Sensitized energy transfer for organic solar cells, optical solar concentrators, and solar pumped lasers

Abstract

The separation of chromophore absorption and excitonic processes, such as singlet exciton fission and photoluminescence, offers several advantages to the design of organic solar cells and luminescent solar concentrators (LSCs) for the end goal of achieving a lower cost solar energy generation. This thesis explores three new device architectures to overcome limited solar absorption in singlet-exciton-fission based solar cells and neodymium based LSCs. The process of singlet exciton fission is de-coupled from photon absorption, exciton diffusion, and charge transport in singlet-exciton-fission based solar cells by inserting a singlet fission material at the donor-acceptor interface of an organic solar cell. Singlet excitons generated in the singlet exciton donor are transferred to the singlet fission material through near field energy transfer. In this device structure, the singlet donor can be chosen for high photon absorption, exciton diffusion, and charge transport, and the singlet fission sensitizer can be selected for high singlet fission efficiency. We demonstrated a doubling of the external quantum efficiency from 12.8% to 27.6% in a singlet donor (TPTPA) through the introduction of thin film singlet fission sensitizer (rubrene) for high efficiency organic solar cells. To reduce the cost of electricity generated by sunlight via LSC systems, replacing the expensive high efficiency visible photovoltaic (PV) elements with cheap, high efficiency, earth abundant near-infrared PV elements made with silicon. This requires replacing within the LSC the visible emitting chromophores with near infrared emitters. Here, we present the use of a lanthanide ion, neodymium--colloidal nanocrystal energy cascade system as a promising LSC emitter scheme for the silicon spectral region. Peak optical quantum efficiencies of 43% in a Nd³+:glass based LSC are demonstrated with simulated high geometric gain performance. With cascade energy transfer, the optical quantum efficiency in the visible of a Nd³+:glass is significantly improved with peak efficiency of 28%. The enhanced solar absorption of Nd³+:glass through cascade energy transfer can be extended into the infrared with more optimal sensitizers. The idea of directly converting broad-band solar radiation into coherent and narrow-band laser radiation could enable many attractive technologies for solar energy. Here, we present an architecture for solar pumped lasers that uses a luminescent solar concentrator to decouple the conventional trade-off between solar absorption efficiency and the mode volume of the optical gain material. We report a 750-[mu]m-thick Nd³+-doped YAG planar waveguide sensitized by a luminescent CdSe/CdZnS (core/shell) colloidal nanocrystal, yielding a peak cascade energy transfer of 14%, a broad spectral response in the visible portion of the solar spectrum, and an equivalent quasi-CW solar lasing threshold of 20 W-cm2 , or approximately 200 suns. The efficient coupling of incoherent, spectrally broad sunlight in small gain volumes should allow the generation of coherent laser light from intensities of less than 100 suns.