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Original article
13 (
1
); 2340-2348
doi:
10.1016/j.arabjc.2018.04.017

Effects of a dianion compound as a surface modifier on the back reaction of photogenerated electrons in TiO2-based solar cells

, , ,
a Convergence Research Center for Solar Energy, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988, Republic of Korea
b Department of Polymer Science and Engineering, Kyungpook National University, Daegu 41566, Republic of Korea
c Department of Optometry and Vision Science, Daegu Catholic University, Gyeongbuk 38430, Republic of Korea
d Department of Advanced Materials and Chemical Engineering, Daegu Catholic University, Gyeongbuk 38430, Republic of Korea

⁎Corresponding authors. monolith@dgist.ac.kr (Dae-Hwan Kim), yshancu@cu.ac.kr (Yoon Soo Han)

Received: , Accepted: ,
© The Author(s) 2018
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
1 Present address: Hayang-ro 13-13, Hayang-eup, Gyeongsan-si, Gyeongbuk 38430, Korea.

Abstract

The TiO2 films were modified with a dianion compound, 1,2-ethanedisulfonic acid disodium salt (ESD), to give a negative charge (ethane sulfonate anion) on the TiO2 surface, i.e., TiO2-O-SO2-CH2-CH2-SO3−), and effects of repulsion between the negative charge and ions (I3−) of the electrolyte on the performance of dye-sensitized solar cells (DSSCs) were investigated. The reference device without any modification showed a power conversion efficiency (PCE) of 9.89%, whereas for the device with ESD(20)-TiO2/FTO, which was prepared by soaking bare TiO2/FTO in an ESD solution for 20 min, the PCE was increased to 10.97%, due to an increase in both short-circuit current (Jsc) and open-circuit voltage(Voc). It was verified from the measurements of electrochemical impedance, open-circuit voltage decay and dark current that the enhancement in the Jsc and Voc values was attributed to the reduced back reaction between photoinjected electrons and I3− ions, resulting from the presence of the ethane sulfonate anions on the TiO2 surface.

Keywords

Dianion compound
1,2-Ethanedisulfonic acid disodium salt
Dye-sensitized solar cell
Back reaction
1

1 Introduction

Dye-sensitized solar cells (DSSCs) based on a mesoporous TiO2 photoanode, a Ru-based dye, an I−/I3− electrolyte and a Pt counter electrode are characterized by their reasonable photovoltaic efficiency, low production cost, and non-vacuum fabrication process (Gong et al., 2017; Kakiage et al., 2015), and thus extensive research into the commercialization of them is being conducted. However, DSSCs still cannot compete with Si- or inorganic semiconductor-based solar cells in the market of large-scale energy production due to an insufficient power conversion efficiency (PCE) and reliability. Many investigators have made great efforts to enhance the PCE of DSSCs by improving the constituents’ properties. These efforts can largely be classified into four categories of development: new sensitizers (Soliman et al., 2017), anodic materials (Hoa et al., 2014; Dao et al., 2014; Imran et al., 2017), electrolytes (Wu et al., 2015), and electrodes (Dao et al., 2015; Dao and Choi, 2016). Recently, water-based DSSCs have been paied much attention due to their eco-friendly nature and higher reliability. Water-based materials such as poly(3,4-ethylenedioxythiophene) (Mustafa et al., 2017), aqueous NaI/I2 mixed solution (Gallianoa et al., 2018), cellulose derivatives (Salvador et al., 2014; Bella et al., 2017, 2014) and poly(glycidyl methacrylate) (Imperiyka et al., 2014) have been employed as a counter electrode or an electrolyte to realize non-tocic and reliable solar cells.

The TiO2 film as a photoelectrode of DSSCs play an important role to adsorb sufficient dye molecules for photoelectron generation and to favorably soak hole-carrying electrolytes, owing to its high surface area, and it is also a good transporter of electrons injected from the photoexcited dyes. Thus, high short-circuit current (Jsc) can be achieved from the TiO2-based cells. However, unlike other solar cells, an electron transporting TiO2 layer is in direct contact with the hole conductor (electrolyte), inducing the back reaction (I3− + 2e− → 3I−) between the photoinjected electrons and ions in the electrolyte. This may cause a loss of approximately 300 mV in the open-circuit voltage (Voc) compared to the theoretical value, which leads to a rapid decrease in the conversion efficiency (Bandaranayake et al., 2004). It has been reported that surface modification of TiO2 photoelectrodes using semiconducting or insulating metal oxides, metal salts, organic co-adsorbents and acids could control the back reaction (Saxena and Aswal, 2015; Sun et al., 2016). The DSSCs with surface-modified TiO2 layers have shown an enhancement in PCE attributed to an improved Voc, Jsc and/or fill factor (FF), resulting from a surface state passivation, formation of energy barrier or formation of surface dipole (O’Regan et al., 2005). And some cases, the PCE improvement by the modifications was due to prevention of direct contact between the TiO2 layer and the hole conductor (electrolyte).

