Alternative technology enables large-area solar-cell production

Much effort has focused on developing dye-sensitized solar cells (DSCs) because of their low production cost and high energy-conversion efficiency.¹ However, to compare favorably with bulk silicon photovoltaic cells, their energy-conversion efficiency must be improved. In addition, researchers need to develop upscaling technology to fabricate large modules.

We have taken a novel approach to achieve these aims. We investigated internal DSC resistance using electrochemical-impedance spectroscopy and observed four internal resistance elements. Further analysis implies that these are related to charge-transfer processes at the platinum counter electrode (R₁), charge transportation at the titanium dioxide (TiO₂)/dye/electrolyte interface (R₂), ionic diffusion in the electrolyte (R₃), and the sheet resistance (R_h) of transparent conducting oxide (TCO). R₂ acts similarly to a diode's resistance, while R₁, R₃, and R_h show series internal resistance behavior. Therefore, we proposed an ‘equivalent circuit’ for DSCs to investigate the underlying operational mechanism (see Figure 1).²

It is well known that the short-circuit current density, open-circuit voltage, and fill factor must be increased to raise the energy-conversion efficiency. We successfully improved efficacy through a strategy of parameter improvement based on our analysis of DSC equivalent circuits. A public test center confirmed the highest efficiency of 11.2% (aperture area: 0.219cm²).^3,4

Figure 1. Equivalent-circuit model for DSCs. R₁, R₃, and R_h: Platinum counter electrode, eletrolyte, and sheet resistance. I_ph and I: Constant-current source and output current. C₁ and C₃: Capacitance elements. R_sh: Shunt resistance. Z₂: Diode.

We also attempted to enlarge the cell size in integrated modules. Integrated modules can use either Z- or W-type connections. Z contacts are generally used in thin-film silicon solar cells. They consist of two opposing electrodes with connections between neighboring cells. W-contact modules do not contain conductors because the neighboring cells are alternately bias cells and connected by TCO. We recently fabricated large-size (25×25 and 5×5cm²) W-type DSC modules (see Figure 2). The module-efficiency record of 8.4% (aperture area: 26.47cm²), confirmed in AIST's (Japan's National Institute of Advanced Industrial Science and Technology) public test center, was obtained⁵ by optimizing the counter electrode, electrolyte composition, TiO₂ film thickness, and unit-cell uniformity (see Figure 3).

Figure 2. 25×25cm²prototype integrated DSC module.

Figure 3. Current-voltage (I-V) characteristics of our integrated DSC module. Short-circuit current: 55.7mA. Open-circuit voltage: 6.35V. Fill factor: 62.6%. Maximum power voltage: 4.53V. Maximum power current: 48.8mA. Maximum power: 221.4mW. Efficiency: 8.4%. The red and green lines represent current and power, respectively.

We recently proposed a new DSC cell structure in the form of back-contact DSCs, in which the TCO is not included and a porous titanium electrode is placed on the side opposite that irradiated (see Figure 4). We obtained an overall conversion efficiency of 8.4% using N719 dye.⁶ Although we achieved an efficiency of 11.2% in DSCs, the efficiency and lifetime must be improved to achieve performance comparable to silicon-based solar cells. Therefore, establishing the principles and mechanisms of power generation in this type of cell is key.

Figure 4. Cross-sectional sketch of a back-contact DSC. hν: Light energy. CE: Contact electrode. BCE: Back-contact electrode. I^-: Iodine ion. e^-: Electron. TiO₂: Titanium dioxide.

The National Institute for Materials Science (NIMS) is Japan's sole independent administrative institution specializing in materials science. Our mission at the Advanced Photovoltaics Center is to construct the next generation, high-efficiency, low-cost solar cells. Our main research objectives are to create new cell structures and intelligent materials. Research areas at the center include dye-sensitized, organic thin-film, quantum-dot, multijunction-compound, and silicon thin-film solar cells. For DSCs, we target a conversion efficiency above 15% and a lifetime of 20 years. Achieving these targets will require innovative ideas, because we have reached the limits of improvement based on existing technology.