Systematic Study via Controlling the Molecular Design for Reducing the Triplet Delayed Lifetime in Thermally Activated Delayed Fluorescence Emitters for Efficient OLEDs Nisha Yadav, Annette Mariya Tedy, Shana Shirin M A, Thamodharan Viswanathan, Arun K Manna, Pachaiyappan Rajamalli ACS Applied Materials and Interfaces, 2026 A strategy for developing high-performance thermally activated delayed fluorescence (TADF) emitters with shorter delayed lifetimes (τD) is highly sought after. To address this, we have systematically constructed three emitters based on open and fused molecular structures by opting for the same 3,6-di-tert-butyl-9H-carbazole (tCz) as the donor unit and varying the acceptor core. NP-tCz (fused), BPh-tCz (open), and PPy-tCz (N-substituted open) showed gradually decreased excited singlet–triplet energy gap (ΔEST) values of 0.48 eV, 0.18 eV, and 0.11 eV, respectively, in a 10 wt % emitter: 3,3′-di(9H-carbazol-9-yl)-1,1′-biphenyl (mCBP) blend. NP-tCz displayed photoluminescence (PL) maxima at 463 nm, exhibiting both room temperature phosphorescence (RTP) and TADF signatures, owing to its large ΔEST and prolonged τD of 40 ms. In contrast, BPh-tCz showed a PL peak at 478 nm with typical TADF behavior and a τD of 456 μs, while PPy-tCz exhibited PL maxima at 498 nm, also characteristic of TADF, with the shortest τD of 116 μs. The emissive layer in the organic light-emitting diodes (OLEDs) comprised 10 wt % emitter blended in mCBP host. The OLEDs based on NP-tCz, BPh-tCz, and PPy-tCz exhibited electroluminescence peaks at 478, 484, and 499 nm, respectively, with maximum external quantum efficiency of 7.2%, 11.9%, and 26.9%. PPy-tCz (open) containing N-substitutional doping is the most efficient among the three emitters, offering the shortest τD and high efficiency. Deeper molecular-level insights into the excited-state deactivation processes of these emitters were obtained from reliable quantum-chemical calculations, which complemented and supported the experimental observations. These findings offer a pathway to control the delayed lifetime and improve efficiency by tailoring molecular structures through open and fused configurations.
Rational Rigid-Core Design Realizes Efficient and Color-Pure Sky-Blue MR-TADF Emission in Boron-Nitrogen Functionalized Dibenzo-Xanthene Thamodharan Viswanathan, Sabyasachi Maity, Upasana Deori, Nikhitha R., Nandish SH, Sandhayarani Pal, Nisha Yadav, Anirban Mondal, Pachaiyappan Rajamalli Small, 2026 In this work, we report the strategic incorporation of a rigid 14H‐dibenzo[a,j]xanthene ( DBX ) core into the BCz‐BN framework, leading to the development of a novel DBX‐BCz‐BN multi‐resonance (MR) thermally activated delayed fluorescence (TADF) emitter. This structural modification preserves the sky‐blue emission color while substantially narrowing the emission profile, reducing the full width at half maximum (FWHM) to 17 nm. In addition, the singlet–triplet energy gap (Δ E ST ) is lowered from 0.17 eV for BCz‐BN to 0.06 eV, thereby promoting more efficient reverse intersystem crossing (RISC) and improved exciton harvesting. As a result, the corresponding OLED device based on DBX‐BCz‐BN exhibits an external quantum efficiency (EQE) of 22.2%, while maintaining the same emission color. These results demonstrate that rigid‐core engineering via DBX incorporation is an effective strategy for achieving narrowband, efficient, and color‐pure sky‐blue TADF emitters.
Step-by-Step Guide for Harnessing Organic Light Emitting Diodes by Solution Processed Device Fabrication of a TADF Emitter Smarak Islam Chaudhury, Savita Chand, Shouvik Bhuin, S. H. Nandish, Nisha Yadav, Pachaiyappan Rajamalli Journal of Visualized Experiments, 2025 The demand for energy-efficient light sources has witnessed a significant surge in the recent era, due to the increasing awareness towards environmental concerns and climate change. Organic Light Emitting Diodes (OLEDs) offer superior display quality, energy efficiency, and sustainability, which makes them an ideal choice for eco-conscious consumers and reduced carbon footprint-based industries. The thinner and more flexible OLEDs have revolutionized the field of smartphones, televisions, automotive displays, and wearable gadgets. Here, in this paper, an efficient OLED device is fabricated starting from the synthesis of a donor-acceptor-based emitter, followed by its purification. The organic synthesis process is explained briefly. For purification, firstly, the product mixture is purified by column chromatography, and then it is sublimed using a temperature gradient vacuum sublimation set-up to obtain a high-purity compound. The ultimate goal is to make a multi-layered OLED device that needs ultra-high-purity emitters. Making an efficient OLED needs precision and expertise; thus, showcasing a detailed step-by-step protocol is important to the science community. Here, the details about the solution-processed OLED device fabrication is unfolded in four steps (substrate cleaning, spin coating of layers, thermal evaporation of layers, and device performance measurement). First, the substrates need to be cleaned using a well-established protocol. Next, the layered architecture is obtained by spin coating and thermal deposition of desired materials to the required thickness. The thermal evaporation is done using a high vacuum thermal evaporator with an integrated glove box setup. Finally, to obtain the device's performance, measurements are to be carried out in a dark environment. This comprehensive work will provide in-depth knowledge of device fabrication and inspire more researchers towards decorating this world with OLEDs.
