PROCESS FOR PRODUCING INVERTED POLYMER PHOTOVOLTAIC CELLS

Abstract

A Process for producing an inverted polymer photovoltaic cell (or solar cell) includes the following steps providing an electron contact layer (cathode); depositing a cathodic buffer layer onto said electron contact layer; depositing a photoactive layer comprising at least one photoactive organic polymer and at least one organic electron acceptor compound onto the cathodic buffer layer; depositing an anodic buffer layer onto the photoactive layer; and providing a hole contact layer (anode).

The step of depositing the cathodic buffer layer includes the steps of forming a layer onto the electron contact layer of a composition comprising having at least one zinc oxide and/or titanium dioxide or a precursor thereof, at least one organic solvent and at least one polymer soluble in the organic solvent; and plasma treating the layer formed onto the electron contact layer so as to form the cathodic buffer layer.

Claims

1. A process for producing an inverted polymer photovoltaic cell (or solar cell), the process includes the following steps: providing an electron contact layer (cathode); depositing a cathodic buffer layer onto said electron contact layer; depositing a photoactive layer comprising at least one photoactive organic polymer and at least one organic electron acceptor compound onto said cathodic buffer layer; depositing an anodic buffer layer onto said photoactive layer; and providing a hole contact layer (anode); wherein the step of depositing said cathodic buffer layer comprises: forming a layer onto said electron contact layer of a composition comprising at least one zinc oxide and/or titanium dioxide or a precursor thereof, at least one organic solvent and at least one polymer soluble in said organic solvent; and plasma treating said layer formed onto said electron contact layer so as to form the cathodic buffer layer.

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

3. The process for producing an inverted polymer photovoltaic cell (or solar cell) according to claim 1, wherein said electron contact layer (cathode) is associated with a support layer that is made of a rigid transparent material such as glass, or flexible material such as polyethylene terephthalate-(PET), polyethylene naphthalate (PEN), polyethyleneimine (PI), polycarbonate (PC), polypropylene (PP), polyimide (PI), triacetyl cellulose (TAC), or their copolymers.

4. The process for producing an inverted polymer photovoltaic cell (or solar cell) according to claim 1, wherein said photoactive organic polymer is selected from: (a) polythiophenes such as poly(3-hexylthiophene) (P3HT) regioregular, poly(3-octylthiophene), poly(3,4-ethylenedioxythiophene), or mixtures thereof; (b) conjugated alternating or statistical copolymers comprising: at least one benzotriazole unit (B) having general formula (Ia) or (Ib): ##STR00003## wherein 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), wherein 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 such as PCDTBT {poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4′, 7′-di-2-thienyl-2′,1′,3′-benzothiadiazole]}, PCPDTBT {poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta [2,1-b; 3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzotiadiazole)]}; (d) conjugated alternating copolymers comprising thieno[3,4-b]pyrazidine units; (e) conjugated alternating copolymers comprising quinoxaline units; (f) conjugated alternating copolymers comprising monomeric silylated units such as copolymers of 9,9-dialkyl-9-silafluorene; (g) conjugated alternating copolymers comprising condensed thiophene units such as copolymers of thieno[3,4-b] thiophene and of benzo [1,2-b: 4,5-b′] dithiophene; (h) conjugated alternating copolymers comprising benzothiadiazole or naphtothiadiazole units substituted with at least one fluorine atom and thiophene units substituted with at least one fluorine atom such as PffBT4T-2OD {poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3′″-(2-octyldodecyl)-2,2′,5′,2″;5″,2″-quaterthiophene-5,5″-diil)]}, PBTff4T-2OD {poly[(2,1,3-benzothiadiazole-4,7-diyl)-alt-(4′,3″-difluoro-3,3″′-(2-octyldodecyl)-2,2′;5′,2″;5″,2″-quaterthiophene-5,5″-diyl)]}, PNT4T-2OD {poly(naphtho[1,2-c:5,-c′]bis [1,2,5] thiadiazole-5,10-diyl)-alt-(3,3′″-(2-octyldodecyl)-2,2′; 5′,2″; 5″,2″-quaterthiophene-5,5′″-diyl)]; (i) conjugated copolymers comprising thieno[3,4-c]pyrrole-4,6-dione units such as PBDTTPD {poly[[5-(2-ethylhexyl)-5,6-dihydro-4,6-dioxo-4H-thieno[3,4-c]pyrrole-1,3-diyl][4,8-bis[(2-ethylhexyl)oxy]benzo-[1,2-b:4,5-b′] dithiophene-2,6-diyl]}; (l) conjugated copolymers comprising thienothiophene units such as 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}}; (m) polymers comprising a derivative of indacen-4-one having general formula (III), (IV) or (V): ##STR00004## wherein: 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 C.sub.1-C.sub.20 alkyl groups; Z and Y, identical or different, represent a nitrogen atom; or a C—R.sub.4 group wherein 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— polyethylenoxyl groups wherein R.sub.5 is selected from linear or branched C.sub.1-C.sub.20 alkyl groups, and n is an integer ranging from 1 to 4, —R.sub.6—OR.sub.7 groups wherein 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— polyethylenoxyl groups wherein 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 wherein 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, preferably identical, 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— polyethylenoxyl groups wherein 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 wherein R.sub.6 and R.sub.7 have the same meanings reported above; —COR.sub.8 groups wherein R.sub.8 has the same meanings as above; or —COOR.sub.9 groups wherein R.sub.9 has the same meanings as 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; or mixtures thereof.

