The field-effect passivation of the interface of thermal oxides on silicon is experimentally investigated by depositing corona charges on the oxide of solar cells and of lifetime test structures. The open circuit voltage of solar cells with interdigitated rear contacts can be increased by +12 mV or decreased by −34 mV, respectively, by depositing positive or negative corona charges on top of the front oxide. The resulting effective surface recombination velocity, $S_{\mathrm{eff}},$ is determined on carrier lifetime test structures for different injection levels and charge densities using microwave-detected photoconductance decay and a new expression for the Auger-limited bulk lifetime. $S_{\mathrm{eff}}$ can be varied between 24 cm/s and 538 cm/s on a 1 $\Omega$ cm p-type wafer with a thermal oxide of 105 nm thickness. The measurements are compared with theoretical predictions of an analytical model for the calculation of the surface recombination. Measured values for the capture cross sections and interface trap densities are used for the calculation. The model predicts an optimum passivation for strong positive compared to strong negative charge densities. This is due to the asymmetry of the capture cross sections for electrons and holes. This prediction is in very good agreement with the measured $S_{\mathrm{eff}}$ values. However, the predicted $S_{\mathrm{eff}}$ values of well below 1 cm/s for 1 $\Omega$ cm p-type silicon cannot be achieved in the experiment. This discrepancy can be explained by an inhomogeneous charge distribution resulting in potential fluctuations and additional loss currents. With a new extended analytical model for the calculation of $S_{\mathrm{eff}}$ the measured $S_{\mathrm{eff}}$ values can be described quantitatively.

We present an experimental approach to extract the dark saturation current density j0e-met at the emitter-metal interface of the front contact. For this purpose, 2×2 cm2 sized silicon solar cells have been realized featuring different metallization fractions FM. By simply applying the one-diode-model, the dark current density j01 is determined from the open circuit voltage Voc. From the slope of the j01 over FM plot, j0e-met is extracted. However, this is only valid if the dominant recombination mechanism at V oc features a diode character that is close to unity. Hence, the local ideality factor m is determined from the suns-Voc-curve indicating the required value close to one. Three main effects are observed. First, the metallization methods which are compared show different influences on j0e-met on the same emitter configuration. Second, an emitter drive-in due to an additional short thermal oxidation lowers j0e-met. Also, the field-effect passivation of the highly n- doped selective emitter decreases j0e-met effectively. By combining the field effect passivation with a short drive-in step the very low value of j0e-met = 386 fA/cm2 is reported

For optimum performance of solar cells featuring a locally contacted rear surface, the metallization fraction as well as the size and distribution of the local contacts are crucial, since Ohmic and recombination losses have to be balanced. In this work we present a set of equations which enable to calculate this trade off without the need of numerical simulations. Our model combines established analytical and empirical equations to predict the energy conversion efficiency of a locally contacted device. For experimental verification, we fabricate devices from float zone silicon wafers of different resistivity using the laser fired contact technology for forming the local rear contacts. The detailed characterization of test structures enables the determination of important physical parameters, such as the surface recombination velocity at the contacted area and the spreading resistance of the contacts. Our analytical model reproduces the experimental results very well and correctly predicts the optimum contact spacing without the use of free fitting parameters. We use our model to estimate the optimum bulk resistivity for locally contacted devices fabricated from conventional Czochralski-grown silicon material. These calculations use literature values for the stable minority carrier lifetime to account for the bulk recombination caused by the formation of boron-oxygen complexes under carrier injection.