Unlocking the bifunctional potential of the NPB:OXD-7 exciplex in organic light-emitting diodes and UV-photodetectors

Abstract

While highly efficient exciplex-host organic light-emitting diodes (OLEDs) have been extensively studied, their untapped potential for multifunctional applications remains largely unexplored. In this study, exciplex emission by utilizing the commonly used hole transport material (HTM) N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) and the electron transport material (ETM) 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl]benzene (OXD-7) was investigated. The dual functionality of the NPB:OXD-7 exciplex for OLEDs and photodetectors was explored, considering the influence of surface potential in organic semiconductor films. The significant surface potential of OXD-7 facilitated spontaneous exciplex dissociation at the molecular interface of NPB and OXD-7, generating a photocurrent, under ultraviolet (UV) exposure to the NPB:OXD-7 mixed layer, allowing the device to function as an efficient UV detector. Devices with higher OXD-7 concentrations exhibited enhanced UV absorption, though at the expense of exciplex emission. Specifically, an NPB : OXD-7 device with a 1 : 3 ratio demonstrated exceptional performance as a UV detector but inferior performance as an OLED. This work pioneers the exploration of multifunctional capabilities in exciplex-type OLEDs, expanding their role beyond light emission.

---

Introduction

Exciplex- and electroplex-based organic light-emitting diodes (OLEDs) have emerged as a promising approach for advancing organic optoelectronic devices due to their unique charge-transfer emission and enhanced exciton utilization.^1–4 These devices leverage intermolecular charge-transfer states formed between donor and acceptor materials, enabling tunable emission characteristics and improved external quantum efficiencies.⁵ Furthermore, exciplex systems offer the advantage of simplified device architecture by reducing the need for multiple emissive layers, thereby simplifying fabrication processes and lowering production costs.⁶ By optimizing energy levels and molecular interactions, researchers can achieve highly efficient and stable OLEDs suitable for next-generation display and lighting applications. The development of exciplex-based OLEDs represents a significant step toward cost-effective, high-performance organic semiconductors, offering new possibilities for energy-efficient and sustainable optoelectronic technologies. Since 2015, the potential of exciplex emission in OLEDs has expanded significantly, leading to numerous studies investigating existing hole and electron transport materials to achieve exciplex emissions.^7–9 However, the selection criteria for suitable transport material combinations remain unpredictable. This is despite the numerous studies that have been done on the design and synthesis of transport materials for OLEDs.^10,11 A few attempts have been made to conduct theoretical calculations for predicting exciplex emissions.^12,13 These theoretical approaches provide valuable insights into exciplex behavior, aiding in the design of optimized materials for OLEDs and other optoelectronic devices.¹⁴

The concept of multifunctionality has recently gained prominence in the development of organic electronic devices. Multifunctional organic electronic devices hold tremendous potential across various applications, promising to transform various sectors through their unique properties and capabilities.^15,16 These devices, which offer advanced functionalities integrated within a single unit, require materials with distinctive photophysical and charge-transporting abilities. In this context, OXD-7 has good electron-transporting capabilities, primarily attributed to the electron-accepting characteristics of the oxadiazole units incorporated within its molecular structure.^17–19 These oxadiazole groups of OXD-7 in conjunction with poly(9-vinylcarbazole) (PVK), an electron-donating polymer, serve as one of the most widely utilized hybrid-type host materials.^20,21 This combination exploits the complementary electron-donating and electron-accepting properties of PVK and OXD-7, thus facilitating balanced charge transport in optoelectronic devices.²² The UV-absorption capability of the NPB:OXD-7 mixed film was first investigated by Zhu et al. in 2014.²³ They fabricated a UV-photodetector by using the NPB:OXD-7 mixed film as the active layer, exploiting its strong UV absorption; TiO₂, which acts as the electron extraction layer, plays a crucial role. After UV photo-excitation, the reduced work function and increased conductivity of the TiO₂ film create low-impedance contact at the carrier-extraction layer–metal interface, facilitating the collection of photogenerated carriers at the electrodes. However, they did not investigate the light emission properties of the NPB:OXD-7 exciplex. Instead, they used it as an active layer specifically for UV detection. The multifunctional nature of such exciplex devices has never been explored before. Herein, the high UV-absorption of the NPB:OXD-7 mixed film and its exciplex-forming capability have been combined to explore the concept of bifunctional devices. This work underscores the potential of multifunctional exciplex-based devices, capable of operating as both OLEDs and UV photodetectors, marking a significant advancement in electronic device miniaturization and integration.

