TechnologyTechnology Conference Conference Report Report

Future perspectives on

19–21 October 2010 Tokyo, Japan Nature Photonics Technology Conference

Look at what you’ve been missing…

OCTOBER 2009 VOL 3 NO 10 www.nature.com/naturephotonics AUGUST 2009 VOL 3 NO 8 www.nature.com/naturephotonics FEBRUARY 2009 VOL 3 NO 2 www.nature.com/naturephotonics

Repulsive forces revealed

PHOTONIC CRYSTAL FIBRE Supercontinua success

NONLINEAR OPTICS Single-photon effects

BIOIMAGING Intracellular insights

On-chip optical isolators

EXCITONICS Switches warm up ULTRAVIOLET SOURCES CLOAKING Boron nitride alternative Invisibility in the infrared

WAVEFORM GENERATION OPTOMECHANICS Ultrafast answer Tuning cavities by light

QUANTUM OPTICS Liquid-crystal microresonators Phase control of single photons

MARCH 2009 VOL 3 NO 3 MAY 2009 VOL 3 NO 5 JANUARY 2009 VOL 3 NO 1 www.nature.com/naturephotonics www.nature.com/naturephotonics www.nature.com/naturephotonics

PLASMONICS Ultrafast modulation

PHOTODETECTORS TERAHERTZ OPTICS SOLAR CELLS High gain—bandwidth Solid-state phase modulator Organics beat recombination SIGNAL PROCESSING TERAHERTZ EMITTERS Analyser on a chip RANDOM LASERS Benefits of confinement Operational insights SUBWAVELENGTH IMAGING Observing colour centres PLASMONICS Electrical detection Quantum cascade Metamaterials with a twist A bright approach to E-paper lasers shine bright

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II Nature Photonics | Technology Conference | www.nature.com/naturephotonics

21916-10NPhoton subscribe_covers 09 FP.indd 1 17/11/09 15:47:41 Nature Photonics Technology Conference

Look at what you’ve been missing…

OCTOBER 2009 VOL 3 NO 10 www.nature.com/naturephotonics AUGUST 2009 VOL 3 NO 8 www.nature.com/naturephotonics FEBRUARY 2009 VOL 3 NO 2 www.nature.com/naturephotonics

Repulsive forces revealed

PHOTONIC CRYSTAL FIBRE Supercontinua success

NONLINEAR OPTICS Single-photon effects

BIOIMAGING Intracellular insights

On-chip optical isolators Speakers Entering the gigawatt era

EXCITONICS Switches warm up Alan J. Heeger University of California ULTRAVIOLET SOURCES CLOAKING Katsuhiko Shirasawa KYOCERA Corporation Boron nitride alternative Invisibility in the infrared Soaring growth in production capacity and materials for improved multijunction cells and Sadao Wasaka NEDO WAVEFORM GENERATION OPTOMECHANICS Ultrafast answer Tuning cavities by light a plethora of emerging alternative cell tech- the use of photonic structures to successfully Antonio Luque Universidad Politécnica de Madrid QUANTUM OPTICS Liquid-crystal microresonators Phase control of single photons Michael Grätzel EPFL nologies are two characteristics of the solar trap light and minimize optical losses. This Yoshitaka Okada The University of Tokyo energy sector that came across particularly short report offers a summary of some of the Joachim Luther SERIS strongly at the recent Nature Photonics opinions expressed about various photovoltaic Technology Conference ‘Future Perspectives technologies at the conference, interviews MARCH 2009 VOL 3 NO 3 MAY 2009 VOL 3 NO 5 JANUARY 2009 VOL 3 NO 1 Tatsuya Takamoto SHARP Corporation www.nature.com/naturephotonics www.nature.com/naturephotonics www.nature.com/naturephotonics Michio Kondo AIST on Photovoltaics’. The three-day event, held in with several speakers and a round-up of what Wim Sinke ECN October 2010 in Tokyo, Japan, brought together was on show at the exhibition. Finally, I would Kazuo Nakajima Kyoto University experts from around the world to discuss the like to thank our co-organizers Impress R&D, Makoto Tanaka SANYO Electric Co. current status and future outlook of the pho- Makoto Konagai from the Tokyo Institute of Hiroshi Komiyama Mitsubishi Research Institute tovoltaic sector. It became clear at the confer- Technology and all the members of the techni- Mitsuo Inoue Mitsubishi Electric Corporation ence that the market dominance of crystalline cal advisory board, as well as our sponsors and Koichi Yamada Japan Science and Technology agency silicon, despite its impressive improvements in exhibitors for all their support in ensuring that Atsushi Masuda AIST performance, may soon be challenged by alter- the event was a success. Ryne Raffaelle NREL native and potentially lower-cost technologies Katsumi Kushiya Solar Frontier K.K. PLASMONICS such as thin-film Si, CIGS, CdTe, polymer cells Oliver Graydon Ultrafast modulation Hiroo Konishi NTT Facilities and dye-sensitized cells. Many research oppor- PHOTODETECTORS Osamu Ikki RTS Corporation Editor, Nature Photonics TERAHERTZ OPTICS SOLAR CELLS High gain—bandwidth Solid-state phase modulator Organics beat recombination tunities still exist, however, particularly at the SIGNAL PROCESSING TERAHERTZ EMITTERS Analyser on a chip RANDOM LASERS Benefits of confinement level of quantum engineering, for example in Operational insights SUBWAVELENGTH IMAGING Observing colour centres PLASMONICS the construction of viable intermediate-band Electrical detection Quantum cascade cells, the fabrication of bandgap-optimized Metamaterials with a twist A bright approach to E-paper lasers shine bright

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Nature Photonics | Technology Conference | www.nature.com/naturephotonics 1

21916-10NPhoton subscribe_covers 09 FP.indd 1 17/11/09 15:47:41 Nature Photonics Technology Conference

Conference report

Solar energy is looking more attractive than ever, thanks to significant improvements in the performance of silicon photovoltaic cells and the emergence of a wide variety of alternative material systems.

