MEDIUM

Technical field

The present disclosure relates to an ion-conducting medium for an electrolyte, an electrolyte for a light-emitting electrochemical cell comprising the ion-conducting medium, and a light-emitting electrochemical cell comprising the electrolyte.

Background

Light-emitting electrochemical cells (LECs) circumvent many of the issues associated with the manufacturing of the more commonplace organic light-emitting diodes (OLEDs), since they can feature low-voltage light emission from simple devices based on solely solution-processable materials. Therefore, LECs are a promising alternative to OLEDs for a wide range of low-cost and/or large-area emissive applications.

The LEC performance is dependent on the properties of mobile ions, i.e. an electrolyte, in its light-emitting active material. Slow turn-on, poor efficiency and limited stability of LECs have, however, commonly been attributed to the use of an inadequate electrolyte (Matyba et al., Organic Electronics 2008, 9, 699-710).

It is, hence, surprising that the number of investigated LEC electrolytes is rather limited. L1CF3SO3 dissolved in high-molecular-weight poly(ethylene oxide) (PEO) as the electrolyte has remained a preferred choice, despite the known drawbacks of such electrolytes, including ambient-temperature crystallization and a limited electrochemical stability window, outside of which the electrolyte is not electrochemically stable.

Alternative LEC electrolytes, such as ionic liquids, polymerizable ion- pair monomers and ion transporters, and oligoether-based electrolytes, including L1CF3SO3 dissolved in hydroxyl-capped trimethylolpropane ethoxylate (TMPE-OH) or in methoxy-capped trimethylolpropane ethoxylate (TMPE-OCH3) (Tang et al., Chemistry of Materials 2014, 26, 5083-5088) have been suggested. However, none of these novel electrolytes has so far been able to deliver LEC devices with sufficient turn-on kinetics, stability and efficiency for most of today's known applications. Summary

An object of the present disclosure is, hence, to provide novel ion- conducting media that form functional and novel electrolytes when mixed with an appropriate salt, and which provide at least some advantages over known electrolytes when employed in LECs. Some specific objects are to provide electrolytes with expanded electrochemical stability window compared to known electrolytes and LECs with reduced turn-on time and/or improved efficiency (being the conversion of input electric energy to radiation exiting the device structure) and/or improved operational lifetime.

The invention is defined by the appended independent claims.

Embodiments are set forth in the appended dependent claims and in the figures.

According to a first aspect, there is provided an ion-conducting medium for an electrolyte containing 5-100%, preferably 50-100%, or 80-100%, by weight, of molecules having the general formula

wherein X is an alkoxy- and/or aryloxy-functional core comprising 2-10, preferably 2- 6, or 3-6, or 3-4, alkoxy and/or aryloxy groups, derived from a polyol having 2-10, preferably 2-6, or 3-6, or 3-4, hydroxyl groups, R ¹ and R ² are respectively linear or branched hydrocarbon chains with 1-15, preferably 2-15 or 3-12, carbon atoms in the chain,

R ³ is selected from a group consisting of H, and a linear or a branched hydrocarbon chain with 1-15, preferably 1-12, or 1-8, carbon atoms in the chain,

(A-O)p, (B-O)q, and (C-O) _s are respectively oligomer chains selected from a group consisting of oligoethers, oligoesters, oligocarbonates and copolymers of one or more thereof, wherein p, q and s are in the range 0-10, preferably 2-4, wherein at most two of p, q and s are simultaneously 0, m + n + o is in the range 2-10, preferably 2-6, or 3-6, or 3-4, wherein at least one of m and n is in the range 1-10 and wherein o is in the range 0-10,

Y and Z are respectively selected from a group consisting of O, CH ₂ , CHR ⁴ , CR ⁴ R ⁵ , NH and NR ⁴ , wherein

R ⁴ and R ⁵ are respectively linear or branched hydrocarbon chains with 1-15, preferably 1-12, or 2-12, or 3-12, carbon atoms in the chain, and wherein the molecules each contain 0 or a maximum of 1 α,β-unsaturated carbonyl group.

By ion-conducting medium ("ion transporter") is here meant a material (e.g. a fluid) in which a salt may be dissolved to form an electrolyte, i.e.

mobile anions and cations, and which assists in the transport of these ions. The ion-conducting medium is, hence, an ion-solvating and ion-transporting medium.

The ion-conducting medium may be used in an electrolyte.

The electrolyte may then be blended with an electroactive material to form the active layer of a light-emitting electrochemical cell (LEC).

The ion-conducting medium comprises 5-100%, preferably 50-100% or 80-100% of the molecules of formula I.

That the ion-conducting medium contains at least 5% and preferably more than 50%, or more than 80%, and less than or equal to 100% of the molecules covered by formula I implies that that the ion-conducting medium may be diluted with other ion-conducting molecules not covered by formula I, such as for example OH- or OCH ₃ -capped trimethylolpropane ethoxylate (TMPE), linear oligoethers, poly(ethylene oxide) or more low-molecular- weight solvents. Such ion-conducting media lack the specified functionalities (e.g. alkyl carboxylate ester and/or alkyl carbonate ester end-groups) of the current ion-conducting medium, and, hence, such ion-conducting media mixtures would obtain characteristics lying in between the ion-conducting media comprising only the current molecules and ion-conducting media comprising only the diluting media.

In various embodiments, the ion-conducting medium may comprise molecules of formula I. Alternatively, the ion-conducting medium may consist essentially of molecules of formula I.

