Photocurrent Generation Properties of Half-Photocell Consisting of Electropolymerized Hierarchical Polythiophene Thin Films as Photoexciting Electron Donor and C60 as Electron Acceptor

Polybithiophene/poly(3,4-ethylenedioxythiophene)/indium-tin-oxide coated transparent glass electrodes were prepared by sequential electrochemical polymerization with cyclic voltammetry. The thickness of polybithiophene films was varied by the scan rate of applied potentials during polymerization. Obtained hierarchical polythiophene films were characterized by absorption spectra, fluorescence spectra, scanning electron microscopy, laser microscopy, and atomic force microscopy. Half-photocell properties of the hierarchical polythiophene thin films were evaluated by a three-electrode electrochemical cell in CH2Cl2 solution of electrolyte and C60 as a sacrificial electron acceptor under irradiation of monochromatic light. The thickness and absorbance of hierarchical polythiophene thin films increased by decreasing of scan rate of applied potentials during the polymerization of polybithiophene. While, maximum peaks of fluorescence intensity and internal quantum yield of cathodic photocurrent were observed at 20 nm thickness of polythiophene film under irradiation from the glass side of the transparent electrode. Almost maximum external quantum efficiencies of cathodic photocurrent were observed from 20 nm to 1.5 μm thicknesses of polythiophene films. These tendencies suggest that CH2Cl2 solution of C60 and electrolyte penetrate into the polybithiophene layer up to 1.5 μm.

Polybithiophene/poly(3,4-ethylenedioxythiophene)/indium-tin-oxide coated transparent glass electrodes were prepared by sequential electrochemical polymerization with cyclic voltammetry. The thickness of polybithiophene films was varied by the scan rate of applied potentials during polymerization. Obtained hierarchical polythiophene films were characterized by absorption spectra, fluorescence spectra, scanning electron microscopy, laser microscopy, and atomic force microscopy. Half-photocell properties of the hierarchical polythiophene thin films were evaluated by a three-electrode electrochemical cell in CH 2 Cl 2 solution of electrolyte and C 60 as a sacrificial electron acceptor under irradiation of monochromatic light. The thickness and absorbance of hierarchical polythiophene thin films increased by decreasing of scan rate of applied potentials during the polymerization of polybithiophene. While, maximum peaks of fluorescence intensity and internal quantum yield of cathodic photocurrent were observed at 20 nm thickness of polythiophene film under irradiation from the glass side of the transparent electrode. Almost maximum external quantum efficiencies of cathodic photocurrent were observed from 20 nm to 1.5 μm thicknesses of polythiophene films. These tendencies suggest that CH 2 Cl 2 solution of C 60 and electrolyte penetrate into the polybithiophene layer up to 1.5 μm. Supplementary material for this article is available online Conductive polymer thin films are important materials for organic electronic devices, such as organic transistors, organic thin-film solar cells, organic electronic luminescence, and so on. [1][2][3][4] Electrochemical polymerization is a suitable method to obtain insoluble conductive polymer films on an electrode with easy control of the thickness of the films. 5,6 This method has been applied to develop many types of conductive polymer films, such as polypyrrole, 6-8 polyaniline, [9][10][11][12] polythiophene, [13][14][15][16] and their functionalized analogs. 17-25 Some of the above researches include that the application of electrochemical polymerized conductive polymer films to photoelectric conversion. 11,12,[15][16][17][18][19][20][26][27][28] Moreover, sequential electrochemical polymerization is good for the preparation of hierarchical layered thin films consisting of different kinds of conductive polymers. 14,[26][27][28] Our previous work reported the fabrication and half-photocell application of double-layered conductive polymer thin-films consisting of polybithiophene (polyBiTh) and poly(3,4-ethylenedioxythiophene) (polyEDOT) by sequential electrochemical polymerization on an indium-tin-oxide (ITO) coated transparent glass electrode. 27,28 Under irradiation from glass side of ITO transparent electrode, we observed decreasing photoelectric conversion efficiencies with increasing thicknesses of the polyBiTh film from 20 nm--1 μm using methylviologen as a sacrificial electron acceptor in an aerobic aqueous electrolyte solution. 28 This photoelectric conversion tendency was mainly due to the absorption loss of the polyBiTh layer, which was not in contact with the aqueous electrolyte solution and a sacrificial electron acceptor. In other words, electron transfer from the polyBiTh moiety to methylviologen occurred only at the surface or near the surface of polyBiTh film, which is explained that aqueous electrolyte solution is difficult to penetrate into hydrophobic polyBiTh film. From the results of these previous studies, we planned to expand the thickness variation of the polyBiTh layers in the polyBiTh/polyEDOT/ITO double-layered polythiophene thinfilms modified electrode and use a hydrophobic electrolyte solution for half-photocell evaluation. This would aid in revealing the detailed photoelectric conversion mechanisms of these systems.
