Electron dynamics in nanomaterials for photovoltaic applications by time-resolved two-photon photoemission
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The impetus of unsustainable consumption coupled with major environmental concerns has renewed our society's investment in new energy production methods. Solar energy is the poster child of clean, renewable energy. Its favorable environmental attributes have greatly enhanced demand resulting in a spur of development and innovation. Photovoltaics, which convert light directly into usable electrical energy, have the potential to transform future energy production. The benefit of direct conversion is nearly maintenance free operation enabling deployment directly within urban centers. The greatest challenge for photovoltaics is competing economically with current energy production methods. Lowering the cost of photovoltaics, specifically through increasing the conversion efficiency of the active absorbing layer, may enable the invisible hand to bypass bureaucracy. To accomplish the ultimate goal of increased efficiency and lowered cost, it is essential to develop new material systems that provide enhanced output or lowered cost with respect to current technologies. However, new materials require new understanding of the physical principles governing device operation. It is my hope that elucidating the dynamics and charge transfer mechanisms in novel photovoltaic material systems will lead to enhanced design principles and improved material selection. Presented is the investigation of electron dynamics in two materials systems that show great promise as active absorbers for photovoltaic applications: inorganic semiconductor quantum dots and organic semiconductors. Common to both materials is the strong Coulomb interaction due to quantum confinement in the former and the low dielectric constant in the latter. The perceived enhancement in Coulomb interaction in quantum dots is believed to result in efficient multiexciton generation (MEG), while discretization of electronic states is proposed to slow hot carrier cooling. Time-resolved two-photon photoemission (TR2PPE) is utilized to directly map out the hot electron cooling and multiplication dynamics in PbSe quantum dots. Hot electron cooling is found to proceed on ultrafast time scales (< 2ps) and carrier multiplication proceeds through an inefficient bulk-like interband scattering. In organic semiconductors, the strong Coulomb interaction leads to bound electron-hole pairs called excitons. TR2PPE is used to monitor the separation of excitons at the model CuPc/C₆₀ interface. Exciton dissociation is determined to proceed through "hot" charge transfer states that set a fundamental time limit on charge separation. TR2PPE is used to investigate charge and energy transfer from organic semiconductors undergoing singlet fission, an analog of multiple exciton generation. The dynamic competition between one and two-electron transfer is determined for the tetracene/C₆₀ and tetracene/CuPc interfaces. These findings allow for the formulation of design principles for the successful harvesting of hot or multiple carriers for solar energy conversion.