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Solution-Chemistry Engineering Toward High-Efficiency Perovskite Solar Cells Yixin Zhao, and Kai Zhu J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/jz501983v • Publication Date (Web): 13 Nov 2014 Downloaded from http://pubs.acs.org on November 17, 2014

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The Journal of Physical Chemistry Letters

Solution-Chemistry Engineering Toward High-Efficiency Perovskite Solar Cells Yixin Zhao a* and Kai Zhu b* a

School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Rd., Shanghai 200240, China b Chemical and Materials Science Center, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, CO 80401, USA

ABSTRACT: Organic and inorganic hybrid perovskites (e.g., CH3NH3PbI3) have emerged as a revolutionary class of light-absorbing semiconductors that has demonstrated a rapid increase in efficiency within a few years of active research. Controlling perovskite morphology and composition has been found critical to developing high-performance perovskite solar cells. The recent development of solution-chemistry engineering has led to fabrication of greater than 15%– 17%-efficiency solar cells by multiple groups, with the highest certified 17.9% efficiency that has significantly surpassed the best reported perovskite solar cell by vapor-phase growth. In this perspective, we review recent progress on solution-chemistry engineering processes and various control parameters that are critical to the success of solution growth of high-quality perovskite films. We discuss the importance of understanding the impact of solution processing parameters and perovskite film architectures on the fundamental charge-carrier dynamics in perovskite solar cells. The cost and stability issues of perovskite solar cells will also be discussed. Table of Contents Image

Keywords: Perovskite, Solar Cell, Solution Chemistry, Charge-Carrier Dynamics

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Organic and inorganic hybrid halide perovskites have become an attractive light-absorber system with a rapid improvement of cell efficiencies from less than 4% in 20091 to a certified 17.9% in 2014. Perovskites (e.g., CH3NH3PbI3 or MAPbI3) were initially used to replace the dye molecules in the conventional configuration of dye-sensitized solar cells (DSSCs) based on liquid electrolyte.1-2 However, the liquid-type perovskite solar cells did not attract much attention because of poor stability and low efficiency levels. In 2012, two reports3-4 on solid-state perovskite solar cells with about 10%–11% efficiencies triggered the explosion of research on perovskite solar cells. Since then, halide perovskites have attracted enormous worldwide attention, focusing on both perovskite material/device development and a fundamental understanding of materials structural and electronic properties, charge-carrier dynamics, and device operation principles.5-24 With these intensive efforts, the efficiencies of perovskite solar cells have skyrocketed to close to 20%, which is comparable to state-of-the-art copper indium gallium diselenide (CIGS) solar cells and approaching commercial monocrystalline silicon solar cells.25 Organic-inorganic lead/tin-based halide perovskite was initially studied by Mitzi and coworkers as an alternative tunable semiconductor material using low-cost solution chemistry for light-emitting diodes and field-effect transistors.26-28 This novel material has a typical ABX3 perovskite structure, as shown in Figure 1. The electronic and optical properties of halide perovskites can be simply tuned by adjusting the composition of A, B, and X. For example, the bandgap can be continuously tuned from about 1.5 eV to 2.2 eV by substituting I with Br ions. One of the most attractive features for perovskite is that the high-performance device can be made with low-cost solution processing. The demonstrated high efficiency and low-cost solution synthesis have made this technology very attractive to researchers from different fields to study various fundamental and applied aspects of perovskites for solar conversion and other optoelectronic applications.


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Figure 1. Schematic illustration of the mesoporous, planar heterojunction, and meso-planar hybrid perovskite film architectures in solar cells and the crystal structure of MAPbI3 perovskite. HTM represents hole-transporting material. There are three primary types of perovskite solar cell structures (based on the perovskite film architecture) as illustrated in Figure 1. The first type involves a mesoporous metal oxide layer (e.g., TiO2 and Al2O3) that is infiltrated with perovskites. Several pioneering reports on high-efficiency solid-state perovskite solar cells were based on this structure using either MAPbI3 or MAPbI3-xClx perovskite absorbers via solution deposition.1-3 It is worth mentioning that the exact composition of MAPbI3-xClx is still under debate. In the first structure, the perovskite morphology is mainly controlled by the underlying mesoporous scaffold.9, 14, 29-38 The mesoporous layer is usually thicker than 500 nm to absorb sufficient light. Because the growth of perovskite is restricted or supported by the mesoporous layer,32 the deposition of perovskite into the mesoporous scaffold is more reproducible. The main disadvantages of mesoporous perovskite solar cells include relatively low open-circuit voltage (Voc) and light absorbance at > 700-nm wavelength.39 The second type is the planar heterojunction perovskite solar cell with a sandwich configuration, which is similar to the organic photovoltaic (OPV) architecture.12, 19, 38, 40-42 The diffusion length of a planar perovskite solar cell is from a few hundreds nanometer to a micrometer;43-44 so the perovskite layer thickness is usually less than 400 nm for better charge collection, which is (fortunately) sufficient for absorbing most sunlight. Most reported planar perovskite solar cells have exhibited higher photovoltage and photocurrent than mesoporous devices. However, the planar perovskite solar cells usually exhibit a severe current-voltage (IV) hysteresis, which is not as common in mesoporous perovskite solar cells.45 Because photocarriers need to transfer through the perovskite layer itself in the planar cell configuration, the perovskite film morphology is expected to be critical to the device operation characteristics.


