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A host-free blue phosphorescent dendrimer in organic light-emitting fieldeffect transistors and equivalent light-emitting diodes: A comparative study Mujeeb Ullah, Kristen Tandy, Andrew J. Clulow, Paul L. Burn, Ian R. Gentle, Paul Meredith, Shih-Chun Lo, and Ebinazar B. Namdas ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b01019 • Publication Date (Web): 13 Mar 2017 Downloaded from http://pubs.acs.org on March 14, 2017
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Article type: Communication
A host-free blue phosphorescent dendrimer in organic light-emitting field-effect transistors and equivalent light-emitting diodes: A comparative study
Mujeeb Ullah,1,† Kristen Tandy,1,† Andrew J. Clulow,2 Paul L. Burn,2 Ian R. Gentle,2 Paul Meredith,1,3 Shih-Chun Lo 2,* and Ebinazar B. Namdas1,* Centre for Organic Photonics & Electronics 1
School of Mathematics and Physics, The University of Queensland, Brisbane, Queensland
4072, Australia 2
School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane,
Queensland 4072, Australia 3
Department of Physics, Swansea University, Singleton Park, Swansea SA2 8PP, Wales,
United Kingdom †
First two authors contributed equally.
*Correspondence to:
[email protected];
[email protected] Keywords: OLED displays, transistors, light-emitting field-effect transistors, charge transport, phosphorescent, electroluminescence quenching
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Light-emitting field-effect transistors (LEFETs) are a promising technology for applications in active matrix displays, communications and electrically pumped lasers [1-2]. These devices operate as a transistor and organic light emitting diode (OLED), combining the switching and emission functionalities in a single device. Most reports on LEFETs describe the use of fluorescent emitters [3-8] and there have only been a few publications in which phosphorescent [9-13] materials have been utilised as the emitting layer. Phosphorescent materials have been successfully used in OLEDs with high external quantum efficiencies (EQEs) due to the fact that the strong spin-orbit coupling of heavy metals enables both the singlet and triplet excitons formed in the device to be harvested [1,14]. However, phosphorescent materials are sensitive to concentration quenching and hence the most efficient phosphorescent OLEDs have been based on evaporated guest-host blends. Likewise, the phosphorescent LEFETs reported thus far have also followed the guest-host blend approach [9,10,12,13]. The disadvantage of the guest-host approach is that it increases the complexity of device fabrication, as it requires optimisation of the guest-host ratio and precise co-deposition of the elements of the blend. Furthermore, in the context of LEFETs, guest-host systems are expected to exhibit less efficient charge transport in the channel due to charge trapping on the guest. In order to avoid using a co-evaporated emissive layers, solution processable dendrimers have been successfully developed for OLED applications. The dendrimer architecture has been shown to be a simple method for overcoming the interchromophore interactions that lead to the luminescence quenching in solid state [15-19]. In this report, we demonstrate solution processed host-free blue phosphorescent dendrimer-based LEFETs and OLEDs, and compare their performance. The LEFETs exhibit excellent electrical and optical characteristics with luminance of 650 cd m-2, which is comparable to the equivalent OLEDs based on the same emissive layer. The LEFETs were fabricated in a heterostructure bilayer configuration using a charge transport layer underneath 2 ACS Paragon Plus Environment
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the emissive layer. The charge transport layer we used was poly(2,5-bis(3-nhexadecylthiophene-2-yl)thieno[3,2-b]thiophene) (PBTTT), which acts as a p-mode material in a unipolar device. Furthermore, equivalent OLEDs (control structures) were fabricated using PBTTT and MoOx as the hole injecting and transporting layers. Notably, negligible EQE roll-off in the LEFETs at high current density and brightness was observed. These results indicate that for phosphorescent materials there could be advantages of LEFETs over OLEDs at high current density or brightness. In addition, we discuss the operating mechanism and effects of solvent used to deposit the dendrimer layer on the interface formed within the active layer heterostructure. X-ray reflectometry showed that the interface between the PBTTT and dendrimer layer changed in the lamellar ordering of the PBTTT layer upon solution deposition of the dendrimer layer. This resulted in a lower charge carrier mobility in the devices formed from such heterostructures. Figure 1 shows the device structures of the LEFETs and equivalent OLEDs with PBTTT and MoOx as the hole-injecting layer, identified as “PBTTT-OLED” and “MoOxOLED”, respectively. The LEFETs were fabricated using a solution-processed PBTTT as a hole transport layer and a host-free dendrimer as the emissive layer. All devices were fabricated on Si++ substrates with a 400 nm thick layer of SiNx on top (Figures 1a, 1b and 1c). In the case of the LEFETs, a 150 nm thick layer of PMMA was deposited on top of the SiNx layer (Figure 1a), followed by a layer of the PBTTT hole transporting material (20 nm). A thin hole injecting Au electrode (15 nm) was thermally evaporated on top of the PBTTT layer for the LEFETs and directly onto the SiNx layers for the both types of OLEDs (Figures 1a, 1b and 1c). In the PBTTT-OLEDs, a PBTTT layer was then spin-coated on top of the Au layer and in the case of the MoOx-OLEDs a thin layer of MoOx (8 nm) was thermally evaporated on top of the Au layer. The light-emitting dendrimer film (75 nm) was then spincoated on top of the PBTTT layer in the case of the LEFETs and PBTTT-OLEDs, and on top
