Polymer-Based LEDs and Solar Cells

Polymer-Based LEDs and Solar Cells$ AC Grimsdale, Nanyang Technological University, Singapore J Jacob, Indian Institute of Technology, New Delhi, Indi...

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Polymer-Based LEDs and Solar Cells$ AC Grimsdale, Nanyang Technological University, Singapore J Jacob, Indian Institute of Technology, New Delhi, India r 2016 Elsevier Inc. All rights reserved.

8.10.1 8.10.2 8.10.2.1 8.10.3 8.10.3.1 8.10.3.2 8.10.3.3 8.10.3.4 8.10.3.4.1 8.10.4 8.10.5 References

General Introduction to LEDs and Solar Cells Device Issues in Electroluminescent Materials and Full Color Displays Device Issues in Organic Solar Cells Material Classes Poly(arylene vinylene)s Polyphenylenes Polycarbazoles Polythiophenes Cyclopentadithiophene, benzodithiophene, and dithienopyran polymers as donors in high efﬁciency solar cells Hybrid Solar Cells Conclusions and Outlook

1 1 4 8 8 12 16 17 19 21 22 22

Polymer-based light-emitting diodes (PLEDs) and solar cells are two important emerging technologies. While there have been comprehensive recent reviews, to which the reader is referred, which seek to list and discuss all the polymers that have been tested in LEDs1 and solar cells,2 in this article we seek to provide an overview only of the most important recent results that have been obtained in the design and synthesis of conjugated polymers for these applications. This will include advances in synthetic methodology and also in the understanding of reaction mechanisms, especially in connection with the formation of defects in the polymer structure that may adversely affect their performance. We will begin with a short overview of the working principles of the LED and solar cell, establishing the crucial parameters by which one may evaluate the performance of a polymer, and then will proceed on to discuss each major category of polymer in turn.

8.10.1

General Introduction to LEDs and Solar Cells

As shown in Figure 1, electroluminescence (EL) results from recombination of charge carriers (holes and electrons) injected into a semiconductor in the presence of an external circuit. If these combine to give a singlet excited state identical to that obtained in photoluminescence (PL) by excitation of an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), then symmetry-permitted relaxation to the ground state may occur with emission of a photon. The color of the emission depends upon the size of the HOMO–LUMO energy gap, which for visible light (380–780 nm) corresponds to 1.5–3.2 eV. The photovoltaic effect is the same process in reverse that is a singlet excited state is produced by excitation with a photon, and then splits to produce positive and negative charges which are transported to the electrodes. In a typical polymer LED (Figure 2), a thin ﬁlm of an emissive polymer is sandwiched between two electrodes, at least one of which is transparent. Generally commercially available indium tin oxide (ITO) on a glass or polymer substrate is chosen as the (transparent) anode, and the cathode consists of a vacuum deposited metal layer. However a variety of other materials have been used as anodes or cathodes. A bulk heterojunction (BHJ) organic solar cell (Figure 3) has essentially the same structure as a single-layer LED, except that the active layer is a mixture of an electron acceptor and an electron donor. By contrast in a bilayer device the electron donor and acceptor are deposited as separate layers. In both cases the devices produce electricity by separation of an excited state (exciton) at the interface between the donor and acceptor, followed by transport of the charges so formed to the electrodes.

8.10.2

Device Issues in Electroluminescent Materials and Full Color Displays

The main criteria for a successful LED are its luminance (brightness), its efﬁciency (i.e., how much light does one get out for a given input of power), and its lifetime which is usually deﬁned as the period taken for the initial luminance to decay to a certain fraction of the original value. The brightness required depends upon the application chosen. For a display application, for example, a laptop computer screen, a luminance of only 100–200 cd m2 (cd m2) may be sufﬁcient, but for lighting applications (especially ☆

Change History: March 2015. A.C. Grimsdale and J. Jacob updated Section 8.10.2.1 ‘Device Issues in Organic Solar Cells,’ with updated highest device efﬁciencies for BHJ cells, added extra reference for DSSCs, and added brief mentions of perovskite-based cells with references, and updated captions for Figures 8 and 10. March 2015: Grimsdale and Jacob updated Section 8.10.3.4 ‘Polythiophenes,’ with new references plus discussion of new thiophene-based polymers including new structures 66 and 69–71 with appropriate references. March 2015: Grimsdale and Jacob updated Section 8.10.4 ‘Hybrid Solar Cells,’ with added brief discussion of perovskite-based cells with references. March 2015: Grimsdale and Jacob updated Section 8.10.5 ‘Conclusions and Outlook’ with updated best efﬁciency data.

Reference Module in Materials Science and Materials Engineering

doi:10.1016/B978-0-12-803581-8.01483-1

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Polymer-Based LEDs and Solar Cells

Figure 1 Operating principles of (a) an LED. An electron is removed from HOMO to anode to form a radical cation or hole (i), while injection of electron from cathode into LUMO produces a radical anion (ii). The charges move through the semiconductor and combine to form an excited state (iii) which relaxes with emission of light; (b) a solar cell. An excited state (iv) is generated by absorption of a photon by donor molecule. This transfers the electron to the acceptor molecule to form a cation and anion (v) which then travel to the electrodes in the reverse of the process in an LED.

Figure 2 Schematic drawing of a single-layer electroluminescence device.

Figure 3 Bilayer and bulk heterojunction of donors and acceptors.

outdoors) brightnesses of 2000–3000 cd m2 may be needed. The efﬁciency may be expressed in a number of different ways. The external quantum efﬁciency (EQE) is the number of photons emitted by the device for each injected electron, and is usually expressed as a percentage. An alternative measurement of efﬁciency is the luminance efﬁciency measured in candelas per Ampere (cd/A) or in lumens per Watt (lm/W). The former is the easier to calculate as one simply divides the luminance by the current

