Dr Saianand Gopalan

Perovskite solar cells (PSCs) have opened a floodgate of diverse research. These highly functional materials have immense potential prospects in renewable energy, optoelectronic applications, to name a few. Most of the perovskites have lead (Pb) as an integral active component. Though the Pb-based organometallic perovskites have shown outstanding performance (over 25%), significant challenges exist including the presence of Pb, its toxicity as well as the instability that can hamper the use of these devices toward practical applications. There are some instances where a complete absence of Pb has been used to achieve respectable device performance, but they face a challenge from the towering success of their counterparts. To offset the issues, significant efforts are being carried out to employ other alternatives (cations) in conjunction with Pb, without distorting the inherent material properties. To that end, tin (Sn) has emerged as a promising alternative to Pb in achieving benchmark performance. Bringing these Sn PSCs into more prominence is a crucial step in taking low Pb or Pb-free PSCs to the next level of research and commercialization. In this topical review, the notable advancements are highlighted, overviewed and recommendations are made for prospects toward environmentally friendly Sn-based perovskite photovoltaics.

This paper presents an efficient and eco-friendly soil stabilization method for binding the soil particles of rammed earth walls. The method uses a combination of a water-soluble acrylic acid¿acrylic amide copolymer and an epoxy system containing a hardener. The results revealed that the unconfined compressive strength (UCS) of soil stabilized with the mixture of the copolymer and epoxy system was five times higher than that of soil treated with water and three times higher than that of stabilized soil (SS) with only the copolymer. The roles of the copolymer and epoxy system in UCS enhancement were determined based on morphology and apparent density (AD) and elucidated using proposed models. The soil stabilized with the copolymer and epoxy system exhibited the lowest AD and highest UCS. Further, its unique web-like morphology is consistent with the formation of an interpenetrating polymer network (IPN). The carboxylic and amide groups in the copolymer contribute to the formation of the IPN through interactions with the epoxy and amine groups of the hardener. The surface hydroxyl groups in the soil promote the coating of the polymer layer on the soil surface and the inter-particle cohesion, resulting in the formation of larger compacted lumps with a decreased AD and increased UCS. The hydrophilic copolymer acts as a stabilizer and plasticizer. The results confirmed that the epoxy system and copolymer significantly increased the UCS by forming an IPN. Thus, this study provides an efficient and eco-friendly water-soluble copolymer and epoxy system for enhancing the UCS of soil.

p-i-n structured semitransparent perovskite solar cells have already been established as promising energy harvesting devices for building-integrated photovoltaics and flexible solar cells due to high transparency and low-cost fabrication. In this study, solution based p-i-n structured semitransparent perovskite solar cells (PSCs) have been developed using thin silver (Ag), zinc oxide (ZnO), and aluminium (Al)-doped ZnO nanoparticles (AZO) as buffer layers in addition to PCBM as an electron transport layer (ETL). The thickness of the ZnO and AZO layers are around ~100 nm. In the case of the thin Ag layer, poor interfacial band alignment and less transparency yield device performance with an inferior PCE of 2.53% when illuminated from the top electrode side. On the contrary, Al-doped ZnO possesses excellent optoelectronic performance as a buffer layer for their better electronic conductivity and interfacial band alignment and yield a photovoltaic device characteristic with a power conversion efficiency (PCE) of 5.87% when illuminated from the top electrode side, whereas the standard device with a metal electrode shows a PCE of 6.4%. The semitransparent device also has an average transparency of 21.8% in the visible region. Graphic Abstract: Inverted structure semitransparent perovskite solar cells have been developed for flexible and building integrated photovoltaic (BIPV) applications. Solution processed perovskite solar cells with a power conversation efficiency of 5.87% and visible transmittance of 21.8% make an excellent candidate for BIPV and smart windows. Aluminum-doped zinc oxide (AZO) nanoparticles are found very effective buffer layer for the sputter grown top electrode over PCBM layer to reduce the interface damage and enhanced the overall photovoltaic performance. [Figure not available: see fulltext.]

