Polymer photovoltaic cell with an inverted structure and process for its preparation

Abstract

Polymeric photovoltaic cell (or solar cell) with an inverted structure comprising: an anode; a first anode buffer layer; an active layer comprising at least one photoactive organic polymer as the electron donor and at least one organic electron acceptor compound; a cathode buffer layer; a cathode; wherein between said first anode buffer layer and said active layer a second anode buffer layer is placed comprising a hole transporting material, said hole transporting material being obtained through a process comprising: reacting at least one heteropoly acid containing at least one transition metal belonging to group 5 or 6 of the Periodic Table of the Elements; with at least an equivalent amount of a salt or a complex of a transition metal belonging to group 5 or 6 of the Periodic Table of the Elements with an organic anion, or with an organic ligand; in the presence of at least one organic solvent selected from alcohols, ketones, esters, preferably alcohols. Said polymer photovoltaic cell (or solar cell) with an inverted structure displays high photoelectric conversion efficiency values (η), i.e. a photoelectric conversion efficiency (η) greater than or equal to 4.5%, and good open circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF) values. Furthermore, said polymer photovoltaic cell (or solar cell) with an inverted structure is able to maintain said values over time, in particular, in terms of photoelectric conversion efficiency (η).

Claims

1. Polymer photovoltaic cell (or solar cell) with an inverted structure comprising: an anode; a first anode buffer layer; an active layer comprising at least one photoactive organic polymer as the electron donor and at least one organic electron acceptor compound; a cathode buffer layer; a cathode; wherein between said first anode buffer layer and said active layer a second anode buffer layer is placed comprising a hole transporting material, said hole transporting material being obtained through a process comprising: reacting at least one heteropoly acid containing at least one transition metal belonging to group 5 or 6 of the Periodic Table of the Elements; with an equivalent amount of at least a salt or a complex of a transition metal belonging to group 5 or 6 of the Periodic Table of the Elements with an organic anion, or with an organic ligand; in the presence of at least one organic solvent selected from alcohols, ketones, esters.

2. Polymer photovoltaic cell (or solar cell) with an inverted structure according to claim 1, wherein said anode is made of metal, or it is constituted by grids of conductive material, and by a transparent conductive polymer or it is constituted by a metal nanowire-based ink.

3. Polymer photovoltaic cell (or solar cell) with an inverted structure according to claim 1, wherein said first anode buffer layer is selected from PEDOT:PSS [poly(3,4-ethylenedioxythiophene):polystyrene sulfonate, and polyaniline (PANI)].

4. Polymer photovoltaic cell (or solar cell) with an inverted structure according to claim 1, wherein said photoactive organic polymer is selected from: (a) polythiophenes; (b) conjugated statistical or alternating copolymers comprising: at least one benzotriazole unit (B) having the general formula (Ia) or (Ib): ##STR00003##  in which the group R is selected from alkyl groups, aryl groups, acyl groups, thioacyl groups, said alkyl, aryl, acyl and thioacyl groups being optionally substituted; at least one conjugated structural unit (A), in which each unit (B) is connected to at least one unit (A) in any of positions 4, 5, 6, or 7; (c) conjugated alternating copolymers comprising benzothiadiazole units; (d) conjugated alternating copolymers comprising thieno[3,4-b] pyrazidine units; (e) conjugated alternating copolymers comprising quinoxaline units; (f) conjugated alternating copolymers comprising monomeric silica units; (g) conjugated alternating copolymers comprising condensed thiophene units; (h) conjugated alternating copolymers comprising benzothiodiazole or naphtathiadiazole units, or conjugated alternating copolymers substituted with at least one fluorine atom, and thiophene units substituted with at least one fluorine atom; (i) conjugated copolymers comprising thieno [3,4-c]pyrrole-4,6-dione units; (l) conjugated copolymers comprising thienothiophene units; (m) polymers comprising a derivative of indacen-4-one having general formula (III), (IV) or (V): ##STR00004## in which: W and W.sub.1, identical or different, represent an oxygen atom; a sulfur atom; an N—R.sub.3 group wherein R.sub.3 represents a hydrogen atom, or is selected from linear or branched alkyl groups of C.sub.1-C.sub.20; Z and Y, identical or different, represent a nitrogen atom; or a C—R.sub.4 group in which R.sub.4 represents a hydrogen atom, or is selected from linear or branched C.sub.1-C.sub.20, alkyl groups, optionally substituted cycloalkyl groups, optionally substituted aryl groups, optionally substituted heteroaryl groups, linear or branched C.sub.1-C.sub.20, alkoxy groups, R.sub.5—O—[CH.sub.2—CH.sub.2—O].sub.n— polyethylenoxy groups in which R.sub.5 is selected from linear or branched C.sub.1-C.sub.m, alkyl groups, and n is an integer ranging from 1 to 4, —R.sub.6—OR.sub.7 groups in which R.sub.6 is selected from linear or branched C.sub.1-C.sub.20 alkyl groups, and R.sub.7 represents a hydrogen atom or is selected from linear or branched C.sub.1-C.sub.20 alkyl groups, or is selected from R.sub.5—[—OCH.sub.2—CH.sub.2].sub.n— polyethylenoxy groups in which R.sub.5 has the same meanings reported above and n is an integer ranging from 1 to 4, —COR.sub.8 groups wherein R.sub.8 is selected from linear or branched C.sub.1-C.sub.20 alkyl groups, COOR.sub.9 groups in which R.sub.9 is selected from linear or branched C.sub.1-C.sub.20 alkyl groups; or represent a —CHO group, or a cyano group (—CN); R.sub.1 and R.sub.2, identical or different, are selected from linear or branched C.sub.1-C.sub.20 alkyl groups; optionally substituted cycloalkyl groups; optionally substituted aryl groups; optionally substituted heteroaryl groups; linear or branched C.sub.1-C.sub.20 alkoxy groups; R.sub.5—O—[CH.sub.2—CH.sub.2—O].sub.n— polyethylenoxy groups in which R.sub.5 has the same meanings reported above and n is an integer ranging from 1 to 4; —R.sub.6—OR.sub.7 groups in which R.sub.6 and R.sub.7 have the same meanings reported above; —COR.sub.8 groups in which R.sub.8 has the same meanings reported above; —COOR.sub.9 groups in which R.sub.9 has the same meanings reported above; or represent a —CHO group, or a cyano group (—CN); D represents an electron donor group; A represents an electron acceptor group; n is an integer ranging from 10 to 500.

