N-type Molecules Research Articles

Design Strategy for the Synthesis of Self-Doped n-Type Molecules.

n-Type organic conductive molecules play a significant role in organic electronics. Self-doping can increase the carrier concentration within the materials to improve the conductivity without the need for additional intentional dopants. This review focuses on the various strategies employed in the synthesis of self-doped n-type molecules, and provides an overview of the doping mechanisms. By elucidating these mechanisms, the review aims to establish the relationship between molecular structure and electronic properties. Furthermore, the review outlines the current applications of self-doped n-type molecules in the field of organic electronics, highlighting their performance and potential in various devices. It also offers insights into the future development of self-doped materials.

Estimation of the Charge Mobility of Phenanthroline derivatives with the view of Density Functional Theory: Reorganization Energy and Charge Transfer Integral are in play

Molecular arrangement and noncovalent interactions in organic materials greatly influence the charge mobility in organic light-emitting diodes (OLEDs), organic photovoltaics (OPVs), and organic field-effect transistors (OFETs). In the light of the this argument, we examined the electronic properties of the phenanthroline derivatives by considering the charge mobility with the combination of density functional theory and Marcus Charge Transfer Theory. The drift electron mobility of the molecule 1 and 2 were determined to 21.13 cm2 V-1 s-1 and 18.00 cm2 V-1 s-1, respectively through J type π⋯π stacking interactions created by small perpendicular distances between the adjacent rings. The effective charge pathways of the molecules were generated with strong π⋯π stacking interactions consolidated by noncovalent interactions in their solid phases. The electron reorganization energy for both molecules were determined smaller than that of holes which means they have n-type semiconductor properties. The charge transfer integrals were calculated with the optimization of molecules’ dimer configurations that the theoretical results demonstrate the charge transfer integral depends on the distance between the stacking rings. High charge transfer integral and small reorganization energy give the high charge mobility fort he semiconductor molecules. Beside the mobility, energy band gap, ionization potential, electron and hole injection barriers of the molecules were interpreted to further understand their electronic properties. Due to the small LUMO values which provide n-type molecule and small electron injection barrier. From the our work both molecules can be effective n type organic semiconductor devices with the high mobility and can be modified for more efficient charge transport in phenanthroline derivatives.

An n-Type All-Fused-Ring Molecule with Photoresponse to 1000nm for Highly Sensitive Near-Infrared Photodetector.

Most of all-fused-ring π-conjugated molecules have wide or medium bandgap and show photo response in the visible range. In this work, an all-fused-ring n-type molecule, which exhibits an ultrasmall optical bandgap of 1.22eV and strong near-infrared (NIR) absorption with an onset absorption wavelength of 1013nm is reported. The molecule consists of 14 aromatic rings and has electron donor-acceptor characteristics. It exhibits excellent n-type properties with low-lying HOMO/LUMO energy levels of -5.48eV/-3.95 eV and high electron mobility of 7.0×10-4 cm2 V-1 s-1 . Most importantly, its thin film exhibits a low trap density of 5.55×1016 cm-3 because of the fixed molecular conformation and consequently low conformation disorder. As a result, organic photodetector (OPD) based on the compound exhibits a remarkably low dark current density (Jd ) of 2.01×10-10 Acm-2 at 0V. The device shows a shot-noise-limited specific detectivity (Dsh *) of exceeding 1013 Jones at 400-1000nm wavelength region with a peak specific detectivity of 4.65×1013 Jones at 880nm. This performance is among the best reported for self-powered NIR OPDs.

Theoretical study of electronic and optical properties of functionalized Indigo and Alizarin as potential organic semi-conductors for solar cells applications

The optical and electronic properties of two conjugated organic molecules, Indigo and Alizarin, as potential organic solar cells were studied theoretically. The electronic properties, reorganization energy (λh and λe), adiabatic Ionization potential (IA), adiabatic Electron affinity (EA),HOMO, LUMO and Energy gap (Eg) were calculated using DFT. The optical properties as the maximum absorption (λmax) along with oscillator strengths (f) at the excited states in vacuum and solvent were also calculated using TD-DFT. The molecules were functionalized by electronegative functional groups to understand the effect of functionalization on the optical and electronic properties. It was found that the functionalization of Indigo and Alizarin has resulted in overall conversion of the materials to better n-type molecules. The Eg for both molecules has been generally reduced upon the introduction of electronegative functional groups. This indicates the possibility of tuning organic molecules optical and electronic properties by introducing functional groups. Also, both studied molecules along with their functionalized molecules show properties that fall among organic semiconductors and thereby, can be potentially used in solar cells.

