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Principles Of Fluorescence Spectroscopy

We show nanoscale phase stabilization of CsPbI 3 quantum dots (QDs) to low temperatures that can be used as the active component of efficient optoelectronic devices. CsPbI 3 is an all-inorganic analog to the hybrid organic cation halide perovskites, but the cubic phase of bulk CsPbI 3 (α-CsPbI 3 )—the variant with desirable band gap—is only stable at high temperatures. We describe the formation of α-CsPbI 3 QD films that are phase-stable for months in ambient air. The films exhibit long-range electronic transport and were used to fabricate colloidal perovskite QD photovoltaic cells with an open-circuit voltage of 1.23 volts and efficiency of 10.77%. These devices also function as light-emitting diodes with low turn-on voltage and tunable emission.

http://science.sciencemag.org/content/sci/354/6308/92.full.pdf

Quantum dot-induced phase stabilization of α-CsPbI3 perovskite for high-efficiency photovoltaics.

The photovoltaics of organic–inorganic lead halide perovskite materials have shown rapid improvements in solar cell performance, surpassing the top efficiency of semiconductor compounds such as CdTe and CIGS (copper indium gallium selenide) used in solar cells in just about a decade. Perovskite preparation via simple and inexpensive solution processes demonstrates the immense potential of this thin-film solar cell technology to become a low-cost alternative to the presently commercially available photovoltaic technologies. Significant developments in almost all aspects of perovskite solar cells and discoveries of some fascinating properties of such hybrid perovskites have been made recently. This Review describes the fundamentals, recent research progress, present status, and our views on future prospects of perovskite-based photovoltaics, with discussions focused on strategies to improve both intrinsic and extrinsic (environmental) stabilities of high-efficiency devices. Strategies and challenges regardi...

Halide Perovskite Photovoltaics: Background, Status, and Future Prospects

Semiconducting lead halide perovskites (LHPs) have not only become prominent thin-film absorber materials in photovoltaics but have also proven to be disruptive in the field of colloidal semiconductor nanocrystals (NCs). The most important feature of LHP NCs is their so-called defect-tolerance—the apparently benign nature of structural defects, highly abundant in these compounds, with respect to optical and electronic properties. Here, we review the important differences that exist in the chemistry and physics of LHP NCs as compared with more conventional, tetrahedrally bonded, elemental, and binary semiconductor NCs (such as silicon, germanium, cadmium selenide, gallium arsenide, and indium phosphide). We survey the prospects of LHP NCs for optoelectronic applications such as in television displays, light-emitting devices, and solar cells, emphasizing the practical hurdles that remain to be overcome.

Properties and potential optoelectronic applications of lead halide perovskite nanocrystals

Lead halide perovskites (LHPs) in the form of nanometre-sized colloidal crystals, or nanocrystals (NCs), have attracted the attention of diverse materials scientists due to their unique optical versatility, high photoluminescence quantum yields and facile synthesis. LHP NCs have a 'soft' and predominantly ionic lattice, and their optical and electronic properties are highly tolerant to structural defects and surface states. Therefore, they cannot be approached with the same experimental mindset and theoretical framework as conventional semiconductor NCs. In this Review, we discuss LHP NCs historical and current research pursuits, challenges in applications, and the related present and future mitigation strategies explored.

/pdf/genesis-challenges-and-opportunities-for-colloidal-lead-3uh77e26ys.pdf

Genesis, challenges and opportunities for colloidal lead halide perovskite nanocrystals

Exciton many-body interaction is the fundamental light–matter interaction that determines the optical response of the new class of colloidal perovskite nanocrystals of the general formula CsPbX3 [X = Cl, Br, or I]. However, the understanding of exciton many-body interactions manifested through the transient biexcitonic Stark effect at the early time scales and the Auger recombination process in this new class of materials still remains rather incomplete. In this Article, we studied the many-body exciton interactions under controlled conditions through ultrafast transient absorption spectroscopy. A large biexcitonic redshift ∼30 meV to the effect of hot excitations on the excitonic resonance is observed at the early time scales. From the fluence-dependent studies, it is evident that the samples have only single and biexciton lifetimes, suggesting that the band edges are 2-fold degenerate. This explicit experimental evidence for the exciton many-body interactions in CsPbBr3 nanocrystals provides a powerful ...

