(604b) Engineering Perovskite Solar Cell Interfaces to Realize > 1000 Hr, Unencapsulated Ambient Stability

Because the power conversion efficiency of organic-inorganic halide perovskite solar cells has reached parity with commercial thin film photovoltaic absorbers, their future commercial prospects appear to hinge on stability. Much recent work has focused on the intrinsic stability of the APbX₃ perovskite absorber materials (A = CH₃NH₃⁺, CH(NH₂)₂⁺, Cs⁺, Rb⁺ and X = Cl^-, Br^-, I^-). Recent work has provided insight into the moisture instability,¹ thermal instability,² phase instability,³ and phase segregation⁴ of halide perovskite materials. To some extent, improved perovskite stability has translated to improved device stability, yet further gains are still necessary, motivating a systematic look at the interfaces of the device stack.

In this study, we systematically investigate the interfaces in the typical n-i-p perovskite solar cell, identify key degradation mechanisms, and systematically engineer these interfaces to improve operational stability. Replacing spiro-OMeTAD with a Li⁺-free hole transport layer (HTL) material, EH44,⁵ we can achieve power conversion efficiencies of approximately 18% in a standard TiO₂/perovskite/HTL/Au device stack. Unencapsulated devices utilizing EH44 show a factor of 4 better ambient operational stability compared to spiro-OMeTAD devices, after a similar initial burn-in of the devices. Analyzing the initial burn-in, Time of Flight Secondary Ion Mass Spectrosocpy (ToF-SIMS) on the device stack shows a redistribution of the perovskite active layer during the initial burn-in which is independent of the HTL. This redistribution, and the corresponding burn-in, is driven by interface effects and is significantly reduced when the TiO₂ electron transport layer (ETL) is replaced with SnO₂. When combined with MoO_x/Al electrodes,⁶ this device stack allows for a ~3 order of magnitude increase in the operational stability of unencapsulated devices in ambient conditions. We observe only a 12% decrease in efficiency after 1000 hrs of continual ambient operation (6% degradation in best device). The ability of these devices to withstand the combined stresses of UV-light, oxygen, and moisture, demonstrates the importance of carefully designed interfaces for realizing true long-term stability.

References

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