The Effect of Pressure, Bending and Annealing Temperature on the Mechanical and Optoelectronic Properties of Perovskite Solar Cells

Abstract

The increasing needs for clean and sustainable energy stimulate the growing interest in photovoltaic (PV) technology using organic-inorganic hybrid perovskite materials. However, perovskite solar cells (PSCs) are faced with stability problems due to cracks or defects within the perovskite absorber and along the interfaces of the multilayered PSC structures. It is therefore important to improve on our understanding of their degradation pathways and mechanical stabilities. This thesis studies the effects of pressure, bending and processing annealing temperature on the mechanical and optoelectronic properties of perovskite solar cells.

First, the effects of pressure on photoconversion efficiencies of perovskite solar cells (PSCs) are studied using a combined experimental and analytical/computational technique. The results show that the application of pressure can significantly improve the crystallization, absorbance, and power conversion efficiencies of PSCs. This leads to the closing-up of voids and the corresponding increase in the interfacial surface contact lengths, which increase with increasing pressure. The improvement in the power conversion efficiencies (9.84 to 13.67%) was observed with increased pressure between 0 and 7 MPa, attributed largely to increased surface contact and the compaction and infiltration of the TiO2 layers with perovskite during the application of pressure. At higher pressure values (> 7 Mpa), the damage due to the sink of the perovskite layers into the mesoporous layers results in reductions in the photoconversion efficiencies of PSCs.

Understanding the variations in the mechanical properties of organic-inorganic hybrid perovskite structures that are processed at different annealing conditions is then studied for ultimate device performance and robustness. We show that the temperature at which perovskite film is annealed affects the mechanical properties of the devices fabricated. The size dependence of hardness is due to the increase in the density of geometrically necessary dislocations (GNDs) with decreasing indentation size. The indentation size effects are characterized between the micron- and nanoscales by a bi-linear strain gradient plasticity (SGP) framework with source-limited and established dislocation substructures. The measured microstructural length scales decrease with increasing annealing temperature to 130oC, after which it began to increase, causing films annealed beyond 130oC to have reduced strengths because the larger microstructural length scales correspond to larger dislocation spacings and weaker dislocation interactions. Perovskite solar devices annealed at temperatures above 130oC have poor performance. The results show that perovskite solar cell devices annealed at 130oC exhibit optimal performance and attractive combinations of mechanical properties.

Finally, the underlying failure mechanisms associated with flexible perovskite solar cells (FPSCs) are elucidated for deformation and cracking under monotonic and cyclic bending. The mechanical robustness of the inverted flexible PSCs is increased with an increasing fraction of polyethylene oxide (PEO) in the double-cation perovskite precursor, which promotes the grain size and passivates the defects of the film. The associated changes in the optical transmittance of the perovskite-PEO absorber and the PCEs of the multilayered FPSCs structures are elucidated under monotonic and cyclic bending. The failure mechanisms of the perovskite films for different radii of bending were observed using a scanning electron microscope before computing the interfacial fracture energies in the multilayer devices using finite element simulations. The failure mechanisms are then used to explain the degradation of the optoelectronic properties of flexible perovskite solar cells.

Keywords: Pressure Effects, Annealing Temperature, Bending, Mechanical Properties, Perovskite Solar Cells and Optoelectronic properties.

Chapter One

BACKGROUND AND INTRODUCTION

1.0. Background and Introduction

1.1. Background

Energy supply and demand determine the course of global development in all human endeavours. Sufficient clean-energy supplies relate to global stability, economic prosperity, and quality of life. One of our most important challenges for the next half-century is the quest for energy sources to meet the growing global demand. Now, the world uses energy at a rate of about 4.1 x 1020 joules/year, equivalent to 13 terawatts (TW) of power consumed continuously (US Department of Energy, 2012). Even with aggressive conservation and energy efficiency measures, the world’s population is estimated to rise to 9 billion by 2050. Together with rapid technological development and worldwide economic growth, the energy demand is estimated to be more than double (to 30TW) by 2050 and more than triple (to 46 TW) by the end of the century ((DOE), n.d.). Reserves of fossil fuels that currently power society will not meet that demand for longer-term purposes as continued use has adverse side effects, including pollution threatening human health and climate change-related greenhouse gases. Therefore, sustainable, and environmentally friendly energy sources are needed. Renewable energy sources can be the solution (Institute for Energy Economics and Financial Economics, 2019), although the cost of exploiting these sources is still a little too high.

