Organic light-emitting transistors with an efficiency that outperforms the equivalent light-emitting diodes

Organic light-emitting transistors with an efficiency that outperforms the...

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We demonstrated the advantages of using an OLET versus an OLED configuration, and enabled OLETs with the highest efficiency reported so far. The new trilayer heterostructure fieldeffect concept unravels the full potential of the light-emitting field-effect technology and restricts the limitation of OLEDs to only materials-related issues. Improvements in the top-layer field-effect mobilityathighcurrentdensitycoupledtotheuseoftripletemitters willenableOLETswithevenhigherEQEandbrightness.Inaddition to the avoidance of the deleterious exciton–charge quenching and injection-electrode photon losses that we demonstrated in this article, our OLET architecture may also guarantee unprecedented light-extraction efficiencies by using ultrathin layers of ITO or Au as gate electrodes28. Moreover, light-outcoupling structures can be easily implemented to extract light from the contact-free top part of the device. The absorption of the generated photons by the charges accumulated within the device is minimized as the thickness of the accumulation layer above the emission layer is only a few nanometres. OLETs also offer an easily processed device architecture that naturally avoids pinholes and shorts between injection contacts, which are among the main technological problems faced by OLEDs.

A critical parameter to be addressed is the device operating voltage. The power efficiency at a given voltage is an essential figure of merit of any light-emitting devices. Present OLETs exhibit their maximum optical power at operating biases of 50–70V. However,farloweroperatingvoltagescouldbeachievedusinghighcapacitance gate insulators46,47. Indeed, we demonstrated FETs based on the semiconductors used in this study operating at |1–4|V, and their implementation in our OLETs is in progress.


Furthermore, the EQE of our OLETs is maximum for balanced electron and hole currents and is, therefore, achieved in a narrow voltage range. However, despite the technical improvements that are needed, we believe that our trilayer OLETs represent a viable route towards practical organic light-emitting devices with unprecedented performance.


Device fabrication. Trilayer heterostructure OLETs were fabricated in the top-electrode configuration (see Fig. 1). The substrates consisted of 1 inch square glass coated with a 150-nm-thick layer of ITO that worked as the gate electrode. A 450-nm-thick PMMA film was spin-coated on the substrate in a nitrogen glove box and annealed for 12h at 120 C. The films composing the trilayer heterostructure were grown by sublimation in high vacuum, at a base pressure of 5×10 mbar, in a home-made chamber directly connected to the nitrogen glove box to prevent sample exposure to air during each step of the device realization. For the first OLET configuration the thicknesses of DFH-4T, Alq :DCM and DH-4T layers were optimized to 7, 40 and 15nm respectively. The growth rate was fixed at 0.1Ås for both DFH-4T and DH-4T thin films, whereas the Alq and DCM molecules were co-sublimed at different growth rates (2Ås for Alq and 3Åmin for DCM) to guarantee a DCM weight concentration of about 3% in the blend. The Au electrode thickness was 50nm. For the reversed OLET configuration the thicknesses of the bottom (DH-4T), central (Alq :DCM) and DFH-4T layers were 7, 40 and 25nm respectively. The top electrodes were made of LiF/Al (1.2/50nm). For both configurations the channel length and width were 150µm and 20cm, respectively. Devices were encapsulated in the glove box using a glass coverslip and an ultraviolet-cured epoxy sealant. Devices with a channel length of 50µm were also measured (see Supplementary Fig. S6) and yielded similar EQE values.

Optoelectronic characterization. Optoelectronic characterization of OLETs and OLEDs was carried out using a SUSS probe station, adapted to carry out optoelectronic investigations, coupled to a B1500A Agilent semiconductor device analyser. An S1337 silicon photodiode (Hamamatsu) with a sensitivity of 0.38AW at 600nm was placed directly in front of the devices and used for simultaneouslight-intensitymeasurements.FortheEQEmeasurementstheemitted photons were collected through an Avantes AVA-SPHERE 50-IRR integrating sphere and measured by an Avantes AVA-SPEC 2048 calibrated spectrometer. The EQE was calculated directly as the ratio between the total emitted photons and the charge flow that formed the drain current. Photoluminescence spectra of the trilayer OLET devices were collected in transmission mode by a Hamamatsu multichannel optical analyser (PMA11) after excitation of the device active area with the 375nm emission of an Oxxius laser diode. Electroluminescence spectra of devices biased using a B1500A Agilent semiconductor device analyser were acquired by a CS200 Konica Minolta spectro-radiometer.

Morphology investigations and optical imaging. CLSM images of the single-layer device channel were carried out with a Nikon Eclipse 2000-E laser scanning confocal microscope. The CLSM images were obtained by exciting the sample with the 485nm emission line of an Ar laser and collecting the photoluminescence emission through the glass substrate with a ×60 magnification objective with 0.7 numerical aperture. Optical micrographs of the OLET device channel and recombination region were taken with the same Nikon Eclipse 2000-E microscope, using a sample holder equipped with electrical connections that placed the optoelectronic device in the focal plane of the microscope. Optical detection was achieved using ×60 or ×90 magnification objectives and a Hamamatsu high-resolution digital camera. AFM topographical images were collected using an NT-MDT Solver Scanning Probe Microscope in the tapping mode with the samples kept in air.

Received 5 November 2009; accepted 17 March 2010; published online 2 May 2010


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Authors kindly acknowledge R. Zamboni, G. Ruani and T. J. Marks for useful discussions, as well as the valuable technical support of M. Murgia. Financial support from Italian MIUR projects FIRBRBIP06YWBH (NODIS), and FIRB-RBIP0642YL (LUCI), Italian MSE project Industria 2015 (ALADIN), and EU projects PF6 035859-2 (BIMORE) and FP7-ICT- 248052 (PHOTO-FET) is acknowledged.

Author contributions

R.C. defined the concept of the trilayer heterostructure, fabricated devices, executed optoelectronic experiments, analysed and interpreted results. S.T. defined the concept of the trilayer heterostructure, executed spectroscopic and photonic experiments, analysed and interpreted results. G.G. carried out AFM measurements, contributed to fabricate devices and to execute optoelectronic experiments. H.U. synthesized DH-4T and DFH-4T. A.F. supervised the synthesis and discussed the results. M.M. defined the concept of the trilayer heterostructure, took part to the key experiments, interpreted results and supervised the entire work. A.F. and M.M. wrote the manuscript.

Additional information

The authors declare no competing financial interests. Supplementary information accompanies this paper on Reprints and permissions information is available online at Correspondence and requests for materials should be addressed to R.C. or M.M.

(Parte 3 de 3)