Thermodynamics and Photonics: a match made in California

Some of the best Science is that of sinple, elegant and seemingly outrageous ideas: the sort that relate to the basics of Physics.

At a recent plenary talk at Photon16 by Professor Shanhui Fan this is exactly what we got to see and hear. The idea of solar cells converting illumination (or what Shanhui calls positive illumiunation) into current is not new.

However he went a step further: negative illumination!

When a cell is dark, and if its temperature is higher than the ambient it must give away the excess energy to maintain thermodynamic equilibrium. Presto- current in the opposite direction compared to the daylight!

Ergo a cell that can generate electrical power in the day and the night!

To me this concept is beuatiful because it uses some fundamental and very simple Physics.

This sort of thinking that focuses not on technological aspects or narrow single disciplines alone but rather sees Science/Physics as a whole is what challenges status quo and leads to exciting new discoveries!


What do we know?

Today the Beeb website carried an article on how plants may be using quantum coherence to improve light harvesting in photosynthesis. You can read it here.

So what do we know?

From our understanding of Physics and the conditions we think determine whether an object operates in the classical/quantum regime, it had been assumed that plants could not sustain quantum states. And so the existence of quantum states/behaviour in plants is big deal. The Science article that the Beeb picks up on is of course not the first to delve into this matter. There has been other research in this area, see refs 1-8 in the Science article. Also you may recall that in my post Monday at Laser world of Photonics I had heard a talk by Nick van Hulst on this very topic!

But why should we care?

Well apart from these findings making us question our understanding of the natural world yet again and opening exciting new investigative journeys, there are huge applications for this in photovoltaics (PV). Renewable energy through solar cells has been a massively important for a number of years and still offers great potential for grid parity and satisfying some of our thirst for electricity. Amongst the issues that dog improvement in PV performance, is how we can cut down on the reflection of sunlight from the solar cell surface, and increase the quantum efficiency or conversion of this energy into electricity.

If we can understand how the absorbed sunlight in plants is transferred to a reaction centre with lower loss and in less time than through classical transport, we may be able to mimic such mechanisms in our solar cells. That would hopefully increase efficiency far beyond what we can achieve today.
Now we may well wonder how easy it would be to transfer such a mechanism of quantum energy transport observed in plants to inorganic structures in say Si solar cells? The answer is I don’t know. But its worth finding out!! And, also if it is more feasible to look to recreate this in organic solar cells though they don’t have proteins like the plants!

Either way I think its tremendously exciting.

Wednesday at the Laser World of Photonics

The highlight of the day for me was the talk by Shanhui Fan of Stanford University on nanophotonics in light management for thermal and solar applications.

He discussed in brief three different concepts. He started with the role of nanophotonic structures on solar cell surfaces and within solar cell structures, to reduce reflection and increase both the short circuit current and open circuit voltage! He then went on to talk about how while our focus is on getting energy from the sun, for applications such as cooling buildings (power for air conditioners) another more feasible approach would be to exploit the properties of a sink! The vacuum that surrounds us has an average temperature of 3K,so we could sink our heat to it. This is such an innovative thought! But how do we do this? Again he showed some results. By making certain nanophotonic gratings of SiC etc. it is possible to give them strong thermal emissivity in 8-13micron range in which the atmosphere is transparent- allowing this radiation to go away into space. This could be an effective way to cool buildings, extract heat! He also presented an idea on how thermal extraction enhancement that without breaking the Stefan Bolztmann law, still manages to improves the performance than a conventional black body!

Some papers cited include:

Nature communications, vol. 4, pp. 1730, 2013

Nano letters,Vol. 13, pp. 1451, 2013

Optics express, Vol. 21, pp. 1209, 2013

Nano Letters, Vol. 13, pp. 1616, 2012.

Happy reading!

The Solar Cell Diary- part II

Set up and benchmarking

This is continuing the experiment that I started with my post The Solar Cell Diary- part 1, on this blog, writing about a research project from its inception. I chose to write about Si solar cells with a surface micro-structure.

