Performance of simple green synthesized Ag incorporated TiO2 nanoparticles based photoanodes by doctor-blade coating as working electrodes for dye sensitized solar cells
Introduction
The global economy is predicted to grow 3-fold between now and 2050, for this to get realized the meeting the energy demand around the world is also going to be a significant challenge. Due to the unfavorable effects of non-renewable energy sources on the environment, there is a specific need to shift focus to renewable sources. One of the most promising candidates for renewable sources is the sun's energy which needs to be converted effectively and reliably to electrical energy through a cost-effective method [1], [2]. Silicon based solar cells has subjugated the solar PV market for the last 2–3 decades with a share of ~90%. Being in the market for long years it still has some disadvantages such as high processing temperatures, growing and sawing of ingots is a highly energy intensive process and much of the energy of higher energy photons, at the blue and violet end of the spectrum, is wasted as heat [3].
This cost-efficient topic associated with these solar cells can be addressed through ways such as, decreasing the material cost or by improving the efficiency of the solar cells. Among different generation of solar cells, the dye sensitized solar cells (DSSCs) is one of the most promising alternatives to the conventional silicon based solar cell for indoor and window applications, because of its semitransparent nature, cost effectiveness and its ability to work under diffused light conditions [4], [5], [6], [7], [8].
In dye-sensitized solar cells, semiconductors with wide bandgap and mesoporous nature are generally used as photoanodes. The work of the photoanode is to transfer the photo exited electron from the chemisorbed dye to outer circuit. Various wide band gap metal oxide semiconductors such as ZnO, Nb2O5, SnO2 and TiO2, have been studied as photoanodes for DSSC [9], [10]. Among different semiconductors TiO2 is an n-type semiconductor, and it has a wide bandgap of 3–3.2 eV and its position of bands suits well for the electron transfer process than its counterparts. Also, the other properties such as chemical stability, cost-effectiveness and non-toxicity, makes TiO2 an efficient electrode material for DSSC [11]. Nevertheless, it has few problems such as high recombination rate of photo generated carriers and its poor light absorption capability in the visible region.
Researchers previously have tried various methods such as doping, making TiO2 composites with different materials among several others to improve its properties [12]. One of the efficient ways, to improve the properties of TiO2 is through modification of its surface with the deposition of the noble metals (Au, Ag, Cu and Pt) [13], [14], [15]. The surface modifications help in both improving the efficiency of the light absorption and in the modification of electronic structure of TiO2. Among different noble metals, Ag has good electron storage properties, and it helps to enhance the electron hole pairs separation in semiconductor-metal alloys [16], [17]. There are several studies, where dye sensitized solar cells were fabricated using Ag doped TiO2 nanoparticle as working electrode material, because it's comparatively cheap and it also helps to tuning the band gap of the TiO2.
Incorporation of Ag creates porous structure at the TiO2 surface and it helped in achieving better adsorption of the dye molecule, and also DSSC using this as working electrode exhibited an efficiency of 6.06% [18]. Enhancement in the light harvesting ability, the improvement in interfacial electron transfer were achieved by using Ag nanowires with TiO2 nanoparticles as composite films, power conversion efficiency of 5.31% was achieved [19].
Rajaei et al. [20] they have synthesized Ag doped TiO2 nanofibers by electrospinning method and they have prepared photo anode using synthesized TiO2 nanoparticles and Ag doped TiO2 nanorods (through calcination of TiO2 nanofibers, nanorods were obtained) with different weight percentage of silver for DSSCs fabrication. They have reported enhancement in the efficiency of 3.6% through silver incorporation. Kaur et al. [21] have reported the improved plasmonic DSSCs with comparatively higher light harvesting ability and reduced recombination of photo-generated charge carriers using low energy (120 keV) Ag ion implanted TiO2 photoanodes at variable fluence. It was attributed to the localized surface plasmon resonance (LSPR) property of metals. Agus Supriyanto et al. have prepared photoanode using TiO2 paste by spin coating method and they have improvised by adding different concentrations of commercial Ag ions mixed directly into the organic triphenylamine dye as sensitizers for DSSCs fabrication and had reported an enhanced efficiency [22].
In this work TiO2 and Ag doped TiO2 have been prepared through green synthesis technique without utilizing any separate reducing agents and chemical solvents. The fruit extracts were used as solvent in the preparation of TiO2 and Ag doped TiO2 nanoparticles and the same fruit extracts also acts as reducing agents. The natural extracts as solvents /reducing agents were used directly, without any separate solvent requirements. The synthesized TiO2 and Ag doped TiO2 nanoparticles were coated over FTO substrates (Fluorine doped Tin oxide) and they were sensitized using two different dyes such as natural dye and commercial N719 dye and its photovoltaic performance as DSSC photoanode were studied.
Section snippets
Materials
Titanium isopropoxide (TTIP, 97%), ethanol, were purchased from Sigma-Aldrich and were used as obtained. Fluorine-doped tin oxide (FTO) plates having 7 Ω as sheet resistance were used.
Instruments
The X-ray diffraction studies has been carried out using X-ray diffractometer XPERT-PRO diffractometer system operating with Cu Ka radiation in a θ–2θ geometry. The surface morphology was analyzed using field-emission scanning electron microscope (FE-SEM, ZEISS, SIGMA, UK). The UV–Vis absorption spectra have been recorded using Perkin Elmer Lambda 35 spectrometer. Raman spectroscopy studies were carried out using a laser confocal Raman microscope (Renishaw, UK, Model: Invia). The efficiency
Conclusions
In this work, TiO2 and Ag incorporated TiO2 nanoparticles were prepared using grapes extracts as solvent/reducing agent. The XRD and Raman analysis revealed the prepared samples are anatase in phase. Absorption in the shorter wavelength region was observed for the bare TiO2 and an enhancement of absorption in the visible region was observed for the Ag incorporated TiO2 samples. The FESEM images of the TiO2 sample exhibited nanorod like structure and Ag incorporated TiO2 showed near spherical
CRediT authorship contribution statement
S. Rajkumar: Conceptualization, Methodology, Writing - Original draft preparation.
M.R. Venkatraman: Conceptualization, characterization of samples, Writing - Original draft preparation.
P. Balraju: Sample characterization and analysis of the data.
K. Suguna: Supervision, Original draft preparation and Editing.
Arivalagan Pugazhendhi: Supervision, Project administration.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Both the authors have equally contributed.