Problem about organic solar cells: In organic PV an exciton needs to be dissocia

Question

Problem about organic solar cells:

In organic PV an exciton needs to be dissociated using a second material next to the absorber due to the low dielectric constants of the organic materials. When solar cells are placed in full sunlight the temperature can rise to 50 °C easily.

a) Calculate what the dielectric constant should at least be to be able to omit the use of a second material in this condition.

b) Then also calculate the distance between the electron and the hole.

Up to now it has not been possible to increase the dielectric constant of organic materials significantly, hence two materials are needed to dissociate the charges in the exciton. After dissociation the charges are transported though their respective materials.
c) Which types of recombination can take place in organic solar cells?
d) Explain why for some bulk heterojunction devices the current that can be extracted from the device increases sublinear to the increase in illumination
intensity.

Explanation / Answer

a)   the dielectric constant should at least = 7.72

b)   the distance between the electron and the hole =   1.32 * 10-6 * 1.54 * 10-6

                                                                                  = 2.0328 * 10-12 m

c)   Electron hole recombination can take place in organic solar cells .

d) The dark current was subtracted from the photocurrent to obtain a corrected photocurrent Jph that excludes the photovoltage-induced injection current and is thus due solely to the flux of incident photons .

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