Efficient Bio-Nano Hybrid Solar Cells via Purple Membraneas 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 foundin the purple membrane of Halobacterium salinarum . Theperformance of bacteriorhodopsin was evaluated as a sensi-tizer in dye-sensitized solar cells (DSSCs). Bacteriorhodopsinwas efficiently immobilized on the titanium dioxide nanopar-ticles and then tested for its ability to convert solar radiation toelectricity. The photovoltaic performance of DSSC based onthe bacteriorhodopsin sensitizer has been examined. UnderAM1.5 irradiation, a short-circuit current of 0.28mA cm−2,open-circuit voltages of 0.51 V, fill factor of 0.62, and anoverall energy conversion efficiency of 0.09 % are achievedemploying platinum as a counter electrode. Carbon was usedas a counter electrode instead of platinum to reduce costs.Based on carbon electrode, a short-circuit current of0.21 mA cm−2 and open-circuit voltages of 0.52 V wereobtained.

Keywords Bio-nano hybrid . Purple membrane . DSSC .

TiO2NPs

1 Introduction

Renewable energy is a crucial topic in our world today. Solarenergy is considered as an alternative energy source because itdoes not destroy our ecosystem and is environmentally friend-ly. One type of solar cell, the dye-sensitized solar cell (DSSC),first reported in 1991 by Grätzel and coworkers [1], is aphotoelectrochemical device that directly converts absorbedsunlight into electrical current. The transparent and low-costDSSCs have been proposed as a promising alternative tosilicon-based photovoltaics [2]. DSSC contains a nanocrystal-line porous semiconductor electrode-absorbed dye, an elec-trolyte, and a counter electrode [3]. The efficiency of DSSCstrongly depends on a dye used as a sensitizer. The absorptionspectrum of the dye, anchorage of the dye to the TiO2 surface,and efficient electron transfer from the highest occupied mo-lecular orbital (HOMO) of the dye to the conduction band ofmesoporous semiconductor are crucial factors that determineDSSC photovoltaic performance [4–6].

In this study, we used the inexpensive light-harvesting bio-molecule sensitizer, bacteriorhodopsin, for sensitizing titaniumdioxide (TiO2) nanoparticles instead of the common expensivesynthetic dyes such as ruthenium-based or organic dyes inDSSCs. Bacteriorhodopsin (bR) is the sole protein (MW26000) found in the purple membrane (PM) of the archaeumHalobacterium salinarum [7, 8]. The bR was discovered in theearly 1970s byOesterhelt and Stoeckenius [9]. Bacteriorhodopsinprotein consists of 248 amino acids, arranged in seven α-helicalbundles inside the lipid membrane [10].

When the bR retinal absorbs a photon, it isomerizes from theall-trans bR to the 13-cis configuration. This triggers aphotocycle, the net effect of which is the transfer of one protonfrom the cytoplasmic to the extracellular side of the membrane.The bacteriorhodopsin photocycle contains BR, K, L, M, N, andO intermediate states. Each intermediate has a distinct absor-bancemaximum; themost studied areBR (568 nm), O (640 nm),and Q (380 nm). Furthermore, both the structure–function

S. Janfaza : J. MehrvandDepartment of Biology, Science and Research Branch, Islamic AzadUniversity, Tehran, Iran

A. Molaeirad (*)Department of Bioscience and Biotechnology, Malek-AshtarUniversity of Technology, Tehran, Irane-mail: molaeirad@gmail.com

R. MohamadpourInstitute of Nanoscience and Nanotechnology, Sharif University ofTechnology, Tehran, Iran

M. KhayatiPharmaceutical and Biologically-Active Compounds ResearchLaboratory, Department of Chemistry, Iran University of Science andTechnology, Tehran 16846-13114, Iran

BioNanoSci. (2014) 4:71–77DOI 10.1007/s12668-013-0118-1

relations of bacteriorhodopsin and ways to modify the bR struc-ture and thus, its functions, have been studied intensively [8, 11].

Research on the photo-electrochemistry of bacteriorhodop-sin has been of great significance due to its variety of practicalapplications ranging from molecular recognition to molecularelectronics. The photo-electrochemistry of bR exhibits bothforward and backward photo-currents, due to isomerizationand reisomerization processes, respectively [12–14].Nanocrystalline TiO2 films have been sensitized by bacterio-rhodopsin for photoelectrochemical [15] application in viewof their low cost, good stability, nontoxicity, and so on.

