Structural and optical characterizations of methyl ammonium lead halide perovskite embedded polymer films

Perovskite solar cells (PSCs) made using methylammonium lead iodide (MAPbI3) perovskite is currently under intensive investigation by the clean and sustainable energy research community. This is because many advantages they offer, such as (i) solution processability and hence lower cost, (ii) feasibility to be fabricated in diverse device designs, (iii) workability under low-light conditions, and (iv) high photovoltaic conversion efficiency (~22%). However, their operational stability is very poor, only few hours under normal operating conditions, and they show a hysteresis in their current – voltage characteristics when the measurements are done at forward and reverse bias conditions. It is hypothesized in this research that the poor operational stability is due to the volatile nature of the methylammonium ions in the crystals and stabilizing them could lead to stable PSCs. Synthetic polymers are very stable under atmospheric conditions and encapsulating the MAPbI3 perovskites in a polymer could be an efficient method to improve their stability. Following this argument, this thesis describes synthesis and characterization of MAPbI3 embedded polyvinylpyrrolidone (PVP) polymeric films. In this research work, four films were produced (i) pure MAPbI3 with 0 wt.% PVP, (ii) MAPbI3 in 5 wt.% PVP, (iii) MAPbI3 in 10 wt.% PVP, (iv) MAPbI3 in 20 wt.% PVP. 20 wt.% of PVP reported retained its optical and structural characteristics in dark for ~2000 h and ~800 h in room light which is noticeably higher than pure perovskite film which fully degraded in 600 h in dark and less than 100 h when exposed to light. The MAPbI3 crystals dissolved in DMF were dispersed in the above amount of polymers and developed into films on 500 nm TiO2 coated glass plates by spin coating. The structural and optical properties of the films as a function of time (up to 2000 h) under light and dark were studied by X-ray diffraction (XRD), Fourier-Transform Infrared spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Ultra-Violet Visible (UV-Vis) absorption spectrometry, and Photoluminescence spectroscopy (PL). Minor degradation in perovskite films stored in humid dark environment were observed whereas upon exposure to light, the films undergo a drastic degradation, primarily owing to the reactive TiO2/perovskite interface and also the surface defects of TiO2. The superior stability of PVP incorporated perovskite films are attributed to improved structural stability of MAPbI3 and also the improved TiO2/perovskite interface upon incorporating a polymer matrix. A charge injection from the polymer embedded perovskite films has also been confirmed by fabricating solar cells using them; thereby providing a promising future research pathway on stable and efficient perovskite solar cells

SnO2–TiO2 Hybrid Nanofibers for Efficient Dye-Sensitized Solar Cells

Pristine SnO2 nanostructures typically result in low open circuit voltage (VOC) <500 mV due to the lower Fermi energy (EF) when employed as a photoanode materials in dye sensitized solar cells (DSSCs). On the other hand, the most successful photoanode material, i.e., TiO2 nanoparticle although provides a high VOC ⩾ 800 mV result in poor charge collection owing to their inferior electron mobility (μn). Herein, we employ nanofiber–nanoparticle composite of SnO2–TiO2 which showed similar VOC and short circuit current density (JSC) to a reference TiO2 based DSSCs. The nanocomposite developed here involves multi-porous SnO2 nanofibers characterized by a lower EF; however, with higher μn and TiO2 nanoparticles of higher EF and lower μn. The TiO2 particles in the pores of SnO2 nanofibers were developed by TiCl4 treatment, whose concentration is optimized for the saturated JSC and VOC. The best performing DSSCs fabricated using the composite electrodes deliver power conversion efficiency (PCE) of ≈7.9% (VOC ≈ 717 mV; JSC ≈ 21 mA cm−2), which is significantly higher than pure SnO2 photoanode with PCE ≈ 3.0% (JSC ≈ 14.0 mA cm−2 and VOC ≈ 481 mV) at similar experimental conditions

