Recombination and bandgap engineering in CdSeTe/CdTe solar cells

Abstract

Selenium compositional grading in CdTe-based thin-film solar cells substantively improves carrier lifetime and performance. However, where and how recombination lifetime improves has not been studied significantly. Here, we deposit a CdSexTe1−x/CdTe bilayer on MgZnO/SnO2/glass, which achieves a short-circuit current density greater than 28 mA/cm2 and carrier lifetimes as long as 10–20 ns. We analyze the grain structure, composition, and recombination through the thickness of the absorber using electron backscatter diffraction, Auger-electron spectroscopy, cathodoluminescence spectrum imaging, and time-resolved photoluminescence microscopy. Despite small CdSeTe grains near the pn-junction and significantly larger CdTe grains in the rest of the film, both time-resolved photoluminescence and cathodoluminescence reveal that the carrier lifetime in CdSeTe alloy regions is longer than in CdTe regions. The results indicate that Se both passivates grain boundaries and improves grain-interior carrier lifetime. However, these effects occur only where there is significant alloying, which is important for bandgap engineering.

Radiative Efficiency and Charge‐Carrier Lifetimes and Diffusion Length in Polycrystalline CdSeTe Heterostructures

“…Here, we observe that the GB contrast is significantly lower than the contrast at the intragrain defects (IGDs). In polycrystalline CdTe, it is more typical to observe a larger GB contrast . The CL results suggest that IGDs in CdSeTe contribute significantly to nonradiative losses and that GBs are relatively benign.…”

“…ERE in CdTe solar cells is only 10 −6 (10 −4 %) . CdTe absorbers alloyed with Se (CdSe x Te 1− x ) used in high‐efficiency solar cells have improved carrier lifetimes, PL, and cathodoluminescence (CL) intensity, but ERE or PLQY in CdSe x Te 1− x has not been reported. One complication for analyzing semiconductor electronic properties is surface and interface recombination.…”

“…Absorber growth was identical to that for magnesium zinc oxide/CdSeTe/CdTe thin‐film solar cells . The CdSe x Te 1− x films were deposited by thermal evaporation using 99.999% 5N+ powders (mesh size: −14/+60) from the source material with x = 0.2.…”

Review of CdTe1−xSex Thin Films in Solar Cell Applications

“…Two-photon excitation TRPL measurements performed on an interdiffused CdTe 1−x Se x /CdTe absorber (close space sublimation, CdCl 2 treated, resulting maximum Se content x ≈ 0.16) have shown lifetimes of around an order of magnitude higher in the Se-rich front layer compared to the CdTe back layer. This lifetime difference within a single absorber was linked to reduced recombination at the grain boundaries in the Se-containing region [51,53]. Se therefore, at least up to x ≈ 0.2, seems to provide both passivation of a bulk defect and of the grain boundaries, resulting in an overall increased lifetime.…”

“…In order to get a reliable measure for the bulk minority carrier lifetime, the surface has to be passivated, e.g., using Al 2 O 3 , which provides field-effect passivation [52]. Another approach is two-photon excitation (2PE) TRPL, which allows for lifetime measurements at a selected depth of the absorber [51,53]. In CdTe 1−x Se x layers (x = 0.2, deposited by close space sublimation and subjected to CdCl 2 treatment) with both surfaces passivated by sputtered Al 2 O 3 , lifetimes of up to 430 ns were measured, an order of magnitude higher than lifetimes in similarly passivated CdTe [23].…”