Charge Generation Kinetics and Transport Mechanisms in Blended

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Charge Generation Kinetics and Transport Mechanisms in Blended Polyfluorene Photovoltaic Devices

2002 Vol. 2, No. 12 1353-1357

Henry J. Snaith, Ana C. Arias, Arne C. Morteani, Carlos Silva, and Richard H. Friend* CaVendish Laboratory, Department of Physics, UniVersity of Cambridge, Madingley Road, Cambridge CB3 0HE, United Kingdom Received August 9, 2002; Revised Manuscript Received October 15, 2002

ABSTRACT We report a compositional analysis of blended hole-accepting and electron-accepting polyfluorene related materials, poly(9,9′-dioctylfluoreneco-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylene-diamine) [PFB] and poly(9,9′-dioctylfluorene-co-benzo-thiadiazole) [F8BT], in films and in photovoltaic devices. We find that photoluminescence quenching is insensitive to blend composition but the photovoltaic quantum yield is strongly composition dependent. This indicates that charge transport, and not charge generation, is the factor limiting device performance. We demonstrate that a meso-length scale phase separation optimizes charge transport properties.

Photovoltaic devices constructed from blended binary systems consisting of hole and electron-transporting organic semiconductor materials have shown high external quantum efficiencies.1-4 Photogeneration of charges within polymer blends occurs by exciton dissociation at polymer heterojunctions, with charge transfer of the electron to one component of the blend and the hole to the other.5 Ideally, a photovoltaic device would have distributed heterojunctions to aid charge separation, and short and direct percolation paths to each electrode within each component of the blend to maximize charge extraction. It has been demonstrated that the device performance can be greatly enhanced by controlling the blend morphology.6,7 This is achieved by altering the device preparation parameters (solution and substrate temperatures, spin-speeds, solvent saturated atmosphere, and solvents). We report on how changing the blend composition affects the external quantum efficiency of the device (percentage of electrons out to incident photons upon the device), and we relate this to the charge separation kinetics and the film morphology. For the PFB/F8BT blends studied here, we show that the charge generation rate from the PFB is sensitive to compositional change. However, the charge generation rate from the F8BT is composition insensitive. This, in conjunction with photoluminescence measurements and device results, gives us an accurate image of the film structure. We suggest here that charge trapping is the main loss mechanism in these devices and that a mesoscale phase separation (i.e., length scales of tens to hundreds of nm) is necessary in order to improve charge transport. * Corresponding author. E-mail: [email protected]. 10.1021/nl0257418 CCC: $22.00 Published on Web 10/29/2002

© 2002 American Chemical Society

The polyfluorene-based polymers used here are poly(9,9′dioctylfluorene-co-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenyl1,4-phenylene-diamine) [PFB] as the high mobility hole transporting polymer,8 and poly(9,9′-dioctylfluorene-cobenzo-thiadiazole) [F8BT] as the electron transporting polymer. The chemical structures are shown in Figure 1 together with the absorption and emission spectra of PFB and F8BT thin films. Due to the relative offset of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), charge separation occurs efficiently at the polymer heterojunction, making these polymer blends good for photovoltaic devices.4 However, due to the low entropy of mixing, the enthalpic term is dominant in the Gibbs free energy of mixing, and the polymers will tend to phase separate,9 reducing the potential number of polymer heterojunctions. When thin films are prepared by spin-coating polymer blends in a common solvent, as the solvent is removed by evaporation, one of the two polymers will become insoluble before the other, and this provides the onset of phase separation. In the present case, F8BT is less soluble in p-xylene than PFB. The nature of phase-separation is highly dependent upon the relative viscosities, molecular weights, concentrations, interactions between the two polymers, and temperature of the substrate and solution. In films spun from p-xylene solutions, two mixed phases are observed with relatively large feature sizes, of up to a few microns, one PFB-rich, the lower phase, and one F8BT-rich, the higher phase10 (Figure 2b). Each phase has been found to penetrate substantially through the film to the underlying substrate.11

Figure 1. (a) Chemical structures of F8BT and PFB. (b) Absorption spectra of thin films of F8BT (solid line), PFB (dot-dashed line) and a blend of 1:5 PFB/F8BT (dotted line). (c) Time integrated photoluminescence (PL) spectra for F8BT (solid line) and PFB (dot-dashed line). The dip at 500 nm is an artifact of the monochromator.

