Jiangeng Xue

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Organization: University of Florida

Department: Department of Materials Science and Engineering

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Organic Chemistry

Organic Solar Cells

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References

Efficient and long-lifetime full-color light-emitting diodes using high luminescence quantum yield thick-shell quantum dots

Co-reporter:Huaibin Shen;Qingli Lin;Weiran Cao;Chenchen Yang;Nathan T. Shewmon;Hongzhe Wang;Jinzhong Niu;Lin Song Li

Nanoscale (2009-Present) 2017 vol. 9(Issue 36) pp:13583-13591

Publication Date(Web):2017/09/21

DOI:10.1039/C7NR04953F

We report full-color quantum-dot-based light-emitting diodes (QLEDs) with high efficiency and long-lifetime by employing high quantum-yield core/shell QDs with thick shells. The increased shell thickness improves the confinement of excitons in the QD cores, and helps to suppress Auger recombination and Förster resonant energy transfer among QDs. Along with optimizing the QD emitting layer thickness and hole transport materials, we achieved significant improvements in device performance as a result of increasing the QD shell thickness to above 5 nm. By using poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB) as a HTL with a 38 nm thick QD layer, these QLEDs show maximum current efficiencies of 18.9 cd A−1, 53.4 cd A−1, and 2.94 cd A−1, and peak external quantum efficiencies (EQEs) of 10.2%, 15.4%, and 15.6% for red, green, and blue QLEDs, respectively, all of which are well maintained over a wide range of luminances from 102 to 104 cd m−2. To the best of our knowledge, this is the first report of blue QLEDs with ηEQE > 15%. Most importantly, these devices also possess long lifetimes with T70 (the time at which the brightness is reduced to 70% of its initial value) of 117 h (red, with an initial luminance of 8000 cd m−2), 84 h (green, 6000 cd m−2) and 47 h (blue, 420 cd m−2). With further optimization of QD processing and device structures, these LEDs based on thick-shell QDs show great promise for use in next-generation full-color displays and lighting devices.

High-Efficiency, Low Turn-on Voltage Blue-Violet Quantum-Dot-Based Light-Emitting Diodes

Co-reporter:Huaibin Shen, Weiran Cao, Nathan T. Shewmon, Chenchen Yang, Lin Song Li, and Jiangeng Xue

Nano Letters 2015 Volume 15(Issue 2) pp:1211-1216

Publication Date(Web):January 12, 2015

DOI:10.1021/nl504328f

We report high-efficiency blue-violet quantum-dot-based light-emitting diodes (QD-LEDs) by using high quantum yield ZnCdS/ZnS graded core–shell QDs with proper surface ligands. Replacing the oleic acid ligands on the as-synthesized QDs with shorter 1-octanethiol ligands is found to cause a 2-fold increase in the electron mobility within the QD film. Such a ligand exchange also results in an even greater increase in hole injection into the QD layer, thus improving the overall charge balance in the LEDs and yielding a 70% increase in quantum efficiency. Using 1-octanethiol capped QDs, we have obtained a maximum luminance (L) of 7600 cd/m2 and a maximum external quantum efficiency (ηEQE) of (10.3 ± 0.9)% (with the highest at 12.2%) for QD-LEDs devices with an electroluminescence peak at 443 nm. Similar quantum efficiencies are also obtained for other blue/violet QD-LEDs with peak emission at 455 and 433 nm. To the best of our knowledge, this is the first report of blue QD-LEDs with ηEQE > 10%. Combined with the low turn-on voltage of ∼2.6 V, these blue-violet ZnCdS/ZnS QD-LEDs show great promise for use in next-generation full-color displays.

Enhancement in Organic Photovoltaic Efficiency through the Synergistic Interplay of Molecular Donor Hydrogen Bonding and -Stacking

Co-reporter:Nathan T. Shewmon;Davita L. Watkins;Johan F. Galindo;Raghida Bou Zerdan;Jihua Chen;Jong Keum;Adrian E. Roitberg;Ronald K. Castellano

Advanced Functional Materials 2015 Volume 25( Issue 32) pp:5166-5177

Publication Date(Web):

