Authors:	Niu, Quan
Title:	Electrical degradation of polymer light-emitting diodes
Online publication date:	29-Sep-2017
Year of first publication:	2017
Language:	english
Abstract:	After 30 years development, organic light emitting diodes have been widely used in recent displays industry and are believed to be the most promising candidate for the next generation of planer and flexible lighting. However, the most critical issue around this technology is the device lifetime and the mechanisms behind device efficiency loss under long-term operation. Although investigated intensively in the past, the complexity of device architectures and materials made the understanding of OLED degradation mechanisms quite difficult. Possible mechanisms such as chemical reactions, formation of traps, accumulation of charges, increase of injection barriers, and decrease of carrier mobilities have been suggested in previous reports. In this thesis, to simply the problem, in Chapter 2 and 3 we have investigated the degradation of polymer LEDs with a simple single layer structure. Another big advantage is that our understanding on PLED operation has been well developed in the past decades. For instance, it has been found the hole transport in PLEDs is space charge limited with a trap-free characteristic with the hole mobility depends on the electric field, the charge density and the temperature. On the other hand, the electron transport is strongly limited by charge trapping. Furthermore, trapped electrons can capture the free holes via non-radiative trap-assisted recombination which competes with the radiative Langevin recombination of free holes with free electrons and thereby play a role of main loss process for the device efficiency. Presently, the JV characteristics and device efficiency of PLEDs can be numerically predicted when the hole mobility and the trap parameters for electrons are known. This numerical model has been used in Chapter 2 to distinguish possible mechanisms for PLED degradation. By individually changing the device parameters, it has been found an increase of injection barriers, a decrease of mobility and increase of electron traps cannot explain the observed experimental phenomena during device aging. The strong decrease of PLED current, which even gets lower than the pristine hole current implies the degradation of hole transport during this process. From numerical simulation, the generation of hole traps simultaneously explained the decrease of JV characteristic and efficiency of degraded PLEDs. The formation of hole traps also induces a shift of the recombination profile towards the anode. Transient electroluminescent measurement further confirms the degradation of hole transport. Subsequently, in Chapter 3 by evaluation of the voltage drift of a PLED upon aging, the density of hole traps as function of aging time has been calculated. With the amount of hole traps known, the luminance decay of the PLED could be predicted. The agreement with experiment demonstrates that the decrease of the radiative recombination under stress is a direct consequence of increased non-radiative recombination of trapped holes with free electrons. It has been found that initially the density of hole traps increases linearly with time, and after a short stress period the trap formation is slowed down to a square root dependence on time. The observed hole trap generation could be linked to the product of exciton density and density of free holes. Hole trap formation due to the interaction between triplet excitons and free holes (polarons) enables unification of the degradation of various PPV derivatives. The information of hole trap generation during PLED aging has then been used to understand the negative contribution to the capacitance observed in the low frequency C-V characteristics of bipolar PLEDs in Chapter 4. It has been found the negative contribution is characterized by a voltage independent relaxation time that is in the same order of magnitude as the inverse rate for trap-assisted recombination. Enhancement of the amount of electron traps by the addition of fullerene molecules leads to a more pronounced NC effect, which can be quantitatively explained by trap-assisted recombination. Furthermore, the enhanced NC effect in degraded PLEDs, in which hole traps are generated by current stress, can be attributed to trap-assisted recombination of free electrons with trapped holes. The absence of NC in a nearly trap-free PLED unambiguously shows that trap-assisted recombination is the only mechanism responsible for the negative contribution to the capacitance in bipolar organic diodes. In Chapter 5 transient electroluminescence measurement has been applied to investigate the charge transport and the degradation of phosphorescent blue OLED with multi-layer structure and double-host mixed with guest (cyclometallated N-heterocyclic carbene iridium complexes) emitting system. In the emissive layer of the OLED, holes are carried by DPBIC hole transport units, whereas the electrons are transported by the guest emitter. The measurements show that for 10% of emitter and 10% DPBIC co-evaporated in an organic host matrix the transport is dominated by electrons. Varying the amount of DPBIC hole transport units between 5% and 10% has no effect on the degradation process of the OLED showing the devices remain electron dominated also during aging. The decrease of the OLED current in aged devices is correlated with the increase of the electron transit time. The degraded electron transport and device efficiency can then be explained by the creation of electron traps which act also as luminous quenchers during device aging. After 30 years development, organic light emitting diodes have been widely used in recent displays industry and are believed to be the most promising candidate