High-Performance Polymer Solar Cells Realized by Regulating the

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High Performance Polymer Solar Cells Realized by Regulating the Surface Properties of PEDOT:PSS Interlayer from Ionic Liquids Liqiang Huang, Xiaofang Cheng, Lifu Zhang, Weihua Zhou, Shuqin Xiao, Licheng Tan, Lie Chen, and Yiwang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09078 • Publication Date (Web): 23 Sep 2016 Downloaded from http://pubs.acs.org on September 25, 2016

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ACS Applied Materials & Interfaces

Article type: Article

High Performance Polymer Solar Cells Realized by Regulating the Surface Properties of PEDOT:PSS Interlayer from Ionic Liquids Liqiang Huanga, Xiaofang Chenga, Lifu Zhanga, Weihua Zhoua,b, Shuqin Xiaoa,b, Licheng Tana,b, Lie Chena,b, Yiwang Chen*a,b a

College of Chemistry/Institute of Polymers, Nanchang University, 999 Xuefu Avenue,

Nanchang 330031, China b

Jiangxi Provincial Key Laboratory of New Energy Chemistry, Nanchang University, 999

Xuefu Avenue, Nanchang 330031, China

Corresponding author. Tel.: +86 791 83968703; fax: +86 791 83969561. E-mail: [email protected] (Y. Chen)

Author contributions. L. Huang and X. Cheng contributed equally to this work.

Abstract: Significant efforts have been dedicated to the interface engineering of organic photovoltaic device, suggesting that the performance and aging of the device are not only dependent on the active layer, but also governed by the interface with electrodes. In this work, controllable interfacial dipole and conductivity have been achieved in ionic liquids (ILs) modified

poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)

(PEDOT:PSS).

We

conclude that an appropriate interfacial conductivity is as essential as the suitable work function for an efficient buffer layer. Through forming favorable dipoles for hole transportation and reducing the film resistance by [HOEMIm][HSO4] treatment, an averaged performance of 8.64% is obtained for OPVs based on PTB7:PC71BM bulk heterojunction with improved stability. However, the improvement of performance is inconspicuous for OPVs based on PTB7-Th:PC71BM bulk heterojunction due to the incompetent energy level of high concentration ILs-modified PEDOT:PSS. The enhanced in-plane conductivity will 1

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reduce shunt resistance, and produce a fake high short-circuit current density (Jsc) with a lower fill factor. We point that the Jsc can be improved by decreasing series resistance, meanwhile the accompanying reduced shunt resistance has an unfavorable effect on device performance. Keywords: controllable work function; ionic liquid; interfacial conductivity; conductive polymers; polymer solar cells

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1. Introduction As the emergence of novel electron donor-electron acceptor (D-A) blend and reasonable device structure, the performance of organic solar cells (OSCs) has exceeded 11%.1-7 The conventional OSCs combining with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) are known for their unsatisfactory stability in ambient atmosphere. Among the emerging hole-transporting layer (HTL) materials including oxidized derivatives of graphene,8-10 self-assembled small molecules,11-12 transition metal oxides,13 and other conductive polyelectrolytes,14-16 PEDOT:PSS still remains ahead for its high work function (5.1 eV), high conductivity and favourable processability in fabricating large-scale, flexible devices.

Significant efforts have been dedicated to the interface engineering of devices, suggesting that the performance and aging of the device are not only dependent on the active layer, but also governed by the interface with electrode.17-20 The thickness and energy level structure of interfacial material are directly relevant to charge transfer efficiency of dissociated carriers. For ultra-thin films with less than 10 nm thickness, they alter the surface properties of substrate such as conductivity, surface tension, and so on. The interface dipole moment forms between interface layer and electrode, which change the work function of substrate. For the film with a thickness more than 10 nm, the surface property is always governed by the interfacial material, while it has weak connection with lower electrode. Recently, a novel high work function material PCPDTBT-SO3K has been developed to compete with PEDOT:PSS.21-22 Our group also employed a series of Cl-substituted self-assembled small molecules to replace traditional PEDOT:PSS and achieve a PCE of 9.2%.12 On the other hand, Cheng et al. developed a conjugated microporous polymer (CMP) film prepared by electrochemical polymerization (EP) with a maximum PCE of 8.42%.23 Nevertheless, the EP process might make it difficult to put into practical application. Besides, Lu et al. introduce a 3