5. Polymer photovoltaic cell (or solar cell) with an inverted structure according to claim 1, wherein said organic electron acceptor compound is selected from: fullerene derivatives; or non-fullerene compounds, optionally polymeric.

6. Polymer photovoltaic cell (or solar cell) with an inverted structure according to claim 1, wherein said cathode buffer layer comprises zinc oxide, or titanium oxide.

7. Polymer photovoltaic cell (or solar cell) with an inverted structure according to claim 1, wherein said cathode is made of a material selected from: indium tin oxide (ITO), tin oxide doped with fluorine (FTO), zinc oxide doped with aluminum (AZO), and zinc oxide doped with gadolinium oxide (GZO); or it is constituted by grids of conductive material selected from silver (Ag), copper (Cu), graphite, and graphene, and by a transparent conductive polymer selected from PEDOT:PSS [poly(3,4-ethylenedioxythiophene):polystyrene sulfonate], and polyaniline (PANI); or it is constituted by a metal nanowire-based ink selected from silver (Ag), and copper (Cu).

8. Polymer photovoltaic cell (or solar cell) with an inverted structure according to claim 1, wherein said cathode is associated with a support layer which is made of rigid transparent material, or flexible material.

9. Polymer photovoltaic cell (or solar cell) with an inverted structure according to claim 1, wherein said at least one heteropoly acid is selected from heteropoly acids having general formula (I):
H.sub.x[A(MO.sub.3).sub.yO.sub.z]  (I) wherein: A represents a silicon atom, or a phosphorus atom; M represents an atom of a transition metal belonging to group 5 or 6 of the Periodic Table of the Elements, preferably selected from molybdenum, tungsten; x is an integer that depends on the valence of A; y is 12 or 18; z is 4 or 6.

10. Polymer photovoltaic cell (or solar cell) with an inverted structure according to claim 1, wherein said at least one heteropoly acid is selected from heteropoly acids having general formula (II):
H.sub.x[A(Mo).sub.p(V).sub.qO.sub.40]  (II) wherein: A represents a silicon atom, or a phosphorus atom; x is an integer that depends on the valence of A; p is 6 or 10; q is 2 or 6.

