Organic bulk heterojunctions combining electron donor and acceptor phases are of great interest for designing organic photovoltaic devices. [1] While impressive advances have been achieved with these systems, so far a deterministic control of their nanoscale morphology has been elusive. It would be a major breakthrough to be able to create model systems with periodic, interpenetrating networks of electron donor and acceptor phases providing maximum control over all structural and electronic features.Herein we report a significant step towards this goal on the basis of the recently discovered class of crystalline covalent organic frameworks (COFs) which are created by condensation of molecular building blocks. [2][3][4][5] Specifically, the stacked layers of two-dimensional COFs permit charge migration through the framework, [6] and several semiconducting structures [7] with high carrier mobilities [8][9][10] have been described. We have created a COF containing stacked thieno[2,3-b]thiophene-based building blocks serving as electron donors (TT-COF), with high surface area and a 3 nm open pore system. This open framework takes up the wellknown fullerene electron acceptor [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM), thus forming a novel structurally ordered donor-acceptor network. Spectroscopic results demonstrate light-induced charge transfer from the photoconductive TT-COF donor network to the encapsulated PCBM phase in the pore system. Moreover, we have created the first working COF-based photovoltaic device with the above components. The organization of the molecular building blocks into a crystalline framework with defined conduction paths provides a promising model system for ordered and interpenetrated networks of donors and acceptors at the nanoscale.The most prominent hole-conducting material used in organic solar cells is poly(3-hexylthiophene) (P3HT), a thiophene-containing polymer with high charge-carrier mobilities. The soluble fullerene derivative PCBM is often used as an electron acceptor in organic photovoltaics. [11] Because of the lack of structural order in the respective bulk heterojunctions it is very difficult to assess the impact of molecular building blocks, bonding motifs, and energy levels on the microscopic processes involving light-induced exciton formation, charge separation, and transport in such systems. Hence ordered charge-transporting networks with a periodicity of several nanometers are of great interest to understand the mechanistic details of the light-induced processes and ultimately to obtain design rules for the creation of efficient and stable organic photovoltaic devices. [12,13] The new TT-COF was synthesized under solvothermal conditions by co-condensation of thieno[3,2-b]thiophene-2,5diyldiboronic acid (TTBA) and the polyol 2,3,6,7,10,11hexahydroxytriphenylene (HHTP; Figure 1 a). Reaction parameters are described in the Supporting Information.As described in the following, the thienothiophene-based COF forms stacks in an AA arrangement, as confirmed by N 2 sorpti...

Covalent organic frameworks (COFs) offer a strategy to position molecular semiconductors within a rigid network in a highly controlled and predictable manner. The π-stacked columns of layered two-dimensional COFs enable electronic interactions between the COF sheets, thereby providing a path for exciton and charge carrier migration. Frameworks comprising two electronically separated subunits can form highly defined interdigitated donor–acceptor heterojunctions, which can drive the photogeneration of free charge carriers. Here we report the first example of a photovoltaic device that utilizes exclusively a crystalline organic framework with an inherent type II heterojunction as the active layer. The newly developed triphenylene–porphyrin COF was grown as an oriented thin film with the donor and acceptor units forming one-dimensional stacks that extend along the substrate normal, thus providing an optimal geometry for charge carrier transport. As a result of the degree of morphological precision that can be achieved with COFs and the enormous diversity of functional molecular building blocks that can be used to construct the frameworks, these materials show great potential as model systems for organic heterojunctions and might ultimately provide an alternative to the current disordered bulk heterojunctions.

Adding cesium (Cs) and rubidium (Rb) cations to FA0.83MA0.17Pb(I0.83Br0.17)3 hybrid lead halide perovskites results in a remarkable improvement in solar cell performance, but the origin of the enhancement has not been fully understood yet. In this work, Time-of-Flight (ToF), Time-Resolved Microwave Conductivity (TRMC), and Thermally Stimulated Current (TSC) measurements were performed to elucidate the impact of the inorganic cation additives on the trap landscape and charge transport properties within perovskite solar cells. These complementary techniques allow for the assessment of both local features within the perovskite crystals and macroscopic properties of films and full devices. Strikingly, Cs-incorporation was shown to reduce the trap density and charge recombination rates in the perovskite layer. This is consistent with the significant improvements in the open-circuit voltage and fill factor of Cscontaining devices. By comparison, Rb-addition results in an increased charge carrier mobility, which is accompanied by a minor increase in device efficiency and reduced current-voltage hysteresis. By mixing Cs and Rb in quadruple cation (Cs-Rb-FA-MA) perovskites, the advantages of both inorganic cations can be combined. Our study provides valuable insights into the role of these additives in multiple-cation perovskite solar cells, which are essential for the design of highperformance devices.

