WO2016000828A2 - Dimeric carbon materials and their use in organic photovoltaic devices - Google Patents

Abstract

Carbon material of general structure (I) A-C(=O)-O-B-O-C(=0)-A' (I) wherein A and A' which may be the same or different are derived from fullerene or fullerene derivatives and B is a C₁-C₃₀ divalent linker. Higher homologues of said carbon material having respectively one central linker of type B and, attached thereto, three or four fullerene or fullerene derivatives of type A. Compositions comprising such materials and their use in organic photovoltaic devices.

Description

Dimeric carbon materials and their use in organic photovoltaic devices

[0001] RELATED APPLICATION

[0002] This application claims priority to European application No. 14175862.3, filed on July 04, 2014, the whole content of this application being

incorporated herein by reference for all purposes.

[0003] BACKGROUND OF THE INVENTION

[0004] The present invention is related to carbon materials and their use in organic photovoltaic devices.

[0005] In the recent years organic photovoltaic (OPV) devices have found increasing interest. Compared to silicon based photovoltaic devices, OPV devices provide the advantage of lower production costs and reduced energy payback time. Furthermore, OPV devices do not require high deposition temperatures or complex processing steps as usually required for inorganic solar cells.

[0006] OPV devices usually consist of a multi-layer structure, featuring a blend of electron donor and electron acceptor material.

[0007] In OPV devices, the morphology of the donor/acceptor phase is of importance for the performance of the device. Morphological instability has been found to be one of the major reasons for a deterioration of the performance properties of OPV devices.

[0008] A common acceptor material frequently used in OPV devices is a molecule known as [6, 6]-Phenyl-C6i -butyric acid methyl ester or PCBM.

[0009] Treat et al., Adv. Energy Mater. 2011 , 1 , 82-89 have observed that PCBM is largely soluble in a solid solution made of a polymer matrix (e.g. P3HT). The rapid diffusion at first is beneficial as same improves the formation of an intertwined network of donor and acceptor and allows an enhanced charge extraction. However, the concurrent phase separation cannot be controlled over time and leads to a massive phase separation and ultimatively to a degradation of the OPV device ( cf. Adv. Funct. Mat. 2005, 15, 1617-1622).

[0010] The dimension of each phase of a donor/acceptor system of an OPV device should be in the range of the exciton diffusion length i.e. the distance travelled by the electron-hole pairs before recombination. The phase separation that occurs during the active layer formation depends on a variety of parameters such as the individual solubility of the polymers in the solvent used, their interaction with the substrate surface, the layer thickness, the method of deposition and the drying and annealing conditions.

[0011] Over time when stored in the dark to avoid light induced stress in conjunction with oxygen it has been observed that PCBM crystallizes out of the polymer matrix of the common donor/acceptor complexes comprising PCBM and a donor polymer. Intense studies have been carried out to understand the reason of this crystallization and it has been concluded that thermal annealing of the device is the main cause for accelerated degradation. Fullerene crystallizes very quickly at temperatures above 170°C and the device characteristics _. degrade dramatically. Accordingly, it has been concluded that heat and storage in the dark should be avoided.

[0012] An attempt to overcome this problem in the prior art was to freeze the morphology by chemically reticulating the film. A reticulating agent is added to the polymer blend or the polymer is functionalized and thereafter exposed to radiation causing the donor polymer to reticulate. This leads to the formation of a cage like material trapping the fullerene inside. Fullerene cannot untrap and crystallize and therefore morphological stability is improved. This method is very effective to conserve the morphology but the drawback is that the electrical properties of the donor material are very often significantly sacrificed in the process resulting in a poorly efficient device.

[0013] Li et al., Nature Communications 4, Article number 2227 (DOI:

10.1038/ncomms3227) describe the performance enhancement of fullerene-based solar cells by light processing. Light soaking of the device is described to cause a dimerization of the PCBM thereby leading to a better retained photo-conversion compared to the devices stored in the dark. It has been observed that when exposed to light for various durations and also heat, the photoactive film is showing no sign of crystallization of PCBM and the properties of the device are not degraded. Whereas this method compared to the reticulation method has the advantage that no additional chemicals are necessary, the disadvantage is the fact that the dimerization achieved is only temporary and reverts when the device is no longer light soaked.

Distler et al., Adv. Energy Mater. 2014, 4, 1300693 report the effect of PCBM dimerization on the performance of bulk heterojunction solar cells. As a result of their experiments they conclude that the photo-induced dimerization of PCBM upon exposure of the films to light in the absence of oxygen is one of the reasons for deterioration of the device performance. As is known from the prior art the photo-dimerization of fullerenes is a [2+2] photo-cycloaddition leading to dimers with two fullerene units directly linked via a common four membered ring. The pendant butyric acid methyl ester groups of PCBM (not shown below) are not involved in this dimerization reaction.

Segura et al., Chem. Soc. Rev. 2000, 29, 13-25 report various types of fullerene dimers, including dimers where two fullerene units are linked via substituent groups attached to the fullerene core. Examples given are butadiynyl-linked dimeric methanofullerene or certain fullerene dimers connected through molecular electroactive bridges. The electroactive bridges are electron donor units attached to the C6o core of the fullerene. Fullerene dimers linked through electron acceptor, substituents at the fullerene core are not disclosed. [0016] Summarizing the foregoing, the methods disclosed in the prior art for improving morphological stability of donor/acceptor systems in OPV devices are not fully satisfactory as they are either not permanent or associated with a deterioration of the device due to chemicals added to the system.

[0017] Thus there still exists a need for materials capable of improving the morphological stability of the donor/acceptor phase in OPV devices which overcome the disadvantages of the prior art at least partly.

[0018] It was thus an object of the present invention to provide materials, in particular acceptor materials, for OPV devices providing or improving the morphological stability of the donor/acceptor phase in such devices.

[0019] This object has been achieved with carbon materials in accordance with claim 1.

[0020] Preferred embodiments of the present invention are the subject of dependent claims and are described in detail in the specification hereinafter.

[0021] A further object of the present invention is to combine the carbon materials in accordance with the present invention with known acceptor materials and the use of the carbon materials or of such compositions in OPV devices.

