70 research outputs found
Manipulation of functional polymers on the molecular and mesoscopic scale
Conferencia Invitada Departamento QuĂmica-FĂsicaThis talk will give an overview over current activities of my research team on the manipulation of semiconducting polymers based on thiophene units on the molecular and mesoscopic scale.
On the molecular scale, we have recently managed to synthesize branched thiophene polymers by different chemical routes and by electropolymerization.[1,2] Relationships between the molecular architecture and functional properties such as absorption behavior and energy levels will be discussed and highlighted.
On the mesoscopic scale our latest results on controlled crystallization of semiconducting polymer thin films will be presented. While our first studies had focused on the work-horse of the solar cell community poly(3-hexylthiophene)[3], we have recently started to work on two donor-acceptor copolymers: the p-type low bandgap (poly{[4,4-bis(2-ethylhexyl)-cyclopenta-(2,1-b:3,4-bâ)dithiophene]-2,6-diyl-alt-(2,1,3-benzo-thiadiazol)-4,7-diyl}) (PCPDTBT)[4] and the n-type poly{[N,Nâ-bis(2-octyldodecyl)-1,4,5,8-naphthalene-dicarboximide-2,6-diyl]-alt-5,5â-(2,2â-bithiophene)} (PNDI2OD-T2)[5]. While PNDI2OD-T2 is known to be highly crystalline, PCPDTBT has long been regarded as marginally crystalline or even âamorphousâ. We show that methods such as solvent-vapor crystallization or shear alignment allow us to induce and control crystalline order over large areas in thin films of both polymers. We find that changes in morphology are closely related to changes in absorption spectra. Furthermore the impact of differently crystallized films on charge transport and solar cell performance is discussed.Universidad de MĂĄlaga. Campus de Excelencia Internacional AndalucĂa Tech
Novel push-pull chromophores to prepare electro-optic modulators
In recent years, a large number of push-pull organic molecules have been proposed as promising candidates for electronic and optical applications. Generally, the main effort has been focused on the design of chromophores with large first hyperpolarizability values (β); this would result in a wide variety of nonlinear optical (NLO) applications, such as modulators.[1,2] In this work, we report an experimental and theoretical investigation of the NLO properties of novel push-pull systems derived from the dicyanomethylene-4H-chromene (DCM) group. Particular attention will be paid to better understand the molecular and electronic properties of these systems by using vibrational spectroscopic techniques and electrochemistry. Furthermore, these materials have been tested in a silicon-organic hybrid modulator based on an integrated dual-mode interferometer.[3]Universidad de MĂĄlaga. Campus de Excelencia Internacional AndalucĂa Tech
In Situ Electrochemical Investigations of Inherently Chiral 2,2â˛-Biindole Architectures with Oligothiophene Terminals
AbstractThe synthesis and characterization of three new inherently chiral N,Nâ˛âdipropylâ3,3â˛âdiheteroarylâ2,2â˛âbiindole monomers, nicknamed Ind2T4, Ind2T6 and Ind2Ph2T4, which differ in the number of thiophenes as terminals, are reported. In addition to a full monomer characterization, stable electroactive oligomeric films were obtained by electroâoxidation upon cycling to potentials which activate the thiophene terminals. Cyclic voltammetry, UVâVisâNIR spectroelectrochemistry and inâ
situ conductance measurements show that oligomeric films of Ind2T6 present the best stability and electrochromic switching performance. Enantioselective tests with a chiral ferrocene amine clearly show the potential as chiral selectors for analytical and sensing purposes
Enhanced photogeneration of polaron pairs in neat semicrystalline donor-acceptor copolymer films via direct excitation of interchain aggregates
We investigate the photogeneration
of polaron pairs (PPs) in neat
films of the semicrystalline donorâacceptor semiconducting
copolymer PCPDTBT. Carefully selecting the solution-processing procedures,
we obtain films with different amounts of crystallinity and interchain
aggregation. We compare the photogeneration of PPs between the films
by monitoring their photoinduced absorption in ultrafast pumpâprobe
experiments, selectively exciting nonaggregated or aggregated polymer
chains. The direct photoexcitation of interchain Ď-aggregates
results in prompt (<100 fs) charge generation. Compared to the
case where nonaggregated chains are excited, we find an 8-fold increase
in the prompt PP to singlet-exciton ratio. We also show that highly
crystalline lamellar nanostructures not containing Ď-stacked
or any light-absorbing aggregates do not improve the efficiency of
PP photogeneration. Our results show that light absorption from interchain
