Authors:	Sajedi Alvar, Mohammad
Title:	Device physics of perovskite solar cells
Online publication date:	10-Feb-2022
Year of first publication:	2022
Language:	english
Abstract:	Lead halide perovskite solar cells (PSCs) emerged as a highly promising photovoltaic technology for converting solar radiation into electricity. Over the past decade, a remarkable progress has been achieved in improving the performance of PSCs and the power conversion efficiency of PSCs has significantly increased from 3.9% to 25.2%. Despite such a rapid and significant progress, the physics of PSCs is not fully understood. In this thesis, the device physics of perovskite solar cells has been comprehensively studied to provide an improved understanding of the operation of PSCs as the most promising photovoltaic technology among the emerging technologies. In this thesis, a combined experimental and simulation approach is used to study the physics of methylammonium lead iodide (MAPbI3) devices, as is the most commonly used perovskite composition. Experimentally, MAPbI3 devices with different configurations, such as parallel-plate capacitors, electron-only devices, hole-only devices, and PSCs were fabricated and a variety of characterizations were performed on the devices. Theoretically, a device model was developed for simulating the operation of MAPbI3 devices as mixed electronic-ionic semiconductor devices. By combining the numerical simulations with the experimental results, various physical properties of MAPbI3 thin films were evaluated and a desirable understanding of the operation of MAPbI3 devices is provided. In the first chapter of this thesis, an introduction to solar energy, various photovoltaic technologies, and different aspects crystalline perovskite materials and PSCs is provided. In the second chapter, the accomplished experimental processing steps for producing high quality MAPbI3 thin films are presented. Additionally, the route toward the fabrication and optimization of MAPbI3 optoelectronic devices such as parallel-plate capacitors, electron-only devices, hole-only devices, and solar cells is provided. For modelling the operation of MAPbI3 optoelectronic devices, an electronic-ionic drift-diffusion device model is developed and provided in the third chapter. The device model is capable of simulating electronic and ionic charge transport in mixed electronic-ionic devices and can provide the time and position dependence of various electrical properties of the device, including the density and transportation of electronic and ionic charges as well as the distribution of electric potential. Experimentally, ferroelectric properties of MAPbI3 thin films were examined by measuring the electric displacement-voltage (D-V) and current-voltage (I-V) of MAPbI3 parallel-plate capacitors and PSCs at different frequencies. No ferroelectric switching was observed in the I-V curves and it was demonstrated that the strong frequency dependence of hysteretic D-V characteristics originates from the migration of ions, rather than ferroelectricity. As the next step, impedance spectroscopy (IS) was introduced as a novel approach to quantify the ionic properties of MAPbI3 thin films in MAPbI3 capacitor configuration. From the characteristic frequencies of impedance spectrum, the density and diffusivity of mobile ions in MAPbI3 thin films were extracted. Additionally, the frequency-dependent permittivity of MAPbI3 thin films showed a significant enhancement at low frequency regime. As a complimentary method for validation of the measured ionic properties, D-V measurements were performed on MAPbI3 capacitors to obtain the ion density and diffusivity in MAPbI3 thin films. The frequency dependent D-V loops were reproduced by numerical simulations by assuming mobile positive ions and uniformly distributed stationary negative ions. From the magnitude and the frequency dependence of the electric displacement, respectively the ion density and the ion diffusion coefficient were obtained, which were in excellent agreement with values obtained from IS. With the knowledge of the ion dynamics, electron- and hole-transport properties of MAPbI3 thin films were independently explored. For this purpose, electron-only and hole-only devices were fabricated and optimized for efficient charge injection. Electron and hole currents were measured at different voltage scan rates and temperatures. Both the electron and hole currents as well as the hysteresis therein depend on temperature and frequency of the applied voltage signal. The temperature-dependent ion diffusion and apparent permittivity were quantified by IS and were used as input parameters for simulating the frequency and temperature dependence of electron and hole currents. It is demonstrated that for the space-charge-limited current (SCLC) analysis of the electron and hole currents in MAPbI3 thin films, the frequency dependence of the permittivity and ion dynamics have to be taken into account. The mobility of electrons and holes in MAPbI3 thin films were obtained from the SCLC analysis and showed no considerable temperature dependence. Additionally, from the direction of the hysteresis in electron and hole currents the sign of mobile ions was inferred to be positive. As the final step, efficient MAPbI3 PSCs with power conversion efficiency of 17.2% were fabricated and characterized, to elucidate their operational mechanism. IS and D-V measurements, as well as temperature- and scan-rate dependent I-V measurements were employed to study the physics of MAPbI3 PSCs in dark conditions and under illumination. The permittivity showed a frequency-dependent behavior in dark and under illumination. The low frequency permittivity under illumination is 2 to 3 orders of magnitude greater than in dark. Comparing the D-V loops in dark and under illumination showed that a hysteresis loop appears at high frequencies due to illumination. By combining the IS and D-V measurements with the numerical simulations, it is demonstrated that the illumination activates the stationary negative ions, giving rise to the enhanced low-frequency permittivity and a high-frequency D-V loop due to high negative ion diffusivity. In addition, despite the hysteresis-free I-V characteristics of MAPbI3 PSC under illumination at low scan rates at room temperature, the I-V characteristics showed temperature and scan-rate dependent hysteresis for lower temperatures and higher scan rates. These characteristics were consistently reproduced by simulations using the experimentally measured values for the frequency-dependent permittivity, positive and negative ion density, positive and negative ion diffusion coefficient, electron mobility, and hole mobility.
DDC:	004 Informatik 004 Data processing 500 Naturwissenschaften 500 Natural sciences and mathematics 530 Physik 530 Physics 540 Chemie 540 Chemistry and allied sciences 550 Geowissenschaften 550 Earth sciences 570 Biowissenschaften 570 Life sciences 600 Technik 600 Technology (Applied sciences) 620 Ingenieurwissenschaften und Maschinenbau 620 Engineering and allied operations 621.3 Elektrotechnik 621.3 Electric engineering 660 Technische Chemie 660 Chemical engineering
Institution:	Johannes Gutenberg-Universität Mainz
Department:	FB 08 Physik, Mathematik u. Informatik
Place:	Mainz
ROR:	https://ror.org/023b0x485
DOI:	http://doi.org/10.25358/openscience-5579
URN:	urn:nbn:de:hebis:77-openscience-a76ee3d9-5c1f-4798-8b0d-63d5372356607
Version:	Original work
Publication type:	Dissertation
License:	In Copyright
Information on rights of use:	http://rightsstatements.org/vocab/InC/1.0/
Extent:	ix, 200 Seiten, Illustrationen, Diagramme
Appears in collections:	JGU-Publikationen