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  • Editor: Bernard Kippelen
  • Vol. 19, Iss. S3 — May. 9, 2011
  • pp: A319–A325
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Ultraviolet electroluminescence from hybrid inorganic/organic ZnO/GaN/poly(3-hexylthiophene) dual heterojunctions

Yungting Chen, Hanyu Shih, Chunhsiung Wang, Chunyi Hsieh, Chihwei Chen, Yangfang Chen, and Taiyuan Lin  »View Author Affiliations


Optics Express, Vol. 19, Issue S3, pp. A319-A325 (2011)
http://dx.doi.org/10.1364/OE.19.00A319


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Abstract

Based on hybrid inorganic/organic n-ZnO nanorods/p-GaN thin film/poly(3-hexylthiophene)(P3HT) dual heterojunctions, the light emitting diode (LED) emits ultraviolet (UV) radiation (370 nm – 400 nm) and the whole visible light (400 nm −700 nm) at the low injection current density. Meanwhile, under the high injection current density, the UV radiation overwhelmingly dominates the room-temperature electroluminescence spectra, exponentially increases with the injection current density and possesses a narrow full width at half maximum less than 16 nm. Comparing electroluminescence with photoluminescence spectra, an enormously enhanced transition probability of the UV luminescence in the electroluminescence spectra was found. The P3HT layer plays an essential role in helping the UV emission from p-GaN material because of its hole-conductive characteristic as well as the band alignment with respect to p-GaN. With our new finding, the result shown here may pave a new route for the development of high brightness LEDs derived from hybrid inorganic/organic heterojuctions.

© 2011 OSA

1. Introduction

Over the past decades, gallium nitride (GaN) has attracted unprecedented attention due to its direct and wide band gap (3.4 eV) and thus has been comprehensively investigated for the use in light emitting diodes (LEDs). In early age, electroluminescence (EL) has been obtained from schottky barrier diodes and the emission may include blue, green, yellow, orange and red light by the doping of group-I, group-II, and rare-earth elements during the synthesis [1

1. T. Ogino and M. Aoki, “Mechanism of Yellow Luminescence in GaN,” Jpn. J. Appl. Phys. 19(12), 2395–2405 (1980). [CrossRef]

5

5. D. S. Lee and A. J. Steckl, “Enhanced blue and green emission in rare-earth-doped GaN electroluminescent devices by optical photopumping,” Appl. Phys. Lett. 81(13), 2331–2333 (2002). [CrossRef]

]. However, the schottky barrier diode has the extremely high turn-on voltage and thus is highly energy-consuming. An intriguing alternative to overcome the difficulty is to develop p-n heterojunction LEDs.

Zinc oxide (ZnO) has long been thought to be a promising material for optoelectronic applications due to its direct and wide band gap (3.3 eV) and large exciton binding energy (60 meV). However, no matter what method is used to prepare ZnO, ZnO is an intrinsically n-type semiconductor due to the existence of oxygen vacancies. Also, it has been indicated that p-type doping in ZnO is tough task to accomplish [6

6. S. Limpijumnong, S. B. Zhang, S. H. Wei, and C. H. Park, “Doping by large-size-mismatched impurities: the microscopic origin of arsenic- or antimony-doped p-type zinc oxide,” Phys. Rev. Lett. 92(15), 155504 (2004). [CrossRef] [PubMed]

,7

7. B. Claflin, D. C. Look, S. J. Park, and G. Cantwell, “Persistent n-type photoconductivity in p-type ZnO,” J. Cryst. Growth 287(1), 16–22 (2006). [CrossRef]

]. Therefore, in order to fabricate ZnO based UV-LEDs, it necessitates a good and reproducible p-type material.

2. Experiment

After the growth, the ZnO sample was spin-coated with the ZEP-520 electron resist, a buffer layer, in order to keep Au/Ti contact from directly touching p-GaN thin film. This method was able to effectively avoid the undesired leakage current. Then, the Au/Ti contact was thermally heated onto the exposed ZnO nanorods. The commercial P3HT was dissolved in chloroform (10 mg mL−1) at a spin speed of 500 rpm for overnight and subsequently dropped onto the GaN part of the sample with one drop of 5 μL. Hereafter, the sample was annealed at 110 °C for 15 minutes in order to remove the residual solvent in P3HT and then the Ag contact was thermally evaporated onto the P3HT sample. The fabrication of P3HT and Ag contact were performed in a nitrogen-filled glove box. The thickness of the P3HT layer was about 3.2 μm according to the SEM image (not shown). It is worth noting that the low preparation temperature of the P3HT layer will not destroy the p-n junction of p-GaN/ZnO and cause unwanted dopant diffusion into p-GaN or ZnO.

