WO2017096434A1 - A method of manufacturing a photovoltaic device - Google Patents

A method of manufacturing a photovoltaic device Download PDF

Info

Publication number
WO2017096434A1
WO2017096434A1 PCT/AU2016/051218 AU2016051218W WO2017096434A1 WO 2017096434 A1 WO2017096434 A1 WO 2017096434A1 AU 2016051218 W AU2016051218 W AU 2016051218W WO 2017096434 A1 WO2017096434 A1 WO 2017096434A1
Authority
WO
WIPO (PCT)
Prior art keywords
layer
copper
light
based light
absorbing
Prior art date
Application number
PCT/AU2016/051218
Other languages
French (fr)
Inventor
Xiaojing Hao
Martin Andrew Green
Wei Li
Jialiang HUANG
Original Assignee
Newsouth Innovations Pty Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from AU2015905135A external-priority patent/AU2015905135A0/en
Application filed by Newsouth Innovations Pty Limited filed Critical Newsouth Innovations Pty Limited
Publication of WO2017096434A1 publication Critical patent/WO2017096434A1/en

Links

Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02367—Substrates
    • H01L21/0237—Materials
    • H01L21/02373—Group 14 semiconducting materials
    • H01L21/02381—Silicon, silicon germanium, germanium
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02367—Substrates
    • H01L21/0237—Materials
    • H01L21/02422—Non-crystalline insulating materials, e.g. glass, polymers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02436—Intermediate layers between substrates and deposited layers
    • H01L21/02439—Materials
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02436—Intermediate layers between substrates and deposited layers
    • H01L21/02439—Materials
    • H01L21/02491—Conductive materials
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02518—Deposited layers
    • H01L21/02521—Materials
    • H01L21/02568—Chalcogenide semiconducting materials not being oxides, e.g. ternary compounds
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02656—Special treatments
    • H01L21/02664—Aftertreatments
    • H01L21/02667—Crystallisation or recrystallisation of non-monocrystalline semiconductor materials, e.g. regrowth
    • H01L21/02675—Crystallisation or recrystallisation of non-monocrystalline semiconductor materials, e.g. regrowth using laser beams
    • H01L21/02683—Continuous wave laser beam
    • 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
    • H01L31/032—Inorganic materials including, apart from doping materials or other impurities, only compounds not provided for in groups H01L31/0272 - H01L31/0312
    • H01L31/0322—Inorganic materials including, apart from doping materials or other impurities, only compounds not provided for in groups H01L31/0272 - H01L31/0312 comprising only AIBIIICVI chalcopyrite compounds, e.g. Cu In Se2, Cu Ga Se2, Cu In Ga Se2
    • 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
    • H01L31/04—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 adapted as photovoltaic [PV] conversion devices
    • H01L31/06—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 adapted as photovoltaic [PV] conversion devices characterised by at least one potential-jump barrier or surface barrier
    • H01L31/072—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 adapted as photovoltaic [PV] conversion devices characterised by at least one potential-jump barrier or surface barrier the potential barriers being only of the PN heterojunction type
    • H01L31/0749—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 adapted as photovoltaic [PV] conversion devices characterised by at least one potential-jump barrier or surface barrier the potential barriers being only of the PN heterojunction type including a AIBIIICVI compound, e.g. CdS/CulnSe2 [CIS] heterojunction solar cells
    • 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
    • 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
    • Y02E10/541—CuInSe2 material PV cells

