An ambipolar semiconductor with a cascade of electronic energy levels generates a long-lived charge separated state for solar cell applications.

Electron excitation and transfer of a triphenylamine derivative, triazine, anthraquinone, oxadiazole, triazine−anthraquinone, and triazine−oxadiazole were studied using density functional theory (DFT) and time-dependent DFT (TDDFT) calculations based on the B3LYP functional. The difference of the excitation wavelengths for anthraquinone using two basis sets, 6-311+G(d,p) and 6-31+G(d,p), was found to be within 4 nm. This indicates 6-31+G(d,p) can produce reasonable results and therefore is appropriate to use in calculations of large systems. The solvent effect on the electron excitation of these six molecules was investigated using three solvents: chloroform, dichloromethane, and ethanol. A red shift in excitation wavelength and enhanced absorption intensity were found in solution with respect to in vacuo condition. Furthermore, we introduced the concept of optically dynamic molecular orbitals (ODMOs) to account for the dynamic effect on electron transfer processes, which is critical in the interpretation of the results. A new dyad, consisting of the triphenylamine derivative−triazine−anthraquinone, is obtained to absorb triple photons of ∼510, ∼400, and ∼320 nm. We predict this new dyad will be a good candidate for use in solar cells.

An ambipolar organic semiconductor with styrene based triphenylamine derivative (MTPA) as an electron donor (D), triazine group (TRC) as an electron acceptor (A 1 ), and 9,10-anthraquinone (AEAQ) as a second electron acceptor (A 2 ) has shown an 8-fold increase in the lifetime of charge separation with a high performance as solar cell materials with respect to the D-A 1 architecture and demonstrated a general D-A 1 -A 2 architecture as a promising materials design strategy for photovoltaics. Here we synthesized and characterized two new D-A 1 -A 2 compounds with perylene bisimide derivatives (PDI and PBI) as A 2 using an integrated experimental and computational method to study and compare the kinetics of three MTPA-TRC-A 2 systems. A two-step sequential decay pathway was observed in both MTPA-TRC-PDI and MTPA-TRC-PBI but a direct decay pathway in MTPA-TRC-AEAQ. The charge separated state with a lifetime of 22 ns in the PDI system and 75 ns in the PBI system relaxes to the corresponding triplet state followed by the decay to ground state in 827 ns and 29.2 μs, respectively. Thus, a triplet state with a lower energy than the charge separated state shortens the lifetime of the charge separated state but increases the overall lifetime of excited states.

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