WO2018150160A1 - Photovoltaic device - Google Patents

Photovoltaic device Download PDF

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Publication number
WO2018150160A1
WO2018150160A1 PCT/GB2018/050349 GB2018050349W WO2018150160A1 WO 2018150160 A1 WO2018150160 A1 WO 2018150160A1 GB 2018050349 W GB2018050349 W GB 2018050349W WO 2018150160 A1 WO2018150160 A1 WO 2018150160A1
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Prior art keywords
photovoltaic device
strain
semiconductor material
photovoltaic
gradient
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PCT/GB2018/050349
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French (fr)
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Marin Alexe
Mingmin YANG
Dong Jik Kim
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The University Of Warwick
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0352—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their shape or by the shapes, relative sizes or disposition of the semiconductor regions
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/02—Details
    • H01L31/0216—Coatings
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/02—Details
    • H01L31/0224—Electrodes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0256—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by the material
    • H01L31/0264—Inorganic materials
    • H01L31/028—Inorganic materials including, apart from doping material or other impurities, only elements of Group IV of the Periodic Table
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/1804—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof comprising only elements of Group IV of the Periodic Table
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/1804—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof comprising only elements of Group IV of the Periodic Table
    • H01L31/1808—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof comprising only elements of Group IV of the Periodic Table including only Ge
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/547—Monocrystalline silicon PV cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the present invention relates to a photovoltaic device.
  • the most common type of solar cell device is based on the most widely- known form of photovoltaic effect employing a semiconductor p-n junction which forms a built-in electric field that is used to separate photo-generated charge carriers.
  • devices of this type are the mainstay of the solar energy industry and produce more than 1% of electricity generated globally, they have the drawbacks that they have an open-circuit voltage that is smaller than the bandgap of their host semiconductor and their conversion efficiency is constrained by the Shockley-Queisser limit.
  • a less widely-known form of photovoltaic effect namely the bulk photovoltaic effect (or “anomalous photovoltaic effect"
  • the bulk photovoltaic effect or “anomalous photovoltaic effect”
  • a more widely-known form of photovoltaic effect has been observed in homogeneous materials having a non-centrosymmetric crystal structure as described in, for example, A. Glass et ah: "High- voltage bulk photovoltaic effect and the photorefractive process in LiNb0 3 ", Applied Physics Letters, volume 25, pages 233 to 235 (1974) and V. Fridkin & B.
  • a photovoltaic device comprising a semiconductor layer, first and second contacts to the
  • a strain-gradient inducer configured to cause or to increase a strain gradient in the semiconductor layer.
  • the strain gradient can be used to induce a bulk photovoltaic type effect (herein referred to as the "flexophotovoltaic effect” or the “local photovoltaic effect”) and so increase the efficiency of photovoltaic conversion in materials, such as silicon or germanium.
  • the semiconductor material may have a centrosymmetric crystal structure, such as silicon or germanium.
  • the semiconductor material may have a non-centrosymmetric crystal structure, such as a III-V semiconductor, a II-VI semiconductor and/ or a semiconductor material having a zinc blende crystal structure.
  • the semiconductor material may be or comprise monocrystalline or polycrystalline semiconductor material.
  • the semiconductor material may be or comprise amorphous semiconductor material.
  • the strain-gradient inducer may be arranged to provide strain at a surface of the semiconductor material layer.
  • the strain-gradient inducer may comprise a die comprising at least one indenter having a tip which is configured to urge into the surface of the semiconductor material layer.
  • the at least one indenter comprises an array of indenters.
  • the at least one indenter maybe coated with a conductive material for providing the first contact.
  • the surface of the semiconductor material layer may be coated with a conductive material for providing the first contact.
  • the at least one indenter may induce a point stress equal to or less than 0.5 GPa.
  • the at least one indenter may induce a point stress less than or equal to 20 GPa.
  • the semiconductor material may have a Young's modulus of between 10 to 500 GPa.
  • silicon may have a have a Young's modulus of between 50 to 200 GPa.
  • the strain may lie between 0.05 to 0.2.
  • the strain may spread over a distance of between 100 to 200 nm.
  • the strain-gradient inducer may be arranged to provide strain inside the
  • the strain-gradient inducer may comprise at least one particle or void arranged to produce a strain gradient.
  • the strain-gradient inducer may be arranged to bend the semiconductor material layer.
  • the strain gradient may be equal to or greater than 5 ⁇ 10 4 , equal to or greater than 1 x 10 5 rrr 1 , equal to or greater than 2 x 10 5 m _1 or equal to or greater than 5 x 10 5 rrr 1 .
  • the strain gradient may be less than or equal to 2 x io? rrr 1 , less than or equal to 1 ⁇ io? rrr 1 , less than or equal to 5 ⁇ io 6 rrr 1 or less than or equal to 2 x 10 6 rrr 1 .
  • a photovoltaic system comprising a photovoltaic device according to the first aspect arranged to be exposed to direct sunlight and a load electrically connected to the photovoltaic device.
  • a method comprising causing or increasing a strain gradient in a semiconductor layer having first and second contacts
  • a method comprising providing a photovoltaic device according to the first aspect of the invention, providing a load which is electrically connected to the photovoltaic device, and arranging the photovoltaic device to be exposed to direct sunlight.