Results and discussion

Absorption and emission studies

The phenomenon of exciplex formation at molecular interfaces plays a crucial role in understanding charge-transfer interactions in organic electronic materials. In this study, we explore the exciplex formation at the interface between N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine (NPB) and 1,3,5-tris(phenyl-2-benzimidazolyl)-benzene (OXD-7), as depicted in Fig. 1. When an excited-state complex is formed at the molecular interface of NPB and OXD-7, it leads to distinctive exciplex emission, due to intermolecular charge transfer between the donor and acceptor molecules. Exciplex emission is governed by critical optoelectronic properties such as charge mobility, dielectric constant, material morphology, etc. This exciplex emission, in turn, plays a significant role in shaping the overall device performance by influencing emission spectra, quantum efficiency, and functional characteristics. Understanding this possibility is fundamental in optimizing molecular architectures for applications in OLEDs and optoelectronic devices.

Fig. 1 Schematic of the exciplex at the molecular interface of NPB and OXD-7 in terms of energy levels.

The absorption and emission spectra of the thin films of the materials were compared. Thin films of NPB, OXD-7, and their blends (NPB : OXD-7) in 1 : 1, 1 : 3, and 3 : 1 ratios were fabricated via thermal evaporation to investigate their optical properties. The corresponding absorption and photoluminescence spectra are shown in Fig. 2(a) and (b), respectively, highlighting the exciplex emission in the NPB:OXD-7 mixed film. The NPB:OXD-7 blend films exhibit the absorption characteristics of NPB and OXD-7, corresponding to their respective proportions in the mixture as depicted in Fig. 2(a). An exciplex, as an excited-state complex, lacks a counterpart in the ground state and therefore does not exhibit a unique absorption spectrum. Instead, it displays the individual absorption features of its constituent molecules. However, the emission spectrum of the mixed film revealed a pronounced red shift and broadening of the emission peak, as shown in Fig. 2(b). Specifically, the peak emission wavelengths of NPB and OXD-7 films were observed at 435 nm and 350 nm, respectively, whereas the emissions from the mixed films were observed at 464 nm for NPB : OXD-7 (1 : 1 and 3 : 1) films and at 456 nm for the 1 : 3 film. The full width at half maximum (FWHM) values broadened from 59 nm for NPB and 65 nm for OXD-7 to 82 nm for the NPB:OXD-7 mixed films. These spectroscopic results confirm the exciplex emission in the NPB:OXD-7 blends, characterized by significant red shifts and emission peak broadening in the mixed films compared to the individual components.^24,25

Fig. 2 Comparison of the (a) absorption and (b) emission spectra of the NPB, OXD-7, and NPB : OXD-7 (1 : 1, 1 : 3, and 3 : 1) blend films.

Surface potential studies

Spontaneous orientation polarization (SOP) is dependent on the molecular orientation within the film and occurs in molecules that possess a permanent dipole moment (PDM). The surface potential (SP) arises from the spontaneous ordering of these PDMs, a process known as SOP. In organic semiconductor films, PDM results from the asymmetric structure of the molecules. SOP can be assessed through surface potential measurements using the Kelvin probe, typically conducted at room temperature, in the dark, and under vacuum conditions.²⁶ SOP is observed in vacuum-deposited films; the dipole alignment happens spontaneously while the film grows layer-by-layer upon vapor deposition. Understanding SOP is crucial for optimizing the design and functionality of organic semiconductor devices. Surface potential measurements demonstrated that OXD-7 exhibits notably high surface potential, whereas NPB does not show surface potential. Consequently, the surface potential of the NPB:OXD-7 blend is predominantly governed by the contribution of OXD-7 to the mixture.²⁷Table 1 shows the results of Kelvin probe measurements on neat films of NPB and OXD-7, as well as the three exciplex mixtures.

