Organic Semiconductor Devices Research Articles

Dopant-induced Morphology of Organic Semiconductors Resulting in High Doping Performance.

Doping plays a crucial role in modulating and enhancing the performance of organic semiconductor (OSC) devices. In this study, the critical role of dopants is underscored in shaping the morphology and structure of OSC films, which in turn profoundly influences their properties. Two dopants, trityl tetrakis(pentafluorophenyl) (TrTPFB) and N,N-dimethylanilinium tetrakis(pentafluorophenyl)borate (DMA-TPFB), are examined for their doping effects on poly(3-hexylthiophene) (P3HT) and PBBT-2T host OSCs. It is found that although TrTPFB exhibits higher doping efficiency, OSCs doped with DMA-TPFB achieve comparable or even enhanced electrical conductivity. Indeed, the electrical conductivity of DMA-TPFB-doped P3HT reaches over 67 S cm-1, which is a record-high value for mixed-solution-doped P3HT. This can be attributed to DMA-TPFB inducing a higher degree of crystallinity and reduced structural disorder. Moreover, the beneficial impact of DMA-TPFB on the OSC films' morphology and structure results in superior thermoelectric performance in the doped OSCs. These findings highlight the significance of dopant-induced morphological and structural considerations in enhancing the film characteristics of OSCs, opening up a new avenue for optimization of dopant performance.

Numerical Simulation, Electrostatic and Physical Compact Modeling of C8-BTBT-C8 Organic Thin Film Transistor

This paper presents numerical simulation and compact modeling of 2,7-Dioctyl {1} benzothieno {3,2-b}{1} benzothiophene (C8-BTBT-C8) organic semiconductor-based TFT. It shows the entire modeling process flow of this organic semiconductor (OSC) and tests the device realization using a ring oscillator. The paper comprises OSC characterisation, band-gap modeling, electrostatic modeling, and capacitance modeling. The TCAD model consists of the Hopping and Pool Frenkel premise and characterizes the Density of State (DOS) for traps in deep and tail states. The findings from this research provide valuable information for improving OTFT models, enhancing their predictive accuracy, and advancing the understanding of organic semiconductor device behavior. The electrostatics demonstrate device structure dependency

High-performance fully solution-processed OLEDs based on carbazole-benzonitrile TADF emitter with long alkyl chains

The introduction of alkyl chains into the luminescent molecules will increase solubility and film-forming performance for fabricating solution-processed organic semiconductor devices. In this work, 5CzBN-9CSP and 5CzBN-12CSP with long alkyl chains were synthesized based on the typical thermally activated delayed fluorescence (TADF) molecule 5CzBN as core, and bipolar 9,9′-spirobi[fluorene] (SP) groups as peripheral dendrons. Not only the excellent solution processability is invested, but also the uniform photophysical property is sustained, including small singlet-triplet splitting energy (ΔEST) and high reverse intersystem crossing (RISC) rate. However, excessive length of alkyl chains degrading thermodynamic stability and carrier transport and balance, results in a maximum external quantum efficiency (EQEmax) of 17.6% for 5CzBN-12CSP-based fully solution-processed non-doped device, which is lower than that for 5CzBN-9CSP counterpart. Furthermore, TADF sensitized (TSF) solution-processed device obtains a high EQEmax of 25.9% within 5CzBN-12CSP host and multiple-resonance (MR) final emitter thanks to an efficient energy transfer.

In-plane oxygen diffusion measurements in polymer films using time-resolved imaging of programmable luminescent tags

Oxygen diffusion properties in thin polymer films are key parameters in industrial applications from food packaging, over medical encapsulation to organic semiconductor devices and have been continuously investigated in recent decades. The established methods have in common that they require complex pressure-sensitive setups or vacuum technology and usually do not come without surface effects. In contrast, this work provides a low-cost, precise and reliable method to determine the oxygen diffusion coefficient D in bulk polymer films based on tracking the phosphorescent pattern of a programmable luminescent tag over time. Our method exploits two-dimensional image analysis of oxygen-quenched organic room-temperature phosphors in a host polymer with high spatial accuracy. It avoids interface effects and accounts for the photoconsumption of oxygen. As a role model, the diffusion coefficients of polystyrene glasses with molecular weights between 13k and 350k g/mol are determined to be in the range of (0.8–1.5) × 10–7 cm2/s, which is in good agreement with previously reported values. We finally demonstrate the reduction of the oxygen diffusion coefficient in polystyrene by one quarter upon annealing above its glass transition temperature.

