Carrier Lifetime Measurements by Photoconductance at Low Temperature on Passivated Crystalline Silicon Wafers
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Carrier Lifetime Measurements by Photoconductance at Low Temperature on Passivated Crystalline Silicon Wafers Guillaume Courtois1,2, Bastien Bruneau2, Igor P. Sobkowicz1,2, Antoine Salomon1 and Pere Roca i Cabarrocas2 1 Total New Energies, R&D Division, Tour Michelet, 24 Cours Michelet – La Défense 10 92069 Paris La Défense Cedex, France 2 LPICM, CNRS – Ecole Polytechnique, Bât 406, Route de Saclay 91128 Palaiseau Cedex, France ABSTRACT We propose an implementation of the PCD technique to minority carrier effective lifetime assessment in crystalline silicon at 77K. We focus here on (n)-type, FZ, polished wafers passivated by a-Si:H deposited by PECVD at 200°C. The samples were immersed into liquid N2 contained in a beaker placed on a Sinton lifetime tester. Prior to be converted into lifetimes, data were corrected for the height shift induced by the beaker. One issue lied in obtaining the sum of carrier mobilities at 77K. From dark conductance measurements performed on the lifetime tester, we extracted an electron mobility of 1.1x104 cm².V-1.s-1 at 77K, the doping density being independently calculated in order to account for the freezing effect of dopants. This way, we could obtain lifetime curves with respect to the carrier density. Effective lifetimes obtained at 77K proved to be significantly lower than at RT and not to depend upon the doping of the a-Si:H layers. We were also able to experimentally verify the expected rise in the implied Voc, which, on symmetrically passivated wafers, went up from 0.72V at RT to 1.04V at 77K under 1 sun equivalent illumination. INTRODUCTION Properties of passivated crystalline wafers involved in solar cell manufacturing have been widely investigated at room temperature (RT). For instance, photoconductance has been widely used since the mid-1990s in order to assess minority carriers effective lifetimes. Among all possible candidates, hydrogenated amorphous silicon (a-Si:H) has proved to ensure very effective surface passivation, hence very long carrier lifetimes obtained in such passivated wafers and high open-circuit voltages in silicon heterojunction solar cells [1]. Yet, passivation mechanisms into play are not yet completely understood, and low temperature measurements may contribute to sharpen knowledge on that topic. Whilst a slight increase in bandgap alongside with a sharp increase in implied Voc at low temperature are well predicted by semiconductor physics, alterations in carrier effective lifetimes are difficult to account for. Experimental studies carried out so far on comparable materials, relying either on microwave-detected photoconductance decay (PCD) [2], or modulated photoluminescence (PL) [3], [4] agree on a decrease in effective lifetime as the temperature diminishes down to 70K. We report here on effective lifetime measurements performed by inductively coupled PCD at 77K on (n)-type mono-crystalline wafers passivated by a-Si:H.
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