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O. Usluer,
C. Kästner,
M. Abbas,
C. Ulbricht,
V. Cimrova,
A. Wild,
E. Birckner,
N. Tekin, N.S. Sariciftci,
H. Hoppe,
S. Rathgeber,
D.A.M. Egbe
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ABSTRACT: This article reports on the synthesis, characterization, and properties of various anthracene-containing poly (p-phenylene-ethynylene)-alt-poly(p- phenylene-vinylene) (PPE-PPV) polymers (AnE-PVs) bearing statistical distributions of various side chains. Primarily, the ratio of linear octyloxy and branched 2-ethylhexyloxy side chains at the poly(p-phenylene vinylene) (PPV) parts was varied, leading to the polymers stat, stat1, and stat2. Furthermore, polymers also containing asymmetric substituted PPV and poly(p-phenylene ethynylene) units (bearing methoxy and 2-ethylhexyloxy side chains) were prepared yielding stat3, stat4, and stat5. These materials exhibit a broad variation in their photovoltaic properties. It is once more shown that side chains and their distribution can crucially affect the photovoltaic device performance. The introduction of units with asymmetric substitution into these systems seems to be harmful for their utilization in photovoltaic applications. Organic field-effect transistors were fabricated to investigate hole mobilities in these new materials. Large variance was observed, falling in the range of almost two orders of magnitude, indicating rather different pi-pi stacking behavior of the polymer backbones owing to side-chain modifications. Moreover, a selection of the new polymeric systems was investigated regarding their potential for light-emitting diode (LED) applications. Polymer LEDs using the polymers AnE-PVstat, -stat3, -stat4, and -stat5, as the active layer showed turn-on voltage of 2 V and exhibited red light emission.
Journal of Polymer Science Part A Polymer Chemistry 02/2013; 50:3425-3436. · 3.92 Impact Factor
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ABSTRACT: The temperature dependence of the field-effect mobility is investigated in vacuum evaporated C60-based organic field-effect transistors. The results show a thermally activated behavior with an activation energy that depends
on the field-induced charge carrier density in the transistor channel. Upon extrapolation of the data in an Arrhenius plot
we find an empirical relation, termed the Meyer–Neldel rule, which states that the mobility prefactor increases exponentially
with the activation energy. Based on this analysis a characteristic temperature is extracted. The possible implications of
this observation in terms of charge transport in fullerene-based field-effect transistors are discussed.
Applied Physics A 04/2012; 97(3):521-526. · 1.63 Impact Factor
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O. Usluer,
C. Kästner,
M. Abbas,
C. Ulbricht,
V. Cimrova,
A. Wild,
E. Birckner,
N. Tekin, N.S. Sariciftci,
H. Hoppe,
S. Rathgeber,
D.A.M. Egbe
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ABSTRACT: Export Date: 13 February 2013, Source: Scopus, CODEN: JPACE, doi: 10.1002/pola.26133, Language of Original Document: English, Correspondence Address: Egbe, D.A.M.; Linz Institute for Organic Solar Cells, Physical Chemistry, Johannes Kepler University Linz, Altenbergerstr. 69, 4040 Linz, Austria; email: daniel_ayuk_mbi.egbe@jku.at, References: Sariciftci, N.S., Smilowitz, L., Heeger, A.J., Wudl, F., (1992) Science, 258, pp. 1474-1476;
Journal of Polymer Science Part A Polymer Chemistry 01/2012; 50:3425-3436. · 3.92 Impact Factor
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ABSTRACT: We report on C(60) based organic field effect transistors (OFETs) that are well optimized for low voltage operation. By replacing commonly used dielectric layers by thin parylene films or by utilizing different organic materials like divinyltetramethyldisiloxane-bis(benzocyclo-butene) (BCB), low density polyethylene (PE) or adenine in combination with aluminum oxide (AlOx) to form a bilayer gate dielectric, it was possible to significantly increase the capacitance per unit area (up to two orders of magnitude). The assembly of metal-oxide and organic passivation layer combines the properties of the high dielectric constant of the metal oxide and the good organic-organic interface between semiconductor and insulator provided by a thin capping layer on top of the AlOx film. This results in OFETs that operate with voltages lower than 500 mV, while exhibiting field effect mobilities exceeding 3 cm(2) V(-1) s(-1).
