Additive engineering for stable halide perovskite solar cells

Halide perovskite solar cells (PSCs) have already demonstrated power conversion efficiencies above 25%, which makes them one of the most attractive photovoltaic technologies. However, one of the main bottlenecks towards their commercialization is their long-term stability, which should exceed the 20-year mark. Additive engineering is an effective pathway for the enhancement of device lifetime. Additives applied as organic or inorganic compounds, improve crystal grain growth enhancing power conversion efficiency. The interaction of their functional groups with the halide perovskite (HP) absorber, as well as with the transport layers, results in defect passivation and ion immobilization improving device performance and stability. In this review, we briefly summarize the different types of additives recently applied in PSC to enhance not only efficiency but also long-term stability. We discuss the different mechanism behind additive engineering and the role of the functional groups of these additives for defect passivation. Special emphasis is given to their effect on the stability of PSCs under environmental conditions such as humidity, atmosphere, light irradiation (UV, visible) or heat, taking into account the recently reported ISOS protocols. We also discuss the relation between deep-defect passivation, non-radiative recombination and device efficiency, as well as the possible relation between shallow-defect passivation, ion immobilization and device operational stability. Finally, insights into the challenge and criteria for additive selection are provided for the further stability enhancement of PSCs

Enhancing Thermal Stability of Perovskite Solar Cells with a Polymer Through Grain Boundary Passivation

Organic-inorganic halide perovskite solar cells have emerged as a promising photovoltaic technology due to their superb power conversion efficiency (PCE) and very low material costs. While perovskite solar cells are expected to eventually compete with existing silicon-based solar cells on the market, their long-term stability has become a major bottleneck. In particular, perovskite films are found to be very sensitive to external factors such as air, UV light, light soaking, thermal stress and others. Among these stressors, light, oxygen and moisture-induced degradation can be slowed by integrating barrier or interface layers within the device architecture. However, the most representative perovskite absorber material, CH3NH3PbI3 (MAPbI3), appears to be thermally unstable even in an inert environment. This poses a substantial challenge for solar cell applications because device temperatures can be over 45 °C higher than ambient temperatures when operating under direct sunlight. In this thesis, the thermal stability of perovskite solar cells was primarily investigated. Initially, we systematically studied the effects of heating and cooling processes on the principal photovoltaic performance of perovskite solar cells by combining temperature-dependent J-V, steady-state PL, UV-VIS and time-resolved lifetime decay measurements. In particular, we have observed the dynamic evolution of degraded crystallinity, increased charge trapping, deep trap depth and PbI2 phase. During the heating process, the thermal degradation of the perovskite film was observed at 70 ° C or higher. An increase in the disordered phase of the perovskite film involved a drastic increase in charge trapping and the development of a deeper trap depth. Interestingly, we observed that the degradation of the perovskite film persisted even after the temperature was dropped, which led to irreversible J-V characteristics of the perovskite solar cell. Later, we introduced a polymer layer of PMMA which improved thermal stability for more than 1000hrs at 85°C. Without PMMA, host-casted MAPbI3 films suffered rapid thermal degradation, forming a number of pin-holes at GBs and then extending into GIs. Rapid thermal degradation of perovskite GBs without PMMA may be due to the rich moisture chemical structure of hydrated (CH3NH3)4PbI6•H2O. At the elevated temperature, hydrated (CH3NH3)4PbI6•H2O grain boundaries might suffer from moisture-assisted decomposition, forming a number of pin-holes at GBs. Conversely, we observed high thermal stability of perovskite films by introducing PMMA to induce marked thermal stability at GBs. It is believed that the excellent hygroscopicity of PMMA played an active role in absorbing moisture from hydrated (CH3NH3)4PbI6•H2O GBs and driving them out through the GB channel. We believe that continuous functionalization of perovskite GBs or crosslinking perovskite GBs with PMMA molecules might drastically render perovskite GBs chemically robust, resilient, and heat-resistant. Moreover, we mixed inorganic cesium (Cs) cation into the perovskite, which improved thermal stability at a higher temperature of 120°C. Finally, we have fabricated perovskite solar cells in an antisolvent method in which the perovskite film does not contain deeper grain boundary like hot-casted perovskite thin film. Also, we introduced a polymer (polyimide) on the top of the perovskite solar cell which has a large contact angle and glass transition temperature. Consequently, perovskite solar cells with polyimide showed thermal stability without any efficiency decrement more than 30 days