Ever since the demonstration of the of wide-scale deployment. The latest growth And it’s not only tech- first working silicon by prediction is 65% for 2010,” commented Ryne nology that has improved over time. There Pearson, Chapin and Fuller at Bell Raffaelle, director of the National Center has also been an explosion in the exploration Labs in the USA in the 1950s, the dream of for Photovoltaics at the National Renewable of new technologies and material systems, harnessing the Sun’s power to generate elec- Energy Laboratory (NREL) in Colorado, including dye-sensitized cells, organic poly- tricity has inspired scientists to develop pho- USA. “If you look at the best technologies, cell mer cells and multijunction cells based on tovoltaic technology. Now, fifty years later, as efficiency has effectively been growing at 1% a compound , as well as the nations around the world establish their ener- year for 30 years.” emergence of thin-film silicon and copper– gy generation plans for the twenty-first cen- Indeed, market statistics show that the – diselenide (CIGS) and CdTe tury and prioritize how to reduce the impact industry has been climbing exponentially systems. Multijunction cells have now reached of fossil fuels on the environment, it seems in terms of production volume. According an amazing conversion efficiency of 40% under certain that is going to play an to figures from the industry magazine small-area concentrated sunlight conditions. increasingly important role in many regions PV News, worldwide photovoltaic produc- In October 2010, Nature Photonics brought across the globe. Indeed, installations of tion almost tripled from 3,746 MW in 2007 to together 20 experts from around the world giant gigawatt-scale solar farms have already 10,660 MW in 2009, with even higher figures to discuss the current status and prospects of begun, with the next big challenge being to expected for 2010. The cumulative installed these various photovoltaic technologies. Here’s introduce photovoltaic technology to build- capacity of Japan alone is expected to grow by a summary of what was discussed, grouped by ings and homes on a wide scale. an order of magnitude over the next decade, technology area. So why has the widespread deployment of from 2.6 GW in 2009 to 28 GW by 2020, final- solar-cell technology on residential proper- ly reaching 53 GW by 2030. This growth is Polymer solar cells ties taken so long? The answer is simply a fuelled by Japan’s long-term energy plans such Alan Heeger from the University of California matter of high cost and low efficiency. Many as PV 2030, PV 2030+ and Cool Earth 50, for- at Santa Barbara in the USA and co-founder experts now believe, however, that the opti- mulated by the New Energy and Industrial of the polymer solar cell developer Konarka mization of silicon technology and the slew Technology Development Organization began his keynote speech on plastic solar cells of low-cost alternatives, including thin- (NEDO). According to Sadao Wasaka, execu- by flexing and throwing a thin plastic solar cell film, polymer and dye-sensitized cells, have tive director of NEDO, the aim is to reach to demonstrate its low weight and robustness. together made the mass deployment of solar a photovoltaic energy generation cost of He explained that the field of polymer solar cells a compelling proposition. ¥14 kWh–1 by 2020, and just ¥7 kWh–1 — on cells originates from the discovery of ultra- “The solar industry used to comprise par with nuclear power — by 2030, through fast (~50 fs) photo-induced electron transfer just 10–20 companies. Now it exceeds 200,” improvements to the technology and manu- between a light-absorbing semiconducting explained Osamu Ikki, president of the RTS facturing techniques. polymer and a fullerene. This discovery rap- Corporation in Japan. “We are now entering From a technological point of view, there is idly led to the development of ‘bulk hetero- the era of gigawatt production. Over the next no doubt that tremendous progress has already junctions’ — solar cells based on a polymer– ten years, gigawatt-level plants will achieve been seen over the past fifty years. Although fullerene mix in the form of an intercon- mass-production, and module prices will fall the first Bell Labs prototype cell in 1954 nected network — which today have reached below 100 yen per watt.” offered a conversion efficiency of only around efficiencies of around 8% and are now being Similar optimism was expressed by speak- 5%, today’s best commercial crystalline silicon commercialized by firms such as Konarka and ers from the USA. “We are now entering an era cells now offer efficiencies of 22–24%. Solarmer. One of the important benefits of this

2 Nature Photonics | Technology Conference | www.nature.com/naturephotonics Nature Photonics Technology Conference technology is that it is processed in solution form, making it compatible with roll-to-roll printing on a plastic substrate — a low-cost, mass-production process. The combination of low cost and high flexibility could help pol- ymer solar cells to reach applications that are not suited to rigid and expensive silicon tech- nology. Because these cells are not only semi- transparent but also insensitive to the angle of incident sunlight, one of their most promising applications is for use in power-generating windows. Heeger says that production speeds of 30 feet per minute for 1.5-m-wide films have now been achieved, and believes that polymer cells will surpass efficiencies of around 10% before 2015.

Dye-sensitized solar cells Another potentially low-cost solar-cell tech- nology that can be manufactured through a print process is the dye-sensitized solar cell, or Joachim Luther, CEO of the SERIS solar insti- owing to its very strong optical absorption. Grätzel cell, named after its inventor Michael tute in Singapore. “The question is how to CIS cells have been successfully commer- Grätzel from École Polytechnique Fédérale reduce costs.” cialized by many firms in conjunction with de Lausanne in Switzerland. First described Wim Sinke from the Energy Research various universities, including Würth Solar, in Nature in 1991, the dye-sensitized solar cell Centre of the Netherlands made a similar Solibro, Miasole, Nanosolar, Avancis, Solar combines a light-absorbing dye with a nanos- point in regard to the strengths of wafer silicon Frontier and Honda Soltec. Kushiya said that tructured porous layer of titanium dioxide and technology. Sinke explained that we already the race is now on to increase the efficiency liquid/gel electrolyte to transport electrons have half a century of manufacturing experi- of commercial modules from 14% to 16%. By and holes out of the cell. ence and a huge technology base of processes 2010, Solar Frontier expects to have estab- “This is the only type of solar cell to use and device designs for manufacturing wafer lished a 1 GW annual production capacity for molecular absorption, thereby mimicking silicon. Furthermore, wafer silicon has an fabricating CIS cells. Michio Kondo, director natural photosynthesis,” explained Grätzel in extensive track record in performance and of the Research Center for Photovoltaics at a presentation on the topic. “Light absorption reliability, boasting the highest power conver- the National Institute of Advanced Industrial and charge transport are decoupled in this sion efficiency for all large-area cell technolo- Science and Technology (AIST) in Japan technique, and this flexibility means that that gies (except multijunction cells, which are commented that the CIGS submodules fabri- light absorption can be performed by dyes, or used in small areas with solar concentrators). cated at AIST have now reached efficiencies alternatively by quantum dots.” The fundamental Shockley–Queisser limit of 15.9% for a 10 cm × 10 cm area comprising Dye-sensitized solar cells are now reaching for single-junction silicon cell operation yields 17 cells. Kondo explained that there is much conversion efficiencies of around 12% in the a theoretical maximum efficiency of around interest in increasing the efficiency of thin- laboratory, and, thanks to the development of 30%, with laboratory- and commercial-scale film devices using multijunction designs new broadband dyes and electrolytes, Grätzel silicon cells already approaching this number. made from a variety of materials, such as believes that efficiencies of 15% are just around Indeed, the best laboratory-scale monoc- Si–Ge, CGS, CIGS and CIS. Other research the corner. rystalline silicon cell efficiency of 25% was opportunities include the investigation of As with polymer solar cells, the attraction of demonstrated in 1999 by Martin Green at the new materials such as wide-bandgap systems this technology is the low cost of the print-based University of New South Wales in Australia. (SnO, CIGSSe and CGS), nanostructures fabrication approach. An important additional Today’s commercial cells from firms such as such as InGaAs quantum dots or carbon benefit of dye-sensitized solar cells is that they Sunpower and Sanyo are now only a few per- nanotubes, and single-crystalline organic also work well under indoor artificial lighting cent behind this figure. materials. Another successful thin-film conditions, suggesting that this technology Sinke also commented that there are sev- material systems is CdTe, which First Solar is will probably find use as a convenient power eral further opportunities for improving wafer now deploying on a wide scale. source for interior portable devices. silicon cell technology, including minimizing One of the initial concerns about dye-sensi- recombination (the unwanted decay/loss of Intermediate-band cells tized solar cells was the practicality of using a photo-excited charge) by improving material Antonio Luque from the Instituto de Energia liquid electrolyte, but the development of suit- quality through better management of impuri- Solar at the Univerisdad Politécnica de able gels seems to have resolved this issue. ties and defects; minimizing optical losses by Madrid in Spain and Yoshitaka Okada from reducing reflections and employing light-trap- the University of Tokyo both gave presenta- Wafer silicon ping technologies such as plasmonic struc- tions on the concept and current status of Several speakers made the important point tures; and minimizing electrical resistive losses intermediate-band solar cells. The principle that existing photovoltaic technology, par- through advanced electrode technologies. idea, as first proposed by Luque in 1997, is ticularly wafer (monocrystalline) silicon, has that conventional solar cells based on a sin- already matured to reach high performance Thin-film cells gle-band transition (valence to conduction levels, and that we don’t necessarily need new Katsumi Kushiya from Solar Frontier gave an band) suffer from an inherent compromise. technologies or higher efficiencies for solar update on the current status of thin-film solar Put simply, solar cells aim to generate as cells to become a great success. Instead, the cells based on CIS/CIGS. Unlike crystalline much power as possible from any incident pressing issue is simply one of bringing down silicon, the layer of active material in a thin- light, which in turn means simultaneously costs. “Price reduction is the key,” commented film solar cell only needs to be 1–2 μm thick, wanting the highest possible open-circuit