As far as we know, dianion compounds as a surface modifier had not been applied to DSSCs. Here, we report effects of a dianion compound, 1,2-ethanedisulfonic acid disodium salt (ESD), on the performance of DSSCs. The TiO2 films were soaked in an aqueous ESD solution to modify their surfaces, and the resulting films (ESD-modified TiO2) were used as the photoelectrodes of the DSSCs. DSSCs with ESD-modified photoelectrodes were fabricated, and the effects of the surface modification on the photovoltaic performance of the cells were examined.

2

2 Experimental details

2.1

2.1 Materials

Commercial fluorine-doped tin oxide (FTO; sheet resistance ∼7 Ω/square) glass (TCO22-7), TiO2 paste for the photoelectrode (Ti-nanoxide T/SP), TiO2 paste for the scattering layer (Ti-nanoxide R/SP), N719 dye (Ruthenizer 535-bisTBA), hot-melt adhesive (SX1170-60PF, Surlyn), and iodide-based electrolytes (AN-50) were purchased from Solaronix. The ESD and TiCl4 were purchased from Sigma-Aldrich Co. LLC. Platinum paste (PT-1, Dyesol-Timo) was selected as the source for the Pt counter-electrode. All the chemicals were used without further purification.

2.2

2.2 Fabrication of DSSCs

The FTO glasses were cleaned in a detergent solution using sonication for 20 min, and then thoroughly rinsed with deionized water and ethanol. After the ultrasonic cleaning, the FTO glasses were immersed in a 40-mM TiCl4 solution at 70 °C for 30 min, and then washed with water and ethanol. One active TiO2 layer, which formed on the FTO glass, was prepared via doctor-blade coating with the TiO2 paste. Additionally, a TiO2 layer composed of approximately 400-nm-diameter particles was deposited on the active TiO2 layer and then calcinated at 500 °C for 60 min to produce the scattering layer. Finally, the TiO2 films were again treated with a 40-mM TiCl4 solution and annealed at 500 °C for 60 min; thus, TiO2/FTO electrodes with scattering layers were prepared. The electrodes were soaked in an aqueous solution (5 × 10−4 M) of ESD for 0–40 min to deposit anions onto the TiO2 layers. Next, the resulting electrodes were rinsed with water and ethanol, and then dried at 65 °C for 10 min to produce the modified photoelectrodes (ESD-TiO2/FTO). The bare TiO2/FTO and ESD-TiO2/FTO photoelectrodes were separately immersed into 0.5 mM of N719 dye solution (acetonitrile/tert-butyl alcohol, v/v = 1) for 24 h to obtain working electrodes.

To prepare the counter-electrode, two holes were formed in the FTO glass with a drill, and then cleaned with the method described above. A Pt layer was formed on the FTO glass via the doctor-blade method using Pt paste, followed by a calcination process at 400 °C for 30 min. The thermally treated platinum counter-electrodes were placed on the photoelectrodes and sealed with a 60-μm-thick sealing material. The electrolyte was introduced into the cells through one of the two small holes drilled on the counter-electrodes to produce DSSCs with a 25 mm2 active area.

2.3

2.3 Measurements

X-ray photoelectron spectroscopy (XPS) was performed using VG Multilab ESCA 2000 (ThermoVG Scientific) with Mg Kα radiation. The C 1s photoelectron peak (binding energy of 284.6 eV) was used as an energy reference. Fourier transform infrared (FT-IR) spectra were recorded on a FT/IR 4100 spectrometer (Jasco) equipped with an attenuated total reflectance (ATR, PRO450-S, Jasco). Field emission scanning electron microscopy (FE-SEM; S-4800, Hitachi High-Technology) was used to study the morphology of the photoelectrodes. The photocurrent–voltage measurement was performed using a CompactStat (Ivium Technologies B.V.) potentiostat and a PEC-L01 solar simulator system equipped with a 150 W xenon arc lamp (Peccell Technologies, Inc.). The light intensity was adjusted to 1 sun (100 mW/cm2) with a silicon photodiode (model PEC-SI01, Peccell Technologies, Inc.). The UV–vis absorption spectra were obtained using a SINCO NEOSYS-2000 spectrophotometer. Both the electrochemical impedance spectroscopic (EIS) analysis and the open-circuit voltage decay (OCVD) measurements were performed using an electrochemical analyzer (CompactStat, Ivium Technologies B.V.). The active areas of the dye-adsorbed TiO2 films were estimated using a digital microscope camera (OLYMPUS SZ61) with image analysis software.

3

3 Results and discussion

3.1

3.1 Adsorption of anions on TiO2 surface

The bare TiO2/FTO was soaked in an aqueous ESD solution for 10–40 min to prepare ESD(10, 20, 30, and 40)-TiO2/FTO, where “(10)” means that the dipping time was 10 min. To confirm the incorporation of anions (–SO3−), an XPS measurement was conducted using the ESD(20)-TiO2/FTO electrode. However, the peaks attributed to sulfur were not detected. This could have occurred because the concentration of sulfonate group on the TiO2 surface is below the detection limit of the XPS instrument. When the dipping time was prolonged for 24 hr, sulfur peaks appeared as shown in Fig. 1. The peaks detected at 167.89 and 169.08 eV corresponds to the 2p3/2 and 2p1/2 binding energies in S, respectively, indicating that, by the simple dipping in each solution, sulfate ions were adsorbed on the TiO2 surface. To further confirm the incorporation of the sulfonate groups, ATR-FTIR spectra were recorded for the ESD-modified TiO2 film. However, absorption peaks measured from the ESD(20)-TiO2 film, which was dipped into the ESD solution for 20 min, were almost similar to those of bare TiO2 film, probably due to the small amount of ESD in ESD(20)-TiO2. Thus, we provide the ATR-FTIR spectrum for 60-min-treated TiO2 [ESD(60)-TiO2] film. Fig. 2 shows the IR spectra of bare TiO2, ESD(20)-TiO2 and ESD(60)-TiO2 films. In contrast to bare TiO2 and ESD(20)-TiO2, the stronger absorption band at 1092 cm−1, assignable to the stretching vibrations of S⚌O⚌S bonds, appeared in the spectrum of the ESD(60)-TiO2 film, strongly supporting the presence of sulfonate groups (Albishri et al., 2017; Lokman et al., 2016). Several research groups have also reported that sulfonate groups can be adsorbed onto TiO2 surface by the soaking process (Khazraji et al., 1999; Zhang and Cole, 2015). It appears the ESD adsorbed on TiO2 surface will be maintained for a certain operation time of the cells since it is not dissolved in the electrolyte and acetronitrile.

XPS spectra for S 2p measured using ESD(20)-TiO2/FTO.
Fig. 1 XPS spectra for S 2p measured using ESD(20)-TiO2/FTO.
ATR-FTIR spectra of bare, ESD(20)- and ESD(60)-TiO2/FTO.
Fig. 2 ATR-FTIR spectra of bare, ESD(20)- and ESD(60)-TiO2/FTO.

In order to investigate the morphological changes by the ESD modification, SEM images were obtained. However, a comparison of the images revealed no noticeable change in morphology as shown in Fig. 3. We believe that the ESD adsorbed with very low concentration does not affect the surface morphology. From the SEM images, it can be seen that the working electrode is consisted of about 7-μm-thick photoanode and 3-μm-thick scattering layer.

Cross-sectional SEM images of (a) bare TiO2/FTO and (b) ESD(20)-TiO2/FTO photoelectrodes; (c) and (d) are also cross-sectional SEM images of TiO2 layer in bare TiO2/FTO and ESD(20)-TiO2/FTO, respectively.
Fig. 3 Cross-sectional SEM images of (a) bare TiO2/FTO and (b) ESD(20)-TiO2/FTO photoelectrodes; (c) and (d) are also cross-sectional SEM images of TiO2 layer in bare TiO2/FTO and ESD(20)-TiO2/FTO, respectively.

