PEDOT: PSS Solar Cell Controlled by Dipole

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Performance of Si/PEDOT:PSS Solar Cell Controlled by Dipole Moment of Additives Toshiki Sakata, Natsumi Ikeda, Tomoyuki Koganezawa, Daisuke Kajiya, and Ken-ichi Saitow J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05144 • Publication Date (Web): 30 Jul 2019 Downloaded from pubs.acs.org on August 1, 2019

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The Journal of Physical Chemistry

Performance of Si/PEDOT:PSS Solar Cell Controlled by Dipole Moment of Additives Toshiki Sakataa, Natsumi Ikedaa, Tomoyuki Koganezawab, Daisuke Kajiyaa,c, Ken-ichi Saitowa,c,*

a Department

of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama,

Higashi-Hiroshima, Hiroshima 739-8526, Japan

b

Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, 1-1-1 Kouto, Sayo, Hyogo 6795198, Japan

c Natural

Science Center for Basic Research and Development (N-BARD), Hiroshima University, 1-3-1

Kagamiyama, Higashi-hiroshima, Hiroshima 739-8526, Japan.

*Corresponding author

Natural Science Center for Basic Research and Development (N-BARD), Hiroshima University, 1-3-1 Kagamiyama, Higashi-hiroshima, Hiroshima 739-8526, Japan

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ABSTRACT: Solar cells composed of silicon (Si) and polymer films have attracted much attention because of their high performance and facile preparation using aqueous solutions. Although a large amount of research has been carried out to further improve their performance, the factors required to achieve this are not yet fully understood from the perspective of physical chemistry. Here we show the effect of additives on the performance

of

Si/poly(3,4-ethylenesioxythiophene):

poly(styrenesulfonate)

(PEDOT:PSS) solar cells using six different additives. The relationships among the solar cell performance, electrical properties, and the structure of the polymer film are examined by changing the additive and conducting cyclic voltammetry, electric conductivity,

and

two-dimensional

grazing

incidence

angle

X-ray

diffraction

measurements. The dipole moment of the additive molecule was determined to be a key factor for improving the performance of Si/PEDOT:PSS solar cells. In particular, an additive with a higher dipole moment ( > 3 D) caused an increase in the rate of charge diffusion and the ratio of face-on to edge-on structures of PEDOT molecules.


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INTRODUCTION Solar cells are renewable energy sources, and various types of solar cells have been developed, not only as rooftop and mega solar cells, but also for indoor applications, stand-alone sensors, and as energy sources for artificial satellites in space. Crystalline silicon (Si) has been the most popular semiconductor for solar cells, due to the ease of production of large single crystals (≅ 30 cm), high mobility (≅ 1500 cm2 V-1 s-1), and the cost-effectiveness of this material. However, high temperatures (≥1000 K) are required to produce Si solar cells to generate p–n junctions, and clean room environments are also needed for solar cell production. Therefore, another type of solar cell consisting of Si wafer and a π-conjugated polymer has attracted much attention due to its high performance and facile preparation by solution processes.1 Solar cells composed of a Si wafer and poly(3,4,-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) exhibited a high power conversion efficiency (PCE) over 17 %.2-5 This PCE is significantly higher than the value of a commercial amorphous-Si solar cell (9 %) and is furthermore approaching to a commercial polycrystalline-Si solar cell (19 %).6,7 Two issues, surface texture and interface engineering, have been important for


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enhancing the performance of Si/PEDOT:PSS solar cells. To address the former issue, the surface of the Si wafer has been nanostructured so that its optical reflectance became lower than 3%, and enabled efficient light harvesting.8-14 With regard to the latter issue, electron and hole transport layers, e.g., SnO2, MoO3, and Cs2CO3, have been found to establish smooth carrier extraction and result in a high PCE.15-21 However, these approaches require either a complex manufacturing process or deposition of an additional layer, both of which complicate the typically simple production process for Si/PEDOT:PSS solar cells. Another method to improve the performance of Si/PEDOT:PSS solar cells is to use an additive in the PEDOT:PSS layer, which involves a simple one-step process via a solution process.23-26 As a typical additive, ethylene glyocol22,23 or dimethyl sulfoxide (DMSO)22,24 has been used. These additives can improve the conductivity of PEDOT:PSS by two orders of magnitude to 600 S cm-1 26 and changing the PCE up to 17 %.2 In addition, an additive mixture, ethylene glycol and methanol, also shows high PCE up to 14.6 %.25 In these studies, a polar molecule was used as an additive, such as ethylene glycol or DMSO, which is considered to effectively separate the PEDOT


