Photoconduction in Alq3

Abstract

Photoelectronic properties of Alq 3 were studied by photoconductivity (PC) measurements in thin film, sandwich (ITO/Alq 3 /LiF/Al) devices. We find that the photocurrent is dominated by bulk generation of carriers for incident photon energies greater than 2.75 eV. The quantum efficiency of photocarrier generation has been measured from carrier collection measurements to be about 10%. The quantum efficiency is largely independent of electric field. This enables a direct measurement of the electric field dependence of mobility using photoconductivity measurements, which is used for quantitative analysis of the dark forward current in these devices. PC measurements were also used to obtain (µ 0n τ n ) product which can be used as a measure of material quality. For Alq 3 , we find that the value of (µ 0n τ n ) product was between 3 x 10 -15 cm 2 /V to 8 x 10 -15 cm 2 /V for different samples. In forward bias, at high field the photocurrent shows saturation accompanied by a phase shift. These effects are attributed to space charge effects in the device.

Influence of organic semiconductor-metal interfaces on the photoresponse of functionalized anthradithiophene thin films

“…14 The choice of the best contacts to use when characterizing photoconductive properties of materials could be difficult. 15,16 For instance, while blocking contacts would ensure low dark current and low contamination of the bulk material with trapped injected charges, they can prevent effective extraction of photoexcited carriers, which would lead to charge accumulation at the interfaces; thereby screen electric field in the film and limit photoresponse. If ohmic contacts are used, however, the material can become charged due to extensive trapping of charge carriers injected from the electrodes, followed by space-charge field formation, which might skew the results of the measurements of intrinsic photoconductivity.…”

Spontaneous and field induced photocarrier generation in TPD and its blend

“…While TPD and AlQ 3 have spontaneous carrier generation efficiencies of 0.1% and 10%, respectively [3,4], the number is enhanced to 30% in the blend. The field induced carrier generation efficiency has also increased from 0.1% (TPD) and 1% (AlQ 3 ) to 8% in the blend.…”

“…g(F) can be expressed [3] as, where G is the exciton generation rate. The field induced carrier generation efficiency, η(F), is also plotted in fig.3.…”

“…In this paper we study the photocarrier generation mechanism in TPD and TPD-AlQ 3 blend. Unlike inorganic semiconductors, where the carrier generation efficiency is close to one, the photocurrent in an organic semiconductor can be limited by both the carrier generation efficiency (η) and transport [3]. The ways to decouple them are explored in this paper.…”

“…The inset shows J PC vs F for small fields. For bulk carrier generation, the photocurrent can be expressed [3,4] as,…”