Spectral imaging method

Abstract

Spectral Imaging Method This description relates to a spectral imaging method of an object (20) comprising the determination, by simulation using a semi-physical model of the spectrum of the luminescence radiation (Llum) emitted by the object (20), of first, second, and third simulated luminescence values ​​of the luminescence radiation through respectively a first, second, and third optical filter, of first and second arrays containing respectively first and second simulated ratio values, the acquisition of first, second, and third luminescence images of the luminescence radiation through respectively the first, second, and third optical filter, the determination of first and second ratio images equal to the ratio respectively of the first and second luminescence images and of the first and third luminescence images,and the determination of an image of the cutoff wavelength λg of the object by comparing the first and second ratio images to the first and second tables. Figure for the abstract: Fig. 1,

Description

Title of the invention: Spectral imaging method technical field

[0001] The present description relates generally to spectral imaging methods. Previous technique

[0002] A spectral imaging method can be implemented to determine possible defects in the semiconductor material of a photovoltaic cell.

[0003] Spectral imaging methods using spectroscopic techniques exist. However, these methods require specific and expensive equipment and are difficult to implement on an industrial scale. Summary of the invention

[0004] One embodiment overcomes all or part of the drawbacks of known spectral imaging methods.

[0005] One embodiment provides a method for spectral imaging of a luminescent object employing an image sensor and first, second, and third optical filters, at least one of the first, second, and third optical filters being a bandpass optical filter, the method comprising the following steps: a) determine, by simulation using a semi-physical model of the 0em spectrum of the luminescence radiation emitted by the object, the first simulated luminescence values ​​of the luminescence radiation emitted by the object through the first optical filter over a first range of a cutoff wavelength Xg of the object, the second simulated luminescence values ​​of the luminescence radiation emitted by the object through the second optical filter over the first range of the cutoff wavelength Xg, and the third simulated luminescence values ​​of the luminescence radiation emitted by the object through the third optical filter over the first range of the cutoff wavelength Xg, a first table containing first simulated ratio values ​​equal to the ratios of the first simulated luminescence values ​​and the second simulated luminescence values,and a second table containing second simulated ratio values ​​equal to the ratios of the first simulated luminescence values ​​and the third simulated luminescence values; b) acquire, by the image sensor, a first luminescence image of the luminescence radiation emitted by the object through the first optical filter; c) acquire, by the image sensor, a second luminescence image of the luminescence radiation emitted by the object through the second optical filter; d) acquire, by the image sensor, a third luminescence image of the luminescence radiation emitted by the object through the third optical filter; e) determine a first image with a ratio equal to the ratio of the first luminescence image to the second luminescence image and a second image with a ratio equal to the ratio of the first luminescence image to the third luminescence image; and f) determine an image of the cutoff wavelength Xg by comparing the first ratio image and the first table and the second ratio image and the second table.

[0006] According to one embodiment, the semi-physical model of the 0em spectrum of the luminescence radiation emitted by the object is according to the following relation:

[0007] [Math.l] 0 em (X)=EQE pv (X)0 bb (A)B

[0008] where X is the wavelength, EQEPV is the external photovoltaic quantum efficiency of the object, 0bb is the blackbody emission, and B is a first multiplicative factor, the external photovoltaic quantum efficiency EQEPV being a function of the bandwidth Xs.

[0009] According to one embodiment, the external photovoltaic quantum efficiency EQEPV of the object is modeled by the following relationship:

[0010] [Math.2] E QE pv (To) =

[0011] where Am is a constant and K is a second multiplicative factor.

[0012] According to one embodiment, in step a), the first simulated luminescence values ​​are further determined over a second range of the bandwidth Xs, the second simulated luminescence values ​​are further determined over a second range of the bandwidth Xs, and the third simulated luminescence values ​​are further determined over a second range of the bandwidth Xs, the method further comprising determining an image of the bandwidth Xs by comparing the first ratio image and the first table and the second ratio image and the second table.

[0013] According to one embodiment, the spectrum of the SP radiation captured by the image sensor is modeled according to the following relationship:

[0014] [Math.3] SP(X)=$ (X)QE (À)FT op t(A) dll V / CIXXX -«■

[0015] where QEcam is the spectral response of the image sensor and any optical elements other than the first, second, and third optical filters, FTopt is the transfer function of one of the first, second, and third optical filters.

[0016] According to one embodiment, the emission of the black body 0bb is modeled by the following relation:

[0017] [Math.4] X5 exp(^)-l

[0018] where h is Planck's constant, c is the speed of light, and T is the absolute temperature.

[0019] According to one embodiment, the method further comprises, after step d) and before step e), a step of acquiring, by the image sensor, a fourth luminescence image of the luminescence radiation emitted by the object through the first optical filter and modifying the second luminescence image and the third luminescence image by an interpolation determined from the first luminescence image and the fourth luminescence image.

[0020] According to one embodiment, at least two of the first, second, and third optical filters are bandpass optical filters.

[0021] According to one embodiment, each of the first, second, and third optical filters is a bandpass optical filter.

[0022] According to one embodiment, the method comprises, between step b) and step c), the automatic replacement of the first optical filter by the second optical filter, and comprises, between step c) and step d), the automatic replacement of the second optical filter by the third optical filter.

[0023] According to one embodiment, the process includes illuminating the object, the object emitting luminescence radiation by photoluminescence.

[0024] According to one embodiment, the method includes supplying the object with electricity, the object emitting luminescence radiation by electroluminescence.

[0025] According to one embodiment, the object includes a photovoltaic cell.

