Andrea Zampetti

Inorganic caesium lead iodide perovskite solar cells

The vast majority of perovskite solar cell research has focused on organic–inorganic lead trihalide perovskites. Herein, we present working inorganic CsPbI3 perovskite solar cells for the first time. CsPbI3 normally resides in a yellow non-perovskite phase at room temperature, but by careful processing control and development of a low-temperature phase transition route we have stabilised the material in the black perovskite phase at room temperature. As such, we have fabricated solar cell devices in a variety of architectures, with current–voltage curve measured efficiency up to 2.9% for a planar heterojunction architecture, and stabilised power conversion efficiency of 1.7%. The well-functioning planar junction devices demonstrate long-range electron and hole transport in this material. Importantly, this work identifies that the organic cation is not essential, but simply a convenience for forming lead triiodide perovskites with good photovoltaic properties. We additionally observe significant rate-dependent current–voltage hysteresis in CsPbI3 devices, despite the absence of the organic polar molecule previously thought to be a candidate for inducing hysteresis via ferroelectric polarisation. Due to its space group, CsPbI3 cannot be a ferroelectric material, and thus we can conclude that ferroelectricity is not required to explain current–voltage hysteresis in perovskite solar cells. Our report of working inorganic perovskite solar cells paves the way for further developments likely to lead to much more thermally stable perovskite solar cells and other optoelectronic devices.

Modeling and simulation of energetically disordered organic solar cells

The aim of this work is to present a consistent model for simulation of organic solar cells (OPV) with a correct description of mobility, density of state, organic-metal contacts, and exciton. We simulate the photoconversion by means of an integration of the optical and electrical part: light absorption is calculated with a Transfer Matrix Model and the charge transport is computed using Drift Diffusion approach including the effect of energetically disorder materials. Most model parameters are directly taken from experiment. The model is used to study the effect of energetic disordered materials and cell thickness on the performance of the cell in terms of short circuit current, open circuit voltage, and fill factor. Based on the results of this model, it will be possible to design and predict the optimal thickness of OPV toward higher efficiencies.

Highly Efficient Solid-State Near-infrared Organic Light-Emitting Diodes incorporating A-D-A Dyes based on α,β -unsubstituted “BODIPY” Moieties

We take advantage of a recent breakthrough in the synthesis of α,β-unfunctionalised 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) moieties, which we symmetrically conjugate with oligothienyls in an unexpectedly stable form, and produce a “metal-free” A-D-A (acceptor-donor-acceptor) oligomer emitting in the near-infrared (NIR) thanks to delocalisation of the BODIPY low-lying lowest unoccupied molecular orbital (LUMO) over the oligothienyl moieties, as confirmed by density functional theory (DFT). We are able to retain a PL efficiency of 20% in the solid state (vs. 30% in dilute solutions) by incorporating such a dye in a wider gap polyfluorene matrix and demonstrate organic light-emitting diodes (OLEDs) emitting at 720 nm. We achieve external quantum efficiencies (EQEs) up to 1.1%, the highest value achieved so far by a “metal-free” NIR-OLED not intentionally benefitting from triplet-triplet annihilation. Our work demonstrates for the first time the promise of A-D-A type dyes for NIR OLEDs applications thereby paving the way for further optimisation.

Airbrush Spray Coating of Amorphous Titanium Dioxide for Inverted Polymer Solar Cells

One of the main topics of organic photovoltaics manufacturing is the need for simple, low cost, and large area compatible techniques. Solution-based processes are the best candidates to achieve this aim. Among these, airbrush spray coating has successfully applied to deposit both active and PEDOT layers of bulk-heterojunction solar cells. However, this technique is not yet sufficiently studied for interfacial layers (electron and hole transporting layers or optical spacers). In this paper, we show that amorphous titanium dioxide () films, obtained with an airbrush from a solution of titanium (IV) isopropoxide diluted in isopropanol, are successfully deposited on glass and PET substrates. Good surface covering results from the coalescence of droplets after optimizing the spray coating system. Simple inverted polymer solar cells are fabricated using as electron transporting layer obtaining encouraging electrical performances (% on glass/FTO and 0.7% on PET/ITO substrates).

