Anion Exchange-Induced Crystal Engineering via Hot-Pressing Sublimation Affording Highly Efficient and Stable Perovskite Solar Cells

Bin Ding, Paul Joseph Dyson, Mohammad Khaja Nazeeruddin, Hujiang Shen
2021
Article

Résumé

Crystalline, dense, and uniform perovskite thin films are crucial for achieving high-power conversion efficiency solar cells. Herein, a universal method of fabricating highly crystalline and large-grain perovskite films via crystal engineering is demonstrated. Anion exchange of Cl- and I-, and annealing perovskite films, in an ultraconfined and uniform temperature enclosed space with saturated MAI (or FAI) vapor using hot-pressing sublimation technology are conducted. This process ensures a rapid crystal growth rate due to fast exchange between the gas phase and the crystalline film to reduce vertically oriented grain boundaries. The generation of the commonly observed PbI2 phase is also suppressed due to the chemical equilibrium state during the thermal annealing process. Using this approach, pinhole-free perovskite films with preferred crystal orientation and micrometer-scale grains are obtained, leading to a high steady-state efficiency of 22.15% based on mixed-cation perovskite composition. In addition, devices based on different perovskite compositions all exhibit enhanced photovoltaic performance based on the crystal engineering method. The device (without encapsulation) has an efficiency loss of about only 4% after 2520 h of aging in ambient conditions and retains 87% of its initial efficiency after 1000 h of continuous 1 Sun light soaking, thus demonstrating considerably improved stability.

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https://infoscience.epfl.ch/record/286192?ln=fr

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Bin Ding, Paul Joseph Dyson, Mohammad Khaja Nazeeruddin, Hujiang Shen
2021
Article

Résumé

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https://infoscience.epfl.ch/record/286192?ln=fr

À propos de ce résultat

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Publications associées (21)

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Compositional Characterization of Organo-Lead tri-Halide Perovskite Solar Cells

Paul Gratia

Perovskite photovoltaics is one of the fastest growing research fields in materials science. Apart from photovoltaics, a wide range of energy-related research fields have gained substan-tial new momentum owing to the fantastic optoelectronic properties of hybrid lead halide per-ovskites. Starting with solar cells showing a mere 3% power conversion efficiency in 2009, researchers have been able to increase this value to over 22% in 2017, comparable to the best monocrystalline silicon solar cells. These improvements are chiefly due to the compositional tuning and mixing of the ABX3 perovskite structure. However, this strategy increases not only the efficiency but also in the same time the complexity of the perovskite formulation. Differ-ent phases, from the nano- to macroscale, can potentially form. In order to rationalize the effi-ciency progress, one needs to understand and characterize the complex perovskite composi-tion and crystal structure in great detail. The main focus of this thesis lies in the in-depth compositional and phase analysis of perovskite thin films as well as full perovskite solar cells, trying to rationalize the consequences of A cation and X anion tuning and mixing. In the first part, it is shown that fabricating solar cells from a non-stoichiometric perovskite precursor composition, namely one with PbI2 excess, leads to increased grain size, enhanced cristallinity, reduced recombination as well as an improved TiO2/perovskite interface. As op-posed to lead iodide phases resulting from film degradation, which is hampering device per-formance, unreacted excess PbI2 phases originating from an excess of PbI2 in the precursor solution are very beneficial for the final solar cell efficiency, reproducibility and stability. The next scientific challenge adressed in this thesis is related to the nano- and microscale composition of the record-breaking mixed cation/mixed anion perovskite composition. Mac-roscale investigations have previously suggested that this formulation results in a single ho-mogenous phase. However, using nanoscale elemental and charge carrier distribution mapping as well as microscale structural and optical film analysis, it is here found that in high efficient solar cells partial phase segregation does take place at the micro- and nanoscale. Moreover, it is shown that mixed cation/anion formulations allow the formation of never re-ported 3D hexagonal lead halide perovskite polytypes, namely 4H and 6H. These polytypes are shown to play a key role during the crystallization process, which is also revealed for the first time here. Indeed, mixed cation/mixed anion perovskite films are fabricated via the crys-tallization sequence 2Hï 4Hï 6Hï 3R(3C). It is demonstrated that the complex crystalliza-tion via these defect-prone hexagonal intermediates can be by-passed by the incorporaton of low amounts of Cs+ cations into the structure. This can explain the improved stability as well as the increased power conversion efficiency and device reproducibility. It gives for the first time a rational explanation as to why the halide perovskite community rapidly adopted the incorporation of Cs+ in mixed perovskites. The last results presented in this thesis are related to the hole-transporting materials (HTM) used in perovskite solar cells. It is shown that carbazole-based small molecule HTMs are a cheap and efficient alternative to the much costlier commercial spiro-OMeTAD.

