Scalable Fabrication of Perovskite Solar Cells to Meet Climate Targets

Scalable Fabrication of Perovskite Solar Cells to Meet Climate Targets

Article Scalable Fabrication of Perovskite Solar Cells to Meet Climate Targets Karsten Bruening, Benjia Dou, John Simonaitis, Yu-Ying Lin, Maikel F.A...

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Scalable Fabrication of Perovskite Solar Cells to Meet Climate Targets Karsten Bruening, Benjia Dou, John Simonaitis, Yu-Ying Lin, Maikel F.A.M. van Hest, Christopher John Tassone [email protected] (K.B.) [email protected] (C.J.T.)

HIGHLIGHTS Our cost model shows competitive perovskite PV requires high-throughput processing We replace conventional annealing with rapid thermal processing without PCE loss The combination of RTA and blade coating establishes a scalable processing route In situ XRD reveals a perovskite conversion mechanism and role of intermediates

Scalable processing of perovskite solar cells from cost modeling to mechanistic understanding.

Bruening et al., Joule 2, 1–13 November 21, 2018 ª 2018 Elsevier Inc.

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Scalable Fabrication of Perovskite Solar Cells to Meet Climate Targets Karsten Bruening,1,6,* Benjia Dou,2,3 John Simonaitis,1,4 Yu-Ying Lin,1,5 Maikel F.A.M. van Hest,2 and Christopher John Tassone1,*


Context & Scale

Cost modeling shows that high-throughput processing of perovskite solar cells is required not only to compete with incumbent technologies in terms of levelized cost of energy, but more importantly, it is the major enabling factor facilitating sustainable growth rates of solar cell manufacturing capacity commensurate with global climate targets. We performed rapid thermal annealing at bladecoating speed to quickly deposit and convert perovskite thin films for scalable manufacturing of perovskite solar cells. In situ X-ray diffraction during film deposition and thermal conversion gave insight into the formation of crystalline intermediates, essential for high-quality films. Parameters were optimized based on the in situ study, allowing perovskite films to be annealed within 3 s with a champion power conversion efficiency of 16.8%. This opens up a clear pathway toward industrial-scale high-throughput manufacturing, which is required to fulfill the projected photovoltaic installation rates needed to reach climate goals.

Metal halide hybrid organicinorganic perovskite solar cells have seen enormous research interest, with more than 10,000 papers published since the report of the first solid-state perovskite cells in 2012. Efficiencies have surpassed 20% in 2015. The major hurdles obviating commercialization are stability and scalability. In this context, we provide a framework bridging domains from economic modeling to materials science. Insights gained by in situ X-ray diffraction allow substitution of spin coating for blade coating and slow thermal annealing for rapid annealing, enabling scale-up without sacrificing cell performance. The high throughputs enabled by this processing route enable costeffective production of perovskite solar cells, which can help to accelerate the shift to renewable energies.

INTRODUCTION To reach climate targets and limit global warming, a transition to renewable energy generation is indispensable, with photovoltaics (PVs) being a major pillar of the future energy mix. In the past 6 years, hybrid organic-inorganic perovskite PVs have seen a meteoric rise in power conversion efficiency (PCE) and research interest, fueling the hope of cheap renewable energy.1–4 In order to deploy up to 10 TW of PVs by 2030, as proposed by Needleman et al.,5 two conditions must be met: (1) the generated energy must be cost competitive with incumbent technologies, i.e., energy generation from fossil fuels; (2) the PV manufacturing output must be ramped up at steep growth rates in a sustainable manner. The first condition is captured by the levelized cost of energy (LCOE), the price per unit of energy.6 With existing silicon and cadmium telluride (CdTe) technology, energy prices for utility-scale PVs have fallen below grid parity under favorable conditions (high insolation). Cost models show that the LCOE is relatively insensitive to the absorber material (unless it involves a paradigm-changing system design7) and that perovskite manufacturing costs are able to keep up with silicon and CdTe if lifetime and PCE criteria are met (Figure 1A).8,9 However, silicon and CdTe counteract the second condition, because they require excessive capital expenditures (capex) to grow the module production at those rates required to fulfill climate targets. The main culprit for the high capex of silicon modules is the complex technology required to make the silicon absorber layer (Figure 1B). This is where solution processable materials, such as perovskites, could drastically lower the capex for cell production.10 Our model shows that reducing capex by an order of magnitude compared with silicon enables sustainable growth rates of ca. 20% annually that

