The porous silicon layer created onto the front and back surfaces of the multicrystalline silicon wafers has a great importance in this work because it has the role of an external trapping site of impurities extracted from the samples. Figure a,b presents an atomic force microscopy (AFM) topography of the porous silicon layer formed by the stain-etching technique.
AFM topography of the porous silicon layer formed by the stain-etching technique. (a) Porous silicon formed on both sides of the samples. (b) 3D AFM topography before treatment with phosphorous.
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
was taken for the calculations under generalized conditions. For 2 Ω cm (the resistivity of the wafers) and ND
, the wafers satisfy the low-level injection condition (Δn
The effectiveness of this gettering process is confirmed by the increase of the minority carrier lifetime, measured at the same carrier density, before and after treatment. Table summarizes the effect of this treatment process on the effective minority carrier lifetime (τeff). We have obtained a very high lifetime value of the minority carrier after a thermal treatment at 900°C. We notice that τeff increases when the annealing temperature increases, as shown in Table .
Measured effective lifetime τeff under low-level injection
Multicrystalline silicon incorporates many impurities and defects that limit the minority carrier lifetime and, thus, the solar cell performance [21
]. 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).
It is important to note that the surface and volume recombination mechanisms coexist. We considered that both front and back surfaces of the wafer are identical since both of them have been subjected to the same treatment. The equation that takes into account all the recombination mechanisms that are present in the wafer is given by [22
Supposing that both front and back surfaces of all samples were subjected to the same passivation effect and have the same surface state, we can neglect the effect of the surfaces [22
]. Consequently, (τbulk
) can be deduced:
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.
It is now interesting to investigate the variation of the defect density at the GBs while changing the treatment conditions, which is a one of the important parameters to evaluate the effect of our gettering process. We have performed four aluminum ohmic contacts on two adjacent grains (as shown in Figure ). The aluminum was deposited by screen printing using Al/Ag containing aluminum and a small amount of silver in order to ensure the welding of the contact’s edges. A DC current passes through the selected GBs between two adjacent grains using two external contacts. Using the two inner contacts, we have measured the voltage U
. Figure shows a schematic illustration of the dark IV
measurements across four selected GBs to evaluate the defect density (NB
Schematic illustration of the dark I-V measurements at a selected grain boundary.
The zero-bias conductance for grain boundary is defined by [24
is the applied voltage. It can be written as [26
The GB potential barrier can be written as following:
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.
The defect density NB
) was determined as a function of the GB potential barrier VB
Identical wafers have been treated and identical GBs in the gettered samples show that defect density decreases when samples were subjected to thermal treatment, as shown in Figure . This significant variation of the defect density at the GBs would indicate that a non-negligible quantity of undesirable impurities have been gettered and removed from the grain boundaries. Thus, the recombination activities at the GBs have been reduced noticeably. These improvements could be attributed to the removal of eventual bulk metal impurities (Fe, Cu, etc.) and their trapping into the n+/PS layer. We notice that high-temperature annealing in infrared furnace has enhanced the impurity diffusion into the sacrificial porous silicon layer. Thus, the phosphorus-rich PS acts as an efficient external gettering site in which the impurities are captured due to the high thermal treatment. The phosphorus diffusion into grain boundaries, at least in the region near the front and back surfaces, can explain the considerable decrease of the defect density and the recombination activities into the GBs, as shown in Figure .
Effect of annealing the PS/phosphorus structure on the defect density at four selected GBs.
To understand the impact of this gettering process on cell performance, the internal quantum efficiency (IQE) measurements were evaluated at various annealing temperatures. All annealing temperatures exhibit an increased blue response in short wavelengths (400 to 750 nm), which is due to an important decrease of the surface recombination velocity and an improvement of the carrier collection at the emitter region, as shown in Figure .
The internal quantum efficiency dependence with the thermal treatment.
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 , 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.
The dark I-V characteristics have been investigated in Figure . After PS treatment, a significant improvement in the typical dark I-V characteristic was observed: the leakage current has considerably decreased, which is due to the reduction of the impurity concentration by gettering undesirable metallic impurities and defects that diffuse at getter sites present at the pore walls of the phosphorous-rich PS layer. The I-V characteristics clearly show a significant improvement of the rectifying behavior and a noticeable decrease of the reverse current after treatment, indicating an efficient passivation of grain and grain boundaries of the mc-Si wafers.
Dark I-V characteristics of the untreated and the PS-gettered samples at 700°C, 800°C, and 900°C.
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 IV
curves, we determined the series resistance (Rs
] and the shunt resistance (Rsh
) of the gettered and untreated cells.
Figure depicts the variation of the series and shunt resistances. We notice a significant increase of the shunt resistance (Rsh) and a diminution of the series resistance (Rs). The improvement of Rsh can be attributed to an efficient passivation of the grain and GBs of the mc-Si wafers and the gettering of unwanted impurities.
Series (Rs) (a) and shunt (Rsh) (b) resistances of the gettered multicrystalline solar cells.
However, the enhancement of Rs
could be due to the gettering effect in adjacent grains [29
]. 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.
In Figure , we carried out the I-V characteristic measurement of obtained solar cells under AM1.5 illumination (100 mW/cm2). Four parameters of solar cells were used to define illuminated solar cells: the short-circuit current (Isc), the open-circuit voltage (Voc), the fill factor (FF), and the efficiency (η).
I-V characteristics under AM1.5 illumination of the gettered multicrystalline solar cells.
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
] of metallic impurities towards the gettering layer. This behavior is confirmed by the degradation of the IV
characteristics for the samples treated at temperatures exceeding the optimum one.
Table shows the variation of the Isc and the Voc after phosphorous gettering at different temperatures. Our gettering process improves the Isc from 74 to 109 mA (sample gettered at 900°C) and the Voc from 0.48 to 0.55 mV (sample gettered at 900°C). The effect of gettering mc-Si using the combination of phosphorous diffusion into a sacrificial PS layer and a thermal treatment becomes apparent when looking at the different cell parameters. The untreated solar cell (reference sample) shows a drastically lower short-circuit current density (Jsc) compared to the cells subjected to the above treatment, especially when annealed at 900°C. The same tendency can be stated for the open-circuit voltage (Voc), which is 70 mV higher in the case of the gettered solar cell at 900°C, compared to the reference wafer (untreated). We conclude that phosphorous gettering process under O2 atmosphere, using a sacrificial PS layer on both sides, improves the electrical parameters and the performances of the cells via the reduction of the carrier recombination activities. This leads to an enhancement of the short-circuit current and the open-circuit voltage and an increase of the conversion efficiency of the mc-Si solar cells.
Comparison of the electrical parameters of phosphorous gettered and ungettered multicrystalline silicon solar cells