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Enhancement of solar cell performance through the formation of a surface dipole on polyacrylonitrile-treated TiO2 photoelectrodes
Journal of Industrial and Engineering Chemistry 73 (2019) 260–267
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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Enhancement of solar cell performance through the formation of a surface dipole on polyacrylonitrile-treated TiO2 photoelectrodes Gun Wook Baeka , Young-Jin Kimb , Kyung-Hye Junga , Yoon Soo Hana,* a b
School of Advanced Materials and Chemical Engineering, Daegu Catholic University, Gyeongbuk 38430, Republic of Korea Department of Biomedical Engineering, Daegu Catholic University, Gyeongbuk 38430, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 12 October 2018 Received in revised form 14 January 2019 Accepted 26 January 2019 Available online 4 February 2019
Polyacrylonitrile (PAN) was adsorbed onto the TiO2 surface, and the resulting photoelectrodes were applied in dye-sensitized solar cells (DSSCs). The DSSC with the PAN-modified TiO2 showed an increase in its short-circuit current (Jsc) and a decrease in its open-circuit voltage (Voc), resulting in a 17% enhancement of its photovoltaic performance when compared to the reference cell. By incorporating PAN onto the surface of the TiO2, a surface dipole was formed, which induced a conduction band edge shift of the TiO2 in a positive direction. This shift led to a considerable improvement in the electron injection efficiency, and hence the Jsc. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Polyacrylonitrile Dye-sensitized solar cell Surface dipole Conduction band edge shift Electron injection efficiency
Introduction It is commonly believed that the sun represents the only viable option in terms of developing a solution to the energy crisis. Photovoltaics could, therefore, be considered a major renewable energy resource, albeit only once the associated manufacturing cost has decreased to an affordable level when compared to the costs of the other available energy resources [1]. Due to being lowcost photovoltaics, dye-sensitized solar cells (DSSCs), polymer solar cells, and perovskite solar cells have all attracted significant research attention in recent years, and they are regarded as thirdgeneration photovoltaics. Since the first report concerning DSSCs based on TiO2 photoelectrodes [2], a power conversion efficiency (PCE) of 14.3% has been achieved in a co-sensitized solar cell featuring a cobalt (II/III) complex redox electrolyte solution and graphene nanoplates as a counter electrode [3]. However, there is still room for further improvement with regard to the theoretical maximum PCE of~20% [4]. The PCE of a cell is given as the ratio of the maximum power (Pmax) to the total intensity of the incident light (Pin = 100 mW/ cm2). The fill factor (FF) of the cell is defined as the ratio of its Pmax to the product of its open-circuit voltage (Voc) and its short-circuit current density (Jsc), which leads to PCE = (Jsc Voc FF) / Pin [where FF = Pmax/(Jsc Voc)]. Thus, in order to achieve increased efficiency,
* Corresponding author. E-mail address: [email protected] (Y.S. Han).
parameters such as the Jsc, Voc, and FF must be increased. As the Jsc value is strongly related to the light absorbance and the absorption range of the dye molecules, many studies have focused on the synthesis of new dyes that are capable of absorbing longer wavelengths of light [5]. It has also been reported that surface modifications made to TiO2 photoelectrodes could serve to improve both the Voc and the Jsc due to the formation of a surface dipole, an energy barrier, or a surface state passivation [6]. By the modification of the TiO2 surface, the conduction band edges (CBEs) could be shifted in a negative direction by the formation of a surface dipole, thereby resulting in an increase in the Voc. For example, certain semiconductors or insulators have been applied to modify TiO2 electrodes, and the resultant DSSCs have shown an enhancement in the PCE, mainly due to the enhancement seen in the Voc [7–10]. Further, surface modifications could lead to a reduction in the interfacial charge recombination between the TiO2 electrode and the electrolytes through the formation of an energy barrier on the TiO2 surface, thereby resulting in the enhancement of both the Voc and the Jsc [11–15]. Several reports have shown that the use of certain inorganic compounds, such as Al2O3, ZnO, and PbS, as a coating on the TiO2 surface led to the passivation of the surface states, which resulted in the retardation of the interfacial charge recombination, and hence an increase in both the Voc and the Jsc [16–18]. As another simple method to improve PCE, it was reported that addition of nanoparticles to the photoelectrode could induce an increase in the light harvesting and a reduction in the charge recombination [19]. Thus, the simple techniques such as the surface modification of the TiO2
https://doi.org/10.1016/j.jiec.2019.01.036 1226-086X/© 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
G.W. Baek et al. / Journal of Industrial and Engineering Chemistry 73 (2019) 260–267
photoelectrode and addition of nanoparticles to the photoelectrode appear to be an effective method for improving the PCE of DSSCs. The methods and materials associated with the surface modification that were discussed above have all been intensively studied, but little attention has previously been paid to the application of polymeric materials as surface modifiers of the photoelectrodes in DSSCs. Thus, we selected polyacrylonitrile (PAN) as a TiO2 surface modifier. The TiO2 surfaces were directly treated with a PAN solution via a simple dipping process, and the resultant electrodes were applied to the photoelectrodes of DSSCs. We expected the PAN to trigger the formation of a surface dipole, an energy barrier, or surface state passivation on the TiO2 surfaces, which was in turn expected to result in an improvement in the performance of the DSSCs. Therefore, DSSCs with PAN-modified photoelectrodes (PAN-TiO2/FTO) were fabricated, and the effects of the surface treatment on the photovoltaic performance of the cells were examined. Experimental details Materials Commercial fluorine-doped tin oxide (FTO; sheet resistance~7
V/square) glass (TCO22-7), TiO2 paste for the photoelectrode (Ti-
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Measurements X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) were performed using a VG Multilab ESCA 2000 apparatus (ThermoVG Scientific), with Al Kα radiation (hv = 1486.6 eV) being used for the XPS and with He I radiation (hv = 21.22 eV) being used for the UPS. The C1 s photoelectron peak (binding energy at 284.6 eV) was used as an energy reference. The Fourier transform infrared (FT-IR) spectra were recorded using an FT/IR 4100 spectrometer (Jasco) equipped with an attenuated total reflectance (ATR) accessory (PRO450-S, Jasco). The photocurrent– voltage measurement was performed using a CompactStat potentiostat (Ivium Technologies B.V.) 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) using a silicon photodiode (PEC-SI01, Peccell Technologies, Inc.). The UV–vis absorption spectra were determined 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 photoluminescence of the dyes adsorbed on the TiO2 surface was measured using a spectrofluorometer (RF-5301PC) produced by the Shimadzu Corporation. The active areas of the
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 all purchased from Solaronix. The PAN and the TiCl4 were purchased from Pfaltz & Bauer, Inc. and Sigma-Aldrich Co. LLC, respectively. Platinum paste (PT-1, Dyesol-Timo) was chosen as the source for the Pt counter electrode. All the chemicals were used without further purification. Fabrication of the DSSCs The sheets of FTO glass were cleaned using a detergent solution by means of sonication for 20 min. They were then thoroughly rinsed with deionized water and ethanol. The cleaned sheets of FTO glass were immersed in a 40-mM TiCl4 solution at 70 C for 30 min, and they were thenwashed with water and ethanol. One active TiO2 layer, which was formed on the FTO glass, was prepared via doctor-blade coating using the TiO2 paste. Additionally, a TiO2 layer composed of particles approximately 400 nm in diameter was deposited on the active TiO2 layer, and it was then calcinated at 500 C for 60 min in order 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 a dimethylformamide solution (5 mM) of PAN for 0–50 min so as to deposit the polymer onto the TiO2 layers. Next, the resultant electrodes were rinsed with water and ethanol, and they were then dried at 65 C for 10 min in order to produce the modified photoelectrodes (PAN-TiO2/FTO). Finally, both the bare TiO2/FTO and the PAN-TiO2/FTO photoelectrodes were separately immersed into 0.5 mM of ethanolic N719 dye solution for 24 h to obtain working electrodes. To prepare the counter electrode, two holes were formed in the FTO glass using a drill, and the glass was then cleaned using the method described above. A Pt layer was formed on the FTO glass via the doctor-blade method using Pt paste, which was followed by a calcination process at 400 C for 30 min. The thermally treated platinum counter electrodes then were placed on the photoelectrodes and sealed using a 60-mm-thick sealing material. The electrolyte was introduced into the cells through one of the two small holes drilled into the counter electrodes in order to produce DSSCs with a 25 mm2 active area.
