
==== Front
Nat Commun
Nat Commun
Nature Communications
2041-1723
Nature Publishing Group UK London

39266527
50358
10.1038/s41467-024-50358-1
Article
Intrinsically stretchable fully π-conjugated polymer film via fluid conjugated molecular external-plasticizing for flexible light-emitting diodes
Zhuo Zhiqiang 1
Ni Mingjian 2
Yu Ningning 1
Zheng Yingying 1
Lin Yingru 1
Yang Jing 1
Sun Lili 3
Wang Lizhi 1
Bai Lubing 1
Chen Wenyu 1
Xu Man 4
http://orcid.org/0000-0002-5318-4267
Huo Fengwei 1
http://orcid.org/0000-0002-2607-3633
Lin Jinyi iamjylin@njtech.edu.cn

1
http://orcid.org/0000-0001-6608-5068
Feng Quanyou 4
http://orcid.org/0000-0001-7004-6408
Huang Wei vc@nwpu.edu.cn

1234
1 https://ror.org/03sd35x91 grid.412022.7 0000 0000 9389 5210 Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Nanjing Tech University (NanjingTech), Nanjing, China
2 https://ror.org/00mcjh785 grid.12955.3a 0000 0001 2264 7233 The Institute of Flexible Electronics (IFE, Future Technologies), Xiamen University, Xiamen, China
3 grid.12981.33 0000 0001 2360 039X School of Flexible Electronics (SoFE) & State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen University, Shenzhen, China
4 https://ror.org/043bpky34 grid.453246.2 0000 0004 0369 3615 State Key Laboratory of Organic Electronics and Information Displays & Institute of Advanced Materials (IAM), Nanjing University of Posts & Telecommunications, Nanjing, China
12 9 2024
12 9 2024
2024
15 799013 11 2023
8 7 2024
© The Author(s) 2024
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.
Fully π-conjugated polymers with rigid aromatic units are promising for flexible optoelectronic devices, but their inherent brittleness poses a challenge for achieving high-performance, intrinsically stretchable fully π-conjugated polymer. Here, we are establishing an external-plasticizing strategy using semiconductor fluid plasticizers (Z1 and Z2) to enhance the optoelectronic, morphological, and stretchable properties of fully π-conjugated polymer films for flexible light-emitting diodes. The synergistic effect of hierarchical structure and optoelectronic properties of Z1 in poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) films enable excellent stretchable deformability (~25%) and good conductivity. PLEDs based on F8BT/Z1 films show stable electroluminescence and efficiency under 15% stretch and 100 cycles at 10% strain, revealing outstanding stress tolerance. This strategy is also improving the stretchable properties of polymers like poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) and poly(2-methoxy-5(2′-ethyl)hexoxy-phenylenevinylene) (Super Yellow), demonstrating its general applicability. Therefore, this strategy can provide effective guidance for designing high-performance stretchable fully π-conjugated polymers films for flexible electronic devices.

The realization of intrinsically stretchable fully π-conjugated polymer film is challenging due to their inherent brittle nature. Here, authors apply semiconductor fluid molecular plasticizers to improve stretchable deformability, achieving high performance flexible polymer light-emitting diodes.

Subject terms

Organic LEDs
Polymers
https://doi.org/10.13039/501100001809 National Natural Science Foundation of China (National Science Foundation of China) 62288102 Huang Wei issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

Emerging flexible electronic devices, as a promising complement and alternative to the traditional rigid electronic components, provided the foundation for developing the next generation of interactive applications due to their unique ultrathin, transparency, lightweight, ease of integration, and compatibility1–8. In general, strong internal stress is easily appeared in the multi-layer’s flexible electronic device under operation deformation, such as bending, stretching and compressing, which may result into intralayer fracture and interlayer separation and further cause device instability and failure9–11. Then, the excellent deformation stability of flexible electronic devices is a prerequisite to ensure long-term operation for their realization of the practical optoelectronic application. Being a crucial element of semiconductor devices, the mechanical property of the active layer is essential to enhance the stress tolerance capacity of flexible electronic devices. As a typical electronic element, fully π-conjugated polymers (FπCPs) present possible and potential viscoelasticity to dissipate internal stress and energy upon the degrees of device deformation, which makes them promising candidates for flexible optoelectronic applications12. However, traditional FπCPs consisted of rigid π-conjugated aromatic structures, which often exhibit a tendency to self-assemble into brittle crystalline and ordered structure13. This leads to deformation of the amorphous phase and fracture of the crystalline phase during deformation, which in turn results in the generation of macroscopic cracks as well as degradation of device performance and stability14. Establishing an effective strategy for manufacturing intrinsically flexible FπCPs nano-layers with excellent optoelectronic properties and stress tolerance capacity simultaneously is essential for flexible electronic devices.

Up to date, two typical strategies are proposed to design and prepare intrinsically flexible FπCPs nano-layers, such as chemical and physical method15,16. Chemical molecular design is defined as the introduction of flexible segments, dynamic bonds and side chains to reduce the large-scale crystalline ability and chain softness of FπCPs, providing energy dissipation centers for nano-layer during deformation processing17–22. However, molecular design often accompanies complex synthesis steps, and its performance is influenced by variations between batches. introducing too many flexible chain segments or supramolecular aggregate leads to a decrease in optoelectronic properties. Later approach involves a physical method to introduce a second elastomer/plasticizer additive23–28. FπCPs chain backbones have the potential to form nanostructures in elastomers or plasticizer blending system, creating a nanoconfinement effect that significantly enhances their stretchability, without compromising or even improving charge carrier mobility12,22,29. However, given the multitude of additive types available, the challenge lies in selecting a specific elastomer and plasticizer that enable the FπCPs to possess both conductivity and stretchability. As the crystalline domains are damaged during the straining process, the interchain charge hopping is hindered, which is macroscopically manifested by the generation of thin film cracks and the degradation of electrical properties14,30. Meanwhile, consideration of the application in flat flexible polymer light-emitting diodes (PLEDs), the insulated solid-state additives, both elastomer and plasticizer, may result in a serious phase separation to cause non-uniform emission and unstable film morphology, disrupt interchain charge transportation to result into low film conductivity, poor color purity and device efficiency. Then, rational molecular designs of molecular additives (both elastomer and plasticizer) for the preparation of intrinsically stretchable light-emitting FπCPs film toward the flexible PLEDs are essential, and inevitably, the hierarchical structure in the condensed phases that govern these mechanical properties, morphological evolution and optoelectronic processes need to be taken into consideration31.

