Organic Electronics 2007 Volume 8 №06

Elsevier. — 171 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.641-647
S. Cheylan, H.J. Bolink, A. Fraleoni-Morgera, J. Puigdollers, C. Voz, I. Mencarelli, L. Setti, R. Alcubilla, G. Badenes. Improving the efficiency of light-emitting diode based on a thiophene polymer containing a cyano group
Abstract:
We report on the overall improvement of a single layer organic light-emitting diode device based on poly{[3-hethylthiophene]-co-3-[2-(p-cyano-phenoxy)ethyl]thiophene} or namely PTOPhCN. This polymer was recently developed by adding a cyano group as a side-chain substituent of the thiophenic backbone onto the main polymer chain and showed promising results for light-emitting diode devices. Using an improved device layout, bright red electroluminescence was obtained at 4 V and showed a luminance of about 400 cd/m² at 8 V with current densities in the order of 6000 A/m².648-654
Philipp Stadler, Kerstin Oppelt, Thokchom Birendra Singh, James G. Grote, Reinhard Schwodiauer, Siegfried Bauer, Heidi Piglmayer-Brezina, Dieter Bauerle, Niyazi Serdar Sariciftci. Organic field-effect transistors and memory elements using deoxyribonucleic acid (DNA) gate dielectric
Abstract:
Deoxyribonucleic acid (DNA) bio-polymers derived from fish waste products are employed as gate dielectric in n-type methanofullerene as well as p-type pentacene based organic field-effect transistors working at low voltage levels and low gate leakage currents. Based on the large hysteresis in the transfer characteristics, operation of the transistor as a non-volatile memory element is shown. Practically hysteresis free operation of DNA based transistors is obtained at low voltage levels by adding an additional aluminium oxide blocking layer between the organic semiconductor and the DNA gate dielectric.655-661
Yi Chen, Ishiang Shih. Fabrication of vertical channel top contact organic thin film transistors
Abstract:
Vertical channel top contact (TC) organic thin film transistors (OTFTs) have been successfully realized on Si substrates with SiO₂ as gate insulators and P3HT(poly-3-hexylthiophene) as organic semiconductors. The active channel region was defined by a steep step through a Si etching method. Source and drain metal contacts were deposited by vacuum evaporation through a shadow mask at a high tilting angle. Top contact transistors with channel lengths 5 lm can be fabricated with a relatively simple and efficient (yield 85%) fabrication process with only two photolithography steps (two photo masks) while no need for high-resolution and precision alignment for channel definition. Measurement results showed that the vertical channel BC (bottom contact) devices have compatible performance with planar BC devices. However, vertical channel TC transistors showed improved performance with double field effect mobilities and three times larger current ON/OFF ratios than vertical channel BC devices.662-672
Cha-Wen Chang, Guey-Sheng Liou. Stably anodic green electrochromic aromatic poly(amine–amide–imide)s: Synthesis and electrochromic properties
Abstract:
A series of new poly(amine–amide–imide)s with pendent 4-methoxy-substituted triphenylamine (TPA) units having inherent viscosities of 0.35–0.45 dL/g were prepared from various aromatic bis(trimellitimide)s and the 4-methoxy-substituted triphenylamine-based aromatic diamine, 4,40-diamino-400-methoxytriphenylamine (I), by direct polycondensation. All the polymers are readily soluble in polar organic solvents. Flexible and amorphous films of these poly(amine–amide–imide)s could be obtained by solution-casting, and showed excellent thermal stability, 10% eight-loss temperatures in excess of 515 °C, and char yields at 800 °C in nitrogen higher than 58% associated with high glass-transition temperatures (297–305 °C). These polymers exhibited a maximum UV–vis absorption at 302–304 nm with fluorescence emission MAXIMA around 360–376 nm in N-methyl-2-pyrrolidinone (NMP) solution. The hole-transporting and electrochromic properties were examined by electrochemical and spectroelectrochemical methods. Cyclic voltammograms of the poly(amine–amide–imide) films cast onto an indium–tin oxide (ITO)-coated glass substrate exhibit a reversible oxidation at 0,79–0,80 V vs. Ag/AgCl in acetonitrile solution, and reveal good stability of electrochromic characteristics with a color change from yellow to green at applied potentials ranging from 0,00 to 0,95 V. These anodically polymeric electrochromic materials not only showed excellent reversible electrochromic stability with good green coloration efficiency (CE = 395 cm²/C) but also exhibited high contrast of optical transmittance change (DT%) up to 78% in 768 nm. After over 100 cyclic switches, the polymer films still exhibited stable electrochromic characteristics.673-682