  • Figure 1 schematically illustrates flexoelectric- mediated correlation between electricity, strain and photon
  • Figure 2 is a schematic diagram of a measurement setup for inducing a
  • AFM atomic force microscope
  • Figure 3a is a graph showing plots of current against voltage for rutile Ti0 2 (001) single crystal using a conductive AFM tip under illumination from a 405-nm laser with a laser intensity of 50 Wcrrr 2 for loading forces of 1 and 15 ⁇ ;
  • Figure 3b is a graph showing a plot of current as a function of loading force for rutile Ti0 2 (001) single crystal using a conductive AFM tip under illumination from a 405-nm laser with a laser intensity of 50 Wcrrr 2 ;
  • Figure 3c is a graph showing a plot of current as a function of light intensity for rutile Ti0 2 (001) single crystal using a conductive AFM tip under illumination from a 405-nm laser with a loading force of 15 ⁇ ;
  • Figure 3d is a graph showing a plot of current as a function of light polarization for rutile Ti0 2 (001) single crystal using a conductive AFM tip under illumination from a 405-nm laser with a laser intensity of 50 Wcrrr 2 and with a loading force of 15 ⁇ ;
  • Figure 4a is a graph showing plots of current against voltage for (ooi)-oriented silicon wafer with a dark resistivity of 15 ⁇ square cm using a conductive AFM tip under illumination from a 405-nm laser with a laser intensity of 100 mWcrrr 2 for loading forces of 0.5, 10 and 15 ⁇ respectively;
  • Figure 4b is a graph showing a plot of current as a function of light polarization for silicon using a conductive AFM tip under illumination from a 405-nm laser with a laser intensity of 100 mWcrrr 2 and with a loading force of 15 ⁇ ;
  • Figure 5 is a schematic diagram of a measurement setup for inducing a
  • Figure 6a is a graph showing plots of current against voltage for silicon using a conductive gold coated tungsten probe with a radius of about 10 ⁇ with and without illumination from a 520-nm laser with a laser intensity of 100 mWcrrr 2 and a loading force of around 2 N;
  • Figure 6b is a graph showing a plot of current as a function of light polarization for silicon using a conductive gold coated tungsten probe with a radius of about 10 ⁇ under illumination from a 520-nm laser with a laser intensity of 100 mWcrrr 2 and a loading force of around 2 N;
  • Figure 7a is a graph showing a plot of local photovoltaic current as a function of time as loading force is varied for (ooi)-oriented SrTi0 3 single crystal using a conductive AFM tip;
  • Figure 7b is a graph showing plots of current against voltage acquired with various loading force under illumination from a 405-nm laser and an intensity of 50 Wcrrr 2 using a conductive AFM tip;
  • Figure 7c is a graph showing a plot of photovoltaic current as function of loading force using a conductive AFM tip
  • Figure 7d is a graph showing plots of current against voltage measured under various light intensity illumination for an applied loading force of 15 ⁇ using a conductive AFM tip;
  • Figure 7e is a graph of a plot of photovoltaic current as a function of the light intensity using a conductive AFM tip
  • Figure 7f is a graph of a plot of current photovoltaic current as a function of light polarization using a conductive AFM tip;
  • Figure 8 is a schematic cross-sectional view of a first flexophotovoltaic device
  • Figure 9 is a schematic cross-sectional view of a second flexophotovoltaic device.
  • Flexoelectricity is a property of a material whereby electrical polarization can be induced in the material by a strain gradient resulting from a bending or point force.
  • the present invention is based on the insight that a strain gradient can be used to break centrosymmetry locally in a centrosymmetric material (such as silicon or germanium) that would otherwise not exhibit a bulk photovoltaic effect.
  • a strain gradient can be used to induce a bulk photovoltaic type effect (herein referred to as the
  • Figure 1 schematically illustrates flexoelectric- mediated correlation between electricity, strain and photon.
  • a strain gradient is used to break a centrosymmetric structure locally into a non-centrosymmetric structure thereby enabling electromechanical coupling between electricity and strain, conversion of photonic energy into electricity and mechanical response to light.
  • centrosymmetric crystals 2 Three different centrosymmetric crystals 2 are investigated, namely a rutile Ti0 2 (ooi) single crystal, a silicon wafer and a SrTi0 3 (ooi) single crystal.
  • the experimental system l includes a conductive atomic force microscope (AFM) 3 including a metal-coated tip 4 having a radius of not more than 30 nm disposed at a distal end of a cantilever arm 6 which is connected to signal measurement system 7 including an in-line amplifier 8, a low-pass filter 9 and a parameter analyser 10.
  • a terminal 11 for example in the form of a metal contact, provides an electrical connection to the crystal 2.
  • a loading force is applied to an upper surface 12 of the crystal 2 using the AFM tip 4 which induces an inhomogeneous strain distribution in a surface region 13 around a point of contact 14. This gives rise to a substantial strain gradient and flexoelectric effect.
  • a laser 15 is used to illuminate the tip-surface contact area 13 with light 16.
  • the photovoltaic current is probed using the conductive tip 4.
  • the magnitude of the photovoltaic current increases with the light intensity and the loading force.
  • the local photovoltaic current exhibits a two-fold dependence on the incident light polarization, thereby demonstrating presence of a bulk photovoltaic effect in semiconductors with centrosymmetric structures through mediation of a strain gradient.
  • Rutile Ti0 2 single crystals possess a centrosymmetric tetragonal structure belonging to the space group of P42/ mmm and a bandgap of 3.05 eV.
  • the conductive AFM tip 4 is placed in contact with the polished (001) surface of a Ti0 2 crystal 2 under various loading force.
  • the sample 2 is illuminated from the top side with a light intensity of about 50 Wcrrr 2 using the blue laser 15.
  • the illuminated Ti0 2 crystal exhibits distinctive local I-V characteristics under different loading force probed by the conductive tip 4.
  • the AFM tip 4 probes the surface with a 1 ⁇ loading force, it shows an asymmetric I-V curve with different onset voltage in the opposite bias polarity.
  • the I-V characteristics changes into an almost linear curve when increasing the loading force up to 15 ⁇ .
  • the I-V curve shows a photovoltaic current of 250 pA at zero bias and an open-circuit voltage, Voc, of 0.2 V, which are absent in the curve measured under 1 ⁇ loading force. Owing to the nanoscale tip radius, this photovoltaic current gives rise to a substantial current density as large as 10 Acrrr 2 at the tip/surface contact area.
  • the local photovoltaic current measured on Ti0 2 crystals increases with the loading force and the light intensity.
  • the point force applied by the AFM tip has a two-fold effect on the local electronic properties.
  • the strain and strain gradient modify the local band structures thereby promoting movement and even separation of the photo-excited carriers.
  • strain gradient breaks the local centrosymmetry thereby inducing a non- centrosymmetry locally and giving rise to a bulk photovoltaic effect under illumination.
  • the photovoltaic current varies sinusoi dally while rotating the light polarization angle by a half-wavelength plate, which can be fitted as:
  • ⁇ . is a third rank bulk photovoltaic tensor, the light intensity, and, e,- and ek the projection of the electrical field of the light.
  • the light polarization-dependent bulk photovoltaic current along a particular crystallographic direction can be transformed into a trigonometric function with a period of i8o°, which is determined by the crystal structure and the direction of the incident light.
  • the tip contact area with a strain gradient-induced non-centrosymmetry it is difficult to predict the exact relationship between the photovoltaic current and the light polarization due to the complicated lattice distortion and unknown BPV tensors of the induced non-centrosymmetric structures. Nevertheless, the light polarization dependence of the local photovoltaic current clearly demonstrates manifestation of a strain gradient-induced bulk photovoltaic effect (i.e. a flexophotovoltaic effect) in the originally centrosymmetric rutile Ti0 2 crystal. In addition to flexophotovoltaic effect, the strain-induced modification of local band structures also affects electronic transport.
