Doo Won Lee

Aluminum Induced Crystallization of Amorphous Silicon Dependent on Annealing Conditions with Graphite Plate

Aluminum induced crystallization (AIC) of amorphous silicon was studied for thin-film solar cell. The AIC have been usually researched on glass substrate which has smooth surface. However, in this paper, the graphite plate was used as a substrate for using thin-film solar cell which has 1μm roughness. The growth silicon layer characteristic could be relatively different with that using glass substrate by the surface roughness. Therefore, the properties of crystallized silicon layer were studied for grain size analysis with variation in temperature and time during the AIC annealing process. The crystalline fraction and crystallinity was analyzed by Optical microscope, X-ray diffraction (XRD), and Raman spectrometer measurement methods. Additionally, the grain size was also relatively analyzed with FWHM results. As a result of measurements, crystalline fraction of grown silicon was increased with the increasing of temperature and time. The maximum crystalline fraction of grown silicon was 92.85% for 2400 minutes of annealing duration at 500°C.

Study of Annealing Temperature for Ni/Cu/Ag Plated Front Contact Single Crystalline Solar Cells

Ni/Cu/Ag contact formed by plating has continuously been studied as a future metallization technique of solar cells due to the lower cost of material and better electrical performance compared with the screen-printed Ag contact. For the metallization of samples, native oxide on a laser-patterned Si area was etched with a buffered oxide etch solution for uniform plating. A Ni seed layer for Cu plating was deposited by using alkaline electroless plating. Afterward, a Cu-Ag metal stack was plated by light-induced plating followed by annealing in a tube furnace with an N2 gas atmosphere. This annealing process forms NiSix, which enhances contact resistance and adhesion. However, Ni penetration through the emitter layer can produce shunting paths, which decrease cell performance. In this experiment, 2.6 N/mm was obtained as the highest adhesion result. In addition, voids that can degrade adhesion were observed at the interface of Cu-Ag due to the Kirkendall effect. According to the experiments results, the suggestion of annealing condition was discussed to have good electrical and physical properties of plated Ni/Cu/Ag front contact.

Poly-crystalline thin-film by aluminum induced crystallization on aluminum nitride substrate

Thin-film polycrystalline silicon (pc-Si) on foreign (non-silicon) substrates has been researched by various research groups for the production of photovoltaic cells. High quality pc-Si deposition on foreign substrates with superior optical properties is considered to be the main hurdle in cell fabrication. Metal induced crystallization (MIC) is one of the renowned techniques used to produce this quality of material. In the current study, an aluminum induced crystallization (AIC) method was adopted to produce pc-Si thin-film on aluminum nitride (AlN) substrate by a seed layer approach. Aluminum and a-Si layer were deposited using an e-beam evaporator. Various annealing conditions were used in order to investigate the AIC grown pc-Si seed layers for process optimization. The effect of thermal annealing on grain size, defects preferentially crystallographic orientation of the grains were analyzed. Surface morphology was studied using an optical microscope. Poly-silicon film with a crystallinity fraction between 95-100% and an FWHM between 5-6 cm−1 is achievable at low temperatures and for short time intervals. A grain size of about 10 micron can be obtained at a low deposition rate on an AIN substrate. Similarly, Focused ion beam (FIB) also showed that at 425 °C sample B and at 400 °C sample A were fully crystallized. The crystalline quality of pc-Si was evaluated using μ-Raman spectroscopy as a function of annealed conditions and Grazing incidence X-ray diffraction (GIXRD) was used to determine the phase direction of the pc-Si layer. The current study implicates that a poly-silicon layer with good crystallographic orientation and crystallinity fraction is achievable on AIN substrate at low temperatures and short time frames.

Past and future of graphene/silicon heterojunction solar cells: a review

Graphene/silicon (Gr/Si) Schottky junction solar cells represent an alternative low-cost, easy fabrication structure in photovoltaic devices. After graphenes emergence in 2004, the first Gr/Si solar cell was fabricated in 2010, and was able to achieve upto 15% efficiency in less than a decade. This breakthrough in cell efficiency was realized by the fact that Gr has tremendous electrical and optical properties for photovoltaic applications. In this review, we highlight some of the recent progress in Gr/Si heterojunction solar cells. The growth processes of 2D graphene using the CVD process are discussed in detail. Afterwards, the key parameters that help to enhance the power conversion efficiency (PCE) of solar cells are considered. The interface of Gr/Si and the effects of chemical doping on the cell parameters were studied. Lastly, the challenges and limitations along with the future developments for Gr/Si solar cells are discussed in detail.

