Cross-Linkable Hole-Transport Materials Improve ... - ACS Publications

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Cross-Linkable Hole-Transport Materials Improve the Device Performance of Perovskite Light-Emitting Diodes Chiung-Fu Huang,† Mukhamed L. Keshtov,‡ and Fang-Chung Chen*,† †

Department of Photonics, National Chiao Tung University, Hsinchu 30013, Taiwan Institute of Organoelement Compounds of Russian Academy of Sciences, Moscow 119991, Russian Federation

‡

S Supporting Information *

ABSTRACT: Hybrid organic/inorganic perovskites are promising candidate materials for use in photovoltaic applications. More recently, they have also become highly attractive as active materials for other optoelectronic devices, including lasers, lightemitting diodes, and photodetectors. Nevertheless, difficulties in forming continuous and uniform films and the existence of a charge-injection barrier between the perovskite layer and the electrodes have hindered the development of highperformance perovskite light-emitting diodes (PeLEDs). In this study, a cross-linked hole-transport layer (HTL) is introduced to improve the hole-injection efficiency of PeLEDs. Furthermore, this layer simultaneously facilitates the formation of smooth perovskite layers, presumably because of the different surface energies. More interestingly, the HTL also exhibits strong solvent effects on the device performance. When the processing solvent for fabricating the HTLs is changed from chlorobenzene to N,N-dimethylformamide (DMF), the perovskite layer becomes more uniform and continuous, leading to better surface coverage and higher device efficiency, presumably because DMF has strong affinity toward the perovskite precursors. The approach presented herein could become a general method for decreasing the hole-injection barrier of PeLEDs and, eventually, lead to higher device performance. KEYWORDS: perovskite, light-emitting, cross-linked, hole-transport, solvent

1. INTRODUCTION Hybrid organic/inorganic perovskites (HOIPs) have attracted considerable attention in recent years as a result of the astonishingly rapid progress that has been made in the field of photovoltaic (PV) technologies using them as photoactive materials.1−8 Because PV devices incorporating HOIPs have achieved power conversion efficiencies exceeding 20%, they are recognized as one of the most promising technologies for developing next-generation energy sources.5 Although PV research remains their dominant field, HOIPs also exhibit many other remarkable properties that make them attractive materials in many other optoelectronic devices, including lasers,9−12 light-emitting diodes (LEDs),13−21 and photodetectors.22 In particular, their high photoluminescence quantum yield, high color purity, low material costs, and solution processability make them promising candidates for use in electroluminescent devices. The earliest LEDs made of HOIPs were very unstable, with bright electroluminescence (EL) observed only when cooled at liquid nitrogen temperatures.23,24 In 1999, Chondroudis and Mitzi reported the first room-temperature EL from HOIP devices incorporating a quaterthiophene dye within the lead halide layers.25 Recently, Tan et al. demonstrated highbrightness LEDs based on solution-processed HOIPs; the emission color could be tuned from green to the near-infrared spectral range through altering the halide composites.13 Nevertheless, the device efficiencies of these perovskite light© 2016 American Chemical Society

emitting diodes (PeLEDs) remained low, presumably because of difficulties in preparing continuous, uniform films and because of the existence of a high charge-injection barrier between the perovskite layer and the electrodes.14 More recently, Cho et al. reported a very high quantum efficiency of 8.52% for PeLEDs by controlling stoichiometry modification and using nanograin enginnering.18 Kim et al. demonstrated green PeLEDs with improved performance when incorporating self-organized polymer hole-injection layers.15 Hoye et al. also employed low-electron-affinity oxides to enhance electron injection in PeLEDs.16 In addition, Wang et al. introduced a multifunctional interfacial layer of polyethylenimine between the oxide electron-transporting layer and the perovskite emissive layer; this approach not only facilitated electron injection but also aided the formation of high-quality perovskite thin films.14 In this present study, we used a cross-linkable holetransport material to improve the device performance of PeLEDs. The incorporation of the cross-linked HTL not only prevented dissolution of the initial layer while processing the subsequent wet depositions but also lowered the hole-injection barrier of the PeLEDs.26 Morphological analysis revealed that the density of the pores was also significantly decreased. Accordingly, the device performance improved substantially. Received: July 6, 2016 Accepted: September 23, 2016 Published: September 23, 2016 27006

