POLYMERIC PHOTOVOLTAIC CELL WITH INVERTED STRUCTURE COMPRISING A CONJUGATED POLYMER COMPRISING AN ANTHRADITHIOPHENE DERIVATIVE

Abstract

There is a polymeric photovoltaic cell (or solar cell) with inverted structure having an anode; an anodic buffer layer; an active layer having at least one photoactive organic polymer as electron-donor and at least one electron-accepting organic compound; a cathodic buffer layer; and a cathode. The at least one photoactive organic polymer is selected from conjugated polymers comprising an anthraditiophenic derivative having a general formula (I):

##STR00001##

The polymeric photovoltaic cell (or solar cell) with inverted structure shows good values of power conversion efficiency (PCE) (η) and can be advantageously used in the construction of photovoltaic modules (or solar modules), either on a rigid support or on a flexible support.

Claims

1. Polymeric photovoltaic cell with inverted structure comprising: an anode; an anodic buffer layer; an active layer comprising at least one photoactive organic polymer as electron-donor and at least one electron-acceptor organic compound; a cathodic buffer layer; and a cathode; wherein: the at least one photoactive organic polymer is selected from conjugated polymers comprising an anthraditiophene derivative having a general formula (I): ##STR00008## in which: Z, equal to or different from each other, represent a sulphur atom, an oxygen atom, or a selenium atom; Y, equal to or different from each other, represent a sulphur atom, or an oxygen atom; R.sub.1, equal to or different from each other, are selected from amino groups —N—R.sub.4R.sub.5 wherein R.sub.4 represents a hydrogen atom, or is selected from alkyl groups C.sub.1-C.sub.20, linear or branched, or is selected from optionally substituted cycloalkyl groups; and R.sub.5 is selected from alkyl groups C.sub.1-C.sub.20, linear or branched, or is selected from optionally substituted cycloalkyl groups, or are selected from R.sub.6—O—[CH.sub.2—CH.sub.2—O].sub.n— polyethylenoxyl groups in which R.sub.6 represents a hydrogen atom or is selected from alkyl groups C.sub.1-C.sub.20, linear or branched, and n is an integer between 1 and 4, or are selected from —O—R.sub.7 groups in which R.sub.7 represents a hydrogen atom or is selected from alkyl groups C.sub.1-C.sub.30, linear or branched; R.sub.2, equal to or different from each other, represent a hydrogen atom or are selected from C.sub.1-C.sub.20, alkyl groups, linear or branched or are selected from —COR.sub.10 groups in which R.sub.10 is selected from C.sub.1-C.sub.20 alkyl groups, linear or branched, or are selected from —COOR.sub.11 groups in which R.sub.11 is selected from C.sub.1-C.sub.20 alkyl groups, linear or branched, or are selected from optionally substituted aryl groups, or are selected from optionally substituted heteroaryl groups; R.sub.3, equal to or different from each other, represent a hydrogen atom or are selected from C.sub.1-C.sub.20, alkyl groups, linear or branched, or are selected from —COR.sub.10 groups in which R.sub.10 is selected from C.sub.1-C.sub.20 alkyl groups, linear or branched, or are selected from —COOR.sub.11 groups in which R.sub.11 is selected from C.sub.1-C.sub.20 alkyl groups, linear or branched, or are selected from optionally substituted aryl groups or are selected from optionally substituted heteroaryl groups; Q, equal to or different from each other, represents a nitrogen atom or represents a C—R.sub.3 group in which R.sub.3 has the same meanings as above; n is an integer between 10 and 500; the at least one organic electron-acceptor compound is selected from a non-fullerene; indacenotiophenes with electron-poor terminal groups; compounds having an aromatic core capable of symmetrically rotating.

2. Polymeric photovoltaic cell with inverted structure according to claim 1, wherein in the general formula (I): Z, equal to each other, represent a sulphur atom; Y, equal to each other, represent an oxygen atom; R.sub.1, equal to each other, represent a —O—R.sub.7 group in which R.sub.7 represents a C.sub.1-C.sub.30 alkyl group; R.sub.2, equal to each other, represent a hydrogen atom; R.sub.3, equal to each other, represent a C.sub.1-C.sub.20; Q, equal to each other, represent a CR.sub.3 group in which R.sub.3 represents a hydrogen atom.

3. Polymeric photovoltaic cell with inverted structure according to claim 1, wherein the organic electron-acceptor compound is selected from the group consisting of 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indacene[1,2-b:5,6-b′]-dithiophene, poly{[N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalene-diimide-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)}, and mixtures thereof.

4. Polymeric photovoltaic cell with inverted structure according to claim 1, wherein the anode is made of metal or takes the form of grids in a conductive material is an ink based on metal nanowires.

5. Polymeric photovoltaic cell with inverted structure according to claim 1, wherein the anodic buffer layer is selected from the group consisting of PEDOT:PSS [poly(3,4-ethylenedioxythiophene):polystyrene sulfonate] and polyaniline (PANI).

6. Polymeric photovoltaic cell with inverted structure according to claim 1, wherein the cathodic buffer layer comprises zinc oxide or titanium oxide.

7. Polymeric photovoltaic cell with inverted structure according to claim 1, wherein the cathode is of a material selected from the group consisting of indium tin oxide (ITO), fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide (AZO), and gadolinium oxide-doped zinc oxide (GZO) or takes the form of grids in a conductive material, or it takes the form of an ink based on metal nanowires.

8. Polymeric photovoltaic cell with inverted structure according to claim 1, wherein the cathode is associated with a support layer which is made of transparent rigid material, or of flexible material.

9. Photovoltaic module (or solar module), either on a rigid support or on a flexible support, comprising at least one polymeric photovoltaic cell with inverted structure according to claim 1.

