Dithienocoronene diimide (DTCDI)-derived triads for high-performance air-stable, solution-processed balanced ambipolar organic field-effect transistors.

Developing ambipolar organic semiconducting materials is essential for use in complementary-like inverters and light-emitting transistors. In this study, three new dithienocoronenediimide (DTCDI)-derived triads, DTCDI-BT, DTCDI-BBT and DTCDI-BNT, were designed and synthesized, in which various sizes of terminal groups, i.e., thiophene (T), benzo[b]thiophene (BT) and naphtha[2,3-b]thiophene (NT) were substituted at the α-positions of the two thiophene rings of DTCDI, respectively. The DFT calculations reveal that the HOMO energy levels of the three triads when compared to that of the parent DTCDI-core (-5.99 eV) are significantly increased to -5.59, -5.59 and -5.45 eV for DTCDI-BT, DTCDI-BBT and DTCDI-BNT, respectively, whereas the LUMO energy levels (-3.07 eV ∼ -3.14 eV) are almost identical with that of the DTCDI-core (-3.10 eV). The results predict that the triads could possess ambipolar transport properties in organic field-effect transistor (OFET) applications. In fact, under an ambient atmosphere, solution-processed bottom-gate top-contact (BGTC) transistors exhibit ambipolar charge transport properties by tuning the HOMOs of the DTCDI-based triads so that they were suitable for hole injection, resulting in balanced maximum electron and hole mobilities of 1.66 × 10-3 and 1.02 × 10-3 cm2 V-1 s-1 for DTCDI-BT, 2.60 × 10-2 and 3.60 × 10-2 cm2 V-1 s-1 for DTCDI-BBT, and 2.43 × 10-3 and 4.15 × 10-3 cm2 V-1 s-1 for DTCDI-BNT, respectively. This is the first time that the DTCDI building block has been used to develop ambipolar small molecular semiconductors, and achieved a device performance comparable to that of the DTCDI-based polymeric semiconductors. In addition, DTCDI-BBT-based complementary-like inverters were made, and the inverter devices operated well in both p-mode and n-mode under ambient conditions. The results show that the DTCDI is a promising π-electron-deficient building block which could be further used to develop ambipolar semiconducting materials for OFET devices.

Design and Synthesis of a Novel n-Type Polymer Based on Asymmetric Rylene Diimide for the Application in All-Polymer Solar Cells.

A novel n-type polymer of PTDI-T based on asymmetric rylene diimide and thiophene is designed and synthesized. The highest power conversion efficiency of 4.70% is achieved for PTB7-Th:PTDI-T-based devices, which is obviously higher than those of the analogue polymers of PPDI-2T and PDTCDI. When using PBDB-T as a donor, an open-circuit voltage (VOC ) as high as 1.03 V is obtained. The results indicate asymmetric rylene diimide is a kind of promising building block to construct n-type photovoltaic polymers.

Two Isomeric Azulene-Decorated Naphthodithiophene Diimide-based Triads: Molecular Orbital Distribution Controls Polarity Change of OFETs Through Connection Position.

Altering the charge carrier transport polarities of organic semiconductors by molecular orbital distribution has gained great interest. Herein, we report two isomeric azulene-decorated naphthodithiophene diimide (NDTI)-based triads (e.g., NDTI-B2Az and NDTI-B6Az), in which two azulene units were connected with NDTI at the 2-position of the azulene ring in NDTI-B2Az, whereas two azulene units were incorporated with NDTI at the 6-position of the azulene ring in NDTI-B6Az. The two isomeric triads were excellently soluble in common organic solvents. Density functional theory calculations on the molecular orbital distributions of the triads reveal that the lowest unoccupied molecular orbitals are completely delocalized over the entire molecule for both NDTI-B2Az and NDTI-B6Az, indicating great potential for n-type transport behavior, whereas the highest occupied molecular orbitals are mainly delocalized over the entire molecule for NDTI-B2Az or only localized at the two terminal azulene units for NDTI-B6Az, implying great potential for p-type transport behavior for the former and a disadvantage of hole carrier transport for the latter. Under ambient conditions, solution-processed bottom-gate top-contact transistors based on NDTI-B2Az showed ambipolar field-effect transistor (FET) characteristics with high electron and hole mobilities of 0.32 (effective electron mobility ≈0.14 cm2 V-1 s-1 according to a reliability factor of 43%) and 0.03 cm2 V-1 s-1 (effective hole mobility ≈0.01 cm2 V-1 s-1 according to a reliability factor of 33%), respectively, whereas a typically unipolar n-channel behavior is found for a film of NDTI-B6Az with a high electron mobility up to 0.13 cm2 V-1 s-1 (effective electron mobility ≈0.06 cm2 V-1 s-1 according to a reliability factor of 43%). The results indicate that the polarity change of organic FETs based on the two isomeric triads could be controlled by the molecular orbital distributions through the connection position between the azulene unit and NDTI.

