Cesium-Containing Triple Cation Perovskite Solar Cells

coatings

Article Cesium-Containing Triple Cation Perovskite Solar Cells

Steponas Ašmontas *, Aurimas Cerškus,ˇ Jonas Gradauskas , Asta Griguceviˇciene,˙ Konstantinas Leinartas , Andžej Luˇcun,Kazimieras Petrauskas, Algirdas Selskis, Algirdas Sužiedelis,˙ Edmundas Širmulis and Remigijus Juškenas˙

Center for Physical Sciences and Technology, LT-10223 Vilnius, Lithuania; [email protected] (A.C.);ˇ [email protected] (J.G.); [email protected] (A.G.); [email protected] (K.L.); [email protected] (A.L.); [email protected] (K.P.); [email protected] (A.S.); [email protected] (A.S.); [email protected] (E.Š.); [email protected] (R.J.) * Correspondence: [email protected]; Tel.: +370-5-2627124

Abstract: Cesium-containing triple cation perovskites are attracting significant attention as suitable tandem partners for silicon solar cells. The perovskite layer of a solar cell must strongly absorb the visible light and be transparent to the infrared light. Optical transmittance measurements of perovskite layers containing different cesium concentrations (0–15%) were carried out on purpose to evaluate the utility of the layers for the fabrication of monolithic perovskite/silicon tandem solar cells. The transmittance of the layers weakly depended on cesium concentration in the infrared spectral range, and it was more than 0.55 at 997 nm wavelength. It was found that perovskite solar cells containing 10% of cesium concentration show maximum power conversion efficiency. Keywords: perovskite; thin film; solar cells; power conversion efficiency; cesium Citation: Ašmontas, S.; Cerškus,ˇ A.; Gradauskas, J.; Griguceviˇciene,˙ A.; Leinartas, K.; Luˇcun,A.; Petrauskas, K.; Selskis, A.; Sužiedelis,˙ A.; 1. Introduction Širmulis, E.; et al. Cesium-Containing Triple Cation Perovskite Solar Cells. The promising features of a perovskite/silicon tandem solar cell have attracted con- Coatings 2021, 11, 279. https:// siderable attention in recent years [1–15]. The interest is inspired by the rapid rise of power doi.org/10.3390/coatings11030279 conversion efficiency (PCE) of a perovskite solar cell from below 3.8% to more than 25.2% in the last decade [16,17]. Moreover, it is known that the crystalline silicon solar cell has Academic Editors: Alicia de Andrés demonstrated a certified power conversion efficiency of 26.6% [18,19] which is close to its and Lucia Nicoleta Leonat theoretical efficiency limit of 33.3% [20]. The efficiency of a solar cell is limited by effective use of photons having energy close to the forbidden energy gap. Photons with higher Received: 28 January 2021 energies create electron–hole pairs, and the excess energy is transmitted to carriers thus, Accepted: 24 February 2021 making them the hot carriers. The heating of charge carriers by light leads to the formation Published: 27 February 2021 of a thermoelectromotive force of hot carriers having a polar opposite to that of the classical photovoltage caused by the electron–hole pair generation [21,22]. As a result, the creation Publisher’s Note: MDPI stays neutral of hot carriers reduces the PCE of a single-junction solar cell [23]. The coating of a silicon with regard to jurisdictional claims in solar cell with a thin perovskite layer absorbing high energy photons allows to considerably published maps and institutional affil- decrease the negative effect of the hot carriers. iations. There are different configurations of tandem solar cells. In the two-terminal (2T) configuration, both solar cells are monolithically integrated [4–10,13], while in the four- terminal (4T) configuration, the two cells are joined together mechanically [20,24–27]. The basic requirements for each configuration are the same: the perovskite layer must be a Copyright: © 2021 by the authors. good absorber of the visible light and must be transparent to the infrared light [17]. Licensee MDPI, Basel, Switzerland. In recent years, various technologies and methods have been developed to improve This article is an open access article PCE and other properties of the perovskite sub-cell. The high-quality performance of a distributed under the terms and perovskite solar cell can be primarily attributed to its excellent optical properties such conditions of the Creative Commons as its high absorption coefficient allowing to use a thin film, high defect tolerance, high Attribution (CC BY) license (https:// carrier mobility, and long carrier diffusion length [17,26–30]. Excellent bandgap tunability creativecommons.org/licenses/by/ allows to use a perovskite layer as a top sub-cell on any bottom cell [31–34]. Currently, the 4.0/).

Coatings 2021, 11, 279. https://doi.org/10.3390/coatings11030279 https://www.mdpi.com/journal/coatings Coatings 2021, 11, 279 2 of 12

best perovskite solar cells use a mixture of formamidinium (FA) and methylammonium (MA) as the monovalent cations [35–41]. Thea addition of cesium makes the triple cation perovskite compositions more thermally stable, as they have less phase impurities and are less sensitive to processing conditions [27,35]. The incorporation of cesium was shown to reduce the trap density and charge recombination rates in the perovskite layer [42]. Triple (Cs/FA/MA) cation-based perovskite materials exhibited average PCE fairly exceeding 20% with a stable performance against the long-time exposure to ambient atmosphere [43]. Triple cation perovskites of the generic form “Csx(MA0.17FA0.83)(1−x)Pb(I0.83Br0.17)3“ developed in [35] can be suitable candidates as tandem companions for silicon solar cells. In this paper, we present an experimental study of structure and optical properties of perovskite Csx(MA0.17FA0.83)(1−x)Pb(I0.83Br0.17)3 layers having different cesium concentration as well as photoelectric properties of solar cells fabricated on their base.