10. Polymeric photovoltaic cell with inverted structure according to claim 1, wherein Z are equal to each other; Y are equal to each other; wherein R.sub.1 are equal to each other; wherein the alkyl groups C.sub.1-C.sub.20 of R.sub.4 are selected from alkyl groups C.sub.2-C.sub.10; wherein the alkyl groups C.sub.1-C.sub.20 of R.sub.5 are selected from alkyl groups C.sub.2-C.sub.10; wherein the alkyl groups C.sub.1-C.sub.20 of R.sub.6 are selected from alkyl groups C.sub.2-C.sub.10; wherein the alkyl groups C.sub.1-C.sub.30 of R.sub.7 are selected from alkyl groups C.sub.2-C.sub.24; wherein R.sub.2 is equal to each other, wherein the alkyl groups C.sub.1-C.sub.20 of R.sub.2 are selected from alkyl groups C.sub.2-C.sub.10; wherein the C.sub.1-C.sub.20 alkyl groups of R.sub.10 are selected from C.sub.2-C.sub.10 alkyl groups; wherein the C.sub.1-C.sub.20 alkyl groups of R.sub.11 are selected from C.sub.2-C.sub.10 alkyl groups; wherein R.sub.3 is equal to each other, wherein the C.sub.1-C.sub.20 alkyl groups of R.sub.3 are selected from C.sub.2-C.sub.10 alkyl groups; wherein Q is equal to each other; and wherein n is an integer between 20 and 300; the non-fullerene is selected from the group consisting of compounds based on perylene-diimides or naphthalene-diimides and fused aromatic rings, indacenotiophenes with electron-poor terminal groups, and derivatives of corannulene or truxenone.

11. Polymeric photovoltaic cell with inverted structure according to claim 2, wherein the C.sub.1-C.sub.30 alkyl group of R.sub.7 is a 2-octyldodecyloxy group; and wherein the C.sub.1-C.sub.20 alkyl group of R.sub.3 is a 2-ethylhexyl group.

12. Polymeric photovoltaic cell with inverted structure according to claim 4, wherein the anode is made of metal, the metal is selected from the group consisting of silver (Ag), gold (Au), and aluminum (Al); wherein the conductive material is selected from the group consisting of silver (Ag), copper (Cu), graphite, graphene, and a transparent conductive polymer; and wherein the metal nanowires include a metal selected from the group consisting of silver (Ag) and copper (Cu).

13. Polymeric photovoltaic cell with inverted structure according to claim 12, wherein the transparent conductive polymer is selected from the group consisting of PEDOT:PSS[poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)] and polyaniline (PANI).

14. Polymeric photovoltaic cell with inverted structure according to claim 5, wherein the anodic buffer layer is PEDOT:PSS [poly(3,4-ethylenedioxythiophene):polystyrene sulfonate].

15. Polymeric photovoltaic cell with inverted structure according to claim 6, wherein the cathodic buffer layer has zinc oxide.

16. Polymeric photovoltaic cell with inverted structure according to claim 7, wherein the conductive material being is selected from the group consisting of silver (Ag), copper (Cu), graphite, graphene, and a transparent conductive polymer; and wherein the metal nanowires include a metal selected from the group consisting of silver (Ag) and copper (Cu).

17. Polymeric photovoltaic cell with inverted structure according to claim 16, wherein the transparent conductive polymer is selected from the group consisting of PEDOT:PSS [poly (3,4-ethylenedioxythiophene):poly(styrene sulfonate)] and polyaniline (PANI).

18. Polymeric photovoltaic cell with inverted structure according to claim 8, wherein the transparent rigid material is glass or a flexible material.

19. Polymeric photovoltaic cell with inverted structure according to claim 18, wherein the flexible material is selected from the group consisting of polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyethylene imine (PI), polycarbonate (PC), polypropylene (PP), polyimide (PI), triacetyl cellulose (TAC), and copolymers thereof.

Description

DESCRIPTION OF THE DRAWINGS

[0063] FIG. 1 shows a current-voltage curve (I-V) obtained in Example 6 (comparative).

[0064] FIG. 2 shows a current-voltage curve (I-V) obtained in Example 7.

[0065] FIG. 3 shows a curve relating to the External Quantum Efficiency (EQE) which was recorded under a monochromatic light in Example 7.

[0066] FIG. 4 below represents a cross-sectional view of a polymeric photovoltaic cell (or solar cell) with inverted structure of the present disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0067] For the purpose of the present description and the following claims, the definitions of the numerical intervals always comprise the extreme values unless otherwise specified.

[0068] For the purpose of the present description and the following claims, the term “comprising” also includes the terms “which essentially consists of” or “which consists of”.

[0069] For the purpose of the present description and the following claims, the term “C.sub.1-C.sub.30 alkyl groups” and “C.sub.1-C.sub.20 alkyl groups” means alkyl groups containing from 1 to 30 carbon atoms and from 1 to 20 carbon atoms, respectively, linear or branched, saturated or unsaturated. Specific examples of C.sub.1-C.sub.20 alkyl groups are: methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, tert-butyl, pentyl, 2-ethyl-hexyl, hexyl, heptyl, n-octyl, nonyl, decyl, dodecyl, 2-octyldodecyl, 2-decyletethradecyl, 2-butyloctyl, 2-hexyldecyl, 3-decylpentadecyl, 4-decylhexadecyl, 3,7-dimethyloctyl.

[0070] For the purpose of the present description and the following claims, the term “cycloalkyl groups” means cycloalkyl groups containing from 3 to 30 carbon atoms. Said cycloalkyl groups can optionally be substituted with one or more groups, equal to or different from each other, selected from: halogen atoms such as, for example, fluorine, chlorine, bromine, preferably fluorine; hydroxyl groups; C.sub.1-C.sub.12 alkyl groups; C.sub.1-C.sub.12 alkoxy groups; C.sub.1-C.sub.12 thioalkoxy groups; C.sub.3-C.sub.24 tri-alkylsilyl groups; polyethylenoxyl groups; cyano groups; amino groups; C.sub.1-C.sub.12 mono- or di-alkylamine groups; nitro groups. Specific examples of cycloalkyl groups are: cyclopropyl, 2,2-difluorocyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, methylcyclohexyl, methoxycyclohexyl, fluorocyclohexyl, phenylcyclohexyl, decalin, abietyl.

