Showing papers by "Junsin Yi published in 2013"

Negative gate-bias temperature stability of N-doped InGaZnO active-layer thin-film transistors

Abstract: Stability of negative bias temperature stress (NBTS) of nitrogen doped amorphous InGaZnO (a-IGZO) thin-film transistor (TFT) is investigated. Undoped a-IGZO TFT stressed at 333 K exhibit a larger negative ΔVTH (−3.21 V) with an unpredictable sub-threshold swing (SS) of hump shaped transfer curve due to the creation of meta-stable traps. Defects related hump formation has disappeared with small ΔVTH (−1.13 V) and ΔSS (0.018 V/dec) in nitrogen doped a-IGZO TFT. It is observed that nitrogen doping enhances device stability by well controlled oxygen vacancy and trap sites in channel and channel/dielectric interface.

Effects of Carrier Concentration, Indium Content, and Crystallinity on the Electrical Properties of Indium-Tin-Zinc-Oxide Thin-Film Transistors

Abstract: We report the effects of carrier concentration (NCH), indium (In) content, and crystallinity (Xc) on the electrical properties of indium-tin-zinc-oxide (ITZO) thin-film transistors (TFTs). The ITZO TFT with the lowest NCH, In content, and amorphous phase at the optimized oxygen flow rate has high field-effect mobility a (μFE) of 37.2 cm2/V·s, high ON/OFF current ratio (ION/IOFF) of ~ 1×107, and low subthreshold swing (S.S) of 0.93. With increasing NCH, In content, and Xc, μFE, ION/IOFF, and S.S surprisingly degraded to 14.4 cm2/V·s, ~ 4×104, and 4.01, respectively. Our high ITZO TFTs with μFE of 37.2 cm2/V·s, obtained thorough control of the NCH, In content, and Xc, was suitable for application to next generation ultrahigh resolution displays as well as high frame rate displays.

Effect of the short collection length in silicon microscale wire solar cells

Abstract: Electrical and optical properties of silicon microscale wire (SiMW) solar cells were investigated. Diverse designs were applied for SiMW geometries as light absorbers. Finite-difference time-domain simulation shows a focused optical field in the wires inducing an optical absorption enhancement in SiMW solar cells. SiMW solar cells provided remarkably higher Voc values (0.597-0.61 V) than that of the planar solar cell (0.587 V). As for the electrical aspects, the position of the space charge region in a SiMW directly affects the carrier collection efficiency according to the SiMW diameter and significantly modulates the photogenerated-currents and voltages in solar cells.

Suppression of temperature instability in InGaZnO thin-film transistors by in situ nitrogen doping

Abstract: We have investigated the effect of nitrogen doping on the behavior of hysteresis curve and its suppression of temperature instability in amorphous InGaZnO thin-film transistors (a-IGZO TFTs). The in situ nitrogen doping reduced the temperature induced abnormal sub threshold leakage current and traps generation. Large falling-rate (FR) ~ 0.26 eV V−1, low activation energy (Ea) ~ 0.617 eV and a small hysteresis compared to the pure a-IGZO TFTs, shows the best immunity to thermal instability. This is mainly attributed to the reduction of interface trap density and oxygen vacancies due to the passivation of defects and/dangling bonds.

Effects of LiF/Al back electrode on the amorphous/crystalline silicon heterojunction solar cells

Abstract: To improve the quantum efficiency (QE) and hence the efficiency of the amorphous/crystalline silicon heterojunction solar cell, we have employed a LiF dielectric layer on the rear side. The high dipole moment of the LiF reduces the aluminum electrode's work–function and then lowers the energy barrier at back contact. This lower energy barrier height helps to enhance both the operating voltage and the QE at longer wavelength region, in turn improves the open-circuit voltage (Voc), short-circuit current density (Jsc), and then overall cell efficiency. With optimized LiF layer thickness of 20 nm, 1 cm2 heterojunction with intrinsic thin layer (HIT) solar cells were produced with industry-compatible process, yielding Voc of 690 mV, Jsc of 33.62 mA/cm2, and cell efficiencies of 17.13%. Therefore LiF/Al electrode on rear side is proposed as an alternate back electrode for high efficiency HIT solar cells.