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density. The luminance efﬁciency can, and usually does, vary with luminance, and so for device performance comparisons it is usually cited for brightnesses of 100 cd m2 or 1000 cd m2, rather than the maximum luminance efﬁciency being cited, as this latter often occurs at very low luminances which are not useful. A value of over 1 cd A is currently regarded as the minimum for a commercially viable device. The efﬁciency in lm/W is calculated by multiplying the device EQE by the relative sensitivity curve, which takes account of the fact that the eye is much less sensitive to red or blue light than to yellow or green light. Here benchmarks for performance are the efﬁciencies of incandescent lamps (B15 lm W1), ﬂuorescent lamps (65–100 lm W1), and state-of-the art inorganic LEDs (B100 lm W1). The efﬁciency of an LED depends upon four factors: (1) the fraction of electrons and holes that recombine with each other; (2) the fraction of recombinations producing excited states that can decay radiatively; (3) the solid state quantum yield of such excited states; and (4) the fraction of photons produced that actually escape the device. For a polymer with refractive index of n, the fraction of photons generated within a basic one-layer LED that actually escape from it is 0.5/n2.3 For a polymer with n¼ 1.58 (a not untypical value) this is 0.2, i.e., 80% of the light is lost inside the device. This fraction can be increased up to 0.4, i.e., the loss can be reduced to 60%, by various methods, including use of microlens arrays,4 photonic crystals,5 surface modiﬁcation,6 or microcavity design.7 As the refractive index of conjugated polymers cannot be signiﬁcantly altered by synthetic design, this last factor is purely a design issue and cannot be controlled by chemical means, whereas the other factors are amenable to the efforts of synthetic chemists. The efﬁciency of an LED depends greatly upon the choice of electrode materials as the efﬁciency of charge injection depends upon the size of the energy barrier between the work function of the electrode and the energy level of the HOMO (for the anode) or LUMO (for the cathode). Since balanced charge injection is required (emission requires a hole and an electron to combine), the energy barriers should also be of comparable sizes. As a result in designing a new polymer for use in an LED, care must be taken to match these orbital energies as closely as possible to the desired electrode materials. These problems may be partially overcome by use of charge-transporting layers (sometimes called level-matching layers) whose energy levels are intermediate between those of the electrode and emissive material. Very high charge carrier mobilities are not necessary for efﬁcient EL, indeed too high mobility would be disadvantageous as the hole or electron might travel through the emissive layer at too high a velocity to be able to interact with the other charge. However, balanced mobilities are desirable so that the charges are likely to recombine in the middle of the emissive layer and not near the electrodes where quenching of luminescence may occur. For an optimally designed device the fraction of charges that recombine should be close to 100%. A high EL efﬁciency from conjugated polymers requires a high solid state PL quantum efﬁciency. While it is currently not possible to predict the value of the latter directly from the polymer structure, there are some steps that can be taken to try to enhance the efﬁciency. For example, the presence of known ﬂuorescence quenching functionalities such as halide or carbonyl groups on the polymer chain as impurities (defects) formed during the reaction pathway should be avoided. As a result, the choice of synthetic pathway or reaction conditions may have a signiﬁcant effect on the PL efﬁciency of the ﬁnal polymer. It is also known that efﬁcient stacking of polymer chains enhances exciton migration, including to potential quenching sites, and thus can signiﬁcantly reduce the PL efﬁciency in the solid state. An example of this is the low PL efﬁciencies seen for ﬁlms of poly(3alkylthiophene)s, which make these polymers of limited use in LEDS.8 While quantum yields in solution can come close to 100% for some polymers, solid state quantum yields for ﬂuorescent polymers range only up to B60%.3 According to spin statistics only 25% of all excited states generated by charge injection should be singlet states capable of spinpermitted radiative decay (ﬂuorescence), and thus the maximum possible EL efﬁciency for a polymer LED should be 25% of the polymer’s solid state PL efﬁciency. Thus the maximum theoretical efﬁciency for a polymer LED using a ﬂuorescent polymer should be only 3–6% (25% of emissive states 60% solid state quantum yield for emissive states 20–40% of photons escaping). However, the EL efﬁciency for some polymers is reported to be higher than 25% of their PL efﬁciency. As a result there has been considerable debate in the physics literature, which has been reviewed elsewhere,9 about the proportion of singlet states in conjugated polymer LEDs, and thus their maximum EL efﬁciency. Here we will simply mention that there have been both experimental10 and theoretical11 reports suggesting that the potential EL efﬁciency of some polymers may be as high as 50% of their PL efﬁciency, although these have since been challenged.12 To overcome this limit, either the proportion of singlet states must be enhanced or some way must be found to obtain emission from the triplet states. With regard to the ﬁrst approach there has been a report that blending cobalt-platinum nanowires with a ﬂuorescent polymer increased the percentage of singlet excitons and thus the overall EL efﬁciency,13 but there have been no further reports of such enhancement. The second approach involving harvesting of energy from the triplet states is an increasingly important goal in research into LEDs, as LEDs utilizing emission from triplet states (phosphorescence), known as electrophosphorescent devices or PHOLEDs, have potentially much higher efﬁciencies than those producing emission only from singlet states. Since phosphorescence from conjugated polymers, if present, is generally very weak, this usually involves the incorporation of phosphorescent metal complexes into the polymers or the blending of such complexes with the polymers. Considerable progress has been made using blends of phosphorescent molecules with polymers so that polymer-based PHOLEDs are already competitive with the conventional ﬂuorescent LEDs.1 The most widely used polymer has been poly(N-vinylcarbazole) (PVK, Figures 4, 1) which is a hole-transporting polymer. Blends of this with various transitionmetal phosphors have been used to make efﬁcient blue, green and red-emitting devices.14 Among conjugated polymers the materials that show most promise as hosts are poly(N-alkyl-3,6-carbazoles) (2).15 While this approach has produced good device results, it suffers the obvious problem of potential phase separation which would reduce device efﬁciencies and/or lifetimes. Attaching the triplet emitting units onto the polymer chain as side chains has been investigated as a way around this but the device results are not as good, possibly because of less efﬁcient charge transport and/or local aggregation of the dyes.1 Incorporating

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Polymer-Based LEDs and Solar Cells

Figure 4 Carbazole-based matrix polymers for PHOLEDs.

phosphors into the chain to produce phosphorescent polymers has been achieved in the case of phenylene-based polymers and will be discussed in more detail when we discuss that class of polymers, but again the results so far reported are not as good as for the blends. The main problem holding back the use of PLEDs is that the lifetimes are often still too short for most commercial applications, generally because of defects due to synthetic byproducts or degradation of the emissive polymers, usually by oxidation.1 While formation of defects by oxidation can be reduced by device design, for example, encapsulation, the formation of defects during synthesis can only be dealt with by attention to the molecular design and the synthetic methodology. In both cases this requires a thorough understanding of the mechanism of the reactions used to make the polymers and also of their potential chemistry within a device, especially at interfaces with metals or metal oxides. The problem of lifetimes is particularly acute for blue-emitting materials which also display problems of color instability, with the appearance of long wavelength emission bands as well as loss of luminescence. We will discuss in more detail the causes and solutions to this problem when we discuss phenylene-based polymers which are the main class of blue-emitting polymers, but in general the problems are related to formation of defects during synthesis or oxidation during device operation, and can be mitigated by careful design and choice of synthetic procedure. The formation of defects also affects device efﬁciencies and we will show for poly(phenylene vinylene) derivatives how this has been surmounted by identiﬁcation of the defects and their minimization by synthetic design. White LEDs are of interest primarily for lighting applications or as liquid crystal display backlights. They present a special problem in that to obtain white emission from a single pixel rather than by combining emission from red, green, and blue pixels, requires simultaneous emission from more than one emissive species since no chromophore is currently known which produces pure white PL. Generally a binary mixture of a blue emitter with an orange emitter or a ternary mixture of a blue emitter with both a red and a green emitter is required to obtain pure white emission. This obviously presents problems in obtaining and retaining color purity as even minor differences in the compositions of the blends due to phase separation or different rates of degradation of the emissive species will cause the color to cease being white. Both blends of polymers or of polymers with molecular emitters have been used to obtain white emission though there is no report on their long term stability, which is probably low for the above reasons.1 The alternative approach, which offers better chances of obtaining stable emission, is to attach the emitters to a blue-emitting polymer backbone such as a polyﬂuorene and this will be covered later when we discuss polyﬂuorenes.

8.10.2.1

Device Issues in Organic Solar Cells

Polymer-based solar cells use a combination of an electron donating polymer with an acceptor that may be another polymer, a small molecule, most commonly a fullerene derivative, or in the case of hybrid solar cells an inorganic nanoparticle.16 Of these the organic molecular acceptors have proven the most successful with the very best devices displaying efﬁciencies close to 8%, whereas the best devices using polymeric or inorganic acceptors are only around 3% in efﬁciency.17 Of the two device geometries shown in Figure 3 above, the BHJ cells are of much greater interest currently, though two-layer devices are still being investigated. This is because they are potentially more efﬁcient as the greater interface area means a higher proportion of excitons can be split into holes and electrons. This is borne out experimentally as the best two-layer devices are only around 2% efﬁciency18 whereas the best BHJ cells are now in range of 10–12% efﬁciency. While the best polymer solar cells still lag behind the best dye-sensitized solar cells (DSSCs)19,20 or metal-organic perovskite-based cells21–26 in efﬁciency, they have a number of potential advantages over the latter. They do not require addition of a reducing agent as is used in DSSCs to regenerate the dye, nor do they require addition of an electrolyte to transport away the holes. One of the major problems holding back development of DSSCs is the problem of encapsulation to avoid electrolyte leakage as to date the best performing cells use a liquid electrolyte which is both corrosive and ﬂammable. By contrast polymer solar cells contain non-volatile components which are not liable to leak from the cell. Unlike the perovskite cells they do not contain toxic heavy metals nor do they have the same sensitivity to water. There are ﬁve measures that are used to characterize a solar cell – the open circuit voltage (Voc), the short-circuit current density (Jsc), the ﬁll factor (FF), the EQE and the power conversion efﬁciency (PCE).16 The open circuit voltage is the y-intercept on the plot of current versus voltage (J–V plot) for the cell under irradiation (Figure 5) and constitutes the maximum voltage that is obtainable from the device. This is typically less than 1 V for an organic photovoltaic cell, but higher potentials are of course available by connecting cells in series. The short-circuit current is the x-intercept on the J–V curve and is the maximum current from the cell. Usually the current density in mA cm2 is cited as the actual current obtained obviously depends upon the size of the device. The maximum power theoretically available from the cell is thus the product Voc Jsc. The ratio of the maximum power obtained from the cell (i.e., the maximum value of V J actually obtained) to this theoretical maximum is called the Fill Factor

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Figure 5 I–V curve for solar cell illustrating calculation of Fill Factor.