Here, in this report, we have fabricated TiO2/bR hybridphotoanode of bio-sensitized solar cell. The satisfactory per-formance of DSSC shows that this novel bio-sensitizer can bea promising low-cost, nontoxic, and abundant candidate ofcommon dyes. Figure 1 shows a schematic diagram of dye-sensitized solar cells using bacteriorhodopsin.

2 Experimental

2.1 Fabrication of Porous TiO2 Electrodes

As working electrode, the fluorine-doped tin oxide conductiveglass (TCO22–15, Solaronix) was first washed in a detergent

solution by using an ultrasonic bath for 15 min, rinsed withethanol and distilled water, and then dried. Ti-Nanoxide DPaste (Solaronix, Co.ltd.) was coated on fluorine-doped tin oxide(FTO) glass by a doctor blade technique. The thickness of theTiO2 film was 9 μm, and the active area of the resulting cellexposed 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 °Cfor 5 min, at 375 °C for 5 min, and at 450 °C for 15 min, andfinally, at 500 °C for 15 min. The TiO2 film was treated with40 mMTiCl4 aqueous solution at 70 °C for 30 min, then washedwith pure water and ethanol, and sintered again at 500 °C for30min. After cooling to 22 °C, the TiO2 electrodewas immersedin 1.5 mL of 1 mg/mL bacteriorhodopsin (Sigma) protein for12 h at 22 °C [16]. The purple color was observed on the surfaceof the film that indicates to the best binding of bacteriorhodopsinonto the surface of nano-TiO2 film. For comparison of thephotovoltaic performance of bR-sensitized solar cell with solarcells using Ru-based dyes, another TiO2 electrode was immersedin a 0.5-mM N719 dye solution at room temperature for 24 h.

2.2 Preparation of PM-Sensitized Solar Cells

The platinum and carbon-coated FTO, sheets were used ascounter electrode in this research. Two holes were drilled inthe FTO glasses by a drill press. The perforated sheets were

Fig. 1 Schematic diagramof DSSC based onbacteriorhodopsin. The workingelectrode (FTO/TiO2/bR) and thecounter electrode (FTO/Pt) arepressed together to form a bio-nano hybrid solar cell. The spacebetween two electrodes is filledwith a liquid electrolyte

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cleaned by ultrasound in an ethanol bath for 10 min. The Ptcatalyst was deposited on the FTO glass by coating with a dropof H2PtCl6 solution (2 mg Pt in 1 mL ethanol), and heattreatment was carried out at 400 °C for 15 min.

Another piece of FTO glass, with the conductive side facingdown, was held about 10 cm above the flame. The carbon fromthe combustion of wax was carried in the smoke and made ablack deposition on the conductive side of the FTO glass.

The bR-covered TiO2 electrode and counter electrode wasassembled into a sealed sandwich solar cell with a hot-meltSurlyn film (30μm thickness) as spacer between the electrodes.A drop of the electrolyte solution was placed on the drilled holein the counter electrode of the assembled cell and was driven

into the cell via vacuum backfilling. Finally, the hole wassealed using additional Surlyn. The electrolyte employed waspurchased from Solaronix (Iodolyte AN-50, Switzerland) [17].

2.3 Measurements

A Unicam UV 300 spectrophotometer was used to recordabsorption spectra of the bacteriorhodopsin protein andN719 dye solution in the range from 350 to 600 nm.

The surface morphology of TiO2 film has been examinedusing atomic force microscopy (AFM). The AFM studiescould furnish the comprehensive information about the sur-face morphology of TiO2 coated on the FTO surface.

Fig. 2 Absorption spectra of30 μM of bacteriorhodopsin (a)and 0.2 mM N719 dye (b) in therange of 350–600 nm

Fig. 3 Evaluation of light-induced pH change ofbacteriorhodopsin immobilizedon titanium dioxide electrode(FTO/TiO2/bR) underillumination of a 40-W tungstenlamp

BioNanoSci. (2014) 4:71–77 73

In order to evaluate the photoactivity of bacteriorhodopsinimmobilized on TiO2, light-induced pH changes were mea-sured under illumination of a tungsten lamp by using aninoLab pH meter. The bR/TiO2 electrode was immersed in afreshly prepared solution of 3MKCl and 80mMMgCl2 at pH7.1, 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 tothe cell and measuring the generated photocurrent under

simulated sunlight (Luzchem) irradiation with a Keithley dig-ital source meter (Keithley 2601, USA). The current–timecurve of the DSSC was recorded with a potentiostat (μSTAT200, DropSens, Spain).

