Solar Cells Constructed with Polythiophene Thin Films Grown along

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Solar Cells Constructed with Polythiophene Thin Films Grown Along Tethered Thiophene-Dye Conjugates via Photoelectrochemical Polymerization Wenyuan Yan, Dianlu Jiang, Qinghua Liu, Qing Kang, and Feimeng Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 26 Apr 2019 Downloaded from http://pubs.acs.org on April 26, 2019

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ACS Applied Materials & Interfaces

Solar Cells Constructed with Polythiophene Thin

Films

Grown

Thiophene-Dye

Along

Tethered

Conjugates

via

Photoelectrochemical Polymerization Wenyuan Yan,† Dianlu Jiang,*,‡ Qinghua Liu,† Qing Kang,*,§ and Feimeng Zhou*,‡

†College

of Chemistry and Chemical Engineering, Central South University,

Changsha, Hunan, P. R. China, 410083 §Institute

of Surface Analysis and Chemical Biology, University of Jinan, Jinan,

Shandong, P. R. China 250022 ‡Department

of Chemistry and Biochemistry, California State University, Los

Angeles, Los Angeles, California 90032

1

ACS Paragon Plus Environment

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

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ABSTRACT: A polythiophene-based solar cell (PTSC) is constructed by photoelectrochemically polymerizing thiophene onto a ultrathin compact TiO2 layer (150

nm

thick)

covered

with

a

sub-monolayer

of

tethered

3-{5-[N,N-bis(4-diphenylamino)phenyl]thieno[3,2-b]thiophen-2-yl}-2-cyano-acrylic acid dye (ca. 10% coverage). The influence of morphology and thickness of the PT film on the photocurrent generated by the PTSC was investigated. With a 270 nm-thick

PT

film

and

2,2,7,7-tetrakis(N,N-di(4-methoxyphenyl)amino)-9,9-

spirobifluorene serving as the hole-transport material, the PTSC exhibited a short-circuit current density Jsc of 12.90 ± 0.63 mA/cm2, an open-circuit voltage Voc of 0.81 ± 0.01 V, and a fill factor FF of 0.72 ± 0.01. The high conversion efficiency (7.52 ± 0.58%) of the PTSC is attributed to the controlled PT growth along the ordered and spatially accessible dye molecules at the compact TiO2 layer, which facilitates charge transfer, prevents the hole/electron recombination, and simplifies the polymer solar cell construction with a stable and easily processible material.

KEYWORDS: dye-initiated photoelectrochemical polymerization, polythiophene, photoactive film thickness, charge transport, polythiophene solar cell

2


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1. INTRODUCTION Solar cells based on semiconducting conjugated polymers have several distinctive features such as low cost, light weight, and good processibility.1-8 Polythiophene (PT) is attractive among the conjugated polymers, due to its high stability of (un)doped states, easy structural modification, tailorable electrochemical properties, and strong absorption in the visible spectrum.9-12 Recently, thin PT films have been used as conducting layers for charge transport, photoactive layers, hole-transfer mediators, and solid electrolytes.13-22 Polythiophene-based solar cells (PTSCs) are considered as a type of promising photovoltaic device. PTSCs can be constructed by simple dip-coating or doctor-blading methods.15, 23-25

For example, Senadeera et al. dip-coated a TiO2/FTO substrate in

poly(thiophenemalonic acid), with the resultant polymer coating serving as a light absorbing material to yield a conversion efficiency of around 1.8%.23 Salatelli et al. obtained a conversion efficiency of 3.5% by doctor-blading a polythiophene/fullerene blend onto an indium tin oxide (ITO) glass slide pre-coated with a hole-transporting hybrid, poly(3,4-ethylenedioxythiophene):polystyrenesulfonic acid (PEDOT:PSS).24 Another widely used method is spin coating,16, 26-28 with which Kim et al. deposited poly(3-hexylthiophene) (P3HT) nanowires and nanocrystals on a PEDOT:PSS/ITO substrate, and obtained a conversion efficiency of 5.5%.16 Although these methods are simple, the connection between the photoactive and hole/electron transport layers is not tight and uniform. A thick, loose and disordered polymer layer can impede charge transport and cause hole/electron recombination. To mitigate the abovementioned 3



problems, electrochemical and photoelectrochemical polymerization methods have been used.17, 29-33 By maintaining 1.2 V vs. Ag/AgCl for 60 s to deposit a PT layer onto TiO2 nanotube arrays, Lan et al. obtained a conversion efficiency of 1.46%.31 Under visible light and using 0.2 V as the deposition potential, Lim et al. grew PT around

cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)rutheniuM(II)

