Semiconductive Polymer Doped PEDOT with High Work Function

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Semiconductive Polymer Doped PEDOT with High Work Function, Conductivity, Reversible Dispersion and Application in Organic Solar Cells Bin Guo, Qingwu Yin, Jiawen Zhou, Wenqiang Li, Kai Zhang, and Yuan Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06215 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 2, 2019

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ACS Sustainable Chemistry & Engineering

Semiconductive Polymer Doped PEDOT with High Work Function, Conductivity, Reversible Dispersion and Application in Organic Solar Cells Bin Guo#, Qingwu Yin#, Jiawen Zhou, Wenqiang Li, Kai Zhang*, Yuan Li* Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China. *Corresponding

author

E-mail addresses: [email protected], [email protected]

ABSTRACT: Considering the intrinsic insulated poly (styrene sulfonate) in conductive PEDOT:PSS Poly (3,4-ethylenedioxythiophene):poly (styrene sulfonate), we investigated the structure homogeneity of PEDOT:PSS on its hole injection/extraction property in our previous work. In order to enhance the structural homogeneity of PEDOT derivatives, conductive dispersant poly (diphenylamine-4-sulfonic acid) (PDAS) was applied to efficiently dope PEDOT to obtain a new water-dispersed conductive polymer PEDOT:PDAS. Compared with traditional PEDOT:PSS, PEDOT:PDAS showed much higher work function (5.38 eV), lower acidity (pH of about 7), outstanding film formation and prospective structural homogeneity. It was worth

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mentioning that the conductivity of PEDOT:PDAS (0.135 S cm-1) was an order of magnitude higher than that of PEDOT:PSS-4083 (0.02 S cm-1) although PEDOT:PDAS exhibited very low PEDOT content. Organic solar cells using PEDOT:PDAS as extract layer showed comparable performance with the PEDOT:PSS control devices. More interestingly, PEDOT:PDAS powder can be re-dispersed for several times due to the weak aggregation among PDAS backbones and our result provides a practically feasible approach for the efficient storage and transportation of PEDOT ink for future industrial application such as antistatic coating.

Keywords: Work function, semiconductor, polydiphenylamine-4-sulfonic acid, radical, organic solar cells

INTRODUCTION Considering the fossil energy crisis, the development of next generation sustainable and green energy has become particularly important. In particular, organic solar cells (OSCs) with light weight, flexibility, roll-to-roll large-area preparation with low cost, have received significant attention in recent years as a form of highly sustainable and inexpensive solar energy production.1-3 Many researchers are committed to improving the performance of organic solar cells by changing the properties of donor and acceptor.4-6 The power conversion efficiency (PCE) of OSC has been significantly improved.7,8 Single junction conventional structure OSCs have been reported to reach PCE over 13%,9-13 and the solution-processed tandem OSC has achieved a PCE of 17.3%.14 Compared with OSCs, perovskite solar cells (PSCs) have become the new hotspot in solar cell research because of their impressive


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properties such as high photovoltaic efficiency, simple preparation and low cost.15-20 At present, the PCE of inverted planar heterojunction PSC has exceeded 21%.21 In the two types of solar cells mentioned above, the normal device structure is ITO/poly (3, 4ethylenedioxythiophene): poly (styrene sulfonate) (PEDOT:PSS)/ active layer of OSCs or PSCs/electron extract layer/cathode. Numerous researchers are committed to improving the performance of OSCs and PSCs via the interfacial engineering.22-24 For organic solar cells, PEDOT:PSS is a well-known anode modifier material and it has been extensively studied in fundamental research due to its commercialization and availability.25-27 Aqueous PEDOT:PSS dispersion exhibit the advantages of good film-forming capability, easy processing, high transmittance and good chemical/optical stability.28,29 One of the most effective applications of PEDOT:PSS is in the OSCs and PSCs as polymer hole transport layer (HTL). PEDOT:PSS is usually applied to the top of the transparent anode, which can effectively improve the performance of the cell.30 However, the high acidity, low work function (WF) and other deficiencies of PEDOT:PSS limit efficiency, performance and its practical application in the solar cell devices.31 In order to overcome the shortcomings of PEDOT:PSS such as the high acidity, strong aggregation of PSS, the relatively low WF and conductivity (pH of 1.75, WF of 5.1 eV, 0.02 S cm-1 for PEDOT:PSS-4083, respectively), as well as poor structure/electronic homogeneity, a series of research work was conducted in this field. There are three approaches to modify the chemical/physical properties of PEDOT derivatives as following: (1) Physical process was widely applied to modify the properties of PEDOT:PSS,32 such as solvent additive and polar-