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are in line with climate targets (Figures 1C and 1D, Supplemental Information section on Cost Modeling). Moreover, perovskite PVs boast a much shorter energy payback time than Si and CdTe.11 In order to exploit this tremendous potential, solution deposition and processing methods compatible with high-throughput manufacturing are required (Figure 1A). So far, the scientific community has focused on material development to increase PCE and lifetime. However, almost all of the reported devices are made in a nonscalable manner, employing techniques such as solvent exchange, spin coating, or prohibitively long annealing steps. The development of scalable fabrication strategies is one of the major hurdles for commercialization. The history of organic PVs has clearly demonstrated that the transferability from laboratory scale to an industrial scale is limited.15 Scalability should be incorporated at an early point in research due to intricate processing-structure-property relationships. This especially applies to perovskites, involving not only kinetically trapped film deposition but also multistage chemical conversion. The goal of this paper is to develop a universally applicable scheme for the scale-up of perovskite processing. The combination of blade coating with in-line rapid thermal annealing (RTA) is identified as a viable pathway to high-throughput deposition and processing (Figure 2A). To accelerate scale-up and to navigate parameter space more efficiently, physical insight is needed (Figure 2B). In situ X-ray diffraction (XRD) can deliver this insight. By performing XRD during blade coating, the film formation starting from the precursor solution can be studied. It also ensures the applicability to scalable deposition techniques. By performing XRD during thermal annealing, the conversion kinetics can be understood, and room for accelerating chemical reactions can be identified to increase throughput. Owing to the crystalline nature of many of the involved species and owing to the generally non-destructive character of XRD, it has become a popular tool to study perovskite materials in situ. Buschbaum and Pearson recently provided reviews on in situ monitoring of perovskite processing.16,17 Several studies have confirmed the presence of transient crystalline precursor species.18–23 The structure of mixed halide precursors has not been clearly determined and their formation kinetics depend on the lead counterion in the precursor.20,21,24,25 Furthermore, previous studies have established that the film formation is a kinetically trapped process, indicating that the reaction coordinates depend on the film processing route (e.g., drop casting, spin coating, blade coating).20 While at first sight this introduces higher complexity, the rate dependency should rather be regarded as an additional knob that can be exploited to optimize film processing. In previous work, we performed in situ XRD during moderately fast annealing of methyl ammonium lead iodide perovskite (MAPI) and mapped out the processing space of formamidinium lead iodide.26,27 Saliba et al. optimized the thermal annealing protocol for lead(II) chloride (PbCl2)-derived perovskite based on in situ XRD, although the proposed ‘‘flash’’ annealing procedure involved a heating protocol of almost 90 min.28 Rossander et al. performed in situ XRD during roll-to-roll coating, which clearly illuminated the challenges of in situ film deposition.29 So far, no in situ experiments during RTA (<10 s) of prototypical MAPI or during the formation of the crystalline precursor during the film drying step have been reported. RTA (also called rapid thermal processing) as well as intense pulsed light annealing (also called flash annealing or photonic curing) are industrially proven processes that


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Materials Science Division, SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA

2National Renewable Energy Laboratory, Golden,

CO 80401, USA 3Department

of Electrical, Computer, and Energy Engineering, University of Colorado Boulder, Boulder, CO 80309, USA


of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61802, USA


of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan 30013, Republic of China



*Correspondence: [email protected] (K.B.), [email protected] (C.J.T.)

