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
