Highly Conducting MXene–Silver Nanowire Transparent Electrodes

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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 25330−25337

Highly Conducting MXene−Silver Nanowire Transparent Electrodes for Flexible Organic Solar Cells Honghao Tang,† Huanran Feng,‡ Huike Wang,† Xiangjian Wan,‡ Jiajie Liang,*,†,‡,§ and Yongsheng Chen*,‡ School of Materials Science and Engineering, National Institute for Advanced Materials, ‡Key Laboratory of Functional Polymer Materials of Ministry of Education, College of Chemistry, and §Tianjin Key Laboratory of Metal and Molecule-Based Material Chemistry and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300350, P. R. China

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†

S Supporting Information *

ABSTRACT: MXene, a new class of two-dimensional materials, offers a unique combination of metallic conductivity and hydrophilicity. This material has shown great promise in numerous applications including electromagnetic interference shielding, sensing, energy storage, and catalysis. In this paper, we report on the fabrication of transparent, conductive, and flexible MXene/silver nanowire (AgNW) hybrid films, resulting in the highest figure of merit (162.49) in the reported literature to date regarding an MXene-based transparent electrode. The hybrid films, prepared via a simple and scalable solution-processed method, exhibit good electrical conductivity, high transmittance, low roughness, work function matching, and robust mechanical performance. Following film fabrication, the hybrid electrodes were demonstrated to function as transparent electrodes in fullerene molecule PTB7-Th:PC71BM and nonfullerene molecule PBDBT:ITIC organic photovoltaics (OPVs). In an effort to further improve the performance of flexible OPVs, a ternary structure of PBDB-T:ITIC:PC71BM was demonstrated, resulting in a power conversion efficiency (PCE) of 8.30%. Mechanical properties were also quantified, with the flexible ternary organic solar cells capable of retaining 84.6% of the original PCE after 1000 bending and unbending cycles to a 5 mm bending radius. These optoelectronic and mechanical performance metrics represent a breakthrough in the field of flexible optoelectronics. KEYWORDS: MXene, silver nanowire, electrostatic interaction, flexible transparent electrodes, flexible organic solar cells

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including carbon nanotubes,11 graphenes,12 conducting polymers,13 metal grids,14 and metal nanowires15 have been investigated. Within these classes of materials, carbon-based and polymer-based materials have been limited by their optoelectronic performance due to the relatively low intrinsic electrical conductivity.16,17 Metal meshes have improved electrical conductivity but suffer from Moiré fringes, which become detrimental to the cosmetic performance of resulting devices.18 Although Moiré effects of metal meshes can be eliminated by reducing the width of the conducting lines by improving process capabilities with techniques such as yellow light manufacturing,19 the corresponding fiscal costs accrue rapidly. Percolation networks of metal nanowires have shown promise regarding electrical performance, optical transparency, and clarity. In contrast to their mesh counterparts, nanowires with their nanometer scale width of conducting networks fully eliminate Moiré interference effects.20 However, when looking

INTRODUCTION In recent years, flexible, transparent, conductive electrodes (FTCEs) have become increasingly popular in wearable, flexible, and semitransparent electronics, spanning applications from various sensors1,2 to touchscreens,3,4 artificial skins,5 transistors6,7 and optical displays.8,9 This rise in popularity can be attributed in part to the development of new technologies with exotic form factors that, in turn, necessitate its components to subsequently be endowed with additional functionalities. When this train of thought is applied to FTCEs, the shortfall of the standard technology is evident when considering the intrinsic limitations of commercially available indium tin oxide (ITO). This TCE, ubiquitously used for conventional devices, faces complications when the same technology is applied toward the next wave of electronics that places a heavy emphasis on low-cost, portable, and wearable applications. Chief among these limitations are the high cost of the raw material, high roughness, and limited mechanical performance due to the brittleness of ITO.10 In the search for low-cost, mechanically stable, and highperforming FTCEs, several alternative conductive materials © 2019 American Chemical Society