[0026] According to one embodiment, the object comprises a photovoltaic cell of type perovskite tandem on silicon. Brief description of the drawings

[0027] These features and advantages, as well as others, will be described in detail in the following description of particular embodiments, given by way of non-limiting example, in relation to the accompanying figures, among which:

[0028] [Fig.1] represents, in a partial and schematic way, an embodiment of a spectral imaging system;

[0029] [Fig.2] represents, in a partial and schematic way, a variant of the spectral imaging system of [Fig.1];

[0030] [Fig.3] represents an example of the luminescence spectrum of a photovoltaic cell;

[0031] [Fig.4] represents curves of evolution of the external photovoltaic quantum efficiency as a function of wavelength according to a sigmoid model for different bandwidths;

[0032] [Fig.5] represents an example of the spectral shape of the radiation captured by an image sensor in the absence of an optical filter;

[0033] Fig. 6, Fig. 7, and Fig. 8 represent examples of spectral shapes of radiation captured by an image sensor through three different optical filters;

[0034] [Fig.9] is a block diagram illustrating one embodiment of a spectral imaging process;

[0035] [Fig. 10] represents curves of evolution of the cutoff wavelength as a function of the bandwidth obtained during the implementation of an embodiment of a spectral imaging process;

[0036] [Fig.1 1] is a block diagram illustrating another embodiment of a spectral imaging method;

[0037] [Fig. 12] and [Fig. 13] represent images of the cutoff wavelength of a photovoltaic cell obtained by the embodiment of the spectral imaging process described respectively in relation to [Fig. 9] and [Fig. 11];

[0038] Figures 14, 15, and 16 represent respectively a luminescence image, a cutoff wavelength image, and a bandwidth image of a photovoltaic cell; and

[0039] Fig. 17, Fig. 18, and Fig. 19 represent respectively a luminescence image, a cutoff wavelength image, and a bandwidth image of another photovoltaic cell. Description of the implementation methods

[0040] The same elements have been designated by the same reference numerals in the different figures. In particular, the structural and / or functional elements common to the different embodiments may have the same reference numerals and may have identical structural, dimensional and material properties.

[0041] For the sake of clarity, only the steps and elements useful for understanding the described embodiments have been represented and are detailed.

[0042] Unless otherwise specified, when referring to two interconnected elements, this means directly connected without intermediate elements other than conductors, and when referring to two elements connected (in English "coupled") to each other, it means that these two elements can be connected or linked through one or more other elements.

[0043] In the following description, when reference is made to absolute position qualifiers, such as the terms "front", "back", "top", "bottom", "left", "right", etc., or relative position qualifiers, such as the terms "above", "below", "superior", "inferior", etc., or to orientation qualifiers, such as the terms "horizontal", "vertical", etc., reference is made, unless otherwise specified, to the orientation of the figures or to a spectral imaging system in a normal operating position.

[0044] Unless otherwise specified, the expressions "approximately", "roughly", and "on the order of" mean to within 10% or 10°, preferably to within 5% or 5°.

[0045] Luminescence is the emission of light by an object resulting from interactions between electrically charged particles. In the case of photoluminescence, the emission of light by the object occurs following the absorption of photons by the object. In the case of electroluminescence, the emission of light by the object occurs in response to an electric current passing through the object, or to an electric field applied to the object.

[0046] A photovoltaic cell comprises a stack of layers, including at least semiconductor layers, and capable of emitting light by photoluminescence or electroluminescence.

[0047] Embodiments of spectral imaging systems and methods for spectral imaging will be described for the detection of defects in a photovoltaic cell. However, it is clear that these embodiments of spectral imaging methods can be implemented for any object exhibiting a luminescence spectrum, for example, a pixel of a light-emitting diode display on a screen.

[0048] Fig. 1 represents, in a partial and schematic way, an embodiment of a spectral imaging system 5 of a photovoltaic cell 20.

[0049] The spectral imaging system 5 comprises: - a support 10 for the photovoltaic cell 20; - an illumination source 11 of the photovoltaic cell 20 configured to project electromagnetic radiation Lm onto the photovoltaic cell 20, the photovoltaic cell 20 emitting electromagnetic radiation Lium by photoluminescence when illuminated by the radiation Lm; - an image acquisition system 12, also called an image sensor; - an optical system 13 comprising an optical filter, the Fl filter in [Fig. 1], one of three optical filters Fl, F2, F3, interposed between the photovoltaic cell 20 and the image sensor 12, the image sensor 12 being configured to perform the acquisition images of a Lium radiation fii which corresponds to the Lium radiation emitted by the photovoltaic cell 20 and filtered by the optical system 13; - a holding device 15 of the optical system 13 between the photovoltaic cell 20 and the image sensor 12, so that the relative positions between the photovoltaic cell 20, the image sensor 12, and the optical filter Fl, F2 or F3 interposed between the photovoltaic cell 20 and the image sensor 12 are fixed; and - a processing device 15 connected to the image sensor 12, the processing device 15 including in particular a memory 16.

[0050] The spectral imaging system 5 may include other optical elements, such as attenuators. The spectral imaging system 5 may include a device for automatically changing optical filters among the three optical filters Fl, F2, F3.

[0051] The spectrum of the illumination radiation Lm is different from the spectrum of the luminescence radiation Lium. The illumination source 11 may include a light-emitting diode, or a set of light-emitting diodes, or a laser source. The image sensor 12 is sensitive to the spectrum of the luminescence radiation Lium. The spectral imaging system 5 may further include a voltmeter, not shown, for measuring the voltage appearing across the terminals of the photovoltaic cell during luminescence. The spectral imaging system 5 may further include an opaque cover surrounding the object 20, the illumination source 11, the image sensor 12, and the optical system 13 to prevent any interference from ambient light. Alternatively, the spectral imaging system 5 may include at least two illumination sources 11 emitting the same illumination radiation Lm, for example, arranged on either side of the photovoltaic cell 20.

[0052] According to one embodiment, the optical axis of the image sensor 12 is orthogonal to the face of the photovoltaic cell 20 from which images are acquired. The illumination radiation Lm is, for example, projected at an angle of inclination of approximately 45° to the optical axis of the image sensor 12.

[0053] Figure 2 represents, in a partial and schematic way, a variant of the system spectral imaging 5 of [Fig.1]. The spectral imaging system 5 shown in [Fig.2] includes all the elements of the spectral imaging system 5 shown in [Fig.1] except that the illumination source 11 is replaced by an electrical source 17 connected to the photovoltaic cell 20, the photovoltaic cell 20 emitting Lium radiation by electroluminescence when powered by the electrical source 17.

[0054] The image sensor 12 is configured to acquire images of the photovoltaic cell 20, for example, of the face of the photovoltaic cell emitting Lium radiation. Each image comprises an array of image pixels. The image sensor 12 includes an array of photodetectors, each providing a signal whose amplitude depends on the light energy received by the photodetector during an integration phase. Each signal corresponds, for example, to an image pixel, and the set of image pixels forms an image.

