•Development of parametrized field-based multiscale solar-cell algorithm.•Application on large-scale polyfluorene-based solar-cell nanodevices.•Inclusion of effects of inter-mixing of donor–acceptor components and photooxidation.•Investigation of effect of chemical details on device performance.•Study of influence of optical absorption on device performance.Polymer electronics has the power of revolutionizing the world of printable flexible electronics through reducing the production costs of large-scale nanoelectronic applications. However, performance and stability of such devices are still generally low compared to their inorganic counterparts, rendering the development of novel multiscale experimental- and theoretical-investigation techniques necessary, to increase the understanding of the causes for performance losses under operation conditions. To this end, we introduce in this paper a novel parametrized field-based multiscale algorithm, which permits to study effects of chemical details, like e.g. inter-mixing of the donor- and acceptor-components and/or photodegradation, on the photovoltaic performance of polymer-based solar-cell nanodevices with sizes of technological relevance. By comparing its results with the ones of atomistic particle-based solar-cell calculations, we demonstrate that the parametrized field-based approach provides a reasonable value for the internal quantum efficiency of a polyfluorene-based blend heterojunction, used for parametrization of the exciton dissociation and charge transfer rates. Moreover, we show that its combination with a modified version of the transfer-matrix method allows the inclusion of the influence of the optical absorption of the individual device components, like e.g. the electrodes and/or nanophases from the photoactive layer, into the algorithm. This full-device solar-cell approach enables us to determine values for the external quantum efficiency of several polymer blend morphologies in good agreement with experimental measurements. Finally, the latter study also reveals, in concordance with experimental observations, that reducing charge-carrier losses is more important than reducing exciton- and photon-losses for optimizing the performance of solar-cell devices.

•Development of a novel particle-based multiscale solar cell algorithm using field-to-particle transformation.•Investigation of inter-mixing of donor- and acceptor-type of monomers in a nanostructured PFB-F8BT blend.•Results show that charge-generation- and -transport occur predominantly inside the nanophases.•Photo-induced keto-defects on F8BT polymers cause intra-chain electron-trapping, reducing electron transport efficiency•Outcome is in agreement with recent electrostatic-force- and photocurrent-microscopy-experiments.Polymer solar cells possess a promising perspective for generating renewable energy at affordable costs, provided their performance and durability can be improved considerably. To this end, several experimental and theoretical techniques have been devised recently, establishing a direct link between local morphology, local opto-electronic properties and device performance. However, their reliability is still unclear to this day. Here, we demonstrate by using a recently developed particle-based multiscale solar cell approach and comparing its results with the ones of a field-based solar cell algorithm that inter-mixing of the electron-donor(D)- and -acceptor(A)-type of segments in a lamellar-like poly(9,9’-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenylene-diamine)-poly(9,9'-dioctylfluorene-co-benzothiadiazole) (PFB-F8BT) blend causes that the major part of the charge generation and charge transport takes place inside the nanophases of the nanostructured polymer solar cells in agreement with recent experimental measurements and not, as commonly believed, at the visible domain boundaries of the DA interface. Moreover, we show that the contribution of the exciton dissociation efficiency to the internal quantum efficiency, due to inter-monomeric mixing, is significant and cannot be neglected in simulation studies at the nanoscale. Finally, we demonstrate that keto-defects on the fluorene moiety of the F8BT phase, induced by photo-oxidation, causes a simultaneous increase of the intra-chain contribution and decrease of the inter-chain contribution to the electronic current density, whereas in the reduced form the difference between both contributions is significantly smaller. This antagonistic effect leads to keto-induced electron trapping, resulting in a deteriorated electronic transport efficiency in devices with a photo-oxidized F8BT phase.

Tapered block copolymers offer an exciting opportunity to tailor the interfacial region between different components by conserving their phase-separated mesoscale structure, which enable the generation of polymer systems with the desired spatio-dynamic properties. Here, we explore their usefulness for optimizing the photovoltaic performance of polymer bulk heterojunctions. To this end, we apply a recently developed particle-based multiscale solar-cell algorithm and investigate the effect of random tapering at the chemical junctions between the electron-donor- (D) and electron-acceptor- (A) blocks on the photovoltaic properties of various lamellar-like polyfluorene-based block-copolymer systems. Our simulation results reveal that introducing a tapered middle block with optimal length leads to a significant increase of the exciton dissociation efficiency, but deteriorates the charge transport efficiency only moderately. This results in a gain of the internal quantum efficiency from 25 up to 39% by increasing the thickness of the active layer of the solar cell from 10 up to 50 nm in direction to the DA interface.