In this work, to maintain charge transport capabilities while achieving a certain degree of mechanical stretchability, we design two semiconductor fluid molecular additives (Fig. 1a, Z1, Z2) with two key characteristics: i). As extrinsic additives, they have low glass transition temperatures and low volatility to increase the probability of polymer chain mobility. ii). Molecules possess π-conjugated structures that may interact with the FπCPs chains, facilitating interchain charge hopping. Since F8BT is widely used in PLEDs and organic solar cells due to its high electron mobility and outstanding emission behavior32, we select F8BT as model FπCP (Fig. 1b) to verify the feasibility of our designed plasticizer. We find that with the introduction of the appropriate conjugated molecular additive Z1, both the stretchability and the electrical properties of the blended films are enhanced, which we attribute to the decrease in crystallinity of the blended films. At the same time, plasticizer Z1 improves the stretchable properties of various typical FπCPs, thus demonstrating the general applicability of plasticizers.Fig. 1 Design principles of semiconductor fluid conjugated molecular plasticizers for flexible electronics.

a Model fluid conjugated molecular plasticizers, named Z1 and Z2, was obtained by adjusting the ratio of the alkyl chains around the “fluorene core”. b For typical F8BT usually have high carrier mobility and brittle properties. In this work, we obtain semiconductor FπCPs films with excellent optoelectronic and stretchable properties by blending semiconductor fluid conjugated additives with typical FπCPs. c Schematic of photoluminescence of a patterned light-emitting polymer film on a finger under deformation conditions.

Results

Optical property of fluid conjugated molecular plasticizer

In order to obtain conjugated molecular additives, we first need to select an appropriate molecular conjugated backbone structure and side-substitution segments. To this end, we designed and synthesized two fluorene-based compounds (Z1, Z2) with distinct alkyl chain lengths. By means of high-resolution mass spectrometry and NMR analysis, there is no residual solvent molecule in the Z1 and Z2 (Supplementary Fig. 1–8). As depicted in Fig. 2a, b, both Z1 and Z2 exhibited intense deep-blue emission under ultraviolet light excitation (365 nm). Both Z1 and Z2 present a viscous fluid state at room temperature (298 K), but Z1 exhibits a larger viscosity than Z2 (Fig. 1a), and Z1 and Z2 have high decomposition temperatures, (Td, Z1 = 384.2 °C, Td, Z2 = 399.5 °C, Supplementary Fig. 9a). From the DSC analysis of Z1 and Z2, we can observe that the glass transition temperature of Z1 is 12.6 °C and that of Z2 is −14.8 °C, which are both below room temperature (298 K), also confirmed their viscous fluid state at room temperature, Supplementary Fig. 9b. And corresponding absorption and emission properties are also shown in Supplementary Table 1. From the Fig. 2a, b and Supplementary Table 1, we can see that the 0-0 vibronic transitions of Z2 compound in dilute solution is 365 nm, and the 0-0 vibronic transitions in thin film is 366 nm, which is only red-shifted by 1 nm, suggested that intermolecular interactions of the fluorene core in Z2 are effectively suppressed under extremely concentrated states, and the optical features manifested in the dilute toluene solution are retained in the solid thin film. Conversely, for Z1, due to the shorter alkyl chains connected around the fluorene core, the luminescent characteristics observed in the dilute solution cannot be reproduced in the “thin-film” state. Therefore, through the photophysical behavior of the molecules in different states, we can deduce that the volume of the Z2 molecule is much larger than that of Z1 due to the linkage of the long alkyl chains around Z231. Interestingly, the photoluminescence quantum yield (PLQY) shows an increasing trend as the doping ratio of conjugated molecular additives (Z1, Z2) increases, as shown in Fig. 2c, increasing from 33% to 49% and 41% for Z1, Z2, respectively. This is closely related to the introduction of Z1 and Z2, which will greatly inhibit interchain aggregation and further suppress the non-radiative transition rate of F8BT in blended films, leading to an increase in the photoluminescence quantum yield33.Fig. 2 Optical properties of F8BT blend films.

a, b Absorption and emission spectra of conjugated molecular plasticizer (Z1 and Z2) in spin-coated film and solution states. c PLQY of the Z1/F8BT or Z2/F8BT blend films increases with the addition ratio. d Spectral overlap between the emission of Z1 and the absorption of F8BT. e, f The absorption and emission spectra of the F8BT/Z1 films at different blending ratios. Source data are provided as a Source Data file.

As conjugated molecular additives with polycyclic aromatic hydrocarbons structures, in addition to considering their inherent optical properties, it is also crucial to assess their influence on the optical performance of FπCPs (F8BT) blend-films. The emission spectrum of Z1 (donor) and the absorption spectrum of F8BT in dilute solution overlap to a large extent (Fig. 2d). In addition, as shown in Fig. 2e, f, with an increasing proportion of the conjugated molecular additives Z1, a new emission peak emerges at 398 nm after blending with F8BT, attributed to the intrinsic emission peak of Z1. The above phenomena show that efficient energy transfer can occur between Z1 and F8BT. Besides, although there is a slight blue-shift in the PL spectrum, there are no significant changes in color purity observed from the CIE coordinates. However, for Z2, with the increasing blending ratio, the CIE coordinates of F8BT show a noticeable blue shift with different emission color purity, we attribute it to incomplete förster energy transfer (Supplementary Fig. 10–12). These observations confirmed the weak phase separation (F8BT interchain aggregation) in F8BT/ Z1 or Z2 blended films.

