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Enhanced polymer solar cells efficiency by surface coating of the PEDOT: PSS with polar solvent
Available online at www.sciencedirect.com
ScienceDirect Solar Energy 129 (2016) 175–183 www.elsevier.com/locate/solener
Enhanced polymer solar cells efficiency by surface coating of the PEDOT: PSS with polar solvent Min Wang a,b, Manxi Zhou a, Lei Zhu a, Qifang Li a,⇑, Chao Jiang b,⇑ a b
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Key Laboratory of Standardization and Measurement for Nanotechnology, National Center for Nanoscience and Technology, Chinese Academy of Sciences, Beijing 100190, China Received 18 August 2015; received in revised form 9 November 2015; accepted 2 February 2016
Communicated by: Associate Editor Sam-Shajing Sun
Abstract Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) films were post-treated with polar solvent dimethylsulfoxide (DMSO) by spin-coating method and polymer solar cells (PSCs) based on poly [N-900 ]-hepta-decanyl-2,7-carbazole-alt-5,5-(40 ,70 -di-2-thie nyl-20 ,10 ,30 -benzothiadiazole) (PCDTBT) : [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) were fabricated to investigate the effect of the treatment. By post-modifying the PEDOT: PSS layer, the conductivity of the PEDOT: PSS film was largely improved. Using electrochemical impedance spectroscopy (IS) we observed that the series resistance of the device decreased greatly after the treatment. With DMSO-treated PEDOT: PSS transport layer, the power conversion efficiency (PCE) of the PSC based on PCDTBT: PC71BM raised from 5.95% to 6.52% with both increase in Jsc and FF. We systematically studied charge transport property via space-chargelimited-current (SCLC) and our results suggest that the increment in device efficiency can be attributed to the increased hole-mobility and thus more balanced charge transport benefits the enhancement of polymer solar cell efficiency. We also noted that if measured without a shadow mask much more overestimation will take place in the DMSO-treated device as a result of lateral electrical conduction. We suggest that when we apply the highly conductive PEDOT: PSS layer in the PSCs, careful measurement should be carried out to avoid inaccuracy. Ó 2016 Elsevier Ltd. All rights reserved.
Keywords: Polymer solar cell; PEDOT: PSS; Dimethylsulfoxide; Charge transport
1. Introduction Over the past decades, polymer solar cells have attracted remarkable attention due to their advantages of low cost, light weight and good mechanical flexibility as well as easy processability (Hoppe and Sariciftci, 2004; Dennler et al., 2009; Li et al., 2012). And extensive efforts have been
⇑ Corresponding authors.
E-mail addresses: qfl[email protected] (Q. Li), [email protected] (C. Jiang). http://dx.doi.org/10.1016/j.solener.2016.02.003 0038-092X/Ó 2016 Elsevier Ltd. All rights reserved.
directed toward this field from designing and synthesizing novel polymers (Chen and Cao, 2009; Cheng et al., 2009; Gunes et al., 2007; Scharber et al., 2006) to optimizing the fabrication of the devices (Krebs, 2009; Kim et al., 2006). Hitherto, power conversion efficiency of PSCs in laboratory scale has exceeded 10% paving a promising way for future commercialization (You et al., 2013). PEDOT: PSS, an electro-conductive polymer solution, has been widely used as a hole transport layer (HTL) in solar cell devices because of its high transparency in the visible region, good thermal stability, high mechanical
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stability, strong hole affinity and high work function (Lim et al., 2014). However, the PEDOT: PSS naturally suffers from low conductivity (less than 1 S/cm) and thus inevitably limits the performance of the PSCs (Kim et al., 2011). Several methods have been implemented to promote its conductivity including: the addition of a suitable percentage of solvents such as surfactant (Fang et al., 2011), glycerol (Lee et al., 2009), ethylene glycol (Li et al., 2015), dimethyl sulfoxide (Thomas et al., 2014), acids (Song et al., 2013; Xia and Ouyang, 2012), graphene oxide (Yang et al., 2015) and isopropanol (Zhang et al., 2014); immersing the films in a polar organic solvent like ethylene glycol (Kim et al., 2012) and DMSO (Unsworth et al., 2014). Through these approaches, the conductivity of PEDOT: PSS was successfully increased by up to 2 or 3 orders of magnitude. It is well accepted that PEDOT: PSS aqueous dispersions are of low conductivity because the PSS chains are rather insulating (Lim, 2013). Special attention has been paid to the variation of the PEDOT: PSS, but rare of the literature discuss about charge transport property in detail and the mechanism about how each method affects the device is still under debate. In this article, we adapted the polar organic solvent DMSO to modify the prepared PEDOT: PSS films by spin-coating and successfully improved their conductivity. As a consequence, we obtained enhanced power conversion efficiency of PCDTBT: PC71BM based solar cells from 5.95% to 6.52%, with simultaneous increase in short current density (Jsc) and fill factor (FF). We also measured charge carriers’ mobility via SCLC and systematically investigated the transporting mechanism with this treatment. 2. Experimental 2.1. Materials and reagents PEDOT: PSS aqueous solution (Clevious P VP AI 4083) was purchased from H. C. Starck. (Germany). PC71BM (purity 99.5%) was acquired from American Dye Inc. and used as received. PCDTBT (Mn = 95 kDa; Mw = 152 kDa; PDI = 1.6) was synthesized ourselves according to the literature (Blouin et al., 2007). All other reagents and chemicals were bought from Sigma–Aldrich and used without further purification. Patterned Indium Tin Oxide (ITO) glass (15 mm 15 mm, 10 X/sq) substrates were received from Nanbo Co. Ltd. (China). 2.2. Preparation and characterization of the PEDOT: PSS films First, ITO substrates were sequential ultrasonic cleaned in detergent, acetone, isopropanol, deionized water and alcohol. And they were dried under nitrogen followed by an ultraviolet-ozone treatment to remove residual organics. PEDOT: PSS solution was filtered through a 0.45 lm PTFE filter and spin coated on the ITO glass at
