Article pubs.acs.org/JPCC
Efficient Hole Extraction from Sb2S3 Heterojunction Solar Cells by the Solid Transfer of Preformed PEDOT:PSS Film Jung Kyu Kim,†,§ Ganapathy Veerappan,†,§ Nansra Heo,† Dong Hwan Wang,‡ and Jong Hyeok Park*,† †
SKKU Advanced Institute of Nanotechnology (SAINT) and School of Chemical Engineering, Sungkyunkwan University, Cheoncheon-dong, Jangan-gu, Suwon 440-746, Republic of Korea ‡ School of Integrative Engineering, Chung-Ang University, 84 Heukseok-Ro, Dongjak-gu, Seoul 156-756, Republic of Korea S Supporting Information *
ABSTRACT: Here, we report significant improvements of Voc and FF in Sb2S3 quantum dot (QD)-based, solid-state heterojunction solar cells prepared from the solid transfer of preformed PEDOT:PSS hole extraction layers. Despite the moderate optical properties of Sb2S3 QDs, the solid state QD solar cells suffer from poor power conversion efficiency (PCE) resulting from the disappointing Voc and the high series resistance since there is inefficient charge extraction from QDs to the metal top electrode. In order to improve the hole extraction performance, a significantly uniform PEDOT:PSS (poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)) layer was transferred on the hole transport layer (P3HT, poly(3-hexylthiophene-2,5-diyl)) by using a simple solid-transfer method. In contrast with conventional spin-cast methods, the hydrophilic PEDOT:PSS layer was uniformly coated on the hydrophobic P3HT layer without any significant detriment to P3HT film properties. Due to improved contact surface for the Au top electrode and hole conductance resulting in significantly improved charge extraction, the power conversion efficiency was dramatically enhanced. Furthermore, the thickness of the PEDOT:PSS film was precisely optimized by layer-bylayer solid transfer, and thereby the PCE of the PEDOT:PSS solid-transfer device (30 nm) was improved by 25.7% in comparison to the PEDOT:PSS spin-cast device and by 76% in comparison to the PEDOT:PSS free device.
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INTRODUCTION As a promising third-generation solar cell, solid-state metal chalcogenide based-quantum dot (QDs)-sensitized heterojunction solar cells (QDHSCs) have gained the attention of researchers worldwide. This increase in interest is due to the unique characteristics of QDs including their facile band gap energy tuning, applicability in solution processing, high absorption coefficient, multiexciton generation, and high potential for hot electron injection.1−3 Recently, Sb2S3 has been increasingly utilized as a QD-sensitizer for highly efficient QDHSCs because of its moderate optical properties: a band gap of 1.7 eV and an absorption coefficient of 1.8 × 105 cm−1.4 However, despite the outstanding properties of Sb2S3 QDs, solid state QD solar cells suffer from low power conversion efficiency (PCE) compared to solid-state dye sensitized solar cells due to the fact that the sulfide radical species in Sb2S3 can induce substantial recombination of photogenerated charges resulting in poor open circuit voltage (Voc) and fill factor (FF) values.3 In order to enhance the PCE, various approaches have been investigated including TiO2 surface modification using transient metal cations, doping metals into the sensitizer, and optimization of the interface properties of QDs.8−11 Unfortunately, the disappointing Voc and the high series resistance in these devices still remain, because inefficient charge extraction from QDs to the metal top electrode acts as a further impediment for realizing high conversion efficiency.5−7,12 © 2014 American Chemical Society
Considering the working principles of the Sb2S3 QDHSCs, photogenerated electrons and holes are dissociated from the excitons in the QDs. Then, electrons are extracted into the fluorine-doped tin oxide (FTO) through the TiO2, and holes are transferred to the metal anode. However, due to surface recombination and short circuit problems, p-type acceptor polymers have been introduced into the interlayer between the QDs and the top electrode (Au). In order to improve charge transfer, hole transporting materials with various highest occupied molecular orbital (HOMO) levels were deposited on the mesoporous QD/TiO2 films. Thus, efficient hole transportation was achieved by using poly(3-hexylthiophene) (P3HT); the HOMO level of P3HT is 5.2 eV when Au (5.0 eV) is deposited on top of it as the top electrode.9 Although the energy level alignments with the valence band (VB) of Sb2S3, the HOMO level of P3HT, and the work function (WF) of Au are well matched, the hole extraction performance from the QDs to the Au electrode can be further enhanced by introducing a poly(3,4-ethylenedioxy thiophene) doped with a polystyrenesulfonate (PEDOT:PSS) film into the interface between the P3HT and Au electrode. This allows the moderate work function of PEDOT:PSS (5.1 eV) to induce a cascade Received: July 29, 2014 Revised: September 8, 2014 Published: September 9, 2014 22672
