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

Raffaella Capelli1*, Stefano Toffanin1, Gianluca Generali1, Hakan Usta2, Antonio Facchetti2 and Michele Muccini1,3*

The potential of organic semiconductor-based devices for light generation is demonstrated by the commercialization of display technologies based on organic light-emitting diodes (OLEDs). Nonetheless, exciton quenching and photon loss processes still limit OLED efficiency and brightness. Organic light-emitting transistors (OLETs) are alternative light sources combining, in the samearchitecture,theswitchingmechanismofathin-filmtransistorandanelectroluminescentdevice.Thus,OLETscouldopen a new era in organic optoelectronics and serve as testbeds to address general fundamental optoelectronic and photonic issues. Here, we introduce the concept of using a p-channel/emitter/n-channel trilayer semiconducting heterostructure in OLETs, providing a new approach to markedly improve OLET performance and address these open questions. In this architecture, exciton–charge annihilation and electrode photon losses are prevented. Our devices are >100 times more efficient than the equivalentOLED,>2×moreefficientthantheoptimizedOLEDwiththesameemittinglayerand>10timesmoreefficientthan any other reported OLETs.

Organic semiconductor-based devices such as OLEDs, solar cells, memories and field-effect transistors (OFETs) are predicted to reduce fabrication costs and enable new functions1–6. The OLET (Fig. 1) is another optoelectronic device having the structure of a thin-film transistor and the capability of light generation7. Bright/multicolour OLETs may allow electroluminescent display fabrication with simpler driving circuits. Furthermore, the most advanced OLETs possess a huge technological potential for the realization of intense nanoscale light sources and highly integrated optoelectronic systems, including the long-researched electrically pumped organic laser8–16. In terms of performance and reliability, OLED technology is by far the most developed, and active matrix OLED displays have been introduced into the market. However, detrimental device-related processes affecting OLED operation under high-injection conditions are the exciton–charge interactions and the photon losses at the electrodes. The proximity (within tens of nanometres) of the contacts to the OLED light-generation region induces losses owing to the absorption of the emitted photons. Moreover, the highly dense electron and hole currents converge to the light-emitting layer, where they form, but spatially coexist with the excitons, and lead to significant exciton–charge quenching17–20. Indeed, this mechanism is predicted to be greater than any other quenching effects21 and should be controlled to enhance the OLED efficiency, brightness and stability even further. Thus, the focus of OLET development is the possibility to enable new display/light source technologies, and exploit a transport geometry to suppress the deleterious photon losses and exciton quenching mechanisms inherent in the OLED architecture. So far, only the exciton–metal interaction has been successfully addressed in OLETs based on ambipolar single layers22–27. Under proper bias conditions, the spatial location of

1Consiglio Nazionale delle Ricerche (CNR), Istituto per lo Studio dei Materiali Nanostrutturati (ISMN), via P. Gobetti 101, I-40129 Bologna, Italy, 2Polyera Corporation, 8045 Lamon Avenue, Skokie, Illinois 60077, USA, 3E.T.C. srl, via P. Gobetti 101, I-40129 Bologna, Italy. *e-mail:r.capelli@bo.ismn.cnr.it; m.muccini@bo.ismn.cnr.it.

the light-emitting area is far from the metal electrodes, preventing exciton–metal quenching. However, in these single-layer devices the charge carrier accumulation and the exciton formation zones largely coincide, leading to severe exciton–charge quenching. Indeed,eveninthemostimpressivedemonstrationsofar,theexternal quantum efficiency (EQE) does not exceed 0.5–0.6% (ref. 28). Consistently, single-layer unipolar OLETs, in which only one type of charge carrier is effectively transported across the channel, reached remarkable results in terms of brightness29,30, but their EQE is only 0.2%, mainly because excitons are subjected to both metal and charge quenching, and electrode-induced photon losses29. A horizontal p–n-heterojunction OLET has also been reported, in which light emission is confined far from the contacts; however, exciton–charge quenching is not avoided31. A bilayer approach has been used to improve OLET brightness, or achieve higher and more balanced charge transport. In the first case a highly efficient luminescent layer is superimposed over a unipolar conducting layer32,whereasinthesecondcasep-typeandn-typetransportfilms are directly in contact with each other33. However, in both cases this device architecture does not offer any control of the exciton quenching and photon losses, as the light-emitting area is in contact with the minority carrier injection electrode. Excitons interact with accumulated charges and the metal electrode, whereas photons are absorbed by the contacts. In this work, we report the first trilayer heterostructure approach for OLETs enabling simultaneous control of electrode-induced photon losses, and exciton–metal and exciton–charge interactions. OLET devices with EQEs of 5% are demonstrated, which exceeds the best OLEDs based on the same emittinglayerandoptimizedtransportlayers(2.2%;ref.34).

