I recently had an opportunity to attend the 39th IEEE Photovoltaics Specialists Conference in Tampa, Florida (http://www.ieee-pvsc.org/PVSC39/). Since I am just starting to work in Photovoltaics, it was a great opportunity for me to get immersed in this very quickly developing field by listening to high quality presentations given by the leaders of the field. The presentations were divided into 11 topic areas, with a few sessions taking place at the same time. I really wanted to be in a few rooms simultaneously, but with my background in quantum theory I decided to focus mostly on attending sessions from research Area 1: Fundamentals and New Concepts of Future Technologies.

I found a talk by Megumi Yoshida from Imperial College London :“Progress towards Realizing Intermediate Band Solar Cell – Sequential Absorption of Photons in a Quantum Well Intermediate Band Solar Cell” particularly interesting. I was familiar with the concept of introducing the intermediate band (IB) into the solar cell to improve the absorption of photons with energy lower than the band gap energy (see Fig 1 (a)), however this talk took this concept one step further. The intermediate band solar cell (IBSC) can be created introducing quantum mechanically confined structures such as quantum wells and quantum dots [1] into the solar cell. In such cases, the IB arises from the confined states of the electrons in the conduction band (CB) potential.  In theory, by introducing the IB the current can be enhanced by two step absorption of long wavelength photons without reducing the voltage, hence leading to higher conversion efficiency (thermodynamical limit of 46.8% at 1 sun [2]), than a single bandgap solar cell (31.0% at 1 sun).

Figure 1 of Anna's blog

Fig. 1. Energy diagram of an (a) IBSC and (b) IBSC with photon rachet band (RB), in which extra photocurrent is produced due to sequential absorption of sub-bandgap photons increasing theoretical limit in the conversion efficiency. Reprinted with permission from [3]. Copyright 2012 AIP Publishing LLC.

 

However, experimentally obtained IBSCs suffer from a significant voltage loss resulting in lower than predicted theoretical conversion efficiency due to the short lifetime of electrons in the intermediate states. The short lifetime is caused by fast radiative and non-radiative recombination of carriers that occurs before the second photon can be absorbed. To achieve a long carrier lifetime of electrons in the IB, the authors suggest the introduction of a non-emissive (optically decoupled from the valance band (VB) [3]) “ratchet band (RB)” at an energy interval ΔE below the IB (shown in Fig. 1(b)). If there is fast thermal transition between the IB and the ratchet band, the photo-excited electrons in the IB rapidly relax into the RB where the lifetime of the carriers can be very long given that the RB is optically isolated from the VB. The increase in lifetime of the electrons enhances the probability of the second optical excitation process from the RB to the CB and increases the IB-CB generation rate. At the same time, the recombination rate of the electrons from the IB to the VB depends on the population in the IB. The presence of the RB reduces the population of carriers in the IB and the same the recombination rate leading to the increase of the photocurrent of the solar cell and its higher efficiency.

Figure 2 of Anna's blog

Fig. 2. Efficiency limit of photon ratchet IBSC of various concentrations as a function of ΔE. Efficiency at ΔE=0 corresponds to conventional IBSCs. Reprinted with permission from [3]. Copyright 2012 AIP Publishing LLC.

Authors calculate the globally optimised limiting efficiency of the photon ratchet IBSC as a function of the energy difference ΔE between the IB and RB. These dependencies are plotted for three solar concentrations in Fig. 2. Even though electrons lose the energy ΔE by transitioning from the IB to the RB, an increase in efficiency is visible as ΔE is increased. At the same time, the presence of the ratchet increases the below-bandgap and thermalization losses, but the associated efficiency gain through reduction in recombination is even larger. At 1 sun illumination, the efficiency of the IBSC is increased from 46.8% (ΔE = 0, conventional IBSC) to 48.5% with a photon ratchet at ΔE = 270 meV. At full concentration however, the introduction of the loss due to the presence of the ratchet band is not compensated by any other mechanism, leading to a decreased efficiency of the IBSC with the RB as compared to the standard IB cell (ΔE = 0).

Although it seems that the implementation of the photon ratchet IBSC is going to be challenging, I think that this concept is very interesting. Authors suggest that the RB can be built out states of indirect bandgap semiconductor separated in momentum space from the IB. Electrons would be first excited into a direct IB state, followed by a relaxation down through phonon emission to an indirect photon ratchet state, which is at a lower energy and separated by momentum k from the IB state, as well as the top of the VB.

