NANO LETTERS
Nanoscale Conjugated-Polymer Light-Emitting Diodes
2005 Vol. 5, No. 1 67-71
Farhad A. Boroumand,† Paul W. Fry,‡ and David G. Lidzey*,† Department of Physics and Astronomy, The UniVersity of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH, United Kingdom, and Department of Electronic and Electrical Engineering, The UniVersity of Sheffield, Mappin Street, Sheffield, S1 3JD, United Kingdom Received October 1, 2004; Revised Manuscript Received November 8, 2004
ABSTRACT We use e-beam lithography to pattern an indium tin oxide (ITO) electrode to create arrays of conjugated-polymer LEDs, each of which has a hole-injecting contact limited to 100 nm in diameter. Using optical microscopy, we estimate that the electroluminescence from a 100 nm diameter LED comes from a region characterized by a diameter of ∼170 nm. This apparent broadening occurs due to current spreading within a PEDOT:PSS layer which was included to aid hole injection.
A number of different approaches have been used to create nanoscale light-sources. In principle, the smallest possible light source is a single fluorescent molecule or a quantum dot (QD). While emission from both single molecules1,2 and QDs3 can be generated following optical excitation, it is clearly advantageous to use electrical excitation to generate light emission, as this would aid device integration with conventional circuits and systems. A number of studies have shown that light emission from both single molecules4 and QDs5 can be generated using a scanning tunneling microscope probe (STM) as a localized excitation source. While this is a proof of principle of an electronically driven “nanoscale” light-source, it does not constitute a practical device. Recent work has shown that electrical emission can be detected from a single QD located within a p-i-n diode.6 However, selective emission from a single QD was not achieved; rather, optical filtration was used to remove the light emission from a number of other QDs that were simultaneously excited. There are in fact significant technological issues involved with scale-down of conventional inorganic semiconductor devices resulting from current spreading (due to high charge carrier mobility) and large exciton diffusion lengths (∼1-2 µm). Problems also arise if inorganic semiconductor surfaces are etched to limit the lateral spreading of charge-carriers, as such etched surfaces are strongly associated with nonradiative recombination. To address this problem, recent work7 has demonstrated that current aperture created by a lateral oxidation technique can be used to restrict the active emissive area of inorganic * Corresponding author. E-mail:
[email protected]. † Department of Physics and Astronomy. ‡ Department of Electronic and Electrical Engineering. 10.1021/nl048382k CCC: $30.25 Published on Web 11/20/2004
© 2005 American Chemical Society
semiconductor LEDs based on InAs quantum dots. Devices having an emissive diameter as small as 100 nm were created using this technique; however, this only proved a reliable method to create devices with an active diameter of g600 nm. Due to the problems associated with the scale-down of conventional inorganic semiconductor devices, a number of alternative material systems have been investigated. Impressive performance has been demonstrated using semiconductor “nanowires”. These are semiconductor whiskers having a diameter of several tens of nanometers. By controlling the composition along the nanowire,8 a nanoscale p-n junction was created from which electrically generated light emission was observed. While this indeed constitutes a nanoscale lightsource, the control and positioning of nanowires is not straightforward, making their routine use in integrated devices difficult. Miniature electrically driven light-sources have also been created using organic semiconductors; photolithography has been used to pattern the ITO (hole-injecting) electrode of an LED based on a low molecular mass dye, creating devices having a stripe geometry, with the width of the stripe as narrow as 0.9 µm.9 However, this is close to the limit in device reduction that can be achieved using conventional optical lithography. Instead of patterning the anode of an organic LED, Lee et al.10 have explored patterning an LED cathode. This was achieved using a patterned elastomeric stamp that had been coated with a thin gold film. When this gold anode was placed in contact with a conjugated polymer film deposited on to ITO, electroluminescence (EL) emission was observed from a stripe having a width estimated to be ∼150 nm. Further reductions in the emissive area have been achieved using self-assembly to create localized conduction
paths through an active organic semiconductor layer. For example, blends of a conjugated polymer and an insulating polymer have been used as the active layer in a conventional LED device.11,12 Here EL from the conjugated polymer domains was observed, with the diameter of the phaseseparated domains being ∼130 nm. Alternatively, template polymerization of a conducting polymer within the 100 nm diameter pores of a membrane filter has also been used to create a medium in which charge conduction could be localized within an LED.13 In two recent studies,14,15 organic LEDs have been created in which the ITO anode is patterned with an insulator having sub-100 nm features. However, in both cases the periodicity of the patterns was well below the resolution limit of conventional optical microscopy, making it impossible to demonstrate that individual lightemitting domains were created. Indeed, it is possible that current flow (and thus EL emission) was not spatially