Table 1 Giant surface potential (GSP) slope of films measured by Kelvin probe, as described in ref. 21. (Raw data showing GSP slopes for the three mixtures are shown in Fig. S1)

Sample	NPB	Ratio of NPB : OXD-7			OXD-7
		3 : 1	1 : 1	1 : 3
GSP slope (mV nm^−1)	0	102	148	148	116

---

All three mixtures show a substantial surface potential, which is comparable to the neat OXD-7 film, even for the 1 : 3 mixture that contains only 25% OXD-7 molecules. This effect is known as the enhancement of SOP by dilution. The mechanism of SOP enhancement by dilution in glassy organic semiconductor mixtures was recently reported by Hofmann et al.²⁷ The dilution of a polar molecule with a non-polar molecule leads to an increase in the mean spacing between the polar molecules, which reduces the dipole–dipole interactions, enabling the dipoles to align freely. The potential energy of dipole–dipole interactions scales inversely with the cube of the distance (∝1/r³), so increasing the spacing directly weakens these interactions; as a result, the molecules are freer to adopt a collective orientation aligned along a specific direction, leading to an enhancement in the SOP.

As described in the study, when OXD-7 is mixed with NPB, the dilution increases the average distance between OXD-7 dipolar molecules. This reduces dipole–dipole interactions, allowing OXD-7 molecules to align their permanent dipole moments (PDMs) in a preferred direction, thus enhancing SOP. As the concentration of OXD-7 increases, the driving force for aligning PDMs in an antiparallel configuration becomes stronger, effectively cancelling spontaneous orientational polarization (SOP).

The optical constants of the films were studied by ellipsometry, and the orientation of the emissive transition dipole moment (of the exciplex) by angle-dependent photoluminescence. As seen in Fig. S2, the mixed films show slightly negative birefringence with Δn = −0.06, indicating that the polarizability of the molecules is slightly larger in the film plane than out of plane. This is commonly referred to as a small deviation from randomly aligned molecules with a tendency toward more “lying molecules”.²⁸ At the same time, the emissive transition dipole moments have an almost isotropic orientation parameter (Θ = 0.30), again with a slight tendency toward a more lying alignment (see Fig. S3). We did not observe significant differences between the mixing ratios in their birefringence and emissive dipole orientation.

Atomic force microscopy (AFM) analysis was performed on thin films of NPB, OXD-7, and their mixed films, NPB : OXD-7 (1 : 1, 3 : 1, and 1 : 3 ratios), over a 5 μm × 5 μm scan area, as shown in Fig. S4. The surface roughness values (R_a) indicate that OXD-7 forms a comparatively smoother film than NPB. This trend is also reflected in the NPB:OXD-7 films, where the roughness appears to correlate with the concentration of OXD-7.

The behavior of exciplexes in organic semiconductor devices is significantly influenced by the presence of an electric field.²⁹ Specifically, molecules with high SOP within the exciplex film generate a surface potential, which plays a crucial role in the dissociation of exciplexes. Exciplexes, which are complexes formed between excited donor and acceptor molecules, are sensitive to such internal electric fields. When molecules in the exciplex film possess high SOP, they create a surface potential due to the orientation of permanent dipole moments. This surface potential can lead to the dissociation of the exciplexes as illustrated in Fig. 3(a). Comparative analyses of the un-normalized PL spectra of the mixed films of NPB:OXD-7 are shown in Fig. 3(b). The 1 : 3 film showed a slight blueshift (∼8 nm) and reduced PL intensity compared to the 1 : 1 and 3 : 1 films. The blueshift of the 1 : 3 film may be due to a high GSP, which inhibits CT interactions. While the 1 : 1 film also has a high GSP, the equimolar blend promotes more exciplex formation, and even with some of the CT interactions being prevented, the overall radiative recombination from the original exciplex exciton is dominant and, hence, no blueshift is observed.

Fig. 3 (a) Illustration of the weakening of the exciplex in the presence of an electric field. (b) Comparison of the PL intensity of the NPB : OXD-7 mixed films (1 : 3, 1 : 1, and 3 : 1).

In NPB:OXD-7 blended films, increasing the concentration of OXD-7 molecules will lead to the dissociation of exciplexes formed in the NPB:OXD-7 mixed film. This enables a balance between exciplex emission and dissociation, controlled by the relative amount of OXD-7 in the films. When exciplex emission dominates, the device operates as an OLED, whereas improved exciplex dissociation enhances its suitability for photodetector applications. This idea offers a promising approach for the development of multifunctional devices.

