We proposed a new design method for single freeform reflective (or refractive) surface tailored to redistribute the radiant flux onto a prescribed illumination pattern. Unlike the conventional optimization approaches based on the grid mapping, in this study we estimated each segmental freeform surface by locally solving a second-order differential equation, which formulates the energy transportation between each domain cell. With finite element method via Hermite element, we validated a series of smooth reflective/refractive surfaces to reallocate the radiant flux from a point source toward a target plane with specific patterns. The proposed technique offers a large flexibility by varying the vectors of each ray with multiple refraction (or reflection), which imposes no restriction on the target distribution, collective solid angle, or even target topography.
We demonstrate the GaAs solar cells which utilize the high-transmittance textured polydimethylsiloxane (PDMS) film can outstanding increase the short circuit current density and power conversion efficiency of solar cells. The transmittance of PDMS film is exceeded 90%, which can pass through almost all the light of GaAs Solar cells can be absorbed. We used a special imprint technology to let the PDMS film possess a highly textured surface. Then we measured the characteristics of textured PDMS film and found out that it has a very excellent Haze performance. The effect of flexible textured PDMS film on the suppression of surface reflection in GaAs solar cells is also investigated. The presented technology provides an inexpensive surface anti-reflection process, which can potentially replace typically complex anti-reflection coating (ARC) layer. The GaAs solar cells with textured PDMS layer can effectively enhance the short-circuit current density from 22.91 to 26.54 mA/cm<sup>2</sup> and the power conversion efficiency from 18.28 to 21.43 %, corresponding to a 17 % enhancement compared to the one without textured PDMS. The open-circuit voltage (V<sub>oc</sub>) and the fill-factor (FF) of GaAs solar cells exhibit negligible change, because the textured PDMS film was pasted up on the surface of GaAs solar cells and did not interfere with the diode operation. At the same time, we observed through the EQE measurement that the textured PDMS film not only proved wonderful light scattering effect but also generated more electron-hole pairs in all absorption spectrum range. Finally, through this simple PDMS process, we believe this technology shall be a great candidate for next generation of highly efficient and low-cost photovoltaic devices.
In recent year, InGaN-based alloy was also considered for photovoltaic devices owing to the distinctive material properties which are benefit photovoltaic performance. However, the Indium tin oxide (ITO) layer on top, which plays a role of transparent conductive oxide (TCO), can absorb UV photons without generating photocurrent. Also, the thin absorber layer in the device, which is consequent result after compromising with limited crystal quality, has caused insufficient light absorption. In this report, we propose an approach for solving these problems. A hybrid design of InGaN/GaN multiple quantum wells (MQWs) solar cells combined with colloidal CdS quantum dots (QDs) and back side distributed Bragg reflectors (DBRs) has been demonstrated. CdS QDs provide down-conversion effect at UV regime to avoid absorption of ITO. Moreover, CdS QDs also exhibit anti-reflective feature. DBRs at the back side have effectively reflected the light back into the absorber layer. CdS QDs enhance the external quantum efficiency (EQE) for light with wavelength shorter than 400 nm, while DBRs provide a broad band enhancement in EQE, especially within the region of 400 nm ~ 430 nm in wavelength. CdS QDs effectively achieved a power conversion efficiency enhancement as high as 7.2% compared to the device without assistance of CdS QDs. With the participation of DBRs, the power conversion efficiency enhancement has been further boosted to 14%. We believe that the hybrid design of InGaN/GaN MQWs solar cells with QDs and DBRs can be a method for high efficiency InGaN/GaN MQWs solar cells.
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<sub>x</sub>) and titanium dioxide (TiO<sub>2</sub>) materials are demonstrated on monolithically grown Ga<sub>0.5</sub>In<sub>0.5</sub>P/In<sub>0.01</sub>Ga<sub>0.99</sub>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.