Exploring the origin of high optical absorption in conjugated polymers

Vezie, Michelle S. et al.The specific optical absorption of an organic semiconductor is critical to the performance of organic optoelectronic devices. For example, higher light-harvesting efficiency can lead to higher photocurrent in solar cells that are limited by sub-optimal electrical transport. Here, we compared over 40 conjugated polymers, and found that many different chemical structures share an apparent maximum in their extinction coefficients. However, a diketopyrrolopyrrole-thienothiophene copolymer shows remarkably high optical absorption at relatively low photon energies. By investigating its backbone structure and conformation with measurements and quantum chemical calculations, we find that the high optical absorption can be explained by the high persistence length of the polymer. Accordingly, we demonstrate high absorption in other polymers with high theoretical persistence length. Visible light harvesting may be enhanced in other conjugated polymers through judicious design of the structure.M.S.V. and S. F. are grateful to the Engineering and Physical Sciences Research Council (EPSRC) for a doctoral training award and a CDT studentship (EP/G037515/1) respectively. G.P. and S.C.H. acknowledge the University of Cyprus for funding through the internal grant "ORGANIC". B.D., A.G. and M.C.Q. acknowledge financial support from the Ministerio de Economía y Competitividad of Spain through projects CSD2010–00044 (Consolider NANOTHERM), SEV-2015_0496 and MAT2012–37776 and the European Research Council through project ERC CoG648901. I.M., R.S.A. and I.McC. acknowledge support from the European Commission FP7 Project ArtESun (604397). J.N. is grateful to the Royal Society for a Wolfson Merit Award, and acknowledges financial support from EPSRC grants EP/K030671/1, EP/K029843/1 and EP/J017361/1. The authors thank Dr. Isabel Alonso for performing supplementary ellipsometric measurements; we thank Prof. Thomas Kirchartz, Dr. Jarvist Moore Frost, Dr. Christian Müller and Dr. Isabel Alonso for helpful discussions.Peer reviewe

Exploring different doping mechanisms in thermoelectric polymer/ carbon nanotube composites

This work compares various methods to prepare polymer/carbon nanotube (CNT) composites for thermoelectric applications, focusing on the different doping mechanisms. We first look at the general trends observed in the Seebeck coefficient and power factor for a large number of composites as a function of electrical conductivity. Then we discuss two methods of nitrogen doping the carbon nanotubes in these composites, namely either during synthesis, or afterwards by ammonolysis. Finally, we discuss doping of the carbon nanotubes through charge transfer from the polymer counterpart, including photo-induced switching of the majority carrier type. As a general remark, we note that processability is negatively influenced by some doping procedures. Best results were achieved for unfunctionalized single-walled carbon nanotubes with a high content of semiconducting CNT species.The authors would like to thank Prof. Michael L. Chabinyc, Prof. Christian Müller and Prof. Alejandro R. Goñi for useful discussions. We are grateful to John D. Craddock, Prof. Matthew C. Weisen- berger and Prof. John E. Anthony for providing the n-MWCNTs. The authors would also like to acknowledge financial support from the Ministerio de Economía y Competitividad of Spain through projects CSD2010–00044 (Consolider NANOTHERM), MAT2015- 70850-P and MAT2014-53500-R; and the European Research Council (ERC) under grant agreement no. 648901. S. Sandoval acknowledges a contract though PIE 201660E013 and P. Kankla the Development and Promotion of Science and Technology Talents Project (DPST). We are grateful to Thomas Swan Co. Ltd for supplying Elicarb1 SWCNTs.Peer reviewe

Comparing different geometries for photovoltaic-thermoelectric hybrid devices based on organics

