High-performance Dye-sensitized Solar Cells Based on Colloid

Subscriber access provided by Eastern Michigan University | Bruce T. Halle Library

Energy, Environmental, and Catalysis Applications

High-performance Dye-sensitized Solar Cells Based on Colloid-solution Deposition Planarized FTO Substrates Jinyin Zhang, Yanyan Lou, Miaomiao Liu, Hualan Zhou, Yin Zhao, Zhuyi Wang, Liyi Shi, Dongdong Li, and Shuai Yuan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01737 • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 11, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

296x209mm (300 x 300 DPI)

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 32

High-performance Dye-sensitized Solar Cells Based on Colloid-solution Deposition Planarized FTO Substrates

Jinyin Zhanga, Yanyan Loua, Miaomiao Liua, Hualan Zhoub*, Yin Zhaoa, Zhuyi Wanga, Liyi Shia, Dongdong Lic, Shuai Yuana*

a

Laboratory for Microstructures and Research Center of Nanoscience and

Nanotechnology, Shanghai University, 99 Shangda Road, Shanghai 200444, China b

School of Medical Instrument and Food Engineering, University of Shanghai for

Science and Technology, Shanghai 200093, China c

Division of Energy & Environment Research, Shanghai Advanced Research Institute,

Chinese Academy of Sciences, Shanghai 201203, China *Corresponding

authors:

E-mail:

[email protected]

(H.Z.).

[email protected] (S. Y.)

Abstract: The transmittance and conductivity of fluorine doped tin oxide (FTO) conductive glasses are the critical factors limiting the performance of dye-sensitized solar cells (DSSCs). Here, the transmittance and conductivity of commercial FTO glasses were improved via a colloid-solution deposition planarization (CSDP) process. The process includes two steps. Firstly, the FTO nanocrystal colloid was deposited on the FTO glasses 1


Page 3 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


by spin-coating. Secondly, the coated glasses were treated by FTO precursor solution. Compare to the bare FTO glasses, the modified FTO glasses by CSDP process achieved 4% increase in transmittance (at 550 nm) and 11% decrease in sheet resistance, respectively. In addition, the modified FTO glasses can reduce the aggregation of Pt nanoparticles and improve the electrocatalytic activity of Pt counter electrodes. When the modified FTO glasses were used to assemble DSSCs, the cells got a photoelectric conversion efficiency as high as 9.37%. In contrast, the efficiency of reference cells using bare FTO substrates was about 8.24%.

Keywords: Fluorine doped SnO2 glasses, transmittance, conductivity, colloid-solution deposition planarization, dye-sensitized solar cells.

1. Introduction Dye-sensitized solar cells (DSSCs) as a kind of clean energy conversion devices have attracted numerous researches in recent years. The transparent conductive substrates, dye-sensitized TiO2 photoanode, iodine redox electrolyte and platinum counter electrode compose a typical sandwich-type DSSC. In order to improve the photoelectric conversion efficiency, most efforts have been done on photoanode construction, dye molecular design and adjustment of redox shuttle. 1-8 As one critical component, the transparent conductive substrates greatly affect both light transmission and charge collection of the 2



Page 4 of 32

cells. 9 However, there are few report on improving the performance of DSSCs from the point of view of transparent conductive glass. Typical transparent conductive substrates utilized in DSSCs are indium tin oxide (ITO) and fluorine doped SnO2 (FTO) conductive glasses. ITO shows outstanding optical and electric performance, in particular lower sheet resistance. 10 However, the ITO film exhibits brittleness, instability at high temperature and high costs due to indium element.11 In comparison, FTO possesses outstanding stability and relatively low cost, and dominates the field of dye-sensitized solar cells. 12-15 But it is worth emphasizing that the trade-off between transmittance and resistance of FTO still limits the enhancement of DSSC performance. On large-scale, the FTO thin films are deposited on glasses by atmospheric pressure chemical vapor deposition (APVCD) or spray pyrolysis. 16-20 The optical transmittance of the conductive films manufactured by these methods is around 80% (at 550 nm). Meanwhile the sheet resistance is about 10 Ω/□. 21 During these fabrication processes, the rough textures and cracks are usually formed on the surface of conductive film due to the low symmetric tetragonal crystal structure of tin oxide. 16 Obviously, a well-connected FTO film is desired to improve light transmittance and charge transport.

22

How to

improve the optical and electrical properties of FTO glasses simultaneously is still a challenge. Previously, our group have developed an easy-reproducible and cost-effectively 3


Page 5 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


method to reduce the surface roughness of substrates with high efficiency, which is called the colloid-solution deposition planarization (CSDP) process.

23

Furthermore, FTO

nanocrystal colloid exhibiting notable electric conductivity, high dispersity and stability has been prepared. 24 Based on these previous works, the CSDP method is trying to be employed to modify the FTO glasses. As shown in Scheme 1, the CSDP process includes two steps. Firstly, the FTO nanocrystal colloid will be deposited on the FTO glasses by spin-coating. Secondly, the coated glasses will be treated by FTO precursor solution in a chemical bath. The modified FTO substrates are going to be utilized to assemble DSSCs. The mechanism of FTO transparent conductive glasses affecting the photoelectric conversion efficiency was investigated in detail.

