Light Manipulation Using Organic Semiconducting Materials for Enhanced Photosynthesis

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 CO<sub>2</sub> 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 π-conjugated 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.


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Technologies that harvest solar energy to generate power and products will comprise a substantial portion 22 of future strategies to moderate greenhouse gas emissions. One such solar technology is the growth of 23 photosynthetic algae to produce a range of bioproducts. However, the efficiency of algae photosynthesis 24 is too low for industrial applications, which presents an opportunity to incorporate innovative 25 technological strategies to improve algae growth. Here we show how combining algae growth with light 26 filtration from electricity-producing organic photovoltaics (OPVs) improves overall system efficiency.

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OPVs consist of organic semiconducting materials that can be tuned to absorb light at desired 28 wavelengths while transmitting wavelengths in other parts of the spectrum, which can then be used for 29 photosynthesis by algae. Based on this, we generated a scalable predictive model and demonstrated that 4 Introduction 55 56 The mass cultivation of photosynthetic microorganisms, collectively referred to herein as "algae" 57 is of interest to agricultural, pharmaceutical, and energy industries. However, challenges including 58 suboptimal photosynthetic efficiency and high operating expenses persist in large scale operations 1,2 . This 59 results in a high price point for algae feedstock, which makes algae biomass currently unsuitable for 60 biofuel applications 3,4 .

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The theoretical maximum efficiency of photosynthesis has been estimated between 8-12%, but in 62 practice, photosynthetic efficiency is often much lower, around 1% 5,6,7 . In algae, high photosynthetic 63 efficiency is only realized at very low light intensities 8 , but can be improved by using red light, at 64 wavelengths close to those absorbed by reaction-center chlorophyll and accessory light-absorbing gets absorbed and attenuates ( Figure S6). algae were grown in an open pond of 20 cm depth, with a dry weight biomass concentration increasing 156 from 0.1 g/L to 0.5 g/L, as explained above. Four locations spanning 30 degrees of latitude in North

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America were chosen, and their average monthly solar radiation and day length were used to estimate 158 photosynthetic output using the model ( Figure 4A). The locations were Calgary Alberta Canada (51° N),

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Chicago Illinois USA (42° N), Phoenix Arizona USA (33° N), and Honolulu Hawaii USA (21°N). We 160 also assumed an intrinsic caloric value for the biomass, based on the high heating value (HHV) of its 161 organic material as a rough translation for its power potential 35,36 . In short, we estimated the average 162 amount of electricity that could be produced by the model OPV with the absorbed light, and we estimated 163 the amount of electricity that could be generated by combustion of the biomass grown with the remaining 164 transmitted light.

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The power analysis showed that the addition of an OPV device always resulted in higher 166 combined OPV and algae power production than with photosynthesis alone ( Figure 4BC, Figure S7) and 167 increased the overall efficiency of the system from less than 2% with algae alone to around 5% with algae 168 and OPV ( Figure 5C). The inclusion of the OPV device resulted in an increase in photosynthetic 169 efficiency to above 2% ( Figure 5A), but a decrease in biomass production that was 43-80% of its 170 predicted maximum levels without light filtration ( Figure 5B). Despite these losses in biomass 171 production, any detriment to photosynthetic growth due to light filtering was more than compensated for 172 by the power generation from the OPV device ( Figure 4BC). Quantitatively, power production was 173 predicted to increase by a factor of 2.2-4.8X when OPVs were added, and this effect was particularly 174 prominent in the summer months when light intensity was highest ( Figure 4B). Predicted power 175 production of both OPVs and photosynthetic growth followed solar radiation trends and was highest in 176 Phoenix, then Honolulu, Chicago, and Calgary ( Figure S7).

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The integration of light filters, particularly in the form of OPVs, into algal bioreactors has been 181 proposed as a potential solution to improve photosynthetic efficiency and limit photoinhibition 37  production presents another avenue to pursue for the incorporation of light-filtering OPV devices, as the 210 use of coloured light has been shown to increase production of certain high valued products, such as 211 pigments or fatty acids 7,37,43 . Therefore, the optimal solution for incorporating OPV devices with 212 photosynthetic growth will be case specific and depend on many factors including the intended product.

