KINETICS OF IONIC AND ELECTRONIC TRANSPORT AT HYBRID PEROVSKITE- LIQUID ELECTROLYTE INTERFACE

Abstract

Hybrid organic-inorganic perovskite (HOIP) materials have achieved remarkable attention in the field of solar photovoltaics owing to their extraordinary opto-electronic properties of high absorption coefficient, large carrier diffusion length, high carrier mobility, ambipolar charge transport, tunable band gap, long carrier lifetime, low-cost solution based processability, etc. The power conversion efficiency (PCE) of perovskite solar cells (PSCs) have skyrocketed to the latest record of 25.7 % (as reported by the National Renewable Energy Laboratory (NREL), U.S., https://www.nrel.gov/pv/assets/pdfs/best-research-cell-efficiencies.20200104.pdf) advancing to the top values accomplished by the single crystalline silicon solar cells, since their inception as visible light sensitizers in photovoltaic cells by Kojima et. al. As a result of considerable interdisciplinary research worldwide from the society of researchers with variegated background, the application of HOIPs have expanded beyond solar cells to numerous other opto-electronic devices such as light emitting diodes, photoelectrochemical cells for water splitting and CO2 reduction, memory devices, supercapacitors, X-ray detectors, and photodetectors. However, despite achieving the PCE of up to 25.5 % in just few decades of research now the commercialization of PSCs is still a challenge due to the Pb toxicity, current voltage hysteresis, ferroelectric effect and instability of these materials to moisture, heat and light. The long-term stability of the HOIP based photovoltaic devices is one of the most important factors which is restricting the industrial usage of this material. Researchers have explored various approaches to address this by optimizing perovskite morphology using numerous strategies, introducing multiple layers between perovskite and electrode material, using the mixed cation or mixed halide approach for perovskite synthesis, accessing the self-healing by putting the device under dark or at low temperature between subsequent operation. Despite such efforts the inherent instability due to ion migration under photo-excitation and applied bias is still creating problems for high efficiency and high stability devices. Further research on the investigation of ion migration in HOIPs is crucial to address this issue. It is well established that, many unusual phenomena such as current-voltage hysteresis, photo voltaic switching, interfacial doping, electrochemical degradation of perovskite devices and anomalous polarization is induced by the ion migration. Ion migration is probed both directly by X-ray photoemission spectroscopy, photothermal induced resonance, and indirectly by photoluminescence mapping, scanning probe microscopy, impedance spectroscopy. Time-of i flight secondary ion mass spectroscopy (ToF-SIMS) have provided additional insights into the chemical dynamic of hybrid perovskites. Although ion migration including chemical changes and halide segregation have been discussed widely using variegated experimental techniques, until now the mystery of ion migration is unresolved. It is to be noted that, most of the investigation are performed on solid-state device architecture. Lateral device geometry (Au/perovskite/Au), conventional solid-state p-i-n and n-i-p perovskite device, and single crystals are some of the mostly used device architecture. In case of conventional perovskite solar cell solid-state device geometry, the selective charge transport layers are also used. These selective charge transport layers highly influence the nature of ion kinetics and the true behavior of ion transport cannot be deciphered. Recently, a lot of charge and ion accumulation and diffusion studies are performed on perovskite-liquid electrolyte device geometry. Kamat and co workers recently investigated the selective expulsion of iodide from the mixed-halide perovskite on electrochemical hole injection and role of the A-site cation in dictating the mobility of the halide ion, providing additional insight into the photoinduced phase segregation in HOIP materials. Moreover, this type of device geometry opens way to modern electrolyte-based devices such as perovskite-based supercapacitors, electrolyte gated FETs, photoelectrochemical water splitting and CO2 reduction devices. Therefore, investigation of charge and ion (mass) kinetics in the hybrid perovskite materials at solid-liquid interface is crucial for discerning of ion migration in HOIPs and exploring the scope of these material in the electrolyte-based modern photovoltaic devices. Here, to achieve this aim we have studied the photophysics of charge and ion transfer at the hybrid perovskite-liquid electrolyte interface. Electrochemical impedance spectroscopy (EIS) is well established technique used extensively to probe the nature of ion migration in hybrid perovskite materials from few years now. In EIS, a small AC signal perturbs the device using a broad frequency range that allows us to decouple the physical processes that occur at different time scales. EIS is one of the most powerful tools for studying the carrier dynamical properties such as ionic diffusion, dielectric relaxation, capacitance, and AC electrical conductivity of the