Effect of substrate roughness on crystallinity and DOS structure of poly(3-hexylthiophene) thin film

In this paper we focus on structural changes of P3HT film due to effect of the substrate roughness and the influence of these changes on the density of states (DOS) function of the material. The crystalline structure was investigated by Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS) and the mapping of DOS in the band gap region was obtained by energy resolved electrochemical impedance spectroscopy (ER-EIS). The investigation of P3HT layers was supplemented with photoluminiscence (PL) measurements to obtain information about optical nature of DOS. We demonstrated the sensitivity of structural features of P3HT polymer films on the roughness of the substrate surface. Even small variance in the surface roughness can result in the formation of various crystal phases, which induces change in the DOS distribution. We found out that P3HT layer grown on ITO substrate with the surface roughness of 7 nm shows two crystalline phases. The first phase, observed also in P3HT layer on p+ c-Si, corresponds to the interchain stack distance s of 1.65 nm. The second crystalline phase with 1.58 nm separation between stacks introduces a defect-state band above the HOMO. PL measurements revealed that this band acts as a nonradiative recombination centre of excitons which can adversely affect solar cell performance. (authors)