arXiv subjects

Pradeep R. Nair

Publications and source records attributed to Pradeep R. Nair.

At least 19 recordsLinked to original sources

arXiv2025

Sub Band Gap Operation Limits for Perovskite Light Emitting Diodes

Ultra low voltage operation of Perovskite light emitting diodes (PeLEDs) has been demonstrated in recent years as high radiance with minimal power consumption is a desired feature. However, the light output at such conditions from PeLEDs is typically very low, and the maximum in external quantum efficiency (EQE) and energy conversion efficiency (ECE) are achieved at large biases with significant power consumption. Here, we explore the possibility of achieving maximums in EQE and ECE at sub band gap voltages for PeLEDs. Our analysis consistently interprets otherwise scattered experimental data from literature, identifies the limits for low voltage operation, and elucidates optimization routes for sub band gap high radiance operation of PeLEDs.

cond-mat.mes-hall↗

arXiv2025

Prospects for market-specific design of Perovskite-Silicon tandem solar cells

The quest for optimal perovskite for tandem cell configurations is challenging as it involves several factors ranging from device level performance under field conditions to degradation rates and cost. Here, we first highlight the limitations of traditional detailed balance or Shockley-Queisser (SQ) analysis towards the design of Perovskite/Silicon tandem solar cells. Through well-calibrated numerical simulations, we evaluate geographic location-specific annual energy yield (EY) and quantify the influence of temperature-dependent material and transport parameters. Our results indicate that the EY scales in a near-identical manner with the top cell band gap(EgT) for various geographic locations. In comparison to SQ analysis, our simulations predict a twofold relaxation in the target degradation rates at which perovskites over a broad range of band gaps could yield a comparable levelized cost of Energy (LCOE). These insights are of broad interest for the development of perovskite materials, and test protocols to evaluate the stability of Perovskite-Silicon tandem solar cells.

cond-mat.mtrl-sci↗

arXiv2024

Photoluminescence efficiency droop in Perovskites

The commercialization prospects of perovskite light emitting diodes depend on its luminescence efficiency under large carrier densities. The decrease in luminescence efficiency under such high injection conditions could lead to an undesired increase in power consumption with associated degradation and stability concerns. Here, through detailed modeling of thermal transport and carrier generation-recombination, we unravel the physical mechanisms that cause luminescence droop under high injection conditions. We show that self-heating leads to a reduction in the radiative recombination (both bimolecular and excitonic). The resultant increase in non-radiative recombination and hence the thermal dissipation acts as a positive feedback mechanism that leads to efficiency droop in perovskites. Our model predictions, well supported by experimental results, could be of broad interest towards the degradation-aware thermal design of perovskite optoelectronics and stability.

cond-mat.mtrl-sci↗

arXiv2024

Influence of Phase Segregation on the Hysteresis of Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells (PSC) have demonstrated impressive performance improvement. Among the various characteristics, the time-dependent current-voltage (J-V) hysteresis allows a direct exploration of various critical phenomena that affect the stability of PSCs. The hysteresis is associated with various spatial heterogeneity-related phenomena, including lifetime, bandgap, and phase segregation. We investigate these phenomena through numerical simulations and quantify how the spatial non-uniformity in the perovskite active layer impacts the hysteresis. Further, we correlate the time dependent device degradation with the hysteresis trends in terms of ion density and effective carrier lifetime.

cond-mat.mtrl-sci↗

arXiv2024

Unraveling the role of excitons in the near ideal performance of perovskite light emitting diodes

Recent reports indicate that perovskite based light emitting diodes (LEDs) have achieved an external quantum efficiency (EQE) of 32% - rather an internal quantum efficiency close to 100%. Much of this improved performance is attributed to the role of excitons. While the experimental trends are encouraging, the recombination parameters estimated through extensive curve-fitting of photoluminescence (PL) transients are often not amenable to reasonable interpretations. In view of the same, through a detailed analysis of free carrier - exciton dynamics in perovskite optoelectronic materials, here we identify a coherent scheme to unambiguously back extract the relevant parameters. The model predictions compare well with the recent experimental results on perovskite LEDs with record EQE thus quantifying the role of excitons. Importantly, this work identifies a physics aware scheme for the design of experiments tailored towards consistent exploration of underlying physical mechanisms and hence could enable a synergistic optimization of process technology and device performance.

