Perspective pubs.acs.org/JPCL
Surface and Interface Aspects of Organometal Halide Perovskite Materials and Solar Cells Luis K. Ono and Yabing Qi* Energy Materials and Surface Sciences Unit (EMSS), Okinawa Institute of Science and Technology Graduate University (OIST), 1919-1 Tancha, Onna-son, Okinawa 904-0495, Japan ABSTRACT: The current challenges (e.g., stability, hysteresis, etc.) in organometal halide perovskite solar cell research are closely correlated with surfaces and interfaces. For instance, efficient generation of charges, extraction, and transport with minimum recombination through interlayer interfaces is crucial to attain high-efficiency solar cell devices. Furthermore, intralayer interfaces may be present in the form of grain boundaries within a film composed of the same material, for example, a polycrystalline perovskite layer. The adjacent grains may assume different crystal orientations and/or have different chemical compositions, which impacts charge excitation and dynamics and thereby the overall solar cell performance. In this Perspective, we present case studies to demonstrate (1) how surfaces and interfaces can impact material properties and device performance and (2) how these issues can be investigated by surface science techniques, such as scanning probe microscopy, photoelectron spectroscopy, and so forth. We end this Perspective by outlining the future research directions based on the reported results as well as the new trends in the field.
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been a common practice to ensure efficient charge extraction from the perovskite absorber layer.35−38 It has been shown that charge recombination processes usually take place at the interfaces due to interfacial defects and/or interfacial doping.39 Furthermore, asymmetry in diffusion lengths for electrons and holes in perovskite solar cells was reported, and thus, much effort has been devoted to the development of interlayer interfacial engineering between perovskites and ETLs.35−38 For example, phenyl-C61-butyric acid methyl ester (PC61BM) has been reported to provide more efficient charge extraction from perovskites than metal oxide layers do.40 To date, a wide range of preparation methods (spin-coating, co-evaporation, vapor-assisted deposition, etc.)22−32,41−47 have been developed to deposit perovskite films. Most of these methods generate perovskite films with polycrystalline grains. Therefore, additional intralayer interfaces among adjacent grains are formed within the perovskite layer. The presence of nonstoichiometric crystals and/or impurities at the grain boundaries may affect perovskite solar cell performance. Furthermore, the unbalanced charge accumulation or depletion between grain boundaries and grain interiors can cause band bending, which affects the separation of photoexcited electron− hole pairs and charge carrier transport across the grain interfaces (referred to as intralayer interfaces hereafter) formed between grains. Therefore, it is crucial to investigate the correlation between local electronic properties and local morphologies.37,38,48−59 In this Perspective, we start describing the atomic resolution imaging of organic−inorganic perovskite (CH3NH3PbI3 and CH3NH3PbBr3) surfaces and the determination of local
rganometal halide organic−inorganic hybrid perovskite materials and solar cells have received intensive attention in the past few years.1 Tremendous progress has been made in terms of perovskite solar cell device performance. Also, applications have been rapidly expanded to other areas, such as energy storage,2 water splitting,3 gas sensors,4 thermoelectric devices,5 lighting,6−8 lasing,9−11 radiation detection,12−14 and memory devices.15,16 Owing to the unprecedented potential for high power conversion efficiency (PCE) and compatibility with low-cost fabrication, organic−inorganic hybrid perovskite-based solar cells have become one of the most attractive contenders for photovoltaic technology. The PCE for perovskite solar cells increased 10 times from 2.2% in 2006 to 22.1% as of today, which is only a few percent lower than the best singlecrystalline silicon solar cells.17−21 The structure of a perovskite solar cell is typically composed of an electron-transport layer (ETL), a mesoporous scaffold layer, a perovskite layer, a holetransport layer (HTL), and a high-work-function (WF) electrode, all sequentially deposited onto a transparent conducting substrate (e.g., fluorine-doped tin oxide, FTO). Alternative structures such as planar structure (n−i−p), HTL-free, carbon counter electrode-based solar cells have also been proposed and demonstrated to reach relatively high device performance.22−32 Another alternative structure is the inverted planar device structure, which employs a HTL, a perovskite layer, an ETL, and a low-WF metal electrode, which are sequentially deposited onto a transparent conducting substrate (e.g., indium-doped tin oxide, ITO).27,33,34 In all device structures, the different layers are all stacked together, producing multiple interfaces. The material properties at these interfaces (referred to as interlayer interfaces hereafter) will influence charge transfer across interfaces, which impacts final device performance. The incorporation of selective contacts (ETL and HTL) has often © 2016 American Chemical Society