Hua-Ping Zhao, Xu-Tang Tao, Ping Wang, Yan Ren, Jia-Xiang Yang, Yun-Xing Yan, Chun-Xue Yuan, Hui-Jun Liu, De-Chun Zou, Min-Hua Jiang. Effect of substituents on the properties of indolo[3,2-b]carbazole-based hole-transporting materials
Abstract:
Three new compounds based on indolo[3,2-b]carbazole,2,8-bis(4-diphenylaminophenyl)-5,11-di-n-octylindolo[3,2-b]carbazole (BTOICZ), 2,8-bis(9,90-di-n-butylfluorenyl)-5,11-di-n-octylindolo[3,2-b]carbazole (BFOICZ) and 2,8-bis[N-(n-butyl)carbazolyl]-5,11-di-n-octylindolo[3,2-b]carbazole (BCOICZ), as hole-transporting materials have been synthesized by Suzuki coupling reaction. The effects of substituents on the optical, thermal and electrochemical properties of indolo[3,2-b]carbazole derivatives have been studied carefully. Electroluminescent (EL) devices using these compounds as hole-transporting layer in combination with Alq₃ as electron-transporting and emitting layer have been fabricated and characterized. The devices based on BTOICZ or BFOICZ showed green emission at 526 nm, while the device based on BCOICZ emitted very weak light. The turn-on voltages are 4,1 and 5,4 V for BTOICZ and BFOICZ, respectively. The maximal luminance efficiencies are 0,738 and 0,767 lm/W for BTOICZ and BFOICZ, respectively. The difference of the EL performances of these compounds reveals that the substituent effects have great effect on the hole-transporting performance of the indolo[3,2-b]carbazole-based compounds.683-689
C.H. Jeong, J.T. Lim, M.S. Kim, J.H. Lee, J.W. Bae, G.Y. Yeom. Four-wavelength white organic light-emitting diodes using 4,4'-bis-[carbazoyl-(9)]-stilbene as a deep blue emissive layer
Abstract:
White organic light-emitting diodes (WOLEDs) with four wavelengths were fabricated by using three doped layers, which were obtained by separating recombination zones into three emitter layers. Among these emitters, blue emissions with two wavelengths (456 and 487 nm) were occurred in the 4,4'-bis(carbazoyl-(9))-stilbene (BCS) host doped with a perylene dye. Also, a green emission was originated from the tris(8-quinolinolato)aluminum (III) (Alq3) host doped with a green fluorescent
of 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-[1]benzopyrano [6,7,8-ij]-quinolizin-11-one (C545T) dye. Finally, an orange emission was obtained from the N,N'-bis(1-naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (NPB) host doped with a 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) dye. The white light could be emitted by simultaneously controlling the emitter thickness and concentration of fluorescent dyes in each emissive layer, resulting in partial excitations among those three emitter layers. Electroluminescent spectra of the device obtained in this study were not sensitive to driving voltage of the device. Also, the maximum luminance for the white OLED with the CIE coordinate of (0,34, 0,34) was 56,300 cd/m² at the applied bias voltage of 11,6 V. Also, its external quantum and the power efficiency at about 100 cd/m² were 1,68% and 2,41 lm/W, respectively.690-694
Dong-Jin Yun, Dong-Kyu Lee, Hwan-Ki Jeon, Shi-Woo Rhee. Contact resistance between pentacene and indium–tin oxide (ITO) electrode with surface treatment
Abstract:
Contact resistance between indium–tin oxide (ITO) electrode and pentacene was studied by transmission line method (TLM). Organic solvent cleaned, inorganic alkali cleaned, and self-assembled monolayer (with OTS: octadecyltrichlorosilane) modified ITO electrode structures were compared. Pentacene layer of 300 Å thickness was vacuum deposited on patterned ITO layer at 70 °C with a deposition rate of 0,3 Å/s. Alkali cleaned and SAM modified ITO gave a lower contact
resistance of about 6,34 x 10⁴ Ω cm² and 1,88 x 10³ Ω cm², respectively than organic solvent cleaned ITO of about 6,58 x 10⁵ Ω cm². Especially with the SAM treatment, the work function of ITO increased closer to the highest occupied molecular orbital (HOMO) level of pentacene, which lowers the injection barrier between ITO and pentacene. It was also believed that pentacene morphology was improved on SAM modified ITO surface due to the lowering of the surface energy. We could obtain the low contact resistance with SAM treatment which is comparable to the measured value of gold–pentacene contact, 1,86 x 10³ Ω cm². This specific contact resistance is still much higher than that of amorphous silicon thin film transistor (0,1–30 Ω cm²).695-701
