Rationalization of excited state energy transfer in D-π-A porphyrin sensitizers enhancing efficiency in dye-sensitized solar cells

Donor-pi-acceptor (D-pi-A) porphyrin based photo-sensitizers are extensively utilized in dye sensitized solar cells (DSSCs). However, investigation on how the intramolecular photoinduced energy/electron transfer influences the device performance is still limited. Herein, we report three new D-pi-A porphyrin sensitizers (LG8, LG9 and LG10) comprising 3-ethynylfluoranthene as the donor and 2,1,3-benzothiadiazole (BTD), 2,3-diphenylquinoxaline (DPQ), and 2,3-di(thiophen-2-yl)quinoxaline (DTQ) as auxiliary acceptors. LG8 showed more efficient and red-shifted energy transfer between the donor and acceptors, which translates into a higher power conversion efficiency (PCE). UV-visible absorption and fluorescence spectra revealed a large bathochromic shift and a significant quenching of the donor emission confirming an efficient intramolecular energy transfer from the fluoranthene-linked porphyrin to the auxiliary acceptor benzothiadiazole in LG8. Density functional theory calculations showed that the LUMO of LG8 and LG10 destabilized and matched with the conduction band edge of the anatase TiO2 nanoparticles. As a result, LG8 and LG10 exhibited the best power conversion efficiency (eta approximate to 3.0%) compared to LG9 (eta = 1.50%) owing to faster electron injection from the dye excited states into the conduction band of TiO2. This work underlines that the modification of the electron donor and the acceptor in D-pi-A porphyrin sensitizers has noticeable influence on the photo-physical properties of pi-conjugated systems, thereby affecting device performances.