Light trapping in thin-film solar cells with randomly rough and hybrid textures

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.

Wave chaos enhanced light trapping in optically thin solar cells

Enhancing the energy output of solar cells increases their competitiveness as a source of energy. Producing thinner solar cells is attractive, but a thin absorbing layer demands excellent light management in order to keep transmission- and reflection-related losses of incident photons at a minimum. We maximize absorption by trapping light rays to make the mean average path length in the absorber as long as possible. In chaotic scattering systems, there are ray trajectories with very long lifetimes. In this paper, we investigate the scattering dynamics of waves in a model system using principles from the field of quantum chaotic scattering. We quantitatively find that the transition from regular to chaotic scattering dynamics correlates with the enhancement of the absorption cross section and propose the use of an autocorrelation function to assess the average path length of rays as a possible way to verify the light-trapping efficiency experimentally.

Chaos: A new mechanism for enhancing the optical generation rate in optically thin solar cells

The photogenerated current of solar cells can be enhanced by light management with surface structures. For solar cells with optically thin absorbing layers, it is especially important to take advantage of this fact through light trapping. The general idea behind light trapping is to use structures, either on the front surface or on the back, to scatter light rays to maximize their path length in the absorber. In this paper, we investigate the potential of chaotic scattering for light trapping. It is well known that the trajectories close to the invariant set of a chaotic scatterer spend a very long time inside of the scatterer before they leave. The invariant set, also called the chaotic repeller, contains all rays of infinite length that never enter or leave the region of the scatterer. If chaotic repellers exist in a system, a chaotic dynamics is present in the scatterer. As a model system, we investigate an elliptical dome structure placed on top of an optically thin absorbing film, a system inspired by the chaotic Bunimovich stadium. A classical ray-tracing program has been developed to classify the scattering dynamics and to evaluate the absorption efficiency, modeled with Beer-Lambert's law. We find that there is a strong correlation between the enhancement of absorption efficiency and the onset of chaotic scattering in such systems. The dynamics of the systems was shown to be chaotic by their positive Lyapunov exponents and the noninteger fractal dimension of their scattering fractals.

Light scattering properties of self-organized nanostructured substrates for thin-film solar cells

We investigate the scattering properties of novel kinds of nano-textured substrates, fabricated in a self-organized fashion by defocused ion beam sputtering. These substrates provide strong and broadband scattering of light and can be useful for applications in thin-film solar cells. In particular, we characterize the transmitted light in terms of haze and angle-resolved scattering, and we compare our results with those obtained for the commonly employed Asahi-U texture. The results indicate that the novel substrate has better scattering properties compared to reference Asahi-U substrates. We observe super-Lambertian light scattering behavior in selected spectral and angular regions due to the peculiar morphology of the nano-textured interface, which combines high aspect ratio pseudo random structures with a one-dimensional periodic pattern. The enhancement of light absorption observed in a prototype thin film semiconductor absorber grown on nano-textured glass with respect to an Asahi-U substrate further confirms the superior light trapping properties of the novel substrate.

Combining light-harvesting with detachability in high-efficiency thin-film silicon solar cells

Efforts to realize thin-film solar cells on unconventional substrates face several obstacles in achieving good energy-conversion efficiency and integrating light-management into the solar cell design. In this report a technique to circumvent these obstacles is presented: transferability and an efficient light-harvesting scheme are combined for thin-film silicon solar cells by the incorporation of a NaCl layer. Amorphous silicon solar cells in p-i-n configuration are fabricated on reusable glass substrates coated with an interlayer of NaCl. Subsequently, the solar cells are detached from the substrate by dissolution of the sacrificial NaCl layer in water and then transferred onto a plastic sheet, with a resultant post-transfer efficiency of 9%. The light-trapping effect of the surface nanotextures originating from the NaCl layer on the overlying solar cell is studied theoretically and experimentally. The enhanced light absorption in the solar cells on NaCl-coated substrates leads to significant improvement in the photocurrent and energy-conversion efficiency in solar cells with both 350 and 100 nm thick absorber layers, compared to flat-substrate solar cells. Efficient transferable thin-film solar cells hold a vast potential for widespread deployment of off-grid photovoltaics and cost reduction.

Novel back-reflector architecture with nanoparticle based buried light-scattering microstructures for improved solar cell performance

A new back-reflector architecture for light-management in thin-film solar cells is proposed that includes a morphologically smooth top surface with light-scattering microstructures buried within. The microstructures are pyramid shaped, fabricated on a planar reflector using TiO2 nanoparticles and subsequently covered with a layer of Si nanoparticles to obtain a flattened top surface, thus enabling growth of good quality thin-film solar cells. The optical properties of this back-reflector show high broadband haze parameter and wide angular distribution of diffuse light-scattering. The n-i-p amorphous silicon thin-film solar cells grown on such a back-reflector show enhanced light absorption resulting in improved external quantum efficiency. The benefit of the light trapping in those solar cells is evidenced by the gains in short-circuit current density and efficiency up to 15.6% and 19.3% respectively, compared to the reference flat solar cells. This improvement in the current generation in the solar cells grown on the flat-topped (buried pyramid) back-reflector is observed even when the irradiation takes place at large oblique angles of incidence. Finite-difference-time-domain simulation results of optical absorption and ideal short-circuit current density values agree well with the experimental findings. The proposed approach uses a low cost and simple fabrication technique and allows effective light manipulation by utilizing the optical properties of micro-scale structures and nanoscale constituent particles.

