NOVELTY - Preparing a photovoltaic thin film having a heterojunction by depositing a composition, including a first organic electron-donor semiconductor (C(P)) and a second organic electron-acceptor semiconductor (C(N)), onto a substrate and carrying out segregation phase, comprises preparing a mixture (M) including (C(N)) and (C(p)) and crosslinking agents with azide groups within a solvent medium adapted to phase segregation to inhibit the precipitation by crosslinking semiconductors by the reaction of crosslinking agents, and disposing the mixture on the surface of the substrate.    USE - The method is useful for preparing a photovoltaic thin film (claimed), which is useful in conjugated polymer-based organic solar cells.    ADVANTAGE - The method is capable of efficiently and economically preparing the photovoltaic thin film with good photovoltaic effect and high growth rate.    DETAILED DESCRIPTION - Preparing a photovoltaic thin film having a heterojunction by depositing a composition, including a first organic electron-donor semiconductor (C(P)) and a second organic electron-acceptor semiconductor (C(N)), onto a substrate and carrying out segregation phase, comprises preparing a mixture (M) including the organic semiconductors (C(N)) and (C(p)) and crosslinking agents with azide groups within a solvent medium adapted to phase segregation under conditions of sufficiently low temperature to inhibit the precipitation by crosslinking one and/or other organic semiconductors by the reaction of crosslinking agents, and disposing the mixture on the surface of the substrate by increasing the temperature during or after phase segregation and/or by subjecting reaction medium to long wavelength UV radiation adapted to be placed under the conditions, where crosslinking additive reacts to form covalent bond with at least a part of organic semiconductors. INDEPENDENT CLAIMS are included for:    (1) formic acid compound of formula (N3-R1-O-C=O-R2-C=O-R1-N3) (I); and    (2) (6,6)-phenyl-butyric acid compound of formulae (II) and (III) obtained by hydrolysis of (6,6)-phenyl-61C-butyric acid methyl ester (PCBM) and (6,6)-phenyl-61C-butyric acid (PCBA).    R1 = linear or branched, optionally saturated hydrocarbyl, 2-18C alkyl or 2-18C alkenyl; and    R2 = linear or branched, optionally saturated hydrocarbyl, 1-18C alkyl or 1-18C alkenyl.