Charge Carriers In Organic Semiconductors Research Articles

Transiently delocalized states enhance hole mobility in organic molecular semiconductors.

Evidence shows that charge carriers in organic semiconductors self-localize because of dynamic disorder. Nevertheless, some organic semiconductors feature reduced mobility at increasing temperature, a hallmark for delocalized band transport. Here we present the temperature-dependent mobility in two record-mobility organic semiconductors: dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]-thiophene (DNTT) and its alkylated derivative, C8-DNTT-C8. By combining terahertz photoconductivity measurements with atomistic non-adiabatic molecular dynamics simulations, we show that while both crystals display a power-law decrease of the mobility (μ) with temperature (T) following μ ∝ T -n, the exponent n differs substantially. Modelling reveals that the differences between the two chemically similar semiconductors can be traced to the delocalization of the different states that are thermally accessible by charge carriers, which in turn depends on their specific electronic band structure. The emerging picture is that of holes surfing on a dynamic manifold of vibrationally dressed extended states with a temperature-dependent mobility that provides a sensitive fingerprint for the underlying density of states.

Regulating Excitonic Effects in Covalent Organic Frameworks to Promote Free Charge Carrier Generation

Strong excitonic effects, induced by the Coulombic interactions between photogenerated electrons and holes, seriously hinder the generation of free charge carriers in organic semiconductors for conducting photocatalysis. Herein, we report a built-in control of the donor–acceptor (D–A) interaction strategy to regulate excitonic effects within benzobisthiazole-bridged covalent organic frameworks (Tz-COFs). Theoretical calculation and ultrafast spectroscopy reveal that strengthening D–A interactions by this built-in control strategy in Tz-COFs can accelerate exciton dissociation, thus generating more long-lived photogenerated charge carriers for photoredox reactions. As a result, the optimized Tz-COF-3 exhibits high photocatalytic H2 evolution activity as high as 43.2 mmol g–1 h–1, with an apparent quantum yield of 6.9% at 420 nm. This work guides the development of COFs from excitonic aspects for photocatalysis.

Lewis acids and bases as molecular dopants for organic semiconductors

AbstractControlling the concentration of charge carriers (mobile electrons and holes) in organic semiconductors is vital to precisely controlling their electronic properties. Significant efforts have gone into understanding how molecular dopants induce charge carriers in organic semiconductors. The most widely used doping mechanisms occur via electron transfer (i.e., oxidation or reduction of the semiconductor) or via reaction with a strong Brønsted acid. Recently, strong Lewis acids have been observed to induce p‐type charge carriers in organic semiconductors with greater efficiency than classical dopants. The mechanism of Lewis‐acid doping could not easily be unified with either classical doping methods and has been under intense scrutiny over the past 5 years. Very recently, the Lewis‐acid doping effects have been shown to be due to water impurities in commercial Lewis acids forming strong Brønsted acids. This means that many studies on doping using Lewis acids may be occurring via a Brønsted‐acid doping mechanism. This recent revelation explains some observations in literature, but not all, and there are still unanswered questions. The nature of the Lewis acid and organic semiconductor can significantly impact the doping mechanism and the doping efficiency. Additionally, strong evidence for alternative doping mechanisms using Lewis acids not involving water has been shown. Lewis‐acid doping has mostly been studied as a p‐type dopant method on Lewis‐basic polymers. There is growing literature showing Lewis bases can also act as n‐type dopants, excluding Brønsted‐acid doping as a possible mechanism. In this tutorial review, we will present a brief overview on molecular doping of organic semiconductors, survey the literature on p‐type and n‐type Lewis doping, outline several proposed mechanisms, and speculate on some possible mechanisms using literature observations.

Balancing Density Functional Theory Interaction Energies in Charged Dimers Precursors to Organic Semiconductors.

