Electromodulated doping of the hole transport layer in a small molecule organic light-emitting diode

Abstract

Abstract. Electromodulation spectroscopy has been used to probe the effect of a polymer hole injection layer on electric fields and charge injection in vacuum-deposited organic light-emitting diodes. The electromodulation spectrum consists of electroabsorption of the transport layers and excited state absorption of trapped cations in the hole transport layer. Field-dependent modulation of trapped charge at the interface between the injection and transport layers substantially modifies the electric field distribution within the device. In reverse bias, the electric field strength is suppressed within the hole transport layer and concentrated in the electron transport layer. In forward bias, field-dependent doping of the hole transport layer dominates the electromodulation spectrum. The field-dependent trap density is calculated to be of order 10 13 cm −2 , equivalent to μC/cm 2 charge density. The built-in potential is estimated to be between 2.2 and 2.5 V, consistent with low carrier injection barriers. C 2011 Society of Photo-Optical Instrumentation Engineers (SPIE).

On the degradation mechanisms of quantum-dot light-emitting diodes

“…Similar to what observed in the degraded red QLED device, the in-phase signal from TFB, 415 nm in this case, is relatively strengthened, resulting in a red-shift of this feature. Spectrum change like this has been reported for small molecule OLED wherein a portion of HTL molecules are permanently oxidized (HTL + ) after lifetime test due to the non-Ohmic contact between HIL and HTL 29 . Since the optical transition energy of HTL + is smaller that of the corresponding HTL molecule, the resultant EA spectrum appears to be red shifted comparing to that of a LT100 sample.…”

“…From the study of OLED, we learned that a hole-injection barrier results in space-charge accumulation and operating-voltage rise in organic HTLs 29,30 . Due to the large ionization potentials, the VBM of Cd-containing QDs cannot be matched by the HOMOs of the HTL materials designed for OLED, therefore the HTL is a possible source of degradation.…”

“…Since the optical transition energy of HTL + is smaller that of the corresponding HTL molecule, the resultant EA spectrum appears to be red shifted comparing to that of a LT100 sample. In our test, the concentration of space-charge is modulated by the reference bias V ac and therefore it contributes charge-modulated signals in addition to the Stark shift 29,32,33 :wherein n is the concentration; σ is the absorption cross section of the charged species. Per Eq.…”

Inverted Polymer Solar Cells with Reduced Interface Recombination

“…The modulation sine wave used in EA measurements was fixed at 855 Hz. The physical mechanism35–37 and experimental details27, 28 of the EA setup were described elsewhere.…”

“…In order to confirm the enhancement of photocurrent is not caused by the change in work function of the ZnO NPs layer after UVO treatment, we studied the change in build‐in potential ( V bi ) by electroabsorption (EA)27, 28 on (ZnO NPs)/polymer/Al devices with and without UVO treatment. The purpose of using this device structure instead of the actual PV devices for the EA measurements is to suppress carrier injection under flat band condition, thus maintaining a uniform field distribution across the polymer layer during the EA measurements.…”