S.k. So

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Organization: Hong Kong Baptist University

Department: Department of Physics and Centre for Advanced Luminescence Materials

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Material Chemistry

OLED

Organic Chemistry

OLED Organic Semiconductor

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References

Using Ultralow Dosages of Electron Acceptor to Reveal the Early Stage Donor–Acceptor Electronic Interactions in Bulk Heterojunction Blends

Co-reporter:Carr Hoi Yi Ho;Sin Hang Cheung;Ho-Wa Li;Ka Lok Chiu;Yuanhang Cheng;Hang Yin;Mau Hing Chan;Franky So;Sai-Wing Tsang;Shu Kong So

Advanced Energy Materials 2017 Volume 7(Issue 12) pp:

Publication Date(Web):2017/06/01

DOI:10.1002/aenm.201602360

Tuning the donor–acceptor (D–A) weight ratio is an essential step to optimize the performance of a bulk heterojunction (BHJ) solar cell. The unoptimized regime with a low acceptor concentration is generally unexplored despite it may reveal the early stage electronic D–A interactions. In this study, PTB7:PC71BM is used to examine factors that limit the device performance in unoptimized regime. The key limiting factor is the creation of traps and localized states originated from fullerene molecules. Photothermal deflection spectroscopy is used to quantify the trap density. Starting with pristine PTB7, addition of small concentration of fullerene increases the electron trap density and lowers the electron mobility. When the D–A weight ratio reaches 1:0.1, fullerene percolation occurs. There is an abrupt drop in trap density and simultaneously a six orders of magnitude increase in the electron mobility. Furthermore, the fill factors of the corresponding photovoltaic devices are found to anticorrelate with the trap density. This study reveals that electron trapping is the key limiting factor for unoptimized BHJ solar cells in low fullerene regime.

Bulk-heterojunction solar cells with enriched polymer contents

Co-reporter:Hang Yin, Ka Lok Chiu, Carr Hoi Yi Ho, Harrison Ka Hin Lee, Ho Wa Li, Yuanhang Cheng, Sai Wing Tsang, Shu Kong So

Organic Electronics 2017 Volume 40(Volume 40) pp:

Publication Date(Web):1 January 2017

DOI:10.1016/j.orgel.2016.10.030

•The OPV devices with enriched polymer contents are re-optimized.•A higher DIO concentration further disperses the fullerene domains.•The enhancements of PCE, hole mobility and optical absorption are observed.In a high performance PTB7:PC71BM bulk-heterojunction (BHJ) solar cell, the commonly optimized polymer:fullerene (D:A) weight ratio is about 1:1.5, when PC71BM is used as the acceptor. This report explores alternative D:A weight ratios. We describe how to enrich the polymer contents of these BHJ solar cells to achieve high power conversion efficiencies (PCEs). The concentration of 1,8-diiodooctane (DIO), a solvent additive for the BHJ precursor solutions, is increased in order to re-optimize the BHJ cells. The PCEs of the re-optimized cells are improved for the PTB7 cells. Detailed charge transport measurements were carried out to examine the polymer-rich BHJs. We observed enhanced hole mobilities for the PTB7 BHJs. Additionally, the electron mobilities are preserved due to the dispersion of fullerene domains by increased DIO concentrations. Two other well-known polymer donors PCDTBT and PDTSTPD have been also investigated, and the improvements of hole mobilities and PCEs can be obtained for both polymer-rich BHJ solar cells.Download high-res image (166KB)Download full-size image

Growth, characterization, and thin film transistor application of CH3NH3PbI3 perovskite on polymeric gate dielectric layers

Co-reporter:Jenner H. L. Ngai;Johnny K. W. Ho;Rocky K. H. Chan;S. H. Cheung;Louis M. Leung;S. K. So

RSC Advances (2011-Present) 2017 vol. 7(Issue 78) pp:49353-49360

Publication Date(Web):2017/10/20

DOI:10.1039/C7RA08699G

Methylammonium-based perovskite compounds are generally grown on conducting or semiconducting substrates for high performance solar cell applications. In this study, we explore the growth of these compounds on insulators and test for their field-effect transistor performance. The key challenge is to find a surface that favors the crystal growth of perovskites without compromising the adhesion of the crystals. A family of methacrylate-based polymers has been identified as the insulators. Onto these insulators, methylammonium lead iodide polycrystalline thin films were grown. Generally, we found that the crystal size in the perovskite layers is well-correlated with the surface hydrophobicity. More hydrophobic polymer layers favor the growth of larger crystals, but result in less favorable adhesion of the perovskite. Methacrylate polymers with a phenyl substituent can give better adhesion and crystal sizes despite their hydrophobic properties. Among the different insulating polymer layers, we found that poly(phenyl methacrylate) (PPhMA), a derivative of the common commercial plastic poly(methyl methacrylate) (PMMA), produces the best perovskite films. The molecular origin of these properties is discussed. To test the electronic properties of these films, we employed them for thin-film transistor applications. Under optimal conditions, the thin-film transistors fabricated on PPhMA produce the best device with an electron mobility of 0.4 cm2 V−1 s−1. Our results are also supported by photothermal deflection spectroscopy investigations of the subgap optical absorptions of these films.

