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Enhancement of the light-harvesting efficiency in poly(3-hexylthiophene)/fullerene derivative (P3HT/PCBM) bulk heterojunction solar cells has been demonstrated by the introduction of near-infrared phthalocyanine molecules as the third component at the P3HT/PCBM interface. The introduction of silicon phthalocyanine derivative (SiPc) increased the short-circuit current density and hence improved the overall power conversion efficiency by 20%, compared to the P3HT/PCBM control device. For P3HT/PCBM/SiPc devices, two distinct external quantum efficiency (EQE) peaks were observed at wavelengths for the absorption bands of SiPc as well as P3HT before and after thermal annealing, suggesting that SiPc molecules are located at the P3HT/PCBM interface because of crystallization of the P3HT and PCBM domains. Furthermore, the EQE for the device increased even at wavelengths for the absorption band of P3HT by the introduction of SiPc molecules. This indicates that P3HT excitons can be dissociated into charge carriers more efficiently in the presence of SiPc molecules at the P3HT/PCBM interface by energy transfer from P3HT to SiPc molecules. These findings suggest that there are two origins for the increase in the photocurrent by the introduction of SiPc; SiPc molecules serve not only as a light-harvesting photosensitizer but also as an energy funnel for P3HT excitons at the P3HT/PCBM interface.
Honda et al. (2009) studied this question.