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In recent years, the power conversion efficiency of perovskite solar cells has increased substantially, reaching today's values of above 25%. This has also placed an increased focus on device stability, which still limits commercial use. Electron transport layers (ETL) are critical for both factors in perovskite solar cells and have received much attention since the research focus boomed for this technology. In this chapter, requirements on ideal ETL are first provided, including electronic and photonic properties, energy band alignment, morphology, defect states, and chemical stability. Different metal oxide and organic electron transport materials used for perovskite solar cells are reviewed, focusing on optimizing ETL using various doping strategies to improve the cells' efficiency. Finally, the progress made on electron transport materials used in mesoscopic perovskite solar cells, as well as normal (n–i–p) and inverted (p–i–n) planar perovskite solar cells is briefly discussed, highlighting device architecture considerations. This chapter reviews the ETL used in perovskite solar cells, emphasizing the considerations needed when embedding such layers inside perovskite solar cells.

Thin-film organic and perovskite solar cells have attracted the interest of research and industry. The main reason behind this is the low price, high tunability, and great potential of these technologies. Lightweight, color tunability, translucence, flexibility, and ease and low cost of processing are a few of the excellent characteristics of these technologies. However, they have a high sensitivity to light, oxygen, temperature, and humidity, which are common environmental conditions. Upon exposure to these environmental stresses, the chemical structure and morphological arrangement undergo complex changes that result in dramatic reduction of device functionalities over time. This chapter collects the approaches currently reported in the field of additive-assisted stabilization, a method in which solar cells’ active layers are blended with additives that promote their stability (e.g., hydrogen donors, radical scavengers, hydroperoxide decomposers, quenchers, UV absorbers).

Photochemical and mechanical stability are critical in the production and application of organic solar cells. While these factors can individually be improved using different additives, there is no example of studies on the combined effects of such additive-assisted stabilization. In this study, the properties of PTB7:[70]PCBM organic solar cells are studied upon implementation of two additives: the carotenoid astaxanthin (AX) for photochemical stability and the silicone polydimethylsiloxane (PDMS) for improved mechanical properties. A newly designed additive, AXcPDMS, based on astaxanthin covalently bonded to PDMS was also examined. Lifetime tests, produced in ISOS-L-2 conditions, reveal an improvement in the accumulated power generation (APG) of 10% with pure AX, of 90% when AX is paired with PDMS, and of 140% when AXcPDMS is added in the active layer blend, as compared to the control devices. Singlet oxygen phosphorescence measurements are utilized to study the ability of AX and AXcPDMS to quench singlet oxygen and its precursors in the films. The data are consistent with the strong stabilization effect of the carotenoids. While AX and AXcPDMS are both efficient photochemical stabilizers, the improvement in device stability observed in the presence of AXcPDMS is likely due to a more favorable localization of the stabilizer within the blend. The mechanical properties of the active layers were investigated by tensile testing and cohesive fracture measurements, showing a joint improvement of the photooxidative stability and the mechanical properties, thus yielding organic solar cell devices that are promising for flexible photovoltaic applications.

Degradation of perovskite solar cells (PSCs) is often found to be partially or fully reversible when the cells are allowed to recover in the dark. Unlike the dynamics of degradation, knowledge about the dynamics of PSC cell recovery is very limited. Here, we demonstrate that the PSC recovery strongly depends on the electrical bias conditions during the light-induced degradation and that it can be manipulated by applying an external electrical bias during the recovery phase. Investigation of the recovery dynamics allows us to analyze the degradation mechanisms in detail. More specifically, we aged a mixed-cation mixed-halide PSC with a n-i-p structure under illumination in open-circuit (OC) or short-circuit (SC) conditions, and periodically measured their characteristics during the recovery. PSCs aged in SC degrade faster and fully recover after the light is switched off, while the performance of the cells aged in OC does not recover but instead further decreases after the light is switched off (“drop-in-dark” effect). With the use of transient photoluminescence, secondary ion mass spectrometry, and drift-diffusion-based simulations, we hypothesize that extrinsic ion migration causes the drop-in-dark effect, by forming an electron extraction barrier at the metal oxide electron transport layer. The applied bias alleviates this effect. Our results are relevant for gaining a deeper understanding of the multiple degradation mechanisms present in perovskite solar cells, and for finding a practical way to assist their recovery.