Humidity Versus Photo-Stability of Metal Halide Perovskite Films in a Polymer Matrix

Despite the high efficiency of over 21% reported for emerging thin film perovskite solar cells, one of the key issues prior to their commercial deployment is to attain their long term stability under ambient and outdoor conditions. The instability in perovskite is widely conceived to be humidity induced due to the water solubility of its initial precursors, which leads to decomposition of the perovskite crystal structure; however, we note that humidity alone is not the major degradation factor and it is rather the photon dose in combination with humidity exposure that triggers the instability. In our experiment, which is designed to decouple the effect of humidity and light on perovskite degradation, we investigate the shelf-lifetime of CH3NH3PbI3 films in the dark and under illumination under high humidity conditions (Rel. H. > 70%). We note minor degradation in perovskite films stored in a humid dark environment whereas upon exposure to light, the films undergo drastic degradation, primarily owing to the reactive TiO2/perovskite interface and also the surface defects of TiO2. To enhance its air-stability, we incorporate CH3NH3PbI3 perovskite in a polymer (poly-vinylpyrrolidone, PVP) matrix which retained its optical and structural characteristics in the dark for ∼2000 h and ∼800 h in room light soaking, significantly higher than a pristine perovskite film, which degraded completely in 600 h in the dark and in less than 100 h when exposed to light. We attribute the superior stability of PVP incorporated perovskite films to the improved structural stability of CH3NH3PbI3 and also to the improved TiO2/perovskite interface upon incorporating a polymer matrix. Charge injection from the polymer embedded perovskite films has also been confirmed by fabricating solar cells using them, thereby providing a promising future research pathway for stable and efficient perovskite solar cells

The Deficiency of CH3NH3PbI3 Solar Cells Performance Under High Relative Humidity

With the consideration of humidity effect, the deficiency of CH3NH3PbI3 solar cells performance under high relative humidity is predicted. The efficiency limitation was studied in order to enhance the perovskite solar cell (CH3NH3PbI3) performance. To understand this convincing prediction, perovskite solar cells were synthesis, fabricated and characterized in high humidity condition. The role of humidity in degradation and then decline the perovskite solar cell performance are also clarified due to the humidity sensitive nature of these perovskite, their thin films are often prepared in-situ which may hinder their mass production

Humidity versus photo-stability of metal halide perovskite films in a polymer matrix

Despite the high efficiency of over 21% reported for emerging thin film perovskite solar cells, one of the key issues prior to their commercial deployment is to attain their long term stability under ambient and outdoor conditions. The instability in perovskite is widely conceived to be humidity induced due to the water solubility of its initial precursors, which leads to decomposition of the perovskite crystal structure; however, we note that humidity alone is not the major degradation factor and it is rather the photon dose in combination with humidity exposure that triggers the instability. In our experiment, which is designed to decouple the effect of humidity and light on perovskite degradation, we investigate the shelf-lifetime of CH3NH3PbI3 films in the dark and under illumination under high humidity conditions (Rel. H. > 70%). We note minor degradation in perovskite films stored in a humid dark environment whereas upon exposure to light, the films undergo drastic degradation, primarily owing to the reactive TiO2/perovskite interface and also the surface defects of TiO2. To enhance its air-stability, we incorporate CH3NH3PbI3 perovskite in a polymer (poly-vinylpyrrolidone, PVP) matrix which retained its optical and structural characteristics in the dark for ∼2000 h and ∼800 h in room light soaking, significantly higher than a pristine perovskite film, which degraded completely in 600 h in the dark and in less than 100 h when exposed to light. We attribute the superior stability of PVP incorporated perovskite films to the improved structural stability of CH3NH3PbI3 and also to the improved TiO2/perovskite interface upon incorporating a polymer matrix. Charge injection from the polymer embedded perovskite films has also been confirmed by fabricating solar cells using them, thereby providing a promising future research pathway for stable and efficient perovskite solar cells.publishe