Fabrizio Cordella

Influence of the bridging atom on the performance of a low-bandgap bulk heterojunction solar cell.

Bulk heterojunction solar cells have attracted considerable attention over the past several years due to their potential for low-cost photovoltaic technology. The possibility of manufacturing modules via a standard printing/coating method in a roll-to-roll process in combination with the use of low-cost materials will lead to a watt-peak price of less than 1 US

Temperature Dependence of Exciton Diffusion in Conjugated Polymers

within the next few years. [1] Despite the low-cost potential, the power conversion efficiency of bulk heterojunction devices is low compared to inorganic solar cells. Efficiencies in the range of 5‐6% have been certified at NREL and AIST usually on devices with small active areas. [2] The current understanding of bulk heterojunction solar cells suggests that the maximum efficiency is in the range of 10‐12%. [3] Several reasons for the power conversion efficiency limitation have been identified. [1] Some of the prerequisites for achieving highest efficiencies are donor and acceptor materials with optimized energy levels [highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)], efficient charge transport in the donor‐acceptor blend, efficient charge generation and limited recombination losses. Power conversion efficiency is strongly dependent on charge transport and charge generation, which are dominated by the phase behavior of the donor and acceptor molecules. The resulting, and often unfavorable, nanomorphology of this two-component blend limits the power conversion efficiency of bulk heterojunction solar cells. Precise control of the nanomorphology is very difficult and has been achieved only for a few systems. [4‐6] The relation between the chemical structure of donor and acceptor materials and the nanomorphology that they form when they are blended is currently not well understood, and as will be shown in this paper, minor changes in the chemical structure can cause major changes in the performance of the materials in organic solar cells. In this work we demonstrate the effect of replacing a carbon atom with a silicon atom on the main chain of the conjugated polymer. The approach has been used previously, and promising materials for field-effect transistors and organic solar cells have been demonstrated. [7‐9] We find that making this simple substitution in poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4b 0 ]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) yields a polysilole, e.g., poly[(4,4 0 -bis(2-ethylhexyl)dithieno[3,2b:2 0 ,3 0 -d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5 0 -diyl] (Si-PCPDTBT), with a higher crystallinity, improved charge transport properties, reduced bimolecular recombination, and a reduced formation of charge transfer complexes when blended with a fullerene derivative. This silole-based polymer is found to form a highly functional nanomorphology when blended with [6,6]-phenyl C71-butyric acid methyl ester (C70-PCBM), and solar cells prepared using this blend gave efficiencies of 5.2%, certified by the National Renewable Energy Laboratory. [1] The presented polymer is the first low-bandgap semiconducting polymer to have a certified efficiency of over 5%. The chemical structure of the subject polymer is shown in Figure 1. The material was synthesized following the procedure described previously. [10] The synthesis and properties of the carbon-bridged polymer have been described before. [11,12] Figure 2a shows the absorbance and photoluminescence (PL) spectra of a thin solid film of the pristine Si-bridged polymer and

Near infrared light emission quenching in organolanthanide complexes

The temperature dependence of the exciton dynamics in a conjugated polymer is studied using time-resolved spectroscopy. Photoluminescence decays were measured in heterostructured samples containing a sharp polymer-fullerene interface, which acts as an exciton quenching wall. Using a 1D diffusion model, the exciton diffusion length and diffusion coefficient were extracted in the temperature range of 4-293 K. The exciton dynamics reveal two temperature regimes: in the range of 4-150 K, the exciton diffusion length (coefficient) of approximately 3 nm (approximately 1.5 x 10 (-4) cm2/s) is nearly temperature independent. Increasing the temperature up to 293 K leads to a gradual growth up to 4.5 nm (approximately 3.2 x 10 (-4) cm2/ s). This demonstrates that exciton diffusion in conjugated polymers is governed by two processes: an initial downhill migration toward lower energy states in the inhomogenously broadened density of states, followed by temperature activated hopping. The latter process is switched off below 150 K.

Charge transfer dynamics in polymer-fullerene blends for efficient solar cells

We investigate the quenching of the near infrared light emission in Er3+ complexes induced by the resonant dipolar interaction between the rare-earth ion and high frequency vibrations of the organic ligand. The nonradiative decay rate of the lanthanide ion is discussed in terms of a continuous medium approximation, which depends only on a few, easily accessible spectroscopic and structural data. The model accounts well for the available experimental results in Er3+ complexes, and predicts an ∼100% light emission quantum yield in fully halogenated systems.

