BioNanoSci. (2014) 4:71–77 DOI 10.1007/s12668-013-0118-1
Efficient Bio-Nano Hybrid Solar Cells via Purple Membrane as Sensitizer Sajad Janfaza & Ahmad Molaeirad & Raheleh Mohamadpour & Maryam Khayati & Jamshid Mehrvand
Published online: 6 December 2013 # Springer Science+Business Media New York 2013
Abstract Bacteriorhodopsin is a heptahelical protein found in the purple membrane of Halobacterium salinarum . The performance of bacteriorhodopsin was evaluated as a sensitizer in dye-sensitized solar cells (DSSCs). Bacteriorhodopsin was efficiently immobilized on the titanium dioxide nanoparticles and then tested for its ability to convert solar radiation to electricity. The photovoltaic performance of DSSC based on the bacteriorhodopsin sensitizer has been examined. Under AM1.5 irradiation, a short-circuit current of 0.28mA cm−2, open-circuit voltages of 0.51 V, fill factor of 0.62, and an overall energy conversion efficiency of 0.09 % are achieved employing platinum as a counter electrode. Carbon was used as a counter electrode instead of platinum to reduce costs. Based on carbon electrode, a short-circuit current of 0.21 mA cm−2 and open-circuit voltages of 0.52 V were obtained. Keywords Bio-nano hybrid . Purple membrane . DSSC . TiO2 NPs
S. Janfaza : J. Mehrvand Department of Biology, Science and Research Branch, Islamic Azad University, Tehran, Iran A. Molaeirad (*) Department of Bioscience and Biotechnology, Malek-Ashtar University of Technology, Tehran, Iran e-mail:
[email protected] R. Mohamadpour Institute of Nanoscience and Nanotechnology, Sharif University of Technology, Tehran, Iran M. Khayati Pharmaceutical and Biologically-Active Compounds Research Laboratory, Department of Chemistry, Iran University of Science and Technology, Tehran 16846-13114, Iran
1 Introduction Renewable energy is a crucial topic in our world today. Solar energy is considered as an alternative energy source because it does not destroy our ecosystem and is environmentally friendly. One type of solar cell, the dye-sensitized solar cell (DSSC), first reported in 1991 by Grätzel and coworkers [1], is a photoelectrochemical device that directly converts absorbed sunlight into electrical current. The transparent and low-cost DSSCs have been proposed as a promising alternative to silicon-based photovoltaics [2]. DSSC contains a nanocrystalline porous semiconductor electrode-absorbed dye, an electrolyte, and a counter electrode [3]. The efficiency of DSSC strongly depends on a dye used as a sensitizer. The absorption spectrum of the dye, anchorage of the dye to the TiO2 surface, and efficient electron transfer from the highest occupied molecular orbital (HOMO) of the dye to the conduction band of mesoporous semiconductor are crucial factors that determine DSSC photovoltaic performance [4–6]. In this study, we used the inexpensive light-harvesting biomolecule sensitizer, bacteriorhodopsin, for sensitizing titanium dioxide (TiO2) nanoparticles instead of the common expensive synthetic dyes such as ruthenium-based or organic dyes in DSSCs. Bacteriorhodopsin (bR) is the sole protein (MW 26000) found in the purple membrane (PM) of the archaeum Halobacterium salinarum [7, 8]. The bR was discovered in the early 1970s by Oesterhelt and Stoeckenius [9]. Bacteriorhodopsin protein consists of 248 amino acids, arranged in seven α-helical bundles inside the lipid membrane [10]. When the bR retinal absorbs a photon, it isomerizes from the all-trans bR to the 13-cis configuration. This triggers a photocycle, the net effect of which is the transfer of one proton from the cytoplasmic to the extracellular side of the membrane. The bacteriorhodopsin photocycle contains BR, K, L, M, N, and O intermediate states. Each intermediate has a distinct absorbance maximum; the most studied are BR (568 nm), O (640 nm), and Q (380 nm). Furthermore, both the structure–function
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relations of bacteriorhodopsin and ways to modify the bR structure and thus, its functions, have been studied intensively [8, 11]. Research on the photo-electrochemistry of bacteriorhodopsin has been of great significance due to its variety of practical applications ranging from molecular recognition to molecular electronics. The photo-electrochemistry of bR exhibits both forward and backward photo-currents, due to isomerization and reisomerization processes, respectively [12–14]. Nanocrystalline TiO2 films have been sensitized by bacteriorhodopsin for photoelectrochemical [15] application in view of their low cost, good stability, nontoxicity, and so on. Here, in this report, we have fabricated TiO2/bR hybrid photoanode of bio-sensitized solar cell. The satisfactory performance of DSSC shows that this novel bio-sensitizer can be a promising low-cost, nontoxic, and abundant candidate of common dyes. Figure 1 shows a schematic diagram of dyesensitized solar cells using bacteriorhodopsin.