Exploring a Fused 2-(Thiophen-2-yl)thieno[3,2-b]thiophene (T-TT) Building Block to Construct n-Type Polymer for High-Performance All-Polymer Solar Cells.

In the field of all-polymer solar cells, exploring new electron-donating units (D) to match with electron-accepting units (A) is an important subject to promote the performance of D-A-type polymer acceptors. Herein, we developed a fused D unit 2-(thiophen-2-yl)thieno[3,2-b]thiophene (T-TT) derivated from the famous 2-(2-(thiophen-2-yl)vinyl)thiophene (TVT) unit. With classical naphthalene diimide (NDI) as A unit, the new D-A polymer PNDI-T-TT exhibits enhanced absorption coefficient, electron mobility, and miscibility with donor polymer in comparison with the analogous PNDI-TVT polymer. These advantages can be attributed to the enlarged conjugation and reduced rotamers due to the fused T-TT unit, leading to a stronger intermolecular interaction. When blending with the donor polymer PBDB-T, both NDI-based polymers can form better interpenetrating nanostructures than the corresponding blend films with the donor polymer J71. Finally, PBDB-T/PNDI-T-TT device achieves a power conversion efficiency of 6.1%, which is much higher than that of PBDB-T/PNDI-TVT device (4.24%). These results demonstrate that n-type polymer based on fused T-TT unit can ameliorate the absorption coefficient, molecular aggregation, and charge-carrier mobility and consequently achieve an improved photovoltaic performance in comparison with the classic TVT unit.

Synthesis and Optical Properties of Donor-Acceptor-Type 1,3,5,9-Tetraarylpyrenes: Controlling Intramolecular Charge-Transfer Pathways by the Change of π-Conjugation Directions for Emission Color Modulations.

In dipolar organic π-conjugated molecules, variable photophysical properties can be realized through efficient excited-state intramolecular charge transfer (ICT), which essentially depends on the π-conjugation patterns. Herein, we report a controllable regioselective strategy for synthesis and optical properties of two donor-acceptor (DA)-type 1,3,5,9-tetraarylpyrenes (i.e., 1,3-A/5,9-D (4b) and 1,3-D/5,9-A (4c)) by covalently integrating two phenyl rings and two p-OMe/CHO-substituted phenyl units into the 2-tert-butylpyrene building block, in which the two phenyl rings substituted at the 1,3-positions act as acceptors for 4b or as donors for 4c and the two p-OMe or p-CHO-substituted phenyl moieties substituted at the K-region of 5,9-positions act as donors for 4b or as acceptors for 4c, respectively. Density functional theory calculations on their frontier molecular orbitals and UV-vis absorption of S0 → S1 transition theoretically predicted that the change of π-conjugation directions in the two DA pyrenes could be realized through a variety of substitution patterns, implying that the dissimilar ground-state and excited-state electronic structures exist in each molecule. Their single-crystal X-ray analysis reveal their highly twisted conformations that are beneficial for inhibiting the π-aggregations, which are strikingly different from the normal 1,3,5,9-tetraphenylpyrenes (4a) and related 1,3,6,8-tetraarylpyrenes. Indeed, experimental investigations on their optical properties demonstrated that the excited-state ICT pathways can be successfully controlled by the change of π-conjugation directions through the variety of substitution positions, resulting in the modulations of emission color from deep-blue to green in solution. Moreover, for the present DA pyrenes, highly fluorescent emissions with moderate-to-high quantum yields both in the thin film and in the doped poly(methyl methacrylate) film were obtained, suggesting them as promising emitting materials for the fabrication of organic light-emitting diodes.