ORGANIC-INORGANIC PEROWSKIT MATERIALS AND METHODS FOR THEIR PREPARATION

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S Provisional Patent Application No. 62 / 554,963 filed September 6, 2017.CONTRACTUAL ORIGIN

[0002] The United States Government has rights in this disclosure under Contract No. DE-AC36-08GO28308 between the United States Department of Energy and Alliance for Sustainable Energy, LLC, the Manager and Operator of the National Renewable Energy Laboratory.BACKGROUND

[0003] Yong Sun et al (Scientific Reports 2017, 7, 46193) describes the non-thermal annealing fabrication of triple-cation mixed-halide perovskites, FA 0.7 MA 0.2 Cs 0.1 PB(I 5 / 6 Br 1 / 6 ) 3 by incorporation of a Pb(SCN) 2 additive. WO2017089819A1 describes the preparation of a crystalline compound comprising (i) cesium, (ii) formamidinium,(iii) one or more metal or metalloid dications [B], and (iv) two or more different halide anions [X]. McMeekin et al (Science 2016, 351(6269), 151-155) describes a mixed-cation lead mixed-halide perovskite absorber for tandem solar cells. Yongzhen Wu et al (Advanced Materials 2017, 29(28), 1701073) relates thermally stable MAPbI 3 perovskite solar cells. WO2016 / 207775A1 relates to functional hole transport materials for optoelectronic and / or electrochemical devices.

[0004] The bandgap tunability from about 1.2 eV to 2.3 eV has enabled various perovskite-related tandem PVs such as perovskite / perovskite, perovskite / Si, perovskite / CIGS tandem solar cells which is strongly considered as an alternative direction to reach over 30% efficiency. At present, the perovskite / perovskite tandem has reached 21.2% for a 4-terminal (4-T) device and 18.5% for a 2-terminal (2-T) structure, whereas the perovskite / Si tandem has reached 26.7% for 4-T and 25.2% for 2-T configurations. Therefore, a key effort for developing these tandem devices is to improve the wide-bandgap (WBG) perovskites with 1.7 eV to 1.9 eV of bandgap. Increasing the bromide portion at the halide anion position of perovskites is the common approach to developing WBG perovskite devices with various their compositions based on MA, FA, Cs or their mixtures on the A site of perovskites. In general, as the amount of substitution of iodide to bromide reaches certain range, the crystallographic, optoelectronic and chemical properties of perovskites exhibit undesired changes such as phase segregation, higher energetic disorder, and inferior light absorption. Thus, there remains a need for WBG perovskite films and devices having improved crystallographic and opto-physicochemical properties and / or performance metrics, as well as a need for improved methods for manufacturing such films and devices.SUMMARY

[0005] An aspect of the present invention is a method comprising treating a liquid comprising a first precursor at a concentration C 1 , a second precursor at a concentration C 2 , a third precursor at a concentration C 3 , a fourth precursor, and an additive at a concentration C 4 , wherein:the first precursor comprises at least one of cesium halide wherein cesium halide is cesium iodide or cesium bromide; the second precursor comprises at least one of formamidinium halide wherein formamidinium halide is formamidinium iodide or formamidinium bromide; the third precursor comprises PbBr 2 ;the fourth precursor comprises PbI 2 ; the additive comprises at least one of methylammonium bromide (MABr), methylammonium chloride (MACl), and methylammonium iodide (MAI);the treating results in a perovskite having the formula FA x Cs 1-x Pb(I y Br 1-y ) 3 , where 0 < x < 1 and 0 < y < 1; each of C 1 , C 2 , and C 3 are independently between 0.001 M and 100 M; and at least one of C 4 / C 1 or C 4 / C 2 equals a ratio greater than zero and less than 0.5.

[0006] A further aspect of the present invention is a perovskite having the formula FA x Cs 1-x Pb z B' z-1 (I y Br 1-y ) 3 , wherein each of x, y, and z are between zero and one, exclusively, and B' is selected from germanium and tin.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Exemplary embodiments are illustrated in referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than limiting. Figure 1 illustrates a structure of an organic-inorganic perovskite, according to some embodiments of the present disclosure. Figure 2 illustrates a method for producing an organic-inorganic perovskite, according to some embodiments of the present disclosure. Figure 3A illustrates UV-vis absorption of organic-inorganic perovskite films prepared from precursors with and without excess methylammonium halides as indicated, according to some embodiments of the present disclosure. Figure 3B illustrates x-ray diffraction (XRD) patterns of organic-inorganic perovskite films prepared from precursors with and without excess methylammonium halides as indicated, according to some embodiments of the present disclosure. Figure 4 illustrates Tauc plots of organic-inorganic perovskite films prepared from precursors with and without excess methylammonium halide additives, according to some embodiments of the present disclosure. When 20% methylammonium bromide (MABr) or methylammonium chloride (MACl) additive was used, the bandgap stayed unchanged at about 1.752 eV. When 20% methylammonium iodide (MAI) additive was used, the bandgap was reduced to about 1.715 eV. Figure 5A illustrates the effect of non-stoichiometric precursor with excess 20% MAI, MABr, or MACl additives on the XRD patterns, the (100) peaks, of the resultant organic-inorganic perovskite layers produced, according to some embodiments of the present disclosure. Figure 5B illustrates the effect of non-stoichiometric precursor with excess 20% MAI, MABr, or MACl additives on the XRD patterns, the (110) peaks, of the resultant organic-inorganic perovskite layers produced, according to some embodiments of the present disclosure. Figure 6 illustrates the morphology of organic-inorganic perovskite films prepared from precursors with and without excess methylammonium halide additives, according to some embodiments of the present disclosure: Panel (a) no additive; Panel (b) 20% MAI; Panel (c) 20% MABr; and Panel (d) 20% MACl (scale bae is 1 µm). Figure 7 illustrates the effect of stoichiometric precursors having an excess of methylammonium halide additives on J-V characteristics, according to some embodiments of the present disclosure. 20% of MAI, MABr or MACl additives were added to the standard stoichiometric precursor. Both reverse and forward scans are shown. Figures 8A, 8B, and 8C illustrate SEM images of perovskite films prepared with 20%, 40%, and 80% MABr, respectively (scale bae is 1 µm) and show the effect of the MABr concentration additive on the final organic-inorganic perovskite films produced, according to some embodiments of the present disclosure Figure 8D illustrates the effect of varying MABr concentration on the UV-vis absorption spectra of the final organic-inorganic perovskite films produced, according to some embodiments of the present disclosure. Figure 8E illustrates the effect of varying MABr concentration on the XRD patterns of the final organic-inorganic perovskite films produced, according to some embodiments of the present disclosure. Figure 8F illustrates the effect of varying MABr concentration on J-V curves, both reverse and forward scans, of the final organic-inorganic perovskite films produced, according to some embodiments of the present disclosure. Figure 9 illustrates the photovoltaic (PV) characteristics of the best-performing device manufactured using 40% MABr additive, according to some embodiments of the present disclosure. J-V curves with both forward and reverse scans as well as the stabilized power output monitored over time near the maximum power point are shown. Figure 10 illustrates a cross-section view of a perovskite solar cell (PSC) having an organic-inorganic perovskite layer produced using non-stoichiometric precursor with excess 40% MABr additive, according to some embodiments of the present disclosure. Figure 11 illustrates an external quantum efficiency (EQE) spectrum of a PSC containing an organic-inorganic perovskite layer, according to some embodiments of the present disclosure. The corresponding J-V curves and stabilized power output are shown in shown in Figure 9. Figure 12A and 12B illustrate performance data for a organic-inorganic perovskite-Si 4-terminal tandem device, according to some embodiments of the present disclosure. Figure 12A illustrates J-V curves of the semi-transparent organic-organic perovskite layer (~1.75 eV) PSC under glass / FTO illumination and the Si cell with and without the semi-transparent organic-inorganic PSC optical filter. Figure 12B illustrates EQE spectra of semi-transparent organic-inrorganic perovskite top cell and a filtered Si bottom cell. Figure 13 illustrates the annealing time dependence of the absorption evolution of an organic-inorganic perovskite film prepared from non-stoichiometric precursor with excess 20% MABr additive, according to some embodiments of the present disclosure. REFERENCE NUMBERS

