FR2949607A1 - METHOD OF PROCESSING PHOTOVOLTAIC CELLS AGAINST DECREASE OF PERFORMANCE DURING ILLUMINATION - Google Patents

Abstract

The invention proposes a method of treating photovoltaic cells against the degradation of the efficiency during illumination, making it possible to reduce the treatment time while improving the stability of the "regenerated" state at temperatures greater than 170 ° C. To this end, the invention relates to a method for treating photovoltaic cells against the degradation of the efficiency during illumination, said method comprising the following steps: providing a substrate made of boron-doped silicon and comprising dimers; oxygen to form a photovoltaic cell; heating said substrate to a temperature of between 300 ° C. and 500 ° C. for a determined treatment period; direct polarization of the photovoltaic cell to a determined potential difference during said determined processing time.

Description

PROCESS FOR TREATING PHOTOVOLTAIC CELLS AGAINST DECREASE IN LIGHT EFFICIENCY. The invention relates to a method for treating photovoltaic cells against the reduction of the yield during illumination.

Photovoltaic cells with crystalline silicon, monocrystalline (sc-Si) and multicrystalline (mc-Si) which contain boron and oxygen atoms, can have an energy conversion efficiency which decreases when they are illuminated. This effect is related to the formation, during illumination, of complexes that associate a boron atom in substitutional position (Bs) and an oxygen dimer (Oi2). Indeed, during illumination, the mobile oxygen dimer diffuses towards the immobile boron atom. The formed complex introduces a deep energy level in the forbidden band of silicon, which allows the recombination of the free charges, and consequently decreases the life of the charge carriers and the efficiency of energy conversion of the cell. The p-type borocrystalline silicon monocrystalline substrates, crystallized by the Czochralsky (Cz) technique, are particularly sensitive to this degradation because of the high concentrations of oxygen, the latter being mainly derived from the silica crucible used for the crystallization. However, this degradation has also been demonstrated in n-type compensated silicides that contain boron, as well as in multicrystalline substrates (which also contain boron). The document WO 2007/107351 aims to solve this problem of decreasing the efficiency of the photovoltaic cell. This document proposes to inject into the silicon substrate, during manufacture, charge carriers (via illumination or a direct polarization of the cell), while heating said substrate to a temperature of between 50 ° C. and 230 ° C. ° C. Under these conditions, it is observed that the efficiency of the cell degrades initially, and then regenerates into

returning to its original properties. The cell then has stable performance under normal operating conditions. The cell is said to be regenerated since it first underwent degradation of its yield and then regeneration. However, this invention has two major disadvantages. Firstly, the complete regeneration times are very high, which makes it difficult to integrate this process during the conventional industrial production of photovoltaic cells. In addition, the regenerated state is not stable beyond a temperature of 170 ° C. This feature can pose significant problems in the manufacture of modules (encapsulation of cells) of photovoltaic cells, since this manufacture of modules may require temperatures well above 170 ° C. The regenerated state can then disappear during the manufacture of the modules. This is all the more true as the research aims to maximize the effects of spectral conversion and decrease of the reflectivity at the module, which could result in the use of higher temperatures during the setting module. The object of the invention is therefore to propose a method of treating photovoltaic cells against the degradation of the output during illumination, making it possible to reduce the processing time while allowing the manufacture of photovoltaic cell modules at temperatures greater than 170.degree. ° C. To this end, the invention relates to a method for treating photovoltaic cells against the degradation of the efficiency during illumination, said method comprising the following steps: providing a substrate made of boron-doped silicon and comprising oxygen dimers to form a photovoltaic cell; Heating said substrate to a temperature of between 300 ° C. and 500 ° C. for a determined treatment period, while

direct polarizing the photovoltaic cell to a determined potential difference. Contrary to what is suggested in document WO 2007/107351, the chosen temperature range not only makes it possible to reduce the treatment time substantially, but it is observed, moreover, that the reduction in the degradation of the efficiency of the cell under illumination is preserved even if the cell has undergone during its preparation or its integration in a module, annealing at a temperature of up to 600 ° C. According to other embodiments: the determined potential difference can be between 0.15 V and 10 V for a doped p-type substrate and may be between 0.06 V and 0.5 V for an n-type doped substrate; The determined treatment time, for a p-type doped substrate whose concentration of free carriers at 300 Kelvin is 5 * 1015 cm-3, can be between 120 and 30 minutes for a potential difference of between 0, 2 and 0.8 volts for a heating temperature of 390 ° C. The invention also relates to a photovoltaic cell obtained by the above method, the material of which has a lifetime tBoi2 of the charge carriers limited by the oxygen-boron dimer complexes, equal to 200 μs. A photovoltaic cell according to the invention has a stable conversion efficiency up to 600 ° C. The invention also relates to a use of a photovoltaic cell obtained by the preceding method, for the manufacture of a module, at a temperature greater than 170 ° C. Other features of the invention will be set forth in the detailed description below, made with reference to the appended figures which represent, respectively:

