The Effect of Nonionic Surfactant Additive in PEDOT:PSS on PFO

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The Effect of Nonionic Surfactant Additive in PEDOT:PSS on PFO Emission Layer in Organic-Inorganic Hybrid Light Emitting Diode Seong Rae Cho, Yoann Porte, Yun Cheol Kim, and Jae-Min Myoung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19267 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on February 26, 2018

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ACS Applied Materials & Interfaces

The Effect of Nonionic Surfactant Additive in PEDOT:PSS on PFO Emission Layer in OrganicInorganic Hybrid Light Emitting Diode Seong Rae Cho,‡ Yoann Porte,‡ Yun Cheol Kim, and Jae-Min Myoung*

Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea

‡

These authors contributed equally to this work.

*E-mail address: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

Poly(9,9-dioctylfluorene) (PFO) has attracted significant interests owing to its versatility in electronic devices. However, changes in its optical properties caused by its various phases and the formation of oxidation defects limit the application of PFO in light emitting diodes (LEDs). We investigated the effects of the addition of triton X-100 (hereinafter shortened as TX) in poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) in order to induce interlayer diffusion between PEDOT:PSS and PFO to enhance the stability of the PFO phase and suppress its oxidation. Photoluminescence (PL) measurement on PFO/TX-mixed PEDOT:PSS layers revealed that, upon increasing the concentration of TX in the PEDOT:PSS layer, the β phase of PFO could be suppressed in favor of the glassy phase, and the wide PL emission centered at 535 nm caused by ketone defects formed by oxidation was decreased considerably. LEDs were then fabricated using PFO as an emission layer, TX-mixed PEDOT:PSS as hole transport layer, and zinc oxide (ZnO) nanorods (NRs) as electron transport layer. As the TX concentration reached 3 wt%, the devices exhibited dramatic increases in current densities, which was attributed to the enhanced hole injection due to TX addition, along with a shift in the dominant emission wavelength from a green electroluminescence (EL) emission centered at 518 nm to a blue EL emission centered at 448 nm. The addition of TX in PEDOT:PSS induced a better hole injection in the PFO layer, and, through interlayer diffusion, stabilized the glassy phase of PFO and limited the formation of oxidation defects.

KEYWORDS: poly(9,9-dioctylfluorene) (PFO), polymer-based organic light emitting diodes (PLED),

interlayer

diffusion,

poly

(3,4-ethylenedioxythiophene):poly(styrenesulfonate)

(PEDOT:PSS), triton X-100,


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INTRODUCTION Conjugated polymers have attracted significant interests in thin film electronic devices owing to their versatile properties, along with low fabrication cost, handiness, and flexibility.1–6 Amongst them, poly(9,9-dioctylfluorene) (PFO) and its derivatives have been extensively studied as a blue emitting layer in polymer-based organic light emitting diodes (PLED),1,2 but also as a photocatalyst,3 laser,4 and hole transport layer (HTL) in photovoltaic devices and light emitting devices (LEDs).5,6 Despite being widely investigated, PFO still possesses several drawbacks preventing its optimized integration in PLED, such as its multiple phases and the formation of ketone defects. The different phases of PFO such as amorphous, glassy, nematic, and β phase determine its optical properties.7,8 Thus, when applied in LEDs as an emitting layer, it is essential to control its phase. The phase of PFO is affected by multiple parameters such as the solution,9 solvent type,10,11 temperature,2 and interaction with other elements in solution.12,13 Moreover, undesirable low energy emissions caused by the formation of oxidation defects on the backbone of the polymer, known as ketone defects, are occasionally observed in PFO.14,15 These ketone defects generally induce an undesirable green emission in PFO and constitute a major obstacle for the application of PFO in the field of optoelectronics. Therefore, to stabilize the optical properties of PFO, it is necessary to control its phase and suppress the formation of oxidation defects. Several research groups addressed this issue by substituting the alkyl chain of PFO, which gave rise to a low energy emission when replaced with oxygen,16,17 or by synthesizing polymers with new units.18,19 They synthesized the polymers with pure blue emission, however, the synthesis process consisted of a series of complex steps. On the other hand, other researchers reported that the optical and electrical properties of the emission layer could be manipulated by interlayer diffusion, which consists in a partial intermixing of two


