Emissive layer for a stretchable organic light emitting diode (OLED) and method of making the emissive layer

Abstract

An emissive layer for a stretchable organic light emitting diode (OLED) comprises a plasticized film including a thermally activated delayed fluorescence (TADF) polymer and a plasticizer distributed within the TADF polymer. A method of making an emissive layer for a stretchable organic light emitting diode (OLED) comprises co-dissolving a plasticizer and a thermally activated delayed fluorescence (TADF) polymer at a predetermined ratio in a solvent to form a precursor mixture, depositing the precursor mixture onto a substrate to form a coating, and removing the solvent from the coating to form a plasticized film comprising the TADF polymer and the plasticizer distributed in the TADF polymer.

Description

PATENT Atty. Dkt. No. 507814.5000516EMISSIVE LAYER FOR A STRETCHABLE ORGANIC LIGHT EMITTING DIODE(OLED) AND METHOD OF MAKING THE EMISSIVE LAYERRELATED APPLICATION

[0001] The present patent document claims the benefit of priority under 35 U. S. C. 119(e) to U. S. Provisional Patent Application No. 63 / 723,773, which was filed on November 22, 2024, and is hereby incorporated by reference in its entirety.FEDERALLY FUNDED RESEARCH OR DEVELOLPMENT

[0002] This invention was made with government support under EB034563 awarded by the National Institutes of Health and 2239618 awarded by the National Science Foundation. The government has certain rights in the invention.TECHNICAL FIELD

[0003] This disclosure relates generally to stretchable polymers and more particularly to an emissive layer for a stretchable organic light emitting diode (OLED).BACKGROUND

[0004] Organic light-emitting diodes (OLEDs) have revolutionized modern display and lighting technologies, offering advantages such as high brightness, wide viewing angle, fast response, low driving voltage, and low power consumption. There is ongoing work to extend these devices into mechanically soft and deformable form factors, most notably stretchable OLEDs, which are emerging as critical components for next- generation wearable and biointegrated devices such as on-skin displays, sensors, imagers, and optical stimulators. Such applications require emissive materials with both high stretchability and high electroluminescent (EL) efficiency. Progress in this area has been hindered, however, by the intrinsic trade-off between mechanical stretchability and high emitting efficiency in existing light-emitting polymers.PATENT Atty. Dkt. No. 507814.5000516

[0005] Thermally activated delayed fluorescence (TADF) polymers are promising candidates for stretchable OLEDs due to their capability in achieving high efficiency from harnessing both singlet and triplet excitons to realize theoretically 100% internal quantum efficiency (IQE). This marks a clear advantage compared to fluorescent emitters, which are limited to 25% IQE. Additionally, TADF polymers do not require heavy metals and thus offer higher biocompatibility than phosphorescent emitters. However, attempts to increase the polymer-chain flexibility of TADF polymers via molecular engineering have resulted in emitter efficiencies lower than that of state-of-the-art TADF polymers.SUMMARY

[0006] An emissive layer for an organic light emitting diode (OLED) comprises a plasticized film including a thermally activated delayed fluorescence (TADF) polymer and a plasticizer distributed within the TADF polymer.

[0007] A method of making an emissive layer for an organic light emitting diode (OLED) comprises co-dissolving a plasticizer and a thermally activated delayed fluorescence (TADF) polymer at a predetermined ratio in a solvent to form a precursor mixture, depositing the precursor mixture onto a substrate to form a coating, and removing the solvent to form a plasticized film comprising the TADF polymer and the plasticizer distributed in the TADF polymer.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The embodiments may be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale.

[0009] FIG. 1A is a schematic of a plasticized film including a plasticizer dispersed in a TADF polymer, and FIG. 1B provides a magnified view of the micro structure while under strain.

[0010] FIG. 1C is a schematic of a “neat” TADF polymer film that does not include a small-molecule plasticizer, and FIG. 1D provides a magnified view of the microstructure.

[0011] FIG. 2 is a schematic of an exemplary flexible OEED that includes an emissive layer comprising the plasticized film shown in FIG. 1A.PATENT Atty. Dkt. No. 507814.5000516

[0012] FIG. 3A shows a schematic of quenching triplet exciton quenching in a neat TADF polymer and an associated energy level diagram.

[0013] FIG. 3B shows a schematic of suppression of triplet exciton quenching in a plasticized TADF polymer, that is, a polymer composite including a TADF polymer with a small-molecular plasticizer dispersed therein.

[0014] FIGS. 4A-4E show the molecular structure of five exemplary TADF polymers, in particular, PDKCE, PTLA, PTT, PDKCM, and PDKCP, respectively.

[0015] FIG. 5 A shows photoluminescence quantum yields (PLQYs) of PDKCE films incorporating varying weight ratios of the DOP plasticizer (0-75 wt.%), which are separated into prompt components and delayed components, where the data are represented as mean values ± s.d. from independent measurements (n = 5), and the corresponding data points are overlaid.

[0016] FIG. 5B shows time-resolved photoluminescence (PL) decays of DOP-incorporated PDKCE films at varying DOP ratios.

[0017] FIG. 5C shows a comparison of PLQY enhancement to PDKCE using plasticizers of different molecular sizes — DOP and DMP (both in 75 wt.%).

[0018] FIG. 5D shows the PLQY of PDKCE dissolved in chlorobenzene at different concentrations, without or with co-dissolved DOP or DMP (75 wt.%).

[0019] FIG. 5E shows the PLQYs of plasticizer- incorporated PDKCE films at different plasticizer:polymer weight ratios.

[0020] FIG. 5F shows the PLQY of SY (representing FL emitters) dissolved in chlorobenzene at different concentrations, without or with co-dissolved DOP or DMP (75 wt.%).

[0021] FIG. 6 shows an exploded view of an exemplary OLED that includes an emissive layer (EML) comprising the plasticized film shown in FIG. 1A.

[0022] FIG. 7 shows current density-luminance-voltage (J-L-V) characteristics of OLEDs with EMLs comprising PDKCE incorporating varying amounts of DOP.