The morphology of ZnO nanorods was characterized by using scanning electron microscopy (SEM) (JSM 6500, JEOL). Photoluminescence (PL) spectra were obtained at room temperature with a SPEX Fluorolog-2 instrument equipped with double-grating monochromator and a R928 photomultiplier tube (PMT). The excitation source was provided by a 325 nm He-Cd laser. In addition, room-temperature electroluminescence (EL) spectra were excited electrically by using a Keithley current source and the emission was detected from the sapphire side of the LED by the same PL measurement system.

3. Results and discussions

The schematic of the dual heterojunction structures is shown in Fig. 1a
Fig. 1 (a) Schematic illustration of the (n-ZnO nanorods)/(p-GaN film)/(P3HT film) dual-heterojunction LED device. (b) Band diagram and transition process responsible for the EL spectra without forward bias. (SD: shallow donor, SA: shallow acceptor, DD: deep donor, DA: deep acceptor)
. On one side of the device, ZnO nanorods and Mg-doped p-type GaN thin film form an inorganic-inorganic heterojunction and the band diagram at thermal equilibrium is depicted in Fig. 1b. On the other side, P3HT and Mg-doped p-type GaN thin film construct an organic-inorganic heterojunction. As shown in Fig. 1b, the band diagram is depicted according to the assumption that the Fermi level at the interface is unpinned [21

21. B.-N. Park, J. J. Uhlrich, T. F. Kuech, and P. G. Evans, “Electrical properties of GaN/poly(3-hexylthiophene) interfaces,” J. Appl. Phys. 106(1), 013713 (2009). [CrossRef]

]. The assumption has been extensively used to describe the transport of electrons or holes through the organic/inorganic interface [22

22. D. C. Olson, S. E. Shaheen, M. S. White, W. J. Mitchell, M. F. A. M. van Hest, R. T. Collins, and D. S. Ginley, “Band-Offset Engineering for Enhanced Open-Circuit Voltage in Polymer–Oxide Hybrid Solar Cells,” Adv. Funct. Mater. 17(2), 264–269 (2007). [CrossRef]

,23

23. T. R. B. Foong, Y. Shen, X. Hu, and A. Sellinger, “Template-Directed Liquid ALD Growth of TiO2 Nanotube Arrays: Properties and Potential in Photovoltaic Devices,” Adv. Funct. Mater. 20(9), 1390–1396 (2010). [CrossRef]

]. Figures 2a
Fig. 2 (a) Top-view and (b) cross-sectional scanning electron microscope (SEM) images of as-grown n-ZnO nanorods on p-GaN thin film.
and 2b show the top-view and the cross-sectional SEM images of ZnO nanorods which are vertically well-aligned and have an average diameter of 100 nm and a length of about 1.2 μm.

Before entering to investigate the photoelectric properties of our designed device, we first measure the room temperature photoluminescence (PL) properties of the individual components of the device since it can demonstrate the possible transitions from each material. As shown in Fig. 3
Fig. 3 Photoluminescence (PL) spectra of n-ZnO nanorods, p-GaN thin film, and poly(3-hexylthiophene)(P3HT) thin film at room temperature.
, the band edge emission centered at 3.3 eV dominates the PL spectrum of ZnO nanorods with low defect emission and thus it exhibits high quality of ZnO nanorods. The PL spectrum of P3HT shows two emission peaks occur near 1.76 eV and 1.9 eV. In addition, the typical PL features of GaN include the yellow luminescence (YL) band, the blue luminescence (BL) band, and the ultraviolet luminescence (UVL) band, corresponding to the peaks centered at around 2.2 eV, 2.75 eV and 3.32 eV, respectively. The YL band can be attributed to transitions from shallow donors to deep acceptors while transitions from deep donors to shallow acceptors are responsible for the BL band. Moreover, the UVL band belongs to conduction band to shallow acceptor (e-A) transitions [24

