J. H. Shieh

The effect of carrier mobility in organic solar cells

The microscopic states and performance of organic solar cell are investigated theoretically to explore the effect of the carrier mobility. With Ohmic contacts between the semiconductor and the metal electrodes there are two origins of carriers in the semiconductor: the photocarriers generated by photon absorption and the dark carriers diffused from the electrodes. The power efficiency of the solar cell is limited by the recombination of a carrier with either the photocarrier or a dark carrier. Near the short-circuit condition the photocarrier recombination in the semiconductor bulk decreases as the mobility increases. Near the open-circuit condition the dark carrier recombination increases with the mobility. These two opposite effects balance with one another, resulting in an optimal mobility about 10−2 cm2/V s which gives the highest power conversion efficiency. The balance of the electron and hole mobilities are not necessary to maintain the optimal efficiency also because of the balance of the photocar...

Polymer photodetector with voltage-adjustable photocurrent spectrum

Polymer photodetectors with voltage-adjustable photoresponse from visible to near infrared range are demonstrated. Poly(3-hexylthiophene) and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) blend is used as the active layer. The photoresponse can be continuously adjusted by the thickness of the active layer as well as the applied voltage bias. The thickness of the active layer is varied from 250 nm to 16.2 μm. The mechanism for the photoresponse adjusted by the thickness can be attributed to the absorption of the photons in the infrared range by thick PCBM layer. The mechanism for the photoresponse adjusted by the applied bias can be attributed to the carrier recombination reduction when the applied bias increases. The adjustable photodetector also has high operating speed up to 10 kHz.

Coexistence of antiferromagnetic order and superconductivity with TN higher than Tc in DyNi2B2C

Abstract A superconducting transition temperature T c of 3.8 K with an onset as high as 5.2–5.5 K was reported for quaternary borocarbide DyNi 2 B 2 C with a high antiferromagnetic Neel temperature T N around 10.1–10.8 K. No reentrant behavior was observed down to 0.3 K demonstrating a perfect coexistence between superconductivity and antiferromagnetic order. A superconducting upper critical field H c2 (0) of 3.0 kG, lower critical field H c1 (0) of 200 G and Ginzburg-Landau parameter κ of 3.7 were derived. A broad T c transition points to a possible small T c varying homogeneity range of DyNi 2 − α B 2 − β C 1 − δ in the phase diagram. The magnetic entropy S m of 10.3 J/mol K or 90% R ln 4 indicates antiferromagnetic ordering of Dy 3+ with a quadruplet ground state in the tetragonal crystal field. Two field-induced weak-ferromagnetic-like transitions with a field of around 1 T were observed at 2 K.

Ionic-size effect on the structure and Tc of T′-(R1−xR′x)1.85Ce0.15CuO4 (R Pr, Nd, Sm and Eu; R′ Gd and Y)

Abstract (R 1− x R′ x ) 1.85 Ce 0.15 CuO 4 (R  Pr, Nd, Sm and Eu; R′  Gd and Y) have been investigated by means of X-ray diffraction, electrical-resistivity, and magnetic-susceptibility measurements. Lattice parameters a , c , and the unit-cell volume V decrease with increasing Gd or Y concentration x , and the decreasing rates with Y are larger than those with Gd. It is found that the critical concentration x c , at which the superconductivity disappears, depends on the ionic size of R, and the T c suppression rate for each system with Y doping is larger than that with Gd doping. Thus the effect of a small ionic radius for Y plays a much more important role than the large magnetic moment for Gd on the suppression of superconductivity in (R 1− x R′ x ) 1.85 Ce 0.15 CuO 4 . From the observations of the deviation of the lattice parameters in R 1.85 Ce 0.15 CuO 4 and the rapid increase of | dln T c /d x | in (Eu 1− x Y x ) 1.85 Ce 0.15 CuO 4 , the structural distortion boundar estimated to lie between Eu 1.85 Ce 0.15 CuO 4 and Gd 1.85 Ce 0.15 CuO 4 . This provides clear evidence that the absence of superconductivity for Gd 1.85 Ce 0.15 CuO 4 is due to weak ferromagnetism in the CuO planes resulting from the too small ionic radius of Gd, which induces a lattice distortion in the T′ structure.

Superconductivity and magnetic ordering in the (Nd1−xGdx)1·85Ce0·15CuO4−δ System

Abstract Superconducting and magnetic ordering temperatures for the (Nd 1−x Gd x ) 1.85 Ce 0.15 CuO 4−δ were reported. Superconducting appears only in the metallic region with metal-insulator transition around x = 0.65 in this oxygen-reduced system. Superconducting transition temperature T c decreases from 24–31 K for x = 0.0 to 10 K for x = 0.6 and no transition observed above 2 K for x = 0.65. In the insulator region (x ≥ 0.7), complex magnetic structures with weak ferromagnetic (canted antiferromagnetic) Curie temperature T C (Cu) of the Cu spins around 158–162 K and low temperature (