cond-mat.mtrl-sci↗

arXiv2024

Extraction of geometric and transport parameters from the time constant of exocytosis transients measured by nanoscale electrodes

Exocytosis is a fundamental process related to the information exchange in the nervous and endocrine system. Among the various techniques, vesicle impact electrochemical cytometry (VIEC) has emerged as an effective method to mimic the exocytosis process and measure dynamic information about content transfer using nanoscale electrodes. In this manuscript, through analytical models and large scale simulations, we develop scaling laws for the decay time constant ($τ$) for VIEC single-exponential transients. Specifically, our results anticipate a power law dependence of $τ$ on the geometric and the transport parameters. This model compares very well with large scale simulations exploring the parameter space relevant for VIEC and with experimental results from literature. Remarkably, such physics based compact models could allow for novel multi-feature based self consistent strategies for back extraction of geometric and transport parameters and hence could contribute towards better statistical analysis and understanding of exocytosis transients and events.

q-bio.SC↗

arXiv2024

Perovskite solar cells dominated by bimolecular recombination -- how far is the radiative limit?

Here, we report an experimental demonstration of perovskite solar cells dominated by bimolecular recombination and critically analyse their performance against radiative limits. To this end, we first establish a set of quantitative benchmark characteristics expected from solar cells limited by bimolecular recombination. Transient as well as steady state intensity dependent measurements indicate that our solar cells indeed operate at such limits with interface passivation comparable to the champion c-Si technology. Further, we identify novel characterization schemes which enable consistent back extraction of recombination parameters from transient optoelectrical and electroluminescence measurements. Remarkably, these parameters predict important features of dark current density vs. voltage characteristics (J-V) and Suns-V_OC measurements, thus validating the estimates and the methodology. Uniquely, this work provides a consistent and coherent interpretation of diverse experimental trends ranging from dark J-V, Suns-V_OC, steady state and transient intensity dependent measurements to electroluminescence quantum yield. As such, insights shared in this manuscript could have significant implications towards fundamental electronic processes in perovskite solar cells and further efficiency optimization towards Shockley-Queisser limits.

cond-mat.mtrl-sci↗

arXiv2023

Emission limited logarithmic and power law transients in pump-probe spectroscopy of perovskites

Optical pump-probe techniques like absorption spectroscopy and microwave conductivity are widely used to characterize the carrier dynamics in perovskites for optoelectronic applications. In contrast to the prevalent assumption of exponentials, here we predict the possibility of trap emission limited logarithmic and power-law transients. These predictions are validated by detailed numerical simulations and well supported by several experimental reports from recent literature. Interestingly, these findings indicate the need to revisit the existing schemes which rely on simplified rate equations and exponential decays to estimate the recombination parameters from pump-probe spectroscopy. Accordingly, we suggest appropriate methodologies to back extract parameters related to trap distribution from such non-exponential transients. Indeed, the insights shared in this manuscript could fundamentally impact the usage and interpretation of transient spectroscopy for emerging materials for optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2023

Geometry aware predictive models for exocytosis

Inter neuron communication happens through the exchange of neurotransmitters at the synapse by a process known as exocytosis. This makes exocytosis a fundamental process of information exchange in the body. The exocytosis process has a distinct geometry as it involves a vesicle that attaches to the cell membrane and then releases the neurotransmitters through a pore. Significant recent research, both experimental and numerical, attempt to understand the time dynamics of exocytosis. In this manuscript, we share an analytical model that predicts the key output parameters of exocytosis based on the geometry of the vesicle and pore. Our analytical predictions are well supported by detailed numerical simulations. This model could help extract geometrical parameters from experimental data and hence could be of broad interest.

physics.bio-ph↗

arXiv2022

Programmable Charge Trap for Junction-less selective extraction of holes in Solar Cells

