Solution Processable Polymer OLED Lighting Panels with 25 lm/W Efficiency

Richard J. Wilson, Martin J. Humphries, Robert A. Archer, David Mohamad, Simon Foxon, Elena Hojas Garcia, Simon King, Thomas Kugler, Colin Baker, Natasha Conway,

Jeremy Burroughes and Ilesh Bidd Cambridge Display Technology Ltd. Building 2020, Cambourne Business Park,

Cambridgeshire CB23 6DW, UK Cambridge Display Technology Ltd (Company Number 02672530)

Abstract Through understanding of the device and photo-physics we have developed a fluorescent and phosphorescent hybrid materials set and device architecture for stable white Polymer OLED (P-OLED) lighting devices yielding 25 lm/W efficiency. We show that this technology can be transferred to solution processed ITO-free lighting panels realizing 32 lm/W efficiency with out-coupling film.

Author Keywords Polymer; OLED; Lighting; fluorescent; phosphorescent; solution processable; high efficiency; color stability; lifetime; panel; SSL.

1. Introduction Lighting consumes a significant fraction of the world’s total energy budget, and the need for more efficient lighting products has been apparent for a number of years. Solid-State Lighting (SSL) technologies promise high efficiency light sources enabling novel luminaire designs. OLEDs with their thin and uniform form factor enable light sources with a uniform emission area which have gained the attention of lighting designers. In addition to achieving high power efficiency with a high color rendering index, it is necessary for a new lighting technology targeting general lighting to be suitable for large volume, low cost manufacture. Polymer OLED devices are well suited to this requirement as they use a simple device structure which minimizes the number of deposition steps. An additional advantage is that the materials are well suited to solution processing which enables efficient material utilization and a reduced number of deposition steps requiring high vacuum, enabling high through-put and larger substrate area.

There has been a wealth of research into OLEDs tailored for lighting and many types of devices have been demonstrated utilizing various combinations of fluorescent and phosphorescent emission [1]. Previously we have reported on White P-OLEDs using a single component white emitting polymer, in which three chromophores of different colors are combined in a single material [2]. These polymers were synthesized using fluorescent blue and green emitters together with phosphorescent red emitters. In order to improved the efficiency we have chosen to develop device structures and materials that facilitate the change from fluorescent to phosphorescent green emitters and preventing the inter-chromophore quenching that is present when phosphorescent green emitters are in proximity to fluorescent blue emitters.

Indium-Tin Oxide (ITO) is a common anode material used for OLEDs due to its excellent combination of electrical conductivity and transparency. However, the ever increasing cost of indium is likely to prohibit the use of ITO for general lighting applications and so we have developed on an ITO-free anode structure. In this paper we demonstrate the combination of solution-processed materials and device structure giving 25 lm/W on a 10mm2 test cell can be used to make ITO-free low cost anode structure 2" polymer OLED lighting panels with 32 lm/W efficiency.

2. Device Structure and Compatible Materials Combining phosphorescent green emitters with fluorescent blue emitters can lead to inter-chromophore quenching of energy from the emissive triplet states of the green to the lower energy non-emissive triplet state of the fluorescent blue, Figure 1a. This results in an undesired loss of energy and hence efficiency. Such a pathway for energy quenching can be avoided by the introduction of a spacer layer between the phosphorescent green and the fluorescent blue, Figure 1b.

Ground State

S1 Blue

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Figure 1. Energy transfer mechanisms in operation for a white OLED device with (a) phosphorescent green in close proximity to fluorescent blue resulting in population of the blue triplet level resulting in quenching and (b) the impact of introducing a spacer layer preventing green triplets transferring to non-emissive blue triplet states.

In practice the use of such a spacer layer introduces the requirement for a couple of additional solution processed layers in our P-OLED devices. A typical device structure is shown in Figure 2.

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Figure 2. Device stack for P-OLED device incorporating a spacer layer to prevent quenching between phosphorescent green and fluorescent blue.

Solution processing of this device structure requires techniques to prevent subsequent polymer layers dissolving the under layers that would otherwise destroy the integrity of the device stack risking intermixing of the layers and potentially limiting the uniformity of emission brightness and color. This can be achieved using cross-solvents (eg. between HIL and interlayer) and with the utilization of cross-linking chemistries (interlayer, phosphorescent green and spacer layer).

In addition to the considerations of the stack integrity, we also found it necessary to develop phosphorescent green materials as a single component polymer as opposed to a blend of green dendrimer emitter/hole-transporter (HT) molecules and electron transport polymer. This was required to prevent the partial

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dissolution of the green emitter/HT when depositing a layer on top of the cross-linked green layer. Our solution, shown in Figure 3, has been to develop chemistries to facilitate the attachment of the green dendrimer to the electron transport polymer without a loss in performance of the dendrimer.

Coat layer on top

Blended green dendrimer Dendrimer washed away

Coat layer on top

Attached green dendrimer Dendrimer stays in place Figure 3. Phosphorescent green light emitting polymers with attached green dendrimer emitters/HT molecules were synthesized in order to prevent the dendrimer washing away during subsequent solution deposition of layers.

We have previously shown that devices made with the device structure shown in Figure 2 and utilizing single component green phosphorescent polymers illustrated in Figure 3 can achieve an efficiency of 13.4 lm/W at a color point of CIE (0.462, 0.426) [3]. In the following section we will describe the development of materials and devices structures that have enabled us to increase the efficiency of test devices to 25 lm/W, as measured in an integrating sphere without optical enhancement films/structures.

