Chemical Interface Tailoring in Hybrid Solar Cells

The proposed project is devoted to fundamental research on hybrid solar cells, a solar cell technology based on conjugated polymers and inorganic semiconducting nanoparticles, containing aspects of nanotechnology and materials science. Hybrid solar cells combine the advantages of organic materials, like easy processability, and high absorption coefficients with the advantages of inorganic solar cell materials like high charge carrier mobilities and robustness. The active layers of hybrid solar cells are only a few hundred nanometres thick and can be prepared on flexible and transparent substrates. The efficiencies of hybrid solar cells increased steadily to about 4 - 5% in recent years, however, the breakthrough concerning power conversion efficiency did not take place so far. The limiting factor is most likely the complex interface between polymer and nanoparticles. Thus, fundamental research on the hybrid organic-inorganic interface is regarded to be the key to realise high efficiencies with this interesting type of solar cells. Consequently, this project aims at chemical tailoring of polymer-nanoparticle interfaces in hybrid solar cells, which are prepared via the so called in situ route, where the nanostructures are synthesised directly in a matrix of a conjugated polymer. Interface engineering in nanocomposite layers prepared via this novel approach, is a completely new and unexplored field. The influence of coordinating molecules with different anchor groups and dipole moments on surface trap states and charge separation will be thoroughly investigated. Special focus will be set on time resolved spectroscopic techniques, such as time-resolved fluorescence and microsecond to millisecond transient absorption spectroscopy for the study of energy and electron transfer processes, which are available at the Nanostructured Materials and Devices Group of the Department of Chemistry at Imperial College London, where the project will be conducted. Thereby, profound knowledge about the physical properties of the interface will be gained and the relations between the chemical tailoring and the optical and electronic properties as well as the morphology of nanocomposite layers will be studied. This fundamental knowledge about interface properties in nanocomposite layers and how they can be influenced could lead to significantly increased photovoltage and photocurrent of in situ prepared hybrid solar cells.

The focus of this research project was set on fundamental research on solution processable polymer/nanocrystal hybrid solar cells. The absorber layers of the investigated solar cells were prepared by a direct synthesis of the nanocrystals in the polymer matrix using metal xanthates as precursors. This has the advantage that these absorber layers are free from capping ligands, which are usually needed to stabilize the nanocrystals. The key factors to further increase the power conversion efficiency of this solar cell technology are mainly the morphology of the absorber layer and the complex interface between the in situ prepared nanocrystals and the conjugated polymer.Regarding the absorber layer morphology, it was found that the morphology of the hybrid films can be efficiently tuned by the design of the used metal xanthates. Xanthates with longer alkyl chains led to more finely mixed morphologies and smaller domain sizes. The different nanomorphologies strongly influence the optoelectronic properties of the absorber layers and also the performance of the hybrid solar cells and a complex interplay between these parameters is disclosed by the performed characterizations.Moreover, the charge generation yields in polymer/copper indium sulfide nanocrystal hybrid films could be significantly increased by a modification of the hybrid interface in the polymer/nanocrystal films with thiols, whereby 1,3-benzenedithiol led to the best results. Also the power conversion efficiencies of the hybrid solar cells could be improved by the treatment of the absorber layer with this organic molecule.Furthermore, alternative metal sulfides with beneficial properties for the application in hybrid solar cells were investigated and a novel solution-based fabrication route for the preparation of polymer/tin sulfide (SnS) hybrid films was developed. The charge generation properties in these films were studied by transient absorption spectroscopy and the fabricated solar cells showed efficient photocurrent generation over a broad spectral range.In addition, a part of the research work was devoted to the stability of metal halide perovskites and the corresponding solar cells, which have recently attracted the attention of many research groups due to their impressing power conversion efficiencies. Thereby, details about the degradation mechanism of these materials were disclosed. Based on that, it was also possible to propose strategies for improving the stability of this exciting material.The fundamental knowledge about the properties of hybrid materials for solar cell applications gained within this project, provides valuable tools for further research in the emerging field of polymer/nanocrystal as well as perovskite-based hybrid solar cells.