2. Perovskite Cell Fabrication and Characterization To fabricate efficient perovskite cells, methods and procedures proposed and detailed in Refs. [35,43,44] were used. Glass substrates of 25 × 25 mm2 size coated with fluorine- doped tin oxide (FTO) layer (TEC 10 (3.3 mm), Ossila, Sheffield, UK) were used for the formation of the perovskite cell. Before the formation of the cell, a ~5 mm wide strip of FTO layer was removed (etched) from one edge of the glass substrate. Zn powder and 4 M HCl solution were used for FTO etching. These and other precursors, unless otherwise noted, were from Sigma-Aldrich. After FTO etching, the substrates were cleaned for 20 min with 2% Helmanex solution in an ultrasonic bath (S40 H, Elmasonic, Singen, Germany). Then, the substrates were several times rinsed thoroughly with deionized water and ultrasonicated in isopropanol for 20 min. The substrates were dried by compressed Ar flow and treated for ~5 min in a low-pressure oxygen plasma cleaner (Atto, Diener electronic GmbH + Co. KG, Ebhausen, Germany). A ~30 nm-thick layer of a compact (dense) TiO2 was formed on the substrates by spraying a solution composed of titanium diisopropoxide (bis) acetylacetonate (Ti (acac)2OiPr2) and isopropanol (1:9 vol. ratio) and then sintering it for 15 min at 450 ◦C on a hotplate (PZ 28-3T Prazitherm, Gestigkeit GmbH, Düsseldorf, Germany). The sintered TiO2-coated substrates were left on the hotplate for ~6 h for natural cooling. After the cooling, a spin-coating method was used to deposit a 150–220 nm-thick layer of a mesoporous TiO2 on the compact TiO2 layer. The mesoporous TiO2 layer was grown using a 30 nm TiO2 particle paste (Dyesol 30NRD/ethanol) diluted in ethanol at a 1:6 wt. ratio. One hundred and fifty microliters (150 µL) of the prepared TiO2 solution was pipetted with an automatic pipette (Eppendorf Research Plus, Eppendorf AG, Hamburg, Germany) on the surface of the substrate placed on a centrifuge table (SPIN 150i POLOS, SPS-Europe B.V.), and the following spinning program was started: 4000 rpm for 20 s with an acceleration of 2000 rpm/s. The mesoporous TiO2 was formed by sintering the layer in a dry air for 30 min at 450 ◦C. Then, the sample was cooled down to 150 ◦C and immediately placed into a glovebox (MBRAUN MB-10-G, M. Braun Inertgas-Systeme GMBH, Garching bei München, Germany) with controlled atmosphere (relative humidity ≤0.5 ppm of N2 was continuously maintained). The perovskite structures were grown on mesoporous TiO2 by a one-step precipitation method from the prepared precursor’s solution. As a solvent, a mixture of anhydrous N, N-dimethylformamide (DMF) and dimethylsulfoxide (DMSO) was used: the vol. ratio of components was 4:1. The contents of the precursors in the prepared DMF/DMSO mixture was: 1.1~1.3 M of PbI2, 0.2 M of PbBr, 1 M of Formamidinium iodide (FAI), 0.2 M of Methylammonium bromide (MABr). Then, Cs ions were added to the prepared precursor’s solution: as a source of Cs a 1.5 M CsI solution in DMSO was used. Concentration of CsI in the precursor’s solution was varied from 0 to 15%. According to the literature, the solution of this composition was used to form a perovskite Csx(FA83MA17)(1−x)Pb(I83Br17)3 compound [44]. To deposit the perovskite layer on the formed mesoporous TiO2, the sample was placed on a centrifuge table, and 150 µL of the prepared precursor’s solution was dropped with an Eppendorf pipette. Then, immediately the spin program consisting of two steps was initiated. At the first step, a spinning Coatings 2021, 11, x FOR PEER REVIEW 3 of 12

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Eppendorf pipette. Then, immediately the spin program consisting of two steps was initiated. At the first step, a spinning at 1000 rpm for 10 s with an acceleration of 200 rpm/s wasat 1000applied. rpm The for parameters 10 s with an of accelerationthe second st ofep 200 were rpm/s 6000 rpm was applied.for 30 s with The an parameters accelera- tionof theof 1000 second rpm/s. step Before were 6000ending rpm the for spin-c 30 soating with anprogram acceleration (~10 s), of 150 1000 µL rpm/s.of chloroben- Before µ zeneending (CB) the were spin-coating dropped on program the surface (~10 of s), the 150 sampleL of to chlorobenzene remove residuals (CB) of were the solution. dropped on the surface of the sample to remove residuals of the solution. The samples were then The samples were then transferred from the centrifuge table on a hotplate (Isotemp Fish- transferred from the centrifuge table on a hotplate (Isotemp Fisherbrand, Thermo Fisher erbrand, Thermo Fisher Scientific, Waltham, MA, USA) and annealed under an inert at- Scientific, Waltham, MA, USA) and annealed under an inert atmosphere for 60 min at 100 ◦C. mosphere for 60 min at 100 °C. After the annealing and natural cooling for ~10 min, a hole After the annealing and natural cooling for ~10 min, a hole transport layer covering the transport layer covering the perovskite layer was formed. The samples were placed again perovskite layer was formed. The samples were placed again on a centrifuge table, poured on a centrifuge table, poured with 150 µL of 70 mM 2-N,2-N,2-N′,2-N′,7-N,7-N,7-N′,7-N′- with 150 µL of 70 mM 2-N,2-N,2-N0,2-N0,7-N,7-N,7-N0,7-N0-octakis(4-methoxyphenyl)- octakis(4-methoxyphenyl)-9,9′-spirobi[fluorene]-2,2′,7,7′-tetramine (Spiro-OMeTAD) so- 9,90-spirobi[fluorene]-2,20,7,70-tetramine (Spiro-OMeTAD) solution and immediately an lution and immediately an appropriate spin-coating program was initiated (4000 rpm for appropriate spin-coating program was initiated (4000 rpm for 25 s with an acceleration of 25 s with an acceleration of 2000 rpm/s). Before this procedure, 17 µL of Li-bis(trifluoro- 2000 rpm/s). Before this procedure, 17 µL of Li-bis(trifluoromethylsulfonyl)imide (Li-TFSI) methylsulfonyl)imide (Li-TFSI) salt in anhydrous acetonitrile and 28.8 µL of 4-tert-bu- salt in anhydrous acetonitrile and 28.8 µL of 4-tert-butylpyridine (TBP) solution were added tylpyridineto the Spiro-OMeTAD (TBP) solution solution. were added Li-TFSI to concentration the Spiro-OMeTAD in the solventsolution. was Li-TFSI 10 mgmL concentra-−1. The −1 tionformed in the perovskite solvent was cells 10 were mgmL turned. The dark formed immediately perovskite after cells preparation were turned and dark kept imme- in Ar diatelyatmosphere after preparation with humidity and <1%. kept in Ar atmosphere with humidity <1%. TheThe morphology morphology and and the the thickness thickness of of the the form formeded cell cell layers layers were were examined examined by by means means ofof a ascanning scanning electron electron microscope microscope (SEM) (SEM) (Helios (Helios NanoLab NanoLab 650, 650, FEI, FEI, Hillsboro, Hillsboro, OR, OR, USA). USA). TheThe chemical chemical composition composition of the layerslayers waswas determineddetermined with with an an attached attached energy energy dispersive disper- siveX-ray X-ray spectrometer spectrometer (EDX) (EDX) (INCA (INCA Energy, Energy, Oxford Oxford Instruments, Instruments, Abingdon, Abingdon, UK). UK). AnAn X-ray X-ray diffraction diffraction (XRD) methodmethod waswas usedused to to study study crystallographic crystallographic structure structure of theof theformed formed perovskite perovskite layers. layers. The The measurements measuremen werets were performed performed with with X-ray X-ray diffractometer diffractometer(SmartLab (SmartLab HR-XRD, HR-XRD, Rigaku, Rigaku, Tokyo, Tokyo, Japan) Japan) equipped equipped with 9 with kW power 9 kW rotatingpower rotating Cu anode Cu X- anoderay source X-ray and source theta/theta and theta/theta goniometer. goniometer. To enable To the enable investigation the investigation of thin perovskite of thin perov- layers skiteand layers to reduce and theto reduce influence the ofinfluence the substrate, of the substrate, the grazing the incidence grazing incidence diffraction diffraction geometry geometrywith an incidence with an incidence angle of Cuangle Kα ofbeam Cu K setα beam to 0.5 ◦setwas to applied.0.5° was Theapplied. XRD The patterns XRD werepat- ternsmeasured were measured with Bragg–Brentano with Bragg–Brentano geometry ingeometry 2Θ range in of2Θ 10 range◦–55◦ of. 10°–55°. OpticalOptical transmission transmission spectra spectra of of the the formed formed perovskite perovskite layers layers were were measured measured in in the the wavelengthwavelength range range of 300–1100300–1100 nmnm using using AvaSpec AvaSpec ULS2048XL ULS2048XL spectrometer spectrometer and and AvaLight- Ava- Light-DH-SDH-S deuterium–halogen deuterium–halogen light sourcelight source (both (both from Avantes,from Avantes, Apeldoorn, Apeldoorn, The Netherlands). The Neth- erlands).During theDuring measurements, the measurements, a 50 ms integrationa 50 ms integration time and time an averaging and an averaging of 100 measured of 100 measuredspectra were spectra used. were used. ToTo measure measure the the efficiency efficiency of of the the formed formed pe perovskiterovskite cells, cells, these these were were supplied supplied with with ~70~70 nm-thick nm-thick Au Au contacts contacts of of special special configuration configuration by by means means of of thermal thermal evaporation evaporation in in the the vacuumvacuum chamber chamber of of VAKSIS VAKSIS PVD PVD Vapor-5S_Th Vapor-5S_Th (Vaksis (Vaksis R&D R&D and and Engineering, Engineering, Ankara, Ankara, Turkey).Turkey). Schematic Schematic cross cross section section of of the the perovs perovskitekite solar solar cell cell and and the the microphotograph microphotograph of of thethe contacts contacts on on the the top top of of Spiro-OMeTAD Spiro-OMeTAD and and FTO FTO are are presented presented in in Figure Figure 11..