solvent post-treatment of PEDOT:PSS.33,34 The other physical methods including heat treatment and light treatment were also reported to enhance conductivity and the WF of PEDOT:PSS.35,36 In addition, the effect of the annealing temperature on the thickness and conductivity of the PEDOT:PSS layer was investigated in details and it showed positive effect on the performance of solar cells.37,38 (2) From the view of chemical modification, several new types of hole transport/extract material were developed to replace the wellknown PEDOT:PSS in organic electronic devices. PEDOT-S (PEDOT-based conjugated polyelectrolyte), sulfonated poly (diphenylamine) with high solubility and good processability and poly (styrene sulfonic acid) grafted with polyaniline (PSSA-g-PANI) were reported in previous work.39-42 PEDOT-S layer can work well in a wide range of thickness because of its high conductivity and good transparency. After ultraviolet ozone treatment, the WF of PEDOT-S can be increased to 5.2 eV.39 Sulfonated poly (diphenylamine) with high transparency in visible region and acceptable conductivity was beneficial as HIL material.40 (3) The novel dispersants were researched to replace PSS. The WF of PEDOT:PSS is mainly determined by the doping extent of PSS on the backbone of PEDOT.PEDOT with enhanced WF is important to improve the performance of OSCs and PSCs.43 The conductivity of the famous PEDOT:PSS-4083 is relatively low (around 0.02 S cm-1) due to the doping of nonconductive PSS (the PEDOT/PSS mass ratio of commercial PEDOT:PSS-4083 is 1:6). Several work on the chemical modification of PEDOT:PSS was reported in previous work. The extremely cheap lignosulfonic acid with three-dimensional structure to replace the linear PSS as dopant of PEDOT, which exhibited comparable performance with PEDOT:PSS in OSCs.44,45. The semiconductive lignosulfonic acid (hole mobility of 3.75 10-6 cm2 V-1 S-1)


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decreases the cost and contributes the conductivity.44 Several novel dopants, such as sulfonated acetone-formaldehyde (SAF), methylnaphthalene sulfonate formaldehyde (MNSF), dopamine (DA), have been designed and prepared to replace PSS as dopant of PEDOT to enhance the performance of solar cell devices. In details, SAF shows decreased acidity and UV-degradation of PSC devices.46 We disclosed the branched chemical structure of MNSF improved the film structural/electrical homogeneity effectively.47 DA doped PEDOT highly increases charge-extraction efficiency. Moreover, DA-copolymerized PEDOT:PSS can significantly improve the durability of the device.48 As mentioned above, we provide and summarize a map for the design dopant for PEDOT as the hole injection/extraction layer ( HIL or HEL) of highly efficient PSC.43-48 In our recent work, we enhanced the structure/electronic homogeneity via the nonconductive and branched dopant MNSF. In this contribution, considering both the MNSF and PSS are insulated, we designed a water soluble, nonlinear and semiconductive polymer polydiphenylamine-4-sulfonic acid (PDAS) to replace PSS in order to enhance the structure/electronic homogeneity. PEDOT:PDAS was readily prepared by one step doping of PDAS on the polymeric PEDOT radical cation backbone. Compared with the traditional insulating PSS dopant, PDAS was semiconductive material that can be self-doped in water. Furthermore, PEDOT:PDAS showed high conductivity, good structure with uniformity of the film. Due to the intrinsic low acidity of PDAS (higher electron donating effect of PDAS than PSS) and the low doping ratio (mass ratio of 1: 2.5), the acidity was relatively weak and the cost was lower. The WF of PEDOT:PDAS was enhanced to 5.38



eV, which was higher than PEDOT:PSS (5.1 eV). Finally, the performance of PEDOT:PSS and PEDOT:PDAS as a hole extract layer in the OSCs was studied and discussed in details.