Received: March 20, 2019 Accepted: June 24, 2019 Published: June 24, 2019 25330

DOI: 10.1021/acsami.9b04113 ACS Appl. Mater. Interfaces 2019, 11, 25330−25337

Research Article

ACS Applied Materials & Interfaces

issue of roughness occurring due to the stacking of hybrid materials. These attributes foreshadow the efficacy of the hybrid electrode. As validation, the MXene/AgNW hybrid FTCE was incorporated in a nonfullerene acceptor and ternary structure PSCs. In this structure, high PV performance was observed, with a demonstrated PCE of 8.3%. Furthermore, bending tests indicate that the ternary structure FOPV can withstand mechanical deformation, capable of retaining 84.6 and 91% of the original PCE after 1000 bending and unbending cycles to 5 and 40 mm bending radii, respectively.

at the macroscopic distribution of nanowires along the FTCE, large voids of insulation exist between the conducting network, limiting the microlevel conductivity of the conductor. The existence of these macrovoids of insulation fundamentally limits the performance of organic semiconductor electronics.21 When considering organic solar cells, the chief limitation attributed to these voids pertains to the nonuniform collection of holes. To remedy this, secondary transparent conductive materials have been introduced to efficiently collect charges both laterally and vertically within the voids of the AgNW network.22−25 Although charge transport can be improved, the inherent tradeoff is the reduction of either transparency or sheet resistance of the FTCE.26 Furthermore, the introduction of secondary materials may result in a mismatch of work function to the organic layers.27,28 In recent years, MXene, a new class of two-dimensional (2D) metal carbides and nitrides, has gained increasing attention due in part to its unique physical and structural properties, which are comparable to or even outperform those of graphene.29 MXene is derived from MAX phases, a moniker for the aforementioned 2D carbides and nitrides. With this nomenclature, the A denotes an A-group element, typically Al or Ga. The final MXene material is obtained by etching away the A-group element and can be represented by the formula Mn+1XnTx, wherein M denotes an early transition metal, X is either carbon or nitrogen, and T is a surface-terminating group. Although multiple different MXene materials have been realized, titanium carbide (Ti3C2Tx) is the most widely studied among this new class of material.30 In this specific MXene material, the surface functional group is oxygen (−O), hydroxyl (−OH), and/or fluoride (−F), which is obtained after selective etching with HF.31,32 These active terminal groups enable the exfoliation of MXene into 2D single layers with a thickness ∼1.6 nm, resulting in high transparency (transmittance of 95% has been observed for 2.8 nm-thick MXene nanosheets). Furthermore, the surface terminal groups are responsible for controlling the work function of MXene, which can be tuned between 1.6 and 6 eV.33 These unique optoelectronic properties, in conjunction with exceptional mechanical properties, render MXene nanosheets a unique material with high potential for various flexible optoelectronic devices. Unfortunately, while MXene exhibits a relatively high electrical conductivity (up to 6500 S cm−1) when considering all classes of materials, its conductivity still lags behind those of metal-based electrodes.34 Consequentially, MXene-based transparent electrodes have lower demonstrated optoelectronic performance when contrasted to that of metal nanowires and ITO-based electrodes. In this work, a AgNW network is combined with MXene nanosheets for the first time to fabricate a flexible transparent electrode, with the electrode subsequently used to fabricate a corresponding state-of-the-art flexible OPV (FOPV) device. The introduction of the AgNW network was based on the following considerations: (1) AgNW networks have high optoelectronic performance, even exceeding that of the ubiquitously used ITO; (2) the absorbed electrostatic force between the MXene nanosheet and the AgNW network results in a negligible delta in ζ-potential, and as a result, mono-to-few layers of the MXene can be adhered with approaching 100% coverage of the void area of the AgNW network while avoiding deleterious impacts to transparency; and (3) the AgNW network is partially imbedded within the surface of a poly(urethane acrylate) (PUA) matrix, which mitigates the