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(conductive polymer) from the PSS (insulator polymer). In our previous study,27 we investigated enhancement of the PCE using DMSO as an additive from the point of view of molecular science. As a result, the reason for the 10-fold enhancement of the PCE by the addition of DMSO was attributed to (i) an increase in π–π stacking, (ii) a reduced distance between π–π planes, (iii) an increase in the quinoid fraction of PEDOT, and (iv) a reduced PEDOT:PSS particle size. Research on additives is important for improving the performance of Si/PEDOT:PSS solar cells. However, there have no reports to date on which additives yield the best performance or how the chemical and/or physical properties of additives control the performance. Thus, a systematic investigation of the effect of additives is very important to understand the performance of Si/PEDOT:PSS solar cells. In the present study, the performance of Si/PEDOT:PSS solar cells using six different additives was investigated (Table 1). Correlations among the additive, film structure, and solar cell performance were investigated using cyclic voltammetry (CV) and two-dimensional grazing-incidence X-ray diffraction (2D-GIXD) measurements of


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PEDOT:PSS films. The results revealed that the dipole moment of additive is a key factor for higher cell performance.

EXPERIMENTAL A PEDOT:PSS solution was prepared by the addition of a 5 wt% additive with a 0.5 wt% aqueous solution of surfactant (Zonyl FS-300, Fluka) into a commercial PEDOT:PSS solution (Clevios PH1000, Heraeus). Before adding the additive, the PEDOT:PSS solution was filtered with a 0.45 m polyvinylidene difluoride (PVDF) filter to remove large polymer aggregates. Pristine PEDOT:PSS without an additive was also prepared for comparison. The six additives used in this work were DMSO, dimethyl acetamide (DMAc), acetone, diiodomethane (DIM), ethanol, and toluene, the dipole moments () of which are listed in Table 1.28 Si/PEDOT:PSS hybrid solar cells were fabricated in the same manner as reported previously.27 Briefly, an n-type Si(100) wafer (20×20 mm2) was cleaned using a solution of 75% sulfuric acid and 25% hydrogen peroxide for 50 min. The native oxide layer on the Si wafer was etched with 5% hydrogen fluoride acid for 3 min. Immediately


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after these processes, a 100 nm thick aluminum anode was deposited onto the back side of the Si wafer using a vacuum evaporation system (SVC-700TM, Sanyu Electron). The Si wafer was stored for 2 h at 120 °C on a hotplate in a glove box (UNILAB2000, MBRAUN) filled with argon gas. The prepared PEDOT:PSS solution was dropped onto the front side of the Si wafer and spin-coated at 700 rpm for 3 s and 2000 rpm for 20 s in the Ar-filled glove box and was annealed at 180 °C for 2 h. Here, we used solutionprocessed silver nanowires to prepare a cathode, because a vacuum process to deposit a cathode in our previous study gave the damage of polymer surface and reduced the PCE of Si/PEDOT:PSS solar cell.27 Thus, a solution of colloidal silver nanowires in 2propanol (0.5 wt%, 40 L) was dropped on the PEDOT:PSS film, followed by heating at 150 °C for 10 min. For all experiments, the PEDOT:PSS film thickness was about 100 nm, which was measured using a confocal laser microscope (OLS4000, Olympus). The current density-voltage (J–V) characteristics of the solar cell were measured using a solar simulator (HAL-C100, 100 mW cm-2, Asahi Spectra), a source meter (2400-C, Keithley), and a four-point probe measurement system (Bunkoukeiki Co.) as reported previously.27 Briefly, the solar cell size was 20×20 mm2 and the light-irradiation


