Space charges and negative capacitance effect in organic light-emitting diodes by transient current response analysis

Guan, Min; Litao, Niu; Zhang, Yang; Liu, Xingfang; Li, Yiyang; Zeng, Yiping

doi:10.1039/c7ra07311a

Cited by 16 publications

(18 citation statements)

References 26 publications

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“…As we can see in Fig. 1a , the turn-on voltage of the three devices is about 3 V. It is relevant to the maximum of the first peak in their capacitance–voltage characteristics, indicating that it makes no difference to the turn-on voltage without AML in D 1 [ 9 – 11 ]. Figure 1b shows the current density–voltage (J-V) characteristics of the three devices in log–log scale, we divide the J-V curves into three regions, (I) leakage or diffusion-limited current caused by Ohmic contact, (II) volume-controlled current with an exponential distribution of traps, and (III) volume-controlled current with partly filled traps [ 20 ].…”

Section: Resultsmentioning

confidence: 99%

“…The inflection point of the first peak in capacitance–voltage (C-V) curves has been reported to be corresponded to the turn-on voltage of OLEDs. It is also a very sensitive probe of carrier accumulation caused by the barrier in the interface of organic layers or the imbalance of charge injection and transport in devices [ 9 – 17 ]. Meanwhile, transient electroluminescence (EL) has also been the subject of intense technological as well as fundamental research, because transient EL studies have generated insight into the internal working mechanism in OLEDs.…”

Section: Introductionmentioning

confidence: 99%

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Efficient Trilayer Phosphorescent Organic Light-Emitting Devices Without Electrode Modification Layer and Its Working Mechanism

et al. 2018

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At present, numerous functional layers are introduced to improve the carrier injection and balance the carrier transport in organic light-emitting devices (OLEDs). Although it may be a good way to enhance the efficiency of devices, the introduction of functional layers would also result in extra process and long manufacture period. Actually, with the enrichment of material system, many appropriate materials could be chosen to share two or even more functions in OLEDs. Here, via impedance spectroscopy and transient electroluminescence analysis, di-[4-(N,N-ditolyl-amino)-phenyl] cyclohexane (TAPC) and 4,7-diphenyl-1,10-phenanthroline (Bphen) are demonstrated to serve as carrier injection and transport layers simultaneously. As a result, efficient trilayer OLEDs are achieved with comparable performances to conventional multilayer devices. Further studies have also been carried out to analyze the recombination and quenching mechanisms in devices. TAPC can block electrons effectively, while Bphen avoids the accumulation of holes. It makes carriers in emitting layer become more balanced, resulting in the reduction of efficiency roll-off.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Efficient Trilayer Phosphorescent Organic Light-Emitting Devices Without Electrode Modification Layer and Its Working Mechanism

et al. 2018

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“…Guan et al pointed out that the C – V curve is affected by the carrier injection rate and consumption rate i.e., the higher C value resulted from the faster carrier injection rate than the carrier consumption rate. The negative capacitance was also reported as a consequence of the synchronized change of the internal accumulated carrier’s transport states, and the applied small ac signal arises from the differences in the mobility of carriers in organic layers . The summary values for the equivalent circuit models are listed in the Supporting Information (Figure S2 and Table S3).…”

Section: Resultsmentioning

confidence: 99%

A Comparative Study via Photophysical and Electrical Characterizations on Interfacial and Bulk Exciplex-Forming Systems for Efficient Organic Light-Emitting Diodes

Amin

Kesavan

Biring

et al. 2020

ACS Appl. Electron. Mater.

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An efficient organic light-emitting diode based on the BCzPh:CN-T2T exciplex as an emitting layer (EML) has been fabricated by exploiting charge balance and favorable molecular orientations. To further understand the details of the exciplex-forming mechanism, time-resolved photoluminescence (TRPL), capacitance-voltage (CV), impedance spectroscopy (IS), and transient electroluminescence (EL) measurements were used to probe the photophysical and electrical characteristics of EL devices by incorporating interfacial (BCzPh/CN-T2T) and bulk (BCzPh:CN-T2T) exciplexes as the emitting layer. Interfacial-and bulk-exciplex devices exhibit a maximum external quantum efficiency (EQE) of 7.7 and 26.4%, respectively. The reason for different device performances was rationalized by comparing the accumulated amount of charge density at the EML's interface responsible for exciplex emission. In addition, the TRPL measurement monitored from short to long wavelengths was used to explore the harvest of nonradiative triplets back to singlets via reverse intersystem crossing and to examine the efficiency of delayed fluorescence. The bulk-exciplex system showed a distinct delayed fluorescence as compared to the interfacial one, which was also corroborated by the observation in the transient EL. The result indicates that the bulk exciplex can reduce the accumulated charge in the EML rapidly, resulting in improvement of EL efficiency. This assumption was further verified by CV and IS measurements. Our results reveal that the accumulated charge density and the bulk resistance of the bulk-exciplex device are much lower as compared to those of the interfacial counterpart device.

