Organic Semiconductor Thin Films Research Articles

Spectroscopic and electronic investigations on tin(II) 2,3-naphthalocyanine/p-Si heterojunction for optoelectronic applications

The spectroscopic and electronic properties of small-molecular organic semiconductors in thin films are a significant aspect that controls the performance of their optical/optoelectronic devices. A heterojunction of thermal vacuum deposited film of tin(II)2,3-naphthalocyanine (SnNc) dye onto a p-type Si wafer (SnNc/p-Si) was fabricated. The morphology of SnNc thin film onto Si wafer was explored using the Field Emission Scanning Electron Microscope (FE-SEM). The specular reflectance and the fluorescent properties of SnNc thin film onto Si wafer were investigated. The PL emission spectra of SnNc/p-Si emission bands are related to the B and Q absorption bands for SnNc. The spectral behavior of the refractive index and the specific polarizability for both SnNc/Glass and SnNc/p-Si were extracted. Under varying temperatures (298–378 K), the electronic properties of the SnNc/p-Si heterojunction were analyzed in dark conditions. The diode parameters of SnNc/p-Si heterojunctions, which exhibited temperature-dependent behavior, were extracted from the current–voltage (I-V) procedure. According to the Cheung-Cheung and Norde procedure, the essential electrical diode parameters of the SnNc/p-Si device, such as the ideality factor, the series resistance, and barrier height, were estimated. The photoconductivity parameters, such as the photocurrent, the sensitivity, and the responsivity of the SnNc/p-Si devices, were estimated. These findings can be helpful and utilized in optoelectronic and organic electronic applications.

Self-Assembly of Organic Semiconductors on Strained Graphene under Strain-Induced Pseudo-Electric Fields.

Graphene is used as a growth template for van der Waals epitaxy of organic semiconductor (OSC) thin films. During the synthesis and transfer of chemical-vapor-deposited graphene on a target substrate, local inhomogeneities in the graphene-in particular, a nonuniform strain field in the graphene template-can easily form, causing poor morphology and crystallinity of the OSC thin films. Moreover, a strain field in graphene introduces a pseudo-electric field in the graphene. Here, the study investigates how the strain and strain-induced pseudo-electric field of a graphene template affect the self-assembly of π-conjugated organic molecules on it. Periodically strained graphene templates are fabricated by transferring graphene onto an array of nanospheres and then analyzed the growth and nucleation behavior of C60 thin films on the strained graphene templates. Both experiments and a numerical simulation demonstrated that strained graphene reduced the desorption energy between the graphene and the C60 molecules and thereby suppressed both nucleation and growth of the C60 . A mechanism is proposed in which the strain-induced pseudo-electric field in graphene modulates the binding energy of organic molecules on the graphene.

Solution slot-die coating perovskite film crystalline growth observed by <i>in situ</i> GIWAXS/GISAXS

Solution method is an important means of fabricating optoelectronic devices. During the thin film sample preparation, organic or inorganic perovskite semiconductor material usually needs to be finished in a glove box. However, most of the traditional experimental characterizations under the air environment, it is hard to reflect the reality of the structure and performance between film and device, therefore it is urgently needed to solve the microstructure evolutions of these semiconductor films based on <i>in situ</i> real-time representation technique. In this work, we report a synchrotron-based grazing incidence wide and small-angle scattering (GIWAXS and GISAXS) <i>in situ</i> real-time observation technique combined with a mini glove box, thereby realizing the standard glove box environment (H<sub>2</sub>O, O<sub>2</sub> content all reached below 1×10<sup>–6</sup>) under remote control film spin coating or slot-die preparation and various sample post-processing. Meanwhile, this technique can real-time monitor the microstructure and morphology evolution of semiconductor film during fabrication. Based on the <i>in situ</i> device and GIWAXS, SnO<sub>2</sub> ETL interface induced perovskite growth crystallization process shows that CQDs additive can result in three-dimensional perovskite, with the random orientation growth changing into highly ordered vertical orientation, meanwhile can effectively restrain the low-dimensional perovskite domain formation, helping to reveal the film microstructure transformation of inner driving force and providing the perovskite device preparation process optimized with experimental and theoretical basis. The conversion efficiency of large-area fully flexible three-dimensional perovskite thin film solar cells prepared by the roll-to-roll total solution slit coating method is increased to 5.23% (the area of a single device is ~15 cm<sup>2</sup>). Therefore, using the <i>in situ</i> synchrotron-based glove box device, the microstructure evolution and the associated device preparation conditions of perovskite and organic semiconductor thin films can be controlled, and the thin film growth interface characteristics and film quality can be further controlled, which is the key technology to control the optimization process conditions of semiconductor thin films and devices.

