Silicon feedstock for the multi-crystalline photovoltaic industry

During the last 5 years the PV industry continues to experience a strong economic growth between 15% and 30% per year. Multi-crystalline silicon became the preferred material for PV production with a share of more than 50% of the shipped PV modules world-wide. For the first time, the available quantity of the classical silicon feedstock sources for the PV industry—electronic grade silicon rejects from the silicon and microelectronics industry—is close to be not sufficient to satisfy the requirements of the PV industry. From this situation arises the need to develop short- and long-term solutions to guarantee a sustainable supply of the PV industry with suitable silicon feedstock at acceptable costs.This paper presents a possible route for short- and long-term solutions to provide solar grade (SoG) silicon feedstock for the PV industry. On a short-term basis a twofold solution is proposed: (i) reduction of silicon consumption by reducing the wafer thickness and the introduction of recycling scenarios for silicon waste produced by the PV industry, (ii) introduction of very low-resistivity silicon (0.1 Ω cm).On long term, a route towards the establishment of a SoG silicon production based on widely available metallurgical grade silicon is proposed. This route includes the development of suitable purification techniques. First results that allowed to lower the impurity tolerances for SoG silicon are presented. The introduction of silicon feedstock with higher impurity concentrations which show a tendency to interact with crystal defects and lead to a degradation of the material performance also requires passivation concepts to achieve highly performing solar cells.     

Metal impurities behaviors of silicon in the fractional melting process

The photovoltaic (PV) rapid growth suffers the severe shortage of silicon. The metallurgical route to solar grade (SoG) silicon is the alternative solution. One of the methods suggested the fractional melting process. Because the metal impurities in the metallurgical grade (MG) silicon such as Fe, Al, Ti and Cu deteriorate the efficiency of the solar cell seriously, it is important to remove those metal elements from MG-Si to upgrade the silicon. The refining behaviors of the metal impurities, however, do not equal in FM process. Cu and Al behaviors in the Si during FM process are studied using SEM, EPMA and ICP-AES. The diffusion coefficient and the grain boundary (GB) enrichment behaviors of the elements are rationalized to cause the difference.     

Multicrystalline silicon wafers prepared from upgraded metallurgical feedstock

A solution to the problem of the shortage of silicon feedstock used to grow multicrystalline ingots can be the production of a feedstock obtained by the direct purification of upgraded metallurgical silicon by means of a plasma torch. It is found that the dopant concentrations in the material manufactured following this metallurgical route are in the 10¹⁷ cm⁻³ range. Minority carrier diffusion lengths L_n are close to 35 μm in the raw wafers and increases up to 120 μm after the wafers go through the standard processing steps needed to make solar cells: phosphorus diffusion, aluminium–silicon alloying and hydrogenation by deposition of a hydrogen-rich silicon nitride layer followed by an annealing. L_n values are limited by the presence of residual metallic impurities, mainly slow diffusers like aluminium, and also by the high doping level.     

Performance improvements of crystalline silicon by iterative gettering process for short duration and with the use of porous silicon as sacrificial layer

In this work, we demonstrate that an efficient purification method of silicon wafers where iterative sequences were used. Each sequence consists of forming porous silicon (PS) on both sides of the samples, followed by thermal annealing in an infrared furnace under N₂/SiCl₄ ambient. Improvements of the electronic parameters were obtained by optimizing the heat treatments temperatures and the number and duration of the iteration sequences. Best results were obtained for temperatures below 980 °C and for three sequences of 20 min each one. After three sequences the mobility of the majority carrier improved from 94 cm² V⁻¹ s⁻¹ (for untreated wafer) to about 374 cm² V⁻¹ s⁻¹. The observed results were explained taking into account the transport properties of the impurities in the porous media and their concentration at the walls at each iteration. It was found that short iterative sequences give almost the same results than one long sequence duration. Silicon solar cells based on iterative gettered silicon wafers exhibit an increase in the short-circuit current and the open-circuit voltage. This fact seems to be important to ameliorate solar grade silicon (SGS) based solar cells performances.     

