In just seven years of development, the efficiency of the perovskite solar cell has increased to the point where it rivals — and could soon overtake — the efficiency of the conventional photovoltaic cell. However, the perovskite structure continues to experience stability issues, causing these cells to degrade quickly and shortening their lifespans. This is a sample of a functioning perovskite-based LED light. Courtesy of OIST. Scientists at Okinawa Institute of Science and Technology (OIST) have identified degradation factors and developed a potential solution to improving perovskite solar cell architecture. The new findings suggest that interactions between components of the solar cell itself are responsible for the rapid degradation of the device, and that interface engineering is important for the fabrication of highly stable and hysteresis-free perovskite solar cells. More precisely, the cell’s titanium oxide (TiO2) layer, which extracts electrons made available through solar energy, was found to cause unwanted deterioration of the neighboring perovskite layer. To achieve a high power conversion efficiency (PCE) and long lifetime simultaneously, an insulating polymer interface layer was deposited on top of the TiO2, preventing direct contact between the TiO2 and the perovskite layers. The polymer layer was insulating but very thin, letting the electron current tunnel through without diminishing the overall efficiency of the solar cell or the integrity of the perovskite structure. “We added a very thin sheet, only a few nanometers wide, of polystyrene between the perovskite layer and the titanium oxide layer,” said Longbin Qiu. “Electrons can still tunnel cross this new layer and it does not affect the light absorption of the cell. This way, we were able to extend the lifetime of the cell four-fold without loss in energy conversion efficiency.” From left to right: Luis Ono, Yan Jiang, Linquiang Meng and Yabing Qi. Courtesy of OIST. The team investigated three polymers, each from a different functional group, to further understand the relation between interface structure and the PCE and stability of the perovskite device. The lifespan of the new perovskite device was extended to over 250 hours. While that’s not enough to compete with the stability of existing commercial photovoltaic cells, it is a step toward fully functional perovskite solar cells and could pave the way for further optimization of the efficiency and stability of perovskite cells. CVD Offers Consistency, Scalability in Manufacture of Perovskite LEDs For the manufacture of perovskite LEDs, OIST researchers investigated an all-vapor growth process by chemical vapor deposition (CVD). According to the team, there are many advantages to preparing perovskites using a vapor-based growth process, including ease of patterning, ability to batch process and material compatibility. “Chemical vapor deposition is already compatible with the industry, so in principle it would be easy to use this technology to produce LEDs,” said professor Yabing Qi. “The second advantage in using CVD is a much lower variation from batch to batch compared to liquid-based techniques. Finally, the last point is scalability: CVD can achieve a uniform surface over very large areas.” Like the solar cell, the perovskite LED comprises many layers that work synergistically. For this approach, an indium tin oxide (ITO) glass sheet and a polymer layer were used to allow electrons into the LED. The chemicals required for the perovskite layer were then successively bound to the sample using CVD. The perovskite layer is composed of nanometer-sized grains, and the size of the grains plays a critical role in the efficiency of the device. “With large grains, the surface of the LED is rough and less efficient in emitting light. The smaller the grain size, the higher the efficiency and the brighter the light,” said Lingqiang Meng. “By changing the assembly temperature, we can now control the growth process and the size of the grains for the best efficiency.” The last step in the process involved the deposition of two additional layers and a gold electrode, forming a complete LED structure. The resulting LED can even be used to create specific patterns using lithography during the manufacturing process. Controlling grain size is not the only challenge for the team that developed this novel technique for assembling LED lights made from perovskites. “Perovskite is great, but the choice in the adjacent layers is really important, too,” said Luis K. Ono. “To achieve high electricity-to-light conversion rates, every layer should be working in harmony with the others.” The all-vapor perovskite growth process by CVD demonstrated luminance up to 560 cd/m2 and resulted in a flexible, thick, film-like LED with a customizable pattern. “Our next step is to improve the luminance a thousand-fold or more,” said Meng. “In addition, we have achieved a CVD-based LED emitting green light but we are now trying to repeat the process with different combinations of perovskite to obtain a vivid blue or red light.” The research into perovskite solar cells was published in The Journal of Physical Chemistry B (doi: 10.1021/acs.jpcb.7b03921).