Fabrication and Photovoltaic Characteristics of Alizarin Dye Based Dsscs

Introduction

The energy consumption of the world increasing day by day due to enhanced in living standard of mankind’s. The energy available from sun is 10000 times greater than the current demand of world’s energy [1]. The energy available in form of solar radiation is nearly 3×1024 J per year [2,3]. The massive needs of energy have enhanced the exhaustion of the fossil fuel and their combustion increases the atmospheric pollution by accumulative the concentration of greenhouse gases [3-5]. Due to these unpreventable worldwide challenges insist of growing more sustainable power sources, i.e. tidal power, solar thermal, hydropower and biomass and solar cell [6]. Solar cells or photovoltaic cells are one of the promising options of renewable energy source because these can produce energy near to end user, avoiding transmission losses and economical. The solar panels work without emissions of greenhouse gas, noise and toxicity which do not affect the environment. The major drawback of Si-based solar panel is high cost of fabrication and materials, which needs auxiliary of this technology [7]. An assortment of structures and materials were being sought after to supplant Si-based and other thin film innovations, in which the dye-sensitized solar cell (DSSC) created by Grätzel keeps on being one of the most effective "next generation" advances having good conversion efficiencies [8,9]. DSSC have the prospective to substitute the Si-based photovoltaic cells due to economical, easy construction and good proficiency [10,11]. The dye sensitizer solar cells consist of following components i.e., photoanode, sensitizer, counter-electrode and electrolyte solution with redox mediator. The photoanode is made-up of nano size metal oxides layer i.e., TiO₂, ZnO, NiO, SrTiO₃, SiO₂, SnO₂, Ga₂O₃, Nb₂O₅ and ZnO having thickness in few micrometers [12-15]. The sensitizer material is the main constituent of dye-sensitized solar cell due to its capability to harvests solar radiation and produce photo-excited electrons at the photoanode interface. The molar extinction coefficient of sensitizer should be high for radiation harvesting in near-infrared and visible region. Sensitizer should have excellent photo-stability, solubility and capability to adjoin with photoanode. With an intention for effective sensitizer many conventional materials, metal-complexes of transition and rare earth metals [16-18] and synthetics dyes [19-23] have been investigated for sensitization of extensive band gap of photoelectrodes. The redox electrolyte iodide–triiodide (I₃⁻/I⁻) has been accepted as most common redox couple due to its excellent rate of electron transfer i.e. rapid oxidation of I^- at interface of photoanode and electrolyte for the regeneration of dye molecule, slow reduction of I₃^- at interface of electrolyte and counter electrode for carrier collection. The redox electrolyte iodide-triiodide (I₃⁻/I⁻) also have easy preparation, economical, high stability and good solubility in various solvents [24]. Another important component of DSSC is counter electrode, it collecting electrons generally made up of platinum and graphite [25]. Platinum have higher electro-catalytic activity as compare to graphite but graphite is more common due to economic cost and excellent stability.

In this research article, the fabrication and investigation of DSSCs using alizarin dye as photosensitizer for the photoelectrochemical, electronic, optical and thermal characteristics. The proficiency of fabricated DSSCs is also studied with various redox electrolytic couples i.e. KI, alkyl quaternary ammonium iodides with I₂ in CH₃CN solvent.

Experimental

Material

The transparent conducting glass purchase from Sigma-Aldrich have layer of FTO with resistance 6-8 Ω/square. For the preparation of photoanode conventional available nano titiania P25 powder (Sigma-Aldrich) have 20% rutile and 80% anatase form of TiO₂ with ~21 nm size was used. Alizarin was used as sensitizers in fabrication of DSSCs. The CH₃CN was used as solvent for redox electrolytic couple; analytic reagent grade potassium iodide (KI), iodine (I₂), tetramethylquaternaryammonium iodide [(CH₃)₄NI], tetraethylquaternaryammonium iodide [(CH₃CH₂)₄NI], tetrapropylquaternaryammonium iodide [(CH₃CH₂CH₂)₄NI] taken from Sigma–Aldrich and used as it is without purification.

Device fabrication

Cleaning and masking of the transparent conducting substrate

The cedepol i.e. neutral soap solution was used to clean the FTO coated transparent conducting substrates of adequate size. The substrates were sonicated for 15 min in neutral soap solution, and then rinsed several times with distilled water. Further reagent grade acetone was used for cleaning of substrate and sonicated for 15 min. The substrates were boiled in propan-2-ol solvent to remove the organic impurities and finally dried in oven at 80ºC. Some portion of the substrates surface was cover to deposit the thin film of nano TiO₂ on desire place. Transparent adhesive elastic tape was utilized to shield the conducting glass in such a manner that a small strip of etched portion was concealed.

Preparation of TiO₂ layer

1 g powder of commercial nano-TiO₂ powder mixed with glacial acetic acid was place in pestle and mortar and further addition of glacial acetic acids in small increments. After grinding with pestle even and swelling free titanium dioxide paste was obtained. The TiO₂ paste was sonicated for 30 min in a small vial. The prepared titanium dioxide paste was spurted onto unmasked conducting glass substrate using doctor blade technique. After evaporation of acetic acid, adhesive tape was removed and then sintered at 300ºC for 2 h.

Instrumentation and characterization

Shimadzu 2450 spectrophotometer with 180-1100 nm range was used for UV–Vis absorption spectra of the alizarin dyes. The STA-7300 thermal analyzer of Hitachi was used to record TGA and DTA. The substance heated with rate 10ºC/min in inert atmosphere of N2 gas. The material was heated in range 35°C to 500°C to obtain TGA and DTA. A digital KEITHLEY 2450 Source meter with KICKSTART software was used to record the current density–voltage (J–V) curves. A 200 w tungsten (W) arc lamp was used for simulation light radiation in DSSCs.

Conclusion

DSSCs were assembled using Alizarin dye as sensitizer and employing four different redox couple electrolytes i.e. (KI), [(CH₃)₄NI], [(CH₃CH₂)₄NI] and [(CH₃CH₂CH₂)₄NI], with I₂ in CH₃CN solvent. The FTO coated conducting transparent glass substrate with nano-titania layer was used as photoanode in fabrication of DSSCs. The dye was used as effective photosensitizer in assemble of DSSCs as shown by absorption spectrum. It was also investigated that the proficiency of the DSSCs can be improved by using different redox couple electrolytes. The efficiency of fabricated DSSCs were obtained in order i.e. (CH₃)₄NI-I₂< KI-I₂<(CH₃CH₂)₄NI-I₂<(CH₃CH₂CH₂)₄NI-I₂ with these electrolytic solution. The dyes could be effectively used as sensitizers because of transfer of electron from excited state to conduction band of nano-TiO₂ layer and therefore the consequent regeneration are viable in this dye as sensitizer.

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