Probing the energy levels of organic bulk heterojunctions by varying the donor content

The performance of organic solar cells is strongly governed by the properties of the photovoltaic active layer. In particular, the energetics at the donor (D)–acceptor (A) interface dictate the properties of charge transfer (CT) states and limit the open-circuit voltage. More generally, energetic landscapes in thin films are affected by intermolecular, e.g., van der Waals, dipole, and quadrupole, interactions that vary with D:A mixing ratio and impact energy levels of free charges (ionization energy, electron affinity) and excitons (singlet, CT states). Disentangling how different intermolecular interactions impact energy levels and support or hinder free charge generation is still a major challenge. In this work, we investigate interface energetics of bulk heterojunctions via sensitive external quantum efficiency measurements and by varying the D:A mixing ratios of ZnPc or its fluorinated derivatives and C60. With increasing donor fluorination, the energetic offset between FxZnPc and C60 reduces. Moving from large to low offset systems, we find qualitatively different trends in device performances with D:C60 mixing ratios. We rationalize the performance trends via changes in the energy levels that govern exciton separation and voltage losses. We do so by carefully analyzing shifts and broadening sEQE spectra on a linear and logarithmic scale. Linking this analysis with molecular properties and device performance, we comment on the impact of charge–quadrupole interactions for CT dissociation and free charge generation in our D:C60 blends. With this, our work (1) demonstrates how relatively accessible characterization techniques can be used to probe energy levels and (2) addresses ongoing discussions on future molecular design and optimal D–A pairing for efficient CT formation and dissociation

Vacuum-deposited donors for low-voltage-loss nonfullerene organic solar cells

The advent of nonfullerene acceptors (NFAs) enabled records of organic photovoltaics (OPVs) exceeding 19% power conversion efficiency in the laboratory. However, high-efficiency NFAs have so far only been realized in solution-processed blends. Due to its proven track record in upscaled industrial production, vacuum thermal evaporation (VTE) is of prime interest for real-world OPV commercialization. Here, we combine the benchmark solution-processed NFA Y6 with three different evaporated donors in a bilayer (planar heterojunction) architecture. We find that voltage losses decrease by hundreds of millivolts when VTE donors are paired with the NFA instead of the fullerene C60, the current standard acceptor in VTE OPVs. By showing that evaporated small-molecule donors behave much like solution-processed donor polymers in terms of voltage loss when combined with NFAs, we highlight the immense potential for evaporable NFAs and the urgent need to direct synthesis efforts toward making smaller, evaporable compounds

Limiting factors for charge generation in low-offset fullerene-based organic solar cells

Free charge generation after photoexcitation of donor or acceptor molecules in organic solar cells generally proceeds via (1) formation of charge transfer states and (2) their dissociation into charge separated states. Research often either focuses on the first component or the combined effect of both processes. Here, we provide evidence that charge transfer state dissociation rather than formation presents a major bottleneck for free charge generation in fullerene-based blends with low energetic offsets between singlet and charge transfer states. We investigate devices based on dilute donor content blends of (fluorinated) ZnPc:C60 and perform density functional theory calculations, device characterization, transient absorption spectroscopy and time-resolved electron paramagnetic resonance measurements. We draw a comprehensive picture of how energies and transitions between singlet, charge transfer, and charge separated states change upon ZnPc fluorination. We find that a significant reduction in photocurrent can be attributed to increasingly inefficient charge transfer state dissociation. With this, our work highlights potential reasons why low offset fullerene systems do not show the high performance of non-fullerene acceptors

Antibody Responses to NY-ESO-1 in Primary Breast Cancer Identify a Subtype Target for Immunotherapy

The highly immunogenic human tumor antigen NY-ESO-1 (ESO) is a target of choice for anti-cancer immune therapy. In this study, we assessed spontaneous antibody (Ab) responses to ESO in a large cohort of patients with primary breast cancer (BC) and addressed the correlation between the presence of anti-ESO Ab, the expression of ESO in the tumors and their characteristics. We found detectable Ab responses to ESO in 1% of the patients. Tumors from patients with circulating Ab to ESO exhibited common characteristics, being mainly hormone receptor (HR)− invasive ductal carcinomas of high grade, including both HER2− and HER2+ tumors. In line with these results, we detected ESO expression in 20% of primary HR− BC, including both ESO Ab+ and Ab− patients, but not in HR+ BC. Interestingly, whereas expression levels in ESO+ BC were not significantly different between ESO Ab+ and Ab− patients, the former had, in average, significantly higher numbers of tumor-infiltrated lymph nodes, indicating that lymph node invasion may be required for the development of spontaneous anti-tumor immune responses. Thus, the presence of ESO Ab identifies a tumor subtype of HR− (HER2− or HER2+) primary BC with frequent ESO expression and, together with the assessment of antigen expression in the tumor, may be instrumental for the selection of patients for whom ESO-based immunotherapy may complement standard therapy

