DEVICE AND METHOD FOR IN-SITU MEASUREMENT OF THE LIFETIME OF MINORITY CARRIERS IN A SEMICONDUCTOR MATERIAL

Description

Technical field :

[0001] The present invention relates to the field of semiconductors and more particularly to photovoltaic cells and more particularly still to the in-situ measurement of the lifetime of minority carriers in semiconductor materials, more particularly photovoltaic cells. Previous technique:

[0002] It is known to study the properties of the material composing a photovoltaic cell by measuring the photoluminescence generated in the structure of the cell. More precisely, the measurement of luminescence is used to study the quality of the material by determining the lifetime of the minority charge carriers.

[0003] A photovoltaic cell has a PN junction. The bandgap E g of the junction is subsequently called the gap in the layer absorbing the incident light radiation. A tandem cell is a stack of two cells. A multi-junction cell consists of several thin semiconductor layers with decreasing bandgap widths.

[0004] Due to the improvement in the quality of crystalline silicon and the current trend to reduce the thickness of silicon wafers, the recombination rate of minority carriers on the surface of photovoltaic cells has become the limiting factor for cell efficiency. For this reason, the surface property of cells is a critical parameter.

[0005] The complexity and number of processes interacting with the surface of photovoltaic cells during their manufacturing have become so important (e.g. surface texturing, passivation, AR coating, encapsulation, co-firing...) that conventional ex-situ techniques for measuring minority charge carrier lifetime are no longer satisfactory. In order to study and understand the effects of each process during the manufacturing of a photovoltaic cell, these ex-situ techniques consist in comparing the minority charge carrier lifetime before and after each step of the manufacturing process. However, this method is time-consuming and complex because it requires several batches of the same sample in order to precisely study a single step of the manufacturing process by repeating the same measurement.

[0006] Many methods for studying in situ the lifetime of minority carriers within photovoltaic materials have been proposed.

[0007] For example, it is known to study the decay of photoconductivity in microwave regime (see TS Horanyi et al, Appl. Surf. Sci. 63 (1993) 306-311). However, this technique uses a high laser intensity which is not compatible to study the properties of photovoltaic cells with a thin passivation layer, for example 5 nm. In addition, the sensitivity of this method decreases when the conductivity of the material increases. Finally, this method has not allowed, to date, to study the lifetime of minority carriers for a specific reference carrier density. Such a result is critical because within crystalline silicon, the lifetime of minority carriers varies greatly depending on the density of minority carriers (variation can range from 100 to 1000 µs or more).This is why, in the photovoltaic or electronics community, the minority carrier lifetime is presented for a specific reference carrier density (e.g. 10 15< cm -3< for a crystalline silicon mono-junction solar cell). This allows comparing the properties of solar cells even though they are manufactured by different methods or in different laboratories.

[0008] Time-resolved measurement of photoluminescence (PL) from a physical model based on the decay of the PL signal allows the study of a photovoltaic cell in situ and allows, for example, to trace back to an electronic trap density or minority carrier lifetime (see US 2013 / 122612 A1 or GD Jenkins et al, Proc. SPIE 9226 (2015)). However, to date, this technique has not made it possible to determine the minority carrier lifetime for a specific carrier density.

[0009] Previous techniques for measuring minority carrier lifetimes in semiconductor materials, and in particular in solar cells, are also described in the following documents: Choi Kwan Bum et al: "Modulated Photoluminescence Lifetime Measurement of Bifacial Solar Cells", IEEE Journal of Photovoltaics, vol. 7, no. 6, 1; Chouffot R et al: "Modulated photoluminescence as an effective lifetime measurement method: Application to a-Si:H / c-Si heterojunction solar cells", Materials Science and Engineering: B, ELSEVIER, AMSTERDAM, vol. 159-160; and Giesecke JA et al: "Understanding and resolving the discrepancy between differential and actual minority career lifetime", Journal of Applied Physics, American Institute of Physics, US vol. 113, no. 7.