Cluster-Based Approach Utilizing Optimally Tuned TD-DFT to Calculate Absorption Spectra of Organic Semiconductor Thin Films.

The photophysics of organic semiconductor (OSC) thin films or crystals has garnered significant attention in recent years since a comprehensive theoretical understanding of the various processes occurring upon photoexcitation is crucial for assessing the efficiency of OSC materials. To date, research in this area has relied on methods using Frenkel-Holstein Hamiltonians, calculations of the GW-Bethe-Salpeter equation with periodic boundaries, or cluster-based approaches using quantum chemical methods, with each of the three approaches having distinct advantages and disadvantages. In this work, we introduce an optimally tuned, range-separated time-dependent density functional theory approach to accurately reproduce the total and polarization-resolved absorption spectra of pentacene, tetracene, and perylene thin films, all representative OSC materials. Our approach achieves excellent agreement with experimental data (mostly ≤0.1 eV) when combined with the utilization of clusters comprising multiple monomers and a standard polarizable continuum model to simulate the thin-film environment. Our protocol therefore addresses a major drawback of cluster-based approaches and makes them attractive tools for OSC investigations. Its key advantages include its independence from external, system-specific fitting parameters and its straightforward application with well-known quantum chemical program codes. It demonstrates how chemical intuition can help to reduce computational cost and still arrive at chemically meaningful and almost quantitative results.

Doping of molecular semiconductors through proton-coupled electron transfer.

The chemical doping of molecular semiconductors is based on electron-transfer reactions between the semiconductor and dopant molecules; here, the redox potential of the dopant is key to control the Fermi level of the semiconductor1,2. The tunability and reproducibility of chemical doping are limited by the availability of dopant materials and the effects of impurities such as water. Here we focused on proton-coupled electron-transfer (PCET) reactions, which are widely used in biochemical processes3,4; their redox potentials depend on an easily handled parameter, that is, proton activity. We immersed p-type organic semiconductor thin films in aqueous solutions with PCET-based redox pairs and hydrophobic molecular ions. Synergistic reactions of PCET and ion intercalation resulted in efficient chemical doping of crystalline organic semiconductor thin films under ambient conditions. In accordance with the Nernst equation, the Fermi levels of the semiconductors were controlled reproducibly with a high degree of precision-a thermal energy of about 25 millielectronvolts at room temperature and over a few hundred millielectronvolts around the band edge. A reference-electrode-free, resistive pH sensor based on this method is also proposed. A connection between semiconductor doping and proton activity, a widely used parameter in chemical and biochemical processes, may help create a platform for ambient semiconductor processes and biomolecular electronics.

Two-Dimensional Van Der Waals Thin Film and Device.

In the rapidly evolving field of thin-film electronics, the emergence of large-area flexible and wearable devices has been a significant milestone. Although organic semiconductor thin films, which can be manufactured through solution processing, have been identified, their utility is often undermined by their poor stability and low carrier mobility under ambient conditions. However, inorganic nanomaterials can be solution-processed and demonstrate outstanding intrinsic properties and structural stability. In particular, a series of two-dimensional (2D) nanosheet/nanoparticle materials have been shown to form stable colloids in their respective solvents. However, the integration of these 2D nanomaterials into continuous large-area thin with precise control of layer thickness and lattice orientation still remains a significant challenge. This review paper undertakes a detailed analysis of van der Waals thin films, derived from 2D materials, in the advancement of thin-film electronics and optoelectronic devices. The superior intrinsic properties and structural stability of inorganic nanomaterials are highlighted, which can be solution-processed and underscor the importance of solution-based processing, establishing it as a cornerstone strategy for scalable electronic and optoelectronic applications. A comprehensive exploration of the challenges and opportunities associated with the utilization of 2D materials for the next generation of thin-film electronics and optoelectronic devices is presented.

