If the Mw of polymer used in OSCs is not high enough to realise good blend film, can I change ration of D/A to get better mophology that the thim film of active layer appears?
Dear Ningbin Mao,
Yes you can.
Furthermore, the blend morphology and device performance are influenced by various treatments. For example the choice of solvent, dissolution of polymers in solvent, annealing, spin coating processes\protocols.
Good luck.
Dear Ningbin,
As far as i know, the length of the polymer chain plays an important role in determining the diffusion length of the photo generated excitons in the donor polymer. As a rule of thumb the exciton diffusion length is more or less equal to the length of the of the polymer molecule. So, as the molecule gets larger it is assumed that the molecule length will be larger and consequently the diffusion length of the excitons.
The other important parameter is that it is important that every donor molecule terminates on an acceptor molecule to make the transfer of electrons from donors to acceptor possible other wise the exciton will recombine again before dissociation.If every polymer molecule is terminated on at least one acceptor molecule and the lumolevel of the accptor molecules lies under the homo level of the donor molecules by the amount of the dissociation energy , then it is probable that all excitons that reach the donor acceptor interface will dissociate.
As the length of the donor polymer increases it absorbs more incident light. So, as molecule length increases the area of the hetero junction decreases leading to lower dark current and higher open circuit voltage.
These are the rules of choosing the donor molecules and building the donor and acceptor interface for more efficient bulk heterojunction solar cells.
Best wishes
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