Highly Efficient Red Organic Light-Emitting Transistors (OLETs) on

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Highly Efficient Red Organic Light-Emitting Transistors (OLETs) on High-k Dielectric Caterina Soldano, Riccardo D'Alpaos, and Gianluca Generali ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00201 • Publication Date (Web): 15 Mar 2017 Downloaded from http://pubs.acs.org on March 17, 2017

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Highly Efficient Red Organic Light-Emitting Transistors (OLETs) on High-k Dielectric

Caterina Soldano*, Riccardo D’Alpaos and Gianluca Generali* ETC s.r.l., viale Italia 77, 20020 Lainate (MI), Italy

* corresponding author(s): E-mail: [email protected] [email protected]

Keywords: organic light emitting transistor (OLET), high-k polymer, organic electronics, dielectric, gate dielectric, P(VDF-TrFE-CFE), terpolymer.

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Abstract The investigation of the optoelectronic properties of a red organic light emitting transistors using a high-k polymer (P(VDF-TrFE-CFE) as gate dielectric are reported. Introducing the high-k polymer strongly reduces the threshold voltages (compared to poly(methyl-methacrylate), PMMA) with improved efficiency, while maintaining comparable output light power. Use of high-k polymer as gate dielectric enables the organic light emitting transistor platform to be exploited in the display market as it can be driven by commercial available electronic and thus enabling low-bias driven organic electronics.

Table of Contents


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Organic semiconductor-based devices such as organic light emitting diodes (OLEDs), solar cells, memories and organic field-effect transistors (OFETs) are expected to reduce fabrication costs and enable novel functionalities with respect to devices and systems based on conventional materials. 1,2,3,4 In the last few years, organic light emitting transistors (OLETs) have been increasingly gaining interest within both the scientific and technological community due to their two-fold functionality of behaving as a thin-film transistor and at the same time being capable of generating light under appropriate bias conditions. 5,6,7,8,9,10 Although this technology platform is only at its early stage of development and it is yet to be fully established and exploited, it has been shown that it can potentially outperform OLED equivalent in terms of device efficiency when using the same set of materials. 11 Further on, the architectural and optoelectronic characteristics of OLETs are particularly suitable to develop flat panel displays with simplified structure both at the backplane level (which allows the use of lower mobility Thin Film Transistor (TFT) technology like a-Si-TFT and Organic TFT) as well as in term of organic stack, where a considerably lower number of organic layers are needed as compared to the state-of-the-art OLEDs, with increased yield and relative reduction in fabrication time and manufacturing costs. 12,13,14,15 To achieve high-performance OLETs 16 , 17 , the integration of high-mobility organic semiconductors with high capacitance insulating films that act as the gate dielectric is rather crucial. The interest in alternative dielectric materials is mainly two-fold: (i) technological, related to the need for reduction of the operation voltage essential for applications in electronics and (ii) market-driven, where reliable and cheap fabrication processes are highly desirable. A key role in the quest for low-bias applications is played by the high permittivity (k) dielectrics that allow overcoming currently faced limitations in commonly used oxides, such as SiO2 18 or low-k polymer based dielectric, such as poly(methyl-methacrylate) (PMMA). 19 3 ACS Paragon Plus Environment

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Preferably, dielectric materials should have a large dielectric constant and should be processable into thin high-quality, flat and defect-free films to form OFETs with low leakage, reduced operating voltage, fast switching speed and large ON/OFF ratio. High-k dielectrics have been widely investigated as gate insulators in field effect devices, with most of the work devoted to inorganic materials 20,21 and in more recent years to OFET 22,23,24

; however the use of high-k dielectrics in OLETs is yet largely unexplored. Although

improvement in the electrical characteristics of this class of devices is likely to be confirmed, none is known about their possible effect on the light output of the devices. 25 In this work, we show a new trilayer red OLET platform comprising an high-k polymer (P(VDF-TrFE-CFE)) as gate dielectric in order to address mainly two issues: (i) improve the performances of the transistor while maintaining similar optical output, as compared to standard PMMA-based counterpart and (ii) introduce an high-k polymer compatible with fabrication processes on flexible substrates, potentially paving the way for the application of organic light emitting transistors in truly flexible commercial devices. 15,26,27


