Keith Reid McIntosh

The optimal tilt β _opt of the modules in a single-axis tracker is often determined by assuming all sunlight is direct and ground reflectance is zero. Prior works, however, have demonstrated that β _opt is smaller when diffuse light is significant. In this article, we determine how β _opt decreases as the conditions change from clear sky to overcast, accounting for many complicated effects like row-to-row shading of diffuse light, ground reflection, structural shading, cell-to-cell mismatch, and angular and spectral dependencies. We find that when compared to monofacial systems, bifacial systems tend to have a higher β _opt when it is cloudier and a lower β _opt when it is sunnier. We also quantify the increase in annual yield that arises when accounting for indirect light at three example sites with different climates, finding it to be ∼30% lower for a bifacial system than a monofacial system; the gain was 0.8%–1.5% for bifacial systems and 1.1%–2.1% for monofacial systems, where the highest gain was attained in the cloudiest climate.

Titania (TiOx) is re-emerging to be a passivating material for the surfaces of high-efficiency crystalline silicon solar cells. Numerous sources in the literature suggest that the surface passivation and thermal stability of TiOx deteriorates with increasing film thickness when the TiOx film is thicker than a sufficient thickness. To circumvent this limitation, this study presents a novel process of Al-doped TiOx (TiOx:Al) film, which demonstrates the potential for improved thermal stability and surface passivation. Based on grazing incident X-ray diffraction and UV-Raman measurements, the incorporation of Al impurity in TiOx effectively restrains the crystal phase transformation of the amorphous TiOx layer during deposition. Furthermore, the TiOx:Al films provide better thermal robustness up to 350 °C, which makes it highly compatible with Si solar cell fabrication processes.

We measure and analyze two commercially available backsheets. One of the backsheets (AGFA’s monoblock backsheet, UniQoat) is significantly more reflective than the other (a typical Fluoropolymer/PET/Fluoropolymer backsheet). We measure their hemispherical reflectance before encapsulation in air, from which we calculate their intrinsic reflectance, and hence the reflectance at an EVA–backsheet interface after lamination. This intrinsic reflectance is considerably higher than the reflectance measured in air (e.g., the reflectance of UniQoat at 1000 nm increases from 85% to 92% after lamination), and it is this intrinsic reflectance rather than the measured reflectance that is required to accurately predict how a particular backsheet affects a module’s optical behavior. With a combination of measurements and ray tracing, we demonstrate that the calculated intrinsic reflectance is consistent with the hemispherical reflectance of glass– EVA–backsheet test samples. The ray tracing is then extended to predict how each backsheet affect a module’s short-circuit current density J_SC as a function of cell spacing. We find, for example, that with a cell spacing of 4 mm, the UniQoat backsheet yields a 2.5% increase in J_SC whereas the less reflective backsheet yields a 2.0% increase.

We apply advanced ray tracing to predict the color of an encapsulated solar cell. Previous studies have predicted the color from the thickness and refractive index of the antireflection coating and encapsulants. Here, we demonstrate how other effects influence the color significantly, applying ray tracing to account for texture, for direct and diffuse insolation, for scattering, and for the observer angle. Despite the sophistication of the ray tracing, solutions for each simulation are attained within seconds. With an accurate and quick way to predict a module’s color, researchers can avoid costly experimentation when (i) predicting how cell color will change after encapsulation, (ii) determining production tolerances to limit color variability within a cell or module and (iii) optimizing for a particular color for BIPV applications.

This work presents the investigation of low pressure in-situ thermal oxidation as the interfacial oxide for n+ polysilicon-oxide passivated contact structure, achieving excellent surface passivation below 1 fA·cm ^-2 and contact resistivity below 1 mΩ·cm ² . The results from the process optimisation are presented in detail, showing the importance of accurate control of oxidation conditions, and presenting the correlation to the electrical properties. Additionally, a method of fabricating contact resistivity structures from symmetrical photoconductance decay lifetime samples, and the extraction of the specific contact resistivity using 3D numerical simulation is presented.

We investigate the light trapping in Si wafers that are textured with conventional random pyramids on their front surface and rounded random pyramids on their rear. It is well established that rounding the pyramids leads to better surface passivation, but whether or not it improves light trapping depends on the cell structure. In this paper, we apply ray tracing, spectrophotometry, and photoluminescence spectroscopy (PLS) to understand and quantify how rounding the rear pyramids might affect the light trapping in back-contact solar cells. We describe how rounding the pyramids leads to two competing optical effects: 1) reduced absorption in the rear films and 2) reduced scattering from the rear texture. The first effect improves light trapping whereas the latter degrades it. We show how the influence of each effect depends on wavelength and how they can be discerned (but not easily quantified) in reflectance curves. With PLS measurements, we conclude that for our sample structure and etch solution, the generation current is approximately constant for etch durations less than ∼60 s, and decreases significantly as the etch duration increases. Thus, by limiting the duration of the rounding etch, superior surface passivation can be attained without degrading the light trapping.

Innovalight's Cougar™ Platform is a selective emitter (SE) process based on a screen-printed patternof Silicon Ink. The Cougar Platform has been licensed to several leading solar companies. The performance gain from the Cougar SE process is well documented at the cell level [1]. In this report we describe the optimization of the Cougar cell for photovoltaic (PV) modules. Ray tracing modeling [2] has been used to optimize the performance of encapsulated Cougar cells, taking into account the optical properties of surface texture, encapsulant materials, and cell anti-reflection coating layers. Anti-reflection coatings are made from silicon nitride (SiN _x ), the optical characteristics of which were determined from ellipsometry and UV-Vis spectroscopy. Modeling predicts different cell-to-module (CTM) values for short-circuit current (I _sc ), depending on the SiN _x characteristics. Mini-modules were constructed to verify modeling results. In addition to optical modeling, fill factor losses in the mini-modules are investigated and analyzed. Accelerated lifetime testing was also performed. The Cougar Platform shows equivalent reliability performance compared to traditional, homogeneous emitter (HE) solar cells.