PD Dr. rer. nat. Andreas Opitz
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Forschungsthemen2
Ladungstransfer an Heterogrenzflächen organischer Halbleiter
Quelle ↗Förderer: DFG Sachbeihilfe Zeitraum: 03/2014 - 12/2017 Projektleitung: PD Dr. rer. nat. Andreas Opitz
Ladungstransfer an Heterogrenzflächen organischer Halbleiter: Zu einer einheitlichen Beschreibung von Szenarien im Übergangsbereich zwischen Grundzustand bzw. angeregtem Zustand
Quelle ↗Förderer: DFG Sachbeihilfe Zeitraum: 05/2019 - 05/2024 Projektleitung: PD Dr. rer. nat. Andreas Opitz
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Publikationen25
Top 25 nach Zitationen — Quelle: OpenAlex (BAAI/bge-m3 embedded für Matching).
Nature Communications · 395 Zitationen · DOI
Ground-state integer charge transfer is commonly regarded as the basic mechanism of molecular electrical doping in both, conjugated polymers and oligomers. Here, we demonstrate that fundamentally different processes can occur in the two types of organic semiconductors instead. Using complementary experimental techniques supported by theory, we contrast a polythiophene, where molecular p-doping leads to integer charge transfer reportedly localized to one quaterthiophene backbone segment, to the quaterthiophene oligomer itself. Despite a comparable relative increase in conductivity, we observe only partial charge transfer for the latter. In contrast to the parent polymer, pronounced intermolecular frontier-orbital hybridization of oligomer and dopant in 1:1 mixed-stack co-crystallites leads to the emergence of empty electronic states within the energy gap of the surrounding quaterthiophene matrix. It is their Fermi-Dirac occupation that yields mobile charge carriers and, therefore, the co-crystallites-rather than individual acceptor molecules-should be regarded as the dopants in such systems.
Angewandte Chemie International Edition · 221 Zitationen · DOI
Molecular doping: The standard model for molecular p-doping of organic semiconductors (OSCs) assumes integer charge transfer between OSC and dopant. This is in contrast to an alternative model based on intermolecular complex formation instead. By systematically varying the acceptor strength it was possible to discriminate the two models. The latter is clearly favored, suggesting strategies for the chemical design of more efficient molecular dopants.
Carbon · 143 Zitationen · DOI
Journal of Applied Physics · 107 Zitationen · DOI
We investigate ambipolar charge transport in organic field-effect transistors (OFETs) with copper-phthalocyanine (CuPc) as active material. It is shown that charge carrier mobilities can be increased by at least one order of magnitude using the long-chain alkane tetratetracontane (TTC) as a passivation layer on top of silicon dioxide. TTC and CuPc films are characterized by atomic force microscopy and x-ray diffraction. TTC forms a highly crystalline layer that passivates electron traps on the SiO2 surface very efficiently and serves as a template for the growth of CuPc films with significantly improved crystallinity. High electron mobilities comparable to the values reported on single crystals are reached. We show that the contact resistance for hole transport as determined by the transmission line method can be reduced considerably by using organic charge-transfer complexes as top contacts in OFETs based on CuPc.
Advanced Energy Materials · 103 Zitationen · DOI
Abstract In organic photovoltaic (PV) cells, the well‐established donor‐acceptor (D/A) concept enabling photo‐induced charge transfer between two partners with suitable energy level alignment has proven extremely successful. Nevertheless, the introduction of such a heterojunction is accompanied with additional energy losses as compared to an inorganic homojunction cell, owing to the presence of a charge‐transfer (CT) state at the D/A interface. Based on the principle of detailed balance, a modified Shockley‐Queisser theory is developed including the essential effects of interfacial CT states, that allows for a quantitative assessment of the thermodynamic efficiency limits of molecular D/A solar cells. Key parameters, apart from the optical gap of the absorber material, entering the model are the energy ( E CT ) and relative absorption strength (α CT ) of the CT state. It is demonstrated how the open‐circuit voltage ( V OC ) and thus the power conversion efficiency are affected by different parameter values. Furthermore, it is shown that temperature dependent device characteristics can serve to determine the CT energy, and thus the upper limit of V OC for a given D/A combination, as well as to quantify non‐radiative recombination losses. The model is applied to diindenoperylene (DIP)‐based photovoltaic devices, with open‐circuit voltages between 0.9 and 1.4 V, depending on the partner, that have recently been reported.
