Photon Up- and Down-Conversion at IEEE PVSC

Another day, another interesting idea. The ideas were not from my own brain, of course, but from the minds of others. A recent trip to the IEEE Photovoltaic Specialists Conference (PVSC) in Tampa led to both ingredients mentioned by Einstein in his recipe for genius: inspiration (from conference speakers) and perspiration (from Tampa weather). Although all of us PVINers had some talks to attend on technology related to our specific research, it was often in the talks on “Fundamentals and New Concepts for Future Technologies”, or “Area 1”, that one found new ideas that could potentially be applied to a wide-variety of technology bases. This was so much the case, that “when in doubt, go to Area 1” became a mantra. Such wandering brought me to the talks on “up-conversion” and “down-conversion”.

Up-Conversion and Down-Conversion of Photons

The conversion of photons of one frequency to those of a different frequency is referred to as either up- or down-conversion. This new frequency can either be a higher frequency (“up-conversion”), or lower frequency (“down-conversion”). A fluorescent material that absorbs UV light and emits visible light is a good example of a down-converter. Down conversion is more readily accomplished, as energy conservation allows (in fact, requires) energy to be lost during the conversion process (through heat, for instance). Up-conversion, however, requires additional energy. In the cases described in the talks at IEEE PVSC, this energy was supplied by a second photon. As an example, the first photon excites the electron into an intermediate state, while the second photon excites the electron from the intermediate state into an even higher energy state. The subsequent decay results in an emitted photon of higher frequency than the initial two photons.

Photon Conversion in Photovoltaics

So why do we care about these types of effects and the materials that exhibit them? In photovoltaics, there are two main reasons:

1. Wavelengths below the band gap of the absorbing layer will not generate carriers.

2. Very short wavelengths are readily absorbed by transparent conducting oxide (TCO) or other buffer layers before they reach the absorber layer.

To address the first issue, up-conversion is a potential approach, while the second can be resolved with a down-conversion.

I found three of the talks given at IEEE PVSC specifically on the topics of up- and down- conversion in photovoltaics particularly engaging. Four talks if you count the excellent plenary talk by Dr. Ekins-Daukes [1]. The first talk was on using up-conversion in erbium-doped yttrium fluoride to increase the photocurrent of silicon solar cells; The second talk was on using quantum well islands (QWIs) as up-converters in InGaAs cells, and the third was about using quantum dots (QDs) as down-converters in GaAs solar cells.

A New Quantum Efficiency Record: Erbium-doped Hexagonal Yttrium Fluoride

In the first (and probably my favorite) talk, Stefan Fischer of the Fraunhofer Institute described some of his work involving the use of trivalent erbium-doped hexagonal yttrium fluoride as an up-converter [2]. This material exhibits at a wide range of wavelengths close to 1523 nm and the resulting emission is primarily of light with a wavelength of 980 nm. Other radiative emission lines occur at 540, 655, and 805 nm. When a sample was illuminated using a 1523 nm laser, the conversion efficiency was about 2.8%. Although that might not seem like much, said quantum efficiency was acknowledged as a new record during the presentation and is a result of some significance. This presentation was given the award of “Best Student Presentation” for Area 1, and I suspect that said result was a major reason why. Silicon does not readily absorb light that has a wavelength of more than 1130 nm. Because trivalent erbium-doped yttrium fluoride is very good at converting photons of wavelengths longer than can be absorbed (~1523 nm) to those of a wavelength that can be absorbed (~980 nm), it has potential use as an up-converter in silicon photovoltaics. When used in said solar cells, the material will be under broad-band (and not laser) illumination. As such, they characterized the absorption and emission under several different illumination scenarios and then calculated the potential increase in current. Although the conversion efficiency drops in broad band cases, the predicted improvement in generated current increases. This was especially true under concentrated illumination. Direct experimental measurement of the improvement achieved in silicon solar cells using this up-conversion technology is ongoing.