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The difference between the mesoporous and planar perovskite cells has become less clear mainly because of the emerging of the third cell architecture, which is based on a hybrid structure with a perovskite capping layer on top of a thin mesoporous perovskite layer.6, 46-48 Such a structure has demonstrated high cell efficiency with negligible IV hysteresis. In this configuration, the mesoporous and capping layers are usually thinner than their respective thicknesses in the corresponding mesoporous and planar perovskite solar cells.

Figure 2. Comparison of scanning electron microscopy (SEM) images. MAPbI3-xClx perovskite films made by (a) co-evaporation and (b) a non-optimized one-step solution process.40 Reproduced with permission from Ref. 40. Copyright 2013 Nature Publishing Group. Vapor-phase deposition techniques have been widely used to prepare high-quality (e.g., uniform thickness and composition) semiconductor thin films for both fundamental research and device development for traditional thin-film solar cells. During the early stage of perovskite development, solution processing showed significant limitation for preparing uniform, pinholefree, large-area perovskite films. Snaith et al. first reported the use of co-evaporation to deposit planar, pinhole-free MAPbI3-xClx film (Figure 2a) leading to >15% cell efficiency.40 In contrast, the best solution-prepared planar MAPbI3-xClx solar cell only exhibited ~12% efficiency around the same time.41, 49 The lower efficiency from solution-deposited cells was mainly attributed to


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the incomplete, uneven coverage of the perovskite film by solution processing (Figure 2b) in comparison to the vapor-deposited perovskite films. The degree of surface coverage is also found to correlate strongly with the extent of the performance variation of perovskite solar cells.40 Because of this drastic difference in film morphology, the vapor-phase deposition clearly demonstrated its advantage over solution processing during the early stage of the development of perovskite solar cells. However, co-evaporation of halide perovskite has certain technical challenges, such as difficulty in controlling the source temperature and the precursor ratio due to the relatively low thermal stability of perovskite materials. For this or perhaps other reasons, there are only a few reports based on vapor-phase deposition for high-efficiency perovskite solar cells. In contrast, solution-chemistry approaches have progressed so much within a very short period of time that a high-quality planar perovskite thin film comparable to that by vapor deposition has been reported by various groups.12, 50-52 Using mesoporous (e.g., TiO2) substrates, uniform perovskite films with high cell efficiencies have also been demonstrated by using either one-step or sequential two-step solution deposition approaches.6, 53-55 Recent advances in perovskite solar cells have been discussed in several reviews and perspectives.56-60 In this perspective, we focus on the recent development of solution-chemistry engineering processes that have significantly improved the structural properties of perovskites for higher device performance. We will provide our perspectives on the importance of understanding the impact of common solution-chemistry engineering approaches on chargecarrier dynamics (e.g., transport) in perovskite solar cells with different architectures. The cost and stability issues of perovskite solar cells will also be discussed.

One-Step Solution Growth of Perovskites. One-step solution process via spin coating is commonly used to deposit perovskite on the substrate with either a planar or mesoporous structure. During spin coating, excess precursor solution will be removed, the precursor left on the substrate will undergo evaporation, and a solid perovskite (or intermediate) film will form. These steps usually occur simultaneously. Finally, a thermal annealing is often required to fully crystallize the perovskite film. Controlling the interactions among the substrate, precursor solution, and processing environment (e.g., air or N2) during spin coating is critical to the final film morphology. These interactions are affected by many factors, including the choice of


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solvent, annealing condition, precursor composition, surface properties of the substrate, and general spin-coating parameters (e.g., spin speed). (a) Solvent Composition. The solvent used in preparing perovskite precursor solution mainly includes γ-butyrolactone (GBL), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) with an oxygen group to coordinate with Pb2+.61 In the first perovskite solar cell paper1 by Miyasaka and coworkers on perovskite-sensitized solar cells using the 10-µm mesoporous TiO2 layer and a liquid electrolyte, the concentrations of MAPbI3 and MAPbBr3 precursor solutions were 8 wt% and 20 wt%, respectively. The concentration of the MAPbI3 solution was later increased to 40 wt% in GBL to increase the absorbance allowing for a thinner mesoporous TiO2 scaffold.2 The 40 wt% seems to be the maximum concentration for a stable GBL solution at room temperature.2 Other solvents such as DMF and DMSO can be used to prepare precursors with higher concentrations up to 60 wt%.62 A high loading of perovskite for a relative thin perovskite film is critical especially for solid-state devices because their diffusion length43-44, 63 is generally shorter than that in liquid-type devices.35 When preparing the MAPbI3 precursor solution using GBL solvent, it is observed that neither MAI nor PbI2 can be easily dissolved alone, but MAI and PbI2 can be dissolved when mixed together in GBL. This observation suggests that MAI and PbI2 might be dissolved in GBL to form cations, ions, or complexes with the solvent molecule. When using either DMF or GBL as solvent, MAPbI3 forms immediately after evaporation of GBL and DMF, which usually occurs within a few minutes of annealing and normally results in the formation of perovskite films with poor surface coverage on a planar substrate.58 However, using a mixture of GBL and DMF (97:3, vol%) as the solvent creates a relatively uniform, smooth perovskite film with interconnected crystalline networks, which is likely caused by an optimized solvent evaporation time. The recent work51 by Seok et al. demonstrates that using the mixed solvent of GBL and DMSO can help retard the rapid crystal reaction because of the formation of strong DMSO-PbI2 complexes, which will be discussed in detail later.