Nature Photonics | Technology Conference | www.nature.com/naturephotonics 3 Nature Photonics Technology Conference

Commercial devices manufactured by Sharp, Emcore, Spectrolab and Azur have efficien- cies of around 35% — significantly higher than single-junction devices. Koichi Yamada, deputy director-general of the Center for Low Carbon Society Strategy at the Japan Science and Technology Agency, comment- ed that a triple-junction cell with bandgaps of 0.74, 1.2 and 1.8 eV would have a theoreti- cal efficiency of 59%. In practice, however, the most commonly employed materials are Ge (0.67 eV), GaAs or InGaAs (1.4 eV), and InGaP (1.85 eV). There is therefore strong motivation to find a new higher-bandgap material to replace Ge and a new lower- bandgap material to replace GaAs. In 2010, by optimizing its cell design and employing InGaP, GaAs and InGAaS for the top, mid- dle and bottom junctions, respectively, Sharp reported the world’s highest conversion effi- voltage and short-circuit current density. conventional p–n junction solar cell. There ciency for a triple-junction solar cell, pro- The problem is that the former rises with are, however, many technical hurdles to be viding 42.1% at 230 suns and 35.8% under increasing bandgap while the latter falls, overcome before an intermediate-band solar one-sun illumination. In October 2010, just a meaning that a compromise must always cell with an advantageous efficiency can be few days before the Nature Photonics confer- be found between these two competing fac- realized. The principal problem is related to ence began, Spire in the USA tors. The idea of an intermediate-band cell is the dynamics of the energy level structure in claimed to have achieved a new triple-junc- to introduce a partially filled band between an intermediate-band solar cell. In particu- tion cell efficiency of 42.3% under 420 suns. the valence and conduction bands, thereby lar, the desired transition between the inter- “Who holds the current world record simultaneously providing the cell with both mediate band and the conduction band tends for efficiency? NREL, Fraunhofer, Emcore, a large and small bandgap. The intermediate to occur at a low rate, whereas unwanted Spectrolab and Spire have all demonstrated band essentially functions as a relay point, recombination between the intermediate efficiencies of over 40%, with numbers differ- allowing lower-energy (longer wavelength) band and the valence band tends to occur ing by much less than the error bars of the meas- photons to excite electrons from the valence relatively often. As a result, electrons that are urements, and often being performed at dif- band to the intermediate band, followed by successfully excited to the intermediate band ferent laboratories with different simulators,” a second transition to the conduction band. from the valence band often escape from the commented Raffaelle from NREL. “Verifying At the same time, higher-energy (shorter quantum dots by thermal or field-assisted efficiencies of over 40%, especially under con- wavelength) photons are still able to excite tunnelling. However, Okada commented centration or with quantum-confined materi- electrons directly from the valence band to that the use of a strain-compensated growth als or advanced photonic structures, demands the conduction band as usual. Intermediate- technique is proving effective for making unprecedented fidelity on solar simulation band cells therefore offer theoretically higher InAs/GaNAs quantum dot superlattices. A and measurement protocols.” conversion efficiencies than conventional 100-layer stack of such a structure has an Efficiencies can of course be increased cells because lower-energy photons can also optical absorption of around 20%, and transi- by using more than three stacked junctions. be put to use. Indeed, calculations suggest tions between the intermediate band and the Yamada and Raffaelle both commented that that values as large as 47% under one sun and conduction band have now been observed at although this is theoretically possible, it is far 67% under concentrated sunlight are theo- room temperature under light from the solar from straightforward. “Increasing the number retically possible. spectrum. Research is now pursuing the of cell layers is an effective way of improv- development of smaller sizes and higher den- ing efficiency. However, the mismatch of the “The solar industry used to sities of quantum dots, which should lead to lattice constant limits the combinations,” increased absorption and compatibility with commented Yamada. “Developing the proc- comprise just 10–20 compa- concentrated sunlight. esses for preparing new materials and forming nies. Now it exceeds 200.” junctions are important.” Multijunction cells To address this need, researchers at the Osamu Ikki, RTS Many of the speakers at the conference dis- University of Tokyo are developing a new cussed the merits and performance of multi- four-junction material based on strain-com- The technology for fabricating interme- junction solar cells. The well-proven design pensated compound semiconductors with a diate-band solar cells is still in its infancy. behind multijunction solar cells exploits a bandgap of around 1 eV. In principle, a mul- Scientists have recently demonstrated the stack of p–n junctions, each made from a tijunction cell comprising AlInGaP (2.0 eV), principle of intermediate-band solar cells in a different set of semiconductors and thus InGaAs (1.4 eV), Ge (0.67 eV) and this new bulk ZnTe:O cell design, although still at low each offering a distinct bandgap and spec- material (1 eV) could yield a four-junction efficiencies. The most promising approach tral absorption profile to cover as much of solar cell with a theoretical efficiency of up seems to be the use of quantum dots (semi- the solar spectrum as possible. Usually either to 52%. conductor nanocrystals that can have their two (‘tandem’) or three (‘triple’) junctions Raffaelle has a pragmatic opinion about bandgap artificially engineered by control- are used. Given the expense and difficulty in the prospects of using more than three band- ling their size). The idea is to ‘sandwich’ such their fabrication, multijunction cells are often gaps to improve efficiencies beyond 50%. quantum dots, which together serve as a cus- used in small-area cells operating under con- “Thermodynamically it can done; practically tom-engineered intermediate band, within a centrated sunlight or in space applications. speaking, probably not.” ■

4 Nature Photonics | Technology Conference | www.nature.com/naturephotonics Nature Photonics Technology Conference Lightweight, low-cost organic approach A new and exciting technology that can produce lightweight, flexible, low-cost solar cells in large quanti- ties from organic semiconducting polymers has a strong future in the field of photovoltaics, says Alan Heeger from the University of California at Santa Barbara in the USA.

efficiency at wavelengths within the absorp- tion band has been demonstrated. But the challenges are to get the absorption band right, and to achieve a self-assembly process that will enable charge collection. Photo-induced elec- tron transfer gives a very high yield of charge separation — that is the foundation on which the whole field is built.

