WO2012107256A1

WO2012107256A1 - Process for producing light absorbing chalcogenide films

Info

Publication number: WO2012107256A1
Application number: PCT/EP2012/050446
Authority: WO
Inventors: Ayodhya N. TIWARI; Yaroslav E. ROMANYUK; Alexander R. UHL; Marc KÄLIN
Original assignee: Empa
Priority date: 2011-02-10
Filing date: 2012-01-12
Publication date: 2012-08-16

Abstract

The present invention relates to a process for producing a light absorbing chalcogenide thin film (5) on a rigid or flexible substrate (4), which can be used in photovoltaic devices. With the presented method the light absorbing chalcogenide thin film (5) and a carbon-rich layer (6) can be produced simultaneously. While the chalcogenide thin film (5) functions as absorber layer, the carbon-rich layer (6) functions as an electrical back contact, which can be used in rigid or flexible photovoltaic devices.

Description

Process for producing light absorbing chalcogenide films

TECHNICAL FIELD

The present invention describes a method for fabrication of a light absorbing chalcogenide film on a substrate and the use of a precursor solution comprising metal-organic complexes for manufacturing chalcogenide films for photovoltaic devices.

STATE OF THE ART

With global expansion of the solar cell market and its rising desire to meet commercially available energy generation, the non-vacuum deposition of chalcogenide (chemical compounds consisting of at least one chalcogen ion, e.g. sulfides, selenides and tellurides) absorber layers have received great attention.

Amongst all thin film technologies, Cu(In,Ga)_x(S,Se)_y (short CIGS) solar cells are considered to have the highest cost reduction potential as being the thin film technology with the highest reported efficiencies, above 20.1% (M .A. Green, Prog. Photovoltaics Res. Appl. 18 (2010), 346). Solar cells based on the abundant element compound Cu(Zn,Sn)_x(S,Se)y (short CZTS) absorbers have shown up to 9.7% efficiency by a wet chemical method (T. K. Todorov, Adv. Mater. 22/20 (2010).

Long term stabi l ity and use of envi ron mental ly ben ig n materials distinguish both, CIGS and CZTS technologies from organic, a-Si and CdTe PV technology. With 1-2 μητι thickness, which is about 100 times thinner than absorbers from more commonly known c-Si technology, chalcogenide films can be deposited on both rigid and flexible substrates. This reduces weight and enables a high throughput production. The direct band gap of this material and its tunability by adjusting the metal ratios or type of chalcogen (elements in group 16 or VIA of the periodic table) makes it an interesting material allowing research from advanced band gap engineering within the fi lm to multijunction solar cells.

In state of the art manufacturing, CIGS is co-evaporated from elemental sources in controlled high-vacuum atmosphere. However, next to high capital investment and a low material utilization of 30- 50%, vast differences between current lab and module efficiencies show the inherent difficulty to scale-up this technique. Different research groups have already shown the feasibility to process chalcogenide absorbers by means of more economical processes. Most routes thereby consist of a two-step process that is comprised of a first deposition step of precursor material and a second conversion step to chalcogenide phase by consecutive heating in chalcogen atmosphere. Precursor deposition can be done by sputtering, electro-deposition, and liquid coating from both, particle dispersions or metal solutions. Whereas sputtering still requires vacuum conditions, the latter techniques especially are believed to excel others with their low capital investment and high throughput.

Following the route of electro-deposition, Solopower has achieved efficiencies up to 14.4% and more than 10% modules (B. M . Ba§ol, 34th IEEE Photovoltaic Specialists Conference, 2009, p.2310). However, differences in redox potentials of individual elements make it difficult to tailor chemical composition of the absorber film or overcome tedious multistep depositions.

Compared to this, improvements in deposition rate, growth homogeneity, and material utilization exceeding 90% are believed to be obtained by the method of liquid coating. A variety of deposition methods can be named such as spin coating, dip coating, spray pyrolysis, doctor blade, ink-jet printing, slit casting, curtain coating, slot die coating, screen printing, flexography and gravure printing which can be divided in lab-scale/batch methods, i.e. spin coating and up-scalable/roll-to-roll processes, i.e. all others.

