Hole transportmolecule (spiro-OMeTAD) was dissolved in chloroben-zene (200 mg mL−1), by heating and stirring the solutionat 80◦C for 30 min. Optimized doping concentrationsof Lithium bis(trifluoromethylsulfonyl)imide salt (Li-TSFI) dissolved in acetonitrile (170 mg mL−1) and4-tert-Butylpyridine (tBp) are 1:19.3 and 1:40.8 in vol-ume ratio respectively. These optimum ratios weredetermined by fine tuning the doping concentration withrespect to that used in the literature, the ratio that yieldedthe highest conductivity for the hole transport layer waschosen. 40 L doped solution was deposited onto eachfilm at room temperature and left for 1 min before spincoating at 3500 RPM for 20 s in air. After spin coating,the films were kept for 24 h in air. 50 nm of gold followedby 50 nm of silver were deposited by thermal evapora-tion to complete the device structure. 50 nm silver wasused mainly to save the amount of Au used. The devicearea, as defined by the overlap between the ITO and Agelectrodes, was 8.6 mm2.2.2