PEDOT: PSS Hybrid Solar Cell Controlled by

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Performance of Si/PEDOT:PSS Hybrid Solar Cell Controlled by PEDOT:PSS Film Nanostructure Natsumi Ikeda, Tomoyuki Koganezawa, Daisuke Kajiya, and Ken-ichi Saitow J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07101 • Publication Date (Web): 09 Aug 2016 Downloaded from http://pubs.acs.org on August 13, 2016

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The Journal of Physical Chemistry

Performance of Si/PEDOT:PSS Hybrid Solar Cell Controlled by PEDOT:PSS Film Nanostructure Natsumi Ikeda,† Tomoyuki Koganezawa,‡ Daisuke Kajiya,§ and Ken-ichi Saitow†,§,* †

Department of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-hiroshima, Hiroshima 739-8526, Japan

‡

Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan

§

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 Telephone & fax: +81-82-424-7487, e-mail address: [email protected]

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ABSTRACT: A hybrid solar cell composed of a crystalline semiconductor and polymer film has attracted much attention due to numerous advantages such as high mobility, long lifetime, and the aqueous solution processing. Recently, the power conversion efficiency (PCE) of the hybrid solar cell of silicon (Si) wafer and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) is reported to be higher than that of a commercial amorphous Si solar cell. Here in the Si/PEDOT:PSS hybrid solar cell was prepared using dimethyl sulfoxide (DMSO) as an additive to the PEDOT:PSS solution. The PCE was increased up to 10-fold by the addition of DMSO at a concentration of 5 wt%. Results from grazing-incidence X-ray diffraction, atomic force microscopy, and Raman spectroscopy indicated the 10-fold enhancement was controlled by the nanostructure of the PEDOT:PSS film. The enhanced performance was attributed to i) an increase of π–π stacking, ii) shortened distances between π–π planes, iii) an increase in the quinoid structure of PEDOT, and iv) reduced PEDOT:PSS particle size. The PCE was also enhanced by a transparent cathode of colloidal Ag nanowires and through the use of a vacuumfree process for preparation of the PEDOT:PSS film.


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INTRODUCTION Crystalline silicon (Si) has been the most popular material for a solar cell, because there are numerous distinct properties such as large single crystal (≅ 30 cm), high mobility (≅ 1500 cm2/Vs), abundance as rock forming element (28 % in the earth crust), and cost effective material. However, the production process of the Si solar cell requires high temperature (≥ 1000 K) to generate p-n junction as well as a clean room. Thus, hybrid solar cells composed of a semiconductor, particular Si wafer, and π-conjugated polymer have attracted much attention due to numerous advantages of Si single crystal and the ease of solution processing with the organic polymer. On the other hand, poly(3,4-ethylene dioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) is a polymer with numerous distinct properties such as high conductivity (1000 S/cm for Clevios PH10001), high transmittance (85% for 100 nm thickness2-4), and water solubility. Accordingly, Si/PEDOT:PSS hybrid solar cell is composed of two distinctive materials and prepared by aqueous solution processing, and thus is recognized as a nextgeneration solar cell.4–19 Since first reported by Williams et al.,5 various researchers have investigated the Si/PEDOT:PSS solar cell.6–21 The power conversion efficiency (PCE) of this cell has been over 14%.20,21 This PCE is higher than that of commercial amorphous Si solar cell (~9 %), and then is comparable to that of commercial polycrystalline Si solar cell (12-18%). In addition, both flexible Si/PEDOT:PSS hybrid solar cells16,17 and lifetimes up to 5 months18 have been reported. Another distinct feature for the Si/PEDOT:PSS hybrid solar cell has been the use of additives to the PEDOT:PSS solution, such as dimethyl sulfoxide (DMSO) or ethylene glycol (EG), which has resulted in significantly enhanced PCE.2-4,14,15,17–25 Ouyang et al. reported an increase of conductivity for PEDOT:PSS films from 0.4 S/cm to 200 S/cm by the addition of


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EG.22 Takano et al. reported an increase of the PEDOT crystallite size from 1.2 nm to 4.5 nm by the addition of EG.2,3 The addition of DMSO was reported to reduce the thickness of the PSS shell of PEDOT particles, and the PCE was increased from 2.4% to 12.0%.25 These studies indicate that the structure and properties of PEDOT:PSS films are sensitively changed by the use of additives. However, the mechanism for the effect of additive on the molecular structure of PEDOT:PSS films has not yet been clarified. Understanding the mechanism of enhanced performance, from the aspect of physical chemistry, is very important for molecular science as well as a practical use of the cell. In the present study, the 10-fold PCE enhancement by the addition of DMSO was found and its mechanism was investigated using two-dimensional grazing-incidence X-ray diffraction (2D-GIXD), atomic force microscopy (AFM), and Raman spectroscopy. As a result, the enhancement of PCE was determined to be due to four factors: i) an increase in π–π stacking in the out-of-plane direction, ii) shortened stacking distances between π–π planes, iii) an increase in the quinoid structure of PEDOT, and iv) reduced PEDOT:PSS particle size. All of these four factors can be optimized by solution process.

