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Recent Progress in the Solution-Based Sequential Deposition of Planar Perovskite Solar Cells Markus Becker, and Michael Wark Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00686 • Publication Date (Web): 27 Jun 2018 Downloaded from http://pubs.acs.org on June 29, 2018

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Crystal Growth & Design

Recent Progress in the Solution-Based Sequential Deposition of Planar Perovskite Solar Cells Markus Becker and Michael Wark* Institute of Chemistry, Chemical Technology 1, Carl von Ossietzky University Oldenburg, Carl-von-Ossietzky Str. 9-11, 26129 Oldenburg, GER. KEYWORDS: Perovskite solar cells; Planar heterojunction configuration; Sequential deposition; Process modification; Film quality

Abstract: Hybrid organic-inorganic metal halide perovskites have probably become the most famous and prospective class of materials for photovoltaic application in the last decade. The extremely fast increase of certified power conversion efficiencies (PCE), from 3.8 to 22.1 % in less than seven years, encourages researchers around the globe and indicates the materials advantageous optoelectronic properties. The most important properties for future commercialization have turned out to be the low cost of precursor materials and the simplicity of solution processability. This article reviews the latest advancements in the solution-based sequential deposition of planar perovskite absorber layers, which show promise to be multifaceted, reproducible and controllable. Strong research interest has recently been devoted to the optimization of the surface coverage and morphology, both of which considerably influence the photovoltaic performance.

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Herein, we first concentrate on the deposition of lead-containing precursors from typical polar solvents and review modifications via additive incorporation and pre-/post-treatment. Afterwards, innovative methods for the conversion reaction in the usually alcoholic solvent are summarized and the influence on the final crystal morphology is highlighted. We conclude with a short discussion about future directions and challenges.

Introduction: The prime energy demand of the worlds society is still covered via fossil fuels, which are characterized by their exceptional high power-contents, quite ease of exploitation and technical versatility. However, its formation took millions of years and the availability is limited. Moreover, the burning of fossil fuels is responsible for the climate change, mainly due to an exorbitant release of the ecologically harmful waste product “CO2”. Among the investigated alternative energy resources with sustainable character, photovoltaics shows up to be very promising, since it directly converts sunlight (which is a practically unlimited source of power) into electricity. Its implementation is enabled via solar cells, which appear in various cell types as well as constructions and mainly dependent on the absorber material. The market dominator is still crystalline silicon with a manufacture proportion of 94 % in 20151. Nevertheless, although its base material is very abundant (quartz and/or silica) and it can be easily n- or p-doped, high power conversion efficiencies of markedly above 20 % are only achieved with monocrystalline silicon. This, in turn, has to be produced from time consuming and cost intensive re-melting processes2. In order to gain a larger market share from crystalline silicon, alternative solar cell technologies have to offer a combination of low processing costs, high photovoltaic performance and sufficient long-term stability. Recently, hybrid organic-inorganic perovskite materials have attracted key attention as they have shown to meet at least two of these conditions in an impressive manner. Intense research efforts all over the globe have enabled the extraordinary fast progress in PCE’s from 3.8 % in 20093 to certified 22.7 % in 20184. This trend not only outperforms other thin film technologies like 2 ACS Paragon Plus Environment

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e.g. dye sensitized solar cells (DSSC), organic photovoltaics (OPV) or quantum dot (QD) sensitized solar cells, but makes also clear the future potential to challenge silicon5. Simple device preparation methods together with advantageous optoelectronic properties such as high optical absorption6, long charge carrier diffusion lengths7 and small exciton binding energies8 certainly support these achievements. Beside the possibility of optimizing the interface between the photoactive phase and the charge selective contacts (i.e. in order to enhance the carrier separation)9-14, the perovskite layer itself represents the heart of the solar cell. In the planar heterojunction configuration, this layer typically exhibits several hundred nanometers of thickness and is deposited without any scaffolding material between an electron and a hole transporting material. Consequently, the perovskite exhibits ambipolar carrier transport characteristics. Its quality greatly affects the performance and it has been shown that recent advancements of multiple fabrication routes for thin films resulted in a rapid and constant increase of the PCE’s to now more than 15 %15-16. Though, these results demonstrated the high significance of a constant thickness and compact property of the absorber layer. Two cell designs are common which depend on the build-up of the stack. When the structure follows glass/transparent conducting oxide (TCO)/electron transporting material (ETM)/perovskite/hole transporting material (HTM)/metal, then the solar cell is illuminated from the n-type side17-18, whereas in the case of glass/TCO/HTM/perovskite/ETM/metal, the light entrances from the p-type side19-20. Since the perovskite deposition is typically carried out at temperatures below ~ 150°C, its application on flexible substrates has become an exciting area of research. Especially the field of OPV (actually a planar perovskite device resembles an organic solar cell) has contributed a lot of know-how in coating techniques, selective contacts and roll-to-roll facilities which can be used for perovskite-based device manufacture21-22. The flexibility not only enables a high throughput production with less processing costs, but also allows energy harvesting in 3 ACS Paragon Plus Environment


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portable devices or on curved surfaces like building-integrated photovoltaics23-24. Polyethylene terephthalate (PET) and naphthalate (PEN) are thereby frequently used flexible substrates for perovskite solar cells due to their high transmittance and low cost, though, these have to be coated with a TCO (e.g. ITO) in order to use them as transparent electrodes25. In the typical n-i-p structure, ZnO has shown the highest PCE’s of 15.96 % when applied as the ETM and was solution processable at T < 150°C26. However, a serious problem with ZnO is that perovskite materials easily decompose during thermal treatment due to the basic surface properties of the former27. Additionally, the high photocatalytic activity of ZnO under UV light gradually degrades the perovskite after extended exposure times, which results in a decrease of the performance28. TiO2 is the most widely applied ETM in planar devices and typically provides high PCE’s when sintered at high temperatures (> 450°C). Nevertheless, recent efforts are devoted to find ways for the film fabrication at milder conditions29-31. The high sintering temperatures of other typical n-type metal oxides present a general issue for flexible substrates, which can be overcome by designing inverted p-i-n planar architectures. The latter comprise materials which are all typically processed at mild conditions (e.g. PEDOT:PSS as HTM and PCBM as ETM). NiOx has been extensively studied in the field of optoelectronics as HTM contact and recent advancements in solution-based thin film deposition techniques now allow the preparation at low temperatures32-34. Planar perovskite devices meanwhile demonstrate efficiencies of more than 20 %35. The methodologies of perovskite thin film manufacture are quite analogous between plastic and solid inorganic substrates. Furthermore, although the reported performances of flexible perovskite solar cells still lack behind those prepared on glass, the use of more transparent and cheap substrates is supposed to lower this gap in the near future. One very attractive feature of hybrid perovskites is their versatile processability, ranging from simple chemical and physical solution-based techniques36-39 to mixed vapor-assisted solution 4 ACS Paragon Plus Environment

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methods40-41 and pure vapor deposition17. Vapor based methods usually result in high quality planar perovskite layers accompanied by good photovoltaic performances. However, their use seems partially impractical due to the limited gas-solid reaction which requires either prolonged reaction times (several hours) or high temperatures for full perovskite conversion40, 42

