Organic solar cell with metallocene-based compounds

Abstract

An organic photovoltaic cell comprising a first and second electrodes; an active layer and an electron transport layer disposed therebetween; wherein the electron transport layer comprises an organic

Description

This invention relates to materials for organic solar cells with metallocene-based compounds. 5 BACKGROUND There are various types of solar cells, such as those based on metal halide perovskites and organic molecules or polymers. Organic solar cells (OSCs) may be particularly advantageous because the organic materials can be deposited on 10 flexible substrates, enabling the creation of lightweight, bendable solar panels. Further, the flexibility and semi-transparency of OSCs may allow OSCs to be integrated into surfaces and devices, such as windows and screens on portable electronics and wearable technology. Additionally, OSCs may be manufactured using low-cost methods and using methods that scale easily. Nevertheless, there 15 remains a need for improved power conversion efficiency (PCE) of OSCs and improved stability of OSCs to provide efficient and stable photovoltaic cells. WO2023203134 discloses a perovskite-based photovoltaic cell, which includes an interface layer comprising a metallocene substituent with a substituent having an 20 O, S, N or P group, for example ferrocene substituted with a thienyl-carboxylate group. It is believed that the electron-rich substituent, e.g. the thienyl group, may form a coordinate bond with Pb of the perovskite and / or that the lone electron pairs of the 0, S, N or P group are capable of binding to uncoordinated metal defects, e.g. Pb defects, at the perovskite surface and / or that the flexibility of metallocenes 25 around the metal-aromatic bond may ameliorate stresses between the electron transport layer and the perovskite layer. Thus, the use of a metallocene interface layer boosts the electron transfer at the perovskite / ETL interface. SUMMARY 30 In a first aspect, the invention provides an organic photovoltaic cell comprising: a first electrode; a second electrode; an active layer and an electron transport layer disposed between the first and second electrodes; 35 wherein the electron transport layer comprises an organic electronic material and a metallocene substituted with at least one substituent R1 comprising at least one of an 0, S, N or P atom. The metallocene may be a compound of formula (I): [Metallocene]p (I) 5 wherein: Metallocene is a metallocene group comprising a metal bound to two aromatic or heteroaromatic groups Ar1; p is at least 1; and at least one Metallocene is substituted with at least one substituent R1. io The compound of formula (I) may have formula (la): Ar1 M—(R2)q Ar1 R3 (la) wherein: 15 M is a metal ion; Ar1 in each occurrence is a monocyclic or polycyclic aromatic or heteroaromatic group; M and the two Ar1 groups form the Metallocene; at least one Ar1 is substituted with at least one R1; 20 R2 is a group for satisfying the valency of M; q is 0 or a positive integer; and R3 in each occurrence is independently H or a substituent. The metallocene may be ferrocene. 25 R1 may be a group of formula (II): -A-B (ID wherein A is a divalent group comprising O, S, N or P; and B is H, C1-12 alkyl, 30 optionally substituted aryl or optionally substituted heteroaryl. A may be selected from formula (III) or formula (IV) -(R5)f-Z-(R5)g- (III) -(R6O)j- (IV) wherein: R5 in each occurrence is independently a hydrocarbon group; 5 f and g are each independently 0 or 1; R6 is a Cl-4 alkylene group, preferably ethylene; and Z is O, S, COO, C(=S)O, C(=O)S, CONR4, CSNR4, 0C(=0)0, 0C(=0)NR4, OC(=O)PR4'NR4, PR4, -OP(=O)(OR4)-O~, -NR4-P(=O)(NR42)-NR4-, wherein R4 is H, optionally substituted C1-12 alkyl or optionally substituted phenyl. 10 The bond between the metallocene and R1 may be a carbon-oxygen bond in which a C atom of the metallocene is bound to an 0 atom of R1. A may be -C(=0)-0-; -0-C(=0)- or -C(=0)-. 15 B may be selected from optionally substituted phenyl and an optionally substituted 5-membered heteroaryl comprising one or more ring atoms selected from 0, S and N. 20 B may be optionally substituted thiophene or optionally substituted furan or optionally substituted phenyl. Optionally, the electron transport layer comprises a conjugated polyelectrolyte (CPE). For instance, the electron transport layer may comprise PFN; PFN-Br; 25 PDINO; NDI-N; PDIN; PDINN; PFN-FP-I; PBDD4T; PBBTSiD; PFN-FP; PFN-I; PFN-FP-I; TFB; F8T2; 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[l,2,5]thiadiazolo[3,4-e]thieno[2",3'’:4',5']thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-lH-indene-2,l-diylidene))dimalononitrile; PolyTPD; 30 PNDI(2HD)T; PNDI(2HD)2T; PDBPyBT; PFO-co-MEH-PPV; PDPP4T-2F; PNF222, and PF8Cz. The electron transport layer may comprise a non-fullerene component that may be one or more of a small molecule, oligomer, polymer, and cross-linked 35 meta structure. The non-fullerene component may include one or more of rhodanine-benzothiadiazole-coupled indacenodithiophene (IDTBR); indacenodithieno[3,2-b]thiophene, IT), end-capped with 2-(3-oxo-2,3- dihydroinden-l-ylidene)malononitrile (INCN) groups (ITIC); indaceno[l,2-b:5,6-b']dithiophene and 2-(3-oxo-2,3-dihydroinden-l-ylidene)malononitrile (IEIC); 2,2'-((2Z,2'Z)-((5,5'-(4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[l,2-b:5,6-b'] dithiophene-2, 7-diyl)bis(4-((2-ethylhexyl)- oxy)thiophene-5,2- 5 diyl))bis(methanylylidene))bis(3-oxo-2,3-di- hydro- IH-indene-2,1-diylidene))dimalononitrile (IEICO); naphthalene diimide (NDI); bay-linked perylene bisimide (di-PBI); perylene bisimide (PBI); Benzotriazole-Containing End-Capped with Thiazolidine-2,4-dione (TD); Naphthalocyanine (NC); Phthalocyanine (PC); Naphtho[l,2-c :5,6- c ']bis[l,2,5]thiadiazole; (2E,2'E)-3,3'-(2,5-dimethoxy-l,4- 10 phenylene)bis(2-(5-(4-(N-(2-ethylhexyl)- 1 ,8-naphthalimide)yl)thiophen-2-yl)acrylonitrile) (NIDCS-MO); thieno[3,4-b] thiophene and 2-(1,1-dicyanomethylene)rhodanine combination (ATT-I); (3,9-bis(4-(l,l- dicyanomethylene)- 3-methylene-2-oxo-cyclopenta[b]thiophen)-5,5,l 1,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d':2,3-d']-s-indaceno[l,2-b:5,6- b']-di 15 thiophene (ITCC); Indanedione; Dicyannovinyl; Benzothiadiazole; Diketopyrolopyrrole; arylene diimide; and IDIC. In a second aspect the invention provides a photovoltaic module comprising a plurality of the organic photovoltaic cells according to the first aspect of the 20 invention, the photovoltaic cells connected in series. LIST OF FIGURES Figure 1 provides a schematic illustration of a conventional solar cell; Figure 2 provides the structures of compounds used in the Examples; 25 Figure 3 shows FTIR. data demonstrating that complex formation occurs between the organic electronic material and the metallocene; Figure 4A provides a schematic illustration of a solar cell according to one of the examples, i.e. comprising a PFN-Br ETL and PM6:BTP-eC9 active layer; Figure 4B shows JV curves for organic photovoltaics with and without a 30 ferrocene-based compound under AM 1.5G illumination; Figure 4C shows the photovoltaic parameters obtained from Figure 4B; Figure 5 shows the results of stability tests based on devices with and without a ferrocene-based compound; Figure 6A provides a schematic illustration of a solar cell according to one of 35 the examples, i.e. comprising a PNDIT-F3N and PM6:L8-BO active layer; Figure 6B shows JV curves for organic photovoltaics with and without a ferrocene-based compound under AM 1.5G illumination; Figure 6C shows the photovoltaic parameters obtained from Figure 6B. DETAILED DESCRIPTION With reference to Figure 1, an example photovoltaic cell (or solar cell) 100 5 comprises several layers. A transparent substrate 102 is provided, which forms the base or support for the solar cell structure 100. Also provided are an active layer 110 and an electron transport layer (ETL) 106. Solar radiation or visible light 116 (such as incident sunlight) enters the solar cell io 100 through the transparent substrate 102. The transparent substrate 102 may be formed of glass, or any other suitable transparent material. The solar radiation or visible light 116 passes through the substrate layer 102 into the active layer 110, whereupon at least a portion of the solar radiation 116 is 15 absorbed by exciting an electron across a semiconductor band gap to enable electrical generation. In particular, the electron is excited from a valence band of the semiconductor, across the bandgap, to a conduction band. The excited electron sits in the conduction band, and a corresponding hole (a vacancy or absence of an electron, rather than a physical particle in and of itself) remains in the valence band 20 of the semiconductor. An asymmetry within the active layer 110 acts to separate the excited electron away from the hole, moving the charge carriers (holes and electrons) away from the point of electron promotion for collection and current generation. 25 k hole transport layer (HTL) 112 comprising or consisting of one or more hole transport materials can also be provided within solar cell 100. Any hole-transport material known to the skilled person may be used. Example hole transport materials include organic hole-transport materials, inorganic hole-transport 30 materials or combinations thereof. Organic hole-transport materials may be polymeric or non-polymeric. Exemplary polymeric hole-transport materials include polythiophenes, for example poly(3-hexylthiophene) (P3HT); poly(arylamines) for example PTTA; and doped PEDOT, for example PEDOT:PSS. Exemplary non-polymeric organic hole-transport materials are compounds containing one or more 35 arylamine groups, for example spiro-OMeTAD and self assemble monolayers, for example (2-(9H-carbazol-9-yl)ethyl)phosphonic acid (2PACz). Exemplary inorganic hole-transport materials include copper-based materials (e.g. CuOx, CuSCN, Cui, etc.), nickel-based materials (e.g. NiOx), two-dimensional layered materials such as chalcogens (e.g. M0S2, WS2, etc.). Hole transport materials may encourage a flow of holes away from the junction 5 within active layer 110, while blocking the movement of electrons. In this way, holes accumulate at a second electrical conductor 114. In use, the second electrical conductor 114 is positively charged due to the accumulation of holes. The second conductor 114 may be any transparent conducting material. In some 10 examples, the second conductor 114 is a transparent conducting film (TCF). In some examples, the TCF is a transparent conducting oxide (TCO) layer. In some examples, the TCO layer comprises indium-tin oxide (ITO), fluorine-doped tin oxide (FTO) or doped zinc oxide. 