DOI: 10.1021/acsami.6b08106 ACS Appl. Mater. Interfaces 2016, 8, 27006−27011

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ACS Applied Materials & Interfaces

surface morphologies of various thin films were measured using a Digital Instrument Dimension 3100 atomic force microscope. The device electrical characteristics of the PeLEDs were measured using a Keithley 2400 source-measure unit and a PR655 spectrascan colorimeter. The photovoltaic properties of the devices were obtained while illuminating the devices with a 150 W Thermal Oriel solar simulator. External quantum efficiency (EQE) spectra were recorded using an EQE measurement system (Enli Technology).27

Moreover, we observed interesting solvent effects of the HTL on the morphology of the perovskite films. We suspect that residual solvent molecules in the cross-linked HTL had strong affinity toward the perovskite precursors, thereby facilitating the formation of uniform perovskite layers.

2. EXPERIMENTAL SECTION 2.1. Materials. 9,9-Bis[4-[(4-ethenylphenyl)methoxy]phenyl]N2,N7-di-1-naphthalenyl-N2,N7-diphenyl-9H-fluorene-2,7-diamine (VB-FNPD) and methylammonium bromide (CH3NH3Br) were purchased from Lumitec. Lead(II) bromide (PbBr2), N,N-dimethylformamide (DMF), and chlorobenzene (CB) were purchased from Sigma−Aldrich. All chemicals were used as received. 2.2. Device Fabrication. PeLEDs were fabricated on patterned indium tin oxide (ITO) glass substrates; the device structure is displayed in Figure 1(a). The ITO substrates were cleaned and

3. RESULTS AND DISCUSSION As illustrated in Figure 1, in this study we employed PEDOT:PSS as the sole hole-injection layer in the PeLEDs. Subsequently, a perovskite light-emitting layer based on CH 3 NH 3 PbBr 3 was deposited on the surface of the PEDOT:PSS buffer from a DMF solution. After thermally annealing at 100 °C for 2 h, a layer of PCBM was deposited as the electron transport layer. To complete the device, Ag was thermally evaporated as the cathode electrode. This simple structure has been suggested as an effective architecture for dual-function devices that can behave as both LEDs and solar cells simultaneously.28 To prepare the additional hole-transporting layer, a solution of VB-FNPD in either DMF or CB26 was spin-coated onto the PEDOT:PSS surface. A two-step annealing processnamely, thermally annealing at 100 °C for 30 min and then at 230 °C for 90 minwas adopted to crosslink the transporting layer. Note that the reference devices are defined hereafter as those prepared without VB-FNPD. Figure 2 displays SEM images of the perovskite layers deposited on various substrates. A very rough surface existed for the perovskite layer deposited directly on the PEDOT:PSS layer (Figure 2a); the under-layer was apparently not entirely covered by CH3NH3PbBr3. For the perovskite layers deposited on the VB-FNPD layer prepared from the CB solution (denoted hereafter as CB-VB-FNPD), the surface structure was highly porous (Figure 2b). We suspected that the different surface energies might have significantly affected the crystal growth process of the perovskite layers. Interestingly, the morphology was totally different after we replaced the processing solvent for preparing the VB-FNPD layer with DMF (denoted hereafter as DMF-VB-FNPD). As revealed in Figure 2c, the surface coverage increased greatly. The entire surface of the under-layer appeared to have been almost covered completely by CH3NH3PbBr3; the rest of the perovskite materials were then deposited on the previous layer, resulting in a “multiple-layer” structure. Cross-sectional SEM images of the completed devices clearly illustrated the differences in the film morphologies (Figure 3; Figure S1, Supporting Information). For the reference device, very large voids appeared in the perovskite layer (Figure 3a). Note that we could hardly identify the PCBM layer as the molecules probably filled the pores and somehow smoothed the perovskite layer. Although the density of the voids decreased slightly for the perovskite film deposited on the CB-VB-FNPD layer, some voids remained (Figure 3b). On the other hand, the perovskite layer had grown continuously on the DMF-VBFNPD surface; significantly fewer voids appeared in the SEM image (Figure 3c). These cross-sectional SEM images reveal that the quality of the CH3NH3PbBr3 films was extremely sensitive to the surface properties of the underlying substrates. Furthermore, the processing solvent for the hole-transport layer probably affected the surface properties, thereby facilitating the formation of high-quality perovskite thin films on the DMFVB-FNPD film.