A study of electrical uniformity for monolithic polycrystalline silicon solar cells

The aim of this work is to investigate the electrical uniformity of monolithic polycrystalline silicon solar cells prepared by various process techniques. By a series of experiments such as P and Al impurity gettering and silicon nitride passivation, a new conclusion is that the application of P and Al gettering as well as silicon nitride passivation enhances the electrical uniformity of small area solar cells diced from the same polycrystalline silicon solar cells, even if impurity gettering is not effective when the dislocation density is above a threshold value of about 10⁶ cm⁻². The experiments give us some hints that when we cut large area polycrystalline silicon solar cells into small pieces needed for application, we should modify production process slightly.     

Research on new method of electron beam candle melting used for removal of P from molten Si

Abstract

A new method, electron beam candle melting (EBCM), is proposed for the removal of P in molten Si, to produce high quality material such as solar grade silicon for photovoltaic applications. EBCM is designed to overcome the shortcomings of electron beam melting while utilising the high saturated pressure of P in molten Si to effect refining. The experimental result showed that it could remove P from Si effectively; in addition, the energy utilisation ratio was experimentally proved to be high. The evaporation coefficient of P removal is in a reasonable region and comparable with the theoretical value, which indicates that EBCM is a feasible method for the removal of P in molten Si in low power.     

Refining of MG-Si by hybrid melting using steam plasma and EMC

In this study, it was aimed to remove boron, one of the major impurities of MG-Si (metallic grade silicon), in molten silicon by a steam plasma method utilizing EMC (electro magnetic casting) process. In order to effectively remove boron, less volatile than silicon, plasma injection gas was systematically varied. As a result, the most effective way to remove boron was additionally injecting hydrogen gas and steam with argon gas, which is a reactive gas of a general plasma injection routine. Although mass loss of silicon increased with increase in plasma injection time, the refining effect of boron and several metallic impurities like aluminium and titanium was improved significantly.     

Stability of thin film solar cells having less-hydrogenated amorphous silicon i-layers

In this study, highly stabilized hydrogenated amorphous silicon films and their solar cells were developed. The films were fabricated using the triode deposition system, where a mesh was installed between the cathode and the anode (substrate) in a plasma-enhanced chemical vapor deposition system. At a substrate temperature of 250 °C, the hydrogen concentration of the resulting film (Si–H=4.0 at%, Si–H₂<1×10²⁰ cm⁻³) was significantly less than that of conventionally prepared films. The films were used to develop the i-layers of solar cells that exhibited a significantly low degradation ratio of 7.96%.     

Porous silicon as an intermediate layer for thin-film solar cell

The potential of porous silicon (PS) with dual porosity structure as an intermediate layer for ultra-thin film solar cells is described. It is shown that a double-layered PS with a porosity of % allows to grow epitaxial Si film at medium temperature (725°–800°C) and at the same time serves as a gettering/diffusion barrier for impurities from potentially contaminated low-cost substrate. A 3.5 μm thin-film cell with reasonable efficiency is realized using such a PS intermediate layer.     

Purification of metallurgical grade silicon by a solar process

The purification of upgraded metallurgical silicon by extraction of boron and phosphorus was experimentally demonstrated using concentrated solar radiation in the temperature range 1550–1700 °C. The process operated with a flow of Ar at reduced pressure (0.05 atm) for elimination of P, and with a flow of H₂O for elimination of B. Impurity content decreased by a factor of 3 after a 50-min solar treatment, yielding Si samples with final average content of 2.1 ppm_w B and 3.2 ppm_w P.     

