Study On The Luminescent Properties Of Fluorene Derivatives

Recently, white OLEDs are of growing interest due to their potential utilization in low-cost, large-area backlights for flat panel displays, and in solid lighting sources. An'pure'white OLED is characterized by an emission that coves the full spectral range of visible region (400 nm 700 nm) and a Commission International d'Eclairage (CIE) chromaticity coordinates of (0.33, 0.33).We introduce two kinds of organic molecules that of incorporating red and green, or just orange chromophores into the main chain, or side chain of blue emissive polymers and white light is achieved by simultaneous emission of blue, green and red, or blue and orange light from these polymers. This two type polymer is fluorene for the nuclear periphery that joined with the groups of carbazole in periphery. Through the photoluminescence and the electroluminescent of the device that made of this series compounds, we explain the essence reason for white light-emitting.The first category is TF-CF1 and TF-CF2, which is carbazole derivatives system, made of the fluorene compounds for nuclear and 9-replace tetrafluoroethane benzene. First of all we have done the photoluminescence spectra of TF-CF1 and TF-CF2, we can see the photoluminescence spectra that compounds in film and in solution (chloroform as solvent) have great distinction. TF-CF1 and TF-CF2 in films relative to that in dilute solution, the emission peak redshift and the width of peak deformation. In order to study the change of TF-CF1 and TF-CF2 in photoluminescence spectra, we made different concentrations of TF-CF1 and TF-CF2 that chloroform as a solvent to see the photoluminescence phenomenon. As to TF-CF1 or TF-CF2, when the concentration of solution is to 6.25*10-6mg/ml, it has the two main emission peaks. Company with increasing the concentration of solution, short-wave emission peak became weaken gradually. When the concentration from 0.1mg/ml to continue to 2mg/ml, in the long -wave direction ,another emission peak strengthened gradually, that peak is similar to the emission peak of pure TF-CF1 (or TF-CF2) in the film. Analysis to the molecular structure of TF-CF1 (or TF-CF2), the Centre fluorene unit is a coplanar biphenyl rigid structure and both sides have four or six groups of carbazole that link by Triple-Bond, the plane of molecular is stronger, excimer(or exciplexes) is more easily to form. Therefore, from the launch of the long-wave peak position and the character of its concentration-dependent, we can judged that the changing electroluminescent spectrogram on the cause of excimer that conform between the molecular at the different concentration, That is the changing peak and peak shape in electroluminescent spectrogram of TF-CF1 (or TF-CF2) lead by excimer. The different of TF-CF1 and TF-CF2 is the number of gathered carbazole modules, TF-CF1 were four carbazole modules and TF-CF2 were six carbazole modules, the number of Carbazole modules not affect its capacity to form shock-association complex. We believe that the main reasons conform excimer is because of the plane and the rigid of these polymers. In order to study the photo physical characters of the system that doping with TF-CF1 (or TF-CF2) and PVK, we investigated the fluorescence spectra made of PVK in film and mixed film of PVK: TF-CF1 (TF-CF2) (1:1) and TF-CF1 (or TF - CF2) in film. From the spectrum, we can see that the photoluminescence of pure TF-CF1 (or TF-CF2) in films spectra have no great difference with the mix. So we conclude that PVK and TF-CF1 (or TF-CF2) have no interaction, PVK only play a small role in molecular membrane matrix and the "dilution" effect. However, under the different concentration, the photoluminescence spectra of mixture of PVK and TF-CF1 (or TF-CF2) changed, it is note that the effective energy transfer appear between PVK and TF-CF1 (or TF-CF2). Based on the above study about the knowledge of spectrum, we produced a single layer device and Multi-layer device that TF-CF1 and TF-CF2 for emitting layer in these devices. The structure of Multi-layer device is ITO/PEDOT/PVK: TF-CF1 (or TF-CF2) / BCP/ AlQ3/Al, AlQ3 for the layer of electronic transmission, BCP for the layer