Low-dimensional layered perovskites (LDLPs) have emerged as promising candidates in photovoltaics owing to their enhanced environmental stability. However, the high exciton binding energy, relatively poor conductivity and low charge carrier concentration in this class of materials limit their application as photoactive materials in photovoltaic devices. One effective method to overcome this bottleneck is to introduce an inherent dipole moment through the fluorination of the organic cation in the perovskite crystal structure. In this work, the structure-property correlation in LDLPs based on the position of the fluorine atom (ortho-, meta-, and para-) in monofluorinated N-methyl-1-phenylmethanaminium (NMPM+) as an A-site cation is investigated. Positional isomerism in the A-site cation results in inter- and intramolecular hydrogen bonding-induced variations in crystal packing, including changes in the interlayer spacing and distortion of the inorganic layers. Among the three LDLP derivatives, the ortho-fluorinated derivative shows a triclinic structure with the highest optical band gap. The meta- and para-fluorinated derivatives are isostructural, both adopting orthorhombic symmetry and having a similar optical band gap. Theoretical calculations also support the variation in optical band gap. Photoluminescence analysis reveals that emission of ortho-substituted derivative is governed by self-trapping of excitons, whereas band-to-band emissions dominate in isostructural meta- and para-derivatives. The positron annihilation spectroscopy (PAS) study suggests that the variation in the concentration of intrinsic defects in the as-synthesised materials results in ferroelectric responses from an otherwise centrosymmetric crystal structure. Furthermore, the photo-ferroelectric behaviour of all three materials is investigated using chronoamperometry and polarisation-electric field (P-E) loop measurements.

Caesium lead bromide (CsPbBr3) perovskite quantum dots (QDs) are one of the promising materials for the fabrication of green light-emitting diodes (LEDs) for their near-unity photoluminescence quantum yield (PLQY) with narrow emission and high brightness. However, its ionic nature makes it susceptible to degradation from moisture, heat and light. Various surface-passivating ligands were attempted, but with limited success in rendering long-term stability. Here, we have developed a novel bisphosphonate ligand, alendronate (ALEN), by alkylating with hexyl chains of acidic protons of alendronic acid, which is otherwise sparingly soluble in common organic solvents. The ligand was characterised by spectroscopic techniques such as NMR, FT-IR, and mass spectroscopies. The CsPbBr3 QDs prepared with ALEN-oleylamine ligand combination showed narrow particle-size distribution as determined by high-resolution transmission electron microscopy and dynamic light scattering measurements. The photostability of the QDs in solution and thin film was analysed by photoluminescence spectroscopy, revealing an interesting phenomenon called positive ageing effect, in which the PLQY of QD dispersion gradually increased by 140% from its initial value of 7 to 16.94% after two and a half months of storage. The defect passivation through multidentate binding sites of ALEN is the most plausible reason for such behaviour in ALEN-stabilised CsPbBr3 QDs.

A new luminescent metal-organic framework [Zn3(H2L1)2(OAc)2] (Zn-MOF) [H2L1= (E)-N'-(3-ethoxy-2 hydroxybenzylidene) isonicotinohydrazide] was synthesized by self-assembly. The single crystal X-ray diffraction of Zn-MOF confirmed an orthorhombic framework. Geometry optimization and calculations of HOMO and LUMO energy levels of Zn-MOF were carried out using DFT. Hirshfeld surface analysis offers a detailed insight into the various weak forces involved in molecular packing and assists in identifying short interatomic interactions. Zn-MOF demonstrates high selectivity and sensitivity in detecting both Fe3+ and Al3+ ions among various inorganic cations in aqueous solution as a turn-off luminescent sensor. Additionally, the limit of detection for Al3+ and Fe3+ ions were found to be 0.1038 mu M and 0.1857 mu M respectively, which indicates Zn-MOF is a promising sensor with a fast response time. This work may provide a strong basis for future applications of zinc- based metal-organic frameworks (MOFs) in detecting various contaminants.