Journal of Applied Physics · 94 Zitationen · DOI
We investigate different parameters influencing the occurrence of s-shaped current voltage (j-V) characteristics in planar heterojunction organic solar cells. It is shown how substrate modification, purity of the active organic material as well as variation of the top contact can affect the shape of the j-V curves. The studies are performed on vacuum-evaporated planar heterojunction solar cells with diindenoperylene (DIP) as electron donor and fullerene C60 as acceptor. The focus is on the fill factor and forward current being the most direct indicators for s-shapes in j-V curves. We find that the main effect of substrate heating during film growth can be assigned to changes in energy barriers rather than to the modification of morphology and crystallinity, which is also influenced by elevated substrate temperatures. The decisive role of the barrier height between the anode work function and the HOMO (i.e., highest occupied molecular orbital) level of the donor is approved by comparing hole-injection layers with different work functions. By using donor materials of different purity we find a correlation between charge carrier mobilities and fill factors. Finally, it is demonstrated that an exciton blocking interlayer is essential to get high fill factors when aluminum is used as top contact, but is dispensable for samarium as cathode material. This finding can be ascribed to the protective effect of the interlayer from aluminum diffusion into the active semiconductor rather than to its role as exciton diffusion barrier.
Organic Electronics · 87 Zitationen · DOI
The Journal of Physical Chemistry C · 81 Zitationen · DOI
The film morphology and device performance of planar heterojunction solar cells based on the molecular donor material α-sexithiophene (6T) are investigated. Planar heterojunctions of 6T with two different acceptor molecules, the C60 fullerene and diindenoperylene (DIP), have been prepared. The growth temperature of the 6T bottom layer has been varied between room temperature and 100 °C for each acceptor. By means of X-ray diffraction and X-ray absorption, we show that the crystallinity and the molecular orientation of 6T is influenced by the preparation conditions and that the 6T film templates the growth of the subsequent acceptor layer. These structural changes are accompanied by changes in the characteristic parameters of the corresponding photovoltaic cells. This is most prominently observed as a shift of the open circuit voltage (Voc): In the case of 6T/C60 heterojunctions, Voc decreases from 0.4 to 0.3 V, approximately, if the growth temperature of 6T is increased from room temperature to 100 °C. By contrast, Voc increases from about 1.2 V to almost 1.4 V in the case of 6T/DIP solar cells under the same conditions. We attribute these changes upon substrate heating to increased recombination in the C60 case while an orientation dependent intermolecular coupling seems to change the origin of the photovoltaic gap in the DIP case.
Surface Science · 73 Zitationen · DOI
Organic Electronics · 69 Zitationen · DOI
Organic Electronics · 59 Zitationen · DOI
IEEE Journal of Selected Topics in Quantum Electronics · 55 Zitationen · DOI
Blends of organic electron and hole conductive materials are widely used for ambipolar charge-carrier transport and donor/acceptor (DA) photovoltaic cells. Thereby, the efficiency of these excitonic solar cells is correlated to the morphology of the interface between the donor and the acceptor materials, which in turns depends on the preparation conditions, the crystallization of the particular materials, and the interaction between the donor and acceptor molecules. In this contribution, the influence of the morphology on the solar cell architecture and performance will be discussed using different molecular DA combinations.
Journal of Applied Physics · 54 Zitationen · DOI
The reliable operation of micro- and nanomechanical devices necessitates a precise knowledge of the water film thickness present on the surfaces of these devices with accuracy in the nanometer range. In this work, the thickness of an ultra-thin water film was measured by distance tunneling spectroscopy and distance dynamic force spectroscopy during desorption in an ultra-high vacuum system, from about 2.5 nm up to complete desorption at 10−8 mbar. The tunneling current and the amplitude of vibration and the normal force were detected as a function of the probe-sample distance. In these experiments, a direct comparison of both methods was possible. It was determined that dynamic force spectroscopy provides the most accurate values. The previously reported tunneling spectroscopy, which requires the application of significantly high voltages generally leads to values that are 25 times higher than values determined by dynamic force spectroscopy.
