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Advertising division: IEK-5 - Photovoltaics
Reference number: 2019M-070

Master Thesis: Aligned metal nanowires as anisotropic transparent contact for thin-film solar modules

Transparent contacts are an inherent ingredient in most photovoltaic cells. The transparent contacts often consist of doped metal oxides (e.g. ITO or ZnO:Al) in some cases supported by a metallic grid. Random networks of metal nanowires are an interesting alternative as transparent contact. The contacts require both a high conductivity and transparency, which generally leads to a tradeoff between the two. With the notable exception of grid supported transparent electrodes, most electrodes exhibit anisotropic conduction. However, in general, given the geometry of the solar cell, the currents through the electrodes follow one fixed direction. Hence the application of anisotropically conductive materials, which only conduct well in one desired direction, would allow a more favorable tradeoff between transparency and conductivity.

It is expected that anisotropic conductive electrodes have an impact on the electrical losses in a solar cell. Electrical losses in solar cells may roughly be divided in current losses (e.g. shunts, weak diodes), and voltage losses (e.g. series resistance, contact barriers). In the case that current losses are inhomogeneously distributed over the solar cell area, anisotropic conduction can mitigate the losses as it hampers the current flow toward the defected areas. On the other hand, losses due to series resistance will be aggravated as anisotropic conduction hampers the current flow around a defected area.

In thin-film solar cells we often predominantly observe current losses due to shunts as a direct consequence of their thin-film nature. Furthermore, we observe that in some cases local shunts get very hot, leading to further, permanent damage to the solar cell. For this reason we expect that, apart from the relaxed trade-off between transparency and conductivity, these types of solar cells may also benefit from mitigating current losses and hot-spot formation.

Some years ago, several processes to align the metal nanowires with anisotropic conductivity have been reported [1-3]. Among these, electrospinning is most interesting because it can result in meter-long nanowires. The dimensions of the wires and the anisotropic conductivity ideal match the requirements for thin film modules.

Aim and approach
The aim of this project is the demonstration of solar modules with anisotropically conductive contacts and the quantification of anisotropy effects on transparency and module performance. The nanowires will be prepared in collaboration with the Cologne University. All further processes and characterization methods are available at the IEK-5 in Jülich. Patterning processes for the nanowire coatings have to be adapted from literature [4]. Amorphous silicon solar cells will be prepared via plasma enhanced chemical vapor deposition of the silicon layers and magnetron sputtering for optional metal or metal oxide contact layers. The nanowires, coatings, and devices will be characterized in detail using a UV-VIS spectrophotometry, current/voltage measurements under various irradiation conditions, and luminescence and infrared imaging. The resulting material properties and solar module performance will serve as input parameters and references for device modeling, respectively.

[1] Wu, H.; Hu, L.; Rowell, M. W.; Kong, D.; Cha, J. J.; McDonough, J. R.; Zhu, J.; Yang, Y.; McGehee, M. D.; Cui, Y., Nano Letters 2010, 10 (10), 4242-4248.
[2] Fuh, Y.-K.; Lien, L.-C., Nanotechnology 2013, 24 (5), 055301.
[3] Kang, S.; Kim, T.; Cho, S.; Lee, Y.; Choe, A.; Walker, B.; Ko, S.-J.; Kim, J. Y.; Ko, H., Nano Letters 2015, 15 (12), 7933-7942.
[4] Henley, S. J.; Cann, M.; Jurewicz, I.; Dalton, A.; Milne, D., Nanoscale 2014, 6 (2), 946-952.

Dr. Jürgen Hüpkes
Institute of Energy and Climate Research – Photovoltaics (IEK-5)
Forschungszentrum Jülich
Telefon: + 49-2461-61-25 94

Send your application to:
Andrea Mülheims
Institute of Energy and Climate Research – Photovoltaics (IEK-5)
Forschungszentrum Jülich