ELECTRON TRANSPORT LAYER FOR PEROVSKITE SOLAR CELL AND PEROVSKITE SOLAR CELL INCLUDING SAME

Abstract

The present disclosure relates to an electron transport layer for a perovskite solar cell, which is tin oxide (SnO.sub.2-x, 0<x<1) comprising oxygen vacancies, wherein the oxygen vacancies are passivated by oxidized black phosphorus quantum dots (O-BPs), and a perovskite solar cell including the same. The electron transport layer for a perovskite solar cell of the present disclosure can prevent phase transition to a structure having semiconductor properties not suitable for solar cells, such as ?-FAPbI.sub.3 or PbI.sub.2, due to the occurrence of iodine interstitials (I.sub.i) in the perovskite structure caused by deficiency of oxygen atoms in SnO.sub.2-x at the interface, by passivating the oxygen vacancies with oxidized black phosphorus quantum dots (O-BPs) containing multiple P?O bonds.

Claims

1. An electron transport layer for a halide perovskite solar cell, which is tin oxide (SnO.sub.2-x, 0<x<1) comprising oxygen vacancies, wherein the oxygen vacancies are passivated by oxidized black phosphorus quantum dots (O-BPs).

2. The electron transport layer for a halide perovskite solar cell according to claim 1, wherein the oxidized black phosphorus quantum dots exhibit peaks at 129-130.5 eV and 132.5-133.5 eV in X-ray photoelectron spectroscopy (XPS) analysis.

3. The electron transport layer for a halide perovskite solar cell according to claim 1, wherein the oxidized black phosphorus quantum dots have a diameter of 4.5-5.5 nm.

4. A perovskite solar cell comprising the electron transport layer for a halide perovskite solar cell according to claim 1.

5. The perovskite solar cell according to claim 4, wherein the perovskite solar cell comprises: a front electrode; an electron transport layer formed on the front electrode, which comprises tin oxide (SnO.sub.2-x, 0<x<1) comprising oxygen vacancies, wherein the oxygen vacancies are passivated by oxidized black phosphorus quantum dots (O-BPs); a halide perovskite photoactive layer formed on the electron transport layer; a hole transport layer formed on the halide perovskite photoactive layer; and a back electrode formed on the hole transport layer.

6. The perovskite solar cell according to claim 4, wherein the halide perovskite of the halide perovskite photoactive layer is represented by Chemical Formula 1: $\begin{array}{cc}AM{X}_3& \left[\mathrm{Chemical}\mathrm{Formula}1\right]\end{array}$ wherein A is CH(NH.sub.2).sub.2.sup.+, CH.sub.3NH.sub.3.sup.+ or NR.sub.4.sup.+, wherein each R is independently a hydrogen atom or a C.sub.1-C.sub.10 alkyl group, M is Pb, Sn, Bi, Ge, Ga, Ti, In, Sb or Mn, and X is I.

7. The perovskite solar cell according to claim 6, wherein the halide perovskite represented by Chemical Formula 1 is FAPbI.sub.3 (formamidinium (FA) lead triiodide).

8. The perovskite solar cell according to claim 5, wherein the electron transport layer exhibits a shoulder peak between 2.0 and 2.5 ? in K-edge XAFS (X-ray absorption fine structure) analysis.

9. The perovskite solar cell according to claim 5, wherein, in the electron transport layer, the oxidized black phosphorus quantum dots (O-BPs) are located throughout the electron transport layer or at the interface of the electron transport layer and the halide perovskite photoactive layer.

10. The perovskite solar cell according to claim 5, wherein the front electrode comprises any one selected from ITO (indium tin oxide), FTO (fluorine-doped tin oxide), GZO (gallium zinc oxide), IZO (indium zinc oxide), IGZO (indium gallium zinc oxide), graphene, molybdenum disulfide (MoS.sub.2), single-walled carbon nanotube (SWCNT), multi-walled carbon nanotube (MWCNT) and metal mesh.

