Improved photovoltaic unit comprising n-integrated photovolaic cells in series

Abstract

A photovoltaic power converter (1N) suitable for use in implanted AIMD's is provided which comprises an insulating substrate (5s) supporting N photovoltaic cells (= PV-cell) (1.1-1.N) physically separated from one another by gaps and conductively coupled in series to one another, wherein N ≥ 2. A blocking layer (4b) is interposed between each of the N PV-cells and the insulating substrate (5s) to reduce photo-leakage caused by photoconductivity of the insulating substrate (5s). Each blocking layer (4b) is formed by M blocking PN-junctions, wherein M ≥ N + 1, each formed between a pair of first and second type p and type n semiconductors (4pi, 4ni, with i = 1 to M), wherein the first blocking PN-junction located closest to the photovoltaic PN-junction has a second polarity of opposite sign from the polarity of the PN junction of the PV cell.

Description

IMPROVED PHOTOVOLTAIC UNIT COMPRISING N-INTEGRATED PHOTOVOLAIC CELLS INSERIESTECHNICAL FIELD

[0001] The present invention concerns a photovoltaic (PV) unit of small dimensions comprising two or more integrated PV cells coupled in series, for transforming optical energy into electrical energy. The size of the PV-unit is configured to power an active implantable medical device (AIMD). The PV unit of the present invention reduces undesired shunt effects which lower the current yield observed when two or more PV cells are coupled in series.BACKGROUND OF THE INVENTION

[0002] Photovoltaic conversion is the conversion of optical energy into electrical energy using semiconducting materials that exhibit the photovoltaic effect. This technology has been extensively developed for converting solar energy into electrical energy. It is also useful for bringing electrical energy to a point from a remote source of energy in applications where electric wires are to be avoided. For example, active implantable medical devices (AIMD) configured to stimulate a tissue with electric current often comprise, on the one hand, a housing (50) holding a source of energy, such as a battery, and an implanted pulse generator (IPG) configured to produce pulses of energy and, on the other hand, an electrode unit (40) of smaller dimensions configured to apply electric pulses to the tissues it is coupled to at a position remote from the housing of larger dimensions; For example, such AIMD’s include but are not limited to neurostimulators configured to stimulate a nerve, such as the vagus nerve or a pacemaker configure to stimulate the cardiac muscle. In a straightforward application, the IPG can be configured to generate electric pulses which are transported to the electrode unit through conductive wires. This simple solution is, however, not compatible with magnetic resonance imaging (MRI) applications, because a current can be induced in the conductive wire exposed to strong magnetic field, generating heat that can burn tissues in contact with the conductive wire.

[0003] An alternative solution to the use of conductive wires is to configure the IPG to generate optical pulses which are transported to the electrode unit via optical fibres. The optical energy is then converted into electrical energy by a PV power converter (1 N) of the electrode unit (40) to feed current to electrodes (7n, 7p) coupled to the tissue to be stimulated. An example of an optoelectronic neurostimulator is described in WO2016131492.and WO2021223839, the latter describing the use of an optoelectronic neurostimulator in MRI applications. Unlike solar cells configured to absorb the full solar spectrum, the PV-cells forming the PV power converter (1 N) for use in AIMD’s are configured to preferentially convert into electrical current a narrow wavelength range of light, corresponding by the wavelength of the IPG. For example, the light can be emitted by a LED or a Laser source.

[0004] A PV-power converter comprises one or more PV-cells (1.1-1 .N) formed by one or more pairsof p-doped and n-doped semiconductor layers forming a PN-junction diode. An n-doped and a p-doped semiconductor refers to a semiconductor material that has been intentionally doped with impurities to change its electrical properties, specifically to increase the number of charge carriers (electrons or holes) for conductivity. An n-doped semiconductor is obtained by doping the semiconductor material with an element that has more valence electrons than the semiconductor material. Because the extra electrons from the dopant are not required for bonding with the semiconductor’s atoms, they are free electrons, forming the negative charge carriers in an n-doped semiconductor. By contrast, a p-doped semiconductor is obtained by doping the semiconductor material with an element that has less valence electrons than the semiconductor material. The missing electrons in the dopant form holes (i.e., an absence of an electron in the crystal lattice), which are the positive charge carriers in a p-doped semiconductor.

[0005] A first semiconductor layer referred to as the emitter layer (2e) is coupled to a second semiconductor layer of opposite doping as the emitter and is referred to as a base layer (2b), forming the PN-junction. Light energy is captured by the PV-cells, generating electron-hole pairs and thus forming a current. Generally, a lateral conductive layer (LCL) (2c) is used to transport the electrons from the base layer (2b) to contacts (3+, 3-) configured to form an electrical circuit to power an element (7).

[0006] PV-power converters in such applications face two major challenges. First, for a desired voltage output, the size of the PV power cell must be very small as it must be coupled to the electrode unit of small dimensions. Second, as the optical IPG is powered by an implanted battery, efficacy of the PV-power converter (1 N) must be maximised, to avoid depleting the battery too rapidly, requiring multiple battery charging sessions, which are cumbersome for the patient.

[0007] PV-power converters of small dimensions can be produced epitaxially by growing on a same semi-insulating substrate (5s) the various layers, sequentially including the lateral conductive layer (2c), the base layer (2b) and the emitter layer (2e). In continuation, when referring to the present invention, the term “substrate" refers to “semi-insulating substrate". The layers can be grown by techniques known in the art, such as by metalorganic chemical vapor deposition (MOCVD). Lithography, etching and metal contacts deposition are used to create the structure of the PV-power converter (1 N). For example, the substrate can be a semiconductor treated to become an insulating material. For this reason, such substrates are referred to as semi-insulating substrates (5s).

[0008] A PV-power converter and the PV-cells (1 .1-1 .N) forming it are characterized by an intensity vs voltage curve or a corresponding power vs voltage curve, as illustrated in Figure 1 a. In Figures 1 a to 1 c, dimensionless intensity, i= I / Isc, voltage, u = U / Uoc, and power, p = P / P0 are used wherein Isc is the short circuit value of the intensity at u = 0, Uoc is the open circuit value of the voltage at i= 0, and pO = Isc x Uoc for a PV-power converter (1 N) comprising N = 2 PV-cells coupled in series. In order to increase the voltage output of the PV-power converter (1 N), N > 2 PV-cells (1.1 , 1.2... 1.N) can be coupled to one another in series, as shown in Figure 1 a (solid line). Similarly, the current output can be increased by coupling to one another Np > 2 PV-cells (1.1 , 1.2... 1.Np) in parallel, as shown inFigure 1 a, dotted line. By coupling in parallel two or more PV-power converters each comprising two or more PV-cells coupled in series, both voltage output and current output can be increased accordingly, as shown in Figure 1 a, long dashed line.

