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, 40129 Bologna, Italy
2Laboratory MIST E-R, via P. Gobetti 101, 40129 Bologna, Italy
3CNR-ISOF, Istituto per lo Sintesi Organica e la Fotoreattività, Consiglio Nazionale delle Ricerche, Via Gobetti 101, 40129 Bologna, Italy
4ETC srl, Via Gobetti 101, 40129 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 lightemitting 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).
(b)
(a)
Fig. 3 Schematic structure
of a single layer bottom
gate-top electrodes OLET
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.
(a)
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 Ntype mobility demonstrates.
4
10
3
10
VDS= 100 V
2
10
IDS (A)
1
10
0
10
-1
10
-2
10
-3
10
-4
10
0
20
40
60
80
100
VGS (V)
Electrical parateres
μN (cm2/Vs)
8,13*10-2
μP (cm2/Vs)
1,07*10-4
80
VtN (V)
49,2
60
VtP (V)
-68,8
Ion/Ioff
105
VGS=0,20,40,60,80,100 V
(b)
120
IDS (A)
100
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 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.
(a)
(b)
(c)
40
20
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)
0
0
20
40
60
80
100
VDS (V)
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
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.
[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, 11840.
[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 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.