Congratulation to Guy Olivier Ngongang Ndjawa on his successfull PhD Defense

Congratulations to Guy Olivier Ngongang Ndjawa on his  successfull  PhD Defense!

10/13/2016
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Title: Impact of Interfacial Molecular Conformation and Aggregation State on the Energetic Landscape and Performance in Organic Photovoltaics
 
 
Abstract: Organic photovoltaics (OPVs) are made of flexible, renewable, inexpensive, and lightweight conjugated organic semiconductors. In these cells, the photovoltaic action is performed at a heterojunction formed through the contact between two organic semiconductors, an electron-donating moiety (D) and an electron-accepting moiety (A), whose energy levels are chosen to have an optimal offset. For an OPV to be efficient, this heterojunction between D and A (D-A interface) should be located close to where the bound photoexcited species (excitons) are generated in the solar cells. This necessity is satisfied by blending the donor and the acceptor such that interfaces between D and A molecules are formed throughout the entire volume of the solar cell, resulting in a structure known as the bulk heterojunction (BHJ). The D-A interfaces are crucial, as they constitute the nexus of the key processes relevant to device operation such as exciton dissociation and free carriers’ recombination and these two processes essentially depend on the interfacial molecular arrangements and the energy landscape. Yet, the inherent disorder in organic materials and the complexity of the BHJ make molecular arrangements at D-A interfaces ill defined and the energy landscape hard to characterize in details. So far, most experimental studies have regarded the BHJ as a molecular ensemble and studies that use well-defined model systems to look in details at the interfacial molecular structure in OPVs and link it to interfacial energy landscape and device operation are critically lacking. Using in situ photoelectron spectroscopy and ex situ x-ray scattering to study D-A interfaces in tailored planar and bulk heterojunctions solar cells based on small molecule donors, we investigate the influence of the molecular structure right at the D-A interface, the resulting energy landscape and the implications in terms of device operation. We chose donors that replicate the wide range of structures, electronic properties and various degrees of miscibility with C60, the most commonly used electron acceptor, which characterize organic semiconductors. In the first place, we used precise control of the molecular orientation of the donor to reveal a new morphological paradigm, bringing to light misconceptions about miscibility at the functional donor-acceptor interface. Second, we systematically measured the energy states present in photoactive blends with a wide range of molecular arrangements and composition. We find that the energy state in the blends can vary enormously in the order of  ~0.5 eV depending on the material composition and extent of order within the blend. These findings allow understanding of how in functional blends, materials structures and composition bring about significant energy level shifts and eliminate the Coulomb binding energy, making separation of bound charges facile and generation of high voltages more likely. Finally, we demonstrate that when both disordered and ordered phases of D coexist at the interface, low-lying energy states form through delocalization effects in ordered phases and significantly limit the voltage output in devices. Overall our work points to the importance of the extent of structural order and conformation states of molecular materials at and near the D-A interface in determining the operation and performance of OPVs. This work shows that the role of D-A interfaces in complex BHJ devices can be unraveled through careful experimental design and by in-depth characterization of planar heterojunction bilayer devices recreating model interfaces.