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By Ebinazar B. Namdas,* Ben B. Y. Hsu, Zehua Liu, Shih-Chun Lo, Paul L. Burn,* and Ifor D. W. Samuel*

Organic light-emitting field-effect transistors (LEFETs)[1–5] are relatively new devices that combine the light emission from an organic light-emitting diode (OLED) with the switching properties of a field-effect transistor in a single device architecture. LEFETs are attractive for new applications, such as simplified pixels in flat panel displays, optoelectronic devices in communications, sensors, and potentially, electrically driven lasers.[6–12] Both unipolar and ambipolar device operation have been demonstrated using a variety of fluorescent materials including small molecules,[13–17] conjugated polymers,[3–4,6–9] and organic single crystals.[11,18–19] Despite this, the light emission performance for LEFETs is poor. This is mainly due to the challenges in developing LEFET materials. To achieve higher performance, it is essential to obtain materials (or material combinations) with both high photoluminescence quantum efficiency and high carrier mobilities. A number of conjugated polymers and small-molecule single crystals have been investigated. Conjugated polymers possess moderate photoluminescence, but the amorphous nature of the thin film usually results in low carrier mobility. Single crystals offer very high carrier mobility due to the well-ordered packing of molecules. However, concentration quenching leads to a significant reduction of photoluminescence quantum efficiency and reduces the device performance. Furthermore the fabrication of single-crystal devices is demanding, so solution-processed materials would offer better prospects for application via low-cost fabrication techniques. Here, we present a new strategy to improve the LEFET performance using phosphorescent materials with thin films

[*] Dr. E. B. Namdas, Prof. I. D. W. Samuel, B. B. Y. Hsu Center for Polymers and Organic Solids, University of California Santa Barbara, CA 93106 (USA) E-mail: [email protected] Prof. P. L. Burn, Dr. S.-C. Lo Centre for Organic Photonics and Electronics School of Chemistry and Molecular Biosciences The University of Queensland Chemistry Building, QLD 4072 (Australia) E-mail: [email protected] Prof. I. D. W. Samuel Organic Semiconductor Centre, SUPA School of Physics and Astronomy, University of St Andrews North Haugh, St Andrews, Fife, KY16 9SS (UK) E-mail: [email protected] Dr. Z. Liu Department of Chemistry, Chemical Research Laboratory University of Oxford, Mansfield Road, Oxford, OX1 3TA (UK)

DOI: 10.1002/adma.200900919

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Phosphorescent Light-Emitting Transistors: Harvesting Triplet Excitons

fabricated by solution processing. Phosphorescent materials have been successfully used to improve the efficiency of OLEDs.[20] The incorporation of heavy atoms into organometallic complexes induces spin–orbit coupling which enables efficient intersystem crossing from the singlet to the triplet states and therefore leads to efficient OLED devices that are able to harvest the emission from both singlet and triplet excitations. The development of materials for LEFETs is, however, more demanding because the channel length is three orders of magnitude larger than the film thickness in OLEDs, meaning that the mobility of phosphorescent OLED materials is insufficient for their direct application in LEFETs. In order to overcome this challenge, we combine three developments: i) doping the phosphorescent materials in a suitable charge-transport host; ii) fabricating bilayered films comprising a charge-transport layer and a light-emitting layer, and iii) depositing ‘‘two color’’ electrodes (source and drain) consisting of a low- and a high-work-function metal for efficient injection of electrons and holes. Figure 1 shows the chemical structures and the device architecture used for the green- and the red-emitting LEFETs. The light-emitting materials for the green-emitting FET were first generation (Ir-G1) and second (Ir-G2) generation dendrimers containing a fac-tris(2-phenylpyridyl)iridium(III) complex core with biphenyl-based dendrons and 2-ethylhexyloxy surface groups.[21–23] For the red-emitting FET, bis(2-(9,9dibutylfluorenyl)-1-isoquinoline)iridium(III)(acetylacetonate) (ADS077RE) was used. The charge-transporting hosts for the green- and red-emitting materials were 4,40 -bis(N-carbazolyl) biphenyl (CBP) and polytriarylamines (PTAA), respectively. A heavily doped n-type silicon wafer functioned as the substrate and as the gate electrode. The hole transporting polymer, poly(2,5bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT; 0.3% in chlorobenzene) was spin-cast onto the nþþ Si substrate (covered by a thermally grown 200 nm layer of SiO2). The light-emitting thin film was spin-cast onto the PBTTT film. The relatively low solubility of the PBTTT layer prevented dissolution during the subsequent processing steps. For dendrimer/CBP blend devices, we used 20 wt% of Ir-G1 and 38 wt% of Ir-G2 in the CBP host. These concentrations provide an equivalent Ir(ppy)3 content to the 6 wt% of Ir(ppy)3 in CBP that has been shown to be optimal for evaporated small-molecule OLEDs.[20] Samples were then mounted onto a silicon shadow mask to complete the device fabrication using angled evaporation of low-work-function metals (Ca, Ba or Yb) followed by Ag deposition.[3] Figure 2a–c shows device output characteristics of neat Ir-G1, neat Ir-G2 and 20 wt% Ir-G1:CBP LEFETs, respectively. The gate

