Organic Electronics 49 (2017) 64e68
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Organic/inorganic F8T2/GaN light emitting heterojunction Y.J. Wu, C.H. Liao, P.M. Lee, Y.S. Liu, C.L. Liu, C.Y. Liu* Department of Chemical and Materials Engineering, National Central University, No. 300, Zhongda Rd., Zhongli District, Taoyuan City, 32001, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 April 2017 Received in revised form 10 June 2017 Accepted 17 June 2017 Available online 20 June 2017
An organic/inorganic white-light emitting F8T2 (9,9-dioctylﬂuorene-co-bithiophene)/GaN heterojunction is reported. The white-light emission is produced by hybridizing the blue light (464 nm) emitted from the GaN MQWs and the yellow/green light (500e650 nm) emitted at the F8T2/p-GaN interface by electroluminescence (EL). The yellow/green light emission in the F8T2 layer is resulted from the carrier accumulation and Frenkel excitons at the F8T2/p-GaN junction interface. It is concluded that the energy barrier and large mobility discrepancy at the F8T2/p-GaN junction interface cause carriers accumulating in the F8T2 side near the F8T2/p-GaN interface. The accumulated carriers at the F8T2/p-GaN interface form Frenkel excitons by Coulombic interaction. Then, the Frenkel excitons recombine to radiate the yellow/green emission in the F8T2 layer. The International Commission on Illumination (CIE) coordinate of the white-light emitted from the present device is at (0.28, 0.30), which is very close to the standard white light (0.33, 0.33). © 2017 Elsevier B.V. All rights reserved.
Keywords: Organic Heterojunction LED GaN Electroluminescence Exciton
1. Introduction Recently, the organic/inorganic hybrid system has been developed to provide a potential platform for the optoelectronic applications, such as tunable photodiodes [1,2], transparent coatings for short-wavelength shielding , light up-conversion devices , hybrid solar cells [5,6]. The current work investigated a potential organic/inorganic light emitting hetero-junction. Numerous research approaches have reported light-emissions from the conjugated polymers. The conjugated polymers have advantages in high photoluminescence (PL) effect and better processing ﬂexibility in a large area over the inorganic semiconductors [7e11]. Also, the broad visible spectrum and low-cost potential of conjugated polymers attract people's serious attention. The conjugated polymer used in this work is F8T2, which is composed of 9,9-dioctylﬂuoren and bi-thiophene. F8T2 not only has a high operational stability in UV and air ambient, but also, a self-ordering nature due to its thermotropic liquid crystalline property [12e14]. The planar, extended, and close-packing backbone of F8T2 leads to fairly good carriers transportation properties, compared to other polymeric semiconductors. In addition, the mobility of F8T2 is reported to be in the range of 0.01e0.02 cm2/V [15e17]. In recent years, GaNbased LEDs (light emitting diodes) have been developed to be the
* Corresponding author. E-mail address: [email protected]
(C.Y. Liu). http://dx.doi.org/10.1016/j.orgel.2017.06.044 1566-1199/© 2017 Elsevier B.V. All rights reserved.
