Nanowire Photovoltaics at the Materials Research Society’s 2013 Spring Meeting

I recently attended the 2013 Materials Research Society’s (MRS) Spring Meeting from April 1^st  to 5^th 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

Ph.D Candidate, Year 2

Electrical and Computer Engineering Department, University of Alberta

Future of Crystalline Silicon Photovoltaics

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

Ph.D Candidate, Year 4

University of Toronto, Toronto, Ontario

How Have We Measured Up?

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 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

Ph.D Candidate, Year 4

Department of Electrical Engineering, University of Alberta

 

CanSIA 2012-Solar Energy within a National Energy Strategy

Panelists:

David Brochu - Vice President Development, North America, Recurrent Energy

F. Michael Cleland - Nexen Executive in Residence, Canada West Fountation

Senator Grant Mitchell - Vice Char, Standing Committee on Energy, the Environment and Natural Resources, Liberal Senator, Alberta, Senate of Canada

Jon Kieran - Director, Development, EDF, EN Canada Inc.

Christian Vachon - President, Enerconcept Technologies

CanSIA concluded with a panel discussion on the development of a national energy strategy. The panelists consisted of David Brochu of Recurrent Energy, F. Michael Cleland of Nexen, Senator Grant Mitchell, Jon Kieran of EDF EN and Christian Vachon of Enerconcept. All members of the panel had the opportunity to express their opinions on how we need to proceed as a nation towards developing our energy strategy.

Four years ago Canada entered the Kyoto protocol in an effort to curb human-generated green house gas (GHG) emissions. Entering Kyoto was a move in the right direction for Canada, but ultimately we developed an unrealistic plan that we could not uphold. After failing to meet target reductions of GHG’s, Canada withdrew from the Kyoto protocol at the end of 2011. Canada needs to learn from its mistakes and work on developing a national energy strategy that will benefit Canadians.

What would a national energy strategy look like, and how would we get there? We could start by increasing engagement and advocation for development of an energy strategy for Canada. Promotion of open discussion to define what is important to Canadians in a energy strategy needs to occur. Why do we need a national energy strategy?  How much do we focus on making renewables a central part of our energy policy? Should we be putting a price on carbon, and would a carbon tax hurt Canada? How can we focus on the longevity and long term views of an energy strategy for Canada? There are many questions to be answered with no readily apparent solutions.

Where do solar energy and other alternative energy sources fit within Canada’s national energy strategy? Following the success of the feed in tariff (FIT) and micro-FIT programs, Canada has developed FIT 2.0  which will put another 160MW of solar energy online. The arrival of FIT 2.0 was not a surprise, but we will likely be seeing less government subsidy of solar projects. We cannot be reliant on a technology that requires subsidy to be sustainable. Fortunately, we have already seen instances where solar energy can be produced at grid parity. With the dropping prices of solar energy and the ever escalating price of non-renewable energies, it is essential for solar to be a large part of Canada’s energy strategy. With the help of the FIT programs, Canada aims to be recognized as a leader in the installation and manufacturing of solar modules.

Canada needs to sculpt an energy strategy that can drive our economy, promoting growth in an underdeveloped sector. An energy strategy that will advance job growth within Canada, and ultimately will lead to a more sustainable country.

 

-Matthew Schuster

M.ASc

Queens University

CanSIA 2012-Integrating Solar Thermal with Geothermal

Thanks to the Photovoltaic Innovation Network, I participated in Solar Canada 2012 in Toronto, Ontario. This conference/exhibition is the largest national solar event in Canada, and is hosted by the Canadian Solar Industry Association (CanSIA). This year the event was quite large, in part due to the fact that it was CanSIA’s 20^th anniversary.

In this conference, I participated in some talks and visited some booths; one of the talks that was really interesting to me was about Solar thermal, Geo-thermal and the opportunity to integrate these two technologies together. Solar thermal installations consist of a solar thermal collector on the roof, a control unit with a pump and a potable water storage tank. The collector absorbs the light from the sun and converts it into heat. This heat is transferred to a liquid which circulates through the collector and down into the solar storage tank (fig-1). There are a lot of solar thermal projects within Canada (as they can easily deployed, even in residential areas), but one of the biggest is at Oxford Gardens retirement home in Woodstock, Ontario. This solar thermal project is saving on air conditioning costs by up to 40%, or approximately $20,000 per year; for heat savings, up to 60% or approximately $40,000 per year, according to Suni Ball from Proterra Solar[1].