This phase separation is generally of the order of microns. However, it is important to note that within these phases is a finer scale of phase separation tens of nm in length.7 Most of the charge generation takes place in this smaller scale of phase-separation, since this matches the short diffusion length of the exciton (a few nm).12 In this letter we examine the problem that charges generated within the bulk of either phase do not always have a direct percolation path to reach an electrode. The smaller length scale phase separation of the F8BT in the PFB-rich phase, and vice versa, gives substantial photoluminescence quenching and charge generation. However, a conduction path for the minor phase back to the collection electrode is 1354

not always present. The PFB-rich and F8BT-rich phases separate out within the plane of the film.11 Thus, in the device, there is an interface between these two phases which runs substantially from one electrode to the other, as illustrated in Figure 2d. We find that charge collection efficiency scales with the surface area of interface between the mesoscale PFB-rich and F8BT-rich phases. We consider this to indicate that effective charge collection requires electrons in the F8BT-rich phase and holes in the PFB-rich phase. This requirement is satisfied when charge generation occurs close to the interface between the mesoscale phases (Figure 2d). We apply atomic force microscopy, in conjunction with time-correlated single photon counting, steady-state photoluminescence efficiency measurements, and photocurrent analysis. We characterize the blend morphology and the charge generation kinetics and deduce the charge transport mechanisms. Films for photoluminescence, atomic force microscopy, ultraviolet-visible (UV-vis) absorption spectroscopy, and time-correlated single photon counting measurements were prepared on Spectrosil substrates. Photoluminescence efficiency and atomic force microscopy measurements were carried out as by Arias et al.7 The time-correlated single photon counting data were obtained using a Kerr-lens mode locked titanium-sapphire laser (KMLabs TiS) as by Russell et al.13 The size of the excitation beam was several hundred microns in diameter, which is much larger than the scale of phase separation in the films. Photovoltaic devices were prepared by spin coating the polymer blend onto a precleaned indium tin oxide coated glass substrate, coated with a thin film of poly(ethylene dioxythiophene) doped with polystyrene sulfonic acid (PEDOT/PSS). All devices were fabricated in a nitrogen-filled glovebox and the electronic performance determined as by Arias et al.7 Polymer solutions were prepared by dissolving the homopolymers in p-xylene at concentrations of 15 mg/mL. Blends of PFB/F8BT were prepared by mixing different quantities of each homopolymer solution, such that the ratios of PFB to F8BT ranged from 500:1 to 1:500 by weight. Polymer films were produced by spin-coating polymer solution onto substrates that were precleaned in an ultrasonic bath, in acetone and 2-propanol, and treated in an oxygenplasma etcher for 10 min. All films were approximately 85 nm thick and fabricated at room temperature in a nitrogen filled glovebox. UV-vis absorption spectra for thin films of PFB and F8BT are shown in Figure 1b. The absorption spectra for the blends (blend ratio 1:5 PFB/F8BT shown) are a superposition of the absorption spectra of the individual components. Photoexcitation accesses the lowest singlet exciton that can decay either radiatively or nonradiatively, or undergo charge separation. The latter occurs efficiently at the interface between PFB and F8BT. Thus the rate at which the photoluminescence is quenched gives information about the structure of the films. Figure 1c shows the time-integrated emission spectra of PFB and F8BT. The emission spectra Nano Lett., Vol. 2, No. 12, 2002

Figure 2. Atomic force microscopy images for thin films of PFB/F8BT blends with ratios (a) 1:5, (b) 1:1, and (c) 5:1. (d) 3-D schematic representation of blended polymer film. The PFB-rich phase is represented by the volume within the dark gray cylinder and the F8BT rich phase is represented by the hollow box on the outside of the dark gray cylinder. Photogenerated charges within a thin cylindrical shell (light gray region) about the interface between the two mesoscale phases (dark gray cylinder), become collected charge at electrodes. t is the film thickness, d is the average diameter of the circular phase, and ∆r is the distance over which charge can migrate, within the minor phase, to reach the interface.

for the blends are a superposition of those from the individual components. The emission from a blend at 468 nm is therefore mainly from PFB, and emission at 538 nm mainly from F8BT. By looking at the emission at these specific wavelengths, the decay of excitons produced in the PFB and those produced in the F8BT can be observed selectively. Atomic force microscopy images of films with blend compositions of 1:5, 1:1, and 5:1 (PFB/F8BT) are shown in Figure 2a, 2b, and 2c. The time-correlated single photon counting data from these films at 468 and 538 nm are shown in Figure 3a and 3b. The photoluminescence decay kinetics at 468 nm, corresponding to the PFB emission, show a clear dependence on blend composition, (Figure 3a). An explanation lies in understanding the relative composition of each phase. We postulate that within the PFB-rich phase the ratio of F8BT to PFB has not reached a maximum in a film spun from a blend solution with composition ratio of 5:1 (PFB/ F8BT). Thus increasing the concentration of F8BT in the blend solution increases the amount of F8BT in the PFBrich phase of the film, creating a higher number of polymer heterojunctions, thereby increasing the decay rate of the PFB Nano Lett., Vol. 2, No. 12, 2002

