Every silicon thin-film solar cell concept is dependent on an excellent optical confinement. As well as texturisation and an anti-reflection coating on the front side, the rear-side needs a reflector for the wavelength region exceeding 600 nm to enhance the long-wavelength response of the solar cell. In our Recrystallised Wafer Equivalent (RexWE) [1] the rear-side of the silicon layer is not accessible during the solar cell process. Therefore, several important features have to be implemented via an intermediate layer: it needs to act as a diffusion barrier of sufficiently high electrical conductivity, an excellent optical reflector, and ideally also as a passivation layer for interface defects. We try to satisfy these requirements with a specially designed reflector. It consists of SiC and SiO2 layers with alternating refractive indices and varying characteristics that can be realised by changing the stoichiometry and layer network. These layer-stacks were implemented into RexWE solar cells by a process sequence including thermal annealing, Si seeding layer deposition, recrystallisation and epitaxial Si growth. To surmount the lack of electrical conductivity of the SiO2 layers we drilled holes through the stacks using a laser. We call this process laser-fired rear access (LFA). The best solar cell incorporating the SiC/SiO2 reflector shows a Jsc of 26.3 mA/cm2 (with front side plasma texture) which constitutes an enhancement of 4 mA/cm2 compared with a single SiC layer. The cell efficiency was thereby increased from 8.8% to 11.1%
Amorphous hydrogenated Si 1-x C x / SiC multilayers consisting of alternating Si 1-x C x and stoichiometric SiC layers were prepared using Plasma Enhanced Chemical Vapour Deposition (PECVD). Annealing at temperatures up to 1100°C was done targeting the size controlled crystallization of Si nanocrystals (NCs) in a SiC matrix. The influence of annealing temperature on the nanostructure of the multilayers was studied using Glancing Incidence X-ray Diffraction (GIXRD), Raman spectroscopy and Transmission Electron Microscopy (TEM). GIXRD reveal the crystallization of Si and SiC, when annealing temperatures exceed 900°C. The crystallization of Si and SiC was confirmed by TEM bright field imaging and electron diffraction. Annealing at 900°C, leads to the formation of Si NCs with a size of 3 nm, whereas the SiC NCs also have a size of 3 nm. However, a large amount of Si is still amorphous as shown by Raman spectroscopy. Annealing at temperatures exceeding 900°C reduces the amorphous phase and a further growth of Si NCs occurs.
The customized substrates for manufacturing 300mm power semiconductors had to be prepared by deposition processes and epitaxial growth on standard substrates since they were not yet available from suppliers in both sufficient quality and quantity. Polysilicon films were deposited on wafer backsides and optimized regarding impurity gettering. Severe modifications of existing epitaxy reactors, the facilitation, and the infrastructure were prerequisite to develop extremely high doped, thick silicon layers with both supreme uniformity of layer thickness and dopant distribution to take full advantage of the productivity advantage of large diameter wafer processing. Simulation supporting these activities was also key to enable furnace processes with extremely tight temperature ranges. For the development of very thick, high resistivity silicon layers a 5-wafer 200mm batch tool was used to exploit the tool and process learning for the design and manufacturing of a similar 300mm batch tool. For pattern transfer the same advanced plasma etch equipment was mandatory to achieve similarly uniformities regarding etch rate, profile shape, and CD as in CD-driven advanced DRAMs and MPUs.
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