Analysis of Polarization Independent Diffused QW Optical Amplifiers

The features of polarization independence of optical amplifiers achieved by using diffused QW are discussed. Our theoretical results successfully explain why polarization independence can achieve in the long wavelength tail of the modal gain and absorption coefficient but not at photon energies above the transition edge. This explanation applies to other tensile-strained QW for polarization independent applications. The understanding is crucial for optimizing polarization independent devices.published_or_final_versio

Optical design of organic/polymer solar cells and light emitting devices

InvitedIn this talk, we will study the effects of the microcavity structures and plasmonic resonances on the quantum efficiencies and photon lifetime. Theoretical and experimental results will be discussed on organic LEDs and solar cells (SCs). © 2011 IEEE.published_or_final_versionThe 11th International Conference on Numerical Simulation of Optoelectronic Devices (NUSOD 2011), Rome, Italy, 5-8 September 2011. In Proceedings of the 11th NUSOD, 2011, p. 1-

Organic Solar Cells with Efficiency of 9% Achieved by Using Multiple Plasmonic Nanostructures

Session 3P4b SC2: Plasmonic Nanomaterials and Nanostrcutures for Photovoltaics and Optoelectronics in Energy 1Polymer-fullerene-based bulk heterojunction (BHJ) solar cells have many advantages including low-cost, low-temperature fabrication, semi-transparency, and mechanical flexibility [1, 2]. However, there is a mismatch between optical absorption length and charge transport scale [3, 4]. These factors lead to recombination losses, higher series resistances and lower fill factors. Attempts to optimize both the optical and electrical properties of the photoactive layer in organic solar cells (OSCs) inevitably result in a demand to develop a device architecture that can enable efficient optical absorption in films thinner than optical absorption length [5, 6]. Here, we report the use of multiple metallic nanostructures to achieve the broad light absorption enhancement, increased short-circuit circuit (Jsc) and improved fill factor (FF) simultaneously based on a new small-bandgap polymer donor of poly {[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2- b:4,5-b’]dithiophene-2,6-diyl]-alt-[2-(2-ethyl-hexanoyl)-thieno[3,4-b]thiophen-4,6-diyl]} (PBDTTT-C-T) in BHJ cells [7]. The multiple metallic nanostructure consists of 2D arrays of metallic nanograting electrode as a back reflector and metallic nanoparticles (NPs) with different geometries embedded into the active layer. Apart from the waveguide modes and diffractions, we simultaneously introduce hybridized surface plasmonic resonances (from Ag nanograting) and localized plasmonic resonances (from Au and Ag NPs [8]) to successfully achieve a broadband absorption enhancement. The detail understanding has been described with our theoretically studies. Consequently, we improve PCE to ∼9% [9] by improving both optical and electrical properties of single-junction OSCs through introducing dual plasmonic nanostructures which contribute to the practical application of OSCs for photovoltaics.published_or_final_versio

New concept to break the intrinsic properties of organic semiconductors for optical sensing applications

Invited paper - Session 9: Radiation-to-Current Transducers: Engineering Materials to Increase Current Generation - no. 9608-44The space charge limit (SCL) effect is a universal phenomenon in semiconductor devices involving light emitting diodes, solar cells, and photodetectors. Typically, the SCL will exist in the condition of (1) unbalanced hole and electron mobility; (2) thick active layer; (3) high light intensity or dense photocarriers (electrons and holes) generation; and (4) moderate reverse bias. Through the study of plasmonic organic solar cells, we will show metallic nanostructures go beyond their optical functions to control recombination, transport, and collection of photocarriers generated from active organic materials. Through spatially redistributing light absorption at the active layer, the proposed plasmonic-electrical concept is fundamentally different from the hot carrier effect where photocarriers are generated from metallic nanostructures. The new plasmonic-electrical effect not only lays a physical foundation but also upgrades electrical properties for semiconductor devices [1]. We will also design different device structures to investigate and demonstrated how plasmonic-electrical [2] and plasmonic-optical [3] effects can be used to enhance device performances such as improving the light absorption of solar cells, increasing emission efficiency of light emitting devices, reducing dark current and enhancing sensitivity of photodetector as well as intensifying the surface enhanced Raman scattering for biosensor applications. Besides the optical (plasmonic) resonances from metal nanostructure, we will also use metal nanostructures to demonstrate electrical resonance which can be used for bistable and memory devices [4]. Consequently, exploiting both plasmonic-optical and plasmonic-electrical effects via metallic nanostructures will open up a more flexible and integrated way to design high-performance optoelectronic nanodevices. [1] W.E.I. Sha, X. Li, W.C.H. Choy, Scientific Reports, vol. 4, p. 6236 (10pp), 2014. [2] F.X. Xie, W.C.H. Choy, W.E.I. Sha, D. Zhang, S. Zhang, X. Li, C.W. Leung, J. Hou, Energy Environ. Sci., vol. 6, pp.3372 – 3379, 2013; D. Zhang, W.C.H. Choy, F. Xie, W.E.I. Sha, X. Li, B. Ding, K. Zhang, F. Huang, and Y. Cao, Adv. Funct. Mat., vol. 23, pp.4255–4261, 2013; 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. [3] X.H. Li, W.C. H. Choy, X. Ren, D. Zhang, H.F. Lu, Adv. Funct. Mat. DOI: 10.1002/adfm.201303384; X.H.Li, W.C.H.Choy, H.F. Lu, W.E.I. Sha, and H. P. Ho, Adv. Funct. Mat., vol.23, pp.2728–2735, 2013; 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, Adv. Mater. vol. 24, pp.3046-3052, 2012; X.H. Li, W. E.I. Sha, W.C.H. Choy, D.D.S. Fung, F. X. Xie, J. of Phys. Chem. C, vol. 116, pp.7200-7206, 2012; C.C.D. Wang, W. C. H. Choy, C. Duan, D.D.S. Fung, W.E.I. Sha, F.X. Xie, F. Huang, and Y. Cao, J. Mater. Chem., vol. 22, pp.1206–1211, 2012. [4] T.H. Zheng, W.C.H. Choy, and Y.X. Sun, vol. 19, pp.2648-2653, 2009; T.H. Zheng, W.C.H. Choy, and Y.X. Sun, Appl. Phys. Lett, vol. 94, 123303 (pp.3), 2009.published_or_final_versio

