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

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Methods/Protocols

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 Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00136 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 28, 2018

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Chemistry of Materials

How to make over 20% efficient perovskite solar cells in regular (n-i-p) and inverted (p-i-n) architectures Michael Saliba,1† Juan-Pablo Correa-Baena,2† Christian M. Wolff,3† Martin Stolterfoht,3 Nga Phung,4b Steve Albrecht,4a Dieter Neher,3 Antonio Abate4b*

1

Adolphe Merkle Institute, Chemin des Verdiers 4, CH-1700 Fribourg, Switzerland.

2

Massachusetts Institute of Technology, Cambridge, MA 02139, USA

3

University of Potsdam, Institute of Physics, Karl-Liebknecht-Str. 24-25, 14476 Potsdam,

Germany 4

Helmholtz-Zentrum Berlin für Materialien und Energie,

a

Young Investigator Group

Perovskite Tandem Solar Cells; b Young Investigator Group Active Materials and Interfaces for Stable Perovskite Solar Cells, Kekuléstraße 5, 12489 Berlin, Germany

†

Authors have contributed equally

*Corresponding

author:

A.A.

[email protected]

[email protected]

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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, GaAs, 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 centre 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 lab scale demonstrated power conversion efficiency (PCE) of ≥ 20%. However, only 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 non-expert laboratories to fabricate highly efficient solar cells. Such research often is not 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 include 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 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. Page 3 of 26 ACS Paragon Plus Environment


Methods

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 coloured following the schematic on the top. Figure 1 displays three commonly-used types of devices for single-junction 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 Page 4 of 26 ACS Paragon Plus Environment

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In meso architectures Planar regular (n-i-p) architectures Planar inverted (p-i-n) architectures Top selective contacts Spiro-OMeTAD in regular (n-i-p) meso and planar architectures PTAA in regular (n-i-p) meso and planar architectures C60 & 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



Perovskite precursor solution Please note that all chemicals for all procedures are listed at the end of Table 6. The preparation of the perovskite precursor solution is only apparently simple. The stoichiometry67

and the formation of coordination complexes8-9 of 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 pipette. In the following we detail the recipe to prepare the (FAPbI3)x(MAPbBr3)1-x “MAFA” solution which is 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 3 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 n-i-p structure, because of a slight modification in p-i-n structure, the difference in details will be reported in SI). First, we prepare stock solutions of 1.5 moles of PbI2 and PbBr2 per L of 4:1 V:V Dimethylformamide (DMF): Dimethylsulfoxide (DMSO). The stock solutions are placed on a hotplate at room temperature and heated to 180 ºC 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 ºC 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:

=

! = = (# + 1)

#= :

: : : : ! :

!' ' , (

( ) *+ ) *+

Molarity of the stock solution, unknown parameter Moles of precursor (PbI2 or PbBr2) in the stock solution, unknown parameter Volume of solution Molecular weight of precursor (PbI2 or PbBr2) Weight of the precursor (PbI2 or PbBr2) in the stock solution Density of the stock solution


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The densities, ! and !' , are calculated from the weight of 100 µL of solution (100 µL is chosen for convenience). With a temperature in the glovebox around 25 ºC, this gives the values reported in Table 1.

Table 1. Densities around 25 ºC of the solvent and stock solutions of PbBr2 and PbI2 measured weighting 100 µL. Solution DMF : DMSO 4:1 V:V PbBr2 1.5 mole per L PbI2 1.5 mole per L

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

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

and added to the vials containing the FAI (MABr) powder to get a stoichiometry of

FAI:PbI2 (MABr:PbBr2) 1:1.09 (i.e. 9% lead excess)13. The solutions are shaken for few minutes until the powder is fully dissolved. An example for the inorganic stock solution is provided in Table 2.

Table 2. Examples of the nominal, inorganic stock solutions Solution PbI2 (DMF:DMSO) PbBr2 (DMF:DMSO) CsI (DMSO) RbI (DMF:DMSO)

Example 1 g in 1.446 mL 1 g in 1.816 mL 1 g in 2.566 mL 1 g in 3.139 mL

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 vapour).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 moles CsI per L of DMSO and 1.5 moles of RbI per L of 4:1 V:V DMF:DMSO. The solutions are heated to 150 ºC 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 Page 7 of 26 ACS Paragon Plus Environment


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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 3.

Table 3. Example of the RbCsMAFA precursor solution.

