The societal pressure to move towards sustainable energy has made photovoltaics the main competitor for cost-competitive energy sources. As of now, the photovoltaics market is dominated by silicon technology and has shown grid parity; however, the energy payback time remains higher in this established technology. With the increasing use of renewable energy, thin-film photovoltaic technology (CIGS, CdTe, Kesterites, Dye, and organic solar cells) will also gradually increase its market share.
The current decade has witnessed the unprecedented rise in the investigation of perovskite materials for solar cell fabrication owing to their excellent semiconducting and light-harvesting behavior. Apart from solar cells, it has also shown success in light-emitting diodes (LEDs) and laser fabrication at lab scale. Solution-processed perovskite solar cells have made stunning advancement in a short time frame, though they also suffer from intrinsic issues such as charge migration and stability.
Device long-term stability, triggered by thermal and moisture-induced degradation, remains a challenging task, and currently, this is seen as a barrier to its technological exploitation. The impulsive loss of iodide is argued to be a potential path in device degradation. By adopting the strategy of using an iodide-rich passivation layer on top of the perovskite layer, several unwanted reactions can be retarded.
Our understanding is that this iodide-rich passivation layer will passivate the surface defects and fill the grain boundaries, which in turn will reduce the non-radiative recombination losses and ultimately push device photovoltaic performance. More importantly, it will keep the performance linear across the time scale. The natural desire in iodide encompassing material such as imidazolium iodide, to lose iodide will act as a good passivant for perovskites. It is a known fact that, due to MAPbI3‘s three-dimensional structure, it rotates across the axis, and this leads to ion migration inside the perovskites.
An additional source of iodine will act as a reservoir to suppress iodide loss and will also fill the grain boundaries. In this article (Nano Energy, 2018) we describe solar cell performance using meticulously engineered material and its integration in working devices. The present investigation demonstrates that the judicious choice of passivation layer can indeed passivate the surface and also solve the issue of ion migration, along with improved photovoltaic performance. This has been achieved through a simple approach by passivating perovskite surface and its molecular encapsulation by imidazolium iodide.
The simple strategy will facilitate other iodide rich imidazolium salts to be further exploited in thin film organic-inorganic perovskite solar cells and push the community to optimize this structure for maximum light harvesting. In a recent report, using a similar approach and covering perovskites with graphene was also found to suppress the iodide loss, and it showed significant improvement in perovskite stability.
In summary, the present work not only provides vital insights to address the daunting challenges for future stable perovskite optoelectrical device development but also put forward the potential of an iodine-rich layer as a multipurpose interfacial spacer. This will lead to the possibilities to explore other halide (iodide) rich spacer layers which have a negative bearing on light-harvesting abilities in perovskites.
These findings are described in the article entitled Surface passivation of perovskite layers using heterocyclic halides: Improved photovoltaic properties and intrinsic stability, recently published in the journal Nano Energy. This work was conducted by Manuel Salado from the Basque Center for Materials and Swiss Federal Institute of Technology Lausanne and Shahzada Ahmad from the Basque Center for Materials and IKERBASQUE, Basque Foundation for Science.