We experimentally investigate the optimum illumination wavelength range and the degradation mechanism by the ultra-violet (UV) light for small-molecule organic solar cells (OSCs) using various long-pass filters with different cutoff wavelengths lambda(c). The OSC structure consists of an indium-tin-oxide (ITO, anode)/copper phthalocyanine (CuPc, donor)/fullerene (acceptor)/bathocuproine (buffer)/Ag (cathode). The initial power conversion efficiency hp is not affected by the use of the long-pass filter with lambda(c) < 400 nm. As lambda(c) increases above 400 nm, the initial hp decreases significantly accompanied by a significant decrease in the short-circuit current, since the total amount of light energy absorbed in the OSC is decreased. The degradation of the OSC is investigated by repeating 30 s cycles, each consisting of 3 s of illumination followed by 27 s in the dark. The curve of the current J(L) vs voltage V under illumination becomes strongly S-shaped at the 100th illumination cycle for lambda(c) < 400 nm, showing degradation. In contrast, for lambda(c) > 400 nm, no distinct degradation appears. Thus, the origin of the degradation is revealed to be UV light with wavelengths less than 400 nm. The degradation mechanism is attributed to the decrease in carrier transport efficiency at the interface between ITO and CuPc film. Reorganization of the CuPc crystal structure occurs in the near-surface region on ITO due probably to the heat generated by the UV light. This would deteriorate the electrical contact at the interface, resulting in an increase in series resistance there. (C) 2016 Elsevier B.V. All rights reserved.