Stephanie Tomasulo

Comparison of GaAsP solar cells on GaP and GaP/Si

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.

Heterogeneous Integration of InGaAs Nanowires on the Rear Surface of Si Solar Cells for Efficiency Enhancement

We demonstrate energy-conversion-efficiency (η) enhancement of silicon (Si) solar cells by the heterogeneous integration of an In(x)Ga(1-x)As nanowire (NW) array on the rear surface. The NWs are grown via a catalyst-free, self-assembled method on Si(111) substrates using metalorganic chemical vapor deposition (MOCVD). Heavily p-doped In(x)Ga(1-x)As (x ≈ 0.7) NW arrays are utilized as not only back-reflectors but also low bandgap rear-point-contacts of the Si solar cells. External quantum efficiency of the hybrid In(x)Ga(1-x)As NW-Si solar cell is increased over the entire solar response wavelength range; and η is enhanced by 36% in comparison to Si solar cells processed under the same condition without the NWs.

Metamorphic GaAsP buffers for growth of wide-bandgap InGaP solar cells

compressively-strained graded GaAsxP1�x buffers on GaP showed nearly-complete strain relaxation of the top layers and no evidence of trenches but possessed threading dislocation densities that were one order of magnitude higher. We subsequently grew and fabricated wide-bandgap InyGa1�yP solar cells on our GaAsxP1�x buffers. Transmission electron microscopy measurements gave no indication of CuPt ordering. We obtained open circuit voltage as high as 1.42 V for In0.39Ga0.61P with a bandgap of 2.0 eV. Our results indicate MBE-grown InyGa1�yP is a promising material for the top junction of a future multijunction solar cell.

Metamorphic GaAsP and InGaP Solar Cells on GaAs

We have investigated wide-bandgap, metamorphic GaAs_1-<i>x</i>P<i>x</i> and In<i>y</i>Ga_1-<i>y</i>P solar cells on GaAs as potential subcell materials for future 4-6 junction devices. We identified and characterized morphological defects in tensile GaAs_1-<i>x</i>P<i>x</i> graded buffers that lead to a local reduction in carrier collection and a global increase in threading dislocation density (TDD). Through adjustments to the graded buffer structure, we minimized the formation of morphological defects and, hence, obtained TDDs ≈ 10⁶ cm^-2 for films with lattice mismatch ≤1.2%. Metamorphic In<i>y</i>Ga_1-<i>y</i> P solar cells were grown on these optimized GaAs_1-<i>x</i>P<i>x</i> graded buffers with bandgaps (<i>Eg</i>) as high as 2.07 eV and open-circuit voltages (<i>Voc</i>) as large as 1.49 V. Such high bandgap materials will be necessary to serve as the top subcell in future 4-6 junction devices. We have also shown that the relaxed GaAs_1-<i>x</i>P<i>x</i> itself could act as an efficient lower subcell in a multijunction device. GaAs_0.66P_0.34 single-junction solar cells with <i>Eg</i> = 1.83 eV were fabricated with <i>V</i>_oc = 1.28 V. Taken together, we have demonstrated that GaAs_1-<i>x</i>P<i>x</i> graded buffers are an appropriate platform for low-TDD, metamorphic GaAs_1-<i>x</i>P<i>x</i> and In<i>y</i>Ga_1-<i>y</i>P solar cells, covering a wide bandgap range.

GaAsP solar cells on GaP substrates by molecular beam epitaxy

We demonstrate molecular beam epitaxy (MBE) of GaAsxP1−x/GaP solar cells over a range of bandgap energies (Eg). Identical GaAs0.66P0.34 cells on GaAs and GaP exhibit similar properties; GaAs0.66P0.34/GaP cells with Eg = 1.82 eV produced an open-circuit voltage (Voc) of 1.24 V, ∼40 mV lower than previous GaAs0.66P0.34/GaAs cells. We then grew GaAs0.56P0.44/GaP cells with Eg = 1.92 eV to investigate their suitability for wide-Eg applications, reaching Voc = 1.27 V. For potential dual-junction integration on Si, we grew Eg = 1.71 eV GaAs0.73P0.27/GaP cells, attaining Voc = 1.15 V. These results indicate that GaAsxP1−x/GaP solar cells by MBE are promising for integration onto Si and for other photovoltaic applications.

