Theoretical study of pyramid sizes and scattering effects in silicon photovoltaic module stacks

Front side pyramids are the industrial standard for wafer based monocrystalline silicon solar cells. These pyramids fulfill two tasks: They act as anti-reflective structure on the one hand and as a light-trapping structure on the other hand. In recent development smaller pyramids with sizes below 1 µm attract more and more interest. In this paper an optical analysis of periodically arranged front side pyramids is performed. The impact on the reflectance as well as on the useful absorption within the solar cell is investigated depending on the pyramids size, the amount of additional scattering in the system and the quality of the rear side reflector. In contrast to other investigations not only the solar cell, but the full photovoltaic (PV) module stack is considered. This can strongly influence results, as we show in this paper. The results indicate that in a PV module stack with realistic assumptions for the amount of scattering as well as for the rear side reflectance only small differences for pyramids with sizes above 600 nm occur. Preliminary conclusions for random pyramids deduced from these results for periodically arranged pyramids indicate that these differences could become even smaller

Impact of Front Side Pyramid Size on the Light Trapping Performance of Wafer Based Silicon Solar Cells and Modules

Smaller pyramid sizes for Silicon solar cell front side textures attract more and more interest. At the same time a very good optical performance of the front side texture is crucial to achieve high PV module efficiencies. In this paper an optical study of the impact of periodically arranged front side pyramids on the useful absorption in a Silicon solar cell is presented. Also a way to account for random pyramids is described. Results for solar cells facing semi-infinite EVA/glass are compared to the modeling results of a full solar module stack with the help of the OPTOS formalism. We found that the impact of different pyramids differs depending on the quality of the planar rear side reflector. It is shown that the impact of the module case is crucial and that it has to be accounted for when predicting the optimal pyramid sizes that are relevant for cell and module manufacturers. We also demonstrate that the use of simple parameters such as a single pass light path enhancement factor for the structured surface can lead to wrong conclusions, and a full modeling is required to predict the real module performance. The results indicate that the overall optical performance of a solar module does almost not vary for pyramid sizes above 600 nm

SOFTWARE PROCESSING FOR THE REAL TIME DATA ACQUISITION SYSTEM (RDAPS)

International Telemetering Conference Proceedings / September 28-30, 1982 / Sheraton Harbor Island Hotel and Convention Center, San Diego, CaliforniaInternational Foundation for TelemeteringProceedings from the International Telemetering Conference are made available by the International Foundation for Telemetering and the University of Arizona Libraries. Visit http://www.telemetry.org/index.php/contact-us if you have questions about items in this collection

Characterizing the degradation of PDMS stamps in nanoimprint lithography

Within this work we investigate the lifetime of soft PDMS stamps as a critical parameter for the up-scaled fabrication of microstructures via nanoimprint lithography. We propose the characterization of the PDMS hardness as a suitable parameter for the stamp degradation. The increasing hardness for a larger number of imprints is attributed to resist diffusion into the PDMS volume and possibly subsequent cross-linking. Force-distance measurements with an atomic force microscope were applied to demonstrate for two acrylic materials that a lower average molecular weight leads to a faster degradation. Using a temperature-assisted UV-imprint process with an epoxy material (SU-8 2002) enabled more than 100 imprints without measurable degradation due to the complex molecular structure which we assume to reduce the resist diffusion. This can be a key development towards a higher stamp lifetime and finally an industrial realization of NIL

Light scattering at random pyramid textures: Effects beyond geometric optics

The currently highest measured efficiency for both side contacted monocrystalline Silicon solar cells is 25.8%. The external quantum efficiency of these solar cells with random pyramids at the front side differs from results achieved with pure ray optical modeling. As is already known from literature, better agreement can be reached by introducing additional scattering into the modeled system. Within this paper we show that this scattering can be well explained using a Phong-like scatterer. What is more, we show that in the investigated high efficiency solar cells scattering at the rear side is a minor effect although it is often assumed as the origin of scattering. The relevant scattering instead is caused by the random pyramids themselves

Angle-dependent reflectance of isotextured silicon

Multicrystalline silicon (mc-Si) is the workhorse of photovoltaics with a market share above 60%, being acidic textured (isotextured) silicon the standard. For optical simulations, the spherical caps model is widely applied, but it was reported that is does not correctly describe the angular dependence of reflection. In this work, our first objective is to verify this. Secondly, we want to obtain an improved model for optics of isotextured mc-Si wafers and solar cells able to describe the light trapping properties of cells and modules correctly. We investigate the reflectance of isotextured silicon by varying the angle of incidence of the light between 8° and 80°. We perform measurements of nine isotextured samples, obtaining reflectance curves with a similar shape, but they differ by up to 60%abs from previously published data. The first simulations are performed using the spherical caps model, which is shown to be inaccurate to describe the angle-dependent reflectance of isotextured silicon. In the second set of simulations we vary the texture morphology, reducing the deviations of the simulation from the measurement from 8% to 1.2% for incident angles smaller than 60°. We conclude that alternative morphologies are worth doing further research

Large-area honeycomb texturing of Si-solar cells via nanoimprint lithography

The highest solar cell efficiencies on mc-Si were realized using photolithographically defined texturing processes. The feasibility of fabricating honeycomb textures using Nanoimprint Lithography (NIL) was already demonstrated and excellent short-circuit current densities of up to 40.7 mA/cm² were achieved on small-area monocrystalline silicon (c-Si) solar cells. In the present work, large-area honeycomb texturing of multicrystalline silicon (mc-Si) substrates based on Roller-NIL and in-line-capable multi-wafer plasma etching using a modified Roth&Rau SiNA system is demonstrated. The realized honeycomb texture shows superior optical properties compared to isotextured mc-Si reference samples and even random pyramids on c-Si substrates, while surface passivation is comparably good. The fabricated 156 x 156 mm² mc-Si solar cells with Al-BSF show an efficiency of 17.8% compared to 17.3% of the isotextured reference. As VOC remains constant for both textures, the efficiency gain of 0.5% absolute results from a Jsc increase of up to 1.3 mA/cm2 (on cut 50 x 50 mm² cells even 2.5 mA/cm2), which confirms the superior optical quality of the honeycomb texture

Interference and nanoimprint lithography for the patterning of large areas

Micro- and nanostructures can be used for reflectance reduction or light guidance in applications like photovoltaic solar cells, LEDs or display technology. The combination of interference lithography and nanoimprint lithography enables the fabrication and replication of high resolution structures on large areas. The origination of master structures, seamlessly patterned on areas as large as 1.2 × 1.2 m2 was shown using interference lithography. Within this work we demonstrate our current results on the up-scaling of the replication process chain based on nanoimprint lithography with in-line capable tools. Application examples in the fields of photovoltaics are demonstrated, e.g. the micron-scale patterning of multicrystalline silicon substrates to increase the solar cell efficiency. Furthermore, the lifetime of soft PDMS stamps is investigated. AFM force-distance measurements are introduced as suitable method to quantify the PDMS hardness as a parameter indicating stamp degradation. This technique is subsequently applied to evaluate two different resist materials. Applying the epoxy material (SU-8) with its more complex molecular structure results in a strongly increased stamp lifetime compared to the acrylate resist (Laromer LR 8996). This is a highly valuable result for further developments towards an up-scaled realization of nanoimprint lithography