Organic Semiconductor Thin Films Research Articles

Microcavity-like exciton-polaritons can be the primary photoexcitation in bare organic semiconductors

Strong-coupling between excitons and confined photonic modes can lead to the formation of new quasi-particles termed exciton-polaritons which can display a range of interesting properties such as super-fluidity, ultrafast transport and Bose-Einstein condensation. Strong-coupling typically occurs when an excitonic material is confided in a dielectric or plasmonic microcavity. Here, we show polaritons can form at room temperature in a range of chemically diverse, organic semiconductor thin films, despite the absence of an external cavity. We find evidence of strong light-matter coupling via angle-dependent peak splittings in the reflectivity spectra of the materials and emission from collective polariton states. We additionally show exciton-polaritons are the primary photoexcitation in these organic materials by directly imaging their ultrafast (5 × 106 m s−1), ultralong (~270 nm) transport. These results open-up new fundamental physics and could enable a new generation of organic optoelectronic and light harvesting devices based on cavity-free exciton-polaritons

Mobility enhancement of DNTT and BTBT derivative organic thin-film transistors by triptycene molecule modification

Interface modification with a particular triptycene molecule that spontaneously forms a highly uniform molecule layer is a promising technique for realizing high-performance flexible organic thin-film transistors (OTFTs). Previous studies have shown that the triptycene-modified polymer surface enhances the crystallinity of the small-molecule organic semiconductor (OSC), DNTT (dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene), resulting in improved mobility. However, to date, the effectiveness of triptycene modification for the other OSCs has not been confirmed. Here, we report on the positive effect of triptycene layers on four different thienoacene-based OSCs (C10-DNTT, DPh-DNTT, C8-BTBT ([1]benzothieno[3,2-b][1]benzothiophene), and DPh-BTBT) in OTFTs. Regardless of the OSC type, triptycene-modified OTFTs are found to improve the field-effect mobility. A maximum effective mobility enhancement of 20-fold was obtained for C10-DNTT OTFTs. Furthermore, detailed observations of the surface morphology of the OSCs via scanning electron microscopy revealed that the crystal grain size of the OSC thin films can be increased when triptycene modification is conducted.

Investigation of internal fields in organic semiconductors in the presence of traps

In an organic semiconductor optoelectronic device, the built-in field within the active layer is typically determined by the difference in contact potentials of the device. However, the presence of space charges and trap states contribute to the electric field within the thin film. Depending on the maximum applied forward voltage, the trap states can be charged, inducing hysteresis in the optoelectronic response of the system. In this work, we investigate the electric fields inside organic photovoltaic device structures, in the presence of traps, using electroabsorption (EA) spectroscopy. Comparing simulations with our experimental results, we explained the origin of hysteresis in the electroabsorption signal as a function of applied DC bias. We solved Poisson’s equation to estimate the densities of trapped carriers in the active layers. The filled trap densities in poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c]-[1,2,5]thiadiazole)] (PPDT2FBT) were found to be ∼1×1017 and ∼6×1016 cm−3, respectively. From the transient EA measurements, the estimated values of energies of the trap states with respect to the HOMO level were 0.82 and 0.76 eV in P3HT and 0.70 and 0.64 eV in PPDT2FBT, which indicated the presence of midgap traps in these organic semiconductor thin films. Such trap induced changes in the internal fields within the active layers, affect the mobility and carrier transport in the organic optoelectronic devices. The midgap traps lead to exciton quenching and also act as non-radiative recombination centers, resulting in reduction in luminescence efficiency of the active layers.

Combining MicroED and GIWAXS for determining structure and orientation of organic semiconductor thin films

Combining MicroED and GIWAXS for determining structure and orientation of organic semiconductor thin films

Carrier Mobilities in Amorphous Organic Semiconductor Films Prepared at Various Film Formation Processes

The hole mobility of an organic semiconductor film that consists of N,N’‐di (1‐naphthyl)‐N,N’‐diphenyl‐benzidine (α‐NPD) is evaluated at various film formation processes, such as vacuum deposition and capillary injection. The mobility varies with the film formation process. From the temperature dependence of mobility, the Poole−Frenkel model and disorder model are discussed. From the analysis based on the Poole−Frenkel model, activation energies of the vacuum‐deposited film, the slowly cooled‐melted film, and the rapidly cooled‐melted film are 0.28, 0.20, and 0.40 eV, respectively. This result suggests that organic semiconductor thin films formed by different film formation methods have different molecular aggregation states.

