Simeon C. Baker-Finch

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.

Pyramidal surface textures for light trapping and antireflection in perovskite-on-silicon tandem solar cells

Perovskite-on-silicon tandem solar cells show potential to reach > 30% conversion efficiency, but require careful optical control. We introduce here an effective light-management scheme based on the established pyramidal texturing of crystalline silicon cells. Calculations show that conformal deposition of a thin film perovskite solar cell directly onto the textured front surface of a high efficiency silicon cell can yield front surface reflection losses as low as 0.52mA/cm(2). Combining this with a wavelength-selective intermediate reflector between the cells additionally provides effective light-trapping in the high-bandgap top cell, resulting in calculated absolute efficiency gains of 2 - 4%. This approach provides a practical and effective method to adapt existing high efficiency silicon cell designs for use in tandem cells, with conversion efficiencies approaching 35%.

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.

A freeware program for precise optical analysis of the front surface of a solar cell

This paper describes a freeware program that computes the optical losses associated with the front surface of a silicon solar cell. The optical losses are not trivial to assess because (i) the refractive index and extinction coefficient of silicon, antireflection coatings (ARCs), and encapsulants varies with wavelength; (ii) cells are usually textured such that the light reflects multiple times from the front surface; (iii) light can be polarised, particularly after the first ‘bounce’ from a textured surface; and (iv) the incident spectrum and the cells quantum efficiency varies with wavelength. The freeware program takes all of these aspects into account to calculate reflection from the solar cell, absorption in the ARCs, transmission into the silicon, and the equivalent current that is generated for any given spectrum. When modelling textured silicon, the program is restricted to normally incident light and pyramidal morphologies. The program computes solutions within one second for regular upright pyramids, regular inverted pyramids, and random upright pyramids—making it much faster than ray tracing. We provide an example of how the freeware can be employed to determine the optimal thickness of an ARC with and without encapsulation. The example demonstrates that the optimal thickness cannot be determined from reflection measurements when absorption in the ARCs is significant. The program is readily adaptable to assess ARCs on glass and thin-film solar cells.

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 ...

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 SiO2 electret 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 electret charge 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,...

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.

Charge Density in Atmospheric Pressure Chemical Vapor Deposition TiO 2 on SiO 2 -Passivated Silicon

The charge density of a TiO 2 film deposited on a SiO 2 -passivated silicon wafer is determined. The TiO 2 is deposited by atmospheric pressure chemical vapor deposition at 400°C, and the Si0 2 is grown thermally at 950°C. This TiO 2 ―SiO 2 stack is a useful coating for the front surface of a silicon solar cell, as it has a high optical transmission and a low density of interface states D it (E) at the SiO 2 ―Si interface. While these properties are beneficial to high efficiency solar cells, so too is a large charge density, as what occurs in Si 3 N 4 ―SiO 2 (+10 12 cm ―2 ) and Al 2 O 3 ―SiO 2 (―10 13 cm ―2 ) stacks. The D it (E) and charge density of TiO 2 -coated and SiO 2 -passivated silicon are evaluated by capacitance―voltage and Kelvin probe measurements. The charge density of the TiO 2 is within the conservative limits of ―8.5 and ―1 × 10 11 cm ―2 after deposition and of ―10 and +1 × 10 11 cm ―2 after a subsequent 800°C oxygen anneal. Photoconductance measurements suggest that the dangling-bond defects at the SiO 2 ―Si interface are predominantly donorlike and, hence, that the change density in the TiO 2 is closer to the upper limits (less negative); this charge is too small to benefit solar cells.

Impact of carrier profile and rear-side reflection on photoluminescence spectra in planar crystalline silicon wafers at different temperatures

The increasing use of spectral photoluminescence as an advanced and accurate diagnostic tool motivates a comprehensive assessment of the effects of some important optical and electrical properties on the photoluminescence spectra from crystalline silicon wafers. In this paper, we present both modeling results and measurements to elucidate the effects of the internal reflectance at the planar wafer surfaces, as well as the carrier profile varying across the sample thickness due to an increased rear-surface recombination velocity, as a function of temperature. These results suggest that the accuracy of existing spectral PL techniques may be improved by using higher temperatures due to the increased effect of the carrier profile at higher temperatures. They also show that changes in the photoluminescence spectrum shape caused by the addition of a rear-side specular reflector offset those caused by changes in the carrier profile due to increased rear surface recombination, and therefore, considerable care needs to be taken when changing the rear-side optics. Finally, the possible impact of variations in the rear-side reflectance on the band-band absorption coefficient and radiative recombination coefficient, which have previously been determined using the spectral photoluminescence technique, is assessed and demonstrated to be insignificant in practice.

Reactive ion etched black silicon texturing: A comparative study

We report on significant progress towards the application of reactive ion etched (RIE) black silicon (b-Si) as an alternative to the most commonly applied front-side textures utilized in the crystalline silicon photovoltaics industry - random pyramids and isotexture. The as-etched b-Si surface displays approximately 1% front side reflectance weighted across the solar spectrum, outperforming both random pyramids (2.83%) and isotexture (6.06%) with optimized anti-reflection coatings. The b-Si front surface reflectance reduces to below 0.4% after the application of an Al2O3 surface passivation layer. At low injection levels, recombination of charge carriers at the b-Si surface poses no limitation on the minority carrier lifetimes of bulk-limited Cz and multicrystalline samples. At higher injection, or with higher quality substrates, additional recombination at the b-Si surface, characterized by a surface Jos of 20 fA.cm-2, may play a more significant role. This study provides a rigorous empirical justification for recent advances in b-Si textured solar cells and indicates pathways for further efficiency gains.