High-Performance Electron Injection Layers with a Wide Processing

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High-Performance Electron Injection Layers with a Wide Processing Window From an Amidoamine-Functionalized Polyfluorene Sebastian Stolz, Martin Petzoldt, Sebastian Dück, Michael Sendner, Uwe H. F. Bunz, Uli Lemmer, Manuel Hamburger, and Gerardo Hernandez-Sosa ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03557 • Publication Date (Web): 10 May 2016 Downloaded from http://pubs.acs.org on May 13, 2016

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ACS Applied Materials & Interfaces

High-Performance Electron Injection Layers with a Wide Processing Window From an Amidoamine-Functionalized Polyfluorene Sebastian Stolz*,≠,‡, Martin Petzoldt√,‡, Sebastian Dück∏, Michael Sendner∫,‡, Uwe H. F. Bunz√,#, Uli Lemmer√,≠,∆ , Manuel Hamburger√,‡,†, Gerardo Hernandez-Sosa*,≠,‡. ≠

Karlsruhe Institute of Technology, Light Technology Institute, Engesserstr. 13, 76131

Karlsruhe, Germany ‡

InnovationLab, Speyerer Str. 4, 69115 Heidelberg, Germany

√

Ruprecht-Karls-Universität Heidelberg, Organisch-Chemisches Institut, , Im Neuenheimer

Feld 270, 69120 Heidelberg, Germany ∏

∫

Cynora GmbH, Werner-von-Siemens-Straße 2-6, 76646 Bruchsal, Germany

Ruprecht-Karls-Universität Heidelberg, Kirchhoff-Institute für Physik, , Im Neuenheimer

Feld 227, 69120 Heidelberg, Germany #

Ruprecht-Karls-Universität Heidelberg, Centre of Advanced Materials, Im Neuenheimer

Feld 225, 69120 Heidelberg, Germany ∆

Karlsruhe Institute of Technology, Institute of Microstructure Technology, Hermann-von-

Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.

ACS Paragon Plus Environment


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KEYWORDS: OLEDs, amino-functionalized polyfluorenes, polyethylenimine, electron injection layers, solution-processing.

ABSTRACT

In this work, we present organic light-emitting diodes utilizing a novel amidoaminefunctionalized polyfluorene (PFCON-C) as an electron injection layer (EIL). PFCON-C consists of a polyfluorene backbone to which multiple tertiary amine side-chains are connected via an amide group. The influence of molecular characteristics on electronic performance and morphological properties were tested and compared to the widely used, literature known, amino-functionalized polyfluorene (PFN) and to polyethylenimine (PEI). PFCON-C reduces the turn-on voltage VON of PPV based OLEDs from ~ 5 V to ~ 3 V and increases the maximum power efficiency from < 2 lm W-1 to > 5 lm W-1 compared to PFN. As a result of its semiconducting backbone, PFCON-C is significantly less sensitive to the processing parameters than PEI, and comparable power efficiencies are achieved for devices where thicknesses of PFCON-C are between 15 and 35 nm. AFM measurements indicate that the presence of non-polar side-chains in the EIL material is important for its film forming behavior while Kelvin probe measurements suggest that the amount of amine groups in the side-chains influences the work-function shift induced by the EIL material. These results are used to suggest strategies for the design of polymeric electron injection layers.

INTRODUCTION The fabrication of organic light-emitting diodes (OLEDs) by high-throughput printing techniques requires the development of solution-processable electron injection layers.1 These materials reduce the work-function of electrodes and can thus be used as cathode layers in