The pursuit of an increasingly accurate description of intermolecular interactions within the framework of Kohn-Sham density functional theory (KS-DFT) has motivated the construction of numerous benchmark databases over the past two decades. By far, the largest efforts have been spent on closed-shell, neutral dimers for which today the interaction energies and geometries can be accurately reproduced by various combinations of dispersion-corrected density functional approximations (DFAs). In sharp contrast, charged, open-shell dimers remain a challenge as illustrated by the analysis of the OREL26rad benchmark set, composed of π-dimer radical cations. Aside from the methodological aspect, achieving a proper description of radical cationic complexes is appealing due to their role as models for charge carriers in organic semiconductors. In the interest of providing an assessment of more realistic dimer systems, we construct a data set of large radical cationic dimers (CryOrel9) and jointly train the 19 parameters of a dispersion corrected, range-separated hybrid density functional (ωB97X-dDsC). The main objective of ωB97X-dDsC is to provide the maximum balance between the treatment of long-range London dispersion and reduction of the delocalization error, which are essential conditions to obtain accurate energy profiles and binding energies of charged, open-shell dimers. The performance of ωB97X-dDsC, its parent ωB97X functional series, and a selection of wave function-based methods is reported for the CryOrel9 data set. The robustness of the reoptimized variant (ωB97X-dDsC) is also tested on other GMTKN30 data sets.

Balancing DFT Interaction Energies in Charged Dimers Precursors to Organic Semiconductors

Accurately describing intermolecular interactions within the framework of Kohn-Sham density functional theory (KS-DFT) has resulted in numerous benchmark databases over the past two decades. By far, the largest efforts have been spent on closed-shell, neutral dimers for which today, the interaction energies and geometries can be accurately reproduced by various combinations of dispersion-corrected density functional approximations (DFAs). In sharp contrast, charged, open-shell dimers remain a challenge as illustrated by the analysis of the OREL26rad benchmark set consisting of pi-dimer radical cations. Aside from the methodological aspect, achieving a proper description of radical cationic complexes is appealing due to their role as models for charge carriers in organic semiconductors. In the interest of providing an assessment of more realistic dimer systems, we construct a dataset of large radical cationic dimers (CryOrel) and jointly train the 19 parameters of a dispersion corrected, range-separated hybrid density functional (wB97X-dDsC), with the objective of providing the maximum balance between the treatment of long-range London dispersion and reduction of the delocalization error. These conditions are essential to obtain accurate energy profiles and binding energies of charged, open-shell dimers. Comparisons with the performance of the parent wB97X functional series and state-of-the-art wavefunction based methods are provided.

Organic Salt Semiconductor with High Photoconductivity and Long Carrier Lifetime

AbstractIntrinsic photogeneration of charge carriers in organic semiconductors is generally attributed to high energy ionization or exciton dissociation by a strong electric field. Here, high bulk photoconductivity is reported in pristine pentamethine cyanine films with photocurrent onset at the band‐edge of the organic semiconductor. Single‐layer cyanine diodes with selective hole and electron contacts show linear dependence of photocurrent with reverse voltage and light intensity. Numerical drift‐diffusion simulations reveal that the linear resistor behavior stems from low and unbalanced carrier mobilities giving rise to negative space charge. Slow bimolecular recombination kinetics of photoinduced charges obtained by time delayed charge extraction measurements show strongly reduced Langevin recombination with long carrier lifetime of the order of a millisecond. Such reduced charge carrier recombination puts forward a materials concept to be exploited in photodiodes and more generally in optoelectronic devices.

Band Transport and Trapping in Didodecyl[1]benzothieno[3,2-b][1]benzothiophene Probed by Terahertz Spectroscopy.

Terahertz electromodulation spectroscopy provides insight into the material-inherent transport properties of charge carriers in organic semiconductors. Experiments on didodecyl[1]benzothieno[3,2-b][1]benzothiophene (C12-BTBT-C12) devices yield for holes an intraband mobility of 9 cm2 V-1 s-1. The short duration of the THz pulses advances the understanding of the hole transport on the molecular scale. The efficient screening of Coulomb potentials leads to a collective response of the hole gas to external fields, which can be well described by the Drude model. Bias stress of the devices generates deep traps that capture mobile holes. Although the resulting polarization across the device hinders the injection of mobile holes, the hole mobilities are not affected.

