A New Paradigm for Solar Energy Conversion: Non-Radiative Energy Transfer

See University of Southern California, A New Paradigm for Solar Energy Conversion, http://nanostructure.usc.edu/research/solar.shtml (as of Nov. 15, 2008, 04:24 GMT).

Siyuan Lu, Anupam Madhukar, "Nonradiative Resonant Excitation Transfer from Nanocrystal Quantum Dots to Adjacent Quantum Channels", Nano Letters, 7, 3443-3451 (2007)

Historically, the earliest photovoltaic solar cells were investigated in silicon p-n junction devices [1] where the absorption and the created electron-hole pair separation and transport are processes that occur in the same medium. With the development of semiconductor epitaxial deposition techniques, the silicon pn junction solar cells have been augmented with III-V compound semiconductor multiple quantum well based solar cells [2] which exhibit the highest efficiencies although remain also the most expensive to produce.

Low cost solar cell with organic absorbers have been and are continuing to be exploited but the high exciton binding energies typical of such materials requires their mixing with an appropriately energy-aligned different material, inorganic or organic, to create internal heterojunctions at which the photo-generated exciton can be split into its electron and hole components for subsequent collection. These have come to be known as excitonic solar cells [3-6] and in the current implementations of these the separated electrons and holes end up in two different media for transport and collection. These media have so far been ones with very low charge carrier motilities, which is a major contributing factor to their hitherto low overall energy conversion efficiency.

Overcoming or bypassing the bottlenecks of charge carrier extraction and transport to collecting electrodes will constitute a major step forward in the quest for the realization of efficient and cost effective solar energy converters. We have thus introduced an new paradigm for solar energy conversion. To learn about this new paradigm see below (or read our full paper: Siyuan Lu and Anupam Madhukar, "Nonradiative Resonant Excitation Transfer from Nanocrystal Quantum Dots to Adjacent Quantum Channels", Nano Letters, 7, 3443-3451 (2007)

Nonradiative Coupling

Figure 1. Schematic showing a new solar cell architecture utilizing nonradiative couplingbetween dipoles for direct transfer of energy from the excitons created in the light absorber to (a) quantum well (b) nanowire high mobility charge carrier transport channels.

For efficient nonradiative transfer of energy, appropriate matching of the absorption and emission spectra of the donor and acceptor species and their separation is a critical consideration [13]. For our studies, PbS NCQDs are employed as the absorbers (donors) and an appropriately designed adjacent InGaAs quantum well buried in GaAs matrix provides the high mobility charge transport channel for accepting the resonantly transferred exciton energy in the form of electron and hole. To shed light on the time scale and efficiency of excitation transfer from the PbS NCQD into the InGaAs near surface quantum well, we have examined the time resolved behavior of the luminescence decay from the NCQDs adsorbed on quantum well containing substrates and compared it to the behavior on control substrates without the buried quantum well.

Figure 2 shows the room temperature time decay behavior of the luminescence peak of the NCQDs at 965nm. Note the considerably fast decay time of ~207ns in the presence of the quantum well as compared to the ~300ns decay time on the GaAs control substrate. The reduction of the NCQD PL decay time from ~300ns to ~207ns is the manifestation of the opening of an excitation transfer channel provided by the one-dimensionally confined states of the quantum well (Fig.2). From the difference between these two measured decay times, we calculate the nonradiative transfer rate to be ~ 1/(690ns), ~1.4 times faster than their radiative decay rate ~ 1/(960ns) measured for the NCQDs dispersed on a glass substrate. This means for a nanocrystal of 100% quantum yield, the efficiency of the nonradiative transfer from the NCQD to the quantum well is ~60% for the 8.2 nm center-to-center separation of the PbS NCQDs adn the InGaAs QW in these experiments. The transfer efficiency is expected to be further improved by reducing the distance between the NCQDs and the NSQW.

Fig. 2. Time resolved PL of PbS NCs on passivated GaAs (blue) and on passivated NSQW (Red). Excited at 900nm (below GaAs bandgap) and detected at 965nm (PbS PL peak). TRPL curves are fitted using stretched exponential function.

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