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In this work, we propose a route to achieve a certified efficiency of up to 24.51% for silicon heterojunction (SHJ) solar cell on a full-size n-type M2 monocrystalline-silicon Cz wafer (total area, 244.53 cm2) by mainly improving the design of the hydrogenated intrinsic amorphous silicon (a-Si:H) on the rear side of the solar cell and the back reflector. A dense second intrinsic a-Si:H layer with an optimized thickness can improve the vertical carrier transport, resulting in an improved fill factor (FF). In order to reduce the plasmonic absorption at the back reflector, a low-refractive-index magnesium fluoride (MgF2) is deposited before the Ag layer; this leads to an improved gain of short circuit current density (Jsc). In total, together with MgF2 double antireflection coating and other fine optimizations during cell fabrication process, ~1% absolute efficiency enhancement is finally obtained. A detailed loss analysis based on Quokka3 simulation is presented to confirm the design principles, which also gives an outlook of how to improve the efficiency further.

A silicon heterojunction solar cell using silicon carbide as front contact is presented, which features the main advantage of high transparency. To enhance this advantage, an optical loss analysis is performed. It is found that reflection losses play an important role for the solar cell, which can easily be reduced by applying an additional MgF2 coating. The deposition of the coating degrades the passivation quality of the contact but can be cured, eventually leading to a certified short circuit current density of 40.9 mA/cm² and efficiency of 23.99%. Afterwards, a roadmap to a theoretical efficiency of 25% is presented.

Silicon heterojunction with nc-SiC(n)/SiO 2 based front transparent passivating contact (TPC) is numerically modeled. The model is then used to study the effect of active dopant concentration at the front and rear contact of the solar cell. A potential of power conversion efficiency above 25 % can be achieved with a suitable acceptor dopant concentration of p-type amorphous silicon at the rear side. Improving fill factor via SiC dopant concentration can enhance the cell power conversion efficiency within a narrow range of active dopant concentration. However, very high doping of SiC can affect the cell performance negatively.

Summary In order to compensate the insufficient conductance of heterojunction thin films, transparent conductive oxides (TCO) have been used for decades in both sides of contacted crystalline silicon heterojunction (SHJ) solar cells to provide lateral conduction for carrier collection. In this work, we substitute the TCO layers by utilizing the lateral conduction of c-Si absorber, thereby enabling a TCO-free design for SHJ solar cells achieving a low series resistivity of 0.32 Ωcm2 and a good fill factor of 80.7% with a conventional finger pitch of 1.8 mm. Achieving high efficiencies in TCO-free SHJ solar cells requires suppressing deterioration of the passivation quality induced by the direct metal-to-a-Si:H contacts. We show that an ozone treatment at the a-Si:H/metal interface suppresses the metal diffusion into the a-Si:H layer and improves the passivation without increasing the contact resistivity. SHJ solar cells with TCO-free front contacts and ozone treatment achieve efficiencies of >22%.

The color produced by visible light that reflects from the photovoltaic modules can influence visual aesthetics for colored photovoltaic applications, such as the building integrated photo-voltaic and the vehicles integrated photovoltaic. How two colors lying close together can be perceived by the human eye is important for aesthetic design. In this article, we investigate the reflectance spectra variation caused by the variation of indium tin oxide thickness and incidence angle of sunlight based on the well-known silicon heterojunction solar cells and modules. By converting the reflectance spectra into the Delta E 2000 value, we quantify whether differences in color can be perceived. The colors are also predicted based on the standard red, green, and blue color space. The results show that the reflectance variation because of an ITO thickness deviation of 5 nm in SHJ solar cells leads to a perceptible color difference, which can be suppressed after encapsulation but is still perceptible on close observation. The ITO thickness deviation should be controlled within 3 nm to produce a nearly imperceptible visual appearance. The color difference of SHJ modules with an ITO thickness of 70 nm is nearly imperceptible if the incidence angle is below 70°. For comparison, the color differences of the passivated emitter and rear contact solar cells using SiNx as an antireflection layer is also investigated

