Amornrat Limmanee

Surface Passivation of Crystalline and Polycrystalline Silicon Using Hydrogenated Amorphous Silicon Oxide Film

Excellent passivation properties of hydrogenated amorphous silicon oxide (a-SiOx:H) prepared by very high frequency plasma-enhanced chemical vapor deposition (VHF PECVD) at a low substrate temperature (170 °C) on crystalline and polycrystalline silicon (Si) wafers are reported. Films were characterized by ellipsometry, Fourier transform infrared spectroscopy (FTIR), ultraviolet–visible (UV–vis) spectrophotometry, and dark-conductivity and photoconductivity measurements. A comparison of the results with those for different passivation layers such as hydrogenated amorphous silicon carbon nitride (a-SiCxNy:H), hydrogenated amorphous silicon nitride (a-SiNx:H), and hydrogenated amorphous silicon (a-Si:H) reveals their superiority as an excellent passivation layer for p-type crystalline Si as well as polycrystalline Si. A maximum effective lifetime of 400 µs was measured for 1–10 Ω cm, 380-µm-thick p-type c-Si using a micro-photocurrent decay (µ-PCD) system. Fixed charge density (Qf) was estimated by high-frequency (1 MHz) capacitance–voltage measurement using a metal–insulator–silicon structure (CV-MIS). The effect of annealing temperature on surface passivation in a nitrogen atmosphere was also studied.

Boron-doped Microcrystalline Silicon Oxide Film for Use as Back Surface Field in Cast Polycrystalline Silicon Solar Cells

We have studied the properties of boron-doped hydrogenated microcrystalline silicon oxide (p++ µc-SiO:H) with the aim of applying this film as the back surface field (BSF) in silicon solar cells. We found that the p++ µc-SiO:H film showed high conductivity and excellent passivation effect. A cast polycrystalline silicon solar cell using the p++ µc-SiO:H film as BSF showed an efficiency of 15.5% (2 ×2 cm2). The high open circuit voltage and quantum efficiency at a wavelength of approximately 1000 nm clearly indicated the excellent BSF properties of the p++ µc-SiO:H film.

Preparation of Hydrogenated Amorphous Silicon Carbon Nitride Films by Hot-Wire Chemical Vapor Deposition Using Hexamethyldisilazane for Silicon Solar Cell Applications

We propose hydrogenated amorphous silicon carbon nitride (a-SiCN:H) films prepared by hot-wire chemical vapor deposition (HWCVD) using hexamethyldisilazane (HMDS) as a new passivation layer of silicon solar cells. The films deposited at various deposition conditions were characterized focusing on optical properties and passivation quality. We found that the effective lifetime of the samples depended on the amount of Si–H bond and had strong correlation with the interface trap densities. After the optimization, the effective surface recombination velocity of n-type single crystalline silicon (c-Si) (1–10 Ω cm) with a-SiCN:H films has become lower than 60 cm/s and that of p-type was approximately 300 cm/s. The efficiency of a cast polycrystalline silicon solar cell using a-SiCN:H as a passivation layer reached 13.7% (2×2 cm2). Experimental results indicate that a-SiCN:H films are promising low-cost antireflection and passivation layers for silicon solar cells.

Effect of Ambient Temperature on Performance of Grid-Connected Inverter Installed in Thailand

The effects of temperature on performance of a grid-connected inverter, and also on a photovoltaic (PV) system installed in Thailand have been investigated. It was found that the maximum efficiency of the inverter showed 2.5% drop when ambient temperature was above 37°C. The inverter performed efficiently in November and December, the months of high irradiance, and monthly average ambient temperature of lower than 35°C, allowing relatively high system performance ratio in this period. Our results show that high temperature provides negative impacts not only on the PV modules, but also on the performance of the inverter. Thus, the effect of temperature on the inverter efficiency should be taken into account when predicting energy yield or analyzing losses of the PV systems—especially in high temperature regions.

High efficiency a-Si:H/a-SiGe:H tandem solar cells fabricated with the combination of V- and U-shaped band gap profiling techniques

Hydrogenated amorphous silicon germanium (a-SiGe:H) films prepared by very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) using a mixture of SiH4, H2, and GeH4 were investigated for their use as the bottom cell of amorphous silicon/amorphous silicon germanium (a-Si:H/a-SiGe:H) tandem solar cell structures. Narrow optical band gaps (Eopt) in the range of 1.5 to 1.6 eV were obtained by varying the GeH4/(SiH4 + GeH4) gas flow rate ratio in low-temperature deposition. The a-SiGe:H films deposited with various GeH4/(SiH4 + GeH4) gas flow rate ratios were used as intrinsic layers for the a-Si:H/a-SiGe:H tandem solar cells with different graded band gaps: V-, VU-, and U-shapes. It was found that using the VU-shape improves the solar cell efficiency owing to a higher Jsc when compared with using V-shape. The VU-shape’s Voc and FF are also improved when compared with the U-shape’s Voc and FF. As a result, a high efficiency of 11.0% (Voc = 1.74 V, Jsc = 9.07 mA/cm2, and FF = 0.70) was successfully achieved with the solar cells fabricated using the VU-shape graded band gap technique.