A novel electrochemical nitrite ion (NO2-) biosensor was designed and fabricated by modifying a glassy carbon electrode (GCE) into three-dimensional nanoarchitectured electrode (3DNE) using a multicomponent nanocomposite (MCNC) film composed of graphene embedded titanium dioxide nanowires (TiO2(G) NWs), thiol-functionalized polyaniline (PANI(SH)), gold nanoparticles (Au), and immobilized cytochrome c (cyt c). The new NO2- biosensor was designated as cyt c/TiO2(G) NWs@PANI(SH)-Au MCNC/3DNE. The assembly of the 3DNE, involved sequential depositions of PANI(SH) and Au over previously synthesized TiO2(G) NWs by electrospinning-hydrothermal processes and immobilization of the cyt c onto the TiO2(G) NWs/PANI(SH)/Au MCNC film. The cyt c/TiO2(G) NWs@PANI(SH)-Au MCNC/3DNE exhibited direct electron transfer from cyt c to the electrode at a high rate constant (25.34 s-1). The fabricated cyt c/TiO2(G) NWs@PANI(SH)-Au MCNC/3DNE showed high selectivity to NO2- ions with excellent sensitivity (9.2 µA/mM), a wide linear concentration range (10 µM-720 mM), and a low detection limit (0.05 µM). Additionally, the cyt c/TiO2(G) NWs@PANI(SH)-Au MCNC/3DNE exhibited high stability with good reproducibility and repeatability.

Improved performance in the bulk heterojunction (BHJ) polymer solar cells (PSCs) can be achieved through multiple mechanisms by tuning the structure of the components and design architecture. Therefore, in this study, we designed and fabricated an electrostatically assembled contact interfacial layer utilizing the electrostatic interactions between positively charged poly(diallyldimethylammonium chloride) (PDDA) and negatively charged sulfonate ions in a polymer, i.e. toluene sulfonic acid incorporated self-doped polyaniline (SPAN(TSA-)) (designated as SPAN(TSA-)/EA-CIL). We investigated the influence of SPAN(TSA-)/EA-CIL on the photovoltaic performance of BHJ PSCs. We adopted a simple "dip-dry" technique to fabricate the SPAN(TSA-)/EA-CIL. Cyclic Voltammetry, X-ray photoelectron spectroscopy, Ultraviolet-visible spectroscopy, water contact angle measurements, electrochemical impedance spectroscopy and atomic force microscopy were employed to characterize the new SPAN(TSA-)/EA-CIL and to elucidate the associated enhancements in the photovoltaic performance. The fabricated PSCs with the SPAN(TSA-)/EA-CILs exhibited an improved photoconversion efficiency (PCE) through enhancement of both the short-circuit current density (JSC) and fill factor compared to the device consisting of only SPAN(TSA-) or PEDOT:PSS. A plausible mechanism is presented here that explains the enhanced JSC and PCE in terms of transient dipole induction at the SPAN(TSA-)/EA-CIL surface. Interestingly, the PCE of devices based on ITO/SPAN(TSA-)/EA-CIL(1:1)/P3HT:PC60BM/Al and ITO/SPAN(TSA-)/EA-CIL(1:1)/PBDTTT-C-T:PC70BM/Al was enhanced by 13% (3.19-3.67%) and 10% (5.11-5.60%) compared to those of devices based on ITO/PEDOT:PSS/P3HT:PC60BM/Al and ITO/PEDOT:PSS/PBDTTT-C-T:PC70BM/Al, respectively. The present work thus identifies a novel class of low-temperature processable interface layer with a potential to fabricate highly efficient photovoltaic devices.

In this report, titanium nitride (TiN) nanoparticles decorated multi-walled carbon nanotube (MWCNTs) nanocomposite is fabricated via a two-step process. These two steps involve the decoration of titanium dioxide nanoparticles onto the MWCNTs surface and a subsequent thermal nitridation. Transmission electron microscopy shows that TiN nanoparticles with a mean diameter of =20nm are homogeneously dispersed onto the MWCNTs surface. Direct electrochemistry and electrocatalysis of cytochrome c immobilized on the MWCNTs-TiN composite modified on a glassy carbon electrode for nitrite sensing are investigated. Under optimum conditions, the current response is linear to its concentration from 1µM to 2000µM with a sensitivity of 121.5µAµM-1cm-2 and a low detection limit of 0.0014µM. The proposed electrode shows good reproducibility and long-term stability. The applicability of the as-prepared biosensor is validated by the successful detection of nitrite in tap and sea water samples.

