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

Tunable Fano resonance in symmetric multilayered gold nanoshells

Fano resonances (FRs) are produced when a discrete state is coupled with a continuum. In addition to fundamental scientific interests, FRs in plasmonic systems give rise to the so-called plasmon-induced transparency. In this work we have studied the evolution of dipole-dipole all-plasmonic FRs in symmetric multilayered nanoshells as the function of their geometrical parameters. We demonstrate that symmetry breaking is not mandatory for controlling the Fano resonance in such multilayered nanoshells. Generation of FRs in these symmetric nanostructures presents clear advantages over their asymmetric counterparts, as they are easier to fabricate and can be used in a wider range of technological applications

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

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

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

Polarization Switching and Light-Enhanced Piezoelectricity in Lead Halide Perovskites

We investigate the ferroelectric properties of photovoltaic methylammonium lead halide CH3NH3PbI3 perovskite using piezoelectric force microscopy (PFM) and macroscopic polarization methods. The electric polarization is clearly observed by amplitude and phase hysteresis loops. However, the polarization loop decreases as the frequency is lowered, persisting for a short time only, in the one second regime, indicating that CH3NH3PbI3 does not exhibit permanent polarization at room temperature. This result is confirmed by macroscopic polarization measurement based on a standard capacitive method. We have observed a strong increase of piezoelectric response under illumination, consistent with the previously reported giant photoinduced dielectric constant at low frequencies. We speculate that an intrinsic charge transfer photoinduced dipole in the perovskite cage may lie at the origin of this effect.Financial support from the Spanish Ministerio de Economia y Competitividad (MINECO) through RyC contracts 2013-12448 and 2011-07726 and projects MAT2013-47192-C3-1-R and MAT2012-3777

Understanding the effect of unintentional doping on transport optimization and analysis in efficient organic bulk-heterojunction solar cells

In this paper, we provide experimental evidence of the effects of unintentional p-type doping on the performance and the apparent recombination dynamics of bulk-heterojunction solar cells. By supporting these experimental observations with drift-diffusion simulations on two batches of the same efficient polymer-fullerene solar cells with substantially different doping levels and at different thicknesses, we investigate the way the presence of doping affects the interpretation of optoelectronic measurements of recombination and charge transport in organic solar cells. We also present experimental evidence on how unintentional doping can lead to excessively high apparent reaction orders. Our work suggests first that the knowledge of the level of dopants is essential in the studies of recombination dynamics and carrier transport and that unintentional doping levels need to be reduced below approximately 7 × 1015 cm-3 for full optimization around the second interference maximum of highly efficient polymer-fullerene solar cells.F. D. and J. R. D. are thankful of the support from the EPSRC APEX Grant No. EP/H040218/2 and SPECIFIC Grant No. EP/1019278. T. K. acknowledges funding by an Imperial College Junior Research Fellowship. We are grateful to the Ministerio de Economa y Competitividad for funding through the project PHOTOCOMB, Reference No. MAT2012-37776.Peer Reviewe

Reduction of the Lattice Thermal Conductivity of Polymer Semiconductors by Molecular Doping

Here we show that molecular doping of polymer thermoelectrics increases the electrical conductivity while reducing the thermal conductivity. A high-throughput methodology based on annealing and doping gradients within individual films is employed to self-consistently analyze and correlate electrical and thermal characteristics for the equivalent of >100 samples. We focus on the benchmark material system poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) doped with molecular acceptor 2,3,5,6-tetrafittoro-7,7,8,8-tetra-cyanoquinodimethane (F4TCNQ). The thermal conductivity of neat PBTTT films is dominated by the degree of crystallinity, with thermal percolation observed for annealing temperatures >170 degrees C. Upon doping the samples with a relatively low amount of F4TCNQ (anion content <1 mol %), the thermal conductivity exhibits a two-fold reduction without compromising the crystalline quality, which resembles the effect of alloy scattering observed in several inorganic systems. The analysis of the relation between thermal and electrical conductivities shows that thermal transport is dominated by a doping-induced reduced lattice contribution.We acknowledge financial support from the Spanish Ministry of Science and Innovation through projects PGC2018-095411-B-I00 and MAT2017-90024-P (TANGENTS)-EI/FEDER, UE projects; the Generalitat de Catalunya through grants 2017SGR488 and AGAUR 2018 PROD 00191; and from the European Research Council (ERC) under grant agreement no. 648901. O.Z.-A. acknowledges CONACYT-SENER for his Ph.D. scholarship (no. 472571). J.M. thanks MCIU for the Ramo ' n y Cajal contract and grant PGC2018-094620-A-I00. We thank Dr. Agusti ' n Mihi for the access to and support with the FTIR equipment. We acknowledge the technical and human support provided by SGIker of UPV/EHU and European funding (ERDF and ESF). We thank Andre ' s Gom ' ez Rodrig ' uez from the Scanning Probe Microscopy Laboratory (ICMAB-CSIC) for a set of AFM 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)

Reduction of the Lattice Thermal Conductivity of Polymer Semiconductors by Molecular Doping

Here we show that molecular doping of polymer thermoelectrics increases the electrical conductivity while reducing the thermal conductivity. A high-throughput methodology based on annealing and doping gradients within individual films is employed to self-consistently analyze and correlate electrical and thermal characteristics for the equivalent of >100 samples. We focus on the benchmark material system poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) doped with molecular acceptor 2,3,5,6-tetrafittoro-7,7,8,8-tetra-cyanoquinodimethane (F4TCNQ). The thermal conductivity of neat PBTTT films is dominated by the degree of crystallinity, with thermal percolation observed for annealing temperatures >170 degrees C. Upon doping the samples with a relatively low amount of F4TCNQ (anion content <1 mol %), the thermal conductivity exhibits a two-fold reduction without compromising the crystalline quality, which resembles the effect of alloy scattering observed in several inorganic systems. The analysis of the relation between thermal and electrical conductivities shows that thermal transport is dominated by a doping-induced reduced lattice contribution.We acknowledge financial support from the Spanish Ministry of Science and Innovation through projects PGC2018-095411-B-I00 and MAT2017-90024-P (TANGENTS)-EI/FEDER, UE projects; the Generalitat de Catalunya through grants 2017SGR488 and AGAUR 2018 PROD 00191; and from the European Research Council (ERC) under grant agreement no. 648901. O.Z.-A. acknowledges CONACYT-SENER for his Ph.D. scholarship (no. 472571). J.M. thanks MCIU for the Ramo ' n y Cajal contract and grant PGC2018-094620-A-I00. We thank Dr. Agusti ' n Mihi for the access to and support with the FTIR equipment. We acknowledge the technical and human support provided by SGIker of UPV/EHU and European funding (ERDF and ESF). We thank Andre ' s Gom ' ez Rodrig ' uez from the Scanning Probe Microscopy Laboratory (ICMAB-CSIC) for a set of AFM 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)