Alternatively, when used in a LEC, the electroactive material of the LEC, with which the ion-conducting medium and salt is blended to form the active layer of the LEC, may contain conjugated light-emitting polymers with ion-conducting side chains, which may be considered as forming part of the ion-conducting medium (see Yang et al., J. Appl. Phys., 1997, 81 , 3294- 3298; and Pei et al., J. Am. Chem. Soc, 1996, 1 18, 7416-7417). Thereby, the ion-conducting medium in such circumstances may be considered to contain less than 10%, or less than 50%, or less than 80%, or less than 100% of the molecules covered by formula I.

X in formula I is a multifunctional core having two or more functional groups to which oligomeric arms are connectable. X is an alkoxy- and/or aryloxy-functional core comprising 2-10, preferably 2-6, or 3-6, or 3-4 alkoxy and/or aryloxy groups derived from a polyol. When X is an aryloxy-functional core, the core is aromatic and may for example contain a (poly)phenol.

It is to be emphasized that the functionality of the molecules of formula

I and the ion-conducting medium are little influenced by the structure of the functional core X. The characteristics of the end-groups and oligomer chains are highly more influential and give the molecules and the ion-conducting medium its characteristics.

(A-O)p, (B-O)q, and (C-O) _s are cation-coordinating oligomeric chains respectively selected from a group consisting of oligoethers, oligoesters, oligocarbonates and copolymers of one or more thereof. Examples of such copolymers, with beneficial cation-solvating and -conducting properties, are poly(trimethylene carbonate-co-£-caprolactone) - see Mindemark et al., Polymer, 2015, 63, 91-98 and Mindemark et al., J. Power Sources, 2015, 298, 166-170 - and poly(ethylene oxide-co-ethylene carbonate) - see Elmer et al., J. Polymer Sci., Part A: Polym. Chem., 2006, 44, 2195-2205.

p, q and s are in the range 0-10, preferably 2-4, wherein at most two of p, q and s are simultaneously 0. In other words 0-2, preferably 0-1 , or 1-2 of p, q and s may be simultaneously 0. The interval 0-10 (preferably 2-4) is the average number of p, q and s for all molecules described with the general formula I in the ion-conducting medium, and, hence, p, q and s are not limited to integers.

m + n + o is in the range 2-10, preferably 2-6, or 3-6, or 3-4. m, n, and o are in the range 0-10, wherein at least one of m and n is in the range 1-10 and wherein o is in the range 0-10, thus ensuring the presence of at least 1 alkyl carboxylate ester or alkyl carbonate ester end-group in the molecule. This means that if none of m, n and o is 0, then there are three different types of arms in the molecule described with formula I. If one of m, n and o is 0, then two of the arms are of the same type and if 2 of m, n and o are simultaneously 0 then all arms are of the same type. An example with three different arm types could be based on a glycerol core with oligo(£- caprolactone), oligo(trimethylene carbonate) and oligo(ethylene oxide) arms, terminated with ethyl carbonate, ethyl carbonate and methoxy end-groups, respectively. An example with two different arm types could be a

trimethylolpropane-derived core with two butyl carbonate-capped

oligo(trimethylene carbonate) arms and one methoxy-capped oligo(ethylene oxide) arm. An example with all arm types the same could be a

trimethylolpropane-derived core with three oligo(ethylene oxide) arms and octyl carbonate end-groups.

Y and Z may respectively consist of O (oxygen), forming carbonate esters.

Alternatively, Y and Z may respectively be selected from a group consisting of CH ₂ , CHR ⁴ , CR ⁴ R ⁵ , forming carboxylate esters. Carbonate esters and carboxylate esters have similar characteristics. Carbonate esters exhibit an improved electrochemical stability against oxidation compared to ethers. This characteristic is also shared by carboxylate esters, see for example Xu et al., Chem. Rev. 2004, 104, 4303-4417.

In yet an alternative, Y and Z are respectively selected from a group consisting of NH and NR ⁴ , forming carbamates.

As the molecules each contain 0 or a maximum of 1 α,β-unsaturated carbonyl groups, such as acrylate, methacrylate and fumarate esters, the formation of chemically crosslinked networks of molecules by free-radical polymerization of these groups is prevented, as opposed to the

oligoethoxylated trimethylolpropane triacrylate ion-conducting medium used by Xiong et al. (see Xiong et al., Mater. Horiz., 2015, 2, 338-343).

Polymerization of this type of highly polymerizable group contained in the molecules of the present invention with concomitant network formation can thus only take place if a separate crossi inking agent is added. Such network formation of molecules is not desired in the present ion-conducting medium.

In the present ion-conducting medium the end-groups of the molecules are free alkyl chains, which are not covalently connected to any other molecules in the ion-conducting medium. Physical crosslinking by e.g.

hydrogen or ion bonding, may, however, exist between molecules in the medium. An electrolyte comprising the ion-conducting medium comprising molecules as defined above demonstrates improved ion mobility and an increased electrochemical window (in which window the electrolyte is electrochemically inert) compared to known ion-conducting media, and when used in a LEC there are recognizable performance improvements of the LEC in terms of faster turn-on time, and/or improved efficiency and/or improved operational lifetime.