Intrinsically mechanical properties and morphology of F8BT films

Generally, plasticizers should be compatible with the rigid FπCPs to ensure efficient and uniform dispersion and avoid the obvious and large-scale phase separation, which are the preconditions to obtain the uniform and stable emission and color purity of PLEDs. In the case of external semiconductor molecular plasticizing, film morphology has a significant impact on the optoelectronic properties of the FπCPs stretchable nano-layer. Then, Atomic Force Microscopy (AFM) is explored here to investigate the morphology of the blend films, as shown in Fig. 3a and Supplementary Fig. 13. Clearly, as the Z1 ratio increases, the surface roughness of the blended film gradually increases with a relatively high Rq, reaching a maximum value of 5.26 nm (at 50 wt%Z1), associated with the enhancement of crystallinity at a high ratio of Z1 content. Additionally, for more visual observation of the morphology of the blend film, we characterized the blend film using scanning electron microscopy (SEM), as shown in Supplementary Fig. 14. With the increase in the content of Z1, the surface of the blend film gradually transitions from its initial smooth state to the appearance of “olive-shaped” nanostructures, which is most prominent at 50 wt%Z1. This is consistent with the discussion above regarding the increase in roughness due to the introduction of Z1 leading to increased crystallinity. In order to understand the effect of the introduction of fluid-conjugated molecular additives on the crystallinity of the blended films, we performed grazing-incidence wide-angle X-ray scattering (GIWAXS), as shown in Fig. 3b. The results show that the diffraction intensity of the blended films doped with 20 wt%Z1 content is significantly lower than that of the reference blends (F8BT and 50 wt%Z1), which provided the possibility of improving the stretchable properties. In addition, from the AFM phase images, there is no noticeable phase separation, suggesting the compatibility of Z1 and F8BT. On the contrary, the surface roughness of the films of the blends remains almost constant as the Z2 ratio increases (Supplementary Fig. 13). Moreover, from the height images, we can observe the emergence of distinct island-like structures in the blend film upon the introduction of Z2. The phase images reveal a slight phase separation between Z2 and F8BT. We speculate that it is due to the weak interaction force between Z2 and F8BT, which leads to their relatively poor compatibility compared to those of Z1. Additionally, the introduction of plasticizers tends to induce morphological changes in blended films over time. We characterized the time dependence of the morphology of blended films with two plasticizers using AFM. Fresh samples were prepared and left for 10 days to observe the morphological changes. As shown in Supplementary Fig. 15, we observed that for the Z1 blended film, there was no significant change in film morphology before and after aging. However, for the Z2 blended film, phase separation became more pronounced over time. Therefore, we concluded that the F8BT/Z1 blended membrane exhibits high stability, with the membrane morphology remaining unchanged over time compared to the F8BT/Z2 blended membrane.Fig. 3 Morphological analysis and mechanical properties of the stretchable F8BT blended nano-films.

a Height (top) and phase images (bottom) of F8BT/Z1 films with different Z1 contents obtained using AFM measurement. b GIWAXS images of blend films of F8BT, 20 wt%Z1, and 50 wt%Z1. Diffraction intensity along qz direction vs. q-values for blend films from GIWAXS measurements. c Crack-onset strain and modulus of stretchable F8BT nano-films with different Z1 contents. d Strain-stress curves of free-standing Z1/F8BT nano-films. Source data are provided as a Source Data file.

It is a common sense that these changes in crystallinity may affect the mechanical properties of the blended films. Therefore, we utilized two distinct characterization methods (Film on Elastomer and Free-standing film tests) to elucidate the impact of conjugated molecular additives on the mechanical properties of the F8BT blended nano-films. First, the crack onset strain and modulus were characterized through the “Film on elastomer” method (Supplementary Fig. 16–19). As shown in Fig. 3c, the crack onset strain and modulus of the films exhibit contrasting trends, whereas 20 wt%Z1 blend ones exhibited the highest crack onset strain (20%) and the lowest modulus (0.09 GPa). From the results exhibited by the data, the F8BT blended films exhibit the best stretchability when the plasticizing ratio of the conjugated molecular additives reaches 20 wt%, but as the blending ratio is further increased, the crack onset strain exhibits a significant decrease, and the modulus is slightly increased. Therefore, this may be due to the fact that the introduction of an appropriate amount of Z1 (20 wt%) may hinder the formation of large-scale crystalline domains in F8BT while maintaining short-range ordered structures. These short-range ordered nano-structures serve as energy dissipation centers during the deformation process, to achieve higher stretchability. With the further increase in the blended ratio of Z1, crystallinity continues to strengthen, leading to a reduction in stretchable behavior. On the contrary, as increasing the content of Z2, the flexibility of the blended film shows significant improvement, with the best stretchable properties achieved at 50 wt%Z2 (crack onset strain of 60% and modulus of 0.02 GPa), (Supplementary Fig. 18–19). Furthermore, we obtained the fracture elongation of the F8BT/Z1 blended film through strain-stress curves, as shown in Fig. 3d. The blended film with 20 wt%Z1 achieved a fracture elongation of 11.3%, whereas the pristine F8BT film only reached 6.4%, this represents nearly a twofold improvement in stretchability. Therefore, it is confirmed that the semiconductor fluid conjugated molecular additives external-plasticizing strategy can effectively enhance the stretchability of FπCPs in solid states.

F8BT blend films for the PLEDs

In high-mobility semi-crystalline FπCPs, the manner of charge transport (intra- and inter-chain) is a key factor in determining the film charge mobility and conductivity. However, the vast majority of the stretching strategies to date involve the introduction of insulating additives, both elastomers or plasticizers, into the FπCPs, which leads to the interchain charge transfer being hindered and will have a detrimental effect on the electrical performance of the flexible electronic devices. In this regard, the intrinsic electrical and semiconducting property of fluid conjugated molecule additives in the stretchable FπCPs blended nano-films can avoid these problems and even enhance the charge mobility and conductivity, which is good for the fabrication of high-performance and stable, flexible optoelectronic devices. Therefore, in order to investigate the effect of the introduction of conjugated molecular additives Z1, Z2 on the electrical properties of the stretchable blended films, we first investigated the hole current densities of the blended films. As shown in Fig. 4a and Supplementary Fig. 20, the hole current density gradually increases with the increase of Z1 content and remains higher than that of the pristine film even when the doping ratio exceeds 50 wt%. Moreover, as illustrated in Supplementary Fig. 20c, d, the hole mobility of the F8BT/Z1 blend film exceeds that of the pristine film by two orders of magnitude, demonstrating good electrical properties. Interestingly, the introduction of Z2 causes the hole current density of the blended films to remain essentially unchanged or even to decrease. We can infer that the interchain transport of carriers is hindered by the introduction of insulating alkyl chains around the Z2 fluorene core. We also prepared Z1-based organic light-emitting diode devices, shown in Fig. 4b, c, to verify the semiconducting properties of Z1. We observed that Z1 still provides a current density of 300 mA cm−2 and a brightness of 100 cd m−2 with a narrow-band ultra-deep blue emission, which suggests that Z1 is inherently charge-transport capacity. Therefore, we reasonably believe that Z1 may act as a “bridge” for carrier interchain transport in the blended films to promote the FπCPs interchain charge transportation, thereby greatly improving the carrier mobility of the blended films. These results indicate that our straightforward molecular design effectively alleviates the long-standing trade-off between electrical and mechanical properties.Fig. 4 Optoelectronic property of stretchable F8BT nano-films.

a Hole current density of F8BT/Z1 or F8BT/Z2 nano-films at 3 V. b Voltage-current density-luminance curves of conjugated molecular plasticizer Z1, (c) along with the electroluminescent spectrum corresponding to a current density of 100 mA cm−2. d Energy level diagram in PLEDs. e Current density-luminance-voltage characteristics of the PLEDs. f EQE/C.E.-current density curves of PLED based on pristine F8BT and F8BT/Z1 blended films. g EL spectra (at 100 mA cm−2) of corresponding PLEDs. h–k The variation of relative current density, luminance, EQE and current efficiency of the corresponding PLEDs based on F8BT blended films with different content of Z1/Z2. Error bars represent SD. Source data are provided as a Source Data file.

