Efficiency enhancement of polymer solar cells via zwitterion doping in PEDOT:PSS hole transport layer

Organic Electronics 27 (2015) 232e239

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Efficiency enhancement of polymer solar cells via zwitterion doping in PEDOT:PSS hole transport layer Zhiqiang Zhao a, Xiang Chen a, Qing Liu a, Qiliang Wu a, Jun Zhu b, Songyuan Dai b, Shangfeng Yang a, * a

Hefei National Laboratory for Physical Sciences at Microscale, Key Laboratory of Materials for Energy Conversion, Chinese Academy of Sciences, Department of Materials Science and Engineering, Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China (USTC), Hefei 230026, China b Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 August 2015 Received in revised form 16 September 2015 Accepted 16 September 2015

Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was doped by a novel zwitterion, 3-(N-morpholino)-2-hydroxypropanesulfonic acid (MOPSO), leading to a dramatic improvement of its conductivity and consequently efficiency enhancement in polymer solar cells (PSCs) based on PEDOT:PSS hole transport layer (HTL) and versatile photoactive systems. Under the optimized MOPSO doping concentration of 20 mmol l
1, the conductivity of PEDOT:PSS film increased by about two orders of magnitude, and this is interpreted by the weakening of the Coulombic attractions between PEDOT and PSS components induced by MOPSO. MOPSO doped PEDOT:PSS was applied as HTL of bulk heterojunction (BHJ) PSC devices based on different photoactive layers including poly(3-hexylthiophene-2,5diyl) (P3HT)/[6,6]-phenyl C61-butyric acid methyl ester (PC61BM), poly[N-900 -hepta-decanyl-2,7carbazole-alt-5,5-(40 ,70 -di-2-thienyl-20 ,10,30 -benzothiadiazole) (PCDTBT)/[6,6]-phenyl C71-butyric acid methyl ester (PCDTBT:PC71BM) and thieno[3,4-b]-thiophene/benzodithiophene (PTB7):PC71BM, leading to the best power conversion efficiency (PCE) of 3.62%, 7.03% and 7.56%, respectively, which are obviously enhanced relative to those of the corresponding reference devices based on pristine PEDOT:PSS HTL. The efficiency enhancement upon MOPSO doping is found to result from the increase of short-circuit current density (Jsc), which is attributed to the increase of the photoabsorption of the photoactive layer and the improved conductivity of PEDOT:PSS HTL. © 2015 Elsevier B.V. All rights reserved.

Keywords: Polymer solar cells Hole transport layer PEDOT:PSS Zwitterion Conductivity

1. Introduction During the past two decades, polymer Solar cells have been attracting increasing attention as an emerging and promising renewable energy source, and showing advantageous potential in low-cost manufacturing, flexible and easy roll-to-roll fabrication [1e9]. Recently power conversion efficiency (PCE) of singlejunction PSCs exceeding 10% has been reported, attributing to the complicated synthesis of novel conjugated polymer donors and fullerene acceptors and/or interface engineering [10e12]. In particular, optimization of the device structure especially the interfaces between donor(acceptor)/electrodes has been extensively studied recently and revealed to be a practical and facile route

* Corresponding author. E-mail address: [email protected] (S. Yang). http://dx.doi.org/10.1016/j.orgel.2015.09.022 1566-1199/© 2015 Elsevier B.V. All rights reserved.

toward further enhancement of PCE, because such interfaces play an important role on efficient charge transport and extraction for PSC device [3,7,13e18]. For a typical bulk heterojunction (BHJ) structure of PSC device comprising an interpenetrating donor/ acceptor network, the energy level offsets between donor(acceptor) and electrodes generally result in pronounced potential loss and consequently the limited performance of BHJ-PSC devices. Thus, interfacial layers or buffer layers are usually introduced between the active layer and electrodes so as to lower such energy level offsets and improve the interfaces between donor(acceptor)/ electrodes by means of promoting charge collection and extraction via inducing interfacial charge redistribution, geometry modifications and/or chemical reactions [3,7,15,19e23]. Hole transport layer (HTL) introduced between the active layer and anode is crucial for selectively transporting holes and blocking electrons [7,21,23]. Up to now versatile HTL materials have been reported, including poly(3,4-ethylene dioxythiophene):poly(styrene