 

    A geothermal heat pump, ground source heat pump (GSHP), or ground heat pump is a central heating and/or cooling system that pumps heat to or from the ground. It uses the earth as a heat source (in the winter) or a heat sink (in the summer). Heat pumps provide winter heating by extracting heat from a source and transferring it into a building. In the summer, the process can be reversed so the heat pump extracts heat from the building and transfers it to the ground. Transferring heat to a cooler space takes less energy, so the cooling efficiency of the heat pump gains benefits from the lower ground temperature (fig-2).

 

The combination of these two systems has many benefits [2]:

-Both systems don’t use fossil fuels at the point of use

-Geothermal is the backup for the solar thermal while the Geo-thermal can also provide cooling.

-Large flexibility in the heating appliances that can be used with both systems.

-Using the geothermal loop field as a storage tank to absorb the excess solar energy in the summer.  This advantage allows you to oversize the solar thermal system and increase the solar thermal contribution to the winter heating.

A study has been done [3] for the viability of a combined system in Milton, Ontario. This study shows that a combined system is feasible for space conditioning. For the house in this study, the seasonal solar thermal energy storage in the ground was sufficient to offset the large amount of Geo-thermal pump system length that would have been required in conventional systems. They showed that the economic benefit of such system depends on climate, as well as borehole drilling cost.

To conclude, a hybrid Solar-Geo-thermal system could be an outstanding solution to the high demand of energy in today’s world. It has a lot of benefits like sustainability, being clean (non-polluting) and having the ability to work all year round. Another important benefit is the possibility of using this system for all kinds of applications such as residential, commercial and industrial.

-Farbod Ghods

Ph.D Candidate, 1st Year

Department of Engineering Physics

McMaster University

References

[1]-
http://oxfordgardenssolarproject.com

[2]-
http://www.dma-eng.com/

[3]- Rad et al, COMBINED SOLAR THERMAL AND GROUND SOURCE HEAT PUMP SYSTEM, Eleventh International IBPSA conference, Glasgow, Scotland, July 2009.

CanSIA 2012-Solar PV Technologies and Innovations: Building an Export Market

Chair
Ian MacLellan, President and CEO, Ubiquity Solar Inc.
Panelists
Nic Morgan, Co-founder and VP Business Development, Morgan Solar
Jan Dressel, President & Managing Director, SPARQ Systems Inc
Ray Morgan, Director Outreach, PV/Solar & Semiconductor, SEMI Americas
Rafael Kleiman, Professor, Director, McMaster University
Clemens van Zeyl, CEO & Co-Founder, ARDA Power Inc.

 

An interesting panel discussion took place on innovation. The panel discussed the meaning of innovation from different points of view. Everyone agreed that Solar is happening faster than everyone expected. In 2001, it was predicted that the world market for new installations in 2010 would be 2.8GW. In 2006, the prediction was increased to 5.5GW. The actual result for new installations in 2010 was 16.8GW. According to PV experience curve, PV module price is estimated to be as low as $0.15/W by 2050. For more information, check out the white paper issued by CanSIA here.

Innovation trends for PV:

• Silicon is and will continue to be the main PV technology, giving a hard time to thin film technology.
• Organic PV might be a player in 20-30 years for specific applications
• Improving reliability in manufacturing yield and PV life time
• Integration of solar systems in commercial buildings by removing the inverter, ie: DC power lines as most instruments work on DC.

Finally, to go the last mile in innovation, it has to be on the system level!

Ahmed Gabr

PhD Candidate, 2nd Year

SUNLAB – University of Ottawa

 

The Thinner the Greener

Several presentations at the European Photovoltaic Conference 2012 in Frankfurt, Germany, including those of Prof. Harry Atwater, illustrate recent breakthroughs in the area of high-efficiency thin film solar cells. One of the most interesting developments is that researchers are beginning to consider materials which have not been used conventionally as a thin film.

