Flexible array of high performance and stable formamidinium-based low-n 2D halide perovskite photodetectors for optical imaging

International audienc

Evolution of Structure and Optoelectronic Properties During Halide Perovskite Vapor Deposition

The efficiency of perovskite-based solar cells has increased dramatically over the past decade to as high as 25%, making them very attractive for commercial use. Vapor deposition is a promising technique that potentially enables fabrication of perovskite solar cells on large areas. However, to implement a large-scale deposition method, understanding and controlling the specific growth mechanisms are essential for the reproducible fabrication of high-quality layers. Here, we study the structural and optoelectronic kinetics of MAPbI$_3$, employing in-situ photoluminescence (PL) spectroscopy and grazing-incidence small/wide-angle X-ray scattering (GI-SAXS/WAXS) simultaneously during perovskite vapor deposition. Such a unique combination of techniques reveals MAPbI$_3$ formation from the early stages and uncovers the morphology, crystallographic structure, and defect density evolution. Furthermore, we show that the nonmonotonous character of PL intensity contrasts with the increasing volume of the perovskite phase during the growth, although bringing valuable information about the presence of defect states

Early-stage growth observations of orientation-controlled vacuum-deposited naphthyl end-capped oligothiophenes

We report on the real-time structure formation and growth of two thiophene-based organic semiconductors, 5,5′-bis(naphth-2-yl)-2,2′-bi- and 5,5''-bis(naphth-2-yl)-2,2′:5′,2''-terthiophene (NaT2 and NaT3), studied in situ during vacuum deposition by grazing-incidence x-ray diffraction and supported by atomic force microscopy and photoabsorption spectroscopy measurements on corresponding ex situ samples. On device-relevant silicon dioxide substrates, for both molecules the growth is observed to transition from two-dimensional (2D) layer-by-layer growth to three-dimensional (3D) growth after the formation of a few-molecule-thick wetting layer. The crystal structure of the NaT2 film is considerably more ordered than the NaT3 counterpart, and there is a significant collective change in the unit cell during the initial stage of growth, indicating strain relief from substrate induced strain as the growth transitions from two to three dimensions. In addition, the orientation of the film molecules are controlled by employing substrates of horizontally and vertically oriented few-layer molybdenum disulfide. Both molecules form needle-like crystals on horizontally oriented MoS$_2$ layers, while the NaT3 molecules form tall, isolated islands on vertically oriented MoS$_2$ layers. The molecules are standing on silicon dioxide and on vertically oriented MoS$_2$, but lying flat on horizontally oriented MoS$_2$. These results demonstrate the importance of film-substrate interactions on the thin-film growth and microstructure formation in naphthyl-terminated oligothiophenes

Early-stage growth observations of orientation-controlled vacuum-deposited naphthyl end-capped oligothiophenes

We report on the real-time structure formation and growth of two thiophene-based organic semiconductors, 5,5′-bis(naphth-2-yl)-2,2′-bi- and 5,5''-bis(naphth-2-yl)-2,2′:5′,2''-terthiophene (NaT2 and NaT3), studied in situ during vacuum deposition by grazing-incidence x-ray diffraction and supported by atomic force microscopy and photoabsorption spectroscopy measurements on corresponding ex situ samples. On device-relevant silicon dioxide substrates, for both molecules the growth is observed to transition from two-dimensional (2D) layer-by-layer growth to three-dimensional (3D) growth after the formation of a few-molecule-thick wetting layer. The crystal structure of the NaT2 film is considerably more ordered than the NaT3 counterpart, and there is a significant collective change in the unit cell during the initial stage of growth, indicating strain relief from substrate induced strain as the growth transitions from two to three dimensions. In addition, the orientation of the film molecules are controlled by employing substrates of horizontally and vertically oriented few-layer molybdenum disulfide. Both molecules form needle-like crystals on horizontally oriented MoS$_2$ layers, while the NaT3 molecules form tall, isolated islands on vertically oriented MoS$_2$ layers. The molecules are standing on silicon dioxide and on vertically oriented MoS$_2$, but lying flat on horizontally oriented MoS$_2$. These results demonstrate the importance of film-substrate interactions on the thin-film growth and microstructure formation in naphthyl-terminated oligothiophenes

Reorientation of π-conjugated molecules on few-layer MoS2_2 films

Small π-conjugated organic molecules have attracted substantial attention in the past decade as they are considered as candidates for future organic-based (opto-)electronic applications. The molecular arrangement in the organic layer is one of the crucial parameters that determine the efficiency of a given device. The desired orientation of the molecules is achieved by a proper choice of the underlying substrate and growth conditions. Typically, one underlying material supports only one inherent molecular orientation at its interface. Here, we report on two different orientations of diindenoperylene (DIP) molecules on the same underlayer, i.e. on a few-layer MoS$_2$ substrate. We show that DIP molecules adopt a lying-down orientation when deposited on few-layer MoS$_2$ with horizontally oriented layers. In contrast, for vertically aligned MoS$_2$ layers, DIP molecules are arranged in a standing-up manner. Employing in situ and real-time grazing-incidence wide-angle X-ray scattering (GIWAXS), we monitored the stress evolution within the thin DIP layer from the early stages of the growth, revealing different substrate-induced phases for the two molecular orientations. Our study opens up new possibilities for the next-generation of flexible electronics, which might benefit from the combination of MoS2 layers with unique optical and electronic properties and an extensive reservoir of small organic molecules

Novel highly substituted thiophene-based n-type organic semiconductor: structural study, optical anisotropy and molecular control

Oligothiophenes and their functionalized derivatives have been shown to be a viable option for high-performance organic electronic devices. The functionalization of oligothiophene-based materials allows further tailoring of their properties for specific applications. We have synthesized a new thiophene-based molecule 1-[5′-(2-naphthyl)-2,2′-bithiophen-5-yl]hexan-1-one (NCOH), and we have studied the optical and structural properties of NCOH thin films. NCOH is a highly substituted member of the oligothiophene family, designed to improve its molecular stacking, where the presence of an electron-withdrawing group enhances its electron transport capabilities. Employing in situ and time-resolved grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements, we determined the NCOH thin film crystallographic structure and its evolution starting from the early stages of the film growth. We observed strong optical anisotropy resulting from a highly oriented crystallographic structure. Additionally, we investigated the substrate-induced changes of the molecular orientation utilizing the few-layer MoS$_2$ with different orientations of the atomic layers. This study, with its primary focus on the fundamentally important n-type molecular semiconductor, contributes to the field of organic-based (opto-)electronics