I am currently a Ph.D. student in the Photonic Materials group, led by Prof. Albert Polman, in the Center for Nanophotonics at AMOLF (Amsterdam, The Netherlands). My work focuses on the theory, fabrication and optical characterization of photonic nano- and microstructures for solar cells and optoelectronic devices.

## Light management for optoelectronic devices at the nanoscale

Optoelectronic devices are found in a very large number of customer markets today. These devices either absorb light and convert it into electricity (solar cells, photodetectors) or they emit light by using electricity (LEDs, displays). Improvements in material quality and device processing have resulted in a consistent increase in efficiency in these devices over the past years. It is now being realized that to these efficiencies can be increased even further by tailoring the surfaces and interfaces of these devices. These novel surfaces and interfaces have textures with features down to the nanometer scale, much smaller than the conventional micrometer-scale designs.

Using nanotextures, the optical response can be improved, but also becomes more complex. Traditionally, light-matter interaction, at the micron-scale and larger, is easily calculated using ray-optics, which is readily available at the industry level. In the ray-optics picture, the wavelength of the light is assumed to be much smaller than the features of the material, and therefore the light propagation is described by light rays. The light rays always travel in linear directions, and have a certain probability to reflect or transmit at an interface. Absorption of light rays occurs according to a fixed probability per unit length. The ray optics model explains reality very well for macroscopic geometries, but when the features of the materials become much smaller than the wavelength, the model breaks down. Instead, light propagation must be described by light waves. In the wave-optics picture, light is described by a wavelength and a propagation phase, which allows for interference between waves. The interference can be constructive or destructive, and as a result the light intensities can vary over distances much smaller than the wavelength. This, in turn, has a major effects on reflection, absorption, and transmission of light.

## Numerical wave-optics simulations

To calculate the propagation in the wave-optics model, one needs to solve Maxwell’s equations at the nanoscale. For regular materials, this can be done analytically. However, for irregular systems such as the nanotextures in optoelectronic devices, this needs to be done with a numerical simulation. Here, the material is modeled by a 3D grid of very small (a few nanometer) cells and Maxwell’s equations are solved to calculate the light fields in each cell using an iterative procedure. The calculations in the wave-optics model are mathematically more involved than those for the ray-optics model and require powerful computer hardware. We have studied the application and validity of analytical models in a recent paper [2], using our numerical wave-optics simulations as a benchmark.

**Get in touch with me through the following links: **

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Light Management in new Photovoltaic Materials research at AMOLF

t.veeken@amolf.nl

**Scientific publications:**

- Two-step sputter-hydrothermal synthesis of NaTaO3 thin films

L. Polak,**T.P.N. Veeken**, J. Houtkamp, M.J. Slaman, S.M. Kars, J.H. Rector, and R.J. Wijngaarden,*Thin Solid Films*,*603**(2016)* - Application and validity of the effective medium approximation to the optical properties of nanotextured silicon coated with a dielectric layer

T.H. Fung,**T. Veeken**, D. Payne, B. Vettil, A. Polman, and M. Abbott, Optics Express**27**, 38645 (2019) - Photovoltaics reaching for the Shockley-Queisser limit
B. Ehrler, E. Alarcon Llado, S.W. Tabernig,

**T. Veeken**, E.C. Garnett, and A. Polman, ACS Energy Lett.**5**, 3029 (2020)

**Conference presentations:**

- Waveguide-based spectrum-splitting concept for parallel-stacked tandem solar cells

**T. Veeken**, J. van de Groep, M.W. Knight, and A. Polman, SPIE Photonics Europe (2018) - Strong directional CdSe/ZnS core-shell quantum dot emission for tetracene/Si singlet-triplet down-conversion photovoltaics

**T. Veeken**, B. Daiber, H. Agrawal, A. Polman, and B. Ehrler, SPIE Photonics Europe (2020)

https://www.youtube.com/watch?v=zCq04la9ZNU&t=1s

- Directional Quantum Dot Emission for Tetracene/Si Singlet-Triplet Down-Conversion Photovoltaics and Luminescent Solar Concentrators

**T. Veeken**, B. Daiber, Kyra Orbons, H. Agrawal, B. Ehrler, and A. Polman, 47th IEEE Photovoltaic Specialists Conference (PVSC) (2020)