Archives for the month of: July, 2013

The Photovoltaic Specialist Conference (PVSC) offers a tremendous opportunity to any photovoltaic (PV) oriented researcher both young and old to convene at a single location and share their latest research results, meet new researchers and potentially new collaborators, catch-up with old colleagues or previous acquaintances in the field, and finally, keep up to date on the recent progress in the field of photovoltaics (or for young researchers, experience an in-depth introduction to the field). For me, it was my second time attending PVSC and I took advantage by participating in the presentation of some of my research group’s latest research results on dilute nitride solar cells, detailed balance predictions for quadruple junction solar cells, and spectral conversion affects with respect to thin film PV devices. I also had the opportunity to learn more about some research progress being made in thin film photovoltaics specifically on Cu(In,Ga)Se2, and on that note, I met a new potential collaborator: Dr. Angus Rockett from the University of Illinois (who happens to have the best name in the field of PV in my opinion).

Over the past year, my colleague Frederic Bouchard and I have embarked on an adventure of exploring the theoretical benefits of a novel multi-junction solar cell architecture which exploits the I-III-VI semiconductor material Cu(In,Ga)Se2 as the bottom sub-cell of a triple-junction solar cell, with the remaining materials composed of III-V semiconductors. A motivation for this novel design is the reduced costs of Cu(In,Ga)Se2 and its strong material properties for PV applications, as illustrated by its cell power conversion efficiencies of >20% demonstrated in the literature on low cost substrates. The first steps in developing a numerical model of Cu(In,Ga)Se2 was studying the material properties as a function of varying the stoichiometry of the material, i.e. changing the In to Ga content, and how these effect the optoelectronic characteristics of PV oriented devices. Throughout our learning process, we encountered a number of publications authored by Dr. Rockett who focused on the growth dynamics of Cu(In,Ga)Se2 using a hybrid sputtering and co-evaporation process under various growth conditions and with different group III ratios, namely the Ga to In content. Working alongside the well known Dr. Shafarman, they explored the performance dependence of polycrystalline Cu(In,Ga)Se2 solar cells as a function of this aforementioned group III ratio, since it greatly influences the optoelectronic properties of the material. Our interest in this was intricately linked to the poorer than expected performance of this material for high Ga content. Based on its bandgap as a function of Ga content, a detailed balance argument predicts that its performance should be higher for a molar fraction of Ga to In closer to 0.6. Alas, solar cells composed of such high Ga content have consistently demonstrated performances lower than those predicted by detailed balance, most likely due to changes in its material properties such as reduced minority carrier lifetimes potentially arising from larger cross-sectional trap states existing within the forbidden bandgap of the material at an energy level closer to the intrinsic level of the material. Modeling devices of different stoichiometries ranging from CuInSe2 to CuGaSe2 could potentially reveal some plausible physical phenomena responsible for this lower than expected performance. Furthermore, we had an important concern regarding the compatibility of Cu(In,Ga)Se2 with GaAs, a III-V semiconductor, using conventional epitaxial growth processes. On this note, Dr. Rockett is the only scientist in the field (as far as we know) that has successfully shown epitaxially grown Cu(In,Ga)Se2 on a GaAs substrate, which further allowed him and his research group to study the optoelectronic properties of the material in its monocrystalline state. This represents a huge stepping stone in achieving a multi-junction solar cell where the Cu(In,Ga)Se2 material would have to be in its monocrystalline state to enable high device efficiencies.

So after a year of research and the preliminary development of a numerical model for both poly- and monocrystalline Cu(In,Ga)Se2 solar cells, Fred and I finally felt confident and knowledgeable enough to communicate directly with Dr. Rockett and request some high level discussion. We thus invited him to a face-to-face discussion, and he agreed! So on Wednesday at around noon after Dr. Rockett’s talk on doping chalcopyrite materials (CuInSe2 and CuGaSe2) using nitrogen, we met and discussed for about an hour some of his work and how it related to our work. We learned a great deal about his motivation for his research and he also gave us some feedback on our approach to the development of our numerical models. We then discussed the novelty and challenges involving the integration of Cu(In,Ga)Se2 with GaAs for multi-junction solar cells. The discussion was very positive and outlined some of the topics Fred and I should focus on in continuing the development of our numerical models. Dr. Rockett also agreed to discuss with his colleague Jim Sykes in order to look into their respective databases of measured device characteristics as a function of Ga content and send us any meaningful data.

Midway through our discussions, a colleague of Dr. Rockett sat nearby, passively listening in to our discussions. Dr. Rebekah Feist, from Dow Chemical, then introduced herself near the end of our discussion and volunteered to assist Fred and I on calibrating our numerical models based on her research group’s effort of understanding the effects of Ga content on polycrystalline Cu(In,Ga)Se2 ­device performance. This was a key moment which was very much unexpected, and opened the door to another potential collaboration which I hope will benefit both parties. Presently, we are all in the process of communicating via email to setup the exchange of relevant data, such as voltage dependent quantum efficiency and temperature dependent current – voltage characteristics. These discussions might prove to be very beneficial to the development of our numerical models in order to study the aforementioned novel multi-junction solar cell architecture.

Alex Walker's picture-Alexandre Walker

Ph. D. Candidate (Year 4)

University of Ottawa’s SUNLAB

Department of Physics

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My name is Bruno and I’m a postdoctoral fellow in the group of Michel Côté  at the University of Montréal. Our group focuses on developing “beyond ab initio” numerical methods in order to model organic molecules, polymers and interfaces for applications in organic photovoltaics.