3500 rpm for 40 s with a thickness of 45 nm approximately. These substrates were then annealed at 150 °C for 30 min on a hot plate in ambient atmosphere. The samples were transferred to a glove box for subsequent procedures. Solvent treatment was performed by spin-casting 25 ll DMSO onto the prepared PEDOT: PSS film at 3000 rpm for 40 s. The films were dried at 100 °C for 15 min. All films were allowed to cool down to room temperature before following processes were performed. The surface roughness and morphology images of the films were characterized using atomic force microscopic (AFM; Veeco Dimension 3100) by tapping mode. The thickness of the PEDOT: PSS films was estimated with an Alpha D-120 stylus profilometer (KLA, Tencor, USA) 2.3. Fabrication and characterization of polymer solar cells The mixture of PCDTBT:PC71BM (1:4, wt%) at a concentration of 7 mg/ml for polymer in orthodichlorobenzene (o-DCB) pre-stirred at 50 °C for eight hours was spin coated at 1500 rpm for 60 s both onto the treated and untreated PEDOT: PSS layer, respectively. The prepared films were thermal annealed at 70 °C for 10 min in the glove box. Two narrow sides (about 1 mm) on both edge of the film were wiped off using methanolwetted swab to allow the contact of ITO and Ca/Al electrodes. The samples were brought into an evaporate chamber afterwards and a 20 nm calcium (Ca) layer followed by a 100 nm aluminum (Al) electrode was deposited on top of the photoactive layer at a base pressure of 106 mbar. The evaporation thickness was controlled by SQC-310C deposition controller (INFICON, Germany). Four devices were fabricated on one substrate and the effective area of each device was 4 mm2 defined by a shadow mask. The current–voltage (J–V) curves were measured with a Keithley 2400 Source-measure unit under the illumination of AM 1.5 G irradiation (100 mW/cm2) using a 150 W solar simulator (Oriel 91159A, Newport, USA) in ambient air. The light intensity was calibrated by a standardized monosilicon cell (Oriel PN 91150V, Newport, USA) prior to the measurements. A 2 mm 2 mm shadow mask was utilized to cover the active area to avoid interference from scattering light and adjacent current leakage. The monochromatic incident photon-to-electron conversion efficiency (IPCE) spectra was obtained using a 500 W Xe lamp-based solar simulator (Oriel lamp 74001) with a lock-in amplifier under short-circuit conditions. A calibrated silicon (Si) photodiode (PRL-12, Newport, USA) was used as a reference. The transmittance spectra of the PEDOT: PSS films were taken with a lambda-950 (Perkin Elmer, USA) spectrophotometer. The impedance spectroscopy (IS) measurement was performed using a Zahner Zennium 40562 electrochemical workstation. X-ray photoelectron spectroscopy (XPS) (Model. ESCALAB250Xi) was applied to measure the sulfur S2p core-level spectra of the PEDOT: PSS films coated on Si/SiO2 substrates.
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2.4. Fabrication and characterization of SCLC devices The hole-only devices were fabricated with ITO/ PEDOT: PSS/PCDTBT: PC71BM/MoO3/Ag structures applying the same processes as the fabrication of photovoltaic devices and the PEDOT: PSS films were untreated and treated with DMSO solvent for comparison. For the electron-only devices, the structures were Al/PCDTBT: PC71BM/Al and here the Al patterned substrates were also untreated and treated with DMSO respectively. The spacecharge-limited current (SCLC) mobility was calculated according to the Mott-Gurney square law J = 9/8 ere0 l(V2/L3), where J is the current density, er is relative permittivity of the polymer, e0 is the vacuum permittivity, l is the hole mobility, V is the applied voltage and L is the thickness of the active layer. The current–voltage (J–V) curves were measured with Keithley 2400 under dark. 3. Results and discussions Scheme 1 presents the chemical structure of materials used in this study and the schematic diagram of solar cell configuration and the energy band diagram. The cell architecture (Scheme 1b) shows the interlayer-like DMSO spin coated between the PEDOT: PSS and active layer. Actually, little of DMSO was left in the device after the evaporation. To evaluate the washing effect of DMSO solvent on the surface of PEDOT: PSS films, we have conducted X-ray photoemission spectroscopy (XPS) studies. Typical S2p core-level spectra of the PEDOT: PSS films before and after DMSO treatment are shown in Fig. 1. The XPS spectra for both samples exhibit high intensity between
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168.0 and 168.9 eV, which is corresponding to the spinsplit components of sulfur atoms of the PSS chains. While between 163.9 and 165.6 eV, we can see another two small peaks for both spectra which are assigned to the sulfur atoms of the PEDOT fragments. These results are in good agreement with the previous report by Schaarschmidt et al. (2009). The XPS spectrum is constrained by the S 2p3/2-S 2p1/2 spin-orbit separation being 1.2 eV, and the intensity ratio of the two peaks of each doublet being 2:1 (Greczynski et al., 2001, 1999). The ratio of the S 2p3/2 areas for PSS (stronger) and PEDOT (weaker) can be used to estimate the content of remaining PSS on the surface. It is calculated that the PSS/PEDOT composition ratio is 8.8 for the pristine sample but drops to 3.75 for that treated with DMSO solvent suggesting that large amount of the insulating PSS has been successfully removed from the PEDOT: PSS films leading to a significant increase in electrical conductivity. To better understand the effect of surface coating of DMSO on PEDOT: PSS layer, the impedance of whole PCDTBT: PC71BM solar cells was carried out in a frequency ranging from 1 MHz to 1 Hz under dark condition. Fig. 2 shows the electrochemical impedance spectra for PCDTBT: PC71BM solar cells with both treated and untreated PEDOT: PSS transport layer, respectively. We can see that the impedance responses of the devices are semicircles in the complex plane giving information about the charge transfer and recombination resistance (Bisquert, 2002). For the pristine devices, the real impedance spectrum is 1217 X, whereas the one treated with DMSO solvent shows the real impedance spectrum of 700 X, a 42% decrease in comparison. The lowered impedance suggests that the charge transfer resistance among
Scheme 1. Chemical structure of the materials (a), the configuration of the solar cell (b) and the energy diagram of the device (c).
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Fig. 1. XPS S2p spectra of PEDOT: PSS film before (a) and after (b) DMSO treatment.
