Lead-free organic-inorganic azetidinium alternating metal cation bromide: [(CH2)3NH2]2AgBiBr6, a perovskite-related absorber.

In the last decade, organic-inorganic hybrid halide perovskite materials have developed into a very large research area in photovoltaics and optoelectronics as promising light harvesters. Lead-free double perovskites have recently been investigated as an environmentally friendly alternative to the lead-containing compositions. However, lead-free organic-inorganic hybrid halide double perovskites have so far rarely been produced due to a certain complexity in their synthesis. A number of small molecular cations have been investigated, but compositions containing azetidinium, which is a 4-membered heterocyclic molecular ring, on the A-site have hardly been considered. This study investigates the potential of [(CH2)3NH2]2AgBiBr6 as an optical absorber in photovoltaics or optoelectronics. The use of this alternative cation changes the crystal symmetry significantly. Columns of alternating metal cation form which are separated by the organic ions. While crystal symmetry is rather different from the perovskites, the overall properties as an absorber are similar. It is thus worthwhile to further investigate alternate hybrid compositions which form into other symmetries than the perovskite base structure.

Vapor-Phase Formation of a Hole-Transporting Thiophene Polymer Layer for Evaporated Perovskite Solar Cells.

Homogeneous layer formation on textured silicon substrates is essential for the fabrication of highly efficient monolithic perovskite silicon tandem solar cells. From all well-known techniques for the fabrication of perovskite solar cells (PSCs), the evaporation method offers the highest degree of freedom for layer-by-layer deposition independent of the substrate's roughness or texturing. Hole-transporting polymers with high hole mobility and structural stability have been used as effective hole-transporting materials (HTMs) of PSCs. However, the strong intermolecular interactions of the polymers do not allow for a layer formation via the evaporation method, which is a big challenge for the perovskite community. Herein, we first applied a hole-transporting terthiophene polymer (PTTh) as an HTM for evaporated PSCs via an in situ vapor-phase polymerization using iodine (I2) as a sublimable oxidative agent. PTTh showed high hole mobility of 1.2 × 10-3 cm2/(V s) and appropriate energy levels as HTM in PSCs (EHOMO = -5.3 eV and ELUMO = -3.3 eV). The PSCs with the in situ vapor-phase polymerized PTTh hole-transporting layer and a co-evaporated perovskite layer exhibited a photovoltaic conversion efficiency of 5.9%, as a proof of concept, and high cell stability over time. Additionally, the polymer layer could fully cover the pyramidal structure of textured silicon substrates and was identified as an effective hole-transporting material for perovskite silicon tandem solar cells by optical simulation.

Semi-Transparent Perovskite Solar Cells with ITO Directly Sputtered on Spiro-OMeTAD for Tandem Applications.

Perovskite silicon tandem solar cells have the potential to overcome the efficiency limit of single-junction solar cells. For both monolithic and mechanically stacked tandem devices, a semi-transparent perovskite top solar cell, including a transparent contact, is required. Usually, this contact consists of a metal oxide buffer layer and a sputtered transparent conductive oxide. In this work, semi-transparent perovskite solar cells in the regular n-i-p structure are presented with tin-doped indium oxide (ITO) directly sputtered on the hole conducting material Spiro-OMeTAD. ITO process parameters such as sputter power, temperature, and pressure in the chamber are systematically varied. While a low temperature of 50 °C is crucial for good device performance, a low sputter power has only a slight effect, and an increased chamber pressure has no influence on device performance. For the 5 × 5 mm2 perovskite cell with a planar front side, a 105 nm thick ITO layer with a sheet resistance of 44 Ω sq-1 allowing for the omission of grid fingers and a MgF2 antireflection coating are used to improve transmission into the solar cells. The best device achieved an efficiency of 14.8%, which would result in 24.2% in a four-terminal tandem configuration.

Novel Low-Temperature Process for Perovskite Solar Cells with a Mesoporous TiO2 Scaffold.

The most efficient organic-inorganic perovskite solar cells (PSCs) contain the conventional n-i-p mesoscopic device architecture using a semiconducting TiO2 scaffold combined with a compact TiO2 blocking layer for selective electron transport. These devices achieve high power conversion efficiencies (15-22%) but mainly require high-temperature sintering (>450 °C), which is not possible for temperature-sensitive substrates. Thus far, comparably little effort has been spent on alternative low-temperature (<150 °C) routes to realize high-efficiency TiO2-based PSCs; instead, other device architectures have been promoted for low-temperature processing. In this paper the compatibility of the conventional mesoscopic TiO2 device architecture with low-temperature processing is presented for the first time with the combination of electron beam evaporation for the compact TiO2 and UV treatment for the mesoporous TiO2 layer. Vacuum evaporation is introduced as an excellent deposition technique of uniform compact TiO2 layers, adapting smoothly to the rough fluorine-doped tin oxide substrate surface. Effective removal of organic binders by UV light is shown for the mesoporous scaffold. Entirely low-temperature-processed PSCs with TiO2 scaffold reach encouraging stabilized efficiencies of up to 18.2%. This process fulfills all requirements for monolithic tandem devices with high-efficiency silicon heterojunction solar cells as the bottom cell.