Dependence of Photocurrent and Conversion Efficiency of Titania-Based Solar Cell on the Qy Absorption and One Electron-Oxidation Potential of Pheophorbide Sensitizer
Titania-based solar cells were fabricated by using six pheophorbide sensitizers having bacteriochlorin, chlorin, and porphyrin macrocycles, to which the carboxyl or allylcarboxyl group is directly attached for binding and electron injection to TiO2. Because of structural similarity, these sensitizers are expected to have similar phys. properties except for electronic-absorption and redox properties. Concerning the former, the state energies, molar extinction coeffs. (), oscillator strengths (f), and transition-dipole moments () of the Soret and Qy absorptions were detd., whereas concerning the latter, one electron-oxidn. potential (Eox) was detd. Alternatively, the performance of pheophorbide-sensitized solar cells, including the incident photon-to-current conversion efficiency, short-circuit c.d. (Jsc), open-circuit voltage, and solar energy-to-electricity conversion efficiency () was detd. It was found that the Jsc and values increased with the increasing Qy absorption and with the decreasing one electron-oxidn. potential (in other words, with the increasing electron-ejection potential). To explain the clear and strong dependence, we built possible models for the pheophorbide-to-TiO2 electron injection and tried to fit the Jsc value in terms of the Qy absorption and the Eox value by using empirical equations. After a no. of fitting trials, two successful models of reasonable fitting emerged. The first one is a model of parallel electron injection, i.e., electron injection upon Qy excitation and redox electron transfer in the ground state, and the other is a model of electron injection simply via the excited state, in which both the Qy absorption and the Qy-state one electron-oxidn. potential can contribute. These models as well as the future strategies for obtaining the real mechanism of electron injection are discussed.
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