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Study of solvent-doped PEDOT: PSS layer on small molecule organic solar cells
Synthetic Metals 164 (2013) 38–41
Contents lists available at SciVerse ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Study of solvent-doped PEDOT: PSS layer on small molecule organic solar cells Chien-Jung Huang a,∗, Kan-Lin Chen b, Yao-Jen Tsao a, Dei-Wei Chou c, Wen-Ray Chen d, Teen-Hang Meen d a
Department of Applied Physics, National University of Kaohsiung, Nanzih, Kaohsiung, Taiwan Department of Electronic Engineering, Fortune Institute of Technology, Kaohsiung, Taiwan Department of Aviation & Communication Electronics, Air Force Institute of Technology, Kaohsiung, Taiwan d Department of Electronic Engineering, National Formosa University, Hu-Wei, Yunlin, Taiwan b c
a r t i c l e
i n f o
Article history: Received 28 September 2012 Received in revised form 30 November 2012 Accepted 11 December 2012 Available online 24 January 2013 Keywords: Organic solar cell Indium tin oxide PEDOT: PSS Solvent
a b s t r a c t Small molecule organic solar cell with an optimized structure of indium tin oxide (ITO)/poly (3,4-ethylenedioxythioxythiophene): poly (styrenesulfonate) (PEDOT: PSS) doped with three solvents, respectively/copper phthalocyanine (CuPc) (10 nm)/CuPc: fullerene (C60 ) mixed (20 nm)/C60 (20 nm)/bathocuproine (BCP) (10 nm)/Ag was fabricated. In this study, the electrical characterization, morphology and optical quality on photovoltaic devices with different solvent-doped PEDOT: PSS layers were discussed. The addition of solvent results in the decided improvement in short-circuit current (JSC ) and fill factor (FF). It is found that the 3 wt% sorbitol-doped PEDOT: PSS film has the optimal quality. The maximum power conversion efficiency (PCE) of 3.37% was obtained under 1sun standard AM 1.5 solar illumination of 100 mW/cm2 . © 2013 Elsevier B.V. All rights reserved.
1. Introduction Photovoltaic cells have played an important role in renewable energy sources of the next generation. In recent years, organic solar cells (OSC) have attracted attention because of their many advantages compared to inorganic devices, such as low production costs, light weight and flexibility [1]. However, their poor power conversion efficiency (PCE) has remained an obstruction to commercial use. Many researchers have made much of effort to improve the PCE of OSC. Among these improvements, poly (3,4ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT: PSS) has been used as the buffer layer to enhance the efficiency in OSC [2]. PEDOT: PSS has been considered a promising conductive polymer due to its structural stability, optical transparency, and process ability. In spite of its numerous advantages, the low conductivity is still a restriction for application in optoelectronic devices. In order to improve this problem, lots of groups find the solvent-doped PEDOT: PSS films contained superior conductivity [3–6]. The conductivity of the PEDOT: PSS film can be increased by addition of the solvent, such as sorbitol, glycerol and dimethyl sulfoxide (DMSO). The effect of the solvents changes the PEDOT to PSS molar ratio at the surface of the PEDOT: PSS films, resulting in an influence
∗ Corresponding author. Tel.: +886 7 5919475; fax: +886 7 5919357. E-mail address: [email protected] (C.-J. Huang). 0379-6779/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2012.12.008
in the sheet resistance (R ) of the films [4]. The larger the PEDOT ratio at the surface of the films is, the larger the conductivity is [7]. The improvement in conductivity is due to morphological changes rather than chemical changes [4]. And this effect makes an increase in the roughness of PEDOT: PSS films. The increase in the roughness of PEDOT: PSS layer induces defects, which may trap the holes to reduce short-circuit current (JSC ) in OSC. Many groups studied solvent-doped PEDOT: PSS film [8–14], but few groups discussed the effect of solvent-doped PEDOT: PSS layer on small molecule organic solar cells. In this study, the difference and the mechanism of different solvent-doped PEDOT: PSS layers on the performance of OSC will be presented. 2. Experimental The cells are fabricated in this structure ITO/solvent-doped PEDOT: PSS/CuPc/CuPc:C60/ C60/ BCP/Ag, as shown in Fig. 1. All ITOglass substrates with R of 7 / were cleaned by ultrasonic treatment in acetone, isopropyl alcohol and deionized water (5 min each) and dried by high-purity nitrogen blow before deposition. Three solvents were sorbitol (98%), glycerol (99.8%) and DMSO (99.9%). The solvent was added to the PEDOT: PSS (Clevios PH500, H.C. Starck, Germany) directly, and then the doped PEDOT: PSS was stirred for 30 min at room temperature. The doped PEDOT: PSS layer was formed by a spin-coating method. The spin-coatings were performed at a rotation rate of 3500 rpm for 20 s. The doped PEDOT: PSS film was heated at 150 ◦ C for 20 min on a hotplate in ambient
C.-J. Huang et al. / Synthetic Metals 164 (2013) 38–41
39
Fig. 1. Schematic structure of the organic photovoltaic cells.