24. U. Kaufmann, M. Kunzer, M. Maier, H. Obloh, A. Ramakrishnan, B. Santic, and P. Schlotter, “Nature of the 2.8 eV photoluminescence band in Mg doped GaN,” Appl. Phys. Lett. 72(11), 1326–1328 (1998). [CrossRef]

26

26. M. A. Reshchikov, Y. T. Moon, X. Gu, B. Nemeth, J. Nause, and H. Morkoc, “Unstable luminescence in GaN and ZnO,” Physica B 376-377, 715–718 (2006). [CrossRef]

]. The above mentioned transitions are depicted in the Fig. 1b. According to the early reports about the PL emission from undoped GaN, the BL band and the UVL band have very low quantum efficiency at room temperature [25

25. M. A. Reshchikov and H. Morkoç, “Luminescence properties of defects in GaN,” J. Appl. Phys. 97(6), 061301 (2005). [CrossRef]

,27

27. M. A. Reshchikov and R. Y. Korotkov, “Analysis of the temperature and excitation intensity dependencies of photoluminescence in undoped GaN films,” Phys. Rev. B 64(11), 115205 (2001). [CrossRef]

]. However, for the lightly Mg-doped GaN, the increase of shallow acceptor states leads to the enhanced emission from the BL band and the UVL band [25

25. M. A. Reshchikov and H. Morkoç, “Luminescence properties of defects in GaN,” J. Appl. Phys. 97(6), 061301 (2005). [CrossRef]

], which is clearly observed in the PL and EL spectra from our experiment.

Similar to the description of power-dependent PL, the current-dependent EL could be described by the same equation [29

29. J. E. Fouquet and A. E. Siegman, “Room-temperature photoluminescence times in a GaAs/AlxGa1-xAs molecular beam epitaxy multiple quantum well structure,” Appl. Phys. Lett. 46(3), 280–282 (1985). [CrossRef]

31

31. L. T. Tung, K. L. Lin, E. Y. Chang, W. C. Huang, Y. L. Hsiao, and C. H. Chiang, “Photoluminescence and Raman studies of GaN films grown by MOCVD,” J. Phys.: Conf. Ser. 187, 012021 (2009). [CrossRef]

]:

I = α Ioβ  .
(1)

In the relation, Io is the injection current density, α is the emission efficiency, and the exponent β represents the recombination mechanisms including the recombination of free excitons and free carriers (1 < β < 2), free excitons recombination (β = 1) and free carriers recombination (β = 2). As shown in Fig. 6a
Fig. 6 Room-temperature, injection-current-density-dependent EL intensity at (a) 3.2 eV in the logarithmic scales, (b) 1.9 eV, 2.2 eV, 2.75 eV and 2.91 eV in the linear scales.
, the slope of the fitting line equals to 1.88 in the logarithmic plot, which indicates the transitions of the UVL band can be attributed to the first case as described above [24

24. U. Kaufmann, M. Kunzer, M. Maier, H. Obloh, A. Ramakrishnan, B. Santic, and P. Schlotter, “Nature of the 2.8 eV photoluminescence band in Mg doped GaN,” Appl. Phys. Lett. 72(11), 1326–1328 (1998). [CrossRef]

]. According to Fig. 6b, beyond the UVL band, the other emissions of the LED can be assigned to the second case. In addition, it is found that the intensities of the emissions at 2.75 eV and 2.91 eV start to decrease when the injection current density exceeds 300 mA cm−2. It seems that the UVL band transition is competing with the 2.75 eV and 2.91 eV transitions at high injection current density, because these transitions share the same shallow acceptors. Based on the recombination processes shown in Fig. 1b, it is reasonable to understand that the UVL band transition has the fastest recombination rate and will dominate the recombination when the supply of shallow acceptors is limited.

4. Conclusions

Acknowledgments

This work was supported by the National Science Council and the Ministry of Education of the Republic of China.

References and links

1.

T. Ogino and M. Aoki, “Mechanism of Yellow Luminescence in GaN,” Jpn. J. Appl. Phys. 19(12), 2395–2405 (1980). [CrossRef]

2.