DOI:10.1002/adfm.201501815

For organic photovoltaic (OPV) cells based on the bulk heterojunction (BHJ) structure, it remains challenging to rationally control the degree of phase separation and percolation within blends of donors and acceptors to secure optimal charge separation and transport. Reported is a bottom-up, supramolecular approach to BHJ OPVs wherein tailored hydrogen bonding (H-bonding) interactions between π-conjugated electron donor molecules encourage formation of vertically aligned donor π-stacks while simultaneously suppressing lateral aggregation; the programmed arrangement facilitates fine mixing with fullerene acceptors and efficient charge transport. The approach is illustrated using conventional linear or branched quaterthiophene donor chromophores outfitted with terminal functional groups that are either capable or incapable of self-complementary H-bonding. When applied to OPVs, the H-bond capable donors yield a twofold enhancement in power conversion efficiency relative to the comparator systems, with a maximum external quantum efficiency of 64%. H-bond promoted assembly results in redshifted absorption (in neat films and donor:C₆₀ blends) and enhanced charge collection efficiency despite disparate donor chromophore structure. Both features positively impact photocurrent and fill factor in OPV devices. Film structural characterization by atomic force microscopy, transmission electron microscopy, and grazing incidence wide angle X-ray scattering reveals a synergistic interplay of lateral H-bonding interactions and vertical π-stacking for directing the favorable morphology of the BHJ.

Hydrogen bond directed assembly of oligothiophene/fullerene superstructures on Au(1 1 1)

Co-reporter:Xueying Zhao, Davita L. Watkins, Johan F. Galindo, Nathan T. Shewmon, Adrian E. Roitberg, Jiangeng Xue, Ronald K. Castellano, Scott S. Perry

Organic Electronics 2015 Volume 19() pp:61-69

Publication Date(Web):April 2015

DOI:10.1016/j.orgel.2015.01.022

•Tailored H-bonds were used to control supramolecular assembly of donor molecules.•STM studies confirmed the formation of ordered H-bonded trimer rosettes on Au(1 1 1).•DFT calculations facilitated the structural understanding of H-bonded assembly.•Observed templated growth of C60 on ordered monolayer of H-bonded donor molecules.We previously reported that a branched quaterthiophene donor chromophore functionalized with a phthalhydrazide hydrogen bonding (H-bonding) unit (MeBQPH) gives twofold more efficient bulk heterojunction organic solar cells (with C60 acceptors) compared to a nearly identical donor incapable of H-bonding (MeBQPME). Here, scanning tunneling microscopy (STM) studies confirm the formation of MeBQPH trimer rosettes on Au(1 1 1) through phthalhydrazide H-bonding interactions that are sufficiently strong to compete with adsorbate/substrate interactions. The MeBQPME comparator molecule void of hydrogen bonding functionality does not similarly assemble on the metal surface. Complementary density functional theory (DFT) calculations facilitate a structural understanding of the MeBQPH donor assemblies and their strong stabilization through formation of six hydrogen bonds. STM studies also reveal the templated growth of C60 on ordered MeBQPH monolayers with C60 molecules preferentially occupying the threefold interstitial sites of the MeBQPH monolayer. This work supports the idea that H-bonding interactions can be used to control the morphology of organic donor–acceptor blends to potentially create efficient and stable bulk heterojunction photovoltaic devices.

Recent progress in organic photovoltaics: device architecture and optical design

Co-reporter:Weiran Cao and Jiangeng Xue

Energy & Environmental Science 2014 vol. 7(Issue 7) pp:2123-2144

Publication Date(Web):17 Mar 2014

DOI:10.1039/C4EE00260A

Research on organic photovoltaic (OPV) materials and devices has flourished in recent years due to their potential for offering low-cost solar energy conversion. With a deepened understanding on the fundamental photovoltaic processes in organic electronic materials and the development of tailored materials and device architectures, we have seen a rapid increase in the efficiency of OPV devices to over 10%, which attracts tremendous commercial interests for further development and manufacturing. Here, we review recent progress in the field of organic photovoltaics, particularly on various innovative device architectures and optical designs to maximize the power conversion efficiency of OPV cells for a given set of photoactive donor and acceptor materials. Following an introduction of the basic device operation of organic photovoltaic cells and the advances in active materials, we firstly present different device architectures that have been used to optimize the charge generation and collection characteristics within the OPV devices. We then discuss various methods to manage and manipulate the light wave propagation in OPV devices for more complete absorption of the incident light, an important area that has been underexplored so far.