Fig. 1. XPS spectra for the N 1 s measured using PAN(40)-TiO2/FTO.
Fig. 2. ATR-FTIR spectra of the bare TiO2/FTO, PAN(40)-TiO2/FTO, and PAN(720)TiO2/FTO.
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Fig. 3. Variations in performance with soaking time in the PAN solution: (a) Jsc, (b) Voc, (c) FF, and (d) PCE of the DSSCs, as measured under AM 1.5 irradiation.
dye-adsorbed TiO2 films were estimated using a digital microscope camera (OLYMPUS SZ61) equipped with image analysis software. Results and discussion Preparation and characterization of the PAN-TiO2/FTO
Fig. 4. J–V characteristics of the DSSCs with bare TiO2/FTO and PAN(40)-TiO2/FTO electrodes.
PAN is a resin synthesized by means of the polymerization of acrylonitrile, and it contains the pendant cyano groups. The active cyano groups allow PAN to adsorb into the five-fold coordinated titanium (Ti5c) atoms located on the surfaces of TiO2 photoelectrodes [20–22]. It has previously been established that the oxygen atoms of bulk crystal TiO2 are three-fold coordinated, while the titanium atoms are six-fold coordinated. Further, the surface corner oxygen atoms are two-fold coordinated, while the titanium atoms are five-fold coordinated [23]. Thus, a strong interaction could occur between the five-fold coordinated Ti5c atoms of TiO2 and the strong electron-withdrawing cyano groups of PAN. Six different photoelectrodes were prepared by soaking the bare TiO2/ FTO in a dilute solution of PAN for 10–50 min, thereby producing PAN(10–50)-TiO2/FTO, where “(10)” indicates that the dipping time was 10 min. In order to confirm the incorporation of the PAN, an
Table 1 Photovoltaic properties of the DSSCs with bare TiO2/FTO and PAN(40)-TiO2/FTO electrodes. Applied electrodes
Jsc (mA/cm2)
Voc (mV)
FF (%)
h (%)
Loaded dye (mol/cm3)
Bare TiO2/FTO PAN(40)-TiO2/FTO
12.13 14.49
712 700
70.62 70.68
6.10 7.17
2.10 105 1.88 105
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XPS measurement was performed using the PAN(40)-TiO2/FTO electrode. In Fig. 1, the peak detected at 400.08 eV corresponds to the 1 s binding energy in nitrogen, which indicates that, via the technique of simple dipping in the solution, the PAN was adsorbed onto the TiO2 surface [24,25]. To further confirm the incorporation of the PAN, the ATR-FTIR spectra were recorded for the PANmodified TiO2 film. Fig. 2 shows the IR spectra of the bare TiO2, PAN (40)-TiO2, and PAN(720)-TiO2 films. In contrast to the bare TiO2, the absorption band seen at 2244 cm1, which could be assigned to the stretching vibrations of the CN bonds, appeared in the spectrum of the ESD(40)-TiO2 film [26,27]. When the dipping time was prolonged to 12 h (720 min), the spectrum of the PAN (720)-TiO2 showed a stronger absorption band at 2244 cm1 due to the CN groups. Based on the XPS and the FT-IR measurements, we can see that PAN can be easily adsorbed onto the surface of TiO2 by means of the soaking process. Further, it is considered that the PAN adsorbed onto the TiO2 surface will be maintained for a certain time during the operation of the cells, since it is not dissolved in acetonitrile, which is a solvent found in the electrolyte. Photovoltaic performance variations of the DSSCs with PAN-TiO2/FTO In order to investigate the effects of PAN on the cell performance, we fabricated DSSCs using the bare TiO2/FTO and the PAN-TiO2/FTO photoelectrodes, and their photovoltaic properties were characterized. Fig. 3 shows the variations in cell performance as a function of the soaking time, that is, the surface modification time. The Jsc values were improved for all the soaking times when compared to the Jsc of the reference cell, and the maximum values were recorded when the surface modification time was 40 min, as shown in Fig. 3(a). However, the Voc values were observed to decrease as the soaking time increased, as shown in Fig. 3(b). There were no meaningful changes in the FF values of the devices. Overall, the PCE values were increased following the incorporation of PAN onto the surface of the TiO2 electrodes, since the improvement in the Jsc overrode the reduction in the Voc. As the highest PCE value was observed when the TiO2/FTO electrode was treated with the PAN solution for 40 min, we focused on this device with PAN(40)-TiO2/FTO in order to investigate the origin of the efficiency enhancement. Fig. 4 shows the current density (J) and voltage (V) curves of the DSSCs with the bare TiO2/FTO and PAN(40)-TiO2/FTO electrodes, and the device performance is compared in Table 1. The DSSCs with the PAN(40)-TiO2/FTO electrode showed a PCE of 7.17%, while the DSSCs with the bare TiO2/FTO electrode showed a PCE of 6.10%. The enhanced PCE was mainly due to the increase in the Jsc.
