Solar Cell Defects Healed by the Sun in the Vacuum of Space

Australian researchers have shown that perovskite solar cells damaged by proton radiation in low-earth orbit can recover their original efficiency in full through annealing in a thermal vacuum.

The process is made possible by careful design of the hole transport material (HTM), a component that moves photo-generated positive charges to the cell’s electrode.

This multidisciplinary project is pioneering in its use of thermal admittance spectroscopy (TAS) and deep-level transient spectroscopy (DLTS) to analyze defects in proton-irradiated and thermal-vacuum recovered perovskite solar cells (PSCs). The study is also the first to employ ultrathin sapphire substrates compatible with high power-to-weight ratios, rendering them suitable for commercial applications.

The results were recently published in the journal Advanced Energy Materials.

Light-weight PSCs are a strong candidate for powering low-cost space hardware thanks to their low manufacturing cost, high efficiency and radiation hardness. 

All previous proton irradiation studies of PSCs took place on heavier substrates thicker than 1mm. Here, to take advantage of high power-to-weight ratios, ultrathin radiation-resistant and optically transparent sapphire substrates of 0.175mm were used by a team based at the University of Sydney. The project was led by Professor Anita Ho-Baillie, who is also an Associate Investigator with the ARC Centre of Excellence in Exciton Science.

The cells were exposed to rapid scanning pencil beam of seven mega-electron-volts (MeV) protons using the high energy heavy ion microprobe at the Centre for Accelerator Science (CAS) at ANSTO, mimicking the proton radiation exposure that the solar cell panels would undergo while orbiting the earth on a satellite in low-earth orbit (LEO) for tens to hundreds of years.

It was found that the type of cells featuring a popular HTM and a popular dopant within its HTM are less radiation tolerant than their rivals. The HTM in question is the compound 2,2′,7,7′-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (Spiro-OMeTAD), while the dopant is lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

Through chemical analysis, the team found that fluorine diffusion from the LiTFSI induced by proton radiation introduces defects to the surface of the perovskite photo-absorber, which could lead to cell degradation and efficiency losses over time.

“Thanks to the support provided by Exciton Science, we were able to acquire the deep-level transient spectroscopy capability to study the defect behavior in the cells,” lead author Dr. Shi Tang said.

The team was able to ascertain that cells free of Spiro-OMeTAD and free of LiTFSI did not experience fluorine diffusion-related damage, and degradation caused by proton-radiation could be reversed by heat treatment in vacuum. These radiation-resistant cells had either Poly[bis(4-phenyl) (2,5,6-trimethylphenyl) (PTAA) or a combination of PTAA and 2,7-Dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8BTBT) as the hole transport material, with tris(pentafluorophenyl)borane (TPFB) as the dopant.

“We hope that the insights generated by this work will help future efforts in developing low-cost light-weight solar cells for future space applications,” Professor Ho-Baillie said.

Reference: “Effect of Hole Transport Materials and Their Dopants on the Stability and Recoverability of Perovskite Solar Cells on Very Thin Substrates after 7 MeV Proton Irradiation” by Shi Tang, Stefania Peracchi, Zeljko Pastuovic, Chwenhaw Liao, Alan Xu, Jueming Bing, Jianghui Zheng, Md Arafat Mahmud, Guoliang Wang, Edward Dominic Townsend-Medlock, Gregory J. Wilson, Girish Lakhwani, Ceri Brenner, David R. McKenzie and Anita W. Y. Ho-Baillie, 22 May 2023, Advanced Energy Materials.
DOI: 10.1002/aenm.202300506