Abstract
Bulk heterojunction solar cells based on conjugated polymer donors and fullerene-derivative acceptors have received much attention in the last decade. Alternative acceptors like organic non-fullerene acceptors or inorganic nanocrystals have been investigated to a lesser extent; however, they also show great potential. In this study, one focus is set on the investigation of the in situ growth of copper indium sulfide nanocrystals in a conjugated polymer matrix. This preparation method allows the fabrication of a hybrid active layer without long-chain ligands, which could hinder charge separation and transport. In contrast, surfactants for the passivation of the nanocrystal surface are missing. To tackle this problem, we modified the absorber layer with 1,3-benzenedithiol and investigated the influence on charge transfer and solar cell performance. Using ToF-SIMS measurements, we could show that 1,3-benzenedithiol is successfully incorporated and homogeneously distributed in the absorber layer, which significantly increases the power conversion efficiency of the corresponding solar cells. This can be correlated to an improved charge transfer between the nanocrystals and the conjugated polymer as revealed by transient absorption spectroscopy as well as prolonged carrier lifetimes as disclosed by transient photovoltage measurements.
Highlights
We initially focused on the investigation of the nanocomposite formation by conversion of the metal xanthates to the CuInS2 nanocrystals in the PCDTBT matrix
We could show for the first time that a post-synthesis modification of in situ and ligand-free prepared polymer/CuInS2 nanocrystal hybrid layers with 1,3-benzenedithiol leads to enhanced photovoltaic properties of these nanocomposite films most likely due to the saturation of non-passivated surface traps of the CuInS2 nanocrystals
The presence of 1,3-benzenedithiol in the polymer/CuInS2 absorber layers, which was confirmed by ToFSIMS measurements, leads to an increase in power conversion efficiency of the corresponding solar cells from 1.9 to 2.5%
Summary
Processing inorganic and organic materials in one step from the same solvent can be problematic due to the inherently different chemical and physical properties of both components This problem is, in the classical approach, tackled by the use of organic capping ligands for the nanocrystals, which facilitate dissolving both components at sufficiently high concentrations to obtain solutions for the coating of the bulk heterojunction layers. For polymer/nanocrystal layers prepared via the classical approach using capping ligands, several molecules, such as amines or thiols, have already been introduced into a solid state ligand exchange process to manage surface traps within the absorber layers or to improve the electronic coupling between the polymer and the nanoparticle phase.[30,31,32] Encouraged by these studies, we investigated how the modification of in situ prepared polymer/CuInS2 nanocrystal absorber layers with 1,3-benzenedithiol influences charge separation, photovoltaic performance and charge recombination dynamics
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