11. The perovskite solar cell according to claim 5, wherein the hole transport layer comprises any one selected from spiro-OMeTAD (2,2,7,7-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9-spirobifluorene), PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate), P3HT (poly(3-hexylthiophene-2,5-diyl)), PTAA (poly(t-arylamine)), PCBTDPP (poly[N-90-heptadecanyl-2,7carbazole-alt-3,6-bis(thiophen-5-yl)-2,5-dioctyl-2,5-dihydropyrrolo[3,4]pyrrole-1,4-dione]), PDPPDBTE (poly[2,5-bis(2-decyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-(E)-1,2-di(2,2-bithiophen-5-yl)ethene]), PCPDTBT (poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4Hcyclopenta[2,1-b:3,4-b]dithiophene-2,6-diyl]]), PCDTBT (poly[N-9-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)]), PFB (poly(9,9-dioctylfluorene-co-bis-N,N-(4-butylphenyl)-bis-N,N-phenyl-1,4-phenylenediamine)), PANI (polyaniline), chloroaluminum phthalocyanine, tetracene, ?-octithiophene, pentacene, lead(II) phthalocyanine, zinc phthalocyanine, copper(II) phthalocyanine, phthalocyanine blue, ?-quaterthiophene, and ?-quinguethiophene.

12. The perovskite solar cell according to claim 5, wherein the back electrode comprises any one selected from gold (Au), silver (Ag), aluminum (Al), graphene, carbon, graphite, single-walled carbon nanotube (SWCNT) and multi-walled carbon nanotube (MWCNT).

13. A method for preparing a perovskite solar cell, comprising: (a) a step of preparing oxidized black phosphorus quantum dots (O-BPs); (b) a step of preparing a tin oxide precursor solution or a tin oxide solution and a solution comprising the oxidized black phosphorus quantum dots (O-BPs); (c) a step of coating the tin oxide precursor solution and the solution comprising the oxidized black phosphorus quantum dots on a front electrode substrate and preparing an electron transport layer comprising tin oxide wherein oxygen vacancies are passivated through heat treatment; and (d) a step of forming a halide perovskite photoactive layer on the electron transport layer.

14. The method for preparing a perovskite solar cell according to claim 13, wherein, the step (a) comprises: a step of preparing black phosphorus quantum dots (BPQDs) by sonicating black phosphorus powder in an organic solvent; and a step of oxidizing the black phosphorus quantum dots (BPQDs).

15. The method for preparing a perovskite solar cell according to claim 13, wherein the sonication is performed sequentially by first sonication at 80-120 W for 8-12 hours and second sonication at 700-900 W for 1-3 hours.

16. The method for preparing a perovskite solar cell according to claim 13, wherein the oxidation is performed by subjecting the organic solvent comprising the black phosphorus quantum dots (BPQDs) to relative humidity 5-15% for 20-50 minutes.

17. The method for preparing a perovskite solar cell according to claim 13, wherein the organic solvent is one or more selected from isopropyl alcohol (IPA), acetone, dimethylacetamide (DMA), acetonitrile, tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), dimethylformamide (DMF) and hexamethylphosphoramide.

18. The method for preparing a perovskite solar cell according to claim 13, wherein, in the step (b), the tin oxide precursor comprised in the tin oxide precursor solution is SnCl.sub.2.

19. The method for preparing a perovskite solar cell according to claim 13, wherein, in the step (b), the tin oxide comprised in the tin oxide solution is SnO.sub.2-x nanoparticles or SnO.sub.2-x quantum dots.

20. The method for preparing a perovskite solar cell according to claim 13, wherein, in the step (c), the tin oxide precursor solution or the tin oxide solution is mixed with the solution comprising the oxidized black phosphorus quantum dots (O-BPs) and coated on the front electrode substrate for passivating the bulk, or the tin oxide precursor solution and the solution comprising the oxidized black phosphorus quantum dots (O-BPs) are coated sequentially on the front electrode substrate for passivating the interface of the electron transport layer and the perovskite photoactive layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] FIG. 1 shows the J-V curves of PSCs obtained in Test Example 1.

[0046] FIG. 2 shows the XPS spectrum of O-BPs obtained in Test Example 1.