(N3)-covered TiO2 nanoparticles to fill the crevices in between. The use of such a thick N3/PT composite layer (12 µm) as light absorbing materials produced a conversion efficiency of 6.44%.33 However, the orientation of the PT backbone in these cells is disordered due to fast PT deposition and the uneven and thick layer of TiO2 nanoparticles, which resulted in a relatively low fill factor and high charge-transfer resistance.33-34 The light-to-energy conversion of PTSCs consists of four steps:10 (1) electron transition from the ground state to the excited state of the polymer layer upon absorption of light; (2) electron diffusion from the excited state to the interface of polymer/electron- or hole-transport layer; (3) hole/electron separation at the interface; and (4) hole and electron movements to and collection by the opposite electrodes. In step (2), efficient charge transport is needed to reduce the electron/hole recombination. If the interior of the polymer layer is not ordered, the diffusion length and charge lifetimes will be reduced. Tremel et al. reported that the electron-transfer rate along the polymer backbone is 10 fold greater than that perpendicular to the backbone.35 Recently Chen and co-workers demonstrated that the edge-on orientation of P3HT with respect to a underlying perovskite layer leads to a high hole-extracting 4


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property.18 This observation was further confirmed by Zhang et al.36 We have shown that one-dimensional electron transfer could also be achieved with a TiO2 nanowire array.37 Therefore, the PT backbone is an excellent one-dimensional nanostructure for electron transport, and can delocalize charges while retarding the electron/hole recombination.

Figure 1. (A) A PT film deposited on a sub-monolayer of dye molecules (denoted by the orange sticks) tethered onto a thin compact TiO2 layer and (B) an enlarged view of the tethered dye and the growth of PT initiated from the dye. The structure of the dye used in this work is shown on the right.

We wish to report a new design to form a layer of highly aligned PT molecules. A PTSC comprising a 270 nm-thick PT film produced a conversion efficiency of 7.52 ± 0.58%, which is better than the values from the studies cited above.16,23,24,31,33 The conductive

PT

film

was

grown

at

a

sub-monolayer

of

3-{5-[N,N-bis(4-diphenylamino)pheny]thieno[3,2-b]thiophen-2-yl}-2-cyano-acrylic acid (C207) dye molecules38 tethered onto a ultrathin compact TiO2 layer (150 nm thick) via photoelectrochemical polymerization of thiophene (Figure 1A). The 2,2,7,7-tetrakis(N,N-di(4-methoxyphenyl)amino)-9,9-spirobifluorene (Spiro-OMeTAD) was coated onto the PT film as the hole-transport layer, followed 5



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by spraying a layer of gold onto spiro-OMeTAD for electrical contact. As depicted in Figure 1B, each C207 dye molecule is tethered onto the compact TiO2 layer via ester formation between its carboxyl group and the hydroxyl group at TiO2 surface.39 From the absorbance value of the dye molecules stripped from the dye/TiO2/FTO layer (red curve in Figure S1A), the dye surface coverage was deduced to be 7.810–11 mol/cm2, which corresponds to about 10% of the density of the hexagonal close-packed monolayer, 7.910–10 mol/cm2.40-42 Formation of such a sub-monolayer of tethered dye molecules is key to the subsequent PT film grown, as space is provided for thiophene monomers to ππ stack onto the dye aromatic moieties.

2. EXPERIMENTAL SECTION Materials

and

Reagents.

