Phosphorus diffusion gettering process of multicrystalline silicon using a sacrificial porous silicon layer
© Lotfi and Hatem; licensee Springer. 2012
Received: 29 April 2012
Accepted: 5 July 2012
Published: 31 July 2012
The aims of this work are to getter undesirable impurities from low-cost multicrystalline silicon (mc-Si) wafers and then enhance their electronic properties. We used an efficient process which consists of applying phosphorus diffusion into a sacrificial porous silicon (PS) layer in which the gettered impurities have been trapped after the heat treatment. As we have expected, after removing the phosphorus-rich PS layer, the electrical properties of the mc-Si wafers were significantly improved. The PS layers, realized on both sides of the mc-Si substrates, were formed by the stain-etching technique. The phosphorus treatment was achieved using a liquid POCl3-based source on both sides of the mc-Si wafers. The realized phosphorus/PS/Si/PS/phosphorus structures were annealed at a temperature ranging between 700°C and 950°C under a controlled O2 atmosphere, which allows phosphorus to diffuse throughout the PS layers and to getter eventual metal impurities towards the phosphorus-doped PS layer. The effect of this gettering procedure was investigated by means of internal quantum efficiency and the dark current–voltage (I-V) characteristics. The minority carrier lifetime measurements were made using a WTC-120 photoconductance lifetime tester. The serial resistance and the shunt resistance carried out from the dark I-V curves confirm this gettering-related solar cell improvement. It has been shown that the photovoltaic parameters of the gettered silicon solar cells were improved with regard to the ungettered one, which proves the beneficial effect of this gettering process on the conversion efficiency of the multicrystalline silicon solar cells.
KeywordsMulticrystalline silicon Porous silicon Defect density Grain boundaries Gettering WTC-120 lifetime tester
Due to the high price of fossil energy source and huge pollution of environmental issue, in the recent years, the development of renewable energy has regained our attention. The development of the solar energy industry is one of the most popular technologies in renewable energy. The majority of solar cells are made from crystalline silicon; more than half of the crystalline silicon solar cell production is based on multicrystalline silicon (mc-Si). The quality of multicrystalline silicon (mc-Si) wafers may become worse in the future for several reasons; the main reason is that lower-quality feedstock will probably be used for cost reduction and availability reasons . Improving conversion efficiency using a low-cost material development is the main research activity in the photovoltaic field [2, 3]. Several experiments demonstrated that the electrical properties of crystalline silicon solar cells can be improved by coating the n + emitter region with a porous silicon layer [2–5], and many technologies have been used for this purpose [6–8], such as using vanadium oxide as an antireflection coating . The possibility of improving the electrical properties of silicon wafers, by extracting impurities from them, using thermal treatment under oxygen atmosphere, or phosphorus diffusion, is well known [10–13]. Besides, the porous silicon (PS) layer may be used as an efficient sacrificial layer for gettering metallic impurities [11, 14]. There are two general classifications of gettering, namely extrinsic and intrinsic. Extrinsic gettering refers to gettering that employs external means to create the damage or stress in the silicon lattice in such a way that extended defects needed for trapping impurities are formed. These chemically reactive trapping sites are usually located at the wafer surfaces, away from the bulk. Intrinsic gettering is using oxygen to enforce precipitation of metallic impurities, such as copper, not at the wafer surface but in its bulk. A preparatory step in this case is a thermal treatment causing precipitation of the excess oxygen which, for a variety of reasons related to the specific properties of oxygen in silicon, forms oxygen precipitates in the bulk of the wafer rather than at its surface. Those oxygen precipitates then act as gettering sites for metallic impurities. PS may also be used as an excellent antireflective coating or as a passivating layer on the emitter of crystalline silicon . Most often, the formation of PS can create defects in the surface of the Si substrate that may enhance the gettering effect as extrinsic gettering effect. A phosphorus gettering model was proposed , which stated that the gettering speed is controlled by two steps. The first step is to limit the gettering temperature by releasing diffusion of metal impurities, and the second step is to control the best gettering temperature by the segregation function. Other authors  proposed that silicon self-interstitial current generated during phosphorus (P) diffusion is an essential factor of the gettering mechanism, and also found that Fermi level-enhanced solubility in the P diffused layer contributes to the gettering effect. The use of an infrared (IR) furnace for the fabrication of p-n junction [18, 19] and gettering impurity through the porous silicon layer  is one of the processes that may be used to reach this goal. In this work, we will investigate the increase of carrier lifetime and the conversion efficiency of multicrystalline silicon solar cells gettered by a phosphorous gettering process using a sacrificial porous silicon layer in which the impurities will be trapped.