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area was 5×5 mm2. Data were collected using 8 solar cells for DMSO and 4 solar cells for other additives. Then, we measured the J-V curves of 4 area of a single solar cell. The parameters analyzed from each curve were averaged, and their standard deviations were calculated to evaluate those parameters. CV measurements were conducted to determine the charge diffusion rate for the PEDOT:PSS film using an electrochemical analyzer (ALS600EB, BAS), a 0.1 M NaCl solution, a PEDOT:PSS film coated on a Pt working electrode, which was followed by coating Nafion, Ag/AgCl (3 M NaCl) as a reference, and a Pt wire as a counter electrode. The scanning range was 1.0 to 1.0 V at a scan rate of 0.1 V s-1 for all PEDOT:PSS films. Data were collected using 3 films of PEDOT:PSS prepared by each additive. The parameters analyzed from the data were averaged, and their standard deviations were calculated. The conductivity of the PEDOT:PSS films was measured by the 4-point probe method. The data were collected using 4 films of PEDOT:PSS prepared by each additive. The data were averaged, and their standard deviations were calculated. GIXD measurements of PEDOT:PSS films on Si wafers were performed at the BL19B2 beamline of SPring-8 as reported previously.27,29-34 Briefly, X-rays with an energy of 12.39 keV (λ = 1.00 Å)


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irradiated the film at an incidence angle of 0.12°. The scattered X-rays were recorded at a camera length of 174.5 mm using a 2D image detector (Pilatus 300K, Dectris) for a period of 60 s. The data were collected by conducting twice measurements. The parameters analyzed from the data were averaged, and their standard deviations were calculated.

RESULTS AND DISCUSSION Figure 1a shows the J-V characteristics of the Si/PEDOT:PSS hybrid solar cells with and without additives. The solar cell performances obtained from the J-V curves are listed in Table 2. The PCE of the solar cell increases with additives in the following order: DMSO > DMAc > DIM > ethanol > toluene, pristine, which is the similar order as the dipole moment of the additives: DMSO ( = 4.0) > DMAc (3.8) >> DIM (1.1) > ethanol (1.7) > acetone (2.9) > toluene (0.4). In Figure 1a, the J-V curve is S-shaped without an additive, but the S-shape is improved with additives, i.e., the S-shape represents a lower fill factor (FF) given by a high series resistance Rs and shunt resistance Rsh, which are due to the electrical resistance of the carrier pathways and the


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leak current, respectively. Figures 1b-f show the relationship between the additive dipole moment and the parameters that characterize the performance of the solar cells. The values of open circuit voltage (Voc) and short-circuit current density (Jsc) did not change significantly with the additives, whereas the fill factor was 1.6 times higher than that for the pristine film. Note that additives with a higher  give a higher FF and a lower series Rs, both of which improve the PCE. Specifically, these parameters significantly change at around the value of ~3 D, according to the profiles of dashed lines guided to eye. To investigate charge diffusion in the PEDOT:PSS films, CV curves were measured, and the results are shown in Figure 2a. Two-step peaks are observed as oxidation and reduction peaks,35 i.e., the 1st step (-0.6 to 0.2 V) is due to a redox reaction between an intrinsic carrier and a polaron, and the 2nd step (0.2 to 0.6 V) is associated with a redox reaction between a polaron and a bipolaron. Note that the peak positions vary for the different additives. The dependence of the separation () between the oxidation and reduction peaks on the additive is listed in Table 3. Figure 2b shows the relation between  and the dipole moment of the additive; it can be seen that 


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decreases as  increases. Based on the smaller , which indicates fast charge diffusion, 36

these results suggest that additives with a high  cause an increase in the carrier

diffusion rate. Figure 2c shows that the PCE for the solar cell increases with decreasing

. Furthermore, higher conductivity was observed for the PEDOT:PSS film prepared using an additive with a higher , as shown in Figure 2d and Table 3. Therefore, the dipole moment of the additive is very important factor for improving the performance of Si/PEDOT:PSS solar cells. In fact, the fastest carrier diffusion (minimum  and the maximum conductivity appear at around  = 3 D in Fig. 2, which is a similar trend as shown in Fig. 1. The film structure that gave a high charge diffusion rate was examined using 2DGIXD. Figures 3a-d show 2D-GIXD images of PEDOT:PSS films, DMSO, DMAc, ethanol and pristine. Diffraction due to (010) -conjugated planes of PEDOT molecules is observed for all the films at around q = 18 nm-1.27 From the intensity of the (010) diffraction in the qz and qxy directions, the ratio of the face-on structure to edge-on structure of PEDOT molecules (details are described in the Supporting Information), as listed in Table 3, was found to increase with increasing . This ratio and the PCE value