One explanation to the improved characteristics of the electrolyte, i.e. the ion-conducting medium comprising molecules of formula I blended with an appropriate salt, and the LEC in which the ion-conducting medium is used, is, although not limited to, the introduction of alkyl carbonate esters on the end- groups, or carbonate esters in the main chains of an oligomer with a plurality of arms (2-10 arms). Inclusion of the alkyl chains render the molecule more hydrophobic and thereby more compatible with a hydrophobic light-emitting compound in the electroactive material of the LEC.

End-group is here in accordance with the lUPAC definition: "A constitutional unit that is an extremity of a macromolecule or oligomer molecule. An end-group is attached to only one constitutional unit of a macromolecule or oligomer molecule."

Main chain is here in accordance with the lUPAC definition: "A linear chain to which all other chains, long or short or both, may be regarded as being pendant."

The optimum structure of the present ion-conducting molecules described by formula I depends on a delicate balance between their ion- conducting properties and their interactions with the light-emitting material. The length and number of the oligomer chains, as dictated by p, q, s, m, n and o, will determine the ion solvation and binding, which is also dependent on the size of the solvated cation and the choice of A, B and/or C. The chain length will also influence the ion transport properties. In particular, it is imperative to keep the oligomer chains sufficiently short so as to avoid molecular entanglements, which reduce the ionic mobility. The structure of the oligomer chains will heavily influence the molecular flexibility, which will have an influence on the ionic conductivity - one of the main determinants of the turn-on time. The structure of the oligomer chains, i.e., the identity of the ion-coordinating groups, will influence the ion coordination strength and ion release kinetics. The identity of the alkyl chains R ¹ , R ² , R ³ , R ⁴ and R ⁵ determine the compatibility with the light-emitting material in the LEC. Thus, for different combinations of salt and light-emitting materials, different ion- conducting media may be preferred for optimum device performance characteristics.

A particular example, which has shown high performance in LEC devices, in combination with the salt UCF3SO3 and the electroactive compound SuperYellow, is a three-armed oligoether with octyl carbonate end- groups, wherein X is derived from trimethylolpropane, m = 3, n = o = 0, A = CH2CH2, p = 2.4, Y = O and R ¹ = (CH ₂ ) ₇ CH ₃ .

The polyol may be selected from a group comprising ethylene glycol, propylene glycol, 1 ,3-propanediol, 1 ,4-butanediol, neopentylene glycol, trimethylolpropane, trimetylolethane, glycerol, pentaerythritol, di- trimethylolpropane, di-pentaerythritol, sugar alcohols.

The oligoether may be selected from a group comprising homo- and co-oligomers of methylene oxide, ethylene oxide, propylene oxide, oxetane, tetrahydrofuran, glycidyl ethers, and functional derivatives thereof, such as 1 ,2-epoxyhexane.

The oligoester may be selected from a group comprising oligomers of γ-butyrolactone, δ-valerolactone, ε-caprolactone, ε-decalactone, dioxanone, and functional derivatives thereof, such as v-octyloxy-E-caprolactone or v-2- [2-(2-methoxyethoxy)ethoxy] ethoxy-£-caprolactone.

The oligocarbonate may be selected from a group comprising oligomers of ethylene carbonate, propylene carbonate, trimethylene

carbonate, tetramethylene carbonate, and functional derivatives thereof, such as 2-heptyloxymethyl-2-ethyltrimethylene carbonate and 4-(butoxymethyl)- 1 ,3-dioxolan-2-one.

The linear hydrocarbon chain may be selected from a group

comprising methyl, ethyl, propyl, butyl, hexyl, octyl, decyl, dodecyl and benzyl.

The branched hydrocarbon chain may be selected from a group comprising isopropyl, isobutyl, sec-butyl, 2-ethylhexyl and 3,7-dimethyloctyl. The ion-conducting medium may comprise molecules having the general formula wherein (A-O) _p is an oligoether.

In this case Y may be O.

A may be a linear or branched hydrocarbon chain with 2-15, preferably 2-3, carbon atoms in the chain. A may for example be CH ₂ CH ₂ , CH2CH2CH2, CH ₂ CH(CH ₃ ) or CH ₂ CH(CH2OCH ₂ CH3).

The linear or branched hydrocarbon chain A may comprise 1-5, preferably 1-3 or 1-2 heteroatoms, i.e. atoms other than C or H, selected from a group consisting of O, N and S. These may additionally be double- bonded to C, so as to form, e.g., carbonyl groups in the specific example of O double-bonded to C.

In a specific example of an oligoether, Y may be O, A may be CH ₂ CH ₂ , m may be 3 and X may be derived from trimethylolpropane, having the general formula

In an oligoether according to formula lla, the oligoether arms will be responsible for the bulk of the ion solvation and conduction when used in an electrolyte, while the alkyl carboxylate ester or alkyl carbonate ester groups will impart an improved electrochemical stability towards oxidation, in addition to taking part in ion coordination. The alkyl chain ends will give an improved compatibility with a hydrophobic light-emitting material when used in a LEC device as well as affect the molecular flexibility, viscosity and ion mobility to an extent that depends on the length and configuration of the alkyl chain end.

The ion-conducting medium may comprise molecules having the general formula

-O^Y-R ¹

(lib) wherein (A-O) _p is an oligocarbonate,

In this oligocarbonate Y may be O.

In this oligocarbonate A may be o

^O-Q- and Q may be a linear or branched hydrocarbon chain with 2-15, preferably 2-3 carbons in the chain.