Fig. 2. Impedance spectra for PCDTBT: PC71BM solar cells without and with DMSO treatment measured in the dark.
the device is reduced due to the solvent treatment on PEDOT: PSS. Parameters of PCDTBT: PC71BM based photovoltaic devices with and without DMSO-treatment under light irradiation are summarized in Table 1. And the corresponding J–V curves of the devices are plotted in Fig. 3a. The Jsc measured without any mask for the DMSO treated device is 26.20 mA/cm2, the FF is 39% and PCE reaches 9.54%. While when we used a shadow mask to cover the non-active area in the measurement, the Jsc decreases to 10.63 mA/cm2 and the FF rises to 65% and PCE reduces to 6.52%, respectively. But the measurements deviation is
not obvious for the pristine device (about 2% between the PCE of 6.07% without a mask and 5.95% with a mask). To verify the accurate value, external quantum efficiency (EQE) of the devices was carried out. The results are shown in Fig. 3b. The respective Jsc values calculated by integrating the EQE data with an AM 1.5G reference spectrum are 9.8 mA/cm2 and 10.5 mA/cm2 for untreated and treated devices. These estimated Jsc values are consistent with those obtained from J–V curves under masks (9.90 mA/cm2 and 10.63 mA/cm2), indicating that the results tested without mask were overestimated especially for the DMSO-treated devices. This is probably because that for the solvent treated device, the conductivity of PEDOT: PSS becomes so high that charge carriers can easily transport laterally from the adjacent region to the effective device area cladded with perpendicular top and bottom electrodes. The mechanism is illustrated in Scheme 2. Comparing the data obtained with different measurements, we can see that with the increment in Jsc the FF drops accordingly. We infer that the Jsc together with the FF can be a reflection of the amounts of the effective charge carriers which are successfully collected by the electrodes. To make it simple, the product of the Jsc and FF can be readily assumed to be proportional to the areas where the effective charge carriers transport. Based on this assumption, we can figure out the critical distance of the outmost effective charge carriers travelling across to the effective area, namely the value of b in Scheme 2. The calculated value of b is 1.50 103 cm for the pristine device and 2.16 102 cm for the DMSO-treated device. The distance of the extra charges transporting to the effective area in the
Table 1 Device parameters of the PSCs based on PCDTBT: PC71BM under the illumination of AM1.5G, 100 mA/cm2. Devices
Mask
Voc (V)
Jsc (mA/cm2)
FF (%)
PCEbest (PCEave) (%)a
Pristine device Pristine device DMSO-device DMSO-device
w/o w w/o w
0.90 0.91 0.91 0.91
11.08 9.90 26.20 10.63
58 63 39 65
6.07 5.95 9.54 6.52
a
The values in parentheses are the average PCE obtained from over 20 devices and w/o is short for without and w for with.
(6.03) (5.87) (9.42) (6.38)
M. Wang et al. / Solar Energy 129 (2016) 175–183
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Fig. 3. (a) J–V characteristics of PCDTBT: PC71BM based photovoltaic devices without (black) and with (red) DMSO-treatment under light irradiation measured without (open) and with (solid) a mask. The inset figure shows the J–V characteristics on a larger scale of Jsc. (b) EQE spectra of PCDTBT: PC71BM based devices without and with DMSO treatment. (c) Dark J–V characteristics of PCDTBT: PC71BM based devices without and with DMSO treatment. (d) Transmittance spectra of the hole transport layers (HTL) and ITO/HTLs without and with DMSO treatment. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Scheme 2. Illustration of the different measurements without and with a mask.
DMSO device is one order higher than that in the pristine device. These results demonstrate that with DMSO-treated PEDOT: PSS layer, the charge carriers will transport more easily from larger area to the effective area resulting in more severely overestimated current density and efficiency. While as the Jsc increases, the FF decreases significantly
without mask and this is probably because with low resistance charge density inside the device increases significantly, as a result, more recombination events are induced. But this explanation is rather empirical, further investigation of this phenomena is still ongoing in our study. Here, to assure the accuracy of the measurement
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we surely applied a shadow mask to cover the extra area so that the lateral electrical conduction effect can be avoided. It can be observed that the pristine device exhibits a PCE of 5.95% with a Jsc of 9.90 mA/cm2 and a FF of 63%. While the DMSO treated device still performs better with a Jsc of 10.63 mA/cm2, a FF of 65% and a PCE of 6.52% measured under a mask. Such a significant improvement in the PCE is induced by the simultaneous increase of Jsc and FF, two of the critical factors that determine the efficiency of the PSCs. As we have known, the Voc is related to the difference between the HOMO of the donor and the LUMO of the acceptor (Heeger, 2014), the relationship was given in Eq. (1) Scharber et al. (2006). Voc ¼ ð1=eÞ EDonor HOMO EPCBM LUMO 0:3 V ð1Þ As is shown in Scheme 1c, the highest occupied molecular orbital (HOMO) of PCDTBT and the lowest unoccupied molecular orbital (LUMO) of PC71BM are 5.5 eV and 4.3 eV, respectively. It is worth noting that there always exists a ca. 0.3 V recombination loss in the voltage value (Street et al., 2010). According to the above explanation the theoretical Voc is 0.9 V consistent well with the experimental value. The Voc is maintained (0.90 ± 0.01 V) before and after the DMSO treatment which indicates that spin-casting DMSO between the PETOT: PSS layer and active layer has no change on the energy alignment of the materials. This can also be confirmed by the dark current versus bias voltage characteristics shown in Fig. 3c. In dark, the two devices exhibit a similar turnon voltage of about 0.9 V, implying that the upper limits of the attainable Voc in both devices are fairly identical. Generally, two aspects of reason can be attributed to the increase in Jsc: (a) enhancement of the light-harvesting ability; (b) improvement in charge transportation and collection efficiency. Fig. 3d is the light transmittance spectra of hole transport layers with and without solvent treatment. Both DMSO treated layers exhibited relatively
Table 2 Summary of the mobility values for PCDTBT: PC71BM blend films without and with DMSO-treatment. DMSO-treatment w/o w
lh (cm2/Vs) 5
7.3 10 3.8 104
le (cm2/Vs) 1.9 104 2.4 104
lower transmission across the visible region, coupled with the EQE measurements, demonstrating that light absorption of the device was enhanced by the DMSO treatment. To gain further insight into the carrier transporting properties among the bulk heterojunction (BHJ) films, both hole-only devices and electron-only devices were fabricated. Hole mobility and electron mobility of PCDTBT: PC71BM (1:4 w/w) single charge carrier devices were estimated via space-charge-limited current (SCLC) model and the related J–V characteristics are found in Fig. 4. The detailed hole-mobility (lh) and electron-mobility (le) extracted from the single charge carriers devices based on Mott–Gurney law are listed in Table 2. At a typical electric field of 6.7 104 V/cm (related to an applied voltage of 1 V across the blend film of 150 nm), the hole mobility for the devices that undergone solvent treatment (3.8 104 cm2/Vs) is five times higher than that without treatment (7.3 105 cm2/Vs). While for the untreated and treated electron-only devices, the electron mobilities are comparable (1.9 104 and 2.4 104 cm2/Vs). The slight increase in electron mobility may be due to the optimized ohmic contact between the anode and active layer by the residual solvent. Both increases in hole and electron mobilities achieving a more balanced transport confirm that the hole injection and charge carriers transporting efficiency are improved after the modification. As is commonly recognized, unbalanced charge transport will cause more charge recombination in the bulk heterojunction films and conduce to low FF and PCE of the PSC (Abbas and Tekin, 2012; Ebenhoch et al., 2015). Therefore we can infer that the higher mobilities and more balanced charge transport should be responsible for the enhanced FF and photovoltaic performance of the PSCs.
