Small Molecular Organic Semiconductors 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.

Solution processable benzo[b]thiophene derivatives as small molecular organic semiconductors for organic field effect transistors

In this study, four new organic semiconductors based on benzo[b]thiophene derivatives were synthesized and characterized as the active layer of organic field-effect transistors (OFETs) to investigate the effect of different isomers and end-group moieties on the electrical performance. The top-contact/bottom-gate OFETs were fabricated via solution-shearing methods and exhibited p-channel activity. In particular, the thin film with a single BT moiety attached via the 6-position showed the highest mobility up to 0.055 cm2/Vs and current on/off ratio of 2.5 × 107. The microstructure and surface morphology of the thin films were analyzed by X-ray diffraction (XRD) and atomic force microscopy (AFM) to investigate the factors contributing to the difference in electrical performance of the devices.

Solution-Processable Benzo[b]thieno[2,3-d]thiophene Derivatives as Organic Semiconductors for Organic Thin-Film Transistors

Two new benzo[b]thieno[2,3-d]thiophene (BTT) derivatives, 2-(benzo[b]thiophen-5-yl)benzo[b]thieno[2,3-d]thiophene (compound 2), and 2-(benzo[b]thieno[2,3-d]thiophene-2yl)dibenzo[b,d]thiophene (compound 3) have been synthesized and utilized as solution-processable small molecular organic semiconductors for organic field-effect transistors (OFETs). The physicochemical characteristics of the recently created substances were analyzed using thermogravimetric analysis (TGA), differential scanning calorimeter (DSC), and UV-vis spectroscopy. Subsequently, the above-mentioned substances were employed as semiconductor layers in bottom-gate/top-contact OFETs through solution shearing methods for device fabrication, and their electrical performances were meticulously evaluated. The outcoming OFET device displayed p-channel behavior, demonstrating hole mobility of up to 0.005cm2/Vs and a current on/off ratio higher than 106.

Flexible all-organic photodetectors via universal water-assisted transfer printing

Transfer printing of small-molecular organic semiconductors often faces challenges due to surface adhesion mismatch. Here, we developed a sacrificing-layer-assisted transfer printing technique for the deposition of small-molecular thin films. High-boiling-point ethylene glycol (EG) was doped in aqueous solution poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as the sacrificing layer to manipulate residual water in film, which allowed chlorobenzene solution to spontaneously spread and form uniform film. The residual water guaranteed film delamination from the stamp, allowing for its transfer onto various substrates and seeding layers. As a proof of concept, laterally conductive organic photodetectors using recyclable EG-PEDOT:PSS electrodes and a small-molecular active layer were consecutively fabricated via transfer printing in ambient air. The resulting device exhibited a high on/off ratio of 711 and a fast rise time of 0.5ms. Notably, the polymer electrode and the bulk heterojunction demonstrated unique repairability and recyclability.

Indoloindole‐Based Hole Transporting Material for Efficient and Stable Perovskite Solar Cells Exceeding 24% Power Conversion Efficiency

AbstractThe new concept of hole transporting materials (HTMs) has inspired researchers to develop high‐performing and stable perovskite solar cells (PSCs). In particular, small molecular organic semiconductors have been extensively studied for HTM due to their high reproducibility and easy synthesis. In this work, a novel linear‐type series of indoloindole (IDID)‐based hole transporting materials comprising a fluorinated IDID core(IDIDF) and multiple thiophene rings is developed. The structure‐property relationship in the IDIDF derivatives is investigated systematically by changing the alkyl position and length of the backbone. The intrinsic properties of the material are significantly different depending on the alkyl position of inner thiophene ring. The optimized material exhibits improved solubility, favorable molecular packing patterns, and superior hole mobility. The champion PSCs using the optimum molecule, IDIDF2, yield a power conversion efficiency of 23.16% in non‐doped and 24.24% in doped conditions, which represent one of the highest performances in n‐i‐p planar device configuration. For the first time, the IDIDF2‐based PSCs achieve outstanding thermal and moisture stabilities under thermal aging (85 °C) and relative humidity of 85%, respectively, for 1500 h.

Binary solvent engineering for small-molecular organic semiconductor crystallization

This article reviews the synergistic effects of engineering binary solvents on the crystallization, morphology and charge transport of organic semiconductors.