(a) (b)

FigureFigure 1. 1. (a()a )Schematic Schematic cross cross section section of of the the perovskite perovskite solar solar cell; (b) top viewview microphotographmicrophotograph of of the theperovskite perovskite solar solar cell. cell.

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Direct current–voltage characteristics and spectra of the perovskite solar cell were measured using Keithley 2602A (Keithley Instruments Inc., Cleveland, OH, US) equipment, and 100 mW/cm2 irradiance was achieved by a AM 1.5 spectral lamp (Newport model 67005, Newport Corp., Irvine, CA, US) placed at an appropriate distance. Continuous wave photoluminescence spectra were measured by means of a usual photoluminescence setup with completely automated 1 m focal length monochromator (FHR–1000, Horiba Jobin Yvon, Kyoto, Japan), thus achieving 0.8 nm/mm spectra disper- sion. For most measurements, the resolution of the photoluminescence system was 0.3 meV at the spectrum maximum and it did not exceed 0.8 meV at the lowest power of excitation. An Ar-ion laser emitting 2.2–2.7 eV photons used for excitation, and its power was varied either by means of the power supply or using neutral glass filters. The diameter of the laser spot on the surface of the sample was 2.5 mm. The photoluminescence was detected by a thermoelectrically cooled gallium arsenide photomultiplier (H7421–50, Hamamatsu, Hamamatsu City, Japan) operating in a photon counting regime. Time resolved photoluminescence experiments were carried out using a pulsed 532 nm diode pumped solid state (DPSS) microchip laser (photon energy 2.3 eV, pulse full width at a half maximum (FWHM) 400 ps, repetition rate 10 kHz, the spot diameter 0.5 mm). The photoluminescence was detected by a thermoelectrically cooled high-efficiency extended- red multi-alkali cathode photomultiplier (PMC-100-20, Becker&Hickl, Berlin, Germany) with an internal GHz preamplifier. The transient photoluminescence was measured with a time correlated single photon counting system.

3. Results and Discussion The inﬂuence of the added Cs ions on the structure of the formed perovskite layers was studied by the XRD method. It should be mentioned that at this stage of investigation, the perovskites were not coated with the hole transporting layer by reason of the higher ac- curacy and convenience of the XRD analysis. The measured XRD patterns of the perovskite layers formed from the solutions without CsI and those containing 5, 10 and 15% of CsI are shown in Figure2. Pronounced peaks at all the XRD patterns unambiguously indicate a clear crystalline structure. In the sample without Cs, the increased intensity of the peaks at 100 and 200 shows prevailing the orientation of perovskite crystallites in (100) direction (the lowest pattern in Figure2). The addition of Cs cations to the perovskite leads to slight shift of these peaks to higher 2Θ values. Modest amount of Cs changes the prevailing orientation of the crystallites from (100) to (110) (pattern 2 in Figure2). However, a further increase in Cs concentration up to 15% makes the orientation (100) prevailing again. The analysis of the XRD results shows that the studied perovskite layers have a cubic crystal lattice. The determined lattice parameter a of the perovskite layers with different Cs content as well as an average size of the crystallites D are presented in Table1.

Table 1. Values of the lattice parameter a and crystalite size D determined by the XRD.

Cs, % a,Å D, nm 0 6.2987 ± 0.0002 48.4 ± 0.8 5 6.2890 ± 0.0002 59.6 ± 1.7 10 6.2899 ± 0.0002 61.1 ± 1.5 15 6.2880 ± 0.0002 53.1 ± 1.7

The most ordered perovskite structure (the largest crystallites are ~61.1 nm) is formed in the solution with 10% of Cs, while the most disordered structure is typical of the sample without Cs. The unit cell volume of the perovskite containing no Cs is equal to a3 = 249.9 Å3, and, according to [45], this value corresponds to the perovskite of composition FA1/6MA5/6PbBr0.5I2.5. CoatingsCoatings2021 2021,,11 11,, 279 x FOR PEER REVIEW 55 of 1212

