Progress on the electrolytes for ...
2241 Pure Appl. Chem., Vol. 80, No. 11, pp. 2241���2258, 2008. doi:10.1351/pac200880112241 �� 2008 IUPAC Progress on the electrolytes for dye-sensitized solar cells* Jihuai Wu���, Zhang Lan, Sanchun Hao, Pingjiang Li, Jianming Lin, Miaoliang Huang, Leqing Fang, and Yunfang Huang The Key Laboratory of Functional Materials for Fujian Higher Education, Institute of Materials Physical Chemistry, Huaqiao University, Quanzhou 362021, China Abstract: Dye-sensitized solar cells (DSSCs) have aroused intense interest over the past decade owing to their low cost and simple preparation procedures. Much effort has been de- voted to the study of electrolytes that enable light-to-electrical power conversion for DSSC applications. This review focuses on recent progress in the field of liquid, solid-state, and quasi-solid-state electrolytes for DSSCs. It is believed that quasi-solid-state electrolytes, es- pecially those utilizing thermosetting gels, are particularly applicable for fabricating high photoelectric performance and long-term stability of DSSCs in practical applications. Keywords: dye-sensitized solar cells liquid electrolytes solid-state electrolytes quasi-solid- state electrolytes photoelectric performance long-term stability. INTRODUCTION The development of new types of solar cells is promoted by increasing public awareness that the earth���s oil reserves could run out during this century. As the energy need of the planet is likely to double within the next 50 years and frightening climatic consequences of the greenhouse effect caused by fossil fuel combustion are anticipated, it is urgent that we develop a new kind of renewable energy to cover the substantial deficit left by fossil fuels. Among the sources of renewable energy, solar energy is consid- ered the most promising. Fortunately, the supply of energy from the sun to the earth is gigantic: 3 �� 1024 J a year, or about 10 000 times more than the global population currently consumes. In other words, covering 0.1 % of the earth���s surface with solar cells with an efficiency of 10 % would satisfy our present needs. But tapping into this huge energy reservoir remains an enormous challenge . Since the prototype of a dye-sensitized solar cell (DSSC) was reported in 1991 by M. Gr��tzel , it has aroused intense interest owing to its low cost, simple preparation procedure, and benign effect on the environment compared with traditional photovoltaic devices [1���4]. In ���Gr��tzel cells���, the functions of light absorption and charge-carrier transportation are separated, which is different from photo- regenerative and -synthetic cells . Although the solar power conversion efficiencies of DSSCs are lower than that of classical crystalline silicon cells, there is a high potential for improvement in effi- ciency, since it still is far from the theoretical efficiency . In DSSCs based on liquid electrolytes, a photoelectric conversion efficiency of 11 % has been achieved [3,4]. However, the potential problems caused by liquid electrolytes, such as the leakage and volatilization of solvents, possible desorption and photodegradation of the attached dyes, and the cor- *Paper based on a presentation at the 3rd International Symposium on Novel Materials and Their Synthesis (NMS-III) and the 17th International Symposium on Fine Chemistry and Functional Polymers (FCFP-XVII), 17���21 October 2007, Shanghai, China. Other presentations are published in this issue, pp. 2231���2563. ���Corresponding author: Tel.: +86 595 22693899 Fax: +86 595 22693999 E-mail: firstname.lastname@example.org
rosion of Pt counterelectrode, are considered as some of the critical factors limiting the long-term per- formance and practical use of DSSCs [4,6���8]. Therefore, much attention has been given to improving the liquid electrolytes or replacing the liquid electrolytes by solid-state or quasi-solid-state electrolytes [5���10]. This review will focus on progress in the development of improved electrolytes, especially quasi-solid-state electrolytes for DSSCs. STRUCTURE AND OPERATIONAL PRINCIPLES OF DYE-SENSITIZED SOLAR CELLS As shown in Fig. 1 [1,3,4], DSSCs include a substrate of fluorine-doped SnO2 conducting glass (FTO), a porous nanocrystalline semiconductor oxide (the most employed is TiO2) film sensitized by a dye (typically bipyridine ruthenium complexes) for absorbing visible light, a redox electrolyte (usually an organic solvent containing a redox system, such as iodide/triiodide couple) layer for deoxidizing oxi- dized dye, and a platinized cathode to collect electrons and catalyze the redox couple regeneration re- action . The light-to-electricity conversion in a DSSC is based on the injection of electron from the photoexcited state of the sensitized dye into the conduction band of TiO2. The dye is regenerated by electron donation from iodide in the electrolyte. The iodide is restored, in turn, by the reduction of tri- iodide at the cathode, with the circuit being completed via electron migration through the external load. The voltage generated under illumination corresponds to the difference between the Fermi level of the electron in the TiO2 and the redox potential of the electrolyte. Overall, the device generates electric power from light without suffering any permanent chemical transformation [1,2,11,12]. The photoelectric chemical process in DSSC can be expressed as eqs. 1���6. The photoexcited elec- tron injects into the conduction band of TiO2 in subpicosecond time scales [13���15]. The dark reaction eqs. 5 and 6 also occur during the light-to-electricity conversion, but do not play a remarkable negative effect on photovoltaic performance of DSSCs owing to their slow reaction speed compared with that of eq. 2 [16���18]. TiO2|S + hv ��� TiO2|S* excitation (1) TiO2|S* ��� TiO2|S+ + e���(cb) injection (2) TiO2|2S+ + 3I��� ��� TiO2|2S + I3��� regeneration (3) J. WU et al. �� 2008 IUPAC, Pure and Applied Chemistry 80, 2241���2258 2242 Fig. 1 Principle of operation of DSSCs.
