Impact of Carrier Concentration and Carrier Lifetime on MgZnO/CdSeTe/CdTe Solar Cells

Pandey, Ramesh; Shimpi, Tushar; Munshi, Amit; Sites, James R.

doi:10.1109/jphotov.2020.3017741

Cited by 43 publications

(29 citation statements)

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“…Assuming that the defect level at the interface stays relatively constant, fewer holes at the interface result in a smaller recombination velocity and would be favorable for cell performance. [ 31 ] At a higher–than‐optimal Mg fraction, however, there are two effects. The one that follows from the progression in Figure 4b is that the conduction band offset will start to block the collection of photogenerated electrons, particularly at voltages approaching maximum power, which will primarily result in a smaller fill factor, as shown in Figure 2.…”

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CdTe‐Based Solar Cells with Variations in Mg Concentration in the MgZnO Emitter

et al. 2021

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The optimal fraction of Mg incorporation in sputter‐deposited MgXZn1−XO (MZO) emitters for thin‐film CdTe‐based solar cells is evaluated by varying it over a range of x from 0 to 0.35. This range allows a variation in the conduction band offset from −0.1 eV (cliff like) to +0.32 eV (spike like). A maximum efficiency of 18.5% for cells with the bilayer CdSeTe/CdTe absorber occurs at x = 0.15, which corresponds to a spike‐like band offset near 0.2 eV, as confirmed by X‐ray photoelectron spectroscopy. In addition, good cell performance is seen over a fairly broad range of x extending from 0.1 to 0.25. The MZO optical bandgap increases with the Mg fraction, consistent with an increasing conduction band offset. Temperature‐dependent current−voltage measurements and time‐resolved photoluminescence show improvement in the emitter/absorber interface with the incorporation of Mg. Capacitance−voltage measurements show that the depletion region extends further into the absorber with more Mg, and X‐ray diffraction confirms a change from a hexagonal‐dominant crystal structure toward zinc blende at x = 0.35.

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CdTe‐Based Solar Cells with Variations in Mg Concentration in the MgZnO Emitter

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“…This is followed by low τ 2 and τ 3 as carriers continue to recombine via easier access to the interface. This effect was predicted in a theoretical study by Pandey et al which explored the effects of insufficient emitter doping when paired with highly doped absorbers at open-circuit voltage . It is possible that interpretation of τ 1 in the current study is more complicated, however, as TRPL measurements were performed without contacting or voltage bias.…”

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“…Recent device modeling has demonstrated that, as bulk properties continue to improve, the front (i.e., emitter/absorber) − and back − interfaces become bottlenecks for performance. Because CdTe-based devices are typically grown in the superstrate configuration, the back interface is relatively accessible for manipulation and study, while the front interface is buried under microns of material and inaccessible.…”

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“…Here, MZO films could not be measured using a Hall system with a sensitivity limit of ∼10 16 cm −3 , meaning that N D < 10 16 cm −3 and may be as low as 10 10 −10 14 cm −3 . 12 For CdSeTe:Cu with N A < 10 14 cm −3 , this has little effect on performance, 13 while for CdSeTe:As with N A ≈ 10 16 cm −3 , N D may be less than or equal to N A . For N A ≈ N D , the effective carrier concentration measured via C−V becomes In this case N D may be effectively greater than N A , but the depletion width would be increased by the thickness of the depleted MZO.…”

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Revealing the Importance of Front Interface Quality in Highly Doped CdSe_xTe_1–x Solar Cells

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As carrier concentration improves in CdTe-based solar cells, modeling shows that the front interface becomes a bottleneck for performance. Optimization of the front interface has proven difficult, however, because it both is buried and evolves substantially during standard superstrate device processing. Here, we address both issues by cleaving state-ofthe-art superstrate CdSe x Te 1−x device stacks at the emitter/ absorber interface and reconstructing in the substrate configuration using a Mg y Zn 1−y O/ZnO:Al front contact. By comparing devices that were either copper-or arsenic-doped, the influence of front interface quality on low-doped and highly doped absorbers was studied, respectively. A much larger performance drop was observed for reconstructed arsenicdoped devices, which was attributed to insufficient emitter doping, formation of compensating defects at the front interface, and increased sensitivity to both effects due to a collapsed depletion region. This work highlights key challenges unique to highly doped CdSeTe:As devices that must be addressed moving forward.

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“…Gallium is believed to be a promising dopant in magnesium zinc oxide (MZO) buffer layer. [5] With such complicated multilayer doped structure, it is necessary to detect PN junction electric field distribution as results of gallium doping in MZO buffer layer. This contribution will utilize 4D-scanning transmission electron microscopy (4D-STEM) to examine the atomic electric field as well as p/n-junction electric field distribution in CdSeTe-GMZO solar cell absorber.…”

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Electric field mapping in CdSeTe solar cell using 4D-STEM

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CdTe is the most promising material in solar cell applications due to its ideal band gap (1.5eV) and high absorption coefficient. However, the CdTe-based solar cells record laboratory efficiencies are still below the theoretical efficiency limit.[1] Research has shown that incorporating selenium into CdTe absorber significantly increases the minority carrier lifetime of device absorber, which is beneficial to solar cell open circuit voltage (Voc) and overall cell performance.[2], [3] In CdTe-based solar cells, carrier concentration in p-type absorber also plays an important role on open circuit voltage and conversion efficiency. Metzger et al.[4] have demonstrated in-situ arsenic doping in CdSeTe solar cells increases carrier concentration to 1016 cm-3, which leads to conversion efficiency of 20.8%. With the increase in carrier concentration in the p-type absorber, it is necessary to increase carrier concentration in n-type buffer layer. In order to avoid interfacial recombination in front interface, with p-type concentration of 1016 cm-3 in absorber, n-type carrier concentration in front buffer layer needs to be 1017 cm-3. Gallium is believed to be a promising dopant in magnesium zinc oxide (MZO) buffer layer.[5] With such complicated multilayer doped structure, it is necessary to detect PN junction electric field distribution as results of gallium doping in MZO buffer layer.

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Impact of Carrier Concentration and Carrier Lifetime on MgZnO/CdSeTe/CdTe Solar Cells

Cited by 43 publications

References 27 publications

CdTe‐Based Solar Cells with Variations in Mg Concentration in the MgZnO Emitter

CdTe‐Based Solar Cells with Variations in Mg Concentration in the MgZnO Emitter

Revealing the Importance of Front Interface Quality in Highly Doped CdSe_xTe_1–x Solar Cells

Electric field mapping in CdSeTe solar cell using 4D-STEM

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Impact of Carrier Concentration and Carrier Lifetime on MgZnO/CdSeTe/CdTe Solar Cells

Cited by 43 publications

References 27 publications

CdTe‐Based Solar Cells with Variations in Mg Concentration in the MgZnO Emitter

CdTe‐Based Solar Cells with Variations in Mg Concentration in the MgZnO Emitter

Revealing the Importance of Front Interface Quality in Highly Doped CdSexTe1–x Solar Cells

Electric field mapping in CdSeTe solar cell using 4D-STEM

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Revealing the Importance of Front Interface Quality in Highly Doped CdSe_xTe_1–x Solar Cells