Organic Photovoltaic Technology
- Sunday, 01 September 2013
Organic photovoltaic (OPV) technology has been rapidly growing in performance and popularity over the past few years and is expected to become a major PV technology within the next decade. Here’s why.
In 2007, U.S. energy consumption surpassed 4 TWh per year, more than 25% of the world’s energy demand. More than 70% of this energy was derived from foreign non-renewable sources, making the need for renewable sources of energy increasingly urgent. The next generation of sustainable energy technologies will revolve around transformational new materials and technologies that convert practically infinite energy sources, e.g., sunlight and nuclear reactions, into electrical power.
Sunlight is the largest carbon-neutral energy source, with 89 petawatthours of sunlight reaching the earth’s surface, or almost 6,000x more than the 15 TWh consumed by humans. Solar electric generation also has the highest power density (global mean of 170 W/m2) among renewable energies. As can be seen in Figure 1, it would take only a relatively small percentage of the earth’s surface area — less than 5% — covered with only moderately efficient solar panels to more than meet energy needs worldwide. Moreover, the demand for such renewable energy sources is growing — projected over $100 billion in China alone by 2025 — and will represent a substantial market for capable manufacturers. Currently, however, only a small fraction of the world’s energy needs are generated by solar technologies. This huge gap between present solar energy production and its enormous potential is due to a number of factors, with manufacturing costs being one of the key barriers.
Thin film photovoltaic technologies offer a means to reduce these manufacturing costs by using smaller materials quantities and unique high-throughput manufacturing processes; however, many contain toxic or non-earth-abundant elements and progress in moving from laboratory to module efficiencies has been slow despite heavy investment. In contrast, thin-film OPV technology retains the benefits of other thin-film technologies but uses low cost, nontoxic, earth-abundant materials and offers the tailorability, manufacturability, flexibility, and light weight of modern plastics.
Comparison of Competing Technologies
The high capital cost of conventional PV systems is the primary reason that, after more than 50 years of development, the contribution of solar energy to global power generation remains at <0.5%. Reducing production cost is absolutely critical for broader market acceptance. Thin film technologies, including a-Si, cadmium telluride (CdTe), copper indium (gallium) diselenide (CIGS, CIS), dye-sensitized solar cells (DSSC), and organic (bulk heterojunction) photovoltaic (BHJ OPV) cells offer the potential for reduced capital costs through the use of smaller materials quantities and unique high throughput manufacturing pro cess es. Each PV technology has advantages and disadvantages, but only OPV offers the unique, transformative combination of low cost, mechanical flexibility, light weight, low-light performance, and optical transparency using materials.
Organic semiconductors are composed of inexpensive, earth-abundant, non-toxic, light elements, and have high optical absorptivity that reduces the need for thick photoactive films, all resulting in reduced module weight and leading to lower transportation and installation costs, along with less disposal waste. These materials can be easily formulated into inks, enabling extremely inexpensive solar cell production over large areas by high-throughput, roll-to-roll printing. Furthermore, all processes are performed at far lower temperatures than silicon (Si), thus dramatically reducing production energy costs and enabling fabrication on inexpensive substrates. Although the theoretical maximum power conversion efficiency (PCE) of OPVs is lower than for inorganic PVs (~ 20% vs 30%), once module efficiencies can achieve 10% or greater, the industry will shift to OPV technology because of the significantly lower-cost of production and installation. These factors will guarantee large-scale energy production at costs far lower than current Si. Moreover, OPV efficiencies have been increasing rapidly, moving from ~3% to Mitsubishi Chemical’s recent announcement of a 10%+ OPV cell in just a few years.
OPVs also exhibit better low-light performance than all other PV technologies, increasing overall output, which effectively increases efficiency beyond its rated value. Additionally, OPVs can be produced in a semi-transparent format and manufactured at high speed for encapsulation in low-cost glass or plastic. The power-producing glass can be used for new construction, replacement windows, atriums, skylights, and greenhouses, and with a power-to-weight ratio more than 3x that of CIGS, OPVs can be more readily integrated into electronics, consumer products, and even clothing, making it a truly transformative and sustainable energy technology.
The technology itself is also quite “green”: OPVs have by far the lowest energy use for the manufacturing of models (MJ/Wp), along with the lowest CO2 footprint (g CO2/Wp). The overall energy payback time (i.e. how long until a module has produced as much energy as went into producing it) is only about 2.5 months, and the capital expenditures required to build an OPV manufacturing plant are extremely small relative to other PV technologies. Consider that the cost of building a 1 gigawatt c-Si fab is approximately $2 billion; for OPV, the cost is about $200 million, or about one tenth the cost.
The architecture of a bulk-heterojunction OPV cell is shown in Figure 2.
• Active Layer. Comprised of both nand
p-type semiconductors. It is where
photons are converted into electrical
• Interlayer. Usually made from conducting polymer PEDOT:PSS. It serves to smooth ITO’s surface and increase its work function.
The photons travel through the contact materials and hit the active layer. At this point, three things may happen for the photons:
1. The photons pass straight through
the active layer.
2. The photons are reflected off the active layer.
3. The photons are absorbed by the active layer if the photons’ energy is higher than the active layer’s bandgap value, which is determined by the particular materials used.