11. Polymer photovoltaic cell (or solar cell) with an inverted structure according to claim 1, wherein said heteropoly acids having general formula (I) and said heteropoly acids having general formula (II) are selected from: phosphomolybdic acid hydrate {H.sub.3[P(MoO.sub.3).sub.12O.sub.4].nH.sub.2O}, phosphomolybdic acid {H.sub.3[P(MoO.sub.3).sub.12O.sub.4]} in alcoholic solution, phosphotungstic acid hydrate {H.sub.3[P(WO.sub.3).sub.12O.sub.4].nH.sub.2O}, phosphotungstic acid in alcoholic solution {H.sub.3[P(WO.sub.3).sub.12O.sub.4]}, silicomolybdic acid hydrate {H.sub.4[Si(MoO.sub.3).sub.12O.sub.4].nH.sub.2O}, silicomolybdico acid {H.sub.4[Si(MoO.sub.3).sub.12O.sub.4]}, in alcohol solution, silicotungstic acid hydrate {H.sub.3[Si(WO.sub.3).sub.12O.sub.4].nH.sub.2O}, silicotungstic acid {H.sub.3[Si(WO.sub.3).sub.12O.sub.4]}, in alcohol solution, phosphomolybdovanadic acid {H.sub.3[P(Mo).sub.6(V).sub.6O.sub.40].nH.sub.2O}, phosphomolybdovanadic acid{H.sub.3[P(Mo).sub.6(V).sub.6O.sub.40]} in alcohol solution, phosphomolybdovanadic acid {H.sub.3[P(Mo).sub.10(V).sub.2O.sub.40} nH.sub.2O}hydrate, phosphomolybdovanadic acid {H.sub.3[P(Mo).sub.10(V).sub.2O.sub.40]} in alcoholic solution, and mixtures thereof.

12. Polymer photovoltaic cell (or solar cell) with an inverted structure according to claim 1, wherein said at least one salt or a complex of a transition metal belonging to group 5 or 6 of the Periodic Table of the Elements with an organic anion or with an organic ligand, is selected from: bis(acetylacetonato)dioxomolybdenum (VI) (Cas No. 17524-05-9), molybdenum(VI) dioxide bis(2,2,6,6-tetramethyl-3,5-heptanedionate (Cas No. 34872-98-5), bis(tert-butylimido)(bis)(dimethylamido)molybdenum (VI) (Cas No. 923956-62-1), 2,6-di-iso-propylphenyl imido-neophilidene molybdenum (VI) bis(tert-butoxide) (Cas No. 126949-65-3), 2,6-di-iso-propylphenylimidoneophilidene molybdenum (VI) bis(hexafluoro-tert-butoxide) (“Schrock's catalyst”) (Cas No. 139220-25-0), adduct of 2,6-di-iso-propylphenylimidoneophylidene-molybdenum (VI)bis(trifluoro-methanesulfonate) dimethoxyethane (Cas No. 126949-63-1), 2,6-di-iso-propylphenylimidoneophylidene-[racemic-BIPHEN] molybdenum (VI) (“rac-Schrock's-Hoveyda catalyst”) (Cas No. 300344-02-9), 2,6-di-iso-propylphenylimidoneophylidene[R-(+)-BIPHEN]molybdenum (VI) [“(R) Schrock's-Hoveyda catalyst” ] (Cas No. 329735-77-5), 2,6-di-iso-propylphenylimidoneophylidene [S-(−)BIPHEN]molybdenum (VI) [“(S) Schrock's-Hoveyda catalyst” ] (Cas No. 205815-80-1), vanadium(V) oxytriisopropoxide (Cas No. 5588-84-1), bis(acetylacetonate) oxovanadium (IV) (Cas No. 3153-26-2), acetylacetonate of vanadium (III), tetrakis(dimethylamino)vanadium (IV) (Cas No. 19824-56-7), tetrakis (diethylamino)vanadium (IV) (Cas No. 219852-96-7), and mixtures thereof.

13. Polymer photovoltaic cell (or solar cell) with an inverted structure according to claim 1, wherein the organic solvent is an alcohol selected from: methanol, ethanol, trifluoroethanol, n-propanol, iso-propanol, hexafluoro-iso-propanol, n-butanol, and mixtures thereof.

14. Polymer photovoltaic cell (or solar cell) with an inverted structure according to claim 1, wherein the organic solvent is a ketone selected from: cyclohexanone, acetone, methyl ethyl ketone, and mixtures thereof.

15. Polymer photovoltaic cell (or solar cell) with an inverted structure according to claim 1, in which the organic solvent is an ester selected from: butyrolactone, ethyl acetate, propyl acetate, butyl acetate, ethyl butyrate, and mixtures thereof.

16. Polymer photovoltaic cell (or solar cell) with an inverted structure according to claim 1, wherein said process is conducted at a temperature ranging from 25° C. to the boiling point of the solvent used, and for a time ranging from 15 minutes to 8 hours.

