Chapter 2. Dicationic iodide for Dye-sensitized solar cells
3. Results and discussions
3.4. Dicationic salts in mixture of γ-Butyrolactone and ethylene carbonate
In addition, mixed solvent of ethylene carbonate (EC) and γ-Butyrolactone (GBL) is used for high temperature stable electrolyte system. The boiling point of EC is 248 ˚C. γ-Butyrolactone (GBL) is also a good candidate for solvent due to its high boiling point of 204 ˚C. In addition, its dielectric constant of 42 and low viscosity of 1.7mPas−1 are preferable characteristics for DSSC applications. A mixture of EC + GBL (30:70, v/v, chosen based on the phase diagram) has been employed as a stable solvent at high temperatures.75,76 Likewise previous experiment, 1-hexyl-2,3-
48
dimethyl imidazolium iodide (HDMII) was used to prepare the reference electrolyte. 0.6 M 1- hexyl-2,3-dimethyl imidazolium iodide (HDMII), 0.1 M lithium iodide (LiI), 0.05 M iodine (I2), 0.1 M guanidinium thiocyanate and 0.5 M tert-butyl pyridine (TBP) dissolved in EC + GBL (30:70, v/v). However, synthesized ionic salts are used half amount of HDMII , 0.3 M concentrations, the other electrolytes compositions are the same of reference electrolyte. Figure 28 and Table 6 show J-V characteristics of the cells with each electrolyte, photovoltaic parameters, respectively. In this experiment, there are little different in Jsc and Voc. However, dicationic salts have better fill factors than HDMII. It caused higher efficiencies. The best performance among them is EBDMIDI electrolyte which is 6.022% with 14.8 mA cm-2 (Jsc), 0.719 V (Voc) and 56.7% (FF).
0.0 0.2 0.4 0.6 0.8
0 2 4 6 8 10 12 14 16 18
Current density (mA/cm2 )
Voltage (V) HDMII
PBDMIDI EBDMIDI PBMPyDI
Figure 28. Current density-voltage (J-V) characteristics of the each devices with different ionic salt in GBL: EC=7:3 (v/v), HDMII (■), PBDMIDI (●), EBDMIDI (▲), PBPyDI (▼) measured under AM1.5G illumination from a calibrated solar simulator with irradiation intensity of 100 mW cm-2
Table 6. Photovoltaic parameters of the devices different salts dissolved in d i f f e r e n t acetonitrile(AcCN): γ-Butyrolactone=7:3 (v/v)
GBL:EC=7:3 Jsc
[mA cm-2]
Voc
[V]
FF [%]
PCE [%]
HDMII 14.6 0.719 48.9 5.139
PBDMIDI 14.8 0.719 55.2 5.853
EBDMIDI 14.8 0.719 56.7 6.022
PBMPyDI 14.5 0.714 54.2 5.596
49 4. Conclusions
DSSCs are recognized as the promising device in the solar industry. These offer the prospective of very low cost fabrication and present attractive features that facilitate market. In this thesis, it has been investigated to improve photovoltaic properties of DSSCs, new dicationic salts based on ionic liquids were synthesized. When small iodide such as LiI and KI are used in electrolyte, cations penetrate into TiO2 surface. It causes band edge shift of TiO2. As the results, Fermi level of nanocrystalline TiO2 working electrode is lowered by cations. For this reason, voltage drop is observed. If the size of cation increases, less cations can reach and occupy the TiO2 surface, for the reason, relatively higher voltage can be expected. Synthesized bulky cationic salts have been used in DSSC electrolyte systems as iodide sources with various solvent (mixture of acetonitrile, ethylene carbonate, propylene carbonate and γ-butyrolactone) systems. In this case, quite comparable results to the conventional imidazolium salts are obtained. For the experiments, dicationic salts with different structures of cation are applied. Possible reasons are suggested for Voc changes, Jsc drops and FF depending on composition of electrolyte solvent. Future directions of dicationic iodides should not only find better molecules than generally used one but also need to investigate mechanisms of dications behavior in the electrolyte.
50
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