Characterisation and optimisation of PECVD SiNx as an antireflection coating and passivation layer for silicon solar cells

In this work, we investigate how the film properties of silicon nitride (SiNx) depend on its deposition conditions when formed by plasma enhanced chemical vapour deposition (PECVD). The examination is conducted with a Roth & Rau AK400 PECVD reactor, where the varied parameters are deposition temperature, pressure, gas flow ratio, total gas flow, microwave plasma power and radio-frequency bias voltage. The films are evaluated by Fourier transform infrared spectroscopy to determine structural properties, by spectrophotometry to determine optical properties, and by capacitance–voltage and photoconductance measurements to determine electronic properties. After reporting on the dependence of SiNx properties on deposition parameters, we determine the optimized deposition conditions that attain low absorption and low recombination. On the basis of SiNx growth models proposed in the literature and of our experimental results, we discuss how each process parameter affects the deposition rate and chemical bond density. We then focus on the effective surface recombination velocity S eff, which is of primary importance to solar cells. We find that for the SiNx prepared in this work, 1) S eff does not correlate universally with the bulk structural and optical properties such as chemical bond densities and refractive index, and 2) S eff depends primarily on the defect density at the SiNx-Si interface rather than the insulator charge. Finally, employing the optimized deposition condition, we achieve a relatively constant and low S eff,UL on low-resistivity (≤1.1 Ωcm) p- and n-type c-Si substrates over a broad range of n = 1.85–4.07. The results of this study demonstrate that the trade-off between optical transmission and surface passivation can be circumvented. Although we focus on photovoltaic applications, this study may be useful for any device for which it is desirable to maximize light transmission and surface passivation.This work was supported by an Australian Research Council Linkage between The Australian National University and Braggone Oy under Grant LP0989593

Temperature dependent carrier lifetime studies of Mo in crystalline silicon

The capture cross sections of both electronsσn and holes σp were determined for interstitialmolybdenum in crystalline silicon over the temperature range of −110 to 150 °C. Carrier lifetimemeasurements were performed on molybdenum-contaminated silicon using a temperature controlled photoconductance instrument. Injection dependent lifetime spectroscopy was applied at each temperature to calculate σp and σn. This analysis involved a novel approach that independently determined the capture cross sections at each temperature assuming a known defect density and thermal velocity. Since the energy state is in the lower half of the bandgap, the determination of σp is unaffected by the defect energy at all temperatures, and σp is found to decrease with temperature in a fashion consistent with excitonic Auger capture. At temperatures below 0 °C, the determination of σn is also unaffected by the defect energy due to the suppression of thermal emission, and σn decreases with temperature as well. It is shown that a projection of σn to higher temperature suggests the defect has an energy of 0.375 eV above the valance band edge of silicon.D.M. likes to thank the Australian Research Council for fellowship and G.C. likes to thank “CrystalClear Integrated Project” Contract No. SES6-CT_2003-502583 funded by the European Commission

Surface passivation of c-Si by atmospheric pressure chemical vapor deposition of Al2O3

Atmospheric pressure chemical vapor deposition of Al₂O₃ is shown to provide excellent passivation of crystalline silicon surfaces.Surface passivation,permittivity, and refractive index are investigated before and after annealing for deposition temperatures between 330 and 520 °C. Deposition temperatures >440 °C result in the best passivation, due to both a large negative fixed charge density (∼2 × 10¹² cm⁻²) and a relatively low interface defect density (∼1 × 10¹¹ eV⁻¹ cm⁻²), with or without an anneal. The influence of deposition temperature on film properties is found to persist after subsequent heat treatment. Correlations between surface passivation properties and the permittivity are discussed

Recombination at textured silicon surfaces passivated with silicon dioxide

The surfaces of solar cells are often textured to increase their capacity to absorb light. This optical benefit is partially offset, however, by an increase in carrier recombination at or near the textured surface. A review of past work shows that the additional recombination invoked by a textured surface varies greatly from one experiment to another. For example, in the most commonly investigated structure—pyramidal textured silicon diffused with phosphorus and passivated with a hydrogenated oxide—recombination ranges from being 1–12 times more than in an equivalently prepared planar {100} surface. Examination of these experiments reveals consistent trends: small increases in recombination occur when the surface is very heavily diffused and dominated by Auger recombination, while larger increases in recombination occur when the surface is lightly diffused and dominated by Shockley–Read–Hall recombination at the surface, making the latter depend critically on surface area and the density of surface states. Comparisons of pyramidal and planar {100} surfaces indicate that when lightly diffused, the difference in recombination is substantially greater than the difference in surface area (1.73) and it is regularly attributed to the pyramid facets having {111} orientations—well known for their higher density of dangling bonds than {100} orientations. This high dangling-bond density makes recombination at pyramidal facets strongly dependent on the passivation scheme, and it is variations in these schemes that led to the wide range of results observed in experimental studies. In addition to surface area and crystal orientation, some experiments suggest a third mechanism that enhances recombination on oxide-passivated pyramids. With capacitance-voltage and photoconductance measurements, we confirm this speculation, showing that oxide-passivated pyramidal textured silicon has a higher density of interface states than can be accounted for by surface area and orientation, and that the additional defects are predominantly acceptorlike when above, or donorlike when below, an energy of 0.3 eV higher than the valence band.This work was funded by an Australian Research Council Linkage Grant between the Australian National University, SierraTherm Production Furnaces, and SunPower Corporation