[0047] FIG. 3 shows the TEM image and size distribution of O-BPs obtained in Test Example 1.

[0048] FIGS. 4A to 4E show results of analyzing the chemical phase of passivation of oxygen vacancies by O-BPs in Test Example 2.

[0049] FIGS. 5A to 5D show results of analyzing the charge transport from FAPbI.sub.3 to SnO.sub.2-x or SnO.sub.2 in Test Example 3.

[0050] FIGS. 6A to 6E show the XRD and morphological analysis results of FAPbI.sub.3 adjacent to SnO.sub.2-x or SnO.sub.2 in Test Example 4.

[0051] FIGS. 7A to 7F show the STEM image analysis results of FAPbI.sub.3/SnO.sub.2-x or SnO.sub.2 interface in Test Example 4.

[0052] FIGS. 8A to 8C show results of analyzing unfavorable phase transition induced between iodine interstitials of tin oxide and organic cation retention capacity in Test Example 5.

[0053] FIG. 9 shows the XPS analysis result of an exfoliated perovskite film in Test Example 6.

[0054] FIGS. 10A and 10B show results of analyzing the performance of a PSC in Test Example 7.

[0055] FIG. 11 shows a result of evaluating the thermal stability of a PSC in Test Example 7.

[0056] FIG. 12 shows a result of evaluating the operational stability of a PSC in Test Example 7.

[0057] FIG. 13 shows J-V curves obtained measuring the long-term stability of PSCs in Test Example 7.

DETAILED DESCRIPTION

[0058] Hereinafter, various aspects and exemplary embodiments of the present disclosure are described in more detail. The exemplary embodiments of the present disclosure will be described in detail with reference to the attached drawings so that those having ordinary knowledge in the art to which the present disclosure belongs can easily carry out the present disclosure. However, the following description is not intended to limit the present disclosure to the specific exemplary embodiments and, in explaining the present disclosure, detailed description of related known technology will be omitted if it is judged that the description may obscure the gist of the present disclosure. The terms used herein are used merely to describe the specific exemplary embodiments and are not intended to limit the present disclosure. Singular expressions include plural expressions unless the context clearly dictates otherwise. In the present specification, the terms such as include, have, etc. are intended to designate the presence of features, numbers, steps, operations, components or combinations thereof described in the specification and are not intended to exclude the possibility of the presence or addition of one or more other features, numbers, steps, operations, components or combinations thereof.

[0059] Hereinafter, an electron transport layer for a perovskite solar cell of the present disclosure is described.

[0060] The electron transport layer for a perovskite solar cell of the present disclosure is tin oxide (SnO.sub.2-x, 0<x<1) including oxygen vacancies, wherein the oxygen vacancies are passivated by oxidized black phosphorus quantum dots (O-BPs).

[0061] Black phosphorus, from which the oxidized black phosphorus quantum dots are derived, is one of the allotropes of phosphorus. It is also called metallic phosphorus and may be obtained by heating white phosphorus at 200? C. under a pressure of 12000 kg/cm.sup.2. It is iron gray with metallic luster and appears similar to graphite. The crystal has a layered lattice structure, with the distance between three neighboring phosphorus atoms being 2.18 ?, the PPP bonding angle being <102? and the interlayer PP distance being 3.68 ?. It has a melting point of 587.5? C., a density of 2.69 g/cm.sup.3 and a vapor pressure of 2.3 cm/357? C. When heated to 550? C., it changes to red phosphorus. It is a good conductor of heat and electricity and is insoluble in carbon disulfide. When yellow phosphorus is heated at high pressure below the temperature where black phosphorus is produced, amorphous black phosphorus is obtained, which may be changed to red phosphorus by heating at 125? C. for a long time.

[0062] The oxidized black phosphorus quantum dots may exhibit peaks at 129-130.5 eV and 132.5-133.5 eV in X-ray photoelectron spectroscopy (XPS) analysis.

[0063] The oxidized black phosphorus quantum dots may have a diameter of 4.5-5.5 nm.

