Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
Organometallic halide perovskite-based solar cells have made rapid progress in recent years. Initially, the perovskite material was mostly utilized to replace the dye in mesoscopic dye-sensitized solar cells. In the meanwhile, it has been demonstrated that perovskite absorber layers work efficiently with solid-state hole-transport layers, even when implemented into planar, scaffold-free devices. In 2014, perovskite-based cell efficiencies have surpassed 16% and open-circuit voltages (Voc) of up to 1.13 V have been reached. Such high Voc values are particularly impressive considering the optical bandgap (EG) of ~1.55 eV for CH3NH3PbI3. Quite generally, the bandgap-voltage offset (EG/q)–VOC, where q is the elementary charge, is a useful measure to assess the electronic quality of the absorber in a solar cell. The small (EG/q)–VOC offset for perovskite-based solar cells thus indicates low detrimental recombination losses, including those via deep defects.
Here, we use highly sensitive photothermal deflection spectroscopy and Fourier-transform photocurrent spectroscopy to reveal the origin of the small (EG/q)–VOCoffset of perovskite-based solar cells. For CH3NH3PbI3 layers at room temperature, we find a band edge with a particularly steep absorption tail, translating into a small Urbach energy of 15 meV, and a sub-bandgap optical absorption below the detection limit (i.e. < 1 cm-1). These results do not only confirm the excellent electronic properties of organometallic halide perovskite thin films but also suggest that the high VOCvalues obtained with perovskite-based solar cells indeed originate from a small defect density and the small structural disorder of the perovskite material. Moreover, we compare these findings with several absorber materials commonly used in solar cells and find a general correlation between the Urbach energy and the (EG/q)–VOC offset.
Finally, we show that the absence of sub-bandgap absorption and the high VOC values make organometallic halide perovskites ideally suited as high-bandgap component cells in tandem solar cells.
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