Slot-Die Coated Triple-Halide Perovskites for Efficient and Scalable Perovskite/Silicon Tandem Solar Cells

Wide bandgap halide perovskite materials show promising potential to pair with silicon bottom cells. To date, most efficient wide bandgap perovskites layers are fabricated by spin-coating, which is difficult to scale up. Here, we report on slot-die coating for an efficient, 1.68 eV wide bandgap triple-halide (3halide) perovskite absorber, (Cs0.22FA0.78)Pb(I0.85Br0.15)3 + 5 mol % MAPbCl3. A suitable solvent system is designed specifically for the slot-die coating technique. We demonstrate that our fabrication route is suitable for tandem solar cells without phase segregation. The slot-die coated wet halide perovskite is dried by a “nitrogen (N2)-knife” with high reproducibility and avoiding antisolvents. We explore varying annealing conditions and identify parameters allowing crystallization of the perovskite film into large grains reducing charge collection losses and enabling higher current density. At 150 °C, an optimized trade-off between crystallization and the PbI2 aggregates on the film’s top surface is found. Thus, we improve the cell stability and performance of both single-junction cells and tandems. Combining the 3halide top cells with a 120 μm thin saw damage etched commercial Czochralski industrial wafer, a 2-terminal monolithic tandem solar cell with a PCE of 25.2% on a 1 cm2 active area is demonstrated with fully scalable processes.

Coating process: the shim protruding the slot-die head was positioned 0.3-0.4 mm above the substrate to be coated, referred to as the coating gap between the slot-die head and substrate. After positioning the die head above the substrate, the precursor solution was pumped into the slot-die head to form a meniscus in between slot-die head and substrate. Then the coating process was started. The slot-die head was moved at speed of 5 mm/s with 300 µl/min pump rate while keeping the coating table and glovebox environment at 21°C. Otherwise, voids can be formed between perovskite and substrate interface leading to coating inhomogeneities illustrated in Figure S3.
Drying process and annealing process: after coating the wet film, the chuck was moved right beneath the air knife which was supplied with 22-24 psi of N2, being located 1-1.5 cm above the substrate, perpendicular pointing towards the surface. The velocity of the substrate moving under the air knife was, as for the coating step, 5 mm/s. We moved the pre-dried film onto the hot plate for annealing under various annealing temperatures, for the duration of 20 min in a N2-filled glovebox.
Afterwards, 23 nm of C60 was thermally evaporated at a rate of 0.15 Å s -1 onto the perovskite film. As indicated, a 1 nm-thick LiF interlayer was deposited between C60 and perovskite, evaporated at a rate of 0.05 Å s -1 , within the same vacuum run as the C60 layer. A thermal atomic layer deposition ALD Arradiance GEMStar reactor was used to deposit 20 nm SnO2. The Sn Precursor was Tetrakis (dimethylamion)tin(IV) (TDMASn) which was held at 60°C in a stainless-steel container. We used water as oxidant, which was passed from a stainless-steel container without active heating. With the deposition temperature of 80°C. The continuous 140 cycles accumulated 20 nm tin oxide. As an alternative, 8 nm BCP, which could be evaporated at 0.15 Å s -1 onto the C60 film, had the same function as SnO2. At the end, either 100 nm Silver (Ag) or 100 nm Copper (Cu) were also deposited with the rate of 1 Å s -1 .

Silicon solar cell preparation (Czochralski)
The fabrication details of our silicon heterojunction (SHJ) cells with rear side p/n junction can be found in 1 . Just briefly: The n-type Czochralski (Cz) wafer, with <100> orientation and 140 µm thickness as well as 5 Ω·cm resistivity, was cleaned and chemically treated for saw damage etching removal and one-side texturing with random pyramids as described in 2 . In order to remove the surface residual oxide, the c-Si wafers were cleaned by RCA procedures and dipped in 1% hydrofluoric acid for minutes. For the 105 µm thick wafer which is used for the session of the slot-die coated perovskite/silicon tandem reproducibility investigation (see Figure S21A), it has an additional front side chemical polishing. Then the intrinsic and doped silicon films were deposited by plasma-enhanced chemical vapor deposition on both sides. An i/p stack (junction) and i/n stack (front surface field) were deposited on the rear and front side, respectively. To enhance light coupling into the bottom cell we used the same nanocrystalline silicon oxide n-layer stack as an optical interlayer 1 . Both front (20 nm) and the rear (150 nm) TCO layers were DC sputtered from a commercial TCO target (nSCOT) at room temperature. 400 nm Ag layer was applied as the rear metallization by sputtering for rear metallization. Both contact-layer stacks of the silicon were deposited using aligned shadow masks with an opening of 1.13×1.13 cm². After fabrication, the silicon bottom cell had been annealed at 210°C for 10 min before perovskite top cell deposition.