Balanced Hole and Electron Transport in Ir(ppy)3:TCTA Blends

Balanced charge injection and transport in organic light-emitting diodes (OLEDs) is essential for highly efficient devices with small efficiency roll-off at high luminance. However, there are few reports on the measurement of charge mobility within the emissive blend layer of an OLED. In this paper, we show that photoexcitation in conjunction with Metal–Insulator–Semiconductor Charge Extraction with Linearly Increasing Voltage (photo-MIS-CELIV) can be used to determine the hole and electron mobilities of the emissive blend layer in a single device architecture. We demonstrate the technique by studying the commonly used emissive blend of fac-tris[2-phenylpyridinato-C2,N]iridium(III) [Ir(ppy)3] and tris(4-carbazoyl-9-ylphenyl)amine (TCTA), as well as neat TCTA and Ir(ppy)3 films. It was found that Ir(ppy)3 and its blend films with TCTA have measurable electron mobilities and critically they are of similar magnitude as their hole mobilities, irrespective of the Ir(ppy)3 doping ratio. Such balanced charge mobility suggests that the transport of both holes and electrons occurs mostly on the Ir(ppy)3 guest molecules in the blend. Additionally, we demonstrate that photo-MIS-CELIV can be used to measure the quantum efficiency of exciton dissociation in organic semiconductor thin films.

Understanding the Microstructure Formation of Polymer Films by Spontaneous Solution Spreading Coating with a High-Throughput Engineering Platform.

An important step of the great achievement of organic solar cells in power conversion efficiency is the development of low‐band gap polymer donors, PBDB−T derivatives, which present interesting aggregation effects dominating the device performance. The aggregation of polymers can be manipulated by a series of variables from a materials design and processing conditions perspective; however, optimization of film quality is a time‐ and energy‐consuming work. Here, we introduce a robot‐based high‐throughput platform (HTP) that is offering automated film preparation and optical spectroscopy thin‐film characterization in combination with an analysis algorithm. PM6 films are prepared by the so‐called spontaneous film spreading (SFS) process, where a polymer solution is coated on a water surface. Automated acquisition of UV/Vis and photoluminescence (PL) spectra and automated extraction of morphological features is coupled to Gaussian Process Regression to exploit available experimental evidence for morphology optimization but also for hypothesis formulation and testing with respect to the underlying physical principles. The integrated spectral modeling workflow yields quantitative microstructure information by distinguishing amorphous from ordered phases and assesses the extension of amorphous versus the ordered domains. This research provides an easy to use methodology to analyze the exciton coherence length in conjugated semiconductors and will allow to optimize exciton splitting in thin film organic semiconductor layers as a function of processing.

Mode Coupling of Whispering Gallery Modes through Organic Semiconductor Thin Films

Mode Coupling of Whispering Gallery Modes through Organic Semiconductor Thin Films

