High-Efficiency Solar Cell on N-Type Silicon
Substrates
In the past few years, there has been an international
collaboration involving universities, research institutes and silicon
wafer manufacturers, aiming to solve the degradation problem associated
with cells on boron doped p-type CZ silicon substrates. Since the
reason for such degradation has been identified as the reaction
of boron with oxygen in the CZ substrates, research has been aiming
either to reduce the boron or the oxygen concentration in silicon
substrates. Substrates have been investigated such as lightly boron-doped
CZ, or gallium-, indium-, and aluminium-doped CZ, which completely
removes boron as the dopant, as well as MCZ and FZ substrates which
have little oxygen in them.
PERT (passivated emitter, rear totally-diffused) cell
structure on n-type silicon substrate.
In the past two years, the High Efficiency Group
at the Centre has been focusing on fabricating solar cells on n-type
CZ silicon substrates, also completely avoiding boron dopant. PERT
cells on CZ and FZ n-type silicon substrates, as shown above, have
demonstrated high efficiencies of 21.1% and 21.9%, respectively
[J. Zhao, A. Wang, P.P. Altermatt, M.A. Green, J.P. Rakotoniaina
and O. Breitenstein, 29th IEEE Photovoltaic Specialist Conference,
New Orleans, p. 218, (2002)].
BBr3 liquid source diffusion is used to
form the boron doped emitters. The front surfaces of these cells
are further passivated by a thermally grown 200Å thin silicon
dioxide, treated by a subsequent “alneal” (aluminium
anneal) process. The alneal process provides excellent
surface passivation; hence these n-PERT cells give high open-circuit
voltages, Voc, close to 700 mV. |
