Hewei Luo

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.

Significant Improvement of Semiconducting Performance of the Diketopyrrolopyrrole–Quaterthiophene Conjugated Polymer through Side-Chain Engineering via Hydrogen-Bonding

Three diketopyrrolopyrrole (DPP)-quaterthiophene conjugated polymers, pDPP4T-1, pDPP4T-2, and pDPP4T-3, in which the molar ratios of the urea-containing alkyl chains vs branching alkyl chains are 1:30, 1:20, and 1:10, respectively, were prepared and investigated. In comparison with pDPP4T without urea groups in the alkyl side chains and pDPP4T-A, pDPP4T-B, and pDPP4T-C containing both linear and branched alkyl chains, thin films of pDPP4T-1, pDPP4T-2, and pDPP4T-3 exhibit higher hole mobilities; thin-film mobility increases in the order pDPP4T-1 < pDPP4T-2 < pDPP4T-3, and hole mobility of a thin film of pDPP4T-3 can reach 13.1 cm(2) V(-1) s(-1) after thermal annealing at just 100 °C. The incorporation of urea groups in the alkyl side chains also has an interesting effect on the photovoltaic performances of DPP-quaterthiophene conjugated polymers after blending with PC71BM. Blended thin films of pDPP4T-1:PC71BM, pDPP4T-2:PC71BM, and pDPP4T-3:PC71BM exhibit higher power conversion efficiencies (PCEs) than pDPP4T:PC71BM, pDPP4T-A:PC71BM, pDPP4T-B:PC71BM, and pDPP4T-C:PC71BM. The PCE of pDPP4T-1:PC71BM reaches 6.8%. Thin films of pDPP4T-1, pDPP4T-2, and pDPP4T-3 and corresponding thin films with PC71BM were characterized with AFM, GIXRD, and STEM. The results reveal that the lamellar packing order of the alkyl chains is obviously enhanced for thin films of pDPP4T-1, pDPP4T-2, and pDPP4T-3; after thermal annealing, slight inter-chain π-π stacking emerges for pDPP4T-2 and pDPP4T-3. Blends of pDPP4T-1, pDPP4T-2, and pDPP4T-3 with PC71BM show a more pronounced micro-phase separation. These observations suggest that the presence of urea groups may further facilitate the assemblies of these conjugated polymers into nanofibers and ordered aggregation of PC71BM.

Solution-processed core-extended naphthalene diimides toward organic n-type and ambipolar semiconductors

In this paper, we report four new core-extended NDI molecules 1, 2, 3 and 4. 1 and 2 entail two benzene rings and two naphthalene rings, respectively; 3 and 4 contain benzene and naphthalene rings as well as 2-(1,3-dithiol-2-ylidene)malonitrile groups. All of them can be easily synthesized with acceptable yields. HOMO/LUMO energies of 1, 2, 3 and 4 were −5.66 eV/−3.93 eV, −5.73 eV/−3.91 eV, −5.88 eV/−4.22 eV and −5.89 eV/−4.23 eV, respectively. OFETs with thin-films of 1, 2, 3 and 4 were successfully fabricated with conventional techniques. Thin-films of 1 and 2 exhibit ambipolar semiconducting properties under N2 atmosphere with relatively balanced hole and electron mobilities, which can further increase after annealing; hole and electron mobilities reach 0.047 and 0.016 cm2 V−1 s−1, respectively for the thin-film of 2 after annealing at 140 °C. Moreover, two identical ambipolar transistors with thin films of 2 were combined into an inverter circuit with a gain of 11. 3 and 4 behave as n-type semiconductors in air, and the electron mobility can reach 0.22 cm2 V−1 s−1 for the thin-film of 3 with an Ion/off of 106 after annealing at 120 °C. XRD and AFM studies are also presented for understanding the variation of carrier mobilities of thin-films of 1, 2, 3 and 4 after annealing at different temperatures.