[0009] Tests on thus produced PV-power converters (1 N) comprising N PV-cells (1.1-1.N) coupled in series, revealed an intensity loss as shown in Figure 1 b, dashed line, where undesired shunt effects were observed at u = 1 / N, resulting in an undesired drop, Ai, of the current intensity. Similarly, as illustrated in Figure 1 c, dotted line labelled “P.A.”, this undesired shunt effect also provokes a drop in the output power, p, of the PV-power converter (1 N) as shown in Figure 1 c, to yield a same power, p, and current, i, measured between the electrodes (7n, 7p) of the electrode unit (40) within a given voltage range, the occurrence of this drop, Ai, of the output current intensity in that voltage range draws more power out of the implanted battery located in the housing, thus requiring more frequent charging sessions, which are long and tedious for the patients. The voltage range for a given target intensity range depends on the impedance of the device (7) to be powered.

[0010] US8742251 identified this problem of current leakage of a PV-cell through the semi-insulating (si-) substrate. To solve this problem, a current blocking layer is interposed between the PV-cell and the substrate. The current blocking layer is a semiconductor layer configured to form with the LCL layer (referred to as buffer layer in the document) a second PN-junction of conductivity type opposite the first PN-junction formed between the emitter and base layers. The current blocking layer described in US8742251 improves the current vs voltage yield, but the effect of the current blocking layer decreases with an increasing number of PV cells connected in series. Because of the additive build-in voltage with each additional PV cell arranged in series, a current drop, Al, is still observed in spite of the presence of the current blocking layer described in US8742251.

[0011] There therefore remains a need for a PV-power converter yielding a given current within a given voltage range in as efficiently as possible, avoiding shunt effects and yet keeping the dimensions thereof compatible for use in applications such as AIMD’s. This is particularly sensitive for AIMD’s, because the PV cells act as current source to the electrodes coupled to a tissue. As such, the delivered current must be as constant as possible over the full range of voltage before the diode redressing effect. In other words, there is a need for a device configured to deliver a targeted current independently of the impedance value of the device to be powered. 3, in the range of voltage that the current source could sustain, as a function of the number N of PV cells coupled in series. The present invention proposes a PV-power converter eliminating undesired shunt effects and of very small dimensions compatible with AIMD’s applications. These and other advantages of the present invention are presented in continuation.SUMMARY OF THE INVENTION

[0012] The present invention is defined in the appended independent claims. Preferred embodiments are defined in the dependent claims. In particular, the present invention concerns a photovoltaic powerconverter configured to transform an optical energy from a monochromatic light of wavelength comprised within a predefined wavelength range and to power an element. With an active area, A < 1 .0 mm2, preferably A < 0.5 mm2, more preferably, A < 0. 2 mm2, the photovoltaic converter is of small dimensions suitable for powering elements within an active implantable medical device (AIMD). The photovoltaic power converter comprises a single insulating substrate supporting N photovoltaic cells (= PV-cell) physically separated from one another and conductively coupled in series to one another, wherein N > 2 and wherein each of the N PV-cells comprises,• an emitter layer made of a type p or type n semiconductor, configured to transmit the monochromatic light, and• a base layer of type n or type p opposite to the emitter layer and forming therewith a photovoltaicPN junction of first polarity, and configured to absorb the monochromatic light transmitted through the emitter layer and to generate a current, i,• first and second contacts coupled to the emitter layer and the base layer, respectively, and configured to be conductively coupled to the element,• a blocking layer is interposed between each of the N PV-cells and the insulating substrate to reduce photo-leakage caused by photoconductivity of the insulating substrate.

[0013] The gist of the present invention is that each blocking layer is formed by M blocking PN- junctions, wherein M > N + 1 , each formed between a pair of first and second type p and type n semiconductors, wherein the first blocking PN-junction located closest to the photovoltaic PN-junction has a second polarity of opposite sign from the first polarity.

[0014] The emitter layer can be made of a material including GaAs, InP, GaN, AlAs, AIGaAs, GalnP, Si, and alloys thereof, preferably GaAs. The emitter layer can have a thickness, T2e, comprised between 0.05 and 2.00 pm, preferably between 0.4 and 1 .00 pm, more preferably between 0.60 and 0.80 pm.

[0015] The base layer can be made of a material including GaAs, InP, GaN, AlAs, AIGaAs, GalnP, Si, and alloys thereof, preferably GaAs. The base layer can have a thickness, T2b, comprised between 0.1 and 5.00 pm, preferably between 1 .00 and 3.50 pm, more preferably between 2.00 and 3.00 pm.

[0016] In a preferred embodiment, the photovoltaic power converter further comprises a lateral conductive layer (= LCL) in conductive contact with the base layer and with the blocking layer configured to transport current to a side of the photovoltaic power converter towards the second contact. The lateral conductive layer can be made of a semiconductor material of same p type or n type as the base layer. The LCL can include GaAs, InP, GaN, AlAs, AIGaAs, GalnP, Si, and alloys thereof, preferably GaAs. The LCL preferably has a thickness, T2c, comprised between 0.10 and 3.00 pm, preferably between 0.50 and 1 .50 pm, more preferably between 1 .40 and 1 .60 pm.

[0017] The first and second type p or type n semiconductors forming each one of the M blockingPN-junctions can be made of a material including GaAs, InP, GaN, AlAs, AIGaAs, GalnP, Si, and alloys thereof, preferably AlxGai-xAs, wherein x = 0 to 0.4, preferably x = 0.1 to 0.3. It is preferred that M = N + 1.

[0018] In a preferred embodiment, the p-doped semiconductors of the blocking layer have a thickness, T4pi, comprised between 0.1 and 1 pm, preferably between 0.15 and 0.20 pm. Alternatively or concomitantly, the n-doped semiconductors (4ni) can have a thickness, T4ni, comprised between 0.2 and 2 pm, preferably between 0.25 and 0.35 pm. It is preferred that each pair of p-doped and n-doped semiconductors forming one of the M PN junctions of the blocking layer has a thickness comprised between 0.3 and 3 pm, preferably between 0.45 and 0.75 pm. The blocking layer preferably has a thickness, T4b, comprised between 0.6 and 20.0 pm, preferably between 1.3 and 2.7 pm.

