Perovskite Solar cells - the Role of Ions, Density of States, and Device Structure
Nir Tessler a, Yana Vaynzof b
a Technion – Israel Institute of Technology
b Heidelberg University, Germany
nanoGe Perovskite Conferences
Proceedings of nanoGe International Conference on Perovskite Solar Cells, Photonics and Optoelectronics (NIPHO19)
International Conference on Perovskite Thin Film Photovoltaics
Jerusalem, Israel, 2019 February 24th - 27th
Organizers: Lioz Etgar and Kai Zhu
Oral, Nir Tessler, presentation 020
Publication date: 21st November 2018

Preventing hysteresis and enhancing stability remain key challenges that could be resolved with the aid of judicious device design. We report numerical study of a solar cell model system that is based on a mixed electron-ion conducting perovskite active layer having various device configurations. In the full picture we allow for both mobile ions and the polarizability due to the easy-rotational methylammonium (MA). We then compare with cells where the MA rotation is frozen and/or the ions are non-existing.

We’ll first show that theses simulations can reproduce experimental results of a set of solar cells that was designed to enhance the open circuit voltage through enhancement of the built in potential. Next, several insights, resulting from such detailed simulations, will be presented.

For example: Even when there is no indication of hysteresis and the device’s characteristics can be modelled using ionic free model, the actual electron and hole distributions may be vastly different to the predictions by ionic free model. The low effective DOS promotes higher Voc but makes it more difficult to overcome energy level mismatch. These are related to the fact that the ionic motion is not only causing the hysteresis, it also allows for large deviations between electron and hole densities. Also, when a large energy mismatch exists between the BL and the perovskite the charge density distribution self-adapt to create an effective dipole at the interface. Such self-induced dipole can compensate for 0.4eV mismatch and thus prevent any loss in Voc. In this context formamidinium is preferred to MA.

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