Large-Scale Flexible and Highly Conductive Carbon Transparent

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Large-scale Flexible and Highly Conductive Carbon Transparent Electrodes via Roll-to-roll Process and Its High Performance Lab-scale ITO-free PSCs Xiaotian Hu, Lie Chen, Yong Zhang, Qiao Hu, Junliang Yang, and Yiwang Chen Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 18 Oct 2014 Downloaded from http://pubs.acs.org on October 18, 2014

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Chemistry of Materials

Large-scale Flexible and Highly Conductive Carbon Transparent Electrodes via Roll-to-roll Process and Its High Performance Lab-scale ITO-free PSCs Xiaotian Hu, Lie Chen, Yong Zhang, Qiao Hu, Junliang Yang, Yiwang Chen*

X. Hu, Prof. L. Chen, Y. Zhang, Prof. Y. Chen School of Materials Science and Engineering/Institute of Polymers, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China Prof. L. Chen, Prof. Y. Chen Jiangxi Provincial Key Laboratory of New Energy Chemistry, College of Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China E-mail: [email protected] (Y. Chen) Q. Hu, Prof. J. Yang Institute of Super-Microstructure and Ultrafast Process in Advanced Materials, Central South University, 932 South Lushan, Changsha 410083, China

Abstract A scalable and highly conductive PEDOT:PSS:CNTs transparent electrode (TE) is demonstrated for high performance optoelectronics. The aligned and uniformly dispersion of electron conduction favored CNTs in the PEDOT:PSS matrix can achieve the rearrangement of the PEDOT chains with more expended conformation via the π-π interaction between CNTs and PEDOT. As a result, PEDOT:PSS:CNTs electrode presents a high conductivity of 3264.27 S cm-1 with a high transmittance over 85%, and ITO-free PSCs based on PEDOT:PSS:CNTs electrode achieves a PCE of 7.47% with high stability. Furthermore, a large-scale flexible electrode was obtained by roll-to-roll technique, which demonstrates an excellent property with a sheet resistance of 17 Ω sq-1 and 80.7% optical transmittance. Combined the flexible and conductive PEDOT:PSS:CNTs film with scalable roll-to-roll process, we anticipate that the commercial production of large-scale transparent electrode, replacing ITO, will be realized in the near future.

ACS Paragon Plus Environment


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Polymer-fullerene solar cells (PSCs) are receiving a great deal of attention due to their potential as a source of renewable energy1,2. For PSCs with a traditional bulk heterojunction (BHJ) device structure, power conversion efficiency (PCE) up to 10% has been achieved owing to the advance on the synthesis of novel polymer donors, interfacial morphology control, optimized device structure and processing optimization3-7. Currently, the state-of-the-art PSCs are mostly based on indium tin oxide (ITO) electrode due to its high transparency for light penetration and high conductivity for charge collection. However, ITO has several intrinsic weaknesses such as limited indium sources, unsuitability for commercial flexible electronic devices, inferior physical properties for high temperature treatment, high mechanical brittleness, expensive cost involved in preparation processes (pulsed laser deposition, sputtering and evaporation) and high portion (more than 50%) in materials consumption of PSCs device preparation8-12. Therefore, it is urgently desirable to find new transparent electrodes (TEs) to replace ITO for PSCs. Stimulated by such a practical need, recently, some conductive materials such as new transparent metal oxide (AZO13 and FTO14, etc.), conducting polymers15-20, functional carbon materials (Graphenes21-26 and carbon nanotubes (CNTs)27-29), nano-metallic materials30-34 have been successfully applied as the alternatives for ITO-free PSC devices. Among these, poly (3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS, Figure 1a) has been considered a promising candidate for TEs due to its mechanical flexibility, high transparency in the visible light spectrum and easy aqueous solution processing35,36. Note that, it also suffers from some fatal problems such as hygroscopy, acidity, sensitivity to oxygen exposure and low primitive conductivity (~1 S cm-1)34,36-38. Over the past decade, two main methods were used to enhance the conductivity (σ) of PEDOT:PSS. Solvent treatment with various organic solvents, surfactants, salts, and acids has been proved to be an efficient approach to improve the σ of PEDOT:PSS by two to three orders of magnitude, which is attributed to morphological changes, chains expansion, and phase separation in PEDOT:PSS18,35,39-43. Recently, a high σ of


