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

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 -3 2 -carboxy-pyropheophorbide- a ( H 2 Chl-1 ), methyl 7-deformyl-7-( trans -carboxyethenyl)-pyropheophorbide- b ( H 2 Chl-2 ), and β -apo-8′-carotenoic acid ( Car ) were investigated as H 2 Chl-1 : H 2 Chl-2 , H 2 Chl-1 : Car , and H 2 Chl-2 : Car combinations. The homogeneously co-sensitized mesoporous TiO 2 ( m -TiO 2 ) layer (TiO 2 – H 2 Chl : 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 TiO 2 sensitized H 2 Chl-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 TiO 2 –( H 2 Chl-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 TiO 2 –( H 2 Chl-1 ). The charge transfer resistance was significantly reduced when TiO 2 –( H 2 Chl-1 : Car ) was applied for BSCs. The best PCE of 2.13% was achieved using TiO 2 –( H 2 Chl-1 : Car  = 10:3), which corresponds to 40% increase of PCE compared to the device using TiO 2 –( H 2 Chl-1 ) without Car .