Preparation of perovskite/silicon for tandem solar cells
The silicon bottom cells were washed with ethanol by spin coating and cleaned for 15 min in an UV-O3 cleaner (FHR UVOH 150 Lab). The same coating process, methods, and steps of HTL, perovskites, LiF, C60 and SnO2 were deposited as described above on the silicon bottom cells. In the next step, 100 nm IZO was deposited by sputtering (Roth&Rau MicroSys 200 PVD, ceramic target purchased from FHR Anlagenbau GmbH). The 2-inch ceramic target consisted of 90%wt. In2O3 and 10%wt. ZnO. We applied 70W RF oscillation power to form a uniform deposition with 0.2%vol gas inlet O2 in the chamber. The optimized layer had mobility, carrier density and resistivity of 43.5 cm² V -1 s -1 , 3.4·1020 cm -3 and 4.2·10 -4 Ω cm, respectively 3 . Subsequently, a 100 nm thick silver frame was thermally evaporated as bus bar on the boundary of the active area to collect the charge carriers. At last, 100 nm LiF was also thermally evaporated as an antireflective coating.

Time-dependent steady-state photoluminescence
The time-dependent steady-state absolute photoluminescence measurements were performed on a home-made setup including a 532 nm laser, and an integrating sphere. The samples were placed at the sphere edge. The PL signal was collected through an optical fiber towards a CCD-array spectrometer (Ocean Optics). The samples were excited by a continuous-wave laser at 532 nm. The photon flux was around 1.2×10 16 photons/s with a spot size of 0.12 cm². This intensity was around 1 sun excitation flux which has been calibrated with a certified silicon reference cell. The spot size was measured by fitting a Gaussian curve to the beam profile extracted from the CCD imaging. Non-absorbed laser light and emitted photoluminescence fluxes were collected by the spectrometer. The PLQY could be calculated by the ratio of the laser absorption and emitted photoluminence. The PL signal was recorded with an integration time of 300 ms with a delay between measurements of one second, and the result was averaged over 10 measurements. Measurements were carried out in air ambient immediately after taking the samples out from the N2-filled glovebox.

Absolute PL and intensity dependent QFLS measurements
520 nm CW laser (Insaneware) was utilized for the PL excitation light source through an optical fiber into an integrating sphere. The light source intensity has been calibrated into 1 sun condition by the active area of 1 cm² perovskite solar cell. The current density under short-circuit condition matched the Jsc under the sun simulator (1.375×10 22 photons m -2 s -1 ). An independent optical fiber was utilized as the output of the integrating sphere to a spectrometer (Andor SR393i-B) which was equipped with silicon CCD camera (DU420A-BR-DD, iDus). Halogen lamp with known spectral irradiance was utilized to calibrate the whole system. Between the spectral output of the detector and the calibrated spectral irradiance of the lamp, a spectral correction factor was utilized. The spectral photon density was calculated from the corrected detector signal by division of the photon energy. Thus, the photon excitation and emission numbers were received from numerical integration by Matlab. At last, three test samples with high known PLQY from Hamamatsu Photonics were utilized to accurately compare and reproduce within a small relative error of less than 5%.
During the measurement, the samples were placed under illumination into the integrating sphere using the same 520 nm CW laser which was described before. To control the intensity, a continuously variable neutral density filter wheel (ThorLabs) was utilized to attenuate the laser input to measure at different intensities. The intensity was also recorded by the additional silicon detectors. The illuminated samples experienced for a variable illumination time using an electrical shutter for a specific intensity. After 1 second illumination time, the PL spectra were received by averaging 30 spectra taken using a 30 µs exposure time by a detector. The electrical shutter was closed, and the filter wheel rotated to a new position. The measurements were repeated and recorded in this way. A home-made LabView code was written to automate the measurement and automate the data evaluation.

Pseudo J-Vs
In previous publication 4 , the pseudo J-V has been introduced from intensity-dependent QFLS or Voc measurements. At a specific light intensity which can also be converted into equivalent intensities as compared to ''suns'', this method calculated the dark-current density from the generated current density. For example, for the wide bandgap 1.68 eV perovskite, 1 sun corresponds to 20.5 mA/cm². 1% of 1 sun corresponds to 0.205 mA/cm². In specific illumination intensity, the measured QFLS or Voc were measured to create a transport/series resistance free dark J-V curve. Then, the curve ws shifted to the corresponding Jsc under certain sun ratios. This method allows to derive pseudo-FF of the partial cell stack, such as bare perovskite film or whole solar cells. Under such investigation, the implied FF is impacted purely by the non-radiative recombination processes without charge transport or resistive losses by the bulk material or the transport layers. Steady-state intensity dependent Voc measurement was measured with a Wavelabs Sinus-70 LED class AAA sun simulator. By tuning the illumination intensity proportionally, the corresponding J-V scans were recorded. The construction method is similar to the explanation before.