  • a strain gradient-induced bulk photovoltaic effect i.e. a flexophotovoltaic effect
  • the strain-induced modification of local band structures also affects electronic transport.
  • a conductivity of semiconductors is normally reduced by compressive strain through a reduced mobility of charge carriers, while the point contact resistance is reduced because the contact area will increase with the loading force.
  • the contact area increase up to 4.6 times with 10 times larger loading force.
  • an increase of the actual tip contact area should be much less than this.
  • the modified band structure may also contribute partially to the local photovoltaic effect which does not depend on light polarization.
  • the flexophotovoltaic effect can occur in centrosymmetric materials due to the universal nature of the flexoelectric effect.
  • the flexophotovoltaic effect can be realized in any semiconductor having a suitable bandgap for absorbing sunlight and can harvest solar energy.
  • the separation of the photo-excited carriers is controlled by the internal symmetry instead of the built- in field as in the conventional photovoltaic effects.
  • This charge separation mechanism may provide a new approach to breaking the Shockley-Queisser limit.
  • a boron-doped silicon wafer 2 with a resistivity of 15 ⁇ square cm is etched by diluted HF solution to remove surface Si0 2 layer and then grounded via an indium contact.
  • the silicon wafer 2 is illuminated from the top side by the linearly polarized 405 nm laser 15 with a light intensity of 100 mWcrrr 2 .
  • the loading force has a dramatic effect on the local electronic properties under illumination.
  • a low loading force namely 0.5 uN
  • the wafer exhibits a low photoconductivity with a negative short circuit current of about -20 pA which probably results from the junction formed at the tip contact area.
  • the photovoltaic current probed by the tip turns to a positive value and is enhanced by one order of magnitude.
  • the photoconductivity at the negative bias polarity is also enhanced by about two order of magnitudes while that of positive bias polarity remains almost unchanged.
  • the light polarization dependence of the local photovoltaic current is studied.
  • the local photovoltaic current shows a two-fold azimuthal dependence upon the incident light polarization, clearly confirming the manifestation of the flexophotovoltaic effect in a silicon wafer.
  • the solar conversion efficiency of the nanoscale photovoltaic cells consisting of AFM tip and strained silicon under 15 ⁇ have also been measured under 1 sun (100 mWcrrr 2 ) AM 1.5 G illumination.
  • the effective area contributing to the flexophotovoltaic effect can be estimated as 3 times of the tip radius, that is about 100 nm.
  • the flexophotovoltaic device With a photovoltaic current of 0.5 nA, an open circuit voltage of Voc of 0.16 V and a fill factor of about 0.2, the flexophotovoltaic device exhibits a power conversion efficiency of about 50% which is about double the efficiency value of the best silicon solar cells based on p-n junctions. This demonstrates that the flexophotovoltaic device effect can effectively overcome Shockley-Queisser limit and boost the solar energy conversion efficiency. Referring to Figure 5, the flexophotovoltaic effect can be realized by using a
  • microscopic indentation system 17 which deforms a top surface 18 of a silicon wafer 19 using a conductive probe 20 with a radius of about 10 ⁇ .
  • the silicon wafer 19 is provided with a metal contact 21 (e.g. indium) on the rear surface 22.
  • Illumination of the top surface 18 with light 23 at a wavelength of 520 nm leads to a photocurrent which can be used to drive a load or measurement system 24.
  • the flexophotovoltaic effect has also been shown in a cubic SrTi0 3 (001) single crystal 2.
  • the SrTi0 3 crystal 2 is illuminated by a linearly- polarized 405 nm laser with an intensity of about 50 Wcrrr 2 .
  • the flexophotovoltaic effect not only offers a new approach to converting solar energy into electricity, but also can plays a role in existing thin-film solar cells. If a strain gradient is induced in thin-film solar cells, then this can be used to generate a significant flexophotovoltaic effect and improve solar cell performance.
  • a tandem solar cell structure can be formed with an upper flexophotovoltaic solar cell and a lower conventional p-n junction solar cell.
  • coupling between the strain gradient and photostriction effect proposed can provide an arrangement by which solar energy can be converted into mechanical energy directly in conventional semiconductors, such as silicon.
  • the photovoltaic device 31 for driving a load 32 is shown.
  • the photovoltaic device 31 employs strain gradients to generate or increase a photovoltaic current using the flexophotovoltaic effect.
  • the photovoltaic device 31 comprises a layer 33 of semiconductor material (or
  • semiconductor layer having first and second opposite faces 34, 35 (herein referred to as top and bottom surfaces respectively).
  • the semiconductor material may have a centrosymmetric crystalline structure, such as a silicon or germanium, or a non- centrosymmetric crystalline structure, such as GaAs and other III-V semiconductors, CdS and other II-VI semiconductors, and any other semiconductor material having a zinc blende crystal structure.
  • the semiconductor material layer 33 is preferably monocrystalline, but can be polycrystalline or amorphous.
  • the photovoltaic device 31 need not include a p-n junction and/or semiconductor heterojunction.
  • the semiconductor layer 33 is provided with first and second contacts 36, 37.
  • the first contact 36 takes the form of a plurality of contacts made to the top surface 34 of the semiconductor layer 33.
  • the second contact 37 is made elsewhere to the semiconductor layer 33, for example, to the bottom surface 35 in the form of a back contact.
  • a surface strain-gradient inducer 38 comprises a die 39 (or “stamp") which is arranged in contact with the top surface 34 and a clamper 40 for urging or holding the die 39 against top surface 35 so as to generate a sufficient strain-gradient.
  • the die 39 comprises a plurality of indenters 41 (or "protrusions") extending from a substrate 42 and which face the semiconductor material layer 33.
  • the indenters 41 and substrate 42 are integrally-formed, for example, by etching a blank substrate (not shown).
  • Each indenter 41 has a tip (which may also be referred to as "point” or “distal end”) 43 which has a suitable shape and dimensions for generating a sufficient strain-gradient at the surface of the semiconductor layer 33 when urged or held by the clamper 40.
  • the indenter 43 may have a tip having a radius less than or equal to 100 ⁇ or a contact area less than or equal to 100 ⁇ 2 .
