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Plasmonic effect of silver nanoparticles intercalated into mesoporous betalain-sensitized-TiO2 film electrodes on photovoltaic performance of dye-sensitized solar cells

Plasmonic effect of silver nanoparticles intercalated into mesoporous betalain-sensitized-TiO2... Mater Renew Sustain Energy (2016) 5:10 DOI 10.1007/s40243-016-0075-z ORIGINAL PAPER Plasmonic effect of silver nanoparticles intercalated into mesoporous betalain-sensitized-TiO film electrodes on photovoltaic performance of dye-sensitized solar cells 1 1 1 1 • • • • Kasim U. Isah Bukola J. Jolayemi Umaru Ahmadu Mohammed Isah Kimpa Noble Alu Received: 21 February 2016 / Accepted: 8 June 2016 / Published online: 18 June 2016 The Author(s) 2016. This article is published with open access at Springerlink.com Abstract Dye-sensitized solar cells (DSSCs) comprising Introduction mesoporous TiO films and betalain pigments extracted from red Bougainvillea glabra flower as natural dye sen- Dye-sensitized solar cells (DSSCs) have remained one of sitizers were fabricated and enhanced by the intercalation the most promising choices for the realization of low-cost of the plasmonic silver nanoparticles (Ag NPs) into the solar cells; it has, in the recent years, attracted remarkable pores of mesoporous TiO electrodes by successive ionic attention owing to simplicity of its fabrication process and layer adsorption and reaction (SILAR) method. The TiO / low manufacturing cost with excellent light absorbing Ag NPs composite films were characterized by SEM and materials [1–3]. Nevertheless, the power conversion effi- UV–Vis spectroscopy. I–V characteristics of the devices ciency (PCE) of the device has barely gone up to 11.9 % were measured by solar simulator (AM1.5 at 100 mW/ [4]. Many technological innovations have been developed cm ). The incorporation of the Ag nanoparticles into the to improve the efficiency and, at the same time, to reduce pores of mesoporous TiO electrodes with one SILAR the cost of production ranging from interfacial modification deposition cycle of the Ag NPs produced the best plas- [5, 6] to material choice and engineering [7]. This devel- monic enhanced-DSSC giving a short-circuit current den- opment leads to the consideration of natural dyes from sity (J ), fill factor (FF), and power conversion efficiency plant sources which are cost-effective, non-toxic, and sc -2 (PCE) of 1.01 mA cm , 0.77, and 0.27 %, respectively. completely biodegradable compared with the much This development amounts to 50 % efficiency enhance- expensive and toxic synthesized counterparts based on ment over the reference DSSC that had a short-circuit metal complex-like ruthenium (II) and metal-free like current density (J ), fill factor (FF), and power conversion porphyrin [8]. Betalain has been regarded by many sc -2 efficiency (PCE) of 0.7 mA cm , 0.57, and 0.18 %, researchers to be a most promising candidate of choice out respectively. of the three most exploited dye pigments from plant sources: chlorophylls, anthocyanins, and betalains [9]. Keywords Plasmonic  Silver nanoparticles  SILAR  Conversely to the anthocyanins, betalains possess essential DSSCs  Betalain  Bougainvillea glabra functional groups (–COOH) to anchor better to the TiO nanoparticles than the functional groups (–OH) present in anthocyanins [10–13]. Indeed, the interaction between TiO film and carboxylic functions should bring a stronger electronic coupling and rapid forward and reverse electron transfer reactions [9]. & Kasim U. Isah Also, the incorporation of plasmonic metal nanoparti- kasim309@futminna.edu.ng cles (NPs) into the electrode (usually m-TiO ) of dye- sensitized solar cells (DSSCs) to boost the light absorption Department of Physics, School of Physical Sciences, Federal University of Technology, Minna, Nigeria due to their localized surface plasmon (LSP) effect has been very outstanding [14–17]. Noble plasmonic metal Physics Advanced Laboratory (PAL), Sheda Science and nanostructures, such as gold (Au), silver (Ag) [18, 19], Technology Complex (SHESTCO), Abuja, Nigeria 123 10 Page 2 of 9 Mater Renew Sustain Energy (2016) 5:10 aluminum (Al) [20], their alloys [16], and their core–shell substrates obtaining about 3.5 lm with polyester mesh formation [21–25], have been incorporated into DSSCs to screen-120. After drying at 55 C for 30 min, the elec- effectively utilize the incident photon and improve cell trodes were sintered at 450 C for 30 min, and then grad- performance. Many methods, both chemical and physical ually cooled down to room temperature. categories, including, wet chemical [26], polyol process [27], seed-mediated growth [28], photochemical [29], Synthesis and incorporation of silver nanoparticles sonochemical [30], electrochemical [31], bioinspired [32], sputtering [33], and laser ablation [34], which have been, Silver nanoparticles (Ag NPs) were prepared by successive hitherto, reportedly employed to synthesize various plas- ionic layer adsorption and reaction (SILAR) method. The monic Ag nanostructures, possess one disadvantage or the cationic precursor for Ag NPs using SILAR method was other, such as toxic reducing agents, impurities, organic diammine silver complex AgðNH Þ solution. This ðaqÞ solvents, or may require special condition like high-tem- solution was prepared using 0.35 g of AgNO (BDH) in perature or low-pressure environment, and sometimes 200 ml of distilled water, to this, ammonia solution expensive and time-consuming procedures may be (NH OH) (ca. 33 wt% NH, Grifflin and George) was added involved [35]. drop by drop until colorless solution was observed. The Successive ionic layer adsorption and reaction (SILAR) anionic (reducing agent) solution was prepared by adding method was used in this work for the intercalation of Ag 2 ml of concentrated HCl (36 %, Loba Chemie) to 2 g of nanoparticles into the pores of mesoporous TiO photo- stannous chloride (SnCl .2H O) (BDH) in 640 ml of dis- 2 2 electrodes of betalain-sensitized DSSCs. It offers simple, tilled water. inexpensive, and time-saving procedures, which can be To intercalate the Ag NPs into the mesoscopic TiO carried out at room temperature with no restrictions on electrodes, the prepared electrode was immersed in the substrate material, dimensions or its surface profile, and the diammine silver complex AgðNH Þ solution for thickness of the film or nanoparticle can be easily con- ðaqÞ trolled [36–38]. 