Photovoltaic and optoelectronic device group

Abstract:

2H-Benzotriazol-2-ylethylammonium bromide and iodide and its difluorinated derivatives are synthesized and employed as interlayers for passivation of formamidinium lead triiodide (FAPbI₃) solar cells. In combination with PbI₂ and PbBr₂, these benzotriazole derivatives form two-dimensional (2D) Ruddlesden-Popper perovskites (RPPs) as evidenced by their crystal structures and thin film characteristics. When used to passivate n-i-p FAPbI₃ solar cells, the power conversion efficiency improves from 20% to close to 22% by enhancing the open-circuit voltage. Quasi-Fermi level splitting experiments and scanning electron microscopy cathodoluminescence hyperspectral imaging reveal that passivation provides a reduced nonradiative recombination at the interface between the perovskite and hole transport layer. Photoluminescence spectroscopy, angle-resolved grazing-incidence wide-angle X-ray scattering, and depth profiling X-ray photoelectron spectroscopy studies of the 2D/three-dimensional (3D) interface between the benzotriazole RPP and FAPbI₃ show that a nonuniform layer of 2D perovskites is enough to passivate defects, enhance charge extraction, and decrease nonradiative recombination.

Abstract:

Piezoelectric materials convert between mechanical and electrical energy and are a basis for self-powered electronics. Current piezoelectrics exhibit either large charge (d₃₃) or voltage (g₃₃) coefficients but not both simultaneously, and yet the maximum energy density for energy harvesting is determined by the transduction coefficient: d₃₃*g₃₃. In prior piezoelectrics, an increase in polarization usually accompanies a dramatic rise in the dielectric constant, resulting in trade off between d₃₃ and g₃₃. This recognition led us to a design concept: increase polarization through Jahn-Teller lattice distortion and reduce the dielectric constant using a highly confined 0D molecular architecture. With this in mind, we sought to insert a quasi-spherical cation into a Jahn-Teller distorted lattice, increasing the mechanical response for a large piezoelectric coefficient. We implemented this concept by developing EDABCO-CuCl₄ (EDABCO = N-ethyl-1,4-diazoniabicyclo[2.2.2]octonium), a molecular piezoelectric with a d₃₃ of 165 pm/V and g₃₃ of ~2110 × 10^-3 V m N^-1, one that achieved thusly a combined transduction coefficient of 348 × 10^-12 m³ J^-1. This enables piezoelectric energy harvesting in EDABCO-CuCl₄@PVDF (polyvinylidene fluoride) composite film with a peak power density of 43 µW/cm² (at 50 kPa), the highest value reported for mechanical energy harvesters based on heavy-metal-free molecular piezoelectric.

Abstract:

Perovskite solar cells represent an emerging and highly promising renewable energy technology. However, the most efficient perovskite solar cells critically depend on the use of lead. This represents a possible environmental concern potentially limiting the technologies' commercialization. Here, we demonstrate a facile recycling process for PbI₂, the most common lead-based precursor in perovskite absorber material. The process uses only hot water to effectively extract lead from synthetic precursor mixes, plastic- and glass-based perovskites (92.6 - 100% efficiency after two extractions). When the hot extractant is cooled, crystalline PbI₂ in high purity (> 95.9%) precipitated with a high yield: from glass-based perovskites, the first cycle of extraction / precipitation was sufficient to recover 94.4 ± 5.6% of Pb, whereas a second cycle yielded another 10.0 ± 5.2% Pb, making the recovery quantitative. The solid extraction residue remaining is consequently deprived of metals and may thus be disposed as non-hazardous waste. Therefore, exploiting the highly temperature-dependent solubility of PbI₂ in water provides a straightforward, easy to implement way to efficiently extract lead from PSC at the end-of-life and deposit the extraction residues in a cost-effective manner, mitigating the potential risk of lead leaching at the perovskites' end-of-life.

Abstract:

Hole-collecting monolayers have drawn attention in perovskite solar cell research due to their ease of processing, high performance, and good durability. Since molecules in the hole-collecting monolayer are typically composed of functionalized π-conjugated structures, hole extraction is expected to be more efficient when the π-cores are oriented face-on with respect to the adjacent surfaces. However, strategies for reliably controlling the molecular orientation in monolayers remain elusive. In this work, multiple phosphonic acid anchoring groups were used to control the molecular orientation of a series of triazatruxene derivatives chemisorbed on a transparent conducting oxide electrode surface. Using infrared reflection absorption spectroscopy and metastable atom electron spectroscopy, we found that multipodal derivatives align face-on to the electrode surface, while the monopodal counterpart adopts a more tilted configuration. The face-on orientation was found to facilitate hole extraction, leading to inverted perovskite solar cells with enhanced stability and high-power conversion efficiencies up to 23.0%.

Abstract:

Owing to the unique chemical and electronic properties arising from 3d‐electrons, substitution with transition metal ions is one of the key routes for engineering new functionalities into materials. While this approach has been used extensively in complex metal oxide perovskites, metal halide perovskites have largely resisted facile isovalent substitution. In this work, it is demonstrated that the substitution of Co2+ into the lattice of methylammonium lead triiodide imparts magnetic behavior to the material while maintaining photovoltaic performance at low concentrations. In addition to comprehensively characterizing its magnetic properties, the Co2+ ions themselves are utilized as probes to sense the local electronic environment of Pb in the perovskite, thereby revealing the nature of their incorporation into the material. A comprehensive understanding of the effect of transition metal incorporation is provided, thereby opening the substitution gateway for developing novel functional perovskite materials and devices for future technologies.