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Intrinsic Polarization and Tunable Color of Electroluminescence from Organic Single Crystal-based Light-Emitting Devices.

Ding R, Feng J, Zhou W, Zhang XL, Fang HH, Yang T, Wang HY, Hotta S, Sun HB - Sci Rep (2015)

Bottom Line: The polarization ratio of 5:1 for the transversal-electric (TE) and transversal-magnetic (TM) polarization at the emission peak of 575 nm, and 4.7:1 for the TM to TE polarization at the emission peak of 635 nm, respectively, have been obtained.The emitting color is tunable between yellow, yellow-green and orange by changing the polarization angle.The polarized EL and the polarization-induced color tunability can be attributed to the anisotropic microcavity formed by the BP3T crystal with uniaxial alignment of the molecules.

View Article: PubMed Central - PubMed

Affiliation: State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun, 130012, People's Republic of China.

ABSTRACT
A single crystal-based organic light-emitting device (OLED) with intrinsically polarized and color-tunable electroluminescence (EL) has been demonstrated without any subsequent treatment. The polarization ratio of 5:1 for the transversal-electric (TE) and transversal-magnetic (TM) polarization at the emission peak of 575 nm, and 4.7:1 for the TM to TE polarization at the emission peak of 635 nm, respectively, have been obtained. The emitting color is tunable between yellow, yellow-green and orange by changing the polarization angle. The polarized EL and the polarization-induced color tunability can be attributed to the anisotropic microcavity formed by the BP3T crystal with uniaxial alignment of the molecules.

No MeSH data available.


(a)–(d) Photographs of BP3T single-crystal-based OLEDs at different driving current of 165, 289, 477 and 643 mA/cm2 under operation. (e) The EL spectra detected at different driving current and the PL spectrum of BP3T single crystal as reference. (f) current density-voltage-luminance characteristics of the BP3T crystal-based OLEDs. Inset in f is the energy level diagram of the devices.
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f2: (a)–(d) Photographs of BP3T single-crystal-based OLEDs at different driving current of 165, 289, 477 and 643 mA/cm2 under operation. (e) The EL spectra detected at different driving current and the PL spectrum of BP3T single crystal as reference. (f) current density-voltage-luminance characteristics of the BP3T crystal-based OLEDs. Inset in f is the energy level diagram of the devices.

Mentions: BP3T single crystal is chosen as the emitting material because of its high hole and electron mobilities of up to 1.64 and 0.17 cm2 V−1 s−1, and high luminescence efficiency of up to 80%26272829303132. Thin and flat BP3T single crystals can be grown by the improved physical vapor transport method37. BP3T crystals have been demonstrated the self-waveguided edge emission, so that light is confined within the crystal and the fringes are brighter than the edge (Fig. 1a). This well-defined PL can be ascribed to the transition dipole moments of organic single crystals in a nearly upright configuration (Fig. 1b)3839. The PL spectrum is broad for low pump power whereas a strong narrow-band emission of amplified spontaneous emission (ASE) from BP3T single crystals will be observed at 575 nm by increasing the pump energy beyond a certain threshold (Fig. 1c)26272829303132. Here BP3T single crystal-based OLEDs are fabricated and characterized based on the technique of template stripping. The Au and Ca/Ag served as anode and cathode are both deposited onto the two sides of the organic single crystals by thermal evaporation, which ensures a compact contact between the electrodes and the crystal (Fig. 1d)16. As the grown condition is controlled precisely, the thickness of the BP3T is around 500 nm. The EL emission from the fringe of the OLEDs is brighter than that from the surface which is shown in Fig. 2a,b, which is coincident with that of the PL and can be attributed to the self-waveguide effect of the BP3T. Homogeneous surface emission can be observed at higher current density as can be seen from the bright picture in Supplementary Figure S1a, and the enlarge view of emission zone indicates a uniform carrier injection by the improved contact (Supplementary Figure S1b). The EL intensity from the device will follow with the increased current density as shown in Fig. 2e. The current density begins to increase at an onset voltage of around 1 V, and a rapid increase at voltage of 6 V indicating the current rectification behavior of the crystal-based OLED (Supplementary Figure S2a). Simultaneously, the EL intensity increases at the same voltage of 6 V observed from the current density-voltage-luminance characteristics (Fig. 2f). The low onset voltage can be ascribed to the improved contact between the electrodes and single crystals and the good energy level matching between the work function of metal electrodes deposited by thermal evaporation and the highest occupied molecular orbital and lowest unoccupied molecular orbital of BP3T single crystals (5.08 eV and 2.76 eV, respectively)29, which shows good performance of homogeneous luminance from the surface. And the energy level diagram of devices is shown in the inset of Fig. 2f. As the current density increases to be 778 mA/cm2, the luminance of the BP3T-based OLED can reach 5 cd/m2 (Supplementary Figure S2b). However, the external EL quantum efficiency (EQE) of the crystal-based OLEDs is still low, which may originate from a poor charge balance with different hole and electron mobilities of BP3T162529. Herein, The reported hole mobility of p-type BP3T single crystals about 1.64 cm2 V−1 s−1 is much higher than the electron mobility of 0.17 cm2 V−1 s−129. The luminance and EQE versus current density characteristics are shown in Supplementary Figure S2b. The EQE of crystal-based OLEDs is calculated by using the EL spectra and luminance according to the measured experiment data, as described in the Methods. Much effort is still needed to improve the EL efficiency of these crystal-based OLEDs in the future.