general light sources for next generation [1,18e20]. When it comes to the polymers used for photoelectrical devices, F8T2 has problems in lack of UV emission capacity and low carrier mobility [1,21e23]. On the other hand, inorganic GaN semiconductor devices can provide UV light and high mobility (~1e102 cm2/Vs). The emitting property of the F8T2/GaN LED hybrid structure is of great interest in present study, because the combination of the broad visible range of F8T2 and the discrete blue light of GaN LED could revitalize another way of lighting. Currently, the parallel and wide-practiced approach to achieve white-light GaN LEDs is to employ down-conversion phosphor layers. The phosphor on the GaN LEDs absorbs the blue emission from GaN LEDs and produces yellow light through ﬂuorescence. A brief comparison between the current white-light LEDs using the “down-conversion phosphor” and the present F8T2/GaN hybrid device is discussed below. The main differences between the downconversion phosphor white-light LEDs and the present F8T2/GaN white-light emitting heterojunction are: (1) Only one active region, MQWs, for the down-conversion phosphor GaN LEDs driven by EL. The yellow light emitted by the phosphor coating is pumped by PL (blue light). However, the present F8T2/p-GaN organic/inorganic hybrid device has two active regions and both blue and green/ yellow light emission are driven by EL. (2) The F8T2/p-GaN interface can harvest carriers that overﬂow through the MQWs of GaN LEDs. (3) Reliability of the polymer (F8T2, in this case) emitting green/yellow for the present F8T2/GaN hybrid device is a concern. (4) Flexible processes and potential color rendering modiﬁcation
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could be advantages for the present F8T2/GaN hybrid device. The F8T2 was coated on the p-GaN of a GaN LED to form an organic/inorganic (F8T2/p-GaN) hetero-junction interface. The interactions of the carriers/excitons in the layered hybrid emitting device of conjugated polymers and III-V GaN-based semiconductors depends on their natural characteristics. We found that the energy barrier at the F8T2/GaN LED interface result in the electron-hole pairs (excitons). The excitons at the F8T2/p-GaN interface are eliminated through the recombination processes to emit light. These hybrid electronic dynamics will be dominated by the Wannier and Frenkel excitons and the physical effects need to be obtained by further analysis [24,25]. In this work, the excitons formation and the carriers transportation at the organic/inorganic interface are studied to understand the lighting mechanism of the F8T2/GaN LED structure.
2. Experimentals In this study, a GaN LED with the GaN epitaxial structure including u-GaN, n-GaN, multi-quantum wells (MQWs), and p-GaN was studied. The detail GaN LED epitaxial layer structure is: a 200nm buffer layer, a 2-mm undoped GaN layer, a 3-mm Si-doped n-GaN layer, twelve pairs of InGaN/GaN multi-quantum wells, and a 500nm Mg-doped p-GaN layer. The carrier concentration of the n-GaN and p-GaN are 1.2 1019 cm 3 and 2.3 1018 cm 3, respectively. F8T2 is a conjugated copolymer composed of 9,9-dioctylﬂuoren and bi-thiophene. Firstly, the F8T2 organic solution was prepared in chlorobenzene (concentration of 10 mg/ml) by 4-h stirring. Then, the F8T2 organic solution was spin-coated (at 1000 rpm) onto the top p-GaN surface of GaN LED epitaxial structure. The thickness of the F8T2 layer was controlled at 70 nm. The cross-section of the white-light F8T2/GaN LED structure is illustrated in Fig. 1(a). Anneal F8T2/GaN LED samples in the vacuum oven at 80e110 C for 1 h. Then, Ni (50 nm)/Au (50 nm) electrodes were fabricated on the F8T2 layer of the F8T2/GaN LED samples by the e-gun thermal process.
Fig. 1. (a) Illustration of the cross-section of the white-light F8T2/GaN LED structure. The photographs of the F8T2/GaN MQWs structure emits white light under (b) 11 V (c) 13 V and (d) 15 V.