2 min (adsorption), then rinsed with distilled water for 1 min (to remove excess adsorbed ions from the diffu- sion layer), the electrode was then transferred to the Experimental stannous chloride solution for 2 min (reduction reaction), and hence, film turns brownish due to the reduction from Preparation of photosensitizer Ag to Ag; it was then treated with acid rinse by immersing it in 0.25 M of HCl (36 %, Loba Chemie) for The photosensitizer was made of natural dye from betalain 30 s (to etch any loosely silver nanoparticles) and finally pigments extracted from red Bougainvillea glabra flower. rinsed with distilled water for 30 s to remove the excess 13.12 g of the fresh flowers was ground and well blended and unreacted species and reaction by-product from the together in 100 ml of distilled water in an electric blender. diffusion layer. The deposition process is shown in Then, the mixture was filtered, and the filtrate was used as Fig. 1. the sensitizing dye without further purification. This process was repeated for another electrode with two SILAR cycles. As soon as the required number of Preparation of active electrode layer cycles was complete, the film was returned to the silver diammine complex solution for 30 s to form a layer of Fluorine-doped tin oxide (FTO: TCO30-8 glass, 8 X/sq, protective oxidized Ag shell on the film and rinsed in 3 mm thick, Solaronix) glass sheets were cleaned with distilled water for another 30 s before drying at 100 Cin detergent (sodium lauryl sulfate), rinsed with distilled an open air until it was fully dried; this leads to the for- water, ethanol, and dried under compressed hot air for mation of a thin silver oxide shell on the film to protect the 7 min at 70 C in a clean container. 0.5 9 0.6 cm area Ag nanoparticles from the iodic electrolyte [16]. To further compact TiO (c-TiO ) paste (Ti-Nanoxide BL/SP, Sola- 2 2 protect the Ag NPs from the corrosive electrolyte, the films ronix) was screen-printed on three 2.5 9 2.5 cm FTO were refluxed in 0.1-M titanium (IV) isopropoxide (Sigma- glass substrates using a 70 mesh screen to obtain about Aldrich) in isopropanol solution for 25 min to form silver– 50-nm thick TiO film. The film was sintered on the hot- titanium dioxide core–shell (Ag@TiO ) nanoparticles and plate at 500 C for 50 min and was allowed to slowly cool sintered at 350 C for 20 min. The preparation process is down to room temperature. Afterward, mesoporous TiO shown in Fig. 2. (m-TiO ) paste (Ti-Nanoxide T300/SP, Solaronix) was The chemical reaction mechanism for the formation of screen-printed on the 0.30 cm of as-deposited c-TiO Ag NPs is given in the following. 123 Mater Renew Sustain Energy (2016) 5:10 Page 3 of 9 10 Fig. 1 Scheme of SILAR cycle process of Ag nanoparticles deposition for DSSC application Fig. 2 Schematic of plasmonic photoanode preparation process Cationic precursor (adsorption step) SnCl þ H O  Sn(OH)Cl þ HCl ð4Þ 2 2 ðsÞ ðaqÞ a clear solution of SnCl requires addition of excess 2(aq) Diammine silver complex cationic precursor is obtained by HCl adding enough ammonia solution (NH OH )to 4 (aq) AgNO . Brownish precipitate appears when a small Sn(OH)Cl þ HCl ¼ SnCl þ 2H O ð5Þ 3(aq) 2 ðsÞ ðaqÞ 2ðaqÞ amount of ammonia solution is added as a result of the where formation of silver (I) oxide (Ag O ): 2 (s) 2þ SnCl ! Sn þ 2Cl ð6Þ 2ðaqÞ 2AgNO þ 2NH OH ! Ag O þ 2NH NO 4 ðaqÞ ðsÞ 4 3ðaqÞ 3ðaqÞ 2 þ H O þ ðlÞ Ag is, reduced to Ag when the films were immersed in 2þ ð1Þ the anionic solution containing Sn as follows: ðaqÞ The precipitate (Ag O ) dissolves in excess ammonia 2 (s) þ 2þ 4þ 2Ag þ Sn ! 2Ag þ Sn ð7Þ ðsÞ ðaqÞ solution to form colorless diammine silver complex ion: Ag O þ 4NH þ H O ! 2 Ag(NH Þ þ2OH ð2Þ Sensitization of photoanode ðsÞ 3 2 ðlÞ 2 3 2 ðaqÞ ðaqÞ The ionic equation of the complex ion is given as: ThebaremesoscopicTiO electrode and mesoscopic þ þ TiO electrodes intercalated with Ag NPs (one and two ½AgðNH Þ   Ag þ 2NH ð3Þ 2 3 3 2 aq SILAR cycles) were each soaked in three different Anionic precursor (reaction step) Petri dishes containing the sensitizing dye for about 20 h. After which, the dye impregnated photoanodes Tin (II) chloride dissolves in less than its own mass of were then rinsed with ethanol (99 %, Sigma-Aldrich), water to form insoluble tin hydroxy chloride specie in a to remove excess dye particles that were not properly reversible hydrolysis, adsorbed. 