Ever growing global energy requirement and depleting level of fossil fuels have accelerated the demand for efficient power generation from solar photovoltaic (PV) cells in recent years(Bach 1998; Meadows et al. 1972; Hosenuzzaman et al. 2015). The environmental impact of the use of fossil fuels is another major concern (Barbir et al. 1990). The current production of photovoltaic (PV) modules is dominated by crystalline silicon modules based on bulk wafers. However, the use of toxic materials and the high production cost of these solar cells have motivated the researchers to find new kinds of less expensive and non silicon-based solar cells to harvest solar energy efficiently (Goetzberger et al. 2003; Alharbi et al. 2011; Lee and Ebong 2017; Yamamoto et al. 2001).
Dye-sensitized solar cells (DSSCs) are a non-conventional photovoltaic technology that has attracted significant attention because of their high conversion efficiencies and low cost. O’Regan. B. & Grätzel reported high efficiency cells using nanoporous titanium dioxide (TiO2) semiconductor electrodes, ruthenium (Ru) metal complex dyes, and iodine electrolyte solutions in the journal of Nature in 1991 (O’Regan and Grätzel 1991). Since then, many studies have been actively carried out on DSSCs and revealed their performance comparable to amorphous silicon thin films (Chiba et al. 2006; Grätzel 2005). These DSSCs have the advantages of low cost, lightweight and easy fabrication, but issues include durability and further improvement of their properties. To respond to these issues, many attempts have been made, such as solidifying electrolytes and improving materials and structures, but there have been no great breakthroughs yet (Chung et al. 2012; Cai et al. 2011).
A dye-sensitized solar cell consists of two conducting glass electrodes in a sandwich arrangement. Each layer has a specific role in the cell. The transparent glass electrodes allow the light to pass through the cell. The titanium dioxide serves as a holding place for the dye and participates in electron transfer. The dye molecules collect light and produce excited electrons which cause a current in the cell. The iodide electrolyte layer acts as a source for electron replacement. The bottom conductive layer is coated with platinum which plays the role of the counter electrode. A schematic structure of a liquid electrolyte DSSC and its working principle is shown in Fig. 1. When light passes through the conductive glass electrode, the dye molecules absorb the photons and the electrons in the dye go from the ground state (HOMO) to an empty excited state (LUMO). This is referred to as photoexcitation. The excited electrons jump to the conduction band of the semiconducting dioxide and diffuse across this layer reaching the conductive electrode. Then they travel through the outer circuit and reach the counter electrode. The dye molecules become oxidized after losing an electron to the semiconductor oxide material. The red-ox iodide electrolyte donates electrons to the oxidized dye molecules thereby regenerating them. When the originally lost electron reaches the counter electrode, it gives the electron back to the electrolyte (O’Regan and Grätzel 1991; Grätzel 2003).
The photovoltaic performance of a DSSC highly depends on all of its components and the fabrication methodology. Therefore, the optimization of every component is extremely crucial to achieve the best performance. Since its introduction into the science community in 1991, the nanocrystalline photoanode in dye-sensitized solar cells have predominantly been comprised of titanium (TiO2) nanoparticles as the semiconducting material (O’Regan and Grätzel 1991; Grätzel 2003; Shao et al. 2011). Many researchers became very interested in studying the dye-sensitized solar cell performance fabricated using alternative semiconducting nanomaterials (Tiwana et al. 2011; Biswas and Chatterjee 2020). Specifically, Zinc Oxide (ZnO) has been an ideal alternative to TiO2 because of having a similar conduction band edge that is appropriate for proper electron injection from the excited dyes; moreover, ZnO provides better electron transport due to its higher electronic mobility. Along with that, ZnO is also highly transparent, which allows greater light penetration (Zhang et al. 2009; Guillén et al. 2011; Quintana et al. 2007; Vittal and Ho 2017; Biswas et al. 2019).
In this study, ZnO nanoparticles were implemented to fabricate the photoanode of the DSSCs and rose bengal dye was utilized as a sensitizer. To obtain better efficiency, the dye molecules must bind tightly to the mesoporous ZnO photoanode surface with the assistance of their anchoring group to ensure proficient electron injection from the LUMO of dye molecule to the conduction band (CB) of ZnO.
To make the DSSCs cost-effective, it is very essential to identify efficient and inexpensive dyes in place of costly ruthenium complexes. In that aspect, rose bengal dye has emerged as a promising alternative candidate. However, dye aggregation on the ZnO surface affects the photoelectron injection and hence limits the overall device performance (Zhang and Cole 2017). The use of additives such as CDCA is a very useful and widely used strategy in lowering the self-aggregation of dye molecules by suppressing unfavourable dye-dye interactions and thereby enhances the photoconversion efficiency (Buene et al. 2020; Kumar et al. 2020; Ismail et al. 2018). However, the strong binding of CDCA molecules to the ZnO surface partially displaces dye molecules and consequently reduces photon harvesting. Therefore, to maximize the positive effect of the co-adsorbent, it is very crucial to carefully optimize the amount of CDCA (Li et al. 2011). Herein, we report the effect of CDCA as co-adsorbent in the performance of Rose Bengal (RB) dye based DSSCs. Different concentrations of CDCA were studied to identify the optimum one for achieving the best device performance.
On the other hand, the mesoporous nature of the ZnO film is very essential to tender high surface area offering more dye loading and thereby generating more photoelectrons. However, small pores present in the nanocrystalline ZnO layer of the photoanode may provide a path for the direct contact between the liquid electrolyte and the FTO substrate. This may allow the electrons of FTO to recombine with the I− 3 ion present in the electrolyte resulting in high recombination current and hence decreased cell performance (Yang et al. 2014; Yeoh and Chan 2019). Therefore, to inhibit the electron back transfer, a promising approach is to modify the FTO/electrolyte interface by adding a compact metal oxide blocking layer. A thin blocking layer (BL) of ZnO was deposited by a facile and cost-effective sol-gel spin coating process before depositing the mesoporous active ZnO layer. In this work, we reported the fabrication and characterization of DSSCs based on ZnO nanoparticle and Rose Bengal dye. The effect of CDCA concentration and the compact ZnO blocking layer in boosting the photovoltaic performance of the device was investigated in terms of photocurrent-voltage (J-V) characteristics and dark current measurement. In addition to that, electrochemical impedance spectroscopy (EIS) analysis was employed to investigate the charge transfer kinetics and electron back reaction of the fabricated cells.