3. Development of 25 lm/W Devices We have applied a electrical drift-diffusion model to the device structure shown in Figure 2 in order to gain insight into the locations of the recombination zones within this device. Inputs to the model include electrical-field dependent electron and hole motilities for each of the active layers. These were experimentally determined from analysis of current-voltage traces measured on single carrier devices made with a range of thicknesses. Using a range of thicknesses allowed for charge injection phenomena to be accounted for and thus enabled the extraction of the fundamental charge mobilities of the materials. The drift-diffusion equations were solved self-consistently in 1D in a similar fashion to that outlined by Barker et al [4] and Martin et al [5].

IL Green Spacer Blue & Red

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Distance from HIL (nm)

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Figure 4. Exciton formation density through the device illustrated in Figure 2 calculated from a drift-diffusion model.

From the calculated electron and hole densities, an exciton formation density could be calculated as shown in Figure 4. Since triplet excitons are long lived, they can diffuse over greater

distances than singlet excitons. Therefore, the exciton emission profile for triplet excitons will be broader than the exciton formation profile in Figure 4, however, such electrical modeling does show that excitons will form within the spacer layer and thus, with this device structure, we are reliant on diffusion of these excitons to either the top or bottom emissive layers in order for them to emit light and contribute to the power efficiency of the device.

To address this issue, we have replaced the spacer layer shown in Figure 2 with a phosphorescent red layer, Figure 5. The triplet energy of the red being lower than both the phosphorescent green triplet energy and the fluorescent blue triplet energy prevents this layer from reducing the quantum efficiency of the device. Any phosphorescent green excitons or fluorescent blue excitons in proximity to the red layer can transfer to the phosphorescent red and then emit from the emitter. Such exciton transfer needs to be managed in order to prevent the device emitting predominantly red light.

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Figure 5. Device stack for P-OLED device where the spacer layer is replaced with a red layer.

Within the red layer we trialed two classes of red emitter, one with an electron trapping nature. We found the stronger electron trapping red enhances the formation of excitons on the red emitters. A comparison of the EL spectrum from two devices using the architecture described in Figure 5, made with either a non-electron trapping red emitter or an electron trapping red emitter is shown in Figure 6. This clearly demonstrates that the electron-trapping red enhances the red emission, resulting in ratios of red, green and blue compatible with overall white emission.

Electron-trapping rednon-electron

trapping red

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Figure 6. EL spectra for devices using the device architecture shown in Figure 5 made with either non-electron trapping red or an electron trapping red.

The final modification to the materials design discussed in this paper is the change to the blue layer that includes a Triplet-Triplet Annihilation (TTA) stabilizer material [6]. This class of fluorescent blue materials facilitates the stable use of the TTA effect to enhance the EQE of the fluorescent blue component up to 10%. The impact of using the TTA-stabilized blue layer is to color tune our highest efficiency devices on the Planckian Locus without losing efficiency.

These approaches have been combined to produce a materials set which yields 25 lm/W devices. Figure 7 shows the EL spectral shape and the data summary for these devices.

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Power Efficiency

Lumen Maintenance (LT70)

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25.3 lm/W 1000 hr 3135K ±0.011 4.28 V

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Figure 7. EL spectrum for >25 lm/W white polymer OLED device made with solution processing and device performance parameters.

4. Lighting Panels The Polymer OLED materials described in the previous section have been applied to lighting panels using an Indium-Tin Oxide-free (ITO-free) anode.

2” Panel

(a) (b)

Figure 8. (a) Example of 2" lighting panel made with solution processed Polymer OLED materials and (b) details of the metal grid-based anode used for the ITO-free panels.

The ITO anode typically used in OLEDs can add cost to the OLED lighting panels and so we have developed a technology to

replace the ITO with a metal grid-based anode. ITO-free panels have been made at 2" size. Figure 8a shows an image of a 2" panel, which has an efficiency of 32 lm/W with external out-coupling. Figure 8b shows the details of the metal grid-based anode used in the lighting panels.

5. Impact In this paper we have highlighted that polymer based materials, which lend themselves to solution processing, continue to develop and improve their efficiency. We have demonstrated 25 lm/W materials set based on fluorescent blue and phosphorescent green and red and measured without out-coupling enhancement. The transition from 10 mm2 test-cells to 2" panels demonstrates scalability of the polymer approach without problems associated with poor film formation issues likely to be faced by other approaches to solution-processed OLED lighting.

6. References [1] K. T. Kamtekar, A. P. Monkman, and M. R. Bryce, "Recent

Advances in White Organic Light-emitting Materials and Devices (WOLEDs)" Adv. Mater. 22, 572 (2010).

[2] [2] T. Pounds, "Developments in Solution Processable Polymers for Lighting" Organic Semiconductors Conference (2009)

[3] M. J. Humphries, R. J. Wilson, O. Fernandez, R. A. Archer; "Developments in solution processable polymer light emitting diodes" J. Photon. Energy 1 011019 (2011)

[4] J. A. Barker, C. M. Ramsdale, and N. C. Greenham, "Modeling the current-voltage characteristics of bilayer polymer photovoltaic devices" Phys. Rev. B 67, 075205 (2003)

[5] S. J. Martin, A. B. Walker, A. J. Campbell, and D. D. C. Bradley, "Electrical transport characteristics of single-layer organic devices from theory and experiment" J. Appl. Phys. 98, 063709 (2005)

[6] M. Roberts et al., "Excited State Interactions in P-OLEDs: Implications For Efficiency And Lifetime", SID 2011 Digest, 56.1, page 820

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