Abstract
To improve the durability of dye-sensitized solar cells (DSCs), monolithic DSCs with ionic liquid electrolyte were studied. Deposited by screen printing, a carbon layer was successfully fabricated that did not crack or peel when annealing was employed beforehand. Optimized electrodes exhibited photovoltaic characteristics of 0.608 V open-circuit voltage, 6.90 cm−2 mA short-circuit current, and 0.491 fill factor, yielding 2.06% power conversion efficiency. The monolithic DSC using ionic liquid electrolyte was thermally durable and operated stably for 1000 h at 80°C.
1. Introduction
To meet the demand for renewable cost-effective energy sources, dye-sensitized solar cells (DSCs), which can be fabricated from low-cost materials (TiO2, dye, etc.) by nonvacuum printing, have attracted attention in academic and industrial research [1–4]. DSCs are normally fabricated by sandwiching TiO2 and Pt counter electrodes. F-doped tin oxide (FTO) glass substrates for these electrodes account for 80% of DSC-fabrication cost. Additionally, Pt used in the counter electrode is also expensive. Hence, to realize further cost reductions, the use of FTO glass and Pt should be minimized. Toward this end, monolithic DSCs have been fabricated on a single FTO glass substrate with a porous carbon counter electrode (Figure 1) [5–12]. A porous carbon layer and a porous ZrO2 layer are set on dyed porous TiO2 layer for the monolithic DSC. The porous ZrO2 layer has two functions: a function as transportation of electrolyte to dyed porous TiO2 layer and a function as spacing insulator to separate dyed TiO2 layer and carbon layer electrically. There are three roles in the porous carbon layer: (1) the catalyst to reduce I3− to I− for the photocurrent flow; (2) the electrical current corrector to external circuits; (3) the transportation of electrolyte to dyed-porous TiO2 and porous ZrO2. By means of capillary force within nanoholes, monolithic porous electrodes prevent electrolyte leakage. Through the use of such porous electrodes, highly durable DSCs are expected.
In research on durable DSCs, industrial applications are now being emphasized. Volatility of the DSC electrolyte has been a major problem because the interface temperature of the cell rises considerably in sunlight. To address the problem of thermal instability, the electrolyte should be nonvolatile and solvent-free, which are notable properties of ionic liquids. Accordingly, ionic liquids have been investigated for use as the electrolyte in DSCs in order to enhance their durability [13–15].
In this paper, we report a screen-printing method for fabricating porous carbon electrodes that do not crack or peel off in monolithic DSCs. In particular, we focused on carbon-paste preparation. Finally, ionic liquid electrolyte was used to prepare thermally durable monolithic DSCs that operated stably for 1000 h at 80°C.
2. Experiments
The process for fabricating carbon paste is presented in Figure 2. Carbon powder and TiO2 powder (P25; Degussa, Germany) were ground in a mortar. The carbon powder was a mixture of Printex L (Degussa) and graphite (Aldrich, Germany) or a mixture of Printex L and active carbon (Degussa, Germany). The total weight of carbon powder was 6 g. Water (10 mL) and ethanol (20 mL) were added to the mortar containing the powder mixture, which was further ground. This mixture was transferred from the mortar to a beaker with an additional 100 mL portion of ethanol. Then, the mixture was agitated with a magnetic stir bar for 1 min and an ultrasonic homogenizer (Vibra Cell 72408 Bioblock Scientific, USA) for 2 min. α-Terpineol (20 g, Tokyo Chemical Industry Co., Ltd., Japan) was added to the carbon dispersion and agitated with a magnetic stir bar for 1 min and an ultrasonic homogenizer for 2 min. Ethyl cellulose solution in ethanol (10 wt%, 30 g) was added to the carbon solution and agitated with a magnetic stir bar for 1 min and an ultrasonic homogenizer for 2 min. The ethyl cellulose solution was prepared beforehand by adding equivalent amounts of two ethyl cellulose powders in ethanol (5 wt% of ethyl cellulose no. 46080 and 5 wt% of ethyl cellulose no. 86480, Tokyo Chemical Industries, Co., Ltd., Japan). Removal of water and ethanol by evaporation at 40°C and 125 m bar afforded the carbon paste.