Z. Zhao et al. / Organic Electronics 27 (2015) 232e239

sulfonate) (PEDOT:PSS) [5,23e25], semiconducting metal oxides such as MoOx [20,26,27], polymers and small-molecule organic materials, self-assembled monolayers and graphene oxides etc [4,28e31]. Among them, since the first application in the late 1990s with effectiveness in hole transporting [32], PEDOT:PSS has been the most widely used HTL for conventional-structure BHJ-PSCs owing to its high optical transparency in the visible light spectrum, easy aqueous solution processing and high work function (4.8e5.2 eV as usually reported) beneficial for the formation of an Ohmic contact with many common donor polymers [5,24,33]. These advantages of PEDOT:PSS enable it as an alternative transparent anode material in ITO-free PSCs as well [6,34]. However, the conductivity of the pristine PEDOT:PSS film is usually very low (<1 S cm
1) because of the existence of insulating PSS moiety [23,35,36], which is unfavorable for its efficient hole transport as HTL or transparent anode material. Therefore, it is highly desirable to improve the conductivity of PEDOT:PSS in order to facilitate hole transport and consequently to enhance the device efficiency. A practical approach for conductivity improvement of PEDOT:PSS film is to dope the aqueous PEDOT:PSS solution with some small-molecule organic compounds such as polar solvents (e.g. N,N-dimethyl formamide (DMF), dimethyl sulfoxide (DMSO), ethylene glycol (EG), diethylene glycol, D-sorbitol), ionic liquid, anionic surfactant, dimethyl sulfate etc., and inorganic salt like CuBr2 [7,23e25,37e41]. The conductivity improvement of PEDOT:PSS film induced by the versatile additives was generally interpreted by the weakening of the Coulombic attractions between the hydrophobic/conductive/positively charged PEDOT and hydrophilic/insulating/negatively charged PSS components due to the preferential interactions of polar additives with individual component [7]. Besides, another approach primarily developed for application of PEDOT:PSS as a transparent anode material in ITO-free PSCs is based on a “dipping” post-treatment of the PEDOT:PSS films with polar organic compounds, zwitterion, surfactant, inorganic salts and acids etc [34e37,42,43]. Such a “dipping” treatment usually leads to conductivity improvement of PEDOT:PSS film as well, but the improvement mechanism was attributed to the removal of the PSS component from PEDOT:PSS mixture [36,42]. In particular, zwitterion as a special molecule carrying both cation and anion with their charges immobilized has been applied in “dipping” treatment of PEDOT:PSS film for ITO-free PSCs, including 1-(N,N-dimethylcarbamoyl)-4-(2-sulfoethyl)pyridinium hydroxide (DMCSP), N-dodecylN,N-dimethyl-3-ammonio-1-propanesulfonate (DDMAP), and N,Ndimethyl-N-[3-(sulfooxy)propyl]-1-nonanaminium hydroxide (DNSPN). All these three zwitterions resulted in moderate conductivity improvement of PEDOT:PSS film. However, PCE of the ITO-free PSC device based on zwitterion-treated PEDOT:PSS film as anode did not follow the same trend of conductivity improvement owing to the influence of the film roughness upon zwitterion “dipping” treatment [35]. These results revealed the complicated effect of zwitterion on PEDOT:PSS film. Besides, few studies reported the applications of conjugated zwitterions as electron-collection interlayers of BHJPSCs, contributing to obvious PCE improvements [44,45]. Hence, the unique amphiprotic nature of zwitterion enables its versatile application in PSCs, and it is desirable to develop novel zwitterion suitable for PSCs and to understand deeply its effect on device performance. In this paper, we introduced a novel zwitterion, 3-(N-morpholino)-2-hydroxypropanesulfonic acid (MOPSO), to dope aqueous PEDOT:PSS solution, resulting in significantly improved conductivity of PEDOT:PSS film and consequently PCE of the BHJ-PSC devices comprised of versatile active layers including poly(3hexylthiophene-2,5-diyl) blended with [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC61BM), poly[N-900 -hepta-decanyl-2,7carbazole-alt-5,5-(40 ,70 -di-2-thienyl-20 ,10,30 -benzothiadiazole) (PCDTBT) blended with [6,6]-phenyl C71-butyric acid methyl ester

233

(PC71BM) (PCDTBT:PC71BM) and thieno[3,4-b]-thiophene/benzodithiophene (PTB7):PC71BM. The doping concentration of MOPSO was optimized, and the effects of MOPSO doping on the conductivity, optical property and morphology of PEDOT:PSS film as well as BHJ-PSC device performance were investigated. Finally the mechanism for the efficiency enhancement upon MOPSO doping was discussed. 2. Experimental 2.1. Materials The indium tin oxide (ITO) glass substrate with a sheet resistance of 10 U/sq was purchased from Shenzhen Nan Bo Group, China. PEDOT:PSS (Clevios P Al4083) was purchased from SCM Industrial Chemical Co., Ltd., for which the PEDOT:PSS concentration is 1.3% by weight and the weight ratio of PSS to PEDOT is 6:1. P3HT, PCDTBT, PTB7, PC61BM and PC71BM were bought from Luminescence Technology Corp., Solarmer Material Inc., 1-material Chemscitech Inc., Nichem Fine Technology Co. and Solenne BV., respectively. MOPSO was purchased from TCI Shanghai Co., Ltd. All chemicals were used as received without further purification. 2.2. Fabrication of BHJ-PSC devices Our detailed fabrication procedure of the P3HT:PCBM BHJ-PSCs has been reported previously [15,22,34,46e49]. In brief, the ITOcoated glass substrate was first cleaned with detergent, then ultrasonicated in acetone and isopropanol, and subsequently dried in an oven overnight. PEDOT:PSS aqueous solution was first filtered by a 0.45 mm polyvinyl difluoride syringe filter. MOPSO was then added into the PEDOT:PSS aqueous solution with variable concentrations (10e30 mmol L
1), and the MOPSO:PEDOT:PSS blend solution was ultrasonicated for 5e10 min and stored at about 4  C in refrigerator prior to film fabrication. The pre-cleaned ITO glasses were subsequently treated by ozone-ultraviolet for 12 min. A thin film of MOPSO:PEDOT:PSS (36 ± 2 nm thick) was spin-coated onto the ITO surface at 3000 rpm for 60 s and then annealed at 120  C for 30 min in air. Pristine (undoped) PEDOT:PSS film was also prepared under identical conditions for comparison. The P3HT:PC61BM (1:0.8 w/w) photoactive blend layer with a thickness of ~80 nm was prepared by spin-coating the chlorobenzene solution at 800 rpm for 60 s. Then, Titanium(IV) isopropoxide diluted by 1:200 in methanol was spin-coated on top of the photoactive layer (5000 rpm for 40 s) as electron transport layer. Finally, the device was transferred into a vacuum chamber (~10
5 Torr), and an Al electrode (about 80 nm thick) was thermally deposited onto the active layer through a shadow mask to define the effective active area of the devices (2  7 mm2). The fabrication procedures of the PCDTBT:PC71BM and PTB7:PC71BM BHJ-PSC devices follow the method we reported previously [34,41] and those reported in literature [3,50], which is slightly different to that for P3HT:PCBM devices. First, MOPSO:PEDOT:PSS films with the same thickness to those used in P3HT:PCBM devices were spin-coated onto the ITO substrate and dried in the ambient atmosphere at 120  C for 30 min. The PCDTBT:PC71BM (1:4 w/w) photoactive blend layer with a thickness of ~80 nm was prepared by spin coating a blend solvent of 1,2dichlorobenzene:chlorobenzene (3:1 v/v) solution at 2500 rpm for 60 s. The PTB7:PC71BM (1:1.5 w/w) photoactive blend layer with a nominal thickness of 100 nm was prepared by spin-coating a blend solvent of chlorobenzene:1,8-diiodoctane (97:3 v/v) solution at 2000 rpm for 2 min. Titanium(IV) isopropoxide diluted by 1:200 in methanol was spin-coated on top of the photoactive layer (5000 rpm for 40 s) as electron transport layer. Then, the films were