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solution by using an ultrasonic bath for 15 min, rinsed with ethanol and distilled water, and then dried. Ti-Nanoxide D Paste (Solaronix, Co.ltd.) was coated on fluorine-doped tin oxide (FTO) glass by a doctor blade technique. The thickness of the TiO2 film was 9 μm, and the active area of the resulting cell exposed in light was approximately 0.25 cm2 (0.5 cm×0.5 cm). The TiO2 film was gradually heated under an air flow at 325 °C for 5 min, at 375 °C for 5 min, and at 450 °C for 15 min, and finally, at 500 °C for 15 min. The TiO2 film was treated with 40 mM TiCl4 aqueous solution at 70 °C for 30 min, then washed with pure water and ethanol, and sintered again at 500 °C for 30 min. After cooling to 22 °C, the TiO2 electrode was immersed in 1.5 mL of 1 mg/mL bacteriorhodopsin (Sigma) protein for 12 h at 22 °C [16]. The purple color was observed on the surface of the film that indicates to the best binding of bacteriorhodopsin onto the surface of nano-TiO2 film. For comparison of the photovoltaic performance of bR-sensitized solar cell with solar cells using Ru-based dyes, another TiO2 electrode was immersed in a 0.5-mM N719 dye solution at room temperature for 24 h.
2 Experimental 2.2 Preparation of PM-Sensitized Solar Cells 2.1 Fabrication of Porous TiO2 Electrodes As working electrode, the fluorine-doped tin oxide conductive glass (TCO22–15, Solaronix) was first washed in a detergent Fig. 1 Schematic diagram of DSSC based on bacteriorhodopsin. The working electrode (FTO/TiO2/bR) and the counter electrode (FTO/Pt) are pressed together to form a bionano hybrid solar cell. The space between two electrodes is filled with a liquid electrolyte
The platinum and carbon-coated FTO, sheets were used as counter electrode in this research. Two holes were drilled in the FTO glasses by a drill press. The perforated sheets were
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Fig. 2 Absorption spectra of 30 μM of bacteriorhodopsin (a) and 0.2 mM N719 dye (b) in the range of 350–600 nm
cleaned by ultrasound in an ethanol bath for 10 min. The Pt catalyst was deposited on the FTO glass by coating with a drop of H2PtCl6 solution (2 mg Pt in 1 mL ethanol), and heat treatment was carried out at 400 °C for 15 min. Another piece of FTO glass, with the conductive side facing down, was held about 10 cm above the flame. The carbon from the combustion of wax was carried in the smoke and made a black deposition on the conductive side of the FTO glass. The bR-covered TiO2 electrode and counter electrode was assembled into a sealed sandwich solar cell with a hot-melt Surlyn film (30 μm thickness) as spacer between the electrodes. A drop of the electrolyte solution was placed on the drilled hole in the counter electrode of the assembled cell and was driven Fig. 3 Evaluation of lightinduced pH change of bacteriorhodopsin immobilized on titanium dioxide electrode (FTO/TiO2/bR) under illumination of a 40-W tungsten lamp
into the cell via vacuum backfilling. Finally, the hole was sealed using additional Surlyn. The electrolyte employed was purchased from Solaronix (Iodolyte AN-50, Switzerland) [17]. 2.3 Measurements A Unicam UV 300 spectrophotometer was used to record absorption spectra of the bacteriorhodopsin protein and N719 dye solution in the range from 350 to 600 nm. The surface morphology of TiO2 film has been examined using atomic force microscopy (AFM). The AFM studies could furnish the comprehensive information about the surface morphology of TiO2 coated on the FTO surface.