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Figure 1. Implications of Manufacturing Throughput on Cost, Capex, Sustainable Growth Rate, and Viable Processing Routes (A) Open circles, manufacturing cost for perovskite modules, based on Song et al.10 Filled circles, capital expenditure on the perovskite module level, based on Powell et al. and Cai et al.7,12 The red dashed line indicates throughput levels attainable by spin coating and conventional (non-rapid) thermal annealing. (B) Breakdown of system-level LCOE and module-level capex into cell-related and other costs (for silicon). Based on data from Powell et al., 7 Lu et al., 13 and Louwen et al. 14 (C) The sustainable growth rate for the production of perovskite modules as a function of throughput, derived from part (A), shown for operating margins of 5% and 10% (based on Powell et al. 7 ). (D) Comparison of targeted annual PV installation growth rate to reach climate goals 5 with sustainable production growth rates for silicon and perovskite PV.

enable much higher throughput than low-temperature conventional annealing and, as opposed to solvent-exchange processes, do not involve additional solvents and are easily controllable. Moreover, due to the short heating time, these processes can be made compatible with heat-sensitive substrates used in roll-to-roll processing. They also allow control over the perovskite texture, although frequently the PCE for devices made with annealing times compatible with high-throughput processing (<10 s) lags behind hot-plate-annealed antisolvent-treated samples.30–33 Very recently, Abate et al. demonstrated the first high-performance devices using RTA.34 Blade coating has been identified as one of the promising scalable film deposition techniques that can be applied in roll-to-roll processing (besides,

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Figure 2. Processing Scheme and Conversion Kinetics of 3 MAI:PbAc2 in DMF (A) Schemes employed in this study: (i) spin coating and hot-plate annealing, (ii) spin coating and rapid thermal annealing (RTA), (iii) blade coating and in-line RTA at coating speed. (B) Unifying workflow for the scale-up of perovskite processing. (C) In situ XRD study of conversion of a spin-coated film heated to 100 C for 5 min. The crystalline precursor disappears within 10 s. The inset shows the full temperature profile and the stability of the plateau of the perovskite peak during the annealing procedure. (D) Conversion and subsequent decomposition at heating rates of 0.3 K/s, 10 K/s, and 56 K/s. With increasing heating rate, conversion and decomposition shift to higher temperatures. See Figure S3 for details.

e.g., slot die coating and spray coating).35–37 Deng et al. reported the first high efficiency (15% PCE) perovskite PV cells using blade coating,38 although the reported process is not strictly scalable due to the hour-long annealing time. The record for blade-coated devices currently stands at 19.5%, using a triple cation and mixed halide recipe requiring a 30 min anneal.39 The film formation and drying kinetics in blade coating are typically slower than in spin coating, and thus recipes optimized for spin coating must be re-evaluated for adaptation to blade coating.35,37,38 Besides being scalable, blade coating has the added benefit of making all stages of the film formation process, starting with the wet film,


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accessible to in situ characterization,21 enabling a systematic approach to optimize perovskite processing. While in the last two years increasingly complex salt compositions, solvent mixtures, and various additives have been introduced to the field of perovskite PVs to enhance the environmental stability and PCE, the prototypical material remains MAPI perovskite. Grancini et al. recently demonstrated that stability issues often encountered with MAPI can be overcome by interface optimization, yielding lifetimes (under simplified conditions) exceeding 10,000 hr.40 Due to the fundamental nature of this study, we used a simple recipe containing a 3:1 molar ratio of lead(II) acetate (PbAc2) and methylammonium iodide (MAI) in dimethylformamide (DMF).18 Typical champion device efficiencies for this recipe are reported around 14%, which can be enhanced by additives or double spin coating.41–43 We chose this recipe for its simplicity and do not suggest it to be actually employed on a large scale without further optimization. (Due to the formation of gaseous byproducts, 22 wt % of the reagents are lost. While not desirable, this loss is small compared with vapor deposition or antisolvent-induced crystallization.) We focus on the scalable deposition of the perovskite, as large-scale deposition of the other layers has been shown elsewhere.44–46 We also show that the scale-up framework laid out in this study is generalizable to other chemistries.