  • the tip 43 may be conical, frustoconical, pyramidal, frustopyramidal, spherical, frustospherical or other suitable shape like.
  • the indenters 41 may be arranged in a regular array, e.g. square or hexagonal, having a pitch equal to or less than 100 ⁇ .
  • the die 39 is formed from silicon nitride or other suitable material.
  • the die 39 comprises a material and/or is sufficiently thin so as to be transparent to light 44 in the visible region of the spectrum.
  • the indenters 41 are coated with a layer of a transparent electrically conductive material, such as indium tin oxide, so as to provide the first contact 36.
  • the first contact 36 may take the form of a layer disposed on the top surface 34 of the semiconductor layer 33.
  • the clamper 40 may take the form of a frame or adhesive.
  • the die 39 is urged against the semiconductor layer 33 by, for example, a vice or press (not shown), and while the die 39 is urged against the semiconductor layer 33, the clamper 40 is fitted to hold the die 39 in place. Force applied by the vice or press (not shown) is then removed, leaving the clamper 40 to compress the layer structure 33, 36, 39.
  • the photovoltaic device 31 is orientated to receive light 44 through the top surface 34. However, if the semiconductor layer 33 is sufficiently thin and a transparent second contact 37 is used, another photovoltaic device 31 may be arranged to receive light 44 through the bottom surface 35.
  • the photovoltaic device 51 for driving a load 52 is shown.
  • the photovoltaic device 51 employs strain gradients to generate or increase a photovoltaic current using the flexophotovoltaic effect.
  • the photovoltaic device 51 comprises a layer 53 of semiconductor material (or
  • semiconductor layer having first and second opposite faces 54, 55 (herein referred to as top and bottom surfaces respectively).
  • the semiconductor material may have a centrosymmetric crystalline structure, such as a silicon or germanium, or a non- centrosymmetric crystalline structure, such as GaAs and other III-V semiconductors, CdS and other II-VI semiconductors, and any other semiconductor material having a zinc blende crystal structure.
  • the semiconductor material layer 53 is preferably monocrystalline, but can be polycrystalline or amorphous.
  • the photovoltaic device 51 need not to include a p-n junction and/or semiconductor heterojunction.
  • the semiconductor layer 53 is provided with first and second contacts 56, 57 disposed on the first and second faces 54, 55.
  • An embedded strain-gradient inducer 58 comprises particles 59 of a material which differs from the semiconductor material or voids.
  • the particles 59 may comprise another, different semiconductor material having a different lattice constant to the host material 53, a dielectric material or a conductive material.
  • the particles 59 may be precipitates, such as precipitated dopants, which may be formed for example, during deposition, such as chemical vapour deposition.
  • the particles 59 may lie in a plane or be distributed in a volume between upper and lower boundaries.
  • the particles 59 may be disposed closer to the top surface 54 than the second surface 55 ⁇
  • the particles 59 may lie, for example, a distance, d, from the top surface 54.
  • the distance, d may be equal to or greater than 10 nm, 20 nm or 50 nm.
  • the distance d may be an upper boundary, i.e. particle may be found in a region between a distance d and d + A, where ⁇ is thickness of the volume, which may be, for example, between 10 nm and 100 nm.
  • the photovoltaic device 51 is orientated to receive light 64 through the top surface 54. Modifications

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  • Condensed Matter Physics & Semiconductors (AREA)
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Abstract

A photovoltaic device is disclosed. The device comprises a layer (33) semiconductor material, first and second contacts (36, 37) to the semiconductor material layer and a strain-gradient inducer (39) configured to cause or to increase a strain gradient in the semiconductor material layer between the first and second contacts.

Description

Photovoltaic device Field of the Invention
The present invention relates to a photovoltaic device.
Background
Considerable effort is being directed to researching and developing alternative sources of energy to fossils fuels. Solar cell devices, which convert solar energy directly into electricity, are of particular interest since the Sun offers a virtually limitless source of energy.
The most common type of solar cell device is based on the most widely- known form of photovoltaic effect employing a semiconductor p-n junction which forms a built-in electric field that is used to separate photo-generated charge carriers. Although devices of this type are the mainstay of the solar energy industry and produce more than 1% of electricity generated globally, they have the drawbacks that they have an open-circuit voltage that is smaller than the bandgap of their host semiconductor and their conversion efficiency is constrained by the Shockley-Queisser limit.
A less widely- known form of photovoltaic effect, namely the bulk photovoltaic effect (or "anomalous photovoltaic effect"), has been observed in homogeneous materials having a non-centrosymmetric crystal structure as described in, for example, A. Glass et ah: "High- voltage bulk photovoltaic effect and the photorefractive process in LiNb03", Applied Physics Letters, volume 25, pages 233 to 235 (1974) and V. Fridkin & B.
Sturman: "The photovoltaic and photorefractive effects in noncentrosymmetric Materials", Gordon & Breach (1992). In a bulk photovoltaic effect device, uniform illumination can generate a voltage which exceeds the bandgap of the host material. Moreover, it has been reported in J. E. Spanier et ah: "Power conversion efficiency exceeding the Shockley-Queisser limit in a ferroelectric insulator", Nature Photonics (2016) that the efficiency of power conversion for this type of device can exceed the Shockley-Queisser limit. This effect, however, only seems to occur in wide bandgap ferroelectrics, such as BaTi03 and BiFe03; the large bandgap results in a low power conversion efficiency. Sum m ary
According to a first aspect of the present invention there is provided a photovoltaic device comprising a semiconductor layer, first and second contacts to the
semiconductor layer and a strain-gradient inducer configured to cause or to increase a strain gradient in the semiconductor layer.
Thus, the strain gradient can be used to induce a bulk photovoltaic type effect (herein referred to as the "flexophotovoltaic effect" or the "local photovoltaic effect") and so increase the efficiency of photovoltaic conversion in materials, such as silicon or germanium.
The semiconductor material may have a centrosymmetric crystal structure, such as silicon or germanium. The semiconductor material may have a non-centrosymmetric crystal structure, such as a III-V semiconductor, a II-VI semiconductor and/ or a semiconductor material having a zinc blende crystal structure.
The semiconductor material may be or comprise monocrystalline or polycrystalline semiconductor material. The semiconductor material may be or comprise amorphous semiconductor material.
The strain-gradient inducer may be arranged to provide strain at a surface of the semiconductor material layer. The strain-gradient inducer may comprise a die comprising at least one indenter having a tip which is configured to urge into the surface of the semiconductor material layer. The at least one indenter comprises an array of indenters. The at least one indenter maybe coated with a conductive material for providing the first contact. The surface of the semiconductor material layer may be coated with a conductive material for providing the first contact.
