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By using absolute electroluminescence measurement, we characterized current-dependent external radiative efficiency of GaInAs/GaAsP multiple quantum well (MQW) single-junction solar cells with and without a back distributed Bragg reflector (DBR). Internal radiative efficiency ( η int) under illuminated condition was quantified for analyzing their difference in photovoltaic performances. It was revealed that MQWs showed an advantage of improved η int compared with a bulk layer, and that a back DBR indeed improved conversion efficiency via double-pass absorption of sun light, but improvement via reduction of rear radiative emission loss toward substrate was small. Efficiency add-on via improved material quality, or η int, in the same structures was predicted.

Photon management plays a vital role in the power conversion efficiency of III-V semiconductor solar cells. However, the photon recycling characteristics of GaAs-based multi-quantum-well (MQW) solar cells employed different optical designs had yet been fully explored. In this work, we investigate the impact of the spectrally selective filter (SSF) and distributed Bragg reflector (DBR) on the photovoltaic characteristics of single-junction, strain-balanced In0.1Ga0.9As/ GaAs0.85P0.15 MQW solar cells. Specifically, the SSFs with cutoff wavelengths of 880, 910, and 940 nm are designed and implemented on MQW solar cells with and without the incorporation of a rear DBR. Photon confinement in the vertical direction is verified based on the characterizations of reflectance, electroluminescence, and external quantum efficiency. We show that the photon confinement reduces the saturation current density, up to 26 times and 3 times for the 880 nm SSF-MQW and SSF-MQW-DBR devices, respectively, compared to that of the 940 nm devices. Furthermore, by comparing the SSF-MQW-DBR solar cells under simulated one-sun and concentrated illumination conditions, the open-circuit voltage exhibits a maximal net increase for the 910 nm SSF due to tradeoff between the short-circuit and saturation current density. The proposed SSF design may offer a viable approach to boost the performance of GaAs-based MQW solar cells.

In this work, we attempt to find the optimal design parameters for the doping concentration, thickness of the base region, the length and pitch of back electrodes by employing a validated Sentaurus TCAD model. Through current-voltage characteristics, one could determine best power conversion efficiency and the correlation between design parameters. While optical shadowing is eliminated in the back-contact design, it is found that electrical shading affects the cell performance and is sensitive to the electrode length and pitch. Through minimizing the length of anode, the efficiency can be elevated significantly. Moreover, a relatively wide cathode results in efficient carrier separation and superior transportation properties. Based on these simulation outcomes, we are able to optimize the design of back-contact back-junction GaAs solar cells

In this work, we present a thin-film GaAs solar cell employing via contacts for forming a back-contacted solar cell to eliminate light blocking effects and enhance photon recycling at the same time. The fabrication procedure and the characterization of the fabricated devices are discussed in this paper. From our measurement, a conversion efficiency of 6.5%, open circuit voltage of 0.95 V, and a current density of 8.6mA/cm2 under one-sun illumination were obtained in a 5×5 mm cell.

We investigate the photon recycling characteristics of single junction solar cells incorporating 50-pairs multiple quantum wells (MQW) to exploit the high radiative recombination rate in one-dimensional carrier-confinement structures. In particular, we systematically compare solar cells with monolithically grown distributed Bragg reflectors (DBRs), as well as selective filters (SFs) with cutoff wavelengths of 880, 910, and 930 nm under standard and concentrated illumination conditions. Optical confinement in the vertical direction is verified based on the reflective spectra and electroluminescence properties. Consequently, the photo-recycling effect manifests under concentrated conditions, where the maximal open circuit voltage difference, by 12mV under 200x illumination, is achieved for MQW solar cells incorporating both the DBR and the 910-nm-cutoff SF, compared to those without the DBR or SFs. It is also revealed that the fill factor (FF) is also affected by the photon recycling effect, where incorporating SF results in the FF enhancement compared to the counterpart without the SF and the enhancement is higher for those with DBRs than without the DBRs.

Multi-junction solar cells offer extremely high power conversion efficiency with minimal semiconductor material usage, and hence are promising for large-scale electricity generation. However, suppressing optical reflection in the UV regime is particularly challenging due to the lack of adequate dielectric materials. In this work, bio-inspired antireflective structures are demonstrated on a monolithically grown Ga0.5In0.5P/In0.01Ga0.99As/Ge triple-junction solar cell, which overcome the limited optical response of reference devices. The fabricated device also exhibits omni-directional enhancement of photocurrent and power conversion efficiency, offering a viable solution to concentrated illumination with large angles of incidence. A comprehensive design scheme is further developed to tailor the reflectance spectrum for maximum photocurrent output of tandem cells. Copyright © 2012 John Wiley & Sons, Ltd.

