How to Make over 20% Efficient Perovskite Solar Cells in Regular (nâi

Methods/Protocols Cite This: Chem. Mater. 2018, 30, 4193−4201

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How to Make over 20% Efficient Perovskite Solar Cells in Regular (n−i−p) and Inverted (p−i−n) Architectures Michael Saliba,†,∇ Juan-Pablo Correa-Baena,‡,∇ Christian M. Wolff,§,∇ Martin Stolterfoht,§ Nga Phung,# Steve Albrecht,∥ Dieter Neher,§ and Antonio Abate*,# †

Adolphe Merkle Institute, Chemin des Verdiers 4, CH-1700 Fribourg, Switzerland Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States § University of Potsdam, Institute of Physics, Karl-Liebknecht-Str. 24-25, 14476 Potsdam, Germany ∥ Young Investigator Group Perovskite Tandem Solar Cells, #Young Investigator Group Active Materials and Interfaces for Stable Perovskite Solar Cells, Helmholtz-Zentrum Berlin für Materialien und Energie, Kekuléstraße 5, 12489 Berlin, Germany

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‡

S Supporting Information *

ABSTRACT: Perovskite solar cells (PSCs) are currently one of the most promising photovoltaic technologies for highly efficient and cost-effective solar energy production. In only a few years, an unprecedented progression of preparation procedures and material compositions delivered lab-scale devices that have now reached record power conversion efficiencies (PCEs) higher than 20%, competing with most established solar cell materials such as silicon, CIGS, and CdTe. However, despite a large number of researchers currently involved in this topic, only a few groups in the world can reproduce >20% efficiencies on a regular n−i−p architecture. In this work, we present detailed protocols for preparing PSCs in regular (n−i−p) and inverted (p−i−n) architectures with ≥20% PCE. We aim to provide a comprehensive, reproducible description of our device fabrication protocols. We encourage the practice of reporting detailed and transparent protocols that can be more easily reproduced by other laboratories. A better reporting standard may, in turn, accelerate the development of perovskite solar cells and related research fields.

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INTRODUCTION Organic−inorganic perovskites are currently at the center of semiconductor research with the promise to deliver the next generation of highly efficient, inexpensive photovoltaic technology.1 In only a few years and with relatively low investments, prototype devices prepared on a lab scale demonstrated power conversion efficiency (PCE) of ≥20%. However, only a few research groups reported perovskite solar cells (PSCs) with PCEs above the “psychological” barrier of 20%, which has been reserved for only the most established materials such as silicon, GaAs, CIGS, and CdTe.2−4 Although efficiencies are now approaching the thermodynamic limit, there is still incomplete knowledge of fundamental working and degradation mechanisms. A deeper understanding is crucial to improving device performances and long-term stability further.1,5 Complex perovskite compositions, including multiple cations, anions, and metals, are becoming more widespread across scientific communities that focus on diverse topics from the chemistry and physics of the material to the device operation. This frequently requires nonexpert laboratories to fabricate highly efficient solar cells. Such research often is not © 2018 American Chemical Society

directly aimed to improve state-of-the-art performances. Nevertheless, the most meaningful insights stem from research that is conducted on high performing and reproducible PSCs that are frequently very different in nature (and thus incomparable) from their less efficient counterparts. Unfortunately, newly reported protocols for the highest PCE are hard to replicate promptly even for very experienced groups. As a result, a significant gap exists between groups that can achieve high-efficiency PSCs and groups that investigate fundamental properties of materials and devices. This slows down progress and may even lead to seemingly contradictory results. In this work, we present comprehensive protocols that yield reproducible, >20% PSCs for the three major, single-junction architectures that are currently used in the community. As depicted in Figure 1, these are the mesoporous and planar regular (n−i−p) structure and the so-called planar inverted (p−i−n) structure. We report detailed preparation steps, commonly encountered problems, and the strategies we have Received: January 11, 2018 Revised: June 11, 2018 Published: June 11, 2018 4193

DOI: 10.1021/acs.chemmater.8b00136 Chem. Mater. 2018, 30, 4193−4201

Chemistry of Materials

Methods/Protocols

Figure 1. Schematics of the three PSC architectures described in this work, comprising planar regular (n−i−p), mesoporous (n−i−p), and planar inverted (p−i−n) architectures. The devices comprise a transparent conductive oxide (TCO) on top of a glass substrate, a compact and mesoporous electron transporting layer (c & mp - ETL), a hole transporting layer (HTL), the perovskite, and the top electrode. The corresponding SEM cross-section images of representative devices are shown below with a scale bar of 200 nm. Half of the SEM images have been colored following the schematic on the top. Perovskite Precursor Solution. Please note that all chemicals for all procedures are listed at the end of Table 1. The preparation of the perovskite precursor solution is only apparently simple. The stoichiometry6,7 and the formation of coordination complexes8,9 within perovskite precursors in solution may largely impact the device performances. Therefore, both the preparation and the storing conditions of the perovskite precursor solution are very delicate steps that affect the reproducibility of highly efficient devices. In general, we recommend a controlled preparation environment, i.e., using a nitrogen-filled glovebox (water and oxygen content below 1 ppm), anhydrous and high-purity solvents, and a recently calibrated balance and pipet. In the following, we detail the recipe to prepare the (FAPbI3)x(MAPbBr3)1−x “MAFA” solution, which has been known to deliver high-efficiency solar cells since 2015.10 We then describe the addition of alkali metal cations (Cs or Rb), which was used for our most efficient cells in all three architectures (see also Figure S2 for an SEM top view of the respective perovskite layers, and Table S1 for all layer thicknesses)2,11,12 Here, we describe the method used for making perovskite of the n− i−p structure; because of a slight modification in p−i−n structure, the difference in details will be reported in the SI). First, we prepare a stock solutions of 1.5 mol of PbI2 and PbBr2 per liter of 4:1 V/V dimethylformamide (DMF)/dimethyl sulfoxide (DMSO). The stock solutions are placed on a hot plate at room temperature and heated to 180 °C within about 10 min. We note that such high temperatures for dissolving the PbI2 and PbBr2 are not often reported in the literature. The PbI2 (PbBr2) powder dissolves within about 15 (5) min resulting in a clear looking solution. Then they are left to cool to room temperature. The stock solutions can be stored for a long time; however, reheating to 180 °C is required each time. The actual molarity (M) of the stock solution is calculated from the density of the solutions and the density of the solvent using the formula:

