We report what we believe to be the first use of organic nanostructures for efficient colour conversion of gallium nitride light emitting diodes (LEDs). The particular nanomaterials, based on star-shaped truxene oligofluorenes, offer an attractive alternative to inorganic colloidal quantum dots in the search for novel and functional ‘nanophosphors’. The truxenes have been formed into a composite with photoresist and ink-jet printed onto microstructured gallium nitride LEDs, resulting in a demonstrator hybrid microdisplay technology with pixel size ~32µm. The output power density of the hybrid device was measured to be ~8.4mW/cm2 per pixel at driving current density of 870.8A/cm2 and the efficiency of colour conversion at drive current of 7mA was estimated to be approximately 50%.
© 2009 Optical Society of America
Recently, there has been considerable interest in developing ‘hybrid’ light-emitting technologies based upon gallium nitride optoelectronics. Electrically injected gallium nitride hetero-structures offer customised direct band-gaps in the ultraviolet to visible spectral range and can selectively and efficiently transfer excitation either non-radiatively (Förster Resonance Energy Transfer or FRET) [1,2] or radiatively  to an overlayer based on alternative light-emitting materials. This approach has important implications for areas including; colour conversion and white-light generation for solid-state lighting , microdisplays , bioscience , instrumentation  and photo-pumped organic semiconductor lasers . It has to date been embodied primarily using conventional inorganic phosphors , inorganic semiconductor nanocrystals (mainly CdSe/ZnS colloidal quantum dots)  and, to a lesser degree, organic polymers . The case of colloidal quantum dots is particularly interesting, because it offers a means of ‘indirect’ electrical injection into nanostructured light emitters, and is a competitive approach to e.g. microdisplays and nanolasers being developed based on direct electrical injection into quantum dot containing conductive composite thin films . Here, we report development of such devices based on organic nanostructures as a technologically and scientifically interesting alternative.
The particular materials we used are star-shaped nanostructures of the truxene oligofluorene type, consisting of oligofluorene arms attached to a central truxene core. These materials [11, 12], related to the polyfluorene polymers used very widely in organic optoelectronics , have engendered considerable interest recently, in terms of their attractive physical properties, processability  and demonstration of laser action . They share the attractions common to their polymeric counterparts of efficient and wavelength-versatile colour conversion, blendability, ready conformability to microstructured surfaces and amenability to a variety of ‘soft’ micro- and nano-patterning techniques. In addition, they also offer advantages of chemical purity, structural uniformity and resistance to degradation . When blended into a photo-resist host, these materials form a composite that we demonstrate can be readily micro-patterned via inkjet printing onto gallium nitride devices, as the basis of a new hybrid device technology.
Our demonstrations of this capability here utilise the specific truxene oligofluorene ‘T3’ , wherein three terfluorenyl arms are attached to the central truxene core, giving a 3.1nm molecular radius. These molecules were blended into a photo-resist based on 1,4-cyclohexyldimethanol divinyl ether (CHDV), and the T3/CHDV composite printed onto 370nm-emitting micro-pixellated AlInGaN light emitting devices. The resulting hybrid devices show per pixel output power density of ~8.4mW/cm2 at driving current density of 870.8A/cm2 and reach colour conversion efficiencies of ~50% at drive current of 7mA.
2. Integration experiments
The structure, fabrication and performance of the gallium nitride devices used here has been described in detail elsewhere, together with their use for micro-display and instrumentation purposes [5,16]. Briefly, these devices were micro-structured quantum well light emitting diodes (LEDs) made from AlInGaN epi-structures grown on sapphire and designed to emit at ~370nm. They were patterned using inductively-coupled plasma dry etching techniques into matrix-addressable arrays of 64×64 micro-pixels, where each pixel had a 16µm emission aperture and the pixel-to-pixel pitch was 50µm. The structure of these pixels is indicated schematically in Fig. 1, wherein it is seen that a Ti/Au annular contact (thickness 250nm, outer diameter 32 µm, inner diameter 16µm) defined the emission aperture, which consisted of a 200nm thick silicon oxide isolation layer above a Ni (3nm)/Au (9nm) current spreading layer. The individual pixels in this work had turn-on voltages of 3.6V, and emitted continuous wave output powers of 48nW at a current of 7mA (see later).