[0064] In addition, the present disclosure provides a perovskite solar cell including the electron transport layer for a halide perovskite solar cell.

[0065] The perovskite solar cell of the present disclosure may include: a front electrode; an electron transport layer formed on the front electrode, which includes tin oxide (SnO.sub.2-x, 0<x<1) comprising oxygen vacancies, wherein the oxygen vacancies are passivated by oxidized black phosphorus quantum dots (O-BPs); a halide perovskite photoactive layer formed on the electron transport layer; a hole transport layer formed on the halide perovskite photoactive layer; and a back electrode formed on the hole transport layer.

[0066] The halide perovskite of the halide perovskite photoactive layer may be represented by Chemical Formula 1.

[0067] [Chemical Formula]

[0068] AMX.sub.3

[0069] In Chemical Formula 1, [0070] A is CH(NH.sub.2).sub.2.sup.+, CH.sub.3NH.sub.3.sup.+ or NR.sub.4.sup.+, wherein each R is independently a hydrogen atom or a C.sub.1-C.sub.10 alkyl group, [0071] M is Pb, Sn, Bi, Ge, Ga, Ti, In, Sb or Mn, and [0072] X is I.

[0073] Specifically, the halide perovskite represented by Chemical Formula 1 may be FAPbI.sub.3 (formamidinium (FA) lead triiodide). When the halide perovskite is used, the finally prepared perovskite solar cell has high thermal stability and a broader photoresponse range in the solar spectrum than MAPbI.sub.3.

[0074] The electron transport layer may exhibit a shoulder peak between 2.0 and 2.5 ? in K-edge XAFS (X-ray absorption fine structure) analysis. The shoulder peak is a SnP peak resulting from the passivation of the oxidized black phosphorus quantum dots (0-BPs).

[0075] In the electron transport layer, the oxidized black phosphorus quantum dots (0-BPs) may be located throughout the electron transport layer or at the interface of the electron transport layer and the halide perovskite photoactive layer. Because the oxygen vacancies may result in interfacial defects, the role of the oxidized black phosphorus quantum dots (O-BPs) located at the interface may be much greater.

[0076] The front electrode may contain any one selected from ITO (indium tin oxide), FTO (fluorine-doped tin oxide), GZO (gallium zinc oxide), IZO (indium zinc oxide), IGZO (indium gallium zinc oxide), graphene, molybdenum disulfide (MoS.sub.2), single-walled carbon nanotube (SWCNT), multi-walled carbon nanotube (MWCNT) and metal mesh. However, any front electrode material that can be used in a perovskite solar cell may be used without limitation.

[0077] The hole transport layer may contain any one selected from spiro-OMeTAD (2,2,7,7-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9-spirobifluorene), PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate), P3HT (poly(3-hexylthiophene-2,5-diyl)), PTAA (poly(t-arylamine)), PCBTDPP (poly[N-90-heptadecanyl-2,7carbazole-alt-3,6-bis(thiophen-5-yl)-2,5-dioctyl-2,5-dihydropyrrolo[3,4]pyrrole-1,4-dione]), PDPPDBTE (poly[2,5-bis(2-decyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-(E)-1,2-di(2,2-bithiophen-5-yl)ethene]), PCPDTBT (poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4Hcyclopenta[2,1-b:3,4-b]dithiophene-2,6-diyl]]), PCDTBT (poly[N-9-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)]), PFB (poly(9,9-dioctylfluorene-co-bis-N,N-(4-butylphenyl)-bis-N,N-phenyl-1,4-phenylenediamine)), PANI (polyaniline), chloroaluminum phthalocyanine, tetracene, ?-octithiophene, pentacene, lead(II) phthalocyanine, zinc phthalocyanine, copper(II) phthalocyanine, phthalocyanine blue, ?-quaterthiophene, and ?-quinguethiophene. However, any hole transport layer material that can be used in a perovskite solar cell may be used without limitation.

[0078] The back electrode may contain any one selected from gold (Au), silver (Ag), aluminum (Al), graphene, carbon, graphite, single-walled carbon nanotube (SWCNT) and multi-walled carbon nanotube (MWCNT). However, any back electrode material that can be used in a perovskite solar cell may be used without limitation.

