Mei-Feng Xu

Plasmon Resonance Enhanced Optical Absorption in Inverted Polymer/Fullerene Solar Cells with Metal Nanoparticle-Doped Solution-Processable TiO2 Layer

This paper investigates the effects of localized surface plasmon resonance (LSPR) in an inverted polymer/fullerene solar cell by incorporating Au and/or Ag nanoparticles (NPs) into the TiO2 buffer layer. Enhanced light harvesting via plasmonic resonance of metal NPs has been observed. It results in improved short-circuit current density (Jsc) while the corresponding open-circuit voltage (Voc) is maintained. A maximum power conversion efficiency of 7.52% is obtained in the case of introducing 30% Ag NPs into the TiO2, corresponding to a 20.7% enhancement compared with the reference device without the metal NPs. The device photovoltaic characteristics, photocurrent properties, steady-state and dynamic photoluminescences of active layer on metal NP-doped TiO2, and electric field profile in metal NP-doped TiO2 layers are systematically investigated to explore how the plasmonic effects of Au and/or Ag NPs influence the OSC performance.

Highly efficient single-layer organic light-emitting devices based on a bipolar pyrazine/carbazole hybrid host material

Organic light-emitting diodes (OLEDs) have attracted tremendous interest and have already become a prevalent technology in MP3 players, smartphones and cameras. In response to the calls for the large-scale application of OLEDs, the complicated and costly processes for preparing a device is a major challenge which should be addressed. Herein, a novel bipolar host material, 26PyzCz, which contains a pyrazine/carbazole hybrid, has been designed and synthesized. 26PyzCz-based single-layer (SL) fluorescent (F)–phosphorescent (P) OLEDs with various colors have been successfully fabricated. Green and orange SL phosphorescent OLEDs (PHOLEDs) have exhibited efficiencies as high as 63.3 and 62.1 cd A−1 at 1000 cd m−2, and 55.7 and 53.8 cd A−1 at 10 000 cd m−2, respectively. Meanwhile, a SL warm white OLED based on fluorescent blue and phosphorescent orange has demonstrated excellent performance, with a maximum current efficiency of 27.5 cd A−1 and a maximum power efficiency of 21.6 lm W−1. In addition, the charge carrier behavior have been evaluated by impedance spectroscopy, which revealed that the dopant trapping effect plays a critical role in charge balance and exciton generation in the SL PHOLEDs.

Improved cation valence state in molybdenum oxides by ultraviolet-ozone treatments and its applications in organic light-emitting diodes

The authors demonstrate a thick MoO3 layer (60 nm) as a good short reduction layer in organic light emitting diodes (OLEDs), which is especially useful for large-area and flexible OLEDs to prevent short circuit issues. The crystallization of organic material and the increase of driving voltage induced by a thick MoO3 layer in OLEDs were resolved by a simple ultraviolet-ozone (UV-ozone) treatment. Ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, contact angle, and atomic force microscope analyses revealed that a longer UV-ozone treatment resulted in an optimized fraction of oxygen vacancies in MoO3, which is responsible for the improved device performance.

Lithium hydride doped intermediate connector for high-efficiency and long-term stable tandem organic light-emitting diodes.

Lithium hydride (LiH) is employed as a novel n-dopant in the intermediate connector for tandem organic light-emitting diodes (OLEDs) because of its easy coevaporation with other electron transporting materials. The tandem OLEDs with two and three electroluminescent (EL) units connected by a combination of LiH doped 8-hydroxyquinoline aluminum (Alq3) and 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) demonstrate approximately 2-fold and 3-fold enhancement in current efficiency, respectively. In addition, no extra voltage drop across the intermediate connector is observed. Particularly, the lifetime (T75%) in the tandem OLED with two and three EL units is substantially improved by 3.8 times and 7.4 times, respectively. The doping effect of LiH into Alq3, the charge injection, and transport characteristics of LiH-doped Alq3 are further investigated by ultraviolet photoelectron spectroscopy (UPS) and X-ray photoemission spectroscopy (XPS).