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of the MoOx layer for the MoOx-OLEDs (Figures 1a, 1b and 1c, respectively). Finally, all the devices were completed by subsequent thermal deposition of TPBI/Cs2CO3/Ag stacks under high vacuum to form the electron-injecting electrode. The respective thicknesses of the TPBI, Cs2CO3 and Ag layers were 40, 6 and 15 nm, respectively. For direct comparison of the LEFETs with the equivalent OLEDs the devices were prepared with the same thickness of charge transport and electrode layers under identical conditions. The chemical structures of the materials used in this study, TPBI, dendrimer and PBTTT, are given in Figures 1d, 1e and 1f, respectively. Full details of the fabrication and testing protocols including error analysis are presented in the Experimental Methods section. The electrical characteristics of the LEFETs are shown in Figure 2a. The LEFETs were operated in p-mode and the hole-mobility (µ) from the transfer characteristics was determined to be 1.4 × 10-2 cm2 V-1 s-1 (Table 1). This mobility is lower than that of OFETs previously reported using only PBTTT in the transistor channel [6,20] in p-mode, where field-effect mobility reached ≈1 cm2 V-1 s-1 for optimised devices [20]. When forming a heterostructure device, the sequential deposition of layers by solution processing can in principle lead to partial dissolution of the layer deposited first, and a mixed phase in between the bulk layers of the two materials. This could potentially lead to a lower mobility in the LEFETs relative to OFETs prepared without the solution processed dendrimer layer using same electrodes (see Figure S1 in supplementary information). To investigate whether a sharp or blended interface occurred between the PBTTT and dendrimer layers, we carried out an X-ray reflectometry study on the bilayer films (Figure S2 in supplementary information). PBTTT has been reported to form a lamellar structure [21] and we observed a similar structure in our neat films of PBTTT by X-ray reflectometry measurements (full description in supplementary information). After spin-coating the dendrimer layer on top of the PBTTT, disruption of the PBTTT lamellar structure was observed (Figure S2), which provides a 4 ACS Paragon Plus Environment
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plausible explanation as to why the hole mobility measured in these devices is lower than if the charges were transported through a pure and undisrupted PBTTT layer. The ON/OFF ratios of the LEFETs was on the order of 105. Figures 2a and S3 show optical transfer and output characteristics of the LEFETs. The results indicate that there is excellent optical modulation with the gate voltage giving a brightness directly proportional to the source–drain current. The maximum luminance of the LEFETs reached 650 cd m-2 at the gate voltage of -100 V. The light emission zone (25 µm) was found adjacent to and under the electron injecting contact (Figure 3a). The location of the emission zone was independent of the gate bias. The photoluminescence (PL) and electroluminescence (EL) spectra of the LEFET devices are given in Figure 3b. It is noteworthy that both the EL and PL of the LEFETs are slightly narrower and red shifted compared to the PL spectrum of the neat film of the blue dendrimer deposited on a quartz substrate [16], which is consistent with a weak micro-cavity effect. In the LEFETs, a weak micro-cavity could be formed by the Ag mirror and refractive index mismatch between the various organic, dielectric (e.g., PMMA and SiNx) and silicon wafer layers. The electrical and optical characteristics of the PBTTT-OLEDs and MoOx-OLEDs are shown in Figures 2b and 2c. The MoOx-OLEDs showed similar current density to the PBTTT-OLEDs. In both devices, the current density reaches a maximum of ≈6 mA cm-2. Further increases in the current density leads to break-down of the devices. The light turn-on voltages of MoOx-OLEDs and PBTTT-OLEDs were found to be 2.7 V and 4.3 V, respectively. The lower turn-on voltage in the MoOx-OLED strongly indicates better energy level matching between the ionisation potentials of the MoOx and light-emitting layers. The photoluminescence (PL) and electroluminescence (EL) spectra of the OLEDs are shown in Figure S5 and the transmission spectra of the electron injecting TPBI/Cs2CO3/Ag electrode is shown in Figure 3b. The EL and PL spectra of the OLEDs were similar, and after taking into 5 ACS Paragon Plus Environment
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account the weak micro-cavity effect in the OLEDs, as discussed above for the LEFETs, we confidently assign the origin of light emitting emission to the triplet excited state of the phosphorescent blue dendrimer. Figure 3a shows the External Quantum Efficiency (EQE) of the three devices as a function of brightness. The EQEs of LEFETs, PBTTT-OLED, and MoOx-OLED were 2.1 ± 0.3% at 650 cd m-2, 1.0 ± 0.2 % at 170 cd m-2, and 1.8 ± 0.3% at 240 cd m-2, respectively. The EQE of the LEFETs were similar to that of the MoOx-OLEDs at a lower brightness (