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devices in combination with high work-function electrodes like Al, Ag, Au or ITO. Today, either alkali salts or low work-function alkaline earth metals like calcium or barium are used as injection layers / cathodes in OLEDs. These materials are highly reactive and cannot be easily solution processed. In order to tackle this problem, different solution-processable electron injection layers (EILs) have been investigated in the context of OLEDs, organic solar cells and organic field effect transistors (OFETs) over the last decade. Self-assembled monolayers (SAMs) have been successfully used as electron injection layer in OFETs.2–7 If applied to OLEDs though, SAMs can only be used in an inverted device architecture. Furthermore, their molecular structure needs to be custom tailored for the used electrode material. Solution-processable Cs2CO3 and alkali metal stearates have been investigated as electron injection layer in OLEDs as well as organic solar cells.8–12 However, it has been shown that the high performance of such devices requires the use of thermally evaporated Al as a top electrode.10 Due to these shortcomings, polymeric electron injection layers, which can be of neutral, cationic, anionic as well as zwitterionic character, have recently, raised interest. These materials can be used in combination with a large variety of electrodes in both regular and inverted device architectures.13–22 In this context, two classes of neutral polymeric amine based injection materials, aliphatic amines and amino-functionalized polyfluorenes, have been extensively investigated during the last few years. On the one hand, aliphatic amines such as polyethylenimine (PEI) and polyethylenimine-ethoxylated (PEIE) are insulators known to strongly reduce the work-function of various metals when spin-cast on top14,15. Both polymers have been successfully used as electron injection layers in OLEDs13– 16

. However, processing of PEI(E) by high-throughput industrial techniques poses an

enormous challenge as film thickness needs to be < 10 nm in order to reach high device performances13–17. On the other hand, amino-functionalized polyfluorenes such as PFN, are semiconductors, potentially enabling the use of thicker layers.18–20 However, while OLEDs



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that use PFN EILs exhibit high current efficiencies, they suffer from limited power efficiencies due to high operational voltages.23,24 In this work, we introduce a novel amidoamine-functionalized polyfluorene (PFCON-C), consisting of a polyfluorene backbone connected to multiple tertiary amine side-chains via an amide, and compare its performance in OLEDs to literature known EIL materials PFN and PEI. Kelvin probe measurements suggest that the work-function shift induced by the EIL polymers to the Ag cathode is correlated to the number of nitrogen-containing groups of the polymers’ side-chains. As PFCON-C and PEI have more amine groups than PFN, higher work-function shifts and, as a result, a better performance as EIL in OLEDs are observed. While PFCON-C performs comparable to PEI as an EIL, it leads to a more than two fold increase in power efficiency compared to PFN. Furthermore, in contrast to PEI, it shows a large processing window with high power efficiencies for layer thicknesses between 15 and 35 nm. This fact, which is a result of its semiconducting conjugated backbone, is very beneficial for industry relevant printing applications. Additionally, we found correlations between the OLED results and the topography as well as electronic properties of the EIL materials. Atomic force microscopy (AFM) images of films of the three EIL materials, which were processed by different parameters, indicate that the presence of non-polar side-chains is beneficial for the wetting, and thus film forming properties, of the polymers on top of the emitting layer.

RESULTS AND DISCUSSION Synthesis and characterization of PFCON-C. As shown in Scheme 1, 6,6'-(2,7-dibromo9H-fluorene-9,9-diyl)bis(N,N-bis(3-(dimethylamino)propyl)hexanamide) (1) and 2,7Bis(4,4,5,5-tetramethyl[1.3.2]dioxaborolan-2yl)-9,9-bis(2-ethylhexyl)-fluorene (2) were prepared according to procedures similar to ones described in literature.25–28 PFCON-C was


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obtained by Suzuki cross coupling polymerization of (1) and (2), detailed information about the synthesis procedure of PFCON-C can be found in the supplementary information. The chemical structure of PFCON-C was confirmed by nuclear magnetic resonance (1H NMR and 13

C NMR). In Figure S1 and S2, the 1H and 13C NMR spectra of (1) and in Figure S3 and S4

the corresponding NMR spectra of PFCON-C are shown. The number molecular weight of PFCON-C was measured by gel permeation chromatography and a value of ~ 7400 g mol-1 with a polydispersity of 2.7 was determined (Figure S5). Cyclic voltammetry (CV) was carried out in order to study the electrochemical properties of PFCON-C (Figure S6). Using ferrocene as the internal standard, the highest occupied molecular orbital (HOMO) of PFCON-C was determined to be –5.7 eV, compared to vacuum. Complementarily, its lowest unoccupied molecular orbital (LUMO) was determined as –2.65 eV, compared to vacuum, from the estimation of the optical band gap (Figure S7). 29,30

Scheme 1. Synthetic route of the copolymer PFCON-C and molecular structures of PEI and PFN.