FigureFigure 2. XRDXRD patterns patterns of ofperovskite perovskite layers layers with with differ differentent content content of Cs: of 1—without Cs: 1—without Cs, 2—with Cs, 2— 5, with3—with 5, 3—with 10, 4—with 10, 4—with15% of Cs. 15% The of indexed Cs. The peaks indexed correspond peaks correspond to the perovskite to the phase perovskite phase FA1/6MA5/6PbBr0.5I2.5. FA1/6MA5/6PbBr0.5I 2.5. The lattice parameter a weakly correlates with the content of Cs: it is lower by ap- Table 1. Values of the lattice parameter a and crystalite size D determined by the XRD. proximately the same value, ~0.01 Å, for all the samples containing Cs in comparison to that withoutCs, Cs.% Most likely, the contractiona, Å of the perovskite lattice aDwas, nm caused by the + + + replacement0 of organic MA (CH3NH 6.29873 ) ± (ionic 0.0002 radius 1.8 Å) cations by 48.4 those ± 0.8 of Cs (ionic ◦ ◦ ◦ radius 1.74 Å).5 The peaks at 2Θ angles 6.2890 of 12.4± 0.0002, 25.5 and 39 (Figure2) should59.6 ± 1.7 be attributed to the PbI2 phase10 (ICDD card 00-007-0235). 6.2899 ± 0.0002 It is seen that increase in Cs 61.1 concentration ± 1.5 leads to a quite steep15 decrease in intensity 6.2880 of the ± 0.0002 PbI2 peaks, and these become 53.1 ± undetectable 1.7 in the samples with 10 and 15% of Cs (patterns 3 and 4 in Figure2). Top SEM images of the morphology of the perovskite layers without and with Cs are The most ordered perovskite structure (the largest crystallites are ~61.1 nm) is formed shown in Figure3. Presence of PbI phase is conﬁrmed by the examination of the images. in the solution with 10% of Cs, while2 the most disordered structure is typical of the sample As cases a and b of Figure3 show, the surface of the layers formed without and with 5% without Cs. The unit cell volume of the perovskite containing no Cs is equal to a3 = 249.9 of Cs is decorated by the bright grains which are attributed to an individual PbI phase. Å3, and, according to [45], this value corresponds to the perovskite of composition2 Their size and quantity on the surface decrease with a higher content of Cs: considerably FA1/6MA5/6PbBr0.5I2.5. smaller and less bright grains of the PbI phase are identiﬁed in the perovskite containing The lattice parameter a weakly correlates2 with the content of Cs: it is lower by ap- 10% of Cs (Figure3c). The PbI crystallites become undetectable at the highest, 15%, Cs proximately the same value, ~0.012 Å, for all the samples containing Cs in comparison to concentration (Figure3d). SEM observation of the perovskites allows concluding that that without Cs. Most likely, the contraction of the perovskite lattice a was caused by the Cs-containing samples can be characterized by a less rough surface as compared with the replacement of organic MA+ (CH3NH3+) (ionic radius 1.8 Å) cations by those of Cs+ (ionic Cs-free ones. The smoothest morphology was observed of the layer formed with 15% CsI radius 1.74 Å). The peaks at 2Θ angles of 12.4°, 25.5° and 39° (Figure 2) should be at- (Figure3d). tributedThe to cross-sectional the PbI2 phase SEM (ICDD images card of 00-007-0235). the same perovskite It is seen cells that are increase shown inin FigureCs concen-4. It istration visible leads that theto a thickness quite steep of alldecrease the examined in intensity samples of the is quite PbI2 similar: peaks, and the total these thickness become undetectable in the samples with 10 and 15% of Cs (patterns 3 and 4 in Figure 2). of the TiO2 layer (both compact and mesoporous) varies from 150 to 200 nm, and that of the perovskiteTop SEM one images is in the of range the morphology of 550 to 600 of nm. the Itpe isrovskite also worth layers noting without that and the with perovskite Cs are grainsshown with in Figure 0 and 3. 5% Presence Cs (Figure of 4PbIa,b)2 phase seem tois confirmed be more integral by the throughout examination the of layer. the images. In the caseAs cases of perovskite a and b of layers Figure containing 3 show, the 10 andsurface 15% of Cs, the the layers grains formed tend towithout grow onand the with top 5% of eachof Cs other is decorated (Figure4 byc,d). the bright grains which are attributed to an individual PbI2 phase. Their size and quantity on the surface decrease with a higher content of Cs: considerably smaller and less bright grains of the PbI2 phase are identified in the perovskite containing 10% of Cs (Figure 3c). The PbI2 crystallites become undetectable at the highest, 15%, Cs concentration (Figure 3d). SEM observation of the perovskites allows concluding that Cs- containing samples can be characterized by a less rough surface as compared with the Cs- free ones. The smoothest morphology was observed of the layer formed with 15% CsI (Figure 3d).

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Figure 3. Top-view SEM images of formed perovskite layers with (a) 0; (b) 5; (c) 10; and (d) 15 % of Cs. Magnification is 100,000×.

The cross-sectional SEM images of the same perovskite cells are shown in Figure 4. It is visible that the thickness of all the examined samples is quite similar: the total thickness of the TiO2 layer (both compact and mesoporous) varies from 150 to 200 nm, and that of the perovskite one is in the range of 550 to 600 nm. It is also worth noting that the perovskiteFigure 3. Top-view grains with SEM images0 and 5% of formedCs (Figure perovskite 4a,b) seem layers to with be (morea) 0; ( bintegral) 5; (c) 10; throughout and (d) 15 %the of layer.FigureCs. Magniﬁcation In3. Top-viewthe case is ofSEM 100,000 perovskite images×. of layersformed contai perovskitening layers10 and with 15% (a )Cs, 0; ( bthe) 5; grains (c) 10; andtend (d to) 15 grow % of onCs. theMagnification top of each is other100,000×. (Figure 4c,d).

FigureFigure 4. 4. Cross-sectionalCross-sectional SEM SEM images images of of perovskite perovskite cells cells formed formed on on fluorine-doped ﬂuorine-doped tin tin oxide oxide (FTO) (FTO)substrate substrate with ( awith) 0, ((ba)) 5,0, ((cb)) 10, 5, ( andc) 10, (d and) 15 ( %d) Cs. 15 % Magniﬁcation Cs. Magnification is 800,000 is 800,000×.×.

InfluenceInﬂuence of of cesium cesium concentration concentration on on optical optical transmittance transmittance of of the the layers layers was was investi- investi- gatedgated with with the the intention intention to to evaluate evaluate their their possible possible utility utility for for the the fabrication fabrication of of monolithic monolithic perovskite/siliconperovskite/silicon tandem tandem solar solar cells. cells. The The measured measured transmittance transmittance spectra spectra of the of theperov- per- skiteovskite layers layers are aredepicted depicted in Figure in Figure 5. 5It. can It can be beseen seen that that the the transparency transparency of ofthe the layers layers in

in the infrared range decreases slightly with the addition of cesium. All the layers show Figuretransmittance 4. Cross-sectional higher than SEM 0.55 images at 997 of perovskite nm wavelength, cells formed thus indicatingon fluorine-doped relatively tin oxide low optical (FTO)loss in substrate the spectral with ( regiona) 0, (b) relevant 5, (c) 10, toand the (d) silicon 15 % Cs. forbidden Magnification energy is 800,000×. gap.

Influence of cesium concentration on optical transmittance of the layers was investi- gated with the intention to evaluate their possible utility for the fabrication of monolithic perovskite/silicon tandem solar cells. The measured transmittance spectra of the perovskite layers are depicted in Figure 5. It can be seen that the transparency of the layers in

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Coatings 2021, 11, x FOR PEER REVIEW 7 of 12 the infrared range decreases slightly with the addition of cesium. All the layers show transmittance higher than 0.55 at 997 nm wavelength, thus indicating relatively low optical loss in the spectral region relevant to the silicon forbidden energy gap. the infrared range decreases slightly with the addition of cesium. All the layers show Coatings 2021, 11, 279 7 of 12 transmittance higher than 0.55 at 997 nm wavelength, thus indicating relatively low optical loss in the spectral region relevant to the silicon forbidden energy gap.

Figure 5. Optical transmittance spectra of the perovskite layers with a different percentage of cesium.

FigureFigureThe 5. 5. OpticalmeasuredOptical transmittance transmittance current–voltage spectra ofcharacteristics the perovskiteperovskite layersoflayers thewith withbest a aproduced different different percentage percentage perovskite of of cesium. solar ce- cellssium. with different cesium concentration are shown in Figure 6. It is visible that the open The measured current–voltage characteristics of the best produced perovskite solar circuit voltage Voc decreases slightly with the addition of cesium, while the short circuit cellsThe with measured different cesiumcurrent–voltage concentration characteristics are shown of in the Figure best 6produced. It is visible perovskite that the opensolar current Jsc first increases with cesium concentration and later drops down. The highest cellscircuit with voltage different Voc cesiumdecreases concentration slightly with are the show additionn in Figure of cesium, 6. It is while visible the that short the circuit open value of the short-current density Jsc demonstrates a solar cell with 10% of Cs concentra- circuitcurrent voltage Jsc ﬁrst V increasesoc decreases with slightly cesium with concentration the addition and of latercesium, drops while down. the short The highestcircuit tion. currentvalue of J thesc first short-current increases with density cesium Jsc demonstrates concentration a solar andcell later with drops 10% down. of Cs concentration. The highest value of the short-current density Jsc demonstrates a solar cell with 10% of Cs concentration.