Organic materials offer great promises for photovoltaic applications, mainly because they would be very cheap to produce. Indeed, we are constantly surrounded by plastics of various kinds, so there already exists a large chemical industry that can handle vats of the stuff. Wouldn’t it be great if you could just buy 100 square meters of rolled up photovoltaic plastic films at a local hardware store, unfurl it on the roof at home and get some of the Sun’s sweet power for (almost) free?

Well that’s still science fiction at this point but our group’s goal is to help make it happen. There are very many possible organic compounds and polymers,and exponentially more possible combinations which could do the trick. It’s like looking for a needle in a haystack, really. My colleagues in organic chemistry tell me it takes a student one year to create a new compound; that’s a lot of resources lost if the compound turns out to be a dud. Chemists of course have great intuition, and can discard some unlikely candidates right away. However, it is not always so obvious from the start whether a given molecule will have all the right properties for photovoltaic applications; that’s where we come in.

It is much cheaper to simulate numerically the properties of a new candidate molecule than to actually cook it in the lab and then measure its properties. The theory behind those models isn’t perfect of course, and experimental measurements have the last word, but efficient simulation tools can allow us to scan large databases of candidates and discard the crazies, leaving only probable candidates for further investigation. The rub lies in having an algorithm which is precise
enough to be useful, but runs fast enough so we can simulate those large organic molecules! We have been working hard on developing such a method, and we hope
to produce exciting results soon!

Colleagues and I visited the group of Professor Holdcroft at Simon Fraser in Vancouver at the beginning of April (see our photojournal of the trip here ). From our very engaging discussions with Professor Holdcroft and his students emerged a fact well known to experimentalists, but perhaps sometimes glossed over by ab initio-ists such as myself: morphology is critical to good performance! Indeed, photovoltaic devices rely on bulk heterojunctions to harvest the Sun’s energy: an electron donor compound is mixed with an electron acceptor (often PCBM), forming an intricate network of domains at whose interfaces excitons must dissociate and through which charge carriers must percolate. Obviously the shape, size and orientations of these domains will affect device performance greatly, and knowing the energy levels of single molecules are not sufficient to fully characterize the performance one can hope to reach with a given device. Furthermore, the ideal material should self-assemble to the optimal geometry without excessive human intervention; the more complicated the steps that are needed, the higher the cost of the final product!

Therein lies a bit of a terra incognita for me. I think we can do a great job at predicting the properties of a single molecule or polymer, but how can we scale up our modeling, and use the appropriate microscopic information to predict the behavior of mesoscopic (or even macroscopic) devices? This is a topic I want to know more about! Thanks for reading, and it was great to see you all in Hamilton at the 2013 Next Generation Solar conference!

Bruno Rousseau

-Bruno Rousseau

Postdoctoral Fellow

Department of Physics, University of Montreal

How can one even begin to describe the APS March Meeting, the biggest material physics international conference of the year?

The sheer magnitude of a whole week with more than 40 simultaneous talks at all time can be vertiginous for its tens of thousands of attendees. Instead of spending this article on a ridiculously long exhaustive list of all the scientific results that were presented, I’ll instead concentrate on the biggest realization I’ve made during the week while preparing my talk. An underlying fact that everyone relates to, but few people ever talk about. The presence of faith in science.

Faith is a word that possesses a strong taboo in the scientific community. Indeed, science relies on verifiable, repeatable facts, and, most often than not, steps away from religion altogether. But faith isn’t really about religion. Faith is a great motivational tool, a strong emotion that scientists tend to forget all too quickly. Faith is at the core of everyone, and is indeed, at the core of science itself.

It’s not easy being a graduate student. One Ph.D. student out of two will never complete their studies. The pressure of one’s advisor or funding agencies to produce meaningful, useful and predictable results can be unbearable, and it’s very easy to lose track of your personal research when there are so many great people out here. And that’s where the paradox lies: there is no possible way to predict the results of scientific research. The hardest truth of all about science is that you don’t know what you’re searching for, or even where to find it.

But that’s where faith comes into play. Scientist as a career choice usually comes from a child’s faith in their desire to invent, to create, to understand and change the world. Every scientific experiment is rooted in faith in the outcome, or in the experiment’s success. Even mathematics as a whole, the very language of the universe, relies on faith in a few axioms on which we can build.

The popularity of networking or job search events at the March Meeting is simply ridiculous. With a great deal of chance and a lot of patience, I found myself participating in one of them in-between presentations. Even though the event was very interesting, there is one thing that couldn’t be shown in the pages of notes that I took, but yet was so visible I couldn’t not notice it: graduate students are scared. All of the ones present at the event, me included, were stuck into communicating in English, which wasn’t our first language. We had to interact constantly with dozens of people of all age and color, trying to befriend our competition and watch our public appearance; all while not knowing our place, our relevance, and for the most part, our future. The hardest challenge of all is the necessity to find faith when the universe seems forgetful and senseless. Because, as with love, it is possible to find unconditional faith without the need for a rational justification. Even for a scientist, whose whole job relies on finding rational justifications.

The good news is that the solution is everywhere. Science communication isn’t all about sharing information, filling forms, finding publishing deals, protecting patents and reading articles. At the core, it’s about meeting other people with similar interests. It’s about seeing passionate people ready to stand up to change the world on every level. People who to tell those who dare doubt them that they haven’t met them yet, and they haven’t met their friends. People talking about their dreams with stars in their eyes and excited about what the constantly uncertain future holds. It’s about people having faith, and trying to share it with the world.

Nicolas Berube

Nicolas Bérubé

Ph.D Candidate, Year 4

Department of Physics, University of Montreal