My experimental partners will be using an ‘integrating sphere’ layout. The first challenge was to see if we could set up our software, Lumerical, to mimic this layout or at least ensure that there is equivalence. After some thought we realised that by allowing the simulations to run for a sufficiently long time in Lumerical, the set up would indeed be equivalent.

Then onto the next step: benchmarking!

Dreaded word…. Yet unless a method, setup etc. are properly benchmarked, one cannot trust the results. The purpose of benchmarking is to test the method (in our case the FDTD in Lumerical) for some known problems to determine the accuracy of the method, its robustness and stability. Only once this is done, is it prudent to use the method on a problem where the solution is not known. Otherwise one risks, dealing with two overlapping signals: is the method inaccurate or is there a problem with the setup/structure/our analysis.

For us benchmarking was broken down into two main steps:

a)      Determining the positions of the source, monitor and PML

b)      Choosing the mesh and other calculation parameters properly.

solar cell- setup

Fig. 1: Set up for simulation of solar cell

For step a, through trial and error, we found the distance between the source, monitor and PML, such that reflections from the cell surface all have time to travel to the monitor (and be recorded), while they do not have enough time to reflect off the PML (even PML will reflect some small amount of incident radiation) and contaminate the readings. Our numerical set up is shown in Fig. 1 and indicates the distances as well as the travel time for pulses.

In Step b, we determined mesh parameters such as spacing between points in the x,y and z directions. Our aim was to find a set/s of values of dx, dy and dz, for which the error was within acceptable limits. To do this, we compared our results for Reflection from a plain solar cell with theoretical as well as known experimental results. We define error as the difference in the reflection we obtain and that of analytical calculations. Figure 2, shows one of these comparisons with a plot of error as a percentage, versus wavelength for different values of dz, while we keep dx and dy constant with a value of 0.25 micron.

benchmarking-planar solar cell

Fig. 2: Error in reflection as a function of wavelength for different dz.

In the figure, it can be seen that increasing dz even slightly produces fairly large error, especially at shorter wavelengths.  By decreasing dz, the resolution and accuracy increase, however so does the computation time and memory requirements. The latter, can increase dramatically. For realistic problems it is extremely important to determine the acceptable error and then find mesh parameters that are feasible computationally. In my next post on this topic I hope to put up some results relating to the actual microstructure on the cell surface.

Meanwhile, please do email me if you have comments!

The Solar Cell Diary – part 1

As I have stated at the bottom of the  ‘my research‘ page, I am keenly interested in solar cell research. I thought I’d do an experiment here on my blog: I’ll report from the very cartoon on solar energy beginning about a piece of research my team are starting on solar cells and provide update on the progress: the difficulties, the issues, successes etc. I hope that those of you who read the posts, will give their views and may be even suggestions.

So, on with it!

The work: Designing a Si solar cell with a microstructure on the surface with the express intention of reducing the reflection of solar light incident.

My team and our task: is to model the reflectance and absorption of light of this solar cell. We will be looking at which microstructure gives optimal performance and searching for those design parameters. We will use the FDTD as a simulation tool to model the evolution of the light as it is made incident on the cell (and then its reflection/absorption).

The key: this work is an amalgam of experimental and theoretical approaches. My collaborators have the facilities to fabricate and experimentally characterize the solar cell. They will make the cell and introduce the microstructure, then measure the Reflectance (R),Transmittance (T) of the cell. My job will be to supply them the cell and microstructure parameters that are the most suitable.

The first challenge: The experimental characterization (measuring R and T) set up uses an ‘integrating sphere’ layout. However, the software we are using, Lumerical, does not have a measuring monitor in that configuration. So we first need to figure out how we will perform simulations that will be equivalent to the experiment my colleagues will perform. If the two are not equivalent, we cannot look for a correspondence between the results.

Together we hope to come up with designs that have higher efficiency, lower costs and are feasible to fabricate.

So, look out for future updates, do write in with your ideas/suggestions/experiences.