Impact of Interface Layers on Luminescence Imaging of Organic Solar Cells: Discriminating ETL from HTL Defects.

Over the last two decades, organic photovoltaic (OPV) devices have seen their efficiency increase, while long-term stability and upscaling have been demonstrated for first-generation modules. Since the maturity level of this technology has now improved, techniques for rapid quality control have become relevant. Imaging techniques such as photo- and electroluminescence have already been used for this purpose. However, defects could only be localized either in the active layer or in interface layers, without being able to distinguish between defects located in the ETL from those within the HTL. Here, we present a simple method to unambiguously discriminate between ETL and HTL defects. Furthermore, we demonstrate the strong impact of HTL thickness on the detected photoluminescence signal. Our approach will help avoid misinterpretations in luminescence experiments and gain an understanding of device failure during processing or aging.

Mechanical Reliability of Flexible Encapsulated Organic Solar Cells: Characterization and Improvement.

The encapsulation of organic photovoltaic (OPV) devices can help mitigate the degradation induced by environmental factors like water and oxygen and thus potential to increase OPV lifetime. Because flexibility is an important parameter for targeted OPV applications, this paper proposes a fundamental study on the impact of the roll-to-roll flexible encapsulation process. Both performance and mechanical reliability of encapsulated devices have been scouted. Furthermore, it has been demonstrated that a relatively simple peeling technique allows understanding the role of the interfaces inside a multilayered OPV device supported by a flexible poly(ethylene terephthalate) substrate. For this purpose, the peeling strengths between each layer were measured using a series of partial devices. This provided a quantitative analysis of the mechanical strength or quality of each interface. Two interfaces revealed pronounced weaknesses: active layer with hole transporting layer and transparent conducting electrode with electron transporting layer. Among various surface treatments applied to improve these interfaces, an optimized UV-ozone (UVO3) treatment proved to modify substantially the surface properties of used zinc oxide (ZnO) and thus improved its adhesion to the neighboring layers. The physicochemical and structural changes of ZnO have been confirmed by IR spectroscopy and contact angle measurements. It has also been shown that better interfaces within the device improve the overall performance of the devices and their resilience to roll-to-roll encapsulation.

Stabilization of well-organized transient micellar phases in CTAB-templated silica and organosilica thin films.

The influence of some previously reported critical parameters (aging of the silica sol, humidity, % of trivalent precursor) controlling the nature and the degree of organization of mesophases is studied in detail for CTAB-templated silica and organosilicate films prepared by rapid evaporation techniques. It is shown that all of these parameters directly influence the silica-condensation kinetics. Concerning films with the 2D-hexagonal structure belonging to the CTAB-water phase diagram, the mesophase is mainly stabilized by interactions between cylindrical micelles, and the reactivity of the silica sol does not drastically modify the extent of the order in films. In contrast, the formation of a rigid silica network just after the micellar organization appears to be crucial to prepare well-organized films with the 3D-hexagonal or the cubic transient structure. In the latter case, the size and reactivity of silica clusters have to be controlled to obtain rapid gelation in the deposited sol.

Molecular engineering of a cobalt-based electrocatalytic nanomaterial for H2 evolution under fully aqueous conditions.

The viability of a hydrogen economy depends on the design of efficient catalytic systems based on earth-abundant elements. Innovative breakthroughs for hydrogen evolution based on molecular tetraimine cobalt compounds have appeared in the past decade. Here we show that such a diimine-dioxime cobalt catalyst can be grafted to the surface of a carbon nanotube electrode. The resulting electrocatalytic cathode material mediates H(2) generation (55,000 turnovers in seven hours) from fully aqueous solutions at low-to-medium overpotentials. This material is remarkably stable, which allows extensive cycling with preservation of the grafted molecular complex, as shown by electrochemical studies, X-ray photoelectron spectroscopy and scanning electron microscopy. This clearly indicates that grafting provides an increased stability to these cobalt catalysts, and suggests the possible application of these materials in the development of technological devices.