In spite of its mechanically inflexible structure, millimeter thick soda-lime glass is generally used for fabrication of Cu2ZnSn(S,Se)(4) (CZTSSe) thin film solar cells; improvements in sintering of the chalcogenide absorber layer from intrinsic sodium doping and subsequent higher device efficiency make soda-lime glass the preferred substrate. Thin and flexible Corning (R) Willow (R) Glass provides a unique advantage in contaminant-free fabrication of flexible solar cells, as it can be processed at the elevated temperatures (c.a. 500 degrees C-550 degrees C) needed for high performance thin film solar cells. In this study, we report successful lab-scale fabrication of CZTSSe solar cells on flexible glass substrates from nanoparticle inks of Cu2ZnSnS4 (CZTS), and identify necessary process changes to bridge device performance to standard devices fabricated with soda-lime glass (SLG). Since the sodium concentration in Willow Glass is negligibly small, supply of sodium to the absorber film during selenization was explored. Soaking coated CZTS films in sodium chloride prior to selenization was found to have a detrimental effect on power conversion efficiency (PCE). Alternatively, sodium doping with a thin NaF film on top of the CZTS nanoparticles films prior to selenization resulted in slightly improved device performance. The addition of a 10 nm thick NaF layer slightly increased the average device efficiency from 6.2 +/- 0.3% to 6.4 +/- 0.4% with a record 6.9% PCE based on total cell area. The increased efficiency results from higher V-oc due to the sodium doping. While these initial results demonstrate the potential of Willow Glass as a flexible support for nanoparticle based photovoltaic devices, there is a need to continue developing improved processing methods to further enhance PCE values.