[0055] Fig. 3 is an example of an evolution curve of the luminescence spectrum 4em of a single junction photovoltaic cell as a function of the wavelength X.

[0056] The luminescence spectrum Δεᵢ depends on the electronic characteristics of the semiconductor material composing the photovoltaic cell 20. Under certain conditions, the luminescence spectrum Δεᵢ is not related to the illumination radiation spectrum Lm in the case of photoluminescence or to the power supply of the photovoltaic cell 20 in the case of electroluminescence. For a single-junction photovoltaic cell, the luminescence spectrum Δεᵢ includes a peak at a wavelength of peak XP. In the case of photoluminescence, the illumination radiation Lm preferably includes wavelengths shorter than the peak XP wavelength. In the case of electroluminescence, the voltage applied to the photovoltaic cell 20 is above a voltage threshold to trigger electroluminescence.

[0057] According to one embodiment, the luminescence spectrum 0em emitted by the photovoltaic cell 20 is modeled as a function of the wavelength X of the Lium radiation by the following relation (1):

[0058] [Math.5]

[0059] where EQEPV(X) is the unitless external photovoltaic quantum efficiency, 4>bb(X) is the blackbody radiance energy, expressed in Wm 3sr *, and B is a multiplicative factor.

[0060] The multiplicative factor B can be given by the following relation (2):

[0061] [Math.6] B=exp(^)-1

[0062] where q is the elementary electric charge (expressed in coulombs), V is the voltage (expressed in volts) across the terminals of the semiconductor material composing the photovoltaic cell, k is the Boltzmann constant (expressed in joules per kelvin), and T is the absolute temperature (expressed in kelvin).

[0063] The emission from the black body 0bb(X) is modeled by Planck's law according to the following relation (3):

[0064] [Math.7]

[0065] where h is Planck's constant and c is the speed of light.

[0066] The external photovoltaic quantum efficiency EQEPV is modeled by a sigmoid function according to the following relation (4):

[0067] [Math. 8] E QEpv(À) = i+exp(K“x-}içJ) / Às)

[0068] where Am is a constant, Xg is the cutoff wavelength, Xs is an expression for the bandwidth, hereafter referred to as the bandwidth, and K is a multiplicative factor. The cutoff wavelength Xg is close to the peak wavelength XP but is not identical to the peak wavelength XP.

[0069] The multiplicative factor K is given by the following relation (5):

[0070] [Math.9] K=ln[ 7+4^3]

[0071] Figure 4 shows evolution curves for the sigmoid model of the external photovoltaic quantum efficiency (EVPE) as a function of wavelength X for different values ​​of bandwidth Xs. The curve C0 has the shape of a step at the cutoff wavelength Xg and is obtained for a bandwidth Xs tending towards 0. The curves C20, C50, C100, C200, C500, and C1000 are obtained respectively for bandwidths Xs equal to 20 nm, 50 nm, 100 nm, 200 nm, 500 nm, and 1000 nm. The curves C20, C50, C100, C200, C500, and C1000 all pass through the point H whose abscissa is equal to Xg and whose ordinate is equal to 50%.

[0072] If the luminescence spectrum 4>em is expressed as a function of the wavelength X, the study near the cutoff wavelength Xg shows that the external photovoltaic quantum efficiency EQEPV is a decreasing function with respect to the wavelength X and the blackbody emission 0bb is an increasing function with respect to the wavelength X. It is the multiplication of these two effects that creates the shape of the luminescence peak of the luminescence spectrum 0em.

[0073] The luminescence spectrum 4>em is therefore completely modeled, up to a multiplicative factor, by means of two parameters, the cutoff wavelength Xg and the bandwidth Xs.

[0074] With the optical system 13 and the image sensor 12, the SP spectrum of the signal captured by the image sensor 12 corresponds to the luminescence spectrum 4>em multiplied by additional terms reflecting the spectral sensitivities of the optical system 13 and the image sensor 12.

[0075] The spectral shape SP of the radiation captured by the image sensor 12 is modeled by the following relation (6):

[0076] [Math. 10] SP(X)=^ (X)QE (X)FT op t(X) F? III VxClXXX J.

[0077] where QEcam is the spectral response (also called quantum efficiency) of the image sensor 12 and FTopt is the transfer function of the optical system 13. The spectral response QEcam and the transfer function FTopt of the optical system 13 are known and generally supplied by the manufacturers of the image sensor 12 and the optical system 13.

[0078] The optical filter Fl has a transfer function FTf[, the optical filter F2 has a transfer function FTf2, and the optical filter F3 has a transfer function FTf3. The transfer function FTopt of the optical system 13 corresponds to the transfer function FTFi, FTf2, or FTf3 of the optical filter Fl, F2, or F3 interposed between the photovoltaic cell 20 and the image sensor 12.

[0079] When the spectral imaging system 5 can include other optical elements, such as attenuators, the spectral response of these optical elements can be included in the QEcam term or can correspond to an additional term added in relation (6).

[0080] According to one embodiment, at least one of the optical filters Fl, F2, and F3 is a bandpass filter, each of the other optical filters Fl, F2, and F3 being a bandpass, lowpass, or highpass filter. Optical filter F1 has a bandwidth AF1, optical filter F2 has a bandwidth AF2, and optical filter F3 has a bandwidth AF3, given that, for a lowpass filter, the lower limit of the bandwidth is 0 Hz and, for a highpass filter, the upper limit of the bandwidth is +∞. According to one embodiment, the selection of the bandwidths of the optical filters Fl, F2, and F3 is adapted in particular to the fact that the analyzed photovoltaic cell 20 is a single-junction or multi-junction cell.In one embodiment, each bandwidth AF1, AF2, AF3 is greater than 20 nm, in particular greater than 30 nm, in particular equal to 50 nm. In one embodiment, the overlap between two bandwidths of two optical filters among the optical filters Fl, F2, and F3 is less than 50 nm, in particular less than 40 nm. In one embodiment, at least two of the optical filters among the optical filters Fl, F2, and F3 are each a bandpass optical filter, the other optical filter among the optical filters Fl, F2, and F3 being a bandpass optical filter. A low-pass optical filter, or a high-pass optical filter. In one embodiment, each optical filter Fl, F2, F3 is a band-pass optical filter. In this case, the first optical filter Fl is centered on a central wavelength XF1, the second optical filter F2 is centered on a central wavelength XF2, and the third optical filter F3 is centered on a central wavelength XF3. The difference between each central wavelength among the central wavelengths XF1, XF2, XF3, and at least one of the other two central wavelengths among the central wavelengths XF1, XF2, XF3 is greater than 10 nm, preferably greater than 20 nm, and in particular equal to 25 nm. The central wavelengths XF1, XF2, XF3 are chosen according to the expected peak wavelength XP for the photovoltaic cell 20.