(FF) and may be expressed either as a number or as a percentage. The best cells fabricated to date produce about two-third of their theoretical maximum power, i.e., their FF is about 0.66. The EQE is the number of charges that come out of the device for every 100 incident photons. The EQE is wavelength dependent, and generally a plot of EQE against wavelength matches the absorption spectra of the materials in the device. The IPCE is the ratio of how much electrical energy is produced within the device compared to the incident light energy that is absorbed and is the best overall measure of the efﬁciency of the device. As the energy of a photon is proportional to its frequency (Planck’s Equation) the PCE depends upon the spectrum of the incident radiation and may also depend upon its intensity. PCE is usually measured in a solar simulator using the AM1.5 solar spectrum (Figure 6) at an intensity of 0.1 Sun (ca. 100 W m2). This is the solar spectrum observed at sea level when the sun is at an angle of 48.21 from the vertical. A silicon diode is used to calibrate the results. Recorded solar cell efﬁciencies are very dependent upon the quality of this calibration so certiﬁcation must be sought from internationally accredited laboratories such as the National Renewable Energy Laboratory (NREL) in the USA. The overall efﬁciency of the cell is dependent upon the Voc, Jsc, and FF. The Voc is determined by the orbital energies of the donor and acceptor as shown in Figure 7. The maximum possible voltage (Voc max) is given by the energy difference between the LUMO of the acceptor and the HOMO of the donor. However, the actual Voc is lower than this as energy has to be used to dissociate the bound exciton at the donor-acceptor interface. For an organic exciton the exciton binding energy is around 0.25 eV, so at least this amount of energy has to be lost during dissociation. In addition there has to be an energy difference (D LUMO) of at least 0.25 eV between the LUMO of the donor and the acceptor for the electron transfer to be efﬁcient. As a result the Voc for a BHJ solar cell must be at least 0.5 eV smaller than the bandgap (Eg) of the donor.27 Whereas in an LED a high charge carrier mobility is not crucial as what is required is balanced charge carrier mobilities, i.e., the holes and electrons should have similar mobilities so as to promote recombination within the emissive layer, in a solar cell the charges need to be extracted expeditiously from the device. As a result one method increasingly used to test the suitability of materials for BHJ solar cells is to measure their charge carrier mobilities, especially the mobilities when blended with the intended other component. Currently a value of at least 103 cm2 Vs1 seems to be accepted as the minimum mobility required for high photocurrent and thus potentially high device efﬁciency. Since efﬁcient charge carrier transport in organic materials depends upon good packing of the molecules within the materials, it is clear that a high degree of order within the donor and acceptor phases of the blend and a relatively low amount of disorder at the interface between the phases within the blend, and between the blend and the electrodes are all conducive to improved photocurrent. Such order may also be of importance in raising the ﬁll factor as a high FF requires the current to show little change over a large voltage range. Factors contributing to a high FF will also include a lack of traps in the blend, so purity of the components should be just as important in solar cells as it is in LEDs, though to date there have been no systematic studies on the effects of molecular impurities or polymer defects on the performance of organic BHJ cells. There is evidence for the molecular mass range and polydispersity of polymers affecting their performance in solar cells, as has also been seen in polymer-based transistors where higher molar masses and lower polydispersities produce better results due to reduction in grain boundaries. The molecular mass must not become too high however, as then solubility and processing starts to become problematical, and also the polymer chains start to become heavily entangled so that the formation of the optimum order within the ﬁlm is retarded.

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Polymer-Based LEDs and Solar Cells

Figure 6 Solar spectrum AM1.5.

Figure 7 Dependence of Voc upon energy levels of donor and acceptor.

There has been considerable research into some of the factors determining the efﬁciency of polymer BHJ solar cells. The ﬁrst factor obviously is that the donor and acceptor must have appropriate energy levels. One great advance in determining these has been the development by workers at Konarka of a model by which the maximum efﬁciency of a given BHJ cell can be predicted using the HOMO and LUMO energy levels of the donor and the offset between the LUMO energies of the donor and acceptor (Figure 8). Originally the model was constructed using the fullerene derivative PCBM (Figures 9, 3) as acceptor28 but it was subsequently extended to any acceptor.27,29 This model predicts that an efﬁciency of over 10% is possible for a device in which the donor bandgap is between 1.4 and 1.8 eV, with a LUMO offset of just over 0.3 eV between donor and acceptor. It should be stressed that possessing the right orbital energies are necessary but not sufﬁcient conditions for obtaining a high efﬁciency, as the calculated efﬁciencies correspond to devices with optimized parameters such as Fill Factor. The values for FF etc. used to calculate the maxima are values that have been shown to be possible in organic solar cells, but if the values actually obtained are lower, then the device efﬁciencies actually obtained will be lower than the calculated maximum values. As HOMO and LUMO energy levels for structures can readily be calculated with a fair degree of conﬁdence by commercially available computational programs, it is possible to use this model in a pre-screening test for proposed new donor-acceptor pairs. What as yet cannot be predicted in advance are factors such as morphology of the blends in a BHJ device. There has been extensive work to show that device efﬁciencies are dependent upon the morphology of the blend.30 This in turn depends upon many factors including concentration, choice of solvent, temperature, etc., and can be altered by control of these factors and by introduction of additives to the solution, or by post-deposition annealing. While there has been extensive work on optimization of certain BHJ blends of PCBM with MDMO-PPV (4) or P3HT (5),27 it is unclear to what extent one may use the knowledge obtained for these blends to optimise devices using other blends, as the behavior of each donor-acceptor pair is likely to be different, so that the optimal conditions for processing each blend will also be different. One problem with organic materials for solar cells is that no currently known organic material shows strong absorption right across the solar spectrum. One way around this problem is to use a tandem cell. In such a cell two cells are stacked on top of each

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Figure 8 Konarka efﬁciency map for single-layer BHJ cells. Reproduced with permission from Dennler, G., Scharber, M.C., Brabec, C.J., 2009. Advanced Materials 21, 1323. © Wiley-VCH 2009.

Figure 9 Some acceptor and donor materials for polymer BHJ solar cells.

other with a transparent interlayer between them. The top cell shows its main absorption at shorter wavelength than the bottom cell, so the high energy radiation is absorbed by the upper cell while the low energy (long wavelength) solar irradiation passes through and is absorbed by the bottom cell. In this way more of the solar spectrum can be covered. Models by the Konarka group suggest that up to 15% efﬁciency may be possible for polymer tandem cells with the optimum combination of individual layers (Figure 10).27 One of the major obstacles still to be overcome in the development of organic solar cells is the need for long lifetimes. Organic materials, especially conjugated polymers, are known to be susceptible to oxidation especially in the presence of ultraviolet light, i.e., in the conditions under which a solar cell is expected to operate. There have been no reviews to date upon the breakdown mechanisms of polymer solar cells but oxidative degradation of the polymers is likely to prove to be a major mechanism for their failure. This is likely to be especially true for low bandgap polymers as most low bandgap organic materials have rather high HOMO energies making them more readily susceptible to oxidation. Since low bandgap materials are needed to efﬁciently harvest solar radiation, and thus obtain high device efﬁciencies, the search of stable low bandgap polymers clearly has a high priority. At the moment the problem of device lifetimes looks like being tackled mainly by advances in device encapsulation as are being used to extend lifetimes of LEDs, but it remains to be seen if this will sufﬁce or if new approaches will need to be adopted as the environments inside a functioning LED and solar cell are different.