Real-time color-tunable electroluminescence from stacked organic LEDs using independently addressable middle electrode

Independently controllable stacked organic light emitting devices (OLEDs) are fabricated by using interconnecting electrode of Al (2 nm)/ WO3 (3 nm)/Au (16 nm) for connecting two primary color OLED units of blue and red. The middle electrode simultaneously functions as the cathode and anode for the bottom and top units respectively with a feature of over 60% optical transmission in a wide wavelength range from 500 to 700 nm such that the color can be tuned in real time from red to blue by changing the bias voltage to the two units. The undistorted primary colors and high efficiency have been obtained through optimizing structure and properly arranging the ordering of the blue and red subpixels. © 2008 IEEE.published_or_final_versio

Improved performance of vertical-cavity modulator through the use of diffused quantum wells

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Theoretical Analysis of Diffused Quantum-well Lasers and Optical Amplifiers

Diffused quantum-well (QW) distributed feedback (DFB) lasers and optical amplifiers will be theoretically analyzed in this paper. For DFB lasers, a design rule will be proposed and the validity of the design rule will be discussed with respect to changes in the injected carrier density. The range of grating period, which can be used in the design, is discussed. As a consequence, the maximum tuning range of the emission wavelength can be estimated without involving the time-consuming self-consistent simulation. The features of polarization independence of optical amplifiers achieved by using diffused QWs are also discussed. Our theoretical results successfully explain why polarization independence can achieve in the long-wavelength tail of the modal gain and absorption coefficient but not at photon energies above the transition edge. This explanation applies to other tensile-strained QWs for polarization-independent applications. The understanding is crucial for optimizing polarization-independent devices. To conclude, our analysis of the diffused QW optical devices demonstrates that QW intermixing technology is a practical candidate for not only realizing monolithic photonic integrated circuit, but also enhancing optical device performance.published_or_final_versio

Interdiffused AlGaAs-GaAs quantum well for improved electroabsorptive modulation

This is a theoretical study of the effects of two asgrown structural parameters on the modulation properties of Al xGa 1-xAs-GaAs quantum wells (QW's), which are the Al concentration in barrier and the thickness of the well layer serving as initial conditions before interdiffusion. The results show that, with a larger Al concentration and a wider well width, the range of interdiffusion for an enhanced electroabsorption (EA) change increases with both of these parameters, while insertion loss increases with the former and decreases with the latter. However, the increase in loss is lower than that of the rectangular QW for the same magnitude of absorption change. The range of a tunable absorption-peak wavelength produced by interdiffussion increases with increasing Al concentration and decreases with increasing well width. Moreover, in a moderately interdiffused QW, the required bias reduces for the same level of EA modulation. For the best device operation, interdiffused QW's with the Al concentration between 0.3 and 0.4 and well width between 10 and 12 nm are must suitable for developing a general-purpose electroabsorptive modulator. When applied in high-speed modulators, the EA of a wide and shallow QW active-region structure can be further enhanced by the use of corresponding interdiffusion.published_or_final_versio

Indium Tin oxide modified by Au and vanadium pentoxide as an efficient anode for organic light-emitting devices

Au/Vanadium pentoxide (V2O5) films on indium tin oxide (ITO) as composite anodes for hole injection in organic light-emitting devices (OLEDs) have been investigated. The device with (6 nm)/V2O5(6 nm) anode shows improved current density-voltage characteristics as compared with the device with ITO/Au as anode. Hole injection is significantly reduced when Au was added on ITO as an anode. However, while a thin V2O5 film is deposited on a Au anode, the barrier height is substantially reduced. The hole injection is facilitated, and the driving voltage of the device decreased by 10 V. In addition, the maximum current efficiency for the ITO/Au/V2O5 OLED is ∼3.5 cd/A, which is higher than that of the ITO/Au anode device (∼1.1 cd/A) and ITO/V2O5 OLED of ∼2.8 cd/A. © 2008 IEEE.published_or_final_versio

The applications of an interdiffused quantum well in a normally on electroabsorptive fabry-perot reflection modulator

A Fabry-Perot reflection-type modulator which uses interdiffused AlGaAs/GaAs quantum wells as the active cavity material has been studied and optimized theoretically. An asymmetric Bragg reflector structure (modeled by transfer matrices), with a doped depletion layer in the heterostructure, has been considered. This is the first study to model such a material system in this type of modulator, and the results show improvement in modulation property over its as-grown rectangular quantum-well modulator. In particular, the change of reflectance in the diffused quantum-well modulator is almost 0.6 to 0.7, which is higher than that of the typically available values (∼0.5 to 0.6), while the OFF-state on-resonance reflectance is almost close to zero. The operation voltage is also reduced by more than half as the interdiffusion becomes extensive. The finesse of the more extensively diffused quantum well also increases. Both of these features contribute to an improvement of the change of reflectance in the modulator. The operation wavelengths can be adjusted over a range of 100 nm. However, the absorption coefficient change of the diffused quantum well increases only when there is a small amount of interdiffusion.published_or_final_versio