Introduction

OLED’s paying more attention scientifically and commercially for the reason of their potential use in full color, high resolution and flat-panel displays [1,2,3,4,5]. To meet the same demands blue, green [6, 7] and red [8, 9] light emitting materials having high electroluminescence efficiencies, excellent thermal stability and good charge carrier abilities attracted more attention. The performance of red and green OLED’s is relatively good as compared to blue OLED’s. Because of their intrinsically wide band gap, the synthesis of highly efficient green light emitters showing good color purity remains a great challenge for the improvement of new OLED’s.

So, Polyaromatic Hydrocarbons (PAHs) have been studied extensively and developed green light emitting materials for OLEDs only because of their excellency in electroluminescence (EL) and photoluminescence (PL) properties.

These excellency in EL and PL is due to electronic excitation energy transfer in PAH crystals. Therefore, these materials are found be suitable for Donor-Acceptor (D-A) pairs in FRET and attracted more attention of new generation. The EET process is verified by fluorescence of donor measured with intentionally doped acceptor concentration. Various factors affect the energy transfer efficiency such as polymer, solvent, concentrations of acceptor and its morphology, etc. [10,11,12]. Out of them, thin film morphology played very important role for efficiency of energy transfer in holding D-A pair at close distance required for EET process [13].

Hence, in this study authors have developed efficient green light emitting materials by very simple doping technique and their doped thin films by using spin coating technique [14] at different concentrations of prepared AN-TN doped p-terphenyl crystalline materials. Anthracene and tetracene are very well known PAH materials used as dopant acting as excellent acceptor in electronic energy transfer processes [15, 24]. Whereas, p-terphenyl which acts as excellent organic semiconductor is used as a host material and also has capacity to solubilize guest impurity. Further, it is found that, p-terphenyl is more stable thermally [16,17,18].

Literature survey revealed that, no work is reported on tetracene and anthracene doped p-terphenyl emitting green light in crystal state and their energy transfer studies in thin films. Systematic quenching of p-terphenyl with instantaneous sensitization of AN-TN and their thin films were observed by fluorescence spectroscopy and electrochemical study by cyclic voltametry technique. Simultaneously structural and morphological study of crystalline materials have been done by using XRD and SEM. Studies on thermal stability have been done by TGA-DSC technique.

Experimental

Pure p-terphenyl, anthracene and tetracene of scintillation grade were brought from Merck-Schuchardt. Then the recrystallization and sublimation method were applied for it’s purification and further confirmed by fluorescence spectra.

Preparation of p-Terphenyl Luminophors and Doped Thin Films

The solid state reaction technique had been employed to prepare polycrystalline p-TP luminophors with different concentrations of AN and TN [19]. This was processed in silica crucible. The solid solution was heated at the temperature of the M.P. of p-TP (214 °C). Further, the melt obtained was slowly cooled to get finely grained powdered polycrystalline luminophors of p-TP. Finally this powder was subjected for characterization.

The polystyrene films doped by tricomponent mixed crystals were prepared by 1% polystyrene solution and spin coating the films on glass substrate [20, 21].

Characterization

The fluorescence spectra of guest doped p-TP were recorded by JOBIN YVON Fluorolog-3-11 spectrofluorimeter, at Indian Institute of Technology, Madras. The XRD spectra of doped and non-doped crystals were recorded with the Philips diffractometer (model PW-3710, Netherlands) with CrKα radiation (2.28) at Solapur University, Solapur. Also, (TGDTA-DSC) TA Inc. SDT-2790 with heating rate of 10 °C per minute under nitrogen atmosphere was used to perform thermogravimetric analysis at Solapur University, Solapur, while surface morphology of samples was studied by scanning electron microscope at SAIF, Indian Institute of Technology, Madras. Cyclic voltammogram was recorded at D.B.F. Dayanand Science College, Solapur.

Results and Discussion

Fluorescence Studies of p-Terphenyl Luminophors in Crystalline State

Figure 1 shows, fluorescence spectra of tetracene and anthracene doped p-terphenyl materials recorded at λmax = 290 nm [22]. The fluorescence spectra of doped p-terphenyl shows very negligible emission bands of p-terphenyl at 320–370 nm. The absence of p-terphenyl emission suggests an efficient transfer of excitation energy from p-terphenyl excitons to anthracene and tetracene. The fluorescence intensity of emission band of tetracene and anthracene doped p-terphenyl observed at 525 nm causes a green shift in emission wavelength. The presence of TN decreases the intensity of AN like emission. The observation led us to assume that, energy of excited AN is trapped by TN moiety before it is emitted radiatively. The possibility of two step energy transfer is confirmed on the basis of energy states of p-terphenyl, AN and TN. The first excited singlet state (S1) of p-terphenyl is higher than those of AN and TN while the first excited state (S1) of AN is lower than p-terphenyl but higher to that of TN. The TN singlet and p-terpenyl singlet reveal a wide gap by which the direct transfer of excitation energy to TN is not possible. However, it is transferred significantly to AN and then to TN. In the present work, we propose a two-step excitation energy transfer from p-terphenyl to the dopants.

