Electronic structure and stability of materials for organic photovoltaic applications

This word was devoted to the stability in various conditions of materials used in the active layer of organic solar cells. The main goals of this work were first to provide deeper understanding about complex mechanisms occurring in the active layer and second to investigate interfacial degradation pathways involving the active layer. A first part was dedicated to the photo and thermal stability of the polymer blend materials which constitute the active layer of the solar cells. A second section focused on the role of the third component of the active layer which can be undesired residual additives coming from the processing or the desired insertion of a stabilizer additive. A third part concerned the delamination issue which takes place at the active layer / hole transporting layer interface. Finally, a last section was devoted to the energy level alignment between the C60 molecules and various electron transporting substrates. The photo and thermal stability of the active layer / electron transporting layer interface was also studied in this section

The Crucial Role of Confined Residual Additives on the Photostability of P3HT: PCBM Active Layers.

International audienceWe have investigated the impact of residual additives such as diiodooctane (DIO) and octanedithiol (ODT) on the photostability of state of the art P3HT:PCBM active layers. A series of active layers prepared with and without additives as well as neat additives were submitted to light irradiation in ambient air and analyzed by UV–vis and IR spectroscopy. We show not only that residues are sensitive to the combined action of light and oxygen but also that their presence can dramatically impact the polymer blend stability. DIO molecules are highly sensitive to light and can directly saturate the polymer conjugated backbone or be trapped by the fullerene moieties. ODT molecules can be photooxidized and may accelerate the intrinsic photooxidation of the active layer. Another important result is that the additives’ impact is directly linked to the presence of a top layer above the active layer. The confinement makes that additives react within the active layer, and thus accelerate its photodegradation, rather than decomposing in the gas phase (irradiation without top layer). Thus, a light-soaking step before top layer deposition could allow a clean removing of additives without affecting the optimized morphology and polymer blend stability. This process would be easily adaptable to industrial scale production

The Crucial Role of Confined Residual Additives on the Photostability of P3HT: PCBM Active Layers.

International audienceWe have investigated the impact of residual additives such as diiodooctane (DIO) and octanedithiol (ODT) on the photostability of state of the art P3HT:PCBM active layers. A series of active layers prepared with and without additives as well as neat additives were submitted to light irradiation in ambient air and analyzed by UV–vis and IR spectroscopy. We show not only that residues are sensitive to the combined action of light and oxygen but also that their presence can dramatically impact the polymer blend stability. DIO molecules are highly sensitive to light and can directly saturate the polymer conjugated backbone or be trapped by the fullerene moieties. ODT molecules can be photooxidized and may accelerate the intrinsic photooxidation of the active layer. Another important result is that the additives’ impact is directly linked to the presence of a top layer above the active layer. The confinement makes that additives react within the active layer, and thus accelerate its photodegradation, rather than decomposing in the gas phase (irradiation without top layer). Thus, a light-soaking step before top layer deposition could allow a clean removing of additives without affecting the optimized morphology and polymer blend stability. This process would be easily adaptable to industrial scale production

The roles of the Electron and Hole Transport Layers on the Degradation Mechanisms in Inverted Organic Solar Cells

International audienc

Crucial Role of the Electron Transport Layer and UV Light on the Open-Circuit Voltage Loss in Inverted Organic Solar Cells

International audienceUnderstanding the degradation mechanisms in organic photovoltaics is crucial in order to develop stable organic semiconductors and robust device architectures. The rapid loss of efficiency, referred to as burn-in, is a major issue to be addressed. This study reports on the influence of the electron transport layer (ETLs) and UV light on the drop of open-circuit voltage (Voc) for P3HT:PC60BM-based devices. The results show that Voc loss is induced by the UV and, more importantly, that the ETL can amplify it, with TiOx yielding a stronger drop than ZnO. Using impedance spectroscopy (IS) and X-ray photoelectron spectroscopy (XPS), different degradation mechanisms were identified according to whether the ETL is TiOx or ZnO. For TiOx-based devices, the formation of an interface dipole was identified, resulting in a loss of the flat-band potential (Vfb) and, thus, of the Voc. For ZnO-based devices, chemical modifications of the metal oxide and active layer at the interface were detected, resulting in a doping of the active layer which impacts the Voc. This study highlights the role of the architecture and, more specifically, of the ETL in the severity of burn-in and degradation pathways