5. The process for producing an inverted polymer photovoltaic cell (or solar cell) according to claim 4, wherein said photoactive organic polymer is selected from: PffBT4T-2OD {poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3′″-(2-octyl-dodecyl)-2,2′,5′,2″;5″,2″-quaterthiophene-5,5″-diyl)]}, PBDTTPD {{poly[[5-(2-ethylhexyl)-5,6-dihydro-4,6-dioxo-4H-thieno[3,4-c]pyrrole-1,3-diyl][4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]}, PTB7{poly({4,8-bis[(2-ethylhexyl)oxo]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyll)carbonyl]thieno[3,4-b]thiophenediyl})}, or mixtures thereof.

6. The process for producing an inverted polymer photovoltaic cell (or solar cell) according to claim 1, wherein said organic electron acceptor compound is selected from fullerene derivatives such as [6,6]-phenyl-C.sub.61-butyric acid methyl ester (PC.sub.61BM), [6,6]-phenyl-C.sub.71-butyric acid methyl ester (PC.sub.71BM), bis-adduct indene-C.sub.60 (ICBA), bis(1-[3-(methoxycarbonyl)propyl]-1-phenyl)-[6,6]C.sub.62 (Bis-PCBM), or mixtures thereof.

7. The process for producing an inverted polymer photovoltaic cell (or solar cell) according to claim 1, wherein said of the present invention, said organic electron acceptor compound is selected from non-fullerene, optionally polymeric, compounds such as compounds based on perylene-diimides or naphthalene-diimides and fused aromatic rings; indacenothiophene with terminal electron-poor groups; compounds having an aromatic core able to symmetrically rotate such as derivatives of corannulene or truxenone; or mixtures thereof; selected from 3,9-bis{2-methylene-[3-(1,1-dicyanomethylene)-indanone]}-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b: 5,6-b′]dithiophene, poly {[N,N′-bis (2-octyldodecyl)-1,4,5,8-naftalenediimide-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)}.

8. The process for producing an inverted polymer photovoltaic cell (or solar cell) according to claim 1, wherein said anodic buffer layer is selected from PEDOT:PSS [poly(3,4-ethylenedioxythiophene):polystyrene sulfonate], polyaniline (PANI).

9. The process for producing an inverted polymer photovoltaic cell (or solar cell) according to claim 1, wherein said anodic buffer layer is selected from hole transporting material obtained through a process comprising: (a) 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 such as 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, silicotungstic acid hydrate {H.sub.4[Si(WO.sub.3).sub.12O.sub.4].nH.sub.2O}, or mixtures thereof; with (b) an equivalent amount of at least one salt or one 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 such as molybdenum(VI) dioxide bis(acetylacetonate) (Cas No. 17524-05-9), vanadium(V) oxytriisopropoxide (Cas No. 5588-84-1), bis (acetylacetonate) oxovanadium (IV) (Cas No. 3153-26-2), or mixtures thereof in the presence of at least one organic solvent selected from alcohols, ketones, esters.

