Enhancing the Efficiency and Stability of Perovskite Solar Cells through Defect Passivation and Controlled Crystal Growth Using Allantoin.

In this study, we present a robust approach that concurrently manages crystal growth and defect passivation within the perovskite layer through the introduction of a small molecule additive─allantoin. The precise regulation of crystal growth in the presence of allantoin yields perovskite films characterized by enhanced morphology, larger grain size, and improved grain orientation. Notably, the carbonyl and amino groups present in allantoin passivate under-coordinated Pb2+ and I- defects, respectively, through molecular interactions. Trap density in the perovskite layer is measured, and it is 0.39 × 1016 cm-3 for the allantoin-incorporated device and 0.83 × 1016 cm-3 for the pristine device. This reduction in defects leads to reduced trap-assisted nonradiative recombination, as confirmed by the photoluminescence, transient photo voltage, and impedance measurements. As a result, when these allantoin-incorporated perovskite films are implemented as the active layer in solar cells, a noteworthy efficiency enhancement to 20.63% is attained, surpassing the 18.04% of their pristine counterparts. Furthermore, devices with allantoin exhibit remarkable operational stability, maintaining 80% of their efficiency even after 500 h of continuous illumination, whereas the pristine device degraded to 65% of its initial efficiency in 400 h. Also, allantoin-incorporated devices exhibited exceptional stability against high humidity and elevated temperatures.

Deciphering the 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, the capacitance frequency technique (CFT) is widely employed to quantify the 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, but not limited to, carrier-selective perovskite-based PIN devices. We show that the electrostatic effect of trapped charges leads to non-linear energy bands in the perovskite layer, which results in the underestimation of the trap density from existing CFT models. Consequently, a parabolic band approximation with effective length (PBAEL) model is developed to accurately predict the trap density for shallow or deep states from CFT analysis. In addition, we demonstrate that the attempt-to-escape frequency, which dictates the trapping dynamics, can be well extracted by eliminating the electrostatic effect at a 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, inorganic, and hybrid semiconductor-based thin-film devices for energy-conversion applications such as solar cells, LEDs, artificial photosynthesis, etc.

Dynamic Tracking Biosensors: Finding Needles in a Haystack.

Accurate detection of target molecules at low concentrations in the presence of undesired molecules in abundance is a major challenge for biosensors. Nonspecific 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. Through a combination of theoretical analysis and detailed statistical simulations, here we show that, with aggressive scaling, DT sensors are capable of fM detection limits even if the undesired molecules are present at nM concentrations, which is several orders of magnitude better than traditional endpoint (EP) biosensors. In addition, we propose a novel unconstrained detection scheme that does not rely on a priori knowledge of the dissociation constants and also allows facile back-extraction of critical parameters. Indeed, this work provides a theoretical basis for DT sensors and demonstrates its suitability to usher in a new paradigm on biosensing in hostile environments.

Novel Boron-Doped p-Type Cu2O Thin Films as a Hole-Selective Contact in c-Si Solar Cells.

p-type Cu2O thin films doped with trivalent cation boron are demonstrated for the first time as an efficient hole-selective layer for c-Si heterojunction solar cells. Cu2O and Cu2O:B films were deposited by rf magnetron sputtering, and the optical and electrical properties of the doped and undoped films were investigated. Boron doping enhanced the carrier concentration and the electrical conductivity of the Cu2O film. The band alignment of the Cu2O:B/Si heterojunction was investigated using XPS and UPS measurements. The Cu2O:B/Si interface has a valance band offset of 0.08 eV, which facilitates hole transport, and a conduction band offset of 1.35 eV, which blocks the electrons. A thin SiOx tunnel oxide interlayer was also explored as the passivation layer. The initial trials of incorporating this Cu2O:B layer as a hole transporting layer in a single heterojunction solar cell with the structure, ITO/Cu2O:B/n-Si/Ag, and a cell area of 1 cm2 yielded an open-circuit voltage of 370 mV, a short-circuit current density of 36.5 mA/cm2, and an efficiency of 5.4%. This p-type material could find potential applications in various optoelectronic applications like organic solar cells, TFTs, and LEDs.

Predictive Modeling of Ion Migration Induced Degradation in Perovskite Solar Cells.

With excellent efficiencies being reported from multiple laboratories 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 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-an observation which is beyond the realm of mere parametric variation in carrier mobility and lifetime, (c) champion devices are more resilient toward the ill effects of ion migration; (d) we propose characterization schemes to determine both magnitude and polarity of ionic species, and finally, (e) we illustrate that ion migration could be differentiated from ion redistribution based on the distinct trends in performance degradation. Our results, supported by detailed numerical simulations and direct comparison with experimental data, are of broad interest and provide a much needed predictive capability toward the research on performance degradation mechanisms in perovskite solar cells.

