Key Factors Affecting the Performance of Sb2S3-sensitized Solar Cells During an Sb2S3 Deposition via SbCl3-thiourea Complex Solution-processing

Yong Chan Choi; Sangl Il Seok; Eunjeong Hwang; Dae-Hwan Kim

doi:10.3791/58062

JoVE Journal > Chemistry

Chimica

Key Factors Affecting the Performance of Sb₂S₃-sensitized Solar Cells During an Sb₂S₃ Deposition via SbCl₃-thiourea Complex Solution-processing

Published: July 16, 2018

doi:

10.3791/58062

Yong Chan Choi¹, Sangl Il Seok², Eunjeong Hwang¹, Dae-Hwan Kim¹

¹Convergence Research Center for Solar Energy,Daegu Gyeongbuk Institute of Science and Technology (DGIST), ²Perovtronics Research Center, School of Energy and Chemical Engineering,Ulsan National Institute of Science and Technology (UNIST)

Summary

This work provides a detailed experimental procedure for the deposition of Sb₂S₃ on a mesoporous TiO₂ layer using a SbCl₃-thiourea complex solution for applications in Sb₂S₃-sensitized solar cells. This article also determines key factors governing the deposition process.

Abstract

Sb₂S₃ is considered as one of the emerging light absorbers that can be applied to next-generation solar cells because of its unique optical and electrical properties. Recently, we demonstrated its potential as next-generation solar cells by achieving a high photovoltaic efficiency of > 6% in Sb₂S₃-sensitized solar cells using a simple thiourea (TU)-based complex solution method. Here, we describe the key experimental procedures for the deposition of Sb₂S₃ on a mesoporous TiO₂ (mp-TiO₂) layer using a SbCl₃-TU complex solution in the fabrication of solar cells. First, the SbCl₃-TU solution is synthesized by dissolving SbCl₃ and TU in N,N-dimethylformamide at different molar ratios of SbCl₃:TU. Then, the solution is deposited on as-prepared substrates consisting of mp-TiO₂/TiO₂-blocking layer/F-doped SnO₂ glass by spin coating. Finally, to form crystalline Sb₂S₃, the samples are annealed in an N₂-filled glove box at 300 °C. The effects of the experimental parameters on the photovoltaic device performance are also discussed.

Introduction

Antimony-based chalcogenides (Sb-Chs), including Sb₂S₃, Sb₂Se₃, Sb₂(S,Se)₃, and CuSbS₂, are considered to be emerging materials that can be used in next-generation solar cells¹^,²^,³^,⁴^,⁵^,⁶^,⁷^,⁸. However, photovoltaic devices based on Sb-Chs light absorbers have not yet reached the 10% power conversion efficiency (PCE) required to demonstrate feasible commercialization.

To overcome these limitations, various methods and techniques have been applied, such as a thioacetamide-induced surface treatment¹, a room temperature deposition method⁴, an atomic layer deposition technique², and the use of colloid dot quantum dots⁶. Among these various methods, the solution-processing based on a chemical bath decomposition exhibited the highest performance¹. However, a precise control of the chemical reaction and the post-treatment are required to achieve the best performance¹^,³.

Recently, we developed a simple solution-processing for high-performance Sb₂S₃-sensitized solar cells using a SbCl₃-thiourea (TU) complex solution³. Using this method, we were able to fabricate a quality Sb₂S₃ with a controlled Sb/S ratio, which was applied to a solar cell to achieve a comparable device performance of 6.4% PCE. We were also able to effectively reduce the processing time since the Sb₂S₃ was fabricated by a single-step deposition.

In this work, we describe the detailed experimental procedure for an Sb₂S₃ deposition on the substrate consisting of mesoporous TiO₂ (mp-TiO₂)/TiO₂ blocking layer (TiO₂-BL)/F-doped SnO₂ (FTO) glass for the fabrication of Sb₂S₃-sensitized solar cells via SbCl₃-TU complex solution-processing³. In addition, three key factors affecting the photovoltaic performance in the course of an Sb₂S₃ deposition were identified and discussed. The concept of the method can be easily applied to other sensitizer-type solar cells based on metal sulfides.

