Electronic Properties Of Organic Semiconductors Research Articles

Extremely Electron‐Withdrawing Lewis‐Paired CN Groups for Organic p‐Dopants

AbstractP‐type chemical doping (p‐doping) is a key technique to modulate the optical, electrical, and electronic properties of organic semiconductors. However, typical functional groups in organic p‐dopants have insufficient electron‐withdrawing strength, and the inevitable diffusion of dopants in host matrices degrades doping stabilities. Herein, we utilize extremely electron‐withdrawing Lewis‐paired CN groups as a new class of building blocks for designing unprecedentedly strong organic p‐dopants with excellent doping stability. Various Lewis acids are paired with CN‐functionalized conjugated molecules in the solution state, which strengthens the electron‐withdrawing properties of CN groups almost twofold. The large dopants afford outstanding doping stability against continuous heating and long‐term atmospheric exposure, which is promising for practical applications in devices. Given the broad applicability of this simple combinatorial approach, it may impact many fields of (opto)electronics.

Extremely Electron-Withdrawing Lewis-Paired CN Groups for Organic p-Dopants.

P-type chemical doping (p-doping) is a key technique to modulate the optical, electrical, and electronic properties of organic semiconductors. However, typical functional groups in organic p-dopants have insufficient electron-withdrawing strength, and the inevitable diffusion of dopants in host matrices degrades doping stabilities. Herein, we utilize extremely electron-withdrawing Lewis-paired CN groups as a new class of building blocks for designing unprecedentedly strong organic p-dopants with excellent doping stability. Various Lewis acids are paired with CN-functionalized conjugated molecules in the solution state, which strengthens the electron-withdrawing properties of CN groups almost twofold. The large dopants afford outstanding doping stability against continuous heating and long-term atmospheric exposure, which is promising for practical applications in devices. Given the broad applicability of this simple combinatorial approach, it may impact many fields of (opto)electronics.

P‐Type Doping of PM6:Y6‐Based Photoactive Layer with Formic Acid

The introduction of molecular doping process is necessary to enhance the optic and electronic properties of organic semiconductors for facilitating charge transport. In particular, since the doping process has a positive influence on the charge transfer interaction between host semiconductor and dopant, improved mobility has been efficiently achieved. Despite its advantages, doping technologies in organic solar cells (OSCs) are emphasized to the development of n‐type dopants used for balancing the electron and hole. In addition, since the bulk‐heterojunction microstructure in OSCs has randomly blended phases of the donor and acceptor, it is important to optimize charge extraction without loss by controlling the morphology. Herein, OSCs by p‐type doping with formic acid (FA) into a photoactive layer is reported. The champion device yields a significantly improved power conversion efficiency from 14.3% to 15.3% with a high fill factor of 71.7%. It is found that the p‐doped photoactive layer exhibits enhanced conductivity, improved carrier mobilities, suppressed charge recombination, and lowered leakage current. The FA simultaneously acts as a film morphology controller of the photoactive layer with enhanced phase separation to transport the charge efficiently. Thus, the doping process with FA can maintain the device performance in stability tests (95.6% remaining).

Are Coarse-Grained Structures as Good as Atomistic Ones for Calculating the Electronic Properties of Organic Semiconductors?

The quality of amorphous molecular morphologies obtained with a recently introduced coarse-grained model, representing molecules in terms of connected anisotropic beads (Phys. Chem. Chem. Phys. 2019, 21, 26195), is benchmarked against reference atomistic data. Typical small-molecule organic semiconductors in their pristine and doped forms are chosen as a challenging and technologically relevant case study for our comparison, which includes both structural features and the resulting electronic properties, such as charge carrier energy levels, energetic disorder, and intermolecular charge transfer couplings. Our analysis shows that our accurate coarse-grained model leads to molecular glasses that are very similar to native atomistic samples, with the discrepancy being further reduced upon back-mapping. The electronic properties computed for back-mapped morphologies are almost indistinguishable from the atomistic reference, especially for multibranched poly(hetero)cyclic hydrocarbons usually employed as organic semiconductors. This study provides a proof of principle for highly accurate large-scale simulations of complex molecular systems at a reduced computational cost.