Figure 1. (a) Device structure of the PeLEDs in this study. (b) Chemical structure of the cross-linkable hole-transporting material VBFNPD. (c) Energy levels of the organic materials and work functions of electrodes. pretreated with UV ozone. Poly(3,4-ethylenedioxythiophene)/polystyrenesulfonate (PEDOT:PSS) was first spin-coated onto the substrates and then baked at 150 °C for 10 min. The samples were then transferred into a N2-filled glovebox. The perovskite precursor solution was prepared by dissolving PbBr2 (0.67 M) and CH3NH3Br (2.2 M) in DMF. The solution was spin-coated on the PEDOT:PSS buffer, and then the resulting film was subjected to thermal annealing at 100 °C for 2 h. The thickness of the perovskite layer was ca. 450 nm. Next, a solution of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in CB (2.0 wt %) was spin-coated onto the perovskite layer, and then the structure was annealed at 100 °C for 15 min. Finally, 80 nm of Ag was deposited as the top electrodes. For the VB-FNPD devices, a solution of VB-FNPD in either DMF or CB was spin-coated on the PEDOT:PSS surface. The sample was thermally annealed at 100 °C for 30 min and then at 230 °C for 90 min. After the thermal cross-linking reaction, the perovskite layers were prepared as described above. The thickness of the VB-FNPD was estimated to be ca. 40 nm. Note that the VB-FNPD, perovskite, and PCBM layers were fabricated in a N2-filled glovebox. Then, all the PeLEDs were encapsulated by cover glasses and sealed with UV-cured epoxy prior to characterization in air. 2.3. Thin-Film and Device Characterization. The device characterization was performed in air. PL spectra were recorded using an Ocean Optics HR4000CG-UV-NIR spectrometer. The 27007


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Figure 2. Top-view SEM images of perovskite films deposited on (a) PEDOT:PSS, (b) CB-VB-FNPD, and (c) DMF-VB-FNPD layers. Scale bars: 1 μm.

Figure 3. Cross-sectional SEM images of perovskite films deposited on (a) PEDOT:PSS, (b) CB-VB-FNPD, and (c) DMF-VB-FNPD layers. Scale bars: 200 nm.

Figure 4(a) displays current density−brightness−voltage (J− B−V) curves of the PeLEDs prepared under the different conditions. For the reference device, the turn-on voltage (VT), defined for a luminance of 1 cd m−2, was 2.1 V. The maximum efficiency was 0.12 cd A−1, comparable with values reported in the literature;13−15 the corresponding voltage and current density were 2.5 V and 13.5 mA cm−2, respectively. Figure 4(c) presents the electroluminescence (EL) spectrum of the reference device. The wavelength of maximum emission was 536 nm with a full width at half-maximum (fwhm) of only 19.6 nm, resulting in a very saturated green emission; its CIE coordinates were (0.231, 0.743) [Figure 4(d)]. The NTSC standard is also displayed, and the color quality of the green PeLED was even better than that of the standard, due to its narrow emission spectrum. Figure 4(e) presents the photoluminescence (PL) spectrum of the CH3NH3PbBr3 thin film. The peak location and fwhm were almost identical to those in the EL spectrum. As displayed in the energy level diagram in Figure 1, a large hole-injection barrier existed between the PEDOT:PSS and CH3NH3PbBr3 layers. Therefore, we suspected that the interface might have limited the device current. Indeed, the CB-VB-FNPD device, as revealed in Figure 4(a), exhibited a

greatly increased current density, presumably resulting from the lower hole-injection barrier. The ladder-type energy structure appeared to promote the hole injection into the perovskite layer. As a result, the device efficiency increased to 0.66 cd A−1, presumably because of better charge balance. We also found that the leakage current was suppressed in the low-bias region, which was probably due to the large band gap of VB-FNPD. To further improve the device performance, we used DMF as the solvent to dissolve VB-FNPD. The J−B−V curves indicate that the current density decreased slightly, but the brightness increased, resulting in a further improvement in efficiency to 0.90 cd A−1. For the champion device, the efficiency reached 1.01 cd A−1. Relative to the previous reference device, the efficiency had improved by approximately 8-fold. From the SEM images, the higher efficiency was probably due to the better morphology of the light-emitting perovskite layer, leading to higher recombination efficiency and/or charge balance. Figure 4(c) also presents the EL spectra of the three different devices. These EL spectra are almost identical, indicating that the enhancement in current efficiency was not related to any spectral changes. The device performance of the PeLEDs in this work is summarized in Table 1. The device 27008