developed to prevent them. Importantly, we extensively describe and discuss experimental details that are commonly ignored in published methods. We encourage other groups to follow this approach and to report very detailed preparation methods in the future. We hope that this will become a new norm in perovskite publications similar to the field of biology, for example, where entire journals are devoted to protocols and methods acknowledging the importance of meticulous reporting.

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METHODS

Figure 1 displays three commonly used types of devices for singlejunction PSCs, which comprise planar and mesoporous heterojunction in regular (n−i−p) and inverted (p−i−n) architectures. In a regular (n−i−p) architecture, the bottom contact is selective for electrons, while the top contact is selective for holes. In the inverted (p−i−n) architecture, the selective hole contact is at the bottom. For the most part, the light reaches the perovskite layer passing through the glass substrate, irrespective of architectures. We grouped the device preparation to show the differences between the three protocols according to the following outline: Perovskite precursor solution Substrate cleaning Bottom selective contacts SnO2 compact layer on FTO for regular (n−i−p) planar architectures TiO2 compact plus mesoporous layer on FTO for regular (n−i−p) architectures PTAA on ITO for inverted (p−i−n) architectures Perovskite film deposition For the mesoporous architecture Planar regular (n−i−p) architecture Planar inverted (p−i−n) architecture Top selective contacts Spiro-OMeTAD in regular (n−i−p) mesoporous and planar architectures PTAA in regular (n−i−p) meso and planar architectures C60 and BCP for inverted (p−i−n) architectures Metal electrode evaporation Gold in regular (n−i−p) planar and mesoporous architecture Copper in inverted (p−i−n) architecture

Msolution = =

mol powder Vsolution

=

Wpowder Vsolution Mwpowder

=

dsolution A (A + 1)Mwpowder

dsolventVsolventusedtopreparethestocksolution Wpowderusedtopreparethestocksolution

where MSolution is the molarity of the stock solution, unknown parameter; molpowder is the moles of precursor (PbI2 or PbBr2) in the stock solution, unknown parameter; VSolution is the volume of the 4194



Methods/Protocols

is fully dissolved. An example for the inorganic stock solution is provided in Table 3.

Table 1. List of All Chemicals, CAS, Product Number, and Producer chemical PbI2 PbBr2 CsI RbI MABr FAI MAI dimethylformamide dimethyl sulfoxide chlorobenzene acetonitrile ethyl acetate toluene co-dopant Li TFSI tBP Spiro-MeOTAD PTAA regular (n−i−p) devices titanium diisopropoxide acetylacetone mercaptoacetic acid urea SnCl2 × 2H2O TiO2 paste (30 NR-D) PTAA inverted (p−i−n) devices C60 BCP copper

producer

CAS

product number

TCI America TCI America abcr GmbH abcr GmbH Dyesol Dyesol Dyesol Acros Acros Acros Acros SigmaAldrich Acros Dyenamo

10101-63-0

L0279

10031-22-8

L0288

7789-17-5 7790-29-6 6876-37-5 879643-71-7 14965-49-2 68-12-2 67-68-5 108-90-7 75-05-8 141-78-6

AB207757 AB203369 MS301000 MS150000 MS101000 34843 34844 44300 36431 270989

108-88-3 1447938-61-5 90076-65-6 3978-81-2

36441 FK209 544094 142379

207739-72-8

SHT-263

17927-72-9

325252

123-54-6

P7754

68-11-1

528056

57-13-6

1.08487

10025-69-1

474762

1333317-99-9

MS002300 702471

99685-96-8 4733-39-5

V0502010 699152

7440-50-8

254177

SigmaAldrich Merck EM Index SigmaAldrich SigmaAldrich SigmaAldrich SigmaAldrich SigmaAldrich Dyesol SigmaAldrich creaphys SigmaAldrich SigmaAldrich

Table 3. Examples of the Nominal, Inorganic Stock Solutions solution

density (g/mL) 0.980 ± 0.001 0.141 ± 0.001 0.151 ± 0.001

g g g g

in in in in

1.446 1.816 2.566 3.139

mL mL mL mL

Table 4. Example of the RbCsMAFA Precursor Solution FAPbI3 solution MAPbBr3 solution