The feasibility of this method was shown by Nanosolar that have published 14.5% active area efficiency for CIGS solar cells prepared by a hybrid method (J. K. J. van Duren, Mater. Res. Soc. Symp. Proc. 1012 (2007) 259). Thereby, a metal precursor layer is deposited on a thin aluminum foil by simple ink-jet printing of a nanoparticle containing paste. Different types of nanoparticles have been investigated such as metal alloys (B. M. Ba§ol, Thin Solid Films, 361 (2000) 514), metal oxides (C. Eberspacher, US 6,268,014, V.K. Kapur, US Patent No. 6127202), metal selenides (J. K. J. van Duren, US2007/0092648, B. M. Sager, US2008/0149176), CIS particles (Guo, Nano Lett 8 (2008) 2982), and CZTS particles (S.C. Riha, J. Am. Chem. Soc.131 (2009) 12054). However, the use of nanoparticle inks poses some inherent difficulties, i.e. the fabrication of stoichiometric- controlled nanoparticles, stabilization to avoid coagulation, protection against oxidation, harsh reducing steps or aggressive selenization in H₂S/H₂Se atmosphere.

Apart from nanoparticle dispersions, true solutions from off-the-shelf metal salts have been utilized to prepare precursor pastes. Pastes were prepared from water based solutions (J. L. Sansregret, US4242374), solutions of organic compounds (K. K. Banger, Inorg. Chem. 42, 7731(2003), H. Ishihara, US5910336), with help of binder materials (M. Kaelin, US2008/0044570, J. K. J. van Duren, US2008/0280030)), or hydrazine based solutions (D. B. Mitzi, US2005/0158909). However, some drawbacks are apparent such as need for reducing atmosphere for conversion of metal oxide/- organic precursor layers into metal precursor films, complicated synthesis of initial organometallic molecules, carbon residuals stemming from binder materials, and use of toxic solvents to enable dissolution of chalcogen. Described oxidation and/or reducing steps that are used to obtain a residual free metal precursor layers demand additional preparation steps and thus, lead to a more complex manufacturing method.

The fabrication of a chalcogenide absorber layer simultaneous with a back contact by incomplete chalcogenization in one production step was previously reported by Tober et al. US6429369. Before the conversion step a deposition of a metal precursor layer has to be carried out which can be converted to build a polycrystalline absorber layer combined with a metal back contact. For the deposition of a metal precursor layer consisting of intermetallic phases of group IB and IIIA metals an production step carried out in vacuum, e.g. by sputtering, vacuum evaporation or a combination thereof, is necessary. This results in high material cost from utilized metals, low deposition speeds of named deposition methods, and again an unfavorable process step in vacuum, which exhibit limitations of this process. In addition to the above mentioned disadvantageous material utilization and possible throughput the capital investment for necessary equipment is high. In respect to inter-diffusion, it can be expected that the presence of a group I or III metal layer adjacent to the absorber can change the composition of the chalcogenide layer and therefore deteriorate its electrical properties. For controll ing the thickness of the metal back contact layer when us i ng th e m eta l a l l oy precu rso r accord i n g to US6429369 the selenization process has to be stopped by adequately cooling down or removing chalcogen. This is difficult and needs sufficient routine. DESCRIPTION OF THE INVENTION

The object of the present invention is to provide a manufacturing method for a chalcogenide film usable in photovoltaic devices, which is reproducible, easy and rapidly workable, without a deposition step in vacuum .

The present invention resolves the aforementioned problems of the reported methods that are described in the prior art so that it:

i) eludes the need of pre-fabricated nanoparticles or organometallic molecules;

ii) does not require addition of organic binder materials to adjust the solution rheology;

iii) omits the need of additional oxidation and reducing steps and thus, reduces the process steps needed to form chalcogenide film;

iv) omits the use of highly toxic and explosive materials.

As an additional effect of the invention the obtainable carbon-rich back contact simultaneously formable with the chalcogenide film, renders the need to prepare a metal back contact by an additional process.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred exemplary embodiment of the subject matter of the invention is described below in conjunction with the attached drawings.

Figure la shows a schematic drawing of the described process to produce a chalcogenide absorber film on a substrate, while Figure lb shows a schematic drawing of a produced chalcogenide absorber film on a substrate with conventional metal back contact. Figure 2 shows an energy dispersive X-ray (EDX) spectrometry line-scan (left) of a cross-section scanning electron micrograph (right) taken from an incompletely selenized precursor layer on a molybdenum back contact. Figure 3 shows the active area quantum efficiency in dependence of wavelength as measured from a solar cell with a chalcogenide film manufactured with the claimed process.