of blocking the hole exaction in regulating the distribution of layers. In the lower voltage, the device showed that the Blue-ray emission, EL and PL spectrum of the mixture of PVK and TF-CF1 (or TF-CF2) in film is similar; it is say that the Blue-ray launch come from excimer generated. As increasing the voltage, a new peak of orange-emitting appeared. This may be form or electroplexes) between the complex of TF-CF1 (or TF-CF2) and PVK (or BCP) or formed intermolecular electromer by TF-CF1 (or TF-CF2) molecular theirselves. In order to study the reason the formation of the new emission peak, we produced a single-layer device, structure as follow: ITO/PEDOT/TF- CF1(or TF-CF2)/LiF/Al. With the voltage increasing, a new orange-emitting peak appeared, the new peak shows that the peak is because electromer generated among moleculars. Because of the blue-emitting light and the orange-emitting light combined to the white-emitting device, it could cover the entire device showed that the white. TF-CF1 (TF-CF2) device fired from the white two aspects: the excimer format blue-emitting light and the electromer format the orange-emitting light. The devices of white-emitting come from the combination of Blue-emitting and orange-emitting. Devices based on TF-CF1, and TF-CF2 showed'pure'white light with CIE coordinates of (0.33, 0.34) and (0.33, 0.33), with a brightnes of 150 cd/m², and 104 cd/m², an efficiency of 0.04 cd/A, and 0.03 cd/A at 16 V, respectively.The second category compounds are F-CF1 and F-CF2 that the system of fluorene compounds for nuclear and 3-replaced fluoro benzene carbazole derivatives. To study these two compounds photoluminescence spectra, we also found similar luminescence phenomenon to the TF-CF1 and TF-CF2. Analysis the photoluminescence spectrum of F-CF1 and F-CF2 in films and different concentrations of chloroform solution, it is showed that the emiting peaks appear quit different change with the chaning concentration of solution. The photoluminescence emit of F-CF1 and F-CF2 in films lead by intermolecular excimer. Compare the photoluminescence spectrum of the mixture of PVK and F-CF1 (or F-CF2) in films to that of F-CF1 (or F-CF2) in films, we can see that between PVK and F-CF1 (or F-CF2) have not been formed exciplexes, PVK only played the formation of small molecules and the role of matrix "diluted" role. The device is maded by PVK doped F-CF1 (or F-CF2) as a luminous layer, AlQ₃ as the layer of electronic transmission At the low voltage, the devices of F-CF1 (or F-CF2) appear the blue (or green) light. Correspond to the electroluminescence spectrum and the fluorescence spectrum, it is showed that the blue (or green) light come from the intermocular excimer. When the voltage is increasing, the spectrum is changing that the width of peak turn wide, the peak red-shift, at the same time, a new orange (or red) emission peak appeared in the long-wave direction. Whether the blue light combine with the orange light or the green light combine with the red light, it is displayed as white-emitting device. The reason of device emitted white light from two aspects. On the one hand, that is excimer formate blue-emitting (or green-emitting). On the other hand is electromer formate of orange emitting (or red-emitting).Whether the blue light combine with the orange or the green light combine with the red light ,the device appear of the combination of white light. Devices based on F-CF1 or F-CF2 showed'pure'white light with CIE coordinates of (0.33, 0.33), and (0.35, 0.37), with a brightness 450 cd/m², and 367 cd/m², an efficiency of 0.04 cd/A, and 0.003 cd/A at 17 V, respectively.Compare the photoluminescence spectrum of single-layer and multi-layer devices based on TF-CF1 or TF-CF2 to that of F-CF1 or F-CF2 device, the performance of device in TF-CF1 or TF-CF2 is better than that of F-CF1 or F-CF2. As to TF-CF1 or TF-CF2, it has joined with 2,3,5,6-tetrafluoroethane phenyl group in external. For TF-CF1 or TF-CF2, it has joined with 4-fluoride benzene group in external. Electronic oxidation of fluorene is strong in TF-CF1, TF-CF2, that is more easily combine with other molecules. So, it can easily formed excimer. Moreover, TF-CF1, TF-CF2 have higher efficiency.