Nanoengineering of perovskite for optoelectronic applications

Abstract

Organic-inorganic lead halide perovskite is one of the most promising optoelectronic semiconductors for alleviating the global energy crisis. Demonstrated perovskite optoelectronic devices include perovskite solar cells, light emitting diodes (LEDs), lasers, visible light and X-ray detectors. The performance of perovskite optoelectronic devices is intimately related to the morphology and nanostructure of perovskite. In this dissertation, we propose several approaches to fabricate perovskite planar thin film, nanograting, nanoplate, and quantum dot for achieving high performance perovskite solar cells and LEDs. Each work is summarized below. 1. Controlled solid-gas reaction approach is proposed to fabricate smooth methylammonium lead triiodide (MAPbI3) thin films. Thermally evaporated methylammonium iodide (MAI) gases react with solid lead iodide (PbI2) thin films and produce MAPbI3 thin films. The MAPbI3 thin films are found to be dependent on the reaction temperature of MAI and PbI2 as well as the evaporation rate of MAI. Under optimal condition, we have obtained smooth MAPbI3 thin films with roughness of 7.37 nm. Using 250 nm smooth MAPbI3 thin films as light absorbing material, we have achieved MAPbI3 solar cells with a power conversion efficiency (PCE) of 10%. This work contributes to the understanding of perovskite formation and crystal growth as well as its influence on perovskite solar cells. 2. Perovskite nanograting is fabricated by solid-liquid-solid phase transformation approach. Solid MAPbI3 thin films react with methylamine (MA) gases to form liquid intermediates MAPbI3.xMA. These liquid intermediates replicate the nanopattern in polydimethylsiloxane (PDMS) mold and maintain the nanopattern after being converted back to solid MAPbI3. MAPbI3 nanograting (size of 17 mm 17 mm) with periods of 735 nm and 1500 nm is demonstrated using this approach. Besides, the obtained nanograting shows improved absorption and photoluminescence (PL) intensity compared with pristine planar MAPbI3 thin film. Using this MAPbI3 nanograting as near-infrared emission material, we have achieved a radiance of 0.53 W/(sr.m2) in nanostructured MAPbI3 LEDs, which is around twice to that (0.29 W/(sr.m2)) in planar MAPbI3 LEDs. This work contributes to fabrication of large-area perovskite periodical nanostructures with different configuration and the improvement of device performance by nanostructures for practical applications of perovskite LEDs. 3. We demonstrate the first all-perovskite white LEDs. The white electroluminescence (EL) is achieved by red PA2CsPb2I7-interlayer-cyan CsPb(Br,Cl)3 emission architecture. The cyan and red colors are provided by CsPb(Br,Cl)3 quantum dot and novel two-dimensional (2D) PA2CsPb2I7 nanoplates, respectively. The perovskite white LEDs exhibit peak radiance of 0.19 W/(sr.m2), commission internationale de l'éclairage (CIE) (0.32, 0.31), and correlated color temperatures (CCT) of 6139 K at 9 V. Furthermore, the perovskite LEDs exhibit stable white emission with CIE (0.311  0.011, 0.307  0.008) under bias from 6.4 V (2.39 mA/cm2) to 10.2 V (66.94 mA/cm2). Besides the all-perovskite white LEDs, an interesting feature of color tunability in novel two-dimensional (2D) PA2CsPb2I7 LEDs via controlling synthesis temperature of PA2CsPb2I7 thin films has been investigated in detail. Consequently, this work contributes to practical applications of perovskite LEDs in lighting and displaying areas.