Vinh Ai Dao

Interface Characterization and Electrical Transport Mechanisms in a-Si:H/c-Si Heterojunction Solar Cells

The fabrication of amorphous silicon/crystalline silicon (a-Si:H/c-Si) heterojunction solar cell and an understanding of the fundamental conduction mechanism in the device are presented. In the first part, the effect of intrinsic amorphous silicon [a-Si:H(i)] layer thickness on the performance of a-Si:H/c-Si solar cells has been studied. The thickness of a-Si:H(i) layer formed on n-type c-Si substrate was controlled accurately with spectroscopy ellipsometry (SE). Based on SE results, we discuss the influence of the a-Si:H(i) thickness on the interface quality and thereby cell performance. Then, in the latter part, we present the temperature-dependent current density―voltage curves, in the dark, in order to elucidate the dominant transport mechanisms in a-Si:H/c-Si heterojunction solar cells with and without incorporation of a-Si:H(i) layers. Finally, using optimum design considerations, we obtained a solar cell efficiency of 17.43%.

Operation mechanism of Schottky barrier nonvolatile memory with high conductivity InGaZnO active layer

Influence of Schottky contact between source/drain electrodes and high conductivity a-InGaZnO active layer to the performance of nonvolatile memory devices was first proposed. The Schottky barrier devices faced to the difficulty on electrical discharging process due to the energy barrier forming at the interface, which can be resolved by using Ohmic devices. A memory window of 2.83 V at programming/erasing voltage of ±13 V for Ohmic and 5.58 V at programming voltage of 13 V and light assisted erasing at −7 V for Schottky devices was obtained. Both memory devices using SiO2/SiOx/SiOxNy stacks showed a retention exceeding 70% of trapped charges 10 yr with operation voltages of ±13 V at an only programming duration of 1 ms.

In and Ga Codoped ZnO Film as a Front Electrode for Thin Film Silicon Solar Cells

Doped ZnO thin films have attracted much attention in the research community as front-contact transparent conducting electrodes in thin film silicon solar cells. The prerequisite in both low resistivity and high transmittance in visible and near-infrared region for hydrogenated microcrystalline or amorphous/microcrystalline tandem thin film silicon solar cells has promoted further improvements of this material. In this work, we propose the combination of major Ga and minor In impurities codoped in ZnO film (IGZO) to improve the film optoelectronic properties. A wide range of Ga and In contents in sputtering targets was explored to find optimum optical and electrical properties of deposited films. The results show that an appropriate combination of In and Ga atoms in ZnO material, followed by in-air thermal annealing process, can enhance the crystallization, conductivity, and transmittance of IGZO thin films, which can be well used as front-contact electrodes in thin film silicon solar cells.

Ultrathin Oxide Passivation Layer by Rapid Thermal Oxidation for the Silicon Heterojunction Solar Cell Applications

It is difficult to deposit extremely thin a-Si:H layer in heterojunction with intrinsic thin layer (HIT) solar cell due to thermal damage and tough process control. This study aims to understand oxide passivation mechanism of silicon surface using rapid thermal oxidation (RTO) process by examining surface effective lifetime and surface recombination velocity. The presence of thin insulating a-Si:H layer is the key to get high by lowering the leakage current (I0) which improves the efficiency of HIT solar cell. The ultrathin thermal passivation silicon oxide (SiO2) layer was deposited by RTO system in the temperature range 500–950°C for 2 to 6 minutes. The thickness of the silicon oxide layer was affected by RTO annealing temperature and treatment time. The best value of surface recombination velocity was recorded for the sample treated at a temperature of 850°C for 6 minutes at O2 flow rate of 3 Lpm. A surface recombination velocity below 25 cm/s was obtained for the silicon oxide layer of 4 nm thickness. This ultrathin SiO2 layer was employed for the fabrication of HIT solar cell structure instead of a-Si:H, (i) layer and the passivation and tunneling effects of the silicon oxide layer were exploited. The photocurrent was decreased with the increase of illumination intensity and SiO2 thickness.

The mechanisms of negative oxygen ion formation from Al-doped ZnO target and the improvements in electrical and optical properties of thin films using off-axis dc magnetron sputtering at low temperature

Transparent conducting aluminum-doped zinc oxide (AZO) films have been prepared on glass substrates by dc magnetron sputtering using ceramic ZnO with 2 wt% Al2O3 target. The mechanism of negative oxygen ion generation on an AZO target surface and its influence on the conductivity of films were discussed. The negative ion generation on an AZO target was contributed by the surface ionization leading to the spot emission from Al atoms adsorbed on the AZO target surface. The contribution of negative ions’ current was mainly from the erosion area of the target due to its higher temperature. To reduce the damage caused by negative ion bombardment to film growth, an off-axis sputtering system was proposed, where the substrates were placed perpendicular to the target. The effects of distance (d) on the electrical properties of films were experimentally verified in detail. A low resistivity of 3.7 × 10 −4 � cm, an average transmittance above 85% in the visible range (300‐800 nm) and reflectance higher than 85% in the infrared range (2500‐4000 nm) were obtained for the films deposited at d = 2.5 cm. The overall analysis revealed that the generation of negative ions on the AZO target has a great influence on film growth, especially in the ultra-low pressure deposition process. Our work demonstrates the feasibility of reducing the negative effects of ion bombardment on the quality of films, which would be of great merit for industrial applications. (Some figures in this article are in colour only in the electronic version)

Hydrogenated Amorphous Silicon Layer Formation by Inductively Coupled Plasma Chemical Vapor Deposition and Its Application for Surface Passivation of p-Type Crystalline Silicon

