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Organic Photovoltaics Summer 2009 Qualifier exam Arun Luykx (107 299 260)

Alternative Energy The future of energy will not be as we know it today. Fossil fuels will run out, at which time alternative sources of energy will have to be utilised. Currently these alternative sources do exist, but for political (nuclear) or economical (solar, wind) reasons, or just the fact they don’t quite work yet (fusion), are not mainstream. Solar panels provide a source of energy with minimal infrastructure changes, a high level of safety, and will provide power as long as the sun shines. As an added benefit privately owned solar panels can return energy to the power grid, making every home a potential mini power-plant. Plans have been set up by power companies offering significant cost benefits for customers who provide power back to the grid. A significant disadvantage of solar panels is their cost. The cost of electricity using a typical solar cell is around $0.106/kWH (cost of purchase + installation + incidentals, averaged over 30 years) [1], whereas grid electricity goes for as little as $0.031/kWH (source: Pepco). Interestingly, after all applicable taxes and franchise fees were added, this author found the price of grid-electricity to be around $0.196/kWH. One way to lower the cost of a solar cell is to mass produce it. Current technologies do allow for this, but other factors come in to play that offset the cost benefits. Research is being done to improve the efficiencies of solar cells by both improving the materials they’re made of, and the way they’re installed [1]. A novel method of solar cell production is by using organic materials. This would allow for ‘plastic solar cells’, which brings other advantages [2]: • Physical flexibility • Cheap production methods • Potentially higher yields Right now, due to lower power outputs, a mentality of ‘cheaper means more’ is being used to advocate research in organic solar cells and their implementation in the market. This paper looks at some of the approaches to designing organic solar cells, and proposes a novel approach to the problem.

Some Physics To illustrate the difference between organic solar cells and inorganic solar cells currently available on the market, we must look at some physics. This section deals with some fundamental differences between the two technologies, and illustrates the compelling


Arun Luykx

Organic Photovoltaics

Qualifier Exam

difference between the two. Charge Transfer Mechanisms Inorganic semiconductors have two energy bands, the conduction band and the valence band. These are separated by an energy gap (hereon referred to as E g). To produce a current, an electron must be excited from the valence band to the conduction band, where it can contribute to current-flow. Photons with energies of E g (or more) can excite the electron so it reaches the conduction band. Figure 1 below illustrates this for an inorganic solar cell.

QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture.

Figure 1, schematic of an inorganic solar cell [1] The heart of the matter lies in the relationship between the energy of the photon (and thus the energy delivered to the electron) and its wavelength: hc E photon = λ where h is Planck’s constant, c the speed of light, and λ the wavelength. From this equation we see that with decreasing wavelength the energy increases. Thus, a semiconductor like Si needs just 1.1 eV to excite an electron. This corresponds to photons with a wavelength of 1128nm. In this case, higher energy photons will also excite electrons, so wavelengths lower than 1128nm will also work. In contrast, ZnO with a bandgap of 3.2 eV would only be suitable for UV wavelengths, which are not very prevalent in sunlight. The goal is to find a material with band gap that will allow the excitation over the spectrum of sunlight. The figure 2 illustrates the relationship between band gap energy and wavelength, and also shows (in arbitrary units) the black body spectrum of normal sunlight on earth (also referred to in industry as the AM 1.5 standard). Si easily fits in this spectrum, ZnO falls out of the usable area. Despite this, ZnO has some other properties that could be beneficial. Inorganic solar cells have been shown to have average power conversion efficiencies (i.e. the amount of sunlight converted to usable power) of around 12%, with some reports as


Arun Luykx

Organic Photovoltaics

Qualifier Exam

high as 23.4%, and one case of 42.8% [3]. Connecting large amounts of inorganic solar cells would produce usable amounts electricity, but due to cost and size concerns this is not feasible. Organics are significantly cheaper to produce, and can be made in flexible formats. Despite lower output efficiencies they are cheap enough that they can be bought in mass amounts, making them an interesting choice for the future of solar cells.

Figure 2, AM1.5 and various materials with their band gaps [modified from 2] Organic photovoltaics Organics operate on a somewhat different platform. In this scenario, the energy of a photon is used to create an exciton (an electron-hole pair) in a polymer. Through the process of diffusion the exciton reaches an interface (of different materials with different electronic affinities, more on this later) where it is energetically favourable for the electron in the exciton to cross the interface. At this point the hole and electron are separated, the hole in the polymer and the electron in another layer, but are still connected due to Coulombic attraction; this is known as a polaron pair. The electron then percolates to a conductor where it is transferred to a circuit [2]. Figure 4(a) illustrates this.


