The most efficient polymer-based photovoltaic material currently known is a class of materials called non-fullerene acceptors NFAs , specifically those based on fused-ring electron acceptor FREA molecules. One of the most promising FREA molecules is called ITIC Indacenodithieno[3,2-b]thiophene-based copolymers , which has shown power conversion efficiencies PCE of over 15% in organic solar cells when combined with suitable donor polymers.The molecular structure of ITIC and other FREA-based NFAs contributes to their high performance in converting solar energy to electrical energy through several factors:1. Strong light absorption: The conjugated -electron system in the fused-ring structure of ITIC allows for strong absorption of sunlight across a broad range of wavelengths, particularly in the visible and near-infrared regions. This enables the material to capture more photons and generate more excitons electron-hole pairs for charge separation.2. Efficient charge separation and transport: The molecular structure of ITIC and other FREA-based NFAs promotes efficient charge separation at the donor-acceptor interface. The electron-rich donor polymer and electron-deficient ITIC acceptor form a type-II heterojunction, which facilitates the transfer of electrons from the donor to the acceptor, reducing the likelihood of recombination. Additionally, the planar and rigid structure of the fused rings in ITIC enables better charge transport within the acceptor phase, leading to higher photocurrents.3. Morphology control: The molecular structure of ITIC and other FREA-based NFAs allows for better control over the morphology of the active layer in the solar cell. A well-ordered, bicontinuous network of donor and acceptor phases is crucial for efficient charge separation and transport. The planar structure of ITIC promotes better phase separation and self-assembly with donor polymers, leading to an optimal morphology for high device performance.4. Energy level alignment: The energy levels of ITIC and other FREA-based NFAs can be easily tuned by modifying their molecular structure, allowing for better alignment with the donor polymer's energy levels. This ensures efficient charge transfer and reduces energy losses in the solar cell.In summary, the molecular structure of ITIC and other FREA-based NFAs contributes to their high performance in converting solar energy to electrical energy through strong light absorption, efficient charge separation and transport, morphology control, and energy level alignment. These factors collectively lead to high power conversion efficiencies in organic solar cells based on these materials.