Organic optoelectronics calls for materials combining bright luminescence and efficient charge transport. The former is readily achieved in isolated molecules, while the latter requires strong molecular aggregation, which usually quenches luminescence. This hurdle is generally resolved by doping the host material with highly luminescent molecules collecting the excitation energy from the host. Here, a novel concept of molecular self-doping is introduced in which a higher luminescent dopant emerges as a minute-amount byproduct during the host material synthesis. As a one-stage process, self-doping is more advantageous than widely used external doping. The concept is proved on thiophene-phenylene cooligomers (TPCO) consisting of four (host) and six (dopant) conjugated rings. It is shown that <1% self-doping doubles the photoluminescence in the TPCO single crystals, while not affecting much their charge transport properties. The Monte-Carlo modeling of photoluminescence dynamics reveals that host-dopant energy transfer is controlled by both excitonic transport in the host and host-dopant Förster resonant energy transfer. The self-doping concept is further broadened to a variety of conjugated oligomers synthesized via Suzuki, Kumada, and Stille crosscoupling reactions. It is concluded that self-doping combined with improved excitonic transport and host-dopant energy transfer is a promising route to highly luminescent semiconducting organic single crystals for optoelectronics.

Two main factors governing the effect of terminal substituents on the properties of thiophene–phenylene co-oligomers are revealed.

Single crystals of thiophene−phenelyne co-oligomers (TPCOs) have previously shown their potential for organic optoelectronics. Here we report on solution growth of large-area thin single-crystalline films of TPCOs at the gas−liquid interface by using solvent−antisolvent crystallization, isothermal slow solvent evaporation, and isochoric cooling. The studied co-oligomers contain identical conjugated core (5,5′diphyenyl-2,2′-bithiophene) and different terminal substituents, fluorine, trimethylsilyl, or trifluoromethyl. The fabricated films are molecularly smooth over areas larger than 10 × 10 μm 2 , which is of high importance for organic field-effect devices. The low-defect structure of the TPCO crystals is suggested from the monoexponential kinetics of the PL decay measured in a wide dynamic range (up to four decades) and from low crystal mosaicity assessed by microfocus X-ray diffraction. The TPCO crystal structure is solved using a combination of X-ray and electron diffraction. The terminal substituents affect the crystal structure of TPCOs, bringing about the formation of a noncentrosymmetric crystal lattice with a crystal symmetry Cc for the bulkiest trimethylsilyl terminal groups, which is unusual for linear conjugated oligomers. Comparing the different crystal growth techniques, it is concluded that the solvent−antisolvent crystallization is the most robust for fabrication of single-crystalline TPCOs films. The possible nucleation and crystallization mechanisms operating at the gas−solution interface are discussed.

This contribution describes recent progress in the design, synthesis and properties of solution-processible star-shaped oligomers and their application in organic photovoltaics. Even though alternative chemistry has been used to design such oligomers, the most successful approach is based on a triphenylamine donor branching center, (oligo)thiophene conjugated spacers and dicyanovinyl acceptor groups. These are mainly amorphous low band-gap organic semiconductors, though crystalline or liquid crystalline ordering can sometimes be realized. It was shown that the solubility, thermal behavior and structure of such molecules in the bulk strongly depend on the presence and position of alkyl groups, as well as on their length. The photovoltaic properties of solution-processed molecules of this type are now approaching 5% which exceeds those of vacuum-sublimed devices. The design rules and future perspectives of this class of organic photovoltaic molecules are discussed.

Thiophene-phenylene co-oligomers (TPCOs) are among the most promising materials for organic light emitting devices. Here we report on record high among TPCO single crystals photoluminescence quantum yield reaching 60%. The solution-grown crystals are stronger luminescent than the vapor-grown ones, in contrast to a common believe that the vapor-processed organic electronic materials show the highest performance. We also demonstrate that the solution-grown TPCO single crystals perform in organic field effect transistors as good as the vapor-grown ones. Altogether, the solution-grown TPCO crystals are demonstrated to hold great potential for organic electronics.