Solar Cell Efficiency: N-type v. P-type
Cover image courtesy of Sweetwater Energy Services, Sugar Land, TX.
The first solar cell fabricated by Bell Laboratories in 1954 was made of an n-type silicon wafer. Despite this head start, much of the research, development and technological advances in solar photovoltaics (PV) were focused around p-type silicon (Si) wafers.
Why the dominance of p-type Si?
In the early days of solar PV production, much of the demand came from space agencies for satellites and manned space exploration. It turns out p-type Si is far more resistant to the degradation from cosmic array. This demand set the tone of the industry and p-type Si solar cells came to dominate the residential and commercial solar markets globally. Recently, however, n-type cells have begun to accumulate market share due to their efficiency and manufacturing benefits.
The difference between p-type and n-type crystalline solar cells
The raw material that precedes the the pulling and cutting of silicon wafers is the same for both p and n-type cells. This raw silicon feedstock is “grown” into ingots (Czochralski process) or cast as bricks and then thinly sliced. These wafers form the basis of a solar cell. It is at this point that p and n-type cells diverge. To create a semiconductor junction that will induce current flow the wafers are doped (coated) with either boron (p-type) or phosphorus (n-type).
The advantages of n-type cells
Monocrystalline p-type solar modules use cells/wafers that are Czochralski-grown (and block cast p-type polycrystalline cells/wafers to a lesser extent) suffer from light induced degradation (LID). LID occurs when oxygen impurities in the silicon wafer react with the doped boron in the first few hours/weeks of illumination of the cell. This effect can reduce cell efficiencies from 2-4% right off the bat. In comparison, n-type cells that rely on phosphorus doping do not see similar drops in efficiency and power output.
Typically, n-type wafers are less sensitive to impurities in the raw silicon. This means producers of n-type cells can rely on using lower quality wafers and still maintain high efficiencies without the impact of LID. Although high efficiency n-type modules cannot currently compete on a cost basis with standard efficiency polycrystalline p-type modules, n-type modules such as the Panasonic HIT 325W are becoming competitive with high efficiency p-type monos that require high grade silicon to produce.
Panasonic n-type cells are composed of monocrystalline and amorphous silicon layers. Amorphous silicon layers in the cells prevent recombinations of electrons, minimizing power loss.
Why consider using module with n-type cells
In short, higher efficiency = lower levelized cost of energy (LCOE) in $/kWh. In the end, higher efficiency modules mean more power per square foot. More power means less modules. Less modules means less space, lower BOS cost and less labor. These reduced costs drive the overall price of the solar electricity generated down. As the price of solar per kWh drops, PV will continue to increase its share of the US energy mix.