Light-trapping structures for planar solar cells inspired by transformation optics.

Optimal light absorption is decisive in obtaining high-efficiency solar cells. An established, if not to say the established, approach is to texture the interface of the light-absorbing layer with a suitable microstructure. However, structuring the light-absorbing layer is detrimental concerning its electrical properties due to an increased surface recombination rate (owing to enlarged surface area and surface defects) caused by the direct patterning process itself. This effect lowers the efficiency of the final solar cells. To circumvent this drawback, this work theoretically explores a transformation optics (TrO) inspired approach to map the nanopatterned texture onto a planar equivalent. This offers a pattern with the same optical functionality but with much improved electrical properties. Schwarz-Christoffel mappings are used for ensuring conformality of the maps. It leads to planar, inhomogeneous, dielectric-only materials for the light trapping structure to be placed on top of the planar light-absorbing layer. Such a design strategy paves a way towards a novel approach for implementing light-trapping structures into planar solar cells.

Strategy for tailoring the size distribution of nanospheres to optimize rough backreflectors of solar cells.

We study the light-trapping properties of surface textures generated by a bottom-up approach, which utilizes monolayers of densely deposited nanospheres as a template. We demonstrate that just allowing placement disorder in monolayers from identical nanospheres can already lead to a significant boost in light-trapping capabilities. Further absorption enhancement can be obtained by involving an additional nanosphere size species. We show that the Power Spectral Density provides limited correspondence to the diffraction pattern and in turn to the short-circuit current density enhancement for large texture modulations. However, in predicting the optimal nanosphere size distribution, we demonstrate that full-wave simulations of just a c-Si semi-infinite halfspace at a single wavelength in the range where light trapping is of main importance is sufficient to provide an excellent estimate. The envisioned bottom-up approach can thus reliably provide good light-trapping surface textures even with simple nanosphere monolayer templates defined by a limited number of control parameters: two nanosphere radii and their occurrence probability.

Feature issue introduction: Light, Energy and the Environment, 2016.

This feature issue highlights contributions from authors who presented their research at the OSA Light, Energy and the Environment Congress, held in Leipzig, Germany from November 14 to 17, 2016.

Feature issue introduction: light, energy and the environment, 2015.

The feature issue highlights contributions from authors who presented their research at the OSA Light, Energy and the Environment Congress, held in Suzhou, China from 2 to 5 November, 2015.

Localized surface plasmon resonance in the IR regime.

Arrays of differently sized disk shaped gold nanoantennas are prepared on glass, which show localized surface plasmon resonance and Rayleigh anomalies in the near infrared and telecom range between 1000 and 1500 nm wavelength. The spectral position of these grating resonances depends critically on the period of the array and the size of the nanoantennas. When PbS quantum dots embedded in PMMA surround the nanoantennas, an up to four fold enhancement of the photoluminescence is observed at the grating resonances due to the constructive diffractive feedback among neighboring antennas. In accordance with the grating resonances a shift of the emission towards smaller wavelengths with decreasing disk diameter is demonstrated.

Photoelectrochemical hydrogen evolution of tapered silicon nanowires.

The origin of the photocurrent enhancement and the overpotential reduction in solar water splitting employing nanostructured silicon is still a matter of debate. A set of tapered Si nanowires (SiNWs) has been designed for clarifying the impact of nanostructured Si on the hydrogen evolution reaction (HER) while precisely tailoring several interference factors such as surface area, light absorption and surface defect density. We find that defect passivation by KOH achieved by tapering is much more beneficial than the optical gain. Surfactant-mediated modification of SiNWs is capable of engineering the band structure. As a result, we suggest a guideline for nanostructured Si photoelectrodes optimized for the HER.

Femtosecond laser-written high-efficiency blazed phase gratings in the volume of soda lime glass for light management in solar modules.

Highly efficient volume phase gratings have been fabricated in low-iron soda lime glass using femtosecond (fs) laser pulses with 1030 nm wavelength and 270 fs pulse duration. Optical simulations based on rigorous coupled-wave analysis theory were performed to determine optimal grating parameters and designs for the application of the gratings for light management in solar modules, suggesting a very effective blazed-like design. Several of such blazed phase gratings have been fabricated and analyzed by measuring their diffraction efficiencies into first and higher orders. Up to 77% of the incoming light in the wavelength region relevant for silicon-based photovoltaics were diffracted by these gratings. Typical induced refractive index changes between 0.002 and 0.006 were derived by comparing the experimental efficiencies with the simulation results.

Influence of black silicon surfaces on the performance of back-contacted back silicon heterojunction solar cells.

The influence of different black silicon (b-Si) front side textures prepared by inductively coupled reactive ion etching (ICP-RIE) on the performance of back-contacted back silicon heterojunction (BCB-SHJ) solar cells is investigated in detail regarding their optical performance, black silicon surface passivation and internal quantum efficiency. Under optimized conditions the effective minority carrier lifetime measured on black silicon surfaces passivated with Al(2)O(3) can be higher than lifetimes measured for the SiO(2)/SiN(x) passivation stack used in the reference cells with standard KOH textures. However, to outperform the electrical current of silicon back-contact cells, the black silicon back-contact cell process needs to be optimized with aspect to chemical and thermal stability of the used dielectric layer combination on the cell.

Automated spray coating process for the fabrication of large-area artificial opals on textured substrates.

3D photonic crystals, such as opals, have been shown to have a high potential to increase the efficiency of solar cells by enabling advanced light management concepts. However, methods which comply with the demands of the photovoltaic industry for integration of these structures, i. e. the fabrication in a low-cost, fast, and large-scale manner, are missing up to now. In this work, we present the spray coating of a colloidal suspension on textured substrates and subsequent drying. We fabricated opaline films of much larger lateral dimensions and in much shorter times than what is possible using conventional opal fabrication methods.