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are correlated with each other; the PCE increases with increasing face-on to edge-on ratio, as shown in Figure 3f. The face-on structure corresponds to  stacking along the out-of-plane direction; therefore, an increase of the face-on structure can provide a carrier diffusion pathway due to -orbital overlapping. The out-of-plane direction corresponds to a current-flow direction in the solar cell. Furthermore, better pathway for carrier diffusion is in good agreement with the higher diffusion rate observed in the CV measurements, as shown in Figure 2. Consequently, the carrier diffusion rate increases with the use of an additive. In conclusion, the FF and Rs values are improved by an increase in the fraction of the face-on structure, as shown in Figures 3g-i. This indicates that the face-on to edge-on ratio for PEDOT molecules determines the performance of the cells and the ratio is controlled by the dipole moments of the additives. Finally, let us discuss the reason why an additive with a larger dipole moment produces high performance and a high ratio of face-on to edge-on structure. A polar additive can effectively remove PEDOT and PSS parts in the PEDOT:PSS polymer by stronger electrostatic interactions, as illustrated in Fig. 4. This has been recognized in previous studies on PEDOT:PSS films using X-ray photoelectron spectroscopy and


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atomic force microscopy measurements.37,38 The PEDOT molecules, removed from PSS of an insulator, can lead to a higher degree of  stacking of PEDOT, which is accomplished by a conformational change from a coiled to a linear structure,38 as illustrated in Figure 4. Therefore, the increase of face-on structure results in the enhancement of  stacking along the out-of-plane direction, followed by the enhancement of solar cell performance. In fact, such a conformational change was observed in the 2D-GIXD measurements (Figure 3). Particularly, these conformational changes appear clearly when an additive with a larger , such as DMSO, is used. Therefore, it was discovered that such a situation is established at around the dipole moment  > 3 D, which is a critical value for enhancing the performance of solar cell.

CONCLUSIONS

The effects of additives on the performance of Si/PEDOT:PSS hybrid solar cells were quantified by J-V, CV, and 2D-GIXD measurements. It was found that the performance enhancement was correlated with the dipole moment  of the additives. A


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large  led to a lower electrical resistance and a higher carrier diffusion rate in the PEDOT:PSS layer, due to an increase in the face-on structure. It was concluded that the  value, (ca  > 3 D), for an additive is a key factor for enhancing solar cell performance.

ASSOCIATED CONTENT

Supporting Information.

Description about 2D-GIXD measurement and a table for the performance of solar cell (PDF)

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS


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KS acknowledges financial support from the Funding Program for the Next Generation World-Leading Researchers (GR073) of the Japan Society for the Promotion of Science (JSPS), a Grant-in-Aid for Scientific Research (A) (15H02001) and (B) (19H02556) from JSPS, the PRESTO Structure Control and Function program of the Japan Science and Technology Agency (JST), and JKA through its promotion funds from AUTORACE. DK acknowledges a Grant-in-Aid for Young Scientists (B) (Nos. 26790015 and 17K14082) from JSPS. 2D-GIXD experiments were performed at the BL19B2 beamline of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2015B1630 and 2016B1575).


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C 2016, 120, 23351–23357. (33) Imanishi, M.; Kajiya, D.; Koganezawa, T.; Saitow, K. Uniaxial Orientation of P3HT Film Prepared by Soft Friction Transfer Method. Sci. Rep. 2017, 7, 5141. (34) Kajiya, D.; Saitow, K. Ultrapure Films of Polythiophene Derivatives are Born on a Substrate by Liquid Flow. ACS Appl. Energy Mater. 2018, 1, 6881–6889. (35) Park, H. S.; Ko, S. J.; Park, J. S.; Kim, J. Y.; Song, H. K. Redox-Active Charge Carriers of Conducting Polymers as a Tuner of Conductivity and Its Potential Window. Sci. Rep. 2013, 3, 2454. (36) Murray, R. W. In Electroanalytical chemistry; Bard, A. J. Ed.; Marcel Dekker: New York, 1984, vol. 13, pp 204−205. (37) Yan, H.; Okuzaki, H. Effect of solvent on PEDOT/PSS Nanometer-Scaled Thin Films: XPS and STEM/AFM Studies. Synth. Met. 2009, 159, 2225−2228.


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(38) Ouyang J.; Xu, Q.; Chu, C. W.; Yang, Y.; Li, G.; Shinar, J. On the Mechanism of Conductivity Enhancement in Poly(3,4-ethylenedioxythiophene):Poly(styrene sulfonate) Film Through Solvent Treatment. Polymer 2004, 45, 8443−8450.