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layers in contact and induces interactions between the two materials.20–22 Amongst these studies, Zhu et al.20 and He et al.22 successfully reported the control of PFO properties by interlayermixing phenomenon with the bottom layer through in-situ spin-coating process. This process involves a simple method compared to the synthesis of new PFO-based polymer and can help controlling the phase of PFO. In this work, we mixed a nonionic surfactant additive, triton X-100 (TX) with a poly (3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) aqueous solution. Researchers have widely studied the effect of PEDOT:PSS when mixed with various additives on its morphology and electrical properties. Nevrela et al. and Crispin et al. focused on improving the conductivity of PEDOT:PSS by using additives such as dimethyl sulfoxide, sorbitol, ethylene glycol, and N,N-dimethylformamide.23–25. Among the various additives investigated, TX has been previously reported to improve the conductivity of PEDOT:PSS.26,27 When mixed with it, TX induced the phase separation of PEDOT:PSS into complexes TX-PEDOT and TX-PSS, along with a structural transition of the PEDOT segments from benzoid to quinoid structure, which improved its π-π stacking. In this case, TX is used to induce interlayer diffusion between the PEDOT:PSS and the PFO layers due to its solubility in the solvent used for the PFO solution. Zaman et al. reported the use of ZnO nanorods (NRs) as electron transport layer (ETL) in LEDs with PFO as the emitting layer.9 ZnO is widely used as ETL in organic LEDs owing to its high electron mobility, transparency, and superior stability for long term use compared to organic ETL.28 As a consequence, LEDs were fabricated using PFO as an emitting layer, TX-mixed PEDOT:PSS as a HTL, and ZnO NRs as an ETL. The electrical and optical properties of the devices were measured and analyzed, and the interlayer diffusion phenomenon was discussed.


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EXPERIMENTAL SECTION Device fabrication. The schematic illustration of the device fabrication process is presented in Figure 1a. The silicon (Si) (100) substrates were cleaned through sonication in acetone, methanol, and deionized (DI) water for 10 min, respectively, and dried with nitrogen (N2) gas. Then, a 50 nm-thick silver (Ag) film was deposited on Si substrate by e-beam evaporation to be used as bottom electrode. PEDOT:PSS solution (Clevious P, Heraeus) was mixed with TX (C14H22O(C2H4O)n (n=9–10), Sigma Aldrich) with different concentrations (0 (pristine), 1, 3, and 5 wt%) and vigorously stirred for 12 h. Subsequently, the PEDOT:PSS solutions with different TX concentrations were spin-coated at 4000 rpm for 1 min on the Ag-deposited Si substrates and then the samples were annealed at 120 °C for 30 min in air on a hot plate. Then, 5 mg/mL PFO solution was prepared by dissolving PFO (C8H9(C29H40)nC8H9, Sigma Aldrich) in toluene, and the PFO solution was spin-coated at 2000 rpm for 1 min on the as-prepared samples to obtain 55 nm-thick PFO film, followed by annealing at 150 °C for 30 min in air on a hot plate. In order to use it as a seed layer for the growth of the n-type ZnO NRs on the PFO layers, ZnO nanoparticles (NPs) with average diameter of 20 nm were synthesized by using a typical method.29 Prepared ZnO NPs were dispersed in DI water. ZnO NP solution was spin-coated on the PFO layers, followed by annealing at 150 °C for 30 min in air. Then, these samples were put in aqueous solution containing 20 mM zinc acetate dihydrate (Zn(CH3COO)2·2H2O, 99.999 %, Sigma Aldrich) and equimolar hexamethylenetetramine (C6H12N4, HMTA), and annealed at 95 °C for 3 h in order to grow the n-type ZnO NRs with average length of 3.3 µm by conventional hydrothermal growth method.30,31 Ag nanowire (NW) solution (average length of 30 µm and average diameter of 25 nm, prepared in ethanol in ratio of 0.1 wt%, Nanopyxis Inc.) was diluted in DI water in volume ratio of 1:20. This diluted solution was then spray-coated on


