Baixe Organic light-emitting transistors with an efficiency that outperforms the equivalent light-emitting diodes e outras Notas de estudo em PDF para Engenharia Elétrica, somente na Docsity! ARTICLES PUBLISHED ONLINE: 2 MAY 2010 | DOI: 10.1038/NMAT2751 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 same architecture, the switching mechanism of a thin-film transistor and an electroluminescent device. Thus, OLETs could open 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 equivalent OLED, >2×more efficient than the optimized OLED with the same emitting layer and >10 times more efficient than any other reported OLETs. Organic semiconductor-based devices such as OLEDs, so-lar 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 elec- troluminescent display fabrication with simpler driving circuits. Furthermore, the most advanced OLETs possess a huge tech- nological 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 pho- ton 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. In- deed, even in themost impressive demonstration so far, the external 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, whereas in the second case p-type and n-type transport films 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 emitting layer and optimized transport layers (2.2%; ref. 34). The trilayer heterostructure OLETs used in this study (Fig. 1a) were fabricated on glass/indium tin oxide (ITO; gate contact, 496 NATURE MATERIALS | VOL 9 | JUNE 2010 | www.nature.com/naturematerials © 2010 Macmillan Publishers Limited. All rights reserved. NATURE MATERIALS DOI: 10.1038/NMAT2751 ARTICLES Source (Au) h+ e¬ Glass AuDCM Alq3 ¬3.3 eV DFH-4T ¬5.8 eV DH-4T ¬2.89 eV ¬3.5 eV Au Drain (Au) Dielectric (PMMA) Gate (ITO) a b ¬5.8 eV ¬5.6 eV ¬6.2 eV ¬5.1 eV O ON Al : Alq3:DCM H3C CH3 NC CN CH3 N3 S S S S F F F F F F F F F F F F F F F F F F F F FFFFFF DFH-4T S S S 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. 150 nm)/PMMA (dielectric, 450 nm) substrates. The active re- gion consists of the superposition of three organic layers. The first, in contact with the PMMA dielectric, and the third lay- ers are field-effect electron-transporting (n-type, 7 nm) and hole- transporting (p-type, 15 nm) semiconductors, respectively, whereas the middle layer is a light-emitting host–guest matrix (40 nm). The device structure is completed by the deposition of the gold (source and drain, 50 nm) contacts. To enable the ver- tical charge diffusion process, the basis of the OLET electro- luminescence 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 occu- pied 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 re- quirements is not trivial and, after several attempts, we identi- fied 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 guest 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)- 4H-pyran (DCM) was used. Figure 1 also shows the chemical struc- ture 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-4T and DH-4T on glass/ITO/PMMA substrates (Au contacts) were fabricated. They exhibited electron and hole mobilities of 0.5 cm2 V−1 s−1 and 0.08 cm2 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 uniform and flat surfacemorphologies35,37–40. 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 (Supplementary Fig. S3a) to evaluate the charge transport properties of these bilayer heterostructures. The FET characteristics (Supplementary Fig. S3b,c) demonstrate an ambipolar behaviour with a state-of-the-art balanced hole (0.01 cm2 V−1 s−1,VTP = +40V) and electron (0.01 cm2 V−1 s−1,VTN=−40V) transport and no hysteresis33,41–43. Importantly, no light emission was collected 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 = 150 µm and W = 20 cm) 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 600 nm in both cases and corresponds to the DCM emission. In the electroluminescence spectrum a shoulder appears at about 530 nm 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. 3a shows the charge transport taking place in theDFH-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 emission32,33. 