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The remarkable optoelectronic properties of metal halide perovskites have generated intense research interest over the last few years. The ability to control and manipulate the crystallisation and stoichiometry of perovskite thin-films has allowed for impressive strides in the development of highly efficient perovskite solar cells. However, being able to effectively modify the interfaces of metal halide perovskites, and to controllably p- or n-type dope the surfaces, may be key to further improvements in the efficiency and long-term stability of these devices. In this study, we use surface doping of the mixed-cation, mixed-halide perovskite FA 0.85 MA 0.15 Pb(I 0.85 Br 0.15 ) 3 (FA – formamidinium; MA – methylammonium) to improve the hole extraction from the perovskite solar cell. By treating the surface of the perovskite film with a strongly oxidizing molybdenum tris(dithiolene) complex, we achieve a shift in the work function that is indicative of p-doping, and a twofold increase in the total conductivity throughout the film. We probe the associated interfacial chemistry through photoelectron and solid-state nuclear magnetic resonance spectroscopies and confirm that charge-transfer occurs between the perovskite and dopant complex. The resulting p-doped interface constitutes a homojunction with increased hole-selectivity. With charge-selective layers, we show that this surface doping enhances the device performance of perovskite solar cells resulting in steady-state efficiencies approaching 21%. Finally, we demonstrate that a surface treatment with this dopant produces the same effect as the commonly employed additive 4- tert butylpyridine ( t BP), allowing us to achieve “ t BP-free” devices with steady-state efficiencies of over 20%, and enhanced thermal stability as compared to devices processed using t BP. Our findings therefore demonstrate that molecular doping is a feasible route to tune and control the surface properties of metal halide perovskites. 

We have considered three two-dimensional (2D) π-conjugated polymer network ( i.e. , covalent organic frameworks, COFs) materials based on pyrene, porphyrin, and zinc-porphyrin cores connected via diacetylenic linkers. Their electronic structures, investigated at the density functional theory global-hybrid level, are indicative of valence and conduction bands that have large widths, ranging between 1 and 2 eV. Using a molecular approach to derive the electronic couplings between adjacent core units and the electron-vibration couplings, the three π-conjugated 2D COFs are predicted to have ambipolar charge-transport characteristics with electron and hole mobilities in the range of 65–95 cm 2 V −1 s −1 . Such predicted values rank these 2D COFs among the highest-mobility organic semiconductors. In addition, we have synthesized the zinc-porphyrin based 2D COF and carried out structural characterization via powder X-ray diffraction, high-resolution transmission electron microscopy, and surface area analysis, which demonstrates the feasibility of these electroactive networks. Steady-state and flash-photolysis time-resolved microwave conductivity measurements on the zinc-porphyrin COF point to appreciable, broadband photoconductivity while transmission spectral measurements are indicative of extended π-conjugation. 

2D polymers (2DPs) are promising as structurally well‐defined, permanently porous, organic semiconductors. However, 2DPs are nearly always isolated as closed shell organic species with limited charge carriers, which leads to low bulk conductivities. Here, the bulk conductivity of two naphthalene diimide (NDI)‐containing 2DP semiconductors is enhanced by controllably n‐doping the NDI units using cobaltocene (CoCp₂). Optical and transient microwave spectroscopy reveal that both as‐prepared NDI‐containing 2DPs are semiconducting with sub‐2 eV optical bandgaps and photoexcited charge‐carrier lifetimes of tens of nanoseconds. Following reduction with CoCp₂, both 2DPs largely retain their periodic structures and exhibit optical and electron‐spin resonance spectroscopic features consistent with the presence of NDI‐radical anions. While the native NDI‐based 2DPs are electronically insulating, maximum bulk conductivities of >10⁻⁴ S cm⁻¹are achieved by substoichiometric levels of n‐doping. Density functional theory calculations show that the strongest electronic couplings in these 2DPs exist in the out‐of‐plane (π‐stacking) crystallographic directions, which indicates that cross‐plane electronic transport through NDI stacks is primarily responsible for the observed electronic conductivity. Taken together, the controlled molecular doping is a useful approach to access structurally well‐defined, paramagnetic, 2DP n‐type semiconductors with measurable bulk electronic conductivities of interest for electronic or spintronic devices.