Archives for the month of: October, 2013

At the 39th 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 4th 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 of Xianqin's blog

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's picture

-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.

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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 GaAsxP1-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's picture

-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

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's picture.

-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.

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

 

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.