- Figure 1, a graph illustrating the potential difference Vb2 to be applied to multiply by two the concentration of holes po of the wafer without external excitation (illumination or polarization) for p-type silicon; - Figure 2, a graph illustrating the potential difference Vb2 to be applied to multiply by two the concentration of holes po of the wafer without external excitation (illumination or polarization) for n-type silicon; and FIG. 3, a graph illustrating the duration of the heat treatment required, as a function of the applied potential difference, to halve the initial concentration of oxygen dimer O 12 at 390 ° C., for a p-type silicon of which the free carrier concentration at 300K is 5X1015 cm-3. The method for treating photovoltaic cells against degradation of the efficiency during illumination according to the invention comprises a first step of providing a boron-doped silicon substrate comprising oxygen dimers, to form a photovoltaic cell. Oxygen dimers Oi2, responsible for the degradation mechanism of the efficiency under illumination, diffuse in silicon by a Bourgoin-Corbett mechanism. Their position in the crystal lattice depends on their state of charge. Thus, they progress in the network e n successively capturing electrons and holes. Consequently, the diffusion coefficient of the dimers depends on the electron capture coefficients and the holes defined respectively by the parameters: nO'n19n and P6p19p n and p are, respectively, the concentrations of electrons and holes; where: - Grn and 6p are the dimer capture cross sections for electrons and holes; and 19net l9p are the thermal velocities of electrons and holes.

During its migration in silicon, the state of charge of the dimer oscillates between Oi2 ++ and Oi2 +. Thus the ability of the dimer to capture electrons is higher than its ability to capture holes, 6n much higher than O.

Thus, even in p-type silicon (with the exception of temperatures close to ambient temperature, under low illumination or low potential difference), it is the capture of the holes that limits the diffusion of the dimer. The diffusion coefficient of the dimer (D (Oi2)) is thus expressed as follows:

D (0i2) = Apapi9p exp (ù -a) (1) kBT Where: - A is a proportionality factor

- E. is the activation energy;

- kB is the Boltzmann constant; and

- p is equal to po + Ap where po is the concentration of holes at thermodynamic equilibrium and Op is the concentration

of holes brought by an external excitation.

The term L of equation (1) can be increased by direct biasing a pn junction. Under the effect of polarization, free carriers are injected into the semiconductor substrate. From the moment when the density Ap of injected carriers is no longer zero, the diffusion coefficient of the dimer increases. The process according to the invention also comprises a step of heating the substrate at a temperature of between 300 ° C. and 500 ° C.

while direct polarizing the photovoltaic cell to a determined potential difference for a given processing time. At these temperatures, and with a polarization, it is observed against all expectation that the efficiency of the cell degrades less under illumination. In addition, this treatment is fast and compatible with the manufacturing processes of photovoltaic cell modules. Oxygen dimers O12 precipitate in the form of thermal donors between 300 and 500 ° C. The precipitates formed are then stable until temperatures close to 600 ° C. On the other hand, the kinetics of precipitation are rather slow. For example, at 420 ° C, 30 hours are required to divide the concentration of oxygen dimer by 1.3. To accelerate the precipitation kinetics of the Goose, the invention proposes, during the heat treatment between 300 ° C. and 500 ° C., to polarize the photovoltaic cell via an external voltage generator, or by any other means. This external polarization makes it possible to increase the diffusion coefficient of the oxygen dimer. By way of example, it has been determined, using numerical simulations, the differences of optimum potential to apply a cell having a doped silicon substrate, with different donor concentrations, and whose modeled lifetime is 30 low. For this purpose, for several treatment temperatures (300 ° C., 400 ° C. and 500 ° C.), the increase in the term D (O12) is studied as a function of the difference in concentration between the donor elements and the acceptor elements of the substrate. Figures 1 and 2 illustrate the results of this study, respectively for a p-type substrate and for an n-type (compensated) substrate. In these figures, the potential difference Vb2 is given to be applied so that the term D (O12) is multiplied by 2. In other words, the potential difference is given which makes it possible to double the initial po concentration in holes, to the balance, without external polarization or illumination. For that, we