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Results and Discussion Ambipolar charge transport is a desirable property for organic light emitting transistors in order to maximize exciton recombination through electron-hole balance and to spatially tune the location of the emissive (recombination) region. Standard approaches include the superposition of n-type and p-type organic semiconductors in a bilayer stack configuration28, however the proximity of the two charge carrier populations, in spite of mutual Coulomb attraction, reduces the probability of exciton formation within the device channel. In this regards, our group has proposed a tri-layer organic stack

11,29

, in which the recombination

layer is inserted between the two organic semiconductor layers to physically separate the transport layers from the region where excitons are generated, therefore drastically removing exciton quenching effects due to interaction with charges. In this work, a Bottom-Gate/Top-Contact (BG-TC) device configuration, as shown in the schematics in Figure 1, is used. The device active region consists of three different organic layers: the first, in direct contact with the dielectric, and the third layers are field-effect holetransporting- (2,7-Dioctyl[1]benzothieno[3,2-b][1]benzothiophene, C8-BTBT, 45 nm) and electron-transporting- (2,5-bis(4-(perfluorooctyl)phenyl)thieno[3,2-b]thiophene, N-F2-6, 45 nm provided by Polyera Inc.) 30 semiconductors, respectively, whereas the intermediate layer, where the electron-hole recombination and emission processes take place, is a host-guest matrix system. In particular, we used a 20% blend of tris(4-carbazoyl-9-ylphenyl)amine (TCTA) and Tris(1-phenylisoquinoline)iridium(III) (Ir(piq)3) [TCTA: Ir(piq)3, 60 nm], material combination which is widely used in organic light emitting devices targeting red emission at, and around 626 nm, in the visible spectrum. 31,32 The choice for the doping percentage of the emissive layer results from the maximization of output power and efficiency (not shown) in the OLET structure. Silver drain and source electrodes (70 nm) are finally deposited on top of 5 ACS Paragon Plus Environment

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the uppermost organic layer. We refer the Reader to [33,34] for general considerations on energetics of the tri-layer hetero-structures and material therein (see also Figure 1.b). Prior to the transistor fabrication, each semiconductor transporting material has been individually tested in single-layer organic thin film transistor (SL-OTFT) configuration in order to determine the optimal thickness and verify intrinsic transport properties (see Supporting Information). In particular, detailed work on the electrical properties of recently developed n-type material will be presented elsewhere. OLETs have been then fabricated using 450 nm-thick high-k polymer (P(VDF-TrFE-CFE)) as gate dielectric, as well as on PMMA (same thickness) for comparison. We referred to Materials and Methods section for details on the fabrication. From now on and throughout the entire text, we will refer to OLETs using high-k polymer or PMMA as high-k- or PMMA-OLET, respectively. Figure 2 shows the saturation transfer curves in our OLETs (both PMMA- and high-k-) in terms of (a) drain-source current, (b) optical power associated to the light emission and (c) external quantum efficiency (EQE) in the limit of VDS = -20 V in both class of devices. It is here important to note that those experimental conditions, although not optimal for the case of PMMA-OLET, are nonetheless used to directly compare the two classes of devices in the limit of the same applied electric fields (lateral and vertical). These measurements have been performed inside a glovebox environment, which ensures that no adsorbed species (i.e. water molecules and/or oxygen) are present on the dielectric surface and/or on the organic layers, which might hinder the correct operation of our devices. We found that in the limit of VDS = 20 V with VGS sweep up to -20 V the PMMA-OLET is hardly in the ON-state, with very little drain-source current and no light detected with consequently not reliable EQE value estimation. Moreover, both devices show negligible current (few tens of nA) flowing between the gate and source electrode through the dielectric layer (see Supporting Information). On 6 ACS Paragon Plus Environment