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~4380 S cm−1 has been achieved through a post-treatment of H2SO418 on PEDOT:PSS. However, the strong acidity of such H2SO4 modified PEDOT:PSS electrodes generally required complicated processes40. Incorporation of high conductive nanotubes or nanofibers into PEDOT:PSS electrode33,44-47 is also found to be another method to enhance its conductivity (σ). Unfortunately, the conductivity improvement is limited by these disordered nano-additives, and the conductivity of these modified PEDOT:PSS TEs are still remarkably lower than that of ITO, which is hard to satisfy the demand of high efficient and air-stable devices. In this communication, a solution-processed PEDOT:PSS TE with high conductivity is demonstrated by simply doped with acidified carbon nanotubes. A well aligned and uniformly dispersion of CNTs can be realized in the PEDOT:PSS matrix, and the highly orientated CNTs enable the rearrangement of the PEDOT chains with more expended conformation surround the aligned CNTs by the π-π interaction between CNTs and PEDOT (Figure 1b). As a result, the conductivity of the CNTs modified PEDOT:PSS TE has been significantly increased to 3264.27 S cm-1 with a reduced sheet resistance of 40.51 Ω sq-1. After modification with polyethylenimine, 80% ethoxylated (PEIE)48 (Figure 1a), work function of the CNTs modified PEDOT:PSS TE is reduced to 4.2 eV, which could satisfy as a cathode for the inverted air-stable devices. Simultaneously, a hole transporting layer (HTL) of solution processed V2O5-RGO (Vanadium Oxide-Reduced Graphene Oxide) is in situ synthesized by graphene oxide (GO) and a V2O5 precursor through one-step growth process. After optimization of the devices (Figure 1c), the power conversion efficiency (PCE) of ITO-free PSC with the novel solution processed TE and HTL reaches 7.47% based on poly[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b]dithiophene-2,6-diyl]-a lt-[2-(2-ethyl-hexanoyl)-thieno[3,4-b]thiophen-4,6-diyl] (PBDTTT-C-T)49:(6,6)-phenyl-C71

butyric

acid

methyl

ester

(PC71BM)

(PBDTTT-C-T:PC71BM) system, which is comparable to the value of the ITO-based device. The novel TE also endows the devices with good stability and universality in different active layers. Furthermore, the PEDOT:PSS:CNTs ink was availably applied in roll-to-roll process for larger-scale flexible electronics (Figure 1d). The film



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demonstrates sheet resistance as low as 17 Ω sq-1 with 80.7% optical transmittance, and exhibits favorable bending reliability, which is superior to commercial ITO transparent electrode. The novel conductive polymer electrode fully meets the requirements of commercial organic optoelectronic devices.

Results The CNTs modified PEDOT:PSS films were prepared from PEDOT:PSS (Clevios PH 1000) aqueous solution doped with 5% (weight concentrate) solution of acid treated CNTs in dimethyl sulfoxide (DMSO). The conductivity (σ) values of the PEDOT:PSS films with various concentrations of CNTs in PEDOT:PSS solution (defined as PEDOT:PSS:0.1%CNTs,

PEDOT:PSS:0.3%CNTs,

PEDOT:PSS:0.5%CNTs,

PEDOT:PSS:0.7%CNTs and PEDOT:PSS:1.0%CNTs) are summarized in Figure 2a and Table S1, and pristine PEDOT:PSS films from 5% (weight concentrate) solution of PEDOT:PSS PH1000 in DMSO was also provided for comparison. After optimization of the thickness (~75nm, see Support Information Figure S1 and Table S2), the modified PEDOT:PSS with 0.5% CNTs loading shows the highest conductivity of 3264.27 S cm-1, which is around five times higher than that of the pristine PEDOT:PSS film (670.91 S cm-1). The corresponding sheet resistance of the electrode also decreases from 136.83 Ω sq-1 for pristine PEDOT:PSS film to 40.51 Ω sq-1 for PEDOT:PSS:0.5%CNTs film, on the same order of magnitude with that of ITO. In addition, the normalized conductivity enhancement also depends on the annealing temperature during the treatment as shown in Figure 2b. Compared to the pristine PEDOT:PSS film, the CNTs can dramatically improve the thermal stability of composite electrodes at high temperature (up to 300 oC). And the optimal annealing temperature is 150 oC, which is compatible to the annealing temperature of the PEDOT:PSS50. High transparency of an electrode is a determinative and vital factor for the devices performance. Figure 2c shows the transmittance (T) spectra of PEDOT:PSS:0.5%CNTs layer in the visible range. The transmittances of PEDOT:PSS:0.5%CNTs and ITO films are 85.1% and 83.5% in visible spectrum, respectively, revealing that CNTs barely affect the transparency of the PEDOT:PSS


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film. The high conductivity of PEDOT:PSS:0.5%CNTs along with the high transparency in the visible range fulfills the requirement as the transparent electrode. Transmission Electron Microscope (TEM) images of the PEDOT:PSS loaded 0.5% CNTs after ultrasonic treatment are displayed in Figure 2d. PEODT:PSS presents regular and homogeneous morphology. Intriguingly, highly oriented CNTs arrange uniformly in PEODT:PSS background. These oriented CNTs can provide a convenient pathway for electrons transportation and collection in PEDOT:PSS:CNTs electrode, which should be responsible for the high conductive electrode. In addition, the conformation change and enrichment of PEDOT on the PEDOT:PSS film surface induced by the oriented CNTs also render a great contribution to the conductivity enhancement (see Discussion Section). The work function (WF) was determined via UPS (Figure S2) and Kelvin probe (Figure S3). The work function of the PEDOT:PSS:0.5%CNTs TE is found to be 5.1 eV, which is not low enough to serve as a cathode for the inverted devices, so PEIE was deposited on the surface of the CNTs modified TE to reduce its WF48. As expected, after deposition of a thin layer PEIE (~10 nm), the neutral amine and hydroxyl groups contained in PEIE create a dipole moment at the interfacial contact48 and makes the WF of PEDOT:PSS:0.5%CNTs downshift to 4.2 eV. The substantial reduced work function is close to the WF of ZnO, as illustrated in Figure 3a, which can avoid the charge from capturing and gathering at the interface to form a recombination center, leading to more effective charge selection and collection for PSCs. Besides high performance TE, an efficient solution processed hole transport layer (HTL) is also critical for high performance and low cost PSCs. A suitable HTL can facilitate the charge carriers extraction, transportation and reduce hole and electron recombination. In view of this, V2O5 is introduced here as HTL, due to its low cost commercialization, compatibility with spin-coating deposition from its precursor, well-defined crystal structure, high current density for device and high work function51. To further tailor the electrical and optical properties of the V2O5 HTL, high conductive graphene is introduced to afford a novel solution processed V2O5-RGO