3.2

3.2 Performance variations of DSSCs with bare and ESD-modified TiO2 photoelectrodes

We expected that the one end (anion) of ESD would be adsorbed on TiO2 surface, and its other anion would be exposed to the electrolyte, i.e., TiO2-O-SO2-CH2-CH2-SO3−, as shown in Fig. 4. The exposed anions (-SO3−) would repulse I3− in the electrolyte, and thus reduce the back reaction between the photoinjected electrons and I3−. This could lead to an improvement in the performance of the DSSCs.

Schematic illustration of the reduced back reaction by the repulsion between sulfonate anions and I3−.
Fig. 4 Schematic illustration of the reduced back reaction by the repulsion between sulfonate anions and I3−.

To confirm the incorporation effects of anions, DSSCs with ESD-modified TiO2/FTO electrodes were fabricated, and their photovoltaic properties (Table. 1) was compared to a reference device with bare TiO2/FTO, i.e., without any surface treatment. Fig. 5 shows performance variations of the DSSCs as a function of the soaking time. The Jsc and Voc values of the DSSCs with ESD-TiO2/FTO were increased for all the soaking time, compared to those of the reference cell, and the maximum values were recorded when the dipping time in the ESD solution was 20 min. There were no meaningful changes in FF values in the devices. Overall, PCE values were increased by the incorporation of ESD on the surface of TiO2 electrodes. Because the highest PCE value was observed when the TiO2/FTO electrode was dipped in the ESD solution for 20 min, we focused on this device with ESD(20)-TiO2/FTO to investigate the origin of the efficiency enhancement.

Table 1 Photovoltaic properties of the DSSCs with bare TiO2/FTO and ESD-modified TiO2/FTO electrodes.
Applied electrodes Jsc (mA/cm2) Voc (mV) FF (%) η (%)
Bare TiO2/FTO 20.36 0.698 69.59 9.89
ESD(10)-TiO2/FTO 21.45 0.704 69.44 10.49
ESD(20)-TiO2/FTO 22.28 0.716 68.79 10.97
ESD(30)-TiO2/FTO 21.63 0.708 69.37 10.62
ESD(40)-TiO2/FTO 21.35 0.704 69.87 10.50
Variations of performance with soaking time in the ESD solution; (a) Jsc, (b) Voc, (c) FF, and (d) PCE of the DSSCs, measured under AM 1.5 irradiation.
Fig. 5 Variations of performance with soaking time in the ESD solution; (a) Jsc, (b) Voc, (c) FF, and (d) PCE of the DSSCs, measured under AM 1.5 irradiation.

The photovoltaic properties of the DSSCs with bare TiO2/FTO and ESD(20)-TiO2/FTO are compared in Fig. 6 and Table 1. The device with ESD(20)-TiO2/FTO showd a PCE of 10.97%. When compared with that (9.89%) of the reference cell with pristine TiO2/FTO, about 11% enhancement in PCE was achieved, which was attributed to an increase in Jsc and Voc.

J–V characteristics of the DSSCs with bare TiO2/FTO and ESD(20)-TiO2/FTO electrodes.
Fig. 6 J–V characteristics of the DSSCs with bare TiO2/FTO and ESD(20)-TiO2/FTO electrodes.

3.3

3.3 Influences of the ESD modification on Jsc

By the ESD modification, Jsc increased from 20.36 for bare TiO2/FTO to 22.28 mA/cm2 for ESD(20)-TiO2/FTO (Table 1). The Jsc value is generally influenced by four efficiency factors, as shown in Eqs. (1) and (2), i.e., the light harvesting (LHE), electron injection (ηinj), dye regeneration (ηreg), and electron collection (ηcoll) efficiencies of the injected electrons to the transparent electrode, where e is the elementary charge and Φph,AM1.5G is the photon flux in AM 1.5 G, 100 mW/cm2 (Hagfeldtt et al., 2010; Arkan et al., 2016).

