Investigation of alternative dopants in MOCVD-grown SnO2:F and their effects on CdTe device performance

Fluorine-doped tin oxide (SnO
₂
:F or FTO) is the most widely used transparent conducting oxide (TCO) in CdTe solar cells due to its low cost, chemical resistance, and thermal stability. Commercial SnO
₂
:F generally has a sheet resistance between 8-15 Ω/sq and fairly high free-carrier absorption due to heavy doping and incorporation of unintentional impurities. We produce very high quality SnO
₂
:F by metal-organic chemical vapor deposition using tetramethyltin (TMT), oxygen, and bromotrifluromethane (CBrF
₃
). TMT is rarely used commercially due to its toxicity, but it is the only chlorine-free organo-tin precursor that is suitable for our reactor. These precursors consistently yield films with mobilities greater than 30 cm
²
/V-s and can produce SnO
₂
:F with mobility up to 50 cm
²
/V-s. However, this process cannot be commercialized because CBrF
₃
(along with many fluorine sources) is banned under the Kyoto Protocol due to its high global warming potential. Here, we investigate the effect of varying the fluorine dopant source on the carrier concentration and mobility in the films. We have produced high mobility films (>;25 cm
²
/V-s) using alternate fluorine precursors including F
₂
, several hydro-fluorinated-ethers, and SF
₆
. Here, we will report on the efficacy of these molecules and other dopants in order to produce high performance SnO
₂
:F with sheet resistances between 10-20 Ω/sq. We will discuss the impact of the novel F dopants on CdTe device performance.