Renewable energy is energy from a naturally regenerating source such as wind, sunlight, tides, waves, or geothermal, (Ellabban et al., 2014). The capacity for different types of renewable energy sources has increased rapidly over the years. Although wind and hydro energy resources dominated during the early years, other renewable energy sources, particularly Solar PV, are growing rapidly in capacity. Solar energy comes as electromagnetic rays from the sun to the earth. Solar energy is one of the leading renewable power sources and has already shown considerable potential for a prime energy source in the future. The potential power extractable from solar energy is approximately 2300 TW/year, which means an efficient solar energy extraction is sufficient to fulfil every human energy requirement. However, there is a reluctance to utilize solar photovoltaics (PV) as a primary energy source because its unit cost is high compared to other renewable energy. The main reason behind the high costs of Solar PV is low extraction and power conversion efficiency. The amount of power extracted from a solar panel for a given solar intensity depends on the power conversion efficiency and the area of the panel. The efficiencies in the conversion of solar panels are still not very high, comparatively costly. Cloudy days and smaller daytime can reduce the power extracted from a given panel. However, for Solar PV, it is a good indication that unit cost has decreased over the years exponentially (IEEFA, 2010). A decline of 74% in total installed costs was observed between 2010 and 2018 (Figure 1.1). Lower solar PV module prices and ongoing reductions in balance-of-system costs remain the main drivers of reductions in the cost of electricity from PV. Solar PV installation costs would decline dramatically from now to 2050 globally, the total installation cost of solar PV projects would continue to decline dramatically in the next three decades, averaging in the range of USD 340 to USD 834/kW by 2030 and 165 to 481/kW by 2050, compared to the average of USD 1210/kW in 2018 (IRENA (2019) & aspects (A Global Energy Transformation: paper), 2019).

Edmund Becquerel was the first to convert light into electricity in 1839 (Szabó, 2017). Photovoltaics (PV) started when a crystalline silicon-based solar cell with a power conversion efficiency (PCE) of 4.5 % originated in Bell lab, USA (Chapin et al., 1954). Since then, researchers have been exploring a low-cost device structure and new materials that display the PV effect. Second-generation solar cells based on III-V device structure, GaAs, CdTe, InP, and CIGs solar cells were introduced in the field of solar photovoltaics. The Dye-sensitized solar cells, the third generation came about in the early 1990s, and in the 2000s, organic photovoltaic cells were

introduced. With increasing interest in the development of nanomaterials, research efforts are towards developing solar materials that are inexpensive and processed at a low cost. Currently, crystalline silicon solar cells dominate the market. Still, considerations like the need for an expensive production method and raw materials are pushing researchers to develop a new PV technology that combines high efficiency with cheap manufacturing costs. Perovskite materials are gaining huge research interest because of their excellent photovoltaic performance, low-cost raw material, and demand for simple manufacturing conditions (Zhou et al., 2018). Perovskite solar cells (PSCs) do not require complicated processing conditions; instead, they may be made in laboratories utilizing wet chemistry and low-cost processes like spin coating, dip coating, screen printing, and dual source evaporation, among others. A flexible substrate can also be used to grow perovskite materials. The rapid increase in power conversion efficiency (PCE) from 3% to 25.2% (28% in tandem architecture) in the last ten years (Best Research-Cell Efficiency Chart | Enhanced Reader, n.d.; Kojima et al., 2009) demonstrates PSCs’ enormous capability, whereas other technologies took nearly 30 years to reach this milestone. Perovskite can be used as both an absorber and a charge transport layer in a solar cell (Mei et al., 2014).

Perovskite materials are promising materials for future-generation PV technology because of their distinctive properties, such as high electron mobility (800 cm2/Vs) (Valverde-Chávez et al., 2015), high carrier diffusion length (exceeding 1 µm) (Miyata et al., 2015), ambipolar charge transport behaviour (Interview with Greatcell Solar Materials’ GM Yanek Hebting, 2019), high absorption coefficient (greater than 105 cm-1) due to s-p antibonding coupling, Wannier type exciton, low exciton binding energy (less than 10 meV) (Miyata et al., 2015), high photoluminescence (PL) quantum efficiency (as high as 70%), high carrier lifetime (exceeding 300 ns), optimum band gap, low surface recombination velocity, tunable bandgap, great structural defect tolerance, and genial grain boundary effect. Despite its many advantages, PSCs’ commercialization is hampered by poor device stability and a short lifetime.