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Page 24 of 31

Fig. 1. (a) J-V curves for Si/PEDOT:PSS hybrid solar cells. Plot of parameters as function of dipole moment of additive: (b) Voc, (c) Jsc, (d) fill factor (FF), (e) PCE, and (f) series resistance (Rs). Data are collected using 8 solar cells for DMSO and 4 solar cells for other additives. Dashed lines are a guide to the eye.


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Fig. 2. (a) CVs of PEDOT:PSS films. (b)  for 1st peak in CV curve as function of dipole moment of additives. (c) PCE as function of  for 1st peak. (d)  as function of dipole moment of additives. Data are obtained for 3 films of PEDOT:PSS prepared by each additive. Error bars are the standard deviation. Dashed lines are a guide to the eye.


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Fig. 3. 2D-GIXD images of PEDOT:PSS films prepared with additives: (a) DMSO, (b) DMAc, (c) ethanol, and (d) without (pristine). (e) Face-on to edge-on structure ratio Iz/Ixy against dipole moment for additives. Performance of solar cells as function of face-on to edge-on structure ratio: (f) PCE, (g) FF, (h) series resistance Rs, and (i)  for CV 1st peak. Iz and Ixy represent (010) diffraction intensity in qz and qxy directions, respectively, corresponding to face-on and edge-on structure. 2D-GIXD data are collected by twice measurements. Dashed lines are a guide to the eye.


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Fig. 4. (a) Schematic illustration of effect of additives on PEDOT:PSS films. The PEDOT molecules, removed from PSS of an insulator, lead to a higher degree of  stacking of PEDOT, which is accomplished by a conformational change from a coiled to a linear structure. (b) Structures of PEDOT (partially positive) and PSS (partially negative).


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Table 1. Chemical structure and dipole moment () of additives used in the present study.

DMSO

 a = 4.0 D a

DMAc

Acetone

Ethanol

DIM

Toluene

3.7 D

2.9 D

1.7 D

1.1 D

0.4 D

Water (Solvent)

1.85 D

ref. 28.


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Table 2. Performance of Si/PEDOT:PSS hybrid solar cells prepared with and without additives. Data are obtained for 8 samples for DMSO and 4 samples for other additives. Data within brackets are maximum values. The pristine sample is that without additives. Additive DMSO DMAc Acetone Ethanol DIM Toluene Pristine

Voc / V

Jsc / mA cm-2

FF

PCE / %

0.51 ± 0.01

28.4 ± 1.0

0.53 ± 0.05

7.8 ± 0.8

(0.52)

(29.1)

(0.59)

(8.9)

0.49 ± 0.03

26.5 ± 1.0

0.41 ± 0.04

5.3 ± 0.9

(0.50)

(27.2)

(0.44)

(6.0)

0.46 ± 0.01

28.0 ± 0.8

0.36 ± 0.02

4.6 ± 0.2

(0.47)

(29.1)

(0.38)

(4.8)

0.47 ± 0.01

28.1 ± 1.2

0.35 ± 0.03

4.7 ± 0.4

(0.48)

(29.1)

(0.39)

(5.3)

0.49 ± 0.01

29.3 ± 0.5

0.36 ± 0.01

5.1 ± 0.2

(0.51)

(30.4)

(0.36)

(5.5)

0.47 ± 0.01

26.0 ± 1.0

0.37 ± 0.02

4.5 ± 0.3

(0.48)

(26.6)

(0.38)

(4.8)

0.46 ± 0.03

28.7 ± 0.4

0.34 ± 0.03

4.5 ± 0.7

(0.49)

(29.3)

(0.37)

(5.2)


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Table 3. List of values obtained from CV, conductivity, and 2-D GIXD measurements. Data are obtained from 3 samples. Additive

1st peak  / V

 / S cm-1

Iqz/Iqxy

DMSO

0.26 ± 0.02

204 ± 31

1.41 ± 0.01

DMAc

0.28 ± 0.05

240 ± 3

1.27 ± 0.01

Acetone

0.29 ± 0.07

2±1

Ethanol

0.40 ± 0.08

11 ± 9

DIM

0.33 ± 0.07

1 ± 0.3

Toluene

0.55 ± 0.08

2±2

Pristine

0.47 ± 0.14

0.6 ± 0.1

1.23 ± 0.04

1.17 ± 0.01


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PEDOT: PSS Solar Cell Controlled by Dipole

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