Photoexcitation and Free-Carrier Transport Phenomena for Organic Optoelectronics and Photovoltaics
laser pulse
I. Biaggio, Department of Physics, Lehigh University
107
ud e it pl
100
106
A m
Time
10–1
In
ral g te 105
ed e p S
2
10–2
104
0 0
20
!"#$%%&'()
40
1016
Investigating the origin of the high photoconductivity of rubrene:
Exciton ionization mechanisms and organic photovoltaics
• After picosecond pulse photoexcitation in rubrene one observes a small • • • •
Build-up rate [s–1]
4
Amplitude Speed
101
Photocurrent
b
*+,!,-.//$0!
large photoconductivity, high quality, large mobility.
rubrene
Photocurrent [µA]
Rubrene crystal:
photocurrent arising during the nanosecond lifetime of the excitons and a large delayed photocurrent that appears ~100 microseconds later. The largest portion of photoexcited excitons are stabilized at defect centers close to the surface that then lead to the delayed photocurrent via thermal excitation. This exciton ionization process has a quantum efficiency close to one. Quadratic recombination of charge carriers released from exciton ionization leads to a dependence of the delayed photocurrent amplitude and build-up speed on the density of photoexcited excitons. The small, fast photocurrent has a different origin, probably related to bulk defects that lead to exciton ionization during its lifetime.
1017
1018
nph [cm–3]
Longer wavelength
1019
Shorter wavelength
oxidized surface layer
Larger current with faster rise time at shorter wavelengths rubrene
rubrene
H. Najafov, B. Lyu, I. Biaggio, V. Podzorov, PRB 77, 125202 (2008) Tuesday, August 4, 2009
