Franky So

Dithienogermole as a fused electron donor in bulk heterojunction solar cells.

We report the synthesis and bulk heterojunction photovoltaic performance of the first dithienogermole (DTG)-containing conjugated polymer. Stille polycondensation of a distannyl-DTG derivative with 1,3-dibromo-N-octyl-thienopyrrolodione (TPD) results in an alternating copolymer which displays light absorption extending to 735 nm, and a higher HOMO level than the analogous copolymer containing the commonly utilized dithienosilole (DTS) heterocycle. When polyDTG-TPD:PC(70)BM blends are utilized in inverted bulk heterojunction solar cells, the cells display average power conversion efficiencies of 7.3%, compared to 6.6% for the DTS-containing cells prepared in parallel under identical conditions. The performance enhancement is a result of a higher short-circuit current and fill factor in the DTG-containing cells, which comes at the cost of a slightly lower open circuit voltage than for the DTS-based cells.

Degradation mechanisms in small-molecule and polymer organic light-emitting diodes.

Degradation in organic light-emitting diodes (OLEDs) is a complex problem. Depending upon the materials and the device architectures used, the degradation mechanism can be very different. In this Progress Report, using examples in both small molecule and polymer OLEDs, the different degradation mechanisms in two types of devices are examined. Some of the extrinsic and intrinsic degradation mechanisms in OLEDs are reviewed, and recent work on degradation studies of both small-molecule and polymer OLEDs is presented. For small-molecule OLEDs, the operational degradation of exemplary fluorescent devices is dominated by chemical transformations in the vicinity of the recombination zone. The accumulation of degradation products results in coupled phenomena of luminance-efficiency loss and operating-voltage rise. For polymer OLEDs, it is shown how the charge-transport and injection properties affect the device lifetime. Further, it is shown how the charge balance is controlled by interlayers at the anode contact, and their effects on the device lifetime are discussed.

Low-Voltage, Low-Power, Organic Light-Emitting Transistors for Active Matrix Displays

Efficient organic light-emitting transistors use carbon nanotubes as the source electrode. Intrinsic nonuniformity in the polycrystalline-silicon backplane transistors of active matrix organic light-emitting diode displays severely limits display size. Organic semiconductors might provide an alternative, but their mobility remains too low to be useful in the conventional thin-film transistor design. Here we demonstrate an organic channel light-emitting transistor operating at low voltage, with low power dissipation, and high aperture ratio, in the three primary colors. The high level of performance is enabled by a single-wall carbon nanotube network source electrode that permits integration of the drive transistor and the light emitter into an efficient single stacked device. The performance demonstrated is comparable to that of polycrystalline-silicon backplane transistor-driven display pixels.

Metal oxides for interface engineering in polymer solar cells

Significant progress in power conversion efficiencies and stabilities of polymer solar cells has been achieved using semiconducting metal oxides as charge extraction interlayers. Both n- and p-type transition metal oxides with good transparency in the visible as well as infrared region make good Ohmic contacts to both donors and acceptors in polymer bulk heterojunction solar cells. Their compatibility with roll-to-roll processing makes them very attractive for low cost manufacturing of polymer solar cells. In this review, we will present the recent results on synthesis and characterization of these metal oxides along with the device performance of the solar cells using these metal oxides as interlayers.

Effects of triplet energies and transporting properties of carrier transporting materials on blue phosphorescent organic light emitting devices

We have studied the effects of the hole transporting layers and electron transporting layers on the device efficiencies of iridium(III) bis[(4,6-di-fluorophenyl)-pyridinato-N,C2′] picolinate (FIrpic) doped 3,5′−N,N′-dicarbazole-benzene (mCP) host blue phosphorescent organic light emitting diodes. We found that the device efficiency is very sensitive to the hole transporting materials used and both the triplet energy and carrier transport properties affect the device efficiency. On the other hand, there is no apparent correlation between the device efficiency and the triplet energy of the electron transporting material used. Instead, the device efficiency is affected by the electron mobility of the electron transporting layer only.

High efficiency blue phosphorescent organic light-emitting device

We have demonstrated a substantial enhancement in the efficiency of iridium (III) bis[(4,6-di-fluorophenyl)-pyridinate-N,C2′]picolinate based blue phosphorescent organic light-emitting devices (PHOLEDs). The efficiencies of PHOLEDs with conventional electron transport materials are low due to their low electron mobilities as well low triplet energies. High triplet energy electron transporting material with high electron mobility was used as a hole blocker to achieve efficient exciton confinement and good charge balance in the device thereby achieving a high current efficiency of 49cd∕A and an external quantum efficiency of 23%.

Quasi-epitaxial growth of organic multiple quantum well structures by organic molecular beam deposition

Multiple quantum well structures consisting of alternating layers of two crystalline organic semiconductors, namely, 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA) and 3,4,7,8 naphthalenetetracarboxylic dianhydride (NTCDA), have been grown by organic molecular beam deposition. The individual layer thicknesses in the multilayer samples were varied from 10 to 200 A. X‐ray diffraction and birefringence data show that there is a strong structural ordering in all layers, as well as across large spatial distances along the sample surface. Thus, the growth is ‘‘quasi‐epitaxial’’ even though the PTCDA and NTCDA crystal structures are incommensurate. From the optical absorption spectra, it was found that the lowest energy PTCDA singlet exciton line shifts to higher energy with decreasing layer thickness. Comparison of these results with a quantum mechanical model based on exciton confinement in the PTCDA layers is proposed to describe the energy shift.

The effect of molybdenum oxide interlayer on organic photovoltaic cells

Both small molecule and polymer photovoltaic cells were fabricated with molybdenum oxide interlayer at the indium tin oxide electrode. Enhancement in power efficiencies was observed in both small molecule and polymer cells. Specifically, the power conversion efficiencies of small molecule cells with the molybdenum oxide interlayer were enhanced by a maximum of 38% due to a significant enhancement in the fill factor. The improved fill factor is attributed to the reduction in series resistance. Our ultraviolet photoemission spectroscopy data indicate that the formation of band bending and the built-in field at the interface due to the interlayer leads to enhancement in hole extraction from the photoactive layer toward the anode.

Recent progress in solution processable organic light emitting devices

Organic light emitting devices (OLEDs) have been the subject of intense research because of their potential for flat panel display and solid state lighting applications. While small molecule OLEDs with very high efficiencies have been demonstrated, solution processable devices are more desirable for large size flat panel display and solid state applications because they are compatible with low cost, large area roll-to-roll manufacturing process. In this review paper, we will present the recent progress made in solution processable OLEDs. The paper will be divided into three parts. In the first part of the paper, we will focus on the recent development of fluorescent polymer OLEDs based on conjugated polyfluorene copolymers. Specifically, we will present results of carrier transport and injection measurements, and discuss how the charge transport and injection properties affect the device performance. In the second part of the paper, we will focus on the recent progress on phosphorescent dye-dispersed nonc...

Highly efficient white organic light-emitting diode

We present a highly efficient white electroluminescence device by the combination of a solution processed blue organic phosphorescence light-emitting diode with appropriate down-conversion phosphor system. The use of this down-conversion system produced an extraordinary enhancement on device performance, resulting in a white electroluminescence device with luminance efficacy of 25lm∕W at luminance efficiency reaching 39cd∕A. The extraordinary enhancement on device performance is attributed to isotropic radiation pattern of the excited phosphor particles, leading to high light extraction properties.