EXPERIMENTAL SECTION Preparation of PDAS and PEDOT:PDAS. Poly (diphenylamine-4-sulfonic acid) (PDAS) was prepared by the chemical oxidation method. The synthetic routes of PDAS and PEDOT:PDAS are shown in Figure S1. Polydiphenylamine-4-sulfonic acid (PDAS) was prepared by the oxidation polymerization with ammonium persulfate (APS) as catalyst. In the same reaction pot, dilute HCl was added to obtain an acid solution with pH of 2.0 and PEDOT:PDAS was then synthesized via the polymerization of EDOT with polydiphenylamine-4-sulfonic acid as the dopant by two steps reaction in a single reaction. The detailed synthesis condition, purification and procedure are provided in the Supporting Information.

RESULTS AND DISCUSSION Structural Characterizations of PDAS and PEDOT:PDAS The synthesized PDAS solution is green, while PEDOT:PDAS solution is blue-black, just like commercialized PEDOT:PSS. The chemical structures of PEDOT:PSS and PEDOT:PDAS are showed in Scheme 1. Considering that PSS is linear insulated dopant and PDAS is non-linear dopant of semiconductor, PEDOT:PDAS show high conductivity and structural homogeneity. In order to identify the structure of PDAS and PEDOT:PDAS, the fourier transform infrared (FT-IR) spectra and UV-vis absorption spectrum were measured. It


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is worth mentioning that the PEDOT:PDAS can also be re-dispersed in water after it has been freeze-dried as a solid. It provides a more convenient and low-cost method for the future transportation and storage of industrialization. Fourier Transform Infrared of PEDOT:PDAS The FT-IR spectra of EDOT, PEDOT:PDAS and PDAS are shown in Figure 1a. The strong band at 892 cm-1 is ascribed to the C-H bending mode and the signal at 2985 cm-1 and 3125 cm-1 are ascribed to the C-H vibration bands in the PEDOT backbone of PEDOT:PDAS polymer, demonstrating the successful formation of PEDOT chains. The adsorption peaks at 1188 cm-1 and 620 cm-1 are corresponded to the stretching modes of the S=O bond of the sulfonic groups. These results show that PDAS and PEDOT:PDAS have been prepared successfully. Compared with the spectra of the EDOT and PDAS, the spectrum of PEDOT:PDAS has wider peaks at 1750 to 500 cm-1 and more intensive absorption at 3700 to 3000 cm-1. Obvious peaks at 2300 cm-1 are found in both the PDAS and PEDOT:PDAS which is in good agreement of the efficient doping of PDAS into PEDOT. UV-vis absorption of PEDOT:PDAS UV-vis absorption spectrum is often used to verify whether the PEDOT chain is successfully synthesized. Figure 1b displays the UV-vis absorption spectra of PEDOT:PSS and PEDOT:PDAS in aqueous dispersions. The broad peak at 600-900 nm is the typical absorption peak of PEDOT. The conjugation and radical cation-qunoid resonance structure on the polymeric PEDOT backbone leads to a double polarized absorption peak around 800 nm. They are both detected in the PEDOT:PSS and PEDOT:PDAS dispersions. This result



indicates the successful preparation of PEDOT:PDAS. However, it is clear from the Figure 1b that the peak at 600 to 900 nm of PEDOT:PDAS is much lower than that of PEDOT:PSS, indicating that the content of PEDOT in PEDOT:PDAS is very low. In addition, PDAS shows significant absorption peaks at 190-250 nm and 260-430 nm, with almost none of absorption peaks from 650 to 900 nm. While PEDOT:PDAS has obvious absorption band at 600-900 nm. Compared with PDAS, the absorption of PEDOT:PDAS has blue shift at 200 nm and red shift between 250-450 nm. The absorption spectra of PEDOT:PSS and PEDOT:PDAS films is showed in the Figure S2. The absorption spectra of PEDOT:PDAS in aqueous dispersions and films are basically the same. For example, the broad peak at 600-900 nm is the typical absorption peak of PEDOT. They are both detected in the PEDOT:PSS and PEDOT:PDAS in the state of aqueous solution and film. In addition, the peak at 300 to 720 nm of PEDOT:PDAS is much higher than that of PEDOT:PSS, indicating that PEDOT:PDAS will absorb more visible light than PEDOT:PSS. This will result in light loss, which may lead to the performance degradation of devices using PEDOT:PDAS as hole transport layer. Electrochemical Behavior and WF of PEDOT:PDAS The cyclic voltammetry curve of PEDOT:PDAS film in 0.1 M Bu4NPF6 in anhydrous dichloromethane (DCM) solution is shown in Figure 2. The CV curve of PEDOT:PDAS shows an obvious oxidation potential at 0.68 V and the WF is estimated as 5.38 eV according to the empirical formula. The enhanced work function of PEDOT:PDAS indicates that PDAS can effectively adjust the WF level of PEDOT dispersion film. It is presumed that the doping