Single-junction GaAsP solar cells grown on SiGe graded buffers on Si

We have investigated the microstructure and device characteristics of GaAs0.82P0.18 solar cells grown on Si0.20Ge0.80/Si graded buffers. Anti-phase domains (APDs) were largely self-annihilated within the In0.39Ga0.61P initiation layer although a low density of APDs was found to propagate to the surface. A combination of techniques was used to show that the GaAs0.82P0.18 cells have a threading dislocation density of 1.2 ± 0.2 × 107 cm−2. Despite these extended defects, the devices exhibited high open-circuit voltages of 1.10–1.12 V. These results indicate that cascading a GaAs0.82P0.18 top cell with a lower-bandgap Si0.20Ge0.80 cell is a promising approach for high-efficiency dual-junction devices on low-cost Si substrates.

Molecular beam epitaxy of metamorphic InyGa1−yP solar cells on mixed anion GaAsxP1−x/GaAs graded buffers

The authors have grown metamorphic InyGa1−yP on optimized GaAsxP1−x/GaAs graded buffers via solid source molecular beam epitaxy (MBE) for multijunction solar cell applications. In this work, the authors show that a previously developed kinetic growth model can be used to predict the composition of mixed anion GaAsxP1−x alloys on GaAs as a function of substrate temperature and group-V flux. The advantages of using a high growth temperature of 700 °C are then described, including the minimized dependence of composition on small temperature variations, a linear dependence of film composition on incident group-V flux ratio, and the ability to attain low threading dislocation densities of ≤106 cm−2. The authors then discuss the effect of faceted trenches, a morphological defect specific to tensile strain relaxation, on minority carrier properties, as well as strategies to eliminate them. Growth temperature effects, phase separation, and difficulties encountered in n-type doping of InAlP:Si are then described i...

Comparison of single junction AlGaInP and GaInP solar cells grown by molecular beam epitaxy

We have investigated ∼2.0 eV (AlxGa1−x)0.51In0.49P and ∼1.9 eV Ga0.51In0.49P single junction solar cells grown on both on-axis and misoriented GaAs substrates by molecular beam epitaxy (MBE). Although lattice-matched (AlxGa1−x)0.51In0.49P solar cells are highly attractive for space and concentrator photovoltaics, there have been few reports on the MBE growth of such cells. In this work, we demonstrate open circuit voltages (Voc) ranging from 1.29 to 1.30 V for Ga0.51In0.49P cells, and 1.35–1.37 V for (AlxGa1−x)0.51In0.49P cells. Growth on misoriented substrates enabled the bandgap-voltage offset (Woc = Eg/q − Voc) of Ga0.51In0.49P cells to decrease from ∼575 mV to ∼565 mV, while that of (AlxGa1−x)0.51In0.49P cells remained nearly constant at 620 mV. The constant Woc as a function of substrate offcut for (AlxGa1−x)0.51In0.49P implies greater losses from non-radiative recombination compared with the Ga0.51In0.49P devices. In addition to larger Woc values, the (AlxGa1−x)0.51In0.49P cells exhibited significan...

Metamorphic 2.1-2.2 eV InGaP solar cells on GaP substrates

We demonstrate ∼2.1–2.2 eV InyGa1−yP (y = 0.18–0.30) solar cells on GaP substrates for potential use in future high-efficiency multi-junction solar cells. Due to increased direct absorption compared to GaP, the InyGa1−yP solar cells exhibited much higher short-circuit current density than indirect gap GaP solar cells with only a slight decrease in open-circuit voltage. As such, the InyGa1−yP solar cells presented here possessed higher efficiency than comparable GaP solar cells. By taking advantage of strong direct-gap absorption, we believe that metamorphic InyGa1−yP will be an ideal top cell material for future multi-junction devices.

Effects of growth temperature and device structure on GaP solar cells grown by molecular beam epitaxy

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.