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where the molar extinction coefficient (e) for N719 in the basic aqueous solution is 1.25 104 M1 cm1 at 500 nm, while b is the width of the quartz cell (1 cm in this study) [30–32]. The averaged dye-loading amounts, as measured using ten cells, for the bare TiO2/FTO and the PAN(40)-TiO2/FTO were 2.10 105 and 1.88 105 mol/cm3, respectively. The amount of adsorbed dye molecules on the PAN(40)-TiO2/FTO decreased by approximately 10% when compared to the amount on the bare TiO2/FTO. This was probably due to the fact that PAN occupies a fraction of the adsorption sites on the TiO2 surface. As less adsorption of organic dyes on the photoelectrodes could generate fewer electrons and fewer holes, the PAN modification of the TiO2 surface could serve to reduce the LHE. In the case of DSSCs, unlike other solar cells, the electrontransporting TiO2 layer is in direct contact with the hole conductor (electrolyte), which induces the charge recombination (I3 + 2 e → 3I) between the photoinjected electrons and the ions in the electrolyte. This causes a reduction in the electron lifetime, and hence in the hcol. Thus, the value of the hcoll is largely related to the lifetime of the electrons injected into the valence band of the TiO2 from the excited dyes, that is, a prolonged electron lifetime could lead to an increase in the hcoll. EIS analysis has been widely used to investigate the kinetics and energetics of charge transport and interfacial charge recombination in DSSCs [33–36]. Fig. 5(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 frequencies (fmax) of 13.4 Hz and 22.6 Hz obtained from the EIS Bode phase plots of the DSSCs with the bare TiO2/FTO
Effects of the PAN adsorbed onto the TiO2 surface on the Jsc As shown in Table 1, the PAN adsorption led to an increase in the Jsc from 12.13 mA/cm2 for the bare TiO2/FTO to 14.49 mA/cm2 for the PAN(40)-TiO2/FTO. The Jsc value is generally influenced by four efficiency factors, as shown in Eqs. (1) and (2), namely the light harvesting (LHE), the electron injection (hinj), the dye regeneration (hreg), and the electron collection (hcoll) efficiencies, where e is the elementary charge and Fph,AM1.5G is the photon flux in AM 1.5 G, 100 mW/cm2 [28,29]. R (1) Jsc = IPCE(l)eFph,AM1.5G(l) dl IPCE(l) = LHE(l)hinj(l) hreghcoll(l)
(2)
The LHE is related to the light absorbance (A) of the adsorbed dyes, that is, LHE = 110A [28,29]. Thus, in order to investigate the effects of the LHE on the Jsc enhancement, the amount of adsorbed dye was first measured using the Beer–Lambert equation (A = ebc),
Fig. 5. EIS spectra of the DSSCs with bare TiO2/FTO and PAN(40)-TiO2/FTO: (a) Bode and (b) Nyquist plots measured at 0.7 V in the dark.
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and the PAN(40)-TiO2/FTO, respectively, the electron lifetime (t n) was estimated using the equation: t n = 1/2pfmax [34,35]. The electron lifetimes were calculated to be 11.9 ms and 7.0 ms for the DSSCs with the bare TiO2/FTO and the PAN(40)-TiO2/FTO electrodes, respectively. The reduced lifetime of the electrons injected from the dyes was observed for the device with the PAN(40)-TiO2/ FTO when compared to situation of the reference cell. Fig. 5(b) shows the Nyquist plots of the EIS spectra for the DSSCs measured at 0.7 V in the dark. The electrochemical parameters depicted in Fig. 5(b) are attributed to the serial resistance, which is determined by the sheet resistance of the FTO and the electrical contact between the 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) [35,36]. It was noted that the PAN modification decreased the impedance component in the R2. When considering that the R2 value refers to the resistance of the charge recombination, the smaller semicircle of R2 seen in Fig. 5(b) indicates that the interfacial charge recombination at the TiO2/dye/ electrolyte interface is stronger [34,35]. Thus, the reduced electron lifetime (Bode plots) and the promoted recombination (Nyquist plots) suggest that the hcol was decreased by the PAN modification. In order to further estimate the electron lifetimes in the DSSCs with the bare TiO2/FTO and the PAN(40)-TiO2/FTO, the OCVD characteristics of the devices were measured. The cells are maintained under constant illumination in an open-circuit condition until they exhibit a steady voltage value. At this point, the light is suddenly switched off, and the photovoltage is measured as a function of time. As some photogenerated electrons cannot be collected by the electrode in the open-circuit condition, they react with the 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, since excess electrons are removed through the charge recombination. More specifically, the recombination rate of photoelectrons is proportional to the rate of photovoltage decay [37–39]. To estimate the electron lifetime (t ) of each device, the corresponding curve for the t versus the voltage [Fig. 6(b)] can be obtained from the OCVD curve
Fig. 6. (a) Curves of the OCVD and (b) the electron lifetime versus the Voc for the DSSCs with bare TiO2/FTO and PAN(40)-TiO2/FTO.
Fig. 7. Schematic illustration of the boosted charge recombination by the electron-withdrawing cyano groups.
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[Fig. 6(a)] 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 [37–39]:
t¼
kB T dVoc 1 ð Þ e dt
ð3Þ
The electron lifetimes seen in the DSSC with the PAN(40)-TiO2/ FTO were shorter than those in the cell with the bare TiO2/FTO film, as shown in Fig. 6(b), which indicates that the PAN adsorbed onto the TiO2 surface boosted the interfacial charge recombination between the photoinjected electrons and the I3, thereby inducing a reduced electron lifetime. This means that the hcol was decreased by the PAN modification, which is consistent with the findings of the EIS measurement. We considered that the insulating polymeric material, namely PAN, would retard the charge recombination. However, the PAN incorporation rather caused rising the recombination. This could be due to the strong electron-withdrawing nature of the cyano groups in the PAN, as illustrated in Fig. 7. As mentioned above, the PAN could be adsorbed onto the TiO2 surface via interactions between the nitrogen atoms (of the cyano groups) with lone pairs and the five-fold coordinated titanium (Ti5c) atoms. During the operation of solar cells, the photoinjected electrons would be pulled by the adsorbed cyano groups, and they would in turn react with the I3 in the electrolyte. Similarly, Harima et al. reported that the cyano groups located close to the TiO2 surface served as good communicators of the photoexcited electrons from the dyes to the conduction band of the TiO2 photoelectrodes [21]. It is, therefore, believed that the reduced electron lifetime could be attributed to the strong electron-withdrawing cyano groups adsorbed onto the TiO2 surface, which boosted the interfacial charge recombination between the photoinjected electrons and the I3. Consequently, a surface dipole, rather than an energy barrier, would be formed by the adsorption of PAN onto the surface of the TiO2, as depicted in Fig. 7. In other words, the incorporation of PAN altered the distribution of the charge across the TiO2/electrolyte interface relative to the non-treated case, meaning that partial positive charges were formed on the TiO2 surface (Ti5c), which served to increase the interfacial recombination [6–10]. In order to understand the effects of the incorporation of PAN on the hinj (efficiency of the electron transfer from the excited dye to the conduction band of the TiO2), the PL spectra of the bare TiO2/ FTO and the PAN(40)-TiO2/FTO electrodes were obtained, as compared in Fig. 8. The amounts of the adsorbed dyes were adjusted to be identical in the two electrodes (the inset in Fig. 8). When excited at their absorption maxima (526 nm), they both showed a PL peak at around 791 nm. It can be clearly observed that the fluorescence from the PAN(40)-TiO2/FTO was quenched, leading to an approximate reduction in the PL intensity of 72%, when compared to the bare TiO2/FTO. This provides evidence of the more efficient charge transfer of the photoexcited electrons from the dye to the TiO2, thereby indicating that the hinj was considerably improved by the PAN incorporation [40,41]. It is also necessary to reveal what actually caused the significant enhancement of the hinj following the PAN modification. To this end, we obtained the UPS valence band and the UV–vis absorption spectra in order to establish the electronic band structures of the bare TiO2/FTO and the PAN(40)-TiO2/FTO. The position of the valence band edge (VBE) of the PAN(40)-TiO2/FTO could be seen at approximately 4.0 eVNHE (NHE = normal hydrogen electrode), which was the same as the position seen in the case of the bare TiO2/FTO, as shown in Fig. 9(a). The energies of the VBEs in the bare TiO2/FTO and the PAN(40)-TiO2/FTO were estimated to be 8.50 eVAVS, since the relationship between the EAVS (AVS = absolute vacuum scale) and the ENHE can be given as EAVS=ENHE-Eo, where Eo (about 4.50 eV) is the energy of the free electrons in the hydrogen scale [42–44]. The absorption edge of the
Fig. 8. PL spectra for the N719 dye adsorbed on the TiO2 surface.