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Figure 3. SEM images showing the effects of (top) annealing temperature and (bottom) mesoporous perovskite film thickness on the final perovskite film morphology.64 Reproduced with permission from Ref. 64. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

(b) Annealing Control. In addition to varying the solvent and concentration of perovskite precursor solution, adjusting the annealing conditions (e.g., temperature, duration, ramping rate, and environment) is also critical to controlling the morphology and composition of the perovskite films. When using the stoichiometric PbI2 and MAI precursor solution in GBL or DMF, the perovskite MAPbI3 forms immediately after the evaporation of the solvent with the annealing temperature ranging from 40o to 160oC,2 although many groups seem to prefer 100oC as the standard annealing temperature. For solution processing of MAPbI3-xClx using DMF precursor solution containing PbCl2 and MAI with a 1:3 molar ratio (denoted as the PbCl2-3MAI precursor), the regular reported annealing temperature is also around 100oC (or sometimes higher).4 Snaith and coworkers have investigated systematically the effect of annealing temperature and initial film thickness on the surface coverage and device characteristics based on the PbCl2-3MAI precursor.64 They found that higher perovskite surface coverage leads to improved photocurrent. As shown in the top row of Figure 3, a lower annealing temperature generally results in better perovskite film coverage, which is preferred for higher device performance as long as the temperature is sufficient to enable full crystallization of the perovskite absorber. It is found that a temperature as low as 60oC is insufficient for effective conversion of perovskites from the PbCl2-3MAI precursor.65 The initial perovskite film thickness is also important to the surface coverage of the final perovskite films (Figure 3, bottom row).


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With increasing initial film thickness, the average pore size increases but the areal density of pores decreases, leading to a higher surface coverage.64 Another study by Wiesner and Snaith et al. shows that a short, rapid annealing at 130oC favors the growth of uniform, micron-sized textured perovskites, whereas a long, slow annealing at 100oC leads to the formation of 100– 1000-nm polycrystalline perovskite grains.66 It is also worth noting that the annealing temperature/duration should be carefully adjusted because the prolonged annealing at relatively high temperature could decompose the perovskites, leading to the formation of PbI2.65 Annealing in humid air could accelerate this process. Because the perovskite crystallization process is very sensitive to moisture, perovskite fabricated under a moisture-free condition (e.g., nitrogen-filled glovebox) may exhibit better film coverage than the one prepared under the ambient condition when preparing planar perovskite thin films.64

Figure 4. SEM images of MAPbI3 films prepared (a) without using and (b) using MACl;38 MAPbI3-xClx films (c) without using and (d) using DIO;67 and FAPbI3 films (e) without using and (f) using HI.52 Reproduced with permissions from Refs. 38，67，and 52. Copyright 2014 American Society of Chemistry, Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, and Copyright 2014 The Royal Society of Chemistry, respectively. (c) Additive-Controlled Growth of Perovskite. We have recently found that using an additive (e.g., MACl) to the standard perovskite precursor (e.g., equimolar mixture of MAI and


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PbI2 in DMF) can significantly slow down the crystallization process of forming MAPbI3.38 The optimum annealing time (at 100oC) required to form MAPbI3 increases from a few minutes for the standard precursor without MACl to about 45 minutes when the precursor contains 2 molar ratio of MACl. With the use of MACl, the film coverage/morphology of MAPbI3 on a planar substrate is substantially improved compared to the one without the use of MACl additive (Figure 4a,b). Consequently, the cell performance is improved from about 2% to 12% for the planar perovskite devices. Using MACl is also found to decrease the recombination resistance for planar cells by 1−2 orders of magnitude. More than two years ago, Snaith and coworkers discovered4 the PbCl2-3MAI precursor composition for making the mixed halide perovskite (MAPbI3-xClx) that has shown much improved surface coverage and electronic properties than MAPbI3 perovskite when perovskite is deposited on a planar substrate. The much improved film morphology for MAPbI3-xClx is presumably associated with the formation of MACl intermediate65 in the precursor or during the film formation process. Jen and coworkers recently showed that adding a small amount (~1 wt%) of 1,8-diiodooctane (DIO) to the standard MAPbI3xClx

precursor solution could further improve the surface coverage and film morphology of

perovskites (Figure 4c,d).67 It was suggested that the DIO additive retards the crystallization rate of perovskite formation through chelation with Pb2+ to allow (or encourage) more defect-free perovskite crystal growth. In preparation of FAPbI3, the addition of a small amount of hydroiodic acid (HI) into the stoichiometric FAI:PbI2 perovskite precursor solution can significantly improve the uniform coverage and crystallization of FAPbI3 (Figure 4e,f).52 The final perovskite film is comparable to the vapor-deposited film. Furthermore, the addition of HI seems to be critical to avoid the formation of an unwanted yellow (impurity) phase.52