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composite HTL by one-step growth process. That is, in-situ growth of V2O5 nanocrystal in GO isopropanol solution from Vanadium triisopropoxide and the reduction of GO occurred simultaneously during thermal annealing. The XPS measurements of V2O5, RGO and V2O5-RGO films are compared in Figure S4a. It can be found that the peaks ascribed to V2O5 (V 2p and O 1s) and RGO (C 1s and O 1s) are clearly presented in the XPS spectra of V2O5-RGO HTL. The formation of the V2O5-RGO can also be verified by TEM and AFM images displayed in Figure 3b,c, and Figure S5a,b,c, where in-situ growth of V2O5 particles are uniformly covered on the RGO surface. Annealing temperature exerts great influence on the morphology and lattice structure of V2O5-RGO. Figure S4b depicts the determination of the C-V-O ratios of V2O5-RGO films with different annealing temperature by XPS spectra. With respect to those of the sample annealing from 100 oC (47.06%) and from 200 oC (46.34%), the content of O element of the sample annealing from 150 oC decreases to 40.01%, suggesting less oxygen defects existing in HTL. At the same time, 150 oC thermal annealing also induce a relatively smooth and homogenous morphology of HTLs, as elucidated in Figure S5b,d,e. In Figure S2 and Figure S3, it can be also discovered that V2O5-RGO exhibits a bit lower work function (5.2 eV) than the V2O5 (5.6 eV), originating from the tightly arrangement of in-situ growth V2O5 on the RGO to lessen the bandgap between Femi level and Vacuum level. The optimized work function is closer to the energy of the highest occupied molecular orbital (HOMO) of the polymer donor, which can facilitate to the hole transport, as shown in Figure 3a. To determine the function of the novel TE on the optoelectronics, the ITO-free inverted

PSCs

based

(P3HT):(6,6)-phenyl-C61

on butyric

P3HT:PC61BM acid

methyl

(poly(3-hexylthiophene) ester

(PC61BM),

TE/ZnO/P3HT:PC61BM/HTL/Ag) were fabricated with the solution-processed highly conductive PEDOT:PSS:0.5%CNTs films as TE and solution-processed 2D V2O5-RGO as HTL. The current density-voltage (J–V) characteristics of inverted cells with various cathodes and HTLs under AM 1.5G irradiation at 100 mW·cm−2 are shown in Figure 3d, and the related electrical parameters are summarized in Table 1.


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The Device A (Glass/PEDOT:PSS/ZnO/P3HT:PC61BM/V2O5/Ag) with the pristine PEDOT:PSS electrode and V2O5 HTL delivers a PCE of 1.03% with a short-circuit current density (Jsc) of 6.94 mA·cm−2, an open circuit voltage (Voc) of 0.54 V and a fill factor (FF) of 27.4%. After the PEDOT:PSS electrode is modified by PEIE, the Voc of the Device B increases to 0.58 V together with improved PCE, Jsc and FF. Since the Voc of PSCs is normally determined by the difference in Fermi level of acceptor and donor, as well as the difference of work function between anode and cathode52, the increase in Voc can be attributed to the better energy alignment through the reduced work function of cathode after PEIE deposition. Moreover, a thin layer of PEIE film also tends to form favorable interfacial dipoles to enhance the charge extraction and reduce the charge combination, leading to the improvement in the Jsc and FF. When the electrode is doped with CNTs, all of the device parameters, e.g. PCE, Jsc, Voc