[0008] 100organic-inorganic perovskite 110A-cation 120B-cation 130X-anion 200method 210combining 212first precursor 214second precursor 216n th< precursor 218additive 219precursor solution 220applying 222liquid film 230treating 232volatiles 234organic-inorganic perovskite layer DETAILED DESCRIPTION

[0009] The present disclosure may address one or more of the problems and deficiencies of the prior art discussed above. However, it is contemplated that some embodiments as disclosed herein may prove useful in addressing other problems and deficiencies in a number of technical areas. Therefore, the embodiments described herein should not necessarily be construed as limited to addressing any of the particular problems or deficiencies discussed herein.

[0010] The present disclosure relates to high efficiency wide-bandgap (WBG) organic-inorganic perovskite materials, crystals, and / or solar cells, including tandem devices such as all perovskite and / or perovskite / Si tandem devices. The use of non-stoichiometric precursor chemistry with excess methylammonium halides (MAX; X = I, Br, or Cl) for preparing high-quality ~1.75 eV FA 0.83 Cs 0.17 Pb(I 0.6 Br 0.4 ) 3 organic-inorganic perovskite solar cells is demonstrated herein ("FA" refers to formamidinium). Among various methylammonium halides, some embodiments of the present disclosure utilized excess MABr in a non-stoichiometric precursor solution resulting in strong improvements to the final organic-inorganic perovskite crystallographic properties and device characteristics, without affecting the organic-inorganic perovskite composition. In contrast, other examples using excess MAI significantly reduced the bandgap of the final organic-inorganic perovskite due to the replacement of bromine with iodine. Using 40% excess MABr, a single-junction organic-inorganic perovskite solar cell was demonstrated having a stabilized efficiency of 16.4%. In addition, 20.3% a 4-terminal tandem device was demonstrated having a 20.3% stabilized efficiency, using a 14.7% semi-transparent WBG organic-inorganic perovskite top cell and an 18.6% unfiltered (5.6% filtered) Si bottom cell.

[0011] As shown herein, the example organic-inorganic perovskite, FA 0.83 Cs 0.17 Pb(I 0.6 Br 0.4 ) 3 , formed from a non-stoichiometric precursor solution having excess MABr demonstrated the highest intensity of main plane peaks in X-ray diffraction pattern (XRD) with the strongest degree of crystal orientation without morphological changes. This suggests that the excess MABr was more effective at healing defects / improving crystallographic properties during the formation of the organic-inorganic perovskite crystals presumably due to the high bromine content, when compared to the use of a precursor solution containing excess MACl. Further, it is confirmed that the use of excess MABr and MACl did not affect the final composition of perovskite, whereas the use of excess MAI in a precursor solution significantly changed the final perovskite composition. The use of the non-stoichiometric MABr precursor solution resulted in an average short-circuit current density (J sc ) and open-circuit voltage (V oc ) for the final organic-inorganic perovskite-containing solar cell of increased by about 0.5 mA / cm 2< and 60 mV, respectively, resulting in average reverse stabilized efficiency increasing from 13.59 ± 0.43% to 15.72 ± 0.56%. The best organic-inorganic perovskite-containing device reached a stabilized PCE of 16.4%.

[0012] Figure 1 illustrates general structures of organic-inorganic perovskites 100 that may organize into cubic crystalline structures, as well as other crystalline structures such as tetragonal and orthorhombic, and may be described by the general formula ABX 3 , where X (130) is an anion and A (110) and B (120) are cations, typically of different sizes (A typically larger than B). In a cubic unit cell, the B-cation 120 resides at the eight corners of a cube, while the A-cation 110 is located at the center of the cube with twelve X-anions 130 centrally located between B-cations 120 along each edge of the unit cell. Typical inorganic perovskites include calcium titanium oxide (calcium titanate) minerals such as, for example, CaTiO 3 and SrTiO 3 . In some embodiments of the present invention, the A-cation 110 may include a nitrogen-containing organic compound such as an alkyl ammonium compound. The B-cation 120 may include a metal and the X-anion 130 may include a halogen.

[0013] Additional examples for the A-cation 110 include organic cations and / or inorganic cations. Organic A-cations 110 may be an alkyl ammonium cation, for example a C 1-20 alkyl ammonium cation, a C 1-6 alkyl ammonium cation, a C 2-6 alkyl ammonium cation, a C 1-5 alkyl ammonium cation, a C 1-4 alkyl ammonium cation, a C 1-3 alkyl ammonium cation, a C 1-2 alkyl ammonium cation, and / or a C 1 alkyl ammonium cation. Further examples of organic A-cations 110 include methylammonium (CH 3 NH 3+< ), ethylammonium (CH 3 CH 2 NH 3+< ), propylammonium (CH 3 CH 2 CH 2 NH 3+< ), butylammonium (CH 3 CH 2 CH 2 CH 2 NH 3+< ), formamidinium (NH 2 CH=NH 2+< ), and / or any other suitable nitrogen-containing organic compound. In other examples, an A-cation 110 may include an alkylamine. Thus, an A-cation 110 may include an organic component with one or more amine groups. For example, an A-cation 110 may be an alkyl diamine halide such as formamidinium (CH(NH 2 ) 2 ). Thus, the A-cation 110 may include an organic constituent in combination with a nitrogen constituent. In some cases, the organic constituent may be an alkyl group such as straight-chain or branched saturated hydrocarbon group having from 1 to 20 carbon atoms. In some embodiments, an alkyl group may have from 1 to 6 carbon atoms. Examples of alkyl groups include methyl (C 1 ), ethyl (C 2 ), n-propyl (C 3 ), isopropyl (C 3 ), n-butyl (C 4 ), tert-butyl (C 4 ), sec-butyl (C 4 ), iso-butyl (C 4 ), n-pentyl (C 5 ), 3-pentanyl (C 5 ), amyl (C 5 ), neopentyl (C 5 ), 3-methyl-2-butanyl (C 5 ), tertiary amyl (C 5 ), and n-hexyl (C 6 ). Additional examples of alkyl groups include n-heptyl (C 7 ), n-octyl (C 8 ) and the like. In still further general embodiments an A-cation 110 may include an inorganic constituent, with examples at least one of a Group I element. In some general embodiments an A-cation 110 may include at least one of cesium and / or rubidium. In some embodiments, an A-cation 110 may include a benzene ring, such as benzylamine and / or phenethylamine.