FigureFigure 6. 6. CurrentCurrent voltage voltage characteristic characteristic of of the the perovski perovskitete solar solar cell cell with with different different cesium cesium concen- concentra- trations.tions.

FigurePhotovoltaicPhotovoltaic 6. Current voltage quantities quantities characteristic of the perovskiteperovskite of the perovski solarsolarte cells solarcells (open cell(open with circuit circuit different voltage, voltage, cesium short-current short-cur- concen- renttrations.density, density, ﬁll factorfill factor (FF) (FF) and and power power conversion conversion efﬁciency) efficiency) with with different different cesium cesium concen- con- centrationstrations are are presented presented in in Table Tabl2e. 2. Obviously, Obviously, the the solar solar cell cell containing containing 10% 10% ofof CsCs has the highestPhotovoltaic value of J quantities, FF and PCE. of the Figure perovskite6 depicts solar the current-voltagecells (open circuit characteristics voltage, short-cur- of solar highest value of Jscsc, FF and PCE. Figure 6 depicts the current-voltage characteristics of rent density, fill factor (FF) and power conversion efficiency) with different cesium con- solarcell withcell with 10% 10% of Cs of measured Cs measured in forward in forward and and reverse reverse directions directions (full (full and and open open triangles, trian- centrations are presented in Table 2. Obviously, the solar cell containing 10% of Cs has the gles,respectively). respectively). It is It seen is seen that that the hysteresisthe hysteresis of the of current–voltagethe current–voltage characteristics characteristics ofthis of highestsolar cell value is also of negligible,Jsc, FF and as PCE. it was Figure observed 6 depicts previously the current-voltage in [35,43]. It should characteristics be noted that of this solar cell is also negligible, as it was observed previously in [35,43]. It should be noted solarhigher cell amount with 10% of Csof largerCs measured than 10% in forward does not and improve reverse the directions performance (full of and the open perovskite triangles,solar respectively). cell. It is seen that the hysteresis of the current–voltage characteristics of this solar cell is also negligible, as it was observed previously in [35,43]. It should be noted

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Coatings 2021, 11, 279 8 of 12 that higher amount of Cs larger than 10% does not improve the performance of the perovskite solar cell.

TableTable 2.2.Photovoltaic Photovoltaic parametersparameters ofof thethe perovskiteperovskite solar solar cells cells with with different different cesium cesium concentration. concentration.

oc sc −−22 Cs,Cs, % % V Voc,VJ, V Jsc,, mAmA·cm·cm FF, % % PCE,% % 00 1.13 1.13 20.220.2 6363 14.414.4 55 1.08 1.08 22.022.0 7575 17.817.8 1010 1.041.04 23.823.8 8181 20.020.0 1515 0.990.99 20.820.8 7272 14.814.8

TheThe continuouscontinuous wavewave photoluminescencephotoluminescence spectraspectra of the perovskite layers with dif- ferentferent cesium concentration concentration are are shown shown in in Figure Figure 7.7 It. is It seen is seen that that the thespectra spectra become become blue- blue-shiftedshifted as the as amount the amount of Cs ofincreases. Cs increases. Peak maxima Peak maxima are 770, are 767, 770, 763 767, and 763 760 and nm, 760 respec- nm, respectively,tively, for 0,for 5, 0,10 5, and 10 and15% 15% of Cs. of Cs.The The blue-shift blue-shift of of10 10 nm nm agrees agrees well well with with the resultsresults observedobserved inin [[35].35]. AnalysisAnalysis ofof PLPL spectraspectra showsshows thatthat eacheach peakpeak consistsconsists ofof twotwo GaussianGaussian peakspeaks (peak center center λλi iandand width width wi warei are presented presented in Table in Table 3). Th3).ey They are related are related to the to band– the band–bandband and excitonic and excitonic transitions. transitions.

FigureFigure 7.7. PhotoluminescencePhotoluminescence spectraspectra ofof thethe perovskiteperovskite layerslayers withwith differentdifferent cesiumcesium concentration.concentration.

Table 3. Results of PL analysis: λi is peak center and wi is full width at half maximum of the Gauss- Table 3. Results of PL analysis: λi is peak center and wi is full width at half maximum of the ian peaks, λmax is the peak maximum, τi and Ai are the decay time and the amplitude of the three- Gaussian peaks, λmax is the peak maximum, τi and Ai are the decay time and the amplitude of the exponential function, and τav is the amplitude weighted average decay time. three-exponential function, and τav is the amplitude weighted average decay time. Cs, % λ1, w1; λ2, w2 (λmax), nm τ1, ns; A1 τ2, ns; A2 τ3, ns; A3 τav, ns λ1, w1; λ2, w2 0Cs, % 767, 26; 775, 55 (770) τ 10;1, ns;0.42A 1 τ 57;2, ns;0.27A 2 τ3 333;, ns; 0.25A3 τav 110, ns (λmax), nm 5 764, 25; 773, 57 (767) 6.1; 0.46 66; 0.12 355; 0.32 139 10 0 761, 767, 28; 26;770, 775, 60 55 (763) (770) 5.1; 10; 0.52 0.42 78; 57; 0.15 0.27 333; 458; 0.25 0.27 110 147 5 764, 25; 773, 57 (767) 6.1; 0.46 66; 0.12 355; 0.32 139 1510 756, 761, 30; 28;765, 770, 60 60 (760) (763) 10; 5.1; 0.38 0.52 47; 78; 0.44 0.15 458; 174; 0.27 0.14 147 50.4 15 756, 30; 765, 60 (760) 10; 0.38 47; 0.44 174; 0.14 50.4 The time-resolved PL decay curves were measured in the center of the PL peak (see Figure 8). The transients have been approximated using different models like stretched The time-resolved PL decay curves were measured in the center of the PL peak (see exponential (or Kohlrausch) and compressed hyperbola (or Becquerel) functions [46] or Figure8). The transients have been approximated using different models like stretched two exponential model, but neither of them provided as good fitting results as the triple exponential (or Kohlrausch) and compressed hyperbola (or Becquerel) functions [46] or exponential approximation (the adjusted R-squared was 0.98 or higher): two exponential model, but neither of them provided as good ﬁtting results as the triple exponential approximation (the adjusted R-squared was 0.98 or higher): (1) 𝐼PL𝑡 = 𝐴𝑒 + 𝐴𝑒 + 𝐴𝑒 t t t − τ − τ − τ with the corresponding intensityIPL(t) =amplitudesA1e 1 + A𝐴2e, and2 + theA 3amplitudee 3 weighted average de-(1) cay times were calculated as with the corresponding intensity amplitudes Ai, and the amplitude weighted average decay times were calculated as ∑ Aiτi τav = (2) ∑ Ai

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∑ 𝐴𝜏 Coatings 2021, 11, 279 𝜏av = 9 of(2) 12 ∑ 𝐴 The deduced decay times and amplitudes are presented in Table 3.