(Invited) Structural and Dynamics Analysis of Organic Semiconducting Materials by Solid State NMR

Recently, our group have successfully developed several blue and green thermally activated delayed fluorescence (TADF) emitters showing high external quantum efficiencies in OLEDs through high-throughput screening based on quantum chemical calculations. Although the conformation, aggregated structure and dynamics of TADF emitters in amorphous states is expected to be the origin of reverse intersystem crossing and resulting high device performance, the experimental approach toward these information is still limited because typical diffraction technique cannot be used to amorphous materials due to the lack of long-range order. Thus we have carried out multiscale simulations and solid-state NMR (ssNMR) analysis of organic amorphous thin films. The ssNMR is one of the powerful technique to obtain the structural and dynamics information of amorphous aggregated materials. In this presentation, we demonstrate structural and dynamics analysis of amorphous organic semiconducting materials using ssNMR.Reference(1) “Analysis of Molecular Orientation in Organic Semiconducting Thin Films Using Static Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy”, Suzuki, K.; Kubo, S.; Aussenac, F.; Engelke, F.; Fukushima, T.; Kaji, H. Angew. Chem. Int. Ed. 2017, 56, 14842.(2) “Efficient blue thermally activated delayed fluorescence emitters showing very fast reverse intersystem crossing”, Ren, Y., Wada, Y., Suzuki, K., Kusakabe, Y., Geldsetzer, J. and Kaji, H. Appl. Phys. Express, 2021, 14, 071003.(3) “Solution-processable thermally activated delayed fluorescence emitters for application in organic light emitting diodes”, Suzuki, K.; Adachi, C.; Kaji, H. J. Soc. Inf. Disp. 2017, 25, 480.(4) “Imidazole Acceptor for Both Vacuum-Processable and Solution-Processable Efficient Blue Thermally Activated Delayed Fluorescence”, Kusakabe, Y.; Wada, Y.; Misono, T.; Suzuki, K.; Shizu, K.; Kaji, H ACS Omega, 7, 16740 - 16745 (2022).(5) “Triarylboron-based Fluorescent Organic Light-emitting Diodes with External Quantum Efficiencies Exceeding 20%”, Suzuki, K.; Kubo, S.; Shizu, K.; Fukushima, T.; Wakamiya, A.; Murata, Y.; Adachi, C.; Kaji, H.; Angew. Chem. Int. Ed. 2015, 54, 15231.(5) “Purely organic electroluminescent material realizing 100% conversion from electricity to light”, Kaji, H.; Suzuki, H.; Fukushima, T.; Shizu, K.; Suzuki, K.; Kubo, S.; Komino, T.; Oiwa, H.; Suzuki, F.; Wakamiya, A.; Murata, Y.; Adachi, C. Nat. Commun. 2015, 6, 8476.

Potential and anion effects on the adsorption of 3′,4′-bis(hexylthio)-2,2′:5′,2′'-terthiophene on Au(1 1 1) electrode characterized by in situ STM

The adsorption of organic molecules on gold electrodes serves as a model to understand the organic/inorganic electrified interface, which is relevant to the study of molecular electronics and organic thin film semiconductors. Our previous study on terthiophene (TT) adsorption on an Au(111) electrode shows that immersing Au(111) crystals in a TT ethanol dosing solution installs an ordered TT adlayer on the sample. The current study addresses the adsorption of 3′,4′-bis(hexylthio)-2,2′:5′,2′'-terthiophene (DTDST), a molecule with a TT backbone attached with two thiolhexyl chains, on an ordered Au(111) electrode. High-quality STM images were obtained to reveal the internal and 2D spatial structures of DTDST admolecules. The potential greatly influenced the organization of DTDST on the ordered Au(111) electrode. Although the pristine DTDST adlayer was disordered, it transformed into ordered Au(111) - (3√3 × 9) and (5√3 × 26) structures after applying a potential more negative than 0 V (vs. Ag/AgCl) in 0.1 M H2SO4 and HClO4, respectively. Shifting the potential more positive than 0.25 V resulted in coadsorption of bisulfate anions and restructuring of the DTDST adlayer. High-quality molecular resolution STM images were collected to reveal the azimuthal orientation of the DTDST admolecule on the Au(111) electrode. The thiolhexyl chains of DTDST admolecules could arrange in such a way that allowed intermolecular van der Waals interactions. Oxidation of adsorbed DTDST molecules to yield oligomers was also revealed by in situ STM.