Definitions

  • the present invention generally relates to a method for manufacturing a photovoltaic device.
  • the present invention relates to a method for forming an absorbing material in a thin film photovoltaic device.
  • CdTe photovoltaic devices One of the drawbacks of CdTe photovoltaic devices is related to the toxicity of Cd which may cause problems once the devices degrade or need to be disposed.
  • Kesterite CZTS, CZTSe or CZTSSe
  • Kesterite is a quaternary compound constituted by copper (Cu) , zinc (Zn) , tin (Sn) and sulphur (S) or selenium (Se).
  • Kesterite has the chemical formula
  • CZTS Cu 2 ZnSn(S, Se) 4 .
  • CZTS or CZTSe are used for Kesterite.
  • kesterite photovoltaic devices are realised on soda lime glass substrates coated with a molybdenum (Mo) layer which functions as a back contact.
  • Mo molybdenum
  • a CZTS(Se) absorber layer is formed by annealing a material containing the precursor elements for CZTS(Se) . This material is usually deposited using PVD, CVD techniques or solution techniques.
  • a front contact consisting of a ZnO/AZO, ITO, BZO layer and a metallic material is normally realised on the absorber layer.
  • kesterite photovoltaic devices also have a CdS
  • the annealing process used to form the absorber layer in kesterite solar cells has been traditionally performed by sulfurisation of a solid state precursor layer in an annealing furnace.
  • This technique has shown problems associated with the thermal stability of soda lime glass substrates, which limits the maximum possible temperature used by conventional furnace or rapid thermal annealing furnaces.
  • the long-time chalcogen annealing is also detrimental to the free carrier transportation due to the increasing of cell's series resistance.
  • precursors may decompose and react with the back contact layer.
  • the present invention provides a method for forming a photovoltaic cell
  • the first layer comprises a metallic material or transparent conductive layer.
  • the intermediate layer and the laser wavelength are selected such that at least 90% of the energy of the laser light that reaches a surface of the intermediate layer is absorbed by the intermediate layer
  • the intermediate layer has a light transmission lower than 10% at a frequency of the continuous wave laser light.
  • intermediate layer comprises a metalli material .
  • the intermediate layer has a thickness comprised between lOnm and lOOnm.
  • the intermediate layer comprises one o a combination of ⁇ 2 ⁇ , T1B 2 , TiN, and C.
  • the present invention provides a method forming a photovoltaic cell comprising the steps of providing a transparent substrate comprising a first transparent conductive layer; forming a copper-based light-absorbing over at least a portion of the first transparent conductive layer; depositing a capping layer arranged over at least a portion of the copper-based light-absorbing layer; exposing the copper-based light-absorbing layer to a continuous wave laser light through the capping layer; depositing a second conductive layer arranged to be electrically coupled with the copper-based light- absorbing layer over at least a portion of the copper-based light-absorbing layer; wherein the capping layer is arranged to affect the energy distribution of the laser light through the copper-based light-absorbing layer.
  • the transparent substrate comprises a TCO covered glass or polymeric substrate or a silicon substrate .
  • the step of forming a copper-based light-absorbing layer comprises the steps of: depositing a plurality of precursors to form a solid state layer of precursors, at least one of the precursors comprising a copper-based material; and annealing a solid state layer of precursors; wherein the annealing of the solid state layer of precursors occurs during the step of exposing the copper- based light-absorbing layer to a continuous wave laser light through the capping layer.
  • the solid state layer of precursors is exposed to the continuous wave laser light through the capping layer.
  • the step of exposing the copper-based light-absorbing layer to a continuous wave laser light through the capping layer is performed in a manner such that electrically active defects present in the copper- based light-absorbing layer are annealed while the copper- based light-absorbing layer is formed.
  • the method further comprises the step of, prior to exposing depositing a second conductive layer, at least partially removing the capping layer.
  • the intermediate layer is arranged in a manner such that at least 90% of the energy of the laser light that reaches a surface of the intermediate layer is absorbed by the intermediate layer.
  • the capping layer has a light
  • the capping layer comprises one or a combination of ZnS, Si0 2 , Sn0 2 , CdS, In 2 S 3 , Cd x Zn ( i- X) O x S .
  • the capping layer has a thickness comprised between lOnm and 200nm.
  • the capping layer has a bandgap arranged to minimise chemical reactions with the light absorbing layer.
  • the capping layer may be also arranged to improve light trapping of the laser light and therefore optimise the efficiency of the laser annealing process.
  • the copper-based light-absorbing layer incurs a transition from a first solid phase to a second solid phase .
  • At least one of the copper-based light-absorbing layer temporary enters a liquid phase.
  • the copper based light-absorbing material comprises defects and the method further
  • the volume density of defects is controlled while the copper-based light-absorbing material is formed.
  • the method further comprises the step of adjusting one or more parameters of the continuous wave laser light to influence the spatial distribution of the defects in the light-absorbing material.
  • the intermediate layer comprises defects and the method further comprises the step of adjusting one or more parameters of the continuous wave laser light to influence the volume density of the defects in the intermediate layer.
  • the continuous wave laser light has a line-focused beam capable of covering the entire
  • the continuous wave laser is scanned across the copper-based light-absorbing layer with a scanning speed in the range of 100-3000 mm/min providing exposure times between 180 to 6 ms .
  • the copper based light-absorbing layer is a kesterite based layer.
  • the copper based light-absorbing layer is a copper-zinc-germanium-tin-chalcogenide based layer
  • the copper based light-absorbing layer is a silver-copper-zinc-tin-chalcogenide based layer.
  • the second conductive layer comprises cadmium sulphide, In 2 S 3 , Cd x Zn ( i-x ) O x S, Zn(0,S) .
  • the present invention provides a photovoltaic cell manufactured in accordance with the method of the first or second aspect.
  • Advantages of embodiments of the inventions include the reduced annealing duration from several tens of minutes to a few microseconds which can greatly reduce the thermal budget .
  • the high-power line-focus CW diode laser used in some embodiments can greatly shorten the processing time comparing with conventional RTP process and is a suitable tool to fulfil the requirement for large area processing of thin film.
  • Diode laser annealing has shown improved quality of kesterite absorber, which offers a scalable higher throughout version of single junction thin film and multi-j unction solar cells technology.
  • the intermediate layer allows reducing the energy transferred from the laser light to the first conductive layer.
  • the capping layer allows
  • Figures 1 and 2 are flow diagrams outlining basic steps required to manufacture a photovoltaic device in
  • Figure 3 schematically shows device being fabricated using methods in accordance with embodiments
  • Figures 4 and 5 show Raman spectra of CZTS layers as- deposited and annealed at different currents in accordance with embodiments
  • Figure 6 shows STEM-EDS maps for Cu, Zn, Sn, and S acquired from CZTS layers as-deposited and annealed at different currents in accordance with embodiments
  • Figures 7 and 9 shows EQE plots of laser annealed samples with a CdS capping layer and A10 x intermediate layer respectively, compared to reference samples;
  • Figure 8 shows cross-sectional TEM-EDS mapping of
  • Figure 10 shows a steady state PL result (a) and the FWHM of PL peaks (b) of laser annealed samples with A10 x barrier layer and reference sample for comparison;
  • Figure 11 and 12 show Raman spectra and a peak analysis performed using a 785 nm excitation on laser annealed samples and a reference samples .
  • Embodiments of the present invention relate to a method for manufacturing a thin film photovoltaic device.
  • Embodiments relate to an annealing method for forming an absorbing material of a thin film photovoltaic device.
  • embodiments of the method allow improving the quality of CZTS epilayer using a novel laser annealing technique .
  • a flow diagram 100 with steps used to manufacture a photovoltaic device in accordance with embodiments.
  • the steps of method 100 allow forming a device on a rigid substrate, such as stainless steel, or a flexible polymeric substrate.
  • a substrate comprising a first conductive layer is provided.
  • An intermediate 'barrier' layer is then deposited on the substrate at step 110.
  • the intermediate layer is in electrical contact with the conductive layer.
  • the absorption coefficient and the thickness of the intermediate layer are designed so that at least 90% of the laser light energy that reaches the intermediate layer is absorbed so that the energy is not transferred to the conductive portion of the substrate.
  • the intermediate layer has a thickness comprised between lOnm and lOOnm.
  • a plurality of precursors comprising a copper-based material is deposited on the intermediate layer to form a copper-based light-absorbing layer.
  • the solid state layer of precursors can be
  • a capping layer is then deposited on the layer of precursors, step 130.
  • the precursors are then exposed to a continuous wave laser light through the capping layer to form a CZTS layer, step 140.