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cooperative plasmonic effect to produce higher performance of 8.67% by mixed Ag and Au nanoparticles into PEDOT:PSS layer.24 The above work all demonstrate that the work function, hole mobility and series resistance (Rs) of the HTL are of great importance. It has been revealed that ionic liquids (ILs) could be an effective additive for PEDOT:PSS to improve the conductivity. It is worth mentioning that Chantal et al. found that PEDOT:PSS PH1000 combined with [EMIM][TCB] attained a σdc up to 2084 S cm-1.25 The nearly infinite combinations of suitable cations and anions lead to the possibility of tailoring the ionic liquid properties with a controllable conductivity in a large field.

The design criteria for an efficient interlayer generally includes the design of suitable energy lever, benign charge selectivity and high carrier mobility. The interlayer conductivity is an additional point should be taken care for designing thick buffer layers. It is corresponding to the carrier mobility, and is closely connected to the charge selectivity of hole and electron. Here, we employed several different kind of facile and low-cost ILs to modify PEDOT:PSS film as HTL. With the addition of a small amount of IL, the in-plane and out-of-plane conductivity of the PEDOT:PSS can be improved to varying degree. Meanwhile, a tunable dipole moment could be created by changing the counterions and concentration of ILs, consequently leading to a lower work function of PEDOT:PSS surface. The conductivity of PEDOT:PSS film improves while the work function reduces with the amount of IL increases. We point out the in-plane and out-of-plane conductivity of HTL should be governed seriously to balance the series and parallel resistance, in order to avoid to miscalculate the performance of device. Devices incorporating with donors of different highest occupied molecular orbital (HOMO) energy level have been studied with HTL of accurately controlled work function. As a result, an averaged PCE of 8.64% has been achieved for PTB7:PC71BM polymer:fullerence bulk heterojunction (BHJ) and a best PCE of 9.21% for PTB7-Th:PC71BM BHJ solar cell. In

4


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addition, the stability of device simultaneously increases above 15% comparing with untreated one after two months.

2. Experimental section Materials.

Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl]

[3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) and Poly[4,8-bis (5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th) were purchased from 1-material, Chemscitech Inc., QC, Canada. The PC71BM and PFN were obtained from Solarmer Materials Inc, China. ITO glass (8–10 Ω/sq) was purchased from Zhuhai Kaivo Optoelectronic Technology Co. Ltd. ILs were obtained form Lanzhou Greenchem ILs, LICP, CAS.

Device Fabrication. The conventional device structure was ITO/PEDOT:PSS/(ILs)/active layer/PFN/Al, and hole-only device was capped with MoO3 and Ag. A thin layer (35 nm) of PEDOT:PSS (Baytron P VP AI 4083) was spin-coated onto a cleaned ITO surface and annealed in air at 140 °C for 20 min. Then, ILs saqueous solution with various concentrations were spin-coated on the top of PEDOT:PSS layer at 2000 rpm for 1 min and annealed in air at 140 °C for 10 min. Next, PTB7:PC71BM and PTB7-Th:PC71BM were cast from a solution with mass ratio of 1:1.5 (PC71BM concentration of 15 mg/mL) in chlorobenzene/ 1,8-diiodoctane (97:3 vol%) mixed solvent at 1000 rpm for 2 min. After dried in nitrogen, PFN interlayer (dissolved in methanol) was spin-coated on the top of active layers at 5000 rpm for 1 min with a thickness within 8 nm. It was completed after deposition of 100 nm Al for conventional device or 8 nm MoO3 and and 90 nm Ag for hole-only device. All the measurements were performed under ambient atmosphere at room temperature. Current density–voltage (J–V) characteristics of the devices were measured using a Keithley 2400 5



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Source Meter (Abet Solar Simulator Sun2000). The light source was calibrated by using silicon reference cells with an AM 1.5 Global solar simulator with an intensity of 100 mW/cm2. An area of 4 mm2 black polytetrafluoroethylene mask was employed to correct the size of cells for some cases.