R. Birkhahn, M. Garter, and A. J. Steckl, “Red light emission by photoluminescence and electroluminescence from Pr-doped GaN on Si substrates,” Appl. Phys. Lett. 74(15), 2161–2163 (1999). [CrossRef]

3.

A. J. Steckl, M. Garter, D. S. Lee, J. Heikenfeld, and R. Birkhahn, “Blue emission from Tm-doped GaN electroluminescent devices,” Appl. Phys. Lett. 75(15), 2184–2186 (1999). [CrossRef]

4.

D. S. Lee, J. Heikenfeld, R. Birkhahn, M. Garter, B. K. Lee, and A. J. Steckl, “Voltage-controlled yellow or orange emission from GaN codoped with Er and Eu,” Appl. Phys. Lett. 76(12), 1525–1527 (2000). [CrossRef]

5.

D. S. Lee and A. J. Steckl, “Enhanced blue and green emission in rare-earth-doped GaN electroluminescent devices by optical photopumping,” Appl. Phys. Lett. 81(13), 2331–2333 (2002). [CrossRef]

6.

S. Limpijumnong, S. B. Zhang, S. H. Wei, and C. H. Park, “Doping by large-size-mismatched impurities: the microscopic origin of arsenic- or antimony-doped p-type zinc oxide,” Phys. Rev. Lett. 92(15), 155504 (2004). [CrossRef] [PubMed]

7.

B. Claflin, D. C. Look, S. J. Park, and G. Cantwell, “Persistent n-type photoconductivity in p-type ZnO,” J. Cryst. Growth 287(1), 16–22 (2006). [CrossRef]

8.

Y. I. Alivov, J. E. Van Nostrand, D. C. Look, M. V. Chukichev, and B. M. Ataev, “Observation of 430 nm electroluminescence from ZnO/GaN heterojunction lightemitting diodes,” Appl. Phys. Lett. 83(14), 2943–2945 (2003). [CrossRef]

9.

W. I. Park and G. C. Yi, “Electroluminescence in n-ZnO Nanorod Arrays Vertically Grown on p-GaN,” Adv. Mater. 16(1), 87–90 (2004). [CrossRef]

10.

D. J. Rogers, F. H. Teherani, A. Yasan, K. Minder, P. Kung, and M. Razeghi, “Electroluminescence at 375 nm from a ZnO/GaN:Mg/c-Al2O3 heterojunction light emitting diode,” Appl. Phys. Lett. 88(14), 141918 (2006). [CrossRef]

11.

M. C. Jeong, B. Y. Oh, M. H. Ham, and J. M. Myoung, “Electroluminescence from ZnO nanowires in n-ZnO film/ZnO nanowire array/p-GaN film heterojunction light-emitting diodes,” Appl. Phys. Lett. 88(20), 202105 (2006). [CrossRef]

12.

M. C. Jeong, B. Y. Oh, M. H. Ham, S. W. Lee, and J. M. Myoung, “ZnO-Nanowire-Inserted GaN/ZnO Heterojunction Light-Emitting Diodes,” Small 3, 568–572 (2007). [CrossRef] [PubMed]

13.

E. Lai, W. Kim, and P. Yang, “Vertical Nanowire Array-Based Light Emitting Diodes,” Nano Res. 1(2), 123–128 (2008). [CrossRef]

14.

J. Y. Lee, J. H. Lee, H. S. Kim, C. H. Lee, H. S. Ahn, H. K. Cho, Y. Y. Kim, B. H. Kong, and H. S. Lee, “A study on the origin of emission of the annealed n-ZnO/p-GaN heterostructure LED,” Thin Solid Films 517(17), 5157–5160 (2009). [CrossRef]

15.

A. M. C. Ng, Y. Y. Xi, Y. F. Hsu, A. B. Djurisić, W. K. Chan, S. Gwo, H. L. Tam, K. W. Cheah, P. W. K. Fong, H. F. Lui, and C. Surya, “GaN/ZnO nanorod light emitting diodes with different emission spectra,” Nanotechnology 20(44), 445201 (2009). [CrossRef] [PubMed]

16.

