Explanation of red spectral shifts at CdTe grain boundaries

Abstract

The best research-cell efficiencies for CdTe thin-film solar cells have recently increased from 17.3% to 20.4%. Despite these impressive recent gains, many improvements in device technology are necessary to reach the detailed-balance efficiency limit for CdTe-based (singlejunction, non-concentrator) solar cells of ~32%. Improvements will increasingly rely on knowledge of the fundamental relationships between processing, electrical properties of defects, and device performance. In this study, scanning electron microscope (SEM)-based cathodoluminescence (CL) spectrum imaging was used to examine these fundamental relationships. In CL spectrum imaging we collect a spectrum per pixel in a 256 × 256 pixel SEM image by synchronizing a cryogenic silicon charge-coupled device with the electron-beam positioning. High spatial resolution photon energy maps obtained with this technique can reveal intricate luminescence phenomena that are not apparent in spectroscopic data.

Enhanced electrical properties at boundaries including twin boundaries of polycrystalline CdTe thin-film solar cells

“…15,16 Additionally, some researchers have found that GBs exhibit higher recombination rates (S x B 10 4 cm s À1 ) than those of the grain interiors. 17 Contrastingly, other researchers have found that after Cl treatment (a key step to improve the CdTe efficiency), 18 the GBs no longer act as recombination centers, but actively contribute to carrier collection. 8,9,[19][20][21][22] The increased carrier collection at the GBs is explained by the presence of a local space charge region with a width of about 100-350 nm, which is beneficial to electron-hole pair separation.…”

Stability and electronic structure of the low-Σgrain boundaries in CdTe: a density functional study

“…They can exist in various forms and play important roles in determining material properties, and thus have been one of the most active research topics in materials science [1][2][3][4][5][6]. For example, segregation of impurities to the GBs is an effective way to purify materials, thus improving material quality for device applications [7][8][9][10][11][12]. GBs also increase phonon scattering, allowing the thermoelectric properties of a material to be improved by intentionally controlling its GB size [13,14].…”

Imaging CdCl2 defect passivation and formation in polycrystalline CdTe films by cathodoluminescence

“…Figure 3 (a) shows a detailed region of a CL map and a CL profile around a GB Figure 3 (b). It can be fit with a simple one-dimensional diffusion model [43][44][45][46] describing the decay of the luminescence intensity from the grain interior (x GI > 0) towards the grain boundary (x = 0).…”