Molecular Electronic Coupling Controls Charge Recombination Kinetics in Organic Solar Cells of Low Bandgap Diketopyrrolopyrrole, Carbazole, and Thiophene Polymers

Low-bandgap diketopyrrolopyrrole- and carbazole-based polymer bulk-heterojunction, solar cells exhibit much faster charge carrier recombination kinetics than that encountered for less-recombining poly(3-hexylthiophene). Solar cells comprising these polymers exhibit energy losses caused by carrier recombination of approximately 100 mV, expressed as reduction in open-circuit voltage, and consequently photovoltaic conversion efficiency lowers in more than 20%. The analysis presented here unravels the origin of that energy loss by connecting the limiting mechanism governing recombination dynamics to the electronic coupling occurring at the donor polymer and acceptor fullerene interfaces. Previous approaches correlate carrier transport properties and recombination kinetics by means of Langevin-like mechanisms. However, neither carrier mobility nor polymer ionization energy helps understanding the variation of the recombination coefficient among the studied polymers In the framework of the charge transfer Marcus theory, it is proposed that recombination time scale is linked with charge transfer molecular mechanisms at,the polymer/fullerene interfaces. As expected for efficient organic solar cells, small electronic coupling existing between donor polymers and acceptor fullerene (V-if < 1 meV) and large reorganization energy (lambda approximate to 0.7 eV) are encountered. Differences in the electronic coupling among polymer/fullerene blends suffice to explain the slowest recombination exhibited by poly(3-hexylthiophene)-based solar, cells. Our approach reveals how to directly connect photovoltaic parameters. as open circuit voltage to molecular properties of blended materials.This work was partially supported by FP7 European Collaborative Project SUNFLOWER (FP7-ICT-2011-7, Contract No. 287594), Ministerio de Educacion y Ciencia (Spain), under Project HOPE CSD2007-00007 (Consolider-Ingenio 2010), and Generalitat Valenciana (Prometeo/2009/058 and ISIC/2012/008 Institute of Nanotechnologies for Clean Energies)

Nouveaux systèmes conjugués linéaires intégrant des motifs 3,4-éthylènedioxythiophènes (EDOT) (synthèse et étude des propriétés électroniques)

Ce travail porte sur l'élaboration et l'étude des propriétés de nouveaux systèmes conjugués linéaires (SCL) intégrant des motifs 3,4-éthylènedioxythiophènes (EDOT). La synthèse d'oligomères variant par la longueur ainsi que par le nombre et la position des motifs EDOTs qu'ils contiennent est décrite. L'analyse des propriétés optiques et électrochimiques montre clairement une influence marquée de l'EDOT sur la structure des oligomères. L'effet donneur du groupement éthylènedioxy augmente sensiblement le niveau de la HOMO et contribue à localiser les charges sur le motif EDOT. Une rigidification des molécules par des interactions intramoléculaires entre les atomes d'oxygène des groupements éthylènedioxy et de soufre d'un thiophène adjacent est observée. Une dernière partie propose deux alternatives à l'utilisation de l'EDOT, son analogue soufré l'EDST et le 3,6-diméthoxy-thiénothiophène, qui constituent des pistes intéressantes pour obtenir des polymères à faible gap plus solubles que le PEDOT.This work concerns the development and the study of the properties of new linear conjugated systems (LCS) built with 3,4-ethylenedioxythiophenes (EDOT) moieties. The synthesis of oligomers varying by their length, the number and the position of the EDOT units in the conjugated chain is described. The analysis of the optical and electrochemical properties clearly shows a marked influence of the EDOT unit on the structure of oligomers. The donor effect of the ethylenedioxy group increases the level of the HOMO and contributes to locate the positive charge on the EDOT moieties. A rigidification of the molecules, by intramolecular interactions between oxygen atoms of ethylenedioxy group and the sulphur atoms of an adjacent thiophene cycle, is observed. The last part proposes two alternatives for replacing the EDOT core, its sulphur analogue, EDST, and the3,6-dimethoxy-thienothiophene, which constitute interesting ways to obtain low gap polymers more soluble than the PEDOT.ANGERS-BU Lettres et Sciences (490072106) / SudocSudocFranceF

Charge transport and recombination in PDPP5T:[70]PCBM organic solar cells: The influence of morphology