Characterization. The morphologies of ILs modified PEDOT:PSS films were investigated by scanning electron microscopy (SEM) using a QuanTA-200F. The atomic force microscopic (AFM) images were measured on a nanoscope III A (Digital Instruments) scanning probe microscope using the tapping mode. The X-ray photoelectron spectroscopy (XPS) studies were performed on a Thermo-VG Scientific ESCALAB 250 photoelectron spectrometer using a monochromated Al Ka (1,486.6 eV) X-ray source. The work function and dipole energy were investigated by ultraviolet photoelectron spectroscopy (UPS) using Thermo-VG Scientific ESCALAB 250 with a He I (21.22 eV) discharge lamp. A bias of −8.0 V was applied to the samples for separation of the sample and the secondary edge for the analyzer. The Kelvin Probe (KP) measurement was performed by Kelvin probe (RHC020, KP Technology Ltd.). The conductivities of the films were measured by the van der Pauw

four-point probe (FPP) technique with a Agilent B1500A source/meter. The ohmic contacts were ensured by pressing Ga-In alloys on the four corners of each PEDOT:PSS film on glass substrate. The electrical impedance spectroscopy (EIS) spectra was obtained using a photo-electrochemical workstation (Zahner CIMPS) in the dark with no bias applied. The UV-Vis absorption and transmission spectra of the films were taken with a Lambda 750 UV-Vis spectrometer. The thickness of film was measured by RISE-Zenith spectroscopic elipsometer system with a light source of L2174-01 xexon lamp from Hamamatsu Photonics (Tokyo) Co., Ltd.

3. Results and discussion 6


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Majority of ILs are based on imidazolium, pyridinium, ammonium, phosphonium, sulfonium, pyrazolium, thiazolium, and oxazolium cations. In general, relatively high conductivities are found for imidazolium-based ILs. As the size and asymmetry of the cation increases, the melting point decreases. By changing the anion, the hydrophobicity, viscosity, density, and solvation of the IL system may be changed.26 In this study, two imidazolium-based cations, 1-(2’-hydroxylethyl)-3-methylimidazolium (HOEMIm) and 1-ehthyl-3-methylimidazolium (EMIm), have been selected for their simple structures and high conductivities, and hydroxyl functional group incorporated in [HOEMIm] is to make it solid-state at room temperature. H2SO4 treatment is widely recognized as an effective method to improve the conductivity of PEDOT:PSS. Hence, [HSO4] is chosen as the counterions to investigate the effect of [HSO4] on property of PEDOT:PSS, at the same time, another acid radical anions [PF6] is also employed as a comparison. By this way, four kinds of ILs were assembled and diluted by water, with varied concentrations from 15 mg/ml to 0.5 mg/ml. Figure 1 shows the process and effects of ILs treatment on PEDOT:PSS film. The morphology and interaction of PEDOT and PSS are depicted according to the reference, which consist of grains with a hydrophobic and highly conductive PEDOT-rich core and a hydrophilic insulating PSS-rich shell. It is also reported that phase separation of PEDOT and PSS is generally found with the insulating PSS grains atop the as-prepared PEDOT: PSS film cast from aqueous PEDOT:PSS solution and it will reduce the surface conductivity.27 As a result of ILs treatment, the modified PEDOT:PSS (m-PEDOT:PSS) film express superior conductivity than pristine PEDOT:PSS film, and the work function (WF) of m-PEDOT:PSS are ranging from 4.4 eV to 5.2 eV.

