Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
Hybrid solar cells with metal-organic perovskite absorbers have gone through a remarkable improvement of their conversion efficiencies over the last years, with a hallmark of 15% reported recently1. Interestingly, devices of different designs have been shown to be functional, such as solar cells with typical mesostructured electron acceptors but also devices with a simple planar layer design. It has further been demonstrated that the morphology of the perovskite itself and the interplay between the absorber and the mesostructured electron acceptor strongly affects the electrical properties and the performance of the device², but no rigorous model of the morphology-function-relationship has been developed so far.
As a first step towards a structure-function model we present a combined study on solution processed solar cells based on mesostructured perovskites. The morphology of the solar cells was visualised by analytical transmission electron microscopy using Electron Energy Loss Spectroscopy (EELS) and Electron Spectroscopic Imaging (ESI) in order to obtain material contrast3.
Samples were prepared as cross-sections of mixed mesostructured TiO2 / perovskite layers using focused ion beam milling. After perovskite deposition the layers were annealed applying different annealing protocols and such layers then showed a significant difference in electrical performance. When analysing the plasmon and core loss region of EEL Spectra of the mixed mesostructured TiO2 / perovskite layers we can identify signals of the typical material components (e.g. TiO2 or Pb2+ ions of the perovskite) and also their spatial distribution within these layers. It should be noted that at present all analysis is done on 2D projection data. However, the cross-sections are relatively thin (below 100nm) and therefore a simple projection gives already significant information about a possible 3D material network.
Our analysis revealed TiO2 and perovskite rich areas, which show significant differences in pore size, filling and materials distribution of the mesostructured layer depending on the annealing ramp applied. The device with a less homogenously distributed mesostructure exhibits a decrease in fill factor and current density, as indicated by correlation with its I-V characteristics.
1 M. Liu et al., Nature 501, 397 (2013) 2 M. M. Lee et al., Science 338, 643 (2012) 3 M. Pfannmöller et al., Nano Lett. 2011, 11, 3099–3107
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