Fine tuning of compact layer for Efficient Quantum Dots Solar Cells

An energy is a fundamental need of every human being to live in. With increase in population, the demand of an energy also increases and accelerated the use of fossil fuels. Coal, oil and natural gas are the main sources of fossil fuels which are common energy sources all over the world. However, with this high demand of fossil fuels, it is estimated that the reserve fossil fuels will be depleted within the couple of decades. On the other hand, fossil fuels emit a high amount of carbon dioxide that results into the climatic change and the environmental pollution. In order to meet the demand of energy as well as clean sources; renewable energy sources like hydro, geothermal, solar and wind energy etc. are the green alternatives for the clean and healthy environment [1, 2]. Among them, solar energy is the safe, quiet adjustable, inexhaustible and easily exploited. In addition, sunlight can be harnessed at domestic as well as at commercial level [3, 4]. The solar energy conversion devices are known as photovoltaic devices or solar cells. Solar cells basically divided into three generations among which the concept of third generation solar cells exploits the solar energy proficiently. Third generation solar cells consist of dye sensitized solar cells (DSSCs), perovskite solar cells (PSCs) and quantum dots solar cells (QDSCs). In all of these, QDSCs has attracted great attention owing to high stability, low cost, ease of fabrication and handling, eco-friendly, efficient optoelectronic properties and absorption coefficient. Moreover, QDSCs have adjustable size, band gap and optical properties of QDs alongwith high extinction coefficient which helps to reduce the dark current [5]. QDs also shows rapid charge separation owing to intrinsic dipole moments and shows extendable photo response in the visible region. QDSCs, basically, comprised of QDs sensitized photoanode and counter electrode with a redox electrolyte in between. In QDSCs, QDs work as a sensitizer, capture the energetic electrons and transfer them to TiO2 material. QDs generates multiple exciton pairs as per absorbed photon which further enhances the absorbance efficiency. Herein, a compact layer of TiO2 (c-TiO2) have been prepared and coated on substrate via spin coating technique. The prepared c-TiO2 have employed in order to reduce series resistance and improve transmittance. The compact layer (CL) have been studied extensively in DSSCs, however, there is no as such report on the optimization of CL in QDSCs as per our knowledge. The CLs have been optimized by spinning speed as well as precursor concentration. Furthermore, CdS QDs have been prepared via successive ionic layer adsorption and reaction (SILAR) process. The as prepared CLs and photoanodes thoroughly characterized using UV-Vis spectroscopy (UV-Vis), Raman, X-ray diffractometer (XRD), scanning electron microscopy (SEM), Photoluminescence spectrophotometer (PL), X-ray photon spectroscopy (XPS) and kelvin probe. The fabricated QDSCs has been studied with current density-voltage (JV) characteristics and electrochemical impedance spectroscopy (EIS). The photovoltaic power conversion efficiency (PCE) of QDSCs found to be significantly improved with the incorporation of CLs and found to be vary with change in thickness of CL.