Highly Ordered Small Molecule Organic Semiconductor Thin-Films Enabling Complex, High-Performance Multi-Junction Devices.

Organic semiconductors have opened up many new electronic applications, enabled by properties like flexibility, low-cost manufacturing, and biocompatibility, as well as improved ecological sustainability due to low energy use during manufacturing. Most current devices are made of highly disordered thin-films, leading to poor transport properties and, ultimately, reduced device performance as well. Here, we discuss techniques to prepare highly ordered thin-films of organic semiconductors to realize fast and highly efficient devices as well as novel device types. We discuss the various methods that can be implemented to achieve such highly ordered layers compatible with standard semiconductor manufacturing processes and suitable for complex devices. A special focus is put on approaches utilizing thermal treatment of amorphous layers of small molecules to create crystalline thin-films. This technique has first been demonstrated for rubrene─an organic semiconductor with excellent transport properties─and extended to some other molecular structures. We discuss recent experiments that show that these highly ordered layers show excellent lateral and vertical mobilities and can be electrically doped to achieve high n- and p-type conductivities. With these achievements, it is possible to integrate these highly ordered layers into specialized devices, such as high-frequency diodes or completely new device principles for organics, e.g., bipolar transistors.

Dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene photoresistor: Generation of photocarriers and charge trapping

Thin films of organic semiconductors have been used as active layer in several electronic devices. The molecular organization of the active layer can be a critical parameter to optimize the performance of these devices. In this work, a thin film photoresistor of Dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) was fabricated using physical vapor deposition (PVD) technique with the most appropriate molecular organization to obtain better performance. The DNTT photoresistor current exhibited a significant increase under ultraviolet (UV) light (λ = 390 nm, Pd = 1.73 W/m2), achieving an IUV/IDark ratio of 1 × 105 at V = −1.5 V. The photoresistor DNTT achieved good values for the most important photodetector's figures of merit (FOMs), such as responsivity of 722.3 mA/W, detectivity of 1.0 × 1013 Jones and external quantum efficiency of 23000%. The measurements of real (Z′) and imaginary (Z″) impedance in the dark indicated that the electrical relaxation process is slow and of the Debye type. Nyquist plots revealed the appearance of negative capacitance (NC) at low frequencies when the photoresistor is under illumination, indicating that the photogenerated carriers can be trapped. The alternating current (AC) conductivity (σAC) measurements under lighting suggest that charge transfer occurs by hopping and those charge carriers can tunnel through the space charge region (SCR) with built-in potential generated by charges trapped. Therefore, the results showed that although the DNTT photoresistor has a good photoconductive response, its performance can be improved by reducing of traps.

Resonance Effect in Brunel Harmonic Generation in Thin Film Organic Semiconductors

AbstractNonlinear optical properties of organic semiconductors (OSCs) have been extensively investigated in the perturbative regime, while strong light induced high‐order processes in solid‐state OSCs are less studied. Here, below‐threshold harmonic generation is examined, both experimentally and theoretically, in two solid‐state thin film OSCs, that is, tetraphenylporphyrin and zinc tetraphenylporphyrin. Results show that the π–π* excitations of the porphyrin ring system generate the harmonic emission. The contribution of the Brunel harmonic to the 5th harmonic emission is uncovered, where the resonant 5‐photon transition (S0 → S2 transition) is found to lead to an early onset of non‐perturbative behavior. A similar resonance effect is expected in Brunel harmonic generation in other organic materials.