Honigwaben unter Strom: Eine Thienothiophen‐basierte kovalente organische Gerüstverbindung (COF) wurde erzeugt, an der nach Beladen mit einem passenden Halbleiter, z. B. einem Fullerenderivat, elektronische Wechselwirkungen beobachtet werden. Das entstehende Donor‐Akzeptor‐System zeigt die spektroskopischen Signaturen eines effizienten Ladungstransfers auf der Nanoskala, und eine Funktionseinheit mit integriertem COF:Fulleren‐Film zeigt photovoltaische Aktivität.

Organic–inorganic metal halide perovskite solar cells have recently attracted considerable attention with reported device efficiencies approaching those achieved in polycrystalline silicon. Key for an efficient extraction of photogenerated carriers is the combination of low nonradiative relaxation rates leading to long carrier lifetimes and rapid diffusive transport. The latter, however, is difficult to assess directly with reported values varying widely. Here, we present an experimental approach for a contactless visualization of the charge carrier diffusion length and velocity in thin films based on time-resolved confocal detection of photoluminescence at varying distances from the excitation position. Our measurements on chloride-treated methylammonium lead iodide thin films, the material for which the highest solar cell efficiencies have been reported, reveal a charge carrier diffusion length of 5.5–7.7 μm and a transport time of 100 ps for the first micrometer corresponding to a diffusion constant of about 5–10 cm2 s–1, similar to GaAs thin films.

Understanding the charge transport characteristics and their limiting factors in organolead halide perovskites is of great importance for the development of competitive and economically advantageous photovoltaic systems derived from these materials. In the present work, we examine the charge carrier mobilities in CHNHPbI (MAPI) thin films obtained from a one-step synthesis procedure and in planar n-i-p devices based on these films. By performing time-of-flight measurements, we find mobilities around 6 cm/V s for electrons and holes in MAPI thin films, whereas in working solar cells, the respective effective mobility values are reduced by 3 orders of magnitude. From complementary experiments on devices with varying thicknesses of electron and hole transport layers, we identify the charge extraction layers and the associated interfaces rather than the perovskite material itself as the major limiting factors of the charge carrier transport time in working devices.

We investigate near-degenerate four-wave mixing in graphene using femtosecond laser pulse shaping microscopy. Intense near-degenerate four-wave mixing signals on either side of the exciting laser spectrum are controlled by amplitude and phase shaping. Quantitative signal modeling for the input pulse parameters shows a spectrally flat phase response of the near-degenerate four-wave mixing due to the linear dispersion of the massless Dirac Fermions in graphene. Exploiting these properties we demonstrate that graphene is uniquely suited for the intrafocus phase characterization and compression of broadband laser pulses, circumventing disadvantages of common methods utilizing second or third harmonic light.

State-of-the-art solar cells based on methylammonium lead iodide (MAPbI3 ) now reach efficiencies over 20 %. This fast improvement was possible with intensive research in perovskite processing. In particular, chloride-based precursors are known to have a positive influence on the crystallization of the perovskite. Here, we used a combination of in-situ X-ray diffraction and charge-transport measurements to understand the influence of chloride during perovskite crystallization in planar heterojunction solar cells. We show that MAPbCl3 crystallizes directly after the deposition of the starting solution and acts as a template for the formation of MAPbI3 . Additionally, we show that the charge-carrier mobility doubles by extending the time for the template formation. Our results give a deeper understanding of the influence of chloride in the synthesis of MAPbI3 and illustrate the importance of carefully controlling crystallization for reproducible, high-efficiency solar cells.