Anna Trojnar's picture

 

-Anna Trojnar, Ph.D

Postdoctoral Fellow

Sunlab, University of Ottawa

 

[1] A. Marti, L. Cuadra, A. Luque, “Quantum dot intermediate band solar cell” Conference Record of the Twenty-Eighth IEEE Photovoltaic Specialists Conference, p940 (2000).

[2] A. Luque and A. Martí, “Increasing the efficiency of ideal solar cells by photon induced transitions at intermediate levels”, Phys. Rev. Lett. vol. 78, p5014 (1997).

[3] M. Yoshida, N. J. Ekins-Daukes, D. J. Farrell, C. C. Phillips, “Photon ratchet intermediate band solar cells,” Appl. Phys. Lett. vol. 100, p263902 (2012)

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The 39th Photovoltaic Specialists Conference in Tampa, Florida was a great conference. Photovoltaic specialists from all over the world gathered at the paradise of recreation, learning the most recent developments in this field delivered by hundreds of high quality presentations and posters. The topics span across eleven general areas from fundamental and new concepts of PV to the supporting of PV innovation with little to none overlap between each area. For me, an organic chemist focusing on developing new polymers to improve the stability of organic photovoltaics (OPV), the area that mostly interested me was, without a doubt, OPV.

Thursday of last week was the day for OPVs. In the morning, Professor Christian Körner from Heliatek GmbH gave a talk on the recent progress of organic solar cells [1].  Like a typical general review of this field, the talk started with some fundamental concepts and theories of organic semiconductors, then moved to some developments that aimed at solving the issues commonly encountered when applying organic materials to photovoltaic devices. He also covered recent improvements on the lifetime and module efficiencies of OPVs, which are not as much discussed as the efficiencies of lab scale devices. Not surprisingly, the materials presented in this talk are highly related to those being developed at Heliatek. With active materials based on small molecules and p-i-n structured devices, they could achieve 12% of photo conversion efficiency on lab scale devices and 9% on 100 cm2 scale modules that last as long as several thousand hours under intensified illumination. However, from an academic point of view, these technologies and accomplishment do not necessarily reflect the general recent progress of OPV in academia, where polymers and bulk heterojunctions are the dominant materials and device structure, respectively. Regarding the progress of OPV using conventional structures and material, there was an interesting presentation in the afternoon session given by an award winning Ph.D. candidate Biswajit Ray from Purdue University [2]. In his research work, he showed that the limiting factor of the charge transportation process are recombinations happening at the neutral, flat region of the band diagram. Conventionally, many people think the limiting factor is charge mobility. Although his evidence is not sufficient enough, it is helpful to look at a problem from a view that opposes the convention.

Apart from the presentation content, a new presentation technique was unveiled at one of the plenary talks. In it, the presenter used a facebook homepage to deliver his talk with the aid of the timeline function. At the end, an internet address that directs to that facebook page made all the slides publically accessible. It is notable that people’s attention was drawn to this presentation even though some of them were not familiar with the field.

It was a wise idea that the conference committee chose to hold it in Tampa Bay. It was the best time of the year to go there and to the beach where the water was warm and the sand was fine and soft like snow. Everyone from the network that went to the beach had a wonderful time there. Also, the Salvador Dalí Museum is definitely worth seeing, since it holds the largest number of Dalí’s masterpieces which will truly stun you.

[1] C. Körner, M. Hermenau, C. Elschner, C. Schunemann, S. Mogck, M. Riede, K. Leo, “Recent progress in organic solar cells: From a lab curiosity to a serious photovoltaic technology” in 2013 39th IEEE Photovoltaic Specialists Conference (PVSC), 2013.

[2] B. Ray and M. Alam “Role of charged defects on organic solar cell performance: Prospect of Heterojunction-free device design” in 2013 39th IEEE Photovoltaic Specialists Conference (PVSC), 2013.

Ben Zhang's picture

Chi Zhang (Ben)

Ph.D. Candidate, Year 3

Simon Fraser University, Burnaby, British Columbia

As Ontario’s flagship renewable energy (RE) incentive program, referred to as the Feed-in Tariff (FIT), enters its third review, it is an appropriate point to assess several recent political and policy changes which have implications for the next iteration of RE incentives. In my previous blog, I wrote about the challenges facing Photovoltaics (PV) with respect to the World Trade Organization’s (WTO) initial ruling against Ontario’s domestic content requirements, growing economic turbulence in the RE sector, political adjustments within the provincial Liberal party  as well as other industrial and policy factors that have created uncertainty surrounding the future deployment and development of PV in Ontario. These events have had lasting impacts on the prospects of PV and continue to influence future policy engagement. In conjunction with prior developments, there are new pressures and policy changes which will have serious implications for PV. Do these changes point to positive change for the future of PV?

