A.-B Prakoso

Chenjin Lu

Rusli

D Cortecchia

C Soci

M Berthe

D Deresmes

B Ayachi

J.-P Vilcot

H Diesinger
email: heinrich.diesinger@univ-lille.fr

Voltage transient analysis as a generic tool for solar junction characterization

Surface photovoltage transients on solar junctions have often been associated with carrier lifetime in the literature. However, the carrier decay in a junction is not governed by a first order carrier decay, but resulting from a differential capacitance interacting with a differential conductivity. This phenomenon is well known as the Kane-Swanson formalism in an engineering context where the carrier density transient is measured by photoconductance with a microwave or infrared beam. In this work, we solve the same differential equations numerically to model the carrier decay in the large signal domain extending over 5 orders of carrier density. Since the surface voltage is linked to the carrier density by a logarithmic relation, we express the carrier decay as surface photovoltage transients. We show how from photovoltage transients, the same information as from photoconductance can be drawn. To demonstrate the method as a generic tool, it is applied to 4 types of solar cells, two monocrystalline silicon cells, a Perovskite solar cell, a transition metal oxide / silicon hybrid junction, and a CIGS solar cell. Acquiring photovoltage transients by Kelvin force microscopy allows working on partial junctions without top contact, speeding up research of future photovoltaic materials. Furthermore, parameters may be mapped with a better lateral resolution compared to the millimeter resolution of microwave photoconductance.

A c c e p t e d M a n u s c r i p t characteristics vary reduces correspondingly, requiring new microscopic characterization tools or at least microscopic equivalents of existing techniques. Transient photovoltage decay (TPV or TPD) or open circuit voltage decay (OCVD) measurements are one approach of adressing carrier density dynamics. They have been applied macroscopically to Perovskite solar cells [START_REF] Pockett | Characterization of planar lead halide perovskite solar cells by impedance spectroscopy, open-circuit photovoltage decay, and intensity-modulated photovoltage/photocurrent spectroscopy[END_REF][START_REF] Gottesman | Dynamic phenomena at perovskite/electron-selective contact interface as interpreted from photovoltage decays[END_REF] and to dye-sensitized solar cells (DSC) with a contactless technique using a high-impedance OpAmp voltage follower [START_REF] Walker | Analysis of photovoltage decay transients in dyesensitized solar cells[END_REF]. Microscopic implementations using KFM as the electrode have been used to determine carrier lifetime in polycristalline solar cells [START_REF] Takihara | Minority carrier lifetime in polycrystalline silicon solar cells studied by photoassisted kelvin probe force microscopy[END_REF], in silicon nanocrystals [START_REF] Borowik | Measuring the lifetime of silicon nanocrystal solar cell photo-carriers by using kelvin probe force microscopy and x-ray photoelectron spectroscopy[END_REF], and organic photovoltaics [START_REF] Gz | Intensity-modulated scanning kelvin probe microscopy for probing recombination in organic photovoltaics[END_REF][START_REF] Paf | Photo-carrier multidynamical imaging at the nanometer scale in organic and inorganic solar cells[END_REF]. These implementations use time-averaging methods to reconstruct a decay curve indirectly by measuring average photovoltage as function of optical intensity modulation frequency.

Another common technique in photovoltaics research and industry is the determination of the emitter saturation current of a solar junction from microwave photoconductance lifetime measurements by tracing the effective decay time over the carrier density [START_REF] Sinton | Contactless determination of current-voltage characteristics and minority-carrier lifetimes in semiconductors from quasi-steadystate[END_REF]. The so-called quasi steady state (QSS) condition means that an effective carrier decay time is acquired for small signal at which the carrier density variation is small compared to the carrier density. The effective decay time derived by the Kane-Swanson (KS) formalism [START_REF] De Kane | Measurement of the emitter saturation current by a contactless photoconductivity decay method[END_REF] is given by [START_REF] Maeckel | On the determination of the emitter saturation current density from lifetime measurements of silicon devices[END_REF]

1 τ eff - 1 τ b = (J oe,front + J oe,back ) N dop + ∆n avg qW n 2 i,eff (1)
Basically, the carrier decay is due to diffusion current under forward bias in response to minority carrier generation. The term τ b combines Auger, radiative and SRH recombination and the injection rate has to be chosen such that those terms remain negligible compared to the diffusion current. Then, plotting 1/τ eff over ∆n av and determining the slope yields the sum of the emitter saturation currents J oe , which is linked to the potential open-circiut voltage V oc by the relation

J sc J oe = exp eV oc kT -1 (2)
where the short-circuit current J sc is determined by the incident photon flux and typically equals around 40 mA cm -2 for standard sun-simulator conditions.

It is common to map the potential open-circuit voltage across the junction, e.g. to visualize edge passivation problems. It can be acquired at different stages of fabrication to observe the quality of the junction alone and its degradation during subsequent processing [START_REF] Pavelka | Carrier lifetime measurements in silicon for photovoltaic applications[END_REF]. The resolution determined by the microwave beam of half a millimeter diameter is sufficient on bulk mono-or polycrystalline silicon cells.

On the other hand, carrier dynamics in solar cells are often investigated by the transient photovoltage technique. It is noteworthy that in this case, the apparent decay lifetime depends on the light intensity used for generation or initial carrier density, as has been mentioned by some authors [START_REF] Gz | Intensity-modulated scanning kelvin probe microscopy for probing recombination in organic photovoltaics[END_REF]. It was also widely accepted since the pioneer work [START_REF] Takihara | Minority carrier lifetime in polycrystalline silicon solar cells studied by photoassisted kelvin probe force microscopy[END_REF] that rising transients were abrupt compared to falling transients which is untypical for exponential decay. Decay constants derived by this technique were therefore associated A c c e p t e d M a n u s c r i p t to a particular illumination intensity [START_REF] Borowik | Measuring the lifetime of silicon nanocrystal solar cell photo-carriers by using kelvin probe force microscopy and x-ray photoelectron spectroscopy[END_REF]. A recent review on the dependence of a first order decay constant on open circuit voltage was published by Kiermasch et al. [START_REF] Kiermasch | Revisiting lifetimes from transient electrical characterization of thin film solar cells; a capacitive concern evaluated for silicon, organic and perovskite devices[END_REF]. In parallel, more sophisticated models of the rise and decay curves, involving different time constants [START_REF] Paf | Photo-carrier multidynamical imaging at the nanometer scale in organic and inorganic solar cells[END_REF], were developed.

2. Adapting the Kane-Swanson formalism to large signal voltage transients

Carrier decay differential equation

The carrier decay addressed by the Kane-Swanson formalism is not an exponential decay such as SRH recombination described by a time constant. It is caused by a forward diode current that results from the forward bias of a junction under illumination, and is due to carrier diffusion in the neutral zones of the junction. The key parameter of this forward current is the emitter saturation current J oe which is determined by the diffusion length and the width of the junction. The KS equation suggests that for small signal variation, dn ≪ n, the decay can be treated as first order exponential decay, with a decay time constant that is inversely dependent on the carrier density. It is clear that if the time constant is not constant but depends itself on the carrier density, the exponential decay is at best a small signal approximation valid for a small range of carrier density variation. We propose to derive a decay law of non-exponential decay with general validity even for large carrier density fluctuation. As in the demonstration of the Kane-Swanson equation, Auger, SRH, and radiative recombination are grouped together in τ b of eq. 1 and are neglected for the base region. Here the KS formalism is briefly resumed and will be applied to the modeling of TPV curves.

