Weiyao Jia

Determining the Origin of Half-bandgap-voltage Electroluminescence in Bifunctional Rubrene/C60 Devices.

Lowering the driving voltage of organic light-emitting diodes (OLEDs) is an important approach to reduce their energy consumption. We have fabricated a series of bifunctional devices (OLEDs and photovoltaics) using rubrene and fullerene (C60) as the active layer, in which the electroluminescence threshold voltage(~1.1 V) was half the value of the bandgap of rubrene. Magneto-electroluminescence (MEL) response of planner heterojunction diodes exhibited a small increase in response to a low magnetic field strength (<20 mT); however, a very large decay was observed at a high magnetic field strength (>20 mT). When a hole-transport layer with a low mobility was included in these devices, the MEL response reversed in shape, and simultaneously, the EL threshold voltage became larger than the bandgap voltage. When bulk heterojunction device was examined, the amplitude of MEL curves presented an anomalous voltage-dependence. Following an analysis of the MEL responses of these devices, we proposed that the EL of half-bandgap-voltage device originated from bimolecular triplet-triplet annihilation in the rubrene film, rather than from singlet excitons that formed via an interface auger recombination. This work provides critical insight into the mechanisms of OLED emission and will help advance the applications of bifunctional devices.

Intersystem Crossing and Triplet Fusion in Singlet-Fission-Dominated Rubrene-Based OLEDs Under High Bias Current

Singlet fission is usually the only reaction channel for excited states in rubrene-based organic light-emitting diodes (OLEDs) at ambient temperature. Intriguingly, we discover that triplet fusion (TF) and intersystem crossing (ISC) within rubrene-based devices begin at moderate and high current densities (j), respectively. Both processes enhance with decreasing temperature. This behavior is discovered by analyzing the magneto-electroluminescence curves of the devices. The j-dependent magneto-conductance, measured at ambient temperature indicates that spin mixing within polaron pairs that are generated by triplet-charge annihilation (TQA) causes the occurrence of ISC, while the high concentrations of triplets are responsible for generating TF. Additionally, the reduction in exciton formation and the elevated TQA with decreasing temperature may contribute to the enhanced ISC at low temperatures. This work provides considerable insight into the different mechanisms that occur when a high density of excited states exist in rubrene and reasonable reasons for the absence of EL efficiency roll-off in rubrene-based OLEDs.

Anomalous temperature dependent magneto-conductance in organic light-emitting diodes with multiple emissive states

The temperature dependence of the magneto-conductance (MC) in organic electron donor-acceptor hybrid and layer heterojunction diodes was studied. The MC value increased with temperature in layer heterojunction and in 10 wt. % hybrid devices. An anomalous decrease of the MC with temperature was observed in 25 wt. %–50 wt. % hybrid devices. Further increasing donor concentration to 75 wt. %, the MC again increased with temperature. The endothermic exciplex-exciton energy transfer and the change in electroplex/exciton ratio caused by change in charge transport with temperature may account for these phenomena. Comparative studies of the temperature evolutions of the IV curves and the electroluminescence and photoluminescence spectra back our hypothesis.

Magnetic field dependence of photocurrent in thermally evaporated rubrene-based devices

The magnetic field response of photocurrent in thermally evaporated rubrene-based single layer devices was investigated, with the results revealing a change of sign in magneto-photocurrent (MPC) with a change of bias. Specifically, under forward bias, there was a positive MPC for |B| > 50 mT, while otherwise there was a negative MPC at smaller fields, forming a “W” shape at low magnetic fields. In contrast, the sign of the MPC was completely changed under reverse bias, forming an “M” shape at low magnetic fields (|B| 50 mT, while otherwise there was a negative MPC at smaller fields, forming a “W” shape at low magnetic fields. In contrast, the sign of the MPC was completely changed under reverse bias, forming an “M” shape at low magnetic fields (|B| < 50 mT). Our study suggests that the observed MPC was not only related to the singlet fission (SF) process itself but also strongly dependent on the subsequent decay routes of triplets in rubrene. The triplet-charge reaction and triplet dissociation superimposed on SF under magnetic fields were proposed to account for the different MPC under forward and reverse bias, respectively. Moreover, the magnitude of both the positive and negative MPC could be promoted by avoiding the triplet loss cha...

Guest concentration, bias current, and temperature-dependent sign inversion of magneto-electroluminescence in thermally activated delayed fluorescence devices

Non-emissive triplet excited states in devices that undergo thermally activated delayed fluorescence (TADF) can be up-converted to singlet excited states via reverse intersystem crossing (RISC), which leads to an enhanced electroluminescence efficiency. Exciton-based fluorescence devices always exhibit a positive magneto-electroluminescence (MEL) because intersystem crossing (ISC) can be suppressed effectively by an external magnetic field. Conversely, TADF devices should exhibit a negative MEL because RISC is suppressed by the external magnetic field. Intriguingly, we observed a positive MEL in TADF devices. Moreover, the sign of the MEL was either positive or negative, and depended on experimental conditions, including doping concentration, current density and temperature. The MEL observed from our TADF devices demonstrated that ISC in the host material and RISC in the guest material coexisted. These competing processes were affected by the experimental conditions, which led to the sign change of the MEL. This work gives important insight into the energy transfer processes and the evolution of excited states in TADF devices.

