Polymers for Electronics and Photonics
The Department is focused on the basic research of novel polymers and polymer systems for potential application in organic electronic devices, particularly in photodiodes, solar cells, light-emitting diodes, organic field-effect transistors, electrochromic windows, sensors and electronic memory devices. This complex interdisciplinary research involves:
- design and syntheses of novel semiconducting conjugated polymers, polymer blends and composites with tailored properties,
- experimental studies of electrical and optical properties using advanced experimental methods, e.g. time-resolved electronic spectroscopy,
- theoretical studies applying various methods of quantum chemistry and mathematical analysis, with the aim to elucidate the photoelectrical energy conversion, charge carrier transport in conjugated polymers, and other related phenomena.
Research Activities
Semiconducting thienothiadiazole-based donor-acceptor copolymers
New donor-acceptor (D–A) copolymers with electron acceptor units - alkyl substituted 4,6-di(thiophen-2′-yl)thieno[3,4-c][1,2,5]thiadiazole (T)- were prepared and investigated as potential active materials for photonic and electronic applications. We demonstrated that the thienothiadiazole-based D–A copolymers with various electron-donor blocks and side-chain combinations possess interesting optical, photovoltaic and electrochromic properties, which are influenced by the donor strength of the electron-donor unit and the nature and length of the alkyl chains attached to the thiophenes of T unit. New D-A copolymers exhibit absorption covering the entire visible spectral region, which is interesting for photovoltaic applications and they also exhibit interesting electrochromic behaviour, some of them display fast response times reaching full optical contrast in 0.3 s.
|
Chemical structures of novel D–A copolymers with 4,6-di(thiophen-2′-yl)thieno[3,4-c][1,2,5]thiadiazole (T) acceptor units. |
|
Example of electrochromic switching in novel D–A copolymers. |
Perylene tetracarboxydiimide based copolymers
D - A copolymers with N,N′-dialkylperylene-3,4,9,10-tetracarboxydiimide acceptor units were prepared. Effects of side chains and their potential applications in photonics were shown. The results of electrochemical and spectroelectrochemical studies, like photoluminescence quenching in the reduced state, indicate that these materials could be used for sensing applications. Hybrid layers of donor-acceptor copolymers containing N,N′-dialkylperylene-3,4,9,10-tetracarboxydiimide electron-acceptor and 9,9-dioctylfluorene electron-donor units with homogenous silver nanoparticle coverage were prepared by electrochemical doping. They exhibit significant improvements in light absorption. Such hybrid layers have great potential for various photonic and electronic applications, particularly in photovoltaics, sensing, or catalysis, and also biological and medical applications.
|
Donor-acceptor copolymers with N,N′-dialkylperylene-3,4,9,10-tetracarboxydiimide as materials for photonics. |
|
Electrochromic switching in novel D–A copolymers. |
Metallo-supramolecular polymers
Metallo-supramolecular polymers (MSPs) are studied as a new category of materials with controllable intermolecular interactions. Several new bis(tpy) unimers with different linkers between the thieno[3,2-b]thiophene-2,5-diyl central unit and terpyridine-4´-yl (tpy) end-groups were prepared and assembled with Fe2+ ions to metallo-supramolecular polymers (Fe-MSPs). The Fe-MSP films show electrochromism dependent on the unimer structure. The best-studied MSPs shows the colouration efficiency CE = 641 cm2 C-1 and fast optical response, making it an excellent candidate for possible use in electrochromic devices. Using time-resolved absorption spectroscopy, the effect of singlet fission was found on Zn-MSPs with ditopic thiophene-bridged terpyridine ligands. Singlet fission is a way to increase the efficiency of solar cells. It allows harvesting high energy photons without dissipating energy to heat.
|
UV/vis transmittance spectra of Fe-MSP electrochromic device in the reduced coloured and oxidized bleached state. Optical response to periodic voltage cycling. |
|
Singlet fission in metallo-supramolecular polymer with ditopic thiophene-bridged terpyridine ligands. |
Influence of various synthetic routes, molecular parameters on properties
Alternating electroluminescent copolymers, poly(9,9-dihexadecylfluorene-2,7-diyl-alt-2,2′-bithiophene-5,5′-diyl)s, were synthesized using two different routes of Suzuki coupling and microwave-assisted or in high boiling point solvents conventional heating. Copolymers with various molecular weights (MW) were obtained depending on the synthesis conditions, and their photophysical, electrochemical and electroluminescent properties were investigated. Impact of MW and of the side chains length on the properties was revealed.
|
Synthesis of poly(9,9-dihexadecylfluorene-2,7-diyl-alt-2,2′-bithiophene-5,5′-diyl)s via Suzuki coupling, photoluminescence and electroluminescence. |
Plasmonic nanocomposites
Localized surface plasmon (LSP) phenomena in metal nanostructures can couple light into photovoltaic devices of sub-wavelength-scale thickness. However, the excitation of LSP is associated with rapid energy dissipation leading to local heating. We found, that ultrashort laser pulse absorbed by the array of metal nanoparticles at the polymer semiconductor surface produces a propagating heatwave, which accelerates the diffusivity of generated excitons by the wavefront of this heatwave. This effect is interesting for the development of polymer light sensors. Our work on the formation of composites made of polymer semiconductors and Ag nanoparticles revealed a rapid formation of three different populations of aggregates rapidly developed after mixing the Ag colloid with the polymer solution. The differences in aggregation led to changes in colour, extent of polymer coating and temporal stability, and allowed tuning wavelength for SERS (Surface Enhanced Raman Spectroscopy) probing.
