Yonghai Li

New Organic Semiconductors with Imide/Amide‐Containing Molecular Systems

Due to their high electron affinities, chemical and thermal stabilities, π-conjugated molecules with imide/amide frameworks have received considerable attentions as promising candidates for high-performance optoelectronic materials, particularly for organic semiconductors with high carrier mobilities. The purpose of this Research News is to give an overview of recent advances in development of high performance imide/amide based organic semiconductors for field-effect transistors. It covers naphthalene diimide-, perylene diimide- and amide-based conjugated molecules and polymers for organic semiconductors.

A Tetrathiafulvalene-Based Electroactive Covalent Organic Framework

Two-dimensional covalent organic frameworks (2D COFs) provide a unique platform for the molecular design of electronic and optoelectronic materials. Here, the synthesis and characterization of an electroactive COF containing the well-known tetrathiafulvalene (TTF) unit is reported. The TTF-COF crystallizes into 2D sheets with an eclipsed AA stacking motif, and shows high thermal stability and permanent porosity. The presence of TTF units endows the TTF-COF with electron-donating ability, which is characterized by cyclic voltammetry. In addition, the open frameworks of TTF-COF are amenable to doping with electron acceptors (e.g., iodine), and the conductivity of TTF-COF bulk samples can be improved by doping. Our results open up a reliable route for the preparation of well-ordered conjugated TTF polymers, which hold great potential for applications in fields from molecular electronics to energy storage.

Dithiazole-fused naphthalene diimides toward new n-type semiconductors

A simple synthetic approach to dithiazole-fused naphthalene diimides 1 and 2 from 2,6-dibromoNDI (2BrNDI) was described. Note that the synthesis did not involve any metallic catalysts. Based on their cyclic voltammograms, the LUMO energies of 1 and 2 were estimated to be −3.99 eV and −3.98 eV, respectively, being lower than that of unsubstituted NDI. Thin films of 1 and 2 show n-type semiconducting behaviors. OFETs of 1 exhibit electron mobility up to 0.15 cm2 V−1 s−1 with high on/off ratio (under ambient conditions) after annealing.

Changes in Chlorophyll Fluorescence in Maize Plants with Imposed Rapid Dehydration at Different Leaf Ages

A comparison of the effects of a rapidly imposed water deficit with different leaf ages on chlorophyll a fluorescence and gas exchange was performed in maize (Zea mays L.) plants. The relationships between photosynthesis and leaf relative turgidity (RT) and ion leakage were further investigated. Leaf dehydration substantially decreased net photosynthetic rate (A) and stomatal conductance (Gs), particularly for older leaves. With dehydration time, Fv/Fm maintained a relatively stable level for youngest leaves but significantly decreased for the older leaves. The electron transport rate (ETR) sharply decreased with intensifying dehydration and remained at lower levels during continuous dehydration. The photochemical quenching of variable chlorophyll fluorescence (qP) gradually decreased with dehydration intensity for the older leaves but increased for the youngest leaves, whereas dehydration did not affect the nonphotochemical chlorophyll fluorescence quenching (NPQ) for the youngest leaves but remarkably decreased it for the older leaves. The leaf RT was significantly and positively correlated with its Fv/Fm, ETR, and qP, and the leaf ion leakage was significantly and negatively correlated with Fv/Fm and NPQ. Our results suggest that the photosynthetic systems of young and old leaves decline at different rates when exposed to rapid dehydration.

New π-conjugated polymers as acceptors designed for all polymer solar cells based on imide/amide-derivatives

In this paper we report two n-type polymeric acceptors P1 and P2 based on imide/amide-derivatives (PDI, NDI and DPP). P1 and P2 exhibit very different optical properties which may be related to the different molecular configuration and polymeric crystallinity. All polymer solar cells were fabricated with conventional configuration based on polymeric donor PBDTTT-C-T and P1/P2. Results reveal that devices based on P1 show the best performance with a PCE of 2.01%, a V-OC of 0.68 V, a J(SC) of 7.06 mA cm 2 and a FF of 41.78%, which is attributed to the appropriate energy levels, relatively high electron mobility and preferable micro-phase separation. The poor performance for P2 ( PCE = 0.30%) was mainly ascribed to its extremely low J(SC) which was probably caused by the small HOMO energy level difference between donor and P2.

Arylacetylene-Substituted Naphthalene Diimides with Dual Functions: Optical Waveguides and n-Type Semiconductors

New arylacetylene-substituted naphthalene diimides (NDIs) 1-6, with both light-emitting and semiconducting functions, are reported. Among them, the crystal structure of 1 was determined. On the basis of their reduction potentials and thin-film absorption spectra, the HOMO/LUMO energies of these modified NDIs were estimated. The results reveal that their HOMO/LUMO energies are slightly affected by the flanking aryl groups. The emission colors of these NDIs vary from green to red, and interestingly, they show aggregation-induced emission enhancement behavior with fluorescence quantum yields reaching 9.86% in the solid state. Microrods of 1, 3, and 5 show typical optical wave-guiding behavior with relatively low optical-loss coefficients. Organic field-effect transistors with thin films of these NDIs were fabricated with conventional techniques. The results indicate that thin films of 2, 4, and 6, with long and branched alkyl chains, show air-stable n-type semiconducting properties with electron mobilities of up to 0.035 cm(2) V(-1) s(-1) after thermal annealing, whereas 1, 3, and 5, with short alkyl chains, behave as n-type semiconductors under a nitrogen atmosphere with electron mobilities of up to 0.075 cm(2) V(-1) s(-1) after thermal annealing.