The Influence of Solubilizing Chain Stereochemistry on Small Molecule Photovoltaics

Co-reporter:Raghida Bou Zerdan;Nathan T. Shewmon;Yu Zhu;John P. Mudrick;Kyle J. Chesney;Ronald K. Castellano

Advanced Functional Materials 2014 Volume 24( Issue 38) pp:5993-6004

Publication Date(Web):

DOI:10.1002/adfm.201401030

Three stereochemically pure isomers and two isomeric mixtures of a solution-processable diketopyrrolopyrrole-containing oligothiophene (SMDPPEH) have been used to study the effect of 2-ethylhexyl solubilizing group stereochemistry on the film morphology and bulk heterojunction (BHJ) solar cell characteristics of small molecule organic photovoltaics. The different SMDPPEH stereoisomer compositions exhibit nearly identical optoelectronic properties in the molecularly dissolved state, as well as in amorphous films blended with PCBM. However, for films in which SMDPPEH crystallization is induced by thermal annealing, significant differences in molecular packing between the different stereoisomer formulations are observed. These differences are borne out in photovoltaic device characteristics for which unannealed devices show very similar behavior, while after annealing the RR- and SS-SMDPPEH enantiomers show blue-shifted peak EQE relative to the SMDPPEH isomer mixtures. Unannealed devices made from the most crystalline stereoisomer, meso RS-SMDPPEH, are not completely amorphous, and show improved photocurrent generation as a result. Unlike the other compounds, after thermal annealing the RS-SMDPPEH devices show reduced device performance. The results reveal that the chirality of commonly used 2-ethylhexyl solubilizing chains can have a significant effect on the morphology, absorption, and optimum processing conditions of small molecule organic thin films used as photovoltaic device active layers.

Consequences of hydrogen bonding on molecular organization and charge transport in molecular organic photovoltaic materials

Co-reporter:Benjamin M. Schulze, Nathan T. Shewmon, Jing Zhang, Davita L. Watkins, John P. Mudrick, Weiran Cao, Raghida Bou Zerdan, Anthony J. Quartararo, Ion Ghiviriga, Jiangeng Xue and Ronald K. Castellano

Journal of Materials Chemistry A 2014 vol. 2(Issue 5) pp:1541-1549

Publication Date(Web):04 Dec 2013

DOI:10.1039/C3TA13529B

Reported is a systematic molecular structure–property relationship study to evaluate the consequences of dedicated H-bonding interactions between molecular electron donors on molecular assembly, absorption, charge collection, and performance in small-molecule bulk heterojunction organic photovoltaic devices. Three families of branched quaterthiophene donor chromophores have been synthesized with members that share nearly identical electronic and optical properties in the molecularly dispersed state but are either capable or incapable of self-association by hydrogen bonding (H-bonding). Phthalhydrazide-functionalized quaterthiophenes are H-bond “active” and show signatures of H-bond promoted assembly in solution (by 1H NMR) and in both neat and blended (with C60) films (by IR). Compared to control compounds with H-bonding “turned off”, the H-bonded derivatives show red-shifted thin film absorption (neat and as blends with C60), different colors as bulk solids, and increased decomposition and melt temperatures. Photovoltaic devices made from blends of H-bonded donor molecules with C60 as the electron acceptor show improved charge collection length and external quantum efficiency resulting in a more than two-fold enhancement in power conversion efficiency relative to non-H-bonding controls, from 0.49% to 1.04%. We anticipate this approach could be generalized to include other donor chromophores with lower optical gap to harvest more longer-wavelength photons and achieve higher power conversion efficiencies.

Tandem small molecule organic photovoltaic cells with broad spectral response up to 1 μm and a high open-circuit voltage

Co-reporter:John P. Mudrick, Weiran Cao, Jian Li, Jiangeng Xue

Organic Electronics 2014 Volume 15(Issue 11) pp:3024-3030

Publication Date(Web):November 2014

DOI:10.1016/j.orgel.2014.08.047

•We demonstrate tandem OPV cells using two donors with complementary absorption.•Broad spectral response from below 400 nm to up to 1 μm is achieved.•Effective series connection leads to high open-circuit voltage of more than 1.5 V.•Optical modeling and selective light biasing are used to study subcell photocurrent.•Tuning the multilayer stack increases photocurrent from the current-limiting cell.We report a series-connected small molecule tandem photovoltaic cell utilizing two donors with complementary photovoltaic characteristics, lead phthalocyanine (PbPc) in the front subcell and boron subphthalocyanine chloride (SubPc) in the back subcell, to achieve both near infrared (NIR) response up to 1 μm and high open-circuit voltage (VOC) of more than 1.5 V in the same device. We find that the C60 layer thickness in the front subcell has a critical impact on the overall optical structure and photovoltaic performance of the tandem device. By combining transfer matrix calculations with subcell-selective spectral measurements, we are able to tune the optical field distribution inside the active layers and increase the photocurrent outputs from both subcells, leading to EQE > 30% over the wavelength range 400 nm < λ < 900 nm. This optimized tandem cell exhibits JSC = (5.5 ± 0.1) mA/cm2, fill factor = 0.54, VOC = 1.53 V, and a power conversion efficiency of (4.5 ± 0.2)%.

