Plasmon-enhanced Performance of Dye-sensitized Solar Cells Based on Electrodeposited Ag Nanoparticles

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Luan Xinning, Wang Ying. Plasmon-enhanced Performance of Dye-sensitized Solar Cells Based on Electrodeposited Ag Nanoparticles. Journal of Materials Science & Technology, 2014, 30(1): 1-7 Copy to clipboard

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Plasmon-enhanced Performance of Dye-sensitized Solar Cells Based on Electrodeposited Ag Nanoparticles

Luan Xinning, Wang Ying^*

Department of Mechanical & Industrial Engineering, Louisiana State University, Baton Rouge, LA 70803, USA

Corresponding author. Prof., Ph.D.; Tel.: +1 225 578 8577; Fax: +1 225 578 9162.

Abstract

In the present work, pulse current deposition is used to deposit evenly distributed and uniformly sized Ag nanoparticles onto a TiO₂ nanotube array as photoelectrode in dye-sensitized solar cells (DSSCs), and the size and amount of loading Ag nanoparticles are controlled by the pulse deposition time. Due to the enhanced light absorption and electron–hole separation caused by plasmon effect, DSSCs based on Ag-modified TiO₂ nanotube arrays show higher energy conversion efficiencies than those based on bare nanotubes with the same tube length. Particularly, DSSC based on nanotubes modified using pulse deposition time 1 s/3 s delivers the highest energy conversion efficiency of 1.68% and the largest short-circuit current of 4.37 mA/cm², while DSSC consisting of bare nanotubes exhibits efficiency of 1.20% and short-circuit current of 2.27 mA/cm², which represents a 40% enhancement of cell efficiency in DSSC based on Ag-modified TiO₂ nanotubes. It is also noted that overly long pulse deposition time will not further increase DSSC efficiency due to agglomeration of Ag particles. For example, when the pulse deposition time is increased to 2 s/6 s, DSSC based on Ag-modified nanotubes exhibits a lower efficiency of 1.42%. Moreover, high-concentration TiCl₄ treatment on TiO₂ nanotube arrays can further increase the energy conversion efficiencies to 3.82% and 2.61% for DSSC based on Ag-modified TiO₂ nanotubes and DSSC based on bare TiO₂ nanotubes, respectively, by significantly creating more surface area for dye loading.

Keyword: TiO₂ nanotube; Dye-sensitized solar cell; Pulse current deposition; Ag nanoparticle; TiCl₄ treatment

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1. Introduction

New generations of solar cells have emerged, as conventional silicon solar cells with high energy conversion efficiencies pose challenges of complicated and high-cost processing and production^{[1] and [2]}. Dye-sensitized solar cells (DSSCs) have attracted increasing attention for their easy processing and low cost, since mesoporous titania with high surface area was used as photo-anode to improved energy conversion efficiency of DSSC as reported by Grätzel et al. in 1991^{[3], [4], [5], [6], [7], [8], [9], [10], [11] and [12]}. In a DSSC, electrons of dye molecules are excited by solar energy absorption and then the electrons from the photo-excited dye inject into the conduction band of photo-anode such as titanium dioxide (TiO₂). The electrons then diffuse through TiO₂ layer to the electrode and subsequently reach the counter electrode through an external circuit. The dye molecules regain electrons from a redox couple iodide/triiodide (I^-/I^3-) in an electrolyte. The iodide is regenerated in turn by the reduction of triiodide at the counter electrode, and this process requires a catalytic functionality of Pt on the cathode surface.

TiO₂ is commonly used as photo-anode material in current DSSCs. One-dimensional TiO₂ nanostructures such as TiO₂ nanotube arrays have attracted much attention as the photo-anode material in DSSCs due to their ordered vertical structure which facilitates filling of new sensitizer molecules or non-volatile electrolytes for improved efficiency and stability of DSSCs, while conventional mesoporous TiO₂ film has a disordered structure that makes infiltration of larger sensitizer molecules or viscous electrolytes difficult and ineffective^{[13], [14], [15], [16], [17], [18], [19], [20], [21] and [22]}. However, DSSC based on TiO₂ nanotube array exhibits limited energy conversion efficiency owning to its lower surface area than mesoporous TiO₂ film^{[23], [24], [25] and [26]}.

Numerous approaches and strategies have been investigated so as to enhance photoelectric conversion efficiency of DSSC by surface modification with noble metal^{[27], [28] and [29]}, using semiconductor quantum dots^{[30] and [31]}, developing new dyes^{[32], [33] and [34]}, and designing new morphology^{[35], [36] and [37]}. The purposes of these methods are to either increase the surface area for dye loading or increase the light harvesting ability and/or suppress the recombination of photogenerated electron–hole pairs. Among these approaches, surface modification by noble metal has been considered as a very effective approach to enhance energy conversion efficiency of DSSC by improving its light harvesting. Surface plasmon induced by noble metals has the potential for enhancing the optical absorption. The surface plasmon effect is caused by light-driven collective oscillations of conduction electrons in metallic nanoparticles. The nanoparticles must be smaller than the wavelength of exciting light for electrons to oscillate with the electric field of light. When these conditions are met, an enhanced electromagnetic field is found nearby the surface of nanoparticle. The enhanced electromagnetic field is highly dependent upon the wavelength of incident light, as well as the shape, size and aggregation state of the nanoparticle. Atwater et al. ^[38] recently outlined three types of plasmonic light-trapping geometries for incorporating metal nanoparticles: i) Far-field scattering. When light is scattered from noble metal nanoparticles in DSSC, light is preferentially scattered and trapped into the semiconductor by multiple and high-angle scattering, causing an increase in the effective optical path length in the cell. ii) Near-field scattering. When the wavelength of the irradiation sources is correlated with the optical absorption of the surface plasmon resonance, an increase in local electromagnetic field is found nearby metal surface. The photo-excited particles' near-field causes the creation of electron–hole (e–h) pairs in semiconductor and suppresses the recombination of electrons in dye and/or electrolyte. iii) Surface plasmon polaritons at the metal/semiconductor interface. A corrugated metallic film on the back of semiconductor can couple sunlight into surface plasmon polaritons at metal/semiconductor interface. Therefore, utilization of plasmon effect for enhancement of light is a promising procedure for improving the efficiency of DSSC.

Till now, surface plasmon arising from metal nanoparticles has mostly been applied to increase optical absorption and/or photocurrent in the field of silicon solar cells^{[39], [40], [41] and [42]} and organic solar cells^{[43] and [44]}. Not many examples of this concept have been introduced and demonstrated in DSSCs. Considering plasmon effect of various noble metals, Ag is one of the most suitable candidates for practical applications due to its easy preparation and lower cost than others. Several methods have been reported to deposit Ag nanoparticles onto TiO₂ such as photo-induced deposition, calcination reduction, and electrochemical deposition. However, none is able to prevent the aggregation of Ag particles and keep structures uniform during the deposition process. Chou et al. synthesized TiO₂/Ag composites via dry coating process and investigated their applications in DSSCs ^[45]. Such a nano-metal modified DSSC yielded a photo conversion efficiency of 0.93%, which was higher than DSSC based on unmodified nanotubes (0.71%) with identical film thickness of 8.0 μm. However, this efficiency of 0.93% was still low, which was ascribed to agglomeration of Ag nanoparticles and a wide particle size distribution of 20–100 nm. Agarwala et al. ^[46] also prepared Ag nanoparticles onto TiO₂ nanotubes by using a photo reduction method, to scatter light and generate surface plasmon effect. The DSSC efficiency was enhanced by 22% after being modified with Ag nanoparticles. However, the particle size was larger than 70 nm and the nanoparticles were mixed with some rod like structures, resulting in lower dye absorption and less scattering effect. On the other hand, Lai et al. developed a simple method using pulse current (PC) deposition to deposit uniformly distributed spherical Ag nanoparticles (2–10 nm) on the surface of prepared TiO₂ nanotube arrays. The generated photocurrent was found to be 36% higher than that of randomly distributed Ag particles prepared by photocatalytic reduction ^[47]. PC electrodeposition is considered as a facile method for controlling the electrocrystallization process and hence depositing materials with unique structures and properties. By tuning current-on time ( t_on) and current-off time ( t_off), instantaneous current density applied by pulse control can lead to ultrafine-grained structures and homogeneous surface appearance of the deposit. Therefore, PC electrodeposition can form evenly distributed and uniformly nano-sized Ag. On the other hand, direct current (DC) electrodeposition results in inhomogeneous Ag with rough surface; hence, it is not a suitable method for producing Ag nanoparticles ^[48].