Fig. 9. (a) UPS valence band and (b) UV–vis absorbance spectra for the bare TiO2/ FTO and PAN(40)-TiO2/FTO.
PAN(40)-TiO2/FTO was observed at 361.4 nm, which could be assigned to a band gap energy of 3.43 eV [Fig. 9(b)]. The bare TiO2/ FTO exhibited an absorption edge of 358.2 nm, corresponding to a band gap energy of 3.46 eV. Based on the identified band gap energies, the energies of the CBEs of the bare TiO2/FTO and the PAN (40)-TiO2/FTO were determined to be 5.04 and 5.07 eVAVS, respectively. As a result, the energy of the CBE in the PAN(40)-TiO2/ FTO was shifted approximately 31 meV in a positive direction when compared to that of the bare TiO2/FTO, as illustrated in Fig. 10. This
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Fig. 10. Schematic energy band diagram of the (a) bare TiO2/FTO and (b) PAN(40)-TiO2/FTO, showing the positive shift in the conduction band edge.
positive shift (away from the vacuum level) of the CBE in the PAN (40)-TiO2/FTO could be attributed to the formation of surface dipoles on the TiO2 surface, and it is more favorable in terms of the injection of photoexcited electrons from the dye to the TiO2 due to the larger potential difference (DEbare < DEPAN) between the LUMO level of the dye and the CBE of the TiO2 photoelectrode, which results in an improvement in the hinj. Similar results were reported in relation to the incorporation of cations on the TiO2 surface inducing an enhancement of the hinj due to the positive shift in the CBE of the TiO2 [45]. The dark currents of DSSCs can be used to measure a conduction band shift or to determine the existence of an energy barrier on the electrode surface [45,46]. Fig. 11 presents the dark current-voltage characteristics of the DSSCs with the bare TiO2/FTO and the PAN(40)TiO2/FTO. The onset potential of the dark current for the bare TiO2/ FTO electrode was measured to be approximately 0.654 V, whereas the onset potential for the PAN(40)-TiO2/FTO electrode was shifted to approximately 0.624 V. Following the PAN incorporation, a lower onset potential was measured in the PAN(40)-TiO2/FTO, which meant that the potential difference between the work function of the FTO and the CBE of the TiO2 in the PAN(40)-TiO2/FTO was smaller than that of the bare TiO2/FTO. Further, the PAN modification induced a shift in the onset potential, and the curves maintained the same shape. This similarity in terms of shape implies that the modification does not form an energy barrier that prevents electron leakage to the electrolyte, but instead forms a surface dipole [47]. The surface dipole formed on the TiO2 surface shifts the conduction band potential in a positive direction, which is consistent with the result of the electronic band structure study. The lower onset potential was, therefore, attributed to the positive shift in the CBE of the TiO2. Based on the measurements concerning the PL quenching, the UPS valence band, the UV–vis absorbance, and the dark currents, it could be concluded that the hinj was significantly improved by the incorporation of PAN, which was attributed to the positive shift in the CBE of the TiO2 that stemmed from the formation of the surface dipole. The dye regeneration process can be expressed as follows: D+ + 2I → D + I2
(4)
2I2 → I + I3
(5)
Therefore, the hreg can be improved when the concentration of the I near the oxidized dye (D+) rises. It has been reported that the adsorption of cations onto the TiO2 induced the formation of (I, I)
ion pairs on the TiO2 surface by means of the electrostatic interactions between the positive charge on the TiO2 and the negative I. The ion pairs led to a faster reaction between the D+ and the 2I [chemical Eq. (4); rate-determining step; electron transfer from the I to the oxidized dye], thereby resulting in an improvement in the hreg [48,49]. However, in our study, the effect of the partial positive charges on the Ti5c is not clear, because partial negative charges formed in the electron-withdrawing cyano groups also present near the surface of the TiO2. In this respect, further studies aimed at elucidating the effects of PAN incorporation on the hreg are currently in progress. Overall, the Jsc value of the DSSC with the PAN(40)-TiO2/FTO was enhanced when compared to that of the reference cell, since the considerable improvement in the hinj outweighed the reduction in both the LHE and the hcol. Effects of the PAN adsorbed on the TiO2 surface on the Voc The Voc value (700 mV) of the DSSC with the PAN(40)-TiO2/FTO decreased relative to that of the reference device (712 mV). This decrease in the Voc can be explained by the positive shift in the CBE of the TiO2 attributed to the formation of a surface dipole. The positive shift induces a narrower potential gap (DVbare DVPAN) between the
Fig. 11. Dark current-voltage characteristics of the DSSCs with bare TiO2/FTO and PAN(40)-TiO2/FTO electrodes.
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CBE and the redox potential of the electrolyte, as shown in Fig.10. As the potential difference between the CBE and the redox potential of the electrolyte is proportional to the Voc value [50], it is considered that the decrease in the Voc value of the device with the PAN(40)TiO2/FTO stems from the positive shift in the CBE of the TiO2 electrode, which results from the formation of the surface dipole. Further, the decrease in the Voc could be elucidated by the boosted charge recombination of the cell with the PAN(40)-TiO2/FTO. The Voc value is given by Eq. (6) [11,12], where k and T are the Boltzmann constant and the absolute temperature, respectively; Iinj is the flux of the charge resulting from the sensitized injection; ncb is the concentration of electrons on the TiO2 surface; and ket and [I3] are the rate constants for the interfacial charge recombination (I3 + 2e → 3I) and the concentration of I3, respectively. According to Eq. (6), the Voc and the ket are inversely correlated, and it is possible to indirectly estimate the ket by measuring the electron lifetime. A shorter electron lifetime implies a higher ket value, that is, a higher possibility that a reduction in the I3 will occur. As described above, the EIS analysis and the OCVD measurements revealed that the interfacial charge recombination was boosted by the incorporation of PAN. This fact indicates that the ket value was increased, thereby inducing a decrease of the Voc based on Eq. (6). Voc ¼
Iinj kT lnð Þ e ncb ket ½I 3
ð6Þ
Conclusions We have investigated the effects of PAN, as a surface modifier, on the performance of DSSCs. PAN-modified TiO2 films were applied to the photoelectrodes of DSSCs. The reference device, which featured no modifications, showed a photovoltaic performance of 12.13 mA/ cm2 for the Jsc, 712 mV for the Voc, and 70.62% for the FF, which led to a PCE of 6.10%, while in the case of the device with the PAN(40)-TiO2/ FTO electrode, which was treated with PAN for 40 min, the PCE was increased to 7.17% (Jsc = 14.49 mA/cm2, Voc = 700 mV, and FF = 70.68%). By means of monitoring the changes in the PL quenching, the UPS valence band, the UV–vis absorbance, and the dark currents, it was confirmed that the electron injection was significantly improved by the incorporation of PAN, which accounted for the enhancement seen in the PCE of the DSSC with the PAN(40)-TiO2/ FTO. Thus, PAN appears to be a promising material for enhancing the conversion efficiency of DSSCs. Acknowledgements This work was supported by research grants from the Daegu Catholic University in 2017. References [1] Z. Abdin, M.A. Alim, R. Saidur, M.R. Islam, W. Rashmi, S. Mekhilef, A. Wadi, Sust. Energ. Rev. 26 (2013) 837. [2] B. O’Regan, M. Grätzel, Nature 353 (1991) 737.