Quantum Well Islands

In the second talk, Itaru Kamiya of the Toyota technological institute described an InGaAs up-conversion technology based on InAs quantum dots [3]. By “under-growing” the QDs (2-3 monolayers), they got broader, shallower structures they termed quantum-well islands. When illuminated by 855 nm light, such islands exhibit emission at around 725 nm, demonstrating that the islands facilitate up-conversion. Unlike in the previous talk where the up-conversion layer is intended to be outside the cell, the QWIs are embedded right in the middle of the absorber structure. An additional difference is that their hypothesized conversion mechanism involves excitons and Auger recombination rather than an electron being directly excited by the additional photons to a higher energy state. In this model, two separate excitons are generated by the longer wavelength light. The energy of one exciton is then transferred to the second exciton via an auger interaction. This additional excitation allows the carriers to “escape” into the absorber layer. However, further collection of evidence to support this hypothesized mechanism is required.

Down-Conversion using Quantum Dots

The final talk on photon conversion was given by Hau-Vei Han of National Chiao Tung University. In it, he described an experiment in which quantum dots were deposited on the surface of a GaAs photovoltaic cell  in order to both enhance the anti-reflection (AR) properties of the solar cell and to act as a down-converter [4]. They demonstrated that a broad spectrum efficiency enhancement was observed due to AR improvements, and additional efficiency improvements were observed in the UV region. This additional improvement in the UV is currently thought to be due to down-conversion from the QDs. Although there was a significant improvement in the overall cell efficiency, it appears that this is mostly due to improvements in the AR-properties, with the down-conversion playing only a minor role.

Overall, it was nice that each talk demonstrated a different approach to photon conversion in photovoltaics.  This allowed me to get some insight into how one might implement conversion in a wide-variety of architectures, and also highlighted some of the potential areas for improvement and further study. I strongly recommend you take a look at the manuscripts given in the references once the conference proceedings are published.

As for me, I’ll be taking some time to investigate this topic in further detail; nose in book and coffee in hand.

Cheers,

 

Andrew Flood

M.A.Sc. Student, 2^nd Year

University of Toronto

References

[1]    N.J. Ekins-Daukes, “Routes to High-Efficiency Photovoltaic Power Conversion,” in 2013 39th IEEE Photovoltaic Specialists Conference (PVSC), 2013.

[2]    S. Fischer, A. Ivaturi, B. Fröhlich, M. Rüdiger, A. Richter, K. Krämer, B.S. Richards, and J.C. Goldschmidt, “Upconversion Silicon Solar Cell Devices for Efficient Utilization of Sub-Band-Gap Photons under Concentrated Solar Radiation,” in 2013 39th IEEE Photovoltaic Specialists Conference (PVSC), 2013.

[3]    I. Kamiya, D.M. Tex, K. Shimomura, F. Yamada, K. Takabayashi1, and Y. Kanemitsu, “InAs Quantum Well Islands – a Novel Structure For Photon Up-conversion From the Near IR To the Visible,” in 2013 39th IEEE Photovoltaic Specialists Conference (PVSC), 2013.

[4]    H. Han, C. Lin, H.C. Chen, Y. Tsai, Y. Yeh, H. Kuo, and P. Yu, “Spectrallu Dependent Performance of Hybrid Colloidal Quantum Dots GaAs Solar Cells,” in 2013 39th IEEE Photovoltaic Specialists Conference (PVSC), 2013.

PVSC 2013-Fun Times in Florida!

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

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

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

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

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

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

-Kevin Boyd

Ph.D Candidate, Year 1

Department of Engineering Physics

McMaster University.

PVSC 2013-Tandem CIGS Solar Cells

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

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

In their poster, the Toshiba corporation group reported achieving n-type CIGS with CdS doping up to a level of 1×10¹⁶ cm^-2. They were able to achieve a high enough doping level to use a CIGS layer as the emitter in a homojunction CIGS solar cell. From this point, they have grown a homojunction CIGS solar cell leading to a power conversion efficiency of 17.2% (See J-V characteristics). This relatively high efficiency device shows the feasibility of a CIGS homojunction. Another talk about n-type CIGS was given by Professor Angus Rockett [2]. This talk was about nitrogen doped CIGS. He was mentioning the possibility in the future to achieve high level of doping in CIGS in order to eventually obtain CIGS tunnel junctions. From a modelling perspective, these two ideas could lead into very interesting designs of a CIGS tandem cell consisting of homojunction subcells. One of the main advantages of CIGS is the fact that it has a tuneable bandgap ranging from 1.0eV to 1.7eV which covers a good portion of the solar spectrum. The change in the bandgap can be achieved by varying the molar fraction x, corresponding to the gallium to indium ratio in CuIn_1-xGa_xSe₂.  A high bandgap CIGS subcell on top of a low bandgap CIGS subcell connected together in series with a CIGS tunnel junction could lead into high power conversion efficiency while potentially reducing the cost compared to III-V based multi-junction solar cell. This type of technology is certainly not achievable in the short term, but it could definitely be an interesting exercise to model this type of device in order to have an idea of what level of efficiency we could potentially achieve from it.