Manipulate organic crystal morphology and charge transport

Organic electronics has become one of the fastest developing fields with exciting advances in discovering novel organic semiconductors, enhancing charge carrier mobility, and exploring high-performance electronic device applications. Nevertheless, the solution-induced growth of small molecular organic semiconductors provokes predominant crystal misorientation, thermal cracks and grain boundary, which poses a tough challenge to maneuver the crystal alignment towards device fabrication. In this article, we conduct an in-depth review of external force-based, additive-based, and binary solvent-based techniques to effectively control the organic semiconductor crystallization, thin-film morphology, and crystal orientations. We first introduce the recent progress and the various challenges of flexible electronics, and discuss how these issues are correlated to charge carrier mobility variations. Next, we discuss the miscellaneous external force alignment methods based on the categories of slot-die coating, blade coating, substrate patterning, and air flow. Furthermore, we investigate the additive-based methods for controlling crystal growth and orientations, including polymeric additives, small molecular additives, and nanostructured additives. Finally, we discuss the binary solvent-based methods to control the crystal morphology and dimensionality. By correlating the charge transport characteristics with the intrinsic merits of those unique methods, this article shares a useful insight into maximizing the performance of organic semiconductors from a new perspective.

Asymmetric dithieno[3,2-b:2′,3′-d]thiophene derivatives as solution-processable small molecular organic semiconductors for organic thin film transistors

In this paper, solution-processable asymmetric dithieno[3,2-d:2′,3′-d]thiophene (DTT) compounds, 2-octyldithieno[3,2-b:2′,3′-d]thiophene (1), 2-octyl-6-(thiophen-2-yl)dithieno[3,2-b:2′,3′-d]thiophene (2), 2-octyl-6-(5-octylthiophen-2-yl)dithieno[3,2-b:2′,3′-d]thiophene (3), and 2-(5-(2-ethylhexyl)thiophen-2-yl)-6-octyldithieno[3,2-b:2′,3′-d]thiophene (4) were synthesized and employed as small molecular organic semiconductors for top-contact/bottom-gate organic thin-film transistors. Physicochemical properties of four compounds including decomposition temperature, melting temperature, maximum absorption wavelength, highest occupied molecular orbital / lowest occupied molecular orbital energy level, and band gap were investigated. All DTT compounds exhibited p-channel activity, and especially compound 2 with additional thiophene group and linear alkyl chain (octyl group) showed superior electrical performance with hole mobility up to 0.07 cm2/Vs and current on/off ratio > 108 in ambient environment. Moreover, film microstructure and morphology showed correlation with the corresponding electrical performance, where high film texture and terrace-like morphology afforded high device performance.

Development of Dithieno[3,2-b:2′,3′-d]thiophene (DTT) Derivatives as Solution-Processable Small Molecular Semiconductors for Organic Thin Film Transistors

Novel solution-processable dithieno[3,2-d:2′,3′-d]thiophene (DTT) derivatives with alkylated thiophene or alkyl chain substituents, 2,6-bis(5-octylthiophen-2-yl)dithieno[3,2-b:2′,3′-d]thiophene (compound 1), 2,6-bis(5-(2-ethylhexyl)thiophen-2-yl)dithieno[3,2-b:2′,3′-d]thiophene (compound 2), and 2,6-dioctyldithieno[3,2-b:2′,3′-d]thiophene (compound 3), have been synthesized and employed as small molecular organic semiconductors for organic field-effect transistors (OFETs). All compounds exhibited good thermal stability over 290 °C, while different side groups of DTT compounds afforded different melting temperatures. The molecular orbital energy levels were experimentally and theoretically calculated, and their trend was almost the same. The developed compounds were employed as active layers for top-contact/bottom-gate OFETs with average charge carrier mobility as high as 0.10 cm2/Vs and current on/off ratio > 107 in ambient atmosphere. Notably, DTT derivative with linear alkyl chain (-octyl) substituents showed the best device performance. High device performance could be attributed to the large grains and continuous surface coverages as well as high film texture of the corresponding semiconductor films.

Organic Semiconductors with Benzoic Acid Based Additives for Solution- Processed Thin Film Transistors