10. The process for producing an inverted polymer photovoltaic cell (or solar cell) according to claim 1, wherein said hole contact layer (anode) is made of metal, said metal being preferably selected from silver (Ag), gold (Au), and aluminum (Al); or it is constituted by grids of conductive material, said conductive material being preferably selected from silver (Ag), copper (Cu), graphite, graphene, and by a transparent conductive polymer, said transparent conductive polymer being preferably selected from PEDOT:PSS [poly(3,4-ethylenedioxythiophene):polystyrene sulfonate], polyaniline (PAM); or it is constituted by a metal nanowire-based ink, said metal being selected from silver (Ag) and copper (Cu).

11. The process for producing an inverted polymer photovoltaic cell (or solar cell) according to claim 1, wherein said zinc oxide precursor is selected from zinc salts and zinc complexes such as: zinc acetate, zinc formiate, zinc acetylacetonate, zinc alcoholates (such as methoxide, ethoxide, propoxide, iso-propoxide, butoxide), zinc carbamate, zinc bis(alkylamide(s), zinc dialkyls or diaryls (such as diethylzinc, diphenylzinc), or mixtures thereof.

12. The process for producing an inverted polymer photovoltaic cell (or solar cell) according to claim 1, wherein said titanium oxide precursor is selected from titanium salts and titanium complexes such as: titanium acetate, titanium formiate, titanium acetylacetonate, titanium alcoholates (such as methoxide, ethoxide, propoxide, iso-propoxide, butoxide), titanium carbamate, titanium bis(alkylamide(s), titanium dialkyls or diaryls (such as diethyltitanium, diphenyltitanium), or mixtures thereof.

13. The process for producing an inverted polymer photovoltaic cell (or solar cell) according to claim 1, wherein said at least one zinc oxide is in the form of colloidal nanoparticles; colloidal zinc oxide nanoparticles have an average particle size ranging from 5 nm to 50 nm.

14. The process for producing an inverted polymer photovoltaic cell (or solar cell) according to claim 1, wherein the amount of said zinc oxide and/or titanium dioxide or precursor thereof, in said composition is ranging from 1% by weight to 30% by weight, with respect to the total weight of the composition.

15. The process for producing an inverted polymer photovoltaic cell (or solar cell) according to claim 1, wherein said at least one organic solvent is selected from: alcohols such as methanol, ethanol, propanol, iso-propanol, butanol, or mixtures thereof; aromatic solvents such as toluene, o-xylene, m-xylene, p-xylene, or mixture thereof; aliphatic solvents such as hexane, heptane, or mixtures thereof; heteroaromatic solvents such as tetrahydrofuran, or mixtures thereof; heterocyclic solvents such as dioxane, or mixtures thereof; oxygenated solvents such as diethyl ether, dimethoxyethane, or mixtures thereof; polar solvents such as acetonitrile, N,N-dimethylformamide, N,N-dimethylacetamide, or mixtures thereof.

16. The process for producing an inverted polymer photovoltaic cell (or solar cell) according to claim 1, wherein said at least one polymer soluble in said organic solvent is selected from: poly(vinyllactames) such as poly(N-vinylpyrrolidone) (PVP), poly(N-vinylcaprolactam), poly(N-vinylbutirolactam), or mixtures thereof; poly(N-acylimines such as poly(N-acetylimine), poly(N-propanoylimine), or mixtures thereof; N,N-dialkyl substituted poly(acrylamides) such as poly(N,N-dimethylacrylamide), poly(N,N-diethylacrylamide), or mixtures thereof; poly[hydroxyalkyl(meth)acrylates such as poly(2-hydroxethylmethacrylate), or mixtures thereof; poly(vinylpyridines) such as poly(2-vinylpiridine), poly(-vinylpiridine), or mixtures thereof; poly(alkylglycols) such as poly(ethyleneglycol), poly(propyleneglycol), or mixtures thereof; or mixtures thereof; the amount of said polymer, in the composition being ranging from 0.02% by weight to 10% by weight, with respect to the total weight of the composition.

