Microstructure Engineered Photon‐Managing Films for Solar Energy to Biomass Conversion

Conversion of solar energy into chemical energy through natural photosynthesis plays a crucial role in sustainable energy transformation, bioresource production, and CO2 biofixation. Nevertheless, the overall solar energy to chemical energy (biomass) conversion efficiency in the photosynthetic organisms is still unsatisfactory because of their inefficient utilization of solar light. Here, a photonic method to improve photosynthesis of a unicellular green microalga, Chlamydomonas reinhardtii, a photosynthetic organism model is reported. For this purpose, an easy‐to‐fabricate microphotonic film is developed to improve the spectral quality of solar light reaching the microalgae through photon management (i.e., simultaneous solar spectral conversion and directional fluorescent emission). This study demonstrates that the short‐term oxygen evolution rate and the long‐duration biomass production of microalgae in 200‐mL laboratory photobioreactors are enhanced by a factor of 38% and 54%, respectively. In 5000‐mL scaled‐up bubble column photobioreactors placed outdoor under natural sunlight and weather conditions, the biomass yield is improved by more than 20% when compared to the control experiments conducted in parallel in an optically clear bubble column photobioreactor. Based on such experimental observations, the work here demonstrates the potential of photon management for promoting the solar energy‐to‐biomass conversion process of microalgae and other photosynthetic organisms.


Introduction
Solar energy has been considered one of the most promising energy alternatives in the context of global warming, air pollution, and energy crisis. [1] During the past decades, tremendous research efforts have been devoted to harvesting and converting solar energy into other useful forms of energy based on photo thermal, photo electric, and photochemical conversions, spectral matching between the light reaching the microalgae and the absorption of the light-harvesting pigments located in the microalgae. [13] Photosynthetic microalgae having different types of light-harvesting pigments generally acclimate to different photon spectra and intensity for effective photosynthesis. [14] Inspired by this fact, photon managing materials have emerged in the past decade. Among these materials, spectral-shifting materials with flat surfaces have been widely applied in the photobioreactors [15] while their effectiveness for augmenting microalgal photosynthesis remains unsatisfactory due to the limited light extraction which restrains the majority of the fluorescently generated light from escaping to free space (i.e., air) for external use due to total internal reflection. The inefficient use of these internally generated light leads to inconsistent results in augmenting the photosynthesis of microalgae. [16] Instead, Wondraczeka et al. proposed a back-light design with the spectral conversion materials positioned behind the flat photobioreactors for improving the light-harvesting efficiency of microalgae while this configuration is useful only for dilute cultures. [17] Conversely, taking advantage of the internally generated light, luminescent solar concentrators are therefore developed, in which the internally generated red light is mostly waveguided and concentrated onto small area photovoltaic (PV) cells to generate electricity [18] for the daily operation of the microalgal cultivation system rather than directly promoting microalgal photosynthesis. From these points of view, there are still great opportunities to augment the photosynthesis of microalgae by further increasing their photosynthetic light utilization efficiency through photon management.
In our previous work, we have demonstrated that photon management can increase the photosynthesis and biomass production of leafy green lettuce plants using the microphotonic film as greenhouse covering materials. [13b] Here, using a unicellular green microalga, C. reinhardtii, as a photosynthetic organism model, we further report a boosted photosynthetic efficiency of microalgae in both laboratory-scale and scaled-up outdoor photobioreactors through photon management. For this purpose, an easy-to-fabricate microstructured photonic film with capability of simultaneous solar spectral conversion and directional fluorescent emission was made by a facile solution method. The microphotonic film efficiently improves the overall spectral quality of sunlight reaching the microalgae in photobioreactors, which increases the conversion efficiency of solar energy to chemical energy, leading to a tremendous augmentation in biomass productivity. This work thus demonstrates the potential for scaling-up applications of this appealing technology relevant to photon management.

