Achieving Compact and Uniform Buried Contact via Sequentially Deposited Hybrid Self-Assembled Monolayers for Efficient Wide-Bandgap Perovskite and All-Perovskite Tandem Solar Cells

Wide-bandgap (WBG) perovskites have emerged as promising materials for all-perovskite tandem solar cells (ATSCs) for their potential to surpass the Shockley–Queisser limit of single-junction perovskite solar cells (PSCs). However, nonradiative recombination at the buried interface of WBG PSCs remains a great challenge, limiting the efficient carrier transport and collection in these devices. Hole selecting materials (HSMs) play a crucial role in charge extraction and growth of the overlying perovskite films. Here, we systematically investigated the deposition of various self-assembled monolayer (SAM) materials on NiO_x to modify the NiO_x/perovskite interface. A sequential deposition of (4-(7H-dibenzo[c,g]-carbazol-7-yl)butyl)phosphonic acid (4PADCB) and [4-(3,6-diphenyl-9H-carbazol-9-yl)butyl]phosphonic acid (Ph-4PACz) is found to result in optimized efficiency, which can be attributed to the ability of sequential deposition to fill the gaps and form a more compact and uniform buried interfacial contact compared to coating each layer separately. Moreover, the modified buried interface notably eliminates the formation of the small grains, which may be caused by random nucleation. This strategy also enhances energy level alignment, decreasing the barrier for carrier transport at the buried interface. As a result, the champion single-junction WBG PSC achieves an open-circuit voltage (V_OC) of 1.33 V and an efficiency of 20.35%. The ATSCs fabricated with the WBG subcells based on the reported strategy achieve an optimized efficiency of 27.03%, exhibiting the great potential of the sequential deposition method for SAMs on future perovskite photovoltaics.