Device fabrication and characterization

The need for the integration of multifunctionality in optoelectronics has been recognized for advancing smart device technologies. From a practical perspective, multifunctional devices hold significant promise for the evolution of smart devices. Multifunctionality in organic electronic devices is due to the incorporation of multiple properties; in this case, both light emission and detection function within the same device. Here, we propose a bifunctional device based on NPB:OXD-7 exciplex combination. Under a forward bias, electrons and holes are injected from the cathode and anode, respectively, and transported towards the emissive layer (EML), where their recombination leads to exciplex formation and subsequent electroluminescence. Thus, the device operates as an exciplex-based OLED. In contrast, under reverse bias and UV illumination from the top side, the exciplex at the EML undergoes dissociation, and the resulting charge carriers are collected at the respective electrodes, generating a measurable photocurrent. Hence, the bifunctional device proposed in this work does behave as an OLED under forward bias and as a UV-detector under reverse bias, as depicted in Fig. 4.

Fig. 4 Illustration of the working mechanism of bifunctional devices.

Blue OLEDs by utilizing NPB:OXD-7 exciplex emission

Blue exciplex OLEDs were fabricated using a blend of NPB and OXD-7 as the EML; 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HAT-CN) served as the hole injection layer (HIL), while an additional layer of NPB was incorporated before the EML to enhance hole transport. The EML was followed by an ETL of 2,2′,2′′-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole (TPBi) for better electron transport. LiF was used as an electron injection layer (EIL) and aluminum (Al) as the cathode. The device configuration is as follows: ITO/HAT-CN (5 nm)/NPB (60 nm)/NPB:OXD-7 (35 nm)/TPBi(45 nm)/LiF (1 nm)/Al (100 nm). Three devices were fabricated using varying ratios of NPB : OXD-7, i.e., 1 : 1, 1 : 3, and 3 : 1. The device architecture and detailed energy level diagram are shown in Fig. 5(a) and (b), respectively.

Fig. 5 (a) Device architecture of the NPB:OXD-7 blue OLED. (b) Detailed energy level diagram of the materials.

The current density and luminance vs. voltage plots of the devices are shown in Fig. 6(a) and (b). It was observed that the highest current density and luminance were obtained in the NPB : OXD-7 (1 : 1) device. The summary of the device performance is shown in Table 2. All the devices exhibited a turn-on voltage of 3 V, which is defined as the voltage at which the luminance reaches 1 cd m⁻².^30,31 The 1 : 1 device achieved a maximum luminance of 1272 cd m⁻², a current efficiency of 0.75 cd A⁻¹, and an external quantum efficiency (EQE) of 0.41%. This observation can be attributed to the balanced proportion of NPB and OXD-7, which promotes the formation of exciplexes at the interfaces, leading to enhanced exciplex emission. The current efficiency comparison also revealed that the maximum efficiency was achieved in the NPB : OXD-7 (1 : 1) device, as shown in Fig. 6(c). Meanwhile, the NPB : OXD-7 (1 : 3) device, which contained a higher proportion of OXD-7, recorded both low current density and luminance, thereby resulting in significantly reduced efficiency. The observed low current density in the NPB:OXD-7 device can be directly attributed to the disparity in charge carrier mobilities between the two materials. OXD-7 exhibits low electron mobility³² in contrast to the high hole mobility of NPB,³³ resulting in an inherent imbalance in charge carriers within the device. This mismatch affects the efficient transport of charge carriers and also compromises the recombination process critical for exciplex emission. Such a mobility mismatch has been reported by our group, particularly in the case of NPB:TAZ exciplex devices, where a similar trend was observed.³⁴ In that instance, the reduced electron mobility of TAZ was identified as the primary factor contributing to the reduced current density in the NPB : TAZ (1 : 3) mixture. This parallel further underscores the impact of intrinsic material properties, such as charge carrier mobilities, on the overall device performance. The charge carrier balance plays a pivotal role in ensuring optimal device operation, and any deviation from this balance, as demonstrated here, inevitably leads to suboptimal outcomes. These findings highlight the need for strategic material selection and compositional tuning to overcome mobility mismatches and achieve efficient exciplex-based devices. The analysis of device characteristics further indicated that the high concentration of OXD-7, coupled with surface potential considerations, contributed to the dissociation of exciplexes formed at the interfaces. This dissociation adversely affected device efficiency, ultimately leading to diminished device performance. Hence, the high surface potential of OXD-7 in the mixture contributes to exciplex dissociation, resulting in a reduced OLED performance. The comparison of the electroluminescence (EL) spectra showed that all devices exhibited NPB:OXD-7 blue exciplex emission, as illustrated in Fig. 6(d). The EL spectra of the 1 : 1 device as a function of applied voltage are given in Fig. S5(a), showing consistent emission profiles across varying voltages, indicating stable color output. Additionally, the plot of CIE coordinates (x,y) versus voltage (Fig. S5(b)) further demonstrates the negligible shift in chromaticity with increasing voltage, confirming the robustness of the device in maintaining color fidelity. The representation of the CIE coordinates in the CIE diagram is shown in Fig. S5(c).