Effects of morphology and charge transport of PDIF-CN2 /graphene TFT

A two dimensional form of carbon, Graphene has captured great interest in many fields due to its attractive physical and electrical properties. Recently, graphene based transistors are produced widely, which becomes in place of silicon in the coming years. Here, we studied the effect of integration n-type molecule with graphene to develop thin film transistors. Morphology and transfer characteristics of N,N′-1H,1H-perfluorobutyldicyanoperylene-carboxydi-imide(PDIF-CN2) islands on graphene are studied and compared the transfer characteristics of graphene TFT and PDIF-CN2/graphene TFT. CVD graphene is used to examine the growth and charge transport of PDIF-CN2/graphene TFT. 6.3 times increase in mobility has noticed after depositing PDIF-CN2 islands on graphene. ION/OFF ratio of PDIF-CN2/graphene TFT is 2 and Threshold voltage is 6.5 ± 1 V. The low ION/OFF ratio is due to the high quantity of PDIF-CN2 molecules. Observed linear behavior of ID at VDS =1 V, shows the absence of interface energy barrier for charge carrier injection, where gold electrodes were deposited on PDIF-CN2/ Graphene. Gold-PDIF-CN2 layer behave as ohmic contact, where the current-voltage (I-V) curve is linear as with Ohm's law. Morphological studies were conducted using Atomic Force Microscopy and Transmission Electron Microscopy. AFM studies reveal that size of PDIF-CN2 islands on graphene. The height of these islands is 35 nm and width is 350 nm. Different crystalline orientation has noticed by Transmission Electron Microscopy.

Enhanced performance of white organic light-emitting devices based on ambipolar white organic single crystals

Organic single crystals are highly promising for applications in optoelectronic devices because of their higher mobility and thermal stability than amorphous thin films. Although white organic single crystals have been fabricated by the double-doped method and applied to realize white organic light-emitting devices (WOLEDs), the unbalanced carrier transport properties of the unipolar crystals severely limit the device performance. Here, ambipolar white organic single crystals are obtained by using mixed p- and n-type molecules as an ambipolar host for the red and green dopants. The white crystal with balanced carrier transport and balanced blue, green, and red emission intensity was applied to the single-crystal WOLEDs. The highest brightness of 1956 cd m−2 and the current efficiency of 1.31 cd A−1 are achieved, which are the best performance of the single-crystal WOLEDs reported to date. A high color rendering index is obtained, which varies between 82 and 87 with increasing driving current. It is expectable that this strategy would support the practical applications of organic single crystal-based OLEDs.

Recent developments on organic single crystal-based light-emitting devices

<p indent="0mm">Organic single crystals with low impurity content and long-range periodic order usually present high carrier mobility, which have attracted increasing interest in the field of optoelectronic devices, such as organic light-emitting transistors, organic field-effect transistors (OFETS), optically pumped lasers, and organic light-emitting devices (OLEDs). They are recognized as ideal materials for OLEDs because of their better thermal stability and higher carrier mobility compared to their amorphous counterparts. This review focuses on the recent developments of organic single crystal-based OLEDs from four aspects, including a summary of widely used organic single crystals, crystal growth methods, fabrication strategies for organic single-crystal OLEDs, and structure optimizations of the devices. Before the device fabrication, it is very important to choose suitable crystal materials with prominent optoelectronic properties. The crystal materials can be classified into three different subjects according to their charge transporting properties, including P-type, N-type, and ambipolar. These crystal materials are summarized with three major properties of charge mobility, emission peak position, and energy level structure. The second is the growth methods for growing organic single crystals with low impurity and defects. Based on the traditional vapor-processing and solution-processing method, some novel crystal growth strategies have been developed for large-area organic single crystals with controllable thickness down to one molecular layer. In view of the fabrication of organic single crystal-based OLED devices, the template stripping method has been introduced to fabricate organic single crystal-based OLEDs with improved contact between metallic electrodes and crystals. Based on this method, organic single crystal-based OLEDs can be realized with bright surface emission and polarized EL behaviors. Molecular doping technique is also employed for the fabrication of crystal-based OLEDs with three primary colors. In addition, WOLEDs based on double-doped organic single crystals are also discovered with high color rendering index, indicating strong potential in the display and lighting field. Later, ambipolar crystals prepared by doping N-type molecules into a P-type crystal matrix for equal mobility are achieved for OLEDs with a recorded brightness and efficiency. Organic single crystal-based OLEDs have made great progress, however, the device performance still lags far behind the traditional amorphous thin-film OLEDs and faces enormous challenges. This review then introduced several potential strategies that can further improve the performance of organic single crystal-based OLEDs in four aspects: Designing new organic single crystal materials with high solid-state luminous efficiency and balanced carrier transport, exploring new growth strategies for precise control of organic single crystal thickness in combination with the semiconductor manufacturing technology, developing various host-guest doping systems for effectively promoting the efficiency of organic single crystal-based OLEDs, and further optimizing the device structure with improved energy levels. In conclusion, organic single crystals have made remarkable progress in the development of electronic and optoelectronic devices, especially for organic single crystal-based OLEDs, which can be ascribed to the development of new material designs, innovative crystal growth methods, and the revolution of device fabrication. Many efforts are required to seek novel strategies for the improvement of organic single crystal-based OLEDs which are expected to be great potential in optoelectronic devices.

