Developing the TiO2 pastes for Low-Temperature Dye-Sensitized Solar Cells and Its Simulation
Date Issued
2011
Date
2011
Author(s)
Tsai, Keng-Wei
Abstract
Recently, research works have focused on developing flexible dye-sensitized solar cells (DSSCs) based on nanocrystalline TiO2 electrodes. The realization of the cost reduction for DSSCs, in particular, can be achieved by using roll-to-roll printing process for electrode fabrications. Moreover, the lightweight flexible DSSC is attractive not only for its attachment on the existing transparent surfaces but also for applications on portable powers.
Unlike the conventional DSSC, a high-temperature sintering process cannot be applied to prepare TiO2 films on plastic substrates. It is generally accepted that the development of low-temperature fabrication methods for DSSCs should overcome two main issues, namely, the incomplete necking of the TiO2 particles and the removal of the residual organics within the film. Therefore, to prepare thick TiO2 films with well-interconnected nanoparticles at low temperature, it is essential to increase the viscosity of the TiO2 colloid solution, without using a polymer binder or thermal sintering process.
In this study, we have prepared a binder-free TiO2 paste by controlling the weight ratio of solid powder to solvent. The binder-free TiO2 paste is mainly composed of P25 (Degussa, 25 nm TiO2, 80% Anatase, 20% Rutile) and a mixed solvent containing DI-water and tert-butanol (with a volume ratio of 1:2). The inter-particle connection of nanocrystalline TiO2 (necking reaction) is assumed to proceed by the dehydration of hydrogen-bonded network of TiO2 nanoparticles, whose surfaces are well-covered with hydroxy groups, heated 120 oC. With the addition of 12.5 wt% P25, the power conversion efficiency reached 4.64% (open-circuit voltage (Voc) = 0.740 V, short-circuit current density (isc) = 8.82 mA/cm2 and fill-factor (FF) = 0.71). Next, we use different amounts of HCl to change the zeta potential of the P25 particles to increase the viscosity of the paste. The best performance was found with the DSSC using the paste at around pH = 4, and the power conversion efficiency of 5.12% (Voc = 0.74 V, isc = 10.16 mA/cm2 and FF = 0.68) was obtained. Moreover, to compare the chemical and thermal sintering effects, the DSSCs were made with the same paste at different sintering temperatures, i.e., 450 oC and 120 oC, for 30 min. The results show the similar performances for the both cases.
Finally, we have focused on the study of the working area effects for the DSSCs with the mentioned low-temperature fabricating process. We have discussed the equivalent circuits for DSSCs, and related it to the electrochemical impedance spectroscopy (EIS) or current-voltage (I-V) characteristic curves. The relation between the resistance and the working area in DSSCs has been clarified by EIS, and the experimental I-V curves have been fitted by the simplifying I-V function obtained with an equivalent circuit. The cell performance of DSSCs with different working areas, from 0.16 cm2 up to 2.56 cm2, has been fitted by a function, from which we can extract the cell parameters, including the photo generation current density (iph), saturation current density (isat), exchange current density (iex), etc, from the experimental I-V curves. These parameters relate only to the working area of the cell. We have also introduced the hydraulic diameter to predict the performance of DSSCs with different working shapes (or aspect ratio). By using the hydraulic diameter and the experiential cell parameters, the performance of DSSCs with different aspect ratios can be predicted.
Subjects
Dye-sensitized solar cells
Equivalent circuit
Electrochemical impedance spectroscopy
Low-temperature fabrication process
Working area effect
Type
thesis
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