History and Function of Polymer-Based Organic Photovoltaic Cells

Thes bandgap describes the area of the solar spectrum that the material absorbs…

Introduction to OPVs and comparison with traditional PV technologies

Introduction

As populations outside of Europe and North America begin to enter the global middle class, one of the most notable effects of that transition will be a skyrocketing worldwide demand for electricity. This can already be observed in the worldwide trends in electricity consumption (Figure 1 [1]). In order to build this new growth on a sustainable foundation, much of this energy must come from renewable, nonpolluting sources.

Figure 1) Worldwide Electricity Consumption 1980-2012 US EIA [1]

Of all renewable sources, solar energy has by far the greatest potential to meet this demand. The total energy solar energy that strikes the earth every hour is greater than the yearly energy consumption worldwide. Solar power, also known as photovoltaics, is the direct conversion of this solar energy to electricity. Solar cells, which accomplish this process, are increasingly being utilized for power generation worldwide. Accordingly, a tremendous amount of academic research is being put toward developing new types of solar cells and improving existing ones. However, current photovoltaic materials are based on elements that are rare, expensive, or otherwise difficult to work with. Photovoltaic materials based on earth-abundant carbon, such as those used in our group and many others worldwide, are one way of meeting this problem.

Solar Cell Operation and Function

Solar cells are made of a semiconducting material (crystalline silicon, or inorganic films or polymeric thin films, more on these types later) sandwiched between two electrodes. In a solar cell, light energy from the Sun excites an electron in a semiconducting material from the occupied valence band to the unoccupied conduction band. These charges can then move to the electrodes to create an electric current. One important property of the semiconducting material which makes up a solar cell is its bandgap. This describes the area of the solar spectrum that the material absorbs. A good solar cell material will ideally absorb as much of the solar spectrum as possible.

Once a solar cell is made, how does it actually generate electricity? Once a solar cell is illuminated, it will respond to different input voltages with different current produced, creating a curve such as the one shown below (Figure 3). This shows several important solar cell parameters. Short circuit current, J_sc represents the maximum current, where no voltage is provided. J_sc relates to the amount of light absorbed. Open circuit voltage, V_oc, similarly, is the voltage at which there is no current. Where the product of these two values are maximized is the operating voltage and current, at which point the power from the cell is maximized. V_oc is related to the bandgap, and thus the energy of the photon absorbed. Fill Factor, FF, is related to the overall quality of the cell and the contact resistances between the various component parts. FF is represented in the diagram below by the hashed area divided by the shaded area. The most important factor, the percent conversion efficiency, is the percent of input power which gets converted into electricity. PCE is the multiple of the previous factors divided by input power:

Comparison of OPV with Traditional PV

Figure 3) Peak Research-Cell Efficiencies chart, NREL [2]

Single junction, non-concentrator solar cells can be divided into three broad categories (Figure 2, blue, green, orange [2]). The first category is solar cells based on crystalline silicon. These represent the vast majority of the commercially available solar cells on the marketplace today. They are very well-established systems with numerous producers. Crystalline silicon solar cells can be expected to achieve ~20% PCE. The downside to these is that they require large amounts of high purity silicon, which makes them costly, heavy, and significantly exposes the manufacturers to any fluctuation in the overall price of silicon [3].

The next category of solar cells is based on thin films of other, rarer inorganic elements. Examples include Cadmium Telluride (CdTe) and Copper Indium Gallium Selenide (CIGS) based solar cells. These solar cells are thinner and lighter than crystalline silicon cells and ultimately give similar efficiencies, although they use rarer, more expensive, and somewhat toxic elements. Still, despite any disadvantages, the first modules are becoming available commercially [4].

