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Aug 7, 2015 - silk fiber reeling process is developed to fabricate layered V2O5/. PEDOT nanowires (VP NWs) by stirring V2O5 powder in an aqueous solut...
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Layered V2O5/PEDOT Nanowires and Ultrathin Nanobelts Fabricated with a Silk Reelinglike Process Chun Xian Guo,† Kuan Sun,‡,§ Jianyong Ouyang,‡ and Xianmao Lu*,† †

Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585 Department of Materials Science and Engineering, National University of Singapore, 7 Engineering Drive 1, Singapore 117574 § School of Power Engineering, Chongqing University, No. 174 Shazhengjie, Shapingba, Chongqing 400044 China ‡

S Supporting Information *

ABSTRACT: For the first time, a method resembling a cocoon-tosilk fiber reeling process is developed to fabricate layered V2O5/ PEDOT nanowires (VP NWs) by stirring V2O5 powder in an aqueous solution of 3,4-ethylenedioxythiophene (EDOT). A mechanistic study indicates that the growth of VP NWs started from the intercalation/polymerization of EDOT within a few V2O5 surface layers, which were then peeled off to produce nanowires. The resulting VP NWs were further exfoliated to form 3.8 nm ultrathin V2O5/PEDOT nanobelts (VP NBs) consisting of V2O5 atomic bilayers intercalated with PEDOT. These VP NBs can be dispersed well in various solvents including water, ethanol, DMF, and acetonitrile for the preparation of transparent thin films as the hole extraction layer (HEL) to replace PEDOT:PSS in solution-processed inverted planar perovskite solar cells (PSCs). Cell efficiency tests over 7 days revealed that PSCs fabricated with VP NBs as HEL retained the initial power conversion efficiency (PCE), while those with PEDOT:PSS as HEL suffered from an efficiency drop of more than 50%.

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INTRODUCTION One-dimensional (1D) nanostructures, including nanowires, nanotubes, and nanobelts, are promising building blocks for nanoscale electronic and optoelectronic devices.1−5 Among various 1D nanostructures, of particular interest are organic/ inorganic nanowires composed of metal oxides and conducting polymers. This class of hybrid nanostructures may offer synergistic effects such as mechanical and electrical robustness inherited from metal oxides, as well as good charge transport properties and solution-processability enabled by conducting polymers.6−10 Therefore, metal oxide/conducting polymer hybrid nanowires have shown a wide range of applications including electrical energy storage, light emitting diodes, and photovoltaic devices.11−13 Very recently, metal oxide/conducting polymer hybrid nanowires composed of V2O5 and poly(3,4-ethylenedioxythiophene) (PEDOT) have received increasing attention.14−17 Because of the good electrical conductivity of PEDOT and electrochemical behavior of V2O5, hybrid V2O5/PEDOT nanowires have been employed as electrode materials for lithium ion batteries (LIBs) and supercapacitors with enhanced energy and power performances.14 Typically, these hybrid nanowires have been synthesized with a two-step growth approach, where preformed V2O5 nanowires or nanobelts are employed as the template for the coating of PEDOT via oxidative or electrochemical polymerization. This method produces 1D V2O5 cores with sizes in the range of 100−200 nm and PEDOT shells with thicknesses larger than 15 nm.8,14,17 Although the resulting hybrid nanowires have © XXXX American Chemical Society

demonstrated good performance as LIB electrodes, the relative thick V2O5 core and PEDOT coating, and hence poor optical transparency, hinder their applications in optoelectronics and photovoltaic devices, despite that the V2O5/PEDOT composite has been shown as a promising hole injection layer in light emitting diodes, while V2O5 and PEDOT are good candidates as the hole transport layer in photovoltaic devices.18,19 Especially as the hole extraction layer (HEL) in solutionprocessed photovoltaic devices including inverted planar perovskite solar cells (PSCs) and polymer solar cells, the materials need to have appropriate energy levels and good optical transparency and be able to form a dense film with a thickness in the range of 10−50 nm.20−22 Conducting polymer PEDOT:PSS fulfills the above requirements and has been widely used as HEL in inverted planar PSCs and polymer solar cells;23−27 but PEDOT:PSS is acidic and also hygroscopic, easily causing degradation of cell performance.28,29 Metal oxide V2O5 can form an efficient hole injection/collection junction with the photoactive layer and has been explored to replace PEDOT:PSS in solar cells.30 Recently 1D V2O5 nanowires as the hole transport layer in solar cells have demonstrated a higher power conversion efficiency than that of V 2 O 5 nanoparticles by providing faster charge transport and higher contact area for the photoactive layer.31 However, V2O5 material is water-soluble (e.g., a solubility of 0.8 g L−1 at 20 Received: July 1, 2015 Revised: August 6, 2015

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Figure 1. (a) Reeling process to produce silk fibers from cocoons. (b) SEM images showing an EDOT-induced reelinglike process to produce layered hybrid V2O5/PEDOT nanowires (VP NWs) from V2O5 microrods. (c) Color change of V2O5 microrods stirred in EDOT aqueous solution at t = 0 and 144 h. (d) Stable dispersions of VP NWs in water, ethanol, dimethylformamide (DMF), and acetonitrile.

