4.4.3 Change crystallization sequence
Some additives have different
solubility for donors and acceptors, allowing them to crystallize at
different stages. Liu et al. reduced the domain size to 11.2 nm by
changing the crystallization sequence of the PBDB-T: PNDI
blends.[120] As shown in the Figure 9c, if the donor and acceptor
crystallize at the same time, it is easy to form a larger domain size
due to the strong phase separation driving force (Process I). The
presence of diphenyl ether (DPE) hindered the preaggregation of PBDB-T,
and only the crystallization process of PNDI was observed. PBDB-T is
enclosed within the PNDI structure during the SVA procedure (Process
II). As a result, an ideal molecular orientation and an optimal domain
size give rise to the formation of a highly crystalline interpenetrating
network.The optimized structure is suitable for a range of processes,
including the separation of excitons, effective movement of charges, and
the prevention of charge recombination between two molecules. In the
end, PCE increased from 6.55% to 7.78%. Liang et al. added TCB to
P3HT:O-IDTBR to change the molecular crystallization order.[121] As
shown in the Figure 9d, due to the selective dissolution and the high
boiling point of TCB, P3HT crystallizes in advance and the film-forming
time is prolonged. To avoid mutual interference, the crystallization
procedures of P3HT and O-IDTBR are conducted separately. Furthermore, by
incorporating TCB, the aggregation of P3HT takes place prior to the L-L
phase separation, resulting in the formation of an interpenetrating
network.By prolonging the film-forming procedure, there is sufficient
time for O-IDTBR to move downwards and for P3HT to ascend, consequently
improving the vertical arrangement of the film. The better morphology
significantly improves charge transfer, resulting in a 7.18% increase
in PCE from 4.45%. Combined with the additive and SVA, the
crystallization sequence can be changed to reduce the domain size.