RESULTS AND DISCUSSION Stepwise Upscaling from Spin-Coated Conventionally Annealed Films to Blade-Coated Rapidly Annealed Films The purpose of this study is to lay out a unifying framework for the scale-up of perovskite processing under industrial and economic boundary conditions. As a starting point, an in-depth understanding of the mechanisms and limitations of the existing process (spin coating and hot-plate annealing) is required (Figure 2B). Next, the prevalent lab-scale processing parameters are compared with industrial processing constraints, and shortcomings are identified. The major shortcomings are scalable deposition and long annealing times, both impeding high throughputs, which are required for economic sustainability (Figure 1). Blade coating is a widely used technique for high-throughput deposition of thin films. However, the drying kinetics are slower than in spin coating, requiring adaptation of existing processes. In addition, the thermal annealing is carefully investigated to map out the reaction coordinates under non-equilibrium conditions as a function of heating rate. This opens up processing space at high ramp rates, which disfavor the overconversion and decomposition reactions. This information is translated to a pulse-like RTA, which can be done at the speed of blade coating. Finally, blade coating and RTA are combined to build an entirely scalable processing scheme. Validation is achieved by benchmarking devices against spin-coated hot-plate-annealed references. In Situ XRD of Spin-Coated Films during Thermal Annealing To obtain a detailed understanding of the conversion mechanism underlying the conventional processing route, we performed in situ XRD during annealing of spin-coated films following the most widely used hot-plate annealing protocol for MAPI films made from a 3:1 MAI:PbAc2 in DMF precursor solution (Figure 2B, step 1).18 Spin-coated films were annealed in situ at the beamline for 5 min at 100 C. As shown in Figure 2C, the as-spun film contains crystalline precursor as well as perovskite. Upon heating, the precursor turns into perovskite within a few seconds, but the longer annealing time is required to achieve satisfactory device performance. Initially, two populations of perovskite are present with distinct orientations with the (110) planes centered around an orientation (1) parallel to the

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substrate and (2) at an angle of 12 to the substrate (Figures S1A–S1C). During heating, only population (1) remains. The texture is to be taken into account when the phase fractions are calculated. Upon cooling, the cubic perovskite transforms into the tetragonal phase at roughly 54 C (Figure S1D).47,48 Conversion and Decomposition Temperatures Depend on the Heating Rate The major shortcomings of the conventional process are long annealing times and non-scalable spin coating (Figure 2B, step 2). Long annealing times are detrimental to capex or throughput, which is paramount for economic sustainability. According to the Arrhenius principle, higher temperatures are expected to speed up the conversion process, as has been observed for example in PbCl2-derived MAPI.28 To retrieve the upper temperature limit of the conversion process, we subjected the spin-coated precursor films to linear heating ramps at rates of 0.3 K/s, 10 K/s and 56 K/s up to 350 C and observed the conversion to perovskite and the decomposition to PbI2 in situ (Figure 2B, step 3; Figure S3). Figure 2D shows that the conversion and decomposition temperatures depend on the heating rate. We hypothesize that the conversion process is governed by the time required to evaporate the solvent, overcompensating the inverse solubility effect.49 Rapid Thermal Annealing of Spin-Coated Films As can be seen in Figure 2D, the decomposition to PbI2 is delayed to higher temperatures when subjecting the film to faster heating ramps. This opens a window for fast conversion of precursor to perovskite at high temperatures, which would degrade the film under conditions closer to the thermodynamic equilibrium. Based on these considerations, a heating profile consisting of a single intense pulse was selected for RTA of spin-coated films, operating the RTA chamber at full power, which would give a heating rate of 50 K/s, for pulse durations between 1.0 s and 4.0 s (Figure 2B, step 4). As can be seen in Figure 3A, the film goes from predominantly precursor phase to the perovskite phase within less than a second under optimum RTA conditions (2.5 s < pulse duration <3.0 s). Here, we define conversion to be proportional to the area of the perovskite XRD peak at q = 1.0 1/A˚, with phase-pure perovskite corresponding to a conversion of 1. For shorter pulse durations, conversion is slower or incomplete. Overconversion manifests itself as a subsequent decrease of the peak area due to degradation of perovskite into lead iodide. For longer pulse duration, overconversion occurs as can be seen from the red hue in the top right corner of Figure 3A. The corresponding temperature profiles are shown in Figures 3B and S4. The cooling is restricted to passive (mostly convective) cooling. Scanning electron microscopy (SEM) analysis confirmed that the grain morphology is optimum for a pulse duration around 3.0 s (see Figure 3C). Application of RTA to Other Recipes In analogy to PbAc2-derived MAPI, we confirmed that similar kinetic considerations apply to other chemistries, e.g., 3:1 MAI:PbCl2 in DMF, 1:1 PbI2:MAI in DMF, and 1:1:1 PbI2:MAI:DMSO in DMF (Figure S5). For PbCl2-derived MAPI, a slightly longer RTA pulse duration of 3.5–4.0 s is required to obtain full conversion due to the higher activation energy.19 It is noted that the temperature goes well into the region in which decomposition would occur under moderate heating rates. These results show that this framework is applicable to the scale-up of various perovskite chemistries, including those with larger thermal budgets. RTA during Blade Coating After establishing RTA for spin-coated films, the next step is to adapt it to bladecoated films at coating speed (Figure 2B, step 5; Figure S6). In situ XRD during blade