The band diagram of a typical photovoltaic junction, under illumination and KFM surface potential measurement, is shown on fig. 1.

It is a typical asymmetric junction with a large low doped absorber called "base" and a thin highly doped counterpart provided with an ohmic contact, referred to as "emitter". It is shown without front contact, either because it is an incomplete partial junction or the Kelvin probe is away from one of those electrodes called "finger". We shall consider that the photogenerated minortiy carriers are all in the neutral region of the base and the fraction of carriers about to recombine in the emitter is negligible. Furthermore, the recombination current of the emitter shall be the only source of carrier loss whereas in the neutral region of the base, the bulk recombination effects SRH, Auger and radiative recombination are neglected. Consequently, in the base, the pseudo-Fermi level and minority carrier density are constant.

If the junction was provided with a front contact, the approximation of constant minority carrier density in the base would still hold: due to asymmetry, low bulk recombination and high mobility in the base, the pseudo-Fermi level would decay only in the very proximity of the front electrode. Therefore and because of the small fraction of surface covered by front contacts, the carrier density in the base could still be considered constant and the respective recombination current would be much smaller than the one A c c e p t e d M a n u s c r i p t Figure 1. Band structure of a photovoltaic junction under illumination: it is assumed that the minority carrier density is constant throughout the base because of its relatively low doping, negligible bulk recombination and high mobility. The main carrier loss mechanism is the recombination current J n at the boundary between the neutral region of the base and the space charge zone at depth W .

of the back emitter. For simplicity, we also renounce to indicating front surface fields in the band structure.

Note that two parasitic parallel components, a capacitor and a resistor, are included. The capacitance to be added is basically the geometric constant capacity, whereas the resistor describes leakage paths. A more detailed description of charge leakage in particular for Perovskite has been presented by Tvingstedt et al. [START_REF] Tvingstedt | Removing leakage and surface recombination in planar perovskite solar cells[END_REF].

The continuity equation is written for the base region, neglecting bulk recombination mechanisms:

d∆n(x, t) dt = G(x, t) -D n d 2 ∆n(x, t) dx 2 (3)
Since the bulk recombination in the neutral base region is neglected, the minority carrier density is constant ∆n(x) = ∆n avg in the neutral area from 0 to W . Integrating over the junction depth W yields

W d∆n avg (t) dt = W G(x, t)dx -D n d∆n(x, t) dx | x=W (4)
The last component is the diffusion current and is set equal to the electron recombination current of the emitter at the boundary of the neutral base and the space charge zone:

D n d∆n(W, t) dx = J oe q n(W, t)p(W, t) -n 2 i,eff (W, t) n 2 i,eff (W, t) = J oe q exp eV kT -1 (5)
With the same approximations also made in [START_REF] Maeckel | On the determination of the emitter saturation current density from lifetime measurements of silicon devices[END_REF] yielding the Kane-Swanson equation, i.e.

p ≈ N dop + ∆n ≫ n 2 i,eff
Renaming the integrated generation g and assuming that the density labeled "average" is roughly constant over the junction depth W leads to

d∆n dt = g - J oe qW ∆n(N dop + ∆n) n 2 i = J oe qW exp eV kT -1 (7)
Showing clearly that there is a component of overexponential decay, since for ∆n ≫ N dop , it simplifies to

d dt ∆n n i = g n i - J oe qW n i ∆n n i 2 (8)
whereas exponential decay is dominant for ∆n < N dop , simplifying eq. 7 to

d dt ∆n n i = g n i - J oe N dop qW n 2 i ∆n n i (9)
The small signal approximation of eq. 1 with an effective exponential decay constant inversely depending on the carrier density ∆n, as found in the literature and applicable to QSS conditions, can easily be derived from eq. 7 by writing

1 τ eff = 1 ∆n d∆n dt = -J oe N dop + ∆n qW n 2 i ( 10
)
if furthermore the generation is set to zero for modeling only the decay branch.

If surface photovoltage (SPV) transients were erroneously interpreted as exponential decay beyond the validity of the small signal approach d∆n ≪ ∆n, the lifetime would apparently depend on generation intensity or initial carrier density and the fit with a simple exponential decay would be unsatisfying. If a SRH-like lifetime was derived from such curve fitting, different time constants would also be obtained for rise and fall transients which would give rise to somewhat speculative interpretations.

In this work we shall solve the general case for large signal decay curves by computing the carrier decay numerically.

Equivalence of optical and electrical injection

While studying decay curves of TMO:Si hybrid solar cells with different injection level and under optical and electrical injection (c.f. section 4.1), the following observations have been made:

1) Optical generation and electrical injection lead to identical voltage transients after switching off the excitation source at t = 0. Fig. 2 forward current and turning it off. The only difference is the additional voltage drop across the contact series resistance for electrical injection.

2) The curves of lower injection are to a large extent superposable to the lower portion of curves with higher injection by shifting their t = 0 origin so that the starting value of the voltage is identical to the voltage of the curve with higher injection at a time t > 0. Therefore we will analyze only the curves with the highest injection.

Note that fig. 2 (b) contains one transient with a higher excitation intensity for 30 mA whose optically excited equivalent could not be reached with our laser intensity. The condition for equal carrier generation g by a current I or by optical generation with power p applied to an area A is :

g = I qW A = p hν W A (11)
or

I[A] = p[W]λ[nm]/1240 (12)
A c c e p t e d M a n u s c r i p t and was roughly satisfied for the applied laser powers and injection currents.

Generally valid solution of the carrier decay differential equation

We shall solve the differential equation 7 so that it covers all ranges of carrier density from overexponential decay at strong injection ∆n > N dop , exponential decay for N dop > ∆n, to the approach of zero voltage.

Steady state

The emitter saturation current J oe is obtained from the steady state where the carrier density is constant by setting dn/dt = 0 in eq. 7:

J oe = gqW exp - eV kT (13)
With optical or electrical excitation g defined by eq. 11.

Infinitesimal decay computation

A voltage transient can be computed by solving the general differential eq. 7 infinitesimally. The carrier density as function of voltage is

∆n = -N dop 2 + N 2 dop 4 + n 2 i exp eV kT (14)
Then we compute the infinitesimal change in charge density d∆n due to diffusion current:

d∆n = dt g - J oe qW exp eV kT -1 (15)
By the derivative of eq. 14, the infinitesimal voltage change

dV = d∆n dV -1 d∆n (16)
is computed and added to the previous value.

Application to solar cells

Open circuit photovoltage decay curves are measured on the samples shown in fig. 3.

Transition metal oxide / silicon hybrid cell

A hybrid silicon:transition metal oxide (Si:TMO) solar cell was fabricated by deposing MoO 3 onto n-type substrate doped N dop = 10 14 cm -3 , shown on fig. 3 (a)). Voltage transients have been acquired macroscopically by oscilloscope under electrical injection.