Protocol

1. Synthesis of the TiO₂-BL Solution

Prepare 2 transparent vials with a 50 mL volume.
Add 20 mL of ethanol to 1 vial (V1) and seal V1.
Transfer V1 to an N₂-filled glove box with a moisture-controlled system of an H₂O level of < 1 ppm.
Add 1.225 mL of titanium (IV) isopropoxide (TTIP) to V1 using a syringe with a 0.45 µm PVDF filter and gently stir the mixture for at least 30 min.
NOTE: This step must be performed in a glove box (or under very low humidity conditions) since TTIP is highly sensitive to moisture. If the TTIP solution is not transparent or white precipitates are observed inside the solution, it should not be used, because an undesirable reaction has already occurred inside the solution.
In the other prepared vial (V2), add 18 μL of HNO₃ (70%) and 138 μL of H₂O to 20 mL of ethanol using a micropipette and gently stir the mixture for at least 30 min.
NOTE: This step must not be performed in a glove box, because H₂O is used.
Mix the 2 solutions by pouring the V2 solution into the V1 solution and stir for more than 2 h to synthesize the transparent 0.1 M TiO₂-BL solution.
NOTE: The final solution must be transparent. If the solution is not transparent, resynthesize it until a transparent solution is obtained. Successfully prepared TiO₂-BL solutions are stable for several days at humidity conditions of < 50%.

2. Synthesis of the SbCl₃-TU Solutions with Various SbCl₃/TU Molar Ratios

NOTE: The synthesis must be performed in the glove box because of the very high sensitivity of SbCl₃ to moisture.

Prepare the SbCl₃ stock solution [1 mmol of SbCl₃ in 1 mL of N,N-dimethylformamide (DMF)] inside the glove box. For example, add 6.486 g of SbCl₃ to 30 mL of DMF for a 32.2 mL stock solution.
Add a proper amount of stock solution to a vial containing a given amount of TU to synthesize the SbCl₃-TU solution with the desired molar ratio of SbCl₃/TU. For example, suppose the 2 vials each contain 0.1 g of TU, add 0.9394 mL of the stock solution to one vial and 0.5637 mL to the other, to synthesize solutions with SbCl₃/TU ratios of 1/1.5 and 1/2.5, respectively.

3. Preparation of the Substrate Consisting of mp-TiO₂/TiO₂-BL/FTO Glass

Wash the FTO-coated glass (FTO glass) of 25 mm x 25 mm in an ultrasonic bath with acetone for 10 min, followed by ethanol.
NOTE: To fabricate the photovoltaic device, use pre-patterned FTO glass, where the 5 – 10 mm x 25 mm FTO surface is completely etched.
Instantly dry the FTO glass by blowing compressed air over the sample.
Treat the FTO glass with a UV/O₃ cleaner for 20 min.
Spin coat ethanol on the FTO glass at 5,000 rpm for 60 s.
Immediately spin coat again with the prepared TiO₂-BL solution under the same conditions of step 3.4.
Dry the FTO glass for 2 min by placing it on a preheated hot plate at 200 °C.
Repeat steps 3.5 and 3.6 to obtain the desired TiO₂-BL thickness.
Deposit the mp-TiO₂ layer on the TiO₂-BL/FTO glass using the screen printing method with TiO₂ paste (50 nm TiO₂ particles) and a polyester mask.
Anneal the mp-TiO₂/TiO₂-BL/FTO glass at 500 °C for 30 min.
Dip the annealed substrates in a transparent aqueous 40 mM TiCl₄ solution after cooling them to room temperature.
NOTE: The 40 mM TiCl₄ solution must be transparent. If the substrates are dipped in the TiCl₄ solution before they are cooled, they can easily break because of the large temperature difference between the substrate and the solution.
Transfer the substrates to an oven at 60 °C and store them for 1 h.
Rinse the substrates several times with warm water and instantly dry them by blowingcompressed air on them.
NOTE: To prevent any cracking of the substrates, use warm water (approximately 60 °C) when rinsing.
Anneal the substrates again at 500 °C for 30 min.

