Polymer Bulk Heterojunction Solar Cells: Nanostructure and Function

Mardi 24 Avril 2012 à 15h00
Salle "Rémy Lemaire" K 223 (1er étage) bât. K de l'institut Néel/CNRS [Plan d'acces]

David LIDZEY (Université de Sheffield, Royaume-Uni)

Conjugated-polymers are promising materials for applications as the active light-harvesting and charge-transporting semiconductor in thin-film organic photovoltaic (OPV) devices. Here, electron-donating polymers are mixed with an electron-accepting fullerene forming a bulk-heterojunction device. The advantage of such systems result from the fact that thin organic-films can be deposited over large areas at low cost, potentially offering an inexpensive route to generate electrical energy from sunlight. In this talk, I briefly review the operational principles of organic PV devices, concentrating on systems based on a blend of a low energy-gap charge-donating conjugated polymer (PCDTBT) and the electron-accepting fullerene derivative (PC70BM). I show that when blended together and cast into a thin film, these materials form the basis of an efficient organic photovoltaic device having (in our labs) a power conversion efficiency of 5.5%.
The nanostructure formed on casting a thin film blend bulk heterojunction is critical to the efficient operation of the device. Such nanostructure controls both the efficiency of photocurrent generation and subsequent charge extraction. Selected material systems (e.g. P3HT:PCBM) have been shown to create OPVs having higher power conversion efficiency once the device has been exposed to a thermal annealing protocol. This thermal annealing process is known to increase the crystallinity of the conjugated polymer and drive aggregation of the fullerene. The polymer PCDTBT is however a largely amorphous material, and thus the effectiveness of such processes are not well understood in PCDTBT:PC70BM based OPVs. To explore this, we have used a number of techniques (neutron reflectivity, spectroscopic ellipsometry and grazing-incidence X-ray scattering) to study the nanostructure of PCDTBT and PCDTBT:PC70BM blends and follow their structure evolution on thermal annealing. We find that the surface of an as-cast film is relatively rich in PC70BM, with a negative PC70BM concentration gradient existing into the film. On annealing at 70°C, we observe limited modification in the depth-dependent concentration of PC70BM. In a pure PCDTBT thin-film, thermal annealing at ≥ 130°C results in a crystallization of the polymer side-chains that disrupts pi-pi stacking. This disruption in the stacking of molecular backbones is consistent with a reduction in hole-mobility observed in thin-film blends annealed at ≥100°C. We show that PCDTBT and PC70BM are highly miscible, however when blend films are annealed at a temperature in excess of 155°C, coarse phase-separation occurs between PCDTBT and PC70BM; an effect that further reduces device efficiency as the volume density of interfaces at which exciton dissociation occurs is reduced. Our measurements demonstrate therefore that although thermal annealing removes casting solvent, it has relatively limited effectiveness in substantially improving the efficiency of PCDTBT:PC70BM devices.
Finally, I briefly address the use of new techniques to deposit thin polymer: fullerene films for device application. In particular, I present recent results on the use of ultra-sonic spray-coating to create the active organic layer in OPVs. Here, I show that PCDTBT:PC70BM films can be spray-coated in air, with very promising device performance achieved. Our results suggest therefore that low-cost coating techniques are highly applicable to high-volume OPV manufacture.

Les séminaires "Nanoélectronique Quantique" sont ouverts à tous et nous espérons que vous serez nombreux à profiter de cette occasion d'échange scientifique de qualité.