Phototropins are photoreceptors regulating the blue-light response in plants and bacteria. They consist of two LOV (light oxygen voltage sensitive) domains each containing a non-covalently bound flavin-mononucleotide (FMN) chromophore, which are connected to a serine/threonine-kinase. Upon illumination, the LOV-domains undergo conformational changes, triggering a signal cascade in the organism through kinase activation. Here, we present results from molecular dynamics simulations in which we investigate the signal transduction pathway of the wildtype LOV1-domain of Chlamydomonas reinhardtii and a methyl-mercaptan (MM) adduct of its Cys57Gly-mutant at the molecular level. In particular, we analyzed the effect of covalent-bond formation between the reactive cysteine Cys57 and the FMN-reaction center, as well as the subsequent charge redistribution, on the spatio-dynamical behavior of the LOV1-domain. We compare the calculation results with experimental data and demonstrate that these adduct state characteristics have an important influence on the response of this photosensor. The light-induced changes implicate primarily an alteration of the surface charge distribution through rearrangement of the highly flexible Cα-, Dα- and Eα-helices including the Glu51-Lys91-salt bridge on the hydrophilic side of the protein domain and a β-sheet tightening process via coupling of the Aβ- and Bβ-strands. Our findings confirm the aptitude of the LOV1-domain to function as a dimerization partner, allowing the green alga to adapt its reproduction and growth speed to the environmental conditions.

LOV domains are the light-sensitive protein domains of plant phototropins and bacteria. They photochemically form a covalent bond between a flavin mononucleotide (FMN) chromophore and a cysteine, attached to the apo-protein, upon irradiation with blue light, which triggers a signal in the adjacent kinase. Although their signaling state has been well characterized through experimental means, their signal transduction pathway as well as dark-state activity are generally only poorly understood. Here we show results from molecular dynamics simulations where we investigated the effect of thermostating and long-range electrostatics on the solution structure and dynamical behavior of the wild-type LOV1 domain from the green algae Chlamydomonas reinhardtii in the dark. We demonstrate that these computational issues can dramatically affect the conformational fluctuations of such protein domains by suppressing configurations far from equilibrium or destabilizing local configurations, leading to artificial changes of the protein secondary structure as well as the H-bond network formed by the amino acids and the FMN. By comparing our calculation results with recent experimental data, we show that the non-invasive thermostating strategy, where the protein solute is only indirectly coupled to the thermostat via the solvent, in conjunction with the particle-mesh Ewald technique, provides dark-state conformers, which are in consistency with experimental observations. Moreover, our calculations indicate that the LOV1 domains can alter the intersystem crossing rate and rate of adduct formation by adjusting the population distribution of these dark-state conformers. This might permit them to function as a modulator of the signal intensity under low light conditions.

Understanding the chemistry and physics of polymer systems challenges scientists from a wide spectrum of research areas, ranging from polymer science to molecular electronic structure theory. One of the characteristic features of polymer systems is that their physics involve a multitude of different length and time scales, which generally render the determination of their structure and physical properties on a detailed level computationally exhaustive. To overcome this difficulty, novel field-theoretic methodologies based on the mean field approximation have emerged recently and have proven to deliver useful results in the calculation of mesoscopic polymer models in the regime of high monomer concentrations. In this review we demonstrate that the field-theoretic approach is not only an useful formalism for treating highly concentrated polymer systems on the mesoscopic level of description, but that it is also a promising theoretical tool, to solve the multiscale problems arising in the calculation of physical properties of a wide variety of neutral and charged polymer materials. To this end, we show that the field-theoretic approach possesses the advantageous property to enable the treatment of all levels of description, spanning from the quantum to the continuum scale, within an unified theoretical framework. On the example of the coupling of the mesoscopic and continuum scale, we show that this specific feature constitutes a crucial advantage of field-theoretic approaches with regard to current state-of-the-art particle-based simulation methodologies for connecting different levels of description. Another major benefit relates to their favorable approximation characteristics, which permit to devise efficient approximation strategies for evaluating sophisticated polymer solution models in the low to moderate regime of monomer concentrations in a reliable way. To show this, we present novel low-cost approximation strategies beyond the mean field level of approximation using effective renormalization concepts, originating from the domain of quantum field theory, and demonstrate their usefulness in the calculation of structure and physical properties of several polymer models, described at various levels of description.

The computation of open many-particle systems at high densities is a major challenge since many decades due to the inherent limitations of grand canonical simulation methods based on particle exchange algorithms. In this paper we report on the statistical convergence behavior in the high density regime of a recently developed alternative called the grand canonical auxiliary field Monte Carlo method. We show on a common soft matter model widely used in polymer simulation that it possesses a more appropriate statistical behavior in the dense regime than the currently employed grand canonical Monte Carlo methods relying on particle exchange algorithms.