[0023] FIG. 8 shows EQEmax as a function of wt.% DOP / PDKCE and electroluminescence (EL) spectra (inset).

[0024] FIG. 9 A shows Young’s modulus, determined using the buckling metrology method, and crack-onset strain (COS) of the plasticized polymer films with different weight ratios of DOP.PATENT Atty. Dkt. No. 507814.5000516

[0025] FIG. 9B shows Young’s modulus, determined using the buckling metrology method, and crack-onset strain (COS) of the plasticized polymer films with different weight ratios of DMP.

[0026] FIG. 10 shows rheological behaviors of PDKCE solutions with different weight ratios of added DOP. The inset shows the apparent viscosity of these solutions at a shear rate of 1 s⁻¹.

[0027] FIGS. 11 A and 1 IB show PLQY and PL transient decays, respectively, of PDKCE films with 75 wt.% DOP; the PL transient decays are shown under different strains.

[0028] FIGS. 12A and 12B show representative J-L-V traces from OLEDs comprising PDKCE: DOP and neat PDKCE emission layers, respectively, under different strains, where the dashed line in FIG. 12B provides a comparison with the 0% strain data of PDKCE: DOP at 75 wt.% DOP.

[0029] FIG. 13 shows EQE-J traces from OLEDs comprising PDKCE: DOP and neat PDKCE emission layers under strains of 0%, 30%, and 60%.

[0030] FIG. 14 shows normalized luminance intensity (L / Lo) of the stretchable OLEDs at different strains, where L / Lo is represented as mean values ± s.d. from three devices.

[0031] FIG. 15 shows EQE (EQE / EQEo) of the of the stretchable OLEDs at different strains, where EQE / EQEo is represented as mean values ± s.d. from three devices.

[0032] FIG. 16 shows representative EQE-J characteristics for fully stretchable OLEDs using DOP-incorporated PDKCE as the EML at different plasticizer:polymer weight ratios.

[0033] FIGS. 17A-17D show the PLQY of TADF polymer composites including PTLA, PTT, PDKCM and PDKCP, respectively, at varying plasticizer / polymer weight ratios.DETAILED DESCRIPTION

[0034] Described in this disclosure is a broadly applicable strategy to enhance both stretchability and light-emitting performance in thermally activated delayed fluorescence (TADF) polymers through the incorporation of optoelectronically inert small-molecule plasticizers. The inventors discovered that physically increasing the intermolecular distance in TADF polymers by using small-molecule plasticizers can reduce local triplet concentrations and suppress triplet-triplet annihilation, thereby increasing the efficiency of TADF emitters. The inventors further recognized that the greater packing distance between polymer chains canPATENT Atty. Dkt. No. 507814.5000516 improve chain mobility, thereby enhancing mechanical ductility and stretchability. Accordingly, this disclosure describes an approach to improve both electroluminescent performance and mechanical compliance by physically increasing the packing distance in TADF polymers through the incorporation of small-molecule plasticizers.

[0035] FIG. 1A is a schematic illustration of a plasticized film 102 that may serve as an emissive layer 202 of an OLED 200, as shown according to one embodiment in FIG. 2. The OLED 200 may be a fully stretchable OLED 212, where the emissive layer 202 and other layers are elastically deformable. Fully stretchable OLEDs 212 may find application in wearable and bio-integrated devices, such as on-skin displays, sensors, imagers and optical stimulators. The emissive layer 202 comprising the plasticized film 102 emits light in response to an electric current via TADF, a phenomenon in which non-emissive, long-lived triplet states of a molecular species are converted into emissive singlet states, allowing for more efficient light generation. This may occur when the triplet excitons gain enough thermal energy to cross back to the singlet state through a process called reverse intersystem crossing (RISC), which then leads to delayed light emission. The ability to harness energy from both singlet and triplet excitons makes TADF a highly efficient mechanism.

[0036] Returning to FIG. 1A, the plasticized film 102 comprises a TADF polymer 104 and a plasticizer 106 distributed within the TADF polymer 104, forming a polymer composite 108 having the micro structure schematically shown in the inset. The arrows indicate polymer chain-to-chain sliding that is facilitated by the plasticizer 106 and which enables high stretchability of the film 102. For comparison, FIG. 1B shows a neat TADF polymer film 122 that consists solely of a polymer 124 without any added plasticizer. In contrast to the plasticized film 102, the neat TADF polymer film 122 tends to crack when deformed, as indicated by the striations 126 in the polymer 124.

[0037] Besides improving stretchability, the presence of the plasticizer 106 also enhances the efficiency of light emission from the TADF polymer 104. The strategy behind dispersing a small-molecule plasticizer in a TADF polymer is illustrated by the schematics of FIGS. 3A and 3B. Triplet-triplet annihilation (TTA) leading to concentration quenching of triplet excitons 302 is illustrated in FIG. 3A, which also shows a schematic of a TADF polymer without plasticizers (left side) and an energy level diagram (right side). In contrast, FIG. 3B shows increased intermolecular distances when plasticizers are incorporated into the TADF polymer (left side schematic) and improved emission efficiency via suppression of TTA (right sidePATENT Atty. Dkt. No. 507814.5000516 energy level diagram). Sn, Si, Ti, and So stand for the higher excited singlet state, lowest singlet state, lowest triplet state, and ground- state energy levels, respectively. RISC and Fa stand for reverse intersystem crossing and delayed fluorescence, respectively. The plasticizers function as molecular spacers within the polymer, expanding free volume to suppress triplet exciton quenching while facilitating stress-dissipative chain mobility, as illustrated in FIG. 1A. In the examples below, emissive layers prepared from such polymer composites achieved near-unity photoluminescence quantum yield (PLQY), stretchability exceeding 110% strain, and improved electroluminescent efficiency, with an external quantum efficiency (EQE) reaching 12.6% in a rigid device and 3.05% in fully stretchable OLEDs.

