Suppressing Deep Traps in PbS Colloidal Quantum Dots via Facile Iodide Substitutional Doping for Solar Cells with Efficiency >10%

Surface passivation of PbS colloidal quantum dots (QDs) with iodide has been used in highly efficient solar cells. Iodide passivation is typically achieved by ligand exchange processes on QD films. Complementary to this approach, herein we present a non-intrusive solution-based strategy for doping QDs with iodide to further optimize solar cell performance. The doping step is applied in-situ at the end of the synthesis of the QDs. The optimum precursor I/Pb ratio is found to be in the 1.5-3% range at which iodide substitutes S without excessively altering the dots´ surface chemistry. This allows for band engineering and decreasing the density of deep trap states of the QDs which taken together lead to PbS QD solar cells with efficiency in excess of 10%.Peer ReviewedPostprint (author's final draft

High Open Circuit Voltage Solar Cells based on bright mixed-halide CsPbBrI2 Perovskite Nanocrystals Synthesized in Ambient Air Conditions

Lead halide perovskite nanocrystals (NCs) are currently emerging as one of the most interesting solution processed semiconductors since they possess high photoluminescence quantum yield (PLQY), and colour tunability through anion exchange reactions or quantum confinement. Here, we show efficient solar cells based on mixed halide (CsPbBrI2) NCs obtained via anion exchange reactions in ambient conditions. We performed anion exchange reactions in concentrated NC solutions with I-, thus inducing a PL red-shift up to 676 nm, and obtaining a high PLQY in film (65%). Solar cell devices operating in the wavelength range 350-660 nm were fabricated in air with two different deposition methods. The solar cells display a photo-conversion efficiency of 5.3% and open circuit voltage (Voc) up to 1.31V, among the highest reported for perovskite based solar cells with band gap below 2eV, clearly demonstrating the potential of this material.Peer ReviewedPostprint (author's final draft

Trap-state suppression and improved charge transport in PbS quantum dot solar cells with synergistic mixed ligand treatments

The power conversion efficiency of colloidal PbS‐quantum‐dot (QD)‐based solar cells is significantly hampered by lower‐than‐expected open circuit voltage (VOC). The VOC deficit is considerably higher in QD‐based solar cells compared to other types of existing solar cells due to in‐gap trap‐induced bulk recombination of photogenerated carriers. Here, this study reports a ligand exchange procedure based on a mixture of zinc iodide and 3‐mercaptopropyonic acid to reduce the VOC deficit without compromising the high current density. This layer‐by‐layer solid state ligand exchange treatment enhances the photovoltaic performance from 6.62 to 9.92% with a significant improvement in VOC from 0.58 to 0.66 V. This study further employs optoelectronic characterization, X‐ray photoelectron spectroscopy, and photoluminescence spectroscopy to understand the origin of VOC improvement. The mixed‐ligand treatment reduces the sub‐bandgap traps and significantly reduces bulk recombination in the devices.Peer ReviewedPostprint (author's final draft

Development of New Adaptive Control Strategies for a Two-Link Flexible Manipulator

