NL2014097B1

NL2014097B1 - Hole transport azomethine molecule.

Info

Publication number: NL2014097B1
Application number: NL2014097A
Authority: NL
Inventors: Leonardus Petrus Michiel; Jacobus Dingemans Theodorus
Original assignee: Univ Delft Tech
Priority date: 2015-01-08
Filing date: 2015-01-08
Publication date: 2016-09-30
Also published as: NL2014097A; WO2016111623A1

Abstract

The present invention is in the field of a azomethine molecule having photovoltaic characteristics and applicable as a hole transporter, in a photovoltaic device, the device further comprising a light absorbing material, such as a perovskite based device, and a solar cell comprising said molecule as a hole transporter or said device.

Description

Hole transport azomethine molecule

FIELD OF THE INVENTION

The present invention is in the field of a azomethine molecule having photovoltaic characteristics and applicable as a hole transporter, in a photovoltaic device, the device further comprising a light absorbing material, such as a perov-skite based device, and a solar cell comprising said molecule as a hole transporter or said device.

BACKGROUND OF THE INVENTION

The present invention is in the field of cost effective solar cells. These cells can be processed from a liquid solution comprising a photovoltaic material, hence the possibility of a simple roll-to-roll printing process, potentially leading to inexpensive, large scale production. In addition, these cells could be beneficial for applications where mechanical flexibility and disposability are important. Current flexible solar cell efficiencies have a low power conversion efficiency (PCE) of 8-10%), compared to inorganic based cells, and practical devices are essentially non-existent. On top of that, costs of organic materials used therein are very high, at least partly due to process costs.

In solar cells perovskites may be used. Perovskites have a general formula of ABC3. A may be an alkaline earth element, a rare earth element, or an organic molecule, such as an amine, such as formamidinum, an ammonium, such as me-thylammonium. B may be a 3d, 4d, and 5d transition metal element, such as Pb, Sn, Ti. C may be a halide, such as C1-, Bh, I”, and F”, and oxygen. A perovskite solar cell is a type of solar cell which includes a perovskite light absorber. Perovskite absorber materials are cheap to produce and simple to manufacture. Solar cell efficiencies of devices using these materials have recently increased by a factor.

It is noted that architectures of perovskite solar cells may vary.

It is noted that perovskite materials can also act as highly efficient, ambipolar charge-conductor. After light absorption and the subsequent charge-generation, both negative and positive charge carrier are transported through the perov- skite to charge selective contacts (typically anode and cathode) .

Power conversion efficiency (PCE) with a thin layer of perovskite on e.g. mesoporous T1O2 as electron-collector was in the order of 5%, which is considerably less than e.g. silicon based solar cells. Recently it was shown that efficiencies of almost 10% were achievable using a T1O2 architecture. New techniques achieved more than 20% efficiency. It was also shown to fabricate perovskite solar cells in what is considered a typical 'organic solar cell' architecture, i.e. an 'inverted' configuration having a hole transporter below and the electron collector above.

In an alternative approach of using solar energy photolysis of water using perovskite solar cells was shown. Thereto two perovskite solar cells were connected.

There are still some limitations to be overcome. For instance the power-conversion efficiency of a solar cell is usually determined by characterizing its current-voltage (JV) behaviour under simulated solar illumination. In contrast to other solar cells it has been observed that the JV-curves of perovskite solar cells show a hysteretic behaviour: there is a discrepancy between the scan from forward-bias to short-circuit (FB-SC) and the scan from short-circuit to forward bias (SC-FB. Another issue is the UV-instability of the perovskite systems. And also the systems are prone to deterioration due to water.

Another class of solar cells, briefly described above already, relates to hybrid cells. Hybrid solar cells are supposed to combine advantages of both organic and inorganic semiconductors, wherein one may act as hole transporter and the other as electron transporter. Hybrid photovoltaics typically have organic materials that consist of e.g. conjugated polymers that absorb light as the donor and transport holes. Inorganic materials in hybrid cells are used as the acceptor and electron transporter in the structure. The hybrid photovoltaic devices have various advantages.

