Quantum Antennae – Light as a Wave

The idea of a quantum antenna is to use the wave nature of light rather than its particle nature [Bailey, 1972]. Incoming light waves oscillate electrons in an antenna that has dimensions such that excited oscillations are resonant for a particular wavelength of light. Each of these oscillations is then rectified for each antennae to give a DC output. The voltage is determined by the built-in voltage of the diode (Уы = 2/3 of the bandgap of the diode semiconductor, depending on the radiative efficiency of the material). The current is determined by the number of electrons in the oscillation that are above the energy barrier determined by the diode built-in voltage. This number is a fraction of the total number of photons absorbed by that antenna, Np = (sin ДУьі (where f is the frequency of the light absorbed).

The short wavelengths of light mean that the devices have to be of the order of a few hundred nm. (For an ideal structure this is half the wavelength, but in practice probably slightly less because of penetration of the wave outside the antennae waveguide.) The broadband polychromatic solar spectrum also requires a wide range of antenna sizes to match all the wavelengths and its incoherent nature the need to arrange these antennae in two orthogonal directions of polarisation. In addition to these practical problems it has been suggested, based on wave theory, that the approach can only achieve 48% even under ideal conditions [Corkish et al., 2003]. Nonetheless, progress has been made on fabricating some of the small diode elements needed for such a device [Grover et al., 2013], although a proof of principle has yet to be demonstrated.

9.5 CONCLUSIONS

Third-generation PV aims to provide high conversion efficiency of photon energy with low manufacturing cost. The combined methodology of using multiple energy thresholds and low-cost processes with abundant nontoxic materials offers significant leverage in achieving this goal. However, the actual efficiencies and ease of optimisation depend on the different physical approaches. Efficiency, spectral robustness, and cost/ease of manufacture are impor­tant for a robust technology that can supply very significant increases in PV implementation.

Advanced nanomaterials offer great potential to optimise absorption, carrier generation, and separation. Low-dimensional quantum confinement provides bandgap flexibility in QW and QD materials. All-Si QDs devices fabricated by thin-film deposition technique have experimentally proved the application as upper cells in the all-silicon tandem solar cells. Efficient MEG has been observed in some semiconductor QDs due to the modified relaxation dynamics of photoexcited excitons. Devices designed to extract such multiple excitons have now shown QEs greater than 1. Work on intermediate-band and hot-carrier devices has also demonstrated that QD materials promise increased efficiencies and greater spectral robustness. Although these devices are still at early stages, implementation of such techniques could dramatically decrease cost per Watt with spectral robustness as they are compatible with conceptually relatively simple thin-film devices.

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