lab conditions. The organic materials used in the device were CuPc (99%), C60 (99.95%) and BCP (99%), which were without further sublimation. The mixed layer was grown by co-deposition from independent thermal evaporation sources, and the ratio of CuPc: C60 was 1:1. Deposition rate of all organic layers were 0.02 and 0.04 nm/s below the pressure of 4.8 × 10−6 Torr. The sliver (Ag) as a cathode was deposited to a thickness of 100 nm by thermal evaporation through a shadow mask. The active area of OSC was 0.06 cm2 . All thickness of the thin film was monitored by an oscillating quartz thickness monitor. In order to analyze the difference of these solvent-doped PEDOT: PSS films, the electrical characterization, morphology and optical quality of OSC with different solvent-doped PEDOT: PSS layers were measured. The current–voltage characteristics were measured with Keithley 2400 sourcemeter, under an illumination of 100 mW/cm2 with an AM1.5G sun simulator. The surface morphology of doped PEDOT: PSS layer was measured by atomic force microscopy (AFM, Park Systems, XE-70). The R of the doped PEDOT: PSS film was measured by four point probe (SRM103, Solar Energy Tech., Taiwan). The transmittance of the doped PEDOT: PSS film was measured by UV/visible spectrometer (HITACHI, U-3900). And external quantum efficiency (EQE) spectra of the devices were measured by QE measurement system (TEO, QE-3000). 3. Results and discussion The performance parameters of devices with various solvent concentrations and the pristine PEDOT: PSS layer were listed in Table 1. These solvents doped in PEDOT: PSS are sorbitoldoped PEDOT: PSS (referred here as S-PEDOT: PSS), glycerol-doped PEDOT: PSS (referred here as G-PEDOT: PSS) and DMSO-doped PEDOT: PSS (referred here as D-PEDOT: PSS). In Table 1, the PCE of device is extremely low by inserting the pristine PEDOT: PSS layer. On the other hand, the PCE of devices with different solvent-doped PEDOT: PSS layers are dramatically improved. The series resistance (RS ) of device with pristine PEDOT: PSS layer is one hundred times higher than that of solvent-doped PEDOT: PSS layers. The difference of PCE is due to the reason that RS is substantially reduced, resulting in a sharp increase in JSC and fill factor (FF). It was found that the R of PEDOT: PSS layer can be decreased by the addition of solvents [4]. Reduced R makes the holes transport more efficient. However, the key point for a decrease in RS may be attributed to the low R of solvent-doped PEDOT: PSS layer for all devices with the same structure. Fig. 2 shows the R and surface roughness (Rrms ) of the various solvent-doped PEDOT: PSS films. By adding different solvent
Fig. 2. Comparison of the sheet resistance and surface roughness with various (a) sorbitol-doped, (b) glycerol-doped, and (c) DMSO-doped PEDOT: PSS concentrations on the ITO-glass substrate.
concentrations, R and surface roughness of the films were changed. In other words, the R of the film decreases with increasing solvent concentration in the PEDOT: PSS. And the surface roughness increases with increasing solvent concentration. This is due to the fact that the amount of the PEDOT to PSS molar ratio at the surface is increased [3,4], and that more PEDOT, which is a conductive material, will be clustered at the surface of the PEDOT: PSS film after doping solvent. However, this clustered phenomenon
40
C.-J. Huang et al. / Synthetic Metals 164 (2013) 38–41
Fig. 3. AFM images of the PEDOT: PSS layer doped with different organic solvent concentrations coated on the ITO, (a) sorbitol, (b) glycerol, and (c) DMSO.