RbCsMAFA solution

FAI powder

PbI2 stock solution(Table 3)

additional (DMF/DMSO 4:1 V/V)

0.1 g MABr powder

0.468 mL PbBr2 stock solution (Table 3)

0.1 g

0.704 mL

0.001 mL additional (DMF/ DMSO 4:1 V/V)

FAPbI3 solution

MAPbBr3 solution

CsI stock solution (Table 3)

0.250 mL

0.050 mL

0.016 mL

0.016 mL RbI stock solution (Table 3) 0.017 mL

Substrate Cleaning. FTO and ITO substrates are used for regular (n−i−p) and inverted (p−i−n) architectures, respectively. We note that each protocol has been developed for the specific substrate and is not interchangeable. Cleaning the FTO or ITO substrates is the most basic step that can already induce variation if conducted improperly. PSCs prepared with regular (n−i−p) or inverted (p−i−n) architectures comprise prepatterned FTO (7 Ohm/sqr, Pilkington TEC7) or ITO (15 Ohm/sqr Lumtec or Automatic Research) glass substrates, respectively. The FTO/ITO are thoroughly cleaned using the following procedure: • Brush as vigorously as possible, without scratching/damaging the FTO or ITO surface, using Hellmanex cleaning solution diluted with water in a 2:98 vol/vol ratio • Ultrasonic bath in 2% Hellmanex solution for 15 min • Rinse with copious amounts of deionized water • Ultrasonic bath in isopropanol for about 15 min • Ultrasonic bath in acetone for about 15 min • Rinse with copious amounts of acetone and then isopropanol • Dry the isopropanol quickly with a strong air or nitrogen flow

Table 2. Densities around 25 °C of the Solvent and Stock Solutions of PbBr2 and PbI2 Measured Weighting 100 μL solution

1 1 1 1

The MAFA precursor solution with the nominal formula of (FAPbI3)83(MAPbBr3)17 is prepared by mixing the FAPbI3 and MAPbBr3 (with a PbI2 excess) solution in a 5:1 V/V ratio. However, the MAFA perovskite is very sensitive to the precise environmental parameters (temperature, solvent vapor).11 The addition of inorganic salts (CsI and RbI) to the MAFA precursor solution resulted in much more reproducible PSCs.2,11 The stock solutions of inorganic salts are prepared by dissolving 1.5 mol of CsI per liter of DMSO and 1.5 mol of RbI per liter of 4:1 V/V DMF/DMSO. The solutions are heated to 150 °C until the powder is completely dissolved and the solution looks clear. Both solutions can be stored long-term without the need to reheat before usage. The CsMAFA precursor solution is prepared by adding 5 vol % (4 vol % for the inverted architecture) of CsI stock solution to the MAFA precursor solution. Accordingly, the quadruple cation composition RbCsMAFA is prepared by adding 5 vol % RbI stock solution to the CsMAFA precursor solution. The solution is stored in the dark in a nitrogen-filled glovebox. The device performances may change with the storing time of the perovskite precursor solution.14,15 We experienced more reproducible results using the precursor solutions within few hours after the preparation. However, this is still subject to ongoing research. We provide a complete example of a RbCsMAFA precursor solution in Table 4.

solution; Mwpowder is the molecular weight of the precursor (PbI2 or PbBr2); Wpowder is the weight of the precursor (PbI2 or PbBr2) in the stock solution; and dsolution is the density of the stock solution. The densities, dsolution and dsolvent, are calculated from the weight of 100 μL of solution (100 μL is chosen for convenience). With a glovebox temperature around 25 °C, this gives the values reported in Table 2. Methylammonium bromide (MABr) and formamidinium iodide (FAI) powders are weighed out in two separate vials. Then the volume of PbI2 (PbBr2) stock solutions is calculated from MSolution and added to the vials containing the FAI (MABr) powder to get a stoichiometry of FAI/PbI2 (MABr/PbBr2) of 1:1.09 (i.e., 9% lead excess).13 The solutions are shaken for few minutes until the powder

DMF/DMSO 4:1 V/V PbBr2 1.5 mol per L PbI2 1.5 mol per L

example

PbI2 (DMF/DMSO) PbBr2 (DMF/DMSO) CsI (DMSO) RbI (DMF:DMSO)

4195



Methods/Protocols

spinning, the layer is dried at 100 °C for a few minutes. Subsequently, the TiO2 is sintered in multiple temperature steps as reported in Table 5. Again, we advise using a hot plate with a lid that enables a