Surface-concentrated light and efficient carrier collection in microhole-patterned Si solar cells.

Abstract: We investigate photovoltaic characteristics of crystalline Si solar cells with microhole-patterned surface. We compare patterned samples with different hole-widths and periods with a planar counterpart. From the finite-difference time-domain simulation, the patterned and planar samples are expected to have similar short circuit current density, J(sc) (difference: 1.2%). In contrast, the difference in the measured J(sc) is as large as 12.6%. The simulated optical field patterns reveal that the sample with more significantly concentrated light near the surface has higher quantum efficiency due to more efficient carrier collection. We report the highest efficiency of 15.6% among the hole-patterned solar cells.

Bias-stability improvement using Al 2 O 3 interfacial dielectrics in a-InSnZnO thin-film transistors

Abstract: We report amorphous-indium–tin–zinc-oxide (a-ITZO) thin-film transistors (TFTs) obtained using an aluminum-oxide (Al2O3) interfacial dielectric using atomic layer deposition between a silicon-nitride (SiNX) gate dielectric and an a-ITZO active channel layer. The effect of the Al2O3 interfacial layer on the suppression of charge trapping in a-ITZO TFTs is presented. In transparent oxide TFTs, reducing the shift in threshold voltage by stress-including negative-bias stress (NBS) is one of the key issues in improving the stability performance of TFTs. The NBS stability of an a-ITZO TFT using an Al2O3/SiNX double-layered dielectric is superior to that using an SiNX single-gate dielectric, because of the smooth surface with a root-mean-square roughness of 0.147 nm and a low defect density of less than 3×1011 eV−1 cm−2, which increases hydrophobicity. The a-ITZO TFTs using the Al2O3 interfacial dielectric show little change in the threshold voltage (~0 V), and a long trapping time of ~5000 s when a gate voltage of −25 V and drain voltage of 1 V are applied for 10 000 s. We show that the gate dielectric has a profound effect on the electrical stability, and suggest a way of improving the stability of a-ITZO TFTs.

Bias–stress-induced threshold voltage shift dependence of negative charge trapping in the amorphous indium tin zinc oxide thin-film transistors

Abstract: Indium tin zinc oxide (ITZO)-based thin-film transistors (TFTs) were fabricated by dc magnetron sputtering in Ar + O2 reactive gas, at room temperature. We present the effect of post-deposition annealing of ITZO thin films on the oxygen vacancies and on the characteristics of TFT devices. When the annealing temperature was increased from room temperature to 350 °C, the resistivity of ITZO film increased from 2.05 × 101 to 2.60 × 103 Ω cm and the interface trap density (Nt) of the TFTs reduced from 3.18 × 1013 to 4.83 × 1011 cm−2. The TFT with the ITZO film which was annealed at 350 °C showed a very small shift in turn-on voltage, even after applying positive bias stress of +12 V for 104 s. The current–voltage characteristics of 350 °C annealing temperature sample indicated that these TFTs were in an enhanced mode of transistor operation with a high on-to-off current ratio of ∼1.26 × 106, high field-effect mobility of 14.17 cm2 V−1 s−1, and low subthreshold slope of 1.23 V/dec. The trapping time reduced from 3720 to 1546 s as the annealing temperature increased from room temperature to 350 °C. These results suggest that thermal annealing played an important role in reducing defects as well as improvement in stability of the TFTs.

Influence of SnO2:F/ZnO:Al bi-layer as a front electrode on the properties of p-i-n amorphous silicon based thin film solar cells

Abstract: The effect of aluminum doped zinc oxide film used between a fluorine doped tin oxide layer and a hydrogenated amorphous silicon carbide layer to improve the open circuit voltage (Voc) and fill factor (FF) for high efficiency thin film solar cells. The efficiency enhancement was accomplished by the insertion of high work-function layers engineered in the interfaces to raise FF as well as Voc. Therefore, we were able to obtain the conversion efficiency of 10.34% at 16.14 mA/cm2 of the current density (Jsc) and 70.37% of FF.

