Organic Monolayers on p-Doped Silicon Affect Photocurrents of Photosystem I Films

Biohybrid solar cells convert light energy to electricity through the use of Photosystem I (PSI), a protein complex that drives photosynthesis in green plants. The near-one-hundred percent quantum efficiency of PSI, in addition to its vast abundance in nature, gives this technology enormous potential as a sustainable source of energy.  Prior work from our group has shown that PSI films on p-doped silicon yield photocurrents that approach 1 mA/cm², far exceeding those for unmodified silicon or for PSI on metals.  In this project, we have investigated the use of organic monolayers deposited between the p-doped silicon semiconductor and the PSI layer to facilitate electron transfer and increase the stability of the PSI film.  These monolayers were prepared by exposing silicon to solutions containing ω-terminated alkynes to produce an alkenyl attachment of the monolayer to silicon and create a surface of the ω-terminated functionality for interaction with PSI.  Of the monolayer surface groups tested, including alcohols, amines, and carboxylic acids, the carboxylic acid terminus most consistently achieved the highest photocurrents, suggesting favorable electrostatic attraction between the negative surface of the monolayer and positive patches on PSI. Carboxylic acids of varying carbon chain lengths—specifically three, six, and seven carbons—were then tested using photochronoamperometry in order to observe the effect of monolayer thickness on photocurrent as well as the stability of the photocurrent over time.  Differences in monolayer thickness did not have a noticeable impact on current, but all carboxylic acid-treated silicon samples retained over 60 percent more of their initial photocurrent density than the plain silicon control after 30 minutes of continuous testing.