Analysis of Cu diffusion in ZnTe-based contacts for thin-film CdS/CdTe solar cells

Abstract

Abstract. Ohmic contacts to thin-film CdS/CdTe photovoltaic devices have been formed using a two-layer contact interface of undoped ZnTe (ZnTe) and Cu-doped ZnTe (ZnTe:Cu), followed by Ni or Ti as an outer metallization. Secondary ion mass spectroscopy (SIMS) is used to study Cu diffusion within this back-contact structure, and also, to monitor Cu diffusion from the contact into the CdTe. When Ni metallization is used, the ZnTe:Cu layer becomes increasingly depleted of Cu, and Ni diffusion into the ZnTe:Cu increases as the contact deposition temperature increases from 100°C to 300°C. Cu depletion is not observed when Ni is replaced with Ti. Diffusion of Cu from the ZnTe:Cu layer into the ZnTe layer also increases with contact deposition temperature, and produces a buildup of Cu at the ZnTe/CdTe interface. High-mass resolution SIMS indicates that, although Cu levels in the CdTe remain low, Cu diffusion from the contact proceeds into the CdTe layer and toward the CdTe/CdS junction region.

Modeling Cu migration in CdTe solar cells under device-processing and long-term stability conditions

“…The device-processing simulations agree qualitatively with observations of Cu segregation in the CdS layer [4,[10][11][12], an effect attributed primarily to enhanced grain-boundary segregation in fine-grained CdS films. For the diffusivity measured in single-crystal CdTe [14], long-term stability simulations show that Cu segregation to the CdS layer equilibrates within about 15 years.…”

“…Therefore, for preliminary modeling purposes, a low solubility activation energy was chosen (0.11 eV) for the BC, such that[Cu]~~X > 10 2 1cm-3 for T > 20°C. Despite the lack of solubility and diffusivity data for ZnTe:Cu, this allows the BC layer to act as Cu reservoir in the simulation, and is consistent with SIMS measurements of sputtered ZnTe:Cu BCs [10][11][12]. The CdTe:Cu diffusivity parameters were also used for the BC, again due to the lack of data.…”

“…Cu solubility and diffusivity data are not available for ZnTe. However, SIMS measurements indicate that high levels of Cu can be incorporated in sputtered ZnTe:Cu BCs [10][11][12]. Therefore, for preliminary modeling purposes, a low solubility activation energy was chosen (0.11 eV) for the BC, such that[Cu]~~X > 10 2 1cm-3 for T > 20°C.…”

First-principles calculation of band offsets, optical bowings, and defects in CdS, CdSe, CdTe, and their alloys

“…The solid dots show the electrical transition energy levels and the shaded dots show the position of the pinning energy ⑀ pin (n) of the closed-shell defect V Cd 2Ϫ .leading to a diffusion of Cu atom from the p-CdTe layer to the n-CdS layer. This type of Cu diffusion has been observed experimentally in CdS/CdTe solar cells 57. ͑ii͒ V Cd formation energy: For neutral V Cd 0 defect the calculated defect formation energy at Cd ϭ0 is 4.10 eV for CdS and 2.30 eV for CdTe.…”

In situ oxygen incorporation and related issues in CdTe∕CdS photovoltaic devices

“…6 In the few reports published so far, secondary ion mass spectrometry ͑SIMS͒ was mostly dedicated to the investigation of back contacts and stability of CdTe/ CdS solar cells. [7][8][9][10] We recently reported a detailed SIMS study of CdTe/ CdS / TCO solar cell structures depth profiled throughout using quantitative SIMS from the back and the front side. 11,12 This study exhibited the distribution and concentration of trace elements in the solar cell structures and led to an insight into their main sources.…”