[0079] In addition, the present disclosure provides a method for preparing a perovskite solar cell including the electron transport layer for a halide perovskite solar cell.

[0080] First, oxidized black phosphorus quantum dots (O-BPs) are prepared (step a).

[0081] Specifically, the oxidized black phosphorus quantum dots (O-BPs) are prepared as follows.

[0082] Black phosphorus quantum dots (BPQDs) are prepared first by sonicating black phosphorus powder in an organic solvent.

[0083] Specifically, the sonication may be performed sequentially by first sonication at 80-120 W for 8-12 hours and second sonication at 700-900 W for 1-3 hours. More specifically, the first sonication may be performed at 90-110 W for 9-11 hours and the second sonication may be performed at 750-850 W for 1.5-2.5 hours. The production yield of the black phosphorus quantum dots may be improved under the above processing conditions.

[0084] Next, the black phosphorus quantum dots (BPQDs) are oxidized.

[0085] Specifically, the oxidation may be performed by subjecting the organic solvent containing the black phosphorus quantum dots (BPQDs) to relative humidity 15% or lower for 20-50 minutes in order to control the degree of oxidation. More specifically, the oxidation may be performed at relative humidity 5-15% for 25-40 minutes. If the relative humidity exceeds 15%, oxidation may occur nonuniformly because it is difficult to control the speed of oxidation.

[0086] The organic solvent may be isopropyl alcohol (IPA), acetone, dimethylacetamide (DMA), acetonitrile, tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), hexamethylphosphoramide, etc. Specifically, isopropyl alcohol (IPA) may be used.

[0087] Next, a tin oxide precursor solution or a tin oxide solution and a solution containing the oxidized black phosphorus quantum dots (O-BPs) are prepared (step b).

[0088] Specifically, the tin oxide precursor contained in the tin oxide precursor solution may be SnCl.sub.2.

[0089] The tin oxide contained in the tin oxide solution may be SnO.sub.2-x nanoparticles or SnO.sub.2-x quantum dots.

[0090] The organic solvent used to prepare the tin oxide precursor solution, the tin oxide solution and the solution containing the oxidized black phosphorus quantum dots may be isopropyl alcohol (IPA), acetone, dimethylacetamide (DMA), acetonitrile, tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), hexamethylphosphoramide, etc. Specifically, isopropyl alcohol (IPA) may be used.

[0091] The tin oxide precursor solution and the solution containing the oxidized black phosphorus quantum dot may be used separately or in admixture depending on the location of the passivation of oxygen vacancies in the electron transport layer in the next step.

[0092] Then, an electron transport layer containing tin oxide wherein oxygen vacancies are passivated is prepared by coating the tin oxide precursor solution and the solution containing the oxidized black phosphorus quantum dots (O-BPs) on a front electrode substrate and heat-treating the same (step c).

[0093] If necessary, bulk passivation may be achieved by mixing the tin oxide precursor solution and the solution containing the oxidized black phosphorus quantum dots (O-BPs) and then coating on a front electrode substrate. Through this, oxygen vacancies may be passivated as the oxidized black phosphorus quantum dots are distributed both inside the electron transport layer and at the interface with the perovskite photoactive layer.

[0094] Alternatively, interfacial passivation may be achieved at the interface of the electron transport layer and the perovskite photoactive layer by sequentially coating the tin oxide precursor solution and the solution containing the oxidized black phosphorus quantum dots (O-BPs) on a front electrode substrate.

[0095] The improvement of interfacial defects through the passivation of oxygen vacancies in the electron transport layer is achieved by suppressing the generation of iodine interstitials (I.sub.i) in the perovskite through passivation of oxygen vacancies at the interface. A sufficient effect of improving interfacial defects can be achieved simply by coating the oxidized black phosphorus quantum dots (O-BPs) at the interface.

[0096] Next, a halide perovskite photoactive layer is formed on the electron transport layer (step d).