Manipulators with thin and light weight arms or links are called as Flexible-Link Manipulators (FLMs). FLMs offer several advantages over rigid-link manipulators such as achieving highspeed operation, lower energy consumption, and increase in payload carrying capacity and find applications where manipulators are to be operated in large workspace like assembly of freeflying space structures, hazardous material management from safer distance, detection of flaws in large structure like airplane and submarines. However, designing a feedback control system for a flexible-link manipulator is challenging due the system being non-minimum phase, underactuated and non-collocated. Further difficulties are encountered when such manipulators handle unknown payloads. Overall deflection of the flexible manipulator are governed by the different vibrating modes (excited at different frequencies) present along the length of the link. Due to change in payload, the flexible modes (at higher frequencies) are excited giving rise to uncertainties in the dynamics of the FLM. To achieve effective tip trajectory tracking whilst quickly suppressing tip deflections when the FLM carries varying payloads adaptive control is necessary instead of fixed gain controller to cope up with the changing dynamics of the manipulator. Considerable research has been directed in the past to design adaptive controllers based on either linear identified model of a FLM or error signal driven intelligent supervised learning e.g. neural network, fuzzy logic and hybrid neuro-fuzzy. However, the dynamics of the FLM being nonlinear there is a scope of exploiting nonlinear modeling approach to design adaptive controllers. The objective of the thesis is to design advanced adaptive control strategies for a two-link flexible manipulator (TLFM) to control the tip trajectory tracking and its deflections while handling unknown payloads. To achieve tip trajectory control and simultaneously suppressing the tip deflection quickly when subjected to unknown payloads, first a direct adaptive control (DAC) is proposed. The proposed DAC uses a Lyapunov based nonlinear adaptive control scheme ensuring overall system stability for the control of TLFM. For the developed control laws, the stability proof of the closed-loop system is also presented. The design of this DAC involves choosing a control law with tunable TLFM parameters, and then an adaptation law is developed using the closed loop error dynamics. The performance of the developed controller is then compared with that of a fuzzy learning based adaptive controller (FLAC). The FLAC consists of three major components namely a fuzzy logic controller, a reference model and a learning mechanism. It utilizes a learning mechanism, which automatically adjusts the rule base of the fuzzy controller so that the closed loop performs according to the user defined reference model containing information of the desired behavior of the controlled system. Although the proposed DAC shows better performance compared to FLAC but it suffers from the complexity of formulating a multivariable regressor vector for the TLFM. Also, the adaptive mechanism for parameter updates of both the DAC and FLAC depend upon feedback error based supervised learning. Hence, a reinforcement learning (RL) technique is employed to derive an adaptive controller for the TLFM. The new reinforcement learning based adaptive control (RLAC) has an advantage that it attains optimal control adaptively in on-line. Also, the performance of the RLAC is compared with that of the DAC and FLAC. In the past, most of the indirect adaptive controls for a FLM are based on linear identified model. However, the considered TLFM dynamics is highly nonlinear. Hence, a nonlinear autoregressive moving average with exogenous input (NARMAX) model based new Self-Tuning Control (NMSTC) is proposed. The proposed adaptive controller uses a multivariable Proportional Integral Derivative (PID) self-tuning control strategy. The parameters of the PID are adapted online using a nonlinear autoregressive moving average with exogenous-input (NARMAX) model of the TLFM. Performance of the proposed NMSTC is compared with that of RLAC. The proposed NMSTC law suffers from over-parameterization of the controller. To overcome this a new nonlinear adaptive model predictive control using the NARMAX model of the TLFM (NMPC) developed next. For the proposed NMPC, the current control action is obtained by solving a finite horizon open loop optimal control problem on-line, at each sampling instant, using the future predicted model of the TLFM. NMPC is based on minimization of a set of predicted system errors based on available input-output data, with some constraints placed on the projected control signals resulting in an optimal control sequence. The performance of the proposed NMPC is also compared with that of the NMSTC. Performances of all the developed algorithms are assessed by numerical simulation in MATLAB/SIMULINK environment and also validated through experimental studies using a physical TLFM set-up available in Advanced Control and Robotics Research Laboratory, National Institute of Technology Rourkela. It is observed from the comparative assessment of the performances of the developed adaptive controllers that proposed NMPC exhibits superior 7performance in terms of accurate tip position tracking (steady state error ≈ 0.01°) while suppressing the tip deflections (maximum amplitude of the tip deflection ≈ 0.1 mm) when the manipulator handles variation in payload (increased payload of 0.3 kg). The adaptive control strategies proposed in this thesis can be applied to control of complex flexible space shuttle systems, long reach manipulators for hazardous waste management from safer distance and for damping of oscillations for similar vibration systems

Comparative P solubilizing efficiencies of five acid soil bacteria incubated with calcium, aluminium and iron phosphates

As acid soils of Odisha have been facing a major constraint in phosphorus availability, application of native P solubilizing bacteria could be promising as well as ecofriendly step towards sustainable P availability for crop growth and development. To address the problem of P availability in acid soil of Bhubaneswar, Odisha, rhizosphere soil samples (pH ≤ 5.50) with rice – pulses (green gram/black gram) cropping system were collected and phosphate solubilising bacteria were isolated. In vitro characterization of the PSB isolates were conducted with calcium, aluminium and iron phosphates to recover soluble P. All the five strains i.e. Bacillus cereus BLS18 (KT582541), Bacillus amyloliquefaciens CTC12 (KT633845), Burkholderia cepacia KHD08 (KT717633), Burkholderia cepacia KJR03 (KT717634), Burkholderia cepacia K1 (KM030037) could solubilize Ca3(PO4)2, AlPO4, FePO4, and Fe3(PO4)2. Higher recovery of soluble P was with Ca3(PO4)2 while the least was with AlPO4. All the strains exibited a trend similar with respect to P recovery i.e. Ca3(PO4)2 > FePO4 > AlPO4 > Fe3(PO4)2. B. amyloliquefaciens CTC12 was most efficient in solubilizing calcium and iron phosphates whereas B. cepacia KHD08 recovered maximum P with aluminium phosphate. All the inorganic salt fortified mediums showed a significant decline in pH which necessitated the identification of compounds present in the mediums. Organic acids viz; acetic, citric, gluconic, lactic, malic, succinic, tartaric acids in the mediums were identified by HPLC. Tartaric acid was only found in the mediums supplemented with AlPO4. B. amyloliquefaciens CTC12 and B. cepacia KHD08 showed promising results in in vitro analysis of P solubilization. The present study is focused on problematic acid soils where phosphorous is unavailable and mostly fixed with aluminium and iron ultimately making it unavailable for the crops to take up. This leads to unbalanced and frequent use of chemical fertilizer. Hence the study is a significant attempt to characterize native PSBs with capacity to solubilize Al-P and Fe-