Fig. 1
figure 1

Fluorescence spectra of tetracene and anthracene doped p-terphenyl

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Concentration effect is also observed. Higher concentrated solid solution (1 × 10−1 mol TN in 1 × 10−1 mol AN / p-TP) giving intense peak as compared to lower concentrated solid solution of TN.

Fluorescence Studies of p-Terphenyl Luminophors Doped with Polystyrene Thin Film

p-TP doped AN-TN mixed crystals showing green emission due to two step EET process from p-TP to TN via AN. These green light emitting luminophors for EL diodes require in thin film form. Polystyrene (PS) thin films doped with p-TP/AN-TN mixed crystals were prepared by spin coating technique in 1% polystyrene solution on glass substrate. p-TP fluoresces in the violet region at excitation wavelength 290 nm. Hence, doped films of p-TP luminophors, selectively excited at 290 nm.

The Fluorescence spectra of p-TP luminophors embedded in PS matrix is shown in Fig. 2. The careful examination of the spectra reveals, complete quenching of p-TP emission with two well separated emissive regions one in the range 385–440 nm and other in the range 475–575 nm. The earlier region corresponds to the AN like emission intensity with partial quenching while later is the TN monomer region. The observed TN monomer peak is very intense and broad, peaking at 525 nm as obtained for crystalline solid luminophors. Hence, present study provides materials suitable for use in EL devices.

Fig. 2
figure 2

Fluorescence spectra of p-terphenyl luminophors doped with polystyrene thin films

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X-Ray Diffraction Studies of p-Terphenyl Luminophors

X-ray diffraction spectra of pure AN and TN doped p-TP is shown in Fig. 3. XRD profile of fine grained powder showed sharp peaks which specified crystallinity. The absence of any new peak in XRD spectra of doped luminophors confirms the homogeneity and formation of solid solution. Microstrain, dislocation density, grain size and stacking fault like structural parameters were calculated from XRD study shown in Table 1. Classical Sherrer formula has been used to estimate the average grain size of p-TP luminophors [23]. We found that, grain size increases with increase in concentration of AN and TN as a dopant into p-TP material. On doping of a pure crystal into host, it deforms by producing defects and imperfection into host lattice. This deformation changes microstrain marginally and dislocation density as expected [24]. The stacking fault observed to be improved with guest moiety in p-TP host material. These results supports the close packed structure of both host and guest molecules.

Fig. 3
figure 3

XRD spectra of p-terphenyl doped AN and TN

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Table 1 Structural parameters of p-terphenyl luminophors
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Surface Morphology of Tetracene and Anthracene Doped p-Terphenyl Luminophors

Fig. 4a indicate the SEM micrographs of pure p-TP and Fig. 4b indicate SEM micrographs of doped p-TP luminophors in crystal state. From SEM micrographs crystallite size observed for TN-AN doped p-TP is of 89.7 nm, required in optoelectronics

Fig. 4
figure 4

SEM images of a) p-terphenyl, b) TN-AN/p-TP

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Thermal Studies of p-Terphenyl Luminophors

For optical applications, thermal stability of the material is essential. So, to study the alteration in thermal properties of p-TP doped AN and TN, thermogravimetric analysis has been done. Figures 5 and 6 show TGA and DSC thermographs of p-TP luminophors (1 × 10−1 mol AN-1 × 10−1 mol TN/ p-TP) under the nitrogen atmosphere within temperature range of 0–300 °C. From Fig. 5, it is observed that, the p-TP luminophors remains thermally stable up to 225 °C and after that the decomposition starts. The stage one from 250 °C to 275 °C, the stage two after 275 °C in which maximum weight loss is observed while decomposition of remaining compound indicate weight loss in third stage. The Fig. 6 show DSC curve which exhibits one endothermic peak at 215 °C respectively.

Fig. 5
figure 5

TGA study of p-terphenyl luminophors

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Fig. 6
figure 6

DSC study of p-terphenyl luminophors

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Electrical Properties of p-Terphenyl Luminophors

Electrochemical properties of AN and TN doped p-TP luminophors were studied by Cyclic Voltammetry (CV) in dichloromethane solution using ferrocene as an internal standard. The cyclic voltamogram is shown in Fig. 7. For the synthesized luminophors the HOMO and LUMO energy levels were observed in the range of 5.82–5.85 eV and 2.94–2.97 eV, respectively. The Eg calculated from the CV was 2.88 eV (Table 2) which found in close proximity with optical band gap.

Fig. 7
figure 7

Cyclic voltammogram

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Table 2 Electrochemical data of doped p-TP luminophors
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Conclusion

Synthesis of highly fluorescent D-A based p-terphenyl luminophors and their thin films with green light emission were achieved. The XRD analysis indicated the formation of homogeneous solid solutions of the host and guest material. The SEM images evidenced crystals of ~ 89.7 nm of size. TGA-DSC and CV study revealed suitability of prepared luminophors used for optoelectronics.