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Review

The Transition of Luminescent Materials and Conductive Electrodes in Upconversion Devices to Flexible Architectures

by
Huijuan Chen
1,
Weibo Feng
1 and
Tianling Qin
2,*
1
School of Optics and Photonics, Beijing Institute of Technology, Beijing 100081, China
2
State Key Laboratory of High Power Semiconductor Laser, Changchun University of Science and Technology, Changchun 130022, China
*
Author to whom correspondence should be addressed.
Photonics 2025, 12(11), 1075; https://doi.org/10.3390/photonics12111075
Submission received: 25 September 2025 / Revised: 21 October 2025 / Accepted: 24 October 2025 / Published: 30 October 2025
(This article belongs to the Special Issue Organic Photodetectors, Displays, and Upconverters)
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Versions Notes

Abstract

Flexible upconversion (UC) devices, owing to their unique combination of high–efficiency optical energy conversion and mechanical flexibility, have attracted increasing attention in the fields of optoelectronics, wearable devices, flexible displays, and biomedical applications. However, significant challenges remain in balancing optical performance, mechanical adaptability, long–term stability, and scalable fabrication, which limit their practical deployment. This review systematically introduces five representative upconversion mechanisms—excited–state absorption (ESA), energy transfer upconversion (ETU), energy migration upconversion (EMU), triplet–triplet annihilation upconversion (TTA–UC), and photon avalanche (PA)—highlighting their energy conversion principles, performance characteristics, and applicable scenarios. The article further delves into the flexible transition of upconversion devices, detailing not only the evolution of the luminescent layer from bulk crystals and nanoparticles to polymer composites and hybrid systems, but also the optimization of electrodes from rigid metal films to metal grids, carbon–based materials, and stretchable polymers. These developments significantly enhance the stability and reliability of flexible upconversion devices under bending, stretching, and complex mechanical deformation. Finally, emerging research directions are outlined, including multi–mechanism synergistic design, precise nanostructure engineering, interface optimization, and the construction of high–performance composite systems, emphasizing the broad potential of flexible UC devices in flexible displays, wearable health monitoring, solar energy harvesting, flexible optical communications, and biomedical photonic applications. This work provides critical insights for the design and application of high–performance flexible optoelectronic devices.

1. Introduction

Upconversion is a nonlinear optical process in which two or more low–energy photons, typically in the near–infrared (NIR) range, are sequentially absorbed and combined to emit a single higher–energy photon at a shorter wavelength than the excitation light. Upconversion devices that circumvent the Stokes shift limitation, enabling near–infrared (NIR) to visible light conversion, have garnered considerable interest in diverse fields such as biomedical imaging, optical anti–counterfeiting, solar energy utilization, optical sensing, and night–vision display technologies [1,2,3,4,5,6,7,8]. With the rapid advancement of upconversion technology, conventional rigid materials–limited by inherent material brittleness and inflexibility–are increasingly inadequate for the demands of emerging flexible electronics and wearable systems [9,10]. These limitations pose a significant barrier to the practical deployment of existing Upconversion devices in advanced applications such as flexible displays, electronic skin, and smart textiles [11,12,13]. Consequently, the integration of upconversion functionality into flexible upconversion represents a pivotal strategy for advancing its transition from laboratory research to real–world applications.
Flexible upconversion devices must not only exhibit outstanding optical performance but also maintain stable electrical and mechanical properties under various deformation conditions, such as bending, stretching, and twisting. These requirements place increased demands on the material selection and structural design of upconversion. Specifically, the realization of high–performance flexible upconversion hinges both on the flexible design of the luminescent material layer and on the compatible evolution of its corresponding conductive electrodes (Figure 1) [14,15,16]. For example, upconversion luminescent materials must retain high optical efficiency while offering sufficient flexibility and devicesability to function under mechanical deformation [17,18]. Flexible electrodes require a careful balance between electrical conductivity and mechanical deformability [9,14,19]. In summary, the upconversion devices have gradually transitioned from rigid to fully flexible material systems, laying a solid foundation for their practical applications in wearable optoelectronics, biomedical devices, and intelligent terminals.
Although the flexible upconversion devices has made promising progress in material design and electrode selection, the integration of these components into a robust and highly efficient system continues to pose a significant challenge [3,20,21]. As multifunctional devices integrating both optical functionality and mechanical flexibility, their performance is determined not only by the optical response of the upconversion materials but also by the mechanical resilience of the conductive electrodes under various deformation conditions [9,22,23]. Key optical parameters include the upconversion quantum yield (UCQY), emission wavelength, and excitation power threshold, which collectively define the efficiency and applicability of the flexible upconversion [2,18,21,24,25]. On the mechanical side, essential indicators such as minimum bending radius, strain limit, and fatigue lifetime (the number of mechanical cycles) directly influence the reliability of the device in wearable or deformable applications [7,17,22,23]. Importantly, these two sets of properties are often intrinsically coupled. Repeated mechanical deformation can alter the local environment of luminescent centers, leading to quenching of emission. Similarly, stress concentration may cause interfacial delamination or structural degradation, compromising overall device stability and operational longevity [26]. To address these issues, comprehensive efforts are required across materials engineering, device architecture design, and multiphysics coupling regulation (optical, electrical, thermal, and mechanical).
This review seeks to investigate the transition of two critical elements in upconversion from rigid to flexible architectures, analyze the interplay of optical and mechanical performance, and thereby establish key future research priorities.
Photonics 12 01075 g001
Figure 1. Applications of Flexible Upconversion [27,28,29,30,31,32]. Copyright 2024, Nano-Micro Letters. Copyright 2025, science advances. Copyright 2022, ACS Applied Materials & Interfaces. Copyright 2024, nature communications. Copyright 2016, Nanoscale. Copyright 2019, Joule.
Figure 1. Applications of Flexible Upconversion [27,28,29,30,31,32]. Copyright 2024, Nano-Micro Letters. Copyright 2025, science advances. Copyright 2022, ACS Applied Materials & Interfaces. Copyright 2024, nature communications. Copyright 2016, Nanoscale. Copyright 2019, Joule.
Photonics 12 01075 g001

2. Mechanisms of Upconversion Emission Devices in UCNPs

The upconversion emission mechanisms of upconversion nanoparticles (UCNPs) play a critical role in determining their overall performance, as they directly influence emission efficiency, stability, and optical properties [3]. Mechanisms such as energy transfer upconversion (ETU), excited state absorption (ESA), triplet–triplet annihilation upconversion (TTA–UC), energy migration upconversion (EMU), and photon avalanche (PA) are central to the optical behavior of upconversion materials. A deep understanding and precise control of these mechanisms are essential not only for significantly enhancing the emission efficiency and stability of UCNPs but also for advancing their practical applications in real–world scenarios.