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Presently, commercially available inorganic photovoltaic modules are achieving efficiency values of 214 10-20%, several times higher than the predicted efficiency for the combined OPV/algae system here 215 (~5%). However, semi-transparent OPV modules and photosynthetic systems offer benefits over opaque 216 inorganic photovoltaics, such as mass production using additive manufacturing methods which have a 217 low energy input and use low capital equipment allowing for localized production 44,45 . OPVs are also 218 primarily comprised of earth abundant and non-toxic materials leading to low cost and safe devices which 219 can be easily disposed of or recycled 46 . OPVs can be tailored to the specific needs of the system, and thus 220 are of interest in alternative applications where some degree of transparency or a certain aesthetic is 221 required (e.g. window coverings and greenhouses) 47-51 . Additionally, the photosynthetic portion of such a  The effect of light attenuation on cultures of different densities was measured using a Li-250A 299 light meter (LI-COR Biosciences, Lincoln, Nebraska, USA) with a submersible spherical light probe. intensity reached less than 100 µmol photons/m 2 /s ( Figure S6). Three, 10-second averages of light 302 intensity were used for each measurement. Attenuation with and without a light filter was measured.

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The light filters were fabricated by slot-die coating tPDI 2 N-EH (PDI) solutions onto PET 307 substrates in air followed by layering a plastic UV-blocking sheet onto the organic layer and finally 308 encapsulation using 3M lamination sheets. Solutions were prepared at 2 mg/mL, 5 mg/mL, and 10 309 mg/mL for LF-1, LF-2, LF-3, respectively. The solvent used was o-xylene. The polymer styrene- 16 biomass)/cm 3 .

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The Beer-Lambert law was fit to empirical data generated from light attenuation measurements giving a 354 mass extinction coefficient (ε) of 750.6 cm 2 /(g dry biomass) and a transmitted fraction (ω) of 0.63576.

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Equations 2 and 3 were combined to give the full algal productivity model (eqn. 4) as a function of depth. Equation 4 can be integrated across the algal mass of the system to give the productivity of the full pond 363 volume (eqn. 5).

(5)
Where P t is the total productivity of the system in μmol O 2 /(g dry biomass·s), and A is the area in cm 2 .

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The values of K 1 and K 2 were determined by fitting the integrated model to empirical productivity data 369 collected at various algal concentrations. The values of K 1 and K 2 were determined to be 199.7 μmol 370 photons · g dry biomass)/(μmol O 2 ·m 2 ) and 0.002564 (g dry biomass · m 2 ·s 2 )/(μmol photons · μmol O 2 ) 371 respectively. Equation five can be normalized (eqn. 6) to a per unit mass basis by dividing equation 5 by 372 the total mass of the system, cV. (6) Where P is the normalized productivity in µmol O 2 /(g dry biomass·s) 378 379 Normalized photosynthetic productivity data (µmol O 2 /g dry biomass/s) was converted to areal 380 productivity (g dry biomass/m 2 /12 hour day) assuming a 1:1 ratio of moles of oxygen produced to moles 381 of biomass produced, which represents the theoretical maximum for a biomass accumulation efficiency 382 value 6 . The molecular weight of biomass was assumed to be 24.6 g/mol (molecular weight for biomass Power production analysis 386 387 Using the modelled relationship between light and biomass production, we aimed to address the 388 feasibility of combining light filtration and the growth of the cyanobacterial consortium in a realistic 389 scenario. To further explore this possibility, we assumed that the light filter tPDI 2 N-EH was part of an  (Table S1).

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Photosynthetically active radiation (PAR), which is the wavelength range used in photosynthesis,

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was calculated from the monthly averages. Watts were converted to µmol photons via a conversion factor 415 of 4.6 µmol photons/m 2 /s = 1 W/m 2 for natural sunlight 57 . PAR radiation was assumed to account for 416 roughly 45% of total spectrum irradiation, and this was also factored into calculations 7 . Additionally, light 417 intensity was normalized per month for average day length in each of the respective cities. These        (Table S1). This power conversion efficiency is appropriate 624 for OPV modules 58,59 . 28 Table S1 Figure 7 shows data for all four ci�es at 0.1 g/L and 0.5 g/L culture density. Algal growth was derived from the photosynthe�c produc�vity model and predicted OPV power was calculated using a power conversion efficiency of 4.2% (Supplementary Table 1).
This power conversion efficiency is appropriate for OPV modules 58,59. Values displayed are the average between 0.1 and 0.5 g/L cultures.

List of Contents
Supplemental Tables  Table S1 -Average and standard deviation of OPV devices metrics Figure S1 -Photographs and transmission spectra of PET/PDI films Figure S2 -Details of OPV device Figure S3 -Photographs of bottle experiments Figure S4 -Photosynthetic electron transport rate of the cyanobacteria consortium under different light intensities Figure S5 -Photograph of culture bottles after experiment with highest light Figure S6 -Data from light attenuation experiments and Beer's Law equation Figure S7 -Predicted power generation of algae and OPV technologies for four North American cities     Bottles are arranged sequentially from left to right: Samples 1→ 12. Samples 1-3: no filter, samples 4-6: