Ethyl cellulose and functionalized carbon nanotubes as a co-gelator for high-performance quasi-solid state dye-sensitized solar cells

Long term device stability and high power conversion efficiency (PCE) are important for practical application of dye-sensitized solar cells (DSCs). Here, we report the use of ethyl cellulose (EC) and acid functionalized multi-walled carbon nanotubes (oMWCNTs) as a co-gelator to gel organic solvents and the application of these gels for quasi-solid state DSCs. The gels are formed by blending EC and oMWCNTs with methoxypropionitrile (MPN) and acetonitrile (ACN). The loadings of EC and oMWCNTs can be as low as 4 wt% and 1.5 wt%, respectively, for the gel formation. The total minimum gelator loading of EC and oMWCNTs is lower than that with EC alone. In the absence of oMWCNTs, the EC loading must be more than 12 wt% for the gel formation. Gel electrolytes were prepared by adding iodine, 1-methyl-3-propylimidazolium iodide (PMII), guanidinium thiocyanate and 4-tert-butyl pyridine into the EC-oMWCNT/ACN-MPN gels, and they were used to fabricate quasi-solid state DSCs. The optimal PCE of the gel DSCs was 6.97% under AM1.5G illumination, quite close to that of control liquid DSCs, while the gel DSCs have better long term stability than their liquid-state counterparts. The former retains more than 98% of the initial PCE after 30 days, whereas the PCE of the latter decreases to about 80%.