Selective extraction of photo-generated carriers is a fundamental challenge in solar cells which is usually achieved through junctions with the associated doping as well as band offset differences. In this context, here we propose a new paradigm for selective extraction for majority carriers through novel usage of the programmable charge trap which comprises of Oxide-Nitride-Oxide (ONO) stack and has the primary function of holding electrically injected charge. Through detailed numerical simulations, here we show that such a charge trap with an additional metal contact can (i) compensate for efficiency loss due to sub-optimal passivation and sub-optimal hole selectivity in homojunction as well as transition metal oxide-based heterojunction solar cells and (ii) can also function as a standalone hole selection scheme. The proposed scheme, with its easy integration and the capability of programmable compensation of performance loss, is of interest to the photovoltaic community.

physics.app-ph↗

arXiv2022

Prospects for Perovskite/Silicon tandem solar cells to outperform c-Silicon solar cells at elevated temperatures

Successful commercialization of Perovskite/Si tandem solar cells (P/Si TSCs) need a-priori estimation of technological benchmarks to outperform c-Si based technologies under field conditions. To this end, through detailed numerical simulations and analytical modeling, here we identify the limits of ion migration and lifetime degradation till which P/Si TSCs remain competitive. Our results unravel a unique scaling law for the evolution of the efficiency and the temperature coefficient of P/Si TSCs which allows us to anticipate the limiting annual degradation rates. Interestingly, we find that 4T cells are potentially more immune to the ill effects of ion migration as compared to 2T cells. These insights are of broad relevance for material/interface engineering approaches and physics based accelerated tests which are focused towards long term stability and module reliability.

cond-mat.mtrl-sci↗

arXiv2021

Deciphering capacitance frequency technique for performance limiting defect state parameters in energy harvesting perovskites

With emerging thin film PIN based optoelectronics devices, a significant research thrust is focused on the passivation of trap states for performance enhancement. Among various methods, capacitance frequency technique (CFT) is often employed to quantify trap state parameters, however, the trapped charge induced electrostatic effect on the same is not yet established for such devices. Herein, we present a theoretical methodology to incorporate such effects in the CF characteristics of well-established carrier selective perovskite-based PIN devices. We show that the electrostatic effect of trapped charges leads to non-linear energy bands in perovskite layer which results in the underestimation of trap density from existing models of CFT. Consequently, a parabolic band approximation with effective length PBAEL model is developed which accurately predicts the trap density for shallow or deep states from CFT analysis. In addition, we demonstrate that the attempt to escape frequency, crucial for trapped charge dynamics with continuum energy bands, can be well extracted by eliminating non-linear effects at reduced perovskite thickness. We believe that our work provides a unified theoretical platform for CFT to extract trap state parameters for a broad class of organic and hybrid materials-based thin film devices for energy conversion applications such as solar cells, LEDs, etc.

physics.app-ph↗

arXiv2021

Is more Phase Segregation better for mixed halide perovskite devices: Spatial randomness, Ion migration, and Non-radiative recombination

Phase segregation is a critical phenomenon that influences the stability and performance of mixed halide perovskite based opto-electronic devices. In addition to the underlying physical mechanisms, the spatial pattern and randomness associated with the nanoscale morphology of phase segregation significantly influence performance degradation a topic which, along with the multitude of parameter combinations, has remained too complex to address so far. Given this, with MAPbI1.5Br1.5 as a model system, here we address the influence of critical factors like the spatial randomness of phase segregation, influence of ion migration, and the effect of increased non radiative recombination at material interfaces. Interestingly, our analytical model and detailed statistical simulations indicate a unique trend morphology evolution with increased phase segregation results, surprisingly, in a recovery in efficiency while non-radiative recombination at grain/domain boundaries results in efficiency degradation. Further, our quantitative and predictive estimates identify critical parameters for interface states beyond which device variability could be an important system level bottleneck. Indeed, these estimates are broadly applicable to systems which undergo phase segregation and have interesting implications to perovskite based optoelectronic devices from stability concerns to engineering approaches that attempt to arrest phase segregation.