[0071] For the purpose of the present description and the following claims, the term “aryl groups” means aromatic carbocyclic groups containing from 6 to 60 carbon atoms. Said aryl groups can optionally be substituted with one or more groups, equal to or different from each other, selected from: halogen atoms such as, for example, fluorine, chlorine, bromine, preferably fluorine; hydroxyl groups; C.sub.1-C.sub.12 alkyl groups; C.sub.1-C.sub.12 alkoxy groups; C.sub.1-C.sub.12 thioalkoxy groups; C.sub.3-C.sub.24 tri-alkylsilyl groups; polyethylenoxyl groups; cyano groups; amino groups; C.sub.1-C.sub.12 mono- or di-alkylamine groups; nitro groups. Specific examples of aryl groups are: phenyl, methylphenyl, trimethylphenyl, methoxyphenyl, hydroxyphenyl, phenyloxyphenyl, fluorophenyl, pentafluorophenyl, chlorophenyl, bromophenyl, nitrophenyl, dimethylaminophenyl, naphthyl, phenylnaphthyl, phenanthrene, anthracene.

[0072] For the purpose of the present description and the following claims, the term “heteroaryl groups” means heterocyclic aromatic, penta- or hexa-atomic groups, also benzocondensed or heterobicyclic, containing from 4 to 60 carbon atoms and from 1 to 4 heteroatoms selected from nitrogen, oxygen, sulfur, silicon, selenium, phosphorus. Said heteroaryl group can optionally be substituted with one or more groups, equal to or different from each other, selected from: halogen atoms such as, for example, fluorine, chlorine, bromine, preferably fluorine; hydroxyl groups; C.sub.1-C.sub.12 alkyl groups; C.sub.1-C.sub.12 alkoxy groups; C.sub.1-C.sub.12 thioalkoxy groups; C.sub.3-C.sub.24 tri-alkylsilyl groups; polyethylenoxyl groups; cyano groups; amino groups; C.sub.1-C.sub.12 mono- or di-alkylamine groups; nitro groups. Specific examples of heteroaryl groups are: pyridine, methylpyridine, methoxypyridine, phenylpyridine, fluoropyridine, pyrimidine, pyridazine, pyrazine, triazine, tetrazine, quinoline, quinoxaline, quinazoline, furan, thiophene, hexylthiophene, bromothiophene, dibromothiophene, pyrrole, oxazole, thiazole, isothiazole, oxadiazole, thiadiazole, pyrazole, imidazole, triazole, tetrazole, indole, benzofuran, benzothiophene, benzooxazole, benzothiazole, benzooxadiazole, benzothiadiazole, benzopyrazole, benzimidazole, benzotriazole, triazolopyridine, coumarin.

[0073] The term “polyethylenoxyl groups” means a group containing oxyethylene units in the molecule. Specific examples of polyethylenoxyl group are: methyloxy-ethylenoxyl, methyloxy-diethyleneoxyl, 3-oxatetraoxyl, 3,6-dioxaheptyloxyl, 3,6,9-trioxadecyloxyl, 3,6,9,12-tetraxohexadecyloxyl.

[0074] The conjugated polymer containing an anthradithiophene derivative having general formula (I) can be prepared according to techniques known in the art as described, for example, in the aforesaid international patent application WO 2019/175367 in the name of the Applicant, incorporated herein by reference. Further details relating to the processes for the preparation of said conjugated polymer containing an anthradithiophene derivative having general formula (I) can be found in the following examples.

[0075] In accordance with a preferred embodiment of the present disclosure, said organic electron-acceptor compound can be selected from 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indacene[1,2-b:5,6-b′]-dithiophene, poly{[N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalene-diimide-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)}, or mixtures thereof.

[0076] More details relating to said non-fullerene compounds can be found, for example, in Nielsen C. B. and others, “Accounts of Chemical Research” (2015), Vol. 48, pages 2803-2812; Zhan C. and others, “RSC Advances” (2015), Vol. 5, pages 93002-93026.

[0077] In accordance with a preferred embodiment of the present disclosure, said anode can be made of metal, said metal being preferably selected, for example, from silver (Ag), gold (Au), aluminum (Al); or it can consist of grids in conductive material, said conductive material being preferably selected, for example, from silver (Ag), copper (Cu), graphite, graphene, and of a transparent conductive polymer, said transparent conductive polymer being preferably selected, for example, from PEDOT:PSS[poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)], polyaniline (PANI); or it can consist of an ink based on metal nanowires, said metal being preferably selected, for example, from silver (Ag), copper (Cu).

[0078] Said anode can be obtained by depositing said metal on top of said anodic buffer layer through the deposition techniques known in the art such as, for example, vacuum evaporation, flexographic printing, knife-over-edge-coating, spray-coating, screen-printing. Alternatively, said anode can be obtained through deposition, above said anodic buffer layer, of said transparent conductive polymer via spin coating, or gravure printing, or flexographic printing, or slot die coating, followed by deposition of said grids in conductive material via evaporation, or screen-printing, or spray-coating, or flexographic printing. Alternatively, said anode can be obtained through deposition, above said anodic buffer layer of said ink based on metal nanowires via spin coating, or gravure printing, or flexographic printing, or slot die coating.

[0079] In accordance with a preferred embodiment of the present disclosure, said anodic buffer layer can be selected, for example, from PEDOT:PSS [poly(3,4-ethylenedioxythiophene):polystyrene sulfonate], polyaniline (PANI), preferably PEDOT:PSS [poly(3,4-ethylenedioxythiophene):polystyrene sulfonate].

[0080] Dispersions or solutions of PEDOT:PSS [poly(3,4-ethylenedioxythiophene):polystyrene sulfonate] which can be advantageously used for the purpose of the present disclosure and which are currently commercially available are the products Clevios™ by Heraeus, Orgacon™ by Agfa.

[0081] In order to improve the deposition and the properties of said anodic buffer layer, one or more additives can be added to said dispersions or solutions, such as, for example: polar solvents such as, for example, alcohols (for example, methanol, ethanol, propanol), dimethyl sulfoxide, or mixtures thereof; anionic surfactants such as, for example, carboxylates, sulfonated α-olefins, sulfonated alkyl-benzenes, alkyl sulfonates, esters of alkyl-ether sulfonates, triethanolamines alkyl sulfonates, or mixtures thereof; cationic surfactants such as, for example, alkyltrimethylammonium salts, dialkyldimethylammonium chlorides, alkylpyridinium chlorides, or mixtures thereof; ampholytic surfactants such as, for example, alkylcarboxybetaines, or mixtures thereof; non-ionic surfactants such as, for example, carboxylic diethanolamides, polyoxyethylene alkyl ethers, polyoxyethylene alkyl phenyl ethers, or mixtures thereof; polar compounds (for example, imidazole), or mixtures thereof; or mixtures thereof. More details relating to the addition of said additives can be found, for example, in: Synooka O. and others, “ACS Applied Materials & Interfaces” (2014), Vol. 6(14), pages 11068-11081; Fang G. and others, “Macromolecular Chemistry and Physics” (2011), Vol. 12, Issue 17, pages 1846-1851.