The thickness of PEDOT:PSS film formed by conventional spin-coating method on ITO was identified to be 40.0 ± 3.0 nm. Due to the relatively lower refractive index (n) of ILs, to study the accurate thickness of ILs modified PEDOT:PSS film, each sample was prepared on polished silicon substrate with an 1.2 nm oxide layer. the PEDOT:PSS film on silicon wafer 7



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showed a thickness of 37.3 ± 2.4 nm, after simple solvent (water) treatment, the thickness of PEDOT:PSS reduced about 30% (Table S1, supporting information). The thickness of [HOEMIm][HSO4] films were 4.8 nm, 12.1 nm and 32.8 nm, when the concentrations were 0.5, 1.0 and 2.0 mg/ml. With different concentration of ILs treatment, the modified PEDOT:PSS films showed thickness of 25.2 nm, 37.2 nm and 52.7 nm, respectively. It could be found that the modified films contained two sub-layer, the lower PEDOT:PSS layer was about 20 nm, the additional thickness was came from upper ILs layer. However, the change in n between pristine PEDOT:PSS film and modified film showed that the modified film is seem to be a homogeneous layer with a reduced n (Figure S1, supporting information), as measured by variable angle spectroscopic ellipsometry (VASE).

To obtain the in-plane resistance (R//) of m-PEDOT:PSS film, the van der Pauw four-point probe (FPP) technique with Ga-In alloys pressed on the four corners of the film was carried out to make sure of ohmic connection.28 The FPP results (Figure 2a) suggest that the ILs treatments can reduce the sheet resistance. The most workable way can bring up the conductivity even to 1 S/cm, for the conductivity of pristine PEDOT:PSS 4083 film is about 4.1×10-4 S/cm. The resistance of pristine PEDOT:PSS film slightly drops after water treatment, for the process washes away a part of PSS and a small amount of PEDOT, similar result has been reported by other group.29 As expected, through ILs treatment, the sheet resistance of each film shows an apparent decline by about 1-3 orders in magnitude. For the films with [HSO4]-based ILs treatment show the lower sheet resistance than those of [PF6]-based ILs treatment, and the lowest sheet resistance is obtained by the [EMIm][HSO4] treated PEDOT:PSS. In the four kind of ILs, [EMIm][HSO4] is the only liquid in room temperature. The flowability of [EMIm][HSO4] let the cations and anions can shift with ease. The shift of cations and anions in film make the morphology of PEDOT:PSS constantly change and promote the carrier transfer between adjacent PEDOT chain segments. 8


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To further investigate the distinction in in-plane conductivity, the surface composition of the ILs-treated PEDOT:PSS films were further characterized by X-ray photoelectron spectroscopy (XPS). The C-N band of imidazolium cation has been captured that indicates the existence of ILs on PEDOT:PSS surface (Figure S2, supporting information). As shown in Figure 2b, two XPS bands between 166 and 172 eV are assigned to the S2p bands of the sulfur atoms in PSS, whereas the two XPS bands between 162 and 166 eV correspond to the S2p bands in PEDOT. The intensity ratio of S2p band of PEDOT to PSS increases by both ILs and water treatment. For the two ILs based on [HSO4], the ratio of PEDOT to PSS seems not so high to the other treated samples, this is because the S2p bands of both HSO4- and SO42- anions contribute to the S2p band of the sulfur atoms in PSS, resulting in hardly distinguished ratio of PEDOT to PSS. In addition, the bands of both PEDOT and PSS obviously shift to the higher or lower binding energy in the spectra of all the ILs-treated films, indicating an interaction between ILs and PEDOT:PSS. Meanwhile, the XPS spectra indicate that there is an amount of PEDOT and PSS existed on film surface, which further prove the modified film is blended with PEDOT:PSS and ILs. Thus, the distinction in in-plane conductivity is not only interrelated to the surface PSS content, but also influenced by the conductivity of superficial ILs.

At the same time, atomic force microscopy (AFM) and scanning electron microscope (SEM) have been performed to examine the surface morphology. The film treated with [HOEMIm][HSO4] expresses a larger root mean square (rms) roughness of 2.60 nm than the pristine PEDOT:PSS film (rms=1 nm) (Figure 2c). The corresponding phase images suggest that in pristine PEDOT:PSS, PEDOT and PSS interconnect with each other without obvious microphase separation structures. After [HOEMIm][HSO4] was spin-coated on the surface of PEDOT:PSS, an additional granular substance can be seen. The more visible phenomenon can 9



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be observed in [EMIm][HSO4] and [HOEMIm][PF6]-treated films (Figure S3, supporting information), that a branch-shaped interpenetrating network structure has formed. Besides, because the PSS can be partly washed away when pristine PEDOT:PSS is directly treated by pure water, the water treatment on the ILs-treated films causes a significant component change, as confirmed by XPS spectrum. The SEM image indicates that the high concentrations of ILs will aggregate on the film surface, and shows bad film-processing ability (Figure S4, supporting information).

