PolyU team traces perovskite cell ageing to grain boundaries; treated lab cells reach 26.55% efficiency
Hong Kong Polytechnic University researchers found that uneven charge losses between neighbouring grains speed up light ageing; an additive slowed it in small lab cells.
Researchers at The Hong Kong Polytechnic University (PolyU), working with a team from Zhejiang University, identified a previously overlooked cause of light-induced ageing in perovskite solar cells, according to polyu.edu.hk. The study, led by Prof. Yang Guang, was published in the journal Watt. The best treated laboratory cell reached a power conversion efficiency of 26.55% polyu.edu.hk
polyu.edu.hkpolyu.edu.hkOpen source ↗.
Why neighbouring grains matter
The active layer consists of many small crystals, or grains, each with a surface orientation called a crystal facet. In mixed-facet cells, grains with different facets sit side by side. The team found that where (100) and (111) grains meet, charge losses on the two sides of the boundary are uneven. In the scinexx account of the work, recombination velocities at such a boundary were about 300 and 800 cm/s.
The (111) side has more defects, mostly vacancies where formamidinium (FA) ions should be, and holes build up there abnormally. On its own, the (111) facet is more resistant to light ageing, yet in the mixed structure these grains degrade faster. The authors conclude that a cell's lifetime depends not only on the stability of each grain but also on how neighbouring grains interact.
Additive at the boundaries and test results
To balance the losses, the team added 3-fluorophenylethylammonium bromide (m-F-PEABr). Its molecules are too bulky to enter the grains and collect at grain boundaries. There the positively charged end takes the place of missing FA ions, while the fluorine-containing part holds the molecule in place polyu.edu.hk
polyu.edu.hkpolyu.edu.hkOpen source ↗. After treatment, recombination velocities on both sides of the boundary became nearly equal.
Treated cells reached 26.55% efficiency, against 25.19% for a control cell. Under light equivalent to one sun, they kept over 90% of initial efficiency after more than 2,200 hours (the T90 operational lifetime). Untreated cells reached that point after about 800 hours. At 85 °C the corresponding time rose from 270 to 1,250 hours.
The tests used small, unencapsulated cells with an active area of about 0.06 cm², in a nitrogen atmosphere. Performance of larger modules, or under humidity and changing weather, was not covered by these tests. Prof. Yang said that balancing charge losses between grains can improve both efficiency and stability. Prof. Nam-Gyu Park of Sungkyunkwan University commended the work in a research highlight in Watt.