[0014] Examples of metal B-cations 120 include, for example, lead, tin, germanium, and or any other 2+ valence state metal that can charge-balance the perovskite halide 100. Examples for X-anions 130 include halogens: e.g. fluorine, chlorine, bromine, iodine and / or astatine. In some cases, the perovskite halide may include more than one X-anion 130, for example pairs of halogens; chlorine and iodine, bromine and iodine, and / or any other suitable pairing of halogens. In other cases, the perovskite halide 100 may include two or more halogens of fluorine, chlorine, bromine, iodine, and / or astatine.

[0015] Thus, the A-cation 110, the B-cation 120, and X-anion may be selected within the general formula of ABX 3 to produce a wide variety of perovskite halides 100, including, for example, methylammonium lead triiodide (CH 3 NH 3 PbI 3 ), and mixed halide perovskites such as CH 3 NH 3 PbI 3-x Cl x and CH 3 NH 3 PbI 3-x Br x . Thus, a perovskite halide 100 may have more than one halogen element, where the various halogen elements are present in non-integer quantities; e.g. x is not equal to 1, 2, or 3. In addition, perovskite halides, like other organic-inorganic perovskites, can form three-dimensional (3-D), two-dimensional (2-D), one-dimensional (1-D) or zero-dimensional (0-D) networks, possessing the same unit structure. Referring again to Figure 1, a perovskite having the basic crystal structure illustrated in Figure 1, having at least one of a cubic, orthorhombic, and / or tetragonal structure, may have other compositions resulting from the combination of the cations having various valence states in addition to the 2+ state and / or 1+ state of lead and alkyl ammonium cations; e.g. compositions other than AB 2+< X 3 (where A is one or more cations, or for a mixed perovskite where A is two or more cations). Thus, the methods described herein may be utilized to create novel mixed cation materials having the composition of a double perovskite (elpasolites), A 2 B 1+< B 3+< X 6 , with an example of such a composition being Cs 2 BiAgCl 6 and Cs 2 CuBiI 6 (embodiments not forming part of the claimed invention). Another example of a composition is described by A 2 B 4+< X 6 , for example Cs 2 PbI 6 and Cs 2 SnI 6 (embodiments not forming part of the claimed invention). Yet another example is described by A 3 B 2 3+< X 9 , for example Cs 3 Sb 2 I 9 (embodiments not forming part of the claimed invention). For each of these examples, A is one or more cations, or for a mixed perovskite, A is two or more cations.

[0016] Thus, the present disclosure relates to methods for producing better performing perovskite materials, having better physical properties, where the method includes the use of at least one an additive alkylammonium halide in excess of the theoretical stoichiometric amounts needed to attain a targeted final perovskite composition. For perovskites having the general formula ABX 3 , the additive may be included in a starting solution that includes at least a first precursor AX at a first concentration C 1 , a second precursor A'X at a second concentration C 2 , and a third precursor BX 2 at a third concentration C 3 , where each of C 1 , C 2 , and C 3 are between 0.001 M and 100 M, or between 0.001 M and 10 M, and the combination of C 1 , C 2 , and C 3 result in a targeted perovskite composition defined by A x A' 1-x BX 3 , where 0 ≤ x ≤ 1, and W*C 1 = x, W*C 2 = 1-x, W*C 3 = 1.0, and W*(C 1 + C 2 + 2*C 3 ) = 3, where 0.01 ≤ W ≤ 1000. W is a scaling factor having units of inverse concentration (e.g. 1 / M, 1 / mol / L, L / mol). The additive may include at least one of AX, A'X, and / or A"X at a concentration C 4 , where at least one of C 4 / C 1 , C 4 / C 2 , and / or C 4 / C 4 is between greater than zero and ten, or between greater than zero and one, and where the additive does not affect the target composition A x A' 1-x BX 3 or the resultant bandgap of the target composition A x A' 1-x BX 3 . Instead, the additive improves at least one physical property and / or performance metric of the target composition A x A' 1-x BX 3 , for example at least one of an improved Tauc plot, an improved current density versus voltage plot, reduced hysteresis, and / or an improved external quantum efficiency (EQE), when the perovskite is used in a solar cell. Similar mixed A-cation perovskites may be produced according to the methods described herein, where the perovskites have the general formula of at least one of (A x A' 1-x ) 2 B 1+< B' 3+< X 6 , (A x A' 1-x ) 2 B 4+< X 6 , and / or (A x A' 1-x ) 3 B 2 3+< X 9 , where an additive is provided in addition to the stoichiometric amounts of the precursors needed to provide the targeted perovskite composition, where the additive does not affect the target composition or its bandgap (embodiments not forming part of the claimed invention).

[0017] In some embodiments of the present disclosure, for perovskites having the general formula ABX 3 , the additive may be included in a starting solution that includes at least a first precursor AX at a first concentration C 1 , a second precursor A'X at a second concentration C 2 , a third precursor BX 2 at a third concentration C 3 , and a fourth precursor BX' 2 at a fourth concentration C 4 , where each of C 1 , C 2 , C 3 , and C 4 are between 0.001 M and 100 M, or between 0.001 M and 10 M, and the combination of C 1 , C 2 , C 3 , and C 4 result in a targeted perovskite composition defined by A x A' 1-x B(X y X' 1-y ) 3 , where 0 ≤ x ≤ 1, and 0 ≤ y ≤ 1, and where W*C 1 = x, W*C 2 = 1-x, W*2*C 3 = y, W*2*C 4 = 1-y, W*(C 1 + C 2 + 2*C 3 + 2*C 4 ) = 3, and W*(C 3 + C 4 ) =1.0, where 0.01 ≤ W ≤ 1000. The additive may include at least one of AX, A'X, and / or A"X, at a concentration C 5 , where at least one of C 5 / C 1 , C 5 / C 2 , and / or C 5 / C 5 is between greater than zero and ten, or between greater than zero and one, and where the additive does not affect the target composition A x A' 1-x B(X y X' 1-y ) 3 or the resultant bandgap of the target composition A x A' 1-x B(X y X' 1-y ) 3 . Instead, the additive improves at least one physical property and / or performance metric of the target composition A x A' 1-x B(X y X' 1-y ) 3 , for example at least one of an improved Tauc plot, an improved current density versus voltage plot, reduced hysteresis, and / or an improved external quantum efficiency (EQE), when the perovskite is used in a solar cell. Similar mixed A-cation, mixed B-cation, and / or mixed anion perovskites may be produced according to the methods described herein, where the perovskites have the general formula of at least one of (A x A' 1-x ) 2 B 1+< B' 3+< (X y X' 1-y ) 6 , (A x A' 1-x ) 2 B 4++< (X y X' 1-y ) 6 , and / or (A x A' 1-x ) 3 B 2 3++< (X y X' 1-y ) 9 , where an additive is provided in addition to the stoichiometric amounts of the precursors needed to provide the targeted perovskite composition, where the additive does not affect the target composition or its bandgap (embodiments not forming part of the claimed invention).

