IONTRONICS IN HALIDE PEROVSKITES FOR OPTOELECTRONICS AND OFF-GRID ENERGY APPLICATIONS

Abstract

Hybrid halide perovskites (HHP) have been the materials of the decade as tremendous advances have been made in optoelectronics due to their solution processability, cheap synthesis methods, and high device performance efficiencies. Since the discovery of the first HHP-based solar cell in 2009 by Miyasaka et al., the power conversion efficiency of perovskite solar cells (PSCs) has increased from 3.8% in 2009 to 25.7% at present. In recent years, HHP is now being spotlighted as the next generation electroluminescent material for display technology due to its wide color gamut and low-cost high-resolution sub-pixel patterning. The external quantum efficiency (EQE) of perovskite light-emitting diodes (PeLEDs) has dramatically exceeded 25%. The reasons behind the great success in these devices come from their unique optoelectronic properties such as high charge carrier mobility with tunable optical bandgap, high absorption coefficient, high quantum yield, high color purity, and long diffusion length. Starting with high-efficiency optoelectronics, perovskites are now used in photodetectors, X-ray detector, sensors, energy storage, photoelectrochemical cells and memory applications and so on. Such multifunctionality of HHP has attracted tremendous attention towards interdisciplinary research across the world. Recently, the scientific community has begun to evaluate the potential use of PSCs and PeLEDs in space and biomedical applications, respectively. However, a major obstacle to commercialization remains the intrinsic instability and toxicity of lead. For toxin free perovskites, recently scientific community started to look for lead-free materials to be employed in photovoltaic and optoelectronic devices. For instance, the power conversion efficiency (PCE) of tin (Sn) based perovskite photovoltaics has already reached to 15 %. However, Sn2+ rapidly oxidizes to Sn4+ when subjected to ambience environment that results in poor stability during device operation. Recently, there has been a search of alternative materials based on antimony and bismuth for perovskite optoelectronics. The ion migration in these mixed electronic-ionic semiconductors is one of the mysterious processes and plays a vital role in the device operation and stability. There are various observed phenomena in PSCs due to ion migration, such as abnormal current-voltage hysteresis, ion redistribution, high negative and positive capacitance at the low-frequency regime, surface polarization, light soaking effect, slow open-circuit voltage (Voc) decay, electronic–ionic coupling and an anomalous dielectric response at low-frequency region. In PeLEDs, electronic ionic mixed dynamics can modulate the charge injection rate and bimolecular charge recombination processes inside the emissive layer. Over the last decade, people have extensively studied these anomalous processes in PSCs. However, it is still not well understood in the field of advanced optoelectronic devices such as perovskite-based light-emitting diodes (PeLEDs). Therefore, the recent research trend is more to elucidate the electronic–ionic kinetics in these wonder materials in order to design efficient and stable optoelectronics. The primary concern in perovskite-based optoelectronic devices is the understanding of electronic dynamics in the presence of ionic transport. In addition, the ion migration capability of the HHPs is a reward for many other emerging applications, such as in memristors, photo electrochemical cells, CO2 capture, and off-grid energy storage devices. Therefore, the aim of this thesis is to understand the electronic-ionic dynamics in perovskite materials and optoelectronic devices using small perturbation frequency domain electrochemical impedance spectroscopy (EIS) and utilize these wonder materials for off-grid energy applications. For this we have structured this dissertation into the following eight chapters- In chapter 1, an introduction of hybrid halide perovskite optoelectronics is presented. The origin and fundamentals of perovskite structure is explained briefly followed by significant highlights on the discovery and history of perovskite photovoltaics. Further, the potentials and the challenges of the hybrid perovskite photovoltaics are discussed. In the next section, ion migration which is one of the major factors responsible for the long-term instability of the perovskites is reviewed in more detail. This is followed by the literature review on the opportunities of ion migration in other applications. Further, we discussed about utilization of ion migration for off grid energy system that can simultaneously harvest and store the energy. This is followed the brief insight into electrochemical impedance spectroscopy that mostly used to investigate electro ionic dynamics in hybrid halide perovskites. The final section briefly describes the literature gap, motivation and the objectives of the present thesis. In chapter 2, we have investigated the effect of ion migration in PeLEDs and demonstrated for the first time that the ideality factor and device