We prepared copper phthalocyanine nanoparticles (CuPc-NPs) by a simple solution process, for use as a buffer layer in organic solar cells (OSCs) based on a poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3- benzothiadiazole-4,7-diyl-2,5-thiophenediyl]/[6,6]-phenyl-C71- butyric acid (PCDTBT/PC71BM) active layer. The optical absorption of such buffer layers was analyzed through UV-visible spectral studies in the wavelength range from 300 to 800 nm. The surface and structure were examined by atomic force microscopy (AFM) and X-ray diffraction (XRD) measurements, respectively. The observed XRD spectra suggested the presence of a stable β-crystallite phase of CuPc-NP films. The effect of the CuPc-NP buffer layer on the photovoltaic parameters was studied. The current density-voltage (J-V) characteristics of photovoltaic cells were measured under 100mW/cm 2 (AM 1.5G). An optimized OSC showed a 5.22% PCE, a 0.465 FF, and a 12.68mA/cm2 Jsc. The improved interface properties and good charge transport properties when using the CuPc-NP layer are believed to be the reasons behind the better device performance. © 2014 The Japan Society of Applied Physics.