Ethanolamine

(redistilled,

99.5+%),

Co(III)-bis(trifluoromethane)sulfonimide or CoTFSI salt, and 4-tert-butylpyridine were purchased from SigmaAldrich (Milwaukee, WI) and used as received. Spiro-OMeTAD was obtained from SES Research Inc. (Houston, TX). Titanium(IV) n-butoxide (99%), lithium bis(trifluoromethane)sulfonimide (LiTFSI, 99%), lithium perchlorate (LiClO4, anhydrous, 99%), polyethylene glycol, thiophene (99.5%), chlorobenzene (99.8%, extra dry), propylene carbonate (99.5%), isopropanol (99.8%, extra dry), tetramethylammonium tetrafluoroborate (TMABF4, >98%), and all other reagents were purchased from Fisher Scientific (Tustin, CA). The FTO substrates (F:SnO2, Tec 15, 10 Ω/square, 1.8  1.5 cm) were acquired from Hartford Glass Company (Hartford, IN). Preparation of Dye Adsorbed TiO2 Layer. The synthetic route of dye, 6


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3-{5-[N,N-bis(4-diphenylamino)pheny]thieno[3,2-b]thiophen-2-yl}-2-cyano-acrylic acid, is shown in Figure S2A, and the NMR results of the purified dye are in agreement with reported values:38 1H-NMR (DMSO-d6, 400 MHz):  6.96 (d, 2H), 6.98 (d, 2H), 7.09 (t, 4H), 7.34 (t, 4H), 7.62 (d, 2H), 7.90 (s, 1H), 8.28 (s, 1H), 8.54 (s, 1H), 13.69 (s, 1H) (Figure S2B). The FTO substrates were etched with zinc powder and HCl (2 M). They were then cleaned by sequential sonication in acetone, 2-propanol, ethanol, and deionized water for 15 min each, and dried under a nitrogen stream. The pre-prepared TiO2 colloidal were spin-coated onto each etched FTO substrate to produce a 150 nm-thick TiO2 layer. The TiO2 layer was then sintered in a furnace at 500 oC for 30 min and subsequently immersed into the dye solution (0.1 mM) for 2 h in dark. The resultant films were rinsed with ethanol to remove any loosely adsorbed dye molecules. Photoelectrochemical Polymerization of the Polythiophene (PT) Films. The photoelectrochemical polymerization reaction was performed following the established procedure.43-44 The dye/TiO2/FTO substrate was used as a working electrode, along with a Pt auxiliary electrode and an Ag/AgCl reference electrode. Under illumination of a 500 W Xe lamp through a high-wavelength pass filter (> 400 nm, 200 mW/cm2), 0.5 V was applied on the dye/TiO2/FTO substrate for 5 min at first, then the potential was cycled between 0.9 and 1.2 V on the substrate at 0.05 V/s in a propylene carbonate solution comprising 0.2 M thiophene and 0.1 M TMABF4. The resultant conducting PT film was washed with ethanol and dried in air. For comparison, a PT film was produced by holding the substrate at 0.5 V. 7



Construction and Characterization of Polythiophene-based Solar Cells (PTSCs). The hole-transport layer was spin-coated onto the PT/dye/TiO2/FTO substrate using a chlorobenzene solution of 0.059 M spiro-OMeTAD, 0.05 M 4-tert-butylpyridine, 0.009 M LiTFSI, and 1% molar ratio of CoTFSI. The anode assembly was completed by sputtering a 90 nm-thick Au coating in the end. The extended edges of the FTO substrate and the thin Au coating served as the cathode and anode of the solar cell, respectively. UV-vis spectra were recorded on a Cary 300 dual-channel UV-visible spectrometer (Agilent Technologies, Santa Clara, CA). Scanning electron microscopy images were acquired from a Quanta FEG 250 field-emission (ZEISS, Germany) and transmission electron microscopy images were obtained with a JEM-2100F microscope (Tokyo, Japan). The Raman spectra were measured on a LabRAM HR800 microscope (Horiba Jobin Yvon) with the excitation wavelength set at 532 nm. A Fluorolog-3 system (Horiba Scientific, Edison, NJ) was used for photoluminescence spectra. The electrochemical experiments were conducted with a CHI440 Electrochemical Analyzer (CH Instruments, Austin, TX). The performance of PTSC was evaluated with a homebuilt solar simulator, which consisted of a 500 W Xe lamp (Newport Corp., Irvine, CA) placed before an AM 1.5G filter. The light intensity was measured by a radiation power meter (Model 70260, Newport Inc.) and adjusted to be 1 sun.

