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In spite of the phenomenal efficiency progression of the organometal halide perovskite-based solar cells (PSCs) over the past few years, detailed understanding of the working mechanisms and effective measures to overcome the main weaknesses such as the long-term instability are of central importance. This paper provides a brief review of our most recent research on high-performance PSCs including the diethanolamine-modification of NiO surface, the mixed solvent engineering and the mesoporous TiO2 nanobowl (NB) array assisted light harvesting.
The effects of TiCl4 post-treatment on the physicochemical properties of porous TiO2 (pTiO2) layers fabricated at 300 °C and 400 °C (denoted as pTiO2(300) and pTiO2(400), respectively) in CH3NH3PbI3 perovskite photovoltaic cells were investigated. Water contents (physisorbed water and water derived from surface hydroxyl groups) of pTiO2(300) and pTiO2(400) before and after TiCl4 post-treatment were measured by using temperature desorption spectroscopy (TDS). Moreover, structural analysis of the CH3NH3PbI3 perovskite part was performed by X-ray diffraction (XRD). In the case of pTiO2(300), the content of water was increased by the TiCl4 post-treatment due to the removal of residual organic compounds that existed before the treatment. It then caused a change in the surface activity of pTiO2(300) and enhancement of solar cell performance and photocurrent density, though suppression of CH3NH3PbI3 perovskite formation occurred. In comparison, contents of water were decreased for pTiO2(400), leading to enhancement of the conversion of PbI2 to CH3NH3PbI3 perovskite. As a result, there were significant increases in short circuit current density (Jscs) and PCEs. The results showed that TiCl4 post-treatment is an effective approach to prepare high-performance CH3NH3PbI3 perovskite solar cells without heat treatment at a very high temperature.
Perovskite based photovoltaics have recently emerged as the forerunner in the next generation photovoltaic technology because of the rapid increase of power conversion efficiency (PCE). However, it is well recognized that the exposure to moisture, heat and light causes the degradation of perovskite  (especially for methylammonium lead iodide (CH3NH3PbI3) which is the most commonly used perovskite material). It makes stability a main issue for the commercialization of perovskite based photovoltaics. Hence, an advanced encapsulation method is one of the keys to improve the stability. Here we present a comparison study between different encapsulation methods. Perovskite based photovoltaics devices were encapsulated using UV epoxy resin, with or without the addition of desiccant and the deposition of SiO2 layer. By minimizing the ingress of moisture and oxygen, devices with storage in ambient air under one sun continuous illumination could retain 94 % of the initial performance (PCE around 13%) after two days.
Zinc oxide thin films were prepared via the sol-gel spin-coating method with the use of a spin processor. The film’s annealing parameters were varied to study their impact on the final film morphology and electrical properties. Characterization of the structural properties of the samples was carried on a X-ray diffractometer (XRD) and scanning electron microscopy. Electrical characterization was obtained with the use of a four point probe. Optical characterization of the samples was carried on a UV-Vis-NIR Spectrophotometer. Samples annealed under a cover are observed to have a higher transmission percentage on the visible light range while having a very small grain size and small relative resistivity. Samples annealed under standard atmospheric conditions show a larger grain size and resistivity, and correlated to it, a smaller transmission percentage. Samples annealed under vacuum prove to have a much more reduced optical, electrical, and structural properties when compared to the rest of the samples.
A new organometallic halide perovskite (OHP) synthesis method, whereby a polymer melt is used to thermodynamically drive the reaction that forms OHP crystallites, is demonstrated. The synthesis method allows for the facile encapsulation of moisture-sensitive OHP without the loss of simplicity during fabrication, which makes OHP materials so attractive for the photovoltaic industry. Degradation of OHP crystallites embedded in a polystyrene matrix was studied using UV-Vis absorbance over a period of several days. The OHP crystallites degrade as a result of the reversible nature of the reaction that forms the crystallites. After the reversion to precursors (PbI2 and CH3NH3I) the CH3NH3I irreversibly degrades  allowing the degradation to be tracked via optical interrogation. Additionally, surface morphology and elemental analysis of fabricated samples was carried out using SEM/EDS techniques.
We studied nonlinear absorption characteristics of exfoliated Molybdenum disulphide (MoS2) dispersion in 1-Methyl-2-pyrrolidinone and demonstrate a dual absorption characteristic at 532nm nanosecond pulsed laser wavelength. A number of recent reports demonstrate a saturable absorption in MoS2 and other 2D materials at low fluences and a deviation from this saturable absorption at higher fluence using open aperture Z scan (OZ scan) technique. It has been suggested that this deviation at higher fluences is due to nonlinear optical scattering. We have recently developed a new technique which combines OZ scan and photoacoustic Z-scan (PAZ-scan). It can measure photoacoustic and optical transmission signals simultaneously. The data obtained from both signals are employed to find nonlinear absorption parameters in non-linear optical materials. Our results reveal that non-linear scattering is not the cause of deviation of 2D materials from saturable absorption at higher fluences. We propose that the optical limiting behavior at higher fluence in these 2D materials is dominated by free carrier absorption.
For optoelectronic application, two-dimensional materials such as molybdenum disulfide (MoS2) are very promising candidate with their interesting electronic and optical properties. The layered structure of these materials makes them amenable to mechanical exfoliation to form scalable 2D atomic crystals. For width range of applications, liquid phase exfoliation using sonication and centrifugation in appropriate solvent is needed. This simple and scalable technique gives high quality of exfoliation of 2D materials without chemical reactions. In this paper, we report an example of the optical and electronic characterizations on MoS2 synthesized by liquid exfoliation in specific solvent.
Transition metal dichalcogenides (TMDs) are emerging among the potential alternatives to graphene. The monolayer of TMDs can easily be exfoliated mechanically and their electronic properties can also be tuned by controlling the number of layers. TMDs possess an advantage over graphene by controlling band gap magnitude appropriate for the electronic and optoelectronic applications. Here we show, mechanically exfoliated TMDs such as NbSe2 and MoTe2 exhibit metallic and fluctuating conductance behavior respectively. Metallic conduction in NbSe2 was investigated under atmospheric conditions and compered with vacuum conditions. Furthermore, NbSe2 resistance was measured at low temperature up to 5.6 K. The above electronic investigations clearly demonstrate ohmic and fluctuating conduction in NbSe2 and MoTe2 respectively which could be applicable for electronic and optoelectronic devices.