EXPERIMENTAL SECTION Si/PEDOT:PSS hybrid solar cells (Fig. 1a) were prepared by the following four procedures. First, the 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. The Si wafer was then aged in air for 2 h to form a thin oxidation layer on the front side of the Si wafer, which provides good band alignment for charge separation at the interface between Si and PEDOT:PSS.6-9 Immediately after these processes, a 100 nm thick aluminum anode was deposited onto the back side of the Si wafer


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using a vacuum evaporation system (SVC-700TM, Sanyu Electron). A PEDOT:PSS solution was prepared by adding DMSO (0, 5, or 10 wt%) with a 0.5 wt% aqueous solution of surfactant (Zonyl FS-300, Fluka) into a commercial PEDOT:PSS solution (PH1000, Clevios). The prepared PEDOT:PSS solution was then dropped onto the front side of the Si wafer and spin-coated at 2000 rpm for 10 s in a glove box (UNILAB2000, MBRAUN) filled with argon. The Si/PEDOT:PSS was heated at 180 °C for 15 min in the glove box. Finally, a solution of colloidal silver nanowires (Ag-NW) in 2-propanol was then dropped on the PEDOT:PSS film and used as a cathode. RESULTS AND DISCUSSION The current density–voltage (J–V) characteristics of the solar cell were measured using a solar simulator (HAL-C100, 100 mW/cm2, Asahi Spectra), a source meter (2400-C, Keithley), and a four-point probe measurement system (Bunkoukeiki Co.). The solar-cell size was 20×20 mm2 and light-irradiation area was 5×5 mm2. GIXD measurements of the PEDOT:PSS film on the Si wafer were measured at the BL19B2 beamline of SPring-8. X-rays with an energy of 12.39 keV (λ = 1.00 Å) were irradiated onto the film at an incident 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 Raman spectrum of the PEDOT:PSS film on Si wafer was measured using a commercial Raman microscope (LabRam HR800, Horiba Jobin Yvon) with a He-Ne laser at an excitation wavelength of 633 nm and with the power of 1 mW at the film surface. The scattered light was collected by the backscattering geometry using a 100×/0.90NA objective (M Plan), a 1800 lines/mm grating, and a CCD camera. The surface morphology of the film was measured using AFM (SPM-9700, Shimadzu) in tapping mode with a microcantilever (OMCL-AC200TS, Olympus).


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RESULTS AND DISCUSSION Figure 1b shows J–V curves for Si/PEDOT:PSS hybrid solar cells with and without DMSO as an additive. The PCEs of the cells obtained from the J–V curves are listed in Table 1. The S shape of the J–V curves is improved by the addition of DMSO into the PEDOT:PSS solution because the series resistance (Rs) is significantly decreased, which corresponds to a 17fold enhancement in the conductivity. In addition, the PCE is enhanced 10-fold by the addition of DMSO at a concentration of 5 wt%, which is due to 3.5 (3.8) and 2.3 (2.0) times increases in the short circuit current density (Jsc) and fill factor (FF), respectively, for 5 wt% (10 wt%) DMSO addition. Figures 2a and 2b show 2D-GIXD images for PEDOT:PSS films and their intensity profiles along the qz, respectively. The strong diffraction observed at around qz = 18 nm–1 is attributed to the π−π stacking of the thiophene ring in PEDOT.26,27 The width of the diffraction becomes narrower, which is interpreted as an increase in the PEDOT crystallite size. In addition, the intensity of the diffraction increases in the qz-direction by the addition of DMSO. The intensity ratio, Iqz/(Iqz+Iqxy), increases from 54.5 ± 0.1 % to 70.5±9.6 % (Table 2), which corresponds to an increase in the face-on orientation composed of out-of-plane π−π stacking of PEDOT. Moreover, the peak position shifts towards the higher q region, which indicates shortening of the distance between π–π planes, as shown in Figure 2c. The particle size of PEDOT:PSS is reduced from 69±12 nm (0 wt%) to 57±11 nm (5 wt%) or 53±8 nm (10 wt%) with increased DMSO addition, as shown in Figure 2d. PEDOT:PSS particles observed using AFM are characterized as nanocrystals composed of a PEDOT core and PSS shell, according to the previous report.2 Based on the 2D-GIXD and AFM results, it can be concluded that DMSO


