We examine the carrier lifetime evolution of block-cast multicrystalline silicon (mc-Si) wafers under illumination (100 mW/cm2) at elevated temperature (75°C). Samples are treated with different process steps typically applied in industrial solar cell production. We observe a pronounced degradation in lifetime after rapid thermal annealing (RTA) at 900°C. However, we detect only a weak lifetime instability in mc-Si wafers which are RTA-treated at 650°C. After completion of the degradation, the lifetime is observed to recover and finally reaches carrier lifetimes comparable to the initial state. To explain the observed lifetime evolution, we suggest a defect model, where metal precipitates in the mc-Si bulk dissolve during the RTA treatment.

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We examine the light-induced carrier lifetime degradation and regeneration at elevated temperature in multicrystalline silicon (mc-Si) wafers of different thicknesses. The experimental results show that the thinner the wafer the less pronounced the degradation is and the faster the regeneration takes place. We interpret this result in the framework of a recently proposed defect model, where the lifetime regeneration is attributed to the diffusion of the recombination-active impurity to the wafer surfaces, where it is permanently trapped. Modeling the measured thickness-dependent lifetime evolutions enables us to determine the diffusion coefficient of the impurity to be in the range (5 AE 2) Â 10 À11 cm 2 s À1 at a temperature of 75 C.Comparing the diffusion coefficient extracted from our measurements with data published in the literature allows us to exclude most impurities.Despite the large uncertainties in the diffusion coefficient data reported in the literature, reasonable agreement is only obtained for nickel, cobalt, and hydrogen. One important practical implication of our study is that mc-Si wafers thinner than 120 μm do not suffer from pronounced light-induced lifetime degradation.

The root cause of “Light and Elevated Temperature Induced Degradation” (LeTID) of the carrier lifetime in multicrystalline silicon (mc‐Si) wafers is investigated by depositing hydrogen‐rich silicon nitride (SiNx:H) films of different compositions on boron‐doped mc‐Si wafers. The extent of LeTID observed in mc‐Si after rapid thermal annealing (RTA) shows a positive correlation with the amount of hydrogen introduced from the SiNx:H layers into the bulk. The concentration of in‐diffused hydrogen is quantified via measuring the resistivity change due to the formation of boron–hydrogen pairs in boron‐doped float‐zone silicon wafers processed in parallel to the mc‐Si wafers. The measurements clearly show that the in‐diffusion of hydrogen into the silicon bulk during RTA depends on both the atomic density of the SiNx:H film as well as the film thickness. Importantly, the impact of SiNx:H film properties on LeTID shows the same qualitative dependence as the hydrogen content in the silicon bulk, providing evidence that hydrogen is involved in the LeTID defect activation process.

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Atomic-layer-deposited (ALD) Al 2 O 3 films with a thickness of a few nanometers have been successfully applied in microelectronics and photovoltaics. [1-5] In particular, in silicon-based solar cells, the introduction of Al 2 O 3 surface passivation layers was a crucial step towards higher efficiencies of industrial solar cells in recent years. The metal contacts in today's industrial silicon solar cells are made by screen-printing of metal pastes in combination with a subsequent rapid thermal annealing (RTA) step at set-peak temperatures in the range between 750 and 850 C for a few seconds. [6] To preserve the excellent passivation quality of the ALD-Al 2 O 3 layers on the silicon surface during the RTA step, the Al 2 O 3 layers are capped by silicon nitride (SiN x) layers. These top layers are grown by means of plasmaenhanced chemical vapor deposition (PECVD), resulting in amorphous SiN x :H layer with very high hydrogen content (typically in the range of 10-20 at%). [7] In contrast, ALD-Al 2 O 3 layers have a hydrogen content in the range of only 1-2 at%. [3] During the RTA step, hydrogen partly diffuses from the hydrogen-rich SiN x layer [8] through the Al 2 O 3 layer to the interface and also into the crystalline silicon bulk, where it is able to passivate defects. [9-11] Interestingly, the hydrogen was also found to be able to create new recombination centers in the silicon bulk, in some cases leading to a severe degradation in solar cell efficiency during illumination. [12-16] Therefore, in photovoltaics, the control of the amount of hydrogen diffusing into the crystalline silicon bulk has turned out to be of utmost importance. There have been conjectures in the literature that Al 2 O 3 layers might severely hamper the in-diffusion of hydrogen from SiN x :H into the silicon bulk. [17,18] However, these studies did not provide any quantitative measurements on how effective Al 2 O 3 actually is as a hydrogen barrier. This letter aims at closing this gap by quantifying the amount of hydrogen diffused into the silicon bulk through Al 2 O 3 layers of different thicknesses (5À25 nm) at varying RTA peak temperatures ϑ peak. Figure 1 shows exemplary measurements for three group A samples (see Experimental Section) with SiN x :H films of different compositions, fired at a measured RTA peak temperature of (792 AE 10) C. Directly after RTA, the hydrogen is mainly present in the form of hydrogen dimers H 2 in the silicon bulk. [19] These H 2 dimers dissociate during low-temperature annealing in darkness (e.g., at 160 C) and the hydrogen atoms subsequently passivate boron dopant atoms. [20] As a consequence, the bulk resistivity ρ of the sample increases as a function of time during dark annealing at 160 C on a hotplate. We measure the resistivities in-between the periods of 160 C-dark annealing using