UC Santa Barbara

Hybrid organic-inorganic perovskite (HOIP) compounds have garnered widespread research attention due to their outstanding optoelectronic properties while being solution-processable semiconductors. When used as the active material in a photovoltaic device, three-dimensionally connected HOIPs based on methylammonium lead iodide (CH₃NH₃PbI₃) and associated alloys have demonstrated power conversion efficiencies that have increased at an unprecedented rate, from an initial value of 3.8%, to present day efficiencies exceeding 25% in just a decade. This has made perovskites competitive with existing thin film solar cell technologies like CdTe and CIGS. Additionally, dispersions of nanoparticles of similar perovskite materials have shown controllable optical emission properties with narrow emission linewidths and large (~95%) photoluminescence quantum yields, making them attractive materials for light emitting applications. However, three-dimensionally connected perovskites typically suffer from poor ambient stability, impeding their feasibility in devices for long term use.

Numerous recent research efforts have shown that lower dimensional perovskites, notably two-dimensional layered structures, possess increased ambient material stability. Layered perovskite compounds correspond to structures in which the 3D perovskite structure has been sliced along a specific crystallographic direction, leading to a loss of connectivity in one dimension. Therefore, champion photovoltaic devices made with layered perovskites have only achieved efficiencies around 15% due to the loss of charge transport in one direction, presenting a fundamental tradeoff between stability and performance. Understanding the optoelectronic and structural properties of layered perovskites is therefore essential for both optimizing device efficiencies and proposing design rules for future materials discovery.

Layered perovskite compounds can be generated through two strategies -- partial anion or partial cation substitution during synthesis. This report will discuss the synthesis and characterization of three different systems -- one generated through the partial anionic substitution of iodide (I^-) anions with the pseudohalide thiocyanate (SCN^-), and two by partial cationic substitution of methylammonium (CH₃NH₃⁺) with butylammonium (C₄H₉NH₃⁺) and 2-(4-biphenyl)ethylammonium (C₆H₅C₆H₄CH₂CH₂NH₃⁺), respectively. Structural characterization with X-ray and electron diffraction reveals the difficulties in producing phase pure samples from solution processing, and the origin and impact of these impurities on electronic properties will be discussed. Transport properties were measured with contactless time-resolved microwave conductivity measurements and compared with values seen in 3D perovskites. We find that the layered perovskite systems possess similar in-plane mobilities to their three-dimensional counterparts, but with increased higher-order carrier recombination, resulting in shorter lifetimes.