Organic-inorganic lead halide perovskite is under intense focus on developing different optoelectronic devices because of its exceptional optical and electrical properties and ease of processing. However, the Pb2+ toxicity and the luminescence instability of these classes of semiconductors require continuous development of newer luminescent variants. Herein, Mn2+ is used as a dopant in luminescent morpholinium lead bromide perovskite, (C4H10NO)PbBr3, to substitute Pb2+ with Mn2+ with varied doping percentage to develop two different analogs of Mn2+-doped (C4H10NO)PbBr3 with varied optical properties. The incorporation of the dopant ion is investigated using various characterization techniques like inductively coupled plasma-optical emission spectroscopy, single-crystal and powder X-ray diffraction, infrared, and electron paramagnetic resonance spectroscopies. The photophysical properties are characterized by absorption and emission spectroscopies. With the increase in Mn2+-dopant amount, the emission maxima showed a blueshift with respect to the pristine sample and exhibited an enhanced (about 40%) photoluminescence quantum yield. Finally, the possibility of utilizing these semiconductors for practical applications is conducted by studying the structural and photophysical properties after embedding them into an optically transparent polymer, polymethyl methacrylate.

Point defects in methylammonium lead iodide (MAPbI3) are believed to be the source of its room-temperature ferromagnetic behavior. The existence of room-temperature ferromagnetism observed through the hysteresis in the M-H (magnetization vs. magnetic field) curve proves that ferromagnetism is possible even at room temperature in the tetragonal phases of MAPbI3 system. Employing positron annihilation spectroscopy, the presence of a significant amount of point defects in the ball-mill ground MAPbI3 sample has been identified. Coincidence Doppler broadening spectroscopy identifies that the point defects are mostly iodine vacancy (V˙I) in MAPbI3. Experimental data supports the theoretical prediction of iodine vacancy (V˙I)-induced ferromagnetism in the cubic phase which exists at a little higher temperature than the room-temperature tetragonal phase for MAPbI3. Moreover, the ab-initio band structure calculation shows the n-/p-type semiconducting behavior due to the vacancy formation in MAPbI3. Ferromagnetism along with semiconducting properties makes it viable for spintronics applications.

Quasi-2-dimensional (2D) halide perovskites have recently attracted attention due to their higher operational stability as alternatives to 3-dimensional (3D) perovskites having exceptional optoelectronic and charge transport properties. To reduce the lead content, here following the double perovskite approach lead is substituted with silver and bismuth simultaneously, and three quasi-2D perovskites, with general formula (C7H10N)2Pb(1-2x)AgxBixBr4, (0 ≤ x ≤ 0.5) were prepared. The optical studies show that the partially lead substituted sample has the lowest optical band gap, aptly supported by the theoretical calculations. The powder X-ray diffraction technique along with field-emission scanning electron microscopy suggests enhancement in crystallinity along with the decrease in grain boundaries with the substitution of lead. The improvement in crystallinity with concomitant reduction in grain boundaries has led to the decrease in point defects as identified from the positron annihilation lifetime spectroscopy and coincidence Doppler broadening analysis. The tuned band gap, improved crystal quality along with lower defects jointly contributed to the enhancement in electrical properties of the perovskites with varying lead percentages. Finally, the photoresponse of all the materials was studied after fabricating metal (Al)-semiconductor (MS) junction thin film photodetector devices.

Methylammonium lead halide-based perovskite has shown excellent optoelectronic properties. But their performances and stability are critically affected by the ionic defects present in the crystal lattice. In this article, we have investigated the presence of ionic vacancy mediated defects formation in ball mill ground methylammonium lead bromide (MAPbBr3) which has applications in tandem solar cell, light emitting diodes and laser devices. The evaluation of those point defects with temperature was analysed by employing the positron annihilation spectroscopic (PAS) studies. The phase transition from tetragonal to cubic phases around 260 K was exactly correlated with the temperature-dependent ‘S parameter’ determination from PAS analysis and with dc conductivity measurement. From coincidence Doppler broadening (CDB) spectroscopy significant proportion of defects arising from lead vacancy was observed whose magnitude reduces from the low-temperature tetragonal phase to higher temperature cubic phases.