In Situ Generation of n‐Type Dopants by Thermal Decarboxylation

AbstractMolecular doping is a powerful and increasingly popular approach toward enhancing electronic properties of organic semiconductors (OSCs) past their intrinsic limits. The development of n‐type dopants has been hampered, however, by their poor stability and high air‐reactivity, a consequence of their generally electron rich nature. Here, the use of air‐stable carboxylated dopant precursors is reported to overcome this challenge. Active dopants are readily generated in solution by thermal decarboxylation and applied in n‐type organic field‐effect transistors (OFETs). Both 1,3‐dimethylimidazolium‐2‐carboxylate (CO2‐DMI) and novel dopant 1,3‐dimethylbenzimidazolium‐2‐carboxylate (CO2‐DMBI) are applied to n‐type OFETs employing well‐known organic semiconductors (OSCs) P(NDI2OD‐T2), PCBM, and O‐IDTBR. Successful improvement of performance in all devices demonstrates the versatility of the dopants across a variety of OSCs. Experimental and computational studies indicate that electron transfer from the dopant to the host OSC is preceded by decarboxylation of the precursor, followed by dimerization to form the active dopant species. Transistor studies highlight CO2‐DMBI as the most effective dopant, improving electron mobility by up to one order of magnitude, while CO2‐DMI holds the advantage of commercial availability.

Direct (hetero)arylation polymerization, electrochemical and optical properties of regioregular 3-substituted polythiophenes with alkylsulphanyl and alkylsulfonyl groups

Regioregular polythiophenes have been widely used in organic semiconducting applications due to their crystallinity and solubility. The alkylsulfanyl and alkylsulfonyl side chains could serve as useful synthetic strategy for adjusting the electronic properties of organic semiconductors. In this paper, head-to-tail coupled and highly regioregular poly(3-octylthiothiophene)s (PTS) and poly(3-octylsulfonylthiophene)s (PTSO2) were synthesized through direct (hetero)arylation polymerization. The head-to-tail (HT) regioregularity of the polymer were determined by the widely used 1H NMR method. The HOMO and LUMO energy levels of poly(3-alkylthiophene) were lowered by adjusting the oxidation state of sulfur from sulfanyl to sulfonyl. The effects of these functional side chains are presented through UV−vis, fluorescence, and electrochemical measurements, as well as crystallographic analysis of these synthetic polymers.

Doping-induced decomposition of organic semiconductors: a caveat to the use of Lewis acid p-dopants

Tuning the electronic properties of organic semiconductors with Lewis acid reagents may go with chemical modifications of the semiconductor structure.

Site-specific chemical doping reveals electron atmospheres at the surfaces of organic semiconductor crystals.

Chemical doping controls the electronic properties of organic semiconductors, but so far, doping protocols and mechanisms are less developed than in conventional semiconductors. Here we describe a unique, site-specific, n-type surface doping mechanism for single crystals of two benchmark organic semiconductors that produces dramatic improvement in electron transport and provides unprecedented evidence for doping-induced space charge. The surface doping chemistry specifically targets crystallographic step edges, which are known electron traps, simultaneously passivating the traps and releasing itinerant electrons. The effect on electron transport is profound: field-effect electron mobility increases by as much as a factor of ten, and its temperature-dependent behaviour switches from thermally activated to band-like. Our findings suggest new site-specific strategies to dope organic semiconductors that differ from the conventional redox chemistry of randomly distributed substitutional impurities. Critically, they also verify the presence of doping-induced electron atmospheres, confirming long-standing expectations for organic systems from conventional solid-state theory.

Molecular Doping Directed by a Neutral Radical.