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Figure 5. (a) Illustration of the solvent engineering process. (b) Cross-sectional view of a device structure and (c) top view of SEM image of a perovskite film using the solvent engineering approach.55 Reproduced with permission from Ref. 55. Copyright 2014 Nature Publish Group. (d) Rapid Precipitation during Spin Coating. Perhaps the most significant, recent advancement in one-step solution process is the demonstration of solvent engineering to prepare extremely uniform, dense perovskite layers that lead to a certified power conversion efficiency of 16.2% with no IV hysteresis.55 Figure 5a shows the schematic of the solvent engineering procedure for preparing perovskite films. The key step involves applying a solvent (e.g., toluene) to the precursor film, during spin coating, that does not dissolve the perovskite precursor film but is miscible with the perovskite precursor solvent. The role of adding toluene is to induce a rapid increase of concentration of perovskite precursor materials uniformly across the entire substrate surface during spin coating when the excess solvent is separated by toluene and rapidly spun off. This process effectively freezes the precursor constituents and forms an intermediate phase that consists of the main perovskite components (e.g., PbI2 and MAI) and sometimes the solvent molecules (e.g., DMSO). Using DMSO in the solvent is found to retard the otherwise rapid reaction between PbI2 and MAI by forming DMSO-PbI2 complexes. The subsequent annealing leads to the formation of highly uniform and crystalline perovskites. The details of this solvent engineering process are given by Seok and coworkers.55 Using this process, the Seok group has recently demonstrated a 8.7%-efficient mini-module consisting of ten serially connected cells with

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glass/ITO/PEDOT:PSS/MAPbI3/PCBM/LiF/Al).51 Cheng and coworkers have independently developed the antisolvent concept in one-step solution-processing perovskite films.50 This antisolvent concept is essentially the same as the solvent engineering concept discussed above except that only DMF is used in this study. A series of 12 solvents were tested; the addition of chlorobenzene, benzene, xylene, and toluene are found to be effective in generating uniform perovskite films over the entire substrate.50

Figure 6. Illustration of inserting a layer of amino acid HI salts between TiO2 and MAPbI3.68 Reproduced with permission from Ref. 68. Copyright 2014 American Society of Chemistry.

(e) Electrode Surface Modification. Modifying the interfacial properties between the electrode (e.g., TiO2) and perovskite (e.g., MAPbI3) could alter the charge separation and back recombination kinetics as is often observed for dye-sensitized solar cells.69 A recent study by Hayase et al. inserted a layer of amino acid HI salts (HOOC-R-NH3+I-; C:n = 1,2,3; Figure 6) between the TiO2 electrode and MAPbI3 absorber and examined the effect on the device characteristics, charge generation kinetics, and recombination lifetimes.68 The results showed that the longer methylene group is more effective for increasing the cell efficiency. Using GABAH+I− (C:n = 3) improves the cell performance from about 7% to 10%. The improvement can be attributed to several factors, including surface passivation, enhanced perovskite crystal growth, faster electron injection, and slower recombination kinetics. Another paper uses a similar concept by using HOOC(CH2)4NH3I (5-AVAI; C:n = 4).18 It shows that using AVAI not only induces the c-axis-oriented growth of perovskite, but also enhances the coverage of perovskite on the electrode surface. Consistent with the report by Hayase et al., a longer recombination lifetime


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and improved cell performance were observed. These studies point out a promising direction for tuning the interface properties for developing perovskites with specific structural and electrical properties.