In this work, we have improved the absorption properties of thin film solar cells by introducing light trapping reflectors deposited onto self-assembled nanostructures. The latter consist of a disordered array of nanopillars and are fabricated by polymer blend lithography. Their broadband light scattering properties are exploited to enhance the photocurrent density of thin film devices, here based on hydrogenated amorphous silicon active layers. We demonstrate that these light scattering nanopillars yield a short-circuit current density increase of +33%rel with respect to equivalent solar cells processed on a planar reflector. Moreover, we experimentally show that they outperform randomly textured substrates that are commonly used for achieving efficient light trapping. Complementary optical simulations are conducted on an accurate 3D model to analyze the superior light harvesting properties of the nanopillar array and to derive general design rules. Our approach allows one to easily tune the morphology of the self-assembled nanostructures, is up-scalable and operated at room temperature, and is applicable to other photovoltaic technologies.

Incomplete photon absorption is one big drawback in solar cells with thin absorbers. Therefore, the development of advanced light-trapping schemes is one major task to reduce the optical losses. The so-called Lambertian scattering is often used as a benchmark for the application of light-trapping schemes with features that are treated by geometrical optics. In our optical simulation study, we introduce two different deterministic light-trapping approaches. We show that in both cases, the absorptance in 60 μm thin c-Si absorbers exceeds the value for Lambertian scattering in a broad spectral range.

We investigate the influence of surface texture on the effectiveness of reflectance by intermediate reflectors in thin-film tandem solar cells. Two distinct angular reflection regimes are identified. For large surfaces with large aspect ratios, frustrated total internal reflection or photon tunneling is found to be the dominating mechanism. We show that the parasitic reflection losses of commonly used randomly textured surfaces are explained by these distinct regimes and introduce a spectral reflectance parameter, which serves as a guideline for designing optimized spectrally selective intermediate reflectors.

Nanophotonic gratings can be used for efficient light trapping in crystalline silicon solar cells. A common challenge is the high quality surface passivation, when applying such gratings. In this article, we report on the prototyping of the application of nanophotonic grating back contacts to planar silicon heterojunction solar cells for improved light trapping. These back contacts are processed after the Si wafer passivation, omitting any direct texturing of the absorber layer. Thereby, maintaining the integrity of the planar passivation layer of high quality and simultaneous light trapping is realized. The induced light-trapping effect is further studied by photoluminescence imaging, external quantum efficiency measurements as well as absorptance measurements of solar cells applying nanophotonic grating back contacts of different grating periods. A clear correlation is found for the three characterization methods which allows the optimal period for efficient light trapping in the investigated solar cells to be identified. For the optimized grating period of the nanophotonic grating back contact an increased short-circuit current density is obtained and the power conversion efficiency is enhanced by 0.4%, absolute.

To improve the infrared (IR) response, a high-refractive-index intrinsic amorphous silicon (a-Si:H) layer is introduced after metallization of bifacial silicon heterojunction (SHJ) solar cells, resulting in a transparent conductive oxide (TCO)/a-Si:H back reflector, which functions like distributed Bragg reflector (DBR). This concept is demonstrated by both Sentaurus Technology Computer-Aided Design (TCAD) simulation and experimental methods. The TCO/a-Si:H back reflector can increase rear internal reflectance by reducing the transmission loss, thus improving the IR external quantum efficiency. The using of Sn-doped In2O3 (ITO)/a-Si:H back reflector in >23.5% efficiency SHJ solar cells can improve short-circuit current density by 0.4 mA cm2 which is quite similar as using the more expensive ITO/Ag back reflector, while keeping a cell bifaciality of 55%. This brings its advantage for monofacial application case. Future studies would be nice to work on higher transparent back reflectors to broaden the application in bifacial case. This back-reflector design promotes IR response of SHJ solar cells with transferring to a wide variety of TCOs.