Fig. 6 (a) Current density vs. voltage plot. (b) Luminance vs. voltage plot. (c) CE vs. current density plot. (d) Normalized EL spectra of NPB : OXD-7 (1 : 1, 1 : 3, and 3 : 1) devices (inset shows the photograph of the 1 : 1 device).

Table 2 Summary of device performance and the maximum luminance, CE and EQE values of NPB:OXD-7 blue exciplex OLEDs with EMLs of NPB : OXD-7 (1 : 1, 1 : 3, 3 : 1)

NPB : OXD-7	Turn-on voltage (V)	Luminance^a (cd m^−2)	Current density^a (mA cm^−2)	Current efficiency^a (cd A^−1)	EQE^a (%)	Maximum luminance, CE, EQE
a Device parameters at 10 V.
1 : 1	3	511	72	0.71	0.39	1272 cd m^−2, 0.75 cd A^−1, 0.41%
1 : 3	3	262	43.5	0.48	0.30	698 cd m^−2, 0.67 cd A^−1, 0.41%
3 : 1	3	545	108	0.52	0.33	939 cd m^−2, 0.74 cd A^−1, 0.47%

---

Organic UV-photodetectors by utilizing NPB:OXD-7

Organic UV-photodetectors, which utilize organic semiconducting materials, are valued for their flexibility, lightweight nature, and cost-effectiveness. These devices convert ultraviolet (UV) light into electrical signals, making them essential for environmental monitoring, medical diagnostics, and security systems. Their tunable optoelectronic properties can be customized through molecular design and material selection. Additionally, the solution-processable nature of organic materials allows for low-temperature fabrication on various substrates, including flexible and transparent ones, facilitating innovative applications in wearable electronics and advanced sensor technologies. The performance of photodetectors is determined by several key parameters, including responsivity (R), detectivity (D), ON/OFF ratio, rise time, and fall time. Responsivity is a crucial parameter that defines the efficiency of a photodetector in converting incident optical power into an electrical signal. It is the ratio of the photocurrent (I_ph) generated to the incident optical power (P₀) from the source lamp, mathematically expressed as follows:

Responsivity is measured in units of amperes per watt (A per W) and indicates how effectively the photodetector converts incoming photons into electrical current. Detectivity is a measure of sensitivity and the ability to detect weak optical signals. It is defined as the reciprocal of the noise equivalent power (NEP), as given in eqn (2), normalized by the active area of the device. The detectivity (D) can also be expressed in terms of responsivity, as given in eqn (3), where q is the electron charge and J_d is the dark current density.

Detectivity is measured in Jones, and a higher detectivity indicates better performance in detecting low levels of light. The ON–OFF ratio is a crucial parameter that quantifies the difference in current between the illuminated (ON) and dark (OFF) states of a photodetector. It is the ratio of the photocurrent to the dark current, which is the current that flows through the photodetector under dark conditions. The ON–OFF ratio is expressed in eqn (4).

A higher ON–OFF ratio indicates a photodetector with better performance, as it shows a significant increase in current upon illumination compared to the dark state. Rise time and fall time describe how quickly a device responds to a change in the input signal, which is particularly important in applications such as high-speed communication, imaging, and sensing. Rise time is the time required to reach from 10% to 90% of the maximum value of the photocurrent, whereas the fall time is the time required to fall from 90% to 10% of the maximum value of the photocurrent.