Foremost among these pressures is the fallout around the WTO decision. In May 2013, the final ruling by the WTO stated that Ontario’s RE policy framework was in violation of international trade law[i]. The decision established that the domestic content requirement, which requires that developers source a certain percentage of RE system components from Ontario companies in order to be eligible for FIT incentives, contravenes international trade agreements. Without protection from international competition, PV companies who have invested in Ontario face an uncertain future. Will PV panel manufacturers be able to compete with Asian firms? Which parts of the PV value chain will Ontario firms occupy? Will we purchase technology from overseas and focus primarily on project installation? These are just some of the questions left unanswered as the government begins to reinvent the FIT.

The provincial government has already communicated significant changes to the FIT, which may portent further revisions presently being contemplated. In June 2013, the government announced that large RE projects (>500 kW) would no longer fall under the FIT mechanism[ii]. Instead, these projects would be contracted using a competitive procurement process. The details of this process have yet to be articulated. Moreover, the annual cap for FIT contracts has been reduced from 250 MW to 200 MW[iii]. Aside from the FIT, the political administration has also renegotiated its deal to procure a significant quantity of RE from Samsung, cutting the contract from 2,500 MW down to 1,369 MW[iv]. From this, it would appear that a deliberate effort is being made to slow further RE procurement and deployment in the province.

Political pressures are also quite notable. First there is the continued uncertainty arising from a minority government situation, where opposing parties have stated they would either abandon[v] or seriously redesign[vi] RE policies. Second, controversial gas plant cancellations continue to plague the governing party, casting shadows over the energy file and the commitment to RE deployment[vii]. Lastly, and perhaps most importantly, there are still many unanswered questions regarding the future of nuclear energy in the province[viii]. If Ontario commits to build new nuclear reactors, there is little impetus to invest in RE and efficiency.

Together, the above factors indicate that there is increasing uncertainty surrounding the RE sector in the province. Recent developments suggest that Ontario is veering away from its commitment to an economy and energy system based on RE. So, the worst may have yet to come for PV in Ontario.

Danny Rosenbloom

Daniel Rosenbloom
Ph.D Candidate

Carleton University


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

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

For a few years now, I have been following high impact research in the field of photovoltaics. I have read hundreds of scientific papers and performed complex calculations. However, I have to admit that I found most of this work relatively straightforward. As a scientist, I am highly interested in new results in my field, especially topics I don’t fully understand. Hence, learning the complexities in the field of renewable energy was second nature for me.

I am currently facing the hardest part of the Ph.D. in my opinion: the process of thesis writing. Initially, I had a lot of difficulty in starting to write since there were so many other things that needed to be done! I concluded that I wasn’t going to write efficiently if I didn’t find a solution. So I stopped my daily routine, sat down and thought about how I could increase my writing throughput. And this is what I want to discuss in this short blog. Instead of writing about my research within the field of photovoltaics in Canada, I decided to talk a little bit about my own writing process, wishing that it might help fellow graduate students in Canada, and even possibly abroad.

To help my writing process, I had to start somewhere. I began some preliminary research online, but it did not lead anywhere. I then decided to change my strategy. I decided to just sit down, and read a book. But it wasn’t any random book. It was “How to write a lot; A practical guide to productive academic writing” by Silvia [1]. I was expecting a long and arduous read. On the contrary, I was pleasantly surprised to find the book written in a personal tone, which made it entertaining and easy to read. It is short enough to be read fairly rapidly, but it is packed with motivating tips and rules to become a good writer. Even though it is written by an academic psychologist, it is applicable to pretty much any field. I believe this book was the turning point in writing my thesis.

The book focuses on many writing topics, but the one I found most important part was creating a writing schedule so as to allocate blocks of time to write during each week. Before I read this book, I usually managed my time another way: I would enter the lab at 9am, sit down, and write a list of things to do for the day. For the last few months, there was always ‘to-do’ point that kept coming back to haunt me: “Start writing the infamous thesis”. However, the actual ‘to-do’ list always seems to be too long as there is so much lab work to be done; I have gradually developed a lot of responsibilities in the research group over the years fixing instrumentation, calling companies, lab mates stopping by to ask questions about general scientific problems, meetings to attend, papers to read, etc… So adding “Write thesis” to the “to-do” list every day achieved nothing. The key to my success was to change this behavior.

