Exciton formation statistics under electrical injection in organic semiconductor thin films

A central question in understanding the electroluminescent properties of organic thin films is the ratio of radiative singlet to triplet Frenkel excitons that are formed by electrical injection. In this paper, we review the physics of the formation process, and describe a simple method for accurately measuring the singlet formation ratio in fluorescent semiconductor thin films. This method eliminates many assumptions and approximations required in previous measurement methods. Following the experimental description, we show that measurements of the singlet–triplet exciton formation ratio in both polymers and small molecular weight organic materials suggest a purely statistical formation ratio of 25% singlets to 75% triplets by electrical injection.