4. Deposition of Sb₂S₃ on the Substrate of mp-TiO₂/TiO₂-BL/FTO Glass

Treat the substrates with a UV/O₃ cleaner for 20 min to clean the surface, and transfer them to the glove box.
Spin coat a DMF solvent on the substrates at 3,000 rpm for 60 s prior to spin coating them with the SbCl₃-TU solution.
Heat the as-coated substrates for 5 min by placing them on a hot plate at 150 °C for a partial thermal decomposition and the amorphous phase formation.
Place the samples on a preheated hot plate at 300 °C for 10 min for the crystalline phase formation.
After cooling the samples to room temperature, remove them from the glove box.

5. Fabrication of Sb₂S₃-sensitized Solar Cells

Add 15 mg of poly(3-hexylthiophene) (P3HT) to 1 mL of chlorobenzene and gently stir them until a clear reddish solution is obtained.
Spin coat chlorobenzene on the Sb₂S₃-deposited substrate at 3,000 rpm for 60 s.
Immediately spin coat again with the prepared P3HT solution under the same conditions as used in step 5.2.
Transfer the samples into a vacuum chamber of the evaporator.
Deposit 100 nm gold with a rate of 1.0 Å/s.

Representative Results

Figure 1 shows a schematic representation of the experimental procedure for the Sb₂S₃ deposition on the substrate of mp-TiO₂/TiO₂-BL/FTO glass. Figure 1d shows the basic properties and scheme of a typical product fabricated by the method described herein. The main X-ray diffraction (XRD) pattern is well matched with that of a stibnite Sb₂S₃ structure¹^,³^,⁴ and impurity phases, such as Sb₂O₃, are not visible except for substrate phases (denoted as T and F). In addition, the absorption edge at approximately 730 nm, as shown in the inset of the XRD pattern, is consistent with the band gap (E_g) of Sb₂S₃ (1.7 eV)¹^,³^,⁴^,⁹. These results confirm that quality Sb₂S₃ can be successfully fabricated through the method presented herein.

To fabricate high-performance Sb₂S₃-sensitized solar cells with a > 5% efficiency using this method, three key deposition steps that significantly affect the quality of the final product should be considered during the Sb₂S₃ deposition. These steps are the TiO₂-BL deposition, the mp-TiO₂ deposition, and the SbCl₃-TU solution deposition. Here, we show the factors during the Sb₂S₃ deposition that affect the photovoltaic (PV) performance.

In the step of the TiO₂-BL deposition (key step 1), the thickness of TiO₂-BL can be controlled by repeating the two steps of spin coating with the TiO₂-BL solution and drying the substrate. Figure 2a shows the cross-sectional field emission scanning electron spectroscopy (FESEM) images of the devices fabricated with different TiO₂-BL thicknesses. The TiO₂-BL thickness linearly increases from 46 to 260 nm as the number of repetition times from 1 to 6 increases, as shown in Figure 2a and 2b. In terms of the PV device performance, as measured by PCE, the highest PCE values were observed at a BL thickness of approximately 130 nm (repetition times of 3).

Figure 3a and 3b show the cross-sectional FESEM images of substrates with different mp-TiO₂ thickness and their current density-voltage (J-V) curves as a function of mp-TiO₂ thickness, respectively. The mp-TiO₂ thickness is controlled by choosing different mesh types of the polyester mask. As the mesh count (per inch) of the mask increases from 250 to 460, the mp-TiO₂ thickness decreases from 1600 to 830 nm, as shown in Figure 3a. The PV performance remained similar in the mp-TiO₂ thickness range of 830 – 1200 nm, but further thickness increase led to a reduced efficiency (Figure 3b).