Arun Luykx

Organic Photovoltaics

Qualifier Exam

(3) (4)

HOMO (1) Photon energy (2) LUMO (4) Figure 3, The energy levels in an organic PV

Taking a closer look at the charge interface one finds two energy levels: the HOMO, or highest occupied molecular orbit, and the LUMO, or lowest unoccupied molecular orbit. One could consider them analogous to the valence and conduction band (resp.) from inorganics. When the photon’s energy is applied to an electron, it jumps up to the LUMO band (steps 1 and 2 in figure 3). Once the exciton has arrived at the interface it crosses over from the LUMO level in the donor to the lower LUMO level in the acceptor (step 3). At this point one has a polaron pair bound by Coulombic forces. The electric field induced by the (appropriately chosen) electrodes forces the electron and the hole to drift to the appropriate electrode, and create a current (step 4). This does not work for all polymers. To do this the polymer must be conjugated; i.e. have delocalised electrons. These are electrons not associated with a single atom or bond, and so can be ‘transferred’. Normally in a p-orbital, after being energised by a photon they are promoted from π to π*. Like in inorganics, the energy of the photon has to correspond to a certain value, in this case the binding energy to create an exciton (and not the energy required to cross the band-gap as with inorganics). Binding energies of excitons in polymers have been reported to be between 0.4 and 1.0 eV [5], making them ideal for solar cell applications. Recombination, where the electron recombines with a hole (and therefore does not exit the cell and contribute to current) is the largest problem with organic PV’s. This is caused mostly due to a short exciton diffusion length of 10nm [6][7]: excitons often do not reach the interface for the charges to split up (figure 3b). Charge traps within the material also cause recombination (figure 3c). The percentage of formed excitons due to photons is known as quantum efficiency, and is a reflection of how well the materials are at absorbing solar energy.


Arun Luykx

Organic Photovoltaics

Qualifier Exam

QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

Figure 4, Charge separation in an organic solar cell [A]

Device structures A typical Si based solar cell works (as discussed before) on the principle of the PN junction. To create an N-type material Si is doped with P, and with B for a P-type material. The extra electron from P and extra hole from B accomplish this. In the simple solar cell, the n-type is on top of the p-type, with electrodes forming a grid on top of the n-type, and a back electrode on the p-type side. Light shines on to the n-type, exciting the electron (as discussed above), which leads to a current. The trick in this scenario is to have as much light absorption on the n-type layer as possible. Factors that reduce the absorption are reflectance, and the size of the top grid, but also intrinsic properties such as the quality of the materials used and the power of the light source. The different characteristics of organics require different architectures, as we will now see. Single Layer PV This is the simplest organic PV, where an organic film is sandwiched between two conductors (as illustrated by the QuickTime™ and a TIFF (LZW) decompressor image to the right [2]). One of the conductors is transparent (ITO, indium tin oxide, for example) which allows for are needed to see this picture. photon absorption. The charge separation occurs at the junction of the polymer and the bottom electrode (and hence is also known as a Schottky device). Though this setup may work, the actual yield is extremely low, often with conversion efficiencies of less than 0.1%. This is due to various factors leading to recombination (and the electron not leaving the solar cell), such as the electric field at the interface not being sufficient to break up an exciton, or electrons recombining with holes on their way towards the electrode. Another common problem is reflection of the incoming photons. There are several significantly more efficient schemes. Double Layer Cells [7]


Arun Luykx

Organic Photovoltaics

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Rather than just using a single polymer, in double layer cells an acceptor polymer (a polymer with more holes) is placed on a donor polymer (a polymer with more electrons). This creates two distinguishable layers, an acceptor (A) and donor (A) layer, which create a charge-interface. This is illustrated in the top right image of figure 5. Here, photons go through the transparent conducting oxide (TCO) and create excitons in the donor material. These excitons diffuse to the D/A interface and break up there: the electron goes into the acceptor layer (as this is energetically favourable; discussed with figure 3 above). From there, due to the induced electric field from the electrodes with differing work functions, the electron moves towards the electrode and goes into a circuit. Important considerations for this are the types of A and D materials to chose. The A material should have high electron affinity, and the D material low ionisation energy. If this is not the case one will still get excitons and even charge transfer, but the excitons will not split. Electrostatic forces are created at the A/D interface which result in an efficient exciton breaking. Typical D materials are P3HT, PPV, etc. An example A type material would be PCBM.

QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture.