123 10 Page 4 of 9 Mater Renew Sustain Energy (2016) 5:10 Fabrication of DSSCs Results and discussions The counter electrodes were first prepared by screen- printing a 0.5 9 0.6-cm thin film of platinum (Pt) paste Scanning electron microscopy (SEM) (Platisol T/SP, Solaronix) on a bare 2.5 9 2.5-cm FTO glass substrate, and then sintered at 450 C for 30 min and SEM images of the bare TiO and the TiO /Ag NPs 2 2 allowed to slowly cool to room temperature. composite films with different SILAR cycles are shown in A sandwich-type DSSCs were fabricated by assembling Fig. 3. The SEM images reveal a random two-dimensional the dye-impregnated photoelectrodes and counter elec- array of Ag NPs with a very broad particle size distribution. trodes in an overlapping manner, so as to establish elec- Figure 3a is the bare TiO electrode, which shows the trical connection between the cells and the photovoltaic absence of Ag NPs, while Fig. 3b, c confirms the interca- measurement equipment. The assembling was achieved lation of Ag NPs in the pores of TiO with one and two using hot-melt sealing gasket of Surlyn-based polymer SILAR cycles, respectively. sheet (SX1170-25PF, Solaronix) and sealed on a heating The morphology of the films appears to be different, stage leaving a pinhole for the electrolyte injection for a which can be attributed to the varied SILAR deposition few seconds. Finally, the iodine-based liquid electrolyte cycles of the Ag NPs. The image of the pure TiO film in (iodolyte) was injected using micropipette before sealing Fig. 3a shows a dense surface, and there are no shinning with the hot-melt sealing gasket. particles observed. Considering heavy elements like Ag, they backscatter electrons more strongly than light ele- Characterizations ments like O and Ti [39], so the metallic silver appears brighter in the image as observed in Fig. 3b, c. At one UV/Vis spectrophotometer (UV752N, Axiom) was used to SILAR cycle, the shining Ag nanoparticles are well dis- measure the optical absorption spectra of the electrode film persed with smaller Ag nanoparticles and more uniformly samples. The measurement was designed to cover the distributed in the pores of TiO , as shown in Fig. 3b. visible region of the electromagnetic spectrum. Scanning Conversely, for higher two SILAR deposition cycles of Ag electron microscope (SEM) (MEL-30000, SCOTECH) was NPs, the Ag nanoparticles became gradually aggregated to employed to record cross-sectional micrographs of the form bigger Ag NPs as observed in Fig. 3c. TiO /Ag NPs films. Also, I–V measurements were per- formed using a solar simulator (Keithley 4200-SCS) under UV–Vis spectroscopy simulated AM 1.5 sunlight at an irradiance of 100 mW/ cm . Absorption spectra of a dye reflect optical transition The photovoltaic parameters: maximum power output probability between the ground state, the excited state, and (P ), fill factor (FF), and conversion efficiency (g) of the the solar energy range absorbed by the dye [13]. The max devices were evaluated according to the following formula comparative optical spectra of the pure betalain dye relationship based on J–V characteristic curves: extract, betalain dye extract adsorbed into TiO (TiO /dye) 2 2 electrode, TiO /Ag NPs (one cycle)/dye, and TiO /Ag NPs 2 2 P ¼ V  J ð8Þ max max max (two cycles)/dye are shown in Fig. 4. P V J max max max The betalain dye adsorbed into TiO (TiO /dye) elec- Fill factor ðFFÞ¼ ¼ ð9Þ 2 2 V J V J oc sc oc sc trode shows a reduced absorbance compared with that of FF  V  J betalain dye extract only, as shown in Fig. 4a. The betalain oc sc Efficiency ðÞ g ¼  100 ð10Þ P extract shows a broad absorption spectrum in the visible in Fig. 3 SEM images of a bare TiO , b TiO /Ag NPs one SILAR cycle, and c TiO /Ag NPs two SILAR cycles 2 2 2 123 Mater Renew Sustain Energy (2016) 5:10 Page 5 of 9 10 Fig. 4 Comparison of absorption spectra of pure betalain dye extract in solution with a betalain dye extract adsorbed on TiO , b dye extract adsorbed on TiO /Ag NPs (one cycle), and c betalain dye extract adsorbed on TiO /Ag NPs (two cycles) Fig. 5 Absorption spectra of a TiO /Ag NPs films of one and two SILAR deposition cycles and b dye impregnated TiO /Ag NPs films of one and two SILAR deposition cycles on soda lime glass substrate light region (ca. 400–700 nm) indicating that betalain has the electrode with two SILAR cycles. A single absorption the ability to function as an efficient sensitizer for wide peak around 390 nm could be detected with an additional bandgap semiconductors [40, 41]. Three absorption peaks shoulder at 530 nm from Fig. 4b. In addition to the peaks at about 480, 535, and 670 nm are observed, which are detected in (b), there are other additional absorption peaks ascribed to the betalain pigments present in the dye [42]. around 420, 470, 490, 510, 560, and 590 nm in (c). Figure 4b, c shows synergistic absorption spectra of dye The comprehensive comparison of optical absorption and Ag NPs on TiO electrode with one and two SILAR spectra of bare, incorporated Ag-TiO , and impregnated 2 2 cycles, respectively, both of which show higher absorption TiO /Ag/dye films on soda lime glass substrate is shown in intensity compared with TiO /dye only in Fig. 4a. This Fig. 5, before and after dye sensitizer adsorption. suggests that the incorporation of Ag NPs with one and two The absorption spectra of TiO film with Ag nanopar- SILAR cycles has significantly improved the optical ticles (TiO –Ag NPs), as shown in Fig. 5a, show broad absorption of the electrode which is attributable to local- peaks centered at 450 nm for both one and two SILAR ized surface plasmons (LSP) effect of the nanoparticles. deposition cycles, which is attributable to localized surface The two SILAR cycles show the highest absorption plasmons (LSP) effect and their intensities increase as intensity indicating the presence of more Ag NPs, which more Ag NPs are loaded onto TiO from one to two SILAR couple with the dye to increase the optical absorption for cycles. This result indicates the excitation of LSP on the 123 10 Page 6 of 9 Mater Renew Sustain Energy (2016) 5:10 TiO –Ag NPs. TiO film with both Ag nanoparticles and