2.1. Excited State Absorption

Excited–state absorption (ESA) is one of the most fundamental upconversion mechanisms, commonly observed in single–ion–dominated luminescent systems. In this mechanism, a single ion undergoes a stepwise absorption of two or more photons. The first photon promotes the ion from the ground state to an intermediate excited state, and subsequent photon absorption further elevates it to higher–lying energy levels. Upon radiative decay from the upper excited state, an upconverted photon with greater energy than the incident excitation is emitted [33,34]. The energy level diagram of the ESA mechanism is shown in Figure 2a.
This mechanism relies exclusively on multi–step photon absorption by a single ion to accumulate the required excitation energy and is highly dependent on the intensity of the excitation source. Typically, high–power laser irradiation is required to achieve efficient multiphoton excitation. Owing to these characteristics, ESA–based upconversion has been widely employed in optoelectronic applications that demand strong excitation fields, such as high–sensitivity fluorescence detection, upconversion lasers, and optical limiting devices [33,35,36].

2.2. Energy Transfer Upconversion

Energy transfer upconversion (ETU) is a well–established and efficient upconversion mechanism in UCNPs, involving sequential energy transfer deviceses between sensitizer and activator rare–earth ions embedded within a host matrix, as illustrated in Figure 2b [3,37,38]. In upconversion materials, a sensitizer efficiently absorbs excitation light (typically NIR) and transfers the energy to an activator, which then emits higher–energy photons in the visible or ultraviolet range. Sensitizers (e.g., Yb3+) act as energy donors, while activators (e.g., Er3+, Tm3+, Ho3+) determine the emission color. Unlike ESA, the ETU mechanism does not rely on multiphoton absorption by a single ion and typically exhibits significantly higher efficiency. It enables upconversion luminescence under relatively low excitation power, making it well–suited for practical applications. However, ETU performance is highly sensitive to the interionic distance and dopant concentration–excessive doping may lead to concentration quenching. By optimizing the sensitizer–to–activator ratio, the emission intensity and overall efficiency can be further enhanced [39].
Due to its high upconversion efficiency and low excitation threshold, ETU has become the foundational mechanism in the material design of flexible upconversion devices. It is particularly effective for near–infrared excitation–visible emission pathways and has recently attracted growing interest in polymer composites and flexible nanofilm architectures [40,41,42,43].

2.3. Triplet–Triplet Annihilation Upconversion

Triplet–triplet annihilation upconversion (TTA–UC) is an upconversion mechanism based on organic molecules or organic–inorganic hybrid systems, as shown in Figure 2c [25]. Its most prominent feature is the ability to achieve efficient anti–Stokes emission under extremely low excitation power densities, such as weak visible light or ambient sunlight. TTA–UC system typically exhibit high upconversion quantum yield, low excitation thresholds, and tunable emission wavelengths [44].
A TTA–UC system consists of a photosensitizer and an annihilator. The photosensitizer efficiently absorbs low–energy excitation light and transfers the energy to the annihilator, while the annihilator undergoes triplet–triplet annihilation to emit higher–energy photons, acting as the emitter in the upconversion process. And their upconversion performance is closely governed by the photophysical properties of both components, as well as the efficiency of the intermolecular triplet–triplet energy transfer devices [45,46]. These unique advantages make TTA–UC highly promising for applications in flexible optoelectronic devices, solar energy conversion, biological imaging, and anti–counterfeiting systems [23,47,48,49].

2.4. Energy Migration Upconversion

Energy Migration Upconversion (EMU) is an upconversion mechanism based on the cooperative interaction of multiple ions, as shown in Figure 2d. It occurs when the excitation energy migrates through a network of sensitizer ions and is ultimately transferred to activator ions capable of radiative emission. This mechanism is particularly effective in multilayered core–shell structured upconversion nanoparticles [50,51]. In such systems, the sensitizer–rich core enables efficient energy harvesting and migration, while the activator–doped shell serves as the emission region, achieving spatial separation between excitation and emission deviceses [3,52].
EMU supports long–range energy migration, which improves emission uniformity, suppresses non–radiative losses, and enhances the overall upconversion efficiency. However, the implementation of this mechanism requires complex ion–doping strategies and sophisticated structural design. In recent years, EMU has been widely explored for applications in anti–counterfeiting fingerprint recognition systems, as well as in flexible electronics and biological imaging platforms [51,53].

2.5. Photon Avalanche

Photon Avalanche (PA) is a highly nonlinear upconversion mechanism characterized by a positive feedback loop initiated by weak photon absorption, which leads to a rapid and pronounced enhancement of upconversion emission. Figure 2e illustrates the representative energy level diagram associated with this mechanism [3,54].
PA typically exhibits a well–defined excitation power threshold, beyond which the system transitions into an “avalanche” regime, resulting in an exponential increase in emission intensity. This threshold–dependent behavior makes PA particularly attractive for applications requiring binary on/off switching or intense optical output, such as high–threshold responsive sensors, laser–triggered microdevices, and optical limiting systems [55,56,57,58].