17. Process for the preparation of the polymer photovoltaic cell (or solar cell) with an inverted structure according to claim 1, comprising: forming the cathode by sputtering; or via electron beam assisted deposition; or through deposition of a conductive transparent polymer via spin coating, or gravure printing, or flexographic printing, or slot die coating, preceded by the deposition of grids of conductive material by evaporation, or screen-printing, or spray-coating, or flexographic printing; or by deposition of a metal nanowire-based ink via spin coating, or gravure printing, or flexographic printing, or slot die coating; forming the cathode buffer layer by spin coating, or gravure printing, or flexographic printing, or slot die above said cathode; forming the active layer via spin coating, or gravure printing, or slot-die, above said cathode buffer layer; forming the second anode buffer layer by spin coating, or gravure printing, or screen-printing, or flexographic printing, or slot-die above said active layer; forming the first anode buffer layer by spin coating, or gravure printing, or screen-printing, or flexographic printing, or slot-die, above said second anode buffer layer; forming the anode by vacuum evaporation, or screen-printing, or spray-coating, or flexographic printing, above said first anode buffer layer; or by deposition of a conductive transparent polymer via spin coating, or gravure printing, or flexographic printing, or slot die coating, followed by deposition of grids of conductive material by evaporation, or screen-printing, or spray-coating, or flexographic printing, above said first anode buffer layer; or by deposition of an metal nanowire-based ink via spin coating, or gravure printing, or flexographic printing, or slot die coating, above said first anode buffer layer.

18. Polymer photovoltaic cell (or solar cell) with an inverted structure according to claim 1, wherein: the anode has a thickness ranging from 50 nm to 150 nm; the first anode buffer layer has a thickness ranging from 10 nm to 2000 nm; the second anode buffer layer has a thickness ranging from 1 nm to 100 nm; the active layer has a thickness ranging from 50 nm to 500 nm; the cathode buffer layer has a thickness ranging from 10 nm to 100 nm; the cathode has a thickness ranging from 50 nm to 150 nm.

19. The polymeric photovoltaic cell (or solar cell) with an inverted structure according to claim 4, wherein the photoactive organic polymer is selected from the group consisting of PffBT4T-20D {poly[(5,6-difluoro-2,1,3-benzothiadiazole-4,7-diyl)-alt-(3,3″′-(2-octyldodecyl)-2,2′;5′,2″;5″,2″′-quaterthiophene-5,5″′-diyl)]}, PBDTTPD {{poly pyrrole-1,3-diyl][4,8-bis [(2-ethylhexyl)oxy]benzo-[1,2-b:4,5-b′]dithiophene-2,6-diyl]}, poly{PTB7({4,8-bis [(2-ethylhexyl) oxy]-benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl} {3-fluoro-2-[(2-ethylhexyl)carbonyl]-thieno[3,4-b]thiophendiyl})}, more preferably PTB7 {poly({4,8 bis[(2-ethylhexyl)oxy]-benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]-thieno[3,4-b]thiophendiyl})}.

Description

EXAMPLE 1

(1) Preparation of Vanadyl Phosphomolybdate in Iso-Propanol

(2) 211 mg of bis (acetylacetonato) oxovanadium (IV) (Cas No. 3153-26-2) (Strem Chemicals) (0.797 mmoles) dissolved in 20 ml of iso-propanol (Aldrich) were placed in a 250 ml flask and, subsequently 4.845 g of a 20% by weight solution of phosphomolybdic acid in ethanol (Aldrich) (0.531 mmoles) and 65 ml of iso-propanol (Aldrich) were added: the mixture obtained was heated to 65° C., for 2.5 hours, gradually obtaining a dark blue-green solution which, after 24 hours, gradually turns pale yellow. The solution obtained was cooled to ambient temperature (25° C.) and transferred into a glass vessel with a plug: one drop of said solution was placed on a strip of wet litmus paper to measure the pH, which was equal to about 6-7.

EXAMPLE 2

(3) Preparation of Vanadyl Silicotungstate in Iso-Propanol

(4) 1.14 g of silicotungstic acid hydrate (Fluka) (0.359 mmoles) dissolved in 20 ml of iso-propanol (Aldrich) were placed in a 250 ml flask and subsequently, 116 g of vanadium(V) oxytriisopropoxide (Cas No. 5588-84-1) (Aldrich) (0.475 mmoles) dissolved in 60 ml of iso-propanol (Aldrich) were added: the mixture obtained was heated to 70° C., for 3 hours, obtaining a colorless solution. The solution obtained was cooled to ambient temperature (25° C.) and transferred into a glass vessel with a plug: one drop of said solution was placed on a strip of wet litmus paper to measure the pH, which was equal to about 6-7.

