Organic-Inorganic Nanoscale Optoelectronics

Department of Electrical and Electronic Engineering, The University of Hong Kong

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High Performance: Dual Plasmonic Solar Cells

X.H. Li, W. C.H. Choy*, L Huo, F.X. Xie, W.E.I. Sha, B. Ding, X. Guo, Y. Li, J. Hou* J. You, Y. Yang, "Dual Plasmonic Nanostructures for High Performance Inverted Organic Solar Cells", Adv. Mater., Adv. Mater. vol. 24, pp.3046-3052, 2012.


Enhancement of Performance: Plasmons

Metallic NPs in both PEDOT and P3HT: PCBM


F.X. Xie, W.C.H. Choy*, C.C.D. Wang, W.E.I. Sha, and D.D.S. Fung, Appl. Phys. Lett., vol. 99, p. 153304, 2011. (selected for the Oct 2011 issue of APL: Organic Electronics and Photonics)
(selected for the Oct 24, 2011 issue of Virtual Journal of Nanoscale Science & Technology)


Metallic NPs in PEDOT:PSS (carrier transport layer)

D.D.S. Fung, L. Qiao, W.C.H. Choy*, C.C.D. Wang, W.E.I. Sha, F. Xie, and S. He, J. Mater. Chem., vol. 21, pp. 16349 – 16356, 2011.
(selected as the back-cover page highlighted articles of an issue in 2011 of J. Mater. Chem.)
(identified as a 'hot article' for Journal of Materials Chemistry and highlighted on http://blogs.rsc.org/jm/)


Multiphysics Modeling of Solar Cells

W.E.I. Sha, W.C.H. Choy*, Y.M. Wu, and W.C. Chew, Opt. Express, 20(3), 2572-2580, 2012.


Hybrid Organic/Nanoparticle Devices with
Strong Resonant Tunneling Behavior (cont’d)

W.C.H. Choy and T.H. Zheng, “Hybrid Organic/Nanoparticle Devices”, U.S. Patent # 61/097,405.
T.H. Zheng, W. C. H. Choy, and Y.X. Sun, Adv. Function. Mat., vol. 19, pp.2648-2653, 2009.
T.H. Zheng, W. C. H. Choy, and Y.X. Sun, Appl. Phys. Lett, Appl. Phys. Lett, vol. 94, 123303 (pp.3), 2009.


Angular Response of Thin-Film Organic Solar Cells With Periodic
Metal Back Nanostrips

Wei E.I. Sha, W.C.H. Choy, and Weng Cho Chew, 2010.


Using Au nanoclusters (no PEDOT) as hole collection interface layer

W.C.H. Choy, 2010.


Magnetic Field Effects on the Electroluminescence of OLEDs:
A Tool to Indicate the Carrier Mobility


B. F. Ding, Y. Yao, Z. Y. Sun, C. Q. Wu, X. D. Gao, Z. J. Wang, X. M. Ding, W. C. H. Choy*, X. Y. Hou,
Appl. Phys. Lett., vol. 97, p,163302 (3 pp), 2010.

Magnetic field can modulate EL of OLEDs, namely magneto electroluminescence (MEL). MEL here is quantitatively expressed as EL/EL=[(EL(B)-EL(0)]/EL(0), where EL(B) and EL(0) are the EL with and without external magnetic field B respectively. As shown in Figure 1, △EL/EL increase at low magnetic field (<15mT) and then saturated at high magnetic field (>24mT).

Question: Why does smaller magnetic field leads to larg MEL? Why does the insertion of an insulating layer LiF enhances MEL? Why do the minority carriers dominate the magnetic field effect in organic semiconductors? These form one basic issue, namely what determines the intensity of MEL in a given magnetic field?

Considering that exciton density in given driving current weakly depends on concentration of NPB in blended layer when no other materials are introduced, so it is ideal to use blended layer based OLED to study the MEL

Results: Concentration Effect
As shown in Figure 2, when the concentration of NPB of the blended layer increases, the △EL/EL ?first increases and then decreases. The maximum value of △EL/EL occurs at the blended ratio of 30%. This trend opposes that of the mobility of the devices (mobility taken from APL 91 142206 (2007).


Enhancement of Solar Cell Performance: Plasmons

Wei E.I. Sha, Wallace C.H. Choy, and Weng Cho Chew, Optics Express, Vol. 18, p.5993, 2010.