The starting material was a p-type (boron-doped) multicrystalline silicon substrate, 400 μm thick with a resistivity of 1 to 2 Ω cm. To avoid changes in grains and grain barriers (GBs) from wafer to wafer, samples were selected from consecutive mc-Si wafers sharpened successively in the same ingot. Similar wafers, which were vertically adjacent to each other in the ingot, were used to compare and confirm the gettering effect. Therefore, differences between them, after different processing, can be interpreted as being due to variations in the process parameters rather than to material variations. PS layers were formed on both sides by the stain-etching technique using HF/HNO3/H2O solution with a 1:3:5 volume composition . A POCl3/acetone liquid source was used for the diffusion process . We optimized the POCl3/acetone ratio to 1:5. The POCl3 spreading out was realized by the spinning technique onto p-type multicrystalline silicon wafers. After drying at 200°C for solvent evaporation, the realized P/PS/Si/PS/P structure undergoes a heat treatment in an IR furnace under an O2 atmosphere. The temperature annealing was varied in the range of 700°C to 950°C for 60 min. This heat treatment has been applied in order to allow P diffusion throughout the PS layer. After annealing the samples, the phosphorous-doped region (the phosphorous-doped PS layer) was removed from both sides using a chemical etching (HF 16%: HNO3 64%:CH3COOH 20%) solution. In order to perform the solar cell fabrication process, the n+/p junction was achieved using a simple phosphorus diffusion technique. The back aluminum (Ag/Al) and the front Ag contacts were screen printed using AMI Presco CP885 screen printer (Affiliated Manufacturers, Inc., North Branch, NJ, USA) and fired at 850°C and 620°C, respectively. The gettering effect has been evaluated by measuring the minority carrier lifetime, the dark and illuminated I V characteristics, and the defect density at the GBs of the mc-Si substrates. The I V measurements under illumination have been performed using PASAN cell tester CT 801 (PASAN Measurement Systems, Neuchâtel, Switzerland). The effective minority carrier lifetime (τeff) in the mc-Si substrates was measured using a WTC-120 photoconductance lifetime tester (Sinton Instruments, Boulder, CO, USA) under the quasi-steady-state lifetime measurement using the generalized analysis condition.
Results and discussion
The porous silicon structure has been treated with phosphorus, followed by heat treatment at various temperatures ranging between 700°C and 950°C. Then, the treated PS layers were removed, and as a result, purified mc-Si substrates have been obtained. To confirm this gettering effect on the electrical properties of the samples, we have investigated the variation of the effective minority carrier lifetime.
The effective minority carrier lifetime of the gettered mc-Si wafers has been measured under the quasi-steady-state lifetime measurement using the generalized analysis condition. The measured area always includes some grain boundaries and thus provides an overall picture of both the intra-grain material and the extra recombination at grain boundaries. It is a combination of mechanisms that determines the performance of practical solar cells. The surfaces of the wafer are always present; we have reduced their possible contribution, but especially for the highest lifetimes measured, they are still likely to have a significant effect. The lifetimes reported here should be considered as effective lifetimes, which include both surface and volume recombination components. In many practical cases, there may be several sources of recombination in a sample, such as recombination through impurities in the wafer bulk or recombination at the surfaces. The effective lifetime represents the combined impact of all of these competing recombination channels. The effective lifetime τeff measured under low-level injection at Δn = 1 × 1014 cm-3 was taken for the calculations under generalized conditions. For 2 Ω cm (the resistivity of the wafers) and ND = 5 × 1014 cm-3, the wafers satisfy the low-level injection condition (Δn < <ND).