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time-of-flight[24] technique. This is mainly due to the good hole-transport of PBTTT. There is a trend of decreasing hole mobility in moving from first generation to the second generation dendrimer and a further decrease when blended layers are used. This arises from the fact that the holes hop through the light-emitting layers from core to core and the spacing of the cores increases in moving from first to second generation dendrimers and to the blend. The electron field-effect mobility extracted from the transfer characteristics (VG between 0 and
20 V) were 1  10
7 and 6  10
7 cm2 V
1 s
1 for Ir-G1 and Ir-G2 devices, respectively. The electron mobility values were in the range of 2  10
7 to 4  10
7 cm2 V
1 s
1 for the 20 wt% Ir-G1 and 38 wt% Ir-G2 dendrimer blends in the CBP (see Table 1). We note that Figure 1. Chemical structures and device architecture used in this study: a) first generation electron field mobility extracted from transfer dendrimer (Ir-G1) with fac-tris(2-phenylpyridyl)iridium(III) complex core and R is b; b) first characteristics are estimates. The green light emitted during transistor generation biphenyl dendron with 2-ethylhexyloxy surface groups; c) second generation dendrimer (Ir-G2) where R is b; d) CBP; e) PBTTT with R ¼ C14H29; f) ADS077RE, g) PTAA, h) LEFET operation was readily visible to the eye for the device architecture. neat Ir-G2, 20 wt% Ir-G1:CBP and 38 wt% Ir-G2:CBP devices. Close examination revealed that the emission was confined to a narrow region electrode was biased negative with respect to the lowadjacent to the edge of the electron-injecting electrode (e.g., Ca, work-function electrode (e.g., Ca, Ba, or Yb), which was Ba, or Yb). The position and width of the light emission zone was grounded. The transistor operates in accumulation mode, with independent of the gate bias. The width (w) was measured by an hole-dominated transport; distinct linear and saturation regimes optical microscope equipped with a digital camera: can be seen in the characteristics. Figure 2d shows the transfer w ¼ 3.5  0.5 mm. The measured emission spectra of Ir-G2, characteristic (source–drain current, IDS, versus gate voltage, VG) 20 wt% Ir-G1:CBP, and 38% Ir-G2:CBP transistors (Fig. 2e) are at a source–drain voltage of VDS ¼
130 V. The gate electrode was the same as the reported phosphorescence spectra for these again biased negative with respect to the grounded lowmaterials, enabling assignment of the electroluminescence to work-function electrode (Ca, Ba or Yb). The data also indicate emission from the triplet state.[22] hole-dominated transport for the VG range of
20 to
130 V and Figure 2a–d show the brightness versus VDS at various gate weak, but measureable electron current (0.8 nA) in the regime voltages and the optical transfer characteristic (brightness versus for VG ¼ 0–(
20 V). The drain current reached a maximum value VG at VDS ¼
130 V) for the neat Ir-G1, neat Ir-G2 and 20 wt% of 5.6, 1.7, and 0.9 mA for Ir-G1, Ir-G2 and 20 wt% Ir-G1:CBP Ir-G1:CBP devices, respectively. The data clearly indicate optical devices, respectively. The FET functions as a gate-controlled modulation by the gate voltage. The brightness increases with switch with an on/off current ratio of > 103. The gate leakage VDS and VG and then reaches saturation. The light turn-on current at high voltage bias (
130 V) was lower than the voltages (defined as the gate voltage required to achieve a source–drain current by a factor of 30. The field-effect mobility brightness of 0.5 cd m
2) for the Ir-G1, Ir-G2, and 20 wt% for holes can be extracted in the saturation regime from the Ir-G1:CBP devices were
80,
70, and
55 V, respectively. The transfer characteristics: maximum brightness reached for each type of device was 0.8, 30, 1 W and 320 cd m
2 for Ir-G1, Ir-G2, and 20 wt% Ir-G1:CBP blend, IDS ¼ mC ðVG
VT Þ2 (1) respectively (see Table 1). The device performance of the 38 wt% 2 L Ir-G2 blend in CBP was similar to that of 20 wt% Ir-G1:CBP. This is due to the fact that the two blends have the same molar where m is the field-effect mobility, C is the capacitance per unit concentration of Ir(ppy)3 in CBP. area of the gate dielectric (17 nF cm
2), W is the channel width Fig. 2f shows the external quantum efficiency (EQE) as a (1000 mm), L is the channel length (23 mm), and VT is the function of gate voltage at VDS ¼
130 V for Ir-G1, Ir-G2, and threshold voltage. The calculated hole mobilities for the Ir-G1 and 20 wt% Ir-G1:CBP device. A gradual increase in EQE was Ir-G2 devices were 4  10
3 and 2  10
3 cm2 V
1 s
1, observed with increasing voltages, indicating electron injection respectively. For the 20 wt % Ir-G1 and 38 wt% Ir-G2 dendrimer from the Ca electrode. We found a striking difference of efficiency blends in the CBP host, the hole mobility values were on the order with the dendrimer generation. The EQE increased by three of 10
4 cm2 V
1 s
1. The hole mobilities were all higher by three orders of magnitude from 0.0005 to 0.11% in going from Ir-G1 to to four orders of magnitude than those previously reported Ir-G2 devices. The EQE was increased by blending either for neat Ir-G1 (9  10
7 cm2 V
1 s
1) and neat Ir-G2 generation into the CBP host. For the 20 wt% Ir-G1:CBP and (3  10
7 cm2 V
1 s
1) dendrimers, as measured by the