electrons in metallic nanoparticles, the nanoparticles must 2 2 dye (TiO /Ag NPs/dye) in Fig. 5b shows lower absorption be smaller than the wavelength of exciting light for elec- intensities for one and two SILAR deposition cycles trons to oscillate with the electric field of light. When these compared with that of Fig. 5a. This result indicates that Ag conditions are met, an enhanced electromagnetic field is nanoparticles incorporation clearly enhances the photon found nearby the surface of nanoparticles. The enhanced harvesting ability of TiO adsorbed with dye. electromagnetic field is highly dependent upon the wave- length of incident light, as well as the shape, size, and Device performance efficiency aggregation state of the nanoparticles [44]. Thus, it is observed, both plasmonic Ag NPs incorpo- The current density–voltage (J–V) and power density– rated DSSCs experience an improvement in their respec- voltage (P–V) characteristics were measured to study the tive photocurrent densities (J ) with the cell with one sc photoelectric performances of both the bare and incorpo- SILAR cycle showing better enhancement from plasmonic rated plasmonic Ag nanoparticles (Ag NPs) dye-sensitized effect. The generated photocurrent density (J ) increases sc solar cells (DSSCs). with the introduction of Ag NPs, which is found to be 44 Figure 6 shows the J–V and P–V characteristic curves of and 19 % higher than the reference device (device 1) for both the bare and intercalated plasmonic Ag NPs DSSCs. one and two SILAR cycles, respectively, as shown in The photovoltaic performances of the cells are determined Table 1; thus, showing the electron transport efficiency in through the photovoltaic parameters (short-circuit current the TiO /dye/electrolyte interface is improved. With the density (J ), open-circuit voltage (V ), fill factor (FF), and increase in the amount of incorporated Ag NPs, increase in sc oc conversion efficiency (g), which are obtained from the J–V the recombination probability arises and leads to the and P–V characteristic curves of the cells. The fill factor decline of photocurrent density [20] and, as a result, (FF) defined as the ratio of P and the product J V reduces the PCE of the DSSCs when the SILAR cycles max sc oc shows curve squareness, and the closer to unity the fill increased from one to two. Considering the size and factor is, the better cell quality will be [43]. aggregation state of Ag NPs as observed in Fig. 3 of SEM The photovoltaic performance parameters: short-circuit analysis, these generate defects at the surface of the active current (J ), open-circuit voltage (V ), fill factor (FF), and layer of the cell. These defects are potential recombination sc oc conversion efficiency (g) of the cells, are shown in Table 1. centers which are capable of trapping generated carriers, As it has been reported that the surface plasmon effect is and thus, the surface recombination pose a negative impact caused by light-driven collective oscillations of conduction on the effective carrier lifetime and, subsequently, on the Fig. 6 J–V and P–V characteristic curves of a DSSC with bare TiO electrode, b DSSC with one SILAR cycle TiO intercalated Ag NPs electrode, and c DSSC with two SILAR cycles TiO intercalated Ag NPs electrode 123 Mater Renew Sustain Energy (2016) 5:10 Page 7 of 9 10 Table 1 Photovoltaic parameters of plasmonic-DSSC -2 -2 Device Photoanode J (mA cm ) V (V) J (mA cm ) V (V) FF g (%) % Increase in g sc oc max max 1 TiO (bare) 0.70 0.45 0.53 0.34 0.57 0.18 0 2 TiO Ag (one cycle) 1.01 0.45 0.77 0.35 0.59 0.27 50 2/ 3 TiO Ag (two cycles) 0.83 0.45 0.60 0.37 0.59 0.22 22 2/ efficiency of the solar cells [21, 32, 44–46]. Also, the maximum short-circuit current density (J ) and PCE of sc -2 recombination reaction creates an internal short-circuit 1.01 mA cm and 0.27 %, respectively. This shows a throughout the bulk of the photoanode layer [32]; as a significant improved power conversion efficiency of 50 % result, the device shows a lower PCE with two SILAR with one SILAR deposition cycle of Ag NPs intercalated cycles. Also, the scattering effect may contribute to the into the photoanode over the bare photoanode reference decline, but the plasmonic effect still dominates because of DSSC cell. the relatively small size of the nanoparticles [20]. Acknowledgments This study was partly supported by grant from The efficiency of the plasmonic-DSSC (device 2) the University Board of Research (UBR) of the Federal University of obtained with one SILAR deposition cycle of Ag NPs onto Technology, Minna, Nigeria. the TiO photoelectrode is 0.27 % indicating an improve- ment of 50 % compared with the efficiency of the reference Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http:// DSSC (device 1) which had an efficiency of 0.18 %. The creativecommons.org/licenses/by/4.0/), which permits unrestricted open-circuit voltage (V ) and fill factor (FF) remain almost oc use, distribution, and reproduction in any medium, provided you give unchanged, while the short-circuit current density (J ) sc appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were indicates a clear increase owed to the light trapping as a made. result of LSP effect of Ag NPs, which make more photons ready and available for dyes to absorb for photoelectrons generation. The intercalation of Ag nanoparticles has been shown not to significantly affect the values of V , because oc References V is determined mainly by the redox potential and the oc Fermi energy of semiconductors and is not significantly 1. Adhyaksa, G.W.P., Baek, S., Lee, G.I., Lee, D.K., Lee, J.-Y., affected with the change in size of the Ag NPs [18, 47, 48]. 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Plasmonic effect of silver nanoparticles intercalated into mesoporous betalain-sensitized-TiO2 film electrodes on photovoltaic performance of dye-sensitized solar cells