[0022] BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 shows the different reaction steps of synthesis of Example 1 in a reaction scheme,

[0024] Figure 2 shows the number density of PCBM crystallites formed in blend films with various concentrations of carbon material (PCB)2C2 in

accordance with the present invention,

[0025] Figure 3 shows optical and AFM images of cast films without and with

addition of the carbon materials in accordance With the present invention, either without or with annealing at 140°C or 85°C,

[0026] Figure 4 shows the initial device performance in terms of current density of a solar cell with and without addition of a carbon material in accordance with the present invention as part of the donor/acceptor system, [0027] Figure 5 shows the degradation at 85°C thermal stress in nitrogen atmosphere of the device performance in terms of normalized PCE of a solar cell with and without addition of a carbon material in accordance with the present invention as part of the donor/acceptor system and

[0028] Figures 6 to 16 show the structures of preferred donor polymers suitable in the organic photovoltaic devices in accordance with the present invention.

[0029] DETAILED DESCRIPTION OF THE INVENTION

[0030] The present invention concerns a carbon material either of general

structure (I)

A-C(=0)-O-B-0-C(=0)-A' (I)

[0031] or of general structure (lb)

(lb)

or of general structure (lc)

wherein

A, A', A" and A'" which may be the same or different, are derived from fullerenes or fullerene derivatives,

B is a C1 -C30 divalent group,

B' is a C2-C30 trivalent group, and

B" is a C2-C30 tetravalent group.

In formula (I), A and A' can be identical to each other, and are preferably [6,6] Phenyl-C61 -propylene group represented by the following structure

In formula (lb), A, A' and A" can be identical to each other, and are preferably a [6,6] Phenyl-C61 -propylene group represented by the following structure

[0035] In formula (lc), A, A', A" and A'" can be identical to each other, and are preferably a [6,6] Phenyl-C61 -propylene group represented by the following structure

[0036] B is advantageously a C1-C30 hydrocarbylene or a C2-C30

heterohydrocarbylene group. Besides, B is advantageously derivable from a diol of formula OH-B-OH.

[0037] B' is advantageously (i) a C2-C30 trivalent group comprising carbon atoms and hydrogen atoms as sole carbon atoms, or (ii) a C2-C30 trivalent group obtained by replacing at least one carbon atom by a heteroatom in a C2- C30 trivalent group comprising carbon atoms and hydrogen atoms as sole carbon atoms. Besides, B' is preferably derivable from a triol of formula B(OH)₃.

[0038] B" is advantageously (i) a C2-C30 tetravalent group comprising carbon atoms and hydrogen atoms as sole carbon atoms or (ii) a C2-C30 tetravalent group obtained by replacing at least one carbon atom by a heteroatom in a C2-C30 tetravalent group comprising carbon atoms and hydrogen atoms as sole carbon atoms. Besides, B" is preferably derivable from a tetrol of formula B(OH) .

[0039] In certain preferred embodiments, the carbon materials in accordance with the present invention are characterized by the general structure (I)

[0040] A-C(=0)-0-B-O-C(=O)-A' (I)

[0041] wherein A and A', which may be the same or different, are derived from fullerenes or fullerene derivatives and

[0042] B is a C2-C30 alkylene group, a C6-C30 arylene group, a C2-C30 alkyleneoxyalkylene group or a C6-C30 aryleneoxyarylene group.

[0043] In the general structures (I), (lb) and (lc), A, A', A" and A'" are derived from fullerenes or fullerene derivatives.

[0044] The term fullerene, as used herein, is intended to denote any molecule composed entirely of carbon in the form of a hollow sphere, ellipsoid, tube or many other shapes.

[0045] Various types of fullerenes have been described in the literature, e.g. buckyball clusters, nanotubes (carbon nanotubes) and megatubes, to name only a few representatives.

[0046] Spherical fullerenes are called buckyballs and cylindrical fullerenes are referred to as nanotubes or carbon nanotubes. Fullerenes are similar in structure to graphite, which is composed of stacked graphene sheets of linked hexagonal rings.

[0047] The smallest member of the buckyballs is C20, which is an unsaturated version of dodecahedrane. The most popular representative of buckyballs is C60 or buckminsterfullerene. Buckminsterfullerene is the smallest fullerene molecule containing pentagonal and hexagonal rings in which no two pentagons share an edge. The structure of C60 is a truncated icosahedron resembling an association football ball of the type made of 20 hexagons and 12 pentagons with the carbon atom at the vertices of each polygon and a bond along each polygon edge. The C60 molecule has two bond lengths. The 6:6 ring bonds (between two hexagons) can be considered double bonds and are shorter than the 6:5 bonds (between a hexagon and a pentagon).

[0048] Other known fullerenes are C70, C72, C76, Cs4 and C100, to name only some examples.

[0049] Nanotubes, also referred to as carbon nanotubes are cylindrical fullerenes.

These cubes of carbon are usually only a few nanometers wide but they can range from less than a few nanometers to several millimeters in length, i.e. their aspect ratio might be very high. They often have closed ends, but can be open ended as well. The unique molecular structure of nanotubes results in extraordinary macroscopic properties. Carbon nanotubes may be single walled or multi-walled, i.e. they may have one cylindrical wall or a multiplicity of cylindrical walls. Respective products are known to the skilled person and have been described in the literature as SWCNT (single walled carbon nanotubes) or MWCNT (multi-walled carbon nanotubes). As a difference to the spherical fullerenes, carbon nanotubes, similar to ideal graphite, comprise only hexagons but no pentagons. Theoretically, nanotubes may have an unlimited length whereas spherical fullerenes are always closed shapes.

[0050] Fullerene derivatives have been the subject of intense research in the recent years. To increase the reactivity of fullerenes active groups have been attached to the surface. Whereas fullerenes are stable, they are not totally unreactive. The characteristic reaction of fullerenes is an electrophilic addition at 6, 6-double bonds which reduces angle strain by changing sp²-hybridized carbons into sp³- hybridized ones.

[0051] In principle any type of fullerene or fullerene derivative is suitable as structural element A, A', A" or A'" in the carbon materials of the present invention. A, A', A" and A'"; in particular, A and A' in formula (I), may be the same or different and preferably are the same. Alternatively, fullerenes of various types may be combined in the carbon materials of the present invention.

