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GaAsP on Si tandem cells represent a promising path towards achieving high efficiency while leveraging the Si solar knowledge base and low-cost infrastructure. However, dislocation densities exceeding 108 cm−2 in GaAsP cells on Si have historically hampered the efficiency of such approaches. Here, we report the achievement of low threading dislocation density values of 4.0–4.6 × 106 cm−2 in GaAsP solar cells on GaP/Si, comparable with more established metamorphic solar cells on GaAs. Our GaAsP solar cells on GaP/Si exhibit high open-circuit voltage and quantum efficiency, allowing them to significantly surpass the power conversion efficiency of previous devices. The results in this work show a realistic path towards dual-junction GaAsP on Si cells with efficiencies exceeding 30%.

Gallium phosphide (GaP) is an attractive candidate for wide-bandgap solar cell applications, possessing the largest bandgap of the III-arsenide/phosphides without aluminum. However, GaP cells to date have exhibited poor internal quantum efficiency (IQE), even for photons absorbed by direct transitions, motivating improvements in material quality and device structure. In this work, we investigated GaP solar cells grown by molecular beam epitaxy over a range of substrate temperatures, employing a much thinner emitter than in prior work. Higher growth temperatures yielded the best solar cell characteristics, indicative of increased diffusion lengths. Furthermore, the inclusion of an AlGaP window layer improved both open-circuit voltage and short wavelength IQE.

We demonstrate metamorphic ∼1.7 eV GaAsxP1−x (x = 0.71 − 0.73) solar cells on high-quality GaP/Si templates and compare them to cells co-grown on bulk GaP. Both n+-emitter/p-base and p+-emitter/n-base polarities are explored. Cells with n-type bases demonstrate current-voltage characteristics that are similar to p-type base cells, but with blue-shifted peak quantum efficiencies. Threading dislocation densities for cells on GaP/Si were 0.92 − 1.3 × 107 cm−2, significantly lower than previous reports but higher than cells grown on bulk GaP. An open-circuit voltage of 1.12 V was obtained for a 1.71 eV cell on Si, leading to a promising bandgap-voltage offset of 0.59 V.

The properties of quantum well carrier escape were studied by varying barrier thicknesses in InGaN/GaN multi-quantum well solar cell devices. The dependence of the photocurrent on applied bias and temperature exhibited properties indicative of the quantum well carrier escape mechanisms of thermionic emission and tunneling, with tunneling dominating for thin barriers and high fields. Simulations using a self-consistent drift-diffusion and Schrodinger solver with analytical formulas extracted carrier escape lifetimes. By employing sufficiently thin barriers, it was found that escape lifetimes can be made small compared to recombination lifetimes, leading to high internal quantum efficiency.

The hole concentration p in Mg-doped GaN films grown by ammonia molecular beam epitaxy depends strongly on the growth temperature TGR. At TGR=760 °C, GaN:Mg films showed a hole concentration of p=1.2 × 1018 cm−3 for [Mg]=4.5×1019 cm−3, while at TGR=840 °C, p=4.4×1016 cm−3 for [Mg]=7×1019 cm−3. Post-growth annealing did not increase p. The sample grown at 760 °C exhibited a low resistivity of 0.7 Ωcm. The mobility for all the samples was around 3−7 cm2/V s. Temperature-dependent Hall measurements and secondary ion mass spectroscopy suggest that the samples grown at TGR>760 °C are compensated by an intrinsic donor rather than hydrogen.

The effect of doping and polarization on carrier collection is investigated for InGaN quantum well solar cells. Energy band diagram simulations of actual devices indicate that spontaneous and piezoelectric polarization sheet charges can inhibit carrier collection unless these charges are screened by sufficient doping. By increasing the doping on both sides of the active region, the polarization-induced barriers to carrier collection were eliminated and the short circuit current density was increased from 0.1 to 1.32 mA/cm2 under 1.5 sun AM1.5G equivalent illumination, leading to devices with an open circuit voltage of 1.9 V and a fill factor of 71%.

We demonstrate high quantum efficiency InGaN/GaN multiple quantum well (QW) solar cells with spectral response extending out to 520 nm. Increasing the number of QWs in the active region did not reduce the carrier collection efficiency for devices with 10, 20, and 30 QWs. Solar cells with 30 QWs and an intentionally roughened p-GaN surface exhibited a peak external quantum efficiency (EQE) of 70.9% at 390 nm, an EQE of 39.0% at 450 nm, an open circuit voltage of 1.93 V, and a short circuit current density of 2.53 mA/cm2 under 1.2 suns AM1.5G equivalent illumination.

High external quantum efficiency (EQE) p-i-n heterojunction solar cells grown by NH3-based molecular beam epitaxy are presented. EQE values including optical losses are greater than 50% with fill-factors over 72% when illuminated with a 1 sun AM0 spectrum. Optical absorption measurements in conjunction with EQE measurements indicate an internal quantum efficiency greater than 90% for the InGaN absorbing layer. By adjusting the thickness of the top p-type GaN window contact layer, it is shown that the short-wavelength (

A comprehensive analysis of forward and reverse current of Ga-face GaN p-n junctions grown by NH3 molecular beam epitaxy (NH3-MBE) on metal organic chemical vapor deposition (MOCVD) GaN:Fe on sapphire is presented. NH3-MBE has a great potential for growing low leakage vertical devices due to its nitrogen-rich growth. Diodes with the lowest n-doping of n∼3.5×1017 cm−3 exhibit an extremely low reverse current of 2.6×10−7 A/cm2 at −40 V and an ideality factor of 1.3, which is lower than the state-of-the-art Ga-face p-n junctions grown on sapphire by MOCVD. An explanation for the difficulty for observing low ideality factors in GaN is presented.