We propose a solution-processable ultraviolet (UV) photodetector with a pn-heterojunction hybrid photoactive layer (HPL) that is composed of poly-n-vinylcarbazole (PVK) as a p-type polymer and ZnO nanoparticles (NPs) as an n-type metal oxide. To observe the effective photo-inducing ability of the UV photodetector, we analyzed the optical and electrical properties of HPL which is controlled by the doping concentration of n-type ZnO NPs in PVK matrix. Additionally, we confirmed that the optical properties of HPL dominantly depend on the ZnO NPs from the UV-vis absorption and the photoluminescence (PL) spectral measurements. This HPL can induce efficient charge transfer in the localized narrow pn-heterojunction domain and increases the photocurrent gain. It is essential that proper doping concentration of n-type ZnO NPs in polymer matrix is obtained to improve the performance of the UV photodetector. When the ZnO NPs are doped with the optimized concentration of 3.4 wt.%, the electrical properties of the photocurrent are significantly increased. The ratio of the photocurrent was approximately 103 higher than that of the dark current.

In this study, we fabricated an ultraviolet (UV) photodetector by blending a hybrid photoactive layer (HPL) that is composed of a hybrid structure containing Carbon Quantum Dots (CQDs) and Zinc Oxide Nanorods (ZnO NRs). To observe the effective photo-inducing abilities of CQDs and ZnO NRs, we analyzed the electrical properties of a UV photodetector using an HPL of CQDs/ZnO NRs. Under an illumination of 365¿nm UV light with an intensity of 1¿mW/cm2, the UV photodetector exhibited a high detectivity of 8.33¿×¿1012 Jones, which is higher than that of a UV photodetector using a HPL of blended poly-n-vinylcarbazole (PVK) and ZnO NRs. Experimental results show that an HPL of blended CQDs/ZnO NRs can induce efficient charge extraction from CQDs and ZnO NRs. In addition, CQDs act as charge controllers that enable hole-electron separation in the device upon UV illumination. These results indicate that synthesized CQDs can substitute for a charge transport polymer (i.e., PVK) and that a UV photodetector using CQDs can exhibit high detectivity.

We demonstrate the first-ever surface modification of green CdSe/ZnS quantum dots (QDs) using bromide anions (Br -) in cetyl trimethylammonium bromide (CTAB). The Br - ions reduced the interparticle spacing between the QDs and induced an effective charge balance in QD light-emitting devices (QLEDs). The fabricated QLEDs exhibited efficient charge injection because of the reduced emission quenching effect and their enhanced thin film morphology. As a result, they exhibited a maximum luminance of 71,000 cd/m 2 and an external current efficiency of 6.4 cd/A, both significantly better than those of their counterparts with oleic acid surface ligands. In addition, the lifetime of the Br- treated QD based QLEDs is significantly improved due to ionic passivation at the QDs surface.

In this study, we performed an analysis of quantum-dot-based light-emitting diodes (QD-based LEDs) to investigate the defects at the interface between the core and the shell of the quantum dots (QDs) using gradient-shell QDs (CdSe/Cd1-xZnxSe1-ySy/ZnS, G-QDs) and single-shell QDs (CdSe/ZnS, S-QDs). QDs of the general core-shell type have defects at the core and shell junction interface due to the different lattice constants. However, G-QDs have a low number of lattice defects in the form of a step function between the core and the shell owing to their chemical composition and can more easily confine electron-hole pairs (EHP). Therefore, we fabricated QD-based LEDs by using two emissive layer (G-QDs and S-QDs) and analyzed their characteristics, including their brightness and efficiency.