[0010] It is also known to carry out the study of a photovoltaic cell by photoluminescence using a sinusoidally modulated laser source by measuring the phase shift between the intensity of the laser source and that of the PL signal (R. Bruggemann et al, J. Non-Cryst. Solids 352 (2006) 188861891), this technique being able to be applied in-situ. However, to date, this technique has not made it possible to determine the lifetime of minority carriers for a specific carrier density.

[0011] Furthermore, the lifetime of minority carriers provided by this technique is the so-called differential lifetime which is imprecise. The invention aims to overcome certain problems of the prior art. To this end, an object of the invention is a method for measuring in situ the lifetime of minority carriers in a photovoltaic cell by measuring a photoluminescence signal generated by light radiation emitted at a plurality of distinct average intensities and successively modulated at a plurality of distinct modulation frequencies. Summary of the invention :

[0012] For this purpose, an object of the invention is a method for measuring in situ the lifetime of minority carriers in a semiconductor material comprising at least a first junction by measuring a first photoluminescence signal, said method comprising: A. illuminating at least the first junction of the semiconductor material with a first light radiation emitted at a plurality of average intensities I1 i , iE[1,m] and at a first wavelength adapted to generate a first photoluminescence signal in said first junction of the semiconductor material; for each i∈[1,m] the sub-steps consisting of: A1- calculating the photogeneration rate G i of charge carriers; A2-modulate said first light radiation successively at a plurality n>1 of distinct modulation frequencies ω jj∈[1,n] and for each j∈[1,n]: A21-detect the first photoluminescence signal and measure the phase shift φ ij between an amplitude of the first incident light radiation and an amplitude of the first photoluminescence signal A3-calculate the lifetime of minority carriers τ i from the measurements of said phase shift φ ij , with j∈[1,n];A4-calculating the minority carrier density Δn i from the minority carrier lifetime τ i and the photogeneration rate G i; B. determining a continuous function, called the first function, representative of the evolution of the minority carrier lifetime as a function of the minority carrier density from the values ​​determined during step A4; C. calculating a value of the corrected minority carrier lifetime for said plurality of average intensities, said calculation being carried out from the derivative of the first function, the calculation of the minority carrier lifetime and the photogeneration rate; D. determining a continuous function, called the second function, calculating the corrected minority carrier lifetime as a function of the minority carrier density, from the values ​​of the corrected minority carrier lifetime determined in step C.;

[0013] According to particular embodiments of the invention:the semiconductor material further comprises a second junction (J2) with a band gap greater than the band gap of the first junction and: step A further comprises illuminating the second junction with a second light radiation (L2 in ) at a second wavelength adapted to generate a second photoluminescence signal (PL2) in said second junction of the semiconductor material; step A21 further comprises detecting said second photoluminescence signal and measuring the intensity of the second photoluminescence signal.the second wavelength is less than the first wavelength and the second light radiation is absorbed by the second junction of the semiconductor material while the first light radiation passes through the second junction to be absorbed by the first junction; at least steps A to A4 are repeated several times during a process of treating the semiconductor material; .

[0014] Another object of the invention is a device for measuring in situ the lifetime of minority carriers in a semiconductor material comprising at least a first junction by measuring a first photoluminescence signal, said device comprising: at least one first laser source emitting a first light radiation at a first wavelength, configured to emit laser radiation at a plurality of distinct average intensities I1 i with i∈[1,m] and configured to be modulated successively at a plurality of distinct modulation frequencies ω j with j∈[1,n] and configured to illuminate a first junction of the semiconductor material so as to generate a first photoluminescence signal in said first junction of the semiconductor material; a detection system connected to said first laser source, adapted to detect the first photoluminescence signal and measure phase shifts φ ij , between the amplitude of each first incident light radiation of intensity I1 i and modulation frequency ω j and the amplitude of the associated first photoluminescence signal;a processing unit connected to the detection system adapted to: calculate, for each distinct average intensity I1 i of the first light radiation, the photogeneration rate G i of minority charge carriers; calculate, for each distinct average intensity I1 i of the first light radiation, the minority carrier lifetime τ i from the phase shift measurements φ ij; calculate, for each distinct average intensity I1 i of the first light radiation, the minority carrier density Δn i from the minority carrier lifetime τ i and the photogeneration rate G i; adjust a continuous function, called the first function, calculating the minority carrier lifetime as a function of the minority carrier density for the plurality of distinct average intensities of the first light radiation;calculating values ​​of the corrected minority carrier lifetime, said calculation being carried out from the derivative of the first function, the calculation of the minority carrier lifetime and the photogeneration rate; determining a continuous function, called the second function, calculating the corrected minority carrier lifetime as a function of the minority carrier density from the calculated values ​​of the corrected minority carrier lifetime. ;