Boosting the photovoltaic thermal stability of fullerene bulk heterojunction solar cells through charge transfer interactions

Co-reporter:Carr Hoi Yi Ho;Huanyang Cao;Yong Lu;Tsz-Ki Lau;Sin Hang Cheung;Ho-Wa Li;Hang Yin;Ka Lok Chiu;Lik-Kuen Ma;Yuanhang Cheng;Sai-Wing Tsang;Xinhui Lu;Shu Kong So;Beng S. Ong

Journal of Materials Chemistry A 2017 vol. 5(Issue 45) pp:23662-23670

Publication Date(Web):2017/11/21

DOI:10.1039/C7TA06530B

Fullerene-based bulk heterojunction organic solar cells (BHJ-OSCs) represent one of the current state-of-the-art organic solar cells. Nonetheless, most of these devices still suffer from adverse performance degradation due to thermally induced morphology changes of active layers. We herein demonstrate that the photovoltaic performance stability of BHJ-OSCs can be profoundly enhanced with an appositely functionalized 9-fluorenylidene malononitrile. The latter, through charge transfer (CT) interactions with a donor polymer, enables the formation of a “frozen” 3-dimensional mesh-like donor polymer matrix, which effectively restrains free movement of embedded fullerene molecules and suppresses their otherwise uncontrolled aggregation. 9-Fluorenylidene malononitrile derivatives with multiple CT interaction sites are particularly effective as preservation of a power conversion efficiency of over 90% under severe thermal stress has been accomplished. The generality of this novel strategy has been affirmed with several common donor polymers, manifesting it to be hitherto the most efficient approach to stabilized fullerene-based BHJ-OSCs.

Improvement of Charge Collection and Performance Reproducibility in Inverted Organic Solar Cells by Suppression of ZnO Subgap States

Co-reporter:Bo Wu, Zhenghui Wu, Qingyi Yang, Furong Zhu, Tsz-Wai Ng, Chun-Sing Lee, Sin-Hang Cheung, and Shu-Kong So

ACS Applied Materials & Interfaces 2016 Volume 8(Issue 23) pp:14717-14724

Publication Date(Web):May 25, 2016

DOI:10.1021/acsami.6b03619

Organic solar cells (OSCs) with inverted structure usually exhibit higher power conversion efficiency (PCE) and are more stable than corresponding devices with regular configuration. Indium tin oxide (ITO) surface is often modified with solution-processed low work function metal oxides, such as ZnO, serving as the transparent cathode. However, the defect-induced subgap states in the ZnO interlayer hamper the efficient charge collection and the performance reproducibility of the OSCs. In this work, we demonstrate that suppression of the ZnO subgap states by modification of its surface with an ultrathin Al layer significantly improves the charge extraction and performance reproducibility, achieving PCE of 8.0%, which is ∼15% higher than that of a structurally identical control cell made with a pristine ZnO interlayer. Light intensity-dependent current density–voltage characteristic, photothermal deflection spectroscopy, and X-ray photoelectron spectroscopy measurements point out the enhancement of charge collection efficiency at the organic/cathode interface, due to the suppression of the subgap states in the ZnO interlayer.

Probing Bulk Transport, Interfacial Disorders, and Molecular Orientations of Amorphous Semiconductors in a Thin-Film Transistor Configuration

Co-reporter:Wai-Yu Sit;Sin Hang Cheung;Cyrus Yiu Him Chan;Ka Kin Tsung;Sai Wing Tsang;Shu Kong So

Advanced Electronic Materials 2016 Volume 2( Issue 3) pp:

Publication Date(Web):

