Michael Smeets

On the geometry of plasmonic reflection grating back contacts for light trapping in prototype amorphous silicon thin-film solar cells

Abstract. We experimentally investigate the light-trapping effect of plasmonic reflection grating back contacts in prototype hydrogenated amorphous silicon thin-film solar cells in substrate configuration. These back contacts consist of periodically arranged Ag nanostructures on flat Ag reflectors. We vary the period, unit cell, and width of the nanostructures to identify design strategies for optimized light trapping. First, a general correlation between the reduction of the period of the nanostructures down to 550 nm and an increase of the absorptance, as well as external quantum efficiency is found for various unit cells formed by nanostructures. Second, increasing the width of the nanostructures from 200 to 350 nm, an enhanced light-trapping effect of the thin-film solar cells is found independent of the period. As a result, we identify a design for improved light trapping for the given solar cell parameters within the considered variations. It consists of thin-film solar cells applying a combination of a period of 600 nm and a structure width of 350 nm. The implementation of back contacts with this configuration yields enhanced power conversion efficiency as compared to reference solar cells processed on conventionally used randomly textured substrates. In detail, the enhancement of the short-circuit current density from initially 14.7 to initially 15.6  mA/cm2 improves the power conversion efficiency from 9.1 to 9.3%.

Angular dependence of light trapping in nanophotonic thin-film solar cells

The angular dependence of light-trapping in nanophotonic thin-film solar cells is inherent due to the wavelength-scale dimensions of the periodic nanopatterns. In this paper, we experimentally investigate the dependence of light coupling to waveguide modes for light trapping in a-Si:H solar cells deposited on nanopatterned back contacts. First, we accurately determine the spectral positions of individual waveguide modes in thin-film solar cells in external quantum efficiency and absorptance. Second, we demonstrate the strong angular dependence of this spectral position for our solar cells. Third, a moderate level of disorder is introduced to the initially periodic nanopattern of the back contacts. As a result, the angular dependence is reduced. Last, we experimentally compare this dependence on the angle of incidence for randomly textured, 2D periodically nanopatterned and 2D disordered back contacts in external quantum efficiency and short-circuit current density.

Light Management in Flexible Thin-Film Solar Cells—The Role of Nanoimprinted Textures and Tilted Surfaces

We present the application of ultraviolet (UV) nanoimprint lithography for the replication of advanced light management schemes in flexible thin-film solar cells. The approach is maintained entirely at low temperatures, which are required for the development of flexible solar cells on low-cost transparent polymer films. Light-scattering properties are significantly improved by this technique, and thin-film silicon solar cells prepared on these substrates show a substantial improvement in performance due to the nanoimprinted texture. We further investigate the effect of various incident angles of the light on the short-circuit current density (Jsc) of the solar cell and evaluate the corresponding performance of a flexible solar cell in a bent state. Our results show that in the case of imprinted texture, the Jsc and efficiency is reduced within 5% in a bent case of a semicircle when a reduction of the effective illumination area with angle is not taken into account. Overall, the solar cell on imprint-textured polyethylene terephthalate (PET) film shows an increased Jsc for the entire range of incident angles and bent states compared with the nonimprinted PET substrate.

Post passivation light trapping back contacts for silicon heterojunction solar cells

Light trapping in crystalline silicon (c-Si) solar cells is an essential building block for high efficiency solar cells targeting low material consumption and low costs. In this study, we present the successful implementation of highly efficient light-trapping back contacts, subsequent to the passivation of Si heterojunction solar cells. The back contacts are realized by texturing an amorphous silicon layer with a refractive index close to the one of crystalline silicon at the back side of the silicon wafer. As a result, decoupling of optically active and electrically active layers is introduced. In the long run, the presented concept has the potential to improve light trapping in monolithic Si multijunction solar cells as well as solar cell configurations where texturing of the Si absorber surfaces usually results in a deterioration of the electrical properties. As part of this study, different light-trapping textures were applied to prototype silicon heterojunction solar cells. The best path length enhancement factors, at high passivation quality, were obtained with light-trapping textures based on randomly distributed craters. Comparing a planar reference solar cell with an absorber thickness of 280 μm and additional anti-reflection coating, the short-circuit current density (JSC) improves for a similar solar cell with light-trapping back contact. Due to the light trapping back contact, the JSC is enhanced around 1.8 mA cm-2 to 38.5 mA cm-2 due to light trapping in the wavelength range between 1000 nm and 1150 nm.