As indicated above, the efficiency of perovskite-based solar cells has increased rapidly in the last five years making this one of the most promising photovoltaic technolo gies and almost being competitive with current silicon-based technologies. Although the perovskite material itself is relatively inexpensive, the best devices prepared to date use an expensive organic hole-conducting material, called spiro-OMeTAD (~$700/gram, see Solaronix: spiro-OMeTAD). This material is expensive to produce because the synthesis requires expensive monomers, transition metal catalysts, inert reaction conditions and extensive product purification.

Many research groups around the world are currently publishing new materials to replace Spiro-OMeTAD. However, these materials are generally synthesized in transition metal catalysed reactions, requiring stringent reaction conditions and significant product workup.

Many molecules that are in principle suited for photovoltaic application, can not be produced or only with difficulty into bulk photovoltaic devices, such as into a film.

The present invention therefore relates to use of an azomethine comprising molecule having photovoltaic characteristics as a hole transport layer in a photovoltaic device, and a solar cell comprising said hole transport layer, which solve one or more of the above problems and drawbacks of the prior art, providing reliable results, without jeopardizing functionality and advantages.

SUMMARY OF THE INVENTION

The present invention relates to a photovoltaic device comprising a hole transporting molecule and optionally at least one hole mobility improver in a hole transport layer, and further a light absorbing material layer, wherein the hole transporting molecule comprises an azomethine molecule according to figure 3, and a solar cell comprising said device according to claim 12.

The present inventors have developed azomethine-based hole-transport materials as a cheap alternative, because they are found to be easily synthesized in a one-step condensation reaction (upon loosing water as only by-product a conjugated chemical bond is formed), do not require expensive catalysts and do not require extensive workup. The estimated price of the present material is some 5$/gram. So far azomethines have not been used as charge transport layer, such as in perov- skite-based photovoltaics.

The azomethine molecules have hole transport capabilities, but these are typically insufficient for the intended application in a photovoltaic device. In order to apply the azomethines in a PV-device at least one hole mobility improver is therefore preferably provided to the hole transport layer. The hole mobility improver is typically of p-type. A typical amount of the first hole mobility improver is 0.1-100 mole %, relative to the azomethine molecule, preferably 0.5-50 mole %, more preferably 1-30 mole %, even more preferably 2-20 mole %, such as 3-5 mole%. The hole mobility improver may be considered as a dopant to the hole transport material; it is noted that the concentration of the hole mobility improver is typically much higher than e.g. dopants in semiconductor devices.

The hole mobility improver is typically capable of oxidizing the hole transport material, and/or promote oxidative reaction thereof; in this respect the hole mobility improver may be considered as an oxidant. In principle any such oxidant could be applied in the present invention. Likewise so-called ionic liquids may be used.

Hole mobility improvers may be selected from earth alkali metal, monovalent salts, hydrogen salts (or acids), N,N-dimethyl- pyrrolidinium, and N-methyl-N-propylpyrrol-idinium salts of bis(tris-fluoromethylsulfonyl)imide (TFSI), such as LiTFSI, AgTFSI, HTFSI, N,N-dimethyl-pyrrolidinium iodide, and tert-butyl pyridines.

In order to improve the conductivity and the hole mobility the hole transport materials are p-type doped. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and monovalent salt equivalents thereof (e.g. Ag+) are attractive additives; also other additives like N(PhBr) 3SbCl6, F4TCNQ and cobalt dopants can be used.

Also other additives for passivating a surface can be used. Generally these additives are combined with the first and/or second hole mobility improver, typically in a concentration of 10-1000 mole%, such as 100 mole%. In an example 4-tert-butylpyridine (TBP) is used; also pyridine and iodopen-tafluorobenzene (IPFB) have shown to improve the device performance, like pyridine, 2,6-lutidine, and 4-methylpyridine. A series of small-molecule azomethines has been synthesized for further investigation. Perovskite based solar cells using these azomethines or likewise a (co-)polymer comprising said molecule as hole transport layer, in combination with the hole mobility improver, have been prepared and measured, showing record efficiencies of around 11% (for sake of comparison, under the same processing conditions Spiro-OMeTAD shows efficiencies up to 13%). Further optimisation will increase the efficiency and reduce hysteresis.

The present azomethine molecules as hole transport material can relate to both small molecules and polymers comprising said azomethine molecules.