[0052] In some cases, buckyballs and in particular buckminsterfullerene C60 or C_/o-fullerene or their derivatives are preferred as monodentate groups A, A', A" or A'". [0053] One well known example of a buckminsterfullererie derivative with active groups attached to the surface, which has been used as electron acceptor in OPV devices, is Phenyl-C61 -butyric acid methyl ester (generally known and referred to hereinafter as PCBM or sometimes ΡΟβ -ΒΜ) which is represented by the following structure

[0054] and which represents a preferred fullerene derivative from which at least one of A, A', A" and A'" is derived. Thus, e.g. in formula (I), A and/or A' can be derived from said preferred fullerene. The monodentate group A or A' or A" or A'" is obtained from PCBM in this case by removing the carboxylic acid ester group and is thus represented by the following structure

[0055] and this is the most preferred group A or A' or A" or A'" in the carbon materials in accordance with the present invention. The asterisk in the structure above shows the carbon atom to which the carboxylic groups are attached.

[0056] Another acceptor material preferred in accordance with the present invention is Phenyl-Cn -butyric acid methyl ester, commonly referred to as PC71 BM, which is represented by the following structure

ps are also suitable and are commonly referred to as Bis-PC62BM and respectively Bis-C72BM.

[0059] The aforementioned preferred fullerene derivatives are commercially available from Solaris Chem. Inc.

[0060] In structure (I) two fullerene or fullerene derivatives A and A' are linked through a group

[0061] -C(=O)-0-B-C(=O)-O-

[0062] wherein B is a C2-C30 alkylene group, a C6-C30 arylene group, a C2-C30 alkyleneoxyalkylene group or a C6-C30 aryleneoxyarylene group.

[0063] Preferred groups B are alkylene and alkyleneoxyalkylene groups.

[0064] The term alkylene group, as used herein, is intended to denote a group (CH2)n, wherein n represents an integer of from 2 to 30, preferably of from 2 to 18. In one or more of the CH2 groups, one or both hydrogen atoms may be replaced by Ci to C 18 alkyl groups. Just by way of example, ethylene, propylene, butylene, hexylene, octylene, decylene, dodecylene and octadodecylene may be mentioned here. [0065] The term alkyleneoxyalkylene, as used herein, is intended to denote alkylene groups with one or more oxygen atoms inserted between two alkylene units, in particular alkylene groups with one or more oxygen atoms inserted between two -CH2- units. Thus, examples for suitable alkyleneoxyalkylene groups are groups -(-CH2-O-CH2-H with n being an integer of from 1 to 15, -(-CH2-CH2-O)n-(CH₂)n"- with n' and n" being an integer of from 1 to 10, and any divalent group having at least two -CH2- units which are linked through an oxygen atom in the main chain. One or both hydrogen atoms of any of the CH2 groups may be replaced by a Ci to C 18 alkyl group.

[0066] The term aryiene group, as used herein, is intended to denote any group derived from a C& to C30 aromatic compound providing two bonding positions at an aromatic ring to be linked to the oxygen atoms as depicted in the structure (I). Thus, suitable examples are phenylene, naphthylene, anthracenylene, to name only a few examples, phenylene being preferred and p-phenylene being much preferred. The aromatic ring may be substituted or unsubstituted.

[0067] The term aryleneoxyarylene, as used herein, is intended to denote groups derived from aryiene groups wherein two or more aromatic rings as defined for aryiene groups are linked through an oxygen atom. A representative exam le would be

[0068] wherein two bonding positions, one in each of the phenyl rings, are provided for the bonding to the oxygen atoms of the group -(C=O)-O- in structure I. [0069] The carbon materials in accordance with the present invention can be obtained in accordance with methods known per se and of which the skilled person is aware of. He will select the appropriate process depending on the type of fullerene or fuderene derivative.

[0070] Just by way of example, suitable processes to obtain carbon materials of structure (I) will be described now for A and A' being derived from the preferred Phenyl-C6i-butyric acid methyl ester (PCBM) as shown above.

[0071] The first step in a suitable process is the conversion of the methyl ester PCBM into the free acid, phenyl-C6i -butyric acid (hereinafter referred to as PCBA). This can be achieved by reacting PCBM with a mixture of hydrochloric acid and acetic acid in a suitable aromatic solvent, e.g. toluene at a temperature in the range of from 0°C to 50 °C. The yield of the apid PCBA is good and normally reaches 70 -90 °/_0j more preferably 85-90 % of the free acid.

[0072] The reaction product PCBA thereby obtained is only very slightly soluble in most solvents which has some influence on the next reaction step.

[0073] A first alternative is the reaction of PCBA with thionyl chloride in the solid phase to convert PCBA into the respective acid chloride of PCBA :

[0074] and thereafter reacting this acid chloride with a dipl of formula HO-B-OH to obtain the carbon materials in accordance with the present invention. Due to the low solubility of the acid PCBA the overall yield is rather low, however and in most case are 10 % or less. Furthermore, there is a risk that the thionyl chloride potentially p-d_.opes the fullerene or fullerene derivative, which is ultimately detrimental for the device perfomance.

[0075] In the course of the present invention it has been found that PCBA can also be converted into the carbon materials in accordance with the present invention through a Steglich esterification reaction which is well known to the skilled person.

[0076] The Steglich Esterification is a mild reaction, which allows the conversion of sterically demanding and acid labile substrates in accordance with the following general reaction scheme

[0077] wherein DCC represents dicyclohexyl carbodiimide and DMAP represents 4-N,N-dimethylaminopyridine (these two components can be replaced by other chemicals having the same effect and function).

[0078] The reaction sequence for PCBA conversion comprises two reaction steps. In the first step, the free acid PCBA is reacted with a dialcohol HO- B-OH (where B is as defined above) to the respective ester

[0079] which then, in the subsequent step reacts as an "alcohol" in a second Steglich esterification reaction with another molecule of PCBA yielding the desired final product

[0080] wherein B is as defined above. Compounds of the general formula above, where B stands for an ethylene (-CH2-CH2), a butylene -(CH2)4-, a hexylene -(CH₂)6- or an octylene -(CH2)e- group or an alkyleneoxyalkylene group -(CH2-0-CH2)n- with n being an integer of from

(III)

1 to 10 have shown to be advantageous in certain cases.

[0081] The reaction product of the first Steglich esterification step has a higher solubility than the respective acid chloride obtained as described above in the other variant and this improves the yield of the desired product significantly compared to the first route.

[0082] The skilled person will chose the appropriate reaction conditions in the subsequent Steglich esterification reactions depending on the actual reactants based on his professional knowledge. [0083] While the reaction route was shown above for PCBM as the preferred fullerene derivative from which the carbon materials in accordance with the present invention are derived, it is readily apparent to the skilled person that he may obtain respective materials based on other fullerenes or fullerene derivatives by suitably replacing the reactants and modifying the reaction conditions in a suitable manner adopted to the reactants.

