Kevin Nay Yaung

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.

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.

Towards high efficiency GaAsP solar cells on (001) GaP/Si

We demonstrate metamorphic 1.66 eV GaAs_0.77P_0.23 solar cells grown by molecular beam epitaxy on exact (001) GaP/Si templates. Cascading such a cell with a 1.1 eV Si junction enables theoretical efficiencies of > 37% under the AM1.5G spectrum. We optimized the initial GaP growth on pseudomorphic GaP/Si templates to promote maximum strain relaxation with minimal nucleation of new threading dislocations. Electron beam-induced current studies of our cells reveal a threading dislocation density (TDD) of 7.8×10⁶ cm^-2, about 18% lower than our prior results on GaP/Si. The lower TDD has contributed to a low bandgap-voltage offset (W_OC=E_G/q-V_OC) of 0.55 V, which is 40 mV lower than our previous report, and represents significant progress for GaAs_yP_1-y/GaP/Si metamorphic solar cells.

GaAsP solar cells on GaP/Si with low threading dislocation density

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%.

GaAsP solar cells on GaP/Si grown by molecular beam epitaxy

We demonstrate metamorphic 1.73 eV GaAs0.72P0.28 solar cells grown by molecular beam epitaxy on high-quality GaP/Si templates and compare them to cells co-grown on bulk GaP. Cascading such a cell with a 1.1 eV Si junction in the substrate could enable a theoretical efficiency of 37% under the AM1.5G spectrum. Electron beam-induced current studies of our cells reveal a threading dislocation density (TDD) of ~1×107 cm-2 for cells on GaP/Si, which is significantly lower than previous reports. We believe that the combination of a highly optimized GaP/Si starting substrate with a well-designed metamorphic buffer enables these relatively low TDDs. Open-circuit voltages as high as 1.10 V were obtained, leading to a bandgap-voltage offset of 0.63 V. This bandgap-voltage offset is also lower than in previous reports, in qualitative agreement with the observation of lower TDD. Direct comparison with cells on bulk GaP confirm the relation between TDD and bandgap-voltage offset, indicating that more investigations to further reduce TDD in GaAsP single-junction cells are required to fulfill the ultimate goal of dual-junction integration on Si.

Direct-gap 2.1–2.2 eV AlInP solar cells on GaInAs/GaAs metamorphic buffers

AlInP offers the highest direct bandgap (Eg) among non-nitride III-V materials, making it attractive for top cell applications in 5-6 junction solar cells. We present novel 2.07-2.19 eV, direct-gap AlInP solar cells, grown on GaInAs/GaAs graded buffers by metal-organic chemical vapor deposition. Despite the high Al content of 36-39% in the active regions, SIMS results indicate oxygen concentrations less than 2.3×1016 cm-3. The AlInP devices we present here exhibit superior photovoltaic performance to GaP and are comparable to metamorphic GaInP solar cells, reaching a Eg-voltage offset of 0.58 V. Design enhancements based on device and material characterization led to improvements of up to 65% in short circuit current density from our first-generation AlInP devices. The promising results in this work provide an alternative path towards realizing high-Eg top junctions with applications in upright metamorphic multijunction solar cells.

GaAsP/Si solar cells and tunnel junctions for III-V/Si tandem devices

The field of III-V integration onto Si for high-efficiency, low-cost tandem photovoltaics has advanced rapidly in recent years. While silicon technology is quite mature, GaAsP solar cells have exhibited relatively low efficiencies <;10%. In this work, we investigated the effect of temperature on the metamorphic growth of single-junction 1.7 eV GaAsP/GaP/Si solar cells, yielding improvements in solar cell TDD down to 5-6×106 cm-2. Our devices yield efficiencies of 11% without anti-reflection coatings. We additionally investigated the use of Si δ-doping to improve the thermal stability of tunnel junction interconnects. The combination of improved solar cell efficiency and tunnel junction stability is promising for III-V/Si tandem cell development.

2.19 eV InGaP solar cells on GaP substrates

We have grown, via molecular beam epitaxy (MBE), the first metamorphic In_0.26Ga_0.74P solar cells with a 2.19 eV direct bandgap on GaP to serve as the top cell in a multi-junction device. Calculations show that the incorporation of a 2.0-2.2 eV top cell into future 4-6 junction cells could enable efficiencies as high as 60%. GaAs_xP_1-x graded buffers enabled a moderate threading dislocation density of 6×10⁶ cm^-2 in the In_0.26Ga_0.74P solar cells. Open circuit voltages (V_oc) as high as 1.42 V were observed under approximate AM1.5G illumination. Little work has been reported on the MBE growth of such high-bandgap In_yGa_1-yP, and we believe that this V_oc can be improved through systematic optimization of growth conditions. Although these devices were not optimized for current collection, we obtain an efficiency of 3.13%, surpassing that of the best GaP solar cells. Finally, as this composition is near the direct-indirect crossover point, we analyzed the low-energy cutoff of the external quantum efficiency spectrum and infer that our In_0.26Ga_0.74P cells are still in the direct regime.

Growth of metamorphic GaAsP solar cells on GaP

In this work, we demonstrate metamorphic GaAs_xP_1-x/GaP solar cells grown by molecular beam epitaxy for potential dual-junction integration with Si. We investigate the appropriate substrate orientation and growth conditions necessary to obtain smooth surface morphology with high open-circuit voltage (V_oc). Growing nearly identical GaAs_xP_1-x/GaP (x=0.65±0.01) cells at three different substrate temperatures allowed us to investigate the dislocation dynamics in the graded buffer, revealing that we are not in the ideal glide-limited regime. We expect this is due to thread interactions with morphological defects. To satisfy the design requirements of the ideal dual-junction device, we grew 1.71 eV GaAs_0.73P_0.27/GaP cells, attaining a high V_oc of 1.15 V. With increased short-circuit current through the addition of a window layer and antireflection coating, the GaAs_xP_1-x cells presented here cascaded with Si could reach efficiencies as high as 30%.