The present azomethine molecules are considered to relate to small, well-defined molecules having well defined properties, such as relatively excellent photovoltaic properties, a molecular mass of 300-3000, preferably 400-1500, more preferably 500-1000, such as 600-750, good film forming properties, low costs, etc. With reference to claim 1 and fig. 3, the present molecules comprise two entities, one aromatic entity A, and one aromatic (optionally heterocycle) bridging entity (B), and a connecting C=N moiety. The present aromatic entity A is non-heterocyclic, i.e. only comprising carbon atoms in the aromatic ring. The C=N moiety may be connected with the nitrogen thereof to entity A or to entity B, and likewise with the carbon thereof to entity B or entity A. Entity B is connected to at least one entity A, or to more than one A entities, such as in a tree like structure, in a chain like structure, and combinations thereof.

The aromatic entity A preferably has a molecular mass of 64-1250, preferably 64-700, such as 64-500, whereas, independently, the aromatic (optionally heterocycle) bridging entity B preferably has a molecular mass of 65-1500, preferably 65-1000, such as 64-750. The aromatic entity A preferably has 1-4 aromatic ring structures, more preferably 1-3, such as 2, whereas, independently, the aromatic (optionally heterocycle) bridging entity B preferably has 1-8 aromatic ring structures, more preferably 1-5, such as 2-4.

For better understanding the background and synthesis of the present molecules the Dutch Patent application NL2012451, filed on 17 March 2014, is referred to, which reference and the contents thereof are incorporated by this reference .

The present azomethine molecules and polymeric equivalents thereof find application as a hole transport material in the present photovoltaic device.

Thereby the present invention provides a solution to one or more of the above mentioned problems and drawbacks.

Advantages of the present description are detailed throughout the description.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates in a first aspect to a photovoltaic device according to claim 1.

The present molecules have excellent properties. For instance a glass transition temperature (Tg) of about 100 °C is obtained, melting points of about 200 °C, a thermal decomposition temperature (Td, 5%) of about 400 °C, and a crystallization temperature Tc of about 150 °C. Such makes the present molecules particularly suited for applications such as hybrid photovoltaic devices that function reliable under ambient conditions.

Further optical properties are also excellent. The molecules exhibit absorbance of light over a range of about 280 nm-about 600 nm, with a λ^χ of about 500 nm, and an onset wavelength Xonset of about 600 nm. The band-gap is in the order of 2.0 eV. These absorbance properties can be improved for the present molecules. Further, a maximum sabs of 30, 000-60, 000 (M~ 1cm_1) is found.

Electrical properties are also fine. For instance an Eoxi of about 1 V (versus Ag/Ag+) , a voltage below vacuum (HOMO) of about -5.0— -6.0 eV, and a LUMO of about -3 eV.

The molecules provide a good solubility and good film forming properties.

In an example of the present azomethine molecule the molecule has an (A-B)n-A structure (fig. 3c) or (B-A)n-B structure, wherein ne[l-5], preferably ne[2-3], or wherein the molecule has an Am-B structure (fig. 3d), wherein me [1-5], preferably me [2-3], or wherein the molecule has a Br-A structure (fig. 3e), wherein re [1-5], preferably re [2-3], or combina tions thereof. A-B-A, A-B, B-A-B, A3B, B3A, A-B-A-B and A-B-A-B-A type molecules are preferred.

In an example of the present azomethine molecule the aromatic (optionally heterocycle) bridging entity comprises one or more of S, N, P, Si, Se and 0, preferably one or more of S, Se and Si, such as a pyrrolidine, a pyrrole, a tetrahy-drofuran, a furan, a thiolane, a thiophene, a phopholane, a phosphole, a sililane, a silole, an azole, such as an imidaz-olidine, a pyrazolidine, an imidazolem a pyrazole, an (is)oxazolidine, an (is)oxazole, a (iso)thiasolidine, an (iso)thiazole, a dioxolane, a dithiolane, a piperidine , a pyridine, an oxane, a pyran, a thiane, a thiopyran, a sa-linane, a saline, a phosphinane, a phosphinine, a piperazine, a diazine, a morpholine, an oxazine, a thiomorpholine, a thia-zine, a dioxane, a dioxine, a dithiane, a dithiine, a triazine a trioxane, an azepane, an azepine, an oxepane, an oxepine, a thiepane, a thiepine, a hmopiperazine, a diazepine and a thi-azepine. The aromatic ring preferably has 5-6 ring members, such as 5-6 ring members. It is further preferred that in the at least one aromatic (optionally heterocycle) entity B at least two, and preferably at least two different, heteroatoms are present, one of which preferably is S.