Coupling thermoelectrics (TE) with photovoltaics (PV) has emerged as an approach to solid-state solar harvesting, directly converting light and infrared heat into electricity. In this work, we compare PV-TE hybrid devices based on organic semiconductors in three different geometries: a reflection geometry, a non-contact transmission geometry, and a contact transmission geometry. The temperature rises of films of common organic thermoelectric materials, including poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), single-walled carbon nanotubes (swCNT), and poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT), were measured in configurations representative of the proposed geometries. Because organic semiconductors possess broadband light absorption and low thermal conductivities, a significant rise in temperature was observed under illumination for all geometries. We find, however, that the best configuration is, in fact, the transmission contact mode because it sums two effects. Operating under 1 sun illumination, the temperature of a commercial organic PV module increased by ≈30 K, which leads to an enhancement in OPV performance compared to room temperature. After attaching a thermoelectric to the OPV module, losses from convection are reduced, and the OPV module heats up even more, further increasing its efficiency while additionally enabling thermoelectric generation. Finally, we calculate theoretical thermoelectric efficiencies for the materials and their respective power densities.The authors acknowledge financial support from the Spanish Ministry Science and Innovation through the “Severo Ochoa” Program for Centers of Excellence in R&D SEV-2015-0496 (FUNMAT) and CEX2019-000917-S (FUNFUTURE), and PGC2018-095411-B-I00 (RAINBOW) projects; from the Generalitat de Catalunya through grants 2017SGR488 and AGAUR 2018 PROD 00191; and from the European Research Council (ERC) under grant agreement no. 648901. This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement no. 713673. J. P. J. has received financial support through the “la Caixa” INPhINIT Fellowship Grant for Doctoral studies at Spanish Research Centers of Excellence (Grant code: LCF/BQ/IN17/11620035), “la Caixa” Banking Foundation (ID100010434), Barcelona, Spain. O. Z. A. acknowledges CONACYT-SENER for his PhD scholarship (no. 472571). J. P. J. acknowledges the departments of Physics, Chemistry and Geology of the Autonomous University of Barcelona (UAB) as coordinators of the PhD programme in Materials Science. The authors thank Dr A. Roig, Dr A. Laromaine and Dr D. Abol-Fotouh (ICMAB-CSIC) for the CNT:cellulose sample preparation and fruitful discussions. The authors thank Dr Aleksandr Perevedentsev for his help with sample preparation, and Mr Pau Molet for his help with the FTIR measurements. We acknowledge support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).Peer reviewe

Photoinduced p- to n-type Switching in Thermoelectric Polymer-Carbon Nanotube Composites

Dörling, Bernhard et al.UV-induced switching from p- to n-type character is demonstrated during deposition of carbon-nanotube–conjugated polymer composites. This opens the possibility to photopattern n-type regions within an otherwise p-type film, which has a potential for complementary circuitry or, as shown here, thermoelectric generators made from a single solution.The authors would like to thank Dr. Isabel Alonso (ICMAB) and Prof. Salvador Ferrer (ALBA) for assisting during the synchrotron sessions as well as for useful discussions. The authors would also like to acknowledge financial support from the Ministerio de Economía y Competitividad of Spain through projects CSD2010–00044 (Consolider NANOTHERM) and MAT2012–37776 and the European Research Council (ERC) under grant agreement nos. 637624 and 648901. C.M. gratefully acknowledges fi nancial support from Formas, the Knut and Alice Wallenberg Foundation through a Wallenberg Academy Fellowship. The transmission X-ray microscopy experiments were performed at MISTRAL beamline at ALBA Synchrotron with the collaboration of ALBA staff. A.E.B. thanks the Egyptian Ministry of Higher Education for funding through the Short-term Scientifi c Mission Postdoctoral program. The authors thank Dr. Elena Bailo (Witec) for assistance with the Raman imaging experiments.Peer reviewe

Interplay Between Fullerene Surface Coverage and Contact Selectivity of Cathode Interfaces in Organic Solar Cells

Interfaces play a determining role in establishing the degree of carrier selectivity at outer contacts in organic solar cells. Considering that the bulk heterojunction consists of a blend of electron donor and acceptor materials, the specific relative surface coverage at the electrode interfaces have an impact on the carrier selectivity. This work unravels how fullerene surface coverage at cathode contacts lies behind the carrier selectivity of the electrodes. A variety of techniques as variable-angle spectroscopic ellipsometry and capacitance-voltage measurements have been used to determine the degree of fullerene surface coverage in a set of PCPDTBT-based solar cells processed with different additives. A full screening from highly fullerene-rich to polymer-rich phases attaching the cathode interface has enabled the overall correlation between surface morphology (relative coverage) and device performance (operating parameters). The general validity of the measurements is further discussed in four donor/acceptor systems: PCPDTBT, P3HT, PCDTBT and PTB7 blended with fullerene derivatives. It is demonstrated that a fullerene rich interface at the cathode is a prerequisite to enhance contact selectivity, and consequently power conversion efficiency

Conductive bacterial nanocellulose-polypyrrole patches promote cardiomyocyte differentiation