• UV ozone or plasma cleaning for 15 min at the maximum available power The substrates should be processed further immediately after the cleaning. We often noticed that letting the surface dry in between the cleaning steps might leave undesired streaks. The glass substrate needs to look spotless and free of streaks. We recommend that all the cleaning solvents should be extra pure or electronic grade. Bottom Selective Contacts. SnO2 Compact Layer on FTO for Regular (n−i−p) Planar Architectures. After cleaning, UV ozone or plasma cleaning is used for 15 min right before depositing the SnO2. The deposition of the compact SnO2 layer on the FTO involves a chemical bath deposition (CBD) step, where 0.5 g of urea is dissolved in 400 mL of deionized water, followed by the addition of 10 μL of mercaptoacetic acid and 0.5 mL of 37% HCl. Finally, 0.1 g of SnCl2 × 2H2O is dissolved in the above solution followed by stirring for 2 min. The solution should be clear. The substrates are horizontally laid in a glass container filled with the solution and heated to 70 °C in a lab oven for 3 h. The temperature of the solution is very important, and therefore, depending on the control of the oven, the temperature of the solution should be monitored with a thermocouple. When using an oven that does not control for temperature very well, we used a higher temperature (80 and 90 °C) to induce the formation of SnO2. In the absence of an oven, the process can also be conducted on a hot plate, but higher temperatures need to be used (e.g., 120−150 °C). After the deposition, the solution turns slightly cloudy. Deionized water replaces the liquid in the container, and it is sonicated for 2 min in an ultrasonication bath, which helps to remove large agglomerations of the metal oxide on the FTO. The CBD process is then followed by an annealing step, which takes place on a hot plate at 180 °C for 1 h. We advise the usage of a hot plate with a lid that enables more uniform temperature distribution over the plate ensuring a gentle flow over the substrate during the whole annealing process. TiO2 Compact Plus Mesoporous Layer on FTO for Regular (n−i− p) Architectures. Immediately after the cleaning, the FTO substrates are transferred to a hot plate and quickly warmed up to 450 °C (within 30 min). The substrates are left for 15 min at 450 °C before depositing the compact TiO2 layer by aerosol spray pyrolysis. The sprayed solution comprises 0.4 mL of acetylacetone and 0.6 mL of titanium diisopropoxide bis(acetylacetonate) in 9 mL of ethanol, which is sufficient to coat an FTO surface of about 10 × 10 cm2. First, the acetylacetone and then the diisopropoxide bis(acetylacetonate) is added to the ethanol. The acetylacetone enhances the solubility of the diisopropoxide bis(acetylacetonate). Inverting the sequence, i.e., adding diisopropoxide bis(acetylacetonate) first, may result in the unwanted formation of TiO2 crystallites. Immediately after the preparation, the solution is shaken for a minute before spraying it using pure oxygen as a carrying gas. The nozzle (in this case purchased from GlasKeller Basel AG called “Chromatografie Zerstäuber”) is about 20 cm far from the FTO surface and at an angle of 45° (see also Video S1). The nozzle is rapidly moved around in a single circle to cover the whole 10 × 10 cm2 FTO surface (already precut for single devices) within a second. This cyclic movement is repeated until finishing the solution with a delay of 20 s between each cycle. Finishing the solution usually takes about 10 min. Then the FTO glasses are left for 10 more minutes at 450 °C before cooling them down slowly to room temperature; i.e., the substrates are not removed from the hot plate until the temperature is below 150 °C. It is important that the hot plate has a uniform temperature distribution, and the aerosol flow is homogeneous and made of small droplets. We often noted that the diisopropoxide bis(acetylacetonate) might degrade if not correctly stored, i.e., at low temperature in inert atmosphere. Samples with TiO2 compact layers can be stored for weeks in a drawer before depositing the mesoporous TiO2 layer. To prepare the mesoporous TiO2 layer, we use 30 N-RD Dyesol TiO2 paste diluted as 150 mg per milliliter of ethanol. The resulting dispersion is left under vigorous stirring overnight before use. The dispersion is stored under continuous stirring. An about 150-nm-thick TiO2 meso-layer can be achieved by spin−coating the above dispersion on the FTO/compact-TiO2 substrates using a speed of 4000 rpm and 2000 rpm/s acceleration for 10 s. Immediately after the

Table 5. Sintering Procedure for the TiO2 Mesoporous Layer ramp (min) temp (°C) hold (min)

