Mark Kerr

General parameterization of Auger recombination in crystalline silicon

A parameterization for band-to-band Auger recombination in silicon at 300 K is proposed. This general parameterization accurately fits the available experimental lifetime data for arbitrary injection level and arbitrary dopant density, for both n-type and p-type dopants. We confirm that Auger recombination is enhanced above the traditional free-particle rate at both low injection and high injection conditions. Further, the rate of enhancement is shown to be less for highly injected intrinsic silicon than for lowly injected doped silicon, consistent with the theory of Coulomb-enhanced Auger recombination. Variations on the parameterization are discussed.

Surface passivation of silicon solar cells using plasma-enhanced chemical-vapour-deposited SiN films and thin thermal SiO2/plasma SiN stacks

Two different techniques for the electronic surface passivation of silicon solar cells, the plasma-enhanced chemical vapour deposition of silicon nitride (SiN) and the fabrication of thin thermal silicon oxide/plasma SiN stack structures, are investigated. It is demonstrated that, despite their low thermal budget, both techniques are capable of giving an outstanding surface passivation quality on the low-resistivity (∼1 � cm) p-Si base as well as on n + -diffused solar cell emitters with the oxide/nitride stacks showing a much better thermal stability. Both techniques are then applied to fabricate frontand rear-passivated silicon solar cells. Open-circuit voltages in the vicinity of 670 mV are obtained with both passivation techniques on float-zone single-crystalline silicon wafers, demonstrating the outstanding surface passivation quality of the applied passivation schemes on real devices. All-SiN passivated multicrystalline silicon solar cells achieve an open-circuit voltage of 655 mV, which is amongst the highest open-circuit voltages attained on this kind of substrate material. The high open-circuit voltage of the multicrystalline silicon solar cells results not only from the excellent degree of surface passivation but also from the ability of the cell fabrication to maintain a relatively high bulk lifetime (>20 µs) due to the low thermal budget of the surface passivation process.

Surface recombination velocity of phosphorus-diffused silicon solar cell emitters passivated with plasma enhanced chemical vapor deposited silicon nitride and thermal silicon oxide

This work was supported by funding from the Australian Research Council. One of the authors ~J.S.! gratefully acknowledges the support of a Feodor Lynen fellowship by the Alexander von Humboldt Foundation of Germany.

Very low bulk and surface recombination in oxidized silicon wafers

Bulk and surface processes determine the recombination rate in crystalline silicon wafers. In this paper we report effective lifetime measurements for a variety of commercially available float-zone silicon wafers that have been carefully passivated using alnealed silicon oxide. Different substrate resistivities have been explored, including both p-type (boron) and n-type (phosphorus) dopants. Record high effective lifetimes of 29 and 32 ms have been measured for 90 Ω cm n-type and 150 Ω cm p-type silicon wafers, respectively. The dependence of the effective lifetime has been measured for excess carrier densities in the range of 1012–1017 cm−3. These results demonstrate that very low bulk and surface recombination rates can be maintained during high-temperature oxidation (1050 °C) by carefully optimizing the processing conditions.

Numerical modeling of highly doped Si:P emitters based on Fermi-Dirac statistics and self-consistent material parameters

P.P.A. is on a Postdoctoral Fellowship from the Australian Research Council ~ARC!. The Center for Photovoltaic Engineering is supported by ARC’s Special Research Centres Scheme. A.C. and M.K. also acknowledge funding by the ARC.

Recombination at the interface between silicon and stoichiometric plasma silicon nitride

The injection level dependence of the effective surface recombination velocity (Seff) for the interface between crystalline silicon and stoichiometric silicon nitride, prepared by high-frequency direct plasma enhanced chemical vapour deposition (PECVD), has been comprehensively studied. A wide variety of substrate resistivities for both n-type and p-type dopants have been investigated for minority carrier injection levels (Δn) between 1012 and 1017 cm−3. Effective lifetimes of 10 ms have been measured for high resistivity n-type and p-type silicon, the highest ever measured for silicon nitride passivated wafers, resulting in Seff values of 1 cm s−1 being unambiguously determined. The Seff(Δn) dependence is shown to be constant for n-type silicon under low injection conditions, while for p-type silicon, there is a clear minimum to Seff for injection levels close to the doping density. Further, the Seff(Δn) dependence for these stoichiometric silicon nitride films appears to be weaker than that for other high-quality, silicon-rich silicon nitride films prepared by remote PECVD.

Generalized analysis of quasi-steady-state and transient decay open circuit voltage measurements

This work has been funded by the Australian Research Council. The authors also thank S. Glunz and S. Rein from the Fraunhofer Institute of Solar Energy Systems, for the use of the solar cell used in Fig. 5.

Millisecond minority carrier lifetimes in n-type multicrystalline silicon

Exceptionally high minority carrier lifetimes have been measured in n-type multicrystalline silicon (mc-Si) grown by directional solidification and subjected to phosphorus gettering. The highest effective lifetimes, up to 1.6 ms averaged over several grains and 2.8 ms within some of them, were measured for relatively lowly doped, 2–3 Ωcm, wafers. The lifetime was found to decrease for lower resistivities, still reaching 500 μs for 0.9 Ωcm and 100 μs for 0.36 Ωcm. Several important findings are reported here: (i) achievement of carrier lifetimes in the millisecond range for mc-Si, (ii) effectiveness of phosphorus gettering in n-type mc-Si, and (iii) demonstration of good stability under illumination for n-type mc-Si.

Recombination and trapping in multicrystalline silicon

Minority carrier recombination and trapping frequently coexist in multicrystalline silicon (mc-Si), with the latter effect obscuring both transient and steady-state measurements of the photoconductance. In this paper, the injection dependence of the measured lifetime is studied to gain insight into these physical mechanisms. A theoretical model for minority carrier trapping is shown to explain the anomalous dependence of the apparent lifetime with injection level and allow the evaluation of the density of trapping centers. The main causes for volume recombination in mc-Si, impurities and crystallographic defects, are separately investigated by means of cross-contamination and gettering experiments. Metallic impurities produce a dependence of the bulk minority carrier lifetime with injection level that follows the Shockley-Read-Hall recombination theory. Modeling of this dependence gives information on the fundamental electron and hole lifetimes, with the former typically being considerably smaller than the latter, in p-type silicon, Phosphorus gettering is used to remove most of the impurities and reveal the crystallographic limits on the lifetime, which can reach 600 /spl mu/s for 1.5 /spl Omega/cm mc-Si. Measurements of the lifetime at very high injection levels show evidence of the Auger recombination mechanism in mc-Si. Finally, the surface recombination velocity of the interface between mc-Si and thermally grown SiO/sub 2/ is measured and found to be as low as 70 cm/s for 1.5 /spl Omega/cm material after a forming gas anneal and 40 cm/s after an anneal. These high bulk lifetimes and excellent surface passivation prove that mc-Si can have an electronic quality similar to that of single-crystalline silicon.

Lifetime and efficiency limits of crystalline silicon solar cells

A new parameterization for Auger recombination in silicon has been determined, which accurately fits the highest available experimental lifetime data for arbitrary injection level and dopant density, for both n-type and p-type dopants. Our analysis confirms that Auger recombination is enhanced above the traditional free-particle rate at both low injection and high injection conditions. The new parameterization is used to show that Coulomb-enhanced Auger recombination imposes the most severe bound on the achievable efficiency of crystalline silicon solar cells, with a maximum limiting efficiency of 29.05% determined for a high resistivity silicon base (90/spl mu/m thick). The limiting efficiency reduces for more heavily doped silicon and is lower for n-type silicon compared to p-type silicon.