Controlling internal and external interfaces in 2D perovskites to overcome intrinsic anisotropy of charge transport in solar cells

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The joint project Köhler-Thelakkat focusses on the question of how to control and manipulate the dimensionality of perovskites by modification of the internal interfaces as well as the extraction of charges through modification of the external perovskite-hole-transport layer interface. The aim is to obtain solar cells that are environmentally stable and exhibit improved vertical charge transport towards the electrodes.In the first part, we address the issue of anisotropy of charge transport in 2D perovskites, since these highly stable layered materials suffer from poor vertical charge transport due to isolating organic interlayers. To overcome the lack of charge percolation through the organic layers, we will synthesize and incorporate organic semiconductor ammonium cations belonging to the class of diketopyrrolopyrroles (DPPs) that fit within the 2D layered perovskite crystalline structure and thus contribute to charge transport and absorption. Particularly the molecular energy levels and HOMO-LUMO gap of these DPP ammonium cations will be tailored relative to the band gap of the inorganic layer. With this, we can address the fundamental question how the electronically active organic layer modifies the quantum well structure, and thus absorption and subsequent energy or charge transfer. In the second part, we address the issues concerning the external interface between perovskite and the p-type extraction layer. Here we envisage the synthesis of novel doped p-type extraction layers by co-evaporation of diverse direct redox dopants and hole conductors in order to control the degree of doping, to avoid uncertain air-oxidation and to guarantee uniform distribution of dopants in hole conductor. This can facilitate the use of less amounts of dopants and a defined interface with improved charge extraction. A well-controlled solvent-free preparation of p-type layers also facilitates the study of spectroscopic features and energetics of such a doped semiconductor material as a function of the degree of doping. We investigate whether the dopants impact on the width of the DOS, trap-filling, Fermi-energy formation, the nature of charge transport (pseudo-percolation) and the resulting charge carrier mobility.Both the novel 2D perovskites and p-type layers will be incorporated in a p-i-n structure of solar cell to evaluate and understand the implications of our innovative approach. The photophysics addresses interactions of excitation energies between organic and inorganic layers as well as the synergy of charge transport and charge extraction in final solar cell devices. We ask whether improved charge extraction by the doped p-type layer indeed reduces recombination in the 2D perovskite film, thus increasing device efficiency, and whether these novel 2D perovskite material improves the device lifetime.