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Abstract

Mater Renew Sustain Energy (2016) 5:10 DOI 10.1007/s40243-016-0075-z ORIGINAL PAPER Plasmonic effect of silver nanoparticles intercalated into mesoporous betalain-sensitized-TiO film electrodes on photovoltaic performance of dye-sensitized solar cells 1 1 1 1 • • • • Kasim U. Isah Bukola J. Jolayemi Umaru Ahmadu Mohammed Isah Kimpa Noble Alu Received: 21 February 2016 / Accepted: 8 June 2016 / Published online: 18 June 2016 The Author(s) 2016. This article is published with open access at Springerlink.com Abstract Dye-sensitized solar cells (DSSCs) comprising Introduction mesoporous TiO films and betalain pigments extracted from red Bougainvillea glabra flower as natural dye sen- Dye-sensitized solar cells (DSSCs) have remained one of sitizers were fabricated and enhanced by the intercalation the most promising choices for the realization of low-cost of the plasmonic silver nanoparticles (Ag NPs) into the solar cells; it has, in the recent years, attracted remarkable pores of mesoporous TiO electrodes by successive ionic attention owing to simplicity of its fabrication process and layer adsorption and reaction (SILAR) method. The TiO / low manufacturing cost with excellent light absorbing Ag NPs composite films were characterized by SEM and materials [1–3]. Nevertheless, the power conversion effi- UV–Vis spectroscopy. I–V characteristics of the devices ciency (PCE) of the device has barely gone up to 11.9 % were measured by solar simulator (AM1.5 at 100 mW/ [4]. Many technological innovations have been developed cm ). The incorporation of the Ag nanoparticles into the to improve the efficiency and, at the same time, to reduce pores of mesoporous TiO electrodes with one SILAR the cost of production ranging from interfacial modification deposition cycle of the Ag NPs produced the best plas- [5, 6] to material choice and engineering [7]. This devel- monic enhanced-DSSC giving a short-circuit current den- opment leads to the consideration of natural dyes from sity (J ), fill factor (FF), and power conversion efficiency plant sources which are cost-effective, non-toxic, and sc -2 (PCE) of 1.01 mA cm , 0.77, and 0.27 %, respectively. completely biodegradable compared with the much This development amounts to 50 % efficiency enhance- expensive and toxic synthesized counterparts based on ment over the reference DSSC that had a short-circuit metal complex-like ruthenium (II) and metal-free like current density (J ), fill factor (FF), and power conversion porphyrin [8]. Betalain has been regarded by many sc -2 efficiency (PCE) of 0.7 mA cm , 0.57, and 0.18 %, researchers to be a most promising candidate of choice out respectively. of the three most exploited dye pigments from plant sources: chlorophylls, anthocyanins, and betalains [9]. Keywords Plasmonic  Silver nanoparticles  SILAR  Conversely to the anthocyanins, betalains possess essential DSSCs  Betalain  Bougainvillea glabra functional groups (–COOH) to anchor better to the TiO nanoparticles than the functional groups (–OH) present in anthocyanins [10–13]. Indeed, the interaction between TiO film and carboxylic functions should bring a stronger electronic coupling and rapid forward and reverse electron transfer reactions [9]. & Kasim U. Isah Also, the incorporation of plasmonic metal nanoparti- kasim309@futminna.edu.ng cles (NPs) into the electrode (usually m-TiO ) of dye- sensitized solar cells (DSSCs) to boost the light absorption Department of Physics, School of Physical Sciences, Federal University of Technology, Minna, Nigeria due to their localized surface plasmon (LSP) effect has been very outstanding [14–17]. Noble plasmonic metal Physics Advanced Laboratory (PAL), Sheda Science and nanostructures, such as gold (Au), silver (Ag) [18, 19], Technology Complex (SHESTCO), Abuja, Nigeria 123 10 Page 2 of 9 Mater Renew Sustain Energy (2016) 5:10 aluminum (Al) [20], their alloys [16], and their core–shell substrates obtaining about 3.5 lm with polyester mesh formation [21–25], have been incorporated into DSSCs to screen-120. After drying at 55 C for 30 min, the elec- effectively utilize the incident photon and improve cell trodes were sintered at 450 C for 30 min, and then grad- performance. Many methods, both chemical and physical ually cooled down to room temperature. categories, including, wet chemical [26], polyol process [27], seed-mediated growth [28], photochemical [29], Synthesis and incorporation of silver nanoparticles sonochemical [30], electrochemical [31], bioinspired [32], sputtering [33], and laser ablation [34], which have been, Silver nanoparticles (Ag NPs) were prepared by successive hitherto, reportedly employed to synthesize various plas- ionic layer adsorption and reaction (SILAR) method. The monic Ag nanostructures, possess one disadvantage or the cationic precursor for Ag NPs using SILAR method was other, such as toxic reducing agents, impurities, organic diammine silver complex AgðNH Þ solution. This ðaqÞ solvents, or may require special condition like high-tem- solution was prepared using 0.35 g of AgNO (BDH) in perature or low-pressure environment, and sometimes 200 ml of distilled water, to this, ammonia solution expensive and time-consuming procedures may be (NH OH) (ca. 33 wt% NH, Grifflin and George) was added involved [35]. drop by drop until colorless solution was observed. The Successive ionic layer adsorption and reaction (SILAR) anionic (reducing agent) solution was prepared by adding method was used in this work for the intercalation of Ag 2 ml of concentrated HCl (36 %, Loba Chemie) to 2 g of nanoparticles into the pores of mesoporous TiO photo- stannous chloride (SnCl .2H O) (BDH) in 640 ml of dis- 2 2 electrodes of betalain-sensitized DSSCs. It offers simple, tilled water. inexpensive, and time-saving procedures, which can be To intercalate the Ag NPs into the mesoscopic TiO carried out at room temperature with no restrictions on electrodes, the prepared electrode was immersed in the substrate material, dimensions or its surface profile, and the diammine silver complex AgðNH Þ solution for thickness of the film or nanoparticle can be easily con- ðaqÞ trolled [36–38]. 