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electrodes at 500 rpm for 20 min and dried at 90 °C for 30 min. As for the transfer device, after a PEDOT:PSS (AI4083) layer was spin coated on the stamping mold, the P3HT coated sample was placed on a hot plate at 90 °C. Then, the PEDOT:PSS layer was immediately transferred from the mold to the surface of the P3HT layer. Approximately 0.1 kgf of pressure was applied to the mold for 10 s before peeling off the mold at a speed of approximately 2 mm/s. We maintained the total thermal treatment time at 90 °C for 30 min. As for the reference device, the PEDOT:PSS solution was diluted with five times the volume of isopropyl alcohol and introduced onto the P3HT layer and dried at 90 °C for 30 min. Finally, devices were completed by depositing Au top electrode (thickness of 100 nm) by thermal evaporation with a base pressure of ∼10−6 Torr. Characterization and Measurements. The surface morphology images were obtained by atomic force microscopy (AFM) measurement using a Dimension 3100 instrument (Veeco, Plainview, NY). In order to obtain cross-sectional transmission electron microscopy (TEM) images, samples were prepared by focused ion beam (FIB) milling using a JIB-4601F apparatus (JEOL). The FIB-etched samples were transferred to the TEM chamber to obtain the cross-sectional images using a JEM ARM 200F instrument (JEOL). The reflectance spectra were obtained by UV−visible spectroscopy using a Cary 5000 instrument (Varian). The J−V characteristics of the devices were measured by using Oriel 91193 (1000 W, 100 mW/cm2) and Keithley 2400 source meters. A 1 sun light was calibrated using an NREL-calibrated Si solar cell (VLSI standards, Oriel 91150 V). An 8.5 mm2 aperture was used as a shadow mask to determine the active area of the cell. The Rs and Rsh values were obtained by using a previously reported process.29 Electrochemical impedance spectroscopy (EIS) measurements were conducted between 0.1 Hz and 100 kHz using a Zahner IM6 under 1 sun irradiation. The IPCE was measured using a specially designed system from PV measurement Inc. with a 75 W xenon lamp as a light source.
hole transportation. Additionally, the use of PEDOT:PSS layer can enhance the adhesion of Au onto the P3HT layer and provide an adequate surface for contact with the Au top electrode.13−16 According to the theoretical calculations by Bisquert’s group, a PCE of approximately 8.5% can be achieved by reducing the hole transport resistance in Sb2S3-based solar cells. This is because the recombination of holes and electrons mostly occurs in the hole transporting layer (HTL) and reduces the charge collecting efficiency.17,18 Conventionally, the PEDOT:PSS hole extraction layer (HEL) is deposited on top of the P3HT layer by a spincasting process. However, because PEDOT:PSS is a water based-solution with strong acidity, the hydroxyl component and the water molecules in PEDOT:PSS can oxidize the polymer chains and aggravate the morphology and crystallinity of P3HT.19,20 Additionally, due to the hydrophilic property of PEDOT:PSS, it can be difficult to obtain a uniform HEL on top of the P3HT layer, which has hydrophobic properties.21 Thus, the hole transport performance might not be improved as significantly as expected. To solve this shortcoming, we successfully deposited a very uniform PEDOT:PSS HEL on top of the P3HT HTL by the solid transfer of preformed PEDOT:PSS films using a rigiflex stamping mold with a very flat surface. In addition, the thickness of the PEDOT:PSS film was precisely tailored via layer-by-layer solid transfer. Owing to the very flat surface of the mold, the transferred PEDOT:PSS film had significant uniformity. This resulted in an improved contact surface for the Au top electrode and charge conductance, which yielded significantly improved charge extraction. As a result, this simple approach considerably improves the Voc and FF values of the device by dramatically reducing the total series resistance.
