Morphology And Miscibility Of Active Layer In Ternary Organic Solar Cells

Thanks to the synthesis of novel donor and acceptor materials as well as the optimization of device structure,the power conversion efficiency（PCE）of organic solar cells increases from 1%to 18%.However,the mechanism of morphology evolution of active layer remains unknown from the aspect of polymer physics.In addition,the organic solar cells are commonly thermally unstable,restricting the commercial application.The article introduced the perylene molecule of PDI-C₁₁ showing of three dimensional structure into the PBDB-T:ITIC system.The results indicate that the device efficiency only keep 56.1%of its original value after thermal annealing at160°C of 12 h.After incorporation of PDI-C₁₁,the PCE value could still remain 80%of the original value.It is believed that PDI-C₁₁ could effectively improve the thermal stability of PBDB-T:ITIC.It is further found that ITIC exhibited an obvious cold crystallization peak around 180°C and the peak disappeared after incorporation of PDI-C₁₁.The enhancement of thermal stability of PBDB-T:ITIC by PDI-C₁₁ is attributing to the restriction of ITIC crystallization.The solvent type will change the pre-aggregation of components in solution and the crystallization of crystalline components.In order to explore the influence of solvent type on the crystallization and miscibility inducing by the pre-aggregation of components,the PDCBT-2F and N2200 serving as the third component were incorporated into the highly efficient PM6:Y6 system.It is found that both of PDCBT-2F and N2200 could enhance the PCE value of PM6:Y6 when using chloroform（CF）as the processing solvent.In contrast,the PCE value sharply reduced as processing from chlorobenzene（CB）,which further decreased upon incorporation of PDCBT-2F.But N2200 could still improve the PCE value.According to the transmission electron microscope（TEM）observation,it is found that Y6 tended to form large aggregates after processing with CB.The incorporation of PDCBT-2F further led to the serious aggregation of Y6 molecules while the existence of N2200 could form the fibrous network,optimizing the performance of solar cells.Combined with the solution UV spectra and surface energy analysis,the Gibbs free energy of the components in CF is close to each other,showing of the matching of pre-aggregation.In the CB solution,the Gibbs free energy of Y6 is relatively low,which is mismatching with the other components.Differential scanning calorimetry（DSC）analysis indicated that the Y6molecules facilitated to form ordered crystalline structure as processing from CF while Y6 tended to form multiple crystal forms corresponding to multiple melting peaks as processing from CB.According to the Flory-Huggins theory and melting point depression method,it is proposed that the multiple melting peaks corresponded to the multipleχvalues.The differentχvalues indicated of the different miscibility,revealing that the miscibility between different components should be determined by severalχvalues and the ratio of melting enthalpy.As processing with CB solvent,there exists the partial mixed phase showing of good miscibility between N2200 and Y6,facilitating the crystallization of Y6.Due to the poor miscibility between PDCBT-2F and Y6,the variation of the active layer morphology and device performance is dependent on the type of third additive.The matching of the pre-aggregation of different components in solvents leading to the change of miscibility could provide theoretical guidelines to optimize active layer morphology and enhance performance of organic solar cells.