Michelle S. Meruvia, Jose A. Freire, Ivo A. Hu¨mmelgen, Jonas Gruber, Carlos F.O. Graeff. Magnetic field release of trapped charges in poly(fluorenylenevinylene)s
Abstract:
We have measured the effect of an applied magnetic field on the current through thin films of two different organic conjugated polymers that have previously shown to exhibit magnetoresistance, poly(9,9-dioctyl-1,4-fluorenylenevinylene) and poly(9,9-dioctyl-2,7-fluorenylenevinylene). The results show that the magnetic field releases trapped charges from inside the material and enhances the current. We have also performed optical absorption experiments on these polymer films under applied voltage and magnetic field. The results show that the magnetic field produces a change in the optical absorption in the low-energy range associated with deep traps and only under conditions when these traps are likely charged. These two results provide a strong case for the release of trapped charges caused by the magnetic field as being the cause of the magnetoresistance in these polymers and possibly in other organic materials where magnetoresistance was recently observed.702-708
Takeaki Sakurai, Ryosuke Fukasawa, Kazuhiro Saito, Katsuhiro Akimoto. Control of molecular orientation of organic p-i-n structures by using molecular templating effect at heterointerfaces
Abstract;
Structural control of the organic intrinsic layer (i-layer), which consists of a mixture of p- and n-type organic semiconductor materials, and the organic p-i-n triple-layer structure has been investigated. Significant modification of these structures, which used metal-free phthalocyanine (H₂Pc) and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) as the p- and n-type materials, respectively, was observed by introducing a modification layer deposited on a glass substrate. The orientation of the molecular planes of both H₂Pc and PTCDA in the i-layer (H₂Pc:PTCDA mixture) and p-i-n structure (H₂Pc/i-layer/PTCDA) formed onto a PTCDA modification layer was parallel to the substrate surface. On the other hand, when H₂Pc was used as the modification layer, the orientation of the molecular planes in the i-layer was found to be random. These results indicate that the internal structure of the organic mixed-layer is strongly affected by the organic layersubstrate interface. The enhancement in optical absorption observed for the orientation controlled p-i-n structure suggests the importance of the control of molecular orientation for improving the optical properties in organic device structures.709-717
Sebastian Scholz, Cathrin Corten, Karsten Walzer, Dirk Kuckling, Karl Leo. Photochemical reactions in organic semiconductor thin films
Abstract:
We investigate the dissociation and the photochemical behavior of different organic semiconductor materials typically employed in organic light emitting diodes (OLED) and other organic electronic devices, using laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS). The study is subdivided into two parts: The first part is an investigation of several hole conductive materials from the class of triarylamines commonly used in OLEDs and organic solar cells. Upon LDI measurements, this class of materials does not show any photochemical reactions. The second part of the work deals with two typical electron conductive materials: tris(8-hydroxyquinolinato)-aluminum (Alq3) and 4,7-diphenyl-1,1'-phenanthroline (BPhen). These materials show photochemical reactions in terms of oligomerization. The individual reaction pathways will be discussed in detail. We compare these reactions by further studying two similar molecules (1,1'-biphenyl-4'-oxy)bis(8-hydroxy-2-methylquinolinato)-aluminum (BAlq) and bathocuproine (BCP). Finally, we discuss a possible relation of the results to degradation mechanisms taking place in organic devices.718-726
Xiao-Hong Zhang, Benoit Domercq, Xudong Wang, Seunghyup Yoo, Takeshi Kondo, Zhong Lin Wang, Bernard Kippelen. High-performance pentacene field-effect transistors using Al₂O₃ gate dielectrics prepared by atomic layer deposition (ALD)
Abstract:
High-performance pentacene field-effect transistors have been fabricated using Al₂O₃ as a gate dielectric material grown by atomic layer deposition (ALD). Hole mobility values of 1,5 ± 0,2 cm²/V s and 0,9 ± 0,1 cm²/V s were obtained when using heavily n-doped silicon (n⁺-Si) and ITO-coated glass as gate electrodes, respectively. These transistors were operated in enhancement mode with a zero turn-on voltage and exhibited a low threshold voltage ( -10 V) as well as a low sub-threshold slope ( 1 V/decade) and an on/off current ratio larger than 10⁶. Atomic force microscopy (AFM) images of pentacene films on Al₂O₃ treated with octadecyltrichlorosilane (OTS) revealed well-ordered island formation, and X-ray diffraction patterns showed characteristics of a ‘‘thin film’’ phase. Low surface trap density and high capacitance density of Al₂O₃ gate insulators also contributed to the high performance of pentacene field-effect transistors.727-734
Mathieu Urien, Guillaume Wantz, Eric Cloutet, Lionel Hirsch, Pascal Tardy, Laurence Vignau, Henri Cramail, Jean-Paul Parneix. Field-effect transistors based on poly(3-hexylthiophene): Effect of impurities
Abstract:
Organic field-effect transistors (OFETs) based on regioregular poly(3-hexylthiophene)s (P3HT)s have been studied as a function of the amount of impurities in the active polymer. P3HTs have been synthesized via a nickel-initiated cross-coupling polymerization and successively purified by a series of Soxhlet extractions with methanol, hexane and chloroform. At each stage, the amount of impurities was quantified by means of 1H nuclear magnetic resonance (NMR) spectroscopy, Rutherford backscattering spectroscopy (RBS) and particle induced X-ray emissions (PIXE). Traces of Ni, Cl, Mg, Ca, Fe and Zn could be detected in non-fully purified P3HTs. The presence of impurities in the different fractions of P3HT is shown to affect not only the characteristics of the OFETs but also the photovoltaic performances.735-742
Jwo-Huei Jou, Ming-Hsuan Wu, Chung-Pei Wang, Yung-Sheng Chiu, Po-Hsuan Chiang, Huei-Ching Hu, Ren-Yang Wang. Efficient fluorescent white organic light-emitting diodes using co-host/emitter dual-role possessed di(triphenyl-amine)-1,4-divinyl-naphthalene
Abstract:
Efficient fluorescent white organic light-emitting diodes with low carrier-injection barriers were fabricated with device structure of indium tin oxide/N,N'-bis-(1-naphthy)-N,N'-diphenyl-1,1'-biphenyl-4-4'-diamine/white emission layer/1,3,5-tris(N-phenyl-benzimidazol-2-yl)benzene/lithium fluoride/aluminium. By blending in the blue host of 1-butyl-9,10-naphthalene-anthracene in the emissive layer an efficient electro-luminescent greenish-blue co-host of di(triphenyl-amine)-1,4-divinyl-naphthalene, with the doping of a trace amount of red dye of 4-(dicyano-methylene)-2-methyl-6-(julolidin-4-yl-vinyl)-4H-pyran, bright and colour-stable white emission with high power-efficiency of 14.6 lm/W at 100 cd/m2 or current efficiency of 19,2 cd/A at 300 cd/m² or 18,7 cd/A at 10,000 cd/m² was obtained. The resulted synergistic increase in brightness and efficiency may be attributed to the presence of cascading new routes with comparatively lower electron injection barrier.743-748
Chaun Gi Choi, Byeong-Soo Bae. Organic–inorganic hybrid materials as solution processible gate insulator for organic thin film transistors
Abstract:
We synthesized organic–inorganic hybrid materials (hybrimers) using a simple non-hydrolytic sol–gel reaction and applied the materials as gate insulators in organic thin film transistors (OTFTs). The hybrimer thin films had smooth and hydrophobic surfaces, and were stable with solvents. In addition, the hybrimer thin films had good electrical properties such as low leakage current and high dielectric strength. The performance of the OTFT with hybrimer gate insulator fabricated by drop casting of regioregular poly(3-hexylthiophene) (P3HT) was similar to that of OTFT with hexamethyldisilazane (HMDS) treated thermally grown SiO₂. The hysteresis of RR-P3HT based OTFT with hybrimer gate insulator was negligible.749-758
Amare Benor, Arne Hoppe, Veit Wagner, Dietmar Knipp. Electrical stability of pentacene thin film transistors
Abstract:
The influence of environmental conditions on the device operation and the stability of polycrystalline pentacene thin film transistors (TFTs) were investigated. Electrical in-situ and ex-situ measurements of staggered pentacene TFTs were carried out to study the influence of dry oxygen and moisture on the device stability. The transistors were fabricated by organic molecular beam deposition on thermal oxide dielectrics. Oxygen exposure of the pentacene films lead to the creation of acceptor-like states in the bandgap. The acceptor-like states cause a shift of the onset of the drain current towards positive gate voltages. The charge carrier mobility and the on/off ratio of the transistor are not affected by the acceptor-like states. Furthermore, the acceptor-like states have an influence on the stability of the TFTs. Devices exposed to oxygen exhibit a shift of the threshold voltage upon prolonged biasing. Transistors characterized under vacuum conditions (no oxygen exposure) do not exhibit a shift of the threshold voltage (bias stress effect) as a consequence of prolonged biasing. The experimental results show a clear correlation between the device behavior upon oxygen exposure and the stability of the devices. The shift of the onset voltage upon oxygen exposure correlates with the shift of the threshold voltage upon prolonged bias. The influence of dry oxygen on the onset voltage, the threshold voltage, and the electrical stability will be described. Furthermore, the influence of bias stress on the operation of organic circuits like an active matrix addressed OLED displays will be discussed.759-766
Hiroshi Wada, Tomohiro Taguchi, Bunpei Noda, Takuya Kambayashi, Takehiko Mori, Ken Ishikawa, Hideo Takezoe. Air stability of n-channel organic transistors based on nickel coordination compounds
Abstract:
A series of tetraphenyl nickel dithiolene complexes, 1, are prepared, and the air stability of the n-channel organic fieldeffect transistors (OFET) is investigated. The fluoro and trifluoromethyl derivatives with reduction potentials higher than 0 V afford reasonably air-stable n-channel OFET. Metallic organic charge-transfer complex (TTF)(TCNQ) (TTF: tetrathiafulvalene and TCNQ: tetracyanoquinodimethane) is used as source and drain electrodes, and realizes n-channel transistor properties even when Al electrodes do not work.767-772
Chiao-Shun Chuang, Fang-Chung Chen, Han-Ping D. Shieh. Organic thin-film transistors with reduced photosensitivity
Abstract:
Organic thin-film transistors with a minimal threshold voltage shift and a more stable photocurrent under illuminated conditions can be made by embedding titanium dioxide (TiO₂) nanoparticles into a polymer dielectric layer. While the studies of pentacene morphology using atomic force microscopy and X-ray diffraction have shown similar results before and after embedding the TiO₂ into the dielectric layer, it has been found that the presence of TiO₂ nanoparticles is essential. The reduced photosensitivity can be attributed to the recombination centers induced by the TiO₂ nanopartilces and this can enhance substantially the recombination process of the electrons trapped in the channel.773-783
Seung-Hwan Oh, Seok-In Na, Yoon-Chae Nah, Doojin Vak, Seok-Soon Kim, Dong-Yu Kim. Novel cationic water-soluble polyfluorene derivatives with ion-transporting side groups for efficient electron injection in PLEDs
Abstract:
Synthesis of cationic water-soluble polyfluorene derivatives with various side groups, which are used as electron injecting layers in polymer light emitting diodes, is described. Neutral polyfluorene derivatives containing bromo-alkyl terminal groups were synthesized by a palladium catalyzed Suzuki coupling reaction. The bromo-alkyl terminal groups in the neutral polyfluorenes were quaternized by treatment with a trimethyl amine solution. When a high work-function metal such as Ag is used as a cathode in a light emitting diode with an ITO/PEDOT:PSS/MEH-PPV/water-soluble polyfluorene/Ag configuration, effects of these water-soluble polyfluorenes on the device performance were investigated. In the case of poly[(9,9-bis((60-(N,N,N-trimethylammonium) hexyl)-2,7-fluorene))-alt-(9,9-bis(2-(2-methoxyethoxy)ethyl)-fluorene)] Dibromide (WPFoxy-F) containing ethylene oxide groups as the electron injecting layer, the electroluminescence efficiency of light emitting devices was significantly enhanced by about two orders of magnitude compared to that of a device without an electron injecting layer because migration of bromide ions via the ethylene oxide side groups led to large space charge. As a result, the injection barrier could be reduced between the emitting layer and Ag cathode resulting high electroluminescence efficiency.784-790
T. Basova, A.G. Gurek, V. Ahsen, A.K. Ray. Electrical properties of dysprosium phthalocyanine films
