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Morphology Control of Photoactive Layer for Highly Efficient Perovskite solar cells

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Title
Morphology Control of Photoactive Layer for Highly Efficient Perovskite solar cells
Author
Jeong, Jaeki
Advisor
Kim, Jin Young
Issue Date
2019-02
Publisher
Graduate School of UNIST
Abstract
Considerable effort to develop renewable energy sources has been expended over the last several decades, leading to the demonstration of several new classes of highly efficient photovoltaic cells. organic, inorganic and hybrid light absorbers such as organic bulk heterojunctions, colloidal quantum-dots and dye-sensitized metal oxide devices have been demonstrated as next generation photovoltaic materials. Among them, hybrid organic-inorganic perovskite materials have attracted substantial attention as photovoltaic light absorbers due to their outstanding electrical and optical properties. Since the processing for obtaining compact and uniform perovskite photoactive layer has intensively studied last few years to achieve high PCEs in solar cells, recent PCEs of perovskite solar cells(PeSCs) have exceeded 23% which is approaching those of commercialized PVs. These achievements have been marked by a constant improvement of deposition techniques by understanding of the crystallization processes. In this perspective, here, we demonstrate several deposition techniques which has a great influence on the crystallization kinetics. First, p-i-n structure PeSCs by optimizing the morphology of perovskite films via solvent mixtures in which all layers are deposited with solution processing at low temperature. we employ a simple device configuration of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):polystyrene sulfonic acid (PEDOT:PSS)/CH3NH3PbI3 perovskite/PCBM/aluminum (Al). Improved morphology is obtained in the perovskite film deposited from mixed solvents, leading to improved exciton dissociation efficiency and reduced recombination losses at the interface between perovskite and PCBM and improvements in device efficiency to over 6%. Second, we employed cesium-doped methylammonium lead iodide perovskites (CsXMA1-XPbI3) photoactive layer to improve the performance of planar heterojunction PeSCs. The CsXMA1-XPbI3 perovskite with optimized 10% Cs doping concentration remarkably improves device efficiency from 5.51% to 7.68% due to increases in short-circuit current density and open-circuit voltage by improving light absorption at optimum device thickness and morphology of perovskite film, and widening the energy difference between energy difference between the valence band of the perovskite and low unoccupied molecular orbital level of PCBM Third suggestion is a bridged ternary halide approach to process materials with the formula MAPbI3-y-xBryClx which yields high PCEs in planar, p-i-n type heterojunction PeSCs. This ternary halide perovskite system improves device performance from 12% to 16% when an optimal concentration of 10% Br is incorporated into the binary Cl – I systems, via increases in short-circuit current density (JSC), open-circuit voltage(VOC) and fill factor(FF) which arise from the formation of homogeneous crystal domains and a subtle widening of the optical band gap. Remarkably, the ternary halide perovskite devices exhibited approximately 100% internal quantum efficiency (IQE) throughout their entire absorption range (400~800 nm). Furthermore, high quality crystal growth of perovskite layer is critical point to enhance device performance. An easy and effective new process for high efficiency p-i-n planar heterojunction structure of PeSCs by handling the compact seed perovskite layer (CSPL). The CSPL assists vertical growth of perovskite crystal and obtains the high crystalline perovksite photoactive layer which leads to the reduction in the charge transfer resistance and longer photoluminescence lifetime. PeSC device with CSPL shows the remarkably improved PCEs from 15.07% to 19.25% with VOC of 1.16 V in p-i-n structure with pure crystal perovskite and negligible current density-voltage hysteresis. And 20.37% was achieved with CSPL assisted n-i-p structure PeSCs Finally, Lightweight and flexible photovoltaic devices have attracted great interest for specific potential applications, such as miniaturized drones, blimps, and aerospace electronics. This study aims to demonstrate ultralight and flexible perovskite solar cells (PSCs) with orthogonal silver nanowire (AgNW) transparent electrodes fabricated on 1.3-μm-thick polyethylene naphthalate foils. The smooth surface morphologies of the orthogonal AgNW transparent electrodes help prevent nonconducting silver halide formation generated by chemical reaction between the AgNWs and iodine in the active layer. The resultant PSCs with orthogonal AgNW transparent electrodes exhibit substantially improved device performance, achieving a power conversion efficiency (PCE) of 15.18%, over PSCs with random AgNW network electrodes (10.3% PCE). Moreover, ultralight and flexible PSCs with the orthogonal AgNW electrodes exhibit an excellent power-per-weight of 29.4 W·g−1, which is the highest value reported for a lightweight solar cell device. These lightweight energy harvesting platforms can be further expanded for various wearable optoelectronic devices.
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Department of Energy Engineering
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