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Development of Annealing-Free, Solution-Processable Inverted Organic Solar Cells with N-Doped Graphene Electrodes using Zinc Oxide Nanoparticles Seungon Jung, Junghyun Lee, Jihyung Seo, Ungsoo Kim, Yunseong Choi, and Hyesung Park Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b05026 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018

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Nano Letters

Development of Annealing-Free, SolutionProcessable Inverted Organic Solar Cells with NDoped Graphene Electrodes using Zinc Oxide Nanoparticles Seungon Jung, Junghyun Lee, Jihyung Seo, Ungsoo Kim, Yunseong Choi, and Hyesung Park* Department of Energy Engineering, School of Energy and Chemical Engineering, Low Dimensional Carbon Materials Center, Perovtronics Research Center, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea

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TABLE OF CONTENTS GRAPHIC

ABSTRACT: An annealing-free process is considered as a technological advancement for the development of flexible (or wearable) organic electronic devices, which can prevent the distortion of substrates and damage to the active components of the device and simplify the overall fabrication process to increase the industrial applications. Owing to its outstanding electrical, optical, and mechanical properties, graphene is seen as a promising material that could act as a transparent conductive electrode for flexible optoelectronic devices. Owing to their high transparency and electron mobility, zinc oxide nanoparticles (ZnO-NP) are attractive and promising for their application as charge transporting materials for low-temperature processes in organic solar cells (OSCs), particularly since most charge transporting materials require annealing treatments at elevated temperatures. In this study, graphene/annealing-free ZnO-NP hybrid materials were developed for inverted OSC by successfully integrating ZnO-NP on the hydrophobic surface of graphene, thus aiming to enhance the applicability of graphene as a transparent electrode in flexible OSC systems. Chemical, optical, electrical, and morphological analyses of ZnO-NPs showed that the annealing-free process generates similar results to those provided by the conventional annealing process. The approach was effectively applied to graphene-based inverted OSCs with notable power conversion efficiencies of 8.16% and 7.41%


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on the solid and flexible substrates, respectively, which promises the great feasibility of graphene for emerging optoelectronic device applications.

KEYWORDS: Annealing-free process, flexibility, graphene electrode, organic solar cells, zinc oxide nanoparticle


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TEXT An organic solar cell (OSCs) is considered as a promising photovoltaic technology owing to its advantages such as low-cost fabrication, light weight, and mechanical flexibility. Numerous efforts have been devoted in the OSC community to improve the device efficiency and operational stability as well as the practical applicability to facilitate the commercialization of OSCs in the near future. The various strategies used to improve the photovoltaic performance, such as the synthesis of novel polymer donors,1−3 modification of charge transporting materials,4−7 or morphology control of the photoactive layer,8,9 have led to a continuous increase in the device performance. To date, several works have reported high-efficiency OSCs with power conversion efficiencies (PCEs) that exceed 10%.10−12 Their capability for practical applications, such as roll-to-roll or printing process, which has been improved as a result of the improvement of manufacturing processes, further facilitated the design and preparation of OSCs on large area flexible substrates.13,14 Herein, the exclusion of the annealing process during fabrication can be particularly advantageous for the practical application and commercialization of OSCs, allowing for a simplified process at room temperature. Recently, graphene has attracted significant attention as a transparent conductive electrode (TCE) owing to its outstanding optical, electrical, and mechanical properties.15−17 To date, several methods to prepare OSCs using graphene-based TCE that is grown by chemical vapor deposition (CVD) have been investigated.18−25 However, the major issue with graphene electrodes in OSCs is commonly related to the use of the organic material-based solutions that are not well-coated on the hydrophobic surface of graphene. For instance, a non-uniform coverage of charge transporting layers on graphene TCEs can cause the degradation of devices; this results in decreased efficiency by providing the leakage current pathways. Different