Highly Sensitive Thin-Film Field-Effect Transistor Sensor for Ammonia with the DPP-Bithiophene Conjugated Polymer Entailing Thermally Cleavable tert-Butoxy Groups in the Side Chains

The sensing and detection of ammonia have received increasing attention in recent years because of the growing emphasis on environmental and health issues. In this paper, we report a thin-film field-effect transistor (FET)-based sensor for ammonia and other amines with remarkable high sensitivity and satisfactory selectivity by employing the DPP-bithiophene conjugated polymer pDPPBu-BT in which tert-butoxycarboxyl groups are incorporated in the side chains. This polymer thin film shows p-type semiconducting property. On the basis of TGA and FT-IR analysis, tert-butoxycarboxyl groups can be transformed into the -COOH ones by eliminating gaseous isobutylene after thermal annealing of pDPPBu-BT thin film at 240 °C. The FET with the thermally treated thin film of pDPPBu-BT displays remarkably sensitive and selective response toward ammonia and volatile amines. This can be attributed to the fact that the elimination of gaseous isobutylene accompanies the formation of nanopores with the thin film, which will facilitate the diffusion and interaction of ammonia and other amines with the semiconducting layer, leading to high sensitivity and fast response for this FET sensor. This FET sensor can detect ammonia down to 10 ppb and the interferences from other volatile analytes except amines can be negligible.

New dithienyl-diketopyrrolopyrrole-based conjugated molecules entailing electron withdrawing moieties for organic ambipolar semiconductors and photovoltaic materials

In this paper we report two DPP-based conjugated molecules DPPBT and DPPTT in which the respective electron withdrawing moieties 2,1,3-benzothiadiazole and thiazolo[5,4-d]thiazole are flanked by two DPP moieties. For comparison, DPPBZ containing 1,4-diethynylbenzene and two DPP moieties were synthesized. HOMO/LUMO energies of DPPBT, DPPTT and DPPBZ were estimated on the basis of cyclic voltammetric data. Owing to the fact that LUMO energies of DPPBT and DPPTT were lowered to ca. −3.5 eV, thin films of both DPPBT and DPPTT exhibit ambipolar semiconducting properties under N2 atmosphere with hole and electron mobilities up to 0.25 cm2 V−1 s−1 and 0.09 cm2 V−1 s−1, respectively. In comparison, thin film of DPPBZ just shows p-type semiconducting property. Notably, ambipolar semiconductors with relatively high carrier mobility are rarely reported for DPP-containing small conjugated molecules. Alternatively, both DPPBT and DPPTT can function as electron donors for photovoltaic materials. Thin films of DPPTT:PC71BM and DPPBT:PC71BM at a weight ratio of 1 : 1 exhibit PCEs of 4.18% and 2.44%, respectively, with VOC higher than 0.95 V.

Electronic tuning effects via cyano substitution of a fused tetrathiafulvalene–benzothiadiazole dyad for ambipolar transport properties

Electronic tuning effects of substituents at the 4- and 8-positions of benzothiadiazole (BTD) within the fused tetrathiafulvalene–BTD donor–acceptor dyad have been studied. The electron acceptor strength of BTD is greatly increased by replacing Br with CN groups, extending the optical absorption of the small dyad into the near-IR region and importantly, the charge transport can be switched from p-type to ambipolar behaviour.

Extended conjugated donor-acceptor molecules with E-(1,2-difluorovinyl) and diketopyrrolopyrrole (DPP) moieties toward high-performance ambipolar organic semiconductors.