[0038] The strategy illustrated in FIG. 3B is believed to be applicable to a range of TADF polymers 104, including PDKCE, PTLA, PTT, PDKCM, and PDKCP, which have the chemical structures shown in FIGS. 4A-4E, respectively. A small-molecular plasticizer is incorporated into each of these TADF polymers at various concentrations in the examples below, and it is found that photoluminescence quantum yield (PLQY) increases for each polymer with increasing concentration of the incorporated plasticizer. Additionally, at the highest plasticizer concentrations, the crack sizes and densities are found to be substantially decreased at strain levels of 50% or 75%, indicating higher stretchability in the polymer composites compared to neat polymer films. As known in the art, strain in terms of a percentage is determined according to the formula (AL / L) 100, where L represents the initial length and AL represents the change in length due to stretching.

[0039] The plasticizer 106 may comprise a small-molecule additive, defined as a low-molecular- weight organic compound (e.g., less than about 1000 g / mol) that enhances flexibility, reduces brittleness, and improves processability by lowering the glass transition temperature of the host polymer. To be effective in TADF polymer systems, the plasticizer should exhibit good miscibility with the TADF polymer to ensure uniform molecular dispersion and maintain optical and mechanical homogeneity. Because TADF polymers are composed of ^-conjugated structures, plasticizers containing aromatic units are believed to promote better compatibility and integration within the polymer matrix. Additionally, the plasticizer should possess a sufficiently large energy bandgap to remain optoelectronically inert — i.e., not interfering with the intrinsic energy levels or excitonic processes of the TADF polymer. When these criteria are satisfied, the incorporation of such plasticizers can enhancePATENT Atty. Dkt. No. 507814.5000516 both mechanical stretchability and electroluminescent performance of the resulting polymer composite.

[0040] In the experiments described below, the larger-molecule phthalate, dioctyl phthalate (DOP, molecular weight - 391 g / mol), produced greater enhancements in both electroluminescent efficiency and mechanical stretchability than the smaller-molecule phthalate, dimethyl phthalate (DMP, molecular weight - 194 g / mol). This finding suggests that, for the tested TADF polymers, plasticizers with larger molecular size can more effectively increase intermolecular spacing and facilitate chain mobility, thereby improving overall device performance. It is therefore believed that each TADF polymer system may possess an optimal plasticizer molecular size that maximizes its combined optoelectronic and mechanical properties. In general, small-molecule plasticizers having relatively higher molecular weights — e.g., at least 250 g / mol, at least 300 g / mol, or preferably at least 350 g / mol, and / or up to 400 g / mol, up to 500 g / mol, or up to 600 g / mol — may be advantageously incorporated into TADF polymers. Exemplary phthalates include, in addition to DOP / DEHP, diisononyl phthalate (DINP, molecular weight (MW) - 419 g / mol), diisodecyl phthalate (DIDP, MW -447 g / mol), ditridecyl phthalate (DTDP, MW - 531 g / mol), di(2-propylheptyl) phthalate (DPHP, MW - 447 g / mol), and butyl decyl phthalate (BDP, MW - 363 g / mol).

[0041] To prepare the plasticized film, the plasticizer 106 may be co-dissolved with a selected TADF polymer 104 at controlled ratios in a solvent (e.g., an organic solvent such as chlorobenzene). The solutions are mixed and subsequently spin-coated or deposited via another solution coating method (e.g., dip coating, spray coating, roll coating, etc.) onto a suitable substrate at room temperature (18-24°C) and atmospheric pressure, followed by drying or removal of the solvent (e.g., by evaporation) to form the plasticized films 102 / emissive layers 202. The thickness of the plasticized polymer films 102 may lie in a range from about 40 nm to about 110 nm. As revealed by grazing-incidence wide-angle x-ray scattering, the plasticized polymer films are amorphous (noncrystalline).

[0042] In this work, the weight ratio of the TADF polymer 104 to the plasticizer 106 is in a range from 8:1 to 4:3, which corresponds to a weight percentage of the plasticizer from 12.5 wt.% to 75 wt.%, relative to the weight of the TADF polymer. More broadly, the plasticizer content may exceed 75 wt.% — for example, reaching as high as 80 wt.% or as high as 85 wt.% — such that, in some cases, the weight ratio of the TADF polymer to the plasticizer may extend to 5:4 or even 20:17. Alternatively, the plasticizer may be present in an amount of atPATENT Atty. Dkt. No. 507814.5000516 least 12.5 wt.%, at least 25 wt.%, at least 50 wt.%, or at least 75 wt.%. Importantly, these performance enhancements are observed under conditions in which the TADF polymer and the plasticizer remain well mixed, without phase separation. Within this stable compositional regime, higher weight fractions of the plasticizer are found to improve both stretchability and photoluminescence quantum yield (PLQY).

[0043] Referring again to FIG. 2, the OLED 200 may include an anode layer 204, a hole transport layer 206 on the anode layer 204, the emissive layer 202 comprising the plasticized film 102 (as shown in FIG. 1A) on the hole transport layer 206, an electron transport layer 208 on the emissive layer 202, and a cathode layer 210 on the electron transport layer 208. The OLED 200 may be a fully stretchable OLED 212. In the examples below, the fully stretchable OLED device structure is fabricated with a silver nanowire (AgNW) as the anode layer 204, poly(3,4-ethylenedioxythiophene): poly (styrenesulfonate): perfluorinated ionomer (PEDOT: PSS_PFI) as the hole transport layer, various embodiments of the emissive layer 202, an electron transport layer 208 comprising a blend of polyethyleneimine ethoxylated and poly[(9,9-bis(3'-((N, N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-a / t-2,7-(9,9-dioctylfluorene)] (PEIE_PFN-Br), and a cathode layer 210 comprising liquid metal eutectic gallium-indium (EGain).

[0044] It is also contemplated that the OLED 200 may be a rigid device 220 that includes rigid or non-stretchable layers in addition to the stretchable emissive layer 202, as illustrated in FIG. 6. The rigid OLED structure 220 of FIG. 6 was utilized in experiments described below with an anode layer 204 comprising indium-tin oxide (ITO), a hole transport layer 206 comprising PEDOT: PSS_PFI, various embodiments of the emissive layer 202, an electron transport layer 208 comprising 2,2',2"-(l,3,5-benzinetriyl)-tris(l-phenyl-l-H-benzimidazole) (TPBi), and a cathode layer 210 comprising aluminum on lithium fluoride (Al / LiF). The OLED 200, whether rigid or stretchable, may in some examples include additional layer(s) not mentioned above and / or may utilize material(s) not mentioned above.

