Ultrafast photomodulation spectroscopy of π-conjugated polymers, nanotubes and organometal trihalide perovskites: A comparison

Abstract

We compare the ultrafast dynamics of the primary photoexcitations in various π-conjugated organic semiconductors, semiconducting single-walled carbon nanotubes (S-NTs) and organometal trihalide perovskites including CH3NH3PbI3 (MAPbI3) and CH3NH3PbI1.1Br1.9 (MAPbI1.1Br1.9), using broadband pump–probe photomodulation spectroscopy in the spectral range of 0.2–2.7eV with 300fs time resolution. The primary photoexcitations in single polymer chains and isolated S-NTs have been found to be quasi-one-dimensional (q-1D) excitons, with characteristic photoinduced absorption (PA) band due to intra-band transitions. This conclusion is in agreement with the large exciton binding energy, Eb in polymers and S-NTs (where Eb>200meV), illustrating the universal optical characteristic features of q-1D excitons. In three dimensional (3D) semiconductors of organometal trihalide perovskites such as MAPbI3 we found that with above-gap excitation both photo-carriers and excitons are photogenerated; but only photocarriers are photogenerated with below-gap excitation. In contrast, mainly excitons are photogenerated in MAPbI1.1Br1.9. The contrast between MAPbI3 and MAPbI1.1Br1.9 is ascribed to the difference of Eb, which is ∼20meV and ∼110meV, respectively. Our work shows that Eb is one of the crucial parameters that determine the photophysics characteristic of semiconductors, that result in universal occurrence of an exciton PA band, regardless if the compound is q-1D carbon based π-conjugated semiconductors, NTs, or 3D crystalline perovskite semiconductors. At the same time, our work also shows that the broadband ultrafast photomodulation spectroscopy is a powerful tool in analyzing the photophysics of semiconductors, and emphasizes the need for a broad probe spectral range in order to decipher the primary photoexcitation species.