Charge transfer state characterization and voltage losses of organic solar cells

A correct determination of voltage losses is crucial for the development of organic solar cells (OSCs) with improved performance. This requires an in-depth understanding of the properties of interfacial charge transfer (CT) states, which not only set the upper limit for the open-circuit voltage of a system, but also govern radiative and non-radiative recombination processes. Over the last decade, different approaches have emerged to classify voltage losses in OSCs that rely on a generic detailed balance approach or additionally include CT state parameters that are specific to OSCs. In the latter case, a correct determination of CT state properties is paramount. In this work, we summarize the different frameworks used today to calculate voltage losses and provide an in-depth discussion of the currently most important models used to characterize CT state properties from absorption and emission data of organic thin films and solar cells. We also address practical concerns during the data recording, analysis, and fitting process. Departing from the classical two-state Marcus theory approach, we discuss the importance of quantized molecular vibrations and energetic hybridization effects in organic donor-acceptor systems with the goal to providing the reader with a detailed understanding of when each model is most appropriate

Exertional hyperckemia might be the first manifestation of a genetic disorder

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Assessing the photovoltaic quality of vacuum-thermal evaporated organic semiconductor blends

Vacuum-thermal evaporation (VTE) is a highly relevant fabrication route for organic solar cells (OSCs), especially on an industrial scale as proven by the commercialization of organic light emitting diode-based displays. While OSC performance is reported for a range of VTE-deposited molecules, a comprehensive assessment of donor:acceptor blend properties with respect to their photovoltaic performance is scarce. Here, the organic thin films and solar cells of three select systems are fabricated and ellipsometry, external quantum efficiency with high dynamic range, as well as OTRACE are measured to quantify absorption, voltage losses, and charge carrier mobility. These parameters are key to explain OSC performance and will help to rationalize the performance of other material systems reported in literature as the authors’ methodology is applicable beyond VTE systems. Furthermore, it can help to judge the prospects of new molecules in general. The authors find large differences in the measured values and find that today's VTE OSCs can reach high extinction coefficients, but only moderate mobility and voltage loss compared to their solution-processed counterparts. What needs to be improved for VTE OSCs is outlined to again catch up with their solution-processed counterparts in terms of power conversion efficiency

Evaluation of a novel spine and surface topography system for dynamic spinal curvature analysis during gait.

INTRODUCTION: The assessment of spinal deformities with rasterstereography can enhance the understanding, as well as can reduce the number of x-rays needed. However, to date this technique only allows measurements under static conditions. Since it would be of great value to be able to also analyze the spine in dynamic conditions, the present study evaluated a novel rasterstereographic system. MATERIALS AND METHODS: A new rasterstereographic device was evaluated in a comparison with the gold standard in motion analysis, the VICON system. After initial testing using 12 flat infrared markers adhered to a solid plate, the two systems were evaluated with the markers adhered onto the backs of 8 test subjects. Four triangles were defined using the markers, and the sides of each triangle were measured under static and dynamic conditions. RESULTS: On the solid plate, the sides of the 4 triangles were measured with a measuring tape and then by the two optical systems. Rasterstereography showed a high accuracy in marker detection on the solid plate. Under dynamic conditions, with the subjects walking on a treadmill, the rasterstereographically-measured side lengths were compared with the lengths measured by the VICON system as an assessment of marker detection. No significant differences (p>0.05) were found between the systems, differing only 0.07-1.1% for all sides of the four triangles with both systems. DISCUSSION: A novel rasterstereographic measurement device that allows surface and spine topography under dynamic conditions was assessed. The accuracy of this system was with one millimeter on a solid plate and during dynamic measurements, to the gold standard for motion detection. The advantage of rasterstereography is that it can be used to determine a three-dimensional surface map and also allows the analysis of the underlying spine