DOI:10.1002/aelm.201500273

Thin film transistors (TFTs) can be used to determine the bulk-like mobilities of amorphous semiconductors. Different amine-based organic hole transporting materials (HTs) used in organic light-emitting diodes have been investigated. In addition, the present study also measures the TFT hole mobilities of two iridium phosphors: Ir(ppy)₃ and Ir(piq)₃. These materials are grown separately on SiO₂ and polystyrene (PS). On SiO₂, the TFT mobilities are found to be 1–2 orders smaller than the bulk values obtained by time-of-flight (TOF) technique. On PS gate dielectric layer, the TFT mobilities are in good agreement TOF data. Only 10 nm of organic semiconductor is sufficient for TFTs to achieve TOF mobilities. Using the Gaussian disorder model, it is found that on SiO₂ surface, when compared to the bulk values, the energetic disorders (s) of the HTs increase and simultaneously, the high temperature limits (µ_∞) of the carrier mobilities decrease. Both s and µ_∞ contribute to the reduction of the carrier mobility. The increase in s is related to the presence of randomly oriented polar Si-O bonds. The reduction of µ_∞ on SiO₂ is related to the orientations of the more planar molecules which tend to lie horizontally on the surface.

Impact of Solvent Additive on Carrier Transport in Polymer:Fullerene Bulk Heterojunction Photovoltaic Cells

Co-reporter:Carr Hoi Yi Ho;Qi Dong;Hang Yin;Winky Wing Ki Leung;Qingdan Yang;Harrison Ka Hin Lee;Sai Wing Tsang;Shu Kong So

Advanced Materials Interfaces 2015 Volume 2( Issue 12) pp:

Publication Date(Web):

DOI:10.1002/admi.201500166

The effects of a solvent additive, 1,8-diiodooctane (DIO), on both hole and electron transport are investigated in a state-of-the-art bulk-heterojunction (BHJ) system, namely PTB7:PC₇₁BM. For a polymer:fullerene weight ratio of 1:1.5, the electron mobility in the blend film increases by two orders of magnitude with the DIO concentration while almost no change is found in the hole mobility. For lower DIO concentrations, the electron mobility is suppressed because of large, but poorly connected PC₇₁BM domains. For higher concentrations of DIO, the electron mobility is improved progressively and the hole mobility becomes the limiting factor. Between 1 and 5 vol%, the electron and hole mobilities are balanced. Using the Gaussian disorder model (GDM), we found that the DIO concentration modifies fundamentally the average hopping distances of the electrons. In addition, there exist alternative donor–acceptor ratios to achieve optimized PTB7:PC₇₁BM based solar cells. It is demonstrated that the fullerene content of the BHJ film can be significantly reduced from 1:1.5 to 1:1 while the optimized performance can still be preserved.

Charge conduction study of phosphorescent iridium compounds for organic light-emitting diodes application

Co-reporter:Wing-Hong Choi, Guiping Tan, Wai-Yu Sit, Cheuk-Lam Ho, Cyrus Yiu-Him Chan, Wenwei Xu, Wai-Yeung Wong, Shu-Kong So

Organic Electronics 2015 Volume 24() pp:7-11

Publication Date(Web):September 2015

DOI:10.1016/j.orgel.2015.05.011

•Thin-film transistor technique is used to investigate a series of iridium-based compounds.•The hole mobilities of Ir-compounds doped CBP films were evaluated.•Different doping concentrations of Ir-compounds were attempted.•Ir-compounds modify carrier transport in the host material of CBP.The charge conduction properties of a series of iridium-based compounds for phosphorescent organic light-emitting diodes (OLEDs) have been investigated by thin-film transistor (TFT) technique. These compounds include four homoleptic compounds: Ir(ppy)3, Ir(piq)3, Ir(Tpa-py)3, Ir(Cz-py)3, and two heteroleptic compounds Ir(Cz-py)2(acac) and FIrpic. Ir(ppy)3, Ir(piq)3 and FIrpic are commercially available compounds, while Ir(Tpa-py)3, Ir(Cz-py)3 and Ir(Cz-py)2(acac) are specially designed to test their conductivities with respect to the commercial compounds. In neat films, with the exception of FIrpic, all Ir-compounds possess significant hole transporting capabilities, with hole mobilities in the range of about 5 × 10−6–2 × 10−5 cm2 V−1 s−1. FIrpic, however, is non-conducting as revealed by TFT measurements. We further investigate how Ir-compounds modify carrier transport as dopants when they are doped into a phosphorescent host material CBP. The commercial compounds are chosen for the investigation. Small amounts of Ir(ppy)3 and Ir(piq)3 (<10%) behave as hole traps when they are doped into CBP. The hole conduction of the doped CBP films can be reduced by as much as 4 orders of magnitude. Percolating conduction of Ir-compounds occurs when the doping concentrations of the Ir-compounds exceed 10%, and the hole mobilities gradually increase as their values reach those of the neat Ir films. In contrast to Ir(ppy)3 and Ir(piq)3, FIrpic does not participate in hole conduction when it is doped into CBP. The hole mobility decreases monotonically as the concentration of FIrpic increases due to the increase of the average charge hopping distance in CBP.