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Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Enhancement of solar cell performance through the formation of a surface dipole on polyacrylonitrile-treated TiO2 photoelectrodes Gun Wook Baeka , Young-Jin Kimb , Kyung-Hye Junga , Yoon Soo Hana,* a b
School of Advanced Materials and Chemical Engineering, Daegu Catholic University, Gyeongbuk 38430, Republic of Korea Department of Biomedical Engineering, Daegu Catholic University, Gyeongbuk 38430, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 12 October 2018 Received in revised form 14 January 2019 Accepted 26 January 2019 Available online 4 February 2019
Polyacrylonitrile (PAN) was adsorbed onto the TiO2 surface, and the resulting photoelectrodes were applied in dye-sensitized solar cells (DSSCs). The DSSC with the PAN-modified TiO2 showed an increase in its short-circuit current (Jsc) and a decrease in its open-circuit voltage (Voc), resulting in a 17% enhancement of its photovoltaic performance when compared to the reference cell. By incorporating PAN onto the surface of the TiO2, a surface dipole was formed, which induced a conduction band edge shift of the TiO2 in a positive direction. This shift led to a considerable improvement in the electron injection efficiency, and hence the Jsc. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Polyacrylonitrile Dye-sensitized solar cell Surface dipole Conduction band edge shift Electron injection efficiency
Introduction It is commonly believed that the sun represents the only viable option in terms of developing a solution to the energy crisis. Photovoltaics could, therefore, be considered a major renewable energy resource, albeit only once the associated manufacturing cost has decreased to an affordable level when compared to the costs of the other available energy resources [1]. Due to being lowcost photovoltaics, dye-sensitized solar cells (DSSCs), polymer solar cells, and perovskite solar cells have all attracted significant research attention in recent years, and they are regarded as thirdgeneration photovoltaics. Since the first report concerning DSSCs based on TiO2 photoelectrodes [2], a power conversion efficiency (PCE) of 14.3% has been achieved in a co-sensitized solar cell featuring a cobalt (II/III) complex redox electrolyte solution and graphene nanoplates as a counter electrode [3]. However, there is still room for further improvement with regard to the theoretical maximum PCE of~20% [4]. The PCE of a cell is given as the ratio of the maximum power (Pmax) to the total intensity of the incident light (Pin = 100 mW/ cm2). The fill factor (FF) of the cell is defined as the ratio of its Pmax to the product of its open-circuit voltage (Voc) and its short-circuit current density (Jsc), which leads to PCE = (Jsc Voc FF) / Pin [where FF = Pmax/(Jsc Voc)]. Thus, in order to achieve increased efficiency,
* Corresponding author. E-mail address: [email protected] (Y.S. Han).
parameters such as the Jsc, Voc, and FF must be increased. As the Jsc value is strongly related to the light absorbance and the absorption range of the dye molecules, many studies have focused on the synthesis of new dyes that are capable of absorbing longer wavelengths of light [5]. It has also been reported that surface modifications made to TiO2 photoelectrodes could serve to improve both the Voc and the Jsc due to the formation of a surface dipole, an energy barrier, or a surface state passivation [6]. By the modification of the TiO2 surface, the conduction band edges (CBEs) could be shifted in a negative direction by the formation of a surface dipole, thereby resulting in an increase in the Voc. For example, certain semiconductors or insulators have been applied to modify TiO2 electrodes, and the resultant DSSCs have shown an enhancement in the PCE, mainly due to the enhancement seen in the Voc [7–10]. Further, surface modifications could lead to a reduction in the interfacial charge recombination between the TiO2 electrode and the electrolytes through the formation of an energy barrier on the TiO2 surface, thereby resulting in the enhancement of both the Voc and the Jsc [11–15]. Several reports have shown that the use of certain inorganic compounds, such as Al2O3, ZnO, and PbS, as a coating on the TiO2 surface led to the passivation of the surface states, which resulted in the retardation of the interfacial charge recombination, and hence an increase in both the Voc and the Jsc [16–18]. As another simple method to improve PCE, it was reported that addition of nanoparticles to the photoelectrode could induce an increase in the light harvesting and a reduction in the charge recombination [19]. Thus, the simple techniques such as the surface modification of the TiO2
https://doi.org/10.1016/j.jiec.2019.01.036 1226-086X/© 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
G.W. Baek et al. / Journal of Industrial and Engineering Chemistry 73 (2019) 260–267
photoelectrode and addition of nanoparticles to the photoelectrode appear to be an effective method for improving the PCE of DSSCs. The methods and materials associated with the surface modification that were discussed above have all been intensively studied, but little attention has previously been paid to the application of polymeric materials as surface modifiers of the photoelectrodes in DSSCs. Thus, we selected polyacrylonitrile (PAN) as a TiO2 surface modifier. The TiO2 surfaces were directly treated with a PAN solution via a simple dipping process, and the resultant electrodes were applied to the photoelectrodes of DSSCs. We expected the PAN to trigger the formation of a surface dipole, an energy barrier, or surface state passivation on the TiO2 surfaces, which was in turn expected to result in an improvement in the performance of the DSSCs. Therefore, DSSCs with PAN-modified photoelectrodes (PAN-TiO2/FTO) were fabricated, and the effects of the surface treatment on the photovoltaic performance of the cells were examined. Experimental details Materials Commercial fluorine-doped tin oxide (FTO; sheet resistance~7
V/square) glass (TCO22-7), TiO2 paste for the photoelectrode (Ti-
261
Measurements X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) were performed using a VG Multilab ESCA 2000 apparatus (ThermoVG Scientific), with Al Kα radiation (hv = 1486.6 eV) being used for the XPS and with He I radiation (hv = 21.22 eV) being used for the UPS. The C1 s photoelectron peak (binding energy at 284.6 eV) was used as an energy reference. The Fourier transform infrared (FT-IR) spectra were recorded using an FT/IR 4100 spectrometer (Jasco) equipped with an attenuated total reflectance (ATR) accessory (PRO450-S, Jasco). The photocurrent– voltage measurement was performed using a CompactStat potentiostat (Ivium Technologies B.V.) 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) using a silicon photodiode (PEC-SI01, Peccell Technologies, Inc.). The UV–vis absorption spectra were determined 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 photoluminescence of the dyes adsorbed on the TiO2 surface was measured using a spectrofluorometer (RF-5301PC) produced by the Shimadzu Corporation. The active areas of the
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 all purchased from Solaronix. The PAN and the TiCl4 were purchased from Pfaltz & Bauer, Inc. and Sigma-Aldrich Co. LLC, respectively. Platinum paste (PT-1, Dyesol-Timo) was chosen as the source for the Pt counter electrode. All the chemicals were used without further purification. Fabrication of the DSSCs The sheets of FTO glass were cleaned using a detergent solution by means of sonication for 20 min. They were then thoroughly rinsed with deionized water and ethanol. The cleaned sheets of FTO glass were immersed in a 40-mM TiCl4 solution at 70 C for 30 min, and they were thenwashed with water and ethanol. One active TiO2 layer, which was formed on the FTO glass, was prepared via doctor-blade coating using the TiO2 paste. Additionally, a TiO2 layer composed of particles approximately 400 nm in diameter was deposited on the active TiO2 layer, and it was then calcinated at 500 C for 60 min in order 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 a dimethylformamide solution (5 mM) of PAN for 0–50 min so as to deposit the polymer onto the TiO2 layers. Next, the resultant electrodes were rinsed with water and ethanol, and they were then dried at 65 C for 10 min in order to produce the modified photoelectrodes (PAN-TiO2/FTO). Finally, both the bare TiO2/FTO and the PAN-TiO2/FTO photoelectrodes were separately immersed into 0.5 mM of ethanolic N719 dye solution for 24 h to obtain working electrodes. To prepare the counter electrode, two holes were formed in the FTO glass using a drill, and the glass was then cleaned using the method described above. A Pt layer was formed on the FTO glass via the doctor-blade method using Pt paste, which was followed by a calcination process at 400 C for 30 min. The thermally treated platinum counter electrodes then were placed on the photoelectrodes and sealed using a 60-mm-thick sealing material. The electrolyte was introduced into the cells through one of the two small holes drilled into the counter electrodes in order to produce DSSCs with a 25 mm2 active area.