Synthetic Metals 10/2011; 161-66(19-20):2058-2062. · 1.83 Impact Factor
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ABSTRACT: Meyer-Neldel rule for charge carrier mobility measured in C(60)-based organic field-effect transistors (OFETs) at different applied source drain voltages and at different morphologies of semiconducting fullerene films was systematically studied. A decrease in the Meyer-Neldel energy E(MN) from 36 meV to 32 meV was observed with changing electric field in the channel. Concomitantly a decrease from 34 meV to 21 meV was observed too by increasing the grain size and the crystallinity of the active C(60) layer in the device. These empiric findings are in agreement with the hopping-transport model for the temperature dependent charge carrier mobility in organic semiconductors with a Gaussian density of states (DOS). Experimental results along with theoretical descriptions are presented.
Synthetic Metals 09/2011; 161(17-18):1987-1990. · 1.83 Impact Factor
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ABSTRACT: The electric field and temperature dependence of the electron mobility is studied comparatively in the bulk of fullerene (C60) diodes and at the interface with dielectric of organic field effect transistors (OFETs). Electron mobility values follow a Poole–Frenkel-type electric field dependence in both types of devices. The activation energy for electron transport is electric field dependent and follows the square root law of field in both devices as predicted by Gill’s law. The same Gill’s energy EGill = 34 meV is measured in diodes and OFETs, which corresponds well to Meyer–Neldel energy (EMN = 35 meV). It is shown that both the electric field and charge carrier concentration must be accounted for the description of disordered charge transport.
Applied Physics Letters 03/2011; 98(9):092114-092114-3. · 3.84 Impact Factor
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ABSTRACT: In this paper, the main photocurrent density and power conversion efficiency limiting mechanisms in bulk heterojunction solar cells are discussed with the emphasis on recombination processes. Charge extraction by linearly increasing voltage, time of flight, and other methods that allow the carrier lifetime and recombination to be studied experimentally in operating solar cells are discussed. It is shown that non-Langevin recombination is required for high-performance organic photovoltaic devices, which typically have low charge carrier mobility. Long charge carrier lifetime, exceeding carrier transit time through the film, can be achieved when non-Langevin recombination is observed. Langevin-type recombination dominates in most low-efficiency solar cells, whereas non-Langevin recombination is present in high efficient, e.g., annealed poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester blend devices. The film nanomorphology plays a crucial role governing the charge transport and the carrier lifetime. Double injection current with non-Langevin carrier recombination is demonstrated in high-efficiency devices, which strongly exceeds the injection current with Langevin recombination, due to the high carrier concentration attainable under non-Langevin recombination. Several different models explaining the non-Langevin recombination in organic solar cells are reviewed. Requirements for charge carrier mobility and recombination to maximize power conversion efficiency in organic photovoltaic devices are outlined.
IEEE Journal of Selected Topics in Quantum Electronics 01/2011; · 3.78 Impact Factor
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ABSTRACT: Charge-carrier transport and recombination in thermally treated and untreated films of poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)propyl-l-phenyl-[6,6]-methanofullerene (PCBM) bulk-heterojunction solar cells (BHSCs) have been measured using various electrooptical techniques. The formation of lamellar structure in P3HT has a large effect on the efficiency, carrier transport, and recombination of photogenerated charge carriers. Treated P3HT/PCBM solar cells show greatly reduced carrier recombination compared to what is typically expected in low-mobility materials and electric-field-independent carrier generation. In untreated films, the recombination is close to Langevin-type with electric-field-dependent quantum efficiency, consistent with the typically observed Onsager-type generation. Furthermore, we observe an increased effective capacitance in treated films, consistent with increased charge screening. The importance of the interface between the lamellar structured P3HT and PCBM is evident from optical spectroscopies showing that 2-D polarons are directly generated using sub-gap excitation. We conclude that the formation of lamellar structures in the polymer donor, and subsequent, derealization of the charges is favorable for making efficient BHSCs.
IEEE Journal of Selected Topics in Quantum Electronics 01/2011; · 3.78 Impact Factor
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Journal of Organometallic Chemistry 01/2011; 696:2252. · 2.38 Impact Factor
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ABSTRACT: The equivalent circuit parameters for a pentacene organic field-effect transistor are determined from low frequency impedance measurements in the dark as well as under light illumination. The source-drain channel impedance parameters are obtained from Bode plot analysis and the deviations at low frequency are mainly due to the contact impedance. The charge accumulation at organic semiconductor–metal interface and dielectric–semiconductor interface is monitored from the response to light as an additional parameter to find out the contributions arising from photovoltaic and photoconductive effects. The shift in threshold voltage is due to the accumulation of photogenerated carriers under source-drain electrodes and at dielectric–semiconductor interface, and also this dominates the carrier transport. The charge carrier trapping at various interfaces and in the semiconductor is estimated from the dc and ac impedance measurements under illumination.
