SOLAR Collaborative: Multiple Exciton Generation and Charge Extraction in All-Inorganic Nanostructured Solar Cells

University Of California-Davis, Davis CA

Investigators

Abstract

TECHNICAL SUMMARY 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.