The out-of-plane electrical measurement was studied by inserting the modified films between ITO and Au electrode to obtain a relative value. The relative out-of-plane resistance (R⊥) can be well depicted by current density vs. voltage characters of devices, as shown in Figure 2d. The resistance of PEDOT:PSS films decreased at first and then increased after concentration of [HOEMIm][HSO4] from 0.5 mg/ml to 2.0 mg/ml aqueous solution treatment. PEDOT:PSS with simple water treatment expressed similar out-of-plane conductivity to pristine one. Incorporating with the film thickness measured by VASE, we can know that PEDOT:PSS film with higher concentration [HOEMIm][HSO4] shows lower resistance per centimeter. Thus, the ILs treatment reduces both in-plane and out-of-plane resistance.

Then, ultraviolet photoelectron spectroscopy (UPS) was carried out to study the energy levels of modified-PEDOT:PSS films. Figure 3a shows UPS spectra of different kind of ILs with a concentration of 2 mg/ml on bare ITO glass and on ITO/PEDOT:PSS. The pristine PEDOT:PSS film on ITO glass delivers a work function of -5.10 eV, and the WFs are reduced in different extent when treated with 2 mg/ml ILs.30-31 The [PF6]-based ILs express more significant than [HSO4]-based ILs on lowing the WF. In addition, the WF of the films modified with ILs cooperated with hydroxyl group are lower. No matter on ITO or on PEDOT:PSS substrate, the variation tendencies of work function are same. Subsequently, for examination 10


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of the influence of doping density of ILs, and [HOEMIm][HSO4] with various concentration was selected for representative, where relevant UPS results are shown in Figure 3b. We find that the change of dipole moment can be adjust in a large range from 0 to 1 eV when the additive amount of ILs increases, and the experimental data were confirmed by Kelvin Probe (KP) measurement (Figure S5, supporting information). The UPS spectra indicate that only small amount of [HOEMIm][HSO4] and [EMIm][HSO4] modified PEDOT:PSS can be qualified for extracting hole from active layer. When the concentration of [HOEMIm][HSO4] and [EMIm][HSO4] are more than 1 mg/ml, the change in work function of modified PEDOT:PSS film cannot be negligible.

Since PEDOT:PSS with controllable work function have been obtained, the ILs modified PEDOT:PSS interlayer would dramatically influence the hole transport and collection. Thus hole-only

devices

with

the

mg/ml)/PTB7:PC71BM/MoO3/Ag

were

configuration fabricated.

of As

ITO/PEDOT:PSS/ILs shown

in

Figure

3c,

(2 the

[HOEMIm][HSO4] modified PEDOT:PSS shows a higher hole mobility than pristine one. However, the hole mobility of [PF6]-based ILs modified PEDOT:PSS drop obviously because the energy levels are mismatched. Unexpectedly, the hole mobility of PEDOT:PSS film treated with [EMIm][HSO4] also goes down even though the energy levels are appropriate. Through experiments, we find out that the PEDOT:PSS films treated with [EMIm][HSO4] with high in-plane and out-of-plane conductivities that express inferior shunt resistance, that resulting in the poor hole selectivity. The OPV devices with [EMIm][HSO4] modified PEDOT:PSS express an extremely high Jsc and a low FF (Table S2, supporting information), just as an inferior electrode inserts between the active layer and ITO electrode. And through the external quantum efficiency (EQE) experiments we cannot acquire the corresponding Jsc value. Therefore, the improvement in interlayer conductivity must be seriously weighed.32

