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. / text

Semiconductor nanocrystal-based photovoltaics are an interesting new technology with the potential to achieve high efficiencies at low cost. CuInSe2 nanocrystals have been synthesized in solution using arrested precipitation and dispersed in solvent to form a “solar ink”. The inks have been deposited under ambient conditions to fabricate photovoltaic devices with efficiency up to 3%. Despite the low cost spray coating deposition technique, device efficiencies remain too low for commercialization. Higher efficiencies up to 7% have been achieved using a high temperature selenization process, but this process is too expensive. New nanocrystal film treatment processes are necessary which can improve the device efficiency at low cost. To this end, CuInSe2 nanocrystals were synthesized using a diphenyl phosphine:Se precursor which allows for precise control over the nanocrystal size. The size is controlled by changing the temperature of the reaction. The smallest size nanocrystals demonstrated extremely high device open circuit voltage. Ligand exchange procedures were used to replace the insulating oleylamine capping ligand used during synthesis with more conductive halide ions or inorganic chalcogenidometallate cluster (ChaM) ligands. These ligands led to improved charge transport in the nanocrystal films. A high-intensity pulsed light processing technique known as photonic curing was used which allows for high temperature sintering of nanocrystal films on temperature-sensitive substrates. High energy pulses cause the nanocrystals to sinter into large grains, primarily through melting and resolidification. The choice of metal back contact has a dramatic effect on the final film morphology, with Au and MoSe2 back contacts providing much better adhesion with the CuInSe2 than Mo back contacts. Nanocrystal sintering without melting can be achieved by replacing the oleylamine ligands with ChaM ligands prior to photonic curing. Low energy photonic curing pulses vaporize the oleylamine ligands without inducing sintering or grain growth. This greatly improved nanocrystal coupling and interparticle charge transport. Multiexcitons were successfully extracted from these nanocrystal films and external quantum efficiencies over 100% were observed. Transient absorption spectroscopy was used to study the multiexciton generation process in CuInSe2 nanocrystal films and colloidal suspensions. The multiexciton generation efficiency, threshold, and Auger lifetimes for CuInSe2 compare well with other nanocrystal materials. / text