The linear hydrocarbon chain may for example be, but is not limited to, ethylene, trimethylene or tetramethylene.

The branched hydrocarbon chain may for example be, but is not limited to, propylene or, CH2CH2(CH2CH3)((CH2)8CH3)CH2.

The linear or branched hydrocarbon chain Q may comprise 1-5, preferably 1-3 or 1-2 heteroatoms selected from a group consisting of O, N and S. These may additionally be double-bonded to C, so as to form, e.g. carbonyl groups in the specific example of O double-bonded to C.

In a specific example of an oligocarbonate, Y may be O and A may be o

o-Q- wherein Q may be CH2CH2CH2, m may be 3, and X may be derived from trimethylolpropane, having the formula

Hence, this is an example of an oligocarbonate based on

oligo(trimethylene carbonate).

In another specific example of an oligocarbonate, Y may be O, A may be o

^O-Q- wherein Q may be CH ₂ C(CH ₂ CH3)(CH2O(CH2)6CH ₃ )CH2 (i.e. a branched chain) containing 1 O heteroatom, m may be 3, and X may be derived from trimethylolpropane, having the formula

(V) This is, hence, an example of an oligocarbonate based on oligo(2- heptyloxymethyl-2-ethyltrimethylene carbonate).

The desired high compatibility of the ion-conducting media in the electrolyte with hydrophobic light-emitting materials in a LEC device is mainly governed by the alkyl-functional end-groups of the molecules, and is as such largely independent of the ion-coordinating oligomer backbone. Alkyl carbonate-capped oligocarbonates according to formula Mb have shown similarly enhanced anodic stability as alkyl carbonate-capped oligoethers, but with additionally improved current efficacy.

The ion-conducting medium may comprise molecules having the general formula wherein (A-O) _p may be an oligoester.

In this oligoester Y may be O.

In this oligoester A may be

wherein D may be a linear or branched hydrocarbon chain containing 2-15, preferably 2-9, or 5-9, carbon atoms in the chain.

D may for example be, but is not limited to, a linear hydrocarbon chain such as trimethylene, tetramethylene or pentamethylene.

D may for example be, but is not limited to, a branched hydrocarbon chain such as (Ch^ChKCh Ch Ch CHs).

The linear or branched hydrocarbon chain D may comprise 1-5, preferably 1-3 or 1-2 heteroatoms selected from a group comprising O, N and S. These may additionally be double-bonded to C, so as to form, e.g. carbonyl groups in the specific example of O double-bonded to C.

In a specific example of an oligoester, Y may be O, A may be wherein D may be (CH ₂ )5, m may be 3, and X may be derived from

trimethylolpropane, having the formula

This is, hence, an example of an oligoester based on oligo(£- caprolactone).

In another specific example of an oligoester, Y may be O, A may be

wherein D may be (Ch ^Ch Ch Ch Ch CHs) (i.e. a branched chain), m may be 3, and X may be derived from trimethylolpropane, having the formula

This is, hence, an example of an oligoester based on oligo(£- decalactone).

As mentioned previously, the properties of the molecules in the ion- conducting medium are largely determined by their end-group. Polyesters share similar properties with polycarbonates and the oligoesters according to formula lie are thus expected to behave similarly in a LEC context as well, although structural differences can give different molecular flexibility and different hydrophobicity.

According to a second aspect there is provided an electrolyte for a light-emitting electrochemical cell comprising an ion-conducting medium as described above and a salt comprising at least one cation and at least one anion, with the molar ratio of cations to molecules in the ion-conducting medium being 0.1-10, preferably 0.5-2 or 0.8-1 .2.

The electrolyte may consist essentially of molecules according to the general formula I but may also be diluted with molecules not having the functionalities specified in formula I, while still retaining the essential characteristics of the present invention.

The at least one cation may be selected from a group comprising Li ⁺ ,

Na ⁺ , K ⁺ , Rb ⁺ , Cs ⁺ , Mg ²⁺ , Ag ⁺ , tetrahexylammonium, tetrabutylammonium, imidazolium, and phosphonium, and the at least one anion may be selected from a group comprising CF3SO3 ^" , CIO ₄ ^" , PF ₆ ^" , BF ₄ ^" , (2,4,4- trimethylpentyl)phosphinate, ethylsulfate, and (CF ₃ SO2)2N ^~ . The electrolyte may have an onset of reduction of < -1 .8 V, preferably < -1 .9 V, and an onset of oxidation of > +0.55 V, preferably > +0.8 V, vs. the Fc7Fc redox couple, e.g. as measured by cyclic voltammetry, using a gold working electrode and a sweep rate of 0.05 V/s.

Such a wide electrochemical stability window suppresses lifetime- limiting side reactions during device operation and leads to a lower

accumulation of insulating side-reaction residues at the electrode-electrolyte interfaces with a concomitant lower drive voltage.

The electrolyte may have a viscosity of 0.16 to 6.9 Pa s, preferably 0.2 to 3 Pa s, 0.5 to 2 Pa s or 0.2 to 1 Pa s. A low electrolyte viscosity translates into a high ionic mobility, contributing to fast device turn-on.

According to a third aspect there is provided a light-emitting

electrochemical cell comprising a first and a second electrode, an

electroactive material and an electrolyte as described above.

The electroactive material may comprise at least one conjugated polymer, at least one conjugated small molecule or at least one ionic transition metal complex.

