High-efficiency AlGaAs-GaAs solar cells with internal Bragg reflector

The work presents an investigation of solar cells (SCs) based on AlGaAs/GaAs heterostructures with internal Bragg reflectors grown by low-pressure MOCVD on n-GaAs substrates in a horizontal resistively heated reactor. The typical structure consists of: Bragg reflector (BR) having 12 periods, n-GaAs base layer with the thickness of 1500-2000 nm, 400-500 nm thick p-GaAs emitter, 70 nm thick p-AlGaAs passivating window and top p-GaAs contact layers. The BR with the reflectance maximum centered at the wavelength of 860 nm consists of twelve pairs of AlAs/GaAs layers. The resulting BR thicknesses is 71.6 nm for AlAs and 59 nm for GaAs. In this case the peak of reflectance spectrum is in the area of 830-900 nm where the reflectance is close to unit. This multi-layer quasi-dielectric stack selectively reflect weakly absorbed photons with energies near the GaAs band gap for a second pass through the photoactive region increasing the photocurrent. The employment of the BR allows to increase the external quantum efficiency in the long wavelength range of the spectrum and to fabricate simultaneously a thinner n-GaAs base layer. The use of the internal BR, Ta/sub 2/O/sub 5/ for antireflecting coating and prismatic cover allowed us to obtain efficiency of 23.4% (17.7 suns, AMO, 25/spl deg/C) and 27.2% (23.4 suns, AM 1.5).