In this study, we report the fabrication and evaluation of a halfphotocell using polyBiTh/polyEDOT/ITO as the photocathode with 5 nm-6.5 μm thickness of polyBiTh. Photoelectric conversion performances of the half-photocells were evaluated in the presence of C 60 as an electron acceptor 29,30 in an aerobic CH 2 Cl 2 -based electrolyte solution.

3,4-Ethylenedioxythiophene
(EDOT) (Tokyo Chemical Industry), 2,2'-bithiophene (BiTh) (Tokyo Chemical Industry), sodium dodecylbenzenesulfonate (DBS, hard type, mixture) (Tokyo Chemical Industry), C 60 fullerene (Frontier Carbon Corporation, nanom purple ST), and other chemicals were used as received. Ultrapure water was used as the aqueous solvent. The fabrication processes of the double-layered polythiophene thin-films consisting of polyEDOT and polyBiTh are summarized in Fig. 1, which are similar to our previous report except for the polymerization condition of BiTh. 28 At first, a polyEDOT film was deposited on an ITO-coated transparent glass (2 × 2 cm 2 ) electrode from an aqueous solution of EDOT (0.05 mol l −1 ) and DBS (0.01 mol l −1 ) by electrochemical polymerization using a potentiostat (ALS, Model 650C) in a twoelectrode (ITO and nickel plate) electrochemical cell under stirring condition at 400-600 rpm. The voltage was scanned between 0 and +2 V (from the ITO electrode to the nickel plate electrode) at a rate of 0.05 V s −1 for one cycle (initial voltage 0 V, switching voltage +2 V, and final voltage 0 V). After polymerization, the ITO electrode was removed from the solution, rinsed with ultrapure water, dried with a stream of nitrogen gas, and annealed at 100°C for 10 min to give a polyEDOT film on the ITO electrode (polyEDOT/ITO).
Then, various thickness polyBiTh films were formed on the polyEDOT/ITO by electrochemical polymerization in a threeelectrode (working: polyEDOT/ITO, reference: Ag wire, and counter: Pt wire) electrochemical cell containing BiTh (1.0 × z E-mail: akiyama.t@mat.usp.ac.jp 10 −3 mol l −1 ) and n-Bu 4 NPF 6 (0.1 mol l −1 ) in CH 2 Cl 2 with various scan rate of applied potential under stirring condition at 400--600 rpm. The applied potential was scanned between 0 and +2 V vs Ag wire at a scan rate of 0.002-1.0 V s −1 for 1 cycle (initial potential 0 V, switching potential +2 V, and final potential 0 V). After polymerization, the working electrode was removed from the solution, rinsed with CH 2 Cl 2 , dried with a stream of nitrogen gas, and annealed at 150°C for 10 min to obtain hierarchically stacked films of polyBiTh/polyEDOT on ITO, denoted as polyBiTh(s)/ polyEDOT/ITO (s = 0.002-1.0 V s −1 , where s is the scan rate of the applied potential for electrochemical polymerization of BiTh).
Before measuring the thickness of the obtained films, the polythiophene layers were partially peeled off to form a clear step between the films and the ITO electrode. Then, the thickness of films was obtained from height difference between the surface of the polymerized films and the surface of ITO electrode.