Background: although solution-processed small molecular organic semiconductors have attracted great attention for organic electronic applications, the intrinsic crystal misorientation of the organic semiconductors still remains a challenging issue. Objective: two benzoic acid-based additives, i.e. 4-propylbenzoic acid (RBA) and 4-octylbenzoic acid (OBA), were employed to regulate the crystal growth and charge transport of organic semiconductors. Methods: RBA and OBA were mixed with a π-conjugated organic semiconductor 6,13- bis(triisopropylsilylethynyl) pentacene (TIPS pentacene), respectively. Organic thin-film transistors (OTFTs) with different bottom-gate, top-contact and bottom-gate, bottom-contact configurations were fabricated to investigate charge transport. Results: RBA and OBA share a similar benzoic acid structure with the same hydrophilic head but differ in the length of the hydrophobic tail. The benzoic acid-based additive forms a stratified selfassembled interfacial layer and maneuvers nucleation seed distribution via synergetic interactions with the silanol groups on silicon dioxide and with the bulky side chains of the semiconductor. The TIPS pentacene film with OBA additive exhibited a 10-fold reduction in misorientation angle as compared to the counterpart with RBA. Conclusion: Distinct thin film morphology in terms of crystal alignment and grain width was observed and correlated to the hydrophobic tail length. In particular, OTFTs incorporating the TIPS pentacene/benzoic acid mixture as the active layer showed a mobility of up to 0.15 cm2/Vs.

(Invited) Side-Chain Engineering of Small-Molecular Organic Semiconductors for Organic Electronic Devices

Molecular orientation and packing structure of small molecular organic semiconductors are very important for the charge carrier mobility of organic thin-film transistors (OTFT). Furthermore, the miscibility of p-and n-type materials and the grain size of each material in bulk heterojunction are critical for the performance of organic photovoltaics (OPVs). Benzoporphyrins (BPs) are one of the outstanding p-type organic semiconductors and have been studied for OTFT and OPV materials [1]. For practical use of organic electronic devices, solution process is desirable but BPs are not soluble in organic solvents.To achieve the solution process of BPs, two strategies have been studied in the last two decades. One is a precursor approach: as-spun films of soluble precursors of BPs are in-situ converted into BP thin-films by retro-Diels–Alder reaction. With this method, the film of intact BP is available but the mixed film of edge-on and face-on grains are obtained because of the too-good crystallinity of BPs. The other is a substituent engineering: proper substituents induced to the BP framework can improve the solubility and processability of the film as well as the morphology and orientation of the molecules. Recently we were successful to obtain edge-on dominant film of BP derivatives by a simple dip-coating. 5,15-triisopropylsilyl-ethynyl(TIPS)-BP could make brickwork-type packing to achieve 1.1 cm2 V-1 s-1 FET charge carrier mobility (m FET) [2].This side-chain engineering was also effective for the control of packing structure of bulk-hetero junction OPV. The alkyl chain length drastically influenced the miscibility of p- and n-materials and the short-circuit current density [3]. We also found open-circuit-voltage shift of over 0.5 V in organic photovoltaic cells induced by a minor structural difference in alkyl substituents of acene-type p-materials [4]. In this presentation, the relation between control of packing structure and the OPV performance will be discussed.

Third-order nonlinear optical properties of the small-molecular organic semiconductor tris (8-Hydroxyquinoline) aluminum by CW Z-scan technique

The linear optical and electroluminescence properties of small molecular organic material tris-(8-hydroxyquinoline) aluminum (Alq3) are known and extensively investigated by many studies. However, the studies on the nonlinear optical (NLO) properties of this material are still scarce. Herein, the third-order NLO (TONLO) properties of Alq3 chloroform solutions with different concentrations have been examined using the Z-scan technique. At different laser powers and different laser wavelengths, the Z-scan signals were recorded using a continuous-wave (CW) laser beam. The open-aperture (OA) and closed- aperture (CA) Z-scan signals revealed strong TONLO properties. Both nonlinear refractive index (n2) and nonlinear absorption coefficient (β) showed negative values, and their smallest values were in the fold of 10−8 cm2/W and 10−2 cm/W, respectively. The negative values of n2 and β suggest thermal nonlinearity and saturable absorption (SA), respectively. The TONLO susceptibility (χ(3)) was calculated and found to be in the order of 10−6 esu. Besides, they increased with increasing the concentrations of Alq3 and laser powers. Therefore, the results indicate that Alq3 could be one of the potential materials as a saturable absorber for starting a passive mode-locking operation.

Engineering the Doping Efficiency in Pentacene Thin Films for High Thermoelectric Performance.

