The document discusses organic semiconductors and their operation in organic solar cells. It describes how organic materials can act as conductors when their molecular structure forms alternating single and double bonds. The source of free electrons in these materials is pi bonds between carbon atoms. When light is absorbed in the materials, excitons are formed which are electron-hole pairs. At the interface between an organic donor material and an organic acceptor material in a solar cell, the excitons can dissociate into free charge carriers. However, the charges remain bound as polarons which need to be separated by an external voltage before being transported to the electrodes.

1. By
Anish Das
.
.
3/6/2015 1

2.  Conductive nature of organic semiconductor.
 Source of free electron in organic materials.
 charge carriers in organic semiconductor.
 Basic mechanism of organic solar cell operation.
3/6/2015 2

3.  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

4. 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.
3/6/2015 4

5. The different types of carriers in
organics materials are.
1. Excitons
2. Solitons
3. polarons
3/6/2015 5

6.  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
3/6/2015 6

7. 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.
3/6/2015 7

8.  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.
3/6/2015 8

9. 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.
3/6/2015 9

10. 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.
3/6/2015 10

11. 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.
3/6/2015 11

12. 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
3/6/2015 12

13. 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.
3/6/2015 13

14. 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.
3/6/2015 14

15. 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
3/6/2015 15

16. Exciplex Exciton
3/6/2015 16

17. Ready to illuminate your purpose.
3/6/2015 17

18. 3/6/2015 18

19. 3/6/2015 19