Silicon heterojunction solar cells on n- and p-type wafers with efficiencies above 20%
A systematic comparison of front- and rear-emitter silicon heterojunction solar cells produced on nand p-type wafers was performed, in order to investigate their potential and limitations for high efficiencies. Cells on p-type wafers suffer from reduced minority carrier lifetime in the low-carrier-injection range, mainly due to the asymmetry in interface defect capture cross sections. This leads to slightly lower fill factors than for n-type cells. However, these losses can be minimized by using high-quality passivation layers. High Vocs were obtained on both types of FZ wafers: up to 735 mV on n- and 726 mV on p-type. The best Voc measured on CZ p-type wafers was only 692 mV, whereas it reached 732 mV on CZ n-type. The highest aperture-area certified efficiencies obtained on 4 cm2 cells were 22.14% (Voc=727 mV, FF=78.4%) and 21.38% (Voc=722 mV, FF=77.1%) on n- and p-type FZ wafers, respectively, demonstrating that heterojunction schemes can perform almost as well on high-quality p-type as on ntype wafers. To our knowledge, this is the highest efficiency for a full silicon heterojunction solar cell on a p-type wafer, and the highest Voc on any p-type crystalline silicon device with reasonable FF.