In this work, we hypothesized and demonstrated a new strategy to tune/modulate the electrochemical, microstructural and opto-electronic properties based on the manipulation of the intentionally included external dopant ion (X^-) within the sulfonated polyaniline (SPANs). Through our new strategy, we developed a different type of SPANs comprising of internal (fixed) and external (mobile) dopant. The X^- included SPANs were prepared through a sequential doping, dedoping and redoping processes and designated as SPAN-R (X^-) (where X^- is the anion of toluene sulfonic acid (TSA) or camphor sulfonic acid (CSA) or napthalene sulfonic acid (NSA)) by modifying the structure of 4-aminodiphenylamine-2-sulfonic acid with additional polyaniline chains to accommodate X^-. SPAN-R(X^-) polymers were characterized by cyclic voltammetry, UV-visible spectroscopy, attenuated total reflectance-Fourier transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Atomic force microscopy to elucidate the influence of X^- on the electrical, optoelectronic, microstructural properties and surface properties on the performance characteristics of polymer solar cells (PSCs) fabricated with SPAN-R(X^-) as a buffer layer. The electrochemical band gap, degree of doping (DD), electrical conductivity and degree of crystallinity (CD) were evaluated and correlated to understand the influence of X^- on them. The power conversion efficiency (PCE) of PSCs featuring SPAN-R(TSA^-) as a buffer layer showed a ~3.2 times improvement in the overall PCE, compared with the PSCs having pristine SPAN (not containing X^-) as a buffer layer and is higher than that of SPAN-R(CSA^-) and SPAN-R(NSA^-) based devices. The superior photovoltaic (PV) characteristics observed for SPAN-R(TSA^-) is due to the synergistic contributions from appropriate energy-level/work function alignment, higher conductivity, higher DD and induced molecular order with the photoactive layer. Importantly, PSCs with SPAN-R(X^-) buffer layer processed at low temperature (30 °C) (without thermal treatment) exhibited improved PV characteristics and better air-stability as compared to the device having poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) (thermally annealed at 150 °C) as buffer layer. As buffer layers, SPAN-R(X^-) polymers, containing fixed and mobile dopants, are most attractive because of low temperature processability and improved solar cell performance.

In this paper, we propose an Au-polypyrrole (Ppy) nanorod gas sensor for the detection of volatile organic compound (VOC) gases. This gas sensor operates on the principle of localized surface plasmon resonance (LSPR). The Au-Ppy nanorods used in this experiment were synthesized using an anodic aluminum oxide template by the electrochemical deposition method. Using field emission scanning electron microscopy, we confirmed that the Au-Ppy nanorod arrays were successfully fabricated with a uniform size. By depositing gold, the Au-Ppy nanorods exhibited both optical and LSPR interference. The gas sensing properties of the fabricated nanorods were tested for VOCs such as acetic acid, benzene, and toluene with a short response time (~1 min). Moreover, the proposed VOC gas sensing system was tested with three types of VOC gases over a wide concentration range from 10 to 100 ppm. Highest sensitivity was observed for acetic acid gas, which had a linear relation with the gas concentration, indicating that the system can be used as a gas sensor.

In this paper, we propose interface engineering between cadmium selenide/zinc sulfide (CdSe/ZnS) quantum dots (QDs) as the emissive layer (EML) and ZnO nanocrystals (NCs) as the electron transport layer (ETL) for reducing the potential barrier in QDs based light-emitting diode (QLED). The n-type ZnO NCs were effective in confining charge to the QDs EML because of their wide band gap. The ZnO NCs were synthesized using a modified sol-gel process and were applied as the ETL in QLED. For comparison, a standard QLED with Tris(8-hydroxyquinolinato)aluminium as the ETL was also fabricated. The standard QLED was shown to have a luminance of 11,240 cd/m2 and current efficiency of 2.3 cd/A. However, QLED with ZnO NCs showed a higher luminance of 28,760 cd/m2 and current efficiency of 4.9 cd/A than the reference structure, and so has more efficient charge transport. Thus, QLED with ZnO NCs not only simplified the process, but also enhanced the luminance and current efficiency by factor of two.