2 min (adsorption), then rinsed with distilled water for 1 min (to remove excess adsorbed ions from the diffu- sion layer), the electrode was then transferred to the Experimental stannous chloride solution for 2 min (reduction reaction), and hence, film turns brownish due to the reduction from Preparation of photosensitizer Ag to Ag; it was then treated with acid rinse by immersing it in 0.25 M of HCl (36 %, Loba Chemie) for The photosensitizer was made of natural dye from betalain 30 s (to etch any loosely silver nanoparticles) and finally pigments extracted from red Bougainvillea glabra flower. rinsed with distilled water for 30 s to remove the excess 13.12 g of the fresh flowers was ground and well blended and unreacted species and reaction by-product from the together in 100 ml of distilled water in an electric blender. diffusion layer. The deposition process is shown in Then, the mixture was filtered, and the filtrate was used as Fig. 1. the sensitizing dye without further purification. This process was repeated for another electrode with two SILAR cycles. As soon as the required number of Preparation of active electrode layer cycles was complete, the film was returned to the silver diammine complex solution for 30 s to form a layer of Fluorine-doped tin oxide (FTO: TCO30-8 glass, 8 X/sq, protective oxidized Ag shell on the film and rinsed in 3 mm thick, Solaronix) glass sheets were cleaned with distilled water for another 30 s before drying at 100 Cin detergent (sodium lauryl sulfate), rinsed with distilled an open air until it was fully dried; this leads to the for- water, ethanol, and dried under compressed hot air for mation of a thin silver oxide shell on the film to protect the 7 min at 70 C in a clean container. 0.5 9 0.6 cm area Ag nanoparticles from the iodic electrolyte [16]. To further compact TiO (c-TiO ) paste (Ti-Nanoxide BL/SP, Sola- 2 2 protect the Ag NPs from the corrosive electrolyte, the films ronix) was screen-printed on three 2.5 9 2.5 cm FTO were refluxed in 0.1-M titanium (IV) isopropoxide (Sigma- glass substrates using a 70 mesh screen to obtain about Aldrich) in isopropanol solution for 25 min to form silver– 50-nm thick TiO film. The film was sintered on the hot- titanium dioxide core–shell (Ag@TiO ) nanoparticles and plate at 500 C for 50 min and was allowed to slowly cool sintered at 350 C for 20 min. The preparation process is down to room temperature. Afterward, mesoporous TiO shown in Fig. 2. (m-TiO ) paste (Ti-Nanoxide T300/SP, Solaronix) was The chemical reaction mechanism for the formation of screen-printed on the 0.30 cm of as-deposited c-TiO Ag NPs is given in the following. 123 Mater Renew Sustain Energy (2016) 5:10 Page 3 of 9 10 Fig. 1 Scheme of SILAR cycle process of Ag nanoparticles deposition for DSSC application Fig. 2 Schematic of plasmonic photoanode preparation process Cationic precursor (adsorption step) SnCl þ H O  Sn(OH)Cl þ HCl ð4Þ 2 2 ðsÞ ðaqÞ a clear solution of SnCl requires addition of excess 2(aq) Diammine silver complex cationic precursor is obtained by HCl adding enough ammonia solution (NH OH )to 4 (aq) AgNO . Brownish precipitate appears when a small Sn(OH)Cl þ HCl ¼ SnCl þ 2H O ð5Þ 3(aq) 2 ðsÞ ðaqÞ 2ðaqÞ amount of ammonia solution is added as a result of the where formation of silver (I) oxide (Ag O ): 2 (s) 2þ SnCl ! Sn þ 2Cl ð6Þ 2ðaqÞ 2AgNO þ 2NH OH ! Ag O þ 2NH NO 4 ðaqÞ ðsÞ 4 3ðaqÞ 3ðaqÞ 2 þ H O þ ðlÞ Ag is, reduced to Ag when the films were immersed in 2þ ð1Þ the anionic solution containing Sn as follows: ðaqÞ The precipitate (Ag O ) dissolves in excess ammonia 2 (s) þ 2þ 4þ 2Ag þ Sn ! 2Ag þ Sn ð7Þ ðsÞ ðaqÞ solution to form colorless diammine silver complex ion: Ag O þ 4NH þ H O ! 2 Ag(NH Þ þ2OH ð2Þ Sensitization of photoanode ðsÞ 3 2 ðlÞ 2 3 2 ðaqÞ ðaqÞ The ionic equation of the complex ion is given as: ThebaremesoscopicTiO electrode and mesoscopic þ þ TiO electrodes intercalated with Ag NPs (one and two ½AgðNH Þ   Ag þ 2NH ð3Þ 2 3 3 2 aq SILAR cycles) were each soaked in three different Anionic precursor (reaction step) Petri dishes containing the sensitizing dye for about 20 h. After which, the dye impregnated photoanodes Tin (II) chloride dissolves in less than its own mass of were then rinsed with ethanol (99 %, Sigma-Aldrich), water to form insoluble tin hydroxy chloride specie in a to remove excess dye particles that were not properly reversible hydrolysis, adsorbed. 