Improving energy harvesting efficiency of dye sensitized solar cell by using cobalt-rGO co-doped TiO₂ photoanode

Highlights

• Cobalt doped anatase photoanode for DSSC.

• Optimized conversion efficiency.

• Enhancement in charge transfer properties.

• Synergetic role of rGO and transition metal.

Abstract

In dye-sensitized solar cells (DSSCs), TiO₂ has long been a popular electron transport material for carrying photo-generated electrons from the dye to the outer circuit. However, increasing the electrical conductivity of TiO₂ while preserving its properties is one of the most significant challenges for increasing DSSC efficiency. Here, cobalt-reduced graphene oxide co-doped TiO₂ nanoparticles were synthesized by a modified sol-gel procfess that together controlled the particle size as well as narrowed bandgap of nanoparticles as determined by XRD and UV-Vis absorption measurements. The SEM and TEM were used to perform in-depth morphological characterization of the newly synthesized nanocomposites while J-V (current-voltage) curves, EIS Nyquist curves, and incident photon to current conversion efficiency (IPCE) spectra were used to examine the photovoltaic parameters. Enhanced short circuit current density (J_sc 12.83 mA cm⁻²), open-circuit voltage (V_oc 0.618 V) and overall power conversion efficiency (PCE, η = 5.24%) of DSSC was obtained with Co/rGO co-doped TiO₂ based photoanode in comparison to bare TiO₂ (η = 3.71%), cobalt doped TiO₂ (η = 4.09%) and rGO doped TiO₂ (η = 4.43%) based DSSCs. Huge improvement in efficiency, 41% higher PEC in Co/rGO co-doped TiO₂, was attributed to better utilization of visible radiations, greater dye adsorption and enhancement in charge transfer properties by suppressing the electron transport resistances. The incorporation of rGO improved electron transfer, which compensated for recombination losses, thereby increasing the DSSC's J_sc. The synergetic role of rGO and transition metal helped in keeping the structure intact in the nano-assembly that enhance the photo-generated carriers.

Introduction

The enhanced energy demand coupled with the depletion of fossil fuels, pollution and global warming, has got much attention from scientists to offer more acceptable solutions in the form of next-generation green energy devices. Among the photovoltaic devices, dye-sensitized solar cells (DSSCs) have got tremendous interest due to their excellent efficiency, simple fabrication method, low production cost, environmentally friendly and working under diffused sunlight [1], [2], [3], [4], [5]. Principally a DSSC is composed of three components that include a semiconductor photoanode (TiO₂, ZnO, WO₃, SnO₂, Nb₂O₅) deposited onto a fluorine-doped tin oxide (FTO) conductive glass, a sensitizer (Ru complex or organic dye) adsorbed on the semiconductor surface, I^-/I₃^- redox electrolyte and a platinum counter electrode [6], [7]. The photoanode is mainly responsible for the photocurrent and efficiency of the device as it transports photo-induced electrons from the excited dye molecules adsorbed on its surface. Owing to its excellent photostability, availability, less toxicity and excellent device efficiency, anatase titanium dioxide (TiO₂) is the most frequently used material for photoanode in DSSC. TiO₂ holds a wide bandgap (3.2 eV) so it mostly absorbs UV radiations which constitute only 5% of solar flux and render its large scale utilization, moreover, the adsorbed sensitizer cannot easily transfer electrons from excited dye molecules due to insufficient driving force. Therefore tuning the properties of TiO₂ photoanode can greatly help to improve the efficiency of DSSC devices [8], [9].