15 The electron transport layer (ETL or n-type charge extraction layer) 106 comprises an organic electron transport material. The organic electron transport material may encourage a flow of electrons away from the junction within active layer 110, while blocking the movement of holes. In this way, electrons accumulate at a first electrical conductor 104. In use, the first electrical conductor 104 is negatively 20 charged due to the accumulation of electrons. When the solar cell is connected to an external load, the electrons leave the solar cell 100 via the first electrical conductor 104. The first conductor 104 may be formed of any suitable conducting material, such as 25 Ag, Au, Cu, etc. An interface layer 108 may be provided between the electron transport layer 106 and the active layer 110. The interface layer 108 may improve the extraction of electrons, increasing the efficiency of the solar cell, and improve the stability of the 30 solar cell 100. The interface layer 108 may interface directly with the active layer 110. In other words, the interface layer 108 may be in direct contact with the active layer. The interface layer 108 can be deposited directly on the active layer 110, or may be 35 otherwise formed. One or more additional layers (not shown) may be provided within the solar cell structure 100. For example, one or more optional hole blocking layers may be provided between the ETL 106 and the first electrical conductor 104 and / or between the interface layer 108 and the ETL 106. Similarly, one or more optional 5 electron blocking layers may be provided between the HTL 112 and the second electrical conductor 114 and / or between the active layer 110 and the HTL 112. Any other layers may be provided within solar cell 100, as appropriate. A plurality of photovoltaic cells can be connected together in series and io encapsulated to form a photovoltaic module (not shown). The photovoltaic modules can be used singly, or a plurality can be connected in series and / or parallel into a photovoltaic array, according to the power demanded by a specific load or application. 15 Electron Transport Layer (ETL) The electron transport layer comprises an organic electronic material and a metallocene substituted with at least one substituent containing an O, S, N or P atom having a lone pair of electrons. 20 The present inventors have surprisingly found that the presence of such a metallocene in electron transporting layers (ETL) in organic photovoltaics (OPV) enhances the stability and performance of OPVs. Without wishing to be bound by any theory, it is believed that the substituted 25 metallocenes form a charge-transfer state complex with the organic electronic material in the ETLs, which enhances charge extraction. Devices comprising OPVs containing an ETL comprising an organic electronic material and a metallocene substituted with at least one substituent containing an 30 O, S, N or P atom having a lone pair of electrons are more stable under photostability stress. The metallocene preferably is a compound of formula (I): [Metallocene]p (I) wherein: Metallocene is a metallocene group comprising a metal bound to two aromatic or heteroaromatic groups Ar1; p is at least 1, optionally 1, 2 or 3; and at least one Metallocene is substituted with at least one substituent R1 5 wherein R1 is a group comprising an 0, S, N or P atom. Optionally, the compound of formula (I) has formula (la): (la) io wherein: M is a metal ion; Ar1 in each occurrence is a monocyclic or polycyclic aromatic or heteroaromatic group; M and the two Ar1 groups form the Metallocene; 15 at least one Ar1 is substituted with at least one R1 wherein R1 is a group comprising an 0, S, N or P atom; R2 is a group for satisfying the valency of M; q is 0 or a positive integer, preferably 0 or 2; R3 in each occurrence is independently H or a substituent; and 20 p is at least 1. Exemplary Ar1 groups include, without limitation, C4-C8 aromatic groups, i.e., cyclobutadiene, cyclopentadienyl, benzene, cycloheptatrienyl or cyclooctatetraene; and C5 heteroaromatic groups, e.g., pyrrole, each of which may be unfused or fused 25 to one or more further rings, preferably one or more benzene rings. Exemplary fused groups Ar1 include benzocyclopentadienyl and fluorenyl. Metallocene preferably comprises a metal M bound to two cyclopentadienyl groups Ar1. Ar1 may consist of the cyclopentadienyl group or the cyclopentadienyl may be 30 fused to one or more further rings, preferably one or more aromatic rings, e.g. one or more benzene rings as in benzocyclopentadienyl or fluorenyl. M may be Fe2+, Co2+, Cr2+, Ni2+ or V2+, preferably Fe2+. For each of these compounds, q is 0. M may be Zr or Ti. For each of these compounds, q is 2 and R2 may be any 5 suitable group capable of bonding to Zr or Ti, for example methyl, ammonia, dialkylamines, phosphines, CO or halogen, e.g. Cl, such as in metallocene dihalides. The two Ar1 groups of the or each Metallocene may be linked - other than through M - by a divalent group, for example a Ci-6 alkylene or a group of formula Si(R3)2 io wherein R3 in each occurrence is independently a Ci 12 hydrocarbyl group, e.g. Ci-12 alkyl or phenyl. It will therefore be understood that compounds of formula (I) include ansa-metallocenes. In a preferred embodiment, M and Ar1 form ferrocene, i.e. M is Fe; each Ar1 is 15 cyclopentadienyl; and y is 0. Preferably, R1 is the only substituent of the Ar1 groups. Preferably, p is 1, 2 or 3, more preferably 1. 20 Compounds of formula (la) may be selected from formulae (lb), (Ic) or (Id): (R1)t1—Ar1 M—(R2)q (R1)ti-Ar1 (lb) (R2)q—M (R'h-Ar1 M---(R2)q Ar1-(R1)t1 (Ic) (R1)t2 (R1)t1—Ar1----Ar1        Ar1—(R1)n (R2)q—M    M-(R2)q  M (R2)q (R1)ti—Ar1    Ar1-------Ar1^(R1)ti (R1)t2 25 (Id) wherein tl is 0, 1 or 2, preferably 0 or 1; t2 is 0 or 1, preferably 1; and at least one of tl and / or t2 is at least 1. In some embodiments, R1 is a group of formula (II): 5 -A-B (II) wherein A is a divalent group comprising O, S, N or P; and B is H, C1-12 alkyl, optionally substituted aryl or optionally substituted heteroaryl. 10 Group A may comprise any group capable of binding to Pb. Exemplary groups A include, without limitation, ethers, thioethers, amines, phosphines, phosphoryl ethers, carbonates, carbamates, carboxylates, amides, thioamides, phosphonamides, thiocarboxylates, aminocarboxylates, and phosphocarboxylates. RI may comprise only one group A. R1 may comprise two or more groups A. 15 Exemplary groups A include groups of formulae (III) and (IV): -(R5)f-Z-(R5)g- (III) -(R6O)j- (IV) wherein: 20 R5 in each occurrence is independently a hydrocarbon group; f and g are each independently 0 or 1; R6 is a Ci 4 alkylene group, preferably ethylene; j is 1-10; and Z is O, S, COO, C(=S)O, C(=O)S, CONR4, CSNR4, OC(=O)O, OC(=O)NR4, 25 OC(=O)PR4' NR4, PR4, -OP(=O)(OR4)-O-, or -NR4-P(=O)(NR42)-NR4-, wherein R4 is H, optionally substituted C1-12 alkyl or optionally substituted phenyl. Hydrocarbon groups R5 are preferably selected from Ci 6 alkylene; optionally substituted phenylene; and Ci 6 alkylene-phenylene. 30 k phenylene group of an R5 group may be unsubstituted or substituted with one or more substituents selected from C1-6 alkyl. In the case where R5 is Ci 6 alkylene-phenylene, the group Z may be bound to 35 either the alkylene or the phenylene group. A particularly preferred group A is -O-C(=O)-, which may be linked to Metallocene through the O atom or the C atom(-C(=O)-O-), preferably through the O atom. A particularly preferred group A is also -C(=O)-. 5 B is preferably an optionally substituted aryl or heteroaryl, more preferably phenyl or a 5-membered heteroaromatic comprising one or more of N, S and O ring atoms, for example furan, thiophene, pyrrole, imidazoles and oxazole. Thiophene and furan are particularly preferred. io Optional substituents of an optionally substituted alkyl or alkylene group as described anywhere herein include F, Cl, OR4 and NR42 wherein R4 is a Ci-6 alkyl. Optional substituents of any optionally substituted aromatic or heteroaromatic group as described anywhere herein, including but not limited to substituents R3 of 15 formula (la), include F, Cl, CN, NO2, C1-6 alkyl wherein one or more H atoms may be replaced with F, OR4 and NR42 wherein R4 is a Ci 6 alkyl. Photovoltaic cell formation Photovoltaic cells as described herein may be formed by any method known to the 20 skilled person. Preferably, the photoactive layer is formed by depositing a solution comprising the photoactive material. Suitable solvents for deposition of the photoactive material include polar solvents such as DMF and DMSO. Preferably, the electron transport layer is formed by depositing a solution comprising the polymer. Suitable solvents for deposition of the ETL material include polar solvents such as 25 methanol, DMF and DMSO. Solutions may be deposited by any method known to the skilled person, for example spin-coating, dip-coating, slot-die coating, doctor blade coating and bar coating. 30 EXAMPLES As described herein, the chemicals used include the following: • PEDOT:PSS (Al 4083) was purchased from Clevios™. • PM6, BTP-eC9, L8-BO; PFN-Br, PNDIT-F3N were purchased from Solarmer 35 Materials (Beijing) Inc. • 1,8-Diiodooctane (DIO) was purchased from Sigma-Aldrich. • The solvents, including dimethylformamide (DMF), dimethyl sulfoxide (DMSO), isopropanol (IPA) and chlorobenzene (CB) were purchased from J&K (China) and used as received. • Acetonitrile (MeCN), dichloromethane (DCM), and hexane were purchased 5 from Sigma-Aldrich and used as received. • High purity silver was purchased from commercial sources. • Glass substrates patterned with ITO (15 Q sq1) were received from Yingkou Shangsheng Business Co., Ltd. (China). io Experimental parameters and measurements • FTIR spectroscopy was conducted by Fourier transform infrared spectrometer (Vertex 70, Germany Bruker). • The J-V characteristics of photovoltaic devices was conducted in a Nz-filled glovebox at room temperature by using a Xenon lamp solar simulator (Oriel 15 Instruments Solar Simulator AAA). The power of the light was calibrated to 100 mW cm 2 by a silicon reference cell (with a KG2 filter). All the devices were measured using a Keithley 2400 source meter under a sweep mode of reverse scan (from -0.1 V to 1 V) No preconditioning was needed before the measurement. The active area was defined and characterized as 0.045 cm2 20 for small-area by metal shadow mask. • NMR spectra were recorded at ambient temperature on a Bruker Avance 400 MHz spectrometer and calibrated to the residual solvent peaks of CDCI3 at 7.26 and 77.2 ppm for XH and “CpH}, respectively. Peak assignment was aided through the use of COSY, edited HSQC and HMBC experiments. 