05 Fakultät Informatik, Elektrotechnik und Informationstechnik
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Item Open Access Characterization of a laser doping process for crystalline silicon solar cells(2010) Ametowobla, Mawuli Francis; Werner, Jürgen H. (Prof. Dr. rer. nat. habil.)This thesis characterizes a process for the laser based formation of highly doped layers on crystalline silicon, which was developed at Institut für Physikalische Elektronik (ipe), University of Stuttgart. A first step analyzes silicon, which is laser irradiated without being doped at the same time. This approach allows for separating the effects of the laser process itself from the influence of employed doping precursors. The irradiation leads to a signicant reduction of the minority carrier lifetime in the processed silicon. A thorough characterization of suchlike treated samples shows the incorporation of the impurities oxygen, carbon and nitrogen, up to concentration levels of C = 1019/cm^3. At the same time, an n-type doping occurs within the irradiated surface layers. The doping concentrations are in the range ND = 10^17/cm^3. In spite of the numerously employed characterization methods, a complete identification of the mechanism, which causes the lifetime reduction, is not achieved. However, there exists the strong indication that the incorporated impurities, which lead to the n-type doping, are at the same time responsible for the lifetime degradation. Due to the low thickness d < 500 nm of the laser melted layers, defect induced local lifetimes t_SRH < 10 ns are required to explain the measured effective lifetimes. The examination of laser irradiated high effciency n-type emitters on p-type silicon substrates, the emitters being pre-fabricated by phosphorous furnace diffusion, yields information about the lifetime SRH of laser induced defects inside the emitters. The values found for t_SRH are in the range of 3 ns < t_SRH < 7 ns. These values are in accordance to the results found for irradiated, undiffused samples. The use of optimized laser parameters allows for obtaining very low emitter saturation current densities J0e = 45 fA/cm^2 after the irradiation. An experimental overview over various liquid and sputtered phosphorous precursors reveals strongly differing J0e values of samples, fabricated with different precursors. Considering the laser induced defects allows for predicting the potential for J0e and the open circuit voltage Voc of solar cells, fabricated with the best precursor, to J0e < 100 fA/cm^2 and Voc = 680 mV. This prediction points out that in the vast majority of cases, the defects, induced by the laser treatment itself, do not limit the performance of emitters, fabricated with the ipe process. Solar cells, which were produced in the course of this work, exhibit significantly lower open circuit voltages of maximum Voc = 635mV. In addition, these cells often exhibit low fill factors FF < 70 % and effciencies < 15 %. However, this lower performance is not in contrast to the predicted Voc potential. An analysis shows that mainly technological problems, leading to doping inhomogeneities, cause the lower efficiencies. An additional characterization of aluminium as a precursor material for p-type emitters on n-type silicon substrates shows exceptionally high doping concentrations ND > 10^21/cm^3. Corresponding emitter saturation current densities reach extremely high values of J0e > 10^11 A/cm^2. Consequently, the open circuit voltage Voc of solar cells, fabricated with aluminium doped emitters, is limited to Voc = 550 mV and their effciency to 7 %.