Scheme. 1 The scheme of (a) bare FTO film, (b) FTO nanocrystal colloid deposition planarized FTO film (CDP-FTO) and (c) FTO nanocrystal colloid and FTO precursor solution deposition planarized FTO film (CSDP-FTO).

2. Experimental 2.1. Materials

4



SnCl4·5H2O, NH4F, NH4OH, H2C2O4, KCl, LiI, I2, LiClO4 and TiCl4 (AR) were purchased from Sinopharm Group Co. Ltd. TiO2 paste (20 nm and 200 nm) and electrolyte were purchased from Heptachroma Co. Ltd. N719 (99%) was purchased from J&K Chemical. H2PtCl6·6H2O and acetonitrile (AR) ware from Aladdin Co Ltd. Surlyn film (60 µm) and FTO (8 Ω/□) were from Wuhan Georg Science Instrument Co. Ltd. 2.2. Synthesis of fluorine doped SnO2 (FTO) colloids and precursor solutions The fluorine doped SnO2 colloids were prepared by sol-hydrothermal method according to the previously description. 24 The typical experimental process is as follows: 11.2g SnCl4·5H2O was dissolved in deionized water. Then, 3.0 M aqueous ammonia solutions were employed to adjust the pH of the solution to 7.5～8.5, leading to the precipitation. In order to ensure the purity of the nano-SnO2 particles, the precipitate was rinsed sufficiently until the ionic conductivity of the filtrate was less than 10 µs/cm. The washed precipitate was dispersed in oxalic acids solution and refluxed at 100°C for 4 h. When the solution was cooled to ambient temperature, NH4F was added proportionally to it under vigorous stirring until completely dissolved. Hydrothermal treatment was then carried out by employed a 100 ml autoclave at 180°C for 48 h. The fluorine doped tin oxide nanocrystals were centrifuged and rinsed with deionized water until the ionic conductivity of the filtrate was less than 10 µs/cm. At last, the FTO nanocrystals colloid with a solid content of 0.5%, 1.0%, and 1.5% were obtained after re-dispersion in proportional ethanol by sonication. 5


Page 6 of 32

Page 7 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The fluorine doped SnO2 precursor solutions was prepared according to the previous method.

25

In a typical process, 4.38g SnCl4·5H2O and 0.23g NH4F were dissolved in

de-ionized water under vigorous stirring at room temperature. 2.3. Fabrication of dye-sensitized solar cells Transparent conductive substrates (FTO glass, 8 Ω/□) were cleaned by sonication in the proper sequence of acetone, ethanol and deionized water. After dried at 70°C for 20 min, the washed FTO conductive substrates were treated by UV-O3 pretreatment. The fluorine doped SnO2 nanocrystal colloid was deposited on FTO glasses by spin-coating and dried at 50°C for 40 min, then calcined at 450°C for 20 min. After the colloidal deposition, the FTO glasses were immersed into the fluorine doped SnO2 precursor solutions for 30 min at 70°C, then calcined again for 20 min at 450°C. The bare and modified conductive substrates were treated with TiCl4 (50 mM) for 30 min at 70°C and subsequently washed in deionized water, then dried at 80°C for 30 min. After cooling to ambient temperature, the commercialized TiO2 pastes (transport layer, scattering layer, respectively) were coated onto the FTO conductive substrates by doctor-blading process. 26 The coated layers were dried at 125°C for 6 min and sintered at 500°C for 30 min. After the porous TiO2 layers were prepared, the glasses were treated by TiCl4 (50 mM) one more time for 30 min at 70°C. Then, the treated glasses were rinsed with deionized water and calcined at 500°C for 30 min. While the glasses were cooling to 80°C, the porous TiO2 films were immersed into 0.3 mM N719 dye solution and 6



Page 8 of 32

sensitizing for 24 h. After sensitized at the N719 dye solutions, the TiO2 photoanode were cleaned by anhydrous ethanol and dried at room temperature. Pt counter electrode (CE) was used due to its relative high stability in / redox system and remarkable catalytic activity. The Pt counter electrodes were prepared by employing pulsed electrodeposition method for 25 circles.

27

One circle composed of 30

mA/cm2 current density for 0.1 s and intermittent 0.3 s. After electrodeposition, these conductive glasses were dried at 80°C for 30 min, and then Pt catalytic counter electrodes were collected. TiO2 photoelectrode and Pt counter electrode were assembled as a sandwich-type configuration by adhered a hot-melt sealing foil (60 µm, Surlyn). The liquid electrolyte is composed of 0.03 M guanidinium thiocyanate, 0.6 M 1,2-dimethyl-3-propylimidazolium iodide, 0.05 M I2, 0.1 M LiI, 0.5 M 4-tert-butylpyridine in acetonitrile. The electrolyte was injected into the gap by pre-punching a hole on the counter electrode. Then, a piece of glass was employed to seal the hole. The active area of the cell was 0.5 cm × 0.5 cm. 2.4. Characterization The surface structure and morphology of transparent conductive glasses modified by fluorine doped SnO2 nanocrystal colloids and precursor solution were thoroughly performed by scanning electron microscopy (SEM, JEOL JSM-7500F) and atomic force microscopy (AFM, Nanofirst-3600A). Sheet resistance was detected by employed a four-point probe (RTS-8). Optical properties were evaluated by UV-Vis spectra 7


