Dries Van Gestel

Intragrain defects in polycrystalline silicon layers grown by aluminum-induced crystallization and epitaxy for thin-film solar cells

Polycrystalline silicon (pc-Si) thin-films with a grain size in the range of 0.1–100 μm grown on top of inexpensive substrates are economical materials for semiconductor devices such as transistors and solar cells and attract much attention nowadays. For pc-Si, grain size enlargement is thought to be an important parameter to improve material quality and therefore device performance. Aluminum-induced crystallization (AIC) of amorphous Si in combination with epitaxial growth allows achieving large-grained pc-Si layers on nonsilicon substrates. In this work, we made pc-Si layers with variable grain sizes by changing the crystallization temperature of the AIC process in order to see if larger grains indeed result in better solar cells. Solar cells based on these layers show a performance independent of the grain size. Defect etching and electron beam induced current (EBIC) measurements showed the presence of a high density of electrically active intragrain defects. We therefore consider them as the reason fo...

Thin-film polycrystalline silicon solar cells with low intragrain defect density made via laser crystallization and epitaxial growth

The case for thin-film polycrystalline silicon (pc-Si) solar cells is strong as it combines the cost benefit of thin-films and the quality potential of crystalline Si technology. The challenge is in making high-quality pc-Si layers on non-Si substrates. By studying layers based on aluminum-induced crystallization (AIC) we previously showed that electrically active intragrain defects are a major limiting factor for thin-film polycrystalline silicon solar cells. This paper investigates the use of a recently proposed novel scanning-laser based mixed-phase solidification (MPS) process which results in large grains with a low intragrain defect density, as well as a narrow grain size distribution and strong surface crystallographic texture [6]. Through subsequent epitaxial growth, absorber layers with the desired doping and thickness can be obtained. Defect etching and TEM measurements demonstrate the drastically decreased intragrain defect density compared to the AIC-based samples. The most efficient solar cell so far has an energy conversion efficiency of 5.4% and open circuit voltage (Voc) of around 500mV. From the preliminary results obtained, we conclude that mixed phase solidification is an attractive technique to crystallize seed layers for thin-film silicon solar cells.

Record Voc-Values for Thin-Film Polysilicon Solar Cells on Foreign Substrates using a Heterojunction Emitter

Thin-film polysilicon solar cells on foreign substrates are often considered as a promising low cost alternative to bulk silicon solar cells. Until now however, the obtained efficiencies and open-circuit voltages are far below those of other technologies. In this paper, we show how the open-circuit voltage can be enhanced significantly using an amorphous silicon-crystalline silicon heterojunction emitter instead of a diffused homojunction emitter. Open-circuit voltages up to 536 mV were obtained for polysilicon layers with a heterojunction emitter. This is the highest open-circuit voltage obtained for polysilicon solar cells with a p-n structure on foreign substrates

Processing and characterization of efficient thin-film polycrystalline silicon solar cells

The price of photovoltaic electricity could be lowered substantially if efficient solar cells could be made from polycrystalline-silicon (pc-Si) thin films on inexpensive substrates. A promising way to make pc-Si layers for solar cells is to use aluminum-induced crystallization (AIC) of amorphous silicon in combination with thermal CVD. To obtain efficient pc-Si solar cells, the material quality has to be optimized and cell processes different from those applied for standard bulk-Si solar cells have to be developed. In this paper, we present our best pc-Si solar cells so far and we discuss some key features of our solar cell process. We also discuss the structural and electronic quality of our pc-Si layers. We obtained efficiencies of up to 8% on polycrystalline-silicon solar cells made by AIC in combination with thermal CVD on alumina substrates. By replacing conventional diffused emitters by a-Si/c-Si heterojunction emitters, much higher V oc values were obtained. The use of plasma texturing led to higher current densities due to an enhanced coupling of light into the pc-Si layers. The efficiency of our pc-Si solar cells is limited by the presence of a high intragrain defect density in our layers. A detailed TEM study revealed that most of these defects are already present in the AIC seed layers prior to epitaxial thickening. To improve the material quality of our pc-Si layers, we will therefore need to improve the seed layer quality.

Thin-Film Polycrystalline-Silicon Solar Cells on Ceramic Substrates Made by Aluminum-Induced Crystallization and Thermal CVD

A considerable cost reduction could be achieved in photovoltaics if efficient solar cells could be made from thin polycrystalline-silicon (pc-Si) layers. Aluminum-induced crystallization (AIC) of amorphous silicon followed by epitaxial thickening is an effective way to obtain large-grained pc-Si layers with excellent properties for solar cells. To obtain efficient solar cells, the electronic quality of the pc-Si material obtained by AIC has to be optimized and the cell design has to be adapted to the material. In this paper, we report on pc-Si solar cells made by AIC in combination with thermal CVD on ceramic alumina substrates. We made pc-Si solar cells on alumina substrates that showed Voc values up to 533 mV and efficiencies up to 5.9%. This is the highest efficiency ever achieved with pc-Si solar cells on ceramic substrates where no (re)melting of silicon was used. We demonstrate that the quality of the pc-Si material can be improved drastically by reducing the substrate roughness using spin-on oxides. We further show that a-Si/c-Si heterojunctions lead to much higher Voc values than diffused homojunctions. A cell concept that incorporates spin-on oxides and heterojunction emitters is therefore best suited to obtain efficient pc-Si solar cells on alumina substrates.