Ultraviolet-violet electroluminescence from highly fluorescent purines

Co-reporter:Yixing Yang, Pamela Cohn, Sang-Hyun Eom, Khalil A. Abboud, Ronald K. Castellano and Jiangeng Xue

Journal of Materials Chemistry A 2013 vol. 1(Issue 16) pp:2867-2874

Publication Date(Web):18 Feb 2013

DOI:10.1039/C3TC00734K

We report efficient ultraviolet (UV)-violet organic light-emitting devices (OLEDs) based on highly fluorescent donor–acceptor purine molecules, which can generate tunable emission from 350 nm to 450 nm in solution by using different electron donor and acceptor arrangements on the heterocycles as reported previously. Here, external quantum efficiencies (EQEs) up to ηEQE = 1.6% are achieved for the multilayer OLEDs based on purine 2, with UV emission peaked at 393 nm, as compared to previously reported purine 1 based OLEDs with ηEQE = 3.1% and peak emission at 433 nm. The efficiencies of the OLEDs based on the two purine molecules are among the highest reported to date with emission peak wavelengths below 450 nm. By using a range of charge transport and host materials, we show that appropriate energy level alignment in multilayer OLED devices is imperative to achieve UV emission and prevent undesired emission from other layers or interfaces.

Enhancing the Efficiency of Solution-Processed Polymer:Colloidal Nanocrystal Hybrid Photovoltaic Cells Using Ethanedithiol Treatment

Co-reporter:Renjia Zhou, Romain Stalder, Dongping Xie, Weiran Cao, Ying Zheng, Yixing Yang, Marc Plaisant, Paul H. Holloway, Kirk S. Schanze, John R. Reynolds, and Jiangeng Xue

ACS Nano 2013 Volume 7(Issue 6) pp:4846

Publication Date(Web):May 13, 2013

DOI:10.1021/nn305823w

Advances in colloidal inorganic nanocrystal synthesis and processing have led to the demonstration of organic–inorganic hybrid photovoltaic (PV) cells using low-cost solution processes from blends of conjugated polymer and colloidal nanocrystals. However, the performance of such hybrid PV cells has been limited due to the lack of control at the complex interfaces between the organic and inorganic hybrid active materials. Here we show that the efficiency of hybrid PV devices can be significantly enhanced by engineering the polymer–nanocrystal interface with proper chemical treatment. Using two different conjugated polymers, poly(3-hexylthiophene) (P3HT) and poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT), we show that treating the polymer:nanocrystal hybrid film in an ethanedithiol-containing acetonitrile solution can increase the efficiency of the hybrid PV devices by 30–90%, and a maximum power conversion efficiency of 5.2 ± 0.3% was obtained in the PCPDTBT:CdSe devices at 0.2 sun (AM 1.5G), which was slightly reduced to 4.7 ± 0.3% at 1 sun. The ethanedithiol treatment did not result in significant changes in the morphology and UV–vis optical absorption of the hybrid thin films; however, infrared absorption, NMR, and X-ray photoelectron spectroscopies revealed the effective removal of organic ligands, especially the charged phosphonic acid ligands, from the CdSe nanorod surface after the treatment, accompanied by the possible monolayer passivation of nanorod surfaces with Cd-thiolates. We attribute the hybrid PV cell efficiency increase upon the ethanedithiol treatment to the reduction in charge and exciton recombination sites on the nanocrystal surface and the simultaneous increase in electron transport through the hybrid film.Keywords: colloidal nanocrystals; conjugated polymers; interface engineering; photovoltaic cells; polymer:nanocrystal hybrid

A universal optical approach to enhancing efficiency of organic-based photovoltaic devices

Co-reporter:Jason D. Myers, Weiran Cao, Vincent Cassidy, Sang-Hyun Eom, Renjia Zhou, Liqiang Yang, Wei You and Jiangeng Xue

Energy & Environmental Science 2012 vol. 5(Issue 5) pp:6900-6904

Publication Date(Web):29 Feb 2012

DOI:10.1039/C2EE21254D

We report a new optical approach that can be used to enhance light harvesting in many different organic-based photovoltaic cells. A transparent polymer microlens array moulded on the light incident surface increases the light path in the active layer and reduces surface reflection, resulting in a 15–60% relative increase in overall cell efficiency.