restricted by such finely patterned structures on the anode surface due to the effects of current spreading, as we discuss below. It is clear, however, that organic semiconductors are in principle a highly promising system in which to explore the creation of miniature light sources. In general, the mobility of charge carriers in disordered organic semiconductors is much smaller compared to those of inorganic semiconductors (e.g., conjugated polymers have a hole mobility ranging between 10-7 and 10-4 cm2/Vs at room temperature,16 which compares with 100 cm2/Vs for p-type Zn-doped GaAs). This relatively low mobility is of direct benefit in restricting current spreading and thus localizing light emission. Furthermore, functional organic materials can be deposited from solution on a number of pre-patterned surfaces,17-19 opening a wealth of possibilities that are not readily possible using more traditional inorganic materials. Here, we use a very high-resolution lithographic tool (an electron-beam writer) to pattern the hole-injecting electrode of a conventional polymeric LED. E-beam lithography has recently been explored to prototype the fabrication of organic devices, for example, by controlling the surface dewetting of functional polymers to create a nanoscale source-drain channel of an organic field effect transistor.19 The advantage of using such a technique to pattern the LED anode is that the dimensions and location of each device can be precisely defined. As we show below, this allows us to create arrays of nanoscale LEDs, with each device having a relatively large physical separation from its neighbors. This permits us to image the emission from individual pixels, and to confirm that we have created a series of electrically driven nanoscale light sources. The generic structure of the LED array that we have created is shown in Figure 1. Here, an insulator layer deposited onto an indium tin oxide anode was patterned using an electron-beam writer. This created a series of current apertures and thus limited the area through which the current can flow into the organic semiconductor. To create this structure, a layer of silicon dioxide (having a thickness of between 100 and 300 nm) was first deposited on an indium tin oxide (ITO) anode by plasma enhanced chemical vapor 68
Figure 1. Schematic of a nanoscale LED array. Here, the silicon dioxide layer is patterned using e-beam lithography to create an array of hole-injecting contacts, each having a diameter of 100 nm. On top of this are coated a thin film of the conducting polymer PEDOT/PSS and a proprietary green-emitting conjugated polymer. A Ca/Al film is used as a device cathode.
deposition. A 100 nm layer of poly(methyl methacrylate) [PMMA] was then spin-coated onto the silicon dioxide. The sample was baked to remove residual solvent, and then a 30 nm thick layer of aluminum was coated onto the surface by thermal evaporation to reduce surface charging during e-beam patterning. The surface was then patterned using an e-beam writer at 10 kV (electron dose 100 µC cm-2) to define a 50 × 50 array of disks having a diameter of 100 or 200 nm on a 10 µm pitch. The aluminum layer was removed by washing the surface in a 10% solution of ‘Decon 90’, followed by a rinse in deionized water. The PMMA that had been exposed (through the Al) to the electron beam was then removed by washing the substrate using 1:3 solution of (methylisobutyl ketone/isopropyl alcohol). Reactive-ion etching using CHF3 was then used to remove the exposed silicon dioxide surface. The substrate was then washed in acetone to lift off the remaining unexposed PMMA, which left an array of holes in the silicon dioxide layer. On top of this patterned electrode, we constructed a standard format LED. The LEDs were based on an ITOPEDOT/PSS (poly(3,4-ethylendioxythiophene)/polystyrenesulfonic acid) anode, which was coated with a 45 nm thick layer of a proprietary green-emitting polyfluorene-based polymer as the active emissive medium. The device was then completed by the thermal evaporation of a composite 20 nm calcium/200 nm aluminum cathode. Here the PEDOT/PSS layer (Baytron P VPCH8000 supplied by H. C. Starck) was used to facilitate hole injection into the polymer. We have deliberately used a formulation of PEDOT/PSS which has a high resistivity of 1000 Ω m.20 This material has been designed for use in active matrix LED displays to reduce unwanted cross-talk between neighboring pixels. The structure was then encapsulated in a nitrogen-filled glovebox using a glass cover-slip and a UV cross-linkable resin. Conventional LEDs having the same polymeric and cathode layers, but with an unpatterned anode, were also fabricated as a control. Figure 2a shows a 950 × 950 nm2 topographic image of a patterned, silicon dioxide surface recorded using a Veeco Explorer atomic force microscope (AFM) in tapping mode. A single 200 nm diameter hole in the silicon dioxide can be seen in this image, whose depth in this case is 300 nm. The depth of the hole was consistent with the thickness of the Nano Lett., Vol. 5, No. 1, 2005
Figure 2. Topography of the nanoscale LED anode, together with emission from an array of devices. Part (a) shows an atomic force microscope image of a single 200 nm diameter hole in a silicon dioxide film defined using e-beam lithography. Note that the thickness of the silicon dioxide film in this case was 300 nm. In all other devices, however, the silicon dioxide thickness was limited to 100 nm. Part (b) shows an image of the EL recorded from an array of 200 nm diameter LEDs. Part (c) shows a magnified view of the EL emission recorded from an array of 100 nm diameter devices.