On the Uniqueness of Ideality Factor and Voltage Exponent of Perovskite-Based Solar Cells.

Perovskite-based solar cells have attracted much recent research interest with efficiency approaching 20%. While various combinations of material parameters and processing conditions are attempted for improved performance, there is still a lack of understanding in terms of the basic device physics and functional parameters that control the efficiency. Here we show that perovskite-based solar cells have two universal features: an ideality factor close to two and a space-charge-limited current regime. Through detailed numerical modeling, we identify the mechanisms that lead to these universal features. Our model predictions are supported by experimental results on solar cells fabricated at five different laboratories using different materials and processing conditions. Indeed, this work unravels the fundamental operation principle of perovskite-based solar cells, suggests ways to improve the eventual performance, and serves as a benchmark to which experimental results from various laboratories can be compared.

A compact analytical formalism for current transients in electrochemical systems.

Micro- and nanostructured electrodes form an integral part of a wide variety of electrochemical systems for biomolecular 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 into the design and optimization of electrochemical systems. In this article, we develop a comprehensive analytical formalism for current transients that not only anticipate the responses 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 the literature, have broad implications in the design and optimization of nanostructured electrodes for healthcare and energy storage applications.

Theory of signal and noise in double-gated nanoscale electronic pH sensors.

The maximum sensitivity of classical nanowire (NW)-based pH sensors is defined by the Nernst limit of 59 mV/pH. For typical noise levels in ultra-small single-gated nanowire sensors, the signal-to-noise ratio is often not sufficient to resolve pH changes necessary for a broad range of applications. Recently, a new class of double-gated devices was demonstrated to offer apparent "super-Nernstian" response (>59 mV/pH) by amplifying the original pH signal through innovative biasing schemes. However, the pH-sensitivity of these nanoscale devices as a function of biasing configurations, number of electrodes, and signal-to-noise ratio (SNR) remains poorly understood. Even the basic question such as "Do double-gated sensors actually resolve smaller changes in pH compared to conventional single-gated sensors in the presence of various sources of noise?" remains unanswered. In this article, we provide a comprehensive numerical and analytical theory of signal and noise of double-gated pH sensors to conclude that, while the theoretical lower limit of pH-resolution does not improve for double-gated sensors, this new class of sensors does improve the (instrument-limited) pH resolution.

Coupled heterogeneous nanowire-nanoplate planar transistor sensors for giant (>10 V/pH) Nernst response.

We offer a comprehensive theory of pH response of a coupled ISFET sensor to show that the maximum achievable response is given by ΔV/ΔpH = 59 mV/pH × α, where 59 mV/pH is the intrinsic Nernst response and α an amplification factor that depends on the geometrical and electrical properties of the sensor and transducer nodes. While the intrinsic Nernst response of an electrolyte/site-binding interface is fundamental and immutable, we show that by using channels of different materials, areas, and bias conditions, the extrinsic sensor response can be increased dramatically beyond the Nernst limit. We validate the theory by measuring the pH response of a Si nanowire-nanoplate transistor pair that achieves >10 V/pH response and show the potential of the scheme to achieve (asymptotically) the theoretical lower limit of signal-to-noise ratio for a given configuration. We suggest the possibility of an even larger pH response based on recent trends in heterogeneous integration on the Si platform.

Flexure-FET biosensor to break the fundamental sensitivity limits of nanobiosensors using nonlinear electromechanical coupling.

In this article, we propose a Flexure-FET (flexure sensitive field effect transistor) ultrasensitive biosensor that utilizes the nonlinear electromechanical coupling to overcome the fundamental sensitivity limits of classical electrical or mechanical nanoscale biosensors. The stiffness of the suspended gate of Flexure-FET changes with the capture of the target biomolecules, and the corresponding change in the gate shape or deflection is reflected in the drain current of FET. The Flexure-FET is configured to operate such that the gate is biased near pull-in instability, and the FET-channel is biased in the subthreshold regime. In this coupled nonlinear operating mode, the sensitivity (S) of Flexure-FET with respect to the captured molecule density (N(s)) is shown to be exponentially higher than that of any other electrical or mechanical biosensor. In other words, while S(Flexure) ~ e(γ1 [square root]Ns-γ2Ns), classical electrical or mechanical biosensors are limited to S(classical) ~ γ(3)N(S) or γ(4) ln(N(S)), where γ(i) are sensor-specific constants. In addition, the proposed sensor can detect both charged and charge-neutral biomolecules, without requiring a reference electrode or any sophisticated instrumentation, making it a potential candidate for various low-cost, point-of-care applications.