The low endogenous regenerative capacity of the heart, added to the prevalence of cardiovascular diseases, triggered the advent of cardiac tissue engineering in the last decades. The myocardial niche plays a critical role in directing the function and fate of cardiomyocytes; therefore, engineering a biomimetic scaffold holds excellent promise. We produced an electroconductive cardiac patch of bacterial nanocellulose (BC) with polypyrrole nanoparticles (Ppy NPs) to mimic the natural myocardial microenvironment. BC offers a 3D interconnected fiber structure with high flexibility, which is ideal for hosting Ppy nanoparticles. BC-Ppy composites were produced by decorating the network of BC fibers (65 ± 12 nm) with conductive Ppy nanoparticles (83 ± 8 nm). Ppy NPs effectively augment the conductivity, surface roughness, and thickness of BC composites despite reducing scaffolds’ transparency. BC-Ppy composites were flexible (up to 10 mM Ppy), maintained their intricate 3D extracellular matrix-like mesh structure in all Ppy concentrations tested, and displayed electrical conductivities in the range of native cardiac tissue. Furthermore, these materials exhibit tensile strength, surface roughness, and wettability values appropriate for their final use as cardiac patches. In vitro experiments with cardiac fibroblasts and H9c2 cells confirmed the exceptional biocompatibility of BC-Ppy composites. BC-Ppy scaffolds improved cell viability and attachment, promoting a desirable cardiomyoblast morphology. Biochemical analyses revealed that H9c2 cells showed different cardiomyocyte phenotypes and distinct levels of maturity depending on the amount of Ppy in the substrate used. Specifically, the employment of BC-Ppy composites drives partial H9c2 differentiation toward a cardiomyocyte-like phenotype. The scaffolds increase the expression of functional cardiac markers in H9c2 cells, indicative of a higher differentiation efficiency, which is not observed with plain BC. Our results highlight the remarkable potential use of BC-Ppy scaffolds as a cardiac patch in tissue regenerative therapies.This work was supported by the Spanish Ministry of Science and Innovation (MICINN) through the National Research Agency (AEI) and European Regional Development Funds (ERDF/FEDER), project BIOCARDIO ref RTI2018-096320–B-C21, project BIOSOFT-REGE ref PID2021-122645OB-I00, the CERCA Program/Generalitat de Catalunya, the ‘Severo Ochoa’ Programme for Center of Excellence in R&D (CEX2019-000917), the Programme/Generalitat de Catalunya (2017-SGR-359), the Severo Ochoa Programme of the Spanish Ministry of Science and Innovation (MICINN─Grant SEV-2014–0425, 2015–2019 and CEX2018–000,789-S, 2019–2023), and the projects FIP-PALOMA, FIP-BEAT, and the PDC2022-133755-I00/AEI/10.13039/501100011033 European Union NextGeneration EU/PRTR. This research was also supported by the European Union’s Horizon 2020 research and innovation program H2020-MSCA-COFUND-2016 (DOC-FAM, Grant Agreement No. 754397). This project also received the support of a La Caixa INPhINIT Fellowship (ID 100010434) with project code LCF/BQ/DR19/11740025. S.Y.S. is enrolled in the Materials Science Ph.D. program of the UAB. S.Y.S. and A.L. participate in the Spanish National Research Council (CSIC) interdisciplinary platform for sustainable plastics towards a circular economy (SusPlast), in the Aerogels COST ACTION (CA 18125), and in CSIC-Conexión Nanomedicine, EPNOE network, and Red Nanocare 2.0. The authors acknowledge the use of Biorender.com.Peer ReviewedPostprint (published version