Page 9 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(Shimadzu UV-2600). J-V characteristic curves of DSSCs were determined under a standard AM 1.5G illumination intensity of 100 mW/cm2 simulated by using a Xenon lamp (Newport, 94063A). The solar simulator was calibrated by a standard silicon solar cell. The measurements of monochromatic incident photon-to-electron conversion efficiency (IPCE) spectra were carried out by IPCE system (QEX10, PV Measurements). The impedance spectra (EIS) and cyclic voltammetry (CV) curves were performed using a modular electrochemical instrument (Autolab 320, Metrohm). In CV measurements, the Pt/FTO, Ag/Ag+ electrodes and platinum foil were used as the working electrodes, reference electrode and counter electrode, respectively. Electrolytes employed in this system composed of 10.0 mM LiI, 1.0 mM I2 and 0.1M LiClO4 in acetonitrile. The electrochemical accessible surface area (ECSA) of Pt counter electrode was performed by the CV measurement in a 0.5 M H2SO4 solution with a saturated calomel electrode (SCE) as the reference. The carrier mobility and carrier concentration were performed by obtained by Hall effect measurements on Accent HL5500PC.

3. Results and discussion 3.1. Surface structure and morphology

8



Figure. 1 FESEM images of (a) bare FTO film and FTO films modified by (b) 0.5%, (c) 1.0%, (d) 1.5% solid content fluorine doped tin oxide (FTO) colloids.

Figure. 1 exhibits the morphologies of FTO conductive films before and after modification by FTO nanocrystal colloids with different solid contents. The bare FTO conductive film shows large FTO crystals of around 300-400 nm with low symmetry and discontinuous surface. As illustrated in Figure. 1(b), (c) and (d), after colloid deposition planarization (CDP) process, FTO nanocrystals with a diameter of around 10 nm are uniformly deposited onto the FTO substrates. According to Figure. 1(b), the texture

9


Page 10 of 32

Page 11 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


surface of FTO film is partially covered by colloid nanoparticles. The rutile tips of FTO are still exposed. With the increasing solid content of FTO nanocrystal colloids, the majority of the substrate surface will be covered homogeneously (Figure. 1(c) and (d)). Due to the deposition of colloidal FTO nanoparticles, the roughness of the films were reduced, which is confirmed by the AFM measurements (Table 1 and Figure. S1). Meanwhile, the FTO nanocrystals will enhance the connection of microsized FTO crystals and improve the conductivity of the films, which is confirmed by the sheet resistance measurements (Table 2).

Figure. 2 FESEM images of 1% FTO colloid modified FTO film before (a) and after

10



chemical bath treatment in FTO precursor solution for (b) 15 min, (c) 30 min and (d) 45 min.

As shown in Figure. 1, there are pores in FTO colloid coatings, which may restrain the carrier transport due to the crystal boundary. In order to further improve the performance of conductive films, the fluorine doped SnO2 precursor solution is adopted to treat the FTO nanocrystal deposited FTO films. According to Figure. 2, the solutions treatment will make the FTO nanocrystals grow up and connect with each other more tightly. Due to the growth of FTO nanocrystals, the pores are partially filled. As shown in Figure. 2(c), when the treatment time is 30 min, the FTO film is more uniform by diminishing the porous structure. When the chemical bath treatment time is prolonged to 45 min, the FTO nanocrystals will grow larger and create more gaps.

Figure. 3 Cross section morphologies of (a) bare FTO film, (b) 1% FTO colloid treated film (CDP-FTO), (c) 1% FTO colloid and 30 min precursor solutions treated film (CSDP-FTO).

11


Page 12 of 32

Page 13 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1 Roughness of bare FTO film, CDP-FTO film and CSDP-FTO film.

Sample

Roughness (nm)

Bare FTO

15.6

CDP-FTO

14.0

CSDP-FTO

13.7

In order to demonstrates the difference of FTO conductive films before and after modification. The cross section morphology and surface roughness were measured by SEM and AFM. The images and results are exhibited in Figure. 3 and Figure. S1, respectively. The roughness values of conductive films were shown in Table 1. The decrease of roughness are agreed with the SEM results.

As illustrated in Figure. 3, the thickness of bare-FTO, CDP-FTO and CSDP-FTO films are 545 nm, 579 nm and 588 nm, respectively. There are plenty of edges and corners on the bare FTO conductive film surface (Figure. 3(a)). The surface and cross section of FTO conductive film become smoother due to the modification by FTO colloids (Figure. 3(b)). Furthermore, the FTO conductive film shows more uniform surface and more tightly cross-section, which confirms that the chemical bath treatment in FTO precursor solution makes the FTO grain connect with each other more perfectly 12



(Figure. 3(c)).

3.2. Electrical and optical properties

Figure. 4 Sheet resistances of FTO films (a) modified by FTO nanocrystal colloids with different solid content and (b) further treated by FTO precursor solutions with different time.

