Molecular Doping Of Organic Semiconductors Research Articles

Effective N‐Doping of Non‐Fullerene Acceptor via Sequential Deposition Enables High‐Efficiency Organic Solar Cells

AbstractCharge transport in the active layer, which can be effectively modulated by molecular doping of organic semiconductors, significantly affects the photovoltaic performance of organic solar cells (OSCs). However, it is difficult to control the dopant distribution in the bulk heterojunction (BHJ) films, which hinders efficient doping in OSCs. Herein, an effective n‐doping strategy is developed via sequential deposition (SD) of D18 donor and doped acceptor. The favorable vertical component distribution in SD films helps to optimize carrier transport pathways. The SD method confines the n‐dopant N‐DMBI to the acceptor layer, allowing positive effects of molecular doping. Consequently, the doped SD device exhibits superior charge transport with suppressed charge recombination, lower trap density, and enhanced charge extraction compared to the undoped one, resulting in a high power conversion efficiency of 19.55% for D18/L8‐BO binary OSCs. In addition, the doping does not affect the thermal stability of the devices, with the doped SD device retaining over 90% of its initial efficiency after 1200 h of heating at 80 °C. The universality of the SD doping method is also verified in other non‐fullerene acceptor systems. These results demonstrate the great potential of SD doping strategy for building high‐performance OSCs with enhanced charge transport.

Surface doping of rubrene single crystals by molecular electron donors and acceptors.

The surface molecular doping of organic semiconductors can play an important role in the development of organic electronic or optoelectronic devices. Single-crystal rubrene remains a leading molecular candidate for applications in electronics due to its high hole mobility. In parallel, intensive research into the fabrication of flexible organic electronics requires the careful design of functional interfaces to enable optimal device characteristics. To this end, the present work seeks to understand the effect of surface molecular doping on the electronic band structure of rubrene single crystals. Our angle-resolved photoemission measurements reveal that the Fermi level moves in the band gap of rubrene depending on the direction of surface electron-transfer reactions with the molecular dopants, yet the valence band dispersion remains essentially unperturbed. This indicates that surface electron-transfer doping of a molecular single crystal can effectively modify the near-surface charge density, while retaining good charge-carrier mobility.

Recent Advances in Molecular Doping of Organic Semiconductors

Organic semiconductors are the key building blocks in the field of organic electronics, which exhibit enormous advantages in low-cost, light-weight, wide variety, solution-processed manufacturing and mechanical flexibility against their inorganic counterparts. As the active component in a broad range of organic devices, including organic field-effect transistors, organic light-emitting diodes, organic solar cells and organic thermoelectrics, there are ubiquitous needs in efficiently and controllably modulating the properties and functionalities of organic semiconductors. Molecular doping, a versatile platform to tune the optoelectrical properties of organic semiconductors, has been one of the central propositions in further development of organic electronics. In this Review, we focus on the recent advances of molecular doping in organic semiconductors, covering both design of dopant molecules and fundamental doping mechanisms. In addition, the up-to-date development in processing techniques and relevant functional applications are also summarized. Finally, the frontier challenges together with future opportunities in this cross-fertilized field are highlighted. We believe this Review will provide more knowledge and insight on how molecular doping plays a critical role in advancing organic electronics for emerging technologies.

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.

Overcoming Coulomb Interaction Improves Free-Charge Generation and Thermoelectric Properties for n-Doped Conjugated Polymers

Molecular doping of organic semiconductors creates Coulombically bound charge and counterion pairs through a charge-transfer process. However, their Coulomb interactions and strategies to mitigate their effects have been rarely addressed. Here, we report that the number of free charges and thermoelectric properties are greatly enhanced by overcoming the Coulomb interaction in an n-doped conjugated polymer. Poly(2,2′-bithiazolothienyl-4,4′,10,10′-tetracarboxydiimide) (PDTzTI) and the benchmark N2200 are n-doped by tetrakis (dimethylamino) ethylene (TDAE) for thermoelectrics. Doped PDTzTI exhibits ∼10 times higher free-charge density and 500 times higher conductivity than doped N2200, leading to a power factor of 7.6 μW m–1 K–2 and ZT of 0.01 at room temperature. Compared to N2200, PDTzTI features a better molecular ordering and two-dimensional charge delocalization, which help overcome the Coulomb interaction in the doped state. Consequently, free charges are more easily generated from charge–counterion pa...