results in a decrease in R and an increase in roughness of surface. The reduced R was able to enhance carriers transport to electrode. And then the increased surface roughness may make the carriers to be trapped in defects. That is to say, we have to strike a balance between the R and surface morphology to obtain optimal performance. The optimal solvent concentration is 3 wt% S-PEDOT: PSS because of its lowest R and good morphology quality. And the PCE of OSC with 3 wt% S-PEDOT: PSS is up to 3.37% which is higher than that of other devices with different solvent concentrations. It is worth noting that the R and surface roughness of PEDOT: PSS layer were considered the main factors, which influenced the device performance. Further confirmation comes from the AFM micrographs. Fig. 3 shows AFM images of the PEDOT: PSS layer doped with various organic solvents such as sorbitol, glycerol and DMSO. Fig. 3(a) shows AFM images of the PEDOT: PSS layer doped
Table 1 The current–voltage characteristics of the devices with various dopant concentrations. Buffer layer
JSC (mA/cm2 )
VOC (V)
FF
RS ( cm2 )
(%)
Without PEDOT:PSS Pristine PEDOT:PSS 1 wt% S-PEDOT:PSS 2 wt% S-PEDOT:PSS 3 wt% S-PEDOT:PSS 4 wt% S-PEDOT:PSS 5 wt% S-PEDOT:PSS 2 wt% G-PEDOT:PSS 4 wt% G-PEDOT:PSS 6 wt% G-PEDOT:PSS 8 wt% G-PEDOT:PSS 5 wt% D-PEDOT:PSS 10 wt% D-PEDOT:PSS 15 wt% D-PEDOT:PSS 20 wt% D-PEDOT:PSS
5.13 0.30 6.31 14.19 15.45 14.73 10.92 13.22 13.94 13.86 12.80 12.36 12.14 12.89 12.63
0.47 0.51 0.47 0.50 0.51 0.48 0.50 0.47 0.47 0.46 0.47 0.46 0.48 0.48 0.48
0.53 0.24 0.28 0.40 0.42 0.38 0.43 0.37 0.41 0.41 0.38 0.41 0.45 0.44 0.44
14.4 1395 58.8 10.8 10.2 10.8 13.2 16.8 10.56 11.82 14.4 9.66 9.36 9.22 9.46
1.28 0.03 0.83 2.89 3.37 2.74 2.37 2.31 2.70 2.62 2.34 2.35 2.64 2.81 2.67
with different sorbitol concentrations: 0 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt% and 5 wt%, that the root-mean-square roughness (Rrms ) of the PEDOT: PSS layer is estimated to be 1.84 nm, 1.98 nm, 2.33 nm, 2.54 nm, 2.89 nm, and 3.25 nm, respectively. Fig. 3(b) shows AFM images of the PEDOT: PSS layer doped with different glycerol concentrations: 0 wt%, 2 wt%, 4 wt%, 6 wt% and 8 wt%, that the rootmean-square roughness (Rrms ) of the PEDOT: PSS layer is estimated to be 1.84 nm, 3.18 nm, 3.24 nm, 3.54 nm, and 3.59 nm, respectively. Fig. 3(c) shows AFM images of the PEDOT: PSS layer doped with different DMSO concentrations: 0 wt%, 5 wt%, 10 wt%, 15 wt% and 20 wt%, that the root-mean-square roughness (Rrms ) of the PEDOT: PSS layer is estimated to be 1.84 nm, 3.68 nm, 4.09 nm, 4.52 nm, and 5.17 nm, respectively. Fig. 4 shows the transmittance of the PEDOT: PSS films doped with different optimal solvent concentrations on the ITO-glass substrate. Three optimal solvent-doped PEDOT: PSS films are 3 wt% S-PEDOT: PSS, 4 wt% G-PEDOT: PSS and 15 wt% D-PEDOT: PSS, respectively. Among these, the optical quality in the visible region for the film of 3 wt% S-PEDOT: PSS is superior to other solventdoped PEDOT: PSS films. The transmittance spectra of 3 wt% S-PEDOT: PSS was different from spectra of 4 wt% G-PEDOT: PSS and 15 wt% D-PEDOT: PSS which is higher transmittance from 400 nm to 700 nm. The superior transmittance will be beneficial to absorb more photons that may obtain higher JSC . The PCE of the device with 3 wt% S-PEDOT: PSS layer is higher than that of other devices because of its better transmittance in the visible region. However, the performance of doped PEDOT: PSS devices was influenced by transmittance besides R and surface roughness of film. To further verify the optical quality of the doped PEDOT: PSS films, QE measurement system is used to measure the EQE spectra of the devices. Fig. 5 shows the EQE of the devices with different optimal solvent-doped PEDOT: PSS layers. The EQE is defined as the ratio of the number of charge carriers collected by the solar cell to the number of photons of a given energy shining on the