[0045] EXAMPLES

[0046] Small-molecule plasticizers are employed to increase the packing distance of TADF polymers as explained above to enhance efficiency by suppressing triplet concentration quenching and improve stretchability by increasing chain mobility. In these examples, two types of phthalate plasticizers of different molecular sizes are employed, dioctyl phthalate (DOP) and dimethyl phthalate (DMP), to prepare polymer composites based on five differentPATENT Atty. Dkt. No. 507814.5000516 TADF polymers with rigid mechanical properties. The larger-size plasticizer, DOP, realizes an increase of TADF polymer’s PLQY by as high as 60% to almost 100%, and an increase of OLED EQE by up to 12.6%, alongside the increase of the stretchability to 110% strain. Using this strategy for the emissive layer, the performance of fully stretchable OLED devices also has a similar extent of improvement.

[0047] Enhancement of photoluminescence and EL properties

[0048] The effect of phthalate plasticizers on the PL and EL performance on a TADF polymer with decent performance, PDKCE (FIG. 4A) is first tested. For thin films prepared from spin-coating, the gradual increase of the DOP amount from 0 (i.e., neat PDKCE film), to 12.5%, and then to 25% (all in weight ratio to PDKCE) led to an increase of PLQY from 60% to 97%, which is an enhancement of 165%, as shown in FIG. 5A. For the further increase of the DOP: PDKCE weight ratios to 50% and 75%, the PLQY is mostly saturated, with just minor increases. Through time-resolved photoluminescence (TRPL) measurements (FIG. 5B) to separate the prompt fluorescence (PF) and the delayed fluorescence (DF) in the PLQY, it is found that the increase mainly comes from the DF. This indicates that DOP’ s molecular spacing effect could primarily serve to suppress triplet-exciton quenching. In comparison, blending the smaller phthalate plasticizer DMP into PDKCE films results in a maximum PLQY increase of only 16% relative to the neat polymer (FIG. 5C). This illustrates the importance of molecular size in this phenomenon. Moreover, the enhancement in emission efficiency can remain stable over time, suggesting a stable morphology of the plasticizer molecules in the PDKCE matrix.

[0049] It was hypothesized that the plasticizers reduce the aggregation of TADF moieties, thereby suppressing concentration quenching of triplet excitons. To verify this, the PLQY of PDKCE in solution using chlorobenzene as the solvent was measured. As shown in Fig. 5D, concentration quenching is evident from the decrease in the PLQY at the higher concentration of PDKCE, starting from a near-unity PLQY at 0.08 mg / mL. With the addition of DOP at a weight ratio of 75% to PDKCE, the PLQY exhibited much less decrease with the increase of the PDKCE concentration. This confirms the DOP’s effect in suppressing the concentration quenching. In comparison, using the smaller-molecule plasticizer DMP led to a weaker suppression effect, which agrees with the trend of the thin-film PLQY testing (FIG. 5E).

[0050] To validate that this effect is mainly on the delayed fluorescence from triplet excitons, the effect of these plasticizers was further tested on Super Yellow (SY), a fluorescent polymer with little triplet emission. In both solid-state thin films and chlorobenzene solutionsPATENT Atty. Dkt. No. 507814.5000516 of different SY concentrations (FIG. 5F), the incorporation of DOP and DMP had no measurable impacts on the fluorescent polymer’s PLQY in all testing conditions. Therefore, this further supports that the primary effect of the phthalate plasticizers is to suppress the concentration quenching of triplet emissions.

[0051] The impact of plasticizer incorporation on the EL performance of TADF polymers in a standard OLED device structure (FIG. 6) was also investigated. As shown in FIG. 7, the incorporation of DOP in PDKCE EML films has little influence on the current density-voltage (J- V) characteristics compared to the pristine polymer, indicating that charge transport remains largely unaffected by the presence of the additive. At the same time, the luminance increases substantially at higher DOP concentrations. As a result, the maximum EQE increased from 9.3% for the neat PDKCE to 12.6% for the EML with DOP in the weight ratio of 75% to PDKCE, which marks an increase by 35.5% (FIG. 8). The electroluminescence spectra remain invariant across plasticizer weight ratios (see inset of FIG. 8), confirming that the emissive states and energy transfer mechanisms are unaltered by the incorporation of DOP (Supplementary Fig. 9). Collectively, these results underscore that spatial dilution via phthalate additives can enhance luminance efficiencies without compromising charge transport or photophysical features.

[0052] Enhancement of mechanical stretchability

[0053] Next, the effect of DOP on the mechanical properties of PDKCE thin films was investigated. As shown in FIG. 9 A, the addition of DOP reduces the Young’s modulus substantially, from 447 MPa to 13.2 MPa at the highest DOP 75% weight ratio to PDKCE. Concurrently, the stretchability improves dramatically, with the crack-onset strain (COS) increasing from 5% to 110%. A similar improvement in COS is observed with DMP, indicating that both plasticizers can enhance mechanical compliance despite their differences in molecular size (FIG. 9B). These effects are likely to stem from the typical plasticizing behavior of phthalates, which increases polymer chain dynamics by weakening interchain packing. This can be observed through rheological measurements of PDKCE solutions in chlorobenzene. Upon the addition of DOP with an increased weight ratio (FIG. 10), the gradual decrease of the viscosity at the low shear rate is observed, which indicates the decreased interactions between the polymer chains. Grazing-incidence wide-angle X-ray scattering (GIWAXS) shows that both pristine and DOP-plasticized PDKCE films are in the amorphous state.PATENT Atty. Dkt. No. 507814.5000516

[0054] During stretching to 100% strain, the PLQY of DOP-PDKCE (75%) thin films remains stable at a near-unity level, as shown in FIG. 11 A, demonstrating that the emission properties are maintained under mechanical deformation. Consistently, PL decay measurements under applied strain reveal that exciton dynamics remain unchanged, as shown in FIG. 11B.

