BY D’Juvayne Christian, Vishal Shrotriya
What if. These two words have been placed at the beginning of questions that are largely responsible for where our world is today. What if we could travel through the air? What if we could interconnect networks and put them on one global system? In response to these ‘What ifs’, the airplane and the Internet changed the world forever. The current energy crisis has many asking “What if solar panels were inexpensive, and installing them was cheap?” California start-up, Solarmer Energy, Inc., is responding with the development of plastic solar panels.
Potential of Plastic Solar Panels
The company’s plastic solar panels, with low-cost fabrication, low specific-weight, mechanical flexibility, variety of colors, transparency, and better performance in low and indirect light, will be launched in the beginning of 2011. This is the first solar technology that has the capability to generate electricity at a cost on par with conventional fuels, making it a cost-effective renewable energy source without government subsidies. The solar panels have the potential to drive the cost of electricity generated by the sun down to 12-15 cents/kWh and direct manufacturing cost to much less than is US$1/W. This low cost, combined with their light weight, flexibility, and attractiveness, makes plastic solar panels capable of being incorporated into any product or material.
Plastic solar cells, based on Bulk-heterojunction (BHJ) technology, is relatively simple. In such a cell, a polymer active layer, consisting of a bulk-heterojunction of donor and acceptor molecules, is sandwiched between two electrodes. The donor and acceptor molecules form an interpenetrating three-dimensional bi-continuous network. The bottom electrode is a high work function Transparent Conducting Oxide (TCO), such as Indium Tin Oxide (ITO), which allows the light to pass through and acts as the anode. The top contact, which acts as the cathode, is a low work function metal, such as aluminum. The donor polymer is the active component in the cell which absorbs light, generates excitons and contributes to hole transport. The acceptor molecule, which is usually a derivative of C60 such as [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), helps in exciton dissociation where the hole remains on the donor molecule and the electron is transferred to the acceptor. Once dissociated, the charges are transported in the respective phases to be collected at the electrodes.
Increasing efficiency is a challenge for plastic solar panels because of a lack of suitable absorber polymers with the required properties. The company’s R&D team is focused on several areas, but its goal is to increase the absorption of polymers in the solar spectrum, especially in the infra-red region. The absorption edge of polymers has to be increased to around 1,100 nm to improve efficiency. Also, the energy lost during exciton separation process between donor and acceptor due to non-optimized band offset results in lower open circuit voltage (Voc). Therefore, the energy levels of the polymer and acceptor molecules have to be optimized. To achieve higher Voc, a donor polymer with a deeper HOMO level or acceptor material with higher LUMO level must be used. Low mobility of conjugated polymers results in poor charge transport and collection, which lowers the external quantum efficiency. From the aspect of molecular design, enhancing π-π stacking or the regularity of molecular structure within polymers can improve mobility. Controlling the morphology of the polymer films can result in better π-π stacking, higher absorption, higher mobility, balanced charge transport and higher efficiency. A much better understanding of morphology is needed to achieve higher efficiency. Solarmer’s R&D team is collaborating with academia (University of Chicago and the University of California, Los Angeles) to address all these issues.
Demand for Solarmer’s panels will greatly increase when 8 to 10% efficiency and product lifetime of 3 to 5 years is reached. Factors that affect the lifetime are temperature, relative humidity, light sources, and cycling of temperature, humidity and light. Also, the stability of packaging, substrates, electrodes, and load conditions affect the lifetime of the panels. As a result, advanced encapsulation technologies need to be developed to protect solar panels from these elements. “Several companies are currently developing barrier films for enhancing the lifetimes of plastic solar panels for practical applications, and we expect a cost effective solution to be available soon” says Managing Director, Dr. Yue Wu. “Historical success with similar materials in this area, plus the progress our team has made, makes me confident that we will have 3 to 5 years lifetime in the near future.”
Low-cost manufacturing will position plastic solar panels well ahead of any other solar panel technology. To realize this cost advantage, manufacturing processes must be developed to enable production at around US$30/m2. The plastic solar panel industry is making amazing progress with the technology: Plextronics, Inc. has deployed organic solar modules on NREL’s outdoor testing facility, Konarka Technologies has launched their first generation of plastic solar panels from their manufacturing line, and Solarmer has recently completed their pilot manufacturing line and will launch their first generation of plastic solar panels within a year.
