An official website of the United States government
Abstract Molecular orientation plays a critical role in controlling carrier transport in organic semiconductors (OSCs). However, this aspect has not been explored for surface doping of OSC thin films. The challenge lies in lack of methods to precisely modulate relative molecular orientation between the dopant and the OSC host. Here, the impact of molecular orientation on dopant–host electronic interactions by large modulation of conjugated polymer orientation via solution coating is reported. Combining synchrotron‐radiation X‐ray measurements with spectroscopic and electrical characterizations, a quantitative correlation between doping‐enhanced charge carrier mobility and the Herman's orientation parameter is presented. This direct correlation can be attributed to enhanced charge‐transfer interactions at host/dopant interface with increasing face‐on orientation of the polymer. These results demonstrate that the surface doping effect can be fundamentally manipulated by controlling the molecular orientation of the OSC layer, enabling optimization of carrier transport.
more »
« less
Polymorphism controls the degree of charge transfer in a molecularly doped semiconducting polymer
When an organic semiconductor (OSC) is blended with an electron acceptor molecule that can act as a p-type dopant, there should ideally be complete (integer) transfer of charge from the OSC to the dopant. However, some dopant–OSC blends instead form charge transfer complexes (CTCs), characterized by fractional charge transfer (CT) and strong orbital hybridization between the two molecules. Fractional CT doping does not efficiently generate free charge carriers, but it is unclear what conditions lead to incomplete charge transfer. Here we show that by modifying film processing conditions in the semiconductor–dopant couple poly(3-hexylthiophene):2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane (P3HT:F4TCNQ), we can selectively obtain nearly pure integer or fractional CT phases. Fractional CT films show electrical conductivities approximately 2 orders of magnitude lower than corresponding integer CT films, and remarkably different optical absorption spectra. Grazing incidence wide-angle X-ray diffraction (GIXD) reveals that fractional CT films display an unusually dense and well-ordered crystal structure. These films show lower paracrystallinity and shorter lamellar and π-stacking distances than undoped films processed under similar conditions. Using plane-wave DFT we obtain a structure with unit cell parameters closely matching those observed by GIXD. This first-ever observation of both fractional and integer CT in a single OSC–dopant system demonstrates the importance of structural effects on OSC doping and opens the door to further studies.
more »
« less
- Award ID(s):
- 1636385
- PAR ID:
- 10065406
- Date Published:
- Journal Name:
- Materials Horizons
- Volume:
- 5
- Issue:
- 4
- ISSN:
- 2051-6347
- Page Range / eLocation ID:
- 655 to 660
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract In both chemical and electrochemical doping of organic semiconductors (OSCs), a counterion, either from the electrolyte or ionized dopant, balances the charge introduced to the OSC. Despite the large influence of this counterion on OSC optical and electronic response, there remains substantial debate on how a fundamental parameter, ion size, impacts these properties. This work resolves much of this debate by accounting for two doping regimes. In the low‐doping regime, the Coulomb binding energies between charge carriers on the OSC and the counterions are significant, and larger counterions lead to decreased Coulomb interactions, more delocalized charge carriers, and higher electrical conductivities. In the high‐doping regime, the Coulomb binding energies become negligible due to the increased dielectric constant of the films and a smoothing of the energy landscape; thereby, the electrical conductivities depend primarily on the extent of morphological disorder in the OSC. Moreover, in regioregular poly(3‐hexylthiophene), rr‐P3HT, smaller counterions lead to greater bipolaron concentrations in the low‐doping regime due to the increased Coulomb interactions. Emphasizing the impact of the counterion size, it is shown that larger counterions can lead to increased thermoelectric power factors for rr‐P3HT.more » « less
-
null (Ed.)Electronic technologies critically rely on the ability to broadly dope the active semiconductor; yet the promising class of halide perovskite semiconductors so far does not allow for significant control over carrier type (p- or n-) and density. The molecular doping approach offers important opportunities for generating free carriers through charge transfer. In this work, we demonstrate effective p-doping of MAPb 0.5 Sn 0.5 I 3 films using the molecular dopant F4TCNQ as a grain boundary coating, offering a conductivity and hole density tuning range of up to five orders of magnitude, associated with a 190 meV Fermi level down-shift. While charge transfer between MAPb 0.5 Sn 0.5 I 3 and F4TCNQ appears efficient, dopant ionization decreases with increasing Pb content, highlighting the need for appropriate energy offset between host and dopant molecule. Finally, we show that electrical p-doping impacts the perovskite optoelectronic properties, with a hole recombination lifetime increase of over one order of magnitude, suggesting passivation of deep traps.more » « less
-
Efficient electrical doping of organic semiconductors relies on identifying appropriate molecular dopants that are capable of ionizing semiconductor molecules with a high yield, thereby creating mobile charges. We explore the suitability of two different material parameters to predict ion pair formation for different sets of semiconductor–dopant combinations: (i) redox-potentials measured by cyclic voltammetry in solution and (ii) ionization energy (IE)/electron affinity (EA) measured on thin films by ultraviolet/inverse photoelectron spectroscopy. Our study suggests, at least for molecular semiconductors and dopants, that redox-potentials are better suited to identify matching material pairs and their ion pair formation yield than IE/EA values. This is ascribed to the dependence of IE/EA values on molecular orientation and film structure on and above the meso-scale. In contrast, cyclic voltammetry measurements, although performed on solutions rather than on thin films, capture dopant–semiconductor energy levels on the molecular scale, which is more relevant for doping even in the case of solid thin films.more » « less
-
null (Ed.)Introducing charge carriers is of paramount importance for increasing the efficiency of organic semiconducting materials. Various methods of extrinsic doping, where molecules or atoms with large/small reduction potentials are blended with the semiconductor, can lead to dopant aggregation, migration, phase segregation, and morphology alteration. Self-doping overcomes these challenges by structurally linking the dopant directly to the organic semiconductor. However, for their practical incorporation into devices, self-doped organic materials must be cast into thin-films, yet processing methods to allow for the formation of continuous and uniform films have not been developed beyond simple drop-casting. Whilst self-doped organic molecules afford the remarkable ability to position dopants with molecular precision and control of attachment mode, their steric bulk inevitably disrupts the crystallization on surfaces. As such, there is great interest in the development of processing modalities that allow deposited molecules to converge to the thermodynamic minimum of a well-ordered and highly crystalline organic thin film instead of getting trapped in local disordered minima that represent metastable configurations. By contrasting drop casting, ultrasonic deposition, and physical vapor deposition, we investigate the free energy landscape of the crystallization of sterically hindered self-doped perylene diimide thin films. A clear relationship is established between processing conditions, the crystallinity and order within the deposited films, the dopant structures and the resulting spin density. We find physical vapor deposition to be a robust method capable of producing smooth, continuous, highly ordered self-doped organic small molecule thin-films with tailored spin concentrations and well-defined morphologies.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/C8MH00223A