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EQE ¼ fcapture  fspin  frad  fescape

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We next consider the light emission and charge-transport mechanisms in the device. Holes are injected from the Ag electrode to the highest occupied molecular orbital (HOMO) of the dendrimer (5.6 eV)[21] and transported subsequently across the channel by the holetransporting layer (PBTTT). At a high voltage bias, electrons are injected from the Ca to the lowest unoccupied molecular orbital (LUMO) of the dendrimer (2.5 eV). Due to poor electron mobility, the injected electrons accumulate near the low-work-function (Ca, Ba, Yb) electrode (see schematic in Fig. 3). This results in significant electron density close to the Ca electrode at the PBTTT/dendrimer interface. The holes moving across the SiO2/PBTTT interface recombine with electrons in the dendrimer leading to the electroluminescence observed close to the edge of the lowwork-function electrode (Ca, Ba, Yb). In the case of the dendrimer:CBP blends, the HOMO and LUMO of the dendrimer falls within the energy levels of the CBP host, so that charge trapping occurs on the dendrimer, assisting charge recombination. The efficiency of light emission is given by,

(2)

Figure 2. Electrical output (blue) and optical output (red) characteristic of a) neat Ir-G1, b) neat Ir-G2, and c) 20 wt% Ir-G1:CBP LEFET at various gate voltages. d) Electrical transfer (blue) and optical transfer (red) characteristics; the gate voltage was scanned from 0 to
130 V while keeping the source–drain voltage at a fixed value of
130 V. e) Electroluminescence (EL) spectrum of Ir-G2 (black) and 20 wt% Ir-G1:CBP (green). The photoluminescence (PL) spectrum of neat Ir-G2 film is shown for comparison (blue). The inset shows the green light emission photographed through a microscope. f) External quantum efficiency (EQE) versus gate voltage of neat Ir-G1, neat Ir-G2, and 20 wt% Ir-G1: CBP. Table 1. Summary of light-emitting, photophysical, and charge-transport properties of the light-emitting field-effect transistors studied. Device

PLQY Brightness

Hole mobility

Electron mobility

[cd m
2]

[%]

[a] [a] [a] [a] [a] [a]

2 150 740 310 480 440

4  10
4 0.11 0.28 0.28 0.45 0.33

4  10
3 2  10
3 2  10
3 4  10
4 4  10
4 2  10
4

1  10
7 6  10
7 4  10
7 3  10
7 2  10
7 2  10
7

49

30

5  10
3

2  10
2

3  10
7

[%] Green LEFET Ir-G1 (Ba /Ag) Ir-G2 (Ba/Ag) 20 wt% Ir-G1:CBP (Ba/Ag) 20 wt% Ir-G1:CBP (Ca/Ag) 20 wt% Ir-G1:CBP (Yb/Ag) 38 wt% Ir-G2:CBP (Ba/Ag) Red LEFET ADS077RE (Ba/Ag)

EQE

65 81 80 80 80 80

[cm2 V
1 s
1] [cm2 V
1 s
1]

[a] From ref. [23].