[0081] According to one embodiment, in the case of the spectral imaging system 5 of [Fig. 1], each optical filter Fl, F2, F3 is further configured to block the illumination radiation Lm. Alternatively, the spectral imaging system 5 of [Fig. 1] may further include an optical filter adapted to block the illumination radiation Lm and disposed upstream of the image sensor 12.

[0082] According to one embodiment, the photovoltaic cell 20 is a multi-junction cell. This is particularly the case for a tandem cell that is subdivided into two sub-cells. In this case, the luminescence spectrum 4>em may include two peaks. The central wavelengths XF1, XF2, XF3 and the bandwidths AF1, AF2, AF3 are chosen according to the peak wavelength XP of the peak of interest in the photovoltaic cell 20, in particular so as not to overlap another peak.

[0083] According to one embodiment, the photovoltaic cell 20 is a multi-junction cell. This is particularly the case for a tandem cell. In this case, the luminescence spectrum Δem may include two peaks. The central wavelengths XF1, XF2, XF3 and the bandwidths AF1, AF2, AF3 are chosen according to the peak wavelength XP of the peak of interest in the photovoltaic cell 20, in particular so as not to overlap another peak.

[0084] Fig. 5 represents an example of the spectral shape SP of the radiation captured by the image sensor 12 in the absence of an optical filter and Fig. 6, Fig. 7, and Fig. 8 represent examples of spectral shapes SP1, SP2, SP3 of the radiation captured by the image sensor 12 respectively when the first optical filter Fl (curve SP1), the second optical filter F2 (curve SP2) or the third optical filter F3 (curve SP3) is interposed between the photovoltaic cell 20 and the image sensor 12 and in the case where the optical filters Fl, F2, F3 are perfect bandpass optical filters.

[0085] A perfect bandpass optical filter has a transmission gain of 1 within its passband and a transmission gain of 0 outside the passband. On the [Fig. 5] The spectral shape SP1 (dashed line), the spectral shape SP2 (dashed line), and the spectral shape SP3 (dashed line) are superimposed on the spectral shape SP. For [Fig. 6], the optical filter Fl is a bandpass optical filter centered on the central wavelength XF1 of 750 nm with a bandwidth AF1 of 50 nm. For [Fig. 7], the optical filter F2 is a bandpass optical filter centered on the central wavelength XF2 of 775 nm with a bandwidth AF2 of 50 nm. For [Fig. 8], the optical filter F3 is a bandpass optical filter centered on the central wavelength XF3 of 800 nm with a bandwidth AF3 of 50 nm.

[0086] The brightness of each pixel of the image captured by the image sensor 12 through each optical filter can be interpreted as a partial integral of the spectral form SP over a wavelength range given by the bandwidth of the optical filter used.

[0087] A first RFi / F2 ratio is determined according to the following relation (7):

[0088] [Math. 11] Fl / F2

[0089] This corresponds to the ratio between doing under the curve of [Fig.6] and doing under the curve of [Fig.7].

[0090] A second RFi / F3 ratio is determined according to the following relation (8):

[0091] [Math. 12] p KF1 / F3— AF3+AF3 / 2 .

[0092] This corresponds to the ratio between doing under the curve of [Fig.6] and doing under the curve of [Fig.8].

[0093] The second ratio RN / F3 can be replaced by the ratio RF 2 / F 3. In the following description, it is therefore possible to replace the ratio RF[ / F3 by the ratio RF 2 / f 3- Similarly, the inverse ratios can also be used in place of those presented in this description.

[0094] In each ratio RN / F2 and RN / F3, the constant Am in the expression for the external photovoltaic quantum efficiency EQEPV and the constant B in the expression for the luminescence spectrum 0em, which are independent of wavelength and present in the numerator and denominator, are simplified. This simplification can only be performed if the Lium radiation is stable over time. If the Lium radiation varies over time, a correction described below must be applied to account for the variation in the Lium radiation and to allow the simplification to be performed.

[0095] In practice, since the optical filters Fl, F2, and F3 are not perfect, the first RFi / F2 ratio is determined according to the following relation (9):

[0096] [Math. 13] F1 / F2

[0097] The first RF ratio [ / F2 corresponds to a first function f 1 (Xg, Xs) which depends on the cutoff wavelength Xg and the bandwidth Xs.

[0098] Similarly, the second RN / F3 ratio is determined by the following relation (10):

[0099] [Math. 14] F1 / F3 ^FT^-EQE^E^

[0100] The second RF ratio [ / F 3 corresponds to a second function f2(Xg, Xs) which depends on the cutoff wavelength Xg and the bandwidth Xs.

[0101] The [Fig.9] is a block diagram illustrating an embodiment of a spectral imaging method of a photovoltaic cell 20 implementing the spectral imaging system 5 of the [Fig.1] or 2.

[0102] In step 50, a first table T1 is determined containing values ​​obtained by simulation of the first RFi / F2 ratio, determined from the equations described previously over a range of cutoff wavelength values ​​Xg and a range of bandwidth values ​​Xs, and a second table T2 containing values ​​obtained by simulation of the second RFi / F3 ratio, determined from the equations described previously over a range of cutoff wavelength values ​​Xg and a range of bandwidth values ​​Xs. The equations used are determined according to the materials intended for the photovoltaic cell 20.As an example, for each of the tables T1 and T2, the RFi / F2 and RFi / F3 ratio values ​​are determined for the cutoff wavelength Xg in a first range from 720 nm to 830 nm with an increment of less than 2 nm, preferably less than or equal to 1 nm, and for the bandwidth Xs in a second range from 12 nm to 30 nm with an increment of less than 1 nm, preferably less than or equal to 0.2 nm. The tables T1 and T2 are stored in memory 16.