Wide-Gap p-c-:H Films and Their Application to Amorphous Silicon Solar Cells

Optimization of p-type hydrogenated microcrystalline silicon oxide thin films (p-c-:H) by very high frequency plasma enhanced chemical vapor deposition 40 MHz method for use as a p-layer of a-Si:H solar cells was performed. The properties of p-c-:H films were characterized by conductivity, Raman scattering spectroscopy, and spectroscopic ellipsometry. The wide optical band gap p-c-:H films were optimized by CO2/SiH4 ratio and H2/SiH4 dilution. Besides, the effects of wide-gap p-c-:H layer on the performance of a-Si:H solar cells with various optical band gaps of p-layer were also investigated. Furthermore, improvements of open circuit voltage, short circuit current, and performance of the solar cells by using the effective wide-gap p-c-:H were observed in this study. These results indicate that wide-gap p-c-:H is promising to use as window layer in a-Si:H solar cells.

Effect of the CO2/SiH4 Ratio in the p-μc-SiO:H Emitter Layer on the Performance of Crystalline Silicon Heterojunction Solar Cells

This paper reports the preparation of wide gap p-type hydrogenated microcrystalline silicon oxide (p-c-SiO:H) films using a 40 MHz very high frequency plasma enhanced chemical vapor deposition technique. The reported work focused on the effects of the CO2/SiH4 ratio on the properties of p-c-SiO:H films and the effectiveness of the films as an emitter layer of crystalline silicon heterojunction (c-Si-HJ) solar cells. A p-c-SiO:H film with a wide optical band gap (E04), 2.1 eV, can be obtained by increasing the CO2/SiH4 ratio; however, the tradeoff between E04 and dark conductivity must be considered. The CO2/SiH4 ratio of the p-c-SiO:H emitter layer also significantly affects the performance of the solar cells. Compared to the cell using p-c-Si:H (CO2/SiH4 = 0), the cell with the p-c-SiO:H emitter layer performs more efficiently. We have achieved the highest efficiency of 18.3% with an open-circuit voltage () of 692 mV from the cell using the p-c-SiO:H layer. The enhancement in the and the efficiency of the solar cells verified the potential of the p-c-SiO:H films for use as the emitter layer in c-Si-HJ solar cells.

Advantages of N-Type Hydrogenated Microcrystalline Silicon Oxide Films for Micromorph Silicon Solar Cells

We report on the development and application of n-type hydrogenated microcrystalline silicon oxide films (n μc-SiO:H) in hydrogenated amorphous silicon oxide/hydrogenated microcrystalline silicon (a-SiO:H/μc-Si:H) micromorph solar cells. The n μc-SiO:H films with high optical bandgap and low refractive index could be obtained when a ratio of carbon dioxide (CO2) to silane (SiH4) flow rate was raised; however, a trade-off against electrical property was observed. We applied the n μc-SiO:H films in the top a-SiO:H cell and investigated the changes in cell performance with respect to the electrical and optical properties of the films. It was found that all photovoltaic parameters of the micromorph silicon solar cells using the n top μc-SiO:H layer enhanced with increasing the CO2/SiH4 ratio up to 0.23, where the highest initial cell efficiency of 10.7% was achieved. The enhancement of the open circuit voltage () was likely to be due to a reduction of reverse bias at subcell connection—n top/p bottom interface—and a better tunnel recombination junction contributed to the improvement in the fill factor (FF). Furthermore, the quantum efficiency (QE) results also have demonstrated intermediate-reflector function of the n μc-SiO:H films.

Study of the structural properties of a-SiCN: H films using hexamethyldisilazane for high-quality silicon surface passivation

We have analyzed the structural and electrical properties of a-SiCN:H films prepared by HWCVD using HMDS and related these to the deposition parameters and its passivating qualities for the purpose of optimizing these films for passivation layers of silicon solar cells. We found that deposition pressure, HMDS flow rate and H2 flow rate had significant effects on the structural properties, chemical composition ratio and bond densities, and influenced passivation effect of a-SiCN:H films. This study indicates that hydrogen concentration derived from Si-H bond in a-SiCN:H films plays an important role in passivating qualities, while the nitrogen content and Si-N bond density are significant parameters that determine the diffusivity of hydrogen in the films. The dependence of anneal characteristics of a-SiCN:H films on nitrogen content was observed. The optimization of the composition ratio and H2 concentration of a-SiCN:H films will lead to the high quality silicon surface passivation

MOCVD ZnO/Screen Printed Ag Back Reflector for Flexible Thin Film Silicon Solar Cell Application

We have prepared Ag back electrode by screen printing technique and developed MOCVD ZnO/screen printed Ag back reflector for flexible thin film silicon solar cell application. A discontinuity and poor contact interface between the MOCVD ZnO and screen printed Ag layers caused poor open circuit voltage () and low fill factor (FF); however, an insertion of a thin sputtered ZnO layer at the interface could solve this problem. The n type hydrogenated amorphous silicon (a-Si:H) film is preferable for the deposition on the surface of MOCVD ZnO film rather than the microcrystalline film due to its less sensitivity to textured surface, and this allowed an improvement in the FF. The n-i-p flexible amorphous silicon solar cell using the MOCVD ZnO/screen printed Ag back reflector showed an initial efficiency of 6.2% with  V,  mA/cm2, and FF = 0.58 (1 cm2). The identical quantum efficiency and comparable performance to the cells using conventional sputtered Ag back electrode have verified the potential of the MOCVD ZnO/screen printed Ag back reflector and possible opportunity to use the screen printed Ag thick film for flexible thin film silicon solar cells.