As summarized above, size of Ag nanoparticles synthesized via photo reduction deposition method is too large and uncontrollable; as a result, these commonly-used methods are not suitable choices for enhancing plasmon effect in DSSCs. However, the enhancement of plasmon effect can be easily achieved by using pulse current deposition method, which can produce evenly distributed nanoparticles with controllable size distribution by tuning pulse current. However, this method has never been used for depositing metallic particles for plasmon effect in DSSCs. The present work is the first effort to employ pulse current deposition method for enhancing plasmon effect in DSSCs. TiO₂ nanotube arrays are prepared via anodization and followed by deposition of uniformly distributed spherical Ag nanoparticles via pulse current deposition. This process is easy to implement and can effectively suppress the agglomeration of Ag particles by taking advantage of high nucleation rate of Ag under the PC condition. DSSCs based on Ag-modified TiO₂ nanotube arrays show significantly enhanced performance than DSSCs consisting of bare TiO₂ nanotube arrays. Efficiency of such DSSCs can also be maximized by tuning pulse deposition time of Ag nanoparticles. Moreover, efficiency of DSSCs can be further improved by treating TiO₂ nanotubes with high concentration TiCl₄ prior to Ag deposition.

2. Experimental

Ti foils (0.25 mm thickness, 99.5 wt% purity, Alfa Aesar) were degreased by sonicating in acetone, deionized (DI) water, and ethanol for 15 min, respectively, then rinsed with ethanol and dried in a nitrogen stream. Electrochemical anodization of the Ti foils was carried out in a two-electrode cell, with a platinum mesh as the counter electrode at room temperature. The electrolyte for anodization was prepared with anhydrous ethylene glycol (EG) with NH₄F (0.3 wt%) and H₂O (5 vol%). The voltage was supplied by a DC power supply with a digital display (Model 1623A, PK Precision). After anodization, the samples were rinsed in deionized water (DI water) and dried in air, followed by heat treatment at 450 °C in air for 3 h to produce anatase phase. Optional TiCl₄ treatment was performed on TiO₂ nanotube arrays by soaking annealed nanotube in 100 mL of 0.2 mol/L TiCl₄ aqueous solution at 70 °C for 30 min and then annealing at 450 °C in air for 30 min.

Pulse-current electrodeposition of Ag nanoparticles was carried out by immersing the as-prepared TiO₂ nanotube in an aqueous electrolyte composed of 10 mmol/L AgNO₃ and 100 mmol/L NaNO₃ at room temperature. The TiO₂ nanotube array and a platinum mesh were used as the working and counter electrodes, respectively. A pulse-current of 10 mA/cm² with different alternating on-time and off-time (0.5 s/1.5 s, 1 s/3 s, 2 s/6 s) for 100 cycles were applied. Fig. 1 shows a scheme showing the pulse current process for electrochemical deposition of Ag nanoparticles with a rectangular pulse shape composed of an alternating on-time and off-time for 100 deposition cycles. The Ag-deposited samples were then rinsed with DI water and dried in vacuum at 50 °C overnight.

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	Fig. 1. Schematic showing the pulse current process for electrochemical deposition of Ag with a rectangular pulse shape composed of alternating on-time and off-time for 100 deposition cycles.

To fabricate DSSCs, TiO₂ nanotube arrays with or without Ag modifications were soaked in anhydrous ethanol containing 0.2 mmol/L N719 dye (Ru[LL′-(NCS)₂], L = 2,2′-bipyridyl-4,4′-dicarboxylic acid, L′ = 2,2′-bipyridyl-4,4′-ditetrabutyl-ammonium carboxylate, Solaronix Co.) and sensitized for 24 h at room temperature. Afterward, these dye-sensitized TiO₂ nanotube arrays with an active area of approximately 0.25 cm² were rinsed with acetonitrile in order to remove physisorbed N719 dye molecules. Platinized counter electrode was fabricated by drop casting 0.5 mmol/L H₂PtCl₆/isopropanol solution on FTO glass substrate that had a hole for electrolyte injection later on, followed by heating at 400 °C in air for 20 min. The dye-sensitized TiO₂ nanotube arrays with or without Ag modifications were sandwiched together with Pt-coated FTO glass by using a 100 μm thick hot-melt sealing film as the spacer (Meltonix 1170-100, Solaronix Co.). DSSCs were sealed by applying heat and pressure with a hot press at 110 °C. A I^-/I^3- based electrolyte, which contained 0.10 mol/L GTC in a mixture of acetonitrile and valeronitrile (85:15 v/v) (No. ES-0004, IoLiTec Inc., Germany) was injected through the hole on the Pt-coated FTO into DSSC.

Crystal structure of TiO₂ nanotube arrays was determined by X-ray diffraction (XRD) using a Rigaku MiniFlex diffractometer with Cu Kα radiation operated at 30 kV and 15 mA at a scan rate of 2°/min. A FEI Quanta 3D FEG scanning electron microscope (SEM) at an accelerating voltage of 5–20 kV was used to characterize the morphology of TiO₂ nanotube arrays. Chemical composition of these nanotubes was studied by energy dispersive X-ray spectroscopy (EDX) on a Hitachi S-3600N SEM. Surface composition of TiO₂ nanotube arrays was analyzed via an X-ray photoelectron spectroscope (XPS) on the AXIS-165 XPS (Kratos Analytical Ltd.) with an AXIS 165 spectrometer from Kratos Analytical LTD by using a twin-anode Al Kα (1486.6 eV) X-ray source. Transmission electron microscopy (TEM, JEM-1011, JEOL LTD.) with an accelerating voltage of 80 kV was used to characterize the morphology of Ag-modified TiO₂ nanotubes. The current–voltage ( J– V) characteristics of DSSCs were recorded using a Keithley 2400 source meter. A solar light simulator (Model: 67005, Oriel) was used to simulate sunlight under one sun AM 1.5 G (100 mW/cm²) illumination provided by a 150 W Xenon arc lamp (Model: 6256, Oriel) and calibrated using a Si solar reference cell (Model: 91150V, Oriel). Backside illumination mode was used, since the photo-anode consists of TiO₂ nanotube array on Ti substrate which was not transparent to light.

3. Results and Discussion

XRD analysis is utilized to examine the crystallinity of TiO₂ nanotubes. Fig. 2 shows XRD patterns of bare TiO₂ nanotube arrays annealed at 450 °C in air for 2 h. The peaks at 2 θ = 25.4° and 2 θ = 48.0° correspond to (101) and (200) in TiO₂ anatase phase, and all of the other dominant peaks correspond to the reflection of Ti foil substrate. This result suggests that the as-prepared anodic TiO₂ nanotubes convert to anatase phase after being heated at high temperatures, which can be used as photo-anodes in DSSCs.

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	Fig. 2. X-ray diffraction pattern of bare TiO₂ nanotube array annealed at 450 °C in air for 2 h.

Fig. 3 shows SEM images and EDX spectrum of TiO₂ nanotube arrays modified with Ag nanoparticles (NPs) via pulse deposition time of 1 s/3 s. The cross-section SEM image in Fig. 3(a) shows that the structure of TiO₂ nanotube arrays preserves well after the PC deposition. Thickness of the TiO₂ nanotube arrays is estimated to be ∼6.1 μm, and the average diameter of these nanotubes is ∼120 nm. It is important to note that Ag NPs are uniformly distributed and anchored along the surface of the tube structure. It is observed from Fig. 3(b) that most of Ag nanoparticles have a size in a range of 10–40 nm. The top-view SEM image in Fig. 3(c) shows Ag NPs deposited on the open entrances of TiO₂ nanotube arrays. It can be seen that Ag NPs deposited on the top open entrances of TiO₂ nanotubes (showing a particle size up to 60 nm) are larger than Ag NPs on the tube walls. Meanwhile, Ag NPs deposited on the top open entrances of TiO₂ nanotubes are denser (without any obvious agglomeration) than Ag NPs on the tube walls. Such a larger particle size and denser distribution of Ag NPs on the top of TiO₂ nanotubes are possibly due to the top open entrances of TiO₂ nanotubes being more exposed to electrochemical deposition. Fig. 3(d) displays the EDX spectrum of these TiO₂ nanotubes arrays modified with Ag NPs. A small amount of Ag element is detected, as evidenced by its weak but discernible peaks in the EDX spectrum.