Figure 4 shows the current density in dependence of voltage for a solar cell with a chalcogenide film from described process.

DESCRIPTION

The present invention relates to an innovative process for producing a light absorbing chalcogenide film 5 on a rigid or flexible substrate 4, which can be used in photovoltaic devices, by deposition of a precursor solution 1 on the substrate 4.

The precursor solution 1 is a true solution of metal salts in at least one polar solvent. The rheology of the precursor solution 1 can be between liquid and more viscous. Metal salts contain at least one IB element and/or IIB element and/or IIIA element and/or IVA element of the periodic table. The solution can additionally contain one or more VIA elements in its elemental form or as a compound. The sol ution provides an ion species, wh ich are capable to form carboxylate chelate complexes with coordinated metal ions. The source for anion species can be:

a) carboxylic acids

b) salts and esters of carboxylic acids

c) alcohols, including monohydric and polyhydric, aliphatic and alicyclic alcohols and combinations thereof.

The metal salt concentration is close to the solubility limit of a salt in a respective solvent and is typically 0.1-lM .

Instead of using ready solutions with carboxylates (e.g. formates, acetates, oxalates, etc.), the formation of carboxylic chelate complexes in the precursor solution 1 can be achieved by oxidation of solvents using :

a) oxidizi ng anions, e .g . n itrate, manganate, chromate, chlorate, hypochlorate or similar anions, or oxidizing agents such as H₂0₂, 0₂ gas, etc. b) solution heating from ambient up to 150°C between minutes and days, with or without stirring

d) acidic solution with pH<7, promoting the chemical reaction The formed chelate complexes have higher evaporation temperatures than initial solvents and/or metal salts. The formed chelate complexes prevent the cation segregation and evaporation during deposition and heating steps. According to figure la on the substrate 4 the precursor solution 1 is deposited in step I forming wet layer/precursor film which is not depicted in Fig. 1.

In step II a drying step leads to a precursor layer 2.

I n the su bsequent step II I the chaicogen ide fi l m 5 is formed by simultaneously forming a carbon-rich layer 6. In contrary to the used substrate 4 of figure la, the substrate 4 in figure lb is coated with a metal back contact 3, known from conventional photovoltaic devices. The light absorbing chaicogenide film 5 is formed on the carbon-rich layer 6 with variable and reproducible thickness, wherein both layers 5, 6 are simultaneously formed upon thermal treatment of the metal- organic contai ni ng precu rsor sol ution 1 i n chalcogen atmosphere without additional oxidation or reduction steps.

The method steps of figure lb are consistent with the steps of figure la, while the carbon-rich layer 6 in both cases is formed on the side of the chaicogenide film 5 facing the substrate 4. Only the thickness of the carbon-rich layer 6 differs from figure la to figure lb what is adjustable by choosing appropriate precursor solution 1 with appropriate amounts of chelate complexes and/or organic material, conversion time and temperature. Depending on the choice of precursor solution 1 and the conversion parameter in step III a pure chalcogenide film 5 without carbon-rich layer 6 is producible. The chalcogenide film 5 functions as an absorber and if present the carbon-rich layer 6 functions as back-contact in photovoltaic devices on rigid and flexible substrates 4, i.e. on glass, metal (e.g. steel or aluminium), ceramic, or polymer (e.g. polyimide) substrates, with or without metal or oxide electrically conductive back contact layer (3).

The chalcogenide film 5 comprises at least one IB element and/or IIB element and/or IIIA element and/or IVA element, and at least one VIA element of the periodic table, e.g. compounds Cu-In-S, Cu-In-Ga-Se, Cu-In-Ga-S-Se, Cd-Te, Cd-S, Cd-Se, Cu-Zn-Sn-S, Cu-Zn-Sn-Se or Cu-Zn-Sn-S-Se, etc.

Step I (deposition)

The precursor solution 1 is applied by an up-scalable, non-vacuum deposition method, i.e. doctor-blade, ink-jet, spray, slot-die, curtain coating, web-coating on a substrate 4 or metal back contact 3 covered substrate 4.

Step II (drying)

The so deposited precursor film is dried at temperatures between ambient or room temperature (RT) - 300°C in i nert or a m bient atmosphere for 30 sec - 10 min to form a dried precursor layer 2, which is a solid amorphous carbon-rich matrix containing coordinated metal ions. The carbon-rich matrix containing coordinated metal ions prohi bits the metal segregation and/or evaporation duri ng heati ng steps.

