Effect of compressed TiO2 nanoparticle thin film thickness on the performance of dye-sensitized solar cells
© Tsai et al.; licensee Springer. 2013
Received: 14 September 2013
Accepted: 10 October 2013
Published: 5 November 2013
In this study, dye-sensitized solar cells (DSSCs) were fabricated using nanocrystalline titanium dioxide (TiO2) nanoparticles as photoanode. Photoanode thin films were prepared by doctor blading method with 420 kg/cm2 of mechanical compression process and heat treatment in the air at 500°C for 30 min. The optimal thickness of the TiO2 NP photoanode is 26.6 μm with an efficiency of 9.01% under AM 1.5G illumination at 100 mW/cm2. The efficiency is around two times higher than that of conventional DSSCs with an uncompressed photoanode. The open-circuit voltage of DSSCs decreases as the thickness increases. One DSSC (sample D) has the highest conversion efficiency while it has the maximum short-circuit current density. The results indicate that the short-circuit current density is a compromise between two conflict factors: enlargement of the surface area by increasing photoanode thickness and extension of the electron diffusion length to the electrode as the thickness increases.
KeywordsMechanism compression Thickness Dye-sensitized solar cells (DSSCs) TiO2 Doctor blading method
Most solar cells are fabricated using Si-based materials ; however, in recent years, new materials have been discovered to replace Si for applications in solar cells. A dye-sensitized solar cell (DSSC) [2–4] is one of the alternatives as it is low cost and lightweight and can be fabricated on flexible substrates to improve portability. DSSC also shows high energy conversion efficiency by using nanoparticle (NP) thin film as photoanode. The film has a nonporous structure, which has an extremely large specific surface area that enhances dye adsorption as well as light harvesting. Titania (TiO2) nanoparticle is stable and nontoxic and has relatively high transmittance in the visible spectrum, thus becomes a promising nanoparticle material for applications in DSSCs. The band gap of rutile- and anatase-phase TiO2 is 3.0 and 3.2 eV, respectively. The anatase phase is specially preferred due to its good photocatalytic properties and wide direct band gap [5, 6].
A dye-sensitized solar cell is composed of three main structures: (1) a dye sensitizer whose function is to harvest solar energy and generate excitons [7, 8], (2) a nanostructured metal oxide to transport electrons efficiently [9, 10], and (3) a redox electrolyte or hole-transporting material [11, 12]. The key element in a DSSC is the photoanode, which is composed of a thin film of TiO2 NPs. Though the nanoparticle thin film has a high specific surface area, electron percolation is hindered by limited interconnected NPs resulting in photoelectron loss due to recombination between the photoelectrons and the oxidized dye molecules or electron-accepting species in the electrolyte. To solve this issue, mechanical compression of the photoanode thin film was adopted to increase the effective interconnection between NPs. The optimal thickness of the mechanically compressed TiO2 nanoparticle thin film was reported.
Deposition of TiO2 thin film as photoanode
TiO2 paste (10 wt%) was prepared by mixing nanocrystalline TiO2 nanoparticles (TG-P25, Degussa, Shinjuku, Tokyo, Japan; the average nanoparticle diameter was about 25 to 30 nm) with tert-butyl alcohol and deionized water. The TiO2 paste was then scraped on a transparent fluorine-doped tin oxide (FTO) glass of 8-Ω/sq resistivity by doctor blading method. The films were mechanically compressed with a pressure of 420 kg/cm2. After the compression, the films were annealed in air by two consecutive steps: 150°C for 90 min and 500°C for 30 min. The 150°C annealing is to decompose residual organic compounds, and the 500°C annealing is to assist the interconnection of TiO2 NPs.
