The p-type solar cell is the solar cell structure everyone knows. It held the biggest size of the market for the last four decades.
The term p-type refers to the fact that the cell is built on a positively charged (hence p-type) silicon base. Indeed, the wafer is doped with boron, which has one electron less than silicium. The top of the wafer is then negatively doped (n-type) with phosphorous, which has one electron more than silicium. This helps form the p-n junction that will enable the flow of electricity in the cell.
n-type solar cells are built the other way around, with the n-type doped side serving as the basis of the solar cell. The first solar cell produced by the Bell Laboratories in 1954 was a n-type, back contact, solar cell. The very first years saw a rapid and important increase of efficiency. Progressively, the p-type structure took the lead, historically because in the early days of its development, the solar technology was mainly used for space applications and it turned out that p-type structure had better resistance to radiations for space applications. Then the industry developed and structured and could enjoy the benefits of economies of scale.
So what are the main differences between the two?
– Efficiency: most powerful solar cells today available on the market are n-type solar cells. The main reason behind their superior efficiency is the higher carrier lifetime. The technology is not prone to what is called boron-oxygen defect. Indeed, when ingots are grown, they tend to have high concentration of dissolved oxygen coming from the quartz of the crucible where was melted the silicon. In presence of boron doped silicon, this oxygen forms a recombination area, known as boron-oxygen defect, that damages efficiency. When using n-type solar cells, doped with phosphorous, this effect disappears. Also, n-type solar cells are less prone to metallic impurities of the silicon.
– Light Induced Degradation effect: n-type solar cells are immune to LID, again because of the absence of the boron-oxygen defect.
– Cost: in general, except minor differences, the process to manufacture ingots does not differ much between p-type and n-type solar cells. Then the scale effect is in favor of p-type solar cells. Also, the process of fabrication afterwards includes more steps and makes a module more expensive to build with n-type solar cells.