The abundance of solar thermal energy and the widespread demands for waste heat recovery make thermoelectric generators (TEGs) very attractive in harvesting low-cost energy resources. Meanwhile, thermoelectric refrigeration is promising for local cooling and niche applications. In this context there is currently a growing interest in developing organic thermoelectric materials which are flexible, cost-effective, eco-friendly and potentially energy-efficient. In particular, the past several years have witnessed remarkable progress in organic thermoelectric materials and devices. In this review, thermoelectric properties of conducting polymers and small molecules are summarized, with recent progresses in materials, measurements and devices highlighted. Prospects and suggestions for future research efforts are also presented. The organic thermoelectric materials are emerging candidates for green energy conversion.

An aromatic heteromacrocycle, selenium bridged neutral annulene 5,16-diphenyltetraselenolo[22]annulene[2,1,2,1] (DPTSA) was synthesized through a convenient four-step procedure. DPTSA is a symmetrical macrocyclic architecture consisting of four selenophene units linked with unsaturated carbon–carbon bonds. DPTSA was shown to have high aromaticity by ¹H NMR and UV/vis absorption spectra. Its electrochemical properties and thermostability were studied. A thin-film transistor based on DPTSA was fabricated to evaluate the possibility of its application in organic field-effect transistors (OFETs). DPTSA performed as a p-type semiconductor with the best hole mobility of 2.8×10⁻³ cm² V⁻¹ S⁻¹ and the on/off ratio of 1.0×10⁵. The thin-film morphology of devices based on DPTSA was also characterized to preliminarily reveal the structure-property relationships.

We report the synthesis, characterization, redox behavior, and n-channel organic field-effect (OFET) characteristics of a new class of thieno[3,2-b]thiophene-diketopyrrolopyrrole-based quinoidal small molecules 3 and 4. Under ambient atmosphere, solution-processed thin-film transistors based on 3 and 4 exhibit maximum electron mobilities up to 0.22 and 0.16 cm² V⁻¹ s⁻¹, respectively, with on-off current ratios (I_on/I_off) of more than than 10⁶. Cyclic voltammetry analysis showed that this class of quinoidal derivatives exhibited excellent reversible two-stage reduction behavior. This property was further investigated by a stepwise reductive titration of 4, in which sequential reduction to the radical anion and then the dianion were observed.

Four novel nonsymmetrical photochromic diarylethene compounds containing dithieno[3,2-b:2′,3′-d]thiophene units were designed and synthesized to investigate their photochromic properties. All these molecules adopt a photoactive antiparallel conformation in single crystals, as revealed by X-ray crystallographic analysis, and exhibit excellent photochromism in solution as well as in the crystalline phase.

A graphite thin film was investigated as the drain and source electrodes for bottom-contact organic field-effect transistors (BC OFETs). Highly conducting electrodes (10² S cm⁻¹) at room temperature were obtained from pyrolyzed poly(l,3,4-oxadiazole) (PPOD) thin films that were prepatterned with a low-cost inkjet printing method. Compared to the devices with traditional Au electrodes, the BC OFETs showed rather high performances when using these source/drain electrodes without any further modification. Being based on a graphite-like material these electrodes possess excellent compatibility and proper energy matching with both p- and n-type organic semiconductors, which results in an improved electrode/organic-layer contact and homogeneous morphology of the organic semiconductors in the conducting channel, and finally a significant reduction of the contact resistance and enhancement of the charge-carrier mobility of the devices is displayed. This work demonstrates that with the advantages of low-cost, high-performance, and printability, PPOD could serve as an excellent electrode material for BC OFETs.

PBBTZ (6H-pyrrolo[3,2-b:4,5-b′]bis[1,4]benzothiazine) was a p-type semiconductor with high field-effect transistor (FET) performance that we have just reported. Two trifluoromethyl substituted PBBTZ derivatives 3a and 3b were synthesized from facile one-pot condensation. They were characterized by means of ¹H NMR, IR, HRMS (EI-TOF) and elemental analysis, furthermore, the crystal structure of 3b was described and discussed. Their optical properties were studied by UV-Vis and fluorescence spectroscopy, electrochemical properties were investigated by cyclic voltammetry (CV), and thermal properties were evaluated by thermal gravimetric analysis (TGA). The energy gaps of 3a and 3b, taken directly from spectroscopic measurements, are as broad as 2.45 and 2.48 eV, leading to bluish green and green photoluminescence. The LUMO and HOMO energy levels are −5.73 and −3.28 eV for 3a, −5.67 and −3.19 eV for 3b, respectively. The low energy levels render them well air-stable, and to be promising n-type semiconductor candidates for use in organic electronics.

Two cyclic carbazolenevinylene dimers 1 and 2 were synthesized by McMurry coupling reactions. A linear compound 3 was also prepared for comparison. Compounds 1–3 were fully characterized by means of NMR spectroscopy, HRMS, elemental analysis, and UV/Vis absorption spectroscopy. Quantum chemical simulations showed that the cyclic compounds possessed smaller HOMO–LUMO gaps and more extended conjugation. The UV/Vis absorption spectra of the cyclic compounds showed blueshifts compared with that of the linear compound 3. Time-dependent DFT (TD-DFT) analysis revealed that this was due to the different selection rules for molecules with cyclic and linear architectures. The cyclic conformation also significantly affected the molecular ordering in the solid state. The X-ray crystal structure of 1 showed partial π–π overlapping between the adjacent molecules. Thin films of 1–3 were fabricated by the vacuum-deposition method on Si/SiO₂ substrates. Multicrystalline thin films were obtained from compounds 1 and 2, but only amorphous thin films could be obtained for the linear compound 3. Another important difference between the cyclic and linear compounds was the reduced reorganization energy for the cyclic compounds. These two facts have resulted in improved field-effect transistor (FET) mobilities for the cyclic compounds compared with the linear compound. In addition, as the substrate temperature has a significant influence on the morphology and the degree of crystallinity of the thin films deposited, the device performance could be optimized by varying the substrate temperature. The FET devices based on 2 gave the highest mobility of 0.013 cm² V⁻¹ s⁻¹. The results showed that carbazole derivatives with cyclic structures might make better FETs.