In this work, ultra-thin n-type hydrogenated nanocrystalline silicon oxide [(nc-SiOx:H (n)] film was used to replace amorphous silicon [a-Si:H (n)] as electron transport layer (ETL) in rear-junction silicon heterojunction (SHJ) solar cell to reduce front parasitic absorption. The contact resistivity between the transparent conductive oxide (TCO) and ultra-thin ETL interface plays an important role on the cell performance. A nanocrystalline silicon (nc-Si:H) contact or seed layer was introduced in the solar cell with ultra-thin nc-SiOx:H and the impact of the nc-Si:H thickness on the cell performance was investigated. To demonstrate scalability, bifacial solar cells with 10 nm ETL were fabricated on the M2 (244 cm2) wafer. The best cell performance is obtained by the solar cell with 5 nm nc-SiOx:H (n) and 5 nm nc-Si:H (n) contact layer and it exhibits open-circuit voltage (Voc) of 738 mV, fill factor (FF) of 80.4%, short-circuit current density (Jsc) of 39.0 mA/cm2 and power conversion efficiency (η) of 23.1% on M2 wafer. Compared to the one with nc-SiOx:H (n), an increase of 3%abs of FF and 0.5%abs of η and lower front contact resistivity is demonstrated for the solar cells with nc-Si:H (n) / nc-SiOx:H (n) double layer, which is caused by the lower energy barrier for electrons, according to the band diagram calculated by the AFORS-HET simulator. A simulation on the solar cell optical and electrical losses was done by the Quokka 3 simulator and shows much lower electrical transport loss and a bit higher front surface transmission loss for the one with double layer than nc-SiOx:H (n) single layer.

The surface patterning of Si heterojunction solar cells is of key importance to enhance the light trapping properties and enable high efficiencies of the emerging thin crystalline silicon solar cells. Herein, periodic inverted pyramids with different periodicity patterns were fabricated by nanoimprint lithography in combination with dry- and wetetching techniques. The impact of their periodicity on the light trapping properties was studied by measuring their reflectance and photoluminescence properties. The inverted pyramids with 700 nm periodicity show excellent anti-reflection and selective light-trapping properties.

Aluminum-doped zinc oxide (ZnO:Al) layers deposited by sputtering and boron-doped zinc oxide (ZnO:B) layers deposited by low-pressure chemical vapor deposition (LPCVD) are well-established materials for front electrodes in thin-film silicon solar cells. In this study, both types of front electrodes are evaluated with respect to their inherent properties and their surface textures in micromorph tandem solar cells in the superstrate configuration. The silicon layer stack investigated here consists of a 220-nm-thick amorphous silicon top cell, a 40-nm-thick intermediate reflector and a 1.1-mu m-thick microcrystalline silicon bottom cell; for this specific silicon layer stack, the LPCVD ZnO:B provides higher power conversion efficiency than its sputtered ZnO:Al counterpart. The growth-friendly surface topography of ZnO:Al yields better electrical performance. However, tandem cells on ZnO:Al suffer from a fundamental optical limitation in terms of light trapping in the top cell. They also show a higher parasitic absorption linked to the relatively high doping concentration of the ZnO:Al layers used here. Detailed analysis of the experimental results allows us to clearly understand the opto-electrical behavior of both types of cells and envisage several possible upgrades to further improve their performance. (C) 2015 Published by Elsevier B.V.

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.

A simple method is developed to determine the light trapping properties of arbitrarily textured solar cells with high accuracy. The method allows for determining the quantum efficiency and short circuit current density of thin film solar cells prepared on randomly nanotextured surfaces. The light trapping of the randomly textured solar cell is described by the area weighted superposition of periodically textured solar cells. The necessary input parameters for the calculations are determined by analyzing the randomly textured surfaces of the solar cells using atomic force microscopy and image processing. The analysis of the atomic force microscope images and the calculation of the quantum efficiency and short circuit current can be determined from current maps, without complex and time-consuming calculations. The calculated solar cell parameters exhibit excellent agreement with experimentally measured quantum efficiencies and short circuit current densities for amorphous and microcrystalline silicon thin film solar cells prepared on randomly textured substrates. Finally, the work contributes to a comparison of random and periodic light trapping structures.

Nanophotonic light management concepts are essential building blocks of advanced thin-film solar cells. These concepts make use of light coupling to waveguide modes that are supported by the photoactive absorber material of the solar cell. In a recent study, we presented a new method based on scanning near-field optical microscopy that enables the direct nanoscale investigation of light coupling to an individual waveguide mode in a nanophotonic thin-film silicon solar cell. Making use of this method, we investigate in this contribution the polarization dependence of the light coupling to a waveguide mode. Based on this polarization dependence, we can attribute the investigated waveguide mode to a transverse electric mode. Moreover, we identify the grating vector, which is responsible for the light coupling to the investigated waveguide mode in the nanopatterned thin-film silicon solar cell.