The at least one indenter may induce a point stress equal to or less than 0.5 GPa. The at least one indenter may induce a point stress less than or equal to 20 GPa. The semiconductor material may have a Young's modulus of between 10 to 500 GPa. For example, silicon may have a have a Young's modulus of between 50 to 200 GPa. Thus, the strain may lie between 0.05 to 0.2. The strain may spread over a distance of between 100 to 200 nm. The strain-gradient inducer may be arranged to provide strain inside the
semiconductor material layer. The strain-gradient inducer may comprise at least one particle or void arranged to produce a strain gradient. The strain-gradient inducer may be arranged to bend the semiconductor material layer.
The strain gradient may be equal to or greater than 5 χ 104, equal to or greater than 1 x 105 rrr1, equal to or greater than 2 x 105 m_1or equal to or greater than 5 x 105 rrr1. The strain gradient may be less than or equal to 2 x io? rrr1, less than or equal to 1 χ io? rrr1, less than or equal to 5 χ io6 rrr1 or less than or equal to 2 x 106 rrr1.
According to a second aspect of the present invention there is provided a photovoltaic system comprising a photovoltaic device according to the first aspect arranged to be exposed to direct sunlight and a load electrically connected to the photovoltaic device. According to a third aspect of the present invention there is provided a method comprising causing or increasing a strain gradient in a semiconductor layer having first and second contacts
According to a fourth aspect of the present invention there is provided a method comprising providing a photovoltaic device according to the first aspect of the invention, providing a load which is electrically connected to the photovoltaic device, and arranging the photovoltaic device to be exposed to direct sunlight.
Brief Description o f the Drawings
Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:
Figure 1 schematically illustrates flexoelectric- mediated correlation between electricity, strain and photon;
Figure 2 is a schematic diagram of a measurement setup for inducing a
flexophotovoltaic effect using an atomic force microscope (AFM) having a conductive AFM tip;
Figure 3a is a graph showing plots of current against voltage for rutile Ti02 (001) single crystal using a conductive AFM tip under illumination from a 405-nm laser with a laser intensity of 50 Wcrrr2 for loading forces of 1 and 15 μΝ;
Figure 3b is a graph showing a plot of current as a function of loading force for rutile Ti02 (001) single crystal using a conductive AFM tip under illumination from a 405-nm laser with a laser intensity of 50 Wcrrr2;
Figure 3c is a graph showing a plot of current as a function of light intensity for rutile Ti02 (001) single crystal using a conductive AFM tip under illumination from a 405-nm laser with a loading force of 15 μΝ;
Figure 3d is a graph showing a plot of current as a function of light polarization for rutile Ti02 (001) single crystal using a conductive AFM tip under illumination from a 405-nm laser with a laser intensity of 50 Wcrrr2 and with a loading force of 15 μΝ;
Figure 4a is a graph showing plots of current against voltage for (ooi)-oriented silicon wafer with a dark resistivity of 15 Ω square cm using a conductive AFM tip under illumination from a 405-nm laser with a laser intensity of 100 mWcrrr2 for loading forces of 0.5, 10 and 15 μΝ respectively;
Figure 4b is a graph showing a plot of current as a function of light polarization for silicon using a conductive AFM tip under illumination from a 405-nm laser with a laser intensity of 100 mWcrrr2 and with a loading force of 15 μΝ;
Figure 5 is a schematic diagram of a measurement setup for inducing a
flexophotovoltaic effect using indentation of a probe tip which has a radius of about 10 μιη;
Figure 6a is a graph showing plots of current against voltage for silicon using a conductive gold coated tungsten probe with a radius of about 10 μπι with and without illumination from a 520-nm laser with a laser intensity of 100 mWcrrr2 and a loading force of around 2 N; Figure 6b is a graph showing a plot of current as a function of light polarization for silicon using a conductive gold coated tungsten probe with a radius of about 10 μηι under illumination from a 520-nm laser with a laser intensity of 100 mWcrrr2 and a loading force of around 2 N;
Figure 7a is a graph showing a plot of local photovoltaic current as a function of time as loading force is varied for (ooi)-oriented SrTi03 single crystal using a conductive AFM tip;
Figure 7b is a graph showing plots of current against voltage acquired with various loading force under illumination from a 405-nm laser and an intensity of 50 Wcrrr2 using a conductive AFM tip;
Figure 7c is a graph showing a plot of photovoltaic current as function of loading force using a conductive AFM tip,
Figure 7d is a graph showing plots of current against voltage measured under various light intensity illumination for an applied loading force of 15 μΝ using a conductive AFM tip;
Figure 7e is a graph of a plot of photovoltaic current as a function of the light intensity using a conductive AFM tip;
Figure 7f is a graph of a plot of current photovoltaic current as a function of light polarization using a conductive AFM tip;
Figure 8 is a schematic cross-sectional view of a first flexophotovoltaic device; and Figure 9 is a schematic cross-sectional view of a second flexophotovoltaic device.
De taile d D e scriptio n o f Ce rtain Em bo d im e n ts
Introduction
Flexoelectricity is a property of a material whereby electrical polarization can be induced in the material by a strain gradient resulting from a bending or point force.
The present invention is based on the insight that a strain gradient can be used to break centrosymmetry locally in a centrosymmetric material (such as silicon or germanium) that would otherwise not exhibit a bulk photovoltaic effect. Thus, the strain gradient can be used to induce a bulk photovoltaic type effect (herein referred to as the
"flexophotovoltaic effect" or the "local photovoltaic effect") and so increase the efficiency of conversion in materials, such as silicon or germanium.
Figure 1 schematically illustrates flexoelectric- mediated correlation between electricity, strain and photon. A strain gradient is used to break a centrosymmetric structure locally into a non-centrosymmetric structure thereby enabling electromechanical coupling between electricity and strain, conversion of photonic energy into electricity and mechanical response to light.
Experiment
Referring to Figure 2, an experimental system l (or "set up") for studying a local photovoltaic effect in a centrosymmetric crystal 2 is shown.
Three different centrosymmetric crystals 2 are investigated, namely a rutile Ti02 (ooi) single crystal, a silicon wafer and a SrTi03 (ooi) single crystal.