Crystal growth control, halide mixing, hetero-elemental combination (Mali et al., 2015), and other methods have been utilized to optimize their chemical compositions and increase crystallinity. In addition, numerous processing procedures have been devised to increase their characteristics and device performance, such as one-step and sequential solution deposition (Jeng et al., 2013), vapour-assisted solution processing (Q. Chen et al., 2014), and solvent engineering (Jeon et al., 2014). Although these chemical and processing methods have shown tremendous promise in improving the performance of this class of functional materials, some difficulties remain, and novel materials design/optimization approaches are urgently needed. Fabrication temperature has proven to be crucial in modifying the structures and properties of these perovskites (Zheng et al., 2015). Pressure, like temperature, is a state parameter that gives an additional dimension for efficiently tuning material properties by altering interatomic distances (Zhang et al., 2020).

Thin-film PV technologies were developed for applications requiring low weight and low cost. In recent years, the growing interest in flexible and wearable electronics has sparked a surge in research into flexible and stretchable solar cells. Integrating these flexible solar cells onto portable products, walls, and windows would revolutionize existing energy production, reduce pollution, and vastly expand energy harvesting situations. Advancing flexible solar cells into stretchable ones will further deepen the compatibility with presently promising portable and wearable electronics.

A Wearable and deformable PV absorber should generate a high PCE and possess excellent tolerance over bending and stretching. Perovskite has these outstanding properties, which makes it suitable for flexible and stretchable solar cells. The application and lifetime of flexible power sources strongly depend on their mechanical deformation tolerance and the electrical property retention under deformation (Mao et al., 2017). Flexible power sources should withstand high strain caused by external mechanical deformation like bending, compressing, stretching, folding, and twisting while retaining their electrochemical performance stability and structural integrity. Hence, the mechanical reliability assessment and electrical property analysis of flexible devices need attention for future applications. Flexible solar devices’ main mechanical deformation characteristic is their tolerance to bending into a specific curvature. Thus far, to evaluate the quality and failure modes of flexible devices, bending characterization criteria and mechanical methods have been developed to determine quality and failure mechanisms of devices. Detailed analysis on bending mechanics combining experimental and theoretical results provide reliable test methods to describe the bending state and guidance for the configuration design of devices against mechanical failure.

Perovskite solar cells’ performance has been improved using a variety of approaches (Cho et al., 2017; Zhao et al., 2016). These include processing conditions (Y. H. Kim et al., 2015), modification, and optimization of layered structures (Tan et al., 2014). In previous research, pressure has improved the performance of organic electronic devices (solar cells and light-emitting devices) (Asare et al., 2016; Du et al., 2014). The application of pressure on perovskite layers (both hydrostatic and non-hydrostatic) affects their molecular packing and structure (Capitani et al., 2016; Oyelade et al., n.d.). Increased pressure on perovskite materials induces crystallization due to a reduction of the bonding length (Lü et al., 2017) which can lead to amorphization at higher pressures (Oyelade et al., n.d.; Swainson et al., 2007). Consequently, pressure-induced crystallization improves multilayered perovskite structures’ structural, electrical, and optical properties (G. Liu et al., 2017). Amorphization also is caused by atom rearrangement (Postorino & Malavasi, 2017; Swainson et al., 2007). Oyelade et. al. have shown that remotely applied pressures on perovskite solar cells can increase the power conversion efficiency, reduce interfacial defects, and induce higher stress at defect sites, which can cause induced crystallization.

Unlike rigid inorganic solar cells, these PSCs’ production temperatures (annealing temperatures) are so low that the constituent materials can be flexible (Lee et al., 2019; Li et al., 2017; Park et al., 2015). The combination of mechanical flexibility and high PCE has stirred up much interest in the field of study. A variety of highly flexible materials are used as the electrodes and electron-and hole-transport layers, the inimitable perovskite material components have organic-inorganic crystalline structures that can be brittle, and the brittleness level must be understood before perovskite materials can be widely used. The annealing temperature affects the structure, morphology, crystallinity, and photoelectric properties of perovskite films (L.-C. Chen et al., n.d.; Dualeh et al., 2014; M. Kim et al., 2017; Peng et al., 2016). The annealing process is also critical in forming perovskite film and consequently on the power conversion efficiency of the assembled devices (Wehrenfennig et al., 2014). Understanding the differences in the mechanical characteristics of perovskite structures processed under different annealing temperatures is crucial for device performance and reliability.