A comprehensive study of the plasmonic thin-film solar cell with the periodic strip structure is presented in this paper. The finite-difference frequency-domain method is employed to discretize the inhomogeneous wave function for modeling the solar cell. In particular, the hybrid absorbing boundary condition and the one-sided difference scheme are adopted. The parameter extraction methods for the zeroth-order reflectance and the absorbed power density are also discussed, which is important for testing and optimizing the solar cell design. For the numerical results, the physics of the absorption peaks of the amorphous silicon thin-film solar cell are explained by electromagnetic theory; these peaks correspond to the waveguide mode, Floquet mode, surface plasmon resonance, and the constructively interference between adjacent metal strips. The work is therefore important for the theoretical study and optimized design of the plasmonic thin-film solar cell.


High Performance OLEDs: Efficiency Enhancement

T. Zheng, and W. C. H. Choy, Adv. Function. Mat., DOI: 10.1002/adfm.200901657.

This paper presents a new strategy to develop efficient organic light-emitting devices (OLEDs) by doping fluorescent- and phosphorescent-type emitters individually into two different hosts separated by an interlayer to form a fluorescence-interlayer-phosphorescence (FIP) emission architecture. One blue OLED with FIP emission structure comprising p-bis(p-N,N-diphenylaminostyryl)benzene (DSA-Ph) and bis[(4,6-di-fluorophenyl)-pyridinate-N,C2']picolinate (FIrpic) exhibiting a peak luminance efficiency of 15.8cd A1 at 1.54?mA cm2 and a power efficiency of 10.2 lm W1 at 0.1mA cm2 is successfully demonstrated. The results are higher than those of typical phosphorescent OLEDs with a single emission layer by 34% and 28%, respectively. From experimental and theoretical investigations on device performance, and the functions of the used emitters and interlayer, such enhancement should ascribe to the appropriate utilization of the two types of emitters. The fluorescent emitter of DSA-Ph is used to facilitate the carrier transport, and thus accelerate the generation of excitons, while the phosphorescent emitter of FIrpic could convert the generated excitons into light efficiently. The method proposed here can be applied for developing other types of red, green, and white OLEDs.


High Performance OLEDs: Efficiency Roll-off Reduction

T.H. Zheng, W. C. H. Choy, C.L. Ho and W.Y. Wong, Appl. Phys. Lett, vol.95, 133304 (pp.3), 2009.
T. Zheng, and W. C. H. Choy, Thin Solid Film, accepted.

Organic light emitting devices (OLEDs) with a fluorescence-interlayer-phosphorescence emission layer structure (FIP EML) has been proposed to solve the efficiency roll-off issue effectively. Efficient green OLED based on FIP EML exhibiting only 26% roll-off in the luminance efficiency, which is lower than the typical roll-off of 51% for conventional phosphorescent OLEDs with single EML operated at 5-150mA/cm2 range, has been demonstrated. Such enhancement should be attributed to the improved carrier balance, the exciton redistribution in recombination zone, the suppression of nonradiative exciton quenching processes, and the elimination of energy transfer loss offered by the FIP EML structure.


Hybrid Organic/Nanoparticle Devices with
Record-high Resonant Tunneling Behavior

T.H. Zheng, W. C. H. Choy, and Y.X. Sun, Adv. Function. Mat., DOI: 10.1002/adfm.200900308.
T.H. Zheng, W. C. H. Choy, and Y.X. Sun, Appl. Phys. Lett, Appl. Phys. Lett, vol. 94, 123303 (pp.3), 2009..

A hybrid nanoparticle/organic device consisting of small molecule organic semiconductors and Ag nanoparticles is reported. The single device exhibits unusual properties of organic resonant tunneling diode (ORTD) at low driving voltage region and offers light emission at high voltage. For ORTD, a strong negative differential resistance behavior is demonstrated at room temperature. The current resonance with the peak-to-valley current ratio of over 4.6 and narrow linewidth of only ~1.4 V is achieved. A detailed operating mechanism of the charging and emission modes is proposed, which can be discussed in terms of the strong charge-trapping effect of Ag nanoparticles. The repeatable operations of hybrid device show the mutual influences between two modes and the light emission properties of the ORTD are also discussed.


