Keith R. McIntosh

Recombination at textured silicon surfaces passivated with silicon dioxide

This work was funded by an Australian Research Council Linkage Grant between the Australian National University, SierraTherm Production Furnaces, and SunPower Corporation.

A freeware 1D emitter model for silicon solar cells

Heavily doped surfaces—often called emitters, diffusions, or back-surface fields—are complicated regions of a solar cell. In these regions, the dopant concentration varies over many orders of magnitude in a short distance, causing large variations in the minority carrier concentration, Auger recombination, Shockley-Read-Hall recombination, carrier mobility and even the band gap. Moreover, when the diffusion is heavily doped, the semiconductor becomes degenerate and the carrier concentrations must be calculated with Fermi-Dirac statistics rather than the simpler Boltzmann statistics. Until now, computer simulation packages that account for all of these aspects are either expensive or not freely accessible, and they do not cater specifically to the PV industry. We therefore present a new freeware computer program that models a 1D emitter in silicon. Given a user-defined dopant profile, a surface recombination velocity, and an incident spectrum, the program calculates recombination as a function of depth within the emitter and as a function of the applied voltage. This permits the computation of the emitter saturation current density, the transparency factor, the collection current density, and the collection efficiency. The program can be applied to both phosphorus and boron diffusions and will assist in their optimisation for practical solar cells. In this paper, we present the equations, the assumptions, and the procedure that are employed by the freeware program.

An optical comparison of silicone and EVA encapsulants for conventional silicon PV modules: A ray-tracing study

Ray-trace simulation is used to quantify the optical losses of photovoltaic modules containing silicon cells. The simulations show that when the modules encapsulant is silicone rather than ethylene vinyl acetate (EVA), the modules short-circuit current density under the AM1-5g spectrum is 0.7–1.1% higher for screen-printed multi-cSi cells, 0.5–1.2% higher for screen-printed mono-cSi cells, and 1.0–1.6% higher for high-efficiency rear-contact cells, depending on the type of silicone. This increase is primarily due to the transmission of short-wavelength light (≪420 nm) and is therefore greatest when used with low UV-absorbing glass and cells of a high IQE at short wavelength. We also quantify absorption in the glass, EVA and silicone at longer wavelengths and describe the influence of an encapsulants refractive index on escape losses.

Input Parameters for the Simulation of Silicon Solar Cells in 2014

Within the silicon photovoltaics (PV) community, there are many approaches, tools, and input parameters for simulating solar cells, making it difficult for newcomers to establish a complete and representative starting point and imposing high requirements on experts to tediously state all assumptions and inputs for replication. In this review, we address these problems by providing complete and representative input parameter sets to simulate six major types of crystalline silicon solar cells. Where possible, the inputs are justified and up-to-date for the respective cell types, and they produce representative measurable cell characteristics. Details of the modeling approaches that can replicate the simulations are presented as well. The input parameters listed here provide a sensible and consistent reference point for researchers on which to base their refinements and extensions.

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

This work was supported by an Australian Research Council Linkage between The Australian National University and Braggone Oy under Grant LP0989593.

Isotextured Silicon Solar Cell Analysis and Modeling 1: Optics

A comprehensive investigation reveals three useful approximations to the optical behavior of isotextured silicon solar cells. First, we confirm experimentally that front-surface reflectance is accurately modeled with “spherical cap” geometry. Second, we find that light reflected from the surface has a Lambertian distribution. Random upright pyramid texturing results in a less favorable distribution so that, when encapsulated, photogeneration in an isotextured cell approaches 99% of that achieved in an equivalent pyramidally textured device. Third, we perform ray tracing simulations to determine the 1-D photogeneration profile beneath isotexture. On their first pass, rays traverse the substrate at angle θ1 with respect to the macroscopic normal such that they are distributed according to cos(3 θ1/2). This approximation to the ray trajectory establishes, for isotexture, a useful simulation tool that has been available for application to pyramidally textured devices for two decades. This paper is followed by a contribution that investigates recombination at isotextured surfaces, coupling results with optical analyses to model the performance of isotextured solar cells.

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 J0s. The dependence of Seff and J0s 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, J0s is superior to the conventional Seff as a metric for quantifying the surface passivation, whereas for dif...

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.