In an example the present azomethine molecule comprises at least one 4-aminotriphenylamine (TPA) according to figure 2. This TPA is readily available.

In an example of the present azomethine molecule the bridging entity is selected from 2,5-thiophenedicarbaldehyde (Th), 2,3-dihydrothieno[3,4-b][1,4]dioxine-5,7-dicarbaldehyde (EDOT), 2,2'-bithiazole-5,5'-dicarbaldehyde (BTz), 4,7-bis(5-formylthiophen-2-yl)-2,1,3-benzothiadiazole (TBT), and combinations thereof.

In an example of the present azomethine molecule the X is independently selected from H, -CN, OAlk, Aik, and combinations thereof, and Aik is selected from alkanes, such as Ci-Cis, preferably C1-C12, such as Me, Et, and combinations thereof.

It is noted that the present azomethine can be obtained by a cheap, simple and reliable method, as detailed in the above NL2012451 application.

In an example the method further comprises one or more steps selected from before spin coating the condensate, thereby forming a solution, wherein the solution is spin coated on a substrate, removing the solvent, and annealing the condensate. Thereby the present product can be applied directly, e.g. to a substrate, without a further intermediate step, which is a big advantage.

Albeit the present conversion efficiencies are somewhat limited (state of the art materials reach efficiencies up to 15%), initial experiments directed to product development and improvement are so promising that these efficiencies are expected to be in reach. At that point the present molecules, films, fibres and device, can be obtained at much lower costs, which is considered to provide market break through opportunities .

The device preparation has not been optimized yet. Optimizing the layer thickness, and solvent/co-solvent will further increase the efficiency.

Generally, a p-type dopant is added to the hole transporting material before spin coating. An oxidant, for example lithium bis-trifluoromethanesulfonimide (LiTFSI), 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) or transition metal based salts like tris(2-(lH-pyrazol-1-yl)pyridine) cobalt(II) bis(hexafluorophosphate) (FK102) or combinations of these, may be used. Also an additive, like 4-tert-butylpyridine (TBP), is added, in order to passivate the surface .

In an example of the present photovoltaic device the hole transport layer comprises a second hole transport improver in an amount of 0.1-15 mole % relative to the azomethine molecule, preferably 0.5-10 mole %, such as 1-5 mole%. Generally it is combined with one of the first hole mobility improver. An examples of a second hole transport improver materials is tris(2-(lH-pyrazol-l-yl) pyridine) cobalt (II) bis(hexafluorophosphate) (FK102), Tris(2-(lH-pyrazol-l-yl)-4- tert-butylpyridine)-cobalt(III)Tris(bis (trifluoromethyl-sulfonyl)imide)) (FK209), SnCl4, LiClCq, 2,3,5,6-Tetrafluoro- 7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), N(PhBr) 3SbCl6, NOBF4 (exposing to vapour), WO3 (evaporating a thin layer on top), and combinations thereof. Likewise further hole transport improvers may be added; typically 2-5 improvers are added, independently of one and another, in similar concentrations as above.

It is noted that at least on first hole mobility improver may be selected, optionally in combination with at least one second hole mobility improver, and so on.

In an example the present photovoltaic device comprises a stack, the stack comprising a substrate, a first optically transparent electrode layer, a second electrode layer, and between the electrode layers, (i) a perovskite layer as light absorbing layer, (ii) the present hole transport layer, and (iii) an intermediate layer. In principle the above sequence of layers is considered an almost minimal set of layers in view of performance, Further (intermediate and protecting) layers may be added, e.g. in order to improve performance.