Probing Charge Carrier Dynamics in Porphyrin-Based Organic Semiconductor Thin Films by Time-Resolved THz Spectroscopy.

To improve the power conversion efficiency of solar cells, it is important to understand the underlying relaxation mechanisms of photogenerated charge carriers in organic semiconductors. In this work, we studied the charge carrier dynamics of diketopyrrolopyrrole-linked tetrabenzoporphyrin thin films where the diketopyrrolopyrrole unit has two n-butyl groups, abbreviated as C4-DPP-BP. We used time-resolved terahertz (THz) spectroscopy to track charge carrier dynamics with excitations at 800 and 400 nm. Compared with tetrabenzoporphyrin (BP), the extension of π-electron delocalization to the diketopyrrolopyrrole peripherals leads to an increase in absorption in the near-infrared region. Following the excitation at 800 nm, we found that the transient THz signals in C4-DPP-BP thin films decay with time constants of 0.5 and 9.1 ps, with small residual components. With excitation at 400 nm, we found that the transient THz signals decay with time constants of 0.4 and 7.5 ps. On the basis of the similarity of the decay profiles of the transient THz signals obtained with excitations at 400 and 800 nm, we considered that the decaying components are due to charge carrier recombination and/or trapping at defect sites, which do not depend on the excess energy of the photoexcitation. In contrast to BP, even without an electron acceptor, we observed the finite offset of the transient THz signals at 100 ps, demonstrating the existence of long-lived charge carriers. We also measured the photoconductivity spectra of C4-DPP-BP thin films with the excitation at both 800 and 400 nm. It was found that the spectra can be fitted by the Drude-Smith model. From these results, it was determined that the charge carriers are localized right after photoexcitation. At 0.4 ps, the product of the quantum yield of charge generation and mobility of charge carriers at 400 nm is approximately twice that obtained at 800 nm. We discuss the implications of the excess excitation energy in organic semiconductor-based devices.

Impact of Molecular Orientation and Packing Density on Electronic Polarization in the Bulk and at Surfaces of Organic Semiconductors.

The polarizable environment surrounding charge carriers in organic semiconductors impacts the efficiency of the charge transport process. Here, we consider two representative organic semiconductors, tetracene and rubrene, and evaluate their polarization energies in the bulk and at the organic-vacuum interface using a polarizable force field that accounts for induced-dipole and quadrupole interactions. Though both oligoacenes pack in a herringbone motif, the tetraphenyl substituents on the tetracene backbone of rubrene alter greatly the nature of the packing. The resulting change in relative orientations of neighboring molecules is found to reduce the bulk polarization energy of holes in rubrene by some 0.3 eV when compared to tetracene. The consideration of model organic-vacuum interfaces highlights the significant variation in the electrostatic environment for a charge carrier at a surface although the net change in polarization energy is small; interestingly, the environment of a charge even just one layer removed from the surface can be viewed already as representative of the bulk. Overall, it is found that in these herringbone-type layered crystals the polarization energy has a much stronger dependence on the intralayer packing density than interlayer packing density.