[0103] According to one embodiment, to obtain the tables T1 and T2, the first values ​​of the spectral shape SP1 of the radiation captured by the image sensor 12 are determined by simulation when the first optical filter Fl is interposed between the photovoltaic cell 20 over the first range of the cutoff wavelength Xg and the second range of the bandwidth Xs. The second values ​​of the spectral shape SP2 of the radiation are further determined by simulation.captured by the image sensor 12 when the second optical filter F2 is interposed between the photovoltaic cell 20 over the first range of the cutoff wavelength Xg and the second range of the bandwidth Xs. Finally, it is determined by simulation of the third values ​​of the spectral shape SP3 of the radiation captured by the image sensor 12 when the third optical filter F3 is interposed between the photovoltaic cell 20 over the first range of the cutoff wavelength Xg and the second range of the bandwidth Xs. The first table T1 is obtained by dividing, for each pair of cutoff wavelength Xg and bandwidth Xs, the first and second values ​​obtained for the same pair, and the second table T2 is obtained by dividing, for each pair of cutoff wavelength Xg and bandwidth Xs, the first and third values ​​obtained for the same pair.The process continues at step 51.

[0104] In step 51, the first optical filter Fl is arranged between the photovoltaic cell 20 and the image sensor 12. The photovoltaic cell 20 is illuminated by Lium radiation for the spectral imaging system 5 of [Fig. 1] or electrically powered for the spectral imaging system 5 of [Fig. 2], and a first image IrnFl of the photovoltaic cell 20 is acquired by the image sensor 12 through the first optical filter FL. The acquired image IrnFl is transmitted to the processing device 15 and stored in the memory 16. The method further includes a step of acquiring an image ImFl' of the photovoltaic cell 20 by the image sensor 12 with the interposition of the first optical filter Fl in the absence of luminescence. The acquired image ImFl' is transmitted to the processing device 15 and stored in the memory 16. The processing device 15 determines a new image IrnFl" equal to the difference between the image ImFl and the image ImFl'.This advantageously allows at least some of the measurement noise to be subtracted from the ImFl image. The process continues in step 52.

[0105] In step 52, the first optical filter Fl is removed and the second optical filter F2 is placed between the photovoltaic cell 20 and the image sensor 12. The photovoltaic cell 20 is illuminated by Lium radiation for the spectral imaging system 5 of [Fig. 1] or electrically powered for the spectral imaging system 5 of [Fig. 2], and a second image ImF2 of the photovoltaic cell 20 is acquired by the image sensor 12 through the second optical filter F2. The acquired image ImF2 is transmitted to the processing device 15 and stored in the memory 16. The method further includes a step of acquiring an image ImF2' of the photovoltaic cell 20 by the image sensor 12 with the second optical filter F2 interposed in the absence of luminescence. The acquired image ImF2' is transmitted to the processing device 15 and stored in memory 16. The processing device 15 determines a new image ImF2" equal to the difference between the image ImF2 and the image ImF2'. This advantageously allows at least some of the measurement noise to be subtracted from the image ImF2. Alternatively, when the noise is essentially electronic in nature, the processing device 15 determines the new image ImF2" equal to the difference between the image ImF2 and the image ImF1' determined in step 51. The process continues in step 53.

[0106] In step 53, the second optical filter F2 is removed and the third optical filter F3 is placed between the photovoltaic cell 20 and the image sensor 12. The photovoltaic cell 20 is illuminated by Lium radiation for the spectral imaging system 5 of [Fig. 1] or electrically powered for the spectral imaging system 5 of [Fig. 2], and a third image ImF3 of the photovoltaic cell 20 is acquired by the image sensor 12 through the third optical filter F3. The acquired image ImF3 is transmitted to the processing device 15 and stored in the memory 16. The method further includes a step of acquiring an image ImF3' of the photovoltaic cell 20 by the image sensor 12 with the third optical filter F3 interposed in the absence of luminescence. The acquired ImF3' image is transmitted to the processing device 15 and stored in memory 16.The processing device 15 determines a new image ImF3" equal to the difference between the image ImF3 and the image ImF3'. This advantageously allows at least part of the measurement noise to be subtracted from the image ImF3. Alternatively, when the noise is essentially electronic in nature, the processing device 15 determines the new image ImF3" equal to the difference between the image ImF3 and the image ImF1' determined in step 51. The process continues in step 54.

[0107] In step 54, the value of the first RFi / F2 ratio is determined for each image pixel at a given position from the image pixel at the given position of the first ImFl image and the image pixel at the given position of the second ImF2 image and the value of the second RFi / F3 ratio is determined for each image pixel at a given position from the image pixel at the given position of the first ImFl image and the image pixel at the given position of the third ImF3 image. According to one embodiment, the first RFi / F2 ratio for each image pixel at a given position is equal to the ratio between the image pixel at the given position of the first ImFl image and the image pixel at the given position of the second ImF2 image, and the second RFi / F3 ratio for each image pixel at a given position is equal to the ratio of the image pixel at the given position of the first ImFl image and the image pixel at the given position of the third ImF2 image.The process continues at step 55.

[0108] In step 55, for each image pixel, the RFi / F2 ratio determined in step 54 is compared to the values ​​in the first table T1. From the first table T1, a first function Xg=L1(Xs) is determined for each image pixel, for which the RFi / F2 ratio in the first table T1 corresponds at least approximately to the value of the RFi / F2 ratio determined in step 54. According to one embodiment, for each value of the cutoff wavelength Xg for which a simulated RFi / F2 ratio is stored in the first table T1, the bandwidth value Xs is selected for which the simulated RFi / F2 ratio stored in the first table T1 is closest to the RFi / F2 ratio determined in step 54. For each image pixel, the RFi / F3 ratio determined in step 54 is compared to the values ​​in the second table T2.From the second table T2, a second function Xg=L2(Xs) is determined for each image pixel, for which the RFi / F3 ratio of the second table T2 corresponds at least approximately to the value of the RFi / F3 ratio determined in step 54. According to one embodiment, for each value of the cutoff wavelength Xg for which a simulated RFi / F3 ratio is stored in the second table T2, the value of the bandwidth Xs is selected for which the simulated RFi / F2 ratio stored in the second table T2 is closest to the RFi / F3 ratio determined in step 54.

