Selective Dye Loading at the Heterojunction in Polymer/Fullerene Solar Cells

Selective dye loading at the polymer/fullerene interface was studied for ternary blend bulk heterojunction solar cells, consisting of regioregular poly(3-hexylthiophene) (RR-P3HT), a fullerene derivative (PCBM), and a silicon phthalocyanine derivative (SiPc) as a light-harvesting dye. The photocurrent density and power conversion efficiency of the ternary blend solar cells were most improved by loading SiPc with a content of 4.8 wt%. The absorption and surface energy measurements suggested that SiPc is located in the disordered P3HT domains at the RR-P3HT/PCBM interface rather than in the PCBM and crystal P3HT domains. From the peak wavelength of SiPc absorption, the local concentration of SiPc ([SiPc]Local) was estimated for the RR-P3HT:PCBM:SiPc ternary blends. Even for amorphous films of regiorandom P3HT (RRa-P3HT) blended with PCBM and SiPc, [SiPc]Local was higher than the original content, suggesting dye segregation into the RRa-P3HT/PCBM interface. For RR-P3HT:PCBM:SiPc blends, [SiPc]Local increased with the increase in the P3HT crystallinity. Such interfacial segregation of dye molecules in ternary blend films can be rationally explained in terms of the surface energy of each component and the crystallization of P3HT being enhanced by annealing. Notably, the solvent annealing effectively segregated dye molecules into the interface without the formation of PCBM clusters.