The development of nonfullerene acceptors (NFAs) has led to dramatic improvements in the device efficiencies of organic photovoltaic (OPV) cells. To date it is, however, still unclear how those laboratory-scale efficiencies transfer to commercial modules, and how stable these devices will be when processed via industrially compatible methods. Herein, the degradation behavior of lab-scale and scalable OPV devices using similar nonfullerene-based active layers is assessed. It is demonstrated that the scalable NFA OPV exhibits completely reversible degradation when assessed in ISOS-O-1 outdoor conditions, which is in contrast to the laboratory-scale devices assessed via the indoor ISOS-L-2 protocol. Results from transient photovoltage (TPV) indicate the presence of charge trap formation, and a number of potential mechanisms are proposed for the selective occurrence of this in laboratory-scale devices tested in ISOS-L laboratory conditions—ultimately concluding that it has its origins in the different device architectures used. The study points at the risk of assessing active layer stability from laboratory-scale devices and degradation studies alone and highlights the importance of using a diverse range of testing conditions and ISOS protocols for such assessment.

Recent efficiency records of organic photovoltaics (OPV) highlight stability as a limiting weakness. Incorporation of stabilizers is a desirable approach for inhibiting degradation-it is inexpensive and readily up-scalable. However, to date, such additives have had limited success. We show that β-carotene (BC), an inexpensive and green, naturally occurring antioxidant, dramatically improves OPV stability. When compared to nonstabilized reference devices, the accumulated power generation of PTB7:[70]PCBM devices in the presence of BC increases by an impressive factor of 6, due to stabilization of both the burn-in and the lifetime, and by a factor of 21 for P3HT:[60]PCBM devices, owing to a longer lifetime. Using electron spin resonance and time-resolved near-IR emission spectroscopies, we probed radical and singlet oxygen concentrations. We demonstrate that singlet oxygen sensitized by [70]PCBM causes the "burn-in" of PTB7:[70]PCBM devices and that BC effectively mitigates it. Our results provide an effective solution to the problem that currently limits widespread use of OPV.

Photo-initiated, oxygen-mediated degradation of the molecules in the active layer of organic photovoltaic, OPV, devices currently limits advances in the development of solar cells. To address this problem systematically and at a molecular level, it is informative to quantify the kinetics of the pertinent processes, both in solution phase and in solid films. To this end, we examined the oxygendependent photophysics and photochemistry of selected functionalized fullerenes, thiophenederivatives, and a subphthalocyanine commonly used in OPV devices. We find that the photosensitized production of singlet molecular oxygen, O2(a1 Δg), by these molecules is a key step in the degradation process. We demonstrate that the addition of either β-carotene or astaxanthin as antioxidants can inhibit degradation by a combination of three processes: (a) deactivation of O2(a1 Δg)to the oxygen ground state, O2(X3 Σg−), (b) quenching of the O2(a1 Δg) precursor, and (c)sacrificial reactions of the carotenoid with free radicals formed in the photo-initiated reactions. For OPV systems in which reaction with O2(a1Δg) contributes to the degradation, the first two of these processes are desired and should have appreciable impact in prolonging the longevity of OPV devicesbecause they do not result in a chemical change of the system.

Embodied Conversational Agents that can express emotions are a popular topic. Yet, despite recent attempts, reliable methods are still lacking to assess the quality of facial displays. This paper extends and refines the work in [6], focusing on the role of the upper and the lower portions of the face. We analysed the recognition rates and errors from the responses of 74 subjects to the presentations of dynamic (human and synthetic) faces. The results points to the possibility of: a) addressing the issue of the naturalness of synthetic faces, and b) a greater importance of the upper part.

This paper reports the results of a preliminary cross-evaluation experiment run in the framework of the European research project PF-Star1, with the double aim of evaluating the possibility of exchanging FAP data between the involved sites and assessing the adequacy of the emotional facial gestures performed by talking heads. The results provide initial insights in the way people belonging to various cultures react to natural and synthetic facial expressions produced in different cultural settings, and in the potentials and limits of FAP data exchange.