14.++Organic+PV+cells+2

14.++Organic+PV+cells+2 - MSE 343 May 2, 2008 Organic...

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MSE 343 May 2, 2008 Organic Photovoltaic Cells 2 Outline • Single layer cells • Bilayer cells -Absorption and interference effects - Exciton diffusion - Electron transfer - Recombination - Charge Transport
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Process 2: Exciton diffusion Excitons typically live about 1 ns before they decay. Excitons are neutral, so an electric field cannot be used to help collect them. In most organic semiconductors excitons have no angular momentum and are called singlets. Singlets hop from one molecule to another by a process known as Forster energy transfer. In some organic semiconductors the excitons have an angular momentum of 1 and are called triplets. Triplets are forbidden to emit and therefore live much longer. They hop around by a slower process known as Dexter energy transfer.
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Förster (resonance) energy transfer The electric field near an excited molecule behaves like a field generated by a classical oscillating dipole. The dipole oscillates at frequencies corresponding to its emission frequencies. The oscillating dipole can induce an oscillating dipole in the acceptor molecule. If the acceptor absorbs light at the frequencies of the oscillation, then it can take the energy from the donor molecule. D A
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Rate of Förster energy transfer The integral quantifies the overlap of the emission of the donor (F D ) with the absorption of the acceptor ( ε A ). Strong emitters put out stronger radiation fields. Strong absorbers are excited more easily. K is related to the degree of allignment between donor and acceptor transition dipole moments. r is the separation between the donor and acceptor. When r = R o , ½ of excitons undergo energy transfer ( 29 ( 29 = λ ε d F K R A D o 4 6 ( 29 6 1 = r R r k o D F τ
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Förster (resonance) energy transfer 2.) The donor and acceptor transition dipole orientations are approximately parallel. 3.) The donor and acceptor molecules are within ~ 10 nm of each other. 1.) There is spectral overlap. The radiation field from an excited dipole can transfer energy to another dipole that oscillates at the same frequency. This occurs when: D o n r e m i s A c p t a b
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Measuring exciton diffusion quencher (TiO 2 , ZnO, etc.) non-quencher (glass, quartz, etc.) thickness (0 – 60 nm) 500 550 600 650 700 750 800 0 1x10 4 2x10 4 3x10 4
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14.++Organic+PV+cells+2 - MSE 343 May 2, 2008 Organic...

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