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Issue Date: 4-May-2011
Authors: Chibbaro, Claudio
Title: Mono-crystalline Silicon Photovoltaic Cells: Innovative Technologies toward low Series Resistance
Abstract: This thesis gives, at first, a collocation of photovoltaic technology inside the picture of world energy production. The need of a transition to a renewables-intensive energy market is reported as a scientific evidence deriving from economical and environmental data analysis. The present state-of-art of photovoltaic technology in terms of research development, manufacturing cost, market status and forecast is illustrated. In spite of emerging new technologies promising higher efficiencies and lower production costs, standard first generation devices (single junction, silicon wafer-based cells) still represent the majority of available photovoltaic equipments because of their cost-effectiveness. However, the necessity to produce more and more competitive photovoltaic cells requires a continuous progress regarding cell design in order to optimize structural parameter toward efficiency improvement and cost reduction. A complete study of all structural parameters characterizing silicon wafer-based photovoltaic cells has been performed by means of a simulation program purposely realized. The simulation program has been used to analyze the most important physical and geometrical factors determining silicon cells performance (as material quality, doping levels, layers thickness) with the purpose of maximize photogeneration and minimize electrical losses. Obtained results quantify the effect of semiconductor quality demonstrating that most effectiveness silicon (in terms of costs/benefits ratio) is the typical Czochralski-grown. It has been also illustrated that best performances could be obtained for an emitter as thin as possible (in the order of few thousands of nanometres) and heavily doped (doping concentration >1e18cm-3). Moreover, has been shown that base thicknesses in the range of 200-300um and doping levels within the interval 1e15-1e16cm-3 optimize bulk design guaranteeing highest efficiencies. Finally, simulations highlight the importance of parasitic resistance effects on device electrical characteristic influencing drastically cells performance. Simulations results have been used inside an industrial R&D project arisen from a collaboration between Meridionale Impianti (M.I.) S.p.A., UniCt (University of Catania), Advanced Technology Solutions (A.T.S.) S.r.l. and Meridionale Impianti Welding Technology (M.I.W.T) S.r.l.. The project purpose was the development of innovative processes and new equipments aimed at production of high efficiency and low-costs first generation silicon photovoltaic cells. It included a pre-competitive development and an industrial research. Thus, the entire industrial standard process used for realize a silicon PV cell has been studied and a revision of every single manufacturing step has been carried out in order to attain a competitive manufacturing line. Data obtained by research and simulation activities have allowed to create a prototype productive line, designed with the principle purposes of product quality, manufacturing cost reduction and energy saving. The main innovation introduced regards the use of a ZnO film deposited by a Pulsed Plasma Deposition method (PPD) as both front contact and passivation layer. Optical and electrical characteristics of the deposited layer have been studied to define best process equipment set up and to optimize film thickness. The effects of the ZnO layer on photovoltaic cell electrical behavior (connected especially to the diminishing of parasitic series resistance value) have been evaluated by the comparison of two prototype cells. It has been demonstrated that innovative process flow with ZnO could improve cell efficiency. Moreover, obtained results highlight the importance of the innovative technique investigated since it is very advantageous because also simplify entire process flow. ZnO deposition performed by PPD developed inside present project opens interesting prospects in the sphere of process flow simplification and it necessities of a dedicated industrialization program directed towards large scale production. The difficulties occurred during the estimation of low series resistances (in the order of few mOhm), in terms of accuracy and repeatability, using standard measurement methods yielded to the development of a new measurement procedure in order to overcome the intrinsic limits of other methods. The presented method does not make use of any particular assumption, so that it can be virtually used for any kind of cell. It is very precise, since it is able to evaluate with good precision and reproducibility series resistance values as low as 1mOhm if the precision of voltage and current measurements in the cell characteristic is of the order of few mV and mA respectively. Moreover the measure of the series resistance is not precluded or made difficult by the presence of a low value shunt resistance . These two facts represent themselves a large improvement of the capabilities and potentialities on respect of the other nowadays used methods.
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