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heated at 80  C for 10 min in the ambient atmosphere. Finally, Al electrode (about 80 nm thick) was evaporated through a shadow mask to define the active area of the devices (2  7 mm2). 2.3. Measurements and characterization Sheet resistances of MOPSO:PEDOT:PSS films were measured by using a four-point probe technique with a source measurement unit (Keithley 2400). The thickness of the film was measured by a KLA-Tencor P6 surface profilometer. The water contact angle measurement was performed using a KSV (Helsinki, Finland) CAM 200 contact angle goniometer. UVevis absorption spectra and transmittance were obtained using a UVeViseNIR spectrophotometer (UV-3600, Shimadzu, Japan). For the absorption spectroscopic measurements, samples were spin-coated on quartz substrate, and air was used as reference. Atomic force microscopy (AFM) measurements were carried out on a Veeco DI-Multimode V scanning probe microscope using tapping mode. Scanning Kelvin probe microscopy (SKPM) measurements were carried out on the same microscope using SKPM mode. All samples were measured by the same tip to avoid any change in the tip's work function. A standard HOPG measurement was done as a reference prior to the measurement of each sample so as to avoid the possible variation resulting from the tip and the atmosphere. The current density-voltage (J-V) characterization of BHJ-PSCs was carried out by using a Keithley 2400 source measurement unit under simulated AM 1.5 irradiation (100 mW cm
2) with a standard xenon-lamp-based solar simulator (Oriel Sol 3A, USA). The solar simulator illumination intensity was calibrated by a monocrystalline silicon reference cell (Oriel P/N 91150 V, with KG-5 visible color filter) calibrated by the National Renewable Energy Laboratory (NREL). All the measurements were carried out in air and a mask with well-defined area size of 14.0 mm2 was attached onto the cell to define effective area so as to ensure accurate measurement and to avoid the so-called “edge effect” [51]. More than ten devices were fabricated and measured independently under each experimental condition to ensure the consistency of the data. The best PCE data were used in the following discussions. 3. Results and discussion 3.1. Optimization of the doping concentration of MOPSO zwitterion and its effect on the conductivity of PEDOT:PSS film It is known that the PEDOT and PSS chains within PEDOT:PSS film are entangled by static interactions, leading to positively charged PEDOT and negatively charged PSS chains. Because the PSS moiety is insulating, the conductivity of the pristine PEDOT:PSS film is usually very low (<1 S cm
1) and dependent on the concentration of PSS moiety [23,35,36,41]. Hence, decreasing the concentration of PSS moiety and/or changing the static interactions between the conducting PEDOT and insulating PSS chains have been reported to result in dramatic improvement of the conductivity of the PEDOT:PSS film [36,38,41,52]. It is thus interesting to investigate whether the amphiprotic nature of MOPSO zwitterion would affect the conductivity of the PEDOT:PSS film or not. In order to optimize the doping concentration of MOPSO zwitterion in PEDOT:PSS aqueous solution, we first studied the effect of MOPSO on the conductivity of PEDOT:PSS film. Fig. 1 shows the dependence of the conductivity of PEDOT:PSS film measured by the four-point probe technique in ambient condition on the doping concentration of MOPSO. For a reliable analysis, the thicknesses of pristine PEDOT:PSS and MOPSO:PEDOT:PSS films are fixed at about 36 nm (see Supporting Information Figure S1). The conductivity of pristine PEDOT:PSS (Clevios P Al4083) film is 1.6  10
4 S cm
1. As shown in

Fig. 1. Conductivities of the pristine and MOPSO doped PEDOT:PSS films with different concentrations of MOPSO (10e30 mmol l
1).