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the n-type ZnO NRs to form 100 nm-thick top electrode using a dot-patterned shadow mask with a diameter of 250 µm.32 This Ag NW electrode has an average sheet resistance of 6.6 Ω/sq. The final device structure is schematically illustrated in Figure 1b and a cross-sectional SEM image showing different layers is presented in Figure 1c. Characterization and measurement. The morphology and thickness of the PEDOT:PSS layers were characterized by scanning electron microscopy (SEM, S-5000, Hitachi). The absorbance of films and the transmittance of Ag NWs were measured by an ultraviolet-visible spectrophotometer (UV-vis-NIR, V-670, JASCO). Sheet resistance of the Ag NW film was examined by a four-point probe measurement system (CMT-SR1000N, AiT). The photoluminescence (PL) spectra of the films and ZnO NRs were measured by a µ-PL measurement system (Dongwoo Optron) using an IK3252R-E He-Cd laser (λ = 325 nm) coupled with a Mono Ra 320i monochromator and an Andor SOLIS simulation package. The depth profiles of the chemical elements from the samples were analyzed by X-ray photoelectron spectroscopy (XPS, K-alpha, Thermo U. K.). The electrical properties of the devices were analyzed by a semiconductor parameter analyzer system (Agilent B1500A, Agilent Technologies). Work function of TX-mixed PEDOT:PSS was measured by surface analyzer (RKEN). The ultraviolet photoelectron spectroscopy (UPS) spectra of TX-mixed PEDOT:PSS were measured by AXISNOVA (Kratos, Inc.). The electroluminescence (EL) spectra were measured in Andor SOLIS simulation software combined with a charge-coupled device (CCD) camera (DV401A-BV). Chromaticity color coordinates of the EL spectra of the devices were analyzed by chromaticity meter (CS-200, Minolta).


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RESULTS AND DISCUSSION

Figure 1. (a) Schematic illustration of the device fabrication process, (b) device structure, and (c) cross-sectional SEM image of the device with scale bar of 2 µm.

In this study, TX-mixed PEDOT:PSS layers and PFO/TX-mixed PEDOT:PSS layers with TX concentrations between 0 and 5 wt% were deposited on glass substrates by spin coating, and their thickness are shown in Figure S1. Figure S1a−h shows the cross-sectional SEM images of all films, and it is confirmed that fairly uniform layers are formed. From Figure S1a−d and i, with increasing the concentration of TX from 0, through 1, 3, to 5 wt%, the thickness of TXmixed PEDOT:PSS layers increased from 45, through 60, 120, to 150 nm. This result is due to the fact that TX increases the viscosity of the TX-mixed PEDOT:PSS solution, as reported by López et al.,33 leading to the thicker films. However, the thickness of PFO/TX-mixed PEDOT:PSS layers exhibited the different trend. As shown in Figure S1e−i, with increasing the


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concentration of TX from 0, through 1, 3, to 5 wt%, the thickness of PFO/TX-mixed PEDOT:PSS layers increased from 100, through 110, 140, to 155 nm. This gradual increase of thickness is due to the increased dissolution of TX-mixed PEDOT:PSS layer at higher TX concentrations by the toluene of PFO.

Figure 2. (a) Normalized light absorbance and (b) normalized PL spectra of PFO film and PFO/TX-mixed PEDOT:PSS layers with different TX concentrations – 0 (pristine), 1, 3, and 5 wt%.