Absorber materials of a high efficiency solar cell typically comprise a significant fraction (~50%) of the total cell cost. One simple way to reduce the cell cost is to use less material. Processing solar cells with thin layers can present handling challenges for some of the materials – breakage being one of the primary issues. Nevertheless, thin materials that are flexible can enable versatility in production, such as roll-to-roll processing, and hence can significantly reduce the processing cost.

Alta Device^[1] fabricates solar cells using a few micron thick gallium arsenide absorber layer. However, gallium arsenide is extremely expensive to use in large area solar cells, and thin films of this material tend to be fragile and difficult to fabricate. This is where Alta enters with its innovation – being able to make cheap solar modules that are practical for most applications using this material. Inventions by two leading academic researchers in photonic materials, Eli Yablonovitch and Harry Atwater, have been integrated to achieve this goal. Eli Yablonovitch developed and patented a technique for creating ultrathin films of gallium arsenide in the 1980s while working at Bell Communications Research. On the other hand, Harry Atwater worked on microstructures and nanostructures to improve the material’s ability to trap light and convert it into electricity. Amalgamation of these two ideas have resulted in efficiency increases at a more reasonable cost while using this material.

Alta’s cells have converted 28.3 percent of sunlight into electricity, which is the highest single junction one sun conversion efficiency record – in contrast, the highest efficiency for a silicon solar cell is 25 percent and commonly used thin-film solar materials don’t exceed 20 percent. Yablonovitch suggests that Alta in due course has the potential of breaking the 30 percent efficiency mark and nearing the theoretical limit of 33.4 percent for cells of this type.

 Flexible power: Alta’s solar cells can be made into bendable sheets. In this sample, a series of solar cells are encapsulated in a roofing material. Credit: Gabriela Hasbun

Unlike gallium arsenide, silicon is a relatively inexpensive material. Interestingly silicon, which is the second most abundant element in the Earth’s crust ( ~28% by mass) after oxygen^[2] _, is also the most commonly used material in the PV industry (85%, multi-crystalline and mono-crystalline silicon combined) – a function of its economics and established processing industry. Nonetheless, efforts are on-going to further reduce cell price of silicon. Recently companies such as Silicon Genesis, Twin Creeks and AstroWatt have developed processes to make ultra-thin silicon wafers. Silicon Genesis and Twin Creeks uses Proton Induced Exfoliation (PIE)^[3] method to isolate ultra-thin (20 micron thin) silicon wafers. In PIE, high-energy protons (or hydrogen ions) are embedded into “donor” wafers, such as thick wafers of silicon, germanium or other single-crystal materials. The ions form a uniform layer beneath the surface of the donor, as shown in the figure below. The depth of the formed layer depends on the energy of the incoming ions. The physical attributes of hydrogen permit the ions to penetrate the surface of the donor wafer without changing its inherent properties and characteristics.

When heated, the ions then lift or exfoliate a uniform ultra-thin layer, called a lamina, from the donor wafer. The lamina becomes a production wafer and can be processed into thin solar cells or semiconductor devices. To use an analogy, the ions act like a scalpel and carve away thin, identical and functional wafers from the donor. A single donor wafer can be reused repeatedly to create multiple laminae.  These ultra-thin wafers contain only a fraction of the material currently used in a standard wafer for solar cells, LEDs or other devices. Twin Creeks reported a maximum cell efficiency of 11% using their 20 micon thin wafers.

Astrowatt^[4] on the other hand uses Semiconductor on Metal (SOM®) kerf-less exfoliation process. A metal layer is deposited on a silicon wafer and then the wafer is subjected to a series of thermal cycles, resulting in residual stresses that exfoliate a thin layer of silicon. Astrowatt recently reported a 15% efficient solar cell using their SOM method.

It is worth noting that there are other solar cell devices that use inherently thin film structures. Examples include copper indium gallium selenide (CIGS) and amorphous silicon (a-Si) solar cells, where maximum cell efficiencies of 19.6% and 10.1% have been reported for CIGS and a-Si solar cells, respectively ^[5].

Zahidur R Chowdhury

Electrical and Computer Engineering, University of Toronto.

