Infrared photodetection based on colloidal nanoparticles is a promising path toward low cost devices. However, mid-infrared absorption relies on interband transitions in heavy metal-based materials, which is a major flaw for the development toward mass market. In the quest of mercury-free infrared active colloidal materials, we here investigate Ag2Se nanoparticles presenting intraband transition between 3 and 15 µm. With photoemission and infrared spectroscopy, we are able to propose an electronic spectrum of the material in absolute energy scale. We also investigate the origin of doping and demonstrate that it results from a cation excess under Ag + form. We demonstrate photoconduction into this material under resonant excitation of the intraband transition. However, performances are currently quite weak with (i) a slow photoresponse (several seconds), and (ii) some electrochemical instabilities at room temperature.

Nanocrystals are promising materials for the design of low cost infrared detectors. Here we focus on HgTe colloidal quantum dots (CQDs) as an active material for detection in the extended shortwave infrared (2.5 µm as cutoff wavelength). In this paper, we propose a strategy to enhance the performances of previously reported photodiodes. In particular we integrate in this diode an unipolar barrier which role is to prevent the dark current injection to enhance the signal to noise ratio. We demonstrate that such unipolar barrier can be designed from another layer of HgTe CQDs with a wider band gap. Using a combination of IR spectroscopy and photoemission, we show that the barrier is resonant with the absorbing layer valence band, while presenting a clear offset with the conduction band. The combination of contacts with improved design and use of unipolar barrier allows us to reach a detectivity as high as 3•10 8 Jones at room temperature with 3 dB cut off frequency above 10 kHz.

Among semiconductor nanocrystals (NCs), 2D nanoplatelets (NPLs) are a special class of nanomaterials with well controlled optical features. So far most of the efforts have been focused on wide band gap materials such as cadmium chalcogenide semiconductors. However, optical absorption can be pushed toward the Infra-Red (IR) range using narrow band gap materials such as mercury chalcogenides. Here we demonstrate the feasibility of a core/shell structure made of a CdSe core with two HgSe external wells. We demonstrate that the optical spectrum of the heterostructure is set by the HgSe wells and this, despite the quasi type II band alignment which makes the band edge energy independent of the inner core thickness. On the other hand, these core/shell NPLs behave, from a transport point of view, as a wide band gap material. We demonstrate that the introduction of a wide band gap CdSe core makes the material less conductive and with a larger photoresponse. Hence the heterostructure presents an effective electric band gap wider than the optical band gap. This strategy will be of utmost interest to design infrared effective colloidal materials for which the reduction of the carrier density and the associated dark current is a critical property.

Lead halide perovskite nanocrystals have attracted attention in the field of nanocrystal based light emitting diodes and solar cells, because their devices showed high performances in only a few years. Among them, CsPbI3 is a promising candidate for solar cell design in spite of a too wide band gap and severe structural stability issue. Its hybrid organic-inorganic counterpart (NH2)2CHPbI3 (FAPI), where the Cs is replaced with formamidinium (FA), presents a smaller band gap and also an improved structural stability. Here, we have investigated the energy landscape of pristine FAPI, and the interface of FAPI with electron and hole selective layers using transport, photoemission, and non-contact surface photovoltage by means of time-resolved photoemission. We have found from transport and photoemission that its Fermi level is deeply positioned in the band gap, enabling the material to be almost intrinsic. Time-resolved photoemission has revealed that the interface of pristine FAPI is bended toward downward side, which is consistent with a ptype nature for the interface (ie hole as majority carrier). Using TiOx and MoOX contacts, as a model for the electron and hole transport layer, respectively, allows the electron transfer from the TiOx to the FAPI and from the FAPI to the MoOx. The latter is revealed by time-resolved photoemission showing inverted band bending for the two interfaces. From these results, we clearly present the energy landscape of FAPI and its interfaces with TiOx and MoOX in the dark and under illumination. These insights are of utmost interest for the future design of FAPI based solar cell.

Organic photovoltaics (OPVs) technology now offers power conversion efficiency (PCE) of over 18% and is one of the main emerging photovoltaic technologies. In such devices, titanium dioxide (TiO x ) has been vastly used as an electron extraction layer, typically showing unwanted charge-extraction barriers and the need for light-soaking. In the present work, using advanced photoemission spectroscopies, we investigate the electronic interplay at the interface between low-temperature-sputtered TiO x and C70 acceptor fullerene molecules. We show that defect states in the band gap of TiO x are quenched by C70 while an interfacial state appears. This new interfacial state is expected to support the favorable energy band alignment observed, showing a perfect match of transport levels, and thus barrier-free extraction of charges, making low-temperature-sputtered TiO x a good candidate for the next generation of organic solar cells.

Molecular thin films of N,N′-di-1-naphthalenyl-N,N′-diphenyl [1,1′:4′,1″:4″,1‴-quaterphenyl]-4,4‴-diamine (4P-NPD) have been demonstrated to function as efficient exciton blocking layers in organic solar cell devices, leading to improved device performance by minimizing exciton losses and by providing hole extraction selectivity. However, the exact mechanisms have been debated, as ultrathin thicknesses of less than 1 nm are required to observe optimized device performance improvements. In this work, we conduct photoelectron spectroscopy to gain information about core levels, HOMO/LUMO levels, and work functions for the hole extraction side of an organic solar cell device consisting of the small molecule tetraphenyldibenzoperiflanthene (DBP) as an electron donor and 4P-NPD for exciton blocking/hole extraction, the latter being in contact with the hole transport layer MoO x . Using in situ deposition and characterization, we demonstrate that a negative HOMO energy offset increases with 4P-NPD thickness on the DBP donor layer, which cannot account for the improvement observed in device performance. Investigation of the 4P-NPD/MoO x interface, on the other hand, reveals shifts of the electronic levels in 4P-NPD and a band alignment that favors hole extraction while blocking for exciton/electron leakage. This appealing behavior is enhanced for ultrathin 4P-NPD films of less than 1 nm. Thus, the exciton blocking/hole extraction behavior of 4P-NPD interlayers in organic solar cell devices is confirmed and understood from the detailed energy level alignment across both interfaces, as extracted from the in situ photoelectron spectroscopy studies.

Two coordination polymers of coinage metals with a rare pyridinium-betainoid L assembling ligand are reported. These polymers are obtained by self-assembly of the linker L and copper(I) or silver(I) ions in acetonitrile. The compounds were characterized by spectroscopic methods and by elemental analysis. The solid-state structures were unambiguously confirmed by single crystal diffraction studies. These assemblies exhibit original helicoidal arrangements. The UV-Vis. absorption and photoluminescence properties are reported as well.