Engineering Cascade Bio-solar Cells Inspired by the Z-Scheme of Oxygenic Photosynthesis: Layered Chlorophyll and Bacterio-chlorophyll Derivatives.

The natural Z-scheme of oxygenic photosynthesis efficiently drives electron transfer from photosystem II (PSII) to photosystem I (PSI) via an electron transport chain, despite the lower energy levels of PSII. Inspired by this sophisticated mechanism, we present a layered cascade bio-solar cell (CBSC) that emulates the Z-scheme. In this design, chlorophyll derivatives (Chl) act as PSI analogs, while bacteriochlorophyll derivatives (BChl) serve as PSII analogs in the active layer. The resulting photocurrent, prominently detected in the near-infrared region, is validated through external quantum efficiency measurements. Sub-nanosecond transient absorption spectroscopy reveals a prolonged charge transfer (CT) state from BChl to Chl (Chl-/BChl+ species) compared to the reverse direction (Chl+/BChl- species). This asymmetry highlights a dominant electron flow from BChl (PSII analog) to Chl (PSI analog) under simultaneous excitation, effectively replicating the natural Z-scheme electron transfer. These findings represent a significant advance in the design of bio-inspired solar cells, paving the way for artificial photosynthesis systems and offering profound insights into improving photovoltaic theory and efficiency.