Molecular doping makes possible tunable electronic properties of organic semiconductors, yet a lack of control of the doping process narrows its scope for advancing organic electronics. Here, we demonstrate that the molecular doping process can be improved by introducing a neutral radical molecule, namely nitroxyl radical (2,2,6,6-teramethylpiperidin-i-yl) oxyl (TEMPO). Fullerene derivatives are used as the host and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazoles (DMBI-H) as the n-type dopant. TEMPO can abstract a hydrogen atom from DMBI-H and transform the latter into a much stronger reducing agent DMBI•, which efficiently dopes the fullerene derivative to yield an electrical conductivity of 4.4 S cm–1. However, without TEMPO, the fullerene derivative is only weakly doped likely by a hydride transfer following by an inefficient electron transfer. This work unambiguously identifies the doping pathway in fullerene derivative/DMBI-H systems in the presence of TEMPO as the transfer of a hydrogen atom accompanied by electron transfer. In the absence of TEMPO, the doping process inevitably leads to the formation of less symmetrical hydrogenated fullerene derivative anions or radicals, which adversely affect the molecular packing. By adding TEMPO we can exclude the formation of such species and, thus, improve charge transport. In addition, a lower temperature is sufficient to meet an efficient doping process in the presence of TEMPO. Thereby, we provide an extra control of the doping process, enabling enhanced thermoelectric performance at a low processing temperature.

Electronic Properties of Branched Molecular Structures Review

The design of new types of macromolecular architecture is one of the main directions in the development of modern polymer physics. As applied to electronics, electroconductive π-conjugated molecules occupy a special place. In this review, the distinctive features of the electronic properties of organic semiconductors when compared with their solid-state analogs are considered. Special attention is paid to the role of electron–electron and electron–phonon interactions in the formation of self-localized excited states, i.e., solitons and polaritons. The constructive and destructive interference effects in molecular structures containing branching nodes and ring groups are discussed. The size of a branched molecule should herewith be small to conserve quantum coherence. Self-localization effects in such molecules are insignificant and transport is determined by electrons and holes injected from contacts. Two approaches to the description of quantum interference in branched molecules are proposed, notably, based on molecular orbitals and in the scope of the pattern of localized atomic orbitals. These approaches make it possible to formulate design rules for molecular structures, in which interference is observed. The latest results on the design of quantum-interference molecular transistors with ultralow energy consumption are presented. Nonconjugated tree-branched polymers, so-called dendrimers, which can be efficient for designing systems of the collection and transformation of electromagnetic radiation are briefly considered.

Aldol Condensation‐Polymerized n‐Doped Conjugated Polyelectrolytes for High‐Performance Nonfullerene Polymer Solar Cells

Doping provides an efficient strategy to control the electronic properties of organic semiconductors. However, compared with the widely reported p‐type doping protocols, n‐type doping of organic semiconductors still remains a challenge. Herein, a series of novel n‐doped conjugated polyelectrolytes (CPEs) with high doping levels and conductivity are designed. These CPEs are synthesized via a facile, metal‐free, and high‐yield aldol condensation protocol from bis‐isatin and bis‐oxindole monomers. The designed CPEs possess a n‐type electron‐deficient and rigid conjugated backbone, resulting in easy charge delocalization, enhanced n‐doping behaviors, and high conductivity. The evolution on the counterions of these CPEs further alters their n‐doping behaviors, charge transporting properties, and work function tunability, etc. By using these CPEs as electron transport materials (ETMs) for nonfullerene polymer solar cells (NF–PSCs), high power conversion efficiencies (PCEs) over 16% can be achieved when PM6:Y6 is used as the active component. Moreover, these CPEs can enable efficient NF–PSCs even if their thicknesses are up to 60 nm, indicating the potential of these CPEs as thickness‐insensitive ETMs for the fabrication of large‐area NF–PSCs.

Electroless‐Plated Gold Contacts for High‐Performance, Low Contact Resistance Organic Thin Film Transistors

AbstractDeposition of metallic electrodes on a semiconductor medium is an indispensable factor in governing carrier injection, and a metal/semiconductor contact that can be formed via solution process is highly desired in printed electronics. However, fine‐patterning the solution processes of metallic electrodes without damaging the excellent electronic properties of organic semiconductors (OSCs) is still a challenge. In this work, electroless plating, a metal coating technique that involves auto‐catalytic reaction in an aqueous solution, is used to fabricate top‐contact organic thin‐film transistors (OTFTs). An electroless‐plated gold pattern with a spatial resolution of 10 micrometers is transferred and laminated on a monolayer of OSCs to serve as a hole‐injection electrode. The fabricated OTFTs exhibit reasonably high field‐effect mobility of up to 13 cm2 V−1 s−1 and decent contact resistance as low as 120 Ω · cm, which implies that an ideal metal/semiconductor contact can be realized. This electroless plating technique can provide possibilities for practical mass production of organic integrated circuits because it is in principle cost‐effective, capable of covering large areas, high‐vacuum free, and environmentally friendly.

