This study is led by Dr. Wu from East China University of Science and Technology. The development of efficient, stable and low-cost hole-transporting materials (HTMs) is crucial for the commercialization of perovskite solar cells, which have been demonstrated with very high sunlight-to-electricity efficiency but still suffer from stability issues. Incorporating chemically anchorable groups onto hole-transporting molecules is a feasible strategy for developing new generation of HTMs that form SAM on transparent conductive oxide (such as indium tin oxide, ITO) to function as efficient hole-selective contact for perovskite solar cells (PSCs). However, the strongly acidic anchor in state-of-the-art SAM is detrimental to the stability of interfaces and devices. In this regard, Huanxin Guo together with lab director Prof. Yongzhen Wu attempted to use alternative anchoring groups to construct hole-selective SAMs.
The team found that the boric acid can establish robust B-O-M (M means metal) linkage on the ITO surface via X-ray photoelectron spectroscopy (XPS). The negative Gibbs free energy of the dissociative adsorption suggested a spontaneous formation of SAM on ITO surface. "This XPS and first-principles simulation support the presence of covalent linkage between the boric acid and ITO surface, and the acidity-weakened boric acid should be competent as a new anchor" Huanxin says.
The researchers examined the effectiveness of boric acid anchoring SAM on inverted PSCs. There is a perfect contact between the boric acid SAM and perovskite layer, resulting a high-quality and coherent perovskite-substrate interface. The PSC device employing boric acid SAM thus achieving a power conversion efficiency over 22% with remarkable fill factor up to 85.2%. More importantly, the boric acid SAM enabled an excellent long-term stability of PSCs when storing in air. The sheet resistance of recycled ITO substrate in aged PSC devices with boric acid SAM showed that the suppressed acidity corrosion.
"These exciting results confirm that the boric acid group will open new avenues for the development and design of hole-selective SAM for efficient and stable PSCs," Huanxin says.
