The Contribution of Planes, Vertices, and Edges to Recombination at Pyramidally 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 √3-fold increase in Seff,UL•. Sec

Temperature dependence of the radiative recombination coefficient in crystalline silicon from spectral photoluminescence

The radiative recombination coefficient B(T) in crystalline silicon is determined for the temperature range 90-363 K, and in particular from 270 to 350 K with an interval of 10 K, where only sparse data are available at present. The band-band absorption coefficient established recently by Nguyen et al. [J. Appl. Phys. 115, 043710 (2014)] via photoluminescence spectrum measurements is employed to compute the values of B(T) at various temperatures. The results agree very well with literature data from Trupke et al. [J. Appl. Phys. 94, 4930 (2003).] We present a polynomial parameterization describing the temperature dependence of the product of B(T) and the square of the intrinsic carrier density. We also find that B(T) saturates at a near constant value at room temperature and above for silicon samples with relatively low free carrier densities

Temperature dependence of the radiative recombination coefficient in crystalline silicon from spectral photoluminescence

The radiative recombination coefficient B(T) in crystalline silicon is determined for the temperature range 90–363 K, and in particular from 270 to 350 K with an interval of 10 K, where only sparse data are available at present. The band-band absorption coefficient established recently by Nguyen et al. [J. Appl. Phys. 115, 043710 (2014)] via photoluminescence spectrum measurements is employed to compute the values of B(T) at various temperatures. The results agree very well with literature data from Trupke et al. [J. Appl. Phys. 94, 4930 (2003).] We present a polynomial parameterization describing the temperature dependence of the product of B(T) and the square of the intrinsic carrier density. We also find that B(T) saturates at a near constant value at room temperature and above for silicon samples with relatively low free carrier densities

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

Near-infrared free carrier absorption in heavily doped silicon

Free carrier absorption in heavily doped silicon can have a significant impact on devices operating in the infrared. In the near infrared, the free carrier absorption process can compete with band to band absorption processes, thereby reducing the number of available photons to optoelectronic devices such as solar cells. In this work, we fabricate 18 heavily doped regions by phosphorus and boron diffusion into planar polished silicon wafers; the simple sample structure facilitates accurate and precise measurement of the free carrier absorptance. We measure and model reflectance and transmittance dispersion to arrive at a parameterisation for the free carrier absorption coefficient that applies in the wavelength range between 1000 and 1500 nm, and the range of dopant densities between ∼10¹⁸ and 3 × 10²⁰ cm⁻³. Our measurements indicate that previously published parameterisations underestimate the free carrier absorptance in phosphorus diffusions. On the other hand, published parameterisations are generally consistent with our measurements and model for boron diffusions. Our new model is the first to be assigned uncertainty and is well-suited to routine device analysis

Rules and tools for understanding, modelling and designing textured silicon solar cells

This thesis is an expansive and in-depth discourse on the textured front surface of silicon solar cells. Systematic and comprehensive analyses of both optical and recombination behaviours are undertaken to arrive at a thorough understanding of the impacts of surface texture. These analyses provide the basis for the development of several novel methods for the precise simulation of photovoltaic devices and modules. A rigorous approach to the optics of surface texture, incorporating polarisation effects, is shown to advance the accuracy of standard reflectance-absorptance-transmittance analyses. Using this approach, assessment of any texture morphology is possible; of particular utility are applications to isotexture and random upright pyramids, as well as extensions to non-ideal morphologies typical on practical devices. The methodology proposed is computationally inexpensive, as it decouples geometric ray tracing from the Fresnel equations, and is hence well suited to routine application. Ray tracing is applied to determine a one-dimensional profile of photogeneration with respect to the distance to an isotextured front surface. An analytical approximation to this profile is developed. The approximation is suitable for routine analysis of multicrystalline devices, and facilitates the simulation of short circuit current in typical wafered silicon solar cells with less than 3.5% error. By comparison with ray traced photogeneration profiles beneath pyramidal texture, an established approximation is validated; application of this widely-used approximation results in the prediction of short circuit current in typical devices to within 6% accuracy. In further exploration of optical behaviour, measurements of the angular distribution of reflection from textured surfaces are shown to provide critical new information for accurate modelling of photovoltaic modules. An example application demonstrates that, in the case that cells are encapsulated, current generation beneath isotexture approaches 99% of that calculated beneath random pyramids. This implies that cast-mono silicon cells should be isotextured, rather than pyramidally textured, when they possess less than 85% monocrystalline surface area. The paucity of experiments dedicated to the recombination mechanisms at textured surfaces is recognised in this work and is rectified by a thorough study. This is particularly pertinent given that the current trend towards cells featuring lighter front diffusions elevates the relative importance of front surface recombination. Compared to a planar surface, it is shown that isotexture incurs little or no recombination penalty. At pyramidal texture, however, increased surface area drives increased recombination according to conventional understanding. It is demonstrated that an additional increase due to the orientation of texture facets occurs when an Si-SiO2 interface is present. Further, results of this study repudiate the common conjecture that vertices and edges necessarily induce supplementary recombination when passivated with common dielectrics; an experimental methodology developed in this thesis is applied to show that any increase in recombination can be avoided by judicious choice of passivation. Finally, it is shown that recombination is commonly greater at a regular array of inverted pyramids than at an equivalently prepared random array of upright pyramids. -- provided by Candidate

Reflection distributions of textured monocrystalline silicon: Implications for silicon solar cells

A common misconception is that alkaline textured silicon solar cell surfaces are characterised by features that are pyramidal and bounded by {111} planes. In preference to the typical approach of observing scanning electron microscope images, we analyse

One-dimensional photogeneration profiles in silicon solar cells with pyramidal texture

The key metric of surface texturing is the short-circuit current Jsc. It depends on front surface transmittance, light trapping and the spatial profiles of photogeneration G and collection efficiencyc. To take advantage of a one-dimensional profile ofc(

Reflection of normally incident light from silicon solar cells with pyramidal texture

A surface texture enhances the capacity of a solar cell to absorb incident radiation. In high efficiency and industry standard designs alike, pyramidal surface textures play the key role of reducing the reflectance of the cell surface. This reduction is achieved by ensuring that incident light rays suffer at least a double reflection in the various facets of the structure. In this work, we define a general expression for the reflectance of a pyramidal texture by identifying discrete paths of reflection and the fraction of reflected light that follows each of these paths. We apply the expression to analyse the reflection of normally incident light at textured surfaces. We examine three common morphologies, finding that a regular array of inverted pyramids just outperforms a random array of upright pyramids, with a regular array of upright pyramids showing poorer capacity to reduce front surface reflection. We extend the analysis to determine the transmittance of the various structures, thus permitting the calculation of a figure of merit that can be used to optimise the thickness of antireflection coatings (ARCs). Finally, by examining the angles at which light is reflected by the pyramidal textures, we find that an encapsulant of refractive index greater than 1.59 gives between 79 and 92% of the initially reflected light a second chance to enter the solar cell