Low Surface Recombination Velocity by Low-Absorption Silicon Nitride on c-Si

We demonstrate that nearly stoichiometric amorphous silicon nitride (SiN<inf>x</inf>) can exhibit excellent surface passivation on both p- and n-type c-Si as well as low absorption at short wavelengths. The key process to obtain such a SiN<inf>x</inf> is the optimized deposition pressure. The effective carrier lifetimes of these samples exceed the commonly accepted intrinsic upper limit over a wide range of excess carrier densities. We achieve a low S<inf>eff,UL</inf> of 1.6 cm/s on 0.85-Ω·cm p-type and immeasurably low S<inf>eff,UL</inf> on 0.47-Ω·cm n-type silicon passivated by the SiN<inf>x</inf> deposited at 290 °C. Capacitance-voltage measurements reveal this SiN<inf>x</inf> has a density of interface states of 3.0 × 10¹¹ eV⁻¹cm⁻² at midgap and an insulator charge of 5.6 × 10¹¹ cm⁻². By comparing the measured injection-dependent S<inf>eff,UL</inf> to calculated S<inf>eff,UL</inf> by an extended Shockley-Read-Hall model, we conclude that either Defect A or B (or both) observed by Schmidt et al. is likely to dominate the surface recombination at our Si-SiN<inf>x</inf> interface. In addition to the outstanding surface passivation, this SiN<inf>x</inf> has a low absorption coefficient at short wavelengths. Compared to Si-rich SiN<inf>x</inf> of an equivalent passivation, the optimized SiN<inf>x</inf> would enhance the photo-generated current density by more than 0.66 mA/cm² or 1.40 mA/cm² for solar cells encapsulated in glass/EVA or operating in air, respectively. The SiN<inf>x</inf> described here is ideally suited for high-efficiency solar cells, which require good surface passivation and low absorption from their front surface coatings.

The contribution of planes, vertices and edges to recombination at pyramidally textured silicon surfaces

We present a methodology by which one may distinguish three key contributors to enhanced recombination at pyramidally textured silicon surfaces. First, the impact of increased surface area is trivial and equates to a √3-fold increase in <i>S</i>_eff,UL. Second, the presence of {1 1 1}-oriented facets drives a fivefold increase in <i>S</i>_eff,UL at SiO₂-passivated surfaces but a small (1.5-fold) increase for SiN_x passivation. A third factor, which is often proposed to relate to stress at convex and concave pyramids and edges, is shown to depend on pyramid period (and, hence, vertex/ridge density). This third factor impacts least on <i>S</i>_eff,UL when the pyramid period is 10 μm. At this period, it results in a negligible increase in <i>S</i>_eff,UL at SiO₂ -passivated textured surfaces but causes at least a sevenfold increase at the Si/SiN_x interface. Finally, we found that <i>S</i>_eff,UL is 1.5-2.0 times higher at inverted pyramid texture than at surfaces featuring a random arrangement of upright pyramids. The results of this study, particularly for the Si/SiN_x system, likely depend on process conditions, but the methodology is universally applicable. We believe this to be the first study to distinguish the impact of {1 1 1} facets from those of vertices and edges. Further, we find that {1 1 1} surfaces, rather than vertices and edges, are chiefly responsible for the poor-quality passivation achieved by thick oxides on textured surfaces.We present a methodology by which one may distinguish three key contributors to enhanced recombination at pyramidally textured silicon surfaces. First, the impact of increased surface area is trivial, and equates to a 1.73-fold increase in Seff, UL. Second, the presence of {111}-oriented facets drives a 5-fold increase in Seff, UL at SiO2 passivated surfaces, but a small (1.5-fold) increase for SiNx passivation. A third factor, often proposed to relate to stress at convex and concave pyramids and edges, is shown to depend on pyramid period (and, hence, vertex/ridge density). This third factor impacts least on Seff, UL when the pyramid period is 10 mm. At this period, it results in a negligible increase in Seff, UL at SiO2 passivated textured surfaces, but causes up to a 4-fold increase at the Si/SiNx interface. That the vertex/ridge density has minimal influence for oxide passivated surfaces is supported by the measurement of Seff, UL on samples with varying oxide thickness: stress at convex and concave features is known to increase with oxide thickness, but appears not to induce additional defects. Instead, it is the dominant {111} oriented surfaces that exhibit thickness dependent passivation quality. Finally, we found that Seff, UL is 1.2–1.5 times higher at inverted pyramid texture than at surfaces featuring a random arrangement of upright pyramids. The results of the present study, particularly for the Si/SiNx system, likely depend strongly on process conditions, but the methodology is universally applicable. We believe this to be the first study to distinguish the impact of {111} facets from those of vertices and edges; its novelty is pronounced among studies of the Si/SiNx:H system.