According to Figure. 4 and Table 2, the sheet resistances of FTO conductive films treated by FTO nanocrystal colloid and subsequent FTO precursor solution show optimized value. To minimize the errors, five pieces of conductive glasses were tested for every sample, and every glass was tested three times. After the FTO nanocrystal colloid modification, the sheet resistance of the FTO conductive films decrease significantly. When the FTO conductive glasses was modified by the colloid contenting 1% FTO

13


Page 14 of 32

Page 15 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nanocrystals, the minimum sheet resistance as low as 7.38 Ω/□ was obtained. If the colloid deposition modified samples were further treated by chemical bath in FTO precursor solution, the sheet resistances of the conductive glasses will firstly decrease then increase with the prolonging time. When the treatment time is 30 min, the sheet resistance is as low as 7.10 Ω/□. Compared with the pristine FTO conductive glasses, the sheet resistances of FTO conductive glasses modified by colloid-solution deposition planarization (CSDP) process decrease more than 11%.

Table 2 The sheet resistances of bare FTO conductive glasses, FTO glasses modified by CDP and CSDP processes. Colloid solid content of

Precursor solutions Sheet Resistance

CDP

treatment time (Ω/□)

(wt. %)

(min)

0

0

8.00 (±0.07)

0.5

0

7.40 (±0.06)

1

0

7.38 (±0.04)

1

15

7.28 (±0.05)

1

30

7.10 (±0.04)

1

45

7.24 (±0.05)

14



1.5

0

Page 16 of 32

7.42 (±0.06)

In order to investigate the mechanism of the CSDP treatment decrease the resistance of FTO films, Hall effects measurements were performed. The test results were shown in Table S1. After CSDP treatment, the carrier mobility decreased from 1.67×102cm2/(V·s) to 0.776×102 cm2/(V·s), this results are disadvantage to the conductivity of the films. However, the carrier concentration was increased remarkably after planarization procedure, which are increased from 0.724×1020 /cm3 of bare-FTO films to 1.61×1020 /cm3 of CSDP-FTO films. It is the enhancement in carrier concentration which reduced the resistivity of CSDP-FTO films.

Figure. 5 (a) The transmittance spectra and (b) reflectance spectra of bare-FTO (black line), CDP-FTO film (red line) and CSDP-FTO film (blue line).

The transmittance for FTO conductive film substrates will affect the light harvesting

15


Page 17 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of dye adsorbed in the TiO2 films.

28

In order to investigate the effect of coatings on the

optical properties of FTO glasses, the transmittance and reflectance spectra were performed. Figure. 5(a) exhibits the transmittance spectra of bare FTO glasses, CDP-FTO and CSDP-FTO glasses. Compared with bare FTO glasses, both CDP-FTO and CSDP-FTO glasses show a clear enhancement on transmittance in the range of 350-700 nm. At 550 nm, the transmittance of modified FTO glasses is 4% higher than that of bare FTO glasses. According to Figure. 5(b), in the range of 350-700 nm, the reflectance ratios of modified FTO glasses are lower than that of bare FTO glass. The results reveal that it is the coatings on the FTO films that results in the lower light loss and permit more light pass through the glasses to irradiate the dyes on TiO2. This enhancement may be due to two aspects: first, the coverage of grain gaps and large-angle grain boundaries by FTO nanocrystals reduce the defects in FTO film which reduces the light scattering and consumption; second, the elimination of the surface texture reduces the reflection loss of the texture-scale wavelength light. 29 The optical absorption spectra were also tested, which is shown in Figure. S2. The variation in optical absorption is insignificant. The enhancements of transmittance are mainly due to the anti-reflectance effect of the CSDP coatings which decreases the reflectance of FTO films.

3.3. Photovoltaic performance

16



Page 18 of 32

Figure. 6 (a) J-V characteristic curves and (b) IPCE spectra and integrated photocurrent IPCE spectra of reference cell (PFTO-CFTO-cell), CSDP treated photoanode substrate cell (PCSDP-FTO-CFTO-cell), CSDP treated counter electrode substrate cell (PFTO-CCSDP-FTO-cell) and

CSDP

treated

both

photoanode

and

counter

electrode

substrates

(PCSDP-FTO-CCSDP-FTO-cell).

Table 3 The photovoltaic parameters of DSSCs

VOC

JSC

η

Cell

FF 2

(V)

(mA/cm )

(%)

PFTO-CFTO-cell

0.71±0.02

16.93±0.33

0.68±0.03

8.24±0.67

PCSDP-FTO-CFTO-cell

0.70±0.04

20.65±0.27

0.63±0.02

9.03±0.62

PFTO-CCSDP-FTO-cell

0.71±0.03

19.70±0.40

0.62±0.03

8.70±0.65

PCSDP-FTO-CCSDP-FTO-cell

0.70±0.03

20.93±0.35

0.64±0.02

9.37±0.67

17


cell

Page 19 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The dye-sensitized solar cells employing CSDP-FTO glasses and bare FTO glasses as photoanode substrate or counter electrode substrate are labeled as PFTO-CFTO-cell, PCSDP-FTO-CFTO-cell, PFTO-CCSDP-FTO-cell, PCSDP-FTO-CCSDP-FTO-cell, respectively.