Asymmetrical impurity effect of C70:C60 mixture in thick small molecule organic photovoltaic cells

Fullerenes, especially C70 and C60, and their derivatives are popular acceptor materials for many organic semiconductor devices, including organic photovoltaic cells. One of the main impurities in C70 is C60, and intentional mixture of C60 and C70, or their derivatives, which have been utilized for maximizing the performance of organic photovoltaic cells or perovskite solar cells. However, the influence of impurities in C70 on the device characteristics has not been fully clarified when incorporating C70 into organic devices. To further clarify the relationship between the fullerene mixture and impurities, the effect of impurities in fullerenes on thick photovoltaic cells was evaluated, and a strange asymmetrical impurity effect of the C60:C70 mixture was demonstrated by using C70 with various purities as an acceptor material and zinc phthalocyanine (ZnPc) as a donor material. It was found that C60, C78, and C84 impurities in C70 caused carrier recombination during electron transport, but the same effect of the C70 impurity in C60 was much smaller. Additionally, the impurity effect of higher fullerenes on C70 was clearly larger than that of C60. The effect of higher fullerene impurities can be explained by electron trapping due to their deep lowest unoccupied molecular orbital (LUMO) levels. For the small difference of LUMO energy between C70 and C60, however, it is suggested that the shallow trapping of C70 impurities does not largely inhibit the transport of photocarriers by C60, while the low energy barrier of C60 impurities largely inhibits the transport of photocarriers by C70. For clear comparison, the photoelectrical conversion layers were crystallized using co-evaporant induced crystallization. An effective purification method for C70 using sublimation purification, which depends on the C70 concentration in the sublimation source, is also reported.

“Steric Armor” Strategy of Blue Fluorescent Emitters against Photooxidation‐Induced Degradation

Comprehensive SummaryStability against oxygen is an important factor affecting the performance of organic semiconductor devices. Improving photooxidation stability can prolong the service life of the device and maintain the mechanical and photoelectric properties of the device. Generally, various encapsulation methods from molecular structure to macroscopic device level are used to improve photooxidation stability. Here, we adopted a crystallization strategy to allow 14H‐spiro[dibenzo[c,h]acridine‐7,9′‐uorene] (SFDBA) to pack tightly to resist fluorescence decay caused by oxidation. In this case, the inert group of SFDBA acts as a “steric armor”, protecting the photosensitive group from being attacked by oxygen. Therefore, compared with the fluorescence quenching of SFDBA powder under 2 h of sunlight, SFDBA crystal can maintain its fluorescence emission for more than 8 h under the same conditions. Furthermore, the photoluminescence quantum yields (PLQYs) of the crystalline film is 327% higher than that of the amorphous film. It shows that the crystallization strategy is an effective method to resist oxidation.

Dilution-induced current-density increase in disordered organic semiconductor devices: A kinetic Monte Carlo study

Dilution-induced current-density increase in disordered organic semiconductor devices: A kinetic Monte Carlo study

Optical Absorption Properties in Pentacene/Tetracene Solid Solutions.

Modifying the optical and electronic properties of crystalline organic thin films is of great interest for improving the performance of modern organic semiconductor devices. Therein, the statistical mixing of molecules to form a solid solution provides an opportunity to fine-tune optical and electronic properties. Unfortunately, the diversity of intermolecular interactions renders mixed organic crystals highly complex, and a holistic picture is still lacking. Here, we report a study of the optical absorption properties in solid solutions of pentacene and tetracene, two prototypical organic semiconductors. In the mixtures, the optical properties can be continuously modified by statistical mixing at the molecular level. Comparison with time-dependent density functional theory calculations on occupationally disordered clusters unravels the electronic origin of the low energy optical transitions. The disorder partially relaxes the selection rules, leading to additional optical transitions that manifest as optical broadening. Furthermore, the contribution of diabatic charge-transfer states is modified in the mixtures, reducing the observed splitting in the 0-0 vibronic transition. Additional comparisons with other blended systems generalize our results and indicate that changes in the polarizability of the molecular environment in organic thin-film blends induce shifts in the absorption spectrum.