[0019] The insulating substrate is made of a material configured to be insulating and can include GaAs, InP, GaN, AlAs, AIGaAs, GalnP, Si, and alloys thereof, preferably GaAs. The substrate preferably has a thickness, T5s, comprised between 100 and 1000 pm, preferably between 200 and 800 pm, more preferably between 300 and 500 pm. The photovoltaic power converter excluding the substrate can have a thickness, T24, comprised between 1.25 and 50.0 pm, preferably, between 2.0 and 40.0 pm, more preferably between 4.0 and 12.0 pm.

[0020] In a preferred embodiment, the photovoltaic power converter is a component of an active implantable medical device (AIMD), preferably a neurostimulating implant, including a vagus nerve stimulating device, a peripheral nerve stimulator, or a brain stimulating device, or a pacemaker. For example, the element can be an electrode unit or a LED of the neurostimulating device, the brain stimulating device, or of the pacemaker.

[0021] A Photovoltaic power converter of the present invention can be coupled in parallel to a number Mp of photovoltaic power converters according to theinvention, with Mp > 1 .

[0022] The present invention also concerns a process for producing a photovoltaic power converter according to the present invention. The process comprises the following steps,• providing a substrate treated to become an insulating material,• sequentially and epitaxially growing on the insulating substrate at least the blocking layer (4b), preferably the lateral conductive layer, the base layer, and the emitter layer, preferably by metalorganic chemical vapour deposition (MOCVD),• shaping the N PV-cells separated from one another by lithographic etching, and• forming metal contacts by metal deposition to couple the N PV-cells in series.

[0023] The present invention also concerns an active implantable medical device (AIMD) comprising,• a housing enclosing an energy source and an implantable pulse generator (IPG) configured to emit towards a window pulses of a monochromatic light of wavelength comprised within a predefined wavelength range,• an optical energy transfer lead comprising an optical fibre comprising a first end coupled to the housing facing the window and a second end,• an electrode unit comprising an insulating substrate supporting electrodes wherein the insulating substrate is configured to be coupled to a tissue of a patient with the electrodes contacting the tissue, and• a coupling element coupling the second end of the optical fibre to the electrode unit and comprising a photovoltaic power converter according to the present invention and a cavity for receiving the second end of the optical fibre facing the photovoltaic power converter to transform the optical energy of the monochromatic light transmitted by the optical fibre from the IPG into electrical energy to power the electrodes,BRIEF DESCRIPTION OF THE FIGURES

[0024] For a fuller understanding of the nature of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawings in which:Figure 1a shows an intensity, i = I / Isc vs voltage, u = U / Uoc, of a single PV-cell (= short dashed line), of two PV-cells coupled in series (= solid line) forming a PV-power converter, two PV-cells coupled in parallel (= dotted line), and two PV-power converter coupled in parallel (=long dashed line). IO is the maximum value of intensity, I, measured at the voltage value and U = 0. Uoc is the maximum value of the voltage, U, measured at I = 0, for a PV-power converter comprising N PV-cells coupled in series.Figures 1 b shows an intensity, i = I / Isc vs voltage, u = U / Uoc curve measured on a PV-power converter comprising N = 2 PV-cells coupled in series: dashed line = prior art, an intensity drop, Ai, can be seen. Solid line = invention, the intensity drop, Ai, observed in the prior art PV-power converter disappeared.Figure 1c: shows an example of a curve (= solid line) intensity, i = I / Isc, vs voltage, u = U / Uoc, and of a curve (= long dashed line), p = P / P0, vs voltage u = U / U0, of a PV-power converter according to the present invention, and the corresponding curves for a PV-power converter of the prior art, exhibiting both intensity drop, Ai, and power drop, Ap, in a voltage range, u > 0.5.Figure 2a: shows a PV-power converter of the prior art comprising two PV-cells coupled in series to one another.Figure 2b: shows a PV-power converter according to US8742251 comprising two PV-cells coupled in series to one another and separated from the substrate by a specific blocking layer.Figure 3: shows a PV-power converter according to the present invention comprising two PV-cells coupled in series to one another and separated from the substrate by a specific blocking layer.Figure 4a: shows an optoelectronic neurostimulator comprising a housing implanted at a remotelocation from an electrode unit coupled to a vagus nerve.Figure 4b: shows an interior of the housing of the AIMDFigure 4c: shows a detail of the coupling of the optical fibre(s) to the electrode unit.Figure 4d: shows an exploded view of a coupling element configured to optically couple an optical fibre to a PV-power converter and to electrically couple the PV-power converter to the electrodes of the electrode unit.Figure 4e: shows a detail of a PV-power converter comprising 5 PV-cells coupled in series, in conductive contact with a LED and with electrical contacts configured to form an electrical circuit with the electrodes of the electrode unit of Figure 4b.Figure 5a: shows a 4-series unit comprising four PV cells coupled to one another in series and enclosed in a circle.Figure 5b: shows i = f(u) curves according to the present invention (solid line) and the prior art without blocking layer (dotted and dashed lines) with different leakage conditions.Figure 6: shows the experimental i = f(u) curves of an 8- and a 12-series unit without blocking layer.DETAILED DESCRIPTION OF THE INVENTION

[0025] As illustrated in Figure 2b, the present invention concerns a photovoltaic power converter (1 N) configured to transform an optical energy from a light electrical energy to power an element (7). The light has a wavelength comprised within a predefined narrow wavelength range spanning over not more than 100 nm. For example, the value of the wavelength of the light can be comprised between 300 nm and 1600 nm, within the narrow wavelength range of 100 nm. The light is preferably a monochromatic light. The photovoltaic power converter (1 N) comprises a semi-insulating substrate (5s) supporting N photovoltaic cells (= PV-cell) (1.1-1.N) physically separated from one another and conductively coupled in series to one another, forming an N-series unit (1 N) wherein N > 2. Each PV-cell sequentially comprises an emitter layer (2e), a base layer (2b), and a lateral conductive layer (LCL) (2c), laid over the substrate (5s). The terms “photovoltaic power converter3’ and “N-series unit3’ are used herein interchangeably as synonyms. The photovoltaic power converter (1 N) of the present invention differs from the prior art in that it comprises a blocking layer (4b) interposed between each of the first and second PV-cells (1.1-1 .N) and the substrate (5s).

