Chaomin Zhang

Material and device characteristics of InAs/GaAsSb sub-monolayer quantum dot solar cells

We have studied the material and photovoltaic characteristics of InAs/GaAsSb sub-monolayer quantum dot solar cells (QDSCs) with different Sb contents of 0%, 5%, 15%, and 20%. All QDSCs exhibit an extended external quantum efficiency (EQE) response in the wavelength range of 960–1000 nm that corresponds to sub-bandgap photon absorption. As Sb content increases from 5% to 20%, the cutoff wavelength in the EQE extends towards longer wavelength whilst the EQE in the wavelength region of 300–880 nm is lowered due to increased defect density. Compared to the QDSC (Sb 0%), an Sb incorporation of 5% enhances the short-circuit current density from 20.65 to 22.15 mA/cm2 induced by Sb surfactant effect. Since the open-circuit voltage and fill factor of the QDSC (Sb 5%) are comparable to those of the QDSC (Sb 0%), an enhancement in solar cell efficiency (10.5%) of the QDSC (Sb 5%) is observed. Further increasing Sb content to 15% and 20% results in the degradation of solar cell performance due to increased nonradiati...

AlGaSb-Based Solar Cells Grown on GaAs: Structural Investigation and Device Performance

GaSb and alloys based on the 6.1 Å family can be grown metamorphically on substrates such as GaAs allowing for the realization of several multijunction solar cell designs. This paper investigates the molecular beam epitaxy growth, crystal quality, and device performance of Al<italic>_x</italic>Ga₁₋<italic>_x</italic>Sb-based single-junction solar cells grown on GaAs substrates. The focus is on the optimization of the growth of Al<italic>_x</italic>Ga₁₋<italic>_x</italic>Sb on GaAs (001) substrates in order to minimize the threading dislocation density resulting from the large lattice mismatch between GaSb and GaAs. Utilizing optimum growth conditions, solar cells with absorbing layers of different Al<italic>_x </italic>Ga₁<italic>_x</italic>Sb compositions are studied and compared to control cells grown on lattice-matched GaSb substrates. GaSb, Al_0.15Ga_0.85Sb, and Al_0.5Ga_0.5Sb solar cells grown on GaAs substrates show open-circuit voltages of 0.16, 0.17, and 0.35 V, respectively. Furthermore, the lattice-mismatched cells demonstrate promising carrier collection with comparable spectral response to lattice-matched control cells grown on GaSb.

Hetero-emitter GaP/Si solar cells with high Si bulk lifetime

III-V/silicon solar cells which have an active silicon bottom solar cell are promising for multi-junction solar cell applications. In such solar cell structures, a high minority carrier lifetime in the bulk silicon substrate is necessary. Annealing silicon wafers at high temperature (> 500oC) in the molecular beam epitaxy (MBE) high-vacuum chamber revealed significant lifetime degradation. In this work, we developed a practical method to maintain high Si bulk lifetime. SiNx layer deposited on Si back side helps maintain millisecond level minority carrier lifetime. By this procedure high minority carrier lifetime in the Si substrate is preserved while high quality thin GaP layer is achieved. We demonstrate GaP as a hetero-emitter layer with high Si bulk lifetime in GaP/Si structure solar cell with 524mV open circuit voltage.

On the source of silicon minority-carrier lifetime degradation during molecular beam heteroepitaxial growth of III-V materials

A major hindrance to the development of devices integrating III-V materials on silicon is the preservation of its electronic quality. In this contribution, we report on the severe decrease in silicon bulk minority-carrier lifetime after heteroepitaxial growth of gallium phosphide, in our molecular beam epitaxy (MBE) system. The drop in lifetime occurs after annealing silicon above 500°C; we assign the increased recombination rate to extrinsic defect originating from highly mobile impurities diffusing from the MBE chamber. We show that the contaminant can be gettered by phosphorous diffusion. We investigate two approaches to protect the Si bulk lifetime by containing the contaminant to a part of the silicon that can be removed by etching. This provides a path to successful III-V growth on silicon.

Influence of high growth rate on GaAs-based solar cells grown by metalorganic chemical vapor deposition

Single-junction GaAs-based solar cell structures are grown by metalorganic chemical vapor deposition system at the growth rates of 14 μm/hr and 56 μm/hr. The X-ray diffraction study reveals that the crystal quality of the structures with varying the growth rates is comparable. From the external quantum efficiency spectra, it is observed that different behaviors exist in the short wavelengths (<; 450 nm) and the long wavelengths (> 500 nm) as the growth rate increases. The short-circuit current densities of the standard and fast grown cells are comparable. However, the open-circuit voltage of the fast grown cell is lower by above 40 mV as a result of the reduced minority carrier lifetime in the base layer, which is estimated by PC1D simulation.

LCOE of residential photovoltaic systems with batteries

Photovoltaics has reached grid parity in many residential locations in the US. However, this levelized cost of electricity (LCOE) is calculated using net metering, which allows all the electricity generated by the photovoltaic system to be used. As the amount of PV on the grid has increased, utilizes are modifying or moving away from net metering. We analyze LCOE of residential PV systems in high solar radiation locations (Phoenix, AZ) and show that the LCOE including battery systems can still achieve residential grid parity.

AlGaSb based solar cells grown on GaAs by Molecular Beam Epitaxy

A goal for concentrating photovoltaics is to realize efficiencies over 50%. Recent 4J bonded solar cells show a path to such high efficiency devices by separately growing the top and bottom solar cells. Present experimental devices use InP-based materials for the bottom junctions. III-Sb solar cells can be good candidates for bottom solar cells. Sb-containing III-V alloys have shown high electron mobility, wide band gap range including small band gaps, flexible band alignment, and small effective electron mass [1]. In addition, GaSb alloys can be grown with low defect densities on GaAs. This paper investigates GaSb-based solar cells. We show AlGaSb based solar cells grown directly on semi-insulator GaAs (001) substrates by Molecular Beam Epitaxy (MBE). Device and structural investigations have been performed to assess the electrical properties and material quality. Devices in the GaSb material system show Woc of 0.30, a very high value for a low band gap solar cell. To control the device properties, GaSb based solar cells grown on GaAs (100) substrates were compared to the devices grown on GaSb substrates.

Efficiency enhancement in InAs/GaAsSb quantum dot solar cells with GaP strain compensation layer

The structural characteristics and device performance of strain-compensated InAs/GaAsSb quantum dot solar cells (QDSCs) with different GaP coverages have been studied. The in-plane (out-of-plane) compressive strain of the QD stacks is reduced from −1.24 (+1.06) to −0.39 (+0.33)% by increasing the GaP coverage from 0 to 4 ML. This strain compensation decreases strain-induced dislocation density and hence enhances the overall crystal quality of the QDSCs. The external quantum efficiency spectra reveal that the increase in the GaP coverage increases the photocurrent from wavelengths shorter than GaAs bandedge of 880 nm, while it decreases the photocurrent from near infrared wavelengths beyond the bandedge. The conversion efficiency of the QDSCs is significantly improved from 7.22 to 9.67% as the GaP coverage is increased from 0 to 4 ML.