In order to investigate the effect of TX-mixed PEDOT:PSS on the optical properties of PFO, a PFO film was deposited on top of the 0, 1, 3, and 5 wt% TX-mixed PEDOT:PSS layers by spin coating. PFO possesses multiple phases with different optical properties, and their formation is regulated by conformational factors. Amongst the various phases of PFO, this work mainly treats about the β phase and glassy phase. The main difference between those two phases lies in the intermonomer torsion angle. The glassy phase has a disordered conformation with a broadly distributed torsion angle. On the other hand, the β phase has a torsion angle of 180° and coplanar orientation.7,8 The formation of the β phase of PFO over its glassy phases results in an undesired


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green-shift in the emission wavelength caused by the geometrical changes and the electronic delocalization.7,8 The normalized absorbance and PL spectra of PFO film and PFO/TX-mixed PEDOT:PSS layers with 0, 1, 3, and 5 wt% are displayed in Figure 2a and b, respectively. All the films exhibited a broad peak around 380 nm in their absorbance spectra, which is characteristic of glassy phase PFO.34 In the case of PFO film only, an additional shoulder peak can be observed at 431 nm in the inset of Figure 2a. This absorption peak has been reported to be characteristic of the β phase PFO and its amplitude is highly dependent upon the fraction of β phase present in the film.8,35 If the PFO film contains some of the β phase chain, it is considered as β phase PFO regardless of the presence of glassy phase.34 While the PFO film without PEDOT:PSS is the only film showing β phase in its absorbance spectrum, the PL data in Figure 2b indicates otherwise. The PL peaks at 438, 466, and 497 nm are characteristic of the β phase PFO and appear not only in the single PFO film but also in the PFO/TX-mixed PEDOT:PSS layers with 0 and 1 wt%.8,35 However, in the case of the 1 wt% TX-mixed PEDOT:PSS film, a shoulder peak appears at 430 nm, which is a characteristic peak of the glassy phase PFO.8,35 As the TX concentration increases further to 3 and 5 wt%, the peaks at 438, 466, and 497 nm are no longer observable while the peak at 430 nm becomes dominant. This indicates that the addition of TX in PEDOT:PSS layer helps retaining the glassy phase of PFO. Upon introducing pure PEDOT:PSS under the PFO layer, the absorbance peak at 431 nm of the β phase PFO disappeared as shown in pristine PFO. However, no change was observed in the PL peaks corresponding to the β phase. This fact indicates that no phase transition occurred in the PFO deposited on pristine PEDOT:PSS, but only a lateral phase separation was observed as reported by Zhu et al.20 Meanwhile, the PL spectra of PFO only and PFO/TX-mixed PEDOT:PSS layers with 0 and 1 wt% exhibited a broad peak at 535 nm. This low energy emission originates from


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ketone defects present in the PFO film.36,37 The alkyl chains of the PFO are replaced by oxygen (O) through oxidation process and the O in the PFO acts as a deep trap for electrons between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO).36,37 The broad peak at 535 nm corresponding to ketone defects was not observed from the PFO/TX-mixed layers with 3 and 5 wt%, implying that the addition of TX in PEDOT:PSS reduces the degree of oxidation in the PFO layer. In order to confirm that the TX treatment in PDEOT:PSS endows PFO with resistance to oxidation and prevents the formation of ketone defects, PFO/TX-mixed PEDOT:PSS layers with 0, 1, 3, and 5 wt% were exposed to ultraviolet-ozone (UVO) treatment for 10 min. Their PL spectra are exhibited in Figure S2. When polyfluorene polymers are exposed to UV radiation, a photo-oxidative degradation occurs, which can be observed in their PL spectra as they exhibit a specific oxidation defect (ketone defect) peak at 535 nm.14 This peak is dominant in the PL spectra of both the 0 and 1 wt% films, with a slight decrease of intensity with the 1 wt% films, confirming the oxidation of the PFO layer. As the TX concentration increased to 3 wt%, the intensity of the ketone peak at 535 nm dropped dramatically, until it disappeared at a concentration of 5 wt%. This confirms that, even though TX is added into the PEDOT:PSS sublayer, TX plays an important role into preventing the formation of ketone defects in PFO. To investigate the effect of the TX treatment on the PFO layer, 1, 3, and 5 wt% TX was directly mixed in the PFO solution and then spin coated on top of the pristine PEDOT:PSS layer. Their absorbance and PL spectra are displayed in Figure S3. As shown in Figure S3a, the absorbance spectra of the films did not show any β phase feature at 431 nm, however, the PL spectra (Figure S3b) revealed the existence of β phase PFO within the film with peaks at 438, 466, and 497 nm regardless of the TX concentration in PFO. Through this information, it can be