. When prepared by simple solution processing, especially at the beginning of perovskite

solar cell research, the surfaces exhibited adverse properties like uncovered voids, pin-holes and large numbers of grain boundaries. These features typically led to decreased charge carrier separation and slower transport characteristics as well as to substantial leakage currents36, 43-44. Effective ways to manipulate the nucleation and crystal growth kinetics of perovskites must be further investigated in order to avoid electrical shunting in these devices. The solution-based preparation can be divided into one-step and two-step deposition techniques. The one-pot synthesis has initially been adopted and has dominated the pioneering time of research on thin film deposition. The precursor materials, usually organic halides like methylammonium iodide (CH3NH3I, MAI) and lead halides (PbX2; X = Cl-, Br- or I-), are dissolved in a polar solvent like γ-butyrolactone (GBL), dimethylsulfoxide (DMSO) or N,Ndimethylformamide (DMF) in order to produce a mixed solution. Film preparation is then conducted via spin-coating of the solution accompanied by a subsequent heating step between 90 – 150°C. Although one-step processing allows the fabrication of devices with a wide range of MAI to PbX2 molar ratios36, 45-46, unfavorable interactions between the substrate and solution lead to substantial de-wetting behavior47. The incomplete surface coverage with needle- or island-like perovskite agglomerates results in increased amounts of shunting paths, lower light absorption and reduced performance. Only after the development of novel solvent engineering techniques, like e.g. the anti-solvent strategy48-49, the one-pot synthesis allowed the preparation of highly efficient planar perovskite solar cells.



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The two-step or “sequential” deposition method is already known since 1998 when Mitzi et al. have reported the preparation of various high and low dimensional perovskite absorber layers37. In the initial period of extensive research on perovskite solar cells (from 2009)3, Liu et al. first have demonstrated the solution-based two-step preparation of a planar perovskite solar cell39. This technique allows to gain a better control over the surface morphology and solar cells typically have shown higher PCE’s compared to devices from the one-pot synthesis. In the first step of this method, a PbI2 precursor layer is deposited via spin-coating a solution from a polar solvent like DMF. Subsequently, the substrate is either immersed into an isopropanolic solution of MAI or it is covered by the latter via spin-coating, followed by a heat treatment at around 100°C for at least 30 min (Figure 1)38-39, 44, 50-52. As has been already reviewed, the ease in control of PbI2 processing offers the possibility for the preparation of uniform and highly covered precursor layers. These, in turn, act as a template for the final coverage of the perovskite films and typically improve their quality16, 53. Shortly after these observations, which have promised enhanced device reproducibility and operability, tremendous work on the sequential deposition process has been conducted. In particular, the conversion reaction mechanism and the crystallite nucleation/growth kinetics have been investigated whereby modified deposition methods have been found that improved the surface quality and device efficiency.


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Figure 1. Schematic illustration of the general sequential deposition method for fabricating planar MAPbI3 absorber layers. The conversion of the PbI2 precursor layer is either conducted by immersing it into a MAI/IPA solution at elevated temperatures or by spin-coating the latter on top with a subsequent heat treatment. The crystal structures of PbI2 and MAPbI3 are depicted on the right side. This article tries to give a thorough review about the actual developments in the modification of the solution-based sequential deposition of planar perovskite solar cells. For that, the first section deals with the application of the lead-based precursor layer and explains how the characteristic plate-shaped morphology can be altered, via deposition techniques or compositions, in order to provide smoother surfaces and/or enhanced guest-molecule diffusion. In the second part, we concentrate on the conversion reaction, where the tailoring of reaction parameters, solvent properties or the inclusion of small additives allow the control of perovskite nucleation and growth. We conclude with a short summary and propose important issues of the two-step synthesis to further enhance the applicability on the preparation of planar absorber layers.



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Film Formation for Planar Cell Structures: As is typical for the two-step synthesis method, the PbI2 precursor is deposited via spin-coating whereas the second precursor can be introduced either by spin-coating the alcoholic MAI solution on top of the PbI2 layer or by immersing the latter into this solution. Both variants induce a solid-liquid reaction via the intercalation of MAI into the metal halide framework according to eq. (1)37:

PbI2 (s) + MAI (sol.) MAPbI3 (s)

(1)

The morphology of the final MAPbI3 layer is hence determined by the templating properties of the PbI2 precursor film, which demonstrates to significantly improve the device performance and repeatability compared to the one-pot synthesis. It is well known that the latter approach produces thermodynamic unstable and de-wetted perovskite films when directly grown from solution36. Furthermore, the two-step synthesis allows to prepare perovskite films at mild conditions enabling the application of a variety of ETM’s. Adopted from the mesoporous cell design38, Liu et al. were the first demonstrating a sequentially deposited planar heterojunction perovskite solar cell with a high performance of 15.7 %39. It was stated that the two-step process potentially reduces the chemical reaction between MAI dissolved in DMF and the underlying ZnO layer, which was expected to be due to the supply of an acidic environment. This success attracted tremendous attention and stimulated intense research due to the ease of operability. However, it has been periodically observed that the resulting surfaces are rough and exhibit a non-uniform crystallite size distribution, like it is shown in Figure 2 c). This is due to a dissolution and recrystallization behavior, called “Ostwald ripening effect”, which was recently disclosed by Yang et al. and ascribed to the conventional IPA solvent as well as to the iodine concentration54-55. Both of these factors influence the chemical reaction dynamics (Figure 2 a). The fluctuated crystal growth (Figure 2 b and c) was found to have adverse effects on the photovoltaic performance. Additionally, 8 ACS Paragon Plus Environment

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the average crystallite size was found to significantly affect the performance of planar solar cells due to alternated charge carrier lifetimes56-59. Although larger crystal dimensions resulted in improved carrier mobilities, high PCE’s were only achieved when the surface coverage was sufficiently high due to decreased recombination between the charge selective contacts as well as reduced surface recombination at the grain boundaries.

Figure 2. a) Schematic illustration of the MAPbI3 formation mechanism in the sequential deposition method via in-situ phase transformation or dissolution-recrystallization. Adapted with permission from Chem. Mater., 2014, 26, 6705−6710. Copyright 2018, American Chemical Society. b) Proposed growth mechanism of MAPbI3 from PbI2 precursor layers where a rapid surface reaction during the initial stage hinders the further diffusion of the second precursor molecules. Prolonged dipping times in high MAI/IPA solution concentrations cause dissolution-recrystallization. Adapted with permission from J. Am. Chem. Soc., 2015, 137, 5810−5818. Copyright (2018) American Chemical Society. c) Top9 ACS Paragon Plus Environment


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view SEM images of a typical MAPbI3 absorber layer grown from PbI2 and demonstrating the rough surface morphology with non-uniform crystallite size distribution. Lead Halide Precursor Layer: The PbI2 precursor material consists of facet-sharing [PbI6] octahedrons forming a hexagonal unit cell60. During the conversion reaction, MAI intercalation results in voids formed between the layered octahedral network leading to a built-up of the corner-sharing 3D structure of MAPbI316. Typically, the precursor deposition is accomplished via spin-coating a PbI2 solution in DMF at elevated temperatures of above 50°C, which is simple, low cost and efficient. The reliance of the perovskite film on the precursor morphology is one of the most useful aspects, since the PbI2 layer is typically smooth and homogeneous. However, when prepared natively, PbI2 forms a slightly porous but rather dense film of compact and plate-shaped crystallites ascribed to the ease of crystallization during solvent evaporation. Consequently, the solid-liquid interfacial reaction between MA+ and PbI2 first proceeds at the outer surfaces of the crystals. Afterwards, the formed crystalline MAPbI3 impedes the further diffusion of MA+ to the underlying PbI2, resulting in an incomplete conversion. As a consequence, too short dipping times (seconds to few minutes) yield MAPbI3-PbI2 mixtures, which must not necessarily affect the device performance but diminish the reproducibility due to inconsistent portions from batch to batch44, 61. Especially in the case of planar devices, which exhibit precursor films of several hundred nanometer thickness, a full conversion requires more than 1 h37. These prolonged dipping times, in turn, lead to increasing Ostwald ripening and sometimes to a layer peel-off from the substrate55, 62. Both factors, i.e. residual PbI2 and dissolution-recrystallization behavior of MAPbI3, result in a deterioration of device performance as well as reproducibility. Several strategies have been evolved to overcome this issue, ranging from the enlargement of the interface contact area for the precursor materials via the suppression of PbI2 crystallization