[0018] Similarly, for perovskites having the general formula ABX 3 , perovskites having the formula A x A' y A" (1-x-y) B(X a X' b X" (1-a-b) ) 3 may be produced using methods described herein, where x, y, a, and b are each greater than or equal to zero and less than or equal to one. For this example, the additive may be included in a starting solution that includes at least a first precursor AX at a first concentration C 1 , a second precursor A'X at a second concentration C 2 , a third precursor A"X at a third concentration C 3 , a fourth precursor BX 2 at a fourth concentration C 4 , a fifth precursor BX' 2 at a fifth concentration C 5 , a sixth precursor BX" 2 at a sixth concentration C 6 , where each of C 1 , C 2 , C 3 , C 4 , C 5 , and C 6 are between 0.001 M and 100 M, or between 0.001 M and 10 M, and the combination of C 1 , C 2 , C 3 , C 4 , C 5 , and C 6 result in a targeted perovskite composition defined by A x A' y A" (1-x-y) B(X a X' b X" (1-a-b) ) 3 , where W*C 1 = x, W*C 2 = y, W*C 3 = 1-x-y, W*2*C 4 = a, W*2*Cs = b, W*2*C 6 = 1-a-b, W*(C 1 + C 2 + C 3 + 2*C 4 + 2*C 5 + 2*C 6 ) = 3.0, and W*(C 4 + C 5 + C 6 ) = 1.0, where 0.01 ≤ W ≤ 1000. The additive may include at least one of AX, A'X, A"X, and or A'"X at a concentration C 7 , where at least one of C 7 / C 1 , C 7 / C 2 , C 7 / C 3 , and / or C 7 / C 7 is between greater than zero and ten, or between greater than zero and one, and where the additive does not affect the composition A x A' y A" (1-x-y) B(X a X' b X" (1-a-b) ) 3 or the resultant bandgap of the composition A x A' y A" (1-x-y) B(X a X' b X" (1-a-b) ) 3 . Instead, the additive improves at least one physical property and / or performance metric of the target composition A x A' y A" (1-x-y) B(X a X' b X" (1-a-b) ) 3 , for example at least one of an improved Tauc plot, an improved current density versus voltage plot, reduced hysteresis, and / or an improved external quantum efficiency (EQE), when the perovskite is used in a solar cell. Similar mixed A-cation, and / or mixed anion perovskites may be produced according to the methods described herein, where the perovskites have the general formula of at least one of (A x A' y A" (1-x-y) ) 2B 1+< B' 3+< (X a X' b X" (1-a-b)6 , (A x A' y A" (1-x-y) ) 2 B 4++< ( X a X' b X" (1-a-b) ) 6 , and / or (A x A' y A" (1-x-y) ) 3 B 2 3++< ( X a X' b X" (1-a-b) ) 9 , where an additive is provided in addition to the stoichiometric amounts of the precursors needed to provide the targeted perovskite composition, where the additive does not affect the target composition or its bandgap (embodiments not forming part of the claimed invention).

[0019] Further, the present disclosure relates to methods for producing better performing perovskite materials having the general composition ABX 3 , having better physical properties, where the method includes the use of at least one an additive alkylammonium halide in excess of the theoretical stoichiometric amounts needed to attain a targeted final perovskite composition. The additive may be included in a starting solution that includes at least a first precursor AX at a first concentration C 1 , a second precursor A'X at a second concentration C 2 , a third precursor BX 2 at a third concentration C 3 , and a fourth precursor B'X 2 at a fourth concentration C 4 , where each of C 1 , C 2 , C 3 , and C 4 are between 0.001 M and 100 M, or between 0.001 M and 10 M, and the combination of C 1 , C 2 , C 3 , and C 4 result in a targeted perovskite composition defined by A x A' 1-x B y B' 1-y X 3 , where 0 ≤ x ≤ 1 and 0 ≤ y ≤ 1, and W*C 1 = x, W*C 2 = 1-x, W*C 3 = y, W*C 4 = 1-y, and W*(C 1 + C 2 + 2*C 3 + 2*C 4 ) = 3, where 0.01 ≤ W ≤ 1000. The additive may include at least one of AX, A'X, and / or A"X at a concentration C 4 , where at least one of C 4 / C 1 , C 4 / C 2 , and / or C 4 / C 4 is between greater than zero and ten, or between greater than zero and one, and where the additive does not affect the target composition A x A' 1-x BX 3 or the resultant bandgap of the target composition A x A' 1-x BX 3 . Instead, the additive improves at least one physical property and / or performance metric of the target composition A x A' 1-x BX 3 , for example at least one of an improved Tauc plot, an improved current density versus voltage plot, reduced hysteresis, and / or an improved external quantum efficiency (EQE), when the perovskite is used in a solar cell. Similar mixed A-cation and / or mixed B-cation perovskites may be produced according to the methods described herein, where the perovskites have the general formula of at least one of (A x A' 1-x ) 2 (B a B' 1-a ) 1+< B" 3+< X 6 , (A x A' 1-x ) 2 (B a B' 1-a ) 3+< B" 1+< X 6 , (A x A' 1-x ) 2 (B a B' 1-a ) 4++< X 6 , and / or (A x A' 1-x ) 3 (BaB' 1-a )2 3++< X 9 , where an additive is provided in addition to the stoichiometric amounts of the precursors needed to provide the targeted perovskite composition, where the additive does not affect the target composition or its bandgap (embodiments not forming part of the claimed invention).

