The UC Davis & Santa Cruz Solar Team will investigate a transformative new paradigm of solar energy conversion: the high efficiency Multiple Exciton Generation (MEG) pathway and the corresponding challenge of charge extraction in all-inorganic nanostructured solar cells. MEG was recently observed in nanoparticles (NPs) and is not subject to the 31% theoretical limit of solar energy conversion. The Solar Team will synthesize pure, doped and alloyed Si and Ge core-shell NPs to analyze their chemistry, quantum states and energetics in a wide range of sizes, dopings, and structures, using PbS NPs as reference. The impact of complex factors such as the relaxation of the NP surface, the various core-shell structures, the exciton-exciton interaction and the NP-NP interaction on the chemistry and spectra of the NPs as well as on the MEG will be analyzed. The tools of the analysis will include photoluminescence and transient absorption studies with femtosecond resolution; and forming fully functional NP based solar cells, complete with embedding charge transport layers. These solar cells will be developed by optimizing the competing design principles of maintaining quantum confinement to preserve the efficiency of the MEG while embedding the NPs into suitably conducting layers for efficient charge extraction and transport. A strong theoretical effort will complement the Team?s experimental work. Density functional theories (DFT) will be used to capture the surface reconstruction and the energetics of NPs; time dependent DFT and Bethe-Salpeter methods to describe the exciton-exciton interaction; and non-equilibrium rate equations to determine the full rate of MEG. Mathematical projects will assist these efforts by developing a Lanczos coefficient extrapolation method, dramatically reducing the computational workload by replacing direct matrix manipulations with matrix by vector products; and by developing global statistical methods to qualitatively improve the analysis and extraction of the hidden dynamics from the noisy, ultra-high dimensional spectrotemporal dataset, obtained by the photoluminescence and transient absorption.
NON-TECHNICAL SUMMARY
Even in theory, the efficiency of solar cells is limited to a disappointing 31%. However, this limit was based on the traditional operation of solar cells, where an incoming solar photon excites only a single electron. A recent breakthrough showed that in nanoparticles one photon may excite several electrons, thus opening a new energy conversion paradigm not constrained by the above limit. The Davis Solar Team will synthesize a wide variety of nanoparticles; perform ultra-fast optical experiments to characterize the energy conversion process in these particles; and construct fully functional solar cells by embedding the nanoparticles into charge transport layers. Path-breaking mathematical work will be performed to accelerate the computational techniques to unprecedented speeds to simulate the energy conversion process with great accuracy. Further, qualitatively new statistical analyses will be developed to uncover the complex factors embedded in the vast amount of data produced by the optical experiments. The improvement of the solar energy conversion efficiency expected to emerge from this project can considerably increase the role solar technologies will play in the US transitioning towards renewable energy sources. The Davis Solar Team will not only develop these new nanoparticle based solar cells, but also plans to chaperon this technology towards the marketplace. This will be pursued through working with the Solar Collaborative of the California Energy Commission (SC-CEC), where the Team played an early leadership role. The Team's industrial collaboration will be developed through one of the PIs who is on the advisory board of a solar company. Besides working toward a wide acceptance of nanoparticle solar technologies, the Team will reach out and serve the solar community at large by analyzing and disseminating the latest academic research to the solar stakeholders: the PV manufacturers, utilities and the regulatory bodies through the SC-CEC. The Team will also develop a "Solarwiki" as a platform for a broad electronic outreach to the interested public. The Team will integrate its work with its activity in the ACS SEED program. Graduate students and postdoctoral fellows will work jointly with the groups of the Solar Team to foster interdisciplinary thinking and to prepare them to join the solar revolution.