EXAMPLE 3

(5) Preparation of Molybdenyl Phosphomolybdate in Iso-Propanol

(6) 260 mg of molybdenum(VI) dioxide bis(acetylacetonate) (Cas No. 17524-05-9) (Strem Chemicals) (0.797 mmoles) dissolved in 50 ml of iso-propanol (Aldrich) were placed in a 250 ml flask: the suspension thus obtained was heated to 80° C. Subsequently, 998 mg of phosphomolybdic acid trihydrate (Aldrich) (0.531 mmoles) dissolved in 50 ml of iso-propanol (Aldrich) were added: the mixture thus obtained was kept at 80° C., for 3 hours, obtaining a very intense emerald green solution. The solution obtained was cooled to ambient temperature (25° C.) and transferred into a glass vessel with a plug: one drop of said solution was placed on a strip of wet litmus paper to measure the pH, which was equal to about 6-7.

EXAMPLE 4 (INVENTION)

(7) Solar Cell with PTB7:PC.sub.71BM, Vanadyl Phosphomolybdate and PEDOT:PSS A polymer-based device was prepared on a glass substrate coated with ITO (indium tin oxide) (Kintec Company—Hong Kong), previously subjected to a cleaning process consisting of manual cleaning, rubbing with a lint-free cloth soaked in a detergent diluted with distilled water. The substrate was then rinsed with distilled water. Subsequently, the substrate was cleaned thoroughly through 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 at 40° C., for a 10 minute treatment. After treatments (iii) and (iv), the substrate was dried with a compressed nitrogen stream.

(8) Subsequently, the glass/ITO was cleaned further in an air-plasma device (Tucano type—Gambetti), straight before proceeding to the next step.

(9) The substrate thus treated was ready for the deposition of the cathode buffer layer. For that purpose, the zinc oxide 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 through 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 the deposition of the cathode buffer layer, the formation of zinc oxide was obtained by thermally treating the device at 140° C., for 5 minutes, on a hot plate in ambient air. The cathode buffer layer thus obtained had a thickness of 30 nm: subsequently, said cathode buffer layer was partially removed from the surface with 0.1 M acetic acid (Aldrich), leaving the layer only on the desired portion of the substrate.

(10) A solution of PTB7 {poly({4,8-bis[(2-ethylhexyl)-oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}-{3-fluoro-2-[(2-ethylhexyl)-carbonyl]-thieno[3,4-b]thiophenediyl})} (Aldrich) and [6,6]-phenyl-C.sub.71-butyric acid methyl ester (PC.sub.7113 M) (Aldrich), 1:1.5 (w:w) in chlorobenzene was prepared with a concentration of PTB7 equal to 10 mg/ml: said solution was left, under agitation, at 45° C., all night. Subsequently, the solution was left to cool to ambient temperature (25° C.) and 1,8-diiodooctane was added (3% by weight with respect to the total weight of the solution): everything was left, under agitation, at ambient temperature (25° C.), for 90 minutes, at the end of which the solution was left to rest, at ambient temperature (25° C.), for 30 minutes. The active layer was deposited, starting from the solution thus obtained, through spin coating, operating at a rotation speed equal to 2000 rpm (acceleration equal to 1000 rmp/s) for 2 minutes. The thickness of the active layer was 90 nm. At the end of the deposition, everything was kept under vacuum (about 10.sup.−2 bar), for about 20 minutes.

(11) The second anode buffer layer was deposited onto the active layer thus obtained, which was obtained starting from the vanadyl phosphomolybdate solution in iso-propanol obtained as described in Example 1, diluted 1:1 in iso-propanol, operating at a rotation speed of 5000 rpm (acceleration equal to 2500 rpm/s), for 30 seconds. The thickness of the second anode buffer layer was 15 nm: subsequently, said second anode buffer layer was partially removed from the surface with 0.1 M acetic acid (Aldrich), leaving the layer only on the desired portion of the substrate.

(12) The first anode buffer layer was deposited onto said second anode buffer layer, through spin coating starting from a suspension comprising PEDOT:PSS [poly(3,4-ethylenedioxythiophene):polystyrene sulfonate] (Clevios™ HTL Solar—Heraeus Co.) with a concentration of PEDOT:PSS equal to 1.2 mg/ml, operating at a rotation speed of 5000 rpm (acceleration equal to 2500 rpm/s), for 90 seconds: straight after the deposition of the anode buffer layer, the device was treated at 120° C., for 3 minutes, on a hot plate in ambient air. The thickness of the first anode buffer layer was 20 nm: subsequently, said first anode buffer layer was partially removed from the surface with 0.1 M acetic acid (Aldrich), leaving the layer only on the desired portion of the substrate.