5 125 5

15 325 5

5 375 5

5 450 30

more uniform temperature distribution over the plate ensuring a gentle flow over the substrate during the whole annealing process. The hot plate is left to cool slowly to 150 °C before removing the samples. The resulting meso-TiO2 film should look uniform and semitransparent. Streaks or spots may indicate a problem with the TiO2 dispersion or deposition. Dust may also cause inhomogeneity. Within the same batch, bad and good looking substrates may occur, and it is advised to discard the bad looking ones. The resulting meso-TiO2 layer is then doped with lithium.16 For this, a solution of 10 mg per mL of bis(trifluoromethane)sulfonimide lithium salt in acetonitrile is prepared. This solution is spin coated onto the meso-layer using 3000 rpm and 1000 rpm/s acceleration for 10 s. The layer is again sintered according to Table 5. The hot plate is left to cool slowly to 150 °C, and the hot samples are rapidly moved into a nitrogen-filled glovebox. We experienced that sintering directly in the glovebox results in significantly lower efficient PSCs because of the formation of oxygen defects as previously addressed.17 PTAA on ITO for Inverted (p−i−n) Architectures. This process takes place in a nitrogen-filled glovebox. Typically 4.5 mg of poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA, Mn = 7000−10 000, PDI = 2−2.2) is dissolved in 3 mL of anhydrous toluene and stirred for 3 h at room temperature in a nitrogen-filled glovebox. The PTAA solution can be stored for a few weeks in the dark and in an inert atmosphere. To prepare the film, 60−80 μL of the solution is uniformly spread on the about 2.5 × 2.5 cm2 ITO substrate and spin−coated at 6000 rpm for 30 s using an acceleration of 2000 rpm/s acceleration for 30 s. The samples are annealed on a preheated 100 °C hot plate for 10 min and then left to cool for 5 min before the next step. This results in an approximately 8-nm-thick PTAA layer as measured with atomic force microscopy.12,18 The PTAA film should be transparent without any spots or cracks, but its presence can be confirmed by scratching along the side of the substrate with, e.g., tweezers, which should be just visible by the eye. Notably, despite the very thin PTAA layer, we observe no shunting which can be directly linked to the thin PTAA layer. However, we have observed that small changes in the PTAA thickness can lead to substantial FF changes as shown in our previous work.12 For instance, spin-coating PTAA from the 1.5 mg/mL solution at 6000 rpm should enable a fill factor of 80% (provided that the device geometry is not limiting the extraction of charges). However, a 5-nm-thicker PTAA film (e.g., obtained from a 3 mg/mL solution) already reduces the FF by an average 4%, which is a result of the increased series resistance due to the PTAA. We also note that higher FFs than 80% are feasible by spin−coating from even lower concentrations (e.g., average 82% using a 0.5 mg/mL solution). However, incomplete coverage of the ITO will result in additional shunts which lower the open-circuit voltage and overall efficiency. Thus, as a troubleshooting guide, the PTAA layer thickness can be increased if shunts are observed, or decreased if the FF is substantially below 80% (see Table S1 for the different layer thicknesses). Perovskite Film Deposition. For the Mesoporous Architecture. We note that the temperature in the glovebox plays a major role in the quality of the perovskite film. The method we describe here has been optimized for temperatures in the glovebox between 22 and 27 °C. We systematically observed much lower efficiency when the glovebox temperature went above 28 °C, even if the perovskite films looked uniform and dark. We hypothesize that a higher temperature may induce crystallites within the precursor before the solution is 4196



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Figure 2. Spin coating process of perovskite films (a) before, (b) during, and (c) after the spinning. (d) Sample after annealing (scale bar in d, 2.5 cm).

Figure 3. Perovskite film of an inverted (p−i−n) planar structure (a) before annealing and (b) after annealing. The scale bar is 2.5 cm. deposited on the substrate, consistent with inverse temperature crystallization of the perovskite,19 which impacts the morphology and thus the quality of the perovskite film. In the glovebox, the samples are left to cool to room temperature and then processed immediately without storing them. The RbCsMAFA or the CsMAFA perovskite film is deposited by spin coating the precursor solution in a two-step protocol: a slow step (10 s at 1000 rpm and 200 rpm/s) to ensure full surface coverage after dropping the solution in the middle of the substrate and a subsequent fast step (20 s at 6000 rpm and 2000 rpm/s). During the second step, the antisolvent is dropped with a pipet in the middle of the rotating substrate. Five seconds before the end of the spinning, 200 μL of chlorobenzene (the antisolvent) is rapidly dropped in the middle of a 2 × 2 cm2 substrate. Dropping the antisolvent is one of the most delicate steps because the right pressure, induced by hand, is required to not wash off the perovskite precursor solution from the substrate. Dropping too rapidly can induce a hole in the middle of the substrate; dropping too slowly can induce cracks in the final film. Additionally, aiming the antisolvent toward the middle avoids a hole in the middle. To acquire the necessary technical skill for this step, manual practice is required (see Videos S2 and S3). Following the spin-coating step, the substrates are placed immediately on a hot plate preheated at 100 °C for 40−60 min. Before annealing, the perovskite film should look semitransparent, brown-like-colored. It should turn opaque black after a few minutes on the hot plate. After annealing, the film surface should look smooth and shiny as shown in Figure 2. Any inhomogeneity, pinholes, cracks, or a hazy-looking surface can affect the device performance. Before starting new samples, sufficient time has to be given to clear the atmosphere in the glovebox from DMSO/DMF vapors. This can be achieved, for example, through a constant flow of nitrogen for an hour; thus the glovebox should be continuously purged during the spinning and the annealing of the perovskite film. Planar Regular (n−i−p) Architecture. The perovskite deposition method for the n-i−p planar architecture follows that of the mesoporous architecture. The only difference is that right before the perovskite deposition (30 min or less), UV-ozone or microwave plasma treatment is used to clean the SnO2 surface. No further

sintering is needed. SnO2 samples can be stored in a drawer for months before the perovskite deposition. Planar Inverted (p−i−n) Architecture. We recommend using a fresh CsMAFA solution (not more than 1 week old), as older solutions may result in pinhole formation. We have observed no significant difference in the perovskite film quality for glovebox temperatures between 20 and 30 °C. However, it is important to note that we use a laminar flow glovebox with a continuous flow of fresh nitrogen to remove DMSO/DMF vapors. Furthermore, we clear the spin-coater atmosphere from residual solvent vapor using, for example, a hairdryer in between processing of the samples. CsMAFA perovskite films are prepared by centrally dropping 120 μL of the perovskite solution on a 2.5 × 2.5 cm2 ITO/PTAA substrate and spincoating at 5000 rpm with 3000 rpm/s acceleration for 35 s. We note the perovskite spin speed can be varied between 2000 and 6000 rpm, with and without the acceleration step at the beginning with little variation in the final PCE (1 Å/s) produce copper layers that are seemingly less shiny and yield poorer performing solar cells. A complete inverted (p−i−n) device is shown in Figure 5. Part

