Organic lead halide perovskite possesses desirable optical and electronic properties for solar cell application resulting in rapid progress in device performance. The ease of fabrication and versatilities in cell design and cell processing and the many facets of material properties make metal halide perovskite solar cells the most researched solar technology in the last few years. I will be talking about some of the work in my research group that seeks to improve performance and stability and to reduce the reliance on lead for perovskite solar cells.

One of the stability issues is hysteresis observed in the current-voltage characteristic. In one of our work, full perovskite cells (not test structures) have been adapted to be compatible with photoluminescence characterizations. By analyzing the dynamic PL lifetime and intensity under different light soaking conditions and correlating these results with the shape of the voltage current curve measured under similar light soaking conditions, we explain the different scenarios of ion migration and accumulation at the interface and in the bulk that result in different hysteresis behaviors.

We have also been part of an inter-comparison of measurement methods for perovskite solar cells. I will present recommendations from this work for identifying the most appropriate measurement method for a given cell, depending on its stabilization and degradation behavior. This allows for more accurate and meaningful efficiency measurements of perovskite solar cells.

In terms of perovskite fabrication, the roles of bulky cations in organic lead halide perovskites are further understood. Depending on how these cations are employed, they can provide surface passivation or grain boundary passivation improving cell performance and stability. Their roles in assisting perovskite formation and their segregation behaviors will be presented.

In terms of deposition method, some of the new insights on two-step sequential solution process will be presented.

Work on inorganic cesium-based perovskites have provided many insights into the different film formation mechanisms when films are fabricated by different deposition methods.  The role of lead-substituting-dopants i) on self-assembled surface passivation layer and ii) on engineering colloidal cluster size in the precursor solution and their subsequent effect of film morphology will be discussed.

We have also developed an electrode design that allows lower resistivity transparent conductive oxide to be used on the substrate while still achieving high efficiency at 19.63% certified on 1.02cm2. 

Recent developments in encapsulation methods allowing perovskite cells to pass the IEC61646 standard damp heat, thermal cycling and humidity freeze tests will be presented.

Finally, details of recent work demonstrating >21% large area (>10cm2)monolithic perovskite/silicon tandem solar cell will be presented. The simplicity of the design (interface layer-free), the ease of scale-up to large area and the use of homo-junction-silicon as the bottom cell make the tandem cell design appealing for commercialization.