[0082] Said anodic buffer layer can be obtained by depositing the PEDOT:PSS [poly(3,4-ethylenedioxythiophene):polystyrene sulfonate], or the polyaniline (PANI), in the form of dispersion or solution, above the anode through the deposition techniques known in the art such as, for example, vacuum evaporation, spin coating, drop casting, doctor blade casting, spin-coating, slot die coating, gravure printing, flexographic printing, knife-over-edge-coating, spray-coating, screen-printing.

[0083] Said active layer can be obtained by depositing, above said cathodic buffer layer, a solution comprising at least one photoactive organic polymer selected from the conjugated polymers comprising an anthraditiophene derivative having general formula (I) above reported and at least one electron-acceptor organic compound selected from the non-fullerene, optionally polymeric, compounds, above reported, using appropriate deposition techniques such as, for example, spin-coating, spray-coating, ink-jet printing, slot die coating, gravure printing, screen printing.

[0084] In accordance with a preferred embodiment of the present disclosure, said cathodic buffer layer can comprise zinc oxide, titanium oxide, preferably zinc oxide.

[0085] Said cathodic buffer layer can be obtained by depositing a precursor solution of zinc oxide on said cathode through deposition techniques known in the art such as, for example, vacuum evaporation, spin-coating, drop casting, doctor blade casting, slot die coating, gravure printing, flexographic printing, knife-over-edge-coating, spray-coating, screen-printing.

[0086] More details in relation to the formation of said cathodic buffer layer starting from a precursor solution of zinc oxide can be found, for example, in Pò R. and others, “Energy & Environmental Science” (2014), Vol. 7, pages 925-943.

[0087] In accordance with a preferred embodiment of the present disclosure, said cathode can be of a material selected, for example, from: indium tin oxide (ITO), fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide (AZO), gadolinium oxide-doped zinc oxide (GZO); or it can consist of grids in conductive material, said conductive material being preferably selected, for example, from silver (Ag), copper (Cu), graphite, graphene, and of a transparent conductive polymer, said transparent conductive polymer being preferably selected, for example, from PEDOT:PSS [poly (3,4-ethylenedioxythiophene):poly(styrene sulfonate)], polyaniline (PANI); or it can consist of an ink based on metal nanowires, said metal being preferably selected, for example, from silver (Ag), copper (Cu).

[0088] Said cathode can be obtained through techniques known in the art such as, for example, sputtering, deposition assisted by electron beam. Alternatively, said cathode can be obtained through deposition of said transparent conductive polymer via spin coating, or gravure printing, or flexographic printing, or slot die coating, preceded by deposition of said grids in conductive material via evaporation, or screen-printing, or spray-coating, or flexographic printing. Alternatively, said cathode can be obtained through deposition of said ink based on metal nanowires via spin coating, or gravure printing, or flexographic printing, or slot die coating. The deposition can take place on the support layer selected from those listed below.

[0089] In accordance with a preferred embodiment of the present disclosure, said cathode can be associated with a support layer which can be made of transparent rigid material such as, for example, glass, or of flexible material such as, for example, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyethylene imine (PI), polycarbonate (PC), polypropylene (PP), polyimide (PI), triacetyl cellulose (TAC), or copolymers thereof.

[0090] As mentioned above, the anode, the cathode, the anodic buffer layer, and the cathodic buffer layer present in the aforesaid polymeric photovoltaic cell (or solar cell) with inverted structure, can be deposited by techniques known in the art. More details related to these techniques can be found, for example in: Pò R. and others, “Interfacial Layers”, in “Organic Solar Cells—Fundamentals, Devices, and Upscaling” (2014), Chapter 4, Richter H. and Rand B. Eds., Pan Stanford Publishing Pte Ltd.; Yoo S. and others, “Electrodes in Organic Photovoltaic Cells”, in “Organic Solar Cells—Fundamentals, Devices, and Upscaling” (2014), Chapter 5, Richter H. and Rand B. Eds., Pan Stanford Publishing Pte Ltd.; Angmo D. and others, “Journal of Applied Polymer Science” (2013), Vol. 129, Issue 1, pages 1-14.

[0091] As said above, said polymeric photovoltaic cells (or solar cells) with inverted structure can be advantageously used in the construction of photovoltaic modules (or solar modules), either on a rigid support or on a flexible support.

[0092] A further object of the present disclosure is therefore a photovoltaic module (or solar module), either on a rigid support or on a flexible support, comprising at least one polymeric photovoltaic cell (or solar cell) with inverted structure described above.

[0093] FIG. 4 below represents a cross-sectional view of a polymeric photovoltaic cell (or solar cell) with inverted structure object of the present disclosure.

[0094] With reference to FIG. 4, the polymeric photovoltaic cell (or solar cell) with inverted structure (1) comprises: [0095] a transparent glass support (7); [0096] a cathode (2) of indium-tin oxide (ITO); [0097] a cathodic buffer layer (3) comprising zinc oxide (ZnO); [0098] a layer of photoactive material (4) comprising regioregular poly(3-hexylthiophene) (P3HT) or a conjugated polymer comprising an anthraditiophene derivative having general formula (I) and 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hexyl-phenyl)-dithieno[2,3-d:2′,3′-d′]-s-indacene[1,2-b:5,6-b′]-dithiophene (IT-4F); [0099] an anodic buffer layer (5) comprising molybdenum oxide (MoO.sub.3); [0100] a silver (Ag) anode (6).

[0101] In order to better understand the present disclosure and to put it into practice, some illustrative and non-limiting examples thereof are reported below.

EXAMPLES

[0102] Characterization of the Polymers Obtained

[0103] Determination of the Molecular Weight

[0104] The molecular weight of the conjugated polymers obtained by operating in accordance with the following examples, was determined by Gel Permeation Chromatography (“GPC”) on a WATERS 150C instrument, using HT5432 columns, with trichlorobenzene eluent, at 80° C.

[0105] The weight average molecular weight (M.sub.w), the number average molecular weight (M.sub.n) and the polydispersity index (“PDI”), corresponding to the M.sub.w/M.sub.n ratio, are given.

[0106] Determination of the Optical Band-Gap

[0107] The conjugated polymers obtained by operating in accordance with the following examples, were characterized by UV-Vis-NIR spectroscopy to determine the energetic entity of the optical band-gap in solution or on thin film according to the following procedure.

[0108] In the case that the optical band-gap was measured in solution, the polymer was dissolved in toluene, chloroform, chlorobenzene, dichlorobenzene, trichlorobenzene, or other suitable solvent. The solution thus obtained was placed in a quartz cuvette and analysed in transmission by means of a double-beam UV-Vis-NIR spectrophotometer and double monochromator Perkin Elmer X 950, in the range 200 nm-850 nm, with a 2.0 nm bandwidth, scanning speed of 220 nm/min and 1 nm step, using as a reference an identical quartz cuvette containing only the solvent used as a reference.

[0109] In the case that the optical band-gap was measured on thin film, the polymer was dissolved in toluene, chloroform, chlorobenzene, dichlorobenzene, trichlorobenzene, or other suitable solvent, obtaining a solution having a concentration equal to about 10 mg/ml, which was deposited by spin-coating on a Suprasil quartz slide. The thin film thus obtained was analysed in transmission by means of a dual-beam UV-Vis-NIR spectrophotometer and double monochromator Perkin Elmer X 950, in the range 200 nm-850 nm, with a 2.0 nm bandwidth, scanning speed of 220 nm/min and 1 nm step, using an identical Suprasil quartz slide as such, as a reference.

[0110] The optical band-gap was estimated from the spectra in transmission by measuring the absorption edge corresponding to the transition from the valence band (VB) to the conduction band (CB). The intersection with the abscissa axis of the straight line tangent to the absorption band at the inflection point was used for the determination of the edge.

[0111] The inflection point (λ.sub.F, y.sub.F) was determined on the basis of the coordinates of the minimum of the spectrum in the first derivative, indicated with λ′.sub.min and y′.sub.min.

[0112] The equation of the straight line tangent to the UV-Vis spectrum at the inflection point (λ.sub.F, y.sub.F) is as follows:

y=y′.sub.minλ+y.sub.F−y′.sub.minλ′.sub.min

[0113] Finally, from the condition of intersection with the abscissa axis φ=0, it was obtained:

λ.sub.EDGE=(y′.sub.minλ′.sub.min−y.sub.F)/y′.sub.min

[0114] Therefore, by measuring the coordinates of the minimum of the first derivative spectrum and the corresponding absorbance value y.sub.F from the UV-Vis spectrum, λ.sub.EDGE was obtained directly by substitution.

[0115] The corresponding energy is:

E.sub.EDGE=hν.sub.EDGE=hc/λ.sub.EDGE

wherein: [0116] h=6.626 10-34 J s; [0117] c=2.998 108 m s.sup.−1;
that is:

E.sub.EDGE=1.988 10−16 J/λ.sub.EDGE (nm).

[0118] Lastly, remembering that 1 J=6.24 1018 eV, we have:

E.sub.EDGE=1240 eV/λ.sub.φEDGE (nm).

[0119] Determination of HOMO and LUMO

[0120] The determination of the HOMO and LUMO values of the conjugated polymers obtained by operating in accordance with the following examples, was carried out using the cyclic voltammetry (CV) technique. This technique makes it possible to measure the values of the potentials of formation of the radical cation and radical anion of the sample under examination. These values, inserted in a special equation, allow the HOMO and LUMO values of the polymer in question to be obtained. The difference between HOMO and LUMO makes the value of the electrochemical band-gap.

[0121] The values of the electrochemical band-gap are generally higher than the values of the optical band-gap since during the execution of the cyclic voltammetry (CV), the neutral compound is charged and undergoes a conformational reorganization, with an increase in the energy gap, while optical measurement does not lead to the formation of charged species.

[0122] The cyclic voltammetry (CV) measurements were performed with an Autolab PGSTAT12 potentiostat (with GPES Ecochemie software) in a three-electrode cell. In the measurements carried out, an Ag/AgCl electrode was used as the reference electrode, a platinum wire as the counter electrode and a glassy graphite electrode as the working electrode. The sample to be analysed was dissolved in a suitable solvent and subsequently deposited, with a calibrated capillary, on the working electrode, so as to form a film. The electrodes were immersed in a 0.1 M electrolytic solution of 95% tetrabutylammonium tetrafluroborate in acetonitrile. The sample was subsequently subjected to a cyclic potential in the shape of a triangular wave. At the same time, as a function of the applied potential difference, the current, which signals the occurrence of oxidation or reduction reactions of the present species, was monitored.

[0123] The oxidation process corresponds to the removal of an electron from HOMO, while the reduction cycle corresponds to the introduction of an electron into LUMO. The potentials of formation of radical cation and radical anion were derived from the value of the peak onset (E.sub.onset), which is caused by molecules and/or chain segments with HOMO-LUMO levels closer to the edges of the bands. The electrochemical potentials to those related to the electronic levels can be correlated if both refer to the vacuum. For this purpose, the potential of ferrocene in vacuum, known in the literature and equal to −4.8 eV, was taken as a reference. The inter-solvent redox pair ferrocene/ferrocinium (Fc/Fc.sup.+) was selected because it has an oxide-reduction potential independent of the working solvent.

[0124] The general formula for calculating the energies of the HOMO-LUMO levels is therefore given by the following equation:

E (eV)=−4,8+[E.sub.1/2 Ag/AgCl(Fc/Fc.sup.+)−E.sub.onset Ag/AgCl (polymer)]

wherein: [0125] E=HOMO or LUMO according to the entered E.sub.onset value; [0126] E.sub.1/2 Ag/AgCl=half-wave potential of the peak corresponding to the redox pair ferrocene/ferrocinium measured under the same analysis conditions as the sample and with the same trio of electrodes used for the sample; [0127] E.sub.onset Ag/AgCl=onset potential measured for the polymer in the anodic area when calculating HOMO and in the cathodic area when calculating LUMO.

