NOVELTY - A photovoltaic cell (100) comprises first and second electrodes (120, 160); a photoactive layer (140) between the electrodes, and containing an organic electron donor material and an organic electron acceptor material; and an optically diffractive layer (105). The first electrode is between the optically diffractive layer and the photoactive layer.    USE - As article useful as a photovoltaic cell and tandem photovoltaic cell used to transfer energy in the form of light into energy in the form of electricity.    ADVANTAGE - The optically diffractive layer can enhance the light path in a photovoltaic cell, thereby increasing the light absorption and efficiency of the photovoltaic cell. The different regions in the optically diffractive layer can have different periods.    DETAILED DESCRIPTION - An article configured as photovoltaic cell (100) or tandem photovoltaic cell, comprises first and second electrodes (120, 160); a photoactive layer (140) between the electrodes, and containing an organic electron donor material and an organic electron acceptor material; and an optically diffractive layer (105). The first electrode is between the optically diffractive layer and the photoactive layer. The optically diffractive layer comprises several features (101) including one-dimensional periodic features, two-dimensional periodic features, or three-dimensional periodic features. The features are periodically arranged along at least one direction. The features have a period of at most 2 mu m and at least 50 nm along the at least one direction; and an average height of 200-2000 (preferably 10-250) nm. The optically diffractive layer comprises a polymer (preferably ultraviolet (UV)-curable polymer, a filler, and a moth-eye layer. The filler comprises a dye or a metal complex.    DESCRIPTION OF DRAWING(S) - The figures show cross-sectional views of the photovoltaic cell.    Photovoltaic cell (100)    Features (101)    Diffractive layer (105)    First electrodes (120)    Photoactive layer (140)    Second electrode (160)