Journal of Polymer Science Part B Polymer Physics · 50 Zitationen · DOI
ABSTRACT The electrical conductivity of organic semiconductors can be enhanced by orders of magnitude via doping with strong molecular electron acceptors or donors. Ground‐state integer charge transfer and charge‐transfer complex formation between organic semiconductors and molecular dopants have been suggested as the microscopic mechanisms causing these profound changes in electrical materials properties. Here, we study charge‐transfer interactions between the common molecular p‐dopant 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane and a systematic series of thiophene‐based copolymers by a combination of spectroscopic techniques and electrical measurements. Subtle variations in chemical structure are seen to significantly impact the nature of the charge‐transfer species and the efficiency of the doping process, underlining the need for a more detailed understanding of the microscopic doping mechanism in organic semiconductors to reliably guide targeted chemical design. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53 , 58–63
Organic Electronics · 50 Zitationen · DOI
Journal of Materials Chemistry A · 48 Zitationen · DOI
Microscopy and spectroscopy correlate efficiency enhancement of TQ1:PC<sub>70</sub>BM solar cells with changes in morphology through optimized solution formulation.
physica status solidi (a) · 47 Zitationen · DOI
Abstract Ambipolar organic semiconductor blends, i.e. mixtures of electron and hole conducting materials, attain growing interest due to their utilization in quasi‐complementary organic field‐effect transistors and organic photovoltaic cells. Many investigations in the latter field have reported an increase of the solar cell efficiency by optimizing the balance between charge carrier transport in phase‐separated structures and exciton dissociation at the interface between these phases. Here we show the implications of blending molecular materials for structural, optical, and electrical properties in two model systems for organic photovoltaic cells. We have investigated blends and neat films of the hole transporting material Cu‐phthalocyanine (CuPc) together with fullerene C 60 and Cu‐hexadecafluorophthalocyanine (F 16 CuPc) as electron transporting materials, respectively. On the one hand, the difference in molecular structure of the spherical C 60 and the planar molecule CuPc leads to nanophase separation in a blend of both of them, causing charge carrier transport being limited by the successful formation of percolation paths. On the other hand, blends of the similar shaped CuPc and F 16 CuPc molecules entail mixed crystalline films, as can be clearly seen by X‐ray scattering measurements. We discuss differences of both systems with respect to their microstructure as well as their electrical transport properties in diodes and field‐effect transistors. Furthermore, we compare the photovoltaic properties of planar‐ and bulk‐heterojunction devices under white light illumination to relate the different morphologies of both material systems to their performance in solar cells. magnified image Sketches of different molecular arrangements in blended systems. The formation of phase‐separated (left) or molecularly mixed crystalline films (right) can occur, depending on the geometry of the involved molecules.
Wear · 47 Zitationen · DOI
Physical Chemistry Chemical Physics · 45 Zitationen · DOI
Molecular doping is a key process to increase the density of charge carriers in organic semiconductors. Doping-induced charges in polymer semiconductors result in the formation of polarons and/or bipolarons due to the strong electron-vibron coupling in conjugated organic materials. Identifying the nature of charge carriers in doped polymers is essential to optimize the doping process for applications. In this work, we use Raman spectroscopy to investigate the formation of charge carriers in molecularly doped poly(3-hexylthiophene-2,5-diyl) (P3HT) for increasing dopant concentration, with the organic salt dimesityl borinium tetrakis(penta-fluorophenyl)borate (Mes<sub>2</sub>B<sup>+</sup> [B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>-</sup>) and the Lewis acid tris(pentafluorophenyl)borane [B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>]. While the Raman signatures of neutral P3HT and singly charged P3HT segments (polarons) are known, the Raman spectra of doubly charged P3HT segments (bipolarons) are not yet sufficiently understood. Combining Raman spectroscopy measurements on doped P3HT thin films with first-principles calculations on oligomer models, we explain the evolution of the Raman spectra from neutral P3HT to increasingly doped P3HT featuring polarons and eventually bipolarons at high doping levels. We identify and explain the origin of the spectral features related to bipolarons by tracing the Raman signature of the symmetric collective vibrations along the polymer backbone, which - compared to neutral P3HT - redshifts for polarons and blueshifts for bipolarons. This is explained by a planarization of the singly charged P3HT segments with polarons and rather high order in thin films, while the doubly charged segments with bipolarons are located in comparably disordered regions of the P3HT film due to the high dopant concentration. Furthermore, we identify additional Raman peaks associated with vibrations in the quinoid doubly charged segments of the polymer. Our results offer the opportunity for readily identifying the nature of charge carriers in molecularly doped P3HT while taking advantage of the simplicity, versatility, and non-destructive nature of Raman spectroscopy.