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Figure 4. (a) Current densities and brightness of PeLEDs prepared under various conditions, plotted with respect to the applied voltage. (b) Current efficiency−driving voltage characteristics of the various devices. (c) EL spectra of the devices. (d) CIE chromaticity diagram of the PeLED. (e) PL spectrum of a perovskite film deposited on DMF-VB-FNPD.

efficiencies are also comparable with the values reported in the recent literature (Table S1, Supporting Information). Table 1. Electrical Characteristics of the PeLEDs Fabricated under Different Conditions device reference CB-VB-FNPD device DMF-VB-FNPD device a

turn-on voltage (VT) [V]

current efficiency [cd A−1]

external QEa [%]

2.1 ± 0.1 2.5 ± 0.1

0.12 ± 0.05 0.66 ± 0.15

0.036 ± 0.015 0.177 ± 0.044

2.5 ± 0.1

0.91 ± 0.09

0.266 ± 0.026

The corresponding external quantum efficiency (QE).

Figure 5. Photovoltaic characteristics of devices prepared with and without a VB-FNPD layer. Processing solvent for VB-FNPD: DMF. Devices were measured under illumination from a standard AM1.5G solar simulator (100 mW cm−2).

From Figure 4(a), it is interesting to note the CB-VB-FNPD device exhibited even lower leakage current at low bias region. Although the perovskite layer in the device had many uncovered regions, it still exhibited a low leakage current. The origin of the difference in the leakage current for the two devices is still unclear, but we suspect that it could be due to the different ion motion behaviors. Because the two devices exhibited different morphologies, the grain boundaries and defects might affect the rate and activation energy, thereby resulting in different ion mobilities.29 Because the open-circuit voltage (Voc) of typical diodes is sensitive to the built-in potential, we conducted photovoltaic measurements of the PeLEDs prepared under the different conditions;30 Figure 5 displays the results. In the absence of the VB-FNPD thin film, the value of Voc was 0.77 V. After incorporating the VB-FNPD layer, it increased to 1.01 V. The higher value of Voc implies an increased built-in potential within the PeLED. Because the cathode material (Ag) was unchanged, the higher photovoltage indicates that the Fermi level at the anode interface had decreased. As displayed in the energy level diagram (Figure 1), the highest occupied molecular orbital (HOMO) of VB-FNPD is lower than the work function of PEDOT:PSS. Therefore, we suspect that the VB-FNPD layer lowered the hole injection barrier between the perovskite layer

and the PEPOT:PSS anode buffer. These results suggest that incorporation of the cross-linked VB-FNPD thin film improved the anode contact. From Figure 5, we also note that the photocurrent improved from 0.40 to 2.65 mA cm−2. Figure S2 (Supporting Information) displays the EQE spectra of the corresponding devices; the higher photoresponse of the VBFNPD device confirmed the increased photocurrent, further suggesting that contact at the anode was improved upon incorporation of the thin VB-FNPD layer. In Figure 4(a), we observed that the VT was increased slightly after the VB-FNPD layers were incorporated. The shift of the “turn-on voltage” may be also correlated to the change of the built-in potential. Previously, VT was defined as the voltage of polymer light-emitting diodes required to reach the “flat-band” condition, which depends on the band gap of the polymer and the work function of the electrodes.31 As we suggested above, the incorporation of the VB-FNPD layer may change the effective Fermi level at the anode, thereby affecting the built-in potential. Therefore, the VT was increased due to the higher built-in potential. This phenomenon is consistent with the previous literature, in which the VT was slightly reduced by 27009


