Tuning morphology of hybrid organic/metal sulfide solar cells

Abstract

This thesis explores the influence that morphology plays in hybrid organic/inorganic solar cells. This is studied for a range of different materials systems. A series of cadmium xanthate complexes were synthesised, for use as in-situ precursors to CdS nanoparticles in hybrid poly(3-hexylthiophene-2,5-diyl (P3HT)/CdS solar cells. The heterojunction morphology of these hybrid P3HT/CdS blends was found to be dependent on the ligand moiety of the precursor used. The formation of CdS domains was studied by time-resolved materials characterisation techniques and directly imaged using electron microscopy. A combination of transient absorption spectroscopy (TAS) and photovoltaic device performance measurements was used to show the intricate balance required between charge photogeneration and having percolated domains in order to effectively extract charges to maximize device power conversion efficiencies. An analogous method was also applied to a P3HT/Sb_2 S_3 system. Following on from the previous work, a non-toxic alternative to CdS and Sb2S3 was explored. Bismuth xanthates were thermally decomposed to form hybrid polymer/Bi_2 S_3 heterojunctions with two distinctly different morphologies. The bismuth xanthates were found to form nanorods in-situ, within the solid-state polymer matrix, as well as mesostructured arrays of Bi_2 S_3 rods that were later infiltrated with a polymer, using a two-step method. TAS was used to study the charge generation yield in both these systems and hybrid photovoltaic devices were also fabricated. Finally, through a collaboration with The Institute of Photonic Sciences (ICFO), TAS was used to study two separate organic semiconductor/Bi_2 S_3 BHJs. The first of which was a P3HT/Bi_2 S_3 nanoparticle blend solar cell. The charge generation yield in this system was investigated and then compared to a novel thiol-functionalised P3HT based block copolymer (P3HT-SH). Secondly, TAS was used to obtain a better understanding of the charge transfer at several interfaces in a vertically structured Bi_2 S_3 nanorod array that was filled with 2,2',7,7'-Tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (SPIRO).