The decay curve for injection with I/A = 50 mA cm -2 along with its modeling is shown in fig. 4. The initial voltage after turning off the current is V = 0.508 V, yielding from eqs. 11 and 13 a saturation current J oe = 1.5 • 10 -10 A cm -2 . The red dashed curve is the voltage transient modeled by taking into account the diffusion current and the diffusion capacitance. It shows a steep onset for overexponential decay and passes into a linear slope for the exponential decay regime (since the voltage is roughly the logarithm of the carrier density). Surprisingly, the experimental curve deviates from the diffusion current model below a certain voltage where its slope becomes steeper again. A way to account for this increase of the slope at lower voltage is taking into account a parallel parasitic resistor R. A relatively low parallel resistor of 6.7 kΩ cm 2 had indeed been measured at reverse bias. This resistance can make the slope steeper at lower junction voltage because the carrier density decreases exponentially with voltage but the current

V (V) t (s)
Figure 4. Transient of TMO:Si solar cell doped N dop = 10 14 cm -3 and electrically injected: black squares experimental, red dashes : infinitesimally computed decay curve with diffusion current and diffusion capacitance, green dots: computed decay curve also including a parallel resistor R = 6.7 kΩ, and blue dash-dot: computed curve also including both the parallel resistor R and a parallel capacitance C = 12 nF. Above t = 0.8 ms, the rising transition is shown for both experimental data and modeled with the same parameters as the decay, including furthermore the generation term.

through the resistor only decreases linearly. Less there are minority carriers in the base, steeper the decay becomes, resulting in a charge breakdown effect. Mathematically the parallel resistance R contributes to the charge decay, eq. 15, as

d∆n = dt g - J oe wq exp eV kT -1 - V RAwq (17)
The subsequent treatment is the same, dV is obtained by eq. 16. The effect of the parallel resistance is shown on fig. 4 as the green curve for R = 6.7 kΩ. The reason why experimentally the breakdown effect is less pronounced in the experimental curve may be explained by the presence of a parallel parasitic capacitance C that smoothes out the queue of the decay and that we will also add to the model by adapting eq. 14:

∆n = V C Awq - N dop 2 + N 2 dop 4 + n 2 i exp eV kT (18)
The charge stored in the capacitor has also been converted into a density by dividing by the junction volume and elementary charge. In the computation of dV, the derivative dn/dV changes accordingly and so does eq. [START_REF] Ayachi | Rapid thermal annealing effect on the spatial resistivity distribution of azo thin films deposited by pulsed-direct-current sputtering for solar cells applications[END_REF].

With the resistor and capacitor in parallel, we can model the experimental transient quite well, illustrated in fig. 4 as the blue dash-dotted curve for a C = 12 nF capacitor and R = 6.7 kΩ. A similar capacitance value of C = 7 nF had been found by AC impedance measurement. A deviation between model and measurement occurs only at the lowest part of the queue. Possibly the model based on fixed parallel resistors A c c e p t e d M a n u s c r i p t and capacitances is not enough sophisticated and in reality the differential capacitance needed for a more accurate model of the parasitic elements would be by itself voltage dependent.

Finally, with the determined parameters it is also possible to model the rising edge of the voltage transient. The start value is set to zero and the generation according to eq. 11 is applied for t > 0.8 ms, shown in fig. 4. The successful modeling of the rising edge clearly demonstrates that it is an inherent feature of the overexponential decay law to yield asymmetry between falling and rising transients despite the same time constant, in contrast to exponential decay law.

We emphasize that the parameters can conveniently be found one after the other:

• In the beginning of the decay where the overexponential and exponential decay approximations are valid, J oe and N dop can be determined.

• The value of the resistor determines the onset of the breakdown effect and can also be measured at small bias.

• The value of the parallel capacitor is progressively increased in curve fitting to soften the breakdown effect and adapt the lower part of the decay to the experimental data. It can be measured at a small AC bias.

Commercial solar cell

We continue the study on a randomly purchased commercial solar cell shown on fig. 3 (b).

The panel is composed of 48 small cells of a surface A = 1.32 cm 2 each, which are apparently connected in series to reach the output voltage rated 12 V. This has been revealed by a dark I(V) curve under forward bias that has a unity ideality factor when dividing the panel voltage by 48. The same I(V) measurement also yields an emitter saturation current density of J oe = 2.60 • 10 -7 A cm -2 given by eq. 13. Furthermore, measurements on the panel revealed a capacitance of 9.7 nF for the panel, corresponding to a capacity per surface C = 353 nF cm -2 , and a parallel resistance of 200 kΩ for the panel corresponding to a surface resistivity of R = 5.5 kΩ cm 2 . Fig. 5 shows the voltage transients for 0.762 mA injection current leading to a starting voltage of 9.6 V for the panel, corresponding to 0.2 V per cell. The red curve is the infinitesimal computation taking into account the differential diffusion conductance and capacitance. Due to the relatively high emitter saturation current, the ratio between differential conductance and differential capacitance is such that the decay is abrupt. Varying the width of the junction from W = 100 µm to W = 300 µm has no noticeable effect. Likewise, the dopant density can be varied from N dop = 10 14 cm -3 to N dop = 10 17 cm -3 without noticeable effect on the modeled decay curve.

Only when the parallel capacitance of C = 353 nF cm -2 is included, a relatively close curve fit is obtained (green dotted line). If furthermore the parasitic resistance is included into the model, the fit becomes perfect. However, the value had to be slightly A cm -2 , W = 100 µm to 300 µm and N dop = 10 14 cm -3 to N dop = 10 17 cm -3 ; green dots: modeling that also includes a parallel capacitance C = 353 nF cm -2 ; blued dash-dotted line: model that also includes C and R = 3.3 kΩ cm -2 . For t > 0.5 ms the rise is shown, measured and modeled (blue dash-dotted) with the same parameters as the decay and a generation term according to eq. 11.

lowered to R = 3.3 kΩ cm -2 versus the 5.5 kΩ cm 2 extracted from measurements performed on the panel. The decay curve is dominated by diffusion current and the charge carriers stored in the parallel capacitance. The parasitic capacitance of C = 353 nF cm -2 appears large. It is more likely that the manufacturer has deliberately connected a capacitor of some nF in parallel to the entire panel to prevent the solar cells from radiating switching noise caused by subsequent converters.

Above t = 0.5 ms, the rising transient is modeled with the same parameters. The generation term for electrical injection has been computed from the steady state according to eq. 11. The large parallel capacitance effectively prevents drawing more information about the junction, as has been done for the TMO solar cell where it was possible to confirm the product of doping level and thickness.

Silicon cell passivated with nitride and featuring an antireflection layer

An Al 2 O 3 passivated silicon cell with 11 percent photoconversion efficiency, as described in [START_REF] Pawlik | Etude de la passivation de surface du silicium cristallin type P par depot de couches atomiques d'alumine pour application aux cellules solaires a haut rendement[END_REF] and shown on fig. 3(c), was then studied. The photovoltage transients were macroscopically measured by oscillography, with an injection current of 100 mA corresponding to 4 mA cm -2 , shown on fig. 6.

The emitter saturation current needed for the decay modeling was obtained from an I(V) measurement yielding J oe = 2.137 • 10 -7 A cm -2 and an ideality factor n = 1.992. The parasitic values obtained by small signal measurements at 100 mV amplitude and freqencies from 1 to 10 kHz were C = 40 nF cm -2 and R = 5 kΩ cm 2 .

