Direct determination of trap density function based on the photoinduced charge carrier extraction technique

Photoinduced charge carrier extraction experiments using a linearly increasing voltage (photo-CELIV) are reported for an organic thin film device from 1.8 to 150 K. This device is composed of an active layer of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). Photo-CELIV data for a zigzag shape voltage sweep provide critical evidence that the CELIV signal reflects the evacuation of charged carriers captured by traps under a high electric field. The data are analyzed using the Poole-Frenkel model. The trap density as a function of escape energy is obtained as ρ()=Dexp{-(-⁰)²/^σ2}, with D = 1.0 × 10²⁴ states m^-3 eV^-1, ⁰=0.087eV, and σ=0.029eV. The carrier drift mobility is estimated to be 2.3 × 10^-6 cm² V^-1 s ^-1 at 1.8 K. As inferred from the light intensity dependence of the photo-CELIV data, geminate pairs are proposed as the origin of traps. This study demonstrates that carrier evacuation from a Coulomb potential effectively plays an important role in the electrical conduction of organic thin films.