Excitonic one-photon absorption of one-and two-dimenstional C60 polymers: Insights from many-body first-principles calculations

By solving the many-body Bethe-Salpeter equation, we explored the excitonic one-photon absorption of one- and two-dimensional C-60 polymers. We studied seven polymeric structures including C-60-chain, two structures with quasi-tetragonal phases (qTP1 and qTP2), and four structures with quasi-hexagonal phases (qHP1, ML-qHP2, BL-qHP2, and bulk-qHP2). The one-photon excitonic transitions were analyzed in detail relative to the independent particle transitions. Significant red-shifts (0.4 and 0.2 eV for ML-qHP2 and BL-qHP2, respectively) were observed in the excitonic one-photon absorption of low-dimensional polymers compared to bulk polymers. The red-shift can be related to the dimensional and interlayer dependences of screening effect which lead to different binding energies (0.61, 0.44, and 0.27 eV for ML-qHP2, BL-qHP2, and bulk-qHP2, respectively). For two qHPs (i.e., qHP1 and ML-qHP2), different connection modes between C-60 molecules lead to different types (direct and indirect for qHP1 and ML-qHP2, respectively) of electronic band gap, and they also have different spectral profiles due to different hole and electron densities of excitons. Finally, the C-60-chain formed by [2 + 2] cycloadditions has a direct band gap of 2.09 eV which is desirable for applications in photoelectric devices based on the solar spectrum.