Organic-Inorganic Halide Perovskite Nanocrystals and Solar Cells

A great challenge facing humanity in the 21st century is finding inexhaustible and inexpensive energy sources to power the planet. Renewable energies are the best solutions because of their abundance, diversity, and pollution-free emission. Solar energy is the cleanest and most abundant renewable energy source available. In the continuing quest for efficient and low-cost solar cells, perovskite solar cells (PSCs) have emerged as a potential replacement for silicon solar cells. Since 2009, the record efficiencies of PSCs have been skyrocketing from 3.8 % to 25.2 % and are now approaching the theoretical limit. Along with the three-dimensional perovskites used for photovoltaics, the layer-structured perovskites also attracted significant attention due to their remarkable optoelectronic properties for application in lasing and light-emitting devices. In this dissertation, the synthesis of two-dimensional (C6H5C2H4NH3)2PbBr4 (PEPB) nanocrystal with thickness as few as 3 unit cell layers was demonstrated. Compare to its bulk crystals, 2D PEPB nanocrystals exhibited a major blue-shifted photoluminescence (PL) peak at 409 nm, which is attributed to the quantum confinement effect, and two new PL peaks at 480 and 525 nm at room temperature. Time-resolved reflectance spectroscopy was used to investigate the exciton dynamics, exhibiting an exciton lifetime of 16.7 ps. The high-quality 2D PEPB nanocrystals are expected to have high PL quantum efficiency and potential applications for light-emitting devices. In the second part, the optimization of planar PSCs is carried out by the engineering of the deposition method for MAPbI3 thin films. We compared the quality of the MAPbI3 films deposited by two different methods. Compared to the film deposited by the Lewis adduct method, perovskite film deposited by the methylamine-gas-assisted method showed larger crystal grains, smoother surface morphology, and a preferred (110) crystal orientation. As for PSC efficiency, the methylamine-gas-assisted method also showed clear advantages over the Lewis adduct method (highest efficiency: 19.28 % vs. 18.17 %; average efficiency: 16.28 % vs. 12.59 %). The methylamine-gas-assisted method, with its potential for upscaling, is no doubt a noteworthy leap towards the roll-to-roll printing of large-area PSCs

Understanding Film Formation Mechanisms of Sequential Solution-Processed Perovskite for Solar Cell Applications

Metal halide perovskite is a promising light-absorber material with excellent optoelectronic properties for solar cell applications as seen by the remarkable progress in terms of power-conversion-efficiency of demonstrated devices, reaching 25.2% in 2019. This thesis focuses on developing deeper understandings of film formation mechanisms of sequential processed mixed-cation perovskites and the role of chlorine (Cl)-additives for such films with the aim of improving film quality and therefore the performance of associated solar devices. Firstly, a dynamic sequential process is developed whereby the two precursors PbI2-CsI-DMSO and MAI-FAI are spin-coated sequentially with no stoppage in between and the second precursor is dispensed while the substrate is in motion for the fabrication of Cs0.15(MA0.7FA0.3)0.85PbI3 perovskite. Compared to the standard sequential process, the dynamic sequential process reliably produces film with better film quality and a 2% absolute improvement in average efficiency of associated devices. The reason for such improvement is investigated systemically. Results of X-ray-diffraction (XRD) measurement and Fourier-transform-infrared (FTIR) spectroscopy confirm the three-stage film formation mechanism i) initial formation; ii) deconstruction; & iii) re-crystallisation. This is consistently observed for sequential process that uses the dimethyl-sulfoxide dimethylformamide (DMSO-DMF) solvent system. The high-quality perovskite film resulted from the dynamic sequential process is due to the earlier onset of DMSO-complex formation aided by the energy provided by the motion dispensing and the absence of stoppage. This DMSO-complex formation “deconstructs” the initial perovskite formed. In dynamically processed films, the earlier onset of “deconstruction” results in slower crystallisation that results in lower amount of non-perovskite phase and better crystallinity in the films. This explains the higher fill factor, current and voltage outputs of associated solar cells. Finally, the effect of Cl-additives on dynamically processed MA0.7FA0.3PbI3 film is studied systemically comparing two types of chlorine-sources (MACl and FACl) and the stages at which the Cl source is added. Results from optical-measurement, XRD and FTIR show that MACl compared to FACl has the advantage of slowing down crystallisation for a more orderly growth. Cl should be added in the 1st step of the process to limit the amount of excess Cl for better film quality