[0015] According to particular embodiments of the invention: the detection system comprises at least one synchronous detection amplifier; the device comprises a second laser source emitting a second light radiation at a second wavelength, said second laser source being configured to illuminate a second junction of the semiconductor material, with a band gap greater than the band gap of the first junction and to generate a second photoluminescence signal in said second junction of the semiconductor material; and an optical splitter component adapted to spatially separate the first photoluminescence signal and the second photoluminescence signal; said detection system being further configured to detect said second photoluminescence signal and measure the intensity of the second photoluminescence signal;the first source and the second laser source are configured such that the second wavelength is less than the first wavelength and the second light radiation is absorbed by the second junction of the semiconductor material while the first light radiation passes through the second junction to be absorbed by the first junction; the detection system comprises at least one photodetector.; Brief description of the figures :

[0016] Other characteristics, details and advantages of the invention will emerge from reading the description given with reference to the appended drawings given by way of example and which represent, respectively: [ Fig. 1 ] a schematic view of a device according to the invention for measuring in situ the lifetime of minority carriers in a photovoltaic cell. [ Fig.2] a method according to the invention for measuring in situ the lifetime of minority carriers in a photovoltaic cell. [ Fig.3 ] a method for measuring in situ the lifetime of minority carriers in a photovoltaic cell, according to a variant of the invention. [ Fig.4 ] a graph representing respectively the lifetime of minority charge carriers and the lifetime of minority charge carriers corrected as a function of minority charge carrier density. [ Fig.5 ] a schematic view of a device for measuring in situ the lifetime of minority carriers in a multi-junction photovoltaic cell according to one embodiment of the invention.

[0017] In the figures, unless otherwise indicated, the elements are not to scale. Detailed description :

[0018] There figure 1illustrates a schematic view of a device Disp according to the invention for measuring in situ the lifetime of minority carriers in a semiconductor material, which may for example be a photovoltaic cell Ec by measuring a first photoluminescence signal PL1. This device is intended to implement the method of the figure 2 which we will detail later.

[0019] In all embodiments of the invention, the photovoltaic cell Ec made of semiconductor material comprises at least a first junction J1 comprising a layer absorbing incident light radiation having a forbidden band width or "gap" E g1. Subsequently, it is considered that the terms junction and semiconductor layer absorbing incident radiation are equivalent. Thus, according to one embodiment of the invention, the photovoltaic cell is a single-junction cell. Alternatively, according to another embodiment of the invention, the photovoltaic cell is a cell comprising several junctions such as a multi-junction cell or a tandem cell (stack of two cells).

[0020] The device Disp comprises a first laser source SL1 consisting of a laser L and a modulator PC. The laser L is a high-intensity monochromatic laser source. Thus, according to one embodiment of the invention, the laser is an LED or a SLED (Superluminescent Diode). Alternatively, according to another embodiment of the invention, the laser L comprises a high-intensity, broadband source coupled to a spectral filter. According to one embodiment of the invention, the modulator is an electronically controlled mechanical shutter. Alternatively, according to another embodiment, the modulator is an electronic modulator controlling the power supply of the laser.

