Hybrid photoanode of TiO₂-ZnO synthesized by co-precipitation route for dye-sensitized solar cell using phyllanthus reticulatas pigment sensitizer

Highlights

• New hybrid photoanode made up of co-precipitate of TiO₂/ZnO was synthesized.

• The TiO₂/ZnO co-precipitates are nanocrystalline and nanoporous structured.

• The TiO₂/ZnO co-precipitates exhibit meritorious properties of both TiO₂ and ZnO.

• The ZnO proportions vary the bandgap energy of TiO₂/ZnO co-precipitates.

• DSSC was fabricated using TiO₂/ZnO and sensitizer from Phyllanthus reticulatas.

Abstract

We report synthesis and characterization of hybrid photoanode made up of Titanium Dioxide (TiO₂) and Zinc Oxide (ZnO) nanoparticles prepared by a co-precipitation process. Though TiO₂ is the most preferred anode for Dye Sensitized Solar Cell (DSSC), the charge recombination between electrode and electrolyte poses a major challenge. ZnO photoelectrode is a potential alternative due to higher carrier mobility but has a slower electron injection than TiO₂. To maximize the benefits of both, a co-precipitation synthesis of TiO₂/ZnO is proposed in this work as a promising alternative. The co-precipitation process produces metal oxides TiO₂ and ZnO and eventually yields the Anatase TiO₂ and Wurtzite ZnO. The experimental investigations of the new co-precipitation process show improved control of particle size, shape, and uniform distribution of ZnO and yet preserve the economy and simplicity of synthesis. The study shows an average crystallite size of 10 nm to 22 nm for TiO₂ and 17 nm to 35 nm for ZnO. Wurtzite phase varies from 4.5% to 10.8% in a co-precipitated photoanode. The deconvolution of the typical Photoluminescence (PL) curve shows two distinct bands for ZnO and TiO₂ cantered around 390 nm and 460 nm. The study shows that the co-precipitated TiO₂/ZnO has a bandgap of less than 3.2 eV, this paves the way for reduced recombination and improved device performance. Further, we fabricated a prototype of a DSSC using photoanode of TiO₂/ZnO and a sensitizer of natural plant pigment extracted from Phyllanthus reticulatas and successfully tested the prototype DSSC.

Introduction

Solar energy has emerged as one of the promising alternates to fossil fuel, among which solar photovoltaics (PV) has gained a lot of research attention over the years. Commercial vehicles may go green in India by 2030 (Vidhi and Shrivastava, 2018) increasing demand for electricity by multiple folds and can be practical only if generation is by alternate energy resources. Silicon wafer (Si-wafer) solar cells form 94% of the total production of the PV industry (Fraunhofer Institute for Solar Energy Systems, 2017). However, new generations of solar cells are being widely explored to reduce the cost and improve the efficiency of the existing solar cells (O’Regan and Grätzel, 1991, Park et al., 2016, Zhao et al., 2016a, Zhao et al., 2016b). Dye-Sensitized Solar Cells (DSSC) are gaining momentum owing to its advantages such as two separate materials for charge transport and charge separation, dye absorption by nanomaterial (O’Regan and Grätzel, 1991), the option of multicolour cells and low-cost production with better performance in diffused light (Hug et al., 2014). Moreover, Building Integrated photovoltaics (BIPV) may contribute towards the increased market share of DSSC in the future. DSSC with natural dye sensitizer makes it greener (Hernández-Martínez et al., 2013, Bhogaita et al., 2016a, Bhogaita et al., 2016b), however, the stability and efficiency of the cell remain a crucial challenge due to liquid electrolyte and natural dye. A recent review on DSSC reports the viability for application in internet of things (IoTs) (Aslam et al., 2020) putting DSSC in the spot light.

DSSC is a photoelectrochemical thin-film solar cell also called Grätzel cell as it was introduced by Michael Grätzel and Brian O’ Regan (O’Regan and Grätzel, 1991) that mimics the process of photosynthesis in plants. DSSC consists of photoelectrode, a counter electrode, sensitizer and redox electrolyte. Semiconductors like TiO₂ (O’Regan and Grätzel, 1991), ZnO (Memarian et al., 2011), SnO₂ (Green et al., 2005), Nb₂O₅ (Lira-Cantu and Krebs, 2006), CeO₂ (Turković and Orel, 1997), WO₃ (Zheng et al., 2010), In₂O₃ (Sharma et al., 2009) etc. have been explored for photoelectrodes in DSSC. A lot of synthetic sensitizers (Nazeeruddin et al., 2005, Sánchez Carballo et al., 2014, Nazeeruddin et al., 1993) have been investigated among which Ru-complex is the most widely explored inorganic sensitizers. However, a wide variety of natural plant pigments have greatly attracted researchers as alternate sensitizers due to its economic viability and bio-degradability. Sensitizer helps in photon absorption and the photoelectrode aids in charge separation.TiO₂ is the commonly used photoelectrode material due to its advantages such as non-toxicity, economics, stability under irradiation, chemical stability in the electrolyte, low-temperature synthesis and variety of nanostructured morphologies (Hagfeldt et al., 2010, Syrrokostas et al., 2014, Dervaux et al., 2015). However, TiO₂ cannot absorb in the visible range as it is a wide bandgap semiconductor (Chen et al., 2015). ZnO demonstrates higher carrier mobility than TiO₂ but its instability in an acidic environment and dye aggregate formation (Minoura and Yoshida, 2008, Horiuchi et al., 2003) poses as major drawbacks. Also, electron injection is slower in ZnO as compared to TiO₂ and hence yields poor photocurrent (Horiuchi et al., 2003, Jan et al., 2015).