Fabrication and Optical Characterization of the Spectral-Shifting Microphotonic Film
Figure 1 illustrates schematically the photon management of solar spectrum through a spectral-shifting microphotonic film with a feature of simultaneous spectral conversion and directional fluorescent emission. The microphotonic film is doped with fluorophores for solar spectral conversion (Figure 1b), leading to an improved photon spectrum that potentially enables more absorption by primary light-harvesting pigments such as chlorophyll a, chlorophyll b, and carotenoids in photosynthetic organisms (Figure 1c). More importantly, we propose an asymmetrical interface design on the spectral-shifting microphotonic film to increase its overall efficiency through a microsized dome structure for enhanced light extraction and directional emission control (Figure 1b).
The spectral-shifting microphotonic film with a microdome array on the top surface can be fabricated using a facile bladecoating process on a replica mold with inverse microstructures ( Figure S1, Supporting Information). These microdomes are closely packed on a square lattice with a period of 400 µm and a height of 65 µm ( Figure S1, Supporting Information), which were in fact designed and optimized using the Monte Carlo ray-tracing method.
[13b] Figure 2a shows transmittance spectra of the fabricated films, which were measured under the emulated one-sun AM1.5 irradiation. A spectroscopic integrating sphere is used to collect both the directly transmitted light and the fluorescently generated light escaped from the film. The spectral-shifting films with the same effective thickness and the same amounts of fluorophores absorb most of the green light and reemit it Figure 1. Schematic illustration of the spectral-shifting microphotonic film for photon management. a) Spectrum of the AM1.5 in the wavelength range of photosynthetically active radiation (PAR). Green light, the least photosynthetically active light, amounts to 35.1% of the total PAR photo ns. b) Cross-sectional schematic of the spectral-shifting microphotonic film. Fluorophores are shown as red dots and the host matrix materials have a refractive index of n. The schematic is not scaled. c) Normalized absorbance spectra of extracted and purified light-harvesting pigments including chlorophyll a (Chl a), chlorophyll b (Chl a), and carotenoids.
in the red light, which results in higher red transmission of over 100% than that of the fluorophore-free film. Remarkably, the spectral-shifting microphotonic film shows a substantial increase in red emission compared to the spectral-shifting planar film. Quantitative analysis indicates that the microdome structures on the spectral-shifting microphotonic film surface increase the forward-viewing external quantum efficiency, expressed as the ratio of surface emission from the flat side of the spectral-shifting microphotonic film to all absorbed photons, by two folds compared to the spectral-shifting planar film. The dramatically improved external quantum efficiency arises from the boosted light extraction efficiency (the ratio of surface emission to all reemitted photons) in the forward viewing direction, from 13% for the spectral-shifting planar film to 65% for the spectral-shifting microphotonic film, due to the microdomes induced photon recycling process. [19] In addition, we also noted that the spectral-shifting microphotonic film decreases the photosynthetic photon flux density by ≈27% due to the nonunity external quantum efficiency while significantly increases the red/green photon flux density ratio from 692:624 of the fluorophore-free planar film to 869:174 under one sun AM1.5 (Table S1, Supporting Information). Besides, the spectral-shifting microphotonic film slightly increases the photon flux density in the far-red waveband (700-750 nm) from 336 to 364 µmol m −2 s −1 . On the other hand, Figure 2a shows that the microdome structures have no detrimental effect on the total transmission of the spectral-shifting microphotonic films. Nevertheless, we noticed a light diffusion effect from the spectralshifting microphotonic film due to the presence of the periodical microdomes. The angular distribution of the scattered light from the fluorophore-free film with microdomes is shown in Figure 2b. The diffusion angle, i.e., the full width at half maximum (FWHM) of the scattering angles, is ≈12°. The inset in Figure 2b shows the scattering pattern that was projected on a wall at a sample-to-wall distance of ≈165 cm. The fluorophorefree film with microdomes has an average total transmittance of 88% and diffuse transmittance of 63%, respectively, and has an average haze of 72%, as shown in Figure 2c. We also experimentally examined the photostability of the spectral-shifting microphotonic films outdoors. The transmittance spectra of the spectral-shifting microphotonic films remained unchanged after 120 d of exposure under natural sunlight and weather conditions ( Figure S2, Supporting Information).

Increased Photosynthetic Activity under Emulated Sunlight
To demonstrate augmented oxygenic photosynthesis with the as-developed spectral-shifting microphotonic films, a unicellular green alga, C. reinhardtii UTEX 90, was used as a model organism. C. reinhardtii is widely distributed in soil and fresh water and has served as a reference organism in fundamental research on aquatic photosynthesis and many other areas. [20] Besides, C. reinhardtii has been extensively exploited to produce high-value compounds, including biopharmaceuticals and biofuels. [21] Figure 3a shows a schematic for tracing the short-term oxygen evolution, an important indicator of photosynthesis, of microalgae in the hermetically sealed photobioreactor (PBR, 250 mL) with the spectral-shifting microphotonic film. Compared to open raceway ponds, the PBRs significantly minimize the likelihood of contamination by other microorganisms or alien microalgae species. [22] The spectral-shifting microphotonic film converts the majority of green light that was poorly captured by C. reinhardtii into the red light overlapped by the absorption of C. reinhardtii ( Figure S3, Supporting Information), thus leading to a modified photon spectrum consisting of less green photons, but more red ones, which potentially increases the effectiveness of the microalgal photosynthesis due to the improved quality of light reaching the microalgae. [23] In this work, two identical photobioreactors were used and immersed in the same water bath side by side ( Figure 3a and Figure S4, Supporting Information) during the experiment, allowing simultaneous comparison between various films at a constant working temperature of 20 °C ( Figure S5, Supporting Information). The head volumes of these two PBRs were kept the same (≈55 mL) and the gaseous oxygen concentrations in the head volumes were measured in real-time (OXF500PT, Pyroscience, Germany). The algae were first cultured in a trisacetate-phosphate (TAP) maintenance medium [20a] and then transferred to the PBRs for tests. To ensure a fair comparison, all short-term oxygen evolution experiments were performed Figure 2. a) Transmittance spectra of the fluorophore-free planar film, the fluorophore-free film with microdomes, the spectral-shifting planar film, and the spectral-shifting microphotonic film. The transmittance is over 100% in the red waveband since the spectral-shifting film reemits red light. b) The scattered light intensity of the fluorophore-free film with microdomes as a function of the scattering angles. Inset shows the light diffusion effect of the same film. c) Total and diffuse transmittance spectra as well as the transmission haze spectrum of the fluorophore-free film with microdomes.
during the exponential growth phase of algae [24] with a concentration of ≈6 × 10 6 mL −1 . A pump was used to circulate the air in the PBRs and stir the algae culture to prevent cells from settling. The film was placed in front of the PBRs but outside the water bath, with the PBRs held against the front wall of the water bath to minimize the gap in between ( Figure 3a and Figure S4, Supporting Information). Figure 3b shows three cycles of oxygen evolution/consumption from algae to illustrate the photosynthetic activity under different optical films. Each experimental cycle was comprised of three alternations of 15 min in darkness and 30 min under emulated solar illumination. The short-term nature of the experiment allows us to keep algae concentrations relatively constant and ensure sufficient carbon dioxide in the sealed photobioreactors. In each experiment, the first and third cycles were performed as controls with the fluorophore-free planar films. During the second experimental cycle, different types of optical films including the fluorophore-free planar film (black), the spectral-shifting planar film (green), the fluorophore-free film with microdome structures (blue), and the spectral-shifting microphotonic film (red) were examined under identical emulated solar illumination, with an impinging photosynthetic photon flux density (PPFD) of 500 µmol m −2 s −1 (equivalent to 0.25 sun in intensity). Impressively, the spectral-shifting microphotonic film showed a significant increase in the short-term oxygen evolution rate of microalgae under solar illumination. In contrast, the use of planar films had a negligible impact on the oxygen evolution rate, which is nearly the same as that in the control cycles. Figure 3c shows the average oxygen evolution rates from three independent experiments. The oxygen production under the spectral-shifting microphotonic film was consistently faster than that under the controls, by 38%. For the case of the spectral-shifting planar films, the differences were insignificant when compared to the controls. It is interesting to observe that the fluorophore-free films with microdome structures also showed a moderate increase in oxygen production, likely because of the diffused light character arising from the microstructures on the top surface of the films. [25] A strong enhancement in the oxygen evolution rates was observed in a broad range of algal concentrations (Figure 3d). The rate peaked at an optical density (OD) at the peak absorption wavelength of 675 nm (OD 675 ) of 1.2, corresponding to an . Short-term oxygen evolution of green algae with the spectral-shifting microphotonic