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Fig. 4 AFM image of nanostructured TiO2 film sintered at 500 °C
In order to evaluate the photoactivity of bacteriorhodopsin immobilized on TiO2, light-induced pH changes were measured under illumination of a tungsten lamp by using an inoLab pH meter. The bR/TiO2 electrode was immersed in a freshly prepared solution of 3 M KCl and 80 mM MgCl2 at pH 7.1, irradiated at 25 °C with a 40-W tungsten lamp. After making the bR-based bio-solar cells, the current– voltage curve was obtained by applying an external bias to the cell and measuring the generated photocurrent under Fig. 5 Current–voltage (I–V) curves of DSSC sensitized with bR, based on platinum- (a) and carbon-coated (b) FTO sheets as counter electrode
simulated sunlight (Luzchem) irradiation with a Keithley digital source meter (Keithley 2601, USA). The current–time curve of the DSSC was recorded with a potentiostat (μSTAT 200, DropSens, Spain). Also a helium–neon laser at 632.8 nm as a red laser and a laser pointer at 532 nm were used to evaluate the activity of bacteriorhodopsin. Both the red and green laser beams were focused separately onto the bR/TiO2 thin film for a period of 10 s, and the current–time curve was measured by a
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Fig. 6 Photocurrent response of DSSCs under green and red light illumination conditions. Upon turning the green light on (10– 20 s), a sharp increase in the photocurrent was observed, followed by a decay after the light was turned off. Contrary to this, DSSC shows no response to the red light (30–40 s)
potentiostat/galvanostat. The output power of the red and green laser diodes were about 5 mW. Afterwards, we investigated the effect of red and green laser irradiation on current generation ability of bacteriorhodopsin-based photovoltaic cells.
3 Results and Discussion 3.1 Structure and Activity of Bacteriorhodopsin The UV–Vis spectra of bacteriorhodopsin N719 dye were measured between 350 and 600 nm. Figure 2 compares the absorption spectra of the bacteriorhodopsin and N719, the main dye for DSSC. The maximum absorbance of bacteriorhodopsin was found at 568 nm which are related to the absorptions of the protonated Schiff-base retinal. The pumping activity of bacteriorhodopsin on TiO2 was investigated by the evaluation of ΔpH changes induced by variations of light intensity, and the light-induced change of bR was observed. The bR adsorbed on TiO2 produces a transient change in pH under illumination. Figure 3 shows pH changes induced by illuminating bacteriorhodopsin immobilized on titanium dioxide. Light absorption isomerizes the retinal from the all-trans to the 13-cis form, followed by a proton transfer from the Schiff base to the proton acceptor Asp-85. To allow vectoriality, reprotonation of the Schiff base from Asp-85 must be excluded. Thus, its accessibility is switched from extracellular to intracellular. The Schiff base is then reprotonated from Asp96 in the cytoplasmic channel. After reprotonation of Asp-96 from the cytoplasmic surface, the retinal reisomerizes thermally, and the accessibility of the Schiff base switches back to extracellular to reestablish the initial state. These steps represent the minimal number of steps needed to account for vectorial catalysis in wild-type bacteriorhodopsin [8]. Proton pumping by
bacteriorhodopsin generates pH gradient across the H. salinarum membrane that can be used to synthesize ATP from inorganic phosphate and ADP [18]. Also, the photoactivity of bR was tested by using a green laser pointer. Atomic force microscopy was used to investigate the surface morphology of nanocrystalline TiO2. The AFM image (2 μm×2 μm surface plots) of the calcined TiO2 nanoparticles is shown in Fig. 4. The film porosity is in the range of 10 nm, which is suitable for photovoltaic applications. Analysis of the AFM images showed that the morphology of samples is very rough and may be beneficial to enhancing the adsorption of bacteriorhodopsin due to its great surface roughness and high surface area. The average diameter of the well-dispersed titanium dioxide particles was found to be close to 15 nm. 3.2 Photoelectrochemical Properties of DSSC The photovoltaic performance of DSSC was investigated by measuring the current–voltage (I–V) curves under irradiation with white light AM1.5 (100 mW cm−2) from the solar simulator lamp. Short-circuit current (J sc), open-circuit voltage (Voc), fill factor (FF), and energy conversion efficiency were measured to investigate the performance of bR as sensitizer in DSSC. The typical I–V curves of the biomolecule sensitizer solar cell using bR are shown in Fig. 5. Our result showed a J sc of 0.28 mA cm−2, a Voc of 0.51 V, a fill factor (FF) of 0.62, and an overall conversion efficiency of 0.09 % when the Table 1 Photovoltaic performance of DSSCs based on different dyes Dye