3. Key Components of Flexible Upconversion

To address the increasing demand for deformable and biocompatible systems in wearable electronics, biomedical diagnostics, and flexible displays, upconversion devices are progressively evolving from rigid architectures to flexible platforms.
The performance and mechanical flexibility of flexible upconversion are critically influenced by their structural configuration and the functional integration of multiple material layers. A typical flexible upconversion consists of several key functional layers, including the upconversion material layer and the conductive electrodes [14,16]. The integrated design of these components is crucial for achieving efficient photon conversion while maintaining mechanical adaptability.

3.1. Upconversion Material Layer

As a key luminescent component in upconversion devices, the upconversion nanoparticle (UCNP) layer has undergone a continuous evolution–from rigid, highly efficient inorganic crystals to flexible, devicesable, and integrable multifunctional nanostructures. In the early stages, UCNPs were primarily based on large–sized rare–earth–doped inorganic crystals. Among them, β–NaYF4 doped with Yb3+/Er3+ or Yb3+/Tm3+ has been widely recognized as one of the most efficient upconversion systems due to its narrow energy band structure, high energy transfer efficiency, and excellent crystallinity [59,60,61]. These nanoparticles typically exhibit diameters ranging from 20 to 100 nm and can emit visible green or red light, as well as near–infrared (NIR) luminescence. However, owing to their oleic acid–capped hydrophobic surfaces, as synthesized UCNPs are generally incompatible with aqueous environments and polymer matrices, which severely limits their direct application in flexible or biological systems [62,63]. To address these interfacial and dispersibility challenges, various surface modification strategies have been developed, including coating or ligand exchange with poly (acrylic acid) (PAA), silica (SiO2), and polyethylene glycol (PEG). These surface treatments significantly improve the colloidal stability, biocompatibility, and adhesion of UCNPs to flexible substrates, thus providing a robust platform for their further integration into functional composite systems.
Rafique et al. synthesized NaYF4:20%Yb3+/3%Er3+ upconversion nanoparticles (UCNPs) via a hydrothermal method and subsequently modified their surface with poly (acrylic acid) (PAA) to enhance aqueous dispersibility and biocompatibility. SEM images (Figure 3a,b) demonstrate that both unmodified and PAA–coated UCNPs exhibit uniform particle size and morphology, with no noticeable increase in diameter following surface modification. The UV–vis absorption spectra (Figure 3c) confirm successful PAA attachment, evidenced by the emergence of a new absorption peak near 290 nm. The right inset further reveals a pronounced absorption band at 980 nm in the uncoated UCNPs, which is characteristic of Yb3+ ion excitation [64]. Guller et al. characterized the structural and optical properties of oleic acid–capped UCNPs (OA–UCNPs). As shown in Figure 3d, transmission electron microscopy (TEM) images of β–NaY0.78Yb0.2Er0.02F4 reveal that most nanocrystals exhibit a well–defined hexagonal morphology, indicating high crystalline quality. The absolute upconversion quantum yield (UCQY) of these particles was measured to be approximately 0.1% at an excitation power density of 10 W/cm2, reaching a maximum of about 1.5% under 300 W/cm2 excitation (Figure 3e) [62].
Thermal decomposition (Rafique) enables the preparation of highly crystalline nanoparticles with excellent luminescence performance; however, the requirement for elevated temperatures and hydrophobic surface ligands complicates subsequent processing into flexible films. Silica coating (Guller) enhances colloidal stability and chemical robustness, yet the chemically inert silica shell can impede interfacial charge transfer, thereby limiting device integration. Ligand exchange and surface engineering (Wang) improve compatibility with flexible organic matrices and facilitate electrical functionality, but can partially disrupt surface passivation, leading to moderate quenching of luminescence.
From an application standpoint, approaches that simultaneously ensure high optical efficiency, facile processability, and mechanical adaptability are the most promising for practical flexible devices. In particular, surface engineering strategies that optimize UCNP–polymer interfacial interactions, combined with scalable low–temperature synthesis, offer significant potential for next–generation wearable and flexible optoelectronic platforms.
Surface coating modifications have effectively mitigated the inherent hydrophobicity of upconversion nanoparticles (UCNPs), thereby improving their dispersibility in aqueous environments and compatibility with polymer matrices. However, it can also introduce problems such as surface quenching and decreased energy transfer efficiency. To address these issues, multi–shell structures have been developed to suppress non–radiative losses and enhance upconversion luminescence [65,66]. For example, wang et al. designed two types of multi–shell UCNPs—NaYF4:Yb/Tm@NaYF4@NaYF4:Yb/Er and NaYF4:Yb/Er@NaYF4@NaYF4:Yb/Tm@NaYF4:Yb, with the corresponding TEM images shown in Figure 3f,g. This structural strategy spatially separates energy donors and acceptors, effectively regulating energy migration pathways and reducing non–radiative quenching induced by surface defects. As shown in Figure 3h, both types of nanoparticles exhibited enhanced sensitivity to excitation power density at the particle surface, further validating the role of shell engineering in energy modulation. To further verify the role of the outermost shell, control samples were synthesized by replacing the outermost NaYF4: Yb shell with an optically inert NaYF4 layer. The results revealed a consistent trend in the blue–to–green emission intensity ratio across different excitation powers (Figure 3i). This observation further confirmed the ability of the inert shell to suppress surface quenching, highlighting the crucial role of multi–shell structures in maintaining high upconversion efficiency [30].
Based on good stability and interfacial compatibility, UCNPs have been gradually embedded into flexible polymer matrices to form luminescent layers that are bendable, stretchable, and transparent [41]. Je Park and colleagues synthesized core/shell structured UCNPs capable of emitting bright green, blue, and red light. The successful formation of the core/shell structures was confirmed by transmission electron microscopy (TEM) and energy–dispersive X–ray spectroscopy (EDS) mapping, as shown in Figure 4a,b. These architectures significantly enhanced the upconversion luminescence intensity. The researchers then embedded the above UCNPs into a bisphenol A ethoxylate dimethacrylate (Bis–EMA) polymer matrix to fabricate highly transparent and flexible optical waveguides (Figure 4c). The resulting composite waveguides exhibited excellent optical transparency, with transmittance exceeding 90% across the spectral range of 443–900 nm. Furthermore, by simply mixing the three types of core/shell UCNPs, the emission color within the waveguide could be precisely tuned. The resulting luminescence could span the entire triangular region defined by the CIE coordinates of blue, green, and red UCNP emission, as illustrated in Figure 4d. These findings demonstrate the great potential of such composite materials for multicolor modulation and integrated photonic applications [43].
In the latest stage of development, UCNPs have been integrated into more complex functional systems by hybridizing with two–dimensional semiconductors, conductive polymers, quantum dots, or biocompatible hydrogels [68,69,70]. These hybrid architectures enable multiphysical responses such as electroluminescence, strain sensing, and photothermal modulation. This evolution signifies a transition of UCNPs from simple luminescent modules to multifunctional interfacial systems, marking an important direction in the advancement of flexible upconversion devices. Abucafy et al. synthesized NaYF4/Yb3+ Er3+ UCNPs via a hydrothermal method, followed by poly (acrylic acid) (PAA) surface coating and subsequent encapsulation with a ZIF–8 shell, forming a core–shell structured UCNP@ZIF–8 nanocomposite. As illustrated in Figure 4e, this composite serves as a near–infrared–responsive drug release system: the UCNPs are embedded within a ZIF–8 shell and loaded with the anticancer drug doxorubicin (DOX). Upon 980 nm laser irradiation, the UCNPs generate upconversion luminescence that triggers the breakdown of the ZIF–8 framework, enabling controlled release of DOX and demonstrating a smart, NIR–triggered release mechanism. Figure 4f compares the emission spectra of UCNP@ZIF–8 and UCNP@ZIF–8–DOX under 980 nm excitation. A significant decrease in green emission intensity is observed upon DOX loading, corresponding to the spectral overlap between the UCNP emission and DOX absorption in the 500–550 nm range. This confirms the presence of an energy transfer devices, in which the UCNPs serve as energy donors to excite DOX, further validating the potential of this system for NIR–triggered drug release and its broader applicability in controlled therapeutic platforms [67].
A systematic comparison of absorber and emitter ions/ligands in flexible and hybrid upconversion systems is summarized in Table 1, highlighting their distinct roles, benefits, and limitations.

