SbSeI and SbSeBr micro-columnar solar cells by a novel high pressure-based synthesis process.

Van der Waals chalcogenides and chalcohalides have the potential to become the next thin film PV breakthrough, owing to the earth-abundancy and non-toxicity of their components, and their stability, high absorption coefficient and quasi-1D structure, which leads to enhanced electrical anisotropic properties when the material is oriented in a specific crystalline direction. However, quasi-1D semiconductors beyond Sb2(S,Se)3, such as SbSeX chalcohalides, have been scarcely investigated for energy generation applications, and rarely synthesised by physical vapor deposition methodologies, despite holding the promise of widening the bandgap range (opening the door to tandem or semi-transparent devices), and showing enticing new properties such as ferroelectric behaviour and defect-tolerant nature. In this work, SbSeI and SbSeBr micro-columnar solar cells have been obtained for the first time by an innovative methodology based on the selective halogenation of Sb2Se3 thin films at pressure above 1 atm. It is shown that by increasing the annealing temperature and pressure, the height and density of the micro-columnar structures grows monotonically, resulting in SbSeI single-crystal columns up to 30 µm, and tuneable morphology. In addition, solar cell prototypes with substrate configuration have shown remarkable Voc values above 550 mV and 1.8 eV bandgap.

High broadband light absorption in ultrathin MoS2 homojunction solar cells.

Transition metal dichalcogenides (TMDCs) have been proposed as light absorber materials for ultrathin solar cells. These materials are characterized by their strong light-matter interaction and the possibility to be assembled into devices at room temperature. Here, we model the optical absorptance of an ultrathin MoS2 absorber embedded in different designs of a 1D optical cavity. We find that up to 87% of the photons contained in the 300-700 nm range of the AM1.5G spectrum can be absorbed employing MoS2 absorbers as thin as 10 nm sandwiched between a h-BN top layer and an optically thick Ag reflector. An h-BN/MoS2/h-BN/Ag cavity produces 0.89 average absorptance for a 57-nm-thick MoS2 slab and it also maximizes the absorption of extremely thin absorbers, between 1 and 9 nm. We also model a possible large-scale device on a glass substrate combined with indium-tin oxide (ITO) whose absorptance is comparable to the other presented structures. The high broadband absorption in these light-trapping structures is caused by the amplification of the zeroth Fabry-Perot interference mode. This study demonstrates that light absorption in ultrathin solar cells based on nanometric TMDC absorbers can compete with conventional photovoltaic technology and provides different simple optical designs to choose from depending on the electronic characteristics of the TMDC junction.

Fluorescence and Electroluminescence of J-Aggregated Polythiophene Monolayers on Hexagonal Boron Nitride.

The photophysics of a semiconducting polymer is manipulated through molecular self-assembly on an insulating surface. Adsorption of polythiophene (PT) monolayers on hexagonal boron nitride (hBN) leads to a structurally induced planarization and a rebalancing of inter- and intrachain excitonic coupling. This conformational control results in a dominant 0-0 photoluminescence peak and a reduced Huang-Rhys factor, characteristic of J-type aggregates, and optical properties which are significantly different to both PT thin films and single polymer strands. Adsorption on hBN also provides a route to explore electroluminescence from PT monolayers though incorporation into hybrid van der Waals heterostructures whereby the polymer monolayer is embedded within a hBN tunnel diode. In these structures we observe up-converted singlet electroluminescence from the PT monolayer, with an excitation mechanism based upon inelastic electron scattering. We argue that surface adsorption provides a methodology for the study of fundamental optoelectronic properties of technologically relevant polymers.

Triplet Excitation and Electroluminescence from a Supramolecular Monolayer Embedded in a Boron Nitride Tunnel Barrier.

We show that ordered monolayers of organic molecules stabilized by hydrogen bonding on the surface of exfoliated few-layer hexagonal boron nitride (hBN) flakes may be incorporated into van der Waals heterostructures with integral few-layer graphene contacts forming a molecular/two-dimensional hybrid tunneling diode. Electrons can tunnel through the hBN/molecular barrier under an applied voltage VSD, and we observe molecular electroluminescence from an excited singlet state with an emitted photon energy hν > eVSD, indicating upconversion by energies up to ∼1 eV. We show that tunneling electrons excite embedded molecules into singlet states in a two-step process via an intermediate triplet state through inelastic scattering and also observe direct emission from the triplet state. These heterostructures provide a solid-state device in which spin-triplet states, which cannot be generated by optical transitions, can be controllably excited and provide a new route to investigate the physics, chemistry, and quantum spin-based applications of triplet generation, emission, and molecular photon upconversion.

Optically triggered infrared photodetector.

We demonstrate a new class of semiconductor device: the optically triggered infrared photodetector (OTIP). This photodetector is based on a new physical principle that allows the detection of infrared light to be switched ON and OFF by means of an external light. Our experimental device, fabricated using InAs/AlGaAs quantum-dot technology, demonstrates normal incidence infrared detection in the 2-6 µm range. The detection is optically triggered by a 590 nm light-emitting diode. Furthermore, the detection gain is achieved in our device without an increase of the noise level. The novel characteristics of OTIPs open up new possibilities for third generation infrared imaging systems ( Rogalski, A.; Antoszewski, J.; Faraone, L. J. Appl. Phys. 2009, 105 (9), 091101).

Self-organized colloidal quantum dots and metal nanoparticles for plasmon-enhanced intermediate-band solar cells.

A colloidal deposition technique is presented to construct long-range ordered hybrid arrays of self-assembled quantum dots and metal nanoparticles. Quantum dots are promising for novel opto-electronic devices but, in most cases, their optical transitions of interest lack sufficient light absorption to provide a significant impact in their implementation. A potential solution is to couple the dots with localized plasmons in metal nanoparticles. The extreme confinement of light in the near-field produced by the nanoparticles can potentially boost the absorption in the quantum dots by up to two orders of magnitude.In this work, light extinction measurements are employed to probe the plasmon resonance of spherical gold nanoparticles in lead sulfide colloidal quantum dots and amorphous silicon thin-films. Mie theory computations are used to analyze the experimental results and determine the absorption enhancement that can be generated by the highly intense near-field produced in the vicinity of the gold nanoparticles at their surface plasmon resonance.The results presented here are of interest for the development of plasmon-enhanced colloidal nanostructured photovoltaic materials, such as colloidal quantum dot intermediate-band solar cells.