Step III (thermal conversion/chalcogenization) The dried precursor layer 2 is thermally treated in an atmosphere containing at least one chalcogen (group VIA elements) for conversion of the precursor layer 2 into a chalcogenide film 5. The thermal conversion treatment is performed at 200°C-800°C_/ typically 400°C- 600°C during 1-60 min. The pressure during chalcogenation ranges between 0.01 mbar and atm pressure, typically 1-10 mbar. During the thermal treatment, the carbon-rich matrix with coordinated metal ions gradually decomposes, thus liberating metal ions which react with elements from group VIA containing atmosphere to form a chalcogenide film 5.

The decomposition of the carbon-rich matrix is accompanied with the simultaneous formation of reducing species, i.e. CO, preventing the formation of metal oxides and thus leading to high phase purity of the obtained chalcogenide film 5. The conversion process to chalcogenide phase is accompanied by migration and diffusion of metal species to the surface of the carbon-rich matrix leading to the formation of both, the chalcogenide film 5 and the carbon-rich layer 6 at the substrate oriented part of the chalcogenide film 5.

The residual carbon-rich layer 6, which is formed underneath the chalcogenide film 5 facing to the substrate 4, provides an electrical contact to the absorber/chalcogenide film 5 and therefore can be used as a back electrical contact in a photovoltaic device.

If the carbon-rich layer 6 is obtained it is not necessary to cover the substrate 4 with a conventional metal back contact 3 or to use conducting substrates 4. The carbon-rich layer 6 facing the substrate 4 ensures necessary electrical conductivity of the back contact in a photovoltaic device.

The evaporation speed of the organic material, the amount of the residual carbon-rich matrix and thickness of the resulting carbon-rich layer 6 is determined by the thermal treatment temperature and environment of heating, type of metal cations and ligands of the intermediate complexes. The amount of the carbon-rich layer 6 can be increased by adding additional organics to the precursor solution 1. In order to increase the possible thickness of the carbon-rich layer 6 one could add also other carbon containing material to the precursor solution 1, i.e. cellulose, polymers (PS, PP, PE, PVC, PVdC and other), polyvinyl alcohols, polysaccharides, etc. The function of these organics is to provide amounts of carbon.

The thickness of the producible chalcogenide film 5 is typically from 0.5 μηη to 5 μητι . After complete decomposition of the carbon-rich matrix the thickness of the carbon-rich layer 6 is 0. If an incomplete conversion step III is carried out the thickness of the carbon-rich layer 6 is greater than 0 and was reproducibly varied up to 20 μηη .

The metal ratio in the formed chalcogenide film 5 is targeted by adjusting metal ratios in the initial precursor solution 1 in order to obtain a suitable solar absorber material, which is typically Cu/In/Ga = 1/0.9/0.4 or Cu/Zn/Sn= 1.6/1/0.8 in case of CIGS or CZTS absorbers, respectively.

A compositional and respective band-gap gradient is created in the formed chalcogenide film 5 because of higher stability of metal-organic complex of some metals (e.g . Gallium) as compared to other metal elements (e.g. Copper) and/or the lower reaction rate of some metals (e.g. Gallium or Zinc) as compared to other metal elements (e.g. Copper) with the VIA elements. The carboxylate chelate complexes are forming in the solution, and/or during the drying phase. EXAMPLE 1 (with conventional metal back contact)

Anhydrous indium (III) chloride is dissolved in a 1 : 2 mixture of ethanol and 1.2-propanediol. Copper (II) nitrate hemi-pentahydrate and hydrated gallium (III) nitrate are added to give a homogeneous, viscous solution with a molar stoichiometry of Cu/In/Ga of 1/0.9/0.4. This precursor solution 1 is deposited on a soda lime glass, as substrate 4, covered with a molybdenum layer, as back contact 3, by doctor blade method with defined distance between surface and blade so that a 10-100 μηη thick wet precursor film is obtained. The covered substrate is immediately transferred to a heating plate and dried in air at temperatures of 100-200°C for 1-10 minutes forming the precursor layer 2. The chelate complexes between metal ions and diol groups are formed in sufficient amount during dissolution of salts and the drying step.