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 DFH-4T OFETs (0.5 cm2 V−1 s−1,VTN = +34V, Supplementary Fig. S1), the hole mobilities are severely degraded with respect to the DH-4T OFETs (5×10−5 cm2 V−1s−1,VTP =−40V). Thus, the insertion of the third Alq3:DCM layer between the DFH-4T and NATURE MATERIALS | VOL 9 | JUNE 2010 | www.nature.com/naturematerials 497 © 2010 Macmillan Publishers Limited. All rights reserved. ARTICLES NATURE MATERIALS DOI: 10.1038/NMAT2751 Drain Drain Drain Drain VGS = 0 V VGS = +30 V VDS = +90 V VDS = +90 V VGS = +90 V VDS = +90 V VGS = +60 V VDS = 0 V 10 µm a c d b Figure 4 | Images of the light-emitting area within the OLET device channel. a, For reference, an optical micrograph of the device channel without bias, to highlight the position of the drain electrode edge that is marked with a yellow line. b–d, Optical micrographs of the emission zone within the device channel of the trilayer heterostructure OLET during a transfer scan at VDS=90 V and VGS values of 30 V (b), 60 V (c) and 90 V (d). Three arrows in b–d indicate the initial position of the recombination and emission zone. ITO Glass ITO Au Au Glass DH-4T Al/LiF Alq3:DCM 3% DHF-4T PMMA ¬4.8 ¬5.2 ¬5.6 ¬6.0 ¬6.4 ¬6.8 Lo g (I D S) Lo g (I D S) VGS (V)VGS (V) 0 20 40 60 80 100 ¬3.0 ¬3.5 ¬4.0 ¬4.5 ¬5.0 ¬5.5 ¬6.0 Al/LiF DHF-4T Alq3:DCM 3% DH-4T PMMA EQ E (% ) EQ E (% ) 0 20 40 60 80 5 4 3 1 0 2 a b 1.0 0.8 0.6 0.4 0.2 0 Figure 5 | EQE as a function of the applied gate voltage for the two trilayer heterostructure OLET configurations. a, The bottom layer and the top layer are thin films of DHF-4T and DH-4T, respectively. b, The layer configuration of a is reversed. The transfer curves with the drain–source current (IDS) plotted on a logarithmic scale are also reported. IDS is measured keeping the drain-source potential constant at 90 V, while sweeping the gate-source potential from 0 to 90 V. is decreased by a factor of 10 with respect to the previous case, as a result of the large threshold voltage (>|50|V) for both charge types. However, our data demonstrate that the achievement of balanced ambipolar transport in devices with current densities similar to those observed in the first trilayer OLET, would enable OLET devices with simultaneous high efficiency and brightness. To enable a direct OLED versus OLET experimental com- parison we fabricated the device schematized in Fig. 6a, with the aim of implementing the OLET trilayer active region in an equivalent OLED structure. The layer sequence, thickness and film growth parameters are exactly those used for the OLET fabrication and the electrodes are ITO (coated with a poly(3,4- ethylenedioxythiophene) (PEDOT) layer, anode) and Au (cath- ode). As the workfunctions of ITO/PEDOT and Au are similar, the charge injection conditions in the OLED configuration mimics the OLET case where both the drain and source electrodes are made of gold. The optoelectronic characteristics of this device (Fig. 6b) fol- low a typical L–I–V OLED behaviour and exhibit a maximum EQE 500 NATURE MATERIALS | VOL 9 | JUNE 2010 | www.nature.com/naturematerials © 2010 Macmillan Publishers Limited. All rights reserved. NATURE MATERIALS DOI: 10.1038/NMAT2751 ARTICLES DH-4T Glass Glass Au (+) Alq3:DCM 3% DHF-4T PEDOT ITO (¬) Al/LiF (¬) DHF-4T Alq3:DCM 3% DH-4T PEDOT ITO (+) C ur re nt ( m A ) Voltage (V) 16 14 12 10 8 6 4 2 0 C ur re nt ( m A ) ¬10 ¬8 ¬6 ¬4 ¬2 0 C ur re nt ( m A ) ¬10 ¬8 ¬6 ¬4 ¬2 0 C ur re nt ( m A ) 120 100 80 60 40 20 0 EL intensity (nW ) EL intensity (nW ) 1,600 1,400 1,200 1,000 800 600 400 200 0 EQ E (% ) EQ E (% ) 4 6 8 10 12 14 Voltage (V) 16 14 12 10 8 6 4 2 0 Voltage (V) Voltage (V) a d b e fc 0 2 4 6 8 10 12 14 0 ¬2 ¬4 ¬6 ¬8 ¬10 ¬12 ¬14 ¬16 ¬18¬20 ¬2 ¬4 ¬6 ¬8 ¬10 ¬12 ¬14 ¬16 ¬18 ¬20 1.2 × 10¬2 1.0 × 10¬2 8.0 × 10¬3 6.0 × 10¬3 4.0 × 10¬3 2.0 × 10¬3 0 2 × 10¬3 1 × 10¬2 8 × 10¬3 6 × 10¬3 4 × 10¬3 0 Figure 6 |Device structure and optoelectronic characteristics of the trilayer OLED in direct and reverse configurations. a, Schematic structure of the trilayer OLED in the direct configuration. b, Optoelectronic characteristics of the OLED sketched in a. c, EQE of the direct heterostructure OLED. d, Schematic structure of the trilayer OLED in the reverse configuration. e, Optoelectronic characteristics of the OLED sketched in d. f, EQE of the reverse heterostructure OLED. of ∼0.012% (Fig. 6c). Therefore, the control over the quenching and loss mechanisms results in an organic electroluminescence- generating device with two orders of magnitude higher efficiency. Furthermore, we fabricated an OLED