123 10 Page 4 of 9 Mater Renew Sustain Energy (2016) 5:10 Fabrication of DSSCs Results and discussions The counter electrodes were first prepared by screen- printing a 0.5 9 0.6-cm thin film of platinum (Pt) paste Scanning electron microscopy (SEM) (Platisol T/SP, Solaronix) on a bare 2.5 9 2.5-cm FTO glass substrate, and then sintered at 450 C for 30 min and SEM images of the bare TiO and the TiO /Ag NPs 2 2 allowed to slowly cool to room temperature. composite films with different SILAR cycles are shown in A sandwich-type DSSCs were fabricated by assembling Fig. 3. The SEM images reveal a random two-dimensional the dye-impregnated photoelectrodes and counter elec- array of Ag NPs with a very broad particle size distribution. trodes in an overlapping manner, so as to establish elec- Figure 3a is the bare TiO electrode, which shows the trical connection between the cells and the photovoltaic absence of Ag NPs, while Fig. 3b, c confirms the interca- measurement equipment. The assembling was achieved lation of Ag NPs in the pores of TiO with one and two using hot-melt sealing gasket of Surlyn-based polymer SILAR cycles, respectively. sheet (SX1170-25PF, Solaronix) and sealed on a heating The morphology of the films appears to be different, stage leaving a pinhole for the electrolyte injection for a which can be attributed to the varied SILAR deposition few seconds. Finally, the iodine-based liquid electrolyte cycles of the Ag NPs. The image of the pure TiO film in (iodolyte) was injected using micropipette before sealing Fig. 3a shows a dense surface, and there are no shinning with the hot-melt sealing gasket. particles observed. Considering heavy elements like Ag, they backscatter electrons more strongly than light ele- Characterizations ments like O and Ti [39], so the metallic silver appears brighter in the image as observed in Fig. 3b, c. At one UV/Vis spectrophotometer (UV752N, Axiom) was used to SILAR cycle, the shining Ag nanoparticles are well dis- measure the optical absorption spectra of the electrode film persed with smaller Ag nanoparticles and more uniformly samples. The measurement was designed to cover the distributed in the pores of TiO , as shown in Fig. 3b. visible region of the electromagnetic spectrum. Scanning Conversely, for higher two SILAR deposition cycles of Ag electron microscope (SEM) (MEL-30000, SCOTECH) was NPs, the Ag nanoparticles became gradually aggregated to employed to record cross-sectional micrographs of the form bigger Ag NPs as observed in Fig. 3c. TiO /Ag NPs films. Also, I–V measurements were per- formed using a solar simulator (Keithley 4200-SCS) under UV–Vis spectroscopy simulated AM 1.5 sunlight at an irradiance of 100 mW/ cm . Absorption spectra of a dye reflect optical transition The photovoltaic parameters: maximum power output probability between the ground state, the excited state, and (P ), fill factor (FF), and conversion efficiency (g) of the the solar energy range absorbed by the dye [13]. The max devices were evaluated according to the following formula comparative optical spectra of the pure betalain dye relationship based on J–V characteristic curves: extract, betalain dye extract adsorbed into TiO (TiO /dye) 2 2 electrode, TiO /Ag NPs (one cycle)/dye, and TiO /Ag NPs 2 2 P ¼ V  J ð8Þ max max max (two cycles)/dye are shown in Fig. 4. P V J max max max The betalain dye adsorbed into TiO (TiO /dye) elec- Fill factor ðFFÞ¼ ¼ ð9Þ 2 2 V J V J oc sc oc sc trode shows a reduced absorbance compared with that of FF  V  J betalain dye extract only, as shown in Fig. 4a. The betalain oc sc Efficiency ðÞ g ¼  100 ð10Þ P extract shows a broad absorption spectrum in the visible in Fig. 3 SEM images of a bare TiO , b TiO /Ag NPs one SILAR cycle, and c TiO /Ag NPs two SILAR cycles 2 2 2 123 Mater Renew Sustain Energy (2016) 5:10 Page 5 of 9 10 Fig. 4 Comparison of absorption spectra of pure betalain dye extract in solution with a betalain dye extract adsorbed on TiO , b dye extract adsorbed on TiO /Ag NPs (one cycle), and c betalain dye extract adsorbed on TiO /Ag NPs (two cycles) Fig. 5 Absorption spectra of a TiO /Ag NPs films of one and two SILAR deposition cycles and b dye impregnated TiO /Ag NPs films of one and two SILAR deposition cycles on soda lime glass substrate light region (ca. 400–700 nm) indicating that betalain has the electrode with two SILAR cycles. A single absorption the ability to function as an efficient sensitizer for wide peak around 390 nm could be detected with an additional bandgap semiconductors [40, 41]. Three absorption peaks shoulder at 530 nm from Fig. 4b. In addition to the peaks at about 480, 535, and 670 nm are observed, which are detected in (b), there are other additional absorption peaks ascribed to the betalain pigments present in the dye [42]. around 420, 470, 490, 510, 560, and 590 nm in (c). Figure 4b, c shows synergistic absorption spectra of dye The comprehensive comparison of optical absorption and Ag NPs on TiO electrode with one and two SILAR spectra of bare, incorporated Ag-TiO , and impregnated 2 2 cycles, respectively, both of which show higher absorption TiO /Ag/dye films on soda lime glass substrate is shown in intensity compared with TiO /dye only in Fig. 4a. This Fig. 5, before and after dye sensitizer adsorption. suggests that the incorporation of Ag NPs with one and two The absorption spectra of TiO film with Ag nanopar- SILAR cycles has significantly improved the optical ticles (TiO –Ag NPs), as shown in Fig. 5a, show broad absorption of the electrode which is attributable to local- peaks centered at 450 nm for both one and two SILAR ized surface plasmons (LSP) effect of the nanoparticles. deposition cycles, which is attributable to localized surface The two SILAR cycles show the highest absorption plasmons (LSP) effect and their intensities increase as intensity indicating the presence of more Ag NPs, which more Ag NPs are loaded onto TiO from one to two SILAR couple with the dye to increase the optical absorption for cycles. This result indicates the excitation of LSP on the 123 10 Page 6 of 9 Mater Renew Sustain Energy (2016) 5:10 TiO –Ag NPs. TiO film with both Ag nanoparticles and electrons in metallic nanoparticles, the nanoparticles