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Figure 3. Conversion of Spin-Coated Film by RTA (A) RTA parameter map with pulse durations ranging from 1.0 to 4.0 s. The RTA heating is turned on at t = 0 s. Conversion is slow or incomplete for pulse durations shorter than 2.0 s. Overconversion occurs for RTA pulse durations of 3.5 s and longer. The optimum duration is around 2.5 s–3.0 s. (B) Corresponding temperature profiles to (A) for RTA pulse durations of 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0 s (bottom to top). (C) SEM images of films made with various RTA pulse durations.

coating revealed that the as-coated film, with the substrate at room temperature, is amorphous (Figure S7A), in contrast to spin-coated films, which exhibit a crystalline precursor. Using an RTA lamp mounted to the blade coater (Figure 2A, iii), the amorphous film was directly converted to perovskite at coating speed without any detectable crystalline intermediate species (direct conversion, Figure S8). Conversion occurs on a timescale of about 2 s. However, the resulting film quality is poor with high roughness and formation of ridges (Figure 4A). These are thought to form due to the large volume change during the concurrent evaporation of DMF and formation of perovskite.50 This is in line with reports about poor morphology resulting from accelerating the drying and crystallization of PbAc2-based perovskite using gas blowing.51 In contrast, films based on MAI:PbI2 precursors, which convert to perovskite without forming byproducts, have been reported to benefit from a direct conversion without going through an intermediate crystalline phase.34,52,53 To overcome the film quality issues in the PbAc2-based films, stepwise conversion from the amorphous phase through the crystalline precursor to the perovskite is required, such that a solid-solid reaction takes place. At room temperature, the formation of the crystalline precursor in a blade-coated film takes about 3–5 min; coating onto a heated substrate reduces the time to less than 20 s, increasing the throughput. Even though deposition onto a heated substrate leads to partial direct conversion into perovskite, both pathways give good-quality films (Figure 4B). Slow heating of an amorphous film to moderate temperatures shows that the crystalline precursor forms rapidly at temperatures above 50 C (Figures S7B and 2B, step 6). Thus, when the substrate is heated to about 60 C, the crystalline precursor is formed without significantly slowing down the film processing, which then can be rapidly

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Figure 4. Blade Coating with RTA at Coating Speed (A) SEM images of blade-coated RTA films without crystalline precursor. (B) SEM images of blade-coated RTA films with crystalline precursor. (C) Waterfall plot of in situ XRD during blade coating and consecutive RTA (precursor, *; PbI 2 , +; perovskite, o; FTO, #). The film is deposited at t = 23 s (blade passing the beam) and the RTA lamp is turned on at t = 0 s. The dark area is due to the printer blocking the X-ray beam. The black line shows the temperature (measured on top of the substrate ex situ). (D) Temperature, voltage, and perovskite XRD peak area (extracted from C) over time. (E and F) J-V curves (E) and performance table (F) for champion devices made using conventional hot-plate annealing and spin-coated and bladecoated RTA.