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of PDAS exhibit the determinable effect on the WF of PEDOT:PDAS. During the oxidation, PDAS in PEDOT:PDAS will form quinoid structure, which is conducive to the transmission of electrons. This is consistent with the Raman spectra. The CV curve of PDAS is shown in Figure S3, which shows good electrochemical stability. The oxidation process of PDAS has very good repeatability, even after 5 run scans, the current value has not changed obviously. Structure Homogeneity of Thin Film The surface morphology of PEDOT:PDAS and PEDOT:PSS films on ITO substrate were studied via atomic force microscopy (AFM), respectively. The height image of PEDOT:PDAS and PEDOT:PSS are shown in Figure S4, respectively. The RMS roughness of PEDOT:PDAS and PEDOT:PSS films were detected to be 1.5 nm and 2.4 nm, respectively. PEDOT:PDAS shows lower surface roughness compared to PEDOT:PSS. In addition, 3D images and phase images of PEDOT:PDAS with different film thickness are shown in Figure S5 and Figure S6, respectively. As the film thickness decreases, the RMS roughness of PEDOT:PDAS increases, With the film thicknesses of 50 nm, 40 nm, 30 nm and 15 nm, the specific RMS values are 0.525 nm, 1.02 nm, 1.34 nm and 1.49 nm, respectively. The maximum roughness was only 1.49 nm. The decrease of RMS roughness mainly contribute to the relatively smaller particle size of PEDOT:PDAS. The underlying mechanism is probably due to the twist chemical structure and intrinsic amorphous property of the polymer PDAS. In contrast, PSS shows linear structure and it will more readily aggregate in solution due to its crystallinity. PSS also has higher molecular weight than that of PDAS and the degree of polymerization of the PEDOT in PEDOT:PDAS is lower than that of the PEDOT:PSS. As a result, PEDOT:PDAS exhibits a



relatively lower roughness. The particles are too small to detect, using dynamic light scattering (DLS) to test the exact particle size distribution of PEDOT:PDAS dispersion. The relatively smooth surface of the PEDOT:PDAS could improve the interfacial contact between hole transport layer (HTL) and PBDB-T: ITIC layer, which is beneficial for charge extraction. The degree of polymerization of PEDOT is low, however, PEDOT:PDAS is still comparable to PEDOT:PSS in terms of the performance of these OSC devices. Acidity and Conductivity of PEDOT:PDAS It is well-known that materials with high acidity can corrode the ITO surface, thereby reducing the stability of the solar cell. Table 1 lists the conductivity and pH of PEDOT:PDAS, PEDOT:PSS and PDAS, respectively. The acidity of PEDOT:PSS and PEDOT:PDAS dispersion were studied by pH meter precisely and pH test paper vividly. The testing concentration of PEDOT:PDAS was 1.0 wt % which achieved the highest device efficiency. The pH value of PEDOT:PDAS was around 7, PEDOT:PDAS shows much lower acidity than PEDOT:PSS, probably because of the low acidity of PDAS itself and its low doping. From the molecule structure, the PDAS contains less -SO3H than PSS. Moreover, the PDAS shows low acidity with pH of around 7.7, due to its electron-donating group (amino group). Consequently, we successfully reduce the acidity of PEDOT:PDAS, which is challenging and rarely reported in previous work.46-48 The conductivity of PEDOT:PDAS is 0.135 S cm-1 which is one order of magnitude higher than that of PEDOT:PSS. As shown in Scheme 1, PSS is linear insulated structure and ultrahigh molecular weight polymer. In addition, the doping ration of PSS for PEDOT:PSS (PVPAI 4083) is 1:6. From these two sides, PEDOT:PSS film possesses a lot of non-conductive PSS areas. Compared with PSS,