The experimental system l includes a conductive atomic force microscope (AFM) 3 including a metal-coated tip 4 having a radius of not more than 30 nm disposed at a distal end of a cantilever arm 6 which is connected to signal measurement system 7 including an in-line amplifier 8, a low-pass filter 9 and a parameter analyser 10. A terminal 11, for example in the form of a metal contact, provides an electrical connection to the crystal 2.
A loading force is applied to an upper surface 12 of the crystal 2 using the AFM tip 4 which induces an inhomogeneous strain distribution in a surface region 13 around a point of contact 14. This gives rise to a substantial strain gradient and flexoelectric effect.
A laser 15 is used to illuminate the tip-surface contact area 13 with light 16. The photovoltaic current is probed using the conductive tip 4. For each of the different materials, the magnitude of the photovoltaic current increases with the light intensity and the loading force. Moreover, the local photovoltaic current exhibits a two-fold dependence on the incident light polarization, thereby demonstrating presence of a bulk photovoltaic effect in semiconductors with centrosymmetric structures through mediation of a strain gradient.
Experiments using a Ti02 crystal
Rutile Ti02 single crystals possess a centrosymmetric tetragonal structure belonging to the space group of P42/ mmm and a bandgap of 3.05 eV. The Ti02 crystal exhibits a high resistivity in dark (p = 1 x 1012 Ω square cm) and a substantial photoconductivity at room temperature under an illumination from a blue laser with a wavelength of 405 nm (hv = 3.07 eV).
The conductive AFM tip 4 is placed in contact with the polished (001) surface of a Ti02 crystal 2 under various loading force. The sample 2 is illuminated from the top side with a light intensity of about 50 Wcrrr2 using the blue laser 15.
Referring to Figure 3a, the illuminated Ti02 crystal exhibits distinctive local I-V characteristics under different loading force probed by the conductive tip 4. When the AFM tip 4 probes the surface with a 1 μΝ loading force, it shows an asymmetric I-V curve with different onset voltage in the opposite bias polarity. In contrast, the I-V characteristics changes into an almost linear curve when increasing the loading force up to 15 μΝ. Surprisingly, the I-V curve shows a photovoltaic current of 250 pA at zero bias and an open-circuit voltage, Voc, of 0.2 V, which are absent in the curve measured under 1 μΝ loading force. Owing to the nanoscale tip radius, this photovoltaic current gives rise to a substantial current density as large as 10 Acrrr2 at the tip/surface contact area.
Referring to Figures 3b and 3c, the local photovoltaic current measured on Ti02 crystals increases with the loading force and the light intensity.
The point force applied by the AFM tip has a two-fold effect on the local electronic properties. First, the strain and strain gradient modify the local band structures thereby promoting movement and even separation of the photo-excited carriers.
Secondly, the strain gradient breaks the local centrosymmetry thereby inducing a non- centrosymmetry locally and giving rise to a bulk photovoltaic effect under illumination.
To help understand the origin of the local photovoltaic current, the magnitude of photovoltaic current as a function of the incident light polarization was studied while keeping the light intensity constant.
Referring to Figure 3d, the photovoltaic current varies sinusoi dally while rotating the light polarization angle by a half-wavelength plate, which can be fitted as:
22cos(20 + 4) - 185 (1) where Ipvi's the value of the photovoltaic current in picoamperes (pA) and Θ is the angle of light polarization. This light-polarization dependence clearly differentiates the point force induced local PV effect from a conventional photovoltaic effect as in a p-n junction. In the conventional photovoltaic effect, where separation of the non- equilibrium photo-generated carriers is based on a gradient of the chemical potential, the photovoltaic current does not depend on the light polarization. This dependence is a unique characteristic of the bulk photovoltaic effect, which can be expressed as:
Ji = lught Pijk ej ek (2) where βφ. is a third rank bulk photovoltaic tensor, the light intensity, and, e,- and ek the projection of the electrical field of the light. In a certain non-centrosymmetric material, the light polarization-dependent bulk photovoltaic current along a particular crystallographic direction can be transformed into a trigonometric function with a period of i8o°, which is determined by the crystal structure and the direction of the incident light.
Regarding the tip contact area with a strain gradient-induced non-centrosymmetry, it is difficult to predict the exact relationship between the photovoltaic current and the light polarization due to the complicated lattice distortion and unknown BPV tensors of the induced non-centrosymmetric structures. Nevertheless, the light polarization dependence of the local photovoltaic current clearly demonstrates manifestation of a strain gradient-induced bulk photovoltaic effect (i.e. a flexophotovoltaic effect) in the originally centrosymmetric rutile Ti02 crystal. In addition to flexophotovoltaic effect, the strain-induced modification of local band structures also affects electronic transport. A conductivity of semiconductors is normally reduced by compressive strain through a reduced mobility of charge carriers, while the point contact resistance is reduced because the contact area will increase with the loading force. In case of an elastic contact, the contact area increase up to 4.6 times with 10 times larger loading force. As this is the upper limit, usually due to a finite size of the tip, an increase of the actual tip contact area should be much less than this. The modified band structure may also contribute partially to the local photovoltaic effect which does not depend on light polarization. In contrast to the bulk photovoltaic effect, the flexophotovoltaic effect can occur in centrosymmetric materials due to the universal nature of the flexoelectric effect. In principle, the flexophotovoltaic effect can be realized in any semiconductor having a suitable bandgap for absorbing sunlight and can harvest solar energy. The separation of the photo-excited carriers is controlled by the internal symmetry instead of the built- in field as in the conventional photovoltaic effects. This charge separation mechanism may provide a new approach to breaking the Shockley-Queisser limit.
Experiments using a silicon wafer
A boron-doped silicon wafer 2 with a resistivity of 15 Ω square cm is etched by diluted HF solution to remove surface Si02 layer and then grounded via an indium contact. The silicon wafer 2 is illuminated from the top side by the linearly polarized 405 nm laser 15 with a light intensity of 100 mWcrrr2.
Referring to Figure 4a, the loading force has a dramatic effect on the local electronic properties under illumination. Under a low loading force, namely 0.5 uN, the wafer exhibits a low photoconductivity with a negative short circuit current of about -20 pA which probably results from the junction formed at the tip contact area. After increase the force to 15 uN, the photovoltaic current probed by the tip turns to a positive value and is enhanced by one order of magnitude. Meanwhile, the photoconductivity at the negative bias polarity is also enhanced by about two order of magnitudes while that of positive bias polarity remains almost unchanged.
