Polymer Brush Guided Formation of Conformal, Plasmonic Nanoparticle-Based Electrodes for Microwire Solar Cells

This report explores the use of sacrificial thin polymer films prepared by surface-initiated polymerization as a template for the fabrication of highly conformal metal nanoparticle solar cell electrodes. As a first proof-of-principle, the use of this method is demonstrated to prepare top electrodes on planar and microwire-based silicon solar cell devices. These metal nanoparticle films are dual functional in that they not only mediate charge transport, but also enhance light capture due to the plasmonic scattering properties of the nanoparticles. Solar cells with a conformal silver nanoparticle-based electrode layer show short circuit currents that are 46% higher as compared to those exhibit by devices coated with standard indium tin oxide as the electrode. It is anticipated that this methodology will contribute to novel electrode concepts in the next generation solar cells.

Computational Modeling of Thin-Film Silicon Solar Cells and Modules

Thomas Andreas Lanz

Solar photovoltaic energy production relies on the direct conversion of sunlight into electricity. Silicon thin-film solar cells represent a reliable and environmentally friendly technology to make photovoltaics economically viable. As these solar cells do not contain any rare or toxic materials they are an ideal candidate for the large scale deployment of solar energy. At the same time photovoltaic energy production is ideal for off-grid, stand-alone installations. This thesis presents new methods and results on computational engineering of thin-film silicon solar cells and modules. Computational engineering employs numerical methods and simulation techniques to address engineering challenges. Predictive numerical simulations allow for device optimizations that otherwise require substantial experimental effort and they may lead to new insights into the device physics which is the basis for further improvements in terms of cell efficiency and lifetime. We discuss the development of two new simulation methods and present several case studies that illustrate their use in solar cell characterization and optimization. The first presented method addresses optical simulations, i.e. the interaction of the solar cell with sunlight. Our optical model is based on the net-radiation method that considers incoherent, ray-like propagation of light. We extend the net-radiation method with thin-film optics to allow for arbitrary sequences of thick and thin layers. Experimentally, advanced techniques such as optimized surface textures have been developed to increase the absorption in the solar cell and thereby to increase the produced current. We illustrate how our optical model takes into account these textures by means of azimuth integrated light scattering properties to formulate a computationally efficient model of light propagation within the solar cell. Our optical model thus integrates coherent and incoherent propagation of light as well as a non-iterative treatment of ligth scattering. It bridges the gap between wave and ray optics that alternative approaches are most often facing. We first apply the method to study a recently proposed rear electrode architecture based on lithium fluoride and aluminum in amorphous silicon solar cells. Our model-based optical analysis identifies the increased reflectivity of this back electrode as its main advantage. Secondly, we then explore the potential performance gains in microcrystalline silicon solar cells that can be reached by eliminating absorption in layers that do not contribute to the photogeneration of solar cell current. Finally, our optical model is also suited for other solar cell technologies and we illustrate its extension and application to light extraction calculations in light emitting diodes (LEDs). Our optical model has been incorporated into the software package setfos, commercialized by Fluxim Inc. It is thus available to the research community. We then turn to address large area solar modules. A new method based on the finite element method (FEM) is presented to calculate the electrical charge and heat transport in these devices and how it is influenced by defects as well as partial shading. We identify such defects with surface temperature measurements by means of lock-in thermography and demonstrate how our model accounts for the thermal signature of localized defects. Taking into account the large aspect ratio of thin-film solar modules we show how geometrical projections can be used to reduce the model complexity, leading to a numerically efficient approach based on the finite element method. We demonstrate this by presenting full current-voltage curves of solar modules under partial shading.

EPFL2013

Influence of crystalline titanium oxide layer smoothness on the performance of inverted organic bilayer solar cells

Roland Hany, Thomas Andreas Lanz, Gaëtan Wicht, Hui Zhang

Due to the small exciton diffusion length in organic materials, the donor-acceptor heterointerface in simple bilayer solar cells must be placed in close proximity to the bottom electrode. This makes great demands on the planarity of the base layer, since a non-uniform topography can cause adverse shorting through overlying layers. We fabricated indium tin oxide (ITO)/titanium oxide (TiOx)/fullerene (C-60)/cyanine dye/molybdenum oxide (MoO3)/silver (Ag) solar cells with TiOx layers deposited via sputtering, coated from a nanoparticle suspension or prepared via a sol-gel process. A power conversion efficiency of 3.7% was measured when using a smooth sol-gel derived TiOx film. (C) 2013 AIP Publishing LLC.

Amer Inst Physics2013

Computational Modeling of Thin-Film Silicon Solar Cells and Modules

Thomas Andreas Lanz

EPFL2013