Multifunctional plasmonic metasurfaces for inverted organic photovoltaics
Creators: name: Petoukhoff, Christopher E.
Publisher: name: No Publisher Supplied
Year: 2017

description: Emerging next-generation photovoltaic devices are fabricated from thin-films, with devices having thicknesses of less than 1 um, because of the reduced material waste, lower embodied energy, and propensity for forming flexible, light-weight devices. However, thin-film photovoltaics have limited absorption of light close to the absorption band edge of the semiconducting photoactive layer. In addition, thin-film photovoltaics fabricated from amorphous semiconductors, such as amorphous Si or organic semiconductors, typically require semiconductor thicknesses of ~100 nm due to their low charge carrier mobilities and correspondingly low charge diffusion lengths. However, by restricting the semiconducting active layer thickness in order to efficiently collect photogenerated charge carriers at the electrodes, incomplete light absorption occurs throughout the visible spectrum. To satisfy these competing constraints on the active layer thickness, light trapping techniques are required to increase the amount of light absorbed in physically-thin active layers. Conventional light trapping in thick, crystalline Si photovoltaics is typically achieved using micron-scale photonic structures that are not suitable for thin-film photovoltaics, which have active layers that are thinner than the height of these structures. Additionally, it is difficult to trap light in active layers with thicknesses below the diffraction limit (thicknesses less than half a wavelength in the material) using conventional photonic designs (e.g. total internal reflection of light scattered from a roughened surface). As such, nanophotonic designs, such as plasmonic nanostructures, are necessary to enhance the amount of light that can be absorbed by thin-film semiconductors.  Here, we propose the use of multifunctional plasmonic metasurfaces to enhance the light trapping and absorption within physically-thin semiconductor active layers. Plasmonic metasurfaces are two-dimensional artificial materials composed of arrays of sub-wavelength metallic nanostructures where the macroscopic electromagnetic properties of the surface arise from the collective response of the individual nanostructures. They support both localized and propagating surface plasmon polaritons, which are hybrid light-charge density waves that exist at metal-dielectric interfaces and have strongly enhanced electric fields near the metal surface. Use of plasmonic metasurfaces in thin-film photovoltaics leads to enhanced absorption via: increased generation of charge carriers by local electric field enhancements; or increased optical path length through the semiconducting active layer either through light scattering from the nanostructures or by coupling the light to an in-plane waveguiding plasmonic mode. As such, thin-films of semiconductors can be both physically and electrically thin (i.e., thinner than the carrier diffusion length), but optically thick when employing plasmonic metasurfaces as electrodes. We gain further control of the properties of the electrode through application of an ultrathin interfacial layer, with thicknesses of less than 5 nm, which allows for tailoring the electronic properties (e.g., surface workfunction) while minimizing the impact on the optical properties of the resulting multifunctional plasmonic metasurface.    In this thesis, we designed and fabricated multifunctional plasmonic metasurfaces with a focus on organic conjugated polymers as thin-film semiconductor active layers. Conjugated-polymer-based organic photovoltaics have shown great potential as alternative energy sources due to their propensity for solution-based processing, rendering devices with the fastest manufacture and energy payback times of all photovoltaic technologies. Conjugated polymers are organic semiconductors composed of primarily earth-abundant elements, and their optical, electronic, and morphological properties can be tuned synthetically. Due to the formation of tightly-bound Frenkel excitons upon photoexcitation, conjugated polymers have strong absorption coefficients, rendering them opaque at film thicknesses on the order of several hundred nanometers. However, like other organic semiconductors, conjugated polymers have low charge mobilities, restricting their thicknesses to less than ~100 nm to minimize charge recombination, thus necessitating the use of nanophotonic light trapping techniques.   Improvements in the efficiency of photovoltaics predominantly arise from increases in the photocurrent or the open-circuit voltage of the device. We begin this work by predicting the optimal planar metal electrode structure by calculating the performance parameters for two types of organic photovoltaic devices (conventional and inverted) with a range of electrode surface workfunctions. We show that highly-efficient and stable inverted organic photovoltaics can be achieved by selecting metal electrodes with low parasitic absorption and high workfunctions, which maximizes the photocurrent and open-circuit voltage of the device, respectively. Based on our calculations, Ag electrodes with ultrathin (less than 5 nm) native AgOx surface layers lead to inverted organic photovoltaic devices with maximal efficiencies due to the low parasitic absorption and high workfunction of AgOx/Ag electrodes. This is the first reported theoretical study that systematically compares the performance parameters of conventional and inverted devices considering a range of different metal electrode types.   Having predicted the optimal metal electrode and photovoltaic device structure, we design and fabricate plasmonic metasurfaces comprised of Ag nanoparticle arrays on Ag films to increase the active layer absorption in thin-film photovoltaics. We demonstrate that plasmonic metasurfaces comprised of low aspect ratio (height-to-diameter fraction) Ag nanoparticles can lead to enhanced absorption in organic active layers. We show that, in addition to the localized surface plasmon resonances (LSPRs) and propagating surface plasmon polaritons (SPPs), absorber-coated plasmonic metasurfaces can support a previously unidentified optical mode type called absorption-induced scattering (AIS). Through our systematic experimental and computational studies, we show that AIS originates from the low energy mode of hybrid plasmon-exciton coupled states, and gives rise to many of the red-edge absorption enhancements frequently observed in plasmon-enhanced organic photovoltaics. We further demonstrate that SPPs with energies less than the AIS mode are out-coupled from absorber-coated metasurfaces for amorphous absorber coatings, but are trapped for semi-crystalline absorber coatings.   In addition to developing a deep understanding of how Ag plasmonic metasurfaces can be employed to enhance sub-wavelength light-trapping and absorption in thin-film organic photovoltaic active layers, we further develop a method of controlling the surface workfunction of plasmonic metasurfaces. We fabricate multifunctional plasmonic metasurfaces comprised of Ag metasurfaces with ultrathin interfacial layers to simultaneously control the optical and electronic properties of the metasurface. We employ monolayer MoS2 and AgOx as ultrathin interfacial layers to minimize changes to the optical properties of the plasmonic metasurfaces. We show that, unexpectedly, the MoS2 interfacial layer contributed to the charge photogeneration process, resulting in the formation of a hybrid MoS2-organic active layer. We demonstrate ultrafast charge transfer between MoS2 and the organic layer, and show that the absorption and total charge generation is enhanced in the presence of the Ag plasmonic metasurface. AgOx, on the other hand, serves as a passive interfacial layer, and does not impact the optical properties of the Ag plasmonic metasurface. Thus, these multifunctional plasmonic metasurfaces allow for control of the optical properties of the electrode through the metasurface designs and the electrical properties through selection of ultrathin interfacial layers, which are expected to give rise to enhanced photocurrent and open-circuit voltage, respectively, in thin-film photovoltaic devices.