[0055] The DOP-plasticized EML was further used to fabricate fully stretchable OLED devices. The device structure, which is shown schematically in FIG. 2, comprises a silver nanowire (AgNW) anode, a stretchable PEDOT: PSS_PFI hole transport layer (HTL), a DOP: PDKCE (75 wt%) EML, a PEIE_PFN-Br electron transport layer (ETL), and a liquid metal eutectic gallium-indium (EGain) cathode. The device exhibits a low turn-on voltage of 3.5 V (FIG. 12A), making it compatible with commercial battery operation, and emits light only from the AgNW side, aligning with typical display requirements. At the initial state, the EQEmax is ca. 3.0% (FIG. 13). During stretching to 60% strain, the devices exhibited stable luminance and current density (FIG. 14). The EQEmax only had a less than 15% decrease (FIG.15). The ultimate failure of the devices at strains higher than 60% is primarily due to electrical shorting between the AgNW and EGain electrodes.

[0056] To show the benefit of DOP to the performance of fully stretchable OLED devices, comparisons were further carried out with another group of devices made with neat PDKCE EMLs. At the initial state, the current density is 58% lower than that of devices with PDKCE: DOP emitters at 6 V, while the luminance is 32% lower (FIG. 12B). The resulting EQEmax is only about 1.5%, which is half of the PDKCE: DOP devices. During the stretching process, the decrease in the EL performance is much more pronounced (FIGS. 13-15). Further expanding the EL performance comparison to three additional DOP ratios — 12.5%, 25%, and 50% — revealed a clear increasing trend in both luminance and EQE with higher DOP content (FIG. 16). This comparison unequivocally shows the dual effects of DOP in enhancing both the EL performance and stretchability of fully stretchable OLED devices.

[0057] Broad applicability to different TADF polymers

[0058] To assess the broad applicability of the plasticizer-induced enhancements in both luminescence performance and stretchability, DOP incorporation was applied to four additional TADF polymers: PTLA, PTT, PDKCM, and PDKCP (see FIGS. 4B-4E). For all tested TADF polymer thin films, PLQY increases with increasing concentrations of incorporated DOP. At the highest tested DOP amount of 75 wt%, the PLQY reaches 146-183% of the neat polymerPATENT Atty. Dkt. No. 507814.5000516 values, indicating substantial enhancement (FIGS. 17A-17D). When DMP is used as the plasticizer, the PLQY increases to a lesser extent, further validating the molecular size effect underlying this phenomenon in these polymers. In parallel, the films at this highest DOP weight ratio (75 wt%) also exhibit much higher stretchability compared to the neat polymer films, as evidenced by the substantially decreased crack sizes and densities at 50% or 75% strain, as observed using optical microscopy. These results demonstrate the broad applicability of DOP in simultaneously enhancing both luminescence performance and mechanical stretchability of diverse TADF polymer systems.

[0059] Experimental Details

[0060] Materials

[0061] Lithium fluoride (LiF) was purchased from Xi’an Polymer Light Technology. Poly[(9,9-bis(3'- ((N, N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PEN- Br) was purchased from 1 -Material. Polyethylenimine, 80% ethoxylated solution (PEIE), and Nafion perfluorinated resin (PEI) solution (527084-25 ml, 5 wt% in a mixture of lower aliphatic alcohols and water) were purchased from Sigma- Aldrich and used without changing the concentration, unless mentioned. PEDOT: PSS solution (Clevios P VP CH 8000, 2.4-3.0 wt% in water) was purchased from Heraeus Deutschland and used without changing the concentration, unless mentioned. SEBS Hl 221 with a volume fraction of poly(ethylene-co-butylene) = 88% was provided by Asahi Kasei. Dow Coming SYLGARD 184 silicone elastomer clear kit was used to prepare polydimethylsiloxane (PDMS) with different base / crosslinker mix ratios. Thermoplastic polyurethane (TPU; Elastollan 1185 A) was provided by BASF. AgNWs (AW045) as suspensions in water were purchased from Zhejiang Kechuang Advanced Materials. Gallium (Ga) and indium (In) for liquid metal were purchased from Amazon. Patterned indium tin oxide (ITO) glass substrates were purchased from Ossila. SY (Super Yellow) and RED (Red Light-Emitting Spiro- Copolymer; Sigma-Aldrich, Product No. 900447).) were purchased from Sigma-Aldrich. Other general reagents and solvents were purchased from Sigma-Aldrich or Fisher Scientific. PDKCE (1), PDKCM (2), PDKCP (2), PTLA P(TMP-l-AcBPAc) (3) and PTT (PNB-TAc-TRZ-5) (4) were synthesized according to reported procedures.

[0062] General characterization

[0063] Absorption of plasticizers was measured using a Shimadzu UV-3600 Plus ultraviolet-visible- near-infrared spectrophotometer. The PL spectra were measured with aPATENT Atty. Dkt. No. 507814.5000516 Horiba spectrofluorometer Fluorolog 3 instrument. Time-resolved emission experiments were performed on samples loaded in a nitrogen cryostat that was evacuated. Time-resolved emission was performed by exciting the sample with ultraviolet pulses prepared by directing the output of a 35-fs Ti: sapphire laser into an optical parametric amplifier, with emission monitored using a streak camera. The PLQY of thin film was measured with an LSM Series high-power light-emitting diode (310 nm, Ocean Optics) as the light source and a fiber integration sphere (FOIS-1) coupled with a QE Pro spectrometer (Ocean Optics) as the spectrometer. The samples were held on a home-made stage to enable the light source to excite the samples, and the emitted light was collected with the integration sphere. The PLQY of the solution was measured under nitrogen (air-free) condition using an FLS1000 spectrometer equipped with an integrating sphere (F-M101), with the solution placed in a quartz cuvette and excited at a 310 nm pulsed Xenon lamp. Static GIWAXS measurements were performed at beamline 11-BM of National Synchrotron Light Source II, Brookhaven National Laboratory. The beam energy is 10 keV. The samples were tilted at an incident angle of 0.14° with respect to the incident beam and exposed for 10 seconds for each scan. The samples were placed under helium protection.

