Light Manipulation Using Organic Semiconducting Materials for Enhanced Photosynthesis

Abstract

Photosynthetic microorganisms, such as algae, are sources of bioproducts and pharmaceuticals. As they require only sunlight and carbon dioxide to grow, they have potential for future mitigation of CO2 emissions. However, inefficiencies in the growth of these organisms remains an issue for realizing these emission reductions, primarily in terms of photosynthetic efficiency, photoinhibition, and photolimitation. Here, we show how the use of light filtration through semi-transparent films comprised of organic πconjugated molecules and subsequent organic photovoltaic devices, has the potential to improve the photosynthetic efficiency of algae, and the total power generation of a combined organic photovoltaic/algae system. Experimental data is used to fit a photosynthetic model predicting algal photosynthetic growth given light intensity and light transmission through an organic photovoltaic device. This work demonstrates the feasibility of using a system combining photosynthetic growth with electricity-producing organic photovoltaics and provides a template for exploring other blended applications of these technologies. 3 Main The mass cultivation of photosynthetic microorganisms, collectively referred to herein as "algae" is of interest to agricultural, pharmaceutical, and energy industries. However, challenges including suboptimal photosynthetic efficiency and high operating expenses persist in large scale operations 1,2. This results in a high price point for algae feedstock, which makes algae biomass currently unsuitable for biofuel applications 3,4. The theoretical maximum efficiency of photosynthesis has been estimated between 8-12%, but in practice, photosynthetic efficiency is often much lower, around 1% 5,6,7. In algae, high photosynthetic efficiency is only realized at very low light intensities 8 , but can be improved by using red light, at wavelengths close to those absorbed by reaction-center chlorophyll and accessory light-absorbing pigment molecules 7,9,10,11,12. For photosynthetic applications of algae, the obvious choice for an abundant and sustainable light source is sunlight. However, direct sunlight encompasses the entire spectrum and has high intensity, causing photoinhibition and decreases in photosynthetic efficiency, in some cases completely killing algae cultures 13. Similar to photosynthesis, photoexcitation of the materials in organic photovoltaic devices (OPVs) induces charge separation, which can subsequently be used for conversion of light energy into usable electricity. OPVs have active layers comprised of organic π-conjugated materials (typically polymers or small molecules) that can be tailored to absorb light at desired wavelengths, while remaining transparent in other parts of the spectrum 14,15,16. Here we investigate if OPVs, when used as electricity-producing light filters, improve the feasibility of algae biotechnology. We explore through experiments and modeling, if this combination of technologies increases solar power conversions, leading to the potential for energetic gains ...