(trifluoromethane)sulfonimide lithium salt (LiTFSI), and 4-tertbutylpyridine (tBP) in chlorobenzene, is prepared using the stock solutions reported in Table 6. First, the spiro-OMeTAD is dissolved in chlorobenzene (70 mM) by shaking the solution for a few minutes at room temperature. Then, the appropriate volumes of FK209 and LiTFSI stock solutions and tBP are added as reported in Table 6. To deposit the spiro-OMeTAD film on top of the perovskite, 50 μL of the solution is rapidly dropped in the middle of a 2 × 2 cm2 substrate while spinning at 4000 rpm. After dropping the solution, the substrate is left spinning for 10 s. The spiro-OMeTAD film on top of the perovskite should look homogeneously distributed over the substrate and smooth. There may be aggregates visible by the eye (see Figure S1), which suggests that the additives were not fully dissolved or were hydrated. In particular, the LiTFSI is highly hygroscopic and should be stored in the glovebox without ever being exposed to air. Overdosing with LiTFSI (by accidentally adding more than prescribed) can lead to a drop in open-circuit voltage (Voc).20,21 The FK209 and LiTFSI stock solutions can be stored for weeks, but the spiro-OMeTAD solution with or without the additives needs to be prepared right before usage. All the preparation takes place in a nitrogen-filled glovebox using anhydrous high-purity solvents. PTAA in Regular (n−i−p) Meso and Planar Architectures. We note that the dopant concentration for the PTAA is relatively small. Therefore, larger volumes of PTAA solution (from 1 mL) may be required to ensure that the volume of dopant solution needed can still be measured accurately with the pipet. The poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) layer is deposited similarly to spiro-OMeTAD, using the same stock solutions of LiTFSI and tBP reported in Table 6 (no FK209 is needed for PTAA doping). The PTAA is dissolved in anhydrous toluene at a concentration of 10 mg per milliliter. If we consider the molecular weight of the monomer (Mw 258.6), as for the spiro-OMeTAD, we can calculate the right volumes of LiTFSI solution and tBP to achieve the molar ratios of 0.082 and 0.390, respectively. Subsequently, 50 μL of the solution is dropped in the middle of a 2 × 2 cm2 substrate before spinning at 4000 rpm for 20 s. 4198



Methods/Protocols

Figure 5. An example of (a) inverted (p−i−n) devices with 6 pixels with an active area of 6 mm2 (scale bar 2.5 cm) and (b) the same device with optional encapsulation. (c) Schematic showing the two copper top electrodes and six silver paste stripes to contact the bottom ITO. The actual solar cell has a 6 mm2 active area, which is defined as the overlap between the copper top electrode and the bottom ITO. The active area measured during the testing is defined by a shadow mask. This device design achieved a record efficiency above 21%. of the ITO bottom layer is exposed for contacting the bottom electrodes (6× at the side) by scratching the perovskite and contact layer films with tweezers. Silver paste is applied for contacting the bottom (ITO) and the top electrodes (copper) of the device as shown in Figure 5. Device Performances and Data Analysis. Figure 6 displays the power conversion efficiency (PCE) of all devices prepared before

of the PCE trend over time to monitor the quality of our protocol and to continue to improve it. Additional Characterization Details of p−i−n Cells. JV curves were obtained in a two-wire source-sense configuration with a Keithley 2400. An Oriel class AAA xenon lamp-based sun simulator was used for illumination providing approximately 100 mW/cm of AM1.5G irradiation, and the intensity was monitored simultaneously with a Si photodiode. The exact illumination intensity was used for efficiency calculations, and the simulator was calibrated with a KG5 filtered silicon solar cell (certified by Fraunhofer ISE). A spectral mismatch calculation was performed based on the spectral irradiance of the solar simulator, the EQE of the reference silicon solar cell, and three typical EQEs of our cells. This resulted in 3 mismatch factors of M = 0.9949, 0.9996, and 0.9976. Given the very small deviation from unity, the measured JSC was not corrected by the factor 1/M.

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CONCLUSION We report detailed protocols to prepare perovskite solar cells for the common perovskite architectures in regular (n−i−p) and inverted (p−i−n) configurations with over 20% efficiency. We describe the most common issues we encountered during the development of these protocols and the potential source of errors that need to be considered during device preparation. Common to all the described protocols/architectures is the control of the environmental parameters during the deposition of the perovskite layer. Particularly, the presence of DMSO/ DMF solvent vapors has been systematically observed to influence the quality of the perovskite films negatively and thus the final device performance. Overheating of the samples during the evaporation of gold metal contacts is a frequent cause of poorly performing devices in the n−i−p configuration. We also find that the high purity of the used solvents and chemicals is fundamental to achieving the highest efficiencies. Finally, we show that practical (manual) experience is highly important to realize the highest efficiency devices reproducibly. This implies that protocols in the future need to be made more robust and that such details are of great importance. With this work, we would also like to encourage reproducibility and transparency in reporting device fabrication and measurement protocols in the perovskite solar cell field. This will be greatly beneficial for the future development of perovskite solar cells but also for perovskite research in general.

