission and thermalization losses. For monochromatic light, these losses can be minimized by matching the photon energy and the absorber material's bandgap energy. Furthermore, for high‐crystal‐quality direct semiconductors, radiative recombination dominates the minority carrier recombination. Light‐trapping schemes can leverage reabsorption of thereby internally generated photons. Such photon recycling increases the effective excess carrier concentration, which, in turn, increases photovoltage and consequently conversion efficiency. Herein, a back surface reflector underneath a GaAs/AlGaAs rear‐heterojunction structure leverages photon recycling to effectively reduce radiative recombination losses and therefore boost the photovoltage. At the same time, resonance in the created optical cavity is tailored to enhance near‐bandgap absorption and, thus, minimize thermalization loss. With a thin film process and a combined dielectric–metal reflector, an unprecedented photovoltaic conversion efficiency of 68.9 ± 2.8% under 858 nm monochromatic light at an irradiance of 11.4 W cm−2 is demonstrated.
GaN-based heterostructure FETs (HFETs) featuring a 2-D electron gas (2DEG) can offer very attractive device performance for power-switching applications. This performance can be assessed by evaluation of the dynamic on-resistance Ron,dyn vs. the breakdown voltage Vbd. In literature, it has been shown that with a high Vbd, Ron,dyn is deteriorated. The impairment of Ron,dyn is mainly driven by electron injection into surface, barrier, and buffer traps. Electron injection itself depends on the electric field which typically peaks at the gate edge towards the drain. A concept suitable to circumvent this issue is the charge-balancing concept which employs a 2-D hole gas (2DHG) on top of the 2DEG allowing for the electric field peak to be suppressed. Furthermore, the 2DEG concentration in the active channel cannot decrease by a change of the surface potential. Hence, beside an improvement in breakdown voltage, also an improvement in dynamic behaviour can be expected. Whereas the first aspect has already been demonstrated, the second one has not been under investigation so far. Hence, in this report, the effect of charge-balancing is iscussed and its impact on the dynamic characteristics of HFETs is evaluated. It will be shown that with appropriate device design, the dynamic behaviour of HFETs can be improved by inserting an additional 2DHG
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.
We present recent results achieved in the field of photonic power conversion, i.e. monochromatic light to electricity conversion, using photovoltaic cells. Based on a thin film processing approach we leverage photon recycling and optical resonance effects with a GaAs/AlGaAs rear-heterojunction photovoltaic cell. A back reflector yields increased effective minority carrier lifetime and, as a consequence, an increase in voltage. Optical resonance in the microcavity yields high absorptance close to bandgap despite weak absorptivity. Hence, high current is reached while thermalization losses are minimized. Maximal spectral response SR=0.653 A/W is measured at 858 nm. At this wavelength, thermalization losses diminish to only 21 meV per photon or 1.5%rel. Based on the calibrated spectral response combined with light I-V measurements under broad band and monochromatic light, we determine a maximum equivalent monochromatic power conversion efficiency at 858 nm of 68.9%±2.8%.
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