Diffused transmission and texture-induced defect with transparent conducting oxide front electrode of amorphous silicon solar cell

Abstract: Highly textured transparent conducting front electrode (TFE) can enhance light trapping; however, we observed an improved performance of a solar cell by nanometer level modification of surface texture. A 40 nm thick aluminum doped zinc oxide was sputter deposited at an oblique flux on the textured fluorine doped tin oxide TFE, and a TFE double layer was used for the front electrode of solar cells. The cells fabricated on these modified TFEs exhibited an increased open circuit voltage, enhancement in short circuit current density and an increase in fill factor that reached up to 74% because of reduced series resistance of the cell. The surface texture of the TFE was modified with various angular fluxes of the zinc oxide sputter particle, which shows lower texture-induced-surface defect, higher work function. These are thought to have helped in improving the performance of the thin-film solar cells. However, at a near normal angular flux of the zinc oxide sputter particle, light trapping was observed to have reduced significantly, leading to a significant reduction in current density of the cell.

High-Efficiency Heterojunction with Intrinsic Thin-Layer Solar Cells: A Review

Abstract: Heterojunction with intrinsic thin layer (HIT) solar cell has attracted attention of photovoltaic research community due to its low process temperature, as compared to crystalline silicon (c-Si) solar cell and relatively high efficiency (η). In this solar cell structure, thin intrinsic amorphous silicon layers are used as surface passivator, which also acts as buffer layer at the top as well as bottom surfaces of n-type crystalline silicon (c-Si) wafer. A thin p-type a-Si top layer acts as an emitter while highly doped n-type bottom layer is used as a back surface field. There have been suggestions that the device performance comes with significant tunnelling transport of the charge carriers across junction barriers, whereas other investigation suggests that the diffusion of the carriers may be adequate. Various experimental as well as theoretical investigations were carried out to characterize and improve performance of HIT solar cell. For example, it was reported that a thicker p-type emitter (up to 40nm) shows higher open circuit voltage, although short circuit current density (Jsc) and cell efficiency decreases. Recent investigation showed that when the p-type and i-type layer thickness increased (within 4 nm) the open circuit voltage (Voc) as well as the efficiency improves. Using thinner c-Si absorbed is another interesting aspect in which a light trapping scheme can be combined to achieve a better performance characteristics. There has been several theoretical as well as experimental investigations on the use of various types of emitter layer and intrinsic layer of the cell, and their role on the Jsc, Voc, η and fill factor (FF). Investigations of HIT solar cell on p-type c-Si wafers have also been carried out. In this article we will present a brief review on some of the important aspects of the HIT solar cell.

High-Efficiency Heterojunction with Intrinsic Thin-Layer Solar Cells

Abstract: Heterojunction with Intrinsic Thin-layer (HIT) solar cells are currently an important subject in industrial trends for thinner solar cell wafers due to the low-temperature of production processes, which is around 200°C, and due to their high-efficiency of 24.7%, as reported by the Panasonic (Sanyo) group. The use of thinner wafers and the enhancement of cell performance with fabrication at low temperature have been special interests of the researchers. The fundamental understanding of the band bending structures, choice of materials, fabrication process, and nano-scale characterization methods to provide necessary understanding of the interface passivation mechanisms, emitter properties, and requirements for transparent oxide conductive layers is presented in this review. This information should be used for the performance characterization of the developing technologies for HIT solar cells.

Light management and efficient carrier generation with a highly transparent window layer for a multijunction amorphous silicon solar cell

Abstract: P-layer of a p-i-n type amorphous silicon solar cell helps in creating a built-in electric field inside the cell; it also contributes to parasitic absorption loss of incident light. Here, we report optimization of these two characteristic contributions of the p-layer of the cell. We used a highly transparent p-type hydrogenated amorphous silicon carbide (p-a-Si 1−x C x :H ) window layer in an amorphous silicon solar cell. With the increased transparency of the p-type layer, the solar cell showed an improvement in short-circuit current density by 17%, along with improvement in blue response of its external quantum efficiency, although further thinner p-layer showed lower open-circuit voltage. Such a cell shows low light-induced degradation and a promise to be used in high-efficiency multijunction solar cell.

