130 research outputs found
Understanding the Double Doping of Organic Semiconductors Via State Energy Renormalization upon Charging
The double ionization of molecular dopants enables the doping efficiency (free charges per dopant molecule) to rise above 100%. However, the current models of doped organic semiconductors based on Fermi–Dirac statistics fail to explain the double ionization of dopants and also the analogous situation of bipolaron formation on a host polymer. Here, we address this shortcoming by considering the renormalization of the state energies upon electron transfer between host and p-dopant. We vary the model parameters─the reorganization energy and evolutions of ionization energies and electron affinities upon charging─and plot the fractions of doubly ionized, singly ionized, and neutral species. The model shows good agreement with experimental measurements of doubly ionized p-dopants and bipolarons on a p-doped polymer. With these insights, we suggest that the state energy renormalization upon charging is the key parameter to be minimized for double ionization of dopants or maximized to avoid formation of bipolarons on the host.Peer Reviewe
Frontier Orbital Degeneracy: A new Concept for Tailoring the Magnetic State in Organic Semiconductor Adsorbates
Kondo resonances in molecular adsorbates are an important building block for
applications in the field of molecular spintronics. Here, we introduce the
novel concept of using frontier orbital degeneracy for tailoring the magnetic
state, which is demonstrated for the case of the organic semiconductor
1,4,5,8,9,11-Hexaazatriphenylenehexacarbonitrile (HATCN, C18N12) on Ag(111).
Low-temperature scanning tunneling microscopy/spectroscopy (LT-STM/STS)
measurements reveal the existence of two types of adsorbed HATCN molecules with
distinctly different appearances and magnetic states, as evident from the
presence or absence of an Abrikosov-Suhl-Kondo resonance. Our DFT results show
that HATCN on Ag(111) supports two almost isoenergetic states, both with one
excess electron transferred from the Ag surface, but with magnetic moments of
either 0 or 0.65 uB. Therefore, even though all molecules undergo charge
transfer of one electron from the Ag substrate, they exist in two different
molecular magnetic states that resemble a free doublet or an entangled spin
state. We explain how the origin of this behavior lies in the twofold
degeneracy of the lowest unoccupied molecular orbitals of gas phase HATCN,
lifted upon adsorption and charge-transfer from Ag(111). Our combined STM and
DFT study introduces a new pathway to tailoring the magnetic state of molecular
adsorbates on surfaces, with significant potential for spintronics and quantum
information science
Impact of morphology on polaron delocalization in a semicrystalline conjugated polymer
We investigate the delocalization of holes in the semicrystalline conjugated
polymer poly(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene) (PBTTT) by
directly measuring the hyperfine coupling between photogenerated polarons and
bound nuclear spins using electron nuclear double resonance spectroscopy. An
extrapolation of the corresponding oligomer spectra reveals that charges tend
to delocalize over 4.0–4.8 nm with delocalization strongly dependent on
molecular order and crystallinity of the PBTTT polymer thin films. Density
functional theory calculations of hyperfine couplings confirm that long-range
corrected functionals appropriately describe the change in coupling strength
with increasing oligomer size and agree well with the experimentally measured
polymer limit. Our discussion presents general guidelines illustrating the
various pitfalls and opportunities when deducing polaron localization lengths
from hyperfine coupling spectra of conjugated polymers
Interplay Between Mixed and Pure Exciton States Controls Singlet Fission in Rubrene Single Crystals
Singlet fission (SF) is a multielectron process in which one singlet exciton
S converts into a pair of triplet excitons T+T. SF is widely studied as it may
help overcome the Shockley-Queisser efficiency limit for semiconductor
photovoltaic cells. To elucidate and control the SF mechanism, great attention
has been given to the identification of intermediate states in SF materials,
which often appear elusive due to the complexity and fast timescales of the SF
process. Here, we apply 10fs-1ms transient absorption techniques to high-purity
rubrene single crystals to disentangle the intrinsic fission dynamics from the
effects of defects and grain boundaries and to identify reliably the fission
intermediates. We show that above-gap excitation directly generates a hybrid
vibronically assisted mixture of singlet state and triplet-pair multiexciton
[S:TT], which rapidly (<100fs) and coherently branches into pure singlet or
triplet excitations. The relaxation of [S:TT] to S is followed by a relatively
slow and temperature-activated (48 meV activation energy) incoherent fission
process. The SF competing pathways and intermediates revealed here unify the
observations and models presented in previous studies of SF in rubrene and