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confirmed that the addition of TX in PFO induced a lateral phase separation of PFO, not a conformational change,20 and that it does not have a direct role either in the formation of glassy phase PFO or in the decrease of ketone defects. Therefore, the changes observed in the optical properties of PFO are linked to the specific addition of TX in PEDOT:PSS, which affects the alignment of PEDOT:PSS and leads to the formation of complexes in nanofibril shape.27,38

Figure 3. (a) XPS spectra for S 2p binding energy of PFO layer/pristine PEDOT:PSS and TXmixed PEDOT:PSS with 5 wt%. XPS depth profiles of (b) PFO/pristine PEDOT:PSS and (c) PFO/TX-mixed PEDOT:PSS with 5 wt%.

It has been reported that the conformation of PFO can be manipulated using the layer underneath by interlayer diffusion, which generally happens during the mixing or diffusion of materials between different layers.31,36 A requirement for this effect to occur is for the coated material to be soluble in the solvent of the deposited solution.20,22 As a nonionic surfactant, TX is soluble in toluene and able to lead the nanofibril complexes to be dissolved in the PFO solution. To further investigate this interlayer diffusion phenomenon, XPS depth profile analyses were carried on PFO/TX-mixed PEDOT:PSS films with 0 and 5 wt% deposited on Ag-covered Si substrate, as shown in Figure 3. Figure 3a shows the binding energy profile of the sulfur (S) 2p


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orbital at a depth of 5 nm for the 0 and 5 wt% films. While no S is detected within the PFO deposited on pristine PEDOT:PSS, the PFO film on 5 wt% TX-mixed PEDOT:PSS shows S 2p peaks at 164.2 and 169 eV, corresponding to PEDOT and PSS respectively.39 PFO does not contain any S in its structure, which means it comes from PEDOT or PSS.40 This observation confirms the diffusion of the nanofibril PEDOT and PSS complexes within the PFO region when adding 5 wt% TX to PEDOT:PSS. Zhu et al.20 observed the disappearance of the β phase PFO after PFO was spin coated on top of poly(9-vinylcarbazole) (PVK). They suggested the interlayer diffusion mechanism where some PVK chains which dissolved into the PFO solution remained partly embedded in the undissolved PVK. As a consequence, when the spin coating was initiated, the free part of the PVK chains would be oriented along the centrifugal force, confining the PFO chains and leading to a homogeneous dispersion of PFO in the PVK matrix. It is believed a similar phenomenon is occurring during the spin coating of PFO on top of TX-mixed PEDOT:PSS caused by the diffusion of TX-PEDOT and TX-PSS complexes in the PFO solution. This effect was not observed with the 1 wt% TX concentration, which is attributed to the insufficient amount of complexes dissolved in PFO. It should be noted that the interlayer diffusion phenomenon is dependent upon the amount or thickness of the first-coated layer, and it cannot occur if the polymers soluble in the PFO solution are entirely washed off during the spincoating process.20,41 The full XPS depth profiles of the PFO/TX-mixed PEDOT:PSS films with 0 and 5 wt% are displayed in Figure 3b and c. As mentioned previously, the PFO structure does not possess any S or O, as opposed to the PEDOT:PSS structure which contains both. Therefore, the PFO and the TX-mixed PEDOT:PSS layers can both be distinguished on the XPS depth scans due to the significant increases in O 1s and S 2p intensity present in PEDOT:PSS. This confirms that the two layers remained distinct in the manner that the PEDOT:PSS layer was not


