Jay Patel1 Qianqian Lin1 Olga Zadvorna1 Christopher Davies1 Laura Herz1 Michael Johnston1

1, University of Oxford, Oxford, , United Kingdom

Hybrid metal-halide perovskite materials show great promise for photovoltaic devices, with power conversion efficiencies (PCE) having recently exceeded 22%. Moreover, the fabrications methods such as thermal evaporation, commonly used currently in industry, can be utilised to fabricate perovskite thin films.[1-2] However, hybrid metal-halide perovskite photovoltaic devices have been affected by anomalous hysteresis, whereby the current-voltage (J-V) characteristics are dependent upon both scan rate and direction.[3] To understand the cause of the anomalous hysteresis, we use optimised co-evaporated perovskite solar cells and then characterise them with high resolution microscopy and current-voltage scans.[4] We demonstrate using transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) that under identical growth conditions the interface affects perovskite morphology and crystallinity. We further correlate this with electron diffraction patterns of the perovskite at the interface to find that the devices that are hysteretic, incorporate amorphous regions of perovskite at the interface. However, when the perovskite is grown on top of an organic layer, such as PCBM, the interface consists of only crystalline perovskite. Not only does this lead to a hysteresis free device, it also leads to the device exhibiting a stabilised power output which is comparable to the measured solar cell efficiency from the J-V characterisation. We then utilise this optimised device structure to track and characterise the temperature-dependence of key current-voltage parameters. The results show that devices exhibit respectable open circuit voltages and short circuit current densities from 295 K to 200 K. However, below 200 K the device shows over 5 orders of magnitude decrease in short circuit current density. Fourier transform photocurrent spectroscopy (FTPS) was then employed to understand and explore spectral features associated with the MAPbI3 based solar cells.

[1] J. B. Patel, R. L. Milot, A. D. Wright, L. M. Herz, M. B. Johnston, J. Phys. Chem. Lett. 2016, 7 (1), 96
[2] M. Liu, M. B. Johnston, H. J. Snaith, Nature 2013, 501 (7467), 395.
[3] H. J. Snaith, A. Abate, J. M. Ball, G. E. Eperon, T. Leijtens, N. K. Noel, S. D. Stranks, J. T.-W. Wang, K. Wojciechowski, W. Zhang, J. Phys. Chem. Lett. 2014, 5, 1511
[4] J. B. Patel, J. Wong-Leung, S. Van Reenen, N. Sakai, J. T. W. Wang, E. S. Parrott, M. Liu, H. J. Snaith, L. M. Herz, M. B. Johnston, Adv. Electron. Mater. 2017, 3, 1600470