Subsequently, we also further fabricated PLEDs based on F8BT blend films, and corresponding device performances are provided in Fig. 4d–k. Firstly, the PLED based on F8BT and F8BT/Z1 (20 wt%) have similar maximum brightness of 6623 cd m−2 and 5314 cd m−2 with a maximum current efficiency (C. E.) of 1.9 and 2.5 cd A−1, indicated their comparable performance (Fig. 4e–g). Meanwhile, F8BT/Z1 (20 wt%)-based device showed a relatively lower Von (4.0 V) than those of F8BT (4.3 V), also confirming the excellent semiconducting property of F8BT/Z1 nanolayers. More interestingly, the current density and luminance of the devices based on the F8BT blend films show a decreasing trend as the doping ratio increases, but unlike the Z2/F8BT blended film, the Z1/F8BT blended film still maintains 70% of the initial luminance at 20 wt%, and this corresponds to the maximal crack onset strain. Simultaneously, with the increasing ratio of Z1, both EQE and C.E. gradually increased, reaching their maximum values at 20 wt%Z1, which were 1.4 times higher than the initial values (Fig. 4h–k). Interestingly, at 20 wt%Z1, the F8BT blended film exhibited optimal stretchability, meaning that the F8BT/Z1 blended film showed good flexible and electrical properties simultaneously at this ratio, which is attributed to the increase in hole mobility, resulting in the matching of the mobility of holes and electrons. However, the F8BT/Z2 film maintains only 40% of the initial luminance at a blending ratio of 20 wt%. Furthermore, with the increase in the proportion of Z2, the device performance showed a significant decrease. We believe that the selection of semiconductor FπCPs that match the energy levels of conjugated molecular additives will lead to further improvement of PLED device performance.

Stretch-stability of the device performance of F8BT/Z1 film for the flexible PLEDs

In flexible electronics, even small changes in the emission layer during strain or deformation effect can have a critical impact on device performance and stability. Since F8BT/Z1 with a blended ratio of 20 wt% (called 20 wt%Z1) has both optimal mechanical and electrical properties, we used it for subsequent studies. Firstly, AFM measurements are explored here to initially characterize the microstructure of the F8BT/Z1 films under stretching effect, as shown in Fig. 5a. For the F8BT/Z1 films, even under significant strain conditions, the films maintain their integrity with intact and smooth surface morphology. There is no obvious crack at the nanoscale in this F8BT/Z1 stretched film.Fig. 5 Stretch-stability of the optoelectronic property of F8BT/Z1 film for the flexible PLEDs.

a The surface morphology of the F8BT/Z1 film with a strain of 15%, as observed under optical microscopic and AFM images. b Performance of flexible PLED based on F8BT/Z1 stretched film (20 wt%Z1). c Schematic of the PLED structure to characterize the device performance of the stretchable F8BT/Z1 film under various strain. d–f The voltage-current density-luminance curves, current density-EQE-C.E. curves and EL spectra of PLEDs based F8BT/Z1 films at different strains. g The trends in C.E. and EQE variations of PLEDs based on the F8BT/Z1 film under different strains. Error bars represent SD. h Normalized relative highest C.E. and EQE of PLEDs based on the F8BT/Z1 (20 wt%) stretchable films under 100 repeated stretching cycles (10% strain). Error bars represent SD.

In addition, we also fabricated flexible PLEDs on a PET substrate and found that the 20 wt%Z1 blend film still exhibited a device brightness of 2000 cd m−2, furthermore, the device maintained higher luminescence even under bending deformation, as shown in Fig. 5b. The insert picture also presents the photo images of operation device. In order to accurately investigate the intrinsic electroluminescence (EL) performance of the stretchable blended films during strain effect, we physically transferred the pre-stretched polymer films onto rigid devices, as shown in Fig. 5c. The F8BT/Z1 films exhibit stable device performance during stretching, even when the film is stretched to 15%, as shown in Fig. 5d. The luminance of the devices based on F8BT/Z1 films reaches approximately 3000 cd m−2, comparable to the pristine ones. Compared to spin-coated ones, transfer devices presented a relatively poor performance, due to interfacial impedance34. The same turn-on voltage (Von: ~4.0 V) and high current density (~200 mA cm−2) are also observed, supporting their good charge transport ability and conductivity (Fig. 5d). Additionally, we extracted the maximum EQE and current efficiency of the devices at various strain levels, and at 15% strain applied to the blend film, the efficiency showed negligible changes, similarly, the EL spectra of all PLEDs to exhibit a sable greenish-yellow emission with a maximum peak at 548 nm, indicated the good stretching emission stability with a higher color purity, as shown in Fig. 5e–g. Meanwhile, in order to further investigate the effect of the stress tolerance on electrical properties, we also fabricated the PLEDs based on F8BT/Z1 films with a different stretching cycle treatment, as shown in Fig. 5h. As we expected, after repeated stretching to 10% strain for 100 cycles, the device performance remained stable without obvious attenuation, demonstrating high strain tolerance. In summary, the blend film with 20 wt%Z1 exhibits good strain tolerance capacity, maintaining structural integrity and stable electroluminescent performance even under substantial strain conditions. We believe that this strategy provides valuable insights for the development of flexible devices.