Fig. 1, the conductivity of MOPSO doped PEDOT:PSS films increases significantly when a small amount of MOPSO is introduced. When the doping concentration of MOPSO reaches 20 mmol L
1, the maximum conductivity of composite film reaches 1.9  10
2 S/cm, which is more than two orders of magnitude higher than that of the pristine PEDOT:PSS (Clevios P Al4083) film. Upon increasing the doping concentration of MOPSO further to 30 mmol L
1, the conductivity decreases to 1.2  10
2 S/cm due to the existence of excess insulating MOPSO in the PEDOT:PSS film, but is still much higher than that of the pristine PEDOT:PSS film as well. Thus, the optimized doping concentration of MOPSO is 20 mmol L
1. To investigate the origin of conductivity improvement of MOPSO:PEDOT:PSS, water contact angles of the pristine PEDOT:PSS film and MOPSO:PEDOT:PSS with different doping concentration of MOPSO are measured (Fig. 2). The film surface of the pristine PEDOT:PSS is obviously hydrophilic with a water contact angle of 16.2 , and this is due to the enrichment of hydrophilic PSS atop of the film. Upon MOPSO doping, water contact angles of MOPSO:PEDOT:PSS films increase to 20.6 , 22.4 and 25.1 for MOPSO doping concentration of 10, 20 and 30 mmol L
1, respectively. These results indicate that the film surface of MOPSO:PEDOT:PSS becomes less hydrophilic than that of the pristine PEDOT:PSS, suggesting that the relative content of the hydrophilic PSS moiety on the film surface of PEDOT:PSS decreases. In other words, an increase of the relative content of the hydrophobic PEDOT moiety on the film surface of PEDOT:PSS is then expected, as confirmed by AFM study below. Such a change on the relative content of the hydrophilic PSS moiety might result from the separation of the PEDOT segments from the entangled PEDOT:PSS domains due to their weakened coulombic interactions induced by MOPSO. Consequently, the separated PEDOT segments could provide a continuous conductive pathway favorable for hole transporting through PEDOT:PSS film [38]. 3.2. Effect of MOPSO doping on the optical property of PEDOT:PSS film UVevis absorption spectra of the pristine and MOPSO doped PEDOT:PSS films are shown in Fig. 3. Obviously MOPSO doping affects the optical absorption of PEDOT:PSS in the UV region only, and the intensities of two absorption bands (193, 225 nm) assigned to PSS moiety [36,52,53] decrease upon MOPSO doping. This phenomenon is similar to the case of PEDOT:PSS treatment by exposure

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235

Fig. 2. Water contact angles of the pristine and MOPSO doped PEDOT:PSS films with different concentrations of MOPSO (10e30 mmol l
1).

to high boiling solvents, mixing with polar solvent, or dipped in the solvent bath reported in literature, for which the decrease on UV absorption intensity was interpreted by loss of PSS moiety [42,52,54]. However, in our present case, MOPSO:PEDOT:PSS film was not rinsed by any solvent, thus loss of PSS moiety seems unlikely, and the decrease on UV absorption intensity should be attributed to MOPSO-induced morphology change of PEDOT:PSS film as discussed in details below, leading to the aggregation of the PSS segments with weakened UV absorption [52]. Fig. 4 compares the optical transmission spectra of MOPSO doped PEDOT:PSS films with that of the pristine PEDOT:PSS film. Apparently MOPSO:PEDOT:PSS films show larger transmittance than that the pristine PEDOT:PSS film in the region of 400e800 nm, which corresponds to the main absorption region of common conducting polymer donors used in the photoactive layers of BHJPSCs. When the doping concentration of MOPSO increases from 10 to 20 mmol L
1, the transmittance intensity slightly increases, while a further increase of the doping concentration of MOPSO to 30 mmol L
1 leads to transmittance intensity drop in the region of 350e450 nm. The optimized MOPSO doping concentration of 20 mmol L
1 in terms of the optical transmittance of PEDOT:PSS film is consistent with that optimized on the basis of conductivity

discussed above. Under such an optimized MOPSO doping concentration, the maximum transmittance of MOPSO:PEDOT:PSS film at ca. 540 nm reaches ~98%, which is increased by about 6% compared to that of the pristine PEDOT:PSS film. It is known that within PEDOT:PSS film PEDOT segments may aggregate and induce a high level of light scattering, resulting in limited transmittance [38,55]. When PEDOT:PSS is doped by MOPSO, the amphiprotic nature of MOPSO zwitterion weakens the coulombic interactions between the hydrophobic PEDOT and hydrophilic PSS moieties, as a result the PEDOT and PSS segments distribute more evenly within PEDOT:PSS film and the light scattering caused by PEDOT segments can be suppressed effectively, thus leading to the increase of transmittance of PEDOT:PSS film.

Fig. 3. UVevis absorption spectra of the pristine and MOPSO doped PEDOT:PSS films with different concentrations of MOPSO (10e30 mmol l
1).

Fig. 4. The transmittance spectra of the pristine and MOPSO doped PEDOT:PSS films with different concentrations of MOPSO (10e30 mmol l
1).