Solution-processed OLEDs. In literature, the electron injecting properties of PEI (Scheme 1, right) and amino-functionalized polyfluorenes have been associated with the lone



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electron pairs of the amine groups in the side-chains of these polymers31. Consequently, PFCON-C was designed to have the same semiconducting backbone as PFN but with a higher number of nitrogen atoms in its side-chains. Whereas there are only two amine groups per monomer in the side-chains of PFN (Scheme 1, right), the side-chains of PFCON-C contain four amine as well as two amide groups. Therefore, PFCON-C has the same amount of nitrogen atoms in its side-chains as PEI. In order to check whether this high amount of nitrogen-containing groups has a positive effect on the electron injecting capabilities of PFCON-C, we used it as EIL in solution-processed OLEDs. These OLEDs were based on a PPV derivative, commonly known as Super Yellow (SY), as the light emitting layer. The complete OLED stack consisted of ITO / PEDOT:PSS / SY / PFCON-C / Ag (Figure 1a). As Ag can be printed by various printing techniques, the used OLED stack is highly relevant to real-world printing applications.32–35 In addition, reference OLEDs, incorporating PEI, PFN or no EIL, were fabricated. For each material, the solid concentration was optimized in order to achieve a maximum device performance. All EILs were prepared by spin-coating from 1Propanol with the optimized solid concentrations of 6 g L-1, 4 g L-1 and 0.5 g L-1 for PFCONC, PFN and PEI, respectively. 1-Propanol was chosen as solvent, as polymeric emitting materials like SY are not soluble in high polar solvents.


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Figure 1. OLED stack (a), LIV-characteristics (b), current efficiency (c) and power efficiency (d) of SY devices that use three different electron injection layers in combination with Ag as cathode. In Figure 1b, 1c and 1d, representative luminance – current density – voltage (LIV) characteristics, and power/current efficiencies of OLEDs incorporating each EIL are shown. In Table 1, a summary of the most important figures of merit of the OLEDs is presented. For each device type, five to eight samples were measured and averaged. As expected, OLEDs, that used a plain Ag cathode, without any EIL, exhibited the lowest performance of all devices. They showed high turn-on voltages (VON) and high operational voltages as well as poor current and power efficiencies. The low performance of these OLEDs indicates a bad charge carrier balance, which can be explained by the lack of an EIL and the use of a high work-function metal like Ag as the cathode. Table 1. Summary of OLED data presented in Figure 1. EIL material

Vona)

Max. Current Efficiency

Power Efficiency @ 10³ cd m-²

Power Efficiency @ 104 cd m-²

-

4.4 V

0.5 cd A-1 [@ 9.9 V]

0.2 lm W-1

-

PFN

4.0 V

6.0 cd A-1 [@ 10.9 V]

1.6 lm W-1

1.8 lm W-1



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PFCON-C

3.1 V

9.9 cd A-1 [@ 7.1V]

5.2 lm W-1

3.7 lm W-1

PEI a)

2.5 V

8.5 cd A-1 [@ 4.9 V]

5.8 lm W-1 -

3.3 lm W-1

Von was determined at a luminance of 1 cd m ²

OLEDs that used PFN as the EIL exhibited very similar luminance-voltage characteristics as OLEDs without an EIL. VON was only marginally lower than for the plain Ag OLED and operational voltages were mostly comparable. Such high operational voltages have also been reported in literature for PFN OLEDs that use Ag as the top contact.23,24 Compared to the plain Ag OLED, though, the use of PFN as an EIL leads to a significantly lower current density across all voltages. In order to understand this behavior, we investigated electron-only devices using ITO covered by a film of aluminum doped zinc oxide (AZO) as the holeinjecting contact and either a PFN/Ag or plain Ag cathode as the top contact (Figure S8). A detailed analysis of these devices is presented in the SI. In short, the PFN/Ag device exhibited a lower current density in the high voltage regime (>11 V), which, in our opinion, can be explained by PFN’s hole trapping/blocking properties, which have been reported recently.36 These hole-blocking properties of PFN can also explain the decreased current density that we observed for the PFN OLED in Figure 1. Compared to the plain Ag cathode, the addition of PFN to the OLED stack leads to a blocking of holes and hence to a more balanced electron-hole current resulting in strongly improved current and power efficiencies. OLEDs with PFCON-C or PEI showed significantly better performance than OLEDs with PFN. Operational voltages were strongly reduced (see Figure 1b and Table 1) with VON decreasing ~ 2 V and ~ 2.5 V in for devices including PFCON-C and PEI, respectively. At the same time, current efficiencies were increased by 40% and 25% compared to PFN. As a result of the decreased operational voltages and the increased current efficiencies, PFCON-C and PEI OLEDs reached power efficiencies of > 5 lm W-1 which corresponds to a more than two-fold increase compared to PFN. In literature, the use of amino N-oxide polyfluorenes as