Finally, the covered substrate is transferred to a nitrogen purged quartz tube which is kept at underpressure (1-10 mbar). The furnace comprises two temperature zones of which one contains a selenium loaded vessel, the other contains the covered substrate. The selenium zone is ram ped up to 300-400°C and substrate temperatures are ramped to 500-600 ° C . Both zones a re ke pt at the res pective temperatures for 10 - 20 minutes. After this treatment, the molybdenum coated substrate 4 is covered with a thin film of polycrystalline Cu(In,Ga)Se₂ chalcogenide film 5 that can be used as an absorber layer in photovoltaic devices.

EXAMPLE 2 (without conventional metal back contact)

Anhydrous indium (III) chloride is dissolved in a 1 : 2 mixture of ethanol and 1.2-propanediol. Copper (II) nitrate hemi-pentahydrate and hydrated gal l i um (III) nitrate are added to give a homogeneous, viscous solution, representing the precursor solution 1 with a molar stoichiometry of Cu/In/Ga of 1/0.9/0.4.

This precursor solution 1 is deposited on soda lime glass as substrate 4 by doctor blade method with defined distance between substrate and blade so that a 10-100 pm thick wet precursor film is obtained. The so prepared substrate 4 is immediately transferred to a heating plate and dried in air at temperatures of 100-200°C for 1-10 minutes while the precursor layer 2 is formed. The chelate complexes between metal ions and diol groups are formed in sufficient amount during dissolution of salts and the drying step.

Finally, the substrate 4 with the precursor layer 2 is transferred to a nitrogen purged quartz tube which is kept at underpressure (1-10 mbar). The furnace comprises two temperature zones of which one contains a selenium loaded vessel, the other contains the prepared substrate 4. The selen i um zone is ra m ped up to 300-400°C and substrate temperatures are ramped to 500-600°C. Both zones are kept at the respective temperatures for 10 - 20 minutes. After this treatment the substrate 4 is covered with a thin film of polycrystalline Cu(In,Ga)Se₂ chalcogenide film 5 on top of a carbon-rich layer 6. Both, the chalcogenide film 5 (more exact the chalcopyrite film) and the carbon-rich layer 6 can be used in photovoltaic devices as absorber layer 5 and an alternative to the known metal back contact 3, respectively.

EXAMPLE 3 (with conventional metal back contact)

Anhydrous tin (IV) chloride is dissolved in a 1 : 2 mixture of ethanol and 1.2-propanediol. Hemi-pentahydrate copper (II) nitrate and zinc (II) n itrate hexa hydrate a re added to g ive a homogeneous, viscous solution, representing the precursor solution 1, with a molar stoichiometry of Cu/Zn/Sn of 1/0.7/0.6.

This precursor solution 1 is deposited on a substrate 4 that is covered with a molybdenum layer, as back contact 3, by doctor blade method with defined distance between surface and blade so that a 10-100 μιτι thick wet precursor film is obtained . The so prepared substrate 4 is immediately transferred to a heating plate and dried in air at temperatures of 100-200°C for 1-10 minutes, while the dried precursor layer 2 is formed. The chelate complexes between metal ions and diol groups are formed in sufficient amount during dissolution of salts and the drying step.

Finally, the substrate 4 with precursor layer 2 is transferred to a nitrogen purged quartz tube which is kept at underpressure (1 - 10 mbar) for the conversion step. The furnace comprises two temperature zones of which one contains a sulfur loaded vessel, the other contains the su bstrate . The su lfu r zone is ra m ped u p to 300-400°C and substrate temperatures are ramped to 500-600°C. Both zones are kept at the respective temperatures for 10-20 minutes. After this treatment the su bstrate 4 is covered with a th i n fi l m of polycrysta l l i ne Cu(Zn,Sn)_xSy chalcogenide film 5 that can be used as an absorber layer in photovoltaic devices.

The process comprises subsequent steps of: forming and using a true solution 1 of metal salts and polar solvents, wherein coordination of metal ions is achieved by carboxylic chelate complexes; applying the solution 1 to the substrate 4 or back contact 3 covered substrate 4 and drying to form a precursor layer 2 which is a solid, amorphous, carbon- rich matrix containing coordinated metal ions; heating of the precursor layer 2 at elevated temperatures that leads to decomposition of the carbon-rich matrix liberating metal ions so an atmosphere containing group VIA elements can convert them into a chalcogenide film 5. Incomplete decomposition of the carbon-rich precursor layer 2 acts as an electrical back contact with or without conventional metal contact 3 in a photovoltaic device.