Coherent random lasing in the deep blue from self-assembled organic nanofibers

Blends of poly(3-hexylthiophene) (P3HT) and the bis-adduct of [6,6]-phenyl-C(61)-butyric acid methyl ester (bisPCBM) show enhanced performances in bulk-heterojunction solar cells compared to P3HT:PCBM thin films due to their higher open-circuit voltage. However, it is not clear whether the decrease of the short-circuit current observed in P3HT-bisPCBM blends originates from the 100 mV reduction of the offset between the lowest unoccupied molecular orbitals of the donor and the acceptor or from a change in the morphology. The analysis of the photoluminescence dynamics of the various bulk heterojunctions provides information on the dependence of the electron transfer process on their microstructure. We find that in solution, where the donor-acceptor distribution is homogeneous, the photoluminescence dynamics is the same for the bis- and PCBM-based blends, while in thin films the first shows a slower dynamics than the second. This result indicates that the reduction of the LUMO offset of approximately 100 meV does not influence the electron transfer efficiency but that the diversity between the photoluminescence dynamics in thin films should be ascribed to the different microstructure of the bulk heterojunctions fabricated with the two acceptors.

Spatial control of 3D energy transfer in supramolecular nanostructured host-guest architectures.

We report on the morphological and nonlinear optical properties of highly ordered para-sexiphenyl nanofibers grown by hot-wall epitaxy on muscovite mica. For suitable substrate temperatures and deposition times, needle-shaped aggregates are obtained. These nanoaggregates, which are formed by regrouping of small islands of the material, can support optical waveguiding and amplification of the deep-blue emission of para-sexiphenyl. We show evidence of low-threshold coherent random lasing in ensembles of nanofibers and study the dependence of lasing on the nanofibers’ film thickness. We also present spatially resolved lasing measurements on individual nanofiber segments. The achievement of isolated nanofibers with suitable morphology and low optical losses could open the route to unprecedented photonic and optoelectronic devices at the submicrometric scale.

An organic laser in the monomolecular regime

Systematic control of 3D energy transfer (ET) dynamics is achieved in supramolecular nanostructured host-guest systems using spacer-functionalized guest chromophores. Quantum chemistry-based Monte Carlo simulations reveal the strong impact of the spacer length on the ET dynamics, efficiency, and dimensionality. Remarkably high exciton diffusion lengths demonstrate that there is ample scope for optimizing oligomeric or polymeric optoelectronic devices.

Organic nanofibers as new media for lasing, waveguiding, and photonic sensing

We demonstrated laser action in the regime of linear recombination of singlet excitons in para-sexiphenyl crystalline films in the form of nanofibers under optical excitation with femto- and nanosecond pulses.

Fast and ultrafast response of aligned organic nanofibers: towards organic nanolasers

We report waveguide amplification of spontaneous emission and coherent random laser action in individual self-assembled organic nanofibers grown by high-vacuum deposition. The interpretation of the experimental results is given on the basis of simple models, including transfer matrix calculations in one-dimensionally disordered structures. We present also the numerical results for light scattering from a nanofiber which can be used as a basis for further experiments.

Solution‐Processable Near‐IR Photodetectors Based on Electron Transfer from PbS Nanocrystals to Fullerene Derivatives

Systematic investigations of luminescence lifetimes of organic phenylene nanofibers are presented as a function of intrinsic parameters such as morphology or bleaching factor as well as extrinsic parameters such as substrate material, coating or excitation intensity. By varying either one of these parameters, the decay times of the electronic excitation can be varied. This should have a strong influence on the efficiency of nanolasing, which is observed by increasing the excitation intensity of a femtosecond pump laser. Lasing action starts at pump fluences as low as a few μJ/cm2 per pulse. In ensemble measurements, the number of lasing modes depends strongly on the density of contributing nanofibers. In spatially resolved measurements, the nonlinear optical response of individual nanofibers is investigated. This enables us to make a correlation between the morphological features of the nanofibers, as deduced from atomic-force microscopy, and their lasing properties.