Highly efficient modulation of the electronic properties of organic semiconductors by surface doping with 2D molecular crystals

Doping is a critically important strategy to modulate the properties of organic semiconductors (OSCs) to improve their optoelectrical performances. Conventional bulk doping involves the incorporation of foreign molecular species ( i.e. , dopants) into the lattice of the host OSCs, and thus disrupts the packing of the host OSCs and induces structural defects, which tends to reduce the mobility and (or) the on/off ratio in organic field-effect transistors (OFETs). In this article, we report a highly efficient and highly controllable surface doping strategy utilizing 2D molecular crystals (2DMCs) as dopants to boost the mobility and to modulate the threshold voltage of OFETs. The amount of dopants, i.e. , the thickness of the 2DMCs, is controlled at monolayer precision, enabling fine tuning of the electrical properties of the OSCs at unprecedented accuracy. As a result, a prominent increase of the average mobility from 1.31 to 4.71 cm2 V−1 s−1 and a substantial reduction of the threshold voltage from −18.5 to −1.8 V are observed. Meanwhile, high on/off ratios of up to 108 are retained.

A drift-diffusion simulation model for organic field effect transistors: on the importance of the Gaussian density of states and traps

The nature of charge transport in organic materials depends on several important aspects, such as the description of the density of states, and the charge mobility model. Therefore specific models describing electronic properties of organic semiconductors must be considered. We have used an organic based drift-diffusion model for the electrical characterization of organic field effect transistors (OFETs) utilizing either small molecules or polymers. Furthermore, the effect of interface traps, bulk traps, and fixed charges on transistor characteristics are included and investigated. Finally, simulation results are compared to experimental measurements, and conclusions are drawn out in terms of transistor performance parameters including threshold voltages, and field-dependent mobilities.

Tuning of electron density distribution on molecular conjugated skeleton to improve intermolecular aggregation style and device memory performance

Abstract Intermolecular packing style in film state has an essential influence on electronic properties of organic semiconductors, and the realization of desired intermolecular packing will largely enhance device performance. In this work, two conjugated molecules with similar spatial configuration are designed to investigate the electrostatic effect on molecular aggregation. UV–Vis and CV show that the molecule without a nitro group is with a J-aggregation and the molecule with the terminal nitro unit is with the H-aggregation, which is indeed induced from the different distribution of charge density of the two molecular. H-aggregation donates the corresponding device with an ordered crystalline orientation, an improved data-storage performance and an increased device stability. This work will help the better understanding of the comprehensive effects on molecular stacking and device performance.

Impact of the molecular quadrupole moment on ionization energy and electron affinity of organic thin films: Experimental determination of electrostatic potential and electronic polarization energies

Energy levels of organic molecular films exert paramount influence on the electronic properties of organic semiconductors. Recently the effect of electrostatic energy was highlighted as an origin of such peculiar phenomena as the molecular-orientation dependence and continuous tuning of energy levels. However, the mechanism has been discussed mostly based on theoretical work and has not been adequately supported by experiments. In this work, we propose a procedure to evaluate the electrostatic and electronic polarization energies in organic films solely from the experimental data obtained by ultraviolet photoelectron and low-energy inverse photoelectron spectroscopies. We apply it to the energy levels of thin films of 6,13-pentacenequinone on $\mathrm{Si}{\mathrm{O}}_{2}$ substrates with three different molecular orientations. The obtained electrostatic energies are fully consistent with the theoretical results at different levels such as the first-principles calculations and the electrostatic energy of charge-multipole interactions. The present work also underlines the importance of the charge-permanent quadrupole interaction which is the leading term of the electrostatic energy in the film of nonpolar molecules.