A c c e p t e d M a n u s c r i p t

The decay of fig. 6 (a) was first modeled with the differential conductance and differential capacitance only (red dashes) for a base thickness of W = 360 µm and base doping levels ranging from 5 • 10 14 to 1 • 10 17 cm -3 . However, for the given saturation current, the diffusion capacitance is so small that the decay appears abrupt irrespective of the doping in the above range of doping levels. Next, the effect of the parasitic capacitance was taken in account. It was necessary to increase this capacitance to 90 nF cm -2 to obtain the decay of the green dotted line, without crossing the experimental decay curve. In the last step, a parallel shunt resistance was added, yielding the blue dash-dotted line that perfectly fits the experimental data. The resistance value had to be lowered to 3.3 kΩ cm 2 vs. the previously measured 5 kΩ cm 2 to obtain a perfect fit.

A c c e p t e d M a n u s c r i p t

The same parameters were then used to model the rising voltage transient, shown on fig. 6 (b) with a significantly smaller timescale, by setting the initial voltage to zero and including a generation term according to eq. 11. A fairly good fit was immediately obtained.

As in the previous silicon solar cell, the differential diffusion capacitance is masked by the parasitic values. Therefore, neither base thickness nor doping level can be determined from the modeling, but the transients with their asymmetry between falling and rising edge can be modeled accurately.

Since the I(V) curve has an ideality factor of 2, the question arises whether in the previous commerical solar cell, there might be 2 parallel sets of 24 cells in series rather than 48 cells in series, if the ideality factor was also 2 instead of unity.

Perovskite photojunction studied by KFM

As third sample, an inverted 3D Perovskite junction, consisting of only the layers ITO, methylammonium lead iodide (MAPbI3) and PCBM, without top contact, has been studied. The surface photovoltage transient of such an incomplete solar cell can be studied only by the Kelvin probe and under optical excitation. Coincidentally, the surface photovoltage decay times were found to be compatible with the response of our Kelvin microscopy (KFM) setup with its 300 Hz bandwidth. A piece of the junction, glued by conductive epoxy on a steel plate, is shown on fig. 3 (d).

A surface photovoltage curve was acquired by KFM on a Perovskite photovoltaic junction under laser illumination of 640 nm wavelength and switched at 2.5 Hz, with 1 mW output power, and directing the beam without further optics onto an area A = 0.05 cm 2 surrounding the KFM probe. The resulting SPV transient curve is shown on fig. 7.

The aim of the following characterization shall be the attribution of parameters in the simple picture of a pn-junction such as the intrinsic carrier density, the doping level, or the junction width that in a monocrystalline silicon solar cell would roughly equal the substrate thickness.

We obtain a steady state forward voltage of V = 0.46 V yielding J oe = 2 • 10 -10 A cm -2 from eq. 13. The voltage decays more than linearly, showing that the carrier decay is overexponential. This can be due to strong carrier injection ∆n ≫ N dop or possibly result from interaction of a constant capacity with the differential conductance of the diffusion current. In the former case, the approximation of eq. 8 is applicable, and the decay modeling is performed by setting the (negligible) dopant density N dop to zero, allowing to determine

J oe qW n i (19)
Therefore, for a given J oe we can further obtain the product of the intrinsic carrier density and the junction width but neither of the factors individually. Under this hypothesis, a particular solution is found with a thickness W = 100 µm and intrinsic carrier density n i = 3 • 10 11 cm -3 , shown by the red dashed line of fig. 7.

For the latter case, the interaction of the parallel capacitance with the diffusion conductance, the decay is modeled with C = 500 nF cm -2 , the previously determined J oe , and with negligible diffusion capacitance, shown by the green dotted curve of fig. 7 (a). Finally, the blue dash-dotted curve contains both contributions and yields a satisfying fit, whereas neither individual contribution fits all over the decay no matter how the values are chosen. This confirms the hypotheses that the strong carrier injection regime is reached here, but also shows a large parasitic capacitance which might be due to the 3D character of the Perovskite junction. However, the accuracy of the parameters cannot be given with the approximate manual decomposition into two components A c c e p t e d M a n u s c r i p t (diffusion and external capacitor). The quality of the curve fit also depends very sensibly on the description of the I(V ) curve by the exponential function. On this incomplete solar cell, an I(V ) curve was not recorded and the saturation current was only determined from the open circuit voltage under one illumination intesity, assuming a unity ideality factor of the diode characteristics. Any deviation from this idealized behavior would affect the modeling of the decay and the parameters, which might explain the unusually high parasitic capacitance. Theroretically the ideality factor can be obtained from recording open circuit voltage versus illumination intensity, but this requires the laser power to be reliably controlled over a certain range of intensities.

The transient is also computed for the rising edge with the same values as for the decay, an optical generation term according to eq. 11, and a non-zero starting value for the SPV since it was not allowed to decay to a negligible value. The modeling reproduces the asymmetry between falling and rising edge fairly well. The model actually predicts a faster rise than the experimental transient as shown on the fast timescale of fig. 7 (b). The 3 ms rise time difference is exactly due to the 300 Hz bandwidth limit of the KFM.

CIGS solar cell

The last sample is a CIGS solar cell, composed of a junction of 2 microns of CIGS with 50 nm of CdS, shown in fig. 3 (e). Its fabrication is described by Ayachi et al. [START_REF] Ayachi | Innovative solution to avoid glass substrate bending in a chalcopyrite solar cell fabrication process[END_REF][START_REF] Ayachi | Rapid thermal annealing effect on the spatial resistivity distribution of azo thin films deposited by pulsed-direct-current sputtering for solar cells applications[END_REF][START_REF] Ayachi | Room temperature pulsed-dc sputtering deposition process for cigs absorber layer: material and device characterizations[END_REF][START_REF] Ayachi | Towards full sputtering deposition process for CIGS solar cells fabrication: from single thin film deposition up to device characterization[END_REF]. It is studied by oscilloscopy under electrical injection of 2.5 mA for a sample surface of 0.45 cm 2 .

A I(V ) curve reveals an emitter saturation current J oe = 2.9 • 10 -6 A cm -2 and an ideality factor of n = 2.37. For these diode characteristics, the parasitic parallel values are difficult to measure even under AC small signal and were estimated to C = 30 nF cm -2 and R = 5 kΩ cm 2 .

Falling and rising voltage transients together with modeled curves are shown on fig.

(a) and (b).

The decay is overexponential as for the previous sample. The black dashed curve shows the decay with the diffusive capacitance and diffusive conductance only, yielding the intrinsic carrier density n i = 1.2 • 10 13 cm -3 . The blue dash-dotted curve contains furthermore the parasitic capacitance. Its value had to be adjusted to C = 40 nF cm -2 during the curve fitting and it reveals that the parasitic resistance is not needed in the model, it can be set to infinity. The rise is obtained with the same parameters as the decay and by including a generation term according to eq. 11. Due to the known thickness, the intrinsic carrier density can be obtained directly in contrast to the previous sample where only the product of thickness and intrinsic density is determined. Again, the large ideality factor makes it difficult to disentangle the parasitic values at low bias from the I(V) caracteristics.