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the other hand, as a result of the large difference in the relative electrical permittivity (kvalue) of the gate dielectric (kPMMA≈3 to khigh-k≈ 27-30), the high-k OLET shows very promising results, with an effective gate modulation of light through the entire sweep range. We refer the Reader to the Supporting Information for a more detailed investigation of the dielectric constant of high-k polymer dielectric films. In fact, we observed the following: (i) it can be driven at much lower bias with a light output corresponding to approximately 17 µW and overall EQE between 3 and 4 %. In particular, we note soon, that compared to our previous work from 2010 11, this value of EQE reaches its maximum value of about 4 % at gate bias of -3 V, corresponding to a condition of balanced ambipolarity and it remains almost constant at around 3.3 % when the device reaches the maximum of light. This outcome can be explained considering the mechanism of the light emission processes in a trilayer OLET device, where the active region can be modeled as a parallel of two OTFTs of opposite polarization. During the p-type ID-VG sweep, we observed two ranges in which light is emitted: the first one in which only the p-type OTFT is operating (right side of the “V” shaped plot of Figure 2(a)) and the efficiency is affected by the same limitations of OLEDs 35, and the second region where both OTFTs are in their ON-state, with balanced charge carriers densities (around the low apex of the plot) and in which the efficiency is increased due to increasing number of minority charge carriers from n-type semiconductor layer towards the recombination area. Details about the n-type transport in the tri-layer structure relative to device in Figure 2(a) (high-k OLET) are reported in the Supporting Information. For a comprehensive explanation of the mechanisms, we refer the Reader to our previous publication.

11

Operation of high-k- and

PMMA-OLETs has been first presented in the limit of same VDS for both devices. Given the same device geometry and same dielectric thicknesses, the devices are operating in the same vertical and horizontal electrical fields so the net difference is mainly related to the properties 7 ACS Paragon Plus Environment

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of the gate dielectric. Figure 2(d) shows that the same PMMA-OLET driven at -100 V (VDS = 100 V), which represents a fully working condition for the device. |100|V is the standard operating bias regime for our PMMA-based OLETs platform as used in the past for research purposes. 11,25,28,36 We here note: (a) drain-source currents are very similar in both devices, however PMMAOLET shows a not negligible hysteretic behavior between the forward and reverse sweep37, which also reflects in the light output of the device itself. This hysteresis is mainly related to an increase of the p-type threshold voltage as a consequence of a hole trapping effect at metal/organic and dielectric/organic interfaces, induced by the high electric field. Further on, the calculated value of EQE ranging between 2 and 3% is smaller than the high-k counterpart. In terms of absolute value of optical power, we found almost double the value of the light, however we have to consider that the field to which the gate and thus the active organic area is subjected is approximately 5 time larger (EG (PMMA-OLET) = |100| V/450 nm = 2.22 MV/cm; EG(high-k OLET)=|20| V/450 nm = 0.44 MV/cm). (b) For the PMMA driven at 100 V, EQE reaches the onset of a plateau of about 2.5 %, corresponding to the maximum of ambipolarity of the transistor at about -30 V and it remains approximately constant for increasing bias. It is well-known, that photoluminescence (PL) and consequently electroluminescence (EL), are both affected by external applied electric fields; in particular, in the limit of high fields (i.e. ≈106 V/cm), quenching phenomena such as exciton-exciton and exciton-charge annihilation and singlet exciton dissociation are often responsible for drop in performances.38 In our case, considering that the light emission is located under one electrode (drain) with vertical fields of PMMA-OLET approximately 5 times larger than the high-k counterpart (≈105 V/cm), we can expect a quenching of about 10% and 0.05% for PMMA and high-k OLETs, respectively. Observed decrease of EQE is thus consistent with field-induced 8 ACS Paragon Plus Environment

