Shrabani Panigrahi

Ordered dispersion of ZnO quantum dots in SiO2 matrix and its strong emission properties

ZnO nanoparticles in the form of quantum dots (QDs) have been dispersed in SiO(2) matrix using StÖber method to form ZnO QDs-SiO(2) nanocomposites. Addition of tetraethyl orthosilicate (TEOS) to an ethanolic solution of ZnO nanoparticles produces random dispersion. On the other hand, addition of ZnO nanoparticles to an already hydrolyzed ethanolic TEOS solution results in a chain-like ordered dispersion. The photoluminescence spectra of the as-grown nanocomposites show strong emission in the ultraviolet region. When annealed at higher temperature, depending on the sample type, these show strong red or white emission. Interestingly, when the excitation is removed, the orderly dispersed ZnO QDs-SiO(2) composite shows a very bright blue fluorescence visible by naked eyes for few seconds indicating their promise for display applications. The emission property has been explained in the light of structure-property relationship.

Morphology driven ultraviolet photosensitivity in ZnO-CdS composite.

Semiconductors in the form of composite yields immense possibilities for the study of charge transfer processes at the interface. We have designed CdS nanoparticles (NPs) capped ZnO nanostructures using two morphologies of the latter namely nanorods arrays (NRAs) and nanocrystalline film to form a composite NRAs and composite films respectively. The photocurrent values in both the composites have been increased but to a different extent when these are illuminated with the ultraviolet (UV) light. More interestingly, the resultant UV photosensitivity in the composite NRAs is decreased while that in the composite films is increased as compared to the values of the respective uncapped samples. A different charge transfer process at the interface is occurred due to the difference in the morphologies resulting in a contrast change in the UV sensitivity. The photoluminescence results also show that the change in the emission property is morphology-dependent. Therefore, our results imply that the choice of the morphology while making a nanocomposite is crucial to tune its UV sensitivity as well as optical properties.

A new hydrogel from an amino acid-based perylene bisimide and its semiconducting, photo-switching behaviour

A new, stable, semiconducting, photo-responsive, amino acid (L-tyrosine) based perylene bisimide derivative (PBI-Y) was found to form pH-sensitive hydrogels. This gelator molecule (PBI-Y) has undergone hydrogelation at 50 mM phosphate buffer at pH 5.00–9.00. Different minimum gelation concentration values (MGC) were recorded in different pHs at the same phosphate buffer strength (50 mM). The hydrogel was also characterized using different techniques, including X-ray diffraction analysis, rheology and FT-IR study. The fluorescence and cyclic voltammetric responses of the PBI-Y hydrogelator can be tuned by changing the pH. The PBI-Y hydrogelator has shown self-assembled nanofibrillar network structures, as it has been evident from the field emission scanning electron microscopic (FE-SEM), as well as transmission electron microscopic analyses (TEM). This PBI-Y hydrogel has also shown interesting semiconducting behaviour. This PBI-Y hydrogelator-based photoconductor was found to exhibit excellent photo-switching behaviour with a high photo-response value. This holds a future promise for the creation of PBI-conjugated functional soft-materials-based photodetectors and photovoltaics.

Encapsulation of 2−3-nm-Sized ZnO Quantum Dots in a SiO2 Matrix and Observation of Negative Photoconductivity

Quantum dots (QDs) of ZnO of 2-4 nm size have been encapsulated within a SiO(2) matrix using aqueous chemically grown ZnO nanoparticles in a precursor of tetraethyl orthosilicate. The microstructure shows almost a uniform embedment of the QDs in the SiO(2) matrix, resulting in a ZnO QDs-SiO(2) composite structure. The photocurrent transients of the composite show an instant fall in the current followed by an exponential decay under ultraviolet (UV) illumination, causing negative photoconductivity (NPC), in contrast to the positive photoconductivity in only ZnO nanoparticles. The interface defect states due to the presence of the SiO(2) network around ZnO act as charge trap centers for the photoexcited electrons and are responsible for the NPC. The presence of interface-trapped charges under UV illumination has been further confirmed from capacitance-voltage measurements.

Solution-processed novel core–shell n–p heterojunction and its ultrafast UV photodetection properties

We demonstrate the feasibility of fabricating a solution-processed novel core–shell coaxial n–p nanorod (NR) heterojunction consisting of an n-type ZnO NR core and a p-type CuS shell through an ion-exchange route. The field emission scanning electron and high resolution transmission electron microscopy experiments show evidence of the formation of core–shell NR arrays manifesting a clear change in the side walls of the NRs, which become rougher due to CuS shell formation over ZnO NRs. The core–shell heterojunction shows diode behavior with an ideality factor of 5.8. The diode shows a very fast response time (<0.3 s) towards 365 nm ultraviolet (UV) light, which is an important criterion for the photodetection property. Furthermore, the main advantages of this solution-based method are its simplicity and low cost. This achievement demonstrates that such solution-processed novel core–shell n–p heterojunctions are promising candidates for applications in electro-optic devices.