The EDX-spectrometry line-scan of Figure 2 and the respective scanning electron micrograph of the cross-section of the same specimen was taken from an incompletely selenized precursor layer 2. On a su bstrate 4 that is coated with a metal back contact 3, the carbon-rich layer 6 adjacent to the chalcogenide film 5 is visible.

Measurements of the quantum efficiency in the active area in dependence of wavelength of a completed solar cell with a chalcogenide film 5 according to example I manufactured with the claimed process, led to the graph according to Figure 3. A band gap of the absorber material of 1.04 eV can be estimated.

As can be proved by the diagram of Figure 4 an efficiency of 7.7 % could be measured in a solar cell comprising the chalcogenide film 5 according to example I.

LIST OF REFERENCE NUMERALS

1 precursor solution

2 precursor layer

3 metal back contact

4 substrate (rigid or flexible)

5 chalcogenide film

6 carbon-rich layer

Claims

PATENT CLAIMS

1. A method for fabrication of a light absorbing chalcogenide film (5) on a substrate (4),

characterized in the steps of

I) deposition of a metal-organic precursor solution (1)

comprising and/or capable of forming carboxylate chelate complexes on the substrate (4), forming a deposited precursor film,

II) drying of the precursor film, forming a precursor layer (2) on the substrate (4) in form of a solid, amorphous carbon-rich matrix comprising coordinated metal ions, by thermal treatment and finally

III) conversion of the precursor layer (2) by thermal treatment in an atmosphere of at least one chalcogen,

resulting in decomposition of the carbon-rich matrix,

chalcogenization of metals and formation of the chalcogenide film (5).

2. A method according to claim 1, in which step III) is performed incompletely by incomplete chalcogenization and incomplete decomposition of the carbon-rich matrix, that a carbon-rich layer (6) adjacent to the substrate (4) is simultaneously formed with the chalcogenide film (5), in such a way that the carbon- rich layer (6) exhibits functionality as an alternative back contact in photovoltaic devices.

3. A method according to one of the preceding claims, in which the metal-organic precursor solution (1) comprises carboxylates like formates and/or acetates and/or oxalates which are capable to form carboxylate chelate complexes before step III).

A method according to claim 1 or 2, in which the precursor solution (1) comprises metal salts and at least one polar solvent, which can provide anion species, which are capable to form carboxylate chelate complexes with coordinated metal ions.

A method according to claim 1 or 2, in which the metal salts contain at least one IB element and/or IIB element and/or IIIA element and/or IVA element of the periodic table. The solution can additionally contain one or more VIA elements in its elemental form or as a compound.

A method according to claim 1 or 2, in which the deposited precursor film is dried at temperatures between RT and 300°C in inert or ambient atmosphere for 30 sec up to 10 min.

A method according to one of the preceding claims, in which the chalcogenide film (5) is formed on a rigid or flexible substrate (4), in particular on glass, metal, ceramic or polymer substrates (4), with or without metal or oxide electrically conductive contact layer (3).

A method according to one of the preceding claims, in which the conversion of step III) is performed at 350°C - 800°C during 1- 60 min in a VIA element comprising atmosphere.

A method according to one of the preceding claims, in which the anions of utilized salts are nitrate and/or manganate and/or chromate and/or chlorate and/or hypochlorate and/or similar anions capable of oxidizing the solvent into species suitable as ligands to form carboxylate chelate complexes with metal ions.

10. A method according to claim 2, in which the amount and thickness of the carbon-rich layer (6) can be controlled by addition of carbon containing material to the precursor solution (1), in particular cellulose, polymers like PS, PP, PE, PVC or PVdC, polyvinyl alcohols and/or polysaccharides.

11. A method according to claim 2, in which the thickness of the carbon-rich layer (6) up to 20 μιτι is obtained.

12. A method according to claim 1 or 2, in which the decomposition of the carbon-rich matrix is accompanied by simultaneous formation of reducing species, preventing the formation of metal oxides and thus leading to high phase purity of the obtained chalcogenide film.

13. A method according to claim 12, in which the reducing species is CO.

14. Use of a precursor solution (1) comprising metal-organics for manufacturing chalcogenide film (5) for photovoltaic devices, characterized in that

the precursor solution (1) comprises carboxylate chelate complexes which are capable of forming a carbon-rich matrix with coordinated metal ions.