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by the addition of species coordinating the Pb2+ ions to the formation of extra smooth precursor layers to enable homogeneous nucleation. Porous PbI2 Precursor Layers: An effective route to enable the complete and/or fast conversion of PbI2 to the perovskite phase is the increase of the contact interface area between both precursor materials by means of introducing porosity. Several methods have been established to fabricate porous PbI2 layers like an additive incorporation or different PbI2 preand post-treatment. For additive-controlled crystallization, Zhang et al. introduced 4-tertbutylpyridine (4-tBP) as accessory agent into the PbI2 precursor solution and formed a porous layer via a self-assembly process (SAP-PbI2). 4-tBP is a nitrogen-donor ligand which forms a coordination complex with PbI2 (PbI2·x4-tBP) at ambient conditions. Via evaporation of the solvent and subsequent heating of the complex to elevated temperatures (~ 70°C), tBP will be released leaving behind small holes at its former positions (Figure 3 a). It was shown that the porous structure enables the fast conversion to the perovskite phase and thus, combined with an increased concentration of MAI, a smooth absorber film can be obtained. The latter demonstrated enhanced long-term stability and boosted the device efficiency from 1.51 % to 16.21 %63. Similarly, Shi et al. published results with 4-tBP as additive in PbI2 precursor solutions. They demonstrated an increase in PCE from 6.71 to 10.62 % (i.e. ~ 60 % enhancement) due to accelerated conversion reactions in randomly crystallized porous PbI2 films64. Yang et al. reported the incorporation of hydrohalide acids (HCl and HI) into PbI2 solutions. With small amounts of 2.5 vol.-% for both additives, the typical rod-shaped crystallization could be successfully inhibited. The resulting films demonstrated enhanced crystal growth with an improved homogeneity as well as porous and hexagonal nucleation characteristics (Figure 3 b). The uniformity and coverage of the final MAPbI3 films were significantly increased, which not only improved the overall stability but also gave rise to a PCE of > 15 % with HCl (a performance gain of roughly ~ 60 % compared to pristine PbI2) 11 ACS Paragon Plus Environment


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and > 12 % with HI (a PCE gain of ~ 30 %)65. Acetonitrile was applied as a weak coordinative species for PbI2 by Li et al.66. The complex interaction between DMF and PbI2 was thereby affected and the relatively low boiling point of acetonitrile helped for a quicker solvent evaporation. Both conditions resulted in a porous PbI2 precursor layer which showed increased grain sizes and an improved PCE after the conversion to MAPbI3.

Figure 3. a) Schematic illustration of the manufacture process for a conventional (c-PbI2) and a self-assembled porous (SAP-PbI2) PbI2 precursor layer. The influence of the immersion time in a 10 mg/ml MAI/IPA solution on the UV-Vis absorbance of a MAPbI3 layer prepared from c-PbI2 and SAP-PbI2 is depicted. The evolution of XRD signals with varying dipping times for perovskite films prepared from c-PbI2 and SAP-PbI2 is also reported. Adapted with permission from Adv. Energy Mater., 2015, 5, 1501354. Copyright (2018) John Wiley and Sons. b) Top-view SEM images, XRD patterns and J-V curves of perovskite layers (bottom row) prepared from pristine PbI2, PbI2 with HI and PbI2 with HCl additive (upper row). Adapted with permission from ACS Appl. Mater. Interfaces, 2015, 7, 14614−14619. Copyright (2018) American Chemical Society. Porosity can also be introduced by means of post-treating the precursor layer. Liu et al. prepared a PbI2 film via natively spin-coating from a solution in DMF, but stored the fresh 12 ACS Paragon Plus Environment

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film inside a petri dish for different times before annealing it to remove the solvent (Figure 4 a)67. They obtained mesoporous PbI2 scaffolds due to heterogeneous nucleation on the supporting layer whereby time and growth control enabled a continuous manipulation of the layer quality. The transport of MAI via the mesoporous channels was stated to be the key for improving the perovskite absorber quality and a faster and more complete conversion could be observed. Accompanied by enlarged average grain sizes, the PCE was improved from less than 3 to more than 15 % in an inverted cell structure. Zheng et al. applied a comparable procedure for a cell design which was free of the compact ETM layer and found an increase of the average PbI2 crystallite size with distinct porosity on bare ITO substrates68. However, they observed a decrease of performance with prolonged dipping times which was attributed to two detrimental effects. First, from 1 to 5 minutes, band doping was caused in the precursor as well as final perovskite layer and second, from 5 to 10 minutes, a recrystallized capping layer was formed on the MAPbI3 surface. With the aid of repeated short-dipping for 1 minute, a highest PCE of 11.4 % could be achieved which was superior compared to the device from pristine two-step synthesis. Zuo et al. similarly conducted the simple storage of a freshly spincoated PbI2 layer under a petri dish, which yielded a mesoporous structure after 15 min69. In combination with a passivating polymer-additive in a mixed MAI/MACl conversion solution, a high PCE of > 20 % was achieved. Pyridine vapor was used to post-introduce porosity into PbI2 films. The PbI2(Py)2 intermediate precursor yielded a highly crystalline and phase pure perovskite after immersion into MAI/IPA with a high PCE of 17.1 %70. An alternative vapor post-treatment was reported by El-Henawey et al., who exposed the freshly spin-coated PbI2 precursor film to N2, chlorobenzene or toluene vapors71. For that, the fumes were directed onto the top of the PbI2 layer during the rotational motion (Figure 4 b). Toluene and chlorobenzene are miscible with DMF and can reduce the evaporation rate of the latter which, together with the heat provided by the vapor, promotes the growth of PbI2 nanocrystals. Increased porosity and larger precursor crystallite sizes were obtained which allowed the easy 13 ACS Paragon Plus Environment


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penetration of MAI and resulted in a more complete conversion reaction as well as larger perovskite grains. The corresponding solar cell in the inverted configuration revealed a performance increase of nearly 100 % compared to the control device. Simple room temperature aging was additionally found to introduce porosity into PbI2 films. Zhou et al. compared different crystalline and/or porous precursors for the formation of FAPbI3 absorbers layers via the interdiffusion method72. Low crystallinity accompanied by porosity was found to be best suited for the formation of phase pure α-FAPbI3. It was stated that the increase in the local density of PbI2 during the aging process leads to a local film shrinkage and subsequently to the porosity generation. Thus, the volume expansion during the conversion reaction could be accommodated and a highest PCE of 13.8 % was achieved. Additionally, a multi-annealing step was used by Zhang et al., where the freshly deposited PbI2 film was gradually heated up to 100°C in 20 K steps for several minutes73. The resulting porous PbI2 enabled the formation of a more uniform and smooth absorber layer and the PCE could be increased from 15.5 to 17.5 % in a conventional cell. Bae et al. reported an easy preparation of a PbI2 layer with a dense bottom and a porous top surface via double spincoating, whereby the second deposition has to be done at a higher spin rate74. The conversion ratio was enhanced and the MAPbI3-xBrx absorber layer had sufficient thickness for light absorption. Compared to the pristine PbI2 precursor film, the PCE was enhanced. In-situ time resolved crystallographic and morphological investigations of PbI2 film formation from DMF solutions were performed by Barrit et al.75. It was found that crystallization occurs via a solgel process and that air drying at room temperature leads to more porosity due to the formation of several subsequent disordered and ordered solvate steps for the precursor material. The resulting precursors are expected to be better suited for conversion to the perovskite phase compared to standard post-annealed films.


