Light-Harvesting Mechanism in Polymer/Fullerene/Dye Ternary Blends Studied by Transient Absorption Spectroscopy

The light-harvesting mechanism in polymer:fullerene:dye ternary blends was comprehensively studied by transient absorption spectroscopy. The ternary blend films consist of poly(3-hexylthiophene) (P3HT), a fullerene derivative (PCBM), and a silicon phthalocyanine derivative (SiPc) as a light-harvesting dye. Upon dye excitation of P3HT:PCBM:SiPc blends, P3HT polarons and SiPc anions were generated from SiPc excitons, and then electrons were shifted from SiPc to PCBM, suggesting that SiPc molecules are selectively located in a disordered P3HT matrix mixed with PCBM molecules at the P3HT/PCBM interface. Upon polymer excitation, P3HT excitons decayed more rapidly in ternary blends than in binary blends with an identically rapid formation of SiPc photobleaching, suggesting efficient energy transfer from P3HT to SiPc because of the large spectral overlap between P3HT emission and SiPc absorption, followed by slightly delayed generation of P3HT polarons and SiPc anions from SiPc excitons. Subsequently, the photobleaching disappeared while the P3HT polaron band remains unchanged, suggesting the charge shift from SiPc anion to PCBM. All these photovoltaic conversion processes are much more rapid than backward reactions, and therefore highly efficient light harvesting is achieved in ternary blend solar cells. This is because the SiPc molecules are selectively localized at the P3HT/PCBM interface with an interfacial coverage of 40%. Remarkably, the SiPc molecules are closely contact to both P3HT and PCBM at the interface in solvent-annealed ternary blends, which is an ideal interfacial structure for the efficient photovoltaic conversion.