Researchers from China's Hangzhou Dianzi University and Jiaxing University have developed a strategy for optimizing the bottom region of perovskite solar cells and designed an inverted perovskite solar cell using the new strategy. The proposed cell was treated with two molecules known as 2-mercaptoimidazole and 2-mercaptobenzimidazole and was based on a hole transport layer relying on a self-assembled monolayer.
“The HTLs prepared with SAM materials not only have negligible parasitic absorption, low material consumption, and stable adhesion but also exhibit inherent passivation of defects on the bottom of perovskites,” the research team explained.
The researchers used two molecules known as 2-mercaptoimidazole (2-MeIM) and 2-Mercaptobenzimidazole (2-MeBIM) as additives for the HTL. According to them, these molecules interact with the phosphonate groups in the so-called MeO-2PACz, which is also known as [2-(3,6-Dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid, inhibiting the aggregation of MeO-2PACz by forming a supramolecular structure, which in turn results in a more homogeneous HTL.
The team designed the cell with a substrate made of glass and indium tin oxide (ITO), a MeO-2PACz layer, the perovskite absorber, a phenylethylammonium bromide (PEA-Br) cation, an electron transport layer based on based phenyl-C61-butyric acid methyl ester (PCBM), a bathocuproine (BCP) buffer layer, and a silver (Ag) metal contact.
They tested the performance of the cell under standard illumination conditions and compared it with that of a reference cell without the additives. The champion solar cell treated with the additives achieved a power conversion efficiency of 24.38%, an open-circuit voltage of 1.181 V, a short-circuit current density of 24.43 mA cm−2, and a fill factor of 84.51%. The control device reached an efficiency of 23.12%, an open-circuit voltage of 1.166 V, a short-circuit current density of 24.09 mA cm−2, and a fill factor of 82.32%.
“Benefitting from the better energy alignment of the target film, the HTL with additives showed better charge carrier extraction capability,” the scientists explained. “The improved properties of the additive-introduced HTLs, such as better homogeneity, enhanced conductivity, and more aligned energy bands, and the improved quality of the bottom of the perovskite films featuring a larger grain size with low defects, together promote the more efficient charge transfer and the suppressed non-radiative recombination energy loss at the interface, leading to the boosted performance of the perovskite solar cells.”
The cells treated with the additives were also able to retain over 89% of their initial efficiency after 450 h, while the reference cell reached a value of only 74%. This was due to the higher quality of the perovskite film in the treated cells, which the group said minimizes their vulnerability to external and internal attacks.
