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Organic light-emitting transistors with simultaneous enhancement of optical power and external quantum efficiency via conjugated polar polymer interlayer Mario Prosa, Emilia Benvenuti, Mariacecilia Pasini, Umberto Giovanella, Margherita Bolognesi, Lorenzo Meazza, Francesco Galeotti, Michele Muccini, and Stefano Toffanin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06466 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 9, 2018

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ACS Applied Materials & Interfaces

Organic light-emitting transistors with simultaneous enhancement of optical power and external quantum efficiency via conjugated polar polymer interlayer Mario Prosa,a,* Emilia Benvenuti,a Mariacecilia Pasini,b,* Umberto Giovanella,b Margherita Bolognesi,a Lorenzo Meazza,b Francesco Galeotti,b Michele Muccinia and Stefano Toffanina,*

a

Consiglio Nazionale delle Ricerche (CNR) - Istituto per lo Studio dei Materiali Nanostrutturati

(ISMN), Via P. Gobetti 101, 40129 Bologna, Italy b

Consiglio Nazionale delle Ricerche (CNR) - Istituto per lo Studio delle Macromolecole (ISMac),

Via Bassini, 15, 20133 Milano, Italy

KEYWORDS. organic light-emitting transistors, conjugated polar polymers, conjugated polyelectrolytes, electron injection, buried interfaces, confocal microscopy

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ABSTRACT

Organic light-emitting transistors (OLETs) show the fascinating combination of electrical switching characteristics and light generation capability. However, to ensure an effective device operation, efficient injection of charges into the emissive layer is required. The introduction of solutionprocessed conjugated polyelectrolytes (CPEs) films at the emissive layer/electrode interface represents a promising strategy to improve the electron-injection process by dipole formation. However, their use in optoelectronic devices involves also some limitations due to the ionic nature of CPEs. In this context, neutral conjugated polar polymers (CPPs) represent a valid alternative to CPEs since the conjugated backbones of CPPs are functionalized with polar non-ionic side groups, thus avoiding ion-dependent drawbacks. By introducing a layer of polyfluorene containing phosphonate groups (PF-EP) underneath the metal electrodes, we here demonstrate a substantial improvement of the electron injection properties into the OLET emissive layer and, accordingly, a more than twofold increased light power and a five-times-higher external quantum efficiency of p-type OLETs in comparison with reference devices without any interlayer. The great benefit of using a transparent glass substrate allowed to selectively investigate the morphological and photoluminescent characteristics of both CPE- and CPP-buried interlayers within complete OLETs by means of an optical scanning probe technique. This, together with a thorough optoelectronic characterization of the figures of merit of working light-emitting devices allowed to disclose the origin of the improved optical performance of CPP-based devices as well as the operation mechanisms of the investigated interlayer in the corresponding OLETs.


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INTRODUCTION

Over recent years, organic light-emitting transistors (OLETs) attracted much attention due to their fascinating capability to combine, in a single device, the electrical switching characteristics of organic field-effect transistors (OFETs) and the light generation functionality typical of organic light-emitting diodes (OLEDs).1,2 The meticulous optimization of the material properties combined with the development of advanced device architectures led to a progressive evolution of this technology, which has been demonstrated to outperform the OLED efficiencies.3 As a result, OLETs have been promoted as effective candidates for a wide variety of applications such as nextgeneration displays, optical communication devices and electrically powered organic lasers.4,5,6 Aiming at obtaining high-performance OLETs, a plethora of device architectures and configurations have been proposed over the years.7,8 Among the variety of explored options, ambipolar OLETs attracted much attention because of their potentially high luminescence efficiency obtained through a relatively simple device manufacture consisting of few production steps.9,10 In ideal ambipolar OLETs, a balanced recombination of holes and electrons occurs by the use of an organic semiconducting material with the ability of transporting, with comparable efficacy, both chargecarrier types. However, the available library of organic materials with good n-type behavior is fairly limited and it is still challenging to find organic semiconductors which are characterized by wellbalanced mobility for both holes and electrons.11,12,13 This leads to unbalanced charge densities in the device which implies that charge recombination takes place predominantly underneath the charge-collecting electrode (drain) with consequent exciton and photon quenching.14 To overcome this limitation, the charge density distribution can be tuned by using asymmetric contacts, where high- and low-work function metals are respectively used as source and drain electrodes.15,16,17 Therefore, the electron-injection is typically improved, the charge density distribution is more balanced in the device and, even in p-type unipolar OLETs where charge transport is largely dominated by holes, the process of exciton formation is effectively promoted.18 3 ACS Paragon Plus Environment