General applicability for the preparation of typical stretchable FπCPs films

To ascertain the range of applications for our fluid plasticizers, we selected two additional typical fluorescent FπCPs (poly(9,9-di-n-octylfluorenyl-2,7-diyl), PFO and poly(2-methoxy-5(2′-ethyl)hexoxy-phenylenevinylene) (MEH-PPV) derivative, Super Yellow), as shown in Fig. 6a, b. We assessed the mechanical and electrical properties of PFO/Z1 and SY/Z1 blend films with varying Z1 contents. For PFO, when the Z1 content reaches 20 wt%, the modulus of the blended film is 0.09 ± 0.01 GPa, which is 17 times lower (1.5 ± 0.6 GPa) than that of the pristine film (0 wt%Z1), and the crack onset strain is increased from 25% (0 wt%Z1) to 50% (20 wt%Z1), (Fig. 6c, Supplementary Fig. 21−22). However, for SY, the fluid plasticizing effect of Z1 was much less pronounced than that of PFO/Z1 (Supplementary Fig. 23−24). Its modulus reached its lowest at 20 wt%Z1, reduced only by a factor of 1.3 compared to the pristine film (0 wt%Z1), and the crack initiation strain increased from 5% (0 wt%Z1) to 10% (20 wt%Z1) (Fig. 6d). Additionally, we characterized the electrical properties of the two FπCPs blend films (Fig. 6e, f). For the PFO/Z1, with the increase in Z1 content, the current efficiency and EQE reached a maximum at 20 wt%, 0.27 cd A−1 and 0.35% respectively (Fig. 6g, h). We attribute this to the opposite changes in current density and luminance, indicating a more balanced charge injection12. Similarly, a comparable trend is observed for SY/Z1 blend films (Fig. 6i, j), which exhibit good electrical performance at 20 wt%Z1. We can infer that PFO obtains the best stretchable properties compared to SY because the repeating unit of PFO is similar to that of Z1, underscoring the importance of aligning the plasticizer structure with the conjugated semiconductor structure. Thus, the superiority of the fluid Z1 plasticizer in maintaining a balance between the mechanical and electrical properties of FπCPs stretchable blended films is further affirmed.Fig. 6 General applicability of Z1 plasticizers for the preparation of typical FπCPs stretchable blended films.

a Chemical structure of PFO. b Chemical structure of Super Yellow (SY). c Crack-onset strain and modulus of stretchable PFO nano-films with different Z1 contents. d Crack-onset strain and modulus of stretchable SY nano-films with different Z1 contents. e, f Energy band diagram of all components in the PLED device. g, h The variation of relative current density, luminance, EQE and current efficiency of the corresponding PLEDs based on PFO blended films with different content of Z1. Error bars represent SD. i, j The variation of relative current density, luminance, EQE and current efficiency of the corresponding PLEDs based on SY blended films with different content of Z1. Error bars represent SD. Source data are provided as a Source Data file.

In summary, we demonstrate a straightforward design strategy for fluid-conjugated molecular additives to obtain efficient and stable intrinsically stretchable FπCPs films for flexible optoelectronics. By introducing fluid-conjugated molecular additives as plasticizers into FπCPs, high mechanical stretchability and charge carrier mobility are achieved simultaneously. More interestingly, the incorporation of conjugated molecular additives into FπCPs can disrupt the formation of large crystalline domains between polymers' main chains while still preserving a certain degree of short-range order, which may serve as stress-dissipation centers to obtain good stress tolerance capacity. Meanwhile, the conjugated molecular additives act as “bridges” during charge transport, providing favorable conditions for interchain charge transportation. After introducing the fluid-conjugated molecular additives, the brightness can still maintain 70% of the initial film’s brightness, but the EQE is improved by 1.4 times. Furthermore, based on the Z1 and model F8BT materials, the PLED device performance based on stretched films can be maintained even at 15% strain. After repeated stretching to 10% strain for 100 cycles, the corresponding PLEDs also showed good emission spectra and efficiency, demonstrating higher strain tolerance capacity. We observed that the designed fluid-conjugated molecular additives can improve the mechanical properties of the typical conjugated polymers (PFO and SY), thus indicating the general applicability of the additives. We believe that there is considerable potential in utilizing fluid-conjugated molecular additives as plasticizers to enhance the stretchability of FπCPs blended films. This signifies a crucial advancement in achieving a balance between the intrinsic stretchability and electrical properties of FπCPs for flexible optoelectronic applications.

Methods

Material

All reagents and solvents were purchased from Sigma-Aldrich, Adamas, and Aladdin. All reagents purchased were used as received. Detailed synthesis steps for compounds Z1 and Z2 are provided in the Supplementary Information. Nuclear magnetic resonance (NMR) spectra were recorded using a JOEL NMR spectrometer (JNM-ECZ400S, 400 MHz Japan), 1H NMR and 13C-NMR spectra were referenced to CDCl3 (7.26 p.p.m.). The Poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) used in this paper was synthesized by members of our group in previous work35. Poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) and poly(2-methoxy-5(2′-ethyl)hexoxy-phenylenevinylene) (Super Yellow) were purchased from Sigma-Aldrich.

UV–vis and PL spectra measurement

For Fig. 1a, the picture of Z1, Z2 was taken under dark with an ultra-violet lamp shining upon the compound with a wavelength of 365 nm. The UV–vis absorption and emission spectra were measured with Shimadzu UV-1750 and Hitachi F-4600 luminescence spectrometer for solution and film samples. The PLQY of blend films was determined with a spectrometer C11347 (Hamamatsu, Japan). All compound films were obtained by spin-coating on quartz substrates at 1500 r.

Mechanical characterizations: free-standing tensile test

Strain-stress curves of all three blended films were characterized using the free-standing tensile test. An approximately 40 nm thick water-soluble sacrificial layer of PEDOT:PSS (Clevios PVP AI 4083, Heraeus) was spin-coated onto a pre-cleaned glass substrate (5 mm × 10 mm) at 3000 rpm for 30 s. Subsequently, the blend solution was dissolved in toluene at 50 mgmL−1 and spin-coated at 1500 rpm for 60 s, yielding ~500 nm of thick film. Then, a glass sheet containing the blended film on the surface was transferred to a PMMA hollow substrate, and the film was released from the glass by immersing the sample in water. Finally, it was carefully removed from the water to prevent rupture from occurring. Detailed dimensional information of the hollow substrate is given in ref. 36. The thickness of these co-blended films was measured using a profilometer (KLA-Tencor P-7). The samples were clamped to a tensile test apparatus for tensile testing. Prior to testing, a soldering iron was used to melt the two edge connections of the hollow PMMA holder. The effective area of the sample was 5 mm × 2 mm (length × width). The strain rate was 0.0025 s−1.

Modulus test

The elastic modulus of a blend film can be obtained by the buckling metrology method, as shown in Supplementary Fig 17.

