Organic Electronics 2012 Volume 13 №02

Elsevier. — 135 p. — ISSN: 1566-1199.«Organic Electronics» is a journal whose primary interdisciplinary focus is on materials and phenomena related to organic devices such as light emitting diodes, thin film transistors, photovoltaic cells, sensors, memories, etc.Papers suitable for publication in this journal cover such topics as photoconductive and electronic properties of organic materials, thin film structures and characterization in the context of organic devices, charge and exciton transport, organic electronic and optoelectronic devices.«Organic Electronics» provides the forum for applied, fundamental and interdisciplinary contributions spanning the wide range of electronic properties and applications of organic materials. A Letters section is included for rapid publication of short articles announcing significant and highly original results.215-221
Ming-Xing Zhang, Shuo Chai, Guang-Jiu Zhao. BODIPY derivatives as n-type organic semiconductors: Isomer effect on carrier mobility
Abstract:
In the present work, two dipyrro-boradiazaindacenes (BODIPY) derivatives functioning as novel high-performance organic semiconductors are investigated by theoretical method. These two isomeric complexes are demonstrated to have large electron-transfer mobility, which means they are favor to be n-type organic semiconductors. The highest electrontransfer mobility appears at the same packing style in two crystals. The intermolecular distances of the packing style are nearly same, 4,994 Å in crystal 1 and 5,283 Å in crystal 2 However, their electron-transfer mobility changes significantly. The mobility of crystal 2 with better planar molecular structure is 0,291 cm² V^-1 s^-1, which is 13 times larger than that of crystal 1 as 0,022 cm² V-1 s^-1. The significant difference of carrier mobility is ascribed to the little structural difference of these two isomers. It has been demonstrated that both crystal 1 and 2 show remarkable anisotropic behavior. This study will undoubtedly provide a new understanding of isomerization on designing novel organic semiconductors.222-229
Toshinori Matsushima, Hideyuki Mura. Molecular orientation induced by high-speed substrate transfer during vacuum vapor deposition of organic films
Abstract:
The authors investigate a relationship between substrate transfer speeds during vacuum vapor deposition and orientation characteristics of organic molecules. Results show that rod-shaped molecules of alpha-sexithiophene (α-6T) are oriented in a substrate transfer direction and an absorption dichroic ratio of 1,44 is obtained from the oriented a-6T molecule film when a high substrate transfer speed of 4 m s^-1 is used. By combining the substrate transfer technique with homoepitaxial growth of a-6T molecules on a rubbed surface, the absorption dichroic ratio further increases to 4,
29. Polarized electroluminescence (EL) characteristics are investigated using rod-shaped molecules of 4,4'-bis[4-(dip-tolylamino)styryl]biphenyl (DPAVBi) as a light-emitting hole-transport layer. An EL dichroic ratio of 2,12 is obtained due to an orientation of DPAVBi molecules caused by combining two techniques.230-234
Ling Li, Jang Hyuk Kwon, Jin Jang. Tail states recombination limit of the open circuit voltage in bulk heterojunction organic solar cells
Abstract:
An analytical theory is presented for bimolecular recombination through tail states and open circuit voltage in bulk heterojunction organic solar cells. It is developed rigorously using the hopping transport and the drift diffusion theory. Based on the proposed model, a variety of temperature, energy disorders of the material and illumination intensity dependencies of the open circuit voltage can be well described. Good agreement between the calculation and recent experimental data is found.235-243
Hylke B. Akkerman, Alice C. Chang, Eric Verploegen, Christopher J. Bettinger, Michael F. Toney, Zhenan Bao. Fabrication of organic semiconductor crystalline thin films and crystals from solution by confined crystallization
Abstract:
Highly crystalline thin films of organic semiconductors processed from solution for electronic devices are difficult to achieve due to a slow and preferential three-dimensional growth of the crystals. Here we describe the development of a processing technique to induce a preferential two-dimensional crystalline growth of organic semiconductors by means of minimizing one dimension and confining the solution in two dimensions into a thin layer. The versatility of the process is demonstrated by processing small molecules (TIPS-pentacene and C₆₀) and a polymer (P3HT), all from solvents with a relatively low boiling point, to obtain crystalline thin films. The thin films show an improved in-plane packing of the molecules compared to films processed under similar conditions by spin coating, which is beneficial for the use in organic field-effect transistors.244-248