Cloaking of contact fingers on solar cells and OLEDs using free-form surfaces designed by coordinate transformations (Conference Presentation)

In transformation optics, coordinate transformations are usually mapped onto equivalent (meta-)material parameter distributions. In 2015, we introduced an approach mapping coordinate transformations onto dielectric free-form surfaces. We presented model experiments on cloaking of reflective contact fingers on solar cells. We now report on the fabrication of masters by 3D laser lithography used for soft imprinting. For prototype silicon heterojunction solar cells investigated under 1-sun illumination, we demonstrate the predicted 9% relative efficiency increase. We additionally show that our approach is adaptable to Lambertian sources, thereby cloaking light-emitting diode contacts to achieve spatially homogeneous emission.

Efficient post passivation light-management concepts for silicon heterojunction solar cells

In the present work, we investigate light-management concepts applied subsequent to the passivation of the Si wafer of planar Si heterojunction solar cells. As a first concept, we apply amorphous silicon based nanophotonic textures at the back side of the Si wafer to realize efficient light trapping. As a second concept, we use a nanoimprint lithography based front side anti-reflection coating to improve the incoupling of light into the solar cell absorber. Both concepts allow for efficient improvements in the short-circuit current densities without degrading the planar passivation layers. As the highlight of this work, we demonstrate planar silicon heterojunction solar cells (thickness ~ 280 μm) with high passivation quality as well as a short-circuit current density of 38.8 mA/cm2.

Optimizing the geometry of plasmonic reflection grating back contacts for improved light trapping in prototype amorphous silicon thin-film solar cells

In this study, we experimentally investigate the light-trapping effect of plasmonic reflection grating back contacts in prototype hydrogenated amorphous silicon thin-film solar cells in substrate configuration. The plasmonic reflection grating back contacts consist of periodically arranged Ag nanostructures on flat Ag reflectors. By varying the geometrical parameters of these back contacts, design strategies for optimized light trapping are identified. First, a general correlation between a reduction of the period of the plasmonic reflection grating back contact and an increase of the absorptance as well as external quantum efficiency is found for various unit cells of the nanostructures i.e. square unit cell, hexagonal unit cell and face-centered unit cell. Second, the width of the nanostructures is varied. With increasing width, an enhanced light-trapping effect of the thin-film solar cells is found independent of the period. As a result, an optimized design for improved light trapping in the studied thin-film solar cells is a combination of a period of 600 nm and a structure width of 350 nm. Solar cells fabricated on plasmonic reflection grating back contacts with this optimized configuration yield enhanced power conversion efficiencies as compared to reference solar cells processed on state-ofthe- art randomly textured substrates. In detail, the power conversion efficiency is enhanced by around 0.2 % from 9.1 % to 9.3 %. This increase is largely due to the enhancement of the short-circuit current density of around 7 % from 14.7 mA/cm2 to 15.6 mA/cm2.

Progress on nanopatterned front electrodes for organic solar cells

We present our recent progress in the development of nanophotonic front electrodes for improved light management in organic solar cell. Experimental results and 3D electromagnetic simulations of prototype nanophotonic solar cell will be presented.

Disordered nanophotonic light management in thin-film photovoltaics

In this contribution, we report our recent results on broadband nanophotonic light management in thin-film silicon solar cells. A systematic experimental study on the impact of disorder in nanophotonic light management concepts of thin-film solar cells will be presented. We show that disorder is conceptually an advantage for nanophotonic light-trapping concepts employing grating coupler in thin-film solar cells. This conclusion holds for a broad range of angles of incidence.