silicon dioxide film that was deposited, demonstrating that the etch process stopped at the ITO surface. Figure 2b shows an image of the EL recorded from an array of 200 nm pixels when driven at a voltage of 10 V. This image was recorded using a Zeiss Axioplan microscope, equipped with a 50x long-working distance lens coupled to a high sensitivity CCD camera. We have made spectral measurements of the emission of individual pixels using a confocal microscope, and find them to be almost identical to emission from the green-emitting polymer when used in a conventional LED. It can be seen in Figure 2b that there appears to be a wide range in brightness between individual devices. This variation in brightness probably results from incomplete etching of the silicon dioxide insulator, as we find that the failure rate of devices increases as their active diameter is reduced. Nevertheless, this failure rate is in many cases low, with for Nano Lett., Vol. 5, No. 1, 2005
Figure 3. Cross-section of the emission intensity through a nanoscale LED, together with the intensity of light transmitted through a nanoscale aperture used to characterize the resolution of the microscope. Part (a) shows an EL intensity cross-section recorded across a representative 100 nm diameter pixel. The fit to the data shown using a line was determined by convolving the optical response of the microscope (a Gaussian of width 580 nm) with a Lorentzian function of width 170 nm. Part (b) shows an intensity cross-section of the light emitted from a nanosource having a known size (a near field microscope probe with a 100 nm diameter aperture). We estimate the optical resolution of our microscope by convolving a Gaussian function of width 580 nm with a top-hat function of width 100 nm (which provides a reasonable approximation of the intensity distribution across the aperture). The intensity cross-sections shown in parts (a) and (b) were recorded under identical conditions.
example only 10% of the 200 nm diameter pixels not emitting any detectable EL. Figure 2c shows a close-up of EL from an array of 100 nm diameter pixels, also driven at 10 V. Here, the emission from each pixel appears as an Airy disk, as its physical dimensions are much less than the spatial resolution of the microscope. The electronic properties of the nanoscale devices are similar to that of conventional LEDs, with the conventional devices emitting light at 3.0 V and the 200 nm diameter devices emitting light at 4.0 V. At present, we have not been able to provide a quantitative measure of the emission brightness from a single nanopixel; however, light emission from arrays of devices are clearly visible under standard laboratory lighting conditions. Figure 3a shows an intensity cross-section taken through a representative 100 nm diameter pixel. Here, the EL emission occurs from an area characterized by a width (full width at half-maximum) of 690 nm. Similar measurements on 200 nm diameter devices indicated that the emissive area was characterized by a width of 1100 nm. This measured size is broadened by the finite spatial resolution of the microscope. To assess the resolution of our microscope when imaging such nanoscale light sources, we 69
have imaged a nanoscale light source of known emissive area (the aperture of a scanning near-field microscope probe). The probes that we have used (supplied by Jasco International Ltd. Tokyo), have a well-defined aperture of approximately 100 nm in diameter, which is pre-characterized using scanning electron microscopy. A white light source was shone into the cleaved end of the probe fiber, and the microscope was used to image the light emitted from the aperture. As shown in Figure 3b, the emission recorded across an image of the SNOM probe aperture has an approximately Gaussian line-shape characterized by a width of 620 nm. To assess the practical resolution of our microscope, we have convolved a Gaussian function of width 580 nm with a top-hat function (of width 100 nm), which we have used to approximate the cross-section of the SNOM aperture. The result of the convolution is shown in Figure 3b using a line. As it can be seen, it provides a very reasonable description of the observed emission from the SNOM probe. We therefore propose that at the limit of resolution, our microscope has an effective spatial resolution of 580 nm. This estimate of the microscope resolution is in good agreement with a conventional calculation of the limit of resolution of an imaging system using the Rayleigh criterion. Using rmin ) 1.22λ/2NA, we calculate that our microscope (having an objective lens of numerical aperture (NA) of 0.6, imaging light at a wavelength of λ ) 530 nm in air) can resolve a minimum feature size of rmin ) 540 nm. We use this estimate of the optical resolution of the microscope to determine the emissive diameter of E-beam patterned devices. We find