@article{RanRolandLoveetal.2017, author = {Ran, Niva A. and Roland, Steffen and Love, John A. and Savikhin, Victoria and Takacs, Christopher J. and Fu, Yao-Tsung and Li, Hong and Coropceanu, Veaceslav and Liu, Xiaofeng and Bredas, Jean-Luc and Bazan, Guillermo C. and Toney, Michael F. and Neher, Dieter and Thuc-Quyen Nguyen,}, title = {Impact of interfacial molecular orientation on radiative recombination and charge generation efficiency}, series = {Nature Communications}, volume = {8}, journal = {Nature Communications}, publisher = {Nature Publ. Group}, address = {London}, issn = {2041-1723}, doi = {10.1038/s41467-017-00107-4}, pages = {9}, year = {2017}, abstract = {A long standing question in organic electronics concerns the effects of molecular orientation at donor/acceptor heterojunctions. Given a well-controlled donor/acceptor bilayer system, we uncover the genuine effects of molecular orientation on charge generation and recombination. These effects are studied through the point of view of photovoltaics-however, the results have important implications on the operation of all optoelectronic devices with donor/ acceptor interfaces, such as light emitting diodes and photodetectors. Our findings can be summarized by two points. First, devices with donor molecules face-on to the acceptor interface have a higher charge transfer state energy and less non-radiative recombination, resulting in larger open-circuit voltages and higher radiative efficiencies. Second, devices with donor molecules edge-on to the acceptor interface are more efficient at charge generation, attributed to smaller electronic coupling between the charge transfer states and the ground state, and lower activation energy for charge generation.}, language = {en} } @article{AsawapiromBulutFarrelletal.2004, author = {Asawapirom, Udom and Bulut, F. and Farrell, Tony and Gadermaier, C. and Gamerith, S. and G{\"u}ntner, Roland and Kietzke, Thomas and Patil, S. and Piok, T. and Montenegro, Rivelino V. D. and Stiller, Burkhard and Tiersch, Brigitte and Landfester, Katharina and List, E. J. W. and Neher, Dieter and Torres, C. S. and Scherf, Ullrich}, title = {Materials for polymer electronics applications semiconducting polymer thin films and nanoparticles}, issn = {1022-1360}, year = {2004}, abstract = {The paper presents two different approaches to nanostructured semiconducting polymer materials: (i) the generation of aqueous semiconducting polymer dispersions (semiconducting polymer nanospheres SPNs) and their processing into dense films and layers, and (ii) the synthesis of novel semiconducting polyfluorene-block-polyaniline (PF-b-PANI) block copolymers composed of conjugated blocks of different redox potentials which form nanosized morphologies in the solid state}, language = {en} } @article{RivnaySteyrleuthnerJimisonetal.2011, author = {Rivnay, Jonathan and Steyrleuthner, Robert and Jimison, Leslie H. and Casadei, Alberto and Chen, Zhihua and Toney, Michael F. and Facchetti, Antonio and Neher, Dieter and Salleo, Alberto}, title = {Drastic control of texture in a high performance n-Type polymeric semiconductor and implications for charge transport}, series = {Macromolecules : a publication of the American Chemical Society}, volume = {44}, journal = {Macromolecules : a publication of the American Chemical Society}, number = {13}, publisher = {American Chemical Society}, address = {Washington}, issn = {0024-9297}, doi = {10.1021/ma200864s}, pages = {5246 -- 5255}, year = {2011}, abstract = {Control of crystallographic texture from mostly face-on to edge-on is observed for the film morphology of the n-type semicrystalline polymer [N,N-9-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diy1]alt-5,59-(2,29-bithiophene)}, P(NDI2OD-T2), when annealing the film to the polymer melting point followed by slow cooling to ambient temperature. A variety of X-ray diffraction analyses, including pole figure construction and Fourier transform peak shape deconvolution, are employed to quantify the texture change, relative degree of crystallinity and lattice order. We find that annealing the polymer film to the melt leads to a shift from 77.5\% face-on to 94.6\% edge-on lamellar texture as well as to a 2-fold increase in crystallinity and a 40\% decrease in intracrystallite cumulative disorder. The texture change results in a significant drop in the electron-only diode current density through the film thickness upon melt annealing while little change is observed in the in-plane transport of bottom gated thin film transistors. This suggests that the texture change is prevalent in the film interior and that either the (bottom) surface structure is different from the interior structure or the intracrystalline order and texture play a secondary role in transistor transport for this material.