  We ask the question, what are the ideal characteristics of a near-field
thermophotovoltaic cell? Our search leads us to a reformulation of near-field
radiative heat transfer in terms of the joint density of electronic states of
the emitter-absorber pair in the thermophotovoltaic system. This form reveals
that semiconducting materials with narrowband absorption spectra are critical
to the energy conversion efficiency. This essential feature is unavailable in
conventional bulk semiconductor cells but can be obtained using low dimensional
materials. Our results show that the presence of matched van Hove singularities
resulting from quantum-confinement in the emitter and absorber of a
thermophotovoltaic cell boosts both the magnitude and spectral selectivity of
radiative heat transfer; dramatically improving energy conversion efficiency.
We provide a model near-field thermophotovoltaic system design making use of
this idea by employing the van Hove singularities present in carbon nanotubes.
Shockley-Queisser analysis shows that the predicted heat transfer
characteristics of this model device are fundamentally better than existing
thermophotovoltaic designs. Our work paves the way for the use of quantum dots,
quantum wells, two-dimensional semiconductors, semiconductor nanowires and
carbon nanotubes as future materials for thermophotovoltaic cells.
