Cesium

Field

The present invention relates to solar cells and/or optoelectronic devices manufactured by deposition of thin-films and more particularly to adding alkali metals when forming layers or the layer stack of optoelectronic devices comprising chalcogenide semiconductors or ABC semiconductive compounds.

Background 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 ABC2 chalcopyrite material, wherein A represents elements in group 11 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 13 of the periodic table including In, Ga, or Al, and C represents elements in group 16 of the periodic table including S, Se, or Te. An example of an ABC2 material is the Cu(ln,Ga)Se2 semiconductor also known as CIGS. The invention also concerns variations to the ordinary ternary ABC compositions, such as copper-indium- selenide 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 ABC2 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 ABC2 photovoltaic devices has been described in much prior art (Rudmann, D. (2004) Effects of sodium on growth and properties of Cu(ln,Ga)Se2 thin films and solar cells, Doctoral dissertation, Swiss Federal Institute of Technology. Retrieved 2012-09-17 from <URL: http://e- collection.ethbib.ethz.ch/eserv/eth:27376/eth-27376-02.pdf&g t;).

Much prior art in the field of thin-film ABC2 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 Contreras et al. (1997) On the Role of Na and Modifications to Cu(ln,Ga)Se2 Absorber Materials Using Thin-MF (M=Na, K, Cs) Precursor Layers, NREL/CP-520-22945), or also EP0787354 by Bodegaard et al., or as well US20080023336 by Basol). 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 (Wuerz et al. (2011 ) CIGS thin-film solar cells and modules on enamelled steel substrates, Solar Energy Materials & Solar Cells 100 (2012) 132-137). Most detailed work has usually focused on adding or supplying sodium at various stages of the thin-film device's manufacturing process For reference, the highest photovoltaic conversion efficiency achieved in prior art for a photovoltaic cell on a polyimide substrate, i.e. on a alkali-nondiffusing substrate, with an ABC2 absorber layer where sodium is added via physical vapor deposition of NaF, is about 18.7%, as reported in Chirila et al. (2011 ) Nature Materials 10, 857-861.

The controlled addition of potassium from sources external to a potassium-nondiffusing substrate has also been described in WO 2014/097112 of the present applicants.

Specific addition, in a controlled manner, of substantial amounts of other alkali elements such as Rb and Cs, or of earth-alkali elements (group II of periodic table) after the growth of the CIGS absorber has not been reported in the prior art.

Prior art has so far not specifically disclosed whether adding, in a controlled manner, substantial amounts of Rb and/or Cs to layers of thin-film ABC2 photovoltaic devices might, especially in combination with sodium and/or potassium, enable the production of a class of photovoltaic devices with superior photovoltaic conversion efficiency. Prior art does not disclose how much Rb and/or Cs might be comprised within devices resulting from a controlled addition and the process parameters. 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 enameled 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 developments with respect to controlled addition of alkali metal combinations, especially upon addition of higher atomic weight alkali elements such as Rb or Cs. Alkali and earth-alkali elements have a low electronegativity, but their atomic size and weight can differ strongly, and yield very different diffusion behavior when interacting with another material that can be used as photovoltaic device. In view of the different influence of Na and K that was described in recent results (see for example P. Reinhard et al., Features of KF and NaF post-deposition treatments of Cu(ln,Ga)Se2 absorbers for high efficiency thin film solar cells, Chemistry of Materials 27, 2015, 5755- 5764), combination with additional alkali elements might lead to a further improvement in photovoltaic efficiency. The following work exploits previously unexplored properties of adding specific combinations of Rb and/or Cs preferably with at least one other alkali metal, such as sodium to a thin-film optoelectronic device, and especially to its absorber layer.

Summary

A major problem of fabricating thin-film photovoltaic cells devices with a roll-to-roll manufacturing apparatus is that the evaporation and deposition process of the ABC chalcogenide materials and other desired added elements require very long time, so that the process is slow, and the substrate foil must be transported between the rolls at an extremely reduced speed, often less than 4 or 2 cm/min. This makes the process not very interesting for an industrial mass production and dramatically increases the cost of the produced photovoltaic cells.

