The past year has seen much progress in the development of perovskite solar cells, with scientists working out new ways to delve deep within the materials and understand the interactions between particles, and how these serve to limit the conversion efficiency or cause rapid degradation.
And there is growing evidence that hydrogen bonds have a key role in limiting the formation of defects and ensuring stability and performance. New research led by the University of California Santa Barbara offers further evidence of this, finding that ‘missing’ hydrogen atoms, or vacancies, can occur deep within the material, causing dramatic reductions in cell efficiency.
To gain these insights, the group headed to the National Energy Research Scientific Computing Center (NERSC)’s Cori supercomputer to model the path of electrons through a perovskite solar cell, using a technique called density functional theory (DFT). “DFT is a very powerful way to treat the behavior of electrons in a material quantum-mechanically,” said UCSB professor Chris Van de Walle. “It makes the problem feasible; but it still requires a lot of central processing unit (CPU) time.”
Organic molecule
The group’s findings are described in full in the paper Minimizing hydrogen vacancies to enable highly efficient hybrid perovskites, published in Nature Materials. The group first modeled methylammonium-lead-iodide; one of the best performing and most commonly researched materials for perovskite solar cells.
The modeling demonstrated that hydrogen vacancies can form at high densities within this material, limiting the cell performance by causing high levels of non-radiative recombination. Meanwhile, modeling of another common perovskite material in solar cell research – formamidinum-lead-iodide, showed far lower levels of hydrogen vacancy formation, and lower non-radiative recombination.
With this knowledge, the researchers theorize that focus on perovskites based on formamidinum, rather than methylammonium, should be the direction of future research. “Our study unveils the critical but overlooked role of hydrogen vacancies in hybrid perovskites and rationalizes why formamidinum is essential for realizing high efficiency in hybrid perovskite solar cells,” the group states. “Minimizing the incorporation of hydrogen vacancies is key to enabling the best performance of hybrid perovskites.”
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