3.3. Effect of MOPSO doping on the performance of PEDOT:PSS HTLbased BHJ-PSC devices The increases of both conductivity and transmittance of MOPSO doped PEDOT:PSS film stimulate us to investigate whether MOPSO doping would affect the performance of PEDOT:PSS HTL-based BHJPSC devices or not. The effect of MOPSO doping on the performance of PEDOT:PSS HTL-based BHJ-PSC devices is then studied

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systematically by applying MOPSO:PEDOT:PSS HTL in different photoactive systems, including P3HT:PC61BM as the most commonly used BHJ-PSC system, low bandgap donor material such as PCDTBT blended with PC71BM (PCDTBT:PC71BM) and PTB7:PC71BM, and their chemical structures together with device architecture are illustrated in Scheme 1 [5,16]. The corresponding MOPSO:PEDOT:PSS HTL-based devices were measured under a simulated AM 1.5 irradiation (100 mW cm
2) in air atmosphere, and the currentevoltage (J-V) curves of the devices based on MOPSO (20 mmol L
1):PEDOT:PSS HTL are compared in Fig. 5, which includes also those of reference devices without any HTL (devices A, D, G) and with pristine PEDOT:PSS HTL (devices B, E, H) for comparison. Table 1 summarized the measured parameters including short-circuit current (Jsc), open-circuit voltage (Voc), fill factor (FF), PCE, series resistance (Rs) and shunt resistance (Rsh) based on the average of more than ten devices fabricated independently under each experimental condition, and the devices with highest PCEs were used for the following discussion. MOPSO:PEDOT:PSS films with different MOPSO doping concentrations were incorporated as HTL into BHJ-PSC devices based on different photoactive layers, P3HT:PC61BM, PCDTBT:PC71BM, PTB7:PC71BM, respectively, and the optimized MOPSO doping concentration is determined to be 20 mmol L
1, under which MOPSO:PEDOT:PSS HTL-based BHJ-PSC devices exhibit the highest PCEs (3.62%, 7.03% and 7.56% for P3HT:PC61BM, PCDTBT:PC71BM, PTB7:PC71BM system, respectively, see Supporting Information Figs. S5eS7 and Tables S2eS4). Again, this optimized MOPSO doping concentration of 20 mmol L
1 agrees well with those determined on the basis of conductivity and optical transmittance of PEDOT:PSS film as discussed above, suggesting the correlation of PCE of MOPSO:PEDOT:PSS HTL-based BHJ-PSC device with the conductivity and optical transmittance of MOPSO:PEDOT:PSS film. For P3HT:PCBM-based devices, the reference device without any HTL (device A) shows a low PCE of 2.34% calculated from a Voc of

0.57 V, a Jsc of 8.22 mA/cm2, and a FF of 49.7%. Such a low PCE is due to the high work function of ITO (4.7 eV) and the direct contact between ITO and PC61BM, leading to a large series resistance and leakage current [21,56,57]. When the pristine PEDOT:PSS was incorporated as HTL (device B), the three parameters, Voc, Jsc, and FF, all increase to 0.63 V, 8.82 mA/cm2, and 60.9%, respectively, resulting in an enhanced PCE of 3.39% due to well-known efficient hole transport of PEDOT:PSS [23,41,57]. Upon incorporating MOPSO:PEDOT:PSS HTL (device C), Jsc increases further to 9.46 mA/cm2, while Voc and FF exhibit negligible changes, consequently PCE reaches 3.62%, which is enhanced by ca. 6.8% compared to that of the control device B based on the pristine PEDOT:PSS HTL. The effectiveness of MOPSO doping of PEDOT:PSS HTL in enhancing the PCE of P3HT:PC61BM BHJ-PSC devices stimulated us to investigate the effect of MOPSO doping on the performance of other devices based on such low bandgap donor materials as PCDTBT and PTB7 (see Scheme 1). Using PCDTBT:PC71BM blend as the photoactive layer, BHJ-PSC devices were fabricated with TiOx incorporated as an ETL [50]. For the reference (without any HTL, device D) and control (with the pristine PEDOT:PSS HTL, device E) devices, PCEs of 4.34% and 6.25% are achieved, respectively. Device E shows dramatically increased Voc (0.92 V), Jsc (10.49 mA/cm2) and FF (64.5%) compared to those of device D (see Table 1), suggesting that the incorporation of PEDOT:PSS HTL improves hole transport by avoiding direct contact between ITO and PC71BM resulting in a large series resistance and leakage current [21,41,56]. The device based on MOPSO:PEDOT:PSS HTL (device F) shows a PCE of 7.03% calculated from a Voc of 0.91 V, a Jsc of 12.16 mA/cm2 and a FF of 63.3%. Compared to the control device E based on the pristine PEDOT:PSS HTL, Jsc of MOPSO:PEDOT:PSS HTL-based device increases dramatically while Voc and FF exhibit negligible decreases, contributing to a PCE enhancement of ca. 12.5%. Interestingly, despite of the difference on the PCE enhancement ratio, the changes of the photovoltaic parameters (Voc, Jsc, FF) between PCDTBT:PC71BM and P3HT:PC61BM systems are quite similar, suggesting that their PCE enhancement mechanisms are same as discussed further below. PTB7 represents another promising low-bandgap conducting polymer donor and PTB7:PC71BM system has been recently studied extensively [9,58]. MOPSO doped (device K) and the pristine (device H) PEDOT:PSS HTL was incorporated respectively into PTB7:PC71BM-based BHJ-PSC devices which contained TiOx as an ETL [59]. Although the reference device (without any HTL, device G) shows a relatively low PCEs of 4.71%, PCE of the control device based on the pristine PEDOT:PSS HTL (device H) increases significantly to 6.66% owing to the increase of all three photovoltaic parameters. For MOPSO:PEDOT:PSS HTL-based device (device K), Voc, Jsc and FF are 0.76 V, 17.02 mA/cm2 and 58.5%, respectively, affording a high PCE of 7.56%, which is enhanced by ca. 13.5% relative to that of the control device H based on the pristine PEDOT:PSS HTL (see Table 1). Noteworthy, similar to the cases of PCDTBT:PC71BM and P3HT:PC61BM systems discussed above, such a PCE enhancement is mainly originated from the increase of Jsc by ca. 13.9% while Voc and FF keep almost constant. 3.4. Effect of MOPSO doping on the surface morphology of PEDOT:PSS film

