Spray-Coated Lead-Free Cs 2 AgBiBr 6 Double Perovskite Solar Cells with High Open-Circuit Voltage

Lead-free Cs 2 AgBiBr 6 double perovskite is considered a promising alternative photovoltaic absorber to the widely used lead halide perovskite due to its easy processability, high stability, and reduced toxicity. Herein, for the ﬁ rst time spray processing for the deposition of Cs 2 AgBiBr 6 double perovskite thin ﬁ lms is reported. Microstructural (X-ray diffraction, scanning electron microscopy) and optoelectronic (absorbance, photoluminescence, photocurrent density versus applied voltage curves, electrochemical impedance spectroscopy) properties of spray-coated ﬁ lm are compared with the spin-coated benchmark. Incorporation of the spray-coated Cs 2 AgBiBr 6 double perovskite thin ﬁ lms in solar cells leads to a 2.3% photoconversion ef ﬁ ciency with high open-circuit voltage of 1.09 V. This study highlights the suitability of ultrasonic spray deposition for the optimization of Cs 2 AgBiBr 6 solar cells in terms of light absorption properties and charge transfer at the Cs 2 AgBiBr 6 /hole transporting layer interface.


Introduction
[3][4] With power conversion efficiencies (PCEs) exceeding 25%, [5] these cost-efficient materials constitute strong alternatives to commonly used Si-based solar cells. [6]However, lead toxicity and poor stability over time are significant obstacles toward their use and commercialization. [7,8][11][12][13][14][15] To diminish the amount of lead in perovskite PV absorber, two options are generally envisaged: the first is to replace a fraction of lead by a non-hazardous metal cation; the second is to completely substitute lead by a nontoxic element such as Ge(II), Mn(II), Cu(II), Bi(III), or Sb(III). [10]ead-free halide double perovskites are considered as one of the promising PV absorber alternatives to lead halide perovskite in view of their nontoxic character and long-term stability. [14]The double perovskite crystal structure A 2 B 0þ B 003þ X 6 (where A and B 0 are monovalent ions, B 00 is a trivalent metal, and X is an halide) is an expansion of the ABX 3 perovskite crystal lattice, where two Pb 2þ on B sites are replaced by one monovalent ion B þ and one trivalent ion B 003þ . [16][18][19][20][21][22][23][24][25][26][27][28][29][30] Cs 2 AgBiBr 6 has an indirect bandgap of 1.95 eV and a direct bandgap of 2.20 eV, which makes the material particularly interesting as wide-gap semiconductor top cell in tandem or even triple junction solar cells. [31]reul et al. first reported Cs 2 AgBiBr 6 films in solar cells. [21]A PCE of 2.4% (short-circuit current density: J sc ¼ 3.93 mA cm À2 ; open-circuit voltage: V oc ¼ 0.98 V, fill factor (FF) ¼ 0.63) was reached using an n-i-p configuration comprising: FTO glass/ compact (c)-TiO 2 /mesoporous (mp)-TiO 2 /Cs 2 AgBiBr 6 /Spiro-OMeTAD/Au.Here, they spin-coated the films from a solution containing the three mixed precursors (CsBr, AgBr, and BiBr 3 ) dissolved in dimethylsulfoxide (DMSO).By contrast, most studies first prepare Cs 2 AgBiBr 6 powder [16][17][18][19][20] -by reaction of CsBr, AgBr, and BiBr 3 in HBr at 120 C for 2 h-which then is further dissolved in DMSO for thin film deposition.However, the resulting solar cells exhibit low PCE values between 1.1% and 2.5 % while being time-consuming in terms of photoactive layer DOI: 10.1002/solr.202100422Lead-free Cs 2 AgBiBr 6 double perovskite is considered a promising alternative photovoltaic absorber to the widely used lead halide perovskite due to its easy processability, high stability, and reduced toxicity.Herein, for the first time spray processing for the deposition of Cs 2 AgBiBr 6 double perovskite thin films is reported.Microstructural (X-ray diffraction, scanning electron microscopy) and optoelectronic (absorbance, photoluminescence, photocurrent density versus applied voltage curves, electrochemical impedance spectroscopy) properties of spray-coated film are compared with the spin-coated benchmark.Incorporation of the spray-coated Cs 2 AgBiBr 6 double perovskite thin films in solar cells leads to a 2.3% photoconversion efficiency with high open-circuit voltage of 1.09 V.This study highlights the suitability of ultrasonic spray deposition for the optimization of Cs 2 AgBiBr 6 solar cells in terms of light absorption properties and charge transfer at the Cs 2 AgBiBr 6 /hole transporting layer interface.
preparation.Some groups have also reported the deposition of Cs 2 AgBiBr 6 by thermal evaporation with 1.4% PCE. [16,25,27]p to now, the best PCE of Cs 2 AgBiBr 6 -based solar cells reached 2.8% through the engineering of multifunctional dye interlayers [23] or the incorporation of single-layered MXene nanosheets into titania (Ti 3 C 2 T x @TiO 2 ) as a multifunctional electron transport layer (ETL). [30]Apart from interlayer design and ETL engineering, it has also been shown that the film processing itself strongly influences the optoelectronic properties of the photoactive layers in classical lead halide perovskites solar cells. [16,19,32,33]he thin film processing must also matter for microstructural and optoelectronic properties of Cs 2 AgBiBr 6 -based solar cells.
In our study, we deposited Cs 2 AgBiBr 6 double perovskite thin films by ultrasonic spray and compared results with the spincoated benchmark.Ultrasonic spray deposition has been shown to be very effective for inorganic-organic lead halide perovskites solar cell manufacturing, with a record PCE of %20% on small area substrates. [34]Due to the production of micron-sized droplets by piezoelectric transducers, ultrasonic spray deposition allows the deposition of very uniform lead halide perovskite thin films with smaller average grain size in comparison with spin-coated films. [35]n addition, ultrasonic spray deposition is a scalable, semiautomated, atmospheric pressure process.In this article, we used ultrasonic spray deposition to process Cs 2 AgBiBr 6 double perovskite thin films.Spray deposition allows the formation of very smooth, uniform, and pinhole-free layers, with smaller and better connected grains compared with the spin-coated benchmark.The corresponding spray-coated devices reached 2.3% efficiency, one of the best performances for spin-coated benchmarks to date.Such performance was directly attributed to the improvement of the Cs 2 AgBiBr 6 interface induced by the spray deposition process and the improvement of the Cs 2 AgBiBr 6 /Spiro-OMeTAD interface.As a consequence of the interface optimization, spray-coated devices led to an impressively high V oc of 1.09 V compared with 0.82 V for the spin-coated benchmark, just surpassing the V oc record for a solution-processed Cs 2 AgBiBr 6 double perovskite layer.