To fabricate monolithic electrodes, FTO glass plates (TEC-15, Nippon Sheet Glass Co., Ltd., Japan) were utilized. The FTO layer was divided into two parts by etching with Zn and HCl (see Figure 1). The etched FTO glass plates were washed with glass-cleaning detergent in an ultrasonic bath for 15 min. The substrates were then rinsed with water and ethanol. Subsequently, the surface of substrate was cleaned in a UV-O3 system for 18 min. Two types of TiO2 pastes—PST-30NRD and PST-400C (JGC-CCIC Co., Ltd., Japan)—were deposited by screen printing on one of the FTO layers (Figure 1) as a transparent nanocrystalline—TiO2 layer (10 μm thick) and a light-scattering layer (4 μm thick), respectively. The resulting TiO2 electrodes had a double-layered structure [16]. The substrate was sintered on a hotplate at 500°C for 2 h. When the substrate was cooled, ZrO2 paste was printed on the TiO2 layer to be 30 μm thick and to cover the porous TiO2 layer. The ZrO2 paste was dried on the hotplate at 125°C. The ZrO2 paste was prepared according to a published procedure [17] using ZrO2 nanoparticles ( = 40–50 nm; Fulka, USA). The substrate was heated on the hotplate at 200°C at 1 h. When the substrate cooled, carbon paste was printed on the ZrO2 layer to form a layer with thickness of 130 μm. At the same time, the carbon layer was connected to the other FTO layer and sintered at 400°C (Figure 1). The fabricated electrodes were immersed in dye solution (0.3 mM in acetonitrile and t-butyl alcohol; Z907 Ru-dye, Solaronix SA) and kept at room temperature for 12 h [18].
The dye-coated electrodes were rinsed with acetonitrile and combined with glass plates by heating a hot-melt glue film at 250°C for 1 min (150 μm thick; Bynel 4164, DuPont, USA). A drop of the electrolyte solution was placed into a hole drilled into the glass of the assembled cell and driven into the cell by vacuum backfilling. Finally, the hole was sealed using additional Bynel film and a glass cover slip (0.1 mm thick). The electrolyte was composed of 0.2 M I2, 0.5 M N-methylbenzimidazole, and 0.1 M guanidinium thiocyanate in PMII and EMIB(CN)4 (13 : 7 v/v), which has been used in highly durability DSCs [18, 19].
Photovoltaic measurements employed an AM 1.5 solar simulator. The power of the simulated light was calibrated to be 100 mW cm−2 by using a reference Si photodiode equipped with an IR-cutoff filter (BS-520, Bunkou-Keiki Co., Ltd., Japan) to limit mismatch in the region of 350–750 nm between the simulated light and AM1.5 to less than 2% [20, 21]. I-V curves were obtained by applying an external bias to the cell and measuring the generated photocurrent with a digital source meter (6240A, ADCMT, Japan). The thermal durability test was performed by keeping DSCs in oven at 80°C. The sheet resistances (four-point probe method), the X-day diffraction patterns, and the electrical impedance spectra were measured using Loresta-EP MCP-T360 (Mitsubishi Chemical Analytech Co., Ltd., Japan), MiniFlex II (Rigaku Co., Ltd., Japan), and SP-150 (Bio-Logic, France), respectively. The impedance spectra were analyzed using software (Z-view 2).
3. Results and Discussion
In past work, to prepare porous carbon counter electrodes for DSCs, colloidal amorphous TiO2 particles have been added as a binder for carbon particles [22]. In the present study, however, gas-synthesized TiO2 nanopowder (80% anatase/20% rutile, P25; Degussa) was added to carbon pastes instead of colloidal amorphous TiO2. When we used colloidal amorphous TiO2 nanoparticles to fabricate thicker porous carbon layers for monolithic DSCs, the annealed carbon layer easily cracked and peeled off the substrate, likely caused by crystallization and shrinkage of the amorphous TiO2 nanoparticles. On the other hand, the gas-synthesized TiO2 nanopowder gave a thicker (>100 μm), more stable porous carbon layer that did not crack or peel, because the gas-synthesized TiO2 nanopowder does not undergo further crystallization and shrinkage.
To optimize the conductivity of the porous carbon layer, the ratio of amorphous nanocarbon (Printex L) to graphite or the ratio of amorphous nanocarbon (Printex L) to active carbon was varied. A carbon layer of 13-14 μm in thickness was fabricated on the glass plate by the doctor blade method. The sheet resistance was measured by four-point probe method (Figure 3). The sheet resistance of amorphous nanocarbon (Printex L) layer was increased by adding the active carbon. In contrast, the sheet resistance of amorphous nanocarbon (Printex L) layer was decreased by adding the graphite, reaching a minimum of 90 Ω/□ at 40% amorphous nanocarbon in graphite (2.4 g amorphous nanocarbon (Printex L) and 3 g graphite). For the experiments below, carbon paste with this mixing ratio was used.