Study of SiOx thickness effects on aluminum-induced crystallization

The thickness effects of SiOx which was deposited as an intermediate layer between aluminum and silicon were studied on Aluminum-induced crystallization (AIC). The SiOx layer thickness varied from 2 nm to 20 nm and affected the crystallization process of the AIC. In the case of the thin SiOx layer, crystallized silicon morphology showed kinetic-limited aggregation. On the other hand, crystallized silicon processed with the thick SiOx layer showed diffusion-limited aggregation due to slow silicon diffusion velocity. Kinetic-limited aggregation showed large grain. The schematic crystallization model was used to describe the relationship between crystallization and grain size in this paper.

A Study of Silicon Crystallization Dependence upon Silicon Thickness in Aluminum-induced Crystallization Process

Aluminum-induced crystallization(AIC) is one of the ways to improve characteristics of thin-film poly-crystalline (TFPC) silicon solar cell since it shows large grain size and good properties for TFPC silicon solar cell. In AIC process, aluminum is firstly deposited on foreign substrate. Then, silicon is deposited on aluminum. Annealing was done below the eutectic temperature of aluminum and silicon(577 oC). Afterward, amorphous silicon layer is crystallized by aluminum-inducing process. In this paper, we report amorphous silicon layer thickness effects to crystallized silicon and hillock characteristics since the amorphous silicon layer is important layer for AIC.Crystallized silicon properties were analyzed dependent upon silicon thickness (50 % 200 % than aluminum layer thickness). In case of the thick amorphous silicon (above 300 nm), hillocks grew on the crystallized silicon layer, Even though silicon hillock showed high crystallinity as a result of Raman spectroscopy, it had not enough grain size (average 4 μm). Therefore, as results, samples which had 113 % 120 % silicon and aluminum thickness ratio showed proper thickness to use as a seed layer. (Received February 19, 2018; Accepted March 16, 2018)

Study on silicon crystallization with aluminum deposition temperature in the aluminum-induced crystallization process using silicon oxide

Aluminum-induced crystallization (AIC) is one process which increases silicon grain size at low temperatures. In this study, we analyzed the effect of silicon crystallization according to the aluminum deposition conditions in the AIC process using silicon oxide. The initial aluminum layer was analyzed using a field emission-scanning electron microscopy (FE-SEM) after cutting the samples with a focused-ion-beam (FIB). Through FE-SEM, we observed that the aluminum grain size of the original aluminum layer increased in proportion to the aluminum deposition temperature. However, not only aluminum grain size but also surface roughness and porosity of the initial aluminum layer were increased. The initial aluminum layer, according to the deposition temperature, significantly affected the crystallized silicon grain size. The silicon grain size was decreased from 16.97 μm to 7.81 μm according to the increase of the aluminum deposition temperature. This was because the Si diffusion area was increased by the increase of the aluminum surface roughness.Aluminum-induced crystallization (AIC) is one process which increases silicon grain size at low temperatures. In this study, we analyzed the effect of silicon crystallization according to the aluminum deposition conditions in the AIC process using silicon oxide. The initial aluminum layer was analyzed using a field emission-scanning electron microscopy (FE-SEM) after cutting the samples with a focused-ion-beam (FIB). Through FE-SEM, we observed that the aluminum grain size of the original aluminum layer increased in proportion to the aluminum deposition temperature. However, not only aluminum grain size but also surface roughness and porosity of the initial aluminum layer were increased. The initial aluminum layer, according to the deposition temperature, significantly affected the crystallized silicon grain size. The silicon grain size was decreased from 16.97 μm to 7.81 μm according to the increase of the aluminum deposition temperature. This was because the Si diffusion area was increased by the increa...

Investigation of Firing Conditions for Optimizing Aluminum-Doped p + -layer of Crystalline Silicon Solar Cells

Screen printing technique followed by firing has commonly been used as metallization for both laboratory and industrial based solar cells. In the solar cell industry, the firing process is usually conducted in a belt furnace and needs to be optimized for fabricating high efficiency solar cells. The printed-Al layer on the silicon is rapidly heated at over 800°C which forms a layer of back surface field (BSF) between Si-Al interfaces. The BSF layer forms p-p + structure on the rear side of cells and lower rear surface recombination velocity (SRV). To have low SRV, deep p + layer and uniform junction formation are required. In this experiment, firing process was carried out by using conventional tube furnace with N₂ gas atmosphere to optimize V oc of laboratory cells. To measure the thickness of BSF layer, selective etching was conducted by using a solution composed of hydrogen fluoride, nitric acid and acetic acid. The V oc and pseudo efficiency were measured by Suns-V oc to compare cell properties with varied firing condition.