• Energy Research

Abstract Accurate and rapid threading dislocation density (TDD) characterization of III–V photovoltaic materials using electron channeling contrast imaging (ECCI) is demonstrated. TDDs measured using ECCI showed close agreement with those from electron beam-induced current mapping (EBIC) and defect selective etching (DSE). ECCI is shown to be well-suited for measuring TDD values over a range of ~5×10 6 –5×10 8 cm −2 . ECCI can distinguish individual dislocations in clusters closer than 0.2 µm, highlighting its excellent spatial resolution compared to DSE and EBIC. Taken together, ECCI is shown to be a versatile and complementary method to rapidly quantify TDD in III–V solar cells.

The highest efficiency heteroepitaxial GaAs solar cells on Si have historically been grown in the p+/n polarity, which was preferred due to the decreased sensitivity of open-circuit voltage in such cells to threading dislocations. The n+/p polarity also has potential advantages due to the higher mobility of electrons than holes in GaAs, and most multi-junction solar cells in the literature are grown in this polarity. Here, we demonstrate n+/p GaAs solar cells on Si with a certified AM1.5G efficiency of 16.8%, approaching the best certified efficiency of 18.1% for p+/n cells in the literature. The high efficiency of our n+/p cells is primarily due to the short-circuit current density of 26.5 mA/cm2, which is significantly higher than prior p+/n record cells. The strong carrier collection results from the use of a highly transparent AlInP window layer, thin n+ emitter, and a relatively high minority electron diffusion length in the p - type base. The high quantum efficiency of these n+/p cells at wavelengths of 700–880 nm makes them promising for future triple-junction devices on Si, where the GaAs will serve as a middle sub-cell.

In this paper, we study the performance of 2.0 eV Al0.12Ga0.39In0.49P and 1.4 eV GaAs solar cells over a temperature range of 25–400 °C. The temperature-dependent ${J_{01}}$ and ${J_{02}}$ dark currents are extracted by fitting current–voltage measurements to a two-diode model. We find that the intrinsic carrier concentration ${n_i}$ dominates the temperature dependence of the dark currents, open-circuit voltage, and cell efficiency. To study the impact of temperature on the photocurrent and bandgap of the solar cells, we measure the quantum efficiency and illuminated current–voltage characteristics of the devices up to 400 °C. As the temperature is increased, we observe no degradation to the internal quantum efficiency and a decrease in the bandgap. These two factors drive an increase in the short-circuit current density at high temperatures. Finally, we measure the devices at concentrations ranging from ∼30 to 1500 suns and observe n = 1 recombination characteristics across the entire temperature range. These findings should be a valuable guide to the design of any system that requires high-temperature solar cell operation.

We report on the molecular beam epitaxy (MBE) growth and device characteristics of Ge solar cells. Integrating a Ge bottom cell beneath a lattice-matched triple junction stack grown by MBE could enable ultra-high efficiencies without metamorphic growth or wafer bonding. However, a diffused junction cannot be readily formed in Ge by MBE due to the low sticking coefficient of group-V molecules on Ge surfaces. We therefore realized Ge junctions by growth of homo-epitaxial n-Ge on p-Ge wafers within a standard III–V MBE system. We then fabricated Ge solar cells, finding growth temperature and post-growth annealing to be key factors for achieving high efficiency. Open-circuit voltage and fill factor values of ~0.175 V and ~0.59 without a window layer were obtained, both of which are comparable to diffused Ge junctions formed by metal-organic vapor phase epitaxy. We also demonstrate growth of high-quality, single-domain GaAs on the Ge junction, as needed for subsequent growth of III–V subcells, and that the surface passivation afforded by the GaAs layer slightly improves the Ge cell performance.

AlInP offers the highest direct bandgap ( $E_{g}$ ) among nonnitride III–V materials, making it attractive for top cell applications in five-to-six-junction solar cells. We present novel 2.05–2.15 eV direct-gap AlInP solar cells, grown on GaInAs/GaAs-graded buffers by metal–organic chemical vapor deposition (MOCVD). Despite the high Al content of 36–39% in the active regions, secondary ion mass spectrometry results indicate oxygen concentrations less than 3.5 × 1016 cm−3. The AlInP devices we present here exhibit superior photovoltaic performance to GaP and similar performance to metamorphic GaInP solar cells, reaching a $E_{g}$ -voltage offset of 0.57 V. Design enhancements based on device and material characterization led to improvements of over ∼4× in short-circuit current density from our first-generation AlInP devices. Our results indicate that a p-i-n device design is necessary to account for low minority carrier diffusion lengths in AlInP solar cells; additionally, diffusion of Zn dopant atoms poses another challenge that must be accounted for in cell design. The effect of offcut on cell performance was also investigated, with improved solar cells on samples offcut toward the A plane. The promising results in this work provide an alternative path toward realizing high- $E_{g}$ top cells with possible applications in upright metamorphic multijunction solar cells.

We demonstrate dual-junction (Al)GaInP/GaAs solar cells designed for operation at 400 °C and 1000x concentration. For the top junction, we compare (Al)GaInP solar cells with room-temperature bandgaps ranging from 1.9 to 2.0 eV. At 400 °C, we find that ∼1.9 eV GaInP solar cells have a higher open-circuit voltage and a lower sheet resistance than higher bandgap (Al)GaInP solar cells, giving them a clear advantage in a tandem configuration. Dual-junction GaInP/GaAs solar cells are fabricated, and we show temperature-dependent external quantum efficiency, illuminated current–voltage, and concentrator measurements from 25 °C to 400 °C. We measure a power conversion efficiency of 16.4% ± 1% at 400 °C and 345 suns for the best dual-junction cell, and discuss multiple pathways to improve the performance further. After undergoing a 200 h soak at 400 °C, the dual-junction device shows a relative loss in efficiency of only ∼1%.

Summary: III–V/Si epitaxial tandems with a 1.7-eV GaAsP top cell promise stable power conversion efficiencies above the fundamental limit of Si single-junction cells. However, III–V/Si epitaxial tandems have suffered from limited minority carrier diffusion length in the top cell, leading to reduced short-circuit current densities (JSC) and efficiencies. While conventional wisdom dictates that dislocation density in III–V/Si tandems must be reduced to boost efficiency, here, we show that heterointerface design and growth sequence also play critical roles in reducing recombination losses. Our improved GaAsP cells make use of a wide-band gap AlGaAsP electron-blocking layer that forms a pristine interface with GaAsP, resulting in a 10%–20% (absolute) boost in quantum efficiency over previous work in the critical red wavelength range (600–725 nm), despite similar dislocation density. Combining the improved top cell carrier collection with Si backside texturing, we obtain 25.0% efficient GaAsP/Si tandem cells with a closely matched JSC of 18.8 mA/cm2.