The final solar cell category concerns newer solar cell technologies which are still mainly in the research stage. Polymer solar cells (PSCs), which the rest of this article will concern, are one of these. PSCs are formed of a thin film of material, like inorganic thin films mentioned above. This means that they are also lightweight and require less material to produce. Unlike inorganic thin films, PSCs are made of organic material. This means that they have the additional advantage of being flexible, and easy to manufacture through solution processing technologies, such as roll-to-roll printing, which would dramatically cut down on manufacturing costs. In addition, though the polymers are used to fabricate them are currently expensive, they are based on carbon which it itself quite common. This means they have the potential to be much cheaper than silicon cells, which use much more material, or inorganic thin films, which use elements that are intrinsically rare.

Despite these advantages, there are some challenges to overcome, as well. The component materials are currently expensive to produce, though this may be improved via streamlined reaction development and economies of scale in the future. Some reactions involve toxic intermediates which require care to work with and dispose of properly. In the final device, PCEs achieved are low, with a maximum of about 12%, about half that of the established silicon devices. This is typically caused by a low V_oc, indicating energy losses within the device. From an academic perspective, work is being done in all these areas to address the deficiencies [5]. As it is, a few companies are already working to commercialize OPV devices [6].

OPV materials History and Development

What are Conjugated Polymers?

Though the field of synthetic polymer science is over a hundred years old, for the vast majority of this period polymers have been the domain of the materials engineer. They found a wide variety of use in products as diverse as building materials, packaging, and textiles. Electronically, however, all these different materials were exactly the same; they act as insulators, making them uninteresting to the electrical engineer.

Polyacetylene, the first example of a conducting polymer, was developed accidentally, by Hideki Shirakawa in 1977. He worked with collaborators Alan MacDiarmid and Alan Heeger to improve the conductance and study the system, and were able to reach conductivities as high as many metals. Though polyacetylene itself proved to be too difficult to work with and unstable to be used commercially, their discovery lead to an entire field of conducting polymers, including poly(p-phenylene vinylene) (PPV) used in some organic light emitting diode (OLED) displays and the polythiophene based materials used in PSCs [7]. This work led to them being awarded the 2000 Nobel Prize in Chemistry.

History and Architecture of a Polymer Solar Cell

Silicon solar cells are composed of a layered structure of p-type semiconductor (positive charge, i.e. hole transporting), n-type semiconductor (negative charge, i.e. electron transporting) and electrodes (Figure 5). P-type materials are also known as electron donor materials and n-type materials are likewise known as acceptor materials. Light excites an electron in one semiconducting material, creating an electron/hole pair known as an exciton. The electron and hole separate at the interface between the two layers and travel through their respective semiconductors to the electrodes (Figure 4). The first organic analogue to this device structure was created in 1986 by C.W. Tang. His solar cell used p-type copper phthalocyanine and n-type bisbenzimidazo[2,1-a:2′,1′-a‘]anthra[2,1,9-def:6,5,10-d‘e‘f‘]diisoquinoline-10,21-dione (Figure 6) [8].

Figure 4) Pathway of electron 1à2 (excitation with light) 2à3 (movement through device) 3à4 (electron reaches electrode)

The Tang cell worked exactly analogously to a crystalline silicon solar cell, but had only 1% PCE. This was good enough for a proof of concept but needed to be much higher for real-world applications. It was found that a serious problem in the cell was that not all excitons reached the interface where they separate into free charges. Due to this problem, little work was done on OPVs until 1995, when a new device architecture known as a bulk heterojunction (Figure 5) was developed [9]. In this case, rather than being stacked in layers, the donor and acceptor material are mixed together into an amorphous structure that has different donor and acceptor regions. Not only does this solve the problem of low surface area, but it is much cheaper to create than a bilayer system in that the bulk heterojunction can be made by simply mixing the various components together in an appropriate solvent and coating them onto an appropriate substrate, such as a glass slide with electrodes attached. As the solvent dries the bulk heterojunction forms spontaneously, assuming the donor and acceptor materials interact properly. This phenomena is unique to organic solar cells, and is highly advantageous in that organic solar cells can be made via printing out a solar ink from a commercial printer. This would decrease manufacturing costs, especially startup costs, tremendously. Incredibly, this bulk heterojunction develops for a wide variety of different organic donor and acceptor material combinations.