must 2 2 dye (TiO /Ag NPs/dye) in Fig. 5b shows lower absorption be smaller than the wavelength of exciting light for elec- intensities for one and two SILAR deposition cycles trons to oscillate with the electric field of light. When these compared with that of Fig. 5a. This result indicates that Ag conditions are met, an enhanced electromagnetic field is nanoparticles incorporation clearly enhances the photon found nearby the surface of nanoparticles. The enhanced harvesting ability of TiO adsorbed with dye. electromagnetic field is highly dependent upon the wave- length of incident light, as well as the shape, size, and Device performance efficiency aggregation state of the nanoparticles [44]. Thus, it is observed, both plasmonic Ag NPs incorpo- The current density–voltage (J–V) and power density– rated DSSCs experience an improvement in their respec- voltage (P–V) characteristics were measured to study the tive photocurrent densities (J ) with the cell with one sc photoelectric performances of both the bare and incorpo- SILAR cycle showing better enhancement from plasmonic rated plasmonic Ag nanoparticles (Ag NPs) dye-sensitized effect. The generated photocurrent density (J ) increases sc solar cells (DSSCs). with the introduction of Ag NPs, which is found to be 44 Figure 6 shows the J–V and P–V characteristic curves of and 19 % higher than the reference device (device 1) for both the bare and intercalated plasmonic Ag NPs DSSCs. one and two SILAR cycles, respectively, as shown in The photovoltaic performances of the cells are determined Table 1; thus, showing the electron transport efficiency in through the photovoltaic parameters (short-circuit current the TiO /dye/electrolyte interface is improved. With the density (J ), open-circuit voltage (V ), fill factor (FF), and increase in the amount of incorporated Ag NPs, increase in sc oc conversion efficiency (g), which are obtained from the J–V the recombination probability arises and leads to the and P–V characteristic curves of the cells. The fill factor decline of photocurrent density [20] and, as a result, (FF) defined as the ratio of P and the product J V reduces the PCE of the DSSCs when the SILAR cycles max sc oc shows curve squareness, and the closer to unity the fill increased from one to two. Considering the size and factor is, the better cell quality will be [43]. aggregation state of Ag NPs as observed in Fig. 3 of SEM The photovoltaic performance parameters: short-circuit analysis, these generate defects at the surface of the active current (J ), open-circuit voltage (V ), fill factor (FF), and layer of the cell. These defects are potential recombination sc oc conversion efficiency (g) of the cells, are shown in Table 1. centers which are capable of trapping generated carriers, As it has been reported that the surface plasmon effect is and thus, the surface recombination pose a negative impact caused by light-driven collective oscillations of conduction on the effective carrier lifetime and, subsequently, on the Fig. 6 J–V and P–V characteristic curves of a DSSC with bare TiO electrode, b DSSC with one SILAR cycle TiO intercalated Ag NPs electrode, and c DSSC with two SILAR cycles TiO intercalated Ag NPs electrode 123 Mater Renew Sustain Energy (2016) 5:10 Page 7 of 9 10 Table 1 Photovoltaic parameters of plasmonic-DSSC -2 -2 Device Photoanode J (mA cm ) V (V) J (mA cm ) V (V) FF g (%) % Increase in g sc oc max max 1 TiO (bare) 0.70 0.45 0.53 0.34 0.57 0.18 0 2 TiO Ag (one cycle) 1.01 0.45 0.77 0.35 0.59 0.27 50 2/ 3 TiO Ag (two cycles) 0.83 0.45 0.60 0.37 0.59 0.22 22 2/ efficiency of the solar cells [21, 32, 44–46]. Also, the maximum short-circuit current density (J ) and PCE of sc -2 recombination reaction creates an internal short-circuit 1.01 mA cm and 0.27 %, respectively. This shows a throughout the bulk of the photoanode layer [32]; as a significant improved power conversion efficiency of 50 % result, the device shows a lower PCE with two SILAR with one SILAR deposition cycle of Ag NPs intercalated cycles. Also, the scattering effect may contribute to the into the photoanode over the bare photoanode reference decline, but the plasmonic effect still dominates because of DSSC cell. the relatively small size of the nanoparticles [20]. Acknowledgments This study was partly supported by grant from The efficiency of the plasmonic-DSSC (device 2) the University Board of Research (UBR) of the Federal University of obtained with one SILAR deposition cycle of Ag NPs onto Technology, Minna, Nigeria. the TiO photoelectrode is 0.27 % indicating an improve- ment of 50 % compared with the efficiency of the reference Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http:// DSSC (device 1) which had an efficiency of 0.18 %. The creativecommons.org/licenses/by/4.0/), which permits unrestricted open-circuit voltage (V ) and fill factor (FF) remain almost oc use, distribution, and reproduction in any medium, provided you give unchanged, while the short-circuit current density (J ) sc appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were indicates a clear increase owed to the light trapping as a made. result of LSP effect of Ag NPs, which make more photons ready and available for dyes to absorb for photoelectrons generation. The intercalation of Ag nanoparticles has been shown not to significantly affect the values of V , because oc References V is determined mainly by the redox potential and the oc Fermi energy of semiconductors and is not significantly 1. Adhyaksa, G.W.P., Baek, S., Lee, G.I., Lee, D.K., Lee, J.-Y., affected with the change in size of the Ag NPs [18, 47, 48]. 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