Improved photovoltaic performance in nano TiO2 based dye sensitized solar cells: Effect of TiCl4 treatment and Sr doping
Graphical abstract
Introduction
Titanium dioxide (TiO2) is an efficient material for photo-electrochemical solar energy conversion [1], photo-catalysis, Li-ion batteries [2], etc. It is widely used as gas sensors, antireflective coatings, photocatalysts and optical filters [3] and as dielectric material in electronic devices [4]. To date, Grätzel et al. [5] and Wu et al. [6] reported the highest power conversion efficiency of up to 12.3% for TiO2 photoanode by using cobalt electrolyte and porphyrin sensitizer. It is known that mesoporous nanocrystalline TiO2 film based DSSCs are gaining significant attention due to their low cost, ease of fabrication, flexibility of design, relatively high PCE, safety, and pollution-free and environmental friendly nature [7], [8], [9]. Till now, TiO2 is the utmost successful wide band gap semiconductor material for energy application compared with other semiconductor oxide materials such as SnO2, ZnO, WO3 and SrTiO3, owing to its high refractive behavior, non-toxicity and stability [9], [10].
It becomes necessary to attune the electronic properties of TiO2 photoanodes for the effective utilization of the solar energy (visible region) i.e., for the effective light absorption and charge collection. This can be achieved by incorporating impurities into the TiO2 matrix or by doping. In spite of the numerous investigations performed on the magnetic, electrical and solar cell activities of TiO2 nanomaterials, many controversial reports are there, particularly in the doping concentration for the better performance of the solar cells. Usually, TiO2 photoanodes are doped with metallic and non-metallic ions, but there are also few works, in which doping is carried out with rare earth and transition metal ions [11], [12]. However, work on TiO2 nanomaterials doped with alkaline earth metal ions for photovoltaic applications is rarely reported [9], [13], [14], [15], [16]. Mehnane et al. investigated the DSSC performance of TiO2 nanoparticles with Sr doping and achieved an efficiency of 7.88% [9]. Peng et al. reported better open circuit voltage by Mg2+ ions substitution into the anatase TiO2 photoanodes [13]. Tang and Yin have achieved an efficiency of 4.7% for Sr-doped TiO2/SrTiO3 nanorod array heterostructure photoanode [14]. Liu et al. observed improved electron transport and hence boosted efficiency in thick layered Ca-doped TiO2 photoanodes [15]. Recently, Ganesh et al. studied Zn/Sr co-doped mesoporous TiO2 photoanode for DSSC and observed reduced recombination with a PCE of 4.6% [16]. In 2013, Mangrola et al. have reported the dielectric behaviour of bulk Sr-doped TiO2 with shortage of Sr2+ ion effect into TiO2 [17]. Reports on dielectric behaviour of Sr-doped TiO2 is sparse in the literature. Nevertheless, researchers have not studied the Sr2+ ion doping into the TiO2 matrix for electrical properties and photovoltaic performance, concurrently. Band gap tuning in TiO2 by doping, that allows to increase the optical absorption properties, which is improve the response of photocatalysis and solar cells. Ferroelectric thin films having thickness less than 300 nm exhibit strong absorption, which can produce more compact photovoltaic devices [18].
The overall light harvesting efficiency of DSSC depends on the geometry of the photoanode (which in turn governs the amount of dye molecules that reach the photoanode) as well as on the scattering of incident light [19]. Hence, introducing an optical scattering layer can increase the light harvesting capability of photoanodes, thus increasing the PCE [20], [21]. Recently, a variety of nanostructures have been used as the scattering layer in different photoanodes with various morphologies. Zhu et al. used nanostructured TiO2 as scattering layer on bare TiO2 photoanode and achieved an efficiency of 7.1% [22]. Yu et al. observed 8.6% efficiency in TiO2 photoanode with ~ 400 nm sized cubic CeO2 nanoparticles as the scattering layer [23].
Further, the soaking of nano-porous TiO2 films into an aqueous TiCl4 solution and then annealing the films in air, the solar-to-electrical conversion efficiency of DSSCs has been improved [24], [25]. On the surface of the TiO2 films, the TiCl4 solution species get converted into TiO2 crystallites by the effect of annealing and these TiO2 crystallites resulted from the TiCl4 treatment are stated for the increase the short circuit current and decrease the dark current [26], [27]. Few earlier works reported, the improvement of DSSC device performance is complicated by the effect of TiCl4 treatment has on the TiO2 conduction band edge position, charge injection, transport, and recombination [25], [27], [28], [29], [30]. As well, It has been shown that traps, which restrict the electron transport and increase the recombination are located predominately on the TiO2 surface [28], [31]. However, due to the TiCl4 treatment of an additional TiO2 layer creation on the surface of the TiO2 film has influenced an electron transport and recombination kinetics. In our previous works, we examined the TiCl4 treatment on the BaSnO3 photoanodes exhibits improved efficiencies by increased photocurrent generation [32], [33], [34]. Similarly, Lee et al. [35] reported the improved efficiency of 4.71% by the effect of TiCl4 treatment (15 mM concentration) on the dye-sensitized TiO2 film. Hence, TiCl4 treatment on the TiO2 surface has enhanced the solar cell performance, by the high photocurrent density and charge-collection behaviour. In this work, the electrical properties of Ti1-xSrxO2 nanoparticles are used to explain the charge separation in DSSC performance. Also, the DSSCs performance are investigated for the effect of Sr doping, TiCl4 treatment and TiO2 scattering layer in TiO2 photoanodes.
