Computational Study of Halide Perovskite-Derived A2_2BX6_6 Inorganic Compounds: Chemical Trends in Electronic Structure and Structural Stability

The electronic structure and energetic stability of A$_2$BX$_6$ halide compounds with the cubic and tetragonal variants of the perovskite-derived K$_2$PtCl$_6$ prototype structure are investigated computationally within the frameworks of density-functional-theory (DFT) and hybrid (HSE06) functionals. The HSE06 calculations are undertaken for seven known A$_2$BX$_6$ compounds with A = K, Rb and Cs, and B = Sn, Pd, Pt, Te, and X = I. Trends in band gaps and energetic stability are identified, which are explored further employing DFT calculations over a larger range of chemistries, characterized by A = K, Rb, Cs, B = Si, Ge, Sn, Pb, Ni, Pd, Pt, Se and Te and X = Cl, Br, I. For the systems investigated in this work, the band gap increases from iodide to bromide to chloride. Further, variations in the A site cation influences the band gap as well as the preferred degree of tetragonal distortion. Smaller A site cations such as K and Rb favor tetragonal structural distortions, resulting in a slightly larger band gap. For variations in the B site in the (Ni, Pd, Pt) group and the (Se, Te) group, the band gap increases with increasing cation size. However, no observed chemical trend with respect to cation size for band gap was found for the (Si, Sn, Ge, Pb) group. The findings in this work provide guidelines for the design of halide A$_2$BX$_6$ compounds for potential photovoltaic applications

Advances in Perovskite Optoelectronics: Bridging the Gap Between Laboratory and Fabrication

In 2019, hybrid halide perovskites celebrated their 10th anniversary as a "wonder material" for optoelectronic applications. Although the parent perovskite structures were elucidated in the late 19th century, the seminal work by Miyasaka et al. exploiting organic‐inorganic hybrid halide perovskites sensitizers for visible‐light conversion in solar cells marked the revisit of these materials and has proven to be a game‐changer in this field. Extensive investigations were undertaken to develop new materials (all inorganic and organic‐inorganic hybrids, in the form of films or alternate morphologies) and deposition techniques, explore interfaces and in‐depth characterization, while engineering devices and testing methods for optimum results. Within a short time span, the power conversion efficiency (PCE) of single‐junction and tandem perovskite solar cells (PSCs) have exceeded 25% and 29% respectively; thus challenging the dominance of silicon solar cells. Building‐integrated photovoltaics (BIPV) is another hot topic in PSCs, where perovskite solar cells are designed to be semi‐transparent for deployment in residential or office building facades. Along with the success in photovoltaics, halide perovskites have also made their impact in light emission, lasing, imaging, spintronics, memristors, and photocatalysis. However, key challenges still lie ahead, particularly on the commercialization of perovskite devices. Poor material and device stability under operational conditions and the lack of reproducibility and scalability have remained problematic; whereas the search for suitable lead‐free perovskites continues

Applications of vacuum vapor deposition for perovskite solar cells: A progress review

Metal halide perovskite solar cells (PSCs) have made substantial progress in power conversion efficiency (PCE) and stability in the past decade thanks to the advancements in perovskite deposition methodology, charge transport layer (CTL) optimization, and encapsulation technology. Solution-based methods have been intensively investigated and a 25.7% certified efficiency has been achieved. Vacuum vapor deposition protocols were less studied, but have nevertheless received increasing attention from industry and academia due to the great potential for large-area module fabrication, facile integration with tandem solar cell architectures, and compatibility with industrial manufacturing approaches. In this article, we systematically discuss the applications of several promising vacuum vapor deposition techniques, namely thermal evaporation, chemical vapor deposition (CVD), atomic layer deposition (ALD), magnetron sputtering, pulsed laser deposition (PLD), and electron beam evaporation (e-beam evaporation) in the fabrication of CTLs, perovskite absorbers, encapsulants, and connection layers for monolithic tandem solar cells

Hot carrier extraction in CH3NH3PbI3 unveiled by pump-push-probe spectroscopy

Halide perovskites are promising materials for development in hot carrier (HC) solar cells, where the excess energy of above-bandgap photons is harvested before being wasted as heat to enhance device efficiency. Presently, HC separation and transfer processes at higher-energy states remain poorly understood. Here, we investigate the excited state dynamics in CH3NH3PbI3 using pump-push-probe spectroscopy. It has its intrinsic advantages for studying these dynamics over conventional transient spectroscopy, albeit complementary to one another. By exploiting the broad excited-state absorption characteristics, our findings reveal the transfer of HCs from these higher-energy states into bathophenanthroline (bphen), an energy selective organic acceptor far above perovskite's band edges. Complete HC extraction is realized only after overcoming the interfacial barrier formed at the heterojunction, estimated to be between 1.01 and 1.08 eV above bphen's lowest unoccupied molecular orbital level. The insights gained here are essential for the development of a new class of optoelectronics

Highly Efficient Thermally Co-evaporated Perovskite Solar Cells and Mini-modules

The rapid improvement in the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has prompted interest in bringing the technology toward commercialization. Capitalizing on existing industrial processes facilitates the transition from laboratory to production lines. In this work, we prove the scalability of thermally co-evaporated MAPbI3 layers in PSCs and mini-modules. With a combined strategy of active layer engineering, interfacial optimization, surface treatments, and light management, we demonstrate PSCs (0.16 cm2 active area) and mini-modules (21 cm2 active area) achieving record PCEs of 20.28% and 18.13%, respectively. Un-encapsulated PSCs retained ∼90% of their initial PCE under continuous illumination at 1 sun, without sample cooling, for more than 100 h. Looking toward tandem and building integrated photovoltaic applications, we have demonstrated semi-transparent mini-modules and colored PSCs with consistent PCEs of ∼16% for a set of visible colors. Our work demonstrates the compatibility of perovskite technology with industrial processes and its potential for next-generation photovoltaics

Coherent Spin and Quasiparticle Dynamics in Solution-Processed Layered 2D Lead Halide Perovskites.