and

FF

are

substantially

enhanced

in

Device

C

(Glass/PEDOT:PSS:0.5%CNTs/PEIE/ZnO/P3HT:PC61BM/V2O5/Ag), resulting from the highly orientated CNTs induced efficient charge collection. Replacing the V2O5 HTL with 2D V2O5-RGO HTL, the PCE of ITO-free device is further improved from 3.54% for Device C to 3.94% for Device D, which is even comparable to the reference Device E with ITO as cathode. The improved efficiency is mainly related to the increased Jsc (9.19 mA cm−2) and FF (68.8%), revealing the function of V2O5-RGO HTL. In good agreement with XPS measurement, V2O5-RGO HTL annealing from 150 oC gives the highest PCE among those with different thermal annealing (Figure S7 and Table S3). It should be noted that 150 oC is exactly the optimal annealing temperature of the active layer, thus the formation of V2O5-RGO HTL and annealing treatment of the active layer can be accomplished by only one-step annealing. Eventually, such high conductive electrode, favorable energy alignment combined with suitable interfacial modification results in the notable PCE of the ITO-free inverted PSC. The Incident photon-to-current efficiency (IPCE) spectra of various devices are shown in Figure 3e. An increase of IPCE at wavelengths between 300 nm and 800 nm was observed for devices with the PEDOT:PSS:0.5%CNTs/PEIE TE, the maximum IPCE is over 70%, indicative of



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efficient photon-to-electon conversion. All the Jsc calculated from integration of the IPCE spectrum from 300 nm to 800 nm is in good agreement with the Jsc obtained from J–V curve. The effectiveness of the PEDOT:PSS:0.5% CNTs TE and V2O5-RGO HTL of P3HT:PC61BM inverted PSCs devices inspire us to apply them in other devices based on low bandgap donor materials, such as PBDTTT-C-T. The J–V curves of the PSCs based on PBDTTT-C-T are shown in Figure 3d and Table 1. Delightfully, the ITO-free

inverted

device

with

a

structure

of

Glass/PEDOT:PSS:0.5%CNTs/PEIE/ZnO/PBDTTT-C-T:PC71BM/V2O5-RGO/Ag (Device F) shows a PCE as high as 7.47%, with a Voc of 0.77 V, a Jsc of 15.76 mA·cm−2, and an FF of 61.9%, and all of the device parameters reach the same level to those of ITO-based device (7.69%, Device G). Figure 3e shows the IPCE spectra of ITO or PEDOT:PSS:0.5%CNTs-PSCs based on PBDTTT-C-T:PC71BM, the values of Jsc via the IPCE measurement well match with the ones from I-V curves. For the Device F, the average device efficiency is more than 7.0% (as shown in Figure S6 ), which shows the good reproducibility of this method for Device F. These results demonstrate the universality of this new PEDOT:PSS:0.5%CNTs and V2O5-GO HTL for versatile ITO-free PSCs devices. Furthermore, fresh solar cells with different electrodes based on P3HT:PC61BM or PBDTTT-C-T:PC71BM were aged in ambient air. In Figure S8, the normalized device parameters averaged over 12 devices are displayed as a function of the ageing time. The PSCs with ITO electrode exhibit relatively high air-stability and maintain over ca.90% of PCE for both P3HT:PC61BM and PBDTTT-C-T:PC71BM after 30 days storage in air. Most PEDOT:PSS-based PSCs suffer from poor stability for its high acidity, hygroscopy and sensitivity to oxygen exposure of the PEDOT:PSS. However, all of the devices based on the new CNTs modified PEDOT:PSS electrode present good ambient-condition stability and the maintained performances after 30 days storage in air are similar to the ITO-based ones. The show decay of the ITO-free devices probably be attributed to the less acidity and hydroscopicity of electrode from morphological changes in the PEDOT:PSS complex, alkalinity of PEIE48. Therefore,


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the new CNTs modified PEDOT:PSS electrode offers the great potential for fabricating long term ambient stable ITO-Free PSCs.

High performance conductive materials processed by roll-to-roll technology (as shown in Figure 4a) are found to be an effective approach to realize the commercial application of transparent conductive film53. Inspired by this, the PEDOT:PSS:CNTs ink was employed into roll-to-roll process to fabricate large-area, flexible TE (Figure 4b).

Table

S4

summarizes

the

characteristics

of

the

gravure-printed

PEDOT:PSS:CNTs electrode grown on flexible PET substrates at different web speed and roll speed to control the thickness of the films (Details in support information including a movie of its operation). Intriguingly, the roll-to-roll processed large-area PEDOT:PSS electrode shows an optimized sheet resistance as low as 17 Ω sq-1 and an optical transmittance of 81.1% at a wavelength of 550 nm with a web speed of 0.3 m/min and a roll speed of 0.33 m/min, which is even superior to common ITO/PET transparent electrode (Table S4 and Figure S9a). The PEDOT:PSS:CNTs electrode exhibits quite low sheet resistance even though it was prepared by gravure printing at room temperature. Figure 4c-e demonstrate the superior flexibility and high transparency of the gravure-printed PEDOT:PSS:CNTs electrode and a picture in Figure 4f shows that the gravure-printed PEDOT:PSS electrode has a sheet resistance of 17 Ω sq-1 measured by a portable four point probe. ITO electrode deposited on the flexible substrate always suffers from crash and breaking undergoing bending, due to the brittleness of ITO, which limits its application in flexible optoelectronic devices. However, besides the high conductivity, the transparent electrode also possesses robust mechanical tolerance to bending stresses. Here we verify that the gravure printed film shows tiny loss in conductivity over 500 bending cycles at a radius of curvature of 2 mm and 5 mm, shown in Figure S9b. Discussion To further complement and understand the origin behind the conductivity of CNTs modified PEDOT:PSS TE, the conformation and composition of PEDOT:PSS:CNTs, were investigated by Raman spectrum and X-ray photoelectron spectroscopy (XPS).



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The Raman spectra of PEDOT:PSS and PEDOT:PSS:0.5%CNTs films are shown in Figure 5a. Compared to the Raman spectrum of the pristine sample, the three peaks at 1496, 1583 and 2882 cm-1 in the modified PEDOT:PSS are assigned to the CNTs 54,55. The intensity of the bands attributed to PEDOT at 1569, 1539, 1367, 1097, 991, 854, 701, 578 and 440 cm-1 56-58 also slightly increase. It should be noted that a strong band of PEDOT at 1435 cm-1 assigned to the ring Cα=Cβ stretching vibration arising from the five-member ring of PEDOT59-61, is red shifted and becomes narrower to 1439 cm-1after addition of CNTs. This change is similar to the case of PEDOT:PSS treated by ethylene glycol38, indicating that the benzenoid structure of PEDOT chains with the coil conformation turns to a quinoid structure with a linear or expanded-coil structure, accompanied by a transition from polarons with a positive charge on a unit to bipolarons with two positive charges delocalized over several units59,62. The phenomena prove the strong interactions between the CNTs and the conjugated thiophene chain63,64 and the formed quinoid structure is very helpful for the conductivity

enhancement.

XPS

measurements

of

PEDOT:PSS

and

PEDOT:PSS:0.5%CNTs films are shown in Figure 5b. The signal XPS band between 166 and 172 eV is associated to the S 2p1 band of the sulfur atoms in PSS, whereas another two bands between 162 and 166 eV with doublet peaks (S 2p and S 2p3) are assigned to the signal from sulfur atoms of PEDOT65,66. Surprisingly, the intensity of S 2p and S 2p3 has a dramatic increase after addition of CNTs, indicating much more PEDOT accumulate on the film surface (the ratio of PEDOT-to-PSS increased from 0.17 to 0.33). The enrichment of PEDOT on the surface can also be observed by Tapping-mode atomic force microscopy (AFM) images. As seen from the AFM images of PEDOT:PSS (Figure 6a, c, e), PEDOT (bright regions) and PSS (dark regions) interconnect with each other with indistinct microphase separation structures. However, incorporation of CNTs into the film, the bright PEDOT domains become larger while the dark PSS domains become smaller (Figure 6b, d, f)20,41. The aggregation of PEDOT on the surface probably results from the assembly of straight PEDOT chains stretched by these aligned CNTs through π-π interaction between CNTs and PEDOT. These continuous PEDOT domains can serve as better percolating


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pathways and play a great contribution to high conductivity. Additionally, the surface of the CNTs modified electrode shows much rougher but homogeneous morphology than that of PEDOT:PSS as revealed by the measured root-mean-square (rms) roughness of PEDOT:PSS and PEDOT:PSS:0.5%CNTs films (1.11 and 1.16 nm, respectively), subsequently providing more imitate contact with the upper layer. On the basis of these results we can conclude that incorporation of the highly orientated CNTs into the PEDOT:PSS can not only effectively improves the conductivity of the electrode, but also promotes the enrichment of the conductive PEDOT on the surface of the electrode. More importantly, the π-π interaction between CNTs and PEDOT could enable the rearrangement of the PEDOT chains with more expended conformation surround the aligned CNTs and facilitate the phase separation between the PEDOT and PSS chains as well, which allows more inter-chain interactions among the PEDOT components for a tremendous enhancement of the conductivity of PEDOT:PSS. A schematic model of the highly orientated CNTs inducing the conformational change of PEDOT is proposed in Figure 1b. Summarily, a solution processed, highly conductive PEDOT:PSS:CNTs composite electrode was demonstrated for printable ITO-free optoelectronics. The CNTs is found to be well aligned and uniformly dispersed in the PEDOT:PSS matrix. And these aligned CNTs not only can favor electron conduction, but also induce a significant conformational change of PEDOT chains and structural rearrangement in the PEDOT:PSS complex. Subsequently, PEDOT:PSS:CNTs electrode presents a high σ of 3264.27 S cm-1 with a sheet resistance of 40.51 Ω sq-1. Combined with a novel solution processed V2O5-RGO HTL, a notable and stable PCE of 7.47% is achieved in ITO-free PSCs with PEDOT:PSS:CNTs electrode. It should be noted that, a large-scale, flexible and robust PEDOT:PSS:CNTs electrode with a remarkable performance with 17 Ω sq-1 and 80.7% optical transmittance is achieved by roll-to-roll technique. Combined the flexible and conductive PEDOT:PSS:CNTs film with scalable roll-to-roll process, we anticipate that the commercial production of large-scale transparent electrode, replacing ITO, will be realized in the near future.