In fluorescence measurement, excitation light irradiated the back ITO-glass side of the polyBiTh(s)/polyEDOT/ITO structure at an incident angle of 45°, and fluorescence emission was measured at an exit angle of 45°. In the case of thickness measurement of the polythiophene thin-films, laser microscopy and atomic force microscopy were used in the case of s = 0.002-0.02 (V s −1 ), and in the case of s = 0.05-1.0 (V s −1 ), respectively. Photocurrent measurements were conducted using a three-electrode photoelectrochemical cell containing 0.1 mol l −1 nBu 4 NPF 6 and 1.0 × 10 −4 mol l −1 C 60 in a CH 2 Cl 2 solution under aerobic conditions at room temperature. A polyBiTh(s)/polyEDOT/ITO electrode, platinum wire, and Ag wire were used as the working, counter, and reference electrodes, respectively. Monochromatic light irradiated the polyBiTh(s)/polyEDOT/ITO electrode (area: 0.28 cm 2 ) from the back ITO-glass side. The photocurrent action spectra were measured by scanning the excitation wavelength, while maintaining the potential at 0 V. The dependence of photocurrent on the applied potential was measured under irradiation with monochromatic light (λ = 460 nm). Light irradiated the back ITO glass side of all the polythiophene electrodes. The photocurrent was recorded using a Huso HECS-318C potentiostat. The external quantum efficiency (EQE) and internal quantum efficiency (IQE) of the photocurrent was evaluated from the irradiation wavelength λ (nm), photocurrent density I (A cm −2 ), irradiation photo power Φ (W cm −2 ), and absorbance(ABS) using the following equation.   The absorption spectra of polyBiTh(s)/polyEDOT/ITO (s = 0.002-1.0 V s −1 ) are shown in Fig. 3. All absorption spectra can be explained that the sum of a broad absorption band in the visible region (400-620 nm) mainly due to polyBiTh and a near-infrared band indicating the polaron absorption mainly due to polyEDOT. The absorbance in the visible region increased with decreasing scan rate s (V s −1 ) during electrochemical polymerization. This tendency indicates that the deposited amount of the polyBiTh increased with decreasing scan rate s (V s −1 ), which corresponds to the image of polyBiTh(s)/polyEDOT/ITO (s = 0.002-1.0 V s −1 ), as shown in Fig. 4. An obvious increase in the baseline is seen in the case of s = 0.002-0.02 (V s −1 ) region, which suggests the formation of a larger structure than μm order.
The thickness of the polyBiTh layer for polyBiTh(s)/polyEDOT/ ITO obtained by laser microscopy (s = 0.002-0.05 V s −1 ) and atomic force microscopy (s = 0.1-1.0 V s −1 , Fig. S1) is shown in Fig. 5. The thickness of the polyBiTh layer decreased with an increasing scan rate of the applied potential during the polyBiTh formation process. This trend can be attributed to the difference in the amount of charge used during electrochemical polymerization (i.e., the difference in practical electrochemical polymerization time).
The SEM images of polyBiTh(s)/polyEDOT/ITO (s = 0.002-1.0 V s −1 ) are shown in Fig. 6. In the case of s = 0.002-0.02 V s −1 , rod-shaped structures with sizes between a few hundred nanometers and a few micrometers are observed. These sized structures seem to scatter light, which can explain the relatively high baseline in the absorption spectra (Fig. 3) of polyBiTh(s)/polyEDOT/ITO (s = 0.002-0.02 V s −1 ). While, in the case of s = 0.05-1.0 V s −1 , structures with diameters ranging from about 100 nm to smaller are observed on the flat electrode surface at low densities. These morphological changes can be explained as follows: 1) relatively flat polyBiTh films are formed on polyEDOT/ ITO at the early stage of electrochemical polymerization, 2) the concentration of the electric field occurs at the micro-rough part of the polyBiTh film, and 3) electrochemical polymerization is accelerated on the micro-rough parts, growing rods of polyBiTh. 28 The fluorescence emission spectra of polyBiTh(s)/polyEDOT/ ITO (s = 0.002-1.0 V s −1 ) are shown in Fig. 7a. In all spectra, the characteristic emission peaks at approximately 620 and 670 nm are attributed to polythiophene films. 31,32 These emissions appear to   originate from the excited state of the polyBiTh layer. Figure 7b shows the emission intensity at 670 nm, the lowest excited state fluorescence of polyBiTh. Maximum emission intensity is observed at s = 0.05 V s −1 . The emission intensities decreased with decreasing scan rate in the region of s = 0.002-0.05 V s −1 . The decrease in fluorescence emission could be due to the light diffusion and multiple reflections of light in the increasingly rough parts of the polyBiTh layer owing to the rod-shaped structures as s decreases. While, in the s = 0.05-1.0 V s −1 region, emission intensities decrease with increasing scan rate s, which is due to simply decreasing amount of polyBiTh with increasing s and the corresponding thickness of the polyBiTh layer (Fig. 5).
