Other types of solar cells

Mesoscopic (Graetzel) organic

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Organic solar cells

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conjugated polymers

three valence electrons form a strong covalent bond, weak bond of 2pz electron

Doping of conjugated polymers

„bandgap” (HOMO-LUMO) energies

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p-doping: [CH]n + 3x/2 I2 --> [CH]nx+ + x I3

- (J, F, Br)n-doping: [CH]n + x Na --> [CH]n

x- + x Na+ (Na, Li, K)

Energy levels in inorganic and molecular semiconductor

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Current transport

o Hole and electron are bound together by Coulomb interaction (exciton)

o Energy of dissociation of electron-hole pair in

organic semiconductors>>crystalline semiconductors

o very small mobility ~10-4 cm2/Vs

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o very small mobility ~10-4 cm2/Vs

o exciton diffusion length ~ 3-10 nm

Width of a device of order of diffusion length is too small to absorb all light!

EXCITON

dielectric constant in organic semicond

Organic molecules used in PV cells

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ZnPc (zinc-phthalocyanine),

Me-Ptcdi (N,N’-dimethylperylene-3,4,9,10-dicarboximide)

the buckminster fullerene C60.

Conjugated polymers

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Absorption coefficients for organic materials

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Operating priciples

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Me-organic seminonductor-Me device

Exciton dissociation at the electrode

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Metal with large work function Metal with small work function

efficiency

heterojunction device

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Exciton dissociation at heterojunction interface

Bulk heterojunction concept

n-type acceptor

p-type donor

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p-type donor

dispersed heterojunction

compositional grading

nanorods

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self-organizing liquid cryatals

dimers absorbing light and helping to separate the carriers

Examples of I-V and QE curves for organic solar cells

I-V QE

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Short-circuit-current densities-voltage (J-V) characteristics and (b) IQE spectra for various devicesfeaturing the blending ratio of PFLAM (P3HT:PCBM:

PFLAM= 1:0.8:X) in wt.%.

•Optimization of the choice of metallic electrodes to achieve good ohmic contacts

on both sides for the collection of oppositely charged photocarriers;

• Optimization of the choice of the donor/acceptor pair (the energy levels of the

HOMO/LUMO influence the VOC);

• Optimization of the band gap and absorption profiles of the semiconducting

polymer for efficient harvesting of the solar spectrum;

• Optimization of the network morphology of the phase separated composite

material to maximize the mobility of the charge carriers within the different

Work in progress

!

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components of the bulk heterojunction.

main problem: stability

Graetzel cell (nanocrystalline-dye, dye-sensitised cell DSC)

dye

semiconductor TiO2

TiO2 (20 nm) covered with dye

1000 x active areaelectrolite

TiO2

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- +

TCO

photosyntesis

immitation

Operation principle

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non-expensive methods of preparation, semitransparent

max efficiency 12.3% (module 4.7%)

Dyes: standard: ruthenium and iodine

new better results: porphyrin and cobalt

stability is an issue!

Co-sensitized mezomorphic cell with η>12%

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two dyes with complimentary absorption

Perovskites ABX3

CaTiO3

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X red (O)

B blue (Ti 4+ )

C green (Ca2- )

Best results:

organic-inorganic perovskite CH3NH3PbI3Current record η=22.2% (unstabilized)

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Perovskite cell architecturelead chloride (PbCl2) and methylammonium iodide (CH3NH3I)

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A – metylamonium

B –Pb

C – I or Cl

TiO2 replaced by Al2O3dye replaced by PVD - perovskite

high Voc, relatively long LD

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mesoporous solar cell thin film solar cell

Problems : soluble in water, unstable (fundamental problem of transformation PbI3→PbI2)

environmental issues

degrades at temperatures ~100oC