17. The process for producing an inverted polymer photovoltaic cell (or solar cell) according to claim 1, wherein said plasma treating is carried out: in the presence of an inorganic gas such as argon (Ar), helium (He), nitrogen (N.sub.2), oxygen (O.sub.2), or mixtures thereof; and/or under a discharge power ranging from 10 W to 1000 W.

18. The process for producing an inverted polymer photovoltaic cell (or solar cell) according to claim 1, wherein in said inverted polymer photovoltaic cell (or solar cell): the electron contact layer (cathode) has a thickness ranging from 50 nm to150 nm; the cathodic buffer layer has a thickness ranging from 10 nm to 100 nm; the photoactive layer has a thickness ranging from 50 nm to 250 nm; the anodic buffer layer has a thickness ranging from 200 nm to 2000 nm; and the hole contact layer (anode) has a thickness ranging from 5000 nm to 15000 nm, preferably ranging from 8000 nm to 12000 nm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0116] The present disclosure will now be illustrated in more detail through an embodiment with reference to FIG. 1 provided below which represents a cross sectional view of an inverted polymer photovoltaic cell (or solar cell) according to the present disclosure.

DETAILED DESCRIPTION OF THE DRAWINGS

[0117] With reference to FIG. 1, the inverted polymer photovoltaic cell (or solar cell) (1) comprises: [0118] a transparent support (7), for example a polyethylene terephthalate (PET); [0119] an electron contact layer (cathode) (2), for example an indium tin oxide (ITO) cathode; [0120] a cathodic buffer layer (3), comprising, for example, a composition comprising colloidal zinc oxide nanoparticle, ethanol and poly(N-vinylpyrrolidone) (PVP) obtained through roll-to-roll (R2R) gravure printing and subjected to plasma treating; [0121] a layer of photoactive material (4) comprising at least one photoactive organic polymer, for example, poly(-hexylthiophene) (P3HT) regioregular and at least one non-functionalized fullerene, for example, methyl ester of [6,6]-phenyl-C.sub.61-butyric acid (PC.sub.61BM) obtained through roll-to-roll (R2R) gravure printing; [0122] an anodic buffer layer (5), comprising, for example, PEDOT:PSS [poly(3,4-ethylenedioxythiophene):polystyrene sulfonate] obtained through roll-to-roll (R2R) rotary screen printing; [0123] an hole contact layer (anode) (6), for example a silver (Ag) anode, obtained through roll-to-roll (R2R) rotary screen printing.

[0124] For the purpose of understanding the present disclosure better and to put it into practice, below are some illustrative and non-limiting examples thereof.

Example 1 (Disclosure)

[0125] Solar Cell with Cathodic Buffer Layer Comprising Zinc Oxide and PVP Plasma Treated

[0126] A polymer-based device was prepared on top of a ITO (indium tin oxide)-coated PET (polyethyleneterephthalate) (Solutia/Eastman) substrate (surface resistivity equal to 40 Ω/sq-60 Ω/sq as disclosed in Välimäki M. et al. above reported. The PET and ITO thicknesses were equal to 125 μm and 0.125 μm, respectively. As a first process step, the ITO was patterned with Isishape HiperEtch 09S Type 40 paste (Merck) as a negative image to the desired pattern. R2R rotary screen printing was performed with a printing speed of 1.1 m/min. After printing, the printed film continued directly into the R2R hot air drying units set to a temperature of 140° C. for 218 seconds. The paste was washed off in baths of water and 2-propanol. After patterning, the surface was ultrasonically washed and dried in the R2R process.

