Recombination Lifetime in Microcrystalline Silicon Absorbers of Highly Efficient Thin-Film Solar Cells
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Recombination lifetime in microcrystalline silicon absorbers of highly efficient thin-film solar cells
Torsten Brammer and Helmut Stiebig Institute of Photovoltaics, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany
ABSTRACT Absorber layers of microcrystalline silicon thin-film solar cells deposited by plasma-enhanced chemical vapor deposition are characterized regarding the recombination lifetime. The characterization is based on a comparison of experimentally determined solar cell characteristics with results from numerical device simulations. Evaluation of the dark reverse saturation current indicates a strong dependence of τ on the hydrogen dilution during the deposition. Close to the transition region to amorphous growth where the highest solar cell efficiencies are observed τ is maximum within the crystalline deposition regime and equals 30 ns.
INTRODUCTION Preferred deposition techniques for thin-film microcrystalline silicon are plasma-enhanced chemical vapor deposition (PECVD) and hot-wire chemical vapor deposition (HW-CVD). Both approaches have proven to yield solar cell efficiencies of about 9 % [1] in single junction cells and above 10% (stabilized) in combination with an amorphous solar cell forming a tandem solar cell [2, 3]. The process gases for thin-film silicon deposition are silane gas (SiH4) and hydrogen gas (H2). The ratio of the two gas fluxes termed silane concentration SC = [SiH4]/([SiH4]+[H2]) is crucial for the microstructure of the deposited silicon film. Previous investigations showed that our vhf-PECVD films deposited at small SC ( 5%, τ needs to be verified by determination of the mobility gap which is the topic of current research. At small SC, tail states might influence the mobility gap. Also shown is τ derived from electron spin densities [6] with equation (3) (ESR, solid symbols, σC was chosen so that the lifetimes match for SC = 2%). The increase in τ corresponding to ESR is less pronounced than concluded from the dark I/V curves. The reason for the change in τ with SC can be attributed to the better passivation of internal surfaces within the silicon layer [17]. In figure 5 lifetimes in Si of different crystalline Si technologies are additionally shown: high temperature CVD Si on glass [18], liquid phase epitaxial (LPE) grown Si on monocrystalline Si, [19], string ribbon grown Si [20], multi- [21] and monocrystalline Si (CZ [22], FZ [23]). The graph shows that τ in doped multi- (mono-) crystalline Si is three (four) orders of magnitude larger than in (undoped) µc-Si:H.
CONCLUSIONS The absorber layer of µc-Si:H solar cells was analysed regarding the recombination lifetime by a comparison of experimentally determined solar cell characteristics with results from numerical device simulations. It was shown that the recombination lifetime can be deduced from the dark diode characteristics at small voltages. For state-of-the-art PECVD µc-Si:H solar cells the effective recombination lifetime is about 30 ns. A6.2.5
ACKNOWLEDGMENTS The authors are grateful for the contributions
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