Hossein Zamani Siboni

Triplet-polaron quenching by charges on guest molecules in phosphorescent organic light emitting devices

We use delayed electroluminescence and photoluminescence measurements to study triplet-polaron-quenching (TPQ) mechanism in phosphorescent organic light emitting devices. Results show that the TPQ mechanism is mainly caused by charges within the bulk of the emission layer (EML) rather than by charges in the hole transport layer (HTL) or at the HTL/EML interface. Furthermore, charges on the guest rather than those on the host are found to be the most efficient in quenching excitons, revealing that guest polaronic species are the most detrimental to device efficiency. The results also show that direct injection of holes from the HTL into the guest material reduces device efficiency.

Luminescence degradation in phosphorescent organic light-emitting devices by hole space charges

We studied electroluminescence degradation in phosphorescent organic light-emitting devices (PHOLEDs) and found that two distinctive mechanisms are responsible for device degradation depending on the device structure. For a device without a hole blocking layer (HBL), excess holes penetrate into the electron transport layer (ETL) and lead to the deterioration of the ETL adjacent to the interface of the emitting layer. The lower electron transport capacity of the degraded ETL alters the balance in hole/electron injection into the emitting layer and results in a decrease in the luminescence efficiency of the PHOLEDs. For a device with a HBL, on the other hand, holes accumulate and become trapped in the emitting layer, and result in a decrease in the luminescence efficiency of the PHOLEDs, likely due to their role in acting as exciton quenchers or as nonradiative charge recombination centers.

Very High Brightness Quantum Dot Light-Emitting Devices via Enhanced Energy Transfer from a Phosphorescent Sensitizer

We demonstrate very efficient and bright quantum dot light-emitting devices (QDLEDs) with the use of a phosphorescent sensitizer and a thermal annealing step. Utilizing CdSe/CdS core/shell quantum dots with 560 nm emission peak, bis(4,6-difluorophenylpyridinatoN,C2) picolinatoiridium as a sensitizer, and thermal annealing at 50 °C for 30 min, green-emitting QDLEDs with a maximum current efficiency of 23.9 cd/A, a power efficiency of 31 lm/W, and a brightness of 65,000 cd/m(2) are demonstrated. The high efficiency and brightness are attributed to annealing-induced enhancements in both the Forster resonance energy transfer (FRET) process from the phosphorescent energy donor to the QD acceptor and hole transport across the device. The FRET enhancement is attributed to annealing-induced diffusion of the phosphorescent material molecules from the sensitizer layer into the QD layer, which results in a shorter donor-acceptor distance. We also find, quite interestingly, that FRET to a QD acceptor is strongly influenced by the QD size, and is generally less efficient to QDs with larger sizes despite their narrower bandgaps.

Causes of driving voltage rise in phosphorescent organic light emitting devices during prolonged electrical driving

We studied the driving voltage stability of typical phosphorescent organic light emitting devices (PHOLEDs) based on 4,4′-bis(carbazol-9-yl)biphenyl and Tris(2-phenylpyridine)iridium(III) host:guest system. The results show that the gradual increase in voltage often observed with prolonged electrical driving is mainly governed by the accumulation of holes at the emission layer/hole blocking layer interface. Reducing the build-up of hole space charges in this region, for example, by means of eliminating guest molecules from the vicinity of the interface, leads to a significant improvement in the stability of PHOLED driving voltage.

The influence of charge injection from intermediate connectors on the performance of tandem organic light-emitting devices

Charge generation in a typical intermediate connector, composed of “n-type doped layer/transition metal oxide (TMO)/hole transporting layer (HTL),” of a tandem organic light-emitting device (OLED) has recently been found to arise from charge transfer at the TMO/HTL interfaces. In this paper, we investigate the effect of hole injection barriers from intermediate connectors on the performance of tandem OLEDs. The hole injection barriers are caused by the offset of the highest occupied molecular orbital (HOMO) energy levels between HTLs contained in the intermediate connector and the top electroluminescence (EL) unit. We also find that although charge generation can occur at the interfaces between the TMO and a wide variety of HTLs of different HOMO values, an increase in the hole injection barrier however limits the electroluminescence efficiency of the top EL units. In the case of large hole injection barriers, significant charge accumulation in the HTLs makes the intermediate connector lose its functionality gradually over operating time, and limits device stability.

Correlation between shift in recombination zone and efficiency roll-off in phosphorescent organic light emitting devices (PHOLEDs)

We study the correlation between the shift in recombination zone and the efficiency roll-off in typical PHOLEDs. To probe the shift in recombination zone, electroluminescence spectra of devices with various architectures at different current densities are studied. Results show that high efficiency at low current density mostly originates from the interfacial emission at the EML/ETL interface due to the formation of host excitons followed by the subsequent energy transfer to the guests. Furthermore, increase in the current density shifts the recombination zone from the EML/ETL interface towards the HTL/EML interface where the device emission is the result of direct charge trapping on the guest sites. The results suggest that the shift in recombination zone and subsequent change in emission mechanism play a main role on the efficiency roll-off.