The performance of organic bulk heterojunction solar cells is strongly dependent on the donor/acceptor morphology. Morphological parameters, such as the extent and the composition of donor- and acceptor-rich domains, influence both the charge generation and the charge transport throughout the active layer. This work focuses on a polymer:fullerene system based on a small bandgap diketopyrrolopyrrole-quinquethiophene alternating copolymer (PDPP5T) mixed with [6,6]-phenyl-C-71-butyric acid methyl ester ([70]PCBM) that is capable of efficiencies higher than 6%. By changing the processing conditions, the morphology can be varied from a coarse separated morphology, with fullerene domains (blobs) embedded in a polymer-rich matrix, to a completely mixed layer. The charge carrier transport and the strength of the bimolecular recombination in PDPP5T:[70]PCBM blends with different morphologies and fullerene concentrations are experimentally characterized. The large difference in electron and hole mobility and the electric field dependency of the electron mobility are identified as the causes that limit the performance of devices with low [70]PCBM content. These effects are not present if the concentration of [70]PCBM is increased while keeping a fine phase separation by the addition of ortho-dichlorobenzene as a cosolvent. For the phase-separated blends, a model is proposed, based on a drift-diffusion approach that combines electrical and morphological parameters; with this model the contribution of each phase to the total current is quantified. Under operating conditions, most of the current comes from the interfacial region between the phases, with holes traveling through the matrix and the electrons through the blobs. This device model consistently connects morphological features to overall device performance. (C) 2014 Elsevier B.V. All rights reserved

Mass Spectrometry Evidence for Self-Rigidification of π-Conjugated Oligomers Containing 3,4-Ethylenedioxythiophene (EDOT) Groups using RRKM Theory and Internal Energy Calibration

International audienceThe self-rigidification of ionized π-conjugated systems based on two combinations of thiophene (T) and 3,4-Ethylenedioxythiophene (E) is investigated using mass-analyzed ion kinetic energy spectrometry (MIKES) of ions produced from electron impact ionization at 70 eV. The m/z 446 radical cations of the two isomers ETTE and TEET lead to detect m/z 418 and 390 daughter ions. The MIKE spectra differ only by the intensities of these fragment ions. As the m/z 418 daughter ion is produced through a same retro-Diels Alder reaction whateve

Solution processed polymer tandem solar cell using efficient small and wide band gap polymer : fullerene blends

Solution processed polymer tandem solar cells that combine wide and small bandgap absorber layers reach a power conversion efficiency of 7% in a series configuration. This represents a 20% increase compared to the best single junction cells made with the individual active layers and shows that the tandem configuration reduces transmission and thermalization losses in converting sunlight

A solution-processed barium hydroxide modified aluminum doped zinc oxide layer for highly efficient inverted organic solar cells

Inverted organic solar cells (iOSCs) with air stable interface materials and top electrodes and an efficiency of 6.01% are achieved by inserting a barium hydroxide (Ba(OH)2) layer between the aluminum doped zinc oxide (AZO) electron extraction layer and the active layer. A low bandgap diketopyrrolopyrrole–quinquethiophene alternating copolymer (pDPP5T-2) and phenyl-C61-butyric acid methyl ester (PC61BM) were chosen as the active layer compounds. Compared to the control device without Ba(OH)2, insertion of a few nm thick Ba(OH)2 layer results in an enhanced VOC of 10%, JSC of 28%, FF of 28% and PCE of 80%. Modification of AZO with a solution processed low-cost Ba(OH)2 layer increased the efficiency of the inverted device by dominantly reducing the energy barrier for electron extraction from PC61BM, and consequently, reduced charge recombination is observed. The drastic improvement in device efficiency and the simplicity of fabrication by solution processing suggest Ba(OH)2 as a promising and practical route to reduce interface induced recombination losses at the cathode of organic solar cells

A universal method to form the equivalent ohmic contact for efficient solution-processed organic tandem solar cells

The highly transparent, conductive and robust intermediate layer (IML) is the primary challenge for constructing efficient organic tandem solar cells. In this work, we demonstrate an easy but generic approach to realize the fully functional, solution-processed IMLs. In detail, solution-processed silver-nanowires are packed at low concentration between hole- and electron-transporting layers to convert an otherwise rectifying interface into an ohmic interface. The IMLs are proven to be of ohmic nature under applied bias, despite the unipolar charge selectivity of the single layers. Ohmic recombination within IMLs is further proven in organic tandem solar cells fabricated by doctor-blading under ambient conditions. The tandem solar cells based on PCDTBT:[70]PCBM as the bottom cell and pDPP5T-2:[60]PCBM as the top cell give a power conversion efficiency of 7.25%, which is among the highest values for solution-processed organic tandem solar cells fabricated by using a roll-to-roll compatible deposition method in air

Diketopyrrolopyrrole polymers as organic semiconductors and optical materials

The present invention relates to polymers comprising diketopyrrolopyrrole repeating units and their use as org. semiconductor in org. devices, esp. a diode, an org. field effect transistor and/or a solar cell, or a device contg. a diode and/or an org. field effect transistor, and/or a solar cell. The polymers according to the invention have excellent soly. in org. solvents and excellent film-forming properties. In addn., high efficiency of energy conversion, excellent field-effect mobility, good on/off current ratios and/or excellent stability can be obsd., when the polymers according to the invention are used in semiconductor devices or org. photovoltaic (PV) devices (solar cells). [on SciFinder (R)