impulse-ed 2-060417

IMPULSE 2017
DEE, RSET
So, Why use CNTs in Solar Cells?
Any solar cell should have good photovoltaic properties to provide maximum number of electronhole
pairs for a given intensity of incident light. The cell should be as strong as possible to improve
its life and should work at maximum possible efficiency. Above all, it should be cheap. Well,
CNTs have many such desirable properties that make it a potential photovoltaic material in the
coming years. When light falls on a solar cell, electrons are generated and transferred to the
external circuit via metallic conductors and connectors. If the photo-reception part of the cell
contains CNTs, a lot of positive differences can be observed. This is due to the electrical properties
of CNTs. They are hollow cylinders and hence electrons can only flow over its surface compared
to the conventional wires. Due to this reason, CNTs offers lesser resistance to the flow of current
and can help reduce losses. As for light harvesting properties, carbon nanotubes possess a wide
range of direct bandgaps matching the solar spectrum and its strong photoabsorption from infrared
to ultraviolet, allows it to absorb light belonging to different frequencies and wavelength. It has
high carrier mobility and reduced carrier transport scattering which allows CNTs to transfer
maximum current to the external load as much as possible with minimum loss of electrons within
the bulk of the cell. What good can a solar cell do if it can’t protect itself from mechanical stresses?
Carbon nanotubes are the strongest and stiffest materials yet discovered in terms of tensile strength
and elastic modulus respectively. In 2000, a multi-walled carbon nanotube was tested to have a
tensile strength of 63 gigapascals (9,100,000 psi). Since carbon nanotubes have a low density for
a solid of 1.3 to 1.4 g/cm 3 , its specific strength of up to 48,000 kN•m/kg is the best of known
materials, compared to high-carbon steel's 154 kN•m/kg.
Recently, SWNTs were directly configured as energy conversion materials to fabricate
thin-film solar cells, with nanotubes serving as both photogeneration sites and a charge carriers
collecting/transport layer. The solar cells consist of a semitransparent thin film of nanotubes
conformally coated on an n-type crystalline silicon substrate to create high-density p-n
heterojunctions between nanotubes and n-Si to favor charge separation and extract electrons
(through n-Si) and holes (through nanotubes). Initial tests have shown a power conversion
efficiency of >1\%, proving that CNTs-on-Si is a potentially suitable configuration for making
solar cells. For the first time, Zhongrui Li demonstrated that SOCl 2 treatment of SWNT boosts the
power conversion efficiency of SWNT/n-Si heterojunction solar cells by more than 60%. Later on
the acid doping approach is widely adopted in the later published CNT/Si works. Even higher
efficiency can be achieved if acid liquid is kept inside the void space of nanotube network. Acid
infiltration of nanotube networks significantly boosts the cell efficiency to 13.8%, as reported by
Yi Jia, by reducing the internal resistance that improves fill factor, and by forming
photoelectrochemical units that enhance charge separation and transport. The wet acid induced
problems can be avoided by using aligned CNT film. In aligned CNT film, the transport distance
is shortened, and the exciton quenching rate is also reduced. Additionally aligned nanotube film
has much smaller void space, and better contact with substrate. So, plus strong acid doping, using
aligned single wall carbon nanotube film can further improve power conversion efficiency (a
record-high power-conversion-efficiency of >11\% was achieved by Yeonwoong Jung).
Mr. SIDHARTH R.S.
S8 EEE