Qinye Bao

Electronic structures of MoO3-based charge generation layer for tandem organic light-emitting diodes

The role of MoO3 in charge generation layers for tandem organic light-emitting diodes is investigated. The electronic structure of a typical MoO3-based charge generation layer, consisting of N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine, MoO3, and Mg doped 4,7-diphenyl-1,10-phenanthroline (NPB/MoO3/Mg:Bphen) is identified to be a p/n/n junction. It is shown that MoO3 can pronouncedly modify the energy level alignment, beneficial to charge separation at the NPB/MoO3 interface and electron injection at the MoO3/Mg:Bphen interface from MoO3 into suitable molecular energy levels of adjacent emission units. Moreover, Mg:Bphen is favorable to block holes flowing from the anode side directly into the adjacent emission unit.

A New Fullerene-Free Bulk-Heterojunction System for Efficient High-Voltage and High-Fill Factor Solution-Processed Organic Photovoltaics

Small molecule donor/polymer acceptor bulk-heterojunction films with both compounds strongly absorbing have great potential for further enhancement of the performance of organic solar cells. By employing a newly synthesized small molecule donor with a commercially available polymer acceptor in a solution-processed fullerene-free system, a high power conversion efficiency of close to 4% is reported.

Mechanism of Cs2CO3 as an n-type dopant in organic electron-transport film

The electronic structures of cesium carbonate (Cs2CO3) doped 4,7-diphenyl-1,10-phenanthroline (BPhen) films with various doping concentration are characterized by in situ ultraviolet and x-ray photoelectron spectroscopies, in an attempt to understand the mechanism of electron-transport enhancement in Cs2CO3-doped organic electron-transport layer for organic optoelectronic devices. The n-type electrical doping effect is evidenced by the Fermi level shift in the Cs2CO3-doped BPhen films toward unoccupied molecular states with increasing doping concentration, leading to increase in electron concentration in the electron-transport layer and reduction in electron injection barrier height. These findings originate from energetically favorable electron transfer from Cs2CO3 to BPhen.

Light out-coupling enhancement of organic light-emitting devices with microlens array

Light out-coupling efficiency of organic light-emitting devices from high-index glass substrate into air is enhanced by attaching ordered microlens arrays, which are fabricated by a roll-to-roll mold transfer process. The dependence of microlens geometries on light extraction is analyzed experimentally and theoretically. An increase of 60% in the light out-coupling with an optimized elliptical microlens array is achieved over a conventional device without affecting the electroluminescent spectrum.

Effects of ultraviolet soaking on surface electronic structures of solution processed ZnO nanoparticle films in polymer solar cells

We systematically show the effect of UV-light soaking on surface electronic structures and chemical states of solution processed ZnO nanoparticle (ZnONP) films in UHV, dry air and UV–ozone. UV exposure in UHV induces a slight decrease in work function and surface-desorption of chemisorbed oxygen, whereas UV exposure in the presence of oxygen causes an increase in work function due to oxygen atom vacancy filling in the ZnO matrix. We demonstrate that UV-light soaking in combination with vacuum or oxygen can tune the work function of the ZnONP films over a range exceeding 1 eV. Based on photovoltaic performance and diode measurements, we conclude that the oxygen atom vacancy filling occurs mainly at the surface of the ZnONP films and that the films consequently retain their n-type behavior despite a significant increase in the measured work function.

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.

A renewable biopolymer cathode with multivalent metal ions for enhanced charge storage

A ternary composite supercapacitor electrode consisting of phosphomolybdic acid (HMA), a renewable biopolymer, lignin, and polypyrrole was synthesized by a simple one-step simultaneous electrochemical deposition and characterized by electrochemical methods. It was found that the addition of HMA increased the specific capacitance of the polypyrrole–lignin composite from 477 to 682 F g−1 (at a discharge current of 1 A g−1) and also significantly improved the charge storage capacity from 69 to 128 mA h g−1.

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.

In Situ Observation of Light Illumination-Induced Degradation in Organometal Mixed-Halide Perovskite Films

Organometal mixed-halide perovskite materials hold great promise for next-generation solar cells, light-emitting diodes, lasers, and photodetectors. Except for the rapid progress in the efficiency of perovskite-based devices, the stability issue over prolonged light illumination has severely hindered their practical application. The deterioration mechanism of organometal halide perovskite materials under light illumination has seldom been conducted to date, which is indispensable to the understanding and optimization of photon-harvesting process inside perovskite-based optoelectronic devices. Here, explicit degradation pathways and comprehensive microscopic understandings of white-light-induced degradation have been put forward for two organometal mixed-halide perovskite materials (e.g., MAPbI3-xClx and MAPbBr3-xClx) under high vacuum conditions. In situ compositional analysis and real-time film characterizations reveal that the decomposition of both mixed-halide perovskites starts at the grain boundaries, leading to the formation of hydrocarbons and ammonia gas with the residuals of PbI2(Cl), Pb, or PbClxBr2-x in the films. The degradation has been correlated to the localized trap states that induce strong coupling between photoexcited carriers and the crystal lattice.

Fully-solution-processed organic solar cells with a highly efficient paper-based light trapping element

We demonstrate the use of low cost paper as an efficient light-trapping element for thin film photovoltaics. We verify its use in fully-solution processed organic photovoltaic devices with the highest power conversion efficiency and the lowest internal electrical losses reported so far, the architecture of which – unlike most of the studied geometries to date – is suitable for upscaling, i.e. commercialization. The use of the paper-reflector enhances the external quantum efficiency (EQE) of the organic photovoltaic device by a factor of ≈1.5–2.5 over the solar spectrum, which rivals the light harvesting efficiency of a highly-reflective but also considerably more expensive silver mirror back-reflector. Moreover, by detailed theoretical and experimental analysis, we show that further improvements in the photovoltaic performance of organic solar cells employing PEDOT:PSS as both electrodes rely on the future development of high-conductivity and high-transmittance PEDOT:PSS. This is due optical losses in the PEDOT:PSS electrodes.