The company’s pilot manufacturing line, recently completed in Los Angeles, California, U.S.A., will be capable of manufacturing small lots of solar panels through a high-speed coating process. A roll-to-roll coating process will be used to manufacture the panels at a speed of at least 10 ft/min. Solarmer will have production grade samples available later 2010 as the product launch approaches. The company plans to complete a full scale manufacturing line in 2011.
The company’s products will be used in three major application areas. The first of the three will be consumer and portable electronics. Cell phones, portable video games, and digital cameras are only getting smaller and more powerful. Unfortunately, the advances in battery technology have not been as fast, leaving consumers with powerful devices that need frequent charging. This explains the growing need for a substitute or supplement to their existing battery technology. Plastic solar panels are best to fulfill this need because of their flexibility/rollability, the low lifetime requirements of these products, and the ability to directly embed the charging function into the device. On average, these applications require around 2-3 watts of power and 18-24 months lifetime, which Solarmer will accommodate with their 4% solar panels, at the beginning of 2011. As technology improves, electronics get lighter, which is another reason the panels, under 10 mg/cm2, are the best supplement or substitute to existing battery technology.
Following portable electronics will be the smart fabrics application area. Smart fabrics are best defined as fabrics with additional functionality, also known as smart textiles. Potential applications include clothing, wallpaper, curtains, and other textile products. The plastic solar panels are ideal for integration into smart fabrics because they are flexible, are available in multiple colors, and can be made in any shape or size. One of the most promising products for plastic solar panels are solar tents. Although we use tents to experience the great outdoors, there are just some devices we cannot do without. These devices need electricity and plastic solar panels are perfect for the job. Tents need to be light weight and flexible to be packed and carried. Plastic solar panels weigh under 100 g/m2, and are the most flexible photovoltaic known to man. Also, a good percentage of tents are bought for infrequent use, which results in consumers shying away from the expensive purchase. Plastic solar panels will be the most inexpensive solar alternative. Furthermore, the panels could be used on military tents, and on uniforms to provide power for lighting, heating, and remote command technologies. With all of these unique features and characteristics, the panels add more benefits by being non-toxic and environmentally friendly. Smart fabrics are expected to grow to US$700 million by 2011, with plastic solar panels playing a major part in its growth.
Building Integrated Photovoltaics (BIPV)
The third targeted application area is Building Integrated Photovoltaics (BIPV). Governments around the world have tried to create a higher demand for the current generation of solar panels to no avail. In most places, solar is looked at as a purchase to help the environment, rather than a money-making investment. The environment is a great reason to buy solar panels, but the reality is that the world has much more investors than environmentalists. Luckily, the plastic solar panels will intrigue both types. The panels will deliver large-scale power to commercial and residential buildings at an affordable price, without compromising aesthetics. With the company’s transparent solar panels, power can be produced by harvesting only a portion of light from the sun. By combining plastic’s flexibility, easy installation, and better performance in low and indirect light conditions, a building’s carbon footprint can be significantly reduced or eliminated. Building integrated photovoltaics is the area the company will enter in 2012 because of the longer lifetime requirements. BIPV will need plastic solar panels to increase their lifetime to at least 5 years lifetime. According to Plastics Technology Online, in the next decade, thin films should grow from 10% of today’s solar market to 40%, which presents a significant opportunity for the plastic solar panels.
A unique characteristic that should open more markets for plastic solar panels is their potential for being semi-transparent or translucent. One limitation of photovoltaic polymers is the limited absorption in the solar spectrum. In addition, the polymer active layer in the solar panels are very thin, about 100-200 nm. As a result, a significant amount of light remains unabsorbed. However, this limitation can be transformed to a benefit by using these polymers to develop transparent solar panels. In transparent cells, a very thin polymer active layer is sandwiched between two transparent electrodes. This type of cell does not rely on a grid-type or mesh-type electrode, which has alternate opaque and transparent areas, but is based on the company’s proprietary transparent electrode technology. In this cell, both electrodes are truly transparent throughout the film surface, with transmittance as high as 85%. This opens up new markets for the technology, such as the power windows.
D’Juvayne Christian is Marketing Communications Coordinator at Solarmer. Dr. Vishal Shrotriya, Director of Business Development, has been active in the field of organic electronic materials and devices for the last 8 years (http://www.solarmer.com/).
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