The dye-sensitized solar cell (DSC) is expected to be a candidate technology for low-cost photovoltaics. These thin film cells feature a semiconductor sandwiched between a titanium dioxide (titania) anode and a platinum cathode. The cells' 3D molecular dye acts like chlorophyll in green plants, absorbing sunlight and exciting electrons in a liquid conductor. Charge flows from the electrolyte into the titania and then toward a transparent electrode.
Standard DSC designs consist of glass layered with a transparent conductive oxide (TCO), a porous titania layer stained with dye molecules, an electrolyte layer of redox species (I− and I3−), and a counter-electrode, such as a Ti plate or TCO-layered glass.1 The latter is indispensable in this approach, because front glass requires light-harvesting and electron-collecting properties. However, the specialized glass is expensive and restricts DSCs to flat shapes: we now report cell designs that do not require it.
We prepared a flexible titania sheet, or T-sheet, consisting of a protected metal mesh stained with dye molecules2 that is completely covered with porous titania materials (see Figure 1). The flexible, composite T-sheet is supported by a protected stainless steel mesh, the surface of which is protected by a blocking layer that retards charge recombination between electrons in the stainless steel and redox species in an electrolyte layer.2 The blocking layer is composed of Ti and TiOx layers with gradient compositions.2,3
Figure 1. T-sheet structure and fabrication process.
Figure 2. Various TCO-less DSC structures incorporating T-sheets.
We prepared TCO-less flexible DSCs by assembling a flexible substrate (polyethylene terephthalate, or PET), a T-sheet, a gel electrolyte sheet, and a layer of Ti foil with a Pt layer.3,4 Assembly does not include high-temperature processes that damage plastic substrates. Instead, the T-sheet was baked at 500°C in advance. The solar cell's efficiency was 4.7% before full optimization.
Also attracting interest now are cylindrical solar cells made of CuInGaSe (CIGS), primarily because they are lightweight and easy to install.5 The cells offer the additional benefit of encapsulating liquid electrolytes more easily. We prepared TCO-less cylindrical cells by encasing a Ti rod (or Ti wire) with a Pt layer, a gel electrolyte sheet, and a T-sheet, followed by a glass tube. The protected stainless steel sheet was connected to a Ti rod counter-electrode to collect current. The assembly obtained 2.7% efficiency before full optimization.
We then fabricated TCO-less fiber DSCs by assembling a glass tube, a T-sheet, a gel electrolyte sheet, and a titania sheet with a Pt layer (see Figure 3).6 Polystyrene particles dispersed in water were placed in the glass tube to scatter light passing through a waveguide (see Figure 4).
Figure 3. Structure of a TCO-less fiber DSC.
Figure 4. Structure and photovoltaic performance of TCO-less fiber DSC with light-scattering wave guide.
We also investigated a tandem structure, which effectively covers a wide range of solar wavelengths. A T-sheet was used as the bottom electrode in a tandem DSC that had electrodes top and bottom. By design, the top electrode—fabricated on TCO-layered glass—generated electrons in visible regions, while the bottom layer generated electrons in IR regions. An experiment using two model dyes demonstrated that the cell successfully converted both wavelengths.7,8
Figure 5. Structure of a flat tandem DSC: the T-sheet serves as the bottom electrode.
TCO layers absorb charge in the IR area because free electrons are present. But TCO-less DSC structures have the advantage of better light-harvesting properties in the IR. So, to address the problem of light-harvesting losses in flat tandem cells, we experimented with a TCO-less vertical design. Each tandem cell is composed of TCO-less DSCs and a waveguide that disperses the light. Tandem properties were verified by using three model dyes. Visible light was split into three wavelengths (450nm, 550nm, and 650nm) by three dichroic mirrors. Cells stained with the appropriate dyes were fabricated on a wall of the waveguide. The IPCE curve had three peaks, which corresponded to each dye-stained cell (see Figure 6). This demonstrated that the three DSCs were active for the three different bands. Voltage of the TCO-less pillar DSC was 1.6V: three times that of the corresponding single DSC (0.5–0.56V). These results strongly demonstrate that TCO-less pillar cells work as tandem cells.
Figure 6. IPCE curves of TCO-less tandem DSC and individual cells.
Low-cost devices with simple structures that can split out the wavelengths are needed to advance cost-effective DSC designs. We have shown that polystyrene particles with different diameters dispersed in water can perform this function. Such configurations can be used instead of dichroic mirrors.
A variety of TCO-less DSCs with T-sheet electrodes should open up new applications. Here we have described how we were able to confirm their fundamental working principles. However, this research remains in its infancy. Improvements in device structures and materials are needed. Developments in IR dyes, in particular, are crucial for the success of high-efficiency cells.
Kyushu Institute of Technology
Professor Shuzi Hayase studies the use of nanomaterials in solar cells, fuel cells, and other energy-conversion devices.
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5. B. Bullar, M. B. Beck. USP 7235736B1.
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7. K. Uzaki, S. S. Pandey, S. Hayase, Tandem dye-sensitized solar cells consisting of floating electrode in one cell, J. Photochem. Photobiol. A. In press.
8. K. Uzaki, S. S. Pandey, Y. Ogimi, S. Hayase, Tandem dye-sensitized solar cells consisting of nanoporous titania sheet, Jpn. J. Appl. Phys. In press.