Conjugated materials for thermoelectrics and photovoltaics

Esta tesis explora cómo las interacciones entre materiales orgánicos pueden ser aprovechadas para obtener funcionalidad adicional de una manera sencilla, sin necesidad de procesamiento complejo. Los resultados obtenidos tienen aplicaciones en el campo de la fotovoltaica y termoeléctrica orgánica. El trabajo se centra en materiales tales como polímeros conjugados y nanotubos de carbono, y procesos de deposición desde disolución simples, como el recubrimiento por cuchilla. La primera parte presenta resultados sobre la caracterización elipsométrica de las propiedades ópticas de los polímeros conjugados y sus mezclas con fullerenos o dopantes. La elipsometría espectroscópica de ángulo variable se utilizó no sólo para caracterizar las constantes ópticas de nuevos polímeros altamente absorbentes, sino también para investigar el efecto de los aditivos disolventes sobre el grado de segregación vertical de fase en mezclas de polímero:fullereno. La segunda parte detalla el trabajo sobre nanocompuestos de polímeros conjugados y nanotubos de carbono, una prometedora clase de materiales termoeléctricos orgánicos. Debido a que los polímeros conjugados permiten desenredar los manojos de nanotubos de carbono de manera eficiente, estos nanocompuestos se pueden preparar fácilmente. Presentan tanto una buena conductividad eléctrica como una baja conductividad térmica, que son requisitos necesarios para un buen rendimiento termoeléctrico. De particular interés son los compuestos de tipo n que contienen nanotubos de carbono dopados con nitrógeno, así como métodos de procesamiento que permiten cambiar el tipo de portador mayoritario. La tercera parte se centra en técnicas sencillas de fabricación de dispositivos fotovoltaicos orgánicos, con el objetivo particular de obtener capas orientadas de polímeros conjugados. Esto se logró controlando localmente la evaporación del disolvente para influir en la cristalización epitaxial direccional de polímeros conjugados en un aditivo disolvente cristalino. El método desarrollado permite preparar dos tipos distintos de películas. Si la nucleación del aditivo se limita a la línea de contacto unidimensional durante el recubrimiento con cuchilla, se obtienen entonces películas uniaxialmente orientadas con una morfología fibrilar. Al limitar la nucleación a un punto, se puede crecer una superestructura cristalina circular conocida como esferulita en cualquier lugar deseado de la película. Los dispositivos fotovoltaicos orgánicos que se prepararon a partir de estas películas orientadas tienen aplicaciones como detectores del estado de polarización de la luz.This thesis explores ways of how interactions between organic materials can be exploited to obtain additional functionality in a simple manner, without the need for complex processing. The obtained results have applications in the field of organic photovoltaics and thermoelectrics. The work focuses on materials such as conjugated polymers and carbon nanotubes, and simple solution-based deposition processes such as blade coating. The first part presents results on the ellipsometric characterization of the optical properties of conjugated polymers and their blends with fullerenes or dopants. Variable angle spectroscopic ellipsometry was used not only to characterize the optical constants of new, highly absorbing polymers, but also to investigate the effect of solvent additives on the degree of vertical phase segregation in polymer:fullerene blends. The second part details the work on nanocomposites of conjugated polymers and carbon nanotubes, a promising class of organic thermoelectric materials. Because conjugated polymers allow for efficient debundling of carbon nanotubes, these nanocomposites can be prepared readily. They exhibit both a good electrical conductivity and a low thermal conductivity, which are necessary requisites for good thermoelectric performance. Of particular interest are n-type composites containing nitrogen-doped carbon nanotubes, as well as processing methods that allow to change the majority carrier type. The third part focuses on simple fabrication techniques for organic photovoltaic devices, with the particular objective of obtaining oriented layers of conjugated polymers. This was achieved by locally controlling solvent evaporation to influence the directional epitaxial crystallization of conjugated polymers on a crystalline solvent additive. The developed method allows to prepare two distinct types of films. If the nucleation of the additive is confined to the one-dimensional contact line during blade-coating, then uniaxially oriented films with a fibrillar morphology are obtained. By confining the nucleation to a point, a circular crystalline superstructure known as a spherulite can be grown at any desired location in the film. The organic photovoltaic devices that were prepared from these oriented films have applications as detectors of the polarization state of light

Investigating the effect of solvent boiling temperature in P3HT:PCBM diffusive bilayer solar cells

Using chlorobenzene as a base solvent for the deposition of the poly(3-hexylthiophene-2,5-diyl) (P3HT) layer in P3HT:phenyl-C61-butyric acid methyl ester diffusive bilayer solar cells, we investigate the effect of adding of small amounts of high-boiling-point solvents with similar chemical structures on the resulting active layer morphologies. The results demonstrate that the crystallinity of the P3HT films as well as the vertical donor–acceptor gradient in the active layer can be tuned by this approach. The use of high-boiling-point solvents improved all photovoltaic parameters and resulted in a 32% increase in power conversion efficiency

A setup to measure the Seebeck coefficient and electrical conductivity of anisotropic thin-films on a single sample

This work documents an all-in-one custom setup, that allows to measure the in-plane Seebeck coefficients and electrical conductivities of anisotropic thin film samples close to room temperature. Both pairs, Sk,σk, and S⊥ and σ⊥ can be measured using four contacts on the same sample, reducing measurement time and minimizing potential sources of error due to aggregating data from several distinct samples. The setup allows to measure the electrical conductivity of isotropic samples using the well-known van der Pauw method. For samples with in-plane anisotropy, the two components σk and σ⊥ can be extracted from the same type of measurements by performing additional calculations. Using the same contacts, the Seebeck coefficient along one direction is measured using a differential state-state method. After rotating the sample by 90◦ , the orthogonal Seebeck component can be measured. In order to show the generality of the method, we measure different types of samples, from metal references to oriented doped conjugated polymers.The authors would like to thank José P. Jurado for helpful discussions and Rafael León for his help designing and machining the sample holder. The authors acknowledge the financial support from the Spanish Ministry of Science and Innovation through the “Severo Ochoa” Program for Centers of Excellence in R&D (Grant No. SEV-2015-0496 and Grant No. PGC2018-095411-B-I00), from the Generalitat de Catalunya, through Grant No. AGAUR 2018 PROD 00191, and from the European Research Council (ERC), under Grant Agreement No. 648901. O.Z.-A. acknowledges CONACYTSENER for his Ph.D. scholarship (Grant No. 472571).Peer reviewe