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Figure 10 Konarka tandem cell efﬁciency map. Reproduced with permission from Dennler, G., Scharber, M.C., Ameri, T., et al., 2008. Advanced Materials 20, 579. © Wiley-VCH 2009.

8.10.3 8.10.3.1

Material Classes Poly(arylene vinylene)s

Poly(1,4-phenylene vinylene) (PPV, Figures 11, 6) was the emissive material in the very ﬁrst PLED.31 Due to its insolubility the polymer ﬁlm had to be prepared by thermal conversion of a soluble precursor. To avoid the potential problems with such precursor routes, for example, corrosion of the electrodes by acidic byproducts, attention has since concentrated on soluble PPV derivatives such as MDMO-PPV (4) and MEH-PPV (7),32 which is probably the most widely studied PPV derivative.

Figure 11 PPV and MEH-PPV.

One of the biggest advances in the study of PPV derivatives has been the unraveling of the reaction mechanisms and in particular an understanding of how polymer defects are produced during the reactions and of how these affect the performance of the polymers in PLEDs. This in turn has enabled investigations of how these can be minimized so as to obtain PPVs with optimal device characteristics. The generally used synthetic routes to PPVs are those developed by Wessling,33 Vanderzande,34 and Gilch,35 which are shown in Scheme 1. In all cases a substituted p-xylene 8 is treated with base to produce a quinodimethane 9. If no more than one equivalent of base is used the polymerization produces the precursor polymer 10, which can be isolated and converted to the fully conjugated PPV 11 by heating under vacuum. In preparation of soluble PPVs such as MEH-PPV usually two or more equivalents of base are used as to produce the ﬁnal polymer 11 without isolation of the intermediate 10. The mechanism of these reactions has been the subject of much debate.36 The intermediacy of the quinodimethane 9 has been established by spectroscopic means, but there was much discussion as to whether the polymerizations proceeded via a radical or an anionic mechanism. Experiments showing that radical trapping reagents suppressed the polymerizations demonstrated that the Wessling,37 and Vanderzande38 processes occurred via radical mechanisms but a non-radical mechanism for the Gilch reaction was

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Scheme 1 Quinodimethane routes to PPVs.

still being proposed as late as 2002.39 Recent studies40–42 have now established that the Gilch reaction also proceeds primarily via a radical pathway, with anionic polymerization being responsible at most only for the formation of low molar mass material. The initiation step is proposed to be the coupling of two quinodimethanes to form a diradical (Figure 12).41

Figure 12 Initiation step for quinodimethane polymerizations.

A key issue in the synthesis of any conjugated polymer is the minimization of defects, as they have a large, usually negative, impact upon the optical and electronic properties of conjugated polymers, by acting as charge traps which reduces charge mobility and conductivity, sites for non-radiative decay of excited states which reduces luminescence efﬁciencies, or as low band gap emissive sites which affects emission color. The formation of defects during the Gilch synthesis of soluble PPVs arises from nonstandard coupling of the quinodimethane intermediates (Scheme 2). Whereas the precursor polymer 12 is formed by coupling of the quinodimethane 13 in the head-to-tail fashion, coupling in head-to-head and tail-to-tail fashions produce respectively diarylethyne (tolane, 14) and bisbenzyl defects (15).43–45 Residual halide defects may also be present as a result of incomplete dehydrohalogenation during formation of 14, or incomplete elimination of 12, and it has recently been suggested that these may be even more important than the tolane or bisbenzyl defects in reducing device efﬁciencies and lifetimes.46 As the initiation step introduces a bisbenzyl defect all polymers made by the Gilch and related routes must, therefore, contain at least one defect. As will be discussed below this understanding of the mechanism of defect formation has enabled new materials to be made with much lower defect levels and hence much better performance in light-emitting diodes (LEDs).42,47 Each of the three unsymmetrical monomers 16–18 can form two isomeric quinodimethanes upon reaction with base (Scheme 3). There is no electronic reason why one of these forms should be favored for 16 and so the two intermediates 19a and 19b are probably formed in equal amount. The steric and electronic effects of the chlorine atoms, however, would strongly favor head-totail coupling of either of these so the level of defects in the polymers 22 is typically only 3%. While there probably is some electronic effect affecting the ratio of the intermediates 20a-b derived from 17, it is not so strong that only one form is produced and steric repulsion between the phenyl groups means that the coupling of 20a and 20b can only proceed in a head-to-head fashion, so that the resulting polymer 23 has a high defect level. The electronic effects of the methoxy group in 18 by contrast mean that the quinodimethane 21 is strongly favored and steric repulsion hinders its head-to-head coupling, so that the defect levels are very low (o0.5%) in the ﬁnal polymer 24. Unfortunately this polymer tends to form gels, so to obtain more processable materials copolymers of 23 and 24 were prepared. Such materials were optimized and marketed by COVION (now a subsidiary of Merck) as ‘Super Yellow’ which was the active material in the ﬁrst commercial polymer LED devices. Another source of defects that must be considered is the geometry of the double bonds. As the elimination step in the Wessling and similar pathways goes via an E2 mechanism, they give predominantly trans-double bonds, but they also produce a small amount of cis-vinylene units. Such cis-vinylenes are considered defects as they cause a twist in the polymer chain which affects the packing (Figure 13). This may not be a problem in LEDs as preventing exciton migration to defect sites or slowing charge movement by disruption of packing may actually enhance EL efﬁciency, but is likely to reduce efﬁciency in OPVs where the charges need to be transported rapidly to electrodes for collection. It has been reported that low molecular weight fractions of MEH-PPV from Gilch polymerizations contain cis-defects which cause them to display blue-shifted emission.48 The cis-vinylenes by producing chain twisting would impair chain growth thus explaining their absence from the higher molar mass fractions.

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Scheme 2 Formation of defects in PAVs during Gilch synthesis.

Scheme 3 Mechanism for formation of PPVs from different monomers.

Besides the quinodimethane-based polymerizations a number of other methods have been used to make PAVs of which the most important are the Wittig, Horner, Knoevenagel, Heck, and metathesis methods (Scheme 4).1 These all have the advantage of avoiding the tolane and bisbenzyl defects obtained from the quinodimethane-based routes, but still suffer the potential problem of cis-vinylene defects. This is a particular problem for polymers made by the Wittig method – the Horner method is generally preferred to the Wittig as it produces much less cis-defects. While the cis-defects can be partially removed by treating the polymer with iodine, this has the undesirable effect of doping the polymer requiring further treatment to remove the dopants. The ﬁrst four of these methods are suitable for making alternating copolymers as the substituents on the two components can be different. The Knoevenagel route is used to make CN-PPVs, which are PPVs with cyano groups on the vinylene units.49 These increase the electron afﬁnity of the polymer greatly as compared to PAVs without these groups, while due to their small size they do not induce torsion of the backbone with a consequent reduction in the conjugation length of the polymers, unlike other groups which have been substituted on the vinylene units in PAVs. The main reason these methods are not used as much as the Gilch route is that the molar masses tend to be only moderate at best, which counteracts the advantage of their much lower defect levels. This is particularly true of metathesis of divinylbenzenes as a route to PPVs, which to date has been reported to give only oligomers.50 As the metathesis of diethynylbenzenes has been used to produce defect-free poly(arylene ethynylene)s with

Polymer-Based LEDs and Solar Cells

11

Figure 13 Cis- (left) and trans- (right) PPV units illustrating their effect upon chain conformation.