[0064] Surface treatment for substrates ( Si wafer, glass or quartz)

[0065] Hexadecyltrimethoxysilane (HTS)-functionalized substrates were prepared as follows. A pre- cleaned substrate was treated with O2 plasma (150 W) for 2 min and then placed in a vacuum oven at room temperature, where it was exposed to HTS vapor overnight to allow surface functionalization. The substrate was then sonicated in toluene for 5 min, followed by thorough cleaning and drying before use.

[0066] Characterization of stretchable TADF polymer thin films

[0067] The TADE polymer was dissolved in chlorobenzene (CB) solution at 60 °C overnight to ensure complete dissolution. A predetermined solid state weight ratio of plasticizer was then introduced into the polymer solution, followed by vigorous vortex mixing to homogenize the blend. The thin films were prepared through the spin coating of the polymer solutions in CB with a concentration of 8 mg / ml, on HTS-modified Si substrates (HTS-Si) at 2000 rpm, followed by annealing at 110 °C for 20 min. This leads to film thicknesses of around 45-100 nm, depending on the plasticizer blending ratio. The films were transferred to PDMS stamps to apply different strains and thentransferred to SEBS-coated Si substrates for characterization. The other parameters for making the films are the same as our stretchablePATENT Atty. Dkt. No. 507814.5000516 polymers. For the time-resolved emission measurement, the films were transferred to SEBS-coated quartz substrates.

[0068] Grazing-Incidence Wide-Angle X-Ray Scattering

[0069] Samples for GIWAXS measurements were prepared on plasma-treated Si substrates by spin coating. Static GIWAXS measurements were performed at beamline 11-BM of National Synchrotron Light Source II, Brookhaven National Laboratory. The beam energy is 10 keV. The samples were tilted at an incident angle of 0.14° with respect to the incident beam and exposed for 10 seconds for each scan. The samples were placed under helium protection.

[0070] Fabrication and testing of conventional rigid OLEDs

[0071] The ITO-coated glasses were first cleaned with 1 vol% Hellmanex solution, isopropyl alcohol (IPA), and deionized water, and then dried and further treated with 02 plasma (150 W, 10 min). The PEDOT: PSS_PEI mixture solution in IPA (PEDOT: PSS: PFI = 1:1:3 (v:v:v)) was spin-coated at 4500 rpm for 1 min and then annealed at 130 °C for 30 min as the hole injection layer / hole transporting layer. Then, TADF EMLs were spin-coated in a glovebox from the polymer solutions of 8 mg / ml in CB. The spin-coating condition is 2000 rpm for 30 s, which is followed by annealing at 110 °C for 10 min. Next, thermal evaporations were carried out to sequentially deposit TPBi as the electron transporting layer, LiF as the electron injection layer and Al as the cathode, with the corresponding deposition rates of 1.0, 0.1, and 10.0 A / s, respectively. The I-V-L measurements were carried out at room temperature in a nitrogen-filled glovebox. A Keithley 2450 source meter and fiber integration sphere (FOIS-1) coupled with a QE Pro spectrometer (Ocean Optics) were used for the measurements. The OLED devices were tested on top of the integrating sphere, where only forward light emission can be collected. The absolute OLED emission was calibrated by a standard visible-nearinfrared light source (HL-3P-INT-CAL plus, Ocean Optics).

[0072] Fabrication of stretchable semitransparent electrodes

[0073] AgNW electrode was fabricated as follows: AgNW solution (AW045, 1 wt% in water) was first vortexed for 10 min to remove big aggregations and then diluted in IPA ( 1: 19 by volume), followed by 30 s bath sonication. Next, the solution was spray-coated on a cleaned OTS-Si substrate at 130 °C until the target sheet resistance of around 20 Q / sq was reached. The pattern was made with Kapton tape masks. After removing the masks, the AgNW on OTS was washed with deionized water to remove the surfactant, and then dried at 110 °C for 5 min. Next, a TPU solution (20 mg / ml in tetrahydrofuran (THF)) was spin-coated onto AgNW at 3,000 rpmPATENT Atty. Dkt. No. 507814.5000516 for 30 s. After drying at 110 °C for 20 min, AgNW covered by TPU was treated with 02 plasma (150 W) for 1 min. Then, the PDMS mixture (base:crosslinker ratio of 15:1) was drop-casted onto TPU and subsequently degassed under a vacuum and cured at 80 °C for 4 h. After the delamination from the OTS-Si substrate, the AgNW_TPU / PDMS electrode was ready to be used for the fabrication of fully stretchable OLEDs.

[0074] Fabrication of stretchable cathodes based on liquid metal

[0075] Gain liquid metal (LM) electrodes were fabricated as follows. LM was first dispersed in a methyl isobutyl ketone (MIBK) solution (1 g / 10 mL) and sonicated at 50% power for 15 s to break up large aggregates. The dispersion was then centrifuged at 3,000g for 5 min, after which the supernatant was discarded, and the LM was washed twice with isopropanol (IPA) to remove residual MIBK. The resulting LM dispersion was diluted in 20-30 mL of IPA and spray-coated onto a glass substrate covered with Teflon tape at 130 °C until the target sheet resistance of approximately 0.5 Q / sq was reached. The patterning was defined using Kapton tape masks, which were removed after deposition. The LM electrodes were then dried at 110 °C for 5 min before being encapsulated. A PDMS mixture (base:crosslinker ratio of 15:1) was drop-cast onto the LM electrodes and subsequently degassed under vacuum before curing at 80 °C for 4 h. After delamination from the Teflon tape substrate, the LM / PDMS electrode was ready for use in the fabrication of fully stretchable OLEDs.