[0127] The substrate thus treated was ready for the deposition of the cathodic buffer layer. For that purpose, to a 500 g of colloidal zinc oxide nanoparticle suspension in ethanol (ZnO Nanoparticles, 5 wt %, 15 nm) (Avantana—Switzerland), 0.51 g of poly(N-vinylpyrrolidone) (PVP) (Aldrich) were added: the whole was maintained, under stirring, at ambient temperature (25° C.), overnight, obtaining an homogeneous suspension which was kept in an ultrasonic bath for 10 minutes before printing. Subsequently, the obtained suspension was deposited through R2R gravure-printing on the substrate, operating at a speed equal to 8 m/min, at nip pressure equal to 1 bar-1.5 bar. The printing cylinder contains engravings with a line density equal to 120 lines/cm. Immediately after the deposition of the cathodic buffer layer, everything was positioned in an in-line plasma unit and was subjected to plasma treating. The plasma treating was performed for the printed and dried cathodic buffer layer in the R2R line at a speed of 2 m/min, using a mixture of N.sub.2/Ar (1/3, v/v) and 200 W discharge power in atmospheric pressure: as the R2R plasma process is not performed under vacuum and the plasma unit is open to air, there is always some (unknown) amount of oxygen also present, which might have an effect on the plasma process.

[0128] The cathodic buffer layer thus obtained had a thickness equal to 25 nm-50 nm.

[0129] A solution of poly(-hexylthiophene) (P3HT) regioregular (Rieke Metals) and [6,6]-phenyl-C.sub.61-butyric acid methyl ester (PC.sub.61BM) (purity 99.5%-Nano-C), 1:0.63 (w:w) in 1,2-dichlorobenzene was prepared with a total concentration of P3HT equal to 0.13 g/ml: said solution was left, under agitation, at 45° C., overnight: subsequently, the solution was left to cool to ambient temperature (25° C.). The photoactive layer was deposited, starting from the solution thus obtained, through R2R gravure-printing, operating at a speed equal to 8 m/min and at nip pressure equal to 1 bar-1.5 bar. The printing cylinder contains engravings with a line density equal to 120 lines/cm. The thickness of the photoactive layer was equal to 175 nm. Straight after printing, the (P3HT):(PC.sub.61BM) layer was dried at 120° C., for 30 seconds, in an oven, in ambient air.

[0130] The anodic buffer layer was deposited onto the photoactive layer thus obtained, starting from a highly viscous suspension comprising PEDOT:PSS [poly(3,4-ethylenedioxythiophene):polystyrene sulfonate] (Orgacon EL-P 5015—Agfa) with a concentration of PEDOT:PSS equal to 5% by weight, trough R2R rotary screen printing performed with a printing speed of 2 m/min: straight after the deposition of the anodic buffer layer, the device was dried, at 130° C., for 2 minutes, in an oven, in ambient air. The thickness of the anodic buffer layer was equal to 1000 nm.

[0131] The silver (Ag) hole contact layer (anode) was deposited onto said anodic buffer layer, starting from the thermoplastic polymer thick-film silver (XPVS-670-PPG Industrial Coatings) trough R2R rotary screen printing using 275 L (meshes/inch) screen (RVS) from Gallus, performed with a printing speed of 2 m/min: straight after the deposition of the hole contact layer (anode), the device was dried, at 130° C., for 2 minutes, in an oven, in ambient air. The thickness of the hole contact layer (anode) was equal to 10000 nm and the active area of the device was ranging from 19 cm.sup.2.

[0132] The obtained device was encapsulated inside a nitrogen glove box using a laminator which activates the pressure sensitive adhesive. The encapsulation material used were: (i) a pressure sensitive adhesive (EL-92734 from Adhesives Research) and (ii) a UV-blocking flexible barrier film (ATCJ from Amcor, wavelengths below 360 nm are blocked), using a copper tape for making the contacts.

[0133] The thicknesses were measured with a Dektak 150 profilometer (Veeco Instruments Inc.).

[0134] The electrical characterization of the device obtained was performed in said 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 a Solartest 1200 (Atlas) solar simulator, able to provide AM 1.5 G radiation with an intensity of 100 mW/cm.sup.2 (1 sun). The obtained power conversion efficiency (PCE) is reported in Table 1, measured using a calibrated reference solar cell (Si-reference solar cell) filtered with KG5 filter.

