From Photons to Excitons and Plasmons: Excited State Processes Hold the Key to New Energy Sources

Sun supplies earth with over 150 Petawatts (1.5X10¹⁷ W) of incident radiation, as a black body source at 6000K. Conversion of a fraction of this energy economically can satisfy the worldwide energy demand which is currently just below 15 Terawatts (1.5X10¹³ W). At present, more than 80 percent of this energy demand is met using non-renewable energy sources. Unfortunately, over 60 per cent of this energy is lost as waste heat. Attempts to harness thermal radiation from sun (or waste heat sources) have been only marginally successful, due to the inability to capture energetically broad distribution of emitted photons. Here, I present an unexplored alternative for converting this incident light into isoenergetic beam of photons, which can be converted into electricity, with near 100% efficiency. Such a strategy involves utilizing a secondary emitter which absorbs the thermal energy and emits a narrow frequency of light, which is then converted into electricity using an infrared semiconductor photocell. This thermophotovoltaic conversion hinges on a two pronged strategy: developing emitters which can tune the frequency and directionality of the emitted light; and fabrication of engineered infrared photocells with high efficiency and tunable bandgap.

Metal nanostructures patterned as three-dimensional photonic crystals, selectively tailor the light-matter interaction near the photonic bandgap, which enhances the emission of a particular frequency of light. Here, I will present results from selective tailoring of thermal emission, at low temperatures, for development of efficient thermophotovoltaic emitters [Nagpal et. al., Nano Letters, 2008]. Another promising route towards tuning thermal emission is utilizing surface plasmons. While these hybrid photon-free electron waves on metal surfaces are typically excited using light or fast electrons, recently we have explored excitation of surface plasmon polaritons using heat. I will discuss recent results on fabricating ultrasmooth patterned surfaces, which reduce scattering of hot electrons on nanoscale surface roughness and grain boundaries, for low-loss propagation of surface plasmons [Nagpal et. al., Science, 2009]. These surface waves, when combined with designed nanostructures, enable modulation of the frequency and directionality of the emitted glow.

Colloidal semiconductor nanocrystals (NCs) are promising candidates for fabrication of cheap, solution-processable, photovoltaic devices with tunable quantum-confined bandgap. However, poor understanding of charge transport in NC assemblies hinders their technological applicability. Conventionally, it was assumed that following exciton dissociation, electrical conduction of photogenerated charges occurs solely through tunneling between quantum confined states. Recently, it was demonstrated that defects on nanocrystal surface participate in conduction by forming a percolated network of mid-gap states [Nagpal et. al., Nature Communications, 2011]. These states affect both the extracted voltage and current output from a NC semiconductor photocell. Electrically doping these films tunes the Fermi-level and modifies the recombination dynamics, tuning the observed photovoltaic and photoconductive response of these films. Such unprecedented control over light-matter interaction, and tunable nanostructured photovoltaics comes from better understanding and utilization of excited state processes. These advances can pave the way towards development of third-generation energy harvesting devices.

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Nagpal P., Klimov V.I., Nature Communications, 2:486, (2011).

Nagpal P., Han S., Stein A., Norris D.J., Nano Letters, 8, 3238, (2008).