Another 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 ABC2 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 some facets of the grains on the surface of some absorbers show a different reactivity. In order to grow a fully covering buffer layer by a wet process, it is necessary to adjust the duration for the slowest growth speed, thereby unnecessarily increasing the thickness of the buffer layer on facets with faster growth speed.

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.

A further problem in the field of thin-film photovoltaic devices is that some alternative buffer layers, such as ZnOS, show a metastable photovoltaic efficiency.

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 40 nm, the photovoltaic device's fill factor and open circuit voltage are substantially lower than with photovoltaic devices having a buffer layer thickness greater than 40 nm. Another problem in the field of thin-film photovoltaic device manufacturing is that the process of addition of alkali elements is a reactive process and requires a minimum amount of time to yield a device with enhanced photovoltaic properties.

Another problem in the field of thin-film photovoltaic device manufacturing is that the minimum amount of time to yield a device with enhanced photovoltaic properties is dependent on the alkali element itself, and that some alkali elements used in prior art such as Na and K require a comparatively long reaction time compared to duration of absorber growth and prevent high-throughput manufacturing.

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.

The above mentioned problems are solved by the invention, as defined in the independent claims.

The dependent claims show specific preferred embodiments of the invention.

The invention proposes a method of fabricating thin-film photovoltaic cells devices preferably with a roll-to-roll manufacturing apparatus at high speed, the method comprising the steps:

(i) providing an alkali-non diffusing substrate delivered between a delivery roll and a take-up roll of a roll-to-roll manufacturing apparatus;

(ii) forming a back-contact layer;

(iii) forming at least one absorber layer, 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 11 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 13 of the periodic table including In, Ga, and Al, and C represents elements in group 16 of the periodic table including S, Se, and Te;

(iv) adding at least one and preferably at least two different alkali metals in elemental form or precursor thereof, the alkali metal(s) being added from a source external to the alkali-non diffusing substrate, preferably during last minutes of absorber growth stage or after growth is finished; and

(v) forming at least one front-contact layer; wherein

said at least one alkali metal(s) comprises Rb and/or Cs .

High speed means that the substrate is delivered between a delivery roll and a take-up roll of a roll-to-roll manufacturing apparatus at a speed higher than 8 cm/min, preferably higher than 12 cm/min, most preferably higher than 20 cm/min.

According to said inventive process, said alkali metal(s) comprise(s) Rb and/or Cs and, following forming said front-contact layer in the interval of layers from back-contact layer, exclusive, to front-contact layer, inclusive, the comprised amounts resulting from adding the alkali metals are advantageously, for Rb and/or Cs, in the range of 200 to 10000, preferably 500 to 2500, atoms per million atoms (ppm) and, where another alkali metal is present, the other alkali metal(s) is/are in the range of 5 to 10000 ppm and at most 3/2, preferably at most ½, and at least 1/2000 of the comprised amount of Rb and/or Cs.

This invention presents in particular a solution to the problem of manufacturing at high speed high efficiency thin-film photovoltaic or optoelectronic devices that comprise an ABC2 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 allows independent control of separate alkali metals during adding to layers of the photovoltaic cell device. Besides aforementioned effects such as doping, passivation of grain boundaries and defects, passivation of surface defects, elemental interdiffusion, and observed optoelectronic characteristics such as enhanced open circuit voltage (thereby lower temperature coefficient of finished device) and fill factor (thereby better low-light performance), 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.

A preferred embodiment of the invention not only specifies a method to add combinations of alkali elements, but also the amount of alkali elements that should remain in the resulting thin-film device and, in case more than one alkali is the ratio of the different alkali elements. A preferred embodiment of the invention also makes use of the enhanced reactivity of the alkali elements in the near-surface region of the absorber layer to enable an increase of the web speed used during the alkali treatment, yielding a higher production throughput. The modified near-surface region also allows a laser patterning step with lower threshold power, saving thereby energy for monolithic interconnection of solar cells to modules.

A preferred embodiment of the invention presents photovoltaic devices that comprise a controlled amount of Rb and/or Cs and describes the characteristics of said devices. A preferred embodiment of the invention also presents a method for manufacturing said devices with the advantage of reduced optical losses and therefore enhanced photovoltaic conversion efficiency. The method is especially advantageous for the high speed production of flexible photovoltaic devices based on plastic substrates. Devices manufactured according to said method have higher photovoltaic efficiency and possibly less unwanted material than equivalent devices manufactured using methods described in prior art. Devices manufactured according to said method have a lower temperature coefficient enabled by a higher open circuit voltage and can be monolithically interconnected at a lower laser power than when using methods described in prior art.