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	Fig. 3. (a, b) Cross-sectional, (c) top-view SEM images, and (d) EDS spectrum of TiO₂ nanotube arrays modified with Ag nanoparticles via pulse deposition time of 1 s/3 s.

To further study the details about Ag NPs deposited on TiO₂ nanotube arrays via pulse deposition time of 1 s/3 s, its TEM image is obtained as shown in Fig. 4. The average diameter of Ag NPs shown in the TEM image is approximately 20 nm, which is consistent with the particle size shown in the SEM images earlier. This homogeneous dispersion of Ag NPs on TiO₂ nanotube arrays is desirable for efficient light harvesting, yielding high plasmon-induced activity for electron transportation.

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	Fig. 4. TEM image of a TiO₂ nanotube modified with Ag nanoparticles via pulse deposition time of 1 s/3 s.

To further confirm the formation of Ag NPs from the pulse deposition, XPS is also employed to study surface composition of Ag-modified TiO₂ nanotubes arrays. The XPS spectrum taken from TiO₂ nanotubes arrays modified with Ag NPs via pulse deposition time of 1 s/3 s exhibits the Ag 3 d peak in Fig. 5(a). In Fig. 5(b), the peaks observed at 368.1 eV and 374.2 eV are ascribed to metallic Ag 3 d_5/2 and Ag 3 d_3/2, respectively, confirming metallic Ag existence on the surface of TiO₂ nanotube arrays. These findings reveal that Ag-modified TiO₂ nanotube arrays are successfully achieved by electrochemical deposition of Ag in AgNO₃-containing solution.

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	Fig. 5. (a) XPS spectra and (b) Ag 3 d XPS peak of TiO₂ nanotubes modified with Ag nanoparticles via pulse deposition time of 1 s/3 s.

Performances of DSSCs based on bare and Ag-modified TiO₂ nanotubes electrodes are examined under 1 sun AM 1.5 simulated sunlight. Fig. 6 presents J– V curves of DSSCs based on 6.1-μm-long TiO₂ nanotubes arrays modified with Ag NPs using different pulse deposition time (0.5 s/1.5 s, 1 s/3 s, and 2 s/6 s). Photovoltaic characteristics of these DSSCs, such as short-circuit current ( J_sc), open-circuit voltage ( V_oc), fill factor (FF), and efficiency ( η) are summarized in . It is evident that the short-circuit currents of all Ag-modified TiO₂ nanotubes based DSSCs (0.5 s/1.5 s, 1 s/3 s, and 2 s/6 s) are larger than that of bare TiO₂ nanotubes based DSSCs, since the Ag NPs on side walls enhance absorption and charge separation, while there are no substantial differences in the photovoltages. DSSC based on Ag@TiO₂ (Ag-modified TiO₂ nanotubes) synthesized via the shorter pulse deposition time of 0.5 s/1.5 s, delivers an energy conversion efficiency of 1.28%, which is just slightly higher than the efficiency (1.20%) of DSSC based on bare nanotube, possibly because the amount of Ag NPs loading is too low to show any plasmon effect. The DSSC based on Ag@TiO₂ 1 s/3 s (Ag-modified TiO₂ via pulse deposition time of 1 s/3 s) delivers the highest energy conversion efficiency (1.68%) exhibiting V_oc of 0.79 V, J_sc of 4.37 mA/cm², and FF of 0.49, which represents a 40.0% increase in cell efficiency compared to DSSC consisting of bare TiO₂ nanotube array (1.20%). In theory, the maximum voltage ( V_max) generated by DSSC is the difference between the (quasi-)Fermi level of the TiO₂ and the redox potential of the electrolyte, about 0.7 V under solar illumination conditions. Since the loaded Ag nanoparticles do not affect the Fermi level of TiO₂ and redox potential of electrolyte much, Ag deposition on TiO₂ would not improve V_oc. The improved J_sc is resulted from higher plasmon resonance effect caused by coverage of Ag NPs on the TiO₂ nanotube surface, which increases the number of photoelectrons generated and improves the effective separation of electron–hole pairs. Therefore, the loaded Ag nanoparticles can improve short-circuit current rather than the open-circuit voltage in these DSSCs. Interestingly, the efficiency of DSSC based on Ag-modified TiO₂ nanotube array decreases again, as the pulse deposition time used for depositing Ag nanoparticles is increased to 2 s/6 s, due to agglomeration of Ag NPs and formation of some particles with larger size resulted from longer pulse deposition time as shown in . Such agglomeration of Ag NPs on TiO₂ nanotubes results in less surface area for dye loading as well as ineffective light absorption and charge separation.

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Fig. 6. J– V characteristics of DSSCs based on different TiO₂ nanotube arrays (Ag@TiO₂ 0.5 s/1.5 s: TiO₂ nanotubes modified with Ag nanoparticles via pulse deposition time of 0.5 s/1.5 s; Ag@TiO₂ 1 s/3 s: TiO₂ nanotubes modified with Ag nanoparticles via pulse deposition time of 1 s/3 s; Ag@TiO₂ 2 s/6 s: TiO₂ nanotubes modified with Ag nanoparticles via pulse deposition time of 2 s/6 s; bare TiO₂: unmodified TiO₂ nanotubes).

Table 1. Photovoltaic characteristics of dye-sensitized solar cells shown in Fig. 6</p>

To further improve energy conversion efficiency of DSSC based on TiO₂ nanotubes, treatment of TiO₂ nanotubes with high concentration TiCl₄ is carried out in this work. It has been reported that TiCl₄ treatment can enhance dye absorption on the surface, improve the electron transport properties in nanocrystalline TiO₂ films, and increase the charge separation efficiency^{[49], [50] and [51]}. After being treated in TiCl₄ aqueous solution, TiO₂ nanotube arrays are sensitized with N719 dye and then integrated into DSSCs. Fig. 7 compares the J– V characteristics of DSSCs based on TiCl₄ treated bare and Ag-modified TiO₂ nanotubes as a function of different pulse deposition time (0.5 s/1.5 s, 1 s/3 s, and 2 s/6 s). Detailed photovoltaic performance parameters are presented in . Compared to untreated bare TiO₂ nanotubes, the energy conversion efficiency of DSSC based on TiCl₄ treated bare TiO₂ nanotubes is increased to 2.61%, and such enhancement is mainly originated from the increased short-circuit current density. Likewise, the efficiency of DSSC based on Ag@TiO₂–TiCl₄ 0.5 s/1.5 s (TiCl₄-treated TiO₂ nanotubes modified with Ag NPs via pulse deposition time of 0.5 s/1.5 s) increase to 2.56% presenting a 100% enhancement of efficiency compared to DSSC based on Ag@TiO₂ 0.5 s/1.5 s (1.28%). The short-circuit current density of DSSC based on Ag@TiO₂–TiCl₄ 0.5 s/1.5 s shows slight enhancement than that of DSSC based on bare TiO₂–TiCl₄ and the enhancement is not significant due to small amount of Ag loading resulted from short pulse deposition time of Ag. In particular, the effect of Ag modification to enhance energy conversion efficiency is more significant for DSSC based on Ag@TiO₂–TiCl₄ 1 s/3 s (TiCl₄ treated TiO₂ modified with Ag NPs via pulse deposition time of 1 s/3 s), which delivers the highest energy conversion efficiency (3.82%) with V_oc of 0.75 V, J_sc of 9.27 mA/cm² and FF of 0.55 ( ), which represents a 46.4% increase in cell efficiency compared to that of DSSC based on bare TiO₂–TiCl₄ (TiCl₄-treated TiO₂ nanotubes) nanotube array (2.61%). These results confirm that Ag NPs loaded on TiO₂ nanotube surface significantly increase the performance of DSSCs.

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Fig. 7. J– V characteristics of DSSCs based on different TiCl₄-treated TiO₂ nanotube arrays (Ag@TiO₂–TiCl₄ 0.5 s/1.5 s: TiO₂ nanotubes modified with Ag nanoparticles via pulse deposition time of 0.5 s/1.5 s; Ag@TiO₂–TiCl₄ 1 s/3 s: TiO₂ nanotubes modified with Ag nanoparticles via pulse deposition time of 1 s/3 s; Ag@TiO₂–TiCl₄ 2 s/6 s: TiO₂ nanotubes modified with Ag nanoparticles via pulse deposition time of 2 s/6 s; bare TiO₂–TiCl₄: unmodified TiO₂ nanotubes).