[0020] Further, the present disclosure relates to methods for producing better performing perovskite materials, having better physical properties, where the method includes the use of at least one an additive alkylammonium halide in excess of the theoretical stoichiometric amounts needed to attain a targeted final perovskite composition. The additive may be included in a starting solution that includes at least a first precursor AX at a first concentration C 1 , a second precursor A'X at a second concentration C 2 , a third precursor BX at a third concentration C 3 , and a fourth precursor B'X' at a fourth concentration C 4 , where each of C 1 , C 2 , C 3 , and C 4 are between 0.001 M and 100 M, or between 0.001 M and 10 M, and the combination of C 1 , C 2 , C 3 , and C 4 result in a targeted perovskite composition defined by A x A' 1-x B y B' 1-y (X z X' 1-z ) 3 , where 0 ≤ x ≤ 1, 0 ≤ y ≤ 1, and 0 ≤ z ≤ 1. In some embodiments of the present disclosure, W*C 1 = x, W*C 2 = 1-x, W*C 3 = y, W*C 4 = 1-y, and W*(C 1 + C 2 + C 3 + C 4 ) =3, where 0.01 ≤ W ≤ 1000. The additive may include at least one of AX, A'X, and / or A"X at a concentration C 4 , where at least one of C 4 / C 1 , C 4 / C 2 , and / or C 4 / C 4 is between greater than zero and tcn, or between greater than zero and one, and where the additive does not affect the target composition A x A' 1-x B y B' 1-y (X z X' 1-z ) 3 or the resultant bandgap of the target composition A x A' 1-x B y B' 1-y (X z X' 1-z ) 3 . Instead, the additive improves at least one physical property and / or performance metric of the target composition A x A' 1-x BX 3 , for example at least one of an improved Tauc plot, an improved current density versus voltage plot, reduced hysteresis, and / or an improved external quantum efficiency (EQE), when the perovskite is used in a solar cell. Similar mixed A-cation, mixed B-cation, and / or mixed anion perovskites may be produced according to the methods described herein, where the perovskites have the general formula of at least one of (A x A' 1-x ) 2 (B a B' 1-a ) 1+< B" 3+< (X b X' b-1 ) 6 , (A x A' 1-x ) 2 (B a B' 1-a ) 3+< B" 1+< (X b X' b-1 ) 6 , (A x A' 1-x ) 2 (B a B' 1-a ) 4++< (X b X' b-1 ) 6 , and / or (A x A' 1-x ) 3 (B a B' 1-a ) 2 3++< (X b X' b-1 ) 9 , where an additive is provided in addition to the stoichiometric amounts of the precursors needed to provide the targeted perovskite composition, where the additive does not affect the target composition or its bandgap (embodiments not forming part of the claimed invention).

[0021] Further, in some embodiments of the present disclosure, the perovskite may be positioned on at least one of a silicon substrate, a CIGS substrate, a CdTe substrate, a III-V alloy, another substrate made of another perovskite material, and / or any other suitable substrate having a bandgap between 0.9 eV and 1.5 eV, inclusively. In accordance with the claimed invention, the final target perovskite includes FAxCs1-xPb(IyBr1-y)3, where x and y are all between greater than zero and less than or equal to one. In some embodiments of the present disclosure, the final target perovskite may include FA 0.83 CS 0.17 Pb(I 0.6 Br 0.4 ) 3 . For any of the above described perovskite materials, produced by the methods described herein, may have a relatively wide bandgap; e.g. greater than or equal to 1.6 eV, or between 1.6 eV and 2.5 eV, inclusively. However, depending on the specifics of the fabrication method, perovskite materials having a relatively low bandgap may also be produced; e.g. less than or equal to 1.5 eV, or between 1.2 eV and 1.5 eV, inclusively.

[0022] Figure 2 illustrates a method 200 for producing an organic-inorganic perovskite layer 234, according to some embodiments of the present disclosure. The method 200 may begin with the combining 210 of various precursors, for example a first precursor 212, a second precursor 214, up to a predetermined number, n, of precursors, thus including an n th< precursor 216. Each of these precursors (212, 214, and 216) may provide one or more of the compounds and / or elements required to achieve the final targeted organic-inorganic perovskite composition, e.g. FA 0.83 Cs 0.17 Pb(I 0.6 Br 0.4 ) 3 . Thus, for this particular example, the first precursor 212 includes CsI, the second precursor 214 includes formamidinium iodide (FAI), a third precursor (not shown) includes PbBr 2 , and an n th< precursor 216 for n equal to four includes PbI 2 . Again for the specific example of an organic-inorganic perovskite defined by FA 0.83 Cs 0.17 Pb(I 0.6 Br 0.4 ) 3 , this final composition may be achieved by combining 210 the desired stoichiometric amounts of each of the first precursor 212 of CsI, the second precursor 214 of FAI, the third precursor (not shown) of PbBr 2 , and the fourth (n=4) precursor 216 of PbI 2 to form a precursor solution 219 having molar concentrations of 0.17 M, 0.83 M, 0.6 M, and 0.4 for each of the four precursors, respectively. In addition, the combining 210 includes an additive 218, which provides an excess (e.g. above the stoichiometric amount) of at least one of element and / or constituents making up the final composition of the organic-inorganic perovskite layer 234. For example, for the case of FA 0.83 CS 0.17 Pb(I 0.6 Br 0.4 ) 3 , the additive 218 includes at least one of MAI, MABr, and / or MACl at a molar concentration between 0.01 M and 1.0 M. The precursor solution 210 may be formed using any suitable solvent, with examples including water and / or organic solvents (not shown). However, this composition is provided for exemplary purposes and other compositions for organic-inorganic perovskites are within the scope of the present disclosure. For example, compositions defined by FA x Cs 1-x Pb(I y Br 1-y ) 3 , where 0 ≤ x ≤ 1 and 0 ≤ y ≤ 1, may be produced according to some of the embodiments described herein. More generally, A x A' 1-x B(X y X' 1-y ) 3 , where A and A' are both A-cations, X and X' are both anions, and B is a cation as described above, 0 ≤ x ≤ 1, and 0 ≤ y ≤ 1, may be produced according to some of the embodiments described herein. Similarly, perovskites having the general formula A x A' y A" (1-x-y) B(X a X' b X" (1-a-b) ) 3 may also be produced using methods described herein, where each of x, y, a, and b are greater than or equal to zero and less than or equal to one.

[0023] The method 200 may then continue by applying 220 the precursor solution 219 to a substrate (not shown), for example by spin-coating, dip-coating, curtain-coating, blade-coating, and / or any other suitable solution-processing method, resulting in the formation of a liquid film 222. After the liquid film 222 has been formed, the method 200 may proceed with treating 230 the liquid film 222, resulting in the removal of volatiles 232 (e.g. water and / or organic solvents) to form the final organic-inorganic perovskite layer 234. For example, the treating 230 may include thermal treating, where the liquid film 222 is heated to a temperature of up to 100 °C to remove the volatiles 232. Further, the treating 230 may include exposing the liquid film 222 to a local pressure that is less than one atmosphere; e.g. a vacuum. In addition, the treating 230 may be performed for a period of time between one minute and one hour. In some embodiments of the present disclosure, the treating 230 may include exposing the liquid film 22 to a gas stream such as nitrogen and / or dry air.