The Effect of Electrostatic Interaction on n‐Type Doping Efficiency of Fullerene Derivatives

AbstractThe molecular doping of organic semiconductors represents a key strategy for advancing organic electronic applications. However, the n‐doping of organic materials is usually less efficient than p‐doping and strategies toward the design of more efficient n‐doping still remain less explored. In this contribution, the impact of electrostatic interaction is explored on the doping efficiency of fullerene derivatives. [6,6]‐Phenyl‐C61‐butyric acid methyl ester (PCBM) and a [60]fulleropyrrolidine with a more polarizable triethylene glycol type side chain (PTEG‐1) are employed for a comparative study. It is found that the doping efficiency of lightly doped PCBM layers is limited to a few percent, while doped PTEG‐1 films exhibit very high doping efficiency approaching 100%. The enhanced n‐doping of PTEG‐1 compared with that of PCBM is further substantiated by Raman and Fourier transform infrared spectroscopic studies. The activation energy for charge generation in doped PTEG‐1 is much smaller than that of doped PCBM, which confirms a higher probability for dissociation of charge transfer complexes in the former compared to the latter. The enhanced molecular n‐doping for PTEG‐1 is attributed to the electrostatic interaction between the charge transfer complex and the polar environment offered by the triethylene glycol diether side chain.

Molecular chemical doping for improvement of performance in organic field effect transistors

In this article we will shortly review molecular doping of organic semiconductors including polymers applied to OFETs The review will cover the main outstanding research achievements reported in the field of organic transistors via doping Before focusing on these achievements we will give a brief theoretical background on the organic semiconducting materials their electronic properties and organic field ndash effect transistors as well as main doping fundamentals We will also discuss some of the reported organic semiconducting materials and doping agents applied for them so far The article will be concluded with a brief summary and outlook over the doping work done recently and its contributions to the development of organic electronics

Controlling Molecular Doping in Organic Semiconductors.

The field of organic electronics thrives on the hope of enabling low-cost, solution-processed electronic devices with mechanical, optoelectronic, and chemical properties not available from inorganic semiconductors. A key to the success of these aspirations is the ability to controllably dope organic semiconductors with high spatial resolution. Here, recent progress in molecular doping of organic semiconductors is summarized, with an emphasis on solution-processed p-type doped polymeric semiconductors. Highlighted topics include how solution-processing techniques can control the distribution, diffusion, and density of dopants within the organic semiconductor, and, in turn, affect the electronic properties of the material. Research in these areas has recently intensified, thanks to advances in chemical synthesis, improved understanding of charged states in organic materials, and a focus on relating fabrication techniques to morphology. Significant disorder in these systems, along with complex interactions between doping and film morphology, is often responsible for charge trapping and low doping efficiency. However, the strong coupling between doping, solubility, and morphology can be harnessed to control crystallinity, create doping gradients, and pattern polymers. These breakthroughs suggest a role for molecular doping not only in device function but also in fabrication-applications beyond those directly analogous to inorganic doping.

Correlated electron-hole mechanism for molecular doping in organic semiconductors

The electronic and optical properties of the paradigmatic F4TCNQ-doped pentacene in the low-doping limit are investigated by a combination of state-of-the-art many-body \emph{ab initio} methods accounting for environmental screening effects, and a carefully parametrized model Hamiltonian. We demonstrate that while the acceptor level lies very deep in the gap, the inclusion of electron-hole interactions strongly stabilizes dopant-semiconductor charge transfer states and, together with spin statistics and structural relaxation effects, rationalize the possibility for room-temperature dopant ionization. Our findings reconcile available experimental data, shedding light on the partial vs. full charge transfer scenario discussed in the literature, and question the relevance of the standard classification in shallow or deep impurity levels prevailing for inorganic semiconductors.