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EXPERIMENTAL SECTION Sb2S3 QD Synthesis. The Sb2S3 inorganic sensitizer was deposited by chemical bath deposition (CBD).3 Next, the prepared TiO2 electrodes were dipped into a CBD mixture containing 650 mg of SbCl3 in 2.5 mL of acetone, 25 mL of a 1 M Na2S2O3 aqueous solution, and 72.5 mL of deionized water at 6 °C. The orange-colored amorphous stibnite layers that formed on the TiO2 electrodes were sintered at 330 °C in an argon atmosphere for 30 min. Solid-Transfer Mold. Trichloro (1H,1H,2H,2H-perfluorooctyl) silane (FOTS) was exploited for the surface treatment of a precleaned silicon wafer to modify its surface energy and form self-assembled monolayers (SAMs) on the surface. Then, a UV-curable polyurethane acrylate (PUA) resin (Norland optical adhesive) was drop-cast on the SAM-treated Si wafer. After covering the PUA-coated Si wafer with a PC film, the sample was cured with UV light (365 nm). The cured PUA/PC film was detached from the wafer and then a UV-ozone surface treatment was carried out for 8 min.22−24 Solar Cell Fabrication. A 70 nm thick TiO2 blocking layer (BL-TiO2) was spin coated onto previously etched and cleaned FTO glass substrates. Then, a commercial nanocrystalline TiO2 paste (∼50 nm, EnB Korea) was deposited by a doctor blading method onto the BL-TiO2/FTO substrate and sintered at 500 °C for 30 min. To improve the contact between TiO2 particles, a 20 mM TiCl4 treatment was carried out at 70 °C for 12 h. Particles were then sintered at 500 °C for 30 min. The optimum photoelectrode thickness was determined to be 1.0 μm. A P3HT solution (15 mg/mL in 1,2-dichlorobenzene, Rieke Metals, Inc.) was spin coated onto Sb2S3-sensitized TiO2
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RESULTS AND DISCUSSION As shown in the schematic image in Figure 1, a P3HT solution was spin coated on top of the Sb2S3 QD-adsorbed TiO2 particulate film. Then, a preformed PEDOT:PSS film, prepared on the stamping mold by a conventional spin coating method, was transferred onto the P3HT layer at 90 °C using a hot plate. Due to the very flat surface of the stamping mold, the transferred PEDOT:PSS film had good uniformity.22−24 Subsequently, the samples were dried at 90 °C for 30 min on the hot plate before the Au electrode was deposited using thermal evaporation at ∼10−6 Torr. In order to disregard effects from the thermal treatment, all samples, including the reference device, were thermally treated for the same period of time at 90 °C. For devices with a double- or triple-layer transfer, the PEDOT:PSS films were stamp-transferred layer-by-layer on top of the as-coated films. For a conventional Sb2S3 QDHSC, the PEDOT:PSS solution is dropped and spin coated on the P3HT layer. However, the hydrophilic nature of PEDOT:PSS hinders its uniform deposition on the surface of hydrophobic P3HT. Thus, many research groups dilute the PEDOT:PSS solution with more than two times the alcohol-based solvents (e.g., methanol or isopropyl alcohol) in order to deposit the PEDOT:PSS layer.12,13,25,26 Due to the polar nature of the alcoholic solvents, this can cause the P3HT polymer chains to deflate at the solid− 22673
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being placed between a silicon wafer and a rigid film. Since the sandwiched resin was peeled off from the very flat silicon wafer, the stamping mold should have a similarly smooth surface. Therefore, the solid transfer of the preformed PEDOT:PSS layer created a uniform coating on the surface of P3HT irrespective of the conflicting polarities of PEDOT:PSS and P3HT. Because of the moderate energy level alignment between the VB of Sb2S3 (−5.4 eV), HOMO level of P3HT (−5.2 eV), WF of PEDOT:PSS (−5.1 eV), and Au (−5.0 eV)9,28 (Figure 4A), all of the performance parameters of the spin coated PEDOT:PSS device (Sp device) were much improved compared to the device without a PEDOT:PSS layer, as shown in Figure 4 and Table 1. Especially, the Voc and FF values of the device fabricated by the solid transfer with a 15 nm thick PEDOT:PSS single layer (1L-T device) were significantly improved compared to the device without PEDOT:PSS and the Sp devices: 0.525 V and 54.9% for the 1L-T device, 0.473 V and 52.0% for the Sp device, and 0.425 V and 50.2% for the PEDOT:PSS-free device (Figure 4B). Furthermore, for the QDHSC device fabricated by the solid transfer of two PEDOT:PSS layers (2L-T device), of which the thickness was approximately 30 nm, the Voc (0.55 V) and FF (61.6%) parameters were significantly improved as shown in Figure 4B. This increase in performance is due to the fact that the uniformly transferred PEDOT:PSS layer provides an enhanced surface for contact with the Au top electrode. Because the metal electrode directly connects to the polymer layer, photogenerated charges can be recombined at the interface of them unless the metal to polymer contact was under ohmic contact.30 Importantly, the charge extraction performance from the polymer layer to metal electrode is strongly affected by the interface energy states and Fermi-level pinning. Therefore, some defects or nonconformal coating of the top layer on the π-conjugated polymer can cause the charge trapping and charge accumulation, thereby resulting in annihilating carriers and reducing Voc.31 As shown in Figure 2, despite the 1L transfer of PEDOT:PSS covers most of the surface of the bottom layer (especially compared to the spin coated PEDOT:PSS), the thickness of ∼15 nm was insufficient to fully cover the bottom layer due to the rough surface of the bottom layer. However, when the second PEDOT:PSS layer was solid-transferred on top of the pretransferred PEDOT:PSS layer, the two transferred layers of PEDOT:PSS (30 nm thick)
Figure 1. Schematic of the fabrication of the Sb2S3 QD-sensitized heterojunction solar cell using the solid transfer of a preformed PEDOT:PSS hole extraction layer.