Figure 8. Photoluminescence decay decay transients transients of of the the pero perovskitevskite layers layers with with a a different different cesium cesium con- con- centration.

ThreeThe deduced decay times decay are times assigned and amplitudes to different are recombination presented in mechanisms, Table3. thus, two of themThree are related decay to times band–band are assigned and excitonic to different transitions. recombination Meanwhile, mechanisms, the third thus, decay two time of couldthem arebe related to various band–band nonradiative and excitonic processes. transitions. Meanwhile, the third decay time couldAs be can related be seen to various from Table nonradiative 3, the average processes. decay time increases with the growth of Cs concentrationAs can be in seen the range from Table0 < x3 ≤, the0.1, averageand it drops decay down time sharply increases in withthe perovskite the growth layer of withCs concentration highest Cs concentration in the range 0(x < = x 0.15).≤ 0.1, The and variation it drops downof the sharplyaverage indecay the perovskitetime with cesiumlayer with concentration highest Cs in concentration the perovskite (x =layer 0.15). correlates The variation well with of the PCE average values decayof the timeper- withovskite cesium solar concentrationcells (see Table in 2). the According perovskite to layer [47], correlatesthe higher wellPCE withpercentage PCE values and longer of the perovskitePL decay times solar are cells stipulated (see Table by 2lower). According trap density to [ in47 ],perovskite the higher layers PCE containing percentage Cs, and as comparedlonger PL decayto ones times without are stipulatedcesium. Lower by lower trap trap density density reduces in perovskite the nonradiative layers containing recombi- nationCs, as comparedlosses which to works ones without in favor cesium. of the higher Lower performance trap density of reduces solar cells the [42,48]. nonradiative recombinationSmall amounts losses of which Cs-cation works can in be favor used of to the influence higher performance the perovskite of solar crystal cells structure [42,48]. by reachingSmall amounts a lower ofeffective Cs-cation tolerance can be usedfactor to leading influence to a the cubic perovskite or pseudo-cubic crystal structure perov- skiteby reaching structure a lower [41,49]. effective This way, tolerance entropic factor stabilization leading to aleads cubic to or the pseudo-cubic desired photo-active perovskite phasestructure obtained [41,49]. at This room way, temperature entropic stabilization [50]. Cesium leads can to effectively the desired suppress photo-active the yellow phase phaseobtained formation at room of temperatureFAPbI3, thus [providing50]. Cesium with can improved effectively and suppress defect-free the perovskite yellow phase thin- filmsformation demonstrating of FAPbI3 ,stabilized thus providing power with conversion improved efficiencies and defect-free exceeding perovskite 21% for up thin-films to 1000 hdemonstrating [35]. The triple-cation stabilized perovskite power conversion is thermally efficiencies more exceedingstable as compared 21% for up to to the 1000 FA/MA h [35]. Theperovskite triple-cation and is perovskite also more is robust thermally against more ambient stable as variations compared such to the as FA/MA different perovskite prepara- tionand isprotocols, also more temperatures, robust against and ambient solvent variations vapors [41]. such Singh as different and Miyasaki preparation [43] confirmed protocols, thistemperatures, remarkable and stability solvent (up vapors to 18 [41 weeks)]. Singh of and triple-cation Miyasaki [ 43perovskites] confirmed fabricated this remarkable under environmentalstability (up to conditions 18 weeks) with of triple-cation a relative hu perovskitesmidity of 25% fabricated showing under a stabilized environmental PCE ex- ceedingconditions 25%. with At aa relativehigher concentration humidity of 25% (x > showing 0.1), cesium a stabilized reduces PCEthe performance exceeding 25%. of the At perovskitea higher concentration solar cell. It can (x > be 0.1), caused cesium by reducesa reduction the performancein the size of ofthe the crystallites perovskite for solar per- ovskitecell. It canwith be a Cs caused concentration by a reduction larger inthan the 10% size (see of theTable crystallites 1) which forinduced perovskite the growth with a Cs concentration larger than 10% (see Table1) which induced the growth of carrier of carrier recombination rate. recombination rate.

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4. Conclusions Experimental study of structure, photoluminescence and optical properties of perovskite Csx(MA0.17FA0.83)(1−x)Pb(I0.83Br0.17)3 layers with different cesium concentration was carried out. It is established that addition of cesium almost does not influence the transparency of the perovskite layers in the infrared range but significantly improves the photovoltaic performance of the perovskite solar cells. It was found that the solar cell with 10% of cesium in the perovskite layer demonstrates the best power conversion efficiency of 20%, and therefore such layers can find application as suitable tandem partners for silicon solar cells.

Author Contributions: Conceptualization and methodology, S.A., K.L. and J.G.; sample fabrication, A.G., K.P. and A.S. (Algirdas Sužiedelis);˙ formal analysis, A.S. (Algirdas Selskis) and A.S. (Algirdas Sužiedelis);˙ experimental investigation, E.Š., K.P., A.C.,ˇ A.S. (Algirdas Selskis), R.J. and A.L.; writing— reviewing and editing, S.A., R.J., J.G. and K.L.; visualization, A.S. (Algirdas Selskis) and K.P.; project administration and supervision, S.A. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Research Council of Lithuania, Grant number 01.2.2-LMT-K- 718-01-0050). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: No new data were created or analyzed in this study. Data sharing is not applicable to this article. Acknowledgments: The authors gratefully acknowledge R. Sedlickas for reading the text of the manuscript and giving valuable suggestions. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. Reynolds, S.; Smirnov, V. Modelling of two-and four-terminal thin-film silicon tandem solar cells. J. Phys. Conf. Ser. 2012, 398, 012006. [CrossRef] 2. Asadpour, R.; Chavali, R.V.K.; Khan, M.R.; Alam, M.A. Bifacial Si heterojunction-perovskite organic-inorganic tandem to produce highly efficient (η*T ~33%) solar cell. Appl. Phys. Let. 2015, 106, 243902. [CrossRef] 3. Mailoa, J.P.; Bailie, C.D.; Johlin, E.C.; Hoke, E.T.; Akey, A.J.; Nguyen, W.H.; McGehee, M.D.; Buonassisi, T. A 2-terminal perovskite/silicon multijunction solar cell enabled by a silicon tunnel junction. Appl. Phys. Lett. 2015, 106, 121105. [CrossRef] 4. Albrecht, S.; Saliba, M.; Baena, J.P.C.; Lang, F.; Kegelmann, L.; Mews, M.; Steier, L.; Abate, A.; Rappich, J.; Korte, L.; et al. Monolithic perovskite/silicon-heterojunction tandem solar cells processed at low temperature. Energy Environ. Sci. 2016, 9, 81–88. [CrossRef] 5. McMeekin, D.P.; Sadoughi, G.; Rehman, W.; Eperon, G.E.; Saliba, M.; Hörantner, M.T.; Haghighirad, A.; Sakai, N.; Korte, L.; Rech, B.; et al. A mixed-cation lead mixed-halide perovskite absorber for tandem solar cells. Science 2016, 351, 151–155. [CrossRef] 6. Werner, J.; Weng, C.H.; Walter, A.; Fesquet, L.; Seif, J.P.; De Wolf, S.; Niesen, B.; Ballif, C. Efficient monolithic perovskite/silicon tandem solar cell with cell area > 1 cm2. Phys. Chem. Lett. 2016, 7, 161–166. [CrossRef] 7. Bush, K.A.; Palmstrom, A.F.; Yu, Z.J.; Boccard, M.; Cheacharoen, R.; Mailoa, J.P.; McMeekin, D.P.; Hote, R.L.Z.; Bailie, C.D.; Leijtens, T.; et al. 23.6%-efficient monolithic perovskite/silicon tandem solar cells with improved stability. Nat. Energy. 2017, 2, 17009. [CrossRef] 8. Werner, J.; Niesen, B.; Ballif, C. Perovskite/silicon tandem solar cells: Marriage of convenience or true love story?– An overview. Adv. Matter. Interf. 2017, 1700731. [CrossRef] 9. Zheng, J.; Lau, C.F.J.; Mehrvarz, H.; Ma, F.J.; Jiang, Y.J.; Deng, X.F.; Soeriyadi, A.; Kim, J.; Zhang, M.; Hu, L.; et al. Large area efficient interface layer free monolithic perovskite/homo-junction-silicon tandem solar cell with over 20% efficiency. Energy Environ. Sci. 2018, 11, 2432–2443. [CrossRef] 10. Chen, B.; Yu, Z.; Liu, K.; Zheng, X.; Liu, Y.; Shi, J.; Spronk, D.; Rudd, P.N.; Holman, Z.; Huang, J. Grain engineering for perovskite/silicon monolithic tandem solar cells with efficiency of 25.4%. Joule 2019, 3, 1–14. [CrossRef] 11. Köhnen, E.; Jošt, M.; Morales-Vilches, A.B.; Tockhorn, P.; Al-Ashouri, A.; Macco, B.; Kegelmann, L.; Korte, L.; Rech, B.; Schlatmann, R.; et al. Highly efficient monolithic perovskite silicon tandem solar cells: Analyzing the influence of current mismatch on device performance. Sustain. Energy Fuels 2019, 3, 1995–2005. [CrossRef] 12. Jaysankar, M.; Raul, A.B.L.; Bastos, J.; Burgess, C.; Weitens, C.; Creatore, M.; Aernouts, T.; Kuang, Y.; Gehlhaar, R.; Hadipour, A.; et al. Minimizing voltage loss in wide-bandgap perovskite for tandem solar cells. ACS Energy Lett. 2019, 4, 259–264. [CrossRef] Coatings 2021, 11, 279 11 of 12

