Improved Carrier Transport Properties Research Articles

Intrinsic carrier mobility limits in the transparent bipolar semiconductor CuInO2

The delafossite semiconductor CuInO2 has shown great potential in transparent electronics for its bipolar dopability. However, little is known about the limiting factors about its carrier mobility, which impedes its further development. Applying abinitio Boltzmann transport formalism, here we calculate the intrinsic, phonon-limited carrier mobility of CuInO2 and study its carrier–phonon coupling mechanisms. The calculated room-temperature electron and hole mobilities along the in-plane direction are μe=97.6 cm2V−1s−1 and μh=1.4 cm2V−1s−1, respectively. We find that the electron mobility is limited by the combination of acoustic phonons and polar longitudinal optical (LO) phonons, while the hole mobility is mainly limited by carrier–acoustic phonon scattering. We further show that the electron effective mass and bandgap of CuInO2 can be tuned through strain engineering for improved carrier transport properties. Our work uncovers the underlying factors that govern the intrinsic carrier mobility of the transparent bipolar semiconductor CuInO2 and sheds light on the design and exploration of bipolar conducting transparent conductive oxides (TCOs) based on delafossite semiconductors.

Achieving thermoelectric performance of rGO/Bi0.5Sb1.5Te3/Cu2Se1–xTex composites through the scattering engineering strategy

Bismuth antimony telluride (Bi2–xSbxTe3) is commonly used for thermoelectric generation at temperatures near ambient temperature. Here, we report incorporating reduced graphene oxide (rGO) and Cu2Se0.98Te0.02 into theBi0.5Sb1.5Te3 (BST) (rGO/Bi0.5Sb1.5Te3-xCu2Se0.98Te0.02, where x = 0.0%, 0.1%, 0.3%, 0.5% and 1.0%, in mass) synthesized by a solid-state technique. The dispersion of rGO and Cu2Se0.98Te0.02 into the BST matrix improved carrier transport properties at the grain boundary interfaces and reduced thermal conductivity. Strong electron scattering at large interface barriers was responsible for increased electrical conductivity. The bulk sample of rGO/BST-0.3% Cu2Se0.98Te0.02 (in mass) possessed a low thermal conductivity of 0.76 W·m−1·K−1 at 497 K. Enhanced phonon scattering at grain boundaries between BST and rGO/Cu2Se0.98Te0.02 caused a low thermal conductivity. At 448 K, the highest zT value for rGO/BST-0.3%Cu2Se0.98Te0.02 (in mass) was 1.64, which is 37% higher than the zT value for pure BST (zT = 1.19). Results suggested that incorporating rGO and Cu2Se0.98Te0.02 into the BST matrix effectively improved thermoelectric power generation.

Enhanced Thermoelectric Performance of Lightly Pb-Doped Sb2Te3 Polycrystalline Alloys for Power Generation in Midtemperature Range

Sb2Te3 alloys are promising thermoelectric materials because of their outstanding electrical transport properties in the midtemperature range of 500–700 K, while codoping with multiple elements has been successful to improve their thermoelectric performance. In this study, enhanced thermoelectric properties with a maximum thermoelectric figure of merit of 0.97 are reported for singly and lightly Pb-doped Sb2Te3 polycrystalline alloys (Sb2– x Pb x Te3). Very light Pb doping in the range 0.005 ≤ x ≤ 0.0125 in the Sb2– x Pb x Te3 alloys yielded significantly improved carrier transport properties and increased electrical conductivity while the Seebeck coefficient is decreased moderately, since the density-of-state effective mass is improved much. As a result, power factor for the Pb-doped Sb2Te3 is largely increased up to 3.7 mW/mK2 at 300 K. The lattice thermal conductivity decreased considerably owing to the additional point defect phonon scattering by the Pb despite slight doping. Consequently, a maximum thermoelectric figure of merit of 0.97 was obtained for Sb1.9875Pb0.0125Te3 ( x = 0.0125 ) at 600 K, which is the highest reported value for singly doped Sb2Te3-based alloys. A maximum energy conversion efficiency was calculated to be 9.0% for a temperature difference of 350 K, which is higher than that for other singly or codoped Sb2Te3 alloys.