3. Results and discussions With the forward-bias operation ranging from 11 V to 15 V, the F8T2/GaN MQWs structure emits white light, as shown in Fig. 1(b)e(d). The white light emits from the F8T2 layer upward toward the ambient. Some blue light escapes from the side of the F8T2/GaN MQWs structure, which can be seen as the blue halo in Fig. 1(b)e(d). Thus, we know that the blue light emitted from GaN LED enters the F8T2 layer and is converted to white light in the F8T2 layer. The white-light conversion should be resulted from the hybridization of the blue light with the yellow/green light. PL spectra of F8T2, GaN MQWs, and F8T2/GaN MQWs structure with a laser excitation source (266 nm) are shown in Fig. 2. GaN MQWs has one discrete PL peak at 464 nm contributed from the InGaN/GaN quantum wells. F8T2 has two primary PL peaks at 514 nm and 543 nm and a broad PL emission peak near 585 nm, which attribute to the band gap and the interstates of the conjugated F8T2 [8,26]. Therefore, we know that the F8T2/GaN MQWs heterojunction interface does not produce any extra PL emissions. The PL emission peaks come from the individual F8T2 layer and GaN MQWs. The above PL spectra conﬁrms that the yellow/green emission attributes to F8T2 (520 nm, 541 nm, and a broad emission near 585 nm), which is hybridized with the blue light from the GaN MQWs to emit the white light. The yellow/green light emission in the F8T2 layer could be either (1) Photoluminescence (PL) effect; the yellow/green light pumped in the F8T2 layer by the blue light from the GaN LED (2) Electroluminescence (EL) effect; the yellow/green light emitted in the F8T2 layer by electron transitions. To verify the main mechanism of the yellow/green emission in the F8T2 layer, 465-nm blue light (from the current studied GaN LED) was used to photoexcite the single F8T2 layer. We found that no obvious yellow/green photoluminescence was generated by blue light, if no current passing through the interface. It means that the visible yellow/ green light excited in the F8T2 layer is not produced by PL. Therefore, the EL effect should be the main mechanism for the yellow/ green emission in the F8T2 layer. Fig. 3(a) shows the EL spectra of the white-light F8T2/GaN MQWs device under the forward voltage ranging from 11 V to 15 V. The inset in Fig. 3(a) presents the EL spectra of the white-light F8T2/ GaN MQWs device under a forward voltage of 11 V. The EL emission peaks are contributed from the MQWs (464 nm) and the F8T2 (521 nm, 542 nm), which correspond to the PL spectra in Fig. 2. The
Fig. 2. PL spectra of F8T2, GaN MQWs, and F8T2/GaN MQWs structure with a 266 nmlaser excitation source.
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Fig. 3. (a) EL spectra of the hybrid white light F8T2/GaN MQWs device under different applied forward voltage from 11 V to 15 V. The inset in (a) shows the EL spectra under a forward voltage of 11 V. (b) The CIE color coordinates of the F8T2/GaN MQWs device under various forward voltages. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)
broad EL peak of the F8T2 layer red-shifts with the external forward voltage. As the forward voltage increases, there is no signiﬁcant change in the emission wavelength. It means that the applied forward voltage has very little effect on the EL emission characteristics in this case. From the above PL and EL analysis, again, it can be concluded that the white-light emission is resulted from the hybridization of the blue light (464 nm) from GaN MQWs with the yellow/green light (500e650 nm) from the F8T2 layer. The CIE coordinates of the white-light emission of the F8T2/GaN MQWs device under various forward voltages are shown in Fig. 3(b). In the CIE chromaticity diagram, we can clearly see that the CIE coordinate of the white-light emission of the F8T2/GaN MQWs device is at (0.28, 0.30), which is close to the CIE coordinate of the standard white-light (0.33, 0.33). With increasing of the applied forward voltage, the CIE coordinates of the white light emission shifts toward the blue region. The light intensity of both blue and yellow/green light emissions increases with the applied forward voltage. However, the increase of the blue light with the forward voltage is larger than that of the yellow/green light. This is
