Search

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.

Mono- and multi-crystalline silicon photovoltaics currently still hold more than 80% market share because of the non-toxic, abundant material resources used, and their long-term stabilities. However, the cost of solar power is still more than three times that of fossil fuels, which necessitates a further reduction to accelerate its widespread use. It has been estimated that cell fabrication consumes 30% of the total manufacturing cost due to energy intensive semiconductor processes, such as high temperature furnace for doping, electrodes co-firing, high-vacuum chemical deposition, etc. Therefore, the organic-inorganic hybrid cell concept has been proposed to take advantage of the solution-based processes for rapid and low-cost production and the wide absorption spectrum of silicon. In this work, we demonstrate a hybrid heterojunction solar cell based on the structure of conductive polymer PEDOT:PSS spun cast on n-type crystalline silicon nanorod (SiNR) arrays with periodic arrangements. The nanorod arrays are fabricated by electron beam (E-beam) lithography followed by reactive-ion etching (RIE), which show capability to enhance light harvesting. In addition, SiNRs and PEDOT:PSS can form core-shell structure that provides a large p-n junction area for carrier separation and collection. We measured the optical and photovoltaic characteristics of these devices under a simulated class A solar simulator with a calibrated illumination intensity of 1000 W/m2 for the AM1.5G solar spectrum. A post-RIE damage removal etching (DRE) is subsequently introduced in order to mitigate the surface recombination issues and also alter the surface reflection due to modifications in the nanorod side-wall profile. Finally, we show that the DRE treatment can effectively recover the carrier lifetime and dark current-voltage characteristics of SiNRs hybrid solar cells to resemble the planar counterpart without RIE damages.

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 (SiN x ) and titanium dioxide (TiO 2 ) materials are demonstrated on monolithically grown Ga 0.5 In 0.5 P/In 0.01 Ga 0.99 As/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.

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.

Biomimetic nanostructures have shown to enhance the optical absorption of Ga(0.5)In(0.5)P/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 (SiO(2)) on top of titanium dioxide (TiO(2)) 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 SiO(2) 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.