3.2. Conductive Electrodes

In flexible upconversion devices, conductive electrodes play a critical role not only in charge injection and collection but also in determining the optical transmittance of the device, mechanical flexibility, and environmental stability. With the growing demand for enhanced performance and structural versatility, electrode materials have evolved from conventional rigid metal films to flexible conductive polymers, metal nanostructures, carbon–based materials, and multifunctional composites. This evolution aims to achieve a synergistic balance of high electrical conductivity, excellent optical transparency, and mechanical compliance, thereby providing essential support for the development of efficient, stretchable, and wearable upconversion devices.
In the early stages, traditional noble metal films such as Au and Ag were widely employed in electrode fabrication due to their excellent electrical conductivity and chemical stability. These metal films are typically deposited onto substrates via techniques such as thermal evaporation and electron beam evaporation, which not only ensure the conductivity of the device but also provide efficient excitation light reflection paths for upconversion luminescence, thereby enhancing the emission efficiency of the device to some extent. However, their high cost, limited ductility, and susceptibility to cracking under repeated mechanical deformation pose significant challenges for the development of flexible electrode materials. Manurung et al. designed a metal–enhanced fluorescence (MEF) structure in the form of a “gold sandwich,” where upconversion nanoparticles (UCNPs, NaYF4:Yb,Er) are embedded between top and bottom metallic layers, as illustrated in Figure 5a. In this configuration, the bottom thick gold film serves as a mirror–like reflector, extending the optical path of the 980 nm excitation light within the UCNP layer and thereby enhancing excitation efficiency, resulting in a 5–8–fold increase in green emission. The top layer, composed of gold nanoislands, exhibits surface plasmon resonance around 550 nm, effectively intensifying the local electric field and further boosting the emission intensity by approximately 2.5 times. Consequently, this “sandwich” architecture achieves a total luminescence enhancement of 13–19 times, as shown in Figure 5b [77].
Further optimization has focused on the construction of carbon–based nanocomposites, such as hybridizing silver nanowires (AgNWs) with reduced graphene oxide (rGO) or carbon nanotubes (CNTs). By leveraging the excellent flexibility and interfacial adhesion properties of carbon materials, these composites enhance the structural integrity and long–term stability of the conductive network. Such architectures not only improve the overall mechanical performance of the device but also optimize interfacial charge transport between the upconversion materials and the electrode. For example, Martinez et al. fabricated flexible, freestanding transparent conductive electrodes (TCEs) by coating AgNWs onto dry–spun multi–walled carbon nanotube (MWNT) aerogels, achieving a tunable transmittance of up to 98% and a low sheet resistance of 11 Ω/sq. Figure 5c illustrates the increase in perpendicular transmittance and the reduction in parallel sheet resistance as the amount of AgNWs deposited within the MWNT aerogel increases. It also highlights three enhancement mechanisms–Densification Effect, AgNWs Effect, and Thermal Annealing Effect–that occur during the spray–coating devices using AgNWs/IPA solution. The SEM image in Figure 5d shows the MWNT aerogel pores formed as a result of the densification effect [65].
To overcome the limitations of traditional noble metal films in flexible applications, researchers have progressively introduced flexible–structured metallic nanowire networks (such as AgNWs and CuNWs) or metal mesh electrodes. These materials offer electrical conductivity comparable to that of conventional metals, while exhibiting excellent mechanical compliance and high optical transparency, representing a key advancement in the development of flexible transparent electrodes (FTEs). For example, Wu et al. employed three techniques—methanol impregnation, argon plasma treatment, and ultraviolet irradiation—to effectively reduce the junction resistance of silver nanowires (AgNWs), thereby optimizing flexible transparent electrodes (FTEs) composed of AgNWs and poly(3,4–ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS). The SEM image of the synthesized AgNPs is shown in Figure 5e. The optoelectronic performance of the FTEs was further enhanced by introducing a co–doped structure of AgNWs and silver nanoparticles (AgNPs), forming a multilayer configuration of PET/AgNWs: AgNPs/PEDOT: PSS/DMSO. As shown in Figure 5g, the optimal doping ratio of 2:1 (AgNWs:AgNPs) yielded the highest figure of merit (FOM) of 1.42 × 10−2 ohm−1 and a low sheet resistance of 13.86 ohm/sq. Notably, after 500 bending cycles, the peak luminance of FOLEDs based on the co–doped FTEs and the AgNWs/PEDOT: PSS/DMSO electrode retained 82% and 70% of their initial luminance, respectively, whereas ITO–based devices failed after only 200 cycles as illustrated in Figure 5h. These results clearly demonstrate the superior flexibility and mechanical stability of AgNWs:AgNPs–based FTEs [78].