because that a stronger electron-hole wave function overlap in GaN MQWs than in the F8T2/p-GaN layer. So, the CIE coordinate of the white-light emission of the F8T2/GaN MQWs device shifts toward the blue region with the applied forward voltage. Fig. 4 shows the current density-voltage-luminance characteristics of the whitelight F8T2/GaN MQWs device. The inset in Fig. 4 shows the power efﬁciency versus current density. The IV curves exhibit clear pn junction characteristics and low leakage current (1.8 10-5 A at 5 V); the working current is 17.12 mA/cm2 at 11 V, and the turnon voltage is about 6 V. As voltage increases, the current density and luminance increases. The power efﬁciency is around 15.7 lm/W and decreases with the current density. To analyze the white-light emission mechanism, the energylevels alignment of the studied white-light F8T2/GaN MQWs device is illustrated in Fig. 5. Fig. 5(a) shows the energy-levels diagram of the white-light F8T2/GaN MQWs device. F8T2 has a LUMO (Lowest Unoccupied Molecular Orbital) level of 3.1 eV and a HOMO (Highest Occupied Molecular Orbital) level of 5.5 eV [27e30]. The band gap, electron afﬁnity, and ionization potential of GaN are 3.39 eV, 4.1 eV, and 7.49 eV, respectively . Fig. 5(b) illustrates the energy-levels alignment between the F8T2 and the GaN LED. Previous researches reported that the carrier exchange at the organic/inorganic interface is quite poor due to the low concentration of thermal carriers, and the non-equilibrium state at the organic/inorganic interface . Hence, the Fermi level of the GaN epi-layers may not align with the F8T2 layer at the organic/inorganic interface. To experimentally deﬁne the energy levels at the organic/inorganic F8T2/p-GaN interface, the ultra-violet photon spectroscopy (UPS) analysis has been performed at this organic/inorganic emitting junction, as shown in the ﬁgure below, i.e., Fig. 6 (a) in the revised manuscript. With the UPS result in Fig. 6, the position of Fermi energy level (Ef) can be estimated as the binding energy at zero. Also, Ev (the lowest energy of the valence electrons) can be obtained with the intercept between the tangent line of the binding-energy curve with the x-axis (binding energy). Thus, with the curve of the intensity of the excited valence electrons against the binding energy (Fig. 6), Ev can be estimated to be about 1.25 eV. The ionization potential (IP) of F8T2 can also be measured by the UPS analysis with F8T2/various-depth-p-GaN etched by Ar sputtering. With those UPS data, the detail energy-level structure at the F8T2/p-GaN interface is illustrated in Fig. 6(b). The energy-level bending effects at the organic/inorganic interface is caused by the formation of the internal dipole energy (eD), as shown in Fig. 6(b). eD (2.34 eV) is calculated by the model reported by J. Bloshwitz
Fig. 4. Current density-voltage-luminance characteristics of the white-light F8T2/GaN MQWs device. The inset shows the power efﬁciency versus current density.
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Fig. 6. (a) Ultra-violet photon spectroscopy (UPS) results of the F8T2/p-GaN interface. (b) Energy level alignment of the F8T2/p-GaN interface derived from UPS.
Fig. 5. (a) Illustration of energy potential of HOMO, LUMO, electron afﬁnity, and ionization potential of the white-light F8T2/GaN MQWs device. The energy-levels alignment (b) and energy potential changes of this white-light F8T2/GaN MQWs device under various forward voltages are shown in (c) to (d). The Frenkel excitons are marked as red dash circles in (c) and (d). (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)
et al.  and the known HOMO energy of F8T2, Fermi energy, and IP of F8T2 and p-GaN. The construction of the energy-level structure at the organic/inorganic F8T2/p-GaN interface also veriﬁes the following factors in this work. The HOMO shift agrees with the broad band gap of F8T2, and the eD at the interface results in a potential difference corresponding to the electron transfer from the p-GaN layer to F8T2. Moreover, the eD contributes the carrier accumulation at the near-interface region and provides the possible efﬁcient recombination at the F8T2/p-GaN junction.
