Fig. 1 The example of organic solar cell
using semiconducting polymers (a) [1] and
silicon based-solar cell (b) [2].
Regarding
the overall energy conversion efficiency in solar cell, several possible
losses should be considered such as losses due to non-absorption of long wave
length , thermalization of the excess energy of photon total reflection , incomplete absorption due to finite thickness , electron-hole recombination, electrode coverage (shading losses), voltage factor, and fill factor.
It can be expressed
as
 |
Fig.2 Typical I-V characteristic of
nanowire dye-sensitized solar cell [3]
Experimentally, the efficiency of solar cells is defined from
I-V measurement result, as depicted in Fig.
2 which contains direct information about fill factor. The maximum power generated by a
solar cell is dependent on this value. In a practical solar cell, the FF
is lower than the ideal value due to voltage drop both by series resistance
and current leakage.
|
b.
High Efficiency-Solar
Cells
Basically, solar cells can be
categorized at least into 5 categories in which each type has efficiency limit
regarding recent progress. There are emerging photovoltaic system (such as dye
sensitized cell), thin film solar cells (such as amorphous Si:H solar cells),
c-Si solar cells, single-junction GaAs,
and multi-junctions solar cells. The researches progress for achieving higher
conversion efficiency for different category of solar cell is summarized in
below table.
Table 1 | Researches toward high
efficiency solar cell
Type
|
Schematic
Devices
|
Descriptions
|
Emerging
Photo-Voltaic Cells
|
Dye-sensitized
Solar Cell (DSSC) [4]
|
DSSC is similar with photosynthesis in the use of a dye as the
light harvester to produce excited electrons, TiO2 replacing
carbon dioxide as the electron acceptor, iodide/tri-iodide (I-/I3-)
replacing water and oxygen as the electron donor and oxidation product and a
multilayer structure (similar to the thylakoid membrane) to enhance both the
light absorption and electron collection efficiency. It was reported that the
efficiency has reached about 11.4 % [5] by improving
the open circuit potential with introduction of co-absorber to avoid dye
aggregation and reduce the charge recombination.
|
Thin
film solar cells
|
Si:H
based solar cells [6]
|
Thin film solar cell is made by depositing one or more
thin layers of photovoltaic
material on a substrate. The thickness range of such a layer is wide and
varies from a few nanometers to tens of micrometers. Reductions in the
requisite thickness are a pathway to reduce
costs for most solar cell materials, as well as a route to higher open
circuit voltages due to decreased bulk recombination. But, offsetting the reduced photocurrent is necessary which can be done
by light trapping [6]. In case of aSi:H based
solar cell the highest stable efficiency so far is 13.4% [7]
|
c-Si
solar cells
|
Passivized Emitter Rear Localized
(PERL) Cell [8]
|
As being the largest market share of solar cell business, the
crystalline silicon PV cell is essentially a diode with a semiconductor structure.
Representative example of high-efficiency mono-crystalline silicon PV cells is
PERL cell. The highest energy conversion efficiency for PERL cell reported so
far is 25% [8]. This value is below the
single crystal Si based solar cell which was reported to have 27.6 % of
conversion efficiency [9].
|
Single-junction
GaAs Solar cells
|
Single
Junction GaAs Solar Cell [10]
|
Owing to its direct band gap and high electron mobility, gallium
arsenide (GaAs) based compound semiconductor has advantages over silicon. In case
of photovoltaic application, limiting the emission angle of a high-quality
GaAs solar cell becomes a feasible route to achieving power conversion
efficiencies above 38% with a single junction [11].
But, experimentally, it was found that highest efficiency was about 28.8% using
thin film crystal [12].
|
Multi-junction
solar cells
|
Schematic diagram of a
multi-junction (MJ) solar cell [13]
|
The
highest conversion efficiency among 5 types of solar cells being discussed
here is multi-junction solar cell which has reached 44 % using terrestrial
concentrator [12]. In MJ solar cell, semiconductors
with different band gaps convert different portions of the solar spectrum to
reduce thermalization losses. Light-management architectures are also applicable
for reaching ultrahigh efficiency [14].
|
c. Low-cost
Solar Cells
As mentioned in previous section, mainly the solar cell that can
be categorized as lowest-cost one is emerging PV especially dye-sensitized
solar cell. In case both fabrication method and cost, this type does not
require complicated manufacturing process compared to silicon based solar cell and
even some of examples can be made by home industry. The issues and challenges
are the conversion efficiency and durability for long-term use. Several
improvements have been done to reach better efficiency as described in Table 2.
Table 2 | Researches toward low-cost solar
cell
Improvements
|
Nanomaterial Design
|
Remark
|
Better nanostructure design for better
collection of charges and higher surface roughness factor for higher dye
loading capacity
|
Hierarchical TiO2 tubular macrochannel arrays
(HTTMAs) [15]
|
229.6 m2g-1
high surface area of HTTMA showed 8.1 % of energy conversion efficiency, a
better efficiency than TiO2 nanoparticles based DSSC. HTTMAs were
produced by one step hydrothermal method without template or electric filed
assistant. This method was the simplification of common alkaline hydrothermal
method which requires anodic aluminum oxide template or anodic oxidation.
|
Low electron recombination rates
by optimizing active layer thickness of electrolyte layers
|
Schematic of thin DSSC [16]
|
Trapping/detrapping processes and
electrons exchange between semiconductor and electrolyte in DSSC determine
recombination rates. Low electron
recombination rates and high electron generation is expected to achieve good
conversion efficiency. It was shown that increasing active layer of DSSC and
decreasing bulk electrolyte layer would further increase the cell efficiency
thus defined the optimum thickness of DSSC.
|
Low electron recombination rates
by increasing energy barrier between ZnO and TiO2
|
Schematic band structure of DSSC using additional energy barrier
[17]
|
Another way to increase
transportation rates of electrons toward decreasing the recombination of
electrons in dye or electrolyte is by creating energy barrier in the working
electrode. ZnO-coated TiO2 electrode was chosen as part of it.
This working electrode was fabricated by using simple dip coating method and
the efficiency calculation showed the increase from 5.45 % (without ZnO
coating) to 6.62 % (with ZnO coating).
|
Better ligand exchange process to
increase hydrophility and anchoring-sites for dyes
|
TEM image of acetic acid-treated TiO2 photo anode [18]
|
The efficiency of DSSC is also
directly related to the quantity of the adsorbed dye on the photo-anode
layer. Thus, TiO2 photo-anode should have lots of anchoring sites for dyes.
The acetic acid treatment on photo electrode was reported to increase
hydrophility which was advantageous for the adsorption of dye molecules. The
result showed there was optimum concentration that suited for DSSC.
|
Adding new functional parts: reflectors
to reduce the energy losses caused by the inactive areas
|
Sketch of reflector [19]
|
The energy losses caused by the inactive
areas decrease the overall efficiency of DSSC. The application of reflectors
to collect more photons so that energy losses due to inactive areas decreased
has been reported. Increasing the reflectance from 0 to 0.9 increased the
total area efficiency around 1.12 %.
|
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