aggregates is highly beneficial for charge photogeneration in semiconducting
polymers and should be taken into account when optimizing film morphologies
for photovoltaic devices
Chemical Doping of Conjugated Polymers with the Strong Oxidant Magic Blue
Molecular doping of organic semiconductors is a powerful tool for the optimization of organic electronic devices and organic thermoelectric materials. However, there are few redox dopants that have a sufficiently high electron affinity to allow the doping of conjugated polymers with an ionization energy of more than 5.3\ua0eV. Here, p-doping of a broad palette of conjugated polymers with high ionization energies is achieved by using the strong oxidant tris(4-bromophenyl)ammoniumyl hexachloroantimonate (Magic Blue). In particular diketopyrrolopyrrole (DPP)-based copolymers reach a conductivity of up to 100 S cmâ1 and a thermoelectric power factor of 10 \ub5W mâ1 Kâ2. Further, both electron paramagnetic resonance (EPR) as well as a combination of spectroelectrochemistry and chronoamperometry is used to estimate the charge-carrier density of the polymer PDPP-3T doped with Magic Blue. A molar attenuation coefficient of 6.0\ua0\ub1\ua00.2
7 103 m2 molâ1 is obtained for the first polaronic sub-bandgap absorption of electrochemically oxidized PDPP-3T. Comparison with chemically doped PDPP-3T suggests a charge-carrier density on the order of 1026 mâ3, which yields a charge-carrier mobility of up to 0.5 cm2 Vâ1 sâ1 for the most heavily doped material
Conductance and spectroscopic mapping of EDOT polymer films upon electrochemical doping
Abstract
This paper deals with the electrochemical doping of different poly(ethylenedioxythiophene) (PEDOT)-based active layers performed in an organic electrochemical transistor configuration through the mapping of in situ conductance trends during electrochemical doping and dedoping. The experiments are complemented by UV/Vis/NIR in situ spectroelectrochemistry in the wavelength range from 400 to 1600 nm, which allow monitoring of the development of the neutral and charged redox species. Both electropolymerized EDOT-based layers and solution-processed chemically synthesized PEDOT films are characterized. In addition to pure electropolymerized PEDOT (e-PEDOT), tris(4-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)phenyl) (TPA-EDOT3) is electrodeposited to generate highly branched networks of P(TPA-EDOT3). The solution-deposited PEDOT films contain poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with ratios of 1:2.5 and 1:6. Overall, we find that e-PEDOT and PEDOT:PSS(1:2.5) behave like classical conjugated polymers with a plateau-like conductance over a wide potential region. In contrast, PEDOT:PSS(1:6) and P(TPA-EDOT3) show rather bell-shaped conductance profiles. The mixed-valence conductivity model is used to interpret the experimental results in terms of the number of accessible redox states. We suggest that the bell-shaped conductance in the case of PEDOT:PSS(1:6) is caused by a high amount of PSS insulator that limits the inter-chain interaction between PEDOT moieties and in the case of P(TPA-EDOT3) by its distorted molecular architecture
ChargeâCompensated NâDoped Ď âConjugated Polymers: Toward both Thermodynamic Stability of NâDoped States in Water and High Electron Conductivity
The understanding and applications of electron-conducting Ď-conjugated polymers with naphtalene diimide (NDI) blocks show remarkable progress in recent years. Such polymers demonstrate a facilitated n-doping due to the strong electron deficiency of the main polymer chain and the presence of the positively charged side groups stabilizing a negative charge of the n-doped backbone. Here, the n-type conducting NDI polymer with enhanced stability of its n-doped states for prospective âin-waterâ applications is developed. A combined experimentalâtheoretical approach is used to identify critical features and parameters that control the doping and electron transport process. The facilitated polymer reduction ability and the thermodynamic stability in water are confirmed by electrochemical measurements and doping studies. This material also demonstrates a high conductivity of 10â2 S cmâ1 under ambient conditions and 10â1 S cmâ1 in vacuum. The modeling explains the stabilizing effects for various dopants. The simulations show a significant doping-induced âcollapseâ of the positively charged side chains on the core bearing a partial negative charge. This explains a decrease in the lamellar spacing observed in experiments. This study fundamentally enables a novel pathway for achieving both thermodynamic stability of the n-doped states in water and the high electron conductivity of polymers
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