A bolaamphiphilic amino acid appended photo-switching supramolecular gel and tuning of photo-switching behaviour

Self-assembled bolaamphiphilic perylene bisimide (PBI) containing an amino acid appended fluorescent semiconducting soft material (hydrogel) has been discovered at physiological pH. This new organic material based on self-assembled perylene bisimide appended amino acid-based bolaamphiphile (PBI-C11-Y) has been well characterized using various techniques including UV-Vis, fluorescence, X-ray diffraction, FT-IR, transmission electron microscopic (TEM) and atomic force microscopic (AFM) studies. Interestingly, the UV-Vis absorption properties of the soft-material are dependent on the pH of the medium. This PBI-conjugated amino acid appended gelator molecule contains a centrally located perylene bisimide moiety as well as an aromatic amino acid l-tyrosine at the side chains, which are extremely useful for interacting with the delocalized large π-surface of GO (graphene oxide) or RGO (reduced graphene oxide) to form a GO/RGO containing hybrid hydrogel. Graphene oxide and reduced graphene oxide have been successfully incorporated into the nanofibrillar network structure of the PBI-C11-Y based gel to make nanohybrid systems. The I-V profile of the semiconducting photo-responsive soft-material of the PBI-C11-Y has been successfully tuned upon the incorporation of GO and RGO within the gel-based soft material. This PBI-C11-Y xerogel based structure shows photo-switching behaviour upon exposure to white light. The ON/OFF ratio of the PBI-C11-Y can be modulated upon the inclusion of GO and RGO within the hydrogel matrix. Furthermore, the OFF state stability of the PBI-C11-Y xerogel material has been increased upon the inclusion of RGO. Regulation of the photo-switching behaviour of the PBI-C11-Y based xerogel holds promise for making PBI-containing amino acid appended biomaterials with interesting properties in future.

Observation of Space Charge Dynamics Inside an All Oxide Based Solar Cell

The charge transfer dynamics at interfaces are fundamental to know the mechanism of photovoltaic processes. The internal potential in solar cell devices depends on the basic processes of photovoltaic effect such as charge carrier generation, separation, transport, recombination, etc. Here we report the direct observation of the surface potential depth profile over the cross-section of the ZnO nanorods/Cu2O based solar cell for two different layer thicknesses at different wavelengths of light using Kelvin probe force microscopy. The topography and phase images across the cross-section of the solar cell are also observed, where the interfaces are well-defined on the nanoscale. The potential profiling results demonstrate that under white light illumination, the photoinduced electrons in Cu2O inject into ZnO due to the interfacial electric field, which results in the large difference in surface potential between two active layers. However, under a single wavelength illumination, the charge carrier generation, separation, and transport processes between two active layers are limited, which affect the surface potential images and corresponding potential depth profile. Because of changes in the active layer thicknesses, small variations have been observed in the charge carrier transport mechanism inside the device. These results provide the clear idea about the charge carrier distribution inside the solar cell in different conditions and show the perfect illumination condition for large carrier transport in a high performance solar cell.

Imaging the Anomalous Charge Distribution Inside CsPbBr3 Perovskite Quantum Dots Sensitized Solar Cells

Highly luminescent CsPbBr3 perovskite quantum dots (QDs) have gained huge attention in research due to their various applications in optoelectronics, including as a light absorber in photovoltaic solar cells. To improve the performances of such devices, it requires a deeper knowledge on the charge transport dynamics inside the solar cell, which are related to its power-conversion efficiency. Here, we report the successful fabrication of an all-inorganic CsPbBr3 perovskite QD sensitized solar cell and the imaging of anomalous electrical potential distribution across the layers of the cell under different illuminations using Kelvin probe force microscopy. Carrier generation, separation, and transport capacity inside the cells are dependent on the light illumination. Large differences in surface potential between electron and hole transport layers with unbalanced carrier separation at the junction have been observed under white light (full solar spectrum) illumination. However, under monochromatic light (single wavelength of solar spectrum) illumination, poor charge transport occurred across the junction as a consequence of less difference in surface potential between the active layers. The outcome of this study provides a clear idea on the carrier dynamic processes inside the cells and corresponding surface potential across the layers under the illumination of different wavelengths of light to understand the functioning of the solar cells and ultimately for the improvement of their photovoltaic performances.

Oxide-Based Solar Cell: Impact of Layer Thicknesses on the Device Performance

A ZnO/Cu2O-based combinatorial heterojunction device library was successfully fabricated by a simple spray pyrolysis technique using ITO-coated glass as the substrate. The combinatorial approach was introduced to analyze the impact of the ZnO and Cu2O layer thicknesses on the performance of the solar cells. The thickness of the ZnO layer was varied from ∼50 to 320 nm, and the Cu2O layer was deposited orthogonal to the ZnO thickness gradient. In the case of Cu2O, the thickness varied from ∼200 to 800 nm. The photovoltaic performance of the cells is strongly dependent on the absorber layer thickness for a particular window layer thickness and reaches a maximum short-circuit current density of 3.9 mA/cm2 when the absorber layer thickness just crosses ∼700 nm. Reducing the thicknesses of the active layers leads to a sharp decrease in the device performance. It is shown that the entire built-in bias of the heterojunction is created in the absorber layer due to low carrier density. The poor performance of the devices having lower thicknesses is attributed to different interfacial phenomena such as optical losses due to the thin Cu2O layer, back-contact recombination of the carriers due to the low layer thickness because a minimum heterojunction thickness is required for the formation of the full built-in bias that slows down the recombination of the carriers, and other factors.

Recombination-tunneling conduction in Cu- and S-doped ZnO nanorods’ core–shell junction: dependence of diode parameters on thermal annealing temperature and role of interfacial defects

We have investigated the effect of thermal annealing on the diode characteristics of solution-processed novel core–shell coaxial n–p nanorods heterojunction consisting of n-type ZnO as a core and p-type CuS as a shell material. The values of turn-on-voltage, rectification ratio, reverse saturation current density, barrier height, and ideality factors have been improved as the as-prepared heterojunction is annealed at higher temperatures owing to the improvement in the interface between ZnO and CuS as evidenced from the high-resolution transmission electron microscopy. The X-ray diffraction results also confirm the improvement in the crystalline quality of ZnO and CuS through annealing. The experimental current–voltage data are consistent with the presence of dominating recombination-tunneling conduction occurring through the interface defect states between ZnO and CuS. The results demonstrate that an annealing process plays a dominant role in the interfacial defects which helps to modify the diode performance paving a way to cheaper electronic nanodevices.