Solution-processed small-molecule bulk heterojunction (BHJ) ambipolar organic thin-film transistors are fabricated based on a combination of [2-phenylbenzo[d,d ']thieno[3,2-b;4,5-b ']dithiophene (P-BTDT) : 2-(4-n-octylphenyl)benzo[d,(d) under bar ']thieno[3,2-b;4,5-b']dithiophene (OP-BTDT)] and C-60. Treating high electrical performance vacuum-deposited P-BTDT organic semiconductors with a newly developed solution-processed organic semiconductor material, OP-BTDT, in an optimized ratio yields a solution-processed p-channel organic semiconductor blend with carrier mobility as high as 0.65 cm(2) V-1 s(-1). An optimized blending of P-BTDT:OP-BTDT with the n-channel semiconductor, C-60, results in a BHJ ambipolar transistor with balanced carrier mobilities for holes and electrons of 0.03 and 0.02 cm(2) V-1 s(-1), respectively. Furthermore, a complementary-like inverter composed of two ambipolar thin-film transistors is demonstrated, which achieves a gain of 115.