  • a furnace annealing step is performed prior to laser annealing to form CZTS layer and laser annealing step 140 is used to improve the properties of the CZTS layer.
  • the device comprising the substrate, the
  • the intermediate layer and the precursors is pre-heated before performing the laser annealing step (140) in order to reduce the thermal mismatch with the CZTS absorbing layer.
  • the deposited capping layer is chemically removed after laser annealing, in alternative embodiments the capping layer dissolves during laser annealing.
  • a second conductive layer is deposited on the CZTS light absorbing layer, step 150.
  • the second conductive material may comprise CdS, In 2 S 3 , Cd x Zn(i-x ) O x S, Zn(0,S) . Subsequently, a top contacting structure is formed to contact the
  • One of the advantages provided by the intermediate layer or 'barrier' layer deposited between the metallic part of the substrate and the CZTS light absorbing layer in method 100 is the reduction of energy transferred from the laser light to the first conductive layer.
  • the capping layer on the other end, allows controlling and optimising the path of the laser light through the solid state layer of precursors.
  • Method 200 allows fabricating a thin film photovoltaic device on a transparent substrate with a transparent conductive layer, such as a TCO covered glass or polymeric substrate.
  • a transparent conductive layer such as a TCO covered glass or polymeric substrate.
  • the layer of precursors is directly deposited onto the conductive portion of the substrate and sulphurised, step 220.
  • a capping layer is deposited at step 230 and the layer of precursors is annealed through the capping layer using a laser (step 240) in a similar manner as in method 100.
  • Method 200 does not permit the use of an intermediate layer between the substrate and the CZTS light absorbing layer. Therefore the annealing step (240) has to be performed in different conditions to annealing step 140.
  • the precursors incur a transition from a first solid phase to a second solid phase.
  • One or more parameters of the continuous wave laser light can be adjusted to control the volume density or spatial distribution of the defects in the light-absorbing material during the transition.
  • the parameters of the continuous wave laser light can be tuned to influence the volume density of the defects in the intermediate layer.
  • the laser is used to anneal the CZTS layer and reduce the density of defect in the grain so that the quality of the material can be improved.
  • the investigation on the phase equilibrium in the Cu 2 S-ZnS-SnS 2 system showed that single phase CZTS crystals can only be grown in a very narrow region.
  • the input laser power is tuned to heat up the CZTS film below the melting point (990°C of CZTS) to avoid decomposition and forming of second phases.
  • the laser annealing process involves only solid-solid phase transition.
  • FIG 3 there are shown two devices 300 and 350 being fabricated in accordance with methods 100 and 200 respectively. Both devices are shown while the layer of precursors is being annealed.
  • a silicon substrate 302 comprising a conductive molybdenum layer 304 is used to fabricate the device.
  • a layer of Al 2 O x is used as intermediate layer.
  • the intermediate layer can be made of TiN, C and TiB 2 .
  • Device 350 instead is deposited on a glass substrate 352 covered with a TCO layer 354.
  • a sulphurised layer of precursors 308 is being annealed by scanning an 808 nm line-focused CW diode laser beam with the dimension of 12x0.27 mm 2 (FWHM) to improve the quality of CZTS epi-layers.
  • the laser annealing is performed through a 150 nm thick ZnS capping layer 310, which is sputtered on the layer 308.
  • ZnS layer 310 is the protection from oxidation, decomposition and evaporation provided to layer 308.
  • Layer 310 is removed by diluted hydrochloric acid after the laser annealing is performed.
  • layer 310 can be made of Si0 2 , Sn0 2 , CdS, In 2 S 3 , Cd x Zn ( i- x) OxS .
  • the annealed portion 316 of layer 308 has improved structural and electrical properties.
  • portion 316 has a lower concentration of structural and electrical defects in comparison to the remaining portions of layer 318.
  • Portion 316 has a size similar to the width of laser beam 314.
  • the laser beam may have a line-focused beam capable of covering the entire longitudinal extension of layer 308 and expose the entire layer 308 by moving once along the layer in a single direction .
  • the ZnS capping layer 310 is designed to transmit at least 90% of the continuous wave laser light and allows
  • ZnS capping layer 310 is designed to improve light trapping of the laser light and therefore optimise the efficiency of the laser annealing process.
  • the refractive index of the ZnS is about 2.3, which is lower than CZTS of 2.6.
  • the ZnS can act as an ARC layer during the laser annealing process.
  • FIG 4 there is shown a plot with Raman curves of CZTS layers as-deposited and annealed with different laser powers.
  • the samples were annealed with a fixed substrate heating temperature of 500 °C, scan velocities of 400mm/min and exposure times of 40ms, a 514 nm excitation wavelength was used for the laser with three different power densities: 44.1 W, 45.7 W and 47.3 W.
  • Cu/Zn disorder accounts for a large part of the voltage deficit with lateral band gap fluctuations. Thus, it is necessary to control Cu/Zn disorder in CZTS. Additionally, it is important to be able to quantify Cu/Zn disorder in the type of thin film samples used for solar cells .
  • the Raman based study of CZTS thin films showed a critical temperature (Tc) of 533 K for the order and disorder transition. At equilibrium, the material is completely disordered if the temperature is above T c . Below T c , the equilibrium ordering degree increases continuously and perfect ordering can be reached only OK.
  • the substrate temperature is 500 °C This temperature is equal to the substrate temperature used for sputtering CZTS and is also the normal growth temperature for CZTS.
  • the ordered kesterite crystal structure cannot exist and the fully disordered structure will form instead.
  • the temperature is much higher when the diode laser is applied, which results in the recrystallization of CZTS thin films. Only when the temperature drops below about 533K during post-synthesis cooling can ordering among Cu and Zn begins. The cooling rate effect was also studied. The slowly cooled (lOK/h) thin films were found to be more ordered than those that were quickly cooled (quenched in ice water) . Further optimization of the diode laser annealing procedure, including power density, substrate heating temperature and cooling rate, is needed in the future.
  • FIG. 6 shows STEM-EDS elemental mapping results for Cu(504), Sn (506), Zn (510), S (512) from the TEM specimens of the annealed sample 650 and as-deposited sample 600.
  • a ZnS can be observed and the total thickness of the ZnS and CZTS bilayer has not changed after annealing.
  • the ZnS layer is tightly packed over the CZTS layer and Cu, Zn, Sn, S elements are uniformly distributed in the CZTS film.
  • some voids are present inside the ZnS layer and are adjacent to the interface of CZTS and ZnS.
  • the diode laser annealing technique proposed by the Applicants is one kind of millisecond annealing that is in thorough use in the microelectronics industry as a replacement for RTA on silicon wafer substrates.
  • the diode laser annealing process heats CZTS thin film over a much shorter time and to a higher temperature than traditional post-sulfurization annealing procedures while allowing the substrate to remain at a much lower temperature and ramp to their peak temperature at a lower rate. In tandem solar cell application, this can prevent the lower cell from being overheated and inducing damage.
  • the fast diode laser annealing process substantially reduces the annealing duration from several tens of minutes to a few microseconds which can greatly reduce the thermal budget. Moreover, more than 90% of the laser energy is absorbed by the CZTS thin film rather than the substrate so the glass substrate can be still kept at a low temperature. CZTS thin films can be heated up quickly to high annealing temperatures, allowing for the removal of the defects at grain boundaries due to crystallographic relaxation, dislocation annihilation through
  • EQE plots of laser annealed samples with a CdS capping layer compared to a reference sample All laser annealed samples have a higher J sc than the reference.
  • the better collection efficiency in the long wavelength range of 550 nm to 800 nm in EQE indicates an increase in either/both depletion region or/and diffusion length.
  • the decrease in V 0 c contributes to two possible reasons. One is the decrease of the bandgap caused by Cd diffusion into the absorber from CdS layer during the annealing process. The other is the damage induced by laser power to the interface of CdS/CZTS hetero-junction.
  • Figure 8 shows the cross-sectional TEM-EDS mapping of the reference sample and the sample received 34 J/cm 2 laser annealing. No decomposition or changing in the element composition after the laser annealing comparing to the reference, even for the sample with highest laser dose investigated .
  • Table 2 shows the result of cell performance of sample received laser annealing with reference cell (without laser treatment) as a comparison.
  • FIG 11 there are shown Raman spectra obtained using a near-resonant excitation (785 nm) Raman measurement.
  • Figure 11 presents the 785 nm excitation Raman result of the reference sample and sample annealed with 24 J/cm 2 , 34 J/cm 2 and 44 J/cm 2 laser dose.
  • Figure 12 shows Raman spectra obtained using a 785 nm excitation with peak fitting of laser annealed samples and reference annealed with laser dose 24 J/cm 2 (a), 34 J/cm 2 (b) , 44 J/cm 2 (c) and reference sample (d) .
  • the Applicant fitted the Raman spectra using 10 symmetric Lorentz peaks by Renishaw's WiRE (Windows-based Raman Environment) software. All of the peaks correspond to kesterite modes reported in the literature.
  • the Q value increased from