Referring to Figure 4b, to verify the contribution of the flexophotovoltaic effect to the enhanced photovoltaic current, the light polarization dependence of the local photovoltaic current is studied. As shown in Figure 3b, the local photovoltaic current shows a two-fold azimuthal dependence upon the incident light polarization, clearly confirming the manifestation of the flexophotovoltaic effect in a silicon wafer. The solar conversion efficiency of the nanoscale photovoltaic cells consisting of AFM tip and strained silicon under 15 μΝ have also been measured under 1 sun (100 mWcrrr2) AM 1.5 G illumination. The effective area contributing to the flexophotovoltaic effect can be estimated as 3 times of the tip radius, that is about 100 nm. With a photovoltaic current of 0.5 nA, an open circuit voltage of Voc of 0.16 V and a fill factor of about 0.2, the flexophotovoltaic device exhibits a power conversion efficiency of about 50% which is about double the efficiency value of the best silicon solar cells based on p-n junctions. This demonstrates that the flexophotovoltaic device effect can effectively overcome Shockley-Queisser limit and boost the solar energy conversion efficiency. Referring to Figure 5, the flexophotovoltaic effect can be realized by using a
microscopic indentation system 17 which deforms a top surface 18 of a silicon wafer 19 using a conductive probe 20 with a radius of about 10 μπι. The silicon wafer 19 is provided with a metal contact 21 (e.g. indium) on the rear surface 22. Illumination of the top surface 18 with light 23 at a wavelength of 520 nm (i.e. in the green region of the electromagnetic spectrum) leads to a photocurrent which can be used to drive a load or measurement system 24.
Referring also to Figures 6a and 6b, in an experiment in which the light 23 is linearly- polarized laser light with an intensity of 100 mWcrrr2 illumination gives rise to a photovoltaic current of about -4 nA with an open circuit voltage of 0.1 V. The photovoltaic current collected by the probe 20 also shows a two-fold azimuthal dependence upon the incident light polarization, which can be fitted with a cosine function, namely IPV = -o.6cos(20 +61) - 4.3. This demonstrates a bulk photovoltaic like effect in silicon, in other words, the flexophotovoltaic effect.
Experiments using SrTi03
Referring to Figures 2 and 7a to 7f, the flexophotovoltaic effect has also been shown in a cubic SrTi03 (001) single crystal 2. The SrTi03 crystal 2 is illuminated by a linearly- polarized 405 nm laser with an intensity of about 50 Wcrrr2.
As clearly shown in Figure 7a, a photocurrent appears when a loading force is increased to 18 μΝ and diminishes when the loading force is decreased to 1 μΝ. The effect of the point force on the local electronic properties is further illustrated in Figures 7b and 7c. With increased loading force, both the photocurrent current and photoconductivity are enhanced, while the value of open circuit voltage, at about 0.25 V, remains almost constant, which is similar to that of the Ti02 crystal.
Additionally, the effect of light intensity is shown in Figures 7d and ye. Similarly, increased light intensity enhances conductivity and photocurrent current while the open circuit voltage remains unchanged.
Meanwhile, the photocurrent probed by the conductive AFM tip 4 under large force exhibits light polarization dependence, confirming manifestation of the
flexophotovoltaic effect in cubic SrTi03 crystal. Thus, the flexophotovoltaic effect has been shown in various centrosymmetric semiconductors where the photoexcited carriers are separated by strain gradient- induced non-centrosymmetry. The flexophotovoltaic effect explored for a silicon wafer exhibits a large solar energy conversion efficiency, exceeding the Shockley-Queisser limit.
The flexophotovoltaic effect not only offers a new approach to converting solar energy into electricity, but also can plays a role in existing thin-film solar cells. If a strain gradient is induced in thin-film solar cells, then this can be used to generate a significant flexophotovoltaic effect and improve solar cell performance. In addition, a tandem solar cell structure can be formed with an upper flexophotovoltaic solar cell and a lower conventional p-n junction solar cell.
Referring again to Figure l, coupling between the strain gradient and photostriction effect proposed (herein referred to as the flexophotostriction effect) can provide an arrangement by which solar energy can be converted into mechanical energy directly in conventional semiconductors, such as silicon.
Device structures
Referring to Figure 8, a photovoltaic device 31 for driving a load 32 is shown. The photovoltaic device 31 employs strain gradients to generate or increase a photovoltaic current using the flexophotovoltaic effect.
The photovoltaic device 31 comprises a layer 33 of semiconductor material (or
"semiconductor layer") having first and second opposite faces 34, 35 (herein referred to as top and bottom surfaces respectively). The semiconductor material may have a centrosymmetric crystalline structure, such as a silicon or germanium, or a non- centrosymmetric crystalline structure, such as GaAs and other III-V semiconductors, CdS and other II-VI semiconductors, and any other semiconductor material having a zinc blende crystal structure. The semiconductor material layer 33 is preferably monocrystalline, but can be polycrystalline or amorphous. The photovoltaic device 31 need not include a p-n junction and/or semiconductor heterojunction.
The semiconductor layer 33 is provided with first and second contacts 36, 37. The first contact 36 takes the form of a plurality of contacts made to the top surface 34 of the semiconductor layer 33. The second contact 37 is made elsewhere to the semiconductor layer 33, for example, to the bottom surface 35 in the form of a back contact.
A surface strain-gradient inducer 38 comprises a die 39 (or "stamp") which is arranged in contact with the top surface 34 and a clamper 40 for urging or holding the die 39 against top surface 35 so as to generate a sufficient strain-gradient.
The die 39 comprises a plurality of indenters 41 (or "protrusions") extending from a substrate 42 and which face the semiconductor material layer 33. In this example, the indenters 41 and substrate 42 are integrally-formed, for example, by etching a blank substrate (not shown).
Each indenter 41 has a tip (which may also be referred to as "point" or "distal end") 43 which has a suitable shape and dimensions for generating a sufficient strain-gradient at the surface of the semiconductor layer 33 when urged or held by the clamper 40. For example, the indenter 43 may have a tip having a radius less than or equal to 100 μπι or a contact area less than or equal to 100 μπι2. The tip 43 may be conical, frustoconical, pyramidal, frustopyramidal, spherical, frustospherical or other suitable shape like. The indenters 41 may be arranged in a regular array, e.g. square or hexagonal, having a pitch equal to or less than 100 μπι.
The die 39 is formed from silicon nitride or other suitable material. The die 39 comprises a material and/or is sufficiently thin so as to be transparent to light 44 in the visible region of the spectrum.