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°C)32 and can be reduced by thermal annealing or some solvents (e.g., isopropyl alcohol),33,34 resulting in relatively poor stability. Hybrid V2O5/PEDOT nanowires, if employed as the hole extraction layer, may eliminate the above-mentioned drawbacks of the individual components to achieve improved device stability via the protection of PEDOT. To fully take this synergistic advantage, however, a new synthetic approach is required to produce 1D hybrid V2O5/PEDOT nanostructures with much reduced thickness. In this work, we present a method similar to cocoon-to-silk fiber reeling process to fabricate layered hybrid V2O5/PEDOT nanowires (VP NWs) by simply stirring V2O5 powder and 3,4ethylenedioxythiophene (EDOT) in aqueous solution. The resulting VP NWs can be facilely exfoliated into layered V2O5/ PEDOT nanobelts (VP NBs) with an average thickness of 3.8 nm, corresponding to two V2O5 atomic bilayers intercalated with two monolayers of PEDOT. To demonstrate their structural and compositional advantages, the VP NBs were explored as HEL in solution-processed inverted planar PSCs. Device tests show that PSCs with the VP NBs exhibited much more stable power conversion efficiencies (PCEs) than those fabricated with PEDOT:PSS.

RESULTS AND DISCUSSION Fabrication and Characterizations of VP NWs. Figure 1a shows the reeling process to produce silk fibers, in which case an external pulling force is applied to silk cocoons so that filaments can be unwound to form silk threads. In our work, layered hybrid VP NWs were synthesized via a similar reelinglike process by mixing commercial V2O5 microrods and EDOT in aqueous solution. With constant stirring at room temperature, nanowires were continuously withdrawn from the ends of V2O5 microrods until all microrods were converted to nanowires (Figure 1b, S1), while the color of the solution gradually changed from yellow to dark green (Figure 1c, S2). The resulting nanowires, having an average width of 45 nm, a thickness of ∼15 nm, and a length up to several micrometers (Figures S3−S5), can form stable dispersions in water and other polar solvents such as ethanol, DMF. and acetonitrile (Figure 1d). Control experiments showed that EDOT plays a key role in forming the nanowires - V2O5 microrods stirred without EDOT did not show obvious morphological change (Figures S6−S8). The growth of the hybrid nanowires was monitored with a transmission electron microscope (TEM). Commercial V2O5 microrods with flat ends show lattice fringes with a d-spacing of 0.26 nm that matches (310) planes of orthorhombic V2O5,35 as shown in Figure 2a. After stirring the V2O5 microrods in an B

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Figure 2. (a-h) TEM images of V2O5 microrods and VP NWs. (a) A V2O5 microrod (inset is the HRTEM image). (b-d) VP NWs obtained at 24, 72, and 144 h, respectively. (e) The layered structure of the VP NWs. (f,g) The tip and (h) side surface of V2O5 microrods during synthesis. (i) XRD patterns of the samples. (j-k) XPS V 2p and S 2p spectra of the VP NWs.