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strategies have been investigated to overcome this issue, such as the use of solvent- or surfactantmodified charge transporting materials,18−23 chemical doping of graphene,24 and insertion of additional metal oxide layer on graphene.25 ZnO nanoparticles (ZnO-NPs), which have been widely used as electron transporting layers (ETL) in OSCs26−30 owing to their high transparency and electron mobility, are one of the suitable materials for flexible OSCs. Apart from the advantages mentioned earlier, ZnO-NPs can be prepared using simple synthesis protocols and can be used for low-temperature processes. Although ZnO ETL materials synthesized by sol–gel approach have been reported,31,32 the development ZnO films by a sol–gel process typically requires post-annealing treatments at hightemperatures (above 200 °C) to form crystalline structures of hexagonal wurtzite.32 As reported earlier,26 OSCs fabricated with ZnO-NP ETLs demonstrated performances similar to those obtained by sol–gel ZnO ETLs despite the lower processing temperatures for the preparation of ZnO ETL. Therefore, it is believed that the low-temperature processed ZnO-NP materials combined with graphene TCE can provide an appropriate material platform for the development of annealing-free, solution-processable and flexible OSCs, thus benefitting the widespread industrial application of OSCs. This study reports on an approach to fabricate flexible OSCs using an annealing-free process that has been successfully developed using graphene and ZnO-NP as the TCE and ETL, respectively. Morphological, chemical, electrical, and optical properties of ZnO-NPs were investigated to examine the operating principle of annealing-free ZnO-NP incorporated into OSCs in comparison with annealed ZnO-NP materials. The uniform coverage of the annealingfree ZnO-NP on graphene electrodes resulted in n-doping of graphene, which facilitates the effective charge transfer from the photoactive layer to graphene electrode. In addition, it was


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demonstrated that graphene-based OSCs with PCEs of 8.16% and 7.41% on glass and polyethylene terephthalate (PET) substrates, respectively, can be successfully prepared. It is expected that the proposed approach will significantly improve the low-temperature and costeffective processability for flexible or wearable optoelectronic devices, thus increasing the scope for practical applications. ZnO-NPs were synthesized by a conventional method,33,34 and a mixture of solvents, i.e., chloroform and methanol, was used for the dispersion of ZnO-NPs. For the optimized conditions of solution-processed annealing-free ZnO-NPs, 1:1 volume ratio of each solvent was used while ZnO-NP films were vacuum-treated to remove the residual solvents. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analyses were performed to investigate the surface chemical composition and crystallinity of the as-synthesized ZnO-NP. The corresponding spectra and diffractograms are illustrated in Figure 1. The curve fitting of O 1s XPS spectrum of annealing-free ZnO-NPs (henceforth referred to as ZnO-NP-AF) and annealed ZnO-NPs (henceforth referred to as ZnO-NP-A) displays three Gaussian peaks for each of them (Figures 1a,b). The first peak at a low binding energy (529.5 eV for ZnO-NP-AF and 529.8 eV for ZnO-NP-A) corresponds to O2– ions in the Zn–O bond of ZnO-NP matrix. The second peak at 531.0 eV (ZnO-NP-AF) and 531.3 eV (ZnO-NP-A) belongs to O21 ions in the oxygen deficient region. The third peak at a high binding energy (532.2 eV for ZnO-NP-AF and 532.4 eV for ZnO-NP-A) is usually related to the chemisorbed oxygen or OH species on the ZnO-NP surface.35 XPS spectra of Zn 2p display two peaks at binding energies of 1022 eV and 1045 eV for ZnO-NP-A that are associated with Zn2+ 2p3/2 and 2p1/2 peaks, respectively. It can be noticed that these two peaks are shifted to low binding energies of 1021 eV and 1044 eV for the ZnO-NP-AF sample (Figures 1c,d), thus indicating the presence of a strong Zn–O bond in the