[0109] Figure 10 represents a CL1 evolution curve of the function Xg=L1(Xs) and a CL2 evolution curve of the function Xg=L2(Xs) for an image pixel. The abscissa XSP and the ordinate XgP of the point P of intersection of the two curves CL1 and CL2 correspond to the values ​​of the bandwidth and cutoff wavelength, respectively, for the image pixel.

[0110] According to one embodiment, a color code or a grayscale code is assigned for the cutoff wavelength values ​​Xg and a color code or a grayscale code is assigned for the bandwidth values ​​Xs. According to one embodiment, the processing device 14 determines a color or grayscale image Im_Xg in which each image pixel represents the XgP value determined for that image pixel and a color or grayscale image Im_A in which each image pixel represents the XSP value determined for that image pixel.

[0111] According to one embodiment, the change of optical filters Fl, F2, F3 can be performed manually by an operator. According to another embodiment, the change of optical filters Fl, F2, F3 can be performed automatically.

[0112] The spectral imaging method advantageously allows obtaining two spectral data Xg and Xs of the semiconductor material, using only three optical filters and a spectral-capacity luminescence image acquisition system 12.

[0113] The use of a semi-physical model to represent the 4>em luminescence spectrum, which employs a physical model for blackbody emission 0bb(X) and a sigmoid representation of the external quantum efficiency EQEPV, provides better modeling than that obtained when the 0em luminescence spectrum is modeled by a purely mathematical curve such as a Gaussian. The simulated RFi / F2 ratios in the first table T1 and the simulated RFi / F3 ratios in the second table T2 can therefore be determined with greater accuracy than that obtained when the 0em luminescence spectrum is modeled by a purely mathematical curve such as a Gaussian. In particular, the semi-physical model is more accurate for wavelengths of the 4>em luminescence spectrum far from the peak wavelength XP.This allows, in particular, for the bandwidth of at least one of the optical filters Fl, F2, F3 to cover an area far from the peak of the luminescence. This makes it possible to broaden the spectral area of ​​analysis for the same set of optical filters without losing precision.

[0114] Furthermore, the cutoff wavelength Xg provides the photovoltaic band gap (also called the photovoltaic gap) of the semiconductor material emitting the luminescence radiation. A definition of the photovoltaic gap can be found, in particular, in the publication by Rau et al. (Uwe Rau, Beatrix Blank, Thomas CM Muller, Thomas Kirchartz, Phys. Rev. Appl. 2017, 7, 044016) entitled "Efficiency Potential of Photovoltaic Materials and Devices Unveiled by Detailed-Balance Analysis". The peak wavelength XP provides the optical band gap (also called the optical gap). A measurement based on optical absorption also provides the optical gap. Although the values ​​of the photovoltaic and optical gaps are close, the photovoltaic gap includes effects due to the entire junction, whereas the optical gap more strictly characterizes the absorbing semiconductor material. The photovoltaic gap includes more effects related to charge and temperature.The photovoltaic band gap is also linked to the geometry of the sample, particularly its thickness. Advantageously, comparing the optical band gap and the photovoltaic band gap can help to distinguish the intrinsic and extrinsic influences on the behavior of the photovoltaic cell 20.

[0115] The use of bandpass optical filters advantageously allows the spectral imaging method to be implemented on multi-junction structures, with sets of three filters defined for the study of each junction such that they exclude the luminescence of the other junctions. This application to multi-junction structures is not possible using only high-pass and low-pass filters because the filtered luminescence then includes other luminescence peaks, which makes The comparison to the luminescence model, whether mathematical or semi-physical, is inaccurate.

[0116] The spectral imaging process advantageously allows for high measurement sensitivity compared to spectroscopic methods because the spectral bands measured through the optical filters Fl, F2, F3 are wider and therefore brighter than those used in the case of spectroscopic methods.

[0117] Advantageously, the implementation of the spectral imaging process does not require high-quality filtration of the optical filters Fl, F2, F3. In one embodiment, an optical density of 2 or more is sufficient. This type of optical filter is inexpensive and readily available among existing technologies. However, when the photovoltaic cell 20 is a multi-junction cell, it may be necessary to treat parasitic emissions elsewhere, notably by adding an optical filter. For example, for a perovskite-on-silicon tandem photovoltaic cell, a low-pass optical filter with an upper limit of 900 nm can be added in series along the optical path. This filter blocks the radiation emitted by the silicon junction and allows only the radiation emitted by the perovskite junction to pass through.Similarly, a high-pass optical filter with an upper limit of 650 nm can be added in series along the optical path. This filter blocks the radiation emitted by the perovskite junction and only allows the radiation emitted by the silicon junction to pass through. The Fl, F2, and F3 optical filters can then be of mediocre quality without causing any problems for the spectral imaging process.

[0118] The spectral imaging process utilizes each pixel of the image sensor 12. The resolution of the spectral imaging process can therefore be measured in megapixels. The spectral imaging process is perfectly suited to both wide-field and near-field applications (using microscope-type optics). In these cases, a pixel represents a significantly smaller area. This is not the case with spectral imaging systems based on scanning by physical displacement of the material or the observation system.

[0119] The spectral imaging method is applicable in both photoluminescence and electroluminescence. This flexibility of use makes it possible to address a wider range of applications because user constraints create situations that sometimes give an advantage to one or the other of these two techniques.

[0120] For certain semiconductor materials, luminescence can evolve over time, reversibly or irreversibly. This can be the case, in particular, for perovskite materials.

[0121] Figure 11 is a block diagram illustrating another embodiment of a spectral imaging method for a photovoltaic cell 20 implementing the system spectral imaging 5 of [Fig.l] or 2 and allowing to take into account the variation of luminescence of the photovoltaic cell 20 over time.

