Question

how is charge separation achieved in crystalline silicon, amorphous silicon and polymer solar cell.

Answer 1

Crystalline Silicon: At the interface there are donor molecules next to the acceptor molecules. lowest energy state is the ground state of the two components. If the exciton is formed within the donor. This exciton will be marked as D* ,excited state in the donor. This exciton must reach the interface. There is a state and energy there that corresponds to this exciton energy above the ground state. Then this exciton at the interface must get into a charge transfer state in which then the electron finds itself on the electron acceptor and therefore, there is also a hole on the donor component. Now we have a charge transfer state D A. The + and -need to move away from one another. This process is called a charge separated state. Amorphous Silicon: In thin film cells e.g amorphous silicon), the diffusion length of minority carriers is usually very small due to the presence of defects so the dominant charge separation is drift, driven by the electrostatic field of the junction. It is extended to the whole thickness of the cell polymer Solar cell In polymer solar cells the light-generated charges are bound together more strongly than in silicon and an additional driving force is required to dissociate the exciton. This is provided by the offset energy levels between donor and acceptor. This offset should be optimized to provide the maximum current and voltage