Tuning the Color Palette of Semi-Transparent Solar Cells via Lateral π-Extension of Polycyclic Heteroaromatics of Donor–Acceptor Dyes

Durable solar cells with tunable color and diaphaneity are very promising for building integrated photovoltaic applications. In this paper we employ donor–acceptor organic dyes U3, U4, U5, and R6 featured by polycyclic heteroaromatics 6,12-dihydroindeno[1,2-b]indeno[2′,1′:4,5]thieno[2,3-d]thiophene (IT2), 7,15-dihydrobenzo[6′,7′]indeno[2′,1′:4,5]thieno[3,2-b]benzo[6,7]indeno[2,1-d]thiophene (BIT2), 7,15-dihydrophenaleno[1,2-b]phenaleno[2′,1′:4,5]thieno[2,3-d]thiophene (PT2), and 9,19-dihydrobenzo[1′,10′]phenanthro[3′,4′:4,5]thieno[3,2-b]benzo[1,10]phenanthro[3,4-d]thiophene (BPT2) to fabricate semitransparent dye-sensitized solar cells (DSSCs). The U3-, U4-, U5-, and R6-based cells are goldenrod, crimson, red, and sapphire blue, with power conversion efficiencies of 3.5%, 8.2%, 7.6%, and 10.1% at the AM1.5G conditions, respectively. Density functional theory calculations reveal that lateral π-extension of the polycyclic heteroaromatic brings forth a downward displacement of lowest unoccupied molecular orbital, affording a high-molar-extinction-coefficient, low-energy-gap blue dye. Femtosecond fluorescence decay measurements of dyed titania and alumina films unravel the electron injection yields of photoexcited dye molecules, which are well correlated with the maximal values of external quantum efficiencies of DSSCs. After 1000 h full sunlight soaking at 60 °C, the red and blue DSSCs exhibit stable photocurrents, owing to the strong bonding and photochemical stability of dye molecules adsorbed on the surface of titania as well as the retention of close-to-unity electron collection yield.