Diketopyrrolopyrrole Derivatives Functionalized with N‐Annulated PDI and Se‐Annulated PDI by Direct (Hetero)Arylation Methods

Effects of solvent vapor annealing on the optical properties and surface adhesion of conjugated D : A thin films

A diketopyrrolopyrrole (DPP) and perylene diimide (PDI)-based molecule, denoted as PDI-DPP-PDI, was investigated as an electron acceptor material in bulk heterojunction (BHJ) solar cells, with poly[[4,8-bis [5-(2-ethylhexyl)-2-thienyl]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl] [2-(2-ethyl-1-oxohexyl)thieno[3,4-b]thiophenediyl]] (PBDTTT-CT) as an electron donor. The donor polymer and the acceptor molecule have complementary absorption spectra, which is an essential feature for energy collection in organic solar cells. However, AFM images indicated the presence of isolated and microsized PDI-DPP-PDI domains along the surface of the films, which reduced the power conversion efficiency. Therefore, to improve the homogenization of the acceptor along the film, a post-deposition treatment, denoted as solvent vapor annealing (SVA), was performed in a saturated atmosphere containing the vapour of an organic solvent for 3-10 minutes. This procedure changed the optical and morphological properties of the PBDTTT-CT : PDI-DPP-PDI active layer, resulting in increased power conversion efficiency values by more than 2.5 times (reaching 5.1%). Theoretical simulation pointed out that the experimental absorbance band localized at 580 nm, which appeared after SVA treatment, is possibly related to an intense simulated band with a maximum at 572 nm, resulting from a pair of transitions starting in the copolymer and ending in PDI-DPP-PDI, in regions where both are stacked at about 3 Å. The most significant natural transition orbitals (NTOs) related to these transitions indicated charge transfer character. Moreover, analyses carried out by power spectrum density (PDS) of images acquired from the SVA-treated film indicated that in the region of larger frequencies, across the length scale at around 30-70 nm, an additional fractal region appeared with a Ds of 0.95, indicating a flattened region, possibly related to changes in the overall conformation after SVA treatment. This indicates an improvement in the molecular packing, a feature not observed in the as-cast film. The analyses by force curve spectroscopy pointed out increased adhesion forces and adhesion energy in the PBDTTT-CT : PDI-DPP-PDI film after SVA treatment; this feature enhanced the interfacial interaction with the top electrodes, reflecting improved charge extraction in the photovoltaic device.

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.