Hydrogen passivation of defects in multicrystalline silicon solar cells

The knowledge of how hydrogen interacts with defects and impurities in silicon is crucial for the understanding of device performance, especially for solar cells made from disordered silicon wafers. Hydrogen can be introduced in silicon by several techniques, but this paper will be focused on hydrogenation by means of plasma enhanced chemical vapor deposition of hydrogen-rich silicon nitride layer on the surface of the wafer. Passivation effects are observed after annealing and evaluated using minority carrier diffusion length measurements and light-beam-induced current scan maps.It was found that individual intragrain defects are well passivated, while deep levels are transformed into poorly recombining shallow levels at grain boundaries and dislocation clusters. In solar cells, the stability of the hydrogen passivation is much higher with this technique than with other hydrogenation techniques. This is probably due to an encapsulation of hydrogen by the frontwall silicon nitride coating layers and by the backside aluminum film.     

Defect passivation of industrial multicrystalline solar cells based on PECVD silicon nitride

Low surface recombination velocity and significant improvements in bulk quality are key issues for efficiency improvements of solar cells based on a large variety of multicrystalline silicon materials. It has been proven that PECVD silicon nitride layers provide excellent surface and bulk passivation and their deposition processes can be executed with a high throughput as required by the PV industry. The paper discusses the various deposition techniques of PECVD silicon nitride layers and also gives results on material and device properties characterisation. Furthermore the paper focuses on the benefits achieved from the passivation properties of PECVD SiN_x layers on the multi-Si solar cells performance. This paper takes a closer look at the interaction between bulk passivation of multi-Si by PECVD SiN_x and the alloying process when forming an Al-BSF layer. Experiments on state-of-the-art multicrystalline silicon solar cells have shown an enhanced passivation effect if the creation of the alloy and the sintering of a silicon nitride layer (to free hydrogen from its bonds) happen simultaneously. The enhanced passivation is very beneficial for multicrystalline silicon, especially if the defect density is high, but it poses processing problems when considering thin (<200 μm) cells.     

UVCVD silicon nitride passivation and ARC layers for multicrystalline solar cells

This work intends to investigate the effectiveness of silicon nitride layers (SiN_x : H) deposited by photochemical vapor deposition (UVCVD) for antireflection and passivation purposes when applied to electromagnetically casted silicon solar cells (EMC). Effective reflectivity of 10.8% is achieved, as well as 66% increase of minority carrier lifetime.     

Surface passivation in high efficiency silicon solar cells

Surface passivation for crystalline silicon solar cells is particularly important for devices with open-circuit voltages in excess of 650 mV. Thick passivating thermal oxides, originally developed for use with buried contact solar cells, are shown to produce the most effective and stable surface passivation particularly in conjunction with lightly phosphorus diffused surfaces. However, for improved optical performance, antireflection coatings are only effective with surface oxide thicknesses reduced to 100–200 Å. Thinner passivating oxides cause significant voltage loss, most of which can be recovered through hydrogen passivation. Throughout this study, variation in surface passivation approaches has produced open-circuit voltages ranging from 620 mV to record voltages of 720 mV.     

Field effect passivation of high efficiency silicon solar cells

In this paper effective surface recombination velocities S_eff at the rear Si---SiO₂ interface of the presently best one -sun silicon solar cell structure are calculated on the basis of measured oxide parameters. A new cell design is proposed allowing for a control of the surface space charge region by a gate voltage. It is shown that the electric field introduced by the positive fixed oxide charge density typically found at thermally oxidized silicon surfaces and the favorable work function difference between the gate metal aluminum and silicon leads to a reduction of S_eff to values well below 1 cm/s at AM1.5 illumination for n-type as well as p-type silicon. At low illumination levels, however, oxidized n-type silicon has much better surface passivation properties than p-type silicon due to the small hole capture cross section (σ_n/gs_p ≈ 1000 at midgap). Only at small illumination intensities for p-type substrates or in the case of poor Si---SiO₂ interface quality the incorporation of a gate electrode on the rear surface is a promising tool for further reducing surface recombination losses.     

Rie-texturing of multicrystalline silicon solar cells

We developed a maskless plasma texturing technique for multicrystalline silicon cells using reactive ion etching that results in higher cell performance than that of standard untextured cells. Elimination of plasma damage has been achieved while keeping front reflectance to extremely low levels. Internal quantum efficiencies as high as those on planar cells have been obtained, boosting cell currents and efficiencies by up to 7% on evaporated metal and 4% on screen-printed cells.     

