Diketopyrrolopyrrole-Based Organic Solar Cells Functionality: The Role of Orbital Energy and Crystallinity

In this work, we investigated diketopyrrolopyrrole (DPP) derivatives as potential donor materials for fullerene:DPP solar cells. The derivatives 3,6-bis­(5-(benzofuran-2-yl)­thiophene-2-yl)-2,5-bis­(2-ethylhexyl)­pyrrolo­[3,4-c]­pyrrole-1,4-dione (DPP­(TBFu)2) and 3,6-bis­(5-(benzothiophene-2-yl)­thiophene-2-yl)-2,5-bis­(2-ethylhexyl)­pyrrolo­[3,4-c]­pyrrole-1,4-dione (DPP­(TBTh)2) were modified by introducing a nitrogen atom into the terminal moiety of the molecule. Our quantum-chemical calculations predicted that this modification would increase the rigidity of the molecular structure and increase the ionization potential relative to the original DPP derivatives. The higher ionization potential primarily supports an enhancement in the open circuit voltage, and a more rigid molecular structure will contribute to reduced nonradiative losses. We experimentally verified the fullerene:DPP solar cell concept based on the coincidence of a smaller driving force for charge separation at the donor/acceptor interface and the crystallinity of the studied DPP derivatives for preparing effective photovoltaic devices. The reduction of the driving force for charge separation could be overcome by more structured/packed donor DPP materials; the delocalization of electrons and holes in such structured materials improves charge separation in OPV devices. Using wide range of experimental methods, we determined the parameters of the studied DPP materials with PC70BM in thin films. This work contributes to practical applications by verifying the concept of this organic solar cell design