Researchers from Japan's Chiba University, Kyoto University and the University of Electro-Communications have developed a universal, physics-based model that clarifies how energy levels align at electrode/hole-collecting-monolayer (HCM)/perovskite interfaces in inverted perovskite solar cells and how this alignment controls hole extraction and device performance.
The new work replaces competing interface models - such as vacuum level alignment, Fermi level alignment, and electrode-modified Schottky models - with a single framework that treats the stack as two coupled but distinct interfaces. At the electrode/HCM contact, the alignment is governed by an interface dipole at a metal/organic interface, where the HCM acts as a dipole layer that shifts the electrode work function. At the HCM/perovskite boundary, both layers are treated as semiconductors and described using semiconductor heterojunction theory, with band offsets and band bending rather than simple vacuum-level matching.
To parameterize the model, the researchers studied three carbazole-derived HCMs - 2PACz, MeO-2PACz, and 3PATAT-C3 - and a mixed perovskite Cs0.05FA0.80MA0.15PbI2.75Br0.25 (MixA-PVK1), using ultraviolet photoelectron spectroscopy (UPS) and low-energy inverse photoelectron spectroscopy (LEIPS) to precisely measure work functions and ionization energies. Based on this, they defined two key interfacial quantities at the HCM/perovskite junction: the work function difference, which sets the direction and magnitude of band bending in the perovskite (ΔΦ), and the ionization energy difference, which defines the interfacial hole barrier height (ΔEV). Efficient hole collection requires both negligible interfacial hole barriers and favorable upward band bending on the perovskite side so that internal electric fields assist hole extraction and limit recombination.
The model was validated experimentally by combining 2PACz, MeO-2PACz, and 3PATAT-C3 with four perovskites: Cs0.05FA0.80MA0.15PbI2.75Br0.25 and Cs0.05FA0.73MA0.22PbI2.31Br0.69 (MixA-PVK1 and MixA-PVK2), MAPbI3, and the lead-free candidate FASnI3. It was then further tested using literature data for structurally diverse non-carbazole HCMs (Me-PhpPACz, Br-2EPT, MeO-BTBT, 4PADCB, ID-Cz, Py3, MPA-BT-XA, and 4-XPBA). Across these systems, the calculated hole collection favorability based on ΔΦ and ΔEV correlated well with reported open-circuit voltage, short-circuit current, fill factor, and power conversion efficiency, demonstrating broad applicability to electrode/HCM/perovskite interfaces.
To turn this physics into design rules, the team analyzed the origin of the required energy parameters. UPS measurements showed that the effective work function of the HCM-coated electrode is largely set by the electrode work function plus the potential step induced by the HCM dipole layer, emphasizing the often-overlooked role of the transparent conductive electrode. The magnitude of this dipole is controlled by molecular dipole moment and orientation, consistent with the Helmholtz equation, and was confirmed by combining UPS and metastable atom electron spectroscopy (MAES) with calculated molecular dipoles. In contrast, the HCM ionization energy stems from the molecular ionization energy modified by solid-state polarization. Together, these insights account for the four central parameters - electrode work function, HCM work function, and the ionization energies of both HCM and perovskite - and provide a practical recipe: choose electrode/HCM/perovskite combinations that produce upward band bending in the perovskite and minimal hole barriers, then fine-tune morphology, wettability, and defect passivation to reach higher and more reproducible device efficiencies.