Two stacks of layers are considered specifically, relating to a so-called inverted and standard perovskite devices (see fig. 4a-b). The substrate 11 therein is typically glass. In the inverted device the anode 21 is an optically transparent electrode, such as FTO, in contact with the substrate. In contact with the anode and the perovskite 31 is the present hole transport layer 41. A metal layer 22 (such as Al) acts as second electrode. Typically an intermediate layer 51, such as a fullerene, is present between the cathode and perovskite.

The perovskite may be CH3NH3Pbl3, CH3NH3PbBr3, a mixed halide, such as CH3NH3PbX13-xX2x, (X1 and X2 being independently selected from halides), such as CH3NH3Pbl3-xClx , and an inorganic perovskite, such as CsSnX3, or mixed inorganic perovskite, such as CsSnX13-xX2x (X, X1 and X2 being independently selected from halides), such as iodides. In the standard perovskite device the cathode 21, such as FTO is on the substrate 11, such as glass. A metal layer (such as Au) acts as second electrode 22. In contact with the cathode and perovskite layer 31 is an intermediate layer 51, such as T1O2. In contact with the anode 22 and the perovskite 31 is the present hole transport layer 41.

The present perovskites, having the general formula of ABC3, preferably relate to A being an alkaline earth ele- ment, a rare earth element, such as Na, K, Li, Ca, Mg, Ce, or an organic molecule, such as an amine, such as formamidinum, and an ammonium, such as methylammonium CH3NH3, and ethylammo-nium C2H5NH3, to B being a 3d, 4d, and 5d transition metal element, such as Pb, Sn, Mn, Ni, Co, Fe, and Ti, C being a halide, such as Cl-, Br-, I-, and F-, and oxygen, as well as mixed minerals thereof. Examples are lead halogenides, such as CH3NH3Pbl3, tin halogenides, such as CfhNHsSnls, titanates such as CaTi03. The band gap of the perovskite is preferably from 1.4-2.5 eV, such as 1.5-2.3 eV.

Note that the first electrode and second electrode in the above two stacks of layers change function, from anode to cathode, and vice versa. Also the nature of the intermediate layer changes. On the other hand the functions of the perovskite layer and hole transport layer remain the same.

In an example of the present photovoltaic device the substrate is selected from glass, silicon, steel, aluminium, silicon oxide, polymers, and combinations thereof, preferably glass .

In an example of the present photovoltaic device the first electrode layer is selected from optically transparent and electrically conducting materials, such as tin doped indium oxide (ITO), graphene, and fluorine doped tin oxide (FTO), preferably ITO.

In an example of the present photovoltaic device the second electrode layer is selected from metals, such as Al,

Cu, Mg, Ba, ZnO, Au, Ag, and Ca, preferably Au or Al, respectively. Au has better properties in view of hole transport, whereas Al has better properties in view of electron transport.

In an example the present photovoltaic device comprises an intermediate layer, typically for electron transport, which layer is preferably selected from fullerenes and T1O2, respectively. The layer may have a thickness of 10-500 nm, preferably 20-300 nm, more preferably 50-200 nm, such as 100-150 nm.

In an example the present photovoltaic device has a sequence of layers comprising one or more of an 20-1000 nm thick first electrode layer, preferably 100-750 nm, more pref- erably 200-500 ran, such as 300-400 nm, such as ITO, a 10-1000 nm thick light absorbing layer, preferably 20-800 nm, more preferably 50-500 nm, such as 200-400 nm, an 10-300 nm thick hole transport layer, preferably 20-200 nm, such as 50-100 nm, such as an azomethine according to the invention and an 0.5-50 nm thick electron transport layer, preferably 1-20 nm, such as 2-10 nm, and a 20-200 nm thick second electrode layer, preferably 30-100 nm, such as 50-75 nm, such as Au. The hole transport layer is preferably not too thin, as holes in the layer may occur and coverage of in particular the perovskite layer may be insufficient. The hole transport layer is preferably not too thick, as it may act as a semi-insulator. In general the hole transport layer can be somewhat thinner than comparable materials. A further intermediate layer between e.g. hole transport layer and second electrode may be present.

In a second aspect the present invention relates to a solar cell according to claim 11.

The one or more of the above examples and embodiments may be combined, falling within the scope of the invention.

EXAMPLES

The below relates to examples, which are not limiting in nature .