Table 2. Photovoltaic characteristics of dye-sensitized solar cells shown in Fig. 7</p>

It is, however, noteworthy that DSSC based on Ag@TiO₂–TiCl₄ 2 s/6 s shows a large decrease in efficiency compared to DSSC based on TiCl₄ treated bare TiO₂ nanotubes. DSSC based on Ag@TiO₂–TiCl₄ 2 s/6 s (TiCl₄ treated TiO₂ nanotubes modified with Ag NPs via pulse deposition time of 2 s/6 s) delivers a low energy conversion efficiency of 1.55% with V_oc of 0.63 V, J_sc of 3.66 mA/cm², FF of 0.67, as summarized in . It can be observed that the drastic efficiency drop of DSSCs based on Ag@TiO₂–TiCl₄ 2 s/6 s is mainly originated from the decreased short-circuit current. The decrease of short-circuit current may be attributed to the following reasons. Long pulse deposition time (2 s/6 s) may cause agglomeration of Ag NPs on the TiO₂ nanotubes surface and this effect is more distinct for TiO₂ nanotubes treated with TiCl₄. TiCl₄ treatment yields very tiny TiO₂ nanoparticles on the surface of TiO₂ nanotubes ( (a and b)), some of which may partially plug the top entrances of TiO₂ nanotubes ( (c)), resulting in more sites for Ag deposition. When the deposition time is very long and since the tops of TiO₂ nanotubes are partially plugged and are much more exposed to electrochemical deposition than tube walls, large agglomerates of Ag NPs would form on the top of TiO₂ nanotubes without much deposition of Ag NPs along tube walls ( (d–f)), leading to reduced plasmon effect and decreased surface area for dye loading. In the case of surface plasmon, metallic nanoparticles must be smaller than the wavelength of the exciting light in order for electrons to oscillate together in response to the electric field of the excitation light. Agglomerates of Ag NPs as well as those with larger particle size (more than 200 nm) are much more difficult to be excited by radical light. Another issue is that Ag agglomerates or larger Ag particles have more severe plasmon decay. The intensity of electrons transferred to TiO₂ depends on the distance that plasmon travel from radical light impact position to TiO₂. Vesseur et al. ^[52] pointed out that plasmon excited by radical light, which are emitted radially from the impact position, are lost through absorption unless they are converted to a target. Therefore, plasmon excited on the surface of TiO₂ decays when transferred to the bottom of nanotube. Hence, plasmon decay is more severe in the case of Ag@TiO₂–TiCl₄ 2 s/6 s, which has large Ag agglomerates or particles on the tops of TiO₂ nanotubes and few Ag nanoparticles along the walls.

It can be seen by comparing Fig. 6 with Fig. 7 that, DSSCs based on TiCl₄ treated TiO₂ nanotubes show higher energy conversion efficiency than DSSCs based on untreated TiO₂ nanotube, due to larger surface area of TiO₂ nanotube resulted from TiCl₄ treatment. It can also be observed that the effect of Ag NPs to improve energy conversion efficiency and short-circuit current is more distinct for DSSCs based on Ag@TiO₂–TiCl₄ 1 s/3 s than DSSCs consisting of TiO₂ nanotubes modified with Ag via other shorter or longer pulse deposition time, because this pulse deposition time (1 s/3 s) yields evenly-distributed and uniformly-sized Ag nanoparticles, leading to enhanced surface plasmon resonance and subsequent improved optical absorption. The particle size of Ag NPs synthesized via commonly-used methods such as photo reduction deposition methods is too large and uncontrollable, while pulse current deposition can provide uniformly distributed nanoparticles which are necessary for plasmon effect in DSSC.

4. Conclusion

In summary, TiO₂ nanotube arrays are synthesized by anodizing Ti foils in EG electrolytes with a small amount of NH₄F and H₂O under alternating-voltage conditions, and subsequent heat treatment transforms them to anatase phase for modification with Ag nanoparticles via electrochemical deposition, for enhanced efficiencies of DSSCs. Ag nanoparticles with a size distribution up to 10–40 nm are formed on the side walls of TiO₂ nanotubes. Metallic Ag formation is confirmed by EDS and XPS analyses. In addition, the loading amount of Ag nanoparticle can be easily optimized by adjusting pulse deposition time. Most of DSSCs based on Ag-modified TiO₂ nanotube arrays show significantly higher energy conversion efficiencies than DSSC consisting of bare nanotube array of the same length. Such enhancement is ascribed to the surface plasmon resonance induced by uniformly deposited Ag NPs. Increasing the Ag deposition content is able to further improve the plasmon effect, but superfluous Ag content yields not as good as or worse photovoltaic performance, and DSSC based on TiO₂ nanotubes modified with Ag via pulse deposition time of 1 s/3 s display the highest energy conversion efficiency. TiCl₄ treatment for TiO₂ nanotubes can further enhance energy conversion efficiency of DSSCs by increasing surface area of TiO₂ nanotubes for more dye loading. Our results confirm that modifying anodic TiO₂ nanotube arrays with noble metal nanoparticles is an effective approach for maximizing efficiency of DSSCs and will advance the DSSC technology.

Acknowledgments

This work is supported by LABOR – RCS grant and BP – Gulf of Mexico Research Initiative (GRI) grant. The authors acknowledge Materials Characterization Center at LSU for using XRD and SEM. X.N. Luan acknowledges LSU Graduate School Enrichment Award.

The authors have declared that no competing interests exist.

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19.	K. Zhu, T. B. Vinzant, N. R. Neale, A. J. Frank, Nano Lett. , 7 (2007), pp. 3739–3746 [Cited within: 1] [JCR: 13.025]
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25.	F. Zhuge, J. Qiu, X. Li, X. Gao, X. Gan, W. Yu, Adv. Mater. , 23 (2011), pp. 1330–1334 [Cited within: 1] [JCR: 14.829]
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2010

11.653

0.0

... IntroductionNew generations of solar cells have emerged, as conventional silicon solar cells with high energy conversion efficiencies pose challenges of complicated and high-cost processing and production^{[1] and [2]} ...

2010

11.653

0.0

1991

38.597

0.0

Oregan

, M.

Grätzel

, Nature, 353 (1991), pp. 737–740

... in 1991^{[3], [4], [5], [6], [7], [8], [9], [10], [11] and [12]} ...

1991

11.016

0.0

Grätzel

, Coord. Chem. Rev. , 111 (1991), pp. 167–174

... in 1991^{[3], [4], [5], [6], [7], [8], [9], [10], [11] and [12]} ...

1993

1.578

0.0

Kay

, M.

Grätzel

, J. Phys. Chem. , 97 (1993), pp. 6272–6277

... in 1991^{[3], [4], [5], [6], [7], [8], [9], [10], [11] and [12]} ...

1993

10.677

0.0

M. K.

Nazeeruddin

, A.

Kay

, I.

Rodicio

, R.

Humphrybaker

, E.

Muller

, P.

Liska

, N.

Vlachopoulos

, M.

Grätzel

, J. Am. Chem. Soc. , 115 (1993), pp. 6382–6390

... in 1991^{[3], [4], [5], [6], [7], [8], [9], [10], [11] and [12]} ...

1994

0.905

0.0

... in 1991^{[3], [4], [5], [6], [7], [8], [9], [10], [11] and [12]} ...

1994

2.978

0.0

Grätzel

, Renew. Energy, 5 (1994), pp. 118–133

... in 1991^{[3], [4], [5], [6], [7], [8], [9], [10], [11] and [12]} ...

2013

1.198

0.293

Jia

, S.

Dang

, H.

Liu

, Z.

Zhang

, T.

, X.

Liu

, B.

, J. Mater. Sci. Technol. , 29 (2013), pp. 415–418

Submicrometer-scale ZnO composite aggregate arrays of nanorods and nanoparticles were prepared by simple wet-chemical route and studied as dye-sensitized solar cells (DSSCs) photoanodes. The ZnO composite aggregate arrays significantly improved the efficiency of DSSCs due to their relatively high surface area, fast electron transport, and enhanced light-scattering capability. A short current density ( J sc ) of 11.7 mA/cm 2 and an overall solar-to-electric energy conversion efficiency ( η ) of 3.17% were achieved for the ZnO composite aggregate DSSCs, which were much higher than those obtained for the monodisperse aggregate DSSCs ( J sc = 6.9 mA/cm 2 , η = 1.51%) and ZnO nanorod array DSSCs ( J sc = 4.2 mA/cm 2 , η = 0.61%).

Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education and Research Center of Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan 030024, China

... in 1991^{[3], [4], [5], [6], [7], [8], [9], [10], [11] and [12]} ...