I decided to use the author’s advice: before doing anything in your workday, schedule 2 hours of writing. Hence, between 8 and 10am, I sit down in front of my computer at home and simply write. This made a huge difference. Forcing myself to follow a planned, scheduled writing time changed everything. Being isolated helped a lot, since working around my colleagues is very distracting. To make those writing sessions more efficient, I had to take it a step further. I personally programmed my computer to not have access to the internet during those hours. Getting lost in your emails and various websites is so easy these days, and with my tendency of having an attention deficit does not help. At the beginning, I thought that I needed the internet to write the thesis. After taking this drastic measure, I realised that it was both true and untrue. I actually know most of the important information to write my thesis on the top of my head. What I need to do is to write as much as I can during the 2 hours session, and note any parts that need some online research. I then write these points on my ‘to-do’ list for the day. Using this schedule, I realised that I was doing as much work in the lab as before, but with the added benefit that my thesis was advancing more quickly than I had initially expected.

One problem that I often faced when I was in undergrad was the ‘blank page syndrome’. I could watch the white computer screen for hours before starting to write. This time, it was different. I didn’t wait for the inspiration to come; I actually forced it on myself. Even though I had no idea what to write, I just wrote freely on the subject. Then, the inspiration came after a few paragraphs. And I have to admit that I was surprised on how the first draft was not as horrible as I was expecting. After a while, I started using a few tricks that worked well for me. Since I consider myself a good communicator, I started using this particular talent for the writing process. To organize my thoughts and the flow of each chapter in my thesis, I am creating a PowerPoint presentation just like I would present for a conference. I then write the chapters much like I would present in front of a crowd of scientists. I just have to edit to make it readable and professional.

I have to admit that here, on a computer screen, it might sound easy. It is not. And I realised that the motivation is probably the biggest driving factor in the writing process. We are human after all, and a little reward is always good. Hence, I decided that I would have the right to a very good bottle of wine after the final touches of each chapter. That makes 5 bottles for the Ph.D. thesis; a good reward in my opinion.

In this adventure that is the Ph.D, I have been very lucky to be a part of the PVIN network with plenty of very interesting people. I am certain that I am not the only one in the Network who is in the writing process. I hope these few tips might help some of you!

Olivier Theriault

Olivier Thériault

Ph.D Candidate, Year 4

Sunlab, University of Ottawa

 

References

[1]. Silvia, Paul J. “How to write a lot: A practical guide to productive academic writing.” American Psychological Association (2007).

I recently attended the 2013 Materials Research Society’s (MRS) Spring Meeting from April 1st  to 5th in San Francisco, California. The MRS brings together members of industry, academia, and government to discuss the latest in materials research across a wide variety of disciplines. There were 56 parallel technical sessions, an exhibit, and a wide variety of tutorial sessions taught by leading scientists and engineers.  I presented a poster entitled, “Flux engineering for height dependent morphological control of branched nanowires” in a section focused on nanostructured semiconductors and nanotechnology. I attended talks primarily focused on nanowire growth and applications. Numerous talks focused on the use of nanowires in photovoltaic devices that I believe are of interest to the Canadian Photovoltaic Innovation Network.  Here I will briefly discuss a couple of highlights.

Results from a paper recently published in Science detailing high performance solar cells consisting of nanowire arrays were presented by a member of The Nanostructure Consortium at Lund University in Sweden.1  P-i-n junction indium phosphide nanowire arrays were employed in the devices, resulting in a maximal efficiency of 13.8% at one sun. InP nanowires have extremely low surface recombination velocities, removing the need for surface passivation as required by nanowire composed of alternative materials (such as Si). Interestingly, the devices exhibited short circuit current densities at 83% of the highest performance planar InP cells, while only covering 12% of the surface (as compared to 100% surface coverage in planar devices). The authors concluded that ray optics is not suitable to model the interaction of light with subwavelength nanostructures due to resonant light trapping. As a result, the authors suggested that nanowire PV devices could potentially reduce the amount of material required to fabricate cells by producing photocurrents comparable to planar devices.

An interesting talk entitled, “Band-gap and structural engineering of semiconductor metal oxides for solar energy conversion,” described the use of 1-D nanostructures (nanowires) to serve as direct pathways for charge extraction in dye-sensitized solar cells (DSSCs).2 In this work, zinc oxide (ZnO) nanowires were used due to their high electron mobility. In a typical nanoparticle film, electrons undergo “zig-zag” transport, increasing transport time and the probability for recombination or trapping. As a result, much of the generated charge carriers are not collected, leading to low performance. Direct “straight-line” conductive pathways are provided for electrons by implanting ZnO nanowires into the nanoparticle film. As a result, charge collection efficiency is significantly improved.  The implementation of ZnO nanowires improved efficiency in DSSC devices by 26.9% in the best performing device.