C.-J. Huang et al. / Synthetic Metals 164 (2013) 38–41
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but also excellent transmittance of the anode buffer layer. All the above results show that the JSC and FF depend on the quality of the device with solvent-doped PEDOT: PSS film. The photons which illuminated the active area of OSC were efficiently converted into charges in the device with 3 wt% S-PEDOT: PSS layer because of its good enough quality in R , surface roughness and transmittance. Thus, the cell with 3 wt% S-PEDOT: PSS layer could reveal the best performance. 4. Conclusion
Fig. 4. The transmittance of different solvent-doped PEDOT: PSS layers with optimal concentration on the ITO-glass substrate.
In summary, the device with solvent-doped PEDOT: PSS layer shows the superior PCE of the OSC. Reduced R makes a sharp decrease in RS , which leads to higher JSC and FF. The improvement in R by adding solvent was attributed to the morphology change, i.e., a higher PEDOT to PSS molar ratio at the surface of PEDOT: PSS film. For the device with 3 wt% S-PEDOT: PSS layer, the lowest RS was obtained, resulting from the optimal balance between the R and the surface roughness. Furthermore, the film of S-PEDOT: PSS contains the best optical transmittance in visible region among all solvent-doped PEDOT: PSS films, resulting in the highest JSC to enhance the PCE. However, the PCE of the device with 3 wt% sorbitol-doped PEDOT: PSS layer is up to 3.37%. Acknowledgment This work was partially supported by the National Science Council of Taiwan (NSCT) under Contract no. NSC-101-2221-E-390-021. References
Fig. 5. The EQE of different solvent-doped PEDOT: PSS layers with optimal concentration on the ITO-glass substrate.
solar cell. The value of EQE is related to JSC of the device. It is found that the spectra of EQE were similar to the transmittance spectra. The device with 3 wt% S-PEDOT: PSS layer has higher EQE value from 400 nm to 700 nm in the visible region. The increase in EQE value is attributed to the superior transmittance of 3 wt% S-PEDOT: PSS film, which makes the device absorb more photons. The 3 wt% S-PEDOT: PSS film provides not only decreased sheet resistance
[1] F.C. Kreds, S.A. Gevorgyan, J. Alstrup, Journal of Materials Chemistry 19 (2009) 5442. [2] X. Xi, Q. Meng, F. Li, Y. Ding, J. Ji, Z. Shi, G. Li, Solar Energy Materials and Solar Cells 94 (2010) 623. [3] J. Huang, P.F. Miller, J.S. Wilson, A.J. de Mello, J.C. de Mello, D.D.C. Bradley, Advanced Functional Materials 15 (2005) 290. [4] S.K.M. Jönsson, J. Birgerson, X. Crispin, G. Greczynski, W. Osikowicz, A.W. Denier van der Gon, W.R. Salaneck, M. Fahlman, Synthetic Metals 139 (2003) 1. [5] J. Ouyang, C.-W. Chu, F.-C. Chen, Q. Xu, Y. Yang, Advanced Functional Materials 15 (2005) 203. [6] O.P. Dimitrieva, D.A. Grinko, Y.V. Noskov, N.A. Ogurtsov, A.A. Pud, Synthetic Metals 159 (2009) 2237. [7] A.M. Nardes, R.A.J. Janssen, M. Kemerink, Advanced Functional Materials 18 (2008) 865. [8] M. Cruz-Cruz, M.A. Reyes-Reyes, A.G. Aguilar-Frutis, R. Rodriguez, LópezSandoval, Synthetic Metals 160 (2010) 1501. [9] H.J. Snaith, H. Kenrick, M. Chiesa, R.H. Friend, Polymer Journal 46 (2005) 2573. [10] C. Liu, B. Lu, J. Yan, J. Xu, R. Yue, Z. Zhu, S. Zhou, X. Hu, Z. Zhang, P. Chen, Synthetic Metals 160 (2010) 2481. [11] J.Y. Kim, J.H. Jung, D.E. Lee, J. Joo, Synthetic Metals 126 (2002) 311. [12] H. Yan, H. Okuzaki, Synthetic Metals 159 (2009) 2225. [13] S. Ashizawa, R. Horikawa, H. Okuzaki, Synthetic Metals 153 (2005) 5. [14] H.J. Lee, J. Lee, S.-M. Park, Journal of Physical Chemistry B 114 (2010) 2660.