Features of obtaining and properties of thin films organic semiconductors

In recent decades, thin films of organic semiconductors (OS), or, as they are also called, organic molecular crystals, have attracted special attention in connection with a number of proposals for their use in functional units of modern electronics, and therefore in modern technological equipment. At the same time, the optical, electrophysical and photoelectric properties of OS are very important, which are determined both by the structure of organic molecules, that is, the starting material, and by the crystal structure of condensates, that is, by the technological conditions of film production. In this regard, this article presents the results of studies of some properties of OS and the technology of obtaining thin films based on them. OS are characterized by weak intermolecular bonds of the Van der Baals type, which causes the low energy of their crystal lattice. In this regard, the electronic structure of individual molecules during the formation of a crystal does not change significantly, and the properties of the crystals almost completely preserve the individual features of the molecules in combination with new properties caused by their collective interaction. This determines the main features of optical and electrophysical properties of OS, their energy structure of neutral and ionized states. In this article, it is experimentally confirmed that the planar structure of atoms of molecular crystals, in particular, linear acenes, and therefore the number of π-electrons responsible for exciton absorption in the visible region of the spectrum determines the position of the edge of their own optical absorption. It was also confirmed that the long-wavelength shift of the optical absorption edge of thin films of phthalocyanines is observed when atoms of heavy elements, for example, lead, are introduced into the molecule. When atoms of lighter elements are introduced into the phthalocyanine molecule, their planarity is not disturbed, and the long-wavelength shift of the edge is much smaller. An explanation of such changes is presented based on the interaction of foreign atoms with the π-electron system of phthalocyanine rings of neighboring molecules. The significant influence of the technological parameters of production, in particular the temperature of the substrate during thermal sputtering, on the crystal structure and optical properties of thin films of linear polyacenes and metallophthalocyanines has been demonstrated and substantiated. The possibility of controlling and presetting the necessary properties of thin-film condensates of molecular crystals is shown.

Revealing intermolecular coupling effects on vibrational spectra with infrared-reflectance absorbance analysis of perylene diimide thin films

Intermolecular interactions govern the optical and electronic properties of organic semiconductor thin films. Optimizing the function of molecular devices such as organic solar cells and light emitting diodes relies on understanding structural effects in a heterogeneous film environment. Because organic thin films are typically more disordered than inorganic crystalline semiconductors, advanced techniques are often needed to sufficiently characterize these materials. However, polarized infrared reflectance measurements are a benchtop method that can provide well-resolved vibrational spectra with specificity in their molecular orientation. By using infrared reflectance-absorbance spectroscopy (IRRAS) we relate the effects of intermolecular packing geometry on thin film vibrational spectra of perylene diimide (PDI), a prototypical molecular semiconductor. We analyze a suite of PDIs with differing sidechain substitutions, which adopt distinct crystal packing arrangements in thin films, by comparison of the reflectance spectra to isotropic KBr mixtures. We observe differences in the degree of Davydov splitting as a function of displacement along the short and long molecular axes and the π-π stacking distance that are evident in either the symmetric or anti-symmetric CO stretching frequencies of the solid. The four PDIs also display changes in relative intensity between the two modes that indicate different average molecular orientations.

Disordered Phase-Assisted Growth of Organic Semiconductor Crystals on Self-Assembled Monolayer Templates.

Achieving high mobility and bias stability is a challenging obstacle in the advancement of organic thin-film transistors (OTFTs). To this end, the fabrication of high-quality organic semiconductor (OSC) thin films is critical for OTFTs. Self-assembled monolayers (SAMs) have been used as growth templates for high-crystalline OSC thin films. Despite significant research progress in the growth of OSC on SAMs, a detailed understanding of the growth mechanism of the OSC thin films on a SAM template is lacking, which has limited its use. In this study, the effects of the structure (thickness and molecular packing) of SAM on the nucleation and growth behavior of the OSC thin films were investigated. We found that disordered SAM molecules assisted in the surface diffusion of the OSC molecules and resulted in a small nucleation density and large grain size of the OSC thin films. Moreover, a thick SAM with disordered SAM molecules on the top was found to be beneficial for the high mobility and bias stability of the OTFTs.