RbCsMAFA solution

FAPbI3 solution

FAI powder 0.1 g

PbI2 stock solution (Table 2) 0.468 mL

MAPbBr3 solution

MABr powder 0.1 g

PbBr2 stock solution (Table 2) 0.704 mL

FAPbI3 solution

MAPbBr3 solution

0.250 mL

0.050 mL

Additional (DMF:DMSO 4:1 V:V) 0.001 mL

Additional (DMF:DMSO 4:1 V:V) 0.016 mL

CsI stock solution (Table 2) 0.016 mL


RbI stock solution (Table 2) 0.017 mL

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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 pre-patterned 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 Hellmanex for 15 min.

•

Rinse with copious amounts of deionised 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.

•

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 without any streaks. We recommend that all the solvents used for cleaning 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 chemical bath deposition (CBD) step, where 0.5 g of urea is dissolved in 400 mL deionised water, followed by the addition of 10 μL of mercaptoacetic acid and 0.5 mL 37% HCl. Finally, 0.1 g SnCl2 x 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 higher temperature (80 and 90 ˚C) to induce the formation of SnO2. In the absence of an oven, the process can also be done on a hotplate, but higher temperatures need to be used (e.g. 120-150˚C). After the deposition, the solution turns cloudy. Deionized water replaces the liquid in the container, and it is sonicated for 2 minutes 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 hotplate with a lid that enables more uniform temperature distribution over the plate ensuring to blow over the substrate with a gentle flow during the whole annealing.

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 ºC (within 30 min). The substrates are left for 15 min at 450ºC 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 x 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 carrying gas. The nozzle (in this case from Glaskeller Basel called “Chromatografie Zerstäuber”) is about 20 cm far from the FTO surface and with an angle of 45 degrees (see also Video S3). The nozzle is rapidly moved Page 10 of 26 ACS Paragon Plus Environment

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around in a single circle to cover the whole 10 x 10 cm2 FTO surface (already cut 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 ºC before cooling them down slowly to room temperature, i.e. the substrates are not removed from the hotplate until the temperature is below 150 ºC. It is important that the hotplate has a uniform temperature distribution and the aerosol flow is homogenous 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 the inert atmosphere. Samples with TiO2 compact layers can be stored for weeks in a drawer before to deposit the mesoporous TiO2 layer. To prepare the mesoporous TiO2 layer, we use 30 N-RD Dyesol TiO2 paste diluted as 150 mg per mL of ethanol. The resulting dispersion is left under vigorous stirring overnight before use. The dispersion is stored under continuous stirring. About 150 nm thick TiO2 meso-layer can be achieved by dropping and then spin coating the above dispersion on the FTO/compactTiO2 substrates using a 4000 rpm and 2000 rpm/s acceleration for 10 s. Immediately after the spinning, the layer is dried at 100 ºC for a few minutes. Subsequently, the TiO2 is sintered in multiple temperature steps as reported in Table 4. Again, we advise using a hotplate with a lid that enables more uniform temperature distribution over the plate ensuring to blow over the substrate with a gentle flow during the annealing.

Table 4. Sintering procedure for the TiO2 mesoporous layer. 15 5 5 Ramp (min) 5 125 325 375 450 Temp (ºC) 5 5 5 30 Hold (min)

The hotplate is left to cool slowly to 150 ºC before removing the samples. The resulting mesoTiO2 film should look uniform and semi-transparent. 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 the same above multistep protocol for meso-TiO2 (Table Page 11 of 26 ACS Paragon Plus Environment


4). The hotplate is left to cool slowly to 150 ºC, 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 defect 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(4phenyl)(2,4,6-trimethylphenyl)amine] (PTAA, Mn = 7000–10 000, PDI = 2–2.2) are dissolved in 3 mL anhydrous toluene and stirred for a 3 hours at room temperature in a nitrogen-filled glovebox. The PTAA solution can be stored for a few weeks in the dark and inert atmosphere. To prepare the film, 60-80 µL of the solution is uniformly spread on the about 2.5 x 2.5 cm2 ITO substrate and spin-coated at 6000 rpm and 2000 rpm/s acceleration for 30 s. The samples are annealed on a pre-heated 100°C hotplate for 10 minutes 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.) a tweezer which should be just visible by 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 1.5mg/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 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 for if the FF is substantially below 80% (see Table S1 for the different layer thicknesses).