[0026] The emitter layer (2e) is made of a type p or type n semiconductor, configured to transmit the monochromatic light and is coupled to the base layer (2b) of type n or type p opposite to the emitter layer (2e) to form therewith a first PN-junction. When photons hit the PV cell, they excite electrons in the semiconductor material, creating electron-hole pairs. The PN-junction formed between the emitter layer (2e) and the base layer (2b) generates an electric field that causes the electrons to move towardthe n-type layer and the holes toward the p-type layer. The electric field prevents recombination of electrons and holes and drives them toward the contacts (3+, 3-) of the cell, allowing the electrons to flow through the external circuit, generating an electric current.

[0027] The base layer (2b) is configured to absorb the monochromatic light transmitted through the emitter layer (2e) and to generate a current, I, and voltage, U.

[0028] The lateral conductive layer (= LCL) (2c) is in conductive contact with the emitter layer (2b). The LCL (2c) is configured to conduct the current, I, generated by the base layer (2b) towards contacts (3+,3-) configured to be conductively coupled to the element (7).

[0029] The blocking layer (4b) is interposed between each of the N PV-cells (1.1-1.N) and the substrate (5s) to reduce photo-leakage caused by photoconductivity of the substrate (5s),

[0030] The photovoltaic power converter (1 N) has dimensions compatible with applications in AIMD’s. In particular, the photovoltaic power converter (1 N) has an active area, A < 1.0 mm2, preferably A < 0.5 mm2, more preferably, A < 0.2 mm2. The active area, A, is defined as the area exposed to irradiation by the monochromatic light configured to convert the optical energy into electrical energy.

[0031] The gist of the present invention is that the blocking layer (4b) is made of M pairs of a type p blocking layer (4pi) and a type n blocking layer (4ni) forming a second PN-junction of opposite conductivity type compared with the first PN-junction, wherein M > N +1 , preferably M = N + 1. The higher number M of pairs (4bi) than the number N of PV-cells ensures that the current is effectively prevented from leaking through the substrate. There is substantially no more current intensity loss nor power loss with the PV-power converter of the present invention, as shown in Figure 1 b (solid line).PV-CELL (1.1-1.N)

[0032] The active portion of the PV-cells (1.1-1.N) of the present invention comprises a sequence of three layers grown epitaxially over the substrate (5s). The three layers are differently doped semiconductors and consist of an emitter layer (2e), a base layer (2b), and a lateral conductive layer (LCL) (2c) as shown in Figure 2b. The PV-cell can comprise other layers well known in the art, but it must comprise the active portion including the foregoing three layers (2e, 2b, 2c). The emitter layer (2e) and base layer (2b) form a PN-junction at their interface. The LCL (2c) is configured to transport the charges formed at the PN-junction towards the contacts (3+,3-). The contacts (3+,3-) can be conductively coupled to an element (7) to be powered and thus form an electrical circuit as shown in Figure 2b. The element (7) can be for instance a LED (7L) or electrodes (7n, 7p) of the electrode unit of an AIMD, as e.g., shown in Figures 3c and 3d.The emitter layer (2e)

[0033] The emitter layer (2e) is made of a semiconductor material, doped with elements to introduceexcess electrons (negative charge carriers) to become an n-type semiconductor or doped with elements to introduce holes (positive charge carriers) to become a p-type semiconductor. For example, the semiconductor can be selected among GaAs, InP, GaN, AlAs, AIGaAs, GalnP, Si, and alloys thereof. For example, the emitter layer (2e) can be made of a semiconductor material comprising an element from column III of the periodic table, such as GaAs. The semiconductor can be positively doped, e.g., with C, or negatively doped, e.g., with Si. For example, GaAs can be p-doped with C-concentration comprised between 1017to 1 O20cm3, preferably between 1018to 1019cm3. GaAs can also be n-doped with Si dopant in amounts comprised between 1017and 1019crrr3, preferably between 2 x 1017to 5 x 1018crrr3.

[0034] The emitter layer (2e) is preferably epitaxially grown. The emitter layer (2e) can have a thickness, T2e, comprised between 0.05 and 2.00 pm, preferably between 0.40 and 1.00 pm, more preferably between 0.60 and 0.80 pm.The base layer (2b)

[0035] Like the emitter layer (2e), the base layer (2b) is made of a semiconductor material, doped with elements to introduce excess electrons (negative charge carriers) to become an n-type semiconductor or doped with elements to introduce holes (positive charge carriers) to become a p-type semiconductor. For example, the semiconductor can be selected among GaAs, InP, GaN, AlAs, AIGaAs, GalnP, Si, and alloys thereof. For example, when the emitter layer (2e) is p-doped, the base layer can be made of a semiconductor material comprising an element from column III of the periodic table, such as GaAs negatively doped preferably with Si. Si dopant can be present at a concentration comprised between 1016and 1019cm3, preferably between 5 x 1016to 5 x 1017crrr3. If the emitter layer (2e) is n-doped, the base layer (2b) is p-doped. For example, the semiconductor material comprising an element from column III of the periodic table can be doped with C, in amounts preferably comprised between 1017to 102° cm3, preferably between 1017to 1018cm3. To limit recombination of electrons and hole across the PN-junction, the base layer (2b) is generally doped less than the emitter layer (2e).This has the effect that the base layer is generally less conductive than the emitter layer (2e).

[0036] The base layer (2b) is preferably thicker than the emitter layer (2e) for absorption optimization and can have a thickness, T2b, comprised between 0.10 and 5.00 pm, preferably between 1.00 and 3.50 pm, more preferably between 2.00 and 3.00 pm.

[0037] The photovoltaic cell preferably further comprises a lateral conductive layer (= LCL) (2c).Lateral conductive layer (= LCL) (2c)

[0038] In a preferred embodiment, the PV-cell (1.1-1.N) further comprises a lateral conductive layer (= LCL) (2c) in conductive contact with the base layer (2b). The presence of an LCL (2c) is not essential in the PV-cell, but the presence thereof is much preferred, because the LCL (2c) collects and transportselectrical current over the PV-cell. As discussed supra, when light hits the photovoltaic cell, it generates electron-hole pairs in the semiconductor material. The charge carriers (electrons in n-type material and holes in p-type material) are separated by the built-in electric field at the PN- junction. As the charge carriers reach a back surface of the base layer (2b), they are collected by the LCL (2c), which spreads the current laterally across the surface towards the second contact (3+, 3-) and the electric circuit.

