Interfacial Dipole Engineering via Boronic Acid-Based Self-Assembled Monolayers in Inverted Tin–Lead Perovskite Solar Cells with Ideal Band Gap

According to the detailed balance limit for a single-junction solar cell, perovskites with a 1.4 eV band gap can theoretically achieve power conversion efficiencies (PCEs) above 33%, but their progress is limited by the hygroscopic nature of PEDOT:PSS and incompatibility with self-assembled monolayers (SAMs) like MeO-2PACz. Using boronic acid (BA)-based SAMs, especially 4-nitrophenyl boronic acid (4-NPBA), the PCE was greatly improved (18.37%). This is attributed to the large molecular dipole moments of the BA-based SAMs significantly increasing the work function (WF) of the FTO, inducing stronger band bending in the perovskite layer. This band bending, whose magnitude is proportional to the difference in the WF between the SAM and the perovskite, facilitated more efficient hole collection. In comparison, MeO-2PACz-based devices yielded only 9.27% and showed an S-shaped current–voltage (I–V) curve, mainly due to the formation of an interfacial energy barrier. Furthermore, the superior performance of the BA-based SAMs even after possessing an interfacial energy barrier can be explained by enhanced hole collection via (i) tunneling aided by short molecular length of BA-based SAMs as calculated by density functional theory (DFT), or (ii) direct hole transfer from perovskite to FTO through the uncovered FTO regions due to lower surface coverage as evidenced by absorption density estimation in X-ray photoelectron spectroscopy (XPS). These findings were further supported by transient absorption (TA) as well as transient photocurrent decay (TPC) analyses, which revealed markedly higher hole extraction rate (3.73 × 10^–3 ps^–1) and lower charge transport time (1.18 μs) when 4-NPBA was used as a SAM layer.