[en] Highlights: • Chitosan was used as cathode interlayer material in inverted organic solar cell by integrating self-assembly techniques. • The self-assembly process formed chitosan films were quite flat and uniform with controlled thickness on a nanometer scale. • Due to the highly ordered layer-by-layer chitosan films, both interfacial dipoles and molecular dipoles were formed. • With the modification of chitosan eLbL films, inverted OSCs exhibited power conversion efficiency (PCE) of 10.18%. In this work, we employed chitosan, which is the second most abundant biomass on earth after cellulose, and its derivatives as cathode interlayer (instead of substrate) materials in inverted organic solar cells (OSCs) by employing electrostatic Layer-by-Layer (eLbL) self-assembly technique. It was found that the eLbL self-assembly technique was a suitable strategy to obtain continuous films with full surface coverage, uniformity and controlled thickness under nanometer scale. What's more, due to the highly ordered layer-by-layer structure, both interfacial dipoles and molecular dipoles were formed and utilized to decrease the work function of the electrode and enhance device performance. With optimized chitosan eLbL films as cathode interlayer, inverted OSCs exhibited a power conversion efficiency (PCE) of 9.34%, which was approximately a 200% improvement over cells with no cathode interlayer. Moreover, the chitosan eLbL films can be utilized to improve the PCE of thick active layer (210 nm) OSCs to 10.18%.