Meniscus‐Guided Deposition of Organic Semiconductor Thin Films: Materials, Mechanism, and Application in Organic Field‐Effect Transistors

Solution-processable organic semiconductors are one of the promising materials for the next generation of organic electronic products, which call for high-performance materials and mature processing technologies. Among many solution processing methods, meniscus-guided coating (MGC) techniques have the advantages of large-area, low-cost, adjustable film aggregation, and good compatibility with theroll-to-roll process, showing good research results in the preparation of high-performance organic field-effect transistors. In this review, the types of MGC techniques are first listed and the relevant mechanisms (wetting mechanism, fluid mechanism, and deposition mechanism) are introduced. The MGC processes are focused and the effect of the key coating parameters on the thin film morphology and performance with examples is illustrated. Then, the performance of transistors based on small molecule semiconductors and polymer semiconductor thin films prepared by various MGC techniques is summarized. In the third section, various recent thin film morphology control strategies combined with the MGCs are introduced. Finally, the advanced progress of large-area transistor arrays and the challenges for roll-to-roll processes are presented using MGCs. Nowadays, the application of MGCs is still in the exploration stage, its mechanism is still unclear, and the precise control of film deposition still needs experience accumulation.

Water‐Surface‐Mediated Precise Patterning of Organic Single‐Crystalline Films via Double‐Blade Coating for High‐Performance Organic Transistors

AbstractApplication‐oriented growth of patterned organic semiconductor (OSC) thin films with a single domain is a nonnegotiable requirement for the manufacturing of high‐performance organic electronic devices. However, the prevalent selective‐wetting patterning method remains a challenge in controlling the density of nucleation events in microscale spaces, resulting in thin films with high grain boundary density and no preferential orientation spherulites. Herein, a simple double‐blade‐coating printing technique using a combination of wetting‐patterned substrates to produce an array of highly crystalline OSC thin films is developed. Specifically, the approach confines the OSC crystallization on a molecular‐flat water surface in specific areas, enabling a significant reduction in the number of nuclei. Consequently, patterned 2,7‐dioctyl[1]benzothieno[3,2‐b] benzothiophene (C8‐BTBT) thin films comprising single‐crystal domains are achieved with an exceptionally high yield of 62.5%. The organic field‐effect transistor array developed from such patterns of C8‐BTBT single‐crystalline films exhibits an excellent average mobility of 11.5 cm2 V−1 s−1 which is 12.5‐fold higher compared to that of the reference sample fabricated via conventional single‐blade coating. It is believed that this approach can be widely applied to other soluble organic materials, thereby opening up opportunities for fabricating multicomponent integrated electronics.

Post-Growth Dynamics and Growth Modeling of Organic Semiconductor Thin Films.

The ability to control the properties of organic thin films is crucial for obtaining highly performant thin-film devices. However, thin films may experience post-growth processes, even when the most sophisticated and controlled growth techniques such as organic molecular beam epitaxy (OMBE) are used. Such processes can modify the film structure and morphology and, thus, the film properties ultimately affecting device performances. For this reason, probing the occurrence of post-growth evolution is essential. Equally importantly, the processes responsible for this evolution should be addressed in view of finding a strategy to control and, possibly, leverage them for driving film properties. Here, nickel-tetraphenylporphyrin (NiTPP) thin films grown by OMBE on highly oriented pyrolytic graphite (HOPG) are selected as an exemplary system exhibiting a remarkable post-growth morphology evolution consistent with Ostwald-like ripening. To quantitatively describe the growth, the height-height correlation function (HHCF) analysis of the atomic force microscopy (AFM) images is carried out, clarifying the role of the post-growth evolution as an integral part of the whole growth process. The set of scaling exponents obtained confirms that the growth is mainly driven by diffusion combined with the presence of step-edge barriers, in agreement with the observed ripening phenomenon. Finally, the results together with the overall approach adopted demonstrate the reliability of the HHCF analysis in systems displaying post-growth evolution.

Synergistic Effect of Solvent Vapor Annealing and Chemical Doping for Achieving High-Performance Organic Field-Effect Transistors with Ideal Electrical Characteristics.