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Figure 4. a) Illustration of a simple procedure for growing mesoporous PbI2 precursor layers via storage under a petri dish in a N2 atmosphere. The growth time could be precisely terminated by placing the samples on a hotplate at 70°C. The Photographs and UV-Vis absorption spectra of PbI2 layers prepared with different growth times are provided. The Photographs and UV-Vis absorbance of perovskite layers after dipping of the corresponding PbI2 precursor films in a 8 mg/ml MAI/IPA solution for 30 s are also presented. Adapted with permission from Adv. Energy Mater. 2016, 6, 1501890. Copyright (2018) John Wiley and Sons b) Schematic illustration of the modified sequential deposition method where organic solvent vapors were applied immediately after the deposition of PbI2. Top-view SEM images of the PbI2 precursor layers (upper row) and the corresponding perovskite films (bottom row) are presented. J-V curves and absorption spectra are additionally provided. Adapted with permission from J. Mater. Chem. A, 2016, 4, 1947–1952. Copyright (2018) Royal Society of Chemistry. Departing from the synopses made above, electrochemical deposition has been found to be another method for fabricating porous layers. Huang et al. applied the environmentally friendly electrodeposition of PbO precursors for the preparation of homogeneous and fully 15 ACS Paragon Plus Environment


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covered MAPbI3 films76. The FTO/compact-TiO2 substrate was used as the working electrode (Pt as counter electrode) in a single chamber containing a lead acetate (PbAc2) solution in DMSO, whereby the thickness and quality of the precursor film could be easily controlled by the deposition parameters. The mesoporous PbO enabled the in-situ conversion reaction to the perovskite phase and resulted in a flat and uniform surface morphology. The best devices yielded a high PCE of 14.59 %. Koza et al. investigated the sequential deposition of MAPbI3 based on electrodeposited and epitaxial grown PbO precursor films on Au substrates77. It was shown that the orientation of the final perovskite film is controlled by PbO, regardless of the Au substrate orientation. Both surface properties, i.e. texture and epitaxial growth, led to lower trap state densities and higher photoluminescence intensities compared to polycrystalline films prepared from spin-coating. It was further argued that the combination of a textured film grown on an inexpensive Au-coated glass substrate provides the ideal morphology for photovoltaic application, since there are very few grain boundaries between the perovskite and the substrate which can act as recombination sides. However, no solar cells were presented. Although not related to solution-based methods, evaporation can also be used for the preparation of porous layers. Fu et al. thermally evaporated PbI2 on a rotating compact FTO/TiO2 substrate and observed nanoplate-shaped crystal growth78. The high inner surface area enabled a fast as well as complete conversion already at room temperature. Furthermore, the amount of unreacted PbI2 could be easily controlled by the concentration of the MAI solution, which was spin-coated on top. Intrinsic crystallographic properties as well as surface characteristics of the underlying substrate were found to strongly influence the growth of the nanoplates. A highest PCE of 8.6 % was achieved. Chai et al. obtained PbI2 nanoplatelets by first spin-coating a thin and homogeneous seed-layer (~ 30 nm) which was subsequently covered by PbI2 vapor79. A nanoplatelet structure was obtained for the precursor layer and 16 ACS Paragon Plus Environment

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enabled a better conversion ratio the perovskite phase after spray-coating of MAI/IPA and increased the absolute efficiency by about 2 %. In summary, several strategies have been reported to introduce porosity into the lead halide precursor films for the two-step synthesis of planar perovskites. An accelerated conversion reaction has been typically observed, independent on the way of manufacture, and the surface morphologies have always been improved. While the prevention of residual PbI2 is expected to enhance the device performance reproducibility, the high-quality perovskite surfaces seem to be responsible for the increase in PCE’s. Avoiding the PbI2 Crystallization: Another route to accelerate the conversion reaction is by means of inhibiting the crystallization of PbI2. The most applied solvent to achieve this goal is strongly coordinative DMSO, where several reports have been published highlighting the severe interaction with Pb2+. Wu et al. successfully utilized DMSO instead of DMF for the preparation of highly reproducible and flat perovskite films with a narrow crystallite size distribution44. The coordination ratio between PbI2 and DMF is 1 : 1, whereas it amounts to 1 : 2 for DMSO. This significantly strengthened interaction retards the crystallization of PbI2 after spin-coating and leads to an amorphous precursor layer, which in turn facilitates the rapid conversion reaction to the perovskite. A highest PCE of 13.5 % was achieved with a superior standard deviation of less than 0.6 % from 120 samples (whereas ~ 2.5 % deviation was observed for DMF based solar cells). Mao et al. reported the usage of DMSO as an additive in a conventional DMF solution80. It was observed that the incorporation of 8 % DMSO resulted in ultrasmooth precursor layers and in homogeneous as well as pin-hole-free perovskite films after conversion. Combined with a mixed EtOH/IPA solvent for MAI, an inverted device with a PCE of more than 15 % was presented. Additionally, Li et al. investigated a series of PbI2(DMSO)x complexes (0 ≤ x ≤ 1.86) average values calculated from XPS data) as precursor materials and observed that the morphology and grain size of the 17 ACS Paragon Plus Environment


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resulting perovskite could be controlled by means of self-assembly81. After conversion, the optimized ratio for the PbI2(DMSO)1.22 complex resulted in ultraflat and superdense absorber layers with a PCE of more than 17 %. A multistep approach for the preparation of a PbI2(DMSO) film was reported by Yang et al.82. For that, first a PbI2(DMSO)2 complex is formed from a mixture of PbI2 and DMSO by adding a nonpolar solvent like toluene. The precipitate is then decomposed to PbI2(DMSO) via heating in a vacuum furnace for 24 h. After resolving the complex in DMF, the mixture is spin-coated with a subsequent immersion into the formamidinium (FAI) solution. Due to the higher affinity of FAI to PbI2 compared to DMSO, intramolecular exchange occurs with negligible volume expansion. High quality and phase pure perovskite layers with large grain sizes were obtained resulting in a PCE higher than 20 %. Zhang et al. reported a combinatorial approach by applying DMSO as the solvent and chloride doping via PbCl2 as additive83. The resultant perovskite film properties were significantly improved, demonstrating a crack-free and uniform surface coverage as well as enhanced light absorption. A high crystal plane orientation and long-range domains were obtained without altering the crystalline structure. The introduction of 30 mol% PbCl2 resulted in a best PCE of 14.42 %, which was roughly ~ 36 % higher than that of the pure PbI2 based system. Nanostructured morphologies could also be obtained from DMSO. Boopathi et al. introduced alkali metal halide salts into the PbI2/DMSO precursor solution and observed rod- or disc-shaped surface features, depending on the type of additive84. The nanostructures were found to greatly influence the perovskite formation, resulting in high quality and uniform absorber layers after spin-coating of the MAI solution (Figure 5 a). It was stated that the halogenated additives chelate with Pb2+ ions during film formation, thereby enhancing crystal growth and morphology. By incorporating a small amount of KCl, the most compact perovskite surfaces were obtained and the PCE of an inverted cell could be increased by about 33 % to 15.08 %. N-Methyl-2-pyrrolidone (NMP) was applied as additive in a conventional DMF solution. Jo et al. demonstrated the formation of an exceptionally smooth 18 ACS Paragon Plus Environment