[0021] The first laser source is configured to emit a first light radiation (or excitation signal) L1 in at a first wavelength λ1, configured to emit laser radiation at a plurality of distinct average intensities I1 i with iE[1,m]. Furthermore, by means of the modulator, the first laser source is configured to be successively modulated at a plurality of distinct modulation frequencies ω j with j∈[1,n].

[0022] The first laser source is adapted to illuminate the first junction J1 of the photovoltaic cell Ec so as to generate a first photoluminescence signal PL1 in the first junction J1 of the photovoltaic cell. The first laser source SL1 is configured to emit laser radiation at a wavelength λ 1 such that h / cλ 1 > E g 1 in order to excite the photovoltaic cell.

[0023] According to one embodiment of the invention, in order to have a more robust alignment, at least part of the optical path of the first light radiation L1 in from the first laser source to the photovoltaic cell is fibered (as shown in figure 1 ). The PE sample holder is suitable for holding the photovoltaic cell fixed during its characterization during the manufacturing process in order to carry out in-situ measurements. For example, according to one embodiment, the sample holder is placed within a reactor during the annealing step of the photovoltaic cell, in order to follow the in-situ evolution of the photovoltaic cell during this step.

[0024] Once excited by the pulsed laser, the solar cell is in a state of disequilibrium. The solar cell returns to equilibrium by emitting light (luminescence of solar cells and more generally of semiconductor materials). In particular, the first light radiation excites the electrons in the valence band of the semiconductor (e.g. crystalline silicon), the latter being in the excited state in the conduction band.

[0025] Then, these electrons relax and recombine with holes (positive charge gaps) located in the valence band. This relaxation is accompanied by an emission of light where the photons have an energy equal to the semiconductor "gap" (bandwidth forbidden). These radiative recombinations are at the origin of the luminescence signal. In summary, the photons of the first light radiation illuminating the first junction will be absorbed by the latter, generating electron-hole pairs and emitting photons forming the first photoluminescence signal PL1.

[0026] Thus, the first laser source SL1 emits a first light radiation at an average intensity I1 i with iE[1,m] modulated at a first modulation frequency ω j with j∈[1,n] and generating a first associated photoluminescence signal which will be called PL1 ij .

[0027] The device of the figure 1further comprises an optical collection system Col1 configured to collect the first photoluminescence signal PL1. According to one embodiment of the invention, this optical device consists of a parabolic mirror and a set of lenses for collimating the first photoluminescence signal and directing it onto a photodiode Phot for detecting the intensity of the first photoluminescence signal. According to another embodiment of the invention, this optical device consists of an achromatic doublet assembly (lens doublet) for collimating the first photoluminescence signal. According to one embodiment, the first collected photoluminescence signal is coupled to an optical fiber and directed onto the photodiode. According to another embodiment of the invention, the first collected photoluminescence signal is focused directly onto the photodiode.

[0028] The first laser source is configured so that the first light beam illuminates the first junction with a sufficient angle of incidence so that the optical collection system does not collect the first light beam reflected by the photovoltaic cell. In addition, a high-pass filter is used to filter the reflection of the first laser source. The first laser source is configured so that the illuminated surface (i.e. photogeneration zone; typically with a diameter of a few millimeters) is larger than the collection surface (i.e. measurement zone; typically a few tens to a few hundreds of microns).

[0029] The device comprises a pre-amplifier Amp configured to amplify the first photoluminescence signal after detection by the photodiode Phot. Alternatively, according to another embodiment the device does not comprise an amplifier.

[0030] After amplification, the first photoluminescence signal is directed to a detection system Det connected to the first laser source SL1 and to a processing unit UT. According to a variant, the detection system comprises at least one synchronous detection amplifier (Lock-In Amplifier in English).

[0031] There figure 2 illustrates a method for measuring in situ the lifetime of minority carriers in a photovoltaic cell according to the invention, implemented by the device of the figure 1 .