A c c e p t e d M a n u s c r i p t

Discussion

The modeling of SPV decay curves is a powerful method for determining unknown solar junction characteristics. The shape of the decay curve is determined by a differential capacitance and a differential conductance. Candidates of capacitances are the diffusion capacitance which is subdivided into strong and weak injection, and a constant parasitic or geometric parallel capacitance. The two possible conductance mechanisms are the diffusion conductance and the parasitic parallel resistance. Hence, six possible RC combinations may be dominating the shape of the decay over its totality or portions of it. Depending on whether an experimental decay curve extends over one or several regimes dominated by a particular set of differential R and C components, the curve fitting allows to eliminate one or more unknown parameters accordingly. Experimentally, we have determined the emitter saturation current J oe and (where applicable) the ideality factor from the steady state SPV under continuous excitation, and estimated the parasitic parallel resistance and capacitance by small signal measurements. These values were used as starting values for the subsequent computation of the decay curve. In the TMO cell, the decay onset is governed by the diffusion capacitance in strong injection regime and diffusion conductance, observed by the overexponential decay. It is followed by the interaction of weak injection diffusion capacitance with the diffusion conductance. The next portion is a breakdown regime, resulting from intercation between weak injection diffusion capacitance and parasitic resistance. This allowed to verify both the base width and the doping (which in this case were known a priori because the cell was fabricated in-house by the co-authors). The two silicon cells are dominated by the parasitic capacitance interacting with the diffusion current. Therefore neither the doping nor the base width could be determined. Nevertheless the modeling correctly reproduced the transients based on differential conductance and parallel parasitic components. In the Perovskite and CIGS solar cells, the prevailing mechanisms are the diffusion capacitance in the strong injection range and the diffusion conductance. However, the parasitic constant capacitance of the Perovskite sample is large and it is difficult to disentangle it from the differential diffusion capacitance. The product W n i , both parameters of which were unknown for this material, could furthermore be determined. In the CIGS cell, an intrinsic carrier density n i could be assigned since its thickness W is known.

Experimental

Surface voltage transients on the complete Si:TMO, Si and CIGS solar cells have been measured directly by a digital storage oscilloscope connected to their contacts, whereas the Kelvin probe was used on the partial Perovskite junction. The optical excitation was achieved by a 640 nm Coherent Obis diode laser modulated by a HP8112A pulse generator. The electrical current injection was applied by switching a BUZ11A MOSFET by a standard function generator. SPV transients on the Perovskite partial junction have been acquired by AM-KFM based on a Omicron VT-AFM controlled by a Specs AFM controller. The bandwidth of the AM-KFM has been set to > 300 Hz [START_REF] Diesinger | Dynamic behavior of amplitude detection kelvin force microscopy in ultrahigh vacuum[END_REF]. The direct measurement of transients is necessarily bandwidth-limited and a rise or fall time constant of less than 3 ms cannot be interpreted as intrinsic feature of the junction. The oscilloscope was set to average over 256 curves during the acquisition of all transients to improve the signal to noise ratio.

Conclusion and perspectives

Inspired by photoconductance measurements of the minority carrier decay in solar cells, the underlaying mechanism of the carrier dynamics, a combination of differential capacitance and conductivity, has been identified. Then, the differential equation has been rewritten so that the carrier decay can be computed infinitesimally in the large signal domain rather than in QSS conditions. In addition, the density of A c c e p t e d M a n u s c r i p t minority carriers stored in the diffusion capacitance has been converted into a surface photovoltage which can be conveniently measured by the Kelvin probe.

It has been shown on different solar cells that surface voltage transients can be fairly well modeled over a voltage range corresponding to a carrier density variation of 5 orders of magnitude. The asymmetry between falling and rising transition is perfectly accounted for by the modeling.

Depending on whether the carrier dynamics are dominated by the differential capacitance and conductance or by external parasitic values, it is possible to determine different missing key parameters of a photovoltaic junction such as the doping level or the width of the base.

In the present work, the transients were computed infinitesimally, but the junction characteristics have been assumed as idealized (abrubt doping profiles and depleted zones). The only non-ideal behavior taken into account was the determining of a diode ideality factor for the silicon and CIGS solar cells of fig. 3 (c) and (e). All approximations from the very beginning of the Kane-Swanson method have been adopted. As perspective, the validity range can be discussed as has previously been done for photoconductance measurements [START_REF] Maeckel | On the determination of the emitter saturation current density from lifetime measurements of silicon devices[END_REF], and numerical methods can be extended to account for real devices similar as has been done for the junction of a bipolar transistor [START_REF] Kwok | Space-charge and injection capacitances of p-n-junctions from small-signal numericalsolutions[END_REF]. Regarding the computational improvement, the curve fitting procedure should ultimately be integrated into an automated least square fit routine rather than searching the best parameters manually by trial and error. This would allow to quantify the errors of the fitting parameters and the junction characteristics.

The largest potential of the technique is its combination with Kelvin microscopy (KFM): the surface photovoltage can be measured by the Kelvin probe on partial solar junctions without top contact. This will speed up conquering future potential photovoltaic materials without building a complete solar cell prior to characterization and allow dissociating the impact of the different contacts on the cell performance. Whereas microwave photoconductance measurements have their resolution limited by the beam diameter, KFM offers better resolution, making it suitable for mapping the junction parameters at the scale of light trapping structuring. In this study the Perovskite sample without top contact was coincidentally, by its slow voltage transient, compatible with the native bandwidth of direct KFM measurement. For acquiring faster SPV transients by KFM on any photovoltaic junction, time averaging methods and ultimately pump-probe techniques will have to be applied.

Acknowledgments

Support from the french Renatech network is gratefully acknowledged.

A c c e p t e d M a n u s c r i p t

Figure 2 .

 2 Figure 2. Voltage transient of a TMO solar cell on substrate doped N dop = 10 14 cm -3 ; (a) optical injection switched off at t = 0, with intensities 5, 10, 20 mW; (b) electrical injection switched off at t = 0, with currents of 1.33, 3.33, 6 and 30 mA.

Figure 3 .

 3 Figure 3. Solar cell samples used in transient voltage spectroscopy: (a) TMO:Si, (b) monochrystalline silicon 12 volt solar panel, (c) monochrystalline silicon cell with surface passivation and antireflection coating, 11 percent efficiency, (d) methylammonium lead iodide Perovskite (MAPbI) junction, (e) CIGS cell.

  A c c e p t e d M a n u s c r i p t

Figure 5 .

 5 Figure 5. Voltage transient of a commercial solar cell, measured by injecting a forward current of 0.76 mA yielding 0.2 V per cell, and turning off the injection at t = 0. The voltage is given per cell. Red dashed curve: decay modeled with diffusion capacitance and diffusion conductance, with J oe = 2.60 • 10 -7 A cm -2 , W = 100 µm to 300 µm and N dop = 10 14 cm -3 to N dop = 10 17 cm -3 ; green dots: modeling that also includes a parallel capacitance C = 353 nF cm -2 ; blued dash-dotted line: model that also includes C and R = 3.3 kΩ cm -2 . For t > 0.5 ms the rise is shown, measured and modeled (blue dash-dotted) with the same parameters as the decay and a generation term according to eq. 11.

Figure 6 .

 6 Figure 6. Voltage transient of a monocrystalline silicon solar cell of fig. 3 (c); (a) decay curve, black squares: measured, red dashes: differential conductance and diffusion capacitance only, green dots: including a parasitic capacitance of 90 nF cm -2 , blue dash-dot : including parasitic capacitance and resistance of 3.3 kΩcm 2 ; (b) rise curve, modeled with the same parameters as the decay curve including R and C.

Figure 7 .

