The impact of molecular orientation on carrier transfer characteristics at a phthalocyanine and halide perovskite interface

Controlling Molecular Orientation of Small Molecular Dopant-Free Hole-Transport Materials: Toward Efficient and Stable Perovskite Solar Cells

Perovskite solar cells (PSCs) have great potential for future application. However, the commercialization of PSCs is limited by the prohibitively expensive and doped hole-transport materials (HTMs). In this regard, small molecular dopant-free HTMs are promising alternatives because of their low cost and high efficiency. However, these HTMs still have a lot of space for making further progress in both efficiency and stability. This review firstly provides outlining analyses about the important roles of molecular orientation when further enhancements in device efficiency and stability are concerned. Then, currently studied strategies to control molecular orientation in small molecular HTMs are presented. Finally, we propose an outlook aiming to obtain optimized molecular orientation in a cost-effective way.

Impact of fluorination on the energy level alignment of an F n ZnPc/MAPbI3 interface

We have studied interactions at an interface between a Methylammonium Lead Iodide (MAPbI3) surface and zinc-phthalocyanine molecules with F substituting peripheral H (F n ZnPc; n = 4, 8, 12, and 16) by employing hybrid density functional theory (DFT) based simulations. These calculations show that F n ZnPc molecules form a stable interface with MAPbI3, whose binding strength is comparable to that of the un-substituted (ZnPc) case. As a consequence of fluorination, an increase in the ionization potential/electron affinity (i.e., a systematic lowering of molecular energy levels), as well as interfacial charge transfer, is observed whose magnitude depends upon the degree of fluorination. In contrast to the common belief of unfavorable hole transfer for excessive fluorination, our work unveils that the valence band offset remains favorable for all ranges of substitution (n); thus, hole transfer from MAPbI3 to F n ZnPc is facilitated while the electron transfer process is suppressed. This unusual behavior originates from the intermolecular interaction and substrate-to-molecule electron transfer at the heterojunction, which gradually suppresses the downward shift of F n ZnPc energy levels by increasing the value of n. Given the beneficial impacts of fluorination, such as hydrophobicity, our work provides valuable insight for exploiting stable F n ZnPc towards high-efficiency perovskite solar cells.