The invention is exemplified by the following illustrations of embodiments of single multiple adding of the alkali metals Rb and/or Cs.

One embodiment involves the adding of Rb alone, Cs alone or a combination of Rb and Cs. For this embodiment, the total amount of comprised Rb and/or Cs is preferably in the upper part 1500 to 2000 of the preferred range 1000 to 2000 ppm. This embodiment provides a lesser improvement of efficiency compared to the other embodiments.

A preferred embodiment includes sodium in the following combinations : Na + Rb; Na + Cs; Na + K + Rb; Na + K + Cs; Na + Rb + Cs. Sodium-containing embodiments have the following advantages : higher open circuit voltage (Voc) and faster addition process of the other alkali elements. The preferred ratio of Na to the other elements is between 1/2000 to 1 .5, preferably 1/40 to 1/5.

A third embodiment comprises potassium in the following combinations : K + Rb; K + Cs; K + Rb + Cs; K + Na + Rb; K + Na + Cs. Potassium-containing embodiments have the following, not necessarily exclusive, advantages : reduction of buffer layer deposition time and broader process window for high efficiency devices. The preferred ratio of K to the other elements is between 1/2000 to 10, preferably 1/5 to 2.

The inventive method can also advantageously comprise adding the alkali earth metals Be, Mg, Ca, Sr and Ba in combination with Rb and Cs and possibly other alkali metals, in the following proportions: 1/20000 to 10, preferably 1/100 to 1/5.

The possible amounts of the various components are summarized in the following Table I. The numbers in ppm are the amounts of Rb and/or Cs. The amounts indicated as fractions are the proportions of other metals to Rb and/or Cs. This Table shows preferred and most preferred ranges.

Table I

Briefly, an aspect of the invention thus pertains to a method of fabricating thin-film photovoltaic devices comprising at least one ABC2 chalcopyrite absorber layer and to adding controlled amounts of Rubidium and/or Cesium preferably in combination with at least one other alkali metal. Said thin-film photovoltaic devices comprise - and we hereby define the term "alkali-nondiffusing substrate" - a substrate that is alkali-nondiffusing and/or comprises means, such as at least one barrier layer, that prevent diffusion of alkali from the substrate into at least said ABC2 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:

- forming a solid deposit where at least some of the provided chemical elements will diffuse into at least one layer of said layer stack, or

- simultaneously providing chemical elements to other chemical elements being deposited, thereby forming a layer that incorporates at least some of the provided chemical elements and the other elements, or

- depositing chemical elements onto a layer or layer stack, thereby contributing via diffusion at least some of the provided chemical elements to said layer or layer stack.

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 Rb and/or Cs preferably in combination with at least one other alkali metal results in devices of higher photovoltaic conversion efficiency than if little or no Rb and/or Cs 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.

In greater detail, the method comprises advantageously providing an alkali-nondiffusing substrate, forming a back-contact layer, forming at least one absorber layer made of an ABC chalcogenide material, adding at least two different alkali metals, and forming at least one front-contact layer, wherein one of said at alkali metals is Rb and/or Cs and where, following forming said front-contact layer, in the interval of layers from back-contact layer, exclusive, to front-contact layer, inclusive, the comprised amounts resulting from adding alkali metals are, for Rb and/or Cs, in the range of 200 to 10000 ppm and, for other alkali metals, in the range of 5 to 10000 ppm and at most 3/2, preferably 1/2, and at least 1/2000 of the comprised amount of Rb and/or Cs. All quantities expressed herein as ppm are by the number of atoms of the alkali metal per million atoms in said interval of layers.

In said method, the comprised amount of Rb and/or Cs in said interval of layers may preferably be kept in the range of 1000 to 2000 ppm. Furthermore, said at least one other alkali metal in said interval of layers may be Na or K, preferably Na, the amount of which is in the range of 5 to 500 ppm. More precisely, in said interval of layers the ppm ratio of Rb/Na and/or Cs/Na may be in the range of 2 to 2000. A narrower proportion for Rb/Na or Cs/Na may be from 10 to 100.

In further detail, forming at least one absorber layer may comprise physical vapor deposition. Forming said absorber layer may comprise physical vapor deposition at substrate temperatures in the range of 100°C to 500°C. Said absorber layer may be

Cu(ln,Ga)Se2. In said method, adding of at least two different alkali metals may comprise separate adding any of said at least two different alkali metals. Furthermore, adding Rb and/or Cs may comprise adding a precursor such as RbF and or CsF. More precisely, after forming said at least one absorber layer, adding Rb and/or Cs may comprise physical vapor deposition of RbF and or CsF at a substrate temperature lower than 700°C. At a narrower temperature range, after forming said at least one absorber layer, adding Rb and/or Cs may comprise physical vapor deposition of RbF and or CsFat a substrate temperature in the range of 300°C to 400°C. Furthermore, adding said two alkali metal(s) may be done in the presence of at least one of said C element. In said method, adding at least two different alkali metals may comprise, at a substrate temperature in the range of 320 to 380°C and after forming said absorber layer, a physical vapor deposition process comprising first adding NaF or KF at a first deposition rate followed by adding RbF and or CsF at a second deposition rate. Said method may comprise forming at least one buffer layer at a step between forming absorber layer and forming front-contact layer. Furthermore, at least one buffer layer may comprise CdS. Forming of said buffer layer may comprise chemical bath deposition resulting in forming at least one buffer layer comprising CdS. Said buffer layer may have a thickness less than 60 nm. In said method, said substrate may be delivered between a delivery roll and a take-up roll of a roll-to-roll manufacturing apparatus. Said substrate may be polyimide.

An aspect of the invention also pertains to a thin-film photovoltaic cell device obtainable by the described method, comprising: an alkali-nondiffusing substrate; a back-contact layer; at least one absorber layer, which absorber layer is made of an ABC chalcogenide material as previously described; and at least one front-contact layer, wherein the interval of layers from the back-contact layer, exclusive, to the front-contact layer, inclusive, comprise controlled amounts of Rb and/or Cs with an amount in the referred range of 500 to 10000 ppm, and the comprised amount of other alkali metals is in the range of 5 to 10000 ppm and is at most 3/2, preferably ½, and at least 1/2000 of the comprised amount of Rb and/or Cs. Said photovoltaic cell device may, when measured under standard test conditions (STC), have a characteristic open circuit voltage which is greater than 680 mV and a short circuit current density greater than 32 mA/cm ² . Advantages

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 an alkali-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:

- Higher speed

- Shorter evaporation and deposition times

- Higher photovoltaic conversion efficiency,

- Thinner buffer layer,

- Faster deposition of buffer layer,

- Enlarged buffer layer deposition process window,

- Enlarged deposition process window for alkali metal doping elements,

- More environmentally-friendly manufacturing process and devices,

- Lower manufacturing costs

- Lower temperature coefficient of the modules

- Shorter minimum alkali deposition time

- Reduced corrosion of back-contact during alkali treatment

- Na + Cs or Na + Rb work better than Na + K.

- Reduction of metastabilities when using buffer layers other than CdS

- Shorter duration of alkali addition treatment possible

- Laser patterning step with a lower threshold power than in prior art.

Further improvements over prior art are expected especially for Na + Rb and Na + Cs, or Na + K + (Rb or Cs). Na is relatively well known as fast diffuser into the bulk of the absorber, where it passivates grain boundaries and electronic defects. Its influence on the surface-region composition of the absorber is however limited. Larger alkali elements such as K, Rb or Cs are not as effective on the bulk of the absorber, probably due to their lower diffusion constant in CIGS because of their larger size. Therefore Na in combination with Rb and/or Cs will be preferred.

It was shown that a main difference between Na and K happens in the surface region of the absorber, where much K is implanted, and remains in the final device. This leads to the formation of a surface with modified composition that is advantageous for conformal growth of a buffer layer with reduced thickness and good electronic band alignment, yielding a junction with improved properties. It is postulated that this difference in influence between bulk and surface properties will be even stronger when utilizing larger alkali elements such as Rb or Cs.