25 • High resolution mass spectrometry (HRMS) data were recorded by direct injection on a Waters LCT time-of-flight (ToF) Mass Spectrometer using electrospray ionization (ESI). • Elemental analyses were analysed using a ThermoFlash 2000 Analyzer. 30 General procedure for the synthesis of l,l'-bis[(2-thienylcarbonyl)oxy]ferrocene (FCTC2), l',l"'-bis[(2-thienylcarbonyl)oxy]-l,l"-biferrocene (FC2TC2), l' / l"'"-bis[(2-thienylcarbonyl)oxy]-l,l":l'",l""-terferrocene (FC3TC2), l,l'-bis[(2- 35 furanylcarbonyl)oxy]ferrocene, l,l'-bis[(lH-2- pyrrolylcarbonyl)oxy]ferrocene and l,l'-bis[(2-phenylcarbonyl)oxy]ferrocene Feb, FC2I2 or FC3I2 (1 equiv), CU2O (1.5 equiv) and respective carboxylic acid (3 equiv) were refluxed for 24 h in CH3CN. The rection mixture was diluted with 5 CH2CI2, filtered and the filtrate washed with saturated aqueous NaHCOa until the aqueous phase became colourless. The organic phase was dried over Na2SO4, filtered and concentrated in vacuo. The crude residue was purified by flash chromatography (n-hexane / CHzCh) and the resulting product recrystallized from CH2CI2 and n-hexane to afford the desired compound. 10 l,l'-Bis[(2-thienylcarbonyl)oxy]ferrocene (FCTC2) - as reported in Z. Li, B. Li, X. Wu, S. A. Sheppard, S. Zhang, D. Gao, N. J. Long and Z. Zhu, Science, 2022, 376, 416-420 15 FcTc2 was prepared according to the general procedure using Feb (158 mg, 0.36 mmol), 2-thiophenecarboxylic acid (139 mg, 1.08 mmol), CU2O (77 mg, 0.54 mmol) and CH3CN (15 mL) to afford orange crystals (108 mg, 68%). XH NMR (400 MHz, CDCI3): 6 7.78 (d, 2H, 3Jhh = 3.8 Hz, H-5'), 7.56 (d, 3Jhh = 4.9 Hz, 2 x H-3'), 7.06 (dd, 3Jhh = 5.0 Hz, 4Jhh = 3.8 Hz, 2 x H-4'), 4.68 (t, 3Jhh = 2.0 Hz, 2 x H-2 20 and H-5), 4.10 (t, 4H, 3Jhh = 2.0 Hz, 2 x H-3 and H-4); NMR (100 MHz, CDCI3): 6 160.34 (2 x C, 2 x C=O), 134.24 (2 x CH, 2 x C-5'), 133.3 (2 x C, 2 x C-2'), 133.22 (2 x CH, 2 x C-3'), 127.99 (2 x CH, 2 x C-4'), 116.58 (2 x C, 2 x C-l), 64.82 (4 x CH, 2 x C-3 and C-4), 62.22 (4 x CH, 2 x C-2 and C-5); MS (ESI+): m / z 437.9672 [M]+ (mCaic 437.9683); Calcd for C2oHi4Fe04S2: C 54.81, H 25 3.22. Found: C 54.62, H 3.29. l,,l,"-Bis[(2-thienylcarbonyl)oxy]-l,l"-biferrocene (FC2TC2) - as reported in B. Li, D. Gao, S. A. Sheppard, W. D. J. Tremlett, Q. Liu, Z. Li, A. J. P. White, R. K. Brown, X. Sun, J. Gong, S. Li, S. Zhang, X. Wu, D. Zhao, C. Zhang, Y. Wang, X. C. Zeng, Z. Zhu and N. J. Long, J. Am. Chern. Soc.f 2024, FC2TC2 was prepared according to the general procedure using FC2I2 (947 mg, 1.52 5 mmol), 2-thiophenecarboxylic acid (585 mg, 4.57 mmol), CU2O (327 mg, 2.28 mmol) and CH3CN (80 mL) to afford orange crystals (652 mg, 69%). Rf 0.32 (CH2CI2 / n-hexane 1:1); JH NMR (400 MHz, CDCI3): 6 7.76 (dd, 2H, 3J = 3.8, 4J = 1.2 Hz, 2 x H-5"), 7.59 (dd, 2H, 3J = 4.9, 4J = 1.3 Hz, 2 x H-3"), 7.11 (dd, 2H, 3J = 5.0, 3J = 3.7 Hz, 2 x H-4"), 4.44 (pseudo-t, 4H, J = 1.8 Hz, 2 x H-3 and H-4), 10 4.39 (pseudo-t, 4H, J = 2.0 Hz, 2 x H-3' and H-4'), 4.16 (pseudo-t, 4H, J = 1.8 Hz, 2 x H-2 and H-5), 3.81 (pseudo-t, 4H, J = 1.9 Hz, 2 x H-2' and H-5'); ^C-pH} NMR (100 MHz, CDCI3): 6 160.4 (2 x C, 2 x C=O), 134.2 (2 x CH, 2 x C-5"), 133.4 (2 x C, 2 x C-2"), 133.0 (2 x CH, 2 x C-3"), 127.9 (2 x CH, 2 x C-4"), 116.3 (2 x C, 2 x 1'), 84.4 (2 x C, 2 x C-l), 69.2 (4 x CH, 2 x C-2 and C-5), 67.7 15 (4 x CH, 2 x C-3 and C-4), 64.5 (4 x CH, 2 x C-2' and C-5'), 62.0 (4 x CH, 2 x C- 3' and C-4'); HRMS (ESI+): m / z 621.9655 [M]+ (mcaic 621.9658); Calcd for C3oH22Fe204S2: C 57.90, H 3.56. Found: C 57.68, H 3.49. l',l""'-Bis[(2-thienylcarbonyl)oxy]-l,l":l'",l""-terferrocene (FC3TC2) - 20 as reported in B. Li, D. Gao, S. A. Sheppard, W. D. J. Tremlett, Q. Liu, Z. Li, A. J. P. White, R. K. Brown, X. Sun, J. Gong, S. Li, S. Zhang, X. Wu, D. Zhao, C. Zhang, Y. Wang, X. C. Zeng, Z. Zhu and N. J. Long, J. Am. Chern. Soc.r 2024, 146, 13391-13398 25 FcsTc2 was prepared according to the general procedure using Fcsl2 (718 mg, 0.89 mmol), 2-thiophenecarboxylic acid (343 mg, 2.67 mmol), Cu2O (191 mg, 1.34 mmol) and CH3CN (100 mL) to afford red crystals (0.36 mg, 50%). Rf 0.42 (CH2CI2 / n-hexane 3:2); JH NMR (400 MHz, CDCI3): 6 7.73 (dd, 2H, 3J = 3.7 Hz, 4J = 1.3 Hz, 2 x H-5"'), 7.58 (dd, 2H, 3J = 5.0 Hz, 4J = 1.3 Hz, 2 x H-3'"), 7.10 (dd, 2H, 3J = 5.0, 3J = 3.7 Hz, 2 x H-4"'), 4.33 (pseudo-t, 4H, J = 2.0 Hz, 2 x H-3" and H-4"), 4.30 (pseudo-t, 4H, J = 1.8 Hz, 2 x H-3 and H-4 or 2 x H-3' and H-4'), 4.21 (pseudo-t, 4H, J = 1.9 Hz, 2 x H-2 and H-5 or 2 x H-2' and H-5'), 4.06 (pseudo-t, 4H, J = 1.9 Hz, 2 x H-3 and H-4 or 2 x H-3' and H-4'), 3.89 (pseudo-t, 4H, J = 1.8 5 Hz, 2 x H-2 and H-5 or 2 x H-2' and H-5'), 3.77 (pseudo-t, 4H, J = 2.0 Hz, 2 x H- 2" and H-5"); NMR (100 MHz, CDCI3): 6 160.4 (2 x C, 2 x C=O), 134.2 (2 x CH, 2 x C-5"'), 133.5 (2 x C, 2 x C-2'"), 133.0 (2 x CH, 2 x C-3'"), 127.9 (2 x CH, 2 x C-4'"), 116.2 (2 x C, 2 x 1"), 85.1 (2 x C, 2 x C-l or 2 x C-l'), 83.3 (2 x C, 2 x C-l or 2 x C-l'), 69.0 (4 x CH, 2 x C-2 and C-5 or 2 x C-2' and C-5'), 69.0 10 (4 x CH, 2 x C-2 and C-5 or 2 x C-2' and C-5'), 67.8 (4 x CH, 2 x C-3 and C-4 or 2 x C-3' and C-4'), 67.5 (4 x CH, 2 x C-3 and C-4 or 2 x C-3' and C-4'), 64.5 (4 x CH, 2 x C-2" and C-5"), 61.9 (4 x CH, 2 x C-3" and C-4"); HRMS (ESI+): m / z 805.9632 [M] + (mcaic 805.9634); Calcd for C4oH3oFe304S2: C 59.58, H 3.75. Found: C 59.28, H 3.49. 