Latest results on semitransparent POWER silicon solar cells

The paper presents the latest results of the polycrystalline wafer engineering result (POWER) silicon solar cell research (G. Willeke, P. Fath, The POWER silicon solar cell, Proceedings of the 12th EPVSEC, Amsterdam, 1994, pp. 766–768). Mono – as well as bifacially active semitransparent silicon solar cells have been created by forming perpendicularly overlapping grooves on the front and the rear side of a silicon wafer resulting in a regular pattern of holes. The developed very simple manufacturing process is fully compatible with an industrial production and uses POCl₃-tube diffusion, PECVD silicon nitride as single ARC and screen-printing metallization. Maximum efficiencies of η=11.2% for monofacial POWER cells on 0.4 Ω cm Cz material with a transparency of 18.2% and η=12.9% for bifacial cells on 1 Ω cm Cz material with a transparency of 16% have been obtained. Results for multicrystalline (mc) semitransparent mono- and bifacially active silicon solar cells are also presented.     

Microcrystalline/micromorph silicon thin-film solar cells prepared by VHF-GD technique

Hydrogenated microcrystalline silicon prepared at low temperatures by the glow discharge technique is examined here with respect to its role as a new thin-film photovoltaic absorber material. XRD and TEM characterisations reveal that microcrystalline silicon is a semiconductor with a very complex morphology. Microcrystalline p–i–n cells with open-circuit voltages of up to 560–580 mV could be prepared. “Micromorph” tandem solar cells show under outdoor conditions higher short-circuit currents due to the enhanced blue spectra of real sun light and therefore higher efficiencies than under AM1.5 solar simulator conditions. Furthermore, a weak air mass dependence of the short-circuit current density could be observed for such micromorph tandem solar cells. By applying the monolithic series connection based on laser patterning a first micromorph mini-module (total area of 23.6 cm²) with 9% cell conversion efficiency could be fabricated.     

Microcrystalline silicon and the impact on micromorph tandem solar cells

Intrinsic microcrystalline silicon opens up new ways for silicon thin-film multi-junction solar cells, the most promising being the “micromorph” tandem concept. The microstructure of entirely microcrystalline p–i–n solar cells is investigated by transmission electron microscopy. By applying low pressure chemical vapor deposition ZnO as front TCO in p–i–n configurated micromorph tandems, a remarkable reduction of the microcrystalline bottom cell thickness is achieved. Micromorph tandem cells with high open circuit voltages of 1.413 V could be accomplished. A stabilized efficiency of around 11% is estimated for micromorph tandems consisting of 2 μm thick bottom cells. Applying the monolithic series connection, a micromorph module (23.3 cm²) of 9.1% stabilized efficiency could be obtained.     

Investigation of the damage as induced by 1.7 MeV protons in an amorphous/crystalline silicon heterojunction solar cell

Current–voltage under illumination and quantum yield characteristics of an amorphous silicon/crystalline silicon hetero solar cell have been measured before and after exposure to high-energy (1.7 MeV) protons. A comparison of the measured wavelength-dependent quantum yield with calculated values enabled to determine the effective electron diffusion length of the crystalline silicon, that dropped from a value of 434 μm before to a value of 4 μm after irradiation with 5×10¹² cm⁻² protons. Good agreement has been obtained between measured and simulated data using DIFFIN,¹ a finite-element simulation program for a-Si:H/c-Si heterojunction solar cells, enabling us to extract the depth profile of the recombination rate and the density of states distribution in the semiconductor layers before and after irradiation.     

Spatial distribution of minority-carrier lifetime and local concentration of impurities in multicrystalline silicon solar cells

Spatial distribution of minority-carrier lifetime (τ) in multicrystalline silicon solar cells was investigated. By mapping of τ, a wide distribution in a higher-efficiency cell and a narrow distribution in a lower-efficiency cell was found, respectively. Based on the relation between τ and the density of grain boundaries, spatial distribution of impurities was characterized by secondary ion mass spectrometry. To clarify the spatial profile near grain boundaries, a method of changing detection area was carried out. Iron (Fe) segregation within 100 μm around a grain boundary was observed. In the lower-efficiency cell, a measurable amount of Fe was contained in a crystalline area as well.     