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Based on the work of above, [HOEMIm][HSO4] was employed in the devices with the structures of ITO/PEDOT:PSS/(ILs)/PTB7:PC71BM/PFN/Al to study the performance of the m-PEDOT:PSS films. Three concentrations (0.5 mg/ml, 1.0 mg/ml, 2 mg/ml) of ILs are adopted for PEDOT:PSS modifier, where Figure 4 presents the current density vs. voltage (J-V) characteristics of conventional devices. Meanwhile, device with solution-processed MoO3 HTL was also investigated as a reference (Figure S6). In view of the increase in conductivity and hole mobility of [HOEMIm][HSO4] modified PEDOT:PSS, the Jsc of the devices with m-PEDOT:PSS is remarkably improved. It is worthy to note that after 1 mg/ml [HOEMIm][HSO4] treated device shows a maximum Jsc up to 17.5 mA/cm2, where the control device expresses a Jsc of 15.5 mA/cm2, with above 13% increase (Table S3, supporting information). However, the m-PEDOT:PSS interlayer does not perform well when it combining with donor which has a lower lying HOMO level such as PTB7-Th. When PEDOT:PSS film modified with [HOEMIm][HSO4] of a low concentration, the Jsc is significantly improved, and the Voc does not show obviously change (Table S4, supporting information). While it modified with [HOEMIm][HSO4] of a concentration up to 2 mg/ml, the Voc and FF of device obviously decline, as expressing a fake ultrahigh Jsc. The ultrahigh Jsc cannot be verified EQE result, and it has exceeded the theoretical value of this kind of device with 100% IQE at the thickness. The fake high Jsc should be originated in the high in-plane conductivity of m-PEDOT:PSS, that make the device captures more carriers in larger area under illumination without mask. Thus, at the moment of improving the out-of-plane conductivity of interlayer, the in-plane conductivity of interlayer should also be seriously considered as well as its work function.

The enhancement in Jsc was confirmed by EQE experiment and consistent with electrical impedance spectroscopy (EIS) results, as shown in Figure 5a. The high-frequency region and low-frequency region of the EIS curve represent the series resistance and charge transfer 12


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resistance of the device.11, 33 It can be found out that the series resistance and charge transfer resistance are reduced with [HOEMIm][HSO4] modifier, which indicates a more superior interface with high charge selectivity and low resistance loss.34-36 The corresponding fitted data were listed in Table S5, supporting information. Moreover, it can be observed in Figure 4 that the EQE of device with modified PEDOT:PSS between 350-500 nm significant improved. The enhanced EQE in this range is mainly attributed to the acceptor, but there were no obvious differences in the absorption spectra of active layer with different HTL. Thus, we studied the wettability property of PEDOT:PSS layer. It can be seen from Figure S7, the contact angle of water on ITO/PEDOT:PSS substrate is 7.5°, after IL treatment, the contact angle is improved to 15°. The change of contact angle can be attributed to the decrease of surface PSS and addition of IL. The variation of contact angle of PEDOT:PSS layer should be taken into account for the improvement of power conversion efficiency. Subsequently, we examined the stability of the devices with m-PEDOT:PSS (Figure 5b). The modified device remains a PCE of about 6.7% while the control device only have 4.8% after 15 days. The enhancement in device stability can be attributed to the ILs treatment rinsing out an amount of acidic PSS, resulting less hydrogen ion left on the surface of ITO. Therefore, ILs treatment can be regard as a workable way to obtain higher PCE and stability.

4. Conclusions In conclusion, we discuss the in-plane and out-of-plane conductivity of interlayer, which play an important role on charge collection and transportation. As well as the energy level of interlayer, the conductivity of interfacial material should be optimized. It is beneficial to enhance the out-of-plane conductivity and control the in-plane conductivity at the same time. We provide a feasible method to optimize the performance of polymer solar cells by incorporation of ILs modified PEDOT:PSS as HTL. High short-circuit current density of 17.5 mA cm-2 with a maximum PCE of 13



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8.75% was obtained in air for PTB7:PC71BM polymer solar cells based on modified PEDOT:PSS with reduced series resistance. And devices based on PTB7-Th:PC71BM polymer solar cells show a maximum PCE of 9.21%. We point out that the interfacial dipoles in ITO/PEDOT:PSS interface for hole transfer can be controllable by simple ionic liquids treatment. Meanwhile, the device stability of these conventional devices are improved for less acid PSS leaving in PEDOT:PSS system. It also provides a useful and universal method to regulate other conjugated polyelectrolytes and other interfaces.