for the next generation of planer and flexible lighting. However, the most critical issue around this technology is the device lifetime and the mechanisms behind device efficiency loss under long-term operation. Although investigated intensively in the past, the complexity of device architectures and materials made the understanding of OLED degradation mechanisms quite difficult. Possible mechanisms such as chemical reactions, formation of traps, accumulation of charges, increase of injection barriers, and decrease of carrier mobilities have been suggested in previous reports. In this thesis, to simply the problem, in Chapter 2 and 3 we have investigated the degradation of polymer LEDs with a simple single layer structure. Another big advantage is that our understanding on PLED operation has been well developed in the past decades. For instance, it has been found the hole transport in PLEDs is space charge limited with a trap-free characteristic with the hole mobility depends on the electric field, the charge density and the temperature. On the other hand, the electron transport is strongly limited by charge trapping. Furthermore, trapped electrons can capture the free holes via non-radiative trap-assisted recombination which competes with the radiative Langevin recombination of free holes with free electrons and thereby play a role of main loss process for the device efficiency. Presently, the JV characteristics and device efficiency of PLEDs can be numerically predicted when the hole mobility and the trap parameters for electrons are known. This numerical model has been used in Chapter 2 to distinguish possible mechanisms for PLED degradation. By individually changing the device parameters, it has been found an increase of injection barriers, a decrease of mobility and increase of electron traps cannot explain the observed experimental phenomena during device aging. The strong decrease of PLED current, which even gets lower than the pristine hole current implies the degradation of hole transport during this process. From numerical simulation, the generation of hole traps simultaneously explained the decrease of JV characteristic and efficiency of degraded PLEDs. The formation of hole traps also induces a shift of the recombination profile towards the anode. Transient electroluminescent measurement further confirms the degradation of hole transport. Subsequently, in Chapter 3 by evaluation of the voltage drift of a PLED upon aging, the density of hole traps as function of aging time has been calculated. With the amount of hole traps known, the luminance decay of the PLED could be predicted. The agreement with experiment demonstrates that the decrease of the radiative recombination under stress is a direct consequence of increased non-radiative recombination of trapped holes with free electrons. It has been found that initially the density of hole traps increases linearly with time, and after a short stress period the trap formation is slowed down to a square root dependence on time. The observed hole trap generation could be linked to the product of exciton density and density of free holes. Hole trap formation due to the interaction between triplet excitons and free holes (polarons) enables unification of the degradation of various PPV derivatives. The information of hole trap generation during PLED aging has then been used to understand the negative contribution to the capacitance observed in the low frequency C-V characteristics of bipolar PLEDs in Chapter 4. It has been found the negative contribution is characterized by a voltage independent relaxation time that is in the same order of magnitude as the inverse rate for trap-assisted recombination. Enhancement of the amount of electron traps by the addition of fullerene molecules leads to a more pronounced NC effect, which can be quantitatively explained by trap-assisted recombination. Furthermore, the enhanced NC effect in degraded PLEDs, in which hole traps are generated by current stress, can be attributed to trap-assisted recombination of free electrons with trapped holes. The absence of NC in a nearly trap-free PLED unambiguously shows that trap-assisted recombination is the only mechanism responsible for the negative contribution to the capacitance in bipolar organic diodes. In Chapter 5 transient electroluminescence measurement has been applied to investigate the charge transport and the degradation of phosphorescent blue OLED with multi-layer structure and double-host mixed with guest (cyclometallated N-heterocyclic carbene iridium complexes) emitting system. In the emissive layer of the OLED, holes are carried by DPBIC hole transport units, whereas the electrons are transported by the guest emitter. The measurements show that for 10% of emitter and 10% DPBIC co-evaporated in an organic host matrix the transport is dominated by electrons. Varying the amount of DPBIC hole transport units between 5% and 10% has no effect on the degradation process of the OLED showing the devices remain electron dominated also during aging. The decrease of the OLED current in aged devices is correlated with the increase of the electron transit time. The degraded electron transport and device efficiency can then be explained by the creation of electron traps which act also as luminous quenchers during device aging.
DDC:	540 Chemie 540 Chemistry and allied sciences
Institution:	Johannes Gutenberg-Universität Mainz
Department:	FB 09 Chemie, Pharmazie u. Geowissensch.
Place:	Mainz
ROR:	https://ror.org/023b0x485
DOI:	http://doi.org/10.25358/openscience-1722
URN:	urn:nbn:de:hebis:77-diss-1000015277
Version:	Original work
Publication type:	Dissertation
License:	In Copyright
Information on rights of use:	https://rightsstatements.org/vocab/InC/1.0/
Extent:	94 Seiten
Appears in collections:	JGU-Publikationen