Photoreflectance analysis of a GaInP/GaInAs/Ge multijunction solar cell

Abstract

We have analyzed the photoreflectance spectra of a GalnP/GalnAs/Ge triple junction solar cell. The spectra reveal signatures from the window layer and middle and top subcells included in the stack. Additional contributions from the multilayer buffer introduced between the mismatched bottom and middle cells have been detected. Franz-Keldysh oscillations (FKOs) dominate the spectra above the fundamental bandgaps of the GalnP and GalnAs absorbers. From the FKO analysis, we have estimated the dominant electric fields within each subcell. In light of these results, photoreflectance is proposed as a useful diagnostic tool for quality assessment of multijunction structures prior to completion of the device or at earlier stages during its processing.Multijunction solar cells currently represent the most efficient approach to photovoltaic energy conversión, with efficiencies over 40%. 1 ' 2 The multijunction design pursues the selective absorption of different parts of the solar spectrum by different absorbers in a layer stack so as to optimize the photogeneration and collection of minority carriers and to reduce energy losses related to thermalization of hot carriers. The characterization of multijunction solar cells is complex and the interpretation of results is usually not straightforward. There are techniques available, e.g., quantum efficiency and spectral response, 3 current-voltage characteristics under illumination and calibration routines thereof, 4 and electroluminescence. However, all these techniques require complete cell structures, including metallization and encapsulation of individual devices. It would be of a great interest to expand the diagnostic tools that reveal the optoelectronic quality of the subcells to earlier stages of device completion or even during processing of the multilayer stack. In this letter, the results of photoreflectance (PR) characterization of a triple junction structure without metallization are presented, and the suitability of PR to this aim is demonstrated. The solar cell structure was grown as an epitaxial, latticemismatched (metamorphic) structure by metalorganic vaporphase epitaxy (MOVPE). More details on the MOVPE growth process can be found elsewhere. 1

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