Fig. 4. J–V characteristics of the (a) hole-only and (b) electron-only diodes of PCDTBT: PC71BM blend films (1:4, w/w) with or without DMSO treatment.
M. Wang et al. / Solar Energy 129 (2016) 175–183
Moreover, since the charge transport property is quite dependent on the nanomorphology of the photoactive layer (Yang et al., 2005; Pivrikas et al., 2011), the surface morphology of PEDOT: PSS films and active layer films were studied with tapping mode AFM. Fig. 5 shows height and phase AFM images of the spin coated films with and without DMSO treatment. Fig. 5b depicts a typical image of PEDOT: PSS film, in which the bright and dark phase correspond to the PEDOT-rich and PSS-rich grains accordingly (Hsiao et al., 2008; Lang et al., 2009; Sun et al., 2007). From the images of the PEDOT: PSS films, we can see distinct agglomeration of PSS grains in pristine samples while in the solvent treated films, the accumulated PSS grains were diminished apparently. The root-mean square (rms) for the solvent treated PEDOT: PSS film is 1.04 nm, slightly higher than that for the as cast film (0.89 nm). It is indicated that upon the removal of PSS grains, the PEDOT nanocrystal size grows and the roughness of the surface increases simultaneously. It is known
Fig. 5. AFM height images (a, c, e, g) and phase images (b, d, f, h) of the films: PEDOT: PSS films without (a, b) and with (c, d) DMSO-treatment; PCDTBT: PC71BM BHJ active layer spin coated on top of untreated PEDOT: PSS (e, f) and solvent-treated PEDOT: PSS (g, h).
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that the increase in the PEDOT nanocrystal size will enhance the close packing and uniform ordering of PEDOT nanocrystals (Thomas et al., 2014) and reducing the intrinsic disorder within the PEDOT-rich regions is favorable for the charge transporting property as well as the conductivity (Sangeeth et al., 2009). According to the morphology of PCDTBT: PC71BM active layers spin coated on the untreated and treated PEDOT: PSS, slight variation in the roughness was also observed. The surface roughness increases slightly from 0.40 nm for the active layer with untreated PEDOT: PSS to 0.44 nm for that with solvent treated PEDOT: PSS. The variation in the roughness of the active layers matches well with that of the PEDOT: PSS layers. These results demonstrate that with the enhancement of PEDOT: PSS nanocrystals, the phase separation in active layer becomes better which facilitates charge transfer across the active layer and benefits the solar cell properties. Finally, to explore the universal utility of this method, another commonly used donor polymer P3HT was employed and the DMSO-treatment effect on P3HT: PC71BM (1:1, w/w, 3% DIO) PSCs were investigated. Fig. 6 shows the J–V characteristics of P3HT: PC71BM photovoltaic devices without and with DMSO-treatment and the corresponding parameters are given in Table 3. As we can see, the DMSO-treated device exhibits a PCE of 4.40% with a Jsc of 10.18 mA/cm2 and a FF of 69% whereas the control device shows a lower PCE of 4.11% with Jsc of 9.77 mA/cm2 and FF of 67%, respectively. The impedance spectra (Fig. S1) show that the solvent treated device possesses lower series resistance in consistent with the increased Jsc compared with the pristine device. Fig. S2 shows that the hole mobility of the post-modified device was higher than that of the control device resulting in the improvement in FF. The same phenomenon has been observed in PCDTBT: PC71BM system. The AFM results (Fig. S3) show that the surface of the DMSO-involved BHJ film was much rougher than the as-cast film producing
Fig. 6. J–V characteristics of P3HT: PC71BM based photovoltaic devices without and with DMSO treatment under light irradiation measured under mask.
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Table 3 Device parameters of the PSCs based on P3HT: PC71BM with and without DMSO treatment under the illumination of AM1.5G, 100 mA/cm2. Devices
Voc (V)
Jsc (mA/cm2)
FF (%)
PCEbest (PCEave)(%)a
Pristine-device DMSO-device
0.60 0.60
9.77 10.18
67 69
4.11 (4.03) 4.40 (4.31)
a
The average PCE is obtained from over 10 devices.
beneficial pathways for charge transportation and collection. Hence, we believe that post-treating PEDOT: PSS layer with DMSO solvent can be a general way to improve the performance of PSCs. 4. Conclusions In conclusion, a post-modification method for hole transport layer to improve performance of polymer solar cells was reported. By modifying the surface of the PEDOT: PSS layer with DMSO solvent, the overall PCE of PCDTBT: PC71BM based solar cell was improved from 5.95% to 6.52%. With this process, hole injection and charge transport were improved a lot. It is considered that spin-coating a drop of DMSO solvent between the PEDOT: PSS and photoactive layer can be an easy and effective way to improve photovoltaic property. And we also suggest that when we apply highly conductive PEDOT: PSS layer to fabricate solar cells careful measurement should be taken to assure the accuracy of the measurement. Acknowledgements The authors gratefully acknowledge the financial support by the National Natural Science Foundation of China (Grant Nos. 11374070, 61327009, and 21432005). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.solener.2016.02.003. References Abbas, M., Tekin, N., 2012. Balanced charge carrier mobilities in bulk heterojunction organic solar cells. Appl. Phys. Lett. 101. Bisquert, J., 2002. Theory of the impedance of electron diffusion and recombination in a thin layer. J. Phys. Chem. B 106, 325–333. Blouin, N., Michaud, A., Leclerc, M., 2007. A low-bandgap poly(2,7carbazole) derivative for use in high-performance solar cells. Adv. Mater. 19, 2295–2300. Chen, J.W., Cao, Y., 2009. Development of novel conjugated donor polymers for high-efficiency bulk-heterojunction photovoltaic devices. Acc. Chem. Res. 42, 1709–1718. Cheng, Y.J., Yang, S.H., Hsu, C.S., 2009. Synthesis of conjugated polymers for organic solar cell applications. Chem. Rev. 109, 5868– 5923. Dennler, G., Scharber, M.C., Brabec, C.J., 2009. Polymer-fullerene bulkheterojunction solar cells. Adv. Mater. 21, 1323–1338.