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EXPERIMENTAL SECTION

Materials. AgNWs were synthesized with an average diameter of 40 nm and an average length of 40 μm according to the previously reported literature.35 Ethoxylated bisphenol A dimethacrylate (EBA SR540) and ethoxylated trimethylolpropane triacrylate (ETPTA SR499) were supplied by Sartomer. A photoinitiator, ethoxylated dimethylolpropane triacrylate (DMPA), chlorobenzene, 1,8-diiodooctane, and methanol were sourced from Sigma-Aldrich. PEDOT:PSS (Clevios VP 4083), PTB7-Th, PC71BM, PrC60MA, and PDINO were obtained from Ossila. PBDB-T and ITIC were purchased from Derthon Co. Ltd. ITO-glass was obtained from Wuhu Crystal Electronics Technology Co., Ltd. Other reagents were of common commercial level and used as received, without further purification. Synthesis of the MXene Nanosheet via Exfoliation of Ti3AlC2. In a typical synthesis,50 concentrated HCl was diluted with distilled water to obtain a 9 M solution (20 mL). LiF powder (2 g) was added into the solution, followed by sonication for 30 min to dissolve the salt. After sonication, 1 g of Ti3AlC2 (400 mesh size) was slowly added into the solution under stirring at 300 rpm and 35 °C for 24 h. The resultant mixture was washed four times with distilled water until the pH was nearly neutral. Finally, the nanosheet dispersion was freeze-dried for 48 h, with the obtained products stored in a glovebox under an Ar atmosphere for further use. Preparation of the MXene/AgNW-PUA Hybrid Electrode. The hybrid electrodes were fabricated using a slot-die coating process. Briefly, a dispersion of AgNWs in isopropanol (5 mg mL−1) was coated on a glass slide using a Meyer rod. After post-treating by annealing at 120 °C for 5 min to solder the junctions, a precursor solution of PUA (SR540, SR499, and DMPA), in a weight ratio of 1:2:0.005 was coated on top to a thickness of 150 μm. The combined AgNW and PUA precursors were cured together under an ultraviolet light for 3 min and then peeled off to reveal a free-standing hybrid electrode. The prepared composite flexible electrode was called AgNW-PUA. The as-prepared hybrid electrode was soaked in a MXene aqueous solution (5 mg mL−1) for 3 min, with the MXene solution stirred at 100 rpm with a stirring magneton. After MXene coating, the FTCE was removed, washed with distilled water, and heated at 80 °C for 5 min to remove the residual water. The prepared composite flexible electrode was called MXene/AgNW-PUA. Fabrication of Flexible PSCs. PSCs were fabricated on ITOglass and the MXene/AgNW hybrid electrode with the following structure: cathode (MXene/AgNW-PUA or ITO-glass/PEDOT:PSS)/PTB7-Th:PC71BM/PrC60MA/AI. The ITO-glass surfaces were cleaned by detergent, deionized water, acetone, and isopropyl alcohol under ultrasonication for 15 min each and dried by a nitrogen blow. PEDOT:PSS was spin-coated onto the electrodes (ca. 40 nm thick), followed by heating at 120 °C for 40 min to remove the residual water. The electrodes were transferred to a glovebox to spin-coat PTB7-Th:PC71BM (1:1.5 w/w, 25 mg mL−1 for PTB7-Th:PC71BM) in chlorobenzene/1,8-diiodooctane (97:3% by volume) with a speed of 1700 rpm for 40 s. To form an electron transport layer, the solution of PrC60MA (1 mg mL−1) in methanol was spin-coated at 3000 rpm for 40 s. Al layers (50 nm) were deposited by thermal evaporation under a vacuum of 1.5 × 10−4 Pa. For nonfullerene binary PSCs, the structure was the following: cathode (MXene/AgNW-PUA or ITO-glass/PEDOT:PSS)/PBDBT:ITIC/PDINO/AI. PEDOT:PSS was spin-coated onto the electro25331