Fig. 1. XPS spectra for the N 1 s measured using PAN(40)-TiO2/FTO.
Fig. 2. ATR-FTIR spectra of the bare TiO2/FTO, PAN(40)-TiO2/FTO, and PAN(720)TiO2/FTO.
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Fig. 3. Variations in performance with soaking time in the PAN solution: (a) Jsc, (b) Voc, (c) FF, and (d) PCE of the DSSCs, as measured under AM 1.5 irradiation.
dye-adsorbed TiO2 films were estimated using a digital microscope camera (OLYMPUS SZ61) equipped with image analysis software. Results and discussion Preparation and characterization of the PAN-TiO2/FTO
Fig. 4. J–V characteristics of the DSSCs with bare TiO2/FTO and PAN(40)-TiO2/FTO electrodes.
PAN is a resin synthesized by means of the polymerization of acrylonitrile, and it contains the pendant cyano groups. The active cyano groups allow PAN to adsorb into the five-fold coordinated titanium (Ti5c) atoms located on the surfaces of TiO2 photoelectrodes [20–22]. It has previously been established that the oxygen atoms of bulk crystal TiO2 are three-fold coordinated, while the titanium atoms are six-fold coordinated. Further, the surface corner oxygen atoms are two-fold coordinated, while the titanium atoms are five-fold coordinated [23]. Thus, a strong interaction could occur between the five-fold coordinated Ti5c atoms of TiO2 and the strong electron-withdrawing cyano groups of PAN. Six different photoelectrodes were prepared by soaking the bare TiO2/ FTO in a dilute solution of PAN for 10–50 min, thereby producing PAN(10–50)-TiO2/FTO, where “(10)” indicates that the dipping time was 10 min. In order to confirm the incorporation of the PAN, an
Table 1 Photovoltaic properties of the DSSCs with bare TiO2/FTO and PAN(40)-TiO2/FTO electrodes. Applied electrodes
Jsc (mA/cm2)
Voc (mV)
FF (%)
h (%)
Loaded dye (mol/cm3)
Bare TiO2/FTO PAN(40)-TiO2/FTO
12.13 14.49
712 700
70.62 70.68
6.10 7.17
2.10 105 1.88 105
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XPS measurement was performed using the PAN(40)-TiO2/FTO electrode. In Fig. 1, the peak detected at 400.08 eV corresponds to the 1 s binding energy in nitrogen, which indicates that, via the technique of simple dipping in the solution, the PAN was adsorbed onto the TiO2 surface [24,25]. To further confirm the incorporation of the PAN, the ATR-FTIR spectra were recorded for the PANmodified TiO2 film. Fig. 2 shows the IR spectra of the bare TiO2, PAN (40)-TiO2, and PAN(720)-TiO2 films. In contrast to the bare TiO2, the absorption band seen at 2244 cm1, which could be assigned to the stretching vibrations of the CN bonds, appeared in the spectrum of the ESD(40)-TiO2 film [26,27]. When the dipping time was prolonged to 12 h (720 min), the spectrum of the PAN (720)-TiO2 showed a stronger absorption band at 2244 cm1 due to the CN groups. Based on the XPS and the FT-IR measurements, we can see that PAN can be easily adsorbed onto the surface of TiO2 by means of the soaking process. Further, it is considered that the PAN adsorbed onto the TiO2 surface will be maintained for a certain time during the operation of the cells, since it is not dissolved in acetonitrile, which is a solvent found in the electrolyte. Photovoltaic performance variations of the DSSCs with PAN-TiO2/FTO In order to investigate the effects of PAN on the cell performance, we fabricated DSSCs using the bare TiO2/FTO and the PAN-TiO2/FTO photoelectrodes, and their photovoltaic properties were characterized. Fig. 3 shows the variations in cell performance as a function of the soaking time, that is, the surface modification time. The Jsc values were improved for all the soaking times when compared to the Jsc of the reference cell, and the maximum values were recorded when the surface modification time was 40 min, as shown in Fig. 3(a). However, the Voc values were observed to decrease as the soaking time increased, as shown in Fig. 3(b). There were no meaningful changes in the FF values of the devices. Overall, the PCE values were increased following the incorporation of PAN onto the surface of the TiO2 electrodes, since the improvement in the Jsc overrode the reduction in the Voc. As the highest PCE value was observed when the TiO2/FTO electrode was treated with the PAN solution for 40 min, we focused on this device with PAN(40)-TiO2/FTO in order to investigate the origin of the efficiency enhancement. Fig. 4 shows the current density (J) and voltage (V) curves of the DSSCs with the bare TiO2/FTO and PAN(40)-TiO2/FTO electrodes, and the device performance is compared in Table 1. The DSSCs with the PAN(40)-TiO2/FTO electrode showed a PCE of 7.17%, while the DSSCs with the bare TiO2/FTO electrode showed a PCE of 6.10%. The enhanced PCE was mainly due to the increase in the Jsc.
263
where the molar extinction coefficient (e) for N719 in the basic aqueous solution is 1.25 104 M1 cm1 at 500 nm, while b is the width of the quartz cell (1 cm in this study) [30–32]. The averaged dye-loading amounts, as measured using ten cells, for the bare TiO2/FTO and the PAN(40)-TiO2/FTO were 2.10 105 and 1.88 105 mol/cm3, respectively. The amount of adsorbed dye molecules on the PAN(40)-TiO2/FTO decreased by approximately 10% when compared to the amount on the bare TiO2/FTO. This was probably due to the fact that PAN occupies a fraction of the adsorption sites on the TiO2 surface. As less adsorption of organic dyes on the photoelectrodes could generate fewer electrons and fewer holes, the PAN modification of the TiO2 surface could serve to reduce the LHE. In the case of DSSCs, unlike other solar cells, the electrontransporting TiO2 layer is in direct contact with the hole conductor (electrolyte), which induces the charge recombination (I3 + 2 e → 3I) between the photoinjected electrons and the ions in the electrolyte. This causes a reduction in the electron lifetime, and hence in the hcol. Thus, the value of the hcoll is largely related to the lifetime of the electrons injected into the valence band of the TiO2 from the excited dyes, that is, a prolonged electron lifetime could lead to an increase in the hcoll. EIS analysis has been widely used to investigate the kinetics and energetics of charge transport and interfacial charge recombination in DSSCs [33–36]. Fig. 5(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 frequencies (fmax) of 13.4 Hz and 22.6 Hz obtained from the EIS Bode phase plots of the DSSCs with the bare TiO2/FTO
Effects of the PAN adsorbed onto the TiO2 surface on the Jsc As shown in Table 1, the PAN adsorption led to an increase in the Jsc from 12.13 mA/cm2 for the bare TiO2/FTO to 14.49 mA/cm2 for the PAN(40)-TiO2/FTO. The Jsc value is generally influenced by four efficiency factors, as shown in Eqs. (1) and (2), namely the light harvesting (LHE), the electron injection (hinj), the dye regeneration (hreg), and the electron collection (hcoll) efficiencies, where e is the elementary charge and Fph,AM1.5G is the photon flux in AM 1.5 G, 100 mW/cm2 [28,29]. R (1) Jsc = IPCE(l)eFph,AM1.5G(l) dl IPCE(l) = LHE(l)hinj(l) hreghcoll(l)
(2)
The LHE is related to the light absorbance (A) of the adsorbed dyes, that is, LHE = 110A [28,29]. Thus, in order to investigate the effects of the LHE on the Jsc enhancement, the amount of adsorbed dye was first measured using the Beer–Lambert equation (A = ebc),
Fig. 5. EIS spectra of the DSSCs with bare TiO2/FTO and PAN(40)-TiO2/FTO: (a) Bode and (b) Nyquist plots measured at 0.7 V in the dark.