High-efficiency multicrystalline silicon solar cells by liquid phase-epitaxy

Thin-film silicon cells produced on crystalline silicon substrates have the potential to achieve high cell efficiencies at low cost. We have used a modified liquid-phase epitaxy growth process to produce very smooth, high-quality silicon films on multicrystalline silicon substrates. Photoconductivity decay measurements indicate that the minority carrier lifetimes in these layers are at least 10 μs, sufficient to achieve cell efficiencies in excess of 16%. This efficiency potential is confirmed in small area cells, which have displayed efficiencies up to 15.4%. Further improvements up to 17% efficiency are possible in the short term, even without the introduction of any light-trapping schemes into the device structure.     

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%.     

Effects of hydrogen plasma on passivation and generation of defects in multicrystalline silicon

Hydrogenation by plasma is a low cost and efficient method to improve the photovoltaic properties of multicrystalline silicon. The role of plasma parameters on the efficiency of hydrogenation was studied using secondary ion mass spectrometry (SIMS), hydrogen effusion, electrochemical impedance spectroscopy and electron beam induced current (EBIC). The experimental results showed a deuterium concentration of 10²⁰ atoms cm⁻³ could be reached in the sample after a 15-min treatment. Optimal treatment time depends on temperature and leads to maximum electrical conductivity and minority carrier diffusion length. The results confirm the reduction of defects densities and potential barriers associated with grain boundaries.     

Efficiency improvement by porous silicon coating of multicrystalline solar cells

Shallow junction multicrystalline Si solar cells have been processed by an anodical etching technique. More than 25% improvement in short-circuit current and photovoltaic energy conversion efficiency was demonstrated. It was shown that improved performance was caused by antireflection action of the porous silicon layer as well as by the cell surface and grain boundary passivation.     

Production technology of large-area multicrystalline silicon solar cells

In 1996 a conversion efficiency of 17.1% had been obtained on 15 cm×15 cm mc-Si solar cell. In this paper, large-scale production technology of the high-efficiency processing will be discussed. Enlarging reactive ion etching (RIE) equipment size, technology of passivation, and fine contact grid with low resistance by screenprinted metallization, which is firing through PECVD SiN, have been investigated.     

Texturisation of multicrystalline silicon solar cells by RIE and plasma etching

Texturing by reactive ion etching (RIE) is demonstrated as an attractive technical solution for lowering of reflectance of multicrystalline silicon solar cells. A suitable sequence of processes is developed to combine the advantage of reactive ion etching with “natural lithography” based on colloidal masks. The RIE single-wafer texturisation is driven to an industrial applicable batch process by plasma etching with a gain in efficiency of 0.3% absolute.     

Characterization of PIII textured industrial multicrystalline silicon solar cells

Optimized textured structure is one of the most important elements for high efficiency multicrystalline silicon solar cells. In this paper, in order to incorporate low reflectance nanostructures into conventional industrial solar cells, structures with aspect ratios of about 1:1 and average reflectance of 8.0% have been generated using plasma immersion ion implantation. A sheet resistance of 56.9 Ω/sq has been obtained by adjusting the phosphorous diffusion conditions, while the thickness of the silicon nitride vary in 70–80 nm by extending the deposition time by 100 s. Under the conventional co-firing conditions, a solar cell with efficiency of 16.3% and short-circuit current density 34.23 mA/cm² has been fabricated.     

Surface passivation of crystalline silicon wafer via hydrogen plasma pre-treatment for solar cells

The carrier lifetime of crystalline silicon wafers that were passivated with hydrogenated silicon nitride (SiN_x:H) films using plasma enhanced chemical vapor deposition was investigated in order to study the effects of hydrogen plasma pre-treatment on passivation. The decrease in the native oxide, the dangling bonds and the contamination on the silicon wafer led to an increase in the minority carrier lifetime. The silicon wafer was treated using a wet process, and the SiN_x:H film was deposited on the back surface. Hydrogen plasma was applied to the front surface of the wafer, and the SiN_x:H film was deposited on the hydrogen plasma treated surface using an in-situ process. The SiN_x:H film deposition was carried out at a low temperature (<350 °C) in a direct plasma reactor operated at 13.6 MHz. The surface recombination velocity measurement after the hydrogen plasma pre-treatment and the comparison with the ammonia plasma pre-treatment were made using Fourier transform infrared spectroscopy and secondary ion mass spectrometry measurements. The passivation qualities were measured using quasi-steady-state photoconductance. The hydrogen atom concentration increased at the SiN_x:H/Si interface, and the minority carrier lifetime increased from 36.6 to 75.2 μs. The carbon concentration decreased at the SiN_x:H/Si interfacial region after the hydrogen plasma pre-treatment.     

