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Solar & Alternative Energy

Advances in monolithic series-interconnected solar-cell development

A new type of dye-sensitized solar-cell module has achieved transparency and color choice, facilitating mass production.
15 April 2009, SPIE Newsroom. DOI: 10.1117/2.1200903.1581

Figure 1. A monolithic series-interconnected transparent dye-sensitized solar-cell (DSC) module.

Dye-sensitized solar cells (DSCs) offer various advantages, including freedom of design as well as lower costs and energy consumption, in production processes as compared to silicon-based solar cells.1,2 Monolithic series-interconnected modules—characterized by a structure similar to that of amorphous-silicon solar-cell architectures—are the most promising DSC-module type for mass production.3 However, these modules neutralize the unique transparency advantage of DSCs, because they use black, carbon-based counter electrodes (CEs) as well as rutile-based opaque separators between the photo-electrodes of porous anatase and the corresponding CEs.

We recently developed transparent CEs composed of platinum (Pt)-loaded tin-doped indium-oxide (In2O3:Sn, or ITO) nanoparticles and separators made of silicon-dioxide nanoparticles. Three-layered electrodes consisting of photo-electrodes, the newly developed separators, and our new CEs were stacked on glass plates covered with conductive oxide layers by successive screen printing, followed by sintering and sensitization with a red dye. A new process was applied to seal the electrodes filled with a liquid electrolyte, using transparent damp-proof films with thermoplastics as back covers. This ensures both good durability and easy production.4, 5 Figure 1 shows the resulting module (95×95mm2), in which 12 cells are series interconnected.

Figure 2. Transmission-electron-microscope image of the platinum (Pt)-loaded tin-doped indium-oxide nanoparticles used in the transparent counter electrodes. The small ( <10nm) dark spheres are Pt islands.

Figure 3. Current-voltage relationships of (a) a small cell and (b) a module after 100h of exposure to full sunlight.

The detailed performance of the new materials and their durability when exposed to full sunlight at 60°C were examined using small, monolithic single cells (not series interconnected). Their fabrication process was the same as that of the full modules, except for the sealing method. Figure 2 shows a transmission-electron-microscope image of the Pt-loaded ITO nanoparticles after sintering. The ITO generates sufficient electric conductivity, while the Pt catalyzes the reaction on the CE,6 i.e., I3+2e → 3 I. Pt-induced light absorption was slight and there was no difference in appearance between the CEs and that of similar ITO layers without Pt. Light scattering by the three-layered electrodes was significantly suppressed after they were filled with electrolyte.

The performance of the small cells improved during the first 100h of exposure to sunlight. The current-voltage relationship under 100mW/cm2 insolation—see Figure 3(a)—was close to that of conventional, parallel DSCs. We performed alternating-current impedance analysis5 and concluded that the improvement was due to the increasing electric conductivity and/or catalytic activity of the CEs. Although the conversion efficiency gradually decreased after 300h of light exposure because of degradation and/or decomposition of components in the electrolyte,7 85% of the maximum current was retained after 2000h.5

The module performance also improved in a similar fashion to that of the single cells. Figure 3(b) shows the current-voltage relationship of the module. Both the short-circuit current density and filling factor were close to those of the small cells, while the open-circuit voltage was approximately 12 times larger than that of the small cells. These results imply that there was no significant short circuit in or poor connection between any of the 12 cells in the module. In addition, the thermal sealing process did clearly not inflict significant damage to either the dye or the electrolyte.

We propose a new application of transparent DSC modules, a surface light-emitting plate driven by solar energy (see Figure 4).6 The plate consists of a transparent DSC module (using a yellow dye), an electroluminescent (EL) panel mounted just below the solar-cell stack, and a rechargeable battery. Electricity generated by the DSC module and stored in the battery during the daytime is supplied to the EL panel at night. Orange emission from the EL panel passes through the DSC module with little absorption loss and illuminates the surroundings: see Figure 4(b). The newly developed modules will broaden the applicability of DSCs.

Figure 4. Prototype of a surface light-emitting plate driven by solar energy, using the transparent DSC module.

We acknowledge the cooperation of IMRA-Europe S. A. S. and Aisin Cosmos Research and Development Co. Ltd.

Yasuhiko Takeda, Naohiko Kato
Toyota Central Research and Development Laboratories Inc.
Nagakute, Japan

Yasuhiko Takeda is a senior researcher. He received his BE and ME in solid-state physics from Osaka University, and his PhD in optically functional materials from Nagoya University.

Naohiko Kato is a senior researcher. He received his BE and ME in photoelectrochemistry from Kyoto University, and his PhD in functional thin films for optoelectronic devices from the Nagoya Institute of Technology. He currently leads the DSC development unit.

Tatsuo Toyoda
Aisin Seiki Co. Ltd.
Kariya-shi, Japan

Tatsuo Toyoda received his BE and ME in material engineering from the Nagoya Institute of Technology. He currently leads the solar-cell development group and is manager of the advanced-technology development department.