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PDAS is intrinsically conductive non-linear polymer. From the UV test of PEDOT:PDAS, the PEDOT content is low, however, PDAS shows higher radical cation content than PEDOT:PSS. As a result, PEDOT:PDAS shows unexpected high conductivity considering the contribution of charge density of PEDOT:PDAS.49 The mechanism of high conductivity of PEDOT:PDAS is explained in more detail in Raman and Electron Spin Resonance (ESR) test. The superior structural homogeneity of PEDOT:PDAS is potentially more conducive to hole extract. The high conductivity and structural homogeneity of PEDOT:PDAS can broaden its range of applications. Raman and Electron Spin Resonance The Raman spectra of PDAS, PEDOT:PDAS and PEDOT:PSS are shown in Figure 3a. Most of the major Raman peaks of PEDOT are observed between 1150 and 1610 cm-1.50 The Raman bands of PEDOT:PSS can be assigned as follows: 1425 cm-1 (Cα-Cβ of quinoid), 1530 and 1568 cm-1 (asymmetric Cα-Cβ stretching modes), which is consistent with the literature.40 For PDAS, the prominent bands are at 1375 (characteristic for dications) and 1620 cm-1 (C-C ring stretching).51 As can be seen from the Figure 3a, the spectrum of PEDOT:PDAS is more like a superposition of PDAS and PEDOT:PSS, in other words, PEDOT:PDAS has all the features of PDAS and PEDOT:PSS. The characteristic peaks of PEDOT:PSS (1145 and 1530 cm-1) are clearly reflected in the spectra of PEDOT:PDAS, while the peaks of 1619 to 1680 cm-1 are attributed to PDAS. However, the maximum band at 1444 cm-1 (corresponding to the benzoid structure in the PEDOT chain) of PEDOT:PSS shifted to 1455 cm-1 in



PEDOT:PDAS. There are two kinds of resonant structure in PEDOT, one is the Cα=Cβ bond of benzoid structure with two conjugated π-electrons, another is the Cα-Cβ of quinoid structure without conjugated π-electron. The Raman spectra of PEDOT:PDAS appears red shift that means the benzene ring structure changes to quinone structure. In benzoid structure with the coil conformation, conjugated π-electron are not delocalized over the entire PEDOT chain. However, quinoid structure possess an expanded-coil or linear conformation. Consequently, radical neighboring thiophene rings in the PEDOT chain are practically in the same plane, which is very conducive to the transport of charge. As a result, PEDOT:PDAS shows higher conductivity than that of PEDOT:PSS. In addition, the addition of the quinoid structure also makes PEDOT:PDAS has a good redox characteristic, which is consistent with the CV curve. The high conductivity of PEDOT:PDAS also can be explained that the very high radical cation content of PDAS contribute to the high conductivity of PEDOT:PDAS. It can be seen from Figure 3b that the ESR intensity of PDAS is more than ten times higher than that of PEDOT:PSS under the same test condition. The high radical-content of PDAS dopant increase the conductivity of PEDOT:PDAS as the conductivity is highly related with the value of radical density, which is reported in this interesting work. For a totally nonconjugated TEMPO-radical-based polymer, it showed much higher conductivity than the famous PEDOT:PSS-4083.49 In addition, small molecule PDAS is beneficial to the accumulation of PEDOT chains. The PEDOT content in PEDOT:PDAS is very low, however, the conductivity of PEDOT:PDAS still is higher than PDAS.


