As part of the run-up to the Photovoltaics Beyond Conventional Silicon conference which ran from 17 to 18 June in the USA, IDTechEx – the organiser of the event – conducted widespread market research and gathered information about technological improvements in the field of solar energy.
According to one comprehensive report that gives a thorough analysis of printed and thin film photovoltaics and batteries - covering the technologies, markets and players - silicon solar cells are seen in many places and represent approximately 95% of current photovoltaic implementations, but the technology is limited. Crystalline silicon will never give tightly rollable devices, let alone transparent ones or even low-cost power generation on flexible substrates.
Fortunately there are many new alternatives. Proprietary nano-particle silicon printing processes are developed by companies such as Innovalight and they could deliver many of the photovoltaic features that conventional silicon can never achieve, eg, they can be printed reel to reel on stainless steel or other high temperature substrates.
Technologies beyond silicon
A lot of the work on the next generation of photovoltaics is directed at printing onto low-cost flexible polymer film and ultimately on common packaging materials. The main contenders are currently:
* CIGS (copper indium gallium selenide).
* CdTe (cadmium telluride).
* DSSC (dye-sensitised solar cells).
* Organic photovoltaics.
Several companies, universities and research institutes are hard at work in different development stages of these technologies with large scale plants being built across the globe.
Printing techniques: adoption and commercialization
Along with other manufacturing techniques, printing (or printing-like) technologies are gradually being adopted, as they can be considered to be some of the fastest, least expensive and highest volume manufacturing techniques. With printed electronics becoming more prevalent, there is an increasing need for power to supply them; printing is amenable to a large number of different types of devices with the possibility of integration (eg, to provide onboard power etc).
Organics + concentrators = organic concentrators
The pursuit of the 'holy grail' of the photovoltaic industry, grid parity - the point at which photovoltaic electricity is equal to or cheaper than grid power - has led to worldwide research in various technologies and approaches that would lead to low-cost manufacturing, low-cost materials, highly efficient photovoltaic cells, or all of the above.
Organic photovoltaics, with their promise of very large, ultra low-cost implementations and with the challenges they are facing (such as increasing efficiency, lifetime and stability), are attracting a lot of attention from both start-up companies and academic institutions that are researching ways of improving their performance characteristics.
Dr Alex Mayer at Stanford University has been researching the behaviour and efficiency of semi-crystalline polymer solar cells by using a highly crystalline polymer in a bulk hetero-junction with fullerenes. Dr Mayer says: "pBTTT is a highly crystalline polymer; we thought that it would never work in a bulk hetero-junction due to this high crystallinity. However, there were some surprises that led to a moderately efficient solar cell".
Plextronics has also been developing organic solar cells and is unveiling its newest conductive inks that would enable a single junction organic solar cell with 5% efficiency.
Brad Hines, CTO and founder of Soliant Energy, believes that grid parity can be achieved with efficiency levels that are currently offered only by concentrator technologies. Fourteen 5-inch silicon solar cells have equal power output to 1 cm² of multijunction solar cells.
According to Brad Hines: "Dense packing of concentrator elements increases module cost but decreases installation cost." The Fresnel lenses used achieve sunlight concentration of 625 times.
At MIT, researchers are combining these two approaches, using concentrating technologies on organic luminescent dyes. The Soft Semiconductor Group of MIT is looking at novel approaches of increasing efficiencies of organic structures and one of them is the study of luminescent solar concentrators.
According to Dr Jon Mapel: "New semiconductors and fabrication techniques alone will not achieve large cost savings", so effort is focused on increasing the efficiency of solar cells. This approach avoids conventional concentrator routes and promises a simple, low-cost construction with a structure that collects and concentrates light on the PV cells at the same time.
Around the world with dye-sensitised solar cells
Annemarie Huijser, during her PhD studies in TU Delft, has succeeded in substantially improving a process in a type of solar cell which is similar to Grätzel cells. In the case of Grätzel cells, however, the dye and semiconductor are so close to each other, they are almost blended. As a result, the excitons do not need to move that far. One disadvantage of this type of cell, however, is the complicated method of charge transport. For this reason, Huijser chose to adopt a different approach and use this simple dual-layer system of dye and semiconductor.
Huijser compares dye molecules to Lego bricks. By varying the way the bricks are stacked and observing how this influences the exciton transport through the solar cells, she studied the best sequence of dye molecules. Excitons need to move as freely as possible through the solar cells in order to generate electricity efficiently. Huijser succeeded in increasing the average distance which the excitons move in the solar cell by 20 times, up to a distance of approximately 20 nanometres, comparable to systems found in nature. This substantially increases the efficiency of the cells.
In another development, the R&D team at Dyesol, led by Dr Hans Desilvestro and Ravi Harikisun, has demonstrated a novel tandem dye solar cell with over 10% efficiency in average light conditions. The importance of this achievement is that the cells are of industrial size and are manufactured using Dyesol's standard glass substrates, pastes and electrolytes.
The device utilises two dyes back to back to capture a much broader spectrum of the light. The first dye is the standard high purity B2 dye manufactured and sold by Dyesol in bulk quantities and the second dye is a novel near-infrared dye. The early success of this R&D project, which was only commenced in March this year, has encouraged the company to establish a new business area in tandem products to further develop this product and other tandem devices.
Earlier in the year, Dyesol and Corus, a leading European steel manufacturer, commenced the joint Welsh Assembly Government (WAG) sponsored project for the application of dye solar cell photovoltaic technology (DSC PV) on steel sheeting as part of a continuous manufacturing process. WAG is providing funding under the SMARTCymru program and the companies are establishing facilities at St Asaph and Shotton to carry out the project.
By using a popcorn-ball design - tiny kernels clumped into much larger porous spheres - researchers at the University of Washington are able to manipulate light and more than double the efficiency of converting solar energy to electricity with DSSCs.
The group made very tiny grains, about 15 nanometres across. They then clumped these into larger agglomerations, about 300 nanometres across. The larger balls scatter incoming rays and force the light to travel a longer distance within the solar cell. The balls' complex internal structure, meanwhile, creates a surface area of about 93 square metres for each gram of material. This internal surface is coated with a dye that captures the light.
The experiments were performed using zinc oxide, which is less stable chemically than the more commonly used titanium oxide but easier to work with. The overall efficiency was 2,4% using only small particles. With the popcorn-ball design, results presented show an efficiency of 6,2%.
The researchers are now hoping to transfer the concept to titanium oxide, which gives the highest efficiency DSSCs.
A claimed breakthrough barrier technology from Singapore A*STAR's Institute of Materials Research and Engineering (IMRE) protects sensitive devices like organic light emitting diodes (OLEDs) and solar cells from moisture, 1000 times more effectively than any other technology available in the market, opening up new opportunities for the up-and-coming plastic electronics sector.
IMRE has already signed agreements with a number of companies to advance the technology into the commercial domain. This includes a collaboration agreement with G24Innovations, based in Wales, in order to look into developing the films for use in solar cells so as to improve the lifetime of DSSCs.
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