Abstract

The present disclosure provides a method for forming a photovoltaic cell and a photovoltaic cell with a copper- based light-absorbing layer. The method comprises depositing an intermediate layer between the conductive substrate and the light-absorbing layer and depositing a capping layer arranged over at least a portion of the light-absorbing layer. The copper-based light-absorbing layer is exposed to a continuous wave laser light through the capping layer. The intermediate layer is arranged to minimise the energy transferred from the laser light to the conductive substrate and the capping layer is arranged to affect the energy distribution of the laser light through the light-absorbing layer.

Description

A METHOD OF MANUFACTURING A PHOTOVOLTAIC DEVICE
Field of the Invention
The present invention generally relates to a method for manufacturing a photovoltaic device. In particular, the present invention relates to a method for forming an absorbing material in a thin film photovoltaic device.
Background of the Invention
Advances in thin film photovoltaics have led to the in popularity of thin film based solar devices such
CdTe based devices .
One of the drawbacks of CdTe photovoltaic devices is related to the toxicity of Cd which may cause problems once the devices degrade or need to be disposed.
Other thin film photovoltaic technologies have been investigated in recent years to design photovoltaic devices which do not contain toxic materials and, at the same time, can provide competitive market performance.
Solar cells with a copper based chalcogenide light- absorbing material represent an important advancement in thin film photovoltaics technology. Kesterite (CZTS, CZTSe or CZTSSe) based thin film solar cells, for example, use earth abundant materials and inexpensive fabrication techniques. Kesterite is a quaternary compound constituted by copper (Cu) , zinc (Zn) , tin (Sn) and sulphur (S) or selenium (Se). Kesterite has the chemical formula
Cu2ZnSn(S, Se)4. Depending on whether the last element is sulphur or selenium the acronyms CZTS or CZTSe are used for Kesterite. By mixing CZTS and CZTSe, absorbers with a direct bandgap, tunable between ~1. OeV and ~1.5eV and a large absorption coefficient can be formed. These
properties are ideal for a thin film photovoltaic device absorber .
Current kesterite photovoltaic devices are realised on soda lime glass substrates coated with a molybdenum (Mo) layer which functions as a back contact. Generally, a CZTS(Se) absorber layer is formed by annealing a material containing the precursor elements for CZTS(Se) . This material is usually deposited using PVD, CVD techniques or solution techniques. A front contact consisting of a ZnO/AZO, ITO, BZO layer and a metallic material is normally realised on the absorber layer. Generally, kesterite photovoltaic devices also have a CdS
intermediate layer between the absorber layer and the front contact .
Although it is widely acknowledged that kesterite solar cells could potentially perform better than other thin film photovoltaic technologies, the current performance of these devices is still below the market average. Record efficiencies of kesterite based solar cells have been reported between 8% and 12.6% compared to, for example, 21.7% for Cu(In, Ga)Se2 (CIGSe) thin film solar cells.
The annealing process used to form the absorber layer in kesterite solar cells has been traditionally performed by sulfurisation of a solid state precursor layer in an annealing furnace. This technique has shown problems associated with the thermal stability of soda lime glass substrates, which limits the maximum possible temperature used by conventional furnace or rapid thermal annealing furnaces. The long-time chalcogen annealing is also detrimental to the free carrier transportation due to the increasing of cell's series resistance. Furthermore, during furnace annealing precursors may decompose and react with the back contact layer.
Summary of the Invention
In accordance with the first aspect, the present invention provides a method for forming a photovoltaic cell
comprising the steps of: providing a substrate comprising a first conductive layer; depositing an intermediate layer over at least a portion of the conductive layer, the intermediate layer being arranged to be electrically coupled with the first conductive layer; forming a copper-based light-absorbing layer over at least a portion of the intermediate layer; depositing a capping layer arranged over at least a portion of the copper-based light-absorbing layer; exposing the copper-based light-absorbing layer to a continuous wave laser light through the capping layer; depositing a second conductive layer arranged to be electrically coupled with the copper-based light- absorbing layer over at least a portion of the copper-bas light-absorbing layer; wherein the intermediate layer is arranged to minimise the energy transferred from the laser light to the first conductive layer and the capping layer is arranged to affect the energy distribution of the laser light through the copper-based light-absorbing layer.
In an embodiment, the first layer comprises a metallic material or transparent conductive layer.
In an embodiment, the intermediate layer and the laser wavelength are selected such that at least 90% of the energy of the laser light that reaches a surface of the intermediate layer is absorbed by the intermediate layer
In an embodiment, the intermediate layer has a light transmission lower than 10% at a frequency of the continuous wave laser light.
In an embodiment, intermediate layer comprises a metalli material .
In an embodiment, the intermediate layer has a thickness comprised between lOnm and lOOnm.
In an embodiment, the intermediate layer comprises one o a combination of Αΐ2θχ, T1B2, TiN, and C.
In accordance with the secori' pect, the present invention provides a method forming a photovoltaic cell comprising the steps of providing a transparent substrate comprising a first transparent conductive layer; forming a copper-based light-absorbing over at least a portion of the first transparent conductive layer; depositing a capping layer arranged over at least a portion of the copper-based light-absorbing layer; exposing the copper-based light-absorbing layer to a continuous wave laser light through the capping layer; depositing a second conductive layer arranged to be electrically coupled with the copper-based light- absorbing layer over at least a portion of the copper-based light-absorbing layer; wherein the capping layer is arranged to affect the energy distribution of the laser light through the copper-based light-absorbing layer.
In an embodiment, the transparent substrate comprises a TCO covered glass or polymeric substrate or a silicon substrate .
In an embodiment, the step of forming a copper-based light-absorbing layer comprises the steps of: depositing a plurality of precursors to form a solid state layer of precursors, at least one of the precursors comprising a copper-based material; and annealing a solid state layer of precursors; wherein the annealing of the solid state layer of precursors occurs during the step of exposing the copper- based light-absorbing layer to a continuous wave laser light through the capping layer. In an embodiment, the solid state layer of precursors is exposed to the continuous wave laser light through the capping layer.
In an embodiment, the step of exposing the copper-based light-absorbing layer to a continuous wave laser light through the capping layer is performed in a manner such that electrically active defects present in the copper- based light-absorbing layer are annealed while the copper- based light-absorbing layer is formed. In an embodiment, the method further comprises the step of, prior to exposing depositing a second conductive layer, at least partially removing the capping layer.
In an embodiment, the intermediate layer is arranged in a manner such that at least 90% of the energy of the laser light that reaches a surface of the intermediate layer is absorbed by the intermediate layer.
In an embodiment, the capping layer has a light