Two-Step Sequential Deposition of Perovskites. As we discussed above for one-step solution deposition of perovskite (e.g., MAPbI3) the morphology of perovskite film is controlled by the solution chemistry and processing conditions. Another way of making uniform, highquality perovskite film is to use a two-step sequential deposition process. Two-step deposition of perovskite film was first developed by Mitzi and his coworkers,26 and was first adapted by Grӓtzel et al. to make >15% perovskite solar cells.53 In a typical two-step deposition, a high concentration of PbI2 solution (e.g., in DMF) is first spin-coated on the mesoporous or planar substrate, followed by dipping into the MAI solution (e.g., in 2-propanol or IPA). During the second step, PbI2 reacts with MAI to form perovskite MAPbI3. The two-step process takes advantage of PbI2 being a layered-structure semiconductor and prone to intercalation reaction with ammonium,34, 70 pyridine,71 and methylamine.26, 53 In comparison to the one-step deposition process, PbI2 can be deposited at a higher concentration into a more compact and uniform film. The subsequent conversion of PbI2 to MAPbI3 leads to a perovskite film with higher absorbance; but it also creates more uniform perovskite films to ensure reproducibility in making highperformance perovskite solar cells. In contrast to the homogenous crystallization reaction in the one-step approach, a typical two-step method is a heterogeneous phase reaction between the PbI2 film and MAI solution. The morphology of the final perovskite is largely determined by the PbI2 film from the first-step deposition. Fortunately, it is relatively easy to grow a uniform PbI2 film because PbI2 tends to form a flat, layered structure. PbI2 can be prepared through either vapor deposition or spin coating. Various groups have adopted the two-step deposition technique to control perovskite film morphology for high-performance perovskite solar cells.6, 12, 19, 53-54, 72 (a) Warm Substrate and Precursor. It has been pointed out that keeping both the substrate and PbI2 precursor solution warm (e.g., 60o–70oC) prior to spin coating is critical to achieve optimum cell performance because the warm substrate and precursor solution help to form a compact and uniform PbI2 film on the substrate. The possible mechanism is that a warm precursor solution and warm substrate can promote the crystallization process of PbI2 film


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through evaporating solvent during the spin-coating process. This step has been commonly adopted to prepare high-quality PbI2 films in two-step deposition by many groups.6, 53-54, 73 (b) Pre-Wetting. It was first reported by Grӓtzel and coworkers that pre-wetting of PbI2coated TiO2 film in IPA prior to the second-step dipping in the solution of MAI and IPA could significantly enhance the solar conversion efficiency. The improvement largely results from a higher photocurrent density, which is likely caused by an enhanced loading of perovskite and light scattering from larger perovskite particle sizes. It is hypothesized that the pre-wetting of IPA decreases the local concentration of MAI, allowing the growth of larger perovskite crystals. Another study found that the pre-wetting time (from 0 to 10 s) strongly affects perovskite morphology and consequently the device performance.73 For example, a longer pre-wetting treatment leads to the formation of larger perovskite grains, which enhances light scattering of perovskite solar cells. The same study also pointed out that the reaction temperature (controlled by the solution temperature) of converting PbI2 into MAPbI3 during the second step is also critical to the PbI2-MAI reaction kinetics, and simultaneously, the perovskite crystallization process.73 It is worth noting that the soaking time required for converting PbI2 on mesoporous TiO2 to perovskite after pre-wetting treatment varies from less than a minute to hours, as reported by different groups.53, 74-75 Although the two-step process has the advantage of forming a more uniform perovskite film compared to the standard one-step solution process, it normally takes quite a long time (up to 2–3 hours) for the complete conversion of compact PbI2 into perovskite on planar substrates using the standard two-step sequential deposition.26 This prolonged soaking time could lead to the dissolution (sometimes peeling off) of perovskites, which significantly limits the reproducibility and potential large-scale fabrication. The long soaking step required for the planar PbI2 film is likely caused by the limited access of diffusion and intercalation of MAI into the compact PbI2 layers to form CH3NH3PbI3. This presents a dilemma for preparing large-area uniform compact perovskite layers. It is generally perceived that a high-quality perovskite needs a compact, uniform PbI2 precursor film; but a compact precursor PbI2 crystal film is generally more difficult to convert into perovskite completely. Therefore, several strategies have been developed to address this challenge as discussed below. (c) Additive. Similar to the one-step solution method, an additive such as MACl can also modify the formation of MAPbI3 perovskite in the two-step method. In addition to the potential


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modification of the electrical properties of perovskite from Cl doping (still a subject of debate), MACl can also work as an additive to control the morphology of the planar MAPbI3-xClx film using the two-step approach. A recent study54 shows that adding 5 wt% of MACl to the MAI solution during the second step of the conversion process significantly improves the device performance from less than 9% to about 15% efficiency. However, using more MACl (10–15 wt%) reduces cell efficiency, mainly due to the deterioration of the film morphology (formation of a large number of gaps between the crystals).

Figure 7. Comparison of using (a,c) crystalline and (b,d) amorphous PbI2 films on the morphology of MAPbI3 films prepared using two-step sequential solution deposition.61 Reproduced with permission from Ref. 61. Copyright 2014 Royal Society of Chemistry. (d) Amorphous PbI2. To deposit the PbI2 layer during the first step, DMF is often used as the solvent for preparing the PbI2 precursor. However, a recent study by Han et al. shows that when using DMF as the solvent, the PbI2 crystallizes very rapidly to form compact, large crystals of PbI2 with various grain sizes even in the absence of annealing (Figure 7a).61 This leads to the formation of MAPbI3 with a large variation of grain size (50 to 330 nm; Figure 7c); but it also inhibits the complete conversion of inner PbI2 to MAPbI3, without having the peeling-off issue. The uncontrolled residue PbI2 causes a large variation of device performance. In contrast, the same study shows that by using the strongly coordinated solvent DMSO, the PbI2 layer from the first step of deposition remains in the amorphous phase (because of the formation of PbI2-DMSO complex) for several months at room temperature. The amorphous PbI2 layer is much more uniform and smooth (Figure 7b), but it also enables the equal probability of all PbI2 to react with MAI, leading to a full conversion of PbI2 to MAPbI3. The resulting perovskite film also exhibits