Highly Efficient Orange Phosphorescent Organic Light Emitting Diodes with Reduced Roll-Off Using Xanthene-Conjugated Carbazole and Acridone as Host Materials Thamodharan Viswanathan, Nisha Yadav, Nandish S H, Sandhyarani Pal, Ashish Kumar Mazumdar, Pachaiyappan Rajamalli Chemistry an Asian Journal, 2025 The development of high‐efficiency orange phosphorescent organic light‐emitting diodes (PhOLEDs) with reduced efficiency roll‐off offers significant benefits, enabling a more simplified method for producing white‐OLEDs through the combination of sky blue and orange OLEDs, in contrast to the conventional use of three primary colors. Herein, we have designed and synthesized two hosts with 9‐(4‐(14H‐dibenzo[a,j]xanthen‐14‐yl)phenyl)‐9H‐carbazole (XaPCz) and 10‐(4‐(14H‐dibenzo[a,j]xanthen‐14‐yl)phenyl)acridin‐9(10H)‐one (XaPAc) derivatives. The singlet and triplet energies for XaPCz and XaPAc are S1=3.65, 3.12 eV, and T1=2.67, 2.70 eV, respectively. These materials are employed as hosts for orange PhOLEDs using bis(2‐phenylquinoline) (acetylacetonate) iridium (III) [Ir(pq)3] as an emitter. XaPCz demonstrates maximum external quantum efficiency (EQEmax) of 20.5 %, and XaPAc shows EQEmax of 22.3 % with Commission Internationale de l′Eclairage (CIE) coordinates of (0.53, 0.46) and (0.54, 0.45), respectively. In addition, the XaPAc‐based device shows a lower turn‐on voltage (2.5 V) and high‐power efficiency of 60.0 lm/W. More importantly, both device shows reduced roll‐off and retain more than 94 % of EQEmax at 1,000 cd/m2. The XaPAc‐based device maintains an EQE of 17.9 % even at 10,000 cd/m2.
Regulating Spatial Configuration in Donor-π-Acceptor for through-Space Exciton Transfer: Concentration-Independent Emitter for OLEDs Nisha Yadav, Upasana Deori, Arun K Manna, Pachaiyappan Rajamalli Advanced Optical Materials, 2025 Thermally activated delayed fluorescence (TADF) emitters have garnered much attention due to 100% exciton utilization and toxic metal‐free design but often experience concentration‐quenching, requiring dispersion into the host matrix. Developing emitters that maintain consistent performance and emission wavelength irrespective of the concentration remains a significant challenge. Herein, two TADF emitters (2BPy‐pTC and 2BPy‐oTC) are designed and synthesized. The nature and energetics of the lowest excited singlet (S1) and triplet (T1) along with the extent of through‐bond exciton transfer (TBET) and through‐space exciton transfer (TSET) are unveiled using reliable quantum‐chemical calculations. While 2BPy‐pTC exhibits pre‐dominantly TBET, a greater extent of TSET is found in 2BPy‐oTC. Both emitters show a low singlet‐triplet energy gap (ΔEST), 0.21 eV for 2BPy‐pTC and 0.02 eV for 2BPy‐oTC. For 2BPy‐pTC, increasing the emitter concentration from 5 to 100 wt.% causes a bathochromic shift in the electroluminescence (EL) peak (467 to 495 nm) and a drop in maximum external quantum efficiency (EQEmax) from 12.0% to 5.5%. In contrast, 2BPy‐oTC maintains EL maxima of 500 nm and consistent EQEmax (24.0% – 27.2%) while increasing emitter concentration, acting as a universal emitter for both doped and non‐doped OLEDs and eliminating the need for tedious co‐deposition.
Systematic Study to Choose Appropriate Materials Combination for Green Hyperfluorescent Organic Light-Emitting Diodes Upasana Deori, Nisha Yadav, Gyana Prakash Nanda, Kishan Lal Kumawat, Pachaiyappan Rajamalli ACS Applied Electronic Materials, 2023 Hyperfluorescent organic light-emitting diodes (HF-OLEDs) combine a thermally activated delayed fluorescence (TADF) molecule with a terminal emitter and have achieved extensive progress as they can fully harness the advantages of both materials. However, the energy differences between the materials and their impact on the efficiency roll-off of the devices need to be investigated to further understand the energy transfer process in detail. In this context, a systematic study was done by combining two efficient TADF materials, 4BPy-mDTC and 3BPy-mDTC, as assistant dopants with various terminal emitters. The resulting HF-OLEDs showed an external quantum efficiency (EQE) above 20%, yet devices experienced roll-off at high luminance when the lowest excited energy level differences between the TADF assistant dopant and terminal emitter are either small or large. 4BPy-mDTC has a small energy difference with Ir(ppy)3 and C545T, showing reasonable performances but high roll-off at high brightness. However, the performance was boosted in the case of 3BPy-mDTC with maximum EQEs of 28 and 23% for Ir(ppy)3 and C545T, respectively. The device with Ir(ppy)3 could retain 87%, and C545T retained 67% of their maximum EQE at 500 cd m–2. Additionally, 3BPy-mDTC showed reduced performance for DTA-AN due to a larger energy difference, while it worked well with 4BPy-mDTC. The results suggest that the energy differences between the materials are vital in improving the device performance and roll-off characteristics by maintaining an optimum energy gap. This work provides insight into developing highly efficient hyperfluorescent OLEDs by modulating the energy difference between the materials.
