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We present an experimental investigation into light trapping within isotextured wafers. Measurements of reflectance and transmittance are performed on Si wafers both before and after the deposition of thin films on their surfaces. The wafers have a variety of surface morphologies that were created by varying the isotexture etch duration. We find that, unlike external reflectance, light trapping in isotextured wafers depends only weakly on the isotexture etch time (for practical etch durations). We also find that the light trapping induced by isotexture is similar to that induced by random pyramids. Moreover, we conclude that the spherical-cap model, which is extended to account for internal rays within the isotexture, can be applied to predict the transmittance and escape reflectance and, hence, the light trapping in isotextured wafers. By way of quantification, the model predicts the AM1.5g generation current to within ±1% and the reflectance and transmittance to within ±2% absolute for λ < 1100 nm (and to within ±4% for λ < 1200 nm for most samples). With an established light-trapping model, photovoltaic researchers can more accurately predict the behavior of isotextured solar cells for various antireflection coatings, encapsulants, and light sources.

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.

Epitaxy can be used to fabricate doped front surface regions that enable high interdigitated back contact (IBC) silicon solar cell efficiency. One- and two-dimensional simulations show that an epitaxial layer with a constant phosphorus dopant concentration on the order of 1017 -1018 cm-3 can possess the properties of an excellent front surface region for an n-type IBC cell. With appropriate control of dopant concentration and thickness, the epitaxially grown region passivates a textured surface, and provides the lateral conductivity necessary to enable high fill factor. The combination of these two factors drives a simulated efficiency improvement above 0.3% absolute over an n-type IBC cell with a typical 200-Ω/sq phosphorus diffusion (e.g., from POCl3). Importantly, the epitaxial front surface region can occupy the entire volume of the pyramidal texture. We, therefore, propose an exemplary process sequence for device fabrication that places texture etching after epitaxial growth.

We extend our analysis of isotextured silicon solar cells by 1) examining experimentally the role played by isotexture in determining the surface recombination velocity at silicon surfaces and 2) combining these experimental results with our model for photogeneration in order to simulate in one dimension typical solar cell devices with isotextured surfaces. We examine both undiffused and diffused n-type isotextured silicon surfaces, and we find that the rate of surface recombination usually decreases with increasing isotexture etch depth. However, when undiffused surfaces are passivated with hydrogenated SiO2 or SiNx, surface recombination velocity is, counterintuitively perhaps, found to be independent of surface texture-this is despite a surface area that is up to 1.9-fold larger than a planar equivalent. We demonstrate the utility of our analysis of isotextured surfaces by simulating various device structures in one dimension. In one case, where device parameters are chosen to approximate a typical screen-printed cell with full-area back surface field, simulation results indicate that the optimal isotexture etch depth is 1-3 μm. This optimum etch depth is slightly below the one deduced from published experimental results, indicating that surface recombination on samples observed in this study is uniquely independent of isotexture morphology.

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.

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. Second, the presence of {1 1 1}-oriented facets drives a fivefold increase in Seff,UL at SiO2-passivated surfaces but a small (1.5-fold) increase for SiNx 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 Seff,UL when the pyramid period is 10 μm. At this period, it results in a negligible increase in Seff,UL at SiO2 -passivated textured surfaces but causes at least a sevenfold increase at the Si/SiNx interface. Finally, we found that Seff,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/SiNx 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.

Perovskite-on-silicon tandem solar cells show potential to reach30% 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 fundamental mechanisms of iron impurities in silicon have been thoroughly studied and are well explained in the literature. Of interest to solar cell manufacturers is to understand how these mechanisms manifest in a production environment and, more importantly, how to quickly diagnose and mitigate iron contamination as it occurs. This paper presents examples of iron contamination using p-type CZ wafers processed in production-style environments. The impact of iron on the IV performance of industrial screen printed solar cells is presented, including the time dependence of these effects and how they manifest in the various characterisation techniques that are typically used to diagnose solar cell performance. Examples are given of potential sources of iron contamination and the impact of subsequent processing on the redistribution of those contaminants. The paper demonstrates that iron contamination can occur in a variety of ways, can spread quickly and is severely detrimental to solar cell efficiency. Additionally, it is shown that the fundamental properties of iron in silicon can be used to quickly identify the root cause of contamination in a production environment.