38 wt% Ir-G2:CBP devices, the EQE was in the range of 0.28–0.45% at 310–740 cd m
2. These good efficiency and brightness values for LEFETs result from utilizing phosphorescent dendrimers in the bilayer structure.

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where Fcapture is the fraction of excitations which recombine, Fspin is a factor to take account of spin statistics (Fspin ¼ 1 in phosphorescent materials), Frad is the fraction of excitons which once formed decay radiatively [approximated by the photoluminescence quantum yield (PLQY)], and Fescape is the fraction of photons which once generated escape from the device (approximately 20%). The use of efficient phosphorescent materials maximizes the Fspin and Frad factors, whilst Fescape, which is related to the refractive index of the light-emitting layer will be similar for all the devices studied. The large differences in efficiency between devices therefore arise primarily from differences in the efficiency of electron–hole capture (Fcapture). A major factor here is the large imbalance in the mobility of electrons and holes. While the mobility of the holes is dominated by the properties of the PBTTT, the electron mobility is limited by that of the light-emitting layer (the electron mobility is three orders of magnitude smaller than that of the holes). For the neat Ir-G1 film, with EQE ¼ 0.0004%, the ratio of hole to electron mobility is 45 000. For the Ir-G2 device, with EQE ¼ 0.11% the ratio of hole to electron mobility is 5000, whilst for the most efficient device, 20 wt% Ir-G1:CBP with EQE ¼ 0.45%, the average ratio of hole to electron mobility was 2000. In addition to enhancing charge capture, reducing the imbalance of electron and hole mobility may enable the light emission to be generated further from the electron-injecting contact, thereby reducing quenching by the contact. In the blend, we estimate the efficiency of electron–hole capture to be 3%,

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Figure 3. a) Energy level diagram showing the relative HOMO and LUMO energies of Ir-G1, Ir-G2, CBP, and PBTTT. The work functions of the source and drain electrodes are also shown. b) LEFET structure in which the hole density extends all the way across the channel. At a high voltage bias, electrons are injected from the low-work-function electrode to the LUMO of the emissive layer. The injected electrons accumulate near the lowwork-function (e.g. Ca, Ba, Yb) electrode. The holes moving across the SiO2/PBTTT interface recombine with electrons in the dendrimer leading to electroluminescence close to the edge of the low-work-function electrode.

indicating that there is considerable scope for improving the efficiency of these devices, e.g., by using an electron transport layer or blending the dendrimer into an electron-transporting host. Next, we examine the red-emitting phosphorescent LEFET, which used ADS077RE. ADS077RE (10 wt%) was blended in the PTAA polymer host, and films were spin-cast on PBTTT/SiO2 layers (see Experimental). Figure 4a–b shows the electrical and optical characteristics of the LEFET. The device exhibits an on/off current ratio exceeding 104 with a hole mobility of 2  10
2 cm2 V
1 s
1. This is the highest hole mobility reported so far for a solution-processed LEFET. The device shows excellent optical modulation by the gate voltage. Both the brightness and current increase and then saturate. Red light emission was observed close to the edge of the Ba electrode. The emission wavelength peaks at 644 nm with a brightness exceeding 30 cd m
2. The lower brightness (relative to the green-emitting devices) is substantially due to the deep red emission in a spectral region where the eye has poor sensitivity. The lower value in light turn-on voltage (
25 V) in the device is an indication of better injection of electrons from the Ba electrode into the light-emitting

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Figure 4. a) Electrical output (blue) and optical output (red) characteristics of 10% ADS077RE:PTAA with Ba/Ag as the source–drain electrodes b) Electrical transfer (blue) and optical transfer (red) characteristics of 10% ADS077RE:PTAA. The upper and lower insets show device image and EL spectra, respectively.