EPFL2018

Chargement

Conversion of sunlight to electricity is the most plausible approach towards the generation of renewable energies. During recent decade lead halide perovskites (LHPs) have proven to be promising candidates for photovoltaic applications such as solar cells. LHPs can be made easily in laboratories around the world via solution process methods and many universities can afford to participate in their development. How-ever, LHPs suffer from instability issues and their efficiencies can be further improved. In this thesis, both instability and efficiency challenges are addressed. We showed by doping the precursor of methylammonium lead iodide (MAPbI3) perovskite with a relatively large cation of 2-Choloroethylammonium (CLEA), perovskites films with CLEA incorporated into their structures are formed. The novel perovskite is able to withstand high temperatures as high as 80°C while achieving efficiencies > 19% in MAPbI3-based perovskite solar cells (PSCs). Various approaches have been employed for the passivation of the defects in perovskite solar cells Vari-ous approaches have been employed to passivate the defects in perovskite films used in perovskite solar cells (PSCs). However, the passivation mechanism is often unclear at a molecular level and the reason for enhanced PSC performance remains elusive. Here, we describe passivation based on the deposition of two dimensional (2D) perovskites (obtained from para-halophenylethylammonium iodide) on the surface of perovskite films. Moreover, the mechanism of passivation was elucidated and was traced to the high work function of the 2D perovskites, which induce band-bending at the surface of perovskite film and therefore reduces the charge carrier recombination at the interface of perovskite and the hole transporting material (HTM). Efficiencies >22% with average voltages >1.15 V were achieved using a typical mesoporous-TiO2 and Spiro-OMeTAD device configuration. This study paves the way for PSCs efficiency improvement by rational design. The employment of 2D perovskites is a promising approach to tackle the stability and voltage issues inherent in perovskite solar cells. It remains unclear however whether other perovskites with different dimensionalities have the same effect on efficiency and stability. Here, we report the use of quasi-3D azetidinium lead iodide (AzPbI3) as a secondary layer on top of the primary 3D perovskite film that results in significant improvements in the photovoltaic parameters. Remarkably, utilization of AzPbI3 leads to the passivation resulting in a power conversion efficiency (PCE) of 22.4 %. The open-circuit voltage obtained is as high as 1.18 V, which is among the highest reported to date for single-junction perovskite solar cells, corresponding to a voltage deficit of 0.37 V for a bandgap of 1.55 eV. Studying the compositional instability of mixed ion perovskites under light illumination is important to understand the mechanisms underlying their efficiency and stability. However, current techniques are limited in resolution and are unable to deconvolute minor ion migration phenomena. Here, we describe a method that enables ion migration to be studied allowing different segregation mechanisms to be elucidated. We applied statistical analysis to cathodoluminescence data to generate compositional distribution histograms. Using these histograms, we identified two different ion migration phenomena, horizontal ion migration (HIM) and vertical ion migration (

EPFL2021

Chargement

Fundamentals of perovskite formation for photovoltaics

Amita Ummadisingu

Perovskite solar cells have become strong contenders in the arena of photovoltaics due to the stunning rise in their efficiency from 3 % to over 20 % in just seven years. In this time, numerous device architectures and thin film deposition methods have been explored. The sequential deposition and anti-solvent methods are among the most widely used for preparing perovskite solar cells. Optimizing these perovskite deposition processes to tune the perovskite film morphology plays a key role in the race for the highest efficiencies as homogeneity of the film correlates with superior photovoltaic performance. To date, the factors controlling perovskite film formation in various deposition methods and the precise mechanism are little understood. In light of this, the aim of the thesis is to unravel the salient aspects of perovskite film formation. I first study the formation of methylammonium lead iodide in sequential deposition using confocal laser scanning fluorescence microscopy (CLSM) and scanning electron microscopy (SEM). I discover that illumination during film formation is a major factor in the reaction as it greatly accelerates perovskite formation and tunes the final film morphology by controlling nucleation of lead iodide. The nucleation density increases logarithmically with the illumination intensity. I uncover the mechanism behind this effect and demonstrate that it is a quantum phenomenon and not merely a heating effect. I posit that absorbed light lowers the surface tension, thereby facilitating nucleation. Not only is this effect present in the main deposition methods â sequential deposition and the anti-solvent methods, but also in perovskites of more complex compositions that are employed in state-of-the-art solar cells. Second, I scrutinize the individual stages in the formation of methylammonium lead iodide perovskite in sequential deposition. SEM-cathodoluminescence imaging identifies the presence of mixed crystalline aggregates composed of methylammonium lead iodide perovskite and lead iodide in forming perovskite films. Using cross-sectional CLSM for the first time, I identify the directionality of perovskite formation. Kinetic monitoring and model fitting bring forward the Avrami models as the most suitable to quantify and represent the formation of perovskite under different conditions of temperature, film thickness and illumination. Finally, I identify a maturation effect in the perovskite film in solar cells stored in the dark and investigate the dynamic spontaneous coalescence of crystals in the film. During maturation, small crystals merge to form larger ones and this reduces the number of grain boundaries and the associated trap states, supressing non-radiative recombination. The photovoltaic performance of high efficiency solar cells thus increases and it is accompanied by a reduction in the hysteresis. Overall, apart from providing insights into the dynamic nature of deposited films, I present a fundamental understanding of the perovskite formation process in different deposition methods and for various compositions. Such a comprehensive picture aids in the achievement of high photovoltaic performance.

EPFL2018

Chargement

Fundamentals of perovskite formation for photovoltaics

Amita Ummadisingu

EPFL2018

Compositional Characterization of Organo-Lead tri-Halide Perovskite Solar Cells

Paul Gratia

EPFL2018

EPFL2021