Scheme 4 Alternative routes to PAVs.

satisfactory molar masses,51 it is likely that this failure of metathesis to produce good PAVs merely reﬂects the use of nonoptimized conditions, and that if the right catalyst system were to be used this could be a way to make very well-deﬁned polymers. The structure-property relationships in PAVs have been extensively studied.1 The main objectives have been to control their emission color and their ability to accept charges. The parent PPV is a yellow-green emitter. Alkoxy substituents in the 2- and 5positions produce red-shifts so that MEH-PPV emits orange-red light. Pure red emission has proved problematic to achieve as even a slight amount of yellow light produces a noticeable orange tinge to the perceived emission color. Near-infra-red emission has been reported from poly(thienylene-vinylene) derivatives.52 Alkyl and silyl substituents in these positions do not produce any redshift in emission so alkyl-, or silyl-PPVs such as 2453 and 2554,55 (Figure 14) are green emitters. Aryl substituents can be more variable in their effects so some 2-aryl-substituted PPVs such as ‘Super Yellow’ are slightly red-shifted in their emission compared to PPV, while others are green emitters. Steric congestion due to 2,3-substitution produces blue-shifts in emission due to twisting of the polymer backbone, so that 2,3-alkoxy-PPVs such as 26 are green rather than orange-red emitters,56 but while blue-green PL and EL has been obtained from polymers such as 27,57 no true blue emission has been obtained from a PAV by such methods. Blue EL is available by reducing the conjugation of a polymer through introduction of non-conjugated segments but this has the effect of reducing the electrical transport within the polymer so that the device results from such materials have not been outstanding. Besides the bandgap and thus the emission color, another major aspect of conjugated polymers that needs to be controlled is their ability to accept charges from electrodes. For PAVs this means adjusting their LUMO energies so as to allow for efﬁcient electron injection from suitable cathode materials, as hole injection from the standard anode material ITO is quite efﬁcient, especially when a layer of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) is used as a charge

12

Polymer-Based LEDs and Solar Cells

Figure 14 Some green or blue-green emitting PAVs.

balancing layer. The use of low work-function cathodes made from metals such as calcium has been used to improve electron injection into PLEDs, but such metals are also highly sensitive toward water or air, and so air stable metals such as aluminum are preferred as electrodes, which means that materials with low lying LUMOs are required to improve the electron injection into the devices. (Composite electrodes containing an inner layer of calcium or barium covered with an outer layer of aluminum are a workable compromise but a pure aluminum electrode would still be preferable from a device stability point-of-view). The ﬁrst major progress toward tackling this problem was the development of CN-PPV 28 (Figure 15) which is a much better electron acceptor than standard PPV.49 Two-layer devices using PPV and CN-PPV were very efﬁcient but displayed low lifetimes which has been attributed to electrochemical instability of the cyano groups. Subsequently the use of more stable electron-accepting groups such as oxadiazoles has been found to be more promising and the highest efﬁciency yet reported for a PAV-based device was observed for one with polymer 29 as active layer.58 The best device using this material was reported to have an efﬁciency of 21.1 cd A1 at a luminance of 5930 cd m2 using aluminum cathodes.

Figure 15 PPV derivatives with improved electron afﬁnity for higher efﬁciency.

PPV derivatives have not to date proved very successful as materials for efﬁcient solar cells. The best results have come from use of MDMO-PPV (4) with fullerene acceptors where efﬁciencies of up to 3% have been obtained.59,60 This system is one which has been extensively investigated to determine the effects of the blend thin ﬁlm morphology upon the device efﬁciency and on the factors such as solvent etc., which in turn control the morphology of the ﬁlms.30 One interesting result has been that the regioregularity of the polymer affects the device performance. When MDMO-PPV is made by the Gilch route starting from the monomer 30 (Scheme 5) one gets a regiorandom polymer, as both quinodimethanes 31a and 32a are produced leading to a regiorandom polymer 4a containing an equal mixture of the possible repeat units. By contrast if the Vanderzande method is used then by choosing either monomer 33a or 33b, one gets selectively only quinodimethane 31b or 32b, since the acidity of the benzylic protons next to the halide and sulfone groups is different, and thus a fully regioregular polymer 4b or 4c is produced. It was found that the fully regioregular polymer is less soluble due to aggregation, so to optimise solubility a 70:30 mixture of the monomers was used. The resulting polymer 4d displayed an improved efﬁciency of 2.7% when combined with PCBM, compared to 1.7–2.5% for the regiorandom polymer from Gilch synthesis.60,61 A class of PPV derivative that have attracted some attention as potential acceptors in BHJ OPV applications are CN-PPVs. As mentioned above these are good electron acceptors which were originally developed as electron-transporting emissive polymers for LEDs.49 A blend of MEH-CN-PPV (Figure 16, 34) with MEH-PPV (7) showed a good photoresponse with an efﬁciency of 0.9% being obtained in what was the ﬁrst reported all-polymer BHJ solar cell.62 Recently a two-layer device using 34 and a polythiophene donor 35 has been reported which shows 2% efﬁciency.18 Rather unexpectedly the efﬁciency of BHJ cells using blends of the polymers was lower than for the two-layer device.

8.10.3.2

Polyphenylenes

Phenylene-based polymers have been extensively investigated as blue-emitting materials for LEDs.63,64 Here three areas of investigation have been of particular importance. The ﬁrst has been the obtaining of pure blue emission. This is difﬁcult to achieve as there is only a fairly narrow spectral window between violet and blue (sometimes called deep blue) light to which the eye is not very sensitive and blue-green emission. Given the much greater sensitivity of the eye toward green than blue light, even a minor tail

Polymer-Based LEDs and Solar Cells

13

Scheme 5 Synthesis of regiorandom and regioregular MDMO-PPV.

Figure 16 Polymeric acceptor and donor for efﬁcient two-layer device.

of the emission spectrum into the green results in a blue-green, rather than pure blue color for the emission. A second problem has been in obtaining stable blue emission as many blue-emitting materials start to show long wavelength emission when an LED has been running for a period, so that the emission color becomes green, yellow or white in appearance. This is often associated with a drop in device efﬁciency and/or luminescence intensity. The third problem has been that due to the high LUMO and low HOMO energies of the wide bandgap polyphenylenes charge injection faces relatively large barriers reducing its efﬁciency and thus the overall device efﬁciency. The synthetic routes to polyarylenes make use of the efﬁcient aryl–aryl bond forming reactions that have been developed over the last century.65 These come in three categories: oxidative coupling, reductive coupling, and transition-metal mediated organometallic cross-coupling reactions (Scheme 6). Of these oxidative coupling, which is here represented by the Kovacic route to poly(para-phenylene) (PPP),66 is the least useful as it is hardest to control, and tends to give a mixture of isomers, for example, in the case of PPP the coupling proceeds mainly 1,4- but a signiﬁcant amount of 1,2-linkages are also formed. Reductive coupling does not produce such defects but suffers from the problem that the efﬁcient coupling method developed by Yamamoto67 uses the rather expensive reagent bis(cyclooctadiene)nickel(0) stoichiometrically so that this method may be too expensive for large-scale production of polyphenylenes. Reductive coupling methods using cheaper reagents are known65 but to date have proven unsatisfactory for polymerizations. The transition-metal mediated cross-coupling reactions have proven the most versatile and widely used method for making polyphenylenes, especially since they can be used to make alternating copolymers as well as random copolymers and homopolymers. Of these methods Suzuki polycondensation68,69 has been the most widely adopted, as it has given the highest molar mass polymers (after optimization), but other procedures such as Kumada, Stille, and Negishi couplings have also been used. The polycouplings are most commonly performed in the so-called AA-BB fashion in which a dihalide is reacted with a bismetal, but the so-called AB method is also sometimes used. The latter suffers from the greater difﬁculty in preparing the desired monomer, but avoids the need for exact stoichiometry required to obtain high molar masses in the AA-BB method, which can be experimentally difﬁcult to achieve, as it requires very pure organometallic monomers. Poly(para-phenylene) (PPP, 36) is a blue emitter70 but it is also insoluble so that ﬁlms of PPP for LEDs need to be made via precursor methods.71 These produce acidic byproducts which can corrode electrodes. For this reason and also because of the low efﬁciencies obtained from PPP-based LEDs, PPP has not been the subject of any recent investigation as an emissive material. Attachment of solubilizing alkyl or alkoxy groups to PPP produces soluble polymers 37 (Figure 17), but due to steric repulsion

14

Polymer-Based LEDs and Solar Cells

Scheme 6 The main synthetic routes to polyphenylenes

Figure 17 Some typical phenylene-based polymers for LEDs.

