Role of Decomposition Product Ions in Hysteretic Behavior of Metal Halide Perovskite

Direct Observation of Contact Reaction Induced Ion Migration and its Effect on Non-Ideal Charge Transport in Lead Triiodide Perovskite Field-Effect Transistors

The migration of ionic defects and electrochemical reactions with metal electrodes remains one of the most important research challenges for organometal halide perovskite optoelectronic devices. There is still a lack of understanding of how the formation of mobile ionic defects impact charge carrier transport and operational device stability, particularly in perovskite field-effect transistors (FETs), which tend to exhibit anomalous device characteristics. Here, the evolution of the n-type FET characteristics of one of the most widely studied materials, Cs0.05 FA0.17 MA0.78 PbI3, is investigated during repeated measurement cycles as a function of different metal source-drain contacts and precursor stoichiometry. The channel current increases for high work function metals and decreases for low work function metals when multiple cycles of transfer characteristics are measured. The cycling behavior is also sensitive to the precursor stoichiometry. These metal/stoichiometry-dependent device non-idealities are correlated with the quenching of photoluminescence near the positively biased electrode. Based on elemental analysis using electron microscopy the observations can be understood by an n-type doping effect of metallic ions that are created by an electrochemical interaction at the metal-semiconductor interface and migrate into the channel. The findings improve the understanding of ion migration, contact reactions, and the origin of non-idealities in lead triiodide perovskite FETs.

Perovskite Solar Cell Performance Boosted by Regulating the Ion Migration and Charge Transport Dynamics via Dual-Interface Modification of Electron Transport Layer

Engineering the buried interfaces of perovskite solar cells (PSCs) is crucial for optimizing the device performance. We herein report a novel strategy by modifying the ETL-FTO interface with MgO, as well as the interface between the perovskite layer (PVKL) and the SnO2 electron transfer layer (ETL) with formamidine bromide (FABr). The dual-interface ETL engineering substantially improved the photoelectric conversion efficiency (19.62 → 22.04%) and suppressed the hysteresis index (14.98 → 1.09%). The structure-activity relationship was explored by using transient photoelectric and time-of-flight secondary-ion mass spectroscopic analyses. It was found that the FABr treatment enhanced the PVKL crystallinity and the PVKL-ETL interaction and that the MgO modification dramatically retarded the ion migration, which together optimized the ETL function. The mechanism underlying the influence of ion distribution on the dynamics of ions and free carriers is discussed, which may be helpful for the rational design of high-performance PSCs.

Disentangling Electronic Transport and Hysteresis at Individual Grain Boundaries in Hybrid Perovskites via Automated Scanning Probe Microscopy

Underlying the rapidly increasing photovoltaic efficiency and stability of metal halide perovskites (MHPs) is the advancement in the understanding of the microstructure of polycrystalline MHP thin film. Over the past decade, intense efforts have been aimed at understanding the effect of microstructures on MHP properties, including chemical heterogeneity, strain disorder, phase impurity, etc. It has been found that grain and grain boundary (GB) are tightly related to lots of microscale and nanoscale behavior in MHP thin films. Atomic force microscopy (AFM) is widely used to observe grain and boundary structures in topography and subsequently to study the correlative surface potential and conductivity of these structures. For now, most AFM measurements have been performed in imaging mode to study the static behavior; in contrast, AFM spectroscopy mode allows us to investigate the dynamic behavior of materials, e.g., conductivity under sweeping voltage. However, a major limitation of AFM spectroscopy measurements is that they require manual operation by human operators, and as such only limited data can be obtained, hindering systematic investigations of these microstructures. In this work, we designed a workflow combining the conductive AFM measurement with a machine learning (ML) algorithm to systematically investigate grain boundaries in MHPs. The trained ML model can extract GBs locations from the topography image, and the workflow drives the AFM probe to each GB location to perform a current-voltage (IV) curve automatically. Then, we are able to have IV curves at all GB locations, allowing us to systematically understand the property of GBs. Using this method, we discovered that the GB junction points are less conductive, potentially more photoactive, and can play critical roles in MHP stability, while most previous works only focused on the difference between GB and grains.

Ionic and electronic polarization effects in horizontal hybrid perovskite device structures close to equilibrium

The electrical response of hybrid perovskite devices carries a significant signature from mobile ionic defects, pointing to both opportunities and threats when it comes to functionality, performance and stability of these devices. Despite its importance, the interpretation of polarization effects due to the mixed ionic-electronic conducting nature of these materials and the quantification of their ionic conductivities still poses conceptual and practical challenges, even for the equilibrium situation. In this study, we address these questions and investigate the electrical response of horizontal devices based on methylammonium lead iodide (MAPI) close to equilibrium conditions. We discuss the interpretation of DC polarization and impedance spectroscopy measurements in the dark, based on calculated and fitted impedance spectra obtained using equivalent circuit models that account for the mixed conductivity of the perovskite and for the effect of device geometry. Our results show that, for horizontal structures with a gap width between the metal electrodes in the order of tens of microns, the polarization behavior of MAPI is well described by the charging of the mixed conductor/metal interface, suggesting a Debye length in the perovskite close to 1 nm. We highlight a signature in the impedance response at intermediate frequencies, which we assign to ionic diffusion in the plane parallel to the MAPI/contact interface. By comparing the experimental impedance results with calculated spectra for different circuit models, we discuss the potential role of multiple mobile ionic species and rule out a significant contribution from iodine exchange with the gas phase in the electrical response of MAPI close to equilibrium. This study helps to clarify the measurement and interpretation of mixed conductivity and polarization effects in hybrid perovskites with immediate relevance to the characterization and development of transistors, memristors and solar cells based on this class of materials as well as other mixed conductors.

Exploring the Relationship of Microstructure and Conductivity in Metal Halide Perovskites via Active Learning-Driven Automated Scanning Probe Microscopy

Electronic transport and hysteresis in metal halide perovskites (MHPs) are key to the applications in photovoltaics, light emitting devices, and light and chemical sensors. These phenomena are strongly affected by the materials microstructure including grain boundaries, ferroic domain walls, and secondary phase inclusions. Here, we demonstrate an active machine learning framework for "driving" an automated scanning probe microscope (SPM) to discover the microstructures responsible for specific aspects of transport behavior in MHPs. In our setup, the microscope can discover the microstructural elements that maximize the onset of conduction, hysteresis, or any other characteristic that can be derived from a set of current-voltage spectra. This approach opens new opportunities for exploring the origins of materials functionality in complex materials by SPM and can be integrated with other characterization techniques either before (prior knowledge) or after (identification of locations of interest for detail studies) functional probing.

Uncovering Aging Chemistry of Perovskite Precursor Solutions and Anti-aging Mechanism of Additives

The aging of precursor solutions is the major stumbling block for the commercialization of perovskite solar cells (PSCs). Herein, for the first time we used the state-of-the-art in situ liquid time-of-flight secondary ion mass spectrometry to molecularly explore the perovskite precursor solution chemistry. We identified that the methylammonium and formamidinium cations and the I^- anion are the motivators of the aging chemistry. Further, we introduced two kinds of Lewis bases, triethyl phosphate (TP) and ethyl ethanesulfonate (EE), as new additives in the solution and unraveled that both of them can protect the reactive cations from aging through weak interactions. Significantly, TP is superior to EE in enhancing long-term solution stability as it can well-maintain the internal interaction structures within the solution phase. The PSC derived from a fresh TP-doped solution delivered a high power conversion efficiency of 23.06 %, 92.23 % of which remained in that from a 21-day-old solution.