Patent Description:
Photovoltaic devices are generally understood as photovoltaic cells or photovoltaic modules. Photovoltaic modules ordinarily comprise arrays of interconnected photovoltaic cells.

A thin-film photovoltaic or optoelectronic device is ordinarily manufactured by depositing material layers onto a substrate. A thin-film photovoltaic device ordinarily comprises a substrate coated by a layer stack comprising a conductive layer stack, at least one absorber layer, optionally at least one buffer layer, and at least one transparent conductive layer stack.

The present invention is concerned with photovoltaic devices comprising an absorber layer generally based on an ABC chalcogenide material, such as an ABC<NUM> chalcopyrite material, wherein A represents elements in group <NUM> of the periodic table of chemical elements as defined by the International Union of Pure and Applied Chemistry including Cu or Ag, B represents elements in group <NUM> of the periodic table including In, Ga, or Al, and C represents elements in group <NUM> of the periodic table including S, Se, or Te. An example of an ABC<NUM> material is the Cu(In,Ga)Se<NUM> semiconductor also known as CIGS. The invention also concerns variations to the ordinary ternary ABC compositions, such as copper-indiumselenide or copper-gallium-selenide, in the form of quaternary, pentanary, or multinary materials such as compounds of copper-(indium, gallium)-(selenium, sulfur), copper-(indium, aluminium)-selenium, copper-(indium, aluminium)-(selenium, sulfur), copper-(zinc, tin)-selenium, copper-(zinc, tin)-(selenium, sulfur), (silver, copper)-(indium, gallium)-selenium, or (silver, copper)-(indium, gallium)-(selenium, sulfur).

The photovoltaic absorber layer of thin-film ABC or ABC<NUM> photovoltaic devices can be manufactured using a variety of methods such as chemical vapor deposition (CVD), physical vapor deposition (PVD), spraying, sintering, sputtering, printing, ion beam, or electroplating. The most common method is based on vapor deposition or co-evaporation within a vacuum chamber ordinarily using multiple evaporation sources. Historically derived from alkali material diffusion using soda lime glass substrates, the effect of adding alkali metals to enhance the efficiency of thin-film ABC<NUM> photovoltaic devices has been described in much prior art (<NPL>).

Much prior art in the field of thin-film ABC<NUM> photovoltaic devices mentions the benefits of adding alkali metals to increase photovoltaic conversion efficiency and, of the group of alkali metals comprising elements Li, Na, K, Rb, Cs, best results have been reported when diffusing sodium from precursor layers (see for example <NPL>, or also<CIT>, or as well <CIT>). More recent prior art provides data regarding diffusion of sodium and potassium from an enamelled substrate while also mentioning that potassium is known to dope CIGS in a similar way as sodium and hinders the interdiffusion of CIGS elements during growth of the absorber layer (<NPL>). Most detailed work has usually focused on adding or supplying sodium at various stages of the thin-film device's manufacturing process. Although often listed among other alkali metals, the beneficial effects of specifically adding, in a controlled manner, very substantial amounts of potassium, possibly in combination with some amount of sodium, has been insufficiently explored in prior art (see for example page <NUM> of Rudmann, D. Section <NUM>. <NUM> of Rudmann, D. (<NUM>) underlines a less pronounced beneficial effect of potassium in comparison to that of sodium. For reference, the highest photovoltaic conversion efficiency achieved in prior art for a photovoltaic cell on a polyimide substrate, i.e. on a potassium-nondiffusing substrate, with an ABC<NUM> absorber layer where sodium is added via physical vapor deposition of NaF, is about <NUM>%, as reported in <NPL>. <CIT> teaches using a potassium-nondiffusing substrate (e.g. stainless steel) for deposition of a single metal Na, Li, or K from a vapour diffusion source.

Prior art has so far not specifically disclosed how adding, in a controlled manner, substantial amounts of potassium to layers of thin-film ABC<NUM> photovoltaic devices can, especially in combination with sodium, enable the production of a class of photovoltaic devices with superior photovoltaic conversion efficiency. Prior art does not disclose how much potassium should be comprised within devices resulting from a controlled addition. In the field of manufacturing of flexible photovoltaic devices, there is a strong need for know-how regarding the controlled addition of alkali metals since some lightweight flexible substrates such as polyimide do not comprise the alkali metals known to passively diffuse out of rigid substrates such as soda-lime glass or enamelled substrates.

Furthermore, most prior art has assumed that sodium and potassium have similar effects on absorber layer and the optoelectronic device, such as doping, passivation of grain boundaries and defects, elemental interdiffusion, the resulting compositional gradients, and observed optoelectronic characteristics such as enhanced open circuit voltage and fill factor. This assumption has hindered inventiveness with respect to controlled addition of alkali metal combinations. This invention exploits previously unexplored properties of adding specific combinations of potassium and at least one other alkali metal, such as sodium, to a thin-film optoelectronic device, and especially to its absorber layer. The invention discloses independent control of separate alkali metals during adding to layers of the optoelectronic device. Besides aforementioned effects such as doping, passivation of grain boundaries and defects, elemental interdiffusion, and observed optoelectronic characteristics such as enhanced open circuit voltage and fill factor, the invention's adding of alkali metals enables manufacturing of a thinner optimal buffer layer. This thinner optimal buffer layer results in reduced optical losses, thereby contributing to increase the device's photovoltaic conversion efficiency. This invention not only specifies a method to add potassium, but also the amount of potassium that should remain in the resulting thin-film device and, in the case sodium is also added, the ratio of potassium to sodium.

Finally, manufacturing of embodiments of photovoltaic devices on polyimide substrates according to the method, and at what a person skilled in the art would consider low and unfavorable temperatures, has resulted in a photovoltaic conversion efficiency that is greater, at filing date, than the highest ever certified using similar absorber layer technology but manufactured at the more favorable high temperature processes allowable by glass substrates. This suggests that the invention contributes a step that may overcome the need for high temperature processes or even benefit them too.

This invention presents a solution to the problem of manufacturing high efficiency thin-film photovoltaic or optoelectronic devices that comprise an ABC<NUM> chalcopyrite absorber layer, especially flexible photovoltaic devices with said absorber layer, and more precisely devices manufactured onto substrates, such as polyimide, that do not comprise within the substrate alkali metals known to augment photovoltaic conversion efficiency.

The invention presents a method for manufacturing photovoltaic devices.

A common problem in the field of thin-film photovoltaic devices relates to doping of the photovoltaic absorber layer for increased efficiency. When manufactured onto glass substrates or possibly onto substrates coated with materials comprising alkali metals, the substrate's alkali metals may diffuse into the absorber layer and increase photovoltaic conversion efficiency. In the case of substrates, such as polyimide, that do not comprise alkali metals, the alkali-doping elements must be supplied via deposition techniques such as, for example, physical vapor deposition. The alkali metals then diffuse during the deposition process within and across various thin-film layers and their interfaces.

Another problem in the field of thin-film photovoltaic devices lies at the interfaces between the absorber layer, the optional buffer layer, and the front-contact. The absorber layer's ABC<NUM> chalcopyrite crystals present substantial roughness that may require the deposition of a relatively thick buffer layer to ensure complete coverage of the absorber layer prior to deposition of the front-contact layer.

A further problem in the field of thin-film photovoltaic devices is that for some buffer layer compositions, the thicker the buffer layer, the lower its optical transmittance and therefore the lower the photovoltaic device's conversion efficiency.

Yet a further problem in the field of thin-film photovoltaic devices is that some buffer layer compositions, such as CdS, comprise the element cadmium, the quantity of which it is desirable to minimize.

Another problem in the field of thin-film photovoltaic device manufacturing is that the process for deposition of the buffer layer, such as chemical bath deposition (CBD), may generate waste. In the case of CdS buffer layer deposition the waste requires special treatment and it is therefore desirable to minimize its amount.

Yet another problem in the field of thin-film photovoltaic devices comprising a CdS buffer layer is that when the buffer layer thickness is less than about <NUM>, the photovoltaic device's fill factor and open circuit voltage are substantially lower than with photovoltaic devices having a buffer layer thickness greater than <NUM>.

Finally, yet another problem in the field of flexible thin-film photovoltaic device manufacturing is that it is desirable to benefit from large process windows for material deposition, and more specifically in relation to this invention, the process window for the adding of alkali metals and subsequent deposition of at least one buffer layer.

Briefly, the invention thus pertains to a method of fabricating thin-film photovoltaic devices comprising at least one ABC<NUM> chalcopyrite absorber layer and to adding very substantial amounts of potassium in combination with at least one other alkali metal. Said thin-film photovoltaic devices comprise - and we hereby define the term "potassium-nondiffusing substrate" - a substrate that is potassium-nondiffusing and/or comprises means, such as at least one barrier layer, that prevent diffusion of potassium from the substrate into at least said ABC<NUM> chalcopyrite absorber layer.

For the purposes of the present invention, the term "adding" or "added" refers to the process in which chemical elements, in the form of individual or compound chemical elements, namely alkali metals and their so-called precursors, are being provided in the steps for fabricating the layer stack of an optoelectronic device for any of:.

An advantageous effect of the invention is that the optimal thickness for an optional buffer layer coating said absorber layer is thinner than the optimal buffer layer needed for prior art photovoltaic devices with comparable photovoltaic efficiency. Another advantageous effect is that adding very substantial amounts of potassium in combination with at least one other alkali metal results in devices of higher photovoltaic conversion efficiency than if little or no potassium had been added. The invention contributes to shortening manufacturing process, reducing environmental impact of manufacturing and of the resulting device, and greater device photovoltaic conversion efficiency.

The invention's features may advantageously solve several problems in the field of thin-film photovoltaic devices manufacturing, and more specifically manufacturing of the absorber and buffer layer of such devices based on a potassium-nondiffusing substrate. The listed advantages should not be considered as necessary for use of the invention. For manufacturing of thin-film flexible photovoltaic devices manufactured to the present invention, the advantages obtainable over devices and their manufacturing according to prior art include:.

Embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, in which:.

In more detail, a "potassium-nondiffusing substrate" is a component, ordinarily a sheet of material, that comprises no potassium or so little potassium that diffusion of potassium elements into the subsequently described layers is considered too small to significantly alter the optoelectronic properties of the device. Potassium-non diffusing substrates also include substrates that comprise means to prevent diffusion of potassium into coatings or layers supported by the substrate. A potassium-nondiffusing substrate may for example be a substrate that has been specially treated or coated with a barrier layer to prevent diffusion of potassium elements into coatings or layers supported by the substrate. Specially treated substrates or barrier-coated substrates ordinarily prevent the diffusion of a broad range of elements, including alkali metals, into coatings or layers supported by the substrate.

For clarity, components in figures showing embodiments are not drawn at the same scale.

<FIG> presents the cross-section of an embodiment of a thin-film optoelectronic or photovoltaic device <NUM> comprising a potassium-nondiffusing substrate <NUM> for a stack of material layers wherein at least two different alkali metals, one of them being potassium, have been added.

Substrate <NUM> may be rigid or flexible and be of a variety of materials or coated materials such as glass, coated metal, plastic-coated metal, plastic, coated plastic such as metal-coated plastic, or flexible glass. A preferred flexible substrate material is polyimide as it is very flexible, sustains temperatures required to manufacture high efficiency optoelectronic devices, requires less processing than metal substrates, and exhibits thermal expansion coefficients that are compatible with those of material layers deposited upon it. Industrially available polyimide substrates are ordinarily available in thicknesses ranging from <NUM> to <NUM>. Polyimide substrates are ordinarily considered as potassium-nondiffusing.

At least one electrically conductive layer <NUM> coats substrate <NUM>. Said electrically conductive layer, or stack of electrically conductive layers, also known as the back-contact, may be of a variety of electrically conductive materials, preferably having a coefficient of thermal expansion (CTE) that is close both to that of the said substrate <NUM> onto which it is deposited and to that of other materials that are to be subsequently deposited upon it. Conductive layer <NUM> preferably has a high optical reflectance and is commonly made of Mo although several other thin-film materials such as metal chalcogenides, molybdenum chalcogenides, molybdenum selenides (such as MoSe<NUM>), Na-doped Mo, K-doped Mo, Na- and K-doped Mo, transition metal chalcogenides, tin-doped indium oxide (ITO), doped or non-doped indium oxides, doped or non-doped zinc oxides, zirconium nitrides, tin oxides, titanium nitrides, Ti, W, Ta, Au, Ag, Cu, and Nb may also be used or included advantageously.

At least one absorber layer <NUM> coats electrically conductive layer <NUM>. Absorber layer <NUM> is made of an ABC material, wherein A represents elements in group <NUM> of the periodic table of chemical elements as defined by the International Union of Pure and Applied Chemistry including Cu or Ag, B represents elements in group <NUM> of the periodic table including In, Ga, or Al, and C represents elements in group <NUM> of the periodic table including S, Se, or Te. An example of an ABC<NUM> material is the Cu(In,Ga)Se<NUM> semiconductor also known as CIGS.

Optionally, at least one semiconductive buffer layer <NUM> coats absorber layer <NUM>. Said buffer layer ordinarily has an energy bandgap higher than <NUM> eV and is for example made of CdS, Cd(S,OH), CdZnS, indium sulfides, zinc sulfides, gallium selenides, indium selenides, compounds of (indium, gallium)-sulfur, compounds of (indium, gallium)-selenium, tin oxides, zinc oxides, Zn(Mg,O)S, Zn(O,S) material, or variations thereof.

At least one transparent conductive layer <NUM> coats buffer layer <NUM>. Said transparent conductive layer, also known as the front-contact, ordinarily comprises a transparent conductive oxide (TCO) layer, for example made of doped or non-doped variations of materials such as indium oxides, tin oxides, or zinc oxides.

Contributing to this invention, the amount of potassium comprised in the interval of layers <NUM> from electrically conductive back-contact layer <NUM>, exclusive, to transparent conductive front-contact layer <NUM>, inclusive, is in the range between <NUM> and <NUM> potassium atoms per million atoms (ppm) and the amount of the other of said at least two different alkali metals is in the range of <NUM> to <NUM> ppm and at most <NUM>/<NUM> and at least <NUM>/<NUM> of the comprised amount of potassium. A thin-film photovoltaic device demonstrating superior photovoltaic conversion efficiency preferably has an amount of potassium comprised in said interval of layers <NUM> in the range between <NUM> and <NUM> potassium atoms per million atoms.

Optionally, front-contact metallized grid patterns <NUM> may cover part of transparent conductive layer <NUM> to advantageously augment front-contact conductivity. Also optionally, said thin-film photovoltaic device may be coated with at least one anti-reflective coating such as a thin material layer or an encapsulating film.

<FIG> presents a method <NUM> comprising material deposition steps to manufacture said thin-film optoelectronic or photovoltaic device <NUM> comprising a potassium-nondiffusing substrate <NUM> for a stack of material layers where at least two different alkali metals, one of them being potassium, have been added. The method is considered to be especially appropriate for substrates considered potassium-nondiffusing or that may comprise at least one barrier layer that prevents the diffusion of alkali metals from the substrate into subsequently deposited coatings. The method as described is especially advantageous for plastic substrate materials such as polyimide.

An exemplary sequence of material layer deposition follows. The purpose of this description is to clarify the context within which adding of alkali metals <NUM>, the main subject of this invention, occurs.

The method starts at step <NUM> by providing a potassium-nondiffusing substrate. Said substrate is considered as potassium-nondiffusing, according to the description provided for substrate <NUM>.

Following step <NUM> and until the step of forming front-contact layer <NUM>, adding of at least two different alkali metals <NUM>, one of them being potassium, occurs as at least one event during step (iii) <NUM> and/or between steps (iii) and (iv) <NUM>. The fact that the adding may occur during or between said interval of steps is represented by dashed arrows emanating from block <NUM> in <FIG>. Each of said alkali metals may be added simultaneously with any of the other of said alkali metals and/or during separate adding events. Adding of each of said alkali metals may comprise any or a combination of adding a layer or precursor layer of at least one of the alkali metals, co-adding at least one of the alkali metals with the forming of any of the method's material layers, or diffusing at least one of the alkali metals from at least one layer into at least one other material layer. Preferably, adding of at least one of said two different alkali metals is done in the presence of at least one said C element. More preferably, adding of potassium, for example by adding via a so-called potassium-comprising precursor such as KF, KCI, KBr, KI, K<NUM>S, K<NUM>Se, is done in the presence of at least one said C element.

At step <NUM>, forming at least one back-contact layer comprises depositing at least one electrically conductive layer. Forming of the back-contact layer may be done using a process such as sputtering, spraying, sintering, electrodeposition, CVD, PVD, electron beam evaporation, or spraying of the materials listed in the description of said electrically conductive layer <NUM>.

At step <NUM>, forming at least one absorber layer comprises coating said electrically conductive layer with at least one ABC absorber layer <NUM>. The materials used correspond to those in the description provided for ABC absorber layer <NUM>. Said absorber layer may be deposited using a variety of techniques such as sputtering, CVD or as a preferred technique for an ABC material, physical vapor deposition. Substrate temperatures during absorber layer deposition are ordinarily comprised between <NUM> and <NUM>. The range of temperatures and temperature change profiles depend on several parameters including at least the substrate's material properties, the supply rates of the materials that compose the ABC material, and the type of coating process. For example, for a vapor deposition process, substrate temperatures during forming of the absorber layer will ordinarily be below <NUM>, and if using substrates requiring lower temperatures, such as a polyimide substrate, preferably below <NUM>, and more preferably in the range from <NUM> to <NUM>. For a co-evaporation vapor deposition process, substrate temperatures during forming of the absorber layer will ordinarily be in the range from <NUM> to <NUM>. Said substrate temperatures may be advantageously used with a polyimide substrate.

For a deposition process such as physical vapor deposition, for example if forming absorber layer <NUM> is done using a physical vapor deposition process, adding of potassium as part of adding at least two different alkali metals <NUM> may be done during and/or in continuation of the physical vapor deposition process by supplying potassium fluoride, KF. This may for example be advantageous when manufacturing with a co-evaporation physical vapor deposition system. Adding the alkali metal potassium will preferably be done in the presence of a flux of element Se supplied at a rate in the range of <NUM> to <NUM>Å/s, preferably at a rate in the range of <NUM> to <NUM>Å/s.

Substrate temperatures for said adding of at least two different alkali metals will ordinarily be greater than <NUM> and less than <NUM>. Substrate temperatures will preferably be greater than <NUM> and less than <NUM>. A person skilled in the art will select appropriate temperatures for said adding of at least two different alkali metals so that they are compatible with the materials deposited, thin-film properties, and substrate. For example, one skilled in the art of physical vapor deposition processes will know that potassium, for example in the form of KF, may be added at higher temperatures than some other alkali metals such as sodium, for example in the form of NaF. The possibility of higher adding temperature for KF may advantageously be used to add alkali metals starting with potassium at temperatures closer to those used at step <NUM> and, as the substrate temperature decreases, to continue with adding of same and/or other alkali metals. A person skilled in the art will also know that adding of at least two different alkali metals may take place with adding of one or more of said at least two different alkali metals at substrate temperatures ordinarily lower than <NUM> and possibly much lower than <NUM>, such as at ambient temperatures of about <NUM> and below. The substrate may then be heated afterwards, thereby facilitating diffusing of said alkali metals to the thin-film layers of the optoelectronic device, possibly in combination with depositing at least one C element.

The amount of potassium added by adding at least two alkali metals <NUM> is such that following forming of front-contact layer <NUM> at later step <NUM>, said amount comprised in the interval of layers <NUM> from back-contact layer <NUM>, exclusive, to front-contact layer <NUM>, inclusive, is in the range between <NUM> and <NUM> potassium atoms per million atoms and the amount of the other of said at least two different alkali metals is in the range of <NUM> to <NUM> ppm and at most <NUM>/<NUM> and at least <NUM>/<NUM> of the comprised amount of potassium. A thin-film photovoltaic device that has a superior photovoltaic conversion efficiency preferably has an amount comprised in said interval of layers <NUM> from about <NUM> to <NUM> potassium atoms per million atoms.

The following steps describe how to complete the manufacture of a working photovoltaic device benefiting of the invention.

At step <NUM>, represented as a dashed box because the step may be considered optional, forming buffer layer comprises coating said absorber layer with at least one so-called semiconductive buffer layer <NUM>. The materials used correspond to those in the description provided for buffer layer <NUM>. Said buffer layer may be deposited using a variety of techniques such as CVD, PVD, sputtering, sintering, electrodeposition, printing, atomic layer deposition, or as a well known technique at atmospheric pressure, chemical bath deposition. Forming of said buffer layer is preferably followed by an annealing process, ordinarily in air or possibly within an atmosphere with controlled composition or even in vacuum, at between <NUM> and <NUM> for a duration of <NUM> to <NUM> minutes, preferably <NUM> for a duration of <NUM> minutes.

To tune the process of forming the buffer layer of step <NUM>, one skilled in the art will ordinarily develop a test suite over a range of buffer coating process durations to manufacture a range of photovoltaic devices comprising a range of buffer layer thicknesses. One will then select the buffer coating process duration that results in highest photovoltaic device efficiency. Furthermore, for the purpose of manufacturing reference devices to be considered as corresponding to prior art devices, one will prepare a range of photovoltaic devices where the step of adding at least two alkali metals <NUM> comprises alkali metals but does not comprise the amount of potassium specified in this invention and a lesser amount of the other alkali metal. Said prior art devices will be coated with said range of buffer layer thicknesses. By comparing said prior art devices with devices manufactured according to the invention, one skilled in the art will notice that the latter have substantially higher photovoltaic conversion efficiency.

At step <NUM>, forming front-contact layer comprises coating said buffer layer with at least one transparent conductive front-contact layer <NUM>. Said front-contact layer ordinarily comprises a transparent conductive oxide (TCO) layer, for example made of doped or non-doped variations of materials such as indium oxide, gallium oxide, tin oxide, or zinc oxide that may be coated using a variety of techniques such as PVD, CVD, sputtering, spraying, CBD, electrodeposition, or atomic layer deposition.

At optional step <NUM>, forming front-contact grid comprises depositing front-contact metallized grid traces <NUM> onto part of transparent conductive layer <NUM>. Also optionally, said thin-film photovoltaic device may be coated with at least one anti-reflective coating such as a thin material layer or an encapsulating film.

The steps may also comprise operations to delineate cell or module components. In the context of superstrate-based manufacturing, the order of the method's manufacturing sequence may be partly reversed in the order comprising forming optional front-contact grid <NUM>, forming front-contact layer <NUM>, forming optional buffer layer <NUM>, forming absorber layer <NUM>, adding at least two alkali metals <NUM>, and forming an electrically conductive back-contact layer.

<FIG> shows a side cross-section of a deposition zone apparatus <NUM> comprised in a section of an apparatus for manufacturing a thin-film optoelectronic or photovoltaic device comprising a potassium-nondiffusing substrate <NUM> for a stack of material layers wherein at least two different alkali metals, one of them being potassium, are being added. Deposition zone apparatus <NUM> is ordinarily comprised inside a vacuum deposition chamber for manufacturing at least the absorber layer of photovoltaic modules. An object to be coated, such as a flat panel or a flexible web, thereafter called web <NUM>, enters deposition zone apparatus <NUM>, travels according to direction <NUM> over a set of sources for forming at least one absorber layer <NUM> and at least one set of sources for adding at least two different alkali metals <NUM>, and then exits deposition zone apparatus <NUM>.

Web <NUM> comprises a substrate <NUM> coated with an electrically conductive back-contact layer, or stack of electrically conductive layers, thereafter called back-contact layer <NUM>. Said substrate, prior to being coated with said stack of electrically conductive layers, is considered as potassium-nondiffusing. For more economical roll-to-roll manufacturing, said substrate is preferably of a flexible material such as coated metal, plastic-coated metal, plastic, coated plastic such as metal-coated plastic, or metal-coated flexible glass. A preferred web polyimide coated with a conductive metal back-contact, where said back-contact layer is preferably Mo although several other thin-film materials such as non-doped, Na-doped, K-doped, Sn-doped variations of materials such as metal chalcogenides, molybdenum chalcogenides, molybdenum selenides (such as MoSe<NUM>), Mo, transition metal chalcogenides, indium oxide (ITO), indium oxides (such as In<NUM>O<NUM>), zinc oxides, zirconium nitrides, tin oxides, titanium nitrides, Ti, Cu, Ag, Au, W, Ta, and Nb may also be used or included.

The set of absorber deposition sources <NUM> comprises a plurality of sources <NUM> generating effusion plumes 331p that, in the case of a preferable co-evaporation setup, may overlap. Said set of absorber deposition sources <NUM> provides the materials to coat web <NUM> with at least one absorber layer <NUM> of ABC material.

In this description, a vapor deposition source, or source, is any device conveying material vapor for deposition onto a layer. The vapor may result from melting, evaporating, or sublimating materials to be evaporated. The device generating the vapor may be at a position that is remote from the substrate, for example providing the vapor via a duct, or near the substrate, for example providing the vapor through nozzles or slit openings of a crucible.

The set of sources for adding at least two different alkali metals <NUM> comprises at least one source <NUM> generating effusion plume 336p adding at least one of two different alkali metals to at least one of the layers of the device prior to it bearing a front-contact. Adding of said alkali metals is preferably done to said absorber layer <NUM>. At least one source <NUM> comprises potassium, preferably in the form of potassium fluoride KF. Preferably at least one source <NUM> comprises sodium, preferably in the form of sodium fluoride NaF. Sources <NUM> may provide other alkali metals, preferably as a co-evaporation setup, and effusion plumes 336p may overlap at least one of effusion plumes 331p. The set of said sources for adding alkali metals <NUM> comprises more than one source <NUM>, the source comprising potassium may be positioned such that its material is added before, at the same time, or after other alkali metals. Furthermore, said apparatus preferably comprises means to provide at least one C element within at least the part of said deposition zone where adding of potassium occurs.

The amount of potassium added by the sources for adding at least two different alkali metals <NUM> is such that, following forming of transparent front-contact layer <NUM>, said amount comprised in the interval of layers <NUM> from back-contact layer <NUM>, exclusive, to front-contact layer <NUM>, inclusive, is in the range of <NUM> to <NUM> potassium atoms per million atoms and, for the other of said at least two different alkali metals, in the range <NUM> to <NUM> ppm and at most <NUM>/<NUM> and at least <NUM>/<NUM> the comprised amount of potassium. A thin-film photovoltaic device that has a superior photovoltaic conversion efficiency preferably has an amount comprised in said interval of layers <NUM> from about <NUM> to <NUM> potassium atoms per million atoms.

The location for forming front-contact layer <NUM> is considered to be outside said deposition zone apparatus <NUM> and the means for forming said front-contact layer are therefore not represented.

<FIG> present characterization data for a set of exemplary photovoltaic device embodiments manufactured according to the method. Said devices comprise a CIGS absorber layer <NUM> and are subjected to adding at least two different alkali metals <NUM>. Said absorber layer of each exemplary device is subjected to a chemical bath deposition (CBD) for forming a CdS buffer layer <NUM>. Said CBD's have different durations so as to generate different buffer layer thicknesses and enable determination of the thickness of the buffer layer <NUM> that maximizes photovoltaic conversion efficiency. <FIG> disclose that devices manufactured according to the invention may have a higher efficiency and/or a thinner buffer layer than devices of prior art.

<FIG> is a sputter depth profiling graph plotting the counts of various elements within the optoelectronic device versus approximate sputter depth. The at least two different alkali metals plotted are potassium <NUM> and sodium <NUM>. The graph also presents data for copper <NUM>, representative of the absorber layer, of zinc <NUM>, representative of the front-contact layer, and of molybdenum <NUM>, representative of the back-contact layer. The graph shows that, at a given depth, the counts of potassium may be over an order of magnitude greater than the counts of sodium. Interval of layers <NUM> from back-contact layer <NUM>, exclusive, to front-contact layer <NUM>, inclusive, is measured from the log-scale plot's half-height of shallowest maximum of back-contact layer to half-height of shallowest maximum of front-contact layer, respectively. The sputter depth profiling graph was obtained using secondary ion mass spectrometry (SIMS). Depth profiling data were obtained with a SIMS system using O<NUM>+ primary ions with <NUM> kV ion energy, <NUM> nA, and 300x300 µm<NUM> spot. The analyzed area was <NUM>×<NUM><NUM> using Bi<NUM>+ with 25kV ion energy.

<FIG> relate to measurements of photovoltaic device external quantum efficiency (EQE) as a function of illumination wavelength. These measurements are useful for tuning the buffer layer coating process to maximize photovoltaic conversion efficiency when manufacturing photovoltaic devices.

<FIG> presents plots of EQE vs. illumination wavelength for a range of photovoltaic devices, each device having a different buffer layer thickness. EQE measurements enable calculation of current density. Buffer layer thickness increases with the duration of the step of forming the buffer layer. In <FIG>, durations of the step of forming the buffer layer range from <NUM> minutes to <NUM> minutes in <NUM> minute increments. Lines for <NUM> and <NUM> durations of the step of forming the buffer layer correspond to buffer layer thicknesses ordinarily used in prior art photovoltaic devices, such as in Chirila (<NUM>).

<FIG> presents plots of current density <NUM> and buffer layer thickness <NUM> as a function of the duration of forming a CdS buffer layer. Data for current density <NUM> are derived from EQE measurements presented in <FIG> obtained with an exemplary embodiment of a photovoltaic cell device manufactured according to the invention and illuminated at wavelengths less than <NUM> under standard test conditions (STC). Current density data is used for subsequent computation of photovoltaic conversion efficiency. <FIG> also presents measurements of CdS buffer layer thickness <NUM> as a function of chemical bath deposition (CBD) duration for forming said buffer layer. Data of <FIG> is useful in combination with <FIG>, <FIG> to illustrate that the main advantageous effect of highest photovoltaic conversion efficiency results from adding at least two different alkali metals, at least one of which is potassium in substantially large amounts according to the invention, in combination with forming a buffer layer of optimal thickness, said buffer layer being substantially thinner than buffer layers ordinarily used in photovoltaic devices manufactured according to prior art. For example, highest efficiency photovoltaic devices manufactured using CBD for forming a CdS buffer layer have a buffer layer thickness greater than about <NUM> and less than about <NUM>.

Measurement of buffer layer thickness was done using inductively coupled plasma mass spectrometry (ICPMS). For ICPMS analysis approximately <NUM><NUM> of material was detached from the thin-film solar cell at the Mo back-contact/absorber layer interface. The solid matter was directly transferred into <NUM> trace metal free polyethylene tubes and fully dissolved in a mixture of <NUM> HNO3 (<NUM>% w/w) and <NUM> HCl (<NUM>% w/w). After filling to <NUM> with <NUM> MΩ·cm deionized water, the sample was not further diluted for analysis. Metal analysis was performed on an inductively coupled plasma mass spectrometer with external calibration using certified metal standards (<NUM>µg/mL). The CdS buffer layer thicknesses are derived from atomic concentrations measured by ICPMS assuming that all measured Cd atoms are incorporated within a perfectly flat CdS layer with a density of <NUM>/cm<NUM>, and neglecting in-diffusion of Cd atoms into the absorber layer. Because some Cd in-diffusion into the absorber layer is occurring and the CdS layer is formed onto an absorber layer with a certain roughness, the actual CdS layer thickness is overestimated by this measurement technique by up to <NUM>% depending on said surface roughness and the extent of Cd in-diffusion. Therefore, the thickness determination by ICPMS provides an upper value for the actual CdS buffer layer thickness. More precise determination can be made by more expensive techniques such as for example transmission electron microscopy (TEM).

<FIG> present data ordinarily used to characterize photovoltaic devices and enable in <FIG> a comparison between durations of chemical bath depositions for forming the CdS buffer layer and in <FIG> a comparison between durations of potassium fluoride (KF) supply after forming of the absorber layer. Standard deviations over a set of photovoltaic device embodiments are indicated by vertical bars. Note that the deposition or supply durations presented are those used to manufacture a prototype device embodiment using laboratory-scale equipment. A person skilled in the art will infer that shorter durations may be obtained with industrial-level equipment. The data presented illustrates how to select the deposition or supply duration that provides highest photovoltaic conversion efficiency.

<FIG> is a graph of open circuit voltage VOC as a function of the duration of the chemical bath deposition for forming the buffer layer. The graph shows that VOC is about constant for durations ranging from about <NUM> to <NUM> minutes which, compared to plot <NUM> in <FIG>, corresponds to a buffer layer thickness ranging from about <NUM> to <NUM>.

<FIG> is a graph of current density JSC as a function of the duration of the chemical bath deposition for forming the buffer layer. The graph shows that JSC decreases for increasing deposition durations ranging from about <NUM> to <NUM> minutes which, compared to plot <NUM> in <FIG>, corresponds to a buffer layer thickness ranging from about <NUM> to <NUM>.

<FIG> is a graph of fill factor FF as a function of the duration of the chemical bath deposition for forming the buffer layer. The graph shows that FF is about constant for deposition durations ranging from about <NUM> to <NUM> minutes.

<FIG> is a graph of photovoltaic conversion efficiency as a function of the duration of the chemical bath deposition for forming the buffer layer. The graph shows that photovoltaic conversion efficiency is maximum for a deposition duration of <NUM> minutes which, compared to plot <NUM> in <FIG>, corresponds to a buffer layer thickness of about <NUM>. Photovoltaic devices manufactured according to prior art ordinarily exhibit highest photovoltaic conversion efficiency with a CdS buffer layer thicker than <NUM>. Photovoltaic devices manufactured according to the present invention therefore exhibit the advantageous effects of having both higher photovoltaic conversion efficiency and thinner buffer layer than prior art.