 7 Figure 7. Surface photovoltage transients (a) measured by KFM on Perovskite sample (black) for a laser modulated with 2.5 Hz. Red dashed curve: modeling of overexponential decay function dominated by diffusion capacitance W = 0.01 cm and n i = 3 • 10 11 cm -3 , green dotted: decay model for negligible diffusion capacitance and parallel capacitance C = 500 nF cm -2 , and blue dash-dotted: model including both diffusion capacitance and parallel capacitance; (b) zoom of the rising photovoltage transient (black solid) and its modeling (blue dash-dot) using the same parameters as for the decay.

Figure 8 .

 8 Figure 8. Voltage transients: (a) decay curve of the CIGS sample of fig. 3 (e). The curve fit uses the values J oe and the ideality factor from the I(V ) curve; black dashed: diffusive capacitance and diffusive conductance only, yielding n i = 1.2 • 10 13 cm -3 , N d = 0; blue dash-dotted curve including furthermore C = 40 nF cm -2 ; (b) rising transient modeled with the same parameters.


Jennifer Merchant

Catherine Vidal

Context

The impact of the environment on human health is a major public health concern.

According to the World Health Organization's (WHO) definition, environmental health includes those aspects of human healthincluding quality of lifethat are determined by the physical, chemical, biological, social, psychosocial and esthetic factors of our environment. Given the scope and complexity of the subject, knowledge of the exposures and health risks is still highly insufficient (The Lancet Commission on Pollution and Health, Landrigan et al. 2017).

In terms of research, many programs and projects have been carried out and are currently being developed in France and internationally. The French Public Health Law of August 9 th 2004 established for a National Environmental Health Action Program (PNSE) every five years. The third such program, carried out between 2014 and 2018, examined the health challenges posed by many environmentally-related illnesses, such as cancer and allergy, as well as reproductive, metabolic, infectious and neurodegenerative diseases. The resulting report underlined the need to consider not only the contribution of physicochemical and biological environmental factors but also factors related to the socioeconomic environment. This approach is the guiding principle for the recent concept of the exposome, which is defined as every exposure (chemical, physical and biological) to which an individual is subject to from conception to adulthood, and incorporates the psychosocial and economic factors surrounding working and living conditions [START_REF] Barouki | Integration of the Human Exposome with the Human Genome to Advance Medicine[END_REF][START_REF] Messing | of Occupational Risk Exposures https[END_REF].

In 2018, a group of experts from the Inserm wrote a report entitled "Préfiguration du volet recherche du 4ème Plan National Santé Environnement" [Preliminaries on the Research Component of the 4th National Environmental Health Action Plan] (PNSE4). This report expands on the concept of the exposome and outlines the methodological basis for its exploration and for its applications in evaluating the environmental impact on health. Emphasis is placed on the importance of cross-referencing various types of exposures with socioeconomic factors and avoiding compartmentalization according to the nature of the agents (chemical, physical, infectious, psychosocial, etc.). From this viewpoint, identifying the health determinants linked to living and working conditions is essential in order to gain insight into the exposome. This particularly involves analyzing the specific vulnerability of certain populations who have certain lifestyle, working or unemployment, physical activity/lack thereof, diet and living conditions, etc.

It has become clear that taking the gender perspective into consideration in differentiated exposure of women and men to health risks is an integral part of this field of research. Paradoxically, in the majority of environmental health reports and action programs, the concept of gender is not considered as such, except when considering sexual and reproductive health. Indeed, the decrease in female and male fertility and the vulnerability of pregnant women and fetuses constitute a major area for research and for prevention policies We, therefore, consider it essential to reflect more broadly on the environmental health of women and men, explicitly taking into account the sociocultural and economic factors related to gender in exposure to health risks, without compartmentalization by disease or country or geographical location. We have chosen to address this issue in two sectors that affect women and men differently: occupational diseases and economic insecurity. We will look at the situation in France, North America and Europe. This comparative analysis will be useful as it contributes research on the exposome and formulates proposals whose end goal is to improve the identification and prevention of health risks for women and men. This document is the first in a series of subsequent and more in-depth reflection that will emerge from expert hearings in the domain of health related to the environment, work and gender.

Gender, occupational health and the environment: The situation in France

Occupational health

The convergence of issues relating to the environment, health and work has been the subject of major research institution and public authority mobilization. Since 2006, the French agency for Food, Environmental and Occupational Health and Safety (ANSES) has been conducting a national "Environmental and Occupational Health" (PNREST) research program.

It covers a broad range of investigations, including the evaluation of occupational health risks that involve risks related to various types of exposure (chemical, biological, physical agents, etc.) as well as psychosocial risks.

Several national surveys (SUMER, RNV3P, SIP) have revealed major differences between women and men in exposure to occupational risks (see the reports by DARES and DREES). One primary cause lies in the segregation of occupations. Men, who account for the majority of industry and construction workers, have health risks related to asbestos, diluent solvents, carrying heavy loads, noise, etc. Women, who account for the majority of workers in the retail, service and care industries, are exposed to pollutants contained in cleaning products, cosmetics, cash register receipts (bisphenol A), etc. That being said, differences in risk exposure are not restricted to physical and chemical environmental factors. They also involve those related to working arrangements and constraints that affect women and men differently (see research by the French Research Institute for Environmental and Occupational Health-IRSET).

Regarding these topics, the French National Agency for the Improvement of Working Conditions (Anact) has demonstrated how physical working conditions, working arrangements, risk exposures and career breaks and discontinuities impact the health of women and men differently. One alarming observation made by Anact is the more marked lack of occupational risk exposure visibility when it comes to women. Indeed, a hierarchy exists in the representations of risks associated with various occupations [START_REF] Chappert | Comment le genre transforme-t-il l'intervention sur les conditions de travail?[END_REF]. Greater attention is paid to the difficulties of male work associated with intense physical effort, carrying heavy loads, noise, etc. Consequently, the mainly female occupations are generally considered easier and less dangerous: a view that contradicts the reality of difficulties linked to many female occupations (Messing 2014). One example among many is the overexposure of women to repetitive weight carrying, which particularly affects nurses, home care employees, store clerks, manual workers, etc. These are all occupations in which loads often exceed the 25 kg French Labor Code limit for women. Meanwhile, psychosocial risks related to working arrangements are often more commonly encountered in female occupations: lowqualified roles, lack of autonomy, fragmented working hours, piecemeal work, psychological tension, sexism, etc. The mental health impact of these risks is not recognized by French State health insurance.

One textbook example of the invisibility of risks incurred by women is the lack of recognition of occupational cancer. Surveys performed by GISCOP93 (a scientific interest group on occupational cancer in the Seine-Saint-Denis region) showed that epidemiological studies on exposure to carcinogens contained bias and prejudice relative to occupational risk inequality in function of gender.

One notable bias is the limited number of studies on the etiology of occupational cancer in women [START_REF] Counil | Que nous enseigne la dernière enquête SUMER sur les expositions cancérogènes professionnelles ?[END_REF][START_REF] Hohenadel | The Inclusion of Women in Studies of Occupational Cancer: A Review of the Epidemiologic Literature from 1991-2009[END_REF]. A review of the literature on lung cancer between 2003 and 2014 showed that the carcinogenic risks remained largely understudied in women (2.9% of the studies) compared with men (55.6%), irrespective of industry [START_REF] Betansedi | Quelle prise en compte du sexe en épidémiologie professionnelle ? Analyse du biais de genre à travers une revue systématique d'études sur les cancers du poumon entre 2003 et 2014[END_REF][START_REF] Betansedi | A Comprehensive Approach of the Gender bias in Occupational Cancer Epidemiology: A Systematic Review of Lung Cancer Studies (2003-2014[END_REF]. Another example is the absence of epidemiological studies on carcinogens used in the cleaning sector, in which women account for the majority of the workers. Tests on the contents of a female office cleaner's trolley revealed the presence of 14 carcinogens which are used with no particular information, warning or protection for the employee [START_REF] Cavet | Les expositions aux produits chimiques cancérogènes en 2010[END_REF].