Dual roles of MoO3-doped pentacene thin films as hole-extraction and multicharge-separation functions in pentacene/C60 heterojunction organic solar cells

The authors demonstrate a pentacene/C60 heterojunction organic solar cell utilizing MoO3-doped pentacene thin films as an interfacial layer at anode and a multicharge-separation layer between pentacene and C60, respectively. The short-circuit current density and the open-circuit voltage were improved simultaneously compared with the reference device, resulting in an improvement in power conversion efficiency from 0.97% to 2.29%. Absorption spectra measurement, surface morphology analysis, and interfacial evaluation at anode side in the hole-dominant devices were carried out to reveal the functions of MoO3-doped pentacene films in OSCs.

Aqueous Solution-Processed GeO2: An Anode Interfacial Layer for High Performance and Air-Stable Organic Solar Cells

A simple and cheap method for depositing solution-processed GeO2 (sGeO2) film is proposed utilizing the weak solubility of GeO2 in water. X-ray photoelectron spectroscopy analysis reveals that a pure GeO2 thin film can be formed by casting its aqueous solution. This method can avoid the difficulty of vacuum evaporation by its high melting point. The sGeO2 film has been used successfully as an anode interfacial layer in poly(3-hexylthiophene) (P3HT) and indene-C60 bisadduct (IC60BA)-based bulk heterojunction organic solar cells with improved power conversion efficiency and device stability compared with that using conventional poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS); the improvement of the power conversion efficiency and the device stability are estimated to be 9% and 50%, respectively. The calculations of optical intensity in a whole cell demonstrate that a thin layer of sGeO2 could function as an optical spacer in the based bulk heterojunction (BHJ) organic solar cells (OSCs) for enhancing the light harvesting in the active layer. Interfacial evaluation by impedance spectroscopy shows that the sGeO2-based cell exists less charge carrier recombination and lower contact resistance. More importantly, the sGeO2 film processing is very simple and environmentally friendly, which has potential applications in green and low-cost organic electronics in the future.

Aqueous solution-processed MoO3 thick films as hole injection and short-circuit barrier layer in large-area organic light-emitting devices

Thick molybdenum trioxide (MoO3) films (ca. 120 nm) were successfully deposited by an aqueous solution-processed method based on direct dissolution of the powder of ammonium molybdate ((NH4)6Mo7O244H2O) precursors in deionized water. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) analyses showed that aqueous solution-processed MoO3 (sMoO3) has almost the same characteristics as the vacuum-evaporated MoO3 (eMoO3). Besides acting as a hole injection material, thicker MoO3 operated as a short-circuit barrier layer in large-scale organic light-emitting devices (OLEDs). The successful fabrication of 150 × 150 mm2 lighting panels confirmed the dual function of thick sMoO3 films in large-area OLEDs.

Work-function tuneable and aqueous solution-processed Cs2CO3 for high-performance polymer solar cells

The authors demonstrate an aqueous solution-processed Cs2CO3 thin film with an adjustable work function via MoO3 and/or Na2WO4 doping. The doped Cs2CO3 as a cathode interfacial layer is successfully used in poly(3-hexyl-thiophene) (P3HT)/indene-C60 bisadduct (IC60BA) heterojunction based solar cells with improved open-circuit voltage and unaffected short-circuit current density. X-ray photoelectron spectroscopy (XPS) evaluation was conducted to verify the formation of the new composites of W–O–Cs and Mo–O–Cs after doping of MoO3 and/or Na2WO4 into Cs2CO3. The change of the work function of MoO3- and/or Na2WO4-doped Cs2CO3 was further confirmed by ultraviolet photoelectron spectroscopy (UPS) measurements.

Efficient optical absorption enhancement in organic solar cells by using a 2-dimensional periodic light trapping structure

We have investigated the effects induced by periodic nanosphere arrays on the performance of organic solar cells (OSCs). Two-dimensional periodic arrays of polystyrene nanospheres were formed by using a colloidal lithography method together with plasma etching to trim down the size to various degrees on the substrates of OSCs. It is found that the devices prepared on such substrates can have improved light harvesting, resulting in as high as 35% enhancement in power conversion efficiency over that of the reference devices. The measured external quantum efficiency and finite-difference time-domain simulation reveal that the controlled periodic morphology of the substrate can efficiently increase light scattering in the device and thus enhance the absorption of incident light.