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Reversible Dispersion Experiment The PEDOT:PDAS and PEDOT:PSS solutions were lyophilized and then dried in a vacuum oven at 75 oC for more than 24 hours to obtain solid powders of PEDOT:PDAS and PEDOT:PSS. A 2-mg sample was placed on an ordinary glass plate and then 0.2 mL of deionized water was added. As shown in Figure 4, PEDOT:PDAS was dissolved in water, while obvious particles were present in the PEDOT:PSS solution. The two solutions were flattened on a glass plate and the PEDOT:PDAS solution was darker and more uniform than the PEDOT:PSS solution. Due to PSS with linear structure and higher molecular weight, the aggregation of the PEDOT backbone and PSS in PEDOT: PSS is more intensive than PEDOT:PDAS.41 The polymer chain of PEDOT:PSS will form more compact aggregate in solid state after being dried as powder, making H2O molecules difficult to disperse PEDOT:PSS. In contrast, the PDAS chain has twist structure and the PEDOT backbone and PDAS are difficult to aggregate in PEDOT:PDAS powder. The H2O molecules can readily insert the PEDOT:PDAS chains and re-disperse the PEDOT:PDAS. Therefore, PEDOT:PDAS can be stored in solid form, which is more conducive to transportation and preservation. Performance of OSCs Device using PEDOT:PDAS as the HTL The OSC with device configuration of ITO/HTI/PBDB-T: ITIC /PFN-Br/Ag using PEDOT:PDAS as the HTL was fabricated. For comparison, the same configuration OSC device with conventional PEDOT:PSS as HTL was also fabricated. The energy levels at each layer of the OSC is shown in Figure 5c. The WF of PEDOT:PDAS is -5.38 eV and the



HOMO of PBDB-T is -5.33 eV, respectively. To evaluate the performance of these OSC devices, the current density-voltage (J-V) was recorded and shown in Figure 5a and 5b. The specific photovoltaic performances are listed in Table 2. The OSC device incorporating PEDOT:PSS as the HTL showed a PCE of 10.35% with a VOC of 0.89 V, a JSC of 16.2 mA/cm2, and a FF of 73%. Using PEDOT:PDAS to replace PEDOT:PSS, the device performance were VOC, JSC, FF and PCE of 0.89 V, 15.85 mA/cm2, 72.0% and 9.75%, respectively. Compared with PEDOT:PSS, the device results of PEDOT:PDAS as a holetransport layer are generally equivalent. PEDOT:PDAS has good electrical conductivity and conductive structure uniformity, however, PEDOT:PDAS possesses much lower PEDOT content. Consequently, the work function of PEDOT: PDAS is enhanced too much. The WF of PEDOT:PDAS with -5.38 eV does not match with the HOMO of PBDB-T (-5.33 eV). To confirm this viewpoint, we conducted the cyclic voltammogram (CV) of PEDOT: PDAS and PDAS and their CV performances are very similar (see Figure 2 and Figure S3). PEDOT: PDAS with higher conductivity can improve the FF of OSCs, however, we failed to obtain enhanced performance using PEDOT:PDAS as HTL in OSCs due to the mismatch of energy levels between PEDOT:PDAS and donor. It is worth mentioning that the thickness of PEDOT:PDAS has little effect on device performance. Photovoltaic parameters of OSCs by using different thicknesses of PEDOT:PSS and PEDOT:PDAS as HTLs are listed in Table S1.

CONCLUSION


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In this work, with intrinsically conductive PDAS as dopant and dispersant of PEDOT, a new water-soluble hole transport layer material PEDOT:PDAS was designed and prepared. PEDOT:PDAS still showed enhanced conductivity with the low PEDOT content compared with PEDOT:PSS. Conductivity and non-linear structure of PDAS promoted the conductivity and homogeneity of PEDOT:PDAS. PDAS doping results in the increase of work function and the decrease of acidity of PEDOT:PDAS. PEDOT:PDAS is the weakest acid PEDOT derivative at present. PEDOT:PDAS based OSC achieved a PCE of 9.75% which is equivalent to PEDOT:PSS controlled OSC (10.35%). More importantly, PEDOT:PDAS solids can be re-dispersed in water and the solution is stable. Our results provide possibility for storage and transportation of PEDOT:PDAS in solid. In addition, the synthesis method of the dispersant PDAS is very simple and the raw materials are cheap and readily available, making PEDOT:PDAS a very promising hole transport layer material. In future work, high conductivity and re-dispersed PEDOT:PDAS might show wide applications in thermoelectric devices,52 dye-sensitized and antistatic coatings.53 PDAS with low cost and re-dispersed can be used to disperse other conductive polymers besides PEDOT, such as polyaniline in the organic light-emitting diodes and antistatic coatings.54 ASSOCIATED CONTENT Supporting Information Experimental section; synthetic route of PDAS and proposed schematic for the polymerization of EDOT using PDAS as the dopant (Figure S1); UV-vis absorption spectra of PEDOT:PSS and PEDOT:PDAS thin films (Figure S2); CV curves of PDAS films on ITO