A roadmap for PERC cell efficiency towards 22%, focused on technology-related constraints

Presently, the crystalline silicon (c-Si) photovoltaic (PV) industry is switching from standard cells to PERC cells to increase cell efficiency from about 18% to about 20%. This paper gives a roadmap for increasing PERC cell efficiency further towards 22%. Which equipment and which process conditions are feasible to go beyond 20% efficiency? To help answer this as generally as possible, we conduct state-of-the-art modelling in which we sweep the inputs that represent major technology-related constraints, such as diffusion depth, metal finger width and height, alignment tolerances, etc. (these are assigned to the x- And y-axes of our graphs). We then predict the optimum device parameters resulting from these restrictions (shown as contour lines). There are many different ways to achieve 22%. Our modelling predicts, for example, that 60 μm wide screen-printed metal fingers are sufficiently narrow if the alignment tolerance (width of the n++ region) is below 90 μm. The rear may be contacted with 30 μm wide openings of the Al2O3/SiNx stack and with local J0,BSF values as high as 900 fA/cm2. If these requirements cannot be met, they may be compensated by improvements in other device parts. Regardless of this, the wafer material requires a SRH lifetime of at least 1 ms at excess carrier densities near 10(14) cm(-3)

On effective surface recombination parameters

This paper examines two effective surface recombination parameters: the effective surface recombination velocity Seff and the surface saturation current density J0 s . The dependence of Seff and J0 s on surface charge Q, surface dopant concentration Ns , and interface parameters is derived. It is shown that for crystalline silicon at 300 K in low-injection, Seff is independent of Ns only when Q²/Ns   1.5 × 10⁷ cm for accumulation and Q¹˙⁸⁵ /Ns  > 1.5 × 10⁶ cm for inversion. These conditions are commonly satisfied in undiffused wafers but rarely in diffused wafers. We conclude that for undiffused silicon, J0 s is superior to the conventional Seff as a metric for quantifying the surface passivation, whereas for diffused silicon, the merit in using J0 s or Seff (or neither) depends on the sample. Experimental examples are given that illustrate the merits and flaws of J0 s and Seff

Degradation of oxide-passivated boron-diffused silicon

Recombination in oxide-passivated boron-diffused silicon is found to increase severely at room temperature. The degradation reaction leads to a 45 fold increase in emitter recombination that saturates in ∼120 days, irrespective of whether the samples received a forming-gas anneal. The degradation was also examined for diffusions stored at 50, 75, and 100 °C. The results indicate that the degradation follows a second-order reaction where the time constant of one component of the reaction is 10–40 times shorter than the other, and where the activation energy of the fast reaction is 0.19±0.05 eV. Subsequent to degradation, annealing in air reduces the recombination with increasing anneal temperature saturating at ∼300 °C to a value that is about four times higher than the predegradation value. A likely cause of this degradation is a reaction of atomic hydrogen at the silicon-oxide-silicon interface

The study of thermal silicon dioxide electrets formed by corona discharge and rapid-thermal annealing

A silicon dioxide (SiO₂) electret passivates the surface of crystalline silicon (Si) in two ways: (i) when annealed and hydrogenated, the SiO₂–Si interface has a low density of interface states, offering few energy levels through which electrons and holes can recombine; and (ii) the electret’s quasipermanent charge repels carriers of the same polarity, preventing most from reaching the SiO₂–Si interface and thereby limiting interface recombination. In this work, we engineer a charged thermal SiO₂electret on Si by depositing corona charge onto the surface of an oxide-coated Si wafer and subjecting the wafer to a rapid thermal anneal (RTA). We show that the surface-located corona charge is redistributed deeper into the oxide by the RTA. With 80 s of charging, and an RTA at 380 °C for 60 s, we measure an electretcharge density of 5 × 10¹² cm⁻², above which no further benefit to surface passivation is attained. The procedure leads to a surface recombination velocity of less than 20 cm/s on 1 Ω-cm n-type Si, which is commensurate with the best passivation schemes employed on high-efficiency Si solar cells. In this paper, we introduce the method of SiO₂electret formation, analyze the relationship between charge density and interface recombination, and assess the redistribution of charge by the RTA

Limitations of a simplified dangling bond recombination model for a-Si:H

The validity of a widely used simple closed-form expression for the recombination associated with dangling bonds in hydrogenated amorphous silicon (a-Si:H) is linked to the relative position of the distribution of the dangling bond states with respect to the quasi-Fermi levels for trapped electrons and holes. However, these quasi-Fermi levels for traps have not been derived before. In this work, we derive the four relevant quasi-Fermi levels for traps associated with dangling bonds in a-Si:H and clarify the limitations of the simple model