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a more uniform distribution of perovskite crystals (size variation about 200±20 nm; Figure 7d). Consequently, high-performance cells with much improved reproducibility are demonstrated.61

Figure 8. (a) Schematic illustration of the vapor-assisted solution process for forming MAPbI3 films. (b) Top-view SEM image of the PbI2 film after dipping in the 10 mg/mL MAI IPA solution for 30 min. (c) Top-view SEM images of the PbI2 film after reaction with MAI vapor at 150°C for 2 h in N2 atmosphere; the inset shows a higher magnification with 1-µm scale bar.12 Reproduced with permission from Ref. 12. Copyright 2014 American Society of Chemistry. (e) Modified Two-Step Sequential Deposition. Another way of avoiding the dissolution and peeling-off issue (especially for making a planar perovskite film) during the second conversion step in the MAI solution is to expose the solution-processed PbI2 film to the MAI vapor during the conversion step. This solution-vapor hybrid approach was first demonstrated by Yang and coworkers to prepare high-quality, pinhole-free planar MAPbI3 films (Figure 8c). The schematic of this approach is illustrated in Figure 8a. In brief, a highly compact and uniform PbI2 film is first deposited by solution processing, which is subject to the MAI vapor at mild temperature under ambient condition to induce the intercalation reaction to form MAPbI3 perovskites. Another advantage of this hybrid method is to avoid the more expensive highvacuum and high-temperature equipment for vapor deposition of MAPbI3. In comparison, the planar MAPbI3 film fabricated from the same PbI2 film with soaking in the standard MAI solution in IPA for 30 min led to the formation of a very coarse, poor-quality MAPbI3 film due to the peeling off/dissolution issue discussed above. Another study with a similar two-step sequential deposition also produces large-area, pinhole-free, planar MAPbI3 films.76 The recent inter-diffusion method (i.e., spin coat a stack of PbI2 and MAI layers, followed by annealing treatment) also avoids the dilemma of controlling the balance between complete PbI2 conversion


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and the peeling-off of the perovskite films,72 leading to the formation of pinhole-free perovskite films. Thus, these modified two-step sequential deposition approaches appear to be promising for growing uniform, large-grain polycrystalline perovskite film for high-performance perovskite solar cells. In addition to the abovementioned solution-chemistry/processing control for the standard/modified two-step sequential deposition, it was reported that repeated deposition of PbI2 during the first step could also improve the surface coverage of the final perovskite films.74 Controlling the PbI2 concentration77 during the first step and the MAI concentration78-79 during the second step also affect the morphology of the perovskite film and consequently the device characteristics. It is interesting to note that a higher concentration of MAI generally leads to the growth of smaller perovskite crystals,79 whereas a higher concentration of PbI2 usually results in the formation of larger perovskite crystals.77 Although the exact perovskite crystal-growth mechanism during the two-step deposition remains to be examined, these solution-chemistry and/or processing controls can be used to adjust perovskite morphologies for specific applications.

Chemical Composition Management in Solution Chemistry. One of the attractive characteristics of inorganic-organic halide perovskites is that their optical bandgap can be tuned to match the different portion of the solar spectrum. In solution-chemistry processing, this can be achieved by simply varying the organic cation (e.g., MA, FA, and EA), metal cation (e.g., Pb and Sn), and/or halide ion (e.g., I and Br) of the precursor components for making perovskites with different compositions.5-6, 9, 14, 21, 52, 80-87 The Seok group and Snaith group have respectively demonstrated the ability of continuous bandgap tuning from 2.3 eV using MA- and FA-based perovskites consisting of mixed iodide and bromide with different ratios.5, 52 Grӓtzel and coworkers have recently reported the use of mixed organic cation perovskite of MAxFA1-xPbI3 with improved light harvesting and stability.81 Park and coworkers have demonstrated a ~16% perovskite solar cells based on a bilayer stack of MAPbI3 and FAPbI3 by doing simple organic cation exchange via solution processing (Figure 9a). This bilayer structure not only improves the photoresponse/light harvesting over a wider spectral range (Figure 9b), but it also demonstrates a


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solution approach for potentially producing multilayers of perovskites with different absorption properties to better match the solar spectrum.

Figure 9. (a) Illustrations and top-view SEM images of FAPbI3 and FAPbI3/MAPbI3 bilayer film structures. The scale bar is 1 µm. (b) Comparison of difference perovskite structures on the incident photon-to-electron conversion efficiency (IPCE) of perovskite solar cells.6 Reproduced with permission from Ref. 6. Copyright 2014 Royal Society of Chemistry. In addition to changing the organic cation and halide ion to tune the bandgap, replacing Pb with Sn has also been used to adjust the bandgap of MAPb1-xSnxI3 to about 1.2 eV.13, 88-90 Because there is concern about the toxicity of lead in perovskites, these studies on replacing Pb with Sn represent one step forward toward Pb-free perovskites for solar conversion applications. It is worth emphasizing that all these reports use solution-chemistry processing to tune the perovskite composition with different optical/electronic properties, which further confirms the effectiveness of solution chemistry for controlling the morphology, composition, and optoelectronic properties of halide perovskite absorbers.

