ELECTRON TRANSFER MECHANISMS OF TETRAPHENYLPORPHYRIN AND ITS DERIVATIVES AS SENSITIZERS IN DYE-SENSITIZED SOLAR CELL

• Main Author: Milana, Phutri
• Format: Dissertations
• Language: Indonesia
• Subjects: Kimia
• Online Access: https://digilib.itb.ac.id/gdl/view/35293
• Institution: Institut Teknologi Bandung

Porphyrins have similar structure with chlorophyll that act as photosynthesis pigment. Porphyrins exhibit high molar extinction coefficient with maximum absorption in the range of 420-450 nm (Soret band) and medium absorption in the range of 500-650 nm (Q band). Grätzel and co-worker reported that porphyrins (i.e. chlorophylls) showed high quantum efficiency, about 80%. It means roughly each porphyrin molecules can produce one electron per absorbed photon. Due to their excellent optical properties, porphyrins can be a good sensitizer candidate for Dye-Sensitized Solar Cell (DSSC). However, porphyrins exhibit poor charge transfer resulting poor cell performance. Poor charge transfer in porphyrins is due to the formation of aggregate on TiO2 surface, the lack of porphyrin adsorption onto TiO2 surface and weakly bound porphyrins onto TiO2 surface. In this research were used some strategies to increase electron transfer from tetraphenylporphyrin (TPP) to TiO2, i.e. by insertion of metal ions into porphyrins ring (formed metalloporphyrin), introduction of linker molecules into metalloporphyrin, also applying the anchoring group to porphyrins structure. These strategies were aimed to reduce the aggregation and to enhance the binding ability between porphyrin and TiO2 surface. Metalloporphyrin is more stable than the free-base porphyrin (TPP). Metalloporphyrin shows higher JSC value than the free-base porphyrin, thus increasing the cell performance. The insertion of metal ion in porphyrin ring had been proven can increase the electron injection from porphyrin excited state to the conduction band of TiO2. This is confirmed by time-resolved photoluminescence results, where ZnTPP generates faster the electron injection rate of 6.88×108 s-1. The insertion of metal ion in porphyrin ring has been shown to reduce the electron recombination and recombination currents, as evidenced by increasing the VOC value. The recombination currents obtained by fitting dark J-V curve using modified Butler-Volmer equation. Due to porphyrins aggregation on TiO2 surface, the electron injection of TPP is slow while the recombination current is enormous. The porphyrin aggregation reduce the electron injection from LUMO to the conduction band of TiO2, due to the internal conversion takes place (nonradiative decay). This is confirmed by the major contribution of slow decay component in TPP. Slow decay component in emission decay can be described as the electron loss process due to internal conversion that occurred among porphyrin molecules. TPP formed J-aggregate onto TiO2 surface as evidenced by red-shift and widening of both absorption and emission peaks\with respect to its solution. The presence of linker molecules in metalloporphyrin increases the electron injection from porphyrin to TiO2. We used linker molecules containing carboxylic groups or hydrazide groups which increase electronic coupling between porphyrin and TiO2 surfaces resulting faster electron injection rate. Pyridine derivatives that were used in this study are isonicotinic acid (LI) and isoniazid (LII) compounds. The enhancement of the electron injection of LI-ZnTPP and LII-ZnTPP was proved by increasing the fast decay component while decreasing the slow decay component. The electron injection rate of LII-ZnTPP is faster than LI-ZnTPP, where the injection rates are 7.69×1010 s-1 and 1.13×109 s-1, respectively. The recombination currents and slow decay component are decreased due to the presence of linker molecules creating a gap between ZnTPP and TiO2 surface. This is confirmed by Raman spectroscopy. The gap between ZnTPP and TiO2 can reduce the electron recombination that proven by the increase of VOC from 0.300 V to 0.450 V for ZnTPP and LII-ZnTPP, respectively. Anchoring group in porphyrins system can affect the adsorption of porphyrin molecules onto TiO2 surface. Dye that slightly adsorbed and did not bind strongly to TiO2 will result in poor cell performance. Free-base porphyrins contain carboxylic group as anchoring will bind stronger onto TiO2. The presence of carboxylic group in porphyrin system can increase the electronic coupling between porphyrin and TiO2, thus improves the electron injection efficiency and cell performance. The position of carboxylic group in porphyrin system also has strong impact on the electron injection and the recombination current. The electron injection rate of 3-TCPP is the fastest (5.26×1011s-1), thus it generates highest short-circuit current (JSC). Despite the recombination currents of 3-TCPP higher than LII-ZnTPP, the VOC value of 3-TCPP is lower than LII-ZnTPP. Dark J-V curves exhibit the difference of electron transfer mechanism between porphyrin and N719. Dark J-V curves show that in porphyrins system, the electron transfer undergoes a “cathodically controlled” mechanism, which means reduction of I3- to I- occurs faster than in N719. However, electron transfer in N719 system undergoes an “anodically controlled” mechanism, which means dye oxidation reaction is faster than in porphyrins. It indicated that the regeneration process is more favourable in porphyrins than in N719 while the electron injection process is better in N719 than in porphyrins system. In conclusion, the electron injection rate of dye is more pronounce than the regeneration rate.