liquid interface, resulting in a decrease in the interface contact energy. Concentrated, submicrometer-scale PEDOT:PSS regions can also form on the surface of P3HT during spin coating of the PEDOT:PSS solution.27 Moreover, by using the spincasting method, controlling the PEDOT:PSS film thickness on top of the P3HT layer can be difficult. Cross-sectional TEM images prepared with FIB milling are shown in Figure 2. These images reveal poor contact of Au electrode onto the P3HT layer; the spin coated PEDOT:PSS was nonuniformly deposited on the P3HT layer, whereas the solid transferred PEDOT:PSS film almost perfectly covered the surface of P3HT. Therefore, the stamp-transferring method allows for the thickness of the PEDOT:PSS film on top of the P3HT layer to be readily controlled. As shown in Figure 3, the pristine P3HT surface (Figure 3A) was very rough; even though the roughness of the P3HT film was slightly alleviated by the spin coated PEDOT:PSS layer (Figure 3B), the stamp-transferred PEDOT:PSS film (Figure 3C) on the P3HT film shows a much smoother surface morphology that resembles the flat surface of the mother substrate (the stamping mold). The surface roughness of the stamping mold was analogous to that of a silicon wafer because the polymer resin was cured after
Figure 2. Cross-sectional TEM images prepared by FIB milling of the Sb2S3 QDHSC devices with spin coated PEDOT:PSS, transferred PEDOT:PSS, and without PEDOT:PSS. The transferred PEDOT:PSS layer was deposited layer-by-layer to form a monolayer (1L), double-layer (2L), or triple-layer (3L). 22674
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Figure 3. Three-dimensional AFM images of the surface of P3HT films (A) without PEDOT:PSS, (B) with spin coated PEDOT:PSS, and (C) with transferred PEDOT:PSS (monolayer) on top of P3HT films that were spin coated on Sb2S3 QDs/TiO2/FTO substrates.
ties between the polymer layers and the Au electrode reduced the series resistance and improved the hole transport performance from the QDs to the top electrode by suppressing charge recombination.14−18 To scrutinize the interface properties of all our devices, EIS measurements were carried out (Figure 4c). The total resistance of the QDHSC was dramatically reduced by introducing PEDOT:PSS with the simple solid transfer method. Additionally, the resistance values were also calculated by fitting the J−V characteristics in Table 1.28,29 The tendency of the parameters was well matched with the EIS results. Furthermore, due to the improved interfacial properties, the leakage current was also decreased, which resulted in an increase in the Rsh. However, when the thickness of the transferred PEDOT:PSS layers increased too much, the charge collecting efficiency decreased; this caused an increase in the series resistance (Rs), thereby reducing the Voc and FF values. Consequently, the Rs value decreased as the number of transferred PEDOT:PSS layers increased from no PEDOT:PSS (10.6 Ω cm2) to the 2L-T device (8.15 Ω cm2). However, the triple-layer transferred device (3L-T device), with an average PEDOT:PSS thickness of approximately 50 nm, had an Rs value of 11.6 Ω cm2. Although the transferred PEDOT:PSS layer provided the QDHSC with an improved surface for contact with the top electrode, the short-circuit current density (Jsc) value of the 2LT device was somewhat lower than that of the Sp device: 14.2
Figure 4. (A) Schematic energy diagram, (B) photocurrent vs voltage (J−V) curves, and (C) Nyquist plots of the electrochemical impedance spectroscopy (EIS) of Sb2S3 QDHSC devices with spin coated PEDOT:PSS, transferred PEDOT:PSS, and without PEDOT:PSS. The transferred PEDOT:PSS layer was deposited layer-by-layer.
