A Study Of Organic Solar Cells: Synchrotron-based X-ray Diffraction Characterization And Performance

Organic solar cells (OSCs) using poly(3-hexyl-thiophene)(P3HT) as the electron-donor and[6,6]-phenyl C61-butyric acid methyl ester (PCBM) as the electron-acceptor have evolvedquickly as promising cost-effective alternatives due to their flexibility, easy fabricating, low cost,and environmental acceptability. However, their photoelectric conversion efficiency (PCE) is stillunsatisfactory which reaches only5-6%in best cases. Conventionally, the PCE of OSCs not onlydepends on the optical absorption efficiency of all the layers between the two electrodes in thesedevices, but also on the morphology and structure of these layers, especially the active layers. Itis well known that better microstructure and nano-morphology as well as a higher opticalabsorption efficiency will lead to better performances of OSCs. On the one hand, we aim toimprove the microstructure and nano-morphology of organic thin films by changing the D/A(donor/acceptor) blend ratios, annealed temperatures, and substrates; on the other hand, we aimto increase the optical absorption of the device by using a complementary absorbing CuPcblended with Au NPs as a buffer layer for P3HT/PCBM OSCs.As the first part of this thesis, we studied the self-organization of P3HT in the P3HT/PCBMthin films annealed at different temperatures by observing the microstructures of P3HT atdifferent depths using synchrotron based grazing incidence X-ray diffraction (GIXRD); and then,we studied the crystallinity, orientation, and nano-morphology of P3HT prepared at differentconditions, such as different concentrations, D/A blend ratios, and substrates, uisng synchrotronbased2dimensional grazing incidence X-ray diffraction (2D-GIXRD), including high/lowconcentrations, D/A blend ratios,and substrates. Based on these studies, we fabricated theoptimized photovoltaic device with enhanced short circuit current density JSC=9.04mA/cm2,open-circuit voltage VOC=0.55V, and photoelectric conversion efficiency (PCE) η=3.43%, incomparison with JSC=6.86mA/cm2, VOC=0.51V, and η=2.23%for a reference device.As another part of this thesis, the complementary absorbing CuPc blended with Au NPs wasused as a buffer layer in P3HT/PCBM OSCs. The fabricated ITO/CuPc:AuNPs/P3HT:PCBM/Au multilayer took advantages of both CuPc and Au NPs to achieve betterperformance. In fact, the photovoltaic device with Au NPs in the buffer layer was found with anincreased short circuit current density JSC=2.39mA/cm2, open-circuit voltage VOC=0.62V, andPCE η=0.31%in comparison with a reference device without Au NPs (JSC=0.70mA/cm2,VOC=0.41V, η=0.06%).