Electrodes for transparent thin film solar cells

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Making perovskite solar cells more efficient

In recent years, hybrid organic-inorganic lead halide-based perovskite materials have been intensively studied for their application in a new generation of thin film solar cells. The certified power conversion efficiency of such perovskite solar cells (PSCs) has already exceeded 25%, approaching the record efficiency of dominant Si photovoltaics (PV) technology. An interesting peculiarity of the perovskite material is its color tunability. Changing its composition enables to vary the absorption bandgap in a well-controlled way. Having the perovskite absorber sandwiched between transparent electrodes enables thereby thin film solar cells that are by itself color tunable and even transparent. This opens new application areas for solar energy generation like placing them in windows of buildings or even cars. Another potential use is to combine such transparent perovskite solar cell as top cell above a Si bottom cell in a tandem configuration, due to its potential of exceeding 30% while combining the attractive properties of perovskites with the well-understood technologies for Si photovoltaics. High transparency of the electrodes in such device architectures is key for optimal user experience as well as performance. The visible part of the spectrum has to be transmitted well for best see-through experience in the one application while for the tandem configuration also the infrared part has to reach the underlying Si cell to achieve highest overall efficiency. Additionally, the transparency needs to be balanced with high conductivity as the electrode must be able to draw efficiently the generated electrical current of the solar cell to an external load. In this master thesis a new conductive oxide material will be investigated for its use as such transparent electrode. While commonly indium tin oxide (ITO) is used, here we will explore the indium zinc oxide (IZO) variant. This material is reported to have higher transparency compared to ITO thanks to its significantly higher charge mobility, which enables comparable conductivity at lower carrier density for reduced free carrier absorption. . A vacuum sputter process will be used to deposit the IZO layer, and parameters like oxygen partial pressure in the O2/Ar mixture, plasma power, sputter rate etc will be varied. Optical and electrical properties of the layers, deposited initially on the glass substrate and later in the effective solar cell architecture, will be characterized by ellipsometry, transmission-reflection, charge carrier mobility and resistivity measurements. When processing larger area devices (>1cm2), the impact of a metal grid enhancing the current collection will be investigated. The width of the metal fingers and their pitch will be varied whereby the solar cell performance is monitored. The aim of this thesis research is to validate IZO, eventually combined with a metal grid, as transparent electrode for perovskite solar cells. Quantifying the transparency of the full solar cell architecture, while maintaining high power conversion efficiency, will enable to select the application it suits best. A comparison with ITO will be made to assess whether it would be a valuable alternative to continue upscaling the process for transparent thin film solar cells and modules.

Type of project: Thesis

Duration: 6-12 months

Required degree: Master of Engineering Technology, Master of Science, Master of Engineering Science

Required background: Energy, Nanoscience & Nanotechnology, Electrotechnics/Electrical Engineering

Supervising scientist(s): For further information or for application, please contact: Yinghuan Kuang (Yinghuan.Kuang@imec.be)

Only for self-supporting students.

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