In organic-inorganic perovskite semiconductor methylammonium lead iodide the occurrence of magnetism has been confirmed from magnetization and H-1 nuclear magnetic resonance (NMR) measurements. From room temperature down to 4 K, the magnetization is a combination of weak ferromagnetism and paramagnetism. H-1 NMR spectra at low temperatures also reveal two components, one with a small chemical shift characteristic of H-1 resonance in nonmetals, and the other with a much larger shift of similar to-100 ppm that is ascribed to the ferromagnetism of the material. The analysis of NMR spectra as well as that of H-1 spin-lattice relaxation rate (1/T-1) indicates that disorder produces an inhomogeneous distribution of magnetic moments that order ferromagnetically where they are dense enough and otherwise behave as random paramagnetic impurities. It is shown that magnetism arises from iodine and lead vacancies present in the orthorhombic and cubic crystal phases of the material.

The defect tolerance nature of organic-inorganic hybrid perovskite is reflected from its stupendous growth in photovoltaic performances. The presence of lattice defect can manipulate or even gives rise to some exceptional properties which otherwise would have remained unseen. One of such properties reported in this article is the experimental observation of defect mediated room temperature ferromagnetism in methylammonium lead halide perovskite for the very first time, ably supported by ab-initio calculations. Theoretical analysis predicts the ferromagnetism principally arises from the iodide vacancies in the orthorhombic and cubic crystal phases but not in the tetragonal phase. The low temperature (100 K) ferromagnetic hysteresis loop was stable even at a high temperature of 380 K substantiating the fact that the origin of magnetism embedded in its defective nature.

Methylammonium lead iodide (MAPbl(3)), a hybrid perovskite has become the semiconductor for case studies in several material properties due to its multi-faceted electronic application. Other than photovoltaics, generation of piezoelectricity from hybrid perovskites has been reported previously, though its origin remained unsubstantiated. In this report, MAPbl(3) with different degrees of crystal defects as determined by positron annihilation spectroscopy are developed through ball-mill grinding procedure and utilized to fabricate flexible piezoelectric nano-generators. We have demonstrated that at room temperature the lattice defects play the pivotal role in governing the ionic polarization which in principle governs the piezo-effect in MAPbl(3). The best device performance is exhibited by maximum defect containing sample having significant amount of Pb2+ defects. A device fabricated with 5 wt% PDMS composite produces piezo-voltage of more than 100 V with a maximum power density of 0.3 mW/cm(3) and can illuminate commercially available 30 blue light emitting diodes.

In this report, bias voltage-dependent dielectric and electron transport properties of ZnS nanoparticles were discussed. ZnS nanoparticles were synthesized by introducing a modified hydrothermal process. The powder XRD pattern indicates the phase purity, and field emission scanning electron microscope image demonstrates the morphology of the synthesized sample. The optical band gap energy (E (g) = 4.2 eV) from UV measurement explores semiconductor behavior of the synthesized material. The electrical properties were performed at room temperature using complex impedance spectroscopy (CIS) technique as a function of frequency (40 Hz-10 MHz) under different forward dc bias voltages (0-1 V). The CIS analysis demonstrates the contribution of bulk resistance in conduction mechanism and its dependency on forward dc bias voltages. The imaginary part of the impedance versus frequency curve exhibits the existence of relaxation peak which shifts with increasing dc forward bias voltages. The dc bias voltage-dependent ac and dc conductivity of the synthesized ZnS was studied on thin film structure. A possible hopping mechanism for electrical transport processes in the system was investigated. Finally, it is worth to mention that this analysis of bias voltage-dependent dielectric and transport properties of as-synthesized ZnS showed excellent properties for emerging energy applications.

