Jin-Peng Yang

The role of cesium carbonate on the electron injection and transport enhancement in organic layer by admittance spectroscopy

The effect of cesium carbonate (Cs2CO3) doping on the electron transport properties of 4,7-diphenyl-1, 10-phenanthroline (BPhen) layer has been investigated in organic light-emitting diodes (OLEDs). Temperature-dependent admittance spectroscopy studies show that the incorporation of Cs2CO3 from 0 to 42 wt. % can decrease the activation energy of the BPhen layer from 1.3 to 0.18 eV, resulting in the enhanced electron injection and transport with respect to reduced injection barrier and increased conductivity of the BPhen layer. This is consistent with the performance improvement in OLEDs, which yields better electrical characteristics and enhanced luminance efficiency.

Role of transition metal oxides in the charge recombination layer used in tandem organic photovoltaic cells

The mechanism of charge recombination in transition metal oxide-based interconnectors for tandem organic photovoltaic cells is investigated, where the interconnector is composed of an abrupt heterointerface between a Mg-doped 4,7-diphenyl-1,10-phenanthroline (Mg:BPhen) layer and a MoO3 film. Based on the results of the interface energetics determined by ultraviolet photoelectron spectroscopy, as well as the corresponding device characteristics, it is revealed that the MoO3 layer pronouncedly modifies the energy level alignment of the interconnector, which is beneficial for the charge recombination process at the interface between MoO3 and the adjacent donor material for electrons and holes injected from stacked subcells. The incorporation of Mg:BPhen is essential for the conduction of the generated electrons from the bottom subcell into the conduction band of MoO3.

Correlation between the electronic structures of transition metal oxide-based intermediate connectors and the device performance of tandem organic light-emitting devices

The impact of electronic structures on the functionality of transition metal oxide-based intermediate connectors for tandem organic light-emitting devices is investigated by studying the interfaces and the corresponding devices. For a typical transition metal oxide-based intermediate connector, consisting of a heterointerface between MoO3 and Mg-doped tris(8-quinolinolato)aluminum (Mg:Alq3), it is identified that MoO3 is essential to the charge generation and separation process, which occurs at the interface between MoO3 and the adjacent hole-transporting layer (HTL) viaelectron transfer from the highest occupied molecular orbital of the HTL into the conduction band of MoO3. In addition, the incorporation of a Mg:Alq3 layer is indispensable to the functionality of the intermediate connector, which not only facilitates the electron injection from MoO3 into the electron-transporting layer of the adjacent electroluminescent (EL) unit, but also blocks the leakage of holes across the intermediate connector into the HTL of the other adjacent EL unit.

Hole-phonon coupling effect on the band dispersion of organic molecular semiconductors

The dynamic interaction between the traveling charges and the molecular vibrations is critical for the charge transport in organic semiconductors. However, a direct evidence of the expected impact of the charge-phonon coupling on the band dispersion of organic semiconductors is yet to be provided. Here, we report on the electronic properties of rubrene single crystal as investigated by angle resolved ultraviolet photoelectron spectroscopy. A gap opening and kink-like features in the rubrene electronic band dispersion are observed. In particular, the latter results in a large enhancement of the hole effective mass (> 1.4), well above the limit of the theoretical estimations. The results are consistent with the expected modifications of the band structures in organic semiconductors as introduced by hole-phonon coupling effects and represent an important experimental step toward the understanding of the charge localization phenomena in organic materials.The charge transport properties in organic semiconductors are affected by the impact of molecular vibrations, yet it has been challenging to quantify them to date. Here, Bussolotti et al. provide direct experimental evidence on the band dispersion modified by molecular vibrations in a rubrene single crystal.

Quantitative Fermi level tuning in amorphous organic semiconductor by molecular doping: Toward full understanding of the doping mechanism

The doping mechanism in organic-semiconductor films has been quantitatively studied via ultrahigh-sensitivity ultraviolet photoelectron spectroscopy of N,N-bis(1-naphthyl)-N,N-diphenyl-1,1-biphenyl-4,4-diamine (α-NPD) films doped with hexaazatriphenylene-hexacarbonitrile [HAT(CN)6]. We observed that HOMO of α-NPD shifts to the Fermi level (EF) in two different rates with the doping concentration of HAT(CN)6, but HOMO distributions of both pristine and doped amorphous α-NPD films are excellently approximated with a same Gaussian distribution without exponential tail states over ∼5 × 1018 cm−3 eV−1. From the theoretical simulation of the HAT(CN)6-concentration dependence of the HOMO in doped films, we show that the passivation of Gaussian-distributed hole traps, which peak at 1.1 eV above the HOMO onset, occurs at ultralow doping [HAT(CN)6 molecular ratio (MR) < 0.01], leading to a strong HOMO shift of ∼0.40 eV towards EF, and MR dependence of HOMO changes abruptly at MR ∼ 0.01 to a weaker dependence for MR...