Three-State Single-Molecule Naphthalenediimide Switch: Integration of a Pendant Redox Unit for Conductance Tuning

We studied charge transport through core-substituted naphthalenediimide (NDI) single-molecule junctions using the electrochemical STM-based break-junction technique in combination with DFT calculations. Conductance switching among three well-defined states was demonstrated by electrochemically controlling the redox state of the pendent diimide unit of the molecule in an ionic liquid. The electrical conductances of the dianion and neutral states differ by more than one order of magnitude. The potential-dependence of the charge-transport characteristics of the NDI molecules was confirmed by DFT calculations, which account for electrochemical double-layer effects on the conductance of the NDI junctions. This study suggests that integration of a pendant redox unit with strong coupling to a molecular backbone enables the tuning of charge transport through single-molecule devices by controlling their redox states.

New alternating electron donor–acceptor conjugated polymers entailing (E)-[4,4′-biimidazolylidene]-5,5′(1H,1′H)-dione moieties

Four new alternating D–A polymers (P1–P4) with (E)-[4,4′-biimidazolylidene]-5,5′(1H,1′H)-dione (BID) as electron accepting moieties were synthesized and characterized. Based on GPC (gel permeation chromatography) data, Mws of P1–P4 are relatively high ranging from 11.8 to 14.7 kg mol−1 with polydispersities of 1.8–1.9. Thin-films of P1–P4 show broad and strong absorptions up to 950 nm. HOMO (−5.11 eV to −5.32 eV) and LUMO (−3.71 eV to −3.81 eV) energies as well as band gaps (1.40 eV to 1.53 eV) were determined based on their onset redox potentials and absorption spectra. Field-effect transistors with thin-films of P1–P4 were successfully fabricated with conventional techniques. The results reveal that thin-films of P1 and P2 exhibit relatively high hole mobilities of up to 6.6 × 10−3 cm2 V−1 s−1.

Naphthalene substituents bonded via the β-position: an extended conjugated moiety can achieve a decent trade-off between optical band gap and open circuit voltage in symmetry-breaking benzodithiophene-based polymer solar cells

Bare naphthalene units bonded via the α- and β-positions as side chain substituents on an asymmetric benzodithiophene (BDT) building block were employed in the design of new light-harvesting polymers for the first time. Accordingly, two D–A type polymers, based on naphthyl-substituted BDT as a D-building block and the well-known 4,7-di(thiophen-2-ethylhexyl)-5,6-difluoro-2,1,3-benzothiadiazole (DTffBT) as an acceptor unit, were synthesized. The polymer PBDTβNPFBT with naphthalene bonded via the β-position exhibits an appropriate π–π distance. The naphthalene rings efficiently broaden the absorption and narrow the optical band gap (Eoptg), by extending the degree of π-conjugation, which is beneficial for capturing more photons and thus improving the short-circuit current density (JSC). Meanwhile, PBDTβNPFBT-based devices also exhibit a desirable high open circuit voltage (VOC), due to the low saturation dark current density (JS) arising from the exactly appropriate π–π distance. As a result, the power conversion efficiency (PCE) of 9.80% for the PBDTβNPFBT/PC71BM-based PSC is the highest efficiency ever obtained among the reported BDT and DTBT backbone photovoltaic polymers. In addition, a PCE of 7.33% was obtained for PBDTβNPFBT/ITIC without any treatment, which is also impressive for non-fullerene PSCs. Thus, it can be concluded that a naphthyl unit bonded via the β-position can lead to a better trade-off between Eoptg and VOC, ultimately increasing the PCE dramatically.

Efficient fullerene-free solar cells with wide optical band gap polymers based on fluorinated benzotriazole and asymmetric benzodithiophene

In this work, α- and β-position naphthalene substituents as side chains on asymmetric BDT were used as donor building blocks to build wide bandgap (WBG) donor materials (PαNBDT-T1 and PβNBDT-T1) with fluorinated benzotriazole (T1) as the acceptor unit. The two co-polymers were used to build polymer solar cells (PSCs) with PC71BM or ITIC acceptor material. In ITIC acceptor material based devices, the PαNBDT-T1 co-polymer with a larger dihedral angle between main backbone and naphthalene ring achieved a higher power conversion efficiency (PCE) of 9.60% with improved short-circuit current density (JSC) and fill factor (FF) compared with PβNBDT-T1, which was ascribed to the excellent morphology of the blended film. The open circuit voltage (VOC) was also maintained at a decent level upon introducing these naphthalene rings due to their high ionization potential and low electron density. Interestingly, for PC71BM based devices, the two polymers show a reverse situation compared with an ITIC system. The PβNBDT-T1 with a small dihedral angle between the main backbone and naphthalene rings shows a slightly higher performance than PαNBDT-T1. However, in the PC71BM system, both polymers did not exhibit ideal optical performance due to their unmatched absorption spectrum. These phenomena indicate that the asymmetric BDTs have great potential towards achieving high optical performance with non-fullerene acceptor materials.