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addition causes an increase in crystallinity, the formation of face-on orientation, a decrease in the π−π stacking distance, and a reduction in the PEDOT:PSS particle size, as illustrated in Figure 2e. An increase in conductivity due to an increase in the PEDOT crystallinity has also been reported in previous strudies.2,3,15 In addition to the increased crystallinity, the formation of π−π stacking with shortened inter-plane distance in the carrier-migration direction was also observed in the present study. To further investigate the additive effect at the molecular scale, Raman spectra of the PEDOT:PSS films were measured. The Cα=Cβ stretching mode of thiophene ring22,29 of PEDOT was measured at around 1420 cm–1 for different DMSO concentrations. Figure 3a shows a typical Raman spectrum for a PEDOT:PSS film on Si wafer, and Figure 3b shows expanded data around 1420 cm–1 for PEDOT:PSS films with and without the DMSO additive. The spectra were analyzed using Gaussian functions (Fig. 3c), and the two spectral components attributed to the quinoid and benzoid forms of PEDOT30–33 are quantified in Table 3. The intensity of the quinoid component (ca. 1412 cm–1) increases, whereas that of the benzoid component (ca. 1445 cm–1) decreases with the addition of DMSO. The quinoid structure has been recognized as the component giving higher conductivity due to the delocalization of conjugated π-electrons in the linear backbone of the structure,30–33 as shown in Figure 3d. Thus, the increased amount of quinoid structure is responsible for the increased conductivity. Here, let us briefly discuss how the quinoid structure is produced. First, DMSO plays a role as a proton acceptor due to the large proton affinity of DMSO, i.e., DMSO•H+.34 Proton transfer from PSS is observed with the addition of DMSO.23,24 Second, the generated DMSO·H+ becomes an electron acceptor from PEDOT. Figure 3d shows an illustration of PEDOT releasing an electron, which results formation of the quinoid structure.30–33


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Since all the data for the additive effects in the present study were described, let us mention which factor would be a main for enhancing the device performance, briefly. Table 4 lists the four ratios before and after DMSO incorporation. Namely, the ratios of the amount of π– π stacking, the π−π inter-plane distance, the amount of quinoid structure, and the PEDOT:PSS particle size before and after the addition of DMSO are estimated as 1.30, 1.02, 1.14, 1.21 folds, respectively. Based on the ratios, the amount of π–π stacking in the z-direction is significantly enhanced by the additive, and then can be evaluated as a principle factor for enhancing the performance of solar cell. Similar result was observed in previous study;35 the conductivity of PEDOT film increases with the ordering of molecular arrangement.

Finally, we briefly note two other distinct features to enhance the performance of the current solar cell. First, the light transmittance is increased 8-fold using the Ag-NW cathode compared with that using a Cu cathode: Fig. 4(a) and Table 5. The high transmittance is achieved by the lower coverage (50.1%) of the Ag-NW cathode on the PEDOT:PSS layer. In addition, the conductivity of the Ag-NW film is almost equal to that of a Cu cathode, as listed Table 5. The similar Ag-NW film was reported in Si/PEDOT:PSS and organic solar cells.13,19,36 Second, the cathode was not prepared by a vacuum evaporation process but by a simple solution process. The better morphology of the PEDOT:PSS layer was obtained by a vacuum-free process(SI, Fig. S2), whereby a 1.3-fold enhancement of the PCE was achieved (Fig. 4(b)).

COUNCLUSION In summary, Si/PEDOT:PSS hybrid solar cells were prepared by spin coating of PEDOT:PSS solution onto a planar Si wafer. A 10-fold enhancement in PCE was observed by the addition of DMSO with a concentration of 5 %. The additive effect was investigated with


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respect to the film structure at the nanoscale. GIXD, Raman, and AFM analyses indicated that the DMSO additive resulted in increased face-on orientation with shortened π−π inter-plane distance, an increased amount of the quinoid structure, and a reduction in the PEDOT:PSS particle size. The solar-cell performance was enhanced by these nanostructural changes. These results are expected to be an important contribution for enhancement of the performance of optoelectronic devices.

ACCOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Raman spectra, properties of cathode, and effects of vacuum process.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Funding Program for the Next Generation World-Leading Researchers (GR073) of the Japan Society for the Promotion of Science (JSPS). GIXD


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experiments were performed at the BL19B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2014B1629 and 2015B1630). The authors acknowledge Mr. Yoshiaki Ishitobi at the machine shop of Hiroshima University for construction of a deposition mask for the Cu cathode to the solar cell.