substrates in CH2Cl2 (Figure S3); AFM images of PEDOT:PSS and PEDOT:PDAS spincoated films on ITO (Figure S4.);3D images of PEDOT: PDAS with different film thickness on ITO (Figure S5); phase images of PEDOT:PDAS with different film thickness on ITO (Figure S6); photovoltaic parameters of OSCs by using different thicknesses of PEDOT:PSS and PEDOT:PDAS as HTLs (Table S1). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] Author Contributions: # Bin Guo and Qingwu Yin contributed equally to this work. ACKNOWLEDGMENTS The work was financially supported by the Pearl River S&T Nova Program of Guangzhou (201710010194), the Natural Science Foundation of China (No. 21490573 and 21875073).

References (1) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-acceptor Heterojunction. Science 1995, 270 (5243), 1789-1791, DOI 10.1126/science.270.5243.1789. (2) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Plastic Solar Cells. Adv. Funct. Mater. 2001, 11 (1), 15−26, DOI 10.1002/1616-3028(200102)11:13.0.co;2-a.


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Scheme 1. Chemical structures and proposed diagrammatic structures of PEDOT:PSS (a) and PEDOT:PDAS (b), respectively.


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Figure 1. (a) FTIR spectra of PEDOT:PSS, EDOT, PDAS and PEDOT:PDAS. (b) UV-vis absorption spectra of PEDOT:PSS, PEDOT:PDAS and PDAS in H2O.




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Figure 2. Cyclic voltammogram of PEDOT:PSS and PEDOT:PDAS films on ITO substrates and the reversible oxidation in the CV test. Electron transfer process of PEDOT:PSS (a) and PEDOT:PDAS (b) during the doping reaction.

Figure 3. (a) Raman spectra of PEDOT:PSS, PEDOT:PDAS and PDAS films, excitation wavelength 532 nm. (b) ESR spectra of PEDOT:PSS and PEDOT:PDAS aqueous solutions (1.0 ωt%).



Figure 4. Reversible dispersion experiment of PEDOT:PSS and PEDOT:PDAS. (a) Diagram of PEDOT:PSS irreversible dispersion (water molecules are difficult to insert the PEDOT:PSS aggregate), (b) Diagram of PEDOT:PDAS reversible dispersion (water molecules can readily insert the PEDOT:PDAS).


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Figure 5. J-V curves of the OSCs with PEDOT:PSS (a) and PEDOT:PDAS (b) as the HTLs in devices. (c) Schematic energy level diagrams of the materials used in the conventional structure OSC.



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Table 1. Conductivity and work function of PEDOT:PSS, PEDOT:PDAS and PDAS films. Acidity (pH value) of PEDOT:PSS, PEDOT:PDAS and PDAS aqueous dispersions (1.0 wt %).

Thickness (nm)

Conductivity (S cm-1)

WF (eV)

pH

PEDOT:PSS

8383

0.02

5.12

1.75

PEDOT:PDAS

2700

0.14

5.38

7

PDAS

3000

0.08

5.32

7.7

Material

Table 2. The best photovoltaic parameters of OSCs by using PEDOT:PSS and PEDOT:PDAS as HTLs.

HTLs

Voc (V)

Jsc (mA cm-2)

FF (%)

PCE (%)

PEDOT:PSS

0.89

16.2

73

10.35

PEDOT:PDAS

0.89

15.85

72

9.75


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For Table of Contents Use Only

Water-soluble and environmentally friendly semiconductive polymer PDAS doped PEDOT and exhibited high performances in organic solar cells.


Semiconductive Polymer Doped PEDOT with High Work Function

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