[0076] Fabrication of fully stretchable OLED devices

[0077] The separately prepared AgNW_TPU / PDMS electrode was laminated onto an OTS-treated glass substrate (OTS-glass) with the AgNW side facing up and treated with O2 plasma (150 W) for 10 s to improve wettability. The PEDOT: PSS_PEI mixture solution in IPA (PEDOT: PSS: PFI = 1:1:3 (v:v:v)) was spin-coated at 3,000 rpm for 1 min and annealed at 130 °C for 30 min, forming a 110 nm film. A PDKCE solution (8 mg / ml in CB) blended with different plasticizer concentrationswas then spin-coated at 2000 rpm for 30 s and annealed at 110 °C for 10 min, yielding a 50-100 nm emissive layer. Next, a PEIE_PEN-Br solution (total 0.25 wt% in methanol at a weight ratio of 1:1) was spin-coated at 3000 rpm for 30 s and annealed at 110 °C for 10 min, forming a 20 nm electron injection / transporting layer. The samples were then released from the Teflon-glass substrate. A separately prepared LM / PDMS electrode was gently laminated on top as the cathode. Finally, the device was heated at 80 °C for 5 min to enhance interlayer adhesion. The performed measurements were the same as those for the rigid devices.PATENT Atty. Dkt. No. 507814.5000516

[0078] This disclosure also includes the following aspects:

[0079] A first aspect relates to an emissive layer for a stretchable organic light emitting diode (OLED), the emissive layer comprising a plasticized film including: a thermally activated delayed fluorescence (TADF) polymer; and a plasticizer distributed within the TADF polymer.

[0080] A second aspect relates to the emissive layer of the first aspect, wherein the TADF polymer is selected from the group consisting of: PDKCE, PDKCM, PDKCP, PTEA, and PTT.

[0081] A third aspect relates to the emissive layer of the first or second aspect, wherein the plasticizer is a small-molecule plasticizer having a molecular weight less than 1000 g / mol.

[0082] A fourth aspect relates to the emissive layer of any of the first through the third aspects, wherein the plasticizer comprises a phthalate, adipate, citrate, and / or bio-based alternative.

[0083] A fifth aspect relates to the emissive layer of any of the first through the fourth aspects, wherein the plasticizer is selected from the group consisting of dioctyl phthalate (DOP), dimethyl phthalate (DMP), diisononyl phthalate (DINP), diisodecyl phthalate (DIDP), ditridecyl phthalate (DTDP), di(2-propylheptyl) phthalate (DPHP), and butyl decyl phthalate (BDP).

[0084] A sixth aspect relates to the emissive layer of any of the first through the fifth aspects, wherein the plasticizer has a molecular weight of at least 250 g / mol, at least 300 g / mol, or at least 350 g / mol.

[0085] A seventh aspect relates to the emissive layer of any of the first through the sixth aspects, wherein the plasticizer has a molecular weight as high as 600 g / mol, as high as 500 g / mol, or as high as 400 g / mol.

[0086] An eighth aspect relates to the emissive layer of any of the first through the seventh aspects, wherein a weight ratio of the TADF polymer to the plasticizer is in a range from 4:3 to 8:1.

[0087] A ninth aspect relates to the emissive layer of any of the first through the eighth aspects, wherein a weight percentage of the plasticizer in the plasticized film is at least 12.5 wt.%, at least 25 wt.%, at least 50 wt.%, and / or at least 75 wt.%.

[0088] A tenth aspect relates to the emissive layer of any of the first through the ninth aspects, wherein a weight percentage of the plasticizer in the plasticized film is no greater than 85 wt.%, no greater than 80 wt.%, or no greater than 75 wt.%.PATENT Atty. Dkt. No. 507814.5000516

[0089] An eleventh aspect relates to the emissive layer of any of the first through the tenth aspects, wherein the plasticized film has an amorphous structure.

[0090] A twelfth aspect relates to the emissive layer of any of the first through the thirteenth aspects, wherein the plasticized film has a thickness in a range from 40 nm to 110 nm.

[0091] A thirteenth aspect relates to the emissive layer of any of the first through the fourteenth aspects, wherein the plasticized film has a reduced density of triplet excitons compared to that of a TADF polymer film without the plasticizer.

[0092] A fourteenth aspect relates to the emissive layer of any of the first through the fifteenth aspects, wherein a stretchability of the plasticized film as represented by crack-onset strain is at least 100%, or at least 110%, and / or up to 150%.

[0093] A fifteenth aspect relates to the emissive layer of any of the first through the fourteenth aspects, exhibiting a photoluminescence quantum yield (PLQY) of at least 90%, at least 95%, or at least 97%, and / or up to 99%.

[0094] A sixteenth aspect relates to the emissive layer of any of the first through the fifteenth aspects, wherein the PLQY is at least 90% at least 95%, or at least 97%, and / or up to 99%, during stretching of the plasticized layer to a crack-onset strain of 100%.

[0095] A seventeenth aspect relates to a stretchable organic light emitting diode (OLED) comprising the emissive layer of any of the first through the sixteenth aspects, wherein, after stretching the OLED to 60% strain, a maximum external quantum efficiency (EQE) is at least 85%, at least 90%, or at least 95% of the maximum EQE at 0% strain.

[0096] An eighteenth aspect relates to the stretchable OLED of the seventeenth aspect, wherein, after stretching the OLED to 60% strain, luminance is at least 85%, at least 90%, or at least 95% of the luminance at 0% strain.

[0097] A nineteenth aspect relates to the stretchable OLED of the seventeenth or eighteenth aspect, wherein, after stretching the OLED to 60% strain, current density is at least 85%, at least 90%, or at least 95% of the current density at 0% strain.

[0098] A twentieth aspect relates to a method of making an emissive layer for an organic light emitting diode (OLED), the method comprising: co-dis solving a plasticizer and a thermally activated delayed fluorescence (TADF) polymer at a predetermined ratio in a solvent to form a precursor mixture; depositing the precursor mixture onto a substrate to form a coating; and removing the solvent from the coating to form a plasticized film comprising the TADF polymer and the plasticizer distributed therein.PATENT Atty. Dkt. No. 507814.5000516

[0099] A twenty-first aspect relates to the method of the twentieth aspect, wherein depositing the precursor mixture comprises spin coating, dip coating or roll coating.