13. Shen, H.; Walter, D.; Wu, Y.; Fong, K.C.; Jacobs, D.A.; Duong, T.; Peng, J.; Weber, K.; White, T.P.; Catchpole, K.R. Monolithic perovskite/Si tandem solar cell: Pathways to over 30% efficiency. Adv. Energy Mater. 2019, 1902840. [CrossRef] 14. Lee, C.; Lee, S.-W.; Bae, S.; Shawky, A.; Devaraj, V.; Anisimov, A.; Kauppinen, E.I.; Oh, J.-W.; Kang, Y.; Kim, D.; et al. Carbon nanotube electrode-based perovskite-silicon tandem solar cells. Sol. RRL 2020, 2000353. [CrossRef] 15. Duong, T.; Pham, H.; Kho, T.C.; Phang, P.; Fong, K.C.; Yan, D.; Yin, Y.; Peng, J.; Mahmud, M.A.; Gharibzadeh, S.; et al. High efficiency perovskite-silicon tandem solar cells: Effect of surface coating versus bulk incorporation of 2D perovskite. Adv. Energy Mater. 2020, 1903553. [CrossRef] 16. Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J. Am. Chem. Soc. 2009, 131, 6050–6051. [CrossRef] 17. Kothandaraman, R.K.; Jiang, Y.; Feurer, T.; Tiwari, A.N.; Fu, F. Near-infrared-transparent perovskite solar cells and perovskite- based tandem photovoltaics. Small Methods 2020, 4, 2000395. [CrossRef] 18. Yoshikawa, K.; Kawasaki, H.; Yoshida, W.; Irie, T.; Konishi, K.; Nakano, K.; Uto, T.; Adachi, D.; Kanematsu, M.; Uzu, H.; et al. Silicon heterojunction solar cell with integrated back contacts for a photoconversion efficiency over 26%. Nat. Energy 2017, 2, 17032. [CrossRef] 19. Yu, C.; Xu, S.; Yao, J.; Han, S. Recent advances in the new perspectives on crystalline silicon solar cells with carrier-selective passivation contacts. Crystals 2018, 8, 430. [CrossRef] 20. Polman, A.; Knight, M.; Garnett, E.C.; Ehler, B.; Sinke, W.C. Photovoltaic materials:present efficiencies and future challenges. Science 2016, 352, aad4424-1–aad442-10. [CrossRef][PubMed] 21. Ašmontas, S.; Gradauskas, J.; Sužiedelis,˙ A.; Šilenas,˙ A.; Širmulis, E.; Vaiˇcikauskas,V.; Vaiˇciunas,¯ V.; Žalys, O.; Fedorenko, L.; Bulat, L. Photovoltage formation across GaAs p-n junction under illuminationof intense laser radiation. Opt. Quantum Electron. 2016, 48, 448. [CrossRef] 22. Ašmontas, S.; Gradauskas, J.; Sužiedelis,˙ A.; Šilenas,˙ A.; Širmulis, E.; Švedas, V.; Vaiˇcikauskas,V.; Žalys, O. Hot carrier impact on photovoltage formation in solar cells. Appl. Phys. Let. 2018, 113, 071103. 23. Gradauskas, J.; Ašmontas, S.; Sužiedelis,˙ A.; Šilenas,˙ A.; Vaiˇcikauskas, V.; Cerškus,ˇ A.; Širmulis, E.; Žalys, O.; Masalskyi, O. Influence of hot carriers and thermal components on photovoltage formation across p-n junction. Appl. Sci. 2020, 10, 7483. [CrossRef] 24. Jaysankar, M.; Filipiˇc, M.; Zielenski, B.; Schmager, R.; Song, W.; Qiu, W.; Paetzold, U.W.; Aernouts, T.; Debucquoy, M.; Gehlhaar, R.; et al. Perovskite-silicon tandem solar modules with optimized light harvesting. Energy Environ. Sci. 2018, 11, 1489–1498. [CrossRef] 25. Charibzadeh, S.; Hossain, I.M.; Fassl, P.; Nejand, B.A.; Abzieher, T.; Schultes, M.; Ahlswede, E.; Jackson, P.; Powalla, M.; Schäfer, S.; et al. 2D/3D heterostructure for semitransporant perovskite solar cell with engineering bandgap enables efficiencies exceeding 25% in four-terminal tandems with silicon and CIGS. Adv. Funct. Mater. 2020, 1909919. [CrossRef] 26. Yeom, K.M.; Kim, S.U.; Woo, M.Y.; Noh, J.H.; Im, S.H. Recent progress in metal halide perovskite-based tandem solar cell. Adv. Mater. 2002, 2002228. [CrossRef] 27. Jena, A.K.; Kulkarni, A.; Miyasaka, T. Halide perovskite photovoltaic: Background, status, and future prospects. Chem. Rev. 2019, 3036–3103. [CrossRef][PubMed] 28. Noh, H.; Im, S.H.; Heo, T.N.; Mandal, T.N.; Seok, S.I. Chemical management for colorful, efficient, and stable inorganic-organic hybrid nanostructured solar cells. Nano Lett. 2013, 13, 1764. [CrossRef] 29. Pham, H.D.; Xianqiang, L.; Li, W.; Manzhos, S.; Kyaw, A.K.K.; Sonar, P. Organic interfacial materials for perovskite-based optoelectronic devices. Energy Environ. Sci. 2019, 12, 1177–1209. [CrossRef] 30. Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J. Electron-hole diffusion lengths > 175 µm in solution-grown CH3NH3PbI3 single crystals. Science 2015, 347, 967–969. [CrossRef][PubMed] 31. Eperon, G.E.; Hörantner, M.T.; Snaith, H.J. Metal halide perovskite tandem and multiple-junction photovoltaics. Nat. Rev. Chem. 2017, 1, 0095. [CrossRef] 32. Leijtens, T.; Bush, K.A.; Prasanna, R.; McGehe, M.D. Opportunities and challenges for tandem solar cells using metal halide perovskite semiconductors. Nat. Energy. 2018, 3, 828–838. [CrossRef] 33. Song, Z.; Chen, C.; Li, C.; Awni, R.A.; Zhao, D.; Yan, Y. Wide-gap, low-bandgap, and tandem perovskite solar cells. Semicond. Sci. Technol. 2019, 34, 093001. [CrossRef] 34. Unger, E.L.; Kegelmann, L.; Suchan, K.; Sorell, D.; Korte, L.; Albrecht, S. Roadmap and roadblocks for the band gap tunability of metal halide perovskite. J. Mater. Chem. A 2017, 5, 11401–11409. [CrossRef] 35. Saliba, M.; Matsui, T.; Seo, J.Y.; Domaski, K.; Correa-Baena, J.P.; Nazeeruddin, M.K.; Zakeeruddin, S.M.; Tress, W.; Abate, A.; Hagfeldt, A.; et al. Cesium-containing triple cation perovskite solar cells: Improved stability, reproducibility and high efficiency. Energy Environ. Sci. 2016, 9, 1989–1997. [CrossRef] 36. Correa-Baena, J.P.; Saliba, M.; Buonnassisi, T.; Grätzel, M.; Abate, A.; Tress, W.; Hagfeldt, A. Promises and challenges of perovskite solar cells. Science 2017, 739–744. [CrossRef][PubMed] 37. Christians, J.A.; Schulz, P.; Tinkham, J.S.; Schloemer, T.T.; Harvey, S.P.; Tremolet de Villers, B.J.; Sellinger, A.; Berry, J.J.; Luther, J.M. Tailored interfacesof uncapsulated perovskite solar cells > 1000 h operational stability. Nat. Energy. 2018, 3, 68–74. [CrossRef] 38. Shen, H.; Omelchenko, S.T.; Jacobs, D.A.; Yalamanchili, S.; Wan, Y.; Yan, D.; Phang, P.; Duong, T.; Wu, Y.; Yin, Y.; et al. In situ recombination junction between p-Si and TiO2 enables high-efficiency monolithic perovskite/Si tandem cells. Sci. Adv. 2018, 4, eaau9711. [CrossRef][PubMed] Coatings 2021, 11, 279 12 of 12