 

-Frédéric Bouchard

Undergraduate student, Year 4

Sunlab, University of Ottawa

References:

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

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

Spectrum Splitting and Thin Film Photovoltaics at PVSC 2013

At the 39^th Photovoltaic Specialist Conference in Tampa, Florida, there were two important and interesting topics which were of particular interest to me.

The first one was covered by Harry A. Atwater, California Institute of Technology (http://daedalus.caltech.edu/research/thinfilmpv.php) “Full Spectrum High Efficiency Photovoltaics” [1]. He was discussing a new concept: splitting the incident solar spectrum into its constituent wavelengths, guiding these different wavelengths into solar cells with different bandgaps, then absorbing them (shown in Figure 1). In theory, the efficiency of such thin film solar cell system can range from around 30% to over 50%. One way of splitting incident light is to use specially engineered nanostructures printed on the surface of a solar cell or planar holographic elements. In the latter case, the solar spectrum is split four ways via a stack of three sinusoidal volume Bragg gratings, where three bands are diffracted at different angles and the 4^th band passes through un-diffracted. Four such stacks guide each band to the appropriate solar cell. Each solar cell is composed of two lattice-matched and current matched III-V subcells grown on either GaAs or InP substrates. In addition, because the diffraction grating is sensitive to the incident angle of incoming light, to achieve high concentration with spectrum splitting, a two-stage compound parabolic concentrator (CPC) is used after the holographic elements. The parameters for the primary and second CPC are carefully optimized.

Figure 1. A scheme illustrating the geometry of eight- junction holographic spectrum-splitting cell with indicated band-gaps and materials

The second topic that was of great interest to me was the progress made in developing flexible thin film solar cells. Since there are an increasing numbers of applications for photovoltaic devices that demand flexible, lightweight solar cells, the research on thin film solar cells on flexible substrates is attracting a lot of attention. The greatest challenge is to lower the cost of production of such devices while maintaining good efficiency in light conversion. There were quite a few talks and posters about this interesting topic during the conference in which the ideas of using tape, metal or polymer as a flexible substrate were discussed [2,3].   I found Kelly Trautz’ talk [2] on epitaxial lift-off (ELO) technology used in MicroLink’s solar cells particularly interesting because it allows flexible solar panels to be made. It also allows one to reuse the substrates on which the cells are grown multiple times.

-Xianqin Meng

Postdoctoral Fellow

Department of Engineering Physics

McMaster University

References

[1] H.A. Atwater et all. “Full Spectrum Ultrahigh Efficiency Photovoltaics”, in 2013 39th IEEE Photovoltaic Specialists Conference (PVSC), 2013.

[2] Kelly Trautz et all, “High Efficiency Flexible Solar Panels”, in 2013 39th IEEE Photovoltaic Specialists Conference (PVSC), 2013.

[3] B. M. Kayes, L. Zhang, R. Twist, I.-K. Ding, and G. S. Higashi, “ Flexible Thin-Film Tandem Solar Cells with >30% Efficiency”, in 2013 39th IEEE Photovoltaic Specialists Conference (PVSC), 2013.

Highlights from PVSC 2013

The IEEE Photovoltaic Specialist Conference (PVSC) is renowned as one of the world’s largest photovoltaics (PV) conferences. It is also probably the oldest conference that is still been held annually. I was fortunate enough to have the opportunity to attend the conference this year, for the second time.

As the PV energy market is evolving from niche to mainstream, I’ve noticed some shift of focus in the topics of this year’s conference. The most noticeable would be the emphasis on the long-term reliability of PV systems. The very first plenary session on Monday morning was dedicated to PV reliability issues, with two talks covering both modeling and analysis of data collected from real field operations.