film. a) Schematic of the experimental setup for shortterm oxygen evolution of microalgae in a hermetically sealed photobioreactor. The film was placed in front of the photobioreactor but outside the water bath. The schematic is not to scale. b) Representative curves of real-time oxygen evolution/consumption of microalgae. In each experiment, the first and third cycles were performed as controls with fluorophore-free planar films. During the second experimental cycle, the fluorophore-free planar film (control, black), the spectral-shifting planar film (green), the fluorophore-free film with microdomes (blue), and the spectral-shifting microphotonic film (red) were examined. c) Average oxygen evolution rates and the corresponding percentages determined from the curves in (b). For reference, the fluorophore-free planar film has a percentage equal to 100%. Data from three independent measurements are presented as means ± SD. Values with the same letter are not statistically different at P = 0.05. d) Averaged oxygen evolution rates of algal suspensions with different concentrations under the spectral-shifting microphotonic film (red circles) and the fluorophore-free film with microdomes (control, blue squares). e) The corresponding relative enhancements between the spectral-shifting microphotonic film and the fluorophore-free film with microdomes. The solid lines are for eye guidance only. f) Comparative studies were performed under the spectral-shifting microphotonic film (red circles), the fluorophore-free film with surface microdomes (blue triangles), and the fluorophore-free planar film (control, black squares), respectively. The solid lines are exponential fittings for eye guidance only.
algal concentration of ≈1.24 × 10 7 mL −1 , showing maximal productivity as a result of balanced photon efficiencies across the depth of the photobioreactor, at which both the incident light and microalgae were used efficiently; [26] however, the relative enhancement of the oxygen evolution rate in Figure 3e started to decrease at this concentration due to the depletion of the red light in the photobioreactor. [26b] Figure 3f shows the oxygen evolution rate as a function of the impinging PPFD under three different films. Light saturation was observed for all three cases. However, the spectral-shifting microphotonic film shows the maximum oxygen evolution rates compared to other films in the studied PPFD range.

Increased Microalgae Proliferation under Emulated Sunlight
Long-term microalgae proliferations were then performed to examine the photosynthetic yield of microalgae. A set of open photobioreactors with different films were investigated under daily 12:12-h light/dark cycles with an impinging PPFD of 500 µmol m −2 s −1 . The photobioreactors were shaken continuously to ensure uniform illumination and to prevent cells from settling. Under these conditions, multiple replications have been performed and Figure 4 shows the main results of experiments done between January 11 and 19, 2019. The algae concentrations were monitored daily over 9 d by measuring the optical density at OD 675 of algal suspensions. After an incubation period of about 4 d, as shown in Figure 4a, all photobioreactors showed a substantial increase in OD 675 due to the rapid proliferation of algae. The photobioreactor with the spectralshifting microphotonic film, however, showed the most rapid increase; the OD 675 and the corresponding cell concentrations measured by cell counting (Marienfeld-SupeRior, Germany) at day 9 are shown in Figure 4b,c, respectively. An increase of 59% in OD 675 and 54% in cell numbers was, respectively, observed when compared to that of the fluorophore-free planar films.

Increased Outdoor Biomass Yield of Microalgae under Natural Sunlight
Using a polymer matrix material for photon management has the advantages of being flexible, lightweight, and easy to laminate on curved surfaces. However, one of the most compelling advantages of developing a fluorophore-doped polymer-based film lies in the possibility of cost-effective scalable fabrication. For scaling-up applications, we further produced the spectralshifting microphotonic film with a size of 150 mm × 240 mm using the same poly(methyl methacrylate) (PMMA) as matrix materials but on a large replica mold by the aforementioned solution method ( Figure S2, Supporting Information). We used Figure 4. Long-term biomass proliferation of green algae with the spectral-shifting microphotonic film. a) Optical density at the peak absorption wavelength of 675 nm (OD 675 ) of algal cultures as a function of time using a spectral-shifting microphotonic film (red circles), a fluorophore-free film with surface microdomes (blue triangles), a spectral-shifting planar film (green hexagons), and a fluorophore-free planar film (black squares). The solid lines are exponential fittings for eye guidance only. b) OD 675 at day 9 and the corresponding percentages for different films. The fluorophore-free planar film has a percentage equal to 100%. Data are means ± SD. Values with the same letter are not statistically different at P = 0.05. c) Cell concentration of algae at day 9 and the corresponding percentages for different films. The fluorophore-free planar film has a percentage equal to 100%. Data are means ± SD. Values with the same letter are not statistically different at P = 0.05.