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well as zwitterionic polyfluorenes has been shown to lead to similar performance improvements compared to PFN-like polyfluorenes.20,28,37 However, in contrast to PFN and PFCON-C, the investigated polyfluorenes had only polar side-chains. Furthermore, Al was used as cathode contact in these studies. Therefore, these results cannot be directly compared to the results of this work. As can be seen in Figure 1d, the PEI containing OLED showed higher power efficiencies for luminances < 2000 cd m-², the OLED with PFCON-C exhibited slightly higher power efficiency for luminances > 2000 cd m-². The reason for this different behavior is not clear, however, we suspect that it is related to the different electronic properties of PEI (insulating) and PFCON-C (semiconducting) and will be the subject of future research. In addition to the OLED data presented in Figure 1, we also measured the electro-luminescence spectra of all devices (Figure S9) and photoluminescence spectra of PFCON-C and PFN (Figure S7 and S13). All EL spectra showed pure SY emission without any feature that could be assigned to the PFN or PFCON-C emission; proving that all radiative recombination takes place in the SY layer. As has been shown in the literature, PEI requires layer thicknesses below 10 nm, over which device performance drops considerably.15,16 This is inconsistent with the stated use as a solution-processable electron injection layer for the future fabrication of OLEDs by highthroughput printing techniques. It has been shown that the preparation of homogeneous films with arbitrary thicknesses is in many cases not feasible by printing techniques due to the inherent restriction of the viscoelastic properties of the inks.38,39 In particular, the preparation of ultra-thin homogeneous layers is problematic due to hydrodynamic instabilities like spinodal dewetting.40 Therefore, the performance of an EIL material at non-optimal processing parameters is equally important as its performance at fully optimized conditions, like in Figure 1.



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In order to test the dependence of the OLED performance on the processing parameters of PFN, PEI and PFCON-C, we built OLEDs with various EIL solid concentrations in order to achieve different EIL thicknesses. In Figure 2a and 2b, the current and power efficiencies of OLEDs with EILs of varying thickness, at a luminance of 1000 cd m-² are plotted against the concentrations used for spin-coating. A reliable thickness measurement of the PEI layers was very challenging, due to its film morphology (see Figure 3, an explanation for the different morphologies will be discussed in detail in a subsequent section). We observed that the highest current and power efficiencies of the PEI samples were achieved at the lowest tested concentration of 0.5 g L-1, however, and with increasing concentration (i.e. thickness), both figures of merit rapidly decreased. When the concentration was increased from 0.5 to 4 g L-1, the current efficiency of PEI OLEDs dropped by ~ 30%, while the power efficiency decreased by ~ 50%. In contrast, PFN and PFCON-C OLEDs exhibited very poor performance at the lowest tested concentration of 1 g L-1. With increasing concentration, however, the current and power efficiencies of both OLED architectures increased and maximum values were achieved at concentrations of 4 g L-1 and 8 g L-1, for PFN and PFCON-C, respectively. When the solid concentration was increased further, a marginal drop in current efficiency and a pronounced drop in power efficiency was observed. Particularly, when compared to PEI, PFCON-C exhibits a significantly lower dependence on the solid concentration used for spin-coating (i.e. thickness). When the solid concentration is varied between 4 and 10 g L-1, the resulting power efficiencies varies by less than 15 %.


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Figure 2. Influence of the processing parameters on the OLED performance. Current efficiencies (a) and power efficacies (b) of OLEDs at a luminance of 1000 cd m-², where various concentrations were used to spin-coat the EIL. When both figures of merit are plotted against the PFN / PFCON-C thickness (c, and inset), the same thickness dependency is observed for both polyfluorenes.



