Design of Next Generation Full Spectrum Solar Cells Using Intermediate Band Semiconductors

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 GaN_xAs_yP_1-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, GaAs_yP_1-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.

[1] Luque, A., Marti, A. & Stanley, C. Understanding intermediate-band solar cells. Nature Photonics 6, 146–152 (2012).

[2] López, N., Reichertz, L., Yu, K.M., Campman, K. & Walukiewicz, W. Engineering the Electronic Band Structure for Multiband Solar Cells. Phys. Rev. Lett. 106, 028701 (2011).

[4] Shan, W. et al. Band anticrossing in GaInNAs alloys. Phys. Rev. Lett. 82, 1221–1224 (1999).

[5]   Yu, K. M. et al. Multiband GaNAsP quaternary alloys. Appl. Phys. Lett 88, 092110 (2006).

[6]   Y. J. Kuang, et al. GaNAsP: An intermediate band semiconductor grown by gas-source molecular beam epitaxy, Appl. Phys. Lett. 102, - (2013).