PhD Candidate (5^th Year)

References:

_[1]
http://www.technologyreview.com/featured-story/426972/alta-devices-finding-a-solar-solution/

[2] Nave, R. Abundances of the Elements in the Earth’s Crust, Georgia State University

[3] Twin Creeks (
http://www.twincreekstechnologies.com/
)

[4] Jawarani et al., ‘Integration and Reliability of Thin Silicon Solar Cells and Modules Fabricated using SOM® Technology’, EU PVSEC 2012, Frankfurt, Germany.

[5] Solar cell efficiency tables (version 40)
http://onlinelibrary.wiley.com/doi/10.1002/pip.2267/abstract

[6] Alta Devices (www.altadevices.com)

[7] AstroWatt (
http://www.astrowatt.com
)

Dangerous times ahead for renewable energy in Ontario

The events of the previous month have raised some serious concerns for renewable energy in Ontario and threaten the survival of the province’s flagship clean energy policies: the green energy and economy act and the feed-in tariff (FIT). First, the World Trade Organization (WTO) is set to rule against the domestic content requirements contained in the FIT. Second, the sudden resignation of Premier Dalton McGuinty over the mismanagement of the energy file has sent tremors throughout the province’s energy landscape. Additionally, delays in implementing the new FIT 2.0 framework, continued media assaults on PV and wind, as well as the growing backlash over rising electricity rates are propelling Ontario’s renewable energy strategy into dangerous waters.

On Monday, October 15^th the WTO ruling backing the EU and Japanese challenge against Ontario’s domestic content requirement was leaked [i]. This ruling will have implications for the longevity of the policy framework surrounding PV in Ontario as well as the regional PV industry. If the domestic content rule is struck down (pending a likely appeal), local module and balance-of-system producers will no longer be sheltered from foreign competition originating from low-cost manufacturers in Asia. In essence, this will expose domestic firms to the same market forces that have transformed the global PV industrial landscape over the last year or so. In turn, plant closures, consolidation and job loss are likely on the horizon. With the regional industrial development impetus for policy support removed, how long will the government continue to pay premium FIT rates for foreign-sourced renewable energy developments?

Adding fire to the flame, Premier Dalton McGuinty – a champion of the current green energy strategy – resigned on the same day as the WTO leak in the face of political fallout stemming from the costly cancellation of new natural gas units during the last election[ii]. His resignation reflects the dangers of tampering with the electricity system for political reasons and highlights the lack of a genuine long-term energy plan for the province. McGuinty’s resignation also poses challenges for the future of renewable energy support. With an election likely on the horizon, will the new Premier seek to distance his or herself from increasingly unpopular support for wind and solar? After all, the last election saw rural voters reject Liberal candidates in part due to wind opposition[iii].

Other issues have also plagued renewable energy policy in the province. Delays in implementing changes to the FIT scheme following the scheduled program review have created difficulties for the domestic industry and investors[iv]. A prominent PV firm has even entered into litigation with the province over the revisions[v]. Moreover, the last several months has seen a ratepayer backlash brewing over electricity rate increases and overgenerous incentives for wind and PV[vi]. Despite the fact that nuclear refurbishments and the rollout of natural gas are primarily to blame for rate increases[vii], the media continues to hammer renewables while giving nuclear and natural gas a relatively free ride.

In many ways, this situation was avoidable. An appropriate renewable energy policy framework with reasonable and justifiable incentives for emerging energy technologies would be far more resilient. The market-based policies in California point to the success of reasonable incentive levels. Although more moderate support may lead to fewer near-term job creation opportunities, it creates a more sustainable market, allowing for a greater degree of certainty for industrial actors and investors. Another key lesson that arises from this unfortunate situation is the need for a less politically interventionist approach to energy planning. An approach that is determined through market mechanisms or an expert bureaucracy with proper authority and regulatory oversight would be far more robust. Legitimacy needs to return to renewable energy support and energy planning in the province.

