We present a comprehensive investigation of the charge-transfer (CT) effect in weakly interacting organic semiconductor mixtures. The donor-acceptor pair diindenoperylene (DIP) and N,N'-bis(2-ethylhexyl)-1,7-dicyanoperylene-3,4/9,10-bis(dicarboxyimide) (PDIR-CN) has been chosen as a model system. A wide range of experimental methods was used in order to characterize the structural, optical, electronic, and device properties of the intermolecular interactions. By detailed analysis, we demonstrate that the partial CT in this weakly interacting mixture does not have a strong effect on the ground state and does not generate a hybrid orbital. We also find a strong CT transition in light absorption as well as in photo- and electroluminescence. By using different layer sequences and compositions, we are able to distinguish electronic coupling in-plane vs out-of-plane and, thus, characterize the anisotropy of the CT state. Finally, we discuss the impact of CT exciton generation on charge-carrier transport and on the efficiency of photovoltaic devices.

Organic small molecule solar cells are used as a test bed to investigate the influence of film morphology on the density of charge-transfer (CT) states. CT states are considered as precursors for charge generation and their energy as the upper limit for the open-circuit voltage in organic donor-acceptor solar cells. In this study the influence of morphology for two perylene donors [crystalline diindenoperylene (DIP) versus amorphous tetraphenyldibenzoperiflanthene (DBP)] with almost identical ionization energy is investigated. As acceptor material, the fullerene C 60 is used. By combining device measurements with optical and low-energy ultraviolet photoelectron spectroscopy, a comprehensive picture is obtained that describes how morphology and the connected density of states (DOS) affect device performance and the spectroscopic signature of CT states. Especially for the crystalline donor material DIP, strong exponential tail states reaching far into the gap are observed, which can be related to the presence of grain boundaries. A voltage-dependent filling of these states is identified as the origin of a blue shift of electroluminescence spectra with increasing applied voltage. Different approaches are compared to study the influence of static and dynamic disorder in the description of CT emission and absorption spectra of organic solar cells. Despite the fact that both donors yield almost identical CT energy (and, thus, the same open-circuit voltage) the Stokes shift between photocurrent and electroluminescence spectra and, concomitantly, the width of the CT DOS varies by more than a factor of 2. We discuss this observation in terms of the donoracceptor reorganization energy as well as an additional line broadening by static disorder. Remarkably, the more crystalline donor DIP shows a significant deviation from a Marcus-type description, while this is not the case for the amorphous DBP. This highlights the importance of film morphology in organic solar cells.

We report controllable anisotropic light emission of photons originating from vertically aligned transition dipole moments in spun-cast films of CsPbBr nanocubes. By depositing films of nanocrystals on precoated substrates we can control the packing density and resultant radiation pattern of the emitted photons. We develop a technical framework to calculate the average orientation of light emitters, i.e., the angle between the transition dipole moment vector (TDM) and the substrate. This model is applicable to any emissive material with a known refractive index. Theoretical modeling indicates that oriented emission originates from an anisotropic alignment of the valence band and conduction band edge states on the ionic crystal lattice and demonstrates a general path to model the experimentally less accessible internal electric field of a nanosystem from the photoluminescent anisotropy. The uniquely accessible surface of the perovskite nanoparticles allows for perturbation of the normally isotropic emissive transition. The reported sensitive and tunable TDM orientation and control of emitted light will allow for applications of perovskite nanocrystals in a wide range of photonic technologies inaccessible to traditional light emitters.

processible PV technology has recently appeared as well. [5] Comparing the different technologies in terms of PCE, which is specified as the product of the short-circuit current density j SC , the open-circuit voltage V OC and the fill factor FF divided by the incoming light intensity under standard AM 1.5G illumination conditions, OPVs can well compete with their inorganic counterparts in terms of j SC or, more precisely, the external quantum efficiency and also with minor trade-off in FF, but clearly suffer from lower V OC at a given energy gap E g of the light absorbing material. While this socalled bandgap-voltage offset can be as low as 0.3-0.4 eV in Si and GaAs [6] and only a little larger in perovskite cells, [7] OPV cells exhibit energy losses of at least 0.6 eV -in many cases, however, this offset can approach and even exceed 1 eV. [8] This is currently one of the main bottlenecks toward making OPVs competitive with inorganic PV cells.In this research news, we provide the required background information on the appearance of energy losses in OPV cells, by which we mean the difference between the equivalent of the optical gap and the measured open-circuit voltage that is frequently also denoted as voltage loss, and discuss recent progress toward better understanding their origin and strategies to reduce them. To keep focused, we will mainly address small molecules as active organic semiconductors, which are being processed into thin films by vacuum deposition techniques. Compared to frequently studied π-conjugated polymers, the synthesis of small molecules is more reproducible. Moreover, a rigorous purification of small molecules is easier, which gives the opportunity to reproducibly investigate well-defined systems. The application of vacuum deposition techniques prevents the use of solvents, which as a third component in wet chemical processing can strongly influence the morphology. [9] Thus, active layers of small molecules prepared by vacuum deposition methods mark a well-controlled model system for fundamental studies such as the origin of energy losses in OPV devices. However, we expect that most of the findings can be transferred to solution-processed OPVs as well, which have considerably higher complexity in terms of local morphology and phase behavior. Excitonic Organic Solar CellsIn order to properly address energy losses in OPVs it is useful to look at their working principles in more detail (see also [10] ).