The indenters 41 are coated with a layer of a transparent electrically conductive material, such as indium tin oxide, so as to provide the first contact 36. The first contact 36, however, may take the form of a layer disposed on the top surface 34 of the semiconductor layer 33.
The clamper 40 may take the form of a frame or adhesive. To fabricate the device 31 the die 39 is urged against the semiconductor layer 33 by, for example, a vice or press (not shown), and while the die 39 is urged against the semiconductor layer 33, the clamper 40 is fitted to hold the die 39 in place. Force applied by the vice or press (not shown) is then removed, leaving the clamper 40 to compress the layer structure 33, 36, 39. The photovoltaic device 31 is orientated to receive light 44 through the top surface 34. However, if the semiconductor layer 33 is sufficiently thin and a transparent second contact 37 is used, another photovoltaic device 31 may be arranged to receive light 44 through the bottom surface 35.
Referring to Figure 9, another photovoltaic device 51 for driving a load 52 is shown. The photovoltaic device 51 employs strain gradients to generate or increase a photovoltaic current using the flexophotovoltaic effect. The photovoltaic device 51 comprises a layer 53 of semiconductor material (or
"semiconductor layer") having first and second opposite faces 54, 55 (herein referred to as top and bottom surfaces respectively). The semiconductor material may have a centrosymmetric crystalline structure, such as a silicon or germanium, or a non- centrosymmetric crystalline structure, such as GaAs and other III-V semiconductors, CdS and other II-VI semiconductors, and any other semiconductor material having a zinc blende crystal structure. The semiconductor material layer 53 is preferably monocrystalline, but can be polycrystalline or amorphous. The photovoltaic device 51 need not to include a p-n junction and/or semiconductor heterojunction. The semiconductor layer 53 is provided with first and second contacts 56, 57 disposed on the first and second faces 54, 55.
An embedded strain-gradient inducer 58 comprises particles 59 of a material which differs from the semiconductor material or voids. The particles 59 may comprise another, different semiconductor material having a different lattice constant to the host material 53, a dielectric material or a conductive material.
The particles 59 may be precipitates, such as precipitated dopants, which may be formed for example, during deposition, such as chemical vapour deposition.
The particles 59 may lie in a plane or be distributed in a volume between upper and lower boundaries.
The particles 59 may be disposed closer to the top surface 54 than the second surface 55· The particles 59 may lie, for example, a distance, d, from the top surface 54. The distance, d, may be equal to or greater than 10 nm, 20 nm or 50 nm. The distance d, may be an upper boundary, i.e. particle may be found in a region between a distance d and d + A, where Δ is thickness of the volume, which may be, for example, between 10 nm and 100 nm. The photovoltaic device 51 is orientated to receive light 64 through the top surface 54. Modifications
It will be appreciated that various modifications may be made to the embodiments hereinbefore described. Such modifications may involve equivalent and other features which are already known in the design, manufacture and use of photovoltaic devices and component parts thereof and which may be used instead of or in addition to features already described herein. Features of one embodiment may be replaced or supplemented by features of another embodiment. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel features or any novel combination of features disclosed herein either explicitly or implicitly or any generalization thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims

Claim s
1. A photovoltaic device comprising:
a layer of semiconductor material;
first and second contacts to the semiconductor material layer ; and
a strain-gradient inducer configured to cause or to increase a strain gradient in the semiconductor material layer between the first and second contacts.
2. A photovoltaic device according to claim 1, wherein the semiconductor material has a centrosymmetric crystal structure.
3. A photovoltaic device according to claim 1 or 2, wherein the semiconductor material is silicon .
4. A photovoltaic device according to claim 1, wherein the semiconductor material has a non-centrosymmetric crystal structure.
5. A photovoltaic device according to claim 1 or 4, wherein the semiconductor material has a zinc blend crystal structure.
6. A photovoltaic device according to any one of claims 1 to 5, wherein the strain- gradient inducer is arranged to provide strain at a surface of the semiconductor material layer.
7. A photovoltaic device according to claim 6 , wherein the strain-gradient inducer comprises a die comprising at least one indenter having a tip which is configured to urge into the surface of the semiconductor material layer.
8. A photovoltaic device according to claim 7, wherein the at least one indenter comprises an array of indenters.
9. A photovoltaic device according to claim 7 or 8 , wherein the at least one indenter is/ are coated with a conductive material for providing the first contact.
10. A photovoltaic device according to claim 7, 8 or 9 , wherein the surface of the semiconductor material layer is coated with a conductive material for providing the first contact.
11. A photovoltaic device according to any one of claims 1 to 5, wherein the strain- gradient inducer is arranged to provide strain inside the semiconductor material layer.
12. A photovoltaic device according to claim 11, wherein the strain-gradient inducer comprises at least one particle or void arranged to produce a strain gradient.
13. A photovoltaic device according to any one of claims 1 to 5, wherein the strain- gradient inducer is arranged to bend the semiconductor material layer.
14. A photovoltaic device according to any preceding claim , wherein the strain gradient is equal to or greater than 5 x 104 m- 1, equal to or greater than 1 x 105 m- 1, equ al to or greater than 2 x 105 m_ 1or equal to or greater than 5 x 105 m- 1.
15. A photovoltaic device according to any preceding claim , the strain gradient is less than or equ al to 2 x 107 m_ 1, less than or equal to 1 x 107 m_ 1, less than or equal to 5 x 106 m_ 1 or less than or equ al to 2 x 106 m_ 1.
16. A photovoltaic system comprising:
a photovoltaic device according to any preceding claim arranged to be exposed to direct sunlight; and
a load electrically connected to the photovoltaic device.
17. A method comprising:
providing a photovoltaic device according to any one of claims 1 to 15 ;
providing a load which is electrically connected to the photovoltaic device; and arranging the photovoltaic device to be exposed to direct sunlight.
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EP3983769A4 (en) * 2019-06-13 2023-08-16 Griffith University Optoelectronic coupling platforms and sensors
US20220173303A1 (en) * 2020-08-31 2022-06-02 Massachusetts Institute Of Technology Flexo-electric broadband photo-detectors and electrical energy generators
CN113778140A (en) * 2021-09-15 2021-12-10 阳光电源（上海）有限公司 Tracking method and device of tracking support and photovoltaic system
CN113778140B (en) * 2021-09-15 2024-04-12 阳光电源（上海）有限公司 Tracking method and device of tracking bracket and photovoltaic system

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