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Physical profilometer measurements were undertaken to determine the thickness of the PFN and PFCON-C films on top of SY. The measured thicknesses of both materials were linearly fitted to estimate the nominal thickness versus concentration, as shown in Figure S10. For the PFN and PFCON-C OLEDs, a normalized representation of the current and power efficiencies vs. EIL thickness is presented in Figure 2c. For both polymers it can be observed that the current and power efficiencies increase with increasing thickness and reach their maximum values at thicknesses of ~ 25-30 nm. When the thickness is further increased, the current efficiency remains constant while the power efficiency begins to drop. This drop is a result of higher operational voltages due to the increasing series resistance of the polyfluorene layers. These results show that, in spite of PEI slightly outperforming PFCON-C in case of fully optimized processing parameters, PFCON-C exhibits a larger processing window with a minimal decrease (~15%) to device performance for PFCON-C layer thicknesses between 15 and 35 nm. This represents a huge advantage for device fabrication scenarios using high-throughput printing techniques. It is important to note that PFCON-C was designed to possess the same semiconducting backbone as PFN but different side-chains. The fact that both materials exhibit the same, in comparison to PEI superior, thickness dependence when used as EIL in OLEDs, highlights that a semiconducting backbone is beneficial for an EIL material. This should be taken into account in the design of future EIL materials.


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Figure 3. AFM images of SY (a) and SY covered by PEI (b, c, d). PEI films were prepared by spin-coating and solid concentrations between 0.5 g L-1 and 4.0 g L-1 were used. The concentration that worked best for OLED devices is marked in green.

Topography of EIL films. In order to understand the dependency of the OLED performance on the solid concentrations used for spin-coating the EIL films, we investigated the topography of the prepared films by AFM. In Figure 3, the topography of SY and SY covered by PEI with various solid concentrations (0.5 g L-1, 1.5 g L-1 and 4 g L-1), are shown. As can be seen, SY forms a very smooth film with a low surface roughness. In contrast, PEI exhibits a strong aggregation for all three concentrations, probably caused by a dewetting process during film drying. At a concentration of 0.5 g L-1, these drop-like clusters had a diameter of about 20 nm and were evenly distributed over the film. When the concentration was increased to 1.5 and 4 g L-1, the diameters of the clusters increased to values of about 200 and 600 nm, respectively. Furthermore, at the higher concentrations the clusters were not evenly distributed over the film, but a polygonal dewetting pattern, that we attribute to the Rayleigh instability of the solution, can be seen.41,42 In literature, dewetting phenomena of polymer films have also been attributed to the presence of water in the polymer solution.43,44 However, as we used anhydrous 1-Propanol as solvent and furthermore spin-cast PEI under a



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nitrogen atmosphere, we do not think that this is the case here. Instead, the dewetting process is probably caused by the high surface tension of PEI, resulting from the high number of polar amine side-chains of the material. In case of the highest concentration of 4 g L-1, the presence of these agglomerates could also be seen in the OLED’s light emission (Figure S11). On the basis of the AFM images, it is not clear whether PEI is only present in the clusters or covers the whole SY surface. However, as we showed in Figure 2, a PEI OLED with a concentration of 4 g L-1 exhibited a much higher power efficiency than a plain Ag OLED (Figure 1). Therefore, we conclude a thin layer of PEI is present between the clusters and that the SY surface is fully covered. The actual thickness of this thin PEI layer is very difficult to measure, as the agglomerates prevent any profilometer measurement from being reliably made. In literature, however, it has been reported that PEI layers need to be in the range of 10 nm for devices to work properly.

13–17

Therefore, we expect it to not be thicker

than 10 nm, even for the highest concentration used in this work.

Figure 4. AFM images of SY (a), SY covered by PFN (b,c,d) and PFCON-C (e, f, g). PFN and PFCON-C films were prepared by spin-coating with solid concentrations between 1.0 g L-1 and 8 g L-1.