In order to investigate the effects of the SbCl₃:TU molar ratio in key step 3, the absorption properties of the samples prepared with different molar ratios of the SbCl₃-TU precursor solutions were examined. As shown in Figure 4a, the absorption remarkably increased with a TU increase in ratio to 1:2.0; however, it gradually decreased with further TU content increases. To investigate the change of E_g, Tauc plots derived from the absorption spectra were investigated¹⁰. The result indicates a different (αhν)² value but the same E_g of 1.7 eV. The best device performance was obtained around the molar ratio of SbCl₃:TU = 1:2.0³, as shown in Table 1.

Figure 1: A schematic diagram of the deposition procedure for the Sb₂S₃ deposition on the substrate. (a), (b), and (c) These panels shows the three key experimental steps. (d) This panel shows the resultant sample composed of (mp-TiO₂ with Sb₂S₃)/TiO₂-BL/FTO glass. In the XRD pattern, the standard stibnite Sb₂S₃ structure (JCPDS No. 42-1393) is plotted as the red column. This figure has been modified from Choi et al.³. Please click here to view a larger version of this figure.

Figure 2: The effects of TiO₂-BL thickness in key step 1. (a) This panel shows cross-sectional FESEM images of photovoltaic devices fabricated with different TiO₂-BL thicknesses. In the images, BL# means the TiO₂-BL fabricated by # of times repetition, and the part of TiO₂-BL is marked with a red rectangle. (b) This graph shows the TiO₂-BL thickness as a function of the repetition number. (c) This panel shows a PCE graph as a function of TiO₂-BL thickness. The symbols and error bars in panel c are averages and standard deviations, respectively, obtained from the PCE data of ten devices. Please click here to view a larger version of this figure.

Figure 3: The effects of mp-TiO₂ thickness in key step 2. (a) This panel shows cross-sectional FESEM images of the substrates with different mp-TiO₂ thicknesses. (b) This panel shows a variation of the J-V curves as a function of mp-TiO₂ thickness. Please click here to view a larger version of this figure.

Figure 4: The effects of the molar ratio of SbCl₃/TU in key step 3. These panels show (a) the absorption, (b) the graph of a Tauc plot, and (c) photos of samples fabricated with different SbCl₃:TU molar ratios. The Tauc plot was obtained by assuming that Sb₂S₃ has a direct E_g. Please click here to view a larger version of this figure.

SbCl₃:TU Ratio	J_SC (mA cm^-2)	V_OC (mV)	FF (%)	PCE (%)	R_SH/R_S (Ω cm²)
1:1.4	12.2	475.4	61.7	3.8	582.4/7.1
1:1.6	12	487.4	66.4	4.1	1135.4/6.5
1:1.8	12.7	493.4	66.5	4.4	1217.3/6.8
1:2.0	13.1	493.4	61.6	4.2	644.7/7.8
1:2.2	13	487.4	59.4	3.9	541.8/8.9

Table 1: The effects of the molar ratio of SbCl₃/TU on the photovoltaic performance. J_SC, V_OC, and FF indicate the short-circuit current density, open-circuit voltage, and fill factor, respectively. The table has been reproduced from Choi et al.³.

Supplementary Figure S1: The effects of the presence of mp-TiO₂. These panels show the typical (a) device performance and (b) absorption properties depending on the presence of mp-TiO₂. The samples were fabricated under the same conditions as those used for Figure 2. Mp-TiO₂ with a 1 μm thickness was used for the comparison. Please click here to download this file.

Discussion

TiO₂-BL is widely used as a hole-blocking layer in solar cells. As shown in Figure 2, a large difference was observed in the device performance depending on the TiO₂-BL thickness. Therefore, its thickness should be optimized to obtain the best overall device performance, because it critically acts as a hole-blocking layer to prevent any direct contact between FTO and hole-transporting materials¹¹. It should be noted that the optimum thickness varies depending on the TiO₂-BL solution species, FTO types, method, light absorbers, and device architectures. In addition to the TiO₂-BL thickness, it should be scanned for annealing conditions including temperature and time in terms of the defect control of TiO₂¹².