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Perovskite film deposition For the mesoporous architecture We note that the temperature in the glove box plays a major role in the quality of the perovskite film. The method we describe here has been optimised for temperatures in the glovebox between 22 and 27 ºC. We systematically observed much lower efficiency when the glovebox temperature went above 28 ºC, even if the perovskite films looked uniform and dark. We hypothesise that a higher temperature may induce crystallites within the precursor before the solution is deposited on the substrate, consistent with inverse temperature crystallisation of the perovskite,19 which impacts the morphology and thus the quality of the perovskite film. r In the glove box, the samples are left to cool to room temperature and then processed immediately without storing them. The RbCsMAFA or the CsMAFA perovskite film are deposited by spin coating the precursor solution in a 2-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 pipette in the middle of the rotating substrate. 5 s before the end of the spinning, 200 µL chlorobenzene (the antisolvent) is rapidly dropped in the middle of a 2 x 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 towards the middle helps to avoid forming a hole in the middle. To acquire the necessary technical skill for this step, manual practice is required (see Videos S1 and S2). 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 coloured. It should turn opaque black after a few minutes on the hotplate. 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 glove box from DMSO/DMF vapours. This can be achieved, for example, through a constant flow of nitrogen for an hour; thus the glove box should be continuously purged during the spinning and the annealing of the perovskite film.



Figure 2. The 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). Planar regular (n-i-p) architecture The perovskite deposition method for the planar architectures 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 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 glove box with a continuous flow of fresh nitrogen to remove DMSO/DMF vapours. Furthermore, we clear the spin-coater atmosphere from residual solvent vapour 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 x 2.5 cm2 ITO/PTAA substrate and spin-coating at 5000 rpm with 3000 rpm/sec acceleration for 35 s. We note the perovskite spin speed can be varied between 2000-6000 rpm, with and without the acceleration step at the beginning with little variation in the final PCE (1 Å s-1) 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 of the ITO bottom layer is exposed for contacting the bottom electrodes (6x at the side) by scratching the perovskite and contact layer films with a tweezer. Silver paste is applied for contacting the bottom (ITO) and the top electrode (Copper) of the device as shown in Figure 5.


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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 2 copper top electrodes and 6 silver paste stripes to contact the bottom ITO. The actual solar cell has 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%.



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Table 6. 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 x 2H2O TiO2 paste (30 NR-D) PTAA inverted (p-i-n) devices C60 BCP Copper

Producer TCI America TCI America abcr GmbH abcr GmbH Dyesol Dyesol Dyesol Acros Acros Acros Acros Sigma-Aldrich Acros Dyenamo Sigma-Aldrich Merck EM Index Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Dyesol Sigma-Aldrich creaphys Sigma-Aldrich Sigma-Aldrich

CAS 10101-63-0 10031-22-8 7789-17-5 7790-29-6 6876-37-5 879643-71-7 14965-49-2 68-12-2 67-68-5 108-90-7 75-05-8 141-78-6 108-88-3 1447938-61-5 90076-65-6 3978-81-2 207739-72-8 17927-72-9 123-54-6 68-11-1 57-13-6 10025-69-1 1333317-99-9 99685-96-8 4733-39-5 7440-50-8

Product number L0279 L0288 AB207757 AB203369 MS301000 MS150000 MS101000 34843 34844 44300 36431 270989 36441 FK209 544094 142379 SHT-263 325252 P7754 528056 1.08487 474762 MS002300 702471 V0502010 699152 254177

Device performances and data analysis Figure 6 displays the power conversion efficiency (PCE) of all devices prepared before to achieve 20% PCE. All data has been collected with slow scanning rates (10 mV s-1) with the aim of minimising 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 degradation. We found it particularly useful to keep track of the PCE trend over time to monitor the quality of our protocol and to continue to improve it. Page 20 of 26 ACS Paragon Plus Environment

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Additional characterisation details of p-i-n cells: JV-curves were obtained in a 2-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-2 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 = 0.9949, 0.9996 and 0.9976. Given the very small deviation from unity, the measured 1SC was not corrected by the factor 1/.

Figure 6. Power conversion efficiency as a function of devices produced in chronological order for planar regular (n-i-p) devices data collected over approximately 1.5 years, mesoporous devices data collected over approximately 2 years, and planar inverted (p-i-n) devices 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 condition at 10 mV s-1 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.



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% power conversion 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 vapours 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 n-i-p configuration. We also find that the high purity of the used solvents and chemicals is fundamental to achieve the highest efficiencies. Finally, we show that practical (manual) experience is highly important to get 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 which will be greatly beneficial for the future development of PSCs.


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Acknowledgements 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.

Supporting Information Figure S1. Spiro film top view picture. Figure S2. SEM top view image of the perovskite films. Table S1. Device Layers thicknesses. Media files: Perovskite film deposition inverted p-i-n devices; Perovskite film deposition regular n-i-p devices; TiO2 films by spray pyrolysis.



TOC


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