Electrodeposited cobalt sulfide hole collecting layer for polymer solar cells

In polymer solar cells based on the blend of regioregular poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester, the hole collecting layer has to be endowed with its ionization potential close to or greater than that of P3HT (∼5 eV). Conductive polymer blends such as poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and metal oxides such as vanadium pentoxide (V2O5) and molybdenum trioxide (MoO3) satisfy this requirement and have been the most common materials used so far in bulk heterojunction structures. We report here cobalt sulfide (CoS) to be a promising hole collecting material deposited by convenient and room temperature electrodeposition. By simply tuning the CoS electrodeposition parameters, power conversion efficiencies similar (within 15%) to a reference structure with PEDOT:PSS were obtained.

Tetraphenylethylene-BODIPY aggregation-induced emission luminogens for near-infrared polymer light-emitting diodes

The aggregation-induced emission (AIE) phenomenon provides a new direction for the development of organic light-emitting devices. Here, we present a new class of emitters based on 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY), functionalized at different positions with tetraphenylethylene (TPE), which is one of the most famous AIE luminogens. Thanks to this modification, we were able to tune the photoluminescence of the BODIPY moiety from the green to the near-infrared (NIR) spectral range and achieve PL efficiencies of ~50% in the solid state. Remarkably, we observed an enhancement of the AIE and up to ~100% photoluminescence efficiencies by blending the TPE-substituted BODIPY fluorophores with a poly[(9,9-di-n-octylfluorene-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,7-diyl)] (F8BT) matrix. By incorporating these blends in organic light-emitting diodes (OLEDs), we obtained electroluminescence peaked in the range 650–700 nm with up to 1.8% external quantum efficiency and ~2 mW/cm2 radiance, a remarkable result for red/NIR emitting and solution-processed OLEDs.

Assembly of graphene nanoflake–quantum dot hybrids in aqueous solution and their performance in light-harvesting applications

Graphene nanoflakes and CdSe/ZnS quantum dots were covalently linked in environmentally friendly aqueous solution. Raman spectroscopy and photoluminescence studies, both in solution and on surfaces at the single nanohybrid level, showed evidence of charge transfer between the two nanostructures. The nanohybrids were further incorporated into solar cell devices, demonstrating their potential as light harvesting assemblies.

Efficient Near-Infrared Electroluminescence at 840 nm with “Metal-Free” Small-Molecule:Polymer Blends

Due to the so-called energy-gap law and aggregation quenching, the efficiency of organic light-emitting diodes (OLEDs) emitting above 800 nm is significantly lower than that of visible ones. Successful exploitation of triplet emission in phosphorescent materials containing heavy metals has been reported, with OLEDs achieving remarkable external quantum efficiencies (EQEs) up to 3.8% (peak wavelength > 800 nm). For OLEDs incorporating fluorescent materials free from heavy or toxic metals, however, we are not aware of any report of EQEs over 1% (again for emission peaking at wavelengths > 800 nm), even for devices leveraging thermally activated delayed fluorescence (TADF). Here, the development of polymer light-emitting diodes (PLEDs) peaking at 840 nm and exhibiting unprecedented EQEs (in excess of 1.15%) and turn-on voltages as low as 1.7 V is reported. These incorporate a novel triazolobenzothiadiazole-based emitter and a novel indacenodithiophene-based transport polymer matrix, affording excellent spectral and transport properties. To the best of knowledge, such values are the best ever reported for electroluminescence at 840 nm with a purely organic and solution-processed active layer, not leveraging triplet-assisted emission.

Electron-collecting oxide layers in inverted polymer solar cells via oxidation of thermally evaporated titanium

A simple and intuitive deposition technique is discussed to obtain titanium oxide used as an electron collecting layer in polymer solar cells based on the thermal evaporation of pristine titanium and further thermal treatment to convert the metal in oxide. Since the degradation of indium-doped tin oxide at high temperatures is an issue, we demonstrate that the combination of glass/fluorine tin oxide and high temperatures represents a promising approach in the fabrication of inverted polymer solar cells with such a titanium oxide electron collecting layer.