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Besides the additional manufacturing step, the evaporation of electrodes with different work functions is still challenging, which clearly denotes an increased complexity in the device fabrication.19 In this context, the introduction of a solution-processed conjugated polyelectrolyte (CPE) film placed between the emissive layer and the electrodes was demonstrated as a simple and efficient method to improve the electron injection in unipolar multilayer OLETs thus avoiding demanding asymmetric metal deposition processes.20,21,22 CPE is a class of π-conjugated polymers endowed with ionic pendant side chains which modify the electrode work-function through interfacial dipoles.23,24,25,26 CPEs have been widely employed in several organic electronic devices ranging from solar cells to OLEDs and lasers.27,28,29,30,31,32 Moreover, their successful use in unipolar OLETs avoids the challenging search for efficient ambipolar organic semiconductors. The great advantage of using CPEs over asymmetric electrodes is their simple and cost-effective deposition from solution in addition to a high transparency of the resulting layer. However, despite the clear advantages in terms of device performance by using CPE layers, a meticulous optimization of the material is typically required since counterions play a crucial role in the resulting interfacial dipole.33,24 Hence, the dependence of the device performance on the chemical nature of the CPE counterions needs to be in-depth investigated. Furthermore, since the ion motion influences the dipole formation, the dimension of the counterions affects the response time of the devices.34 Moreover, migration of ions in the emissive layer of polymer LEDs (PLEDs) was demonstrated to gradually quench the luminescence thus affecting the device stability over time.27 These reported limitations, combined with the susceptibility of CPE layers to thermal treatments,35 rise the need for non-ionic alternatives. In this context, conjugated polar polymers (CPPs) have been demonstrated as efficient electron injection materials in polymer LEDs

36,37,38

but their use in OLETs in place of CPEs has not been

explored, yet. CPPs is a class of neutral polymers where the conjugated backbones are modified with polar non-ionic side groups, which can be deposited from alcoholic solutions.39 Among them, 4 ACS Paragon Plus Environment

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polyfluorenes (PFs) are the most employed backbones for their suitable optical and electrical properties, their high chemical and thermal stability as well as their facile functionalization without affecting the optoelectronic properties.40 In particular, phosphonate-functionalized PFs, beyond a high solubility in polar solvents, showed peculiar properties at the interface with metal electrodes, which resulted in an efficient electron-injection behavior when introduced in PLEDs and organic solar cells (OSCs).41,42,43,44 In sight of this, phosphonate-functionalized PFs could be considered as potential non-ionic alternatives to CPEs for OLET applications. In this work, we report on unipolar p-type bilayer OLETs incorporating ethanol-soluble poly[9,9bis(6’-diethoxylphosphoryl-hexyl)fluorene] (PF-EP) as phosphonate-functionalized PF electroninjecting layer (EIL). The introduction of PF-EP at the interface between the emissive layer and the source/drain electrodes showed OLETs with a more-than-twofold increased intensity of the emitted light and a five-times-higher external quantum efficiency in comparison with devices where no additional electrode interlayer was used. The enhanced electroluminescence in PF-EP incorporated OLETs, ascribed to an improved electron injection from the drain contact to the emissive layer, resulted even superior to that of corresponding OLETs containing a CPE interlayer of poly[(2,7(9,9′-dioctyl)fluorene)-alt-(2,7-(9,9′-bis-(5″-trimethylammonium

bromide)hexyl)fluorene)]

(PFN+Br-). The benefit of using a transparent glass/ITO substrate in the OLET structure opened towards a direct investigation of the buried interfaces between both PF-EP and PFN+Br- interlayers and the emissive layer by means of Confocal Laser Scanning Microscopy (CLSM). Indeed, being both PFEP and PFN+Br- layers photoluminescent but spectrally well-separated from the emissive layer (i.e. TCTA:Ir(ppy)3), CLSM is an effective tool for imaging the pseudo-morphology of the interfaces between the EIL and the exciton formation layer. To note that in the OLET fabrication protocol both the transporting and emissive layers were deposited by thermal evaporation while the EILs were processed from solution. Aiming at fully solution-processable and high-performance multilayer devices, we believe that this hybrid dry/wet 5 ACS Paragon Plus Environment


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manufacturing strategy represents a first step towards a cost-effective approach for the exploitation of the full potential of the OLET technology.