The at least one conjugated polymer may be selected from a group comprising poly(para-phenylene), poly(para-phenylene vinylene),

poly(fluorene) and neutral and ionic derivatives thereof, and any type of copolymer structure thereof.

The conjugated polymer may be SuperYellow.

The ionic transition metal complex may be Ru(bpy)3 ²⁺ (X ^" )2, wherein bpy may be 2,2'-bipyridine and X ^" may be a molecular anion such as CIO ₄ ^" or PF ₆ ^" , or combination thereof.

The conjugated small molecule may be rubrene.

The light-emitting electrochemical cell may have a turn-on time of <16 s, preferably <13 s, to attain >300 cd nrf ² when driven galvanostatically at a current density of 7.7 mA cm ^-2 .

The light-emitting electrochemical cell may have a power conversion efficiency of >7.5 Im W ^"1 , preferably >10 Im W ^~1 .

The light-emitting electrochemical cell may have an operational lifetime at >300 cd cm ^"2 of >130 h, preferably >300 h. Preferably, the light-emitting electrochemical cell may have a turn-on time of <16 s, preferably <13 s, to attain >300 cd nrf ² , a power conversion efficiency of >7.5 Im W ^~1 , prefereably >10 Im W ^"1 and an operational lifetime at >300 cd cm ^"2 of >130 h, preferably >300 h.

Brief Description of the Drawings

Fig. 1 is a Fourier Transform Infrared Spectroscopy (FTIR) spectrum of a TMPE-OC:LiCF ₃ SO ₃ electrolyte.

Fig. 2 is a schematic view of a surface cell LEC device wherein the active material effectively bridges two coplanar electrodes.

Fig. 3 is a schematic view of a sandwich cell LEC device wherein the two electrodes sandwich the active material.

Fig. 4 is a graph showing the turn-on kinetics for LEC devices having an electrolyte comprising the ion-conducting medium TMPE-OC, TMPE-BC or TMPE-EC, as specified in the inset.

Fig. 5 is the general formula (I) of the molecules of the ion-conducting medium.

Synthesis of ion-conducting medium

The synthesis methods used for producing the different groups of molecules covered by the general formula I (Fig. 5) depend on the starting material, i.e. the functional core X, and on the arms and the functional end- groups that are to be added to the functional core.

Below follows an example synthesis of an oligoether-based ion- conducting medium. The oligoether, having a functional core X derived from trimethylolpropane, was synthesized using a commercially available oligoether (TMPE-OH) as the starting material.

Example syntheses of two oligocarbonate-based ion-conducting media are also shown, TMP-oTMC-EC and TMP-oHEC-EC. Butyl carbonate-capped TMPE (TMPE-BC)

TMPE-OH TMPE-EC: R = Et

TMPE-BC: R = n-Bu

TMPE-OC: R = n-octyl

Synthesis of oligoether-based ion-conducting medium. All chemicals were obtained from commercial sources and used as received. Hydroxyl-capped trimethylolpropane ethoxylate (TMPE-OH) had an average molecular weight, M _n , of -450 according to the supplier (Aldrich).

TMPE-OH (2.7 g, equivalent to 18 mmol of hydroxyl end-groups) was dissolved in anhydrous DCM (40 ml). Butyl chloroformate (2.8 ml, 22 mmol) was added and the solution was cooled in an ice bath. Pyridine (3.5 ml, 44 mmol) was gradually added over a period of 20 min. After an additional 30 min, the ice bath was removed and the reaction was allowed to proceed at room temperature overnight. Λ/,/V-dimetylethanolamine (1 ml, 10 mmol) was added to quench the reaction and the reaction mixture was washed with deionized water (40 ml), 1 M hydrochloric acid (2x40 ml) and saturated aqueous NaHCO3 (40 ml). The organic phase was dried with MgSO ₄ , filtered and the solvent evaporated. Solvent residues were removed in a vacuum oven at -40 °C over P ₂ O ₅ to yield 4.0 g of a slightly yellowish and slightly viscous liquid. The successful conversion of the hydroxyl end-groups may be confirmed by ¹ H NMR spectroscopy.

Synthesis of TMP-oTMC-EC and TMP-oH EC-EC

2-Heptyloxymethyl-2-ethyltrimethylene carbonate (HEC) was synthesized as described elsewhere (Mindemark et al., Electrochim. Acta 2015, 175, 247-253). Trimethylolpropane (TMP; Perstorp) was recrystallized from ethyl acetate and dried under vacuum over P2O ₅ .1 ,8- Diazabicyclo[5.4.0]undec-7-ene (DBU; Acros Organics) was distilled under reduced pressure. Trim ethylene carbonate (TMC; Boehringer Ingelheim) was handled and stored in a glovebox under Ar. All other chemicals were obtained from commercial sources and used as received.

Ethyl carbonate-capped three-armed oligo(trimethylene carbonate) (TMPE-

Synthesis of oligocarbonate-based ion-conducting medium.