Ultrafast Exciton Dynamics in Colloidal CsPbBr3 Perovskite Nanocrystals: Biexciton Effect and Auger Recombination

Long term stability of the black perovskite phase of CsPbI3 nanocrystals under ambient conditions is an important challenge for their optoelectronic applications in real life. The nanocrystalline size is found to improve the stability of the black phase at room temperature. Furthermore, doping Mn is proposed to improve the stability of the black perovskite phase of CsPbI3 nanocrystals (NCs). However, the undoped and Mn-doped CsPbI3 NCs are prepared in different batches under somewhat different synthesis conditions thus obliterating the role of Mn in the stability of the black phase of CsPbI3 NCs. Here, we elucidate the effect of Mn doping on the surface and lattice energy of CsPbI3 NCs, stabilizing the black phase. For this purpose, we employ a postsynthesis doping strategy which has an advantage that the initial host remains the same for both undoped and Mn-doped samples. Uncertainties in the size/shape, surface energy, and structure through direct synthesis of undoped and Mn-doped NCs in different batches can be neglected in our postsynthesis doping strategy, allowing us to study the effect of dopants in a more controlled manner. Our postsynthesis Mn-doping in CsPbI3 NCs shows that the black phase stability under ambient conditions improves from few days for the undoped sample to nearly a month's time for the Mn-doped sample. We found that though surface passivation with a dopant precursor improves both colloidal and phase stability of black CsPbI3 NCs, it is the contraction of the lattice upon Mn-doping that mainly stabilizes the films of black phase CsPbI3 NCs. Similarly, we found that Mn-doped CsPbBr3 NCs show improved ambient stability of photoluminescence compared to the undoped sample.

Postsynthesis Mn-doping in CsPbI3 nanocrystals to stabilize the black perovskite phase

Green photoluminescence (PL) from CsPbBr3 nanocubes (∼11 nm edge-length) exhibits a high quantum yield (>80%), narrow spectral width (∼85 meV), and high reproducibility, along with a high molar extinction coefficient (3.5 × 10(6) M(-1) cm(-1)) for lowest energy excitonic absorption. In order to obtain these combinations of excellent properties for blue (PL peak maximum, λ max 80%) was also maintained, but the spectral width increased and became asymmetric for blue emitting CsPbBr3 nanocubes. Furthermore, PL was unstable and irreproducible for samples with λ max ∼ 460 nm, exhibiting multiple features in the PL. These problems arise because smaller (<7 nm) CsPbBr3 nanocubes have a tendency to form nanoplatelets and nanorods, eventually yielding inhomogeneity in the shape and size of blue-emitting nanocrystals. Reaction conditions were then modified achieving nanoplatelets, with strong quantum confinement along the thickness of the platelets, yielding blue emission. But inhomogeneity in the thickness of the nanoplatelets again broadens the PL compared to green-emitting CsPbBr3 nanocubes. Therefore, unlike high quality green emitting CsPbBr3 nanocubes, blue emitting CsPbBr3 nanocrystals of any shape need to be improved further.

https://iopscience.iop.org/article/10.1088/0957-4484/27/32/325708/pdf

Excellent green but less impressive blue luminescence from CsPbBr3 perovskite nanocubes and nanoplatelets.

High-performance and high-reliability photodiodes are demonstrated by using α-CsPbI3 perovskite nanocrystals (NCs) phase-stabilized with a low-temperature solution-treated active layer. In addition to the high charge mobility, the high absorption coefficient, and IR-blind characteristics of the perovskite material, the defect-tolerant nature of α-CsPbI3 perovskite NCs are combined to realize hysteresis-free and high detectivity photodiodes. To further minimize interface defects originating from multi-layer photodiode construction, a poly(3-hexylthiophene) layer is strategically introduced as a passivation and electron blocking layer, resulting in a low diode ideality factor of 1.5 and a noise equivalent power of 1.6 × 10−13 W Hz−0.5. As a result, high detectivity of 1.8 × 1012 Jones is demonstrated with near-zero hysteresis. Furthermore, the optimized photodiode exhibits excellent stability under high humidity conditions owing to the intrinsic nature of the defect-tolerant α-CsPbI3 perovskite NCs.

Phase Stabilized α-CsPbI3 Perovskite Nanocrystals for Photodiode Applications

Metal chalcogenide perovskites were proposed as potential solar cell material in 2015. The theoretical maximum solar cell efficiencies of some chalcogenide perovskites are ∼30%, similar to CH3NH3Pb...

Abhishek Swarnkar

Papers

Ultrafast Exciton Dynamics in Colloidal CsPbBr3 Perovskite Nanocrystals: Biexciton Effect and Auger Recombination

Postsynthesis Mn-doping in CsPbI3 nanocrystals to stabilize the black perovskite phase

Excellent green but less impressive blue luminescence from CsPbBr3 perovskite nanocubes and nanoplatelets.

Phase Stabilized α-CsPbI3 Perovskite Nanocrystals for Photodiode Applications

Are Chalcogenide Perovskites an Emerging Class of Semiconductors for Optoelectronic Properties and Solar Cell