G. Agostinelli

Influence of stoichiometry of direct plasma-enhanced chemical vapor deposited SiNx films and silicon substrate surface roughness on surface passivation

In this article, we report on the use of direct plasma-enhanced chemical vapor deposited silicon nitride (SiNx) films deposited at low excitation frequency (440 kHz) on low-resistivity (1.5 Ω cm) p-type Czochralski silicon substrate surfaces with different textures, to elucidate the influence of microroughness of the substrate surface on the surface-passivating properties of thin SiNx films. Whereas flat surfaces get the best passivation from Si-rich SiNx films, the optimum passivation shifts towards stoichiometric nitride as the microroughness increases, which points to the increasing relative importance of a charge-induced field effect. When short high-temperature (firing) treatments are applied upon passivation layer deposition, the process window to yield good surface passivation broadens, although very Si-rich films tend to suffer from blistering.

Rear Surface Passivation for Industrial Solar Cells on Thin Substrates

The use of hydrogenated silicon nitride is one of the most significant technological evolutions that has taken place in solar cells industry, due to its ability to act simultaneously as antireflective coating as well as a source of hydrogen for surface and bulk passivation. These very same properties make it an ideal candidate for rear surface passivation in structures with local contacts, yet its application has led so far to results below expectation. This work analyses limits and phenomena that prevent the use of standard nitride as rear surface passivation layer in commercial solar cells and presents a convenient process that can be used to overcome these problems and allows the fabrication of industrial, fully screen printed, PERC-type solar cells on ultrathin substrates. By means of this technology the cells open circuit voltage shows a significant improvement with respect to the conventional aluminum BSF and is retained all the way down to 100 mum thick devices

Impact of Rear-Surface Passivation on MWT Performances

Back-contact metal-wrap-through (MWT) solar cells are very attractive for implementation into industrial production lines. They combine the advantages of back-contact cells and the potential of easy integration into the production lines of standard cells. Nonetheless, they tend to show lower fill factors and open-circuit voltages than conventional cells. This is attributed to a non-linear shunt behavior under the emitter busbars and is believed to arise from a too-deep penetration of the silver paste printed on the emitter region on the rear during the firing step. In order to improve the MWT solar cells performances, we propose to deposit on the rear-surface a full coverage layer of a dielectric material. This layer is used first to protect the emitter during the firing step; but if it is smartly chosen, it can also be used as passivating layer for the base surface. In this work, we have processed 12.5times12.5 cm2 mc-Si wafers into 220-mum-thick MWT cells, including the deposition of a passivating dielectric layer on the rear surface. By means of dark lock-in thermography measurements, we observe that the shunting effect in the resulting cells is greatly reduced compared to neighboring cells processed into MWT with an Al-BSF rear-surface passivation. The dielectric plays in addition its role of surface passivation, according to the nearly 7 mV increase observed on the open-circuit voltage even on thick wafers. We also observe a 1.4% FF absolute increase, resulting in a 0.6% absolute efficiency increase

Advanced dry processes for crystalline silicon solar cells

Significant cost reduction of bulk crystalline silicon solar cells requires the removal of the technological barriers that impede the development of a high throughput, low cost, and reliable industrial process on thin substrates. Present industrial surface conditioning and rear surface passivation processes do not meet the requirements for yield and performance on thin substrates. In addition, large-scale production brings about the issue of the environmental impact of PV processing and its related externalities, which may contribute a significant part of the final costs and are so far, underestimated or belittled. In this paper we present an advanced, plasma-based processing technology, suitable for industrial production of bulk silicon solar cells on thin substrates and capable of meeting the PV market growth challenge with a low environmental impact and a competitive cost. The modified processing steps include plasma etching and texturing, dielectric passivation and rear side local contact schemes. Each step is compatible with the standard process sequence, can be integrated separately in the production line and leads to improved performance and/or cost reduction.

Deposition and Properties of the Pseudobinary Alloy (Al2O3)x(TiO2)1-x and Its Application for Silicon Surface Passivation

The electrical properties of (Al2O3)x(TiO2)1-x thin films, obtained from sol solution by spin coating on Si substrates (c-Si or mc-Si), have been studied. By varying the ratios between Al2O3 and TiO2 components, the optical and dielectric characteristics can be changed. This deposition method can be used for effective engineering of physical properties of the dielectric layer. Surface recombination velocities as low as 150 cm/s have been obtained using (Al2O3)x(TiO2)1-x layers on 1 Ωcm Czochralski (CZ) silicon wafers. Low surface recombination is achieved by field induced surface passivation due to a high density of negative fixed charges.

