Organic Thin Film Transistors Based on Highly Dipolar Donor–Acceptor Polymethine Dyes

Abstract

Organic thin fi lm transistors (OTFTs) of a series of twenty dipolar donoracceptor-substituted polymethine dyes (D-A dyes, dipole moments from 3-15 D) are investigated. The employed merocyanine dyes contain a dimethine bridge that is substituted with 1-alkyl-3,3-dimethylindolin-2-ylidene ("Fischer base"), 3-alkyl-2,3-dihydrobenzothiazol-2-ylidene or 1,3-benzodithiole-2-ylidene, respectively, as electron-donating unit and various acceptor heterocycles. These studies show that thin fi lms formed by these D-A dyes upon deposition in high vacuum are all composed of antiparallel π-stacked dimers. However, they are either amorphous, discontinuous or highly crystalline due to the interplay between molecule-substrate and dimer-dimer interactions. With the help of single crystal X-ray analysis, out-of-plane X-ray studies (XRD), selected area electron diffraction (SAED), and atomic force microscopy (AFM), a correlation between the molecular structure, fi lm ordering, and hole charge transport ability can be established. The mobility values are compared to Bässler's disorder charge transport theory and a fi lm growth mechanism is proposed based on DFT calculations and single crystal structures. The results show that with carefully adjusted bulky substituents and high dipolarity an intimate centrosymmetric packing with a slipped, but tight π-stacking arrangement could be realized. This provides two-dimensional percolation pathways for holes and ultimately results in charge carrier mobilities up to 0.18 cm 2 V −1 s −1 .

Influence of Solid-State Packing of Dipolar Merocyanine Dyes on Transistor and Solar Cell Performances

“…In Figure 3 the packing arrangements of representative dyes 1, 4 and 9 40 are shown. The data of all obtained crystal structures can be found in the SI ( Figure S9).…”

“…In accordance with an earlier study for another series of merocyanine dyes, the low mobility values can be attributed to the formation of amorphous thin films. 40 Moreover, the fact that dyes 6, 7 and 8 showed no field effect behavior on Si/SiO 2 substrates can be explained by the formation of small discontinuous droplet-like regions. 40 Hence, no percolation pathways are provided for charge carrier transport.…”

Structure–Property Relationships for Exciton and Charge Reorganization Energies of Dipolar Organic Semiconductors: A Combined Valence Bond Self-Consistent Field and Time-Dependent Hartree-Fock and DFT Study of Merocyanine Dyes

“…For the MD376 series structures 2, 3, 5, and 6 would be 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 3.3 Correlation between electronic structures and charge reorganization energies: Experimental results also seem to indicate a relationship between the cyanine limit and the charge transport efficacy. [3][4][5][6][7]…”

“…From these quantities the ܿ ଶ value can be calculated as [3][4][5][6][7][8][14][15][16][17][18][19][20][21][22][23] c 2 = ½ [1-∆μ (4(μ ag ) 2 + ∆μ 2 ) -1/2 ]…”

Head‐to‐Tail Zig‐Zag Packing of Dipolar Merocyanine Dyes Affords High‐Performance Organic Thin‐Film Transistors

“…Despiteof their successful use in bulk heterojunction (BHJ) solar cells,d ipolar donor-acceptor (DA) substituted psystems are still not considered as ideal organic semiconductors.T hus,t he success of merocyanines [1,2] and related DA dyes [3,4] in organic photovoltaics with power conversion efficiencies larger than 6% is more attributed to their outstanding absorption properties than to their hole transport capabilities.T os hed more light onto the latter we initiated aresearch program on the use of highly dipolar merocyanine dyes in organic thin-film transistors (OTFTs) [5] and dedicated particular efforts on the elucidation of packing arrangements of merocyanine dyes in the solid state. [6,7] This work has so far reached to the following insights:The most common arrangement of merocyanine dyes in the solid state consists of antiparallel (centrosymmetric) dimers at close van-der-Waals distance (d < 3.5 ), further packing of which is not easy to predict and may even lack additional p-contacts to neighbor molecules or show one-dimensional p-stacking,a lbeit with larger distances to the second neighbor (p-stack with alternating p-contacts,see Figure 1a,left). [8,9] Closer inspection of adozen of such packing motifs [10] suggests that most typically the closest neighbor interaction is optimized at the expense of the next neighbor.A ccordingly one very close and electrostatically highly favorable contact is formed and any flexible substituents are directed towards the other face,t hereby reducing the chance for close contacts to the other neighbor molecules as required for the most desirable two-dimensional brickwork or at least an equidistant one-dimensional columnar packing arrangement.…”