Based on the above energy-levels alignment of the F8T2/GaN MQWs device, the white-light emission process of the F8T2/GaN MQWs device is described in the following steps. Firstly, under the forward voltage, the carriers are transporting to the GaN MQWs. The transporting electrons and holes would be conﬁned in the quantum wells and recombine to emit the blue light (464 nm). A portion of the carriers overﬂow through the GaN MQWs and are transporting to the F8T2/p-GaN interface, as shown in Fig. 5(c). The energy barrier at the F8T2/p-GaN interface would cause the carriers accumulating at the F8T2/p-GaN junction interface. Also, note that the carrier mobility of GaN is much larger than that of F8T2 by two to three orders . The difference of mobility manifest the electricity nature of organic and inorganic materials for their carrier transport mechanism: hopping among the overlapping wave function region in p-system for conjugated organic semiconductor, and transfer through periodically lattice structure. The microstructure change increases the diffusion distance as well as decrease the electron-hole overlapping wave function for excitons or charge carriers. As the carriers crossing the F8T2/p-GaN interface, the carriers would slow down and accumulate in the F8T2 side, as shown in Fig. 5(c). Thus, the large mobility difference between the F8T2 and the GaN further enhances the carrier accumulation in the F8T2 side near the F8T2/p-GaN interface. Note that lattice defects, impurity atoms, and electricity difference would cause potential wells (traps) at the organic/inorganic interface. The carrier loses its kinetic energy due to localization at these traps and form bound excitons. It means that the lattice defect structure, impurity atoms, and electricity difference at the hetero-interface promote the bound exciton formation. The luminescence efﬁciency of the bound exciton is substantially higher than that of free excitons. It is known that Frenkel excitons would
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form in the organic semiconductor with a restricted spatial extension . So, we would expect that the accumulated electrons in the F8T2 side near the F8T2/p-GaN interface would form Frenkel excitons with the holes in HOMO of F8T2 by the Coulumbic interaction. The Frenkel excitons are marked as red dash circles in Fig. 5(c) and (d). Especially for the organic semiconductors (F8T2 in this study), their low dielectric constants, large disorder, and trapping effects would result in a stronger Coulombic interaction, which promotes the formation of Frenkel excitons. Besides, the charge transfer exitons also likely to form at the organic/inorganic interface. The charge transfer exitons has large radius and lower binding energy than Fenkel excitons. As the electrons pass through the F8T2/p-GaN, the large difference of lattice structure enhances the formation of charge transfer exciton, which can contribute to long-wavelength emission. It is the recombination of Frenkel excitons dominantly radiates the yellow/green emission in the F8T2 layer. Then, the yellow/green emission from the F8T2/p-GaN interface is hybridized with the blue light emitted from the GaN MQWs to emit the white light of the present F8T2/GaN MQWs device. Under a relatively larger forward voltage, the minimum energy level of the conduction band of the n-GaN would be higher than the LUMO level of F8T2. The energy level of the valences band of the nGaN would also be raised and get closer to the HOMO level of F8T2. Besides, the energy level of F8T2 polymer is in a broad range corresponding to its PL emission color range in Fig. 2. Therefore, the electrons injected from the n-GaN layer as well as the holes from the F8T2 would gain a sufﬁcient kinetic energy and have a high possibility to overcome the energy barrier at the interface, as shown in Fig. 5(d). As a result, the current through the F8T2/p-GaN interface increases with the applied voltage, which agrees with the increase of the EL intensity with the applied voltage, as seen in Fig. 3(a). The above result proves that the keys for the white-light emission of the F8T2/GaN MQWs device are the carrier accumulation and Frenkel excitons formation at the organic/inorganic F8T2/p-GaN interface. 4. Conclusion In conclusion, we have demonstrated a white-light emitting device, which is an organic/inorganic F8T2/GaN MQWs hybrid structure. We found that the primary mechanism for this whitelight device is EL based on the carrier accumulation and Frenkel excitons at the organic/inorganic F8T2/GaN interface, and can allow further optimization of organic/inorganic optoelectronic devices. The energy barrier and large mobility discrepancy at the F8T2/pGaN junction interface would cause the carrier accumulation in the F8T2 side near the F8T2/p-GaN interface. The strong Coulombic interaction in F8T2 layer promotes the accumulated carriers form Frenkel excitons and radiate the yellow/green emission. Strikingly, there are two emission regions coexist in this hybrid structure to hybridize white light. The white light emission is produced by hybridizing the blue light (464 nm) emitted from the GaN MQWs with the yellow/green light (500e650 nm) emitted at the F8T2/p-GaN interface by electroluminescence (EL). The CIE coordinate of the white-light emission from the present device is at (0.28, 0.30), which is very close to the standard white light (0.33, 0.33). It is noted that the F8T2/p-GaN interface creates unique and efﬁcient carrier recombination and can combine with the traditional GaN light emitting diode for a potential extension of emission color range. These results not only embark further understanding on the characteristics of carrier transport of hybrid interface but also provide an attractive white light electroluminescence device, and extend the possibility of large-area hybrid white light GaN-based
Acknowledgments This work was supported in part by the program MOST 1042221-E-008 -112 -MY3, MOST 105-3113-E-008 -008 -CC2 and MOST 105-2221-E-008 -104 -MY3.
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