Solution-processed, nanostructured hybrid solar cells with broad spectral sensitivity and stability

Co-reporter:Renjia Zhou, Ying Zheng, Lei Qian, Yixing Yang, Paul H. Holloway and Jiangeng Xue

Nanoscale 2012 vol. 4(Issue 11) pp:3507-3514

Publication Date(Web):20 Mar 2012

DOI:10.1039/C2NR30210A

Hybrid organic–inorganic solar cells, as an alternative to all-organic solar cells, have received significant attention for their potential advantages in combining the solution-processability and versatility of organic materials with high charge mobility and environmental stability of inorganic semiconductors. Here we report efficient and air-stable hybrid organic–inorganic solar cells with broad spectral sensitivity based on a low-gap polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and spherical CdSe nanoparticles. The solvents used for depositing the hybrid PCPDTBT:CdSe active layer were shown to strongly influence the film morphology, and subsequently the photovoltaic performance of the resulted solar cells. Appropriate post-deposition annealing of the hybrid film was also shown to improve the solar cell efficiency. The inclusion of a thin ZnO nanoparticle layer between the active layer and the metal cathode leads to a significant increase in device efficiency especially at long wavelengths, due to a combination of optical and electronic effects including more optimal light absorption in the active layer and elimination of unwanted hole leakage into the cathode. Overall, maximum power conversion efficiencies up to 3.7 ± 0.2% and spectral sensitivity extending above 800 nm were achieved in such PCPDTBT:CdSe nanosphere hybrid solar cells. Furthermore, the devices with a ZnO nanoparticle layer retained ∼70% of the original efficiency after storage under ambient laboratory conditions for over 60 days without any encapsulation.

Flexible organic solar cells using an oxide/metal/oxide trilayer as transparent electrode

Co-reporter:Weiran Cao, Ying Zheng, Zhifeng Li, Edward Wrzesniewski, William T. Hammond, Jiangeng Xue

Organic Electronics 2012 Volume 13(Issue 11) pp:2221-2228

Publication Date(Web):November 2012

DOI:10.1016/j.orgel.2012.05.047

Organic solar cells (OSCs) have attracted much attention as a clean and renewable energy convention system, owning to the low-cost and easy-processing nature of organic semiconductors. While indium tin oxide (ITO) is commonly used in OSCs as the transparent conductive electrode, the rising cost of indium, the high temperature process and the poor flexibility of ITO, make it incompatible with large-scale roll-to-roll manufacture of OSCs. In this paper, the MoO3/thin metal/MoO3 trilayer structure was used to replace the ITO electrode in OSCs. The optical and electrical properties of the trilayer were shown to depend on the material and thickness of the intermediate metal layer. The maximum power conversion efficiency of up to 2.5% under simulated 1 sun AM 1.5 solar illumination was achieved for OSCs based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), compared to a maximum efficiency of 3.1% for the ITO-based devices. Moreover, due to the flexible nature of the trilayer structure, the OSCs with the trilayer electrode exhibited good mechanical flexibility. The efficiency of the flexible device was only reduced by ∼6% from its original performance after 500 bending cycles with a bending radius of 1.3 cm. Therefore, the performance of the ITO-free devices on rigid/flexible substrates suggests that this oxide/metal/oxide trilayer electrode is a promising ITO replacement in OSCs.Graphical abstractHighlights► Oxide/thin metal/oxide trilayer structure shows high transparency and conductivity. ► Flexible organic solar cells with oxide/metal/oxide trilayer electrode are demonstrated. ► Oxide/metal/oxide trilayer electrodes exhibit good mechanical flexibility.

