Intermediate reflecting layers (IRL) in multijunction solar cells improve the spectral light management and thereby increase the overall energy conversion efficiency. Nevertheless, the incorporation of an IRL can increase parasitic losses in the device mainly by three effects: (i) parasitic absorption in the IRL, (ii) increased parasitic absorption in the transparent front electrode due to the light reflection at the IRL, and (iii) reflection losses due to insufficient absorption of light in the top solar cell that is reflected at the IRL. Here, we investigate the different contributions of these three loss mechanisms on the external quantum efficiency and short-circuit current density by a combination of experiment and optical simulation. The parasitic absorption in the transparent front electrode is varied experimentally by a post-deposition laser annealing process on the front electrode layer. The experimental work is supplemented by rigorous optical simulations which allow to extract and vary the parasitic absorption losses in the IRL by a variation of the optical material properties. Furthermore, the cell reflectance is studied by experiment and simulation for different layer configurations. Our comparative study reveals that the dominating parasitic losses are absorption within the intermediate reflector and reflection out of the solar cell. From this study, we show one possible solution for advanced light management in tandem solar cells with intermediate reflectors.

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.

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.

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.

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.

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.

The optical and electrical properties of transparent conductive oxides (TCOs), traditionally used in thin-film silicon (TF-Si) solar cells as front-electrode materials, are interlinked, such that an increase in TCO transparency is generally achieved at the cost of reduced lateral conductance. Combining a highly transparent TCO front electrode of moderate conductance with metal fingers to support charge collection is a well-established technique in wafer-based technologies or for TF-Si solar cells in the substrate (n-i-p) configuration. Here, we extend this concept to TF-Si solar cells in the superstrate (p-i-n) configuration. The metal fingers are used in conjunction with a millimeter-scale textured foil, attached to the glass superstrate, which provides an antireflective and retroreflective effect; the latter effect mitigates the shadowing losses induced by the metal fingers. As a result, a substantial increase in power conversion efficiency, from 8.7% to 9.1%, is achieved for 1-μm-thick microcrystalline silicon solar cells deposited on a highly transparent thermally treated aluminum-doped zinc oxide layer combined with silver fingers, compared to cells deposited on a state-of-the-art zinc oxide layer.

We investigate the impact of intermediate reflectors on textured thin-film tandem solar cells. The electric near-field intensity distribution inside the layer stack is simulated by rigorous scattering theory. From those results, we extract the angular scattering intensity distribution in the top and bottom solar cell. The improvement in top cell quantum efficiency for various different intermediate reflectors is correlated with the coupling efficiency to leaky waveguide modes for a periodic triangular grating as a model system. In a further step, we show that the same approach can be applied to randomly textured substrates where light couples to a broad distribution of waveguide modes. The integrated scattering intensity above the critical angle of total internal reflection of this system is successfully correlated with the measured external quantum efficiency of tandem solar cells with various IRs. A local investigation of trapped light is presented to identify good light-trapping structures.

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 SnO2: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 tandem solar cells are composed of an amorphous silicon top cell and a microcrystalline silicon bottom cell, stacked and connected in series. In order to match the photocurrents of the top cell and the bottom cell, a proper photon management is required. Up to date, single-layer intermediate reflectors of limited spectral selectivity are applied to match the photocurrents of the top and the bottom cell. In this paper, we design and prototype multilayer intermediate reflectors based on aluminum doped zinc oxide and doped microcrystalline silicon oxide with a spectrally selective reflectance allowing for improved current matching and an overall increase of the charge carrier generation. The intermediate reflectors are successfully integrated into state-of-the-art tandem solar cells resulting in an increase of overall short-circuit current density by 0.7 mA/cm(2) in comparison to a tandem solar cell with the standard single-layer intermediate reflector.

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/cm(2) compared to the standard random texture and about 6 mA/cm(2) compared to a flat solar cell.

Periodic or random nanostructures implemented into thin-film solar cells at the front, rear and as interlayer enhance solar cell efficiencies. The underlying effect of dielectric or plasmonic light scattering will be discussed combining experiments (ARS, EQE, SNOM) and theory (FEM, FDTD).