Batch-to-Batch Variation of Polymeric Photovoltaic Materials: its Origin and Impacts on Charge Carrier Transport and Device Performances

Co-reporter:Harrison Ka Hin Lee;Zhao Li;Iordania Constantinou;Franky So;Sai Wing Tsang;Shu Kong So

Advanced Energy Materials 2014 Volume 4( Issue 16) pp:

Publication Date(Web):

DOI:10.1002/aenm.201400768

A detailed investigation of the impact of molecular weight distribution of a photoactive polymer, poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT), on photovoltaic device performance and carrier transport properties is reported. It is found that different batches of as-received polymers have substantial differences in their molecular weight distribution. As revealed by gel permeation chromatography (GPC), two peaks can generally be observed. One of the peaks corresponds to a high molecular weight component and the other peak corresponds to a low molecular weight component. Photovoltaic devices fabricated with a higher proportion of low molecular weight component have power conversion efficiencies (PCEs) reduced from 5.7% to 2.5%. The corresponding charge carrier mobility at the short-circuit region is also significantly reduced from 2.7 × 10⁻⁵ to 1.6 × 10⁻⁸ cm² V⁻¹ s⁻¹. The carrier transport properties of the polymers at various temperatures are further analyzed by the Gaussian disorder model (GDM). All polymers have similar energetic disorders. However, they appear to have significant differences in carrier hopping distances. This result provides insight into the origin of the molecular weight effect on carrier transport in polymeric semiconducting materials.

Fluorinated 9,9′-bianthracene derivatives with twisted intramolecular charge-transfer excited states as blue host materials for high-performance fluorescent electroluminescence

Co-reporter:Yue Yu, Bo Jiao, Zhaoxin Wu, Zhanfeng Li, Lin Ma, Guijiang Zhou, Wai Yu, S. K. So and Xun Hou

Journal of Materials Chemistry A 2014 vol. 2(Issue 44) pp:9375-9384

Publication Date(Web):03 Sep 2014

DOI:10.1039/C4TC01166J

We report the electroluminescent (EL) performance of a series of fluorinated 9,9′-bianthracene derivatives (BAnFs) to serve as host materials for selected dopants in organic light-emitting diodes (OLEDs). Different F substitution patterns strongly affect the EL performances. In particular, 10,10′-bis(4-fluorophenyl)-9,9′-bianthracene (BAn-(4)-F) works as an excellent fluorescent host for N,N-diphenylamino phenyl vinyl biphenyl (DPAVBi) and 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-benzo[l]pyrano(6,7,8-ij)quinolizin-11-one (C545T) dopants to obtain high-performance OLEDs with excellent external quantum efficiencies (EQEs) of 5.43% and 5.20%, respectively. We demonstrate that the improved EQEs are due to the enhanced occurrence of singlet excitons in BAn-(4)-F-based EL devices. The BAn-(4)-F host molecule with a particular twisted intramolecular charge-transfer (TICT) excited state, which participates in a charge-transfer (CT) intersystem crossing mechanism, realizes transitions from the triplet to singlet CT-states. At the dopant site, the available singlet excitons formed via Förster energy transfer from host BAn-(4)-F to dopant are greatly increased. We conclude that the BAn-(4)-F host based on 9,9′-bianthracene can significantly enhance the singlet generation fraction in EL devices.

Achieving time-of-flight mobilities for amorphous organic semiconductors in a thin film transistor configuration