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quenching, however a more precise assessment should also take into account additional quenching phenomena. We recall that values of mobilities and thresholds are calculated from the p-type and ntype locus curve VDS = VGS = [|020 or 100| V] (not shown). It is interesting to note that high-k OLET does not show an appreciable hysteresis in the saturation transfer (Figure 2(a)) as one would expect in the case of several fluorinated polymers. On the other hand, it has been shown

that

copolymerization

of

chlorofluoroethylene

(CFE),

or

similarly

chlorotrifluoroethylene (CTFE), in the co-polymer P(VDF-TrFE) has the effect of strongly reducing the polarization hysteresis (due to heating) at room temperature, making the terpolymer (P(VDF-TRFE-CFE) a relaxor ferroelectric. 39,40 Prior to optical characterization which was performed in atmosphere, as-fabricated devices were encapsulated inside the glove-box environment by using a glass coverslip and an ultraviolet-cured epoxy sealant. A getter (Dryflex, provided by SAES-Getters S.p.a.) was also used to prevent any deterioration of the sample upon exposure to oxygen and moisture. Figure 3 shows normalized electroluminescence spectra of both PMMA- and high-k-OLETs, where the normalization has been considered to take into account the different biases condition of the two transistors (PMMA-OLET has been driven at |100| V). As expected, since both PMMA and high-k materials are optically inactive in the visible range the two spectra are equal, presenting a very broad emission peak centered at around 626 nm. Photoluminescence spectrum of a TCTA:Ir(piq)3 20 % blend thin film is also reported for reference (dashed line). Optical image on the right shows the high-k- OLET while mounted on a sample holder in its ON state (as compared to OFF-state in inset of main panel of EL), with all devices lit at the same time (top and bottom not lit devices are test capacitor devices used to verify on each single substrate the capacitance of the dielectric layer). 9 ACS Paragon Plus Environment

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Table 1 summarizes the electro-optical properties of both types of OLETs where, for the PMMA one, we have reported values (where possible) referred to the device driven at |20| V for a direct comparison with the high-k and the standard |100| V bias characterization, for which electrically and optically the two transistors behave similarly. In more details: i.

PMMA-OLET reaches its full ON state when driven at -100 V, while for the high-k one

the same ON state is obtained at -20 V, as suggested by similar p-type saturation mobilities. ii.

when PMMA-OLET is driven at -20 V, the device is not completely in its ON state, as

shown by the lower mobility value and the only p-type unipolar operating regime. Nevertheless, the device shows a threshold voltage similar to its high-k counterpart. iii.

The introduction of the high-k dielectric clearly enables the device full turning-on at

low threshold voltage (and consequently light emission). Furthermore, even if the amount of current is much lower with respect to the PMMA-OLET at -100 V, the device shows an improvement in terms of EQE. One main result of our work is that implementing the OLET platform with a high-k polymer materials as gate dielectrics, enable low-bias driving device with compatibility with flexible substrates, enabling numerous applications in the field of flexible organic electronics, as shown in [15]. Moreover, the devices driven at lower biases are less sensitive to stresses due to the high electric fields, thus enabling device with a longer lifetime with respect to their PMMA counterparts.


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In conclusion, we have demonstrated the feasibility of a red trilayer OLET using a high-k dielectric instead of PMMA as gate dielectric. High-k OLET device has shown higher EQE and more than 4 times smaller driving voltages with respect to the PMMA counterpart, while maintaining comparable output power for the light emission. Even though, additional developments must be performed to further improve this novel organic electroluminescent platform, the results here presented pave the way towards enabling this new technology to be potentially exploited in the display market as it can be driven by commercial available electronic. Moreover, using high-k polymers allows for diverse applications of OLET-based technology in low-power consumption organic electronics.