Structure and Morphology of Cs 2 AgBiBr 6 Thin Films
Prior to solar cell assembly, X-ray diffraction (XRD) measurements were performed on spin-coated and spray-coated Cs 2 AgBiBr 6 films to confirm the synthesis of phase-pure films.Cs 3 Bi 2 Br 9 (reflection at 12.8 and 30.9 ) and AgBr (reflection at 44.2 ) are two undesired side phases that can be formed during the film deposition. [21]To avoid such side phases formation, an annealing temperature above 250 C was required (best conditions: 285 C) for the complete conversion of the precursors into the desired Cs 2 AgBiBr 6 double perovskite crystal phase. [21]igure 1 shows the XRD patterns of the Cs 2 AgBiBr 6 double perovskite deposited by spin-coating and spray-coating on mp-TiO 2 /c-TiO 2 /FTO glass substrates.Three main peaks at 15.7 , 22.5 , and 31.8 were observed, representative of the (200), (220), and (400) reflections of the Cs 2 AgBiBr 6 (JCPDS file number: 01-084-8699) double perovskite, respectively, along with (100), (311), (222), and (331) reflections at 13.7 , 26.5 , 27.5 , and 35.7 , respectively.Specific reflections of residual Cs 3 Bi 2 Br 9 or AgBr were not observed.Cs 2 AgBiBr 6 average crystallite size was calculated from Scherrer equation on the (400) peak signal.No significative difference was observed between spin-coated and spray-coated samples, with 28 and 34 nm average values extracted, respectively.
The morphology of the Cs 2 AgBiBr 6 double perovskite films was evaluated by scanning electron microscopy (SEM) analysis.For the spin-coated benchmark (Figure 2a-c), %350 nm diameter grains were observed on top of the mp-TiO 2 layer, with a poorly connected Cs 2 AgBiBr 6 network.Ultrasonic spray deposition (Figure 2d-f ) allowed for the formation of a very smooth and uniform Cs 2 AgBiBr 6 film with smaller grains (%150 nm), as previously observed for ultrasonic spray-processed lead halide perovskite compared with a spin-coated counterpart. [35]In the literature, density functional theory (DFT) calculations determined the spectroscopic limited maximum efficiency (SLME) of Cs 2 AgBiBr 6 , which is 7.92% for a 200 nm-thick film. [36]hicker overlayers would result in increased recombination due to the limited diffusion length of electrons, without significant enhancement of light absorption.The strength of optical absorption and the nature of the bandgap influence the SLME. [17]By adjusting the concentration of the spin-coated and spray-coated solutions, an average overlayer of 200 nm thickness was obtained for both samples.However, there were more variations in spin-coated films due to its higher roughness (Figure 2c and S3, Supporting Information).