Crack-on-set strain test

Cracks-on-set strains were tested by transferring polymer films from the OTS substrate to the PDMS substrate. Then, the polymer film on PDMS was stretched using a tensile stage, and finally, the cracks were observed by optical microscopy.

PLEDs fabrication

The PLEDs devices were fabricated onto pre-patterned indium-tin oxide (ITO) whose sheet resistance is 10 Ω per square. The structure of the device is ITO/PEDOT:PSS/compounds film/TPBi/LiF/Al. A thin hole-injection layer (20 nm) of PEDOT:PSS (AI 4083) was spin-coated on the transparent conductive ITO at a spin speed of 3000 rpm for 60 s. After thermal treatment in air for 15 min, solutions of Z1/Z2 (8 mg mL−1) and F8BT (8 mg mL−1) in Tol with different weight ratios were then spin coated onto the PVK layer with films thickness around 20 nm. The background pressure of the chamber was under 10−6 Torr during the deposition process. EL spectra and CIE coordinates of the devices were used Spectra Scan PR655 spectra radiometer to measure. All the devices were characterized without encapsulation under ambient condition. The device’s current-voltage luminescence characteristics were recorded by Keithley source meter (model 2602) and PR655 spectra radiometer. The structure of hole-only devices is ITO/PEDOT:PSS/compounds film/MoO3/Al.

Fabrication of PLEDs with stretched emissive layer

The ITO glasses were washed with acetone, isopropanol, and deionized water before use. To fabricate the PLED, the ITO glass was first treated by O2 plasma, followed by spin coating of a PEDOT:PSS (AI 4083) solution at 3000 rpm for 60 s in air, and subsequent annealing at 120 °C for 10 min. Subsequently, Z1/F8BT was spin coated onto OTS treated silica substrate at 1500 rpm for 30 s. And then transferred to the PDMS stamps. Polymer films on PDMS stamps were then stretched to desired strain levels and directly transferred from the PDMS stamp onto the surface of the PEDOT:PSS/PVK, and subsequently annealed at 120 °C in the glove box for 10 min. Finally, a TPBi (20 nm), LiF (1 nm) and Al layer (100 nm) was thermally deposited at a pressure of 10−6 Torr.

Supplementary information

Supplementary Information

Peer Review File

Source data

Source Data

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-024-50358-1.

Acknowledgements

This work is financially supported by the National Key R&D Program of China (No. 2020YFA0709900); W. Huang, J. Lin, and L. Bai acknowledge support from the National Natural Science Foundation of China (No. 62288102, 61874053, 22075136, 22105099, 62105262, and 62205141); Z. Zhuo acknowledges support from the Cultivation Program for The Excellent Doctoral Dissertation of Nanjing Tech University (No.2024−15); L. Sun acknowledge support from the Shenzhen Science and Technology Program (No. RCBS20231211090610014) and China Postdoctoral Science Foundation funded project (No. 2024M753737). We also greatly acknowledged Professor Y. Yang (Yingguo Yang) from Fudan University for his professional GIWAXS experiments.

Author contributions

Jinyi Lin and Zhiqiang Zhuo conceived and designed the project and wrote the manuscript and supplementary materials. Mingjian Ni characterized and analyzed the photoluminescence quantum yield of the blended films. Ningning Yu assisted with the synthesis of compounds Z1 and Z2. Yingying Zheng characterized and analyzed the AFM tests of the blended films. Yingru Lin provided assistance with SEM characterization. Jing Yang helped with NMR spectroscopy testing of the synthesized compounds. Lili Sun and Lizhi Wang assisted with the analysis of absorption and PL spectra. Lubing Bai helped test and analyze the mechanical properties. Wenyu Chen provided assistance in data processing. Man Xu, Fengwei Huo, and Quanyou Feng contributed to the data analysis. Jinyi Lin and Wei Huang initiated the project, provided ideas, and offered substantial funding support. All authors discussed the results and commented on the manuscript at all stages.

Peer review

Peer review information

Nature Communications thanks the anonymous reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

Data availability

The authors declare that all data supporting the current findings of this study are provided in the main manuscript or in the Supplementary information. The Source data generated in this study have been deposited in the FigShare [10.6084/m9.figshare.26146189]. Source data are provided with this paper.

Competing interests

The authors declare no competing interests.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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References