DOI: 10.1021/acsami.9b04113 ACS Appl. Mater. Interfaces 2019, 11, 25330−25337

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ACS Applied Materials & Interfaces Scheme 1. Fabrication Process of MXene-Based Flexible Transparent Electrode

des (ca. 40 nm thick), followed by heating at 120 °C for 40 min to remove the residual water. The electrodes were transferred to a glovebox to spin-coat PBDB-T:ITIC (1:1 w/w, 20 mg mL−1 for PBDB-T:ITIC) in chlorobenzene/1,8-diiodooctane (99.5:0.5% by volume) at a speed of 2500 rpm for 40 s. The device was annealed at 120 °C for 5 min. To form an electron transport layer, PDINO (1 mg mL−1) in methanol was spin-coated at 3000 rpm for 40 s. Al layers (50 nm) were deposited by thermal evaporation under a vacuum of 1.5 × 10−4 Pa. For ternary PSCs, the structure was (MXene/AgNW-PUA or ITOglass/PEDOT:PSS)/PBDB-T:ITIC:PC71BM/PDINO/AI. PEDOT:PSS was spin-coated on the electrodes (ca. 40 nm thick), followed by heating at 120 °C for 40 min to remove the residual water. The electrodes were transferred to a glovebox to spin-coat PBDB-T:ITIC:PC71BM (1:0.7:0.3 w/w, 20 mg mL−1 for PBDBT:ITIC:PC71BM) in chlorobenzene/1,8-diiodooctane (99.5:0.5% by volume) at a speed of 2500 rpm for 40 s. The device was annealed at 120 °C for 5 min. To form an electron transport layer, PDINO (1 mg mL−1) in methanol was spin-coated at 3000 rpm for 40 s. Al layers (50 nm) were deposited by thermal evaporation under a vacuum of 1.5 × 10−4 Pa. Characterization. AFM images were obtained using a Dimension Icon from Bruker. The scanned area of AFM images was 5 × 5 and 20 × 20 μm2. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) studies were performed on a Thermo Scientific ESCALAB 25Xi photoelectron spectrometer using a monochromated Al Kα (1486.6 eV) X-ray source. Scanning electron microscopy (SEM) and elemental mapping characterization were carried out using a field-emission scanning electron microscope (JSM7800) at an accelerating voltage of 5.0 kV. Optical transmission measurements were performed over a wavelength range of 300−800 nm using a UV−vis/near-infrared spectrophotometer (Cary 5000, Agilent). The current density−voltage (J−V) characteristics of photovoltaic devices were obtained using a Keithley 2400 source measure unit. The photocurrent was measured under simulated 100 mW cm−2 AM 1.5G irradiation using a xenon-lamp-based solar simulator [Oriel 96000 (AM 1.5G)] in an argon-filled glovebox. Simulator irradiance was characterized using a calibrated spectrometer, and the illumination intensity was set using a certified silicon diode. External quantum efficiency (EQE) values of the devices were measured using a Stanford Research Systems SR810 lock-in amplifier. The effective area of each cell was 4 mm2 for the solar cell devices. The transmission electron microscopy (TEM) image in Figure S1a shows the prepared MXene nanosheet. The detailed elemental analysis and conductivity information of the prepared MXene are illustrated in Figure S2. The AFM image in Figure S1b shows the morphology of the prepared MXene nanosheet; the thickness of the MXene nanosheet was between 2 and 3.5 nm, and its length was on the order of microns (Figure S1d). The conductivity of the prepared MXene nanosheet was 5320 S cm−1, as shown in Figure S1c. The