We may therefore conclude that in many occupational health epidemiology publications, the authors tend to generalize the results obtained from studies conducted only in men.

Recommendations

The invisibility of risk factors and the difficulties of work carried out by women represents a major obstacle to the recognition of occupational diseases and the implementation of preventive measures. To counteract this, Anact insists on the need to produce gender statistics, which continue to remain insufficiently documented. The concept of gender must be explicitly included in epidemiological studies of occupational risks. Such data are essential in order to evaluate the occupational risk exposures of men and women, and to supplement the existing criteria in order to improve the consideration given to female working conditions.

Economic insecurity

People in situations of economic insecurity accumulate social and economic difficulties that impact physical and psychological health and impedes access to healthcare. Precarious living conditions lead to increased health risks compared to the general population: substandard housing, poor diet, lack of physical activity, difficult and unstable nature of work.

Substandard housing can cause or worsen many diseases: poisoning from lead paint, respiratory diseases linked to humidity, infectious diseases brought about by unsanitary living conditions. This leads to a deterioration in lifestyle linked to poor diet, alcohol and tobacco consumption and lack of physical activityall these factors promote obesity, diabetes, cardiovascular diseases, depression, etc. It thus follows that the lack of financial means is one of the primary reasons for forgoing healthcare.

Women currently account for the majority of the people living in precarious conditions.

A broad and comprehensive analysis of this reality was provided in the 2017 report by the French High Council for Equality (HCE) entitled "Santé et accès aux soins : une urgence pour les femmes en situation de précarité" [Health and Healthcare access: An Emergency for Women Living in Precarious Conditions] by Danièle Bousquet and Geneviève Couraud.

Women account for 70% of all impoverished workers, occupy 82% of all part-time jobs and represent 85% of all single-parent families, of the latter, one in three living below the poverty line. Obesity is much more common in women experiencing economic insecurity (35%) than in men (17.6%). Among women who have low incomes and receive supplementary health coverage (Couverture Maladie Universelle Complémentaire -CMU-C), 15% are overweight compared to 9% of the general population.

Female manual workers present a cardiovascular mortality rate that is three times higher than for female executives, due to the increased risk related to lifestyle, less frequent screenings and delayed treatment (see Inserm Memo). The women most confronted with economic insecuritylow-level employees and manual workersare by far the most affected by musculoskeletal disorders (MSDs). The frequency rate of MSDs in female manual workers is 8.4 per million working hours versus 2.4 for male manual workers.

Working nights is also known as a risk factor in the development of breast cancer, as was recently demonstrated in an international study carried out by a team from the Inserm (Cordina-Duverger 2018). The health risk is 26% higher in women who worked nights compared to women working days. Women in a situation of economic insecurity for whom working nights is more common are the most exposed: 14% of female manual workers usually or occasionally work nights versus 8% of executives [START_REF] Menegaux | Night Work and Breast Cancer: A Population-Based Case-Control Study in France (the CECILE study)[END_REF]. More generally, women in precarious situations attend screenings for specific female cancers less often and survive them less often.

It must be noted that many environmental health studies and action programs do take into account the vulnerability of populations of low socioeconomic level. However, very rare are the reports that extend their analysis to include the situation of women who are nevertheless the primary victims of economic insecurity.

Recommendations

The promotion of living and working conditions conducive to good health requires taking into account socioeconomic inequalities in relation to environmental risks and paying particular attention to the condition of women experiencing economic insecurity. Failure to seek healthcare for socioeconomic reasons remains too common. It is imperative to inform vulnerable populations of environmental risks and improve access to health care and social rights, particularly for women. Inserm is a partner of the NCSE within the Science Alliance on Environmental Health. We hope that among the missions of the Alliance, the issue of "Gender and Occupational Health" will be the subject of particular attention when it comes to creating prevention policies for vulnerable populations. Research Gender team" within the CIHR, in order to develop approaches and methods for incorporating gender in studies that look at the impact of the environment and working conditions on health. This research on the differences of male-female exposure to occupational risks led the Canadian health authorities (Canadian Centre for Occupational Health and Safety CCHST, Acts and Regulations Section, 2019) to review criteria defining the difficulties of work and the recognition of occupational illnesses in the following domains: exposure to carcinogens, dermatological diseases, musculoskeletal disorders, exposure to neurotoxins, diseases related to working in a standing position [START_REF] Antle | Development of discomfort and vascular and muscular changes during a prolonged standing task[END_REF], impact of multiple chemical sensitivities related to multiple daily low-dose exposures to cocktails of chemical substances in domestic and professional environments of which women are the primary victims.

Canada

In Canada, the environmental health research programs are distributed among various This Directive guarantees minimum safety and health requirements throughout Europe, and Member States are also allowed to establish more stringent measures (EU-OSHA, 1989).

The "Gender and OSH" issue is not addressed as such in this Directive. It includes only a few guidelines by employment sector (EU-OSHA, 2003, updated in 2019).

In 2017, a proposal for a new European Directive was submitted, entitled Work-Life Balance for Parents and Carers (Eur-Lex, 2017). The objective is to increase the participation of women in the labor market and promote work-life balance. It particularly involves enabling women and men to take time off for family commitments and to benefit from more flexible working arrangements. This proposal is still being discussed (European Commission, Employment, Social Affairs and Inclusion, 2019).

The European Agency for Safety and Health at Work (EU-OSHA)

This agency, created in 1996, gives special attention to research on "Gender and Occupational Safety and Health (OSH)". Under the auspices of the EU-OSHA, the European Risk Observatory provides a statistical overview of trends in terms of employment and working conditions, exposure to risk, work accidents and health problems for women. It particularly explores combined exposures, occupational cancer, access to rehabilitation, women and informal employment and female occupations in the home care and domestic work sectors.

Several reports on "Gender and OSH" have been published (EU-OSHA, 2014[START_REF] Chappert | Comment le genre transforme-t-il l'intervention sur les conditions de travail?[END_REF]and 2019). The principal conclusions and recommendations are as follows:

 Occupational risks to the health and safety of women have been underestimated and neglected in relation to those of men, both in terms of research and prevention. This imbalance should be corrected in the realm of research, awareness-raising and prevention.

 Research and monitoring can be improved by taking systematic account of the gender aspect in data collection and by basing the evaluation of the exposure on actual research.

 Epidemiological methods should be evaluated in order to detect any gender bias.

 European directives in terms of health and safety do not cover domestic workers (who are mainly women). Women working informally, for example the wives or partners of men managing a family farm, are also not always covered by the legislation. Gender-related incidences should be evaluated for current and future directives in terms of OSH, and mechanisms of defining standards and modes of compensation should be adapted.

 Women are under-represented at all levels in decision-making structures relating to occupational health and safety. They should be more directly involved, and their opinions, experience, knowledge and skills should be taken into consideration when formulating and implementing OSH strategies.