Co-reporter:Cyrus Y.H. Chan, K.K. Tsung, W.H. Choi, S.K. So

Organic Electronics 2013 Volume 14(Issue 5) pp:1351-1358

Publication Date(Web):May 2013

DOI:10.1016/j.orgel.2013.02.007

We observe bulk-like hole transport in amorphous organic semiconductors in a thin film transistor (TFT) configuration. Five different organic hole transporters (HTs) commonly used in organic light-emitting diodes are investigated. When these HTs are deposited on SiO2 gate dielectric layer, the TFT mobilities are 1–2 orders of magnitude smaller than those obtained from bulk films (3–8 μm) using time-of-flight (TOF) technique. The reduction of hole mobilities can be attributed to the interactions between the organic HTs and the polar SiO bonds on the gate dielectric layer. Detailed temperature dependence studies, employing the Gaussian disorder model, indicate that the SiO2 gate dielectric contributes between 60 and 90 meV of energetic disorder in the charge hopping manifold. Besides SiO2 gate dielectric, similar effects can also be observed for other polar insulators including polymeric PMMA and BCB, or HMDS-modified SiO2. However, when a common non-polar polymer, polystyrene (PS), is employed as the dielectric layer, the dipolar energetic disorder becomes negligible. Holes effectively experience bulk-like transport on the PS gate dielectric surface. TFT mobilities extracted from all five organic HTs are in excellent agreements with TOF mobilities. The present study should have broad applications in the transport characterization of amorphous organic semiconductors.Graphical abstractHighlights► Charge transport in five amorphous organic hole transporters are examined by thin film transistor (TFT) technique. ► The mobilities extracted from TFT grown on SiO2 differ greatly from those obtained from time-of-flight (TOF) experiments. ► By using a non-polar polymeric dielectric layer, TOF mobilities are found to be achievable in a TFT.

Charge transport study of semiconducting polymers and their bulk heterojunction blends by capacitance measurements

Co-reporter:Kevin K. H. Chan;S. W. Tsang;Harrison K. H. Lee;F. So;S. K. So

Journal of Polymer Science Part B: Polymer Physics 2013 Volume 51( Issue 8) pp:649-658

Publication Date(Web):

DOI:10.1002/polb.23236

Abstract

We use frequency dependent capacitance measurements to probe carrier mobilities and transport parameters of six representative semiconducting polymers and some of their bulk heterojunction (BHJ) blends. With a suitable choice of a hole injection layer, well-defined signals for hole transport characterization can be obtained for the pristine polymers [J. Appl. Phys. 99, 013706 (2006)]. However, ill-defined signals with negative capacitances, arising from undesirable electron leakages, are obtained for the BHJ blends. The problem of electron leakage can be circumvented by inserting an electron blocking and trapping layer under the cathode. As a result, hole transport properties of BHJ blends can be obtained. For the BHJ of poly(3-hexylthiophene) blended with [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC₆₁BM), the hole mobilities seem to be insensitive to the composition of the BHJ, indicating the P3HT component in the BHJ is well connected. On the other hand, for poly[N-9“-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadia zole)] doped with [6,6]-phenyl-C71-butyric acid methyl ester (PCDTBT:PC₇₁BM), a clear reduction of the hole mobility is observed as the polymer composition is reduced. Temperature dependent experiments were performed. The data are analyzed by the Gaussian Disorder Model. We found that the energetic disorder is independent of the composition of the BHJ. Organic photovoltaic performances of BHJ blends are also measured in this contribution. The correlation between device performance and energetic disorder of the BHJ will be discussed. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013

Role of electron blocking and trapping layers in transport characterization of a photovoltaic polymer poly(3-hexylthiophene)

Co-reporter:Harrison K.H. Lee, Kevin K.H. Chan, S.K. So

Organic Electronics 2012 Volume 13(Issue 4) pp:541-544

Publication Date(Web):April 2012

DOI:10.1016/j.orgel.2011.12.023

Hole injection and transport in films (300–350 nm) of poly(3-hexylthiophene) (P3HT) were investigated by dark-injection space-charge-limited current (DI-SCLC) technique. For samples with a nominally hole-only configuration of anode/P3HT/Au, the DI current transients depart significantly from the theory, and the signals cannot be used for reliable carrier mobility extraction. The origin of the departure can be attributed to electron leakage from the Au cathode. We outline a means of suppressing electron leakage by inserting an interlayer between the P3HT and the cathode. This interlayer has dual functions of blocking and trapping electrons. Using this interlayer, we obtain well-defined DI-SCLC signals for reliable carrier mobility determination. With a suitable interlayer to suppress undesirable carrier injection and transport, DI-SCLC technique should find broad applications in the transport characterization of narrow gap photovoltaic polymers.Graphical abstractHighlights► DI-SCLC technique is used to probe hole transports in thin films of P3HT. ► Ill-defined signal is observed in hole-only device due to electron leakage. ► Interlayer for blocking and trapping electrons is added to suppress electron leakage. ► With interlayer, well-defined signal is obtained for reliable mobility extraction.