|
The aggregation kinetics of borate-stabilized silver nanoparticles induced by the cationic poly{3-[6-(1-methylimidazolium-3-yl)hexyl]thiophene-2,5-diyl bromide}. |
|
Gold nanoparticles on polymer thin film, excited with pulsed laser excitation, generate heat waves that accelerate the diffusion of excitons. |
Energy transfer
Various phenomena related to fast energy relaxation and transfer in organic semiconductors are studied. We developed a method for extraction of the time evolution of the diffusion coefficient of photoexcited species (excitons, polarons, etc.) during their bi-molecular collisions events from the transient absorption spectroscopy. We also focused on the search for materials with efficient singlet fission. The process yields two triplet excited states from a single photon absorbed in an organic compound and allows harvesting high energy photons without dissipating excessive energy to heat. Singlet fission is considered as a possible way to overcome the Shockley–Queisser energy conversion efficiency limit of solar cells.
|
Determination of the time dependences of the diffusion coefficient of photoexcited species was determined from transient absorption. |
|
Proces of the singlet fission process after the photoexcitation in molecular pairs in single-crystal structures of N,N'-alkylated derivatives of bis(thienyl)diketopyrrolopyrroles (TDPPs) with adamantyl (a,d), butyl (b,e) and hexyl (c,f) substituents. |
Charge transport in organic semiconductors
Our theoretical research is devoted mainly to linear conjugated polymers, which have perspective use in future printed organic electronic devices. The charge carrier transport in these materials consists of the relatively fast motion of charge carriers on the conjugated chains and subsequent much slower inter-chain hopping, which is often a bottle-neck of the transport determining the macroscopic properties. Because of this principal difference between the on-chain motion and the inter-chain hopping, the mobility of charge carriers cannot be successfully described by previous models elaborated either for inorganic, or for disordered organic low-molecular-weight materials. Our unique hybrid quantum-mechanical and semi-classical approach explained the observed mobility degradation at high gate voltage and elucidated the behaviour of OFETs with a vertical nanobrush architecture. The latter shows a non-trivial dependence of charge carrier mobility on the gate voltage, with a significant formation of the second resonant peak.
|
End-on orientation of polymer chains leads to the negative transverse-field dependence of charge mobility due to reduction of the effective chain conjugation. |
|
Simulation of the OFETs with a vertical nanobrush architecture proposed formation of resonant peaks in the mobility dependence at higher gate voltages. |
|
OFET characteristics in polymers with concentration-dependent charge carrier mobility can be described by the 1-D model with the mobility and coupled contact resistance controlled by local equivalent surface charge concentration. |
|
Charge transport in BHJ solar cells is modulated by the hybridization transfer integral between the polymer and admixed acceptor. |
Application of theoretical models on devices
We contributed to the understanding of mutual interactions of the polar polymer gate dielectric and organic semiconductor in OFETs. The conditions during solidification of a high-k dielectric layer containing strong polar side groups, like cyanoethylated polyvinylalcohol (CEPVA), can significantly affect the charge transport in the semiconductor layer. In contrast to the previous literature, we attributed the reduced mobility to the broader distribution of the semiconductor density of states (DOS) to a significant dipolar disorder in the dielectric layer. The formation of a rigid hydrogen network limits the dipolar disorder in the dielectric layer and leads to a significantly narrowed distribution of DOS and, hence, to higher charge carrier mobility in the OFET active channel.
|
Schematic representation of the cyanoethylated polyvinylalcohol (CEPVA) surface dipoles and their impact on the density of state of the adjacent semiconductor layer. |
Cooperation
- Central South University, Changsha, China (Prof. Y. Zou)
- Clemson University, South Carolina, U.S.A. (Prof. S. Foulger)
- Hanyang University, Seoul, Republic of Korea (Prof. Y. Kang)
- Johannes Kepler University, Linz, Austria (Prof. D. A. M. Egbe)
- Lodz University of Technology, Poland (Prof. J. Ulański)
- Nanyang Technological University, Singapore (Prof. G. Lisak)
- National University of Kaohsiung, Taiwan, Republic of China (Prof. S. J. Sun)
- The University of Colorado, Boulder, U.S.A. (Prof. J. Michl)
- Warsaw University of Technology, Poland (Prof. M. Zagórska, Prof. A. Proń)
- Wroclaw University of Technology, Poland (Prof. W. Bartkowiak, Prof. A. Miniewicz)
- Faculty of Chemistry, Brno University of Technology, Czech Republic (Prof. M. Weiter)
- Faculty of Science, Charles University, Prague, Czech Republic (Prof. J. Vohlídal, Prof. B. Vlčková)
Recent Funding Support
- “Study of polymer memristors based on methacrylate polymers with carbazole pending rings“, MEYS – INTEREXCELLENCE, 2020 – 2022
- “Polymer - perovskite hybrid materials for photonics”, CSF(GAČR) – international project, 2020 – 2021
- “Hybrid polymer systems for photonics and electronics”, CSF(GAČR) – international project, 2018 – 2019
- “Polymer memristors for organic printed electronics“, CSF(GAČR), 2017 – 2019
- “Electron-deficient [n]phenacenes as the new organic semiconductors“, CSF(GAČR), 2017 – 2019
- “Study of optoelectronic processes in hybrid systems for third-generation solar cells”, MEYS – COST, INTEREXCELLENCE, 2017 – 2019
- “Autonomous identification system for detection and verification of products in the system Industry 4.0 – Label 4.0”, TAČR, 2017 – 2019
- “Flexible printed electronics based on organic and hybrid materials - FLEXPRINT” TAČR – Centre of Competence, 2012 – 2019
- “Progressive polymers for photonics”, CSF(GAČR), 2013 – 2017
- “Study of electronic processes in molecular systems for organic photonics”, CSF(GAČR), 2015 – 2017