 Adopting a holistic approach in terms of OSH, taking into account the interactions between personal and professional life and the broader issues of work and employment arrangements, can improve the prevention of professional risks, in the interests of both women and men. Such is the aim of the proposal of the new European Directive Work-Life Balance for Parents and Carers (Eur-Lex, 2017).

The Horizon 2020 European Research Program

The Horizon 2020 program incorporates the issue of gender and male/female equality in its seven societal challenges. The first of its seven research projects is "Health, Demographic Change and Wellbeing" aimed at understanding the determinants of the European population's health in the professional, social environment, and climate contexts and, of course, emphasizes gender.

The 2018-2010 Horizon 2020 program includes funding and tender opportunities for research projects on "The Human Exposome Project: A Toolbox for Assessing and Addressing the Impact of the Environment on Health". The concept of gender is explicitly mentioned. The topic description stipulates that "effective preventive action will need to be designed, building on knowledge of various risk factors, including exposure to pollutants in daily life, individual behavior and the social context, taking into account gender issues" (Horizon 2020, The Human Exposome Project, Funding and Tender Opportunities).

The Health and Environment-Wide Associations Based on Large Population

Surveys/HEALS European program

This project, launched in 2014, brings together a consortium of 15 European countries to study the health impact of environmental stress on the European scale. Its objective is to conduct a vast survey that includes the various exposome research activities: genome, epigenome, transcriptome, proteome, metabolome, sociocultural environment and economic status. The ongoing survey is being conducted on a cohort of 300,000 people followed over a period of 5 years.

One of the Work Packages (WP no. 10) is responsible for studying the health impact of the social, cultural and economic environment, with special attention to vulnerable groups: underprivileged socioeconomic status, ethnic minorities, children and the elderly.

It is worthy to note that issues related to working conditions and the situation of women are not explicitly mentioned in the objectives. We hope that they will be taken into consideration as the research of the WP (HEALS, Working Package 10) progresses.

Conclusion and Perspectives

Taking social and economic living conditions into account in order to address the health impact of environmental risks forms an integral part of the exposome concept. Studies conducted in France and abroad all agree that exposure to occupational risks and work difficulties are underestimated, and that particularly for female-dominated occupations, it is rare that these factors are recognized. Women are the primary victims of economic insecurity, which is linked to an increased exposure to environmental risks. Hence, an ethical perspective approach is needed.

It is indeed time to broaden our approaches to the exposome: the convergent topics of "environmental health" and "occupational health" should be linked to "gender health". The objective is to enrich knowledge in research and produce cross-disciplinary interrogations by encouraging dialogue among various scientific communities. Another important objective is to refine our perspectives on prevention initiatives and providing information to vulnerable populations, in particular women.

We indeed consider it desirable that the theme "Gender, Occupational Health and the Environment" should be explicitly integrated into future research plans on environmental health where Inserm teams are stakeholders: PNSE4, IFRES (French Environmental Health Research Initiative), HERA (European Health Environment Research Agenda), International Science Alliance on Environmental Health, etc.

The expert hearings that the Inserm Ethics Committee Gender and Health Research Group will hold in 2020 will enable us to consider in greater detail and formulate proposals to improve the identification and prevention of health risks for women and men.

(

  Reproduction et Environnement: expertise collective INSERM 2011 [Reproduction and the Environment: A Collective Expert Review by the Inserm, 2011]). Another gender-related concern is the increased vulnerability of women in the developing countries in the midst of ongoing climate change: drought, food shortages, migration, etc. (UN Women 2018 report, "Women, Gender Equality and Climate Change").

  the US Congress created the Occupational Safety and Health Administration/OSHAa federal agency falls under the auspices of the Department of Labor. Its role is to ensure safe and healthy working conditions for public and private sector workers through the establishment of safety standards and access to prevention and assistance resources. "Gender, Occupational Health and the Environment" is a research area specific to the National Institute for Occupational Safety and Health (NIOSH) that works under the auspices of the Centers for Disease Control (CDC). Several reports have been published: Gender and Toxicity (1997), Working Women Face High Risks From Work Stress, Musculoskeletal Injuries, Other Disorders (2000), Women's Safety and Health Issues at Work (2001), Gender Differences in Acute Pesticide-Related Illnesses and Injuries (2012). It must be noted that due to the workings of the US Federal system, labor law depends on the policy of each state. The OSHA and NIOSH recommendations concern only the national level and are not binding on the states. In 1969, the National Institute of Environmental Health Sciences (NIEHS) was founded in order to promote environmental health research in the USA. The concept of the exposome has been the subject of specific programs since 2012 (Exposome: Time for Transformative Research (2012), The Exposome: Partnerships for Environmental Public Health (2014), Toward Greater Implementation of the Exposome Research Paradigm Within Environmental Epidemiology (2017). The exposome is a research priority of the NIEHS's 2018-2030 Strategic Plan. The objective of the National Council for Science and the Environment (NCSE) created in 1990 is to advance research on the impact of environmental changes on health by bringing together scientific experts, civil society representatives and political decision-makers. The

  Since 2008, the Canadian Institutes of Health Research (CIHR: the equivalent of the Inserm) support the study of "Gender, Occupational Health and the Environment". Two researchers are pioneers in the field: Karen Messing, biologist, professor at the UQAM and Chair of the International Ergonomics Association Technical Committee on Gender and Work; and Katherine Lippel, legal specialist and holder of the Canada Research Chair on Occupational Health and Safety Law. Together they created the "Canadian Institutes of Health

  federal agencies: the Canadian Institutes of Health Research, Health Canada, the Public Health Agency of Canada, the Canadian Food Inspection Agency. In the 2014-2019 strategic program (Capturing Innovation to Produce Better Health and Health Care for Canadians), the impact of the environment on health are addressed according to the nature of the agents involved (chemical, physical, infectious and psychosocial) with little mention, though, of the exposome concept. What is nonetheless worthy to note is the 2019 launch of The Social Exposome Research Cluster Project by the University of British Columbia which aims to understand the influence of social and environmental factors on children's health and development. Europe 1. The European Framework Directive on Safety and Health at Work (OSH, Directive 89/391 EEC).

NIH/NIESH (National Institute of Environmental Health Sciences), 2014. Miller G., "The Exposome: Partnerships for Environmental Public Health": https://www.niehs.nih.gov/research/supported/translational/peph/podcasts/2014/jan6_exposo me/index.cfm NIOSH/National Institute for Occupational Safety and Health: https://www.cdc.gov/niosh/ NIOSH, 1997. "Gender and Toxicity": https://www.cdc.gov/niosh/docs/97-119/default.html : NIOSH, 2000. "Working Women Face High Risks From Work Stress, Musculoskeletal Injuries, Other Disorders": https://www.cdc.gov/niosh/updates/womrisk.html NIOSH, 2001. "Women's Safety and Health Issues at Work": https://www.cdc.gov/niosh/docs/2001-123/ OSHA/Occupational Safety and Health Administration (L'Agence fédérale de la sureté et la santé au travail) : https://www.osha.gov OSHA, 1999. "Women in the Construction Workplace: Providing Equitable Safety and Health Protection": https://www.osha.gov/doc/accsh/haswicformal.html OSHA, 2008. "Chemical Hazards and Toxic Substances": https://www.osha.gov/SLTC/hazardoustoxicsubstances/hazards.html Stingone, J. et. al. 2017. "Toward Greater Implementation of the Exposome Research