1. Ma R Chou SY Xie Y Pei Q Morphological/nanostructural control toward intrinsically stretchable organic electronics Chem. Soc. Rev. 2019 48 1741 1786 10.1039/C8CS00834E 30601498
Ma, R., Chou, S. Y., Xie, Y. & Pei, Q. Morphological/nanostructural control toward intrinsically stretchable organic electronics. Chem. Soc. Rev. 48, 1741–1786 (2019).30601498 10.1039/C8CS00834E
2. Yang JC Electronic skin: Recent progress and future prospects for skin-attachable devices for health monitoring, robotics, and prosthetics Adv. Mater. 2019 31 e1904765 10.1002/adma.201904765 31538370
Yang, J. C. et al. Electronic skin: Recent progress and future prospects for skin-attachable devices for health monitoring, robotics, and prosthetics. Adv. Mater. 31, e1904765 (2019).31538370 10.1002/adma.201904765
3. Oh JY Bao Z Second skin enabled by advanced electronics Adv. Sci. 2019 6 1900186 10.1002/advs.201900186
Oh, J. Y. & Bao, Z. Second skin enabled by advanced electronics. Adv. Sci. 6, 1900186 (2019).10.1002/advs.201900186
4. Zhao Z Intrinsically flexible displays: key materials and devices Natl Sci. Rev. 2022 9 nwac090 10.1093/nsr/nwac090 35711242
Zhao, Z. et al. Intrinsically flexible displays: key materials and devices. Natl Sci. Rev. 9, nwac090 (2022).35711242 10.1093/nsr/nwac090
5. Lee H Stretchable organic optoelectronic devices: design of materials, structures, and applications Mater. Sci. Eng. R. Rep. 2021 146 100631 10.1016/j.mser.2021.100631
Lee, H. et al. Stretchable organic optoelectronic devices: design of materials, structures, and applications. Mater. Sci. Eng. R. Rep. 146, 100631 (2021).10.1016/j.mser.2021.100631
6. Wang GJN Gasperini A Bao ZA Stretchable polymer semiconductors for plastic electronics Adv. Electron Mater. 2018 4 1700429 10.1002/aelm.201700429
Wang, G. J. N., Gasperini, A. & Bao, Z. A. Stretchable polymer semiconductors for plastic electronics. Adv. Electron Mater. 4, 1700429 (2018).10.1002/aelm.201700429
7. Matsuhisa N Chen X Bao Z Someya T Materials and structural designs of stretchable conductors Chem. Soc. Rev. 2019 48 2946 2966 10.1039/C8CS00814K 31073551
Matsuhisa, N., Chen, X., Bao, Z. & Someya, T. Materials and structural designs of stretchable conductors. Chem. Soc. Rev. 48, 2946–2966 (2019).31073551 10.1039/C8CS00814K
8. Missale E Frasconi M Pantano MF Ultrathin organic membranes: can they sustain the quest for mechanically robust device applications? iScience 2023 26 105924 10.1016/j.isci.2023.105924 36866039
Missale, E., Frasconi, M. & Pantano, M. F. Ultrathin organic membranes: can they sustain the quest for mechanically robust device applications? iScience 26, 105924 (2023).36866039 10.1016/j.isci.2023.105924
9. Ge X Mechano-graded electrodes mitigate the mismatch between mechanical reliability and energy density for foldable lithium-ion batteries Adv. Mater. 2022 34 2206797 10.1002/adma.202206797
Ge, X. et al. Mechano-graded electrodes mitigate the mismatch between mechanical reliability and energy density for foldable lithium-ion batteries. Adv. Mater. 34, 2206797 (2022).10.1002/adma.202206797
10. Li Y Achieving tissue-level softness on stretchable electronics through a generalizable soft interlayer design Nat. Commun. 2023 14 4488 10.1038/s41467-023-40191-3 37495580
Li, Y. et al. Achieving tissue-level softness on stretchable electronics through a generalizable soft interlayer design. Nat. Commun. 14, 4488 (2023).37495580 10.1038/s41467-023-40191-3
11. Kang J Tough-interface-enabled stretchable electronics using non-stretchable polymer semiconductors and conductors Nat. Nanotechnol. 2022 17 1265 1271 10.1038/s41565-022-01246-6 36357793
Kang, J. et al. Tough-interface-enabled stretchable electronics using non-stretchable polymer semiconductors and conductors. Nat. Nanotechnol. 17, 1265–1271 (2022).36357793 10.1038/s41565-022-01246-6
12. Zhang Z High-brightness all-polymer stretchable led with charge-trapping dilution Nature 2022 603 624 630 10.1038/s41586-022-04400-1 35322250
Zhang, Z. et al. High-brightness all-polymer stretchable led with charge-trapping dilution. Nature 603, 624–630 (2022).35322250 10.1038/s41586-022-04400-1
13. Yao ZF Ordered solid-state microstructures of conjugated polymers arising from solution-state aggregation Angew. Chem. Int Ed. 2020 59 17467 17471 10.1002/anie.202007589
Yao, Z. F. et al. Ordered solid-state microstructures of conjugated polymers arising from solution-state aggregation. Angew. Chem. Int Ed. 59, 17467–17471 (2020).10.1002/anie.202007589
14. Mun J A design strategy for high mobility stretchable polymer semiconductors Nat. Commun. 2021 12 3572 10.1038/s41467-021-23798-2 34117254
Mun, J. et al. A design strategy for high mobility stretchable polymer semiconductors. Nat. Commun. 12, 3572 (2021).34117254 10.1038/s41467-021-23798-2
15. Zheng Y Zhang S Tok JB Bao Z Molecular design of stretchable polymer semiconductors: Current progress and future directions J. Am. Chem. Soc. 2022 144 4699 4715 10.1021/jacs.2c00072 35262336
Zheng, Y., Zhang, S., Tok, J. B. & Bao, Z. Molecular design of stretchable polymer semiconductors: Current progress and future directions. J. Am. Chem. Soc. 144, 4699–4715 (2022).35262336 10.1021/jacs.2c00072
16. Root SE Mechanical properties of organic semiconductors for stretchable, highly flexible, and mechanically robust electronics Chem. Rev. 2017 117 6467 6499 10.1021/acs.chemrev.7b00003 28343389
Root, S. E. et al. Mechanical properties of organic semiconductors for stretchable, highly flexible, and mechanically robust electronics. Chem. Rev. 117, 6467–6499 (2017).28343389 10.1021/acs.chemrev.7b00003
17. Ni M Intrinsically stretchable and stable ultra-deep-blue fluorene-based polymer with a high emission efficiency of ≈90% for polymer light‐emitting devices with a ciey = 0.06 Adv. Funct. Mater. 2021 32 2106564 10.1002/adfm.202106564
Ni, M. et al. Intrinsically stretchable and stable ultra-deep-blue fluorene-based polymer with a high emission efficiency of ≈90% for polymer light‐emitting devices with a ciey = 0.06. Adv. Funct. Mater. 32, 2106564 (2021).10.1002/adfm.202106564
18. Liu W High-efficiency stretchable light-emitting polymers from thermally activated delayed fluorescence Nat. Mater. 2023 22 737 745 10.1038/s41563-023-01529-w 37024592
Liu, W. et al. High-efficiency stretchable light-emitting polymers from thermally activated delayed fluorescence. Nat. Mater. 22, 737–745 (2023).37024592 10.1038/s41563-023-01529-w
19. Li XC Intrinsically stretchable electroluminescent elastomers with self-confinement effect for highly efficient non-blended stretchable oleds Angew. Chem. Int Ed. 2022 62 e202213749 10.1002/anie.202213749