Example 1

Preparation of 2,5-dibromobenzene-1,4-dicarbaldehyde Having Formula (II)

[0128] ##STR00004##

[0129] In a 100 ml flask, equipped with magnetic stirring, thermometer and coolant, in an inert atmosphere, N-bromosuccinaldehyde (Aldrich) (11.57 g; 65 mmoles) was added in small portions, over 15 minutes, to a solution of terephthaldehyde (Aldrich) (4.02 g; 30 mmoles) in sulfuric acid (Aldrich) (40 ml): the reaction mixture obtained was left, in an inert atmosphere, under stirring, at room temperature (25° C.), for 3 hours. Subsequently, the reaction mixture was placed in water and ice and the white precipitate obtained was recovered by filtration obtaining a solid. The solid was dissolved in dichloromethane (Aldrich) (200 ml) and the solution obtained was placed in a 500 ml separating funnel: the whole was extracted with a saturated sodium bicarbonate solution (Aldrich) (3×100 ml) obtaining an acid aqueous phase and an organic phase. The entire organic phase (obtained by combining the organic phases deriving from the three extractions) was washed to neutral with distilled water (3×50 ml) and subsequently anidrified on sodium sulphate (Aldrich) and evaporated obtaining a solid which was further purified by crystallization with ethyl acetate (Aldrich). The crystals obtained were collected by filtration obtaining 6.57 g of 2,5-dibromobenzene-1,4-dicarbaldehyde having formula (II) (yield 75%).

Example 2

Preparation of bis(2-octyldodecyl)[1,2-b:5,6-b]dithiophene-4,10-dicarboxylate Having Formula (III)

[0130] ##STR00005##

[0131] In a 100 ml flask, equipped with magnetic stirring, thermometer and coolant, in an inert atmosphere, 2,5 dibromobenzene-1,4-dicarbaldehyde having formula (II) obtained as described in Example 1 (0.292 g; 1.0 mmol) and potassium carbonate (K.sub.2CO.sub.3) (Aldrich) (0.691 g; 5.0 mmol) were added to a mixture of 3-thiopheneacetic acid (Aldrich) (0.312 g; 2.2 mmol), triphenylphosphine (Aldrich) (0.026 g; 0.1 mmol), palladium(II)acetate [Pd(OAc).sub.2] (0.112 g; 0.5 mmol) in anhydrous N,N-dimethylformamide (DMF) (Aldrich) (5 ml): the resulting reaction mixture was heated to 80° C. and left under stirring, at said temperature, for 24 hours. Subsequently, 1-bromo-2-octyldodecane (Sunatech) (0.795 g; 2.2 mmol) was added in a single portion: the reaction mixture obtained was left, under stirring, at 80° C., for 24 hours. Subsequently, after cooling to room temperature (25° C.), the reaction mixture was placed in a 500 ml separating funnel: an ammonium chloride (NH.sub.4Cl) 0.1 (Aldrich) (3×100 ml) solution was added to said reaction mixture and the whole was extracted with ethyl acetate (Aldrich) (3×100 ml) obtaining an aqueous phase and an organic phase. The entire organic phase (obtained by combining the organic phases deriving from the three extractions) was separated and subsequently anidrified on sodium sulphate (Aldrich) and evaporated. The residue obtained was purified by elution on a chromatographic column of silica gel [(eluent: n-heptane/ethyl acetate 98/2) (Carlo Erba)], obtaining 0.752 g of bis(2-octyldodecyl)anthra[1,2-b:5,6-b′]dithiophene-4,10-dicarboxylate having formula (III) as a waxy yellow solid (yield 80%).

Example 3

Preparation of bis(2-octyldodecyl)-2,7-bis-(tributylstannyl)anthra[1,2-b: 5,6-b′]dithiophene-4,10-dicarboxylate Having Formula (IV)

[0132] ##STR00006##

[0133] In a 250 ml flask, equipped with magnetic stirring, the following were charged, under an argon flow, in the order: bis(2-octyldodecyl)anthra[1,2-b:5,6-b′]dithiophene-4,10-dicarboxylate having formula (III) (0.470 g; 0.5 mmoles) obtained as described in Example 2, and 40 ml of anhydrous tetrahydrofuran (THF) (Aldrich). The reaction mixture obtained was placed at −78° C. for about 10 minutes. Subsequently, 4.4 ml of a lithium di-iso-propylamine (LDA) solution was added by dripping in a mixture of tetrahydrofuran (THF)/hexane (1:1, v/v) 2.0 M (0.182 g; 1.7 mmoles) (Aldrich): the reaction mixture obtained was kept at −78° C. for 3 hours. Subsequently, 0.678 ml of tri-butyl tin chloride (1.302 g; 4 mmoles) were added by dripping: the reaction mixture obtained was placed at −78° C., for 30 minutes and, subsequently, at room temperature (25° C.), for 16 hours. Subsequently, the reaction mixture was placed in a 500 ml separating funnel: said reaction mixture was diluted with a 0.1 M sodium bicarbonate solution (Aldrich) (200 ml) and extracted with diethyl ether (Aldrich) (3×100 ml), obtaining an acid aqueous phase and an organic phase. The entire organic phase (obtained by combining the organic phases deriving from the three extractions) was washed to neutral with water (3×50 ml) and subsequently anidrified on sodium sulphate (Aldrich) and evaporated. The residue obtained was purified by elution on a basic alumina chromatographic column (Aldrich) [(eluent: n-heptane) (Aldrich)], obtaining 0.607 g of bis(2-octyldodecyl)-2,7-bis(tributylstannyl)-anthra[1,2-b:5,6-b′]dithiophene-4,10-dicarboxylate having formula (IV) as straw yellow oil (yield 80%).