annealed at coating speed yielding high-quality films (Figure 2B, steps 7 and 8). If the substrate is heated beyond 70 C, complete direct conversion without formation of a crystalline precursor takes place, with the above-mentioned implications for the morphology. A waterfall plot of an in situ deposition onto a substrate heated to 60 C and subsequent rapid conversion is shown in Figure 4C. Temperature and perovskite XRD intensity are displayed in Figure 4D. The formation of the crystalline precursor is evident from the transient diffraction peaks at q = 0.46 1/A˚ and q = 0.69 1/A˚ (around 8 s). We conclude that the formation of a crystalline precursor is an essential prerequisite for the formation of a high-quality PbAc2-derived perovskite film.


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Device Performance Finally, devices were made from PbAc2-derived MAPI based on in situ parameter refinement (Figure 2B, steps 9 and 10). Reference devices were made from spincoated films using conventional hot-plate annealing at 100 C for 5 min, yielding a PCE up to 16.2%. Spin-coated films were also rapidly annealed using an RTA pulse duration of 3.0 s (PCE up to 16.3%). Blade-coated films were prepared with the substrate temperature set to 60 C to rapidly form a crystalline precursor and then underwent RTA at coating speed, yielding PCE up to 16.8%. Figure 4E shows current density-voltage (J-V) scans (under AM1.5G 100 mW cm 2 illumination, reverse scan; see Figure S9 for performance statistics). Considering the absence of additives or additional treatment steps, the devices achieve a relatively high PCE for a prototypical MAPI recipe (Figure 2B, step 11). The rapidly annealed devices are at least on par with the conventionally annealed devices. The lower reproducibility of the blade-coated devices is likely due to the transient zone of film formation extending into the active area and is expected to improve when using substrates longer than 25 mm. As expected, the PV performance of bladecoated films without crystalline precursor formation was rather poor with a maximum PCE of 6.5%. To our knowledge, this is the first report of high-efficiency devices made by a simple thermal high-throughput compatible deposition and processing scheme for the absorber layer. We hypothesize that rapid crystallization is beneficial for the performance of perovskite thin films, as has generally been observed in solvent-exchange recipes, which lead to rapid crystallization comparable with RTA. Conclusion We have investigated the economic case and accompanying constraints for how perovskite absorbers can enable PV installation rates that allow us to meet global climate targets. We have developed an approach to transfer lab-based fabrication processes to high-throughput compatible deposition strategies. First, the parameter space for solution processing of perovskite thin films is mapped out using in situ XRD. Using linear heating ramps combined with in situ XRD, a rate-dependent component of the perovskite formation and decomposition reactions was identified. Using a fast heating ramp, the conversion time could be reduced to less than 1 s using a 3 s RTA pulse, outrunning the thermal decomposition that would occur at these temperatures under moderate heating rates for longer times. The performance of devices made under these RTA conditions proved to be at least on par with those obtained from conventional hot-plate annealing. We demonstrated that the RTA approach is universal as it can be applied to various chemistries. The short pulse duration allows RTA to be performed at blade-coating speed. Coupling blade coating and RTA allowed us to deposit the film and convert it to perovskite at a relatively high speed using scalable techniques without sacrificing device performance, obtaining up to 16.8% PCE for a prototypical lead-acetate-based methylammonium lead iodide perovskite device. We thus propose RTA as a viable processing scheme for high-throughput large-scale manufacturing of perovskite solar cells.

EXPERIMENTAL PROCEDURES Solution Preparation MAI (Dyesol) and lead acetate trihydrate in a molar ratio of 3:1 were dissolved in DMF and stirred at room temperature for 1 hr. For spin coating, the concentration was 40 wt %. A more concentrated solution of 55 wt % was chosen for blade coating to obtain a similar film thickness and to reduce solvent evaporation time. All chemicals were supplied by Sigma and used as received unless noted otherwise.

