Date of Award
Physics and Astronomy
Sean E. Shaheen, Ph.D.
CELIV, Charge transport, Disordered materials, Organic photovoltaics, Perovskite, Resistance photovoltage
This dissertation looks at the charge transport properties of solution processed solar cells and how these properties are effected by the microstructure and morphology of the material. It begins with an introduction and history of the field of solution processed solar cells, focusing on organic photovoltaics and perovskites. The physics and issues of charge transport in these materials are reviewed, and the transport probing techniques of photo-Charge Extraction via Linearly Increasing Voltage (photo-CELIV), Metal Insulator Semiconductor-CELIV (MIS-CELIV), and Resistance Photovoltage (RPV) are detailed.
The photo-CEILV technique is used to probe P3HT devices of varying molecular weights. This material is known to have a microstructure which varies depending on the molecular weight of the polymer. By measuring samples over a range of molecular weights, the mobilities and relative recombination rates are found and a relationship between the microstructure and charge transport is proposed.
The photo-CELIV technique is again used, along with MIS-CELIV and RPV, to measure the mobility of devices made with two related high performing polymers, PTB7 and PBDTTT-EFT. These polymers differ only by the addition of a thiophene ring on the side chain yet Grazing Incidence Wide Angle X-ray Scattering measurements show a significant change in their packing. It is proposed that the decrease in packing distance with the addition of the thiophene ring is responsible for the measured increase in mobility of PBDTTT-EFT which in turn increases the device performance.
The effect of changes to the functional groups of fullerenes on bulk heterojunction morphology is then explored. Samples of three new fullerene derivatives are prepared and compared to PCBM. The changes in film morphology with these materials are studied with Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM) and UV-Vis measurements in order to investigate the origin of their increased open circuit voltage.
Finally, a modified version of the transport probing RPV technique is adapted and used to measure mobility in Perovskite solar cells. Devices treated with two different annealing times are compared showing a doubling in mobility.
Dixon, Alexander G., "The Effect of Morphology on Carrier Dynamics in Solution Processed Solar Cells" (2016). Electronic Theses and Dissertations. 1224.
Received from ProQuest
Alexander G. Dixon
Physics, Materials Science, Electrical Engineering