(13) The silver (Ag) anode was deposited onto said first anode buffer layer, the anode having a thickness of 100 nm, through vacuum evaporation, appropriately masking the area of the device so as to obtain an active area of 0.25 mm.sup.2.

(14) The deposition of the anode was performed in a standard vacuum evaporation chamber containing the substrate and an evaporation vessel equipped with a heating element containing 10 shots of silver (Ag) (diameter 1 mm-3 mm) (Aldrich). The evaporation process was performed under vacuum, at a pressure of about 1×10.sup.−6 bar. After evaporation, the silver (Ag) was condensed in the non-masked parts of the device.

(15) The thicknesses were measured with a Dektak 150 profilometer (Veeco Instruments Inc.).

(16) The electrical characterization of the device obtained was performed in a (nitrogen) controlled atmosphere in a glove box at ambient temperature (25° C.). The current-voltage curves (I-V) were acquired with a Keithley® 2600A multimeter connected to a PC for data collection. The photocurrent was measured by exposing the device to the light of an ABET SUN® 2000-4 solar simulator, able to provide AM 1.5G radiation with an intensity of 90 mW/cm.sup.2 (0.9 suns), measured with an Ophir Nova® II powermeter connected to a 3A-P thermal sensor. Table 1 shows the characteristic parameters as mean values.

EXAMPLE 5 (INVENTION)

(17) Solar Cell with PTB7:PC.sub.71BM, Molybdenyl Phosphomolybdate and PEDOT:PSS

(18) A polymer-based device was prepared on a glass substrate coated with ITO (indium tin oxide) (Kintec Company—Hong Kong), previously subjected to a cleaning process operating as described in Example 4.

(19) The deposition of the cathode buffer layer, the deposition of the active layer and the deposition of the first anode buffer layer were performed as described in Example 4; the composition of said cathode buffer layer, the composition of said active layer and the composition of said first anode buffer layer are the same as those reported in Example 4; the thickness of said cathode buffer layer, the thickness of said active layer and the thickness of said first anode buffer layer are the same as those reported in Example 4. The second anode buffer layer was deposited onto the active layer obtained, through spin coating starting from the molybdenyl phosphomolybdate solution in iso-propanol obtained as described in Example 3, diluted in iso-propanol, operating at a rotation speed of 5000 rpm (acceleration equal to 2500 rpm/s), for 30 seconds. The thickness of the second anode buffer layer was 15 nm: subsequently, said second anode buffer layer was partially removed from the surface with 0.1 M acetic acid (Aldrich), leaving the layer only on the desired portion of the substrate.

(20) The deposition of the silver (Ag) anode was performed as described in Example 4:the thickness of said silver anode is the same as that reported in Example 4.

(21) The thicknesses were measured with a Dektak 150 profilometer (Veeco Instruments Inc.). The electrical characterization of the device, the current-voltage curves (I-V) and the photocurrent, were measured as described in Example 4. Table 1 shows the characteristic parameters as mean values.

EXAMPLE 6 (COMPARATIVE)

(22) Solar Cell with PTB7:PC.sub.71BM and Molybdenum Oxide (MoO.sub.3)

(23) A polymer-based device was prepared on a glass substrate coated with ITO (indium tin oxide) (Kintec Company—Hong Kong), previously subjected to a cleaning process operating as described in Example 4.

(24) The deposition of the cathode buffer layer and the deposition of the active layer were performed as described in Example 4; the composition of said cathode buffer layer and the composition of said active layer are the same as those reported in Example 4; the thickness of said cathode buffer layer and the thickness of said active layer are the same as those reported in Example 4.

(25) The anode buffer layer was deposited onto the active layer obtained, the buffer layer being obtained by depositing molybdenum oxide (MoO.sub.3) (Aldrich) through a thermal process: the thickness of the anode buffer layer was 10 nm. The silver (Ag) anode was deposited onto the anode buffer layer, the anode having a thickness of 100 nm, through vacuum evaporation, appropriately masking the area of the device so as to obtain an active area of 0.25 mm.sup.2.