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Substrate Preparation For in situ experiments of spin-coated films, microscope glass slides (Fisher Scientific) cut to 12 3 12 3 1 mm3 were used as substrates. For in situ blade coating, fluorine-doped tin oxide (FTO) glass (Sigma) cut to 75 3 25 3 2.2 mm3 was used. Devices were made on custom-patterned FTO glass of 25 3 25 3 1 mm3 size (Thin Film Devices). All substrates were sonicated in Hellmanex solution, water, acetone, and isopropanol for 15 min each, blow dried, and treated by UV-ozone for 20 min. Spin Coating Films were spin coated in a nitrogen glove box on a Laurell spin coater for 60 s at 2,000 rpm followed by 10 s at 6,000 rpm using 35 mL of precursor solution per film. In Situ XRD The RTA chamber and the blade coater were mounted on beamline 7-2 at the Stanford Synchrotron Radiation Lightsource (SSRL).54 The X-ray energy was set to either 12.7 or 14.0 keV. The incident angle was 2.5 . A Dectris Pilatus 300K detector was used at a distance of roughly 200 mm. Spin-coated films were transferred to the RTA chamber in a sealed vial from the nitrogen glove box. During loading, the chamber was briefly opened to air. The chamber was then purged with helium. The RTA chamber was controlled using Labview software, which was triggered by the beamline control software. In order to minimize beam damage, XRD images were acquired at a frame rate of 10 Hz for 15 s, followed by 1 Hz for 10 s, and finally at 0.5 Hz for 160 s. The exposure time was 95 ms with a 5 ms deadtime. The heat pulse started 1 s after the image acquisition started. Conventional annealing (100 C for 5 min) was carried out in the RTA chamber as well in order to simulate the temperature profile that a sample experiences when placed on a pre-heated hot plate and in order to capture the early stages of the annealing process. To exclude potential artifacts from delays in thermocouple readings, XRD peak shifts of crystalline standards were used to calibrate the thermocouple readings (Supplemental Information section on Temperature Calibration). In situ blade coating and RTA were done on a custom-designed miniaturized blade coater enclosed in a helium-containing enclosure (relative humidity <1%). The substrate is fixed with respect to the X-ray beam and the blade (glass slide) moves at a speed of 5 mm/s. The blade gap was adjusted to 120 mm. Then, 25 mL of precursor solution was deposited onto one end of the substrate using a remote-controlled syringe pump. The blade then swept across the substrate, depositing the film. XRD image acquisition started at the same time as the blade movement. XRD images were acquired at a frame rate of 10 Hz for 19 s and at 1 Hz for 30 s. The exposure time was 95 ms. For direct conversion, the RTA lamp was turned on 1 s after the blade started moving and was turned off after the blade had swept across the length of the substrate in order to simulate a steady-state process. The distance of 23 mm between the blade and the lamp resulted in a delay of ca. 4 s between coating and rapid annealing (direct conversion). To simulate larger distances between coating and RTA, the print head (along with the lamp) was moved back, the blade was ejected, and the lamp made a second pass across the substrate. The X-ray impinged on the middle of the substrate. To check for beam damage, which would show as sample heterogeneity, the beam was scanned across the sample after the in situ experiment, and in the case of slow experiments, also during the experiment. We note that the blade speed of 5 mm/s is slow compared with industrial roll-to-roll printing. This rate is not an inherent limitation of the combined blade coating and