Because of the high mobility and Seebeck coefficient, pentacene (PEN) is a promising candidate for organic small-molecule thermoelectric (TE) materials. However, the low intrinsic conductivity impedes its application in thermoelectricity. In this work, hexacyano-trimethylene-cyclopropane (CN6-CP) is employed as the dopant for PEN via constructing bilayer-structured thin films. The almost intact crystallinity and high charge carrier generation efficiency of these interface-doped PEN films ensure their high conductivity. Time of flight secondary ion mass spectrometry was applied to demonstrate the diffusion of dopant molecules into the PEN layer. UV-vis spectral analysis reveals that integral charge transfer happens between the PEN and CN6-CP molecules. The doping process is further characterized by electron spin-resonance, ultraviolet photoelectron spectroscopy, and X-ray photoelectron spectroscopy analysis. Under optimized conditions, the conductivity of the PEN film deposited on the SiO2/Si substrate can reach up to 10.1 S cm-1. To the best of our knowledge, this is the highest conductivity ever reported for doped PEN thin films. The optimal TE performance with a power factor of 36.4 μW m-1 K-2 can be achieved in the PEN/CN6CP thin film with a Seebeck coefficient and conductivity of 199 μV K-1 and 9.2 S cm-1, respectively. This result shows that interface doping with a strong electron acceptor is a promising approach for optimizing the TE performance of small molecular organic semiconductors.

Effect of Polymer Molecular Weight on Morphology and Charge Transport of Small-Molecular Organic Semiconductors

The utilization of polymer additives provides an intriguing pathway to tune the electrical performance of solution-grown, small-molecular organic semiconductors. In this study, we report the effect of different polymer molecular weight on the crystal growth, phase segregation and charge transport of the organic semiconductors. A semicrystalline polymer additive polyethylene oxide (PEO) with 8000 and 100 K molecular weight was blended with a well-known organic semiconductor 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene). Distinctively different thin film morphology of the resultant TIPS pentacene/PEO blend film was identified and quantitatively characterized. In particular, PEO with a higher molecular weight of 100 K exerted a stronger effect on enhancing film formation, crystal coverage and likely mechanical properties, whereas PEO with an 8000 molecular weight yielded more superior crystal alignment and larger crystal sizes. Bottom-gate, top-contact TIPS pentacene/PEO OTFTs were fabricated to test the charge transport, which indicated loading the PEO polymer with molecular weight of 8000 demonstrated a five-fold enhancement in the extracted hole mobilities as compared to the 100 K counterpart. This work of using polymer additives with different molecular weight can be used to tune the crystallization of other solution-processed high-mobility small-molecular organic semiconductors.

Annealing Behavior with Thickness Hindered Nucleation in Small-Molecule Organic Semiconductor Thin Films

We introduce a comprehensive overview of spontaneous and thermal induced structure changes in thin films of the high performance small-molecular organic semiconductor 5,11-bis(triethyl silylethynyl) anthradithiophene (TES-ADT). Most importantly, we find that nucleation of the TES-ADT high temperature beta phase can be induced only for films thicker than a critical thickness, while it is hindered for thinner films. The described limited crystal phase nucleation, the circumstances of which are unique in the realm of small-molecule organic semiconductors, is explained in terms of a two step nucleation mechanism while taking into account the anisotropic surface energy of crystal grains. Furthermore, we identify changes in the optical absorption spectra observed during aging of as deposited films with a spontaneous transformation from the amorphous phase to the alpha(2) phase via the alpha(1) phase of TES-ADT. The transformation takes place on the time scale of several days. Finally, we determine temperature dependence of lattice parameters of the alpha(1) and alpha(2) phases and the lattice parameters of the beta phase. Thermal expansion of the alpha(1) phase is found to be much lower than that of the alpha(2) phase.

Indene‐Based Donor‐Acceptor Type Small Molecular Semiconductors for High‐Performance n‐Channel Transistors

AbstractIt is a great challenge to design n‐channel donor‐acceptor (D−A) type small molecules because of the lack of strong electron withdrawing acceptor unit. Herein, 3‐(dicyanomethylidene)‐indan‐1‐one (IC) and 1,3‐bis(dicyanomethylidene)‐indene (IDC) based D−A type small molecular organic semiconductors IC–DDTPTI and IDC‐DDTPTI are synthesized and characterized. Both compounds have low‐lying LUMO energy levels, especially for IDC‐DDTPTI with LUMO energy level lower than −4.0 eV. Thin film transistor characteristics show IC–DDTPTI displays ambipolar transistor behavior with balanced hole and electron mobility. The electron mobility is 0.40 cm2 V−1 s−1 and the hole mobility reaches 0.18 cm2 V−1 s−1. IDC‐DDTPTI exhibits unipolar n‐channel character with an electron mobility of 0.027 cm2 V−1 s−1. These results highlight the applications of IC and IDC units in high performance D−A type n‐channel organic semiconductors.

Optimization of the thermoelectric performance of layer-by-layer structured copper-phthalocyanine (CuPc) thin films doped with hexacyano-trimethylene-cyclopropane (CN6-CP).