I3��� + 2e���(Pt) ��� 3I��� reduction (4) I3��� + 2e���(cb) ��� 3I��� recaption (dark reaction) (5) TiO2|S+ + e���(cb) ��� TiO2|S recombination (dark reaction) (6) It can be seen that DSSCs are a kind of complex system for light-to-electricity conversion. As a basic component, the electrolyte plays an important role in the process of light-to-electricity conversion in DSSCs. The electrolytes employed in DSSCs can be classified as liquid, solid-state, or quasi-solid- state. Several aspects are essential for any electrolytes in a DSSC [5,10]. (1) The electrode must be able to transport the charge carrier between photoanode and counter- electrode. After the dye injects electrons into the conduction band of TiO2, the oxidized dye must be reduced to its ground state rapidly. Thus, the choice of the electrolyte should take into account the dye redox potential and regeneration of itself. (2) The electrode must be able to permit the fast diffusion of charge carriers (higher conductivity) and produce good interfacial contact with the porous nanocrystalline layer and the counterelectrode. For liquid electrolytes, it is necessary to prevent the loss of the liquid electrolyte by leakage and/or evaporation of solvent. (3) The electrolyte must have long-term stability, including chemical, thermal, optical, electro- chemical, and interfacial stability, which does not cause the desorption and degradation of the dye from the oxide surface. (4) The electrolyte should not exhibit a significant absorption in the range of visible light. For the electrolyte containing I���/I3��� redox couple, since I3��� shows color and reduces the visible light ab- sorption by the dye, and I3��� ions can react with the injected electrons and increase the dark cur- rent. Thus, the concentration of I���/I3��� must be optimized. LIQUID ELECTROLYTES The first DSSC was reported in 1991 by M. Gr��tzel  using organic liquid electrolyte containing LiI/I2, which obtained an overall light-to-electricity conversion efficiency of about 7.1 % under irradi- ation of AM 1.5, 100 mW cm���2. Later, many kinds of liquid electrolytes containing iodide/triiodide redox couple and high dielectric constant organic solvents such as acetonitrile (AcN), ethylene carbon- ate (EC), 3-methoxypropionitrile (MePN), propylenecarbonate (PC), ��-butyrolactone (GBL), and N-methylpyrrolidone (NMP) were investigated, and some DSSCs with high photovoltaic performance were obtained [6,19���22]. Research during the past decade shows that each component of liquid elec- trolyte such as solvent, redox couple, and additive plays a different role in the photovoltaic performance of DSSCs. Organic solvents The organic solvent is a basic component in liquid electrolytes, it gives an environment for iodide/tri- iodide ions��� dissolution and diffusion. The physical characteristics of organic solvent including donor number, dielectric constants, viscosity, etc. affect the photovoltaic performance of DSSCs. Especially, the donor number of solvent shows obvious influence on the open-circuit voltage (Voc) and short-cir- cuit current density (Jsc) of DSSCs. The donor���acceptor reaction between nonaqueous solvents and iodide to generate triiodide in DSSCs was investigated, the results showed  that the extent of trans- formation from iodide to triiodide ions in a given solvent could be predicted according to the donor number of the solvent. Therefore, the Voc increased and Jsc decreased with the increase of the donor number of solvent in liquid electrolytes in DSSCs  as shown in Fig. 2. The similar influence of the �� 2008 IUPAC, Pure and Applied Chemistry 80, 2241���2258 Electrolytes for dye-sensitized solar cells 2243