with the structure ITO/PEDOT/DH-4T/Alq3:DCM/DFH-4T/LiF/Al. In this reverse configuration, hole injection from ITO into the DH-4T hole transport layer, and electron injection from Al into the DFH-4T electron transport layer are optimized. The EQE of this OLED is <0.01% (see Fig. 6f), which is about 500 times lower than that of the corresponding OLET. Finally, the comparison of our OLET with a thoroughly optimized OLED based on Alq3:DCM as the emitting layer34, provides an important figure of merit to fully appreciate the advantages of the trilayer OLET configuration with respect to conventional OLEDs. Note that the reported EQE of an optimized OLEDbased on the same emitting layer is 2.2% (ref. 34). All of these data clearly indicate that the quenching and loss processes in our trilayer OLET devices are minimized. 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 field- effect 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 mobility at high current density coupled to the use of triplet emitters will enableOLETswith even higher EQE and brightness. In addition 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, far lower operating voltages could be achieved using high- capacitance 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. NATURE MATERIALS | VOL 9 | JUNE 2010 | www.nature.com/naturematerials 501 © 2010 Macmillan Publishers Limited. All rights reserved. ARTICLES NATURE MATERIALS DOI: 10.1038/NMAT2751 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. Methods 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 12 h at 120 ◦C. The films composing the trilayer heterostructure were grown by sublimation in high vacuum, at a base pressure of 5×10−7 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, Alq3:DCM and DH-4T layers were optimized to 7, 40 and 15 nm respectively. The growth rate was fixed at 0.1Å s−1 for both DFH-4T and DH-4T thin films, whereas the Alq3 and DCM molecules were co-sublimed at different growth rates (2 Å s−1 for Alq3 and 3Åmin−1 for DCM) to guarantee a DCM weight concentration of about 3% in the blend. The Au electrode thickness was 50 nm. For the reversed OLET configuration the thicknesses of the bottom (DH-4T), central (Alq3:DCM) and DFH-4T layers were 7, 40 and 25 nm respectively. The top electrodes were made of LiF/Al (1.2/50 nm). For both configurations the channel length and width were 150 µm and 20 cm, respectively. Devices were encapsulated in the glove box using a glass coverslip and an ultraviolet-cured epoxy sealant. Devices with a channel length of 50 µm 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−1 at 600 nm was placed directly in front of the devices and used for simultaneous light-intensitymeasurements. For the EQEmeasurements the emitted 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 375 nm 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 485 nm 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 References 1. Yan, H. et al. A high-mobility electron-transporting polymer for printed transistors. Nature 457, 680–687 (2009). 2. Forrest, S. R. The path to ubiquitous and low-cost organic electronic appliances on plastic. Nature 428, 911–918 (2004). 3. Malliaras, G. & Friend, R. H. An organic electronics primer. Phys. Today 58, 53–58 (2005). 4. Park, S. H. et al. Bulk heterojunction solar cells with internal quantum efficiency approaching 100%. Nature Photon. 3, 297–303 (2009). 5. Margapoti, E. et al. Excimer emission in single layer electroluminescent device based on [Ir(4,5-diphenyl-2-methylthiazolo)2 (5-methyl-1,10-phenanthroline)]+[PF6]−. J. Phys. Chem. C 113, 12517–12522 (2009). 6. Chua, L-L. et al. General observation of n-type field-effect behaviour in organic semiconductors. Nature 434, 194–199 (2005). 7. Muccini, M. A bright future for organic field-effect transistors. Nature Mater. 5, 605–613 (2006). 8. Hepp, A. et al. Light-emitting field-effect transistor based on tatracene thin film. Phys. Rev. Lett. 91, 157406 (2003). 9. Rost, C. et al. Ambipolar light-emitting organic field-effect transistor. Appl. Phys. Lett. 85, 1613–1615 (2004). 10. Takenobu, T. et al. High current density in light-emitting transistors of organic single crystals. Phys. Rev. Lett. 100, 066601 (2008). 