Investigation of the morphology and electrical characteristics of hybrid blends based on poly(3-hexylthiophene) and colloidal CuInS2 nanocrystals of different shapes

Abstract

Colloidally synthesized CuInS2 nanocrystals are a promising candidate for hybrid solar cell applications due to suitable optical and transport properties of copper indium disulfide and being it an eco-friendly material. However, as opposite to solar cells where CuInS2 is synthesized in situ in a conductive polymer matrix, advances in the field of hybrid solar cells containing colloidal CuInS2 nanocrystals that are blended after synthesis with a polymer are still negligible. Here, we report about the influence of pyridine, alkylamine, and hexanethiol stabilizing ligands on the morphology of the active layer and the electrical characteristics of solar cells based on elongated and pyramidal CuInS2 nanocrystals blended with poly(3-hexylthiophene) (P3HT). All CuInS2 nanocrystals used within this study had a wurtzite crystal structure as revealed by X-ray diffraction. With pyridine as ligand, the morphology was found to depend strongly on the shape of the nanocrystals. Strong agglomeration was observed in the case of elongated nanocrystals and explains the low performance of corresponding solar cells. Employment of hexanethiol as ligand resulted in an improvement of the morphology of the CuInS2/P3HT layers and enhancement of the rectification ratio of the laboratory solar cells. Nevertheless, it was found that morphology of the active layer is not the main limiting factor in the CuInS2/P3HT system. According to cyclic voltammetry measurements, unsuitable alignment of the energy levels for CuInS2 nanocrystals and P3HT was observed. Taking this fact into account, appropriate donor materials for CuInS2 based bulk heterojunctions are discussed.