Additive engineering is used to passivate defects in perovskite to reduce the nonrelative recombination loss and enhance the PCE of the PSCs (J.-Q. Chen et al., 2019; Lei Guo, 2019). Various additives such as urea and polymer were developed and incorporated into the precursor solution for perovskite fabrication. Rubidium cation (Rb) can create a perovskite structure with outstanding photoelectric characteristics by incorporating it into a “cation cascade.” Zhou and coworkers effectively introduced sodium fluoride into a perovskite absorber to improve PSC efficiency and stability by strengthening chemical bonding (Wu et al., 2015). The mentioned strategy aims at improving the film morphology and decreasing defects of the perovskite in flexible PSCs to improve their performance.

The mechanical robustness of inverted flexible PSCs was increased by introducing a small amount of novel additive polymer, polyoxyethylene (PEO) to the double-cation perovskite precursor, to promote the grain size and passivate the defects of the film. The PEO additive improved the quality of the perovskite film on the PET/ITO substrates. Specifically, both the grain size and crystallinity were significantly enhanced. We use a combined experimental and theoretical approach to study the failure and fatigue behavior of flexible perovskite solar cells.

1.2 Unresolved issues

The perovskite-based photovoltaic is a promising candidate for future energy technologies. But despite the number of advantages, PSCs remain within laboratories. PSC technology can progress to the industrial stage and begin commercialization if some significant concerns are addressed. The following are current obstacles in the commercialization of PSCs, as well as potential solutions to these issues:

1. Scale up

There are numerous reports on module production; however, not all these reports have included scalable manufacturing methods. The development of large-area processing utilizing industry-compatible technologies and the fabrication of perovskite PV modules with many serially interconnected sub-cells transfer from laboratory to industrial manufacturing. The control and understanding of the nucleation and crystal growth mechanisms are critical in scaling up perovskite manufacture (Hu et al., 2020; C. Liu et al., 2020). Controlling crystal development will enable the production of high-quality perovskite films, which is the most critical aspect of the scalable method. Temperature, solvent composition, the surface qualities of the substrates, and, most importantly, post-deposition treatment can influence it. The ability to create large-area perovskite films opens doors for PSC technologies to scale up and become more industrialized.

2. Stability

Another impediment to the commercialization of perovskite PV technology is the long-term stability of PSCs. Environmental variables such as water (or moisture), oxygen, heat, and UV light (Schileo & Grancini, 2020), as well as ion migration mechanisms (Domanski et al., 2017), induce PSC degradation. Therefore, knowing the degradation mechanisms in perovskite-based devices is the only way to attain long-term stability. The addition of alkali cations such as caesium and rubidium to PSC improves its stability substantially (Domanski et al., 2017; Saliba et al., 2016). 2D perovskites demonstrate high stability against environmental factors because of the densely packed structure and hydrophobic nature of the used cation (Grancini et al., 2017). SAM deposited on the perovskite layer increased environmental stability (Wolff et al., 2020), and the inorganic transport layers are the most promising materials for stable PSCs (Coll et al., 2019).

3. Stability and scale

Layer homogeneity across a large area influences the stability of big-area devices. The stability of large-area modules depends on the fabrication of consistent pinhole-free and crack-free layers (J. Kim et al., 2017). For high-stability perovskite modules, research should focus on identifying intrinsically stable device stacks, understanding, and eliminating degradation processes at the cell and module levels, and developing an encapsulation strategy that will allow for high extrinsic stability and a long module lifetime.

4. Lead toxicity

Lead is a popular substance used in perovskite-based solar cells that are highly efficient (Pb). The environmental impact of lead-based PSCs is a source of challenge for their commercialization. Pb has been replaced or reduced in the ABX3 perovskite structure with less hazardous elements e.g., Ge, Sn, and Bi [50, 51 Debbi]. With a bandgap of roughly 1.3eV, Sn exhibits good optoelectronic characteristics. Sn2+ oxidizes to Sn4+, causing p-type doping of the material. Despite significant advances, the efficiency of Pb-free PSCs still lags considerably below that of traditional Pb-based systems. With appropriate encapsulation methods and recycling processes, the hazardousness of lead in the environment can be reduced.