While crystalline silicon is still the dominant technology, exploration into new materials and concepts has never been slowed down. It is the same with this year’s conference. It is my area of interest to discover potential new technologies that can bring fundamental improvement to the conversion efficiency of the solar cells, or dramatically reduce the cost per watt. I’ve noticed that there was a session dedicated to III-V on silicon solar cells. This is very exciting since I’ve been working in the same area for nearly my entire postdoc period. Previously there has never been a separate session for this topic. Although still nothing revolutionary was reported even with a dedicated III-V on Si session, at least it shows that people are realizing the great potential of substituting expensive III-V or Ge substrates in a traditional multi-junction solar cell with a much cheaper Si substrate.

To give more insights on the latest development in this area, I have summarized some of the highlights from the III-V on Si sessions. The most noticeable one would be the big picture outlined by Alexander Haas et al. from Emcore and Ohio State University. They predicted 39% efficiency in the near-term for III-V on Si 3J solar cells with active Si bottom cells, and GaAsP and GaInP top cells, grown by monolithic approach, with GaP and GaAsP graded buffers between the GaAsP and Si sub-cells. In my opinion, this efficiency is shockingly high considering that the most successful monolithic multi-junction solar cells involving Si as the active cell reported so far are only 21% in efficient [1]. If it is true that 39% can be achieved in the near term, this may be one of the most exciting breakthroughs in multi-junction solar cell development in nearly two decades!

On the more practical experimental front, Andreas W.  Bett’s group from Fraunhofer reported that direct epitaxial growth of a GaInP/GaAs dual-junction solar cell on a GaAs_xP_1-x buffer on silicon yielded a 1 sun efficiency of 16.4% (AM1.5g), and a similar device fabricated by semiconductor wafer bonding on n-type inactive Si reached already efficiencies of 26.0 % (AM1.5g). S. A. Ringer et al. from Ohio State University and University of New South Wales are tapping into the field of transitioning the buffered growth technique of GaInP/GaAsP on Si from MBE to MOCVD for potential high volume production capacity. More details on this topic can be found in references [2-4].

Among some other sessions that captured my attention, one would be the Fundamental and New Concepts session on Tuesday afternoon. Dr. Alex Zunger from University of Colorado presented a systematic approach to identify new PV materials with suitable properties. He proposed to filter candidate materials from tens of thousands of possible materials combinations from the periodic table with a first principle approach and then try to experimentally synthesize these candidate materials. As a promising example, he and his collaborators have succeeded in discovering a new transparent conductive oxide (TCO) material with this approach. See reference [5] for more details.

Also, as one of the best student presentation award finalist, Aaron Martinez from Colorado School of Mines presented his results on the synthesis of silicon clathrates. This material is essentially Si, but with a very special crystal structure, neither diamond structure nor amorphous structure as is typical. The most attractive feature of the silicon clathrates is that it can be tuned into a direct bandgap material, which means dramatically improvement in the efficiency if the material can be made into a perfect shape. Interested readers can find more details in reference [6].

There were a lot of takeaways from this conference. Tampa is a beautiful city with nice communities and beaches. This was an unforgettable experience.

-Jingfeng Yang

Research Associate

Department of Engineering Physics

McMaster University

References

[1] M. Umeno et al., Solar Energy Materials and Solar Cells, vol. 50, pp. 203–212, Jan. 1998.

[2] A. Haas et al., PVSC 39, Area 3-246, June 18, 2013

[3] F. Dimroth et al., PVSC 39, Area 3-245, June 18, 2013

[4] S. Ringel et al., PVSC 39, Area 1-942, June 21, 2013

[5] A. Zunger, PVSC 39, Area 1-235, June 18, 2013

[6] A. Martinez et al., PVSC 39, Area 1-236, June 18, 2013

PVSC 2013-Discussions on Photovoltaic System Implementation at the 2013 Photovoltaic Specialist Conference

The 39th IEEE photovoltaic specialist conference was held between June 16th and 21st at the Tampa bay convention center in Tampa, Florida. It was a congregation of industry experts, and research giants. Researchers from NREL, Sandia National Laboratories and Universities across the globe graced the occasion to present their latest studies on photovoltaic system design, implementation and reliability of on-sun PV modules.. The program was significantly all encompassing. Besides the presentations, social activities and mixer programs were held to allow attendees to interact, network and share knowledge. Of notable interest was the presentation of the cherry award to Keith Emery. Previously unknown to me, I found that he is renowned for his contribution to photovoltaic research for his design, development and implementation of IV characterization methods. He pioneered the first generation of hardware, software and procedures to measure current-vs.-voltage characteristics as a function of temperature, spectrum and intensity for single and multi-junction cells and modules.