Multifunctional asymmetrical molecules for high-performance perovskite and organic solar cells

Asymmetrical n-type molecules are high-performance multifunctional materials simultaneously used as ETMs or ILMs in PVKSCs and electron acceptors in OSCs.

Eliminating Light-Soaking Instability in Planar Heterojunction Perovskite Solar Cells by Interfacial Modifications

Stability remains as a challenge of perovskite solar cells although encouraging progress has been made toward their maximum achievable power conversion efficiency in recent years. Light-soaking issue, where the device performance improves upon continuous light illumination and reduces upon storage in dark, is widely observed and marked as the early-stage instability during device operation. In this work, we have employed conjugated polymer PCDTBT as the dopant-free hole-transport layer to fabricate devices without hysteresis but with reversible light-soaking instability. The introduction of n-type molecules, either organic molecule PDI2 or fullerene derivative PC61BM, as the interfacial layer between TiO2 and perovskite layers can effectively reduce or eliminate this instability owing to the efficient charge transport and defect passivation at the electron-transport layer interface, accompanied with an efficiency of 15.7 and 17.7%, respectively. We conclude that the light-soaking instability of these perovskite solar cells is mainly originated from the charge accumulation at the TiO2/perovskite interface and can be eliminated once the interfacial charge can be suppressed by interfacial modifications to improve charge transport at the interface.

Electrostatic Polarization Energies of Charge Carriers in Organic Molecular Crystals: A Comparative Study with Explicit State-Specific Atomic Polarizability Based AMOEBA Force Field and Implicit Solvent Method.

The electrostatic polarization plays an important role in determining the energy levels of charge carriers in organic solids, which is controlled by the atomic polarizability in AMOEBA polarizable force field. QTAIM-based space partitioning of molecular polarizability is utilized to uniformly parametrize the state-specific atomic polarizability (SSAP) of π-conjugated organic small molecules to avoid fitting molecular polarizability of some artificial training set. Herein, the SSAPs are applied to explicitly extrapolate the electrostatic polarization energy ( E pol) of the charge carriers of nine π-conjugated organic crystals including six p-type transfer materials, oligoacenes and TIPS-substituted oligoacenes, and three n-type transfer materials, F-substituted oligoacenes and TCNQ. Our results demonstrate that the electrostatic polarization energies of the hole carrier ( E+pol) are smaller than that of the electron carrier ( E-pol) for p-type molecules while E+pol are larger than E-pol for n-type molecules. SSAP-based E pol values of oligoacenes behave as a nearly unvaried feature with the increase of conjugation length which is similar to implicit polarizable continuum model (PCM) results, while E pol obtained from the default atomic polarizability behaves with a notable decrease. Implicit PCM can correctly capture most of electrostatic polarization of ions in bulk system although it slightly underestimates the gap between the electrostatic polarization of hole and electron carriers in oligoacene crystals. Our results demonstrate that this unified parametrized SSAP provides a reliable and cheap tool to estimate the energy landscape of charge carriers in condensed-phase organic solids.

Organic Photovoltaic Devices Based on Oriented n-Type Molecular Films Deposited on Oriented Polythiophene Films.