Cyclopentadithiophene-based Conjugated Polymers for Organic Thermoelectric Devices and Other Applications

The 4H-cyclopenta[2,1-b:3,4-b’]dithiophene (CPDT)-based conjugated polymers (CPs) have garnered significant attention in various fields of organic electronics due to their strong electron-donating properties, extended π-plane, and rigid, planar chemical structure. These unique features enable CPDT-based CPs to be highly advantageous for use in a range of organic semiconductor devices. While CPDT-based CPs have been extensively investigated and utilized as electron donors in various organic semiconductor devices, there is limited literature discussing the electrochemical properties of CPDT building blocks and the representative examples of CPDT-based CPs. In this mini-review, the authors outline the electrochemical properties of the CPDT building block, which stem from its rigid and planar chemical structure, facilitating the use of CPDT derivative materials in the field of organic semiconductors, such as organic photovoltaics (OPVs), organic thin film transistors (OTFTs), and organic photodetectors (OPDs). Furthermore, the authors highlight the advantages of CPDT-based CPs, particularly, for organic thermoelectric applications (OTEs) such as strong electron-donating properties and extended π-conjugation, which lead to facile p-type doping characteristics in CPDT-based CPs. The authors discuss the basic working principles of OTEs, including several key parameters of OTE devices such as the Seebeck coefficient (S) and power factor (PF). Additionally, the authors address the main challenge in OTEs: the trade-off relationship between electrical conductivity and the Seebeck coefficient. The review presents several strategies to overcome these trade-off limitations, focusing on CPDT and other CPs for OTE applications.

Carrier density and delocalization signatures in doped carbon nanotubes from quantitative magnetic resonance.

High-performance semiconductor materials and devices are needed to supply the growing energy and computing demand. Organic semiconductors (OSCs) are attractive options for opto-electronic devices, due to their low cost, extensive tunability, easy fabrication, and flexibility. Semiconducting single-walled carbon nanotubes (s-SWCNTs) have been extensively studied due to their high carrier mobility, stability and opto-electronic tunability. Although molecular charge transfer doping affords widely tunable carrier density and conductivity in s-SWCNTs (and OSCs in general), a pervasive challenge for such systems is reliable measurement of charge carrier density and mobility. In this work we demonstrate a direct quantification of charge carrier density, and by extension carrier mobility, in chemically doped s-SWCNTs by a nuclear magnetic resonance approach. The experimental results are verified by a phase-space filling doping model, and we suggest this approach should be broadly applicable for OSCs. Our results show that hole mobility in doped s-SWCNT networks increases with increasing charge carrier density, a finding that is contrary to that expected for mobility limited by ionized impurity scattering. We discuss the implications of this important finding for additional tunability and applicability of s-SWCNT and OSC devices.

A drift‐diffusion based electrothermal model for organic thin‐film devices including electrical and thermal environment

AbstractWe derive and investigate a stationary model for the electrothermal behavior of organic thin‐film devices including their electrical and thermal environment. Whereas the electrodes are modeled by Ohm's law, the electronics of the organic device itself is described by a generalized van Roosbroeck system with temperature‐dependent mobilities and using Gauss–Fermi integrals for the statistical relation. The currents give rise to Joule heat which together with the heat generated by the generation/recombination of electrons and holes in the organic device occur as source terms in the heat flow equation  that has to be considered on the whole domain. The crucial task is to establish that the quantities in the transfer conditions at the interfaces between electrodes and the organic semiconductor device have sufficient regularity. Therefore, we restrict the analytical treatment of the system to two spatial dimensions. We consider layered organic structures, where the physical parameters (total densities of transport states, reference energy levels , disorder parameter, basic mobilities, activation energies, relative dielectric permittivity, heat conductivity) are piecewise constant, and we work in a setting for some . We prove the existence of weak solutions using Schauder's fixed point theorem and a regularity result for strongly coupled systems with nonsmooth data and mixed boundary conditions that is verified by Caccioppoli estimates and a Gehring‐type lemma.

Charge Transport Mechanism of Organic Semiconductors Based on Molecular Dynamics Simulation

The aim of this paper is to investigate the charge transport mechanism in organic semiconductors based on molecular dynamics simulation. Molecular dynamics simulation, as an effective computational method, can reveal the microscopic mechanism of charge transport in organic semiconductors. The basic principles and methods of molecular dynamics simulation will be introduced in the paper, and its application in studying charge transport in organic semiconductors will be discussed. Through the simulation analysis, the effects of key parameters such as intermolecular interactions, carrier mobility and electron transport on the performance of organic semiconductor devices can be revealed, providing guidance for the design and optimization of organic semiconductor devices.

Resolving Nonlinear Recombination Dynamics in Semiconductors via Ultrafast Excitation Correlation Spectroscopy: Photoluminescence versus Photocurrent Detection.