Optical Properties of Cs 2 AgBiBr 6 Thin Films
The optical absorbance curves of the spin-coated and spraycoated Cs 2 AgBiBr 6 thin films are shown in Figure 3a.A strong absorption peak is present at an %440 nm wavelength, as previously observed for Cs 2 AgBiBr 6 and other double perovskite materials such as Cs 3 Bi 2 I 9 . [19]However, the origin of this peak is still not very clear; [37] it could be associated with trapped excitonic transitions below the high-energy direct bandgap intraband transitions [38] or even color centers in the material. [39]The absorbance is globally higher for the spray-coated film than the spin-coated benchmark, but this is attributed to less uniform thickness for spin-coated films as seen in cross-sectional images (Figure 2c and S3, Supporting Information).Tauc plots (Figure 3b) allow for the determination of a direct band gap of %2.20 and %2.30 eV, respectively, for spray-coated and spin-coated samples.The bandgaps determined for the spraycoated and spin-coated samples are in accordance with values previously reported in the literature for similar synthesis conditions. [20,40]These absorbance data highlight the strong interest in ultrasonic spray-coating to improve the Cs 2 AgBiBr 6 films morphology and structures and therefore their photoabsorbing properties, which is of strong importance for PV purposes.
The photoluminescence (PL) spectra of the spray-coated and spin-coated Cs 2 AgBiBr 6 films are shown in Figure 4a.Most importantly, the PL emission is broad and centered around 645 nm (%1.9 eV) for both samples, which is attributed to the indirect bandgap of about 1.9 eV, [40] without significant contribution from the direct bandgap around 560 nm (2.2 eV).The PL spectra shown in Figure 4a are therefore dominated by recombination though the indirect gap, though recent low-temperature studies suggest that PL emission might arise from color centers in Cs 2 AgBiBr 6 rather than band-to-band transitions. [39]The PL peak intensity for the spray-coated film is higher than for the spin-coated benchmark.An increase in PL intensity is usually attributed to a reduced number of defect sites or less active defect sites responsible for trap-assisted recombination. [37]However, the higher PL intensity can be explained by the more continuous layer for spray-coated film and so the more absorption of excitation light too.Figure 4b shows a PL image with high spatial resolution confirming the more uniform and smooth spray-coated film compared with the less connected spin-coated network.Indeed, the PL intensity is higher in each area with a better homogeneity on the microscale for spray-coated film and the distribution is a little broader for spin-coated film compared with spray-coated film (Figure S4, Supporting Information).We further note that zoomed-out PL images (Figure S5, Supporting Information) reveal inhomogeneities on larger length scales that we attribute to thickness fluctuations across longer distances across the film.To complement our steady-state PL characterization, we recorded the PL decay kinetics of the two films (Figure S6, Supporting Information).However, we cannot observe significant difference between the two films.