RESULTS AND DISCUSSION

The materials and the OLET architecture used in this work are shown in Figure 1. All the devices were fabricated onto transparent glass/Indium Tin Oxide (ITO) substrates with a bottom-gate/topcontact configuration using reflective Ag source/drain electrodes to direct and collect the emitted light at the bottom side of the devices. Reference OLETs present a bilayer structure consisting of 2,7-dioctyl[1]benzothieno[3,2b][1]benzothiophene (C8-BTBT) as p-type charge-transport material and a host-guest green emissive layer (EML) based on a matrix of tris(4-carbazoyl-9-ylphenyl)amine (TCTA) doped with the emitter tris[2-phenylpyridine]iridium(III) (Ir(ppy)3). All the layers except the poly(methyl methacrylate) (PMMA) dielectric have been thermally evaporated. Further details on the materials and device fabrication are reported in the Experimental Section. Unipolar bilayer OLETs have been widely investigated in the literature and can be considered as reference systems.45 However, the use of symmetric Ag contacts, i.e. source and drain, directly on top of the EML is expected to present sub-optimal charge injection due to energy levels misalignment between the metal and the organic layer (Figure 1b).46 On the one side, a good injection of majority charges, i.e. holes, from the source contact to C8-BTBT is ensured as revealed from the optimal operation of corresponding organic FET (OFET) devices (Figure S1), where the top electrodes directly cover the charge transporting C8-BTBT layer. Moreover, the optimal energy level alignment between C8-BTBT and TCTA (Figure 1b) ensures a good hole injection into the 6 ACS Paragon Plus Environment

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EML. On the other side, the occurrence of an energy barrier at the drain contact/EML for the injection of minority charges, i.e. electrons, (Figure 1b) represents a limitation for the processes of light generation,47,48 since the drastically unbalanced density of opposite charges would affect the exciton formation. In order to improve the injection of electrons into the emissive layer, an ethanol-soluble PF-EP interlayer was introduced, by spin-coating, between the EML and the Ag electrodes (Figure 1a). PFEP is a neutral CPP based on a phosphonate-functionalized PF (Figure 1c) with the energy levels located at -2.2 eV and -5.7 eV, respectively for the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) (Figure 1b).40,41,43 Due to the suitable energetics, PF-EP has been widely used in OLEDs and OSCs to reduce the electron barriers at the cathode through an interfacial dipole effect. In those devices, the performance enhancement was attributed to the formation of favorable interfacial dipoles and to the good contact with the electrodes, thanks to the coordination ability of phosphonate groups to the metal atoms at the PF-EP/electrode interface. Moreover, the multilayered structure effectively prevented metal atoms diffusion into the emissive layer, thus avoiding exciton quenching at the cathode interface. In this work, the performance of OLETs including the PF-EP interlayer is compared with that of analogous devices where neutral PF-EP was replaced by PFN+Br-, one of the most commonly employed CPEs (Figure 1). PFN+Br- is a polyelectrolyte based on alternating ionic and neutral fluorene-based monomers, which has already been successfully employed for the interfacial engineering of different optoelectronic devices.28,49,50 The corresponding chemical structure is showed in Figure 1d. The solubility in alcohol of both interlayer materials allows the wet deposition on top of the EML without dissolving/damaging the underlying layers. In addition to this advantage it is worth mentioning that, in fluorene based polymers as PF-EP and PFN+Br-, the functionalization with polar or ionic side chains does not interfere with the conjugated π-orbitals of the polyfluorene backbone.51,52 Therefore, no significant differences between the HOMO and LUMO levels of 7 ACS Paragon Plus Environment


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PFN+Br- and PF-EP are revealed (Figure 1b). Accordingly, the corresponding thin films show similar features both in their absorption and emission spectra (Figure S2). Note that, the resulting electrochemical band gaps of PFN+Br- and PF-EP (Figure 1b) are slightly larger than their respective optical band gaps (Figure S2a). This is associated to the different sample preparation in the two characterization techniques53 as well as to the fact that in conjugated polymers, after light absorption, the electron-hole couple is electrostatically bound in the excited state in contrast to the ionized state in the electrochemical measurements.54 In addition to that, it is worth mentioning that the reported HOMO and LUMO values do not take into account the dipole effect occurring during device operation.