Detailed Studies of Absolute Optical Properties
(Refractive Index and Absorption Coefficient)

W.C.H. Choy, and H.H. Fong, J. Phys. D, vol. 41, p.155109 (7 pp.), 2008.

The dispersive absorption coefficient and refractive index are the key optical functions in optimizing the performance of organic optoelectronic devices. The optical functions of organic materials have been generally determined and reported by, for example, using ellipsometry and photoluminescence excitation. However, these methods cannot provide much physical understanding of the optical functions. In addition, conclusive studies on the origins of the optical functions of the organic materials are limited. In this paper, we first determine the absolute optical functions by using ellipsometry against different oscillator models. We can then find out the better oscillator model to be used with ellipsometry together for determining the experimental optical functions. The next step is to determine and explain the origins of the experimentally determined optical functions by investigating the molecular orbitals and electronic transitions of organic molecules through time-dependent density functional theory. This comprehensive study is conducted on two popular materials of tris(8-hydroxyquinoline) aluminium (Alq3) and 4,4' bis[N-(1-napthyl)-N-phenyl-amino]-biphenyl (NPD).


Real-time Color Tunable Organic LEDs

pic

C.J. Liang, W.C.H. Choy Appl. Phys Lett, vol. 89, pp.251108, 2006
H.M. Zhang and W.C.H. Choy, IEEE Photon Technol. Lett., vol. 20, pp. 1154-1156, 2008
H.M. Zhang, Wallace C.H. Choy, J. Phys. D (fast track), vol. 41, 062003(4 pp.), 2008.
Wallace C. H. Choy, J.H. Niu, W.L. Li, P. C. Chui, J. Phys. D, vol. 41 p. 025106, 2008.

Using the rare-earth special feature of a sharp emission spectrum, voltage-controlled continuous color tuning of organic light-emitting diodes is achieved. Europium(dibenzoylmethanato)3(bathophenanthroline) is used as the strategic starting point close to the red corner of the Commission International de I’Eclairage chromaticity diagram for a wide color tuning. The end point and path of the color tuning can be engineered by doping the hole-transport emitting layer with dyes. The mechanisms of color tuning have been investigated and explained by the efficiency reduction of the europium complex and the extension of carrier recombination zone with driving voltage. The effect of exciplex on the color tuning is also studied.


High Performance OLEDs:
Theoretical Analysis of Device Structures


X.W. Chen, W.C.H.Choy, et al, IEEE Display Technol., vol.3, pp.110-117, 2007
W.C.H. Choy and Y.C. Ho, OSA Opt. Express, vol. 15, pp. 13288-13294, 2007

The changes of emission peak wavelength and angular intensity with viewing angles have been issues for the use of microcavity OLEDs. We will investigate Distributed Bragg Gratings (DBRs) constructed from largely dispersive index materials for reducing the viewing angle dependence. A DBR stack mirror, aiming at a symmetric structure and less number of grating period for a practical fabrication, is studied to achieve a chirp-featured grating for OLEDs with blue emission peak of 450nm. For maximizing the compensation of the viewing angle dependence, the effects of dispersive index, grating structure, thickness of each layer of the grating, grating period and chirp will be comprehensively investigated. The contributions of TE and TM modes to the angular emission power will be analyzed for the grating optimization, which have not been expressed in detail. In studying the light emission of OLEDs, we will investigate the Purcell effect which is important but has not been properly considered. Our results show that with a proper design of the DBR, not only a wider viewing angle can be achieved but also the color purity of OLEDs can be improved.


Extract Critical Parameters of LEDs
Internal Quantum Efficiency

X.W.Chen, W. C. H. Choy, et al., Appl. Phys Lett., vol. 91, p.221112, 2007.

A comprehensive analysis is given on the modifications of the exciton lifetime and internal quantum efficiency (ηint) for organic light-emitting devices (OLEDs). A linear relation is derived between the exciton lifetime and ηint, which is difficult to measure directly. The internal quantum efficiency can thus be estimated easily through the measurement of the exciton lifetime. The exciton lifetimes for OLEDs with weak or strong microcavity are studied experimentally and theoretically. The modification of the exciton lifetime is well explained through the microcavity effect and surface plasmon resonance. An excellent agreement between the experimental and theoretical results is achieved.