[0097] The halide perovskite photoactive layer may contain the halide perovskite represented by Chemical Formula 1 as described above. For specific details, refer to the above description.

[0098] In addition, since the front electrode and the back electrode can be formed according to a common method for preparing a perovskite solar cell, detailed description thereof will be omitted.

[0099] Particularly, although not described explicitly in the examples described below, perovskite solar cells were prepared while varying the sonication condition and relative humidity during the oxidation in the step (a), the concentration of the tin oxide precursor solution, the concentration of the oxidized black phosphorus quantum dot (0-BP) solution and the volume ratio of the two solutions in the step (b) and the heat treatment condition in the step (c) of the method for preparing a perovskite solar cell according to the present disclosure. As a result of identifying the characteristics of the positive electrode active materials, iodine interstitials were improved at the interface of the electron transport layer and the perovskite photoactive layer, the solar efficiency of the solar cell was remarkably high with little phase transition to structures with semiconductor properties unsuitable for a solar cell, such as ?-FAPbI.sub.3 or PbI.sub.2, and thermal and operational stability were improved when all of the following conditions were satisfied.

[0100] In the step (a), the sonication is performed sequentially by first sonication at 90-110 W for 9-11 hours and second sonication at 750-850 W for 1.5-2.5 hours. In the step (b), the concentration of the tin oxide precursor solution is 50-100 mM for chemical bath deposition, the concentration of the oxidized black phosphorus quantum dot (0-BP) solution is 0.1-0.2 mg mL.sup.?1 and the volume ratio of the two solutions is 100:1-100:3. In the step (c), the heat temperature is performed at 180-220? C.

[0101] Hereinafter, the present disclosure is described specifically through examples.

EXAMPLES

Preparation Example 1: Preparation of Oxidized Black Phosphorus Quantum Dots (O-BPs)

[0102] After adding 0.5 g of black phosphorus powder to 150 mL of IPA (isopropanol), oxygen was removed from the IPA by pumping argon through the solution for 10 minutes. Then, after performing sonication at 100 W for 10 hours, the obtained dispersion was transferred to an iron cup for further sonication (at 800 W for 2 hours). Argon was pumped through the dispersion consistently to prevent oxidation of the black phosphorus. Black phosphorus quantum dots (BPQDs) were prepared after the sonication. After centrifuging the BPQDs in the IPA solution, the purity of the dispersion was increased by washing several times with IPA.

[0103] Then, the solution was placed in a humidity-controlled room (RH<15%) for 30 minutes to control oxidation and oxidized black phosphorus quantum dots (O-BPs) were synthesized. The final concentration of the O-BP-dispersed IPA solution was 0.12 mg mL.sup.?1.

Example 1: Preparation of Perovskite Solar Cell

[0104] A partially etched FTO (fluorine-doped tin oxide) (?7 ?sq.sup.?1) glass substrate was washed sequentially with a detergent solution, acetone and ethanol for 20 minutes. After conducting ultraviolet (UV)-ozone (03) treatment for 20 minutes, a solution of 75 mM SnCl.sub.2.Math.2H.sub.2O dissolved in IPA was spin-coated on a substrate at a speed of 3000 rpm for 30 seconds and then annealed at 200? C. for 30 minutes. In order to prepare tin oxide (SnO.sub.2) with oxygen vacancies passivated, 20 ?L of the O-BP dispersion prepared in Preparation Example 1 was added to 1 mL of the SnCl.sub.2.Math.2H.sub.2O solution.

[0105] In order to prepare deposited SnO.sub.2-x in a chemical bath, 220 mg of SnCl.sub.2.Math.2H.sub.2O, 1 g of urea, 1 mL of HCl and 20 ?L of TGA were dissolved in DI water and the FTO substrate was placed in a chemical bath for 6 hours. After the reaction, the substrate was washed with IPA and DI water in an ultrasonic bath for 5 minutes and then annealed at 170? C. for 1 hour.

[0106] In order to prepare SnO.sub.2-x nanoparticles (SnO.sub.2-x NPs), a tin(IV) oxide dispersion diluted with deionized water (Alfa Aesar) (1:5; v:v) was coated on ITO (indium-doped tin oxide) at 3000 rpm for 30 seconds and then annealed at 150? C. for 1 hour.