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and homogeneous PbI2/NMP precursor layer, which was ascribed to the intercalation of NMP into the PbI2 framework. The latter resulted in a strong expansion of the c-axis interlayer space85. The NMP based precursor films with a molar ratio of 1 : 1 (PbI2 : NMP) demonstrated a surface morphology even smoother than that of DMSO based systems (Figure 5 b). The subsequent intramolecular exchange with FAI molecules yielded dense and uniform FAPbI3 films with significantly enlarged grain sizes. A highest PCE of 19.5 % was achieved (compared to 17.6 % with PbI2/DMSO precursor layers). The incorporation of MAI as an additive was also reported to improve the final absorber properties. Zhang et al. prepared a mixed PbI2(+MAI)/DMF precursor solution and obtained very smooth layers by avoiding the full PbI2 crystallization59. Adding small amounts of MAI (10 – 20 %) was found to preexpand the PbI2 film into PbI2:xMAI with adjustable morphology. The about ten times faster conversion reaction kinetics evaded residual PbI2 and minimized negative impacts of the IPA solvent on the MAPbI3 phase during intercalation/conversion. By applying a PbI2:0.15MAI precursor ratio, a best PCE of 17.22 % could be obtained which was significantly higher than the 6.11 % observed from the standard two-step approach. In general, the usage of strong coordinative species for Pb2+ cations provides an encouraging way to avoid the fast crystallization of the precursor during the film deposition. The following conversion reaction seems to be highly accelerated and the homogeneous nucleation of perovskite grains results in more smooth and compact absorber layers which, in turn, improve the device performance of planar solar cells.



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Figure 5. a) J–V characteristics, Nyquist plots (with a bias of 0.8 V) and top-view SEM images of PbI2 films (upper row) and perovskite films (bottom row) prepared without any additives and with KCl, NaCl and LiCl as salt additives. The scale bar is 2 µm. Adapted with permission from J. Mater. Chem. A, 2016, 4, 1591–1597. Copyright (2018) Royal Society of Chemistry. b) XRD signals, theoretical crystal structures and top-view SEM images of PbI2 films prepared from a pristine PbI2/DMF solution, a PbI2(DMSO) in DMF solution and a PbI2(NMP) in DMF solution. DMSO and NMP intercalate into the PbI2 framework. Adapted with permission from Adv. Mater. Interfaces, 2016, 3, 1500768. Copyright (2018) John Wiley and Sons. 20 ACS Paragon Plus Environment

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Compact PbI2 for Homogeneous Nucleation: It was observed that the preparation of densely crystallized (i.e. compact) PbI2 precursor layers can also assist the growth of high quality perovskite films. Wu et al. introduced a small amount of water into the PbI2/DMF mixture and demonstrated a clear appearance of the solution which resulted in an extremely smooth, crystalline and fully covered precursor surface after spin-coating86. It was suggested that H2O as additive induces the homogeneous nucleation by modifying the PbI2/PEDOT:PSS interfacial energy. By applying 2 % of H2O into the precursor solution, highly pure, dense and pin-hole free perovskite films were obtained after conversion. The best cell demonstrated a PCE of approximately 18 % with a remarkably high FF of 0.85. A similar study was reported by Nawaz et al., who added different amounts of H2O (0 – 3 %) to the PbI2/DMF solution87. In the range of 1 – 2 % of H2O, the precursor solutions became more homogeneous and the resulting perovskite films after conversion changed from the rod-like surface morphology to much more uniform hexagonal plates. The water-assisted homogeneous nucleation was stated to be the main reason for the improved surface characteristics, whereby two additive characteristics were indicated as important: 1) higher boiling point compared to the host material and 2) strong interaction with the precursor materials. Zhang et al. modified the sequential deposition procedure by introducing a small amount of PbCl2 (~ 5 %) into the PbI2/DMF solution and spin-coating it on top of a pure PbI2 precursor layer88. The low chlorine fraction was stated to generate a more suitable Egap for efficient light absorption and a better control over the surface morphology of the perovskite film. However, the UV-Vis spectra were not compared to own results for pure iodide compounds and should be therefore regarded with care, since chlorine doping is not expected to change the Egap89. A PCE of 9.54 % was achieved, which was significantly higher than that of the standard MAPbI3 solar cell. A relatively large amount of HCl (33 %) was used to prepare compact and uniform HCl-PbI2 precursor layers90. This intermediate stage immediately converted to the perovskite phase within 10 s at room temperature, whereby the obtained surface morphology was considerably 21 ACS Paragon Plus Environment


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smooth and demonstrated a high moisture tolerance as well as a PCE of roughly ~ 14 %. Liu et al. added FAI into the PbI2/DMF solution to form FA0.4PbI2.4 intermediate complexes91. By post-annealing the precursor at 135°C, homogeneous and uniform films were obtained which turned into ordered and well-crystallized MA0.6FA0.4PbI3 absorber layers after the conversion reaction. The high-quality surface was attributed to homogeneous nucleation as well as enhanced growth kinetics. The PL lifetimes were considerably long (78 ns) and the PCE of the mixed cation perovskite was superior (11.5 vs. 10.6 %, resp.) compared to those from pure MAPbI3. Summarizing, it is expected that the formation of compact PbI2 precursor layers potentially leads to improved homogeneities for the nucleation of perovskite grains. However, the dense surface morphology may complicate the control of unreacted PbI2 in the final absorber. As a consequence, work should be directed more towards intermediate complexes, since the latter typically enhance guest molecule diffusion and accelerate the conversion reaction. Further Methodologies: A rather innovative approach was published by Bi et al.92. The growth of perovskite grains with high aspect ratio of 2.3–7.9, which is needed for reduced carrier recombination at the crystal boundaries, was achieved on different HTM’s (polyvinyl alcohol (PVA), PEDOT:PSS, crosslinked N4,N4’-bis(4-(6-((3-ethyloxetan-3yl)methoxy)hexyl)phenyl)-N4,N4’-diphenylbiphenyl-4,4’-diamine (c-OTPD), poly(bis(4phenyl)(2,4,6-trimethylphenyl)amine) (PTAA) and poly(N-9’-heptadecanyl-2,7-carbazole-alt5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)) (PCDTBT) (Figure 6 a). It was stated that the non-wetting surface properties of several HTM’s suppress heterogeneous nucleation of the precursor grains, thereby enlarging the nucleus spacing. The resulting perovskite films demonstrated minor grain boundary area and improved crystallinity after conversion, which resulted in 10 – 100 fold reduced trap state density (measured by thermal admittance spectroscopy; tDOS) and high efficiencies of more than 18 %. It was mentioned that the 22 ACS Paragon Plus Environment

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significantly improved optoelectronic properties give rise to applications in other fields of research like transistors, photodetectors, lasers and LED’s. Similarly, Hou et al. reported the use of a hydrophobic lead phthalocyanine (PbPc) HTM which resulted in a highly porous PbI2 precursor film (Figure 6 b - d)93. Accordingly, the formation of high quality and large grained perovskite layers was quite rapid in the inverted cell structure. The preparation could be conducted in humid environment and the long-term stability was significantly enhanced compared to devices based on more hydrophilic PEDOT:PSS.