Film Fabrication of Perovskites and their Derivatives for Photovoltaic Applications via Chemical Vapor Deposition

In recent decades, metal halide perovskites have attracted much attention after showing great potential in photovoltaic (PV) applications. With the rapid progress of perovskites, various thin-film fabrication methods have been studied intensively. However, a film deposition method with controllability, cost efficiency, scalability, and uniformity is required to obtain perovskite films with the desired morphologies and properties and achieve large-scale manufacture. Chemical vapor deposition (CVD) stands out among the various deposition methods because of its unique advantages. In this review, perovskite films for PV applications deposited by diverse CVD methods are discussed, and a summary of the development and investigation of CVD processes utilized is provided

Rapid annealing of Perovskite solar cell thin film materials through intense pulse light.

Perovskite solar cells (PSCs) have garnered a great attention due to their rapid efficiency improvement using cheap and solution processable materials that can be adapted for scalable high-speed automated manufacturing. Thin film perovskite photovoltaics (PVs) are typically fabricated in an inert environment, such as nitrogen glovebox, through a set of deposition and annealing steps, each playing a significant role on the power conversion efficiency (PCE), reproducibility, and stability of devices. However, atmospheric processing of PSCs would achieve lucrative commercialization. Therefore, it is necessary to utilize materials and methods that enable successful fabrication of efficient PSCs in the ambient environment. The lab scale experiments have been dominated using deposition methods, such as spin-coating or thermal evaporation in vacuum, which are not adaptable for automation; hence, taking advantage of scalable deposition methods, such as inkjet printing, is necessary for automation. Besides deposition, post process annealing is a pivotal aspect which crystallizes the thin film materials and determines the performance and stability of PSCs. Therefore, it is necessary to further investigate this step and develop new methods and utilize potential materials vi that are amenable for scalable, high-throughput, and cost-effective automated manufacturing of PSCs. Conventional methods have successfully resulted in efficient labscale PSCs using prolong and high temperature annealing; however, industrialization requires rapid annealing methods that allow for scalable, high-speed, and cost-effective manufacturing of efficient PSCs in the ambient environment. Intense pulse light is a rapid annealing method (IPL) that allows for the lucrative, scalable, and high throughput atmospheric processing of PSCs; thus, it is necessary to study the photothermal impact on the morphology and phase evolution of the thin film materials and develop ambient processable precursors that yield efficient perovskite modules. IPL exerts intermittent millisecond(s) duration flashes carrying energetic photons to anneal the material, and the parameters of flash energy, duration, count, and interval time between flashes determine the annealing extent and affect the PV performance of PSCs. This dissertation investigates the impact of these parameters on the morphology, phase change, and conductivity of the potential PSC thin films using various material characterization techniques of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), photoluminescence (PL), impedance spectroscopy (IS), X-ray photoelectron spectroscopy (XPS), UV-Vis, as well as voltammetry, by introducing a novel additive and annealing approaches which allow for rapid fabrication of PSCs, and is applicable for rapid, cost-effective, and scalable automated fabrication of PSCs