[0084] Similarly, while the reaction route was shown above for the preparation of carbon materials of structure (I) (i.e. fullerene dimers), it is readily apparent to the skilled person that he may obtain carbon materials of formula (lb) or of formula (lc) by replacing the diol HO-B-OH by a suitable amount

(typically, an equivalent amount) of triol B'(OH)3 or tetrol B"(OH)₄ respectively in the above two proposed reaction schemes, namely the scheme including the reaction with the acid chloride of PCBA and the scheme including Steglich esterification reactions, and, if needed, modifying the reaction conditions in a suitable manner adopted to the reactants. Just for example purposes, trimethylolpropane can be a suitable triol while pentaerythritol can be a suitable tetrol.

[0085] Since Steglich esterification reactions allow for the preparation of carbon materials in accordance with the invention with a higher yield than the acid choride route, a preferred method for the preparation of said carbon materials is one which comprises causing one or more acids of general structure A-C(=O)-OH to react with a polyol chosen from diols of general structure HO-B-OH, triols of general structure B'(OH)3 and tetrols of general structure B"(OH)₄ under a Steglich Esterification mechanism, in particular in the presence of dicyclohexyl carbodiimide (DCC) and 4-N,N- dimethylaminopyridine (DMAP), it being understood that these two components can be replaced by other chemicals having the same effect and function. As used here for the invented method, A, B, B' and B" have the same meanings as respectively A, B, B' and B" used for defining the carbon material. It goes without saying that when a diol is used, a carbon material of general structure (I) is obtained; when a triol is used, a carbon material of general structure (lb) is obtained; when a tetrol is used, a carbon material of general structure (lc) is obtained. [0086] The carbon materials in accordance with the present invention may advantageously be used to improve the morphological stability of fullerenes or fullerene derivatives in donor/acceptor systems in OPV devices, in particular in so called bulk heterojunction (BHJ) devices, with fullerenes or fullerene derivatives as electron acceptors.

[0087] For this purpose the carbon materials in accordance with the present invention may be used as such or they may be mixed in accordance with a preferred embodiment with electron acceptors selected from fullerene or fullerene derivatives different from structures (I), (lb) and (lc) which are commonly used in respective donor/acceptor systems in OPV devices.

[0088] In accordance with a preferred embodiment of the present invention compositions are provided which comprise a carbon material of structure (I), (lb) or (lc) in accordance with the present invention and a fullerene or fullerene derivative of general structure (II)

A^*-C(=O)-O-R' (II) wherein A* may have the meaning as defined for A in claim 1 and R' is a Ci-Ci8-alkyl group. A particularly preferred fullerene derivative in such compositions is PCBM as defined above.

[0089] The weight ratio of the carbon material of structure (I), (lb) or (lc) and the material of structure (II) is not subject to particular limitations but in certain cases amounts of material of structure (I), (lb) or (lc) based on the combined weight of material of structure (I), (lb) or (lc) and material of structure (II) in the range of from 1 to 50 %, preferably of from 2 to 40 % particularly preferred in the range of from 4 to 30 % have proved to provide some advantages. In some cases amounts of material of structure (I), (lb) or (lc) not exceeding 20 wt%, based on the combined weight of material of structure (I), (lb) or (lc) and material of structure (II), have shown to provide advantages.

[0090] Furthermore, in some cases it has been observed that amounts of material of structure (I), (lb) or (lc) exceeding 25 wt%, based on the combined weight of material of structure (I), (lb) or (lc) and material of structure (II), may lead to a decrease in efficiency of the devices. [0091] A particulary preferred composition comprises the dimer of PCBM as shown above in structure (III) in combination with the corresponding PCBM monomer itself.

[0092] Experiments have shown that amounts as low as 5 wt%, based on the combined weight of material of structure (I), (lb) or (lc) and material of structure (II), of dimer in the composition can prevent the formation of crystallites in systems comprising a common donor polymer and a composition comprising a carbon material of formula (I), (lb) or (lc) with a fullerene or fullerene derivative of general formula (II) used as donor/acceptor combination in an OPV device. Reduction of crystallite formation of 50 % can be obtained with only 5 wt% of dimer in the composition.

[0093] In some cases no more than 20 wt%, based on the combined weight of material of structure (I), (lb) or (lc) and material of structure (II), of material of structure (I), (lb) or (lc) in the composition are needed to achieve a practically quantitative suppression of the formation of crystallites.

[0094] The carbon materials of the present invention or the compositions as defined above are suitable for use as electron acceptors in organic electronic devices, preferably in combination with a donor material in so called bulk heterojunction (BHJ) solar cells.

[0095] Suitable donor materials for such use in OPV devices have been described in the literature and are known to the skilled person and thus no detailed information is principally necessary here.

[0096] Conjugated polymer based electron donor materials have been mainly used as electron donor materials for the manufacture of bulk heterojunction OPV devices in the literature.

[0097] The energy bandgaps and HOMO and LUMO energy levels of such polymers are the most important parameters influencing the performance of the polymer solar cells. Photon flux density of the solar spectrum is highest in the wavelength range from red to near infrared and thus suitable polymers should absorb light efficiently in this area in order to get the best results. Thus, the polymers should have low bandgaps and high absorption coefficients. [0098] The best investigated strategy to reduce the energy bandgap of conjugated polymers is to increase quinoid structures in the conjugated polymer backbone. The quinoid form distributes pi-electrons through the polymer main chain by transforming double bonds into single bonds and synchronously single bonds into double bonds. Due to the fact that the quinoid structure has a higher ground state energy than the aromatic form, polymers showing a high tendency to form a quinoid structure exhibit usually smaller energy bandgaps. In addition, the quinoid form enables more effective derealization of pi-electrons along the polymer backbone which increases the planarity of the polymer.

[0099] As already indicated, Figures 6 to 16 show the structures of preferred donor polymers suitable in the organic photovoltaic devices in accordance with the present invention. In the shown structures, and as commonly known to the skilled person, n represents an integer of at least 2 denoting the number of repeat units contained in the preferred donor polymers.

[00100] Poly(3-hexyl-thiophene), also known as P3HT (shown in figure 6), is the most commonly used polymeric electron donor material in photovoltaic cells. It is easy to synthesize, has a high charge carrier mobility and a good processability. The main disadvantage of P3HT is its large bandgap and its high LUMO level, which are detrimental to a good near IR photon absorption. Thus, there have been attempts to narrow the bandgap and to downshift the HOMO level compared to P3HT.