Li, X. C. et al. Intrinsically stretchable electroluminescent elastomers with self-confinement effect for highly efficient non-blended stretchable oleds. Angew. Chem. Int Ed. 62, e202213749 (2022).10.1002/anie.202213749
20. Liu D Tuning the mechanical and electric properties of conjugated polymer semiconductors: Side‐chain design based on asymmetric benzodithiophene building blocks Adv. Funct. Mater. 2022 32 2203527 10.1002/adfm.202203527
Liu, D. et al. Tuning the mechanical and electric properties of conjugated polymer semiconductors: Side‐chain design based on asymmetric benzodithiophene building blocks. Adv. Funct. Mater. 32, 2203527 (2022).10.1002/adfm.202203527
21. Mun J Effect of nonconjugated spacers on mechanical properties of semiconducting polymers for stretchable transistors Adv. Funct. Mater. 2018 28 1804222 10.1002/adfm.201804222
Mun, J. et al. Effect of nonconjugated spacers on mechanical properties of semiconducting polymers for stretchable transistors. Adv. Funct. Mater. 28, 1804222 (2018).10.1002/adfm.201804222
22. Xu J Highly stretchable polymer semiconductor films through the nanoconfinement effect Science 2017 355 59 64 10.1126/science.aah4496 28059762
Xu, J. et al. Highly stretchable polymer semiconductor films through the nanoconfinement effect. Science 355, 59–64 (2017).28059762 10.1126/science.aah4496
23. Peng Z Thermoplastic elastomer tunes phase structure and promotes stretchability of high‐efficiency organic solar cells Adv. Mater. 2021 33 2106732 10.1002/adma.202106732
Peng, Z. et al. Thermoplastic elastomer tunes phase structure and promotes stretchability of high‐efficiency organic solar cells. Adv. Mater. 33, 2106732 (2021).10.1002/adma.202106732
24. Guan Y-S Elastic electronics based on micromesh-structured rubbery semiconductor films Nat. Electron 2022 5 881 892 10.1038/s41928-022-00874-z
Guan, Y.-S. et al. Elastic electronics based on micromesh-structured rubbery semiconductor films. Nat. Electron 5, 881–892 (2022).10.1038/s41928-022-00874-z
25. Guan YS Air/water interfacial assembled rubbery semiconducting nanofilm for fully rubbery integrated electronics Sci. Adv. 2020 6 eabb3656 10.1126/sciadv.abb3656 32938663
Guan, Y. S. et al. Air/water interfacial assembled rubbery semiconducting nanofilm for fully rubbery integrated electronics. Sci. Adv. 6, eabb3656 (2020).32938663 10.1126/sciadv.abb3656
26. Kim JH Park JW Intrinsically stretchable organic light-emitting diodes Sci. Adv. 2021 7 eabd9715 10.1126/sciadv.abd9715 33627424
Kim, J. H. & Park, J. W. Intrinsically stretchable organic light-emitting diodes. Sci. Adv. 7, eabd9715 (2021).33627424 10.1126/sciadv.abd9715
27. Selivanova M Branched polyethylene as a plasticizing additive to modulate the mechanical properties of π-conjugated polymers Macromolecules 2019 52 7870 7877 10.1021/acs.macromol.9b01697
Selivanova, M. et al. Branched polyethylene as a plasticizing additive to modulate the mechanical properties of π-conjugated polymers. Macromolecules 52, 7870–7877 (2019).10.1021/acs.macromol.9b01697
28. Savagatrup S Plasticization of pedot:Pss by common additives for mechanically robust organic solar cells and wearable sensors Adv. Funct. Mater. 2014 25 427 436 10.1002/adfm.201401758
Savagatrup, S. et al. Plasticization of pedot:Pss by common additives for mechanically robust organic solar cells and wearable sensors. Adv. Funct. Mater. 25, 427–436 (2014).10.1002/adfm.201401758
29. Liu Y A self-assembled 3d penetrating nanonetwork for high-performance intrinsically stretchable polymer light-emitting diodes Adv. Mater. 2022 34 e2201844 10.1002/adma.202201844 35488389
Liu, Y. et al. A self-assembled 3d penetrating nanonetwork for high-performance intrinsically stretchable polymer light-emitting diodes. Adv. Mater. 34, e2201844 (2022).35488389 10.1002/adma.202201844
30. Yu X Intrinsically stretchable polymer semiconductors with good ductility and high charge mobility through reducing the central symmetry of the conjugated backbone units Adv. Mater. 2023 35 e2209896 10.1002/adma.202209896 36772843
Yu, X. et al. Intrinsically stretchable polymer semiconductors with good ductility and high charge mobility through reducing the central symmetry of the conjugated backbone units. Adv. Mater. 35, e2209896 (2023).36772843 10.1002/adma.202209896
31. Xu J Tuning conjugated polymer chain packing for stretchable semiconductors Adv. Mater. 2022 34 e2104747 10.1002/adma.202104747 34558121
Xu, J. et al. Tuning conjugated polymer chain packing for stretchable semiconductors. Adv. Mater. 34, e2104747 (2022).34558121 10.1002/adma.202104747
32. Chappell J Correlating structure with fluorescence emission in phase-separated conjugated-polymer blends Nat. Mater. 2003 2 616 621 10.1038/nmat959 12923529
Chappell, J. et al. Correlating structure with fluorescence emission in phase-separated conjugated-polymer blends. Nat. Mater. 2, 616–621 (2003).12923529 10.1038/nmat959
33. Cadby AJ Imaging the fluorescence decay lifetime of a conjugated‐polymer blend by using a scanning near‐field optical microscope Adv. Mater. 2006 19 107 111 10.1002/adma.200601715
Cadby, A. J. et al. Imaging the fluorescence decay lifetime of a conjugated‐polymer blend by using a scanning near‐field optical microscope. Adv. Mater. 19, 107–111 (2006).10.1002/adma.200601715
34. Hamilton I Optimizing interfacial energetics for conjugated polyelectrolyte electron injection layers in high efficiency and fast responding polymer light emitting diodes ACS Appl Mater. Interfaces 2022 14 24668 24680 10.1021/acsami.2c05640 35583466
Hamilton, I. et al. Optimizing interfacial energetics for conjugated polyelectrolyte electron injection layers in high efficiency and fast responding polymer light emitting diodes. ACS Appl Mater. Interfaces 14, 24668–24680 (2022).35583466 10.1021/acsami.2c05640
35. Bai L Steric poly(diarylfluorene‐co‐benzothiadiazole) for efficient amplified spontaneous emission and polymer light‐emitting diodes: Benefit from preventing interchain aggregation and polaron formation Adv. Opt. Mater. 2020 8 1901616 10.1002/adom.201901616
Bai, L. et al. Steric poly(diarylfluorene‐co‐benzothiadiazole) for efficient amplified spontaneous emission and polymer light‐emitting diodes: Benefit from preventing interchain aggregation and polaron formation. Adv. Opt. Mater. 8, 1901616 (2020).10.1002/adom.201901616
36. Bai L Unveiling the effects of interchain hydrogen bonds on solution gelation and mechanical properties of diarylfluorene-based semiconductor polymers Research 2020 2020 3405826 10.34133/2020/3405826 33083787
Bai, L. et al. Unveiling the effects of interchain hydrogen bonds on solution gelation and mechanical properties of diarylfluorene-based semiconductor polymers. Research 2020, 3405826 (2020).33083787 10.34133/2020/3405826
