@article{SiniSchubertRiskoetal.2018, author = {Sini, Gjergji and Schubert, Marcel and Risko, Chad and Roland, Steffen and Lee, Olivia P. and Chen, Zhihua and Richter, Thomas V. and Dolfen, Daniel and Coropceanu, Veaceslav and Ludwigs, Sabine and Scherf, Ullrich and Facchetti, Antonio and Frechet, Jean M. J. and Neher, Dieter}, title = {On the Molecular Origin of Charge Separation at the Donor-Acceptor Interface}, series = {Advanced energy materials}, volume = {8}, journal = {Advanced energy materials}, number = {12}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1614-6832}, doi = {10.1002/aenm.201702232}, pages = {15}, year = {2018}, abstract = {Fullerene-based acceptors have dominated organic solar cells for almost two decades. It is only within the last few years that alternative acceptors rival their dominance, introducing much more flexibility in the optoelectronic properties of these material blends. However, a fundamental physical understanding of the processes that drive charge separation at organic heterojunctions is still missing, but urgently needed to direct further material improvements. Here a combined experimental and theoretical approach is used to understand the intimate mechanisms by which molecular structure contributes to exciton dissociation, charge separation, and charge recombination at the donor-acceptor (D-A) interface. Model systems comprised of polythiophene-based donor and rylene diimide-based acceptor polymers are used and a detailed density functional theory (DFT) investigation is performed. The results point to the roles that geometric deformations and direct-contact intermolecular polarization play in establishing a driving force ( energy gradient) for the optoelectronic processes taking place at the interface. A substantial impact for this driving force is found to stem from polymer deformations at the interface, a finding that can clearly lead to new design approaches in the development of the next generation of conjugated polymers and small molecules.}, language = {en} } @article{KurpiersFerronRolandetal.2018, author = {Kurpiers, Jona and Ferron, Thomas and Roland, Steffen and Jakoby, Marius and Thiede, Tobias and Jaiser, Frank and Albrecht, Steve and Janietz, Silvia and Collins, Brian A. and Howard, Ian A. and Neher, Dieter}, title = {Probing the pathways of free charge generation in organic bulk heterojunction solar cells}, series = {Nature Communications}, volume = {9}, journal = {Nature Communications}, publisher = {Nature Publ. Group}, address = {London}, issn = {2041-1723}, doi = {10.1038/s41467-018-04386-3}, pages = {11}, year = {2018}, abstract = {The fact that organic solar cells perform efficiently despite the low dielectric constant of most photoactive blends initiated a long-standing debate regarding the dominant pathways of free charge formation. Here, we address this issue through the accurate measurement of the activation energy for free charge photogeneration over a wide range of photon energy, using the method of time-delayed collection field. For our prototypical low bandgap polymer:fullerene blends, we find that neither the temperature nor the field dependence of free charge generation depend on the excitation energy, ruling out an appreciable contribution to free charge generation though hot carrier pathways. On the other hand, activation energies are on the order of the room temperature thermal energy for all studied blends. We conclude that charge generation in such devices proceeds through thermalized charge transfer states, and that thermal energy is sufficient to separate most of these states into free charges.}, language = {en} } @article{HoermannZeiskePiersimonietal.2018, author = {H{\"o}rmann, Ulrich and Zeiske, Stefan and Piersimoni, Fortunato and Hoffmann, Lukas and Schlesinger, Raphael and Koch, Norbert and Riedl, Thomas and Andrienko, Denis and Neher, Dieter}, title = {Stark effect of hybrid charge transfer states at planar ZnO/organic interfaces}, series = {Physical review : B, Condensed matter and materials physics}, volume = {98}, journal = {Physical review : B, Condensed matter and materials physics}, number = {15}, publisher = {American Physical Society}, address = {College Park}, issn = {2469-9950}, doi = {10.1103/PhysRevB.98.155312}, pages = {9}, year = {2018}, abstract = {We investigate the bias dependence of the hybrid charge transfer state emission at planar heterojunctions between the metal oxide acceptor ZnO and three donor molecules. The electroluminescence peak energy linearly increases with the applied bias, saturating at high fields. Variation of the organic layer thickness and deliberate change of the ZnO conductivity through controlled photodoping allow us to confirm that this bias-induced spectral shift relates to the internal electric field in the organic layer rather than the filling of states at the hybrid interface. We show that existing continuum models overestimate the hole delocalization and propose a simple electrostatic model in which the linear and quadratic Stark effects are explained by the electrostatic interaction of a strongly polarizable molecular cation with its mirror image.}, language = {en} } @misc{ShoaeeStolterfohtNeher2018, author = {Shoaee, Safa and Stolterfoht, Martin and Neher, Dieter}, title = {The Role of Mobility on Charge Generation, Recombination, and Extraction in Polymer-Based Solar Cells}, series = {dvanced energy materials}, volume = {8}, journal = {dvanced energy materials}, number = {28}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1614-6832}, doi = {10.1002/aenm.201703355}, pages = {20}, year = {2018}, abstract = {Organic semiconductors are of great interest for a broad range of optoelectronic applications due to their solution processability, chemical tunability, highly scalable fabrication, and mechanical flexibility. In contrast to traditional inorganic semiconductors, organic semiconductors are intrinsically disordered systems and therefore exhibit much lower charge carrier mobilities-the Achilles heel of organic photovoltaic cells. In this progress review, the authors discuss recent important developments on the impact of charge carrier mobility on the charge transfer state dissociation, and the interplay of free charge extraction and recombination. By comparing the mobilities on different timescales obtained by different techniques, the authors highlight the dispersive nature of these materials and how this reflects on the key processes defining the efficiency of organic photovoltaics.}, language = {en} } @article{StolterfohtWolffMarquezetal.2018, author = {Stolterfoht, Martin and Wolff, Christian Michael and Marquez, Jose A. and Zhang, Shanshan and Hages, Charles J. and Rothhardt, Daniel and Albrecht, Steve and Burn, Paul L. and Meredith, Paul and Unold, Thomas and Neher, Dieter}, title = {Visualization and suppression of interfacial recombination for high-efficiency large-area pin perovskite solar cells}, series = {Nature Energy}, volume = {3}, journal = {Nature Energy}, number = {10}, publisher = {Nature Publ. Group}, address = {London}, issn = {2058-7546}, doi = {10.1038/s41560-018-0219-8}, pages = {847 -- 854}, year = {2018}, abstract = {The performance of perovskite solar cells is predominantly limited by non-radiative recombination, either through trap-assisted recombination in the absorber layer or via minority carrier recombination at the perovskite/transport layer interfaces. Here, we use transient and absolute photoluminescence imaging to visualize all non-radiative recombination pathways in planar pintype perovskite solar cells with undoped organic charge transport layers. We find significant quasi-Fermi-level splitting losses (135 meV) in the perovskite bulk, whereas interfacial recombination results in an additional free energy loss of 80 meV at each individual interface, which limits the open-circuit voltage (V-oc) of the complete cell to similar to 1.12 V. Inserting ultrathin interlayers between the perovskite and transport layers leads to a substantial reduction of these interfacial losses at both the p and n contacts. Using this knowledge and approach, we demonstrate reproducible dopant-free 1 cm(2) perovskite solar cells surpassing 20\% efficiency (19.83\% certified) with stabilized power output, a high V-oc (1.17 V) and record fill factor (>81\%).}, language = {en} }