■■ Do you see this technology dra- matically coming down in price over the next five years? What could help make this happen? It’s always dangerous to predict the future; cre- ating the future is a better idea. It’s clear from what I have seen that there are significant opportunities for reducing the cost from what we have today, but this is a great challenge. I am confident that the cost will come down when Alan Heeger foresees that the low-cost roll-to-roll manufacturing of polymer solar cells may allow we get the efficiency into the range of 10% or them to succeed where other solar-cell technologies have failed. greater. I think the actual cost will be very low by then. As we heard in the conference, the ■■ What is a polymer solar cell and why must be achieved at the same time. When we cost of installation and the cost of the inverter is it attractive? do that, I am confident that higher efficiency all come into the total cost of the solar cell. It’s a new idea, and new ideas are always can be achieved. Cost is also a serious chal- Although the goal of getting the cost down to exciting. The polymer solar cell is a new con- lenge. We can already fabricate polymer solar an acceptable level involves many challenges, cept that will allow us to convert sunlight to cells, although currently only as a demonstra- I believe that polymer solar cells will be a low- electricity using semiconducting polymer tion, not as a commercial product. However, cost technology in the future. materials. Mixing semiconducting polymers we foresee the ability to make polymer solar with suitable acceptors allows ultrafast pho- cells in large areas at high throughput. On the ■■ How do you expect the market share to-induced electron transfer to be achieved. other hand, roll-to-roll coating is inherently of polymer solar cells to look in 5–10 This method is capable, in principle, of very a low-cost manufacturing process. The high- years time? large-area, fast and low-cost production, est efficiencies of 7–8% were achieved by a This is a question that I am not equipped to which makes it very exciting. However, the company in the USA called Konarka. Konarka answer. We see many unique opportunities science is still not completely understood. has a roll-to-roll manufacturing facility that at the moment. Lightweight, flexible, rugged There are major challenges that I am enjoying is being developed for product introduction, technology provides opportunities in devel- working on and that people all over the world and the company has demonstrated that this oping countries for relatively small power are currently engaged in. process is scalable. That in itself is exciting. generation in individual houses, which could Over the past 3–4 years, we’ve only been able change the lives of billions of people. Semi- ■■ What do you think are the biggest to make small devices at very low efficiencies. transparent solar cells for use in windows challenges facing polymer solar-cell Now we are able to fabricate devices measur- and building-integrated applications are also technology today? ing many square metres. I think there is a real major opportunities. The fact that we can Of course, there are challenges. We need to future here, and we are all working hard to achieve significant power generation even improve the efficiency, and already under- make it happen. under tungsten light demonstrates that these stand why the efficiency is currently in the devices can generate electricity even without range of 7–8%. We know that we have to cre- ■■ Do you believe we have reached the direct sunlight. Polymer solar cells therefore ate new materials that have absorption bands maximum cell efficiency? have many applications and many advantages mimicking those of silicon. We also have to We have certainly not reached the maximum. over today’s leading technologies. I am sure absorb solar radiation from the infrared wave- The materials are not optimum for the appli- there will be opportunities in which polymer length range. We understand that we have to cations, and there have been some very seri- solar cells can flourish. For example, light- tune the energy levels of both the donors and ous analyses of the potential for this field. The weight structures that cannot support heavy acceptors to achieve a higher open-circuit basic physics of polymer solar cells is essen- silicon solar-cell panels could support poly- voltage. We understand we have to mini- tially the same as that of inorganic solar cells. mer solar cells, which can be easily removed mize recombination to get a higher fill factor. The prediction is that we should be able to at a later date. However, understanding is not the same as achieve a cell efficiency of 15–20%. Indeed, we doing. In an ideal system, all of these things have a specific example in which that kind of Interview by Rachel Won

Nature Photonics | Technology Conference | www.nature.com/naturephotonics 5 Nature Photonics Technology Conference Intermediate-band boost First proposed in 1997, the intermediate-band solar cell has attracted significant attention from the photovoltaic community for its potentially very high efficiency, says Antonio Luque from Universidad Politécnica de Madrid in Spain.

■■ What is an intermediate-band solar cell and why is it attractive? The intermediate-band solar cell uses a mate- rial that has an additional permitted band within the main bandgap of the semiconduc- tor. This additional band can be achieved through several methods, such as the addi- tion of impurities, which leads to deeper energy levels, or by introducing quantum dots and then using their energy levels as the intermediate band. In principle, this inter- mediate band allows you to produce addi- tional current by using lower-energy photons that cannot directly pump electrons from the valence band to the conduction band; instead, two small lower-energy photons can use the intermediate band as a ‘relay’ for the current. The nice thing is that it should be possible to produce voltages higher than the energy of the photons you are using. Of course, more than one photon is required, but in principle you can extract the energy at a voltage above the photon energy. The maximum efficiency of this concept is 63%, whereas for ordinary solar cells using the same hypothesis the thermodynamic limit is 40%. This concept Antonio Luque expects that intermediate-band solar cells will come to fruition after 2030, with impor- is therefore attractive because it allows for tant applications arriving by 2050. potentially very high efficiency, although getting close to the thermodynamic limit will understood, even if we are still using unop- band solar cells can provide higher efficiency be very difficult. timized structures and therefore still at low at almost the same cost. efficiencies. At the moment we are only using ■■ What is the highest efficiency structures that are good for our research, ■■ What do you think the pho- reached so far, and how much higher such as InAs quantum dots in GaAs. tovoltaic industry will look like in could it rise? 5–10 years time? Today, the best efficiency is 18%. In 20–30 years ■■ Is cost one of the challenges? Over the next 10 years, I predict the propor- from now, the efficiency could reach 50% by It is too early to talk about cost. There are tion of the world’s energy generated by pho- using two cells in tandem; the thermodynam- two potential ways of applying this cell. One tovoltaics will rise to around 2%. I believe ic limit is 72% in such a set-up. If we apply the will be in concentrators, just like for multi- the intermediate-band solar cell will start to same ‘efficiency rule’ used for triple junction junction solar cells, in which case the cost of be industrialized in 2020, but will still com- cells — that the maximum efficiency is two- the device becomes very small because very prise only a small fraction of photovoltaic thirds of its thermodynamic limit — I think little material is needed. Such a potentially technology. The intermediate-band solar cell we could reach efficiencies of 50%. high efficiency — in the range of 45–50% — will truly come to fruition after 2030. It may means the cost of the tiny cell doesn’t matter become a serious contender to other tech- ■■ What is the biggest challenge for very much. The other option is to use it in the nologies by 2050 if the efficiency reaches the this technology? fabrication of thin-film solar cells together level I mentioned earlier, but probably not The biggest challenge today is still to under- with other thin-film concepts. For example, by 2020. In general, photovoltaic technology stand how the cell works. We believe that the it would be relatively cheap to put titanium needs to have a fast learning curve. The good sub-bandgap light is quite well-absorbed, but or even quantum dots in such a structure. If thing is that photovoltaic devices are based only a small part of it is converted into cur- this concept is to be successful — which it on twenty-first century science and tech- rent. Although we explain this as recombi- might — you could increase the efficiency of nology. By the end of the century, I expect nation, ‘recombination’ is just a word — we these intermediate-band solar cells from 20% photovoltaic technology to become the main don’t yet understand the specific mechanisms to 25–28%. The cost of making the whole cell provider of the world’s electricity — that is that prevent the extraction of the mode cur- would not change much as such additional what we are aiming towards with our inter- rent. This is the main challenge to be tackled processes are relatively cheap. Although mediate-band solar cell. over the next five years. I hope that by 2015 these cells face the same cost issues as all the intermediate-band solar cell will be well- other solar cell technologies, intermediate- Interview by Rachel Won

6 Nature Photonics | Technology Conference | www.nature.com/naturephotonics Nature Photonics Technology Conference New dyes, new opportunities Joint efforts within the research community to develop innovative dyes will provide dye-sensitized solar cells with many new opportunities, explains Michael Grätzel from École Polytechnique Fédérale de Lausanne in Switzerland.