New tailored organic semiconductors thin films for optoelectronic applications

In this study, we have investigated new tailored organic semiconductor materials for optoelectronic application, such as organic solar cells. The carbon-based organic semiconductor material has promising advantages in organic thin-film form. Moreover, due to its low cost, organic thin films are suitable and cheaper than inorganic thin-film. The bandgap of organic semiconductors materials can be tuned and mostly lies between 2.0 eV and 4 eV and the optical absorption edge of organic semiconductors typically lies in between 1.7 eV and 3 eV. They can be easily tailored by modifying the carbon chain and legends and looks promising for engineering the bandgap to harness the solar spectrum. In this work, with new tailored organic semiconductors, the solution route is explored which is a low-cost processing method. (Anthracen-9-yl) methylene naphthalene-1-amine; 4-(anthracen-9-ylmethyleneamino)-1,5dimethyl-2-phenyl-1H-pyrazol-3-one and N-(anthracen-9-ylmethyl)-3, 4-dimethoxyaniline thin-films are processed by spin coating method with changing concentration such as 0.05 wt.% and 0.08 wt.%. Thin films of organic semiconductors were prepared on the glass substrate and annealed at 55 °C. The structural and optical behavior of (Anthracen-9-yl) methylene naphthalene-1-amine, 4-(anthracen-9-ylmethyleneamino)-1,5dimethyl-2-phenyl-1H-pyrazol-3-one, and N-(anthracen-9-ylmethyl)-3, 4-dimethoxyaniline organic semiconductors thin films is studied by X-ray diffraction (XRD), Scanning electron microscopy (SEM) and UV-Visible spectroscopy technique. The XRD data of the synthesized sample suggests the nano crystallinity of the organic layers. And, the SEM micrographs show the dense packing when we increase the wt.% 0.05 to 0.08. Additionally, analysis of the optical absorption measurements found that the engineered bandgap of synthesized thin films are 2.18 eV, 2.35 eV, 2.36eV, 2.52eV, and 2.65eV which suggest suitability for applications of optoelectronic devices such as solar cell. Such lightweight, eco-friendly and disposable new carbon-based materials seem to have the potential to replace other traditional hazardous heavy materials for future eco-friendly flat fast electronics.

Organic Semiconductors: Solutal‐Marangoni‐Flow‐Mediated Growth of Patterned Highly Crystalline Organic Semiconductor Thin Film Via Gap‐Controlled Bar Coating (Adv. Funct. Mater. 28/2021)

In article number 2100196, Boseok Kang, Kilwon Cho, and co-workers report a novel bar-coating approach using a solutal-Marangoni flow to form an array of single-crystal-like organic semiconductor thin-film patterns at a high rate in a consecutive manner. The use of an appropriate solvent additive enhances molecular mass transport and induces vertical phase separation between a small-molecule semiconductor and a polymeric semiconductor.

Relation between Spherulitic Growth, Molecular Organization, and Charge Carrier Transport in Meniscus‐Guided Coated Organic Semiconducting Films

AbstractMeniscus‐guided coating (MGC) is an efficient and promising route to grow small molecule and polymer organic semiconductors (OSCs) into highly ordered and uniaxially orientated thin films for electronic applications. In this work, the impact of domain size and molecular order on the charge carrier transport in field‐effect transistors for a molecular organic semiconductor 4‐tolyl‐bithiophenyl‐diketopyrrolopyrrole (DPP(Th2Bn)2) is investigated. The spherulitic domain growth of DPP(Th2Bn)2 in thin films is controlled in the evaporative regime of zone‐casting by varying the substrate velocity. The decrease of coating velocity leads to a lower nucleation density and larger domain size of DPP(Th2Bn)2. At sufficiently low velocity, the spherulitic domains first elongate and then uniaxially grow in the coating direction. Although at the same time the molecular order decreases due to higher film thickness, the charge carrier transport improves for larger domain size and reduced density of boundaries in the transistor channel. These results provide insight on the relation between domain growth, molecular organization, and charge carrier transport in zone‐cast OSC thin films that are important for the upscaling of the technique for practical applications.

Optical Index Matching, Flexible Electrospun Substrates for Seamless Organic Photocapacitive Sensors

Planar, rigid glass substrates support the formation of organic small-molecular semiconductor thin films with extended crystalline texture and distinct pleochroism (top in the cover picture), visible by cross-polarized microscopy. Woven, soaking electrospun nanofiber mat substrates disrupt the crystalline growth, leading to the formation of submicron-sized crystallites embedded in the fiber mesh (bottom). The benefits of the tissue-like nanofiber mats are that they become virtually transparent in wet, physiological environment. Therefore, they are considered by Timo Grothe, Andrea Ehrmann and co-workers (article number 2000543) as alternative substrates for future visual all-organic neuro-prosthesis.

