Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Selected organic dyes (carminic acid, pyrocatechol violet and dithizone) sensitized metal (silver, neodymium) doped TiO2/ZnO nanostructured materials: A photoanode for hybrid bulk heterojunction solar cells
Graphical abstract
Introduction
Due to their low cost and high power conversion efficiency, DSSCs (dye sensitized solar cells) have become a common alternative to traditional silicon-based solar cells. [1], [2]. Many semiconductors, such as ZnO, SnO2, TiO2, ZnO, Nb2O5, and SrTiO3, have been investigated as photoanode materials in the development of high-performance DSSCs [3], [4], [5], [6]. However, in spite of all other oxides, TiO2 continues to be the best semiconductor electrode material due to its exceptional charge transfer performance and chemical stability. The photoelectric conversion efficiency of DSSCs using TiO2 (anatase) as the photoanode materials has been reported to exceed 11% [7], [8].Titanium dioxide (TiO2) is still a standout, amongst effectively studied adaptable materials, in light of the variety of its utilizations, as photovoltaic material, photo catalyst, structural ceramics, gas sensor, electrical circuit, optical coating and biocompatible material for inserting in bone [9], [10], [11], [12]. The mesoporous TiO2 film is a key candidate for many devices. Anatase Titania band gap is 3.2 eV, and has the tendency to absorb the organic dyes which make it a suitable candidate in DSSCs (Dye sensitized solar cells) [13]. The TiO2 plays three crucial roles in DSSCs: (i) the best dye adsorption substrate, (ii) accepting the electrons from the dye's excited state, and (iii) finally transporting the electron to the FTO. Further its higher band gap also reduces the recombination processes. On the other hand, the main problem is that higher band gap confines its absorption to the UV region only [14], [15], [16]. To overcome this issue the band gap tuning of anatase TiO2 utilizing various transition metals is always an influential approach. This extends the absorption spectrum to the visible region. Different studies have been reported on deformity creation and its impact on electrical, optical properties and performance of TiO2 in DSSCs. Doping is the process in which small amounts of external components are incorporated at a controlled rate into a semiconductor to change its electronic properties, usually by creating small bands in between valance band and anti-bonding molecular orbital [17], [18], [19].
Therefore, the band gap changes enables the device to use photons of different wavelengths for the excitation of electrons. For doping of TiO2, several transitions and non-transition metals have been widely studied in literature such Cr, Nd, Au, Ag, Ni, Fe, Co, La, Pt, Cu, and Ru [20], [21]. The modification of TiO2 by doping has improved the visible light driven properties by the incorporation of metals [22], [23], [24]. Neodymium is one of the lanthanide ions that can be applied to increase the photo activity of TiO2,a series of Nd–TiO2 nanotubes by sol–gel technique and hydrothermal treatment have been developed[25], [26]{Bokare, 2013 #2434;Hewer, 2011 #2435}. Moreover, Silver ions are particularly attractive as a dopant in dye sensitized solar cell applications owing to their excellent electron transport properties, ease of synthesis, and low cost. Ag is used to change the surface morphology of anatase TiO2 and to create defects in the Ti lattice, such as oxygen vacancies and surface defects that allow for effective charge separation [27], [28]. Different techniques have been utilized for TiO2 to tune its absorption toward visible light [29]. Ag incorporation in TiO2 distinctly shifts its absorption band edge towards the visible region and also reduces the resistance against charge transfer[30]. It has been investigated that integration of ZnO with TiO2 utilizes the high reactivity, capability of TiO2 and the high mobility of electrons in ZnO, which enhances the electron-hole transfer process between the conduction and valence bands[31]. Recently various experimental works have been focused on the synthesis of TiO2 metal oxide nanocomposite through efficient charge separation such as ZnO/Al2O3, Al2O3/TiO2, TiO2/ZnO, ZnO/SnO2 and MgO/TiO2. High stability and improved photovoltaic properties have been obtained because of the special modification in electronic structure of TiO2 [32], [33], [34], [35], [36], [37].
This work focuses on solid state DSSCs fabrication by changing the dopant (silver, neodymium) concentration in TiO2 through the sol–gel technique. We also report the incorporation of ZnO in TiO2 i.e. bare and silver or neodymium doped TiO2/ZnO nanocomposites. The tuning of band gap and bathochromic effect in absorption spectrum due to Ag and Nd in TiO2 NPs as well as synthesis of nanocomposites showed better conversion efficiency in DSSCs. This was done to compare the power conversion efficiencies of pyrocatechol violet, carminic acid and dithizone dyes (photosensitizers) functionalized silver and neodymium doped TiO2/ZnO nanostructured under the same experimental conditions. The photovoltaic response of these heterojunctions (Ag and Nd doped TiO2/ZnO) showed better photovoltaics efficiency as compared to bare TiO2. Additionally, the dopants and ZnO layer reduce the charge carrier recombination and also enhances the fast free electron transport.
Section snippets
Materials and method
All of the chemicals reagents were used in analytical grade. For the preparation of solutions of these chemicals deionized and doubly distilled water was used. C12H28O4Ti (Titanium tetraisoproxide), Nd (NO3)3·6H2O (Neodymium (III) nitrate hexahydrate), AgNO3 (Silver nitrate), ZnC4H6O4·2H2O (Zinc acetate dehydrate), 2-Propanol, Sodium hydroxide were purchased from sigms aldrich and used without further purification.
UV/VIS studies
Fig. 4a represents the absorption spectra of bare TiO2 and silver doped TiO2 along with their respective nanocomposites by coupling with ZnO. The appearance of a sharp band with a steep edge was observed at 300 nm for TiO2 that corresponds to the characteristic peak of wide gap semiconductor. The shift in optical edge to higher wavelength was observed for 2%, 3% and 4% doping of silver. While a further bathochromic shift was found to appear (Fig. 5a) in case of the TiO2/ZnO and Ag (2%, 3%, 4%)
Conclusions
TiO2 is a wide band gap semiconductor, used as photoanode in DSSCs, with the highest photo conversion efficiency till date. However, its large band gap and electron-hole recombination decreases its durability and versatile use, therefore the metal ion doping, and composites synthesis was employed to improve the photovoltaic performance of TiO2. In this study, TiO2 was doped with two metals (i.e. silver and neodymium) by using a sol gel method and later on the respective nanocomposite were
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.
Acknowledgment
We are highly thankful to Higher education commission of Pakistan under project No-6169/Federal//NRPU/R&D/HEC/16 for financial support and Quaid-i-Azam University, Islamabad, for providing laboratory and other facilities.
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2023, Spectrochimica Acta - Part A: Molecular and Biomolecular SpectroscopyCitation Excerpt :Consequently, various efforts have so far been done to enhance the DSSCs performances [22,23]. Among different methods, modifying TiO2 photoanodes using several semiconductor nanomaterials (e.g. metal oxides) can effectively boost efficiencies [24,25]. In fact, such modifications enhance light harvesting potential and charge transfer but diminish recombination of charges [4,26].