Passivating Defects of Perovskite Solar Cells with Functional Donor-Acceptor-Donor Type Hole Transporting Materials

Mohammad Khaja Nazeeruddin, Kasparas Rakstys, Jianxing Xia, Yi Zhang
WILEY-V C H VERLAG GMBH, 2022
Journal paper

Abstract

In this study, a series of donor-acceptor-donor (D-A-D) type small molecules based on the fluorene and diphenylethenyl enamine units, which are distinguished by different acceptors, as holetransporting materials (HTMs) for perovskite solar cells is presented. The incorporation of the malononitrile acceptor units is found to be beneficial for not only carrier transportation but also defects passivation via Pb-N interactions. The highest power conversion efficiency of over 22% is achieved on cells based on V1359, which is higher than that of spiro-OMeTAD under identical conditions. This st shows that HTMs prepared via simplified synthetic routes are not only a low-cost alternative to spiro-OMeTAD but also outperform in efficiency and stability state-of-art materials obtained via expensive cross-coupling methods.

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https://infoscience.epfl.ch/record/297463?ln=en

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Conversion of sunlight to electricity is the most plausible approach towards the generation of renewable energies. During recent decade lead halide perovskites (LHPs) have proven to be promising candidates for photovoltaic applications such as solar cells. LHPs can be made easily in laboratories around the world via solution process methods and many universities can afford to participate in their development. How-ever, LHPs suffer from instability issues and their efficiencies can be further improved. In this thesis, both instability and efficiency challenges are addressed. We showed by doping the precursor of methylammonium lead iodide (MAPbI3) perovskite with a relatively large cation of 2-Choloroethylammonium (CLEA), perovskites films with CLEA incorporated into their structures are formed. The novel perovskite is able to withstand high temperatures as high as 80°C while achieving efficiencies > 19% in MAPbI3-based perovskite solar cells (PSCs). Various approaches have been employed for the passivation of the defects in perovskite solar cells Vari-ous approaches have been employed to passivate the defects in perovskite films used in perovskite solar cells (PSCs). However, the passivation mechanism is often unclear at a molecular level and the reason for enhanced PSC performance remains elusive. Here, we describe passivation based on the deposition of two dimensional (2D) perovskites (obtained from para-halophenylethylammonium iodide) on the surface of perovskite films. Moreover, the mechanism of passivation was elucidated and was traced to the high work function of the 2D perovskites, which induce band-bending at the surface of perovskite film and therefore reduces the charge carrier recombination at the interface of perovskite and the hole transporting material (HTM). Efficiencies >22% with average voltages >1.15 V were achieved using a typical mesoporous-TiO2 and Spiro-OMeTAD device configuration. This study paves the way for PSCs efficiency improvement by rational design. The employment of 2D perovskites is a promising approach to tackle the stability and voltage issues inherent in perovskite solar cells. It remains unclear however whether other perovskites with different dimensionalities have the same effect on efficiency and stability. Here, we report the use of quasi-3D azetidinium lead iodide (AzPbI3) as a secondary layer on top of the primary 3D perovskite film that results in significant improvements in the photovoltaic parameters. Remarkably, utilization of AzPbI3 leads to the passivation resulting in a power conversion efficiency (PCE) of 22.4 %. The open-circuit voltage obtained is as high as 1.18 V, which is among the highest reported to date for single-junction perovskite solar cells, corresponding to a voltage deficit of 0.37 V for a bandgap of 1.55 eV. Studying the compositional instability of mixed ion perovskites under light illumination is important to understand the mechanisms underlying their efficiency and stability. However, current techniques are limited in resolution and are unable to deconvolute minor ion migration phenomena. Here, we describe a method that enables ion migration to be studied allowing different segregation mechanisms to be elucidated. We applied statistical analysis to cathodoluminescence data to generate compositional distribution histograms. Using these histograms, we identified two different ion migration phenomena, horizontal ion migration (HIM) and vertical ion migration (

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EPFL2021