In the device created with this protocol, the mp-TiO₂ plays a crucial role in achieving a high performance for two reasons. First, devices with mp-TiO₂ generally have higher J_SC values than those without mp-TiO₂, due to the higher absorption characteristics obtained from the Sb₂S₃ deposited on mp-TiO₂, as shown in Supplementary Figure S1. Second, the Sb₂S₃ fabricated via this protocol is easily formed into an island shape rather than a compact thin film on a planar surface¹³. This leads to an undesirable direct contact between the HTM and the TiO₂-BL in planar solar cells. Therefore, it is essential to use mp-TiO₂ in the device introduced here and to find the optimum thickness of mp-TiO₂ for achieving a high performance. For the solar cells fabricated with mp-TiO₂, the mp-TiO₂ thickness is considered as a key factor for obtaining solar cells of high performance and varies depending on the types of materials deposited on the surface of mp-TiO₂. For example, mp-TiO₂ with a thickness of 5 – 30 μm and < 200 nm is typically applied in dye-sensitized¹⁴ and hybrid perovskite solar cells¹⁵^,¹⁶^,¹⁷, respectively, to achieve a good device performance. In the current Sb₂S₃-sensitized solar cells, the thickness of mp-TiO₂ of approximately 1 μm is more suitable for the best performance³, but the optimum thickness may vary and mp-TiO₂ may not be needed depending on the method².

Determining the ideal SbCl₃:TU molar ratio is critically important because it strongly affects the absorption properties of the light sensitizer, which are closely related to J_SC, as shown in Figure 4. In addition, an optimized ratio can aid in forming high-purity Sb₂S₃ without impurities or residues. For the samples fabricated with higher TU ratios, elemental sulfur is formed on the surface, which interrupts the charge flow in the device³. Therefore, to obtain improved devices, the molar ratio should be optimized.

In this study, we have demonstrated three key experimental factors in the course of an Sb₂S₃ deposition and their effects on the PV device performance of Sb₂S₃-sensitized solar cells. The protocol presented here can be applied to other sensitizer type PV systems based on Sb₂Se₃⁵, Sb₂(S/Se)₃⁷, and CuSbS₂⁸. We strongly believe that this method provides guidance on accessing novel materials for PV systems.

Divulgazioni

The authors have nothing to disclose.

Acknowledgements

This work was supported by the Daegu Gyeongbuk Institute of Science and Technology (DGIST) R&D Programs of the Ministry of Science and ICT, Republic of Korea (Grants No. 18-ET-01 and 18-01-HRSS-04).

Materials

Ethyl alcohol, Pure, >99.5%	Sigma-Aldrich	459836
Titanium(IV) isopropoxide 97%	Aldrich	205273
Nitic acid, ACS reagent, 70%	Sigma-Aldrich	438073
Antimony(III) chloride	Sigma-Aldrich	311375
Thiourea	Sigma-Aldrich	T7875
N,N-Dimethylformamide, anhydrous, 99.8%	Sigma-Aldrich	227056
TiO2 paste with 50 nm particles	ShareChem	SC-HT040
Poly(3-hexylthiophene)	1-Material	PH0148
Chlorobenzene	Sigma-Aldrich	284513
FTO/glass (8 Ohmos/sq)	Pilkington
Spin coater	DONG AH TRADE CORP	ACE-200
Hot plate	AS ONE Corporation	HHP-411
Glove box	KIYON	KK-021AS
UV OZONE Cleaner	AHTECH LTS	AC-6
Furnace	WiseTherm	FP-14
UV/Vis Absorption spectroscopy	PerkinElmer	Lambda 750
Multifunctional evaporator with glove box	DAEDONG HIGH TECHNOLOGIES	DDHT-SDP007

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Choi, Y. C., Seok, S. I., Hwang, E., Kim, D. Key Factors Affecting the Performance of Sb₂S₃-sensitized Solar Cells During an Sb₂S₃ Deposition via SbCl₃-thiourea Complex Solution-processing. J. Vis. Exp. (137), e58062, doi:10.3791/58062 (2018).