Admittance spectroscopy and DLTS measurements on multication mesoscopic perovskite solar cells
Alexander Polyakov a, Nickolay Smirnov a, Ivan Shcemerov Shchemerov a, Danila Saranin b, Anna Pozniak c, Ali Sehpar Shikoh a b, Sergey Didenko a, Denis Kuznetsov c, Antonio Agresti d, Sara Pescetelli d, Fabio Matteocci d, Aldo Di Carlo b d
a Department of Semiconductor Electronics and Device Physics, National University of Science and Technology MISiS, Leninsky Avenue, 4, Moskva, Russian Federation
b LASE–Laboratory for Advanced Solar Energy, National University of Science and Technology MISiS, Leninsky Avenue, 6, Moskva, Russian Federation
c Department of Functional Nanosystems and Hightemperature Meterials, NUST MISiS, Leninsky Avenue, Moskva, Russian Federation
d CHOSE - Centre for Hybrid and Organic Solar Energy, University of Rome ‘‘Tor Vergata’’, Via del Politecnico, 1, Roma, Italy
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Oral, Alexander Polyakov, presentation 070
Publication date: 11th February 2019

Several questions still remain open regarding the electronic structure of perovskite-based solar cells (PSCs): i) the nature, levels position and concentration of centers pinning the Fermi level in halide perovskite (PS) films in dark conditions, ii) the possible contribution to the admittance of ionic conductivity in PS layers and the role of ions movement in PSCs' degradation, iii) the type of deep traps present in various parts of PS layer and the way these deep spectra are affected by varying the PS film composition and iv) the possible role of various deep level defects in photoelectrical performance. To shed light on these issues, we studied trap levels of multication lead halide perovskites in mesoscopic PSCs by means of capacitance-voltage (C-V), capacitance-frequency (C-f), admittance spectra (AS) and deep levels transient spectroscopy (DLTS) analysis.  The investigation was performed on several PSCs with and without the use of 2D materials to tune cell’s performances. For all PSCs, with and without 2D materials, the dark conductivity in the PS layer is determined by the presence of relatively deep centers with levels near 0.2 eV below the conduction band edge (Ec) showing a strong freeze-out in AS spectra at temperatures below 200K. These centers invariably demonstrate strong concentration build-up near the interface with the electron transport layer while the bulk of the PS films is almost fully depleted for an applied bias of 0 V. The low frequency capacitance of PSCs showed a strong increase with frequency indicating considerable contribution of the ionic conductivity. Although with similar thickness, all PS layers showed a considerable spread in the apparent thickness as determined from C-V profiling under the assumption that the PS permittivity is constant. This suggests that, even at high frequencies where the capacitance shows a plateau, the actual permittivity could be different, possibly due to different impact of ionic conductivity or that the thickness of the accumulation layer near the interface varies strongly for different OSCs. DLTS spectra are always dominated by 1-2 hole traps with levels near ~Ev+0.6 eV or ~Ev+0.8 eV and 1-2 electron traps with levels near ~Ec-0.7 eV and Ec-0.9 eV [1] in reasonable agreement with theoretical results [2]. All the centers show a measurable dependence of respective DLTS peaks magnitude on the injection pulse length, which is not easily compatible with the apparent capture cross sections determined from standard DLTS analysis. This suggests that some of the traps possess a sizable barrier for capture of charge carriers.   From recent theoretical calculations [3] one would expect that only the electron and hole traps near midgap should significantly limit the lifetime of charge carriers created by light. This conclusion is partly corroborated by the results of DLTS analysis performed on the different PSCs where we observe some correlation between changes in cell efficiency and the signal from the Ec-0.9 eV electron traps. We also report on a serious changes in C-V profiles occurring after several temperature runs under applied bias during DLTS and optical DLTS (ODLTS). These changes are somewhat tentatively attributed to ion movement in PS film. In ODLTS spectra such movement results in the appearance of a strong hole-trap-like peaks with activation energies exceeding the bandgap of the PS films. We discuss possible means of quantitative assessment of the activation energies involved in ions movement and its impact on the high-frequency dielectric permittivity of PS films.

The authors gratefully acknowledge the financial support of the Ministry of Education and Science of the Russian Federation in the framework of Megagrant No. 14.Y26.31.0027.

© Fundació Scito
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