A chitosan binder-based TiO2 photoelectrode is used in dye-sensitized solar cells (DSSCs). Field-emission scanning electron microscope (FE-SEM) images revealed that the grain size, thickness, and distribution of TiO2 films are affected by the chitosan content. With addition of 2.0 wt% chitosan to the TiO2 film (D2), the surface pore size became the smallest, and the pores were fairly evenly distributed. The electron transit time, electron recombination lifetime, diffusion coefficient, and diffusion length were analyzed by IMVS and IMPS. The best DSSC, with 2.0 wt% chitosan addition to the TiO2 film, had a shorter electron transit time, longer electron recombination lifetime, and larger diffusion coefficient and diffusion length than the other samples. The results of 2.0 wt% chitosan-added TiO2 DSSCs are an electron transit time of
Since the Grätzel group discovered dye-sensitized solar cells (DSSCs), many researchers have become interested in them [
In this present research, chitosan was adopted as a new electrode binder for DSSCs. Chitosan is a polysaccharide composed mainly of
The chitosan used in this study was kindly supplied by Sehwa Co., Korea. The degree of deacetylation and molecular weight of the chitosan were 85% and 5.2 × 105 g/mol, respectively. Chitosan sol with different concentrations (1.5, 2.0, 2.5, and 3.0 wt%) were prepared by dissolving the proper amount of chitosan in 2 mL of 3% (v/v) aqueous acetic acid solution. The solution was mixed using a shaking incubator for 24 h at 250 rpm, and then the solution was left to stand for 24 h at room temperature, for complete hydration of the polymer and removal of bubbles.
The chitosan-based TiO2 paste was prepared in the following way: nitric acid treated TiO2 (P-25, Deagesa) power was added to 2 mL of prepared chitosan colloidal solution, and then stirring was maintained until the TiO2 colloid was well mixed with the chitosan solution slurry. For increased dispersal of the TiO2 paste, a three-roll mill (DEA WHA TECH., EXAKT50i) was used for about 7 h. The three-roll mill makes it possible to simultaneously mill and mix the paste. Table
Composition of the as-prepared chitosan sol-based TiO2 pastes used in this study.
Sample name | Chitosan |
TiO2 |
Water |
Acetic acid |
---|---|---|---|---|
D1 | 0.03 | 1 | 2 | 0.06 |
D2 | 0.04 | 1 | 2 | 0.06 |
D3 | 0.05 | 1 | 2 | 0.06 |
D4 | 0.06 | 1 | 2 | 0.06 |
The prepared TiO2 paste was cast on precleaned FTO (Pilkington FTO glass, 8 Ω/cm2), using the squeeze printing method [
The Pt electrode was placed over the dye-adsorbed TiO2 electrode, and the edges of the cell were sealed. The sealing was accomplished by hot-pressing two electrodes together at 110°C. The redox electrolyte was injected into the cell through two small holes drilled in the counter electrode. The
The thermogravimetric (TGA) analyses of the chitosan sol were performed with an RIS diamond TG-DTA (PerkinElmer) analyzer. The chitosan sol was loaded into alumina pans and heated from room temperature to 530°C.
Field-emission scanning electron microscopy (FE-SEM) (Hitachi S-4700, Japan) was used to examine the film morphology, such as the surface of TiO2 films and thickness.
The electron transit time and electron recombination lifetime were measured by intensity modulated photocurrent spectroscopy (IMPS) and intensity modulated photovoltage spectroscopy (IMVS). Blue light-emitting diodes (LEDs, 475 nm) were used as the light source. The light intensities were modulated (10%), by modulating the bias applied to the LED with sine waves, in a frequency range typically from 0.1 Hz to 1000 Hz.
The photovoltaic properties were investigated by measuring the photocurrent-voltage characteristics under illumination, with an air mass (AM) of 1.5 (100 mW/cm2) simulated sunlight. The charge transport characteristics were investigated by intensity-modulated photovoltage spectroscopy (IMVS). The IMVS was measured using red light-emitting diodes (LED, 635 nm). The light intensities were modulated by 10%, in a frequency range typically from 0.01 to 100 Hz.
The thermal weight loss curves for the pure chitosan sol (2.0 wt%) and chitosan sol-based TiO2 paste (D2) are shown in Figure
Thermal weight loss (TGA) curve of the pure chitosan sol (2.0%) and chitosan sol-based TiO2 paste (D2).
The surface morphologies of the pure TiO2 photoelectrode and TiO2 photoelectrode containing 1.5 wt% (D1), 2.0 wt% (D2), 2.5 wt% (D3), and 3.0 wt% (D4) of chitosan were obtained by FE-SEM and are depicted in Figure
FE-SEM images of the surface morphologies of chitosan sol-based TiO2 films.
Figure
FE-SEM images of the cross-section of chitosan sol-based TiO2 films.
Figure
Photocurrent-voltage data of the chitosan sol-based DSSCs.
Sample | Parameter | |||
---|---|---|---|---|
|
|
FF (%) |
|
|
D1 | 0.69 | 9.48 | 58.58 | 3.83 |
D2 | 0.69 | 10.15 | 59.41 | 4.16 |
D3 | 0.68 | 9.59 | 60.35 | 3.94 |
D4 | 0.69 | 8.48 | 56.20 | 3.29 |
Charge transport parameter of the chitosan sol-based DSSCs.
Sample | Parameter | ||||
---|---|---|---|---|---|
Thickness |
|
|
|
|
|
D1 | 6.8 | 3.121 | 3.998 | 2.359 | 9.71 |
D2 | 6.9 | 2.189 | 6.336 | 3.463 | 14.81 |
D3 | 6.8 | 2.295 | 4.317 | 3.208 | 11.77 |
D4 | 6.6 | 4.056 | 2.522 | 1.710 | 6.57 |
Photocurrent-voltage curves of chitosan sol-based DSSCs.
Intensity modulation photocurrent spectroscopy (IMPS) and intensity modulation photovoltage spectroscopy (IMVS) of chitosan sol-based DSSCs.
Diffusion coefficient of chitosan sol-based DSSCs.
DSSCs were fabricated using different chitosan binder sol (1.5~3.0 wt%)-based photoelectrodes. The chitosan binder sol had low calcination temperature (~150°C) and successfully prepared low temperature TiO2 photoelectrodes. The D2 (~2.0 wt%)-based DSSCs had faster electron transit time and slower electron recombination time than the other samples (D1, D3, and D4). The D2-based DSSC exhibited higher solar conversion efficiencies than D1, D3, and D4-based DSSCs, because the electron recombination is more than 1.5~3 times slower in the D2-based DSSC. The electron diffusion coefficient of the D2-based DSSC was 3.463 × 10−5 cm2 s−1, and the diffusion length was 14.81
This research was supported by Specialized Local Industry Development Program through Korea Institute for Advancement of Technology (KIAT) funded by the Ministry of Trade, Industry and Energy (1415127923-R0002040). And the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (2012010655).