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

(27) The thicknesses were measured with a Dektak 150 profilometer (Veeco Instruments Inc.). The electrical characterization of the device, the current-voltage curves (I-V) and the photocurrent, were measured as described in Example 4. Table 1 shows the characteristic parameters as mean values.

EXAMPLE 7 (COMPARATIVE)

(28) Solar Cell with PTB7:PC.sub.71BM and PEDOT:PSS

(29) A polymer-based device was prepared on a glass substrate coated with ITO (indium tin oxide) (Kintec Company—Hong Kong), previously subjected to a cleaning process operating as described in Example 4.

(30) The deposition of the cathode buffer layer, the deposition of the active layer and the deposition of the first anode buffer layer were performed as described in Example 4, the composition of said cathode buffer layer, the composition of said active layer and the composition of said first anode buffer layer are the same as those reported in Example 4; the thickness of said cathode buffer layer, the thickness of said active layer and the thickness of said first anode buffer layer are the same as those reported in Example 4. The deposition of the silver (Ag) anode was performed as described in Example 4: the thickness of said silver anode is the same as that reported in Example 4.

(31) The deposition of the second anode buffer layer between the active layer and the silver (Ag) anode was not performed.

(32) The thicknesses were measured with a Dektak 150 profilometer (Veeco Instruments Inc.). The electrical characterization of the device, the current-voltage curves (I-V) and the photocurrent, were measured as described in Example 4. Table 1 shows the characteristic parameters as mean values.

EXAMPLE 8 (COMPARATIVE)

(33) Solar Cell with PTB7:PC.sub.7iBM and Vanadyl Phosphomolybdate

(34) A polymer-based device was prepared on a glass substrate coated with ITO (indium tin oxide) (Kintec Company—Hong Kong), previously subjected to a cleaning process operating as described in Example 4.

(35) The deposition of the cathode buffer layer, the deposition of the active layer and the deposition of the second anode buffer layer were performed as described in Example 4; the composition of said cathode buffer layer, the composition of said active layer and the composition of said second anode buffer layer are the same as those reported in Example 4; the thickness of said cathode buffer layer, the thickness of said active layer and the thickness of said second anode buffer layer are the same as those reported in Example 4. The deposition of the silver (Ag) anode was performed as described in Example 4: the thickness of said silver anode is the same as that reported in Example 4.

(36) The deposition of the first anode buffer layer between the second anode buffer layer and the silver (Ag) anode was not performed.

(37) The thicknesses were measured with a Dektak 150 profilometer (Veeco Instruments Inc.). The electrical characterization of the device, the current-voltage curves (I-V) and the photocurrent, were measured as described in Example 4. Table 1 shows the characteristic parameters as mean values.

EXAMPLE 9 (COMPARATIVE)

(38) Solar Cell with PTB7:PC.sub.71BM and Mixture of Vanadyl Phosphomolybdate/PEDOT:PSS

(39) A polymer-based device was prepared on a glass substrate coated with ITO (indium tin oxide) (Kintec Company—Hong Kong), previously subjected to a cleaning process operating as described in Example 4.

(40) The deposition of the cathode buffer layer and the deposition of the active layer were performed as described in Example 4; the composition of said cathode buffer layer and the composition of said active layer are the same as those reported in Example 4; the thickness of said cathode buffer layer and the thickness of said active layer are the same as those reported in Example 4.

(41) An anode buffer layer was deposited onto the active layer obtained through spin coating starting from the solution obtained by mixing, at ambient temperature (25° C.), for 120 minutes, 0.8 ml of a suspension comprising PEDOT:PSS [poly(3,4-ethylenedioxythiophene):polystyrene sulfonate] (Clevios™ HTL Solar—Heraeus Co.) with a PEDOT:PSS concentration of 1.2 mg/ml, 0.1 ml of iso-propanol, and 0.1 ml of the solution of vanadyl phosphomolybdate in iso-propanol obtained as described in Example 1, operating at a rotation speed of 5000 rpm (acceleration equal to 2500 rpm/s), for 30 seconds: straight after the deposition of the anode buffer layer, the device was treated at 120° C., for 3 minutes, on a hot plate in ambient air. The thickness of the anode buffer layer was 15 nm: subsequently, said anode buffer layer was partially removed from the surface with 0.1 M acetic acid (Aldrich), leaving the layer only on the desired portion of the substrate.

(42) The deposition of the silver (Ag) anode was performed as described in Example 4: the thickness of said silver anode is the same as that reported in Example 4.

(43) The thicknesses were measured with a Dektak 150 profilometer (Veeco Instruments Inc.). The electrical characterization of the device, the current-voltage curves (I-V) and the photocurrent, were measured as described in Example 4. Table 1 shows the characteristic parameters as mean values.