Progress in the metallisation of n-type multicrystalline silicon solar cells

A study on the optimisation of front contacts of n-type multicrystalline silicon solar cells is presented. In this study the same cell processing was applied to two types of wafers: electronic grade (EG-Si) and metallurgic grade (MG-Si) silicon. The contact firing temperature was optimised, by measuring the contact resistivity of the front and back contacts for different firing temperatures. The front contacts were improved by deposing silver using an electrochemical process. The solar cells were characterised before and after the silver deposition. For all cells processed the line resistance was reduced by over 90% after the silver deposition. After the contact improvement, EG-Si cells showed an absolute efficiency improvement of 2.6%, but MG-Si cells suffered a reduction on the cell efficiency, an effect related to parasitic shunting existent in these cells.     

Recent results obtained at IMEC on multicrystalline silicon solar cells

In order to optimize the efficiency of multicrystalline silicon solar cells, the influence of specific process steps and sequences were studied. Therefore clean-room high efficiency as well as industrial screen-printed cells were fabricated. Benefits are found in choosing a substrate with lower base resistivity, using front and rear oxide passivation, using hydrogen passivation for bulk and surfaces, the use of Si₃N₄ with a double function i.e. as an anti-reflection and passivation layer and the use of mechanical V-grooving. Efficiencies of 17% are found on 4 cm² clean-room fabricated cells and 15.2% has been obtained on 100 cm² V-grooved screenprinted industrial cells.     

Solar cells prepared on multicrystalline silicon subjected to new gettering and passivation treatments

New combined gettering and passivating procedures for solar cells prepared from multicrystalline silicon (mc-Si) have been considered. Passivation has been performed by (i) diamond-like carbon films deposition onto front or rear side of the wafers with following annealing, or (ii) hydrogen plasma treatments. Gettering region has been formed by deposition of Al film on specially prepared Si with developed surface. The advantages of such a gettering process in comparison with traditional gettering with Al are demonstrated. The improving influence of the treatments on diffusion length in mc-Si and efficiency of prepared solar cells have been found out. Physical mechanisms responsible for the observed effects of gettering and passivation are discussed.     

Overview of solar cell technologies and results on high efficiency multicrystalline silicon substrates

Fabrication technologies for multicrystalline silicon (mc-Si) solar cells have advanced in recent years with efficiencies of mc-Si cells exceeding 18%. Intense efforts have been made at laboratory level to improve process technology, growth methods, and material improvement techniques to deliver better devices at lower cost. Deeper understanding of the physics and optics of the device led to improved device design. This provided a fruitful feedback to the industrial sector. Both screenprinting and buried-contact technologies yield cells of high performance. An increasingly large amount of research activity is also focussed on the fabrication of thin solar cells on cheap substrates such as glass, ceramic, or low quality silicon. Success of these efforts is expected to lead to high efficiency devices at much lower costs. Efforts are also being put on low thermal budget processing of solar cells based on rapid thermal annealing.     

A simple passivation technique for the edge area of silicon solar cells improves the efficiency

The efficiency of silicon solar cells (SC) can strongly be degraded by localized defects especially at the edge of SC (e.g. scratches) which are introduced during the production of the SC and may cause local shunts. A new optimized chemical etching procedure has been developed which allows a very effective passivation of shunts at the SC edges without reducing the surface area, i.e. without a reduction of the I_sc current. In contrast to other techniques like plasma etching (“coin staking”) or cutting off the edges, this procedure could be implemented cheaply in a large-scale production.The newly developed passivation method always leads to an improvement in the efficiency η of slightly or severely degraded SCs which is typically around 10–30%, but can be as large as 100%, while good SCs are totally unaffected with respect to η but still showing an improvement of the leakage current.     

Optimum two-dimensional short circuit collection efficiency in thin multicrystalline silicon solar cells with optical confinement

The two-dimensional short-circuit AM1.5 collection efficiency is studied in thin multicrystalline silicon solar cells with optical confinement. The collection efficiency is calculated by linking an optical analytical generation profile with the two-dimensional collection probability in pn junction solar cells. The calculations are carried out for variable grain boundary recombination velocity, cell thickness, grain width, diffusion length, and back surface recombination velocity. The role of optical confinement leading to a strong dependence of the collection efficiency on the cell thickness in very thin cells is confirmed. The optimum cell thickness for maximum collection efficiency increases in cells with low back reflection or poor back surface passivation. Also, the optimum thickness in very thin cells increases significantly with increasing the diffusion length. It is also found that the effect of grain boundary recombination is predominant if the cell thickness is larger than the diffusion length and if the diffusion length is larger than half the grain width, especially, in cells with unpassivated grain boundaries. On the other hand, back surface recombination dominates the response in cells with unpassivated back surface if the thickness is smaller than or comparable to the diffusion length.