Hua-Hsien Liao

Highly efficient inverted polymer solar cell by low temperature annealing of CS2CO3 interlayer

We demonstrate a highly efficient inverted bulk heterojunction polymer solar cell based on regioregular poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester with a low temperature annealed interfacial buffer layer, cesium carbonate (Cs2CO3). This approach improves the power conversion efficiency of the inverted cell from 2.3% to 4.2%, with short-circuit current of 11.17mA∕cm2, open-circuit voltage of 0.59V, and fill factor of 63% under AM1.5G 100mW∕cm2 irradiation. This result is comparable to the previous regular structure device on the same system. Ultraviolet photoelectron spectroscopy shows that the work function of annealed Cs2CO3 layer decreases from 3.45to3.06eV. Further x-ray photoelectron spectroscopy results reveal that Cs2CO3 can decompose into low work function, doped cesium oxide Cs2O upon annealing, which is accountable for the work-function reduction and device efficiency improvement.

Dynamics and reversibility of oxygen doping and de-doping for conjugated polymer

We perform comprehensive long-time monitoring of the p-doping and de-doping of poly(3-hexyl thiophene) under changing external conditions of oxygen, light, and temperature. They are shown to be controlled by the complex adsorption and desorption process with time scales ranging from seconds to weeks. The oxygen doping at atmospheric pressure takes several hours in the dark. The doping is dramatically accelerated to be within seconds with light of wavelength of 500–700 nm. Even at low oxygen pressure of 10−4 torr doping occurs within minutes with light. The de-doping by oxygen desorption takes as long as weeks at room temperature and vacuum of 10−4 torr, but when the temperature is raised to near the polymer glass temperature of 370 K, the de-doping is accelerated to minutes as the enhanced chain motion releases the trapped oxygen. Even though visible and near infrared light causes very efficient doping within seconds or minutes depending on vacuum level, such light-induced doping is not a chemical reactio...

General method to solution-process multilayer polymer light-emitting diodes

An intermediate liquid buffer layer is introduced to overcome the dissolution problem of solution-processed multilayer conjugated polymer light-emitting diodes. This method can be applied to arbitrary combinations of polymers with no restriction on solvents. As an example, a hole-blocking layer is successfully spin coated on the common p-type emissive polymer layers. One green- and two blue-emitting polymers are chosen as the emissive layers. The electron-hole balance and efficiency are significantly improved by the addition of hole-blocking layer. The electroluminescence efficiency can be increased up to nine times, while the luminance up to seven times. In particular, 1.5cd∕A is obtained for deep blue emission from poly(9,9-dioctyl-fluorene) with 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene spin coated as the hole-blocking material.

Integration of polymer light-emitting diode and polymer waveguide on Si substrate

We integrate a polymer light-emitting diode (PLED) and a polymer waveguide on a Si substrate. The light emitted from the PLED is coupled to the waveguide by a diffuser and a reflection layer with coupling efficiency about 1%. There is no delay nor distortion between PLED emission and the light propagation in the waveguide. Good direct modulation characteristics of the waveguide output are demonstrated up to 200kHz. The device structure and processes are based on easy spin coating and are compatible to Si technology.

Current injection and transport in polyfluorene

A comprehensive numerical model is established for the electrical processes in a sandwich organic semiconductor device with high carrier injection barrier. The charge injection at the anode interface with 0.8eV energy barrier is dominated by the hopping among the gap states of the semiconductor caused by disorders. The Ohmic behavior at low voltage is demonstrated to be not due to the background doping but the filaments formed by conductive clusters. In bipolar devices with low work function cathode it is shown that near the anode the electron traps significantly enhance hole injection through Fowler-Nordheim tunneling, resulting in rapid increases of the hole carrier and current in comparison with the hole-only devices.

High-efficiency blue multilayer polymer light-emitting diode based on poly(9,9-dioctylfluorene)

A highly efficient blue polymer light-emitting diode based exclusively on commercial poly(9,9-dioctylfluorene) and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-s-butylphenyl)) diphenylamine)] is demonstrated. High electroluminescent efficiency is achieved by enhancing electron currents and making devices in multilayered structures. CsF∕Al is used as the efficient electron injection cathode, and the fabrication process is in the glove box to enhance electron mobility by reducing oxygen adsorption. The multilayer structure is prepared by the liquid buffer layer technique. The maximum efficiency is 2.5 cd∕A at deep blue with the corresponding external quantum efficiency of 2%.

Large enhancement of intersystem crossing in polyfluorenes by iridium-complex doping

Photoinduced absorption is used to study the intersystem crossing (ISC) rate of two polyfluorenes doped with iridium (III) tris[2-(4-totyl)pyridinato-N,C2]. It is found that the triplet exciton lifetime of polyfluorene is reduced by the dopants. But instead of decreasing, the population density of polyfluorene triplet exciton increases by almost one order of magnitude. The finding shows that the ISC rate can increase over 100 times due to the spin-orbital interaction with the Ir ion even though it is covered by the ligands. Specifically, the ISC lifetime changes from the intrinsic value of 62ns to as short as 0.28ns upon 10% doping.