(1)
J sc = ∫ IPCE ( λ ) e Φ ph , AM 1.5 G ( λ ) d λ
(2)
IPCE ( λ ) = LHE ( λ ) η inj ( λ ) η reg η coll ( λ )

The LHE is related to the light absorbance (A) of the adsorbed dyes, i.e., LHE = 1–10−A (Hagfeldtt et al., 2010; Arkan et al., 2016). Thus, to investigate the effects of the LHE on the Jsc enhancement, the amount of adsorbed dyes was first measured using the Beer-Lambert equation (A = εbc), where the molar extinction coefficient (ε) for N719 in the basic aqueous solution is 1.25 × 104 M−1 cm−1 at 500 nm, and b is the width of the quartz cell (1 cm in this study) (Alarcόn et al. (2007); Lee et al., 2012; Kim and Han, 2014). The average dye loading amounts, measured using 10 cells, for the bare TiO2/FTO and the ESD(20)-TiO2/FTO electrodes were 6.21 × 10−5 and 6.33 × 10−5 mol/cm3, respectively. The amount of adsorbed dye molecules on the ESD(20)-TiO2/FTO electrode was almost similar to that on the bare TiO2/FTO film, indicating that the ESD modification of TiO2 surface did not affect the LHE, and thus the Jsc value.

The ηinj value could be varied by the shift of conduction band edge (CBE). To confirm the CBE shift, we measured UV–visible absorption and valence band spectra of bare TiO2/FTO and ESD(20)-TiO2/FTO (Fig. 7). The absorption edge of ESD(20)-TiO2/FTO was at 370.8 nm, which could be assigned to a band gap of 3.36 eV. Bare TiO2/FTO exhibited an absorption edge of 367.4 nm corresponding to the band gap of 3.38 eV. The position of the valence band edge (VBE) of ESD(20)-TiO2/FTO could be seen at about 3.83 eVNHE (NHE = Normal Hydrogen Electrode), which was the same as that of bare TiO2/FTO. The energies of the VBE of ESD(20)-TiO2/FTO and bare TiO2/FTO were estimated to be −8.33 eVAVS, because the relationship between ENHE and EAVS (AVS = Absolute Vacuum Scale) is EAVS = ENHE − Ee, where Ee (about 4.50 eV) is the energy of free electrons in the hydrogen scale (Sun et al., 2014). Based on the band gap energies, the energies of the CBE of ESD(20)-TiO2/FTO and bare TiO2/FTO were determined to be −4.97 and −4.95 eVAVS, respectively. Overall, the energies of CBE in both ESD(20)-TiO2/FTO and bare TiO2/FTO was almost the same. It is probably because amount of ESD adsorbed on TiO2 surface was very small, and anions (TiO2-O-SO2-CH2-CH2-SO3−) were far from the TiO2 surface. Thus the ESD modification of TiO2 surface did not affect the Jsc value.

UV–visible absorbance (a) and UPS valence band (b) spectra for bare and ESD(20)-TiO2/FTO.
Fig. 7 UV–visible absorbance (a) and UPS valence band (b) spectra for bare and ESD(20)-TiO2/FTO.

The value of ηcoll depends on the ratio of the charge transport through TiO2 to the back reaction of photoinjected electrons, i.e., ηcoll = Kt/(Kt + Kb) = 1/(1 + Kb/Kt), where Kb and Kt are the rate constants for the back reaction and the charge transport, respectively (Kim et al., 2015; Park et al., 2012; Yu et al., 2017). This equation shows that ηcoll can be improved by decreasing Kb and by increasing Kt. The Kb value is related to the lifetime of electrons injected from the dyes, i.e., a prolonged electron lifetime indicates a decrease in Kb.