3. RESULTS AND DISCUSSION

8


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Figure 2. (A) Linear scan voltammograms recorded in propylene carbonate solution containing 0.1 M TMABF4 and 0.2 M thiophene at a dye/TiO2/FTO or TiO2/FTO substrate in dark and under irradiation. (B) Amperometric i-t curves recorded at the same substrates at 0.5 V vs. Ag/AgCl under irradiation. Inset: cyclic voltammograms of photoelectrochemical polymerization of thiophene. For both panels the scan rate was 0.05 V/s. (C) Absorption spectra of a dye/TiO2/FTO substrate before and after photoelectrochemical polymerization of thiophene. (D) Raman spectra of a dye/TiO2/FTO substrate and the same surface deposited with a 270 nm-thick PT film. Inset in (D) is an enlarged Raman spectrum of the dye. Excitation wavelength = 532 nm.

Figure 2A is an overlay of linear scan voltammograms collected at a dye/TiO2/FTO substrate in dark (black curve) and under irradiation (red curve). In dark, the onset potential is about 0.6 V vs. Ag/AgCl, which initiates the electrochemical polymerization of thiophene. In contrast, the onset potential decreases to 0.6 V under irradiation. As for the TiO2/FTO substrate, the onset potential is about 1.5 V both in dark (blue curve) and under irradiation (green curve), and the polythiophene molecules do not adhere to the compact layer. These results support our contention that the thiophene oxidation and polymerization were initiated by the 9



tethered dye molecules. As shown in Figure 2B, when the substrate was biased at 0.5 V and under irradiation, thiophene polymerization proceeded continuously. The photocurrents increase with the dye surface coverage (cf. Figure S3). The PT film can also be deposited via cyclic voltammetry (inset of Figure 2B). When the potential was cycled to 1.2 V, the PT film thickness increased. When the potential was cycled to ‒0.9 V, the dopant, TMA+, was incorporated into the PT film. Figure 2C is an overlay of absorption spectra of a dye/TiO2/FTO substrate (black curve), along with the same substrate covered with a photoelectrochemically deposited 270 nm-thick PT film (red curves). After polymerization, the PT/dye/TiO2/FTO substrates displayed a broad adsorption band between 370 to 650 nm, which can be assigned to the  LUMO) transition in the PT backbone.28,45 The PT film and the dye molecules constitute 96.4% and 3.6% of the total absorbance, respectively. Thus, the PTSC relies almost exclusively on the PT photoactive film to generate photocurrent. Figure 2D compares a Raman spectrum of the dye molecules adsorbed onto the compact, 150 nm-thick TiO2 layer (black curve) to that after polymerization (red curve). The peaks at 144, 398, 517, and 638 cm1 are ascribed to anatase TiO2,46-47 and the peaks between 1300 and 1700 cm1 (inset) are assigned to the dye. The significant attenuation of these peaks in the red curve confirms the deposition of a PT film through the space between the tethered dye molecules to eventually cover the entire dye sub-monolayer (cf. Figure 1A). Upon PT film formation, the symmetric stretching of the thiophene ring within the polymer skeleton emerged at 1457 cm1.48-49 10


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Figure 3. SEM images of dye/TiO2/FTO substrates deposited with 270 nm- (A) and 450 nm-thick (B) PT films with 10% surface coverage of dye C207. Insets are cross-sectional views of the different layers. A TEM image (C) and the EDX elemental mapping patterns (DE) of N and S elements across a piece of the PT/dye/TiO2 layer scraped off the substrate. Absorption spectra (F) of PT films of different thicknesses.

SEM images of two dye/TiO2/FTO substrates, covered with 270 nm- (Figure 3A) and 450 nm-thick (Figure 3B) PT films, respectively, revealed markedly different morphologies. The 270 nm-thick PT film has a smooth and uniform surface, and the boundary between the PT film and the 150 nm-thick TiO2 layer is distinctive (inset of Figure 3A). The uniform PT film is in tight contact with the underlying compact TiO2 layer, suggesting that the thin and compact TiO2 layer with a small dye coverage is favorable to the orderly growth of the PT film. However, the PT film has a rougher morphology and contains larger and more irregular granules when its thickness exceeds 270 nm (e.g. 450 nm shown in Figure 3B). The irregular and rough surface of 450 nm-thick PT film, shown in the cross-sectional view, indicates that the PT film 11