Magneto-conductance characteristics of trapped triplet–polaron and triplet–trapped polaron interactions in anthracene-based organic light emitting diodes

The effects of a magnetic field on the dissociation of triplet excitons by free charges (TCI) are well understood. However, the magneto-conductance (MC) characteristics of trapped triplet-polaron interactions (TtPI) and triplet-trapped polaron interactions (TPtI) within organic light emitting diodes (OLEDs) are not well understood. We have studied these interactions in an anthracene-based OLED. The electroluminescence spectra, current-voltage characteristics and magneto-electroluminescence indicated that the anthracene layer contained many defects that could trap either triplet excitons or polarons, which led to TPtI and TtPI. The MC curves at low temperature exhibited a complex line shape, which indicated that intersystem crossing, TPtI, TtPI, and TCI occurred simultaneously in the device. The individual MC characteristics of TPtI and TtPI were extracted from temperature dependant MC curves by fitting them to three empirical Lorentzian functions and one non-Lorentzian function. The MC of TPtI exhibited a negative sign, while that of TtPI exhibited a positive one, with characteristic magnetic fields (B0) of ∼10.5 and ∼15 mT, respectively. Both processes were prominent below 150 K and weakened with increasing temperature. TPtI was neglected above 200 K, while TtPI was observed even at ambient temperature. These results add significant insight into the magnetic field effects on triplet-polaron interactions.

Abnormal temperature dependent behaviors of intersystem crossing and triplet-triplet annihilation in organic planar heterojunction devices

Anomalous temperature dependent magneto-electroluminescence was observed at low and high magnetic field strength from organic planar heterojunction devices incorporated common phosphorescent host materials of N,N′-dicarbazolyl-3,5-benzene (mCP) or 4,4′-N,N′-dicarbazole-biphenyl (CBP) as an emissive layer. We found that intersystem crossing became stronger with decreasing temperature and that triplet-triplet annihilation (TTA) occurred at room temperature but ceased at low temperature. Analyses of the electroluminescence spectra of these devices and their temperature dependences indicated that the population of exciplex states increased at low temperature, which caused the abnormal behavior of intersystem crossing. Additionally, long lifetime of the excitons within mCP or CBP layer may allow TTA to occur at room temperature, while the reduced population of excitons at low temperature may account for the disappearance of TTA even though the excitons had increased lifetime.

Triplet-triplet annihilation process in exciplex-based organic light-emitting diodes

The triplet-triplet annihilation process in exciplex-based devices is hardly considered to occur at all because it is a four-intermolecular- interaction process. In this work, the annihilation process was observed in an exciplex-based device with a higher donortriplet energy and a greater energy difference between the singlet and triplet states by using the magnetic field effects (MFEs) as a new non-contact measurement approach. The red-shifted emission of the spectrum suggests that the exciplex state did indeed form. The MFE results show that the magneto-electroluminescence (Δ EL / EL ) does not decrease remarkably under higher magnetic fields ( B >40 mT) at temperatures higher than 100 K ( T >100 K); however, decreasing Δ EL / EL will only appear when the measurement temperature is reduced to the minimum (20 K) temperature. In addition, the decreasing Δ EL / EL also depends on the current density. The evolution of Δ EL / EL at different voltages and temperatures further confirmed the existence of the annihilation process. The annihilation process formation conditions for exciplex-based devices were also discussed based on the experimental results.

Influence of BCP inserted-layer and working temperature on the sign inversion of magneto-conductance at high magnetic field in organic light-emitting diodes

Organic light-emitting diodes with structure of ITO/CuPc/NPB/Alq 3 /BCP(x)/Al were fabricated in this work. The thickness of BCP is 40, 80 and 120 nm, respectively. The magnetic field effects on the conductance (magneto-conductance, MC) of these diodes were measured at temperature range from 300 to 15 K, respectively. Results show that the MC responses of all the three devices reveal a positive component under low magnetic field at all working temperatures. However, the MC effects under high magnetic field of the BCP(x)-inserted devices show a strong dependence on the temperature and device thickness. In particular, the MC of BCP (120nm) device shows a sign inversion as decreasing the temperature. However, this phenomenon can not be observed in the BCP (40, 80nm) devices. Only negative component of MC is existed in the two devices at low temperatures. We suggest that the positive MC in low magnetic field is caused by hyperfine interaction. Moreover, the sign inversion of MC at high magnetic field is caused by the interaction between the triplet excition with long lifetime at low temperature and holes which are intercalated by the BCP inserted layer.