Structurally modified bacteriorhodopsin as an efficient bio-sensitizer for solar cell applications

Structurally modified bacteriorhodopsin (BR) was prepared by simple surfactant treatment using Cetyl trimethylammonium bromide (cationic; CTAB), Sodium dodecyl sulphate (anionic; SDS) and Triton X-100 (nonionic; TX-100). In the UV-visible absorption spectrum, the characteristic absorption band of native BR at 560 nm is hyperchromically (CTAB, due to induced aggregation), bathochromically (SDS, BR solubilisation and partial unfolding) and hypsochromically (TX-100, BR monomerizes) shifted after chemical treatment and the structural modifications were further confirmed by Raman spectra. Theoretical calculations based on optical absorption support an enhancement of BR optical and electrical conductivity via structural modification. Bio-sensitized solar cells (BSSCs) with structurally altered BR as sensitizer were fabricated and their photovoltaic performance was measured. We obtained the maximum short-circuit photocurrent and photoelectric conversion efficiency with TX-100-treated BR (0.93 mA cm^-2, 0.47%), with a quasi-Fermi level and a 124-ms lifetime of photogenerated electrons in TX-100-treated BR-sensitized BSSCs, two times higher than that observed in BSSCs with native BR. A single-diode equivalent circuit model reveals favorable BSSC parameters such as high reverse saturation current (I₀ = 55 nA), low series resistance (R_s = 22.9 Ω) and high shunt resistance (R_sh = 3765.5 Ω) with TX-100-treated BR-based BSSCs. As TX-100 does not alter the BR carboxyl terminus during its monomerization, maximum anchoring to the BSSC occurs which results in enhanced photocurrent generation. Thus, monomerized BR-sensitized BSSCs with their excellent photovoltaic parameters suggest the possibility of replacing native BR with TX-100 BR and this opens up the possibility of reduced cost manufacture of bio-sensitized solar cells.