[0032] This method comprises a first step A consisting of illuminating, by the first laser source SL1, at least the first junction J1 of the photovoltaic cell Ec with a first light radiation L1 in emitted at a plurality of average intensities I1 i , i∈[1,m] and at a first wavelength λ 1 adapted to generate a first photoluminescence signal PL1 in said first junction of the photovoltaic cell Ec.

[0033] For each i∈[1,m] we carry out the following sub-steps A1-A4: A step A1 consists of calculating, using the processing unit connected to the detection system itself connected to the first laser source SL1, the photogeneration rate G i of charge carriers. We assume that this is equal to the photon flux F i = I i λ 1 / hc of the first light radiation assuming that an absorbed photon generates an electron-hole pair.

[0034] A step A2 consisting of modulating, using the modulator PC, the first light radiation emitted by the first laser source SL1 successively at a plurality n>1 of distinct modulation frequencies ω j jE[1,n]. The modulation frequencies are selected as a function of the first laser source SL1, the preamplifier and the lock-in amplifier. Typically, they are between 20 Hz and 100 kHz.

[0035] For each j∈[1,n] the sub-step A21 is carried out, consisting of detecting the amplitude, with the detection system Det, of the first photoluminescence signal PL1 ij associated with the first light radiation of average intensity I1 i and modulation frequency ω j and measuring the phase shift φ ij between the amplitude of the first incident light radiation and the amplitude of the first photoluminescence signal PL1. The measurement of the phase shift is for example carried out by the synchronous detection amplifier of the detection system, said synchronous detection amplifier being synchronized to the first laser source SL1.

[0036] The method of the figure 2comprises another step A3 consisting of calculating, using the processing unit Ut connected to the detection system, the lifetime of minority carriers τ i from the measurements of the phase shift φ ij , with j∈[1,n] between the intensity of the first incident light radiation and the intensity of the first photoluminescence signal. The relationship between the lifetime of minority carriers τ i for an average intensity I1 i , the phase shift φ ij and the modulation frequency ω j is given by the following equation: tan ϕ ij = τ i ω j

[0037] The lifetime of minority carriers τ i is therefore calculated, by the processing unit, from the slope of the line representing the value of the tangent of the phase shift tan( φ ij ) depending on the modulation frequency ω j, this line drawn for all modulation frequencies ω j , j∈[1,n]. It is necessary to use a plurality of modulation frequencies in order to improve the reliability of the lifetime calculation. Preferably, at least 3 modulation frequencies are used.

[0038] The method of the figure 2 includes a final step A4, carried out by the processing unit, consisting of calculating the density of minority carriers Δn i from the lifetime of minority carriers τ i and the photogeneration rate G i from the following equation: Δ n i = τ i × G i

[0039] The method of the figure 2allows to calculate a density Δn i and a lifetime τ i of minority charge carriers for each average intensity I1 i , i∈[1,m] of the first light radiation. It is interesting to calculate the lifetime of the minority carriers for a plurality of distinct average intensities because, as we will explain later, this constitutes a necessary step for the determination of the lifetime for a specific reference carrier density. However, this determination is critical because the lifetime varies greatly depending on the density of minority carriers (variation can go from 100 to 1000 µs or more for crystalline silicon). This is the reason why, in the photovoltaic or electronic community, the lifetime of the minority carriers is presented for a specific carrier density (eg 10 15< cm -3< for a mono-junction solar cell in crystalline silicon).This allows comparing the properties of solar cells even though they are manufactured by different methods or in different laboratories. However, the laser intensity is related to the photogeneration rate and also varies the carrier density. Therefore, it is very difficult to measure the carrier lifetime at a desired carrier density. It is therefore necessary to measure the carrier lifetime for several laser intensities in order to calculate the lifetime τ at a desired density Δn (eg 10 15< cm -3< ) by interpolation or extrapolation using the method of . figure 3 detailed further.

[0040] Moreover, the method of the figure 2 can be carried out in situ by the device of the figure 1by repeating all of steps A-A4 several times during a photovoltaic cell treatment process in order to observe how the minority carrier lifetime evolves during the process.