The satisfactory surface passivation properties of hydrogenated amorphous silicon (a-Si:H) prepared by inductively coupled plasma chemical vapor deposition (ICP-CVD) at a low temperature (400 °C) on p-type crystalline silicon wafers are reported. Certain parameters, such as SiH4/H2 ratio, annealing temperature, film thickness, and substrate temperature, were varied to determine their optimal levels. Completely amorphous layers with a broad transverse optic (TO) mode peak at approximately 480 cm-1 were identified by Raman spectroscopy, and an optical band gap of approximately 1.6 eV was determined from optical absorption data. A maximum carrier lifetime of 53 µs for an a-Si:H thickness of 15 nm and an annealing temperature of 450 °C was measured for 525-µm-thick p-type crystalline silicon (c-Si) substrates with a resistivity in the range of 1–20 Ω cm by the quasi-steady-state photoconductance (QSSPC) method. The lowest value of interface trapped charge density (Dit) of approximately 3.34 ×1011 cm-2 eV-1 was estimated by capacitance–voltage (C–V) measurement using a metal–insulator–semiconductor (MIS) structure. Furthermore, simple processing with satisfactory results can be achieved with substrate heating at 400 °C during deposition. The optimal conditions of a SiH4/H2 gas ratio of 1/1 and a substrate temperature of 400 °C were implemented for a passivation layer thickness of 5 nm.

Effect of high conductivity amorphous InGaZnO active layer on the field effect mobility improvement of thin film transistors

High mobility thin film transistors (TFTs) with a high conductivity amorphous InGaZnO (a-IGZO) active layer were successfully fabricated. The operation of the high-carrier-IGZO thin film transistor with a Schottky barrier (SB) was proposed and clearly experimentally explained. The switching characteristic of SB-TFT does not rely on the accumulation process but due to the Schottky barrier height control. Leakage current can be reduced by Schottky contact at the source/drain (S/D), while it was as high as the on current so that the switch properties could not achieve in ohmic ones. The a-IGZO SB-TFTs with Ag S/D contact express the high performance with μFE of 20.4 cm2 V−1 s−1, Vth of 5.8 V, and ION/IOFF of 2 × 107 @ VD = 1V. The introduction of operating mechanism for TFTs using high conductivity a-IGZO promises an expansion study for other active layer materials.

Study of stacked-emitter layer for high efficiency amorphous/crystalline silicon heterojunction solar cells

A modified emitter, of stacked two layer structure, was investigated for high-efficiency amorphous/crystalline silicon heterojunction (HJ) solar cells. Surface area of the cells was 181.5 cm2. The emitter was designed to achieve a high open circuit voltage (Voc) and fill factor (FF). When doping of the emitter layer was increased, it was observed that the silicon dihydride related structural defects within the films increased, and the Voc of the HJ cell decreased. On the other hand, while the doping concentration of the emitter was reduced the FF of the cell reduced. Therefore, a combination of a high conductivity and low defects of a single emitter layer appears difficult to obtain, yet becomes necessary to improve the cell performance. So, we investigated a stacked-emitter with low-doped/high-doped double layer structure. A low-doped emitter with reduced defect density was deposited over the intrinsic hydrogenated amorphous silicon passivation layer, while the high-doped emitter with high conductivity w...

Performance of hetero junction with intrinsic thin-layer solar cell depending upon contact resistivity of front electrode

Abstract. Low temperature curing of Ag paste for electrode formation in silicon hetero junction (SHJ) solar cells is important for providing better device characteristics. Ag paste is composed of solvent, various organic materials, and additives; hence its electrical and mechanical adhesion properties depend on the curing conditions. The adhesion of the Ag paste was determined by scratch test, whereas the specific contact resistance was measured using the transfer length method. Various Ag electrodes were formed at various curing temperatures within the temperature range of 160°C–240°C, at 20°C intervals. The curing time was also varied for another set of Ag electrodes. With 200°C temperature and for 20-min curing, the critical load of 20.06 N and specific contact resistance of 19.61  mΩ·cm2 were observed. Using the same conditions, the efficiency of the SHJ solar cell was found to be improved by 3.8%.

Al2O3/SiON stack layers for effective surface passivation and anti-reflection of high efficiency n-type c-Si solar cells

Excellent surface passivation and anti-reflection properties of double-stack layers is a prerequisite for high efficiency of n-type c-Si solar cells. The high positive fixed charge (Q f) density of N-rich hydrogenated amorphous silicon nitride (a-SiNx:H) films plays a poor role in boron emitter passivation. The more the refractive index ( n ) of a-SiNx:H is decreased, the more the positive Q f of a-SiNx:H is increased. Hydrogenated amorphous silicon oxynitride (SiON) films possess the properties of amorphous silicon oxide (a-SiOx) and a-SiNx:H with variable n and less positive Q f compared with a-SiNx:H. In this study, we investigated the passivation and anti-reflection properties of Al2O3/SiON stacks. Initially, a SiON layer was deposited by plasma enhanced chemical vapor deposition with variable n and its chemical composition was analyzed by Fourier transform infrared spectroscopy. Then, the SiON layer was deposited as a capping layer on a 10 nm thick Al2O3 layer, and the electrical and optical properties were analyzed. The SiON capping layer with n = 1.47 and a thickness of 70 nm resulted in an interface trap density of 4.74 = 1010 cm−2 eV−1 and Q f of −2.59 = 1012 cm−2 with a substantial improvement in lifetime of 1.52 ms after industrial firing. The incorporation of an Al2O3/SiON stack on the front side of the n-type solar cells results in an energy conversion efficiency of 18.34% compared to the one with Al2O3/a-SiNx:H showing 17.55% efficiency. The short circuit current density and open circuit voltage increase by up to 0.83 mA cm−2 and 12 mV, respectively, compared to the Al2O3/a-SiNx:H stack on the front side of the n-type solar cells due to the good anti-reflection and front side surface passivation.