November 29-December 4, 2015 | Boston
Meeting Chairs: T. John Balk, Ram Devanathan, George G. Malliaras, Larry A. Nagahara, Luisa Torsi
Excitonic solar cells including dye-sensitized solar cells, quantum dot-sensitized solar cells, bulk heterojunction organic photovoltaics, are built upon nanostructures of various functional materials. Nanostructures are essential for the high power conversion efficiency, for example, in dye-sensitized solar cells and quantum dot-sensitized solar cells, mesoporous photoanodes made of nanoparticles offer large specific surface area for loading a large amount of dyes or quantum dots so as to capture a sufficient fraction of photons. However, the large surface area of such nanostructures also provide easy pathways for charge recombination, and surface defects and connections between adjacent nanoparticles may retard effective charge injection and charge transport, leading to a loss of power conversion efficiency. Surface facets and chemistry may also affect the conformal coating and adhesion of dye molecules and polymer layers. In this presentation, I will present and discuss our recent work on the design and control of (1) nanostructures and surface chemistry of photoanodes for quantum dot - sensitized solar cells, (2) nonstoichiometric composition, doping, and allignment of quantum dots in quantum dot-sensitized solar cells, and (3) the incorporation of plasmonic nanocrystals. Our research has demonstrated that the power conversion efficiency can be significantly enhanced with excellent device stability when both nanostructures and interface chemistry are properly enginnered.
A high efficiency device requires that the useful process, such as charge extraction in the case of a solar cell, be faster than the loss of energy as heat, which occurs as electrons release their energy in the form of mechanical vibration quanta (‘phonons&’). While electron-phonon coupling is well understood in bulk crystalline semiconductors, where electron and phonon density of states have been measured and calculated and coupling rates measured, electron-phonon coupling in nanomaterials is not understood.Here, we perform inelastic neutron scattering (INS) and density-functional-theory calculations to study the electron-phonon interactions in nanocrystal-solids. We show that mechanical weakness of the nanocrystal surface enables the strong coupling of low-energy surface-phonon modes to electronic transitions. Using thermal admittance spectroscopy to quantify the electronic transition rates in nanocrystal-based diodes, we provide evidence that these low-energy phonon modes can drive large energy transitions (~1 eV) at unusually high rates, due to large entropy change in these transitions.Our findings explain the role of surface defects and passivation on nanocrystal-based device performance and guide the bottom-up engineering of next generation semiconductors, where optical, electronic, and phononic properties can be tailored.