Scheme 1. (a) Device architecture of the BHJ-PSC devices based on different photoactive systems. (b) Chemical structures of MOPSO and photoactive layer materials used in this study.

Since it is known that the performance of a BHJ-PSC device is usually dependent on the film morphology of different layers, it is necessary to investigate the effect of MOPSO doping on the surface morphology of PEDOT:PSS film, which was measured by atomic force microscopy (AFM) in tapping mode (Fig. 6). Fig. 6aed illustrate the height images of PEDOT:PSS films doped by MOPSO with different doping concentrations (10, 20 and 30 mmol L
1 shown in

Z. Zhao et al. / Organic Electronics 27 (2015) 232e239

237

Fig. 5. JeV curves of the BHJ-PSC devices without any HTL (A, D, G), with pristine PEDOT:PSS HTL (B, E, H), and with MOPSO (20 mmol L
1):PEDOT:PSS HTL (C, F, K). Photoactive system: (a) P3HT:PC61BM, (b) PCDTBT:PC71BM, (c) PTB7:PC71BM. The measurements were carried out under AM 1.5 illumination at an irradiation intensity of 100 mW cm
2.

Table 1 Photovoltaic parameters of the BHJ-PSC devices with different HTLs and photoactive layers under the illumination of AM 1.5 (100 mW/cm2). System

P3HT:PC61BM

PCDTBT:PC71BM

PTB7:PC71BM

a b c d

Device

A B C D E F G H K

[MOPSO]a (mmol
1 L)

ed 0 20 ed 0 20 ed 0 20

Voc (V)

0.57 0.63 0.63 0.89 0.92 0.91 0.70 0.76 0.76

Jsc (mA/cm2)

8.22 8.82 9.46 9.14 10.49 12.16 12.60 14.94 17.02

FF (%)

49.7 60.9 60.7 53.2 64.5 63.3 53.2 58.6 58.5

PCE (%)

Rsc (U$cm2)

Rshc (U$cm2)

24.7 12.9 11.4 14.8 11.1 10.1 13.0 12.1 9.4

605.0 945.0 1007.4 561.2 986.1 829.8 506.2 597.2 609.0

b

Best

Average

2.34 3.39 3.62 4.34 6.25 7.03 4.71 6.66 7.56

2.26 3.27 3.48 4.27 6.18 6.99 4.67 6.59 7.42

± ± ± ± ± ± ± ± ±

0.02 0.01 0.01 0.02 0.03 0.02 0.02 0.03 0.02

MOPSO doping concentration in PEDOT:PSS HTL. Averaged over ten devices. Rs and Rsh are given by the PCE measurement system. Without any HTL.

images b, c and d, respectively) in comparison with that of the pristine PEDOT:PSS film (image a). Obviously, the surfaces of MOPSO doped PEDOT:PSS films are rougher than that of the former

as revealed by the increase of the measured root-mean-square (RMS) roughness from 1.03 nm (pristine) to 1.35e1.66 nm (see Supporting Information Figure S2). As the doping concentration of

Fig. 6. AFM height (I) and phase (II) images of pristine (a, e) and MOPSO doped PEDOT:PSS films with different MOPSO doping concentrations of 10 (b, f), 20 (c, g), and 30 (d, h) mmol L
1.