between the sidegroups the backbone of such polymers is twisted, reducing the conjugation so that they display blue-violet emission.63 Conversely some of these polymers show yellow emission in the solid state due to the formation of emissive aggregates.72 The problem of backbone torsion can be overcome by linking the phenylene units together by methane bridges to produce ladder-type PPPs (LPPPs).63,64,73,74 The parent polymer 3875 is a blue emitter in solution, but the emission from LEDs was found to be unstable with a yellow emission band appearing rapidly during operation of devices. Originally attributed to aggregates this has more recently been explained by the presence of emissive defects on the chain produced by oxidation of some of the bridges.76 The color instability can be overcome by substituting the bridgeheads with methyl groups as in Me-LPPP (39)77 or phenyl groups (Ph-LPPP, 40),74 both of which display stable blue-green emission. In the case of Ph-LPPP a long wavelength emission band has been seen which arises from phosphorescence of palladium complexes incorporated into the polymer during synthesis.78 Another problem with LPPPs is that the ﬁnal step is a polymer-analogous Friedel–Crafts ring-closure reaction, in which quantitative reaction is required to avoid formation of defects. Nuclear magnetic resonance and mass spectra of LPPPs suggest complete ring closure is achieved, but the results from X-ray and neutron diffraction experiments,79 the anomalously low persistence length obtained from dynamic light scattering studies80 compared to those from step-ladder polymers81 and the evidence for discrete chromophores of varying length seen by single molecule spectroscopy analysis,82 all indicate that there exist signiﬁcant amounts of defects due to incomplete ring closure in LPPPs, so that rather than being rigid double-stranded ribbon-like molecules they are worm like structures with ladderized segments joined by single stranded linkages. Since stable blue emission has not been obtained from fully ladderized PPPs, attention has concentrated on so-called ‘step-ladder’ polymers in which only some of the aromatic rings are linked by bridges. These can be random step-ladder polymers, which are random copolymers of LPPP and substituted PPPs, or regular step-ladder polymers such as polydialkylﬂuorenes (PDAFs, 41)83,84 or polytetra alkylindenoﬂuorenes

Polymer-Based LEDs and Solar Cells

15

(PIFs, 42).85 PDAFs have been of particular interest as the monomers are extremely easy to make by alkylation of ﬂuorene under basic conditions, and synthetic protocols for obtaining the polymers by Suzuki or Yamamoto methods have been developed.63,64 While relatively stable blue emission has been obtained from the random polymers,86 the regular polymers have been the focus of more research interest as their properties are more controllable and predictable, especially as the bridges are formed during monomer preparation at which stage incompletely ring-closed materials can be removed by standard chromatographic and other puriﬁcation techniques. The emission from PDAFs and PIFs is violet-blue (deep blue) rather than pure blue, but pure blue emission was obtained ﬁrst from the poly(ladder-pentaphenylene) 43 (Figure 18),87 and later from a poly(ladder-tetraphenylene) 44.88

Figure 18 Pure blue emitting step-ladder polyphenylenes.

Just as with LPPP 38 above a problem of color stability arises when ﬁlms of polymers 41–44 are heated in air or upon operation of an LED, with the rapid appearance of a green emission band. It has been shown both for PDAFs 41,89 and PIFs 42,90 that this emission arises from ketone defects at bridgehead sites. These appear to arise when incompletely alkylated bridgeheads are oxidized during or after synthesis. This problem has been overcome for polyﬂuorenes by the development of a method for synthesizing completely alkylated dialkylﬂuorene monomers,91 and also by attaching aryl instead of alkyl substituents to the ﬂuorene. Polydiarylﬂuorenes (Figure 19) produce stable emission, as was ﬁrst shown for polymer 45 with phenylene dendron substituents,92 and later for polymers such as 4693 and 4794 with substituted phenyl rings as substituents. These aryl substituents are not attachable directly to ﬂuorene by alkylation and so the monomers are made by reaction of a 2-biphenylcarboxylate with aryllithiums or by reaction of ﬂuorenone with highly activated arenes (phenol or aniline derivatives).63,64

Figure 19 Polydiarylﬂuorenes with stable blue emission.

This concept was then extended to other bridged phenylene polymers where it has been found that the polytetraarylindenoﬂuorene (Figure 20, 48),95 and the fully arylated pentaphenylene 4996 and tetraphenylene 5097 polymers all show stable blue emission. In the case of 49 no green emission was seen even after a ﬁlm was heated at 200 1C in air for 24 h. The problem of inefﬁcient charge injection into polyﬂuorenes has been overcome by the attachment of charge accepting groups as substituents. This was ﬁrst demonstrated in polymer 46 bearing hole-accepting triphenylamine substituents.93 These improved hole injection into the polymer so that hole-transporting layers did not increase the efﬁciency of LEDs based on 46, whereas they do for LEDs using PDAFs 41.98 Electron injection has been improved by the incorporation of ozadiazole substituents as in polymer 51 (Figure 21).99 The oxadiazole units were attached by nucleophilic substitution of a benzonitrile by ﬂuorenyl anions, followed by conversion of the nitrile to an oxadiazole unit. Finally it has been shown that incorporation of both types of charge accepting units in the copolymer 52 leads to much higher device efﬁciency.100 As phenylene-based polymers have large bandgaps when emissive chromophores with smaller bandgaps are incorporated into the main chain or attached as end- or sidegroups, efﬁcient energy transfer results in emission from the lower bandgap chromophores. This process also explains why even very low levels of emissive defects can lead to strong green or yellow emission from

16

Polymer-Based LEDs and Solar Cells

Figure 20 Poly(tetraarylindenoﬂuorene) for stable blue emission.

Figure 21 Fluorene copolymers with electron-accepting units at C9.

phenylene-based polymers. Incorporation of 1–5 mol% of perylene based chromophores into PDAFs has been used to produce ﬂuorene-based polymers with emission spectra ranging across the entire visible spectrum.101 Phosphorescent chromophores have been incorporated into polyﬂuorenes to utilize the triplets arising from spin-independent charge injection.102 If less than 1 mol% of the small bandgap chromophore is incorporated then the blue emission from the polymer is not suppressed which has been used as a way of obtaining white emission. The highest efﬁciency white LED used a ﬂuorene copolymer containing a green ﬂuorescent (0.005–0.05 mol%) and a red phosphorescent (0.1–0.5 mol%) comonomer.103

8.10.3.3

Polycarbazoles

Carbazole is isoelectronic with ﬂuorene but the nitrogen bridge improves its hole-accepting abilities (raises the HOMO) and also, unlike the carbon bridge in ﬂuorene, cannot be oxidized to a ketone. As a result poly(2,7-carbazole)s (Figure 22, 53) have been investigated as potentially stable blue emitters,104,105 whereas the less conjugated poly(3,6-carbazole)s (2) have been proposed as hosts for emitters due to their high triplet energies.15 A problem with 2,7-carbazole-based materials is that, unlike the 3,6carbazole analogues, they cannot be synthesized directly from carbazole but efﬁcient synthetic routes to 2,7-dihalocarbazole monomers have been developed (Scheme 7).104,106,107 While the 2,7-carbazole polymers do not display degradation due to oxidation of the bridgehead there is evidence that they are photo- and electrochemically unstable due to the nitrogen atoms stabilizing the formation of charges at the free 3- and 6positions.105 This can be avoided by substituting those positions, so that the polymer 54 displays stable blue emission.88 Recently 2,7-carbazole-based polymers have become of interest as electron donating materials for BHJ solar cells.108 While the carbazole homopolymers 53 produce efﬁciencies of less than 1% due to their high bandgaps,109–111 carbazole copolymers (Figure 23) have displayed considerable potential for obtaining high device efﬁciencies when used as electron donors. The copolymer 55 containing a dithienylbenzothiadiazole unit, when combined with PCBM (3) produced an efﬁciency of 3.6%.112 Subsequent device optimization led to an efﬁciency of 6.1% being achieved, which was the ﬁrst polymer-based OPV to go above 6% efﬁciency.113 An efﬁciency of 5.4% has been obtained from a similar copolymer 56,114 but to date no other carbazole copolymers have been reported to produce efﬁciencies of over 3%. Copolymers have been designed and made which satisfy the criteria of bandgap and LUMO energy offset required by the Konarka model to be theoretically capable of up to 10% efﬁciency when blended with fullerene acceptors, but none has come close to attaining these efﬁciency levels.115 This seems to be due to

Polymer-Based LEDs and Solar Cells

17

Figure 22 Carbazole-based homopolymers.

