MORPHOLOGY OPTIMIZATION AND PHOTOPHYSICAL STUDY OF PEROVSKITE SOLAR CELLS

Abstract

Fossil fuels are limited, and we can not depend on them forever. The inevitable need of energy for daily life has triggered the scientific community to work on renewable energy. The solar energy as an important class of renewable energy has drawn considerable research interest. The various different generations of solar cells have evolved over the time due to the discovery of new materials. Starting from highly efficient first generation crystalline silicon to decent second generation polycrystalline silicon solar cells or flexible third generation organic solar cells, the trade off between efficiency and cost per watt always remains an issue for commercialization of this technology. In recent years, organic-inorganic hybrid perovskite solar cell (PSCs) as a third generation solar cells have emerged as a shining star in the horizon of various photovoltaic technologies. Due to its high efficiency and low cost fabrication method it has received significant research interests. The PCE has increased phenomenally from 3.8% to over 22.7% in a short span of time. In pursuing high efficiency of PSCs interface modification, compositional engineering and morphology tailoring have been the major research focuses. The perovskite film morphology and photophysics are found to be crucial factors in fabricating highly efficient and stable solar cells. Non-uniform and irregular film leads to the pin hole formation and enhance the surface roughness which inhibits efficient charge transport across the film. The morphology of the prepared sample control the device photophysics like nature, location (bulk vs surface), and concentration of defects. The reproducible optimized morphology and photophysical properties of the perovskite film are a major concern with perovskite material which limits its excited state dynamics. Apart from perovskite, recently, organic nanomaterials with rich photophysical properties have drawn considerable research interest in organic photovoltaics (OPV). The photophysical properties in different nanostructure geometries provide an excellent approach to tune their optoelectronics properties. Page ii In this dissertation work, the morphology optimization and photophysics study of perovskite solar cells (both lead perovskites and lead free perovskites solar cells) and organic nanoparticles solar cells have been studied in details. The work presented in this thesis is structured into five different chapters. Chapter 1 gives an introductory discussion on solar energy, physics of solar cells, the description of different generations of solar cells mainly focusing on perovskite solar cells and organic nanoparticles based solar cells. This chapter also describes the motivation and relevance of the present study with respect to understanding the morphology and photophysics in perovskites and organic nanoparticles solar cells. Chapter 2 describes the morphology optimization and photophysics study in lead based organic inorganic CH3NH3PbI3 perovskites film at room temperature by tuning the dielectric constant (εr) of the precursor solvents and the corresponding air processed device fabrication strategy. The extraction of highly polar solvent over localized area in this dual solvent elimination method (DSEM) leads to larger grain size and better crystallinity at room temperature. The X-ray diffractograms (XRD) confirms the perovskite crystal structure and AFM microscopy reveals the lower surface roughness of these films implying the formation of uniform film over large grain sizes. The correlated confocal PL and scanning electron microscopy (SEM) depicts minimal distribution in local PL intensity across this uniform film. Photolumisicience spectroscopy and fluorescence microscopy were utilized to probe the underlying photophysics of the perovskite film. Finally, the photovoltaic device fabricated outside the glove box is found to have power conversion efficiency (PCE) of 6% with JSC value of 12.5 mA and VOC of 900 mV. In Chapter 3, we report, the preparation of highly reproducible perovskite CH3NH3PbI3 thin films using antisolvent coating method. The films were characterized using Scanning Electron Microscopy (SEM), X Ray Diffraction (XRD) and optical absorption and photolumiscience (UVVis and PL) spectroscopy. The morphology of the antisolvent treated film is optimized and a detailed photophysical investigation is performed to understand the charge carrier dynamics. The average lifetime of these solvent annealed films does not vary significantly with excitation fluence confirming the formation of almost defect free perovskite crystallites. The local reduction of Page iii shallow trap densities as observed in PL dynamics was correlated with scanning electron microscopy (SEM) and X-ray spectroscopy. The air processed devices with ITO/PEDOT:PSS/MAPbI3/C60/BCP/Al structure is prepared and characterized. The stability of thin films is checked by recording UV-Vis absorption spectra of thin film for 30 days. In Chapter 4, the morphology optimization and photophysical studies are performed for lead free Bi based perovskite CH3NH3BiI3. Here, We have fabricated lead-free methylammonium bismuth iodide (CH3NH3BiI3) perovskite films and have studied the effect of solvent annealing on the local optoelectronic properties of the films as measured via (photo)conductive atomic force microscopy ((p)c-AFM)). We have found solvent annealing to significantly increase electrical conductivity in the out-of-plane direction. Photoconductivity in both solvent-annealed and thermally-annealed MBI films are boosted in the grain interiors versus the grain boundaries (GBs). It is observed that DMF induced solvent annealing impacts charge transport through the film which can be a unique design parameter for optimizing local optoelectronic properties. By studying how solvent annealing changes the ways in which charge is transported through the film, we have developed a better understanding of how local optoelectronic properties are affected by anti-solvent annealing. In Chapter 5, we describe the morphology optimization and photophysics study in organic nanoparticles based solar cells. Here, we report, two different hybrid nanostructures namely intradyad and interdyad using all small molecules (DPP-TPA)/(NTCDI)/PCMB hetero-dyads. DPP-TPA molecules have been used earlier in BHJ solar cells because of high and strong fluorescent performance and exceptional stability. Due to the favorable energetics, it is used here as donor molecule for cascaded energy transfer in ternary heterodyad geometry. NDI can play a bridging role between DPP and PCBM and thus provide efficient charge transfer at the D/A interface. Moreover, as non-fullerene acceptor, NDI has high electron mobility and strong electron affinity and can tune the active layer morphology through the effect of small molecules. The intradyad nanostructure contain DPP, NDI and PCBM molecules inside the same matrix and in interdyad nanostructure individual nanoparticles are attached together by electrostatic force. The morphology optimization in hybrid nanostructures is performed by AFM, DSC and TGA studies. Page iv The steady state and time resolved photolumiscience spectroscopy reveal interesting photophysics about this ternary heterostrcuture.