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“…So the responsible mechanism for NC is trap-mediated recombination, [18], [20], [59], [60] where the charge carrier consumption rate is faster than the replenishment rate of the charge carriers. [13], [18], [19], [25], [29], [31], [61] B. Evidence of trap states in the SiQD-LEDs…”

Section: A Negative Capacitance In Siqd-ledsmentioning

confidence: 99%

“…In particular, its behavior at different temperature and frequency modulations is important. [11], [29] In this contribution, we provide a comprehensive study of the capacitance characteristics of hexyl functionalized SiQD-LEDs. In doing so, we found an NC in SiQD-LEDs for the first time.…”

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confidence: 99%

Revealing the Negative Capacitance Effect in Silicon Quantum Dot Light-Emitting Diodes via Temperature-Dependent Capacitance-Voltage Characterization

et al. 2022

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In this study, quantum dot light-emitting diodes based on non-toxic silicon quantum dots functionalized with hexyl and dodecyl organic ligands showed a negative capacitance effect. Current density-voltage (J −V ) measurements revealed the charge transport mechanisms in the QLEDs. The capacitancevoltage (C − V ) characteristics were measured with an LCR meter over a wide range of frequencies (200 Hz to 1 MHz) and at temperatures from −40 • C to 60 • C. The classical heterojunction theory can describe the operation of quantum LEDs, but an effect not predicted by Shockley's theory was observed. Negative capacitance values were recorded in both fabricated LEDs, which were not observed in SiQD-LEDs before. Hence, we investigate the negative capacitance origin and its influence on the device performance. We attribute the negative capacitance to trapmediated recombination, where charges at defect sub-bandgap states contribute to the recombination but cannot be replenished fast enough. As a result, a current flows to re-establish the equilibrium, which lags behind the applied voltage, and the NC appears. By comparing the two functionalizations, we also observed a different temperature dependence of the positive capacitance peak and a stronger negative capacitance effect for dodecyl functionalized SiQDs. This effect is attributed to their improved charge carrier confinement abilities.Index Terms-Silicon quantum dot (SiQD), light-emitting diode (LED), negative capacitance (NC). I. INTRODUCTIONQ ANTUM dot light-emitting diodes (QLEDs) are highly researched as promising electroluminescence (EL) devices. QDs itself combine the advantages of inorganic and organic materials by offering size-tunable emission, [1] high color purity, [2] easy processibility, [3] and high photoluminescence quantum efficiency. [4] Additionally, QLEDs are expected to have improved stability compared to OLEDs because of their inorganic crystalline materials. [5] Many conventional QDs consist of toxic or heavy metal materials, such as Cd or Pb, and therefore, alternative materials are researched. A promising alternative is nanosized silicon,

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Space charges and negative capacitance effect in organic light-emitting diodes by transient current response analysis

Abstract: Space charge capacitance and the physical mechanism of negative capacitance in organic light-emitting diodes (OLEDs) by transient current response analysis are investigated for the first time.

Cited by 16 publications

References 26 publications

Efficient Trilayer Phosphorescent Organic Light-Emitting Devices Without Electrode Modification Layer and Its Working Mechanism

Efficient Trilayer Phosphorescent Organic Light-Emitting Devices Without Electrode Modification Layer and Its Working Mechanism

A Comparative Study via Photophysical and Electrical Characterizations on Interfacial and Bulk Exciplex-Forming Systems for Efficient Organic Light-Emitting Diodes

Revealing the Negative Capacitance Effect in Silicon Quantum Dot Light-Emitting Diodes via Temperature-Dependent Capacitance-Voltage Characterization

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