One-Sun AM0 36% Solar Cell Enhanced by Engineered 2.15 eV Homojunction Top Subcell

Abstract

Optimal bandgap combination to deliberately split the solar spectra is the key to high efficiency multijunction solar cell design. III-V Multijunction solar cells with more than four junctions require an inverted top subcell with specific bandgap wider than 2.1 eV to absorb the short-wavelength photons (< 600 nm) effectively. (Al_xGa_1-x)_0.5In_0.5P alloy lattice-matched to GaAs substrate, with x ≥ 0.31, is usually preferred. However, it has been a challenge to achieve high performance inverted 2.1 eV AlGaInP homojunction solar cells for the severe decrease of short-wavelength (especially for wavelength < 450 nm) quantum efficiency. Thus, until now, a compromise structure replacing the n-type emitter with a narrow bandgap material (for instance, 1.91 eV GaInP), which is called the heterojunction solar cell, is widely employed. However, this structure would decrease the open-voltage for increasing energy loss of short-wavelength photons and limit the multijunction device efficiency. Here, we investigate the underlying mechanisms besides those commonly known results and present arguments that two new mechanisms should be attributed to the degradation of short-wavelength quantum efficiency: (1) bulk AlGaInP quality degradation resulting from the underneath AlInP window surface morphology and (2) additional optical absorption of the intermediate layers formed by III-V atom intermixing. Based on these findings, an interface Induced lifetime decrease model, and an intermixing layer model are introduced into the numerical simulations to time-resolved photoluminescence and internal quantum efficiency, achieving nice agreement between measured and modelled data with reasonable input parameters. Consequently, two strategies, 1) thermal treatment for AlInP layers and 2) minor compressive strain in P—containing materials, are suggested not only for the inverted 2.1 eV AlGaInP homojunction solar cells but also for the inverted 2.1 eV/1.7 eV/1.4 eV triple-junction solar cell. The subcells with these two strategies show higher quantum efficiency than the normal ones despite the bandgaps changing from 2.12 eV to 2.15 eV (e.g., IQE@400 nm increases from 69% to 76%). Meanwhile, the fill factor of inverted triple junction solar cells is slightly enhanced from 0.85 to 0.87. This improved triple junction solar cell is bonded with a 1.1 eV/0.83 eV dual-junction solar cell to form a five-junction solar cell, achieving an outstanding one-sun AM0 efficiency of 36.06% (Voc:4.904 V, Jsc:11.51 mA/cm², FF:0.8645). The efficiency gain, compared with those previously reported, is attributed to the thermalization loss reduction of short-wavelength photons for using a high spectral response 2.15 eV homojunction top cell.