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Materials and Methods Device Fabrication Trilayer organic heterostructure OLETs were fabricated in bottomgate/top-contact configuration. The substrate consists of 25 mm × 25 mm glass coated with a 120-nm thick patterned ITO serving as a gate electrode. A PMMA film (450 nm) was spin coated on the substrate and annealed in vacuum at 90 °C for 18 h. P(VDF-TrFE-CFE) from Piezotech is dissolved in cyclopentanone and then spin-coated onto substrate and annealed in air on a hot plate at 110°C. The organic multilayer stack and metal electrodes were deposited by thermal evaporation by means of shadow masks in a home-made high vacuum deposition chamber at room temperature at a base pressure of 10−7 mbar. Devices have the following characteristics: 12 mm channel width, 70 μm channel length and 500 µm wide source and drain electrodes. Thicknesses of dielectric, semiconducting, and metal layers were measured by means of a KLA Tencor P6 profilometer. The deposition chamber is directly connected to a glovebox to prevent sample exposure to air during the fabrication process and allow preliminary electrical and optical characterization. Devices are encapsulated inside the glovebox using a glass coverslip and an ultraviolet-cured epoxy sealant. A getter (Dryflex, provided by SAES-GETTERS) was also used to prevent any deterioration of the sample. Electrical and Optical Characterization. Optoelectronic characterization of our devices was carried out using a SUSS probe station, coupled with a B1500A Agilent semiconductor parametric device analyzer. The measurement setup was equipped with an S1337 silicon photodiode (Hamamatsu) with a sensitivity of 0.38A W−1 at 600 nm, placed in contact with the devices to enable simultaneous electrical characterization and collection of all emitted photons. Electroluminescence spectra have been measured by a Minolta CS-2000 commercial spectro-radiometer.


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Supporting Information [Organic semiconductor electrical characterization in single-layer OTFT configuration, additional OLET characterization, organic layer morphology, dielectric properties of high-k] The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.XXXXXXX Acknowledgement The authors would like to thank V. Biondo and G. Turatti (ETC s.r.l.) for their continuous and very helpful support and Dr. A. Facchetti and Dr. W. Zhao from Polyera Corp. for providing high-k polymer.


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Figure 1 (a) Schematic representation of the tri-layer OLET device and (b) energy-level diagram of the entire hetero-structure used in the present work. The dielectric layer is either PMMA (reference polymer platform) or high-k polymer (P(VDF-TrFE-CFE)). The active region is constituted by blend of TCTA and Ir(piq)3 (20 %) sandwiched between a (bottom) p- and (top) n-type organic semiconductor layers.


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Figure 2 Saturation transfer curves of PMMA- and high-k OLETs in the limit of VDS = -20 V in terms of (a) drain-source current (IDS), (b) optical power associated to the light emitted and (c) external quantum efficiency (EQE) of both classes of devices. (d) Saturation transfer curve as in panel (a-c) for PMMA-OLET driven at VDS = -100 V, representing standard operation regime for PMMA-based transistors.


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Figure 3 (Left) Normalized electroluminescence spectra of both organic light emitting transistors __ __ fabricated with ( ) PMMA and ( ) high-k as dielectric materials, respectively.

Photoluminescence of the blend TCTA:Ir(piq)3 20 % is also reported as reference. (Inset) optical image of a representative device while mounted on a sample holder in its OFF-state. (Right) Same sample as the inset in the OFF state (VDS = VGS = -20 V) with all device in the ONstate.


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Table 1 Summary of the electrical and optical properties of high-k and PMMA-OLET, including threshold voltages and mobilities for both charge carriers, maximum current and output power for light emission and corresponding EQE values at the maximum driving bias.

High-k OLET

PMMA OLET

Dielectric Thickness (nm)

V (V)

max

V p (V)

th

µp 2 (cm /Vs)

V n (V)

th

µn 2 (cm /Vs)

IDS (µA)

EL (µW)

EQE @ max VDS (%)

450

|20|

-10.2

0.42

12.5

7.7 x 10-3

-276

17.1

3.3

|20|*

-11.5

3.8 x 10

-

-

-0.16

-

-

-833

40.5

2.6

450

|100|

-51.9

-3

0.53

29.7

3.7 x 10

* No light emission is obtained for PMMA-OLET driven at |20|V.


-3

max

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Responses

in

Terpolymer

of

trifluoroethylenechlorofluoroethylene) Adv. Mat. 2002, 14, 1574-1577.


Poly(vinylidene

fluoride-

Highly Efficient Red Organic Light-Emitting Transistors (OLETs) on

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