PMMA because of its excellent mechanical and chemical resistance, offering potentially long lifetimes for outdoor use. [27] Compared with indoor cultivation on the laboratory scale under relatively stable conditions, outdoor cultivation under natural sunlight and weather conditions is desired for largescale cultures of microalgae for commercial purposes in a costeffective way due to its less operational costs. [28] In this view, we further examined the general use of the spectral-shifting microphotonic films for outdoor microalgal cultivation in scaledup photobioreactors. We constructed four bubble column photobioreactors (Figure 5a) of 90.0 cm heights and 12.0 cm inner diameters for comparative cultivation studies. Previous studies have demonstrated that the bubble column photobioreactor with a properly large diameter, generally between 10 and 20 cm, could be a realistic option for producing large quantities of microalgae biomass outdoors. [29] The optically transparent photobioreactors are covered by four different films using a 30-µm-thick, optically clear double-sided acrylic film (PSF25015, MyBinding, OR, USA) as an adhesive interlayer, including the fluorophore-free planar film, the spectral-shifting planar film, the fluorophore-free film with microdome structures, and the spectral-shifting microphotonic film, respectively. Each photobioreactor was filled with 5000 mL algae suspension with an initial OD 675 around 0.04. An air sparger was placed at the bottom of each photobioreactor to supply the fresh air for algae growth with a constant flow rate of 2 L min −1 , to strip out the accumulated oxygen, and to circulate the algae suspensions to prevent cells from settling. [30] Real-time temperatures of algae suspension in each photobioreactor were recorded by precalibrated K-type thermocouples (0.3 °C accuracy). Before The photobioreactors from left to right were covered by four different films using a 30-µm-thick, optically clear double-sided acrylic adhesive film, including the fluorophore-free planar film, the spectral-shifting planar film, the fluorophore-free film with microdome structures, and the spectralshifting microphotonic film, respectively. All photobioreactors were placed on the ground with a nearly 55° tilt angle to the south. b) Daily PPFD during the experiment period. c) Daily temperature profiles of microalgal solution in four photobioreactors surrounded by four different films, respectively. d) OD 675 of algal cultures as a function of time. The solid lines are linear fittings for eye guidance only. e) Biomass yield of microalgae in photobioreactors with the fluorophore-free planar film (black), the spectral-shifting planar film (green), the fluorophore-free film with microdome structures (blue), and the spectral-shifting microphotonic film (red), respectively. Data are means ± SD. Values with the same letter are not statistically different at P = 0.05. the comparative study, another test was first carried out to show that the four-bubble column photobioreactors are truly identical for algae cultivation when no films were installed, and the thermocouples were calibrated before use.