[0039] The LCL (2c) must be more conductive than the base layer (2b). Conductive materials can be used to form the LCL (2c). In the present invention, however, it is preferred to use a semiconductor doped with same sign as the base layer (2b) so that the charge carriers can freely pass from the base layer (2b) to the LCL (2c). The semiconductor and dopant of the LCL (2c) is preferably the same as the base layer (2b), but with a higher content of dopant as in the base layer (2b) to yield a LCL (2c) having a higher conductivity than the base layer (2b). For example, if the base layer is an GaAs semiconductor n-doped with Si at a concentration of about 1017cm-3, the LCL (2c) can be formed of GaAs n-doped with Si at a concentration of about 1018cm3.

[0040] The LCL (2c) can have a thickness, T2c, comprised between 0.10 and 3.00 pm, preferably between 0.50 and 1 .50 pm, more preferably between 1 .40 and 1 .60 pm.N PV-CELLS (1.1-1.N) IN SERIES

[0041] As shown in Figure 1 a, a PV-cell (1 N) is characterized by a given i = f(u) curve (cf. Figure 1 a, short-dashed curve). By coupling two (or N > 2) PV-cells in series a 2- (or N-)series unit (1 N) is obtained characterised by a normalised voltage, u, approximately multiplied by a factor two (or by a factor N) (cf. Figure 1 a, continuous line). By coupling two (or P > 2) PV cells in parallel a 2- (or P-)parallel unit (1 p) characterised by a normalised intensity, i, multiplied by a factor two (or a factor P) (cf. Figure 1 a, dotted line). By coupling in parallel two 2-series units (1 N), an PV-unit (1 sp) is obtained characterised by both normalised voltage, u, and current, i, multiplied by a factor two (or by a factor N or P, respectively) (cf. Figure 1 a, long-dashed line).

[0042] N PV cells (1 .1-1 .N) can be coupled to one another in series to form the N-series unit (1 N). The N PV cells can be arranged linearly or in an array if the source of light is large enough. This is the case, e.g., with solar panels activated by the sunlight. In small electronic devices, such as active implantable medical devices (AIMD) the PV cells generally face an optical fibre whence a narrow light beam illuminates the PV cells with a circular output shape. For this reason, as illustrated in Figures 3, 4e and 5a, the PV cells are preferably coupled to one another in series forming a circular N-series unit of diameter similar to the diameter of the spot light emitted out of the optical fibre it faces so as to ensure that all the PV cells (1 .1-1 .N) are identically illuminated.

[0043] As shown in Figure 2a, a 2-series unit (1 N) comprising two PV cells (1.1 , 1 .2) coupled in series configured for powering of an AIMD, such as for example the electrodes (7n, 7p) or a control LED (7L)in an electrode unit (40) of a neurostimulator (cf. Figures 4a to 4e) comprises a semi-insulating substrate (5s) on which are epitaxially grown the emitter and base layers (2e, 2b) of two PV cells separated from one another by a gap (1_2) exposing a portion of the semi-insulating substrate (5s) to the light (photons) (10L). As discussed supra, a current leakage of the PV-cells is, however, observed through the semiinsulating substrate, reducing the efficacy of the 2-series unit (1 N). As illustrated in Figure 2b, to solve this problem US8742251 proposed to interpose between the PV-cells coupled in series and the substrate (5s) a current blocking layer (4b) made of a semiconductor doped opposite the base layer (2b) or LCL (2c) it is adjacent to, to form a second PN-junction with the PV cell. If this solution, reduces the current drop, Ai, it does not, however, suffice to substantially remove the current leakage, in particular for N-series units (1 N) comprising a large number, N, of PV cells, such as at least three (N > 3) or at least four (N > 4) PV cells in series or, as illustrated in Figure 6, N = 8 or N = 12 PV cells in series.

[0044] Figure 5a illustrates a 4-series unit (1 N) comprising four PV cells (1 .1 -1.4), each characterised by a same i = f(u) curve, coupled to one another in series and arranged in a circle of diameter similar to (preferably same as) the diameter of the optical fibre light spot it is configured to face. The gaps separating two adjacent PV cells (1 .i, 1 .j) are identified by the reference (i J). The efficiency of a 4-series unit (1 N) exposed to 24 mW of optical power was measured to 51 %. The same measurement was repeated with a 2-series unit (1 N) composed of two PV cells (1.1 , 1.2) grown side by side on a 230 pm thick undoped GaAs si-substrate (5s) and comprising a single gap (1 _2) separating them and exposed to the same optical power of 24 mW yielded an efficiency of 56% instead of 51 % for the corresponding 4-series unit (1 N). Similar results were observed by comparing an 8-series unit comprising 8 PV cells in series with a 12-series unit comprising N = 12 PV cells in series as illustrated in Figure 6. It can be seen that the current drop, Ai(N = 12) = 28%, observed with the 12-series unit (cf. dashed line in Figure 6) is greater than the current drop, Ai(N = 8) = 20%, observed with the 8-series unit (cf. solid line in Figure 6) (i.e., Ai(N = 12) = 28% > 20% = Ai(N = 8)).

[0045] This simple experiment suggests that the current losses between a N1 -series unit compared with a N2-series unit, with N1 > N2, is mainly due to some shunt defects that become predominant when a larger number N of PV cells are connected in series. The current loss difference between the 4- and the 2-series units was estimated around 5%, and between the 8- and the 12-series units was estimated around 8%.

[0046] Without wishing to be bound by any theory, it is proposed that the N-dependent current loss can be attributed to photoleakage located in the substrate (5s). A shunt path is activated in the substrate (5s) upon exposure to light at a voltage of about 1 V, provoking current losses and reducing the final conversion efficiency. The 4-series unit is more affected than the 2-series unit by these current losses because the shunt paths are activated in the substrate (5s) at the level of the gaps (ij, with i and j = 1 to 4)), and the 4-series unit comprises 4 linear gaps between the 4 PV cells compared with a single gap (1 _2) in the 2-series unit.