In this paper, we demonstrate a simple strategy for the incorporation of gold nanodots (GNDs) into poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films, towards fabrication of an efficient anode interfacial layer in order to improve the performance of bulk heterojunction (BHJ) solar cells that use a blend of poly(3-hexyl thiophene) and [6,6]-phenyl-C71 butyric acid methyl ester as the organic active layer. We synthesized citrate-stabilized GNDs, with sizes in the range of ~20-30 nm, by initially blending them into PEDOT:PSS by aqueous dispersion. The influence of GNDs in the PEDOT:PSS layer on the photovoltaic characteristics of BHJ solar cells was investigated. Our results show that the improved performance is due to the enhanced conductivity and increased interfacial contact area between the PEDOT:PSS and organic active layer, owing to the inclusion of GNDs into the PEDOT:PSS. The BHJ solar cell included with GNDs (0.02 wt%) into PEDOT:PSS exhibited a power conversion efficiency (PCE) of 2.92% with an open circuit voltage of 0.61 V, fill factor of 50%, and a short-circuit current density of 9.51 mA/cm2, whereas the pristine device exhibited a PCE of 2.52%.

The electrical characteristics of quantum dots (QDs) can vary depending on the surface modulation, which can change the luminance and efficiency of electroluminescent devices. Thus, understanding surface ligand is essential in improving the performance of QDs-based light-emitting diodes (LEDs). We analyzed the performance of QDs-based LEDs with respect to the QD surface volume. On the QD surfaces, the 1.1 nm-long tryoctylphosphine oxide (TOPO) ligand with three neck-type structure was replaced with a 1.7 nm-long oleic acid (OA) ligand with a one neck-type structure to evaluate the dependence of the LED properties on the ligand length. With all other conditions being identical, the luminance and efficiency of the QDs-based LEDs with an OA ligand were approximately 1,000 cd/m2 greater and 1.5 times higher, respectively, than those of the QDs-based LEDs with a TOPO ligand. These results show that if the physical length of the surface ligand is relatively long, decreasing the surface area would result in increased injection of electrons and holes into the QDs, increasing the luminance and efficiency.

Graphene and metal nanoparticles were incorporated into titanium dioxide nanowires. The heterojunction generated in the titanium dioxide nanowires was beneficially utilized for photocatalytic degradation of Rhodamine-B.

We propose a nanobiosensor to evaluate a lung cancer-specific biomarker. The nanobiosensor is based on an anodic aluminum oxide (AAO) chip and functions on the principles of localized surface plasmon resonance (LSPR) and interferometry. The pore-depth of the fabricated nanoporous AAO chip was 1 µm and was obtained using a two-step electrochemical anodization process. The sensor chip is sensitive to the refractive index (RI) changes of the surrounding medium and also provides simple and label-free detection when specific antibodies are immobilized on the gold-deposited surface of the AAO chip. In order to confirm the effectiveness of the sensor, the antibodies were immobilized on the surface of the AAO chip, and the lung cancer-specific biomarker was applied atop of the immobilized-antibody layer using the self-assembled monolayer method. The nanoporous AAO chip was used as a sensor system to detect serum amyloid A1, which is a lung cancer-specific biomarker. The specific reaction of the antigen-antibody contributes to the change in the RI. This in turn causes a shift in the resonance spectrum in the refractive interference pattern. The limit of detection (LOD) was found to be 100 ag/mL and the biosensor had high sensitivity over a wide concentration range.

Design of power-efficient medium access control (MAC) protocols in wireless body area networks (WBANs) is important as it helps to prolong the network lifetime. This paper attempts to survey recent works of power efficient MAC protocols for WBANs and presents a comparison of the approaches pursued. We investigate a few MAC protocols devised for WBAN by highlighting their salient features. Finally, a number of open research issues are discussed. © 2010 IEEE.

In this paper, we provide a comprehensive survey of recent energy-efficient medium access control (MAC) protocols for wireless body area networks (WBANs) and presents a comparison of the various approaches pursued. At the outset, we outline the crucial attributes for a good MAC. Several sources that contribute to the energy inefficiency are identified. Then, we investigate few MAC protocols devised for WBAN by emphasizing their salient features. As a conclusion, we put forward a number of open research challenges with regard to prospects of medium access techniques and other issues. ©2010 IEEE.