Figure 1. Schematic representation of OLET architecture (a) and the energy level diagram (b) and the chemical structures of all the employed materials (c-g) including: the polymers PF-EP (c) and PFN+Br- (d) used as EILs in the OLET devices, the host (e) and guest (f) emissive materials and the p-type organic semiconductor C8-BTBT (g). 8 ACS Paragon Plus Environment

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In order to evaluate the performance of the polar polymer PF-EP as EIL, the optoelectronic properties of corresponding OLETs were measured. As reference devices, OLETs without any interlayer were fabricated and characterized. In addition, devices where the PF-EP interlayer was replaced by the polyelectrolyte counterpart, i.e. PFN+Br-, were tested as a comparative system. To note, the thickness of PF-EP and PFN+Br- interlayers was intentionally fixed around 10 nm to avoid limitations arising from their typical low vertical conductivity while ensuring a valid comparison between the two different materials.51 Evidently, the fine-tuning of the EMLs thickness in order to achieve independently the best optoelectronic performance for each compound typology is out of the scope of the present work. Figure 2 shows the current – voltage (I – V) and the electroluminescence (EL) characteristics of the three different types of OLETs, respectively endowed with: no interlayer (Figures 2a and 2d), PFEP (Figures 2b and 2e) or PFN+Br- as EIL (Figures 2c and 2f). As shown from the saturation transfer I – V plots (Figures 2a, 2b and 2c), all the devices reported a clear p-type behavior (negatively biased gate) due to the use of C8-BTBT as charge transporting material. By sweeping the gate bias (VGS) up to -100 V, a good quadratic I – V behavior was shown by all the OLETs with an ON/OFF current in the range of 105-106 and a hole mobility (µh) close to 0.2 cm2 V-1 s-1 (Table 1), in agreement with the characteristics reported by the corresponding OFET devices (Figure S1). Accordingly, by inverting the gate polarization to a positive bias, none of the devices showed any evidence of n-type behavior. In addition to that, all the devices reported similar as well as optimal output characteristics, as highlighted by an initial linear increase of the current with |VDS| at fixed VGS (for |VGS| > |VT, h|, where VT, h is the threshold for holes conduction) then reaching a saturation regime when exceeding the channel pinch-off point (Figures 2d, 2e, 2f and Table 1). Despite the similar I –V characteristics of all the investigated OLETs, the incorporation of an electrode interlayer partially influenced the VT,

h

of the corresponding devices, which slight 9

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increased from -34 V for the reference bilayer OLETs to -46 V and -40 V for PF-EP- and PFN+Br-incorporating OLETs, respectively. This can be associated to a partially hampered hole injection in the channel due to the occurrence of a small hole injection barrier at the source contact as a result of the presence of the EIL under both the source and drain electrodes. On the contrary, from the output characteristics (Figures 2d, 2e and 2f) at |VDS| < 20 V, PF-EP as well as PFN+Br- based OLETs show a linear trend, a clear indication that, by introducing an EIL, the corresponding OLETs are not affected by additional contact resistances. From the I – V characteristics of the reported OLETs, the overall contact resistance of the devices cannot be decoupled into the single contributions arising either from the source or from the drain contact resistance. It is reasonable to assume that, in EILincorporating OLETs, the presence of a small hole-injection barrier at the source contact is balanced by an enhanced electron-injection behavior at the drain electrode. The use of EILs, according to their chemical structures, strongly influenced the light emission ability of the corresponding OLETs. In particular, the presence of PF-EP as EIL greatly improved the amount of photons emitted from the corresponding OLETs (Figure 2 and Table 1). Notably, when compared to the response of the reference device without any interlayer, a more than twofold increase of light power (from 340 nW to 750 nW) and a five-times-higher external quantum efficiency (EQE) (from 0.02% to 0.11%) were detected at VDS = -100 V and VGS = -100 V by the insertion of the PF-EP interlayer between the EML and the Ag electrodes (Table 1). In relation to the non-negligible hysteresis present in the saturation transfer curve of the PF-EP-based OLET (Figure 2b), we highlight that an effective and suitable optimization of the EML thickness would surely increase the overall optoelectronic performance of the multilayer device. By contrast, the use of PFN+Br- partially hampered the light response of the device, which showed almost halved light power (from 340 nW to 190 nW) and EQE (from 0.02% to 0.01%), if compared to the reference devices without EIL (Table 1).


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Figure 2. p-type saturation transfer curves at VDS = -100 V (a - c) and output curves recorded at VGS ranging from 0 V to -100 V at intervals of 20 V (d - f) (blue dots) with the relative electroluminescent characteristics (pink dots) of OLETs without any source/drain interlayer (a, d) or incorporating PF-EP (b, e) or PFN+Br- (c, f) as EIL.