[0047] Referring to Figures 5a and 5b, for a 4-series unit comprising 4 PV cells arranged in series to form the four quarters of a circle, the voltage, U(1_4), between the PV cell (1.1) and the PV cell (1 .4) across the gap (1_4) is the largest and is equal to 4 x U(PV) (i.e., U(1_4) = 4 x U(PV)), wherein U(PV) is the tension obtained by each PV cell upon a given light exposure. The voltage between, on the one hand, the PV cells (1.1 and 1 .3) and, on the other hand, the PV cells (1 .2 and 1 .4) is equal to 3 x U(PV) (i.e., U(1_3) = U(2_4) = 3 x U(PV)). Finally, the voltages between the PV cells (1.1 and 1 .2), (1 .2 and 1 .3), (1 .3 and 1 .4) across the gaps (1_2), (2_3), and (3_4), respectively, are each equal to twice U(PV) (i.e., U(1_2) = U(2_3) = U(3_4) = 2 x U(PV)).

[0048] Under given conditions, e.g., controlled by the value of the leakage (Lij) between the PV cells (1.i and 1.j), the profile of the i = f(u) varies. For example, as shown in Figure 5b, it was observed experimentally that in case the conditions are such that L1_4 < L1_2 < L2_3 < L3_4, three separate current drops are observed as shown with the dotted curve of Figure 5b. By contrast, in case the conditions are such that L1_4 = L1_2 = L2_3 = L3_4, a single current drop of same magnitude as the sum of the three drops discussed earlier is observed, as shown with the dashed curve of Figure 5b. The conditions on L1_4, L1_2, L2_3, and L3_4 can be varied if the gaps have different widths or if the four PV cells are not illuminated homogeneously, as shown in Figure 5b with the shaded area representing a light beam which is perfectly centred on all four PV cells (cf. left insert in Figure 5b) or is offset illuminating some PV cells and the corresponding gaps more than others. Regardless of the number of current drops in the i = f(u) curve, the resulting observed current drop, Ai, is a serious drawback for applications where the source of energy is restricted, such as with batteries. Rechargeable batteries can be recharged, but in case of implanted AIMD’s, the charging sessions of an empty battery are a serious burden for the patient and should be spaced in time as much as possible for the patient’s comfort.BLOCKING LAYER (4B)

[0049] To substantially eliminate the current loss, Ai, observed when N PV cells are coupled in series, the concept of a blocking layer (4b) as proposed in US8742251 is interesting, although it experimentally proved insufficient to yield satisfactory results, in particular in the stringent conditions of implanted AIMD’s. To solve this problem, the present invention proposes a more efficient blocking layer (4b).

[0050] As illustrated in Figure 3, the blocking layer (4b) according to the present invention is formed by M blocking PN-junctions, wherein M > N + 1 , preferably M = N + 1 . In the embodiment illustrated in Figure 3, N = 2 and M = 3. Each of the M PN-junctions is formed between a pair of first and second type p and type n semiconductors (4pi, 4ni, with i = 1 to M), wherein the first blocking PN-junction located closest to the photovoltaic PN-junction has a second polarity (i.e., positively or negatively doped) of opposite sign from a first polarity of the base layer (2b) and, if present, of the LCL (2c). Including M > N PN-junctions to form the blocking layer (4b) ensures that no charge carrier can passthrough the blocking layer (4b) in spite of the high voltages between adjacent PV cells.

[0051] The first and second type p or type n semiconductors (4ni, 4pi) forming each one of the M blocking PN-junctions can be made of a material including GaAs, InP, GaN, AlAs, AIGaAs, GalnP, Si, and alloys thereof, preferably AlxGai-xAs, wherein x = 0 to 0.4, preferably x = 0.1 to 0.3. The p-doped semiconductor (4pi) is positively doped, e.g., with C and the n-doped semiconductor (4ni) is negatively doped, e.g., with Si. In a preferred embodiment, the semiconductors (4pi, 4ni) forming the blocking layer (4b) can be made of a semiconductor material comprising an element from column III of the periodic table, such as GaAs or AlxGai-xAs. All p-doped semiconductors (4pi) of the blocking layer (4b) are preferably identical to one another. Similarly, all n-doped semiconductors (4ni) of the blocking layer (4b) are preferably identical to one another. For example, AlxGai-xAs, with x = 0.3 or 0.4, can be p-doped to form the p-doped semiconductors (4pi) with C-concentrations comprised between 1018and 1019cm3, preferably between 2 x1018and 5 x 1018cm3. AlxGai-xAs, with x = 0.3 or 0.4 can also be n-doped to form the n-doped semiconductors (4ni) with Si dopant in amounts comprised between 1017and 1018cm3, preferably between 2 x 1017to 5 x 1017cm-3.

[0052] The p-doped semiconductors (4pi) can have a thickness, T4pi, comprised between 0.1 and 1 pm, preferably between 0.12 and 0.5 pm, more preferably between 0.15 and 0.20 pm. The n-doped semiconductors (4ni) can have a thickness, T4ni, comprised between 0.10 and 2.00 pm, preferably between 0.15 and 0.50 pm, more preferably between 0.25 and 0.35 pm.

[0053] Each pair of p-doped and n-doped semiconductors (4pi, 4ni) forming one of the M PN junctions of the blocking layer (4b) can have a thickness comprised between 0.3 and 3 pm, preferably between 0.4 and 1 pm, more preferably between 0.45 and 0.75 pm. The blocking layer (4b) can have a thickness, T4b, can be comprised between 0.6 and 20.0 pm, preferably between 0.9 and 18.0 pm, more preferably between 1 .3 and 2.7 pm. In a preferred embodiment, all pairs of p-doped and n-doped semiconductors (4pi, 4ni) forming the M PN junctions have same thickness, and the blocking layer (4b) has a thickness, T4b = M x (T4ni + T4pi).EXAMPLE

[0054] A 4-series unit comprising N = 4 identical PV cells coupled in series and arranged to form a circle as illustrated in Figure 5a. Each PV cell (1.1 to 1.4) is separated from one another by a corresponding gap (1_2, 2_3, 3_4, 1_4). Table 1 lists the compositions and thicknesses of the emitter layer (2e), base layer (2b), LCL (2c) and of the M = 4 pairs of semiconductors (4pi, 4ni) forming the M = 4 PN junctions of the blocking layer (4b). Other layers well known to a person skilled in the art and not essential to the present invention, including front and back surface field layers (FSF, BSF) and buffer layer, were also included in the 4-series unit, but not listed in Table 1 for sake of clarity and conciseness. It can be seen from Table 1 below that Figures 2a, 2b, and 3 are not to scale and the thicknesses of the various layers were selected for clarity’s sake rather than for faithfully reproducingthem to scale.

