Stable White Light Electroluminescence from Highly Flexible Polymer/ZnO Nanorods Hybrid Heterojunction Grown at 50°C
© The Author(s) 2010
Received: 7 April 2010
Accepted: 24 May 2010
Published: 4 June 2010
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© The Author(s) 2010
Received: 7 April 2010
Accepted: 24 May 2010
Published: 4 June 2010
Stable intrinsic white light–emitting diodes were fabricated from c-axially oriented ZnO nanorods (NRs) grown at 50°C via the chemical bath deposition on top of a multi-layered poly(9,9-dioctylfluorene-co–N-(4-butylpheneylamine)diphenylamine)/poly(9,9dioctyl-fluorene) deposited on PEDOT:PSS on highly flexible plastic substrate. The low growth temperature enables the use of a variety of flexible plastic substrates. The fabricated flexible white light–emitting diode (FWLED) demonstrated good electrical properties and a single broad white emission peak extending from 420 nm and up to 800 nm combining the blue light emission of the polyflourene (PFO) polymer layer with the deep level emission (DLEs) of ZnO NRs. The influence of the temperature variations on the FWLED white emissions characteristics was studied and the devices exhibited high operation stability. Our results are promising for the development of white lighting sources using existing lighting glass bulbs, tubes, and armature technologies.
Zinc oxide (ZnO) is a II–VI semiconductor material with a wide bandgap of about (3.37 eV) together with a high exciton binding energy of (60 meV) both at room temperature rendering ZnO to receive global attention especially in connection with the emerging nanotechnology paces toward functionality . Moreover, ZnO possesses deep levels that emit light covering the whole visible spectrum . ZnO nanostructures family has gained substantial interest due to their simple fabrication routes, along with low cost and self organization growth behavior enabling the growth of ZnO nanostructures on any substrate material regardless of lattice matching issues . Many groups have fabricated and studied ZnO nanorod-based devices including LEDs e.g. by our group recently [2–7], random laser based on ZnO nanorods has also been demonstrated and showed a potential of ZnO for photonics applications . The most challenging problem of ZnO-based photonic devices is the lack of stable and reliable p-type doping; mainly due to the self compensation property of ZnO . Therefore, using heterojunction strategy for photonic devices based on ZnO nanorods (NRs) has been a feasible way to obtain good performance LED based on ZnO NRs [2–7].
Organic polymers light-emitting diodes (PLED) have been intensively investigated for optoelectronic applications such as flat panel displays and solar cell, to name a few. The main advantageous features of polymer-based devices are low cost, low power consumption, and simple processability etc. Due to the self organized growth property of ZnO, it is possible to grow ZnO nanorods on polymeric substrates. The combination of ZnO NRs and polymers to form hybrid junction will add the advantage of achieving large area LEDs using a single contact, which is an advantage not possible to gain by using PLED configuration. The first hybrid organic/ZnO NRs LED has been reported by Könenkamp et al. , their LED composed of electrodeposited ZnO NRs on F-doped SnO2 glass substrate acting as a cathode and applying PEDOT:PSS as a p-type contact . Although the device demonstrated rational LED characteristics, it has a drawback regarding stability . White light–emitting diodes based on organic/ZnO NRs have also been studied by our group [4, 6, 7] different multilayer and blended polymers were utilized to fabricate LEDs on glass substrates. In those studies, ZnO NRs were grown at a temperature of 95°C, and the emitted light was dominated by the polymer emissions (mainly the blue line) leading to bluish white emission LEDs [4, 6, 7]. Fabrication of white organic/inorganic LEDs on flexible substrates on the other hand represents an additional step toward the realization of ZnO/polymer hybrid heterojunctions LEDs. Nevertheless, there are many issues to be solved to achieve this goal. For instance, the growth temperature of ZnO NRs has to be lowered to a large extent to permit the fabrication of LEDs on flexible substrates. It is important to mention that all published results on hybrid organic/ZnO NRs heterojunctions white electroluminescence (EL) were obtained for cases where the growth was performed at 80°C or higher [4, 6, 7, 9–11]. Moreover, it is of interest to demonstrate a white EL from such hybrid heterojunctions at lower temperatures, to enable the use of a variety of flexible plastic as a substrate. Such achievement will lead to the possibility of integrating this class of white LEDs with existing glass bulbs, tubes, and armature technologies.
In this paper, a novel stable white light–emitting diode fabricated on highly flexible plastic substrate is reported. This flexible white light–emitting diode (FWLED) was fabricated on commercial PEDOT:PSS flexible plastic and composed of a vertically aligned ZnO NRs grown by chemical bath deposition route at a low temperature of 50°C, on multi-layered blue emitting polymer poly(9,9-dioctyl-fluorene) (PFO) and hole transporting polymer poly(9,9-dioctylfluorene-co–N-(4-butylpheneylamine)diphenylamine) (TFB) in between the PFO and the PEDOT:PSS. The highly flexible and stretchable light-emitting diode yielded a stable white broad emission band covering the entire visible spectrum.
All materials used in the growth of ZnO nanorods were purchased from (Sigma–Aldrich) and were applied as-received without further purification. The PFO and TFB polymers were purchased from American Dye Source, Canada. All polymer solutions were prepared by dissolving 4 mg/ml in toluene. A commercial PEDOT:PSS on plastic foil was chosen as a substrate in the subsequent fabrication of the hybrid LED due to the facts that the PEDOT:PSS on plastic is flexible, transparent to the visible light with reasonable electrical properties and can be used for large-scale production.
All measurements on the fabricated hybrid LED device were performed at room ambient conditions, only in the case of temperature stability the measurements were carried out at different temperatures. Field emission scanning electron microscopy (SEM) was used to study the morphology of the grown ZnO NRs. Room temperature photoluminescence (PL) was investigated using (coherent MBD 266) with λ = 266 nm as excitation source, and the PL spectra were collected with a CCD detector. The current–voltage (I–V) characteristics were measured with Agilent 4155B semiconductor parameter analyzer. The EL behavior of the fabricated FWLED device was examined with Keithley 2400 source meter, while the EL spectra were assembled via Andor Shamrock 303iB spectrometer supported with Andor-Newton DU-790N CCD. Temperature-dependent stability EL measurements of the fabricated hybrid FWLED was determined with Thorlab TED 200C temperature controller.
In summery, an intrinsic white-emitting diode was fabricated by growing well-aligned ZnO NRs at a temperature as low as 50°C following chemical bath deposition strategy. The ZnO NRs were grown on multi-layered polymers spun coated on commercially available flexible PEDOT:PSS/plastic substrate. The FWLED showed excellent I–V characteristics combined with single intrinsic white light extending from 420 to 800 nm. The light emission was clearly observed by the naked eye at a bias voltage of 14 V. The blue light emission from the PFO polymer layer was completely intermixed with the deep level emissions from ZnO NRs to generate single broad white light emission band. The influence of the ambient temperature on the fabricated FWLED was investigated, and the results demonstrated that the device is quite stable at elevated temperatures without showing any severe depreciation of the output light characteristics. The fabricated device was bent at large angles (>60o) and still retained its electro-optical characteristics. This FWLED can fit well to a wide varieties of lighting applications, for instance as the substrate is highly flexible, the use of the FWLED in decoration and in-door lighting using existing armature technologies become feasible to achieve.
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