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Whilst organic photovoltaics (OPV) offer several opportunities such as easy-to-tailor molecular properties, low-cost fabrication and mechanical flexibility, an extensive boost of power conversion efficiency (PCE) is of fundamental necessity to realize the commercialization of OPV devices. This thesis focuses on two approaches, incorporation of (i) titanium suboxide (TiOx) interfacial layer and (ii) plasmonic gold nanowires (Au-NWs), to improve the device efficiencies via modulation of optical and charge transporting properties in OPV devices. The PCE of ~ 6% was achieved in PCDTBT:PC71BM and PTB7:PC61BM The sol-gel derived TiOx layer was integrated at the cathode interface of P3HT:PCBM, PCDTBT:PCBM and PTB7:PCBM devices. Firsdy, the role of TiOx optical spacer was investigated using the finite difference time domain ( FDTD) optical simulation technique. The calculated and experimental absorption profiles show that optical spacer effect was more pronounced in the thinner active layer and diminished with increasing active layer thickness. The optical advantage of TiOx interfacial layer was not achieved once the active layer thickness has been optimized. The enhanced current density 0 SC) with TiOx interlayer was attributed to the combined effect of optical spacer and charge transport across cathode interface. The first part o f the thesis highlights the decoupling of optical and electrical contributions of TiOx to Jsc by using the optical reflectance and internal quantum efficiency (IQE) measurements. Decoupling of optical and electrical effects of interlayer would support further device optimization and designs including the evaluation of new interfacial layers. School of Materials Science and Engineering i devices. ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library NANYANG TECHNOLOOCAL UMVERSTTY Abstract The second part of thesis explores an implication of the modified contact properties (at polymer/TiOx/cathode interface), surface segregation o f BHJ films, and Voc enhancements in OPV devices. Next, the effect of TiOx interlayer on charge transport and recombination across the cathode interface was studied by the dark current analysis. It was found that TiOx interlayer increased the hole-blocking barrier and suppressed the charge recombination at polymer/cathode interface of P3HT:PC61BM and PCDTBT:PC71BM devices. Upon integrating TiOx, Voc increased in PCDTBT:PC71BM device ( PCBM-rich surface) while Voc remained constant in P3HT:PC61BM and PTB7:PC61BM devices polymer-rich surface). Thus, the electron transport at the PCBM/TiOx/cathode interface may have a stronger impact on the Voc enhancement than the hole-blocking at the polymer/TiOx/cathode interface. Solution processed zinc tin oxide (ZTO) film was also exploited as an efficient electron transporting layer in the P3HT:PCBM inverted solar cell and the PCE as high as 3.05% was achieved. Compared to the device with TiOx, higher Jsc in the device with ZTO is attributed to the efficient electron transport via higher conductivity of ZTO layer. Finally, Au-NWs were used as plasmonlc antennae in the P3HT:PCBM devices and the photocurrent Jsc was increased by 23.2 %. The physical origin of Jsc enhancement was examined by optical and time-resolved photoluminescence spectroscopy. The primary enhancement mechanism is an increased plasmon excitation field that enhances the active layer absorption, thereby increasing photocurrent in the Au-NWs integrated devices. Finally, the integrations of Au-NWs as plasmonic antennae and TiOx as optical spacer and electron transporting buffer are promising strategies for realization of the physically thin but optically thick organic photovoltaic devices. School of Materials Science and Engineering ii ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library

Investigation of photophysical and charge transport properties of optical spacers and plasmonic nanostructures in organic photovoltaics