Copper-induced recrystallization and interdiffusion of CdTe/ZnTe thin films

Abstract

ZnTe is commonly employed as a buffer layer between CdTe and the metallization layer at the back contact of state-of-the-art CdTe solar cells. Here, the critical role of Cu in catalyzing recrystallization and interdiffusion between CdTe and ZnTe layers during back contact activation is presented. Several CdTe/ZnTe:Cu thin-film samples were prepared with varying levels of copper loading and annealed as a function of temperature and time. The samples were characterized by x-ray diffractometry, scanning electron microscopy, transmission electron microscopy, and energy dispersive x-ray spectroscopy. The results show that stress is present in the as-deposited bilayers and that negligible interdiffusion occurs in the absence of Cu. The presence of Cu facilitates rapid interdiffusion, predominantly via Cd migration into the ZnTe phase. Zn migration into CdTe is limited to areas around defects and grain boundaries. Ternary CdxZn1-xTe interlayers are formed, and the extent of alloy formation ranges from 0.08 < x < 0.5 throughout the whole ZnTe layer. The level of Cu loading controls the composition of the CuxTe clusters observed, while their size and migration is a function of annealing conditions.

Distribution of Copper States, Phases, and Defects across the Depth of a Cu-Doped CdTe Solar Cell

“… a The phases are chosen based on the plausible reactions between Cu atoms and the elements present in each layer of the device. The details of their crystal structures can be found in the Supporting Information. b Cu x Te phases are known to exist in Cu-doped CdTe. ,,− ,, c The existence of Cu x O in CdTe was reported by Wu et al, Liu et al , d The charge states of these defects are neutral unless stated otherwise. e Point defects in CdTe bulk were chosen based on previous reports of Yang et al, Krasikov and Sankin. , f The models of Cu-segregated grain boundaries were chosen as their formation is energetically favorable as discussed in Yang et al, Tong and McKenna. , g Cu point defects in CdS from previous reports Roehl et al, Nishidate et al , …”

“…59 These findings are in agreement with previous reports showing the formation of Cu x Te at the ZnTe|CdTe interface. 1,11,13,15,52 Among the Cu−Te phases, the Cu 1.4 Te is the most beneficial for the device performance because of its stability and electrical properties. 1,15,60 Unlike the Cu−Te phases with a high Cu/Te ratio (e.g., Cu 2 Te), it has lower probability to decompose to metallic Cu and lower-stoichiometric Cu x Te, creating vacancies which can reduce the mobility of the free carriers in the material.…”

“…These findings are in agreement with previous reports showing the formation of Cu x Te at the ZnTe|CdTe interface. ,,,, Among the Cu–Te phases, the Cu 1.4 Te is the most beneficial for the device performance because of its stability and electrical properties. ,, Unlike the Cu–Te phases with a high Cu/Te ratio (e.g., Cu 2 Te), it has lower probability to decompose to metallic Cu and lower-stoichiometric Cu x Te, creating vacancies which can reduce the mobility of the free carriers in the material . However, with Cu/Te ratio >1.25, both Cu 1.4 x Te structures can cause metallic Cu precipitates as predicted by Da Silva et al, which explains why even when the more stable Cu–Te phases were formed at the back contact, the Cu segregation at the front contact (around the CdTe|CdS) is still observed (Figure c).…”

“…The good match above the rising edge for all structures validates the use of simulated XANES spectra for the chemical structures whose experimental reference spectra are unavailable. The formation of Cu x Te at the ZnTe|CdTe interface has been previously reported ,,− , as well as the presence of Cu x O phases inside the CdTe absorber. , Moreover, in order to analyze the spectra measured from the CdS layer, we also considered the Cu atoms interacting with the Cd atoms as well as the Sn atoms from the nearby SnO 2 layer. Their corresponding simulated XANES spectra are shown in Figure b.…”

“…Copper has been utilized as a p-type dopant for decades to enhance the V oc of PV devices. , In fact, it has been shown that CdTe absorbers with insufficient Cu concentration perform poorly (efficiency less than 1%) compared to a CdTe absorber with an optimized amount of Cu (efficiency greater than 13%) . Cu is typically deposited as a part of the back contact as it not only serves as a dopant but also helps to create a low-resistance and stable Ohmic contact, thereby increasing the conversion efficiency. ,, For the particular case of a ZnTe back contact, Cu can dope ZnTe and react with excess Te atoms, forming a series of Cu x Te phases (predominantly Cu 2 Te, Cu 1.4 Te, and CuTe), where the stoichiometry of Cu is dictated by the Cu/Te ratio and temperature. − It has also been shown that the control of the Cu x Te stoichiometry is critical to achieve high-efficiency devices. ,, …”