X. M. Zhang, M. Y. Lu, Y. Zhang, L. J. Chen, and Z. L. Wang, “Fabrication of a High-Brightness Blue-Light-Emitting Diode Using a ZnO-Nanowire Array Grown on p-GaN Thin Film,” Adv. Mater. 21(27), 2767–2770 (2009). [CrossRef]

17.

C. H. Chen, S. J. Chang, S. P. Chang, M. J. Li, I. C. Chen, T. J. Hsueh, and C. L. Hsu, “Electroluminescence from n-ZnO nanowires/p-GaN heterostructure light-emitting diodes,” Appl. Phys. Lett. 95(22), 223101 (2009). [CrossRef]

18.

H. Sirringhaus, P. J. Brown, R. H. Friend, M. M. Nielsen, K. Bechgaard, B. M. W. Langeveld-Voss, A. J. H. Spiering, R. A. J. Janssen, E. W. Meijer, P. Herwig, and D. M. de Leeuw, “Two-dimensional charge transport in self-organized, high-mobility conjugated polymers,” Nature 401(6754), 685–688 (1999). [CrossRef]

19.

Y. Kim, S. Cook, S. M. Tuladhar, S. A. Choulis, J. Nelson, J. R. Durrant, D. D. C. Bradley, M. Giles, I. Mcculloch, C. S. Ha, and M. Ree, “A strong regioregularity effect in self-organizing conjugated polymer films and high-efficiency polythiophene:fullerene solar cells,” Nat. Mater. 5(3), 197–203 (2006). [CrossRef]

20.

G. Dennler, M. C. Scharber, and C. J. Brabec, “Polymer-Fullerene Bulk-Heterojunction Solar Cells,” Adv. Mater. 21(13), 1323–1338 (2009). [CrossRef]

21.

B.-N. Park, J. J. Uhlrich, T. F. Kuech, and P. G. Evans, “Electrical properties of GaN/poly(3-hexylthiophene) interfaces,” J. Appl. Phys. 106(1), 013713 (2009). [CrossRef]

22.

D. C. Olson, S. E. Shaheen, M. S. White, W. J. Mitchell, M. F. A. M. van Hest, R. T. Collins, and D. S. Ginley, “Band-Offset Engineering for Enhanced Open-Circuit Voltage in Polymer–Oxide Hybrid Solar Cells,” Adv. Funct. Mater. 17(2), 264–269 (2007). [CrossRef]

23.

T. R. B. Foong, Y. Shen, X. Hu, and A. Sellinger, “Template-Directed Liquid ALD Growth of TiO2 Nanotube Arrays: Properties and Potential in Photovoltaic Devices,” Adv. Funct. Mater. 20(9), 1390–1396 (2010). [CrossRef]

24.

U. Kaufmann, M. Kunzer, M. Maier, H. Obloh, A. Ramakrishnan, B. Santic, and P. Schlotter, “Nature of the 2.8 eV photoluminescence band in Mg doped GaN,” Appl. Phys. Lett. 72(11), 1326–1328 (1998). [CrossRef]

25.

M. A. Reshchikov and H. Morkoç, “Luminescence properties of defects in GaN,” J. Appl. Phys. 97(6), 061301 (2005). [CrossRef]

26.

M. A. Reshchikov, Y. T. Moon, X. Gu, B. Nemeth, J. Nause, and H. Morkoc, “Unstable luminescence in GaN and ZnO,” Physica B 376-377, 715–718 (2006). [CrossRef]

27.

M. A. Reshchikov and R. Y. Korotkov, “Analysis of the temperature and excitation intensity dependencies of photoluminescence in undoped GaN films,” Phys. Rev. B 64(11), 115205 (2001). [CrossRef]

28.

H. K. Fu, C. L. Cheng, C. H. Wang, T. Y. Lin, and Y. F. Chen, “Selective Angle Electroluminescence of Light-Emitting Diodes based on Nanostructured ZnO/GaN Heterojunctions,” Adv. Funct. Mater. 19(21), 3471–3475 (2009). [CrossRef]

29.

J. E. Fouquet and A. E. Siegman, “Room-temperature photoluminescence times in a GaAs/AlxGa1-xAs molecular beam epitaxy multiple quantum well structure,” Appl. Phys. Lett. 46(3), 280–282 (1985). [CrossRef]

30.

