Time-Resolved Infrared Spectroscopy of Dye Sensitized TiO2 Photovoltaic Devices.

Todd Heimer and Edwin J. Heilweil
Optical Technology Divison (844)
National Institute of Standards and Technology
Dye sensitization has become an important method for extending the spectral response of wide bandgap semiconductors, to increase their utilization of the solar spectrum. One of the key parameters affecting the efficiency of photovoltaic devices using this technology is the rate of electron transfer from the sensitizer to the semiconductor. Time-resolved infrared spectroscopy in the 6 micrometer region was employed to study the excited state properties of the sensitizer [Ru(4,4'-(COOCH2CH3)2-2,2'-bipyridine)(2,2'-bipyridine)2]+2 in solution and anchored to nanostructured thin films of TiO2. For the molecules attached to TiO2, a transient absorption appears which is attributed to electrons injected into TiO2. This absorption appears within the instrumental time resolution (ca. 30 ps) yielding an approximate 20 ps upper limit time constant for electron transfer from the sensitizer excited state to TiO2. Preliminary experiments with shorter light pulses indicate an approximate 1 ps electron injection time with a possible longer component for Ru(4,4'-(COOH2)2-2,2'-bipyridine)2(NCS)2 anchored to TiO2. For these systems, electron injection dominates the microsecond sensitizer excited state decay, resulting in a near unit quantum yield for interfacial electron transfer.