Table 1. Summary of the optoelectronic properties of OLETs without EIL or incorporating PF-EP or PFN+Br- as EIL at the EML/Ag interface. All the parameters are averaged over 5 devices. µh

VT, h

[nm]

[cm2V-1s-1]

[V]

-

0.19

- 34

EIL OLETs

w/o EIL

I /I ON

OFF

6

10

IDS MAX.a) EQEa) ELMAXa,b)

λ ELa)

CIE

[µA]

[%]

[nW]

[nm]

(x,y)

- 480

0.02

340

522

(0.34, 0.61)

w/ PF-EP

10

0.19

- 46

6

10

(0.35, - 290

0.11

750

522 0.61)

w/ PFN+Br-

a)

10

0.22

- 40

5

10

- 460

0.01

435,457,

(0.33,

522

0.57)

190

Measured at VGS = -100 V; VDS = -100 V; b) measured at the emission peak of the EML (522 nm).



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Operation mechanism of CPP and CPE interlayers

In order to shed light on the different behavior in terms of emitted light of PF-EP- and PFN+Brbased OLETs, their EL spectra and the color coordinates according to the Commission Internationale de l’Éclairage (CIE) diagram were acquired. As shown in Figure 3a, both the reference and the PF-EP-incorporating OLETs are characterized by an EL band peaked at 522 nm (Table 1), which perfectly overlaps with the photoluminescence (PL) spectrum of the corresponding thin film of TCTA:Ir(ppy)3. This clearly indicates that: i) the light emission of PF-EP-incorporating OLETs exclusively arises from the EML and ii) the characteristics of the emissive layer are not affected by the deposition of the overlying solution-processed PF-EP. As a result, the twofold increase of EL intensity in comparison with the reference bilayer OLETs is likely due to a greater amount of charges converted into excitons. Indeed, the OLET EQE results five times higher when PF-EP interlayer is used (Table 1). This means that the presence of PF-EP promotes a better electron-injection from the drain electrode to the EML where opposite charges more efficiently recombine to form excitons and finally photons. In a simplified view, the increased OLET emission arising from the PF-EP insertion can be schematized in two interlinked physical processes: i) electrons are favored to be injected from the drain electrode to the EML due to favorable interfacial dipoles; ii) holes are partially hampered to flow into the EIL and hence opposite charges meet in the EML thus generating excitons that, without any EIL, would not have been generated due to non-radiative recombination of opposite charge-carriers at the EML/drain interface (charge recollection). On the contrary, despite the EL band centered at 522 nm, PFN+Br- incorporated devices present two additional emission peaks at 435 nm and 457 nm, respectively. Accordingly, the corresponding CIE coordinates result slightly shifted in comparison with those of both the reference bilayer and PF-EP incorporated devices (Figure 3b). Since the current flowing into the devices is similar to that of 12 ACS Paragon Plus Environment

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reference bilayer OLETs, the exciton formation process of PFN+Br- incorporating OLET is affected by a reduced hole blocking capability of the EIL, where holes drift and recombine with the incoming electrons. Indeed, the emission peaks at 435 nm and 457 nm perfectly correspond to those of the thin film of pristine PFN+Br- (Figure S2b). This means that part of the electrons which are injected from the drain electrode through the EIL do not reach the EML but they recombine in the PFN+Br- via less efficient radiative paths (e.g. in the proximity of the drain contact), as evidenced by: i) the device EL intensity, which is lower than the reference OLET without any interlayer (Table 1) and ii) the corresponding device EQE, which halves passing from the reference bilayer OLET to PFN+Br- incorporating OLET (Table 1). As a result, not only the light generation process is affected, but also the color coordinates of the device are altered.

Figure 3. a) EL spectra measured at VGS = -100 V and VDS = -100 V of reference OLETs without any EIL (red dots), PF-EP- (blued dots) and PFN+Br-- (green dots) incorporated devices; the PL of the EML layer (black line) is reported for comparison. b) CIE coordinates of the three different OLETs under investigation.

A relevant difference of operation is clearly observed when PF-EP or PFN+Br- is used as EIL. A possible explanation concerns the different organization of the two polymeric materials in the solid state. Indeed, possible differences in the morphology and/or structural arrangement of PF-EP and 13 ACS Paragon Plus Environment