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ScienceDirect Solar Energy 129 (2016) 175–183 www.elsevier.com/locate/solener
Enhanced polymer solar cells efficiency by surface coating of the PEDOT: PSS with polar solvent Min Wang a,b, Manxi Zhou a, Lei Zhu a, Qifang Li a,⇑, Chao Jiang b,⇑ a b
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Key Laboratory of Standardization and Measurement for Nanotechnology, National Center for Nanoscience and Technology, Chinese Academy of Sciences, Beijing 100190, China Received 18 August 2015; received in revised form 9 November 2015; accepted 2 February 2016
Communicated by: Associate Editor Sam-Shajing Sun
Abstract Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) films were post-treated with polar solvent dimethylsulfoxide (DMSO) by spin-coating method and polymer solar cells (PSCs) based on poly [N-900 ]-hepta-decanyl-2,7-carbazole-alt-5,5-(40 ,70 -di-2-thie nyl-20 ,10 ,30 -benzothiadiazole) (PCDTBT) : [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) were fabricated to investigate the effect of the treatment. By post-modifying the PEDOT: PSS layer, the conductivity of the PEDOT: PSS film was largely improved. Using electrochemical impedance spectroscopy (IS) we observed that the series resistance of the device decreased greatly after the treatment. With DMSO-treated PEDOT: PSS transport layer, the power conversion efficiency (PCE) of the PSC based on PCDTBT: PC71BM raised from 5.95% to 6.52% with both increase in Jsc and FF. We systematically studied charge transport property via space-chargelimited-current (SCLC) and our results suggest that the increment in device efficiency can be attributed to the increased hole-mobility and thus more balanced charge transport benefits the enhancement of polymer solar cell efficiency. We also noted that if measured without a shadow mask much more overestimation will take place in the DMSO-treated device as a result of lateral electrical conduction. We suggest that when we apply the highly conductive PEDOT: PSS layer in the PSCs, careful measurement should be carried out to avoid inaccuracy. Ó 2016 Elsevier Ltd. All rights reserved.
Keywords: Polymer solar cell; PEDOT: PSS; Dimethylsulfoxide; Charge transport
1. Introduction Over the past decades, polymer solar cells have attracted remarkable attention due to their advantages of low cost, light weight and good mechanical flexibility as well as easy processability (Hoppe and Sariciftci, 2004; Dennler et al., 2009; Li et al., 2012). And extensive efforts have been
⇑ Corresponding authors.
E-mail addresses: qfl[email protected] (Q. Li), [email protected] (C. Jiang). http://dx.doi.org/10.1016/j.solener.2016.02.003 0038-092X/Ó 2016 Elsevier Ltd. All rights reserved.
directed toward this field from designing and synthesizing novel polymers (Chen and Cao, 2009; Cheng et al., 2009; Gunes et al., 2007; Scharber et al., 2006) to optimizing the fabrication of the devices (Krebs, 2009; Kim et al., 2006). Hitherto, power conversion efficiency of PSCs in laboratory scale has exceeded 10% paving a promising way for future commercialization (You et al., 2013). PEDOT: PSS, an electro-conductive polymer solution, has been widely used as a hole transport layer (HTL) in solar cell devices because of its high transparency in the visible region, good thermal stability, high mechanical
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stability, strong hole affinity and high work function (Lim et al., 2014). However, the PEDOT: PSS naturally suffers from low conductivity (less than 1 S/cm) and thus inevitably limits the performance of the PSCs (Kim et al., 2011). Several methods have been implemented to promote its conductivity including: the addition of a suitable percentage of solvents such as surfactant (Fang et al., 2011), glycerol (Lee et al., 2009), ethylene glycol (Li et al., 2015), dimethyl sulfoxide (Thomas et al., 2014), acids (Song et al., 2013; Xia and Ouyang, 2012), graphene oxide (Yang et al., 2015) and isopropanol (Zhang et al., 2014); immersing the films in a polar organic solvent like ethylene glycol (Kim et al., 2012) and DMSO (Unsworth et al., 2014). Through these approaches, the conductivity of PEDOT: PSS was successfully increased by up to 2 or 3 orders of magnitude. It is well accepted that PEDOT: PSS aqueous dispersions are of low conductivity because the PSS chains are rather insulating (Lim, 2013). Special attention has been paid to the variation of the PEDOT: PSS, but rare of the literature discuss about charge transport property in detail and the mechanism about how each method affects the device is still under debate. In this article, we adapted the polar organic solvent DMSO to modify the prepared PEDOT: PSS films by spin-coating and successfully improved their conductivity. As a consequence, we obtained enhanced power conversion efficiency of PCDTBT: PC71BM based solar cells from 5.95% to 6.52%, with simultaneous increase in short current density (Jsc) and fill factor (FF). We also measured charge carriers’ mobility via SCLC and systematically investigated the transporting mechanism with this treatment. 2. Experimental 2.1. Materials and reagents PEDOT: PSS aqueous solution (Clevious P VP AI 4083) was purchased from H. C. Starck. (Germany). PC71BM (purity 99.5%) was acquired from American Dye Inc. and used as received. PCDTBT (Mn = 95 kDa; Mw = 152 kDa; PDI = 1.6) was synthesized ourselves according to the literature (Blouin et al., 2007). All other reagents and chemicals were bought from Sigma–Aldrich and used without further purification. Patterned Indium Tin Oxide (ITO) glass (15 mm 15 mm, 10 X/sq) substrates were received from Nanbo Co. Ltd. (China). 2.2. Preparation and characterization of the PEDOT: PSS films First, ITO substrates were sequential ultrasonic cleaned in detergent, acetone, isopropanol, deionized water and alcohol. And they were dried under nitrogen followed by an ultraviolet-ozone treatment to remove residual organics. PEDOT: PSS solution was filtered through a 0.45 lm PTFE filter and spin coated on the ITO glass at