As

shown in Figure. 6(a) and Table 3, the photoelectric conversion efficiencies are enhanced remarkably when modified conductive substrates are used. There is a significant increase on JSC. Meanwhile, the difference on VOC is negligible. Compared to the control sample PFTO-CFTO-cell, the increases on JSC for modified FTO glasses as photoanode or counter electrode substrates are 21% or 16% respectively. When the modified FTO conductive substrates are used as the substrates of both photoanode and counter electrode, the JSC is improved as high as 23%. The corresponding incident monochromatic photon-to-electron conversion efficiency (IPCE) spectra in Figure. 6(b) reveals the improvement on current density. Compare to the control sample PFTO-CFTO-cell, these cells employing modified FTO glasses exhibit higher IPCE in the range of 350-700 nm. The increase in IPCE value is consistent well with the variation of JSC. Figure. 5(b) also shows the calculated accumulated photocurrent by integrating the IPCE curves. The integrating of IPCE data was refer to the publicly available data for AM1.5G solar irradiation30. Due to the discrepancy of light intensity between solar simulator and single wavelength irradiation of IPCE measurements, the theoretical

18



Page 20 of 32

calculated JSC are underestimate than the actual values tested from JV characteristic. However, the integrated JSC from IPCE are still consistent with the trend of the actual measured values. The short circuit current density of the solar cell is given by the following equations: 31

= ()()()

(1)

= ×

(2)

!"#

×

$%&

× '( ))

Where is the elemental charge, LHE is the light harvesting efficiency,

*+,

is the

injection efficiency of electrons from the dye lowest unoccupied molecular orbital (LUMO) level to the TiO2 conduction band (CB),

$%&

is the regeneration efficiency of

oxidized dye, and '-.// is the collection efficiency of the injected electrons at the conductive substrate (FTO glass). The light harvesting efficiency LHE greatly relies on the dye loading amount on TiO2 mesoporous layer and the molar extinction coefficients of dye. 32-33 In this work, the effect of substrate on the dye loading is not obvious (Table S2). For the cells assembled with modified FTO glass, the transmittance of the photoanode are enhanced. Higher transparency will facilitate more light penetrate the substrate and trapped by the dyes attached on the TiO2 surface. The optical absorption spectra of TiO2-dye@FTO and TiO2-dye@CSDP-FTO are shown in Figure. S3, which confirms that the increase of photoanode absorption can lead to a higher light harvesting efficiency. In general, the 19


Page 21 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


higher IPCE results in more electrons injected into conduction band (CB) of the TiO2 and thus lead to a higher JSC of the cell. Furthermore, the smooth CSDP coatings can form a well contact interface layer between FTO substrates and TiO2 absorbers,34 which is helpful to enhance the charge collection efficiency '-.// . The increment of JSC in IPCE consistent well with the results of in the J-V characteristic curve. It should be noticed that PFTO-CCSDP-FTO-cell also shows higher IPCE than the control sample PFTO-CFTO-cell, which indicates that the counter electrode made from modified FTO glasses may improve the

012 .

The reason will be investigated by the following electrochemical analysis.

3.4. Electrochemical analysis

Figure. 7 (a) Cyclic voltammetry curve and (b) electrochemical accessible surface area of platinum deposited on bare FTO glass (black line) and CSDP-FTO glass (red line).

In Figure. 7(a), two pairs of oxidation and reduction peaks in the CV curves can be

20



observed for both electrodes. The left pair of redox peaks corresponding to the reaction: + 25 ←→ 3 . 35-37 The main function of counter electrode in DSSC is to reduce to ions at the CE/electrolyte interface. 38 The reduction peak current density and the peak to peak separation (Epp) are significant factors for analyzing the catalytic activity of the counter electrode. The reduction peak current density of Pt@CSDP-FTO CE (IRed1 = 2.05 mA/cm2) is larger that of Pt@ FTO CE (IRed1 = 1.74 mA/cm2). In addition, the 9 potential value of and are 0.225 V and 0.210 V for Pt@FTO CE and

Pt@CSDP-FTO CE, respectively, indicating a higher rate constant of the / redox reaction on Pt@CSDP-FTO CE.39 To reveal the main factor affecting the electrocatalytic activities of counter electrode, the electrocatalytic accessible surface area (ECSA) corresponding to the effective surface area of Pt in contact with the electrolyte, was measured by CV technique in 0.5M H2SO4. According to Figure. 7(b), the ECSA of Pt@CSDP-FTO CE is significant larger than that of Pt@FTO CE. This result confirms that the Pt@CSDP-FTO CE possesses larger effective area and catalytic ability for reaction. The increase in effective surface of Pt@CSDP-FTO CE should be ascribed to the reduced Pt nanoparticle size, which can be confirmed by the SEM images in Figure. S4.

21


Page 22 of 32

Page 23 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure. 8 The (a) Nyquist plots and (b) Bode plots of symmetry cells.

