NOVELTY - Structure comprises: an electron donor; an electron acceptor composed of a nanoscale electron acceptor material (a form of aggregation or clusters) and a mesoscale mixture of electron donor/acceptor material; and a multi-substituted fullerene derivative composed of a first fullerene structure and a multi-substituted derivative connected to the first fullerene structure.    USE - The structure is useful for: improving thermal stability of bulk heterojunction solar cells; and making a bulk heterojunction photovoltaic apparatus (all claimed).    ADVANTAGE - The structure: utilizes the multi-substituted fullerene derivative to prevent the mesoscale mixture from a large-scale segregation of acceptor material over a specific temperature for a specific period, preferably by heating the photovoltaic apparatus at a temperature higher than 110 degrees C for more than 30 minutes or lower than 100 degrees C for more than 5 hours, thus maintaining the thermal stability and the sizes of the nanoscale acceptor material and mesoscale mixture (all claimed); exhibits good power conversion efficiency and high-performance; and is cost effective and is simple for making.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) a bulk heterojunction photovoltaic apparatus (2) comprising a photoelectric conversion layer (22) for converting an incident beam into many hole-electron pairs, and the structure; and    (2) making the bulk heterojunction photovoltaic apparatus comprising: providing a solution of photoelectric material, while the solution of photoelectric material comprises the photoelectric conversion layer and the structure; coating the solution of photoelectric material on a first electrode (20) so as to form the photoelectric conversion layer; and forming a second electrode (21) on the photoelectric conversion layer.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the photovoltaic apparatus.    Photovoltaic apparatus (2)    First electrode (20)    Second electrode (21)    Photoelectric conversion layer (22)    Conductive layer (201)