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

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 c