The molecular orientation of π-conjugated molecules has been reported to significantly affect the performance of organic photovoltaic devices (OPVs) based on molecular films. Hence, the control of molecular orientation is a key issue toward the improvement of OPV performance. In this research, oriented thin films of an n-type molecule, 3,4,9,10-Perylenetetracarboxylic Bisbenzimida-zole (PTCBI), were formed by deposition on in-plane oriented polythiophene (PT) films. Orientation of the PTCBI films was evaluated by polarized UV-vis spectroscopy and 2D-Grazing incidence X-ray diffraction. Results indicated that PTCBI molecules on PT film exhibit nearly edge-on and in-plane orientation (with molecular long axis along the substrate), whereas PTCBI molecules without PT film exhibit neither. OPVs composed of PTCBI molecular film with and without PT were fabricated and evaluated for correlation of orientation with performance. The OPVs composed of PTCBI film with PT showed higher power conversion efficiency (PCE) than that of film without PT. The experiment indicated that in-plane orientation of PTCBI molecules absorbs incident light more efficiently, leading to increase in PCE.

Effect of acceptor strength in new acceptor–donor–acceptor-type molecules on their miscibility with donor polymers for bulk-heterojunction fullerene-free solar cells

We designed and synthesized two kinds of spirobifluorene (SP)-based acceptor–donor–acceptor (A–D–A)-type acceptor materials, denoted by SP-IC and SP-TCF, which have an A–D–A structure consisting of a 3D tetrathienospirobifluorene donor core and 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC) or 2-(3-cyano-4,5,5-trimethylfuran-2(5H)-ylidene)malononitrile (TCF) acceptors as terminal units, respectively. Their LUMO levels (−3.91 eV for SP-IC and −3.97 eV for SP-TCF) are suitable as n-type molecules for PTB7-Th-based polymer solar cells. However, their morphologies in blend films with a conjugated donor polymer, Poly([2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene]{3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7-Th), exhibited considerable differences. The PTB7-Th:SP-IC blend film exhibited a better-defined nanophase-separated morphology than PTB7-Th:SP-TCF blend film. The corresponding bulk heterojunction polymer solar cell based on PTB7-Th:SP-IC showed four times higher power conversion efficiency (PCE = 4.50%) than that of PTB7-Th:SP-TCF. The higher PCE was well explained by investigating the morphology, which was affected by the surface energy of the acceptor molecules at the donor–acceptor interface.

Doping Versatile n-Type Organic Semiconductors via Room Temperature Solution-Processable Anionic Dopants.

In this study, we describe a facile solution-processing method to effectively dope versatile n-type organic semiconductors, including fullerene, n-type small molecules, and graphene by commercially available ammonium and phosphonium salts via in situ anion-induced electron transfer. In addition to the Lewis basicity of anions, we unveiled that the ionic binding strength between the cation and anion of the salts is also crucial in modulating the electron transfer strength of the dopants to affect the resulting doping efficiency. Furthermore, combined with the rational design of n-type molecules, an n-doped organic semiconductor is demonstrated to be thermally and environmentally stable. This finding provides a simple and generally applicable method to make highly efficient n-doped conductors which complements the well-established p-doped organics such as PEDOT:PSS for organic electronic applications.

Charge transport network dynamics in molecular aggregates

Due to the nonperiodic nature of charge transport in disordered systems, generating insight into static charge transport networks, as well as analyzing the network dynamics, can be challenging. Here, we apply time-dependent network analysis to scrutinize the charge transport networks of two representative molecular semiconductors: a rigid n-type molecule, perylenediimide, and a flexible p-type molecule, [Formula: see text] Simulations reveal the relevant timescale for local transfer integral decorrelation to be [Formula: see text]100 fs, which is shown to be faster than that of a crystalline morphology of the same molecule. Using a simple graph metric, global network changes are observed over timescales competitive with charge carrier lifetimes. These insights demonstrate that static charge transport networks are qualitatively inadequate, whereas average networks often overestimate network connectivity. Finally, a simple methodology for tracking dynamic charge transport properties is proposed.

Strong n-type molecule as low bias negative differential resistance device predicted by first-principles study

A first-principles study of the transport properties of two thiolated pentacenes sandwiching ethyl is performed. The thiolated pentacene molecule shows strong n-type characteristics when contact Ag lead because of low work function about metal Ag. A strong negative differential resistance (NDR) effect with large peak-to-valley ratio of 758% is present under low bias. Our investigations indicate that strong n- or p-type molecules can be used as low bias molecular NDR devices and that the molecular NDR effect based on molecular-level leaving not on molecular-level crossing has no hysteresis.