There are two related challenges for integrating the organic materials onto such gallium nitride devices; one is how to process the organics and another is how to deposit the materials precisely, uniformly and reproducibly onto such small (32µm diameter) device elements:
The main technical challenge of processing organic light-emitting materials is the incompatibility with conventional photolithography methods . Recently, we have shown that incorporation of the oligofluorene truxene T4 with novel UV-transparent organic photoresist materials has opened up alternative approaches to processing and micro-patterning organic fluorescent nanocomposites . The polymerisation of oligofluorene truxene blends is in this case photo-induced and leads to a cross-linked network allowing the creation of optical structures. The UV-transparent negative photoresist matrix used, and exploited further in the current work, is 1,4-cyclohexanedimethanol divinyl ether (CHDV)(Sigma-Aldrich) with the photo-acid generator (PAG): p-(octyloxyphenyl) phenyliodonium hexafluoroantimonate (ABCR, Karlsruhe, Germany). Our study suggested a reduction of oxygen diffusion and photo-oxidation after encapsulating the fluorescent molecules with this polymer matrix , which is one of the attractions of this approach. The photo-polymerization mechanism has also been studied in the previous report . The T3 molecules used here (see Fig. 2(a)) exhibit high blue photoluminescence quantum yields both in solution and solid state (83% and 60% respectively) . We prepared various T3/CHDV blends, with T3 concentrations ranging from 0.5wt% to 10wt%. For reference spectroscopic measurements, blends with 0.5wt% PAG were drop-cast onto a quartz substrate and cured under ~370nm wavelength irradiation at 15mW/cm2. The absorption maximum for the blend was at ~368nm (matching the emission wavelength ~368nm of the gallium nitride LEDs) whilst the emission maximum was at ~408nm, ~430nm and ~460nm. These spectral characteristics are similar to those shown by T3 in solution and spin-coated films . Figure 2(b) shows that the LED emission matches very well with the T3/CHDV blend absorption, which allows sufficient colour down-conversion.
In order to address another challenge of integration, we explored the inkjet printing technique using the T3/CHDV blends to create oligofluorene truxene microstructures on the top of GaN-LED micro pixels to achieve colour down-conversion. The Dimatix DMP-2800 inkjet printing system used consists of a heated vacuum platen with XYZ control, built-in drop jetting observation, fiducial camera for pattern monitoring and alignment. The piezo-driven cartridges with silicon micro-electro-mechanical-systems (MEMS) fabricated print-heads could deliver ~1pL or ~10pL droplets with which we have found it possible to create microstructures down to ~12µm in diameter. The jettable fluid viscosity in this piezoelectric drop-on-demand print head is ~2–30 cP, and the viscosity of oligofluorene truxene blends (0.5wt% T3 concentration) was measured to be ~15 cP, which is in the viscosity range required. It was observed that varying the concentration of T3 had little effect on the viscosity. Precise alignment and controllable flow are required for this experiment. Thanks to the intelligent functioning of the instrument (computer interfacing, active position monitoring and repeatable stepper control accuracy of ±1µm), we could successfully achieve accurate alignment during the deposition. In addition, we required the printed organic droplet to remain stable on the top of the pixel prior to photopolymerisation without any excess flowing into the channels between pixels. As the printing was carried out onto the exposed SiO2 insulation layer within the ring-shaped GaN device p-contact (see Fig. 1), we applied a treatment with 1H,1H,2H,2H-perfluorooctyldimethylchlorosilane (ABCR, Karlsruhe, Germany) via vapour deposition at room temperature and atmospheric pressure. This formed a monolayer to change the surface energy and thus the contact angle of the organic droplet generated from the nozzle of the inkjet printing system (see Fig. 1) . By using this treatment, the measured contact angle of truxene blends on the SiO2 surface was altered from ~2° to ~39°, which effectively prevents the oligofluorene truxene drop from overflowing.
A T3/CHDV blend with 10 wt% T3 concentration and 0.5 wt% PAG to initiate the photopolymerisation was prepared for the fabrication of hybrid inorganic/organic LED devices. It is estimated that, at this concentration, there are around 1013 T3 molecules in 1 pL of blend. Due to the high miscibility of T3 molecules in the CHDV matrix, it is not necessary to add any solvent into the blend to obtain a phase-uniform solution. This solventless blend facilitated the fabrication of uniform microstructures by preventing the ‘coffee ring’ stain formation due to solvent evaporation . After printing the blend droplets onto the top of micro-LED pixels, the whole packaged device was exposed under an ultraviolet (370 nm in wavelength) lamp at an energy density of ~15mW/cm2 for 15min in order to fully polymerize the CHDV monomers.
Figure 3(a) is a plan-view optical micrograph under white light illumination, which shows that microstructures of 10wt% T3 in CHDV matrix have been successfully inkjet printed onto the treated LED micropixels. For illustration purposes, we chose here to print alternate pixels to form a simple 3×3 hybrid array. It can be seen that the diameter of the organic structures is ~40µm and that each has not flowed, before photocuring, beyond the boundary of the respective underlying GaN pixel. The close-up image of the microstructure taken by scanning electron microscopy (SEM) (Fig. 3(b)) shows the printed polymer microstructure has a smooth surface and well defined edges, demonstrating the effect of the surface treatment and the accurate printing alignment. The shape of the microstructure is dome-like and the thickness is approximately 2.9µm.
Figure 4(a) is an optical micrograph comparing a turned on bare GaN pixel (upper) with a turned on T3/CHDV integrated pixel (lower). The bare pixel emits UV light at ~368nm wavelength (see the unconverted light in Fig. 5 later). The integrated pixel emitted blue light from the oligofluorene truxene molecules photo-pumped by the underlying UV LED, showing colour conversion to the visible in an integrated device format. Figure 4(b) shows a demonstration combination of multiple illuminated hybrid pixels.