Mechanism for doping induced p type C60 using thermally evaporated molybdenum trioxide (MoO3) as a dopant.

Thermally evaporated molybdenum trioxide (MoO3) doped C60 films, which could change n type features of pristine C60 to form a p type mixed C60 layer, are investigated by x-ray and ultraviolet photoelectron spectroscopy. It is found that C60 HOMO progressively shifts closer to the Fermi level after increased MoO3 doping concentration, and final onset of C60 HOMO is pinned at binding energy of 0.20 eV, indicating the formation of p type C60 films. It is proposed that in charge transfer induced p type C60 formation, due to large electron affinity of MoO3 (6.37 eV), electrons from HOMO of C60 could easily transfer to MoO3 to form cations and therefore increase hole concentration, which could gradually push C60 HOMO to the Fermi level and finally form p type C60 films. Moreover, clear different types of C60 species have been confirmed from UPS spectra in highly doped films.

Suppressed polarization effect and enhanced carrier confinement in InGaN light-emitting diodes with GaN/InGaN/GaN triangular barriers

In this study, with the objective of lowering the polarization without degrading the carrier confinement in the active region, an InGaN light-emitting diode with GaN/InGaN/GaN triangular (GIGT) quantum barriers (QBs) was proposed and studied systematically. When traditional GaN QBs were replaced by the GIGT QBs, the output power at 150 mA rose from 82.05 to 155.99 mW, and the efficiency droop was decreased from 51.0% to 16.5%. Simulation results indicated that these improvements could result from the effectively suppressed polarization field in the quantum wells and the markedly enhanced carrier confinement in the active region because of the appropriately modified energy band structure.In this study, with the objective of lowering the polarization without degrading the carrier confinement in the active region, an InGaN light-emitting diode with GaN/InGaN/GaN triangular (GIGT) quantum barriers (QBs) was proposed and studied systematically. When traditional GaN QBs were replaced by the GIGT QBs, the output power at 150 mA rose from 82.05 to 155.99 mW, and the efficiency droop was decreased from 51.0% to 16.5%. Simulation results indicated that these improvements could result from the effectively suppressed polarization field in the quantum wells and the markedly enhanced carrier confinement in the active region because of the appropriately modified energy band structure.

Improved carrier injection and confinement in InGaN light-emitting diodes containing GaN/AlGaN/GaN triangular barriers

In this study, an InGaN lighting-emitting diode (LED) containing GaN/AlGaN/GaN triangular barriers is proposed and investigated numerically. The simulation results of output performance, carrier concentration, and radiative recombination rate indicate that the proposed LED has a higher output power and an internal quantum efficiency, and a lower efficiency droop than the LED containing conventional GaN or AlGaN barriers. These improvements mainly arise from the modified energy bands, which is evidenced by analyzing the LED energy band diagram and electrostatic field near the active region. The modified energy bands effectively improve carrier injection and confinement, which significantly reduces electron leakage and increases the rate of radiative recombination in the quantum wells.

A striking mobility improvement of C60 OFET by inserting diindenoperylene layer between C60 and SiO2 gate insulator

Gap states in organic semiconductors play a crucial role in determining Energy-Level Alignment and in many cases they act as charge trapping centers to result in serious lowering of charge mobility. Thus origin of gap states has gained increasing attention in order to realize higher mobility organic devises [1-4]. Bussolotti et al. have demonstrated recently that gap states in a pentacene thin film increase even by exposing the film to inert gas and confirmed that the gas exposure mediates structural defects in the film thus gap states [4]. The results have also indicated that preparation of highly-ordered organic thin film is necessary to improve the device performance, namely to decrease trapping states. To improve the ordering of molecule in the film, deposition of a template molecular underlayer is one of the simplest methods to increase the domain size of overlayer film and its crystallinity, and thus we expect improvement of the charge mobility [5]. Hinderhofer et al. reported recently that diindenoperylene (DIP; Figure 1a) could be used as a template layer to grow highly ordered and oriented C60 film with its (111) plane parallel to the SiO2 substrate [6]. Considering the hole mobility of DIP single crystal, which is quite low (~0.005 cm2 V-1S-1 at room temperature [7]), it is expected for the DIP template C60 thin film system that lower drain current would be achieved to improve the on/off ratios based on n type C60 transistor and its electron mobility (especially on the negative Vgs region, compared to PEN modified C60 transistors [8]).