transmittance and sheet resistance of the FTCE were judged by the coating content of the AgNWs and MXene, which could be adjusted by changing the coating times and the concentration of AgNW dispersion. Figure S3a shows the transmittance curve of the prepared AgNW-PUA and MXene/AgNW-PUA films with different AgNW coating contents. Table S1 provides corresponding specific points of transmittance on the transmittance curve and sheet resistance of different FTCEs in a greater detail. Figure S3b provides the summary of sheet resistance and transmittance changes of various AgNW-PUA and MXene/AgNW-PUA films. Figure S3c provides the summary of figure of merit (FOM) of various MXene/AgNW-PUA films. Figure S4 provides the summary of the AFM image of prepared films, and Table S2 provides corresponding Ra and Rq values of prepared films. Figure S5 provides the AFM image of the flexible electrode before and after coating with MXene at 20 μm range. Table S3 provides roughness values obtained using tapping-mode AFM images of AgNW-PUA and optimized MXene/AgNW-PUA at 20 μm range. Figure S6 provides the detailed information of UPS to calculate the work function of the MXene hybrid electrode.

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RESULTS AND DISCUSSION Fabrication and Characterization of Ti3C2Tx MXeneBased FTCE: MXene/AgNW-PUA. In this work, we propose a novel approach to prepare MXene-based hybrid FTCEs via an all-solution process, as depicted in Scheme 1. AgNWs with a length-to-diameter aspect ratio of ∼1000 were used to construct the framework of the electrical conducting network. The synthesis and purification of the as-used AgNWs can be found in our previously reported work.35 Briefly, silver nanowires were first coated on a release substrate through a slot-die coating process. Following nanowire coating, a PUA precursor comprising ethoxylated bisphenol A dimethacrylate (EBA SR540), ethoxylated trimethylolpropane triacrylate (ETPTA SR499), and a photoinitiator, ethoxylated dimethylolpropane triacrylate (DMPA), mixed in a ratio of 1:2:0.005, was coated onto AgNW films and subsequently polymerized through ultraviolet curing. The polymerization process embeds the AgNWs on the surface of the PUA matrix, allowing the composite electrode to be completely peeled off from the release glass substrate. Following the peeling process, the AgNW-PUA films were subsequently immersed in a MXene nanosheet dispersion. To theoretically gauge the compatibility of the two materials, the ζ-potentials of the AgNWs and the MXene nanosheet found in the literature are −7.8 and −29 mV (at pH 7),36 respectively. Because the absolute value of the ζ-potential of the AgNW is smaller than that of the MXene nanosheet, the attractive forces would exceed the electrostatic repulsion between the nanosheets and nanowires, resulting in 25332

DOI: 10.1021/acsami.9b04113 ACS Appl. Mater. Interfaces 2019, 11, 25330−25337

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Z0σopt yz ij zz T = jjj1 + zz j 2 R σ (1) s DC { k where T denotes the transmittance at 550 nm, Rs represents the sheet resistance, and Z0 is the free space impedance (377 Ω). The highest recorded FOM value of MXene/AgNW-PUA was 162.49 (details are in Figure S3c in the Supporting Information), which represents the highest FOM of all reported MXene transparent films to date.39−45 Although the optoelectronic performance of the electrode may have high performance metrics, organic devices additionally have strict requirements regarding the roughness of electrodes. Figure 1a,b illustrates the morphology of the conducting surface, while Table 2 provides the corresponding −2

MXene nanosheets being attracted to the surface of the AgNW network.37 However, because the MXene sheet-to-sheet ζpotential is the same, the force balance would be repulsive, allowing only a single layer or a few layers to be adhered to the AgNW network. This thin coverage would cover almost 100% of the void area of AgNW network but lead to a negligible decrease in transparency, as observed in the transmission curves before and after MXene nanosheet coating in Figure 1f

Table 2. Roughness Values Obtained Using Tapping-Mode AFM Images of AgNW-PUA and Optimized MXene/AgNWPUA 5.0 × 5.0 μm2 image samples

Rq (nm)

Ra (nm)