[0055] The measured i = f(u) curve measured with the 4-series unit (1 N) as listed in Table 1 is represented in Figure 5b, solid line labelled “INV”. No current loss was observed demonstrating that the blocking layer (4b) formed by M = N + 1 PN junctions efficiently blocked all charge carriers from straying along shunt paths through the substrate (5s). This is very important for AIMD’s since eliminating the current drops, Ai, observed when coupling N PV cells in series allows prolonging the service life of a rechargeable battery between two successive charging sessions by a proportional amount of time, to the benefit of the comfort of the patients.

[0056] To increase the voltage of the N-series units, Mp > 1 N-series units (1 N) of the present invention can be coupled to one another in parallel as shown in Figure 1 a to form an array (1 sp) of Mp N-series units (1 N) each comprising N PV cells. If all PV cells (1 ;i) are identical, the array (1 sp) yields a current approximately N times the current of each PV cell (1 .i) and a voltage approximately Mp times the voltage of each PV cell constituting the array (&sp) (cf. Figure 1 a, long dashed line).Table 1: compositions and thicknesses of the layers forming a 4-series unit comprising N = 4 PV cells, each comprising a blocking layer (4b) formed by M = 5 PN junctions defined between p-doped andPROCESS FOR PRODUCING A PHOTOVOLTAIC POWER CONVERTER

[0057] The present invention also concerns a process for producing a photovoltaic power converter (1 N) as described supra, comprising N PV cells coupled in series, the process comprising the following steps,• providing a substrate (5s) treated to become an insulating material.• sequentially and epitaxially growing on the insulating substrate preferably by metalorganicchemical vapour deposition (MOCVD), at least, o the M pairs of n-doped and p-doped semiconductors (4ni, 4pi) to form the blocking layer (4b), o preferably the lateral conductive layer (2c), o the base layer (2b), and o the emitter layer (2e),• shaping the N PV-cells (1.1-1 .N) separated from one another by lithographic etching, and• forming metal contacts by metal deposition to couple the N PV-cells in series to form the PV power converter (or N-series unit) (1 N).AIMD COMPRISING A PHOTOVOLTAIC POWER CONVERTER (1 N)

[0058] The higher energy efficacy provided by the photovoltaic power converter (1 N) of the present invention makes it particularly suitable for applications in active implantable medical devices (AIMD) comprising conversion of an optical energy into electrical energy and powered by an implanted battery, preferably a rechargeable battery. An example of AIMD illustrated in Figures 4a to 4e is an optoelectronic neurostimulator configured to stimulate a vagus nerve (VN), of the type described, e.g., in WO2016131492 or WO2021139887. Figure 4a shows a general view of the optoelectronic neurostimulator implanted in a patient such as to electrically stimulate a vagus nerve (VN). It comprises a housing (50), an electrode unit (40) and an optical energy transfer lead (30) for transferring an optical energy from the housing (50) to the electrode unit (40) which is coupled to the vagus nerve (VN). As shown in Figure 4b, the housing encloses an energy source (51 b), such as a battery, preferably a rechargeable battery, an electronic circuit board (51 pcb) and an implantable pulse generator (IPG) (51g) configured to emit towards a window (51 w) pulses of a monochromatic light of wavelength comprised within a predefined wavelength range. As visible in Figures 4b to 4d, the optical energy transfer lead (30) comprises an optical fibre (31s, 31 L) comprising a first end coupled to the housing facing the window and a second end. As shown in Figures 4c and 4d, the electrode unit (40) comprises an insulating substrate (41s) supporting electrodes (7n, 7p). The insulating substrate is configured to be coupled to the vagus nerve (Vn) of the patient with the electrodes (7n, 7p) contacting the vagus nerve.

[0059] A coupling element (10) coupling the second end of the optical fibre (31s, 31 L) to the electrode unit (40) is fixed to the electrode unit (40). As shown in Figure 4d, the coupling element (10) comprises a photovoltaic power converter according to the present invention and a cavity (11s, 1 1 L) for receiving the second end of the optical fibre facing in perfect alignment therewith the photovoltaic power converter to transform the optical energy of the monochromatic light transmitted by the optical fibre from the IPG (51g) into electrical energy to power the electrodes (7-, 7+). Figures 4d and 4e show an N = 5-seriesunit (1 N) which faces an optical fibre (31 s) from which it receives optical energy and which is coupled to an electrical circuit, 13n-14n-44n-7n-VN-7p-44p-14p-13p-7L, to power the electrodes (7n, 7p) to stimulate the vagus nerve (VN) and to power a LED (7L) facing a second optical fibre (31 L) to send an optical feedback signal to the housing (50) that the electrical pulses have been transmitted to the vagus nerve (VN). The optical fibre (31s) facing the 5-series unit (1 N) and the optical fibre (31 L) facing the feedback LED (7L) are preferably enclosed in a sheath (35) forming the optical energy transfer lead (30).

[0060] It is clear that increasing the efficacy of the N-series unit (1 N) of an implanted AIMD is crucial. If the battery (51 b) is non-rechargeable, then when the battery is empty, the patient must be operated to replace the spent battery with a new one. Not a rejoicing option. If the battery (51 b) is rechargeable, typically by generating an inductive current between a first coil enclosed in the housing (not shown) and a second coil of an external charger facing the first coil, then time-spacing apart two successive charging sessions is particularly advantageous for the patient who must repeat this cumbersome and time consuming operation its whole lifetime. Eliminating the current drop, Ai, observed when N > 2 PV cells integrated on a same semi-insulating substrate (5s) substantially contributes to increasing the efficacy of the N-series unit (1 N) and therefore solving the above issues.

Claims

CLAIMS1. A photovoltaic power converter (1 N) configured to transform an optical energy from a monochromatic light of wavelength comprised within a predefined wavelength range and to power an element (7), the photovoltaic power converter (1 N) comprising an insulating substrate (5s) supporting N photovoltaic cells (= PV-cell) (1.1-1.N) physically separated from one another and conductively coupled in series to one another, wherein N > 2 and wherein each of the N PV-cells comprises,• an emitter layer (2e) made of a type p or type n semiconductor, configured to transmit the monochromatic light, and• a base layer (2b) of type n or type p opposite to the emitter layer (2e) and forming therewith a photovoltaic PN junction of first polarity, and configured to absorb the monochromatic light transmitted through the emitter layer (2e) and to generate a current, i,• first and second contacts (3+, 3-) coupled to the emitter layer (2e) and the base layer (2b), respectively, and configured to be conductively coupled to the element (7),• a blocking layer (4b) is interposed between each of the N PV-cells and the insulating substrate(5s) to reduce photo-leakage caused by photoconductivity of the insulating substrate (5s),Characterized in that, the photovoltaic power converter (1 N) has an active area, A < 1.0 mm2, preferably A < 0.5 mm2, more preferably, A < 0. 2 mm2, and in that, each blocking layer (4b) is formed by M blocking PN-junctions, wherein M > N + 1 , each formed between a pair of first and second type p and type n semiconductors (4pi, 4ni, with i = 1 to M), wherein the first blocking PN-junction located closest to the photovoltaic PN-junction has a second polarity of opposite sign from the first polarity.