■■ What is a dye-sensitized solar cell and why is it attractive? The dye-sensitized solar cell is the only solar cell that mimics the photosynthesis process in a green leaf. It uses molecules as sensitiz- ers or dyes, just like the leaf does with chlo- rophyll molecules, which absorb light and use it to generate electric charge. This reaction is entirely mimicked by the dye-sensitized solar cell. The difference, however, is that in the green leaf the charges are immediately con- verted into energy by the plant, whereas in dye-sensitized solar cells the electric charges are drawn out to make a photovoltaic device.

■■ What do you think are the biggest challenges facing dye-sensitized solar- cell technology today? The dye-sensitized solar cell has achieved a module efficiency of over 10%. This remarkable Michael Grätzel points out that the dye-sensitized solar cell has to be competitive in terms of price and figure was achieved through small-laboratory conversion efficiency to rival other solar-cell technologies. cell research, and we’ve already reached effi- ciencies of up to 12%. The losses are very small, to 1,000 nm and converting about 90% of the nice product. For example, I am sure there is a which is quite unusual. This shows that the photocurrent into electric current. This shows market for the solar lampshade made by Sony, fabrication of dye-sensitized solar cells can eas- that it is possible to fabricate a dye-sensitized although this could be a niche application. ily be scaled up. The challenge in the research solar cell from a single dye, and it’s an amazing The dye-sensitized solar cell is a technology community now is to increase this 12% effi- demonstration of the power of this technol- that will find widespread use through niche ciency to over 15% or 20%. The higher the ogy. To get efficiencies of over 15%, we need applications. efficiency, the more competitive the cell will to use this type of dye to produce a high cur- be. As there is strong competition from other rent. We also have to improve the output volt- ■■ How do you expect the photovoltaic solar-cell technologies, the dye-sensitized solar age of this technology. Fortunately there is an sector to look, in terms of deployed cell must be competitive in terms of price, capa- active research community working in this technologies and market share, in 5–10 bility and conversion efficiency. Fortunately, area, allowing new discoveries to be gathered years time? this solar cell has specific applications that are from all around the world. If someone invents I think the dye-sensitized solar cell will face unachievable using other photovoltaic tech- a new dye, he or she could become very pros- very strong competition from China because nologies. For example, it is the only solar cell perous by commercializing it. Unlike other more than 50% of the solar cells produced in that can be made into transparent glass while photovoltaic technologies that have thou- China are made from silicon. It will be diffi- being used as an electric power source — this is sands of people working on a single material, cult for the dye-sensitized solar cell to corner something that other silicon or thin-film pho- we have a wide choice of materials and every the market; there are always new materials tovoltaic technologies cannot offer. We foresee new discovery can bring a jump in conversion in this type of solar cell. The market cannot many opportunities in the design of buildings efficiency. Whoever achieves the jump will be easily be cornered by one individual with a and other creative applications. There are also rewarded — that is the great thing about the single material. But by 2020, we will certainly other applications that would benefit from the dye-sensitized solar cell. see many more applications that use dye- flexible, lightweight configuration of the dye- sensitized solar cells, perhaps even in solar sensitized solar cell, such as for charging mobile ■■ Has this cell been commercialized, farm applications. However, I expect that telephones. This is certainly very attractive, and and is the fabrication process difficult? solar farms will continue to be dominated by sales in the marketplace have started with this We have already started to commercialize silicon panel cells. I don’t think solar cells in kind of product. the dye-sensitized solar cell with a company general will dominate the renewable energy called G24 Innovations. The initial product market; there is still a huge need for atomic ■■ Do you believe we have reached the — a solar-cell-equipped backpack — is sell- energy, and I think there will be space for all maximum cell efficiency? ing very well. The Presidents of Germany technologies. It’s not necessarily a cut-throat The recent discovery of a new dye by the and Switzerland visited us recently and were competition — what we really need is more Segawa group at the University of Tokyo has delighted to receive a backpack equipped cooperation to meet the energy challenges revolutionized the whole field. The photocur- with these flexible dye-sensitized solar cells that all of us are facing. rent response curve of this dye looks like sili- for their next mountain excursion. I think con, capturing the whole visible spectrum up the consumer will decide whether you have a Interview by Rachel Won

Nature Photonics | Technology Conference | www.nature.com/naturephotonics 7 Nature Photonics Technology Conference Upbeat silicon Although crystalline silicon solar cells will dominate the market, thin-film cells have great prospects in terms of efficiency and cost, says Michio Kondo from the National Institute of Advanced Industrial Sci- ence and Technology in Japan.

common in the short term. In the long term, if crystalline-silicon-based solar cells cannot exceed the physical limit of 30%, I would be pessimistic about their future.

■■ Do you see the technology drasti- cally coming down in price over the next five years? What could help make this happen? Yes, and I think thin-film technology will be of great help. I think that the efficiency and the manufacturing technology, includ- ing equipment and process control, will help reduce the manufacturing cost drastically. We will be able to achieve a cost of US$0.50– 0.70 per watt for thin-film silicon modules, and the price of crystalline silicon solar cells will be around US1.00 per watt. It could be difficult for crystalline silicon solar cells to reach US$0.50 per watt in the next five years.

■■ Do you think the emergence of new materials will help reduce the price? Michio Kondo is positive about the future of the thin-film silicon solar cell. Of course, in the long term, the answer is ‘yes’. For example, silicon–germanium is an old material but thin films are a new technology. ■■ How many different types of silicon challenge is to reach a price of US$0.50 per Its use will help improve the efficiency and solar cell are there? watt for a solar module. This is comparable reduce the cost. In the next five years, I think There are two major types of silicon solar cell. to the competing technologies. These are the the multijunction, thin-film silicon solar cell One is wafer-based, bulk crystalline silicon big challenges of crystalline silicon solar cell is a realistic target. solar cells, which can be categorized into single- technology. However, the challenge of thin- crystalline and multicrystalline solar cells. This film silicon solar cells is more one of efficiency ■■ In five to ten years’ time, how do you type of cell can be formed in a conventional than cost. see the sector for silicon solar cells look- sandwich structure or a back-contact structure. ing in terms of deployed technologies? The other major type is thin-film silicon solar ■■ What is the typical efficiency now? This is a very big question. Even in the crys- cells. There are also some other types of silicon Have we reached the limit? talline silicon solar cell, there are many differ- solar cell, such as silicon-ball solar cells and It depends on the technology. For crystalline ent varieties of structure, for example back- concentrator-type solar cells. They are available silicon solar cells, the lowest range of module contact, heterojunction, single-crystalline but are not so popular. efficiency is around 12–13% and the highest and multicrystalline structures. Therefore, is around 17–20%. If we want to achieve an it is very difficult to predict which technol- ■■ Why are silicon solar cells attractive? efficiency of 20% in a crystalline silicon solar ogy is going to be viable in the future. But if The greatest advantage of silicon solar module, we need a cell efficiency of about we look at the recent trend, single-crystalline cells is silicon’s natural abundance and 23%. At the moment, the highest efficiency in and back-contact silicon solar cells that have environmental friendliness, but the driv- the laboratory cell is 25%. In this sense, the higher efficiencies will dominate the market. ing force behind silicon solar cell technol- research-level solar cell has already reached ogy comes from its established and robust the efficiency needed. However, many peo- ■■ Any comment on the market share in manufacturing technology. ple argue that the practical limit of crystalline terms of deployed technology? silicon solar cells can go up to about 28–29%. The total production of silicon solar cells is ■■ What do you see as the biggest increasing rapidly. In 2002 the market share challenges today facing silicon solar cell ■■ You mentioned during your talk that percentage was about 3%, and in 2009 it was technology? people are expecting to see efficiencies 20%. In the future, it will be around 25–30%. From the technological point of view, the big- of >40% in 2050. Is it going to take In the next 20 years, crystalline silicon solar gest challenge is to exceed the physical limit that long? cells will dominate the market and the share of of the crystalline silicon solar cell, namely The short-term target is to get module effi- thin-film silicon solar cells will increase. the 30% efficiency level, which is the limit for ciencies of 20% for crystalline silicon solar any single-junction solar cell. The industrial cells. I think this efficiency will become more Interview by Rachel Won