The concept of photonic random textures for application as a light-trapping scheme in thin-film solar cells is introduced. Those textures consist of a randomly textured interface, as commonly applied in thin-film solar cells, which is superimposed with a two-dimensional grating structure. The light-scattering properties of those textures are investigated by scalar scattering theory for transmission into the absorber layer and reflection at the back contact. A quantity to describe the light-trapping efficiency is derived and verified by rigorous diffraction theory. The photonic random textures outperform the random textures and the grating structures significantly.

Thin-film silicon tandem solar cells consist of an amorphous silicon top cell and a microcrystalline silicon bottom cell stacked in series. In order to match the photocurrents of the top cell and the bottom cell, a proper photon management is essential. In this regard, we present the conceptual design and optical simulations of an intermediate reflector consisting of a stack of microcrystalline silicon oxide layers of different, alternating refractive indices. In contrast to 1-layer intermediate reflectors, the spectral and directional selectivity of these intermediate reflectors result in a gain for the top cell current while simultaneously increasing the charge carrier generation in the bottom cell.

To achieve higher efficiencies in solar cells one possibility is to integrate angular selective filters, with the aim of decreasing losses caused by radiative recombination. In fact, thermodynamically, angular selectivity is equivalent to concentration. In both cases the Shockley-Queisser-Limit of solar cells is overcome by manipulating the ratio of incoming and outgoing radiation represented by the angles of incidence and emission. In concentrating systems the angle of incidence is increased, whereas in systems with an angular confinement the angle of emission can be decreased. Another possibility to achieve highest efficiencies is to combine both, concentration and angular confinement. Starting with a given concentrating system, photonic angularly selective filters such as thin film stacks are investigated and optimized for the use in this system. We present results of wave optical simulations of these filters and show some of their characteristics. The goal of this study is, however, not only to optimize optical filters but also to consider the whole system. One approach is to use results from optical simulations as input values for detailed balance simulations of the solar cell. So, the main advantage is, that in fact not the optical characteristics are optimized separately, but rather the whole system is taken into account, which allows predictions of theoretical efficiency enhancement.

The scattering of light by the textured transparent conductive oxide (TCO) in thin-film silicon solar cells is frequently described by transmission haze and angular intensity distribution (AID) at the interface between the TCO and air. The scattering is expected to improve the light trapping and, therefore, the absorption of the solar cell. Using these scattering properties as input parameters for the electrical modeling of thin-film solar cells leads to significant deviations from the measurements for short circuit current densities. The major disadvantage of the AID measurement at the TCO/air interface is that in real thin-film silicon solar cells the TCO/Si interface is relevant. We use a model that is based on scalar scattering theory to calculate the scattering properties at the transition into air and into silicon. The model takes into account the measured surface topography and the optical constants of the adjacent media. For a series of μc-Si:H cells on ZnO:Al with different surface topographies, AID and the transmission haze into a μc-Si:H half space are calculated. From these results, a quantity is derived that describes the scattering efficiency. This quantity is compared to the short circuit current densities of μc-Si:H solar cells showing good agreement. It will be shown that for artificially modified textures an increase in the short-circuit current density and thus, the efficiency of thin-film silicon solar cells can be achieved.

The European project HELATHIS, executed by the five project partners signing this article, is dedicated to the improvement of the efficiency of very large area (5.7m 2 ) silicon thin film photovoltaic (PV) modules. Optical confinement has been identified by the project partners as a major source for efficiency improvement of thin film silicon PV modules. One reason of the performance gap between highly efficient laboratory solar cells and industrial modules is the poorer electrical and optical performance of the industrial TCO-covered front glass substrate (TCO glass) which is investigated in this work. The glass substrate in industrial PV modules is about 3 times thicker than in laboratory solar cells where frequently Asahi U-type TCO glass (about 1 mm thick) is used. Therefore, the glass quality has an important impact on the transmission of industrial TCO glass. Reducing the iron-content in the float glass substrate increases the integrated transmission in the wavelength range from 400-800nm by nearly 2%. The electrical properties, namely the electrical carrier mobility, of the industrial TCO layer of AGC has been increased by about 15% by improving the industrial deposition process, resulting in a thinner TCO layer with higher transmission by maintaining the sheet resistance of about 9-10 /sq. Combining both developments the integrated transmission of industrial TCO glass has been increased by more than 2%. The TCO layer properties of Asahi U-type and standard industrial TCO glass (AGC AN10) have been investigated by SEM, AFM, XRD and ARS, showing that the Asahi U TCO scatters red light more effectively into larger angles.