An aperture-type scanning near-field optical microscope (a-SNOM) is readily used for the optical and optoelectronic characterizations of a wide variety of chemical, biological and optoelectronic samples with sub-wavelength optical resolution. These samples mostly exhibit nanoscale topographic variations, which are related to local material inhomogeneity probed either by an optical contrast or by secondary effects such as photoconductivity or photoluminescence. To date, in the interpretation and evaluation of the measurement results from a-SNOM or derived methods, often only the local material inhomogeneity is taken into account. A possible influence of the optical interaction between the scanning probe and the surface topography is rarely discussed. In this paper, we present experimental and theoretical investigation of the effects of nanoscale topographic features on a-SNOM measurement results. We conduct local photocurrent measurements on a thin-film solar cell with an a-SNOM as the illumination source. A clear correlation between the photocurrent response and local topography is observed in all measurements with a signal contrast of up to ∼30%, although the sample features homogeneous permittivity and electrical properties. With the help of finite-difference time-domain (FDTD) simulations, this correlation is reproduced and local light coupling is identified as the mechanism which determines the local photocurrent response. Our results suggest that a-SNOM-based measurements of any sample with material inhomogeneity will be superimposed by the local light-coupling effect if surface topography variation exists. This effect should always be taken into consideration for an accurate interpretation of the measurement results.

High-efficiency thin-film silicon solar cells require advanced textures at the front contacts for light management. In this contribution, the influence of the texture of various transparent conductive oxides (TCO) on the effectiveness of an intermediate reflector layer (IRL) in a-Si:H/μc-Si:H tandem solar cells is investigated. The employed front side TCOs include several types of sputter-etched ZnO:Al, LPCVD ZnO:B and APCVD SnO 2 :F. The topographies after different stages of the deposition process of the tandem solar cell, at the front TCO, after deposition of the amorphous top cell and after the deposition of the microcrystalline bottom cell, were characterized by atomic force microscopy at precisely the same spot. The external quantum efficiency of the fabricated solar cells were measured and successfully reproduced by a finite-difference time-domain method applying the measured topographies at each interface of the solar cell. With these simulations, the impact of structure type and feature size on the effectiveness of the IRL is investigated. The highest IRL effectiveness in a tandem solar cell was found for double-textured ZnO:Al. In this contribution, we study the interplay between interface textures and parasitic losses. Our findings are relevant for the design of topography for optimized IRL performance.

Thin-film silicon solar cells often apply a metal back reflector (BR) separated from the silicon layers by a thin rear dielectric of thickness around 80 nm or a white paint combined with a thick rear dielectric of several micrometers. In this work, we investigate the optical performance of microcrystalline silicon (μc-Si:H) solar cells applying BRs of various topographies. In contrast to a standard 80 nm-ZnO/Ag BR design, for which the BR nearly strictly follows the texture of the underlying μc-Si:H layers, placing the Ag BR far from the μc-Si:H layers allows for a variation of the BR topography. Irrespective of the investigated BR topographies and also for a conventional white paint BR, long distances (of several micrometers) between the BR and the μc-Si:H layers are found to be detrimental for the light trapping. Optical simulations based on both rigorous and scalar scattering theory have been performed to understand the impact of the diverse BR designs on the optical cell performance.

Impact of periodicity was studied for micro- and nano-pyramids fabricated by low-cost nanoimprint lithography process. Investigations by photoluminescence imaging displayed excellent anti-reflection and wavelength-selective light-trapping properties.

Light scattering OLED substrates relying on disordered self-assemblies are fabricated by microsphere and polymer blend lithography and used for light extraction. We report on a device efficiency enhancement of up to 50 %.