Iodine ion modification enables Ag nanowire film with improved carrier transport properties and stability as high-performance transparent conductor

Iodine ion modification enables Ag nanowire film with improved carrier transport properties and stability as high-performance transparent conductor

Improved carrier transport properties by I-doping in n-type Cu0.008Bi2Te2.7Se0.3 thermoelectric alloys

The addition of Cu becomes essential in polycrystalline n-type Bi2(Te,Se)3-based alloys since it is known to enhance stability of carrier transport properties as well as thermoelectric performance. However, a way to further optimize is necessary owing to the limited controllability of transport parameters by Cu addition. Herein, we present improved carrier transport properties via I-doping in Cu0.008Bi2Te2.7Se0.3. Weighted mobility and effective mass are increased simultaneously by small amount doping of I at Te/Se-site. As a result, ~15% increased power factor and high average thermoelectric figure of merit (zT) of 0.79 were obtained in a wide temperature range.

Carrier Transport in CH3NH3PbI3 Films with Different Thickness for Perovskite Solar Cells

The typical broad absorption features have enabled halide perovskite to be a promising candidate of the next generational solar cell materials. However, the fundamental properties, upon which the photoelectric performance of perovskite device is based, are currently still not clear. Herein, the photovoltaic efficiencies in perovskite films with various thicknesses have been investigated to reveal a direct correlation between internal structure factors, such as crystal orientation, grain size, and photoelectric performance of perovskite films. It is found that the photovoltaic efficiency of perovskite films, especially with the optimal thickness around 300 nm, is significantly increased, which can be ascribed to the improved carrier transport properties resulting from the preferred crystal structure. When the film thickness diverges from 300 nm, the extra charge recombination with decreasing mobility leads to the reduction of photovoltaic efficiency again in perovskite solar cells. These results demonstrate crystal structure as one of the decisive roles in device properties, which are helpful to improve photovoltaic performance of perovskite solar cells.

Improved Mobility and Bias Stability of Thin Film Transistors Using the Double-Layer a-InGaZnO/a-InGaZnO:N Channel.

The amorphous indium-gallium-zinc oxide (a-IGZO) thin film transistors (TFTs) were demonstrated based on a double-layer channel structure, where the channel is composed of an ultrathin nitro-genated a-IGZO (a-IGZO:N) layer and an undoped a-IGZO layer. The double-layer channel device showed higher saturation mobility and lower threshold-voltage shift (5.74 cm2/Vs, 2.6 V) compared to its single-layer counterpart (0.17 cm2/Vs, 7.23 V). The improvement can be attributed to three aspects: (1) improved carrier transport properties of the channel by the a-IGZO:N layer with high carrier mobility and the a-IGZO layer with high carrier concentration, (2) reduced interfacial trap density between the active channel and the gate insulator, and (3) higher surface flatness of the double-layer channel. Our study reveals key insights into double-layer channel, involving selecting more suitable electrical property for back-channel layer and more suitable interface modification for active layer. Meanwhile, room temperature fabrication amorphous TFTs offer certain advantages on better flexibility and higher uniformity over a large area.

The influence of sequential ligand exchange and elimination on the performance of P3HT:CdSe quantum dot hybrid solar cells

We report on a sequential ligand exchange and elimination process for the fast and easy surface modification of CdSe quantum dots (QDs) in order to improve the electronic interaction between poly(3-hexylthiophene) (P3HT) and CdSe QDs in P3HT:CdSe hybrid solar cells. We systematically investigated the influence of surface treatment on the insulating ligand shell of CdSe QDs using 1H-NMR analysis, and correlated their influence on the photovoltaic properties of P3HT:CdSe hybrid solar cells. A decrease in the average thickness of the ligand shells directly improved carrier transport properties. Moreover, the presence of remnant 1-hexylamine ligands provided efficient surface trap passivation. As a result, overall solar cell performance (especially fill factor and power conversion efficiency) was enhanced and the recombination mechanism was dominated by monomolecular recombination due to enhanced carrier collection length (lC0).