capacitance are strongly dependent on perovskite film morphology. Relatively large ideality factor and anomalous capacitive behavior observed in perovskite light emitting diode is due to the presence of strong coupling between ions and electrons near the electrode interface. Therefore, ideality factor as well as anomalous capacitance at mid-frequency regime can be decreased by minimizing electronic-ionic coupling in textured perovskite film while light out-coupling can be improved significantly. In chapter 3, we discussed the role of different A-cation and X-halide anion, and grain boundaries and trap states on the dielectric properties and the AC ionic conductivity of perovskite optoelectronics. We have measured ionic conductivity as a function of frequency to demonstrate that the interplay between A-site cation and X-site halide ion leads to opposite behavior in the low frequency regime upon increasing bias voltage. This could be due to two competing processes: interfacial polarization and cation-halide hydrogen bonding in two samples. In last section of this chapter, we have studied the electro-ionic dynamics in the perovskite single crystals (SCs). The device capacitances are temperature independent, revealing the temperature is only accelerating the ions (MA+ and Br-) migration not increasing the number of ion migrations in single crystals. Moreover, the ions in perovskite single crystals are not trapped at the grain boundaries, and hence all the ions participate in the conduction; this leads to a higher increase in conductivity resulting in the super-linear power law (SPL) region at a higher frequency In chapter 4, we have synthesized the mixture of 2D/3D perovskites by incorporating sulfur doped graphene quantum dots (SGQDs) and demonstrated that the optical as well as electrical properties of the hybrid system can be tuned by controlling the ion conductivity through the active layer. SGQDs at the grain boundaries of 2D/3D perovskites prohibit the ion migration through active layer, therefore, electronic-ionic coupling is reduced. This results in increasing recombination resistance with the increasing applied bias. In chapter 5, we have fabricated methylammonium lead tri-bromide perovskite-based electrode by spin-coating as well as from the single crystal for electrolyte-based capacitors and demonstrated that the device performance is strongly dependent on the electrode morphology. Our experimental results show that the modified electrode from the metal halide perovskites (MHPs) single crystal has a 500-time higher volumetric capacitance (~ 429.1 F cm-3 @ 5 mV s 1 ) compared to the spin-coated thin-film-based capacitors (~ 0.8 F cm-3 @ 5 mV s-1) having same electrolyte and device structure. Our modified powder electrode-based supercapacitors show significant improvement in terms of cyclic stability over 97% as well as coulombic efficiency over 91% after 1500 cycles of operation. In chapter 6, we have fabricated porous electrodes from 3D-bulk and 2D-layered perovskite single crystals and demonstrated that the ion migration could play a significant role in determining the overall performance of the electrochemical supercapacitor. We have estimated the amount of diffusion-controlled charge storage to that of electric double-layer capacitance and surface redox reaction (pseudo-) capacitance from the power-law relation in both the samples. The major difference is observed at low field regime where ionic conductivity in 3D-bulk perovskites is significantly higher than 2D-layered perovskites mainly due to strong electron-ion coupling. Therefore, in 3D perovskite-based supercapacitor, only 2% is diffusion-controlled iii charge storage compared to 40% in 2D sample at a low field regime. With the increasing applied voltage, both capacitive as well as diffusion-controlled charge storage become comparable in both the samples. 3D sample stability is ~98%, while 2D sample stability is almost 100% even after 1000 cycles of operation. Current approaches for off-grid power separate the processes for energy conversion from energy storage. With the right balance between the electronic and ionic conductivity and a semiconductor that can absorb light in the solar spectrum, we can combine energy harvesting with storage into a single photoelectrochemical energy storage device. In chapter 7, We reported here such a device, a halide perovskite-based photo-rechargeable supercapacitor. This device can be charged with an energy density of 30.71 Wh kg-1 and a power density of 1875 W kg-1. By taking advantage of the semiconducting and ionic properties of halide perovskites, we report a method for fabricating photo-rechargeable supercapacitors having photo-charging conversion efficiency (ƞ) is about ~0.02% and photo-energy density is ~160 mWh kg-1 under 20 mW cm-2 intensity white light source. We also report a detailed analysis of the photoelectrode to understand the working principles, stability, limitations, and prospect of halide perovskite-based photo rechargeable supercapacitors. In the chapter 8, we summarize the findings of all the work presented in the previous chapters and propose future perspectives.