could fully cover the rough bottom layer. Therefore, the 2L transferred PEDOT:PSS layer enhanced hole extraction from P3HT to the Au electrode and resulted in significantly improved contact properties. The enhanced interfacial proper-
Table 1. Performance Parameters of the Sb2S3 QDHSC Devices Fabricated by Spin Coating; Single-, Double-, and Triple-Layer Transferring PEDOT:PSS; and without a PEDOT:PSS Layer VOC (V) spin coated without 1L transferred 2L transferred 3L transferred
0.473 0.425 0.525 0.550 0.527
± ± ± ± ±
0.05 0.02 0.01 0.03 0.05
JSC (mA/cm2) 14.2 12.0 12.6 13.2 13.1
± ± ± ± ±
0.12 0.32 0.10 0.24 0.22
FF (%) 52.0 50.2 54.9 61.6 55.8
± ± ± ± ±
0.07 0.02 0.03 0.02 0.03
PCE (%)a
Rs (Ω cm2)b
Rsh (Ω cm2)b
± ± ± ± ±
10.6 11.7 9.01 8.15 11.6
422 244 504 826 915
3.5 2.5 3.6 4.4 3.8
0.20 0.18 0.10 0.15 0.22
a A shadow mask with an 8.5 mm2 aperture was placed on the FTO glass side of the device to determine the exact operating area during measurement of device performance. The parameters are average values resulting from measuring five devices. bThe series resistance (Rs) and the shunt resistance (Rsh) values were calculated by fitting the J−V characteristics of the devices, which had the highest performance.14
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mA/cm2 for the Sp device and 13.2 mA/cm2 for 2L-T device. This was confirmed via measurement of the incident photocurrent conversion efficiency (IPCE), as shown in Figure S1 (Supporting Information). However, the better resistance performance (Rs and Rsh) of the 2L-T device, compared to the Sp device, reveals that the ∼7% lower Jsc value may not be related to charge transport or recombination. Instead, the uniform morphology of the transferred PEDOT:PSS might be diminishing the amount of light reaching the QDs. When the incident light is irradiated on the FTO side of the device, the visible light must pass through the FTO and TiO2 layers before arriving at the QDs. Furthermore, unabsorbed light continues to pass through the PEDOT:PSS layer and then reaches the top electrode. Because the metal electrode can reflect this light, reflected light can be absorbed by the QDs again. However, due to the uniform morphology of the transferred PEDOT:PSS layer, the intensity of the reflected light at the surface of the Au electrode could be decreased compared to the amount of light reflected in the Sp device; the PEDOT:PSS layer was formed on the P3HT layer in patches and nonuniformly in the Sp device, as shown in Figure S2. Therefore, the decrease in the amount of light reflected from the Au metal electrode could result in a lower Jsc value for the layer transferred device compared to the spin coated device. In any case, despite the slightly lower Jsc value, the dramatically improved Voc and FF parameters dominated the resultant PCE; the 2L-T device was improved by more than 25.7% compared to the Sp device.
CONCLUSION In summary, we demonstrated that the simple solid transfer of preformed PEDOT:PSS between the top electrode and the polymer layer (the HTL) in Sb2S3 QD-based heterojunction solar cells could significantly enhance device performance. Due to the very flat surface of the stamping mold, the transferred preformed PEDOT:PSS layer had better uniformity compared to the spin coated PEDOT:PSS layer on top of the hydrophobic conventional HTL layer. Therefore, both the charge transport performance and the surface contact with Au top electrode were dramatically improved. We believe that this work helps to address one of the serious processing problems facing QDHSCs. ASSOCIATED CONTENT
S Supporting Information *
UV−vis reflectance and IPCE curves. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions §
J.K.K. and G.V. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the NRF of Korea Grant funded by the Ministry of Science, ICT & Future Planning (NRF2013R1A2A1A09014038, 2009-0092950, 20110023215, 20090083540, 2014M3A7B4051747). J.H.P. acknowledges the support from the MKE (20123010010070). 22676
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The Journal of Physical Chemistry C
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dx.doi.org/10.1021/jp507652r | J. Phys. Chem. C 2014, 118, 22672−22677
