Impact of Metal Contamination in Silicon Solar Cells

Abstract

The impact of the transition metals iron, chromium, nickel, titanium and copper on solar‐cell performance is investigated. Each impurity is intentionally added to the silicon feedstock used to grow p‐type, directionally solidified, multicrystalline silicon ingots. A state‐of‐the‐art screen‐print solar‐cell process is applied to this material. Impurities like iron, chromium and titanium cause a reduction in the diffusion length. Nickel does not reduce the diffusion length significantly, but strongly affects the emitter recombination, reducing the solar‐cell performance significantly. Copper has the peculiarity of impacting both base‐bulk recombination as well as emitter recombination. Two models based on the Scheil distribution of impurities are derived to fit the degradation along the ingot. Solar‐cell performances are modelled as a function of base‐bulk recombination and emitter‐bulk recombination. The model fits the experimental data very well and is also successfully validated. Unexpectedly, the distribution of impurities along the ingot, due to segregation phenomena (Scheil distribution), leaves its finger‐print even at the end of the solar‐cell process. A measure of impurity impact is defined as the level of impurity that causes a degradation in cell performance of less than 2% up to 90% of the ingot height. The advantage of this impurity‐impact metric is that it comprises the different impurities’ physical characters in one single parameter, which is easy to compare.

Degradation mechanism(s) of GaAs solar cells with Cu contacts

“…Unfortunately, most of the available literature describes copper diffusion in bulk semiconductor material at high temperatures (> 500 • C). There is only limited literature available on the effects of copper in actual devices and it mostly describes the effects of deliberate copper doping or contamination [29][30][31][32] , rather than the effect of gradual diffusion over time.…”

Precipitated iron: A limit on gettering efficacy in multicrystalline silicon

“…1,2 Silicon-based photovoltaic devices are no exception, with conversion efficiencies typically decreasing as metal impurities exceed atomic concentrations of parts per billion. 3,4 Iron, in particular, limits the bulk minority carrier lifetime of most as-grown p-type silicon wafers, 5,6 in part because of its large electron capture cross section, 7,8 but also because of its inevitable presence in feedstocks, 9 crystal growth crucibles and their linings, 10 and throughout the industrial growth environment. 11 Recently, several authors have investigated the macroscopic device-level effects of iron contamination, [12][13][14] updating foundational studies of metal contamination in silicon solar cells (e.g., the Westinghouse study of Davis, Jr. et al…”

Influence of n+ and p+ doping on the lattice sites of implanted Fe in Si

“…13, many fundamental questions on the basic underlying microscopic mechanisms are still poorly understood, among them the detailed knowledge of Fe lattice sites in different types of silicon, or when forming complexes with other impurities, such as dopants, or implantation defects, such as vacancies. For instance, the P-diffusion gettering process, which is nowadays widely used in the fabrication of n-p-junction Si solar cells [13][14][15][16][17][18][19][20] is known to create on top of a low p-doped multi-crystalline Si wafer highly-doped n + -regions, in which Fe is trapped. It has been recently suggested 20 that the high concentration of vacancies created during P-diffusion is a key ingredient in the gettering process; at the same time it is expected from theoretical grounds that Fe becomes less electrically active when it is trapped by Si vacancies 21 .…”