Future Outlook: Issues and Challenges. Solution-chemistry engineering has clearly progressed to be competitive with vapor-phase approaches for developing large-area, uniform, high-quality perovskite films for low-cost and high-performance polycrystalline thin-film perovskite solar cells. The diversity of solution approaches with various solution/control parameters as discussed in previous sections demonstrates the flexibility and potential of solution approaches for this emerging perovskite research field; but it also creates complexity and a challenge for fully understanding the perovskite system prepared with different solution approaches. It is expected that various factors—including the microstructure, film architecture, material composition, impurity phase, defect/doping, and interfacial properties of perovskites—


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will affect the optical and electrical properties of perovskites and consequently influence the design and optimization of perovskite solar cells. These factors depend on various solution processing conditions for making perovskite films. To further improve the device performance of perovskite solar cells and make this PV technology competitive in the future PV market place, it is critical understand the impact of these and other factors on the fundamental charge-carrier dynamics and device operation principles, such as the charge generation, separation, and collection processes. Some fundamental studies have already been conducted and showed useful guidance for our improved understanding of perovskites as referenced in earlier sections. Here, we want to emphasize one factor (i.e., perovskite film architecture) that we believe to be critical and should receive attention when studying the fundamental properties of perovskite solar cells. Because perovskite (e.g., MAPbI3) cannot only serve as the light absorber but can also conduct electrons and/or hole, it is important to understand the impact of the three different perovskite film architectures (i.e., mesoporous, planar, and meso-planar hybrid structures as shown in Figure 1) on charge-carrier dynamics and device characteristics. We have recently shown by using frequency-resolved intensity-modulated photocurrent and photovoltage spectroscopies that the transport and recombination properties in perovskitesensitized solar cells based on a mesoporous TiO2 scaffold are similar to those in the solid-state dye-sensitized solar cells.63 This observation is similar to an earlier study by Park et al. for a comparison study of perovskite- and dye-sensitized solar cells.2 We further found that when the perovskite composition is varied from MAPbI3 to MAPbI2Br to MAPbIBr2 to MAPbBr3, transport rates and recombination lifetimes remain similar in the mesoporous perovskite cell structure.37 These results suggest that perovskites in this film structure merely act as the light absorber and that charge transport and recombination is dominated by the underlying mesoporous TiO2 network. However, these results contrast significantly with a recent paper by Mora-Sero et al. on the recombination study of combined halides perovskite solar cells based on a 250-nm-thick mesoporous TiO2 conducting scaffold.82 This study, using impedance spectroscopy, shows that the composition of perovskite is critical to recombination kinetics. The incorporation of Br and Cl in the perovskite structure is shown to reduce recombination kinetics, leading to improved photovoltage.82 The different observations on the effect of perovskite composition on charge recombination from these two studies37, 82 likely results from the different perovskite film architectures as depicted in Figure 1. Because we used a relatively thick (~650


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nm) TiO2 scaffold,37 the perovskite film architecture is properly described as the mesoporous film structure (Figure 1a). In contrast, Mora-Sero et al. used a relatively thin (~250 nm) TiO2 scaffold, and consequently, the perovskite film architecture is properly described by the mesoplanar hybrid structure (Figure 1c), which is also evidenced by the perovskite capping layer on top of the mesoporous perovskite/TiO2 layer.82 The difference on the impact of perovskite film architecture via solution processing on the charge-carrier dynamics is perhaps better summarized in a recent study by Snaith et al. on the importance of pore filling on perovskite solar cells.48 It shows that when TiO2 layer thickness is systematically varied from 750 to 260 nm, the perovskite solar architecture is changed from one where the perovskite acts only as sensitized with incomplete surface coverage to one where perovskites fill the pores and act not only as the light absorber but also as the charge transporter.48 The pore filling with perovskite could reduce charge recombination from TiO2 to the hole-conducting materials (e.g., Spiro-MeOTAD). The key measures for a viable PV technology are its efficiency, cost, and reliability. With cell efficiencies rapidly approaching 20%, most people are convinced that perovskite solar cells will have no problem being competitive on efficiency. The material and fabrication costs of perovskite absorbers are low due to the Earth abundance of individual elements of perovskite and the low-cost, scalable solution processing. However, the conventional hole-transport material (i.e., spiro-MeOTAD) is quite expensive. In addition, noble/precious metals (e.g., Au or Ag) are typically used as the top contact to extract photogenerated holes. A promising research direction for perovskite solar cells would be to replace these expensive materials/components with lowcost alternatives or to use new device architectures to avoid certain expensive cell material/component. Some groups have already demonstrated progress in this direction. We have recently found that a bilayer of MoOx/Al can replace Au or Ag as the top hole-collection contact without affecting the device performance.42 Low-cost inorganic hole-conducting materials (e.g., CuI and CuSCN) have also been incorporated in perovskite cells with reasonable efficiencies.11, 91-92