Scheme 7 Efﬁcient syntheses of 2,7-dihalocarbazoles.

Figure 23 Carbazole copolymers for efﬁcient solar cells.

problems with their packing reducing the efﬁciency of charge transport in the blends, indicating that possession of the right orbital energies may be necessary for attaining high efﬁciency but is certainly not sufﬁcient. Copolymers 57116,117 and 58118 containing diketopyrrolopyrrole groups have appropriate energy levels and good charge transport properties, suggesting that though the device efﬁciencies reported to date using them are only around 2%, they might with device optimization similar to that performed for 55 be capable of producing quite high efﬁciencies.

8.10.3.4

Polythiophenes

Poly(3-hexylthiophene) (P3HT, 5) has been the most intensively studied of all polythiophenes. Some very innovative synthetic methods have been developed to make regioregular P3HT, i.e., a polymer in which as many as possible of the thiophene units are linked in head-to-tail (HT) fashion, as such a regular arrangement facilitates good chain packing and with it high charge carrier mobility.119 Routes to regioregular P3HT (Scheme 8) were ﬁrst developed by the groups of McCullough120,121 and Rieke,122,123 who prepared 2-halo-5-metallo-3-hexylthiophenes 59a and 59b which then underwent nickel-catalysed coupling reactions to produce P3HT with over 95% HT linkages.

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Polymer-Based LEDs and Solar Cells

Scheme 8 Rieke and McCullough routes to regioregular P3HT.

A disadvantage of these methods is that very pure monomers 59 are required to obtain high molar masses and the conditions for the synthesis and handling of these monomers are rather demanding. More recently the McCullough group have developed a much simpler method for preparing regioregular P3HT.124,125 The Grignard metathesis (GRIM) method (Scheme 9) involves reacting the readily available dibromide 60 with a methylmagnesiumhalide. Nickel-catalysed cross-coupling of the resulting mixture of thiophene Grignard intermediates 61 and 62 then produces regioregular P3HT. That the mixture of metallothiophenes produces only HT coupling is explicable by a combination of thermodynamic and kinetic factors. This method is experimentally very straightforward and can be carried out on a large scale and is currently being developed for the industrial scale preparation of P3HT.

Scheme 9 GRIM route to regioregular P3HT.

A particularly interesting feature of the GRIM synthesis is that it is quasi-living and so allows good control of the molar mass of the polymer produced,126 and selective endcapping with functional groups is possible.127,128 Such endcapping can be useful for making block copolymers or for assisting the polymers to bind to a surface or to an inorganic semiconductor nanoparticle. This feature of the process is explained by the mechanism shown in Scheme 10. One interesting point to note is that the very ﬁrst thiophene–thiophene coupling occurs tail-to-tail so every P3HT chain thus contains a defect – a situation reminiscent of what has been found in the Gilch synthesis of poly(arylene vinylene)s.

Scheme 10 Mechanism of the GRIM polymerization.

The GRIM route is an example of a chain-growth condensation polymerization, and has been successfully applied to the synthesis of polyphenylenes and polyﬂuorenes.129 Thus the advances in the synthesis of P3HT may produce advances in the synthesis of other well-deﬁned conjugated polymers. While P3HT has proved of little use as an LED material, due to its low solid state PL efﬁciency,8 its relatively modest bandgap (ca. 2.1 eV) has made it of interest as a solar cell material, and the combination of P3HT as electron donor with PCBM as acceptor

Polymer-Based LEDs and Solar Cells

19

is one of the more intensively studied BHJ combinations in polymer solar cell research.16,27 It has been reported that the solar cell performance of P3HT is enhanced by increased molecular weight130 and/or increased regioregularity.131 Conversely it has been reported that a small decrease in the effective regioregularity of P3HT appears to confer more stability to the BHJ solar cell with PCBM as acceptor.132 Efﬁciencies of up to 5% have been obtained from devices using a regioregular P3HT:PCBM blend, and there has been considerable investigation into methods for enhancing the photocurrent by methods such as thermal or solvent annealing, which aim to increase the order of the polymer chains and promote charge transport within the blend.27 However, the theoretical model developed by workers at Konarka suggests that the maximum possible efﬁciency from these blends is only a little above 5% so any further improvement in efﬁciency from them is likely to be very limited. Nonetheless the insights gained into the behavior of polymer-acceptor blends, on the relationships between the blend morphology, the degree of order in the blend, and the device efﬁciencies, and most importantly on the methods by which these can be enhanced have been of great value in the ongoing efforts to improve the efﬁciency of organic solar cells.

8.10.3.4.1

Cyclopentadithiophene, benzodithiophene, and dithienopyran polymers as donors in high efﬁciency solar cells

As the limits of potential efﬁciency in devices using polythiophenes appear to have been reached, attention has switched to other classes of polymers with the potential for even better efﬁciencies. We have already discussed carbazole-based polymers, which have attained 6.1% efﬁciency, but two classes of thiophene-related polymers have also been successful in producing efﬁciencies of over 5% in single-layer solar cells. The ﬁrst of these to be investigated were cyclopentadithiophene copolymers. The dithienylbenzodithiazole copolymer PCPDTBT (Figure 24, 63) is a low bandgap (B1.45 eV) material which has produced an efﬁciency of 5.2% when blended with PCBM.133,134 This polymer has a rather high HOMO ( 5.2 eV) which reduces the potential voltage from such cells to 0.6–0.7 V, but the use of an acceptor with a higher lying LUMO would surmount this problem. A tandem solar cell comprising two BHJ blends of 63 and 5 with fullerene acceptors stacked on top of each other with a conducting interlayer, produced an efﬁciency of 6.5%.135 Models by the Konarka group suggest that by suitable choice of the other layer tandem cells using 63 in one layer could attain up to 12% efﬁciency,29 and up to 15% efﬁciency may be possible for polymer tandem cells with the optimum combination of individual layers.27 The second class of thiophene-related polymers which have been used to make particularly high efﬁciency solar cells are benzo[1,2-b;4,5-b´]dithiophene copolymers. Copolymers containing thieno[3,4-b]thiophene derivatives when blended with

Figure 24 Cyclopentadithiophene, benzodithiophene, dithienopyran, and other copolymers for high efﬁciency solar cells.

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Polymer-Based LEDs and Solar Cells

fullerene acceptors have been found to produce efﬁciencies above 5%. The ﬂuorinated copolymer 64, was the ﬁrst material to produce efﬁciencies of over 7% (NREL certiﬁed value of 6.77%).136 Even better results have been obtained from the similar polymer PTB7 (65) which was ﬁrst reported to produce efﬁciencies of up to 8.37% with a conﬁrmed certiﬁed value of 7.65%.137 Later an improved efﬁciency of over 9% was reported from a cell with a metallopolymer as cathode interlayer.138 Further modiﬁcation of the devices to incorporate a dual-doped zinc oxide nanoﬁlm produced an efﬁciency of 10.31%.139 An efﬁciency of 10.1% has also been obtained by using a nano-patterned ﬁlm of PTB7 and PC71BM.140 The ﬁrst devices to show efﬁciencies above 10% however, used a dithienopyran-based donor-acceptor polymer PDTP-DFBT 66 (Figure 24) containing a ﬂuorinated-acceptor. A single-layer cell using 66 plus PC71BM produced 7.9% efﬁciency while a tandem cell with P3HT:ICBA produced 10.6% efﬁciency which made it the ﬁrst polymer based solar cell to exceed 10% efﬁciency.141 The best single-layer device yet reported for 66 showed up to 8.1% efﬁciency using an inverted structure with MoO3 as hole transport layer.142 A tandem cell consisting of two layers of 66 plus PC71BM of different thickness, with a MoO3-based interlayer displayed up to 10.2% efﬁciency. Triple-junction cells using layers of P3HT, PTB7, and 66 are reported to produce efﬁciencies of 11.55%,143 while an even higher value of 11.83% has been reported from a triple-layer cell using layers of copolymer 67, PTB7, and the copolymer 68.144 The exceptional results obtained from these structures show that there remains much scope for synthetic design, and leads to the hope that 12% efﬁciency from single-layer devices and 14% from tandem cells may be attainable in the near future. That such complicated comonomers are needed for very high efﬁciencies has been disproved by the recent report that efﬁciencies of over 10% can be obtained from the relatively simple dialkylquaterthiophene copolymers 69–71.145 This is reportedly due to optimization of the ﬁlm morphology, suggesting that with appropriate treatment very high efﬁciencies may be obtainable from a range of other thiophene-based copolymers. Other thiophene-based donor-acceptor type copolymers which have shown efﬁciencies of over 4.2% include poly(ﬂuorenedicyclopentathiophene-alt-benzothiadiazole) (72, Figure 25),146 poly(carbazole-dicyclopentathiophene-alt-benzothiadiazole)

Figure 25 Other thiophene-based polymers and copolymers with relatively good efﬁciency.