calculate the p-value at the center of the wafer thickness and determine the potential difference so that p = 2 ". For a p-type substrate (FIG. 1), the difference in potential Vb 2 to be applied increases when the difference between the concentration NA of acceptor elements and the concentration ND of donor elements increases, whatever the temperature applied. According to the invention, the determined potential difference to be applied is between 0.15 V and 10 V for a p-type doped substrate. For an n-type substrate (FIG. 2), and for temperatures of 400 ° C. and 500 ° C., the difference in potential Vb 2 to be applied decreases when the difference between the concentration ND in donor elements and the concentration NA in acceptor elements increases. At 300 ° C, Vb2 is substantially constant. According to the invention, the determined potential difference to be applied directly is between 0.06 V and 0.5 V for an n-type doped substrate. In order for the method according to the invention to be implemented, it suffices that a collection structure (junction) for the minority carriers is present. This may be a homojunction np, but also a heterojunction, for example amorphous silicon on crystalline silicon, or a metal-semiconductor junction. The evolution of the concentration of Oi2 dimers can be measured during a heat treatment at 390 ° C. of a wafer without junction and without polarization. It is shown that the initial concentration of dimers O12 is divided by 2 after a period of 4 hours. This means that after 4 hours of heat treatment at 390 ° C., the service life of load carriers LBOi 2 is multiplied by 2. In other words, by carrying out a heat treatment at a temperature of between 300.degree. 500 ° C., the formation of Bore-O12 complexes is favored, which reduces the concentration of oxygen dimer O 12.

The direct polarization of a junction makes it possible to increase the diffusion coefficient of the 0i2 dimers, and thus to accelerate the formation of Boron - Goose complexes. FIG. 3 shows that if the diffusion coefficient of the goose dimer is doubled by polarizing an n + p junction with a potential difference of 800 mV, 30 minutes of heat treatment at 390 ° C. suffice to divide by two. the initial concentration of Oi2 dimers (2-fold increase in 'CBOi2) compared to 4 hours in the absence of polarization. Direct polarization of a junction thus makes it possible to reduce the duration of heat treatment. This shows the time saving provided by the invention while allowing its adaptation to the industrial manufacturing process of cells and modules. The choice of temperature used for dimer trapping is a compromise between the density of thermally formed donors and their formation kinetics. It is therefore extremely useful to propose a means of accelerating the formation kinetics of thermal donors, since these heat treatments can then be carried out at a lower temperature and to obtain a large quantity of donors formed while maintaining acceptable heat treatment times. . The invention therefore makes it possible to attenuate the effects of degradation under illumination of the conversion efficiency of the cells with crystalline silicon containing boron and oxygen. One of the particular advantages provided by the invention is that the physical mechanism responsible for these limited degradation effects is stable up to a temperature of 600 ° C., which provides a great deal of flexibility with respect to the choice of the modulating method. cells used. Another advantage, and that the invention makes it possible to increase the kinetics of precipitation or agglomeration of Oi2 dimers within the thermal donors, which facilitates the integration of the process in the industrial manufacturing of photovoltaic cells and modules.

An exemplary embodiment of the invention of a photocell is given below. A conventional p-type photoelectric cell with homojunction n + p is used. The concentration of free carriers in the substrate at 300 Kelvin is 5x1015 cm-3. This material is not compensated and is doped with boron. The concentration [B] of the boron is 5x1015 cm-3. The concentration [O;] of interstitial oxygen in this material is equal to 1018 cm-3. At the end of the manufacturing process of the cell, before placing the cells in a module, the cell is positioned on a hot plate at 390 ° C., and forward biased at a voltage of 800 mV for 30 minutes. As mentioned above, these experimental conditions make it possible to multiply the lifetime ('MBO; 2) of charge carriers limited by the BOi2 complexes by two. Without polarization heat treatment, at the end of cell fabrication, CBO1 2 should be 100 ps.

The invention makes it possible to obtain a value of LBoi2 equal to 200 ps. Such an increase in the lifetime of the charge carriers makes it possible to obtain a gain on the conversion efficiency of the cell of 0.3% absolute, and the gain obtained is stable as long as the temperature of the cell does not exceed 600. ° C.20

Claims (6)

REVENDICATIONS1. A method of treating photovoltaic cells against the degradation of the output during illumination, characterized in that it comprises the following steps: - providing a boron-doped silicon substrate comprising oxygen dimers, to form a photovoltaic cell having a collection structure for minority holders; heating said substrate to a temperature of between 300 ° C. and 500 ° C. for a determined treatment duration, while direct polarizing the photovoltaic cell to a determined potential difference. The method for processing photovoltaic cells according to claim 1, wherein the determined potential difference is between 0.15 V and 10 V for a p-type doped substrate and between 0.06 V and 0.5 V for an n-type doped substrate. 3. Process for treating photovoltaic cells according to any one of claims 1 or 2, wherein the determined potential difference is between 0.2 and 0.8 volts at a heating temperature of 390 ° C, for a substrate doped p-type whose concentration of free carriers at 300 Kelvin is 5 * 1015 cm-3, in order to obtain a determined duration of treatment of between 120 and 30 minutes. 4. Photovoltaic cell obtained by the method according to any one of claims 1 to 3, characterized in that it has a TBOi2 lifetime charge carriers limited by the boron-oxygen dimer complexes substantially equal to 200 las . 5. Photovoltaic cell obtained by the method according to any one of claims 1 to 3, characterized in that it has a stable conversion efficiency up to 600 ° C. 6. Use of a photovoltaic cell obtained by the method according to any one of claims 1 to 3, for the manufacture of a module at a temperature greater than 170 ° C.