The current–voltage (I–V) measurements on Al/ZnS Schottky barrier diodes in the temperature range 303 –423 K by the step of 15 K were carried out. The forward I–V characteristics were analyzed on the basis of the thermionic emission theory. The temperature dependence I-V parameters such as ideality factor (n) and barrier height (ϕb0) have been explained on the basis of inhomogeneity. An abnormal increase of apparent barrier height and decrease of ideality factor with increasing temperature have been explained due to the barrier height inhomogeneities on the basis of the thermionic emission theory with Gaussian distribution. Experimental results reveal the existence of a single Gaussian distribution with apparent barrier height value (ϕb0‾) of 1.091 eV and standard deviations (σs) of 0.18 V. Richardson constant (A*) was obtained as 8.49 x 10-2 A.m-2K-2 from the ln(I0/T2) vs. q/kT plot, which is far from the calculated value of 5.6 x 105 A.m-2K-2. The modified Richardson plotof ln(I0/T2) – (q2σs2/2k2T2) gives ϕb0‾ and A* values as 1.093 eV and 6.07 x 105 A.m-2K-2, without using the temperature coefficient of the barrier height. This obtained value of A* is extremely close to the previously calculated value. So, the temperature dependence of the forward bias I-V characteristics of the Schottky device can be successfully explained on the basis of the thermionic emission mechanism with a single Gaussian distribution of the barrier heights.

Inability to early diagnosis is a major concern for the treatment of fatal disease like cancer. Early diagnosis and treatment enhances the scope of disease curability. The accurate identification, realtime monitoring and targeting the cancerous tissues in a precise manner hold the key for longer progression free survival of a patient. Among different diagnostic techniques, fluorescence based minimally invasive bio-imaging techniques are considered to be ideal to have clear understanding about the physiological processes of the infected tissues as well as to reduce physical and mental stress of a patient. Super-resolution fluorescence microscopy has improved the spatial optical resolution of biological molecules, living cells and tissues with the use of highly fluorescent inorganic semiconductor nanocrystals, also known as quantum dots (QDs), with sizes ranging from 2 nm to 15 nm [1]. These nanocrystals comprised of elements belong to groups II–VI (eg, CdSe and CdTe), groups III–V (eg, InP), groups IV–VI (eg, PbS and PbSe) [2]. The QDs have several unique optical properties which make them far superior than organic chromophores as fluorescent probes in fluorescence microscopy. The exciting optical properties of QDs exhibiting due to their size which is smaller than their exciton Bohr radius [3]. For example, QDs have high molar extinction coefficients, high quantum efficiency (>50%), narrow emission spectra, higher excited state lifetimes and high resistance to photo-bleaching [4]. In addition to this, QDs are at least 15 times brighter than organic dyes under the same experimental conditions [5]. Such unique optical properties are utilized to enhance the signal-to-background ratio for microscopic imaging [6]. Moreover, the optical band gap and in turn, emission wavelength of QDs can be systematically tuned from the visible to near-infrared (NIR) spectral region by simply manipulating their size, shape, composition, and structure as shown in Figure 1 [7]. Thus, optical tunability of QDs renders opportunity in multiplexed and real-time imaging [8]. Due to large absorption cross section, QDs are also potential candidates for twophoton imaging as compared to some organic dyes [9]. Semiconductor Quantum Dots as In-Vivo Imaging Agent

In this report, we have synthesized Bornite (Cu5FeS4) material by hydrothermal synthesis technique. The interface characteristics of Al/Cu5FeS4/FTO Schottky barrier diode (SBD) are investigated by using ac impedance spectroscopy (IS) analysis (under dark condition) and dc current-voltage (I-V) measurements (under dark and light both condition). IS is a powerful tool to identify the interface regions of SBDs. Ac impedance spectra of Al/Cu5FeS4 SBD are recorded in the frequency range 40 Hz-20 MHz during dc bias scanning from -0.6 V to 0.6 V under dark condition. The diode parameter including ideality factor and barrier height is calculated from the conventional I V measurement based on thermionic emission (TE) theory. Space charge limited current (SCLC) theory has been employed to further exemplify the improved performance of Cu5FeS4 based SBD, which points out that the carrier mobility is enhanced similar to 2-fold after irradiation of light.