When the energy from the photon is absorbed, electrons and holes slightly decouple to form excitons. These excitons then diffuse to the interface between the n-type and p-type semiconductors (active layer), where electrons and holes fully separate from each other, producing direct current (DC).
This current is then carried out of the cell through connections. In the end, arrays of cells convert light into DC electricity.
The performance of an OPV cell is measured by its power-conversion efficiency, or just efficiency, and is designated by the Greek letterη(eta). η measures the amount of power incident on the cell converted to electric power.
The formula for calculating efficiency is:
η = Jsc x Voc x FF,
where Jsc is the short-circuit current (when there is maximum current flowing and no voltage difference across the circuit), Voc is the open-circuit voltage (when there is no current flowing — a break in the circuit), and FF is the fill-factor (the actual power relative to the theoretical power produced by the cell).
To increase the efficiency of OPV cells, we need to improve these 3 factors.
Jsc is primarily affected by:
• band-gap: lower band-gaps will lead to more photon absorption;
• film morphology: an intimate mixing of the p- and n-type materials is needed to allow for efficient exciton dissociation;
• carrier mobility: good charge carrier mobility is needed to extract charges from the active layer.
Voc is primarily affected by:
• the molecular orbitals of the p and n materials; the difference between the highest molecular orbital (HOMO) of the p-type and the lowest molecular orbital (LUMO) of the n-type determine the Voc.
FF, is primarily affected by:
• balanced charge carrier mobility of pand n-type materials to make sure that both charge carriers are efficiently extracted and there is no build-up on charge carrier in the active layer;
• efficient charge separation and low charge carrier recombination in the active layer and at the interfaces.
Another common indicator of OPV performance is external quantum efficiency (EQE). This is the ratio of the number of charge carriers generated in an OPV cell relative to the number of photons of a given energy incident upon it. It measures the response of a cell to a given wavelength (i.e. energy) of light. The ideal shape for the EQE would be a square, where nearly all photons within the cells absorption range would be converted. In practice, however, a number of factors make this difficult to achieve.
As mentioned previously, when a photon is absorbed by the active layer and excitons are formed, it is important that they are separated from each other quickly before they have time to recombine. The way to do this is to have both the n-type and p-type materials close to exciton-generation sites, so that the ntype materials can transport the electrons away and the p-type can transport the holes away.
This requires that an exciton-generation site is usually close to a p-n interface. As a result, the morphology of the active layer becomes very important. Currently, the most common activelayer architecture is known as bulk heterojunction (BHJ). In a BHJ, the electron donor (p-type) and acceptor (ntype) materials are blended together and put in a mixture that then separates into distinct regions. Each region is separated by only several nanometers, a distance optimized for chargecarrier diffusion.
Although a significant improvement over planar designs (in which the n- and p-type materials were each in a separate layer), BHJs require sensitive control over materials morphology on a nano scale. Moreover, many variables, including materials, solvents, and the donor-acceptor weight ratio, can dramatically affect the BHJ structure. Therefore a thorough optimization of BHJs is required to maximize OPV performance.
Though efficiency is obviously a key technical parameter, module reliability and/or lifetime is of paramount importance in real-world applications. OPV materials are typically quite sensitive to oxygen and humidity, as well as photooxidation and photo-bleaching. While further development on the photoactive materials can be done to improve intrinsic stability, further development of module packaging is needed. Additionally, the stability of the device architecture itself plays a significant role, so further work is being done in this area as well. While glass/glass configurations typically show the longest lifetimes, obviously this limits the low cost, light weight, and flexibility of the device, which are some of the strongest advantages of OPVs.
Markets & Applications
The photovoltaic market is currently estimated to be between $2-6 billion, with expected growth rates of 20-30% per year. While currently at a nascent stage of market penetration, OPVs have the potential to achieve true all-in-cost competitiveness and widespread adoption. The broad classes of markets that can be addressed by OPVs are as follows:
• Niche Products. Products like energy harvesting cells, portable solar chargers for mobile applications, visible and near-IR phovtodetection, and other niche products require module powerconversion efficiencies of only 2-3%. As such, OPVs are able to address these markets today.
• Emerging Markets. Because of the lack of energy generation and transportation infrastructure, many less developed countries can benefit from the low-cost, lightweight energy sources like OPVs, particularly in conjunction with new business models like pay-as-you- go.
• Building-Integrated PV. Products like solar capture roofing and semi-transparent solar-panel windows that directly augment building power and reduce business and consumer energy bills require module efficiencies of around 5%.
• Cost-Competitive Grid Power. Enabling cost-competitive, solar-generated grid power can revolutionize the energy industry and change the face of energy production worldwide.
OPV technologies are clearly at a very early stage of maturity, but rapid advancements have been made over the past few years. Continued progress will depend on the development of new organic materials for both improved efficiencies and lifetimes, the availability of low-cost, lightweight, effective barrier films, and further work in translating lab-scale cell results to fab-scale module manufacturing. While continued investments in R&D will be needed to achieve commercial success in the future, OPV technologies can become serious players in PV, and will almost certainly play leading roles in areas where their properties such as optical semi-transparency, light weight, flexibility, and low-cost give them an inherent advantage.