In a study inventors have synthesized three azome-thine-based small molecules via a simple condensation reaction (Figure 1). 4-Aminotriphenylamine (TPA, 1) was reacted with two conjugated dialdehydes. 2,5-Thiophenedicarbaldehyde (Th, 2a) was used since thiophenes have been studied extensively and possess good charge transport properties. Besides that, both materials are readily available from commercial sources. 2,3-dihydrothieno[3,4-b][1,4]dioxine-5,7-dicarbaldehyde (EDOT, 2b) was used because it is found to have a large overlap with the solar spectrum, high mobilities and good film-forming abilities. Also 4-amino-4',4''-dimethoxytriphenylamine (TPA(OMe)2/ lb) was reacted with 2,3-dihydrothieno[3,4-b][1,4]dioxine-5,7-dicarbaldehyde (EDOT, 2b) because the alkoxy groups are considered to improve the interfaces with the perovskite. The reaction was performed in chloroform and p-toluenesulfonic acid was used as a catalyst. The products (3a-c) were precipitated, and treated with diluted triethyla-mine to neutralize the azomethine bond. The small molecules were obtained in good yields (>80%) and characterized using 1H and 13C NMR, FTIR, and mass spectrometry where possible. The thermal properties of the new molecules were assessed using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). All small molecules show excellent thermal stabilities with degradation temperatures (Td5ts) above 350 °C. Comparing TPA-Th-TPA to its vinyl analogue, inventors found that the degradation temperature is approximately 40 °C higher, confirming the superior thermal stability of the present azomethines. DSC experiments showed a glass transition temperature (Tg) in the range of 86 and 113 °C. The azomethine based small-molecule TPA(OMe)2-EDOT-TPA(OMe)2 shows a higher Tg compared to its fully conjugated analogue (H101, Tg = 73 °C), which is considered to be a more thermal stable hole transporting layer. TPA-Th-TPA and TPA-EDOT-TPA also showed a cold crystallization exotherm during the second heating at 143 °C and 171 °C, respectively. All small molecules melt above 200 °C and form isotropic melts, as was confirmed by hot-stage optical microscopy.

Absorption spectra of the small molecules were recorded in solution and in the solid state and the data are summarized in Table 1. The absorption peak around 310 nm can be ascribed to the triphenylamine moiety and is found for all small molecules. The absorption spectra of the small molecules in the solid state show no large shifts as compared to the solution spectra (Table 1). The band-gap of the small molecules could be estimated from the onset of the absorption spectra, which is around 600 nm for all small molecules, resulting in band gaps in the range of 2.0 to 2.2 eV.

The electrochemical properties of the small molecules in solution were investigated using cyclic voltammetry. Contrary to prior art azomethines inventors found that oxidation is reversible. However, when applying a higher potential (>1.2 V) the small molecules are oxidized to their 2+ form, and this second oxidation appears to be irreversible. It is well-known from literature that oxidation of triphenylamines results in the formation of a cationic radical, which can dimerise via the para-position. The oxidation potential was found between 0.1 and 0.5 V (vs Fc/Fc+) . The methoxy functionalized small-molecule shows the lowest oxidation potential, which is ascribed to the electron donating methoxy groups. The highest occupied molecular orbital (HOMO) energy levels of the small molecules were estimated by using the oxidation onset of the first oxidation and were found in the range of -5.2- -5.5 eV below vacuum. The relatively deep HOMO energy levels are expected to give a good energy alignment with the perovskite.

The lowest unoccupied molecular orbital (LUMO) energy levels were estimated by combining the optical band-gap with the obtained HOMO energy level. The azomethine moiety was found to slightly deepen the HOMO energy level compared to the vinyl and fully aromatic analogues, which are about 0.1-0.3 eV higher .

Planar perovskite based photovoltaics with the azomethine as hole transporting layer show power conversion efficiencies (PCE) exceeding 10% (Table 2). This efficiency is only slightly lower compared state of the art materials, whereas optimisation of the chemical structure and processing conditions is found to further increase the PCE. As azomethines can be prepared in a simple and clean condensation reaction under near ambient conditions we believe this approach offers a cost effective route towards hole transporting materials.