Journal of Applied Physics 12/2010; 108(11):113703-113703-7. · 2.17 Impact Factor
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ABSTRACT: Temperature-activated charge transport in disordered organic semiconductors at large carrier concentrations, especially relevant in organic field-effect transistors OFETs, has been thoroughly considered using a recently developed analytical formalism assuming a Gaussian density-of-states DOS distribution and Miller-Abrahams jump rates. We demonstrate that the apparent Meyer-Neldel compensation rule MNR is recovered regarding the temperature dependences of the charge carrier mobility upon varying the carrier concentration but not regarding varying the width of the DOS. We show that establishment of the MNR is a characteristic signature of hopping transport in a random system with variable carrier concentration. The polaron formation was not involved to rationalize this phenomenon. The MNR effect has been studied in a OFET based on C 60 films, a material with negligible electron-phonon coupling, and successfully described by the present model. We show that this phenomenon is entirely due to the evolution of the occupational DOS profile upon increasing carrier concentration and this mechanism is specific to materials with Gaussian-shaped DOS. The suggested model provides compact analytical relations which can be readily used for the evaluation of important material parameters from experimentally accessible data on temperature dependence of the mobility in organic elec-tronic devices. Experimental results on temperature-dependent charge mobility reported before for organic semiconductors by other authors can be well interpreted by using the model presented in this paper. In addition, the presented analytical formalism predicts a transition to a Mott-type charge carrier hopping regime at very low temperatures, which also manifests a MNR effect.
Physical Review B 01/2010; 80(81):045202. · 3.69 Impact Factor
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ABSTRACT: We report a detailed investigation of the electrical properties of organic field-effect transistors (OFETs) and metal-insulator-semiconductor (MIS) capacitors formed from highly ordered thin films of C <sub>60</sub> as the active semiconductor and divinyltetramethyl disiloxane-bis(benzocyclobutene) (BCB) as the gate dielectric. Current-voltage measurements show the OFETs to be n-channel devices characterized by a high electron mobility (∼6 cm <sup>2</sup>/ V s ) . An equivalent circuit model is developed which describes well both the frequency and voltage dependences of the small-signal admittance data obtained from the corresponding MIS capacitors. By fitting the circuit response to experimental data, we deduce that increasing gate voltages increases the injection of extrinsic charge carriers (electrons) into the C <sub>60</sub> . Simultaneously, the insulation resistance of the BCB decreases, presumably by electron injection into the insulator. Furthermore, the admittance spectra suggest that the capacitance-voltage (C-V) behavior originates from a parasitic, lateral conduction effect occurring at the perimeter of the capacitor, rather than from the formation of a conventional depletion region.
Journal of Applied Physics 01/2010; · 2.17 Impact Factor
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D. A. M. Egbe,
S. Türk,
F. Kühnlenz,
R. Jadhav,
A. Wild,
E. Birckner,
G. Adam,
A. Pivirkas,
V. Cimrova,
G. Knör, N. S. Sariciftci,
S. Rathgeber,
H. Hoppe
Macromolecules 01/2010; 43:1261. · 5.17 Impact Factor
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ABSTRACT: We report the dynamic admittance, both in the dark and under illumination, of heterojunc-tions made of poly(3-hexyl thiophene)/1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C 61 (P3HT:PCBM) blends, which are used in efficient organic solar cells. In the dark there appears a huge low frequency negative capacitance which we associate with slow electron hole bimolecular recombination at the heterojunction interfaces. Surprisingly, under pho-toexcitation the negative capacitance gradually disappears with increasing light intensity. We attribute this positive photoinduced capacitance to the combined effect of (1) long lived photogenerated charges at the P3HT:PCBM interfaces that increase electron-hole bimolecular recombination rate, which in turn renders the capacitance less negative and (2) trapped photogenerated charges that increase the capacitance upon re-emission.
Organic Electronics 01/2009; 10:115-118. · 4.05 Impact Factor
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ABSTRACT: Due to the limited spectral width of absorption in organic dyes, it is necessary to look for strategies
to enhance the spectral photon harvesting in organic solar cells. Investigations of the energy transfer
between zinc-phthalocyanine (ZnPc) and CdSe/ZnS core shell nanocrystals are performed and show ahighly
efficient energy transfer and no sign of acompeting charge transfer mechanism. The dependence of
the energy transfer efficiency on the length of the alkyl spacers around the nanoparticles is investigated.