Multiple groups7, 74, 93 have even demonstrated hole-conductor-free perovskite cells. Inverted

device architectures using PEDOT:PSS and PCBM as hole- and electron-transport materials have also shown promising results.51, 72, 94 Another interesting area is to replace spiro-MeOTAD with carbon-based hole-collecting contact materials,18,

95-96

which is expected to attract more

attention in the near future. The most critical challenge in the next few years is to demonstrate the long-term stability


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of perovskite devices under operating conditions (including accelerated aging tests). MAPbI3 perovskite is generally accepted to be sensitive to moisture and high temperature, both of which can change the crystal structure of MAPbI3. We even find that the NH3 gas can rapidly induce the phase transformation of MAPbI3 under room temperature.34 Despite these known stability issues, there are only a limited number of studies that address the stability issues of perovskites; most perovskite-related studies have focused on cell efficiency and device architectures. Key degradation pathways and the underlying mechanisms have yet to be thoroughly examined and understood. While one study indicates that the bulk crystal MAPbI3 is stable up to >200°C,97 another study shows that about 140°C annealing is enough to decompose the CH3NH3PbI3 into PbI2.98 Park and Grӓtzel et al., in their first report of solid-state perovskite sensitized solar cells, showed stable device performance of mesoporous-TiO2-based perovskite solar cells of over 500 h when stored in air at room temperature without cell encapsulation.3 However, work by Snaith et al. indicates that the use of TiO2 might degrade MAPbI3 by active oxygen generated from UVexcited TiO2; this issue could be overcame by using mesoporous Al2O3 film instead of TiO2.39 These reports clearly suggest that more fundamental investigations are necessary to understand the degradation mechanisms of perovskites and their dependence on materials synthesis and device architecture. Recently, a porous carbon electrode has been used to demonstrate the stability of perovskite cells under continuous light soaking for over 1000 h.18 In addition, chemically modified perovskites, such as FAPbI3 and MAPbI3-xBrx, have shown improved stability compared to standard MAPbI3.5, 52 All these reports suggest that the stability issue has begun to attract increasing attention. However, more studies are required to evaluate perovskite at both the material and device scales to identify key factors determining cell stability. Fundamental studies on material properties and principal photophysics and their relationship to the device characteristics under various operating conditions need to be established. Moreover, various cell encapsulation schemes will need to be examined to minimize the ingress of water/moisture for enhancing the lifetime of perovskite solar cells. In summary, solution-chemistry engineering has demonstrated rapid progress in a short period of time for producing high-quality perovskite films, leading to high-performance singlejunction 15%–20%-efficient perovskite solar cells that have already attracted industrial interest.25 Further understanding and improving the solution chemistry is expected to lead to perovskite solar cells with efficiencies greater than 20%. Continued advances in perovskite device


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performance will be based on a thorough understanding of the impact of various solution processing parameters and perovskite film architectures on fundamental charge-carrier dynamics. Other issues such as improving the reproducibility of solution-processed perovskite solar cells and understanding the impact of the device architecture and various solution processing controls/conditions on the stability of perovskite devices should also be fully examined and understood to make perovskite solar cells a relevant PV technology.

AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected] (YZ); [email protected] (KZ).

Biographies: Yixin Zhao is an associate professor at Shanghai Jiao Tong University and his research interest is on solar energy and nanomaterials for environmental remediation. He obtained his PhD from Case Western Reserve University in 2010 followed by working as postdoctoral fellow at Penn State University and National Renewable Energy Laboratory. Kai Zhu is a senior scientist in the Chemical and Materials Science Center at the National Renewable Energy Laboratory (NREL). He received his PhD degree in physics from Syracuse University in 2003. His recent research is focused both basic and applied studies on perovskite solar cells, including material development, device fabrication/characterization, and basic understanding of charge carrier dynamics in these cells.

ACKNOWLEDGMENT YZ is thankful for the support of NSFC (Grant 51372151). KZ acknowledges the support by the U.S. Department of Energy/National Renewable Energy Laboratory’s Laboratory Directed Research and Development (LDRD) program under Contract No. DE-AC36-08GO28308.

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Quotes to highlight in paper:

Mesoporous-planar hybrid perovskite cell structure is promising for high solar conversion efficiency with negligible IV hysteresis.

Solution-chemistry approaches have progressed so much within a very short period of time that a high-quality perovskite thin film comparable to that by vapor deposition has been reported by various groups, leading to the record efficiency of perovskite solar cells.

Controlling the interactions among the substrate, precursor solution, and processing environment during spin coating and post-growth treatment is critical to the final perovskite film morphology.

Adjusting the PbI2-CH3NH3I reaction kinetics and the perovskite crystallization process is important to two-step sequential growth of high-quality perovskite films.

The most critical challenge in the next few years is to demonstrate the long-term stability of perovskite devices under operating conditions (including accelerated aging tests).


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Solution Chemistry Engineering toward High ... - ACS Publications

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