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Therefore, it was difficult to remove it completely after the thermal annealing process when forming the cross-linked VBFNPD layer. The residual DMF in the thin films somehow affected the surface properties of the VB-FNPD thin films, leading to stronger affinity to the perovskite precursor solutions. Notably, strong interactions through coordination between the lead halide and solvent molecules (e.g., dimethyl sulfoxide, DMF) has been well documented.32 We suspect that strong coordination between the perovskite precursors and DMF was responsible for the unique solvent effects. The improved device performance of the PeLEDs could be attributed two factors. First, from the electrical properties and the results of the photovoltaic measurements, we could see that the cross-linked VB-FNPD layer increased the hole-injection efficiency of the PeLEDs, thereby improving the charge balance. Furthermore, this layer also facilitates the formation of smooth perovskite layers. The better morphology probably led to higher recombination efficiency and reduced charge losses, and the rather continuous thin films could also improve the charge transport, further affecting the balance of charge carriers.

using electrodes that were less matched to the energy levels of the light-emitting semiconductors, at the expense of the device efficiency and operating voltage.31 To investigate the effect of the processing solvent of VBFNPD, we used atomic force microscopy (AFM) to study the surfaces of the cross-linked polymer films. As revealed in Figure S3 (Supporting Information), the morphologies were quite similar in both cases. The root-mean-square (RMS) roughness, however, decreased from 1.562 to 0.576 nm after changing the solvent from CB to DMF. We suspect that the smoother surface could be attributed to the higher boiling point of DMF. We also performed contact angle measurements to study the surface properties of the VB-FNPD films prepared with the different solvents. We observed hardly any differences in the contact angles when using pure solvents (water, DMF, CB) as the test droplets. Interestingly, the angles changed after we deposited the perovskite precursor solution onto the VB-FNPD films prepared using the different methods. As displayed in Figure 6, the contact angle was 30° for the sample prepared

4. CONCLUSIONS Incorporating a single cross-linked hole-transporting layer into PeLEDs based on CH3NH3PbBr3 improves the device performance. The cross-linked VB-FNPD layer lowered the hole-injection barrier, resulting in a higher device current and improved charge balance. As a result, the device efficiency increased from 0.12 to 0.90 cd A−1. The processing solvent of the VB-FNPD layers affected the device performance significantly. When the solvent was changed from CB to DMF, the perovskite layer became smoother, leading to improved surface coverage and higher device efficiency. Residual DMF molecules in the VB-FNPD film probably had strong affinity toward the perovskite layers. We anticipate that the approach presented herein could become a general method for lowering the hole-injection barriers of PeLEDs, eventually leading to even higher device performance.

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Figure 6. (a,b) Photographs of the contact angle measurements on (a) CB-VB-FNPD and (b) DMF-VB-FNPD substrates. Test droplets: perovskite precursor solution. (c,d) Photographs of PeLED samples prepared using (c) CB and (d) DMF as the processing solvent for the VB-FNPD layer. Red lines indicate the areas covered with perovskite materials. The film on the bottom-right corner of the device in (d) was intentionally scraped out for electrode contact.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b08106. The cross-sectional SEM images of the completed devices fabricated with the VB-FNPD layers, the EQE spectra of the devices under AM1.5G illumination, and the AFM images of the cross-linked VB-FNPD films prepared with different solvents (PDF)

with CB; it decreased to 16° after changing the solvent to DMF, suggesting that the affinity between the VB-FNPD surface and the perovskite precursor solution had increased. Figures 6c and 6d also present pictures of the two different devices. The corners of the CB-VB-FNPD device were often exposed because of the poor adhesion of the perovskite materials to the HTL surface (Figure 6c). The perovskite layer could, however, readily cover the entire surface of the sample when using DMF as the processing solvent (Figure 6d). The different surface properties clearly indicate that the use of DMF as the processing solvent did indeed improve the compatibility between the VB-FNPD layer and the perovskite films. The underlying mechanism of the solvent effects remains unclear; relevant studies are ongoing. We suspect, however, that DMF has a strong affinity toward the VB-FNPD molecules.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +886 3 5131484. Fax: +886 3 5735601. 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. 27010


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ACKNOWLEDGMENTS This study was supported financially by the Ministry of Science and Technology of Taiwan (grant nos. MOST 104-2221-E009-094-MY3, MOST 103-2923-E-009-001-MY3, and MOST 102-2221-E-009-130-MY3) and the Ministry of Education of Taiwan (through the ATU program). M. L. Keshtov thanks the support by RFBR (grant number: 14-03-92003).

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