Example 4

Preparation of the Conjugated Copolymer Containing an Anthradithiophene Derivative Having Formula (Ia)

[0134] ##STR00007##

[0135] In a 250 ml flask, equipped with magnetic stirring, thermometer and coolant, in an inert atmosphere, the following were charged in the order: bis(2-octyl-dodecyl)-2,7-bis(tributylstannil)anthra[1,2-b:5,6-b′]dithiophene-4,10-dicarboxylate having formula (IV) obtained as described in Example 3 (1.517 g; 1.05 mmoles), chlorobenzene (Aldrich) (100 ml), 1,3-bis(5-bromothiophene-2-yl)-5,7-(bis(2-ethylhexyl)benzo-[1,2-c:4,5-c′]dithiophene-4,8-dione (Sunantech) (0.767 g; 1.00 mmoles), tris(dibenzylideneacetone)dipalladium(0) [Pd.sub.2(dba).sub.3] (Aldrich) (0.018 g; 0.02 mmoles) and tris(ortho-tolyl)phosphine [P(o-tol).sub.3] (Aldrich) (0.024 g; 0.08 mmoles). Subsequently, the reaction mixture was heated under reflux and left under stirring for 18 hours. The colour of the reaction mixture turned violet after 3 hours and turned dark violet at the end of the reaction (i.e. after 18 hours). Subsequently, after cooling to room temperature (25° C.), the reaction mixture obtained was placed in methanol (Aldrich) (300 ml) and the precipitate obtained was subjected to sequential extraction in a Soxhlet apparatus with methanol (Aldrich), acetone (Aldrich), n-heptane (Aldrich) and, finally, chloroform (Aldrich). The residue left inside the Soxhlet apparatus was dissolved in dichlorobenzene (Aldrich) (50 ml) at 80° C. and, subsequently, the hot solution obtained was precipitated in methanol (300 ml) (Aldrich). The obtained precipitate was collected and dried under vacuum at 50° C. for 16 hours, obtaining 1.23 g of a dark violet solid product (80% yield), corresponding to the conjugated polymer comprising an anthradithiophene derivative having formula (Ia).

[0136] Said solid product was subjected to determination of the molecular weight by Gel Permeation Chromatography (“GPC”) operating as described above, obtaining the following data: [0137] (M.sub.w)=74141 Dalton; [0138] (PDI)=6.369.

[0139] The values of the optical band-gap, operating as described above, both in solution (E.sub.g.sup.opt.sub.solution), and on thin film (E.sub.g.sup.opt.sub.film) and the HOMO value were also determined: [0140] (λ.sub.EDGE sol)=650 nm; [0141] (λ.sub.EDGE film)=654 nm; [0142] E.sub.g.sup.opt.sub.film=1.90 eV; [0143] E.sub.g.sup.opt.sub.solution=1.91 eV; [0144] HOMO=−5.49 eV.

Example 5 (Comparative)

Solar Cell Comprising Regioregular Poly-3-Hexylthiophene (P3HT)

[0145] For this purpose, a polymeric solar cell with inverted structure was used, schematically represented in FIG. 4.

[0146] For this purpose, a polymer-based device was prepared on an ITO (indium-tin oxide) coated glass substrate (Kintec Company—Hong Kong), previously subjected to a cleaning procedure consisting of a manual cleaning, rubbing with a lint-free cloth soaked in a detergent diluted with tap water. The substrate was then rinsed with tap water. Subsequently, the substrate was thoroughly cleaned using the following methods in sequence: ultrasonic baths in (i) distilled water plus detergent (followed by manual drying with a lint-free cloth); (ii) distilled water [followed by manual drying with a lint-free cloth]; (iii) acetone (Aldrich) and (iv) iso-propanol (Aldrich) in sequence. In particular, the substrate was placed in a beaker containing the solvent, placed in an ultrasonic bath, left at 40° C., for a treatment of 10 minutes. After treatments (iii) and (iv), the substrate was dried with a compressed nitrogen flow.

[0147] Subsequently, the glass/ITO was further cleaned in an air plasma device (Tucano type—Gambetti), immediately before proceeding to the next step.

[0148] The substrate thus treated was ready for the deposition of the cathodic buffer layer. For this purpose, the zinc oxide (ZnO) buffer layer was obtained starting from a 0.162 M solution of the complex [Zn.sup.2+]-ethanolamine (Aldrich) in butanol (Aldrich). The solution was deposited by rotation on the substrate operating at a rotation speed equal to 600 rpm (acceleration equal to 300 rpm/s), for 2 minutes and 30 seconds, and subsequently at a rotation speed equal to 1500 rpm, for 5 seconds. Immediately after deposition of the cathodic buffer layer, zinc oxide formation was obtained by thermally treating the device at 140° C. for 5 minutes on a hot plate in ambient air. The cathodic buffer layer thus obtained had a thickness equal to 30 nm and was partially removed from the surface with 0.1 M acetic acid (Aldrich), leaving the layer only on the desired surface.

[0149] The active layer, comprising regioregular poly-3-hexylthiophene (P3HT) (Plexcore OS) and methyl ester of the [6,6]-phenyl-C.sub.61-butyric acid (PC61BM) (Aldrich), was deposited on the cathodic buffer layer thus obtained by spin coating of a 1:0.8 (v/v) solution in o-dichlorobenzene (Aldrich) with a P3HT concentration equal to 10 mg/ml which had been kept under stirring overnight, operating at a rotation speed of 300 rpm (acceleration equal to 255 rpm/s), for 90 seconds. The thickness of the active layer was found to be 250 nm.

[0150] On the active layer thus obtained, the anodic buffer layer was deposited, which was obtained by depositing molybdenum oxide (MoO.sub.3) (Aldrich) through a thermal process: the thickness of the anodic buffer layer was equal to 10 nm. A silver (Ag) anode, having a thickness equal to 100 nm, was deposited on the anodic buffer layer by vacuum evaporation, appropriately masking the area of the device in order to obtain an active area equal to 25 mm.sup.2.

[0151] The depositions of the anodic buffer layer and of the anode were carried out in a standard evaporation chamber under vacuum containing the substrate and two evaporation vessels equipped with a heating resistance containing 10 mg of molybdenum oxide (MoO.sub.3) in powder and 10 (Ag) silver shots (diameter 1 mm-3 mm) (Aldrich), respectively. The evaporation process was carried out under vacuum, at a pressure of about 1×10.sup.−6 bar. The molybdenum oxide (MoO.sub.3) and silver (Ag), after evaporation, are condensed in the unmasked parts of the device.

[0152] The thicknesses were measured with a Dektak 150 (Veeco Instruments Inc.) profilometer.