[0024] Figure 3A illustrates the UV-vis absorption spectrum for organic-inorganic perovskite films prepared with and without the use of excess of methylammonium halides in the precursor solution. For this example, the pristine organic-inorganic perovskite sample (FA 0.83 Cs 0.17 Pb(I 0.6 Br 0.4 ) 3 ), which was prepared according to the procedure described below, used a stoichiometric perovskite precursor consisting of 0.17M CsI, 0.83 M FAI, 0.6 M PbBr 2 , 0.4 M PbI 2 in DMF solution with HI and HBr acid additives, exhibited the absorption spectrum corresponding to an optical bandgap of 1.752 eV. The absorption spectra were barely affected using 20% excess of MABr or MACl additives in the precursor solution, suggesting that the perovskite structure was not altered. This suggests that non-stoichiometric solution strategies having excess methylammonium halides may be used to improve the characteristics and / or properties of organic-inorganic mixed-halide (e.g. bromine and iodine) perovskites. In contrast, the use of 20% of MAI additive, resulted in a significant red shift of the absorption onset by about 20 nm. Tauc plots in Figure 4 further confirm the decrease of the optical bandgap of the organic-inorganic perovskite materials from 1.752 eV (for the reference and MABr / MACl samples) to 1.715 eV when 20% MAI additive was used. The degree of bandgap shift (~0.037 eV) is consistent with the incorporation of about additional 0.2 iodide per 3 halides (I and Br). The X-ray diffraction (XRD) patterns in Figure 3B and Figures 5A and 5B also agreed with the bandgap change as reflected from the UV-vis spectra. The main peak position of the sample produced using excess MAI additive shifted slightly to a lower diffraction angle by about 0.05°. These results suggest that when excess MAI is used as an additive in the precursor solution to produce Br-I (e.g. (X y X' 1-y ) 3 ) mixed perovskite materials, the iodide ion may be more stable in the lattice than the bromide ion. Consequently, the final amount of iodide ion in I-Br mixed-halide organic-inorganic perovskite was increased depending on the amount of iodide ion in the precursor solution. In contrast, the excess bromide (or chloride) resulting from the MABr additive (or the MACl additive) was less stable than iodide when competing for the lattice sites, and consequently, the final bandgap of the resultant organic-inorganic perovskite prepared using excess MABr or MACl additives was not affected by the removal of the bromine and iodine during the subsequent treating, e.g. annealing. This is important for developing alloyed perovskite samples (especially for WBG organic-inorganic perovskite materials having a high bromine content) since the bandgap may be easily affected by halide-based precursor precursors and / or additives.

[0025] It is noteworthy that the intensity of the main organic-inorganic perovskite (100) peak was enhanced with the use of excess methylammonium halides additives, especially for the MABr additive, which leads to an order of magnitude increase in the peak intensity. In comparison to the use of the MACl additive, the MABr additive presumably reduces to a lesser degree the perovskite lattice structure with a bromide rich condition during the transition from a precursor state to the intermediate phase, and ultimately to the final organic-inorganic perovskite crystal structure. In addition, the chlorine in the MACl additive could form a relatively stable Br-Cl alloy state during the intermediate film formation stage. The enhanced XRD peak intensity is normally associated with higher crystallinity and / or more aligned grain orientation (texture). The full-width half-maximum (FWHM) of the (100) plane decreased from 0.153° for the reference sample, to 0.117° with 20% MAI additive in the precursor solution, 0.105° with 20% MACl additive, and 0.099° for 20% MABr additive (see Figure 5A and Table 1). In general, the sharp XRD peak with small FWHM means high crystallinity, increased crystal size, and decreased bulk defect. Thus, the organic-inorganic perovskite prepared from non-stoichiometric precursor, e.g. containing excess MABr additive, showed the most improved crystallographic properties in comparison to using other excess halide-containing additives. The degree of grain orientation is another parameter that shows the crystallographic properties of polycrystalline thin films. Through determining the intensity ratio of peaks for the (100) plane to the (110) plane (see Table 1), the degree of orientation was qualitively assessed. The (100) / (110) intensity ratio increased from about 20 for the pristine organic-inorganic perovskite film using stoichiometric amounts of precursors to 181 for the film prepared using excess MABr additive in the precursor solution. Table 1. Effect of non-stoichiometric precursor with excess 20% MAI, MABr, or MACl on the (100) and (110) XRD peak intensity, full-width half-maximum (FWHM) of (100) planes, and (100) / (110) peak intensity ratio.(100) ~14.3° (100) FWHM (110) ~20.3° (100) / (110) ratio Reference66530.1533332020% MAI169160.1173784520% MABr449480.09924718220% MACl223050.10535064

[0026] Figure 6 shows a top view of SEM images of organic-inorganic perovskite films prepared with and without the use of excess methylammonium halide additives (20% MAI, MABr, or MACl), according to some embodiments of the present disclosure. Analysis of these images shows that the average grain size was about 1.2 ± 0.6 µm for all these films. There is no clearly discernable difference in the average grain size among these films. This could be caused by the use of an acid additive (e.g., HI or HBr) in the precursor chemistry (not shown in Figure 2). It is often found that larger grain size leads to improved carrier lifetime, higher carrier mobility, and improved device characteristics. This is frequently attributed to the reduction of the number of grain boundaries. However, the larger grain size is also frequently connected to enhanced film crystallinity and preferred orientation, which also affect device performance.

[0027] Figure 7 shows the typical photocurrent density-voltage (J-V) curves for perovskite solar cells (PSCs) using organic-inorganic perovskite films prepared with and without the use of excess methylammonium halide additives (20% MAI, MABr, or MACl), according to some embodiments of the present disclosure. When the MAI additive was used, the J sc was clearly higher than that of the other devices. This is consistent with the reduced bandgap as we discussed above in connection with the absorption spectra shown in Figure 3A and Figure 4. Because of the minimum impact to the bandgap when 20% MABr or MACl additive was used, the J sc values of these devices are essentially the same as the reference device. Regardless which methylammonium halide additive was used, the device hysteresis was clearly reduced. This can be attributed to the improved crystallographic properties as discussed in connection with Figure 3B and Figure 5B. The details of the PV parameters including J sc , V oc , fill factor (FF), and power conversion efficiency (PCE) are summarized in Table 2. Based on these results, it is clear that the MABr additive had the strongest positive effect on the final device performance, especially the V oc , without changing the organic-inorganic perovskite composition (and bandgap). This is consistent with the structure effect as discussed above. It should be noted that the use of excess MAI additive can also significantly improve device performance and reduce hysteresis, but at a cost of shifting the bandgap from 1.752 to 1.715 eV. Table 2. Effect of non-stoichiometric precursor with excess methylammonium halides on the statistics of PV parameters of WGB PSCs.J sc (mA / cm 2< ) V oc (V) FF PCE (%) Referencereverse17.47 ± 0.261.071 ± 0.0070.726 ± 0.02113.59 ± 0.43forward17.39 ± 0.321.047 ± 0.0060.505 ± 0.0249.20 ± 0.5420% MAIreverse18.88 ± 0.121.085 ± 0.0050.727 ± 0.02114.90 ± 0.45forward18.88 ± 0.121.067 ± 0.0200.616 ± 0.02712.40 ± 0.7120% MABrreverse17.87 ± 0.181.102 ± 0.0130.765 ± 0.02415.08 ± 0.60forward18.01 ± 0.101.092 ± 0.0170.570 ± 0.05311.20 ± 0.9220% MAClreverse17.81 ± 0.181.064 ± 0.0360.779 ± 0.01614.76 ± 0.28forward17.84 ± 0.191.028 ± 0.0460.590 ± 0.03110.83 ± 0.86

