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- Ji, Fuxiang, 1991-
(författare)
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Bandgap Engineering of Lead-Free Halide Double Perovskites
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Lead-free halide double perovskites (HDPs, A2BIBIIIX6) with attractive optical and electronic features are regarded as one of the most promising alternatives to overcome the toxicity and stability issues of lead halide perovskites. They provide a wide range of possible combinations and rich substitutional chemistry with interesting properties for various optoelectronic devices. However, the performance of state-of-the-art lead-free HDPs is not yet comparable to that of lead halide perovskites, especially in the photovoltaic field. One of the main reasons for this is that HDPs usually have large and/or indirect bandgaps, which limit their optical and optoelectronic properties in the visible and infrared region. In this thesis, we attempt to modify the bandgap and optical properties of HDPs using metal doping/alloying and crystallization control, as well as provide detailed understanding of the alloying at the atomic level. We also observe significant changes of the bandgap of HDPs at different temperatures (i.e., thermochromism) and uncover the reasons behind it. We first adopt the metal doping/alloying strategy to alter the absorption properties of benchmark HDPs Cs2AgBiBr6. By introducing Cu as the dopant in Cs2AgBiBr6, we significantly broaden the absorption edge from around 610 nm to around 860 nm. Systematic characterizations indicate that Cu doping introduces defect states (sub-bandgap states) in the bandgap, without changing the bandgap of Cs2AgBiBr6. Interestingly, these sub-bandgaps can generate considerable amount of band carriers upon optical excitation, making these double perovskites promising for near-infrared light detection. In parallel with the material modification using the metal doping/alloying strategy, the fundamental understanding of these doped/alloyed double perovskite is also of critical importance. In the second paper, we reveal the atomic-level structure of alloyed double perovskites by presenting a series of double perovskite alloys with the chemical formula Cs2AgIn1-xFexCl6 (x = 0-1) showing tunable bandgaps in the range of 2.8-1.6 eV. Our results show that Fe3+ substitutes In3+ in the lattice with the formation of [FeCl6]3−·[AgCl6]5− domains, which grow larger gradually as the Fe3+ concentration increases. It is noted that these domains could be further connected to form microscopically segregated Fe3+-rich phases in the double perovskite alloys. To narrow the bandgap of Cs2AgBiBr6, we also develop a crystallization control approach, where high temperature is employed to assist the single crystal growth. By simply increasing the crystal growth temperature from 60 oC to 150 oC, the bandgap of Cs2AgBiBr6 crystals can be reduced from 1.98 eV to 1.72 eV, which is the lowest reported bandgap for Cs2AgBiBr6 at ambient conditions. The underlying reason is hypothesized to be related to the increased level of Ag–Bi disorder in the crystal structure. Lastly, we observe an interesting reversible thermochromic behavior in HDPs Cs2NaFeCl6. Specifically, the optical bandgap of Cs2NaFeCl6 is reduced from 2.06 eV to 1.86 eV when the temperature increases from RT to 150 oC and turns back to its original value after cooling. Meanwhile, we observe lattice expansion during the heating/ cooling process without phase transition. Our first-principles calculation indicates that the underlying mechanism for the thermochromic phenomenon in Cs2NaFeCl6 is mainly related to the electron-phonon coupling. Although the development of HDPs is in its early stages, we believe that HDPs with impressive optical and electronic properties and rich substitutional chemistry have a bright future in optoelectronic and multifunctional applications. Our findings shed new light to the absorption and bandgap modulation of HDPs and provide new insights into the atomic-level structures of DPAs, which can help to develop efficient optoelectronic devices.
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| 2. |
- Ji, Fuxiang, et al.
(författare)
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The atomic-level structure of bandgap engineered double perovskite alloys Cs2AgIn1-xFexCl6
- 2021
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Ingår i: Chemical Science. - : Royal Society of Chemistry. - 2041-6520 .- 2041-6539. ; 12:5, s. 1730-1735
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Tidskriftsartikel (refereegranskat)abstract
- Although lead-free halide double perovskites are considered as promising alternatives to lead halide perovskites for optoelectronic applications, state-of-the-art double perovskites are limited by their large bandgap. The doping/alloying strategy, key to bandgap engineering in traditional semiconductors, has also been employed to tune the bandgap of halide double perovskites. However, this strategy has yet to generate new double perovskites with suitable bandgaps for practical applications, partially due to the lack of fundamental understanding of how the doping/alloying affects the atomic-level structure. Here, we take the benchmark double perovskite Cs2AgInCl6 as an example to reveal the atomic-level structure of double perovskite alloys (DPAs) Cs2AgIn1-xFexCl6 (x = 0-1) by employing solid-state nuclear magnetic resonance (ssNMR). The presence of paramagnetic alloying ions (e.g. Fe3+ in this case) in double perovskites makes it possible to investigate the nuclear relaxation times, providing a straightforward approach to understand the distribution of paramagnetic alloying ions. Our results indicate that paramagnetic Fe3+ replaces diamagnetic In3+ in the Cs2AgInCl6 lattice with the formation of [FeCl6](3-)center dot[AgCl6](5-) domains, which show different sizes and distribution modes in different alloying ratios. This work provides new insights into the atomic-level structure of bandgap engineered DPAs, which is of critical significance in developing efficient optoelectronic/spintronic devices.
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