Absorption Enhancement in Solar Cells With Periodic Interface Textures of Asymmetric Shape

We investigate light scattering interface textures for solar cells with the intention to obtain a more fundamental understanding of the absorption enhancement. Therefore, we study simplified structures with the periodic interface texture of sinusoidal and blazed shape, and we employ a theoretical model with moderate complexity on the basis of the Rayleigh method. We test our model by comparing diffraction effects with theoretical predictions for the two possible polarization directions. The intensity of first-order diffraction is virtually insensitive to the asymmetry of the grating for both s- and p-polarizations. However, in the latter case, the absorption signature of the surface plasmon resonances does show a mild asymmetry. Subsequently, we extend the model to multilayers such as a full solar cell stack, which allows us to relate the measured absorption enhancement to the interface texture.

Angular behavior of the absorption limit in thin film silicon solar cells

Christophe Ballif, Corsin Battaglia, Franz-Josef Haug, Hans Peter Herzig, Ali Naqavi, Vincent Paeder, Toralf Scharf, Karin Söderström

We investigate the angular behavior of the upper bound of absorption provided by the guided modes in thin film solar cells. We show that the 4n2 limit can be potentially exceeded in a wide angular and wavelength range using two-dimensional periodic thin film structures. Two models are used to estimate the absorption enhancement; in the first one, we apply the periodicity condition along the thickness of the thin film structure but in the second one, we consider imperfect confinement of the wave to the device. To extract the guided modes, we use an automatized procedure which is established in this work. Through examples, we show that from the optical point of view, thin film structures have a high potential to be improved by changing their shape. Also, we discuss the nature of different optical resonances which can be potentially used to enhance light trapping in the solar cell. We investigate the two different polarization directions for one-dimensional gratings and we show that the transverse magnetic polarization can provide higher values of absorption enhancement. We also propose a way to reduce the angular dependence of the solar cell efficiency by the appropriate choice of periodic pattern. Finally, to get more practical values for the absorption enhancement, we consider the effect of parasitic loss which can significantly reduce the enhancement factor.

Wiley-Blackwell2014

Single and multi-junction thin film silicon solar cells for flexible photovoltaics

Thomas Söderström

This thesis investigates amorphous (a-Si:H) and microcrystalline (μc-Si:H) solar cells deposited by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) in the n-i-p or substrate configuration. It focuses on processes that allow the use of non transparent and flexible substrates such as plastic foil with Tg < 180°C like poly-ethylene-naphtalate (PEN). In the first part of the work, we concentrate on the light trapping properties of a variety of device configurations. One original test structure consists of n-i-p solar cells deposited directly on glass covered with low pressure chemical vapour deposition (LP-CVD) ZnO. For this device, silver is deposited below the LP-CVD ZnO or white paint is applied at the back of the substrate as back reflector. This avoids the parasitic plasmonic absorptions in the back reflectors, which are observed for conventional rough metallic back contacts. Furthermore, the size and morphology of the LP-CVD ZnO is varied and the relation between the substrate morphology and the short circuit current density (Jsc) is experimentally explored. As a result, the Jsc can be increased by 23% for a-Si:H and 28% for μc-Si:H solar cells compared to the case of flat substrate and the role of the size and shape can be clearly separated. We also explore the optical behavior of single and multi-junction devices prepared with different back and front contacts. The back contact consists either of a 2D periodic grid with moderate slope, or of LP-CVD ZnO with random pyramids of various sizes. The front contacts are either a 70 nm thick, nominally flat ITO or a rough 2 microns thick LP-CVD ZnO. We observe that, for a-Si:H, the cell performance is critically dependent on the combination of thin flat or thick rough front TCOs and the back contact. Indeed, for a-Si:H, a thick LP-CVD ZnO front contact provides more light trapping on the 2D periodic substrate. The Jsc relatively increases by 7 % with LP-CVD ZnO compared to ITO. Then, we study the influence of the thick and thin TCOs in conjunction with thick absorbers like triple junction or mc-Si:H solar cells. Because of the different nature of the optical systems, thick (> 1 micron) against thin (