15 l,l'-Bis[(2-furanylcarbonyl)oxy]ferrocene l,l'-Bis[(2-furanylcarbonyl)oxy]ferrocene was prepared according to the general procedure using Fcl2 (311 mg, 0.71 mmol), 2-furoic acid (239 mg, 2.13 mmol), 20 Cu2O (152 mg, 1.07 mmol) and CH3CN (30 mL). The crude residue was purified by flash chromatography (CH2CI2 / n-hexane 1:1 -» CH2CI2 neat) and recrystallised from CH2CI2 / n-hexane to afford orange-yellow crystals (205 mg, 71%). RrO.43 (CH2CI2 / n-hexane 4:1); XH NMR (400 MHz, CDCI3): 6 7.56 (s, 2H, 2 x H-5'), 7.19 (d, 2H, 3Jhh = 3.5 Hz, 2 x H-3') 6.49-6.48 (m, 2H, 2 x H-4'), 4.69 (s, 4H, 2 x H-2 25 and H-5), 4.09 (s, 4H, 2 x H-3 and H-4); 13C{XH} NMR (100 MHz, CDCI3): 6 156.7 (2 x C, 2 x C=O), 146.9 (2 x CH, 2 x C-5'), 144.2 (2 x C, 2 x C-2'), 118.9 (2 x CH, 2 x C-3'), 116.3 (2 x C, 2 x C-l), 112.1 (2 x CH, 2 x C-4'), 64.8 (4 x CH, 2 x C-3 and C-4), 62.2 (4 x CH, 2 x C-2 and C-5); MS (ESI+): m / z 406.0144 [M] + (mcaic 406.0140); Calcd for C2oHi4Fe06: C 59.14, H 3.47. Found: C 59.40, H 3.47. l,l'-Bis[(lH-2-pyrrolylcarbonyl)oxy]ferrocene l,l-Bis[(lH-2-pyrrolylcarbonyl)oxy]ferrocene was prepared according to the general procedure using Fcl2 (113 mg, 0.26 mmol), 2-furoic acid (86 mg, 0.77 5 mmol), CU2O (55 mg, 0.39 mmol) and CH3CN (10 mL). The crude residue was purified by flash chromatography (CH2CI2 neat) and recrystallised from acetone / n-hexane to afford yellow crystals (29 mg, 26%). Rt 0.14 (CH2CI2 neat); XH NMR (400 MHz, (CD3)2CO): 6 11.03 (br s, 2H, 2 x NH), 7.11 (dd, 2H, 3Jhh = 2.5 Hz, 4Jhh = 1.5 Hz, 2 x H-51), 6.91 (dd, 2H, 3Jhh = 3.8 Hz, 4Jhh = 1.5 Hz, 2 x H-3'), 6.23 (dd, 2H, 10 3Jhh = 3.8 Hz, 4Jhh = 2.5 Hz, 2 x H-4'), 4.60 (t, 4H, 3Jhh = 2.0 Hz, 2 x H-2 and H-5), 4.08 (t, 4H, 3Jhh = 2.0 Hz, 2 x H-3 and H-4); ^C^H} NMR (100 MHz, (CD3)2CO): 6 159.5 (2 x C, 2 x C=O), 125.2 (2 x CH, 2 x C-5'), 125.1 (2 x C, 2 x C-2'), 117.3 (2 x C, 2 x C-l), 117.1 (2 x CH, 2 x C-3'), 110.9 (2 x CH, 2 x C-4'), 65.2 (4 x CH, 2 x C-3 and C-4), 63.0 (4 x CH, 2 x C-2 and C-5); MS (ESI+): m / z 15 404.0458 [M]+ (mcaic 404.0459). Calcd for C2oHi6FeN204: C 59.43, H 3.99, N 6.93. Found: C 59.64, H 4.03, N 6.80. l,l'-Bis[(2-phenylcarbonyl)oxy]ferrocene 20 l,l-Bis[(2-phenylcarbonyl)oxy]ferrocene was prepared according to the general procedure using Fcl2 (108 mg, 0.25 mmol), 2-furoic acid (90 mg, 0.74 mmol), Cu2O (53 mg, 0.37 mmol) and CH3CN (10 mL). The crude residue was purified by flash chromatography (CH2CI2 / n-hexane 1:1 -» CH2CI2 neat) and recrystallised from CH2CI2 / n-hexane to afford yellow-orange crystals (95 mg, 89%). RrO.44 (CH2CI2 / n- 25 hexane 1:1); XH NMR (400 MHz, CDCI3): 5 8.02 (d, 4H, 3J = 7.1 Hz, 2 x H-2' and H-6'), 7.54 (t, 2H, 3J = 7 A Hz, 2 x H-4'), 7.36 (t, 4H, 3J = 7.8 Hz, 2 x H-3' and H-5'), 4.70 (t, 4H, 3J = 2.0 Hz, 2 x H-2 and H-5), 4.12 (t, 4H, 3J = 2.0 Hz, 2 x H-3 and H-4); ^C-pH} NMR (100 MHz, CDCI3): 6 165.0 (2 x C, 2 x C=O), 133.4 (2 x CH, 2 x C-4'), 130.0 (4 x CH, 2 x C-2' and C-6'), 129.7 (2 x C, 2 x C-l'), 128.53 (4 x CH, 2 x C-3' and C-5'), 116.6 (2 x C, 2 x C-l), 64.8 (4 x CH, 2 x C-3 and C-4), 62.3 (4 x CH, 2 x C-2 and C-5); MS (ESI+): m / z 426.0564 [M]+ (mca,c 426.0554); Calcd for Cz^isFeCM: C 67.63, H 4.26. Found: C 67.51, H 4.10. 5 General procedure for the synthesis of l,l'-bis(2-thienylcarbonyl)ferrocene, l,l'-bis(2-furanylcarbonyl)ferrocene and 1,1'-bis(2-phenylcarbonyl)ferrocene io Ferrocene (1 equiv) was added to a stirred solution of AICH (5 equiv) and respective acid chloride (5 equiv) in CH2CI2 and stirred rt for 18 h. The reaction mixture was cooled to 0 °C and ice-cooled water added. The mixture was separated, and the aqueous phase extracted with CH2CI2 (x3). The combined organic layers were washed with satd. aq. NaHCOs, water and brine, dried over Na2SO4, filtered and 15 concentrated in vacuo. The crude residue was purified by flash chromatography (n-hexane / EtOAc) and the resulting product recrystallized to afford the desired compound. l,l'-Bis(2-thienylcarbonyl)ferrocene- as reported in A. A. O. Sarhan, M. S. 20 Ibrahim, M. M. Kamal, K. Mitobe and T. Izumi, Monatshefte fur Chemie -Chemical Monthly, 2009, 140, 315-323 l,l'-Bis(2-thienylcarbonyl)ferrocene was prepared according to the general procedure using AICI3 (4.44 mg, 33.3 mmol), benzoyl chloride (3.56 mL, 33.3 25 mmol), ferrocene (1.24 g, 6.67 mmol) and CH2CI2 (50 mL). The crude residue was purified by flash chromatography (CH2CI2 neat) and the resulting product recrystallized from CH2Cl2 / n-hexane to afford l,l'-bis(2-thienylcarbonyl)ferrocene (1.49 g, 55%) as red-pink fluffy needles. RrO.lO (CH2CI2 neat); XH NMR (400 MHz, CDCI3): 6 7.84 (d, 2H, 3Jhh = 3.8 Hz, 2 x H-3'), 7.63 (d, 2H, 3Jhh = 5.0 Hz, 2 x H- 30 5'), 7.14-7.12 (m, 2H, 2 x H-4'), 5.06 (s, 4H, 2 x H-3 and H-4), 4.60 (s, 4H, 2 x H-2 and H-5); “C^H} NMR (100 MHz, CDCI3): 6 188.4 (2 x C, 2 x C=O), 143.9 (2 x C, 2 x C-l'), 132.7 (4 x CH, 2 x C-5'), 132.1 (4 x CH, 2 x C-3'), 128.0 (2 x CH, 2 x C-4'), 80.5 (2 x C, 2 x C-l), 74.8 (4 x CH, 2 x C-2 and C-5), 72.8 (4 x CH, 2 x C-3 and C-4); MS (ESI+): m / z 406.9849 [M + H]+ (mcaic 406.9863); Calcd for C20Hi4FeO2S2: C 59.12, H 3.47. Found: C 58.80, H 3.09. 