Thin-film silicon solar cells often rely on a metal back reflector separated from the silicon layers by a thin rear dielectric as a back reflector (BR) design. In this work, we aim to obtain a better insight into the influence of the rear-dielectric/Ag BR design on the optical performance of hydrogenated microcrystalline silicon (µc-Si:H) solar cells. To allow the application of a large variety of rear dielectrics combined with Ag BRs of diverse topographies, the solar cell is equipped with a local electrical contact scheme that enables the use of non-conductive rear dielectrics such as air or transparent liquids of various refractive indices n. With this approach, detached Ag BRs having the desire surface texture can be placed behind the same solar cell, yielding a direct and precise evaluation of their impact on the optical cell performance. The experiments show that both the external quantum efficiency and the device absorptance are improved with decreasing n and increasing roughness of the BR. Calculations of the angular intensity distribution of the scattered light in the µc-Si:H are presented. They allow for establishing a consistent picture of the light trapping in the solar cell. Copyright © 2013 John Wiley & Sons, Ltd.

We present three different working modes of scanning near-field optical microscopy which provide complementary information about the electro-optical properties of solar cells. Those working modes allows to study the local light scattering and trapping in textured solar cells, the local light propagation inside the cell and the impact of lateral inhomogeneity on a nanoscopic scale. Due to the presence of evanescent light modes, the presented techniques give access to important physical effects which is only possible in the optical near-field.

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.

The optical properties of textured surfaces in the micro and nanometer range are of interest in manifold topics, such as thin-film silicon photovoltaics. Light scattering models, which are based on Fourier transform techniques, are applied to calculate both, the angularly resolved scattering and the haze. Therein, topography, measured by atomic force microscopy, and the refractive index are used as input data. In this study, these models are applied to zinc oxide/air interfaces and to zinc oxide/hydrogenated amorphous silicon interfaces. Results obtained from zinc oxide/air interfaces are compared to the measured scattering properties.

We investigate the scattering behavior of nano-textured ZnO–Air and ZnO–Silicon interfaces for the application in thin film silicon solar cells. Contrary to the common approach, the numerical solution of the Maxwell's equations, we introduce a ray tracing approach based on geometric optics and the measured interface topography. The validity of this model is discussed by means of scanning near-field optical microscopy (SNOM) measurements and numerical solutions of the Maxwell's equations. We show, that the ray tracing model can qualitatively describe the formation of micro lenses, which are the dominant feature of the local scattering properties of the investigated interfaces. A quantitative analysis for the ZnO–Silicon interface at λ = 488 and 780 nm shows that the ray tracing model corresponds well to the numerical solution of the Maxwell's equations, especially within the first 1.5 µm distance from the interface. Direct correlations between the locally scattered intensity and the interface topographies are found. Copyright © 2011 John Wiley & Sons, Ltd.

In thin-film silicon solar cell devices randomly textured interfaces are used to achieve light scattering sufficient for efficient light trapping. We use near-field scanning optical microscopy (NSOM) for visualizing wave guiding mechanisms experimentally by measuring the evanescent modes. Their impact on the light trapping efficiency and the link to topographic structures will be addressed.

Light scattering at randomly textured interfaces in tandem solar cells consisting of hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (μc-Si:H) is studied in the optical near-field experimentally by near-field scanning optical microscopy (NSOM) and theoretically by a rigorous solution of Maxwell's equations using a Fourier ansatz. The work concentrates on the influence of evanescent light modes on the absorption in the a-Si:H top cell. It is found that the improvement of solar cell efficiency due to the surface texturing is borne by evanescent light modes. For both, the theoretical and the experimental study, techniques are developed to extract the evanescent part of the light from the obtained data. From the theoretical results, the principal mechanism of light scattering in the optical near-field regime is studied and, in particular, light which is trapped in the a-Si:H top cell by total internal reflection is visualized. From the experimental results, a map of the local light trapping efficiency is generated and the correlation to morphological properties is studied.

Light trapping in thin-film silicon solar cell arising from randomly textured ZnO front contact layers is investigated in the optical near-field experimentally and theoretically. The experimental data are obtained from near-field scanning optical microscopy, theoretical data are calculated using rigorous diffraction theory. The surface of the randomly textured ZnO consists of statistically distributed craters with different shapes. Along the rims of the craters light localizations are found. Photon jets emerge from the vertices of the surface profile. From the theoretical study, the local absorption enhancement is calculated. The results provide important insight into the local effects of light trapping in thin-film optoelectronic devices and strategies for optimizing the external quanturn efficiency in thin-film silicon solar cells are discussed.