formation of the VP NWs. The composition of the VP NWs was also analyzed with X-ray photoelectron spectroscopy (XPS) and energy-dispersive spectroscopy (EDS). XPS spectra in Figure 2j-k exhibit V 2p and S 2p signals that can be attributed to V2O5 and PEDOT, respectively. The deconvolution of the V 2p2/3 peak shows two binding energies at 516.0 and 517.5 eV, corresponding to V4+ and V5+, respectively.40,41 V4+ should be formed from the reduction of V5+ during the EDOT intercalation and oxidative polymerization. EDS indicates that the weight percentage of PEDOT in the VP NWs is around 26% (Figure S9). The VP NWs exhibited different light absorption behavior (Figure S10) and optical band gap (Figure S11) from those of V2O5 microrods. UV−vis spectrum of V2O5 microrods showed an absorption band at 250 nm that is related to V5+ in the tetrahedral coordination state42,43 with an optical bandgap of 3.78 eV. While VP NWs displayed the similar band at 260 nm, an additional band was observed at 410 nm with an optical band gap of 2.58 eV, which may originate from intercalated PEDOT and/or the interaction between PEDOT and V2O5. Mechanistic Study on the Formation of VP NWs. It is known that V2O5 can be slightly dissolved in water. For V2O5 microrods stirred for 144 h in water without EDOT, a slight truncation of the corners can be observed (Figures S7−S8). However, no nanowires were formed. Upon mixing V2O5 microrods and EDOT in water, EDOT started to intercalate into the surface layers of V2O5 microrods. Eventually two intercalated monolayers of PEDOT can be formed between V2O5 bilayers. With continuous stirring, the friction between the microrods and the solvent may peel the intercalated surface layers of the microrods to form nanowires (Figure 2f, S12), similar to the cocoon-to-silk fiber reeling process. Due to the relatively strong interaction between EDOT and V2O5,35,44 EDOT in the solution may attach to the peeled nanowires, generating a PEDOT layer via surface polymerization. In the meantime, dissolved V2O5 from the ends of microrods may also

aqueous solution in the presence of EDOT for 24 h, nanowires started to extrude along the axial direction of the microrods (Figure 2b). With the increase of reaction time, the nanowires continued to grow, while the length and the diameter of the microrods gradually reduced (Figure 2c, S1). Before V2O5 microrods disappeared after 144 h (Figure 2d), the flat ends of the microrods became truncated (Figure 2f), but the tip still displayed the same lattice fringes as that of V2O5 microrods (Figure 2g). However, the side surface of the microrods showed a layered structure with an interlayer spacing of 1.86 nm (Figure 2h), the same as the nanowires formed after the complete conversion of the microrods (Figure 2e). This interlayer distance is close to the reported value for interlayer spacing of V2O5 bilayers intercalated with two monolayers of PEDOT,36 indicating that the nanowires consist of alternating the two-monolayer PEDOT and bilayer V 2 O 5 . These observations demonstrate that the formation of VP NWs started from the intercalation/polymerization of PEDOT within the outer layers of the V2O5 microrods, followed by reeling the PEDOT-intercalated V2O5 layers from the ends of the microrods to produce layered hybrid VP NWs. With the growth of the nanowires, the length and diameter of the V2O5 microrods gradually reduced, resembling a cocoon-to-silk fiber reeling process. The X-ray diffraction pattern (XRD) of V2O5 microrods in Figure 2i shows an orthorhombic crystal structure (PDF 01072-0433) with a strong diffraction peak at 20.31°, corresponding to stacked V2O5 bilayers with a d-spacing of 4.37 Å.37−39 After the microrods were converted into VP NWs, two diffraction peaks appeared at 4.74 and 9.36°, corresponding to d-spacings of 18.62 and 9.44 Å, respectively. The d-spacing of 18.62 Å is close to the interlayer spacing of V2O5 bilayers intercalated with two monolayers of PEDOT,36 consistent with the TEM result. On the other hand, XRD of V2O5 stirred without EDOT shows the same pattern as that of V2O5 microrods, indicating the EDOT intercalation is key for the C

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Figure 3. (a) Schematic illustration of the reelinglike process to produce VP NWs from V2O5 microrods. (b) Exfoliation of the VP NWs to produce ultrathin V2O5/PEDOT nanobelts (VP NBs). (c) AFM images and height profiles of the VP NBs. (d) TEM image of the VP NBs.

Figure 4. (a) Device architecture of solution-processed inverted planar PSCs. (b) 3D AFM image and the corresponding height profile of VP NBs on ITO glass. (c) J−V characteristics of PSCs with V2O5, PEDOT:PSS, and VP NB as HEL. (d) Stability test of PSCs over 7 days.

attach to the newly formed nanowires from the solution. This process can also be revealed from the time-sampled XRD patterns (Figure S14). With the increase in reaction time, two peaks with 2θ at 4.74° and 6.38° became more pronounced.