[0041] To work, the method illustrated in the figure 2 requires a sufficiently long minority charge carrier lifetime. Thus, the first junction J1 is made of a material such that the minority charge carrier lifetime is greater than 1 µs, preferably greater than 10 µs. Thus, according to one embodiment, the first junction J1 is made of crystalline silicon (with a gap E g1 ) and the first laser source is configured to emit a first radiation at a first wavelength λ 1 < hc / E g . Thus, in an example given without limitation, the first laser source is configured to emit at a wavelength λ 1 =785 nm.

[0042] There figure 3illustrates a method for measuring in situ the lifetime of minority carriers in a photovoltaic cell according to a variant of the invention, implemented by the device of the figure 1 . This method is identical to that illustrated in the figure 2 except that it comprises three steps B, C and D carried out successively after steps A-A4 carried out for a plurality of average intensities I1 i , i∈[1,m] of the first light radiation.

[0043] Step B consists of the determination by the processing unit of a first continuous function f 1 calculating the lifetime of minority carriers as a function of the density of minority carriers from the values ​​of the lifetime and the density of minority carriers determined during step A4. This step therefore allows the determination of a function calculating the lifetime of minority carriers for a specific reference carrier density. In addition, this continuous function allows us to have the correct parameters used for the correction of the lifetime calculation carried out in step C.

[0044] Step C, carried out after step B, consists of calculating, with the processing unit, the minority carrier lifetime corrected from the derivative of the function f 1 , the calculation of the minority carrier lifetime and the photogeneration rate. Indeed, the calculation of the minority carrier lifetime carried out during step A3 is a so-called differential lifetime because the phase shift measurement technique is itself a dynamic measurement technique of the charge carrier recombination properties based on a relative variation in the illumination. That is to say, a global parameter of the photovoltaic cell (the charge carrier lifetime τ i for an average intensity I1 i ) is determined by comparing measurements where only the modulation frequency ω j of the cell illumination varies.The calculation of the corrected minority carrier lifetime τ ci , carried out for each average intensity I1 i , i∈[1,m] of the first light radiation, is given by the following equation: . τ ci = τ i 1 − G i dτ dΔn Δn i

[0045] With dτ dΔn Δn i the derivative of the function f 1 for the carrier density Δn i calculated for the intensity I1 i .

[0046] Finally, the method according to a first variant of the invention comprises a step D, after step C, of ​​determination by the processing unit of a second continuous function f 2 calculating the corrected minority carrier lifetime as a function of the minority carrier density from the values ​​of the corrected minority carrier lifetime τ ci determined during step C.

[0047] Step D allows the determination of a function calculating the minority carrier lifetime corrected for a specific carrier density. The lifetime calculated by the function f 2 is the corrected lifetime, the inverse of the lifetime calculated by the function f 1 which allows the differential lifetime to be calculated. The corrected lifetime is a standard lifetime used in the photovoltaic and microelectronics community to characterize the properties of a semiconductor material. According to an embodiment of this first variant of the invention, only step B is carried out, steps C and D being omitted.

[0048] There figure 4 is a graph representing the first function f 1 (curve 31), determined during step B of the method of the first variant of the invention, calculating the evolution of the lifetime of minority carriers as a function of the density of minority carriers. The figure 4further represents the second function f 2 (curve 32), determined during step D of the method of the first variant of the invention, calculating the evolution of the minority carrier lifetime corrected as a function of the minority carrier density.

[0049] Curve 32 therefore makes it possible to determine the lifetime of minority charge carriers corrected for a specific carrier density.

[0050] In the example of the figure 4 the number of distinct average intensities I1 i of the first light radiation is 6. This example is given as a non-limiting example and according to an embodiment of the first variation of the invention, a different number of 6 distinct average intensities is used when carrying out the method illustrated in figure 3 by the Disp device of the figure 1In this example the laser intensities range from 2 mW to 50 mW and the modulation frequencies are between 225 Hz and 1025 Hz.