To build mechanically robust and stable perovskite solar cells for deformable applications, an understanding of the underlying physics linking the structure of perovskite solar cells to their characteristics and mechanical stability is required.

1.3 Scope of the Thesis

This research aims to find solutions to some of the above-unresolved issues to enable the design of mechanically robust and stable perovskite solar cells for deformable and stretchable applications. An understanding of the underlying physics linking the structure of perovskite solar cells to their characteristics and mechanical stability is required. This thesis uses a combined analytical, computational, and experimental method to study the effects of pressure on the photoconversion efficiencies of perovskite solar cells. An analytical model was used to predict the effects of pressure on interfacial contact in the multilayered structures of PSCs. A range of pressure values was applied to the devices to improve their interfacial surface contacts. The implications of the results are discussed for the fabrication of efficient PSCs. The effect of annealing temperature on the mechanical properties of hybrid organic-inorganic perovskite (HOIPs) was explored using a combined experimental and theoretical approach. A mechanism-based strain gradient (MSG) theory was used to explain the indentation size effects (ISE) in films at different annealing temperatures. The implications of the results are then discussed in the design of mechanically robust and stable perovskite solar cells (PSCs). And finally, the failure mechanisms in flexible perovskite solar cells are studied under monotonic and cyclic bending using experiments and simulations to gain insight into the interfacial cracking that can occur in the bi-layers and the multi-layers of the perovskite solar cells. The effects of monotonic and cyclic loading on the optical transmittance, power conversion efficiencies and fatigue lifetimes were studied. The implications of the results were discussed for potential applications in flexible and stretchable power sources for electronic structures and devices.

Following the introduction and background in chapter 1, detailed literature reviews on prior work are presented in the chapter. Prior work discussed includes types of solar cells, operation principles of solar cells, perovskite solar cells, flexible perovskite solar cells, pressure effects on multilayered solar cell structures, annealing effects on the mechanical properties of PSCs, the fundamentals of fracture mechanics in electronic systems and the bending mechanics.

Chapter 3 presents the results of the effects of pressure on photoconversion efficiencies of perovskite solar cells (PSCs) using a combined experimental and analytical/computational study. First, an analytic model was used to predict the effects of pressure on the interfacial contact in the multilayered structures of PSCs. The PSCs were fabricated, and a pressure range of 0-10 MPa was applied to improve the interfacial surface contacts. The result shows that the application of pressure can significantly improve the crystallization, absorbance, and power conversion efficiencies of PSCs. This results in the closing-up of voids and the corresponding increase in the interfacial surface contact lengths, which increases with increasing pressure. The improvement in the power conversion efficiencies was observed with increased pressure between 0 and 7 MPa, attributed largely to the effects of increased surface contact and the compaction and infiltration of the TiO2 layers with perovskite during the application of pressure. At higher pressure values (> 7 Mpa), the damage due to the sink of the perovskite layers into the mesoporous layers results in reductions in the photoconversion efficiencies of PSCs.

Chapter 4 presents the effects of annealing temperature on the mechanical properties of hybrid organic-inorganic perovskite (HOIPs). The mechanical properties (hardness and Young’s modulus), microstructural, and surface topography of the HOIPs film at different annealing temperatures ranging from 80 to 170oC were examined. A mechanism-based strain gradient (MSG) theory was used to explain the indentation size effects (ISE) in films at different annealing temperatures. The intrinsic film yield strengths and hardness values (deduced from the MSG theory) are then shown to exhibit a Hall–Petch dependence on the inverse square root of the average grain size. The implications of the results were discussed for the design of mechanically robust perovskite solar cells (PSCs).

Chapter 5 presents the effects of PEO on failure mechanisms of flexible perovskite solar cells under monotonic and cyclic bending. The failure mechanisms for different radii were observed using a scanning electron microscope before the interfacial fracture energies in the multilayer devices are computed using finite element simulations. The failure mechanisms are then used to explain the degradation of the optical transmittance and current-voltage characteristics of flexible perovskite solar cells.

Finally, salient conclusions arising from this work are summarized in Chapter 6. Suggestions for future work are also presented.

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