Light management in thin-film silicon solar cells

Solar energy can fulfil mankind’s energy needs and secure a more balanced distribution of primary sources of energy. Wafer-based and thin-film silicon solar cells dominate todays’ photovoltaic market because silicon is a non-toxic and abundant material and high conversion efficiencies are achieved with silicon-based solar cells. To stay competitive with bulk crystalline silicon and other thin-film solar cell technologies, thin-film silicon solar cells have to achieve a conversion efficiency level of 20% on a laboratory scale. In this respect, light management techniques are essential for enhancing the efficiency of such solar devices since the energy of solar radiation is effectively used, absorption in the absorber layers is maximized and optical losses are minimized. Among several light management techniques presented in Chapter 2, light trapping is especially important for fabricating thinner cells with high efficiency and stable performance. The most basic light trapping scheme is given by the random texture at internal interfaces coupled with an efficient back reflector. The aim of this doctoral thesis is to introduce, analyse, model and employ in real solar devices novel types of textures and back reflectors. After Chapter 3, in which deposition techniques, characterization setups and modelling tools are introduced and discussed, four thematic chapters are reported. They are focussed, respectively, on periodic diffraction gratings and modulated surface textures as light scatterers, and dielectric distributed Bragg reflectors and Flattened Light-Scattering Substrates as efficient rear reflectors. In Chapter 4, transparent rectangular-like 1-D periodic gratings that were used as angle-selective scattering substrates in pin solar cells are described. These gratings were characterized from both morphological and optical perspectives. By making use of the Harvey-Shack scattering model, it was found that the geometrical parameters and the shape of such gratings play a considerable role in light scattering. Afterwards, such 1-D gratings were for the first time employed in the fabrication of pin single junction a-Si:H solar cells. These devices delivered up to +13.4% higher short-circuit current density than the solar cell with flat interfaces and exhibited, for wavelengths longer than 550 nm, a spectral response slightly higher than that of the solar cell deposited on the reference randomly-textured substrate. In order to find an optimal combination of period and height of rectangular-like 1-D gratings, and to explore the potential of 2-D gratings, 3-D optical modelling was employed. Solar-cell structures on 1-D (2-D) grating textures with different periods and heights were simulated. The best combinations led to a percentage increase of +25.5% (+32.5%) in the short-circuit current density with respect to the flat cell. In the framework of wave guide theory, this result indicates that solar cells on 2-D gratings can be optically better than random textures based on the same transparent conductive oxide. The recent possibility to texture transparent substrates over large areas opens the way for the industrial optimization of thin-film silicon solar cells on periodic gratings. In Chapter 5, modulated surface texturing is introduced for efficient light scattering at long wavelengths for multi-junction solar-cell applications. Based on the combination of two or more classes of textures on the same substrate, this concept paves the way for the realization of textures showing increased broadband light scattering. Different types of modulated surface textures were fabricated on etched glass, on 1-D gratings and on etched polycrystalline silicon wafers. The possibility to optimize the optical performance by manipulating the surface texturing was demonstrated by using wet etching processes. Single junction solar cells deposited on such modulated surface textures showed initial conversion efficiencies up to 9.74% and exhibited very good yield despite severe surface roughness. From these experiments, the best modulated surface texture could be selected, namely a combination of etched glass with large micrometer-scale features and etched ZnO:Al with nanometer-scale features. The same type of etched glass coated with textured ZnO:B was used as substrate for the fabrication of a tandem micromorph silicon device at IMT-PVLAB (Switzerland). The result was a state-of-the-art solar cell (11.6% initial conversion efficiency), where the redistribution of the light absorption between top and bottom cell occurred with a broadband increase in the (red) spectral response. Given such proofs of concept, the next step will be implementation of replicated modulated surface textures coated with low absorption front transparent conductive oxide for triple junction solar cells. In Chapter 6, Distributed Bragg reflectors are studied as dielectric mirrors for thin-film silicon solar-cells applications. Physical properties like photonic band gap, condition of omni-directionality and modulation were analysed and practical rules for appropriate design were reviewed. Distributed Bragg reflectors based on pairs of a-SiNx:H and a-Si:H and optimized for high internal reflectance in solar cells were designed employing advanced optical modelling. Afterwards, using a continuous plasma-assisted process at low temperature, these mirrors were fabricated, optically characterized and finally applied at the rear side of flat and textured single junction solar cells. From spectral and electrical measurements, solar cells with Distributed Bragg reflectors performed as well as the reference cells with Ag reflector. Future studies on these dielectric mirrors will focus on two main objectives: the concurrent usage of omni-directionality and modulation concepts for multi-junction applications and the development of patterning methods that do not make use of photolithography. In Chapter 7, Flattened Light Scattering Substrates are optimized for high efficiency single, double and triple junction solar cells. This type of substrate, based on 2-D photonic crystals proposed by AIST (Japan), is specifically developed for effective diffuse internal reflectance at long wavelengths and for deposition of high quality nc-Si:H. The study was based on a hybrid opto-electrical model that allowed to efficiently simulate both the optical situation and the electrical performance of thin-film multi-junction silicon solar cells. A potential initial conversion efficiency of 11.6%, 14.2%, and 16.0% for single, double, and triple junction solar cells on the optimized Flattened Light Scattering Substrates, respectively, was reported. Further studies on this matter will involve the modelling of asymmetric 2-D photonic crystals, usage of low-absorption supporting materials and application in multi-junctions with intermediate reflectors.