Search

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., Comment: Accepted in ACS Energy Letters

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.

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.

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.

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., Comment: 7 pages, 4 figures

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.

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.

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.

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., Comment: 4 pages, 4 figures, Updated version accepted for publication in IEEE Transactions on Electron Devices

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., Comment: revised the abstract and updated the article by extending our approach to 2-D geometries

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., Comment: journal, 5 pages, 5 figures

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.

Carrier Selective (CS) Silicon solar cells are increasingly explored as a low cost alternative to PN junction Silicon solar cells. While the recent trends on power conversion efficiency are encouraging, the temperature coefficient and hence the power output under elevated temperatures are not well explored for such solar cells. Here, we address this issue through detailed numerical simulations to explore the influence of interface and material parameters on the temperature coefficient. Our results indicate that irrespective of the interface quality, the temperature coefficient of CS solar cells improves with an increase in band discontinuities. Interestingly, contrary to the trends related to efficiency, our results indicate that the temperature coefficient of CS solar cells is more critically affected by the interface quality of the minority carrier extraction layer than the majority carrier extraction layer. These insights have important implications towards the choice of optimal material and processing conditions for Si based CS solar cells.

Capacitance-Voltage (CV) measurements along with the Mott-Schottky (MS) analysis are widely used for characterization of material and device parameters. Using a simple analytical model, supported by detailed numerical simulations, here we predict that the capacitance of thin film devices scale as V^-2 (V is the applied potential), instead of the often used V^-0.5 dependence of MS analysis- with significant implications towards extraction of parameters like doping density, built-in voltage, etc. Surprisingly, we find that the predicted trends are already hidden in multiple instances of existing literature. As such, our results constitute a fundamental contribution to semiconductor device physics and are directly applicable and immediately relevant to a broad range of optoelectronic devices like organic solar cells, perovskite-based solar cells, and LEDs, thin film a-Si devices, etc., Comment: 6 Pages, 4 Figures

While the performance enhancement witnessed in the field of perovskite solar cells over the recent years has been impressive, it is now evident that further optimization beyond the existing literature would require detailed analysis of various loss mechanisms. Here we address the same through detailed numerical simulations of optical and electrical characteristics. We quantify the various losses like optical losses (5-6%), recombination losses (3-4%), and resistive losses against the Auger limited practical efficiency limits. Moreover, we illustrate the schemes that result in reduction of these losses and eventual increase in efficiency. In addition, we extend the analyses to identify the optimum thickness of perovskite and the factors affecting the optimum thickness have been discussed in detail.

Carrier selective (CS) silicon solar cells are increasingly explored using a variety of different materials. However, the optimum properties of such CS materials are not well understood. In this context, through detailed analytical and numerical modeling, here we provide several interesting insights on the efficiency tradeoff with CS material properties. First, we show that perfect band alignment is a desirable feature only if the interface is devoid of any trap states. Otherwise, a band offset of around 0.2eV-0.4eV provides sufficient band bending to reduce the effect of interface recombination, thus improving the performance. Surprisingly, the interface passivation quality for the minority carrier extraction layer is found to be far less demanding than that for the majority carrier extraction layer. Additionally, doping density and dielectric constant of CS layers have a similar effect as band offset on solar cell performance. Our results have obvious implications toward the selection of appropriate materials as carrier selective layers and hence are of broad interest to the community., Comment: 12 pages, 11 figures and 1 table

With excellent efficiencies being reported from multiple labs across the world, device stability and the degradation mechanisms have emerged as the key aspects that could determine the future prospects of perovskite solar cells. However, the related experimental efforts remain scattered due to the lack of any unifying theoretical framework. In this context, here we provide a comprehensive analysis of ion migration effects in perovskite solar cells. Specifically, we show, for the first time, that (a) the effect of ionic charges is almost indistinguishable from that of dopant ions, (b) ion migration could lead to simultaneous improvement in Voc and degradation in Jsc - a unique observation which is beyond the realm of mere parametric variation in carrier mobility and lifetime, (c) champion devices are more resilient towards the ill effects of ion migration, and finally (d) we propose unique characterization schemes to determine both magnitude and polarity of ionic species. Our results, supported by detailed numerical simulations and direct comparison with experimental data, are of broad interest and provide a much needed predictive capability towards the research on performance degradation mechanisms in perovskite solar cells, Comment: 8 pages, 5 figures

