Room Temperature Ligand-engineered Perovskites for Improving the Solar Cell Performance and Stability

Professor Choy, Wallace Chik Ho   (Principal Investigator (PI))

Professor Yan Yanfa   (Co-Investigator)

Professor Jen Alex K.-Y.   (Co-Investigator)

42

2018-01-01

875000

Room Temperature Ligand-engineered Perovskites for Improving the Solar Cell Performance and Stability

ligand treatment, Perovskite solar cells, post-device treatment, room temperature, solution process

Photonics

Engineering (E)

17204117

General Research Fund (GRF)

2017

Completed

1 Achieving a new scheme of room-temperature ligand-promoted formation of high-quality organic-inorganic hybrid/ all-inorganic perovskites. We will demonstrate a new scheme of room-temperature ligand-promoted formation of perovskites (e.g., CH3NH3 PbxSn1-xI3, HC(NH2)2CH3NH3PbxSn1-xI3, and CsPbxSn1-xI3) featuring pinhole-free, impurity-free and high crystalline structures. The scheme includes two stages: (I) ligand-induced formation of nanostructure-based mixed MX2(L)y complex films (M= Pb, Sn, Bi or alloy; L=ligand for perovskite formation; X = I, Br, Cl, or mixtures, 0≤y≤ 2) from MX2 film and ligand vapors at room temperature that favors the perovskite transformation; (II) ligand-facilitated formation of perovskite film through ligand exchange reactions between MX2(L)y complex and organic ammonium halide or inorganic halide salts with advantage of ligand exchange at room temperature. Using our method for the crystallization of perovskite films at room-temperature, we aim to construct high-performance PSCs that achieve a power conversion efficiency (PCE) toward 20% (the current best PSCs based on perovskite fabricated at room temperature have a PCE of <16%). This room-temperature scheme can (i) avoid the unexpected generation of defects/trap states caused by thermal annealing; and (ii) allow all steps of device fabrication to take place at room temperature (through leveraging with other room-temperature solution-processed layers in the multilayered PSC structure) toward the realization of large-scale and low-cost production of rigid and flexible PSCs; 2 Demonstrating a new approach of post-device ligand-induced regional modification of perovskites for improving PSC stability. We propose a new approach for the post-device ligand treatment of perovskites using ligand vapors to chemically modify the non-operated regions of the perovskite layer in fabricated PSC devices to improve their stability. We will (I) realize ligand-enhanced PSC stability for different perovskites, (II) identify the origins of the improved moisture stability, and (III) investigate the changes of electrical and optical properties of PSCs due to the ligand treatment. Consequently, the post-device treatment can improve PSC stability and reduce the risk of introducing unwanted impurities during the fabrication, and can be easily adopted to other perovskite devices. We also aim to demonstrate that the treated PSCs (without encapsulation) will retain 90% of the initial efficiency after 500hrs of the standard moisture stability test; 3 Theoretically understanding the mechanisms of ligand-promoted high-quality perovskite formation and post-device ligand-induced perovskite modification. We will elucidate the mechanism of ligand-enhanced high-quality perovskite formation through thermodynamics and kinetics studies. Thermodynamically, we will study and minimize the formation enthalpy of perovskite from MX2(L)y complex (∆Hf) to achieve the spontaneous perovskite formation from MX2(L)y complex. Kinetically, we will study the formation rate of perovskite by investigating the activation energy (Ea) of perovskite formation via different pathways. We will also elucidate the mechanism of post-device ligand-induced perovskite modification through thermodynamics and molecular dynamic studies. Thermodynamically, we will minimize the formation enthalpy of MX2(L"")y complex from perovskite (∆Hd) where L"" is the ligand for post-device treatment to realize spontaneous MX2(L"")y formation. We will also study the adsorption energy, diffusion pathways, and diffusion barriers of water molecules in MX2(L"")y complex for understanding the improved PSC stability; 4 Establishing ligand selection rules for high-quality perovskite formation and stability-improved perovskite modification. For the ligand-promoted perovskite formation, we will establish ligand selection rules through (i) understanding the reactivity of various ligands (obtained from Objective 3) on the formation kinetics; and (ii) investigating the effects of the ligand properties (e.g., solubility, molecular size, and volatility) on the morphology and film quality of perovskites. For the post-device ligand treatment, we will identify the rules through (i) systematically analyzing the reactivity of different ligands (obtained from Objective 3), and (ii) understanding the effects of the water-proofing properties of MX2(L"")y complex films and the ligand properties on the moisture stability of PSCs. ***Note: as proof of concept and to show feasibility of the project, we have conducted preliminary studies on the ligand-enhanced perovskite formation and the post-device ligand-induced perovskite modification. By using the CH3NH3PbI3 perovskite formed using our room temperature approach, we fabricated PSCs that could reach PCE > 17% (see ""Research Plan and Methodology"" for details).