4.4.4 Solid additive
Solid additives can reduce the surface energy between donor and acceptor, inhibit the increase of the active layer phase separation structure at high temperature, and achieve high stability.[116, 122-124] Fan et al. used a photoinitiator, bifunctional benzopenone, added to PBDB-T:ITIC films as a non-volatile solid additive.[125] AFM measurements show that solid additives have little effect on the surface morphology of the blend film. Nevertheless, they are effective in enhancing the π-π stacking of the polymer donor, promoting a more favorable face-on orientation (Figure 10a-i). With the addition of BP-BP, the efficiency of the PBDB-T:ITIC device increased from 10.61% to 11.89%.Yan et al. introduced volatile solid additives into J51:N2200 blend system.[126] The additive 4,40 bipyridine was dissolved in chloroform together with the active layer and was removed after hot annealing at 130 ° C for 10 minutes. Bipy can enhance the π-π packing of N2200 and increase the connectivity between the crystal layers. In the process of volatilization, the authors demonstrate that bipy interacts with N2200, resulting in the buildup of N2200 on the surface of the film. The blend experiences a more advantageous vertical phase separation as a consequence of this interaction (Figure 10j). A decrease in the number of donor/acceptor interfaces led to a small decline in J sc, but a notable rise in V oc and FF, indicating a distinct mechanism of action compared to other volatile solid additives.