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completely washed off during the spin coating process. Increasing the TX concentration in the TX-mixed PEDOT:PSS solution leads to the formation of thicker films, as observed in Figure 3b and c, due to an increase in the viscosity of the solution.33 The lower thickness observed in the XPS depth profile of the 5 wt% TX-mixed PEDOT:PSS layer, compared to the as-deposited film, is due to the solubility of TX in toluene, which causes a part of the PEDOT:PSS film to be washed off during the spin coating of the PFO solution.20 However, although the thickness of TX-mixed PEDOT:PSS decreased after PFO spin-coating, the thickness of TX-mixed PEDOT:PSS after spin coating can be thicker than that of not treated PEPDOT:PSS due to being only partially dissolved in the PFO solvent and to the increased viscosity of the TX-mixed PEDOT:PSS as shown in Figure S1. Ultraviolet photoelectron spectroscopy (UPS) analyses were performed on the PFO/TX-mixed PEDOT:PSS for 0 and 5 wt.% to measure the position of the HOMO of PFO. The cut-off region and HOMO edge measurements are displayed in Figure S4 along with the optical band gap extracted by Tauc plot method from absorbance measurements. Figure S4a shows the secondary electron cut-off energy (Esec) region and Figure S4b shows HOMO edge (Eedge) region of UPS He I spectra. The Esec of PFO on TX-mixed PEDOT:PSS for 0 and 5 wt% corresponds to 17.14 and 16.66 eV respectively, while the Eedge of PFO on TX-mixed PEDOT:PSS for 0 and 5 wt% is 1.70 and 1.22 eV respectively. The ionization potential (IP), corresponding to the HOMO energy of PFO, was then extracted using the following equation:42

IP = hv – (Esec – Eedge)


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where hv is the energy of the incident He I rays, i.e. 21.20 eV. Regardless of the TX concentration in PEDOT:PSS, the IP value of PFO was measured to be 5.76 eV in both cases, which means that the addition of TX into PEDOT:PSS did not affect the HOMO position of PFO. Similarly, the band gap of PFO did not show significant variations for TX concentrations of 0, 1, 3, and 5 wt% which exhibited band gaps of 2.90, 2.91, 2.90, and 2.91 eV, respectively, as shown in Figure S4c.

Figure 4. (a) J–V and L–V characteristics, (b) current efficiency, (c) band energy diagram, and (d) I-t characteristics of the devices using TX-mixed PEDOT:PSS layers with different concentrations – 0 (pristine), 1, 3, and 5 wt%.


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In order to further investigate the effect of the TX addition, LED devices were fabricated. The fabrication process is illustrated in Figure 1a and detailed in the experimental part. The PEDOT:PSS layers with different concentrations of TX were incorporated as the HTL with PFO as the emitting layer. Here, ZnO was incorporated as ETL owing to its transparency and good electrical property.28 The ZnO NRs used in the devices were grown for 3 h. When using growth time below 3 h, the device exhibited poor J–V behavior. Figure S5a and b shows the J–V characteristics and cross-sectional SEM images of ZnO NRs grown for 1 and 2 h, respectively. The lengths of ZnO NRs grown for 1 and 2 h were 1.2 and 2.5 µm, respectively, and were shorter than that of ZnO NRs grown for 3 h (3.3 µm), which may induce undesirable electrical path of charge carrier, including contact not only between ZnO NRs and Ag NWs, but also between PFO and Ag NWs. On the other hand, the device with ZnO NRs grown for 3 h didn’t exhibit the phenomenon and, therefore, we fabricated the device with these ZnO NRs. The current density (J)–voltage (V) and the luminance (L)–V curves of the devices are displayed in Figure 4a. As the TX concentration increases, J increases as well and is further amplified when it reaches 3 wt%. Moreover the luminance of the device increased with increasing TX concentration. The luminance of the devices with 0, 1, 3, and 5 wt% TX-mixed PEDOT:PSS was 20.4, 70.1, 267.5, and 985.1 cd∙m-2, respectively, at 7 V. The current efficiency was calculated from the J–V and L– V curves of the devices as exhibited in Figure 4b. The overall current efficiency slightly increased in the device with 3 wt% TX-mixed PEDOT:PSS with a maximum current efficiency of 0.83 cd∙A-1. However, in the case of the device with 5 wt% TX-mixed PEDOT:PSS, the current efficiency dramatically increased, with maximum current efficiency of 2.55 cd∙A-1.To explain this behavior a band diagram of the device is represented in Figure 4c, using the UPS