Meanwhile, conductive polymer electrodes (such as PEDOT:PSS, PANI, and PPy) have emerged as ideal candidates for constructing flexible electronic devices due to their excellent flexibility, good film–forming ability, and high optical transparency. These polymers can be used alone or combined with metallic nanoconductors or carbon–based materials to further enhance the electrode’s electrical conductivity and environmental stability. Moreover, their compatibility with solution devicesing makes them suitable for large–area, low–cost fabrication of flexible devices. Hu et al. developed a conductive polymer network electrode that exhibits outstanding electrical conductivity (>4000 S/cm) and maintains over 80% transmittance in the wavelength range of 400–900 nm, demonstrating excellent optical properties (Figure 6a). Figure 6b shows the normalized conductivity of flexible PEDOT:PSS electrodes under conditions of 85 °C and 85% relative humidity (RH). Compared to conventional PEDOT:PSS films, the PEDOT:PSS:CFE electrodes show significantly improved moisture stability, retaining over 90% of their original conductivity even after 30 days, indicating excellent environmental durability. As shown in Figure 6c, bending cycle tests were conducted with a curvature radius as small as 3 mm. Even after 5000 cycles, the sheet resistance of the PEDOT:PSS:CFE electrode shows negligible change, highlighting its exceptional mechanical endurance. Furthermore, by analyzing the limit deflection curves of various flexible electrodes (Figure 6d), PEDOT:PSS:CFE is found to possess a higher deformation limit compared to other electrode materials, further confirming its mechanical advantages and strong potential for application in flexible electronic devices [31].
As flexible electronics continue to advance toward highly deformable applications such as wearable and bio–integrated devices, research has progressively shifted toward the design of stretchable conductive composite structures. These structures achieve a balance among high stretchability, mechanical recoverability, and stable electrical conductivity by incorporating elastic substrates (e.g., PDMS, Ecoflex) with conductive fillers (e.g., AgNWs, CNTs, liquid metals). Such electrodes not only exhibit highly stable electrical performance under repeated stretching or twisting but also provide a robust material foundation for the development of flexible upconversion devices in complex application scenarios including wearable technologies and bio–integrated systems. Li et al. fabricated PDMS/AgNW–AgNP flexible transparent electrodes (Figure 6e). Compared with conventional AgNW electrodes, the PDMS/AgNW–AgNP electrodes exhibited significantly lower increases in normalized resistance (R/R0) after 2000 bending cycles, demonstrating superior bending stability (Figure 6f). Moreover, under strains ranging from 0% to 40%, the resistance variation in PDMS/AgNW–AgNP remained consistently lower than that of AgNW, indicating better strain adaptability (Figure 6g). Furthermore, during 200 continuous stretching cycles at 20% strain, the PDMS/AgNW–AgNP electrodes maintained a relatively low resistance change and were able to successfully light up an LED after deformation (Figure 6h), confirming their excellent mechanical durability and electrical reliability for applications in flexible electronic devices [79].
In summary, the development of flexible electrode materials has evolved from rigid metal films to metal networks, carbon–based composites, conductive polymers, and finally to stretchable composite structures. This progression reflects continuous innovation in multiscale structural design, interfacial engineering, and performance optimization. To enable the stable operation of wearable electronic devices under complex and dynamic conditions, flexible electrodes must not only exhibit excellent electrical conductivity and optical transparency, but also possess high flexibility, stretchability, and environmental stability. Looking ahead, the design of flexible electrodes will place greater emphasis on synergistic material integration and multifunctionality to meet the diverse requirements of next–generation smart wearable technologies, including biosensing, energy harvesting, and flexible displays.
The comparative analysis of the three flexible electrodes, as summarized in Table 2, reveals a clear trade–off between mechanical endurance, electrical performance, and long–term stability. The AgNW–AgNP/L–His electrode [79] demonstrates superior flexibility, withstanding 100,000 bending cycles while maintaining excellent sheet resistance (~17.5 Ω/sq) and high transparency. In contrast, the PEDOT:PSS:CFE electrode [31] excels in long–term environmental stability, retaining its performance for over 180 days, and maintains good functionality after 5000 bending cycles. The Laser rGO/CNT electrode [80], while offering the lowest number of bending cycles (500) and short–term stability (7 days), stands out in capacitive energy storage applications due to its outstanding capacitance retention (98.2%) after bending. Consequently, the choice of electrode is highly application–dependent: AgNW–AgNP/L–His is ideal for devices requiring extreme mechanical durability, PEDOT:PSS:CFE is suited for applications where long–term operational stability is critical, and Laser rGO/CNT is an excellent candidate for flexible supercapacitors where energy retention under deformation is paramount.

