Jun-Ho Youn

Inverted quantum-dot light-emitting diodes with solution-processed aluminium–zinc oxide as a cathode buffer

We report an inverted structure of quantum-dot light emitting diodes (QLEDs) with a metal oxide as a cathode buffer. The Al doped zinc oxide (AZO) with Al concentration from 0 to 30% was used as a charge transport layer in QLEDs which were processed at the temperature of 225 °C. It is found that the conductivity of AZO decreases with increasing Al concentration, but the luminance intensity increases from 6380 to 26 700 cd m−2. The current and power efficiencies at 1000 cd m−2 increase from 3.03 to 4.63 cd A−1 and 2.75 to 3.64 lm W−1, respectively, as the Al concentration increases from 0 to 30%. The luminance intensity for the QLED with 30% AZO increases further to 31 030 cd m−2 and the current efficiency to 5.21 cd A−1 by increasing the thickness of the electron transport layer from 33 to 50 nm. It is concluded therefore that a solution processed AZO can be an effective cathode buffer for an inverted structure of QLEDs.

Optical Modeling and Analysis of Organic Solar Cells with Coherent Multilayers and Incoherent Glass Substrate Using Generalized Transfer Matrix Method

We present optical modeling and physical analysis results of thin-film organic solar cells (OSCs) based on a generalized transfer matrix method, which can calculate, with a simple matrix form, the mixed coherent and incoherent interaction of an incoherent glass substrate with other coherent layers. The spatial distribution of the electric field intensity, power density, and power dissipation are calculated in both coherent and incoherent layers with respect to the optical spacer thickness. By decomposing the power density and power dissipation into forward-propagating, backward-propagating, and their interference components, we demonstrate that the dependence of the spacer thickness on the total device reflectance plays an important role in determining the light absorption efficiency of the OSC.

Inverted quantum-dot light emitting diodes with cesium carbonate doped aluminium-zinc-oxide as the cathode buffer layer for high brightness

We report an inverted structure of quantum-dot light emitting diodes (QLEDs) with cesium carbonate (Cs2CO3) doped aluminum-zinc-oxide (AZO) as the cathode buffer. The Cs2CO3 doped AZO with Cs2CO3 blending ratios (AZO : Cs2CO3) from 4 : 1 to 2 : 1 was used as an electron transport layer in QLEDs. It is found that the conductivity of the AZO : Cs2CO3 blended solution increases with the increase in the concentration of Cs2CO3 until a particular concentration, and the luminance intensity increases sharply from 9949 to 57 350 cd m−2. It is concluded therefore that Cs2CO3 doped AZO could be a good cathode buffer for inverted QLEDs.

Effect of illumination and annealing on electrical properties of indium tin oxide/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)/poly(3-hexylthiophene)/Al device

We have studied the modification of electrical properties in poly(3-hexylthiophene) (P3HT) thin film by means of illumination and annealing. The hole mobility of P3HT was monitored by impedance spectroscopy and the density of localized states in the band gap near the Fermi level was calculated using the step-by-step method based on the space-charge-limited-current model. With the illumination, we found that the bulk trap density increased from 2.51 to 2.85×1016 cm−3 eV−1 and the hole mobility of P3HT decreased from 1.88×10−4 cm2 V−1 s−1 to 4.22×10−5 cm2 V−1 s−1. With the annealing at 70 °C for 30 min, it is found that the mobility then partially recovers to 1.18×10−4 cm2 V−1 s−1.

Effect of the active-layer thickness on the short-circuit current analyzed using the generalized transfer matrix method

The effect of the active-layer thickness on the short-circuit current of organic solar cells (OSCs) was numerically investigated using a new optical modeling approach, the generalized transfer matrix method, where the optical interaction of an incoherent glass substrate with coherent layers is analyzed with simple matrix forms. The effect of the optical-spacer thickness on the spatial distribution of the electric-field intensity, time-averaged Poynting vector, and absorptivity was calculated. By incorporating the spectral absorptance with the solar irradiance, the short-circuit current was obtained at various thicknesses of the poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) active layer to show how the variation of the P3HT:PCBM active-layer thickness affects the performance of the OSC.

Degradation of Poly(3-hexylthiophene):Methano-Fullerene [6,6]-Phenyl-C71 Butyric Acid Methyl Ester Bulk Heterojunction Solar Cells and Annealing Effect on the Degraded Devices

We have studied the degradation of poly(3-hexylthiophene):methano-fullerene [6,6]-phenyl-C71 butyric acid methyl ester (P3HT:PC71BM) bulk heterojunction BHJ solar cells and the annealing effect on the degraded devices. Firstly, the modification of electrical properties of the BHJ solar cells was monitored as a function of illumination time. The degradation of BHJ solar cell is dominated by the filling factor reduction and an irradiation to the solar cell accelerates the gradual degradation. Secondly, we found that inserting LiF between Al cathode and active layer, alleviating degradation, improves the device properties of the solar cell as well as its stability. Moreover, annealing on the degraded device helps partially recover the properties of the solar cell and the diode function. We speculated that this partial recovery caused by annealing is attributed to the rearrangement of P3HT as well as the aggregation of PC71BM.