Proceedings of nanoGe September Meeting 2017 (NFM17)
Publication date: 20th June 2016
Jeffrey A. Christians,1 Philip Schulz,1 Erin M. Sanehira,1,2 Jonathan S. Tinkham,3 Tracy H. Schloemer,3 Steven P. Harvey,1 Bertrand J. Tremolet de Villers,1 Alan Sellinger,1,3 Kai Zhu, 1 Joseph J. Berry,1 and Joseph M. Luther1
1NREL, Golden, CO
2University of Washington, Seattle, WA
3Department of Chemistry, Colorado School of Mines, Golden, CO
The efficiency of halide perovskite solar cells has reached parity with commercially available thin film photovoltaic absorbers. Because of this, their future commercial prospects appear to hinge upon their long-term operational stability. Recent work has provided insight into the moisture instability, thermal instability, phase instability, and phase segregation of the halide perovskite absorbers themselves. This work has translated into much improved device-level operational stability, yet the combined effects of light (including UV-light), oxygen, and moisture remain problematic.
In this work, we investigate the performance of n-i-p perovskite solar cells which are held, unencapsulated, under continual simulated solar illumination in ambient conditions. We demonstrate that degradation is driven by the heterointerfaces in the device stack and systematically engineer the interfaces in the device to improve operational stability. Replacing the Li+-containing spiro-OMeTAD with a Li+-free hole transport material (HTM), EH44, we achieve comparable power conversion efficiency and a factor of 4 better operational stability. Using Time of Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) of devices at various stages of degradation, we observe an irreversible redistribution of components in the perovskite layer when TiO2 is used as the electron transport layer (ETL) which correlates to the initial burn-in which the devices exhibit under illumination. We find that this redistribution and the burn-in which the devices experience is driven by the TiO2/perovskite interface and can be significantly reduced by the use of a nanoparticle SnO2 ETL in place of TiO2. By utilizing MoOx/Al electrodes in place of Au, which can migrate into the devi ce stack, we develop a SnO2/perovskite/EH44/MoOx/Al device stack with very promising stability. This systematic approach to stability results in a device which retains 94% of its peak power conversion efficiency despite 1000 hrs of continual, unencapsulated operation in ambient conditions. This represents a >3 order of magnitude improvement over standard TiO2/perovskite/spiro-OMeTAD/Au devices with the same perovskite absorber layer. This dramatic improvement in stability, despite the combined stresses of UV-light, oxygen, and moisture, demonstrates the importance of carefully designed interfaces for realizing true long-term perovskite solar cell stability.
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