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MOPSO increases from 10 to 30 mmol L
1, the RMS roughness of the film surface increases from 1.35 to 1.66 nm (see Supporting Information Figure S2). Moreover, in the image of the pristine PEDOT:PSS film, the bright bulged grains seem to aggregate to large domains or stripes, which are however more evenly distributed in images of MOPSO:PEDOT:PSS films (images b-d). This tendency is more clearly seen in the 3D height image in which the bulged grains on the surface of MOPSO:PEDOT:PSS films appear more uniform that those on the pristine PEDOT:PSS film (see Supporting Information Figure S2) [60]. In the corresponding phase images shown in Fig. 6eeh, the PEDOT-rich (hard segments) and PSS-rich regions (soft segments) are observed as bright and dark regions, respectively due to the microphase separation structure as reported in literature [34,53]. While the pristine PEDOT:PSS film shows relatively small and indistinct PEDOT-rich grains of 5e10 nm, the size of the PEDOT-rich grains increase to about 25 nm when the doping concentration of MOPSO increase to 20 mmol L
1. Such morphological changes suggest that MOPSO doping induces a more distinct phase separation featured by the formation of a continuous PEDOT phase composed of larger grains. This continuous PEDOT phase provides better charge transport pathways that result in higher conductivity as discussed above [34,53]. 3.5. Mechanism of the efficiency enhancement of BHJ-PSC devices based on MOPSO doped PEDOT:PSS HTL According to the above analysis, independent on the photoactive systems of P3HT:PC61BM, PCDTBT:PC71BM and PTB7:PC71BM, MOPSO doping of PEDOT:PSS HTL leads to efficiency enhancements of all of these BHJ-PSC devices, and the efficiency enhancement is mainly due to the increase of Jsc. Among the determinative parameters of PCE of BHJ-PSCs, Jsc is dependent not only on the multiplication of the photo-induced charge carrier density and the charge carrier mobility within the active material but also on the interface properties between the active layer and the electrodes [48]. For a given donor/acceptor photoactive layer system, the photoabsorption under the fixed AM 1.5 illumination should be same in principle. However, the increase of transmittance of PEDOT:PSS film as discussed above leads to the slight increase of the photoabsorption of the photoactive layer as confirmed by UVevis absorption measurements of all three photoactive films (see Supporting Information Figure S3). Interestingly, the optimized MOPSO doping concentration (20 mmol L
1) in terms of the highest PCE for versatile photoactive systems of P3HT:PC61BM, PCDTBT:PC71BM and PTB7:PC71BM is same to that optimized for the conductivity of MOPSO:PEDOT:PSS film as discussed above. Thus, it is reasonable to attribute the improved conductivity of PEDOT:PSS film upon MOPSO doping as a major factor to the efficiency enhancement of BHJ-PSC devices based on MOPSO doped PEDOT:PSS HTL. The proposed correlation of the performance of BHJ-PSC device with the conductivity of PEDOT:PSS HTL is consistent with those reported in literature [38,61]. For instance, a treatment of PEDOT:PSS film by polar solvent doping was reported to result in the improved conductivity of PEDOT:PSS film and the obvious increase of Jsc as well [38]. In order to investigate the influence of MOPSO doping on the interface between the active layer and PEDOT:PSS HTL, we carried out scanning Kelvin probe microscopy (SKPM) measurements, which can directly probe the interfacial dipole induced by interfacial layer and can be further used to estimate the work function of interfacial material [15,22,41,62e64]. The surface potential of MOPSO (20 mmol L
1):PEDOT:PSS HTL is found to be 170 mV more positive than that of the prisitine/undoped PEDOT:PSS HTL (see Supporting Information Figure S4 and Table S1). Such a change of the surface

potential value upon MOPSO doping can be directly correlated to the change of the work function of MOPSO:PEDOT:PSS HTL by assuming that the surface potential of the film is uniform in macroscopic scale [15,22,41,62e64]. Accordingly the work function of MOPSO (20 mmol L
1):PEDOT:PSS HTL is estimated to decrease by ca. 170 meV relative to that of the pristine PEDOT:PSS HTL, which was reported to be 5.0 eV (see Supporting Information Table S1) [4,50]. Furthermore, we found that varying the doping concentration of MOPSO results in slight change of the work function of the MOPSO:PEDOT:PSS HTL, which all decrease compared to that of the pristine PEDOT:PSS HTL (see Supporting Information Table S1). Such a decrease of the work function upon MOPSO doping is presumably due to the decrease of the relative content of the PSS moiety within PEDOT:PSS domains resulted from the weakened coulombic interactions between PEDOT and PSS moieties as discussed above, which may induce an inward directed surface potential dipole and consequently increase the work function of the films [24]. With such an decrease of the work function of PEDOT:PSS HTL upon MOPSO doping, the energy offset between the work function of the MOPSO:PEDOT:PSS HTL and the highest occupied molecular orbital (HOMO) level of the polymer donor (5.1, 5.5, 5.2 eV for P3HT, PCDTBT, PTB7, respectively, see Scheme 2) increases, unfavorable for hole collection from the polymer donor to PEDOT:PSS HTL and consequently hole extraction by the ITO anode. In this sense, such an adverse effect of MOPSO doping competes with its beneficial effect in improving the conductivity of PEDOT:PSS film, with the latter playing a more dominant role contributing to the increase of Jsc. If a zwitterion dopant would lead to an increase of the work function of the PEDOT:PSS HTL facilitating hole collection from the polymer donor to PEDOT:PSS HTL instead, we expect that a further enhancement of PCE would be achieved, and this is underway in our lab.

4. Conclusions In summary, MOPSO zwitterion was applied to dope PEDOT:PSS for the first time, which was incorporated as HTL in BHJ-PSC devices based on P3HT:PC61BM, PCDTBT:PC71BM and PTB7:PC71BM, resulting in significantly enhanced conductivity of PEDOT:PSS and consequently enhanced PCE of the BHJ-PSC devices. Under the optimized doping concentration of MOPSO of 20 mmol L
1, the conductivity of PEDOT:PSS film increased by about two orders of magnitude, and this is interpreted by the weakening of the Coulombic attractions between PEDOT and PSS components induced by MOPSO. The best PCEs of the MOPSO:PEDOT:PSS HTLbased P3HT:PC61BM, PCDTBT:PC71BM and PTB7:PC71BM BHJ-PSC devices reach 3.62%, 7.03% and 7.56%, which are improved by ~6.8%, ~12.5% and ~13.5%, respectively, compared to those of the reference devices based on pristine PEDOT:PSS HTL. The improved

Scheme 2. Energy level diagram of ITO/MOPSO:PEDOT:PSS/donor:acceptor/TiOx/Al BHJ-PSC devices with pristine and MOPSO doped PEDOT:PSS HTL. The HOMO/LUMO levels of P3HT, PCDTBT, PTB7, PC61BM and PC71BM and the work functions of ITO, pristine PEDOT:PSS and Al were referred to refs. 3, 13, 16.

