Hybrid Organic Inorganic Perovskites (HOIPs) have attracted a lot of attention since in 2009 a thin-film solar cell was produced with a PCE of almost 4%. Since then record after record in PCEs of perovskite solar cells using so-called 3D hybrid perovskites has been broken, reaching nowadays a PCE of more than 25%. The main drawback relates to the limited stability of performances by defects and/or ion mobility in the inorganic lattice. In an effort to improve stability and reduce hysteresis of the solar cells quasi 2D hybrid perovskites have been explored, quite often using besides methylammonium cations larger cations, e.g. butylammonium or more often phenylethylammonium cations. In recent years our research group is exploring the use of more complex and potentially functional larger cations composed of pyrene or carbazole subunits. We could demonstrate a substantial improvement of the thermal stability and under high relative humidity (77%) of active layers for perovskite solar cells composed of such quasi 2D hybrid perovskites. In this contribution, we discuss the basic mechanism of this stability effect. Furthermore, we present some recent results obtained using oligothiophene and a benzothieno[3,2-b]benzothiophene (BTBT) alkylammonium cation into the organic layer of a 2D layered lead iodide perovskite. Structural characterization, phase stability, and photoconductivity measurements of (n=1) and quasi 2D perovskites will be presented. Extraordinary high stability was observed for such layers under thermal stress (>240°C) and under high moisture conditions. Potential mechanisms and implications for alternative structures will be discussed.
Hybrid organic-inorganic perovskites (HOIPs) have received a lot of research attention over the past decade, related to the rapid increase in the power conversion efficiency of perovskite solar cells. The materials used for solar cells are mainly three-dimensional (3D) HOIPs, with a general formula of ABX3 with A being a small monovalent organic cation, B a divalent metal ion, and X a halide anion. More recently, the related material class of 2D HOIPs, with a general formula of (A*)2BX4, is receiving increased attention by combining a generally enhanced material stability compared to 3D HOIPs with a much higher degree of compositional flexibility. 2D HOIPs can accommodate bulkier organic cations (A*) with a conjugated organic core. Depending on the relative alignment between the frontier energy levels of the organic core and the inorganic framework, energy/charge transfer between the components of the hybrid is possible. We built in a carbazole derivative as the organic cation into a 2D HOIP. Through electron paramagnetic resonance experiments combined with computational calculations, we show that excitons generated in the inorganic layer undergo charge transfer at the organic−inorganic interface, resulting in a positive polaron delocalized over several carbazole moieties. In another material system, we incorporate an organic charge-transfer complex (CTC) with a pyrene derivative as the donor and TCNQ as the acceptor into the organic layer of a 2D HOIP. Based on time-resolved spectroscopy, we show that holes are transferred to the inorganic layer upon excitation of the CTC while electrons stay localized on TCNQ acceptor molecules.
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