4. Opportunities and Challenges for Flexible Upconversion Devices

Flexible upconversion (UC) devices, which leverage unique anti–Stokes photophysics to emit higher–energy photons under near–infrared (NIR) excitation while maintaining stable performance under mechanical deformation, are propelling optoelectronics toward flexible and multifunctional paradigms. These systems show significant promise across a range of applications, including wearable health monitors, skin–conformal light–emitting therapeutics, implantable photomedical devices, and flexible displays with intelligent optical interfaces. For instance, flexible UCNP–based implantable light guides have enabled wireless, bendable photodynamic therapy (PDT) in the mouse brain, demonstrating potential for long–term deep–tissue treatment [81]. In another approach, embedding UCNPs into transparent polymer waveguides has yielded core/shell UC films exhibiting over 90% optical transmittance and robust bending stability, suitable for flexible displays and optical interfaces [42]. Furthermore, hybrid architectures integrating UC materials with metal–organic frameworks (MOFs) offer a promising route to enhance both luminescence efficiency and multifunctional integration [82].
However, despite the enormous application potential of flexible upconversion (UC) technologies, multiple bottlenecks remain in materials, device engineering, and standardization. At the materials level, poor nanoparticle dispersion, concentration quenching and weak particle–matrix interfaces are routinely reported to cause rapid loss of upconversion emission when UCNPs are incorporated at high loading or exposed to hostile media. For example, concentration–quenching and energy–migration losses are well documented in the UCNP literature and set practical limits on achievable brightness at high dopant or particle loading [83]. More directly related to environmental stability, Zhou et al. exposed core–shell UCNPs to several simulated biological fluids and observed rapid particle degradation and corresponding intensity drops that strongly depended on fluid composition (phosphate–rich media produced pronounced fragmentation and intensity loss over days), showing that moisture/ionic environments can quickly degrade UCNP optical performance unless properly protected [84].
At the composite/device level, several studies show that mechanical deformation and inadequate encapsulation lead to optical failure pathways. Embedding UCNPs into elastomeric matrices without optimized surface chemistry or interface design can lead to nanoparticle aggregation and interfacial debonding under cyclic strain; such microstructural changes increase scattering of both excitation and emission light and open nonradiative channels, producing substantial luminous loss in hours to days or after repeated deformation cycles (degree and timescale depend on particle surface treatment, filler loading, and encapsulation) [83]. Conversely, works that apply robust encapsulation strategies (e.g., siloxane/sol–gel encapsulation) demonstrate markedly improved tolerance to heat and moisture, illustrating that failure under humid/thermal or mechanical stress is often an engineering (encapsulation/interface)—not an intrinsic—limitation [85]. Practical device studies further illustrate these points: stretchable polymer/UCNP optical fibers and films can operate stably under moderate strain when ratiometric detection or careful composite design is used (demonstrated in stretchable UC–polymer sensors), but the literature also contains multiple reports where poor dispersion, high loading, or insufficient encapsulation produced rapid brightness loss or irreversible damage under harsh environmental or mechanical cycling [86].
Taken together, these works underline two clear messages for flexible UC device development: (1) quantitative, standardized durability testing (e.g., defined cyclic bending/stretching protocols combined with damp–heat aging) is urgently required so that different studies can be compared on a common basis; and (2) interface and encapsulation engineering (surface shells, siloxane or sol–gel encapsulation, ligand strategies, and optimized loading levels) are effective and necessary countermeasures to avoid the rapid performance losses that have been observed in poorly protected systems.

5. Conclusions

In summary, flexible upconversion (UC) devices have advanced significantly, with diverse mechanisms (ESA, ETU, EMU, TTA–UC, and PA) and evolving architectures from rigid bulk crystals to multifunctional flexible composites. Despite these advances, major challenges remain, including limited upconversion efficiency in flexible matrices due to nanoparticle aggregation and interfacial incompatibility, insufficient mechanical durability under repeated deformation, and lack of standardized, quantitative evaluation under harsh environmental conditions. Future research should focus on designing biocompatible, self–healing materials with high UC efficiency, developing advanced fabrication strategies such as 3D and inkjet printing for scalable, robust devices, and constructing multiscale composite systems that integrate UC nanoparticles with organic semiconductors, perovskites, and two–dimensional materials to achieve synergistic optical, electrical, and mechanical performance. Overcoming these challenges will be crucial to realizing practical applications in wearable sensing, soft robotics, personalized phototherapy, solar energy harvesting, and next–generation flexible displays.