Enhancing photovoltaic response of organic solar cells using a crystalline molecular template

Co-reporter:Wei Zhao, John P. Mudrick, Ying Zheng, William T. Hammond, Yixing Yang, Jiangeng Xue

Organic Electronics 2012 Volume 13(Issue 1) pp:129-135

Publication Date(Web):January 2012

DOI:10.1016/j.orgel.2011.10.016

We demonstrate that a crystalline pentacene molecular templating layer considerably changes the morphology of the subsequently deposited lead phthalocyanine (PbPc) layer, resulting in an improved crystallinity at the early stages of growth of the PbPc film and a higher content of the triclinic phase. For bilayer PbPc (20 nm)/C60 (40 nm) organic solar cells with or without the pentacene templating layer, the use of the pentacene templating layer leads to a 48% enhancement in the short-circuit current without noticeably affecting the solar cell open-circuit voltage or fill factor. A copper or zinc phthalocyanine molecular templating layer also leads to enhanced photovoltaic response from the PbPc/C60 cells, though less significant than the pentacene template. The improved device performance originates from stronger absorption by the triclinic PbPc phase in the near infrared and the enhanced internal quantum efficiency over the entire spectrum where PbPc absorbs.Graphical abstractHighlights► Nucleation and growth of PbPc films is significantly modified by molecular templates. ► Higher content of infrared absorbing PbPc phase is obtained with pentacene template. ► A 40–50% higher PbPc/C60 cell efficiency is obtained with a pentacene template.

Hybrid PolymerNanocrystal Materials for Photovoltaic Applications

Co-reporter:Renjia Zhou ; Jiangeng Xue

ChemPhysChem 2012 Volume 13( Issue 10) pp:2471-2480

Publication Date(Web):

DOI:10.1002/cphc.201101016

Abstract

Hybrid polymer–nanocrystal photovoltaic (PV) cells have received much attention during the past decade as promising low-cost solar energy harvesting devices, and showed significant progress with power conversion efficiency reaching 5 % recently. This review starts from the introduction of hybrid materials to their application in electronic devices, with particular focus on bulk-heterojunction hybrid polymer–nanocrystal PV devices. The synthesis, surface chemistry, and electronic properties of colloidal inorganic nanocrystals are described. The recent development of hybrid PV devices will be discussed from the perspective of tailoring both inorganic nanocrystals and conjugated polymers, controlling polymer–nanocrystal hybrid morphology, engineering polymer–nanocrystal interface, and optimizing device architecture. Finally, future directions for further advancing hybrid PV technology to potential commercialization are also discussed.

Extended Conjugation Platinum(II) Porphyrins for use in Near-Infrared Emitting Organic Light Emitting Diodes

Co-reporter:Kenneth R. Graham, Yixing Yang, Jonathan R. Sommer, Abigail H. Shelton, Kirk S. Schanze, Jiangeng Xue, and John R. Reynolds

Chemistry of Materials 2011 Volume 23(Issue 24) pp:5305

Publication Date(Web):November 23, 2011

DOI:10.1021/cm202242x

A family of π-extended platinum(II) porphyrins has been synthesized and incorporated into solution processed polymer light emitting diodes (PLEDs) and vapor deposited multilayer organic light emitting diodes (OLEDs), giving rise to devices with peak emission ranging from 771 to 1005 nm. The longest wavelength emitter, platinum(II)-5,10,15,20-(3,5-di-tert-butylphenyl)tetraanthroporphyrin (Pt-Ar4TAP), shows an emission maximum at 1005 nm, an external quantum efficiency (EQE) of 0.12%, and a maximum radiant emittance (Rmax) of 0.23 mW/cm2 in single layer PLED architectures, which is enhanced to an EQE of 0.20% with an Rmax of 0.57 mW/cm2 upon vapor deposition of an electron transport layer. In an effort to understand substituent effects and enhance the performance of π-extended Pt-porphyrins in PLEDs and OLEDs, a family of Pt-tetrabenzoporphyrins (Pt-TBPs) with varying functionality was investigated. The luminescent lifetimes of the Pt-TBPs in solution and in films were measured, and a strong correlation was demonstrated between the film lifetimes and the PLED and OLED efficiencies. An improvement in external quantum efficiency (EQE) from 2.07 to 2.49% for PLEDs and from 8.0 to 9.2% for OLEDs was observed between the less substituted Pt-tetraphenyltetrabenzoporphyrin and the more substituted Pt-5,10,15,20-(3,5-di-tert-butylphenyl)tetrabenzoporphyrin. The PLED EQEs were further enhanced to 3.02% with the disubstituted Pt-5,15-(3,5-di-tert-butylphenyl)tetrabenzoporphyrin; however, this increase was not observed for the OLEDs where an EQE of 7.8% was measured.Keywords: electroluminescence; near-infrared; organic light emitting diode; platinum(II) porphyrin;

