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.

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-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-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)

PVSC 2013-A long awaited meeting with Cu(In,Ga)Se2 expert Dr. Rockett!

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)Se_{2 ­}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.

-Alexandre Walker

Ph. D. Candidate (Year 4)

University of Ottawa’s SUNLAB

Department of Physics

The Challenges of Cheap Organic Photovoltaics

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

Postdoctoral Fellow

Department of Physics, University of Montreal

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)