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Exhibition report

OPTO RESEARCH CORPORATION which have high transparency but low thermal production facility, which it claims to be the Fast and precise spectroradiometer stability, and typical polyamides/polyimides, world’s largest production plant. The facil- which have high thermal stability but low ity, in Miyazaki, Japan, is scheduled to com- OPTO Research Corporation showcased its transparency, the polyimide film exhibited by mence operation in 2011, and aims to bring pulse analysis spectroradiometer for measur- Mitsui Chemicals offers both high transpar- the production capacity of Solar Frontier to ing and evaluating the spectral distribution of ency and high thermal stability, thanks to its the gigawatt level. In addition, the company pulsed light at both high speed and high preci- proprietary design technology and monomer also announced that it had recently achieved a sion. The unit, the OKL-HSSR1300, is ideal for database. Three types of polyimide films are record-high conversion efficiency of 16.29% verifying solar simulator compliance with IEC available: type A, B and C, which have glass in CIS-based thin-film photovoltaic solar 60904-96 and Japanese Industrial Standards. transition temperatures of 290 °C, 260 °C and submodules by optimizing the absorber The principle of pulsed light solar simulator 280 °C, tensile moduli of 2.0 GPa, 2.8 GPa and thickness and circuit design. emissions for production lines is to create an 4.5 GPa, and tensile elongations of 8%, 18% www.solar-frontier.com/jp electrical discharge between poles on a lamp by and 15%, respectively. The total transparencies emitting a charged electrical load to the capaci- achieved are 90% for type A and B, and 87% Sharp tor. There is a resulting variation in emission for type C. The polyimide films produced by Thin-film solar module timing of 10–100 μs, which makes performing Mitsui Chemicals have relatively high and con- accurate measurements problematic. To solve stant transparency over the entire visible range. With 50 years’ development history in photo- this problem, OPTO Research Corporation For instance, the type A film has a transparency voltaics, Sharp continues to bring solar cells to teamed up with AIST, Nisshinbo, Yamashita of 90% in the range of ~360–800 nm, whereas practical use. One of the products exhibited by Denso and OK Labs to develop a trigger cir- typical polyimide films have a slowly increas- Sharp at the exhibition was its thin-film pho- cuit required for high-speed synchronous ing transparency profile from 0% at ~460 nm tovoltaic module, NA-HSL8G. Unlike normal measurements using the spectroradiometer. to <80% at 800 nm. crystalline silicon solar cells, which have a bulk The spectroradiometer has dimensions of http://jp.mitsuichem.com silicon layer of ~0.2 mm, the silicon thin-film 340 mm × 300 mm × 280 mm and can measure layer used in the NA-HSL8G is only around wavelengths in the range of 350–1,300 nm with Solar Frontier 0.002 mm. In addition, the cell is made in a tan- a wavelength resolution of 4 nm and an accu- Gigawatt-level production capacity dem configuration that includes both an amor- racy of ±1 nm. phous silicon layer and a thin film of silicon, www.optoresearch.co.jp Established in the 1970s, Solar Frontier’s mis- based on Sharp’s unique proprietary technol- sion is to create the world’s most economi- ogy. The amorphous silicon layer absorbs vis- Mitsui Chemicals cal and ecological solar energy solutions. ible light, whereas the silicon thin film absorbs Thermally stable transparent polyimide Its proprietary copper–indium–selenide infrared light. This tandem configuration (CIS)-based thin-film photovoltaic technol- extends the absorption range of the thin-film Mitsui Chemicals is a provider of many photo- ogy produces solar cells with high efficiency cell, hence increasing its overall conversion voltaic materials, including encapsulants, back- and low production costs, as well as supe- efficiency. The NA-HSL8G thin-film module sheet adhesives and edge-seal materials with rior reliability, stability, sustainability, non- measures 1,419 mm × 1,009 mm × 46 mm, and long-term reliability. The company showcased toxicity and low overall energy consumption weighs 19 kg. It has a nominal maximum out- a thermally stable and transparent polyimide throughout the entire manufacturing proc- put power of 128 W at a voltage of 45.4 V and a film that is still under development. Unlike ess. Solar Frontier took the opportunity at the current of 2.82 A. poly(methyl methacrylate) and polycarbonate, exhibition to announce its third photovoltaic www.sharp.co.jp/sunvista

Nature Photonics | Technology Conference | www.nature.com/naturephotonics 9 Nature Photonics Technology Conference

Kyosemi Nisshinbo Mechatronics Underwriters Laboratories Japan Three-dimensional light capture Non-destructive analysis Promoting product safety