The optimized condition of all-back-contact solar cells is studied to eliminate the problem of light blocking by the front contacts in conventional solar cells. The influences of layer thickness and pitch distance are studied. The optimized structure is obtained with a 2 μm thick base layer and a 631 μm contact pitch, and the efficiency can reach to 26.15%.

Enhancing the photon recycling process in a solar cell can boost the open-circuit voltage (Voc). In this work, we employ a selective filter realized by alternative TiO2 and SiO2 dielectric layers with different cutoff wavelengths as the front surface structures, and a distributed Bragg reflector (DBR) grown by epitaxy on the rear side of GaAs solar cells to form an optical cavity. The idea is to suppress the radiative recombination emission loss at the material bandgap toward the front and rear side by increasing photon-recycling via a cavity design with minimal auxiliary impact on light current. From our experiment, when cut-off wavelength was designed at 840nm, the Voc of the cell increase by 1.1mV compared with the device without the selective filter. Moreover, we successfully developed an optical model that combines a rigorous couple wave analysis (RCWA) and photon recycling calculation NREL developed recently to quantify the Voc enhancement from different optical design on GaAs solar cells. Based on theoretical optimizations of the experimental data, if we can optimize the back DBR reflector or using metal as the back mirror, we can obtain a large Voc enhancement by 36.4mV using the selective filter with a cut-off wavelength of 840 nm. However, although we can use selective filter as top structure to improve the Voc, the power conversion efficiency is still limited due to the implementation of selective filters has affected light absorption at the desired spectral range. Nevertheless, this concept of light management may be further improved by using angular filters in order to prevent the loss of light current while improving the Voc and power conversion efficiency at the same time.

High-efficiency III–V solar cells typically incorporate an indirect wide-bandgap semiconductor as a passivation layer to limit surface recombination at higher photon energies. The poor extraction efficiency of the carriers photogenerated in this window layer limits the performance of the devices in the high-energy region of the spectrum. To address this problem, a resonance energy transfer (RET)-mediated luminescent down-shifting (LDS) layer is engineered by depositing an epilayer of colloidal quantum dots (QDs) on an InGaP solar cell. In this configuration, while the QDs act as a standard LDS layer, excitons are also funneled from the window layer to the QD epilayer using near-field RET. The luminescence energy of the QDs is tuned below the bandgap of the window layer and the emitted light is absorbed in the p–n junction, where carriers are generated and efficiently extracted. The overall performance of the solar cell is found to be significantly improved after hybridization, with a large 14.6% relative and 2% absolute enhancement of the photon conversion efficiency.

Biomimetic nanostructures have shown to enhance the optical absorption of Ga₀.₅In₀.₅P/GaAs/Ge triple junction solar cells due to excellent antireflective (AR) properties that, however, are highly dependent on their geometric dimensions. In practice, it is challenging to control fabrication conditions which produce nanostructures in ideal periodic arrangements and with tapered side-wall profiles, leading to sacrificed AR properties and solar cell performance. In this work, we introduce compound biomimetic nanostructures created by depositing a layer of silicon dioxide (SiO₂) on top of titanium dioxide (TiO₂) nanostructures for triple junction solar cells. The device exhibits photogenerated current and power conversion efficiency that are enhanced by ~8.9% and ~6.4%, respectively, after deposition due to their improved antireflection characteristics. We further investigate and verify the optical properties of compound structures via a rigorous coupled wave analysis model. The additional SiO₂ layer not only improves the geometric profile, but also serves as a double-layer dielectric coating. It is concluded that the compound biomimetic nanostructures exhibit superior AR properties that are relatively insensitive to fabrication constraints. Therefore, the compound approach can be widely adopted for versatile optoelectronic devices and applications.

In this work, we demonstrate the device design, characterization, fabrication and analysis of biomimetic antireflective structures on III–V multi-junction solar cell. The cell without antireflective structure has a severe change of refractive index between the cell and air, causing a huge reflection (about 30%) and decrease the cell efficiency significantly. We can reduce light reflection by coating a proper material on the cell, diminishing the refractive index difference between cell and air interface. The optimization of single antireflection coating and biomimetic structure will be discussed and the experimental results will also be presented.