<FIG> are graphs of open circuit voltage VOC, current density JSC, fill factor FF, and photovoltaic conversion efficiency, respectively, as a function of the duration of potassium fluoride supply as a PVD process for adding potassium after forming the absorber layer. All devices were subjected to a <NUM> minutes CBD providing them with a buffer layer that is about <NUM>. <FIG> respectively show that VOC, JSC, FF, and photovoltaic conversion efficiency reach a maximum at about <NUM> minutes of KF supply. The values remain about constant for durations ranging from about <NUM> to <NUM> minutes.

<FIG> present exemplary substrate temperature <NUM> and supply rate <NUM>, <NUM> as a function of time during said adding of at least two different alkali metals. In this example of manufacturing a photovoltaic device on a polyimide substrate, the substrate temperature <NUM> decreases from the about <NUM> used for said forming of the absorber layer to the about <NUM> used for adding of at least two different alkali metals. Note that the supply durations and rates <NUM>, <NUM> presented are those used to manufacture a prototype device using laboratory scale equipment. A person skilled in the art will infer that shorter durations and greater supply rates <NUM>, <NUM> may be obtained with industrial-level equipment.

In the example of <FIG> the adding of at least two different alkali metals uses a physical vapor deposition process where alkali metal potassium, for example in the form of KF potassium-comprising precursor <NUM>, is supplied at a rate equivalent to an effective layer deposition ranging from about <NUM>Å/s to <NUM>Å/s, preferably <NUM>Å/s, for a duration of <NUM> minutes. Adding at least two alkali metals <NUM> is done in the presence of Se. Adding of at least one other alkali metal is not represented in <FIG> as it may, for example, be added prior to or during forming of the absorber layer.

In the example of <FIG> the adding of at least two different alkali metals uses a physical vapor deposition process where sodium, for example in the form of NaF sodium-comprising precursor <NUM>, is first added at a rate of about <NUM>Å/s for a duration of <NUM> minutes and followed, possibly as part of a co-evaporation process, by adding of potassium, for example in the form of KF potassium-comprising precursor <NUM>, at a rate of about <NUM>Å/s for a duration of <NUM> minutes. At least one adding of at least two different alkali metals is done in the presence of Se.

<FIG> respectively present EQE as a function of illumination wavelength and current density as a function of voltage for an exemplary embodiment of an optoelectronic device with high photovoltaic conversion efficiency manufactured according to the invention. Said device comprises a polyimide substrate and, between back- and front-contacts, a CIGS absorber layer subjected to adding at least K in the amounts in the range of <NUM> to <NUM> ppm and Na in the amounts in the range of <NUM> to <NUM> ppm, and a buffer layer. The device's officially certified photovoltaic efficiency is <NUM>%. The device's open circuit voltage and short circuit current under standard test conditions (STC) was measured at <NUM> mV and <NUM> mA/cm<NUM>, respectively. The fill factor is <NUM>%.

Claim 1:
A method (<NUM>) of fabricating thin-film optoelectronic devices (<NUM>) comprising at least two different alkali metals (<NUM>), the method comprising the steps:
(i) providing a substrate (<NUM>, <NUM>);
(ii) forming a back-contact layer (<NUM>, <NUM>);
(iii) forming at least one absorber layer (<NUM>, <NUM>), which absorber layer is made of an ABC chalcogenide material, including ABC chalcogenide material ternary, quaternary, pentanary, or multinary variations, wherein A represents elements of group <NUM> of the periodic table of chemical elements as defined by the International Union of Pure and Applied Chemistry including Cu and Ag, B represents elements in group <NUM> of the periodic table including In, Ga, and Al, and C represents elements in group <NUM> of the periodic table including S, Se, and Te; and
(iv) forming at least one front-contact layer (<NUM>, <NUM>)
characterized in that
the substrate (<NUM>,<NUM>) is a potassium-nondiffusing substrate; and that selectable quantities of at least two different alkali metals (<NUM>) are added during step (iii) and/or between steps (iii) and (iv) by vapour deposition, the selectable quantities of alkali metals being added from vapour deposition sources external to the potassium-nondiffusing substrate (<NUM>,<NUM>);
wherein
one of said at least two different alkali metals is potassium (K) and where, following forming said front-contact layer (<NUM>), in the interval of layers (<NUM>) from back-contact layer (<NUM>), exclusive, to front-contact layer (<NUM>), inclusive, the comprised amounts resulting from adding the selectable quantities of at least two different alkali metals are, for potassium, in the range of <NUM> to <NUM> atoms per million atoms (ppm) and, for the other of said at least two different alkali metals, in the range of <NUM> to <NUM> ppm and at most <NUM>/<NUM> and at least <NUM>/<NUM> of the comprised amount of potassium.