Study of various evaporation rates of the mixture of Alq3: DCM in a single furnace crucible
Introduction
Organic light emitting devices (OLEDs) are promising candidates for displays and light sources due to their unique features such as high brightness, wide color range, high contrast, wide viewing angle, rapid response, and low fabrication cost. In addition, it is worth mentioning that they can be used to make thin and flexible displays [1], [2], [3]. Each kind of luminescent materials possesses its own characteristics which have to be compromised to show better optical and electrical characteristics [4], [5]. Therefore, a lot of efforts have focused on the synthesis and using variety of improved synthetic electroluminescent (EL) materials for the construction of reliable OLEDs which can be highly suitable in different aspects like quantum efficiency, charge carrier mobility, thermal stability and processability. Among these materials Tris(8-hydroxyquinolinato)aluminum (Alq3) which, due to its specific properties, is used as a combined electron transport and emitter layer is a common component for organic light-emitting diodes [6], [7], [8]. Furthermore, Alq3 has also been shown to work as a host material in organic devices in which Alq3 provides a polar environment for a highly polar excited state of the dye molecule. Thus, it can cause the reduction of concentration quenching [9]. Among the blue, green and red-light-emitting materials required for full-color displays, red-light-emitting materials remain one of the greatest challenges. In a red OLED, Alq3 is used as host material and 4-dicyanomethylene-2-t-butyl-6-1,1,7,7-tetra- methyljulolidyl-9-enyl-4H-pyran(DCJTB) is used as dopant. However, the emission from such a host–guest system is often contaminated by the residual green emission fromAlq3 [10]. To improve the device performance, various methods have been developed, such as using 5,6,11,12-tetra- phenylnathacene(rubrene) as an assist dopant or using a cohost emitter system [11], [12], [13], [14]. Electroluminescence with colors tunable from yellow–green to red can be obtained with DCM doped in Alq3 layers as the host material because of its high stability and good carrier-transport properties. [15], [16]. Based on resonance energy transfer theory [17], an efficient Forster energy transfer requires a large overlap between host emission and guest absorption. For this reason, emission from Alq3:DCM can be achieved through direct excitation of the DCM fluorophores or through Forster energy transfer from the excited Alq3 host matrix [16]. The common mechanism for doping DCM in Alq3 layer is based on the usage of two separate furnace crucibles for Alq3 and DCM.
Eventhough the effects of the evaporation rate of Alq3 on electroluminescence performance of OLEDs have been investigated in many researches [18], [19], [20], but there has been no report on the study of evaporation rate of dye doped Alq3 when a single crucible is applied. In our previous work, we reported the deposition of the mixture of Alq3 with porphyrin compounds with additional functional groups [21]. Indeed, the most serious problem in using two separate crucibles is to monitor and control two evaporation rates simultaneously. In particular, the emitter with the lowest HOMO-LUMO energy has to be evaporated with a precise rate because of the fact that small fluctuations in its low concentration can significantly change the emitting color of its OLED device. In the present work, instead of evaporating Alq3 and DCM separately by two evaporative systems, the evaporation is occurring simultaneously in a single sublimation crucible. The advantage of this mixture of dyes with using single furnace crucible is the rate control. In the case of two crucibles, control of evaporation rate and the subsequent temperature control of the evaporation sources are very difficult, expensive and lowering the overall yield. In case of the single crucible, because the rate of the dye source is significantly higher, the difference in concentration of dyes does not need to be monitored but is determined by the weight ratio within the crucible. This allows for much easier evaporation rate control which potentially decreases the process complexity for large scale industrial production. Although using two separate crucibles to evaporate DCM and Alq3 has the advantages of better control on evaporation concentrations, however, low cost of fabrication and more homogenous layer are advantages of this method. Here, by changing the evaporation rate of Alq3:DCM, the emission wavelength and energy transfer between Alq3 and DCM have been investigated.
Section snippets
Materials
poly(3,4-ethylenedi-oxythiophene):poly(styrenesulfonate)(PEDOT:PSS), polyvinylcarbazole (PVK), tris(8-hydroxyquinolinato)aluminum(Alq3) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) were obtained from Sigma-Aldrich and used without any further purification. Fig. 1 gives the structure of the materials used in this study.
Physical measurements
Thermal properties were measured by using differential scanning calorimetry (DSC)—Perkin Elmer Pyris Diamond—in alumina caps under a nitrogen flow at
Results and discussion
The results of TGA reveals that Alq3 is quite stable up to 430 °C but DCM exhibit onsets for degradation in the range of 305–310 °C. The DSC data of the glass transition temperature (Tg) for the Alq3 was estimated at 204 °C and melting point (Tm) of 414 °C (Fig. 3), while for DCM a significant sharp Tm of 223 °C was detected (Fig. 4), indicating that it represents a more crystalline phase than Alq3 in the blend. Since Alq3 did not decompose until 430 °C, the onset temperatures of the weight loss at
Conclusion
The optical and transport properties of charge carriers in doped amorphous Alq3 films fabricated using a single furnace crucible were investigated. Furthermore, the current–voltage, EL and PL were measured. Device 3 exhibited orange emission with high color purity. And also it could be observed that by increasing the evaporation rate, the driving voltage and the turn-on voltage will shift to higher voltages. In addition, the maximum luminescence was observed for the Alq3:DCM with an evaporation
Acknowledgment
The authors would like to thank the Vice-President's Office for Research Affairs of Shahid Beheshti University and the Iran National Science Foundation: INSF for supporting this work.
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