TMC (2.21 g, 21 .6 mmol) and TMP (403 mg, 3.00 mmol, for DP = 7.2) were charged under an inert atmosphere in an oven-dried round-bottom flask. The flask was sealed with a septum and heated under stirring in a 50 °C oil bath until a homogenous melt had formed, 2-(dimethylamino)ethyl benzoate (57 μΙ, 0.30 mmol) of was added and the flask was heated for 7 h for polymerization. The crude polymer was dissolved in anhydrous

dichloromethane (20 ml) and ethyl chloroformate (1 .0 ml, 1 1 mmol) was added. The resulting solution was cooled in an ice bath and pyridine (1 .8 ml, 22 mmol) was added dropwise over the course of 20 min and the reaction was allowed to proceed for 30 min under cooling, the ice bath was removed and the reaction left overnight. The reaction was quenched with of N,N- dimethylethanolamine (0.5 ml, 5 mmol). The reaction mixture was extracted with deionized water (20 ml), 1 M HCI (2x20 ml) and saturated aqueous NaHCO3 (20 ml). The organic phase was dried with MgSO ₄ and filtered, the solvent was evaporated and any remaining solvent residues were removed in a vacuum oven at -40 °C over P2O ₅ . Yield: 1 .61 g. The successful

polymerization and conversion of the hydroxyl end-groups may be confirmed by ¹ H NMR spectroscopy.

TMP-oTMC-EC Ethyl carbonate-capped three-armed oligo(2-heptyloxymethyl-2- ethyltrimethylene carbonate) (TMP-oHEC-EC)

An oven-dried 8 ml glass vial was charged under an inert atmosphere with HEC (1 .86 g), TMP (134 mg, 1 .00 mmol, for DP = 7.2) and DBU (30 μΙ, 0.2 mmol). The vial was sealed and heated under stirring in a 60 °C heating block for 21 ½ h. The crude polymer was dissolved in 20 ml of anhydrous dichloromethane and the solution was cooled in an ice bath. Ethyl

chloroformate 0.35 ml (3.7 mmol) was added dropwise, followed by the dropwise addition of pyridine (0.59 ml, 7.3 mmol) over the course of 10 min. The reaction was allowed to proceed for 1 h under cooling, the ice bath was removed and the reaction left overnight. The reaction was quenched with Λ/,/V-dimethylethanolamine (0.17 ml, 1 .7 mmol) and the reaction mixture was extracted with deionized water (20 ml), 1 M HCI (2x20 ml) and saturated aqueous NaHCO3 (20 ml). The organic phase was dried with MgSO ₄ and filtered, the solvent was evaporated and any remaining solvent residues were removed in a vacuum oven at -40 °C over P2O ₅ . Yield: 1 .22 g. The successful polymerization and conversion of the hydroxyl end-groups may be confirmed by ¹ H NMR spectroscopy.

TMP-oH EC-EC

Other molecules can be synthesized using the same methodology, by starting from other polyol cores and using other monomers to form the oligomer chains with appropriate polymerization catalysts. AlkyI carboxylate ester end-groups can be obtained by substituting acyl chlorides for the alkyl chloroformates used to form the alkyl carbonate end-groups and alkyl carbamate end-groups can, e.g., be obtained using alkyl isocyanate reagents.

Preparation of electrolytes

Electrolytes were prepared by dissolving UCF3SO3 salt in the different synthesized ion-conducting media. The molar ratio of cations (Li ⁺ ) to molecules in the ion-conducting medium was 0.1-10. The molar ratio could alternatively be 0.5-2 or 0.8-1 .2.

Other salts which may be used may comprise cations such as Na ⁺ , K ⁺ , Rb ⁺ , Cs ⁺ , Mg ²⁺ , Ag ⁺ , tetrahexylammonium, tetrabutylammonium, imidazolium, or phosphonium, and anions such as CF3SO3 ^" , CIO ₄ ^" , PF ₆ ^~ , BF ₄ ^" , (2,4,4- trimethylpentyl)phosphinate, ethylsulfate, or (CF ₃ SO2)2N ^~ . Electrolyte characterization

Cyclic voltammetry

The electrochemical stability windows (ESW) of the electrolytes were measured with the aid of cyclic voltammetry (CV). CV measurements were carried out at a sweep rate of 0.05 V/s with an Autolab PGSTAT302 potentiostat using the GPES software. An Au-coated glass plate was the working electrode, a Pt rod was the counter electrode, an Ag wire was the quasi-reference electrode, and 0.1 M UCF3SO3 and 0.2 M ion-conducting medium in anhydrous CH ₃ CN was used as the electrolyte. Directly after each CV scan, a calibration scan was run with a small amount of ferrocene added to the electrolyte. All CV potentials are reported vs. the ferrocene/ferrocenium ion (Fc/Fc ⁺ ) reference. The reduction/oxidation onset potentials were defined as the intersection of the baseline with the tangent of the current at the half- peak-height. CV sample preparations and measurements were performed under N ₂ atmosphere in a glovebox ([O2] < 1 ppm, [H ₂ O] < 0.5 ppm). Table 1 below presents the redox potentials, as derived from the CV measurements, for electrolytes containing the following ether oxygen-based ion-conducting media: TMPE-BC (butyl carbonate-capped TMPE), TMPE-EC (ethyl carbonate-capped TMPE) and TMPE-OC (octyl carbonate-capped TMPE). The table also presents redox potentials for electrolytes containing the following oligocarbonate-based ion-conducting molecules: TMP-oTMC-EC and TMP-oHEC-EC. For reference, the table also shows the redox potentials for TMPE-OH (hydroxyl-capped TMPE), TMPE-OCH3 (methoxy-capped TMPE) and SuperYellow (a conjugated polymer often used as the

electroactive material in LECs). Table 1

Reduction onset Oxidation onset

Material potential vs. potential vs.

