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The term defect tolerance is widely used in literature to describe materials such as lead-halides perovskites, where solution-processed polycrystalline thin films exhibit long non-radiative lifetimes of microseconds or longer. Studies...

The most peculiar property, however, is an excellent luminescence quantum efficiency [4][5][6][7][8][9][10] that substantially exceeds that of solution processed polycrystalline films made of other material families. It is this last property that has started extensive investigations into the defect tolerance [11][12][13][14][15] of the material class and has enabled highly efficient light emitting diodes as well as solar cells with open-circuit voltages [7,[16][17][18] that so closely approach the radiative limit that they are only overcome by monocrystalline GaAs solar cells. [19] Halide perovskite devices so far have been made by employing a large library of contact materials [20][21][22][23][24] borrowed from organic solar cells, organic LEDs and dyesensitized solar cells. The key concept of device making is based on the idea that the actual light absorbing or emitting layer is fairly intrinsic and that electron and hole injection or extraction has to be ensured by contacts with suitable work functions, electron affinities and ionization potentials. [25] The classical approach of doping the active layer as done in Si, III-Vs or other inorganic semiconductors has so far only rarely been pursued and currently there is no evidence that a functional perovskite-based pn-junction [26] solar cell or LED can be made using the classical approach. Nevertheless, unintentional doping via shallow intrinsic point defects is an important topic of research, [27] triggered to a large degree by the evidence for mobile ionic charges [28,29] that lead to reversible transient effects and features like the JV curve hysteresis. [29][30][31][32] Mobile ions may appear in Frenkel pairs (i.e., for each positive ion there would be a negative ion) without actually doping the semiconductor by creating an excess of one type of charge. Therefore, it is not entirely obvious whether halide perovskites should be considered as doped semiconductors. Given the typical thicknesses and permittivities, a halide perovskite solar cell would be considered doped from doping densities of roughly >10 16 cm -3 as found in ref [32]. There is clear evidence in the most frequently studied composition methylammonium lead-iodide (MAPI) that the material behaves like an intrinsic semiconductor [33] with extremely low bulk charge densities (<10 12 cm -3 ) measured in thick crystals. [2] Furthermore, in transient photoluminescence of thin films of MAPI, [8] the signature of quadratic radiative recombination has been observed [8] down to (low) charge densities of 10 14 cm -3 , suggesting doping densities that must be even lower. There are a variety of Mott-Schottky Most traditional semiconductor materials are based on the control of doping densities to create junctions and thereby functional and efficient electronic and optoelectronic devices. The technology development for halide perovskites had initially only rarely made use of the concept of electronic doping of the perovskite layer and instead employed a variety of different contact materials to create function...