Multi-junction-solar-cell designs and characterizations based on detailed-balance principle and luminescence yields

Abstract

We developed a straightforward method based on detailed balance relations to analyze individual subcells in multijunction solar cells via measuring absolute electroluminescence quantum yields. This method was applied to characterization of a InGaP/GaAs/Ge 3-junction solar cell for satellite use. In addition to subcell I-V characteristics and internal luminescence yields, we derived balance sheets of energy and carriers, which revealed respective subcell contributions of radiative and nonradiative recombination losses, junction loss, and luminescence coupling. These results provide important diagnosis and feedback to fabrications. We calculated conversion-efficiency limit and optimized bandgap energy in 2-, 3-, and 4-junction tandem solar cells, including finite values of sub-cell internal luminescence quantum yields to account for realistic material qualities in sub-cells. With reference to the measured internal luminescence quantum yields, the theoretical results provide realistic targets of efficiency limits and improved design principles of practical tandem solar cells.

Defect‐induced current coupling in multi‐junction solar cells revealed by absolute electroluminescence imaging

“…To analyze the effect of local NR loss on cell performance, an essential indicator, the external radiative efficiency (ERE), is usually employed to quantify the contribution of NR rate (R NR_i ) in each subcell, [27][28][29]…”

“…To analyze the effect of local NR loss on cell performance, an essential indicator, the external radiative efficiency ( ERE ), is usually employed to quantify the contribution of NR rate ( R NR_i ) in each subcell, 27–29 where subscripts i = 1 and 2 represent the top and bottom subcells, R ext_i denotes the radiative emission rate via the front cell surface to the air, and R ext_i → i +1 indicates the radiative emission rate toward the bottom cell (namely, luminescent coupling or LC). Both global and local R ext_i can be derived directly from the absolute EL images, while in the anti‐reflection (AR) condition R ext_i → i +1 can be approximately modeled by where n i denotes the refractive index of the subcell; in this study, we assign n 1 = 3.5 and n 2 = 3.55.…”

“…27,28 Therefore, EL imaging is not only conducive to an in-depth understanding of electrical or manufacturing origin for local defects but also helpful for theoretically predicting efficiency improvements or degradation due to design and fabrication changes. 29 Current research works of EL-based defect diagnosis mainly focus on single-junction solar cells. Fuyuki et al 23 distinguished intrinsic and extrinsic defects in Si solar cells via EL images at different temperatures.…”

“…Moreover, to avoid extra parametric adjustments or fittings in the relative EL measurement, the absolute EL imaging technique with calibration is developed to access quantitative characterization mappings of subcells, such as radiative/non‐radiative recombination rates, energy losses, and efficiency degradations 27,28 . Therefore, EL imaging is not only conducive to an in‐depth understanding of electrical or manufacturing origin for local defects but also helpful for theoretically predicting efficiency improvements or degradation due to design and fabrication changes 29 . Current research works of EL‐based defect diagnosis mainly focus on single‐junction solar cells.…”

External quantum efficiency artifacts in partial‐irradiated GaInP/GaAs/Ge solar cells by protons and electrons

“…High energy electrons enhance nonradiative recombination (SRH recombination) current by a factor of k (from J 0 ′ to kJ 0 ′). Assuming V oc = 1.40 V, J sc = 13.0 mA/cm 2 (by integral of EQE), J 0 = 5.36 × 10 −26 mA/cm 2 and J 0 ′ = 2.64 × 10 −11 mA/cm 2 before radiation (calculated by e = 0.001 from EL data [16,17]. Common luminescence efficiency [14,15] of GaInP is 0.0001~0.01, so e = 0.001 was used).…”