To exploit the exciplex dissociation at the molecular interface of NPB and OXD-7, we conducted UV photodetector characterization on the same devices, highlighting their bifunctionality. We employed a straightforward setup to characterize a UV-photodetector, utilizing a UV-lamp and conducting measurements under dark conditions. The characterization was done under UV-illumination and reverse bias. The IV characteristics were measured both in the dark and under 365 nm UV light. All three devices demonstrated UV organic photodetector (OPD) characteristics, with the highest photocurrent generation observed in the 1 : 3 device configuration. The steady-state IVs of the photocurrent of the devices are compared in Fig. 7(a). At zero bias, the 1 : 3 device exhibited a higher photocurrent compared to the other devices, which exhibited relatively lower photocurrent levels. The 1 : 3 device showed a high responsivity of 17 mA W⁻¹, an ON/OFF ratio of ∼10³, and a high detectivity of 4 × 10¹¹ Jones at zero bias. The increase in the photocurrent of the 1 : 3 device is evident in the transient photoresponse comparison illustrated in Fig. 7(b). The photodetector parameters of the devices at 0 V were compared and are summarized in Table 3. It may be noted that the performances of these photodetectors are comparable or even better than the UV-photodetectors reported by our group with a NPB/ZnO system.³⁵

Fig. 7 (a) Comparison of the steady state IV of the photocurrent under 365 nm UV light of the NPB : OXD-7 (1 : 1, 1 : 3, and 3 : 1) devices. (b) Comparison of the cyclic photoresponse of the devices.

Table 3 Summary of the performance of UV-photodetectors using NPB:OXD-7 blend films as the active layer

NPB : OXD-7	ON/OFF ratio^a	Responsivity^a (mA W^−1)	Detectivity^a (Jones)
a Device parameters at 0 V.
1 : 1	1.00 × 10^2	1.2	1.81 × 10^10
1 : 3	3.50 × 10^3	17.00	4.00 × 10^11
3 : 1	0.15 × 10^2	0.92	6.00 × 10^9

---

Upon UV illumination, the bulk active layer forms the NPB:OXD-7 exciplex as it absorbs light from the 365 nm UV source. A comparative analysis of the absorption spectra (Fig. 2(a)) revealed that the 3 : 1 film, containing a higher concentration of NPB, exhibited higher UV absorption. This can be attributed to the dominant absorption of NPB within the 365 nm spectral region. Under conventional assumptions, where UV absorption is the primary determinant of photodetector response, the 3 : 1 composition should have exhibited superior performance. However, the experimental results suggest that some additional factors play a crucial role in governing detector performance. In this context, the significant surface potential of OXD-7 combined with the built-in potential of the heterojunction collectively facilitates exciplex dissociation. This enhanced dissociation process facilitates more efficient charge carrier generation and transport, leading to the superior UV detection performance of the 1 : 3 device compared to its counterparts.

In BHJ-based organic photodetectors, there exists an optimum donor : acceptor (D : A) ratio that facilitates balanced charge generation and transport, thereby improving device performance. In our system, NPB functions as the donor and OXD-7 as the acceptor. Several studies have reported that a low donor concentration in the BHJ matrix leads to reduced dark current and enhanced detectivity.³⁶ In our case, the 1 : 3 ratio, which corresponds to a low donor concentration, can be considered the optimum composition. This ratio appears to support efficient charge extraction of the dissociated exciplex excitons, contributing to the improved photodetector performance.

Comparative analyses of the photocurrent response of the devices under 254 nm and 365 nm UV sources, along with their corresponding dark currents, are shown in Fig. S6(a)–(c), respectively. At zero bias, the photocurrent generated under 254 nm UV illumination remains consistently low for all the devices. This observation confirms that the responsivity of NPB:OXD-7-based UV-OPDs is significantly higher in the 365 nm UV region compared to 254 nm.

The comparison of the OLED and photodetector performance revealed that the high SOP of OXD-7, combined with the strong UV absorption of NPB:OXD-7 bulk layer, plays a crucial role in device performance, contributing to its bifunctionality. Notably, the self-powered UV detector with a 1 : 3 ratio exhibited impressive performance while also maintaining satisfactory OLED performance.

