Identification of electronic traps in perovskite solar cells by thermally stimulated current analysis

Andreas Baumann^a, Vladimir Dyakonov^a^,^b, Stefan Väth^b, Kristofer Tvingstedt^b, Philipp Rieder^b

^aBavarian Center for Applied Energy Research, Energy Efficiency, Am Galgenberg 87, Würzburg, D-97074, Germany

^bJulius-Maximilian University of Würzburg, Experimental Physics 6, Am Hubland, Würzburg, D-97074, Germany

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Oral, Andreas Baumann, presentation 167
Publication date: 5th February 2015

Hybrid perovskite solar cells are progressing very fast showing extraordinary performance competing with inorganic thin-film technologies. Open-circuit voltages close to and even exceeding 1.0 V can be measured, which is closer to their maximum possible value defined by the Shockley-Queisser limit than many other PV technologies. [1] However, there is a lack of fundamental understanding of the photovoltaic properties and working principles of this class of solar cells. For example, an anomalous behavior like the current-voltage hysteresis is often observed in perovskite solar cells being either an interface phenomenon or related to perovskite bulk. Other interesting issues are polarization effects either related to reorientation of organic molecules or are due to trapped charges or migration of charged species. To address the possible influence of electronic traps on the devices performance and to identify the energy levels of such states we performed thermally stimulated current (TSC) measurements on solution processed methylammonium lead iodide perovskite (MAPbI₃) solar cells. [2] In this experimental technique the current is very precisely measured while heating the sample with a constant heating rate from 10 K to 300 K. The current flow is related to charge carriers being released from trap states in the semiconductor in case these states are filled optically or electrically before the heating ramp. From a peak in the TSC current a distribution of defect states can be determined and from the peak position its energetic depth. To separate between bulk and interface traps, we varied the device configuration using different transport layers in normal and inverted device geometry and also studied pure perovskite layers. We observed several peaks in the TSC measurements. TSC peak at low temperature (between 20K and 50K) is indicative for very shallow traps near the conduction or valence bands. The TSC peak at high temperature (between 190K and 210K) is assigned to deep traps in the band gap of perovskite. In case of perovskite solar cells with organic transport layers like PCBM we observed an increased number of shallow trap states which can be related to the organic layers. What is even more intriguing, we observed a very asymmetric peak at around T=162K which we assigned to the reported phase change of the perovskite crystal from orthorhombic to tetragonal crystal lattice structure.

[1] Tvingstedt, K; Malinkiewicz, O; Baumann, A.; Deibel, C; Snaith, H. J.; Dyakonov, V.; Bolink, H. J. Radiative Efficiency of Lead Iodide Based Perovskite Solar Cells. Sci. Rep. 2014, 4, 6071.

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