transmission higher than 90% at the wavelength of the continuous wave laser light. In an embodiment, the capping layer comprises one or a combination of ZnS, Si02, Sn02, CdS, In2S3, CdxZn ( i-X) OxS .
In an embodiment, the capping layer has a thickness comprised between lOnm and 200nm.
In an embodiment, the capping layer has a bandgap arranged to minimise chemical reactions with the light absorbing layer. The capping layer may be also arranged to improve light trapping of the laser light and therefore optimise the efficiency of the laser annealing process. In an embodiment, during the step of exposing the copper- based light-absorbing layer to a continuous wave laser light, the copper-based light-absorbing layer incurs a transition from a first solid phase to a second solid phase .
In an embodiment, during the step of exposing the copper- based light-absorbing layer to a continuous wave laser light at least one of the copper-based light-absorbing layer temporary enters a liquid phase.
In an embodiment, the copper based light-absorbing material comprises defects and the method further
comprises the step of adjusting one or more parameters of the continuous wave laser light to control the volume density of the defects in the light-absorbing material.
In an embodiment, the volume density of defects is controlled while the copper-based light-absorbing material is formed.
In an embodiment, the method further comprises the step of adjusting one or more parameters of the continuous wave laser light to influence the spatial distribution of the defects in the light-absorbing material.
In an embodiment, the intermediate layer comprises defects and the method further comprises the step of adjusting one or more parameters of the continuous wave laser light to influence the volume density of the defects in the intermediate layer.
In an embodiment, the continuous wave laser light has a line-focused beam capable of covering the entire
longitudinal extension of the copper-based light-absorbing layer and expose the copper-based light-absorbing layer by moving relative to the photovoltaic cell in a single direction .
In an embodiment, the continuous wave laser is scanned across the copper-based light-absorbing layer with a scanning speed in the range of 100-3000 mm/min providing exposure times between 180 to 6 ms .
In an embodiment, the copper based light-absorbing layer is a kesterite based layer.
In an embodiment, the copper based light-absorbing layer is a copper-zinc-germanium-tin-chalcogenide based layer
In an embodiment, the copper based light-absorbing layer is a silver-copper-zinc-tin-chalcogenide based layer.
In an embodiment, the second conductive layer comprises cadmium sulphide, In2S3, CdxZn(i-x)OxS, Zn(0,S) .
In accordance with the third aspect, the present invention provides a photovoltaic cell manufactured in accordance with the method of the first or second aspect.
Advantages of embodiments of the inventions include the reduced annealing duration from several tens of minutes to a few microseconds which can greatly reduce the thermal budget .
The high-power line-focus CW diode laser used in some embodiments can greatly shorten the processing time comparing with conventional RTP process and is a suitable tool to fulfil the requirement for large area processing of thin film. Diode laser annealing has shown improved quality of kesterite absorber, which offers a scalable higher throughout version of single junction thin film and multi-j unction solar cells technology.
Further, the intermediate layer allows reducing the energy transferred from the laser light to the first conductive layer. At the same time, the capping layer allows
controlling and optimising the path of the laser light through the solid state layer of precursors.
Brief Description of the Drawings
Features and advantages of the present invention will become apparent from the following description of
embodiments thereof, by way of example only, with
reference to the accompanying drawings in which:
Figures 1 and 2 are flow diagrams outlining basic steps required to manufacture a photovoltaic device in
accordance with embodiments;
Figure 3 schematically shows device being fabricated using methods in accordance with embodiments;
Figures 4 and 5 show Raman spectra of CZTS layers as- deposited and annealed at different currents in accordance with embodiments;
Figure 6 shows STEM-EDS maps for Cu, Zn, Sn, and S acquired from CZTS layers as-deposited and annealed at different currents in accordance with embodiments;
Figures 7 and 9 shows EQE plots of laser annealed samples with a CdS capping layer and A10x intermediate layer respectively, compared to reference samples; Figure 8 shows cross-sectional TEM-EDS mapping of
reference sample (a) and laser annealed CZTS sample with laser dose of 34J/cm2 (b) ;
Figure 10 shows a steady state PL result (a) and the FWHM of PL peaks (b) of laser annealed samples with A10x barrier layer and reference sample for comparison; and
Figure 11 and 12 show Raman spectra and a peak analysis performed using a 785 nm excitation on laser annealed samples and a reference samples . Detailed Description of Embodiments
Embodiments of the present invention relate to a method for manufacturing a thin film photovoltaic device.
Embodiments relate to an annealing method for forming an absorbing material of a thin film photovoltaic device. In particular, embodiments of the method allow improving the quality of CZTS epilayer using a novel laser annealing technique .
Referring now to figure 1, there is shown a flow diagram 100 with steps used to manufacture a photovoltaic device in accordance with embodiments. The steps of method 100 allow forming a device on a rigid substrate, such as stainless steel, or a flexible polymeric substrate. At step 102, a substrate comprising a first conductive layer is provided. An intermediate 'barrier' layer is then deposited on the substrate at step 110. The intermediate layer is in electrical contact with the conductive layer.
The absorption coefficient and the thickness of the intermediate layer are designed so that at least 90% of the laser light energy that reaches the intermediate layer is absorbed so that the energy is not transferred to the conductive portion of the substrate. Generally, depending on the material, the intermediate layer has a thickness comprised between lOnm and lOOnm.
Subsequently, at step 120, a plurality of precursors comprising a copper-based material is deposited on the intermediate layer to form a copper-based light-absorbing layer. The solid state layer of precursors can be
sulphurised and a capping layer is then deposited on the layer of precursors, step 130. The precursors are then exposed to a continuous wave laser light through the capping layer to form a CZTS layer, step 140. In some embodiments a furnace annealing step is performed prior to laser annealing to form CZTS layer and laser annealing step 140 is used to improve the properties of the CZTS layer. The device comprising the substrate, the
intermediate layer and the precursors is pre-heated before performing the laser annealing step (140) in order to reduce the thermal mismatch with the CZTS absorbing layer. In some embodiments, the deposited capping layer is chemically removed after laser annealing, in alternative embodiments the capping layer dissolves during laser annealing. After removal of the capping layer a second conductive layer is deposited on the CZTS light absorbing layer, step 150. The second conductive material may comprise CdS, In2S3, CdxZn(i-x)OxS, Zn(0,S) . Subsequently, a top contacting structure is formed to contact the
photovoltaic device. One of the advantages provided by the intermediate layer or 'barrier' layer deposited between the metallic part of the substrate and the CZTS light absorbing layer in method 100 is the reduction of energy transferred from the laser light to the first conductive layer. The capping layer, on the other end, allows controlling and optimising the path of the laser light through the solid state layer of precursors.
Referring now to figure 2, there is shown a flow diagram 200 with steps used to manufacture a photovoltaic device in accordance with an alternative embodiment. Method 200 allows fabricating a thin film photovoltaic device on a transparent substrate with a transparent conductive layer, such as a TCO covered glass or polymeric substrate. There are a number of advantages associated with depositing the device on a transparent substrate, for example, light may be absorbed by the device through the substrate. In method 200, the layer of precursors is directly deposited onto the conductive portion of the substrate and sulphurised, step 220. A capping layer is deposited at step 230 and the layer of precursors is annealed through the capping layer using a laser (step 240) in a similar manner as in method 100. Method 200 does not permit the use of an intermediate layer between the substrate and the CZTS light absorbing layer. Therefore the annealing step (240) has to be performed in different conditions to annealing step 140.
In some embodiments of the method, during step 140 the precursors incur a transition from a first solid phase to a second solid phase. One or more parameters of the continuous wave laser light can be adjusted to control the volume density or spatial distribution of the defects in the light-absorbing material during the transition. In addition, the parameters of the continuous wave laser light can be tuned to influence the volume density of the defects in the intermediate layer.
In the methods described above, the laser is used to anneal the CZTS layer and reduce the density of defect in the grain so that the quality of the material can be improved. The investigation on the phase equilibrium in the Cu2S-ZnS-SnS2 system showed that single phase CZTS crystals can only be grown in a very narrow region.
Therefore, during the laser annealing process, the input laser power is tuned to heat up the CZTS film below the melting point (990°C of CZTS) to avoid decomposition and forming of second phases. As a result, in advantageous embodiments, the laser annealing process involves only solid-solid phase transition.
Referring now to figure 3, there are shown two devices 300 and 350 being fabricated in accordance with methods 100 and 200 respectively. Both devices are shown while the layer of precursors is being annealed. A silicon substrate 302 comprising a conductive molybdenum layer 304 is used to fabricate the device. A layer of Al2Ox is used as intermediate layer. In alternative embodiments, the intermediate layer can be made of TiN, C and TiB2.
Device 350 instead is deposited on a glass substrate 352 covered with a TCO layer 354. In both figure 3(a) and figure 3 (b) a sulphurised layer of precursors 308 is being annealed by scanning an 808 nm line-focused CW diode laser beam with the dimension of 12x0.27 mm2 (FWHM) to improve the quality of CZTS epi-layers. The laser annealing is performed through a 150 nm thick ZnS capping layer 310, which is sputtered on the layer 308. One of the advantages of ZnS layer 310 is the protection from oxidation, decomposition and evaporation provided to layer 308. Layer 310 is removed by diluted hydrochloric acid after the laser annealing is performed. In alternative embodiments, layer 310 can be made of Si02, Sn02, CdS, In2S3, CdxZn(i- x) OxS .
The annealed portion 316 of layer 308 has improved structural and electrical properties. In particular, portion 316 has a lower concentration of structural and electrical defects in comparison to the remaining portions of layer 318.
Portion 316 has a size similar to the width of laser beam 314. In some alternative embodiments, the laser beam may have a line-focused beam capable of covering the entire longitudinal extension of layer 308 and expose the entire layer 308 by moving once along the layer in a single direction .
The ZnS capping layer 310 is designed to transmit at least 90% of the continuous wave laser light and allows
minimising chemical reactions with the CZTS absorbing layer 308. In addition to this, ZnS capping layer 310 is designed to improve light trapping of the laser light and therefore optimise the efficiency of the laser annealing process. The refractive index of the ZnS is about 2.3, which is lower than CZTS of 2.6. As a result, the ZnS can act as an ARC layer during the laser annealing process.
Devices were fabricated by the Applicants to investigate the effects of the laser power density, bearn speed and substrate temperature on the annealed CZTS absorbing layer 308. The laser annealing steps were completed in air without S incorporation and the samples were naturally cooled after laser treatment.
Referring now to figure 4, there is shown a plot with Raman curves of CZTS layers as-deposited and annealed with different laser powers. The samples were annealed with a fixed substrate heating temperature of 500 °C, scan velocities of 400mm/min and exposure times of 40ms, a 514 nm excitation wavelength was used for the laser with three different power densities: 44.1 W, 45.7 W and 47.3 W.
All of the spectra are characterized by the presence of four main peaks at about 257, 288, 338 and 370 cm-1, which are from CZTS. After laser annealing, the FWHM of the main peak at 338 cm-1, i.e., dominating A mode (miA) , decreases, indicating the improvement in quality of the CZTS epi- layers . Furthermore, the peak at 288 cm-1, i.e., secondary A mode (ΙΪΙ2Α) , is more distinguishable, especially in the spectrum of CZTS annealed at laser power of 45.7 W, which has not been observed in our previous research on post- annealing of CZTS thin films. The variation in m2A after annealing is related to the decrease of Zn/Cu disorder, i.e., the overall random distribution of Cu and Zn at both Cu and Zn sites in the z=l/4 and z=3/4 planes.
Currently, theoretical and experimental investigations have highlighted the possible negative effects of Cu/Zn disorder on CZTS-based solar cell performance. Cu/Zn disorder accounts for a large part of the voltage deficit with lateral band gap fluctuations. Thus, it is necessary to control Cu/Zn disorder in CZTS. Additionally, it is important to be able to quantify Cu/Zn disorder in the type of thin film samples used for solar cells . The intensity ratio between peaks at 288 (ΙΪΙ2Α) and 303 cm-1 (m3A) (Q = I m2A/I m3A) can be regarded as an order
parameter to estimate the magnitude of random distribution of Cu and Zn. The higher the Q factor, the lower the ability of Cu(Zn) atoms to occupy Zn(Cu) atomic sites in a random way. The Applicants have measured the samples using a 785nm laser excitation line to quantify the disorder- order change . Referring now to figure 5, there are shown Raman spectra 500 and 550 of CZTS layers that were as-deposited and annealed with a laser power of 45.7 W. The Raman spectra were fitted using nine symmetric Lorentz-Gaussian peaks by Renishaw's WiRE (Windows-based Raman Environment)
software. All of the peaks correspond to reported
kesterite modes. The Q value was increased from 0.94 to 1.51 and the FWHM of miA decreased from 11.8 to 8.7 cm-1, suggesting a decrease in Cu/Zn disorder and improved symmetry of the crystalline structure. The Raman based study of CZTS thin films showed a critical temperature (Tc) of 533 K for the order and disorder transition. At equilibrium, the material is completely disordered if the temperature is above Tc. Below Tc, the equilibrium ordering degree increases continuously and perfect ordering can be reached only OK. In this laser annealing process, the substrate temperature is 500 °C This temperature is equal to the substrate temperature used for sputtering CZTS and is also the normal growth temperature for CZTS.
Accordingly, in this situation, the ordered kesterite crystal structure cannot exist and the fully disordered structure will form instead. The temperature is much higher when the diode laser is applied, which results in the recrystallization of CZTS thin films. Only when the temperature drops below about 533K during post-synthesis cooling can ordering among Cu and Zn begins. The cooling rate effect was also studied. The slowly cooled (lOK/h) thin films were found to be more ordered than those that were quickly cooled (quenched in ice water) . Further optimization of the diode laser annealing procedure, including power density, substrate heating temperature and cooling rate, is needed in the future.
Referring now to figure 6, there are shown cross sections of samples probed by STEM-EDS. Figure 6 shows STEM-EDS elemental mapping results for Cu(504), Sn (506), Zn (510), S (512) from the TEM specimens of the annealed sample 650 and as-deposited sample 600. In Figure 6, a ZnS can be observed and the total thickness of the ZnS and CZTS bilayer has not changed after annealing. In the as- deposited sample 600, the ZnS layer is tightly packed over the CZTS layer and Cu, Zn, Sn, S elements are uniformly distributed in the CZTS film. In the annealed sample 650 some voids are present inside the ZnS layer and are adjacent to the interface of CZTS and ZnS. Also, in the Cu mapping 604, there is a Cu-rich neighbouring the interface between the CZTS film and the substrate. However, in the same area, there is no obvious absence of Zn, Sn and S in their elemental mapping images in 650. These observations demonstrate that ZnS is most likely to be incorporated to form a new CZTS layer which is more Zn-rich during annealing, and that the incorporation of Zn may drive Cu to gather at the interface of the CZTS. Traditional high-temperature sulfurization, such as tube furnace annealing and rapid thermal annealing (RTA) , involves the addition of toxic (N2+H2S) gas and sulphur vapour, and the entire substrate must be heated to the required temperature. This places stricter requirements on the substrate material that has to withstand such
temperatures and thus limits the maximum available annealing temperature. The diode laser annealing technique proposed by the Applicants is one kind of millisecond annealing that is in thorough use in the microelectronics industry as a replacement for RTA on silicon wafer substrates. The diode laser annealing process heats CZTS thin film over a much shorter time and to a higher temperature than traditional post-sulfurization annealing procedures while allowing the substrate to remain at a much lower temperature and ramp to their peak temperature at a lower rate. In tandem solar cell application, this can prevent the lower cell from being overheated and inducing damage.
The fast diode laser annealing process substantially reduces the annealing duration from several tens of minutes to a few microseconds which can greatly reduce the thermal budget. Moreover, more than 90% of the laser energy is absorbed by the CZTS thin film rather than the substrate so the glass substrate can be still kept at a low temperature. CZTS thin films can be heated up quickly to high annealing temperatures, allowing for the removal of the defects at grain boundaries due to crystallographic relaxation, dislocation annihilation through
crystallographic glide planes, and microtwins elimination. With the reduction and even removal of these
crystallography defects, the structural, electronic qualities and associated solar cell performance of the kesterite thin films are expected to be improved.
CdS capping layer
Devices with a 50 nm CdS layer deposited by CBD (chemical bath deposition) were fabricated by the Applicants. The CdS allows protecting the CZTS absorber from oxidation, decomposition and evaporation. Table 1 shows cell
performance of samples that received laser annealing compared with a reference cell (without laser treatment). The samples with 24 J/cm2 and 29.4 J/cm2 laser annealing archived higher cell efficiency of 5.6% and 4.93% compare with 4.55% of the reference. The increase in cell
efficiency mainly contributes to the improvement in FF and Jsc despites that the V0c shows a decrease compare with the reference sample.
Figure imgf000020_0001
Table 1 Device parameters of laser annealed samples with CdS capping layer
Referring now to figure 7, there are shown EQE plots of laser annealed samples with a CdS capping layer compared to a reference sample. All laser annealed samples have a higher Jsc than the reference. The better collection efficiency in the long wavelength range of 550 nm to 800 nm in EQE indicates an increase in either/both depletion region or/and diffusion length. The decrease in V0c contributes to two possible reasons. One is the decrease of the bandgap caused by Cd diffusion into the absorber from CdS layer during the annealing process. The other is the damage induced by laser power to the interface of CdS/CZTS hetero-junction.
Figure 8 shows the cross-sectional TEM-EDS mapping of the reference sample and the sample received 34 J/cm2 laser annealing. No decomposition or changing in the element composition after the laser annealing comparing to the reference, even for the sample with highest laser dose investigated . ΑΙΟχ intermediate layer
Devices with a sputtered 3nm A10x layer between the Mo back contact layer and the CZTS precursor were fabricated by the Applicants . No capping layer was used for these samples and different laser dose was tested by changing the output power the laser beam.
Table 2 shows the result of cell performance of sample received laser annealing with reference cell (without laser treatment) as a comparison. The samples with 24 J/cm2 and 34 J/cm2 laser annealing archived higher cell
efficiency of 7.14% and 7.33% compare with 6.74% of the reference .
Laser Dose Voc FF Jsc Efficiency
[J/ cm2] [mV] [%] [mA/cm2] [%] 24.50 682.9 54.13 19.31 7.14
34.30 664.1 55.99 19.72 7.33
44.10 486.5 44.37 17.50 3.78 reference 674.24 55.51 18.00 6.74
Referring now to figure 9, there are shown EQE plots of laser annealed samples with an A10x intermediate layer compared to a reference sample. All laser annealed samples have a higher Jsc than the reference. The boost in efficiency mainly contributes to the increase in Jsc and the EQE gives more insight of the current enhancement due to the significant improvement at the long wavelength.
Steady State PL and Reman measurement
The Applicants have performed steady state room
temperature PL and Resonance Raman measurement on finished device of samples have received laser annealing. Results are shown in figure 10(a) which shows the steady state PL result of laser annealed samples with different laser dose. Comparing with the reference sample, laser annealed samples with lower dose have narrower PL peak. The full width half maximum of each sample is determined and showed in figure 10 (b) . The figure shows a clear trend which indicates a sharper absorption edge and less severe bandgap fluctuation for the laser annealed samples.
Referring now to figure 11, there are shown Raman spectra obtained using a near-resonant excitation (785 nm) Raman measurement. Figure 11 shows that the intensity ratio between peaks at 288 (m2A) and 303 cm"1 (m3A) (Q = I m2A/I m3A) can be regarded as an order parameter to estimate the magnitude of random distribution of Cu and Zn. The higher the Q factor, the lower the ability of Cu (Zn) atoms to occupy Zn (Cu) atomic sites in a random way. Figure 11 presents the 785 nm excitation Raman result of the reference sample and sample annealed with 24 J/cm2, 34 J/cm2 and 44 J/cm2 laser dose.
Figure 12 shows Raman spectra obtained using a 785 nm excitation with peak fitting of laser annealed samples and reference annealed with laser dose 24 J/cm2 (a), 34 J/cm2 (b) , 44 J/cm2 (c) and reference sample (d) . The Applicant fitted the Raman spectra using 10 symmetric Lorentz peaks by Renishaw's WiRE (Windows-based Raman Environment) software. All of the peaks correspond to kesterite modes reported in the literature. The Q value increased from
1.08 to 1.48 at 24 J/cm2 and 1.17 at 34 J/cm2, besides, the FWHM of mlA decreased from 6.65 cm-1 to 6.13 cm-1 at 24 J/cm2 and 6.35 cm-1 at 34 J/cm2, suggesting a decrease in Cu/Zn disorder and improved symmetry of the crystalline structure.
The term "comprising" (and its grammatical variations) as used herein are used in the inclusive sense of "having" or "including" and not in the sense of "consisting only of".
It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are,
therefore, to be considered in all respects as
illustrative and not restrictive.