ASSOCIATED CONTENT Supporting Information Text gives experimental details of characterization; The refractive index spectra, XPS spectra, AFM images, SEM images, Kelvin probe spectra, contact angle of modified PEDOT:PSS films, and performance of device with MoO3 buffer layer. The Table of film thickness, photovoltaic performance of PSCs based on different ILs modified PEDOT:PSS, parameters of the PSCs equivalent circuit with m-PEDOT:PSS under illumination, values of hole mobility are included. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Tel.: +86 791 83968703; fax: +86 791 83969561. E-mail: [email protected] (Y. Chen). Author Contributions L. Huang and X. Cheng contributed equally to this work. Notes Competing financial interests. The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Science Fund for Distinguished Young Scholars (51425304), National Natural Science Foundation of China (51673091 and 51473075), and National Basic Research Program of China (973 Program 2014CB260409). 14


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15. Zhou, H.; Zhang, Y.; Mai, C.-K.; Seifter, T.; Nguyen, T.-Q.; Bazan, G. C.; Heeger, A. J., Solution-processsed pH-neutral Conjugated Polyelectrolyte Improves Interfacial Contact in Organic Solar Cells. ACS Nano 2015, 9 (1), 371-377. 16. Choi, M.-H.; Lee, E. J.; Han, J. P.; Moon, D. K., Solution-processed pH-neutral Conjugated Polyelectrolytes with One-atom Variation (O, S, Se) as a Novel Hole-collecting Layer in Organic Photovoltaics. Sol. Energy Mater. Sol. Cells 2016, 155, 243-252. 17. Lu, L.; Yu, L., Understanding Low Bandgap Polymer PTB7 and Optimizing Polymer Solar Cells Based on It. Adv. Mater. 2014, 26 (26), 4413-4430. 18. Wang, F.; Tan, Z. a.; Li, Y., Solution-processable Metal Oxides/Chelates as Electrode Buffer Layers for Efficient and Stable Polymer Solar Cells. Energy Environ. Sci. 2015, 8 (4), 1059-1091. 19. Iwan, A.; Boharewicz, B.; Hreniak, A.; Tazbir, I.; Chmielowiec, J., Polymer Solar Cells with a TiO2:Ag Layer. J. Mod. Opt. 2014, 61 (21), 1767-1772. 20. Lee, E. J.; Heo, S. W.; Han, Y. W.; Moon, D. K., An Organic–inorganic Hybrid Interlayer for Improved Electron Extraction in Inverted Polymer Solar Cells. J. Mater. Chem. C 2016, 4 (13), 2463-2469. 21. Zhou, H.; Zhang, Y.; Mai, C. K.; Collins, S. D.; Nguyen, T. Q.; Bazan, G. C.; Heeger, A. J., Conductive Conjugated Polyelectrolyte as Hole-transporting Layer for Organic Bulk Heterojunction Solar Cells. Adv. Mater. 2014, 26 (5), 780-785. 22. Choi, H.; Mai, C. K.; Kim, H. B.; Jeong, J.; Song, S.; Bazan, G. C.; Kim, J. Y.; Heeger, A. J., Conjugated Polyelectrolyte Hole Transport Layer for Inverted-type Perovskite Solar Cells. Nat. Commun. 2015, 6, 7348. 23. Gu, C.; Chen, Y.; Zhang, Z.; Xue, S.; Sun, S.; Zhong, C.; Zhang, H.; Lv, Y.; Li, F.; Huang, F.; Ma, Y., Achieving High Efficiency of PTB7-Based Polymer Solar Cells via Integrated Optimization of Both Anode and Cathode Interlayers. Adv. Energy Mater. 2014, 4 (8), 1301771. 24. Lu, L.; Luo, Z.; Xu, T.; Yu, L., Cooperative Plasmonic Effect of Ag and Au Nanoparticles on Enhancing Performance of Polymer Solar Cells. Nano Lett. 2013, 13 (1), 59-64. 25. Badre, C.; Marquant, L.; Alsayed, A. M.; Hough, L. A., Highly Conductive Poly(3,4-ethylenedioxythiophene):Poly (styrenesulfonate) Films Using 1-Ethyl-3-methylimidazolium Tetracyanoborate Ionic Liquid. Adv. Funct. Mater. 2012, 22 (13), 2723-2727. 26. Mohammad, A.; Inamuddin, Green Solvent II, Properties and Applications of Ionic Liquids. Springer, 2012. 27. Lang, U.; Müller, E.; Naujoks, N.; Dual, J., Microscopical Investigations of PEDOT:PSS Thin Films. Adv. Funct. Mater. 2009, 19 (8), 1215-1220. 28. Xia, Y.; Sun, K.; Ouyang, J., Solution-processed Metallic Conducting Polymer Films as Transparent Electrode of Optoelectronic Devices. Adv. Mater. 2012, 24 (18), 2436-2440. 29. Delongchamp, D. M.; Vogt, B. D.; Brooks, C. M.; Kano, K.; Obrzut, J.; Richer, C. A.; Kirillov, O. A.; Lin, E. K., Inflence of a Water Rinse on the Structure and Properties of Poly(3,4-ethylene dioxythiophene):Poly(styrene sulfonate) Films. Langmuir 2005, 21 (24), 11480-11483. 30. Fu, P.; Huang, L.; Yu, W.; Yang, D.; Liu, G.; Zhou, L.; Zhang, J.; Li, C., Efficiency Improved for Inverted Polymer Solar Cells with Electrostatically Self-assembled BenMeIm-Cl ionic Liquid Layer as Cathode Interface Layer. Nano Energy 2015, 13, 275-282. 31. Yu, W.; Huang, L.; Yang, D.; Fu, P.; Zhou, L.; Zhang, J.; Li, C., Efficiency Exceeding 10% for Inverted Polymer Solar Cells with a ZnO/Ionic Liquid Combined Cathode Interfacial Layer. J. Mater. Chem. A 2015, 3 (20), 10660-10665.