Figure 5) Solar Cell Architectures

Historical Material Systems

Early on, the search for bulk heterojunction materials for PSCs focused on three different classes of materials. PPV-based materials, mentioned before due to their use in OLEDs, were one such candidate, such as MEH-PPV, which was used in the first bulk heterojunction solar cell. Related materials such as nitrile-substituted CN-PPV were also used. Another class of materials was based on fluorene, such as PFDTBT. Finally, polythiophenes such as P3HT were also used (figure 6). These polymers had various problems that kept PCE below 5%, limiting their use. PPV derivatives generally exhibited good V_oc, indicating good energy level matching, but poor J_sc, due to poor absorption. In contrast, P3HT and Fluorene-derivative cells exhibited poor V_oc but good J_sc. P3HT eventually won out when Professor Y. Yang at UCLA achieved a PCE of 4.4% by modifying fabrication conditions [10]. This was soon improved to over 6% with minor modifications to the system. This high PCE combined with the easy fabrication resulted in the P3HT system being an early standard in PSC research, with the Yang paper being cited over 3500 times and P3HT acting as the basis for most attempts to commercialize PSCs.

Figure 6) Early OPV materials

Though P3HT demonstrated for the first time that PSCs might have real commercial value, several difficulties with the system remained. Chief among these was the wide bandgap of P3HT, which in turn limits the amount of the solar spectrum which may be absorbed. One attempt to address this was made in our group with renewed interest in alternating copolymers, such as were explored early on with PFDTBT. In this class of material, a strong electron donor subunit and a moderate electron acceptor subunit are combined to make a polymer that is overall an electron donor. This culminated in the synthesis of PTB7. Developed in 2009, PTB7 had a PCE of 7.4%, which was a record at the time. The renewed interest in donor-acceptor copolymers resulted in significant academic interest in the field, and since then the record has been broken several times, with several current examples of polymers having over 10% PCE [13].

Stability and Commercial Viability of Organic Solar Cells

Moving from an academic to commercial perspective, not only is the efficiency and cost of a solar cell device an important parameter, but so too is the stability of the product. Early Silicon photovoltaic panels were sold with only about 5 year warranty, but currently that has increased to about 25 years [14]. As such, this shows that consumers will have an expectation that a solar cell product will have a significant lifetime. On the other hand, this shows that stability of devices might start slowly and increase as the industry becomes more mature.

Concerning stability, there are two different parts of the solar cell we might examine independently. These two parts are the active layer materials discussed in the previous section, which form the bulk heterojunction, and the other parts of the device such as the electrodes and interlayer materials, which connect to the bulk heterojunction and collect electricity from it. Both of these parts of the cell have different engineering requirements to optimize them for long term performance.

The electrodes and interlayer have received relatively little research effort as compared to the active layer materials. Research on stability has generally shown that these parts of a solar cell are a major target of environmental factors such as air and moisture. Oxygen from the air will easily oxidize the metallic electrodes. Similarly, the interlayers are highly hydroscopic and acidic which will cause damage to the cell when exposed to moisture from the air. There are several ways to combat these problems. Research is being undertaken into new interlayer materials that are more stable [13]. An alternate device structure known as an inverted cell (more on this in the following section) uses more stable metals for the electrodes and prolongs cell lifetime [14]. Finally, simply encapsulating the cell in a layer of glass or plastic will prevent most environmental damage to cell if done properly.

As compared with the electrode and interlayer materials, the active layer materials are relatively stable toward air and moisture. Thermal- and photo-oxidation of the active layer materials may occur over time, though both of these processes are slower than the air and moisture damage described previously. These problems may be minimized with high quality polymers and well-constructed devices. In addition to these processes, over long periods of time, materials may move around in the bulk heterojunction, damaging the organization. Techniques to create cross-linked structures post-fabrication may be able to hold the bulk heterojunction in place without sacrificing efficiency [15]. A recent test by Krebs and coworkers [16] found that properly encapsulated solar cells could retain up to 95% efficiency after 1 year of continuous operation, which is similar to silicon cells during the initial stages of commercialization.