Figure 6. a) Chemical structures of the different HTM’s used as substrates for sequentially deposited planar perovskite layers. Adapted from Nat. Commun., 2015, 6, 7747. b – g) Topview SEM images of PbI2 precursor layers and the corresponding MAPbI3 absorber films prepared on b) and e) PEDOT:PSS, c) and f) on 20 nm PbPc and d) and g) on 50 nm PbPc. The insets show the wetting capability of H2O on the respective substrate. h) XRD patterns of MAPbI3 layers on PEDOT:PSS and PbPc substrates. Adapted with permission from Sol. Energy Mater Sol. Cells., 2016, 157, 989-995. Copyright (2018) Elsevier.



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Bag et al. incorporated Na+ ions into the PbI2/DMF precursor solution94. Combined with a MAPbI3 post annealing in DMF vapor, they obtained highly compact and large grained absorber films. However, the distinct role of the additives in the first deposition step could not be clarified. It was suggested that Na+ might act as nucleation sites leading to large PbI2 grain growth. With the addition of 2 mol% of Na+, the PCE of an inverted cell could be increased to 14.2 % (3 - 4 % absolute efficiency higher than without Na+). Apparently, increasing the precursor nucleus spacing by hydrophobic substrates or ionic additives represents alternative routes to introduce porosity into PbI2 films. The conversion ratio as well as the absorber surface qualities can thereby be significantly enhanced. The Conversion Reaction Step: The first application of the two-step synthesis of MAPbI3 in a planar configuration was reported by Liang et al., where a simple bath immersion method was used to convert the spin-coated PbI2 precursor layer on a simple ash or quartz glass substrate37. Although the complete transformation to the 3D perovskite phase needed 1 – 3 h, the versatile and promising characteristics of this technique were highlighted. However, the first report during the era of hybrid perovskite solar cells was published by Liu et al., who used a compact ZnO nanoparticular electron transport layer in a high efficiency planar perovskite solar cell (PCE > 15 %)39. After that, several research groups have adopted the native two-step method for the preparation of highly efficient planar cell designs50, 58, 95-105. An alternative route was found by spin-coating a MAI film on top of the PbI2 precursor accompanied by a subsequent heating step. This process is often declared as interdiffusion of precursor stacking layers where the heat treatment initiates the solid-solid reaction52, 101-102, 106109

. However, from both methods and with optimization of the processing conditions, the

resulting absorber surfaces were typically too rough due to the non-uniform crystallite size distribution and large number of grain boundaries. As a consequence, the preparation of really


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compact films with higher quality like those from e.g. vapor assisted techniques is prohibited. To address this issue, various strategies have been evolved regarding the conversion solution. Solid Additive Incorporation: Apparently, solid additive incorporation has shown to be one of the most often used process modifications. For example, Wu et al. improved the final absorber morphology by introducing small amounts of acetate salts (NH4Ac and NaAc) into the MAI/IPA immersion solution110. These chemicals increased the surface coverage whereby an optimized NH4Ac concentration of 10 % resulted in a PCE as high as 17.02 %, which was ~ 23 % higher than that of the pristine device without additives (Figure 7 a). Although the improved surface morphology was additionally obtained with NaAc, no efficiency enhancement was observed. The latter was attributed to the retaining of NaAc in the final perovskite phase as an insulating and non-volatile salt. MACl was used as additive in a conventional cell design. Docampo et al. prepared a planar cell by simply immersing the PbI2 precursor into the mixed MAI/MACl/IPA solution and achieved nearly 15 % efficiency with an exceptional short circuit current density of more than 22 mA/cm² (a gain of ~ 10 % over state-of-the-art devices)50. However, the superior device performance was attributed to enhanced photoluminescence lifetimes due to the presence of chloride and rather not to improved surface properties. On the other hand, Ip et al. introduced MACl into the conversion solution in the interdiffusion process and observed significant improvements in the surface morphology111. The higher uniformity and compactness of the resulting films prevented device short-circuiting and yielded a stable efficiency exceeding 11 % when combined with a PCBM passivation layer. An acetamidine based salt was also used in the interdiffusion method of conventional and planar perovskite cells. Zheng et al. introduced acetamidine hydrochloride (C2H3N2H4Cl, AaHc) into the MAI solution and observed an enhanced crystallinity of the resulting MAPbI3 film as well as a smooth, uniform and well covered surface112. Longer charge carrier lifetimes were obtained compared to a standard device (τ1 = 25 ACS Paragon Plus Environment


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29.4 vs. 6.8 ns and τ2 = 292.2 vs. 20.7 ns, resp.) and suggested to be due to suppressed recombination at surfaces and in the bulk. The best PCE of 16.54 % was also higher than that of the control device without additives. Watthage et al. employed divalent metal salts (ZnCl2, CdCl2 and HgCl2) inside the conversion solution113. It was found that a concentration of 10 mM for all metals in the interdiffusion process significantly improved the morphology due to larger grain sizes (Figure 7 b – e). However, only Cd2+ resulted in improved charge carrier lifetimes compared to the control device (τmean = 258 vs. 96 ns, resp.) whereas the inclusion of Zn2+ or Hg2+ adversely affected the optoelectronic properties of the perovskite. It was stated that the surface trap densities due to Zn or Hg impurities led to increased non-radiative recombination rates. Cd2+ incorporation resulted in a higher degree of crystal orientation in the absorber films. Photovoltaic efficiencies were not reported. Zuo et al. introduced different polymer additives in a mixed MAI/MACl solution for the conversion of mesoporous PbI2 layers69. The binding interaction energies between the functional groups and the perovskite were found to affect the passivation of trap states. Poly(4-vinylpyridine) (PVP) revealed the highest binding energy and yielded superior photovoltaic performance. A solar cell with a high PCE of 20.2 % was presented. To sum up, solid additive incorporation represents a simple method to influence the nucleation and crystal growth dynamics during the conversion reaction to the perovskite phase. Thereby, the main object is to obtain compact absorber layers with enlarged grain sizes, which typically demonstrate enhanced charge carrier mobilities. However, care should be taken when applying non-volatile chemicals. These may adversely affect the optoelectronic properties by introducing trap states etc..


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Figure 7. a) NH4Ac additive incorporation into the MAI/IPA conversion solution and associated perovskite morphology change. The PCE increases with better compactness of the absorber films. Adapted with permission from ACS Appl. Mater. Interfaces, 2016, 8, 15333−15340. Copyright (2018) American Chemical Society. b) – e) Top-view and crosssectional SEM images of MAPbI3 layers prepared by the interdiffusion technique b) without additive and c) with ZnCl2, d) with CdCl2 and e) with HgCl2 additives in the MAI/IPA conversion solution. Adapted with permission from MRS Advances, 2017, 2, 1183-1188. Copyright (2018) Cambridge University Press. Liquid Additive Incorporation: Liquid additives have also been utilized. Mo et al. applied a polar solvent additive like DMF or GBL in the interdiffusion method of a mixed halide (I/Cl) perovskite in an inverted structure114. Especially DMF is popular for dissolving PbI2 but has never been used in the MAI solution before that work. It was stated that the formation of a “wet” environment could be advantageous for the diffusion of MAI molecules to deeper lying PbI2 precursor material (Figure 8 a). They observed a strong promotion of the PbI2 conversion reaction and significantly improved surface properties like e.g. reduced pin-holes, increased grain sizes and enhanced optical absorption strengths. The efficiency was thereby boosted to over 19 % when 0.9 % DMF was added to the MAI solution. Morphology alternations could also be achieved by converting the PbI2 to perovskite with ether additives. The glycol ethers 2-methoxyethanol (ME), 2-ethoxyethanol (EE) and 2-propoxyethanol (PE) were added to the conversion solution in small amounts of 10 – 50 µl/ml by Ugur et al.115. By this, the reaction dynamics were changed resulting in enhanced conversion ratios. Furthermore, the perovskite 27 ACS Paragon Plus Environment