Table 4 Parameters of Pt symmetry cells. Electrode of

Rs

Rct

CPE

Zn

Rall

:%

symmetry cell

(Ω)

(Ω)

(µF)

(Ω)

(Ω)

(s)

Pt@FTO

28.7

12.7

7.9

3.5

44.9

5.04×10-5

Pt@CSDP-FTO

15.3

7.0

17.7

2.8

25.1

1.26×10-5

Figure. 8(a) shows the EIS Nyquist plots of symmetry cells with two Pt@FTO or Pt@CSDP-FTO counter electrodes. There are two semicircles in each plot. The semicircle in high frequency region corresponds to the charge transfer impedance on the counter electrode/electrolyte interface, represented by resistance (Rct) and capacitance (Cµ). The low frequency semicircle is related to the Nernst diffusion impedance (Zn) of the electrolyte in the solution.9, 40 The high frequency intercept on the real axis is ascribed 22



Page 24 of 32

to series resistance (Rs) of the circuit contacts. The impedance parameters fitting the inset equivalent circuit in Figure. 8(a) are summarized in Table 4. The series resistance Rs of Pt@CSDP-FTO CE is significantly lower than that of Pt@FTO CE, which is favorable for the charge transport. Furthermore, the Rct of Pt@CSDP-FTO CE is smaller than that of Pt@FTO CE, which means that the CSDP-CE benifits the charge transfer from Pt to . 41 As shown in Table 4, the fitted chemical capacitance (Cµ) of Pt@CSDP-FTO CE is 17.7µF which is much higher than that of Pt@FTO CE. The results reveals that the electrode/electrolyte interfaces of Pt@CSDP-FTO CE is larger than that of Pt@FTO CE, which agrees well with the measurement of electrochemical accessible surface area of platinum. The reduction of Rct confirms that the CSDP coatings can act as an interface layer to hold Pt nanocrystals of large catalytic area. The Bode diagram belongs to the symmetrical cell Nyquist plots are shown in Figure. 8(b). The peak at high frequency range is corresponding to the charge transfer at counter electrode/electrolyte interface. Compared with the Pt@FTO CE symmetry cell, the Pt@CSDP-FTO CE cell exhibits a larger phase at a higher frequency. The electron lifetime calculated by the following expression: ;

:% =

(3)

?@A

Where B%CD represents the peak frequency of the high frequency arc. :% of Pt@FTO CE symmetry cell is about 5.04×10-5 s, and that of Pt@CSDP-FTO CE symmetry cell is about 1.26×10-5 s. The result confirms that the charge transfer at the 23


Page 25 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electrolyte/Pt@CSDP-FTO interface is significantly faster than at the electrolyte/Pt@ FTO interface. Due to the fast electrode/electrolyte interface charge transfer, the generation ratio of is improved, which is beneficial for the dye regeneration efficiency

012 .

4. Conclusion A simple method to improve the transmittance and conductivity of commercial FTO glasses was investigated. This method is called the colloid-solution deposition planarization (CSDP) process. In this work, the FTO nanocrystal colloid was deposited on the FTO glasses by spin-coating, followed by the chemical bath treatment in the FTO precursor solution. Compare to the bare FTO glasses, the modified FTO glasses by CSDP process achieved 4% increase in transmittance and 11% decrease in sheet resistance, respectively. In addition, the modified FTO glasses can reduce the aggregation of Pt nanoparticles and improve the electrocatalytic activity of Pt counter electrodes. When the modified FTO glasses were used to assemble DSSCs, the cells got a photoelectric conversion efficiency as high as 9.37%. In contrast, the efficiency of reference cells using bare FTO substrates was about 8.24%. The J-V, IPCE and EIS measurement results reveal that the improvement should be ascribed to the higher light harvest efficiency, electron collection efficiency and dye regeneration efficiency.

24



Supporting Information AFM images and Hall effect parameters of FTO transparent conductive glasses before and after modification; UV-vis spectra and dye loading amount of photoanodes; FESEM images of Pt counter electrodes.

Acknowledgments The authors acknowledge support of the National Natural Science Foundation of China (Grants 51472154).

Reference 1.

Wu, W. Q.; Xu, Y. F.; Su, C. Y.; Kuang, D. B., Ultra-long anatase TiO2 nanowire

arrays with multi-layered configuration on FTO glass for high-efficiency dye-sensitized solar cells. Energy Environ. Sci. 2014, 7 (2), 644-649. 2.

Hamann, T. W.; Jensen, R. A.; Martinson, A. B. F.; Van Ryswyk, H.; Hupp, J. T.,

Advancing beyond current generation dye-sensitized solar cells. Energy Environ. Sci. 2008, 1 (1), 66-78. 3.

Saygili, Y.; Soderberg, M.; Pellet, N.; Giordano, F.; Cao, Y.; Munoz-Garcia, A. B.;

Zakeeruddin, S. M.; Vlachopoulos, N.; Pavone, M.; Boschloo, G.; Kavan, L.; Moser, J. E.; Gratzel, M.; Hagfeldt, A.; Freitag, M., Copper Bipyridyl Redox Mediators for Dye-Sensitized Solar Cells with High Photovoltage. J. Am. Chem. Soc. 2016, 138 (45), 25


Page 26 of 32

Page 27 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15087-15096. 4.

Bella, F.; Galliano, S.; Gerbaldi, C.; Viscardi, G., Cobalt-Based Electrolytes for

Dye-Sensitized Solar Cells: Recent Advances towards Stable Devices. Energies 2016, 9 (5), 384. 5.

Yao, Z.; Zhang, M.; Li, R.; Yang, L.; Qiao, Y.; Wang, P., A metal-free N-annulated

thienocyclopentaperylene dye: power conversion efficiency of 12% for dye-sensitized solar cells. Angew. Chem., Int. Ed. Engl. 2015, 54 (20), 5994-5998. 6.