Transient photoluminescence
The transient photoluminescence measurements were performed in confocal setup using a pulsed laser at 125 kHz repetition rate and center wavelength of 655 nm. The photon flux per excitation pulse was adjusted to approximately 3x10 11 cm -2 , which should induce a carrier concentration that is comparable to steady-state conditions under solar illumination. For detection a single photon counting system with a Geiger-mode avalanche photodiode was used. The laser was filtered out by a 700 nm long pass filter.

Single junction solar cell characterization (current-voltage curves, EQE)
The J-V measurements of single-junction solar cells were recorded in the nitrogen atmosphere with the solar simulator (Oriel LCS-100) and Keithley 2400 source-measured unit. The measurement system was controlled by a LabView program. The sun simulator was calibrated to AM1.5G as 1 sun equivalent with a filtered KG3 Silicon reference solar cell which has been calibrated by Fraunhofer ISE. The spectral mismatch was around 0.997 within the measurement error. J-V scans were performed in a 2-point-probe configuration with a step size of 20 mV, an integration time of 20 ms and a settling time of 20 ms (250 mV/s). There is no pretreatment of the perovskite solar cells before the measurement. All the J-V curve measurements were taken several times for better statistics. Shunted devices, showing significantly lower Voc values were not considered in the statistical analysis. EQE spectra were measured with an Oriel Instruments QEPVSI-b system with a Newport 300W Xenon arc lamp, utilizing the TracQ-Basic software. A calibrated silicon reference cell was applied before every measurement to calibrate the light intensity. The electrical response of the device under illumination was recorded with a Stanford Research SR830 Lock-In simplifier with the time constant of 0.3 s and evaluated in TracQ. We generally find good agreement between the integrated external quantum efficiency times AM1.5G irradiance and the Jsc from J-V scan values.

Tandem solar cell characterization (current-voltage curves, EQE)
A Wavelabs Sinus-70 LED class AAA sun simulator was used for the measurement of the tandem solar cells in air condition under AM1.5G (1sun) equivalent illumination. No preconditioning of the tandem solar cells was applied before measurement. With the current density obtained by EQE, the limited junction can be determined. The intensity was changed slightly by sample to sun simulator distance so that the Jsc of the limited sub cell can be reached.
The backside of the solar cell was contacted onto a metal vacuum chuck at 25°C. On the front side, two Au probes contacted the front silver electrode. A metal laser-cut aperture mask covered the substrate with the exposure area as an active area. The J-V measurement and MPP tracking were recorded using a home-build LabView software. The EQE spectra were recorded with a homemade setup by using 78 Hz chopped monochromatic light from a Xe and He light source respectively. For the measurement of perovskite top junction EQE, the silicon bottom cell was saturated by using an 850 nm peak emission LED. In order to maintain the short circuit conditions, a 0.6V bias voltage was applied. For the silicon junction EQE, the silicon bottom cell EQE was measured by saturating the perovskite top cell by 455 nm blue LED and an additional 1 V bias voltage.

XRD
X-ray diffraction patterns (XRD) were measured using a Bruker D8 diffractometer in Bragg-Brentano geometry with a custom-made inert sample holder at room temperature. For all measurements, the Cu K-alpha radiation emitted from an X-ray tube operated at 40 mA and 40 kV acceleration voltage was used.

GIXRD
Grazing incidence X-ray diffraction (GIXRD) measurements were carried out using a Bruker D8 diffractometer. The height of the films was calibrated before each measurement. Diffraction patterns were collected with a step size of 0.02 degrees, for 5 seconds at each step, with the measurement conducted in air.

UV-VIS
For transmission and reflection measurements a PerkinElmer Lambda -1050 UV/VIS/NIR spectrophotometer with an integrating sphere was used.