Contents lists available at SciVerse ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Study of solvent-doped PEDOT: PSS layer on small molecule organic solar cells Chien-Jung Huang a,∗, Kan-Lin Chen b, Yao-Jen Tsao a, Dei-Wei Chou c, Wen-Ray Chen d, Teen-Hang Meen d a
Department of Applied Physics, National University of Kaohsiung, Nanzih, Kaohsiung, Taiwan Department of Electronic Engineering, Fortune Institute of Technology, Kaohsiung, Taiwan Department of Aviation & Communication Electronics, Air Force Institute of Technology, Kaohsiung, Taiwan d Department of Electronic Engineering, National Formosa University, Hu-Wei, Yunlin, Taiwan b c
a r t i c l e
i n f o
Article history: Received 28 September 2012 Received in revised form 30 November 2012 Accepted 11 December 2012 Available online 24 January 2013 Keywords: Organic solar cell Indium tin oxide PEDOT: PSS Solvent
a b s t r a c t Small molecule organic solar cell with an optimized structure of indium tin oxide (ITO)/poly (3,4-ethylenedioxythioxythiophene): poly (styrenesulfonate) (PEDOT: PSS) doped with three solvents, respectively/copper phthalocyanine (CuPc) (10 nm)/CuPc: fullerene (C60 ) mixed (20 nm)/C60 (20 nm)/bathocuproine (BCP) (10 nm)/Ag was fabricated. In this study, the electrical characterization, morphology and optical quality on photovoltaic devices with different solvent-doped PEDOT: PSS layers were discussed. The addition of solvent results in the decided improvement in short-circuit current (JSC ) and fill factor (FF). It is found that the 3 wt% sorbitol-doped PEDOT: PSS film has the optimal quality. The maximum power conversion efficiency (PCE) of 3.37% was obtained under 1sun standard AM 1.5 solar illumination of 100 mW/cm2 . © 2013 Elsevier B.V. All rights reserved.
