All-solution-processed flexible organic solar cells (FOSCs) with high power conversion efficiency (PCE) are the prerequisite for application in low-cost, large-area, flexible, photovoltaic devices. In this work, high-performance, top-illuminated FOSCs using ultrathin Ag-modified graphite-coated poly(ethylene terephthalate) (PET) substrates are demonstrated. The ultrathin Ag-modified graphite/PET substrates have excellent electric conductivity, mechanical flexibility, and easy processability for FOSCs. A PCE of 5.31% for FOSCs, based on the blend system poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo [1,2-b:4,5-b′]dith-iophene- co -3-fluorothieno[3,4-b]thiophene-2-carboxylate]: [6,6]-phenyl-C7l-but-yric acid methyl ester, having a bilayer of MoO x /Ag upper transparent anode is demonstrated. Top-illuminated FOSCs with a transparent upper electrode of solution-processed Ag nanowires also yielded a PCE of 3.76%. All-solution-processed FOSCs exhibit excellent mechanical flexibility and retain >81% of the initial efficiency after 500 cycles of bending test. Furthermore, graphite-based electrodes demonstrate good heat-insulation properties. The outcomes of this work offer an alternative to fabricate high-performance, all-solution-processable, top-illuminated FOSCs, providing a commercially viable approach for application in large-area solar cells that can be prepared by printing and roll-to-roll fabrication processes.