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Luca Bertoluzzi1 Andrea Bowring1 Kevin Bush1 Brian O'Regan2 Michael McGehee1

1, Stanford University, Stanford, California, United States
2, Sunlight Scientific, Berkeley, California, United States

The race to commercialize hybrid perovskite solar cells (HPSCs) is currently limited by device stability, which is influenced by the structure of the perovskite compound, its interaction with the environment (moisture, oxygen, etc) and its intrinsic electrochemical stability.1 It is only recently that electrochemical ionic processes have been suspected by several groups for triggering reversible efficiency losses.2,3 However, under operational conditions the direct observation of ionic dynamics is often blurred by other processes associated with free and trapped carriers. To this extent, reverse bias in the dark is an interesting regime that does not involve thermally injected or photogenerated carriers and allows us to isolate the role of mobile ions accumulating at the contacts. In addition, by studying a solar cell in these conditions, one can simulate cell shading in a solar module. When a cell is shaded in a module, the current flowing through the cell drops and a negative voltage builds up within the shaded cell as the illuminated cells try to push current through it.Since it is challenging to incorporate bypass diodes into thin-film solar panels, reverse bias breakdown of shaded cells can be a serious reliability problem.
In this talk, we will discuss the effect of reverse bias on HPSCs with different contacts.4 We will present various electrochemical measurements which show that at relatively low values of the applied reverse bias (of the order of -1V to -4V), mobile ions accumulating at the contacts induce the formation of a few-nanometer-thick depletion layer, which triggers electron tunneling. We will then demonstrate that at prolonged reverse bias, an ionic electrochemical reaction takes place and leads to reversible performance losses. Finally, we will discuss the implications of these findings for perovskite solar cells and modules.

1Leijtens, T et al. J. Mater. Chem. A. (2017) 5, 11483- 11500
2Denf X. et al. J. Mater. Chem. C, 2016, 4, 9060-9068
3Domanski K. et al. Energy Env. Sci., 2017,10, 604-613
4Bowring A. et al. Adv. Energy Mater., 2017, just accepted.

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