layer. The holes are injected efficiently from the Ag electrodes into the HOMO of PTAA (5.1 eV).[25] We believe that energy transfer from the PTAA host to ADS077RE is taking place in the device. The working principle of the device is similar to that of the dendrimer devices described in the earlier sections. The device shows EQE in the range of 0.005–0.006%. We attribute the lower EQE value mainly to an imbalance of hole and electron mobility ratio (the ratio is 70 000). We estimate the electron–hole capture probability of the red device to be 0.06%. In summary, we have demonstrated a new strategy to improve light-emitting transistor performance by harvesting both singlet and triplet excitons. Our approach combines the use of phosphorescent complexes with a bilayer transistor structure, and retains the simplicity of solution-deposition of the organic layers. The results presented here open a new opportunity for LEFET research using phosphorescent materials. The devices show green emission with peak EQE of 0.45% at 480 cd m
2. For the red emission, we obtained a brightness exceeding 30 cd/m2 with a relatively high hole mobility of 2.52  10
2 cm2 V
1 s
1.

Experimental The phosphorescent LEFETs were fabricated on a highly doped n-type silicon wafer, which functioned as the substrate and gate electrode. The nþþ Si gate electrode was first coated with 200 nm thick, thermally grown SiO2. The SiO2 surface was cleaned by sonication in acetone followed by an isopropanol rinse and further sonication in isopropanol, then dried under a

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Acknowledgements It is a pleasure to acknowledge assistance and helpful discussions with Prof. A. J. Heeger and Dr. D. Moses. Support for research was provided by the Air Force Office of Scientific Research (Charles Lee, Program Officer), the National Science Foundation (Polymer program, NSF-DMR-0602280) and the EPSRC (Senior Research Fellowship for IDWS). A portion of this work was done in the UCSB nanofabrication facility, part of the NSF-funded NNIN network. We thank Dr. Martin Heeney (Queen Mary, University of London) and Prof. Iain McCulloch (Imperial College London) for supplying the BPTTT-C14 material for our use (the PBTTT-C14 was synthesized at Merck). PLB is a recipient of an Australian Research Council Federation Fellowship (project number FF0668728).

Received: March 17, 2009 Revised: May 28, 2009 Published online: August 24, 2009

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[1] A. Hepp, H. Heil, W. Weise, M. Ahles, R. Schmechel, R. H. von Seggern, Phys. Rev. Lett. 2003, 91, 157406. [2] M. Muccini, Nat. Mater. 2006, 5, 605. [3] J. S. Swensen, C. Soci, A. J. Heeger, Appl. Phys. Lett. 2005, 87, 253511. [4] J. Zaumseil, R. H. Friend, H. Sirringhaus, Nat. Mater. 2006, 5, 69. [5] N. Suganuma, N. Shimoji, Y. Oku, S. Okuyama, K. Matsushige, Org. Electronics 2008, 9, 834. [6] T. Sakanoue, E. Fujiwara, R. Yamada, H. Tada, Appl. Phys. Lett. 2004, 84, 3037. [7] J. S. Swensen, J. Yuen, D. Gargas, S. K. Buratto, A. J. Heeger, J. Appl. Phys. 2007, 102, 013103. [8] J. Zaumseil, C. L. Donley, J. S. Kim, R. H. Friend, H. Sirringhaus, Adv. Mater. 2006, 18, 2708. [9] E. B. Namdas, J. S. Swensen, P. Ledochowitsch, J. D. Yuen, D. Moses, A. J. Heeger, Adv. Mater. 2008, 20, 1321. [10] E. B. Namdas, P. Ledochowitsch, J. D. Yuen, D. Moses, A. J. Heeger, Appl. Phys. Lett. 2008, 92, 83304. [11] T. Takenobu, S. R. Bisri, T. Takahashi, M. Yahiro, C. Adachi, Y. Iwasa, Phys. Rev. Lett. 2008, 100, 066601. [12] E. B. Namdas, M. Tong, P. Ledochowitsch, S. R. Mednick, J. D. Yuen, D. Moses, A. J. Heeger, Adv. Mater. 2009, 21, 799. [13] C. Rost, S. Karg, W. Riess, M. A. Loi, M. Murgia, M. Muccini, Appl. Phys. Lett. 2004, 85, 1613. [14] K. Yamane, H. Yanagi, A. Sawamoto, S. Hotta, Appl. Phys. Lett. 2007, 90, 162108. [15] S. De Vusser, S. Schols, S. Steudel, S. Verlaak, J. Genoe, W. D. Oosterbaan, L. Lutsen, D. Vanderzande, P. Heremans, Appl. Phys. Lett. 2006, 89, 223504. [16] F. Dinelli, R. Capelli, M. A. Loi, M. Murgia, M. Muccini, A. Facchetti, T. J. Marks, Adv. Mater. 2006, 18, 1416. [17] M. A. Loi, C. Rost-Bietsch, M. Murgia, S. Karg, W. Riess, M. Muccini, Adv. Funct. Mater. 2006, 16, 41. [18] H. Nakanotani, R. Kabe, M. Yahiro, T. Takenobu, Y. Iwasa, C. Adachi, Appl. Phys. Exp. 2008, 1, 091801. [19] T. Takahashi, T. Takenobu, J. Takeya, Y. Iwasa, Adv. Funct. Mater. 2007, 17, 1623. [20] C. Adachi, M. A. Baldo, M. E. Thompson, S. R. Forrest, J. Appl. Phys. 2001, 90, 5048. [21] S. C. Lo, E. B. Namdas, P. L. Burn, I. D. W. Samuel, Macromolecules, 2003, 36, 9721. [22] E. B. Namdas, A. Ruseckas, I. D. W. Samuel, S. C. Lo, P. L. Burn, J. Phys. Chem. B 2004, 108, 1570. [23] J. C. Ribierre, S. G. Stevenson, I. D. W. Samuel, S. V. Staton, P. L. Burn, IEEE. J. Disp. Tech. 2007, 3, 233. [24] J. P. J. Markham, I. D. W. Samuel, S. C. Lo, P. L. Burn, M. Weiter, H. Bassler, J. Appl. Phys. 2004, 95, 438. [25] S. G. J. Mathijssen, M. Colle, H. Gomes, E. C. P. Smits, B. de Boer, I. McCulloch, P. A. Bobbert, D. M. de Leeuw, Adv. Mater. 2007, 19, 2785. [26] N. C. Greenham, I. D. W. Samuel, G. R. Hayes, R. T. Phillips, Y. Kessener, S. C. Moratti, A. B. Holmes, R. H. Friend, Chem. Phys. Lett. 1995, 241, 89.

