The film morphology and device performance of planar heterojunction solar cells based on the molecular donor material α-sexithiophene (6T) are investigated. Planar heterojunctions of 6T with two different acceptor molecules, the C 60 fullerene and diindenoperylene (DIP), have been prepared. The growth temperature of the 6T bottom layer has been varied between room temperature and 100 °C for each acceptor. By means of X-ray diffraction and X-ray absorption, we show that the crystallinity and the molecular orientation of 6T is influenced by the preparation conditions and that the 6T film templates the growth of the subsequent acceptor layer. These structural changes are accompanied by changes in the characteristic parameters of the corresponding photovoltaic cells. This is most prominently observed as a shift of the open circuit voltage (V oc ): In the case of 6T/C 60 heterojunctions, V oc decreases from 0.4 to 0.3 V, approximately, if the growth temperature of 6T is increased from room temperature to 100 °C. By contrast, V oc increases from about 1.2 V to almost 1.4 V in the case of 6T/DIP solar cells under the same conditions. We attribute these changes upon substrate heating to increased recombination in the C 60 case while an orientation dependent intermolecular coupling seems to change the origin of the photovoltaic gap in the DIP case.

Owing to the excitonic nature of photoexcitations in organic semiconductors, the working mechanism of organic solar cells relies on the donor-acceptor (D/A) concept enabling photoinduced charge transfer at the interface between two organic materials with suitable energy-level alignment. However, the introduction of such a heterojunction is accompanied by additional energy losses compared to an inorganic homojunction cell due to the presence of a charge-transfer (CT) state at the D/A interface. By careful examination of planar heterojunctions of the molecular semiconductors diindenoperylene (DIP) and C 60 we demonstrate that three different analysis techniques of the temperature dependence of solar-cell characteristics yield reliable values for the effective photovoltaic energy gap at the D/A interface. The retrieved energies are shown to be consistent with direct spectroscopic measurements and the D/A energy-level offset determined by photoemission spectroscopy. Furthermore, we verify the widespread assumption that the activation energy of the dark saturation current E and the CT energy E CT may be regarded as identical. The temperature-dependent analysis of open-circuit voltage V OC and dark saturation current is then applied to a variety of molecular planar heterojunctions. The congruency of E and E CT is again found for all material systems with the exception of copper phthalocyanine/C 60 . The general rule of thumb for organic semiconductor heterojunctions, that V OC at room temperature is roughly half a volt below the CT energy, is traced back to comparable intermolecular electronic coupling in all investigated systems.

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.

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] ).

We compare the gain in power conversion efficiency (PCE) achieved by inserting either amorphous or crystalline exciton blocking layers at the anode interface for planar (PHJ) and planar-mixed heterojunction (PM-HJ) organic solar cells based on Tetraphenyldibenzoperiflanthene and fullerenes. For PHJ devices, there is a gain of more than 37% for both types of blocking layers, mainly due to an increase in photocurrent, indicating that this gain can be solely ascribed to the exciton blocking effect. A templating effect as proposed in literature for crystalline blocking layers cannot be affirmed. On the contrary, it is shown that there is a connection between the choice of acceptor (C60/C70) and the blocking effect on the anode side. Moreover, we can show that also for PM-HJ devices a remarkable efficiency enhancement is possible. The insertion of suitable blocking layers at the anode interface can alter the effective work function and thus the open-circuit voltage, leading to a maximum PCE of 5.8% in single junction cells.

Diindenoperylene (DIP) and tetraphenyldibenzoperiflanthene (DBP) are two commonly used donor materials in organic solar cell devices. Despite their structural similarities, DIP films are crystalline, exhibiting good charge and exciton transport, whereas DBP films are amorphous and have lower carrier mobility and a short exciton diffusion length. However, DBP reveals a distinctly higher absorption due to the lying orientation of its transition dipole moments. In this paper, we investigate the influence of solvent vapor annealing (SVA) on the solar cell performance of both materials. In general, SVA induces a partial re-solubilization of the material leading to enhanced crystallinity of the treated layer. For DBP, extended annealing times result in a strong aggregation of the molecules, creating inhomogeneous layers unfavorable for solar cells. However, in DIP cells, SVA leads to an increase in fill factor (FF) and also a slight increase in short-circuit current density (J SC ) due to interface roughening. The best results are obtained by combining solvent vapor annealed DIP layers with strongly absorbing DBP and C 70 on top. Through this device architecture, we obtain the same increase in FF in addition to a higher gain in J SC , elevating the power conversion efficiency by a factor of 1.2 to more than 4 %. FIG. 1. Round crystallites formed in a 50 nm thick DBP-layer by annealing for 10 minutes in chloroform vapor.could be observed. Therefore, in-plane (grazing incident X-ray diffraction) measurements were recorded, but also in this configuration no peaks indicating DBP crystallinity are observable. The results of both, out-of-plane and in-plane measurements are shown in the supplementary material. Other groups have already reported on "more crystalline" DBP, achieved through different techniques. Growing on a crystalline template 38 or on a heated substrate 40 as well as applying organic vapor phase deposition (OPVD) using a hot inert carrier gas 21 were reported to result in DBP layers of higher order. However, in these cases the crystallinity could neither be visualized by means of XRD 38 , nor via reflection high energy electron diffraction 40 (RHEED) nor by selected area electron diffraction 21 (SAED).Next, we investigated the surface properties of 15 nm films of DBP annealed for various durations via AFM. There was no difference for layers growing either on ITO/HIL1.3 or on glass so the reorganization of the molecules is independent of the substrate. These layers were treated by SVA for 4, 8 and 12 minutes, respectively, and then compared with each other as well as with an untreated sample. As reported previously 23,40 , the pristine DBP layer has an extremely smooth surface with a root-mean-square (RMS) roughness of merely RMS = 0.63 nm. However, SVA causes a strong aggregation of the DBP molecules. After 4 minutes of SVA treatment, the RMS roughness increases more than thirtyfold to 21.65 nm. 7 Evaluating the performance of solar cells fabricated with annealed DBP films confirms these problems. For elevate...

Organic photovoltaic devices utilizing α-sexithiophene (6T) as a donor and tetraphenyldibenzoperiflanthene (DBP) as an acceptor were fabricated and compared to devices utilizing DBP as a donor and C 60 as an acceptor. The 6T/DBP devices exhibit substantially higher open circuit voltage, 1.27 V compared to 0.86 V for DBP/C 60 , as a consequence of the higher energy charge transfer state formed. The 6T/DBP devices yield short-circuit current of 3.9 mA/ cm 2 , open-circuit voltage of 1.27 V, and fill factor of 0.55, resulting in a power conversion efficiency of 2.8%. Atomic force microscopy studies show that 6T forms textured films on indium−tin oxide, and subsequent deposition of DBP infills the surface. Optical modeling provides insight into the ideal active layer and transport layer thicknesses. A power conversion efficiency of close to 3% is achieved for a fairly large process window of layer thickness combinations. The high open-circuit voltage in conjunction with absorption out to a wavelength of 650 nm make this material combination especially attractive for tandem devices.