L. Basiricò, P. Cosseddu, A. Scidà, B. Fraboni, G.G. Malliaras, A. Bonfiglio. Electrical characteristics of ink-jet printed, all-polymer electrochemical transistors
Abstract:
We report on the fabrication and characterization of inkjet-printed, all-Organic Electro-Chemical Transistors (OECTs) entirely realized by a conducting polymer, namely poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonic acid) (PEDOT:PSS). The transistors utilized saline as the electrolyte and exhibited output characteristics typical for operation in depletion regime. The transfer characteristics could be tuned on the basis of device geometry, with the ratio between the area of the channel and the area of the gate electrode determining the transconductance. This work paves the road for the low-cost, print-on-demand fabrication of circuits for applications in bio-sensors and disposable electronics.249-251
Minlu Zhang, Hui Wang, C.W. Tang. Tandem photovoltaic cells based on low-concentration donor doped C₆₀
Abstract:
We demonstrate that enhanced efficiency can be achieved in organic tandem photovoltaic cells using identical bulk heterojunction subcells based on 1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane doped C60 in series. Power conversion efficiencies greater than 4% have been achieved in 2- and 3-stack tandem cells, an improvement of at least 30% over the single-stack cell.252-263
G.D. Sharma, J.A. Mikroyannidis, Surya Prakash Singh. Photovoltaic properties of low band gap copolymers based on phenylenevinylene donor and cyanovinylene 4-nitrophenyl acceptor units
Abstract:
Two novel copolymers P1 and P2 having phenylenevinylene donor and cyanovinylene 4-nitrophenyl acceptor units, were synthesized by heck coupling and employed as electron donor along with PCBM or modified PCBM (F) as electron acceptor for the fabrication of bulk heterojunction (BHJ) photovoltaic devices. These copolymers P1 and P2 showed broad band absorption around 640–700 nm and optical band gap of 1,60 eV and 1,72 eV, respectively. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) estimated from cyclic voltammetry measurement reveals that these values are well suitable for the use of these copolymers as electron donor along with PCBM derivatives as electron acceptor for BHJ active layer. The suitable LUMO off set allows efficient photo-induced charge transfer at the donor/acceptor interfaces in the BHJ photovoltaic device and resulting power conversion efficiency (PCE) of 2,8% and 3,29% for P1 and P2, respectively, when PCBM is used as acceptor. This value has been improved up to 3,52% and 4,36% for the devices based on P1 and P2 when F is used as electron acceptor, instead of PCBM. We have also investigated the effect of solvent annealing on the photovoltaic performance of device based on P1: F and P2: F blends and found that the over all PCE of the devices is 4.36% and 4.88%, respectively. The increase in PCE is mainly due to the improvement in the Jsc, which is due to the increased charge transport in the annealed device as compared to as cast device.264-272
Yun Li, Chuan Liu, Akichika Kumatani, Peter Darmawan, Takeo Minari, Kazuhito Tsukagoshi. Large plate-like organic crystals from direct spin-coating for solution-processed field-effect transistor arrays with high uniformity
Abstract:
Solution-processed organic crystals are important in field-effect transistors because of their highly ordered molecular packing and ease of device fabrication. For practical applications, the patterning of organic crystal transistor arrays is critical. However, uniformity, which concerns the variation in electrical performance among devices fabricated simultaneously on the same substrate, is a common consideration in the commercial applications of the solution-processed organic crystal transistor arrays. Here, a simple approach for fabricating field-effect transistor arrays based on organic plate-like crystals is reported. Through this method, a direct spin-coating process from a mixture solution of organic semiconductor and polymer dielectric can produce organic plate-like crystals. The grain size of the crystals is observed to be hundreds of micrometers. By controlling the concentrations of the active materials, the transistor arrays exhibit high uniformity and good