5 l,l'-Bis(2-furanylcarbonyl)ferrocene (F4) - as reported in A. A. O. Sarhan, M. S. Ibrahim, M. M. Kamal, K. Mitobe and T. Izumi, Monatshefte fur Chemie - Chemical Monthly, 2009, 140, 315-323 l,l'-Bis(2-furanylcarbonyl)ferrocene was prepared according to the general io procedure using AICI3 (10.8 g, 81.1 mmol), 2-furoyl chloride (8.0 mL, 81.1 mmol), ferrocene (3.02 g, 16.2 mmol) and CH2CI2 (120 mL). The crude residue was purified by flash chromatography (n-hexane / EtOAc 4:1 -» 1:1) and the resulting product recrystallized from CH2CI2 and n-hexane to afford l,l'-bis(2-furanylcarbonyl)ferrocene (3.52 g, 58%) as red-orange crystals. Rf 0.37 (n- 15 hexane / EtOAc 1:1); XH NMR (400 MHz, CDCI3): 6 7.55 (s, 2H, 2 x H-5'), 7.27 (d, 2H, 3Jhh = 3.6 Hz, 2 x H-3') 6.54 (s, 2H, 2 x H-4'), 5.17 (s, 4H, 2 x H-2 and H-5), 4.57 (s, 4H, 2 x H-3 and H-4); ^C-pH} NMR (100 MHz, CDCI3): 6 153.6 (2 x C, 2 x C=O), 145.8 (2 x C, 2 x C-2'), 145.8 (2 x CH, 2 x C-5'), 117.3 (2 x CH, 2 x C-3'), 112.3 (2 x CH, 2 x C-4'), 79.3 (2 x C, 2 x C-l), 74.4 (4 x CH, 2 x C-3 and C- 20 4), 72.6 (4 x CH, 2 x C-2 and C-5); HRMS (ESI+): m / z 375.0320 [M + H]+ (mcaic 375.0320); Calcd for C20Hi4FeO2: C 64.20, H 3.77. Found: C 64.27, H 3.50. l,l'-Bis(2-phenylcarbonyl)ferrocene - as reported in A. A. O. Sarhan, M. S. Ibrahim, M. M. Kamal, K. Mitobe and T. Izumi, Monatshefte fur Chemie - 25 Chemical Monthly, 2009, 140, 315-323 l,l'-Bis(2-phenylcarbonyl)ferrocene was prepared according to the general procedure using AICI3 (1.79 g, 13.4 mmol), benzoyl chloride (1.56 mL, 13.4 mmol), ferrocene (502 mg, 2.69 mmol) and CH2CI2 (20 mL). The crude residue was purified by flash chromatography (n-hexane / EtOAc 9:1 -» 1:1) and the resulting product recrystallized from n-hexane to afford l,l'-bis(2-phenylcarbonyl)ferrocene (745 mg, 70%) as red fluffy needles. Rf 0.63 (n-hexane / EtOAc 1:1); XH NMR (400 MHz, 5 CDCI3): 6 7.78 (d, 4H, 3Jhh = 7.0 Hz, 2 x H-2' and H-6'), 7.55 (t, 2H, 3Jhh = 7.4 Hz, 2 x H-4'), 7.43 (t, 4H, 3Jhh = 7.6 Hz, 2 x H-3' and H-5'), 4.92 (pseudo-t, 4H, J = 2.0 Hz, 2 x H-3 and H-4), 4.58 (pseudo-t, 4H, J = 2.0 Hz, 2 x H-2 and H-5); 13C{XH} NMR (100 MHz, CDCI3): 6 198.1 (2 x C, 2 x C=O), 139.3 (2 x C, 2 x C-1'), 132.1 (2 x CH, 2 x C-4'), 128.5 (4 x CH, 2 x C-3' and C-5'), 128.3 (4 x CH, 2 10 x C-2' and C-6'), 79.6 (2 x C, 2 x C-l), 74.8 (4 x CH, 2 x C-2 and C-5), 73.3 (4 x CH, 2 x C-3 and C-4); HRMS (ESI+): m / z 395.0717 [M + H]+ (mcaic 395.0734); Calcd for C24Hi8FeO2: C 73.12, H 4.60. Found: C 72.92, H 4.46. Solar Cell Preparation 15 PFN-Br was prepared by dissolving 0.5 mg / mL in methanol. PNDIT-F3N was prepared by dissolving the same concentration in methanol with 0.5 %(v / v) of acetic acid. The doped solutions were arranged in the mentioned molar ratio by calculating from a monomer unit and left overnight to regulate the reaction. 20 All devices were fabricated in a conventional geometry of ITO / PEDOT:PSS / active layer / ETL / Ag. The patterned indium tin oxide (ITO) substrates (15 Q sq x) were cleaned by sequential ultrasonic cleaning in water, acetone, and isopropyl alcohol for 10 minutes each, and dried nitrogen gun. These substrates were transferred to UV Ozone and treated for 30 minutes. Then PEDOT:PSS was spin-coated at 25 4000 rpm for 40 s, and dried on a hot plate at 150°C for 10 minutes. The photoactive layer solutions, i.e., PM6:BTP-eC9 and PM6:L8-BO were prepared by dissolving 15.4 mg / mL the donor:acceptor weight ratios of 1:1.2 in chloroform with 0.25 %(v / v) of DIO. Then the solutions were spun at 3000 rpm for 30 s to achieve films thickness around 100 nm. Then ETL solutions were cast on the photoactive 30 layer at a speed of 2000 rpm for 20 s to form 5 nm-thick films. Afterwards, the devices were transferred to the evaporator and 100 nm Ag layer was thermally evaporated under high vacuum (3xl0-6 mbar) through the shadow mask of which the area is 0.045 mm2. 35 RESULTS: Pristine and doped PFN-Br ETL with PM6:BTP:eC9 Active layer Figure 3 shows FTIR data comparing PFN-Br (middle line), l,l'-bis(2-furanylcarbonyl)ferrocene (bottom line, labelled "F4") and a mixture 50 mol% 1,1'- bis(2-furanylcarbonyl)ferrocene (top line, labelled "50 mol% F4(New)"). The peaks at ~1750 cm1 (C=O stretching) and at ~1400 cm4, ~1300cm-1, and ~1000cm-1 (C-0 stretching in the furan) for l,l'-bis(2-furanylcarbonyl)ferrocene are reduced in the mixture, confirming that a complex was formed. 