[0100] A twenty-second aspect relates to the method of the twentieth or twenty-first aspect, wherein the predetermined ratio of the TADF polymer to the plasticizer is in a range from 20: 17 to 8:1.

[0101] A twenty-third aspect relates to the method of any of the twentieth through the twenty-second aspects, wherein the predetermined ratio of the TADF polymer to the plasticizer is in a range from 4:3 to 2:1.

[0102] A twenty-fourth aspect relates to the method of any of the twentieth through the twenty-third aspects, wherein a thickness of the plasticized film is in a range from 40 nm to 110 nm.

[0103] A twenty-fifth aspect relates to the method of any of the twentieth through the twenty-fourth aspects, wherein the substrate comprises a hole transport layer of a stretchable OLED.

[0104] To clarify the use of and to hereby provide notice to the public, the phrases "at least one of , ,... and < N>" or "at least one of , ,... or < N>" or "at least one of , ,... < N>, or combinations thereof" or ", ,... and / or < N>" are defined by the Applicant in the broadest sense, superseding any other implied definitions hereinbefore or hereinafter unless expressly asserted by the Applicant to the contrary, to mean one or more elements selected from the group comprising A, B,... and N. In other words, the phrases mean any combination of one or more of the elements A, B,... or N including any one element alone or the one element in combination with one or more of the other elements which may also include, in combination, additional elements not listed. Unless otherwise indicated or the context suggests otherwise, as used herein, "a" or "an" means "at least one" or "one or more."

[0105] While various embodiments have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible. Accordingly, the embodiments described herein are examples, not the only possible embodiments and implementations.

[0106] In addition to the features mentioned in each of the independent aspects enumerated above, some examples may show, alone or in combination, the optional features mentioned in the dependent aspects and / or as disclosed in the description above and shown in the figures.

Claims

PATENT Atty. Dkt. No. 507814.5000516 CLAIMSWhat is claimed is:

1. An emissive layer for a stretchable organic light emitting diode (OLED), the emissive layer comprising:a plasticized film comprising:a thermally activated delayed fluorescence (TADF) polymer; and a plasticizer distributed within the TADF polymer.

2. The emissive layer of claim 1, wherein the TADF polymer is selected from the group consisting of: PDKCE, PDKCM, PDKCP, PTEA, and PTT.

3. The emissive layer of claim 1, wherein the plasticizer is a small-molecule plasticizer having a molecular weight less than 1000 g / mol.

4. The emissive layer of claim 1, wherein the plasticizer comprises a phthalate, adipate, citrate, and / or bio-based alternative.

5. The emissive layer of claim 1, wherein the plasticizer is selected from the group consisting of dioctyl phthalate (DOP), dimethyl phthalate (DMP), diisononyl phthalate (DINP), diisodecyl phthalate (DIDP), ditridecyl phthalate (DTDP), di(2-propylheptyl) phthalate (DPHP), and butyl decyl phthalate (BDP).

6. The emissive layer of claim 1, wherein the plasticizer has a molecular weight of at least 250 g / mol.

7. The emissive layer of claim 1, wherein the plasticizer has a molecular weight as high as 600 g / mol.

8. The emissive layer of claim 1, wherein a weight ratio of the TADF polymer to the plasticizer is in a range from 4:3 to 8:1.PATENT Atty. Dkt. No. 507814.50005169. The emissive layer of claim 1, wherein a weight percentage of the plasticizer in the plasticized film is at least 12.5 wt.%.

10. The emissive layer of claim 1, wherein a weight percentage of the plasticizer in the plasticized film is no greater than 85 wt.%.

11. The emissive layer of claim 1, wherein the plasticized film has an amorphous structure.

12. The emissive layer of claim 1, wherein the plasticized film has a thickness in a range from 40 nm to 110 nm.

13. The emissive layer of claim 1, wherein the plasticized film has a reduced volumetric concentration of triplet excitons compared to that of a TADF polymer film without the plasticizer.

14. The emissive layer of claim 1, wherein a stretchability of the plasticized film as represented by crack-onset strain is at least 100%.

15. The emissive layer of claim 1 exhibiting a photoluminescence quantum yield (PLQY) of at least 90%.

16. The emissive layer of claim 1, wherein the PLQY is at least 90% during stretching of the plasticized layer to a crack-onset strain of 100%.

17. A stretchable organic light emitting diode (OLED) comprising:the emissive layer of claim 1; andwherein, after stretching the OLED to 60% strain, a maximum external quantum efficiency (EQE) is at least 85% of the maximum EQE at 0% strain.PATENT Atty. Dkt. No. 507814.5000516 18. The stretchable OLED of claim 17, wherein, after stretching the OLED to 60% strain, luminance is at least 85% of the luminance at 0% strain.

19. The stretchable OLED of claim 17, wherein, after stretching the OLED to 60% strain, current density is at least 85% of the current density at 0% strain.

20. A method of making an emissive layer for a stretchable organic light emitting diode (OLED), the method comprising:co-dissolving a plasticizer and a thermally activated delayed fluorescence (TADF) polymer at a predetermined ratio in a solvent to form a precursor mixture;depositing the precursor mixture onto a substrate to form a coating; and removing the solvent from the coating to form a plasticized film comprising the TADF polymer and the plasticizer distributed in the TADF polymer.

21. The method of claim 20, wherein depositing the precursor mixture comprises spin coating, dip coating or roll coating.

22. The method of claim 20, wherein the predetermined ratio of the TADF polymer to the plasticizer is in a range from 20:17 to 8:1.

23. The method of claim 20, wherein the predetermined ratio of the TADF polymer to the plasticizer is in a range from 4:3 to 2:1.

24. The method of claim 20, wherein a thickness of the plasticized film is in a range from 40 nm to 110 nm.

25. The method of claim 20, wherein the substrate comprises a hole transport layer of a stretchable OLED.

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