Perspectives for a-Si/c-Si heterojunction solar cells with p or n type base

In this paper, an overview is presented of amorphous silicon-crystalline silicon heterojunction solar cells on different base materials. We also tried to make a comparison with the classically used thermal diffusion. Cells were made with efficiencies of 15.8% and open circuit voltages up to 620 mV on p-type 1 /spl Omega/cm Fz material without the use of high efficiency features. When switching to 0.5 /spl Omega/cm p-type Fz material V/sub oc/ values increased up to 650 mV with a maximum efficiency of 16.4%. To be able to compare the p or n type base for the heterojunction approach, we also investigated the heterojunction emitter on n type Cz material with a thermally in-diffused phosphorus BSF. The highest efficiency achieved so far on this n-type material was 14.1% with a maximum V/sub oc/ of 630 mV. Despite the known shortcomings of p-type base hetero-junction solar cells compared to n-type base hetero-junction solar cells, they might find application in thin film solar cells. First result on thin freestanding films yielded efficiencies up to 9.6%. Using the heterojunction emitter instead of the diffused emitter higher V/sub oc/ were obtained on several investigated materials.

IIb-2 – Low Cost Industrial Technologies of Crystalline Silicon Solar Cells

Publisher Summary This chapter discusses industrial technologies for manufacturing crystalline silicon solar cells at low costs. Typical efficiency of commercially produced crystalline silicon solar cells lies in the range of 14%–17%. Because the efficiency of the cell influences the production cost at any production stage, considerable efforts are directed toward efficiency improvement of solar cells. The required near future efficiency goals for industrial cells are 18–20% on monocrystalline, and 16–18% on multicrystalline silicon. Based on laboratory scale achievements, one can consider that production type cells able to fulfill the efficiency goal should have features including front surface texturing, optimized emitter surface concentration and doping profile, effective front surface passivation, fine line front electrode, and front electrode passivation. This chapter discusses the concepts of cell processing, substrates, etching, texturing, optical confinement, and junction formation in detail. It then explains front surface passivation and functions of antireflection coating. Techniques of gettering by phosphorous diffusion and gettering by aluminum treatment are also discussed. Concepts related to screen-printed solar cells, buried contact solar cells, solar cells on silicon ribbons, and back-contacted solar cells are explained in detail in the chapter.

Selective junction separation techniques for multi-crystalline silicon solar cells

This paper addresses the need for new junction separation techniques that are low-cost, selective, have a high throughput and high-yield. Pre- as well as post-diffusion selective approaches as integrated into an existing low cost screenprinted metallisation processing scheme are discussed and compared to state-of-the-art technology.

IIb-2 – Low cost industrial technologies of crystalline silicon solar cells

Publisher Summary This chapter proposes a cheap and good-quality solar-grade polysilicon feedstock material to increase the sizes of substrates, to reduce the kerf loss in slicing, and to decrease the thickness of the substrates below 200 μm. Silicon substrates used in commercial solar cell processes contain a near-surface saw-damaged layer, which has to be removed at the beginning of the process. The thickness of the damaged layer depends on the technique used in wafering of the ingot. The silicon surface after saw-damage etching is shiny and reflects more than 35% of incident light. An important step in solar cell processing therefore consists of texturing the front surface—to create a structure that causes reflected rays to get a second chance to be coupled into the cell. The reflection losses in commercial solar cells are reduced mainly by random chemical texturing. Monocrystalline silicon substrates with a surface orientation can be textured by anisotropic etching at temperature of 70–80℃ in a weak solution of sodium hydroxide or potassium hydroxide with addition of isopropanol.

Progress on the Pseudo-Binary (Al2O3)3(TiO2)2-System for Surface Passivation of P-Type Silicon

We have reported before on the use of (Al2O3)3(TiO2)1-x pseudo-binary alloys, deposited by spin coating and thermally treated, for field-induced surface passivation of p-type silicon. These layers show fixed negative charge densities in the order of 1012 cm -2 and yield low surface recombination velocities, but the passivating properties are degrading with time. It was found now that a treatment at high temperature (720degC-1000degC) in oxygen substantially reduces this degradation. The influence of temperature, oxygen flow and duration of the treatment on the effective lifetime is studied, as are effects occuring at the PBA/Si interface. A first optimization of this process leads to surface recombination velocities below 150 cm/s on low-resistivity FZ material and around 300 cm/s on CZ months after deposition. Experiments on the exact nature of the degradation mechanism are ongoing