Defect Formation and Healing at Grain Boundaries in Lead-Halide Perovskites

in Journal of Materials Chemistry A (2022)

Polycrystalline perovskite solar cells show high power conversion efficiencies despite the presence of grain boundaries (GBs). The benign nature of GBs on the electronic properties and structural ... [more ▼]

Polycrystalline perovskite solar cells show high power conversion efficiencies despite the presence of grain boundaries (GBs). The benign nature of GBs on the electronic properties and structural stability in metal-halide perovskites contradicts their propensity for point defect formation, a controversy that is far from being understood. In this work, we combine ab initio molecular dynamics and density functional theory calculations on the S5[130] GB of cesium lead iodide, CsPbI3, to shed light on the structural and electronic properties of such GBs. Our results present the first evidence of structural healing of GBs driven by the facile migration of iodine ions, resulting in stabilized GB structures with reduced hole trap states and shallow electron trap states by strain-induced Pb–Pb dimers. Drift-diffusion simulations reveal that shallow electron trap states in GB mainly lower open-circuit voltage by enhanced non-radiative recombination. Finally, we observe the spontaneous formation of iodine Frenkel defects with reduced formation energies compared to the perovskite bulk. Overall, our study reveals a controversy of GBs showing a moderate impact on the electronic properties by structural healing but a detrimental impact on the point defect densities, both being connected to the facile migration of iodine ions in GBs. [less ▲]

Methylamine Gas Treatment Affords Improving Semitransparency, Efficiency, and Stability of CH3NH3PbBr3-Based Perovskite Solar Cells

in Solar RRL (2021)

High bandgap semitransparent solar cells based on CH3NH3PbBr3 perovskites are attractive for building integration, tandem cells, and electrochemical applications. The lack of control of the CH3NH3PbBr3 ... [more ▼]

High bandgap semitransparent solar cells based on CH3NH3PbBr3 perovskites are attractive for building integration, tandem cells, and electrochemical applications. The lack of control of the CH3NH3PbBr3 perovskite growth limit the exploitation of CH3NH3PbBr3-based perovskite solar cells. Herein, a posttreatment is carried out after the initial CH3NH3PbBr3 crystallization based on methylamine gas that drastically enhances the perovskite quality leading to a highly crystalline film with improved average visible transmittance (AVT) close to 56%. Opaque devices showed outstanding results in terms of open-circuit voltage and power conversion efficiency (PCE) reaching 1.54 V and 9.2%, respectively. These achievements are ascribed to a film with reduced morphological defects and better interface quality and reduced nonradiative pathways. For the first time, the fabrication of semitransparent CH3NH3PbBr3-based solar cells is demonstrated reaching a maximum PCE equal to 7.6%, an AVT of the full stack device of 52%, and an excellent light stability at maximum-power point tracking. [less ▲]

Tuning halide perovskite energy levels

in Energy and Environmental Science (2020)

Solar energy is playing a significant role in the development of a world powered by clean energy sources. In this context, halide perovskite solar cells (PSCs) are considered one of the most promising ... [more ▼]

Solar energy is playing a significant role in the development of a world powered by clean energy sources. In this context, halide perovskite solar cells (PSCs) are considered one of the most promising research lines thanks to their high efficiencies and flexibility, combined with an easy and cheap fabrication process. The possibility of combining different materials and compositions is an excellent advantage of PSCs. However, still, a big limit is posed by the need for a proper energy level alignment between the layers of materials comprising devices. Therefore, it is of utmost interest to develop methods allowing to tune the energy levels of the different materials. In semiconductors physics, a common technique to achieve this purpose is to functionalize the surface of the materials with dipolar molecules. Nevertheless, this has been rarely applied to perovskites because of the highly rough surface of the films. In this study, we show that it is possible to use this technique in hybrid organic–inorganic perovskite semiconductors systematically and tune the direction and magnitude of the shift by controlling the deposition process. These findings offer a toolbox to simplify the application of halide perovskites in optoelectronic devices. [less ▲]

Interface Electrostatics of Solid-State Dye-Sensitized Solar Cells: A Joint Drift-Diffusion and Density Functional Theory Study

in The Journal of Physical Chemistry C (2019)

Dye-sensitized solar cells (DSCs) have gained great attention in recent years due to their low-cost fabrication, flexibility, and high power conversion efficiency. In a DSC, due to interfaces between the ... [more ▼]

Dye-sensitized solar cells (DSCs) have gained great attention in recent years due to their low-cost fabrication, flexibility, and high power conversion efficiency. In a DSC, due to interfaces between the dye and the chargetransport materials, the interface electrostatics becomes a key factor determining the overall performance of the cell. Liquid-electrolyte-based DSCs suffer from low stability, electrolyte leakage, and, in some cases, electrode corrosion. Replacing liquid electrolyte with a solid semiconducting material leads to poor interfacial contacts, hence the interface electrostatics becomes one of the limiting factors. In this work, we present a drift-diffusion and density functional theory (DFT) study of solid-state DSCs to investigate the electrostatics at the TiO2/organic dye/Spiro-OMeTAD interface and its impact on the adsorbed dye energy levels, its absorption spectrum, and the related charge injection. In our three-dimensional drift-diffusion model, we solve a set of drift-diffusion equations coupled to Poisson equation for electrons, holes, doping impurities, and interface traps simultaneously. After that, we use first-principles DFT modeling of dye-sensitized interfaces in the presence of the calculated electric fields. We find that interface traps located below the conduction band edge of mesoporous TiO2 influence the accumulation of photogenerated holes and built-in electric field near the interface. The built-in electric field leads to change in the energetics at the dye/TiO2 interface, leading to poor charge injection from excited dye into TiO2. The simulations were carried out for different electronic trap densities in TiO2 and different doping levels in the Spiro-OMeTAD hole-transport layer. This study helps to a better understanding of the interface electrostatics and its role in the charge injection mechanism of solid-state DSCs. [less ▲]

Drift-diffusion and Machine Learning for High Efficiency Perovskite-Perovskite based Tandem Solar Cells

Poster (2019, May)