Oral and poster presentations at the conference were grouped into eleven categories which ran in parallel beside keynote or plenary sessions. Personally, I attended sessions in the categories of advanced PV module concepts and designs and PV modules and terrestrial systems. From the presentations, I deduced that there is a significant amount of attention being given to system performance evaluation and energy yield assessments of photovoltaic systems. As such, there is a growing interest in research on concepts for data collection which is a necessary input for energy assessments. There were also presentations on the design of experiments for photovoltaic system assessments. Particularly, I found some modeling techniques used to evaluate PV system performance to be of interest. A few of them include:

Validation of the PVLife Model Using 3 Million Module-Years of Live Site Data [1]

In this article, SunPower corporation (the manufacturers of SunPower PV modules) presented their experiments and results on long term system degradation analysis. An interesting fact is that they performed their analysis using a relatively new approach. Rather than using high fidelity diagnosis methods, they settled for noisy large statistical samples that represent records from a large number of installed systems to estimate the median system degradation rate of PV modules. As a key player in the PV module industry, the company aimed to consolidate their understanding and confidence in system degradation trends and hence they’ve developed a model called the “PVLife model” which is used to simulate module degradation characteristics. Their PVLife model operates with inputs of weather data and cell characteristics to determine degradation factors such as UV induced cell degradation, encapsulant browning, bypass diode and solder joint failures.

For comparison, degradation analysis was carried out on a total of 445 systems.  226 systems were comprised of SunPower modules which had an installed capacity of 86MW. These systems had been operating for up to 5.5 years.  There were also included 149 systems of non SunPower modules which were as old as 11.5 years with an installed capacity of 42MW. Altogether, the total fleet-wide modules representing 3.2 million module-years of monitored data were used to determine degradation rates. Following a plethora of statistical analytics, they found the PVLife model to be in very good agreement with the compared module degradation rates. It was further claimed that the model results were used to develop better modules with lower degradation rates. Attention was focused on the relationship between degradation rates and the placement of the module contacts. According to their studies, it was found that front contact modules for a variety of reasons had a higher degradation rate when compared to back contact modules.

Overall, the work by SunPower suggests that they have successfully developed a working system to model the degradation mechanisms due to several factors in PV system operation. Validation of its results against a large dataset of on-sun measurements was shown to be in very good agreement

Simulations of Energy Yield Improvement in Utility-Scale PV Plants Using Distributed Power Point Trackers [2]

Researchers from First Solar Inc. presented their research on the use of distributed maximum power point trackers (MPPT). It was identified that any energy losses in utility scale PV installations decrease the financial value of the system. They aimed to analyze methods that might reduce the losses in utility scale PV installation such as partial cloud cover induced mismatch loss in the system. Since similar loss mechanisms due to building shading has been analyzed in detail, their focus was directed at non-uniform irradiance patterns created by cloud edges on utility-scale PV installations.  A model was developed to simulate the mismatch loss. The model simulates a PV array with a variable number of strings on a mounting structure. It simulates the movement of a cloud edge over the PV array whilst outputting the array IV characteristics based on voltage and current relationships. A study was then conducted on varying string lengths in the arrays for 10 and 15 modules in a string. Results from the analysis using their model showed a decrease in net energy loss when multiple maximum power point trackers were used. The key energy losses were found to be dependent on the length of the cloud edge. Measurement of the cloud edge in correlation the utility –scale system performance was prescribed to be conducted to further assess the impact of distributed MPPT in decreasing net energy losses.

Overall, the conference was a great learning experience. Its success encourages me to look forward to the next annual conference, which is scheduled to be held in Denver, Colorado

.

