Organic solar cell

Organic solar cell


An organic solar cell or plastic solar
cell is a type of polymer solar cell that uses organic electronics, a branch
of electronics that deals with conductive organic polymers or small
organic molecules, for light absorption and charge transport to produce
electricity from sunlight by the photovoltaic effect.
The plastic used in organic solar cells has low production costs in high
volumes. Combined with the flexibility of organic molecules, organic solar
cells are potentially cost-effective for photovoltaic applications. Molecular
engineering can change the energy gap, which allows chemical change in these
materials. The optical absorption coefficient of organic molecules is
high, so a large amount of light can be absorbed with a small amount of
materials. The main disadvantages associated with organic photovoltaic
cells are low efficiency, low stability and low strength compared to inorganic
photovoltaic cells. Physics
A photovoltaic cell is a specialized semiconductor diode that converts
visible light into direct current electricity. Some photovoltaic cells
convert infrared or ultraviolet radiation into DC. A common
characteristic of both the small molecules and polymers used in
photovoltaics is that they all have large conjugated systems. A conjugated
system is formed where carbon atoms covalently bond with alternating single
and double bonds; in other words these are chemical reactions of hydrocarbons.
These hydrocarbons’ electrons pz orbitals delocalize and form a
delocalized bonding π orbital with a π* antibonding orbital. The delocalized π
orbital is the highest occupied molecular orbital, and the π* orbital is
the lowest unoccupied molecular orbital. The voltage separation between HOMO and
LUMO is considered the band gap of organic electronic materials. The band
gap is typically in the range of 1–4 eV. When these materials absorb a photon, an
excited state is created and confined to a molecule or a region of a polymer
chain. The excited state can be regarded as an electron-hole pair bound together
by electrostatic interactions, i.e. excitons. In photovoltaic cells,
excitons are broken up into free electron-hole pairs by effective fields.
The effective fields are set up by creating a heterojunction between two
dissimilar materials. Effective fields break up excitons by causing the
electron to fall from the conduction band of the absorber to the conduction
band of the acceptor molecule. It is necessary that the acceptor material has
a conduction band edge that is lower than that of the absorber material.
Junction types=Single layer=
Single layer organic photovoltaic cells are the simplest form. These cells are
made by sandwiching a layer of organic electronic materials between two
metallic conductors, typically a layer of indium tin oxide with high work
function and a layer of low work function metal such as Aluminum,
Magnesium or Calcium. The basic structure of such a cell is illustrated
in Fig 2. The difference of work function between
the two conductors sets up an electric field in the organic layer. When the
organic layer absorbs light, electrons will be excited to the LUMO and leave
holes in the HOMO, thereby forming excitons. The potential created by the
different work functions helps to split the exciton pairs, pulling electrons to
the positive electrode and holes to the negative electrode.
Examples In 1958 the photovoltaic effect or the
creation of voltage of a cell based on magnesium phthalocyanine—a macrocyclic
compound having an alternating nitrogen atom-carbon atom ring structure—was
discovered to have a photovoltage of 200 mV. An AlAg cell obtained photovoltaic
efficiency of 0.01% under illumination at 690 nm.
Conjugated polymers were also used in this type of photovoltaic cell. One
device used polyacetylene as the organic layer, with Al and graphite, producing
an open circuit voltage of 0.3 V and a charge collection efficiency of 0.3%. An
AlPt cell had an external quantum yield of 0.17%, an open circuit voltage of 0.4
V and a fill factor of 0.3. An ITOAl cell showed an open circuit voltage of 1
V and a power conversion efficiency of 0.1% under white-light illumination.
Issues Single layer organic solar cells do not
work well. They have low quantum efficiencies (

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