In this study, we performed multiple replications of experiments for outdoor microalgae cultivation in the bubble column photobioreactors with the spectral-shifting microphotonic film, respectively, on October 16-22, 2021 ( Figure S6, Supporting Information), October 24-31, 2021 ( Figure S7, Supporting Information), and November 04-11, 2021. Figure 5b showed a 7-d experiment of our most recent test, which was mostly sunny and showed a natural day length of ≈10 h (Figure 5b). Solar light radiation at the site reached ≈1200 µmol photons m −2 s −1 on the middle day. No artificial light was supplied to the cultures at night during the growth experiments. The environmental temperatures experienced a significant variation between day and night ( Figure S8, Supporting Information) and were comparatively high in the first three days while dropping dramatically in the following four days. Algal solution temperatures in four photobioreactors with various films are plotted in Figure 5c. It is noticeable that the algal solution temperature fluctuated upon the change in the environmental temperatures. Compared to the fluorophore-free planar film, the spectral-shifting microphotonic film decreased the maximum temperatures of the algal solution by nearly 2 °C probably due to the light diffusion effect and the partial absorption of sunlight by the film. Nonetheless, the upper limit of the solution temperature in the first three days was up to 40 °C in all four PBRs, which was far beyond the optimum temperature range for the growth of C. reinhardtii, [31] probably resulting in a detrimental effect on the microalgae growth rate due to the heat stress [32] regardless of the films. The spectral-shifting microphotonic film decreasing the temperature of algae cultures might reduce the heat stress; however, this temperature effect on algae growth in the outdoor bubble column photobioreactor needs to be further investigated, especially when it is used for year-round mass cultures. To trace the algal growth in the PBRs, we monitored the algae concentrations daily by measuring the OD 675 . The growth curves in Figure 5d showed that the lag phase was not detected and microalgal cells were able to quickly adapt to the medium probably because of the relatively high culture temperature. [33] More importantly, the PBR with the spectral-shifting microphotonic films showed the most rapid increase in microalgal concentration. An increase of 18.5% in OD 675 was observed when compared to that of the controls (fluorophore-free planar films). On the other hand, 100 mL algal suspensions from each PBR were collected daily from November 6th, and dry biomass of algae was obtained by concentrating using a centrifuge and subsequently drying in an oven till no weight change. The spectral-shifting microphotonic films showed a substantial increase in biomass yield of microalgae by 23.2% relative to the fluorophore-free planar film (Figure 5e). The relatively high volumetric productivity suggested the great potential for outdoor mass cultures. [34] Variable weather conditions outdoors significantly alter the algal growth, and especially a high level of solar irradiance or an extreme culture temperature might result in a decrease in algal productivity. [35] Besides, biomass production of microalgae depends largely on the photobioreactor type, geometry, and other operating conditions like mixing. [36] The augmented biomass yield in the PBRs with the spectral-shifting microphotonic films is most likely attributed to the improved quality of the sunlight reaching the microalgae according to our indoor and outdoor experimental observations.

Conclusions
Our experimental results demonstrated that improving sunlight spectral quality through photon management by an easyto-fabricate spectral-shifting microphotonic film increases the conversion efficiency of solar energy into chemical energy via natural photosynthesis of microalgae and thereby augments their biomass production in photobioreactors both indoors under emulated sunlight and outdoors under natural sunlight, respectively. The augmentation of biomass production is highly dependent on algal concentrations. The spectral-shifting microphotonic films here use low-cost and commercially available polymers as matrices and, more importantly, are easy to manufacture at scale by a solution processing method (e.g., doctor-blade coating). These characteristics make this technology appealing for large-scale deployment of photobioreactors for augmenting biomass production in near future. With the demands for sustainable and renewable algae-derived bioproducts, our work here paves a way for promoting microalgal production in photobioreactors for scaling-up applications by efficient utilization of solar energy.

Experimental Section
Fabrication and Optical Characterization of the Spectral-Shifting Microphotonic Film: The spectral-shifting microphotonic film was fabricated using a similar solution-based method. [13b] First, the fluorophore-doped polymer solution was prepared by dissolving 0.1 wt% Lumogen F Red 305 (LF305, BASF, Germany) and PMMA in N, N-dimethylformamide (DMF) at ambient temperature for at least 24 h with mechanical stirring. The spectral-shifting microphotonic film was then cast-molded with an automated blade coating film applicator (MSK-AFA-II, MTI Corp.) using a polyether ether ketone (PEEK) replica mold. The as-casted film was subsequently dried at 60 °C for 90 min and then released from the mold in water at ambient temperature. Similar procedures were performed for the fabrication of the fluorophore-free film. A glass plate was used as the substrate to fabricate the planar films. For diffusion angle measurement, a normally incident laser beam (λ = 532 nm) with a waist of 2 mm was used. A power meter (PM100D, Thorlabs, Inc., NJ, USA) was placed at a fixed distance of 15 cm away from the film but can be scanned at different scattering angles to collect the scattered light. An iris was placed in front of the power meter to increase the angular resolution. Total transmittance (T t ) and diffuse transmittance (T d ) were measured by a UV-vis spectrophotometer (UV-3101 PC, Shimadzu Inc.) equipped with an integrating sphere. Haze, describing the amount of light scattered when light passes through a sample, is defined as Haze (%) = T d /T t × 100%.