One additional advantage of Rb or Cs is that their catalytic activity on surface modification is stronger than Na and K, and therefore a shorter reaction time can be enabled, speeding up the alkali treatment process duration significantly.

Addition of Rb or Cs should also improve the temperature coefficient of the modules due to increased open circuit voltage, Voc.

Brief description of figures

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

FIG. 1 is a cross-section of an embodiment of a thin-film photovoltaic cell device.

FIG. 2 presents steps in a method to manufacture a thin-film photovoltaic cell device.

FIG. 3 is a side cross-section of a vapor deposition zone of an apparatus for manufacturing a thin-film photovoltaic cell device.

Fig. 4 shows the photovoltaic parameters of submodules manufactured with NaF and optimized RbF concentration (triangles) and without RbF (squares). Submodules were manufactured on a roll-to-roll manner. Each data point corresponds to one submodule on a same roll.

Fig. 5 shows the major photovoltaic (PV) parameters of submodules with fast (triangle) and slow (square) web speed showing a) no decrease of submodule efficiency when NaF and RbF is applied and b) significant decrease of submodule efficiency when NaF and KF are applied. This is especially linked with a decrease of Voc and FF, and shows the increased process window due to the beneficial effect of alkali treatment with RbF over KF.

Fig. 6 shows SIMS spectra showing that Rb is present in device treated with NaF and RbF post deposition treatment (PDT). Detailed description

In more detail, an "alkali-nondiffusing substrate" is a component, ordinarily a sheet of material, that comprises no potassium or other alkali metals or so little thereof that diffusion of potassium or other alkali elements into the subsequently described layers is considered too small to significantly alter the optoelectronic properties of the device. Alkali-nondiffusing substrates also include substrates that comprise means to prevent diffusion of alkali into coatings or layers supported by the substrate. An alkali-nondiffusing substrate may for example be a substrate that has been specially treated or coated with a barrier layer to prevent diffusion of alkali 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. 1 presents the cross-section of an embodiment of a thin-film optoelectronic or photovoltaic device 100 comprising an alkali-nondiffusing substrate 110 for a stack of material layers wherein at least two different alkali metals, one of them being Rb and/or Cs, have been added.

Substrate 1 10 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 7 μηη to

150 μηη. Polyimide substrates are ordinarily considered as alkali-nondiffusing.

At least one electrically conductive layer 120 coats substrate 110. 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 110 onto which it is deposited and to that of other materials that are to be subsequently deposited upon it. Conductive layer 120 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 MoSe2), 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 130 coats electrically conductive layer 120. Absorber layer 130 is made of an ABC material, wherein A represents elements in group 11 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 13 of the periodic table including In, Ga, or Al, and C represents elements in group 16 of the periodic table including S, Se, or Te. An example of an ABC2 material is the Cu(ln,Ga)Se2 semiconductor also known as CIGS.

Optionally, at least one semiconductive buffer layer 140 coats absorber layer 130. Said buffer layer ordinarily has an energy bandgap higher than 1.5 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,0)S, Zn(0,S) material, or variations thereof.

At least one transparent conductive layer 150 coats buffer layer 140. 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 Rb and/or Cs comprised in the interval of layers from electrically conductive back-contact layer 120, exclusive, to transparent conductive front-contact layer 150, inclusive, is in the range between 20 or preferably 500 and 10000 Rb and/or Cs atoms per million atoms (ppm) and the amount of other alkali metals when present is in the range of 5 to 10000 ppm and at most preferably 1/2 and at least 1/2000 of the comprised amount of Rb and/or Cs. A thin-film photovoltaic device demonstrating superior photovoltaic conversion efficiency preferably has an amount of Rb and/or Cs comprised in said interval of layers 470 in the range between 1000 and 2000 Rb and/or Cs atoms per million atoms.

Optionally, front-contact metallized grid patterns 160 may cover part of transparent conductive layer 150 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. 2 presents a method 200 comprising material deposition steps to manufacture said thin-film optoelectronic or photovoltaic device 100 comprising an alkali-nondiffusing substrate 110 for a stack of material layers where one and preferably at least two different alkali metals, one or both of them being Rb or Cs, have been added. The method implies the use of substrates considered alkali-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, the main subject of this invention, occurs.