We explore the application of excitation correlation spectroscopy to detect nonlinear photophysical dynamics in two distinct semiconductor classes through time-integrated photoluminescence and photocurrent measurements. In this experiment, two variably delayed femtosecond pulses excite the semiconductor, and the time-integrated photoluminescence or photocurrent component arising from the nonlinear dynamics of the populations induced by each pulse is measured as a function of inter-pulse delay by phase-sensitive detection with a lock-in amplifier. We focus on two limiting materials systems with contrasting optical properties: a prototypical lead-halide perovskite (LHP) solar cell, in which primary photoexcitations are charge photocarriers, and a single-component organic-semiconductor diode, which features Frenkel excitons as primary photoexcitations. The photoexcitation dynamics perceived by the two detection schemes in these contrasting systems are distinct. Nonlinear-dynamic contributions in the photoluminescence detection scheme arise from contributions to radiative recombination in both materials systems, while photocurrent arises directly in the LHP but indirectly following exciton dissociation in the organic system. Consequently, the basic photophysics of the two systems are reflected differently when comparing measurements with the two detection schemes. Our results indicate that photoluminescence detection in the LHP system provides valuable information about trap-assisted and Auger recombination processes, but that these processes are convoluted in a nontrivial way in the photocurrent response and are therefore difficult to differentiate. In contrast, the organic-semiconductor system exhibits more directly correlated responses in the nonlinear photoluminescence and photocurrent measurements, as charge carriers are secondary excitations only generated through exciton dissociation processes. We propose that bimolecular annihilation pathways mainly contribute to the generation of charge carriers in single-component organic semiconductor devices. Overall, our work highlights the utility of excitation correlation spectroscopy in modern semiconductor materials research, particularly in the analysis of nonlinear photophysical processes, which are deterministic for their electronic and optical properties.

Investigating the Effect of Cross-Conjugation Patterns on the Optoelectronic Properties of 7,7′Isoindigo-Based Materials

Isoindigo (IID) is widely used as a building block for the fabrication of organic semiconductor devices. Understanding the impact of cross-conjugation and linear conjugation on the optoelectronic properties of disubstituted IID is of great importance for the design of improved materials. In this study, phenyl and thienyl groups were substituted at the cross-conjugated 7,7′ position of IID to generate three novel organic semiconductor structures with a donor–acceptor architecture. The optoelectronic properties of this IID derivative were investigated and compared with those of the 6,6′ linearly conjugated IID analogs using UV–Vis spectroscopy and cyclic voltammetry. The experimental results were compared using density functional theory calculations to provide structure–property relationships based on substitution types and attachment sites for IID. The frontier orbital energy levels of the material did not vary dramatically with the position of the substituent, while the type of substituent showed a more significant influence on the HOMO’s energy level and oscillator strength. Phenyl-disubstituted 7,7′ IID (7Ph7′Ph) and thienyl-disubstituted 7,7′ IID (7Th7′Th) materials were used as electron transport layers in perovskite solar cells with a power conversion efficiency of 5.70% and 6.07%, respectively. These observations enhance our understanding of the electronic structure and optoelectronic properties of IID, guiding the design of the next generation of IID-based semiconductors.

Epitaxial growth of crystalline thin-films of perfluorinated copper phthalocyanine on the single-crystal copper phthalocyanine

Understanding and controlling the growth structures of molecular heterojunctions are important for the pursuit of highly efficient organic semiconductor devices. In this study, a well-defined molecular heterojunction was constructed by the deposition of an n-type organic semiconductor molecule, perfluorinated copper phthalocyanine (F16CuPc), on single-crystals of a typical p-type material, copper phthalocyanine (CuPc). The heterojunction was characterized using synchrotron radiation surface X-ray diffraction techniques to unveil the epitaxial growth of crystalline thin-films of F16CuPc on the CuPc surface. The inter-lattice relationships between F16CuPc and CuPc deduced from the present results imply that the in-plane orientation of the F16CuPc crystal lattice was predominantly governed by the molecular-level alignments of the π-backbones of the F16CuPc and CuPc molecules at the nucleation center for crystal growth at the interface.

Influence of Traps and Lorentz Force on Charge Transport in Organic Semiconductors.