High field-effect mobility amorphous InSnZnO thin-film transistors with low carrier concentration and oxygen vacancy

Abstract: The influence of carrier concentration and oxygen vacancy on the performance of amorphous-indium–tin–zinc-oxide (a-ITZO) thin-film transistors (TFTs) is reported. The ITZO TFT with lowest carrier concentration and oxygen vacancy has a high field-effect mobility (μ FE) of 37.2 cm2/V•s, a high on/off current ratio (I ON/I OFF) of ∼1 × 107 and a low subthreshold swing (SS) of 0.93 V/decade. By increasing the carrier concentration and oxygen vacancy, μ FE, I ON/OFF and SS were surprisingly degraded to 14.4 cm2/V•s, ∼4 × 104 and 4.01 V/decade, respectively. By controlling the carrier concentration and oxygen vacancies of ITZO bulk, improvement of the performance in TFT devices can be achieved. The proposed high μ FE of 37.2 cm2/V•s is enough for the application of next-generation displays requiring ultra-high resolution and high-frame-rate displays.

Enhancing light trapping properties of thin film solar cells by plasmonic effect of silver nanoparticles.

Abstract: The preparation of thin film silicon solar cells containing Ag nanoparticles is reported in this article. Ag nanoparticles were deposited on fluorine doped tin oxide coated glass substrates by the evaporation and condensation method. a-Si:H solar cells were deposited on these substrates by cluster type plasma enhanced chemical vapor deposition. We discuss the double textured surface effect with respect to both the surface morphology of the substrate and the plasmonic effect of the Ag nanoparticles. Ag nanoparticles of various sizes from 10 to 100 nm were deposited. The haze values of the Ag embedded samples increased with increasing particle size whereas the optical transmittance decreased at the same conditions. The solar cell with the 30 nm size Ag nanoparticles showed a short circuit current density of 12.97 mA/cm 2 , which is 0.53 mA/cm 2 higher than that of the reference solar cell without Ag nanoparticles, and the highest quantum efficiency for wavelengths from 550 to 800 nm. When 30 nm size nanoparticles were employed, the conversion efficiency of the solar cell was increased from 6.195% to 6.696%. This study reports the application of the scattering effect of Ag nanoparticles for the improvement of the conversion efficiency of amorphous silicon solar cells.

Reduction of tail state on boron doped hydrogenated amorphous silicon oxide films prepared at high hydrogen dilution.

Abstract: In this report, we have investigated on the defect state of diborane (B2H6) doped wide bandgap hydrogenated amorphous silicon oxide (p-type a-SiO:H) films prepared using silane (SiH4), hydrogen (H2) and nitrous oxide (N2O) in a radio frequency (RF) plasma enhanced chemical vapor deposition (PECVD) system with different hydrogen dilutions. The films prepared with higher hydrogen dilution show lower Urbach energy (Eu), lower microstructure (R*), lower short and medium range disorder (omegaTO, Gamma(TO), I(TA)/I(TO), I(LA)/I(TO)), higher dark conductivity (sigma d) and higher refractive index (n) with high optical gap (Eg). Eu decreases from 248 meV to 153 meV, and R* decreases from 0.46 to 0.26, Raman peak omegaTO-TO mode position shifts from 480.24 to 483.28, GammaTO-full width half maximum of omegaTO decreases from 78.16 to 63.87, I(TA)/I(TO)-the ratio of integrated area of TA and TO mode decreases from 0.624 to 0.474, I(LA)/I(TO)-the ratio of integrated area of LA and TO mode deceases from 0.272 to 0.151, sigma d increases from 4.6 x 10(-7) S/cm to 1.1 x 10(-6) S/cm, n increases from 3.70 to 3.86. Reduced Nd, Eu and R* at wide Eg indicates that the films are more useful for solar cell window layer. Applying this layer to a single junction solar cell shows open circuit voltage (Voc) = 0.80 V, short circuit current density (Jsc) = 16.3 mA/cm2, fill factor (FF) = 72%, efficiency (eta) = 9.4%.