Enhancement of power conversion efficiency by chlorophyll and carotenoid co-sensitization in the biosolar cells

Abstract

Photosynthetic pigment-based biosolar cells (BSCs) have attracted much attention over the past few years in the field of artificial photosynthesis. To further improve the power conversion efficiency (PCE) of chlorophyll (Chl)-based bilayer BSCs, co-sensitization effects of three carboxylated chromophores, methyl trans-32-carboxy-pyropheophorbide-a (H2Chl-1), methyl 7-deformyl-7-(trans-carboxyethenyl)-pyropheophorbide-b (H2Chl-2), and β-apo-8′-carotenoic acid (Car) were investigated as H2Chl-1:H2Chl-2, H2Chl-1:Car, and H2Chl-2:Car combinations. The homogeneously co-sensitized mesoporous TiO2 (m-TiO2) layer (TiO2–H2Chl:Car) serves as the photoactive and electron extraction layer, while a Chl-a derivative, zinc methyl 3-devinyl-3-hydroxymethyl-pyropheophorbide-a (ZnChl) forms J-type self-aggregation film and acts as electron donor layer. The light absorption and electron injection efficiency of the TiO2 sensitized H2Chl-1:Car blends are better than those of other types of co-sensitization after optimizing the blend ratio of each type. The electron absorption spectrum of TiO2–(H2Chl-1:Car) at the best mass ratio of 10:3 (molar ratio of 12:5) showed wider absorption band in 350–450 nm region compared to the spectrum of sole TiO2–(H2Chl-1). The charge transfer resistance was significantly reduced when TiO2–(H2Chl-1:Car) was applied for BSCs. The best PCE of 2.13% was achieved using TiO2–(H2Chl-1:Car = 10:3), which corresponds to 40% increase of PCE compared to the device using TiO2–(H2Chl-1) without Car.