39. Mateen, M.; Arain, Z.; Liu, X.; Yang, Y.; Ding, Y.; Ma, S.; Ren, Y.; Wu, Y.; Tao, Y.; Shi, P. High-performance mixed-cation mixed-halide perovskite solar cells enabled by a facile intermediate technique. J. Power Sources 2020, 448, 227386. [CrossRef] 40. Liu, P.; Liu, Z.; Qin, C.; He, T.; Li, B.; Ma, L.; Shaheen, K.; Yang, K.; Yang, H.; Liu, H.; et al. High-performance perovskite solar cells based on passivating interfacial and intergranular defects. Sol. Energy Mater. Sol. Cells. 2020, 212, 110555. [CrossRef] 41. Ünlü, F.; Jung, E.; Haddad, J.; Kulkarni, A.; Öz, S.; Choi, H.; Fischer, T.; Chakraborty, S.; Kirchartz, T.; Mathur, S. Understanding the interplay of stability and efficiency in A-site engineered lead halide perovskite. APL Mater. 2020, 8, 070901. [CrossRef] 42. Hu, Y.; Hutter, E.M.; Rieder, P.; Grill, I.; Hanisch, J.; Aygüler, M.F.; Hufnagel, A.G.; Handloser, M.; Bein, T.; Hartschun, A.; et al. Understading the role of cesium and rubidium additives in perovskite solar cells: Trap states, charge transport, and recombination. Adv. Energy Mater. 2018, 1703057. [CrossRef] 43. Singh, T.; Miyasaka, T. Stabilizing the efficiency beyond 20% with the mixed cation perovskite solar cell fabricated in ambient air under controlled humidity. Adv. Energy Mater. 2018, 8, 1700677. [CrossRef] 44. Domanski, K.; Alharbi, E.A.; Hagfeldt, A.; Grätzel, M.; Tress, W. Systematic investigation of the impact of operation conditions on the degradation behaviour of perovskite solar cells. Nat. Energy 2018, 3, 61–67. [CrossRef] 45. Jacobsson, J.T.; Baena, J.P.C.; Pazoki, M.; Saliba, M.; Schenk, K.; Grätzel, M.; Hagfeldt, A. An exploration of the compositional space for mixed lead halogen perovskites for high efficiency solar cells. Available online: https://pubs.rsc.org/en/content/ getauthorversionpdf/C6EE00030D (accessed on 18 February 2021). 46. Berberan-Santos, N.M. A luminescence decay function encompassing the stretched exponential and the compressed hyperbola. Chem. Phys. Lett. 2008, 460, 146–150. [CrossRef] 47. Solanki, A.; Yadav, P.; Turren-Cruz, S.H.; Lim, S.S.; Saliba, M.; Sum, T.C. Cation influence on carrier dynamics in perovskite solar cells. Nano Energy 2019, 58, 604–611. [CrossRef] 48. Sutton, R.J.; Eperon, G.E.; Miranda, L.; Parrott, E.S.; Kamino, B.A.; Patel, J.B.; Hörantner, M.T.; Johnston, M.B.; Haghighirad, A.A.; Moore, D.T.; et al. Bandgap-tunable lead halide perovskite with high thermal stability for efficient solar cells. Adv. Energy Mater. 2016, 6, 1502458. [CrossRef] 49. Wu, C.; Guo, D.; Li, P.; Wang, S.; Liu, A.; Wu, F. A study on the effects of mixed organic cations on the structure and properties in lead halide perovskites. Phys. Chem. Chem. Phys. 2020, 22, 3105–3111. [CrossRef] 50. Yi, C.; Luo, J.; Meloni, S.; Boziki, A.; Ashari-Astani, N.; Grätzel, C.; Zakeeruddin, M.; Röthlisberger, U.; Grätzel, M. Entropic stabilization of mixed A-cation ABX3 metal halide perovskites for high performance perovskite solar cells. Energy Environ. Sci. 2016, 9, 656–662. [CrossRef]