AgNW-PUA optimized MXene/AgNW-PUA

1.21 2.30

0.72 1.52

root-mean-square roughness (Rq) and average roughness (Ra). From this data, it is evident that the surface is smooth after the application of the MXene nanosheet. For compositional analysis, X-ray photoelectron spectroscopy (XPS) was used to characterize the top conducting surface. The bands at a binding energy region spanning 380−387 eV are attributed to the Ag 3d XPS bands from the exposed AgNWs at the surface. The Cl 2p XPS bands between 200 and 201 eV and the Ti 2p XPS bands between 453 and 460 eV are attributed to the MXene nanosheet. SEM and elemental analysis were further used to investigate the uniformity of the MXene surface coverage and are presented in Figure 1c,d. The elemental analysis shows that the silver and titanium components in the electrode have a uniform distribution. Work function matching is yet another critical consideration for organic semiconductor devices. Because this parameter is so critical, the ability to adjust the work function of the MXene through surface functional group tuning represents another important attribute of MXene nanosheets.29 Ultraviolet photoelectron spectroscopy (UPS) was utilized to characterize the work function of the prepared MXene/AgNW-PUA electrodes; the Fermi edge and cutoff edge of the electrode were 20.61 and 4.67 eV, respectively; the work function was calculated to be −5.28 eV (Figures 2a and S6). This value falls within the range typically required for anodes utilized in organic optoelectronics. As the last characterization and demonstration for the stand-alone film, Figure 2b illustrates

Figure 1. (a, b) AFM images of the AgNW-PUA and MXene/AgNWPUA films. (c, d) TEM images of the AgNW-PUA and MXene/ AgNW-PUA films. (e) XPS spectra of the prepared neat PUA, AgNW-PUA, and MXene/AgNW-PUA films. (f) Transmittance spectra of neat PUA, MXene-PUA, AgNW-PUA, optimized MXene/AgNW-PUA, and ITO-glass.

and Table 1. Additionally, it is evident that the optimized MXene/AgNW-PUA film has optoelectronic performance comparable to that of ITO-glass. Furthermore, because single-layer MXene has a conductivity significantly higher than that of the traditionally used conducting polymer (PEDOT:PSS), charges can be efficiently transported along both the lateral and vertical direction void regions of the AgNW network. Figure of merit (FOM), which is defined as the ratio of electronic (σDC) to optical (σopt) conductivity, was used to quantify the performance of the TCE. The equation governing FOM can be written as38

Table 1. Summary of Typical Transmittance and Sheet Resistance Values of Neat PUA, MXene-PUA, AgNW-PUA, Optimized MXene/AgNW-PUA, and ITO-Glass Transparent Electrodes transmittance (%) samples

@400 nm

@550 nm

@800 nm

Ω/sq

neat PUA MXene-PUA AgNW-PUA optimized MXene/AgNW-PUA ITO-glass

89.63 89.00 70.58 68.54 79.78

91.68 91.05 84.78 83.32 87.39

91.87 91.12 85.53 84.03 83.53

null null 26.3 26 15

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of the subsequently prepared PSCs. Herein, the hybrid electrode has a work function of −5.28 eV, by ultraviolet photoelectron spectroscopy analyses, which matches well with that of PEDOT:PSS. MXene/AgNW-PUA3, with its excellent FOM (Figure S3c), was selected as the FTCE. To evaluate the impact of the electrode, reference FTCEs with only AgNWs and AgNWs/MXene hybrid electrodes were also used to fabricate FOPVs. ITO-glass with a sheet resistance of 15 Ω and transmittance at 550 nm was also used as a reference. (Details of the photoelectric property of the ITO-glass are provided in Table 1.) In the device structure, the classical organic solar donor and acceptor PTB7-Th and PC71BM, respectively, were used with the following final structure: MXene/AgNW-PUA/ PEDOT:PSS/PTB7-Th:PC71BM/PrC60MA/Al. An optical photograph of the resulting FOPV is shown in Figure 4a. Detailed descriptions of the fabrication process of the PSCs are presented in the Experimental Section. The performance of the PVs is illustrated in Figure 3d,g and Table 3. It was observed that the PSC based on AgNW-PUA has a low open-circuit voltage (VOC), low fill factor (FF), and short current density (JSC), which can be attributed to the high prevalence of voids among the conducting network. In this structure, the hole transport layer is in direct contact with the insulating surface, impacting the ability of the anode to collect holes, because the AgNW conducting network only occupies small conducting areas on the surface of conducting films. This fundamental issue can be addressed with the addition of the MXene nanosheets. In contrast to the AgNW-PUA electrode, FOPVs based on the MXene/AgNW-PUA electrode exhibit comparable VOC and FF to those of ITO-glass. Furthermore, the JSC of the MXene-based FTCE is lower than that of ITOglass, as a result of the differences in the conductive and