1. Introduction Photovoltaic cells have played an important role in renewable energy sources of the next generation. In recent years, organic solar cells (OSC) have attracted attention because of their many advantages compared to inorganic devices, such as low production costs, light weight and flexibility [1]. However, their poor power conversion efficiency (PCE) has remained an obstruction to commercial use. Many researchers have made much of effort to improve the PCE of OSC. Among these improvements, poly (3,4ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT: PSS) has been used as the buffer layer to enhance the efficiency in OSC [2]. PEDOT: PSS has been considered a promising conductive polymer due to its structural stability, optical transparency, and process ability. In spite of its numerous advantages, the low conductivity is still a restriction for application in optoelectronic devices. In order to improve this problem, lots of groups find the solvent-doped PEDOT: PSS films contained superior conductivity [3–6]. The conductivity of the PEDOT: PSS film can be increased by addition of the solvent, such as sorbitol, glycerol and dimethyl sulfoxide (DMSO). The effect of the solvents changes the PEDOT to PSS molar ratio at the surface of the PEDOT: PSS films, resulting in an influence
∗ Corresponding author. Tel.: +886 7 5919475; fax: +886 7 5919357. E-mail address: [email protected] (C.-J. Huang). 0379-6779/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2012.12.008
in the sheet resistance (R ) of the films [4]. The larger the PEDOT ratio at the surface of the films is, the larger the conductivity is [7]. The improvement in conductivity is due to morphological changes rather than chemical changes [4]. And this effect makes an increase in the roughness of PEDOT: PSS films. The increase in the roughness of PEDOT: PSS layer induces defects, which may trap the holes to reduce short-circuit current (JSC ) in OSC. Many groups studied solvent-doped PEDOT: PSS film [8–14], but few groups discussed the effect of solvent-doped PEDOT: PSS layer on small molecule organic solar cells. In this study, the difference and the mechanism of different solvent-doped PEDOT: PSS layers on the performance of OSC will be presented. 2. Experimental The cells are fabricated in this structure ITO/solvent-doped PEDOT: PSS/CuPc/CuPc:C60/ C60/ BCP/Ag, as shown in Fig. 1. All ITOglass substrates with R of 7 / were cleaned by ultrasonic treatment in acetone, isopropyl alcohol and deionized water (5 min each) and dried by high-purity nitrogen blow before deposition. Three solvents were sorbitol (98%), glycerol (99.8%) and DMSO (99.9%). The solvent was added to the PEDOT: PSS (Clevios PH500, H.C. Starck, Germany) directly, and then the doped PEDOT: PSS was stirred for 30 min at room temperature. The doped PEDOT: PSS layer was formed by a spin-coating method. The spin-coatings were performed at a rotation rate of 3500 rpm for 20 s. The doped PEDOT: PSS film was heated at 150 ◦ C for 20 min on a hotplate in ambient
C.-J. Huang et al. / Synthetic Metals 164 (2013) 38–41
39
Fig. 1. Schematic structure of the organic photovoltaic cells.
lab conditions. The organic materials used in the device were CuPc (99%), C60 (99.95%) and BCP (99%), which were without further sublimation. The mixed layer was grown by co-deposition from independent thermal evaporation sources, and the ratio of CuPc: C60 was 1:1. Deposition rate of all organic layers were 0.02 and 0.04 nm/s below the pressure of 4.8 × 10−6 Torr. The sliver (Ag) as a cathode was deposited to a thickness of 100 nm by thermal evaporation through a shadow mask. The active area of OSC was 0.06 cm2 . All thickness of the thin film was monitored by an oscillating quartz thickness monitor. In order to analyze the difference of these solvent-doped PEDOT: PSS films, the electrical characterization, morphology and optical quality of OSC with different solvent-doped PEDOT: PSS layers were measured. The current–voltage characteristics were measured with Keithley 2400 sourcemeter, under an illumination of 100 mW/cm2 with an AM1.5G sun simulator. The surface morphology of doped PEDOT: PSS layer was measured by atomic force microscopy (AFM, Park Systems, XE-70). The R of the doped PEDOT: PSS film was measured by four point probe (SRM103, Solar Energy Tech., Taiwan). The transmittance of the doped PEDOT: PSS film was measured by UV/visible spectrometer (HITACHI, U-3900). And external quantum efficiency (EQE) spectra of the devices were measured by QE measurement system (TEO, QE-3000). 3. Results and discussion The performance parameters of devices with various solvent concentrations and the pristine PEDOT: PSS layer were listed in Table 1. These solvents doped in PEDOT: PSS are sorbitoldoped PEDOT: PSS (referred here as S-PEDOT: PSS), glycerol-doped PEDOT: PSS (referred here as G-PEDOT: PSS) and DMSO-doped PEDOT: PSS (referred here as D-PEDOT: PSS). In Table 1, the PCE of device is extremely low by inserting the pristine PEDOT: PSS layer. On the other hand, the PCE of devices with different solvent-doped PEDOT: PSS layers are dramatically improved. The series resistance (RS ) of device with pristine PEDOT: PSS layer is one hundred times higher than that of solvent-doped PEDOT: PSS layers. The difference of PCE is due to the reason that RS is substantially reduced, resulting in a sharp increase in JSC and fill factor (FF). It was found that the R of PEDOT: PSS layer can be decreased by the addition of solvents [4]. Reduced R makes the holes transport more efficient. However, the key point for a decrease in RS may be attributed to the low R of solvent-doped PEDOT: PSS layer for all devices with the same structure. Fig. 2 shows the R and surface roughness (Rrms ) of the various solvent-doped PEDOT: PSS films. By adding different solvent
Fig. 2. Comparison of the sheet resistance and surface roughness with various (a) sorbitol-doped, (b) glycerol-doped, and (c) DMSO-doped PEDOT: PSS concentrations on the ITO-glass substrate.