FcVFc (V) FcVFc (V)

SuperYellow -2.20 +0.42

As is seen from Table 1 , electrolytes containing the new ion-conducting media TMPE-BC, TMPE-EC, TMPE-OC, TMP-oTMC-EC and TMP-oHEC-EC have an expanded ESW in comparison to TMPE-OH and TMPE-OCH3, particularly on the anodic (oxidation) side.

Alkyl carbonate-capping of TMPE results in an expansion of the anodic stability by -0.5 eV, whereas the cathodic stability remains essentially intact - an indication that the cathodic stability is no longer limited by the ion- conducting medium but by the CF3SO3 ^" anion. A similar improvement in anodic stability is also seen for the alkyl carbonate-capped oligocarbonates TMP-oTMC-EC and TMP-oHEC-EC.

Given the well-known electrochemical properties of carbonate and carboxylate esters versus ethers (see e.g. Xu et al., Chem. Rev. 2004, 104, 4303-4417) the same improvement is expected for other combinations of similar end-groups and oligomeric backbones.

IR spectroscopy Fourier Transform Infrared Spectroscopy (FTIR) spectra were recorded on a PerkinElmer Spectrum One FT-IR spectrometer using an attenuated total reflectance (ATR) accessory.

Polyether-based ion-conducting media are known to dissolve salts such as UCF3SO3 through a multi-dentate coordination of ether oxygens to the cation, whereas the anion is left effectively "free". The effects of the addition of a carbonate group on the Li-ion coordination were investigated by IR spectroscopy. The carbonyl (C=O) stretch mode of a TMPE-OC:LiCF ₃ SO ₃ electrolyte, as presented in Fig. 1 , is sensitive to ion coordination, and the emergence of a shoulder at ~1723 cm ^-1 following the addition of the salt reveals that a fraction of the Li cations are additionally coordinated to a carbonyl group, effectively leading to a mixed ether/carbonate coordination. This conclusion was also supported by NMR data (not shown). For oligoether- based ion-conducting media, this mixed coordination is anticipated to partially disrupt the chelating effects of the oligoether coordination, leading to a weaker binding of the Li cation to its parent ion transporter, and that the release of the cations during, e.g., the LEC turn-on, as a consequence, will be more facile and fast. For oligocarbonate- and oligoester-based ion-conducting media, the ion coordination is intrinsically weaker, as indicated by the considerably higher cation transference numbers seen in polymer electrolytes with such configurations (see Tominaga et al., Chem. Commun., 2014, 50, 4448-4450; Mindemark et al., J. Power Sources, 2015, 298, 166-170);

hence, the end-group effects will naturally be less significant in this particular respect.

Viscosity

The ionic mobility of an electrolyte scales with the inverse of the viscosity, as derived by the classical Stokes-Einstein equation and as stated by Walden's rule. The viscosity of electrolytes based on TMPE-BC, TMPE-EC and TMPE-OC was found to be lower than for the TMPE-OH-based

electrolyte but higher than the TMPE-OCH3 equivalent. The first observation can be rationalized by that the intermolecular hydrogen bonding in the TMPE- OH system is eliminated through the capping of the hydroxyl end-group. The second observation is due to the larger size of TMPE-BC, TMPE-EC and TMPE-OC compared to TMPE-OCH ₃ . The viscosity was also found to decrease with increasing length of the alkyl end-group, suggesting that the end-group acts to shield complexes formed between the cations and the ion transporter molecules from each other, and that this shielding is more effective for longer alkyl chains.

Viscosity data for TMPE-based electrolytes, as determined by rotational viscometry using a cone-and-plate geometry (20 mm, 1 ° stainless steel cone) at a shear rate from 10-300 s ^"1 , are shown in Table 2. All samples showed Newtonian behavior, indicating a lack of molecular entanglements.

Table 2

Material Viscosity (Pa s)

Light-emitting electrochemical cell (LEC) device fabrication

A LEC may comprise a first and a second electrode, and an active material comprising an electroluminescent electroactive material and an electrolyte.

Figs 2 and 3 show different variants of LEC devices fabricated on a substrate 5. The schematic in Fig. 2 shows a surface cell device with the active material 1 effectively bridging two coplanar electrodes 2, 3. The schematic in Fig. 3 shows a sandwich cell device where the two electrodes 2 ^' , 3 ^' sandwich the active material 1 . The active material may comprise at least one conjugated polymer, at least one conjugated small molecule, or at least one ionic transition metal complex. The at least one conjugated polymer may for example be poly(para- phenylene), poly(para-phenylene vinylene), poly(fluorene) and neutral and ionic derivatives thereof, or any type of co-polymer structure thereof. One specific example of a conjugated polymer is SuperYellow (SY). A specific example of the ionic transition metal complex is Ru(bpy)3 ²⁺ (X ^" )2, wherein bpy is 2,2'-bipyridine and X ^" is a molecular anion such as CIO ₄ ^" or PF ₆ ^" , or a combination thereof. A specific example of the conjugated small molecule is rubrene.

The active material further comprises an electrolyte.

Master solutions were prepared by separately dissolving the

constituent materials in anhydrous tetrahydrofuran at a concentration of 6.5 mg/ml (SY) and 10 mg/ml (ion-conducting medium and UCF ₃ SO ₃ ). The molar ratio between SY and the ion-conducting medium was constant, which resulted in the following mass ratios for the active-material inks: SY:ion- conductive medium:LiCF ₃ SO ₃ = 1 :x:0.03, x = 0.14 (TMPE-EC), x = 0.17 (TMPE-BC), x = 0.20 (TMPE-OC).