[0051] There Figure 5 illustrates a schematic view of a device Disp according to an embodiment of the invention for measuring in situ the lifetime of minority carriers in a photovoltaic cell Ec. In this embodiment, the photovoltaic cell comprises at least two junctions, a first junction J1 with a gap E g1 and a second junction J2 with a gap E g2 > E g1 . The second junction of the photovoltaic cell can be made of a group III-V semiconductor material, perovskite or CIGS. Thus, in this embodiment, the photovoltaic cell is a cell comprising at least two junctions such as a multi-junction cell or a tandem cell (stack of two cells).

[0052] The device of the embodiment of the Figure 5is identical to that of the figure 1 except that it comprises a second laser source SL2 emitting a second light radiation L2in at a second wavelength λ 2 at constant intensity. The second laser source is configured to illuminate the second junction J2 of the photovoltaic cell and to generate a second photoluminescence signal PL2 in said second junction of the photovoltaic cell. The second laser source SL2 is a high intensity monochromatic laser source. Thus, according to one embodiment the second laser source is an LED or an SLED.

[0053] In the embodiment of the Figure 5, the first source and the second laser source are configured so that the second wavelength is less than the first wavelength (λ 2 <λ 1 ) and so that the second light radiation is totally absorbed by the second junction of the photovoltaic cell while the first light radiation passes through the second junction to be absorbed by the first junction.

[0054] The device of the Figure 5 further comprises an optical separator component Sep adapted to spatially separate the first photoluminescence signal and the second photoluminescence signal from the photovoltaic cell. According to one embodiment, the optical separator component is a dichroic mirror.

[0055] In the embodiment of the Figure 5, the detection system further comprises a photodetector configured to detect and measure the intensity of the second photoluminescence signal. According to one embodiment, the photodetector of the detection system is a photodiode or an avalanche photodiode.

[0056] The device of the Figure 5 is intended to implement the same method as that of the figure 2 except that step A further comprises illuminating the second junction J2 of the photovoltaic cell with the second light radiation L2 in at a second wavelength adapted to generate a second photoluminescence signal PL2 in said second junction of the photovoltaic cell and step A21 further comprises detecting the second photoluminescence signal and measuring the intensity of the second photoluminescence signal.

[0057] Measuring the intensity of the second photoluminescence signal makes it possible to trace the electrical properties of the second junction of the photovoltaic cell. In addition, this measurement is carried out simultaneously with the measurement of the phase shift between the amplitude of the first incident light radiation and the amplitude of the first photoluminescence signal PL1. Thus, it is possible to characterize the first junction and the second junction of the photovoltaic cell simultaneously and independently.

Claims

1. A method for in situ measuring the minority carrier lifetime in a semiconductor material (Ec) comprising at least a first junction (J1) by measuring a first photoluminescence signal (PL1), said method comprising: A. illuminating at least the first junction (J1) of the semiconductor material (Ec) with a first light radiation (L1in) emitted at a plurality of average intensities I1i, i∈[1,m] and at a first wavelength adapted to generate a first photoluminescence signal (PL1) in said first junction of the semiconductor material; for each i∈[1,m] we perform the sub-steps consisting of: A1- calculating the photogeneration rate Gi of charge carriers; A2-modulating said first light radiation successively at a plurality n>1 of distinct modulation frequencies ωj j∈[1,n] and for each j∈[1,n]: A21-detecting the first photoluminescence signal (PL1) and measuring the phase shift Φij between an amplitude of the first incident light radiation (L1in) and an amplitude of the first photoluminescence signal (PL1) A3-calculating the minority carrier lifetime Ti from measurements of said phase shift Φij, with j∈[1,n]; A4-calculating the minority carrier density Δni from the minority carrier lifetime Ti and the photogeneration rate Gi, B. determining a continuous function, called the first function, representative of the evolution of the minority carrier lifetime as a function of the minority carrier density from the values determined in step A4, C. calculating a corrected minority carrier lifetime value for said plurality of average intensities, said calculation being performed from the derivative of the first function, the calculation of the minority carrier lifetime and the photogeneration rate, D. determining a continuous function, the so-called second function, calculating the corrected minority carrier lifetime as a function of the minority carrier density, from the corrected minority carrier lifetime values determined in step C.