5 Normal architecture OPVs were prepared based on ITO / PEDOT:PSS / Active layer (PM6:BTP:eC9) / metallocene:ETL / Ag, illustrated in Figure 4A. The OPVs were measured under AM 1.5 G simulated solar illumination. The current density-voltage (JV) curves are shown in Figure 4B. The extracted photovoltaic parameters are io depicted in Figure 4C and provided in Table 1 below. Table 1 F4 / mol% l / oc / V Jsc / mA cm-2 FF / % PCE / % Max Avg Max Avg Max Avg Max Avg 0 0.852 0.850 ±0.003 27.3 26.6 ±0.4 75.9 75.6 ±0.7 17.5 17.1 ±0.3 1.25 0.851 0.850 27.8 26.9 76.0 76.1 18.0 17.4 ±0.002 ±0.8 ±0.7 ±0.5 2.5 0.856 0.855 28.1 27.4 77.3 77.3 18.6 18.1 ±0.003 ±0.5 ±0.3 ±0.3 5 0.844 0.845 ±0.002 27.2 26.7 ±0.5 75.1 75.1 ±1.0 17.2 16.9 ±0.3 The OPVs containing 1.25 mol% and 2.5 mol% l,l'-bis(2-furanylcarbonyl)ferrocene 15 (F4), exhibited increased open-circuit voltage (Voc), increased short-circuit current density (Jsc), increased fill factor (FF) and increased power conversion efficiency (PCE). The superior performance of the OPV containing 2.5 mol% l,l'-bis(2-furanylcarbonyl)ferrocene (F4) was due to the improved FF. 20 Stability The stability of the OPVs was determined under AM 1.5 G illumination, at maximum power point (MPP) and under a N2 atmosphere. The OPV containing the ferrocene-based complex was more stable overtime, as shown in Figure 5. The OPV containing the ferrocene-based complex showed greater normalized efficiency than 25 the comparative OPV. Indeed, the OPV demonstrated outstanding stability with smaller initial reduction in the normalized efficiency than the comparative OPV and greater retention of the normalized efficiency overtime. RESULTS: Pristine and doped PNDIT-F3N ETL with PM6:L8-BO Active layer 5 OPVs were prepared based on ITO / PEDOT:PSS / Active layer (PM6:L8-BO) / metallocene:ETL / Ag, illustrated in Figure 6A. The OPVs were measured under AM 1.5 G simulated solar illumination. The current density-voltage (JV) curves are shown in Figure 6B. The extracted photovoltaic parameters are depicted in Figure 6C and provided in Table 2 below. io Table 2 F4 / mol%) l / oc / V Jsc / mA cm-2 FF / % PCE / % Max Avg Max Avg Max Avg Max Avg 0 0.880 0.885 ±0.004 26.68 26.01 ±0.73 78.08 78.08 ±0.38 18.34 18.02 ±0.53 2.5 0.883 0.885 27.56 26.99 79.65 79.24 19.38 18.92 ±0.002 ±0.43 ±0.48 ±0.29 The OPVs containing l,l'-bis(2-furanylcarbonyl)ferrocene (F4) exhibited increased short-circuit current density (Jsc), increased fill factor (FF) and increased power 15 conversion efficiency (PCE). While the present invention has been described in connection with specific forms and embodiments, it will be appreciated that various modifications can be made without departing from the spirit or scope of the invention.

Claims

1. An organic photovoltaic cell comprising:a first electrode;5 a second electrode;an active layer and an electron transport layer disposed between the first and second electrodes;wherein the electron transport layer comprises an organic electronic material and a metallocene substituted with at least one substituent R1 comprising at leastio one of an O, S, N or P atom.

2. The organic photovoltaic cell according to claim 1 wherein the metallocene substituted with at least one substituent R1 comprising at least one of an 0, S, N or P atom is a compound of formula (I):15 [Metallocene]p(I)wherein:Metallocene is a metallocene group comprising a metal bound to two aromatic or heteroaromatic groups Ar1;20 p is at least 1; andat least one Metallocene is substituted with at least one substituent R1.

3. The organic photovoltaic cell according to claim 2 wherein the compound offormula (I) has formula (la):25Ar1M—(R2)qAr1R3(la)wherein:M is a metal ion;Ar1 in each occurrence is a monocyclic or polycyclic aromatic or30 heteroaromatic group;M and the two Ar1 groups form the Metallocene;at least one Ar1 is substituted with at least one R1;R2 is a group for satisfying the valency of M;q is 0 or a positive integer; andR3 in each occurrence is independently H or a substituent.5 4. The organic photovoltaic cell according to any one of the preceding claimswherein the metallocene is ferrocene.

5. The organic photovoltaic cell according to any one of the preceding claims wherein R1 is a group of formula (II):io -A-B(II) wherein A is a divalent group comprising O, S, N or P; and B is H, C1-12 alkyl, optionally substituted aryl or optionally substituted heteroaryl.15 6. The organic photovoltaic cell according to claim 5 wherein A is selected fromformula (III) or formula (IV):-(R5)f-Z-(R5)g- (III)-(R6O)j- (IV)wherein:20 R5 in each occurrence is independently a hydrocarbon group;f and g are each independently 0 or 1;R6 is a Cl-4 alkylene group, preferably ethylene;j is 1-10; andZ is O, S, 0(=0), COO, C(=S)O, C(=O)S, CONR4, CSNR4, 00(=0)0,25 OC(=O)NR4, OC(=O)PR4' NR4, PR4, -OP(=O)(OR4)-O-, -NR4-P(=O)(NR42)-NR4-, wherein R4 is H, optionally substituted C1-12alkyl or optionally substituted phenyl.

7. The organic photovoltaic cell according to claim 5 or 6 wherein A30 is -C(=0)-0-; -0-0(=0)- or -0(=0)-.

8. The organic photovoltaic cell according to any one of claims 5-7 wherein B is selected from optionally substituted phenyl and an optionally substituted 5-membered heteroaryl comprising one or more ring atoms selected from 0, S and 35 N.

9. The organic photovoltaic cell according to any one of claims 5-8 wherein B is optionally substituted thiophene or optionally substituted furan or optionally substituted phenyl.5 10. The photovoltaic cell according to any one of the preceding claims whereinthe active layer does not comprise perovskite and / or a metallocene substituted with at least one substituent R.1 comprising at least one of an O, S, N or P atom11. A photovoltaic module comprising a plurality of the organic photovoltaic cells io according to any one of the preceding claims, the photovoltaic cells connected in series.

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