Simultaneous P and B diffusion, in-situ surface passivation, impurity filtering and gettering for high-efficiency silicon solar cells

Abstract

A technique is presented to simultaneously diffuse boron and phosphorus in silicon, and grow an insitu passivating oxide in a single furnace step. It is shown that limited solid doping sources made from P and B Spin-On Dopant (SOD) films can produce optimal n+ and p' profiles simultaneously without the deleterious effects of cross doping. A high quality passivating oxide is grown in-situ beneath the thin (-60 A) diffusion lass, resulting in low J, values below 100 fNcm for transparent (-100 W o ) phosphorus and boron diffusions. For the first time it is shown that impurities present in the boron SOD film can be effectively filtered out by employing separate source wafers, resulting in bulk lifetimes in excess of 1 ms for the sample wafers. The degree of lifetime degradation in the sources is related to the gettering efficiency of boron in silicon. This novel simultaneous diffusion, in-situ oxidation, impurity filtering and gettering technique was successfully used to produce 20.3% Fz, and 19.1% Cz solar cells, in one furnace step. 9

Silicon PV devices based on a single step for doping, anti-reflection and surface passivation

“…The only disadvantage of this method was the long oxidation time (200 min) required for achieving quarter wave thickness. Similar work was also reported, where diffusion was obtained by doping the wafer that was kept in close proximity to a solid state dopant source [9].…”

“…We believe that the mildly oxidizing atmosphere in which the doping is carried out aides the growth of a thin interfacial oxide, leading to passivation of surface dangling bonds or the oxide in the SOD film passivates the surface. Similar passivation schemes have also been shown to reduce emitter saturation current density, which is a direct indicator of passivation [9,10]. It can also be seen that open circuit voltage for the chemically textured Fz sample is reduced by about 20 mV as compared to the non-textured case.…”

University Crystalline Silicon Photovoltaics Research and Development

“…An appropriate spin-on SiO 2 / PECVD SiN x stack was found to be effective in reducing or controlling the inversion of the rear surface, which is important for reduced parasitic shunting in the LBSF structures. The use of spin-on phosphorous for limited source diffusion has also been demonstrated at Georgia Tech [21]. Spin-on limited source diffusion provided high quality emitters, while providing flexibility with profile engineering.…”

Recent Developments in High-Efficiency PV Cells

“…The process has led to a new record value of 22% efficiency for CZ-Si. A novel approach involving simultaneous diffusion of B and P in Si from a spin on solid source and growth of an in situ passivating oxide in a single-step process has been successfully used to achieve 20.1% in FZ-grown Si (Krygowski et al, 1997).…”