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Figure 4 shows AFM topography images of SY (a), SY covered by PFN (b, c, d) and PFCON-C (e, f, g) with concentrations of 1 g L-1, 4 g L-1 and 8 g L-1. In contrast to PEI, no formation of agglomerates was observed in the case of PFN. Instead, the PFN films are homogenous, with a low surface roughness. The better film forming properties of PFN is assumed to be the result of better wetting properties on top of SY. Compared to PEI, which only contains polar amine side-chains, PFN also possesses non-polar alkyl side-chains (see Scheme 1). Therefore, the reduced polarity of PFN probably leads to a better wetting on top of SY and hence to more homogeneous layers. Like PFN, PFCON-C has both polar and nonpolar side-chains and its film forming behavior is therefore expected to be similar to the one of PFN. The topography measurements of the PFCON-C films, presented in Figure 4 (e-g), confirm this hypothesis. Similar to PFN, PFCON-C forms closed layers with low surface roughness. However, in contrast to PFN, the formation of small agglomerates on top of the films is observed. The size of these PFCON-C agglomerates is two orders of magnitude smaller than the size of the PEI agglomerates observed in Figure 3, underlining that the presence of non-polar alkyl side-chains is beneficial for the film formation on top of the emissive layer. Contact potential difference induced by EIL films. In order to understand the difference in OLED performance, the work-function shift induced by the three EIL materials was measured by kelvin probe (KP). In Figure 5, the KP results of these measurements are shown. As can be seen, all materials exhibited negative contact potential differences (CPDs) and hence reduced the work-function of the underlying Ag substrate. Furthermore, the absolute values of the CPDs, i.e. work-function shifts, increased moderately with increasing concentration. However, the work-function shifts observed for PEI and PFCON-C are significantly higher than those observed for PFN. The maximum shifts observed are ~ -0.70 eV, -0.75 eV and -0.35 eV for PEI, PFCON-C and PFN, respectively. This result



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demonstrates that the work-function shift is closely linked to the number of polar, nitrogencontaining groups on the polymers’ side-chains. As both PEI and PFCON-C have more polar groups than PFN, a higher work-function shift is observed. However, a detailed investigation of the link between the number of nitrogen-containing groups and the work-function shift falls outside the scope of the present work, and thus, will be carried out in future work. Furthermore, the KP measurements are in good agreement with the OLED results presented in Figure 1 where PFCON-C and PEI, clearly outperformed OLEDs with PFN due to a lower injection barrier.

Figure 5. Kelvin probe measurements of Ag samples covered by films of PEI, PFN and PFCON-C. The determined contact potential difference (CPD) relative to a plain Ag substrate was considerably larger for PEI and PFCON-C than for PFN. Schematics of the energy levels of the cathode / SY contacts in case of PEI and PFCON-C / PFN are shown in Figure 6a and 6b. The vacuum level and work-function of Ag ( ~ 4.3 eV) and the CPDs of Figure 5, depicted as ∆, were used to determine the vacuum level positions of the EIL layers.45 For the EIL / SY contact, a constant vacuum level alignment was assumed. The shift of the EIL vacuum level ∆ moves the HOMO / LUMO levels of the EIL materials and of SY relative to the Ag Fermi level downwards, and thus, lowers the energy barrier for electron injection. The HOMO / LUMO levels of PFCON-C and PFN were


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determined by CV measurements and UV-Vis spectra (Figure S6, S7 and Figure S12, S13) while for SY literature values where used.46 According to the energy levels in Figure 6, the barrier for electron injection ΦB is considerably smaller for PEI and PFCON-C than for PFN which explains the lower operational voltages as well as the higher power efficiencies compared to the PFN OLED (see Figure 1). The presence of a high injection barrier ΦB in case of PFN furthermore underlines our finding that the increased current and power efficiencies in comparison to plain Ag are mainly due to the hole trapping characteristics of PFN.36

Figure 6. Proposed model of charge carrier injection at the cathode contact in case of nonconjugated (a) and conjugated (b) electron injection layers. The energy diagrams were deduced by the vacuum level shifts ∆, measured by KP, and the HOMO / LUMO levels of the materials which were determined by CV and absorption measurements.

In addition to the height of the injection barriers ΦB, Figure 6 also illustrates the different injection mechanisms for the two kinds of EIL. Since PEI is an insulator, electrons must tunnel through it in order to be injected into the LUMO of SY (Figure 6a). When the thickness of the PEI layer is increased, the tunneling and hence electron injection probability is expected to decrease, causing the observed strong reduction in the current and power efficiencies of the devices. In contrast, PFN and PFCON-C are semiconductors, therefore electrons can be injected into their LUMOs, transported through the polymer and subsequently injected into SY. Consequently, the efficiency of electron injection is



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significantly less dependent on the layer thickness for the polyfluorenes than for PEI as observed in Figure 2.