EIS has been widely used for investigating the kinetics and energetics of charge transport and interfacial charge recombination in DSSCs (Sarker et al., 2017; Kim et al., 2012; Mohamed et al., 2016; Wang and Hu, 2015; Zhao et al., 2012; Tian et al., 2010; Park et al., 2011; Xu et al., 2014). Fig. 8(a) shows the Bode phase plots of the EIS spectra for the DSSCs with the corresponding photoelectrodes measured at −0.7 V in the dark. Using the peak frequency (fmax) of 13.38 and 11.24 Hz obtained from the EIS Bode phase plots of the DSSCs with bare TiO2/FTO and ESD(20)-TiO2/FTO, respectively, the electron lifetime (τn) was estimated from the equation: τn = 1/2πfmax (Kim et al., 2012; Mohamed et al., 2016). The electron lifetime was calculated to be 11.89 and 14.16 ms for the DSSCs with bare TiO2/FTO and ESD(20)-TiO2/FTO electrodes, respectively. A prolonged lifetime (approximately 19% increase) of the electrons injected from the dyes was obtained for the device with ESD(20)-TiO2/FTO compared to that of the reference cell. Fig. 8(b) shows the Nyquist plots of the EIS spectra for the DSSCs measured at −0.7 V in the dark. Electrochemical parameters are attributed to the serial resistance, which is determined by the sheet resistance of FTO and electrical contact between TiO2/FTO interface (Rs), the redox reaction at the platinum counter-electrode (R1), the electron transfer at the TiO2/dye/electrolyte interface (R2), and the carrier transport by ions within the electrolytes (R3) (Mohamed et al., 2016; Wang and Hu, 2015). It was noted that the ESD modification increased the impedance component in R2. The larger semicircle of R2 measured in the dark indicates that the back reaction at the TiO2/dye/electrolyte interface is weaker (Zhao et al., 2012; Tian et al., 2010). Thus, the prolonged electron lifetime (Bode plots) and the retardation of the back reaction (Nyquist plots) indicate that Kb was reduced by the ESD modification. Fig. 8(c) shows the Nyquist plots of the EIS spectra for the DSSCs in an open-circuit condition under illumination by simulated AM 1.5 solar light (100 mA/cm2). When the EIS measurements are conducted in an open-circuit condition, a smaller semicircle in the medium-frequency region indicates a more efficient charge transfer process at the TiO2/dye/electrolyte interface (Kim et al., 2012; Park et al., 2011; Xu et al., 2014). However, the impedance components of R2′ in the DSSCs with bare TiO2/FTO and ESD(20)-TiO2/FTO were measured to be approximately 10.9 Ω. This result demonstrates that the Kt values in both DSSCs are almost similar, i.e., a similar charge transfer process occurs at the TiO2/dye/electrolyte interface. As a result, we can conclude that the ESD modification induced a decreased Kb, while the Kt value was almost maintained. A decrease in Kb, caused by delayed back reaction, led to an increase of the electron lifetime, which can contribute to the enhancement of ηcoll. This is likely to be because the ethane sulfonate anions on the TiO2 surface push out the I3− in the electrolyte.

EIS spectra of the DSSCs with bare TiO2/FTO and ESD(20)-TiO2/FTO. (a) Bode and (b) Nyquist plots measured at −0.7 V in the dark and (c) Nyquist plots measured under illumination.
Fig. 8 EIS spectra of the DSSCs with bare TiO2/FTO and ESD(20)-TiO2/FTO. (a) Bode and (b) Nyquist plots measured at −0.7 V in the dark and (c) Nyquist plots measured under illumination.

To further confirm the prolonged electron lifetimes in the DSSC with ESD(20)-TiO2/FTO, the OCVD characteristics of the devices were measured. Typically used to study recombination kinetics in DSSCs, OCVD is a technique that monitors the subsequent decay of the photovoltage (Voc) after stopping the illumination in a steady state (Park et al., 2011; Xu et al., 2014; Lamberti et al., 2013). The cells are maintained under constant illumination in an open-circuit condition until they reach a steady voltage value. At this point, the light is suddenly switched off, and the photovoltage is measured as a function of time. Because some photogenerated electrons cannot be collected by the electrode under the open-circuit condition, they react with I3− in the electrolyte at an approximately constant rate, thereby reducing the photovoltage. Consequently, the photovoltage decay rate is directly related to the electron lifetime because excess electrons are removed through the back reaction. Specifically, the recombination rate of photoelectrons is proportional to the photovoltage decay rate. To estimate the electron lifetime (τ) of the devices, the corresponding curves for τ versus voltage (Fig. 9(b)) can be obtained from the OCVD curves (Fig. 9(a)) by using Eq. (3), where kB is the Boltzmann constant, T is the absolute temperature, e is the electron charge, and dVoc/dt is the first derivative of the open-circuit voltage transient (Hagfeldtt et al., 2010; Park et al., 2011; Xu et al., 2014; Lamberti et al., 2013):

(3)
τ = - k B T e dVoc dt - 1
Curves of OCVD (a) and electron lifetime versus Voc (b) for the DSSCs with bare TiO2/FTO and ESD(20)-TiO2/FTO.
Fig. 9 Curves of OCVD (a) and electron lifetime versus Voc (b) for the DSSCs with bare TiO2/FTO and ESD(20)-TiO2/FTO.

