Effect of Injection Layer Sub-Bandgap States on Electron Injection in

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Effect of Injection Layer Sub-Bandgap States on Electron Injection in Organic Light-Emitting Diodes Carsten Hinzmann,†,‡ Osnat Magen,§ Yvonne J. Hofstetter,†,‡ Paul E. Hopkinson,†,‡ Nir Tessler,§ and Yana Vaynzof*,†,‡ †

Kirchhoff Institute for Physics and ‡Centre for Advanced Materials, Heidelberg University, Heidelberg, Germany Sara and Moshe Zisapel Nano-Electronic Center, Department of Electrical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel

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S Supporting Information *

ABSTRACT: It is generally considered that the injection of charges into an active layer of an organic light-emitting diode (OLED) is solely determined by the energetic injection barrier formed at the device interfaces. Here, we demonstrate that the density of surface states of the electron-injecting ZnO layer has a profound effect on both the charge injection and the overall performance of the OLED device. Introducing a dopant into ZnO reduces both the energy depth and density of surface states without altering the position of the energy levelsthus, the magnitude of the injection barrier formed at the organic/ ZnO interface remains unchanged. Changes observed in the density of surface states result in an improved electron injection and enhanced luminescence of the device. We implemented a numerical simulation, modeling the effects of energetics and the density of surface states on the electron injection, demonstrating that both contributions should be considered when choosing the appropriate injection layer. KEYWORDS: surface states, electron injection, organic light-emitting diode, modeling, drift-diffusion Poisson

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INTRODUCTION In the past 25 years, the field of organic light-emitting diodes (OLEDs) has attracted much attention from both the academic and industry research communities.1−4 Significant advances in the development of materials, processing, and device physics resulted in OLEDs being the first of organic electronics devices to enter commercial application. Polymer light-emitting diodes (PLEDs) are of particular interest due to the ease of processability from solution and the feasibility of mass scale deposition by printing.5,6 It is well-known that the performance of the light-emitting device is tightly bound to the optical, electronic, and transport properties of not only the organic active layer but also the charge injection layers that sandwich it. In particular, the energetic barrier for injection of charges is considered to be the determining factor for the choice of charge injecting layers.7,8 For efficient electron injection the energetic barrier between the injecting layer’s work function and the lowest occupied molecular orbital (LUMO) of organic active layer material should be as small as possible. In OLEDs fabricated in a standard architecture, this is achieved by using low work function metals for the evaporation of the cathode9−11 or insertion of alkali metal halide interlayers,12,13 with little to no control over the interfacial properties of the organic/cathode interface. In devices fabricated in the so-called “inverted architecture”, the properties of the electron-injecting layer can © 2017 American Chemical Society

be modified prior to the deposition of the organic active layer. For polymer light-emitting diodes, various solution processed injection layers have been investigated in order to comply with printing requirements.14 One example of a commonly used solution processed electron injection layer is ZnO,15,16 which attracted significant attention due to its low work function, high electron mobility, and low temperature processing from solution. It has also been demonstrated that ZnO can be used to modify a single contact in an organic light-emitting field-effect transistor to enhance electron injection and overall efficiency.17 Various types of ZnO modifiers including selfassembled monolayers and both thin polymer and inorganic interlayers have been shown to increase the performance of the OLED.18−20 Typically, these layers lower the work function of ZnO, thus reducing the electron injection barrier.20,21 ZnO has also been extensively researched as an electron extracting layer in bulk heterojunction solar cells22−24 or as the inorganic acceptor in hybrid photovoltaics.25−27 For the latter, it has been recently shown that charge carrier delocalization is significantly suppressed at hybrid ZnO/organic interfaces28,29 and that the density of sub-bandgap surface state on the ZnO layer plays a critical role in determining the efficiency of Received: November 14, 2016 Accepted: January 18, 2017 Published: January 18, 2017 6220