Algal Culture: Unicellular green algae C. reinhardtii (UTEX 90) was purchased from the Culture Collection of Algae at the University of Texas at Austin, which is commonly used as a model organism amenable to studying chloroplast-based photosynthesis. [20a] Algae cells were cultured in tris-acetate-phosphate (TAP) medium [20b] in 500 mL flasks. The cultures were shaken at 145 rpm on a J-KEM BTS 3000 oscillating mixer (St. Louis, MO) to ensure uniform light exposure and to prevent cells from settling. Parafilm was used for covering the flasks to minimize contamination and reduce the evaporation of cultures. Algal cultures were maintained at ≈20 °C under a PPFD of 120 µmol m −2 s −1 provided by cool-white LEDs (Model 6A191440DA, Color temperature ≈4000 K, Great Eagle Lighting Corporation, Porter Ranch, CA). [37] A 12-h/12-h light/dark photoperiod was used and all outside light sources were eliminated to establish the photoperiod. [38] OD of algal suspensions was monitored frequently with UV-vis-NIR spectrophotometers (UV-3101 PC, Shimadzu Inc.). A cuvette with a 1 cm path length was used for OD measurement. The corresponding algal cell concentrations were measured by cell counting in commercial chambers with V-slash at the exterior sides of the chamber bottom (Marienfeld-SupeRior, Germany). The algal morphology was observed using an optical microscope (Nikon Eclipse LV150N, Japan).
Algal Short-Term Oxygen Evolution/Consumption in Photobioreactors Indoors under Emulated Sunlight: UTEX 90 algae are microscopic unicellular organisms, capable of producing carbohydrates through photosynthesis and releasing oxygen as a byproduct. This provides a means to quantify photosynthesis by tracing the oxygen concentration variations in a sealed photobioreactor (PBR). All shortterm oxygen evolution experiments were performed in sealed PBRs during the exponential growth phase of algae with a concentration of ≈6 × 10 6 mL −1 , [24] corresponding to an OD 675 of ≈0.6 at the algae absorption peak wavelength of 675 nm. Two identical PBRs with 120 mL algae suspension solutions were used and submerged in the same water bath throughout the experiment, allowing simultaneous comparison between different films at a constant working temperature. The head volumes of both PBRs were kept the same at ≈55 mL and the gaseous oxygen concentrations in the head volumes were measured in real-time (OXF500PT, Pyroscience Inc., Germany) with a resolution of 0.05% O 2 in the gas phase. A diaphragm pump was used to circulate the air in the sealed PBRs and to stir the algae culture to prevent cells from settling. The light from the solar simulator (91192A Solar simulator, Newport Inc.) was angled by a 45° mirror to illuminate the PBRs. A short-pass filter with a cut-off wavelength at 710 nm was used to block the infrared light and to reduce solar heating inside the PBRs and, if needed, neutraldensity filters were employed to attenuate the photon flux below 1.0 sun. The illumination areas were limited to 50 mm × 50 mm for each PBR (using rectangular-shaped apertures). Different films were placed in front of the PBRs but on the outside of the water bath, with the PBRs held against the front wall of the water bath to minimize the gap in between. During the experiment, the water bath was surrounded by 2 in.-thick polystyrene foams to reduce the heat exchanges between the water bath and the environment. A feedback-controlled precision heater (FEBOTE sous vide, Okeba Industries Inc., CA, USA) was introduced to maintain the working temperature in PBRs at 20 °C. Each oxygen evolution/consumption experimental cycle was comprised of three alternations of 15 min in darkness and 30 min under emulated solar illumination (91192A Solar simulator, Newport Inc.). The short-term nature of the experiment allowed to keep algae concentrations relatively constant and ensure sufficient carbon dioxide in the sealed PBRs. In each experiment, the first and third cycles were performed as controls with identical fluorophore-free planar films. During the second experimental cycle, different optical films including the fluorophore-free planar film, the spectral-shifting planar film, the fluorophore-free film with microdome structures, and the spectralshifting microphotonic film were examined under the identical emulated solar illumination. The short-term oxygen evolution data were presented as the average of three independent measurements. Statistical significance was straightforwardly determined by unpaired Student's t-test or one-way ANOVA at P < 0.05. Values with the same letter are not statistically different at P = 0.05.