Author Contributions

Conceptualization, T.Q. and H.C.; investigation, H.C. and W.F.; data curation, H.C.; writing—original draft preparation, H.C.; writing—review and editing, T.Q. and H.C.; supervision, T.Q.; funding acquisition, T.Q. and W.F. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Beijing Natural Science Foundation (No. QY25274).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 2. Mechanisms of Upconversion Emission Devices in UCNPs. (a) Excited State Absorption (ESA). (b) Energy Transfer Upconversion (ETU). (c) Triplet–Triplet Annihilation Upconversion (TTA–UC). (d) Energy Migration Upconversion (EMU). (e) Photon Avalanche (PA). G: Ground State, the most stable state of particles with the lowest energy. E1: First Excited State, an excited state that particles can transition to after absorbing energy. E2: Higher Excited State. Pink upward arrows: The process in which particles absorb energy and transition from lower energy levels to higher energy levels. Pink downward dashed arrow: The process where Ion radiatively transitions from an excited state to the ground state (releasing energy). Green dashed arrows: The process of energy transfer from Ion1 to Ion2. Blue downward dashed arrow: The process in which particles transition from higher energy levels to lower energy levels with radiation (energy release). S0: Singlet Ground State, the most stable state with the lowest energy for both the Sensitizer and Annihilator. S1: First Excited Singlet State. T1: First Excited Triplet State.
Figure 2. Mechanisms of Upconversion Emission Devices in UCNPs. (a) Excited State Absorption (ESA). (b) Energy Transfer Upconversion (ETU). (c) Triplet–Triplet Annihilation Upconversion (TTA–UC). (d) Energy Migration Upconversion (EMU). (e) Photon Avalanche (PA). G: Ground State, the most stable state of particles with the lowest energy. E1: First Excited State, an excited state that particles can transition to after absorbing energy. E2: Higher Excited State. Pink upward arrows: The process in which particles absorb energy and transition from lower energy levels to higher energy levels. Pink downward dashed arrow: The process where Ion radiatively transitions from an excited state to the ground state (releasing energy). Green dashed arrows: The process of energy transfer from Ion1 to Ion2. Blue downward dashed arrow: The process in which particles transition from higher energy levels to lower energy levels with radiation (energy release). S0: Singlet Ground State, the most stable state with the lowest energy for both the Sensitizer and Annihilator. S1: First Excited Singlet State. T1: First Excited Triplet State.
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Figure 3. SEM images of (a) UCNP (b) UCNP@PAA and (c) UV/Visible absorbance spectra of UCNP, PA, and UCNP@PAA; insets show the more detailed UV/Vis spectra of UCNP, PAA, UCNP@PAA (left) and bared UCNP (right) [64]. Copyright 2009, Journal of Materials Chemistry (d) Transmission electron microscopy (TEM) image of as–synthesized β–NaY0.78Yb0.2Er0.02F4 nanoparticles. (e) Absolute conversion efficiency of OA–UCNPs [62]. Copyright 2018, ACS Biomaterials science & engineering. (f,g) TEM images of NaYF4:Yb/Tm@NaYF4@NaYF4:Yb/Er@NaYF4:Yb and NaYF4:Yb/Er@NaYF4@NaYF4:Yb/Tm@NaYF4:Yb nanoparticles, respectively. (h) Power dependent blue–to–green upconversion emission intensity ratio of NaYF4:Yb/Tm@NaYF4@NaYF4:Yb/Er@NaYF4 and NaYF4:Yb/Er@NaYF4@NaYF4:Yb/Tm@NaYF4 UCNPs upon 975 nm laser irradiation. (i) Schematic representation showing pump–power–dependent luminescence of a sensitizer/emitter codoped nanoparticle in the presence or absence of an energy distributor [30]. Copyright 2016, Nanoscale.
Figure 3. SEM images of (a) UCNP (b) UCNP@PAA and (c) UV/Visible absorbance spectra of UCNP, PA, and UCNP@PAA; insets show the more detailed UV/Vis spectra of UCNP, PAA, UCNP@PAA (left) and bared UCNP (right) [64]. Copyright 2009, Journal of Materials Chemistry (d) Transmission electron microscopy (TEM) image of as–synthesized β–NaY0.78Yb0.2Er0.02F4 nanoparticles. (e) Absolute conversion efficiency of OA–UCNPs [62]. Copyright 2018, ACS Biomaterials science & engineering. (f,g) TEM images of NaYF4:Yb/Tm@NaYF4@NaYF4:Yb/Er@NaYF4:Yb and NaYF4:Yb/Er@NaYF4@NaYF4:Yb/Tm@NaYF4:Yb nanoparticles, respectively. (h) Power dependent blue–to–green upconversion emission intensity ratio of NaYF4:Yb/Tm@NaYF4@NaYF4:Yb/Er@NaYF4 and NaYF4:Yb/Er@NaYF4@NaYF4:Yb/Tm@NaYF4 UCNPs upon 975 nm laser irradiation. (i) Schematic representation showing pump–power–dependent luminescence of a sensitizer/emitter codoped nanoparticle in the presence or absence of an energy distributor [30]. Copyright 2016, Nanoscale.
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Figure 4. (a) NaGdF4:Yb,Tm/NaGdF4:Eu UCNPs. (b) EDS map superposed with Yb Lα (cyan) and Eu Lα (red) for NaGdF4:Yb,Tm/NaGdF4:Eu UCNPs. (c) Transmittance spectra of stripe–type polymer waveguides. Inset in (c) shows photographs of (i) the fabricated polymer waveguides and (ii) luminescent polymer waveguides coupled with a 980 nm NIR laser (d) CIE color coordinates of blue–, green–, and red–emitting C/S UCNP solutions. (e) A light–responsive drug release system based on UCNP@ZIF–8 [43]. Copyright 2017, science reports (f) DOX UV–visible absorption spectrum and upconversion luminescence emission spectra of solutions of UCNP@ ZIF– 8 and UCNP@ZIF–8– DOX in water, using λexc = 980 nm and power energy of 200 mW cm−2 [67]. Copyright 2025, ACS applied bio materials.
Figure 4. (a) NaGdF4:Yb,Tm/NaGdF4:Eu UCNPs. (b) EDS map superposed with Yb Lα (cyan) and Eu Lα (red) for NaGdF4:Yb,Tm/NaGdF4:Eu UCNPs. (c) Transmittance spectra of stripe–type polymer waveguides. Inset in (c) shows photographs of (i) the fabricated polymer waveguides and (ii) luminescent polymer waveguides coupled with a 980 nm NIR laser (d) CIE color coordinates of blue–, green–, and red–emitting C/S UCNP solutions. (e) A light–responsive drug release system based on UCNP@ZIF–8 [43]. Copyright 2017, science reports (f) DOX UV–visible absorption spectrum and upconversion luminescence emission spectra of solutions of UCNP@ ZIF– 8 and UCNP@ZIF–8– DOX in water, using λexc = 980 nm and power energy of 200 mW cm−2 [67]. Copyright 2025, ACS applied bio materials.