-Jafaru Mohammed

M.A.Sc Electrical and Computer Engineering

Department of Electrical Engineering and Computer Science

SUNLAB Research Group

University of Ottawa, Canada.

References

[1]         E. Hasselbrink, M. Anderson, Z. Defreitas, M. Mikofski, Y. Shen, S. Caldwell, D. Kavulak, Z. Campeau, D. Degraaff, S. Corporation, R. Robles, and S. Jose, “Validation of the PVLife Model Using 3 Million Module-Years of Live Site Data,” in Photovoltaic Specialists Conference, 2013. PVSC’13. 39th IEEE, 2013.

[2]         A. Pope, J. E. Schaar, M. Schenck, F. Solar, and S. Francisco, “Simulations of Energy Yield Improvement in Utility – Scale PV Plants Using Distributed Maximum Power Point Trackers,” in Photovoltaic Specialists Conference, 2013. PVSC’13. 39th IEEE, 2013.

PVSC 2013-Luminescent Coupling

In June, I had the opportunity to attend the IEEE Photovoltaic Specialists’ Conference in Tampa, FL.  This is a huge academic conference covering the entire field of photovoltaics, and has been at the center of photovoltaic research since 1961.

One topic that got a lot of discussion this year was ‘luminescent coupling’, a process where energy that is lost through photons radiated from one part of a solar cell can be recovered by absorption in another part of the same cell [1,2].  This has potential to change the way that solar cells – especially very high-efficiency multi-junction solar cells – are designed, either through careful control  of the internal optics of the cell, or by manipulating materials so that photons are emitted in particular directions where they have a high probability of being recovered.  In this way radiative loss, which is an important loss mechanism in multi-junction cells, can be partially suppressed.

There is an added benefit to designing cells for very efficient luminescent coupling, in that they tend to be less sensitive to changes in the solar spectrum.  Multi-junction cells have traditionally been very carefully optimized to work best under a specific spectrum, but designing for strong luminescent coupling reduces the need to do this, allowing the cell to operate at high efficiency under a wide range of spectral conditions.

At this point, it isn’t clear how to approach designing cells to take maximum advantage of luminescent coupling, or even how to evaluate the performance of cells incorporating it.  There is likely to be a lot of discussion of this topic over the next year, and it will be very interesting to see how solar cell designs change as a result.

 

 

Matt Wilkins

Ph.D Candidate, Year 1

University of Ottawa

 

[1] O. D. Miller, E. Yablonovitch, and S. R. Kurtz, “Strong Internal and External Luminescence as Solar Cells Approach the Shockley–Queisser Limit,” IEEE J. Photovolt., 2 (3), pp. 303–311, Jul. 2012.

[2] M. A. Steiner and J. F. Geisz, “Non-linear luminescent coupling in series-connected multijunction solar cells,” Appl. Phys. Lett. 100 (25), p. 251106, 2012.

PVSC 2013-CdTe thin films progress

I had the opportunity to go to PVSC 39 in Tampa, Florida with fellow Highly Qualified Personnel (HQP). There were a lot of interesting speeches but I will only focus on a couple of them here – particularly those focusing on CdTe thin films progress. CdTe is one of the most attractive materials for production of low cost thin film solar modules [1]. The record efficiency for CdTe solar cells has been established to be 16.7% for 10 years. In the past 2 years, the CdTe record was broken several times and increased from 16.7% to 18.7%. However, there has been no significant change in the open-circuit voltage which was in the range of 840-860 mV for over 20 years. Many arguments have been made to justify the apparent V_oc limitation, most frequently: poor hetero-interface with CdS, the difficulty in doping polycrystalline CdTe, midgap defect levels, or non-uniformities at the nano- or micro-scale. Paths for open-circuit voltage above 900 mV are:

• Doping: Increasing doping level of CdTe is believed to increase the built-in potential and reduce recombination at the back-surface. Present doping levels are of the order 10¹⁴ /cm³ and different ways are proposed to increase them.
• Lifetime: Higher lifetime(s) are expected to be a sign of less recombination in the junction and quasi-neutral region, and, hence, improved V_oc and carrier collection. With higher lifetime, it is expected that a greater fraction of the recombination may occur at the back-contact due to increased electron diffusion through the absorber.

Gloeckler from First Solar announced a new record efficiency of 19.1% for CdTe, although not yet certified by NREL.