The electron lifetimes in the DSSC with ESD(20)-TiO2/FTO were longer than those in the reference device with bare TiO2/FTO film, as shown in Fig. 9(b). This indicates that the back reaction in the former DSSC was delayed, thereby inducing a prolonged electron lifetime, which also improved ηcoll in that device.

The dark current values can be used to estimate the back reaction in DSSCs (Chen et al., 2001; Diamant et al., 2003). Fig. 10 shows the dark currents of DSSCs with bare TiO2/FTO and ESD(20)-TiO2/FTO as a function of the applied potential. Throughout the measured potential range, the dark current values of the device with ESD(20)-TiO2/FTO were lower than those of the reference device, which indicated that the back reaction between injected electrons and I3− ions was retarded by the incorporation of ESD with a terminal anion. The observation of the delayed back reaction (or the prolonged electron lifetime) was consistent with the results of the EIS and OCVD measurements.

Dark current–voltage characteristics of DSSCs with bare TiO2/FTO and ESD(20)-TiO2/FTO.
Fig. 10 Dark current–voltage characteristics of DSSCs with bare TiO2/FTO and ESD(20)-TiO2/FTO.

Overall, it is believed that the enhancement in the Jsc value of the DSSC with ESD(20)-TiO2/FTO was mainly attributed to the improvement in ηcoll. It is because the exposed anions (SO3−) repulse I3− of the electrolyte, and thus reduce the back reaction between the photoinjected electrons and I3−.

3.4

3.4 Influences of the ESD modification on Voc

The Voc value (0.716 V) of the DSSC with the ESD(20)-TiO2/FTO electrode also increased compared with the reference device (0.698 V), which contributed to the increase of the cell efficiency. Under constant illumination, the Voc value of DSSCs corresponds to the increase of the quasi-Fermi level (EFn) of the semiconductor with respect to the dark value (EF0), and thus it can be expressed as Eq. (4) (Peng et al., 2015; Zaban et al., 2003):

(4)
V oc = E Fn - E F 0 e = k B T e ln n n 0 where kBT is the thermal energy (4.11 × 10−21 J at 25 °C). For more details, kB and T are the Boltzmann constant and the absolute temperature, respectively. e is the positive elementary charge (1.602 × 10−19 C), n0 is the concentration in the dark, and n is the free electron density in the TiO2 photoelectrode under illumination.

Eq. (4) suggests that Voc and n are directly correlated, and n is affected by both the electron injection from the light-absorbed dyes to the conduction bands of TiO2 and the back reaction between photoinjected electrons and ions (I3−) in the electrolyte. Accordingly, higher electron injection and lesser back reaction are required to increase n, which can improve the Voc value. In our case, it is considered that the back reaction is predominant to influence the Voc value, probably because the electron injection efficiency is not altered by the ESD modification due to the similar amounts of adsorbed dyes. As presented above, EIS, OCVD and dark current measurements revealed that the ESD modification (i.e., the incorporation of ethane sulfonate anions) increased the lifetime of the photoinjected electrons, and therefore lowered the back reaction. This indicates that the n value in Eq. (4) increased. Consequentially, the ESD modification led to an increased n, thereby inducing an enhancement in Voc based on Eq. (4).

4

4 Conclusions

To reduce the back reaction between photoinjected electrons and I3− in the electrolyte, a negative charge (ethane sulfonate anions) was formed on TiO2 surface by soaking bare TiO2/FTO film to an ESD solution. The modified TiO2 layer was applied to the photoelectrodes of the DSSCs. The DSSC with ESD(20)-TiO2/FTO showed an increase in Jsc and Voc, resulting in a PCE of 10.97% (Jsc = 22.28 mA/cm2, Voc = 716 mV and FF = 68.79%), compared to that of the reference device with bare TiO2/FTO (PCE = 9.89%, Jsc = 20.36 mA/cm2, Voc = 698 mV and FF = 68.59%). The incorporation of the ethane sulfonate anions on TiO2 surface led to about 11% improvement in PCE, because the anions could repulse I3−, and thereby reduce the back reaction. It is worth mentioning that the anion incorporation on TiO2 surface is a good strategy to increase performance of DSSCs by a retardation of the back reaction.

Acknowledgments

This work was supported by the DGIST R&D Program of the Ministry of Science, ICT, and Future Planning of Korea (18-ET-01). This research was also supported by the Basic Science Research Program of the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT (NRF-2016R1A2B1015037).

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