The peak at 4.74° corresponds to interlayer distances of 18.62 Å for V2O5 bilayers intercalated with the two-monolayer PEDOT. The increase in its peak intensity should be due to a larger portion of the side surface of V2O5 microrods D

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PEDOT:PSS, the PCE dropped more than 50% - an efficiency of only 5.3% was obtained (Figure 4d). It is known that photoactive perovskite materials are sensitive to environmental conditions such as moisture.46,47 When PEDOT:PSS is used as the HEL, its acidic and hygroscopic nature28,29 can accelerate the degradation of PSCs. In contrast, VP NBs are pH neutral and stable in aqueous solution. Therefore, VP NB as HEL may overcome the drawbacks of PEDOT:PSS, leading to improved stability of PSCs. However, further studies are necessary to elucidate the mechanism.

intercalated with the two-monolayer PEDOT. The peak at 6.38° with an interlayer spacing of 13.84 Å agrees well with V 2 O 5 bilayers intercalated with the one-monolayer PEDOT.35,36 As further increase of reaction time from 96 to 144 h, the peak at 6.38° disappeared and only the peak at 4.74° remained, suggesting further intercalation of PEDOT into the layered V2O5 causes the formation of stacked V2O5 bilayers each intercalated with the two-monolayer PEDOT. The EDOT-induced reelinglike process to produce VP NWs from V2O5 microrods is schematically summarized in Figure 3a. Exfoliation of VP NWs To Produce VP NBs. The VP NWs can be exfoliated into ultrathin V2O5/PEDOT nanobelts (VP NBs) by sonicating the NWs in aqueous solution at room temperature (Figure 3b). The exfoliated VP NBs have an average thickness of 3.8 nm (Figure 3c), much thinner than that of the VP NWs. Considering 18.62 Å for the interlayer spacing of V2O5 bilayers intercalated with two monolayers of PEDOT, the exfoliated VP NBs should contain two such layers, as schematically shown in Figure 3b. The exfoliated VP NBs have an average width of 15 nm (Figure 3d) and the same composition as that of the VP NWs (Figure S15). In addition, the VP NBs exhibited slightly different UV−visible light absorption behavior and optical band gaps from those of the VP NWs (Figure S16). The optical band gap of VP NBs is 2.71 eV, slightly larger than that of the VP NWs (2.58 eV). This relatively larger band gap of VP NBs might be related to the quantum confinement effect due to their thin layered structure. VP NBs have another optical band gap at 3.41 eV, close to that (3.45 eV) of the VP NWs, which should be attributed to V2O5 bilayers, further proving the retained V2O5 bilayer structure after exfoliation. More importantly, aqueous solution of the VP NBs has a near neutral pH of ∼6.8 at a concentration of ∼2.0 mg mL−1, which avoids an acidic nature as observed for PEODT:PSS aqueous solutions (a pH of 1.2−2.2 with a concentration of 1.3−1.7 wt %). VP NBs as the Hole Extraction Layer in PSCs. Solutionprocessed inverted planar perovskite solar cells (PSCs) are an upcoming member of the third-generation photovoltaics.45 This type of PSCs needs transparent hole extraction layers at the active layer/electrode interfaces to favor charge collection for high efficiency.23−25 Since the VP NBs have unique characteristics of neutral pH, ultrathin thickness, and solution processability, they were explored as the hole extraction layer in PSCs. Figure 4a shows the device architecture employed in this work. The hole extraction layer was formed by spin coating VP NBs onto clean ITO glass. It was observed that VP NBs can form a uniform and dense layer on ITO glass (Figure 4b). The average thickness of the VP NB layer is around 40 nm, with an average transmittance more than 80% between 450 and 800 nm. Control PSCs with V2O5 or the PEDOT:PSS hole extraction layer were also fabricated. The J−V characteristics and photovoltaic performance parameters are shown in Figures 4c and S17. The PSCs with VP NB as HEL exhibited a shortcircuit current density (Jsc) of 13.59 mA/cm2, an open-circuit voltage (Voc) of 1.05 V, and a fill factor (FF) of 0.59, leading to an overall power conversion efficiency (PCE) of 8.4%. The photovoltaic performance was much better than that of PSCs with the V2O5 buffer layer (Jsc of 9.30, Voc of 0.88, FF of 0.62, and PCE of 5.1%). Although the PCE of PSCs with VP NBs is slightly lower than that of PEDOT:PSS (Figure S17), VP NBs offered a much better stability. After storing in sealed flasks for 7 days, PSCs with VP NBs retained a PCE of 9.0%, even slightly higher than the initial PCE. While for PSCs based on

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CONCLUSIONS To summarize, we report a cocoon-to-silk fiber reelinglike process to fabricate layered hybrid V2O5/PEDOT nanowires (VP NWs) by mixing V2O5 powder and EDOT in aqueous solution under constant stirring at room temperature. The VP NWs can be facilely exfoliated under sonication to form 3.8 nm ultrathin V2O5/PEDOT nanobelts (VP NBs), each consisting of two V2O5 atomic bilayers that are wrapped with two PEDOT monolayers. The VP NBs disperse well in water, ethanol, DMF, or acetonitrile to form stable dispersions, which can be drop cast onto substrates to form thin films with good optical transmittance. When employed as the hole extraction layer in solution-processed inverted planar PSCs, the VP NBs offered better stability (initial PCE: 8.4%, after 7 days: 9.0%) than that of PEDOT:PSS (initial PCE: 11.3%, after 7 days: 5.3%).