A design concept supported by numerical device modeling is presented for a p-type IBC cell with screen-printed aluminum for both contact polarities. Applying such a design to Cz silicon appears to offer cell efficiency exceeding 20%. The key enabling feature for this design is cleaving each cell into strips held together by tape. These strips are then connected electrically in series using extrusion printing after the strips are laminated to the module glass. The resulting module technology is called SPLICE, for “Screen Printed Locally Interdigitated Contact Elements”.

The freeware program OPAL 2 computes the optical losses associated with the front surface of a Si solar cell. It calculates the losses for any angle of incidence within seconds, where the short computation time is achieved by decoupling the ray tracing from the Fresnel equations. Amongst other morphologies, OPAL 2 can be used to assess the random-pyramid texture of c-Si solar cells, or the ‘isotexture’ of mc-Si solar cells, and to determine (i) the optimal thickness of an antireflection coating with or without encapsulation, (ii) the impact of imperfect texturing, such as non-ideal texture angles, over-etched isotexture, and flat regions, and (iii) the subsequent 1D generation profile in the Si. This paper describes the approach and assumptions employed by OPAL 2 and presents examples that demonstrate the dependence of optical losses on texture quality and incident angle.

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 reflection distributions from various pyramidal textures and find that {111} faceted pyramids are a poor approximation to the features on such surfaces. We conclude that features are hillocks, with an octagonal base. Furthermore, the characteristic base angle of the texture depends on the etchant and is closer to 50–52° than the commonly accepted value of 54.74°. Analyses of antireflection, light trapping, photogeneration and surface recombination properties of textured surfaces should take this feature morphology into account. The base angle has a strong influence on the hemispherical reflectance of the textured surface, with higher angles resulting in reduced reflectance. The influence of this reflection enhancement upon device performance is smallest when an optimised antireflection coating is applied; compared with an array of {111} faceted pyramids, a hillock morphology with 50° base angle results in a 0.2% reduction in photogenerated current in a typical cell. Additionally, as base angle is reduced, an encapsulant of increasingly higher refractive index is required to drive internal reflection at the air–glass interface of light initially reflected from the cell surface. The development of texturing processes resulting in higher base angles is encouraged. Copyright © 2012 John Wiley & Sons, Ltd.

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 S eff, UL . Second, the presence of {111}-oriented facets drives a 5-fold increase in S eff, UL at SiO 2 passivated surfaces, but a small (1.5-fold) increase for SiN x 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 S eff, UL when the pyramid period is 10 mm. At this period, it results in a negligible increase in S eff, UL at SiO 2 passivated textured surfaces, but causes up to a 4-fold increase at the Si/SiN x interface. That the vertex/ridge density has minimal influence for oxide passivated surfaces is supported by the measurement of S eff, 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 S eff, 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/SiN x 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/SiN x :H system.

Photoluminescence (PL) imaging is used to identify a contamination source that is introduced during a thermal oxidation of crystalline p-type silicon. The PL images indicate that the contamination severely decreases the carrier lifetime from ∼350 ms to 50 ms, and that it is localised and radially symmetric with a diameter of ∼10 mm. By taking PL images before and after the wafers are ‘light-soaked’, the contamination source is shown to be Fe and its net concentration is quantified. By solving the steady-state quasi-neutral semiconductor equations in cylindrical coordinates, the size of the original contaminant Fe particles is determined. This approach takes into account the out-diffusion of the Fe during thermal processing, as well as the lateral flow of carriers into the contaminated region; the latter is important due to the rapid and large change in lifetime in the vicinity of the defected region. We find that if the original particles were pure Fe, their diameter would be just 150–180 nm. Finally, we analyse the PL images of a comprehensive experiment to identify that the Fe particulates were deposited onto the wafer during the ‘HF dip’, which followed the RCA clean, and preceded the loading of the wafers into the furnace. Thus, we provide a methodology for the analysis of PL images of wafers affected by localised, lifetime-degrading contaminants; a proper treatment of carrier transport in such regions allows us to more accurately determine the size of the contaminating particles.

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 cell's 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.

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.

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.

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,...

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.

This paper describes how capacitance–voltage (CV) and Kelvin probe (KP) measurements can be combined to determine the magnitude and centroid of the electric charge in a thin-film insulator. The technique is demonstrated on three films of relevance to silicon solar cells: aluminium oxide, amorphous silicon nitride and silicon dioxide. Since the charge within these films is of different magnitudes, locations and polarity, they offer a good selection with which to demonstrate the combined CV and KP technique.