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Figure 1. Schematic illustration of the process of ILs treatment on the PEDOT:PSS film. Structures and abbreviations of the cations and anions in ILs are inserted.

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Figure 2. (a) Column diagram of square resistance of pristine and modified PEDOT:PSS films. (b) XPS spectra of the pristine and modified PEDOT:PSS films. (c) 3µm×3µm AFM micrographs (tapping-mode) and corresponding phase images of pristine PEDOT:PSS film (left) and [HOEMIm][HSO4]-modified PEDOT:PSS film (right). The root-mean-squared roughness of each film is indicated in nm. (d) Current density versus voltage (J-V) characteristics of modified PEDOT:PSS film.

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Figure 3. (a) UPS spectra of ILs on bare ITO and ITO/PEDOT:PSS. (b) UPS spectra of different concentration of [HOEMIm][HSO4] on ITO/PEDOT:PSS. (c) Hole mobility of hole-only device with different ILs modified PEDOT:PSS.

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Figure 4. The effect of the [HOEMIm][HSO4] modified PEDOT:PSS (m-PEDOT:PSS) on PTB7:PC71BM solar cell performance, (a) current density versus voltage (J-V) characteristics of conventional PSCs and (b) external quantum efficiency (EQE) of the devices. The effect of the m-PEDOT:PSS on PTB7-Th:PC71BM solar cell performance, (c) J-V characteristics of conventional PSCs and (d) EQE of the devices.

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The table of content

The in-plane and out-of-plane conductivity of interlayer, which play an important role on charge collection and transportation. A high performance of 8.64% has been achieved for

conventional

polymer

solar

cell

with

ionic

liquids

(ILs)

modified

poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as hole transport layer with enhanced interfacial conductivity.

Keyword: controllable work function, ionic liquid, interfacial conductivity, conductive polymers, polymer solar cells Liqiang Huang, Xiaofang Cheng, Lifu Zhang, Weihua Zhou, Shuqin Xiao, Licheng Tan, Lie Chen, Yiwang Chen* Title High Performance Polymer Solar Cells Realized by Regulating the Surface Properties of PEDOT:PSS Interlayer from Ionic Liquids

TOC figure

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High-Performance Polymer Solar Cells Realized by Regulating the

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