2012

1.198

0.293

S. L.

Chung

, C. M.

Wang

, J. Mater. Sci. Technol. , 28 (2012), pp. 713–722

Three different types of TiO 2 nano powders were synthesized by a solution combustion synthesis (SCS) method using three different fuels and for comparison, another type of TiO 2 nano powder was synthesized by calcination of titanyl hydroxide. These TiO 2 nano powders were used to fabricate photoelectrodes for the dye-sensitized solar cell (DSSC) and their performance was compared to that of the DSSC fabricated with Degussa P25 TiO 2 . The results showed that the SCS TiO 2 could work well as photoelectrode for DSSC. The SCS TiO 2 contained impurities of C and/or S, thus exhibiting visible light absorption and reduced band gap. The open circuit voltage and the fill factor both varied little among the various TiO 2 and thus both had little effect on the photoelectrical conversion efficiency (μ). However, the variation of μ was seen to be in quite a good agreement with that of the short circuit current ( I sc ), suggesting that μ was dominated by I sc . I sc was found to be enhanced by light scattering effect due to the presence of large particles but reduced by high impurity content due to an increase in electron transfer resistance. In addition, the specific surface area of the powders was found to be an important factor affecting the I sc and thus the μ.

1) Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan, China 2) Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 70101, Taiwan, China

... in 1991^{[3], [4], [5], [6], [7], [8], [9], [10], [11] and [12]} ...

2013

1.198

0.293

Wang

, X.

Cui

, Y.

Zhang

, X.

Gao

, Y.

Sun

, J. Mater. Sci. Technol. , 29 (2013), pp. 123–127

The uniform cauliflower-like ZnO films were deposited on the conducting substrate by a chemical bath deposition in urea/water solution. The film structure and morphology were characterized by X-ray diffraction, thermogravimetric differential thermal analysis, energy dispersive spectroscopy, selected area electron diffraction, field emission scanning electron microscopy and high resolution transmission electron microscopy. The average diameter of ZnO nanoparticles and the petal thickness were 25 nm and 8 mm, respectively. Dye-sensitized solar cells based on the cauliflower-like ZnO film electrode showed the short-circuit current density of 6.08 mA/cm 2 , the open-circuit photovoltage of 0.66 V, the fill factor of 0.55 and the overall conversion efficiency of 2.18%. The equivalent circuit of cells based on the ZnO film electrodes was measured by the electrochemical impedance spectroscopy. Furthermore, the analysis of equivalent circuit provided the relationship between the cell performance and the interfacial resistance, such as the shunt resistance and the series resistance.

1) School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189,China 2) School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, China 3) School of Science, Hebei University of Engineering, Handan 056038, China

... in 1991^{[3], [4], [5], [6], [7], [8], [9], [10], [11] and [12]} ...

2011

1.198

0.293

Ranjusha

, P.

Lekha

, K. R. V.

Subramanian

, V.

Nair Shantikumar

, A.

Balakrishnan

, J. Mater. Sci. Technol. , 27 (2011), pp. 961–966

Vertical ZnO nanotube (ZNT) arrays were synthesized onto an indium doped tin oxide (ITO) glass substrate by a simple electrochemical deposition technique followed by a selective etching process. Scanning electron microscopy (SEM) showed formation of well-faceted hexagonal ZNT arrays spreading uniformly over a large area. X-ray diffraction (XRD) of ZNT layer showed substantially higher intensity for the (0002) diffraction peak, indicating that the ZnO crystallites were well aligned with their c -axis. Profilometer measurements of the ZNT layer showed an average thickness of ~7μm. Diameter size distribution (DSD) analysis showed that ZNTs exhibited a narrow diameter size distribution in the range of 65-120 nm and centered at ~75 nm. The photoluminescence (PL) spectrum measurement showed violet and blue luminescence peaks that were centered at 410 and 480 nm, respectively, indicating the presence of internal defects. Ultra-violet (UV) spectroscopy showed major absorbance peak at ~348 nm, exhibiting an increase in energy gap value of 3.4 eV. By employing the formed ZNTs as the photo-anode for a dye-sensitized solar cell (DSSC), a full-sun conversion efficiency of 1.01% was achieved with a fill factor of 54%. Quantum efficiency studies showed the maximum of incident photon-to-electron conversion efficiency in a visible region located at 520{550 nm range.

Nano Solar Division, Amrita Center for Nanosciences, Kochi 682 041, Kerala, India

... in 1991^{[3], [4], [5], [6], [7], [8], [9], [10], [11] and [12]} ...

2011

1.149

0.0

... One-dimensional TiO₂ nanostructures such as TiO₂ nanotube arrays have attracted much attention as the photo-anode material in DSSCs due to their ordered vertical structure which facilitates filling of new sensitizer molecules or non-volatile electrolytes for improved efficiency and stability of DSSCs, while conventional mesoporous TiO₂ film has a disordered structure that makes infiltration of larger sensitizer molecules or viscous electrolytes difficult and ineffective^{[13], [14], [15], [16], [17], [18], [19], [20], [21] and [22]} ...

2005

35.749

0.0

Law

, L. E.

Greene

, J. C.

Johnson

, R.

Saykally

, P. D.

Yang

, Nat. Mater. , 4 (2005), pp. 455–459

Excitonic solar cells 1 —including organic, hybrid organic–inorganic and dye-sensitized cells (DSCs)—are promising devices for inexpensive, large-scale solar energy conversion. The DSC is currently the most efficient 2 and stable 3 excitonic photocell. Central to this device is a thick nanoparticle film that provides a large surface area for the adsorption of light-harvesting molecules. However, nanoparticle DSCs rely on trap-limited diffusion for electron transport, a slow mechanism that can limit device efficiency, especially at longer wavelengths. Here we introduce a version of the dye-sensitized cell in which the traditional nanoparticle film is replaced by a dense array of oriented, crystalline ZnO nanowires. The nanowire anode is synthesized by mild aqueous chemistry and features a surface area up to one-fifth as large as a nanoparticle cell. The direct electrical pathways provided by the nanowires ensure the rapid collection of carriers generated throughout the device, and a full Sun efficiency of 1.5% is demonstrated, limited primarily by the surface area of the nanowire array.

2011

3.499

0.0

J. Y.

Liao

, B. X.

Lei

, Y. F.

Wang

, J. M.

Liu

, C. Y.

, D. B.

Kuang

, Chem. Eur. J. , 17 (2011), pp. 1352–1357

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275 (P.R. China), Fax: (+86) 20-8411-3015 * MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275 (P.R. China), Fax: (+86) 20-8411-3015

> One-dimensional and quasi-one-dimensional semiconductor nanostructures are desirable for dye-sensitized solar cells (DSSCs), since they can provide direct pathways for the rapid collection of photogenerated electrons, which could improve the photovoltaic performance of the device. Quasi-1D single-crystalline anatase TiO 2 nanostructures have been successfully prepared on transparent, conductive fluorine-doped tin oxide (FTO) glass with a growth direction of [101] through a facile hydrothermal approach. The influences of the initial titanium n -butoxide (TBT) concentration, hydrothermal reaction temperature, and time on the length of quasi-1D anatase TiO 2 nanostructures and on the photovoltaic performance of DSSCs have been investigated in detail. A power conversion efficiency of 5.81 % has been obtained based on the prepared TiO 2 nanostructure photoelectrode 6.7 μm thick and commercial N719 dye, with a short-circuit current density of 13.3 mA cm −2 , an open-circuit voltage of 810 mV, and a fill factor of 0.54.

2010

4.814

0.0

B. X.

Lei

, J. Y.

Liao

, R.

Zhang

, J.

Wang

, C. Y.

, D. B.

Kuang

, J. Phys. Chem. C, 114 (2010), pp. 15228–15233

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, People’s Republic of China * To whom correspondence should be addressed. Phone: +86 20 84113015 . Fax: +86-20-84113015 . E-mail address: kuangdb@mail.sysu.edu.cn .

The fabrication of highly ordered one-dimensional TiO 2 nanotube arrays on transparent conductive glass is of key interest for constructing dye-sensitized solar cells (DSSCs) from the front-side illumination mode. We report the formation of large-scale free-standing TiO 2 nanotube arrays via sonication of a TiO 2 nanotube on Ti foil prepared by an anodization process, which can be further transferred to the fluorine-doped tin oxide conductive glass via a drop of TiO 2 sol containing Ti(OBu) 4 and polyethylene glycol. The photovoltaic performance of DSSCs based on 20.8 μm length TiO 2 nanotube arrays on FTO glass reached 8.07%, which is higher than that of a TiO 2 nanoparticle electrode (7.58%) because of the reduced electron combination and efficient light-harvesting efficiency for the former. It is also observed that the power conversion efficiency of DSSCs measured from the front-side illumination mode (8.07%) is higher than that of the back-side illumination mode (7.29%) owing to the light absorption by the iodine electrolyte and light reflection by the Pt counter electrode for the latter.

2007

4.814

0.0

T. P.

Chou

, Q.

Zhang

, G.

Cao

, J. Phys. Chem. C, 111 (2007), pp. 18804–18811

Materials Science and Engineering, University of Washington, 302 Roberts Hall, Box 352120, Seattle, Washington 98195

In this paper, we report the significant effects of dye loading conditions on the overall light conversion efficiency of zinc oxide (ZnO) film electrodes in dye-sensitized solar cells. A comparison of the ZnO film electrodes was also made with TiO 2 film electrodes prepared with similar dye loading conditions. It was found that using a higher and lower dye concentration requires a shorter and longer immersion time, respectively, for optimal sensitization of ZnO to obtain maximum efficiencies. A similar trend was found for the TiO 2 film electrode as well; however, smaller differences in the overall light conversion efficiencies were observed with varying dye concentration and immersion time. It was found that the chemical stability was an issue for the ZnO film electrodes but was not pertinent for the TiO 2 film electrodes. The film quality and structure of the ZnO film differed after prolonged immersion in high dye concentration, where the formation of N3 dye and Zn 2+ aggregates and/or the deterioration of the ZnO colloidal spheres and nanoparticles on the surface may have occurred. On the other hand, the film quality and structure of the TiO 2 film was not appreciably affected by prolonged immersion in high dye concentration, where the nanoparticle structure was not affected.

2006

13.025

0.0

G. K.

Mor

, K.

Shankar

, M.

Paulose

, O. K.

Varghese

, C. A.

Grimes

, Nano Lett. , 6 (2006), pp. 215–218

Department of Electrical Engineering and Department of Material Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802

We describe the use of highly ordered transparent TiO 2 nanotube arrays in dye-sensitized solar cells (DSCs). Highly ordered nanotube arrays of 46-nm pore diameter, 17-nm wall thickness, and 360-nm length were grown perpendicular to a fluorine-doped tin oxide-coated glass substrate by anodic oxidation of a titanium thin film. After crystallization by an oxygen anneal, the nanotube arrays are treated with TiCl 4 to enhance the photogenerated current and then integrated into the DSC structure using a commercially available ruthenium-based dye. Although the negative electrode is only 360-nm-thick, under AM 1.5 illumination the generated photocurrent is 7.87 mA/cm 2 , with a photocurrent efficiency of 2.9%. Voltage-decay measurements indicate that the highly ordered TiO 2 nanotube arrays, in comparison to nanoparticulate systems, have superior electron lifetimes and provide excellent pathways for electron percolation. Our results indicate that remarkable photoconversion efficiencies may be obtained, possibly to the ideal limit of 31% for a single photosystem scheme, with an increase of the nanotube-array length to several micrometers.

2007

13.025

0.0

Zhu

, T. B.

Vinzant

, N. R.

Neale

, A. J.

Frank

, Nano Lett. , 7 (2007), pp. 3739–3746

National Renewable Energy Laboratory, Golden, Colorado 80401-3393

We report on the influence of morphological disorder, arising from bundling of nanotubes (NTs) and microcracks in films of oriented TiO 2 NT arrays, on charge transport and recombination in dye-sensitized solar cells (DSSCs). Capillary stress created during evaporation of liquids from the mesopores of dense TiO 2 NT arrays was of sufficient magnitude to induce bundling and microcrack formation. The average lateral deflection of the NTs in the bundles increased with the surface tension of the liquids and with the film thicknesses. The supercritical CO 2 drying technique was used to produce bundle-free and crack-free NT films. Charge transport and recombination properties of sensitized films were studied by frequency-resolved modulated photocurrent/photovoltage spectroscopies. Transport became significantly faster with decreased clustering of the NTs, indicating that bundling creates additional pathways via intertube contacts. Removing such contacts alters the transport mechanism from a combination of one and three dimensions to the expected one dimension and shortens the electron-transport pathway. Reducing intertube contacts also resulted in a lower density of surface recombination centers by minimizing distortion-induced surface defects in bundled NTs. A causal connection between transport and recombination is observed. The dye coverage was greater in the more aligned NT arrays, suggesting that reducing intertube contacts increases the internal surface area of the films accessible to dye molecules. The solar conversion efficiency and photocurrent density were highest for DSSCs incorporating films with more aligned NT arrays owing to an enhanced light-harvesting efficiency. Removing structural disorder from other materials and devices consisting of nominally one-dimensional architectures (e.g., nanowire arrays) should produce similar effects.

2010

6.233

0.0

2012

6.233

0.0

2012

1.198

0.293

Chang

, L.

, X.

, W.

, J. Mater. Sci. Technol. , 28 (2012), pp. 594–598

Rutile TiO 2 nanowire array films with multi-walled carbon nanotube (MWCNT) inclusion perpendicularly grown on fluorine-doped tin oxide (FTO) substrate were prepared by a facile hydrothermal method. The absorption edges of the TiO2 nanowire array films are blue-shifted with increasing MWCNT content. The resistance of the TiO2 nanowire array film is decreased by MWCNT inclusion. The optimum TiO 2 /MWCNT molar ratio in the feedstock is 1:0.1. For the TiO 2 nanowire array film with MWCNT inclusion served as electrode in dye-sensitized solar cell (DSSC), an overall 194% increase of photoelectric conversion efficiency has been achieved.

1) Key Laboratory of Renewable Energy and Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China 2) Graduate University of Chinese Academy of Sciences, Beijing 100049, China 3) Environment Engineering Co. Ltd, China Shipbuilding Industry Corporation, Wuhan 430064, China

1961

0.71

0.0

... However, DSSC based on TiO₂ nanotube array exhibits limited energy conversion efficiency owning to its lower surface area than mesoporous TiO₂ film^{[23], [24], [25] and [26]} ...

2010

9.765

0.0

H. J.

Snaith

, Adv. Funct. Mater. , 20 (2010), pp. 13–19

Department of Physics Clarendon Laboratory University of Oxford Parks Road, Oxford OX1 3PU (UK) * Department of Physics Clarendon Laboratory University of Oxford Parks Road, Oxford OX1 3PU (UK).

> For an ideal solar cell, a maximum solar-to-electrical power conversion efficiency of just over 30% is achievable by harvesting UV to near IR photons up to 1.1 eV. Dye-sensitized solar cells (DSCs) are, however, not ideal. Here, the electrical and optical losses in the dye-sensitized system are reviewed, and the main losses in potential from the conversion of an absorbed photon at the optical bandgap of the sensitizer to the open-circuit voltage generated by the solar cell are specifically highlighted. In the first instance, the maximum power conversion efficiency attainable as a function of optical bandgap of the sensitizer and the “loss-in-potential” from the optical bandgap to the open-circuit voltage is estimated. For the best performing DSCs with current technology, the loss-in-potential is ∼0.75 eV, which leads to a maximum power-conversion efficiency of 13.4% with an optical bandgap of 1.48 eV (840 nm absorption onset). Means by which the loss-in-potential could be reduced to 0.4 eV are discussed; a maximum efficiency of 20.25% with an optical bandgap of 1.31 eV (940 nm) is possible if this is achieved.

... However, DSSC based on TiO₂ nanotube array exhibits limited energy conversion efficiency owning to its lower surface area than mesoporous TiO₂ film^{[23], [24], [25] and [26]} ...

2011

14.829

0.0

Zhuge

, J.

Qiu

, X.

Gao

, X.

Gan

, W.

, Adv. Mater. , 23 (2011), pp. 1330–1334

State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences (SIC, CAS); Dingxi Rd. 1295, Shanghai, 200050, P. R. China * State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences (SIC, CAS); Dingxi Rd. 1295, Shanghai, 200050, P. R. China.

... However, DSSC based on TiO₂ nanotube array exhibits limited energy conversion efficiency owning to its lower surface area than mesoporous TiO₂ film^{[23], [24], [25] and [26]} ...

2011

3.794

0.0

... However, DSSC based on TiO₂ nanotube array exhibits limited energy conversion efficiency owning to its lower surface area than mesoporous TiO₂ film^{[23], [24], [25] and [26]} ...

1.067

0.0

... Numerous approaches and strategies have been investigated so as to enhance photoelectric conversion efficiency of DSSC by surface modification with noble metal^{[27], [28] and [29]}, using semiconductor quantum dots^{[30] and [31]}, developing new dyes^{[32], [33] and [34]}, and designing new morphology^{[35], [36] and [37]} ...

2000

0.0

Westphalen

, U.

Kreibig

, J.

Rostalski

, H.

Luth

, D.

Meissner

, Sol. Energy Mater. Sol. Cells, 61 (2000), pp. 97–105

2008

3.794

0.0

2010

0.0

Mora-Sero

, J.

Bisquert

, J. Phys. Chem. Lett. , 1 (2010), pp. 3046–3052

Grup de Dispositius Fotovoltaics i Optoelectrònics, Departament de Física, Universitat Jaume I, 12071 Castelló, Spain *To whom correspondence should be addressed. E-mail: sero@fca.uji.es .

Semiconductor sensitized solar cells have attracted growing interest in the past few years. Starting from quite low conversion efficiencies, these have grown very rapidly to values of around 4−5%. This Perspective analyzes the optimization of three aspects toward an increase in cell performance, (i) materials, including not only light-absorbing material but also electron and hole conductors and counter electrodes, (ii) control of recombination and band alignment by surface treatments, and (iii) development of absorbing nanocomposites with enhanced light-harvesting and -collecting properties. We argue that these key topics could promote major breakthroughs in the design and development of semiconductor-sensitized solar cells.

2010

0.0

S. J. B.

Sambur

, T.

Novet

, B. A.

Parkinson

, Science, 330 (2010), pp. 63–66

1 Department of Chemistry and School of Energy Resources, University of Wyoming, Laramie, WY 80271, USA. 2 Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA. 3 Voxtel Incorporated, Beaverton, OR 97006, USA. ↵ *To whom correspondence should be addressed. E-mail: bparkin1@uwyo.edu

2009

20.833

0.0

Grätzel

, Acc. Chem. Res. , 42 (2009), pp. 1788–1798

Laboratory of Photonics and Interfaces, Institute of Chemical Science and Engineering, Faculty of Basic Science, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland * To whom correspondence should be addressed. E-mail: michael.graetzel@epfl.ch .

Perhaps the largest challenge for our global society is to find ways to replace the slowly but inevitably vanishing fossil fuel supplies by renewable resources and, at the same time, avoid negative effects from the current energy system on climate, environment, and health. The quality of human life to a large degree depends upon the availability of clean energy sources. The worldwide power consumption is expected to double in the next 3 decades because of the increase in world population and the rising demand of energy in the developing countries. This implies enhanced depletion of fossil fuel reserves, leading to further aggravation of the environmental pollution. As a consequence of dwindling resources, a huge power supply gap of 14 terawatts is expected to open up by year 2050 equaling today’s entire consumption, thus threatening to create a planetary emergency of gigantic dimensions. Solar energy is expected to play a crucial role as a future energy source. The sun provides about 120 000 terawatts to the earth’s surface, which amounts to 6000 times the present rate of the world’s energy consumption. However, capturing solar energy and converting it to electricity or chemical fuels, such as hydrogen, at low cost and using abundantly available raw materials remains a huge challenge. Chemistry is expected to make pivotal contributions to identify environmentally friendly solutions to this energy problem. One area of great promise is that of solar converters generally referred to as “organic photovoltaic cells” (OPV) that employ organic constituents for light harvesting or charge carrier transport. While this field is still in its infancy, it is receiving enormous research attention, with the number of publications growing exponentially over the past decade. The advantage of this new generation of solar cells is that they can be produced at low cost, i.e., potentially less than 1 U.S. $/peak watt. Some but not all OPV embodiments can avoid the expensive and energy-intensive high vacuum and materials purification steps that are currently employed in the fabrication of all other thin-film solar cells. Organic materials are abundantly available, so that the technology can be scaled up to the terawatt scale without running into feedstock supply problems. This gives organic-based solar cells an advantage over the two major competing thin-film photovoltaic devices, i.e., CdTe and CuIn(As)Se, which use highly toxic materials of low natural abundance. However, a drawback of the current embodiment of OPV cells is that their efficiency is significantly lower than that for single and multicrystalline silicon as well as CdTe and CuIn(As)Se cells. Also, polymer-based OPV cells are very sensitive to water and oxygen and, hence, need to be carefully sealed to avoid rapid degradation. The research discussed within the framework of this Account aims at identifying and providing solutions to the efficiency problems that the OPV field is still facing. The discussion focuses on mesoscopic solar cells, in particular, dye-sensitized solar cells (DSCs), which have been developed in our laboratory and remain the focus of our investigations. The efficiency problem is being tackled using molecular science and nanotechnology. The sensitizer constitutes the heart of the DSC, using sunlight to pump electrons from a lower to a higher energy level, generating in this fashion an electric potential difference, which can exploited to produce electric work. Currently, there is a quest for sensitizers that achieve effective harnessing of the red and near-IR part of sunlight, converting these photons to electricity better than the currently used generation of dyes. Progress in this area has been significant over the past few years, resulting in a boost in the conversion efficiency of the DSC that will be reviewed.

2007

0.0

M. K.

Nazeeruddin

, T.

Bessho

, L.

Cevey

, S.

Ito

, C.

Klein

, F.

De Angelis

, S.

Fantacci

, P.

Comte

, P.

Liska

, H.

Imai

, M.

Grätzel

, J. Photochem. Photobiol. A, 185 (2007), pp. 331–337

2003

35.749

0.0

Wang

, S. M.

Zakeeruddin

, J. E.

Moser

, M. K.

Nazeeruddin

, T.

Sekiguchi

, M.

Grätzel

, Nat. Mater. , 2 (2003), pp. 402–407

Dye-sensitized nanocrystalline solar cells (DSC) have received considerable attention as a cost-effective alternative to conventional solar cells. One of the main factors that has hampered widespread practical use of DSC is the poor thermostability encountered so far with these devices. Here we show a DSC with unprecedented stable performance under both thermal stress and soaking with light, matching the durability criteria applied to silicon solar cells for outdoor applications. The cell uses the amphiphilic ruthenium sensitizer cis -RuLL′(SCN) 2 (L = 4,4′-dicarboxylic acid-2,2′-bipyridine, L′ = 4,4′-dinonyl-2,2′-bipyridine) in conjunction with a quasi-solid-state polymer gel electrolyte, reaching an efficiency of >6% in full sunlight (air mass 1.5, 100 mW cm −2 ). A convenient and versatile new route is reported for the synthesis of the heteroleptic ruthenium complex, which plays a key role in achieving the high-temperature stability. Ultramicroelectrode voltammetric measurements show that the triiodide/iodide couple can perform charge transport freely in the polymer gel. The cell sustained heating for 1,000 h at 80 °C, maintaining 94% of its initial performance. The device also showed excellent stability under light soaking at 55 °C for 1,000 h in a solar simulator (100 mW cm −2 ) equipped with a ultraviolet filter. The present findings should foster widespread practical application of dye-sensitized solar cells.

2012

4.814

0.0

Luan

, D.

Guan

, Y.

Wang

, J. Phys. Chem. C, 116 (2012), pp. 14257–14263

Department of Mechanical Engineering, Louisiana State University , Baton Rouge, Louisiana 70803, United States *E-mail: ywang@lsu.edu , Tel: 225-578-8577 , Fax: 225-578-5924 .

We report fast synthesis of bamboo-type TiO 2 nanotube arrays via anodization of Ti in nonhazardous electrolyte under alternating voltage condition for applications as photoanodes in high-efficiency dye-sensitized solar cells (DSSCs). Formation mechanism of bamboo-type nanotubes is also explored. The low-voltage step reduces pH and ion-diffusion gradient inside TiO 2 nanotubes and induces formation of bamboo ridges on outer tube walls when a second high-voltage step is conducted. Ridge spacing and length of bamboo-type nanotubes can be facilely tuned by adjusting time of high-voltage step and electrolyte composition. All DSSCs based on bamboo-type TiO 2 nanotube arrays show higher efficiencies than those based on smooth-walled nanotubes with the same tube length due to enhanced surface area of bamboo-type nanotubes for dye loading. It is also noted that DSSC efficiency increases with the ridge density of bamboo-type nanotubes of the same length. For example, DSSC based on bamboo-type nanotubes (8 μm long) with the highest ridge density exhibits an efficiency of 5.64%, higher than the efficiency of 3.90% exhibited by DSSC based on smooth-walled nanotubes of the same length. Moreover, the DSSC efficiency can be further increased to 6.8% by growing significantly longer bamboo-type TiO 2 nanotubes (16.5 μm long) via decreasing water content in electrolyte.