Kakiage, K.; Aoyama, Y.; Yano, T.; Oya, K.; Fujisawa, J.; Hanaya, M.,

Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxy-anchor dyes. Chem. Commun. 2015, 51 (88), 15894-15897. 7. Sil, M. C.; Sudhakar, V.; Kavungathodi, M. F. M.; Punitharasu, V.; Nithyanandhan, J., Orthogonally Functionalized Donor/Acceptor Homo- and Heterodimeric Dyes for Dye-Sensitized Solar Cells: An Approach to Introduce Panchromaticity and Control the Charge Recombination. ACS Appl. Mater. Interfaces 2017, 9 (40), 34875-34890. 8.

Kim, J. S.; Kim, B. M.; Kim, U. Y.; Shin, H.; Nam, J. S.; Roh, D. H.; Park, J. H.;

Kwon, T. H., Molecular Engineering for Enhanced Charge Transfer in Thin-Film Photoanode. ACS Appl. Mater. Interfaces 2017, 9 (40), 34812-34820. 9.

Han, Q.; Wang, H.; Liu, Y.; Yan, Y.; Wu, M., Highly effective carbon sphere counter

electrodes based on different substrates for dye-sensitized solar cell. J. Colloid Interface Sci. 2017, 506, 518-523. 26



10. Chen, C. Y.; Lee, W. K.; Chen, Y. J.; Lu, C. Y.; Lin, H. Y.; Wu, C. C., Enhancing Optical Out-Coupling of Organic Light-Emitting Devices with Nanostructured Composite Electrodes Consisting of Indium Tin Oxide Nanomesh and Conducting Polymer. Adv. Mater. 2015, 27 (33), 4883-4888. 11. Yoo, B.; Kim, K.; Lee, D. K.; Ko, M. J.; Lee, H.; Kim, Y. H.; Kim, W. M.; Park, N. G., Enhanced charge collection efficiency by thin-TiO2-film deposition on FTO-coated ITO conductive oxide in dye-sensitized solar cells. J. Mater. Chem. 2010, 20 (21), 4392. 12. Benck, J. D.; Pinaud, B. A.; Gorlin, Y.; Jaramillo, T. F., Substrate selection for fundamental studies of electrocatalysts and photoelectrodes: inert potential windows in acidic, neutral, and basic electrolyte. PLoS One 2014, 9 (10), e107942. 13. Gonçalves, L. M.; Bermudez, V. D.; Ribeiro, H. A.; Mendes, A. M., Dye-sensitized solar cells: A safe bet for the future. Energy Environ. Sci. 2008, 1 (6), 655. 14. Zhu, B. L.; Liu, F.; Li, K.; Lv, K.; Wu, J.; Gan, Z. H.; Liu, J.; Zeng, D. W.; Xie, C. S., Sputtering deposition of transparent conductive F-doped SnO2 (FTO) thin films in hydrogen-containing atmosphere. Ceram. Int. 2017, 43 (13), 10288-10298. 15. Geiger, S.; Kasian, O.; Mingers, A. M.; Mayrhofer, K. J. J.; Cherevko, S., Stability limits of tin-based electrocatalyst supports. Sci. Rep. 2017, 7 (1), 4595. 16. Gao, Q.; Jiang, H.; Li, C.; Ma, Y.; Li, X.; Ren, Z.; Liu, Y.; Song, C.; Han, G., Tailoring of textured transparent conductive SnO2:F thin films. J. Alloys Compd. 2013, 574, 427-431. 27


Page 28 of 32

Page 29 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


17. Zhao, Q. N.; Wu, S.; Miao, D. K., Effect of Substrate Temperature on the Haze and Properties of SnO2: F Thin Film Coated on Glass (FTO) by Spray Pyrolysis Process. Adv. Mater. Res. 2011, 150-151, 1043-1048. 18. Ren, Y.; Zhao, G.; Chen, Y., Fabrication of textured SnO2:F thin films by spray pyrolysis. Appl. Surf. Sci. 2011, 258 (2), 914-918. 19. Mehmood, U.; Afzaal, M.; Al-Ahmed, A.; Yates, H. M.; Hakeem, A. S.; Ali, H.; Al-Sulaiman, F. A., Transparent Conductive Oxide Films for High-Performance Dye-Sensitized Solar Cells. IEEE J. Photovolt. 2017, 7 (2), 518-524. 20. Karthick, P.; Vijayanarayanan, D.; Sridharan, M.; Kurnar, R. T. A.; Sanjeeviraja, C.; Jejadheepan, K., Optimization of substrate temperature and characterization of tin oxide based transparent conducting thin films for application in dye-sensitized solar cells. Thin Solid Films 2017, 631, 1-11. 21. Tang, P.; Qiu, C.; He, J.; Zhang, T.; Feng, X.; Luo, X., Effect of the substrate temperature on the properties of spray-deposited SnO2:F thin films. J. Mater. Sci.: Mater. Electron. 2014, 25 (10), 4369-4374. 22. Choi, J.; Song, S.; Horantner, M. T.; Snaith, H. J.; Park, T., Well-Defined Nanostructured, Single-Crystalline TiO2 Electron Transport Layer for Efficient Planar Perovskite Solar Cells. ACS Nano 2016, 10 (6), 6029-6036. 23. Zhou, H.; Chai, F.; Fang, J.; Shi, L.; Cai, C.; Yuan, S., Highly efficient colloid– solution deposition planarization of Hastelloy substrate for IBAD-MgO film. Res. Chem. 28



