Photoinduced Charge Transport Spectra for Porphyrin and Naphthalene Derivative-based Dendrimers

Dendrimers are important chemical structures for harvesting charge. We prepared model dendrimers using two porphyrin derivatives and a naphthalene derivative. Films of these porphyrin derivatives have a strong Soret band ($\sim$430nm) and four significant Q-bands; the naphthalene derivative has strong absorption at 365 and 383nm. Two kinds of photovoltaic cell structures [ITO/BaytronP/(thick or thin) dendrimer/Al] are constructed to investigate the optical response spectra of dendrimers under electric potential($V$) on the cell (range from -1V to 2V). To obtain pure optical responses, incident light is modulated with an optical chopper and a lock-in amplifier is used to measure current ($I_{AC}$) and phase ($\theta$). For the excitation of the Soret band, $I_{AC}$ and $\theta$ do not change substantially with change of sign and amplitude of $V$. For Q-bands and naphthalene absorption bands, $\theta$ nearly follows the polarity of V on the cells and $I_ {AC}$ is linear with $V$. Hence, $I_{AC}$ is nearly ohmic for Q- band although there are shifts due to built-in-potential. $I_ {AC}$ for Soret band is almost same for thick and thin active layer cells. In contrast, $I_{AC}$ increases with thickness increase for Q bands. Mechanisms of photogeneration and charge transport will be discussed.