device performance. The results presented in this work promise that this approach is a comparable technology to hydrogenated amorphous silicon-based FETs and is a great candidate for practical applications in electronic devices.273-282
Pranabesh Dutta, Wooseung Yang, Seung Hun Eom, Soo-Hyoung Lee. Synthesis and characterization of triphenylamine flanked thiazole-based small molecules for high performance solution processed organic solar cells
Abstract:
Two new small molecules, 5,5-bis(2-triphenylamino-3-decylthiophen-2-yl)-2,2-bithiazole (M1) and 2,5-bis(2-triphenylamino-3-decylthiophen-2-yl)thiazolo[5,4-d]thiazole (M2) based on an electron-donor triphenylamine unit and electron-acceptor thiophene–thiazolothiazole or thiophene–bithiazole units were synthesized by a palladium(0)-catalyzed Suzuki coupling reaction and examined as donor materials for application in organic solar cells. The small molecules had an absorption band in the range of 300–560 nm, with an optical band gap of 2,22 and 2,25 for M1 and M2, respectively. As determined by cyclic voltammetry, the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of M1 were -5,27 eV and -3,05 eV, respectively, which were 0.05 eV and 0,02 eV greater than that of M2 Photovoltaic properties of the small molecules were investigated by constructing bulk-heterojunction organic solar cell (OSC) devices using M1 and M2 as donors and fullerene derivatives, 6,6-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) and 6,6-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as acceptors with the device architecture ITO/PEDOT:PSS/M1 or M2:PCBM/LiF/Al. The effect of the small molecule/fullerene weight ratio, active layer thickness, and processing solvent were carefully investigated to improve the performance of the OSCs. Under AM 1,5 G 100 mW/cm² illumination, the optimized OSC device with M1 and PC71BM at a weight ratio of 1:3 delivered a power conversion efficiency (PCE) of 1,30%, with a short circuit current of 4,63 mA/cm², an open circuit voltage of 0,97 V, and a fill factor of 0,29 In contrast, M2 produced a better performance under identical device conditions. A PCE as high as 2,39% was recorded, with a short circuit current of 6,49 mA/cm², an open circuit voltage of 0,94 V, and a fill factor of 0,39283-289
Yi-Hao Chen a, Ping-Tsung Huang a,⇑, Keng-Ching Lin b, Yu-Jui Huang c, Chin-Ti Chen. Stabilization of poly(3-hexylthiophene)/PCBM morphology by hydroxyl group end-functionalized P3HT and its application to polymer solar cells
Abstract:
A new concept to stabilize the morphology of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) blend through H-bond formation by using a hydroxyl group end-functionalized P3HT (HOC-P3HT-COH) as a compatibilizer is presented. Domain size of the PCBM crystals in the annealed P3HT/PCBM film is diminished with addition of HOC-P3HT-COH. Surface roughness of the P3HT/PCBM film also becomes smoother with addition of HOC-P3HT-COH. Thermal stability of solar cell device is improved significantly through the H-bond formation between HOC-P3HT-COH and PCBM. A high performance and thermal stable polymer solar cell with 4,06% power conversion efficiency under AM1,5G irradiation is fabricated with 5% HOC-P3HT-COH in P3HT/PCBM layer.290-296
Ali Veysel Tunc, Antonietta De Sio, Daniel Riedel, Felix Deschler, Enrico Da Como, Jürgen Parisi, Elizabeth von Hauff. Molecular doping of low-bandgap-polymer:fullerene solar cells: Effects on transport and solar cells
Abstract:
We show how molecular doping can be implemented to improve the performance of solution processed bulk heterojunction solar cells based on a low-bandgap polymer mixed with a fullerene derivative. The molecular dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) is introduced into blends of poly[2,6(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b0]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) via co-solution in a range of concentrations from 0% to 1%. We demonstrate that the hole conductivity and mobility increase with doping concentration using field-effect measurements. Photoinduced absorption (PIA) spectroscopy reveals that the polaron density in the blends increases with doping. We show that the open circuit voltage and short circuit current of the corresponding solar cells can be improved by doping at 0,5%, resulting in improved power conversion efficiencies. The increase in performance is discussed in terms of trap filling due to the increased carrier density, and reduced recombination correlated to the improvement in mobility.297-301