The trilayer heterostructure OLETs used in this study (Fig. 1a) were fabricated on glass/indium tin oxide (ITO; gate contact,

NATUREMATERIALSDOI:10.1038/NMAT2751 ARTICLES

Glass

AuDCMAlq3 ¬3.3 eV

DFH-4T ¬5.8 eV

DH-4T

¬2.89 eV ¬3.5 eV

Drain (Au)

Dielectric (PMMA) Gate (ITO)

¬5.8 eV ¬5.6 eV

¬6.2 eV

¬5.1 eV

O ON Al

Alq3:DCM

DFH-4T

DH-4T S

¬3.0 eV

Figure 1 | Trilayer OLET device structure and active materials forming the heterostructure. a, Schematic representation of the trilayer OLET device with the chemical structure of each material making up the device active region. The field-effect charge transport and the light-generation processes are also sketched. b, Energy-level diagram of the trilayer heterostructure. The energy values of the HOMO and LUMO levels of each molecular material are indicated together with the Fermi level of the gold contacts.

150nm)/PMMA (dielectric, 450nm) substrates. The active region consists of the superposition of three organic layers. The first, in contact with the PMMA dielectric, and the third layers are field-effect electron-transporting (n-type, 7 nm) and holetransporting (p-type, 15nm) semiconductors, respectively, whereas the middle layer is a light-emitting host–guest matrix (40nm). The device structure is completed by the deposition of the gold (source and drain, 50nm) contacts. To enable the vertical charge diffusion process, the basis of the OLET electroluminescence mechanism, energetic compatibility between the materials forming the heterostructure is required. The lowest unoccupied molecular orbital (LUMO) of the n-type transport layer should be equal to or higher than the LUMO of the guest matrix in the central layer, whereas the highest occupied molecular orbital (HOMO) of the p-type transport layer should be equal to or lower than the guest matrix HOMO level. Furthermore, the morphology of these films must allow the formation of a continuous multistack. Meeting these requirements is not trivial and, after several attempts, we identified the α,ω-disubstituted-quaterthiophenes with hexyl (DH-4T, Polyera ActivInk P0400) and perfluorohexyl (DFH-4T, Polyera ActivInk N0700) chains as the hole and the electron transporting films, respectively35. To realize the central light-formation layer a blend of host tris(8-hydroxyquinolinato)aluminium (Alq3) and guest4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-

4H-pyran (DCM) was used. Figure 1 also shows the chemical structure and the electrochemically derived energy-level diagram of the organic heterostructure, clearly showing the energetic compatibility of these materials.

As part of the preliminary study, single-layer OFETs based on

DFH-4TandDH-4Tonglass/ITO/PMMAsubstrates(Aucontacts) were fabricated. They exhibited electron and hole mobilities of 0.5cm2 V−1 s−1 and 0.08cm2 V−1 s−1, respectively, 2–5 times larger than those reported on the Si/SiO2 and Al/PMMA substrates35,36.

Atomic force microscopy (AFM) of the films on Si/SiO2 substrates evidenced a bidimensional layer-by-layer film growth, resulting in

uniformandflatsurfacemorphologies35,37–40.Importantly,confocal laser scanning microscopy images (CLSM, see Supplementary Fig. S2) indicate that similar flat surface morphologies are obtained on glass/ITO/PMMA substrates although the DH-4T domains are significantly larger than those of DFH-4T. This condition is essential for the realization of our multilayer OLETs (see below). DFH-4T/DH-4T bilayer OFETs were also fabricated(SupplementaryFig.S3a)toevaluatethechargetransport properties of these bilayer heterostructures. The FET characteristics (Supplementary Fig. S3b,c) demonstrate an ambipolar behaviour from these bilayer devices. The good electrical properties of these oligothiophene FETs, coupled with the possibility to control the growth morphology, make them good candidates for trilayer heterostructure OLET fabrication.

Figure 2 shows an optical image of a lit trilayer OLET

(L = 150µm and W = 20cm) as well as a zoom of the OLET channel with the light generated within it. Differently from the bilayer FET, the trilayer OLET generates light when switched on. In Fig. 2c, the OLET photoluminescence and electroluminescence spectra are compared. The central emission peak is positioned at 600nm in both cases and corresponds to the DCM emission. In the electroluminescence spectrum a shoulder appears at about 530nm that can be attributed to the residual Alq3 emission44. The OLET optoelectronic characteristics are reported in Fig. 3.

The curves shown in Fig. 3a and b were obtained by operating the device within the unipolar regime (|VDS| = |VGS|) and correspond to the field-effect transport of only electrons and holes, respectively.

Thus,Fig. 3ashowsthechargetransporttakingplaceintheDFH-4T layer and Fig. 3b shows the charge transport occurring in the DH-4T film. Light emission is collected in correspondence of the electron transport (Fig. 3a). When charge carriers recombine at the drain electrode through one, or more, upper layers, a diode-like mechanism gives rise to light emission32,3. This process is characterized by a linear correlation between the electroluminescence and current intensity, clearly observable in Fig. 3a, and by the spatial localization of the emission region at the drain electrode region. A large difference between the hole and the electron currents is observed in this OFET structure. Although the electron mobilities are comparable to those of the single-layer

ARTICLES NATUREMATERIALSDOI:10.1038/NMAT2751

Source

Drain Drain 25 µm

Normalized EL/PL (a.u.) a b c PL EL

Figure 2 | Optical micrographs of the lit trilayer OLET and its emission spectra. a, Optical micrograph of the interdigitated trilayer heterostructure

OLET biased with VDS =VGS =90V. Channel length and channel width are 150µm and 20cm, respectively. b, Optical micrograph of the OLET channel when no bias is applied to the device, and when the applied bias is

VDS =VGS =90V. The schematic representation of the trilayer heterostructure OLET showing the expected location of the light-generation area is reported in the inset. c, Comparison between the electroluminescence (EL) and photoluminescence (PL) spectra of the trilayer heterostructure OLET.

the DH-4T films strongly affects the transport properties of the top DH-4T layer.