Close-packed hemispherical microlens arrays for light extraction enhancement in organic light-emitting devices

Co-reporter:Sang-Hyun Eom, Edward Wrzesniewski, Jiangeng Xue

Organic Electronics 2011 Volume 12(Issue 3) pp:472-476

Publication Date(Web):March 2011

DOI:10.1016/j.orgel.2010.12.021

Close-packed hemispherical microlens arrays have been fabricated using a soft lithography method and used to enhance the light extraction efficiency in organic light-emitting devices (OLEDs). A close-packed monolayer of polystyrene microspheres was used to produce a soft template of polydimethylsiloxane for fabricating microlens arrays from a photopolymerizable optical adhesive. The microlens contact angle and array fill-factor increase with the polystyrene microsphere size, reaching (85° ± 5°) and (85 ± 3)% for 100 μm size microspheres, respectively. Attaching such a large-area (2 cm in diameter) microlens array on the glass substrate of an OLED leads to up to 70% efficiency enhancement, which also shows dependencies on the area and detailed structure of the OLED.Graphical abstract.Research highlights► We have fabricated close-packed hemispherical microlens arrays using a soft lithography method. ► The microlens contact angle and array fill-factor increase with polystyrene microsphere size. ► We have applied such microlens array to enhance light outcoupling in OLEDs. ► Up to 70% efficiency enhancement is achieved with the microlens array attached to OLEDs. ► Light outcoupling enhancement depends on the area and detailed structure of the OLED.

Effects of nanocrystal size and device aging on performance of hybrid poly(3-hexylthiophene):CdSe nanocrystal solar cells

Co-reporter:Jihua Yang, Aiwei Tang, Renjia Zhou, Jiangeng Xue

Solar Energy Materials and Solar Cells 2011 95(2) pp: 476-482

Publication Date(Web):

DOI:10.1016/j.solmat.2010.09.005

Blue-Violet Electroluminescence from a Highly Fluorescent Purine

Co-reporter:Yixing Yang, Pamela Cohn, Aubrey L. Dyer, Sang-Hyun Eom, John R. Reynolds, Ronald K. Castellano and Jiangeng Xue

Chemistry of Materials 2010 Volume 22(Issue 12) pp:3580

Publication Date(Web):May 25, 2010

DOI:10.1021/cm100407n

Electroluminescence from light-emitting polymer/ZnO nanoparticle heterojunctions at sub-bandgap voltages

Co-reporter:Lei Qian, Ying Zheng, Kaushik R. Choudhury, Debasis Bera, Franky So, Jiangeng Xue, Paul H. Holloway

Nano Today 2010 Volume 5(Issue 5) pp:384-389

Publication Date(Web):2010

DOI:10.1016/j.nantod.2010.08.010

The turn-on voltage of a light-emitting diode is generally equal to or greater than its bandgap voltage (or the bandgap energy divided by the electron charge). In contrast, we have found that electroluminescence at sub-bandgap voltages can be observed in polymer light-emitting devices with a heterojunction based on ZnO nanoparticles (NPs). This sub-bandgap electroluminescence is attributed to an Auger-assisted energy up-conversion process at the polymer/ZnO NPs interface, the observation of which depends strongly on the size of the nanoparticles.Graphical abstractResearch highlights▶ Light emission from polymer light emitting devices (PLEDs) was observed at energies less than the photon energy. ▶ Energy up-conversion Auger process resulted in photon emission at sub-bandgap energy (driving voltages). ▶ A layer of <5 nm diameter ZnO nanoparticles was necessary to observe sub-bandgap emission.

Efficient Near-Infrared Polymer and Organic Light-Emitting Diodes Based on Electrophosphorescence from (Tetraphenyltetranaphtho[2,3]porphyrin)platinum(II)

Co-reporter:Jonathan R. Sommer, Richard T. Farley, Kenneth R. Graham, Yixing Yang, John R. Reynolds, Jiangeng Xue and Kirk S. Schanze