Process induced efficiency variation is a major concern for all thin film solar cells, including the emerging perovskite based solar cells. In this manuscript, we address the effect of pinholes or process induced surface coverage aspects on the efficiency of such solar cells through detailed numerical simulations. Interestingly, we find the pinhole size distribution affects the short circuit current and open circuit voltage in contrasting manners. Specifically, while the Jsc is heavily dependent on the pinhole size distribution, surprisingly, the Voc seems to be only nominally affected by it. Further, our simulations also indicate that, with appropriate interface engineering, it is indeed possible to design a nanostructured device with efficiencies comparable to that of ideal planar structures. Additionally, we propose a simple technique based on terminal IV characteristics to estimate the surface coverage in perovskite solar cells.

Despite the exciting recent research on perovskite based solar cells, the design space for further optimization and the practical limits of efficiency are not well known in the community. In this manuscript, we address these aspects through theoretical calculations and detailed numerical simulations. Here, we first provide the detailed balance limit efficiency in the presence of radiative and Auger recombination. Then, using coupled optical and carrier transport simulations, we identify the physical mechanisms that contribute towards bias dependent carrier collection, and hence low fill factors of current perovskite based solar cells. Curiously, we find that while Auger recombination is not a dominant factor at the detailed balance limit, it plays a significant role in device level implementations. Surprisingly, our device designs indicate that it is indeed possible to achieve efficiency and fill factor greater than 25% and 85%, respectively - even in the presence of Auger recombination.

Micro and nanostructured electrodes form an integral part of a wide variety of electrochemical systems for biomolecule detection, batteries, solar cells, scanning electrochemical microscopy, etc. Given the complexity of the electrode structures, the Butler-Volmer formalism of redox reactions, and the diffusion transport of redox species, it is hardly surprising that only a few problems are amenable to closed form, compact analytical solutions. While numerical solutions are widely used, it is often difficult to integrate the insights gained to the design and optimization of electrochemical systems. In this article, we develop a comprehensive analytical formalism for current transients that not only anticipate the response of complex electrode structures to complicated voltammetry measurements, but also intuitively interpret diverse experiments such as redox detection of molecules at nanogap electrodes, scanning electrochemical microscopy, etc. The results from the analytical model, well supported through detailed numerical simulations and experimental data from literature, have broad implications for the design and optimization of nanostructured electrodes for healthcare and energy storage applications., Comment: 27 pages, 7 figures

Polymer based organic photovoltaic (OPV) technology offers a relatively inexpensive option for solar energy conversion provided its efficiency increases beyond the current level (6-7%) along with significant improvements in operational lifetime. The critical aspect of such solar cells is the complex morphology of distributed bulk heterojunctions, which plays the central role in the conversion of photo-generated excitons to electron-hole pairs. However, the fabrication conditions that can produce the optimal morphology are still unknown due to the lack of quantitative understanding of the effects of process variables on the cell morphology. In this article, we develop a unique process-device co-simulation framework based on phase-field model for phase separation coupled with self-consistent drift-diffusion transport to quantitatively explore the effects of the process conditions (e.g., annealing temperature, mixing ratio, anneal duration) on the organic solar cell performance. Our results explain experimentally observed trends of open circuit voltage and short circuit current that would otherwise be deemed anomalous from the perspective of conventional solar cells. In addition to providing an optimization framework for OPV technology, our morphology-aware modeling approach is ideally suited for a wide class of problems involving porous materials, block co-polymers, polymer colloids, OLED devices etc.

Despite tremendous potential of highly sensitive electronic detection of bio-molecules by nanoscale biosensors for genomics and proteomic applications, many aspects of experimentally observed sensor response (S) are unexplained within consistent theoretical frameworks of kinetic response or electrical screening. In this paper, we combine analytic solutions of Poisson-Boltzmann and reaction-diffusion equations to show that the electrical response of nanobiosensor varies logarithmically with the concentration of target molecules, time, the salt concentration, and inversely with the fractal dimension of sensor surface. Our analysis provides a coherent theoretical interpretation of wide variety of puzzling experimental data that have so far defied intuitive explanation., Comment: 7 pages, 2 figures