[0028] The impact of different MABr additive concentrations on the final organic-inorganic perovskites, including film morphology, optical absorption, crystal structure, and device characteristics were also studied. Figures 8A, 8B, and 8C show SEM images of the organic-inorganic perovskite films produced using different amounts of MABr additives, from 20% to 80% excess of stoichiometric amounts. There was no significant variation of the grain morphology when MABr additive was changed from 20% to 40%. However, when 80% MABr additive was used, the film morphology was changed significantly with clear evidence of void formation. This is likely caused by the release of large amounts of MABr additive during the film formation process. The deterioration of film morphology with 80% MABr additive is consistent with the much higher base line of the absorption spectrum shown in Figure 8D. In contrast, the films prepared with 20% and 40% MABr additive showed similar optical absorption spectra as the reference film. Figure 8E shows that the XRD peak intensity increased with the amount of MABr additive without clearly affecting the peak locations. The typical J-V curves of devices based on organic-inorganic perovskites using non-stoichiometric precursor with different amounts of MABr additive are shown in Figure 8F. The corresponding statistics of PV parameters are summarized in Table 3 below. When the MABr additive concentration was increased from 20% excess to 40% excess, there was a clear increase in both V oc and FF, leading to improved device efficiency and reduced J-V hysteresis. With a further increase to 80% MABr additive, the average device efficiency dropped substantially from about 15.72% to 7.90%. This is consistent with the observed deterioration of the film morphology when 80% MABr was used. Figure 9 shows the J-V curves of the best-performing device using 40% MABr additive with both forward and reverse scans. This device was composed of about 430-nm organic-inorganic perovskite and a cross-section of the device stack is shown in Figure 10. The reverse and forward J-V scans yield conversion efficiencies of 16.77% and 13.37%, respectively. Because of the hysteresis, the stabilized power output (SPO) near the maximum power point under continuous one-sun illumination was also evaluated. The result is shown in the inset of Figure 9, where the SPO of ~16.4% is established. The SPO value is close to the efficiency determined from the reverse J-V scan. The EQE spectrum of the champion device is shown in Figure 11. The integrated current density from EQE spectrum is 17.53 mA / cm 2< , which is consistent with the J sc value (~17.97 mA / cm 2< ) from the J-V curve. Table 3. Effect of MABr concentration on the statistics of PV parameters for both reverse and forward J-V scans.J sc (mA / cm 2< ) V oc (V) FF PCE (%) Referencereverse17.47±0.261.071±0.0070.726±0.02113.59±0.43forward17.39±0.321.047±0.0060.505±0.0249.20±0.5420% MABrreverse17.87±0.181.102±0.0130.765±0.02415.08±0.60forward18.01±0.101.092±0.0170.570±0.05311.20±0.9240% MABrreverse17.99±0.281.134±0.0230.771±0.01415.72±0.56forward18.07±0.141.127±0.0140.622±0.03712.67±0.8380% MABrreverse16.32±0.090.93±0.0580.520±0.0097.90±0.55forward16.32±0.110.93±0.0580.505±0.0147.69±0.25

[0029] In order to investigate the contribution of the organic-inorganic perovskites used in tandem solar cells, an organic-inorganic perovskite-Si 4-terminal tandem cell was prepared by mechanically stacking a Si bottom cell with a semi-transparent wide-bandgap (~1.75 eV) organic-inorganic perovskite top cell. Figure 12A and Table 4 show the corresponding J-V curves and PV parameter summary. For the tandem study, SnO 2 was used rather than TiO 2 as the electron transport material for the PSC. The use of SnO 2 can further increase the V oc . For the transparent top contact a stack of 15 nm MoO x , 10 nm Au and 15 nm MoO x was used. A stack of MoO x / Au / MoO x not only facilitates the maximization of semi-transparent top cell efficiency due to the formation of continuous ultrathin Au metal layer and reasonable conductivity, but also enables the relatively high transmittance for the top electrode / cell. The semi-transparent WBG PSC exhibited a PCE of 14.7% with a V oc of 1.185 V, a J sc of 17.8 mA cm -2< , and a FF of 69.7% when measured under reverse voltage scan. With the filter of the semi-transparent WBG top solar cell, the efficiency of the Si bottom cell decreases from 18.6% to 5.6% due to the large reduction in the incident light harvesting by Si active layer. The perovskite-Si 4-terminal tandem cell yields a PCE of 20.3%, which is higher than the semitransparent perovskite top cell (14.7%) and the unfiltered Si bottom cell (18.6%). As shown in Figure 12B, the measured EQE-integrated J sc 's of the organic-inorganic perovskite top cell and the filtered Si bottom cell are 17.45 and 13.38 mA cm -2< , respectively, in agreement with the J sc 's obtained from their respective J-V curves. It is worth noting that these perovskite-Si 4-terminal tandem cells still have potential to be further improved by using more transparent eletrode stack for semi-transparent perovskite top cells, Si bottom cell with higher efficiency, and more effective optical coupling scheme. Table 4. PV parameters of perovskite top cell, Si bottom cell, and perovskite-Si 4-terminal tandem device.Cells J sc (mA / cm 2< ) V oc (V) FF PCE (%) Perovskite cell17.81.1850.69714.7Si cell38.80.6930.69218.6Filtered Si cell13.00.6390.6745.64-Terminal tandem cell20.3

[0030] In summary, the use of non-stoichiometric precursor solutions having excess methylammonium halides additives (MAI, MABr, and MACl) for preparing high-quality organic-inorganic perovskite thin films (~1.75 eV FA 0.83 Cs 0.17 Pb(I 0.6 Br 0.4 ) 3 ) for solar cell applications has been demonstrated. In contrast to the standard iodine-based perovskites, the composition of the perovskites containing high bromine content can be affected significantly by the use of excess methylammonium halides additives due to the competition of different halides in the organic-inorganic perovskite lattice. For organic-inorganic mixed-halide perovskites based on Br-I mixtures, excess MAI additive significantly reduced the bandgap due to more iodide incorporation, whereas the use of both MABr and MACl additives showed little impact on the organic-inorganic perovskite composition and crystal structures. Interestingly, excess MABr additive (rather than excess MACl additive) in the non-stoichiometric precursor showed the strongest effect on improving both the crystallographic properties (e.g., crystallinity and orientation) and the device characteristics.Methods:

