Improving stability and efficiency of perovskite solar cells by 1-ethyl-3-methyl imidazolium tetracyanoborate ionic liquid

Ionic liquids are being used to increase the stability and efficiency of perovskite solar cells (PSCs). Ionic liquids not only help to enlarge grain size of perovskite due to their high thermal stability and low volatility, but also passivate defects on the perovskite surface, increasing the stability and efficiency. 1-ethyl-3-methylimidazolium tetracyanoborate ionic liquid is one of the excellent ionic liquids, which greatly increased the growth of perovskite crystal particles and enhanced the stability of PSCs. From calculation by Gaussian 09 program, electron density of N in the B(CN)₄⁻ ion is about 6.3. So, B(CN)₄⁻ can form chelate bonds or hydrogen bonds with Pb²⁺ or I⁻. The power conversion efficiency (PCE) of the three-layer structured PSCs fabricated using this ionic liquid was 13.8 % for the maximum, and it was maintained above 80 % of the initial efficiency under full-spectrum sunlight at 60–65 °C in air with relative humidity ranging from 40 to 50 % after 240 h.