The integration of semiconductor nanoparticles into aphotodiode based on ZnPc yields increased sensitivity
of the device in the green spectral range.
09/2008: pages 16-20;
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Advanced Materials 02/2008; 20(5):1018 - 1022. · 13.88 Impact Factor
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ABSTRACT: Organic electronics (a cross point of organic chemistry, condensed matter physics, materials science, and device physics)
[1, 2] has made remarkable technological breakthroughs for the last decade, enabling the realization of viable devices such
as organic light-emitting diodes (OLEDs) [3, 4], organic field-effect transistors (OFETs) [5], organic solar cells [6] as
well as memory circuits [7, 8]. Some products such as medium resolution emissive OLED displays [9, 10] are already commercially
available and others are now in various stages of commercialization. On the other hand, nanoscale technology has clear advantages
to be gained from exploiting self-organized growth, as it avoids the need for highly sophisticated patterning of surfaces
with nanometersize objects. Ideally, in organic applications the functional properties can be obtained essentially in one
self-assembled molecular layer, so that organic electronics in principle would offer a maximum degree of miniaturization.
12/2007: pages 263-300;
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ABSTRACT: The charge carrier transport and recombination in two types of thermally treated bulk-heterojunction solar cells is reviewed: in regioregular poly(3-hexylthiophene) (RRP3HT) mixed with 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene (PCBM) and in the blend of poly[2-methoxy-5-(3,7-dimethyloctyloxy)-phenylene vinylene] (MDMO-PPV) mixed with PCBM. The charge carrier mobility and bimolecular recombination coefficient have been comparatively studied by using various techniques including Time-of-Flight (ToF), Charge Extraction by Linearly Increasing Voltage (CELIV), Double Injection (DI) transients, Current–Voltage (I–V) technique. It was found that the carrier mobility is at least an order of magnitude higher in RRP3HT/PCBM blends compared to MDMO-PPV/PCBM. Moreover, all used techniques demonstrate a heavily reduced charge carrier recombination in RRP3HT/PCBM films compared to Langevin-type carrier bimolecular recombination in MDMO-PPV/PCBM blends. As a result of long carrier lifetimes the formation of high carrier concentration plasma in RRP3HT/PCBM blends is demonstrated and plasma extraction methods were used to directly estimate the charge carrier mobility and bimolecular recombination coefficients simultaneously. A weak dependence of bimolecular recombination coefficient on the applied electric field and temperature demonstrates that carrier recombination is not dominated by charge carrier mobility (Langevin-type recombination) in RRP3HT/PCBM blends. Furthermore, we found from CELIV techniques that electron mobility in RRP3HT/PCBM blends is independent on relaxation time in the experimental time window (approx. hundreds of microseconds to tens of milliseconds). This reduced carrier bimolecular recombination in RRP3HT/PCBM blends implies that the much longer carrier lifetimes can be reached at the same concentrations which finally results in higher photocurrent and larger power conversion efficiency of RRP3HT/PCBM solar cells. Copyright © 2007 John Wiley & Sons, Ltd.
Progress in Photovoltaics Research and Applications 11/2007; 15(8):677 - 696. · 5.79 Impact Factor
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ABSTRACT: We report negative capacitance at low frequencies in organic semiconductor based diodes and show that it appears only under bipolar injection conditions. We account quantitatively for this phenomenon by the recombination current due to electron-hole annihilation. Simple addition of the recombination current to the well established model of space charge limited current in the presence of traps, yields excellent fits to the experimentally measured admittance data. The dependence of the extracted characteristic recombination time on the bias voltage is indicative of a recombination process which is mediated by localized traps.
07/2007;
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ABSTRACT: The authors report on the application of luminescent concentrators on organic solar cells fabricated zinc-phthalocyanine and C <sub>60</sub> . These solar cells have their main absorption in the wavelength range between 600 and 800 nm . Below 600 nm , the low absorption limits the quantum efficiency. Luminescence concentrators are used to overcome this limitation by spectrally shifting blue and green light towards the red and waveguiding it to the solar cell. With a second solar cell harvesting the red light transmitted through the concentrator, the photocurrent density is increased compared to a single solar cell of equal active area from about 8.5 up to 10 mA / cm <sup>2</sup> .
Applied Physics Letters 05/2007; · 3.84 Impact Factor
