Manoj Jadhav and N Sekar Dyestuff Technology Department Institute of Chemical Technology Matunga, Mumbai 400019
THE increasing demand of energy is one of the vital challenge of today's generation which can be answered with the help of renewable source of energy. The major challenge is to develop a technology which can meet the increasing demand without sacrificing our future environment.“Energy can neither be created nor be destroyed, but it can be converted from one form to another form” a well known first law of thermodynamics has helped researchers to focus on the technologies which can convert one form of energy into another form. Solar energy being the most abundant source of energy has gained significant attention in past few decades due to possibility of being clean, environmental friendly and cost effective energy source. Solar cell is a device which converts solar energy into electrical energy. The silicon based solar cells are being used commercially on larger scale which will result in huge demand of pure silicon in near future. Organic dye based solar cells can be next cost effective, eco- friendly, efficient alternative for solar industry. Dye sensitized solar cells (DSSCs) have gained the lime-light after the major breakthrough reported in 1991 by Michael Grätzel for a ruthenium based photovoltaic device with 7.1% efficiency. At present ruthenium dye based solar cells have achieved 12-13% total conversion efficiency. In last two decades many efforts have been taken to understand the basic principle of DSSCs by synthesizing and investigating dyes for DSSCs. Dye sensitized solar cells can be broadly classified in metal based dye sensitized solar cell and metal free dye sensitized solar cells. Metal free dye sensitized solar cell can be prepared by cost efficient, established synthetic route. They show broad absorption, high molar extinction coefficient with tunable photo- physical and electrochemical properties. Irrespective of type, the working principle and aim of DSSCremains the same i.e. to enhance the conversion efficiency at low cost to provide a cost-effective sustainable energy.
Components of DSSC
A typical dye sensitized solar cell device contains four major components. Sensitizer is a light absorbing material which upon photo-excitation promotes its electron from HOMO level to LUMO level. The photo anode i.e. working electrode containing porous thin TiO2 layer on ITO/FTO doped conducting glass and counter cathode electrode having thin platinum layer on conducting glass separated by electrolyte. Typically iodide/tri-iodide redox couple in acetonitrile is employed as electrolyte. The light absorbing layer of sensitizer formed on working electrode by adsorbing the dye on electrode. The schematic representation of typical DSSCs is given above.
Working of DSSC
A typical DSSC working involves three major process, light absorption, charge separation and charge collection. When a cell is not exposed to sun light the potential difference between two electrodes is zero hence no electrical work is performed, but when cell is exposed to light source, sensitizer absorbs the photon and promotes its electron to from HOMO to LUMO level (Step1). The photo-excited dye injects the electron into TiO2 working electrode if LUMO energy level of dye lies well above the conduction band of TiO2 creating a hole in dye at ground state. Charge separation is achieved when electron is located in the TiO2 and hole is created in oxidized dye (Step2). These electrons diffuse through porous layers of TiO2 of working electrode (Step 3). Which creates a potential difference between two electrodes resulting in electrical work by migration of electron from working anode to counter cathode through external circuit. The migrated electron at counter electrode reduces redox electrolyte which acts as a mediator between two electrodes (Step 4). Which in turn reduces the oxidized dye to complete the circuit (Step 5). The working of DSSCs involves simultaneous occurrence of thermodynamic and kinetic processes. The availability of required energy level is major a driving force for electron transfer from excited dye to TiO2 and redox electrolyte mediator tooxidized dye. The excited dye can inject electron on TiO2 the conduction band of TiO2 . Whereas for efficient dye regeneration by accepting an electron from electrolyte the HOMO energy level of oxidized dye needs to be below the energy level of electrolyte. In case of favorable thermodynamic conditions the kinetics associated with the electron transfer governs course of electron transfer as other competing processes occurs simultaneously with desired electron transfer processes. When electron is in excited state of dye it has two choices either to get injected into TiO2 or relax back to ground sate dye by non-radioactive decay. If the time required for electron injection is sufficiently smaller in comparison with time required for non-radiative de-excitation of dye then kinetically favorable electron injection proceeds with faster rate. After successful electron injection on TiO2 the electron can diffuse through porous layers of TiO2 or it can undergo recombination where electron is transferred to ground state HOMO directly or via redox electrolyte. If the rate of diffusion is faster than the rate of recombination of dye then diffusion process occurs significantly. Regeneration is a process of accepting an electron from redox electrolyte.
The dye de-excitation and dye recombination are as losses in the system inhibiting the electrons from performing electrical work in the external circuit. These undesired process competes simultaneously with desired processes. The kinetic of these process governs the outcome of conversion efficiency.
Design of Sensitizer
In last 15 years metal free dye sensitized solar cell has received much attention due to possibility of being more cost effective and environmental friendly. The sensitizer used in metal free DSSC can be synthesized well established synthetic routes. The Molecular design of sensitizer contributes significantly in the efficiency of DSSCs. Hence lot of efforts have been taken to synthesize novel colorants for DSSCs. To be an efficient sensitizer a dye should have broad absorption spectrum in whole visible region and near IR region with high molar extinction coefficient. Most of the sensitizer employed in DSSCs contain a push- pull chromophores where donor and acceptor is separated by pi-bridge. In recent years several push-pull chromophores having nitrogen, oxygen containing donors like carbazole, triphenyl amines, n,n-dialkyl amines, biphenyl amines, nitrogen containing fused heterocyclic compounds, phenothiazine, phenoxazine, hybrid donor ( combination of two or more donors) have been synthesized and evaluated in DSSCs. Dyes containing triphenyl amine, phenothiazine show better efficiency in comparison with rigidized carbazole donors. Utilization of functionalized two or morehybrid donor system in DSSC has resulted in enhancement of efficiency. Selection of a proper donor and its functionalization can be tricky in improvement of efficiency. Strategic use of long alkyl chains in design of donors has significantly reduced dye aggregation and enhanced photo-stability of some sensitizers. Ethenyl, acetenyl, phenyl, thiophene and their functionalized molecules are being extensively used as a pi-bridge to tune the absorption spectrum of the photosensitizer by adjusting HOMO-LUMO band gap. Acceptor pulls the electron density from donor via pi-bridge and injects the electron onto TiO2 through anchoring group. Cynoacetic acid has been used 2 extensively as acceptor in many sensitizer. Cyno group being strong electron withdrawing groups pulls the electron density from donor upon excitation resulting in charge separation, whereas carboxylic acid has very good affinity for TiO2 binds with nano porous layer of TiO2 . There are many other acceptor are being utilized in DSSC like rohdanine-3-acetic acid, dimer of rohdanine-3-acetic acid quanternized benzthiazole, benzimidazole, quinoline, indol etc. Among these cynoacetic acid based sensitizers show enhanced photo stability, ease in synthesis with high efficiency.
The performance of DSSCs is determined by thermodynamic energy levels of dye, TiO2 and redox electrolyte and kinetics associated electron transfer. A better efficient sensitizer can be designed based on the working principle to full fill the needs. Efficient synthetic routes can be utilized to synthesize designed molecules to achieve better efficiency.
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