Charge injection and transport studies of poly(2,7-carbazole) copolymer PCDTBT and their relationship to solar cell performance

Co-reporter:Kevin K.H. Chan, S.W. Tsang, Harrison K.H. Lee, Franky So, S.K. So

Organic Electronics 2012 Volume 13(Issue 5) pp:850-855

Publication Date(Web):May 2012

DOI:10.1016/j.orgel.2012.01.030

The charge injection and transport properties of a high performance semiconducting polymer for organic photovoltaic (OPV) applications, poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT), are investigated by time-of-flight (TOF) and dark-injection space-charge-limited current (DI-SCLC) techniques. OPV cells employing PCDTBT are known to possess power conversion efficiency (PCE) exceeding 6% and . While TOF probes only the hole mobilities of a thick film, DI-SCLC is shown to be useful down to a sample thickness of ∼200 nm, which is comparable to thicknesses used in OPV cells. We show that for pristine PCDTBT, the hole mobilities for both thick used in TOF and thin films for DI-SCLC are essentially the same, and they are in the range of 0.4–3.0 × 10−4 cm2/Vs at room temperature. Both poly(3,4-ethylene dioxythioplene) doped with poly(strenesulfonate) (PEDOT:PSS) and molybdenum (VI) oxide (MoO3) form quasi-Ohmic contacts to PCDTBT with better hole injection from MoO3. Furthermore, the Gaussian Disorder Model (GDM) was employed to analyze the hopping transport of PCDTBT thin films. We show that PCDTBT possesses a relatively large energetic disorder (σ) of ∼129 meV, which is significantly higher than the σ of poly(3-hexylthiophene) (P3HT) processed under similar conditions. The correlation between σ and OPV device performance is addressed.Graphical abstract.Highlights► The charge injection and transport properties of a poly(2,7-carbazole) copolymer, PCDTBT, is probed by DI-SCLC technique. ► The hole mobilities are in the range of 0.4–3.0 × 10−4 cm2/Vs. ► MoO3 is a better hole injection layer than PEDOT:PSS for PCDTBT. ► PCDTBT possesses a large energetic disorder of ∼129 meV, which is higher than that of P3HT. ► The correlation between energetic disorder and OPV device performance is addressed.

Using transistor technique to study the effects of transition metal oxide dopants on organic charge transporters

Co-reporter:Cyrus Y.H. Chan, C.M. Chow, S.K. So

Organic Electronics 2011 Volume 12(Issue 8) pp:1454-1458

Publication Date(Web):August 2011

DOI:10.1016/j.orgel.2011.04.023

Organic thin-film transistor (OTFT) technique was used to investigate the effects of doping on N’-diphenyl-N,N’-bis(1-naphthyl)(1,1’-biphenyl)-4,4’diamine (NPB). Different transition metal oxides (TMOs) including molybdenum oxide (MoO3), vanadium oxide (V2O5), tungsten oxide (WO3) were employed as dopants. Using temperature dependent OTFT measurement, the carrier mobility and carrier concentration of the doping system can be extracted simultaneously. Generally, all TMOs form p-dopants and the conductivities increase drastically after doping. Among the TMOs, MoO3 appears to be the most effective p-type dopant. It generates the largest free carrier concentration (1.4 × 1017 cm−3) and has the least activation energy (∼138 meV) for modest doping concentration of ∼5 vol.%. Detailed carrier transport analysis indicates that the carrier mobilities were slightly reduced. It appears that the increase of free carrier concentration is the deciding factor in the conductivity enhancement in TMO-doped NPB.Graphical abstractHighlights► Effects of doping transition metal oxides into organic hole transporter was examined by organic thin film transistor technique. ► The conductivity was improved after doped with TMOs. ► The improved conductivity was realized by the increased free carrier concentration after doping. ► MoO3 was found to be the best p-dopant among three TMOs candidates (MoO3, WO3 and V2O5).

Role of air exposure in the improvement of injection efficiency of transition metal oxide/organic contact

Co-reporter:C.H. Cheung, W.J. Song, S.K. So

Organic Electronics 2010 Volume 11(Issue 1) pp:89-94

Publication Date(Web):January 2010

DOI:10.1016/j.orgel.2009.10.003

Oxygen or air exposure to transition metal oxides (TMOs) was demonstrated to be essential in improving the hole injection (HI) efficiency at the contact formed by TMOs and small organic hole transporter. Current–voltage (J–V) and dark-injection space-charge-limited current (DI-SCLC) techniques were used to cross-examine the TMO/organic contacts. The hole transporter under investigation was N,N′-diphenyl-N,N′-bis(1-naphthyl)(1,1′biphenyl)-4,4′diamine (NPB). The improvement was attributed to the reduction in the energy barrier at TMO/NPB interface, which was a consequence of the work function enhancement of TMO by the oxidation of oxygen in air.

Can an organic phosphorescent dye act as a charge transporter?

