Light trapping in solar cells: numerical modeling with measured surface textures

We present and experimentally validate a computational model for the light propagation in thin-film solar cells that integrates non-paraxial scalar diffraction theory with non-sequential ray-tracing. The model allows computing the spectral layer absorbances of solar cells with micro-and nano-textured interfaces directly from measured surface topographies. We can thus quantify decisive quantities such as the parasitic absorption without relying on heuristic scattering intensity distributions. In particular, we find that the commonly used approximation of Lambertian scattering intensity distributions for internal light propagation is violated even for solar cells on rough textured substrates. More importantly, we demonstrate how both scattering and parasitic absorption must be controlled to maximize photocurrent. (C) 2015 Optical Society of America

L'oxyde de zinc par dépôt chimique en phase vapeur comme contact électrique transparent et diffuseur de lumière pour les cellules solaires

Sylvie Faÿ

Zinc oxide (ZnO) is a material that belongs to the family of Transparent Conductives Oxides (TCO). Its non-toxicity and the abundant availability in the Earth's crust of its components make it an ideal candidate as electrical transparent contact for thin-film amorphous and/or microcrystalline silicon solar cells. The Low-Pressure Chemical Vapour Deposition (LP-CVD) method allows one to obtain rough ZnO layers, which can effectively scatter the light that passes through them. This high scattering capacity increases the path of the light within the solar cell and therefore enhances its probability to be absorbed by the solar cell, and consequently the photogenerated current. This thesis work studies in detail the LP-CVD technology used for deposition of ZnO layers, which are employed as TCO layers for amorphous, microcrystalline and "micromorph" (microcrystalline/amorphous tandem) solar cells developped at the Institute of Microtechnology of the University of Neuchâtel. The chapter 3 is a detailed study of the growth of ZnO layers on a glass substrate. This study reveals a columnar microstructure of the ZnO layers, which are composed of large conical monocrystals. The tops of these monocrystals emerge at the surface of the ZnO layers as pyramids, which give to such layers their rough aspect and therefore their capacity of scattering the light. The two following chapters represent a study of the effect of variations in deposition parameters on the optical properties (transparency and scattering power), electrical properties (conductivity), and structural properties of ZnO layers. The deposition parameters varied thereby are the various gas flows, the substrate temperature, and the process pressure. In particular, a strong influence of the substrate temperature on the microstructure of the ZnO layers is observed. A variation of ten degrees of this deposition temperature leads to abrupt variations of the optical and electrical properties of the layers. From these various studies, a strong correlation is established, between the scattering power, the electrical properties, and the microstructure of the ZnO layers deposited by LP-CVD: The higher the pyramidal grains at the surface of the layers, the higher the scattering capacity of these layers. The larger the monocrystals within the bulk of the layers, the higher the mobility of the free electrons. This enhanced mobility leads to an increase of the conductivity of the layers. Finally, the chapter 6 of this thesis work describes the various situations where LP-CVD ZnO layers are used as transparent electrical contacts and light scattering layers in thin-film silicon solar cells. It is schown that, by incorporating ZnO layers developped in this work in the photovoltaic devices, an increase of 8% in the photogenerated current is obtained, compared to the performances of solar cells using standard commercially available TCO.

EPFL2003

Optical Layers for Thin-film Silicon Solar Cells

Peter Cuony

In this work we develop and analyze optical layers for use in Micromorph solar cells, a tandem configuration with an amorphous silicon top cell and a microcrystalline silicon bottom cell. The morphology of the front electrode has a decisive role in maximizing the efficiency of a solar cell. To reach a better understanding of the requirements for the front electrode surface, we present a wide range of morphologies that can be obtained with as-grown rough zinc oxide (ZnO) and post-deposition argon plasma surface treatments. We correlate the morphological parameters to light scattering in transmission and reflection, and identify the inclination angles of ZnO pyramids as the most pertinent parameter for thin-film silicon solar cells. We show that there are no reflection losses at the interface between as-grown rough ZnO and silicon, and we quantify the reflection losses for smoother interfaces. With titanium dioxide, produced by reactive magnetron sputtering, we demonstrate that for flat interfaces reflection losses of up to 7.4% can be canceled at 550 nm wavelength. We show that in microcrystalline silicon cells p-type silicon sub-oxide (SiOx) can also be used to reduce reflection losses, and at the same time can act as field-creating window layer. We develop a wide range of mixed-phase SiOx layers and with energy-filtered transmission electron microscopy we reveal the filamentous nanostructure of films produced with high hydrogen dilution of the precursor gas mixture. A partial decoupling of electrical and optical parameters is achieved for such films, where the transparency and the refractive index are determined mostly by the silicon oxide matrix, while sufficient transverse conductivity for the use in solar cells is maintained by few-nanometer-wide silicon filaments. In thin-film silicon solar cells, p-type SiOx~0.5 shows the best results as a window layer with enhanced transparency, while strongly phosphorous-doped n-type SiOx~1 is best suited for use as an intermediate reflecting layer in tandem cells due to a low refractive index of 1.8 with sufficient transverse conductivity. The tunable resistance of doped SiOx layers can be used at various locations in a Micromorph cell to quench shunts or bad diodes in spatially non-uniform devices. Implementation of SiOx with these three functionalities in Micromorph cells has significantly contributed to increasing the best initial efficiency from 11.8% in 2006 to 13.5% in 2010. While rough ZnO front electrodes and SiOx-based intermediate reflecting layers both help to improve Micromorph cell efficiencies, they render the devices more sensitive to water-vapor-induced degradation. With lock-in thermography measurements we demonstrate that water contamination affects mainly the cell borders resulting in non-uniform distribution of the local open circuit voltage. Finally we show how free-carrier absorption and surface plasmon polariton absorption can reduce the reflectivity of the back reflector, and we present preliminary results aimed toward the realization of an absorption-free back reflector making use of polished ZnO and sputtered silver layers.

EPFL2011

Comparison of periodic and random structures for scattering in thin-film microcrystalline silicon solar cells

Corsin Battaglia

Random structures are typically used for light trapping in thin-film silicon solar cells. However, theoretically periodic structures can outperform random structures in such applications. In this paper we compare random and periodic structures of similar shape. Both types of structure are based on atomic force microscopy (AFM) scans of a sputtered and etched ZnO layer. The absorption in a solar cell on both structures was calculated and compared to external quantum efficiency (EQE) measurements of samples fabricated on the random texture. Measured and simulated currents were found to be comparable. A scalar scattering approach was used to simulate random structures, the rigorous coupled wave analysis (RCWA) to simulate periodic structures. The length and height of random and periodic structures were scaled and changes in the photocurrent were investigated. A high height/length ratio seems beneficial for periodic and random structures. Very high currents were found for random structures with very high roughness. For periodic structures, current maxima were found for specific periods and heights. An optimized periodic structure had a period of Λ = 534 nm and a depth of d = 277 nm. The photocurrent of this structure was increased by 1.6 mA/cm2 or 15% relative compared to the initial (random) structure in the spectral range between 600 nm and 900 nm.