[00101] To realize these goals, several synthetic strategies have been developed and proven to be effective: (1) construct the backbone using alternating electron-rich (donor) and electron-deficient (acceptor) units to form the D-A co-polymers; (2) stabilize the quinoid resonance structure (as mentioned above); (3) incorporate strong electron withdrawing substituents such as carbonyl group or fluorine atoms; (4) attach conjugated side chains on the polymer main chains.

[00102] One example is the carbazole and benzothiadiazole (BT) based polymer (referred to as PCDTBT and shown in Figure 7) reported by Leclerc et al., Adv. Mater. 2007, 19, 2295. The optical bandgap of this polymer is 1.88 eV with a low-lying HOMO energy level of -5.50 eV. To further lower the bandgap, cyclopenta[2,1-b;3,4-b']dithiophene (CPDT) and dithieno[3,2- b:2',3'-d]silole (DTS) units with stronger electron-donating properties were independently synthesized (Yang et al., J. Am. Chem. Soc. 2008, 130, 16144 and Brabec et al., Macromolecules 2007, 40, 1981). The polymers PCPPTBT and PSBTBT (as shown in Figure 8) show small bandgaps of appr. 1.5 eV due to the strong intra-molecular donor-acceptor interactions. The performance of PCPDTBT based devices can be further enhanced significantly by adding 3% 1 ,8-diiodooctane (DIO) as a processing additive to tune the morphology, whereas no comparable effect is observed for PSBTBT.

[00103] Another building block which has been investigated is diketopyrrolopyrrole (DPP), which was first described in OPV in 2008 (Janssen et al., Adv. Mater. 2008, 20, 2556). The electron-withdrawing effect of the lactam units causes the chromophore to have a high electron affinity and thus, it can be used as a strong electron-withdrawing unit. When polymerized with a thio- phene unit via Suzuki cross-coupling, a low bandgap polymer PDPP3T (shown in Figure 9) was obtained (Janssen et al., J. Am. Chem. Soc. 2009, 131 , 1661). The polymer shows a very low bandgap of 1.31 eV and a deep HOMO level of -5.17 eV.

[00104] These three polymers represent an important family of photovoltaic materials due to their relatively small optical bandgaps, which are useful in a tandem solar cell device.

[00105] Yu et al., J. Am. Chem. Soc. 2009, 131 , 56 describe the thieno[3,4- b]thiophene (TT) building block, which can stabilize the quinoid structure to reduce the bandgap. Yang et al., Macromolecules 2008, 41 , 6012, reported the application of the benzo[1 ,2-b;4,5-b']dithio-phene (BDT) unit in OPV materials. When combining these two units together and adding a fluorine atom on the TT ^: unit to lower the HOMO level several high performing polymers, such as PTB7 and PBDT-TT-CF (structures shown in Figures 10 and 11 respectively) have been obtained (Yu et al., J.Am. Chem. Soc. 2009, 131 , 7792 and Yu et al., Adv. Mater. 2010, 22, E135). They have similar bandgaps of around 1.6 eV and HOMO levels of around -5.2 eV. Hou et al., Adv. Mater. 2012, 24, 3383, reported modifications on this family (PBDTTT-C-T and PBDTTT-S-T) with conjugated thiophene side chains on the BDT unit.

[00106] Another strong electron acceptor unit, thieno[3,4- c ]pyrrole-4,6-dione (TPD), was also sythesized. A first polymer wherein this acceptor unit was combined with BDT is PBDT-TPD (structure shown in Figure 12). The polymer shows a bandgap of 1 .81 eV and a deep HOMO level of -5.57 eV. To lower the bandgap of the TPD based polymers, TPD was copolymerized with the DTS unit and a polymer PDTS-TPD (structure shown in Figure 13) with a bandgap of 1.73 eV was obtained.. Further optimi-zation on this structure was reported by Reynolds et al. J. Am. Chem. Soc. 201 1 , 133, 10062, who replaced the silicon atom in the DTS unit with a germanium atom to form the dithienogermole (DTG) unit. The new polymer RDTG-TPD (structure also shown in Figure 13) shows a slightly lower bandgap of 1.69 eV.

[00107] Still another preferred donor polymer suitable in the organic photovoltaic devices in accordance with the present invention is PBDT-DTffBT the structure of which is shown in Figure 14.

[00108] To absorb more photons in the NIR region, a systematic investigation on the BDT and DPP based low bandgap polymers was conducted. A first polymer which has been described in this regard is PBDTT-DPP (structure shown in Figure 15), which shows a small bandgap of 1.46 eV, deep HOMO level, and high charge carrier mobility. By replacing the oxygen atoms attached to the BDT unit with thiophene moieties to form the thienylbenzodithiophene (BDTT) unit, the HOMO level of PBDTT-DPP is shifted deeper (-5.30 v.s. -5.16 eV). Bulkier 2-ethylhexyl side chains on BDTT and 2-butyloctyl side chains on DPP increase the solubility of the resulting polymers and thus allow to obtain much higher molecular weights. Compared to PBDT-DPP, the new polymer shows improved solubility, higher molecular weight, and higher carrier mobility. Further improvement can be accomplished by substituting the sulfur atoms on the DPP unit with selenium atoms. The resulting polymer PBDTT-SeDPP (Energy gap ~ 1.38 eV, structure also shown in Figure 15) showed also excel-lent photovoltaic performance. [00109] Dou and Yoshinura et^' al., Macromolecules 2013, 46, 3384 reported a new family of low bandgap polymers using an asymmetric electron rich dithieno[3,2-b:2',3'-d] pyran (DTP) unit. The electron-donating property of the DTP unit was found to be the strongest among the most frequently used donor units, such as BDT, DTS or CPDT. When the DTP unit was polymerized with the strongly electron-deficient DFBT unit, a regiorandom polymer ( PDTP-DFBT, bandgap = 1.38 eV, structure shown in Figure 16) was obtained. It was found that the DTP based polymer PDTP-DFBT shows significantly improved solubility and processability compared to the BDT or CPDT based polymers with same alkyl side chains.