Polymer-Based LEDs and Solar Cells

21

73,146 and poly[9,9-dialkylﬂuorene-alt-(bis-thienylene)benzothiadiazole] 74–76.13,147,148 Some other thiophene-based polymer designs which have attracted attention recently and illustrate some of the new design concepts which may prove of importance in the future, though as yet not challenging the best materials in efﬁciency, are also illustrated in Figure 25. Reducing the number of alkyl chains on a polythiophene backbone can enhance the polymer packing, though care must be taken with retaining solubility, which is usually achieved by using slightly longer alkyl chains. As an example of this approach, the random copolymer poly(3dodecylthiophene-co-thiophene) 78 has been made; a blend with PCBM achieved an efﬁciency of 1.84%.149 Poly(thienylenevinylene) (PTV) is a well-known low bandgap material, so naturally there has been interest in studying PTV derivatives as donors in solar cells. The best results to date have come from PTV derivatives with 79–81 conjugated side chains which have been found to be very promising materials with reported efﬁciencies of over 3.18%.150,151 The device results for the polymers shown in Figure 17 are summarized in Table 1.

Table 1

Electronic energy levels and efﬁciency of some new thiophene-based copolymers

Polymer

Acceptor

HOMO (eV)

LUMO (eV)

PCE (%)

Reference

72 73 74 75 76 77 78 79 80 81

PC71BM PC71BM [C60]PCBM PCBM PCBM PCBM PCBM PCBM PCBM PCBM

5.32 5.38

3.55 3.61

4.96 4.94 4.93

2.97 2.97 2.96

2.8 3.7 4.2 2.2 2.84 4.4 1.84 1.71 2.57 3.18

140 141 142–143 143–143 142–143 144 144 145–146 145–146 145–146

8.10.4

Hybrid Solar Cells

Hybrid solar cells where a conjugated polymer is blended with an inorganic nanoparticle, have also been the subject of intense research in recent years. BHJ hybrid solar cells have been fabricated by blending inorganic materials like ZnS, TiO2, CdSe, etc., in the form of nanoparticles, nanorods, or nanowires with a semiconductor polymer into a thin ﬁlm.152–154 It is important to note that this is different from the use of such materials in DSSCs.19,20 Nor should hybrid cells be confused with the previously mentioned metal-organic perovskite cells in which the absorbing material is a hybrid crystalline material containing metal and organic cations with inorganic anions.21–25 Conjugated polymers such as P3HT have been used as hole-transporting materials in such devices but with only moderate performance.26 The high absorption coefﬁcients together with the well-deﬁned and size tunable properties of inorganic semiconductor nanoparticles are highly advantageous for achieving high efﬁciency. The size of the nanoparticles is of the order of the exciton wavelength, which creates quantum conﬁnement and allows for the tuning of optoelectronic properties, such as bandgap and electron afﬁnity. Nanoparticles have a large area to volume ratio, which presents more area for charge transfer to occur. Combining these properties with the ready processability and good physical properties of polymers offers a chance to get excellent device performance. Interest in hybrid solar cells was sparked by the report of a power conversion efﬁciency of 1.8% from a blend of P3HT and CdSe nanorods and nanoparticles.155–157 Later it was shown that branched nanorods of CdSe improve charge transport and the efﬁciency can be increased up to 2.4%.155,158 Most recently, an efﬁciency of 3.13% was obtained from a blend of the cyclopentadiene copolymer 63 with CdSe tetrrapods.17 Due to the toxicity of cadmium salts, less toxic metal oxides particularly ZnO (zinc oxide) and TiO2 (titanium dioxide) have been used extensively in hybrid solar cells, though the efﬁciencies reported to date have been less than outstanding. TiO2 has been utilized in various forms such as nanoparticles159, nanotubes160, porous network,161 and by in situ generation of TiO2 from appropriate precursors.148,162,163 Solar cells based on P3HT and TiO2 have shown an efﬁciency of up to 0.45% whereas blends of MDMO-PPV and TiO2 (made from Ti(i-PrO)4 precursor) produced an efﬁciency of only 0.2%. One reason cited for the poor efﬁciency is that the conversion of Ti(i-PrO)4 does not give crystalline TiO2. Crystalline TiO2 can only be obtained at temperatures greater than 350 ˚C, at which temperature most organic materials are likely to suffer signiﬁcant degradation.164 In contrast to TiO2, ZnO is known to generate crystalline nanoparticles at much lower temperature. One of the major hurdles in hybrid cells is the incompatibility of the two components leading to poor mixing. The effects of P3HT inﬁltration technique, solvent and annealing conditions on the performance of hybrid solar cells have been investigated,165 and the performance has been improved by a variety of means. Improved photovoltaic performance has been seen in a nanostructured ZnO/P3HT blend,166 while soluble crystalline ZnO nanoparticles (nc-ZnO) have been made that can be blended with conjugated polymers without the use of surfactants.167,168 While the hybrid BHJ cells still have not lived up to their potential, these and other advances suggest that they are likely to improve markedly in the future.

22

8.10.5

Polymer-Based LEDs and Solar Cells

Conclusions and Outlook

The prospects for polymer LEDs look bright with increasing numbers of products, for example, displays, TVs, entering production each year. The efﬁciencies of PLEDs are already competitive with small molecule OLEDs and device lifetimes, even for blueemitting devices, are now acceptable for most display applications. There is every reason to believe that the market share for PLED displays will increase dramatically over the next few years. The use of PLEDs for lighting applications is still problematic but it looks likely that here too there is some reason for optimism. One area that remains still to be fully explored is the possibility of fabricating polymer laser diodes, i.e., PLEDs that produce laser light. Optically stimulated lasing from conjugated polymers is well known and has been extensively investigated.169 However, the obtaining of electrically pumped lasing from conjugated polymers remains an elusive, but much sought after goal. Another area that is becoming of interest is the use of a three-electrode transistor rather than a two-electrode diode structure for emissive devices – organic light-emitting transistors have recently been reported which outperform OLEDs fabricated using the same emissive materials.170 The prospects for polymer OPVs are still unclear. The recent advances in efﬁciency to over 10% and the ongoing industrial research, most notably at Konarka, into developing fabrication methods for polymer solar cells leave real grounds for believing that if lifetimes can be improved that the low fabrication cost of these devices will make them competitive for at least some applications within the immediately foreseeable future. While the hybrid cells using mixtures of polymers and inorganic particles still lag behind their all-organic counterparts, they are steadily improving, and may yet surprise us. The use of tandem cells looks like being an approach of particularly great promise for the obtaining of high efﬁciency, though the optimization of such cells is inherently more difﬁcult than for single-layer devices. Much still remains to be done before polymer solar cells become signiﬁcant players in the energy supply market, but with the growing and increasingly urgent need for alternative energy sources, they will continue to attract signiﬁcant academic and industrial attention, and there is every reason to be optimistic about their future relevance and importance.

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Polymer-Based LEDs and Solar Cells

Polymer-Based LEDs and Solar Cells

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