DOI: 10.1021/acsami.6b14594 ACS Appl. Mater. Interfaces 2017, 9, 6220−6227

Research Article

ACS Applied Materials & Interfaces photoinduced charge separation.30,31 The density of subbandgap surface states has been correlated to the yield of bound charge pairs formed at the hybrid ZnO/polymer interface due to electron trapping at the ZnO surface states. However, the effect of these states on the electron injecting properties of ZnO has not been previously investigated. Herein, we investigate the role of surface states of the electron-injecting layer and demonstrate that suppressing the effect of subbandgap states results in an improved electron injection into the OLED active layer, thereby improving device performance. To elucidate the role of these states in determining the electron injection efficiency, we model the ZnO/polymer hybrid interface using a numerical simulation. The combination of our experimental results with the results of modeling demonstrates that for efficient charge injection both the injection barrier and the density of surface trap states must be considered.

we performed ultraviolet photoemission spectroscopy (UPS) measurements. Figures 2a and 2b show the spectra acquired for undoped ZnO and Cs doped ZnO (Cs:ZnO) at different doping levels. The photoemission onset does not change with the introduction of the dopant, and the work function remains at 3.6 ± 0.05 eV. It is important to point out that during the UPS measurements (under UV illumination) the Fermi level of ZnO is close to its conduction band,17,25 meaning that all the sub-bandgap states of ZnO are filled and can be measured directly by UPS. The results show that there are two energetic regions where changes in the sub-bandgap density of states can be seennear the conduction band (binding energy range 0−1 eV) and near the valence band (range 2.5−3.5 eV). The n-type nature of ZnO suggests that the states near the valence band are always filled and thus are unlikely to be affecting electron injection. However, the states near the conduction band are likely to be empty without UV light, meaning that the actual Fermi level of ZnO is slightly further from the conduction band than that measured by UPS during UV illumination, in agreement with previous measurements.45,46 This is also in agreement with the previously reported enhancement of ZnO conductivity upon illumination with UV light.47,48 We postulate that these states can act as electron traps for injection at the ZnO/F8:F8BT interface. Doping of the ZnO layer results in shallower trap states of reduced density. The origin of the sub-bandgap density of states near the conduction band is likely to be related to oxygen vacancies of the ZnO layer. Both computational49 and experimental results50,51 show that neutral and single ionized oxygen vacancies are located 0.5−1 eV below the conduction band. We believe that the incorporation of Cs2CO3 into the films results in a decreased amount of oxygen vacancies. This is supported by the results of XPS measurements (not shown here) on doped and undoped films that showed that the stoichiometry (defined as O−Zn/Zn) of the doped films is closer to 1 as compared to the undoped films. Despite the changes in the density of sub-bandgap states, the electron injection barrier to F8BT remains unchanged: the energetic distance between the Fermi level and the highest occupied molecular orbital (HOMO) of F8BT (2.40 ± 0.05 eV) does not change within the experimental error (Figure 2c). Using a 2.6 eV bandgap of F8BT,17,52 the electron injection barrier is equal to 0.2 eV for both undoped and Cs doped ZnO, with no dependence on the doping level. The energetic alignment at the interface is summarized in Figure 2d. UPS measurements on F8/ZnO (not shown here) have shown that the injection barrier for electrons into F8 is 0.9 eV, significantly higher than that for F8BT. This is in agreement with previous observations that electron injection into F8:F8BT blends occurs preferentially via F8BT molecules.34 Electron Injection in Unipolar Diodes. To investigate the efficiency of electron injection from ZnO and Cs:ZnO into the active layer, we fabricated electron-only diodes in which the MoO3/Ag contact was substituted by Ca/Ag (Figure 3a). Figure 3b shows the electron-only current density injected from the ZnO contact. Despite the fact that the electron injection barrier into F8:F8BT is not affected by the incorporation of Cs dopants in ZnO, there is a stark difference in the magnitude of the electron current density. In the low voltage regime (