Comparison of large area high ohmic emitter silicon solar cells with standard screen-printed contacts

Significant improvement in screen-printing technology and the advancement in commercial Ag pastes has attracted considerable attention of PV community due to the possibility to contact lowly doped emitters to boost cell efficiencies without need of complex processing steps and device structures (eg. selective emitter solar cell). In this paper, our focus is on high ohmic emitters (R
sheet
∼ 100 Ω/sq.) formed using different approaches. These emitters possess either high doping level (N
s
∼ 2.5 × 10
²⁰
cm
⁻³
) and are shallow in depth or a very low doping level (N
s
∼ 1.6 × 10
¹⁹
cm
⁻³
) and are deeper/Gaussian in nature. Thermal oxidation processes have been used to optimise such emitter profiles. Hence emitter formation and passivation was accomplished in a single step. By changing the oxidation temperature surface concentration significantly decreases while the emitter depth increases. The solar cells of this investigation were fabricated on large area p-type Cz-Si wafers and have been screen-printed with commercially available Ag paste at front and full area Al-BSF at rear. Despite the low doping levels, all solar cells found to have a very good ohmic contact (FF > 78%). The highest cell efficiency for this simple and low cost approach, featuring low temperature oxide (LTO) and SiN
x
stack at front is 18,33% with a V
oc
of 640 mV, J
sc
of 37,03 mA/cm
²
and a fill factor of 79,14%.