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and the PAN(40)-TiO2/FTO, respectively, the electron lifetime (t n) was estimated using the equation: t n = 1/2pfmax [34,35]. The electron lifetimes were calculated to be 11.9 ms and 7.0 ms for the DSSCs with the bare TiO2/FTO and the PAN(40)-TiO2/FTO electrodes, respectively. The reduced lifetime of the electrons injected from the dyes was observed for the device with the PAN(40)-TiO2/ FTO when compared to situation of the reference cell. Fig. 5(b) shows the Nyquist plots of the EIS spectra for the DSSCs measured at 0.7 V in the dark. The electrochemical parameters depicted in Fig. 5(b) are attributed to the serial resistance, which is determined by the sheet resistance of the FTO and the electrical contact between the 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) [35,36]. It was noted that the PAN modification decreased the impedance component in the R2. When considering that the R2 value refers to the resistance of the charge recombination, the smaller semicircle of R2 seen in Fig. 5(b) indicates that the interfacial charge recombination at the TiO2/dye/ electrolyte interface is stronger [34,35]. Thus, the reduced electron lifetime (Bode plots) and the promoted recombination (Nyquist plots) suggest that the hcol was decreased by the PAN modification. In order to further estimate the electron lifetimes in the DSSCs with the bare TiO2/FTO and the PAN(40)-TiO2/FTO, the OCVD characteristics of the devices were measured. The cells are maintained under constant illumination in an open-circuit condition until they exhibit a steady voltage value. At this point, the light is suddenly switched off, and the photovoltage is measured as a function of time. As some photogenerated electrons cannot be collected by the electrode in the open-circuit condition, they react with the 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, since excess electrons are removed through the charge recombination. More specifically, the recombination rate of photoelectrons is proportional to the rate of photovoltage decay [37–39]. To estimate the electron lifetime (t ) of each device, the corresponding curve for the t versus the voltage [Fig. 6(b)] can be obtained from the OCVD curve
Fig. 6. (a) Curves of the OCVD and (b) the electron lifetime versus the Voc for the DSSCs with bare TiO2/FTO and PAN(40)-TiO2/FTO.
Fig. 7. Schematic illustration of the boosted charge recombination by the electron-withdrawing cyano groups.
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265
[Fig. 6(a)] 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 [37–39]:
t¼
kB T dVoc 1 ð Þ e dt
ð3Þ
The electron lifetimes seen in the DSSC with the PAN(40)-TiO2/ FTO were shorter than those in the cell with the bare TiO2/FTO film, as shown in Fig. 6(b), which indicates that the PAN adsorbed onto the TiO2 surface boosted the interfacial charge recombination between the photoinjected electrons and the I3, thereby inducing a reduced electron lifetime. This means that the hcol was decreased by the PAN modification, which is consistent with the findings of the EIS measurement. We considered that the insulating polymeric material, namely PAN, would retard the charge recombination. However, the PAN incorporation rather caused rising the recombination. This could be due to the strong electron-withdrawing nature of the cyano groups in the PAN, as illustrated in Fig. 7. As mentioned above, the PAN could be adsorbed onto the TiO2 surface via interactions between the nitrogen atoms (of the cyano groups) with lone pairs and the five-fold coordinated titanium (Ti5c) atoms. During the operation of solar cells, the photoinjected electrons would be pulled by the adsorbed cyano groups, and they would in turn react with the I3 in the electrolyte. Similarly, Harima et al. reported that the cyano groups located close to the TiO2 surface served as good communicators of the photoexcited electrons from the dyes to the conduction band of the TiO2 photoelectrodes [21]. It is, therefore, believed that the reduced electron lifetime could be attributed to the strong electron-withdrawing cyano groups adsorbed onto the TiO2 surface, which boosted the interfacial charge recombination between the photoinjected electrons and the I3. Consequently, a surface dipole, rather than an energy barrier, would be formed by the adsorption of PAN onto the surface of the TiO2, as depicted in Fig. 7. In other words, the incorporation of PAN altered the distribution of the charge across the TiO2/electrolyte interface relative to the non-treated case, meaning that partial positive charges were formed on the TiO2 surface (Ti5c), which served to increase the interfacial recombination [6–10]. In order to understand the effects of the incorporation of PAN on the hinj (efficiency of the electron transfer from the excited dye to the conduction band of the TiO2), the PL spectra of the bare TiO2/ FTO and the PAN(40)-TiO2/FTO electrodes were obtained, as compared in Fig. 8. The amounts of the adsorbed dyes were adjusted to be identical in the two electrodes (the inset in Fig. 8). When excited at their absorption maxima (526 nm), they both showed a PL peak at around 791 nm. It can be clearly observed that the fluorescence from the PAN(40)-TiO2/FTO was quenched, leading to an approximate reduction in the PL intensity of 72%, when compared to the bare TiO2/FTO. This provides evidence of the more efficient charge transfer of the photoexcited electrons from the dye to the TiO2, thereby indicating that the hinj was considerably improved by the PAN incorporation [40,41]. It is also necessary to reveal what actually caused the significant enhancement of the hinj following the PAN modification. To this end, we obtained the UPS valence band and the UV–vis absorption spectra in order to establish the electronic band structures of the bare TiO2/FTO and the PAN(40)-TiO2/FTO. The position of the valence band edge (VBE) of the PAN(40)-TiO2/FTO could be seen at approximately 4.0 eVNHE (NHE = normal hydrogen electrode), which was the same as the position seen in the case of the bare TiO2/FTO, as shown in Fig. 9(a). The energies of the VBEs in the bare TiO2/FTO and the PAN(40)-TiO2/FTO were estimated to be 8.50 eVAVS, since the relationship between the EAVS (AVS = absolute vacuum scale) and the ENHE can be given as EAVS=ENHE-Eo, where Eo (about 4.50 eV) is the energy of the free electrons in the hydrogen scale [42–44]. The absorption edge of the
Fig. 8. PL spectra for the N719 dye adsorbed on the TiO2 surface.