2011

0.0

Zhang

, C.

, X.

Yang

, W.

Cao

, J. Inorg. Organomet. Polym. Mater. , 21 (2011), pp. 43–49

1. State Key Laboratory of Chemical Resource Engineering, Institute of Modern Catalysis, Beijing University of Chemical Technology, Beijing, 100029, China 2. Hainan Institute of Science and Technology, Haikou, 571126, China 3. BOE Technology Group Co., Ltd., Beijing, 100029, China

Zn,N-TiO 2 powder was prepared using TiCl 4 as the precursor. XRD, TEM and CV characterizations show Zn,N have been doped into nano-crystal TiO 2 successfully, and the absorbance range of TiO 2 anode red-shift to the visible region. Zn,N doped-TiO 2 anode film was first prepared by combined technology. Ultraviolet–Visible (UV–Vis) chararterization proved there is a complementarity in UV–Vis absorbance range between poly (3-dodecylthiophene) (P3DDT) and N719. Solar cell based on Zn,N-TiO 2 was first co-sensitized by P3DDT and N719. A solar-to-electric energy conversion efficiency of 3.25% was attained with the system. The photoelectric conversion efficiency, V oc and J sc have been all increased compare to the single dye-sensitized solar cell.

2010

0.0

Han

, J.

Zhang

, X.

Yang

, H.

Zhu

, W.

Cao

, Sol. Energy Mater. Sol. Cells, 94 (2010), pp. 755–760

2010

35.749

0.0

H. A.

Atwater

, A.

Polman

, Nat. Mater. , 9 (2010), pp. 205–213

The emerging field of plasmonics has yielded methods for guiding and localizing light at the nanoscale, well below the scale of the wavelength of light in free space. Now plasmonics researchers are turning their attention to photovoltaics, where design approaches based on plasmonics can be used to improve absorption in photovoltaic devices, permitting a considerable reduction in the physical thickness of solar photovoltaic absorber layers, and yielding new options for solar-cell design. In this review, we survey recent advances at the intersection of plasmonics and photovoltaics and offer an outlook on the future of solar cells based on these principles.

... ^[38] recently outlined three types of plasmonic light-trapping geometries for incorporating metal nanoparticles: i) Far-field scattering ...

1998

3.794

0.0

... Till now, surface plasmon arising from metal nanoparticles has mostly been applied to increase optical absorption and/or photocurrent in the field of silicon solar cells^{[39], [40], [41] and [42]} and organic solar cells^{[43] and [44]} ...

2005

3.794

0.0

2006

3.794

0.0

2007

0.71

0.0

2011

12.062

0.0

J. L.

, F. C.

Chen

, Y. S.

Hsiao

, F. C.

Chien

, P.

Chen

, C. H.

Kuo

, M. H.

Huang

, C. S.

Hsu

, ACS Nano, 5 (2011), pp. 959–967

† Department of Photonics and Institute of Electro-optical Engineering, National Chiao Tung University , Hsinchu 30010, Taiwan ‡ Department of Photonics and Display Institute, National Chiao Tung University , Hsinchu 30010, Taiwan § Research Center for Applied Sciences, Academia Sinica , Taipei 11529, Taiwan Department of Chemistry, National Tsing Hua University , Hsinchu 3001, Taiwan Department of Applied Chemistry, National Chiao Tung University , Hsinchu 30010, Taiwan *Address correspondence to fcchen@mail.nctu.edu.tw .

We have systematically explored how plasmonic effects influence the characteristics of polymer photovoltaic devices (OPVs) incorporating a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM). We blended gold nanoparticles (Au NPs) into the anodic buffer layer to trigger localized surface plasmon resonance (LSPR), which enhanced the performance of the OPVs without dramatically sacrificing their electrical properties. Steady state photoluminescence (PL) measurements revealed a significant increase in fluorescence intensity, which we attribute to the increased light absorption in P3HT induced by the LSPR. As a result, the rate of generation of excitons was enhanced significantly. Furthermore, dynamic PL measurements revealed that the LSPR notably reduced the lifetime of photogenerated excitons in the active blend, suggesting that interplay between the surface plasmons and excitons facilitated the charge transfer process. This phenomenon reduced the recombination level of geminate excitons and, thereby, increased the probability of exciton dissociation. Accordingly, both the photocurrents and fill factors of the OPV devices were enhanced significantly. The primary origin of this improved performance was local enhancement of the electromagnetic field surrounding the Au NPs. The power conversion efficiency of the OPV device incorporating the Au NPs improved to 4.24% from a value of 3.57% for the device fabricated without Au NPs.

2004

0.71

0.0

2009

2.024

0.0

C. S.

Chou

, R. Y.

Yang

, C. K.

Yeh

, Y. J.

Lin

, Powder Technol. , 194 (2009), pp. 95–105

... synthesized TiO₂/Ag composites via dry coating process and investigated their applications in DSSCs ^[45] ...

2012

2.04

0.0

Agarwala

, G. W.

, J. Solid State Chem. , 189 (2012), pp. 101–107

... ^[46] also prepared Ag nanoparticles onto TiO₂ nanotubes by using a photo reduction method, to scatter light and generate surface plasmon effect ...

2010

2.966

0.0

... The generated photocurrent was found to be 36% higher than that of randomly distributed Ag particles prepared by photocatalytic reduction ^[47] ...

1980

0.223

0.0

... hence, it is not a suitable method for producing Ag nanoparticles ^[48] ...

2004

0.0

L. Y.

Zeng

, S. Y.

Dai

, K. J.

Wang

, X.

Pan

, C. W.

Shi

, L.

Guo

, Chin. Phys. Lett. , 21 (2004), pp. 1835–1837

sydai@ipp.ac.cn 1 Institute of Plasma Physics, Chinese Academy ofSciences, PO Box 1126, Hefei 230031 2 School of Chemical Engineering, Hefei University ofTechnology, Hefei 230009

Performance of dye-sensitized solar cells can be improved by treating the nanoporous TiO 2 films with titanium tetrachloride (TiCl 4 ) aqueous solution. We explore the reason why the performance of dye-sensitized solar cells is enhanced by this method. It is found that the effect of TiCl 4 treatment not only reduces the films surface area and improves the electronic contact, but also enhances the binding of N719 with the TiO 2 films surface.

... It has been reported that TiCl₄ treatment can enhance dye absorption on the surface, improve the electron transport properties in nanocrystalline TiO₂ films, and increase the charge separation efficiency^{[49], [50] and [51]} ...

1997

2.107

0.0

2006

3.607

0.0

P. M.

Sommeling

, B. C.

O'Regan

, R. R.

Haswell

, H. J. P.

Smit

, N. J.

Bakker

, J. J. T.

Smits

, J. M.

Kroon

, J. A. M.

van Roosmalen

, J. Phys. Chem. B, 110 (2006), pp. 19191–19197

Energy Research Centre of The Netherlands (ECN), Unit Solar Energy, Westerduinweg 3, 1755 LE Petten, The Netherlands, and Shell Global Solutions, Badhuisweg 3, CM 1031 Amsterdam, The Netherlands

In this study, the influence of the TiCl 4 post-treatment on nanocrystalline TiO 2 films as electrodes in dye-sensitized solar cells is investigated and compared to nontreated films. As a result of this post-treatment cell efficiencies are improved, due to higher photocurrents. On a microscopic scale TiO 2 particle growth on the order of 1 nm is observed. Despite a corresponding decrease of BET surface area, more dye is adsorbed onto the oxide surface. Although it seems trivial to match this finding with the improved photocurrent, this performance improvement cannot be attributed to higher dye adsorption only. This follows from comparison between incident photon to current conversion efficiency (IPCE) and light absorption characteristics. Since the charge transport properties of the TiO 2 films are already more than sufficient without treatment, the increase in short circuit current density J SC cannot be related to improvements in charge transport either. Transient photocurrent measurements indicate a shift in the conduction band edge of the TiO 2 upon TiCl 4 treatment. It is concluded that the main contribution to enhanced current originates from this shift in conduction band edge, resulting in improved charge injection into the TiO 2 .

2012

5.024

0.0

... ^[52] pointed out that plasmon excited by radical light, which are emitted radially from the impact position, are lost through absorption unless they are converted to a target ...