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PCE is mainly due to the increase of Jsc, which is attributed to the conjunct factors including the increase of the photoabsorption of the photoactive layer and the improved conductivity of PEDOT:PSS HTL. Improving the performance of BHJ-PSC device via doping the commonly used PEDOT:PSS HTL by a zwitterion is facile and universal for different photoactive systems, thus being promising for the high-efficiency BHJ-PSCs. Acknowledgments This work was partially supported by National Natural Science Foundation of China (Nos. 21132007, 21371164), Key Project of Hefei Center for Physical Science and Technology (No. 2012FXZY006) and the Fundamental Research Funds for the Central Universities (WK3430000002). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.orgel.2015.09.022. References [1] G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270 (1995) 1789e1791. [2] S. Guenes, H. Neugebauer, N.S. Sariciftci, Chem. Rev. 107 (2007) 1324e1338. [3] C.E. Small, S. Chen, J. Subbiah, C.M. Amb, S.-W. Tsang, T.-H. Lai, J.R. Reynolds, F. So, Nat. Photonics 6 (2012) 115e120. [4] Z. He, H. Wu, Y. Cao, Adv. Mater. 26 (2014) 1006e1024. [5] G. Li, R. Zhu, Y. Yang, Nat. Photonics 6 (2012) 153e161. [6] J.E. Carle, M. Helgesen, M.V. Madsen, E. Bundgaard, F.C. Krebs, J. Mater. Chem. C 2 (2014) 1290e1297. [7] Z.Q. Zhao, W.F. Zhang, X.M. Zhao, S.F. Yang, Interfacial Materials toward Efficiency Enhancement of Polymer Solar Cells, in: Q.Q. Qiao (Ed.), Organic Solar Cells: Materials, Devices, Interfaces, and Modeling, CRC Press, London, 2015, pp. 201e246. [8] X.T. Hu, L. Chen, Y. Zhang, Q. Hu, J.L. Yang, Y.W. Chen, Chem. Mater. 26 (2014) 6293e6302. [9] Y. Yang, W. Chen, L. Dou, W.-H. Chang, H.-S. Duan, B. Bob, G. Li, Nat. Photonics 9 (2015) 190e198. [10] S.H. Liao, H.J. Jhuo, P.N. Yeh, Y.S. Cheng, Y.L. Li, Y.H. Lee, S. Sharma, S.A. Chen, Sci. Rep. 4 (2014) 6813. [11] Y. Liu, J. Zhao, Z. Li, C. Mu, W. Ma, H. Hu, K. Jiang, H. Lin, H. Ade, H. Yan, Nat. Commun. 5 (2014) 5293. [12] Z. He, B. Xiao, F. Liu, H. Wu, Y. Yang, S. Xiao, C. Wang, T.P. Russell, Y. Cao, Nat. Photonics 9 (2015) 174e179. [13] J.B. You, L.T. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C.C. Chen, J. Gao, G. Li, Y. Yang, Nat. Commun. 4 (2013) 1446. [14] X.G. Guo, N.J. Zhou, S.J. Lou, J. Smith, D.B. Tice, J.W. Hennek, R.P. Ortiz, J.T.L. Navarrete, S.Y. Li, J. Strzalka, L.X. Chen, R.P.H. Chang, A. Facchetti, T.J. Marks, Nat. Photonics 7 (2013) 825e833. [15] X.M. Zhao, C.H. Xu, H.T. Wang, F. Chen, W.F. Zhang, Z.Q. Zhao, L.W. Chen, S.F. Yang, ACS Appl. Mater. Interfaces 6 (2014) 4329e4337. [16] S.H. Liao, H.J. Jhuo, Y.S. Cheng, S.A. Chen, Adv. Mater. 25 (2013) 4766e4771. [17] F.L. Zhang, K.G. Jespersen, C. Bjorstrom, M. Svensson, M.R. Andersson, V. Sundstrom, K. Magnusson, E. Moons, A. Yartsev, O. Inganas, Adv. Funct. Mater. 16 (2006) 667e674. [18] C.K. Song, K.A. Luck, N. Zhou, L. Zeng, H.M. Heitzer, E.F. Manley, S. Goldman, L.X. Chen, M.A. Ratner, M.J. Bedzyk, R.P.H. Chang, M.C. Hersam, T.J. Marks, J. Am. Chem. Soc. 136 (2014) 17762e17773. [19] J.B. You, C.C. Chen, Z.R. Hong, K. Yoshimura, K. Ohya, R. Xu, S.L. Ye, J. Gao, G. Li, Y. Yang, Adv. Mater. 25 (2013) 3973e3978. [20] S. Chen, J.R. Manders, S.W. Tsang, F. So, J. Mater. Chem. 22 (2012) 24202e24212. [21] H. Ma, H.L. Yip, F. Huang, A.K.Y. Jen, Adv. Funct. Mater. 20 (2010) 1371e1388. [22] S.X. Qu, M.H. Li, L.X. Xie, X. Huang, J.G. Yang, N. Wang, S.F. Yang, ACS Nano 7 (2013) 4070e4081. [23] R. Po, C. Carbonera, A. Bernardi, N. Camaioni, Energy Environ. Sci. 4 (2011) 285e310.

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