}, language = {en} } @article{IlnytskyiNeherSaphiannikova2011, author = {Ilnytskyi, Jaroslav M. and Neher, Dieter and Saphiannikova, Marina}, title = {Opposite photo-induced deformations in azobenzene-containing polymers with different molecular architecture molecular dynamics study}, series = {The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr}, volume = {135}, journal = {The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr}, number = {4}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0021-9606}, doi = {10.1063/1.3614499}, pages = {12}, year = {2011}, abstract = {Photo-induced deformations in azobenzene-containing polymers (azo-polymers) are central to a number of applications, such as optical storage and fabrication of diffractive elements. The microscopic nature of the underlying opto-mechanical coupling is yet not clear. In this study, we address the experimental finding that the scenario of the effects depends on molecular architecture of the used azo-polymer. Typically, opposite deformations in respect to the direction of light polarization are observed for liquid crystalline and amorphous azo-polymers. In this study, we undertake molecular dynamics simulations of two different models that mimic these two types of azo-polymers. We employ hybrid force field modeling and consider only trans-isomers of azobenzene, represented as Gay-Berne sites. The effect of illumination on the orientation of the chromophores is considered on the level of orientational hole burning and emphasis is given to the resulting deformation of the polymer matrix. We reproduce deformations of opposite sign for the two models being considered here and discuss the relevant microscopic mechanisms in both cases.}, language = {en} } @article{KniepertSchubertBlakesleyetal.2011, author = {Kniepert, Juliane and Schubert, Marcel and Blakesley, James C. and Neher, Dieter}, title = {Photogeneration and recombination in P3HT/PCBM solar cells probed by time-delayed collection field experiments}, series = {The journal of physical chemistry letters}, volume = {2}, journal = {The journal of physical chemistry letters}, number = {7}, publisher = {American Chemical Society}, address = {Washington}, issn = {1948-7185}, doi = {10.1021/jz200155b}, pages = {700 -- 705}, year = {2011}, abstract = {Time-delayed collection field (TDCF) experiments are performed on bulk heterojunction solar cells comprised of a blend of poly(3-hexylthiophene) and [6,6]-phenyl C-71 butyric acid methyl ester. TDCF is analogous to a pump-probe experiment using optical excitation and an electrical probe with a resolution of < 100 ns. The number of free charge carriers extracted after a short delay is found to be independent of the electric field during illumination. Also, experiments performed with a variable delay between the optical excitation and the electrical probe do not reveal any evidence for the generation of charge via field-assisted dissociation of bound long-lived polaron pairs. Photocurrent transients are well fitted by computational drift diffusion simulations with only direct generation of free charge carriers. With increasing delay times between pump and probe, two loss mechanisms are identified; first, charge-carriers are swept out of the device by the internal electric field, and second, bimolecular recombination of the remaining carriers takes place with a reduced recombination coefficient.}, language = {en} } @article{KuehnPingelBreusingetal.2011, author = {Kuehn, Sergei and Pingel, Patrick and Breusing, Markus and Fischer, Thomas and Stumpe, Joachim and Neher, Dieter and Elsaesser, Thomas}, title = {High-Resolution Near-Field Optical Investigation of Crystalline Domains in Oligomeric PQT-12 Thin Films}, series = {Advanced functional materials}, volume = {21}, journal = {Advanced functional materials}, number = {5}, publisher = {Wiley-Blackwell}, address = {Malden}, issn = {1616-301X}, doi = {10.1002/adfm.201001978}, pages = {860 -- 868}, year = {2011}, abstract = {The structure and morphology on different length scales dictate both the electrical and optical properties of organic semiconductor thin films. Using a combination of spectroscopic methods, including scanning near-field optical microscopy, we study the domain structure and packing quality of highly crystalline thin films of oligomeric PQT-12 with 100 nanometer spatial resolution. The pronounced optical anisotropy of these layers measured by polarized light microscopy facilitates the identification of regions with uniform molecular orientation. We find that a hierarchical order on three different length scales exists in these layers, made up of distinct well-ordered dichroic areas at the ten-micrometer-scale, which are sub-divided into domains with different molecular in-plane orientation. These serve as a template for the formation of smaller needle-like crystallites at the layer surface. A high degree of crystalline order is believed to be the cause of the rather high field-effect mobility of these layers of 10(-3) cm 2 V(-1) s(-1), whereas it is limited by the presence of domain boundaries at macroscopic distances.}, language = {en} } @article{BlakesleyNeher2011, author = {Blakesley, James C. and Neher, Dieter}, title = {Relationship between energetic disorder and open-circuit voltage in bulk heterojunction organic solar cells}, series = {Physical review : B, Condensed matter and materials physics}, volume = {84}, journal = {Physical review : B, Condensed matter and materials physics}, number = {7}, publisher = {American Physical Society}, address = {College Park}, issn = {1098-0121}, doi = {10.1103/PhysRevB.84.075210}, pages = {12}, year = {2011}, abstract = {We simulate organic bulk heterojunction solar cells. The effects of energetic disorder are incorporated through a Gaussian or exponential model of density of states. Analytical models of open-circuit voltage (V(OC)) are derived from the splitting of quasi-Fermi potentials. Their predictions are backed up by more complex numerical device simulations including effects such as carrier-density-dependent charge-carrier mobilities. It is predicted that the V(OC) depends on: (1) the donor-acceptor energy gap; (2) charge-carrier recombination rates; (3) illumination intensity; (4) the contact work functions (if not in the pinning regime); and (5) the amount of energetic disorder. A large degree of energetic disorder, or a high density of traps, is found to cause significant reductions in V(OC). This can explain why V(OC) is often less than expected in real devices. Energetic disorder also explains the nonideal temperature and intensity dependence of V(OC) and the superbimolecular recombination rates observed in many real bulk heterojunction solar cells.