L. Bergman, X. B. Chen, J. L. Morrison, J. Huso, and A. P. Purdy, “Photoluminescence dynamics in ensembles of wide-band-gap nanocrystallites and powders,” J. Appl. Phys. 96(1), 675–682 (2004). [CrossRef]

31.

L. T. Tung, K. L. Lin, E. Y. Chang, W. C. Huang, Y. L. Hsiao, and C. H. Chiang, “Photoluminescence and Raman studies of GaN films grown by MOCVD,” J. Phys.: Conf. Ser. 187, 012021 (2009). [CrossRef]

OCIS Codes
(160.4890) Materials : Organic materials
(230.3670) Optical devices : Light-emitting diodes
(160.4236) Materials : Nanomaterials

ToC Category:
Light-Emitting Diodes

History
Original Manuscript: February 17, 2011
Revised Manuscript: April 7, 2011
Manuscript Accepted: April 14, 2011
Published: April 18, 2011

Citation
Yungting Chen, Hanyu Shih, Chunhsiung Wang, Chunyi Hsieh, Chihwei Chen, Yangfang Chen, and Taiyuan Lin, "Ultraviolet electroluminescence from hybrid inorganic/organic ZnO/GaN/poly(3-hexylthiophene) dual heterojunctions," Opt. Express 19, A319-A325 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-S3-A319


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References

  1. T. Ogino and M. Aoki, “Mechanism of Yellow Luminescence in GaN,” Jpn. J. Appl. Phys. 19(12), 2395–2405 (1980). [CrossRef]
  2. R. Birkhahn, M. Garter, and A. J. Steckl, “Red light emission by photoluminescence and electroluminescence from Pr-doped GaN on Si substrates,” Appl. Phys. Lett. 74(15), 2161–2163 (1999). [CrossRef]
  3. A. J. Steckl, M. Garter, D. S. Lee, J. Heikenfeld, and R. Birkhahn, “Blue emission from Tm-doped GaN electroluminescent devices,” Appl. Phys. Lett. 75(15), 2184–2186 (1999). [CrossRef]
  4. D. S. Lee, J. Heikenfeld, R. Birkhahn, M. Garter, B. K. Lee, and A. J. Steckl, “Voltage-controlled yellow or orange emission from GaN codoped with Er and Eu,” Appl. Phys. Lett. 76(12), 1525–1527 (2000). [CrossRef]
  5. D. S. Lee and A. J. Steckl, “Enhanced blue and green emission in rare-earth-doped GaN electroluminescent devices by optical photopumping,” Appl. Phys. Lett. 81(13), 2331–2333 (2002). [CrossRef]
  6. S. Limpijumnong, S. B. Zhang, S. H. Wei, and C. H. Park, “Doping by large-size-mismatched impurities: the microscopic origin of arsenic- or antimony-doped p-type zinc oxide,” Phys. Rev. Lett. 92(15), 155504 (2004). [CrossRef] [PubMed]
  7. B. Claflin, D. C. Look, S. J. Park, and G. Cantwell, “Persistent n-type photoconductivity in p-type ZnO,” J. Cryst. Growth 287(1), 16–22 (2006). [CrossRef]
  8. Y. I. Alivov, J. E. Van Nostrand, D. C. Look, M. V. Chukichev, and B. M. Ataev, “Observation of 430 nm electroluminescence from ZnO/GaN heterojunction lightemitting diodes,” Appl. Phys. Lett. 83(14), 2943–2945 (2003). [CrossRef]
  9. W. I. Park and G. C. Yi, “Electroluminescence in n-ZnO Nanorod Arrays Vertically Grown on p-GaN,” Adv. Mater. 16(1), 87–90 (2004). [CrossRef]
  10. D. J. Rogers, F. H. Teherani, A. Yasan, K. Minder, P. Kung, and M. Razeghi, “Electroluminescence at 375 nm from a ZnO/GaN:Mg/c-Al2O3 heterojunction light emitting diode,” Appl. Phys. Lett. 88(14), 141918 (2006). [CrossRef]
  11. M. C. Jeong, B. Y. Oh, M. H. Ham, and J. M. Myoung, “Electroluminescence from ZnO nanowires in n-ZnO film/ZnO nanowire array/p-GaN film heterojunction light-emitting diodes,” Appl. Phys. Lett. 88(20), 202105 (2006). [CrossRef]
  12. M. C. Jeong, B. Y. Oh, M. H. Ham, S. W. Lee, and J. M. Myoung, “ZnO-Nanowire-Inserted GaN/ZnO Heterojunction Light-Emitting Diodes,” Small 3, 568–572 (2007). [CrossRef] [PubMed]
  13. E. Lai, W. Kim, and P. Yang, “Vertical Nanowire Array-Based Light Emitting Diodes,” Nano Res. 1(2), 123–128 (2008). [CrossRef]
  14. J. Y. Lee, J. H. Lee, H. S. Kim, C. H. Lee, H. S. Ahn, H. K. Cho, Y. Y. Kim, B. H. Kong, and H. S. Lee, “A study on the origin of emission of the annealed n-ZnO/p-GaN heterostructure LED,” Thin Solid Films 517(17), 5157–5160 (2009). [CrossRef]
  15. A. M. C. Ng, Y. Y. Xi, Y. F. Hsu, A. B. Djurisić, W. K. Chan, S. Gwo, H. L. Tam, K. W. Cheah, P. W. K. Fong, H. F. Lui, and C. Surya, “GaN/ZnO nanorod light emitting diodes with different emission spectra,” Nanotechnology 20(44), 445201 (2009). [CrossRef] [PubMed]
  16. X. M. Zhang, M. Y. Lu, Y. Zhang, L. J. Chen, and Z. L. Wang, “Fabrication of a High-Brightness Blue-Light-Emitting Diode Using a ZnO-Nanowire Array Grown on p-GaN Thin Film,” Adv. Mater. 21(27), 2767–2770 (2009). [CrossRef]
  17. C. H. Chen, S. J. Chang, S. P. Chang, M. J. Li, I. C. Chen, T. J. Hsueh, and C. L. Hsu, “Electroluminescence from n-ZnO nanowires/p-GaN heterostructure light-emitting diodes,” Appl. Phys. Lett. 95(22), 223101 (2009). [CrossRef]
  18. H. Sirringhaus, P. J. Brown, R. H. Friend, M. M. Nielsen, K. Bechgaard, B. M. W. Langeveld-Voss, A. J. H. Spiering, R. A. J. Janssen, E. W. Meijer, P. Herwig, and D. M. de Leeuw, “Two-dimensional charge transport in self-organized, high-mobility conjugated polymers,” Nature 401(6754), 685–688 (1999). [CrossRef]
  19. Y. Kim, S. Cook, S. M. Tuladhar, S. A. Choulis, J. Nelson, J. R. Durrant, D. D. C. Bradley, M. Giles, I. Mcculloch, C. S. Ha, and M. Ree, “A strong regioregularity effect in self-organizing conjugated polymer films and high-efficiency polythiophene:fullerene solar cells,” Nat. Mater. 5(3), 197–203 (2006). [CrossRef]
  20. G. Dennler, M. C. Scharber, and C. J. Brabec, “Polymer-Fullerene Bulk-Heterojunction Solar Cells,” Adv. Mater. 21(13), 1323–1338 (2009). [CrossRef]
  21. B.-N. Park, J. J. Uhlrich, T. F. Kuech, and P. G. Evans, “Electrical properties of GaN/poly(3-hexylthiophene) interfaces,” J. Appl. Phys. 106(1), 013713 (2009). [CrossRef]
  22. D. C. Olson, S. E. Shaheen, M. S. White, W. J. Mitchell, M. F. A. M. van Hest, R. T. Collins, and D. S. Ginley, “Band-Offset Engineering for Enhanced Open-Circuit Voltage in Polymer–Oxide Hybrid Solar Cells,” Adv. Funct. Mater. 17(2), 264–269 (2007). [CrossRef]
  23. T. R. B. Foong, Y. Shen, X. Hu, and A. Sellinger, “Template-Directed Liquid ALD Growth of TiO2 Nanotube Arrays: Properties and Potential in Photovoltaic Devices,” Adv. Funct. Mater. 20(9), 1390–1396 (2010). [CrossRef]
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