EXAMPLE 10 (COMPARATIVE)

(44) Solar Cell with PTB7:PC.sub.71BM, Vanadium(V) Oxytriisopropoxide and PEDOT:PSSS

(45) A polymer-based device was prepared on a glass substrate coated with ITO (indium tin oxide) (Kintec Company—Hong Kong), previously subjected to a cleaning process operating as described in Example 4.

(46) The deposition of the cathode buffer layer, the deposition of the active layer and the deposition of the first anode buffer layer were performed as described in Example 4; the composition of said cathode buffer layer, the composition of said active layer and the composition of said first anode buffer layer are the same as those reported in Example 4; the thickness of said cathode buffer layer, the thickness of said active layer and the thickness of said first anode buffer layer are the same as those reported in Example 4. The second anode buffer layer was deposited onto the active layer obtained, the buffer layer being obtained through spin coating starting from a solution of vanadium(V) oxytriisopropoxide (Cas No. 5588-84-1) (Strem) in iso-propanol (Aldrich) at a concentration of 6.9 mg/ml, operating at a rotation speed of 5000 rpm (acceleration equal to 2500 rpm/s), for 30 seconds: straight after the deposition of the second anode buffer layer, the device was treated at 120° C., for 1 minute, on a hot plate in ambient air. The thickness of the second anode buffer layer was 15 nm: subsequently, said second anode buffer layer was partially removed from the surface with 0.1 M acetic acid (Aldrich), leaving the layer only on the desired portion of the substrate.

(47) The deposition of the silver (Ag) anode was performed as described in Example 4: the thickness of said silver anode is the same as that reported in Example 4.

(48) The thicknesses were measured with a Dektak 150 profilometer (Veeco Instruments Inc.). The electrical characterization of the device, the current-voltage curves (I-V) and the photocurrent, were measured as described in Example 4. Table 1 shows the characteristic parameters as mean values.

(49) TABLE-US-00001 TABLE 1 Voc.sup.(2) Jsc.sup.(3) η.sup.(4) Example FF.sup.(1) (mV) (mA/cm.sup.2) (%) 4 (invention) 0.59 0.77 11.09 5.05 (after 1 month) 0.59 0.76 10.78 4.84 5 (invention) 0.61 0.77 12.21 5.71 (after 1 month) 0.61 0.76 11.76 5.48 6 (comparative) 0.62 0.73 11.33 5.09 7 (comparative) 0.46 0.73 11.85 3.99 8 (comparative) 0.47 0.66 11.65 3.68 9 (comparative) 0.40 0.34 9.34 1.27 10 (comparative) 0.50 0.62 3.77 1.16 .sup.(1)fill factor; .sup.(2)open circuit voltage; .sup.(3)short-circuit photocurrent density; .sup.(4)photoelectric conversion efficiency.

(50) From the data reported in Table 1 it can be deduced that: the solar cell having a first anode buffer layer comprising PEDOT:PSS and a second anode buffer layer comprising vanadyl phosphomolybdate in accordance with the present invention [Example 4 (invention)], has comparable, if not higher, performance levels, in particular in terms of photoelectric conversion efficiency (η), which remain stable 1 month after the solar cell has been manufactured, both with respect to those of solar cells having a single anode buffer layer comprising molybdenum oxide (MoO.sub.3) [Example 6 (comparative)] or PEDOT:PSS [Example 7 (comparative)] or vanadyl phosphomolybdate [Example 8 (comparative)] or a mixture of PEDOT:PSS and vanadyl phosphomolybdate [Example 9 (comparative)], and with respect to those of a solar cell having a first anode buffer layer comprising PEDOT:PSS and a second anode buffer layer comprising vanadium(V) oxytriisopropoxide [Example 10 (comparative)]; the solar cell having a first anode buffer layer comprising PEDOT:PSS and a second anode buffer layer comprising molybdenyl phosphomolybdate in accordance with the present invention [Example 5 (invention)], has higher performance levels, in particular in terms of photoelectric conversion efficiency (η), which remain stable 1 month after the solar cell has been manufactured, both with respect to those of solar cells having a single anode buffer layer comprising molybdenum oxide (MoO.sub.3) [Example 6 (comparative)] or PEDOT:PSS [Example 7 (comparative)] and with respect to those of a solar cell having a first anode buffer layer comprising PEDOT:PSS and a second anode buffer layer comprising vanadium(V) oxytriisopropoxide [Example 10 (comparative)].