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morphology was improved by revealing compact and pin-hole free characteristics (Figure 8 b). Especially when ME was used in the spin-spin process, the grain sizes increased to nearly 1 µm with a vertical alignment on the surface and a decreased number of grain boundaries. Combined with a PbI2-DMF complex precursor layer, the average PCE could be enhanced from 13.5 to 15.9 % (by using 3 % v/v ME). EtOH has also been used as a simple and cheap additive. Mao et al. used mixed MAI/IPA-EtOH conversion solutions with EtOH contents ranging from 0 to 100 % to produce planar cells by the interdiffusion process80. By optimizing this ratio (i.e. 25 % EtOH content), the J-V-curve parameters were substantially enhanced compared to the control device (JSC = 19.19 vs. 16.90 mA/cm², VOC = 1.026 vs. 1.013 V and FF = 75.22 vs. 63.86, resp.), due to improved morphologies and charge carrier transfer efficiencies. When this strategy was combined with a DMSO modified PbI2/DMF precursor film, a maximum PCE of 15.76 % could be achieved, which was ~ 50 % higher than that of a standard sequentially deposited cell. Related to solid additives, liquid chemicals can influence the crystal growth behavior in the conversion solution and may result in improved surface morphologies for the final absorber layer.

Figure 8. a) Process scheme of the interdiffusion method with DMF additive leading to compact, large-grained and pin-hole free perovskite layers. Adapted with permission from J. Mater. Chem. A, 2017, 5, 13032–13038. Copyright (2018) Royal Society of Chemistry. b) Schematic representation and top-view SEM images of the interdiffusion method with 28 ACS Paragon Plus Environment

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different glycol ethers as additives in the conversion solution. The glycol ethers are 2methoxyethanol (ME), 2-ethoxyethanol (EE), and 2-propoxyethanol (PE). Adapted with permission from ACS Energy Lett. 2017, 2, 1960−1968. Copyright (2018) American Chemical Society. Alternative Solvents: Yang et al. reported that high quality perovskite films can be obtained by the judicious choice of the solvent for the conversion reaction54. By synthesizing micrometer crystals from precursor dispersions in different alcohols (1-propanol (PA), 1butanol (BA), 1-hexanol (HA), IPA, tert-butanol (TBA) and tert-pentanol (TPA)), the intensity of PbI2 dissolution could be controlled via the molecular structure and polarity of the solvent. Typically, branched structures provided the MAI in-situ intercalation into the PbI2 framework, thereby inhibiting Ostwald ripening. Based on these findings, TBA was used to produce planar perovskite solar cells via the interdiffusion method. Whereas crystallinity and absorbance remained nearly the same compared to pristine IPA, the surface morphology was significantly changed (Figure 9 a). The films prepared from TBA showed much more uniform and flat properties with a few nano-sized holes. Furthermore, the surface roughness was comparable to vapor-assisted films. Solar cells based on these layers delivered a PCE of 14.61 %, which was much higher than that from the pristine two-step synthesis. We investigated the effects of the static relative permittivity of different alcoholic solvents (EtOH, PA, IPA, BA and 2-butanol (2-BA)) on the perovskite morphology56. It was observed that the average crystallite size of MAPbI3 increased with increasing values for the permittivity. Additionally, we found that the permittivity, as a macroscopic feature, is better suited to describe a correlation to the average grain size compared to the dipole moment, since the latter does not account for e.g. polarizability effects. By using mixed IPA/PA solutions in the immersion step, the crystallite size and surface coverage could be fine-tuned whereby larger grains typically showed enhanced photoluminescence lifetimes (Figure 9 b). With a PA content of 29 ACS Paragon Plus Environment


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80 %, the PCE of a conventional planar cell was increased by nearly threefold. Generally, the physical properties of the solvent for the conversion reaction provide an elaborate pathway to determine the perovskite nucleation as well as the extent of dissolution-recrystallization.

Figure 9. a) XRD patterns, UV-Vis absorbance, top-view SEM images and AFM surface roughnesses of perovskite films prepared from IPA and tert-butanol (TBA) solvents. The scale bars of the left and right SEM images are 2 µm and 500 nm, respectively. The digital photography inset in the absorption spectrum demonstrates the mirror-like appearance of 30 ACS Paragon Plus Environment

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MAPbI3 layers prepared from TBA. Adapted with permission from RSC Adv., 2015, 5, 69502–69508. Copyright (2018) Royal Society of Chemistry. b) Top-view SEM images and photoluminescence decays of sequentially deposited MAPbI3 films prepared at 60°C from mixed IPA/1-PrOH solvents. The IPA : 1-PrOH ratios are denoted in the pictures. The PL decay time constants were mono-exponentially fitted and reveal a static improvement of carrier lifetimes with increased average grain sizes. Adapted with permission from Org. Electron., 2017, 50, 87-93. Copyright (2018) Elsevier. Post Treatment: Xiao et al. applied a subsequent DMF solvent vapor annealing step after the perovskite was deposited by interdiffusion in the inverted cell structure116. For that, the substrate was simply placed on a hotplate under a petri dish together with a small amount of solvent and heated to 100°C for 1 h (Figure 10 a). It was expected that DMF penetrates into the perovskite and aids the growth of crystalline domains. The obtained average MAPbI3 grain size was substantially increased as well as the crystallinity. A highest PCE of 15.6 % was achieved due to improved optoelectronic properties compared to a control device (i.e. hole-conductivity, external quantum efficiency (EQE) and trap-state density). A combination of DMF solvent vapor annealing and the interdiffusion process was reported by Dong et al.117. The crystallized PbI2 precursor layer was spin-coated with a mixed MAI/MACl/IPA solution for multi-cycle coatings (Figure 10 b), where every three seconds a minor amount of 10 µl of the solution is dropped onto the spinning substrate. Afterwards the perovskite is exposed to DMF vapor at elevated temperatures. An abnormal grain growth was observed accompanied by the appearance of partially very large domains, which was attributed to the presence of chloride. The large crystallites contribute to improved carrier lifetimes and an increased PCE of 18.9 % (~ 20 % higher than for MAPbI3). Likewise, Chae et al. confirmed the promising features of this technique and presented high efficiency inverted cells with PCE’s > 15 %118. These post-treatments offer a facile way to improve the surface morphologies of the absorber 31 ACS Paragon Plus Environment


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layer, whereby crystal qualities can be obtained which resemble that of vapor-assisted techniques.

