Dye-Sensitized Solid-State Heterojunction Solar Cells
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Dye-Sensitized
Solid-State Heterojunction Solar Cells Michael Grätzel
Abstract The dye-sensitized solar cell (DSSC) provides a technically and economically viable alternative concept to present-day p–n junction photovoltaic devices. In contrast to conventional silicon systems, where the semiconductor assumes both the task of light absorption and charge carrier transport, these two functions are separated in DSSCs. The use of sensitizers having a broad absorption band in conjunction with wide-bandgap semiconductor films of mesoporous or nanocrystalline morphology permits harvesting a large fraction of sunlight. There are good prospects that these devices can attain the conversion efficiency of liquid-electrolyte-based dye-sensitized solar cells, which currently stands at 11%. In this article, we present the current state of the field, the realm of our review being restricted to the discussion of organic molecular hole conductors, which have demonstrated the best performance to date. Keywords: dye-sensitized solar cells, mesoscopic junctions, solid-state heterojunctions.
Introduction In this review, we focus our attention on solid-state solar cells based on interpenetrating network junctions.1–5 In particular, we shall discuss devices that contain inorganic oxides as electron conductors and a molecular hole-conducting material. The phenomenal progress realized recently in the fabrication and characterization of nanocrystalline materials and the advent of new organic hole conductors has opened up vast new opportunities for these hybrid systems. Devices based on interpenetrating networks of mesoscopic semiconductors have shown strikingly high conversion efficiencies, which compete with those of conventional devices. This is contrary to expectation, as the corrugated interface produces electronic surface states that enhance recombination of the charge carriers produced by light. The prototype of this family of devices is the dye-sensitized solar cell (DSSC), which accomplishes the dual functions of optical absorption and charge separation by combining a sensitizer as a light-absorbing material with a wide-
MRS BULLETIN • VOLUME 30 • JANUARY 2005
bandgap semiconductor of mesoporous or nanocrystalline morphology.
Operational Principles of Mesoscopic Sensitized Heterojunction Solar Cells Figure 1 shows a basic schematic for the sensitized heterojunction photovoltaic cell. A monolayer of dye is attached to the surface of a mesoscopic film of a widebandgap oxide semiconductor, such as TiO2, and serves to harvest solar light. Upon excitation, an electron is injected into the conduction band of the semiconductor oxide. The electrons migrate across the nanoparticle network to the current collector (anode). They subsequently pass through the external circuit, where they perform electrical work and then proceed to the counter electrode (cathode). An organic hole conductor serves to regenerate the sensitizer and transport the positive charges to the counter electrode, where they recombine with the electrons. Overall, there i
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