Figure 6. Power conversion efficiency as a function of devices produced in chronological order for planar regular (n−i−p) device data collected over approximately 1.5 years, mesoporous device data collected over approximately 2 years, and planar inverted (p−i−n) device data collected over approximately 1.5 years. Data also include a minority of devices that have been prepared for testing different materials and procedures, thus not perfectly conformal to the described procedure. PCEs were extracted from current density− voltage curves collected from forward bias to short circuit conditions at 10 mV/s under simulated 1.5 AM solar light. The temperature was kept at 25 °C (using a thermocouple) to measure the planar inverted (p−i−n) devices. The black lines display the average batch performances; the red dots are the individual cells.

achieving 20% PCE. All data have been collected with slow scanning rates (10 mV/s) with the aim of minimizing the impact of hysteresis on the calculated PCE as discussed elsewhere.23 For regular mesoporous and planar architectures (n−i−p), before testing, the devices are left resting overnight or longer in the dark and dry air (below 1% relative humidity). We detailed elsewhere that the device efficiency may improve after as little as a few hours to several days resting in the dark and dry air.24 We observe that the learning curve to develop the described protocol is long and required dedicated teamwork to produce such a large number of devices in a relatively short time. We note that once the protocol was established, an underperforming batch of devices might still occur. Most of the time we found that poor performance or shunting was linked to obvious errors made during the preparation, to chemical/solvent contamination, or to degradation. We found it particularly useful to keep track

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b00136. Figure S1: Spiro film top view picture. Figure S2: SEM top view image of the perovskite films. Table S1: Device layers thicknesses (PDF) 4199



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Methods/Protocols

TiO2 films by spray pyrolysis (MPG) Perovskite film deposition inverted p−i−n devices (MPG) Perovskite film deposition regular n−i−p devices (MPG)

Iodide Perovskite Precursor Solutions. Chem. Mater. 2017, 29, 1315− 1320. (9) Yan, K.; Long, M.; Zhang, T.; Wei, Z.; Chen, H.; Yang, S.; Xu, J. Hybrid halide perovskite solar cell precursors: Colloidal chemistry and coordination engineering behind device processing for high efficiency. J. Am. Chem. Soc. 2015, 137, 4460−4468. (10) Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I. Solvent engineering for high-performance inorganic−organic hybrid perovskite solar cells. Nat. Mater. 2014, 13, 897−903. (11) Saliba, M.; Matsui, T.; Seo, J.-Y.; Domanski, K.; Correa-Baena, J.-P.; Nazeeruddin, M. K.; Zakeeruddin, S. M.; Tress, W.; Abate, A.; Hagfeldt, A.; Gratzel, M. Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency. Energy Environ. Sci. 2016, 9, 1989−1997. (12) Stolterfoht, M.; Wolff, C. M.; Amir, Y.; Paulke, A.; PerdigonToro, L. P.; Caprioglio, P.; Neher, D. Approaching the Fill Factor Shockley Queisser Limit in Stable, Dopant-Free Triple Cation Perovskite Solar Cells. Energy Environ. Sci. 2017, 10, 1530−1539. (13) Bi, D.; Tress, W.; Dar, M. I.; Gao, P.; Luo, J.; Renevier, C.; Schenk, K.; Abate, A.; Giordano, F.; Baena, J.-P. C.; et al. Efficient luminescent solar cells based on tailored mixed-cation perovskites. Sci. Adv. 2016, 2, 2375−2548. (14) Tsai, H.; Nie, W.; Lin, Y. H.; Blancon, J. C.; Tretiak, S.; Even, J.; Gupta, G.; Ajayan, P. M.; Mohite, A. D. Effect of Precursor Solution Aging on the Crystallinity and Photovoltaic Performance of Perovskite Solar Cells. Adv. Energy Mater. 2017, 7, 1602159. (15) McMeekin, D. P.; Wang, Z.; Rehman, W.; Pulvirenti, F.; Patel, J. B.; Noel, N. K.; Johnston, M. B.; Marder, S. R.; Herz, L. M.; Snaith, H. J. Crystallization Kinetics and Morphology Control of Formamidinium−Cesium Mixed-Cation Lead Mixed-Halide Perovskite via Tunability of the Colloidal Precursor Solution. Adv. Mater. 2017, 29, 1607039. (16) Giordano, F.; Abate, A.; Correa Baena, J. P.; Saliba, M.; Matsui, T.; Im, S. H.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Hagfeldt, A.; Graetzel, M. Enhanced electronic properties in mesoporous TiO2 via lithium doping for high-efficiency perovskite solar cells. Nat. Commun. 2016, 7, 10379. (17) Sivaram, V.; Crossland, E. J.; Leijtens, T.; Noel, N. K.; Alexander-Webber, J.; Docampo, P.; Snaith, H. J. Observation of annealing-induced doping in TiO2 mesoporous single crystals for use in solid state dye sensitized solar cells. J. Phys. Chem. C 2014, 118, 1821−1827. (18) Wolff, C. M.; Zu, F.; Paulke, A.; Toro, L. P.; Koch, N.; Neher, D. Reduced Interface-Mediated Recombination for High OpenCircuit Voltages in CH3NH3PbI3 Solar Cells. Adv. Mater. 2017, 29, 1700159. (19) Saidaminov, M. I.; Abdelhady, A. L.; Murali, B.; Alarousu, E.; Burlakov, V. M.; Peng, W.; Dursun, I.; Wang, L.; He, Y.; Maculan, G.; et al. High-quality bulk hybrid perovskite single crystals within minutes by inverse temperature crystallization. Nat. Commun. 2015, 6, 7586. (20) Abate, A.; Leijtens, T.; Pathak, S.; Teuscher, J.; Avolio, R.; Errico, M. E.; Kirkpatrik, J.; Ball, J. M.; Docampo, P.; McPherson, I.; Snaith, H. J. Lithium salts as “redox active” p-type dopants for organic semiconductors and their impact in solid-state dye-sensitized solar cells. Phys. Chem. Chem. Phys. 2013, 15, 2572−2579. (21) Correa-Baena, J.-P.; Tress, W.; Domanski, K.; Anaraki, E. H.; Turren-Cruz, S.-H.; Roose, B.; Boix, P. P.; Grätzel, M.; Saliba, M.; Abate, A.; Hagfeldt, A. Identifying and suppressing interfacial recombination to achieve high open-circuit voltage in perovskite solar cells. Energy Environ. Sci. 2017, 10, 1207−1212. (22) Domanski, K.; Correa-Baena, J.-P.; Mine, N.; Nazeeruddin, M. K.; Abate, A.; Saliba, M.; Tress, W.; Hagfeldt, A.; Grätzel, M. Not all that glitters is gold: Metal-migration-induced degradation in perovskite solar cells. ACS Nano 2016, 10, 6306−6314. (23) Correa-Baena, J.-P.; Abate, A.; Saliba, M.; Tress, W.; Jesper Jacobsson, T.; Grätzel, M.; Hagfeldt, A. The rapid evolution of highly efficient perovskite solar cells. Energy Environ. Sci. 2017, 10, 710−727.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. ORCID