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Figure 5. (a) Schematic of the proposed ‘gold sandwich’ for metal enhanced fluorescence in UCNPs. (b) SEM image of the gold sandwich structure [77]. Copyright 2016, RSC Advances (c) The three enhancing mechanisms occurring during spray–coating of AgNWs/IPA to MWNTs aerogels; Densification Effect, AgNWs Effect and Thermal Annealing Effect from which the transmittance is increased while the sheet resistance of the AgNWs/MWNT aerogel is reduced. Inset depicts the optical transmittance of the AgNWs/MWNTs hybrids TCE over the visible spectrum. (d) SEM image of MWNT aerogel apertures formed as consequence of the densification effect [65]. Copyright 2019, ACS Applied Materials & Interfaces (e) The SEM image of the synthesized Ag NPs. (f) Real product picture and schematic diagram of FTE structure. (g) The average sheet resistance and FOM values of Ag NWs: Ag NPs for each co–doping. (h) The bending test diagram of the prepared FOLED device [78]. Copyright 2024, materials.
Figure 5. (a) Schematic of the proposed ‘gold sandwich’ for metal enhanced fluorescence in UCNPs. (b) SEM image of the gold sandwich structure [77]. Copyright 2016, RSC Advances (c) The three enhancing mechanisms occurring during spray–coating of AgNWs/IPA to MWNTs aerogels; Densification Effect, AgNWs Effect and Thermal Annealing Effect from which the transmittance is increased while the sheet resistance of the AgNWs/MWNT aerogel is reduced. Inset depicts the optical transmittance of the AgNWs/MWNTs hybrids TCE over the visible spectrum. (d) SEM image of MWNT aerogel apertures formed as consequence of the densification effect [65]. Copyright 2019, ACS Applied Materials & Interfaces (e) The SEM image of the synthesized Ag NPs. (f) Real product picture and schematic diagram of FTE structure. (g) The average sheet resistance and FOM values of Ag NWs: Ag NPs for each co–doping. (h) The bending test diagram of the prepared FOLED device [78]. Copyright 2024, materials.
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Figure 6. (a) The diffusive transmittance spectra of different flexible electrodes. (b) Normalized conductivity of different flexible electrodes under 85 °C and 85% RH conditions. (c) Normalized resistance of flexible electrodes as a function of bending cycles with a radius of 3 mm. (d) Limit deflection curves of different flexible electrodes [31]. Copyright 2019, Joule (e) Schematic diagram of the water–assisted peeling devices for PDMS/AgNW–AgNP. (f) Variations in Rsh of FTEs versus bending cycles at a bending radius of 4 mm. (g) Variations in Rsh of FTEs versus strain. (h) Variations in Rsh of FTEs versus strain cycles under the continuous 200 cycles with 20% S–6 S–7 strain. Insert: illuminated LED biased at 2 V using PDMS/AgNW–AgNP FTEs after 200 cycles with 20% strain [79]. Copyright 2024, ACS Applied Materials & Interfaces.
Figure 6. (a) The diffusive transmittance spectra of different flexible electrodes. (b) Normalized conductivity of different flexible electrodes under 85 °C and 85% RH conditions. (c) Normalized resistance of flexible electrodes as a function of bending cycles with a radius of 3 mm. (d) Limit deflection curves of different flexible electrodes [31]. Copyright 2019, Joule (e) Schematic diagram of the water–assisted peeling devices for PDMS/AgNW–AgNP. (f) Variations in Rsh of FTEs versus bending cycles at a bending radius of 4 mm. (g) Variations in Rsh of FTEs versus strain. (h) Variations in Rsh of FTEs versus strain cycles under the continuous 200 cycles with 20% S–6 S–7 strain. Insert: illuminated LED biased at 2 V using PDMS/AgNW–AgNP FTEs after 200 cycles with 20% strain [79]. Copyright 2024, ACS Applied Materials & Interfaces.
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Table 1. Systematic comparison of the advantages and disadvantages of absorber ions/ligands and emitter ions/ligands in upconversion systems.
Table 1. Systematic comparison of the advantages and disadvantages of absorber ions/ligands and emitter ions/ligands in upconversion systems.
TypeAdvantagesDisadvantagesPerformance in Flexible/Hybrid SystemsReferences
Absorber ions Large absorption cross–sections; high energy transfer efficiencyWeak intrinsic emission; excessive doping may lead to concentration quenchingIon spacing in flexible matrices can be optimized for energy transfer; non–radiative losses may lower overall efficiency[71,72]
Emitter ionWell–defined energy levels with tunable multicolor emission; high photostabilityNarrow absorption necessitates sensitization; emission is sensitive to the host matrixEmission in flexible/hybrid systems is enhanced by local field or ligand effects, but mechanical stress can reduce stability[73,74]
Absorber ligandsBroaden absorption to enhance visible/NIR light harvesting; energy levels tunable via molecular designEnergy transfer efficiency is lower than inorganic absorbers; environmental stability is limitedEnables efficient energy transfer in flexible/hybrid matrices, but stability and mechanical durability are limited[75]
Emitter ligandsImproves dispersion, reduces quenching, and enables emission wavelength/quantum yield tuningOrganic ligands may degrade under bending or heat; interactions with matrices can induce defectsImproves optical homogeneity and interfacial stability in flexible/hybrid systems; molecular design is needed to balance flexibility and luminescence[76]
Table 2. Comparative Analysis of High–Performance Flexible Electrodes.
Table 2. Comparative Analysis of High–Performance Flexible Electrodes.
DimensionLaser rGO/CNT [80]AgNW–AgNP/L–His [79]PEDOT:PSS:CFE [31]
Sheet Resistance–~17.5 Ω/sq~24 Ω/sq
Conductivity13.51 × 103 S/m–>4000 S/cm
Bending test500 cycles100,000 cycles5000 cycles
Performance Retention after BendingCapacitance: 98.2% Luminance: 90%PCE: 85–90%
Stability7 days30 days>180 days
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Chen, H.; Feng, W.; Qin, T. The Transition of Luminescent Materials and Conductive Electrodes in Upconversion Devices to Flexible Architectures. Photonics 2025, 12, 1075. https://doi.org/10.3390/photonics12111075

AMA Style

Chen H, Feng W, Qin T. The Transition of Luminescent Materials and Conductive Electrodes in Upconversion Devices to Flexible Architectures. Photonics. 2025; 12(11):1075. https://doi.org/10.3390/photonics12111075

Chicago/Turabian Style

Chen, Huijuan, Weibo Feng, and Tianling Qin. 2025. "The Transition of Luminescent Materials and Conductive Electrodes in Upconversion Devices to Flexible Architectures" Photonics 12, no. 11: 1075. https://doi.org/10.3390/photonics12111075

APA Style

Chen, H., Feng, W., & Qin, T. (2025). The Transition of Luminescent Materials and Conductive Electrodes in Upconversion Devices to Flexible Architectures. Photonics, 12(11), 1075. https://doi.org/10.3390/photonics12111075

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