[001 10] P3HT, its structural analogs with the hexyl groups replaced by butyl, oetyl or decyl groups, PCDTBT, PCPDTBT, PCPDTTBTT (Poly[2,6-(4,4-bis-(2- ethylhexyl)-4H-cyclopenta [2,1-Β;3,4-Β'] dithiophene)-alt-4, 7(2,1 ,3-benzo- thiadiazole)] and in particular P3HT and its structural analogs and PCDTBT are particularly preferred donor polymers for the organic photovoltaic devices. All these preferred polymers are commercially available from Solaris Chem Inc.

[001 1 1] Another embodiment of the present invention is an organic photovoltaic device comprising a carbon material or a composition in accordance with the present invention as acceptor material.

[001 12] Preferred OPV devices are so called bulk heterojunction devices. In this type of OPV devices donor and acceptor materials are intimately mixed to form a three dimensional interpenetrating network. The majority of bulk heterojunction devices comprise at least one component which is a polymer semiconductor, usually a conjugated polymer as described above. Ideally, the bicontinuous network of donor and acceptor is organized on a nanometer scale thereby providing a large interface area so that the excitons created can reach a donor/acceptor interface within their diffusion length.

[001 13] All the polymers described above and shown in Figures 6 to 16 are in principle suitable for the purposes of the present invention to be combined with the carbon materials of structure I or the compositions comprising the carbon materials of structure I described above as donor/acceptor systems in organic photovoltaic devices, in particular in bulk heterojunction solar cells. There are further suitable donor polymers known to the skilled person, but the polymers described above have proven advantageous in a number of cases. The skilled person will select the best suited polymer dependent on the actual application situation.

[001 14] Particularly preferred OPV devices in accordance with the present invention are solar cells.

[001 15] The carbon materials or compositions in accordance with the present invention can be advantageously used in bulk heterojunction OPV devices to provide increased stability and performance compared to the prior art devices of the respective type.

[001 16] Working Examples (Reaction scheme shown in Fig. 1)

[001 17] Example 1 - Synthesis of carbon materials of structure I

[001 18] a) Synthesis of PCBA (Phenyl-C₆i-butyric acid)

[001 19] Acetic acid (150 ml) and HCI (60 ml) were added to a solution of PCBM (1 mmol) dissolved in 150 ml of toluene. The biphasic solution was heated to reflux under vigorous stirring for 12 hours. After reaction the solution was cooled to room temperature and the fine black solid was filtered off. The recovered solid was washed with water, methanol, toluene and diethyl ether. The blackish solid was collected and dried under reduced pressure. H NMR (400 MHz, chloroform-^ δ ppm 7.65 (d, J - 7.2 Hz, 2H), 7.33-7.15 (m, 3H), 2.72-2.62 (m, 2H), 2.67 (t, J = 7.3 Hz, 2H), 2.04- 1.89 (m, 2H). MS (MALDI-ToF): /77/ calcd for C71 H 12O2 (M-) 896.1 found 896.1.

[00120] b) Synthesis of esters of PCBA

[00121] [6,6]-Phenyl-C61 -butyric acid (PCBA) (600 mg, 0.67 mmol) was added to

100 mL of anhydrous o-dichlorobenzene and sonicated during 20 minutes. After sonication the brownish suspension was heated to 110°C for two hours in order to fully dissolve the PCBA. The resulting black solution was cooled to 0°C in an ice-bath before the diol (10 eq) and 4- Dimethylaminopyridine (0.1 eq) were added. At last Ν,Ν'- Dicyclohexylcarbodiimide (1.1 eq) was added to the reaction mixture, which was allowed to warm to room temperature and stirred overnight. The solvent was removed from the reaction mixture by rotary evaporation. The recovered crude product was purified by column chromatography on silica gel using a mixture of toluene:pyridine (95:5) as mobile phase. The recovered fractions were concentrated and precipitated into cold methanol. The title compound was recovered as a brown solid after filtration. The diols used corresponded to the general formula HO-(CH2)n-OH with n being 2, 4, 6 or 8.

[00122] n=2: Yield 39%. ¹H NMR (400 MHz, chloroform-a) δ ppm 7.97 - 7.91 (m, 2 H) 7.60 - 7.46 (m, 3 H) 4.27 - 4.21 (m, 2 H) 3.88 - 3.77 (m, 2 H) 3.01 - 2.87 (m, 2 H) 2.59 (t, J=7.3 Hz, 2 H) 2.29 - 2.15 (m, 2 H) 1.80 (br s, 1 H). MS (MALDI-ToF): m/zca\cd for C73H 16O3 (M -) 940.1 found 940.1

[00123] n=4: Yield 28%. ¹H NMR (400 MHz, tetrachloroethane-c ?) δ ppm 7.98 (d, J=7.3 Hz, 2 H), 7.63 - 7.51 (m, 3 H), 4.14 (t, ^6.4 Hz, 2 H), 3.69 (t, J=6A Hz, 2 H), 2.99 - 2.91 (m, 2 H), 2.58 (t, J=7.3 Hz, 2 H), 2.27 - 2.16 (m, 2 H), 1.78 - 1.54 (m, 5 H). MS (MALDI-ToF): 77/zcalcd for C75H20O3 (M-) 968.1 found 968.1

[00124] n=6: Yield 32%. H NMR (400 MHz, chloroform-^ δ ppm 7.96 - 7.93 (m, 2 H), 7.59 - 7.47 (m, 3 H), 4.09 (t, ^6.8 Hz, 2 H), 3.66 (t, J=6.6 Hz, 2 H), 2.96 - 2.90 (m, 2 H), 2.56 - 2.50 (m, 2 H), 2.25 - 2.16 (m, 2 H), 1.69 - 1.50 (m, 5 H), 1.50 - 1.30 (m, 4 H). MS (MALDI-ToF): /77/ calcd for C77H24O3 (M -) 996.2 found 996.2.

[00125] n=8: Yield 32%. H NMR (400 MHz, chloroform-o) δ ppm 7.96 - 7.93 (m, 2 H), 7.59 - 7. 47 (m, 3 H), 4.08 (t, ^=6.8 Hz, 2 H), 3.66 (t, J=6.6 Hz, 2 H), 2.96 - 2.90 (m, 2 H), 2.53 (t, ^7.3 Hz, 2 H), 2.25 - 2.16 (m, 2 H), 1.66 - 1.45 (m, 5 H), 1.40 - 1.30 (m, 8 H). MS (MALDI-ToF): m/z calcd for C79H28O3 (M-) 1024.2 found 1024.2.