Fig. 9. (a) UPS valence band and (b) UV–vis absorbance spectra for the bare TiO2/ FTO and PAN(40)-TiO2/FTO.
PAN(40)-TiO2/FTO was observed at 361.4 nm, which could be assigned to a band gap energy of 3.43 eV [Fig. 9(b)]. The bare TiO2/ FTO exhibited an absorption edge of 358.2 nm, corresponding to a band gap energy of 3.46 eV. Based on the identified band gap energies, the energies of the CBEs of the bare TiO2/FTO and the PAN (40)-TiO2/FTO were determined to be 5.04 and 5.07 eVAVS, respectively. As a result, the energy of the CBE in the PAN(40)-TiO2/ FTO was shifted approximately 31 meV in a positive direction when compared to that of the bare TiO2/FTO, as illustrated in Fig. 10. This
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Fig. 10. Schematic energy band diagram of the (a) bare TiO2/FTO and (b) PAN(40)-TiO2/FTO, showing the positive shift in the conduction band edge.
positive shift (away from the vacuum level) of the CBE in the PAN (40)-TiO2/FTO could be attributed to the formation of surface dipoles on the TiO2 surface, and it is more favorable in terms of the injection of photoexcited electrons from the dye to the TiO2 due to the larger potential difference (DEbare < DEPAN) between the LUMO level of the dye and the CBE of the TiO2 photoelectrode, which results in an improvement in the hinj. Similar results were reported in relation to the incorporation of cations on the TiO2 surface inducing an enhancement of the hinj due to the positive shift in the CBE of the TiO2 [45]. The dark currents of DSSCs can be used to measure a conduction band shift or to determine the existence of an energy barrier on the electrode surface [45,46]. Fig. 11 presents the dark current-voltage characteristics of the DSSCs with the bare TiO2/FTO and the PAN(40)TiO2/FTO. The onset potential of the dark current for the bare TiO2/ FTO electrode was measured to be approximately 0.654 V, whereas the onset potential for the PAN(40)-TiO2/FTO electrode was shifted to approximately 0.624 V. Following the PAN incorporation, a lower onset potential was measured in the PAN(40)-TiO2/FTO, which meant that the potential difference between the work function of the FTO and the CBE of the TiO2 in the PAN(40)-TiO2/FTO was smaller than that of the bare TiO2/FTO. Further, the PAN modification induced a shift in the onset potential, and the curves maintained the same shape. This similarity in terms of shape implies that the modification does not form an energy barrier that prevents electron leakage to the electrolyte, but instead forms a surface dipole [47]. The surface dipole formed on the TiO2 surface shifts the conduction band potential in a positive direction, which is consistent with the result of the electronic band structure study. The lower onset potential was, therefore, attributed to the positive shift in the CBE of the TiO2. Based on the measurements concerning the PL quenching, the UPS valence band, the UV–vis absorbance, and the dark currents, it could be concluded that the hinj was significantly improved by the incorporation of PAN, which was attributed to the positive shift in the CBE of the TiO2 that stemmed from the formation of the surface dipole. The dye regeneration process can be expressed as follows: D+ + 2I → D + I2
(4)
2I2 → I + I3
(5)
Therefore, the hreg can be improved when the concentration of the I near the oxidized dye (D+) rises. It has been reported that the adsorption of cations onto the TiO2 induced the formation of (I, I)
ion pairs on the TiO2 surface by means of the electrostatic interactions between the positive charge on the TiO2 and the negative I. The ion pairs led to a faster reaction between the D+ and the 2I [chemical Eq. (4); rate-determining step; electron transfer from the I to the oxidized dye], thereby resulting in an improvement in the hreg [48,49]. However, in our study, the effect of the partial positive charges on the Ti5c is not clear, because partial negative charges formed in the electron-withdrawing cyano groups also present near the surface of the TiO2. In this respect, further studies aimed at elucidating the effects of PAN incorporation on the hreg are currently in progress. Overall, the Jsc value of the DSSC with the PAN(40)-TiO2/FTO was enhanced when compared to that of the reference cell, since the considerable improvement in the hinj outweighed the reduction in both the LHE and the hcol. Effects of the PAN adsorbed on the TiO2 surface on the Voc The Voc value (700 mV) of the DSSC with the PAN(40)-TiO2/FTO decreased relative to that of the reference device (712 mV). This decrease in the Voc can be explained by the positive shift in the CBE of the TiO2 attributed to the formation of a surface dipole. The positive shift induces a narrower potential gap (DVbare DVPAN) between the
Fig. 11. Dark current-voltage characteristics of the DSSCs with bare TiO2/FTO and PAN(40)-TiO2/FTO electrodes.
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CBE and the redox potential of the electrolyte, as shown in Fig.10. As the potential difference between the CBE and the redox potential of the electrolyte is proportional to the Voc value [50], it is considered that the decrease in the Voc value of the device with the PAN(40)TiO2/FTO stems from the positive shift in the CBE of the TiO2 electrode, which results from the formation of the surface dipole. Further, the decrease in the Voc could be elucidated by the boosted charge recombination of the cell with the PAN(40)-TiO2/FTO. The Voc value is given by Eq. (6) [11,12], where k and T are the Boltzmann constant and the absolute temperature, respectively; Iinj is the flux of the charge resulting from the sensitized injection; ncb is the concentration of electrons on the TiO2 surface; and ket and [I3] are the rate constants for the interfacial charge recombination (I3 + 2e → 3I) and the concentration of I3, respectively. According to Eq. (6), the Voc and the ket are inversely correlated, and it is possible to indirectly estimate the ket by measuring the electron lifetime. A shorter electron lifetime implies a higher ket value, that is, a higher possibility that a reduction in the I3 will occur. As described above, the EIS analysis and the OCVD measurements revealed that the interfacial charge recombination was boosted by the incorporation of PAN. This fact indicates that the ket value was increased, thereby inducing a decrease of the Voc based on Eq. (6). Voc ¼
Iinj kT lnð Þ e ncb ket ½I 3
ð6Þ
Conclusions We have investigated the effects of PAN, as a surface modifier, on the performance of DSSCs. PAN-modified TiO2 films were applied to the photoelectrodes of DSSCs. The reference device, which featured no modifications, showed a photovoltaic performance of 12.13 mA/ cm2 for the Jsc, 712 mV for the Voc, and 70.62% for the FF, which led to a PCE of 6.10%, while in the case of the device with the PAN(40)-TiO2/ FTO electrode, which was treated with PAN for 40 min, the PCE was increased to 7.17% (Jsc = 14.49 mA/cm2, Voc = 700 mV, and FF = 70.68%). By means of monitoring the changes in the PL quenching, the UPS valence band, the UV–vis absorbance, and the dark currents, it was confirmed that the electron injection was significantly improved by the incorporation of PAN, which accounted for the enhancement seen in the PCE of the DSSC with the PAN(40)-TiO2/ FTO. Thus, PAN appears to be a promising material for enhancing the conversion efficiency of DSSCs. Acknowledgements This work was supported by research grants from the Daegu Catholic University in 2017. References [1] Z. Abdin, M.A. Alim, R. Saidur, M.R. Islam, W. Rashmi, S. Mekhilef, A. Wadi, Sust. Energ. Rev. 26 (2013) 837. [2] B. O’Regan, M. Grätzel, Nature 353 (1991) 737.
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