Coaxial TiO₂/ZnO nanotube arrays demonstrated improved charge separation, reduced recombination and 40% higher efficiency as compared to only the TiO₂ electrode (Xie et al., 2011). ZnO coated TiO₂ nanoparticles showed enhancement in efficiency from 0.7% to 1.21% due to reduced recombination between electrolyte and electrode facilitated by the presence of ZnO (Kim et al., 2005). In a study of TiO₂, ZnO and dual-layer of TiO₂/ZnO, the bilayer had reduced recombination and thus higher efficiency as compared to individual semiconductors (Rani and Tripathi, 2013). Though few reports are available as mentioned above on combining TiO₂ and ZnO, no reports on co-precipitated TiO₂ / ZnO are available in combination with natural plant to the knowledge of the authors.

To harness the merits of the two most promising photoelectrodes used in DSSC, this work presents the preliminary investigation and novel attempt of combining TiO₂ and ZnO using the co-precipitation technique. Meticulous studies have been carried out by varying the composition of the semiconductors to obtain deeper insight into the material feasibility for the DSSC application.

“Natural versus synthetic sensitizers for DSSC”: In recent literatures, it is reported that the DSSC based on photoanodes of TiO₂ and ZnO using synthetic dye as sensitizer have efficiency of 5.31% (Song et al., 2014), 5.92% (Zhao et al., 2016a, Zhao et al., 2016b, Zhao et al., 2016c), 8.2 ± 0.2% (Matos et al., 2019) , 9.16 ± 0.58% (Nien et al., 2020) significantly higher than natural sensitizer. Ru complexes with TiO₂ photoanode has recorded 11.0% efficiency (Gao et al., 2008). However, Ru is the sixth rarest metal found on earth (Ablialimov et al., 2014) and hence its expensive pricing is a biggest challenge on scalability to meet growing energy demand. Extraction process of Ru from its ore and purification process involve series of complex steps, followed by synthesis of Ru complexes to suit for DSSC application, can be considered as a second drawback. Comparatively natural plant pigments are abundant and usually extraction process is less complex and it can be used directly after extraction. Application of natural plant pigment as a sensitizer indicates feasibility of green energy solution to the real world energy needs.

“Recent progress on natural sensitizer based DSSC”: Natural plant pigments like carotenoid, betalain, anthocyanin and chlorophyll extracted from plants have been widely studied (Calogero et al., 2018, Obi et al., 2020, Patni et al., 2020, Dhafina et al., 2020). Recently natural pigment extracted from bacteria has also been reported (Silva et al, 2019) opening the novel avenues of research towards natural sensitizers. Similarly natural plant sensitizer from microalgae has been reported as a novel approach (Orona-Navar et al., 2020). Reviews on DSSC based on natural plant pigments indicate focus and development towards application of environment friendly materials (Iqbal et al., 2019). Several TiO₂ based DSSC with natural sensitizers have been reported recently (Chandra Maurya et al., 2019, Arulraj et al., 2019, Omar et al., 2020, Carvalho et al., 2020, Yildiz et al., 2019). Although TiO₂ has been widely explored, we also find ZnO based DSSC with natural sensitizers reported recently (Dinesh et al., 2019, Saeidi et al., 2019, Adedokun et al., 2019, Shiyani et al., 2020). TiO₂ and carboxylic functions interaction facilitates desirable dye anchoring due to formation of quasi-covalent linkage of the carbonyl group of the dye with the TiO₂ surface (Tabacchi et al., 2019).

Investigation of environment friendly materials for DSSC do not end with natural sensitizers alone but also extends to green synthesis of photoanodes like TiO₂ (Maurya et al., 2019) and ZnO (Sharmila et al., 2019).

The objective of this experimental study is to synthesize novel nano metal oxide semiconductor for natural sensitizer based DSSC prototype harnessing advantages of two widely explored oxides TiO₂ and ZnO. Room temperature Non-Hydrolytic Sol-Gel (NHSG) synthesis route has been followed to co-precipitate these two oxides TiO₂ and ZnO. NHSG is a known bottom-up technique following liquid phase sol–gel synthesis. As compared to hydrolytic sol–gel synthesis, NHSG has a water-free reaction, a slower rate of reaction and solvent acts as an oxygen donor making it possible to achieve high pores volume in a single step. NHSG also provides good control over morphology and structure while the phenomenon of pore collapse is not observed justifying its selection for our experiments. Apart from numerous parameters like pH, precursor, solvent, annealing, etc., the role of surfactant being significant, a preliminary study to optimize the ratio of surfactant to precursor is carried out. The focus remains towards the identification of optimized structural and optical properties of co-precipitates by various characterization techniques like X-Ray Diffraction (XRD), UV–visible spectroscopy, PL Spectroscopy and Scanning Electron Microscopy (SEM) and High Resolution Transmission Electron Microscopy (HRTEM).