}, language = {en} } @article{LangeBlakesleyFrischetal.2011, author = {Lange, Ilja and Blakesley, James C. and Frisch, Johannes and Vollmer, Antje and Koch, Norbert and Neher, Dieter}, title = {Band bending in conjugated polymer layers}, series = {Physical review letters}, volume = {106}, journal = {Physical review letters}, number = {21}, publisher = {American Physical Society}, address = {College Park}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.106.216402}, pages = {4}, year = {2011}, abstract = {We use the Kelvin probe method to study the energy-level alignment of four conjugated polymers deposited on various electrodes. Band bending is observed in all polymers when the substrate work function exceeds critical values. Through modeling, we show that the band bending is explained by charge transfer from the electrodes into a small density of states that extends several hundred meV into the band gap. The energetic spread of these states is correlated with charge-carrier mobilities, suggesting that the same states also govern charge transport in the bulk of these polymers.}, language = {en} } @article{SchubertCollinsMangoldetal.2014, author = {Schubert, Marcel and Collins, Brian A. and Mangold, Hannah and Howard, Ian A. and Schindler, Wolfram and Vandewal, Koen and Roland, Steffen and Behrends, Jan and Kraffert, Felix and Steyrleuthner, Robert and Chen, Zhihua and Fostiropoulos, Konstantinos and Bittl, Robert and Salleo, Alberto and Facchetti, Antonio and Laquai, Frederic and Ade, Harald W. and Neher, Dieter}, title = {Correlated donor/acceptor crystal orientation controls photocurrent generation in all-polymer solar cells}, series = {Advanced functional materials}, volume = {24}, journal = {Advanced functional materials}, number = {26}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1616-301X}, doi = {10.1002/adfm.201304216}, pages = {4068 -- 4081}, year = {2014}, abstract = {New polymers with high electron mobilities have spurred research in organic solar cells using polymeric rather than fullerene acceptors due to their potential of increased diversity, stability, and scalability. However, all-polymer solar cells have struggled to keep up with the steadily increasing power conversion efficiency of polymer: fullerene cells. The lack of knowledge about the dominant recombination process as well as the missing concluding picture on the role of the semi-crystalline microstructure of conjugated polymers in the free charge carrier generation process impede a systematic optimization of all-polymer solar cells. These issues are examined by combining structural and photo-physical characterization on a series of poly(3-hexylthiophene) (donor) and P(NDI2OD-T2) (acceptor) blend devices. These experiments reveal that geminate recombination is the major loss channel for photo-excited charge carriers. Advanced X-ray and electron-based studies reveal the effect of chloronaphthalene co-solvent in reducing domain size, altering domain purity, and reorienting the acceptor polymer crystals to be coincident with those of the donor. This reorientation correlates well with the increased photocurrent from these devices. Thus, effi cient split-up of geminate pairs at polymer/polymer interfaces may necessitate correlated donor/acceptor crystal orientation, which represents an additional requirement compared to the isotropic fullerene acceptors.}, language = {en} } @misc{MouleNeherTurner2014, author = {Moule, Adam J. and Neher, Dieter and Turner, Sarah T.}, title = {P3HT-Based solar cells: structural properties and photovoltaic performance}, series = {Advances in Polymer Science}, volume = {265}, journal = {Advances in Polymer Science}, editor = {Ludwigs, S}, publisher = {Springer}, address = {Berlin}, isbn = {978-3-662-45145-8; 978-3-662-45144-1}, issn = {0065-3195}, doi = {10.1007/12_2014_289}, pages = {181 -- 232}, year = {2014}, abstract = {Each year we are bombarded with B.Sc. and Ph.D. applications from students that want to improve the world. They have learned that their future depends on changing the type of fuel we use and that solar energy is our future. The hope and energy of these young people will transform future energy technologies, but it will not happen quickly. Organic photovoltaic devices are easy to sketch, but the materials, processing steps, and ways of measuring the properties of the materials are very complicated. It is not trivial to make a systematic measurement that will change the way other research groups think or practice. In approaching this chapter, we thought about what a new researcher would need to know about organic photovoltaic devices and materials in order to have a good start in the subject. Then, we simplified that to focus on what a new researcher would need to know about poly-3-hexylthiophene: phenyl-C61-butyric acid methyl ester blends (P3HT: PCBM) to make research progress with these materials. This chapter is by no means authoritative or a compendium of all things on P3HT: PCBM. We have selected to explain how the sample fabrication techniques lead to control of morphology and structural features and how these morphological features have specific optical and electronic consequences for organic photovoltaic device applications.}, language = {en} }