An experimental study on the molecular organization and exciton diffusion in a bilayer of a porphyrin and poly(3-hexylthiophene)

The exciton root-mean-square displacement (𝛬𝐷) in regioregular poly(3-hexylthiophene) (P3HT) deposited onto meso-tetrakis (𝑛-methyl-4-pyridyl) porphyrin tetrachloride (H2TMPyP) has been determined from the photovoltaic response of a device based on these materials in a bilayer configuration. Excitons formed on illumination that reach the interface between H2TMPyP
and P3HT can undergo interfacial charge separation by electron injection into the H2TMPyP and hole injection into the P3HT. The incident photon to current efficiency (IPCE) exceeds 20% over a broad wavelength regime. The theoretical analysis of the IPCE values gives a value for 𝛬𝐷 in H2TMPyP that amounts to 14 nm, while for P3HT a value of 18 nm is obtained. The latter value exceeds literature values reported for P3HT by almost a factor of 3. X-ray diffraction analysis shows that in the studied bilayer the P3HT backbones are aligned parallel to the interface with H2TMPyP. In contrast, in the case of P3HT deposited onto TiO2, for which 𝛬𝐷 has been reported to equal only 7 nm, hardly any organization of the P3HT backbones is observed. The excitonic coupling between P3HT backbones deposited onto H2TMPyP is as high as 125cm−1
, a factor of 3 larger than the excitonic coupling between the disordered P3HT backbones that amounts to 47cm−1. The difference illustrates the importance of controlling the molecular organization for the realization of efficient energy transfer in organic optoelectronics.