Future Directions in PSCs

Other than improvements in donor materials, there are a number of directions in current OPV research attempting to utilize the intrinsic advantages of OPVs. One of the most active areas of research currently is use of ternary solar cells. Ternary solar cells use multiple donor or acceptor components to absorb more strongly over the solar spectrum, which increases performance. This is in many was much like a tandem solar cell (figure 7) which may be made of either inorganic or organic materials. However, one of the drawbacks of a tandem cell is it requires additional engineering work to enable the different solar cells to operate in parallel. In contrast, in a ternary solar cell, all components are mixed together and cast identically to a normal, single junction cell, however, the increased absorption provided enables the cell to have higher efficiency than a regular cell, but without any increased production cost [17].

Figure 8) A) Tandem Cell Architecture (NREL SJ3 Concentrator Cell [2]) B) Ternary Bulk Heterojunction Cell

Another modification to the solar cell architecture is the use of an inverted device structure. A conventional device structure uses the cathode as the transparent electrode and a metal such as Calcium or Aluminum as the anode. This may be problematic because these metals are easily oxidized. Recent research has swapped the nature of the electrodes and instead used a transparent anode and metallic cathode. In addition to providing much greater stability toward oxidation, these inverted cells have proved to be equal or slightly higher in efficiency as compared to similar conventional devices [14].

Finally, efforts are underway to streamline the production process of the polymeric materials. These polymers are generally produced academically via reactions such as the Stille, Suzuki, or Kumada coupling reactions. These reactions are very strong and versatile in enabling the production of a wide variety of materials. Unfortunately all of them use organometallic intermediates which create harmful waste and add to production costs. Recent research into Direct Arylation Polymerization [18] is enabling the production of the same materials without the organometallic intermediates, making production of OPV polymers both more streamlined and greener.

Conclusion

Organic bulk-heterojunction polymer solar cells are a topic that is currently receiving much attention in the academic realm. They have strong promise as a next-generation solar energy technology because the plastics involved in making them have the potential to bring costs down below that of current solar cell technology if they were produced on a mass scale. In addition, there are several advantages to PSCs that current technologies do not share. As a thin film technology, PSCs are flexible, lightweight, and use only a small amount of active layer material, giving them an advantage over the current silicon devices. Unlike CdTe thin films, PSCs use cheap, nontoxic carbon and can be fabricated using very simple roll-to-roll printing. In addition, some other possibilities are unique to OPVs, such as the use of a ternary structure which could enable the same advantages of multilayer cells with none of the increased demands to device fabrication.

Against these advantages there are several problems which need to be overcome before the technology can be commercialized. Low PCE compared to traditional cells, due mainly to problems with low V_oc is currently the biggest issue from the efficiency perspective. Newer PSC materials should address this point. Similarly, work is being done to make polymer synthesis greener and cheaper. Finally, efforts are already underway to develop encapsulation techniques for these materials which would enable them to function in the real world.

Solar energy is a booming field, and with good reason; it is a completely renewable technology with great potential waiting to be harvested. However, the current technologies do have room for improvement. Traditional silicon solar cells are well established but are bulky and could become expensive if the price of silicon peaks. The usual competitors to silicon cells, CdTe- and CIGS-based devices, use rare and toxic elements. OPVs have the potential to solve all these issues. The problems left to be addressed in the OPV world are well known and solving them is receiving much attention in the field. It is our hope that, once these concerns are addressed, OPVs will take their place as an important renewable energy technology for a greener world.

Acknowledgements

I would like to thank my advisor, Prof. Luping Yu, for his support and knowledge throughout my graduate career thus far. Without him, this article would not be possible. Similarly, I would like to thank my coworkers in the Yu Research Group for help and camaraderie during my time here, especially my colleague Donglin Zhao.

Doctor Alexander M. Schneider, PhD in Chemistry at The University of Chicago. Doctor Schneider is specialised in research on polymer solar cells.

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