The active layer analysis of interpenetrating heterojunction organic thin-film solar cells by X-ray photoelectron spectroscopy (XPS) has been studied. The interpenetrating heterojunction structure based on poly(3-hexylthiophene) (P3HT) and fullerene (C-60) could be fabricated by spin-coating P3HT onto a deposited C-60 film. The composition ratios of P3HT and C-60 in the interpenetrating heterojunction active layer in the depth direction and the interface morphology between the P3HT and C-60 layers have been clarified by XPS with argon ion etching and scanning electron microscopy. It has been found that the donor-acceptor interface of the interpenetrating heterojunction active layer is formed by the rod-shaped C-60 particles with a width of approximately 100 nm and C-60 grains with a size of 10-50 nm. (C) 2013 Elsevier B.V. All rights reserved.