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ZnO-NP-AF film.36 Table S1 summarizes the calculated Zn/O atomic ratios for ZnO-NP-AF and ZnO-NP-A that are 1.29 and 1.88, respectively, which illustrates the improved atomic stoichiometry in the ZnO-NP-AF film with low oxygen deficiency. Figure 1f shows the XRD pattern of the ZnO-NP-A sample, which matches well with that of the pure ZnO in hexagonal wurtzite crystal structure according to a JCPDS-ICDD card (No. 361451). As shown in Figure 1e, the XRD pattern of ZnO-NP-AF also shows similar diffraction peaks to those of ZnO-NP-A, thus indicating that the crystalline nature and degree of crystallinity of ZnO-NP are not significantly affected by the annealing process. All the identified peaks in the XRD pattern are summarized in Table S2. Typically, the intrinsic hydrophobicity of the surface of CVD-grown graphene leads to nonuniform distribution of organic materials on the graphene surface owing to the difference in energy at the interface.23 In this study, the wettability of a ZnO-NP solution on the graphene surface was investigated via several microscopic analytic techniques such as atomic force microscopy (AFM), scanning electron microscopy (SEM), and optical microscopy (OM). As shown in SEM and OM images from Figures 2a–c and Figure S1, ZnO-NPs dispersed in an optimized mixture of chloroform and methanol solvents with relatively low surface tensions (27.5 and 22.7 dyne cm−1, respectively)37 allowed homogeneous coverage of ZnO-NPs on graphene, which has surface energy of 62.2 mJ m-2,38 without any pinhole for both the annealed and annealing-free processed ZnO-NPs. The surface morphology of ZnO-NPs on graphene was further investigated by AFM as shown in Figures 2d–f. Generally, uniform ZnO-NP films were obtained on the graphene surface regardless of the annealing process. The surface roughness of the graphene sheet increased slightly after coating with ZnO-NP-AF (root-mean square (RMS) roughness of 0.85 and 2.50 nm, respectively), which has an average particle size and a film


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thickness of 30 and 50 nm, respectively. Interestingly, ZnO-NP-A films display agglomerated ZnO-NP domains and result in a high RMS roughness of 2.79 nm; this is likely owing to the coalescence of nanoparticles induced by the annealing process,39 which may alter the device performance in OSCs by increasing the density of trap sites at the interface between ZnO-NPs and photoactive layer.40,41 Optical properties of ZnO-NPs were established based on the absorption spectra shown in Figure 3a. Optical band gaps of 3.17 eV for both ZnO-NP-AF and ZnO-NP-A films were calculated using the Tauc relation. The value of the band gap is obtained from the intercept of the straight line resulted by plotting (αhν)1/2 vs (hν) (where α is the absorption coefficient), which crosses the X-axis. This result indicates that both types of ZnO-NPs manifest similar optical properties. Next, electronic energy states of ZnO-NPs were analyzed by ultraviolet photoemission spectroscopy (UPS). As shown in Figure 3b, both the ZnO-NP-AF and ZnO-NPA films deposited on graphene show higher binding energies than those of the pristine graphene in the cut-off region as well as in the onset region. Accordingly, the work function of graphene was shifted from 4.27 eV to 4.03 eV (ZnO-NP-AF) and 4.01 eV (ZnO-NP-A), which indicates that the graphene layer was effectively n-doped by the ZnO-NP. As the film thickness of ZnONP-AF decreased, the work function of graphene/ZnO-NP-AF was shifted from 4.03 eV (50 nm) to 4.12 eV (35 nm, Figure S2a). We note that the work function of ZnO NPs was measured to be 3.68 eV for both the ZnO-NP-AF and ZnO-NP-A (Figure S2b), which corroborates that the observed shift in the work function of graphene was induced upon the deposition of ZnO-NPs. The doping behavior of graphene can also be corroborated from the transfer characteristics of graphene field-effect transistors (GFETs). As shown in Figure 3c, we fabricated GFET using the method from our previous work.42 The Dirac point (charge neutrality point) of GFET with