concentrations, R and surface roughness of the films were changed. In other words, the R of the film decreases with increasing solvent concentration in the PEDOT: PSS. And the surface roughness increases with increasing solvent concentration. This is due to the fact that the amount of the PEDOT to PSS molar ratio at the surface is increased [3,4], and that more PEDOT, which is a conductive material, will be clustered at the surface of the PEDOT: PSS film after doping solvent. However, this clustered phenomenon
40
C.-J. Huang et al. / Synthetic Metals 164 (2013) 38–41
Fig. 3. AFM images of the PEDOT: PSS layer doped with different organic solvent concentrations coated on the ITO, (a) sorbitol, (b) glycerol, and (c) DMSO.
results in a decrease in R and an increase in roughness of surface. The reduced R was able to enhance carriers transport to electrode. And then the increased surface roughness may make the carriers to be trapped in defects. That is to say, we have to strike a balance between the R and surface morphology to obtain optimal performance. The optimal solvent concentration is 3 wt% S-PEDOT: PSS because of its lowest R and good morphology quality. And the PCE of OSC with 3 wt% S-PEDOT: PSS is up to 3.37% which is higher than that of other devices with different solvent concentrations. It is worth noting that the R and surface roughness of PEDOT: PSS layer were considered the main factors, which influenced the device performance. Further confirmation comes from the AFM micrographs. Fig. 3 shows AFM images of the PEDOT: PSS layer doped with various organic solvents such as sorbitol, glycerol and DMSO. Fig. 3(a) shows AFM images of the PEDOT: PSS layer doped
Table 1 The current–voltage characteristics of the devices with various dopant concentrations. Buffer layer
JSC (mA/cm2 )
VOC (V)
FF
RS ( cm2 )
(%)
Without PEDOT:PSS Pristine PEDOT:PSS 1 wt% S-PEDOT:PSS 2 wt% S-PEDOT:PSS 3 wt% S-PEDOT:PSS 4 wt% S-PEDOT:PSS 5 wt% S-PEDOT:PSS 2 wt% G-PEDOT:PSS 4 wt% G-PEDOT:PSS 6 wt% G-PEDOT:PSS 8 wt% G-PEDOT:PSS 5 wt% D-PEDOT:PSS 10 wt% D-PEDOT:PSS 15 wt% D-PEDOT:PSS 20 wt% D-PEDOT:PSS
5.13 0.30 6.31 14.19 15.45 14.73 10.92 13.22 13.94 13.86 12.80 12.36 12.14 12.89 12.63
0.47 0.51 0.47 0.50 0.51 0.48 0.50 0.47 0.47 0.46 0.47 0.46 0.48 0.48 0.48
0.53 0.24 0.28 0.40 0.42 0.38 0.43 0.37 0.41 0.41 0.38 0.41 0.45 0.44 0.44
14.4 1395 58.8 10.8 10.2 10.8 13.2 16.8 10.56 11.82 14.4 9.66 9.36 9.22 9.46
1.28 0.03 0.83 2.89 3.37 2.74 2.37 2.31 2.70 2.62 2.34 2.35 2.64 2.81 2.67
with different sorbitol concentrations: 0 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt% and 5 wt%, that the root-mean-square roughness (Rrms ) of the PEDOT: PSS layer is estimated to be 1.84 nm, 1.98 nm, 2.33 nm, 2.54 nm, 2.89 nm, and 3.25 nm, respectively. Fig. 3(b) shows AFM images of the PEDOT: PSS layer doped with different glycerol concentrations: 0 wt%, 2 wt%, 4 wt%, 6 wt% and 8 wt%, that the rootmean-square roughness (Rrms ) of the PEDOT: PSS layer is estimated to be 1.84 nm, 3.18 nm, 3.24 nm, 3.54 nm, and 3.59 nm, respectively. Fig. 3(c) shows AFM images of the PEDOT: PSS layer doped with different DMSO concentrations: 0 wt%, 5 wt%, 10 wt%, 15 wt% and 20 wt%, that the root-mean-square roughness (Rrms ) of the PEDOT: PSS layer is estimated to be 1.84 nm, 3.68 nm, 4.09 nm, 4.52 nm, and 5.17 nm, respectively. Fig. 4 shows the transmittance of the PEDOT: PSS films doped with different optimal solvent