2. The method according to the preceding claim, wherein the semiconductor material further comprises a second junction (J2) of bandgap width greater than the bandgap width of the first junction, and wherein: - step A further comprises illuminating the second junction, with second light radiation (L2in) at a second wavelength adapted to generate a second photoluminescence signal (PL2) in said second junction of the semiconductor material; - step A21 further comprises detecting said second photoluminescence signal and measuring the intensity of the second photoluminescence signal.

3. The method according to the preceding claim, wherein, during illumination, the second wavelength is shorter than the first wavelength and the second light radiation is absorbed by the second junction of the semiconductor material, while the first light radiation passes through the second junction to be absorbed by the first junction.

4. The method according to any one of the preceding claims, wherein at least steps A to A4 are repeated several times during a process of treating the semiconductor material.

5. A device (Disp) for in situ measuring the minority carrier lifetime in a semiconductor material (Ec) comprising at least a first junction (J1), by measuring a first photoluminescence signal (PL1), said device comprising: - at least one first laser source (SL1) emitting a first light radiation (L1in) at a first wavelength, configured to emit laser radiation at a plurality of distinct average intensities I1i with i∈[1,m] and configured to be successively modulated at a plurality of distinct modulation frequencies ωj with j∈[1,n] and configured to illuminate a first junction of the semiconductor material so as to generate a first photoluminescence signal (PL1) in said first junction (J1) of the semiconductor material; - a detection system (Det) connected to said first laser source (SL1), adapted to detect the first photoluminescence signal (PL1) and to measure phase shifts Φij, between the amplitude of each first incident light beam (L1in) of intensity I1i and modulation frequency ωj and the amplitude of the associated first photoluminescence signal (PL1); - a processing unit (UT) connected to the detection system suitable for: - calculating, for each distinct average intensity I1i of the first light beam, the rate of photogeneration Gi of minority charge carriers; - calculating, for each distinct average intensity I1i of the first light beam, the minority carrier lifetime Ti from phase shift measurements Φij; - calculating, for each distinct average intensity I1i of the first light beam, the minority carrier density Δni from the minority carrier lifetime Ti and the photogeneration rate Gi, - adjusting a continuous function, called the first function, calculating the minority carrier lifetime as a function of the minority carrier density for the plurality of distinct average intensities of the first light radiation - calculating values for the corrected minority carrier lifetime, said calculation being based on the derivative of the first function, the calculation of the minority carrier lifetime and the photogeneration rate, - determining a continuous function, the so-called second function, calculating the corrected minority carrier lifetime as a function of the minority carrier density from the calculated corrected minority carrier lifetime values.

6. The device according to claim 5, wherein the detection system comprises at least one synchronous detection amplifier.

7. The method according to either one of the two preceding claims, comprising: - a second laser source (SL2) emitting a second light beam (L2in) at a second wavelength, said second laser source being configured to illuminate a second junction (J2) of semiconductor material, with a bandgap greater than the bandgap of the first junction, and to generate a second photoluminescence signal (PL2) in said second junction of semiconductor material, - a beamsplitter (Sep) adapted to spatially separate the first photoluminescence signal and the second photoluminescence signal; said detection system being further configured to detect said second photoluminescence signal and to measure the intensity of the second photoluminescence signal.

8. The device according to the preceding claim, wherein the first and second laser sources are configured so that the second wavelength is shorter than the first wavelength and the second light radiation is absorbed by the second junction of the semiconductor material, while the first light radiation passes through the second junction to be absorbed by the first junction.

9. The device according to either one of the two preceding claims, wherein the detection system comprises at least one photodetector.