Received: August 29, 2016 Accepted: October 28, 2016 Published: October 28, 2016 4764
DOI: 10.1021/acs.jpclett.6b01951 J. Phys. Chem. Lett. 2016, 7, 4764−4794
The Journal of Physical Chemistry Letters
Perspective
energy levels. We then discuss the investigation of energetics at the surfaces/interfaces in perovskite solar cells, perovskite film morphology and its influence on solar cell performance, surface and interface engineering in perovskite solar cells, and physicochemical properties of spiro-MeOTAD HTL studied by surface science. In the outlook, we outline the surface/interfacerelated topics that warrant further investigation. Microscopic Imaging of Organic−Inorganic Perovskite Surfaces. Surfaces and/or interfaces are phase boundaries that delimit distinguishable regions of materials. For instance, as shown in Figure 1a, in a planar device configuration22,51,61 employing
The polycrystalline nature, grain sizes, and grain boundaries of perovskite films can have a strong impact on charge generation and transport, carrier dynamics, and overall solar cell performance. Systematic studies on such surfaces/interfaces influences provide vital insight into innovation and optimization of perovskite optoelectronic devices. technique,51,60 which clearly reveals additional distinctive interfaces between the grains with different crystal orientations. The polycrystalline nature, grain sizes, and grain boundaries of perovskite films can have a strong impact on charge generation and transport, carrier dynamics, and overall solar cell performance. Systematic studies on such surfaces/interfaces influences provide vital insight into innovation and optimization of perovskite optoelectronic devices. Figure 1b−d shows high-resolution TEM images of MAPbI3 perovskite films that were directly deposited on TEM grids using the solvent−solvent extraction method.60 Interplanar spacings of ∼0.64, ∼0.44, and ∼0.32 nm were assigned to (110) or (002), (020), and (004) planes, respectively, in the β-MAPbI3 phase (tetragonal structure with lattice parameters of a = b = 8.849 Å, c = 12.642 Å). These assignments were further confirmed by selected area diffraction patterns consistent with the β-MAPbI3 with an I4/mcm space group.63 Xiao et al.64 presented atomically resolved TEM images within the MAPbI3 grain and measured the lattice fringe spacing of 0.31 ± 0.01 nm indexed as (004) or (220) of the β-MAPbI3 phase. Baikie et al.65 also reported atomically resolved TEM images on a slice of single-crystal MAPbI3, which showed a highly ordered honeycomb structure, but they were unable to identify unambiguously the phase structure possibly due to complications resulting from the surface reconstruction phenomena. In Table 1, we summarize the different MAPbI3 crystal structures investigated mainly by X-ray diffraction (XRD).66,67 As seen from Table 1, there are conflicting results regarding the reported space groups for the same perovskite material (e.g., MAPbI3).68 Further insights into the lattice parameters and structures extracted from scanning tunneling microscopy (STM) studies are described in the next section. The MAPbI3 perovskite material widely studied for solar cell applications has a tetragonal structure at room temperature (RT, Table 1). It is also known to undergo a structural phase transition from the tetragonal phase to an orthorhombic phase below 161.4− 162.2 K and to a cubic phase above 327.4−330.4 K.69,70 Atomic Resolution Imaging of Perovskite Surfaces by Scanning Tunneling Microscopy. Scanning probe microscopy (SPM) techniques allow real-space visualization of surface morphologies and dynamics. In particular, the invention of STM and atomic force microscopy (AFM) allowed one to achieve true atomic-scale imaging of surfaces as well as to visualize physicochemical dynamical processes.76−82 In STM measurements, a sharp metallic tip raster-scans across the sample surface. The sample−tip distance is small (178.8 172.9−178.8 236.9 155.1−236.9 149.5−155.1 327.4 162.2−327.4