[0135] Furthermore the obtained device was subjected to accelerated ageing test in Atlas XXL+ weathering chamber and frequently electrically characterized during 7000 hours (offline measurement, AM 1.5). The voltage range for the measurements was from −1 V to 14 V. The aging conditions were 65° C. and 50% relative humidity (R.H.), under constant sunlight at an exposure irradiance level of 42 W/m.sup.2 (300 nm-400 nm), according to the ISOS-L-3 protocol disclosed in Roesch R. et al., “Advanced Energy Materials” (2015), Vol. 5, 1501407.

[0136] The photocurrent was measured offline from −1 V to 14 V as reported above up to 7000 hours: the obtained power conversion efficiency (PCE) is reported in Table 1.

Example 2 (Comparative)

[0137] Solar Cell with Cathodic Buffer Layer Comprising Zinc Oxide Plasma Treated

[0138] A polymer-based device was prepared operating according to Example 1, the only difference being the cathodic buffer layer which is made from colloidal zinc oxide nanoparticle suspension in ethanol (ZnO Nanoparticles, 5 wt %, 15 nm) (Avantama) without the addition of poly(N-vinylpyrrolidone) (PVP).

[0139] The obtained device was subjected to the characterizations reported in Example 1: the obtained power conversion efficiency (PCE) is reported in Table 1.

Example 3 (Comparative)

[0140] Solar Cell with Cathodic Buffer Layer Comprising Zinc Oxide and PVP

[0141] A polymer-based device was prepared operating according to Example 1, the only difference being the cathodic buffer layer is not subjected to plasma treating.

[0142] The obtained device was subjected to the characterizations reported in Example 1: the obtained power conversion efficiency (PCE) is reported in Table 1.

Example 4 (Comparative)

[0143] Solar Cell with Cathodic Buffer Comprising Zinc Oxide

[0144] A polymer-based device was prepared operating according to Example 1, the only difference being the cathodic buffer layer which is made from colloidal zinc oxide nanoparticle suspension in ethanol (ZnO Nanoparticles, 5 wt %, 15 nm) (Nanograde) and is not subjected to plasma treatment.

[0145] The obtained device was subjected to the characterizations reported in Example 1: the obtained power conversion efficiency (PCE) is reported in Table 1.

[0146] The data reported in Table 1 represent the mean values obtained from the characterization of three devices for each example. Moreover, the data reported in Table 1 were obtained normalizing, for each example, all the data taking as a reference the power conversion efficiency (PCE) measured just after exposing the device to light soaking, i.e. by exposing the device to the light of a Solartest 1200 (Atlas) solar simulator, able to provide AM 1.5 G radiation with an intensity of 100 mW/cm.sup.2 (1 sun), for 62 minutes.

TABLE-US-00001 TABLE 1 PCE.sup.(3) Plasma T.sub.50.sup.(2) (%) EXAMPLE PVP.sup.(1) treatment (hours) (after 7000 hours) 1 (invention) yes yes >8000 59 2 (comparative) no yes 7000 50 3 (comparative) yes no 3500 below threshold 4 (comparative) no no 1700 below threshold .sup.(1)PVP: poly(-N-vinylpyrrolidone); .sup.(2)hours of accelarating ageing test at which the power conversion efficiency (PCE) was 50% of the starting value; .sup.(3)PCE is the power conversion efficiency (PCE) of the device calculated according to the following formula: [00001]$\frac{V_{\mathrm{OC}}.Math.J_{\mathrm{SC}}.Math.\mathrm{FF}}{P_{\mathrm{in}}}$ [0147] wherein the FF (fill factor) is calculated according to the following formula:

[00002]$\frac{V_{\mathrm{MPP}}.Math.J_{\mathrm{MPP}}}{V_{\mathrm{OC}}.Math.J_{\mathrm{SC}}}$ [0148] wherein V.sub.MPP and J.sub.MPP are current tension and current density corresponding to the point of maximum power, respectively, V.sub.OC is the open circuit voltage and J.sub.SC is short-circuit photocurrent density and Pin is the intensity of the light incident on the device.

[0149] The data reported in Table 1 clearly show that the inverted polymer photovoltaic cell (or solar cell) according to the present disclosure is endowed with good power conversion efficiency (PCE) and, in particular, is able to maintain said power conversion efficiency (PCE) stable over time.