Figure 2. (a) UPS spectra of the optimized MXene/AgNW-PUA films. (b) Optical images of the optimized MXene/AgNW-PUA films. (c) Normalized resistance of the MXene/AgNW-PUA films under different bending radii. (d) R/R0 of MXene/AgNW-PUA films as a function of bending and unbending cycles to a bending radius of 5 mm.

an optical image of a bent MXene/AgNW-PUA film. The flexibility of the hybrid electrode was quantified through a bending test. As visualized in Figure 2c,d, the normalized resistance of the films increases only slightly after 1000 bending−unbending cycles to a 5 mm bending radius. Fabrication of PSCs Based on MXene/AgNW-PUA Films. The work functions of the carrier transport layers and electrodes were investigated from the reported literature and taken into consideration for the design of the device in this research.46,47 Figure 3a−c illustrates the energy-level diagram

Figure 3. Energy-level diagrams, J−V curves, and EQE of PSCs based on PTB7-Th:PC71BM (a, d, g), PBDB-T:ITIC (b, e, h), and PBDBT:ITIC:PC71BM (c, f, i). 25334

DOI: 10.1021/acsami.9b04113 ACS Appl. Mater. Interfaces 2019, 11, 25330−25337

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ACS Applied Materials & Interfaces ε=

H 2R·

(2)

wherein H represents the thickness of the transparent film, ε is the strain, and R is the bending radius. An optical photograph of the PSCs in a bent state is illustrated in Figure 4a. Figure 4c indicates that the strain of the PSCs ranges from 0.0025 to 0.015. For the ternary structure FOPV, the PCE retains 98% of its original value after being bent to a 5 mm bending radius (Figure 4b). Cycling tests were also conducted at 5 mm, where the PCE was observed to retain 84.6% of its original value after 1000 bending and unbending cycles (Figure 4d). At a higher bending radius of 40 mm, 90% of the original PCE was retained after 1000 bending and unbending cycles.

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CONCLUSIONS A highly conductive MXene-based hybrid FTCE was fabricated and used to demonstrate the highest FOM reported to date of 162.49. The MXene coating was applied via coating by electrostatic interactions, with the resultant MXene/AgNWbased hybrid FTCE exhibiting good electrical conductivity, high transmittance, low roughness, and robust mechanical performance. The MXene FTCE was further inocorporated in flexible OPVs using a fullerene-based PTB7-Th:PC71BM system, a nonfullerene-based PBDB-T:ITIC system, and a ternary structure of PBDB-T:ITIC:PC71BM, achieving a PCE of 8.30%. In addition to optoelectronic performance, the MXene-based FOPV had robust mechanical performance, retaining 84.6% of its initial PCE after 1000 bending and unbending cycles to a 5 mm bending radius. These results represent a new breakthrough in the research of twodimensional materials for optoelectronic devices.