Assembly and Characterization of Cs 2 AgBiBr 6 Double Perovskite Solar Cells
The PV performance of the Cs 2 AgBiBr 6 films prepared by spincoating or ultrasonic spray deposition is evaluated in PV devices with the following architecture: FTO glass/c-TiO 2 /mp-TiO 2 /Cs 2 AgBiBr 6 /Spiro-OMeTAD/Au (Figure S2 and S3, Supporting Information).As shown in Figure 5, the cells exhibit a very low hysteresis.The champion spin-coated and spraycoated devices present PCEs of 2.3% (Table 1).The PCEs reported in this work are among the highest efficiencies reached to date, which is noted given it is achieved using a highly processable ultrasonic spray deposition technique for the preparation of Cs 2 AgBiBr 6 -based solar cells.As expected from PL results, the spray-coated Cs 2 AgBiBr 6 device shows V oc enhancement (1.09 vs 0.82 V for the spin-coated benchmark).Indeed, the more uniform and covering spray-coated films lead to lowered recombination in spray-coated devices.Fewer electrons are trapped in the intermediate energy levels, which further results in the increased open-circuit potential.This V oc value of 1.09 V is so far the highest reported for Cs 2 AgBiBr 6 -based solar cells with a solution process; only a higher V oc (1.12 V) was obtained by sequential vapor deposition. [25]Yan et al. were the first to exceed 1 V for direct and reverse measures (V oc ¼ 1.02 and 1.06 V, respectively) by using a spin-coated dye interlayer to suppress the carrier recombination among others. [23]In our study, ultrasonic spray process allows for the significant improvement of the open-circuit voltage for direct and reverse measures, without additional interlayer engineering.We therefore demonstrate that the processing method used for the deposition of the Cs 2 AgBiBr 6 photoactive layer strongly influences its charge transfer properties.Despite the higher absorbance and the better connectivity (Figure 2d and 3a) of the Cs 2 AgBiBr 6 grains in the photoactive layer, the spray-coated device shows reduced current density ( J sc ) compared with the spin-coated benchmark and this trend is corroborated by the external quantum efficiency (EQE) measurements (Figure S8, Supporting Information).The same conclusions can be drawn on average PV parameters (Figure S7, Supporting Information, 20 devices average).The difference between champion and others devices comes from reduced average J sc .Indeed, others parameters (V oc and FF) are similar.
To further analyze the charge transfer properties of spincoated and spray-coated Cs 2 AgBiBr 6 -based solar cells,  electrochemical impedance spectroscopy (EIS) is conducted.EIS results are presented as Nyquist plots in Figure 6.Data were fitted with the equivalent circuit model shown in the inset [41] and are shown in Table 2.The equivalent electrical circuit consists of: an R 1 resistance associated with wires and contacts (high frequency); an R 2 //Q 2 element (with Q as constant phase element modeling an imperfect capacitor) characteristic of the selective contacts, including c-TiO 2 and mp-TiO 2 for electron-selective contacts and the Spiro-OMeTAD for hole-selective contacts, respectively (medium frequency); and an R 3 //Q 3 element for interfacial recombination between electrons and holes (low frequency).From data fitting (Table 2), the interfacial recombination resistance R 3 of the spray-coated device is 367.1 Ω, which is higher than R 3 value of the spin-coated device (254.4Ω).As the recombination rate at the Cs 2 AgBiBr 6 /Spiro-OMeTAD interface is inversely proportional to R 3 , it reveals faster interfacial recombination in the spin-coated device.Therefore, it can be assumed that charge transfer at the Cs 2 AgBiBr 6 /Spiro-OMeTAD interface is improved in the spray-coated device.This is in agreement with V oc values obtained for both devices: high interfacial recombination increases the forward bias diffusion current, which, in turn, reduces the open-circuit voltage. [42]ooking at R 2 selective contacts resistance, we determine lower R 2 value for the spin-coated device (177.7 Ω) compared with the spray-coated device (191.2Ω), corresponding to more efficient charge transfer into the selective contact for the spin-coated device.Because of the centrifugal force inherent in the deposition process, spin-coating can allow for a better infiltration of Cs 2 AgBiBr 6 within the mp-TiO 2 layer, leading to improved charge collection at the electron selective contact and thus increased J sc .Moreover, the larger grains reported for the spin-coated sample reduce the amount of grain boundaries and so a decrease in the recombination rate is expected.The EIS data thus corroborate PL and J-V results.