3500 rpm for 40 s with a thickness of 45 nm approximately. These substrates were then annealed at 150 °C for 30 min on a hot plate in ambient atmosphere. The samples were transferred to a glove box for subsequent procedures. Solvent treatment was performed by spin-casting 25 ll DMSO onto the prepared PEDOT: PSS film at 3000 rpm for 40 s. The films were dried at 100 °C for 15 min. All films were allowed to cool down to room temperature before following processes were performed. The surface roughness and morphology images of the films were characterized using atomic force microscopic (AFM; Veeco Dimension 3100) by tapping mode. The thickness of the PEDOT: PSS films was estimated with an Alpha D-120 stylus profilometer (KLA, Tencor, USA) 2.3. Fabrication and characterization of polymer solar cells The mixture of PCDTBT:PC71BM (1:4, wt%) at a concentration of 7 mg/ml for polymer in orthodichlorobenzene (o-DCB) pre-stirred at 50 °C for eight hours was spin coated at 1500 rpm for 60 s both onto the treated and untreated PEDOT: PSS layer, respectively. The prepared films were thermal annealed at 70 °C for 10 min in the glove box. Two narrow sides (about 1 mm) on both edge of the film were wiped off using methanolwetted swab to allow the contact of ITO and Ca/Al electrodes. The samples were brought into an evaporate chamber afterwards and a 20 nm calcium (Ca) layer followed by a 100 nm aluminum (Al) electrode was deposited on top of the photoactive layer at a base pressure of 106 mbar. The evaporation thickness was controlled by SQC-310C deposition controller (INFICON, Germany). Four devices were fabricated on one substrate and the effective area of each device was 4 mm2 defined by a shadow mask. The current–voltage (J–V) curves were measured with a Keithley 2400 Source-measure unit under the illumination of AM 1.5 G irradiation (100 mW/cm2) using a 150 W solar simulator (Oriel 91159A, Newport, USA) in ambient air. The light intensity was calibrated by a standardized monosilicon cell (Oriel PN 91150V, Newport, USA) prior to the measurements. A 2 mm 2 mm shadow mask was utilized to cover the active area to avoid interference from scattering light and adjacent current leakage. The monochromatic incident photon-to-electron conversion efficiency (IPCE) spectra was obtained using a 500 W Xe lamp-based solar simulator (Oriel lamp 74001) with a lock-in amplifier under short-circuit conditions. A calibrated silicon (Si) photodiode (PRL-12, Newport, USA) was used as a reference. The transmittance spectra of the PEDOT: PSS films were taken with a lambda-950 (Perkin Elmer, USA) spectrophotometer. The impedance spectroscopy (IS) measurement was performed using a Zahner Zennium 40562 electrochemical workstation. X-ray photoelectron spectroscopy (XPS) (Model. ESCALAB250Xi) was applied to measure the sulfur S2p core-level spectra of the PEDOT: PSS films coated on Si/SiO2 substrates.
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2.4. Fabrication and characterization of SCLC devices The hole-only devices were fabricated with ITO/ PEDOT: PSS/PCDTBT: PC71BM/MoO3/Ag structures applying the same processes as the fabrication of photovoltaic devices and the PEDOT: PSS films were untreated and treated with DMSO solvent for comparison. For the electron-only devices, the structures were Al/PCDTBT: PC71BM/Al and here the Al patterned substrates were also untreated and treated with DMSO respectively. The spacecharge-limited current (SCLC) mobility was calculated according to the Mott-Gurney square law J = 9/8 ere0 l(V2/L3), where J is the current density, er is relative permittivity of the polymer, e0 is the vacuum permittivity, l is the hole mobility, V is the applied voltage and L is the thickness of the active layer. The current–voltage (J–V) curves were measured with Keithley 2400 under dark. 3. Results and discussions Scheme 1 presents the chemical structure of materials used in this study and the schematic diagram of solar cell configuration and the energy band diagram. The cell architecture (Scheme 1b) shows the interlayer-like DMSO spin coated between the PEDOT: PSS and active layer. Actually, little of DMSO was left in the device after the evaporation. To evaluate the washing effect of DMSO solvent on the surface of PEDOT: PSS films, we have conducted X-ray photoemission spectroscopy (XPS) studies. Typical S2p core-level spectra of the PEDOT: PSS films before and after DMSO treatment are shown in Fig. 1. The XPS spectra for both samples exhibit high intensity between
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168.0 and 168.9 eV, which is corresponding to the spinsplit components of sulfur atoms of the PSS chains. While between 163.9 and 165.6 eV, we can see another two small peaks for both spectra which are assigned to the sulfur atoms of the PEDOT fragments. These results are in good agreement with the previous report by Schaarschmidt et al. (2009). The XPS spectrum is constrained by the S 2p3/2-S 2p1/2 spin-orbit separation being 1.2 eV, and the intensity ratio of the two peaks of each doublet being 2:1 (Greczynski et al., 2001, 1999). The ratio of the S 2p3/2 areas for PSS (stronger) and PEDOT (weaker) can be used to estimate the content of remaining PSS on the surface. It is calculated that the PSS/PEDOT composition ratio is 8.8 for the pristine sample but drops to 3.75 for that treated with DMSO solvent suggesting that large amount of the insulating PSS has been successfully removed from the PEDOT: PSS films leading to a significant increase in electrical conductivity. To better understand the effect of surface coating of DMSO on PEDOT: PSS layer, the impedance of whole PCDTBT: PC71BM solar cells was carried out in a frequency ranging from 1 MHz to 1 Hz under dark condition. Fig. 2 shows the electrochemical impedance spectra for PCDTBT: PC71BM solar cells with both treated and untreated PEDOT: PSS transport layer, respectively. We can see that the impedance responses of the devices are semicircles in the complex plane giving information about the charge transfer and recombination resistance (Bisquert, 2002). For the pristine devices, the real impedance spectrum is 1217 X, whereas the one treated with DMSO solvent shows the real impedance spectrum of 700 X, a 42% decrease in comparison. The lowered impedance suggests that the charge transfer resistance among
Scheme 1. Chemical structure of the materials (a), the configuration of the solar cell (b) and the energy diagram of the device (c).
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Fig. 1. XPS S2p spectra of PEDOT: PSS film before (a) and after (b) DMSO treatment.