Co-reporter:W.H. Choi, C.H. Cheung, S.K. So

Organic Electronics 2010 Volume 11(Issue 5) pp:872-875

Publication Date(Web):May 2010

DOI:10.1016/j.orgel.2010.02.001

The hole transport property of a widely used phosphorescent dye, tris(2-phenylpyridine) iridium or Ir(ppy)3 was investigated by thin film transistor (TFT) technique. The field effect (FE) mobility of Ir(ppy)3 was found to be 1.7 × 10−5 cm2 V−1s−1. This value is actually comparable to NPB and CBP, two popular hole transporting materials for fluorescent and phosphorescent organic light-emitting diodes (FOLED and POLED), respectively. In addition, temperature dependent measurements were carried out to study the energetic disorder (σ) of Ir(ppy)3. The extracted σ ∼ 88 meV is comparable to those of other common amorphous organic hole transporters, which are in the range of 80–90 meV. Our findings indicate that the dye can directly act as a hole transporting component in POLEDs.

High temperature carrier mobility as an intrinsic transport parameter of an organic semiconductor

Co-reporter:K.K. Tsung, S.K. So

Organic Electronics 2009 Volume 10(Issue 4) pp:661-665

Publication Date(Web):July 2009

DOI:10.1016/j.orgel.2009.02.014

The high temperature limit of hole mobility (μ∞) in N,N′-diphenyl-N,N′-bis(1-naphthyl)(1,1′-biphenyl)-4,4′diamine (NPB) has been studied by time-of-flight technique. The effect of dopants on μ∞ was also investigated. It was found that the μ∞ is independent of the nature of dopants. The common μ∞ can be applied to estimate the full temperature dependence of zero-field mobility (μ0), if μ0 at one temperature is known. We demonstrate this concept by predicting the room temperature μ0 of NPB-doped with copper phthalocyanine (CuPc). The mobility prediction of CuPc-doped-NPB was then verified by the classic work of Hoesterey and Letson [D.C. Hoesterey, G.M. Letson, J. Phys. Chem. Solids 24 (1963) 1609].

PEDOT:PSS polymeric conducting anode for admittance spectroscopy

Co-reporter:S.W. Tsang, S.C. Tse, K.L. Tong, S.K. So

Organic Electronics 2006 Volume 7(Issue 6) pp:474-479

Publication Date(Web):December 2006

DOI:10.1016/j.orgel.2006.06.002

The carrier mobilities of two hole transporting organic materials were evaluated by admittance spectroscopy (AS). The materials were 4,4′,4″-tris{N,-(3-methylphenyl)-N-phenylamino}triphenylamine (m-MTDATA) and N,N′-diphenyl-N,N′-bis(1,1′-biphenyl)-4,4′-diamine (NPB). They were sandwiched in a configuration of anode/organic material/cathode. The anode was either indium-tin-oxide (ITO) or poly(3,4-ethylenedioxythiophene) doped with polystrenesulphonic acid (PEDOT:PSS). It is shown that PEDOT:PSS can, respectively, form Ohmic and quasi-Ohmic contact with m-MTDATA and NPB. Using PEDOT:PSS as the anode, the average hole mobilities of m-MTDATA and NPB were extracted by AS through susceptance analysis. The results are in excellent agreement with those obtained by an independent time-of-flight (TOF) technique. With PEDOT:PSS, the application of AS for characterizing carrier mobilities can be extended to hole transporting organic materials with highest occupied molecular orbital (HOMO) energy levels down to 5.4 eV.

Heterojunction OLEDs fabricated by Eu ternary complexes with conducting secondary ligands

Co-reporter:M.K. Lam, K.L. Kwok, S.C. Tse, S.K. So, J.B. Yuan, Louis M. Leung, M.L. Gong

Optical Materials 2006 Volume 28(6–7) pp:709-713

Publication Date(Web):May 2006

DOI:10.1016/j.optmat.2005.09.017

A series of Eu ternary complexes with charge conducting secondary ligands were examined as potential light-emitting materials for organic light-emitting diodes (OLEDs). The Eu complex contains an imidazole-based secondary ligand and is electron-conducting. Simple heterojunction OLEDs can be fabricated with a structure ITO/CuPc/TPD/Eu complex/cathode where ITO = indium–tin-oxide, CuPc = copper phthalocyanine, TPD = N,N-diphenyl-N,N′-bis(3-methylphenyl)(1,1′-biphenyl)-4,4′-diamine. The Eu complex plays the dual role of electron-transporting and light-emitting layer. Sharp red emission can be observed at about 612 nm. The OLEDs have a current efficiency of about 0.5 cd/A (at about 20 mA/cm2) and a maximum luminance between 150 and 200 cd/m2. The effects of various substituted imidazole moieties on the electrical properties of the on the OLEDs will be discussed. It is shown that the Eu complex with unsubstituted imidazole has electron mobility ∼10−5 cm2 V−1 s−1, and should be a very useful material for fabricating red OLEDs with simple device structure.