(Invited) Importance of Interface Engineering in Organic Spintronics

Organic semiconductors provide a route to revolutionary changes in many areas of device technology with their unique electronic properties and functionalities, which can be easily tailored by chemical synthesis and molecular engineering.[1] In particular, an emerging research area referred to as organic spintronics has initiated a range of device concepts by exploiting spin-related properties in organic semiconductors.[2] Currently, there are two major interests in organic spintronics technology.[3] One of them is to realize a system enabling the injection/transport/detection of spin polarized charge carriers in organic semiconductors.[3] The other one is to understand and explore the magnetic field effects (MFEs) of organic semiconductors for the development and improvement of organic-based devices such as organic light emitting diodes and magnetic sensors.[3] It has been known that in organic-based devices, the interfaces between active materials play a critical role in their performance and functionality because the interfaces greatly affect the injection/transport/extraction of charge carriers in organic semiconductors.[4] Recent findings suggest that the interface engineering is also very crucial in organic spintronics technology such as enabling efficient spin-polarized charge carrier injection and tuning of organic MFEs.[5-7] In this presentation, we will discuss our efforts towards the realization of efficient spin injection and transport in organic semiconductors as well as the modification of organic MFEs by interface engineering. We experimentally show the importance of band alignment between a metal electrode and an organic semiconductor at their interface for efficient spin injection/detection.[6] We will also show how one can utilize a molecular self-assembled monolayer by inserting it between a metal electrode and an organic semiconductor in order to change the characteristics of spin injection into an organic semiconductor from a ferromagnetic metal as well as in order to tune the organic MFEs.[7] 1. Editorial, Nat. Mater. 12, 591 (2013). 2. C. Boehme and J. M. Lupton, Nat. Nano. 8, 612 (2013). 3. W. Wagemans, et al., SPIN, 1, 93 (2011). 4. H. Ma, et al., Adv. Funct. Mater. 20, 1371 (2010). 5. S. Sanvito, Nat. Phys. 6, 562 (2010). 6. H.-J. Jang, et al., Appl. Phys. Lett. 101, 102412 (2012). 7. H.-J. Jang, et al., ACS Nano 8, 7192 (2014).

Theory of pulsed reaction yield detected magnetic resonance

We propose pulse sequences for Reaction Yield Detected Magnetic Resonance (RYDMR), which are based on refocusing the zero-quantum coherences in radical pairs by non-selective microwave pulses and using the population of a radical pair singlet spin state as an observable. The new experiments are analogues of existing EPR experiments such as the primary echo, Carr-Purcell, ESEEM, stimulated echo and Mims ENDOR. All pulse sequences are supported by analytical results and numerical calculations. The pulse sequences can be used for more efficient and highly detailed characterization of intermediates of chemical reactions and charge carriers in organic semiconductors.

Spin-Polarized Carriers Injection from Ferromagnetic Metal into Organic Semiconductor

Charge carriers in organic semiconductor are different from that of traditional inorganic semiconductor. Based on three-current model, considering electrical field effect, we present a theoretical model to discuss spin-polarized injection from ferromagnetic electrode into organic semiconductor by analyzing electrochemical potential both in ferromagnetic electrode and organic semiconductors. The calculated result of this model shows effects of electrode's spin polarization, equilibrium value of polarons ratio, interfacial conductance, bulk conductivity of materials and electrical field. It is found that we could get decent spin polarization with common ferromagnetic electrode by increasing equilibrium value of polarons ratio. We also find that large and matched bulk conductivity of organic semiconductor and electrode, small spin-dependent interfacial conductance, and enough large electrical field are critical factors for increasing spin polarization.

Concentration dependence of the transport energy level for charge carriers in organic semiconductors

The concept of the transport energy (TE) has proven to be one of the most powerful theoretical approaches to describe charge transport in organic semiconductors. In the recent paper L. Li, G. Meller, and H. Kosina [Appl. Phys. Lett. 92, 013307 (2008)] have studied the effect of the partially filled localized states on the position of the TE level. We show that the position of the TE is essentially different to the one suggested by L. Li, G. Meller, and H. Kosina [Appl. Phys. Lett. 92, 013307 (2008)] We further modify the standard TE approach taking into account the percolation nature of the transport path. Our calculations show that the TE becomes dependent on the concentration of charge carriers n at much higher n values than those, at which the carrier mobility already strongly depends on n. Hence the calculations of the concentration-dependent carrier mobility cannot be performed within the approach, in which only the concentration dependence of the TE is taken into account.