[00126] c) Synthesis of Dimers (as shown in Figure 1)

[00127] PCBCnOH (190 mg, 0.20 mmol) was dissolved in 100 mL of anhydrous o- dichlorobenze and cooled to 0°C. PCBA (2 eq) and 4- Dimethylaminopyridine (0.1 eq) were added, before Ν,Ν'- Dicyclohexylcarbodiimide (1.1 eq) was added. The reaction mixture was allowed to warm to room temperature and stirred overnight. Afterwards the solvent was removed from the reaction mixture and the recovered crude product was purified by column chromatography on silica gel using toluene as eluent. The recovered fractions were concentrated and precipitated into methanol. The title compound was recovered as a dark brown solid after filtration.

[00128] n=2: Yield 16%. ¹H NMR (400 MHz, 1 ,1 ,2,2-tetrachloroethane-a¾ δ ppm 7.91 - 7. 86 (m, 2 H) 7.55 - 7.42 (m, 3 H) 4.22 (s, 2 H) 2.91 - 2.80 (m, 2 H) 2.51 (t, J=7.3 Hz, 2 H) 2.18 -1.94 (m, 2 H). MS (MALDI-ToF): w/ calcd for Ci ₄H₂60₄ (M-) 1819.2 found 1818.7.

[00129] n=4: Yield 19%. ¹H NMR (400 MHz, 1 ,1 ,2,2-tetrachloroethane-fly δ ppm

7.97 (d, J=7.3 Hz, 4 H), 7.63 - 7.50 (m, 6 H), 4.1 1 (t, J=6.3 Hz, 4 H), 2.99 -

2.89 (m, 4 H), 2.57 (t, J=7.3 Hz, 4 H), 2.26 - 2.14 (m, 4 H), 1.73 - 1 .67 (m, 4 H). MS (MALDI-ToF): m/z calcd for Ci₄₆H₃₀O (M ) 1847.2 found 1846.8.

[00130] n=6: Yield 24%. ¹H NMR (400 MHz, 1 ,1 ,2,2-tetrachloroethane-o¾ δ ppm 7.99 - 7.96 (m, 4 H), 7.62 - 7. 50 (m, 6 H), 4.08 (t, ^=6.6 Hz, 4 H), 2.98 - 2.91 (m, 4 H), 2.57 (t, ^=7.6 Hz, 4 H), 2.25 - 2.16 (m, 4 H), 1.68 - 1.60 (m, 4 H), 1 .41 - 1.36 (m, 4 H). MS (MALDI-ToF): m/z calcd for Ci ₈H₃40₄ (M-) 1875.2 found 1874.7.

[00131] n=8: Yield 22%. H NMR (400 MHz, 1 ,1 ,2,2-tetrachloroethane-^) δ ppm

7.98 (d, ^=7.3 Hz, 4 H), 7.63 - 7. 50 (m, 6 H), 4.08 (t, =6.6 Hz, 4 H), 2.99 -

2.90 (m, 4 H), 2.57 (t, J=7.3 Hz, 4 H), 2.20 (quin, J=7.8 Hz, 4 H), 1.67 - 1.62 (m, 4 H), 1.38 - 1.30 (m, 8 H). MS (MALDI-ToF): m/z calcd for Ci5oH₃8O₄ (M-) 1903.3 found 1902.8.

[00132] [Example 2 - Influence of compound of structure I as prepared in [Example

1 on active layer morphology

[00133] A blend of PCBM (as fullerene) and Poly[N-9'-heptadecanyl-2,7-carbazole- alt-S.S^'J'-di^-thienyl^'.l'.S'-benzothiadiazole), commonly known as PCDTBT was used as the model system. PCDTBT is a high performing amorphous polymer with a glass transition temperature (T_g ~ 106°C) in neat films, which is below the temperatures used here for examination of thermal stability. To determine the minimum weight percentage of the dimer obtained in accordance with Example 1 (n being 2, this dimer referred to hereinafter as (PCB)2C2)) necessary to stabilize the PCDTBT CBM blend under thermal stress, blends of PCDTBT:fullerene (1 :2) were spin coated on silicon substrates from chlorobenzene and thermally annealed at 140°C for 1 hour. The effect of dimer content in the fullerene component was varied while maintaining the overall weight ratio of polymenfullerene constant. Figure 2 shows that the addition of dimer leads to a more thermally stable blend. The formation of micron-sized PCBM crystallites at temperatures above the T_g of the pure polymer is completely impeded upon addition of 20% of (PCB)2C2, and the number- density of the PCBM crystallites is suppressed by over 50% by adding only 5% of dimer to the blend.

The effect of (PCB)2C2 on the nanomorphological behavior upon thermal stress analogous to solar cell operating conditions was also investigated. The evolution of the blend morphology upon thermal annealing at 85°C, customary for standard thermal stress tests, on PEDOT:PSS substrates is shown in Figure 3. Optical images of a) PCDTBT:PCBM as-cast blend films; b) PCDTBT:PCBM:(PCB)₂C2 (20%) as-cast films; c) PCDTBT: PCBM blend films after thermal annealing at 140°C for 1 h and d) PCDTBT:PCBM:(PCB)₂C2 (20%) blend films after thermal annealing at 140°C for 1 h on SiO_x substrates (scale bar, 200 pm) are shown. The insets c) and d) highlight the drastic differences in PCBM crystal formation in the annealed blend films with and without the (PCB)2C2 (scale bar is 40 μιτι). Furthermore, Fig. 3 shows AFM images of e) PCDTBT: PCBM as-cast blend films; f) PCDTBT:PCBM:(PCB)₂C₂ (20%) as-cast films; g) PCDTBT:PCBM blend films after thermal annealing at 85°C for 1 h; and h) PCDTBT: PCBM :(PCB)₂C₂ (20%) blend films after thermal annealing at 85°C for 1 h on PEDOTPSS ((Poly(3,4-ethylenedioxythiophene/ polystyrene sulfonate) substrates (scale bar is 5.7 pm). As revealed by the AFM images, as cast samples are relatively smooth and featureless, with a similar surface roughness for both with and without (PCB)2C₂. Upon thermal annealing at 85°C for 1 h, the samples without (PCB)2C2 exhibit densely formed PCBM crystallites of the order of 150 nm in diameter and 30 nm in height. In clear contrast, the samples with 20% (PCB)₂C2 show drastically suppressed PCBM aggregation with no measurable change in film surface morphology after thermal treatment.