Defect Study of Polycrystalline–silicon Seed Layers made by Aluminum Induced Crystallization

A polycrystalline silicon (pc-Si) thin film with large grains on a low-cost non-Si substrate is a promising material for thin-film solar cells. One possibility to grow such a pc-Si layer is by aluminum-induced crystallization (AIC) followed by epitaxial thickening. The best cell efficiency we have achieved so far with such an AIC approach is 8%. The main factor that limits the efficiency of our pc-Si solar cells at present is the presence of many intra-grain defects. These intra-grain defects originate within the AIC seed layer. The defect density of the layers can be determined by chemical defect etching. This technique is well suited for our epitaxial layers but relatively hard to execute directly on the seed layers. This paper presents a way to reveal the defects present in thin and highly-aluminum-doped AIC seed layers by using defect etching. We used diluted Schimmel and diluted Wright etching solutions. SEM pictures show the presence of intra-grain defects and grain boundaries in seed layers after defect etching, as verified by EBSD analyses. The SEM images of diluted Wright etched pc-Si seed layer shows that grain boundaries become much better visible than with diluted Schimmel etch.

Processing and Characterization of Efficient Thin-Film Polycrystalline-Silicon Solar Cells

The price of photovoltaic electricity could be lowered substantially if efficient solar cells could be made from polycrystalline-silicon (pc-Si) thin films on inexpensive substrates. A promising way to make pc-Si layers for solar cells is to use aluminum-induced crystallization (AIC) of amorphous silicon in combination with thermal CVD. To obtain efficient pc-Si solar cells, the material quality has to be optimized and cell processes different from those applied for standard bulk-Si solar cells have to be developed. In this paper, we present our best pc-Si solar cells so far and we discuss some key features of our solar cell process. We also discuss the structural and electronic quality of our pc-Si layers. We obtained efficiencies of up to 8% on polycrystalline-silicon solar cells made by AIC in combination with thermal CVD on alumina substrates. By replacing conventional diffused emitters by a-Si/c-Si heterojunction emitters, much higher Voc values were obtained. The use of plasma texturing led to higher current densities due to an enhanced coupling of light into the pc-Si layers. The efficiency of our pc-Si solar cells is limited by the presence of a high intragrain defect density in our layers. A detailed TEM study revealed that most of these defects are already present in the AIC seed layers prior to epitaxial thickening. To improve the material quality of our pc-Si layers, we will therefore need to improve the seed layer quality.

Defect characterization of polycrystalline silicon layers obtained by aluminum-induced crystallization and epitaxy

To reduce the harmful influence of grain boundaries in polycrystalline Si layers we make absorber layers on foreign substrates with columnar grains with a grain width larger than the grain thickness. Such layers with a grain size in the range of ~1-100 µm can be obtained by aluminum-induced crystallization and epitaxy. Until now however, the open-circuit voltage of solar cells made from such layers was quasi-independent of the grain size. To understand this fact, defect etching and Electron Backscattered diffraction (EBSD) measurements were performed to investigate the crystallographic defects. A very large density (~ 10 9 cm -2 ) of intra-grain defects (IGD) was found. Room temperature Electron Beam Induced Current (EBIC) measurements were done to localize and investigate the electrically active defects. The intra-grain defects found with defect etching showed a strong recombination activity. These results indicate that the unexpected quasi-independence on the grain size of the open-circuit voltage of our pc-Si solar cells is due to the presence of numerous electrically active intra-grain defects.

Toward Efficient Thin-Film Polycrystalline-Silicon Modules using Interdigitated Top Contacts

Efficient thin-film polycrystalline-silicon (pc-Si) modules on inexpensive substrates could substantially lower the price of photovoltaic electricity. To obtain efficient pc-Si modules, processes different from those applied for standard bulk-Si solar cells have to be developed. In this paper we report on a monolithic module process for thin-film pc-Si solar cells in substrate configuration. In this process all contacts are made on top of the cells in an interdigitated pattern. Individual pc-Si cells with interdigitated top contacts showed very good fill factors up to 75% in combination with Voc values well above 500 mV. First module tests showed that cell isolation is the most crucial step to obtain properly working pc-Si modules using our process. These results indicate that a module concept based on interdigitated top contacts could lead to highly efficient pc-Si modules