Two-Photon Absorption in ${\mathrm{GaAs}}_{1-x-y}{\mathrm{P}}_y{\mathrm{N}}_x$ Intermediate-Band Solar Cells

We demonstrate that a two-photon absorption process occurs in a ${\mathrm{GaAs}}_{1-x-y}{\mathrm{P}}_y{\mathrm{N}}_x$-based solar cell in which a ${\mathrm{GaAs}}_{0.31}{\mathrm{P}}_{0.68}{\mathrm{N}}_{0.01}$ layer is implemented in the heterostructure and the $E_{-}$ energy band is isolated using an AlP blocking layer. The observed transition energies in external quantum-efficiency spectra correspond to the $E_{-}$ and $E_{+}$ energy bands of the ${\mathrm{GaAs}}_{0.31}{\mathrm{P}}_{0.68}{\mathrm{N}}_{0.01}$ alloy and agree with the photoreflectance results. With photon energy smaller than the transition between the valence band and the $E_{-}$ band, the absorption of IR light increases the quantum efficiency by over 10% at room temperature and significantly more at lower temperatures due to the suppression in the thermionic emission. The effects of two-photon absorption on the $I\text{−}V$ characteristics and output power of the fabricated ${\mathrm{GaAs}}_{1-x-y}{\mathrm{P}}_y{\mathrm{N}}_x$ solar cell are studied by current transport simulations. We show how doping of the intermediate band and current transport between the intermediate band and surrounding regions affect the operation of ${\mathrm{GaAs}}_{1-x-y}{\mathrm{P}}_y{\mathrm{N}}_x$ solar cells. Simulations under 1 Sun illumination result in an enhancement of 6% in the power density compared to a corresponding reference single-junction cell.

Figure 1

Schematics of the structure of sample SC1. The structure of samples R1 and R2 is otherwise similar except the 40 nm thick AlP layer is missing.

Figure 2

Comparison between (a) the photoreflectance spectrum of a 20-nm-thick ${\text{GaAs}}_{0.31}{\mathrm{P}}_{0.68}{\mathrm{N}}_{0.01}$ layer and the EQE spectrum of samples (b) R2, (c) R1, and (d) SC1.

Figure 3

Increase in quantum efficiency with IR light illuminating the sample.

Figure 4

Simulated band diagrams at short-circuit condition for samples (a) R2, (b) R1, and (c) SC1.

Figure 5

Temperature-dependent behavior of (a) the EQE and (b) the increase in quantum efficiency with IR light of the sample SC1. The inset of (b) shows the thermionic emission rate over the AlP blocking layer as a function of temperature. (c) The ratio between the integrated EQE of the energy range corresponding to the $E_{-}$ and the $E_{+}$ energy transition [i.e., the shaded region of (a)].

Figure 6

(a) Total generation rates (G) of different band-to-band transitions and (b) the photocurrent induced by the total generation rate as a function of $n$-type doping concentration in the ${\text{GaAs}}_{0.31}{\mathrm{P}}_{0.68}{\mathrm{N}}_{0.01}$ layer.

Figure 7

Simulated $I\text{−}V$ curves of the six limiting cases of solar-cell operation. Dotted curves show the $I\text{−}V$ curves so that all the photogenerated electrons from the valence band end up either in $E_{-}$ or $E_{+}$, from which they can escape freely to the buffer layers. The open-circuit voltage is determined mainly by the radiative recombination rate, which exceeds the generation rate roughly at voltages corresponding to the $E_{-}$ or the $E_{+}$ band gap. Solid curves illustrate the effect of $n$-type doping to $I\text{−}V$ curves by accounting all the generation and recombination processes in the model. Dashed curves show the single-junction limits of the device with corresponding band gaps.