2. Photovoltaic power converter according to claim 1 , wherein the emitter layer (2e) is made of a material including GaAs, InP, GaN, AlAs, AIGaAs, GalnP, Si, and alloys thereof, preferably GaAs, and preferably has a thickness, T2e, comprised between 0.05 and 2.00 pm, preferably between 0.4 and 1.00 pm, more preferably between 0.60 and 0.80 pm.

3. Photovoltaic power converter according to anyone of the preceding claims, wherein the base layer (2b) is made of a material including GaAs, InP, GaN, AlAs, AIGaAs, GalnP, Si, and alloys thereof, preferably GaAs, and preferably has a thickness, T2b, comprised between 0.1 and 5.00 pm, preferably between 1.00 and 3.50 pm, more preferably between 2.00 and 3.00 pm.

4. Photovoltaic power converter according to anyone of the preceding claims, further comprising a lateral conductive layer (= LCL) (2c) in conductive contact with the base layer (2b) and with the blocking layer (4b) configured to transport current to a side of the photovoltaic power converter towards the second contact (3+,3-).

5. Photovoltaic power converter according to the preceding claim, wherein the lateral conductivelayer (2c) is made of a semiconductor material of same p type or n type as the base layer (2b) and including GaAs, InP, GaN, AlAs, AIGaAs, GalnP, Si, and alloys thereof, preferably GaAs, and preferably has a thickness, T2c, comprised between 0.10 and 3.00 pm, preferably between 0.50 and 1.50 pm, more preferably between 1 .40 and 1 .60 pm.

6. Photovoltaic power converter according to anyone of the preceding claims, wherein the first and second type p or type n semiconductors (4ni, 4pi) forming each one of the M blocking PN-junctions are made of a material including GaAs, InP, GaN, AlAs, AIGaAs, GalnP, Si, and alloys thereof, preferably AlxGai-xAs , wherein x = 0 to 0.4, preferably x = 0.1 to 0.3, wherein preferably M = N + 1 .

7. Photovoltaic power converter according to the preceding claim 6, wherein,• the p-doped semiconductors (4pi) of the blocking layer (4b) have a thickness, T4pi, comprised between 0.1 and 1 pm, preferably between 0.15 and 0.20 pm, and I or• the n-doped semiconductors (4ni) of the blocking layer (4b) have a thickness, T4ni, comprised between 0.2 and 2 pm, preferably between 0.25 and 0.35 pm.

8. Photovoltaic power converter according to the preceding claim 7, wherein• each pair of p-doped and n-doped semiconductors (4pi, 4ni) forming one of the M PN junctions of the blocking layer (4b) has a thickness comprised between 0.3 and 3 pm, preferably between 0.45 and 0.75 pm and wherein,• the blocking layer (4b) has a thickness, T4b, comprised between 0.6 and 20.0 pm, preferably between 1 .3 and 2.7 pm.

9. Photovoltaic power converter according to anyone of the preceding claims, wherein the insulating substrate (5s) is made of a material configured to be insulating and including GaAs, InP, GaN, AlAs, AIGaAs, GalnP, Si, and alloys thereof, preferably GaAs, and preferably has a thickness, T5s, comprised between 100 and 1000 pm, preferably between 200 and 800 pm, more preferably between 300 and 500 pm.

10. Photovoltaic power converter according to anyone of the preceding claims, wherein the photovoltaic power converter excluding the substrate (5s) has a thickness, T24, comprised between 1.25 and 50.0 pm, preferably, between 2.0 and 40.0 pm, more preferably between 4.0 and 12.0 pm.

11. Photovoltaic power converter according to anyone of the preceding claims, wherein the photovoltaic power converter is a component of an active implantable medical device (AIMD), preferably a neurostimulating implant, including a vagus nerve stimulating device, a peripheral nerve stimulator, or a brain stimulating device, or a pacemaker.

12. Photovoltaic power converter according to the preceding claim 11 , wherein the element (7) is an electrode unit (7p, 7n) or a LED (7L) of the neurostimulating device, the brain stimulating device, or of the pacemaker.

13. Photovoltaic power converter (1 N) according to anyone of the preceding claims, coupled in parallel to a number Mp of photovoltaic power converters (1 N) according to anyone of the preceding claims, wherein Mp > 1.

14. Process for producing a photovoltaic power converter according to anyone of the preceding claims comprising the following steps,• providing a substrate (5s) treated to become an insulating material,• sequentially and epitaxially growing on the insulating substrate at least the blocking layer (4b), preferably the lateral conductive layer (2c), the base layer (2b), and the emitter layer (2e), preferably by metalorganic chemical vapour deposition (MOCVD),• shaping the N PV-cells (1.1-1 .N) separated from one another by lithographic etching, and• forming metal contacts by metal deposition to couple the N PV-cells in series.

15. An active implantable medical device (AIMD) comprising,• a housing (50) enclosing an energy source (51 b) and an implantable pulse generator (IPG) (51g) configured to emit towards a window (51w) pulses of a monochromatic light of wavelength comprised within a predefined wavelength range,• an optical energy transfer lead (30) comprising an optical fibre (31 s, 31 L) comprising a first end coupled to the housing facing the window (51w) and a second end,• an electrode unit (40) comprising an insulating substrate (41s) supporting electrodes (7n, 7p) wherein the insulating substrate is configured to be coupled to a tissue (VN) of a patient with the electrodes (7n, 7p) contacting the tissue, and• a coupling element (10) coupling the second end of the optical fibre (31s, 31 L) to the electrode unit (40) and comprising a photovoltaic power converter and a cavity (11s, 11 L) for receiving the second end of the optical fibre facing the photovoltaic power converter to transform the optical energy of the monochromatic light transmitted by the optical fibre from the IPG into electrical energy to power the electrodes (7-, 7+), characterized in that, the photovoltaic power converter is according to anyone of claims 1 to 13.

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