Section snippets
Materials and synthesis of Ti1-xSrxO2 nanoparticles
Titanium (IV) isopropoxide (Ti[OCH(CH3)2]4–99%) was purchased from Himedia. Strontium chloride hexahydrate (SrCl2·6H2O-99%), and isopropyl alcohol were purchased from Merck. Unless otherwise stated, all of the chemicals were obtained commercially and used directly without purification. Undoped and Sr doped TiO2 nanoparticles were prepared by chemical hydrolysis method and reported our previous work [36].
Preparation of Ti1-xSrxO2 film
Ti1-xSrxO2 nanoparticle films were prepared on FTO glass substrates from a homemade paste
Structural characterization
Fig. 1a shows the XRD patterns of undoped and Sr doped TiO2 nanoparticles. The patterns show diffraction peaks corresponding to the (101), (0 0 4), (2 0 0), (2 1 3), and (2 1 5) planes of tetragonal anatase crystalline phase of TiO2 (JCPDS card No.: 89–4921) along with the (2 1 1), (3 0 1) planes of rutile (JCPDS card No.: 89–4920) phase. No other characteristic diffraction peaks related to metallic Ti, Sr, SrO or any other impurities are observed. The detailed discussion about this XRD spectra are
Conclusions
In summary, we examined the photovoltaic performance of dye-sensitized solar cells based on undoped and Sr-doped TiO2 photoanodes with TiO2 scattering layer and TiCl4 treatment. The efficiency initially started from 2.74% for bare undoped TiO2 photoanode and reached a higher efficiency of 9.6% after Sr doping and TiCl4 treatment. The increased efficiency is due to several factors. Sr-doped TiO2 sample shows fast charge separation, which is confirmed by dielectric, EIS and IPCE measurements.
CRediT author statement
Nagalingam Rajamanickam: Conceptualization, Funding acquisition, Writing - original draft, Data curation, Formal analysis, Investigation, Methodology, Resources, Visualization. Kathirvel Ramachandran: Conceptualization, Funding acquisition, Writing - original draft, Project administration, Supervision, Validation, Writing - review & editing.
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.
Acknowledgement
The author NR acknowledges the SERB (DST) – National Postdoctoral Fellowship (N’PDF) (PDF/2015/000689) and KR acknowledges UGC, New Delhi, for the financial support given in the form of Emeritus fellowship.
References (56)
- et al.
Mesoporous TiO2 nano networks: Anode for high power lithium battery applications
Electrochem. Commun.
(2009) - et al.
Enhanced conversion efficiency of dye-sensitized titanium dioxide solar cells by Ca-doping
J. Alloy. Compd.
(2013) - et al.
Zn and Sr co-doped TiO2 mesoporous nanospheres as photoanodes in dye sensitized solar cell
Mater. Chem. Phys.
(2019) - et al.
Enhanced power conversion efficiency of dye-sensitized solar cells with multifunctional photoanodes based on a three-dimensional TiO2 nanohelix array
Sol. Energy Mater. Sol. Cells
(2015) - et al.
Boosting photo charge carrier transport properties of perovskite BaSnO3 photoanodes by Sr doping for enhanced DSSCs performance
Electrochim. Acta
(2019) - et al.
Influence of Sr doping on structural, optical and magnetic properties of TiO2 nanoparticles
Mater. Lett.
(2015) - et al.
Size effects in the Raman spectra of TiO2 nanoparticles
Vib. Spectrosc.
(2005) - et al.
Synthesis, characterization, and visible light activity of new nanoparticle photocatalysts based on silver, carbon, and sulfur-doped TiO2
J. Colloid Interface Sci.
(2007) - et al.
Enhanced performance of dye-sensitized solar cells aided by Sr, Cr co-doped TiO2 xerogel films made of uniform spheres
J. Colloid Interface Sci.
(2015) - et al.
Efficient photocatalytic degradation of brilliant green using Sr-doped TiO2 nanoparticles
Ceram. Int.
(2015)