Fig. 2. Impedance spectra for PCDTBT: PC71BM solar cells without and with DMSO treatment measured in the dark.
the device is reduced due to the solvent treatment on PEDOT: PSS. Parameters of PCDTBT: PC71BM based photovoltaic devices with and without DMSO-treatment under light irradiation are summarized in Table 1. And the corresponding J–V curves of the devices are plotted in Fig. 3a. The Jsc measured without any mask for the DMSO treated device is 26.20 mA/cm2, the FF is 39% and PCE reaches 9.54%. While when we used a shadow mask to cover the non-active area in the measurement, the Jsc decreases to 10.63 mA/cm2 and the FF rises to 65% and PCE reduces to 6.52%, respectively. But the measurements deviation is
not obvious for the pristine device (about 2% between the PCE of 6.07% without a mask and 5.95% with a mask). To verify the accurate value, external quantum efficiency (EQE) of the devices was carried out. The results are shown in Fig. 3b. The respective Jsc values calculated by integrating the EQE data with an AM 1.5G reference spectrum are 9.8 mA/cm2 and 10.5 mA/cm2 for untreated and treated devices. These estimated Jsc values are consistent with those obtained from J–V curves under masks (9.90 mA/cm2 and 10.63 mA/cm2), indicating that the results tested without mask were overestimated especially for the DMSO-treated devices. This is probably because that for the solvent treated device, the conductivity of PEDOT: PSS becomes so high that charge carriers can easily transport laterally from the adjacent region to the effective device area cladded with perpendicular top and bottom electrodes. The mechanism is illustrated in Scheme 2. Comparing the data obtained with different measurements, we can see that with the increment in Jsc the FF drops accordingly. We infer that the Jsc together with the FF can be a reflection of the amounts of the effective charge carriers which are successfully collected by the electrodes. To make it simple, the product of the Jsc and FF can be readily assumed to be proportional to the areas where the effective charge carriers transport. Based on this assumption, we can figure out the critical distance of the outmost effective charge carriers travelling across to the effective area, namely the value of b in Scheme 2. The calculated value of b is 1.50 103 cm for the pristine device and 2.16 102 cm for the DMSO-treated device. The distance of the extra charges transporting to the effective area in the
Table 1 Device parameters of the PSCs based on PCDTBT: PC71BM under the illumination of AM1.5G, 100 mA/cm2. Devices
Mask
Voc (V)
Jsc (mA/cm2)
FF (%)
PCEbest (PCEave) (%)a
Pristine device Pristine device DMSO-device DMSO-device
w/o w w/o w
0.90 0.91 0.91 0.91
11.08 9.90 26.20 10.63
58 63 39 65
6.07 5.95 9.54 6.52
a
The values in parentheses are the average PCE obtained from over 20 devices and w/o is short for without and w for with.
(6.03) (5.87) (9.42) (6.38)
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Fig. 3. (a) J–V characteristics of PCDTBT: PC71BM based photovoltaic devices without (black) and with (red) DMSO-treatment under light irradiation measured without (open) and with (solid) a mask. The inset figure shows the J–V characteristics on a larger scale of Jsc. (b) EQE spectra of PCDTBT: PC71BM based devices without and with DMSO treatment. (c) Dark J–V characteristics of PCDTBT: PC71BM based devices without and with DMSO treatment. (d) Transmittance spectra of the hole transport layers (HTL) and ITO/HTLs without and with DMSO treatment. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Scheme 2. Illustration of the different measurements without and with a mask.
DMSO device is one order higher than that in the pristine device. These results demonstrate that with DMSO-treated PEDOT: PSS layer, the charge carriers will transport more easily from larger area to the effective area resulting in more severely overestimated current density and efficiency. While as the Jsc increases, the FF decreases significantly
without mask and this is probably because with low resistance charge density inside the device increases significantly, as a result, more recombination events are induced. But this explanation is rather empirical, further investigation of this phenomena is still ongoing in our study. Here, to assure the accuracy of the measurement
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we surely applied a shadow mask to cover the extra area so that the lateral electrical conduction effect can be avoided. It can be observed that the pristine device exhibits a PCE of 5.95% with a Jsc of 9.90 mA/cm2 and a FF of 63%. While the DMSO treated device still performs better with a Jsc of 10.63 mA/cm2, a FF of 65% and a PCE of 6.52% measured under a mask. Such a significant improvement in the PCE is induced by the simultaneous increase of Jsc and FF, two of the critical factors that determine the efficiency of the PSCs. As we have known, the Voc is related to the difference between the HOMO of the donor and the LUMO of the acceptor (Heeger, 2014), the relationship was given in Eq. (1) Scharber et al. (2006). Voc ¼ ð1=eÞ EDonor HOMO EPCBM LUMO 0:3 V ð1Þ As is shown in Scheme 1c, the highest occupied molecular orbital (HOMO) of PCDTBT and the lowest unoccupied molecular orbital (LUMO) of PC71BM are 5.5 eV and 4.3 eV, respectively. It is worth noting that there always exists a ca. 0.3 V recombination loss in the voltage value (Street et al., 2010). According to the above explanation the theoretical Voc is 0.9 V consistent well with the experimental value. The Voc is maintained (0.90 ± 0.01 V) before and after the DMSO treatment which indicates that spin-casting DMSO between the PETOT: PSS layer and active layer has no change on the energy alignment of the materials. This can also be confirmed by the dark current versus bias voltage characteristics shown in Fig. 3c. In dark, the two devices exhibit a similar turnon voltage of about 0.9 V, implying that the upper limits of the attainable Voc in both devices are fairly identical. Generally, two aspects of reason can be attributed to the increase in Jsc: (a) enhancement of the light-harvesting ability; (b) improvement in charge transportation and collection efficiency. Fig. 3d is the light transmittance spectra of hole transport layers with and without solvent treatment. Both DMSO treated layers exhibited relatively
Table 2 Summary of the mobility values for PCDTBT: PC71BM blend films without and with DMSO-treatment. DMSO-treatment w/o w
lh (cm2/Vs) 5
7.3 10 3.8 104
le (cm2/Vs) 1.9 104 2.4 104
lower transmission across the visible region, coupled with the EQE measurements, demonstrating that light absorption of the device was enhanced by the DMSO treatment. To gain further insight into the carrier transporting properties among the bulk heterojunction (BHJ) films, both hole-only devices and electron-only devices were fabricated. Hole mobility and electron mobility of PCDTBT: PC71BM (1:4 w/w) single charge carrier devices were estimated via space-charge-limited current (SCLC) model and the related J–V characteristics are found in Fig. 4. The detailed hole-mobility (lh) and electron-mobility (le) extracted from the single charge carriers devices based on Mott–Gurney law are listed in Table 2. At a typical electric field of 6.7 104 V/cm (related to an applied voltage of 1 V across the blend film of 150 nm), the hole mobility for the devices that undergone solvent treatment (3.8 104 cm2/Vs) is five times higher than that without treatment (7.3 105 cm2/Vs). While for the untreated and treated electron-only devices, the electron mobilities are comparable (1.9 104 and 2.4 104 cm2/Vs). The slight increase in electron mobility may be due to the optimized ohmic contact between the anode and active layer by the residual solvent. Both increases in hole and electron mobilities achieving a more balanced transport confirm that the hole injection and charge carriers transporting efficiency are improved after the modification. As is commonly recognized, unbalanced charge transport will cause more charge recombination in the bulk heterojunction films and conduce to low FF and PCE of the PSC (Abbas and Tekin, 2012; Ebenhoch et al., 2015). Therefore we can infer that the higher mobilities and more balanced charge transport should be responsible for the enhanced FF and photovoltaic performance of the PSCs.