Figure 5, Various inorganic PV cell architectures [2] An issue with this setup is that the excitons typically only have a diffusion length of 10nm, so the D layer dimensions should be in the same range. Otherwise the exciton may never reach the interface. On the other hand, a thickness of at least 100nm is necessary to absorb enough light. Still, this is much more efficient than the single layer setup, with efficiencies reported from 0.04% up to 1% [2][7]. Blend cells [8][9] In the double layer setup the A/D interface is limited by the surface of the device. In a blend the A and D materials are interspersed, creating a much larger interface. There is no ‘one line’ between the two, rather there are many interfaces all wriggling around each other. This creates many more opportunities for excitons to break up, as excitons are now


Arun Luykx

Organic Photovoltaics

Qualifier Exam

more likely to reach a charge interface. Ideally, the A material would be attached to the bottom electrode, and the D to the TCO, thereby allowing for a much more easy flow of current. The top left of figure 5 illustrates this, as does figure 4. The most pressing concern with this is that it is very possible a portion of A material does not reach an electrode, causing the electron to recombine with a hole (this would be a charge trap, figure 3c). However the benefits outweigh the disadvantages, with reported efficiencies of 2.9% [2][8][9]. A disadvantage of this method is that both layers need to be soluble in the same solvent to allow for the layers to mix together. A new form of blended cells are polymer matrix blends. A donor material is blended with fullerenes (typically C60) or another nano-particle. The way the fullerenes are constructed, and the fact they are like nano-spheres, means the charge interface area is significantly larger. Cells constructed in this way have shown improvements over polymer blends, but there is still room for improvement. [10][11][12] Laminated Devices [2][13] These are a combination of the previous two schemes, with an A/D mixture is placed inbetween an A and a D layer (figure 4, bottom). Like this charge separation occurs in the blend layer, and electrons and holes are conducted to their respective electrodes with more ease. The key to this technique lies in the A and D layers being prepared individually, and then brought together (either by heat treatment or some other method). This allows the A and D layers to be made through different processes, and gives some control to the blend layer thickness. Some defining characteristics of organic solar cells are: • Large charge interface: This allows not only for more excitons to split up, but also increases the odds of an exciton diffusing to an interface instead of recombining. • TCO: It is beneficial to have a top conducting layer. ITO is very commonly used, though SnO2, In2O3, and ZnO are also used. • Work function: the top and bottom electrodes must be chosen with differing work functions. This creates an internal electric field that forces electrons to go to the cathode and holes to the anode. • When blends or layers are used, these need to have sufficiently different electronic affinities to create a charge boundary. Without this the exciton will simply recombine. Dye-sensitised Cells These cells use a somewhat different approach than the others discussed, where a photon is used to give rise to an QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture.


Arun Luykx

Organic Photovoltaics

Qualifier Exam

electron in a pigment1 layer. This electron is injected to a highly n-type material (TiO 2 in the classic case by Gratzel), and from there goes on to the rest of the circuit. The image to the right illustrates this [14]. These cells are particularly known for very high quantum efficiencies. This is in part due to how the pigments are placed, namely adsorbed on the surfaces of the nanomaterials. The rougher the surface, the larger the overall surface area, and thus the more pigment that can be adsorbed. A logical next step is to use nanomaterials of which the dimensions can be characterised during growth. There are several complications in this that will be addressed in the next section.These cells have been shown to have quantum efficiencies of 80%, with conversion efficiencies of up to 12% in daylight. [1][14][15][16]

Novel ZnO Nanowire-based Dye-sensitised Solar Cells The concept of using ZnO nanowires in dye-sensitised solar cells (DSSC) is based on the relative ease of growth of ZnO nanostructures. ZnO is also a wide-band gap semiconducture (3.2 eV), and is n-type, meaning it could be a direct replacement for TiO2 which has already proven to be quite effective [14]. In the past ordered arrays of nanowires have been used to create a large surface area for adsorption to happen on. However, these arrays are not the most efficient design, and do not allow for back-side illumination. I propose to use ZnO nano-stars (see figure 6; own research) for improved conversion efficiency.

Figure 6, ZnO nano-stars In an array of nanowires, such as in the case of [15] and [16], the density of the area of nanowires versus volume is about 0.0786. By using stars such as the one in figure 6, a much more dense area can be attained, estimated at 42.41. This is an increase of 540 times. Furthermore, these stars can be loose from the substrate and do not depend on it for rigidity, so are free to be placed anywhere. An advantage of this is that backside 1

Pigments differ from dyes in that they are insoluble. Both are, however, oligomers or monomers that can absorb light.


Arun Luykx

Organic Photovoltaics

Qualifier Exam

illumination is now possible, as there is no bottom support layer or precursor layer – which is the case for nanowires. Quantum efficiency of ZnO nanowire arrays have been reported to be around 70% [16], which will be continued here. However, now the low conversion efficiency of 1.5% [15] will be increased due to more pigments being excited due to solar radiation.

