Influence of dopant on the electronic structure of spiro-oligophenyl-based disordered organic semiconductors

The electronic structure of the propeller-shaped spiro-type molecules 2,2', 7,7'-tetraphenyl-9,9'- spirobifluorene (spiro-quarterphenyl) and 2,2', 7,7'-tetrakis(biphenyl-4-yl)-9,9-spirobifluorene (spiro-sexiphenyl) and its evolution upon doping with Li atoms or Na atoms have been studied by means of photoelectron spectroscopy. With lithium atoms as the dopant, two charges are stored on the same spiro branch in the form of bipolarons, for both spiro-quarterphenyl and spiro-sexiphenyl. In contrast, for doping with sodium atoms, the size of the counter ion makes it less energetically desirable to store two charges onto a single branch, and the charged species are polarons independent of the level of doping; this result is confirmed by optical absorption data. (C) 2002 American Institute of Physics.