Journal of the American Chemical Society · 45 Zitationen · DOI
We present a comprehensive investigation of the charge-transfer (CT) effect in weakly interacting organic semiconductor mixtures. The donor-acceptor pair diindenoperylene (DIP) and N,N'-bis(2-ethylhexyl)-1,7-dicyanoperylene-3,4/9,10-bis(dicarboxyimide) (PDIR-CN<sub>2</sub>) has been chosen as a model system. A wide range of experimental methods was used in order to characterize the structural, optical, electronic, and device properties of the intermolecular interactions. By detailed analysis, we demonstrate that the partial CT in this weakly interacting mixture does not have a strong effect on the ground state and does not generate a hybrid orbital. We also find a strong CT transition in light absorption as well as in photo- and electroluminescence. By using different layer sequences and compositions, we are able to distinguish electronic coupling in-plane vs out-of-plane and, thus, characterize the anisotropy of the CT state. Finally, we discuss the impact of CT exciton generation on charge-carrier transport and on the efficiency of photovoltaic devices.
Advanced Science · 44 Zitationen · DOI
Molecular doping allows enhancement and precise control of electrical properties of organic semiconductors, and is thus of central technological relevance for organic (opto-) electronics. Beyond single-component molecular electron acceptors and donors, organic salts have recently emerged as a promising class of dopants. However, the pertinent fundamental understanding of doping mechanisms and doping capabilities is limited. Here, the unique capabilities of the salt consisting of a borinium cation (Mes<sub>2</sub>B<sup>+</sup>; Mes: mesitylene) and the tetrakis(penta-fluorophenyl)borate anion [B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>-</sup> is demonstrated as p-type dopant for polymer semiconductors. With a range of experimental methods, the doping mechanism is identified to comprise electron transfer from the polymer to Mes<sub>2</sub>B<sup>+</sup>, and the positive charge on the polymer is stabilized by [B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>-</sup>. Notably, the former salt cation leaves during processing and is not present in films. The anion [B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>-</sup> even enables the stabilization of polarons and bipolarons in poly(3-hexylthiophene), not yet achieved with other molecular dopants. From doping studies with high ionization energy polymer semiconductors, the effective electron affinity of Mes<sub>2</sub>B<sup>+</sup>[B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>-</sup> is estimated to be an impressive 5.9 eV. This significantly extends the parameter space for doping of polymer semiconductors.
Journal of Materials Chemistry C · 43 Zitationen · DOI
For molecularly doped poly(3-hexyl-thiophene) solvated individual chains can be unambiguously differentiated from aggregated ones by diagnostic polaron absorption.
