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.