Charge transport characteristics in organic semiconductor devices become altered in the presence of traps due to defects or impurities in the semiconductors. These traps can lead to a decrease in charge carrier mobility and an increase in recombination rates, thereby ultimately affecting the overall performance of the device. It is therefore important to understand and mitigate the impact of traps on organic semiconductor devices. In this contribution, the influence of the capture and release times of trap states, recombination rates, and the Lorentz force on the net charge of a low-mobility organic semiconductor was determined using the finite element method (FEM) and Hall effect method through numerical simulations. The findings suggest that increasing magnetic fields had a lesser impact on net charge at constant capture and release times of trap states. On the other hand, by increasing the capture time of trap states at a constant magnetic field and fixed release time, the net charge extracted from the semiconductor device increased with increasing capture time. Moreover, the net charge extracted from the semiconductor device was nearly four and eight times greater in the case of the non-Langevin recombination rates of 0.01 and 0.001, respectively, when compared to the Langevin rate. These results imply that the non-Langevin recombination rate can significantly enhance the performance of semiconductor devices, particularly in applications that require efficient charge extraction. These findings pave the way for the development of more efficient and cost-effective electronic devices with improved charge transport properties and higher power conversion efficiencies, thus further opening up new avenues for research and innovation in this area of modern semiconductor technology.

Polymeric Benzothiadiazole, Benzooxadiazole, and Benzoselenadiazole Photocathodes for Photocatalytic Oxygen Reduction to Hydrogen Peroxide

AbstractVisible‐light‐driven semiconductor photoelectrodes are promising new devices for on‐demand photocathodic generation of hydrogen peroxide. Herein, the fabrication of organic polymeric photocathodes employing poly(4,7‐di(thiophen‐2‐yl)benzo[c][1,2,5]thiadiazole) (pThBTD), and comparatively seven other related derivatives is reported. The monomer dithienobenzodithiazole can be directly polymerized on an electrode via two methods: electropolymerization or iodine‐vapor‐assisted polymerization. Both give polymers with wide visible light absorption and suitable stability for photoelectrodes. These methods yield different active layer morphologies, with electropolymerization yielding photocathodes with better performance. Critical issues affecting oxygen reduction photocurrents are evaluated, namely thickness, wettability, and pH. Photocathodic oxygen reduction currents, as well as photovoltages, are among the highest reported for an organic photoelectrocatalyst, and pThBTD films can stably produce H2O2 with high faradaic yield over at least 8 h. This study shows that single‐component organic semiconductor devices can be highly competitive versus more complex heterostructures and that such low‐bandgap organic polymers can afford remarkable stability.

Effect of trap position on the electrical properties of unipolar organic semiconductor devices

We investigated the electrical properties of a unipolar organic device by intentionally inserting traps at specific positions within the device. Our findings demonstrate that the trap position has a considerable impact on the charge distribution, electric field, and charge transport behavior of the device. In particular, when the traps are positioned closer to a charge-injection electrode, the organic layer containing the traps experiences more significant band bending, resulting in a higher charge injection barrier. We propose an electrical model that fully explains the observed changes in the electrical properties of the device as a function of trap position.

Subgap Absorption in Organic Semiconductors.

Organic semiconductors have found a broad range of application in areas such as light emission, photovoltaics, and optoelectronics. The active components in such devices are based on molecular and polymeric organic semiconductors, where the density of states is generally determined by the disordered nature of the molecular solid rather than energy bands. Inevitably, there exist states within the energy gap which may include tail states, deep traps caused by unavoidable impurities and defects, as well as intermolecular states due to (radiative) charge transfer states. In this Perspective, we first summarize methods to determine the absorption features due to the subgap states. We then explain how subgap states can be parametrized based upon the subgap spectral line shapes. We finally describe the role of subgap states in the performance metrics of organic semiconductor devices from a thermodynamic viewpoint.

Ohmic Electron Injection into Organic Semiconductors by Solution‐Processed and Evaporated Organic Interlayers

AbstractEfficient electron injection from an electrode into an organic semiconductor remains a problem to solve for efficient organic semiconductor devices. In this work, a general method is presented to form an ohmic electron contact by inserting a thin organic interlayer between the metal electrode and the organic semiconductor. It is demonstrated that inserting an interlayer of a few nanometers of an organic semiconductor with a lower electron affinity than the transport material can improve the injected electron current by over three orders of magnitude. The electron current becomes space‐charge limited, demonstrating that the interlayer‐enhanced contact is ohmic. The ohmic‐contact formation by inserting a thin interlayer is ascribed to the elimination of barrier formation as a result of direct contact between the metal and organic semiconductor. Additionally, it is demonstrated that it is possible to achieve solution processing of such interlayers on top of organic semiconductors. The method is generalized for different interlayer materials as well as for different organic semiconductors, providing a general method for ohmic electron injection in organic devices.