Grain size estimation
The SEM images have been processed by MATLAB script which has been modified for our images' adaption 5 . Figure S1. Efficiency vs. time for state-of-the-art perovskite single junction and perovskite/silicon tandem cells with various coating techniques and perovskite precursors. The coating techniques included spin coating, inkjet printing, blade coating and slot-die coating. There was no inkjet printed perovskite for tandem solar cells. For publications 5 and 6, the corresponding single-junction was shown for comparison without extra label and information. All data is collected below in the Table S1. Cells with low methylammonium iodide (MAI) concentration have been highlighted with *. Table S1. Literature overview of state of the art single and tandem solar cells using various coating techniques and perovskite precursors, as shown in Figure S1.       with 7 elements for 3 annealing conditions. The decrement of the Chloride trend follows the same bandgap increment. The lead concentration trend can also be observed. In the low energy band 0.5 keV, the iodine shows higher signal while the 3.9 keV shows no difference. The iodine statistic increment is contributed from the surface. It draws our attention towards the surface compositional investigation. The data was collected by the scanning area of 20 µm² in total.

The investigation of the reproducibility issue of the slot-die coated perovskite silicon solar cell
It has been reported previously that a silver contact layer can degrade because halides from the perovskite layer could react with silver into silver halides 11 . Due to our coating process and the 5 µm back textured silicon, the perovskite solution could easily infiltrate into the silicon rear side during coating which could not be perfectly sealed. Therefore, the rear electrode could be easily eroded in the slot-die process. Although we have tried ethanol and EDTA to clean the degraded electrode and brushed additional silver paste, no conductive enhancement has been observed which meant the residue was still attached and indissoluble. The significant series resistance reduction leads to a direct FF improvement.
For a better understanding of this effect, we conducted the Transfer Length Method (TLM) to examine the sheet resistance and contact resistance between silver and rear side TCO underneath the silver. This data is displayed in Figure S20. After the extraction of the total resistance along various contact distances, the slope of the two linear relations can represent the sheet resistance of the TCO. This sheet resistance increases 16 times after being in contact with the perovskite solution. At the same time, the increased contact resistance has been increased from 1.81 Ohm to 8 Ohm. The contact resistance indicates that the silver surface has deteriorated into silver halides inducing for higher resistivity. To proof our assumption and prevent the undesired silver reaction, we sputtered an additional 25 nm of ITO onto the rear side to protect the silver back electrode. However, as seen in the SEM images in Figure S19, it showed that the ITO couldn't stop the penetration. In a second approach we used 25 nm SiOx to protect the rear side of the silicon bottom cell. The approach was successful: 1 % diluted hydrofluoric acid (HF) has been used to remove this protection layer after the whole tandem solar cells was completed. The new device architecture is illustrated in Figure S21A. The performance deviation has been narrowed down from 12% to 4% after introducing this protective and sacrificial SiOx layer on the back side. Within this batch, with confirmed current density by EQE in Figure S21B, the best performance cell's J-V scans are shown in Figure S22. Again, the silicon bottom cell limits the matched current to 17.8 mA/cm² due to a silicon wafer thickness of only 105 µm. When we implemented the variated spectral illumination, the best FF achieves 76.13 for the perovskite junction. According to the previous reported result 6 , our slot-die coated perovskite silicon tandem solar cell using 2PACz as HTL has approached the medium performance and reached the same charge extraction as spin coated perovskite tandem cells.  Then 150 nm nSCOT and 400 nm Ag electrode was sputtered on wafer (Cz/SiN/nSCOT/Ag). The perovskite solution was dropped on the top directly. After removing extra solution by gravity, the silver side was attached onto hotplate to simulate the standard annealing condition. The color lines were solid lines before halide perovskites erosion while the dashed lines were after halide perovskites erosion and 20 min annealing at 150°C. The distance from 1 mm to 3 mm corresponds to the Ag pads distance. Right: extraction of the data from TLM measurement. The regression indicates the contact resistance between silver and TCO increases from 1.815 Ohm to 8 Ohm. And the slope indicates the sheet resistance increases almost 16 times which may indicate the halide perovskites have also reacted with the TCO layer. Figure S21. (A) Another 25 nm SiOx has been added by PECVD to protect the back side. After the whole fabrication process, 1% HF solution has been used to remove the SiOx protection layer. The 100 nm silver electrode has also been slightly damaged. In Figure S16, the silver paste has been brushed after erasing to achieve better contact. (B) The EQE comparison between 120 µm and 105 µm silicon wafer. The current density of perovskite layer was almost the same. However, the current mismatch is caused by the insufficient silicon absorption. (C) Here was a batch which has been measured before and after HF etching. The light intensity has been the same for both measurements. Afterwards, the light intensity has been calibrated as silicon limited performance with 17.8 mA/cm². The lower current generated by silicon was due to the reduced silicon thickness from 120 µm to 100 µm. The series resistance values are calculated at the slope of the Voc intersect points. Figure S22. The obtained pseudo J-V curve of the tandem solar cell to quantify the implied efficiency potential in absence of any transport or serial resistance losses. The maximum power point is marked in red in panel.