AFM images carried out on each layer of the heterostructure

(Fig. 3d–f) account for the degraded hole transport in these OLETs. Figure 3e shows that DFH-4T films vapour-deposited on the PMMA surface form two-dimensional islands which then coalesce completely in the first monolayer. An incipient three-dimensional growth with the formation of thick elongated needles is also present in the 7-nm-thick film. Figure 3f shows the morphology of a 40-nm-thick Alq3:DCM film grown on the previous DFH-4T layer. The surface of this layer is composed by three-dimensional globular aggregates with 100–300nm diameters. Clearly the surface roughness and the presence of voids and protuberances partially prevent the layer-by-layer growth of the 20-nm-thick DH-4T film on the optical layer (Fig. 3f). The DH-4T film topology mirrors that of the underlying Alq3:DCM layer, resulting in an inhomogeneous and poorly connected film that limits transport efficiency45.

When the device bias conditions allow simultaneous charge injection from both the source and drain electrodes (|VGS|<|VDS|) the OLET is in the ambipolar operation regime, representative transfer curves of which are reported in Fig. 3c (linear scale). The strong unbalanced transport in the DFH-4T/Alq3:DCM/DH-4T heterostructures hinders the ‘V’ shape characteristic of ambipolar transistors operated in transfer mode. Consequently, the ambipolar (Fig. 3c) and unipolar (Fig. 3a) electrical transfer curves are very similar. However, despite the current similarity, the comparison between Fig. 3a and c clearly shows that a new mechanism of electroluminescence generation is taking place in these OLETs when operated in the ambipolar regime. The shaded area in Fig. 3c highlights the electroluminescence produced by the ambipolar current.

The optical micrographs of the device channel (Fig. 4) show the position of the light-emitting region with respect to the edge of the drain electrode as a function of the applied voltages. The first frame (Fig. 4a) shows the optical image of the reference device channel, where the drain electrode edge is clearly recognizable and marked by a yellow line. It can be observed in Fig. 4b that in the ambipolar operation mode the narrow light-emitting stripe is located far from the drain electrodes, at a distance of ∼8µm. By increasing the gate voltage, the emitting region broadens and shifts towards the drain electrode. Figure 4 therefore confirms that the ambipolar light-formation process takes place well inside the channel, far from the electrodes, thus preventing photon losses at the injection electrodes and the exciton–metal quenching. Moreover, as in the trilayer structure the light-emitting layer is physically separated from the charge flows, exciton–charge quenching is also simultaneously prevented. The light-generation process is based on the charge percolation of electrons and holes moving from the respective transport layers to the Alq3:DCM layer, where excitons are formed. The transverse electric field generated by electron and hole accumulation in the DFH-4T and DH-4T films, respectively, promotes charge percolation. The nature of the force driving this process, which is correlated to the electron–hole annihilation capability of the central recombination film, leads to a self-regulated equilibrium between the amount of charges located in the transport layers and those entering the optical layer. It is known that in ambipolar OLETs, where the recombination occurs well within the channel, all injected electrons and holes must recombine, because the charges cannot move through several micrometres of an accumulation layer of opposite charge without recombining23. In the trilayer OLET this holds true for the charges entering the recombination layer. It is expected that exciton–charge quenching is prevented in the trilayer OLET because: (1) there is no overlap of the opposite charge accumulation layers; (2) the recombination layer has a thickness of 40nm, which decreases the spatial density of excitons and charges; and (3) excitons do not interact with trapped charges eventually localized at the dielectric interface that are likely to be luminescence quenching sites28.

A quantitative analysis of the optical properties of the trilayer

OLETwithoutconsideringanyexciton–chargequenching(seeSupplementary Information) shows that the OLET light outcoupling efficiency is ∼27%, which is 30% higher than that of the typical OLED structure (20%). This finding is in agreement with the avoidance of losses at the metal cathode. Any sizable exciton–charge quenching in the trilayer OLET would result in outcoupling efficiencies exceeding 30%, which is unlikely because of the losses at the ITO/PMMA structure. Therefore, the trilayer OLET does not show any sizable exciton–charge quenching and can be regarded as a contactless OLED where exciton–charge quenching is intrinsically prevented. Despite the degraded mobility of the hole minority carriers, the maximum EQE (Fig. 5a) of ambipolar OLETs in this trilayer configuration is higher than 1% and is greater than the maximum EQE estimated for an ideal single-layer OLET (ref. 28). Note that in determining the EQE of the trilayer OLET we did not introduce any corrections related to the device geometry, but calculated the efficiency directly as the ratio between the total

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EL intensity (nW)

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