Chung-Yu Hong

Compound biomimetic structures for efficiency enhancement of Ga(0.5)In(0.5)P/GaAs/Ge triple-junction solar cells.

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.

Antireflective scheme for InGaP/InGaAs/Ge triple junction solar cells based on TiO 2 biomimetic structures

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.

Optimization of all-back-contact GaAs solar cells

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%.

Diagnosis of GaInAs/GaAsP Multiple Quantum Well Solar Cells With Bragg Reflectors via Absolute Electroluminescence

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 (<italic>η</italic>_int) under illuminated condition was quantified for analyzing their difference in photovoltaic performances. It was revealed that MQWs showed an advantage of improved <italic>η</italic>_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 <italic>η</italic> _int, in the same structures was predicted.

Photovoltaic characteristics of multiple quantum well solar cells with distributed Bragg reflector and selective filters

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.

Back-contacted thin-film 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.

Photovoltaic characteristics of GaAs solar cells with selective filters

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.

Towards High-Efficiency Triple-Junction Solar Cells with Bio-Inspired Nanostructures

Triple-junction solar cells offer extremely high power conversion efficiency with minimal semiconductor material usage, and hence are promising for large-scale electricity generation. To fully exploit the broad absorption range, antireflective schemes based on biomimetic nanostructures become very appealing due to sub-wavelength scale features that can collectively function as a graded refractive index (GRIN) medium to photons. The structures are generally fabricated with a single-type dielectric material which guarantees both optical design robustness and mechanical durability under concentrated illumination. However, surface recombination and current matching issues arising from patterning still challenge the realization of biomimetic nanostructures on a few micrometer thick epitaxial layers for MJSCs. In this presentation, bio-inspired antireflective structures based on silicon nitride (SiNx) and titanium dioxide (TiO2) materials are demonstrated on monolithically grown Ga0.5In0.5P/In0.01Ga0.99As/Ge triple-junction solar cells. The nano-fabrication employs scalable polystyrene nanosphere lithography, followed by inductively-coupled-plasma reactive-ion-etching (ICP-RIE). We show that the fabricated devices exhibit omni-directional enhancement of photocurrent and power conversion efficiency, offering a viable solution to concentrated illumination with large angles of incidence. Moreover, a comprehensive design scheme is also presented to tailor the reflectance spectrum of sub-wavelength structures for maximum photocurrent output of tandem cells.