cond-mat.mtrl-sci↗

arXiv2021

Ion Mobility Independent Large Signal Switching of Perovskite Devices

The presence of mobile ions in perovskites is well known to influence the device electrostatics leading to a wide variety of anomalous characteristics related to hysteresis, efficiency degradation, low frequency capacitance, large signal switching, etc. Accordingly, the ion mobility is understood to a have a critical influence on the associated time constants/delays. Quite contrary to this broadly accepted thought, here we show that the time delays associated with large signal switching show a universal behavior dictated by electronic dipoles, rather than ionic dipoles. Due to the resultant sudden and dramatic collapse of contact layer depletion region, switching delays are independent of ion mobilities! Further, our detailed numerical simulations, well supported by experimental results, indicate that terminal currents show near steady state behavior well ahead of the relaxation of ionic distributions to their steady state conditions. These results have interesting implications towards the understanding and optimization of perovskite based electronic devices, including solar cells and LEDs.

cond-mat.mtrl-sci↗

arXiv2021

Material and Process Tolerant High Efficiency Solar Cells with Dynamic Recovery of Performance

Low cost, highly efficient, and stable solar cells demand low temperature processing, less stringent criteria on materials, and possibility of dynamic recovery from long term degradation: a combination of features unachievable from the perspectives of current cSi technology. To this end, here we propose a novel solar cell architecture with an additional control gate. Our simulation results indicate that the proposed device can achieve excellent efficiency even if the back-surface passivation is sub-optimal; thus allowing exploration of a wide variety of materials and low temperature fabrication processes. Importantly, such solar cells can dynamically offset efficiency loss due to elevated temperature and interface degradation associated with long term field operation and hence could be of broad interest to the PV community.

physics.app-ph↗

arXiv2020

Electronic Circuit Inspired Optimization of Nanogap Electrochemical Biosensors

Electrochemical biosensors and the related concept of redox detection at nanogap electrodes are increasingly explored for ultra-sensitive detection of biomolecules. While experimental demonstrations have been encouraging, the associated design and optimization of electrode geometry, beyond the simple one-dimensional architectures, is inherently challenging from multiple aspects related to numerical complexity. Here we develop a facile simulation scheme to address this challenge using well established electronic circuit analysis techniques that are available as open source-ware. Based on this approach, we show that electrode geometry, especially nano-structured redox electrodes on a planar surface, has interesting implications on the detection limits and settling time of electrochemical biosensors. The methodology we developed and the insights obtained could be useful for electrode optimization for a wide variety of problems ranging from biosensors to electrochemical storage.

physics.bio-ph↗

arXiv2019

Dynamic Tracking Biosensors: Unconstrained Detection and Performance Limits

Accurate detection of target molecules at low concentrations in the presence of a high concentration of undesired molecules is a major challenge for End Point (EP) assays. Non-specific binding of undesired molecules to receptors limits the minimum detectable concentration of the target significantly. Dynamic tracking (DT) of binding and unbinding events allows us to overcome this challenge and provides a remarkable improvement in the minimum detectable target concentration, as demonstrated recently. In this manuscript, we propose a novel unconstrained detection scheme which does not rely on a priori knowledge of the reaction constants. This scheme allows facile back extraction of various critical sensor parameters as well. Further, through a combination of theoretical analysis and detailed statistical simulations, we show that DT sensors could be several orders of magnitude better than $EP$ biosensors. This work identifies and establishes the functional dependence of critical parameters on the performance of DT sensors and hence could be of broad interest to the community towards further optimization.

physics.chem-ph↗

arXiv2018

Ion Induced Passivation of Grain Boundaries in Perovskite Solar Cells

Demonstration of high-efficiency large area cells with excellent stability is an important requirement towards commercialization of perovskite solar cells (PSC). With reports of high-quality perovskite grains, it is evident that the performance of such large area cells will be strongly influenced by phenomena like carrier recombination and ion migration at grain boundaries (GBs). Here, we develop a modeling framework to address performance limitation due to GBs in large area PSCs. Through detailed numerical simulations, we show that photo-carrier recombination has a non-trivial dependence on the orientation of GBs. Interestingly, we find that ions at GBs lead to significant performance recovery through field effect passivation, which is influenced by critical parameters like density and polarity of ions, and the location of GB. These results have interesting implications towards long-term stability and hence are relevant for the performance optimization of large area polycrystalline based thin film solar cells such as PSCs, CIGS, CZTS, etc.

physics.app-ph↗