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Methods Sample Preparation: For the PEDOT:PSS:CNTs films is fabricated by a facile method. The multi-wall carbon nanotubes (CNTs) with a diameter of ～15 nm (purchased from Sigma Aldrich) was acidified by hydrochloric acid/nitric acid mixture and sulfuric acid, in sequence. After this acid process, the acid CNTs present more pure and smooth, as shown in TEM images (Figure S10). For the raman spectrum (Figure S11), the relative peak intensity of D/G was increased after acid process, which means CNTs were successfully modified a certain amount of hydrophilic group. Then the acid treated CNTs were dispersed in dimethyl sulfoxide (DMSO, 0.5 mg/ml) through 2 h ultrasonic treatment. For the post-treatments, the various volume ratio (0-1%) modified CNTs dispersion was immersed into PEDOT:PSS aqueous solutions (Clevios PH 1000, were purchased from Heraeus Ltd.), then was filtered using a hydrophilic syringe filter (0.45 µm) to remove large-size particles. Zinc acetate dihydrate [Zn(CH3COO)•2H2O] (Aldrich, 99.9%) with 0.1 M concentration was first dissolved in anhydrous ethanol [CH3CH2OH] (99.5+% Aldrich) and rigorously stirred for 2-3 h at 60

o

C. Subsequently,

ethanolamine was added to the solution as sol stabilizer followed by thorough mixing process with magnetic stirrer for 12 h at room temperature. For the Graphene Oxide (GO) synthesis a modified Hummers method67 was used. Typically, Graphite flakes (purchased from Sigma Aldrich.) were subjected to thermal annealing at 250 oC in humid air atmosphere overnight, and then were washed with hydrochloric acid to remove the catalyst residual. Thereafter, the Graphite flakes were rinsed sequentially with aqueous Na2CO3, H2O, and methanol, followed by dried at 50 oC overnight. A mixture of the purified Graphite flakes (500 mg) and 98% H2SO4 (150 mL) was ultrasonicated in a water bath for 30 minutes and stirred overnight at room temperature. To the Graphite flakes dispersion, KMnO4 (1.5 g) was then added in five portions and stirred at room temperature for 2 h. After the resulting mixture was stirred at 70 °C for 3 h, extra KMnO4 (600 mg) was added in several portions, which was accompanied by color change from dark to light violet-red. Upon cooling down


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to room temperature, the reaction mixture was carefully poured into a mixture of 400 g ice, 600 g water, and 15 mL H2O2 under stirring. Finally, the resultant dispersion was centrifuged (22000 rpm, 20 min) to give crude GO as dark precipitate (550 mg). Vanadium(V) triisopropoxide solution (96% w/v in isopropanol, IPA, Alfa Aesar) was diluted with IPA at 1:70 volumetric ratio, and the powder of GO was dispersed in IPA (0.5 mg mL−1) by ultrasonication for 1 h. Vanadium(V) triisopropoxide-GO solution was prepared at 1:1 volumetric the ratio, and then thermal annealing at 100~200 oC to afford V2O5-RGO HTL during device fabrication (see below). Device Fabrication: The polymer solar cells were fabricated on glass substrates. The glass was cleaned with sequential ultrasonic treatment in acetone, detergent, deionized water, and isopropanol, and dried by nitrogen flow followed by plasma treatment (low level, 6.8 W) for 15 minutes. After cleaning, the PEDOT:PSS:CNTs aqueous solutions was spin-coated at 2000 rpm for 60 s on glass substrates. The obtained films (~75 nm thickness) were annealed on a hot plate in the ambient atmosphere at 140 oC for 15 min. The Polyethylenimine, 80% ethoxylated (PEIE; Mw = 70,000 g/mol), was dissolved in H2O with a concentration of 35-40 wt% when received from Aldrich. Then it was further diluted with 2-methoxyethanol to a weight concentration of 0.4 %. The solution was spin coated on top of the electrode films at a speed of 5000 rpm for 1 min and an acceleration of 1000 rpm/s. Spin-coated PEIE films were annealed at 100 oC for 10 min on a hotplate in ambient air. The prepared ZnO sol-gel was spin-coated on the ITO-coated glass substrate with 3000 rpm. The ZnO films were annealed at 200 oC for 1 h in the air. The thickness of ZnO film is approximately 30 nm, determined by a profilometer (Alpha-Step-IQ). Subsequently, the modified samples were transferred to the nitrogen-filled glove-box and the P3HT based photoactive

blend

layer

was prepared

by

spin

coating

(800

rpm) the

1,2-dichlorobenzene solution of P3HT and fullerene derivative (PC61BM) (1:1 w/w, polymer concentration of 20 mg mL−1) on the modified PEDOT:PSS:CNTs electrodes for 30 s then annealed on hot plate. The thickness of the photoactive layer was around 200 nm. The D-A copolymer-containing photoactive layer was prepared by spin



