Tien-Lung Chiu

High Photoelectric Conversion Efficiency of Metal Phthalocyanine/Fullerene Heterojunction Photovoltaic Device

This paper introduces the fundamental physical characteristics of organic photovoltaic (OPV) devices. Photoelectric conversion efficiency is crucial to the evaluation of quality in OPV devices, and enhancing efficiency has been spurring on researchers to seek alternatives to this problem. In this paper, we focus on organic photovoltaic (OPV) devices and review several approaches to enhance the energy conversion efficiency of small molecular heterojunction OPV devices based on an optimal metal-phthalocyanine/fullerene (C60) planar heterojunction thin film structure. For the sake of discussion, these mechanisms have been divided into electrical and optical sections: (1) Electrical: Modification on electrodes or active regions to benefit carrier injection, charge transport and exciton dissociation; (2) Optical: Optional architectures or infilling to promote photon confinement and enhance absorption.

Orthogonally Substituted Benzimidazole-Carbazole Benzene As Universal Hosts for Phosphorescent Organic Light-Emitting Diodes

The novel ambipolar hosts of o-CbzBz and o-DiCbzBz contain carbazole and benzimidazole through an ortho-connection. The orthogonal conformations cause the triplet state to be confined at the carbazole units to secure efficient energy transfer. The phosphorescent organic light-emitting diodes (PhOLEDs) show a high current efficiency, power efficiency, and low efficiency roll-off. o-DiCbzBz can be used as a host for sky-blue, green, and orange-red PhOLEDs, giving 57.5, 78.4, and 60.3 cd/A, respectively.

Novel Benzimidazole Derivatives as Electron-Transporting Type Host To Achieve Highly Efficient Sky-Blue Phosphorescent Organic Light-Emitting Diode (PHOLED) Device

The development of benzimidazole substituted biphenyls as electron-transporting hosts for bis[2-(4,6-difluorophenyl)pyridinato-C(2),N](picolinato)iridium(III) is reported. Under the optimized conditions, the organic light-emitting diode (OLED) achieves the maximum current efficiency of 57.2 cd/A, power efficiency of 50.4 lm/W, and external quantum efficiency 25.7%.

Novel ambipolar orthogonal donor-acceptor host for blue organic light emitting diodes.

Ambipolar triplet hosts comprising 1,2,4-triazole and carbazole in ortho-positions have been developed. The blue PHOLED has a high current efficiency of 47.1 cd A(-1), power efficiency of 41.2 lm W(-1), and low efficiency roll-off. The high efficiency was attributed to the successful control of π-conjugation through orthogonal arrangement of the substituents so that a wide T1-S0 gap could be maintained.

Quantum efficiency enhancement in selectively transparent silicon thin film solar cells by distributed Bragg reflectors.

This work demonstrated a-Si:H thin-film solar cells with backside TiO(2)/ SiO(2) distributed Bragg reflectors (DBRs) for applications involving building-integrated photovoltaics (BIPVs). Selectively transparent solar cells are formed by adjusting the positions of the DBR stop bands to allow the transmission of certain parts of light through the solar cells. Measurement and simulation results indicate that the transmission of blue light (430 ~500 nm) with the combination of three DBR mirrors has the highest increase in conversion efficiency.

Oxadiazole host for a phosphorescent organic light-emitting device

In this paper, we demonstrate a phosphorescent organic light-emitting device (OLED) with enhanced current efficiency (in terms of cd/A) based on an oxadiazole (OXD) derivative as the electron-transporting host of the emitting layer (EML) doped with a phosphorescent dopant, iridium(III)bis[4,6-(di-fluorophenyl)-pyridinato-N, C2′] picolinate (FIrpic). The maximum current efficiency of OXD-based OLEDs was 13.0 cd/A. Compared to the phosphorescent OLED with a conventional hole-transporting host, 1,3-bis(carbazol-9-yl)benzene (mCP) with 11.1 cd/A in maximum current efficiency, 17.2% improvement was achieved. However, in terms of external quantum efficiency (EQE), the OXD- and mCP-based OLEDs were 4.01 and 4.66%, respectively, corresponding to a 13.9% decrease. Such a discrepancy can be understood from the electroluminescence (EL) variation. Contrary to the hole-transporting mCP, OXD exhibited electron transporting characteristics which shifted the recombination zone toward the anode. The optical interference e...