The electric field intensity above the surface of opal photonic crystals (PCs) and its alteration due to ‘crystallographic’ defects is investigated by using near-field scanning optical microscopy (NSOM). The photonic crystals are developed by dip coating in a liquid solution with PMMA opals. Highly regular hexagonal planes with lattice constants of about 260 nm grow on the glass substrate. During the drying process several crack lines are formed that correspond to defects in the crystal structure. The transmitted light intensity at wavelengths inside and outside of the stop band of the PC is studied with NSOM using a tapered fiber tip scanning in all three dimensions. By this technique, a 3D image of the electric field intensity can be measured with a resolution better than 100 nm. The results show that the local optical field distribution is strongly dominated by the defect states in all directions in space over a length scale of several μm. Above the crack lines, the intensity of light is strongly reduced. Beams of light are observed emerging from the edges of the crack lines and propagate in air with heights of more than 3 μm. In between two different crack lines, periodic repetitions of the beams are observed. These results are interpreted as light diffraction on a microscopic scale.

We report on disordered nano-structured scattering layers fabricated by lateral phase separation of two immiscible polymers, and demonstrate a short-circuit current density enhancement of +13.5%rel in hydrogenated amorphous silicon solar cells.

Photonic crystals provide new concepts for light management. Here, we fabricate 3DPCs via spray coating, which is a fast, cheap, and scalable technique, and apply them as backside structures in solar cells and solar modules.

The successful implementation of light management concepts processed after the planar passivation of SHJ solar cells is demonstrated. We present prototype solar cells, improved performance and discuss promising approaches for future development.

We report on near-field scanning optical microscopy measurements on randomly textured ZnO thin films. These films are commonly used as transparent conducting oxide in thin-film solar cells. Textured interfaces are used to increase the scattering of light, which leads to a better light trapping in the solar cell. Here, both the topography and the local transmission are measured with a tapered fiber tip with very high spatial resolution. By varying the distance of the tip and the wavelength of the incident light, the optical profile is visualized and reveals a strong confinement of light on a subwavelength scale which corresponds to ridges in the surface structure. The confinement of light results from guided optical modes in the ZnO which are accompanied by a modulated evanescent field in air. No corresponding structure to this modulation is found in the topography. These results give new insight for further improvement of light trapping in solar cells.

In thin optoelectronic devices, like organic light emitting diodes (OLED) or thin-film solar cells (TFSC), light propagation, which is initiated by a local point source, is of particular importance. In OLEDs, light is generated in the layer by the luminescence of single molecules, whereas in TFSCs, light is coupled into the devices by scattering at small surface features. In both applications, light propagation within the active layers has a significant impact on the optical device performance. Scanning near-field optical microscopy (SNOM) using aperture probes is a powerful tool to investigate this propagation with a high spatial resolution. Dual-probe SNOM allows simulating the local light generation by an illumination probe as well as the detection of the light propagated through the layer. In our work, we focus on the light propagation in thin silicon films as used in thin-film silicon solar cells. We investigate the light-in-coupling from an illuminating probe via rigorous solution of Maxwell's equations using a Finite-Difference Time-Domain approach, especially to gain insight into the light distribution inside a thin layer, which is not accessible in the experiment. The structures investigated include at and structured surfaces with varying illumination positions and wavelengths. From the performed simulations, we define a "spatial sensitivity" which is characteristic for the local structure and illumination position. This quantity can help to identify structures which are beneficial as well as detrimental to absorption inside the investigated layer. We find a strong dependence of the spatial sensitivity on the surface structure as well as both the absorption coefficient and the probe position. Furthermore, we investigate inhomogeneity in local light propagation resulting from different surface structures and illumination positions.