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coating (900 rpm) the dichlorobenzene solution of D-A copolymer (PBDTTT-C-T) and PC71BM (1:1.5 w/w, polymer concentration of 10 mg mL−1) with 3% volume ratio of DIO additive on the modified PEDOT:PSS:CNTs electrode. The thickness of the photoactive layer was about 90 nm. Then, the V2O5 precursor and GO with optimal volume ratio of 1:1 in IPA solution was spin-cast in air on top of the polymer:fullerene composite layer. Subsequently, the sample was then heated for 3 min inside the glove box at different temperature (100~200oC). Finally, the device was pumped down in vacuum (< 10-7 torr; 1 torr~133 Pa), and a~90 nm thick Ag electrode was deposited on top. The deposited Ag electrode area defined the active area of the devices as 19.7 mm2 (all the areas were tested with an aperture). Roll-to-Roll process: The Roll-to-Roll coating was carried out on a GTB150B-0602E Multi-functional coating machine from SHENZHEN SHINING AUTOMATION EQUIPMENT CO. LTD (CHINA). All coating was performed with tension control on the web using the coating roller to drive the web at fixed speed. The system comprised: unwinder, purge unit, drying unit, double roller edge guiding system, gravure printing system and rewinder. The width of the PET substrate is 12 cm, and five 1 cm wide lines are evenly coated on the PET substrate. The strain of the three tension roller is 30 N, 40 N and 30 N, respectively. Characterization: Sheet resistances of electrodes were measured by using a four point probe setup with a source measurement unit (Keithley 2400). Current-voltage (J-V) characteristics were characterized using Keithley 2400. The currents were measured in the dark and under 100 mW·cm-2 simulated AM 1.5 G irradiation (Abet Solar Simulator Sun2000). All the measurements were performed under ambient atmosphere at room temperature. The scan range is form 0 V to 1V, and 6.7 mV for each step. All the J–V curves are based on 150 points. The incident photo-to-electron conversion efficiency spectrum (IPCE) were detected under monochromatic illumination (Oriel Cornerstone 260 1/4 m monochromator equipped with Oriel 70613NS QTH lamp), and the calibration of the incident light was performed with a monocrystalline silicon diode. Transmittance


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spectra were analyzed by UV-vis spectroscopy (Perkin Elmer Lambda 750). The morphologies of PEDOT:PSS:CNTs films were investigated by atomic force microscopy (AFM) using a Digital Instrumental Nanoscope 31 operated in the tapping mode. The thicknesses of all the spin-coating layers were measured by surface profilometer (Alpha-Step-IQ). The thickness of R2R-prepared PEDOT:PSS/CNT composite layers were measured by surface profilometer (AMBIOS TECHNOLOGY ltd. XP-2). XPS studies were performed on a Thermo-VG Scientific ESCALAB 250 photoelectron spectrometer using a monochromated AlKa (1,486.6 eV) X-ray source. All recorded peaks were corrected for electrostatic effects by setting the C−C component of the C 1s peak to 284.8 eV. The base pressure in the XPS analysis chamber was 2 × 10-9 mbar. For the UPS measurements, He I (21.22 eV) radiation line from a discharge lamp was used, with an experimental resolution of 0.15 eV. All the UPS measurements of the onset of photoemission for determining the work function were done using standard procedures with a -5 V bias applied to the sample.

Supporting Information The detailed experimental sections and the other characterization of devices are in Supporting Information. This information is available free of charge via the Internet at http://pubs.acs.org

Acknowledgements This work was supported by the National Natural Science Foundation of China (51273088, 51263016 and 51473075) and Doctoral Programs Foundation of Ministry of Education of China (Grants 20133601110004).



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Figure 1．．(a) The chemical structure of PEDOT:PSS, The chemical structure of PEIE. (b) Schematic structure of acidified CNTs inducing the conformational orientated change of PEDOT, (c) Device optimization, (d) Roll-to-roll process.



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Figure 2. (a) Electrical properties of PEDOT:PSS films with different CNTs concentration, (b) Normalized conductivities of PEDOT:PSS films after tested in different temperature, (c) The transmittance spectra of ITO and PEDOT:PSS:CNTs films in the visible range, insets show the comparison of transmittances of ITO and PEDOT:PSS:CNTs, (d) Transmission Electron Microscope (TEM) image of PEDOT:PSS:CNTs film electrode.


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Figure 3. (a) The schematic energy diagram of the interfacial layers involved in the PSCs, (b) TEM image of RGO, (c) TEM image of V2O5-RGO, (d) Current (J)–voltage (V) characteristics of cells based on different devices, (e) Incident photon-to-current efficiency (IPCE) of photovoltaic cells based on different devices. Device A: Glass/PEDOT:PSS/ZnO/P3HT:PC61BM/V2O5/Ag;

Device

B: Glass/PEDOT:PSS

/PEIE/ZnO/P3HT:PC61BM/V2O5/Ag; Device C: Glass/PEDOT:PSS:0.5%CNTs/PEIE /ZnO/P3HT:PC61BM/V2O5/Ag; Device D: Glass/PEDOT:PSS:0.5%CNTs/PEIE/ZnO /P3HT:PC61BM/V2O5-RGO (150 oC)/Ag; Device E: Glass/ITO/ZnO /P3HT:PC61BM /V2O5-RGO(150

o

C)/Ag;

Device

F:

Glass/PEDOT:PSS:0.5%CNTs/PEIE/ZnO

/Glass/PBDTTT-C-T:PC71BM/V2O5-GO (150 oC)/Ag; Device G: Glass/ITO/ZnO /PBDTTT-C-T:PC71BM/V2O5-GO (150 oC)/Ag.