Cathodic photocurrents were observed in polyBiTh(s)/ polyEDOT/ITO (s = 0.002-1.0 V s −1 ) under light irradiation from the back ITO-glass side. Figure 8a shows the external quantum efficiency (EQE) of the cathodic photocurrents of all polyBiTh(s)/ polyEDOT/ITO at 0 V vs Ag wire. In the case of polyBiTh(s)/ polyEDOT/ITO (s = 0.05 and 0.1 V s −1 ), a broad EQE peak is observed at 440-460 nm, which correlates with the absorption spectra of the polyBiTh layer of polyBiTh(s)/polyEDOT/ITO (s = 0.05 and 0.1 V s −1 ) (Fig. 3). This indicated that cathodic photocurrents from polyBiTh(s)/polyEDOT/ITO (s = 0.05 and 0.1 V s −1 ) are generated via the photoexcited state of polyBiTh. The cathodic photocurrents of polyBiTh(s)/polyEDOT/ITO (s = 0.2, 0.5, and 1.0 V s −1 ) are small during the present samples, which Is due to a small amount of polyBiTh was deposited on the polyEDOT layer. In the cases of polyBiTh(s)/polyEDOT/ITO (s = 0.01 and 0.02 V s −1 ), their EQE peaks are broadened and limited at 0.9%-1.0%, which is the highest EQE value in these samples. While, there is a decrease in the EQE values and changes in the EQE profiles in the cases of polyBiTh(s)/polyEDOT/ITO (s = 0.002 and 0.005 V s −1 ) compared with s = 0.01-0.1 V s −1 . This decreasing of photocurrent may be due to the absorption with those of irradiated light by areas of the polyBiTh moiety, which is much far from the electrolyte solution containing C 60 as a sacrificial electron acceptor and residual oxygen. Alternatively, it may be produced the deep trapping of the carriers by  rough morphology of thicker polyBiTh films. This is another possibility to decrease photocurrent at around over 1.5 μm. Figure 9 shows the applied potential dependence of the EQE values of all polyBiTh(s)/polyEDOT/ITO films under 460 nm light irradiation. In each polyBiTh(s)/polyEDOT/ITO electrode, except in the case of s = 0.5 and 1.0 V s −1 , it is observed that large cathodic photocurrents are generated at negatively applied potentials. Negligibly small cathodic photocurrents are observed in polyBiTh (s)/polyEDOT/ITO (s = 0.5, 1.0 V s −1 ), which also suggests that a minimal amount of polyBiTh was deposited on the polyEDOT layer in all samples. The above-mentioned photocurrent generation properties suggest the photocurrent generation diagram of the present system is a combination of sequential electron transfer reactions, as shown in Fig. 10.
EQE values at 460 nm irradiation on and 0 and −0.5 V applied potentials for the thickness of polyBiTh are plotted in Fig. 8b. In the case of 0 V applied potential, EQE values increased to 0.9%, respectively, with increasing thickness of polyBiTh from 5 to 20 nm. Then, EQE values increased gently to 0.95%, with increasing polyBiTh thickness from 20 nm to 1.5 μm. As further increasing of the polyBiTh thickness from 1.5 to 6.5 μm, EQE and IQE values decreased to almost 0.1%. This behavior indicates that the hydrophobic electrolyte solution containing C 60 as a sacrificial electron acceptor penetrates from bulk to 1.5 μm thickness into the polyBiTh layer, which allows sufficient sequential electron transfer shown in Fig. 10. While, in the case of a thicker polyBiTh layer than 1.5 μm, it seems that electron transfer from the ITO electrode to polyBiTh moiety contributing photocurrent generation is suppressed. In the case of −0.5 V applied potential, the tendency of EQE values change for the thickness of polyBiTh is similar to the case of 0 V applied potential, which also suggests penetration of the present electrolyte solution into polyBiTh over even μm. These tendencies of EQE values for the thickness of polyBiTh are different from to use of aqueous electrolyte solution with methylviologen as a sacrificial electron acceptor, in our previous report (Fig. S2). In the previous report, EQE values increased with decreasing of the polyBiTh layer, which may be mainly due to occurring photoinduced electron transfer from polyBiTh to sacrificial electron acceptor only at the outermost moiety of polyBiTh to bulk aqueous electrolyte solution. In this research, the combined use of hydrophobic CH 2 Cl 2 solvent and nonpolar C 60 as a sacrificial electron acceptor allows penetration of the electrolyte solution with electron acceptor into polyBiTh, which contributes to efficient use of polyBiTh layer for photocurrent generation.

Conclusions
Hierarchical structured polythiophene films modified electrode polybithiophene/poly(3,4-ethylenedioxythiophene)/indium-tin-oxide were prepared by sequential electrochemical polymerization. The thickness of the polybithiophene layer was varied from 5 nm to 6.5 μm. By using these modified electrodes as part of a halfphotocell, it was suggested that penetration of electrolyte solution into the polybithiophene layer occurred over even μm to contribute photocurrent generation.