CONCLUSION In this article, we report on the use of a novel functionalized polyfluorene (PFCON-C) with multiple tertiary amine side-chains connected via an amide, as an EIL material in OLEDs. By comparing its properties to the literature known EIL materials PEI and PFN, we find correlations between the molecular structure of the EIL materials, their performance and processability. It is shown that, due to their semiconducting backbones, PFCON-C and PFN possess significantly larger processing windows than PEI since a change in layer thickness between 15 and 35 nm only has a marginal influence on the OLEDs’ power efficiency (~15 %). This result is relevant in the context of the potential processing of EIL materials via highthroughput printing or coating techniques. OLEDs with PFCON-C as the EIL perform significantly better than OLEDs with PFN, while performing similarly to PEI containing OLEDs. We show that these differences in performance can be attributed to the side-chains of the polymers. Whereas PFN only has two nitrogen atoms in its side-chains per monomer, PFCON-C and PEI have six. KP measurements indicate that this higher number of polar sidechains leads to a larger work-function shift induced by the EIL material and consequently to a better OLED performance. AFM measurements indicate that such an increase in polar sidechains can result worse film forming properties of the EIL material, however, this negative effect can be countered by the addition of non-polar side-chains. We believe that the positive influence of additional polar groups on the EIL performance is universal for polymeric EIL materials. Therefore, it should be considered for the design of future EIL materials.


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EXPERIMENTAL SECTION Sample preparation. PEI and polyfluorene solutions were prepared by dissolving the polymers in 1-Propanol at concentrations between 0.5 and 8 g L-1. In the case of PFN, 1 vol.% of acetic acid was added in order to increase the solubility. PFN and PFCON-C solutions were filtered with a 0.45 µm PVDF filter prior to use. PDY-132 (Super Yellow, light emitting polymer), acquired from Merck KGaA, was dissolved in Toluene with a concentration of 5 g L-1. Aluminum doped zinc oxide (AZO) nanoparticle solution (N-21X, supplied by nanograde AG) was diluted 1:1 with 1-Propanol and filtered with a 0.45 µm PVDF filter prior to use. PEDOT:PSS (VPAi 4083, acquired from Heraeus) was filtered with a 0.45 µm PVDF filter prior to use. Substrates (either bare glass or glass covered by 180 nm of ITO, 10 Ohm sq-1 from Kintec) were subsequently cleaned in Acetone and Isopropanol under sonication for 15 min, respectively, and treated by O2 plasma for 5 min. PEDOT:PSS was spin-cast and annealed (135°C for 20 min) under ambient conditions. Spin-coating parameters were ω = 3800 rpm, a = 1000 rpm s-1 and t = 30 s such that layers of 30 nm were obtained. AZO was spin-cast and annealed (135°C for 10 min) under ambient conditions. Spin-coating parameters were ω = 4000 rpm, a = 500 rpm s-1 and t = 30 s. PDY-132 was spin-cast in an inert atmosphere. Spincoating parameters were ω = 2000 rpm, a = 1000 rpm s-1 and t = 60 s so that a layer of 65 nm was obtained. PEI, PFN and PFCON-C were spin-cast and annealed (120°C for 10 min) in inert atmosphere. Spin-coating parameters were ω = 5000 rpm, a = 1000 rpm s-1 and t = 60 s for PEI and ω = 3000 rpm, a = 1000 rpm s-1 and t = 60 s for PFN and PFCON-C. Ag layers were thermally evaporated in a vacuum system with a base pressure of 1·10-7 mbar at a rate of 0.2 nm s-1.



