Charge-carrier transport and photogeneration processes in pyronine (G) (PYR) sensitized-TiO2 photovoltaic device

Steady-state current–voltage characteristics in dark and under illumination, along with impedance characteristics at different biasing voltages, were investigated on solid film of pyronine (PYR) sensitized-TiO2, sandwiched between ITO and Al electrodes. The J–V characteristics of the material in dark were explained by the formation of percolation network of nano particles between the electrodes. The device ITO/TiO2–PYR/Al device shows rectification below 2.5 V and becomes symmetrical at higher voltages. Dark current increases by two orders, which indicates that charge-carrier transport occurs not only in the organic phase, but also involves processes in nano particles. The impedance characteristics (variation of imaginary impedance with real impedance) at different biasing voltages show two semicircles at voltage below 2.5 V, and beyond which only one semicircle was observed. This also indicates that at low voltages the device exhibits rectification, and above 2.5 V, both the interfaces behave as Ohmic contact. This observation is also confirmed from impedance spectroscopy. The photoaction spectra of the device-based-composite containing PYR-sensitized-TiO2 is different from the photoaction spectra of the device employing pure PRY. The charge collection efficiencies for the PYR–TiO2 composite increases under both forward bias and reverse bias. This indicates that the same process governs photocurrent multiplication for both bias directions.