(Invited) Structural Analysis of Organic Semiconducting Materials Using Solid-State NMR

Our research group is currently conducting basic research on organic light-emitting diodes (OLEDs). Recently, we have successfully developed a series of blue and green TADF emitters for OLEDs realizing high external quantum efficiencies through high-throughput screening based on quantum chemical calculations. However, even using highly efficient emitting materials, the device performance depends on the device structure and aggregated state of organic molecules in the device. To understand the origin of the device performance, both theoretical and experimental approaches are important. In this regards, we have also carried out multiscale simulations and solid-state NMR (ssNMR) analysis of organic amorphous thin films. The ssNMR is the powerful technique for the detailed experimental analysis of amorphous aggregated materials, which has been difficult by typical diffraction methods because organic molecules in OLEDs are in the amorphous state. In this presentation, we demonstrate the structural analysis of organic semiconducting materials using ssNMR. Also, we we show the analysis of molecular orientation of an organic semiconducting material in an amorphous thin film state using DNP-ssNMR, which is one of the most powerful sensitivity enhancement technique of NMR.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) “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.(3) “Combined Inter- and Intramolecular Charge-Transfer Processes for Highly Efficient Fluorescent Organic Light-Emitting Diodes with Reduced Triplet Exciton Quenching”, Moon, C.-K.; Suzuki, K.; Shizu, K.; Adachi, C.; Kaji, H.; Kim, J.-J. Adv. Mater. 2017, 131, 6614.(4) “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.

Charge Carrier Inversion in a Doped Thin Film Organic Semiconductor Island.

Inducing an inversion layer in organic semiconductors is a highly nontrivial, but critical, achievement for producing organic field-effect transistor (OFET) devices, which rely on the generation of inversion, accumulation, and depletion regimes for successful operation. Here, we develop a pulsed bias technique to characterize the dopant type of any organic material system, without prior knowledge or characterization of the material in question. We use this technique on a pentacene/PTCDI heterostructure and thus deduce that pentacene is exhibiting n-doped like response. The source of the additional charges in the pentacene island can be identified by charging rings in the dissipation channel of the noncontact atomic force microscopy (AFM) signal, a typical signature for localized charge transfer from the AFM tip to the sample. Additionally, through tip-induced band-bending, we generate inversion, depletion, and accumulation regimes over a 20 nm radius, three monolayer thick n-doped pentacene island. Our findings demonstrate that nanometer-scale lateral extent and thickness are sufficient for an OFET device to operate in the inversion regime.

Vibrational Spectroscopy Analysis of Oligothiophene Blend Film

One of the challenges in fabricating organic semiconductor thin film is to produce bettermolecular ordering that compromise its electronic properties. Molecular ordering of amorphous thin film can be improved in many ways. Here, high molecular weight polylactic acid (PLA) is introduced as binding matrix to promote 3'''-didodecyl-2,2':5',2'':5'',2'''-quaterthiophene (4T) film’s homogeinity across indium tin oxide (ITO) surface. Molecular ordering of the spin coated biodegradable PLA and 4T blend film processed at ambient atmosphere was studied using two vibrational spectroscopy methods. The complementary analysis of infrared absorption spectrum and Raman spectrum had identified several vibrational modes contributed by thiophene rings and alkyl functional groups. The Raman analysis implied there is a slight change of thiophene ringsʼ molecular orientation due to compressive stress after introduction of polymer. Microscopic characteristics of oligothiophenes especially at the π-π conjugated backbones contained crucial information in order to exploit the oligothiophene as flexible electronics devices.

Stretchable Mesh‐Patterned Organic Semiconducting Thin Films on Creased Elastomeric Substrates

AbstractRecently, many researchers have tried to develop stretchable semiconducting thin films that can maintain their electrical performance under stretching. However, the fabrication processes have not been sufficiently practical and feasible to be used for soft electronics. Here, a stretchable high‐performance organic semiconducting thin film is fabricated by exploiting simultaneous patterning and pinning of a polymer semiconductor solution on an elastomeric substrate in which creasing‐instability has occurred. As a result, a mesh‐like polymer semiconducting thin film having vacant regions in the crease centers and surrounding crystalline regions near them can be fabricated. Due to the mesh‐like morphology and the percolated crystalline regions, the polymer semiconducting thin film shows superior stretchability and charge‐transport performance compared to the reference flat polymer thin film. When incorporated into organic thin‐film transistors, the DPP‐DTT polymer semiconducting thin film maintains its high field‐effect carrier mobility (0.53 ± 0.03 cm2 (V s)−1) under a strain ε of 80% and is highly stable under repeated stretching cycles at an ε of 50%.