Also a helium–neon laser at 632.8 nm as a red laser and alaser pointer at 532 nm were used to evaluate the activity ofbacteriorhodopsin. Both the red and green laser beams werefocused separately onto the bR/TiO2 thin film for a period of10 s, and the current–time curve was measured by a

Fig. 4 AFM image ofnanostructured TiO2 film sinteredat 500 °C

Fig. 5 Current–voltage (I–V)curves of DSSC sensitized withbR, based on platinum- (a) andcarbon-coated (b) FTO sheets ascounter electrode

74 BioNanoSci. (2014) 4:71–77

potentiostat/galvanostat. The output power of the red and greenlaser diodes were about 5 mW. Afterwards, we investigated theeffect of red and green laser irradiation on current generationability of bacteriorhodopsin-based photovoltaic cells.

3 Results and Discussion

3.1 Structure and Activity of Bacteriorhodopsin

The UV–Vis spectra of bacteriorhodopsin N719 dye weremeasured between 350 and 600 nm. Figure 2 compares theabsorption spectra of the bacteriorhodopsin and N719, themain dye for DSSC. The maximum absorbance of bacterio-rhodopsin was found at 568 nm which are related to theabsorptions of the protonated Schiff-base retinal.

The pumping activity of bacteriorhodopsin on TiO2 wasinvestigated by the evaluation of ΔpH changes induced byvariations of light intensity, and the light-induced change of bRwas observed. The bR adsorbed on TiO2 produces a transientchange in pH under illumination. Figure 3 shows pH changesinduced by illuminating bacteriorhodopsin immobilized on tita-nium dioxide. Light absorption isomerizes the retinal from theall-trans to the 13-cis form, followed by a proton transfer fromthe Schiff base to the proton acceptor Asp-85. To allowvectoriality, reprotonation of the Schiff base from Asp-85 mustbe excluded. Thus, its accessibility is switched from extracellularto intracellular. The Schiff base is then reprotonated from Asp-96 in the cytoplasmic channel. After reprotonation of Asp-96from the cytoplasmic surface, the retinal reisomerizes thermally,and the accessibility of the Schiff base switches back to extra-cellular to reestablish the initial state. These steps represent theminimal number of steps needed to account for vectorial catal-ysis in wild-type bacteriorhodopsin [8]. Proton pumping by

bacteriorhodopsin generates pH gradient across the H.salinarum membrane that can be used to synthesize ATP frominorganic phosphate and ADP [18]. Also, the photoactivity ofbR was tested by using a green laser pointer.

Atomic force microscopy was used to investigate the sur-face morphology of nanocrystalline TiO2. The AFM image(2 μm×2 μm surface plots) of the calcined TiO2 nanoparticlesis shown in Fig. 4. The film porosity is in the range of 10 nm,which is suitable for photovoltaic applications. Analysis of theAFM images showed that the morphology of samples is veryrough and may be beneficial to enhancing the adsorption ofbacteriorhodopsin due to its great surface roughness and highsurface area. The average diameter of the well-dispersed tita-nium dioxide particles was found to be close to 15 nm.

3.2 Photoelectrochemical Properties of DSSC

The photovoltaic performance of DSSC was investigated bymeasuring the current–voltage (I–V) curves under irradiationwith white light AM1.5 (100 mW cm−2) from the solar sim-ulator lamp. Short-circuit current (J sc), open-circuit voltage(Voc), fill factor (FF), and energy conversion efficiency weremeasured to investigate the performance of bR as sensitizer inDSSC. The typical I–V curves of the biomolecule sensitizersolar cell using bR are shown in Fig. 5. Our result showed aJ 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

Fig. 6 Photocurrent response ofDSSCs under green and red lightillumination conditions. Uponturning the green light on (10–20 s), a sharp increase in thephotocurrent was observed,followed by a decay after the lightwas turned off. Contrary to this,DSSC shows no response to thered light (30–40 s)

Table 1 Photovoltaic performance of DSSCs based on different dyes

Dye Isc (mA cm−2) Voc (V) Efficiency (%)

Bacteriorhodopsin 0.28 0.52 0.09

N719 9.05 0.77 5.9

BioNanoSci. (2014) 4:71–77 75

platinum was used as cathode. A power conversion efficiencyof 5.9 % was obtained for DSSC based on the N719 dye.

