Photoconductivity and crystal structure of organic molecular complexes

Abstract

The steady state semiconductivity and photoconductivity of a series of single crystals of 24 molecular complexes, in which the electron donor (D) and acceptor (A) molecules were varied systematically, has been studied. Resistivity trends, semiconduction activation energies and Seebeck coefficients suggest extrinsic semiconduction due to A^– ions. Spectral response of photoconduction is little affected by impurities, and is discussed in terms of crystal electronic states and interaction between charge transfer and D or A singlet excitations. The magnitude of photoconduction varies markedly within the series. Independent parameters relating to photoconduction quantum yield are derived from a consideration of the steady state conditions and from measured photoconduction activation energies. Both parameters show the same trends, which are interpreted in terms of two structural requirements for efficient photoconduction: (a)π-orbitals of adjacent D or A molecules (but not both) should overlap appreciably, and (b) the orientation of D with respect to A should not be ideal for overlap of orbitals involved in the charge transfer transition leading to photoconduction. These requirements may be used for prediction of photoconduction efficiency in molecular complexes.