Abstract:
Forty-four nanometers thick films of alkyl-substituted dysprosium phthalocyanine molecules were prepared on carefully cleaned indium tin oxide (ITO) substrates by the spin casting method. UV–vis absorption spectra and dc conduction characteristics were obtained in order to investigate the structure–conduction property relationship. The conduction is Ohmic at low applied voltages but the space charge limited mechanism is prevalent at relatively high voltages. The conductivities of both as-deposited and heat treated films are found to show directionally anisotropic behaviour. The columnar conductivity was found to be significantly large in the liquid crystalline state.791-795
S. Sax, G. Mauthner, T. Piok, S. Pradhan, U. Scherf, E.J.W. List. Intrinsic electrochemical doping in blue light emitting polymer devices utilizing a water soluble anionic conjugated polymer
Abstract:
We report on light emitting device related properties of the water soluble blue light emitting anionic poly{1,4-phenylene-[9,9-bis(4-phenoxy-butylsulfonate)]fluorene-2,7-diyl} copolymer (PBS-PFP) with alkyl sulfonic acid sodium side chains. Compared to organic solvents water has the advantage of being environmentally friendly, which simplifies the handling during device fabrication by novel preparation techniques such as ink-jet printing. Light emitting devices fabricated from this polymer in thin film configuration show stable blue emission with an emission maximum at approximately 420 nm. Investigations concerning the current–density/voltage and the luminance/voltage device characteristics as well as the current–density over operation time at constant biases allow the identification of intrinsic electrochemical doping effects caused by ionic groups attached on the side chains of the polymer. Due to this fact, firstly the electroluminescence onset voltage for devices using high work-function electrodes for electron injection such as aluminum, which has the advantage of being air stable, can be distinctly reduced, and secondly devices fabricated from this polymer showed luminance in forward bias direction as well as in reverse bias direction.796-800
Bjorn Winther-Jensen, Maria Forsyth, Keld West, Jens Wenzel Andreasen, Gordon Wallace, Douglas R. MacFarlane. High current density and drift velocity in templated conducting polymers
Abstract:
Conducting polymers prepared by a templated vapour phase polymerisation process involving solid phase transition metal complexes are found to produce polymers with charge carriers that exhibit maximum drift velocity in the range of 1 m/s. This super-mobility seems to be related to a high degree of ordering in the materials as evidenced by the X-ray diffraction data. This may result from a templated polymerisation process. The high mobility manifests itself as a capacity to sustain very high current densities ( 10000 A/cm²); such high current densities are of importance in thin film conductor applications.801-812
Bimlesh Lochab, Paul L. Burn, Aaron Barkhouse, Kiril R. Kirov, Hazel E. Assender, David J. Keeble, Elizabeth A. Thomsen, Andrew J. Lewis, Ifor D.W. Samuel. Electronically asymmetric poly(1,4 phenylenevinylene)s for photovoltaic cells
Abstract:
We report a study on the feasibility of using an intramolecular dipole to enhance charge separation for organic photovoltaic applications. We have developed a poly(1,4-phenylenevinylene) derivative, poly{2-[(9,9-di-n-propyl-9H-7-nitrofluoren-2-yl)]-5-(2-ethylhexyloxy)-1,4-phenylenevinylene} 12, which has electronically asymmetric chromophores as a result of the para attachment of alkoxy and nitrofluorenyl groups on the phenyl units of the polymer backbone. The luminescence of the polymer with the electronically asymmetric chromophores was found to be smaller by a factor of eight when compared to the equivalent polymer but without the nitro group attached to the fluorenyl unit. Light-induced electron spin resonance experiments showed that the polymer with the electronically asymmetric chromophores had more photo-induced spins, showing that the quenching of the luminescence was due to intramolecular charge separation. Single layer photovoltaic devices containing neat films of 12 showed efficiencies similar to other single layer devices and this was ascribed to poor transport of the separated charges. This was confirmed by a strong improvement in device power efficiency to 0,52% at AM1,5 by blending the polymer with an electron transporting material.