Device fabrication

As a substrate FTO glass was etched with zinc powder and HC1 (2M) and cleaned afterwards. A compact layer of T1O2 was deposited by spin-coating a titanium isopropyl solution. It was heated to 500 °C for 45 minutes. After cooling down the substrate were transferred to a nitrogen filled glove-box. A solution of 1M Pbl2 and 1M methylammoniumiodide was spincoated dynamically (at 5000 rpm, total 15 sec) onto the substrate. After 5 seconds 100 pL of chlorobenzene was added on top of the spinning substrate. Thereafter the substrate was placed on a hotplate at 70 °C for 10 minutes. After cooling to ambient temperature the present hole transporter was spin-coated on top. In an example TPA (OMe) 2-EDOT-TPA (OMe) 2 (10 mg mlh1) , and Spiro-OMeTAD (75 mg mlh1) were dissolved in chlorobenzene. Lithium bis(tris-fluoromethylsulfonyl)imide (Li-TFSI; 5 pL mlh1 of a 15 mg mlh1 acetonitrile solution) and Tert-butylpyridine (TBP; 30 pL mlh1) were added to the hole transport material solutions. As a Co-dopant tris(2-(lH-pyrazol-l-yl) pyridine) cobalt(III) bis(hexafluorophosphate) (FK102) was pre-dissolved in acetonitrile and added to the hole transport material solution at a ratio of 0-15 mole%.

The thickness of the hole transport layer was found to be 20nm for TPA(OMe)2-EDOT-TPA(OMe)2 and ~250nm for Spiro-OMeTAD. Thereafter a top electrode was deposited by thermal evaporation of gold under vacuum (at 1CT6 mbar), with a thickness of 40 nm.

In conclusion, two conjugated small-molecule azome-thines were synthesized via simple and cheap condensation chemistry. The small molecules both exhibit reversible oxidation behaviour, a fairly deep lying HOMO energy level, and a band-gap between 1.9 to 2.2 eV. Inventors demonstrated that this chemistry enables the fabrication of hole transport materials for perovskite-based photovoltaics. Because azomethine chemistry is easy, clean and proceeds under near ambient conditions inventors consider that this approach has the ability to reduce materials and production costs of organic photovoltaic devices.

The invention is further detailed by the accompanying figures, which are exemplary and explanatory of nature and are not limiting the scope of the invention. To the person skilled in the art it may be clear that many variants, being obvious or not, may be conceivable falling within the scope of protection, defined by the present claims.

FIGURES

The invention although described in detailed explanatory context may be best understood in conjunction with the accompanying figures.

Fig. 1 shows a synthesis route and molecular structures of the present small-molecule azomethines.

Fig. 2 shows a TPA molecule with an optional group X.

Fig. 3a-e show basic configurations of the present azomethine molecules .

Fig. 4a-b relate to a so-called inverted perovskite device and standard perovskite device, respectively. DETAILED DESCRIPTION OF THE FIGURES The figures have been detailed throughout the description .

TABLES

Table 1. Summary of the optoelectronic properties of TPA-X-TPA small molecules UV-vis solution3 UV-vis filmb

Compound ?tmax 1 ?tmax 2 ?omax 1 ?omax 2 bonset Eg Eoxi, onset HOMO LUMO ;nm) ;nm) ;nm) ;nm) ;nm) (eV)c (V) (eV)d (eV)e TPA-Th-TPA 306 467 314 463 565 2.19 0.45 5.55 3.36 TPA-EDOT-TPA 305 476 308 472 575 2.16 0.37 5.48 3.32 TPA(OMe)2- EDOT- 306 495 306 505 625 1.98 0.17 5.28 3.28 TPA(OMe)2 aMeasured in a quartz cuvette in dichloromethane. bSpin cast on a quartz substrate from chloroform. cEstimated from the onset of the absorption spectrum of the film, 1240Aonset· dDetermined by cyclic voltammetry, with Fc/Fc+ at 5.1 eV below vacuum. eEstimated by subtracting the band-gaps from the HOMO energy levels .

Table 2: J-V characteristics of the photovoltaic devices with different HTMs.