Shou-Yuan Ma, Yu-Min Shen, Po-Ching Yang, Chao-Shuo Chen, Ching-Fuh Lin. Morphological modification induced by external electric field during solution process of organic solar cells
Abstract:
An electric field was externally applied on the poly(3-hexylthiophene)/[6,6]-phenyl C₆₁ butyric acid methyl ester (P3HT:PCBM) blend film during the fabrication of the bulkheterojunction (BHJ) solar cells to induce morphological modification. It influences the vertical ratio of P3HT:PCBM molecules. Because the field is applied externally to the device, its direction can be altered. When the electric field of 5,0 x 10⁵ Vm^-1 was applied with the specific direction, it formed a P3HT-rich and rougher surface, compared with that of pristine active layer, to improve the performance of the inverted polymer solar cells. Hence, the current density was improved from 9,15 mA cm^-2 to 9,83 mA cm^-2, and power conversion efficiency increased from 3,16% to 3,51%. This finding provides guidance for morphology engineering in organic materials for higher power conversion efficiency of organic solar cells.302-308
Feng Xu, Wen Qing Zhu, Long Yan, Hong Xu, Ling Hao Xiong, Jia Heng Li. Single walled carbon nanotube anodes based high performance organic light-emitting diodes with enhanced contrast ratio
Abstract:
Commercially-available single walled carbon nanotubes (SWCNTs) were used to fabricate SWCNT sheets for anodes of organic light-emitting diodes (OLEDs) by spray-coating process without any use of surfactant or acid treatment. A layer of DMSO doped PEDOT:PSS was spray-coated on the SWCNT sheets to not only lessen the surface roughness to an acceptable level, but also improve the conductivity by more than three orders of magnitude. For our SWCNT-based OLEDs of tris-(8-hydroxquinoline) aluminum (Alq₃) emission layers, a maximum luminance 4224 cd/m² and current efficiency 3,12 cd/A were achieved, which is close to the efficiency of ITO-based OLEDs. We further found out that our OLEDs based on the PEDOT:PSS covered SWCNT anodes tripled the contrast ratio of the conventional indium tin oxide (ITO) based OLEDs.309-319
Kaname Kanai, Masato Honda, Hisao Ishii, Yukio Ouchi, Kazuhiko Seki. Interface electronic structure between organic semiconductor film and electrode metal probed by photoelectron yield spectroscopy
Abstract:
We present an investigation of the interface between organic semiconductor films and metal substrates (organic/metal interface) using photoelectron yield spectroscopy (PYS) as the probing technique. PYS studies were conducted on the pentacene/Au, copper phthalocyanine (CuPc)/Au, and perfluorinated zinc phthalocyanine (F₁₆ZnPc)/Au, and the results were compared with literature results obtained using conventional ultraviolet photoemission spectroscopy (UPS). PYS is advantageous for probing the electronic structure of the organic/metal interface because of the relatively long mean free path of photoexcited electrons with very low kinetic energy in PYS, which enables the detection of the photoelectrons from the metal substrate buried deep in the organic film. We demonstrate herein that the use of PYS reduces the significance of the final state effect of the electronic density surrounding the photohole at the organic molecule generated after the photoemission; this effect is known as the electric polarization effect. Although this effect has a significant influence on the results obtained using conventional UPS, the reduced influence of the final state effect in PYS makes it possible to construct an energy level diagram at the organic/metal interface with greater accuracy than can be achieved with UPS. In addition, a novel mechanism of the photoelectron detection for PYS enables us to apply PYS to very thick organic films, and therefore, PYS provides a reliable value of ionization energy for organic films without the influence of the substrate. Because the interface electronic structure has a significant influence on the carrier injection properties of organic devices, the increased reliability of the information obtained by PYS will render it very useful for the improvement of device performance as well for understanding their operation principles.320-328
Xiaoyang Cheng, Mario Caironi, Yong-Young Noh, Christopher Newmand, Jianpu Wang, Mi Jung Lee, Kal Banger, Riccardo Di Pietro, Antonio Facchetti, Henning Sirringhaus. Downscaling of n-channel organic field-effect transistors with inkjet-printed electrodes
