Sacrificial Porous Silicon Layer Research Articles

Laser ablation microgrooving and nanostructured silicon surfaces for an effective gettering process and reduced optical losses

This work investigates the effect of laser microgrooved monocrystalline silicon surfaces to improve the gettering effectiveness of Si substrates for enhanced solar cell electrical properties. A combination of microgrooves and a grown sacrificial porous silicon layer (PSL), subjected to a heat treatment at different temperatures was considered to point out the crucial role of the microgrooves during the gettering procedure. This treatment leads to a significant decrease of the impurities concentration and recombination activities, in addition to a noticeable enhancement of light trapping, enabling an increased carrier collection probability. The microgrooves were made by means of NWR 213 laser ablation system. The PS layers were performed by a thermal activated stain etching method that will act as sacrificial trapping layers of the gettered impurities. The topography of the patterned surfaces with and without PSLs was investigated using field emission scanning electron microscopy (FESEM). As we expected, the combination of microgrooves with higher density at both surfaces and PSL improved obviously the optoelectronic properties of the processed solar cells. The minority carrier lifetime measured by means of the photoconductive decay method (PCD) was significantly increased from 9.2 μsec to 89 μsec. The Hall Effect measurement shows a significant increase of the carrier mobility. Moreover, the dark IV characteristics and the internal quantum efficiency (IQE) found to be improved. An apparent enhancement of the open circuit voltage Voc and current density Jsc was obtained indicating improved solar cell electrical properties.

Effect of ionic Ag+ transfer on localization of metal-assisted etching of silicon surface

The specific features of fabrication of 3D silicon structures via local formation of a sacrificial porous silicon layer by metal-assisted chemical etching with 50- and 100-nm-thick silver films as a catalyst are studied. It is found that the mass transfer of Ag+ ions, caused by the temperature gradient, affects the surface morphology of the structure being formed, depending on the linear size of the mask-catalyst.

Minority carrier lifetime and efficiency improvement of multicrystalline silicon solar cells by two-step process

Impurities and defects are of significant interest in multicrystalline silicon, due to the detrimental effect they can have on carrier lifetimes and electrical properties. In view of that, it is important to incorporate certain processing steps to decrease the recombination activities. In this study, a novel experiment was applied as a beneficial approach to improve the electronic quality of low-resistivity mc-Si substrates via a two-step process. Initially, the first step involves gettering multicrystalline substrates using sacrificial porous silicon layer on both sides, which was introduced as a simple sequence for efficient extrinsic gettering schemes. The gettering experiment was performed at 600–900 °C, and optimum results were obtained at 900 °C. Then, the second step involves coating the front surface of gettered mc-Si at 900 °C with vanadium oxide that serves as an excellent antireflection layer and leads to improve furthermore the electrical properties. Significant improvements were obtained after the deposition of vanadium oxide antireflection coating, in view of the fact that gettered mc-Si substrate at 900 °C provides the highest minority carrier lifetime and the lowest effective surface recombination velocity. An overall increase of the electrical properties was obtained after the described two-step process. The conversion efficiency increases from 6% (reference) and reached 13.7%.

A novel method to enhance the gettering efficiency in p-type Czochralski silicon by a sacrificial porous silicon layer

A new two-step phosphorous diffusion gettering (TSPDG) process using a sacrificial porous silicon layer (PSL) is proposed. Due to a decrease in high temperature time, the TSPDG (PSL) process weakens the deterioration in performances of PSL, and increases the capability of impurity clusters to dissolve and diffuse to the gettering regions. By means of the TSPDG (PSL) process under conditions of 900°C/60 min + 700°C/30 min, the effective lifetime of minority carriers in solar-grade (SOG) Si is increased to 14.3 times its original value, and the short-circuit current density of solar cells is improved from 23.5 o 28.7 mA/cm2, and the open-circuit voltage from 0.534 to 0.596 V along with the transform efficiency from 8.1% to 11.8%, which are much superior to the results achieved by the PDG (PSL) process at 900°C for 90 min.