Despite these promising results, the gap between solar cell and module efficiencies is still wide (3-5%) [2]. This so-called “solar gap” constitutes a major challenge for commercial viability of photovoltaics. One explanation, proposed by M. Alam and his group at the University of Purdue, says that the “solar gap” is due to the monolithic series connection of thin films that causes shunt leakage current. Analysis of the shunt leakage current show that an I-V curve can be modeled by the diode equation and the shunt current, which has a non-linear relation with the voltage, as shown in Fig. 1. It was shown by Alam et al, that as a consequence of the series connection of cells, large shunts have a twofold impact on module performance. First, they modify the operating point of their neighboring good sub-cells, thereby lowering their output power. Second, a large fraction of this (already reduced) power, generated by the neighbours, is consumed by the shunted sub-cell. Interestingly this phenomenon is not unique to CdTe photovoltaics but more of a universal phenomenon and studies on CdTe, CIGS, OPV and amorphous silicon thin films show the same behaviour. At PVSC they have described a post-deposition scribing technique for electrically isolating these distributed shunts in monolithic thin film PV modules. The localized scribes minimize the losses due to defective shunts by restricting lateral current drain from its (otherwise defect-free) neighbors.

Fig. 1 Measured I_Dark (squares) can be represented by a parallel combination of diode with series resistance I (green), and a parasitic shunt component (red), with a symmetric (around V = 0) non-Ohmic voltage dependence. Reproduced from [3] with permission of The Royal Society of Chemistry.

References:

[1] W. N. Shafarman, “What’s next for Cu(InGa)Se2 Thin Film PV: Opportunities and Challenges”, 39th IEEE Photovoltaic Specialists Conference, 2013

[2] S. Dongaonkar, M. A. Alam, “Reducing the Cell to Module Efficiency Gap in Thin Film PV using In-line Post-Process Scribing Isolation”, 39th IEEE Photovoltaic Specialists Conference, 2013

[3] S. Dongaonkar, S. Loser, E. J. Sheets, K. Zaunbrecher, R. Agrawal, T. J. Marks, and M. A. Alam, “Universal statistics of parasitic shunt formation in solar cells, and its implications for cell to module efficiency gap,” Energy & Environmental Science, vol. 6, pp. 782–787, 2013

-Ahmed Gabr

Ph.D Candidate, Year 3

SUNLAB, University of Ottawa

PVSC 2013 in Tampa, Florida

This year, the Photovoltaics Specialists Conference was held in Tampa.  In the middle of June in Florida, you could really feel the sun.  It was hot.  The temperature accounting for humidity was easily into the 40’s each day.   And this was awesome for me, since I almost feel perpetually cold in Ottawa.  An ultra-hot day feels like a blessing so I didn’t mind it at all. We conveniently stayed right across the street from the Tampa Convention Centre, which was also conveniently connected to our hotel with a bridge.  All it took was a quick 30 seconds in the heat and it was back to the frigidity of an air conditioned building.

 

The actual conference was much more along the interests of the students in the network.  And you could tell.  Students were picking out their sessions as soon as they got their hands on a physical copy of the conference schedule.  I was amazed by the sheer volume of talks and posters at the conference.  It seems as though each of these presenters had a unique research topic as well.  I was completely unaware of the breadth of the research activity in the field of solar energy.  Practically any strategy you could think of had a researcher working on exactly that.

The organization of the conference was also equally impressive.  Talks were kept on time.  The seating was great.  Even the coffee was in adequate supply!  The conference centre itself was a great venue too.  There was enough space for everyone to roam about freely (and then some!).

The PVIN booth was manned as usual by quite a few students at a time.  It seems being at the booth allowed you to interact with more people (it also helped being close to the coffee). While at the booth, it was nice to meet other researchers, industry professionals and manufacturers of solar products and characterization tools as well.  Being a research-oriented conference, it really felt like the place to be as a student studying photovoltaics and I couldn’t have asked for a better conference experience.

 

 

-Ross Cheriton

Ph.D Candidate, Year 1

SUNLAB, University of Ottawa

PVSC 2013-Intermediate Band Solar Cells

I recently had an opportunity to attend the 39^th 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).

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.

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, 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)