electronic−ionic mixed conduction materials. In 2017, Li et al. have analyzed capacitance and impedance spectra of the interface between the electrolyte and spin-coated and sprayed MAPbI3, FA0.85Cs0.15PbI3, and MAPbBr3 perovskite films. The EIS spectra revealed the presence of exceptionally high capacitance at a frequency of <1 kHz owing to ion diffusion. However, the photophysics and dependance of this ion diffusion on various factors including film morphology, device architecture, operating conditions, material composition, nature of interface etc., was not investigated further. Therefore, in this thesis the charge and ion diffusion kinetics at the perovskite-liquid electrolyte interface and its dependance ii on various device structure and operating parameters is investigated using electrochemical impedance spectroscopy technique. The investigation of the kinetics of ionic and electronic charge transport at hybrid perovskite electrolyte interface carried out in this dissertation is structured into the following seven chapters. In chapter 1, brief introduction of HOIP photovoltaics is presented including the fundamentals of perovskite structure, significant highlights of the discovery and history followed by the potentials and challenges of the hybrid perovskite photovoltaics. Ion migration, one of the major factors responsible for the long-term instability of the perovskites is reviewed in more detail. Further, the complexity in the understanding of ion migration due to investigations in a conventional solid-state device geometry is described and hybrid perovskite-liquid electrolyte interface is proposed to be a promising device architecture for additional insight. A theoretical review of the charge dynamics at the semiconductor-liquid electrolyte interface is presented followed by a brief insight into EIS technique. The final section describes briefly the motivation and the objectives of the present thesis. In chapter 2, methylammonium lead tri-iodide perovskite has been synthesized by spin coating of lead acetate trihydrate and methylammonium iodide precursor on preheated substrate. Significant difference in film morphology has been observed as the temperature was varied from room temperature to 120 °C prior to spin coating. Nucleation and growth mechanism is revisited to find out the optimum substrate temperature for fabricating uniform perovskite films and is attributed to the fast homogeneous nucleation followed by delayed growth process. EIS measurement at the perovskite−liquid electrolyte interface reveals the impact of film morphology on the anomalous diffusion behavior observed at the low frequency regime. In chapter 3, we have investigated charge transport through a perovskite/electrolyte interface using EIS under illumination and applied bias conditions. Similar EIS trends observed with positively biased as well as negatively biased working electrodes, indicate ambipolar charge transport through the perovskite layer. Ionic conductivity upon photo-illumination plays a significant role in modulating charge transport at the interface and an anomalous charge transport resistance is observed under illumination at around 400 to 600 mV applied bias. Electric field induced UV-Vis absorption spectroscopy shows a decrease in absorption when both positive and negative bias voltages are applied to a perovskite coated ITO working electrode, indicating the occurrence of excited state charge transfer at the electrolyte interface. iii Hybrid perovskite materials are mixed electronic−ionic conductors and hence complete understanding of ionic conductivity along with electronic conductivity is crucial. In chapter 4, we employed photoelectrochemical impedance spectroscopy (PEIS) on a perovskite/electrolyte interface-based device to investigate the role of an A-site cation and X-site halide ion in dictating the charge and ionic conductivity of the perovskite material. It was noted that ionic conductivity of the perovskite material can be tuned either by changing the A-site cation (MA+/FA+) or by changing the X-site halide ion (I−/Br−). Photoinduced ionic conductivity can be significantly different (opposite) in different cation perovskites or different halide-based perovskites. Therefore, mixed-cation perovskites can be utilized to reduce photoinduced ion conductivity. The influence of ion accumulation and migration on the charge storage and transport property of the device is analyzed using vacancy hopping and the jump relaxation model. Ionic conductivity spectra revealed Jonscher’s law dependence in the mid- and low-frequency range with a constant plateau at high frequencies. It can be concluded that the interplay of ion migration and accumulation decides the resulting conduction and storage property of the complete device. Performing EIS in perovskite devices presents new challenges related to multilayer solid-state device geometry and complicated material properties. The ions in the perovskite behave in a specified manner, which is dictated by the energy levels of the transport layer. Electronic-ionic coupling is one of the major challenges to understand ion transport kinetics in solid-state devices. In chapter 5, we have performed impedance measurements in both solid-state (S-S) and liquid electrolyte (L-E) device geometry to unfold the effect of charge