Figure 10. a) Schematic of the interdiffusion method combined with DMF solvent annealing for grain size increase. The top-view SEM images show perovskite films with thicknesses of 250, 430 and 1015 nm, respectively, prepared by thermal annealing (left) and solvent annealing (right). The scale bar is 2 µm. The single-path absorption and photocurrent characteristics of perovskite films with different thicknesses are provided. Adapted with permission from Adv. Mater. 2014, 26, 6503–6509. Copyright (2018) John Wiley and Sons. 32 ACS Paragon Plus Environment

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b) Illustration of the multicycle coating method used to fabricate the MAPbI3-yCly perovskite layers with a thickness of 575 nm by sequential deposition. The deposition of a PbI2 layer is followed by multiple cycles of MAI:MACl layers. The SEM image shows an absorber film after 6 deposition cycles. The XRD patterns show an air-dried film (red) and the same film after annealing at 110 °C for 10 s (black) and 150 min (blue). Typical photocurrent and EQE spectra of MAPbI3-xClx and MAPbI3 based devices are provided. Adapted with permission from Nano Lett., 2015, 15 (12), 8114–8121. Copyright (2018) American Chemical Society. The main strategy for improving the optoelectronic properties and device performances of planar perovskite solar cells is the preparation of compact and large grained absorber layers. Several methods have been investigated which tackle both precursor deposition treads in the two-step synthesis. For the lead-based material, the typical plate-shaped crystallization from DMF solvent should be avoided whereby an alternated crystal growth can help to induce a faster conversion and/or a more homogeneous nucleation for the perovskite phase. In the case of the alcoholic conversion solution, additive incorporation as well as post-treatments have been found to easily allow the manipulation of the surface morphology. The resulting films have demonstrated significantly enhanced qualities and the PCE’s have been frequently increased to values approaching the 20 % margin. Conclusions and Issues: The planar heterojunction configuration represents a promising cell design due to the ease in practical implementation and may lower the general processing costs when compared to the mesoscopic structure of perovskite solar cells. The main disadvantages of the latter is the high temperature annealing step needed to obtain the scaffold material, which renders it unsuitable for flexible substrate applications and increases the processing costs and complexity. The simpler planar device architecture greatly relaxes manufacture conditions. However, the film formation of the absorber layer is a key factor for high efficiencies and careful control over the reaction between the organic and inorganic species 33 ACS Paragon Plus Environment


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must be taken. The solution-based sequential deposition has shown much promise to overcome this hurdle and a lot of advances have been achieved in the recent years. Several types of process modifications were reviewed here and were systematically separated into the two steps of deposition. For the PbX2 precursor layer, the native crystallization should be avoided in order to promote a more complete conversion in shorter time periods. This will prevent the typical dissolution-recrystallization of perovskite crystallites in the alcoholic AXsolution. The corresponding strategies encompass the introduction of porosity for an increased contact surface area, which enables an accelerated conversion and usually improved surface morphologies. Porosity can be introduced by additive incorporation, electrochemical deposition of PbO or by different pre- and post-treatment processes. The inhibition of the PbX2 crystallization by coordinating species represents another route to accelerate the conversion reaction. Most commonly, DMSO is applied as additive or alternative solvent due to the strong coordinating properties for Pb2+. The better homogeneity for the perovskite nucleation results in an enhanced compactness for the absorber layer. Smoother precursor layers were additionally found to positively influence the formation of high-quality perovskite films. However, the complicated control of unreacted PbI2 may decrease the general device reproducibility. During the conversion reaction, various solid and liquid additives were applied in order to affect the crystal growth dynamics. Furthermore, the use of solvents with different permittivities was demonstrated to significantly affect the average grain size as well as the intensity of dissolution-recrystallization. Finally, post-treatment strategies were investigated and found to improve the uniformity and compactness of the resulting absorber films. However, there still remain some ongoing challenges and future research orientations for promoting commercialization of perovskite solar cells. For example, mixed cation and halide perovskite combinations (i.e. containing different stoichiometries of Cs+, FA+ and MA+ in the 34 ACS Paragon Plus Environment

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A-position and Cl-, Br- and I- in the X-position) partly demonstrated improved moisture or air stability compared to the pure counterparts50, 66, 91, 103, 114, 117-127. Furthermore, in some cases a better matched band alignment in terms of relative energetic positions of the perovskite’s VBM and CBM to the selective contacts may suppress the carrier recombination and enhance the light absorption. However, while it is easy to control the stoichiometric ratio in the onepot synthesis, the fast ionic exchange renders it hard to control it in the sequential deposition method128-129. As a consequence, further experimental and theoretical work has to be done on the dynamics of ion exchange in hybrid perovskites. The rather soft crystal structure of organic-inorganic ABX3 compounds poses a general problem on device stability and reproducibility. In an environment containing moisture, the hygroscopic organic species can easily escape, which is accelerated at elevated temperatures. Growing larger and defect-free crystal domains can somewhat reduce this issue65, 119. Additionally, doping with inorganic elements125, 130 and the preparation of water-resistive surfaces via functionalization with hydrophobic molecules131 have shown to improve the stability. A further challenge to address is the large-scale fabrication of perovskite photovoltaic devices, since the most reported performances are limited to very small areas (typically < 1 cm²). Spin-coating is still the most widely used method for solution deposition and has yielded the highest efficiencies, but it is not suitable for production in bulk. Some well-known large-area manufacture technologies have recently attracted notice in the perovskite solar cell community, which are for example doctor blading, spray-coating, electrochemical deposition, vapor deposition, bath conversion or slot-die coating. However, upscaling perovskite layers with these methods to several cm² resulted in drastic drop downs of efficiencies and the surfaces showed partially inferior crystal qualities in research labs. Consequently, they have to be exploited more intensively17, 76-77, 132135

. Slot-die coating, in this respect, represents a simple technique which uses an accurate

metering of the coating-solution and dispensing at a controlled rate, while the device is precisely moved relative to the substrate. It seems to be the most promising route in terms of 35 ACS Paragon Plus Environment


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crystallinity, film uniformity, technical reproducibility, economic and environmental friendliness136. To summarize up, stability and efficiency improvements are expected in the near future bearing in mind the unequaled progress in understanding of film formation and material features. The excellent optoelectronic properties as well as the easy and low-cost fabrication possibilities render perovskites particularly promising for the next generation of photovoltaics. AUTHOR INFORMATION Corresponding Author *Corresponding Author Phone (+49) 441 798 3675; e-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests. References 1. Frauenhofer Institute for Solar Energy Systems, "Photovoltaics Report". https://www.ise.fraunhofer.de/de/downloads/pdf-files/aktuelles/photovoltaics-report-in-englischersprache.pdf (accessed 30.04.2018). 2. Saga, T., Advances in Crystalline Silicon Solar Cell Technology for Industrial Mass Production. Npg Asia Mater 2010, 2, 96-102. 3. Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T., Organometal Halide Perovskites as VisibleLight Sensitizers for Photovoltaic Cells. J Am Chem Soc 2009, 131, 6050-6051. 4. Renewable Energy Laboratory (Nrel). http://www.nrel.gov/ncpv/images/efficiency_chart.jpg (accessed 24.04.2018). 5. Battaglia, C.; Cuevas, A.; De Wolf, S., High-Efficiency Crystalline Silicon Solar Cells: Status and Perspectives. Energy Environ Sci 2016, 9, 1552-1576. 6. de Wolf, S.; Holovsky, J.; Moon, S. J.; Löper, P.; Niesen, B.; Ledinsky, M.; Haug, F. J.; Yum, J. H.; Ballif, C., Organometallic Halide Perovskites: Sharp Optical Absorption Edge and Its Relation to Photovoltaic Performance. J Phys Chem Lett 2014, 5, 1035-1039. 7. Giorgi, G.; Fujisawa, J. I.; Segawa, H.; Yamashita, K., Small Photocarrier Effective Masses Featuring Ambipolar Transport in Methylammonium Lead Iodide Perovskite: A Density Functional Analysis. J Phys Chem Lett 2013, 4, 4213-4216.


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"For Table of Contents Use Only" Recent Progress in the Solution-Based Sequential Deposition of Planar Perovskite Solar Cells Markus Becker and Michael Wark

The latest advancements in the solution-based two-step synthesis of planar perovskite solar cells are reviewed. Strong focus is devoted to the optimization of the surface coverage, compactness and morphology, all of of which significantly influence the photovoltaic performance.


Recent Progress in the Solution-Based Sequential ... - ACS Publications

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