Enhanced mobility of hydrogenated MO-LPCVD ZnO contacts for high performances thin film silicon solar cells
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Enhanced mobility of hydrogenated MO-LPCVD ZnO contacts for high performances thin film silicon solar cells L. Ding, M. Benkhaira, S. Nicolay, C. Ballif Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of Microengineering (IMT), Photovoltaics and Thin Film Electronics Laboratory, Rue A.-L. Breguet 2, CH-2000 Neuchâtel ABSTRACT In this contribution, we study the increase in metalorganic-low pressure chemical vapor deposited (MO-LPCVD) ZnO thin films conductivity by hydrogen plasma post-treatment. We show that this improvement is linked to defect passivation at grain boundaries, decreasing the electron traps density and resulting in the almost complete suppression of the electron scattering at grain boundaries. For a 2 μm thick non-intentionally doped ZnO layer, electron mobility reaches after treatment values close to 60 cm2V-1s-1 (corresponding to an increase of 100%), with a carrier density still as low as 3 x1019 cm-3 (+1.5 x1019 cm-3). Such layers have an absorbance below 2-3% in the range of 400 to 1100 nm making them among the most transparent and conductive materials reported so far. In addition, we demonstrate that hydrogen plasma posttreated ZnO layers can be used as front electrode for producing highly transparent and conductive electrodes. Eventually, it is shown that hydrogen plasma treatment can also be used on the complete thin film solar cell stack (back contact and silicon device) to improve the cell performances. INTRODUCTION Polycrystalline metalorganic-low pressure chemical vapor deposited (MO-LPCVD) zinc oxide (ZnO) films present, under certain growth conditions [1, 2], V-shaped elongated grains resulting in a self-textured rough surface providing efficient light scattering when used as electrodes in thin film silicon (TF Si) solar cells. The polycrystalline structure of the film implies a strong limitation in electrical conductivity: transport of the charge carriers is indeed mainly restricted by scattering at the potential barrier present at the defective region constitutive of grain boundaries (GB) [3-5]. One way to overcome this limit is to increase the n-type doping, e.g. by addition of group III elements, such as boron. But doping has the inconvenience of increasing the free carrier absorption (FCA), which is detrimental for ZnO transparency. In addition, high doping leads to a higher ionized impurities concentration, also acting as electron scattering centers, which then decreases the intra-grain mobility and hence the conductivity and transparency of the films. In this work, we apply hydrogen plasma (pH2) post-treatment onto MO-LPCVD ZnO films to improve film conductivity and to relax the conductivity-transparency trade-off. As a matter of fact, this treatment was reported not only to increase the electron concentration by creating shallow donors, but more particularly to strongly reduce defects density at GB [6-8]. Finally, we show the benefits of pH2 post-treatment applied to front and back electrodes of a TF Si solar cell, whereas the benefits could be valid for other types of
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