Enlightened People: The Journey of Prof. Henry Snaith’s (FRS) perovskite optoelectronics – Light: Science & Applications

The first use of perovskite materials in a DSSC in 2009 showed only a few percent success, but this work laid the groundwork for future research. Since we reported our findings, almost everyone working on organic PVs has shifted their focus to the structured heterojunction structure. PSC operations increased rapidly, exceeding 20% ​​within a few years. The first efficient perovskite LEDs were reported around 2014.

During the twenty years, another important discovery we made, probably in 2017, was related to the configuration of the device. Initially, we used what we call a nip structure, where the n-type material is usually TiO₂was at the bottom, and the p-type material, an organic hole conductor, was at the top. To explore other possibilities, we also created pin cells, where the pe-type material was placed at the bottom, and the o-type material was at the top. The first materials we used in the pin cells were PEDOT:PSS and PCBM fullerene, which were familiar to natural PVs. This format worked well, although it did not match the performance of the nip format. However, in 2017, we found that the pin configuration was much more stable than the nip configuration. As a result, we have turned ~90% of my research group’s effort into cell typing. This configuration is also very compatible with multijunction cells, which I will talk about shortly.

Besides stability, another important factor is efficiency, especially in the case of existing silicon technology. The industry has made great progress, and when perovskites have reached the performance levels of silicon in single-layer configurations, it is difficult to develop a new technology that matches the success of silicon. To truly compete, we need to deliver a very high level of efficiency. Realizing this early on, we focused on tandem solar cells. In fact, our first patents, submitted before we published our first paper, identified high-energy perovskites that are particularly suitable for tandem solar cells. These papers included claims about using perovskites in tandem solar cells, and we have continued to work on this idea ever since.

One of the biggest problems in this area has been to fix the band gap of perovskites, where mixed halides have proven to be very important. By adjusting the ratio of iodide to bromide, we can achieve a suitable band gap for bonding with silicon. In addition, by changing from lead to tin, we can reduce the group gap of perovskites, which enables the creation of “perovskite tandem” cells or triple-junction cells using film materials thin with well-ordered band gaps. Starting in 2014, a large part of our research is focused on improving these tools and implementing toolkits for integrated cells.

Over the past decade, some major changes have occurred. Stability has become an increasingly important topic, especially in understanding how to reduce photo-induced damage, which occurs due to heat, light and electric fields. Initially, there were many questions about the role of ions in perovskites and whether our reported success was correct. At first, we reported the current voltage analysis, but soon we realized the need to report the steady state performance. As a result, we developed stable parameters for perovskites, as well as following a number of energy factors. Today, all efficiency records are recorded as the highest power tracked or sustained.

Although we now have a better understanding of the effect of ions on efficiency, the effect of mobile ions on the long-term stability of the process is still a hot topic of research. When these materials are exposed to high temperatures, sunlight, and various environmental conditions, more mobile ions can be produced in perovskites. These ions can affect the electronic properties and overall performance of the device. Although we have gained a lot of understanding in this matter, there is still a lot of work that can be done.