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PFN+Br- layers could cause a different dipole formation/distribution at the electrode interface, which strongly influences the electron injection mechanism. The use of a transparent substrate in the here-investigated devices allowed for a thorough analysis of the EILs organization in complete OLETs by means of CLSM.55,56 As shown in Figure 4, CLSM images of PL were recorded for both PF-EP (Figures 4a and 4c) and PFN+Br- (Figures 4b and 4d) incorporating OLETs. The different spectral regions of emission characterizing the EML and EIL allowed to collect PL images selectively either from one or the other layer. Indeed, while the EML emission was recorded by the photomultiplier tube (PMT) detector with spectral range 488 nm – 543 nm, the EIL emission was collected in the region 408 nm – 488 nm, in agreement with the EML (Figure 3a) and EIL (Figure S2b) PL spectra. As evidenced in Figures 4a and 4b, the EML morphology for both the PF-EP and PFN+Br- incorporating OLETs is featureless at the microscope resolution, similarly to the PL image in the same spectral region of the reference bilayer device without any additional electrode interlayer (Figure S3). As a consequence, it can be confirmed that the deposition of an overlying EIL from solution did not dissolve or affect the morphology and PL properties of the underlying EML. On the contrary, CLSM images of the PF-EP and PFN+Brinterlayers in complete OLETs showed a morphology which is different from that of the underlying EML layer, with bright aggregates of micrometric and sub-micrometric dimensions. It is worth mentioning that CLSM images were taken in the area inside the device channel, while in an optoelectronic device the polymer layer organization can be affected by both the upper and lower interfaces.57 In particular, the EIL morphology could depend on both the EML and the capping electrode, i.e. the charge collection region, organization. For this reason, the morphology of both PF-EP and PFN+Br- layers was also investigated in the area under the drain electrode. As shown in Figure S4, the presence of a capping electrode did not influence the aggregate distribution and size in both EILs at the microscopic resolution if compared to CLSM images inside the channel (Figures 4c and 4d). Hence, for the sake of clarity, CLSM images recorded in the area inside the device


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channel are preferred to those under the drain electrode, which are characterized by a lower signal intensity to noise ratio due to scattering effects of the top electrode. By comparing CLSM images of the two EILs (Figure 4c and 4d), a higher amount of bright domains was evidenced in PF-EP (Figure 4c) while, PFN+Br- showed a finer textured morphology with fewer aggregates (Figure 4d). This may be correlated to the different chemical structure of the two materials, which induces a higher degree of aggregation in PF-EP with respect to PFN+Br-. Despite no relevant differences are evidenced between the two EIL morphologies, which are overall characterized by a similar and low-coarse morphology with few aggregates, the better performing PF-EP showed a slightly greater aggregation as also evidenced by atomic force microscopy analyses (Figure S5). This counterintuitive result suggests that the greater electron-injection behavior of PF-EP comparing to PFN+Br- may not have a morphological origin.

Figure 4. CLSM images, recorded in the area inside the channel, in the emission spectral range of the EML (a, b) or of the EILs (c, d) from PF-EP (a, c) and PFN+Br- (b, d) incorporating OLETs. Laser excitation wavelength: 405 nm. Image dimensions: 50 x 50 µm.



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Aside from the analysis of the morphology, the optical properties as well as the energy level positioning resulted similar for both PE-EP and PFN+Br- (Figure 1b). This, together with their diverse chemical structures, raises the question on the origin of the different optoelectronic behavior of the two interlayers, which may be due to either a different bulk conductivity or a different dipole induced injection mechanism.23,25 The former hypothesis, which would also involve charge transport limitations, can be excluded since the thickness of both EILs was intentionally confined to 10 nm. In addition, the process of electron injection by ohmic conduction diverges from the reported working principles of OLETs.47 We rather believe that the origin of the different behavior of the EILs can be ascribed to a different dipole formation at the EML/electrode interface. To evaluate this hypothesis, the surface polarity of PF-EP and PFN+Br- films was investigated through contact angle measurements on glass, which revealed indeed a relevant difference between the two EILs, 63° and 78° respectively for PF-EP and PFN+Br- (Figure S6). This result is in agreement with the optoelectronic properties of the corresponding OLETs and indicates the presence of a stronger dipole on the surface of the fully polar PF-EP film with respect to that formed on PFN+Br-. Indeed, it is worth mentioning that, despite endowed with an ionic monomer characterized by a high dipole moment, PFN+Br- is an alternating copolymer, for which the presence of an apolar fluorene co-monomer functionalized with two non-polar octyl side chains may produce an overall reduced polarity of the film. The measured different polarity together with the presence of polar/ionic groups respectively in the PF-EP and PFN+Br- films is therefore supposed to induce a different injection mechanism at the EML/electrode. In particular, PF-EP incorporating OLETs reported a more than twofold increased optical power and five-times-higher EQE with respect to the reference bilayer devices, while PFN+Br- incorporating OLETs showed a slightly lower EL than references. Indeed, in the case of PFN+Br-, some charge carriers recombine radiatively in the EIL itself rather than in the EML, which lowered the emissive excitons density. On the contrary, the partial hole blocking capability, in conjunction with the improved electron injection property of PF-EP EIL due to the favorable 16 ACS Paragon Plus Environment

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interfacial

dipoles

orientation,

results

in

hybrid-processed

OLETs

with

enhanced

electroluminescence. Despite the relatively high operating voltages, it is worth noting that the use of PF-EP in combination with high permittivity dielectric systems (such as a bilayers comprised by a high-k oxide layer and a planarizing topping polymeric layer) would open towards efficient and low-biased OLETs.