Conclusion
Cs 2 AgBiBr 6 double perovskite is considered as a promising leadfree alternative to commonly used lead halide perovskites for solar cell applications, due to its easy processability, high Table 2. Resistance (R) and constant phase element (Q ) fitting parameters obtained from EIS data measured on the spin-coated and spray-coated Cs 2 AgBiBr 6 -based solar cells.stability, and nontoxicity.In this study, we report for the first time spray process for the deposition of highly processable Cs 2 AgBiBr 6 double perovskite thin films.Ultrasonic spray-coating allows for the deposition of very smooth and uniform thin films, with smaller and better connected grains in comparison with the spin-coated benchmark.In addition to microstructural improvements, spray-coated films show more suitable light absorption properties than the spin-coated ones.The further incorporation of the spray-coated Cs 2 AgBiBr 6 double perovskite thin films in full devices leads to a 2.3% photoconversion efficiency, which is one of the highest values for spin-coated benchmarks.Such beneficial effect is directly correlated to the improvement of the Cs 2 AgBiBr 6 interface induced by the spray-coating process.Indeed, from EIS data, spray-coated devices show lower interfacial recombination rates than spin-coated ones.As a consequence of the interface optimization, spraycoated devices lead to an impressively high V oc of 1.09 V compared with 0.82 V for the spin-coated benchmark, and slightly surpassing the V oc reached by a solution-processed Cs 2 AgBiBr 6 double perovskite layer with additional interface optimization by dye interlayer engineering.However, further work will be needed to ensure better infiltration of the spraycoated analogue into the mesoporous electrodes and expect to surpass spin-coated electrodes.This study motivates strong interest in ultrasonic spray deposition for the optimization of The PL maps were acquired using a wide-field, hyperspectral imaging microscope (Photon etc. IMA VIS), equipped with a 20Â or 100Â objective, and detected with a front-illuminated, low-noise CCD camera.A continuous wave (CW), power-tunable 405 nm laser was used as the PL excitation source with an incident photon flux of 9 Â 10 20 or 9 Â 10 21 photons m À2 s À1 These values correspond well to the generation rate under 1 or 10 suns of n 1sun ¼ ∫ 630 nm 350 nm AM1.5GðλÞ ¼ 9 Â 10 20 m À2 s À1 .All data were acquired in ambient atmospheric conditions.Time-resolved PL spectra were recorded using a confocal single-photon counting fluorescence microscope from Picoquant.Excitation was performed at λ ¼ 405 nm using a 20Â air objective at an intensity of 1.3 μJ cm À2 for each pulse using a repetition rate of 0.25 MHz.The PL was collected through a dichroic mirror by a λ ¼ 450 nm long-pass filter onto a single-photon counting Single Photon Avalanche Diode detector.
EQE spectra were recorded under N 2 atmosphere using a 100 W halogen lamp, mechanically chopped at 80 Hz and a double monochromator as light source, coupled into an optical fiber.Calibration was performed using an UV-enhanced Si photodetector (Newport).The photogenerated current was measured using a lock-in-amplifier from Stanford Research.

Figure 4 .
Figure 4. a) Steady-state PL and b) PL images of spin-coated and spray-coated Cs 2 AgBiBr 6 films recorded with 100Â objective.

Cs 2
AgBiBr 6 solar cells in terms of light absorption properties and charge transfer at the Cs 2 AgBiBr 6 /Spiro-OMeTAD interface.Most importantly, this deposition method can be adapted for the development of large-area solar cells devices and promote the scaled commercialization of perovskite-like solar cell technologies, without losing conversion efficiency.current variation response was recorded.A frequency range of 3-85 MHz with 10 mV sinusoidal modulation was applied for the EIS data acquisition for the spray-coated and spin-coated devices.Measurements were performed at room temperature under standard 1 sun illumination (AM1.5 filter) and in open-circuit potential (OCP) conditions.

Table 1 .
Photovoltaic parameters of the champion spin-coated and spraycoated Cs 2 AgBiBr 6 -based solar cells.