This proposal investigated a transformative new paradigm of solar energy conversion: (1) the high efficiency Multiple Exciton Generation (MEG) pathway; the corresponding challenge of (2) charge extraction; in (3) all-inorganic, nano-structured solar cells. MEG was recently observed in quantum confined nanoparticles (NP), and is not subject to the low, 31% theoretical limit of solar energy conversion. We pursued the proposed research in multiple dimensions, each spanned by synergistically organized triads. Our project dimension is spanned by the triad of (P1 ) synthesizing pure, doped and alloyed Si and Ge NPs to explore systematically wide ranges of size, composition and structure and their effect on quantum confinement, (P2 ) exploring the impact of the reconstruction of the NP surface, the various core-shell structures and the NP-NP interaction on the MEG, and (P3 ) investigate NPs embedded in solar cell architectures to track the persistence of MEG while optimizing NP layers for charge extraction. The interdisciplinary triad connects (I1 ) the chemical effort to synthesize and characterize a wide variety of NPs; (I2 ) the material science effort to compute the single NP spectrum with high accuracy, model the MEG and the charge transfer from the NPs to the electrodes, and optimize the photovoltaic (PV) properties on nanostructured surfaces; and (I3 ) the mathematical effort to invent transformative mathematical breakthroughs for a qualitatively better description of the exciton-exciton interactions and to develop methods of global statistical analysis to extract previously hidden dynamics from ultra-high dimensional spectrotemporal datasets. Group IV semiconducting nanoparticles (Si and Ge) have been explored in the last two decades targeting applications in wide ranging fields such as solar energy conversion, LED’s, and bio-imaging and are highly regarded as potential green replacements for the heavy metal composed II-VI, IV-VI and III-V semiconductors. In particular, germanium has attracted attention due to its larger exciton Bohr radius (~ 24 nm) and a smaller band gap of 0.67 eV, thus making it possible to achieve optically useful band gaps when prepared in nanoscale due to quantum confinement effects. A variety of synthetic strategies have been reported in the literature for its colloidal preparation, such as the reduction of germanium halides in high boiling solvent/surfactant systems, metathesis reactions using Zintl salts, decomposition of organogermane precursors in high boiling solvents and super critical fluids, solid state reductions and plasma synthesis. However, so far, none have risen to the level of ease and reproducibility and therefore popularity of more conventional II-VI semiconductor nanoparticles. The difficulty in the synthesis of Group IV nanocrystals has been well documented in literature and therefore at the time that this project was initiated, there was still significant room for improvement in order to be able to document the benefits of the nanoscale in applications such as photovoltaics. In the P1 project/I1 interdisciplinary/E1 experimental dimensions, Kauzlarich, with postdoc Elayaraja Muthuswamy, was able to demonstrate a simple, scalable synthesis of Ge Nanocrystals (NCs). The stability of these NCs were improved with thiol ligand capping and the quantum confinement investigated with surface photovoltage spectroscopy. Ge NCs samples were supplied to the Sue Carter group (UCSC) and the Delmar Larsen group (UCD) to sustain the existing collaborative projects. The research efforts were assisted by graduate students and several undergraduate students who were participants in Mentorships for Undergraduate Research Participants in Mathematical and Physical Sciences, MURPPS at UCD. We developed control in nanocrytallite size in the range of 3-15 nm by employing new ideas in nanoparticle synthesis. Microwave heating have become a popular method to prepare nanomaterials and a variety of materials have already been reported in literature. Germanium nanoparticles however have not yet been prepared via microwave heating. We were able to successfully synthesize Ge nanoparticles by carrying out the reduction of germanium iodides in oleylamine by microwave heating. The reduction of Ge4+ precursors by oleylamine have previously not been established in literature and we have shown that the size control during this reduction reaction in oleylamine can be achieved by controlled addition of Ge4+ iodide as part of the precursor. PXRD measurements indicate that the as prepared samples are crystalline and TEM images show that samples with low polydispersity index can be achieved. In the case of Si nanoparticles, Bradley Nolan (UCD graduate student) in collaboration with Prof. Thomas Fässler's research group (TU München), utilized Rb7NaSi8 and A12Si17 (A = K, Rb, Cs) Zintl phases as precursors towards Si nanoparticle production. The metathesis reaction of NaSi with SiI4 was also studied and resulted in 10 times the best concentration of Si nanoparticles obtained from the reaction with NH4Br. Si nanoparticles were supplied to the Carter group for fabricating Si nanoparticle thin films. Several undergraduate students assisted the research efforts on Si nanoparticle synthesis.