Kyosemi demonstrated their light, durable The mission of Underwriters Laboratories and reusable micro-solar cell, Sphelar. Sphelar (UL) Japan is to contribute to society by pro- technology is based on tiny single-crystal sil- moting the safe product growth of renewable icon spheres measuring only 1–2 mm across. energy equipment as a power measure for cop- The spherical cells are interconnected with a ing with global warming. In September 2010 thin filament, and the strings are lined up in a the company opened a photovoltaics labora- grid pattern and encased in either clear glass tory in Ise, Japan, to complement its facilities or a clear, flexible membrane that serves as in the USA, China and Germany, allowing it to the final product. Unlike flat, conventional serve customers at regions close to its manufac- solar cells, Sphelar allows light absorption turing and deployment locations. Apart from from every direction, maximizing power providing performance and safety testing for generation without any tracking mechanism. photovoltaic equipment at its Japanese facility, It exhibits high pliability, allowing it to fit UL Japan also provides technical support for in various types of see-through, flexible or Japan’s photovoltaic equipment manufacturers irregularly shaped modules. Because each when developing their businesses and enter- cell is discrete, any combination of serial ing overseas markets. At the exhibition, UL and/or parallel connections in a mesh-wiring Japan presented its services for the photovolta- scheme is available, overcoming partial shad- ics industry under its global network, which ing problems. With the features of optical includes product safety, field evaluation, verifi- transparency, bifacial response and flexibil- Nisshinbo Mechatronics creates materials cation services and global market access. ity in layout, Sphelar allows for a wide range and equipment to make photovoltaic modules http://ul.com/jp of innovative designs and applications that with lifetimes of around 100 years. It analyses cannot be realized by flat cells. previous photovoltaic modules to reveal the DKSH Japan www.kyosemi.co.jp relationship between lifetime and three pro- Market expansion service provider duction factors: materials, equipment and TRUMPF production recipe. It also works on inspec- Formerly known as Nihon SiberHegner, DKSH Laser systems for solar modules tion methods for module production and Japan is the Japanese branch of the Switzerland- non-destructive photovoltaic module analysis based DKSH Group. Operating for more than TRUMPF, one of the world’s leading manu- technologies. At the exhibition, the company 140 years in Japan, DKSH Japan is the country’s facturers of laser technology, has developed explained their concepts for next-generation leading provider of market expansion services, lasers that produce pulses of suitable duration materials and equipment that are based on focusing on consumer and luxury goods, speci- and intensity for fabricating solar modules. their analysis. ality chemicals, pharmaceuticals, food ingredi- One of these — the TruMicro Series — is most www.nisshinbo.co.jp ents, and technology products and services. It suitable for microprocessing, and is optimized for high speed and high accuracy. There are three TruMicro Series available — the 3000, Mitsubishi Electric Corporation 5000 and 7000. TruMicro Series 3000 ena- Monocrystalline module bles selective and precise material ablation with high feed rates, and has lasers operat- Mitsubishi Electric Corporation showcased ing at wavelengths of 532 nm (TruMicro two photovoltaic modules — one for resi- 3220) and 1,064 nm (TruMicro 3120) with dential use, and the other for public/indus- maximum average powers of 12 W and 8 W, trial use — together with a photovoltaic respectively. The powerful picosecond lasers inverter for residential use. The company of the TruMicro Series 5000 can vapor- launched the photovoltaic module for resi- ize most materials quickly enough to leave dential use on 20 October 2010, during no heat-affected zone. Models operating at the conference. The module is based on wavelengths of 343, 515 and 1,030 nm at dif- monocrystalline silicon solar-cell technolo- ferent maximum average powers in the range gy, using lead-free for lower environ- of 5–50 W are available in this series. Thanks mental impact, and incorporating photo- to the combination of short pulses and high voltaic cells with an efficiency of more than average output, TruMicro Series 7000 offers 5%. Four bus bars (instead of the standard rapid, large-area material ablation with ult- two) are used to increase the output from rahigh throughput times. TruMicro 7050 each cell. The module comes in different operating at 1,030 nm has an output power of shapes — rectangular (PV-MA2000B), 750 W with a pulse duration of 30 ns, whereas square (PV-MA1000HB), right-trapezi- large power output means that fewer mod- TruMicro 7250 operates at 515 nm and has um (PV-MA1000RB) and left-trapezium ules are needed to build a system, helping an output power of 400 W with a pulse dura- (PV-MA1000LB) — accommodating to reduce the total system cost. The module tion of 300 ns. These lasers can help solar-cell rooftops with differently shaped areas. costs ¥134,400, ¥72,765 and ¥79,380 yen for manufacturers and facility builders to fulfil The rectangular-shaped panel contains 50 rectangular-, square- and trapezium-shaped jobs such as patterning, edge deletion, edge cells, producing an output power of 200 W, panels, respectively. The company expects insulating, removing, structuring, cutting, whereas the other models contain 25 cells, to sell 100,000 modules per year. drilling and marking. producing an output power of 100 W. The www.mitsubishielectric.co.jp www.trumpf-laser.com

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(AIST), is dedicated to performing compre- Kyocera hensive research into photovoltaics, from High-efficiency multicrystalline module materials research to large-scale system design and characterization. Its primary goal Established on 1 April 1959, Kyocera have is to develop the fundamental technologies advanced their cell-processing technology required to decrease the cost of photovolta- and automated production facilities to pro- ic technologies to one seventh of their 2004 duce a highly efficient multicrystalline pho- level by 2030. The centre comprises six teams, tovoltaic module, the KD235GH-2PB. The namely the Advanced Crystalline Silicon unit is useful for grid-connected systems for Team, the Novel Silicon Material Team, Thin residential, public and industrial use, as well Film Compound Semiconductor Team, the as for standalone solar-power systems in Characterization, Testing and System Team, microwave/radio repeater stations, villages the Advanced Organic Material Team and and medical facilities in rural areas. Launched the Strategic Industrialization Team. AIST in July 2010, the KD235GH-2PB, measur- and a backsheet-covered pottant to provide presented its recent results relating to the ing 1,662 mm × 990 mm × 46 mm, contains protection from severe environmental con- materials, processes, devices, modules and 60 squared multicrystalline silicon solar cells ditions. The entire laminate is installed in an systems of various solar cells, including crys- with efficiencies of >16%. The unit can pro- anodized aluminium frame to provide struc- talline silicon, thin-film silicon, compound duce a maximum power of 235 W. The cells are tural strength and ease of installation. thin-film, dye-sensitized, organic film and encapsulated between a tempered glass cover www.kyocera.co.jp quantum-dot solar cells. AIST also presented results from collaborative studies with pri- vate companies, including the Consortium provides sourcing, marketing, distribution and practical method for characterizing multi- Study on Fabrication and Characterization of sales, and after-sales services in many areas to compound junction interface structures. The Solar Cell Modules with Long Life and High help small and medium-sized companies grow microscope provides direct visualization of Reliability, and the R&D on Innovative Solar their businesses in Japan. DKSH Japan intro- atomic arrangements at atomic-level resolu- Cells with NEDO. duced their range of services at the exhibition. tion. Another way to improve the character- http://unit.aist.go.jp/rcpv/ci www.dksh.jp istics of CIGS solar cells is to study the cor- relation between the microstructures and NEDO DuPont electrical properties of the absorber layer of Moulding a photovoltaic future Solutions for photovoltaic infrastructure the cell. By using scanning spread resistance microscopy and electron-beam-induced cur- As Japan’s public management organiza- Dupont’s vision is to be the world’s most dynam- rent, MST showed that the resistance and elec- tion promoting research and development ic science company, creating sustainable solu- trical potential distributions in the cell can be while disseminating industrial, energy and tions essential for improving the health, safety visualized, allowing the relationship between environmental technologies, New Energy and quality of life for people around the world. the microstructures and CIGS characteristics and Industrial Technology Development Dupont offers a wide range of innovative prod- to be investigated. Organization (NEDO) aims not only to ucts and services for markets such as agricul- www.mst.or.jp address global energy and environmental ture, nutrition, electronics, communications, problems but also to enhance Japan’s indus- safety and protection, home and construction, AIST trial competitiveness. NEDO presented the transportation and apparel. Its broad and grow- Photovoltaics research centre contributions it had made to the development ing portfolio of solutions is key to the manufac- of photovoltaic power generation technology, turing of both crystalline silicon and thin-film The Research Center for Photovoltaics, beginning with the New Sunshine Project in solar cells. Dupont collaborates closely with founded in 2004 by the National Institute of 1974, to the present day. NEDO also show- other companies and institutions throughout Advanced Industrial Science and Technology cased a history of various photovoltaic mod- the photovoltaic industry to help increase the power output per manufacturing dollar for photovoltaic technology, while also increasing Soma Optics the efficiency and lifetime of solar modules. Portable solar spectrometer The company introduced its advanced prod- ucts and applications at the exhibition. The portable and high-performance spec- www.dupont.co.jp trometers showcased by Soma Optics — the S-2440, S-2441 and S-2442 — are especially MST designed for measuring sunlight and light Analysing CIGS thin-film solar cells from solar simulators. They are compact, with dimensions of 90 mm × 110 mm × 170 The Foundation for Promotion of Material mm, and can perform wavelength measure- Science and Technology of Japan (MST) pre- ments with a resolution of 5 nm in the range sented technical information regarding the of 300–1,100 nm, covering the operational structure and performance of copper–indium– wavelengths of silicon, dye-sensitized and gallium–selenide (CIGS) thin-film solar organic thin-film solar cells. Light detection cells. Atomic-level characterization of local is through the use of a cosine-type diffuse trometers can be easily operated using a structures is important for realizing advanced reflector. The response time is 2.5–1,000 ms computer running Windows XP/Vista/7, high-efficiency CIGS solar cells. MST showed for the S-2440 and S-2441 models, and and are equipped with a USB 2.0 interface. that high-angle annular dark-field scanning 1–1,000 ms for the S-2442 model. The spec- www.somaopt.co.jp transmission electron microscopy is the most