HTM Jsc Voc FF PCE (mA cm'2) (V) (%) (%) TPA-Th-TPA 12.6 1.00 56 7.1 TPA-EDOT-TPA 16.9 0.90 61 9.4 TPA(OMe)2-EDOT-TPA(OMe)2 18.6 0.95 61 11.0

Spiro-OMeTAD 17.9 1.00 66 11.9

Claims

A photovoltaic device comprising a hole-transporting molecule in a hole-transporting layer, and further a light-absorbing material layer, the hole-transporting molecule comprising an azomethine molecule according to Figure 3

wherein the azomethine molecule comprises at least one aromatic unit (A) comprising (Ai) at least one nitrogen atom directly connected to the aromatic entity (Fig. 3a), or (Aii) at least one carbon atom directly connected to the aromatic entity (Fig. 3b) ), at least one aromatic heterocyclic bridging entity (B), wherein the bridging and aromatic entity have a connecting N = C bridge (azomethine), wherein the bridging entity comprises (Bi) (in the case of Ai) at least one carbon atom directly connected to the aromatic heterocyclic unit and to the nitrogen atom (Fig. 3a), or (Bii) (in the case of Aii) at least one nitrogen atom directly connected to the aromatic heterocyclic unit and to the carbon atom (Fig. 3b).

A photovoltaic device according to claim 1, comprising at least one hole mobility enhancer in the hole transport layer, wherein a first hole mobility enhancer is present in an amount of 0.1-100 mol% based on the azomethine molecule.

A photovoltaic device according to any one of the preceding claims, wherein the molecule has an (AB) nA structure (Fig. 3c) or (BA) nB structure, wherein ne [1-5], or wherein the molecule has an Am-B structure (Fig. 3d), where me [1-5], or wherein the molecule has a Br-A structure (Fig. 3e), where re [1-5], or combinations thereof.

The photovoltaic device according to any of the preceding claims, wherein the aromatic heterocyclic bridging unit comprises one or more of S, N, P, Si, Se and 0.

A photovoltaic device according to any one of the preceding claims, comprising at least one 4-aminotriphenyl amine X 2 (TPA) according to Figure 2,

and wherein the bridging entity X is selected from 2,5-thiphen-dicarbaldehyde (Th), 2,3-dihydrothieno [3,4-b] [1,4] dioxin-5,7-dicarbaldehyde (EDOT), 2,2 '-bithiazole-5,5'-dicarbaldehyde (BTz), 4,7-bis (5-formylthiophen-2-yl) -2,1,3-benzothiadiazole (TBT), and combinations thereof.

The photovoltaic device of claim 5, wherein X is independently selected from H, -CN, OAlk, Alk, and combinations thereof, and wherein Alk is selected from alkanes, and combinations thereof.

A photovoltaic device according to any one of claims 1-6, wherein the hole transport layer comprises a second hole transport improver in an amount of 0.001-20 mol% based on the azomethine molecule.

A photovoltaic device according to any one of claims 1-7, comprising a stack of a substrate, a first optically transparent electrode layer, functioning as an anode or cathode, a second electrode layer, functioning as a cathode or anode, and between the electrode layers (i) a perovskite layer as a light-absorbing layer, (ii) a hole transport layer, wherein the hole transport layer is in contact with the anode layer and the perovskite layer, and (iii) an intermediate layer, wherein the intermediate layer is in contact with the cathode layer and the perovskite layer.

The photovoltaic device of claim 8, wherein the substrate is selected from glass, silicon, steel, aluminum, silicon oxide, polymers, and combinations thereof, and wherein the first electrode layer is selected from tin-doped indium oxide (ITO), graphene, fluorine-doped tin oxide (FTO), and wherein the second electrode layer is selected from metals, such as Al, Cu, Mg, Ba, ZnO, Au, Ag, and Ca, and wherein the interlayer is selected from T102 and fullerees.

The photovoltaic device according to any of claims 8-9, further comprising an electron transport layer.

A photovoltaic device according to any one of claims 8-10, with a sequence of layers comprising one or more of a 20-1000 nm first electrode layer, such as ITO, a 10-1000 nm light-absorbing layer, a 10-300 nm hole transport layer, a 0.5-50 nm electron transport layer, and a 20-200 nm second electrode layer.

A solar cell comprising a photovoltaic device according to any one of claims 1-11.