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 first mixture (M1) including (C(N)) and (C(p)) within a solvent medium, adding a crosslinking additive with azide groups to (M1) to inhibit the precipitation by crosslinking semiconductors to form the mixture (M), and disposing the mixture (M) 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 first mixture (M1) including (C(N)) and (C(p)) within a solvent medium, adding a crosslinking additive having at least two azide groups and optionally solvent to (M1) under conditions of sufficiently low temperature to inhibit the precipitation by crosslinking one and/or other organic semiconductors with crosslinking additive to form the mixture (M) including (C(N)) and (C(p)) and a solvent adapted to phase segregation, and disposing the mixture (M) 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 react to form covalent bond with at least a part of organic semiconductors. INDEPENDENT CLAIMS are included for:    (1) the photovoltaic thin film produced by the method; and    (2) formic acid compound of formula (N3-R1-O-C=O-R2-C=O-R1-N3) (I).    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.