Abstract:
In this contribution we demonstrate for the first time a downscaled n-channel organic field-effect transistors based on N,N'-dialkylsubstituted-(1,7:1,6)-dicyanoperylene-3,4:9,10-bis(dicarboximide) with inkjet printed electrodes. First we demonstrate that the use of a high boiling point solvent is critical to achieve extended crystalline domains in spincoated thin films and thus high electron mobility 0,1 cm² V^-1 s^-1 in top-gate devices. Then inkjet-printing is employed to realize sub-micrometer scale channels by dewetting of silver nanoparticles off a first patterned gold contact. By employing a 50 nm crosslinked fluoropolymer gate dielectric, ~200 nm long channel transistors can achieve good current saturation when operated 5 V with good bias stress stability.329-334
V.K. Chandra, B.P. Chandra. Transient electroluminescence in heavy metal complex-based phosphorescent organic light-emitting diodes
Abstract:
In phosphorescent organic light-emitting diodes (PHOLEDs), both the rise time and decay time decrease with increasing amplitude of the applied voltage pulse. The rise time sr of the transient electroluminescence (TEL) increases linearly with increasing value of the ratio of voltage V to the current j, that is, with V/j. Using the equations for the dynamics of charge carriers an expression is derived for the rise time т_r of the TEL in OLEDs. It is shown that т_r should increase with increasing values of the ratio (V/j), dielectric constant ε, and area of cross-section of the emission layer, however, it should decrease with the thickness of emission layer. For higher values of the applied voltage nonlinearity occurs in the sr versus V/j plot because the increase in mobility of carriers at high electric field causes increase in the current flowing through the OLEDs. In fact, the rise time of TEL is related to the product of capacitance and effective resistance of the OLED. Considering the rate of generation and decay of radiative triplet excitons in the emission layer, an expression is derived for the decay time of TEL in PHOLEDs and it is shown that, for higher values of the time-constant of OLED, the decay time should be equal to the time-constant, however, for lower values of the time-constant, the decay time should be equal to the lifetime of radiative triplet excitons in the emission layer. A good agreement is found between the theoretical and experimental results.335-340
Tetsuro Hori, Tetsuya Masuda, Naoki Fukuoka, Takeshi Hayashi, Yasuo Miyake, Toshiya Kamikado, Hiroyuki Yoshida, Akihiko Fujii, Yo Shimizu, Masanori Ozaki. Non-peripheral octahexylphthalocyanine doping effects in bulk heterojunction polymer solar cells
Abstract:
The improvement of long-wavelength sensitivity in bulk heterojunction organic thin-film solar cells based on poly(3-hexylthiophene) (P3HT) by the addition of the soluble phthalocyanine derivative, 1,4,8,11,15,18,22,25-octahexylphthalocyanine (C₆PcH₂) is reported. C₆PcH₂ possesses near-infrared absorption and can be mixed with a P3HT:1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6)C₆₁ (PCBM) bulk heterojunction active layer. By doping C₆PcH₂, the photosensitivity in the long-wavelength region was improved, and the energy conversion efficiency reached 3,0% at a composition ratio of P3HT:C₆PcH₂:PCBM = 10:3:10 We discuss the principle of photoconversion in the bulk heterojunction solar cell based on the P3HT:C₆PcH₂:PCBM active layer by taking into consideration the existence of both highly ordered P3HT domains and hexagonal columnar structures of C₆PcH₂, and the microphase separation of P3HT and C₆PcH₂ in the active layer.341-349
Chang Woo Seo, Ji Hwan Yoon, Jun Yeob Lee. Engineering of charge transport materials for universal low optimum doping concentration in phosphorescent organic light-emitting diodes
Abstract:
A universal low optimum doping concentration of below 5% was demonstrated in phosphorescent organic light-emitting diodes (PHOLEDs) by managing the energy levels of charge transport materials. The device performances of PHOLEDs could be optimized at a low doping concentration of 3% irrespective of the host material in the emitting layer. The suppression of charge trapping and hopping by the dopant through charge transport layer engineering optimized the device performance at low doping concentration. In addition, it was revealed that PHOLEDs with low optimum doping concentration show better quantum efficiency, low efficiency roll-off and low doping concentration dependency of the device performance.