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RTA process; rather this is the maximum speed of the particular linear stage employed for the blade motion. The film quality generally improved with coating speed. For instance, to enable a web speed of 4.2 m/min (corresponding to a throughput of 2.5 m2/min on a 60-cm-wide web), the RTA heating zone would have to be extended to a length of 25 cm to obtain a similar thermal profile. The data were analyzed using self-written routines based on the pyFAI and pygix modules.55,56 The relative phase fractions were obtained from the most prominent diffraction peak of the corresponding species. The conversion is obtained by normalizing the maximum of the peak area over time to unity, provided there is no other phase present at that time. Overconversion, leading to the formation of lead iodide, thus leads to conversion values less than unity. Film Characterization SEM was done on an FEI Sirion at 5 keV in SE mode using an ET or TTL detector. Devices The same precursor recipes as for the in situ samples were used to make planar n-i-p devices. Titania precursor solution (271 mL of titanium diisopropoxide bis(acetylacetonate)) in isopropanol 75 wt % solution was mixed in 3.72 mL of 1-butanol and stirred and filtered with a 200 nm filter. The solution was spin coated in air at 700 rpm for 10 s, 1,000 rpm for 10 s, and finally 2000 rpm for 30 s, using 150 mL per sample. The resulting film was wiped off in the contact areas using a cotton swap dipped in isopropanol. The samples were then put onto a hot plate set to 130 C for 5 min and stored at room temperature for up to 1 hr before being sintered in a furnace at 500 C for 1 hr. Samples were then UV-ozone treated and the perovskite precursor film was deposited as described above. The hole transport layer was prepared as follows: 72.3 mg of Spiro-OMeTAD (1-material), 28.8 mL of TBP (tert-butylpyridine), and 17.5 mL LiTFSI (lithium bis(trifluoromethylsulfonyl)imide solution, 520 mg/mL in acetonitrile) were dissolved in 1 mL of chlorobenzene under stirring in a nitrogen glove box and spin coated at 3,000 rpm for 30 s using 120 mL per sample. The samples were stored overnight in dry air (relative humidity <1%). The devices were completed by thermally evaporating 15 nm of molybdenum oxide and 150 nm of aluminum as the top contact. The device active area was 0.1 cm2. The J-V scan rate was 100 mV/s with a reverse scan from 1.3 V to 0.2 V. A metal aperture of 0.06 cm2 was used.

SUPPLEMENTAL INFORMATION Supplemental Information includes Supplemental Experimental Procedures, nine figures, and one table and can be found with this article online at 10.1016/j.joule.2018.09.014.

ACKNOWLEDGMENTS The work at the Stanford Synchrotron Radiation Lightsource was funded by the Department of Energy under contract no. DE-AC02-76SF00515. Stanford Synchrotron Radiation Lightsource at the SLAC National Accelerator Laboratory is a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. Part of this work was performed at the Stanford Nano Shared Facilities (SNSF), supported by the National Science Foundation under award ECCS-1542152. K.B. thanks Doug van Campen (SSRL) for support with the RTA chamber and Bart Johnson (SSRL) for beamline support. K.B. and C.J.T. thank M. Woodhouse (NREL) for discussion of cost modeling. The part of the work at

Joule 2, 1–13, November 21, 2018


Please cite this article in press as: Bruening et al., Scalable Fabrication of Perovskite Solar Cells to Meet Climate Targets, Joule (2018), https://

the National Renewable Energy Laboratory is supported by the Solar Energy Research Institute for India and the U.S. (SERIIUS) funded jointly by the U.S. Department of Energy subcontract DE AC36-08G028308 (Office of Science, Office of Basic Energy Sciences, and Energy Efficiency and Renewable Energy, Solar Energy Technology Program, with support from the Office of International Affairs) and the Government of India subcontract IUSSTF/JCERDC-SERIIUS/2012 dated 22 November, 2012.

AUTHOR CONTRIBUTIONS Conceptualization, K.B. and C.J.T.; Methodology, K.B. and J.S.; Software, K.B.; Investigation, K.B., B.D., J.S., and Y.-Y.L.; Resources, K.B., B.D., M.F.A.M.v.H., C.J.T.; Writing – Original Draft, K.B. and B.D.; Writing – Review & Editing, K.B., B.D., J.S., Y.-Y.L., M.F.A.M.v.H., and C.J.T.; Visualization, K.B.; Supervision, K.B., M.F.A.M.v.H., and C.J.T.; Project Administration and Funding Acquisition, M.F.A.M.v.H. and C.J.T.

DECLARATION OF INTERESTS The authors declare no competing interests. Received: May 21, 2018 Revised: July 12, 2018 Accepted: September 12, 2018 Published: October 9, 2018

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