In this work, polyaniline (PANI) protonated with various levels of camphor sulfonic acid (HCSA) has been used as a hole-transport layer (HTL) in organic bulk-heterojunction solar cells. Polyaniline with three different protonation levels was inserted between poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) and the indium-tin oxide (ITO) glass transparent electrode to explore the effects of varying the protonation level to optimize the hole-transport properties. The three protonation concentrations (in molar ratios) of PANI are zero-protonated (PANI/HCSA, 1:0), half-protonated (PANI/HCSA, 1:1), and fully protonated (PANI/HCSA, 1:2) thin films. Current–voltage measurements under AM 1.5 conditions revealed that a conversion efficiency of 1.3 % was achieved if half-protonated PANI was used as the HTL. Several analytical methods were utilized for characterizing PANI to understand the effects of the protonation level on the electrical, optoelectronic, and structural characteristics, and their correlation with final device properties.