Try doping with zinc.  An unusual enhancement in electrical conductivity was observed for SnO2 films upon zinc doping. Films with Zn/(Zn + Sn) reaching 0.48 were grown by pulsed spray-evaporation chemical vapor deposition.

(http://pubs.rsc.org/en/Content/ArticleLanding/2011/CP/c0cp00816h#!divAbstract)

X-Ray diffraction (XRD) shows that pure and zinc-doped SnO2 films grow in the tetragonal rutile-type structure. Within the low doping concentration range, Zn leads to a significant decrease of the crystallite size and electrical resistivity. Increasing Zn doping concentration above Zn/(Zn + Sn) = 0.12 leads to an XRD-amorphous film with electrical resistivity below 0.015 Ω cm at room temperature. Optical measurements show transparencies above 80% in the visible spectral range for all films, and doping was shown to be efficient for the band gap tuning.

Mirza Arshad Ali Beg

Former Director General PCSIR

Research & Development Consultants

The conductivity of tin oxide can be changed by changing the stoichiometry of SnO2, that is either by increasing the oxygen vacancies or by tin interstitial .

Incorporating, tin interstitial is not the first option as tin ionic radius is very large. There by, effective way is incorporating oxygen vacancies is easier.

Furthermore, the conductivity can be increased by doping Sb at Sn site or F at O site. This will drastically, of 3-4 orders of magnitude, increase the conductivity.

You can use commercial product Solaronix (SnO2:F) with high electrical conductivity. Link: https://shop.solaronix.com/tco10-10.html