The oxidative doping performed by B (B, BBr3/BCl3 or B2H6 or BN), Al (Al, Al2O3) and P (POCl3) componds is used to to achieve longer lifetimes,uniformity of the sheets and increased % efficiency of solar cells. Boron componds are best used because of its very high solubility into Si (B= 10^20 cm^-3 at 1050C; Al= 10^18cm^-3 at 1200C), self cleansing effect, better gettering effect and passivation of the oxide layer.
The process involving both diffusion and in situ oxidation (dry and wet) of boron can be best explained by introducing the vapours of the above named compounds like BBr3/BCl3 (though having some limitations) into a diffusion furnace where first N2/Ar is bubbled under pressure to create an inert atmosphere. Then O2 is reacted at 850-1150C (dry oxidation) to form:
4BX3+3O2--2B2O3+6X2(X=Br/Cl).
2B2O3+3Si---- 3SiO2+4B.
In wet thermal oxidation (900-1000C), O2 is led through a bubbler vessel filled with heated water (95 C), so that in addition to O2, water is present as steam.
Si + 2H2O→SiO2 + 2H2
It leads to fast growth but is of inferior quality than dry oxidation.
The B diffuses into Si.Excess B forms an alloy called BRL(Boron Rich Layer) with formula SiBx(x=4-6) while B2O3 and SiO2 form borosilicate glass (BSG) and .The excess BSG can be removed by HF while the alloy lying under BSG remains unaffected but is oxidized by O2. X2 reacts with the excess metal and thus can cause ‘self cleaning and accounts for the maximum use of boron dopant source in fabricating highly efficient PV devices (equal to or more than 20% efficiency; even producing most efficient PERL (Passivated Emitter Rear Cell) single junction Si solar cell till date).
For the oxidative doping of conductive polymer is given in detail in the paper at the link: http://rsta.royalsocietypublishing.org/content/314/1528/3.
The conductivity of certain organic polymers can be raised to metallic levels by chemical or electrochemical ߢp-doping’ (oxidation), or ‘n-doping’ (reduction).
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