Multiscale transparent electrode architecture for efficient light management and carrier collection in solar cells

The challenge for all photovoltaic technologies is to maximize light absorption, to convert photons with minimal losses into electric charges, and to efficiently extract them to the electrical circuit. For thin-film solar cells, all these tasks rely heavily on the transparent front electrode. Here we present a multiscale electrode architecture that allows us to achieve efficiencies as high as 14.1% with a thin-film silicon tandem solar cell employing only 3 μm of silicon. Our approach combines the versatility of nanoimprint lithography, the unusually high carrier mobility of hydrogenated indium oxide (over 100 cm(2)/V/s), and the unequaled light-scattering properties of self-textured zinc oxide. A multiscale texture provides light trapping over a broad wavelength range while ensuring an optimum morphology for the growth of high-quality silicon layers. A conductive bilayer stack guarantees carrier extraction while minimizing parasitic absorption losses. The tunability accessible through such multiscale electrode architecture offers unprecedented possibilities to address the trade-off between cell optical and electrical performance.

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.

EPFL2013

Low-Pressure Chemical Vapor Deposited Zinc Oxide Films

Laura Ding

Polycrystalline zinc oxide (ZnO) films, prepared by low-pressure chemical vapor deposition are investigated in this thesis. ZnO belongs to the class of transparent conductive oxide materials, as it is transparent to light from the visible to the near-infrared range and is simultaneously electrically conductive. These two properties allow ZnO films to be integrated in thin-film silicon solar cells, as well as other opto-electronic devices. In particular, thin-film solar cells require a contact layer covering the whole active layer in order to efficiently collect the photogenerated electrical current, and therefore, the film electrode at the front of the cell has to be a window to the sunlight, in addition to being able to transport current. ZnO’s electrical conductivity is usually improved by doping with impurities like boron, but the film transparency is reduced, and consequently, the current that the solar cell can produce decreases. Thus, the first point investigated in this thesis is to find ways to relax the trade-off between transparency and conductivity. Instead of doping, which increases the electron concentration, the electron mobility has to be improved. To do so, the electron transport is first studied and modeled for standard films, in order to better define and understand its limitations. Then, a new way of preparing ZnO bilayer films is shown to lead to enhanced transparency and light scattering, for a maintained conductivity compared to uniformly doped films. Alternatively, various treatments (hydrogen plasma, vacuum annealing and ultraviolet exposure) are tested, all of which reduce the density of defects that cause electron scattering in the ZnO; treated films present an improved conductivity and very high mobilities close to 60 cm2V−1s−1, which corresponds to the mobility value in single crystals of equivalent doping. In addition, these films absorb less than 2% of the visible-to-infrared light, which makes them very promising ultra-transparent electrodes, for solar cells or other applications. For use in thin-film silicon solar cells, two additional criteria have to be met besides trans- parency and conductivity. First, because the front electrode is also the substrate for the deposition of the rest of the cell layers, its surface morphology has to be favorable to the growth of silicon. Moreover, this surface has to present a rough texture capable of diffusing and trapping the light into the cell, and increasing its chance of being absorbed. These charac- teristics can be incompatible, but are nevertheless necessary to reach thin-film solar cells with high efficiencies. So, the second point of interest of this thesis is to adapt the surface of ZnO layers by investigating their preparation method. Combinations of different growth regimes in multilayers are proposed to enable a smoother controllable of the surface without resort to additional etching treatments, leading to improved solar cell electrical performance. Finally, the stability of ZnO films exposed to various conditions that modify their electrical and optical properties is investigated. First, ZnO is exposed to different oxidizing conditions (storage and annealing in air, exposure to moisture, plasma of carbon dioxide). The loss in conductivity from the interaction of the oxygen species with ZnO is characterized by a new infrared spectroscopic method and a hydrogen plasma treatment is investigated as a means to reverse or partially prevent the degradation in conductivity. Then, the effect of simulated sunlight exposure on ZnO film properties in solar cells is investigated. The resistivity of the front ZnO electrode decreases due to ultraviolet light absorption, and the resistivity of the back electrode increases due to the chemisorption of oxygen species, in absence of ultraviolet light. The front electrode transparency is also degraded; this effect is more pronounced for doped ZnO and its impact on the current photogeneration of solar cells is demonstrated. Therefore, a specific optimization of the front and back electrodes appears necessary. This thesis contributes on the one side to an in-depth understanding of the opto-electrical properties of ZnO films prepared by low-pressure chemical vapor deposition. On the other side, the different proposed approaches enable more flexibility in the development of more efficient and stable electrodes, not only for solar cells, but for many other applications such as electroluminescent diodes, flat-panel displays, touch-screens or transparent transistors.

EPFL2013

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

EPFL2013

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

Thomas Söderström

University of Neuchâtel2009

Low-Pressure Chemical Vapor Deposited Zinc Oxide Films

Laura Ding

EPFL2013