Conclusion

In this work, we successfully demonstrated the bifunctionality of an exciplex by using NPB as the donor and OXD-7 as the acceptor, highlighting its potential for both OLED and UV-photodetector applications. The significant surface potential of OXD-7 facilitated exciplex dissociation at the molecular interface of NPB and OXD-7 with the strong UV-absorption, generating a photocurrent under UV illumination. This enables the device to have dual functions, both as an OLED and a UV photodetector, achieving a balance between exciplex emission and dissociation. The 1 : 3 (NPB : OXD-7) device performed better as a UV detector but showed poor performance as an OLED. Conversely, the 3 : 1 device excelled as an OLED but was a poor UV detector. This study demonstrates that the chosen exciplex combination, NPB:OXD-7, has the potential for bifunctionality, enabling the devices to serve as both OLEDs and UV photodetectors. Many exciplex combinations remain unexplored, offering the potential for multifunctional devices. By considering factors such as surface potential development, UV absorption, etc., in the transport materials, these combinations could be effectively utilized for enhanced device performance. By tailoring the SOP of the materials used, it is possible to enhance either the dissociation of exciplexes for OPDs or the emission of exciplexes for OLEDs, thus improving the performance of these devices for their respective applications. Therefore, the interplay of exciplex formation, UV absorption, and SOP of the conjugate materials critically influences the ability of the device to exhibit bifunctional behavior.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the main text and SI.

Kelvin probe measurement of the contact potential difference vs. film thickness; optical constants (n & k) obtained from spectroscopic ellipsometry for the example of a 1 : 1 mixture; angle-dependent photoluminescence in s- and p-polarization; AFM images of the NPB, OXD-7 and NPB:OXD-7 mixed films; EL spectra vs. applied voltage plot (b) CIE coordinates (x,y) vs. voltage plot (c) CIE diagram with the CIE coordinates of the NPB:OXD-7 (1 : 1) device; steady state IV of photo current under 365 nm, 254 nm UV source and under dark conditions of the NPB:OXD-7 devices (a) 1 : 1, (b) 1 : 3, (c) 3 : 1; experimental details. See DOI: https://doi.org/10.1039/d5tc01944c

Acknowledgements

K. R. acknowledges a PhD Fellowship from CSIR and NU acknowledges support from the CSIR FIRST Project (MLP-0066), SERB-CRG Project (CRG/2020/003699) and the DST Nanomission, Govt. of India. A. K. S. acknowledges financial support in the form of an INSPIRE Ph D Fellowship from DST, Govt. of India. IN and NU acknowledge support from DST-INT/DAAD/P-12/2023. WB thanks the Deutscher Akademischer Austauschdienst (DAAD) for funding under project no. 57683451.