Platinum-loaded conducting glass has been used widely ascounter electrode. In this study, we investigated the effective-ness of the carbon-coated FTO glass as counter electrode thathad an acceptable performance. Under illumination, thecarbon-based DSSC exhibited a J sc of 0.21 mA cm−2 and aVoc of 0.52 V (Fig. 5). It is important to mention that the costof carbon is considerably cheaper than that of the platinum. Byusing the carbon electrode instead of platinum electrode, thecost of the photocell can be decreased.

A photoactivity of bR was tested by using a green laserpointer at 532 nm. The green laser beam was irradiated to thebacteriorhodopsin adsorbed on nano-TiO2 film, and theresulting current was increased considerably upon illuminationbecause of bR photocycle. The illumination of the bio-solar cellby light from the red laser did not significantly affect thecurrent–time curve (Fig. 6). The bacteriorhodopsin photocyclecontains BR, K, L, M, N, and O intermediate states. The bRnative state (BR) has a characteristic absorbance maximum ataround 568 nm. The absorption of light around 570 nm resultsin conversion all-trans to 13-cis photoisomerization of theBR’s retinal chromophore. Therefore, upon absorption of greenlight by the retinal, the bacteriorhodopsin alters its structure,and the current is intensified [12].

The DSSCs using efficient dyes like N719, N3, N749, andZ907 as sensitizers have been obtained with relatively highconversion efficiencies. Table 1 shows the performances ofdye-sensitized solar cells using various dyes. However, theDSSC sensitized by bacteriorhodopsin in this work did notoffer high conversion efficiencies. But the Voc of bacteriorho-dopsin is comparable to that of the DSSC sensitized by N719.Furthermore, we replaced toxic and expensive dyes with non-toxic, completely biodegradable, and inexpensive protein assensitizer for DSSC. Unlike many other synthetic dyes, bacte-riorhodopsin is biocompatible and environmentally friendly. Itis very easy and cheap to produce in practically unlimitedquantities. H. salinarum is able to grow under industrial con-ditions not requiring expensive growth requirements, and bR iseasy to separate and purify. Unlike most proteins, the isolatedbacteriorhodopsin is extremely stable, and solutions or driedfilms with unlimited activity can be readily produced [19, 20].

Proteins like rhodopsin and bacteriorhodopsin are used asbiosensors as they have the ability to convert photons intoenergy, undergoing structural changes once every few milli-seconds, and it can do this for hundreds of millions of timeswithout becoming denatured. It has been previously demon-strated that nanoparticles exhibit unique chemical, physical,and electronic properties, and variety of nanoparticles such asmetal, oxide, semiconductor, or composite nanoparticles canbe used in biosensors [21]. Moreover, different kinds of nano-particles may play different roles in different biosensor sys-tems. The nanocrystalline TiO2-based DSSC, using

bacteriorhodopsin as the bio-sensitizer, also opens a potentialpathway for further development of novel biosensor devices.

4 Conclusion

Bacteriorhodopsin was adsorbed on nanostructured TiO2, andthe feasibility and efficiency of bR for use in bio-solar cells wasinvestigated. The AFM results confirmed that the morphologyof TiO2 electrodes is very rough and has highly disordered pores,which may enhance the adsorption of bR. The activity of bRadsorbed on TiO2 was assayed by measuring the current–timeand pH–time curves under irradiation. Photocurrent generationduring irradiation with green light and light-induced pH changeclearly proved the successful immobilization of bR on TiO2

electrode. Also, to reduce costs, we used carbon in the counterelectrode instead of platinum. Finally, photoelectrochemical per-formance of the DSSCs based on bacteriorhodopsin was mea-sured, and a Voc of 0.51 V, a Jsc of 0.28 mA cm−2, a fill factor of0.62, and an efficiency of 0.09 % were obtained. Our resultsshow that the purple membrane as a sensitizer of DSSC ispromising because of its environmental friendliness and low-cost production.

Conflict of Interest We declare that we have no conflicts of interest inthe authorship or publication of this contribution.

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