Advances in Nanoimprint Lithography

Nanoimprint lithography (NIL), a molding process, can replicate features <10 nm over large areas with long-range order. We describe the early development and fundamental principles underlying the two most commonly used types of NIL, thermal and UV, and contrast them with conventional photolithography methods used in the semiconductor industry. We then describe current advances toward full commercial industrialization of UV-curable NIL (UV-NIL) technology for integrated circuit production. We conclude with brief overviews of some emerging areas of research, from photonics to biotechnology, in which the ability of NIL to fabricate structures of arbitrary geometry is providing new paths for development. As with previous innovations, the increasing availability of tools and techniques from the semiconductor industry is poised to provide a path to bring these innovations from the lab to everyday life.

Experimental quantification of useful and parasitic absorption of light in plasmon-enhanced thin silicon films for solar cells application

A combination of photocurrent and photothermal spectroscopic techniques is applied to experimentally quantify the useful and parasitic absorption of light in thin hydrogenated microcrystalline silicon (μc-Si:H) films incorporating optimized metal nanoparticle arrays, located at the rear surface, for improved light trapping via resonant plasmonic scattering. The photothermal technique accounts for the total absorptance and the photocurrent signal accounts only for the photons absorbed in the μc-Si:H layer (useful absorptance); therefore, the method allows for independent quantification of the useful and parasitic absorptance of the plasmonic (or any other) light trapping structure. We demonstrate that with a 0.9 μm thick absorber layer the optical losses related to the plasmonic light trapping in the whole structure are insignificant below 730 nm, above which they increase rapidly with increasing illumination wavelength. An average useful absorption of 43% and an average parasitic absorption of 19% over 400–1100 nm wavelength range is measured for μc-Si:H films deposited on optimized self-assembled Ag nanoparticles coupled with a flat mirror (plasmonic back reflector). For this sample, we demonstrate a significant broadband enhancement of the useful absorption resulting in the achievement of 91% of the maximum theoretical Lambertian limit of absorption.

Scattering analysis for random antireflective structures on fused silica in the ultraviolet

Random antireflective structures are fabricated on fused silica by the thermal dewetting process and reactive ion etching, which shows a broadband antireflective effect over the whole visible wavelength. However, the transmittance in the ultraviolet is limited by the scattering from the etched structures. A graded refractive index model ignoring the scattering in the visible range is applied to extract the etched profile. Then the Lubachevsky-Stillinger algorithm is used to reconstruct the random antireflective structures with the extracted profile. Bidirectional scattering distribution for the reconstructed structures is simulated with the finite-difference time-domain method, which indicates the importance of transmissive scattering the scattering directivity. The scattering directivity is explained well with an effective grating model. The period of the effective grating can guide the prepared technique in the ultraviolet.

Complex Photonic Structures for Light Harvesting

Over the last few years, micro- and nanophotonics have roused a strong interest in the scientific community for their promising impact on the development of novel kinds of solar cells. Certain thin- and ultrathin-film solar cells are made of innovative, often cheap, materials which suffer from a low energy conversion efficiency. Light-trapping mechanisms based on nanophotonics principles are particularly suited to enhance the absorption of electromagnetic waves in these thin media without changing the material composition. In this review, the latest results achieved in this field are reported, with particular attention to the realization of prototypes, spanning from deterministic to disordered photonic architectures, and from dielectric to metallic nanostructures.

A fair comparison between ultrathin crystalline-silicon solar cells with either periodic or correlated disorder inverted pyramid textures

Fabrication of competitive solar cells based on nano-textured ultrathin silicon technology is challenging nowadays. Attention is paid to the optimization of this type of texture, with a lot of simulation and experimental results published in the last few years. While previous studies discussed mainly the local features of the surface texture, we highlight here the importance of their filling fraction. In this work, we focus on a fair comparison between a technologically realizable correlated disorder pattern of inverted nano-pyramids on an ultrathin crystalline-silicon layer, and its periodically patterned counterpart. A fair comparison is made possible by defining an equivalent periodic structure for each hole filling fraction. Moreover, in order to be as realistic as possible, we consider patterns that could be fabricated by standard patterning techniques: hole-mask colloidal lithography, nanoimprint lithography and wet chemical etching. Based on numerical simulations, we show that inverted nano-pyramid patterns with correlated disorder provide typically greater efficiency than their periodic counterparts. However, the hole filling fraction of the etched pattern plays a crucial role and may limit the benefits of the correlated disorder due to experimental restrictions on pattern fabrication.

15.7% Efficient 10-μm-thick crystalline silicon solar cells using periodic nanostructures

Only ten micrometer thick crystalline silicon solar cells deliver a short-circuit current of 34.5 mA cm(-2) and power conversion efficiency of 15.7%. The record performance for a crystalline silicon solar cell of such thinness is enabled by an advanced light-trapping design incorporating a 2D inverted pyramid photonic crystal and a rear dielectric/reflector stack.

Broadband photocurrent enhancement in a-Si:H solar cells with plasmonic back reflectors

Plasmonic light trapping in thin film silicon solar cells is a promising route to achieve high efficiency with reduced volumes of semiconductor material. In this paper, we study the enhancement in the opto-electronic performance of thin a-Si:H solar cells due to the light scattering effects of plasmonic back reflectors (PBRs), composed of self-assembled silver nanoparticles (NPs), incorporated on the cells' rear contact. The optical properties of the PBRs are investigated according to the morphology of the NPs, which can be tuned by the fabrication parameters. By analyzing sets of solar cells built on distinct PBRs we show that the photocurrent enhancement achieved in the a-Si:H light trapping window (600 - 800 nm) stays in linear relation with the PBRs diffuse reflection. The best-performing PBRs allow a pronounced broadband photocurrent enhancement in the cells which is attributed not only to the plasmon-assisted light scattering from the NPs but also to the front surface texture originated from the conformal growth of the cell material over the particles. As a result, remarkably high values of J(sc) and V(oc) are achieved in comparison to those previously reported in the literature for the same type of devices.