Polar Side Chains Enhance Processability, Electrical Conductivity, and Thermal Stability of a Molecularly p-Doped Polythiophene.

Molecular doping of organic semiconductors is critical for optimizing a range of optoelectronic devices such as field-effect transistors, solar cells, and thermoelectric generators. However, many dopant:polymer pairs suffer from poor solubility in common organic solvents, which leads to a suboptimal solid-state nanostructure and hence low electrical conductivity. A further drawback is the poor thermal stability through sublimation of the dopant. The use of oligo ethylene glycol side chains is demonstrated to significantly improve the processability of the conjugated polymer p(g4 2T-T)-a polythiophene-in polar aprotic solvents, which facilitates coprocessing of dopant:polymer pairs from the same solution at room temperature. The use of common molecular dopants such as 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) is explored. Doping of p(g4 2T-T) with F4TCNQ results in an electrical conductivity of up to 100 S cm-1 . Moreover, the increased compatibility of the polar dopant F4TCNQ with the oligo ethylene glycol functionalized polythiophene results in a high degree of thermal stability at up to 150 °C.

Molecular doping in organic semiconductors: fully solution-processed, vacuum-free doping with metal–organic complexes in an orthogonal solvent

A novel route of molecular doping in organic semiconductors is demonstrated to achieve high carrier concentration.

N-Type Molecular Doping in Organic Semiconductors: Formation and Dissociation Efficiencies of a Charge Transfer Complex

Electrical doping is an important method in organic electronics to enhance device efficiency by controlling the Fermi level, increasing the conductivity, and reducing the injection barrier from the electrode. To understand the charge generation process of the dopant in doped organic semiconductors, it is important to analyze the charge transfer complex (CTC) formation and dissociation into a free charge carrier. In this paper, we correlate the charge generation efficiency with the CTC formation and dissociation efficiency of an n-dopant in organic semiconductors. The CTC formation efficiency of Rb2CO3 linearly decreases from 82.8% to 47.0% as the doping concentration increases from 2.5 to 20 mol %. The CTC formation efficiency and its linear decrease with the doping concentration are analytically correlated with the concentration-dependent size and number of dopant agglomerates by introducing the degree of reduced CTC formation. Lastly, the behavior of the dissociation efficiency is discussed on the basis...

Molecular Electrical Doping of Organic Semiconductors: Fundamental Mechanisms and Emerging Dopant Design Rules.

Today's information society depends on our ability to controllably dope inorganic semiconductors, such as silicon, thereby tuning their electrical properties to application-specific demands. For optoelectronic devices, organic semiconductors, that is, conjugated polymers and molecules, have emerged as superior alternative owing to the ease of tuning their optical gap through chemical variability and their potential for low-cost, large-area processing on flexible substrates. There, the potential of molecular electrical doping for improving the performance of, for example, organic light-emitting devices or organic solar cells has only recently been established. The doping efficiency, however, remains conspicuously low, highlighting the fact that the underlying mechanisms of molecular doping in organic semiconductors are only little understood compared with their inorganic counterparts. Here, we review the broad range of phenomena observed upon molecularly doping organic semiconductors and identify two distinctly different scenarios: the pairwise formation of both organic semiconductor and dopant ions on one hand and the emergence of ground state charge transfer complexes between organic semiconductor and dopant through supramolecular hybridization of their respective frontier molecular orbitals on the other hand. Evidence for the occurrence of these two scenarios is subsequently discussed on the basis of the characteristic and strikingly different signatures of the individual species involved in the respective doping processes in a variety of spectroscopic techniques. The critical importance of a statistical view of doping, rather than a bimolecular picture, is then highlighted by employing numerical simulations, which reveal one of the main differences between inorganic and organic semiconductors to be their respective density of electronic states and the doping induced changes thereof. Engineering the density of states of doped organic semiconductors, the Fermi-Dirac occupation of which ultimately determines the doping efficiency, thus emerges as key challenge. As a first step, the formation of charge transfer complexes is identified as being detrimental to the doping efficiency, which suggests sterically shielding the functional core of dopant molecules as an additional design rule to complement the requirement of low ionization energies or high electron affinities in efficient n-type or p-type dopants, respectively. In an extended outlook, we finally argue that, to fully meet this challenge, an improved understanding is required of just how the admixture of dopant molecules to organic semiconductors does affect the density of states: compared with their inorganic counterparts, traps for charge carriers are omnipresent in organic semiconductors due to structural and chemical imperfections, and Coulomb attraction between ionized dopants and free charge carriers is typically stronger in organic semiconductors owing to their lower dielectric constant. Nevertheless, encouraging progress is being made toward developing a unifying picture that captures the entire range of doping induced phenomena, from ion-pair to complex formation, in both conjugated polymers and molecules. Once completed, such a picture will provide viable guidelines for synthetic and supramolecular chemistry that will enable further technological advances in organic and hybrid organic/inorganic devices.