Impact of Algal Concentrations on Short-Term Oxygen Evolutions of Microalgae Indoors under Emulated Sunlight:
To elucidate the impact of algal concentrations, a set of short-term oxygen evolution experiments were carried out using algal suspensions of different concentrations. Their optical densities at 675 nm (OD 675 ) were 0.3, 0.6, 1.2, and 2.3, corresponding to cell concentrations of 0.14 × 10 17 , 0.42 × 10 17 , 1.24 × 10 17 , and 3.42 × 10 17 mL −1 , respectively, measured by cell counting (Marienfeld-SupeRior, Germany). For comparison, the fluorophore-free film with the same microdome structures was used as a control. The hermetically sealed PBRs were illuminated with emulated sunlight (91192A Solar simulator, Newport Inc.) with an identical impinging PPFD of 500 µmol m −2 s −1 . The other experimental parameters were kept the same as those in all other short-term oxygen evolutions experiments.
Long-Term Algal Proliferation in Photobioreactors Indoors under Emulated Sunlight: Long-term biomass proliferation was performed in open PBRs under emulated solar irradiation with an average PPFD of 500 µmol m −2 s −1 (≈0.25 sun). A 12:12-h light/dark photoperiod was used for the experiments. The algal cultures were shaken continuously at 145 rpm on a J-KEM BTS 3000 oscillating mixer (St. Louis, MO) to ensure uniform illumination and to prevent cells from settling. Parafilm was used for covering the flasks to minimize contamination and reduce the evaporation of cultures. On day 1, fresh algal solutions were prepared by sampling and diluting from the maintaining culture. The initial OD 675 was all kept at ≈0.04. The algae concentrations were monitored daily over 9 d by measuring the OD 675 of algal suspensions using a UV-vis-NIR spectrophotometer (UV-3101 PC, Shimadzu Inc.). The corresponding algal cell concentrations were measured by cell counting in commercial chambers with V-slash at the exterior sides of the chamber bottom (Marienfeld-SupeRior, Germany). At least three separate measurements from the same culture were performed per day for both optical density and cell concentrations. The algal morphology was observed using an optical microscope (Nikon Eclipse LV150N, Japan).

Long-Term Algal Proliferation in Bubble Column Photobioreactors Outdoors under Natural Sunlight and Weather Conditions:
Algal proliferations were also performed outdoors under natural sunlight with variable weather conditions. For this purpose, four bubble column photobioreactors of 90.0 cm heights and 12.0 cm inner diameters were constructed, and various films including the fluorophore-free planar film, the spectral-shifting planar film, the fluorophore-free film with microdome structures, and the spectral-shifting microphotonic film were used to surround the photobioreactors using a 30-µm-thick, optically clear double-sided acrylic film (PSF25015, MyBinding, OR, USA) as an adhesive interlayer. For comparative studies, each photobioreactor was first filled with 5000 mL algae suspension solutions with an initial OD 675 around 0.04. An air sparger was placed at the bottom of each photobioreactor to supply the fresh air in the reactor with a constant flow rate of 2 L min −1 , to strip out the accumulated oxygen, and to circulate the algae suspensions to prevent cells from settling. [39] The temperatures of algae suspension solution in each photobioreactor were recorded by precalibrated K-type thermocouples (0.3 °C accuracy). Before the comparative study, another test was first carried out to show that the four column reactors are truly identical for algae cultivation when no films were installed, and the thermocouples were calibrated before use. The real-time PPFD was measured at the photobioreactor level using a PAR quantum meter (MQ-501, Apogee Instruments).