Figure 4 shows X-ray diffraction spectra of carbon pastes. The carbon paste containing graphite shows an intense peak attributed to the graphite structure, which is not seen in the paste using active carbon. Although the graphite can give high conductivity due to the high crystallinity, the graphite particle was so large that the connection between the graphite particles was not enough to achieve the high conductivity. On the other hand, since the amorphous nanocarbon (Printex L) was very small, the connection between the particles can be very strong. However, because of the low crystallinity of amorphous nanocarbon (Printex L), the pure amorphous nanocarbon layer was not able to improve the conductivity. Hence, the addition of amorphous carbon improved the connection between the graphite particles, resulting in the improvement of conductivity. These are the reasons why the minimum sheet resistance was obtained with 40% amorphous nanocarbon (Printex L) and 60% graphite as shown in Figure 3.
To fabricate porous carbon layers on porous TiO2/ZrO2 layers, the annealing process should be optimized. Under simple heating at 400°C for 30 min, the carbon layer cracked and peeled (Figure 5(a)). This cracking and peeling was found to occur on the ZrO2 layer, but not on the FTO glass plate. Therefore, peeling of the carbon layer was likely attributable to shrinkage of ZrO2. To prevent ZrO2 shrinkage from damaging the carbon layer, ZrO2 was pre-annealed at 200°C for 20 min. Thereafter, carbon paste was printed on the surface of the porous ZrO2 layer, resulting in a crack-free porous carbon layer (Figure 5(b)).
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Figure 6 shows the current-voltage curve of monolithic and standard sandwich-type DSCs. The Monolithic cell exhibited a short-circuit photocurrent density of 6.90 mA cm−2, open-circuit photovoltage of 0.608 V, and fill factor of 0.491, yielding a power conversion efficiency of 2.06% under AM1.5 irradiation. This photovoltaic performance was comparable to that of standard sandwich-type DSCs using Pt and FTO as counter electrodes (short-circuit photocurrent density, 6.91 mA cm−2; open-circuit photovoltage, 0.544 V; fill factor, 0.592; power conversion efficiency, 2.22%). The prepared monolithic DSCs performed similarly to standard sandwich-structured DSCs in terms of short-circuit photocurrent density. However, the fill factor of the monolithic cells was slight lower than that of standard sandwich-structured DSCs, because the resistance of the carbon layer was higher than that of the FTO layer.
Figures 7(a) and 7(b) show the impedance spectra of monolithic and sandwich-structured DSCs, respectively. The structure of a sandwich DSC was a standard one: <glass/FTO/porous TiO2/dye/electrolyte/Pt/FTO/glass> [16]. The spacer between FTO-glass substrates was 35 μm, where the electrolyte was filled without porous-ZrO2 layer. These spectra were analyzed using an equivalent circuit (Figure 7(c)), resulting in a series resistance (), parallel resistances (), and constant-phase elements (CPE, imperfect capacitance). Each element was summarized in Table 1. The CPE is a kind of capacitance which indicates the depressed semicircles to fit the experimental spectra. In monolithic DSC, the series resistance and interface resistance between the carbon electrode and electrolyte were 35.3 Ω and 10.5 Ω, respectively. For Pt/FTO counter electrodes, these values were 16.0 Ω and 6.24 Ω, respectively. In addition, the diffusion resistance in the electrolyte was increased from sandwich-structured DSC (7.04 Ω) to monolithic-structured DSC (33.1 Ω) due to the addition of the ZrO2 layer (30 μm). Thus, the deterioration of fill factor was due to the incensement of resistivity by the carbon counter electrode layer and the ZrO2 layer. Further optimization is needed to improve the photovoltaic characteristic of the carbon electrode and ZrO2 layer. Figure 8 shows stability test results for an ionic liquid monolithic DSC at 80°C in the dark. Although the efficiency varied slightly at first, it was relatively stable in a 1000 h thermal test at 80°C [18].
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In conclusion, we have reported a method using TiO2 nanopowder, instead of TiO2 colloid (P25), to prepare carbon paste that does not crack and peel. In addition, cracking due to shrinkage of the ZrO2 layer was prevented by preannealing the electrode at 200°C for 30 min before printing the carbon layer. As a result, thermally durable monolithic DSCs using ionic liquid electrolyte were prepared that operated stably at 80°C for 1000 h.