Contact resistance and charge trapping are two key obstacles, often intertwined, that negatively impact on the performance of organic field-effect transistors (OFETs) by reducing the overall device mobility and provoking a nonideal behavior. Here, we expose organic semiconductor (OSC) thin films based on blends of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT-C8) with polystyrene (PS) to (i) a CH3CN vapor annealing process, (ii) a doping I2/water procedure, and (iii) vapors of I2/CH3CN to simultaneously dope and anneal the films. After careful analysis of the OFET electrical characteristics and by performing local Kelvin probe force microscopy studies, we found that the vapor annealing process predominantly reduces interfacial shallow traps, while the chemical doping of the OSC film is responsible for the diminishment of deeper traps and promoting a significant reduction of the contact resistance. Remarkably, the devices treated with I2/CH3CN reveal ideal electrical characteristics with a low level of shallow/deep traps and a very high and almost gate-independent mobility. Hence, this work demonstrates the promising synergistic effects of performing simultaneously a solvent vapor annealing and doping procedure, which can lead to trap-free OSC films with negligible contact resistance problems.

The impact of film deposition and annealing on the nanostructure and dielectric constant of organic semiconductor thin films.

The strategy of using a bulk-heterojunction light-absorbing layer has led to the most efficient organic solar cells. However, optimising the blend morphology to maximise light absorption, charge generation and extraction can be challenging. Homojunction devices containing a single component have the potential to overcome the challenges associated with bulk heterojunction films. A strategy towards this goal is to increase the dielectric constant of the organic semiconductor to ≈10, which in principle would lead to free charge carrier generation upon photoexcitation. However, the factors that affect the thin film dielectric constants are still not well understood. In this work we report an organic semiconductor material that can be solution processed or vacuum evaporated to form good quality thin films to explore the effect of chromophore structure and film morphology on the dielectric constant and other optoelectronic properties. 2,2'-[(4,4,4',4'-Tetrakis{2-[2-methoxyethoxy]ethyl}-4H,4'H-{2,2'-bi[cyclo-penta[2,1-b:3,4-b']dithiophene]}-6,6'-diyl)bis(methaneylylidene)]dimalononitrile [D(CPDT-DCV)] was designed to have high electron-affinity end groups and low ionisation-potential central moieties. It can be processed from solution or be thermally evaporated, with the film morphology changing from face-on to a herringbone arrangement upon solvent or thermal annealing. The glycol solubilising groups led to the static dielectric constant (taken from capacitance measurements) of the films to be between 6 and 7 (independent of processing conditions), while the optical frequency dielectric constant depended on the processing conditions. The less ordered solution processed film was found to have the lowest optical frequency dielectric constant of 3.6 at 2.0 × 1014 Hz, which did not change upon annealing. In contrast, the more ordered evaporated film had an optical frequency dielectric constant 20% higher at 4.2 and thermal annealing further increased it to 4.5, which is amongst the highest reported for an organic semiconductor at that frequency. Finally, the more ordered evaporated films had more balanced charge transport, which did not change upon annealing.

Reducing Trap Density in Organic Solar Cells via Extending the Fused Ring Donor Unit of an A-D-A-Type Nonfullerene Acceptor for Over 17% Efficiency.

The high trap density (generally 1016 to 1018 cm-3 ) in thin films of organic semiconductors is the primary reason for the inferior charge-carrier mobility and large nonradiative recombination energy loss (ΔEnr ) in organic solar cells (OSCs), limiting improvement in power conversion efficiencies (PCEs). In this study, the trap density in OSCs is efficiently reduced via extending the donor core of nonfullerene acceptors (NFAs) from a heptacyclic unit to a nonacyclic unit. TTPIC-4F with a nonacyclic unit has stronger intramolecular and intermolecular interactions, affording higher crystallinity in thin films relative to its counterpart BTPIC-4F. Thus, the D18:TTPIC-4F-based device achieves a lower trap density of 4.02×1015 cm-3 , comparable to some typical high-performance inorganic/hybrid semiconductors, with higher mobility and inhibited charge-carrier recombination in devices. Therefore, the D18:TTPIC-4F-based OSC exhibits an impressive PCE of 17.1% with a low ΔEnr of 0.208eV, which is the best known value for A-D-A-type NFAs. Therefore, extending the donor core of NFAs is an efficient method for suppressing trap states in OSCs for high PCEs.