Optical and electrical characteristics of Ag-doped perylene diimide derivative

In this paper, a highly conductive and strongly absorptive organic thin film by doping Ag into N,N′-bis (2,6-di-isopropylphenyl)-1,7-bis (4-methoxy-phenyl) perylene-3,4,9,10-tetracarboxydiimide (MPPDI) was demonstrated. Strong absorption resulted from the broadband absorption of MPPDI at visible range and plasmon-enhanced absorption around 420 nm of Ag nanoparticles. Ag dopants in MPPDI acted as quenchers, which resulted in a dramatic decrease in photoluminescence intensity of MPPDI. Besides, J-V characteristics of Ag:MPPDI thin film changed from trapped-charge-limited current to Ohmic conduction with increasing Ag concentrations. Conductivity of 1.15×10−6u2002Ωu2009cm was achieved when MPPDI/Ag=5:1. This organic thin film has potential applications for low-reflectance organic light-emitting diode and organic photovoltaic device.

Modification of silver anode and cathode for a top-illuminated organic photovoltaic device

We have demonstrated a top-illuminated organic photovoltaic device with a thick Ag anode and a thin Ag cathode capped with an α-naphthylphenylbiphenyl diamine (NPB) thin film. The surface of the Ag anode was oxidized by UV–ozone which improved the carrier collection and reduced the exciton quenching. Compared with the control device with an indium tin oxide anode, a 15.59 times reduction in the serial resistance and a 1.72 times increase in the shunt resistance were observed with a fill factor of 0.61 in such a device. The NPB capping layer not only improved the light transmission from the semitransparent cathode, but also hindered the formation of Ag island growth and thereby improved the device stability.

Absorptive and conductive cavity cathode with silver nanoparticles for low-reflection organic light-emitting devices

We have successfully fabricated a low-reflection organic light-emitting diode (LR-OLED) by replacing the highly reflective Al cathode of a conventional OLED with a Fabry?P?rot (FP) cavity cathode, which is simultaneously responsible for absorption, plasmonic absorption and destructive interference. The FP cavity cathode consisted of a front semi-transparent double thin metal layer (Al/Ag), an inserted organometallic black layer (BL) in the middle and a highly reflective Al mirror. The organometallic BL contained a high-mobility electron transport and broadband absorptive organic matrix, N,N-bis(2,6-diisopropylphenyl)-1,7-bis(4-methoxy-phenyl)perylene-3,4,9,10-tetracarboxydiimide (MPPDI), and Ag-nanoparticle (NP) dopants which contributed not only to the plasmonic absorption but also to the metallic conductivity. By adjusting the thickness and Ag-NP concentration of the organometallic BL, one can optimize the destructive interference cavity effect. LR-OLEDs fabricated using the aforementioned characteristics of the Ag-NP yielded superior electrical performance and low reflection across almost the entire visible spectrum. With the exemption of surface reflection (air/glass ~4%), a lowest reflection of 0% near 750?nm and an average reflection of 1.39% for the entire visible spectrum were obtained for a LR-OLED with a 65?nm organometallic BL (mixing ratio of MPPDI?:?Ag = 10?:?1). With the additional structural cavity cathode, the LR-OLEDs nonetheless exhibited similar electrical performances and continuous operational lifetimes to those of control devices with a traditional highly reflective Al cathode.

Exciplex-Sensitized Triplet-Triplet Annihilation in Heterojunction Organic Thin-Film

A new concept for organic light-emitting diodes (OLEDs) is presented, which is called exciplex-sensitized triplet-triplet annihilation (ESTTA). The exciplex formed at the organic heterojunction interface of 4,4,4″-tris(N-3-methyphenyl-N-phenyl-amino) triphenylamine and 9,10-bis(2-naphthyl) anthracene (ADN) is used to sensitize the triplet-triplet annihilation (TTA) process on the ADN molecules. This results in a turn-on voltage (2.2 V) of the blue emission from the OLED below the bandgap (2.9 eV). From the transient electroluminescence measurement, blue emission totally came from the TTA process without direct recombination on the ADN molecules. The blue singlet exciton from the TTA process can be quenched by energy transfer to the exciplex, as revealed by transient photoluminescence measurements. This can be prevented by blocking the energy transfer path and improving the radiative recombination rate of blue emission. With the insertion of the triplet diffusion and singlet blocking (TDSB) layer and the incorporation of the dopant material, an ESTTA-OLED with external quantum efficiency of 5.1% was achieved, which consists of yellow and blue emission coming from the exciplex and ESTTA process, respectively.