[0122] The embodiment of the spectral imaging method illustrated in [Fig. 11] comprises all the steps of the embodiment of the spectral imaging method illustrated in [Fig. 9] and further includes an additional step 53' between steps 53 and 54 described previously. In addition, the acquisition times of the images IrnF1, ImF2, and ImF3 are determined. By way of example, the acquisitions of the images ImF1, ImF2, and ImF3 are time-stamped.

[0123] At step 53', the third optical filter F3 is removed and the first optical filter Fl is again placed between the photovoltaic cell 20 and the image sensor 12. The photovoltaic cell 20 is illuminated by Lium radiation for the spectral imaging system 5 of [Fig. 1] or electrically powered for the spectral imaging system 5 of [Fig. 2], and a fourth image Im4 of the photovoltaic cell 20 is acquired by the image sensor 12 through the first optical filter FL. The acquisition time of the image ImF4 is determined. As an example, the acquisition of the image ImF4 is time-stamped. The acquired ImF4 image is transmitted to the processing device 15 and stored in the memory 16. The process further includes a step of acquiring an ImF4' image of the photovoltaic cell 20 by the image sensor 12 with interposition of the first optical filter Fl in the absence of luminescence.The acquired ImF4' image is transmitted to the processing device 15 and stored in memory 16. The processing device 15 determines a new ImF4" image equal to the difference between the ImF4 image and the ImF4' image. This advantageously allows at least part of the measurement noise to be subtracted from the ImF4 image. Alternatively, the processing device 15 determines the new ImF4" image equal to the difference between the ImF4 image and the ImF4' image determined in step 51.

[0124] According to one embodiment, the processing device 15 determines an interpolation function that interpolates the evolution of the luminescence of the photovoltaic cell 20, image pixel by image pixel, as a function of time. The processing device 15 uses the interpolation function thus determined to modify each image pixel of the image ImF2" and each image pixel of the image ImF3", so that the images ImF2" and ImF3" are theoretically those that would have been obtained if the images ImF1, ImF2, and ImF3 had been acquired at the same instant.

[0125] Alternatively, if the spectral imaging system 5 of [Fig. 1] is used, the voltage across the photovoltaic cell 20 is measured at the time of acquisition of each image ImF1, ImF2, ImF3, and ImF4. The processing device 15 determines an interpolation function that interpolates the evolution of the luminescence of the photovoltaic cell 20, pixel by pixel. depending on the voltage across the photovoltaic cell. The processing device 15 uses the interpolation function thus determined to modify each image pixel of the image ImF2" and each image pixel of the image ImF3", so that the images ImF2" and ImF3" are those which would have been obtained if the images ImF1, ImF2, and ImF3 had been acquired with the same voltage across the photovoltaic cell 20.

[0126] Tests were carried out on first, second, and third photovoltaic cells of the perovskite-on-silicon tandem cell type. For these tests, the first, second, and third photovoltaic cells were illuminated by radiation emitted by an array of light-emitting diodes (LEDs) whose radiation has a spectrum centered at 520 nm. For the first photovoltaic cell, the spectral imaging method according to the embodiments described previously in relation to [Fig. 9] and [Fig. 11] was implemented. For the second and third photovoltaic cells, the spectral imaging method according to the embodiment described previously in relation to [Fig. 11] was implemented. For the tests, the first, second, and third optical filters Fl, F2, F3 are bandpass optical filters having a bandwidth of 50 nm. The first bandpass optical filter Fl is centered at 750 nm.The second F2 bandpass optical filter is centered at 775 nm. The third F3 bandpass optical filter is centered at 800 nm.

[0127] Figure 12 shows an image Iml_Xg of the cutoff wavelength of the first photovoltaic cell obtained during the implementation of the spectral imaging method described above in relation to Figure 9, i.e., without a correction step to account for the temporal variation of the luminescence. Figure 13 shows the same image Iml_Xg obtained during the implementation of the spectral imaging method described above in relation to Figure 11, i.e., with the correction step 53' to account for the temporal variation of the luminescence. Figure 12 highlights the modification of the Iml_Xg image of the cutoff wavelength when the temporal variation of the luminescence is taken into account.

[0128] Fig. 14, Fig. 15, and Fig. 16 represent respectively an Im2 luminescence image of the perovskite cell of the second photovoltaic cell 20 obtained with the first optical filter Fl, an Im2_Xg image of the cutoff wavelength of the second photovoltaic cell 20, and an Im2_Xs image of the bandwidth of the second photovoltaic cell 20.

[0129] Fig. 17, Fig. 18, and Fig. 19 respectively represent an Im3 luminescence image of the third photovoltaic cell obtained with the first filter optical Fl, an Im3_Xg image of the cutoff wavelength of the third photovoltaic cell, and an Im3_Xs image of the bandwidth of the third photovoltaic cell.

[0130] Comparison of images in classical photoluminescence (Im2, Im3) and images of cutoff wavelength and bandwidth (Im2_Xg, Im3_Xg, Im 2 / ,,, Im3_ / „) allows for a more precise diagnosis of defects in a luminescent semiconductor material.

[0131] By way of example, for the second photovoltaic cell, the area in the lower right corner appears darker in conventional photoluminescence (image Im2), whereas spectral analysis shows that only its edge appears to be affected by a shift towards a higher cutoff wavelength Xg (image Im2_Xg) while having a narrower luminescence peak (image Im2_ / „), which indicates a smaller number of defects than would be suggested by image Im2. Conversely, the area near the upper edge also appears darker in conventional photoluminescence (image Im2), but spectral imaging here concludes that there is a shift in the cutoff wavelength Xg across the entire surface (image Im2_Xg). Two similar areas in image Im2 are therefore interpreted differently thanks to spectral imaging performed by the spectral imaging method according to the invention.

[0132] By way of example, for the third photovoltaic cell, certain dark areas in photoluminescence (image Im3) are barely visible in the spectral image of the cutoff wavelength Xg (image Im3_Xg), particularly the circles around the vertical edges of the metallization. This establishes that the decrease in luminescence is not due to an intrinsic problem in the semiconductor material of the studied sub-cell of the tandem cell. However, two dark circles of this type are an exception because they both exhibit a broadening of the luminescence peak (image Im3_ / „), i.e., an increase in defects in the semiconductor material. Yet, only one of them shows a shift in the cutoff wavelength Xg (image Im3_Xg), specifically towards shorter wavelengths.Here again, the Im3_Xg and Im3_ / „ images allow us to distinguish between perovskite-independent phenomena (circles not appearing in the spectral image) and perovskite-related phenomena. The latter fall into two categories: a shift in the cutoff wavelength Xg and a broadening of the luminescence peak, which can be interpreted respectively as a modification of the crystal and as an increase in the density of electronic defects.