Comparative Study of the N‐Type Doping Efficiency in Solution‐processed Fullerenes and Fullerene Derivatives

Molecular doping of organic semiconductors and devices represents an enabling technology for a range of emerging optoelectronic applications. Although p‐type doping has been demonstrated in a number of organic semiconductors, efficient n‐type doping has proven to be particularly challenging. Here, n‐type doping of solution‐processed C60, C70, [60]PCBM, [70]PCBM and indene‐C60 bis‐adduct by 1H‐benzimidazole (N‐DMBI) is reported. The doping efficiency for each system is assessed using field‐effect measurements performed under inert atmosphere at room temperature in combination with optical absorption spectroscopy and atomic force microscopy. The highest doping efficiency is observed for C60 and C70 and electron mobilities up to ≈2 cm2/Vs are obtained. Unlike in substituted fullerenes‐based transistors where the electron mobility is found to be inversely proportional to N‐DMBI concentration, C60 and C70 devices exhibit a characteristic mobility increase by approximately an order of magnitude with increasing dopant concentration up to 1 mol%. Doping also appears to significantly affect the bias stability of the transistors. The work contributes towards understanding of the molecular doping mechanism in fullerene‐based semiconductors and outlines a simple and highly efficient approach that enables significant improvement in device performance through facile chemical doping.

Controlled formation of charge depletion zones by molecular doping in organic pin-diodes and its description by the Mott-Schottky relation

Molecular doping of organic semiconductors is a key technology for highly efficient organic light-emitting diodes. Nevertheless, the underlying fundamental mechanisms are under discussion. This is because of the complex situation of structural disorder and strong polaronic coupling in such systems. We provide for the first time a systematic study of the formation of charge depletion zones in organic pin-diodes comprising molecular doped hole and electron transport layers. Impedance spectroscopy is employed to study the capacitance of these depletion zones. In particular, we show that the voltage dependent capacitance function obeys the Mott-Schottky relation concerning the influence of doping and the effect of an additional depletion zone given by the intrinsic interlayer. From temperature dependent measurements of the depletion capacitance, we can deduce the amount of active dopant states, their activation energy, and the spatial field distribution within the junction. The measured activation energy of the dopant states and the overall value of active dopant molecules suggest a highly efficient doping process, where dopant molecules act as shallow acceptor states.

Unraveling the Mechanism of Molecular Doping in Organic Semiconductors

The mechanism by which molecular dopants donate free charge carriers to the host organic semiconductor is investigated and is found to be quite different from the one in inorganic semiconductors. In organics, a strong correlation between the doping concentration and its charge donation efficiency is demonstrated. Moreover, there is a threshold doping level below which doping simply has no electrical effect.

Novel Approach for Alternating Current (AC)‐Driven Organic Light‐Emitting Devices

AbstractA novel approach for alternating current (AC)‐driven organic light‐emitting devices is reported, which uses the concept of molecular doping in organic semiconductors. Doped organic charge‐transport layers are used to generate charge carriers within the device, hence eliminating the need for injecting charge carriers from external electrodes. Bright luminance of up to 1000 cd m−2 is observed when the device is driven with an AC bias. The luminance observed is attributed to charge‐carrier generation and recombination, leading to the formation of excitons within the device, without injection of charge carriers through external electrodes. A mechanism for internal charge‐carrier generation and the device operation is proposed.