Daniel Rosenbloom

Research Associate in Sustainable Energy Policy

Graduate from the MA program in Public Policy and Administration at Carleton University

---

[i] 
http://www.thestar.com/opinion/editorialopinion/article/1173543–rising-electricity-prices-have-little-to-do-with-renewable-energy

[ii]
http://www.thestar.com/news/canada/article/1271913–premier-dalton-mcguinty-resigns

[iii]
http://www.betterfarming.com/online-news/did-wind-turbines-blow-rural-liberal-seats-away-4561

[iv]
http://solarindustrymag.com/e107_plugins/content/content.php?content.11382

[v]
http://www.cbc.ca/news/canada/toronto/story/2012/07/14/toronto-solar-power-lawsuit-ontario.html

[vi]
http://www.cbj.ca/mobile/business_news/canadian_business_news/ontario_electricity_subsidies_should_be_zapped_study.html

[vii] 
http://www.thestar.com/opinion/editorialopinion/article/1173543–rising-electricity-prices-have-little-to-do-with-renewable-energy

Experiences at EUPVSEC 2012

Hello everyone! I’m here to tell you all about my trip to Frankfurt, Germany for the 27^th European Photovoltaic Solar Energy Conference and Exhibition!  I flew out of Toronto at 1pm on Saturday afternoon, and I landed in Germany at 6am Sunday morning; that’s 12am Toronto time. This was my first time travelling alone, so I was admittedly nervous about the whole experience. Flying alone into a foreign country where I don’t speak the language, and where I can’t read street signs, made the prospect of navigating my own way from the airport to my hotel a rather daunting task, but I made it okay!  At the hotel, I met up with the other members of the Photovoltaic Innovation Network (PVIN), and we set out to explore the city before our week-long conference. Frankfurt is one of the financial hubs of Europe, and it features hundreds of different banks from all over the world. It is a very international city, but also a very quiet one. At night, you don’t see too many people walking in the streets; living in Hamilton, and having spent time in Toronto, this is not something to which I am accustomed!

Frankfurt, being such an international city, made an appropriate setting for the conference. 4024 people from across 76 countries participated either by giving talks, presenting posters, or simply listening to the presentations. The conference brought a lot of different people together, and with them came a lot of different ideas. During the first few days, the presentations focused on solar cell devices, and I must say that the diversity of solar cell designs is quite amazing. There were talks on traditional silicon cells, organic cells, heterojunction cells, multijunction cells, Dye and Hybrid cells, concentrator systems and so much more! My own work involves silicon, so I was particularly interested in these talks. Silicon has been around for a long time in photovoltaics, and it is a very mature technology. I observed that a lot of the current interest in silicon is not about improving devices, but about finding ways of making them cheaper while maintaining the same performance.  For example, there were many talks on replacing the silver in silicon cells with cheaper metals such as nickel and copper. While silver has nice electrical properties that make it attractive for use in solar cells, it is also expensive and the price is somewhat unpredictable.

Later into the week, the presentations focused on the ‘big picture’ issues of solar energy. There were talks on large-scale photovoltaic power plants, grid integration, manufacturing and processing, and solar cell markets, just to name a few. These talks were interesting because they covered issues that we don’t often think about at the research level. For one, we often don’t consider the manufacturability of the devices we make in the lab. It is one thing to be able to make high efficiency solar cells one at a time in a laboratory, but it is quite another to be able to produce them in higher quantities at a large-scale factory. As a researcher, the talks I saw later in the week really put things in perspective. I saw that solar energy is not just about the research; research is only the first link in a long chain. Included in the chain are the manufacturers that mass produce cells, the engineers who build and install the systems, and the policy makers that study and regulate the deployment of these systems. There are so many contributors from across numerous disciplines working in solar energy, and the conference really reinforced that idea in me.

