HTL210, (3-((9-ethyl-9H-carbazol-3-yl)oxy)propyl)phosphonic acid, is a self-assembled monolayer material designed for high-efficiency inverted perovskite or perovskite/silicon tandem solar cells. HTL210 features an asymmetric molecular structure with an ethylated carbazole terminal, a flexible alkyloxy linker, and a phosphonic acid anchor. With an electron donating carbazole terminal, HTL210 serves as hole selective layer in between the substrate and the perovskite active layer. The novel alkyloxy linker of HTL201 yields minimized steric hindrance and improved coverage on the transparent conductive oxide (TCO) recombination layer, while comparing with other self-assembled monolayers with typically a nitrogen-bonded phosphonic acid group, such as Me-4PACz or MeO-4PACz.
With a dipole moment (μ) of 3.48 D, HTL210 shows better coordination interaction with the perovskite film to effectively reduce non-radiative recombination at the buried interface for better charge extraction efficiency. Moreover, the perfect energy-level alignment between HTL201 and the perovskite warrants an impressive voltage of nearly 2 V for perovskite/silicon tandem solar cells, giving a certified power conversion efficiency of up to 34.58% based on a silicon heterojunction solar cell, with JSC of 20.80 mA, and FF of 83.63% for a 1.004 cm2 device.
Serving as hole selective contact for organic solar cells and perovskite solar cells, HTL201 is an alternative to PEDOT:PSS with superior performance and the convenience of solution deposition at low concentration, i.e. 1 mM.