Layered 2D halide perovskites with their alternating organic and inorganic atomic layers that form a self-assembled quantum well system are analogues of the purely inorganic 2D transition metal dichalcogenides. Within their periodic structures lie a hotbed of photophysical phenomena such as dielectric confinement effect, optical Stark effect, strong exciton-photon coupling, etc. Detailed understanding into the strong light-matter interactions in these hybrid organic-inorganic semiconductor systems remains modest. Herein, the intricate coherent interplay of exciton, spin, and phonon dynamics in (C6H5C2H4NH3)2PbI4 thin films using transient optical spectroscopy is explicated. New insights into the hotly debated origins of transient spectral features, relaxation pathways, ultrafast spin relaxation via exchange interaction, and strong coherent exciton-phonon coupling are revealed from the detailed phenomenological modeling. Importantly, this work unravels the complex interplay of spin-quasiparticle interactions in these layered 2D halide perovskites with large spin-orbit coupling

Signatures de l'injection optique et électrique de charges dans des monocristaux de rubrene

Thèse en anglaisOrganic single crystals are of particular fundamental interest as tools in probing the intrinsic electrical properties and the upper limit of performance for a given organic semiconducting molecule devoid of disorder. Rubrene single crystals are of particular interest in the field of organic electronics due to the high levels of charge carrier mobilities measured in transistors constructed of the same. In this thesis, we explore the properties of rubrene single crystal transistors. The photocurrent properties of rubrene single crystals are measured in ‘air-gap' transistors whose unique structure allows the measurement of photocarrier dynamics without the influence of a dielectric that can act as a source for traps. This structure has allowed us to identify phenomenon like persistent photoconductivity associated with the creation of oxygen related traps on the rubrene surface. Transient studies of the photocurrent reveal the presence of bimolecular recombination of the charge carriers. In addition, we have also performed optical spectroscopy studies including Raman spectra measurements which revealed the presence of endoperoxide related signature on the surface of the crystal while also confirming the low levels of intermolecular coupling present between the molecules. We have also explored the extrinsic factors that determine the surface conductivity of the rubrene crystal, particularly the presence of oxide related compounds on the surface of the crystal using XPS and photoluminescence measurements. The impact of photo-oxidation of the rubrene crystal on the surface conductivities were evaluated by a novel experiment involving the gradual photo-oxidation of the rubrene surface using a focussed laser. The creation of a deep acceptor state that can trap electrons indicates that the electrical properties of the rubrene surface like high unipolar p-type II conductivity and photoconductivity may be modulated by the presence of these oxygen induced states

The origin of high efficiency in low-temperature solution-processable bilayer organometal halide hybrid solar cells

This work reports a study into the origin of the high efficiency in solution-processable bilayer solar cells based on methylammonium lead iodide (CH3NH3PbI3) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). Our cell has a power conversion efficiency (PCE) of 5.2% under simulated AM 1.5G irradiation (100 mW cm−2) and an internal quantum efficiency of close to 100%, which means that nearly all the absorbed photons are converted to electrons and are efficiently collected at the electrodes. This implies that the exciton diffusion, charge transfer and charge collection are highly efficient. The high exciton diffusion efficiency is enabled by the long diffusion length of CH3NH3PbI3 relative to its thickness. Furthermore, the low exciton binding energy of CH3NH3PbI3 implies that exciton splitting at the CH3NH3PbI3/PC61BM interface is very efficient. With further increase in CH3NH3PbI3 thickness, a higher PCE of 7.4% could be obtained. This is the highest efficiency attained for low temperature solution-processable bilayer solar cells to date

Signatures optique et électronique de l'injection de charge dans un monocristal de rubrene

Au cours de cette thèse nous avons exploré les propriétés d un monocristal de rubrene dans une configuration de type transistor. Les mesures ont été réalisées dans une configuration à "gap d'air" qui permet la caractérisation des transporteurs de charge en mode dynamique sans subir l'influence d'un milieu diélectrique. Nous avons identifié différents phénomènes comme la persistance de la photoconductivité associée à la création du piégeage d'oxygène en surface ainsi que la présence d'une recombinaison bimolecular des transporteurs de charges. Les spectroscopies optique et Raman ont permis de révéler la présence de peroxyde en surface et ont montré le faible couplage intermoléculaire. L'impact de la photo-oxydation du rubrene sur les propriétés de transport a été déterminé en utilisant une nouvelle expérience de photooxydation graduelle sous rayonnement laser. La création d'un état accepteur qui permet le piégeage d électrons indique que le caractère de conduction de type p hautement unipolaire et la photoconductivité de surface du rubrene sont modulés par la présence de l'oxygènePARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF