Daniele Di Nuzzo

Efficient Solar Cells Based on an Easily Accessible Diketopyrrolopyrrole Polymer

A new easily accessible, high molecular weight, alternating dithieno-diketopyrrolopyrrolophenylene copolymer provides high electron and hole mobilities exceeding 0.02 cm2 V-1 s-1 in FETs and AM1.5 power conversion efficiencies of 4.6% and 5.5% in solar cells when combined with [60]PCBM and [70]PCBM. The performance of the solar cells strongly depends on the use of a processing agent.

Improved Film Morphology Reduces Charge Carrier Recombination into the Triplet Excited State in a Small Bandgap Polymer-Fullerene Photovoltaic Cell

The use of diiodooctane as processing additive for construction of PCPDTBT:PCBM solar cells results in a profound change in photophysical behavior of this blend. In the improved morphology obtained with the additive, recombination of charge carriers to the lowest triplet excited state is suppressed. This contributes to the boost in solar power conversion efficiency induced by the use of the processing agent.

Improving the Stability and Performance of Perovskite Light-Emitting Diodes by Thermal Annealing Treatment

A perovskite LED with a perovskite film treated under optimum thermal annealing conditions exhibits a significantly enhanced long-term stability with full coverage of the green electroluminescence emission due to the highly uniform morphology of the perovskite film.

Amine-Based Passivating Materials for Enhanced Optical Properties and Performance of Organic–Inorganic Perovskites in Light-Emitting Diodes

The use of hybrid organic-inorganic perovskites in optoelectronic applications are attracting an interest because of their outstanding characteristics, which enable a remarkable enhancement of device efficiency. However, solution-processed perovskite crystals unavoidably contain defect sites that cause hysteresis in perovskite solar cells (PeSCs) and blinking in perovskite light-emitting diodes (PeLEDs). Here, we report significant beneficial effects using a new treatment based on amine-based passivating materials (APMs) to passivate the defect sites of methylammonium lead tribromide (MAPbBr3) through coordinate bonding between the nitrogen atoms and undercoordinated lead ions. This treatment greatly enhanced the PeLEDs efficiency, with an external quantum efficiency (EQE) of 6.2%, enhanced photoluminescence (PL), a lower threshold for amplified spontaneous emission (ASE), a longer PL lifetime, and enhanced device stability. Using confocal microscopy, we observed the cessation of PL blinking in perovskite films treated with ethylenediamine (EDA) due to passivation of the defect sites in the MAPbBr3.

Improved performance of perovskite light-emitting diodes using a PEDOT:PSS and MoO3 composite layer

We demonstrate the enhanced performance of perovskite light-emitting diodes (PeLEDs) using a solution-processable MoO3 and PEDOT:PSS (PEDOT:MoO3) composite layer as the hole transport layer (HTL). The PEDOT:MoO3 composite layer presents improved hole injection through a reduction in the contact barrier between the HTL and the CH3NH3PbBr3 layer and enhanced crystallinity of the perovskite film. The optimized PeLEDs with the PEDOT:MoO3 composite film showed enhanced external quantum efficiency (EQE) and maximum luminous efficiency, compared to a PeLED using a pristine PEDOT:PSS layer.

High Circular Polarization of Electroluminescence Achieved via Self-Assembly of a Light-Emitting Chiral Conjugated Polymer into Multidomain Cholesteric Films

We demonstrate a facile route to obtain high and broad-band circular polarization of electroluminescence in single-layer polymer OLEDs. As a light-emitting material we use a donor-acceptor polyfluorene with enantiomerically pure chiral side-chains. We show that upon thermal annealing the polymer self-assembles into a multidomain cholesteric film. By varying the thickness of the polymer emitting layer, we achieve high levels of circular polarization of electroluminescence (up to 40% excess of right-handed polarization), which are the highest reported for polymer OLEDs not using chiral dopants or alignment layers. Mueller matrix ellipsometry shows strong optical anisotropies in the film, indicating that the circular polarization of luminescence arises mainly after the photon has been generated, through selective scattering and birefringence correlated in the direction of the initial linear polarization of the photon. Our work demonstrates that chirally substituted conjugated polymers can combine photonic and semiconducting properties in advanced optoelectronic devices.

Growth of Nanosized Single Crystals for Efficient Perovskite Light-Emitting Diodes

Organic-inorganic hybrid perovskites are emerging as promising emitting materials due to their narrow full-width at half-maximum emissions, color tunability, and high photoluminescence quantum yields (PLQYs). However, the thermal generation of free charges at room temperature results in a low radiative recombination rate and an excitation-intensity-dependent PLQY, which is associated with the trap density. Here, we report perovskite films composed of uniform nanosized single crystals (average diameter = 31.7 nm) produced by introducing bulky amine ligands and performing the growth at a lower temperature. By effectively controlling the crystal growth, we maximized the radiative bimolecular recombination yield by reducing the trap density and spatially confining the charges. Finally, highly bright and efficient green emissive perovskite light-emitting diodes that do not suffer from electroluminescence blinking were achieved with a luminance of up to 55 400 cd m-2, current efficiency of 55.2 cd A-1, and external quantum efficiency of 12.1%.

Ultrafast Charge and Triplet State Formation in Diketopyrrolopyrrole Low Band Gap Polymer/Fullerene Blends: Influence of Nanoscale Morphology of Organic Photovoltaic Materials on Charge Recombination to the Triplet State

Femtosecond transient absorption spectroscopy of thin films of two types of morphologies of diketopyrrolopyrrole low band gap polymer/fullerene-adduct blends is presented and indicates triplet state formation by charge recombination, an important loss channel in organic photovoltaic materials. At low laser fluence (approaching solar intensity) charge formation characterized by a 1350 nm band (in ~250 fs) dominates in the two PDPP-PCBM blends with different nanoscale morphologies and these charges recombine to form a local polymer-based triplet state on the sub-ns timescale (in ~300 and ~900 ps) indicated by an 1100 nm absorption band. The rate of triplet state formation is influenced by the morphology. The slower rate of charge recombination to the triplet state (in ~900 ps) belongs to a morphology that results in a higher power conversion efficiency in the corresponding device. Nanoscale morphology not only influences interfacial area and conduction of holes and electrons but also influences the mechanism of intersystem crossing (ISC). We present a model that correlates morphology to the exchange integral and fast and slow mechanisms for ISC (SOCT-ISC and H-HFI-ISC). For the pristine polymer, a flat and unstructured singlet-singlet absorption spectrum (between 900 and 1400 nm) and a very minor triplet state formation (5%) are observed at low laser fluence.

Research data supporting "High Circular Polarization of Electroluminescence Achieved via Self-Assembly of a Light-Emitting Chiral Conjugated Polymer into Multidomain Cholesteric Films"

Left and right-handed circularly polarized components of Electro-Luminescence and corresponding dissymmetry factors.

Solvatochromic covalent organic frameworks

Covalent organic frameworks (COFs) are an emerging class of highly tuneable crystalline, porous materials. Here we report the first COFs that change their electronic structure reversibly depending on the surrounding atmosphere. These COFs can act as solid-state supramolecular solvatochromic sensors that show a strong colour change when exposed to humidity or solvent vapours, dependent on vapour concentration and solvent polarity. The excellent accessibility of the pores in vertically oriented films results in ultrafast response times below 200 ms, outperforming commercially available humidity sensors by more than an order of magnitude. Employing a solvatochromic COF film as a vapour-sensitive light filter, we demonstrate a fast humidity sensor with full reversibility and stability over at least 4000 cycles. Considering their immense chemical diversity and modular design, COFs with fine-tuned solvatochromic properties could broaden the range of possible applications for these materials in sensing and optoelectronics.Covalent organic frameworks (COFs) find increasing application as sensor material, but fast switching solvatochromism was not realized. Here the authors demonstrate that combination of electron-rich and -deficient building blocks leads to COFs which fast and reversibly change of their electronic structure depending on the surrounding atmosphere.