The necessary length of carbon nanotubes required to optimize solar cells
Majid Vaezzadeh
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Mohammad Reza Saeedi
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Tirdad Barghi
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Mohammad Reza Sadeghi
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Address: K.N. Toosi University, Department of Physics
, 41, Shahid Kavian St., 15418-49611 Tehran,
Iran
Background: In recent years scientists have been trying both to increase the efficiency of solar cells, whilst at the same time reducing dimensions and costs. Increases in efficiency have been brought about by implanting carbon nanotubes onto the surface of solar cells in order to reduce the reflection of sunrays, as well as through the insertion of polymeric arrays into the intrinsic layer for charge separation. Results: The experimental results show power rising linearly for intrinsic layer thicknesses between 0-50 nm. Wider thicknesses increase the possibility of recombination of electrons and holes, leading to perturbation of the linear behaviour of output power. This effect is studied and formulated as a function of thickness. Recognition of the critical intrinsic layer thickness can permit one to determine the length of carbon nanotube necessary for optimizing solar cells. Conclusion: In this study the behaviour of output power as a function of intrinsic layer thicknesses has been described physically and also simulated. In addition, the implantation of carbon nanotubes into the intrinsic layer and the necessary nanotube length required to optimize solar cells have been suggested.
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Background
Developing inexpensive and renewable energy sources is
one of the most important scientific and technological
challenges of our time. Solar energy is an inexhaustible
energy source that can be harnessed to meet our growing
future energy needs. However, traditional photovoltaic
(solar-to-electric conversion) technology is too expensive
to be the suitable alternative for fossil fuels or even other
competing renewable energy sources. A significant leap in
the scientific and technological progress of renewable
energy sources will be required to displace proven, but
unsustainable energy production methods.
Nanotechnology is driving new interesting developments
in photovoltaic technology. Advances in organic synthesis
and characterization techniques allow us to coax a
photocurrent from organic, 'soft' molecules in a process that
mimics photosynthesis in plants, thus potentially
opening up the way to cheap, ubiquitous solar cells. Power
production resulting in zero greenhouse gas emissions is
economically and environmentally desirable. Direct
photovoltaic conversion of sunlight into electricity is
therefore the highly attractive alternative to unsustainable
energy sources such as fossil fuels.
Although silicon solar cells have gained a considerable
market share and commercial success, high production
and up-front installation costs still limit their commercial
viability. In this study, we explore the use of low-cost
advanced materials for photovoltaic energy production
and the mechanism of photovoltaic action in a new class
of solar cell, that is, the heterojunction photovoltaic.
These are constructed from a thin film of cheap composite
material, a mixture of carbon nanotubes and conductive
polymer [1].
Results
We now aim to address the issues that determine the
thickness of the absorber (or "intrinsic") layer. Figure 1
illustrates a computational calculation that shows how
the output power of an a-Si-based pin cell varies with
intrinsic layer thickness. The curves differ in the specified
cFCeiogllmuarpseuat1efurnccatlicounlaotifointorifntshice lpayoewretrhiocuktnpeusts from a pin solar
Computer calculation of the power output from a pin solar
cell as a function of intrinsic layer thickness. The differing
curves indicate results for monochromatic illumination with
absorption coefficients from 5000/cm to 100 000/cm; for
typical a-Si: H, this range corresponds to a photon energy
range from 1.8 to 2.5 eV. Solid symbols indicate illumination
through the p-layer and open symbols indicate illumination
through the n-layer. Incident photon flux = 2 1017/cm2s;
no back reflector. The data are obtained from real
experiments, with the curves calculated using methods outlined in
the references [3-8].
absorption coefficients for a monochromatic
illumination using varying photon energies. All curves were
calculated for the same photon flux. Such illumination
conditions might be achieved by experiment using a laser
whose photon energy can be tuned from 1.8 to 2.3 eV,
however the presence of sunlight, of course, presents a
much more complex situation.
We will first discuss the results for illumination through
the p-layer (solid symbols in the figure). For intrinsic
layers that are sufficiently thin, the power is proportional to
the number of photons absorbed (i.e. to the product of
the thickness, d and the absorption coefficient, ). Within
this limit the fill factors have nearly ideal values around
0.8. As the thickness of the cell increases, the power
saturates. In figure 1 the first plot ( = 100 000/cm
corresponding photon energy of about (...truncated)