Copper-phthalocyanine (CuPc), as a classical small molecular organic semiconductor, has been applied in many fields. However, the low intrinsic conductivity limits its application in thermoelectricity. Here, hexacyano-trimethylene-cyclopropane (CN6-CP), a strong electron acceptor, is synthesized as dopant for CuPc thin films to improve their conductivities. Multilayer thin films constructed from alternate thermally evaporated CuPc and CN6-CP thin layers are investigated. Under the optimized condition, the doped CuPc film with a conductivity of 0.76 S cm−1 and a Seebeck coefficient of 130 μV K−1, shows a high power factor of 1.3 μW m−1 K−2 and the carrier concentration is estimated to be 2.8 × 1020 cm−3. Considering the relatively superior performance, the CN6-CP doped CuPc film is a promising small molecular organic thermoelectric (OTE) material. In addition, for those highly crystalline materials with poor solubility, the layer-by-layer structure offers a general strategy for investigation and optimization of their TE performance.

Solution-processable small molecules for bulk heterojunction ambipolar thin-film transistors and complementary-like inverters

Solution-processable thioalkylated bithiophene derivatives with different lengths of side chains (SBT-n; n = 10, 12, 14, 16, 18) have been synthesized and characterized as p-channel organic semiconductors for thin-film transistors (TFTs). Based on a combination of the highest performing SBT derivative (SBT-14) and quinoidals (DTTRQs) as p- and n-channel materials, solution-processed small-molecular bulk heterojunction (BHJ) ambipolar thin-film transistors were fabricated, and the resulting devices showed air-stable and high ambipolar performance with well-balanced electron and hole mobilities as high as 0.70 cm2 V−1 s−1 and 0.21 cm2 V−1 s−1, respectively. Furthermore, complementary-like inverters comprising two ambipolar thin-film transistors were demonstrated, which exhibited a high voltage gain of up to 81. Our study clearly demonstrated that side chain engineering of small molecular organic semiconductors had a significant influence on the electrical performance of TFTs and BHJ transistors, as well as complementary-like inverters.

Synthesis and Optoelectrical Characterization of Novel Squaraine Dyes Derived from Benzothiophene and Benzofuran.

Synthesis and photophysical characterizations of two novel small molecules SQ-BEN-THI and SQ-BEN-FUR with D–A–D molecular structure consisting of squaraine as central unit and benzothiophene and benzofuran as end groups are being reported. Apart from very sharp and intense light absorption by these molecular sensitizers in near-infrared (NIR) wavelength region, their possibility as small molecular organic semiconductor was also explored after fabricating organic field-effect transistors (OFETs). Results obtained from photophysical, electrochemical, and quantum chemical studies were combined to elucidate the structural and optoelectronic properties. Electrical characterization pertaining to the charge-transport properties carried after OFET fabrication exhibited field-effect mobilities of 4.0 × 10–5 and 5.4 × 10–5 cm2/(V s) for SQ-BEN-THI and SQ-BEN-FUR, respectively. After thermal annealing at 130 °C, the field-effect mobility was found to increase for both squaraine dyes. Relatively facile carrier transport in SQ-BEN-FUR compared to that of SQ-BEN-THI could be attributed to relatively higher backbone planarity as indicated from optimized molecular structure obtained after density functional theory calculations. This work may guide for further molecular design and synthesis of novel squaraine dyes for high-performance OFET applications.

Thienoisoindigo-Based Semiconductor Nanowires Assembled with 2-Bromobenzaldehyde via Both Halogen and Chalcogen Bonding

We fabricated nanowires of a conjugated oligomer and applied them to organic field-effect transistors (OFETs). The supramolecular assemblies of a thienoisoindigo-based small molecular organic semiconductor (TIIG-Bz) were prepared by co-precipitation with 2-bromobenzaldehyde (2-BBA) via a combination of halogen bonding (XB) between the bromide in 2-BBA and electron-donor groups in TIIG-Bz, and chalcogen bonding (CB) between the aldehyde in 2-BBA and sulfur in TIIG-Bz. It was found that 2-BBA could be incorporated into the conjugated planes of TIIG-Bz via XB and CB pairs, thereby increasing the π − π stacking area between the conjugated planes. As a result, the driving force for one-dimensional growth of the supramolecular assemblies via π − π stacking was significantly enhanced. TIIG-Bz/2-BBA nanowires were used to fabricate OFETs, showing significantly enhanced charge transfer mobility compared to OFETs based on pure TIIG-Bz thin films and nanowires, which demonstrates the benefit of nanowire fabrication using 2-BBA.