Intrinsic Polarization and Tunable Color of Electroluminescence from Organic Single Crystal-based Light-Emitting Devices.

Ding R, Feng J, Zhou W, Zhang XL, Fang HH, Yang T, Wang HY, Hotta S, Sun HB - Sci Rep (2015)

(a)–(d) Photographs of BP3T single-crystal-based OLEDs at different driving current of 165, 289, 477 and 643 mA/cm2 under operation. (e) The EL spectra detected at different driving current and the PL spectrum of BP3T single crystal as reference. (f) current density-voltage-luminance characteristics of the BP3T crystal-based OLEDs. Inset in f is the energy level diagram of the devices.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4513548&req=5

f2: (a)–(d) Photographs of BP3T single-crystal-based OLEDs at different driving current of 165, 289, 477 and 643 mA/cm2 under operation. (e) The EL spectra detected at different driving current and the PL spectrum of BP3T single crystal as reference. (f) current density-voltage-luminance characteristics of the BP3T crystal-based OLEDs. Inset in f is the energy level diagram of the devices.
Mentions: BP3T single crystal is chosen as the emitting material because of its high hole and electron mobilities of up to 1.64 and 0.17 cm2 V−1 s−1, and high luminescence efficiency of up to 80%26272829303132. Thin and flat BP3T single crystals can be grown by the improved physical vapor transport method37. BP3T crystals have been demonstrated the self-waveguided edge emission, so that light is confined within the crystal and the fringes are brighter than the edge (Fig. 1a). This well-defined PL can be ascribed to the transition dipole moments of organic single crystals in a nearly upright configuration (Fig. 1b)3839. The PL spectrum is broad for low pump power whereas a strong narrow-band emission of amplified spontaneous emission (ASE) from BP3T single crystals will be observed at 575 nm by increasing the pump energy beyond a certain threshold (Fig. 1c)26272829303132. Here BP3T single crystal-based OLEDs are fabricated and characterized based on the technique of template stripping. The Au and Ca/Ag served as anode and cathode are both deposited onto the two sides of the organic single crystals by thermal evaporation, which ensures a compact contact between the electrodes and the crystal (Fig. 1d)16. As the grown condition is controlled precisely, the thickness of the BP3T is around 500 nm. The EL emission from the fringe of the OLEDs is brighter than that from the surface which is shown in Fig. 2a,b, which is coincident with that of the PL and can be attributed to the self-waveguide effect of the BP3T. Homogeneous surface emission can be observed at higher current density as can be seen from the bright picture in Supplementary Figure S1a, and the enlarge view of emission zone indicates a uniform carrier injection by the improved contact (Supplementary Figure S1b). The EL intensity from the device will follow with the increased current density as shown in Fig. 2e. The current density begins to increase at an onset voltage of around 1 V, and a rapid increase at voltage of 6 V indicating the current rectification behavior of the crystal-based OLED (Supplementary Figure S2a). Simultaneously, the EL intensity increases at the same voltage of 6 V observed from the current density-voltage-luminance characteristics (Fig. 2f). The low onset voltage can be ascribed to the improved contact between the electrodes and single crystals and the good energy level matching between the work function of metal electrodes deposited by thermal evaporation and the highest occupied molecular orbital and lowest unoccupied molecular orbital of BP3T single crystals (5.08 eV and 2.76 eV, respectively)29, which shows good performance of homogeneous luminance from the surface. And the energy level diagram of devices is shown in the inset of Fig. 2f. As the current density increases to be 778 mA/cm2, the luminance of the BP3T-based OLED can reach 5 cd/m2 (Supplementary Figure S2b). However, the external EL quantum efficiency (EQE) of the crystal-based OLEDs is still low, which may originate from a poor charge balance with different hole and electron mobilities of BP3T162529. Herein, The reported hole mobility of p-type BP3T single crystals about 1.64 cm2 V−1 s−1 is much higher than the electron mobility of 0.17 cm2 V−1 s−129. The luminance and EQE versus current density characteristics are shown in Supplementary Figure S2b. The EQE of crystal-based OLEDs is calculated by using the EL spectra and luminance according to the measured experiment data, as described in the Methods. Much effort is still needed to improve the EL efficiency of these crystal-based OLEDs in the future.

Bottom Line: The polarization ratio of 5:1 for the transversal-electric (TE) and transversal-magnetic (TM) polarization at the emission peak of 575 nm, and 4.7:1 for the TM to TE polarization at the emission peak of 635 nm, respectively, have been obtained.The emitting color is tunable between yellow, yellow-green and orange by changing the polarization angle.The polarized EL and the polarization-induced color tunability can be attributed to the anisotropic microcavity formed by the BP3T crystal with uniaxial alignment of the molecules.

View Article: PubMed Central - PubMed

Affiliation: State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun, 130012, People's Republic of China.

ABSTRACT
A single crystal-based organic light-emitting device (OLED) with intrinsically polarized and color-tunable electroluminescence (EL) has been demonstrated without any subsequent treatment. The polarization ratio of 5:1 for the transversal-electric (TE) and transversal-magnetic (TM) polarization at the emission peak of 575 nm, and 4.7:1 for the TM to TE polarization at the emission peak of 635 nm, respectively, have been obtained. The emitting color is tunable between yellow, yellow-green and orange by changing the polarization angle. The polarized EL and the polarization-induced color tunability can be attributed to the anisotropic microcavity formed by the BP3T crystal with uniaxial alignment of the molecules.

No MeSH data available.