In this work, we have rationally designed and synthesized a novel thiophene-diketopyrrolopyrrole (TDPP)-vinyl-based dimer. We have investigated the optical and electronic properties and have probed the photophysical dynamics using transient absorption to investigate the possibility of singlet exciton fission. These revealed extremely rapid decay to the ground state (<50 ps), which we confirm is due to intramolecular excitonic processes rather than large-scale conformational change enabled by the vinyl linker. In all cases, the main excited state appears to be "dark", suggesting rapid internal conversion into a dark 2Ag-type singlet state. We found no evidence of triplet formation in TDPP-V-TDPP under direct photoexcitation. This may be a consequence of significant singlet stabilization in the dimer, bringing it below the energy needed to form two triplets. Our studies on this model compound set valuable lessons for design of novel triplet-forming materials and highlight the need for more broadly applicable design principles.

This review highlights recent advancement in developing ambient stable organic molecular semiconductors from the theoretical and experimental perspectives.

We have investigated the origin of ionic conductivity in methylammonium lead iodide (MAPbI(3)) by positron annihilation lifetime spectroscopy (PALS), supplemented by coincidence Doppler broadening spectroscopic (CDBS) techniques which reveal the presence of methylammonium (MA(+)) defects in the perovskite crystal lattice. Crystallinity and the defect concentration vary with the perovskite synthesis process, which in turn governs the magnitude of ionic conductivity. Single-crystalline perovskite contains lesser defects with equal probability of developing both cationic and anionic (halide) vacancies, whereas the polycrystalline perovskite sample developed through mechanical process carries mainly cationic, i.e., MA(+) vacancy (V'(MA)) in its crystal lattice as indicated by direct current (dc) polarization experiment.

Organic-inorganic hybrid perovskite has appeared as one of the leading materials for realizing solution-based high-performing optoelectronic devices. The charge transport properties in this class of material are quite intriguing and still need to be carefully investigated. The temperature-dependent electrical property of methylammonium lead iodide (CH3NH3PbI3) has been investigated by employing positron annihilation spectroscopy (PAS), which unambiguously reveals the gradual formation of open volume defects with the enhancement in temperature. The high-temperature ionic conductivity is due to the generation of both cationic (CH3NH3+) and anionic (I-) vacancies, possibly because of the elimination of methylammonium iodide (CH3NH3I) as identified from the coincidence Doppler broadening (CDB) of the positron annihilation spectroscopy. Further, the evolution of temperature-dependent defect density and corresponding electrical responses has been correlated with the structural phase transitions of CH3NH3PbI3. This is the first ever report of temperature-dependent PAS measurement on hybrid lead halide perovskites to understand the nature and the origin of its electrical characteristics arising due to the variation in temperature.

Here we have presented the results of large area (30 × 30 cm2) silicon-hydrogen alloy material and solar cell by argon dilution method. As an alternative to hydrogen dilution, argon dilution method has been applied to develop single junction solar cell with appreciable stability. Optimization of deposition conditions revealed that 95% argon dilution gives a nanostructured material with improved transport property and less light induced degradation. The minority carrier diffusion length (L d ) and mobility-lifetime (μτ) product of the material with 95% argon dilution degrades least after light soaking. Also the density of states (DOS) below conduction level reveals that this material is less defective. Solar cell with this argon diluted material has been fabricated with all the layers deposited by argon dilution method. Finally we have compared the argon diluted solar cell results with the optimized hydrogen diluted solar cell. Light soaking study proves that it is possible to develop stable solar cell on large area by argon dilution method and that the degradation of argon diluted solar cell is less than that of hydrogen diluted one.

The mono-alkylation of DPP derivatives leads to cofacial π-π stacking via H-bonding unlike their di-alkylated counterparts, which exhibit a classical herringbone packing pattern. Single crystal organic field-effect transistor (OFET) measurements reveal a significant enhancement of charge carrier mobility for mono-hexyl DPP derivatives.