ACS Applied Materials & Interfaces 2009 Volume 1(Issue 2) pp:274

Publication Date(Web):February 3, 2009

DOI:10.1021/am800236x

The new metalloporphyrin Pt(tptnp), where tptnp = tetraphenyltetranaphtho[2,3]porphyrin, has been prepared and subjected to photophysical and electrooptical device studies. In degassed toluene solution at room temperature Pt(tptnp) features efficient phosphorescence emission with λmax 883 nm with a quantum efficiency of 0.22. The complex has been used as the active phosphor in polymer and organic light-emitting diodes. Polymer light-emitting diodes based on a spin-coated emissive layer consisting of a blend of Pt(tptnp) doped in poly(9-vinylcarbazole) and 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole exhibit near-IR emission with λmax 896 nm, with a maximum external quantum efficiency (EQE) of 0.4% and a maximum radiant emittance of 100 μW/cm2. Organic light-emitting diodes prepared via vapor deposition of all layers and that feature an optimized multilayer hole injection and electron blocking layer heterostructure with an emissive layer consisting of 4,4′-bis(carbazol-9-yl)biphenyl (CBP) doped with Pt(tptnp) exhibit a maximum EQE of 3.8% and a maximum radiant emittance of 1.8 mW/cm2. The polymer and organic light-emitting diodes characterized in this study exhibit record high efficiency for devices that emit in the near-IR at λ >800 nm.Keywords: electrophosphorescence; metalloporphyrin; near-infrared; organic light emitting diodes; polymer light emitting diodes

Effect of electron injection and transport materials on efficiency of deep-blue phosphorescent organic light-emitting devices

Co-reporter:Sang-Hyun Eom, Ying Zheng, Edward Wrzesniewski, Jaewon Lee, Neetu Chopra, Franky So, Jiangeng Xue

Organic Electronics 2009 Volume 10(Issue 4) pp:686-691

Publication Date(Web):July 2009

DOI:10.1016/j.orgel.2009.03.002

We investigate the performance of FIr6-based deep-blue phosphorescent organic light-emitting devices (PHOLEDs) with three different electron transport materials, bathocuproine (BCP), 4,7-diphenyl-1,10-phenanthroline (BPhen), and tris[3-(3-pyridyl)mesityl]borane (3TPYMB), and study the effect of doping alkaline metals (Li and Cs) into these charge transport materials. External quantum efficiency (ηEQE) of (20 ± 1)% and peak power efficiency (ηP) of (36 ± 2) lm/W were achieved maintaining Commission Internationale de L’Eclairage (CIE) coordinates of (x = 0.16, y = 0.28) in p-i-n dual-emissive-layer (D-EML) deep-blue PHOLEDs with 3TPYMB as the electron transport material and 3TPYMB:Cs as the electron injection layer. The high efficiencies are attributed to the high triplet energy of 3TPYMB as well as the increased conductivity of 3TPYMB:Cs.

Photocarrier behavior in organic heterojunction photovoltaic cells

Co-reporter:Jason D. Myers, Teng-Kuan Tseng, Jiangeng Xue

Organic Electronics 2009 Volume 10(Issue 6) pp:1182-1186

Publication Date(Web):September 2009

DOI:10.1016/j.orgel.2009.05.023

We have investigated the bias dependence of photocurrent in several organic heterojunction cells to elucidate the behavior of photogenerated charge carriers. Both the planar and planar-mixed heterojunction devices are shown to always have negative photocurrent even at large forward biases; this phenomena has been attributed to an increased driving force for carrier diffusion away from the heterointerface as the applied bias increases. In contrast, the drift current generally dominates in mixed heterojunction devices due to distributed nature of charge generation throughout the active layer, leading to a photocurrent that is highly dependent on the internal electric field. This dependence gives rise to the reversal of the photocurrent direction at high biases when compared to that at the short-circuit condition. However, the voltage yielding zero photocurrent shows appreciable wavelength dependence, which is strongly correlated to the detailed charge carrier generation profile within the active layer.

Organic photovoltaic cells with vertically aligned crystalline molecular nanorods

Co-reporter:Ying Zheng, Robel Bekele, Jiaomin Ouyang, Jiangeng Xue

Organic Electronics 2009 10(8) pp: 1621-1625

Publication Date(Web):

DOI:10.1016/j.orgel.2009.08.009

Ultraviolet-violet electroluminescence from highly fluorescent purines

Co-reporter:Yixing Yang, Pamela Cohn, Sang-Hyun Eom, Khalil A. Abboud, Ronald K. Castellano and Jiangeng Xue

Journal of Materials Chemistry A 2013 - vol. 1(Issue 16) pp:NaN2874-2874

Publication Date(Web):2013/02/18

DOI:10.1039/C3TC00734K

Consequences of hydrogen bonding on molecular organization and charge transport in molecular organic photovoltaic materials

Journal of Materials Chemistry A 2014 - vol. 2(Issue 5) pp:NaN1549-1549