The role of charge-transfer integral in determining and engineering the carrier mobilities of 9,10-di(2-naphthyl)anthracene compounds

Co-reporter:S.C. Tse, S.K. So, M.Y. Yeung, C.F. Lo, S.W. Wen, C.H. Chen

Chemical Physics Letters 2006 Volume 422(4–6) pp:354-357

Publication Date(Web):10 May 2006

DOI:10.1016/j.cplett.2006.02.079

Abstract

The charge transporting properties of t-butylated 9,10-di(2-naphthyl)anthracene (ADN) compounds have been investigated experimentally and computationally in relation to their molecular structures. The ADN compounds are found to be ambipolar with both electron and hole mobilities in the range of 1–4 × 10⁻⁷ cm² V⁻¹ s⁻¹ (electric field 0.5–0.8 MV/cm). As the degree of t-butylation increases, the carrier mobility decreases progressively. The mobility reduction was examined by Marcus theory of reorganization energies. All ADN compounds possess similar reorganization energies of ∼0.3 eV. The reduction of carrier mobilities with increasing t-butylation can be attributed to a decrease in the charge-transfer integral or the wavefunction overlap.

Transport and luminescence in naphthyl phenylamine model compounds

Co-reporter:K.L. Tong, S.K. So, H.F. Ng, L.M. Leung, M.Y. Yeung, C.F. Lo

Synthetic Metals 2004 Volume 147(1–3) pp:199-203

Publication Date(Web):7 December 2004

DOI:10.1016/j.synthmet.2004.06.036

The effects of polar side groups (methoxy, fluoro, and chloro substitution) on the transport and luminescence properties of a series of naphthyl phenylamine (NPA) compounds were examined. The compounds are (i) N,N′-diphenyl-1-naphthylamine, (ii) N-(4-methoxyphenyl),N′-phenyl-1-naphthylamine, (iii) N-(4-chlorophenyl),N′-phenyl-1-naphthylamine, and (iv) N-(4-fluorophenyl),N′-phenyl-1-naphthylamine. Time-of-flight measurements were used to measure the hole mobilities of the NPA compounds. The hole mobilities were found to correlate with the dipole moments of the compounds. The least polar compound N,N′-diphenyl-1-naphthylamine has the highest mobility in the range of 0.5–2 × 10−4 cm2 V−1 s−1, whereas the most polar compound N-(4-chlorophenyl),N′-phenyl-1-naphthylamine has the lowest mobility in the range of 0.2–2 × 10−6 cm2 V−1 s−1 at room temperature. The dipolar disorder model proposed by Dieckmann and Young is found to be applicable to describe the data. The NPA compounds were further incorporated into a multilayer organic light-emitting diode (OLED) as the light-emitting layer. The OLEDs emit in the range 420–440 nm, and the polar side groups appear to have no clear influence on the emission wavelength of the molecules in solid state.

Effects of tertiary butyl substitution on the charge transporting properties of rubrene-based films

Co-reporter:H.H. Fong, S.K. So, W.Y. Sham, C.F. Lo, Y.S. Wu, C.H. Chen

Chemical Physics 2004 Volume 298(1–3) pp:119-123

Publication Date(Web):8 March 2004

DOI:10.1016/j.chemphys.2003.11.008

Abstract

The charge transporting properties of rubrene (5,6,11,12-tetraphenylnaphthacene or RB), and a new rubrene-based complex, tetra(t-butyl)-rubrene [2,8-di(t-butyl)-5,11-di[4-(t-butyl)phenyl]-6,12-diphenylnaphthacene or TBRB], were examined in the form of amorphous films as functions of electric field and temperature by means of time-of-flight technique. At room temperature, the hole mobility μ for RB is 7–9 × 10⁻³ cm² V⁻¹ s⁻¹ whereas μ for the more bulky TBRB is about 2 × 10⁻³ cm² V⁻¹ s⁻¹. The microscopic conduction mechanism in both materials can be modeled by the Gaussian disorder model in which hopping conduction occurs through a manifold of sites with energetic and positional disorder. The energetic disorder in RB and TBRB is almost identical and is about 78 meV in each case, and is mainly controlled by van der Waals interaction. The t-butyl groups in TBRB induce large fluctuations in the spatial separation among TBRB molecules and result in an increase in the positional disorder, and hence a reduction in the hole mobility.