by Prof. Ashutosh Tiwari
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 cells2,3 (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
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
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 involved6,7 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 oxides8,9 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
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
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
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.
1. Schipper, L.; Meyer, S.; Howarth, R.; Steiner, R., Energy
Efficiency and Human Activity: Past Trends, Future Prospects (Cambridge
University Press, Cambridge, 1997).
2. O'Regan, B.; Grätzel,
M., A low cost, high efficiency solar cell based on dye sensitized colloidal
TiO2 films, Nature 1991, 353, 737-739.
3. Grätzel, M.,
"Photoelectrochemical cells", Nature 2001, 414, 338-344.
Tiwari, A.; Snure, M., "Synthesis and Characterization of ZnO Nano-Plant-Like
Electrodes" Journal of Nanoscience and Nanotechnology 2008, 8, 3981-3987.
5. Li, B.; Wang, L. D.; Kang, B. N.; Wang, P., Qiu, Y., "Review of
recent progress in solid-state dye-sensitized solar cells. Solar Energy
Materials and Solar Cells" 2006, 90, 549-573.
6. O'Regan, B. ;
Lenzmann, F. ; Muis R.; Wienke, J., "A solid-state dye-sensitized solar cell
fabricated with pressure-treated P25-TiO2 and CuSCN: Analysis of pore filling
and IV characteristics", Chemistry of Materials 2002, 14, 5023-5029.
7. Sirimanne, P. M.; Jeranko, T. ; Bogdanoff, P. ; Fiechter, S. ;
Tributsch, H. , "On the photo-degradation of dye sensitized solid-state TiO2/dye
CuI cells", Semiconductor Science and Technology 2003, 18, 708-712.
8. Bandara, J.; Weerasinghe, H., "Solid-state dye-sensitized solar
cell with p-type NiO as a hole collector", Solar Energy Materials and Solar
Cells 2005, 85, 385-390.
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.
10. Snure, M.; Tiwari, A., "CuBO2-A p-type transparent oxide",
Applied Physics Letters 2007, 91, 092123 1-3.
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,
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