The method starts at step 210 by providing an alkali-nondiffusing substrate. Said substrate is considered as alkali-nondiffusing, according to the description provided for substrate 110.

Following step 210 and until the step of forming front-contact layer 250, adding preferably at least two different alkali metals, one of them being Rb and/or Cs, occurs as at least one event during and/or between any of steps comprised in the interval from step 210, exclusive, to step 250, exclusive. The fact that the adding may occur during or between said interval of steps is represented by dashed arrows emanating from block 235 in FIG. 2. 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 Rb and/or Cs, for example by adding via a so-called Rb and/or Cs-comprising precursor such as RbF, RbCI, RbBr, Rbl, Rb2S, Rb2Se, is done in the presence of at least one said C element.

At step 220, 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 120.

At step 230, forming at least one absorber layer comprises coating said electrically conductive layer with at least one ABC absorber layer 130. The materials used correspond to those in the description provided for ABC absorber layer 130. Said absorber layer may be deposited using a variety of techniques such as sputtering, spraying, sintering, CVD, electrodeposition, printing, or as a preferred technique for an ABC material, physical vapor deposition. Substrate temperatures during absorber layer deposition are ordinarily comprised between 100°C and 650°C. 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 600°C, and if using substrates requiring lower temperatures, such as a polyimide substrate, preferably below 500°C, and more preferably in the range from 100°C to 500°C. For a co-evaporation vapor deposition process, substrate temperatures during forming of the absorber layer will ordinarily be in the range from 100°C to 500°C. 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 230 is done using a physical vapor deposition process, adding of Rb and/or Cs as part of adding alkali metals may be done during and/or in continuation of the physical vapor deposition process by supplying Rb and/or Cs fluoride, RbF and/or CsF. This may for example be advantageous when manufacturing with a co-evaporation physical vapor deposition system. Adding the alkali metal Rb and/or Cs will preferably be done in the presence of a flux of element Se supplied at a rate in the range of 5 to 100 A/s, preferably at a rate in the range of 20 to 50 A/s.

Substrate temperatures for said adding of at least two different alkali metals will ordinarily be greater than 100°C and less than 700°C. Substrate temperatures will preferably be greater than 300°C and less than 400°C. 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 Rb and/or Cs, for example in the form of RbF and/or CsF, 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 RbF and/or CsF may advantageously be used to add alkali metals such as potassium or sodium at temperatures closer to those used at step 230 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 700°C and possibly much lower than 350°C, such as at ambient temperatures of about 25°C 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 Rb and/or Cs added by adding at least two alkali metals is such that following forming of front-contact layer 150 at later step 250, said amount comprised in the interval of layers 470 from back-contact layer 120, exclusive, to front-contact layer 150, inclusive, is typically in the range between 200 and 10000 Rb and/or Cs atoms per million atoms and the amount of the other alkali metals is in the range of 5 to 10000 ppm and at most 3/2, preferably1/2, and at least 1/2000 of the comprised amount of Rb and/or Cs A thin-film photovoltaic device that has a superior photovoltaic conversion efficiency preferably has an amount comprised in said interval of layers from about 1000 to 2000 Rb and/or Cs atoms per million atoms.

In a variation of the invention, alkali earth metals can also be added in the same way as the alkali metals.

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

At step 240, 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 140. The materials used correspond to those in the description provided for buffer layer 140. 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 100°C and 300°C for a duration of 1 to 30 minutes, preferably 180°C for a duration of 2 minutes.

To tune the process of forming the buffer layer of step 240, 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 alkali metals that comprises alkali metals but does not comprise the amount of Rb and/or Cs specified in this invention and a lesser amount of other alkali metal(s). 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 250, forming front-contact layer comprises coating said buffer layer with at least one transparent conductive front-contact layer 150. 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 260, forming front-contact grid comprises depositing front-contact metallized grid traces 160 onto part of transparent conductive layer 150. 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

260, forming front-contact layer 250, forming optional buffer layer 240, forming absorber layer 230, adding at least two alkali metals 235, and forming an electrically conductive back-contact layer.