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measurements on PFO along with the work function of the TX-mixed PEDOT:PSS films, which has been previously extracted in Figure S6a. The work function of Ag and Ag NWs were measured to be 4.82 and 4.5 eV, respectively, and the band energy structure of ZnO is reported from Yang et al.43 As the TX concentration in PEDOT:PSS increased from 0 to 5 wt%, the work function of PEDOT:PSS gradually increased from 5.05 to 5.32 eV respectively. The deeper work function of the 5 wt% TX-mixed PEDOT:PSS allows to reduce the energy gap with the HOMO of PFO, thus decreasing the hole injection energy barrier between the two layers, which can lead to a better hole injection into PFO.44 Moreover, with increasing TX concentration, the conductivity of PEDOT:PSS has been shown to improve considerably, as shown in Figure S6b.26,27 This result confirms the effect of TX on the conductivity of PEDOT:PSS, with a drastic increase from 0.31 S/cm for pristine PEDOT:PSS to 29.10 S/cm for 5 wt.% TX-mixed PEDOT:PSS. Therefore, the larger J, L, current efficiency observed are attributed to the conductivity and work function improvement in the addition of TX to the PEDOT:PSS layer.45 Furthermore, the current (I) – time (t) curves displayed in Figure 4d show that all the devices were stable under a forward bias of 7 V and did not exhibit significant fluctuations in I during the entire operation time. The stability of the devices was not affected by the addition of TX in the PEDOT:PSS layer.


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Figure 5. (a) EL spectra of the devices employing TX-mixed PEDOT:PSS layers with different concentrations - 0, 1, 3, and 5 wt%. Inset: Normalized EL spectra. (b) CCD images of the same devices with scale bar of 100 µm at 7 V and (c) CIE 1931 chromaticity diagram of the same devices.

Figure 5a shows the electroluminescence (EL) data of the devices under a forward bias of 7 V and the inset graph shows the normalized intensities. As the concentration of TX increased, the EL intensity of the devices increased considerably. This is a direct result of the conductivity enhancement and deeper work function of the PEDOT:PSS layer, resulting in larger J in device. Upon variation of the TX concentration, a change in the emission wavelength was observed, resulting in different color profiles. The EL spectra of the 0 and 1 wt% devices exhibited a dominant peak at 528 nm and a secondary peak at 450 nm. The emission at 528 nm is a consequence of the ketone defects present in the 0 and 1 wt% devices, and is an issue when PFO is used as blue emitting layer in PFO-based LEDs.36,37 The ketone defects have been previously