Experimental The ZnO portion of this DSSC will be grown by chemical vapour phase deposition. The exact parameters will still have to be determined, though this has been proven to work (as the images above illustrate). Zn particles are placed along with the substrate (next paragraph) in a boat, in a quartz tube, which is located in a (tube) furnace. Argon and oxygen flow through the tube at a given rate while the furnace is heated to around 600C. Due to the high temperature ITO will not suffice as a substrate, as its resistance increases with increasing temperature [1]. Rather, we shall be using fluorine doped SnO2, which has been shown to be able to withstand high temperatures [15]. The remaining procedures will be taken from [16], as their individual nanowires are not unlike those of the individual strands of the nano-stars. The nanostars are placed in an oxygen plasma for 10 minutes at room temperature, followed by a heating to 100C to remove any water. They are then immersed in dye N7192 for 30 minutes. Another fluorine doped SnO2 electrode with a 20nm Pt layer (by e-beam evapouration) is used as a counter electrode. The electrodes are separated by a 20 micrometer polypropylene spacer and kept together with clips. An electrolyte 3 is introduced by capillary forces. The surface area of the device will be around 5mm2.

Characterisation Power output measurements taken over wavelengths from 1000nm to 280nm (using a monochromator attached to a Xe lamp) reveal the absorption spectra of the device. With a properly calibrated power measurement device (such as the ones from EDTM, Inc.) we can compare the output from a known solar cell to these, thereby giving immediate information on solar conversion efficiency. Photoluminescence measurements taken either using the monochromator, or a laser, show quantum efficiency yields. The amount of reflected light shows how many pigments are not actually being excited. IV measurements, both in the dark and illuminated, return short circuit current and open circuit voltage numbers. These can be used in determining the fill factor and the overall conversion efficiency of the device as well. 2

cis-bis(isothiocyanato)bis(2,2’-bipyridyl-4,4’-dicarboxylato)-ruthenium(II) bistetrabutylammonium 3 0.5 M tetrabutylammonium iodide, 0.05 M I2, and 0.5 M 4-tertbutylpyridine in acetonitrile


Arun Luykx

Organic Photovoltaics

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Expected outcome Due to the increased area-volume density we expect a significantly better conversion efficiency now that more light is able to penetrate. Furthermore, the added area also allows for more electrons to percolate to the electrodes.

Summary Organic solar cells have a very promising future, mainly due to cost benefits but also due to beneficial construction methods. Though mostly complex in nature, depending heavily on specifically chosen compounds, some cells such as DSSCs can be relatively easy to conjecture. The addition of cheap, abundant, and environmentally conscious materials such as ZnO will engrain it’s future in the marketplace. References: 1. A. Luque, S. Hegedus, “Handbook of Photovoltaic Science and Engineering”, Wiley and Sons (2003) 2. K. Petrisch, “Organic Solar Cell Architectures” (2000) 3. A. Barnett et al, “Milestones Toward 50% Efficient Solar Cell Modules” Presented at 22nd European Photovoltaic Solar Energy Converence (2007) 4. R.N. Marks, J.J. M. Halls et al, J. Phys.: Condens. Matter 6, 1379-1394 (1994) 5. J.J.M. Halls, K. Pichler, et al, Appl. Phys. Lett. 68, 3120-3122 (1996) 6. J.J.M. Halls, R. H. Friend, Synth. Met. 85, 1307-1308 (1996) 7. C.W. Tang, Appl. Phys. Lett. 48(2) (1986) 8. Halls J.J.M. et al, Nature 376, 498-500 (1995) 9. G. Yu, A.J. Heeger, et al, J. Appl. Phys. 78, 4510-4515 (1995) 10. G. Yu, Heeger, et al, Science 270, 1789-1791 (1995) 11. Roman, Inganas et al, Adv Mater. 9, 1164-1168 (1997) 12. M. W. Rowell, M. A Topinka, et al, Appl. Phys. Lett. 88 (2006) 13. M. Granstrom, R. H. Friend, et al, Nature 95, 257-260 (1998) 14. B. O’Regan, M. Gratzel, Nature 353, 737-740 (1991) 15. M. Law, L. Greene, Nature Materials 4, 455-459 (2005) 16. J. Baxter, E. Adyil, Appl. Phys. Lett. 86 (2005) Illustrations: A: C. Deibel, “How Do Organic Solar Cells Function – Part II” <http://blog.disorderedmatter.eu> (2008)


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