Graded-band-gap pGa1−xAlxAs-nGaAs heterojunction solar cells

Abstract
The theoretical spectral response of the graded‐band‐gap pGa1−xAlxAs‐nGaAs solar cell is calculated by solving the continuity equations. The electric fields produce a collection efficiency two times larger than that of the abrupt heterojunction diodes at wavelengths less than 700 nm for surface recombination velocities of 106–107 cm/sec. The graded‐band‐gap diode can be operated at high frequencies because of the reduced transit time of minority carriers in the p‐type layer. A graded‐band‐gap pGa1−xAlxAs‐nGaAs solar cell is fabricated by utilizing the diffusion of Al in the Ga melt. The gradient of the energy gap is evaluated by photoresponse and photoluminescence measurements. The photoluminescence shows a broad emission peak from 700 to 800 nm at 77 K.