Michael Saliba: 0000-0002-6818-9781 Juan-Pablo Correa-Baena: 0000-0002-3860-1149 Christian M. Wolff: 0000-0002-7210-1869 Antonio Abate: 0000-0002-3012-3541 Author Contributions ∇

Authors have contributed equally

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank all the people who prepared thousands of perovskite solar cells. We thank the HyPerCells (Hybrid Perovskite Solar Cells) joint Graduate School. S.A. acknowledges funding by the BMBF within the project “Materialforschung für die Energiewende” (grant no. 03SF0540), as well as from the German Federal Ministry for Economic Affairs and Energy (BMWi) through the “PersiST” project (grant no. 0324037C). J.-P.C.-B. acknowledges the support of a Department of Energy (DOE) EERE Postdoctoral Research Award.

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REFERENCES

(1) Saliba, M.; Correa-Baena, J.-P.; Graetzel, M.; Hagfeldt, A.; Abate, A. Perovskite Solar Cells: From the Atomic Level to Film Quality and Device Performance. Angew. Chem., Int. Ed. 2018, 57, 2554−2569. (2) Saliba, M.; Matsui, T.; Domanski, K.; Seo, J.-Y.; Ummadisingu, A.; Zakeeruddin, S. M.; Correa-Baena, J.-P.; Tress, W. R.; Abate, A.; Hagfeldt, A.; Gratzel, M. Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance. Science 2016, 354, 206−209. (3) Tan, H.; Jain, A.; Voznyy, O.; Lan, X.; Garcia de Arquer, F. P.; Fan, J. Z.; Quintero-Bermudez, R.; Yuan, M.; Zhang, B.; Zhao, Y.; et al. Efficient and stable solution-processed planar perovskite solar cells via contact passivation. Science 2017, 355, 722−726. (4) Shin, S. S.; Yeom, E. J.; Yang, W. S.; Hur, S.; Kim, M. G.; Im, J.; Seo, J.; Noh, J. H.; Seok, S. I. Colloidally prepared La-doped BaSnO3 electrodes for efficient, photostable perovskite solar cells. Science 2017, 356, 167−171. (5) Ibn-Mohammed, T.; Koh, S. C. L.; Reaney, I. M.; Acquaye, A.; Schileo, G.; Mustapha, K. B.; Greenough, R. Perovskite solar cells: An integrated hybrid lifecycle assessment and review in comparison with other photovoltaic technologies. Renewable Sustainable Energy Rev. 2017, 80, 1321−1344. (6) Roldan-Carmona, C.; Gratia, P.; Zimmermann, I.; Grancini, G.; Gao, P.; Graetzel, M.; Nazeeruddin, M. K. High efficiency methylammonium lead triiodide perovskite solar cells: the relevance of non-stoichiometric precursors. Energy Environ. Sci. 2015, 8, 3550− 3556. (7) Chang, J.; Zhu, H.; Xiao, J.; Isikgor, F. H.; Lin, Z.; Hao, Y.; Zeng, K.; Xu, Q.-H.; Ouyang, J. Enhancing the planar heterojunction perovskite solar cell performance through tuning the precursor ratio. J. Mater. Chem. A 2016, 4, 7943−7949. (8) Sharenko, A.; Mackeen, C.; Jewell, L.; Bridges, F.; Toney, M. F. Evolution of Iodoplumbate Complexes in Methylammonium Lead 4200



Methods/Protocols

(24) Roose, B.; Ummadisingu, A.; Correa-Baena, J.-P.; Saliba, M.; Hagfeldt, A.; Graetzel, M.; Steiner, U.; Abate, A. Spontaneous crystal coalescence enables highly efficient perovskite solar cells. Nano Energy 2017, 39, 24−29.

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