Recent Trends in Dye-Sensitized Solar Cell Technology

The very foundation of modern civilization lies on the abundant supply of electrical energy. For the last two centuries, most of our electricity needs have been fulfilled by fossil fuel sources such as coal, natural gas and petroleum. However, the global electricity demand is continuously increasing. The continuous increase in energy demand is forcing our society to search for environmentally clean, sustainable and renewable energy sources.¹

Several alternate sources of energy such as wind, solar, hydro and biomass have been explored over the last several decades. Among all these unconventional energy sources, solar energy has emerged as a most practical alternative to conventional fossil-fuel based energy sources. However, even with the continuously increasing interest in solar energy, it is still not able to compete fully with the conventional fossil energy sources because of a number of material challenges. For example, the conventional silicon based solar cells require high purity defect free silicon. The cost of producing such high purity silicon is very high. Because of the high material cost and low energy conversion efficiency, the cost of power produced by these cells is several times more than that produced by conventional sources.

In recent years, dye-sensitized solar cells^2,3 (DSSCs) have received considerable attention as a cost-effective alternative to conventional solar cells. DSSCs operate on a process that is similar in many respects to photosynthesis, the process by which green plants generate chemical energy from sunlight. Central to these cells is a thick semiconductor nanoparticle film (electrode) that provides a large surface area for the adsorption of light harvesting organic dye molecules. Dye molecules absorb light in the visible region of the electromagnetic spectrum and then "inject" electrons into the nanostructured semiconductor electrode. This process is accompanied by a charge transfer to the dye from an electron donor mediator supplied by an electrolyte, resetting the cycle.

Because of the low cost of production, DSSCs have potential to revolutionize the solar cell industry. However, until recently the most common DSSC systems under investigation were based on electrodes consisting of sintered semiconducting nanoparticles (mostly TiO₂ or ZnO). These nanoparticle-based DSSCs rely on trap-limited diffusion through the semiconductor nanoparticles for the electron transport.

This is a slow transport mechanism that limits device efficiency, especially at longer (less energetic) wavelengths, because recombination events become more likely. Moreover sintering of nanoparticles requires high temperature (~450°C) which restricts the fabrication of these cells only on nonflexible solid substrates. Very recently our group has shown that significant increase in the efficiency of DSSC can be achieved if the sintered nanoparticle electrode is replaced by a specially designed electrode possessing an exotic 'nanoplant-like' morphology (see fig.1).

Figure 1. Schematic diagram of ZnO nanoplant-based DSSC developed.

Professor Ashutosh Tiwari and his team at the Nanostructured Materials Research Laboratory demonstrated that the direct electrical pathway, provided by the interconnected nanoplants, ensures the rapid collection of carriers generated throughout the device, which significantly enhances the conversion efficiency of the system. Semiconducting ZnO nanoplants used in above DSSC were grown using a low temperature (<150°C) technique invented by our group.⁴ Because of the low temperature nature of our processing technique, these structures can be grown on polymer substrates by slight modifications in the processing parameter. ZnO nanoplant-based polymer substrates can be used for fabricating flexible solar cells.

DSSCs based on liquid electrolytes have reached efficiency as high as 11% under AM 1.5 (1000 W m^-2) solar illumination. However, a major problem with these DSSCs is the evaporation and possible leakage of the liquid electrolyte from the cell. This limits the stability of these cells and also poses a serious problem in the scaling up of DSSC technology for practical applications.

Recently the use of p-type semiconductors as solid-state hole-collectors in DSSCs has been proposed.⁵ However, because of the scarcity of suitable hole collectors having proper band-gap and band positions, not much progress has yet been made on solid state (SS) DSSCs. Most of the work performed so far in this field involved^6,7 the use of CuSCN or CuI as hole-collectors. Although CuSCN and CuI possess an appropriate band gap and band positions, both lack stability and tend to degrade in a short time.

In terms of stability, inorganic oxide semiconductors are good candidates However, they have seldom been utilized as hole-collectors in SS-DSSC to date mostly because of the scarcity of p-type oxide semiconductors and difficulties of fabrication of an oxide semiconductor layer on dye coated TiO₂. NiO and CuAlO₂ are among the very few oxides^8,9 which have been shown to possess suitable band-gap and band-position for application in SS DSSC. Though the NiO and CuAlO₂ based SS-DSSC showed quite high stability, the efficiency of the cells was still very low.

The poor performance of these solar cells was attributed to the: (i) lower intrinsic conductivity and hole mobility of NiO and CuAlO₂, and (ii) bigger particle sizes of NiO and CuAlO₂ compared to that of TiO₂ pores, hindering the penetration of the hole collector into entire dyed mesoporous TiO₂ film, which results in weak contact between the hole collector and dye. Despite the lower conversion efficiency, these SS DSSCs were very stable.^8,9 If the efficiency of SS DSSCs can be made comparable to liquid electrolyte based DSSCs, then they will definitely have significant impact on the solar cell technology.

In order to be useful in DSSCs, the prospective p-type semiconductor (hole-collector) and the dye are required to have following special properties: (i) The p-type material must be transparent throughout the visible spectrum, where the dye absorbs light, (ii) A method must be available for depositing the p-type material without dissolving or degrading the monolayer of dye on TiO₂ nanocrystallites, (iii) The dye must be such that its excited level is located above the bottom of the conduction band of TiO₂ and the ground level below the upper edge of the valence band of the p-type material.

Very recently we have shown that CuBO₂, a new p-type oxide discovered by our group,¹⁰ fulfills most of above requirements. It is transparent over a wide spectral range with an indirect bandgap of 2.6 eV and a direct bandgap of 4.5 eV. It exhibits high conductivity and hole mobility compared to all other known p-type oxides. For example the room temperature electrical conductivity of the polycrystalline CuBO₂ film10 was 1.65 S-cm^-1, about 65% higher than the corresponding value (~1 S-cm^-1) reported by Kawazoe et al¹¹ for CuAlO₂ (see Fig. 2).

Figure 2. Electrical conductivity of CuBO2. Inset shows the Thermoelectric power of the material.

Hall coefficient and thermoelectric power measurements showed the CuBO₂ to be of p-type with carrier density of the order of 1017 cm^-3. Hall mobility estimated from the electrical conductivity and Hall measurements was ~100 cm² V^-1 s^-1, about 10 times higher than the corresponding value (~10 cm² V^-1 s^-1) reported by Kawazoe et al¹¹ for CuAlO₂. High electrical conductivity and hole mobility of CuBO₂ suggests that it could be a very good candidate for hole collector application in solid-state DSSCs.

References

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9. Bandara J.;Yasomanee, J. P., "p-type oxide semiconductors as hole collectors in dye-sensitized solid-state solar cells", Semiconductor Science and Technology 2007, 22, 20-24.
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11. Kawazoe, A. H.; Yasukawa, M.; Hyodo, H.; Kurita, M.; Yanagi, H.; Hosono, H., "P-type electrical conduction in transparent thin films of CuAlO2", Nature 1997, 389, 939-942.