Luminescent and electronic properties of perovskite solar cells

Perovskite solar cells show great promise to serve as an alternative for traditional silicon-based solar cells, however several problems present obstacles for their commercialization. Standard perovskite cells contain lead, which is toxic to humans and harmful to the environment. Alternative perovskite compositions offer the opportunity to replace lead with non-toxic metals. Lead can be directly replaced by substituting it for tin. Unfortunately, tin-based perovskite solar cells do not show the same efficiency or stability as their lead-based counterparts. Using numerical modeling, a study of a current optimized, tin-based solar cell was performed to understand its efficiency limitations and where the most gains can be acquired. While the short-circuit current of the device is high, its limiting factors arise from those already known, primarily the alignment of the electron transport layer with the perovskite conduction band.

Another option for replacing lead is by using a double perovskite. This class of perovskites replaces lead with two different metals that maintain charge neutrality. While providing more compositional freedom compared to the direct replacement of lead, double perovskites offer their own challenges. To better understand transport phenomenon between the transport layers and the perovskite, an investigation into the temperature dependent photoluminescence of Cs2AgBiBr6 with different hole and electron transport layers was performed. No clear trends appear allowing a comparison of the transport layers, but some speculations are provided to explain the results common amongst all the samples.

Lead is not the only roadblock for perovskites though, as they also show instability across multiple domains. Two of the most prominent ways instability shows up are closely linked -- ionic motion and reversible degradation. A continuation of an insightful paper by Domanski \emph{et. al.} was performed to further elucidate the effects of ionic movement on reversible degradation processes. We show that these processes can be controlled by the application of a bias voltage and that the enhancement of these processes by illumination is not significant. We continued the examination of these reversible processes by studying their effects under real world conditions. Several perovskite solar cells were subjected to real world conditions in a controlled environment during which we tracked their performance. By comparing the measured data to those calculated if the cells had remained pristine we are able to quantify and separate the reversible from irreversible processes.

Finally, a more fundamental investigation was made into the internal mechanisms of the perovskite cells. A temperature dependent study correlating photoluminescence spectra and current-voltage curves was performed revealing different two regimes operation. The potential mechanisms underlying each regime are explored.