Large enhancement of the Hall mobility of the majority carrier in p-type Czochralski silicon after porous silicon damage: solutions for gettering efficiency improvement

The gettering of Czochralski silicon with a sacrificial porous silicon layer (PSL) is shown to enhance external and chemical gettering efficiency. The study of the majority charge carrier mobility of holes proves the effectiveness of the repeating iterative gettering process under a SiCl4/N2 atmosphere in comparison with external conventional gettering techniques. For aluminium and phosphorous gettering with interfacial PSL, following the segregation thermodynamics theory, it is established that the superposition of the gettering effects has occurred. This can, indeed, be explained by the obtained results, which agreed well with the theoretical model of segregation. Furthermore, for iterative and repeating gettering under SiCl4, based on gettering efficiency, it is found that a two- and three-times process with a given gettering layer dg is more efficient than one run with 2dg and 3dg.

New solar cell production process may improve efficiency and reduce cost

Photovoltaics is a renewable energy that can reduce pollution and climate change effects. A serious alternative to fossil energy, the solar industry is showing exponential growth in production. Currently about 90% of fabricated solar modules are made of crystalline silicon. Materials costs represent about 45% of the total module price. One way to make the photovoltaic industry more competitive is to reduce the consumption of silicon and increase the efficiency of solar cells. Approaches to trimming materials costs include replacing crystalline silicon with silicon thin films deposited on cheap substrates at low temperatures. However, the efficiency of solar cells made using this technique is limited by the low quality of the material. Cutting thinner wafers of crystalline silicon from the ingot can also decrease costs, but mechanical problems limit this method. We are developing silicon thin films that are epitaxially grown on a sacrificial layer and transferred to a cheap foreign substrate.1 To maximize the potential of this material, we combine it with rear contact solar cell technology. Figure 1 illustrates the process we use to create these solar cells. First, we create a sacrificial porous silicon layer on top of a monocrystalline silicon substrate. A high-quality single-crystal silicon film is then grown by epitaxy on this surface. The rear contact solar cell is created on this film. All metallic contacts are deposited on the rear side of the cell to avoid shadowing effects and to allow high efficiencies. A low-cost substrate (glass or ceramic) is then stuck on the back side of the cell. Using the sacrificial layer, the cell is separated from the initial wafer, which can be recycled several times.2 Figure 1. Process to create solar cells with lower cost and higher efficiency.

Impacts of phosphorus and aluminum gettering with porous silicon damage for p-type Czochralski silicon used in solar cells technology

In this paper, porous silicon damage (PSD) is introduced like a simple sequence for efficient extrinsic gettering schemes. The technique consists to create a sacrificial porous silicon layer on both sides of the silicon substrates with randomly hemispherical voids. Then, two main sample types are processed. In the first group, thin aluminium layers (≥ 1 μm) are thermally evaporated followed by photo-thermal annealing at 700 and 800 °C, under N 2 atmosphere. In the second group, phosphorous is continually diffused during heating at one of several temperatures ranging between 750 and 950 ° C for 1 h in a solid phase from POCl 3 solution, in N 2/O 2 ambient. Hall Effect and Van Der Pauw methods prove the existence of an optimum temperature in the case of phosphorus gettering equal to 900 °C yielding a hall mobility of about 982 cm 2 V − 1 s − 1 . FTIR investigations show an increase of interstitial oxygen enhancing the precipitation phenomena and proving that extrinsic and intrinsic gettering are in work at the same time. However, in the case of aluminum gettering, there isn't a gettering limit in the as mentioned temperature range. Moreover, silicon solar cells are processed to clarify this effect.

Enhancement of microelectronic device performances by photothermal annealing under SiCl 4 ambient

The use of low cost silicon wafers seems to be very attractive for photovoltaic and microelectronic devices. However, this material is widely contaminated by different impurities particularly transitions metals, which deteriorate the lifetimes and the bulk diffusion lengths of the minority charge carriers. One possible way to overcome this undesirable behavior is to include an efficient purification technique in the process of device fabrication. In this work, we present the effect of photothermal treatments of monocrystalline Czochralski silicon substrates under SiCl 4/N 2 atmosphere using a thin sacrificial porous silicon layer. The main results show a decrease of the resistivity over 40 μm depth. The Hall mobility of the majority charge carriers is improved from 300 to 1417 cm 2 V − 1 s − 1 . The capacitance–voltage ( C– V) characteristics of metal/SiO 2/Si (MIS) structures indicate a decrease of carrier concentration which confirms the results obtained by Hall Effect and Van Der Pauw method. The reduction of boron concentration in Czochralski silicon may reduce boron- and oxygen related metastable defect centers.