In this paper we present a study of the growth of quaterthiophene (4T), a linear conjugated oligomer of thiophene behaving as a p-type semiconductor, on n-doped GaAs and Si substrates to form hybrid heterojunction solar cells. We focus on the influence of the substrate and the different surface preparations on the morphology of the organic thin films. The 4T deposited by thermic evaporation on GaAs forms films consisting of grains of some hundred of nanometers and large lamella defects. However, in the case of Si as substrate we demonstrate almost defect-free high ordered films with grain sizes of several micrometers up to a film thickness of 250 nm. The photovoltaic characteristics of the hybrid solar cells made by the heterojunction organic–inorganic semiconductor systems are investigated.