Research Projects

The metal halide perovskites have come to occupy the centre stage of photovoltaics in a short period of slightly over a decade, an interest that started with the work from the Miyasaka group finding power conversion efficiencies of 3.8%. This number today is around 25.2%, and is comparable with commercially available solar cells based on Si or Copper Indium Gallium Selenide. However their widespread use is limited by their stability under ambient conditions. Within the perovskite layers, ion migration, especially of the anions is believed to be a key factor in undermining the stability, often leading to phase separation. This has been partially overcome by using a passivating layer involving a two-dimensional perovskite or a molecular layer over a three dimensional perovskite. While the anion migration has been found to reduce significantly with certain choices of the overlayers, graphene being one such example, this approach is limited because of the non availability of large area graphene. So it is essential to understand the reasons why a particular choice of passivating overlayer works well in reducing ion migration while simultaneously allowing for the movement of the charge carriers. These ideas would help identify alternate materials which could be used. Detailed investigations complementing charge transport studies with microscopy have revealed that small compositional variations are tolerated and don’t affect the performance. In contrast, large compositional variations as well as structural disorder are found to lead to degradation during solar cell operation. While a lot of effort has been spent in optimizing each of the components of the stack that make up a solar cell, the interface is found to be the weakest link to device stability. We will explore the growth of various two dimensional perovskite overlayers on three dimensional perovskites, examining if a conformal layer can be achieved with a favourable formation energy. In addition, the band offsets will be examined. Although the rise of the halide perovskite based photovoltaics may be largely attributed to the defect tolerance of these materials, shallow defects are still present. These then provide a route to non radiative recombination processes, reducing the efficiency from the Shockey-Queisser limit. We will explore routes to reduce the non radiative transitions, and hence improve the efficiency of the devices. So, all in all, the present approach aims to examine solar cell stability by looking at the device as a whole, building on the knowledge that has been gained by optimizing individual components of the device.