Since 1991, when mesoporous TiO₂ has been introduced as a photoanode for the DSSC, extensive research efforts have been carried out and energy conversion efficiency greater than 14.3% have been obtained but scientists are still unsatisfied with this efficiency. Currently, many methodologies have been adopted to enhance the power efficiency including the surface treatment with TiCl₄ [10], [11], coupling semiconductor photoanode with other metal oxides (ZnO, Fe₂O₃, Al₂O₃, Nb₂O₅) [12], [13], [14], doping TiO₂ with transition metal ions [Nb, Cr, Co, Ni, Ag, W, Sn, Fe, Zr etc] and coupling with non-metals like N, F and carbon [7], [15]. Fuzhi et al. synthesized Zn doped TiO₂ using agarose gel as a template and got higher photocurrent in DSSC than bare TiO₂ based DSSC [16]. Similarly, non-transition metals like Mg, Sr, Bi and rare-earth-doped TiO₂ photoanodes have been applied for DSSC efficiency improvement [7], [17], [18], [19]. Metal ions insertion into TiO₂ lattice had been recognized to modify the bandgap and shift absorption edge towards visible region thus accelerating photocurrent and lowering electron carrier recombination in DSSC [7].

The recent decade has led to a huge interest in applying carbonaceous materials like carbon nanotubes (CNTs) and graphene into TiO₂ photoanode and obtained superior device efficiency [20]. Graphene, a two dimensional (2D) material with carbon atoms packed in honeycomb fashion has got huge attention since its discovery in 2004 due to its high surface area, excellent thermal conductivity, superior electrical and mechanical properties. Owing to its unique properties graphene has extensively been used in recent years in sensors, capacitors, batteries, solar cells, inorganic and polymeric materials. In DSSCs, graphene films have been used as a transparent electrode as well as a counter electrode in conjunction with the commonly employed platinum counter electrode, due to its high transparency, raised electronic mobility and excellent electrochemical properties. The reduced graphene oxide (rGO) has also been applied in the semiconductor photoanodes and fairly high charge transport and overall DSSC efficiency have been reported [21], [22]. Chen et al. prepared graphene-TiO₂ nanocomposites by simultaneous reduction hydrolysis method and fabricated photoanode that showed considerably higher efficiency (η = 7.1%) which was attributed to the improvement in short-circuit current density (J_sc) [23]. Liu et al. successfully prepared few-layer graphene/TiO₂ nanocomposites by an in-situ method using C₂₈H₁₆Br₂ as a carbon precursor. The synthesized composites were fabricated for DSSC as a photoanode and yielded 8.25% efficiency which was 65% higher than pure TiO₂ nanoparticles-based DSSC (η = 5.01%) [24].

Doping of TiO₂ with a single element often leads to issues like lowering of thermal stability and incident photon to current conversion efficiency (IPCE). While doping with only non-metals may cause the creation of trap states between valence and conduction band of TiO₂ which accelerates recombination rate thus lowering the DSSC performance [25], [26]. So, a combination of co-doping of metals, non-metals and composites has also been explored [27], [28], [29]. Park et al. prepared Zr/N doped TiO₂ and Cu/N doped TiO₂ nanoparticles for photoanode and harvested fairly high DSSC power conversion efficiency (PCE) of 12.62% and 11.35% respectively [11], [30]. Wang et al. reported nitrogen and yttrium co-doped TiO₂ nanoparticles for DSSC applications and obtained 18% enhancement in device performance as compared to the undoped TiO₂ based DSSC [29]. Yu et al. demonstrated 37% improvement in power conversion efficiency from Ho³⁺/ Yb³⁺/ F^- tri-doped TiO₂ nanoparticle-based DSSC [31]. Cu-S co-doped TiO₂ nanoparticles employed as a photoanode in DSSC exhibited superior efficiency (η = 10.44%) in comparison to undoped TiO₂ (η = 6.37%), which was credited to its higher surface area, greater dye adsorption and enhanced J_sc [32].

Keeping in mind these facts the current approach has been devoted to the synthesis of Co/rGO co-doped TiO₂ nanoparticles and the fabrication of these electrodes as photoanodes for DSSC applications. To the best of our knowledge, this novel combination of hetero doping has not been utilized as a photoanode in DSSC so far. The photovoltaic parameters of the fabricated DSSCs measured by J-V characteristics, IPCE and electrochemical impedance spectroscopy (EIS) analysis proved it a better candidate as a working electrode than the bare TiO₂, Co-TiO₂ and rGO -TiO₂-based photoanode systems.