Ideally, the electrical properties of the inorganic light emitter should not be influenced by the integration of the organic fluorescent microstructures. We therefore first characterized the current-voltage (I-V) performance of the LED pixels before and after the integration of T3/CHDV blend microstructures. Our measurements show that the turn-on voltage (3.6V) and I-V characteristics of the representative micro-LED pixels did not change after T3/CHDV integration. As the aim of the integration is to down convert the UV light emitted from the inorganic LED to the visible wavelength via optically pumping the organic light emitting molecules, we have carried out spectral and optical power measurements to investigate the colour conversion of the integrated device. Figure 5 is the normalized photoluminescence (PL) spectrum of a representative T3/CHDV blend microstructure pumped by the AlInGaN LED pixel underneath operated at an injection current of 7mA. This spectrum was measured by using a home-built micro-PL system, which allowed the emission from a single target pixel to be imaged and analysed. It is observed that the emission spectrum is composed of unconverted and/or leakage/scattered LED pump light peaked at 368nm together with the characteristic emission of the T3 organic material showing vibronic peaks at 408nm, 428nm and 458nm respectively. As the CHDV matrix is UV-transparent after photocuring , the unconverted UV light transmits through the matrix, forming the predominant peak in the spectrum. This observation clearly demonstrates the optical pumping of the organic light emitting molecules by the UV micro-LED pixel to achieve UV to blue colour conversion.
To further study the emitting performance of the hybrid device, it is necessary to investigate the colour conversion of the T3/CHDV microstructures under various driving currents. Figure 6 shows the experimental spectra of the T3/CHDV blend microstructure pumped by the underlying GaN micropixel under increasing currents. Over this range of currents (1-7mA) the spectra show no obvious peak wavelength shift or broadening at either UV or blue wavelengths. Moreover, the integrated intensity of the 408nm (integrated from 390nm to 418nm) blue peak (ST3) is plotted versus the driving current in the inset of Fig. 6, showing no strong saturation effects over the range though there is some modest saturation observed higher than 2.5mA.
To determine the colour conversion efficiency of this hybrid device, the optical output powers of the bare UV LED pixel and T3/CHDV integrated LED pixel were measured. During the measurements, a calibrated UV-sensitive Si photo-detector (of active area 78.5 mm2) was placed ~2mm from the LED emitter. The light output powers were measured under different injection currents and are plotted in Fig. 7. It is shown that the bare UV pixel emission area of 16µm in diameter gives a maximum measured output power of 48nW when driven by 7mA current (~23.8mW/cm2 optical power density at a driving current density of 870.8A/cm2, current injected through the whole pixel area, 32µm in diameter). It is noted that the output power is somewhat low compared to our previous report  as this work was conservatively carried out on a non-optimized device to establish the principle of this pixellated colour conversion scheme. After printing a T3/CHDV blend microstructure on the same pixel, the pixel output power was measured again. As only the emitting power from T3 was desired to be characterised, a coloured glass long-pass filter with a cut-off wavelength at 390nm and 70% transmission at 415nm was used to remove the unconverted UV light, and the measured powers adjusted accordingly. The measured output powers of the integrated pixel are also plotted in Fig. 7. It is shown that the output power of the integrated LED pixel is 17nW at a driving current of 7mA (~8.4mW/cm2 optical power density at a driving current density of 870.8A/cm2). Thus, the colour conversion efficiency of the T3/CHDV blend microstructure under 7mA driving current is calculated to be 35.4% (50.5% at 7mA if the compensation of the loss from the filter is taken into account). This promising, but non-optimised demonstration shows the promise of these organic nanostructure colour converters. The conversion efficiency may be increased by further increasing the concentration of T3 in the CHDV matrix.
We have demonstrated that organic nanostructures, here represented by the ‘T3’ type of truxene oligofluorenes, can be successfully used as efficient colour converters for gallium nitride optoelectronics. When incorporated into a novel form of photocurable polymer matrix, these materials are suitable for solventless ink-jet printing to form controlled microstructured nanocomposites on device surfaces. By utilising a micro-pixellated and matrix-addressable format of underlying ultraviolet gallium nitride light-emitting diode, we provide an ideal photopumping device underlayer for the printed organics, allowing a simple hybrid micro-display technology to be demonstrated. This type of approach is anticipated to be of both fundamental interest, in delivering ‘indirect’ electrical excitation to organic nanostructures, and technological interest as a form of micro-display competitive with other approaches . As a further interest, the recent demonstration of photo-pumped laser action in the truxene oligofluorenes  offers the prospect, using the advances demonstrated here, of integrated photo-pumped organic lasers.
This research work is supported by the EPSRC project ‘HYPIX’ and an EPSRC Science & Innovation Award on “Molecular Nanometrology”. The authors thank the supports from M. McGrady for the contact angle measurements, Dr O.J. Rolinski for absorption spectrum measurement and Y.F. Zhang, Dr P.R. Edwards and Prof R.W. Martin for the scanning electron microscopy.
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