CONCLUSIONS The effective injection of electrons into the emissive layer is of fundamental importance to ensure efficient light emission in p-type OLETs. In this context, CPEs is a class of versatile EIL materials, which can bring significant benefits for optoelectronic applications. However, as here demonstrated, they also show some limitations due to the strong dependence of their electrical properties on the chemical structures. Indeed, it is well known that the counterion plays a relevant role on the operation of CPEs as EILs in optoelectronic devices. In view of these considerations, we here report for the first time the effective insertion of a solution-processed CPP, as non-ionic counterpart of CPEs, in OLETs to achieve simultaneous enhancement of electroluminescence intensity and efficiency. The hybrid concept of combining wet and dry deposition processes in the fabrication protocol represents a first step for future development of efficient and low-cost multilayer devices. Interestingly, the use of phosphonate moiety as lateral polar group in polyfluorene backbone proves to be more effective than the use of ionic trimethylammonium bromide. Indeed, it allows achieving a favorable alignment of dipole moments with the electrodes resulting in more than twofold increased light-power and five-times-higher EQE of p-type OLETs in comparison with the benchmark device without any interlayer. By in-depth investigation of the operation mechanisms of complete OLETs we here demonstrated that, despite characterized by similar optoelectronic properties of PF-EP, the insertion of ionic PFN+Br- limits the exciton formation process of the corresponding devices by favoring 17 ACS Paragon Plus Environment


Page 18 of 30

recombination of opposite charges in the EIL itself rather than in the emissive layer. Diversely, the use of polar PF-EP opened to the possibility to obtain a dipole-induced injection of electrons using a transparent and solution-processed material, which avoids the need of charged functionalized side chains. This represents a great benefit to improve the device performance while reducing the ionicdependent drawbacks.

EXPERIMENTAL SECTION Synthesis and characterization of polar and electrolytic polymers Poly[9,9-bis(6’-diethoxylphosphoryl-hexyl)fluorene] (PF-EP) was synthesized using a modified literature procedure.40 Briefly, 2,7-dibromo-9,9-bis(6’-bromohexyl)fluorene (0.087 g, 0.13 mmol) purchased from TCI Chemicals, 2,7-bis(pinacolato)diboron-9,9-(6’-dibromohexyl)-fluorene (0.100 g, 0.13 mmol) synthesized in our lab according to the literature,58 and Pd(PPh3)2Cl2 (0.001 g, 0.0013 mmol) were introduced in a Schlenk tube under nitrogen atmosphere. 3 mL of degassed toluene and 1.5 mL of Na2CO3 2M aqueous solution were added and the resulting mixture was stirred for 8 h at 82 °C. The end groups were capped by further 4 h reaction after addition of bromobenzene (0.065 mmol) and further 4 h after addition of phenylboronic acid pinacol ester (0.065 mmol). After cooling to room temperature, the organic phase was precipitated in 100 mL of methanol, to afford the poly[9,9-bis(6’-bromohexyl)fluorene] precursor polymer with a yield of 72%. 1

H NMR (CDCl3, 600 MHz, δ, ppm): 7.61 (2H, m, fluorenyl group), 7.45 (4H, m, fluorenyl group),

3.30 (4H, t, –CH2Br), 2.10 (4H, m, –CH2–) 1.68 (4H, m, –CH2–), 1.15 (8H, m, –CH2–), 0.76 (4H, m,–CH2–). GPC (THF, polystyrene standard) analysis showed a Mw = 10200 and PDI = 4.09. To obtain the phosphonated polymer PF-EP, 0.030 g of poly[9,9-bis(6’-bromohexyl)fluorene] were dissolved in 2 mL of triethyl phosphite and stirred for 20 h at 140 °C. Then the excess triethyl phosphite was distilled by rotavapor. PF-EP was obtained as a yellow solid with a yield of 96%. 18 ACS Paragon Plus Environment

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1

H NMR (CDCl3, 600 MHz, δ, ppm): 7.84 (2H, m, fluorenyl group), 7.68 (4H, m, fluorenyl group),