High-k dielectric Al2O3 nanowire and nanoplate field effect sensors for improved pH sensing.

Over the last decade, field-effect transistors (FETs) with nanoscale dimensions have emerged as possible label-free biological and chemical sensors capable of highly sensitive detection of various entities and processes. While significant progress has been made towards improving their sensitivity, much is yet to be explored in the study of various critical parameters, such as the choice of a sensing dielectric, the choice of applied front and back gate biases, the design of the device dimensions, and many others. In this work, we present a process to fabricate nanowire and nanoplate FETs with Al(2)O(3) gate dielectrics and we compare these devices with FETs with SiO(2) gate dielectrics. The use of a high-k dielectric such as Al(2)O(3) allows for the physical thickness of the gate dielectric to be thicker without losing sensitivity to charge, which then reduces leakage currents and results in devices that are highly robust in fluid. This optimized process results in devices stable for up to 8 h in fluidic environments. Using pH sensing as a benchmark, we show the importance of optimizing the device bias, particularly the back gate bias which modulates the effective channel thickness. We also demonstrate that devices with Al(2)O(3) gate dielectrics exhibit superior sensitivity to pH when compared to devices with SiO(2) gate dielectrics. Finally, we show that when the effective electrical silicon channel thickness is on the order of the Debye length, device response to pH is virtually independent of device width. These silicon FET sensors could become integral components of future silicon based Lab on Chip systems.

Silicon field effect transistors as dual-use sensor-heater hybrids.

We demonstrate the temperature mediated applications of a previously proposed novel localized dielectric heating method on the surface of dual purpose silicon field effect transistor (FET) sensor-heaters and perform modeling and characterization of the underlying mechanisms. The FETs are first shown to operate as electrical sensors via sensitivity to changes in pH in ionic fluids. The same devices are then demonstrated as highly localized heaters via investigation of experimental heating profiles and comparison to simulation results. These results offer further insight into the heating mechanism and help determine the spatial resolution of the technique. Two important biosensor platform applications spanning different temperature ranges are then demonstrated: a localized heat-mediated DNA exchange reaction and a method for dense selective functionalization of probe molecules via the heat catalyzed complete desorption and reattachment of chemical functionalization to the transistor surfaces. Our results show that the use of silicon transistors can be extended beyond electrical switching and field-effect sensing to performing localized temperature controlled chemical reactions on the transistor itself.

Theoretical detection limits of magnetic biobarcode sensors and the phase space of nanobiosensing.

A scaling theory of the sub atto-molar (aM) detection limits of magnetic particle (MP) based biosensors (e.g., bio-barcode assays) is developed and discussed. Despite the dramatic differences of sensing protocols and detection limits, the MP-based sensors can be interpreted within the same theoretical framework as any other classical biosensor (e.g., nanowire sensors), except that these sensors are differentiated by the geometry of diffusion and the probe (ρ(MP))/target (ρ(T)) density ratio. Our model predicts two regimes for biomolecule detection: For classical biosensors with ρ(MP) ≤ ρ(T), the response time t(s) proportional to 1/ρ(T); while for MP-based biosensors with ρ(MP) > ρ(T), t(s) proportional to 1/ρ(MP). The theory (i) explains the performance improvement of MP-sensors by ρ(MP)/ρ(T) (order of 10(3)-10(6)), broadly validating the sub-aM detection limits reported in literature, (ii) offers intuitive interpretation for the counter-intuitive ρ(T)-independence of detection time in MP-sensors, (iii) shows that statistical fluctuations should reduce with ρ(T) for MP sensors, and (iv) offers obvious routes to sensitivity improvement of classical sensors.

Theory of "Selectivity" of label-free nanobiosensors: A geometro-physical perspective.

Modern label-free biosensors are generally far more sensitive and require orders of magnitude less incubation time compared to their classical counterparts. However, a more important characteristic regarding the viability of this technology for applications in genomicsproteomics is defined by the "Selectivity," i.e., the ability to concurrently and uniquely detect multiple target biomolecules in the presence of interfering species. Currently, there is no theory of Selectivity that allows optimization of competing factors and there are few experiments to probe this problem systematically. In this article, we use the elementary considerations of surface exclusion, diffusion limited transport, and void distribution function to provide guidance for optimum incubation time required for effective surface functionalization, and to identify the dominant components of unspecific adsorption. We conclude that optimally designed label-free schemes can compete favorably with other assay techniques, both in sensitivity as well as in selectivity.

Localized heating on silicon field effect transistors: device fabrication and temperature measurements in fluid.

