M. Wohlgenannt (1), X.M. Jiang (1), Z.V. Vardeny (1), R.A.J. Janssen (2), Kunj Tandon (3), S. Ramasesha (3) and S. Mazumd (4)

(1) Department of Physics

University of Utah

Salt Lake City, Utah 84112

USA

(2) Eindhoven University

The Netherlands

(3) Indian Institute of Science

Bangalore, India

(4) Department of Physics

University of Arizona

Tucson, AZ

USA

The spin dependent exciton formation cross-sections are
directly related to the singlet exciton yield in organic LEDs
(OLED), which determines the maximum quantum efficiency for
OLEDs. We report on an experimental study of the ratio r
=sigma_S/sigma_T. Here sigma_S is the formation
cross-section of the singlet exciton channel and \sigma_T is
the cross-section of any one of the three triplet exciton
channels in the charge-transfer reaction between spin 1/2
positive and negative polarons. We developed an all-optical
technique for measuring r, which combines steady state
photoinduced absorption (PA) and a spin-dependent
recombination technique, namely PA-detected magnetic resonance
(PADMR). Upon magnetic resonance the PADMR spectra generally
show a reduction of polaron and triplet populations, and a
single positive band that we assign to enhanced singlet
population. This directly demonstrates that r>1 in
pi-conjugated polymers. Moreover the fractional reduction,
d_n/n in polaron density, n is directly related to r. We
have obtained r for a large variety of pi-conjugated
oligomers and polymers and studied the dependencies of r on
optical gap and conjugation length, CL. We use a direct and
universal spectroscopic method, which we developed, to
determine the average CL of a polymer. Our main result is the
discovery of a universal relationship between r and CL
largely independent of backbone and sidegroup structure. Our
results also show that polymers have an advantage over small
molecules for their use in OLED. We advance possible
explanations for the universal dependence r(CL).