I must say that my favourite presentations were those by Dr. Harry Atwater from the California Institute of Technology, and Dr. Martin Green from the University of New South Wales. Both men are very prominent researchers in the field of photovoltaics, and each lead a large group of graduate students and post docs that work at the frontier of photovoltaic research. Dr. Green in particular has done some of the pioneering work in silicon cells, including making a cell that is 25% efficient. This cell holds the world record for efficiency! A neat little bit of trivia is that Dr. Green graduated with a PhD from McMaster University, where I did my undergrad and am currently working on my Master’s degree. The talks given by Drs. Atwater and Green focused on the exciting work being done to incorporate the field of plasmonics into photovoltaics. Plasmonics is a field that deals with the interaction of light with nano-sized metal structures, and the strange phenomena that can result. For example, thin sheets of metal with nanoscopic holes can be made to transmit light in one direction, but not in the opposite. For a solar cell, this means that light could enter a cell, but not escape it. A useful application, indeed!  As solar cells are made thinner, in a push to reduce cost and the amount of material used, their ability to capture light is compromised. Techniques for manipulating and trapping light then become necessary in order to maintain comparable performance with thicker cells. The field of plasmonics happens to be brimming with these techniques. I should mention that my own research involves plasmonics. I am working with ultra-thin silicon cells, and using nano-sized particles of silver as a scheme for light trapping. This is why I was especially interested in these talks. Seeing Drs. Atwater and Green speak at the conference was truly a pleasure! Considering that they are co-authors on many of the research journals that I read, it felt like I was at a concert seeing my favourite musician play!

Plasmonic nanoparticles.

 

Alongside the conference there was a solar energy exhibition, in which companies and research institutes ran booths that showed off what each had to offer. Obviously there were many companies offering just solar panels, and some had put a decorative spin on their design. For example, I saw one company that had designed the cell to have a waterfall trickling down it! Definitely something that would look nice in a backyard! Other companies were offering equipment for research or manufacturing, and these booths were the most prominent at the exhibition. The most impressive booth, in my opinion, was from a company that had actually brought a solar cell assembly line into the building. There were periodic demonstrations in which cells would be produced, ready to be used in a panel. The operators were quite secretive about the assembly line, and photography was prohibited. Our own NSERC Photovoltaic Innovation Network (http://www.pvinnovation.ca) also had a booth at the exhibition, which displayed posters that highlighted the research that we do, and the Universities and companies that are involved. Manning the booth was one of my favourite parts of the trip. At the booth, we (representatives of the Network) were able to talk one-on-one with so many interesting people, and teach them a little about what we do in Canada, and what we have to offer. We talked with students, with members of industry, and other researchers that were interested in our work. The PVIN was (to my knowledge) the only Canadian presence at the conference. I felt truly honoured to be standing there at an international convention, representing our entire country and the research that we do, all while piquing the interest of experts from all over the world. It was an absolutely amazing experience. Being there in person also showed me that interest in solar energy is a truly global phenomenon. It’s really no surprise though; the sun is essentially an unlimited source of energy!

Overall, my trip to Germany was a fantastic experience. I learned a lot at the conference, talked with many interesting people, and made some new friends with other members of the Network. I was very grateful that we all stayed in the same hotel together. Very few people had a working cell phone, so organizing trips for dinner or walks around the city would have been a nightmare had we not stayed under the same roof. The city of Frankfurt was a neat place, and according to the locals, very boring! It was mostly banks and retail stores, but we always found something to do and were never bored. Aside from the conference, food was the highlight of the trip! I don’t think I will ever be able to eat schnitzel again, because it would just pale in comparison to authentic German schnitzel. Although the trip was fun, it was also exhausting, and after a week of being away, I was excited to get home. As I mentioned before, this was my first time travelling by myself and I learned a very valuable lesson: indirect flights are no fun! I flew from Frankfurt to New Jersey, and then I had to get a taxi to New York, where I could fly back to Toronto. After 19 hours of travelling and waiting and four airports later, I finally arrived home safe and sound at Pearson! I will definitely choose my flights more wisely in the future!

Suffice to say, I really enjoyed Germany and the conference, and can’t wait for the next one! Before I go, I’d like to thank Jennifer and Sandra for all of their help in organizing the trip and making it possible! Dankeschön!

-Kevin Boyd (Year 1 of M.A.Sc in Engineering
Physics at McMaster University, Hamilton Ontario)

“3D” Solar Cells Can Enhance Light Absorption

Scanning electron microscope image of endview of Solar3D prototype

Solar3D Inc, a small research company founded in 2010, has recently fabricated a working prototype of a three-dimensional (3D) silicon solar cell [1]. Conventional solar cells employ a 2D geometry for capturing light. That is, light enters the cell through a 2D plane and is absorbed inside. A 3D solar cell uses 3D structures (the ridge-like structures pictured above) to capture light. An obvious advantage of this geometry is that there is a greater surface area for light absorption, and thus more light can enter a cell. The concept of a 3D cell is not new; cells that use thin nanowire structures to capture light have been known and studied for some time now. However, to the author’s knowledge, Solar 3D’s cell is the first of its kind to employ the ridge-like structures pictured above. Solar3D claims that their cell design offers many advantages over conventional 2D cells [2].