propose alternative strategies for the development of SF-enhanced photovoltaic
materials
Intermolecular CT excitons enable nanosecond excited-state lifetimes in NIR-absorbing non-fullerene acceptors for efficient organic solar cells
State-of-the-art Y6-type molecular acceptors exhibit nanosecond excited-state
lifetimes despite their low optical gaps (~1.4 eV), thus allowing organic solar
cells (OSCs) to achieve highly efficient charge generation with extended
near-infrared (NIR) absorption range (up to ~1000 nm). However, the precise
molecular-level mechanism that enables low-energy excited states in Y6-type
acceptors to achieve nanosecond lifetimes has remained elusive. Here, we
demonstrate that the distinct packing of Y6 molecules in film leads to a strong
intermolecular charge-transfer (iCT) character of the lowest excited state in
Y6 aggregates, which is absent in other low-gap acceptors such as ITIC. Due to
strong electronic couplings between the adjacent Y6 molecules, the iCT-exciton
energies are greatly reduced by up to ~0.25 eV with respect to excitons formed
in separated molecules. Importantly, despite their low energies, the iCT
excitons have reduced non-adiabatic electron-vibration couplings with the
electronic ground state, thus suppressing non-radiative recombination and
allowing Y6 to overcome the well-known energy gap law. Our results reveal the
fundamental relationship between molecular packing and nanosecond excited-state
lifetimes in NIR-absorbing Y6-type acceptors underlying the outstanding
performance of Y6-based OSCs
High operational and environmental stability of high-mobility conjugated polymer field-effect transistors through the use of molecular additives.
Due to their low-temperature processing properties and inherent mechanical flexibility, conjugated polymer field-effect transistors (FETs) are promising candidates for enabling flexible electronic circuits and displays. Much progress has been made on materials performance; however, there remain significant concerns about operational and environmental stability, particularly in the context of applications that require a very high level of threshold voltage stability, such as active-matrix addressing of organic light-emitting diode displays. Here, we investigate the physical mechanisms behind operational and environmental degradation of high-mobility, p-type polymer FETs and demonstrate an effective route to improve device stability. We show that water incorporated in nanometre-sized voids within the polymer microstructure is the key factor in charge trapping and device degradation. By inserting molecular additives that displace water from these voids, it is possible to increase the stability as well as uniformity to a high level sufficient for demanding industrial applications.We gratefully acknowledge financial support from Innovate UK (PORSCHED project) and the Engineering and Physical Sciences Research Council though a Programme Grant (EP/M005141/1). I.N. acknowledges studentship support from FlexEnable Ltd. K.B. gratefully acknowledges financial support from the Deutsche Forschungsgemeinschaft (BR 4869/1-1). B.R., M.K.R., and J.L.B. thank the financial support from King Abdullah University of Science and Technology (KAUST), the KAUST Competitive Research Grant program, and the Office of Naval Research Global (Award N62909-15-1-2003 );This is the author accepted manuscript. The final version is available from Nature Publishing Group via https://doi.org/10.1038/nmat478
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Revealing the crystalline packing structure of Y6 in the active layer of organic solar cells: the critical role of solvent additives
The bulk heterojunction (BHJ) morphology of photovoltaic materials is crucial to the fundamental optoelectronic properties of organic solar cells (OSCs). However, in the photoactive layer, the intrinsic crystalline packing structure of Y6, currently the hallmark molecule among Y-series non-fullerene acceptors (NFAs), has not been unambiguously determined. Here, employing grazing-incidence wide-angle X-ray scattering (GIWAXS), we managed to uncover the intrinsic crystalline packing structure of Y6 in the BHJ active layer of OSCs, which is found to be different from its single-crystal structure reported previously. Moreover, we find that solvent additive 1-chloronaphthalene (CN) can induce highly ordered packing of Y6 in BHJ thin films. With the help of atomistic molecular dynamics simulations, it is revealed that π-π interactions generally exist between naphthalene derivatives and IC terminals of Y6 analogues, which would essentially improve their long-range ordering. Our work reveals the intrinsic crystalline packing structure of Y6 in the BHJ active layer as well as its crystallization mechanism in thin films, thus providing direct correlations between this crystalline packing and the device characteristics and photophysical properties.Knut och Alice Wallenbergs StiftelseImmediate accessThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]
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