Design of Next Generation Full Spectrum Solar Cells Using Intermediate Band Semiconductors
Authors: Luce Alexander , Kuang Y J , Dubon Oscar , Wu Junqiao , Tu C W , Yu Kin Man , Walukiewicz Wladek 
Published: February 2020

The intermediate band solar cell (IBSC) is an attractive concept to achieve photovoltaic conversion efficiencies exceeding the Shockley-Quiessar efficiency limit for a single junction cell. In an IBSC, a partially occupied energy band located within the bandgap region of a semiconductor serves as a ‘stepping stone’ for the absorption of lower energy photons that are nominally not absorbed by the cell. In theory, the addition of this ‘intermediate band’ would provide a high photocurrent while maintaining a high output voltage.1 Recently, an IBSC device using the highly-mismatched semiconductor GaNAs alloy (HMA’s) has demonstrated optical activity from the three energy bands and is a proof of principle for the IBSC concept.[2] It has been proposed that the GaAsPN dilute nitride alloy system is another promising candidate material for IBSC’s.3 Calculations based on the band anticrossing model4 show that the intermediate band formed by the localized N level in GaNxAsyP1-x-y alloys with y<0.7 will be narrow and completely separated from the conduction band and hence more suitable as an intermediate band semiconductor for IBSC’s.3 The location of the IB (E-) and the conduction band (E+) of this material system can be tuned more effectively for optimal absorption of the solar spectrum by changing the As/P ratio (y).. Furthermore, GaAsyP1-y exhibits a direct-indirect gap crossover at approximately y=0.5, and for alloys with compositions y>0.5 the photon absorption occurs at the lower energy Γ band, yet electron transport may take place in the indirect X band. This could result in an IBSC absorber material with a high absorption coefficient that retains long carrier lifetimes.5

This work presents the design and characterization of a GaAsPN-based IBSC device. In order to fabricate a functioning IBSC device, first p-type, and n-type GaNAsP were grown and characterized in terms of their optical and electrical properties. We found that efficient p and n-type doping of GaNAsP can be achieved with Be and Si, respectively. Next, GaAsPN-based IBSC devices were grown by gas-source molecular beam epitaxy (MBE) on GaP substrate with a thick (~1.5µm) compositionally graded GaAsP buffer layers. Both a blocked intermediate band structure, where the intermediate band is electrically isolated from the valence and conduction bands, and an unblocked intermediate band structure (as a reference structure) were considered. The design of the structure was optimized to achieve efficient charge-carrier extraction, yet still demonstrate absorption from the three possible band-to-band transitions.The external quantum efficiency, both with and without a white-light bias, was used to evaluate the spectral response of the devices and the optical activity of the intermediate band. I-V measurements using both AM 1.5 and under 30x concentration were used to evaluate the PV device performance.

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