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Page 24 of 28

Table 1. Performance of PSCs (P3HT:PC61BM) with different devices under the illumination of AM1.5G, 100 mW/cm2. Jsc

Voc

FF

PCE

(mA cm−2)

(V)

(%)

(%)

(Device A)

6.94

0.54

27.43

1.03

Glass/PEDOT:PSS/ZnO/P3HT:PC61BM/V2O5/Ag

±0.75

±0.03

±4.3

±0.32(1.35) a

(Device B)

8.53

0.58

54.28

2.67

Glass/PEDOT:PSS/PEIE/ZnO/P3HT:PC61BM/V2O5/Ag

±0.55

±0.03

±3.79

±0.43(3.10) a

9.34

0.60

62.72

3.54

±0.54

±0.02

±2.14

±0.28(3.82) a

9.48

0.60

68.81

3.94

±0.57

±0.02

±1.8

±0.34(4.28) a

(Device E)

9.93

0.62

66.77

4.09

Glass/ITO/ZnO /P3HT:PC61BM/V2O5-RGO(150 oC)/Ag

±0.61

±0.02

±2.26

±0.52(4.61) a

15.76

0.77

61.97

7.47

±0.62

±0.02

±1.88

±0.66(8.16) a

15.91

0.77

63.13

7.69

±0.60

±0.02

±2.19

±0.79(8.48) a

Device

(Device C) Glass/PEDOT:PSS:0.5%CNTs/PEIE/ZnO/P3HT:PC61BM /V2O5/Ag

(Device D) Glass/PEDOT:PSS:0.5%CNTs/PEIE/ZnO/P3HT:PC61BM/ o

V2O5-RGO (150 C)/Ag

(Device F) Glass/PEDOT:PSS:0.5%CNTs/PEIE/ZnO/ o

Glass/PBDTTT-C-T:PC71BM/V2O5-GO (150 C)/Ag

(Device G) Glass/ITO/ZnO/PBDTTT-C-T:PC71BM/ V2O5-GO (150 oC)/Ag *

All values represent averages from twelve 19.7 mm2 devices on a single chip, and the

areas were tested with an aperture.a best device PCE.


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Figure 4. (a) The roll-to-roll machine, (b) The roll-to-roll process of PEDOT:PSS:CNTs printing, the wet PEDOT:PSS:CNTs film (dark blue), (c)-(e) Picture of the gravure-printed PEDOT:PSS:CNTs electrode with superior flexibility and high transparency, the corresponding dried films (light blue), (f) The transparent PEDOT:PSS:CNTs electrode coated onto PET substrate showing the best sheet resistance of 17 Ω sq-1. For a movie of its operation see Supplementary Information.



(a)

(b)

S 2p1

PEDOT:PSS:0.5% CNTs PEDOT:PSS

S 2p S 2p3

Intensity (a.u.)

PEDOT:PSS:0.5% CNTs PEDOT:PSS

Intensity (a.u.)

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Page 26 of 28

500

1000

1500

2000

2500 -1

Raman shift (cm )

3000

174

172

170

168

166

164

162

160

Binding Energy (eV)

Figure 5. (a) Raman spectra of PEDOT:PSS and PEDOT:PSS:CNTs films. Raman spectroscopy was performed using an InVia Raman Microscope system (Renishaw, Inc.), with an Ar+ ion laser operating at 613 nm and 1.2 mW, (b) S 2p XPS spectra of PEDOT:PSS and PEDOT:PSS:0.5%CNTs films.


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Figure 6. Tapping-mode atomic force microscopy (AFM) images of (a) PEDOT:PSS height image, (b) PEDOT:PSS:CNTs height image, (c) PEDOT:PSS phase image, (d) PEDOT:PSS:CNTs phase image, (e) PEDOT:PSS three-dimensional image, (f) PEDOT:PSS:CNTs three-dimensional image, (scan range:3µm×3µm).



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Table of Content Large-scale Flexible and Highly Conductive Carbon Transparent Electrodes via Roll-to-roll Process and Its High Performance Lab-scale ITO-free PSCs Xiaotian Hu, Lie Chen, Yong Zhang, Qiao Hu, Junliang Yang, Yiwang Chen*

A scalable PEDOT:PSS:CNTs transparent electrode with a high conductivity of 3264.27 S cm-1 is demonstrated for high performance optoelectronics. ITO-free PSCs based on PEDOT:PSS:CNTs electrode achieve a high and stable PCE of 7.47%. Furthermore, a large area flexible electrode was obtained by roll-to-roll technique, which demonstrates an excellent property with a sheet resistance of 17 Ω sq-1 and 80.7% optical transmittance, providing a feasible way to replacing ITO.

Graphical abstract


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Large-Scale Flexible and Highly Conductive Carbon Transparent

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