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stream of a nitrogen gun. The SiO2 was then cleaned in an ozone cleaner for 20 min. The PBTTT (0.3% in chlorobenzene) was then spin-cast onto the substrate at 3000 rpm to serve as a hole-transporting layer. The samples were annealed at 150 8C for 10 min in a glovebox. Neat films of Ir-G1 and Ir-G2 were made using dendrimer solutions with a concentration of 20 mg mL
1 in chloroform at a spin speed of 2500 rpm for 60 s, giving films with thicknesses of 120–130 nm. Dendrimer CBP blend films (20 and 38 wt%) were spin-coated from 20 mg mL
1 chloroform solutions to give film thicknesses of approximately 100 nm. For the red-emitting phosphorescent LEFET, ADS077RE was used. ADS077RE (10 wt%) was blended in the PTAA polymer host, and films were spin-coated from 10 mg mL
1 toluene solutions. Prior to spin-coating, the solution was filtered. After spin-coating, the multilayer samples were mounted onto a silicon shadow mask to complete the device fabrication by using the angled evaporation of the low-work-function metal (Ca, Ba, Yb) first, followed by Ag. This fabrication process is described in detail in an earlier publication [3]. The phosphorescent LEFET was tested using a Signatone probe station that was housed in a nitrogen glovebox. All electrical and optical measurements were done in a nitrogen glovebox at < 1.0 ppm oxygen. A Keithley 4200 system was used to gather the electrical data; light emission was collected simultaneously with a Hamamatsu photomultiplier (PMT). The brightness was calculated by comparing the photocurrent in a PMT with a device of known brightness (79 cd m
2) and light-emission area (500 mm  500 mm). A Super Yellow light-emitting electrochemical cell (LEC) was fabricated and used for this comparison. The light-emitting area was 2 mm  1000 mm. The photocurrent in the PMT was corrected for the effective light-emitting area to get the correct brightness value. The EQE was calculated from the brightness, drain current, and emission spectrum of the device. The uncertainties in the EQE and brightness measurements were 20%. The PLQY of the solid films was measured in an integrating sphere under a flowing nitrogen atmosphere, in accordance with Greenham et al. [26] using 363 nm excitation.

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