We demonstrate electrically addressable localized heating in fluid at the dielectric surface of silicon-on-insulator field-effect transistors via radio-frequency Joule heating of mobile ions in the Debye layer. Measurement of fluid temperatures in close vicinity to surfaces poses a challenge due to the localized nature of the temperature profile. To address this, we developed a localized thermometry technique based on the fluorescence decay rate of covalently attached fluorophores to extract the temperature within 2 nm of any oxide surface. We demonstrate precise spatial control of voltage dependent temperature profiles on the transistor surfaces. Our results introduce a new dimension to present sensing systems by enabling dual purpose silicon transistor-heaters that serve both as field effect sensors as well as temperature controllers that could perform localized bio-chemical reactions in Lab on Chip applications.

Device considerations for development of conductance-based biosensors.

Design and fabrication of electronic biosensors based on field-effect-transistor (FET) devices require understanding of interactions between semiconductor surfaces and organic biomolecules. From this perspective, we review practical considerations for electronic biosensors with emphasis on molecular passivation effects on FET device characteristics upon immobilization of organic molecules and an electrostatic model for FET-based biosensors.

Screening-limited response of nanobiosensors.

Despite tremendous potential of highly sensitive electronic detection of biomolecules by nanoscale biosensors for genomics and proteomic applications, many aspects of experimentally observed sensor response (S) are difficult to understand within isolated theoretical frameworks of kinetic response or electrolyte screening. In this paper, we combine analytic solutions of Poisson-Boltzmann and diffusion-capture equations to show that the electrostatic screening within an ionic environment limits the response of nanobiosensor such that S(t) approximately c1(ln(rho0) - ln(I0)/2 + ln(t)/ D F + c2[pH]) + c3 where c i are geometry-dependent constants, rho0 is the concentration of target molecules, I0 the salt concentration, and D F the fractal dimension of sensor surface. Our analysis provides a coherent theoretical interpretation of a wide variety of puzzling experimental data that have so far defied intuitive explanation.

Electrical detection of the biological interaction of a charged peptide via gallium arsenide junction-field-effect transistors.

GaAs junction-field-effect transistors (JFETs) are utilized to achieve label-free detection of biological interaction between a probe transactivating transcriptional activator (TAT) peptide and the target trans-activation-responsive (TAR) RNA. The TAT peptide is a short sequence derived from the human immunodeficiency virus-type 1 TAT protein. The GaAs JFETs are modified with a mixed adlayer of 1-octadecanethiol (ODT) and TAT peptide, with the ODT passivating the GaAs surface from polar ions in physiological solutions and the TAT peptide providing selective binding sites for TAR RNA. The devices modified with the mixed adlayer exhibit a negative pinch-off voltage (V(P)) shift, which is attributed to the fixed positive charges from the arginine-rich regions in the TAT peptide. Immersing the modified devices into a TAR RNA solution results in a large positive V(P) shift (>1 V) and a steeper subthreshold slope ( approximately 80 mVdecade), whereas "dummy" RNA induced a small positive V(P) shift ( approximately 0.3 V) without a significant change in subthreshold slopes ( approximately 330 mVdecade). The observed modulation of device characteristics is analyzed with analytical modeling and two-dimensional numerical device simulations to investigate the electronic interactions between the GaAs JFETs and biological molecules.

Dimensionally frustrated diffusion towards fractal adsorbers.

Diffusion towards a fractal adsorber is a well-researched problem with many applications. While the steady-state flux towards such adsorbers is known to be characterized by the fractal dimension (D{F}) of the surface, the more general problem of time-dependent adsorption kinetics of fractal surfaces remains poorly understood. In this Letter, we show that the time-dependent flux to fractal adsorbers (1

Anomalous resonance in a nanomechanical biosensor.

The decrease in resonant frequency (-Deltaomega(r)) of a classical cantilever provides a sensitive measure of the mass of entities attached on its surface. This elementary phenomenon has been the basis of a new class of bio-nanomechanical devices as sensing components of integrated microsystems that can perform rapid, sensitive, and selective detection of biological and biochemical entities. Based on classical analysis, there is a widespread perception that smaller sensors are more sensitive (sensitivity approximately -0.5omega(r)/m(C), where m(C) is the mass of the cantilever), and this notion has motivated scaling of biosensors to nanoscale dimensions. In this work, we show that the response of a nanomechanical biosensor is far more complex than previously anticipated. Indeed, in contrast to classical microscale sensors, the resonant frequencies of the nanosensor may actually decrease or increase after attachment of protein molecules. We demonstrate theoretically and experimentally that the direction of the frequency change arises from a size-specific modification of diffusion and attachment kinetics of biomolecules on the cantilevers. This work may have broad impact on microscale and nanoscale biosensor design, especially when predicting the characteristics of bio-nanoelectromechanical sensors functionalized with biological capture molecules.