Solutal‐Marangoni‐Flow‐Mediated Growth of Patterned Highly Crystalline Organic Semiconductor Thin Film Via Gap‐Controlled Bar Coating

AbstractApplication‐oriented patterned growth of organic semiconductor (OSC) thin films with single crystalline domains is crucial for fabricating sophisticated high‐performance organic‐electronic and optoelectronic devices; however, fabricating these patterned nanometer‐thick crystals in a simple, fast, and effective manner is a difficult task with a roll‐to‐roll printing process. Here, a simple bar‐coating approach to form an array of single‐crystal‐like OSC thin‐film patterns at a rate of a few millimeters per second is introduced. To this end, the processing parameters of a gap‐controlled bar‐coating method is optimized, including coating speed, crystal nucleation, and solution fluidics, which allow a high degree of morphological control of bar‐coated OSC films in an area of several centimeters. In particular, it is demonstrated that the solutal‐Marangoni flow induced by a suitable solvent additive can considerably improve molecular mass transport and induce favorable vertical phase separation. Thus, organic transistors based on the OSC patterns fabricated with the additive‐assisted bar coating show a field‐effect mobility of up to 20 cm2 V−1 s−1 and superior operational stability. The proposed bar coating method will facilitate an industry‐level application of organic electronics.

Nano‐Ground Glass as a Superhydrophilic Template for Printing High‐Performance Organic Single‐Crystal Thin Films

AbstractThin‐film devices are typically fabricated through a bottom‐up approach, wherein the constituents are deposited sequentially from the bottom to top layer. This method requires the precise management of the heterointerfaces, which leads to complicated integration issues particularly in solution‐processed organic thin‐film transistors (OTFTs). This limitation arises because a surface suitable for the printing of semiconductors is not necessarily suitable for maximizing the electronic properties of OTFTs. To overcome this, a transfer technique of organic semiconductor (OSC) thin films has been studied. This enables facile transfer of the OSC thin film from a hydrophilic template to any given substrate; thus, the printing substrate and destination substrate can be optimized individually. Here, a nano‐ground glass (NGG) is developed whose surface is chemically etched using a mild base. The NGG functions as a thermally stable, superhydrophilic template for printing high‐quality single‐crystal OSC thin films. To evaluate the practical applicability of the NGG, an n‐type OSC, which requires a relatively high temperature of around 150 °C during crystal growth, is fabricated. The fabricated OTFTs exhibit an outstanding electron mobility of 2.2 cm2 V−1 s−1. The NGG proposed in this study can be utilized for the fabrication of a wide variety of printable materials.

Linear and Non-Linear Optical Properties for Organic Semiconductor (CuPc) Thin Films

Thin films of CuPc of various thicknesses (150,300 and 450) nm have been deposited using pulsed laser deposition technique at room temperature. The study showed that the spectra of the optical absorption of the thin films of the CuPc are two bands of absorption one in the visible region at about 635 nm, referred to as Q-band, and the second in ultra-violet region where B-band is located at 330 nm. CuPc thin films were found to have direct band gap with values around (1.81 and 3.14 (eV respectively. The vibrational studies were carried out using Fourier transform infrared spectroscopy (FT-IR). Finally, From open and closed aperture Z-scan data non-linear absorption coefficient and non-linear refractive index have been calculated respectively using He-Ne laser which have beam waist of (24.2 μm), wave-length of (632.8 nm) and Rayleigh thickness was 2.9 mm.

Epitaxy of an Organic Semiconductor Templated by Molecular Monolayer Crystals

Molecular template growth is an efficient approach to enhance the crystallinity of organic semiconductor materials deposited by physical vapor deposition methods. Lattice matching, large grain size, and large monolayer coverage are critical figures of merits to evaluate effective templates. Here, solution-processed molecular monolayer crystals (MMCs) with 100% monolayer coverage and large single-crystalline domain size present the optimum form of molecular templates, which are successfully utilized for the growth of organic semiconductor thin films. Commensurate epitaxy growth over a large area was observed, and the reason for widely observed lamella structures in thin films was disclosed. In addition, the MMCs are proven effective in templating the growth of various high-performance organic semiconductor materials into orientated thin films. The MMC template provides an efficient approach to enhance the crystallinity and regulate the molecular orientation of organic semiconductor thin films.