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EXPERIMENTAL SECTION

Material Synthesis. To synthesize hybrid V2O5/PEDOT nanowires, 3.5 g of commercial V2O5 powder (Sigma-Aldrich) was dispersed in 100 mL of deionized water (18.2 MΩ·cm, Milli-Q) under vigorous stirring at room temperature, followed by adding 1.5 mL of 3,4-ethylenedioxythiophene (EDOT, Aldrich) dropwise. The mixture was stirred continuously. With the progress of the reaction, the color of the mixture gradually changed from yellow to light green and finally dark green. Samples (∼1 mL) were taken out at different reaction times and centrifuged to remove excess EDOT before material characterizations. Exfoliation was carried out by sonicating diluted VP NWs aqueous solution for 3 h to obtain ultrathin V2O5/ PEDOT nanobelts (VP NBs). Material Characterizations. Field emission scanning electron microscopy (FESEM) images were obtained using JEOL JSM-6700F at an acceleration voltage of 5.0 kV. Transmission electron microscopic (TEM) and high-resolution transmission electron microscopic (HRTEM) images were recorded by a JEOL JEM-2010F microscope at an acceleration voltage of 200 kV. Height profiles of samples were observed from Bruker Dimension ICON-PT atomic force microscope (AFM) with the scanning model. AFM samples were prepared by dip coating of aqueous solution on a mica substrate. X-ray diffraction (XRD) patterns were measured using X-ray diffractometer (GADDS XRD system, Bruker AXS) with a CuKα source (λ = 1.54 Å). X-ray photoelectron spectroscopy (XPS) characterizations were performed on a PHI Quantera X-ray photoelectron spectrometer with a chamber pressure of 5 × 10−9 Torr, a spatial resolution of 30 μm, and an Al cathode as the X-ray source to determine composition of the nanoparticles. Elemental composition and distribution were investigated by energy dispersive X-ray spectroscopy (EDS) from JEOL JSM-6700F equipped with an Oxford/INCA EDS. The adsorption profile of material solutions and optical behaviors of the VP NBs coated ITO glass were obtained with a Shimadzu UV-2450 spectrophotometer. Solar Cell Fabrication and Characterizations. Planar perovskite solar cells (PSCs) were fabricated on prepatterned and cleaned ITO glass substrates. A thin layer (ca. 30 nm thick) of PEDOT:PSS (Clevios 4083, Heraeus GmbH) or VP NBs (0.2 mg mL−1) or V2O5 (0.2 mg mL−1) was spin coated on ITO substrates at 1000 rpm for 3 E

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Chemistry of Materials min. VP NBs instead of NWs were employed for the film preparation because the NWs are too thick (∼15 nm) to form uniform 40 nm thin films. The substrate was then baked on a hot plate at 140 °C for 10 min in air. A perovskite photoactive layer was deposited by spin coating (1000 rpm for 20 s and then 4000 rpm for 60 s) a solution containing 0.14 M (39 mg) PbCl2, 1.26 M (581 mg) PbI2, and 1.3 M (209 mg) methylammonium iodide (MAI) in 1 mL of a cosolvent of dimethyl sulfoxide:γ-butyrolactone (3:7 vol. ratio), followed by annealing at 100 °C for 20 min. A 50 nm-thick layer of PC61BM was deposited on the perovskite layer by spin coating (20 mg mL−1 in chlorobenzene, 1500 rpm 40s). Subsequently, 0.5 mg mL−1 rhodamine 101/IPA was spin-cast at 2000 rpm onto the PCBM layer in order to form a surface dipole layer for better electron extraction. The films were then transferred to a metal evaporation chamber, where LiF (1 nm) and Ag (100 nm) were deposited. The photovoltaic performance of the PSCs was measured with a computer-programmed Keithley 2400 source/meter under a Newport’s Oriel class A solar simulator, which simulated the AM1.5G sunlight with energy density of 100 mW cm−2. The active area of each device was 0.11 cm2.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b02512. Detailed material characterizations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Education, Singapore (R279-000-391-112, R279-000-298-112) and the National Research Foundation CRP program (R279-000-337281).

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REFERENCES

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DOI: 10.1021/acs.chemmater.5b02512 Chem. Mater. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.chemmater.5b02512 Chem. Mater. XXXX, XXX, XXX−XXX