Firstly, conventional cells suffer from unavoidable reflection losses. Techniques exist to reduce these losses, but inevitably some light is always reflected away. A 3D cell is not immune to these losses either, however by virtue of the ridge-structures in such a cell, reflected light may be bounced back and forth between adjacent ridges, increasing the chances of it being absorbed. As well many 2D cells have metal contacts on the front to harvest the electrical current created by sunlight; these contacts are not transparent, so they block some of the sunlight from reaching the cell. In Solar 3D’s cell, the metal contacts run below the light-capturing ridges so that light-absorption is not compromised.

Another advantage offered by Solar 3D’s cell are thin absorbing regions. In a conventional silicon cell, the cell is made relatively thick to promote light-absorption. However, if a cell is too thick the electrons generated by sunlight may not make it out of the cell before being reabsorbed. Solar 3D’s cell overcomes this by using many thin absorbing regions. The thinness allows electrons to be efficiently collected, and the fact that there are many of these regions allows light to be efficiently absorbed.

Finally, Solar3D claims their cell has superior light-collection throughout the day (pictured above). Conventional cells operate most efficiently when the sun is directly overhead. This means that these cells will only perform optimally for a small part of the day. Many solar cell installations mount cells on trackers so that they can follow the sun throughout the day and overcome this disadvantage. According to Solar3D, their cell can collect light effectively over a range of angles, obviating the need for tracking systems, and allowing their cells to operate efficiently throughout most of the day.

The company fabricated their cell in July, 2012 and so far have not released any concrete details about the prototype’s performance. According to Solar 3D’s Director of Technology, Dr. Changwan Son, “When measured relative to a conventional solar cell design, our working prototype produces electricity beyond our previous expectations. First, we fabricated our working prototype. Then we created a simple cell based on the conventional design, using the same fabrication environment, to serve as a control sample. By measuring the side-by-side power output of both cells, we were able to determine the relative performance under a number of conditions, ranging from bright sunlight to lower, diffuse light. In each test, our 3D Solar Cell consistently outperformed the control cell and produced at least 2½ times the amount of electricity under the same conditions.” Unfortunately, the company does not provide any information on what particular “conventional solar cell design” they are comparing their 3D cell to. In the context of silicon, a “conventional cell” can refer to a myriad of devices covering a broad range of performance, and so these claims are rather ambiguous. The company has stated that simulations they have run on their cell design show that it should have an efficiency of 25.47%. This is a rather ambitious claim, considering that the record efficiency achieved in practice for a silicon cell is 24.7% [3]. The top 10 commercially available silicon cells have efficiencies ranging from 19.1%-22.5% [4]. Solar3D has not yet quoted an efficiency for their working prototype.

According to Dr. Son, Solar 3D’s next step is to improve the fabrication process so that they can drive down the cost of manufacturing their 3D devices. Son says “We believe that the result will be a 50% reduction in the cost of solar electricity. Perhaps the installed system cost savings will be even greater.” Again, this is a rather ambitious claim, considering how remarkable a 50% reduction of cost would be for solar energy. Solar 3D’s cell is certainly an interesting design, and the purported advantages it offers (mentioned above) are quite plausible. However, until the company releases detailed performance characteristics for their device, it is difficult not to remain skeptical regarding the company’s more ambitious claims.

-Kevin Boyd (Year 1 of M.A.Sc in Engineering Physics at McMaster University)

 

 

References

[1]. http://www.pv-tech.org/news/solar3d_reveals_working_prototype_of_3d_silicon_solar_cell

[2].
http://www.solar3d.com/technology.php

[3].
http://www.sciencedaily.com/releases/2008/10/081023100536.htm

[4].
http://www.renewableenergyworld.com/rea/news/article/2012/03/sunpower-tops-in-mono-c-si-solar-cell-efficiency