that by convoluting a Lorentzian function of width 170 nm (720 nm) with a Gaussian of width 580 nm, we can reproduce the observed shape and width of the central bright disk of the 100 nm (200 nm) pixels. Here, the use of a Lorentzian function to fit the intensity distribution does not necessarily imply any underlying physical mechanism, rather we find that it simply provides a better fit to the data than is achieved using a Gaussian. It is clear that the width of the region from which EL emission occurs in the 100 nm device is below the resolution limit of our microscope; however, our comparative approach using a nanosource of known size (the SNOM fiber-probe aperture) gives us confidence that we can at least place a lower limit on the emissive diameter of our devices. We have therefore demonstrated that the emissive diameters of the nanoscale LEDs are larger than their holeinjection contact (170 nm compared to 100 nm), indicating that current spreading effects occur even within systems that are usually associated with low carrier-mobility. Current spreading is most likely to occur in the PEDOT/PSS layer as its conductivity is significantly greater than that of the conjugated polymer. We estimate20 that, at a drive voltage of 6.0 V (i.e., well above turn-on), the conjugated polymer used in this study has an effective ‘resistivity’ of 30 000 Ω m. The PEDOT/PSS used in these devices has a resistivity of 1000 Ω m,20 some 30 times smaller than that of the conjugated polymer. Thus we anticipate some lateral component of the electric field within the PEDOT/PSS layer, 70
while the field distribution in the conjugated polymer is mainly normal to the plane of the device. This will permit limited spreading of holes within the PEDOT/PSS layer before they are injected into the lower conductivity conjugated polymer. We note that the width of the emission zone of the 200 nm LED is significantly broader than that observed in the 100 nm diameter device (720 nm compared to 170 nm). The reason for this apparent nonlinearity in recombination zone size not understood; however, finite element analysis modeling is planned to resolve this issue. In conclusion, we have used E-beam lithography to pattern 100 nm injection contacts for polymer LEDs and have created devices whose individual emissive diameters are as small as 170 nm. The width of this recombination zone is broadened by current spreading within the device. Such current spreading could be significantly reduced by replacing the higher conductivity PEDOT/PSS layer from the LED with a self-assembled molecular monolayer to facilitate hole injection.22 Organic nanoscale light LEDs might find possible applications in nanoscale assay systems,23 in near-field opticalcommunication and storage devices,24,25 or as a source of single photons on demand.6 Furthermore, our devices offer an intriguing possibility to create an LED where emission comes from few molecules. For example, by doping the conjugated polymer with a low concentration of acceptor molecules (at which charge trapping and subsequent recombination occurs),26 the number of emissive states could be reduced to a point where single-molecule effects become important. Single molecules have recently been demonstrated to act as an anti-bunched source of photons at room temperature.27 Thus, a nanoscale LED doped with a small number of molecular emitters would be directly analogous to the structures made using quantum-dot-based systems5 and may act as an electrically driven single-photon light source for quantum cryptography and communication systems. Acknowledgment. We thank the UK EPSRC for funding this research under grant GR/R25712/01 ‘High performance light sources based on conjugated polymer light emitting diodes’ and through the award of an Advanced Research Fellowship to D.G.L. We also thank Cambridge Display Technology for the provision of the green-fluorescent polymer and Simon Martin and Edward Daw for useful comments and suggestions made during the preparation of this manuscript. References (1) Betzig, E.; Chichester, R. J. Science 1993, 262, 1422-1425. (2) Mu¨ller, J. G.; Lemmer, U.; Raschke, G.; Anni, M.; Scherf, U.; Lupton, J. M.; Feldmann, J. Phys. ReV. Lett. 2003, 91, 267403. (3) Vuckovic, J.; Fattal, D.; Santori, C.; Solomon, G. S.; Yamamoto, Y. Appl. Phys. Lett. 2003, 82(21) 3596-3598. (4) Qiu, X. H.; Nazin, G. V.; Ho, W. Science 2003, 299, 542-546. (5) Yamanaka, K.; Suzuki, K.; Ishida, S.; Arakawa, Y. Appl. Phys. Lett. 1998, 73(11), 1460-1462. (6) Yuan, Z.; Kardynal, B. E.; Stevenson, R. M.; Shields, A. J.; Lobo, C. J.; Cooper, K.; Beattie, N. S.; Ritchie, D. A.; Pepper, M. Science 2002, 295, 102-105. (7) Fiore, A.; Chen, J. X.; Ilegems, M. Appl. Phys. Lett. 2002, 81(10), 1756-1758. (8) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Nature 2002, 415, 617-620.
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