Electrical transport and optical model of GaAs-AlInP core-shell nanowires

GaAs nanowires were passivated by AlInP shells grown by the Au-assisted vapor-liquid-solid method in a gas source molecular beam epitaxy system. Transmission electron microscopy confirmed a core-shell GaAs-AlInP structure. Current-voltage measurements on ensemble nanowires indicated improved carrier transport properties in the passivated nanowires as compared to their unpassivated counterpart. Similarly, individual nanowires showed improved photoluminescence intensity upon passivation. A detailed model is presented to quantify the observed improvements in nanowire conduction and luminescence in terms of a reduction in surface charge trap density and surface recombination velocity upon passivation. The model includes the effects of high-level injection, bulk recombination, and surface recombination. The model can be used as a tool for assessing various passivation methods.

Lg=100 nm InAs PHEMTs on InP substrate with record high frequency response

A report is presented on 100 nm and 200 nm InAs PHEMTs on an InP substrate with a record fT performance. This result was obtained by reducing a parasitic delay associated with the extrinsic gate capacitances of the device, as well as by using an InAs sub-channel to improve carrier transport properties. In particular, a 100 nm InAs PHEMT exhibits excellent performance, such as gm,max=2 S/mm, fT=421 GHz and fmax=620 GHz at VDS=0.7 V. The device also shows a well-balanced fT and fmax in excess of 400 GHz, even at VDS=0.5 V. In addition, the device gains about 70 % improvement in fT as Lg shrinks down from 200 to 100 nm. The results obtained in this work should make this technology of great interest to a multiplicity of applications and guide a realistic path in trying to achieve a 1 THz fT from III-V HEMTs in the future.

Field-effect hole mobility of poly(3-hexylthiophene-block-3-(2-ethylhexyl)thiophene)s

The field-effect hole mobility of a series of highly regioregular all-conjugated P(3HT-b-3EHT) diblock copolymers was investigated. Although disordered segments of P3EHT were introduced, thin films of P(3HT-b-3EHT)s with 83mol% P3HT exhibited field-effect hole mobilities up to 0.14cm2V−1s−1 without any thermal treatment. In contrast, the corresponding random copolymers P(3HT-ran-3EHT)s showed a monotonic decrease in mobility upon introduction of 3EHT monomer units, owing to the decrease in polymer crystallinity. Absorption spectra of the films suggest the formation of improved structural order induced by the P3EHT block, which contributed to improved carrier transport properties in P(3HT-b-3EHT)s.

Hydrogenated amorphous and nanocrystalline silicon solar cells deposited by HWCVD and RF-PECVD on plastic substrates at 150 °C

In this work we present a study of the structural, optoelectronic and transport properties of a series of Si films deposited in a parameter region (namely hydrogen dilution) corresponding to a transition from amorphous-to-nanocrystalline silicon by hot-wire (HW) and radio-frequency plasma enhanced chemical vapor deposition (RF) on plastic substrates at 150°C. To achieve a higher deposition rate of Si films by RF we used a relatively high power density (350mW/cm2) and deposition pressure (1.5Torr). For certain hydrogen dilution values, these deposition conditions can lead to the formation of Si crystals in the silane plasma and to a growth of polymorphous silicon film. This material has improved carrier transport properties (ambipolar diffusion length=220nm) and very high photosensitivity (>5×106). The best HW amorphous silicon films exhibited lower photosensitivity (7×104) and an ambipolar diffusion length of only 100nm. For solar cell fabrication, we optimized the RF deposition conditions to produce very thin amorphous and nanocrystalline phosphorous and boron doped silicon layers. Our best n–i–p solar cell, with a polymorphous Si intrinsic layer deposited on plastic, has an efficiency of 5.5%, FF=52.5%, VOC=920mV, JSC=11.6mA/cm2. For solar cells with a nanocrystalline Si active layer deposited on glass the following results were achieved: efficiency=3.4%, FF=43.5%, VOC=460mV, JSC=17.2mA/cm2; and on plastic substrate: efficiency=2.2%, FF=32.7%, VOC=397mV, JSC=17.2mA/cm2.