had grown disorderly. To gather further evidence of the ordered PT growth along the tethered dye molecules, we scraped small pieces of the PT/dye/TiO2 off an FTO surface, and analyzed them with TEM and energy-dispersive X-ray (EDX). The TEM image indicates that a smooth PT film is tightly connected to the TiO2 layer, and the EDX mapping clearly shows that the dye molecules (containing N atoms; Figure 3D) are evenly scattered across the entire TiO2 surface. These results are consistent with the sub-monolayer dye coverage deduced from the absorbance measurement. In contrast, the PT film (containing S atoms; Figure 3E) is much denser. Thus, the scattered and tethered dye molecules initiated the photochemical polymerization and the PT film growth is ordered and uniform. In addition, we investigated the effect of surface coverage of dye C207 at TiO2/FTO substrates on the quality of PT films. Compared with the PT film formed with 10% surface coverage of dye C207 (cf. Figure 3A), films with higher and lower coverages have rougher morphology and agglomerated particles (Figure S4). At coverages less than 10%, ππ stacking between the dye molecules and thiophene monomers is insufficient, which leads to a disordered PT film. At coverages higher than 10%, the growth of PT film becomes accelerated, which leads to agglomeration of PT particles. These results indicate that the dye surface coverage plays an important role in growing orderly PT films. Notice in Figure 3F that the PT absorbance increases with the film thickness. The X-ray diffraction (XRD) patterns of dye/TiO2/FTO substrates covered with PT films of different thicknesses are shown in Figure S5. The broad peak around 2 of 2024 is characteristic of amorphous PT, which decreases with the PT film thickness and is eventually overwhelmed by the diffraction peaks of anatase TiO2 and FTO substrate.

12


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Figure 4. The impact of PT film thickness on the PTSC photovoltaic parameters: (A) VOC, (B) JSC, and (C) FF. (D) JV characteristics of PTSCs, measured under illumination of a Xe lamp at 100 mW/cm2 (simulated AM 1.5G).

Table 1. Photovoltaic Parameters of PTSCs Constructed with Different PT Films Average PT film thickness (nm)

VOC (V)

JSC (mA/cm2)

FF

 (%)

150 210 270 330 450

0.78 ± 0.01 0.79 ± 0.01 0.81 ± 0.01 0.79 ± 0.01 0.77 ± 0.01

10.28 ± 0.42 11.61 ± 0.56 12.92 ± 0.63 12.13 ± 0.52 11.29 ± 0.43

0.72 ± 0.01 0.72 ± 0.01 0.72 ± 0.01 0.61 ± 0.03 0.49 ± 0.03

5.77 ± 0.40 6.61 ± 0.49 7.53 ± 0.58 5.85 ± 0.63 4.26 ± 0.49

We investigated the relationship between the PT film thickness and the photovoltaic parameters. The short-circuit current densities (JSC), open-circuit potentials (VOC), fill factors (FF), and the conversion efficiencies () of these cells are listed in Table 1. Note that JSC increases with the PT film thickness from 150 to 270 nm. This is expected as thicker photoactive layers absorb more light (cf. Figure 3F). 13



However, JSC, VOC and FF all began to decrease beyond 270 nm (Figure 4A‒C and Table 1). Notice that the PTSC comprising a 270 nm-thick PT film yielded a JSC of 12.9  0.63 mA/cm2, a VOC of 0.81  0.01 V, and a FF of 0.72 ± 0.01. The corresponding  is 7.53 ± 0.58%, which, to the best of our knowledge, is the highest among all the solar cells constructed solely with PT.10 The shapes of the J‒V characteristics of PT films ranging from 150 to 270 nm (black, red and blue curves in Figure 4D) are better defined, leading to higher fill factors. This behavior can be rationalized by the fact that the PT film grown orderly along the tethered dye molecules facilitates unidirectional charge transport and effectively prevents the hole/electron recombination. However, the rougher morphology and disordered structure associated with PT films thicker than 270 nm (cf. Figure 3B and inset) hinder the charge transport and cause more rapid hole/electron recombination, causing degradation of the photovoltaic performance (green and purple curves in Figure 4D). As shown in Figure S6A, it is evident that the PT film significantly improved the Jsc and Voc, due to increased light absorption and decreased recombination. Moreover, the PT film produced photoelectrochemically at a constant potential of 0.5 V exhibits a JSC of 10.34  0.29 mA/cm2, a VOC of 0.78  0.01 V, and a FF of 0.67 ± 0.01. The corresponding  is 5.4 ± 0.31%, a decrease of 28.28% when compared to that of the film prepared with cyclic voltammetry. The cyclic voltammogram of dye C207 adsorbed on a TiO2/FTO substrate shows an oxidation peak at 0.80 V and a reduction peak at 0.71 V (cf. Figure S6B). This decrease is attributed to doping of the PT films by cations from the electrolyte solution. The effect of dye surface coverage on the PTSC performance is shown in Figure S7. The JSC, VOC, FF and  all increase with the surface coverage from 1.7 to 10%, then decrease beyond 10%. This result is consistent with the quality of PT films formed with different surface coverages in 14


