Self-assembly monolayers boosting organic-inorganic halide perovskite solar cell performance
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Lijian Zuoa) Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195, USA (Received 29 September 2017; accepted 11 December 2017)
Organic–inorganic halide perovskite solar cells (OIHPSCs) offer a fantastic opportunity to harness solar energy in a low cost and efficient way. This ambition for commercialization has been greatly encouraged by the surge in device performance from 3.8% in 2009 to the state-of-the-art 22.7%. For high device performance, tailoring the interfacial properties is demonstrated essentially important. Being in a molecular scale, the self-assembly monolayers (SAMs) are proved a facile but effective tool for interface modification. And lots of studies have demonstrated that SAMs have a variety of positive effects for perovskite solar cells, including mediating the morphology, improving energy level alignment, passivating trap states, etc. In this mini review, we give an insightful summary on the recent application of SAMs in OIHPSCs, analyze the mechanisms to improve device performance, and provide guidance to SAM-boosted perovskite solar cells for high performance and practical application. Finally, a landscape is depicted for future application of SAMs in perovskite solar cells.
I. INTRODUCTION
Organic–inorganic halide perovskite solar cells (OIHPSCs) show promise as renewable energy sources in the future.1–4 A distinct feature with OIHPSCs is their combined merits of low-cost solution processability and extraordinary optoelectronic properties, including long carrier lifetime,5 large absorbing coefficient,6 high tolerance for trap states,7 and high dielectric constant.4,8,9 Within a quite short time span, the device performance of perovskite solar cells was rapidly improved from 3.8% (2009)10 to 22.7%,11 as a result of intensive research on compositions,12,13 device architecture designs,14 morphologies,15,16 and interfacial engineering.17 The structure of the perovskite active layers can be described as ABX3, where A is a cation, e.g., methylammonium (MA1), formamidinium (FA1), Cs1, etc., B is a metal cation, e.g., Pb21, Sn21, etc., and X is an anion, e.g., I, Br, Cl, SCN, etc.12,18,19 To form a stable threedimensional (3D) structure, the selection of component ions can be predicted by the Goldschmidt tolerance factor, t, RX þ RA t ¼ pffiffiffi 2ðRX þ RB Þ
Contributing Editor: Sam Zhang a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.477
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where RA, RB, and RX are the radius of A, B, and X ions. It was revealed that a cubic perovskite is formed when 0.9 , t , 1, while a hexagonal or tetragonal structure is formed with t .1 and an orthorhombic or rhombohedral structure is formed with 0.71 , t , 0.9.20 It has been demonstrated that the perovskite film can form through the self-assembly of the mixed precursors via annealing,21,22 indicating a dynamically or kinetically stable process for the formation of the perovskite structure. In the organic–inorganic halide perovskite film, the organic cations determine the sel
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