Organic & Hybrid Electronics Device Laboratory (OHEDL)
Research Portfolio:
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The primary research interest of our group is to explore new dimensions for next generation flexible and wearable electronics such as polymer (OPV) and perovskite solar cell, OLED, OFET, bio-sensors, detectors, memory devices etc. Our group at “Organic & Hybrid Electronic Device Laboratory (OHEDL)”, DESE IIT-Delhi, is a multi-disciplinary research group that studies the physics, material science and device engineering with the aim to develop innovations that will lead to better device performance and higher device efficiencies. We also investigate the fundamental photo-physics of such new class of devices to improve their performances and determining the performance limitation of the material in question. Our techniques are primarily experimental, although we also collaborate with various national and international theoretical and computational experts to understand the results. The devices studied in our group include polymer, perovskite and quantum dot based photovoltaic devices, photodiodes for light detection, noninvasive wearable sensors, and organic memory devices etc. Some of our ongoing major activities are:
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Ultrasonic spray coated scalable OPVs:
We are investigating various effects of in-situ annealing and acoustic substrate on nanoscale phase segregation and photovoltaic properties of ultrasonic spray deposited scalable OPV devices. It has been found that ultrasonic spray coating at moderate substrate temperature, well below the solvent boiling point, results in the best quality films with comparable properties to spin coated samples. The similarities in film characteristics and device performances between spin and ultrasonic spray deposited samples indicates its practical feasibility as a scalable technique for large area applications.
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Graphene like 2D materials for photovoltaic applications:
We have explored the role of graphitic carbon nitride quantum dots (g-C3N4 QDs) on polymer solar cell performances. Almost 40% performance improvement was obtained compared to the pristine devices, due to the multifunctionality of the incorporated g-C3N4 QDs as crystallanity improver and energy harvesting booster. Forster resonance energy transfer (FRET) between the QDs and host polymer was observed. The enhanced over all power conversion efficiency attributed to the combined effect of improved morphology and increased photon capturing due to FRET effect. This study opens new prospects for developing high-efficiency solution processable OPV devices using g-C3N4 QDs as the third component of the active layer.
Nano materials for hybrid solar cell:
The present work has provided complete study from systematic synthesis of phase pure FeS2 NCs through hot injection to fabrication of solution processed polymer fullerene solar cell incorporated FeS2 NCs with varying loading amount (weight %) as well as performance analysis and charge carrier dynamics. The devices with optimum doping showed performances better than any other reported devices with similar material systems so far. The detailed analysis of structure-property relationship of such low dimensional material systems using battery of characterization techniques such as AFM, XRD, TPC-TPV, Photocurrent and I-V measurements have enabled us to understand the effect formation and influences of FeS2 NCs on sub bandgap localized energy states and charge career dynamics of the devices in great details.
Functionalized 2D Materials: Plasmonic OPV:
We have utilized the synergistic plasmonic effects of multi-shaped Au nanostructures (diameter/edge length ~ 50 nm) hybridized with few-layer WS2 nanosheets in improving the photocurrent of organic solar cells (OSCs). An efficiency enhancement of more than 15% and an external quantum efficiency improvement in a broad wavelength range of 350-700 nm were demonstrated by incorporating these Au-WS2 nanohybrids as an interlayer between hole-transport and photoactive layers. Finite-difference time-domain simulations were performed to comprehensively study the plasmonic responses, such as scattering efficiency, direction-dependent scattering, and near-field effects of individual nanostructures. More than 50% forward light scattering indicated that these hybridized Au nanostructures are highly suitable candidates to couple the incident light into the active layer, hence attaining a significant improvement in device photocurrent.
ML Assisted High Efficiency NFA OPV:
Our approach to predict and screen the photovoltaics performance parameters of OSCs based on various polymer:NFA combinations by employing a data-driven machine learning (ML) and successively validated this predictivity by fabricating a set of highly efficient devices with a PCE up to 15.23%. A dataset of 1242 experimentally verified donor:acceptor (D/A) combinations was constructed, and the corresponding material descriptors were generated to train and test five different supervised ML models using a unique combination of both frontier molecular orbital (FMO) and RDKit descriptors. Performance predictions was verified by SHapley Additive exPlanations (SHAP) analyses to discard any over or under estimation. The proposed ML framework guided by these new descriptors will be indeed fruitful for designing new molecules and screening and predicting suitable D/A combinations to accelerate the development of highly efficient OSCs.
Perovskite solar cell:
We have carried out systematic investigations on the energetic distribution of trap states influencing performance of inverted perovskite solar cells by temperature dependent transient photocurrent analysis. Low-temperature measurement enabled us to understand the structure-property relationship of such novel material systems. Detailed analysis of the recombination process of photo-generated charge carriers through shallow and deep trap states and their effects on various device parameters have been investigated thoroughly, which is crucial for the fundamental understanding of device physics, material designing, and performance stabilities. We have observed significant changes in the density of trap states (tDoS) for different perovskite material phases at different temperatures.
We have developed an intermediate ion-complex method to form 1.52eV bandgap “phase-pure” FA0.95Cs0.05PbI3 composition by incorporating judicious amount of smaller and more volatile ions (such as Cl-/I- and MA+/Cs+) into the host FAPbI3 system. These ions were observed to persist in the “transitional-phase” and “stimulate” the formation of α-phase. A high-quality perovskite thin-film with enhanced optoelectronic quality, such as a 10x improvement in charge carrier lifetime, was developed. Furthermore, their photovoltaic efficiency recorded as 20.21% with negligible PCE degradation when measured for about 1000 hours. We quantified non-radiative recombinations by estimating and comparing quasi-fermi energy level splitting (QFLS), and obtained over 90% of the theoretical limit for open-circuit voltage (Voc).
Adding a judicious amount of smaller ionic radii element Mg and TAEA leads to greatly enhanced optoelectronic properties along with enhanced phase and ambient stability. The presence of Mg leads to the reduction of the formation temperature of the γ-CsPbI3 phase. The presence of TAEA on the surface is very high, which turns the surface more resistant to water ingress. Owing to the large and comparable bond length of the TAEA molecule, it is expected to cross-link one CsPbI3 unit on the surface to the adjacent one making H-bond with the halides. This strong interaction between –NH2 and the halide, suppresses I- ion migration. The cross-linkage between adjacent units will inhibit the spontaneous octahedral tilting and rotating thus restricting the rapid phase transition from the photoactive black phase to the non-photoactive yellow phase at room temperature and in ambient humid conditions.