How Ionic Accumulation Affects the Ideality Factor for Perovskite Solar Cells

Nicola Courtier^a

^aUniversity of Southampton, Southampton, United Kingdom

International Conference on Hybrid and Organic Photovoltaics
Proceedings of Online International Conference on Hybrid and Organic Photovoltaics (OnlineHOPV20)

Online, Spain, 2020 May 26th - 29th

Organizers: Tracey Clarke, James Durrant, Annamaria Petrozza and Trystan Watson

Poster, Nicola Courtier, 098
Publication date: 22nd May 2020

ePoster:

Unusual values of the ideality factor have been reported for perovskite solar cells [1,2,3].

A simplified expression for the current density, as a function of the applied voltage, has been systematically derived from a charge transport model, based on drift-diffusion theory, that includes ion migration in the perovskite layer [4,5]. The simplified model is verified against numerical simulations performed using the free and open-source Matlab code named IonMonger [6].

According to this model, mobile ions induce strong electric fields at the interfaces which control the recombination rate.

While the ideal case remains the same (i.e. the ideality factor equals one for direct bimolecular recombination), other values for the "ideality factor" are found to depend on an interplay between the type of recombination and the extent of ionic accumulation at the perovskite/transport layer interfaces. This interplay can explain the numerous reports of non-integer "ideality factors" in the literature.

The simplified model suggests that the "ionic ideality factor" is intrinsically voltage-dependent (even when measured under steady-state conditions) and approximately equal to:
the total potential difference across the cell (V - Vbi) divided by the potential barrier to recombination (which depends on the type of recombination and ionic accumulation at the interfaces).

In conclusion, it is crucial to consider the distribution of the electric field when trying to diagnose the performance-limiting recombination mechanism from measurements of the "ideality factor".

References:

[1] W. Tress, M. Yavari, K. Domanski, P. Yadav, B. Niesen, J. P. C. Baena, A. Hagfeldt, and M. Graetzel. Interpretation and evolution of open-circuit voltage, recombination, ideality factor and subgap defect states during reversible light-soaking and irreversible degradation of perovskite solar cells. Energy Environ. Sci. 2018, 11, 151–165.

[2] O. Almora, K. T. Cho, S. Aghazada, I. Zimmermann, G. J. Matt, C. J. Brabec, M. K. Nazeeruddin, and G. Garcia-Belmonte. Discerning recombination mechanisms and ideality factors through impedance analysis of high-efficiency perovskite solar cells. Nano Energy 2018, 48, 63–72.

[3] P. Calado, D. Burkitt, J. Yao, J. Troughton, T. M. Watson, M. J. Carnie, A. M. Telford, B. C. O’Regan, J. Nelson, and P. R. F. Barnes. Identifying dominant recombination mechanisms in perovskite solar cells by measuring the transient ideality factor. Phys. Rev. Appl. 2019, 11, 4, 044005.

[4] N. E. Courtier, J. M. Cave, J. M. Foster, A. B. Walker, and G. Richardson. How transport layer properties affect perovskite solar cell performance: insights from a coupled charge transport/ion migration model. Energy Environ. Sci. 2019, 12, 396–409.

[5] M. T. Neukom, A. Schiller, S. Züfle, E. Knapp, J. Ávila, D. Pérez-del Rey, C. Dreessen, K. Zanoni, M. Sessolo, H. J. Bolink, and B. Ruhstaller. Consistent device simulation model describing perovskite solar cells in steady-state, transient and frequency domain. ACS Appl. Mater. Interfaces 2019, 11, 26, 23320–23328.

[6] N. E. Courtier, J. M. Cave, A. B. Walker, G. Richardson, and J. M. Foster. IonMonger: A free and fast planar perovskite solar cell simulator with coupled ion vacancy and charge carrier dynamics. J. Comput. Electron. 2019, 18, 4, 1435–1449.

Acknowledgements:

NEC would like to thank Dr Giles Richardson, Prof. Juan A. Anta and Laurence Bennett for their useful comments. This work was supported by an EPSRC Doctoral Prize (ref. EP/R513325/1).

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