I sc (mA cm−2)
Voc (V)
Efficiency (%)
Bacteriorhodopsin N719
0.28 9.05
0.52 0.77
0.09 5.9
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platinum was used as cathode. A power conversion efficiency of 5.9 % was obtained for DSSC based on the N719 dye. Platinum-loaded conducting glass has been used widely as counter electrode. In this study, we investigated the effectiveness of the carbon-coated FTO glass as counter electrode that had an acceptable performance. Under illumination, the carbon-based DSSC exhibited a J sc of 0.21 mA cm−2 and a Voc of 0.52 V (Fig. 5). It is important to mention that the cost of carbon is considerably cheaper than that of the platinum. By using the carbon electrode instead of platinum electrode, the cost of the photocell can be decreased. A photoactivity of bR was tested by using a green laser pointer at 532 nm. The green laser beam was irradiated to the bacteriorhodopsin adsorbed on nano-TiO2 film, and the resulting current was increased considerably upon illumination because of bR photocycle. The illumination of the bio-solar cell by light from the red laser did not significantly affect the current–time curve (Fig. 6). The bacteriorhodopsin photocycle contains BR, K, L, M, N, and O intermediate states. The bR native state (BR) has a characteristic absorbance maximum at around 568 nm. The absorption of light around 570 nm results in conversion all-trans to 13-cis photoisomerization of the BR’s retinal chromophore. Therefore, upon absorption of green light by the retinal, the bacteriorhodopsin alters its structure, and the current is intensified [12]. The DSSCs using efficient dyes like N719, N3, N749, and Z907 as sensitizers have been obtained with relatively high conversion efficiencies. Table 1 shows the performances of dye-sensitized solar cells using various dyes. However, the DSSC sensitized by bacteriorhodopsin in this work did not offer high conversion efficiencies. But the Voc of bacteriorhodopsin is comparable to that of the DSSC sensitized by N719. Furthermore, we replaced toxic and expensive dyes with nontoxic, completely biodegradable, and inexpensive protein as sensitizer for DSSC. Unlike many other synthetic dyes, bacteriorhodopsin is biocompatible and environmentally friendly. It is very easy and cheap to produce in practically unlimited quantities. H. salinarum is able to grow under industrial conditions not requiring expensive growth requirements, and bR is easy to separate and purify. Unlike most proteins, the isolated bacteriorhodopsin is extremely stable, and solutions or dried films with unlimited activity can be readily produced [19, 20]. Proteins like rhodopsin and bacteriorhodopsin are used as biosensors as they have the ability to convert photons into energy, undergoing structural changes once every few milliseconds, and it can do this for hundreds of millions of times without becoming denatured. It has been previously demonstrated that nanoparticles exhibit unique chemical, physical, and electronic properties, and variety of nanoparticles such as metal, oxide, semiconductor, or composite nanoparticles can be used in biosensors [21]. Moreover, different kinds of nanoparticles may play different roles in different biosensor systems. The nanocrystalline TiO 2 -based DSSC, using
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bacteriorhodopsin as the bio-sensitizer, also opens a potential pathway for further development of novel biosensor devices.
4 Conclusion Bacteriorhodopsin was adsorbed on nanostructured TiO2, and the feasibility and efficiency of bR for use in bio-solar cells was investigated. The AFM results confirmed that the morphology of TiO2 electrodes is very rough and has highly disordered pores, which may enhance the adsorption of bR. The activity of bR adsorbed on TiO2 was assayed by measuring the current–time and pH–time curves under irradiation. Photocurrent generation during irradiation with green light and light-induced pH change clearly proved the successful immobilization of bR on TiO2 electrode. Also, to reduce costs, we used carbon in the counter electrode instead of platinum. Finally, photoelectrochemical performance of the DSSCs based on bacteriorhodopsin was measured, and a Voc of 0.51 V, a J sc of 0.28 mA cm−2, a fill factor of 0.62, and an efficiency of 0.09 % were obtained. Our results show that the purple membrane as a sensitizer of DSSC is promising because of its environmental friendliness and lowcost production. Conflict of Interest We declare that we have no conflicts of interest in the authorship or publication of this contribution.
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