emission. The decay of the PFB emission from the F8BTrich phase remains composition independent. The PFB emission will always decay quickly as PFB is the minor component within this phase. On the other hand, the photoluminescence decay kinetics at 538 nm, corresponding to F8BT emission, show no clear dependence on blend composition, (Figure 3b). Once again, an explanation for this lies in understanding the relative composition of each phase. We postulate that within the F8BT-rich phase, the ratio of PFB to F8BT has reached a maximum at a blend composition ratio of less than 1:5 (PFB/ F8BT). Thus, increasing the concentration of PFB in the blend solution does not increase the density of PFB within the F8BT-rich phase of the film. The F8BT emission from the PFB-rich phase decays quickly, regardless of the blend composition, within the range 1:5 to 5:1 (PFB/F8BT), as the F8BT is predominantly surrounded by PFB. The charge separation rate therefore does not increase, and consequently the F8BT emission decay kinetics remain composition independent. These results imply that within these films the PFB-rich phase can contain more F8BT than the F8BT-rich phase contains PFB. 1355

Figure 3. PL decay for thin polymer films of PFB/F8BT at: (a) 468 nm, PFB (circles) and (b) 538 nm, F8BT (circles), with blend ratios 1:5 (squares), 1:1 (solid diamond), and 5:1 (triangles). Instrument response (crosses).

Photoluminescence efficiencies for thin films of the individual homopolymers and the blends are shown in Figure 4a. The photoluminescence efficiencies were used to study the composition dependence of the charge-transfer efficiency. The degree of charge generation can be inferred from the degree of photoluminescence quenching. Figure 4b shows the external quantum efficiency of the photovoltaic devices at the peak absorption wavelengths of PFB (400 nm) and of F8BT (335 nm and 480 nm). The most efficient devices are those made from blends of ratio 1:5 (PFB/F8BT). This is in contrast to the highest photoluminescence quenching yield, which occurs at a blend ratio of between 5:1 to 10:1(PFB/F8BT). For a blend of 1:1, the photoluminescence is quenched from 55% for the pure F8BT film, and 38% for the pure PFB film, to 16%. However, the external quantum efficiency is less than 2% at 400 nm, suggesting that the limiting factor to device performance is not charge generation but charge transport to the electrodes. This is in agreement with findings by Pacios et al.14 Considering the device results (Figure 4b) in conjunction with the time-correlated single photon counting data (Figure 3a), there is slightly higher charge separation yield in the 1:5 (PFB/F8BT) blend than in the 1:1 and 5:1 blends; 1356

Figure 4. (a) Dependence of photoluminescence efficiency on PFB/ F8BT blend composition. (b) Dependence of external quantum efficiency on blend composition, with excitation wavelengths of 480 nm (dark gray bar), 400 nm (open bar), and 335 nm (light gray bar).

however, this is evident only from the emission of the PFB. Since the most efficient devices consist of less than 20% PFB, this effect is unlikely to be large enough to explain the dramatic increase in external quantum efficiency from the 1:1 to the 1:5 (PFB/F8BT) device. We consider that the increase in the external quantum efficiency is due to improved charge transport within the blend. The interfacial area between the mesoscale phaseseparated regions in the blends can be estimated from the atomic force microscope images. In a simple model, we assume the interface to be cylindrical in shape, with height of the thickness of the film, t, and of the average diameter of the circular phase, d, as observed in the AFM image, (Figure 2d). We find that the 1:5 (PFB/F8BT) blend has an interfacial area of 0.46 µm2 per µm2 of film, which is three times higher than that of the 1:1 blend. The external quantum efficiency at 400 nm is two times higher in the 1:5 blend than in the 1:1 blend. The external quantum efficiency for Nano Lett., Vol. 2, No. 12, 2002

Figure 5. External quantum efficiency at 400 nm illumination for devices of blend ratios 1:1, 4:5, 3:5, 2:5, and 1:5 PFB/F8BT, (from left to right on graph), versus interfacial area between the mesoscale phases. The line is the best fit straight line. These devices were fabricated in a different batch to those in Figure 4, explaining the slight discrepancy in the values of the external quantum efficiency.