Characterizations and photoelectrochemical measurement
The structures and morphologies of the TiO2 NP thin films were studied using a field emission scanning electron microscope (FESEM; JSM-7500F, JEOL, Akishima-shi, Japan). The ultraviolet–visible (UV–vis) transmittance spectrum of the sample was observed using a UV–vis spectrophotometer (U-2900, Hitachi High-Technologies Corporation, Tokyo, Japan). Electrochemical impedance spectroscopy (EIS; Zahner Zennium, Kronach, Germany), which is a standard method to measure the current response under an ac voltage of various frequencies, was used to characterize the carrier transport behavior of the DSSCs. The frequencies ranged from 10 mHz to 100 kHz. The measurement was under illumination of air mass 1.5 global (AM 1.5G) at an applied bias of open-circuit voltage. The incident photon-to-current conversion efficiency (IPCE), which was determined by the light-harvesting efficiency of the dye, the quantum yield of electron injection, and the efficiency of collecting the injected electrons, was recorded using an IPCE instrument equipped with a 1,000-W xenon arc lamp as the light source composed of a compact 1/8-m monochromator (CM110, Spectral Products, Putnam, CT, USA), a color filter wheel (CFW-1-8, Finger Lakes Instrumentation, Lima, NY, USA), and a calibrated photodiode (FDS1010-CAL, Thorlabs Inc., Newton, NJ, USA). The IPCE data were taken using a source meter (2400, Keithley Instruments, Inc., Cleveland, OH, USA) with lluminating monochromatic light on the solar cells (with the wavelength from 300 to 800 nm). The current–voltage characteristics of the samples were measured using the Keithley 2400 source meter under a simulated sunlight (SAN-EI XES-40S1, San Ei Brand, Higashi-Yodogawa, Japan), with AM 1.5G radiation at 100 mW/cm2.
Results and discussion
Characteristics of DSSCs composed of the compressed TiO 2 NP thin film as photoanode
In general, the photocurrent density of DSSCs is influenced by three factors: (1) the number of photoexcited electrons, which is dominated by the adsorptive capacity of dye molecules, (2) the recombination rate at the interface of dye/TiO2 NP or TiO2 NP/electrolyte, and (3) the redox of I-/I3- in the electrolyte. In this study, the TiO2 NP thin film is compressed before heat treatment. The procedure enhances the interconnection between the NPs, hence decreases the recombination probability. The performance of the DSSCs is improved. Besides, a thick photoanode induces a large surface area enhancing dye molecules to adsorb on it. Hence, a thick photoanode captures more light to generate photoexcited electrons. However, the JSC requires that these electrons successfully transport to the FTO substrate (electrode) without recombination at the dye/photoanode or photoanode/electrolyte interfaces; therefore, electron diffusion length is also a key point that needs to be considered. Though a thick photoanode enhances the generation of photoexcited electrons, a long electron diffusion length is inevitable for those photoexcited electrons generated in the deep layer. Thus, the JSC is a compromise between the two conflict factors: enlarged surface area by increasing photoanode thickness and increased thickness resulting in a long electron diffusion length. The experimental results indicate that the optimized thickness is 26.6 nm. The probability of recombination of injected electrons and the iodides in the electrolyte is smallest in this case. Therefore, sample D has the highest photo-to-electron conversion efficiency of 9.01%. The results also agree with those of EIS and IPCE, as shown in the inset of Figure 6.
The effect of TiO2 NP photoanode thickness on the performance of the DSSC device was studied. The TiO2 NP photoanode thin film was fabricated by mechanical compression before thermal treatment. The final film was uniform and dense. The UV–vis spectrophotometer analysis indicates that the absorbance increases with the increase of the thickness of TiO2 NP thin film due to the large surface area enhancing the adsorption of dye molecules. However, the optimal incident photon-to-current conversion efficiency and total energy conversion efficiencies were found in the TiO2 NP photoanode film with a thickness of 26.6 μm under an incident light intensity of 100 mW/cm2. The results indicate that there are two conflict factors acting together so that an optimal thickness is observed. The two factors are as follows: (1) increasing the photoanode thickness could enlarge the surface area and enhance the adsorption of dye molecules which improves the light absorbance as well as the generation of photoexcited electrons and (2) a thick photoanode results in a long electron diffusion distance to the FTO substrate (electrode) which increases the probability of recombination and thus degrades the efficiencies.
Dye-sensitized solar cells
Electrochemical impedance spectroscopy
Field emission scanning electron microscopy
Fluorine-doped tin oxide
Incident photon-to-current conversion efficiency
Indium tin oxide
This work was partially supported by the National Science Council of Taiwan, the Republic of China, and Core Facilities Laboratory in Kaohsiung-Pingtung area.
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