References

1. Wallentin, J. et al. Science 339, 1057, (2013).

2. Bai, Y. et al. Advanced Materials 24, 5850, (2012).

Allan Beaudry

-Allan Beaudry

Ph.D Candidate, Year 2

Electrical and Computer Engineering Department, University of Alberta

Last year I had the opportunity to attend two big international Photovoltaics (PV) conferences.  The keynote speeches at both conferences discussed the future of crystalline silicon (c-Si) and ideas for high efficiency low cost c-Si PV technology. With less expensive organic PVs in the market and efficiency mark of thick crystalline silicon cells jammed at ~ 26% , these issues have been the hottest break-time discussion topics among people working in c-Si PV.

Recently, a very interesting article on “exotic silicon” by researchers at University of California, Davis on January 25, 2013 in Physical Review letters stirred up excitement in c-Si PV world. This exotic silicon, also called BC8 silicon, is a type of silicon that can be formed under extremely high pressure and is still capable of maintaining its stability at normal pressures. So what’s exotic about this silicon?? It can produce multiple electron hole pairs per incident photon in contrast with one e-h pair/photon generation in normal c-Si! The simulations run through the National Energy Research Scientific Supercomputing Center at the Lawrence Berkeley Laboratory predicted ~ 42 % efficiency in BC8 silicon solar cells under one sun that can be increased to ~ 70% by concentrating sunlight on the cell.

Wondering if you can actually make this exotic silicon? The answer is yes! Joint research between MIT and Harvard University shows that one can convert ordinary c-Si into high efficiency exotic silicon merely by shining it with laser light or by applying chemical pressure.

Conclusion: Silicon is an exotic element.  I think c-si will keep holding its share in future PV market with its surprising properties and contribution from researchers.

Check out the links and papers below for more information on this exciting research

  1. Lin, Yu-Ting,  Sher, Meng-Ju, Winkler, Mark T., Mazur, Eric,  Gradecak, Silvija Pressure-induced phase transformations during femtosecond-laser doping of silicon, Journal of Applied Phyics 5-110 (2011)
  2. Sher, Meng-Ju, Franta, Benjamin, Lin, Yu-Ting, Mazur, Eric, Gradecak, Silvija The origins of pressure-induced phase transformations during the surface texturing of silicon using femtosecond laser irradiation, Journal of Applied Phyics 8-112 (2012)
  3. http://www.energymatters.com.au/index.php?main_page=news_article&article_id=3572

Kitty Kumar

-Kitty Kumar

Ph.D Candidate, Year 4

University of Toronto, Toronto, Ontario

We are constantly inundated with projections, estimates, and forecasts of our future global and photovoltaic (PV) energy requirements. How much energy will we need by 2050? How much of our energy should come from PV? How much PV capacity can we install by 2020? Will that be enough?

I thought it would be interesting to see how we’ve done on our last decade of projections for global PV installations. The European Photovoltaic Industry Association (EPIA) is among various organizations that project and report the status of the PV market. From a series of EPIA’s Solar Generation (SG) reports ranging from SG1 (2001) to SG6 (2011) I looked at year-over-year projections for total global installed PV capacity, and plotted them alongside the actual installed values (see Figure 1).

Figure 1 for Ryan Tucker's blog

Figure 1 Cumulative installed photovoltaic capacity values and estimates from several EPIA reports.

It turns out that we’ve consistently beat the projections by large margins. In fact, the 2010 installed value (>39 GW) is more than a factor of three higher than the 2001 projections for that year (~13 GW). We can see from the plot in logarithmic scale that the installed capacity keeps jumping off of the exponential prediction curves.

The industry has made well on its promises. We must realize that the solar industry is in an enormous state of growth, but only because we are so far behind. This exponential growth is not sustainable for the industry. Certainly there has been amazing growth in the past decade, and we hope that this trend will continue for the following decade or two. Beyond that, let’s just hope that the total capacity is a significant portion of the global energy supply, and the industry can happily transition into a more steady growth state. In the meantime, I’m happy to take every megawatt of PV that we can get!

 

Ryan Tucker

-Ryan Tucker

Ph.D Candidate, Year 4

Department of Electrical Engineering, University of Alberta