Abstract
Photosystem I (PSI) complexes isolated from three different species were electrodeposited on FTO conducting glass, forming a photoactive multilayer of the photo-electrode, for investigation of intricate electron transfer (ET) properties in such green hybrid nanosystems. The internal quantum efficiency of photo-electrochemical cells (PEC) containing the PSI-based photo-electrodes did not exceed ~ 0.5%. To reveal the reason for such a low efficiency of photocurrent generation, the temporal evolution of the transient concentration of the photo-oxidized primary electron donor, P+, was studied in aqueous suspensions of the PSI complexes by time-resolved absorption spectroscopy. The results of these measurements provided the information on: (1) completeness of charge separation in PSI reaction centers (RCs), (2) dynamics of internal charge recombination, and (3) efficiency of electron transfer from PSI to the electrolyte, which is the reaction competing with the internal charge recombination in the PSI RC. The efficiency of the full charge separation in the PSI complexes used for functionalization of the electrodes was ~ 90%, indicating that incomplete charge separation was not the main reason for the small yield of photocurrents. For the PSI particles isolated from a green alga Chlamydomonas reinhardtii, the probability of ET outside PSI was ~ 30–40%, whereas for their counterparts isolated from a cyanobacterium Synechocystis sp. PCC 6803 and a red alga Cyanidioschyzon merolae, it represented a mere ~ 4%. We conclude from the transient absorption data for the PSI biocatalysts in solution that the observed small photocurrent efficiency of ~ 0.5% for all the PECs analyzed in this study is likely due to: (1) limited efficiency of ET outside PSI, particularly in the case of PECs based on PSI from Synechocystis and C. merolae, and (2) the electrolyte-mediated electric short-circuiting in PSI particles forming the photoactive layer, particularly in the case of the C. reinhardtii PEC.Graphical abstract
Highlights
Photosynthetic proteins, e.g., Photosystem I (PSI) and purple bacteria reaction centers, catalyzing light-induced charge separation are often used as the photoactive modules of semi-artificial solar cells and photo-electrochemical devices [1,2,3,4,5,6]
We examined whether upon complete charge separation, photo-generated electrons are efficiently transferred to the exogenous electron transfer (ET) mediators, or whether charge recombination inside PSI reaction centers (RCs) competes with the external ET process, which may significantly limit the efficiency of photocurrent generation
Since the open circuit potential was applied between a working electrode (WE) and reference electrode (RE), the dark current equaled zero, whereas the illumination of the samples resulted in the generation of negative currents in a 300–700 nA/cm2 range, which disappeared after switching off the light and were similar to Photocurrent Absorbance
Summary
Photosynthetic proteins, e.g., Photosystem I (PSI) and purple bacteria reaction centers, catalyzing light-induced charge separation are often used as the photoactive modules of semi-artificial solar cells and photo-electrochemical devices [1,2,3,4,5,6]. This is because these complexes show an exceptionally high efficiency of conversion of absorbed photons into a current of near 100% [7, 8]. The last parameter can be directly compared to the ~ 100% intrinsic “IQE” value characteristic of the photosynthetic proteins
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