Improved Molecular Stacking and Data‐Storage Performance of Pyridine‐ and Pyrimidine‐Substituted Small Molecules

AbstractThe molecular stacking in film state has been confirmed to have an important influence on electronic properties of organic semiconductors. Thus, how to improve the molecular stacking and the corresponding device performance through molecular design is a difficult question that researches want to figure out. In this work, three small molecules with different bridge‐spacers, phenyl ring, pyridine, and pyrimidine, are designed to study the effect of nitrogen amount on molecular stacking in film state. The infrared spectroscopy in solution and film states are first measured to consider the hydrogen bond between the neighboring molecules, which can induce ordered molecular stacking through intermolecular interaction. X‐ray diffraction, atomic force microscopy, and grazing incidence small angle X‐ray scattering measurements confirm the ordered molecular stacking and crystalline orientation of nitrogenous heterocycles substituted molecules in film state. Additionally, nitrogenous heterocycles substituted molecules exhibit the excellent ternary data‐storage properties with low threshold voltage and high current ratio. This result presents the strategy for designing novel small molecules with excellent molecular stacking in film state.

Polymer-Assisted Single Crystal Engineering of Organic Semiconductors To Alter Electron Transport.

A new crystal phase of a naphthalenediimide derivative (α-DPNDI) has been prepared via a facial polymer-assisted method. The stacking pattern of DPNDI can be tailored from the known one-dimensional (1D) ribbon (β phase) to a novel two-dimensional (2D) plate (α phase) through the assistance from polymers. We believe that the presence of polymers during crystal growth is likely to weaken the direct π-π interactions and favor side-to-side C-H-π contacts. Furthermore, β phase architecture shows electron mobility higher than that of the α phase in the single-crystal-based OFET. Theoretical calculations not only confirm that β-DPNDI has an electron transport performance better than that of the α phase but also indicate that the α phase crystal displays 2D transport while the β phase possesses 1D transport. Our results clearly suggest that polymer-assisted crystal engineering should be a promising approach to alter the electronic properties of organic semiconductors.

Insight into doping efficiency of organic semiconductors from the analysis of the density of states in n-doped C60 and ZnPc.

Doping plays a crucial role in semiconductor physics, with n-doping being controlled by the ionization energy of the impurity relative to the conduction band edge. In organic semiconductors, efficient doping is dominated by various effects that are currently not well understood. Here, we simulate and experimentally measure, with direct and inverse photoemission spectroscopy, the density of states and the Fermi level position of the prototypical materials C60 and zinc phthalocyanine n-doped with highly efficient benzimidazoline radicals (2-Cyc-DMBI). We study the role of doping-induced gap states, and, in particular, of the difference Δ1 between the electron affinity of the undoped material and the ionization potential of its doped counterpart. We show that this parameter is critical for the generation of free carriers and influences the conductivity of the doped films. Tuning of Δ1 may provide alternative strategies to optimize the electronic properties of organic semiconductors.

Conjugated-Backbone Effect of Organic Small Molecules for n-Type Thermoelectric Materials with ZT over 0.2.

Conjugated backbones play a fundamental role in determining the electronic properties of organic semiconductors. On the basis of two solution-processable dihydropyrrolo[3,4-c]pyrrole-1,4-diylidenebis(thieno[3,2-b]thiophene) derivatives with aromatic and quinoid structures, we have carried out a systematic study of the relationship between the conjugated-backbone structure and the thermoelectric properties. In particular, a combination of UV-vis-NIR spectra, photoemission spectroscopy, and doping optimization are utilized to probe the interplay between energy levels, chemical doping, and thermoelectric performance. We found that a moderate change in the conjugated backbone leads to varied doping mechanisms and contributes to dramatic changes in the thermoelectric performance. Notably, the chemically doped A-DCV-DPPTT, a small molecule with aromatic structure, exhibits an electrical conductivity of 5.3 S cm-1 and a high power factor (PF373K) up to 236 μW m-1 K-2, which is 50 times higher than that of Q-DCM-DPPTT with a quinoid structure. More importantly, the low thermal conductivity enables A-DCV-DPPTT to possess a figure of merit (ZT) of 0.23 ± 0.03, which is the highest value reported to date for thermoelectric materials based on organic small molecules. These results demonstrate that the modulation of the conjugated backbone represents a powerful strategy for tuning the electronic structure and mobility of organic semiconductors toward a maximum thermoelectric performance.