devices fabricated from a range of blends, with ratios between 1:5 and 1:1 PFB/F8BT, are plotted against their respective interfacial area in Figure 5. We observe a linear trend, giving substantial evidence that the external quantum efficiency scales with the interfacial area between the mesoscale phase separated regions. We consider that charges generated within a certain range of the interface between the two mesoscale phases, ∆r, can be easily transported to an electrode. Charge produced near a mesoscale interface can diffuse into the preferential rich phase, where it will have a direct percolation path to the collection electrode. This will give an addition, from the interface, to the external quantum efficiency. Electrons (holes) produced in the bulk of the PFB-rich (F8BT-rich) phase do not have a direct percolation path to their respective electrode, and charge trapping occurs. The fraction of these charges transported to the electrodes is likely to be low, although this will form part of the external quantum efficiency. This relationship, however, is not observed with devices fabricated from blend solutions containing more PFB than F8BT. We believe this to be due to the asymmetry of the device. PEDOT/PSS coated ITO is the semitransparent anode, which collects holes. Due to the high absorption coefficient of the polymers, when light is absorbed there is a concentration gradient of excitons from the anode to the cathode with the highest concentration being at the anode. This results in a longer average path length to the electrode for dissociated electrons than for holes. This leads to devices with more F8BT having an improved balance in charge transport to the electrodes, thus higher efficiencies. This

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contributes to the effect from the mesoscale interfaces, biasing the performance of devices with more F8BT. Work is currently being carried out to investigate this phenomenon. In conclusion, charges generated within the bulk of either phase do not have much influence on the device performance, as the majority of the charges recombine, geminately after photogeneration, or after being trapped in the bulk. Higher external quantum efficiencies are achieved by increasing the interfacial area between the mesoscale phases and thus improving percolation paths to the electrodes, which is the factor limiting device performance. Having a mesoscale phase separation of the order of the thickness of the film could optimize the morphology of these polymer-blend devices. This will maximize the interfacial area between the phases while still allowing each phase to penetrate substantially through the film to the substrate, thus optimizing charge transport to the electrodes. Further work is presently being undertaken changing the molecular weight of one of the components of the blend. This will allow the morphology dependence of the external quantum efficiency to be studied without varying the blend composition. Further quasi-steadystate spectroscopy is being carried out in order to understand the behavior of the charges within each phase. Acknowledgment. This work was funded by the Engineering and Physical Sciences Research Council, (EPSRC), UK. H.J.S. thanks Catherine Ramsdale and Lukas SchmidtMende for valuable discussions. A.C.M. thanks the Gates Cambridge Trust for funding. C.S. is an EPSRC Advanced Research Fellow. References (1) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater. 2001, 11, 15. (2) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (3) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (4) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498. (5) Greenham, N. C.; Peng, X.; Alivisatos, A. P. Phys. ReV. B 1996, 54, 17628. (6) Halls, J. M.; Arias, A. C.; Mackenzie, J. D.; Wu, W.; Inbasekaran, M.; Woo, E. P.; Friend, R. H. AdV. Mater. 2000, 12, 498. (7) Arias, A. C.; MacKenzie, J. D.; Stevenson, R.; Halls, J. M.; Inbasekaran, M.; Woo, E. P.; Richards, D.; Friend, R. H. Macromolecules 2001, 34, 6005. (8) Redecker, M.; Bradley, D. D. C.; Inbasekaran, M.; Wu, W. W.; Woo, E. P. AdV. Mater. 1999, 11, 241. (9) Utraki, L. A. Polymer Alloys and Blends; Hanser: New York, 1989; Ch. 2.4. (10) Stevenson, R.; Arias, A. C.; Ramsdale, C.; MacKenzie, J. D.; Richards, D. Appl. Phys. Lett. 2001, 79, 2178. (11) Ramsdale, C. M.; Bache, I. C.; MacKenzie, J. D.; Thomas, D. S.; Arias, A. C.; Friend, R. H.; Greenham, N. C. Physica E 2002, 14, 268. (12) Stevens, M. A.; Silva, C.; Russell, D M.; Friend, R. H. Phys. ReV. B 2001, 63, 165213. (13) Russell, D. M.; Arias, A. C.; Friend, R. H.; Silva, C.; Ego, C.; Grimsdale, A. C.; Mullen, K. Appl. Phys. Lett. 2002, 80, 2204. (14) Pacios, R.; Bradley, D. D. C. Synth. Met. 2002, 127, 261.

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