transport layers on the ac ionic conductivity in perovskite materials. We have modelled the impedance spectra using the electrical equivalent circuit (EEC) and compared the behavior of ions in different controlling environments. It was concluded that the ac as well as dc ionic conductivity and the accumulation of ions in the perovskite material are highly influenced by the nature of the interface in different device geometry. Charge accumulation in the S-S device gives rise to large polarization, thereby negative capacitance or an inductive loop can be observed in the Nyquist plot while in the L-E device the presence of an electric double layer at the perovskite/electrolyte interface reduces the surface polarization effect. The numerous assorted accounts to the fundamental questions of ion migration in hybrid perovskites are making the picture further intricate. The review of photo-induced ion migration using small perturbation frequency domain techniques other than impedance spectroscopy is more crucial now. Here in chapter 6, we probe more into this by investigating perovskite electrolyte (Pe-E) and polymer-aqueous electrolyte (Po-aqE) interface using intensity modulated iv photocurrent spectroscopy (IMPS) in addition to PEIS. We found that the electronic-ionic interaction leading to the processes at different time scales can be more easily separated by IMPS than PEIS. The spiral trajectory of the IMPS Nyquist plot for perovskite-electrolyte interface depicts three distinct ion kinetics going on at different time scales which was otherwise not clear by PEIS measurements. Hence, IMPS is a promising alternative to PEIS. We estimated charge transfer efficiency (Qste) from the Rate Constant Model. The Qste at low-frequency for Pe-E interface exceeds unity due to ion migration induced modified potential across the perovskite active layer. Hence, ion migration and mixed electronic-ionic conductivity of HOIPs are responsible for the extraordinary properties of this material which can be useful for application in next generation photo-rechargeable electrochemical energy storage, photoelectrochemical water splitting and carbon capture devices. The chapter 7 summarizes the findings from all the work presented in the previous chapters. We concluded from the chapter 1 that uniform, smooth and full coverage morphology of the spin coated perovskite films is crucial for the high open circuit voltage essential for good photovoltaic device performance. The morphology of the perovskite films can be modified by tuning the time gap between nucleation of particles and their growth. Second chapter is focused on the tunable ambipolar nature of the hybrid perovskite materials. We noted that the polarity of the hybrid perovskite material can be tuned by changing the transport layer used in the device architecture or operating conditions. Two distinct electric field induced bleaching bands have been observed at around 480 nm and 750 nm, similar to the two photobleaching bands due to the dual nature of the excited states in lead halide perovskites. In third chapter, it was found that ionic conductivity of the perovskite material can be tuned either by changing the A-site cation (MA+/FA+) or by changing the X-site halide ion (I−/Br−). Later, the chapter 4 provides the investigation of role of device architecture in the charge accumulation and extraction at the interface. It was concluded that the ac as well as dc ionic conductivity and the accumulation of ions in the perovskite material are highly influenced by the nature of the interface in different device geometry. The fifth chapter provides more insight into the interplay of electronic and ionic charge carriers in the perovskite material and their influence on the device kinetics by comparing with polymer-electrolyte interface using intensity modulated photocurrent spectroscopy. It is concluded that IMPS is a promising alternative technique to inspect the hidden ion kinetics in HOIP materials which wasn’t otherwise possible by PEIS. Later this chapter provides the brief introduction to the future scope of the perovskite-electrolyte interface-based devices in hybrid perovskite-based CO2 capture and reduction, v photoelectrochemical water splitting, supercapacitors, batteries and liquid electrolyte gated FETs. Finally, the possibility of hybrid perovskite assisted reduction and oxidation of anthraquinone derivative for photoelectrochemically mediated carbon capture and release technology is explored. In this section, we have examined HOIP based working electrodes under illumination for reduction and oxidation of anthraquinone (AQ) derivatives. The kinetics of the charge and ion transport during the redox reaction is inspected using cyclic voltammetry (CV), Mott-Schottky analysis and PEIS measurements. The stability and the reversibility of the system is studied for different CV electrochemical window and working electrode architecture. The possibility of using anthraquinone derivative in form of thin films in electrochemical working electrode as well as in conventional solid-state device geometry is explored. It can be concluded that although the photoelectrochemically mediated reduction and oxidation of redox couples is possible through HOIP materials, the stability and reversibility of the system needs further optimization.