1. Electrical characterization Optoelectronic characterization
Thieno(bis)imide-based semiconductor as active multifunctional material in single layer ambipolar light emitting transistors
E. Benvenuti1, S. D. Quiroga1, M. Natali1, C. Bettini2, G. P. Donati1, E. Bonaretti1, M. Melucci3, S. Toffanin1 and M. Muccini1,4
1CNR-ISMN, Istituto per lo Studio dei Materiali Nanostrutturati, Consiglio Nazionale delle Ricerche, Via Gobetti 101, Bologna, Italy
2Laboratory MIST E-R, via P. Gobetti 101, Bologna, Italy
3CNR-ISOF, Istituto per lo Sintesi Organica e la Fotoreattività, Consiglio Nazionale delle Ricerche, Via Gobetti 101, Bologna, Italy
4ETC srl, Via Gobetti 101, Bologna, Italy
Introduction
Organic molecular semiconductors are key components for a new generation of low cost, flexible, and large area electronic devices. In particular, ambipolar semiconductors endowed with electroluminescent properties have the potential to enable a photonic field-effect technology platform, whose key building blocks are the emerging organic light-emitting transistor (OLET) devices [1].
To this aim, the design of innovative molecular compounds combining effective electrical and optical properties in thin-films is highly desirable.
Fig. 2 Molecular structure of NT4T
Recently, we report the implementation of a novel oligothiophene derivative bearing 2,3-thienoimide symmetric ends (2,20-(2,20-bithiophene-5,50-diyl)bis(5-butyl-5H-thieno[2,3-c]pyrrole-4,6)-dione, NT4N, as active material in both Organic Thin Film Transistors (OTFT) and OLETs.[4,5].
The newly developed material enabled the fabrication of single layer molecular ambipolar OLETs with optical power comparable to that of the equivalent polymeric single layer devices based on poly(9,9-dioctylfluorenealt-benzothiadiazole) (F8BT).[2]
Fig. 1 Schematic of an organic light-emitting field-effect transistor
Experimental
The devices were fabricated by vacuum sublimation on top of a Glass/ITO/PMMA substrate.
A layer of PMMA was deposited by spin-coating thecnique as a dielectric layer .
Organic material and gold electrodes were depositated for sublimation in hight vacuum chamber (10-7mbar).
Fig. 3 Schematic structure of a single layer bottom gate-top electrodes OLET
(a)
(b)
The DTF calculation shows that both the HOMO and LUMO energy levels are delocalized over the entire molecule.
The 2,3 thienoimide symmetric end-sostitution promoted π-π stacking paking motif, which is commonly related to enanced charge transport capability, instead of the herring bone structure which is commonly found in oligothiophenes.
Fig. 4 (a) Isodensity plots of the HOMOs (down) and LUMO (top) of NT4N. (b) Arbitrary view of the crystal packing showing the π-π stacks
Electrical characterization
Optoelectronic characterization
The electrical characterization shows an ambipolar behavior.
During the electrical characterization of OFETs, intense electroluminescence emission from the devices was also measured, highlighting the potential of these ambipolar materials for single layer ambipolar OLETs devices. The emission accours inside the channel, closer to the drain electrode as a consequence of the difference between hole and electron mobilities. However, the electroluminescence generation area is well separated from the drain edge, preventing optical coupling of the emitted light with the metal electrodes.
The insertion of the thieno(bis)immide switches the tipycal unipolar p-type behaviour of the oligothiophene semiconductor to ambipolar and electroluminescence materials
The electron contribution is the most relevant as the high value of the N-type mobility demonstrates.
(a)
The EL spectrum and the PL spectrum are very similar, confirming that the EL emission originates from NT4N.
The maximum EQE relevant value is about 0.2%, at 50V.
Electrical parateres
(b)
(c)
(a)
μN (cm2/Vs)
8,13*10-2
μP (cm2/Vs)
1,07*10-4
VtN (V)
49,2
VtP (V)
-68,8
Ion/Ioff
105
(b)
Fig. 6 (a) EQE eficiency curve for NT4N based OLET. The blue line is the drain current, the purple line is the EQE (b) Electroluminescence spectrum of OLET based on NT4N (red line) compared to the PL spectrum (blue line) (c) Image of a device in the OFF state and in the ON state (d) Optical microscope image of a working NT4N.
(d)
Fig. 5 (a) Electrical characteristic of a single layer OFET based on NT4N (a) n-type transfer saturation curve (b) n-type multiple output curves
Conclusion
Reference
[1] Muccini, M.; Koopman, W.; Toffanin, S. Laser Photonics Rev. 2012, 6, 258.
[2] Gwinner, M. C.; Kabra, D.; Roberts, M.; Brenner, T. J. K.; Wallikewitz, B. H.; McNeill, C. R.; Friend, R. H.; Sirringhaus, H. Adv. Mater. 2012, 24, 2728.
[3] Bao, Z.; Locklin, J. Organic Field-Effect Transistors; CRC Press: New York, 2007; p 139.
[4] Melucci, M.; Zambianchi, M.; Favaretto, L.; Gazzano, M.; Zanelli, A.; Monari, M.; Capelli, R.; Troisi, S.; Toffanin, S.; Muccini, M. Chem. Commun. 2011, 47,
[5] Melucci M., Favaretto L., Zambianchi M., Durso M., Gazzano M., Zanelli A., Monari M., Lobello M.G., De Angelis F., Biondo V., Generali G., Troisi S., Koopman W., Toffanin S., Capelli R., Muccini M. Chem. Mater. 2013, 25, 668.
The insertion of the thieno(bis)immide group into linear thiophene oligomers leads to an orbital distribution and molecular packing pattern able to promote ambipolar charge transport and electroluminescence in the result semiconductors.
The optical power of our NT4N-based OLETs is comparable to that of the state of art ambipolar OLET based on F8BT polymer (taking into account the specific geometry used for our device).
These results demostrates the suitability of the small molecular approach for the development of ambipolar single layer OLETsdevices.