Magnetic behavior in Pr-containing Tl- and Pb-based cuprates

Abstract Magnetic ordering among Pr ions occurs at a relatively high temperature of 17 K in PrBa2Cu3O7, but is absent in Pr2CuO4 and Bi2Sr2PrCu2O8 above 1.6 K. This work extends the study of magnetic properties to other Pr-containing cuprates including TlBa2PrCu2O7, (Pb.6Cu.4)Sr2PrCu2O7 and Pb2Sr2PrCu3O8. Low temperature calorimetric and SQUID measurements show broad peaks in C and dχ/dT in all cases. The onset temperatures are near 10 K, while the maxima in C as well as minima in dχ/dT prevail at about 8 to 5 K. The heat capacity data provides additional information concerning the entropy associated with the magnetic anomalies and large contributions seemingly linear with temperature.

Magnetic ordering of Pr in Pb2Sr2PrCu3O8

The magnetic ordering of the Pr ions in Pb2Sr2PrCu3O8 has been studied using neutron‐diffraction and ac‐suspectibility measurements. An imperfect three‐dimensional magnetic ordering of the Pr spins was observed at a temperature well below its Neel temperature of TN≊7 K. The magnetic intensities observed at T=1.4 K can be explained by assuming long‐range order in the ab plane with short‐range correlations, of correlation length ξc=20 A, along the c axis and a moment directed along the c‐axis direction. The magnetic ordering is 2D in nature, and the basic magnetic structure consists of nearest‐neighbor spins that are aligned antiparallel along all three crystallographic directions. The magnetic transition is also evident in the ac‐susceptibility versus temperature measurements, where a cusp that is typical of antiferromagnetic ordering is clearly observed, which matches the TN obtained by neutron diffraction.

Comparative studies of Gd-ordering in various cuprate systems

Abstract Gd-ordering near 2.2 K in both superconducting and nonsuperconducting GdBa2Cu3O7−y has been well documented. This report summarizes our results from comparative studies on other Gd-containing cuprates (with well-defined Ne´el temperatures in parentheses) including Bi2Sr2GdCu2O8 (1.6 K), TlBa2GdCu2O7 (2.2 K), TlSr2GdCu2O7 (2.4 K), (Pb.5Cu.5)Sr2GdCu2O7 (2.3 K), Pb2Sr2GdCu3O8 (2.3 K), Gd2CuO4 (6.6 K) and (Gd1.85Ce0.15)CuO4 (5.3 K). The Ne´el temperature values for the Bi-, Tl-, and Pb-based cuprates remain close to that for PrBa2Cu3O7−y having similar layered structures but somewhat different Gd-Gd distance along the c-axis, whereas the T′ (214) compounds become ordered at higher temperatures. Also to be noted is the general occurrence of a broad shoulder below the heat capacity peak, which needs to be taken into account for obtaining the expected magnetic entropy ΔS=Rln8 associated with the ordering.

SYSTEMATIC VARIATION OF TN(PR) FOR THE TWO-CUO2-LAYER CUPRATE M212 (M=1, 2, 3) SYSTEMS

The systematic variation of anomalous Pr antiferromagnetic ordering temperature T N (Pr) for the two‐CuO2‐layer cuprate system MA2PrCu2O y (M=Cu, Hg, Tl, Pb; A=Ba, Sr) was verified through the observation of a new Hg‐1212 HgSr2PrCu2O6+δ tetragonal compound with a T N (Pr) of 6 K and a Pr—O bond length of 2.492 A between Pr and oxygen in the adjacent CuO2 layers. For the tetragonal/orthorhombic 1212 compounds including the PrBa2Cu3O7−y system, T N (Pr) was found to decrease monotonically with increasing Pr—O distance. The importance of Pr‐O‐Pr superexchange magnetic coupling is expected for all two‐CuO2‐layer M m A2PrCu2O y m212‐type (m=1, 2, 3) compounds. A similar Pr anomaly in the 2212‐type system was first observed on the new tetragonal (Pb0.5Cu0.5)2(Ba0.5Sr0.5)2PrCu2O8 compound with a high T N (Pr) of 9 K.

FIELD DEPENDENCE OF MAGNETIC TRANSITIONS FOR MAGNETIC SUPERCONDUCTORS RNI2B2C (R=DY, HO)

High field (up to 5 T) magnetization measurements were performed on both bulk and field‐aligned powder samples of RNi2B2C (R=Dy, Ho) magnetic superconductors. For Tc=3.8 K, a DyNi2B2C superconductor with antiferromagnetic Neel temperature TN of 10–11 K, and anomalous field‐induced weak‐ferromagnetic transitions were observed above the superconducting upper critical field Hc2(2 K) of 1.75 kG. The aligned powders show distinct hysteresis characteristics for both fields applied parallel and perpendicular to the tetragonal basal plane. For Tc=7.8 K, HoNi2B2C has an incommensurate magnetic ordering temperature Tm of 6 K and TN of 5.2 K. Anomalous field‐induced weak‐ferromagnetic transitions were also observed below TN. No hysteresis can be detected for TN