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Figure S4. Hence, 10% is the optimal surface coverage of dye for PTSCs.

Figure 5. (A) Open-circuit voltage decay curves of PTSCs with different thicknesses of PT films. Inset shows the electron lifetimes. (B) Photoluminescence spectra of PT films of different thicknesses.

As mentioned in the Introduction, the second step of the light-to-energy conversion in PTSCs is the diffusion of electrons from the photoactive film to the TiO2 layer. The electron-hole recombination within PT film is an undesired path, while the transfer through the external circuit to the electron-collecting electrode is desired.37 The recombination process is the only path under the open-circuit condition.37 Voc decays exponentially with time right after termination of irradiation, as shown in Figure 5A. The derivatives of the decay curves are an indirect way of measuring the electron lifetimes n (inset of Figure 5A).37,50-51 The reverse logarithm-linear

dependence

suggests

that

the

recombination

process

is

charge-transfer controlled, as it follows the Butler-Volmer equation.37,50 We found that n decreases in the order of 270 < 210 < 150 < 320 < 450 nm. The result is in good agreement with the J-V curves and FF values (Figure 4AC), indicating that the hole/electron recombination increases as the PT film becomes rougher and less ordered. Photoluminescence (PL) emission is a useful tool for probing the electron 15



transfer and electron-hole recombination behaviors.52-54 The trend revealed by Figure 5B is also consistent with that shown in Figure 5A. Compared with the film directly coated onto FTO, the PL intensity of a 270 nm-thick PT film on dye/TiO2/FTO is quenched by about 90%, indicating that the hole/electron recombination has been largely suppressed (Figure S8). To further investigate the influence of the PT film thickness on the PTSCs performance, we performed electrochemical impedance spectroscopy (cf. Figure S9 for the Nyquist plots of PT films with different thicknesses).55-56 The electron transfer resistance decreased inversely with PT films between 150 and 270 nm, but increased when the film thickness exceeded 270 nm. Thus, a thin and ordered PT film improves the charge transfer, whereas a PT film thicker than 270 nm retards the charge transfer. Based on these data, we believe that it is possible to produce a thicker and more orderly PT film with the use of a dye containing a longer chain of multiple aromatic moieties.

4. CONCLUSIONS Controlled photoelectrochemical growth of ordered PT thin films was achieved at compact TiO2 layers partially covered with tethered dye molecules. The resultant layered structure was used as the cathode of the PTSC, with PT acting as the predominant photoactive material. A PTSC constructed with a 270 nm-thick PT film has a conversion efficiency of 7.52 ± 0.58%, which is the highest among solar cells based solely on PT. The ordered and uniform PT film ensures unidirectional charge transport and efficient hole/electron separation. Our PTSC is simple in design and can be constructed with stable and easily processable materials. 16


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ASSOCIATED CONTENT Supporting Information Supporting Information is available free of charge on the ACS Publications website: UV-vis spectra of solution comprising the stripped dye, synthetic route and 1H-NMR spectra of the C207 dye, the photocurrent, SEM images of PT films, JV curves of dye C207 with different surface coverages, redox potentials of dye C207 attached onto TiO2, the X-ray diffraction pattern and electrochemical impedance spectra of different PT films. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] ORCID Feimeng Zhou: 0000-0002-2568-765X Qing Kang: 0000-0001-9549-5576 Notes The authors declare no competing financial interest. ACKNOWLEDGEMNTS This work was partially supported by a 2011 Collaborative Innovative Grant of Hunan Province of China, and the Nature Science Foundation of China (No. 21802051), and the Center for Research Excellence in Science and Technology program at CSULA (NSF No. 1112105). 17



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Solar Cells Constructed with Polythiophene Thin Films Grown along

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