Measured effective lifetime τ eff under low-level injection
Multicrystalline silicon incorporates many impurities and defects that limit the minority carrier lifetime and, thus, the solar cell performance . This significant variation of the minority carrier lifetime would indicate that a non-negligible quantity of unwanted impurities has been gettered and removed from the Si material and a decrease of the grain and GBs carrier recombination activities. Considering that the measurement area includes several grain boundaries, the results presented here indicate a very low recombination activity at the grain boundaries when comparing treated wafers to untreated one (reference).
Therefore, we can determine the bulk recombination properties of the wafer from the effective minority carrier lifetime measurements. As a result, the obtained improvement indicates that impurities in the bulk of the treated wafers have been removed, and the recombination activities have been decreased noticeably.
where K is the Boltzmann constant, T is the ambient temperature, e is the elementary charge, A is the Richardson constant, and S is the grain boundary surface crossed by the current flow. Nv and Na are the effective densities of the valence states and the doping concentration, respectively.
A significant increase of the IQE in the long-wavelength range 700 to 1,000 nm (red response) is observed. This improvement can be explained by the important reduction of the carrier recombination activities in the bulk of the treated wafers, which is proven by the obtained minority carrier lifetime values in Table 1, and the significant reduction of the defect density at the GBs, at least in the region near the front and back surfaces. The observed behavior of the spectral response indicates that our gettering process leads to an efficient surface and bulk passivation and indicates an extended effect deep into the bulk of the substrate, which we suggest to be considered especially at the grain boundaries. The gettered solar cell at 900°C shows the highest IQE, which is not surprising because the effective lifetime of the minority carrier in the wafer treated at 900°C proved to be the highest.
In multicrystalline silicon solar cells, the saturation current is essentially due to short circuiting via GBs. The current–voltage characteristic of the gettered samples shows an enhancement of the saturation current, which is a sign of the passivation effect of this gettering process at GBs. From these dark I V curves, we determined the series resistance (Rs)  and the shunt resistance (Rsh) of the gettered and untreated cells.
However, the enhancement of Rs could be due to the gettering effect in adjacent grains . The deep recombination centers present at the GBs and bulk defects have been reduced, and an important decrease of the surface recombination velocity has been obtained. Gettering at 950°C seems to have a bad effect on the electrical properties of the cells, which could be due to the deep diffusion of phosphorus into the mc-Si substrate when annealing the sample at 950°C.
The optimal temperature is about 900°C. This optimum could be the result of the competition between the release of impurities from the bulk and a capture of impurities in the gettering layer, which has been removed after the thermal treatment. Below the optimum temperature, gettering process is limited by the release or the diffusion [28, 30] of metallic impurities towards the gettering layer. This behavior is confirmed by the degradation of the I V characteristics for the samples treated at temperatures exceeding the optimum one.
Comparison of the electrical parameters of phosphorous gettered and ungettered multicrystalline silicon solar cells
The application of a sacrificial porous silicon layer, followed by a deposition of thin POCl3 liquid film and then subjecting to a thermal treatment, has been proven to be able to getter undesirable impurities from the mc-Si substrate. The best results were achieved after gettering at 900°C for 60 min. This gettering process has led to a significant increase of the minority carrier lifetime and a noticeable decrease of the defect density at grain boundaries. The improvement of the internal quantum efficiency confirms the beneficial effect of the above treatment. Impurities and defects at the surfaces and at the GBs have been trapped inside the P-doped PS layer after the heat treatment. Obtained results show a significant improvement of the electrical performances of the mc-Si solar cells, and the conversion efficiency has been increased from 5.9% to 11.6% in the sample gettered at 900°C.
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