Solar Cells Utilizing Hot Carriers for Enhanced Conversion Efficiency

In 1982, thermodynamic calculations [4] showed for the first time that the same high conversion efficiency of solar irradiance into free energy in a tandem stack of different bandgaps can be also be obtained by utilizing the total excess kinetic energy of hot photogenerated carriers in a single bandgap semiconductor before they cooled to the lattice temperature through electron-phonon scattering; in the limit of a carrier temperature of 3,000K the conversion efficiency also reaches 67%, the same value as for a tandem PV cell with a multiple stack of bandgaps matched to the solar spectrum. This can be achieved by transporting the hot carriers to carrier-collecting contacts with appropriate work functions (either into an electrolyte redox system in a photoelectrochemical fuel producing cell [5] or a solid state ohmic contact in a PV cell [6], before the carriers cool. These cells are called hot carrier solar cells [2,5-8].

Another approach to beneficially utilize hot electron-hole pairs is to use their excess kinetic energy to produce additional electron-hole pairs, and thus increase the possible photocurrent. However, this approach yields a lower maximum con­version efficiency of about 45% at one-sun intensity. This lower efficiency occurs because to satisfy energy conservation the photon energies between 1 and 2 Eg are lost through electron-phonon scattering and produce heat. In bulk semiconductors, this process is called impact ionization [9] and is an inverse Auger type of process. However, impact ionization (I. I.) cannot contribute to improved quantum yields (QYs) in present solar cells based on bulk semiconductors such as Si, CdTe, CuInxGai_xSe2, or III-V semiconductors in a multi-junction, tandem structure because the maximum QY for I. I. does not produce extra carriers until photon energies reach the ultraviolet region of the spectrum (hu > 3.5 eV), where solar photons are absent. In bulk semiconductors, the threshold photon energy for I. I. exceeds that required for energy conservation alone because crystal momentum (k) must also be conserved [9]. Additionally, the rate of I. I. must compete with the rate of energy relaxation by phonon emission through electron-phonon scattering. It has been shown that in bulk semiconductors the rate of I. I. becomes competitive with phonon scattering rates only when the kinetic energy of the electron is many multiples of the bandgap energy (Eg) [10-12]. In bulk semiconductors, the observed transition between inefficient and efficient I. I. occurs slowly; for example, in Si the I. I. efficiency was found to be only 5% (i. e. total quantum yield = 105%) at hv ^ 4eV (3.6 Eg), and 25% at hv ^ 4.8eV(4.4 Eg) [9,13].

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