Organic metal halide perovskites are an emerging semiconducting material for photovoltaics. They are low-cost materials with tunable bandgaps and high charge carrier mobilities that can be solution-processed. Interfaces in perovskite photovoltaics are important, as they provide pathways for charge carrier recombination and device degradation. The use of surface ligands to treat the perovskite surface is a promising approach to minimize charge recombination and degradation in perovskite photovoltaics. In this study we use two families of extended π-conjugated surface ligands, including ammonium-functionalized thiophene ligands and perylene diimide (PDI) derivatives, to determine their influence on MAPbI₃ surface energetics, charge transfer, and photovoltaic performance. Three different thiophene ligands are compared by changing the length of the aromatic unit to vary the ionization energy and the electron affinity. The PDI derivatives have larger electron affinities and ionization energies than the thiophene derivatives and varying numbers of terminal amine groups. We find that when the thiophene ligands are used in inverted (p-i-n) devices at the MAPbI₃/C60 (i-n) interface, a significant decrease in the power conversion efficiencies (PCEs) is obtained. Whereas, when they are used in the MAPbI₃/spiro-OMeTAD (i-p) interface in regular (n-i-p) devices an increase in PCE is obtained. When PDI derivatives are used in p-i-n devices at the MAPbI₃/C60 interface we see a marginal decrease in PCE and a more severe decrease in PCE when applied to the MAPbI₃/ spiro-OMeTAD interface in n-i-p devices. Ultraviolet and inverse photoelectron spectroscopy (UPS and IPES) show that the thiophene ligands introduce an energy barrier for electron transfer at the MAPbI₃/C60 interface, which leads to PCE loss, whereas no energy barrier is formed for hole transfer at the MAPbI₃/spiro-OMeTAD interface. This study demonstrates the importance of extended π-conjugated surface ligand energetics on photovoltaic performance in both n-i-p and p-i-n device architectures.

Ke Ma

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