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Kelvin probe measurements. The Kelvin probe characterization was performed using a KP020 single point Kelvin probe system by KPTechnology. The system is equipped with a gold tip with a diameter of 2 mm and its energetic resolution is ~ 20 meV. Atomic force microscopy images. AFM images were recorded with a DME DS 95 Dualscope AFM in ambient conditions. We measured in AFM tapping mode using highly doped silicon cantilever from NanoWorld (Arrow NCR). These cantilevers have resonance frequencies of about 285 kHz and tip radii of less than 10 nm. Additionally, a Nanoscope IV AFM (Bruker, former Digital Instruments) was utilized in tapping mode under ambient conditions. For this AFM, µmasch cantilever (NSC15) with a resonance frequency of 325 kHz and a tip radius below 10 nm were used. Device characterization. Devices were characterized with a Botest LIV Functionality Test System. The driving voltages of the system can be between -20 and 40 V and the current measurement resolution is as low as 5 nA. Emission spectra were recorded with an Ocean Optics Jaz Spectrometer.

ASSOCIATED CONTENT Supporting Information. Synthetis details. NMR Spectra. Cyclic Voltammetry. UV-Vis Spectra. Electron only devices. OLED Emission Spectra. Optical Microscopy. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; [email protected] Present Addresses † Performance Materials, Merck KGaA, Frankfurter Str. 250, DE-64293 Darmstadt, Germany.

ACKNOWLEDGMENTS The authors are thankful to Dr. Florian Golling and Christine Rosenauer for measuring gel permeation chromatography. The authors are furthermore thankful to Dr. Anthony J. Morfa for fruitful discussion. Nanograde Ltd. is acknowledged for providing AZO nanoparticle suspension, 3M Display Materials & Systems Division for providing barrier film for device


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encapsulation. The authors acknowledge financial support from the German Ministry of Education and Re-search (BMBF) under grants FKZ13N13691, FKZ13N12793, FKZ13N12794. REFERENCES

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Table of Contents artwork


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b)

a) Page 25 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

c)


Ag Injection layer* 65 nm

SuperYellow

25 nm

PEDOT:PSS ITO Glass

* PEI / PFN / PFCON-C

d)


a) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51

ACS Applied Materials & Interfaces Page 26 of 32

b)

c)


z0

a)Page SY 27 b) SY/PEI& Interfaces ACS of 32 Applied Materials

1 2 3 4 z0 = 5 nm z0 = 60 nm 10mm 5 rms = 0.4 nm 0.5 g/l rms = 7.7 nm 6 d) SY/PEI c) SY/PEI 7 8 9 10 11 ACS Paragon Plus Environment 12 z0 = 200 nm z0 = 600 nm 13 1.5 rms = 31 nm 4.0 g/l rms = 72 nm 14 g/l

0 nm

z0

b) SY/PFNACS Applied Materials c) SY/PFN & Interfaces 1 2 a)3 SY 4 5 6 7 8 9 10 2.5mm 11 12 13 14 15 16 17

1 g/l

z0 = 4 nm rms = 0.4 nm 4 g/l

e) SY/PFCON-C

d) SY/PFN

z0 = 10 nm rms = 0.7 nm 8 g/l

f) SY/PFCON-C

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z0 = 12 nm rms = 1.1 nm

g) SY/PFCON-C

z0 = 4 nm rms = 0.4 nm


1 g/l

z0 = 12 nm rms = 1.4 nm 4 g/l

z0 = 12 nm rms = 1.4 nm 8 g/l

z0 = 12 nm rms = 1.7 nm 0 nm

Page 29 ACS of 32 Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13


a) E

vac

D =-0.65 eV

1 2 3 -2.95 eV 4 F B 5 EF Ag 6 SY PEI 7 WF = 4.30 eV 8 -5.45 eV 9 10 11

b)ACS E Applied Materials & Interfaces D =-0.70 eV vac

eV30 of 32 D =-0.35 Page

Evac

-2.35 eV -2.65 eV

FB

EF Ag

PFCON-C

-2.95 eV

-2.95 eV

FB

EF SY

WF = 4.30 eV

Ag

PFN

SY

WF = 4.30 eV

ACS Paragon Plus Environment -5.45 eV -5.70 eV

-5.40 eV

-5.45 eV

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Scheme 1. Synthetic route of the copolymer PFCON-C and molecular structures of PEI and PFN. 344x161mm (300 x 300 DPI)



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PFCON-C High OLED performance

Wide processing window

ACS

Ag EIL SY ParagonPEDOT:PSS Plus Environment ITO Glass

High-Performance Electron Injection Layers with a Wide Processing

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