LECs were fabricated by spin-coating the active-material ink (at 2000 rpm for 60 s) onto carefully cleaned indium-tin-oxide (ITO) coated glass substrates (20 Ω/square, Thin Film Devices, USA). The dry thickness of the active material was 100 nm for all ion-conductive media. Al cathodes were deposited on top of the active material by thermal evaporation (at p <

5x 10 ^"4 Pa). The light-emission area, as defined by the size of the cathode, was 0.85x0.15 cm ² . The devices were characterized using a computer- controlled source-measure unit (Agilent U2722A) and a calibrated

photodiode, equipped with an eye-response filter (Hamamatsu Photonics), connected to a data acquisition card (National Instruments USB-6009) via a current-to-voltage amplifier. All of the above procedures, except for the cleaning of the substrates, were carried out in two interconnected N ₂ -filled glove boxes ([O2] < 1 ppm, [H ₂ O] < 0.5 ppm). TMP-oTMC-EC- and TMP-oH EC-EC-based LEC structures were also prepared. These comprised ITO/SY:ion-conductive medium:LiCF ₃ SO3/AI in a sandwich-cell structure (see Fig. 3).

LECs comprising the ion-conducting media TMPE-EC, TMPE-BC, TMPE-OC exhibited a fast turn-on of ~10 s to 300 cd nrf ² during constant current-density driving at j = 7.7 mA cm ^-2 . In Fig. 4 the turn-on kinetics for TMPE-OC, TMPE-BC and TMPE-EC are shown, which represent a significant improvement in comparison to previous TMPE-based LECs. The improved turn-on kinetics might seem counterintuitive considering that the ion mobility of the "pure" new electrolytes is lower than that of the corresponding TMPE- OCH ₃ electrolyte (as indicated by the viscosity data (see above)), but the LEC turn-on is dependent on not only the ion mobility of the pure electrolyte but also on the phase morphology of the mixture of the electrolyte and the light emitter (the electroactive material), where an intimate blending is preferable, as well as on the strength of the ion solvation, where an excessively strong solvation by the ion-conducting medium will make the ion release during LEC turn-on difficult. In both these aspects, these new ion-conducting media appear to represent a clear improvement over the TMPE-OCH ₃ -based system. It was also observed that the subsequent increase to peak luminance is fastest for the electrolyte containing the largest molecule, TMPE-OC, and slowest for the ion-conducting media containing the smallest molecule, TMPE-EC, and this is attributed to the higher ionic mobility (lower viscosity) and improved phase compatibility with SY as the length of the alkyl chain increases. Table 3 summarizes the turn-on times of LECs as a function of ion- conducting media. Table 3

TMPE-EC 8 8.3 8.1 155

TMPE-BC 12 10.4 10.7 312

TMPE-OC 9 10.6 10.7 355

TMPE-OCHs 16 8.4 7.1 130

TMPE-OH 1740 7.1 5.9 130

TMP-oTMC-

28440 8.9 5.6 140

EC

TMP-oH EC-

50 13.4 9.8 139

EC

Table 3 also shows the long-term performance of the LECs comprising the different ion-conducting media during galvanostatic driving at

7.7 mA cm ^-2 . It is again found that the new ion-conducting media result in notably improved performances, with the TMPE-OC based LEC attaining a 355 h operational lifetime at a high luminance of >300 cd nrf ² . By using a driving protocol with a prebias at a higher current density, followed by low- current-density steady-state driving, an operational lifetime of 1400 h at >100 cd nrf ² was attained for the same type of device. These data were recorded on non-encapsulated devices operating in a N ₂ -filled glove box. Recent results suggest that an even better stability can be obtained from properly encapsulated devices operating under ambient air. This prolonged stability of the new LECs results from: (i) an improved phase compatibility between the electrolyte and the light-emitter of the electroactive material, and (ii) the expanded ESW of the new electrolytes. The stability of a LEC has been demonstrated to be dependent on the ion concentration and the concomitant doping concentration. As such, a homogenous distribution of ions in a well-blended active material will result in a more stable operation. The expanded ESW of the electrolyte will in turn suppress electrolyte-induced side reactions at the electrode interfaces that limit the operational lifetime.

A homogenous optimized doping concentration in a well-mixed system will also lead to minimized exciton quenching, and a lowered amount of electrolyte side-reaction residues will result in a lowered drive voltage. Both of these effects lead to improved power conversion efficiency (PCE). The former effect is also reflected in the current efficacy, which is markedly improved (to > 10 cd/A), see Table 3, for the two larger of the oligoether ion-conducting media, thereby confirming that the improved phase morphology results in a better harvesting of the excitons. An even higher efficiency of 13.4 cd/A was obtained with the oligocarbonate-based TMP-oHEC-EC. This ion-conducting medium features additional compatibilizing long alkyl ether side chains compared to TMP-oTMC-EC, an ion-conducting medium that does not show as high current efficacy. An even more important efficiency parameter is the PCE. Both the TMPE-BC and TMPE-OC devices feature an impressive PCE value well above 10 Im W ^"1 at a high luminance of 800 cd nrf ² and the TMP- oTMC-EC device shows a PCE close to 10 Im W ^"1 (Table 3).