The Journal of Physical Chemistry C · 42 Zitationen · DOI
We exploited the thermal annealing of poly(3-hexylthiophene) (P3HT) molecularly p-doped with the strong electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) as a tool for tuning the doping concentration as a quasi singular parameter. Via directed dopant desorption, we could unravel the complex microstructure of this semicrystalline system, leading to a detailed growth model solely based on complementary experimental evidence from scattering and spectroscopic techniques. We find the crystalline portion of p-doped P3HT to comprise regions, where dopant anions pack with the polymer chains in a metastable, cocrystalline structure with additional ionized dopants dispersed in the alkyl chain region of P3HT. Simultaneously, regions exist where the pristine polymer backbones closely pack. The dedoping via dopant desorption through thermal annealing reveals the dopants within the mixed crystalline phase to be thermally least stable. Notably, their initial desorption does not alter the thin film conductivity, which indicates this phase to be not crucial for charge transport. Upon further dopant desorption, the pristine P3HT backbone phase prevails with dopant anions remaining still dispersed in the alkyl chain region of the film. During the entire dedoping, we did not observe indications for the presence of neutral F4TCNQ. Only upon completing the dedoping at 120 °C are both the conductivity and the microstructure of pristine P3HT recovered. We demonstrate that the temperature-induced dedoping provides valuable information on the microstructure of doped organic semiconductors, which remains inaccessible otherwise because of the intrinsic structural and energetic complexity of such systems.
Physics Letters A · 42 Zitationen · DOI
Chemistry of Materials · 41 Zitationen · DOI
Controlling the electrical conductivity of organic semiconductors is a key asset for organic electronics, nowadays realized mostly by molecular dopants. Two doping mechanisms have been reported – charge-transfer complex (CTC) and ion pair (IPA) formation. However, their occurrence depending on molecular structure, energy levels, and structure of thin films remains elusive. Here, we study p-type doping of the planar organic semiconductor dibenzotetrathiafulvalene (DBTTF) in combination with the electron acceptors tetracyanonaphthoquinodimethane (TCNNQ) and hexafluorotetracyanonaphthoquinodimethane (F6TCNNQ) as planar dopants. The conductivity of DBTTF films increases by more than two orders of magnitude upon doping with F6TCNNQ and only slightly with TCNNQ. The highest conductivity is reached at about 10 mol % dopant concentration as a result of two counteracting effects: (1) increasing carrier concentration and (2) reduced carrier mobility due to the growing density of structural defects. We identified two different CTCs for DBTTF:TCNNQ blends and both types of charge-transfer interactions (CTC and IPA) in films of DBTTF doped with F6TCNNQ from absorption measurements. No signature of the charge-transfer interaction is found for DBTTF and TCNNQ in solution, whereas IPA formation only is observed for DBTTF and F6TCNNQ. Many-body perturbation theory calculations of the electronic and optical properties of one-dimensional stacks complement the experimental data and help in understanding the behavior of CTCs. The degree of charge transfer turns out to be higher for the DBTTF:F6TCNNQ complexes than for DBTTF:TCNNQ, as derived from the C≡N stretching mode softening in infrared absorption. We discuss the different fundamental semiconductor–dopant interactions in solution as compared to the solid state with the aid of the state-of-matter-dependent energy levels of the materials. The presence of both charge-transfer mechanisms in the material combinations investigated here gives us access to their doping efficiency, which is higher for IPA than for CTC formation. Avoiding the CTC formation by structural imperfections seems to be a way to increase the doping efficiency for crystalline materials. The determination of energy levels both in solution and in thin films is beneficial for understanding charge-transfer behavior.
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Ladungstransfer an Heterogrenzflächen organischer Halbleiter: Zu einer einheitlichen Beschreibung von Szenarien im Übergangsbereich zwischen Grundzustand bzw. angeregtem Zustand
university
Ladungstransfer an Heterogrenzflächen organischer Halbleiter: Zu einer einheitlichen Beschreibung von Szenarien im Übergangsbereich zwischen Grundzustand bzw. angeregtem Zustand
university
Stammdaten
Identität, Organisation und Kontakt aus HU-FIS.
- Name
- PD Dr. rer. nat. Andreas Opitz
- Titel
- PD Dr. rer. nat.
- Fakultät
- Mathematisch-Naturwissenschaftliche Fakultät
- Institut
- Institut für Physik
- Arbeitsgruppe
- Struktur, Dynamik und elektronische Eigenschaften molekularer Systeme
- Telefon
- +49 30 2093-82286
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- 26.4.2026, 01:10:05