Two diketopyrrolopyrrole (DPP)-based donor-acceptor (D-A) conjugated molecules, DPP-F and DPP-2F, which contain E-(1,2-difluorovinyl) moieties, are reported. The LUMO energies of DPP-F and DPP-2F were estimated to be -3.49 and -3.70 eV, respectively, based on their redox potentials and absorption spectral data; these values were clearly lowered because of the incorporation of electron-withdrawing E-(1,2-difluorovinyl) moieties. Organic field-effect transistors (OFETs) with thin films of DPP-F and DPP-2F were successfully fabricated with conventional techniques. Based on the respective transfer and output characteristics measured in an inert atmosphere, thin films of DPP-2F display ambipolar semiconducting behavior with hole and electron mobilities reaching 0.42 and 0.80 cm(2) V(-1) s(-1), respectively. The as-prepared OFET of DPP-2F already shows high hole and electron mobilities that are not influenced remarkably by thermal annealing. For thin films of DPP-F, only p-type semiconducting behavior was observed in both an inert atmosphere and air, and the hole mobility increased to 0.1 cm(2) V(-1) s(-1) after thermal annealing. XRD and AFM studies were performed with thin films of DPP-F and DPP-2F after annealing at different temperatures.

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.

New Conjugated Molecules with Two and Three Dithienyldiketopyrrolopyrrole (DPP) Moieties Substituted at meta Positions of Benzene toward p- and n-Type Organic Photovoltaic Materials

Two conjugated molecules, TADPP3 and TADPP2-TT, are reported, in which three and two dithienyldiketopyrrolopyrrole (DPP) moieties, respectively, are substituted at the meta positions of benzene. Based on cyclic voltammetry and absorption data, TADPP3 and TADPP2-TT possess similar HOMO and LUMO energies of about -5.2 and -3.4 eV, respectively. Thin films of TADPP3 and TADPP2-TT exhibit p-type semiconducting behavior with hole mobilities of 2.36×10(-3) and 3.76×10(-4)  cm(2)  V(-1)  s(-1) after thermal annealing. Molecules TADPP3 and TADPP2-TT were utilized as p-type photovoltaic materials to fabricate organic solar cells after blending with phenyl C71 butyric acid methyl ester (PC71BM) and phenyl C61 butyric acid methyl ester (PC61BM). The relatively low JSC and fill factor values can be attributed to poor film morphologies based on AFM and XRD studies. A solar cell with a thin film of TADPP3 with PC71BM in a weight ratio of 1:2 exhibits a high open-circuit voltage (VOC) of 0.99 V and a power conversion efficiency (PCE) of 2.47 %. Interestingly, TADPP3 can also be employed as an n-type photovoltaic material. The blended thin film of TADPP3 with P3HT in a weight ratio of 1:2 gave a high VOC of 1.11 V and a PCE of 1.08 % after thermal annealing.

Thiepin‐Fused Heteroacenes: Simple Synthesis, Unusual Structure, and Semiconductors with Less Anisotropic Behavior

The simple one-pot syntheses of sulfur-rich thiepin-fused heteroacences with an alkylidene-fluorene framework, THA1 and THA6 (thiepin-fused heteroacene 1 or 6, in which the thiepin is conjugated at both ortho positions with SCH3 or SC6 H13 , respectively), is reported. Based on electrochemical studies and theoretical calculations, their LUMO energies are relatively low (-3.26 eV), and their HOMO and HOMO-1 orbitals are nearly degenerate. The thiepin ring contributes mainly to HOMO-1 and LUMO orbitals, however, HOMO orbitals dominantly reside on thienoacence rings. Within the crystal of THA1, the molecules adopt a herringbone arrangement and multiple intermolecular interactions lead to the formation of a 2D network. Interestingly, THA6 shows totally different intermolecular arrangements. Organic field-effect transistor (OFET) devices show both compounds exhibiting p-type semiconducting behavior. Thin films or microcrystals of THA1 possess relatively high hole mobility. Moreover, the mobilities of the microcrystal of THA1 along three directions are in the same order, thus the hole-carrier transporting within the hexagonal-plane of microcrystal of THA1 exhibits less anisotropic behavior. In comparison, both thin films and microrods of THA6 show low hole mobilities. This agrees well with the intermolecular arrangements and interactions within crystal of THA6. Further theoretical calculations reveal that significant intermolecular electronic coupling among HOMO-1 orbitals and sulfur atoms play an important role in intermolecular electronic coupling for THA1.