Energy is an essential resource for supporting everyday life and economic development. Among numerous approaches which people use to collect energy, photovoltaics stands out for two factors: it allows to obtain electricity by exploiting an abundant source of solar energy and it does it in an environmentally friendly way. In recent years, the development of organic solar cells gained a large interest as this technology offers low-cost, light-weight and flexible devices. Moreover, in contrast to inorganic semiconductors, organics offers a variety of materials with optoelectronic properties tailored in a wide range. To further increase the solar cell efficiency, it is important to study charge-carrier transport, that is strongly influenced by the presence of trap states. Organic semiconductors are particularly prone to the formation of such states due to the weak attraction between molecules. No investigation of trap states has been done for oligothiophenes so far in spite of their excellent performance in organic solar cells. In this work, the blend of the dicyanovinyl end-capped oligothiophene DCV5T-Me and C60 is studied on the presence of trap states. This material showed high efficiencies in vacuum-processed small-molecule organic solar cells with a PCE of the best single-junction cell of 8.3% and a fill factor (FF) of 65.8%. The traps are investigated by using impedance spectroscopy (IS) and thermally stimulated currents (TSC) measurements. The blend DCV5T-Me:C60 (2:1, 80°C) contains two types of electron and a set of hole trap states. A deep Gaussian distributed electron trap at 470 meV (with respect to the transport level) is observed in the blend by IS measurements. Its origin is attributed to the distortion of the natural morphology in the C60 phase due to the intermixing of donor and acceptor molecules. Moreover, a shallow Gaussian distributed electron trap at 100 meV (with respect to the transport level) is observed in neat C60 by IS measurements. Finally, a distribution of shallow trap states with depth below 200 meV (with respect to the transport level) and overall trap density of Nt > 8.7E+16 cm^−3 is indicated in the blend by TSC measurements. The majority of these defects is attributed to hole trap states in the DCV5T-Me phase. The deep electron traps at 470 meV reduce the free charge carrier density and act as recombination centers, leading to trap-assisted recombination. According to drift-diffusion simulations, these deep traps lead to the relative reduction of FF of about 10%. The hole trap states in DCV5T-Me can explain a reduced hole mobility of μh=7E−5 cm^2/(Vs), which is limiting for the solar cell performance as it is two orders of magnitude lower than the electron mobility