4.03 (8H, q, –CH2O–), 2.12 (4H, m, –CH2–), 1.58 (12H, m,–CH2–), 1.46 (4H, m, –CH2–), 1.25 (12H, t, –CH3), 0.83 (4H, m, –CH2–). Poly[(2,7-(9,9’-dioctyl)fluorene)-alt-(2,7-(9,9’-bis(5”-trimethylammonium bromide)pentyl)fluorene)] (PFN+Br-) was synthesized according to the literature.28 1

H NMR (CD3OD, 600 MHz, δ, ppm): δ 7.91-7.76 (m, 12H, fluorenyl group), 3.15 (m, 4H, -CH2N),

2.99 (s, 18H, -NCH3), 1.32-1.22 (m, 8H, H-alkyl), 1.65-1.56 (m, 6H, H-alkyl), 1.27-1.13 (m, 28H, H-alkyl), 0.83-0.74 (m, 12H, H-alkyl). GPC (THF, polystyrene standard) analysis showed a Mw = 54000 and PDI = 3.1.

Device fabrication and characterization OLETs were fabricated on 25 mm x 25 mm transparent glass/ITO substrates covered with 450 nmthick PMMA layer as gate dielectric according to the literature procedure.45 The organic active region consists of a stacked bi-layer of i) a high-mobility p-type semiconductor C8-BTBT (45 nm) covering the dielectric layer and ii) a 60 nm-thick host-guest emissive layer of TCTA (host) and Ir(ppy)3 (guest) (doping percentage: 20%). The organic bilayer stack was then covered by 70 nm thick source and drain silver electrodes deposited by thermal evaporation in a home-made highvacuum deposition chamber (base pressure of 10−6 mbar) using shadow masks. Concerning EILincorporated OLETs, both PF-EP and PFN+Br- were spin-coated at 2000rpm for 60 seconds without further treatments. The resulting devices present the following characteristics: 12 mm channel width (W), 70 µm channel length (L), 500 µm wide source and drain electrodes. Electrical and optical measurements were performed in inert atmosphere inside a nitrogen filled glove box. The light output was measured at the bottom side of the substrates (i.e. through the gate electrode) with a silicon photodiode (sensitivity of 0.38 A W−1 at 600 nm) directly in contact with the devices to enable collection of all emitted photons.



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The electrical measurements were carried out inside the glovebox using a standard SUSS probe station coupled to a B1500A Agilent semiconductor device analyzer. The field-effect mobility in the saturation regime (µsat) was calculated using the equation IDS = (W/2L)Ciµsat(VG - VT)2, where Ci is the capacitance per unit area of the insulating layer and VT is the threshold voltage extracted from the square root of the drain current (IDS1/2) versus gate voltage (VG) characteristics. The optical characterization of OLETs was carried out in air on epoxy-resin glass/glass encapsulated devices to avoid possible degradation of the samples. EL spectra and CIE coordinates were measured by using a commercial CS2000 Konica Minolta spectroradiometer. CLSM images were carried out with a Nikon TE2000 optical microscope, equipped with a 60x objective with 0.70 numerical aperture, connected with a Nikon EZ-C1 confocal scanning head using an excitation wavelength of 405 nm. UV-vis absorption spectra were obtained with a Perkin Elmer Lambda 900 spectrometer. PL spectra were obtained with a SPEX 270 M monochromator equipped with a N2 cooled CCD by exciting with a monochromated 450 W Xe lamp and corrected for the instrument response. Contact angle measurements were performed with an OCA20 instrument (Dataphysics Co., Germany), equipped with a photocamera CCD and with a 500 mL Hamilton syringe to dispense liquid droplets. Static measurements were made at room temperature by means of the sessile drop technique. At least ten measurements were performed at different places on each sample and results were averaged. Distilled water was used as probe liquid, and the delivered volume was 1.5 mL. Measurements were carried out with a time interval of 1 s between drop deposition and first measurement. The thickness of the OLET layers was measured by profilometer (KLA Tencor, P-6) and Atomic Force Microscopy (AFM). All the AFM investigations were performed using a NT-MDT NTEGRA instrument in semi-contact mode in ambient conditions.


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ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Saturation transfer curve of p-type bottom-gate topcontact C8-BTBT OFET. Absorption and emission spectra of PF-EP and PFN+Br- thin films. CLSM image of reference bilayer OLET. CLSM images of PF-EP and PFN+Br- incorporated OLETs in the area under the drain electrode. AFM images of PF-EP and PFN+Br- incorporated OLETs.

Contact

angles

of

PF-EP

and

PFN+Br-

thin

films.

AUTHOR INFORMATION Corresponding authors *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No780839, MOLOKO project. The authors gratefully



Page 22 of 30

acknowledge V. Ragona, F. Prescimone for the technical contribution and F. Mercuri for stimulating discussions.


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