References

1. H. Du, Y. Hao, X. Zhang, S. Zhang, Q. Liu and Z. Pang, Org. Electron., 2025, 136, 107159.
2. S. Muruganantham, Y. H. Jung, H. R. Kim, J. H. Ham, R. Braveenth, K. R. Naveen, M. Y. Chae and J. H. Kwon, J. Mater. Chem. C, 2025, 13, 2923–2931.
3. H. Y. Zhang, M. Zhang, H. Zhuo, H. Y. Yang, B. Han, Y. H. Zheng, H. Wang, H. Lin, S. L. Tao, C. J. Zheng and X. H. Zhang, Chem. Sci., 2024, 15, 14651–14659.
4. C. K. Vipin, A. Shukla, K. Rajeev, M. Hasan, S.-C. Lo, E. B. Namdas, A. Ajayaghosh and K. N. N. Unni, J. Phys. Chem. C, 2021, 125, 22809–22816.
5. K. K. Kesavan, J. Jayakumar, M. Lee, C. Hexin, S. S. Swayamprabha, D. K. Dubey, F.-C. Tung, C.-W. Wang and J.-H. Jou, Chem. Eng. J., 2022, 435, 134879.
6. J. Shao, C. Chen, W. Zhao, E. Zhang, W. Ma, Y. Sun, P. Chen and R. Sheng, Micromachines, 2022, 13, 298.
7. Q. Wang, Q.-S. Tian, Y.-L. Zhang, X. Tang and L.-S. Liao, J. Mater. Chem. C, 2019, 7, 11329–11360.
8. S. K. Jeon and J. Y. Lee, Org. Electron., 2020, 76, 105477.
9. M. Zhang, C. J. Zheng, H. Lin and S. L. Tao, Mater. Horiz., 2021, 8, 401–425.
10. S. S. Swayamprabha, M. R. Nagar, R. A. K. Yadav, S. Gull, D. K. Dubey and J.-H. Jou, J. Mater. Chem. C, 2019, 7, 7144–7158.
11. A. Sebastian, S. Achankunju, S. Nair, K. Rajeev, A. V. Gopinath, V. K. Maka, S. Kunnikuruvan, K. N. N. Unni, K. N. Parida and I. Neogi, J. Mater. Chem. C, 2025, 13, 11800–11813.
12. F. Claveria-Cadiz, M. Rojas-Poblete, A. Muñoz-Castro, E. Schott and R. Guajardo-Maturana, J. Photochem. Photobiol., A, 2024, 452, 115547.
13. R. A. Krueger and G. Blanquart, Phys. Chem. Chem. Phys., 2019, 21, 10325–10335.
14. R. Keruckiene, M. Guzauskas, L. Lapienyte, J. Simokaitiene, D. Volyniuk, J. Cameron, P. J. Skabara, G. Sini and J. V. Grazulevicius, J. Mater. Chem. C, 2020, 8, 14186–14195.
15. T. Pan, S. Liu, L. Zhang, W. Xie and C. Yu, Light: Sci. Appl., 2022, 11, 59.
16. Y. Chen, Y. Yao, N. Turetta and P. Samorì, J. Mater. Chem. C, 2022, 10, 2494–2506.
17. J. Saghaei, T. Leitner, V. N. Mai, C. S. K. Ranasinghe, P. L. Burn, I. R. Gentle, A. Pivrikas and P. E. Shaw, ACS Appl. Electron. Mater., 2021, 3, 4757–4767.
18. B. B. Meer, D. Sharma, S. Tak, G. G. Bisen, M. D. Shirsat, K. G. Girija and S. S. Ghosh, RSC Adv., 2023, 13, 33668–33674.
19. P. L. dos Santos, P. Stachelek, Y. Takeda and P. H. Pander, Mater. Chem. Front., 2024, 8, 1731–1766.
20. V. Jankus, K. Abdullah, G. C. Griffiths, H. Al-Attar, Y. H. Zheng, M. R. Bryce and A. P. Monkman, Org. Electron., 2015, 20, 97–102.
21. F. Teixeira, J. C. Germino and L. Pereira, Appl. Sci., 2023, 13, 12020.
22. M. Kumar and L. Pereira, Nanomaterials, 2020, 10, 101.
23. H. L. Zhu, W. C. H. Choy, W. E. I. Sha and X. G. Ren, Adv. Opt. Mater., 2014, 2, 1082–1089.
24. Y. Miao, G. Wang, M. Yin, Y. Guo, B. Zhao and H. Wang, Chem. Eng. J., 2023, 461, 141921.
25. L. Lu, Y. Guo, B. Zhao, H. Wang and Y. Miao, Mater. Today, 2024, 74, 109–120.
26. E. Ito, Y. Washizu, N. Hayashi, H. Ishii, N. Matsuie, K. Tsuboi, Y. Ouchi, Y. Harima, K. Yamashita and K. Seki, J. Appl. Phys., 2002, 92, 7306–7310.
27. A. Hofmann, A. Cakaj, L. Kolb, Y. Noguchi and W. Brutting, J. Phys. Chem. B, 2025, 129, 779–787.
28. A. Hofmann, M. Schmid and W. Brütting, Adv. Opt. Mater., 2021, 9, 2101004.
29. Y. Ueda, H. Nakanotani, T. Hosokai, Y. Tanaka, H. Hamada, H. Ishii, S. Santo and C. Adachi, Adv. Opt. Mater., 2020, 8, 2000896.
30. H. Iwasaki, Y. Majima and S. Izawa, Synth. Met., 2024, 309, 117772.
31. J. Song, F. Zhang, L. Yang, K. Chen, A. Li, R. Sheng, Y. Duan and P. Chen, Mater. Adv., 2021, 2, 3677–3684.
32. V. Bozkus, E. Aksoy and C. Varlikli, Electronics, 2021, 10, 725.
33. J. Song, C. Wang, Y. Guan, X. Bao, W. J. Li, L. Chen and L. Niu, RSC Adv., 2023, 13, 23619–23625.
34. K. Rajeev, C. K. Vipin, A. K. Sajeev, A. Shukla, S. K. M. McGregor, S. C. Lo, E. B. Namdas and K. N. Narayanan Unni, Front. Optoelectron., 2023, 16, 46.
35. B. Hanna, K. P. Surendran and K. N. Narayanan Unni, RSC Adv., 2018, 8, 37365–37374.
36. T. Zhang, J. Schröder, J. Wolansky, K. Leo and J. Benduhn, Appl. Phys. Lett., 2024, 124, 19.

---