Plant growth promoting activities of P solubilizing bacteria and their impact on disease resistance in groundnut, Arachis hypogaea L. against soil borne fungal pathogens

Plant growth promoting (PGP) activities of soil bacteria directly help plants in taking up the nutrients, attuning the growth hormones and indirectly safeguard by inhibiting diverse groups of fungal pathogens. In this study, we explored the native P solubilizing bacteria (PSB) isolated from the acid soils (pH < 5.5) of Odisha for selection of efficient PGPR with antifungal potential. Five PSB strainswere checked for their P solubilization efficiencies with Ca3(PO4)2,AlPO4,FePO4 and Fe3(PO4)2. The bioconversion of P by all the five strains in the broth medium followed the order Ca-P > Fe(III)-P > Fe(II)-P >Al-P. The strains interestingly showed potential plant growthpromoting properties including indole acetic acid (IAA) andsiderophore production in in vitro tests. These five strains also exhibited antifungal activities against fungal pathogens(Pythium aphanidermatum, Fusarium oxysporum, Pythium debaryanum, Thanatephorus cucumeris and Aspergillus niger)of groundnut. A field study was carried out with two of the above PSB strains [identified as Bacillus amyloliquefaciens(KT633845) and Burkholderia cepacia (KT717633)] with groundnut. Both the stains significantly influenced the plantgrowth (plant height, nodule no. and nodule dry weight) and pod yield. However, these two strains inoculated along withdoses of inorganic phosphate (SSP, single super phosphate) resulted in significantly higher pod yield as well as residual soil P.Additionally; the prevalence of both seedling mortality and plant mortality due to collar rot and stem rot were found to bereduced significantly in the inoculated plots. The findings substantiate the growth promoting ability of the two P solubilizingstrains, and thus qualifies to be used as biofertilizers either alone or as components of INM practice

Colloidal Quantum Dot Tandem Solar Cells Using CVD Graphene as An Atomically Thin Intermediate Recombination Layer

Two-terminal tandem cell architectures are believed to be an effective way to further improve the power conversion efficiency in solution processed photovoltaics. To design an efficient tandem solar cell, two key issues need to be considered. First, subcells with well-matched currents and complementary absorption characteristics are a prerequisite for high efficiency. Second, identifying the appropriate intermediate layer (IML) to connect the subcells is necessary to minimize the optical and electronic losses. PbS colloidal quantum dots (CQDs) are a notable choice for the subcells due to their low cost, solution processability, and remarkable wide range band gap tunability. Single-layer graphene (Gr) has been proposed to be a promising IML due to its high transparency and conductivity. Here, as a proof of concept, we demonstrate a solution-processed, two-terminal PbS CQDs tandem solar cell employing chemical vapor deposited Gr as the IML. In doing so, we report a PbS CQD cell comprising subcells with bandgaps of 1.4 and 0.95 eV that delivers power conversion efficiency in excess of 7%, substantially higher than that of previously reported CQD tandem cells.Peer ReviewedPostprint (author's final draft

Plant growth promoting activities of P solubilizing bacteria and their impact on disease resistance in groundnut, <i>Arachis hypogaea</i> L. against soil borne fungal pathogens

606-616Plant growth promoting (PGP) activities of soil bacteria directly help plants in taking up the nutrients, attuning the growth hormones and indirectly safeguard by inhibiting diverse groups of fungal pathogens. In this study, we explored the native P solubilizing bacteria (PSB) isolated from the acid soils (pH 3(PO4)2,AlPO4,FePO4 and Fe3(PO4)2. The bioconversion of P by all the five strains in the broth medium followed the order Ca-P > Fe(III)-P > Fe(II)-P >Al-P. The strains interestingly showed potential plant growthpromoting properties including indole acetic acid (IAA) andsiderophore production in in vitro tests. These five strains also exhibited antifungal activities against fungal pathogens(Pythium aphanidermatum, Fusarium oxysporum, Pythium debaryanum, Thanatephorus cucumeris and Aspergillus niger)of groundnut. A field study was carried out with two of the above PSB strains [identified as Bacillus amyloliquefaciens(KT633845) and Burkholderia cepacia (KT717633)] with groundnut. Both the stains significantly influenced the plantgrowth (plant height, nodule no. and nodule dry weight) and pod yield. However, these two strains inoculated along withdoses of inorganic phosphate (SSP, single super phosphate) resulted in significantly higher pod yield as well as residual soil P.Additionally; the prevalence of both seedling mortality and plant mortality due to collar rot and stem rot were found to bereduced significantly in the inoculated plots. The findings substantiate the growth promoting ability of the two P solubilizingstrains, and thus qualifies to be used as biofertilizers either alone or as components of INM practices