By

Anish Das

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3/6/2015 1

Conductive nature of organic semiconductor.

Source of free electron in organic materials.

charge carriers in organic semiconductor.

Basic mechanism of organic solar cell operation.

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At early days organics materials are widely used as

insulating materials specially at semiconductor

industry.

Alan Heeger was the 1st person to propose that under

certain circumstances organic materials can act as

conducting materials.

And the condition in congutaed polymer, i.e.

alternating double and single bonds. Simplest exam is

poly acetylene. It is available in trans and sis form.

trans form is very conductive like silvery powder, more

conductive than Si.3/6/2015 3

The source of free electron in

polymer is the π bond electron, which is loosely bound

due to the lateral overlap of electron orbitals and is

perpendicular to the sigma bond. Sigma bond is strong

bond cause covalencey is there.

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The different types of carriers in

organics materials are.

1. Excitons

2. Solitons

3. polarons

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An excitonis an excited

quasiparticle in a solid, which is

formed by a Coulomb-bound

electron-hole pair.

It is more prominent in organic semiconductors as compared to their inorganic counterparts: as the dielectric constant is lower in organics

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Soliton is a structural distortion in

organic materials and forms when two single bonds meet.

Single bonds moves through out the

polymer chain, i.e. soliton wave propagates, i.e electron is

propagating through out the chain give rise to the current.

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Polaron is the coupling of electron/hole is

the lattice.

Because organic materials does not have

prominent periodicity like wise in crystal.

So the charge carriers see potential

difference at bound with the lattice.

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Use a blend of acceptor(fullerene) and donor(conjugated

polymer).

Light absorption in donor and stable exciton generation.

Exciton diffusion/ Polaron production.

Polaron pair dissociation and free 𝑒−-ℎ+ pairs generation.

Charge transport and get current.

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Here we are using two kind of

materials. One is donor to produce exciton and acceptor

dissociate exciton attract electrons. It’s a kind of

interfacing device.

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As stated above that light is absorbed

by donor material or conjugated polymer & exciton is

generated. This exciton is stable due to low K and high

binding energy.

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Excitons are electrically neutral so it

will come to the interface of donor and acceptor by only

diffusion and see different potential or atmosphere, known

as exciplex

Exciplex

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At the interface of donor and acceptor

materials the exciplex will see different atmosphere i.e.

different potential i.e exciplex will feel potential

difference. Then the electron will be drifted to the acceptor

lattice and hole to the donor latttice i.e. exciplex

dissociation occurs. And forms polarons pairs. These pair

is still in coulomb bound.

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Polaron pairs formed at the acceptor

and donor lattice is still in coulomb attraction. So we have

to apply external voltage to dissociate them. This is one of

the draw back of organic solar cell.

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As the polaron pairs are separated by applipled volrage

then they will move towards the metal electrods by

hopping transport.

Hopping transport is very slow and depends on 𝐸2−𝐸1

𝑇.

E1 E2

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Exciplex Exciton

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Ready to illuminate your purpose.

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