11. Verlaak, S., Cheyns, D., Debucquoy, M., Arkhipov, V. & Heremans, P. Numerical simulation of tetracene light-emitting transistors: A detailed balance of exciton processes. Appl. Phys. Lett. 85, 2405–2407 (2004). 12. Gehlhaar, R., Yahiro, M. & Adachi, C. Finite difference time domain analysis of the light extraction efficiency in organic light-emitting field-effect transistors. J. Appl. Phys. 104, 331161–331165 (2008). 13. Santato, C. et al. Tetracene light-emitting transistors on flexible plastic substrates. Appl. Phys. Lett. 86, 1411061–1411063 (2005). 14. Cicoria, F. et al. Organic light-emitting transistors based on solution-cast and vacuum-sublimed films of a rigid core thiophene oligomer. Adv. Mater. 18, 169–174 (2006). 15. Capelli, R. et al. Investigation of the opto-electronic properties of organic light emitting transistors based on an intrinsically ambipolar material. J. Phys. Chem. C 112, 12993–12999 (2008). 16. Yuen, M-Y. et al. Semiconducting and electroluminescent nanowires self-assembled from organoplatinum(II) complexes. Angew. Chem. Int. Ed. 47, 9895–9899 (2008). 17. Yamamoto, H., Oyamada, T., Sasabe, H. & Adachi, C. Amplified spontaneous emission under optical pumping from an organic semiconductor laser structure equipped with transparent carrier injection electrodes. Appl. Phys. Lett. 84, 1401–1403 (2004). 18. Baldo, M. A., Holmes, R. J. & Forrest, S. R. Prospects for electrically pumped organic lasers. Phys. Rev. B 66, 035321 (2002). 19. List, E. J. W. et al. Interaction of singlet excitons with polarons in wide band-gap organic semiconductors: A quantitative study. Phys. Rev. B 64, 155204 (2001). 20. Staudigel, J., Stößel, M., Steuber, F. & Simmerer, J. A quantitative numerical model of multilayer vapor-deposited organic emitting diodes. J. Appl. Phys. 86, 3895–3910 (1999). 21. Gärtner, C., Karnutsch, C. & Lemmer, U. The influence of annihilation processes on the threshold current density of organic laser diodes. J. Appl. Phys. 101, 231071–231079 (2007). 22. Swensen, J. S., Soci, C. & Heeger, A. J. Light emission from an ambipolar semiconducting polymer field-effect transistor. Appl. Phys. Lett. 87, 253511 (2005). 23. Zaumseil, J., Friend, R. H. & Sirringhaus, H. Spatial control of the recombination zone in an ambipolar light-emitting organic transistor. Nature Mater. 5, 69–74 (2006). 24. Zaumseil, J., Donley, C. L., Kim, J-S., Friend, R. H. & Sirringhaus, H. Efficient top-gate, ambipolar, light-emitting field-effect transistors based on a green-light-emitting polyfluorene. Adv. Mater. 18, 2708–2712 (2006). 25. Bisri, S. Z. et al. High mobility and luminescent efficiency in organic single-crystal light-emitting transistors. Adv. Funct. Mater. 19, 1728–1735 (2009). 26. Wang, Y., Kumashiro, R., Nouchi, R., Komatsu, N. & Tanigaki, K. Influence of interface modifications on carrier mobilities in rubrene single crystal ambipolar field-effect transistors. J. Appl. Phys. 105, 124912 (2009). 27. Schidleja, M., Melzer, C. & Seggern, H. Electroluminescence from a pentacene based ambipolar organic field-effect transistor. Appl. Phys. Lett. 94, 123307 (2009). 28. Zaumseil, J. et al. Quantum efficiency of ambipolar light-emitting polymer field-effect transistors. J. Appl. Phys 103, 064517 (2008). 29. Ke, T-H. et al. High efficiency blue light emitting unipolar transistor incorporating multifunctional electrodes. Appl. Phys. Lett. 94, 1533071–1533073 (2009). 30. Namdas, E. B. et al. Gate-controlled light emitting diodes. Adv. Mater. 20, 1321–1324 (2008). 31. Suganuma, N., Shimoji, N., Oku, Y. & Matsushige, K. Novel organic light-emitting transistors with PN-heteroboundary carrier recombination sites fabricated by lift-off patterning of organic semiconductor thin-films. J. Mater. Res. 22, 2982–2986 (2007). 32. Namdas, E. B., Ledochowitsch, P., Yuen, J. D., Moses, D. & Heeger, A. J. High performance light emitting transistors. Appl. Phys. Lett. 92, 183304 (2008). 33. Dinelli, F. et al. High-mobility ambipolar transport in organic light-emitting transistors. Adv. Mater. 18, 1416–1420 (2006). 34. Matsushima, T. & Adachi, C. Extremely low voltage light-emitting diodes with p-doped alpha-sexithiophene hole transport and n-doped phenyldipyrenylphosphine oxide electron transport layers. Appl. Phys. Lett. 89, 253506 (2006). 502 NATURE MATERIALS | VOL 9 | JUNE 2010 | www.nature.com/naturematerials © 2010 Macmillan Publishers Limited. All rights reserved.