[00135] Example 3 - Influence on device performance

[00136] Bulk heterojunction solar cells with PCDTBT:PCBM and PCDTBT: PCBM: PCBM dimer of Example 1 (n=2) with a dimer content of 20 wt% in the mixture of PCBM and dimer as donor/acceptor were manufactured. A substrate layer of transparent indium tin oxide (ITO) was spin coated with a hole transport layer of PEDOT:PSS (Poly(3,4- ethylenedioxythiophene/polystyrene sulfonate), then annealed above 150 °C prior to the spin coating of the photoactive layer made of a PCDTBT:PCM + dimer mixture with various content of (PCB)2C2- A cathode made of a calcium underlayer with a thickness of 20 nm and a layer of aluminum with a thickness of 100 nm was finally deposited on the photoactive layer via thermal evaporation under reduced pressure. A standardized thermal stress of 85 °C was applied for various periods of time. Figure 4 shows the results through a comparison of the initial J-V characteristics of optimized conventional PCDTBT: PCBM devices containing different weight percentages of (PGB)2C2 prior to thermal stability test (left graph), while Figure 5 shows degradation of solar cell PCE as a function of time at 85°C thermal stress in nitrogen atmosphere of optimized conventional PCDTBT: PCBM devices containing different weight percentages of (PCB)2C2. It is immediately apparent that the power conversion efficiency degrades much faster in the device without dimer, i.e. the addition of the dimer leads to a significantly improved stability of the device. In the absence of the dimer, the device performance decreases by 20 % within the first 25 minutes, whereas the same reduction in performance is only observed after 2000 minutes in the device with the dimer added. It is thus apparent that the device thermal stability can be enhanced by at least one order of magnitude.

[00137] Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the - present description shall take precedence.

Claims

Claims

1. Carbon material either of general structure (I)

A-C(=O)-0-B-0-C(=0)-A (I) or of general structure (lb)

or of general structure (lc)

wherein

A, A, A" and A^s" which may be the same or different, are derived from fullerenes or fullerene derivatives,

B is a C1 -C30 divalent group, B' is a C2-C30 trivalent group, and

B" is a C2-C30 tetravalent group.

2. Carbon material in accordance with claim 1 wherein:

- in formula (I), A and A' are identical to each other, and are preferably a [6,6] Phenyl-C61 -propylene group represented by the following structure

- in formula (lb), A, A' and A" are identical to each other, and are preferably a [6,6] Phenyl-C61 -propylene group represented by the following structure

and

- in formula (lc), A, A', A" and A'" are identical to each other, and are preferably a [6,6] Phenyl-C61 -propylene group represented by the following structure

3. Carbon material in accordance with claim 1 or 2 wherein:

- B is (i) a C1-C30 hydrocarbylene group or (ii) a C2-C30 heterohydrocarby!ene group, and B is preferably derivable from a diol of formula OH-B-OH,

- B' is (i) a C2-C30 trivalent group comprising carbon atoms and hydrogen atoms as sole carbon atoms, or (ii) a C2-C30 trivalent group obtained by replacing at least one carbon atom by a heteroatom in a C2-C30 trivalent group comprising carbon atoms and hydrogen atoms as sole carbon atoms, and

B' is preferably derivable from a triol of formula B(OH) 3 , and

- B" is (i) a C2-C30 tetravalent group comprising carbon atoms and hydrogen atoms as sole carbon atoms or (ii) a C2-C30 tetravalent group obtained by replacing at least one carbon atom by a heteroatom in a C2-C30 tetravalent group comprising carbon atoms and hydrogen atoms as sole carboh atoms, and B" is preferably derivable from a tetrol of formula B(OH) .

4. Carbon material in accordance with any one of the preceding claims, which is a carbon material of general structure (I)

A-C(=O)-0-B-0-C(=0)-A' (I) wherein A and A' which may be the same or different, are derived from fullerenes or fullerene derivatives and

B is a C2-C30 alkylene group, a C6-C30 arylene group, a C2-C30

alkyleneoxyalkylene group or a C6-C30 aryleneoxyarylene group.

5. Carbon material in accordance with claim 4 wherein A and A' are identical.

6. Carbon material in accordance with claim 4 or 5 wherein A and/or A' is a [6,6] Phenyl-C61 -propylene group represented by the following structurewherein ^* denotes the carbon atom linked to the carboxyl group.

7. Carbon material in accordance with any of claims 4 to 6 wherein B is a C2-C18- alkylene or a C₂-Ci8-alkyleneoxyalkylene group.

8. Composition comprising the carbon material in accordance with any of the preceding claims and a fullerene or fullerene derivative having a structure different from general structures (I), (lb) and (lc).

9. Composition in accordance with claim 8 wherein the fullerene or fullerene derivative has the general structure (II)

A^*-C(=0)-0-R' (II) wherein A* has the same meaning as the meaning specified for A in claim t or 6 and wherein R' is a Ci-Ci8-alkyl group.

10. Composition in accordance with claim 8 or 9 comprising 1-50 % by weight of the carbon material, based on the combined weight of the carbon material and the fullerene or fullerene derivative, in particular compositions comprising 4-20 % by weight of the carbon material, based on the combined weight of the carbon material and the fullerene or fullerene derivative.

1 1. Use of the carbon material in accordance with any of claims 1 to 7 or of the composition in accordance with any of claims 8 to 10 as acceptor in an organic photovoltaic device.

12. Use in accordance with claim 1 1. in combination with a donor polymeric material.

13. Organic photovoltaic device comprising the carbon material in accordance with any of claims 1 to 7 or the composition in accordance with any of claims 8 to 10, such as a bulk heterojunction solar cell.

14. Organic photovoltaic device in accordance with claim 13 comprising a combination of (i) an acceptor material selected from the carbon material in accordance with any of claims 1 to 7 and the composition in accordance with any of claims 8 to 10 and (ii) a donor polymer material.

15. Method for the preparation of the carbon material in accordance with any of claims 1 to 7, said method comprising causing one or more acids of general structure A-C(=O)-OH to react with a polyol chosen from diols of general structure HO-B-OH, triols of general structure B'(OH) 3 and tetrols of general structure B"(OH)₄, under a Steglich Esterification mechanism, in particular in the presence of dicyclohexyl carbodiimide (DCC) and 4-N,N- dimethylaminopyridine (DMAP), wherein A, B, B' and B" have the same meanings as A, B, B' and B" used for defining the carbon material.