Designing III-V multijunction solar cells on silicon

Abstract

Single junction Si solar cells dominate photovoltaics but are close to their efficiency limits. This paper presents ideal limiting efficiencies for tandem and triple junction multijunction solar cells subject only to the constraint of the Si bandgap and therefore recommending optimum cell structures departing from the single junction ideal. The use of III-V materials is considered, using a novel growth method capable of yielding low defect density III-V layers on Si. In order to evaluate the real potential of these proposed multijunction designs, a quantitative model is presented, the strength of which is the joint modelling of external quantum efficiency and current-voltage characteristics using the same parameters. The method yields a single parameter fit in terms of the Shockley-Read-Hall lifetime. This model is validated by fitting experimental data of external quantum efficiency, dark current, and conversion efficiency of world record tandem and triple junction cells under terrestrial solar spectra without concentration. We apply this quantitative model to the design of tandem and triple junction solar cells, yielding cell designs capable of reaching efficiencies without concentration of 32% for the best tandem cell and 36% for the best triple junction cell. This demonstrates that efficiencies within a few percent of world records are realistically achievable without the use of concentrating optics, with growth methods being developed for multijunction cells combining III-V and Si materials.

Modeling lattice-matched InP-based multijunction solar cells

“…The multijunction model is tested on a tandem InGaP/GaAs solar cell [8] and, as seen from Table 2, the calculated data nearly fitted the experimental data. SRH coefficients and surface recombination terms are taken from [23]. [27].…”

“…After testing the single junction model, two, three, and four subcells are assumed to be serially connected via tunnel junctions. Similar to [23] we assumed the tunnel junction as an absorbing layer while calculating photocurrent and the electrical loss due to tunnel junction is taken into account as a series resistance element.…”

“…Optical parameters (n-k parameters) and electrical properties of GaAs are taken from [22]. SRH recombination life times of electrons and holes are assumed to be 25 ns and surface recombination rates Sn and Sp are assumed to be 2 × 10 3 and 10 2 cm/s, respectively [23]. After testing the single junction model, two, three, and four subcells are assumed to be serially connected via tunnel junctions.…”

“…[27]. In all designs, double layer antireflection coating [23] and AlP 0.39 Sb 0.61 window layer are used. To best of the author's knowledge, the quadruple AlAsSb/InP/InGaAsP/GaAsSb structure is proposed and analyzed for the first time in this study.…”

III–V Multijunction Solar Cell Integration with Silicon: Present Status, Challenges and Future Outlook

“…Efficiencies exceeding 33% seems feasible at a realistic TDD of 10 6 cm −2 under 200-sun AM1.5d (1,000 W/m 2 ) spectrum ). Using areal current-matching (ACM), a 2J GaInP/GaAs connected onto an enlarged bottom Si subcell is predicted to have 3J efficiencies exceeding 43% under 1-sun AM1.5g spectrum (Connolly et al 2013). 4J AlGaAs/ GaAs/Si/InGaAs tandem solar cells utilizing Si as an intermediate subcell could achieve efficiencies exceeding of 45% (Mathews et al 2012).…”