Nature Photonics | Technology Conference | www.nature.com/naturephotonics 11 Nature Photonics Technology Conference

Lintec Durable backsheet

Lintec, a leading Japanese manufacturer of adhesives, exhibited LIPREA, a high-quality durable backsheet for photovoltaic module applications. Made using Lintec’s proprie- tary vacuum heating and pressurizing proc- esses, LIPREA’s high durability is due to an outer coating of fluorocarbon. Two types of backsheet were exhibited at the exhibition: LIPREA-PKT and LIPREA-TFB. LIPREA- PKT is a multilayered laminate film suitable for long-term outdoor use. It has high reflec- tivity, providing high electricity generation, and a partial discharge of >1,000 V based on the IEC-60664-1 standard. IPREA-TFB has a high water resistance of <0.005 g m–2 per day — lower than LIPREA-PKT’s value of 2 g m–2 per day — owing to the additional layer of aluminium foil placed underneath the fluorocarbon coating. LIPREA-TFB is high reflectivity, providing high electricity bility, a good moisture barrier, electrical especially suitable for use in thin-film pho- generation. LIPREA offers the necessary insulation, dielectric properties and out- tovoltaic modules. In both types, backsheets functions required for high-performance standing adhesion to encapsulants. with white-coloured surface demonstrate photovoltaic modules, such as high dura- www.lintec.co.jp ules, showing the evolution of photovoltaic Technology Research and Development which aims to achieve conversion efficien- technologies such as monocrystalline, mul- Program — which will contribute to devel- cies of more than 40% by exploring novel ticrystalline, amorphous thin-film, com- opments in low-carbon technology. JST also concepts such as concentrator multijunction pound thin-film, CIS, iii–v, dye-sensitized presented some of the latest achievements solar cells, quantum structure tandem solar and organic thin-film solar cells and modules. made in its Basic Research Program, such as cells and hybrid-materials solar cells, as well With the aim of accomplishing power genera- prototype samples of organic thin-film pho- as through non-silicon-based approaches tion costs of 7 yen kW–1 h–1 by 2030, NEDO tovoltaics, microbial solar cells and polymer/ such as dye-sensitized, organic and iii–v- introduced “a photovoltaic power generation low-molecular hybrid cells. based solar cells. roadmap” (PV2030+), which shows the direc- www.jst.go.jp/alca www.rcast.u-tokyo.ac.jp/en tion in which photovoltaic technology devel- opment must progress. The University of Tokyo Nihon Entegris www.nedo.go.jp SolarQuest Easing the phototransformation process

JST As the only university-based research insti- Nihon Entegris focuses on improving the Realizing a low-carbon society tute at the exhibition, the Research Center phototransformation efficiency of solar cells for Advanced Science and Technology of the while also reducing manufacturing costs. University of Tokyo introduced SolarQuest, The company showcased its filtration/puri- an international research centre for energy fication products and new shipping contain- and environmental technology designed ers at the exhibition, emphasizing its two key to tackle global environmental problems. technologies of contamination control and SolarQuest pursues comprehensive activities plastic moulding. Contamination control related to energy and environmental tech- technology is for the filtration and purifica- nologies through research and development, tion of chemicals in different gases and envi- development strategy and diffusion strat- ronments. Entegris introduced a diffuser egy. Its emphasis is on partnerships among filter required in the vent chamber for proc- universities, companies and governments, esses involving environmental transforma- The Low Carbon Society is a research cen- both inside and outside of Japan. Currently, tion from the vacuum to the atmosphere. The tre established by the Japanese Science and its main subject of research is diffuser filter is needed to ensure that lami- Technology Agency (JST) to develop strate- generation, in which it aims to improve the nar flow — instead of turbulent flow, which gies and scenario proposals for realizing a low- efficiencies of optical-to-electrical energy stirs all particles in the chamber — keeps carbon society. Its research aims to address conversion and solar power generation sys- all particles at the bottom of the chamber. several key factors in the development of low- tems, as well as the performance of light con- Entegris’s plastic moulding technology is for carbon technologies — photovoltaics, for centrators, silicon-refining technologies and the shipping, storage and process transport example — by looking at the progress needed new cooling systems. SolarQuest presented of wafers and cells. Its main function is to for their widespread diffusion. JST present- “Post-silicon solar cells for ultra-high effi- reduce breakages during shipping, as well as ed the overall challenges faced by the Low ciencies”, an innovative research and devel- to reduce the time needed for taking wafers Carbon Society and explained its new funding opment project supported by NEDO and the and cells in and out of shipping containers. project in 2010 — the Advanced Low Carbon Ministry of Economy, Trade and Industry, www.entegris.com

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MARCH 2010 VOL 4 NO 3 www.nature.com/naturephotonics

EfficientEffffiiicientt sinsingle-photongle-photon sousourcesrces

OPTICAL MEMORY Silicon flip-flop SEMICONDUCTOR LASERSASSERRSS Fibonacci feedback LASER COOLING Cryogenic regime

Interested in lasers, Lasers, LEDs and light sources are some of LEDs and light sources? the many topics that Nature Photonics covers, papers published to date include: Nature Photonics is a monthly journal dedicated to research in all areas of light generation, manipulation • Soliton–similariton fi bre laser and detection. Coverage extends from research into the Bulent Oktem et al. (Volume 4, No 5) fundamental properties of light and how it interacts with matter through to the latest designs of optoelectronic • Random distributed feedback fi bre laser Sergei K. Turitsyn et al. (Volume 4, No 4) devices and emerging applications that exploit photons. • Light extraction from organic light-emitting diodes enhanced Research areas covered in the journal: by spontaneously formed buckles Won Hoe Koo et al. (Volume 4, No 4) • Lasers, LEDs and other light sources • Imaging, detectors and sensors • Quasi-periodic distributed feedback laser • Optoelectronic devices and components Lukas Mahler et al. (Volume 4, No 3) • Novel materials and engineered structures • A highly effi cient single-photon source based on a quantum dot • Physics of light propagation, interaction & behaviour in a photonic nanowire • Quantum optics and cryptography Julien Claudon et al. (Volume 4, No 3) • Ultrafast photonics • Highly power-effi cient quantum cascade lasers • Biophotonics Peter Q. Liu et al. (Volume 4, No 2) • Optical data storage • Spectroscopy • Generation of molecular hot electroluminescence by resonant • Plasmonics nanocavity plasmons • Nonlinear optics Z. C. Dong et al. (Volume 4, No 1) • Fibre optics and optical communications • A passively mode-locked external-cavity semiconductor laser • Solar energy and photovoltaics emitting 60-fs pulses • Displays Adrian H. Quarterman et al. (Volume 3, No 12) • Terahertz technology • Far-ultraviolet plane-emission handheld device based on • Nanophotonics hexagonal boron nitride • X-rays Kenji Watanabe et al. (Volume 3, No 10)

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20860-07 Lasers LEDs and light sources.indd 1 17/6/10 11:34:54 Future perspectives on photovoltaics