Evolution of microstructure and phase in amorphous, protocrystalline, and microcrystalline silicon studied by real time spectroscopic ellipsometry

Real time spectroscopic ellipsometry has been applied to develop deposition phase diagrams that can guide the fabrication of hydrogenated silicon (Si:H) thin films at low temperatures (<300°C) for highest performance electronic devices such as solar cells. The simplest phase diagrams incorporate a single transition from the amorphous growth regime to the mixed-phase (amorphous+microcrystalline) growth regime versus accumulated film thickness [the a→(a+μc) transition]. These phase diagrams have shown that optimization of amorphous silicon (a-Si:H) intrinsic layers by RF plasma-enhanced chemical vapor deposition (PECVD) at low rates is achieved using the maximum possible flow ratio of H₂ to SiH₄ that can be sustained while avoiding the a→(a+μc) transition. More recent studies have suggested that a similar strategy is appropriate for optimization of p-type Si:H thin films. The simple phase diagrams can be extended to include in addition the thickness at which a roughening transition is detected in the amorphous film growth regime. It is proposed that optimization of a-Si:H in higher rate RF PECVD processes further requires the maximum possible thickness onset for this roughening transition.     

Extension of the a-Si:H electronic transport model to μc-Si:H: use of the μτ product to correlate electronic transport properties and solar cell performances

The aim of this communication is to show that it is possible to extend the model of the electronic transport developed for amorphous silicon (a-Si:H) to microcrystalline silicon (μc-Si:H). By describing the electronic transport with the μ⁰τ^R products (mobility×recombination time) as a function of the Fermi level, we observed the same behaviour for both materials, indicating a similar type of recombination. Moreover, applying the normalised μ⁰τ⁰ product (mobility×life-time) obtained by combining the photoconductivity (σ_photo) and the ambipolar diffusion length (L_amb) measured in individual layers, we are able, as in the case of a-Si:H, to predict the quality of the solar cells incorporating these layers as the active i layer.     

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.     

Light scattering coefficients for textured SnO

Transparent electrodes that are used in amorphous silicon solar cells are textured to provide light scattering. We studied the light scattering behavior in transmission and reflection and found that in order to describe the measured spectra thickness variations of 50–60 nm over several micrometers have to be assumed. This is in qualitative agreement with measured rms roughnesses as determined with atomic force microscopy (AFM). It is important to include these thickness variations in the modeling of amorphous silicon solar cells. The wavelength dependence of the light scattering in transmission at the TCO–air interface was found to be λ⁻³ for light incident from both sides. Scattering of the weakly absorbed long wavelength light at the back contact is therefore essential in order to obtain high solar cell efficiencies.     

Amorphous silicon/p-type crystalline silicon heterojunction solar cells

We have investigated the photovoltaic (PV) characteristics of both glow discharge deposited hydrogenated amorphous silicon (a-Si:H) on crystalline silicon (c-Si) in a n⁺ a-Si:H/undoped a-Si:H/p c-Si type structure, and DC magnetron sputtered a-Si:H in a n-type a-Si:H/p c-Si type solar cell structure. It was found that the PV properties of the solar cells were influenced very strongly by the a-Si/c-Si interface. Properties of strongly interface limited devices were found to be independent of a-Si thickness and c-Si resistivity. A hydrofluoric acid passivation prior to RF glow discharge deposition of a-Si:H increases the short circuit current density from 2.57 to 25.00 mA/cm² under 1 sun conditions.DC magnetron sputtering of a-Si:H in a Ar/H₂ ambient was found to be a controlled way of depositing n type a-Si:H layers on c-Si for solar cells and also a tool to study the PV response with a-Si/c-Si interface variations. 300 Å a-Si sputtered onto 1–10 ω cm p-type c-Si resulted in 10.6% efficient solar cells, without an A/R coating, with an open circuit voltage of 0.55 V and a short circuit current density of 30 mA/cm² over a 0.3 cm² area. High frequency capacitance-voltage measurements indicate good junction characteristics with zero bias depletion width in c-Si of 0.65 μm. The properties of the devices have been investigated over a wide range of variables like substrate resistivity, a-Si thickness, and sputtering power. The processing has focused on identifying and studying the conditions that result in an improved a-Si/c-Si interface that leads to better PV properties.