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observed in the PL data, and result in this case in a green-color emission visible to the naked eye, as shown in Figure 5b. Here, the dark image around the center area is due to the low transmittance (∼62.5%) of the top Ag NW electrode, as shown in Figure S7. On the other hand, as the concentration of TX increased to 3 and 5 wt%, the devices exhibited a dominant emission peak at 448 nm and a secondary peak at 518 nm. The green emission switches to a blue emission visible to the naked eye, Figure 5b, suggesting a reduced amount of ketone defects in the PFO film when the TX concentration reaches 3 wt%. A slight shift in the emission wavelength from 450 nm to 448 nm is also observed as the amount of TX increases above 3 wt%, which is due to the phase shift from β phase to glassy phase in PFO,8,34,35 as observed in the PL data (Figure 2b). The difference in relative intensity between the emission peaks at 450 and 518 nm as the concentration of TX increases reveals that above 3 wt% the impact of the ketone defects is significantly reduced. The optical properties of n-type ZnO NRs are presented in Figure S8 and our previous report.46 A strong near-band edge emission (NBE) centered at 379 nm and broad deep-level emission (DLE) centered at 534 nm, which are related to excitonic transitions and defect-related radiative transitions, respectively, are observed.46 Therefore, the resulting emission profile of the device is not influenced by the PL spectrum of ZnO NRs. The quantitative information on the luminescence profile of the devices is given in Figure 5c. The chromatic coordinates of the EL spectra of the devices are reported in the Commission Internationale de l’Eclairage (CIE) 1931 diagram. As the concentration in TX increased, the color emission of the devices moved to the blue region. This blue shift is even more significant between 1 and 3 wt% TX concentrations, which appears to be a key point in controlling the optical properties of the PFO film. These results confirm what has been previously observed during the analysis of the PFO/TX-mixed PEDOT:PSS films. The addition of TX to the PEDOT:PSS layer increases its


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conductivity and work function, enabling a better hole injection into the PFO layer, but also indirectly alters the optical properties of PFO by favoring the formation of its glassy phase and preventing the formation of ketone defects.


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CONCLUSIONS The interactions between PFO and TX-mixed PEDOT:PSS films have been investigated in this study. When mixed with PEDOT:PSS, TX separates PEDOT from PSS by weakening the ionic interaction and forms complexes with PEDOT and PSS independently, and improves the π–π stacking of the PEDOT chains, leading to an increase in conductivity. As-prepared PEDOT:PSS solutions containing TX were spin-coated on glass substrates and PFO solution was then spincoated on the PEDOT:PSS layer. During the PFO coating process, the PEDOT and PSS complexes diffused into PFO, owing to the solubility of TX in the PFO solution, and induced a change in the structure of PFO from β phase to glassy phase. Moreover, the addition of TX to the PEDOT:PSS layer endowed PFO with resistance to oxidation, preventing the formation of ketone defects. As a result, the optical properties of PFO were modified, as revealed in PL analyses. The treated layers were then incorporated in LED devices. An increase of the current density was observed, which was attributed to the increase of conductivity and work function in the PEDOT:PSS layer upon addition of TX. As the concentration of TX in PEDOT:PSS increased from 1 to 3 wt%, the emission wavelength shifted from a dominant green peak at 518 nm to a dominant blue peak at 448 nm. This change in emission wavelength is attributed to a decrease of ketone defects formed by oxidation of the PFO layer, as well as to a phase shift leading to the glassy phase PFO instead of β phase. In this study, we successfully controlled the phase of PFO and limited the formation of oxidation defects by taking advantage of the simple interlayer diffusion phenomenon between the TX-mixed PEDOT:PSS and the PFO layer. Therefore, even though more efforts are still required to optimize PFO in LED application, this research allowed to improve the stability of its optical properties with a blue emission centered at 448 nm without altering the fabrication process.


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ASSOCIATED CONTENT Supporting Information SEM images and thickness of TX-mixed PEDOT:PSS films and PFO/TX-mixed PEDOT:PSS films. PL spectra of PFO film and the PEDOT:PSS/PFO layers as a function of TX concentration in PEDOT:PSS after UVO treatment. Normalized light absorbance profile and normal PL spectra of TX-mixed PFO layers as a function of TX concentration in PFO. UPS spectra and Tauc plot of PFO as a function of TX concentration in PEDOT:PSS. J–V behaviors and SEM images of the devices with ZnO NRs grown for 1 and 2 h. Work function and conductivity of TX-mixed PEDOT:PSS as a function of TX concentration. Transmittance of Ag NW electrode. PL spectrum of ZnO NRs.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by Samsung Research Funding Center of Samsung Electronics under Project Number SRFC-MA1301-07.


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The Effect of Nonionic Surfactant Additive in PEDOT:PSS on PFO

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