Intermed. 2015, 42 (5), 4751-4758. 24. Wu, S.; Yuan, S.; Shi, L.; Zhao, Y.; Fang, J., Preparation, characterization and electrical properties of fluorine-doped tin dioxide nanocrystals. J. Colloid Interface Sci. 2010, 346 (1), 12-16. 25. Aouaj, M. A.; Diaz, R.; Belayachi, A.; Rueda, F.; Abd-Lefdil, M., Comparative study of ITO and FTO thin films grown by spray pyrolysis. Mater. Res. Bull. 2009, 44 (7), 1458-1461. 26. Rothenberger, G.; Comte, P.; Gratzel, M., A contribution to the optical design of dye-sensitized nanocrystalline solar cells. Sol. Energy Mater. Sol. Cells 1999, 58 (3), 321-336. 27. Liu, H.; Lou, Y.; Yuan, S.; Liu, M.; Zhou, H., Depositing Pt nanoparticles by pulse electrodeposition for DSSCs counter electrode with high electrocatalytic activity. Res. Chem. Intermed. 2017, 43 (8), 4881-4892. 28. Pazoki, M.; Cappel, U. B.; Johansson, E. M. J.; Hagfeldt, A.; Boschloo, G., Characterization techniques for dye-sensitized solar cells. Energy Environ. Sci. 2017, 10 (3), 672-709. 29. Giusti, G.; Consonni, V.; Puyoo, E.; Bellet, D., High performance ZnO-SnO2:F nanocomposite transparent electrodes for energy applications. ACS Appl. Mater. Interfaces 2014, 6 (16), 14096-14107. 30. Christians, J. A.; Manser, J. S.; Kamat, P. V., Best Practices in Perovskite Solar Cell 29


Page 30 of 32

Page 31 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Efficiency Measurements. Avoiding the Error of Making Bad Cells Look Good. J. Phys. Chem. Lett. 2015, 6 (5), 852-857. 31. Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H., Dye-Sensitized Solar Cells. Chem. Rev. 2010, 110 (11), 6595-6663. 32. Mozaffari, S.; Nateghi, M. R.; Zarandi, M. B., An overview of the Challenges in the commercialization of dye sensitized solar cells. Renewable Sustainable Energy Rev. 2017, 71, 675-686. 33. Martín, C.; Ziółek, M.; Douhal, A., Ultrafast and fast charge separation processes in real dye-sensitized solar cells. J. Photochem. Photobiol., C 2016, 26, 1-30. 34. Duan, J.; Xiong, Q.; Feng, B.; Xu, Y.; Zhang, J.; Wang, H., Low-temperature processed SnO2 compact layer for efficient mesostructure perovskite solar cells. Appl. Surf. Sci. 2017, 391, 677-683. 35. Yu, C.; Meng, X.; Song, X.; Liang, S.; Dong, Q.; Wang, G.; Hao, C.; Yang, X.; Ma, T.; Ajayan, P. M.; Qiu, J., Graphene-mediated highly-dispersed MoS2 nanosheets with enhanced triiodide reduction activity for dye-sensitized solar cells. Carbon 2016, 100, 474-483. 36. Abdulkarim, S.; Elbohy, H.; Adhikari, N.; Hasan, M. N.; Yue, W.; Qiao, Q., Urea-Treated Electrolytes for Higher Efficiency Dye-Sensitized Solar Cells. J. Phys. Chem. C 2017, 121 (39), 21225-21230. 37. Wang, X.; Xie, Y.; Bateer, B.; Pan, K.; Jiao, Y.; Xiong, N.; Wang, S.; Fu, H., 30



Selenization of Cu2ZnSnS4 Enhanced the Performance of Dye-Sensitized Solar Cells: Improved Zinc-Site Catalytic Activity for I3. ACS Appl. Mater. Interfaces 2017, 9 (43), 37662-37670. 38. Tang, Q.; Zhang, H.; Meng, Y.; He, B.; Yu, L., Dissolution Engineering of Platinum Alloy Counter Electrodes in Dye-Sensitized Solar Cells. Angew. Chem., Int. Ed. Engl. 2015, 54 (39), 11448-11452. 39. Park, J. T.; Lee, C. S.; Kim, J. H., High performance electrocatalyst consisting of CoS nanoparticles on an organized mesoporous SnO2 film: its use as a counter electrode for Pt-free, dye-sensitized solar cells. Nanoscale 2015, 7 (2), 670-678. 40. Gong, F.; Wang, H.; Xu, X.; Zhou, G.; Wang, Z. S., In situ growth of Co(0.85)Se and Ni(0.85)Se on conductive substrates as high-performance counter electrodes for dye-sensitized solar cells. J. Am. Chem. Soc. 2012, 134 (26), 10953-10958. 41. Wu, J.; Tang, Z.; Huang, Y.; Huang, M.; Yu, H.; Lin, J., A dye-sensitized solar cell based on platinum nanotube counter electrode with efficiency of 9.05%. J. Power Sources 2014, 257, 84-89.

31


Page 32 of 32

High-performance Dye-sensitized Solar Cells Based on Colloid

Recommend Documents