January's Paper of the Month

The efficiency of lead halide perovskites has increased significantly since their introduction in 2009. The high performance as well as cost-effective manufacture make them ideal for photovoltaic applications. However, lead halide perovskites are known for their instability when subjected to changes in light, oxygen, temperature as well as issues with homogeneity. As such it becomes important to test their stability in a practical situation, such as in a device stack, to better understand the degradation mechanics. 

Raman spectroscopy is a powerful probing tool to study the degradation of individual perovskite layers as well as the degradation kinetics. Raman mapping techniques can also be used to investigate the homogeneity of the perovskite film. 

January’s Paper of the Month comes from the University of Swansea. They used a Linkam RH95 and THMS600-H to conduct in-situ Raman Spectroscopy and further understand the degradation kinetics of perovskite materials when temperature and humidity were altered. 

The device stack was first tested for thermal degradation by heating to 150°C. While photo-degradation was found to be dependent on top and bottom layers, thermal degradation was shown to be non-reversible and affects the entire MAPI film. It is determined by the homogeneity of the film rather than structural layers. Raman signals from in-situ humidity experiments show the dihydration of the perovskite to be almost completely reversible once drying occurs. However further analysis of the raman peaks showed dihyration remained in the Au region, suggesting some moisture remained trapped in this region. Device performance may be fully recovered if trapped moisture can be removed.
 

 Figure 1: The in situ raman spectra highlights the reversible dihydration when relative humidity is decreased but also indicates the presence of a dehydrated species in the Au region. 

Figure 1: The in situ raman spectra highlights the reversible dihydration when relative humidity is decreased but also indicates the presence of a dehydrated species in the Au region. 

Understanding how humidity affects perovskite layers differentially within a stack has led to the conclusion that targeted drying would help improve and regain device performance. Optimising such performances will have great benefits for the future development of microelectronics and telecommunication.

By Tabassum Mujtaba

Tsoi et al., Probing the degradation and homogeneity of embedded perovskite semiconducting layers in photovoltaic devices by Raman spectroscopy. Phys.Chem.Chem.Phys. 19, 5246 (2017).