Efficient, stable perovskite solar cells enabled by electrode interface engineering and nanoscale phase stabilization
Sanehira, Erin Mariko
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Semiconducting metal halide perovskites have emerged as a promising solution-processable, photovoltaic material with research cell power conversion efficiencies now exceeding 22% under simulated sunlight. The prototypical composition of this “ABX3” semiconductor is CH3NH3PbI3, in which organic methylammonium cations charge stabilize lead iodide octahedra. Research is underway on mixed component systems with A-site cation combinations of methylammonium, formamidinium, cesium, and rubidium; B-site cations of Pb2+ and Sn2+; and iodide, bromide and chloride anions. Although perovskite solar cells with low-cost fabrication methods have demonstrated impressive power conversion efficiencies, device durability remains a key concern of the technology. In this dissertation, the effect of the anode electrode material on the device lifetime is characterized under constant operating conditions. It is demonstrated that MoOx/Al electrodes are more stable than commonly used Au or Ag electrodes. Interestingly, the enhanced stability of MoOx/Al electrodes is due to the formation of an oxide at the MoOx/Al interface, which likely prevents ion migration between the device layers, as opposed to encapsulation from environmental agents. I also demonstrate a more stable photoactive layer comprised of CsPbI3 quantum dots (QDs). CsPbI3 is the lowest bandgap, all-inorganic lead halide perovskite, and has shown remarkable chemical and thermal stability up to 400 °C. However, bulk and thin film CsPbI3 transitions to the undesired orthorhombic phase when cooled to room temperature. CsPbI3 QDs have unique surface properties which alter the phase transitions and successfully maintain the photoactive cubic phase at room temperature and even well below. In addition to reporting the first demonstration of an all-inorganic CsPbX3 nanocrystal solar cell, I also detail new QD surface treatments that improve the short circuit current density of the devices by doubling the QD film mobility. These advancements led to an NREL-certified QD solar cell efficiency of 13.43% that is currently the record efficiency reported for a QD solar cell of any material system. In this dissertation, I assess operational stability of thin film organic-inorganic perovskite solar cells, fabricate more durable electrodes, develop novel CsPbI3 QD photovoltaic devices and discover new surface modifications to improve charge transport in efficient perovskite QD solar cells.
- Electrical engineering