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Table 1. Performance of Si/PEDOT:PSS hybrid solar cells fabricated with and without the DMSO additive. DMSO 0 wt% 5 wt% 10 wt% a

VOC / V

JSC / mA cm-2

FF

PCE / %

0.40±0.01a (0.41)b 0.51±0.01 (0.53)

7.9±0.2 (8.1) 28±1 (30)

0.24±0.02 (0.26) 0.54±0.05 (0.59)

0.78±0.05 (0.84) 7.8±0.8 (8.9)

0.49±0.01 (0.50)

30±1 (31)

0.49±0.02 (0.52)

7.1±0.3 (7.5)

Rs / Ω 384 22.6 32.8

Average data and bmaximum data for 4 sample measurements.


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Table 2. Intensities (I), positions (q), and distance (d) obtained from 2D-GIXD data for the π−π stacking planes of PEDOT. Iqxy and Iqz denote the integrated intensities from χ = 0° to 15° and from χ = 75° to 90°, respectively, where χ is the pole angle.28 DMSO

I qz (Face-on)

I qxy (Edge-on)

I qz / (I qz+I qxy)

q / nm-1

d / nm

0 wt%

1112

930

0.545

18.0±0.04

0.349

5 wt%

1444

605

0.705

18.4±0.04

0.341

10 wt%

1927

914

0.678

18.6±0.04

0.338


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Table 3. Relative ratios of Raman bands for the quinoid structure. Iquinoid and Ibenzoid denote the integrated intensities of the spectral components at 1412 cm-1 and 1445 cm-1, respectively. All Raman spectra are shown in Figure S1 in SI. DMSO

Iquinoid Ibenzoid + Iquinoid

0 wt%

5 wt%

10 wt%

Integrated intensity

0.424

0.485

0.487


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Table 4. The ratios of the amount of π–π stacking, the PEDOT:PSS particle size, the amount of quinoid structure, and the π−π inter-plane distance before and after the addition of DMSO. DMSO 5wt% DMSO 10wt% the amount of π–π stacking

1.30 fold

1.25 fold

the PEDOT:PSS particule size

1.21 fold

1.30 fold

the amount the quinoid structure

1.14 fold

1.15 fold

the π–π inter-plane distance

1.02 fold

1.03 fold


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Table 5. Transmittance and resistance of cathodes.

a b

Visible light transmittance (%)

Conductivitya (S)

Cu cathodeb

8.2

0.105

Ag nanowire cathode

62.5

0.100

The conductivity is obtained as the reciprocal value of resistance in 10 mm interval. The Cu electrode is deposited using a vacuum evaporation system (SVC-700TM, Sanyu Electron).


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Figure 1. (a) Schematic diagram of Si/PEDOT:PSS hybrid solar cell, and top-view optical microscope image of the Ag-NW film. (b) J–V curves for Si/PEDOT:PSS hybrid solar cells fabricated with and without the DMSO additive.


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Figure 2. (a) 2D-GIXD images of PEDOT:PSS films prepared with DMSO concentrations of 0, 5, and 10 wt%. (b) GIXD patterns along the qz direction. Vertical lines on the diffraction patters indicate the peak positions of π–π stacking in PEDOT. (c) Schematic diagram for decrease of the π–π inter-plane distance. Blue sheets denote π-stacking planes. Red, yellow, gray, and white spheres represent oxygen, sulfur, carbon, and hydrogen atoms, respectively. (d) AFM height images of PEDOT:PSS films. (e) Schematic illustration of the PEDOT:PSS particle summarizes the results from 2D-GIXD and AFM analyses, i.e., an increase in crystallinity, decrease in the π−π inter-plane distance, and reduced particle size.


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Figure 3. (a) Typical Raman spectrum of PEDOT:PSS film on Si wafer. (b) Raman spectra for the Cα−Cβ stretching mode of PEDOT in PEDOT:PSS films prepared with addition of DMSO at concentrations of 0, 5, and 10 wt%. (c) Spectrum for PEDOT:PSS film with 5 wt% DMSO analyzed using Gaussian functions (top), and the residual between the data and the fitting curve (bottom). The spectra for PEDOT:PSS films with 0 and 10 wt% DMSO are shown in Figure S1 in Supporting Information (SI). (d) PEDOT structures for the benzoid (top) and quinoid (bottom) structures.


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Figure 4. (a) Transmittance spectra of AgNW cathode and Cu cathode. (b) J-V characteristics for Si/PEDOT:PSS hybrid solar cells fabricated with and without vacuum processes. The PCE for the cell fabricated without the vacuum process is 1.3 times that with the vacuum process.


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TOC GRAPHICS


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PEDOT: PSS Hybrid Solar Cell Controlled by

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