Understanding the environmental degradation of methylammonium lead iodide Perovskite

Hybrid lead halide perovskite semiconductors have accelerated to the forefront of pho- tovoltaics. These materials possess highly desirable features including, fast charge trans- port, high extinction coefficients, large spectral overlap and solution process-ability. As a result of these, low-cost devices have emerged boasting impressive power conversion ef- ficiencies in excess of 20%. The rapid development of this technology is in part due to the materials versatility allowing numerous device configurations and fabrication techniques to be employed. Unfortunately, the excitement surrounding perovskites is hampered by their inability to withstand environmental stress. These systems have been found to ex- hibit significant performance losses and undergo irreversible material degradation when ex- posed to oxygen and light. Highlighted from these initial findings is that under these condi- tions, the reactive oxygen species superoxide can form and breakdown the perovskite crystal. A greater understanding of the mechanistic action leading to the generation of superoxide has been achieved through a powerful combination of experimental and computational results. The work has examined the role of material selection in the fabrication of devices. In addition the role of morphology of the perovskite has also been examined, where electron extraction from the perovskite layer is critical in achieving long term stability. The driving force for separation and the velocity at which electrons can be extracted are critical components in the effective- ness of an electron extraction layer in aiding stability enhancements. Rapid oxygen diffusion and iodide vacancies have been identified as key contributors to the mechanistic formation of superoxide. In order to achieve this a unique combination of isothermal gravimetric analysis and Time-of-Flight secondary ion mass spectrometry were employed. Critically these showed the rapid uptake of oxygen and the ubiquitous presence of these species after exposure to air. Inspired by these results, new methods have been developed to generate perovskite solar cells with increased performance life-time. The work herein, has also identified the impact of the selection of the organic cation and exchanging the halide upon the stability towards oxygen and light. Furthermore, the consequence of introducing moisture into the equation has been consid- ered and revealed greater detail about the mechanistic formation of superoxide from these species. The generation of superoxide in perovskite materials for photovoltaic applications is highly undesirable and persists as a key issue regarding their commercial employment. However, inspired by the fact photo-absorbers can generate superoxide a new application where the generation of the species could be used in a productive way is explored. To this end, the generation of superoxide from the organic polymer P3HT is explored. The production of superoxide from films is then harnessed to react with another species in a solution media. This simulation, leads to potential application where a contaminated solution, for example with a biological species, could be cleaned by addition of a P3HT film, oxygen and light. Here, the superoxide species would form from the film and then react and denature the contaminant.Open Acces

Understanding the Enhanced Stability of Bromide Substitution in Lead Iodide Perovskites

Lead halide perovskites have rapidly emerged as candidate materials for high-performing solar cells, but show serious issues related to long-term stability. Methylammonium (MA) lead perovskites with mixed iodide-bromide compositions, MAPb(I1-xBrx)3, are reported to exhibit improved stability, but the origin of such behavior is not fully understood. Here, we report new insights into the degradation properties of MAPb(I1-xBrx)3 using ab initio simulations and a range of spectroscopic techniques. Absorbance spectroscopy shows that as the Br content increases, the material stability toward oxygen and light increases. Isothermal gravimetric analysis and time-resolved single photon counting show that the amount of oxygen incorporation into perovskite films decreases significantly with increasing Br content. Ab initio simulations indicate that the degradation reaction involving superoxide species is energetically exothermic for pure MAPbI3 but becomes less favorable with increasing Br content with an endothermic energy for pure MAPbBr3, suggesting that the degradation of MAPbBr3 in the presence of oxygen and light is unfavorable. The simulations indicate shorter N-H...Br hydrogen bonds between the MA+ cation and Br ions, which would promote greater structural stability upon bromide substitution. Thin-film passivation with iodide salts is shown to enhance the stabilities of mixed-halide perovskite films and solar cell devices. The greater fundamental understanding of mixed iodide-bromide systems gained from this study is important for the future design of stable perovskite solar cells.</p

Light and oxygen induced degradation limits the operational stability of methylammonium lead triiodide perovskite solar cells

Here, we demonstrate that light and oxygen-induced degradation is the main reason for the low operational stability of methylammonium lead triiodide (MeNH3PbI3) perovskite solar cells exposed to ambient conditions. When exposed to both light and dry air, unencapsulated MeNH3PbI3 solar cells rapidly degrade on timescales of minutes to a few hours. This rapid degradation is also observed under electrically bias driven current flow in the dark in the presence of O2. In contrast, significantly slower degradation is observed when the MeNH3PbI3 devices are exposed to moisture alone (e.g. 85% relative humidity in N2). We show that this light and oxygen induced degradation can be slowed down by the use of interlayers that are able to remove electrons from the perovskite film before they can react with oxygen to form O2−. These observations demonstrate that the operational stability of electronic and optoelectronic devices that exploit the electron transporting properties of MeNH3PbI3 will be critically dependent upon the use of suitable barrier layers and device configurations to mitigate the oxygen sensitivity of this remarkable material

Toward Improved Environmental Stability of Polymer:Fullerene and Polymer:Nonfullerene Organic Solar Cells: A Common Energetic Origin of Light- and Oxygen-Induced Degradation

With the emergence of nonfullerene electron acceptors resulting in further breakthroughs in the performance of organic solar cells, there is now an urgent need to understand their degradation mechanisms in order to improve their intrinsic stability through better material design. In this study, we present quantitative evidence for a common root cause of light-induced degradation of polymer:nonfullerene and polymer:fullerene organic solar cells in air, namely, a fast photo-oxidation process of the photoactive materials mediated by the formation of superoxide radical ions, whose yield is found to be strongly controlled by the lowest unoccupied molecular orbital (LUMO) levels of the electron acceptors used. Our results elucidate the general relevance of this degradation mechanism to both polymer:fullerene and polymer:nonfullerene blends and highlight the necessity of designing electron acceptor materials with sufficient electron affinities to overcome this challenge, thereby paving the way toward achieving long-term solar cell stability with minimal device encapsulation

CH3NH3PbI3 films prepared by combining 1- and 2-step deposition: how crystal growth conditions affect properties

The growth of CH3NH3PbI3 films is transformed using a new combined one-step and two-step deposition method which results in controlled morphology and property changes.</p