[0133] Generally, in a multi-junction cell, spectral imaging is used to analyze one of the cell's junctions. A defect in a junction other than the one being analyzed can affect the luminescence of the junction analyzed, particularly when the luminescence of the analyzed junction is obtained by electroluminescence. The spectral imaging process makes it possible to distinguish phenomena independent of the analyzed junction from phenomena related to the analyzed junction.

[0134] Various embodiments and variations have been described. A person skilled in the art will understand that certain features of these various embodiments and variations could be combined, and other variations will become apparent to a person skilled in the art.

[0135] Finally, the practical implementation of the embodiments and variants described is within the reach of a person skilled in the art, based on the functional indications given above.

Claims

1. Demands A method for spectral imaging a luminescent object (20) employing an image sensor (12) and first, second, and third optical filters (F1, F2, F3), at least one of the first, second, and third optical filters (F1, F2, F3) being a bandpass optical filter, the method comprising the following steps: a) determining, by simulation using a semi-physical model of the 0em spectrum of the luminescence (Lium) radiation emitted by the object (20), the first simulated luminescence values ​​of the luminescence (Lium) radiation emitted by the object (20) through the first optical filter (F1) over a first range of a cutoff wavelength Xg of the object (20), the second simulated luminescence values ​​of the luminescence (Lium) radiation emitted by the object (20) through the second optical filter (F2) over the first range of the cutoff wavelength Xg,and the simulated third luminescence values ​​of the luminescence radiation (Lium) emitted by the object (20) through the third optical filter (F3) over the first range of the cutoff wavelength Xg, a first table containing simulated first ratio values ​​equal to the ratios of the simulated first luminescence values ​​and the simulated second luminescence values, and a second table containing simulated second ratio values ​​equal to the ratios of the simulated first luminescence values ​​and the simulated third luminescence values; b) acquire, by the image sensor (12), a first luminescence image of the luminescence radiation (Lium) emitted by the object (20) through the first optical filter (Fl); c) acquire, by the image sensor (12), a second luminescence image of the luminescence radiation (Lium) emitted by the object (20) through the second optical filter (F2); d) acquire, by the image sensor (12), a third luminescence image of the luminescence radiation (Lium) emitted by the object (20) through the third optical filter (F3); e) determine a first image with a ratio equal to the ratio of the first luminescence image and the second luminescence image, and a second image with a ratio equal to the ratio of the first luminescence image and the third luminescence image; and f) determine a cutoff wavelength image Xg by comparing the first ratio image and the first table and the second ratio image and the second table.

2. A spectral imaging method according to claim 1, wherein the semi-physical model of the 4>em spectrum of the luminescence radiation (Lium) emitted by the object (20) is according to the following relation: [Math. 15] 4>em(X)=EQEpv(A)<|)bb(X)B where X is the wavelength, EQEPV is the external photovoltaic quantum efficiency of the object (20), 0bb is the blackbody emission, and B is a first multiplicative factor, the external photovoltaic quantum efficiency EQEPV being a function of the bandwidth Xs.

3. Spectral imaging method according to claim 2, wherein the external photovoltaic quantum efficiency EQEPV of the object (20) is modeled by the following relation: [Math. 16] EQE^A^g^^ where Am is a constant and K is a second multiplicative factor.

4. Spectral imaging method according to claim 2 or 3, wherein, in step a), the first simulated luminescence values ​​are further determined over a second range of the bandwidth Xs, the second simulated luminescence values ​​are further determined over a second range of the bandwidth Xs, and the third simulated luminescence values ​​are further determined over a second range of the bandwidth Xs, the method further comprising determining an image of the bandwidth Xs by comparing the first ratio image and the first array and the second ratio image and the second array.

5. A spectral imaging method according to any one of claims 2 to 4, wherein the spectrum of the SP radiation captured by the image sensor (12) is modeled according to the following relation: [Math. 17] where QEcam is the spectral response of the image sensor (12) and any optical elements other than the first, second, and third optical filters (F1, F2, F3), FTopt is the transfer function of one of the first, second, and third optical filters (F1, F2, F3).

6. A spectral imaging method according to any one of claims 2 to 5, wherein the blackbody emission 4>bb is modeled by the following relation: [Math. 18] where h is Planck's constant, c is the speed of light, and T is the absolute temperature.

7. A spectral imaging method according to any one of claims 1 to 6, further comprising, after step d) and before step e), an acquisition step, by the image sensor (12), of a fourth luminescence image of the luminescence radiation (Lium) emitted by the object (20) through the first optical filter (Fl) and the modification of the second luminescence image and the third luminescence image by an interpolation determined from the first luminescence image and the fourth luminescence image.

8. Spectral imaging method according to any one of claims 1 to 7, wherein at least two of the first, second, and third optical filters (F1, F2, F3) are bandpass optical filters.

9. Spectral imaging method according to claim 8, wherein each of the first, second, and third optical filters (F1, F2, F3) is a bandpass optical filter.

10. A spectral imaging method according to any one of claims 1 to 9, comprising, between step b) and step c), the automatic replacement of the first optical filter (F1) by the second optical filter (F2), and comprising, between step c) and step d), the automatic replacement of the second optical filter (F2) by the third optical filter (F3).

11. A spectral imaging method according to any one of claims 1 to 10, comprising illuminating the object (20), the object (20) emitting luminescence radiation (Lium) by photoluminescence.

12. A spectral imaging method according to any one of claims 1 to 11, comprising powering the object (20), the object (20) emitting luminescence radiation (Lium) by electroluminescence.

13. Spectral imaging method according to any one of claims 1 to 12, wherein the object (20) comprises a photovoltaic cell.

14. Spectral imaging method according to claim 13, wherein the object (20) comprises a perovskite tandem silicon photovoltaic cell.

Images