Acenaphtho[1,2-b]quinoxaline diimides derivative as a potential small molecule non-fullerene acceptor for organic solar cells

We designed and synthesized a small molecule acenaphtho[1,2-b]quinoxaline diimide derivative AQI-T2 as an electron-accepting material for non-fullerene organic solar cells. This molecule exhibits a relatively broad absorption band from 300 to 650 nm, with a moderately low-lying lowest unoccupied molecular orbital energy level of −3.64 eV. Non-fullerene organic solar cells with conventional structure using PTB7-Th as the electron donor and AQI-T2 as the electron acceptor exhibited moderate photovoltaic performances. The best performance was attained from the pristine device, which showed a power conversion efficiency of 0.77% with a relatively high open-circuit voltage of 0.86 V, a short circuit current of 2.04 mA cm−2 and a fill factor of 43.98%. These results indicated that this n-type molecule can be a promising electron-accepting material for non-fullerene organic solar cells.

Bis(thienothiophenyl) diketopyrrolopyrrole-based conjugated polymers with various branched alkyl side chains and their applications in thin-film transistors and polymer solar cells.

New thienothiophene-flanked diketopyrrolopyrrole and thiophene-containing π-extended conjugated polymers with various branched alkyl side-chains were successfully synthesized. 2-Octyldodecyl, 2-decyltetradecyl, 2-tetradecylhexadecyl, 2-hexadecyloctadecyl, and 2-octadecyldocosyl groups were selected as the side-chain moieties and were anchored to the N-positions of the thienothiophene-flanked diketopyrrolopyrrole unit. All five polymers were found to be soluble owing to the bulkiness of the side chains. The thin-film transistor based on the 2-tetradecylhexadecyl-substituted polymer showed the highest hole mobility of 1.92 cm2 V(-1) s(-1) due to it having the smallest π-π stacking distance between the polymer chains, which was determined by grazing incidence X-ray diffraction. Bulk heterojunction polymer solar cells incorporating [6,6]-phenyl-C71-butyric acid methyl ester as the n-type molecule and the additive 1,8-diiodooctane (1 vol %) were also constructed from the synthesized polymers without thermal annealing; the device containing the 2-octyldodecyl-substituted polymer exhibited the highest power conversion efficiency of 5.8%. Although all the polymers showed similar physical properties, their device performance was clearly influenced by the sizes of the branched alkyl side-chain groups.

Horizontally oriented molecular thin films for application in organic solar cells

The authors investigate the influence of molecular orientation of p-type molecules of alpha-sexithiophene (α-6T) and n-type molecules of 3,4,9,10-perylene tetracarboxylic bisbenzimidazole (PTCBI) on organic solar cell (OSC) performance. Deposition of α-6T and subsequently PTCBI on an α-6T buffer surface rubbed with a nylon cloth allows their horizontal orientations to be formed in separate layers. Power conversion efficiency and operation stability of a rubbed OSC are markedly improved when compared with an unrubbed OSC. The improved OSC performance is confirmed to be due to increases in light absorption, exciton diffusion length, energy difference between the p-type and n-type layers, and carrier collection by electrodes, which are caused by the rubbing-induced double horizontal orientations.

All-organic vertical transistor in an analogous n-semiconductor/metal/p-semiconductor trilayer structure

We report on the development and characterization of an all-organic vertical transistor, consisting of a p-type conjugated polymer, poly(bithiophene), as collector material, a poly(ethylene dioxythiophene)/poly(styrene sulfonate) layer as base material and a n-type molecule layer, Alq3, as emitter material. The transistor is operated under direct bias in the common-emitter mode, being its operation based upon charge recombination of the injected majority carriers at the emitter–base interface. Experimental results have yielded current amplification factors up to 7. We also demonstrate that tunneling controls the transport characteristics of the device.

Band Gap Tunable N-Type Molecules for Organic Field Effect Transistors

A series of four novel n-type molecules has been synthesized. Unlike previous approaches, the end group of these molecules was fixed and the molecular core was varied. The resulting materials were thoroughly analyzed. Electronic properties were derived from photoemission spectroscopy, optical properties were derived with the help of optical spectroscopy, and the structure of thin films on Au(111) was derived by scanning tunneling microscopy (STM). In addition, prototypical organic field-effect transistors (OFETs) (forming n-channels in OFETs) have been fabricated and tested. The correlation between the device performance of the respective OFETs (i.e., electron mobility) and their electronic as well as structural properties was investigated. It turned out that a combination of beneficial electronic and structural properties provides the best results. These findings are important for the design of new materials for future device applications.