University of Neuchâtel2009

Coupling light into thin silicon layers for high-efficiency solar cells

Karin Söderström

Thin-film solar cells based on amorphous and microcrystalline silicon require thin photoactive layers to ensure a satisfactory collection of the photogenerated carriers. The small thickness is advantageous in terms of raw material consumption and industrial throughput but results in poor light absorption at long wavelengths. Most of the time, textured substrates are used for the deposition of solar cells inducing scattering of light and increased light absorption in the silicon layer which enhance the short-circuit current density (Jsc) but also inducing the growth of silicon layers with defects that limit the open-circuit voltage (Voc) and the fill factor (F F ). Therefore, a major challenge is the design and realisation of structures that allow proper growth of the material while providing efficient light coupling. In a first part of this thesis a better understanding of the light in-coupling mechanism via interface textures is achieved. Angle- and polarisation-resolved analysis of the external quantum efficiency of a cell grown on a one-dimensional grating structure demonstrates that light management can be viewed as the excitation of guided modes that are supported by the silicon layers. Defined peaks of enhanced photocurrent in the weakly absorbing region were observed for this cell, and these absorption phenomena were related to dispersion curves calculated for guided-modes in an equivalent flat multilayer system. This allows an intuitive understanding of photocurrent enhancement via interface texturing and provides new insights into the features that are required for efficient light trapping. Then, a novel means of substrate texturing was required in order to emancipate the thin-film silicon solar cells from the standard textures used for light management, and open the road for the implementation of novel photonic designs that improve light trapping in high-efficiency devices. To achieve both of these goals, the replication of textures by UV nano-imprinting was investigated and developed. The remarkable replication fidelity obtained is such that the original texture cannot be distinguished from the replicated one by measurements such as atomic force microscopy and scanning electron microscopy. Solar cell efficiencies as high as on standard electrodes were demonstrated by texturing both plastic substrates for the n-i-p configuration and glass substrates for both the n-i-p and p-i-n configurations. In addition, the nano-imprinting process enabled the development of a novel tool called nanomoulding which permits the selected shaping of the surface of zinc oxide layers. Furthermore, it was observed that nano-imprinting of micro-metric features at the front of n-i-p and p-i-n devices boosts the Jsc by more than 4% by providing an anti-reflection effect. The manifold applications which use UV nano-imprinting in this thesis show its extraordinary versatility and demonstrate that it is a suitable experimental platform for thin-film silicon solar cells. The reduction of parasitic absorption in inactive layers is another means to enhance Jsc and solar cell efficiency. Therefore, parasitic absorption that takes place in rough metallic back reflectors which are commonly used in the n-i-p configuration is investigated and reduced. It is shown that the addition of a zinc oxide buffer layer of optimal thickness between the metallic layer and the silicon layers helps to mitigate parasitic absorption both in the metal and in the adjacent doped layer. Then, parasitic absorption related to the quality of the silver back reflector when deposited on rough nano-textures was reduced using a thermal annealing at low temperature. The combination of texturing on plastic by UV nano-imprinting with the deposition of a high-quality silver reflector led to a flexible a-Si:H/a-Si:H device with remarkable initial and stable efficiencies of 11.1% and 9.2%, respectively, using less than half a micron of silicon. Eventually, to circumvent the strong limitation of Voc and F F due to the defective growth of silicon on textured substrates, a novel type of substrate was realised and studied. This substrate decouples the optically rough interface that allows high Jsc, from the growth surface which is made flat to allow the growth of devices with good-quality silicon and high Voc and F F . Triple-junction n-i-p a-Si:H/μc-Si:H/μc-Si:H solar cells were realised on this substrate, and a stable efficiency of 13% was obtained, which is the highest stable efficiency reported so far for thin-film silicon solar cells by our laboratory. This novel approach is very promising as it demonstrates that the usual morphology trade-off can be overcome through the use of a single flat light-scattering substrate that fulfills all the requirements to push even further thin-film silicon solar cell efficiencies. To conclude, this thesis brought improvements both in understanding and in devices. A better understanding of the features required for efficient light coupling was first found by showing that light coupling via interface texturing is due to the excitation of guided modes. Then it was demonstrated that UV nano-imprinting allows the introduction of novel photonic designs for better light management in high-efficiency solar cells and also allows the introduction of textures suitable for efficient light management on different substrates. This is illustrated by the flexible a-Si:H/a-Si:H device exhibiting 9.2% stable efficiency on a plastic which is, to the knowledge of the author, the highest reported efficiency for a-Si:H on plastic substrate. Also, novel flat light-scattering substrates which allow the growth of excellent material quality were developed and introduced in solar cells. This led to the realisation triple-junction n-i-p solar cell with a record stable efficiency of 13% that is close to the current world record of 13.4% reported in 2012 by LG Solar using the p-i-n configuration.