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pristine graphene is located at gate bias of 49 V showing the typical p-type behavior of graphene, which generally originates from the transfer processes.43 After the graphene was coated by ZnONP-AF, large shift of Dirac point was observed from 49 V to -57 V which indicates the n-doping effect of graphene upon the deposition of ZnO-NP-AF. This doping effect was further confirmed by Raman spectroscopy. As shown in Figure 3d, G bands of ZnO-NP-AF and ZnO-NP-A films on graphene were both up-shifted from that of the pristine graphene, which implies the presence of electron doping. A low ratio of 2D/G peak intensities also reveals doping in graphene.44,45 It is expected that such a decreased work function of graphene by ZnO-NPs lowers the interfacial energy barrier to the electron transport from the photoactive layer to graphene electrode by favoring the required ohmic contact. To evaluate the performance of a ZnO-NP ETL in OSCs first, OSCs with inverted structure were fabricated by using poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) and [6,6]-phenylC71-butyric acid methyl ester (PC71BM) as donor and acceptor materials, respectively for the bulk heterojunction photoactive layer. The devices were completed by thermal evaporation of MoO3 and Ag onto the photoactive layer with a thickness of 20 and 100 nm, respectively. The schematic of the device structure and corresponding flat-band energy level diagram are presented in Figures 4a,b. Three layers of stacked CVD-grown graphene were used as the TCE with an average sheet resistance and transmittance (Figure S3) of 305 ± 17 Ω sq−1 and 92.9%, respectively, at 550 nm and prepared on target substrates using the wet transfer method.46,47 It is worth re-emphasizing that no annealing process is performed while fabricating graphene/ZnONP-AF-based OSCs. The details of fabrication are further provided in the Experimental Section. The current density versus voltage (J–V) characteristics for graphene electrode-based OSCs


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developed by using ZnO-NP-AF film as ETL were established under simulated AM 1.5G illumination at an irradiance of 100 mW cm−2 (Figure 4c). The corresponding photovoltaic parameters are summarized in Table 1. The graphene-based champion PTB7:PC71BM device shows a PCE of 7.37% with a short-circuit current density (Jsc) of 15.25 mA cm−2, open-circuit voltage (Voc) of 0.72 V, and fill factor (FF) of 67.5%, thus approaching 90% of the performance of indium tin oxide (ITO)-based reference device (PCE = 8.21%). The slightly low Jsc and FF of graphene-based OSCs are attributed to the higher sheet resistance of graphene than that of ITO (~20 Ω sq−1). Such low Jsc values are also reflected in the results of external quantum efficiency (EQE) measurements as shown in Figure 4d. High values of EQE from the ITO-based device in the range of 480–600 nm originate from the higher transmittance of ITO than graphene in the same wavelength range (Figure S3). In addition, the effect of the annealing process on the formation of ZnO-NP ETL for the OSC application using the ITO-based reference device platform was investigated. Despite the negligible difference in the optical transmittance of ZnO-NP-AF and ZnO-NP-A films, each displaying high transmittance values of 97.3% and 97.1% at 550 nm (Figure S3), ZnO-NP-AFbased devices showed slightly better performance than the ZnO-NP-A-based devices (Figure S4 and Table S3). Furthermore, the ZnO-NP-based device generally showed a better performance than the conventional sol–gel ZnO-based device, which was processed under high temperature annealing treatments for the formation of crystalline ZnO films. To further improve the efficiency of graphene-based OSCs and also to demonstrate the versatility of the proposed approach, a low-band gap polymer donor, poly[4,8-bis(5-(2ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b’]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-(3fluorothieno[3,4-b]thiophene-)-2-carboxylate-2,6-diyl)] (PTB7-Th) was introduced with the


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following

device

configurations:

glass

or

PET/ITO

or

graphene/ZnO-NP-AF/PTB7-

Th:PC71BM/MoO3/Ag. Measurements of J–V characteristics are shown in Figure 4e, while the corresponding calculated photovoltaic parameters are summarized in Table 2. The graphenebased device onto glass substrate showed notable PCE of 8.16% with a Jsc of 16.26 mA cm−2, Voc of 0.76 V, and FF of 66.4%, also approaching 90% of the performance of ITO reference device (PCE = 9.13%). The graphene-based flexible OSC on PET substrate also showed a high PCE of 7.41%. The bending stability of the flexible graphene-based device was further studied with a bending radius of 3 mm (Figure 4f). After 100 bending cycles, the initial device performance was still maintained over 80% of the initial efficiency, while that of the flexible ITO-based device was reduced below 30% after only 20 times bending. These results suggest that the proposed annealing-free process for the fabrication of OSCs represents an attractive and promising technology in advancing flexible electronic devices by preventing the possible damage of the substrates, which typically occurs at temperatures above the glass transition point (e.g., for PET at 70 °C–80 °C) when the entire device can be deformed. In this study, annealing-free ZnO-NPs were successfully used as ETLs to develop graphene electrode-based inverted OSCs. Annealing of ZnO-NPs can affect the oxygen vacancy state and surface roughness of the as-prepared film, which can potentially degrade the device performance. The annealing-free ZnO nanoparticles yielded uniform coating on the graphene surface while effectively n-doping the graphene electrode. Thus, notably high PCEs of 7.37% and 8.16% were achieved using the graphene film as TCE for PTB7-based OSCs and PTB7-Th-based OSCs, respectively. On the basis of mechanical flexibility of graphene and capability of the annealingfree process of ZnO-NPs, graphene-based flexible OSCs were demonstrated on PET substrates, thus yielding a PCE of 7.41% and PCE retention over 80% after 100 bending cycles. The


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graphene/ZnO-NP (annealing-free) configuration developed in this work promises a simple and low-cost fabrication process for OSCs and may find applications in flexible and wearable optoelectronic devices. Methods. Graphene Electrode Preparation. Graphene was synthesized on a 25-µm thick copper foil via low-pressure CVD method. The copper foil was loaded into the CVD chamber and annealed at 1000 °C for 30 min under hydrogen gas. Subsequently, methane gas was introduced for 30 min for graphene growth, after which the chamber was cooled to room temperature. Graphene sheets were then transferred onto the target substrates by poly(methyl methacrylate) (PMMA)-assisted transfer method, and the supporting PMMA layer was finally removed by acetone. The transfer process was repeated to obtain four-layer stacked graphene electrodes. Solar Cell Fabrication. ITO-coated glass substrates were cleaned by sonication in soap water, deionized water, acetone, and isopropanol and subsequently underwent oxygen plasma treatment. Graphene-transferred glass and PET substrates were rinsed with acetone and isopropanol. ZnO-NP solutions were spin-coated on ITO or graphene at 2000 rpm for 1 min. Annealing treatment was conducted at 100 °C for 10 min under ambient atmosphere for the annealed ZnO nanoparticles. PTB7, PTB7-Th, and PC71BM were dissolved in a mixture of chlorobenzene:1,8-diiodooctane (97: 3 vol%) solvents at concentrations of 12, 12, and 40 mg mL–1, respectively. The blended solutions of PTB7:PC71BM (2: 1 vol%) and PTB7-Th:PC71BM (2:1 vol%) were spin-coated at 900 rpm for 2 min under nitrogen environment. MoO3 and Ag were thermally evaporated under high vacuum conditions at base pressure of 8 × 10−7 Torr. The actual device area defined by the overlap of the bottom and top electrode was 4.2 mm2.


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Measurements and Characterization. J–V characteristics of solar cells were measured using a Xenon arc lamp (300 W) solar simulator and Keithley 2635A source measurement unit in a nitrogen-filled glove box. A custom-made shadow mask was used during the measurement to prevent any potential overestimation over the actual device area. EQE measurements were performed using the QE system (QEX7, PV measurements) under ambient conditions. The analysis of the surface morphology was performed using dimension AFM (DI-3100, Veeco) operating in tapping mode, SEM (S-4800, Hitach), and OM (Eclipse LV150, Nikon). XPS and UPS measurements were conducted using monochromated Al-Kα radiation source and He I (21.2 eV) discharge lamp, respectively, (ESCALAB 250Xi, Thermo Fisher Scientific) under ultra-high vacuum condition (

Development of Annealing-Free, Solution ... - ACS Publications

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