concentrations on the ITO-glass substrate. Three optimal solvent-doped PEDOT: PSS films are 3 wt% S-PEDOT: PSS, 4 wt% G-PEDOT: PSS and 15 wt% D-PEDOT: PSS, respectively. Among these, the optical quality in the visible region for the film of 3 wt% S-PEDOT: PSS is superior to other solventdoped PEDOT: PSS films. The transmittance spectra of 3 wt% S-PEDOT: PSS was different from spectra of 4 wt% G-PEDOT: PSS and 15 wt% D-PEDOT: PSS which is higher transmittance from 400 nm to 700 nm. The superior transmittance will be beneficial to absorb more photons that may obtain higher JSC . The PCE of the device with 3 wt% S-PEDOT: PSS layer is higher than that of other devices because of its better transmittance in the visible region. However, the performance of doped PEDOT: PSS devices was influenced by transmittance besides R and surface roughness of film. To further verify the optical quality of the doped PEDOT: PSS films, QE measurement system is used to measure the EQE spectra of the devices. Fig. 5 shows the EQE of the devices with different optimal solvent-doped PEDOT: PSS layers. The EQE is defined as the ratio of the number of charge carriers collected by the solar cell to the number of photons of a given energy shining on the
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but also excellent transmittance of the anode buffer layer. All the above results show that the JSC and FF depend on the quality of the device with solvent-doped PEDOT: PSS film. The photons which illuminated the active area of OSC were efficiently converted into charges in the device with 3 wt% S-PEDOT: PSS layer because of its good enough quality in R , surface roughness and transmittance. Thus, the cell with 3 wt% S-PEDOT: PSS layer could reveal the best performance. 4. Conclusion
Fig. 4. The transmittance of different solvent-doped PEDOT: PSS layers with optimal concentration on the ITO-glass substrate.
In summary, the device with solvent-doped PEDOT: PSS layer shows the superior PCE of the OSC. Reduced R makes a sharp decrease in RS , which leads to higher JSC and FF. The improvement in R by adding solvent was attributed to the morphology change, i.e., a higher PEDOT to PSS molar ratio at the surface of PEDOT: PSS film. For the device with 3 wt% S-PEDOT: PSS layer, the lowest RS was obtained, resulting from the optimal balance between the R and the surface roughness. Furthermore, the film of S-PEDOT: PSS contains the best optical transmittance in visible region among all solvent-doped PEDOT: PSS films, resulting in the highest JSC to enhance the PCE. However, the PCE of the device with 3 wt% sorbitol-doped PEDOT: PSS layer is up to 3.37%. Acknowledgment This work was partially supported by the National Science Council of Taiwan (NSCT) under Contract no. NSC-101-2221-E-390-021. References
Fig. 5. The EQE of different solvent-doped PEDOT: PSS layers with optimal concentration on the ITO-glass substrate.
solar cell. The value of EQE is related to JSC of the device. It is found that the spectra of EQE were similar to the transmittance spectra. The device with 3 wt% S-PEDOT: PSS layer has higher EQE value from 400 nm to 700 nm in the visible region. The increase in EQE value is attributed to the superior transmittance of 3 wt% S-PEDOT: PSS film, which makes the device absorb more photons. The 3 wt% S-PEDOT: PSS film provides not only decreased sheet resistance
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