| Popis: |
In this thesis, novel in-situ CVD processes have been investigated that promise to decrease the costs and increase cell efficiencies at the same time. The central approach is the epitaxial wafer-equivalent cell structure, consisting of an epitaxial layer deposited on a low-cost silicon substrate. This epitaxial wafer-equivalent (EpiWE) is then processed using a standard solar cell process. The main goal of this thesis project was to improve the quality and the electrical properties of the deposited films for a future industrial-type application of the crystalline silicon thin-film solar cells. Two major aspects were considered: the silicon deposition with adapted doping profiles and the functional HCl etching for optical confinement and substrate gettering.Special attention was paid to improve the efficiencies of the crystalline silicon thin-film solar cells with conventional emitters made by POCl3 diffusion. The doping level of the base has a major impact on the efficiency and optimum constant doping levels slightly higher than conventional wafer solar cells were found. Simulations showed that the influence of positive drift fields in the base is relevant for low minority carrier lifetimes and no light confinement. It was found that an epitaxial BSF decreases the short circuit current density when the substrate lifetime is higher than 0.1 µs. For substrates with shorter lifetimes or high interface recombination rates, however, a BSF is essential to hinder the minority carriers from diffusing into the substrate, where they are likely to recombine. From comparisons with simulation results, carrier lifetimes between 1 and 5 µs for epitaxial base layers on Cz substrates have been identified. Epitaxial layers with high base lifetimes showed a large gain in short circuit current density for thicker bases. By applying these optimisations, efficiencies of crystalline silicon thin-film solar cells up to 16.1% on Cz and 14.5% on mc substrates were achieved.One main focus of this thesis was the application of silicon epitaxy for the emitter formation. This can either be performed ex-situ for wafer cells or in-situ for crystalline silicon thin-film solar cells. Emitter formation by epitaxial growth has many advantages; the most important are the in-situ process for the epitaxial wafer-equivalents, the fast process realisation and the possibility to design the emitter profile as desired. The performance of the emitters was first tested on wafers. Boron-doped emitters were grown on n-type wafers and resulted in efficiencies up to 15.9%. However, a high surface recombination velocity was observed and better passivation layers than SiO2 are still required. In contrast, the passivation of SiO2 on n+-type surfaces is excellent and the quality of the epitaxial emitter could be investigated.A highly innovative process for emitter formation was established in this work, where a blue sensitive emitter can be easily prepared. In order to prevent the phosphorus from out-diffusing from the epitaxial emitter layer, the wafers are cooled in a PH3/H2 atmosphere. This not only prevents the out-diffusion but also increases the doping concentration at the surface. This is advantageous as high surface concentrations are necessary for most metallisation methods in order to achieve low contact resistances. EpiWE solar cells with photolithographic grid definition and evaporated contacts showed efficiencies up to 15.2% on highly-doped Cz substrates and small area. Even on large area cells on Cz material, an efficiency of 14.9% with a high open circuit voltage of 655 mV and a fill factor of 79.9% was achieved, exceeding the values of the reference sample with a 120 Ω/sq. POCl3 emitter. Furthermore, efficiencies up to 13.6% were reached on highly-doped multicrystalline substrates. It was found that the fill factors of the multicrystalline cells with epitaxial emitters are partly limited due to the inhomogeneous growth of the emitter on different grain orientations. Furthermore, simulation results and dark saturation current densities indicate that the recombination within the space charge region is even lower than on corresponding POCl3 diffused crystalline silicon thin-film solar cells. The growth of an in-situ epitaxial emitter can substitute the conventional POCl3 emitter for the crystalline silicon thin-film solar cells.Screen-printed metallisations, which are widely used in the PV industry, require high surface doping concentrations to achieve a low contact resistance. The combination of screen-printed contacts and epitaxial emitters was performed for the very first time within this work. It was found that a good contact is possible on surfaces with phosphorus concentration of approximately 1x1020 cm-3, when the cells were fired two times. Efficiencies up to 12.1% were reached on highly-doped multicrystalline substrates, with a high 78.2% fill factor and nearly 620 mV open circuit voltage. These results are similar to POCl3 diffused cells. Epitaxial emitters have a high potential since the doping profile can be considerably improved compared with industrial-type emitters.The second main topic of the thesis was the application of functional in-situ HCl etching of silicon for solar cells. Depending on the process parameters, rough surfaces could be etched independent on the grain orientation. The resulting texture showed a predominant diffuse reflectance. Structures such as front-side texturing were investigated, however the electrical properties showed still high surface recombination velocities. After annealing of the rough surfaces, pores of 1-5 µm were created in the substrate. By adjustment of the process parameters pore densities up to 10000 pores/mm2 were obtained. The porous intermediate layer was investigated for internal reflection, but it has not yet shown a clear gain in the short circuit current density. However, the implementation of a porous layer into the epitaxial wafer-equivalent by simply HCl gas etching prior to the epitaxy, was successfully demonstrated for the first time.HCl gas additionally reacts with impurities to form highly-volatile chlorides. The advantage is obvious, as impurities in the substrate could be extracted before the epitaxial deposition. The gettering effect of gaseous HCl was applied to lowly-contaminated multicrystalline and higher contaminated metallurgical silicon wafers. Measurements showed a clear increase in the wafer lifetime by the HCl gettering at a temperature of 850°C. The total interstitial iron concentration could be decreased by more than two orders of magnitudes compared to annealed samples. Lifetimes as high as in reference phosphorus gettered wafers were achieved. On metallurgical silicon, neutron activation analysis also showed a decrease up to 70% of the impurity concentrations. These are the first results presented so far about the gettering effect of hot HCl gas diluted in hydrogen for solar cell application. Gettering at high temperatures was performed successfully with subsequent epitaxial deposition. An increased growth of whiskers, spikes and wires induced by the presence of metals from the metallurgical silicon was found. Nevertheless, crystalline silicon thin-film solar cells were fabricated on metallurgical substrates with efficiencies up to 7.4%.This work proves that in-situ processes can be applied with comparatively low effort to crystalline silicon thin-film solar cells. These processes can simplify the actual fabrication sequence and may enable the use of low-cost silicon substrates. With such improvements, the crystalline silicon thin-film solar cells are brought closer to an industrial application. |