Figure 4. (a) Optical photograph of flexible PSCs. (b) Normalized PCEs of flexible PSCs with MXene/AgNW-PUA transparent electrodes under different bending radii. (c) Normalized PCE of flexible PSCs with MXene/AgNW-PUA transparent electrodes under different strains. (d) Normalized PCE of flexible PSCs with MXene/ AgNW-PUA transparent electrodes at different bending radii as a function of the number of bending cycles.

electrical properties of MXene/AgNW-PUA and ITO-glass. These differences are highlighted in Figure 1f and Table 1. As a further validation of the improvement of the performance using the hybrid electrode, higher EQE values at 550 nm can be seen in Figure 3g and higher calculated JSC can be observed in Table 3. As the research in the field of organic solar cells continues to advance, nonfullerene molecules have attracted increasingly more attention. To evaluate the MXene hybrid electrode in these classifications of devices, the classical nonfullerene acceptor ITIC and the polymer donor PBDB-T were used as active layers in the construction of additional FOPVs. The resulting device performance was similar to that of the PTB7Th:PC71BM devices, with a recorded PCE value of 7.70%. To further improve the device performance, the ternary structure PBDB-T, ITIC, and PC71BM was used as the active layer,47 resulting in a PCE of 8.30% when the hybrid electrode was used as the FTCE. The flexibility of the FOPV was also probed through bending tests. The thickness of the prepared MXene/AgNWPUA films was measured to be approximately 150 μm. The strain under different bending radii can be represented by the following equation48,49

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b04113. Synthesis of the MXene nanosheet, fabrication of MXene hybrid films, and flexible polymer solar cells; detailed characterization of the MXene nanosheet and MXene hybrid films; TEM images; AFM images; EDS elemental mapping; transmittance spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.L.). *E-mail: [email protected] (Y.C.).

Table 3. Device Performance Metrics for Devices Using ITO-Glass, AgNW-PUA, and MXene/AgNW-PUA Electrodes Based on a PTB7-Th:PC71BM Active Layer, PBDB-T:ITIC Active Layer, and PBDB-T:ITIC:PC71BM Ternary Active Layer active Layer PTB7-Th:PC71BM

PBDB-T:ITIC

PBDB-T:ITIC:PC71BM

anode ITO-glass AgNW-PUA MXene/AgNW-PUA ITO-glass AgNW-PUA MXene/AgNW-PUA ITO-glass AgNW-PUA MXene/AgNW-PUA

JSC [mA cm−2] 15.77 7.1 14.62 15.04 8.06 13.98 15.85 10.08 14.85

± ± ± ± ± ± ± ± ±

0.31 0.57 0.34 0.31 0.79 0.28 0.31 0.87 0.14

VOC [V] 0.81 0.76 0.79 0.89 0.82 0.86 0.90 0.83 0.88

± ± ± ± ± ± ± ± ±

0.01 0.05 0.02 0.01 0.05 0.02 0.01 0.05 0.02

FF [%]

JSC [cal.] [mA cm−2]

± ± ± ± ± ± ± ± ±

15.24

61 20 61 63 22 64 64 23 63

1.2 5.4 1.7 1.3 4.7 1.8 1.3 4.3 1.9

13.97 14.85 13.54 15.04 14.32

PCEmax(PCEavea) [%] 7.87 1.09 7.16 8.55 1.46 7.70 9.19 1.91 8.30

(7.77) (1.03) (7.04) (8.53) (1.39) (7.61) (9.02) (1.67) (8.21)

a

PCEave is calculated from the average of 20 devices. 25335

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Research Article

ACS Applied Materials & Interfaces ORCID

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Jiajie Liang: 0000-0003-2112-6721 Yongsheng Chen: 0000-0003-1448-8177 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.

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ACKNOWLEDGMENTS This work was supported by NSFC (51872146, 91633301, 51673099, and 21421001) and Tianjin Municipal Science and Technology Commission in China (17JCZDJC30200).

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DOI: 10.1021/acsami.9b04113 ACS Appl. Mater. Interfaces 2019, 11, 25330−25337

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DOI: 10.1021/acsami.9b04113 ACS Appl. Mater. Interfaces 2019, 11, 25330−25337