Fig. 4. J–V characteristics of the (a) hole-only and (b) electron-only diodes of PCDTBT: PC71BM blend films (1:4, w/w) with or without DMSO treatment.
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Moreover, since the charge transport property is quite dependent on the nanomorphology of the photoactive layer (Yang et al., 2005; Pivrikas et al., 2011), the surface morphology of PEDOT: PSS films and active layer films were studied with tapping mode AFM. Fig. 5 shows height and phase AFM images of the spin coated films with and without DMSO treatment. Fig. 5b depicts a typical image of PEDOT: PSS film, in which the bright and dark phase correspond to the PEDOT-rich and PSS-rich grains accordingly (Hsiao et al., 2008; Lang et al., 2009; Sun et al., 2007). From the images of the PEDOT: PSS films, we can see distinct agglomeration of PSS grains in pristine samples while in the solvent treated films, the accumulated PSS grains were diminished apparently. The root-mean square (rms) for the solvent treated PEDOT: PSS film is 1.04 nm, slightly higher than that for the as cast film (0.89 nm). It is indicated that upon the removal of PSS grains, the PEDOT nanocrystal size grows and the roughness of the surface increases simultaneously. It is known
Fig. 5. AFM height images (a, c, e, g) and phase images (b, d, f, h) of the films: PEDOT: PSS films without (a, b) and with (c, d) DMSO-treatment; PCDTBT: PC71BM BHJ active layer spin coated on top of untreated PEDOT: PSS (e, f) and solvent-treated PEDOT: PSS (g, h).
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that the increase in the PEDOT nanocrystal size will enhance the close packing and uniform ordering of PEDOT nanocrystals (Thomas et al., 2014) and reducing the intrinsic disorder within the PEDOT-rich regions is favorable for the charge transporting property as well as the conductivity (Sangeeth et al., 2009). According to the morphology of PCDTBT: PC71BM active layers spin coated on the untreated and treated PEDOT: PSS, slight variation in the roughness was also observed. The surface roughness increases slightly from 0.40 nm for the active layer with untreated PEDOT: PSS to 0.44 nm for that with solvent treated PEDOT: PSS. The variation in the roughness of the active layers matches well with that of the PEDOT: PSS layers. These results demonstrate that with the enhancement of PEDOT: PSS nanocrystals, the phase separation in active layer becomes better which facilitates charge transfer across the active layer and benefits the solar cell properties. Finally, to explore the universal utility of this method, another commonly used donor polymer P3HT was employed and the DMSO-treatment effect on P3HT: PC71BM (1:1, w/w, 3% DIO) PSCs were investigated. Fig. 6 shows the J–V characteristics of P3HT: PC71BM photovoltaic devices without and with DMSO-treatment and the corresponding parameters are given in Table 3. As we can see, the DMSO-treated device exhibits a PCE of 4.40% with a Jsc of 10.18 mA/cm2 and a FF of 69% whereas the control device shows a lower PCE of 4.11% with Jsc of 9.77 mA/cm2 and FF of 67%, respectively. The impedance spectra (Fig. S1) show that the solvent treated device possesses lower series resistance in consistent with the increased Jsc compared with the pristine device. Fig. S2 shows that the hole mobility of the post-modified device was higher than that of the control device resulting in the improvement in FF. The same phenomenon has been observed in PCDTBT: PC71BM system. The AFM results (Fig. S3) show that the surface of the DMSO-involved BHJ film was much rougher than the as-cast film producing
Fig. 6. J–V characteristics of P3HT: PC71BM based photovoltaic devices without and with DMSO treatment under light irradiation measured under mask.
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Table 3 Device parameters of the PSCs based on P3HT: PC71BM with and without DMSO treatment under the illumination of AM1.5G, 100 mA/cm2. Devices
Voc (V)
Jsc (mA/cm2)
FF (%)
PCEbest (PCEave)(%)a
Pristine-device DMSO-device
0.60 0.60
9.77 10.18
67 69
4.11 (4.03) 4.40 (4.31)
a
The average PCE is obtained from over 10 devices.
beneficial pathways for charge transportation and collection. Hence, we believe that post-treating PEDOT: PSS layer with DMSO solvent can be a general way to improve the performance of PSCs. 4. Conclusions In conclusion, a post-modification method for hole transport layer to improve performance of polymer solar cells was reported. By modifying the surface of the PEDOT: PSS layer with DMSO solvent, the overall PCE of PCDTBT: PC71BM based solar cell was improved from 5.95% to 6.52%. With this process, hole injection and charge transport were improved a lot. It is considered that spin-coating a drop of DMSO solvent between the PEDOT: PSS and photoactive layer can be an easy and effective way to improve photovoltaic property. And we also suggest that when we apply highly conductive PEDOT: PSS layer to fabricate solar cells careful measurement should be taken to assure the accuracy of the measurement. Acknowledgements The authors gratefully acknowledge the financial support by the National Natural Science Foundation of China (Grant Nos. 11374070, 61327009, and 21432005). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.solener.2016.02.003. References Abbas, M., Tekin, N., 2012. Balanced charge carrier mobilities in bulk heterojunction organic solar cells. Appl. Phys. Lett. 101. Bisquert, J., 2002. Theory of the impedance of electron diffusion and recombination in a thin layer. J. Phys. Chem. B 106, 325–333. Blouin, N., Michaud, A., Leclerc, M., 2007. A low-bandgap poly(2,7carbazole) derivative for use in high-performance solar cells. Adv. Mater. 19, 2295–2300. Chen, J.W., Cao, Y., 2009. Development of novel conjugated donor polymers for high-efficiency bulk-heterojunction photovoltaic devices. Acc. Chem. Res. 42, 1709–1718. Cheng, Y.J., Yang, S.H., Hsu, C.S., 2009. Synthesis of conjugated polymers for organic solar cell applications. Chem. Rev. 109, 5868– 5923. Dennler, G., Scharber, M.C., Brabec, C.J., 2009. Polymer-fullerene bulkheterojunction solar cells. Adv. Mater. 21, 1323–1338.
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