Claims

The Claims as defined in the invention are as follows :
1. A method for forming a photovoltaic cell comprising the steps of: providing a substrate comprising a first conductive layer; depositing an intermediate layer over at least a portion of the conductive layer, the intermediate layer being arranged to be electrically coupled with the first conductive layer; forming a copper-based light-absorbing layer over at least a portion of the intermediate layer; depositing a capping layer arranged over at least a portion of the copper-based light-absorbing layer; exposing the copper-based light-absorbing layer to a continuous wave laser light through the capping layer; depositing a second conductive layer arranged to be electrically coupled with the copper-based light- absorbing layer over at least a portion of the copper-based light-absorbing layer; wherein the intermediate layer is arranged to minimise the energy transferred from the laser light to the first conductive layer and the capping layer is arranged to affect the energy distribution of the laser light through the copper-based light-absorbing layer.
2. The method of any one of the preceding claims, wherein the first conductive layer comprises a metallic material or a transparent conductive material.
3. The method of any one of the preceding claims wherein the intermediate layer and the laser wavelength are selected such that at least 90% of the energy of the laser light that reaches a surface of the intermediate layer is absorbed by the intermediate layer. . The method of any one of the preceding claims wherein the intermediate layer has a light transmission lower than 10% at a frequency of the continuous wave laser light.
5. The method of any one of the preceding claims wherein the intermediate layer comprises a metallic material.
6. The method of any one of the preceding claims wherein the intermediate layer has a thickness between lOnm and lOOnm.
7. The method of any one of the preceding claims wherein the intermediate layer comprises one, or a combination, of Al2Ox, TiE>2 , TiN, and C.
8. A method for forming a photovoltaic cell comprising the steps of: providing a transparent substrate comprising a first transparent conductive layer; forming a copper-based light-absorbing layer over at least a portion of the first transparent conductive layer; depositing a capping layer arranged over at least a portion of the copper-based light-absorbing layer; exposing the copper-based light-absorbing layer to a continuous wave laser light through the capping layer; depositing a second conductive layer arranged to be electrically coupled with the copper-based light- absorbing layer over at least a portion of the copper-based light-absorbing layer; wherein the capping layer is arranged to affect the energy distribution of the laser light through the copper-based light-absorbing layer.
9. The method of claim 8 wherein the transparent substrate comprises a TCO covered glass or a polymeric substrate or a silicon substrate.
10. The method of any one of the preceding claims wherein the step of forming a copper-based light-absorbing layer comprises the steps of: depositing a plurality of precursors to form a solid state layer of precursors, at least one of the precursors comprising a copper-based material; and annealing a solid state layer of precursors; wherein the annealing of the solid state layer of precursors occurs during the step of exposing the copper- based light-absorbing layer to a continuous wave laser light through the capping layer.
11. The method of any one of the preceding claims wherein the solid state layer of precursors is exposed to the continuous wave laser light through the capping layer.
12. The method of any one of the preceding claims wherein the capping layer is arranged to minimise chemical reaction of the precursors with air and evaporation of the precursors .
13. The method of any one of the preceding claims wherein the capping layer is arranged such that at least a portion of the capping layer evaporates during the exposure step.
1 . The method of any one of the preceding claims wherein the step of exposing the copper-based light-absorbing layer to a continuous wave laser light through the capping layer is performed in a manner such that electrically active defects present in the copper-based light-absorbing layer are annealed while the copper-based light-absorbing layer is formed.
15. The method of any one of the preceding claims wherein the method further comprises the step of, prior to depositing a second
removing the cappin'
16. The meth'od of a
the intermediate la
at least 90% of the
a surface of the in
intermediate layer .
17. The method of any one of the preceding claims wherein the capping layer has a light transmission higher than 90¾ at the wavelength of the continuous wave laser light.
18. The method of any one of the preceding claims wherein the capping layer comprises one or a combination of ZnS, Si02, Sn02, CdS, In2S3, CdxZn(1_x)OxS .
19. The method of any one of the preceding claims wherein the capping layer has a thickness comprised between lOnm 200nm. of the precedi
ged to improve
se the efficie
annealing process.
21. The method of any one of the preceding claims wherein during the step of exposing the copper-based light- absorbing layer to a continuous wave laser light, the copper-based light-absorbing layer incurs a transition from a first solid phase to a second solid phase.
22. The method of any one of the preceding claims wherein during the step of exposing the copper-based light- absorbing layer to a continuous wave laser light at least one of the copper-based light-absorbing layers temporary enters a liquid phase .
23. The method of any one of the preceding claims wherein the copper based light-absorbing material comprises defects and the method further comprises the step of adjusting one or more parameters of the continuous wave laser light to control the volume density of the defects in the light-absorbing material.
24. The method of claim 23 wherein the volume density of defects is controlled while the copper-based light- absorbing material is formed.
25. The method of claim 23 or claim 24 wherein the method further comprises the step of adjusting one or more parameters of the continuous wave laser light to influence the spatial distribution of the defects in the light- absorbing material.
26. The method of any one of the preceding claims wherein the intermediate layer comprises defects and the method further comprises the step of adjusting one or more parameters of the continuous wave laser light to influence the volume density of the defects in the intermediate layer .
27. The method of any one of the preceding claims wherein the continuous wave laser light has a line-focused beam capable of covering the entire longitudinal extension of the copper-based light-absorbing layer and expose the copper-based light-absorbing layer by moving relative to the photovoltaic cell in a single direction.
28. The method of claim 27 wherein the continuous wave laser is scanned across the copper-based light-absorbing layer with a scanning speed in the range of 100-3000 mm/min providing exposure times between 180 to 6 ms .
29. The method of any one of the preceding claims wherein the copper based light-absorbing layer is a kesterite based layer.
30. The method of any one of claims 1 to 28 wherein the copper based light-absorbing layer is a copper-zinc- germanium-tin-chalcogenide based layer
31. The method of any one of claims 1 to 28 wherein the copper based light-absorbing layer is a silver-copper- zinc-tin-chalcogenide based layer.
32. The method of any one of the preceding claims wherein the second conductive layer comprises cadmium sulphide,
In2S3, CdxZn( i_x)OxS, Zn(0,S) .
33. A photovoltaic cell manufactured in accordance with the method of any one of claims 1 to 32.
PCT/AU2016/051218 2015-12-11 2016-12-09 A method of manufacturing a photovoltaic device WO2017096434A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
AU2015905135 2015-12-11
AU2015905135A AU2015905135A0 (en) 2015-12-11 A method of manufacturing a photovoltaic device

Publications (1)

Publication Number Publication Date
WO2017096434A1 true WO2017096434A1 (en) 2017-06-15

Family

ID=59012471

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/AU2016/051218 WO2017096434A1 (en) 2015-12-11 2016-12-09 A method of manufacturing a photovoltaic device

Country Status (1)

Country Link
WO (1) WO2017096434A1 (en)

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20120193349A1 (en) * 2011-01-28 2012-08-02 Greentech Solutions, Inc. Heating layers containing volatile components at elevated temperatures
US20130065355A1 (en) * 2011-09-12 2013-03-14 Intermolecular, Inc. Laser annealing for thin film solar cells
WO2014210451A2 (en) * 2013-06-28 2014-12-31 First Solar, Inc. Method of manufacturing a photovoltaic device

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20120193349A1 (en) * 2011-01-28 2012-08-02 Greentech Solutions, Inc. Heating layers containing volatile components at elevated temperatures
US20130065355A1 (en) * 2011-09-12 2013-03-14 Intermolecular, Inc. Laser annealing for thin film solar cells
WO2014210451A2 (en) * 2013-06-28 2014-12-31 First Solar, Inc. Method of manufacturing a photovoltaic device

Similar Documents

Publication Publication Date Title
Zhuk et al. Critical review on sputter-deposited Cu2ZnSnS4 (CZTS) based thin film photovoltaic technology focusing on device architecture and absorber quality on the solar cells performance
Tao et al. 7.1% efficient co-electroplated Cu 2 ZnSnS 4 thin film solar cells with sputtered CdS buffer layers
US10109755B2 (en) Capping layers for improved crystallization
US8501526B2 (en) Synthesizing photovoltaic thin films of high quality copper-zinc-tin alloy with at least one chalcogen species
Vigil-Galán et al. Route towards low cost-high efficiency second generation solar cells: current status and perspectives
US20100319777A1 (en) Solar cell and method of fabricating the same
Mkawi et al. Substrate temperature effect during the deposition of (Cu/Sn/Cu/Zn) stacked precursor CZTS thin film deposited by electron-beam evaporation
US20120222742A1 (en) Compound thin film solar cell and method for manufacturing the same
US8709861B2 (en) Method of fabricating CIGS by selenization at high temperature
EP2504864A2 (en) Chalcogenide absorber layers for photovoltaic applications and methods of manufacturing the same
JP2011129631A (en) Method of manufacturing cis thin film solar cell
Sanchez et al. Effect of post annealing thermal heating on Cu2ZnSnS4 solar cells processed by sputtering technique
Acciarri et al. CIGS thin films grown by hybrid sputtering-evaporation method: Properties and PV performance
US8859323B2 (en) Method of chalcogenization to form high quality cigs for solar cell applications
Rawat et al. Thermal annealing induced modification on structural and optical properties of Cu2ZnSnS4 thin films for solar cell application
EP2755242B1 (en) Method for producing cigs film, and method for manufacturing cigs solar cell using same
WO2015120512A1 (en) A photovoltaic cell and a method of forming a photovoltaic cell
Li et al. Preparing Cu2ZnSnSe4 thin film solar cells based on selenium containing precursors
US20130192667A1 (en) Photovoltaic devices and method of making
EP2851961A2 (en) Method for manufacturing photoelectric conversion device
US8809105B2 (en) Method of processing a semiconductor assembly
WO2017096434A1 (en) A method of manufacturing a photovoltaic device
KR20150023103A (en) Thin film solar cell and method of fabricating the same
Nkuissi et al. Cu2ZnSnS4 thin-film solar cells
Gordillo et al. Fabrication of solar cells based on Cu2ZnSnS4 films grown with optimized chemical composition homogeneity

Legal Events

Date Code Title Description
121 Ep: the epo has been informed by wipo that ep was designated in this application

Ref document number: 16871830

Country of ref document: EP

Kind code of ref document: A1

NENP Non-entry into the national phase

Ref country code: DE

122 Ep: pct application non-entry in european phase

Ref document number: 16871830

Country of ref document: EP

Kind code of ref document: A1