[0031] Organic-Inorganic perovskite and device fabrication. Fluorine doped tin oxide (FTO, TEC15, Hartford glass Co) was patterned using Zn powder and HCl (~5M). Compact TiO 2 (c-TiO 2 ) film was deposited on thoroughly cleaned substrate by spray pyrolysis using 0.2 M titanium diisopropoxide bis(acetylacetonate) in 1-butanol solution. PCBM (20mg / ml in dichlorobenzene) was span at 4000 rpm for 30 s on top of c-TiO 2 , followed by 1 hr annealing at 100°C. Organic-inorganic perovskite precursor solutions, targeting a final organic-inorganic perovskite composition of FA 0.83 Cs 0.17 Pb(I 0.6 Br 0.4 ) 3 , was composed of 0.17M CsI, 0.83 M FAI, 0.6 M PbBr 2 , 0.4 M PbI 2 precursors in a DMF solution. 50 µl of HI acid and 27 µl of HBr acid were added into 1 ml precursor. Different amounts of additives, eg. 0.2 M MAI, MABr, MACl, were added to the prepared precursor solution. The precursor solution was deposited by spin-coating 100 µl precursor at 3000 rpm for 30s. The resultant transparent yellow liquid film was transferred onto a hotplate at 65°C for 5 min, and then at 185°C with a petri-dish covered for 10 min, resulting in the final, solid organic-inorganic perovskite layer. A hole transport layer (HTL) was spin-coated at 4000 rpm for 30 s with a HTL solution consisting of 80 mg 2,2',7,7'-tetrakis(N,N-dip-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD; Merck), 30 µL bis(trifluoromethane) sulfonimide lithium salt stock solution (500 mg Li-TFSI in 1 mL acetonitrile), and 30 µl 4-tert-butylpyridine (TBP), and 1 mL chlorobenzene solvent. Finally, a 150 nm Ag layer was deposited on the HTL layer by thermal evaporation for the top contact.

[0032] Semi-transparent device fabrication. For the (~1.75 eV) semi-transparent top organic-inorganic perovskite cell, SnO 2 was used as the electron transport material (ETM). The SnO 2 ETM was deposited onto pre-patterned and cleaned FTO substrates by plasma-enhanced atomic layer deposition (Ensure Scientific Group AutoALD-PE V2.0). The substrate was then sequentially deposited by a C 60 -SAM layer, organic-inorganic perovskite film, and spiro-OMeTAD as the hole transport material (HTM). Finally, the sequence of 15 nm MoO x , 10 nm Au, and 15 nm MoO x were thermally evaporated through a patterned mask onto the HTM.

[0033] Film and device characterization. The optical absorption spectra of the organic-inorganic perovskite films were measured using a UV-vis spectrophotometer with the aid of an integrated sphere (Cary-6000i, Agilent) The morphologies of the prepared organic-inorganic perovskite films and the cross-sectional structure and thickness of the solar cells were investigated using a feld-emission scanning electron microscopy (FESEM, Quanta 600, FEI). J-V curves were measured in air under 100 mW / cm 2< simulated AM1.5G solar irradiation with a Keithley 2400 Source Meter. The light intensity for J-V measurements was calibrated by a standard Si solar cell. EQE spectra were measured from 300 to 800 nm for perovskite solar cells and from 300 to 1200 nm for Si cells using a QE system from PV Measurements Inc. All characterizations and measurements were performed in the ambient condition. The stabilized current and power output were measured using a potentiostat (Princeton Applied Research, VersaSTAT MC).

[0034] 4-terminal tandem cell measurements. The measurements were performed using the standard methods. In brief, the J-V curves of semitransparent top organic-inorganic perovskite cells were measured under 100 mW / cm 2< AM1.5G solar irradiation. EQE spectra were performed on a QE system. Each semitransparent wide-bandgap top cell consists of multiple subcells with small and large active areas as defined by the areas of the metal electrodes. The small subcells have similar active areas as the bottom cells and are used for J-V measurements.

[0035] The large subcells are used to filter the bottom cells for easy cell alignment. The J-V curve and EQE spectrum of Si bottom cell were taken by putting such a semitransparent wide-bandgap top cell with a large active area on top as a filter.

[0036] As used herein, the term "about" accounts for naturally occurring errors in measuring any quantitative value. For example, the phrase "C 1 *W is about equal to x" refers to C 1 *W not only equaling x, but also equaling values close to x within reasonable limits, for example x ± L, where L may be between 0.01 and 0.1 inclusively.

Claims

1. A method comprising: treating a liquid comprising a first precursor at a concentration C1, a second precursor at a concentration C2, a third precursor at a concentration C3, a fourth precursor, and an additive at a concentration C4, wherein: the first precursor comprises at least one of cesium halide wherein cesium halide is cesium iodide or cesium bromide; the second precursor comprises at least one of formamidinium halide wherein formamidinium halide is formamidinium iodide or formamidinium bromide; the third precursor comprises PbBr2; the fourth precursor comprises PbI2; the additive comprises at least one of methylammonium bromide (MABr), methylammonium chloride (MACl), and methylammonium iodide (MAI); the treating results in a perovskite having the formula FAxCs1-xPb(IyBr1-y)3, where 0 < x < 1 and 0 < y < 1; each of C1, C2, and C3 are independently between 0.001 M and 100 M; and at least one of C4 / C1 or C4 / C2 equals a ratio greater than zero and less than 0.5.

2. The method of claim 1, wherein the perovskite is FA0.83Cs0.17Pb(I0.6Br0.4)3.

3. The method of claim 1 or claim 2, wherein the perovskite has a bandgap greater than or equal to 1.6 eV.

4. The method of any one of claims 1-3, wherein: W*C1 is about equal to x, W*C2 is about equal to (1-x), W*2*C3 is about equal to one, 0.01 ≤ W ≤ 1000, and W is a scaling factor having units of inverse concentration.

5. The method of any one of claims 1-4, wherein the liquid further comprises a solvent.

6. The method of any one of claims 1-5, wherein the treating further comprises thermal treating, for example heating the liquid to remove the solvent.

7. The method of any one of claims 1-5, wherein the treating further comprises exposing the liquid to a gas stream.

8. A perovskite having the formula FAxCs1-xPbzB'z-1(IyBr1-y)3, wherein: each of x, y, and z are between zero and one, exclusively, and B' is selected from germanium and tin.