Limitations of P gettering during shallow emitter formation
Marstein, E.S.
,
Mayandi, J.
,
Syre, M.
,
Olaisen, B.R.
,
Vines, L.
,
Holt, A.
European Photovoltaic Solar Energy Conference and Exhibition, 26, Hamburg, 2011-09-05--09-11. EU PVSEC Proceedings, 26, 1248-1251
- Publ. year
- 2011
- Publ. type
- paper
- Abstract
- The electronic quality of a Si wafer strongly affects solar cell efficiency. Although Si crystallization processes aim at producing high lifetime material directly, it is well known that the lifetime of many Si materials used in solar cell production can be further increased by the use of gettering. Phosphorous (P) gettering, a process that takes place during the emitter formation step, is a commonly used and effective way of purifying the bulk of wafers, thereby increasing lifetime. Initially, solar cells were made with relatively deep and heavily doped emitters. However, a range of new solar cell designs aiming at increased efficiency are currently being implemented on industrial scale. In several of these, a shallow emitter is a key component. In order to form a shallow emitter without introducing new etching steps, lower diffusion temperatures, shorter diffusion times or reduced surface concentrations of the P source can be used. In the present work, the effect of reducing diffusion times in order to obtain emitters with high values of sheet resistance (>100 Ω∙cm) upon lifetime is investigated. The P diffusion and gettering experiments reported here were performed both in a tube furnace using POCl3 as a dopant source, as well as in an in-line belt furnace using a set of liquid P sources. The lifetime in the Si wafers was determined by microwave photoconductance decay (μ-PCD) measurements. Lifetime changes have been correlated with impurity level measurements performed using a secondary ion mass spectrometry (SIMS) measurement setup. The results clearly show that the effectiveness of the gettering process, as determined by lifetime increases, is strongly dependent on the process parameters and that there are threshold values for time, temperature and surface concentration below which efficient gettering is no longer obtained.
2BV.1.8.pdf
—
PDF document,
592 kB (606717 bytes)