FIG. 3 shows a side cross-section of a deposition zone apparatus 300 comprised in a section of an apparatus for manufacturing a thin-film optoelectronic or photovoltaic device comprising an alkali-nondiffusing substrate 1 10 for a stack of material layers wherein at least two different alkali metals, one of them being Rb and/or Cs, are being added. Deposition zone apparatus 300 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 325, enters deposition zone apparatus 300, travels according to direction 315 over a set of sources for forming at least one absorber layer 130 and at least one set of sources for adding at least two different alkali metals 335, and then exits deposition zone apparatus 300.

Web 325 comprises a substrate 1 10 coated with an electrically conductive back-contact layer, or stack of electrically conductive layers, thereafter called back-contact layer 120.

Said substrate, prior to being coated with said stack of electrically conductive layers, is considered as alkali-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 MoSe2), Mo, transition metal chalcogenides, indium oxide (ITO), indium oxides (such as 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 330 comprises a plurality of sources 331 s generating effusion plumes 331 p that, in the case of a preferable co-evaporation setup, may overlap. Said set of absorber deposition sources 330 provides the materials to coat web 325 with at least one absorber layer 130 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 335 for adding alkali metals comprises at least one source 336s generating effusion plume 336p adding at least one alkali metal 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 130. At least one source 336s comprises Rb and/or Cs, preferably in the form of Rb fluoride, RbF, or Cs fluoride, CsF. Preferably at least one source 336s comprises sodium, preferably in the form of sodium fluoride NaF. These sources may provide other alkali metals, preferably as a co-evaporation setup, and effusion plumes 336p may overlap at least one of effusion plumes 331 p. If the set of said sources 335 for adding alkali metals comprises more than one source 336s, the source comprising

Rb and/or Cs 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 Rb and/or Cs occurs.

The amount of Rb and/or Cs added by the sources for adding alkali metals is such that, following forming of transparent front-contact layer 150, said amount comprised in the interval of layers from back-contact layer 120, exclusive, to front-contact layer 150, inclusive, is preferably in the range of 500 to 10000 Rb and/or Cs atoms per million atoms and, for the other alkali metals, in the range 5 to 2000 ppm and at most 1/2 and at least 1/2000 the comprised amount of Rb and/or Cs. A thin-film photovoltaic device that has a superior photovoltaic conversion efficiency preferably has an amount comprised in said interval of layers from about 1000 to 2000 Rb and/or Cs atoms per million atoms.

The location for forming front-contact layer 150 is considered to be outside said deposition zone apparatus and the means for forming said front-contact layer are therefore not represented.

Experiments

Combinations of NaF and RbF, or RbF were used to produce the measured specimens, which have been then accurately tested.

Figure 4 shows the photovoltaic parameters of submodules manufactured with optimized RbF concentration (triangles) and without RbF (squares). Submodules were manufactured on a roll-to-roll manner. Each data point corresponds to one submodule on a same roll. Measured Rb concentration was 1500 ppm.

Measured Na concentration was 300 ppm.

The sub-module efficiency is the ratio of electrical generated energy per solar incident energy, as measured under standard test conditions (STC).

Voc is the open circuit voltage, as measured under standard test conditions (STC).

Jsc is the short circuit current, as measured under standard test conditions (STC).

The filling factor (FF) is calculated from the voltage-current characteristic of the cell as Vmp Jmp / Voc Jsc, where Vmp and Jmp are respectively the maximum power voltage and maximum power current.

The improved photovoltaic (PV) parameters of submodules produced with Rb are evident.

Figure 5 shows the major photovoltaic (PV) parameters of submodules with fast (triangle) and slow (square) web speed showing a) no decrease of submodule efficiency when NaF and RbF is applied and b) significant decrease of submodule efficiency when NaF and KF are applied. This is especially linked with a decrease of Voc and FF, and shows the increased process window due to the beneficial effect of alkali treatment with RbF over KF.

Web speeds were 6 cm/min for triangles and 12 cm/min for squares. Estimated deposition time would be ca. 6 min (6 cm/min) and 3 min (12 cm/min).

The positive effect of Rb on the speed compared to K is evident, dramatic, unexpected and surprising.

Figure 6 shows SIMS spectra showing that Rb is present in device treated with NaF and

RbF post deposition treatment (PDT).