Archives for the month of: November, 2013

I recently had the opportunity (and pleasure!) of attending the 39th Photovoltaics Specialists Conference (PVSC) in Tampa, Florida. The conference, which is put on by the Institute of Electrical and Electronics Engineers (IEEE), is an annual meeting of scientists and engineers who work in the field of solar energy. PVSC attracts people from all over the world to come and share their research on some of the cutting-edge topics in the field. In this entry I will be providing some highlights of the trip, especially topics that were of interest to me.

I once again had the honour of hosting (alongside other HQP) the Photovoltaic Innovation Network (PVIN) booth. As I have mentioned in previous blog entries, the booth is where we get to represent Canada’s research in photovoltaics. A lot of companies and Universities outside of Canada do not know exactly what we do up in Canada in regards to photovoltaic research, so the booth gives us an opportunity to educate them and get them interested in our work. There are always interesting discussions to be had at the booth with researchers from all over the world!

The conference itself had a plethora of talks which covered almost every aspect of photovoltaic research. It really is amazing just how many different technologies there are for capturing light from the sun and turning it into electricity; so many that it can make one’s head spin! There were discussions on existing technology that has been around since the dawn of photovoltaics such as crystalline silicon, as well as proposals and theories for ideas that have not yet come to fruition, such as hot carrier solar cells. Two talks in particular stood out to me because of their relevance to my own research (light trapping in ultra-thin crystalline silicon). One talk, given by Nicholas Hylton from the Imperial College of London, was on using aluminum nanoparticles (microscopic spheres that are only 100 nm in diameter) for light trapping in solar cells. This type of light trapping is known as plasmonic light trapping, and a lot of research has been devoted to the field in recent years. The novelty of their approach was to use aluminum instead of the more commonly used silver and gold. The issue with using silver and gold is that although they are useful for trapping light towards the red end of the solar spectrum, they are detrimental at the blue end since they soak up a lot of the light there. Aluminum does not have this issue, and the researchers were able to demonstrate an enhancement in light absorption across the whole spectrum!

Another talk that interested me was given by Joao Serra from Universidade de Lisboa in Portugal. His talk, titled “Comparative Study of Stress Inducing Layers to Produce Kerfless Thin Wafers by the Slim-Cut Technique”, focused on the fabrication of ultra-thin (thinner than 100 micrometers) silicon wafers from thicker wafers. Using ultra-thin wafers for silicon solar cells has become attractive in recent years because of the potential to cut costs by using less silicon. Serra’s talk discussed a method for making such wafers. The process involves laying down a layer of epoxy on top of a thick wafer, then heating and cooling opposite sides of the wafer. During the heating/cooling process, a thin layer (he discussed 60 micrometer thick layers in his talk) peels away from the wafer. The advantage to this process is that thin silicon wafers can be fabricated without losing any silicon in the process. Standard wafers are usually made by sawing from a large silicon ingot; the sawing process naturally destroys useful silicon in the process.

I would have to the say my favourite technical aspect of the trip was a discussion that Martin Gerber and I had with Keith Emery, the winner of this year’s William R. Cherry Award. The award is given to “an individual engineer or scientist who devoted a part of their professional life to the advancement of the science and technology of photovoltaic energy conversion”. Keith gave me a very useful suggestion for an undergraduate lab we run here at McMaster University. The lab allows undergrads to fabricate a PERL cell, the solar cell that holds the world record efficiency for single junction silicon cells. Being a record breaking cell, it is naturally very complicated to make and students have had very little success in achieving reasonable performance from them. We shared this with Keith and he suggested that a PERL cell is far too ambitious for an undergraduate lab. He suggested instead that we make far simpler cells. Although it may appear to be a simple suggestion, it really got my thinking about how we can make the lab a better learning experience for students. I will now be working on redesigning this lab around the concept of a simpler solar cell, and owe my inspiration to Keith!

The conference was by far not the only fun part of the trip. I mean, it is summer time and we were in Florida, you can’t really get much better than that! When we weren’t attending the conference we were at the pool, wandering about Tampa, or dining at the many restaurants they had down there. On one day we went down to the beach in the neighbouring city of Clearwater. I’ve never swam in the ocean before, and I don’t think I’ve ever swam in a body of water that was that warm! Overall the trip to Florida was great, and sometimes it felt more like we were on vacation and not on a business trip! I would like to thank Jennifer Briand for organizing this excursion for us, and for all the HQP who attended for the great company and discussions we had.

Kevin's picture

-Kevin Boyd

Ph.D Candidate, Year 1

Department of Engineering Physics

McMaster University.

A subject that caught my attention during the 39th edition of the Photovoltaic Specialist Conference (PVSC) was a poster from Toshiba Corporation [1] about the study of a homojunction CIGS (Copper Indium Gallium Selenium) solar cell. CIGS solar cells are gaining more and more interest in the photovoltaic community as a thin film solar cell due to the material’s high absorptivity, low cost and relatively high power conversion efficiency. 

Standard CIGS solar cell consists of a p-type CIGS base, n-type CdS emitter and a ZnO transparent conductive oxide. This heterojunction between CIGS and CdS results in a conduction band offset. The heterojunction structure is used due to the fact that it is hard to get high enough levels of n-type doping in CIGS. P-type doping in CIGS is usually done intrinsically through Cu vacancies, which act as acceptors. To achieve n-type doping, a donor material would need to be introduced into CIGS. 

In their poster, the Toshiba corporation group reported achieving n-type CIGS with CdS doping up to a level of 1×1016 cm-2. They were able to achieve a high enough doping level to use a CIGS layer as the emitter in a homojunction CIGS solar cell. From this point, they have grown a homojunction CIGS solar cell leading to a power conversion efficiency of 17.2% (See J-V characteristics). This relatively high efficiency device shows the feasibility of a CIGS homojunction. Another talk about n-type CIGS was given by Professor Angus Rockett [2]. This talk was about nitrogen doped CIGS. He was mentioning the possibility in the future to achieve high level of doping in CIGS in order to eventually obtain CIGS tunnel junctions. From a modelling perspective, these two ideas could lead into very interesting designs of a CIGS tandem cell consisting of homojunction subcells. One of the main advantages of CIGS is the fact that it has a tuneable bandgap ranging from 1.0eV to 1.7eV which covers a good portion of the solar spectrum. The change in the bandgap can be achieved by varying the molar fraction x, corresponding to the gallium to indium ratio in CuIn1-xGaxSe2.  A high bandgap CIGS subcell on top of a low bandgap CIGS subcell connected together in series with a CIGS tunnel junction could lead into high power conversion efficiency while potentially reducing the cost compared to III-V based multi-junction solar cell. This type of technology is certainly not achievable in the short term, but it could definitely be an interesting exercise to model this type of device in order to have an idea of what level of efficiency we could potentially achieve from it.

Fred Bouchard

 

-Frédéric Bouchard

Undergraduate student, Year 4

Sunlab, University of Ottawa

References:

[1] N. Nakagawa, et al., “Feasibility study of homojunction CIGS solar cells”, 39th IEEE PVSC, 2013.

[2] A. Rockett, et al., “Nitrogen doped chalcopyrites as contacts to CdTe photovoltaics”, 39th IEEE PVSC, 2013.