Nanophotonic light management concepts are essential building blocks of advance photovoltaic technologies. These concepts make use of light coupling to waveguide modes supported by the photoactive absorber material of the solar cell. In this contribution, we will explain on the basis of our recently published results the development of scanning near field optical microscopy as a new method that enables the direct nanoscale observation of light coupling to an individual waveguide mode in a nanophotonic thin-film silicon solar cell. Beyond this, we present a new detailed study based on this new method on the polarization dependence of the light coupling to an individual waveguide mode.

We investigate the influence of surface texture on the effectiveness of reflectance by intermediate reflectors in thin-film tandem solar cells. Two distinct angular reflection regimes are identified. For large-angle textures, frustrated total internal reflection or photon tunneling is the dominating mechanism. We show that the parasitic reflection losses of commonly used randomly textured surfaces are explained by these distinct regimes and introduce a spectral reflectance parameter which serves as a guideline for designing optimized spectrally selective intermediate reflectors.

We have developed inkjet-printed scattering layers for enhanced light extraction in organic light emitting diodes (OLEDs). These layers are based on scattering polymer/nanoparticle composites, which are prepared from a solvent-free process and used for the outcoupling of waveguide modes to free propagating modes, thus improving the device efficiencies. Two different monomers are examined and an inkjet-printing process is developed for each of them. We first analyze the inks' rheological properties, followed by the optical and morphological properties of the resulting layers. Upon integration of these printed layers into an OLED stack, the device efficiencies are increased by up to 40% with respect to an unpatterned reference device. Furthermore, we show that these internal light extraction layers improve the angular and spectral stability of the devices.

Nontransparent contact fingers on the sun-facing side of solar cells represent optically dead regions which reduce the energy conversion per area. We consider two approaches for guiding the incident light around the contacts onto the active area. The first approach uses graded-index metamaterials designed by two-dimensional Schwarz–Christoffel conformal maps, and the second uses freeform surfaces designed by one-dimensional coordinate transformations of a point to an interval. We provide proof-of-principle demonstrators using direct laser writing of polymer structures on silicon wafers with opaque contacts. Freeform surfaces are amenable to mass fabrication and allow for complete recovery of the shadowing effect for all relevant incidence angles.

3D photonic crystals (3DPCs) provide light management for solar cells. Here, we fabricate 3DPCs via spray coating, which is a fast, cheap, and scalable technique and apply them as backside structures for solar cells.

Cellulose substrates for PV applications present a fibrous surface texture that is not suitable for the uniform deposition of thin-film solar cells causing poor device performance. However, uniform thin-film deposition and efficient light management for solar cells can be achieved on cellulose substrates by transferring well-known surface textures that provide an adequate surface for thin film solar cell deposition and also, provide light scattering properties into the cellulose surface. In this work, we study the properties of crater-like textures transferred onto cellulose substrates by nanoimprint lithography and the corresponding effect on the J–V and EQE characteristics of amorphous silicon thin-film solar cells. The prototype solar cells are deposited on cellulose substrates and the results are compared with the results of such solar cells deposited on flat glass substrates. The results show that the J–V characteristics of solar cells deposited on planar as well as textured glass substrates are well reproduced. Due to the process routine, the solar cells on the cellulose substrate with nanoimprinted textures show an increase in the short circuit current density and power conversion efficiency over previous results in our laboratory.

We investigated the light coupling to waveguide modes in nanophotonic thin-film solar cells exhibiting a tailored disordered grating texture by rigorous optical simulations. Such disordered nanophotonic light trapping concepts have demonstrated enhanced short-circuit current density compared to ordered nanophotonic thin-film solar cells. This observation is commonly explained by a spectral broadening of the resonant light coupling to waveguide modes. In this work, we investigated the origin of this spectral broadening. We identified two basic mechanisms that lead to a spectral broadening of the light coupling to waveguide modes: (1) the broadening of the wave vector transferred by the disordered interface texture and (2) the broadening of the waveguide mode due to the distortion of the wave guiding absorber layer. Depending on the type of disorder, the contribution from each of the mechanisms varies.