[0153] The electrical characterization of the device obtained was carried out in a controlled atmosphere (nitrogen) in a glove box, at room temperature (25° C.). The current-voltage curves (I-V) were acquired with a Keithley®2600A multimeter connected to a personal computer for data collection. The photocurrent was measured by exposing the device to the light of an ABET SUN® 2000-4 solar simulator, capable of providing 1.5G AM radiation with an intensity equal to 100 mW/cm.sup.2 (1 sun), measured with a Ophir Nova® II powermeter connected to a 3A-P thermal sensor. The device, in particular, is masked before said electrical characterization, so as to obtain an effective active area equal to 16 mm.sup.2: Table 1 shows the four characteristic parameters as average values.

Example 6 (Comparative)

Solar Cell Comprising Conjugated Polymer Comprising an Anthradithiophene Derivative Having Formula (Ia)

[0154] A polymer-based device was prepared on an ITO (indium-tin oxide) coated glass substrate (Kintec Company—Hong Kong), previously subjected to a cleaning procedure operating as described in Example 5.

[0155] The deposition of the cathodic buffer layer and the deposition of the anodic buffer layer were carried out as described in Example 5; the composition of said cathodic buffer layer and the composition of said anodic buffer layer are the same as the ones in Example 5; the thickness of said cathodic buffer layer and the thickness of said anodic buffer layer are the same as the ones in Example 5.

[0156] The active layer, comprising the conjugated polymer comprising an anthradithiophene derivative having formula (Ia) obtained as described in Example 4 and methyl ester of the [6,6]-phenyl-C.sub.61-butyric acid (PC61BM) (Aldrich), was deposited on the cathodic buffer layer thus obtained by spin coating of a 1:1.5 (v/v) solution in o-dichlorobenzene (Aldrich) with a concentration of conjugated polymer comprising an anthradithiophene derivative having formula (Ia) equal to 7 mg/ml which had been kept under stirring overnight, at a temperature of 100° C., operating at a rotation speed equal to 1000 rpm (acceleration equal to 2500 rpm/s), for 30 seconds. The thickness of the active layer was found to be 178 nm.

[0157] The deposition of the silver (Ag) anode was carried out as described in Example 5: the thickness of said silver anode (Ag) is the same as the one reported in Example 5.

[0158] The thicknesses were measured with a Dektak 150 (Veeco Instruments Inc.) profilometer.

[0159] The electrical characterization of the obtained device was carried out as described in Example 5: Table 1 shows the four characteristic parameters as average values.

[0160] FIG. 1 shows the current-voltage curve (I-V) obtained [the abscissa shows the voltage in millivolts (V); the ordinate shows the short circuit current density (Jsc) in milliamps/cm.sup.2 (mA/cm.sup.2)].

Example 7 (Disclosure)

Solar Cell Comprising [Conjugated Polymer Having Formula (Ia)]

[0161] A polymer-based device was prepared on an ITO (indium-tin oxide) coated glass substrate (Kintec Company—Hong Kong), previously subjected to a cleaning procedure operating as described in Example 5.

[0162] The deposition of the cathodic buffer layer and the deposition of the anodic buffer layer were carried out as described in Example 5; the composition of said cathodic buffer layer and the composition of said anodic buffer layer are the same as the ones in Example 5; the thickness of said cathodic buffer layer and the thickness of said anodic buffer layer are the same as the ones in Example 5.

[0163] The active layer, comprising the conjugated polymer comprising an anthradithiophene derivative having formula (Ia) obtained as described in Example 4 and 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hexyl-phenyl)-dithieno[2,3-d:2′,3′-d′]-s-indacene[1,2-b:5,6-b′]dithiophene (IT-4F) (Ossila), was deposited on the cathodic buffer layer thus obtained by spin coating of a 1:1 (v:v) solution in o-dichlorobenzene (Aldrich) with a concentration of conjugated polymer comprising an anthradithiophene derivative having formula (Ia) equal to 7 mg/ml which had been lett under stirring overnight, operating at a rotation speed equal to 2000 rpm (acceleration equal to 2500 rpm/s), for 30 seconds. The thickness of the active layer was found to be 102 nm.

[0164] The deposition of the silver (Ag) anode was carried out as described in Example 5: the thickness of said silver anode (Ag) is the same as the one reported in Example 5.

[0165] The thicknesses were measured with a Dektak 150 (Veeco Instruments Inc.) profilometer.

[0166] The electrical characterization of the obtained device was carried out as described in Example 5: Table 1 shows the four characteristic parameters as average values.

[0167] FIG. 2 shows the current-voltage curve (I-V) obtained [the abscissa shows the voltage in millivolts (V); the ordinate shows the short circuit current density (Jsc) in milliamps/cm.sup.2 (mA/cm.sup.2)].

[0168] FIG. 3 shows the curve relating to the External Quantum Efficiency (EQE) which was recorded under a monochromatic light (obtained using the TMc300E-U (I/C)—Triple grating monochromator and a double source with a Xenon lamp and a halogen lamp with quartz) in an instrument from Bentham Instruments Ltd [the abscissa shows the wavelength in nanometers (nm); the ordinate shows the External Quantum Efficiency (EQE) in percent (%)].

TABLE-US-00001 TABLE 1 V.sub.OC.sup.(2) J.sub.SC.sup.(3) PCE.sub.av.sup.(4) EXAMPLE FF.sup.(1) (V) (mA/cm.sup.2) (%) 5 (comparative) 0.57 0.56 10.10 3.30 6 (comparative) 0.68 0.86 11.47 6.68 7 (disclosure) 0.69 0.82 16.79 9.39 .sup.(1)FF (Fill Factor) is calculated according to the following equation: [00001]$\frac{V_{\mathrm{MPP}}.Math.J_{\mathrm{MPP}}}{V_{\mathrm{OC}}.Math.J_{\mathrm{SC}}}$wherein V.sub.MPP and J.sub.MPP are voltage and current density corresponding to the point of maximum power, respectively, V.sub.OC is the open circuit voltage and J.sub.SC is the short circuit current density; .sup.(2)V.sub.OC is the open circuit voltage; .sup.(3)J.sub.SC is the short circuit current density; .sup.(4)PCE.sub.av is the device efficiency calculated according to the following equation: [00002]$\frac{V_{\mathrm{OC}}.Math.J_{\mathrm{SC}}.Math.\mathrm{FF}}{P_{\mathrm{in}}}$ [0169] wherein V.sub.OC, J.sub.SC and FF have the same meanings reported above and P.sub.in is the intensity of the incident light on the device.