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.

Effective charge separation is one of the key determinants for the photovoltaic performance of the dye-sensitized solar cells (DSSCs). Herein, two charge-separated (CS) sensitizers, MTPA-Pyc and YD-Pyc, have been synthesized and applied in DSSCs to investigate the effect of the CS states of the sensitizers on the device's efficiency. The CS states with lifetimes of 64 and 177 ns for MTPA-Pyc and YD-Pyc, respectively, are formed via the photoinduced electron transfer (PET) from the 4-styryltriphenylamine (MTPA) or 4-styrylindoline (YD) donor to the pyrimidine cyanoacrylic acid (Pyc) acceptor. DSSCs based on MTPA-Pyc and YD-Pyc exhibit high internal quantum efficiency (IQE) values of over 80% from 400 to 600 nm. In comparison, the IQEs of the charge transfer (CT) sensitizer cells are 10-30% lower in the same wavelength range. The enhanced IQE values in the devices based on the CS sensitizers are ascribed to the higher electron injection efficiencies and slower charge recombination. The results demonstrate that taking advantage of the CS states in the sensitizers can be a promising strategy to improve the IQEs and further enhance the overall efficiencies of the DSSCs.

Three systems were designed, synthesized, and characterized to understand decay processes of photoinduced charge separation in organic semiconductors that are imperative for efficient solar energy conversion. A styrene-based indoline derivative (YD) was used as donor moiety (D), a triazine derivative (TRC) as the first acceptor (A1), and 9,10-anthraquinone (AEAQ) as a second acceptor (A2) in constructing two systems, YD-TRC and YD-TRC-AEAQ. The lifetime of the photoinduced charge-separated states in YD-TRC, a D–A1 system, was found to be 215 ns and that in YD-TRC-AEAQ, a D–A1–A2 system, to be 1.14 μs, a 5-fold increase with respect to that of the YD-TRC. These results show that YD is a more effective donor in YD-TRC and YD-TRC-AEAQ systems at forming long-lived charge-separated states compared to a previously reported atriphenylamine derivative (MTPA) that generated charge-separated states with a lifetime of 80 ns in MTPA-TRC and 650 ns in MTPA-TRC-AEAQ. The third system was constructed using a metal-free porphyrin derivative (MHTPP) to form a MHTPP-TRC-AEAQ structure, a D–L (linker)–A system with a charge separation lifetime less than 10 ns. Therefore, the D–A1–A2 architecture is the best at generating long-lived charge-separated states and thus is a promising design strategy for organic photovoltaics materials.

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