Nanophotonic light management concepts are on the way to advance photovoltaic technologies and accelerate their economical breakthrough. Most of these concepts make use of the coupling of incident sunlight to waveguide modes via nanophotonic structures such as photonic crystals, nanowires, or plasmonic gratings. Experimentally, light coupling to these modes was so far exclusively investigated with indirect and macroscopic methods, and thus, the nanoscale physics of light coupling and propagation of waveguide modes remain vague. In this contribution, we present a nanoscopic observation of light coupling to waveguide modes in a nanophotonic thin-film silicon solar cell. Making use of the subwavelength resolution of the scanning near-field optical microscopy, we resolve the electric field intensities of a propagating waveguide mode at the surface of a state-of-the-art nanophotonic thin-film solar cell. We identify the resonance condition for light coupling to this individual waveguide mode and associate it to a pronounced resonance in the external quantum efficiency that is found to increase significantly the power conversion efficiency of the device. We show that a maximum of the incident light couples to the investigated waveguide mode if the period of the electric field intensity of the waveguide mode matches the periodicity of the nanophotonic two-dimensional grating. Our novel experimental approach establishes experimental access to the local analysis of light coupling to waveguide modes in a number of optoelectronic devices concerned with nanophotonic light-trapping as well as nanophotonic light emission.

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.

We investigate light-scattering textures for the application in thin-film solar cells which consist of a random texture, as commonly applied in thin-film solar cells, that are superimposed with a two-dimensional grating structure. Those textures are called photonic random texture. A scalar optical model is applied to describe the light-scattering properties of those textures. With this model, we calculate the angular resolved light scattering into silicon in transmission at the front contact and for reflection at the back contact of a microcrystalline silicon solar cell. A quantity to describe the light- trapping efficiency is derived and verified by rigorous diffraction theory. We show that this quantity is well suitable to predict the short-circuit current density in the light-trapping regime, where the absorptance is low. By varying the period, height and shape of the unit cell, we optimize the grating structure with respect to the total generated current density. The maximal predicted improvement in the spectral range from 600-900 nm is found to be about 3 mA/cm² compared to the standard random texture and about 6 mA/cm² compared to a flat solar cell. Different light-trapping concepts were applied in silicon-based thin-film solar cells to overcome the limitations due to the low absorptance near the band gap for flat devices 1,2 and the limited device thickness due to light-induced degradation. Differently textured interfaces improve light coupling into the absorber material and provide light guidance in the layer. Mostly, random textures were incorporated that scatter incoming light diffusely prolonging the effective light path. 3-6 Recent work was done by several groups where periodic structures, e.g. gratings or photonic crystals, were incorporated at different interfaces of the device. 7-12 The complex surface structures demand for rigorous optical models with high computational effort and huge memory in order to describe their impact on light absorption. With such models, optimizations are only possible in strictly limited parameter spaces to fix into computer capacities availably nowadays. We recently demonstrated that the angular resolved scattering in transmission and reflection inside the silicon absorber material can be sufficiently described by a simple scalar approach. 13-15 This simple model allows optimization of the interface structure in a huge parameter space with low computational effort. We found that the amount of light scattered beyond the critical angle of total internal reflection correlates nicely to the external quantum efficiency. Therefore, this investigation was done in the spectral range, where light trapping is mostly efficient (600-900 nm) and the short-circuit current density was derived for this spectral range (jsc,600-900). We interpret this quantity as a benchmark for the efficiency of the light-trapping texture and carry out the optimum. In this work, we investigated in detail the combination of both, random texture and periodic structure. Starting with a randomly textured ZnO:Al layer, that is well-known to provide high-efficiency microcrystalline silicon (µc-Si:H) solar cells, we superimposed a two-dimensional periodic structure with optimized period and height to the random texture. The design of the structure was done by applying the scalar approach. Our results from the simple scalar approach were verified by Finite-Difference Time-Domain (FDTD) simulations of the real layer stack for selected structures. In detail, we optimized the period and height of different unit cell geometries. Those gratings were applied to flat interfaces, leading to photonic textures, and the randomly textured ZnO:Al layer, leading to photonic random textures. The jsc,600-900 for the flat and random reference cell were found to be 6.48 mA/cm² and 9.32 mA/cm², respectively. The largest short-circuit current density we found is jsc,600-900=12.40 mA/cm². This is an improvement of about 6 mA/cm² compared to the flat cell and still about 3 mA/cm² compared to the standard random texture.