[0107] In order to synthesize SnO.sub.2-x quantum dots (SnO.sub.2-x QDs), 80 ?L of SnCl.sub.4 was added to 6 mL DI water and stirred uniformly on a hot plate at 100? C. for 30 minutes. After the synthesis, the solvent of the solution was changed to IPA (10 mL) using a centrifuge and 30 ?L of oxidized black phosphorus quantum dots (O-BPs) dispersed in IPA was added to 1 mL of the solution. The SnO.sub.2-x QDs dispersed in IPA were coated on an ITO substrate at 3000 rpm for 30 seconds and then annealed at 200? C. for 1 hour.

[0108] For the SnO.sub.2-x NPs and QDs, the ITO substrate was used instead of the FTO substrate because of the high roughness of the FTO substrate. The post-processing of the O-BPs in SnO.sub.2-x was achieved in an O-BP dispersion (1 mL of the solution in 9 mL of IPA) diluted by spin coating (at 3000 rpm for 30 seconds).

[0109] UV-O.sub.3 treatment was performed for 15 minutes before deposition of a perovskite film. A perovskite precursor solution was prepared by mixing 1.4 M FAI, 1.4 M PbI.sub.2, 0.023 M MABr, 0.023 M PbBr.sub.2, 0.023 M CsI and 0.5 M MACl in a mixed solvent of DMF and DMSO (85:15 (v:v)). The a phase of FAPbI.sub.3 was stabilized further by adding MAPbBr.sub.3 and CsI.

[0110] The precursor solution was spin-coated through a two-step process (1st: at 1000 rpm for 10 seconds, 2nd: at 5000 rpm for 20 seconds). 1 mL of diethyl ether was poured 5 seconds before the ending of the spin coating process. For passivation of perovskite, 150 ?L of a solution of 10 mM MeO-PEAI (4-methoxyphenethylammonium iodide) dissolved in IPA was loaded on a substrate and spin-coated at 5000 rpm for 30 seconds. The passivated film was annealed at 100? C. for 5 minutes. Subsequently, it was annealed at 150? C. for 15 minutes and then at 100? C. for 10 minutes.

[0111] For a hole transport layer (HTL), 90 mg of spiro-OMeTAD (2,2,7,7-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene) was dissolved in 1 mL of chlorobenzene (CB). Then, 23 ?L of a Li-TFSI solution (520 mg mL.sup.?1 in acetonitrile), 10 ?L of a FK209 Co(III) TFSI solution and 39 ?L of tBP (4-tert-butylpyridine) were added to the spiro-OMeTAD solution as additives. The resulting solution was spin-coated on the perovskite surface at 3000 rpm for 30 seconds.

[0112] For measurement of thermal stability, 10 mg of CuPC (copper(II) phthalocyanine) was dissolved in 1 mL of CB. Then, 7.5 ?L of a Li-TFSI solution (170 mg mL.sup.?1 in acetonitrile) and 5 ?L of tBP were added to the CuPC solution as additives. Finally, 120 nm of an Au counter electrode was deposited by thermal evaporator in a high-vacuum chamber of 10.sup.?6 torr or lower at a speed of 0.8-1.2 ? s.sup.?1.

Comparative Example 1

[0113] A perovskite solar cell was prepared in the same manner as in Example 1 except that the O-BPs prepared in Preparation Example 1 were not used.

Comparative Example 2

[0114] A perovskite solar cell was prepared in the same manner as in Example 1 except that 25 ?L of PA (phosphoric acid) added to 1 mL of a SnCl.sub.2.Math.2H.sub.2O solution was used as a passivating agent instead of the O-BPs prepared in Preparation Example 1.

Comparative Example 3

[0115] A perovskite solar cell was prepared in the same manner as in Example 1 except that 2 mg of DPPO (diphenylphosphine oxide) added to 1 mL of a SnCl.sub.2.Math.2H.sub.2O solution was used as a passivating agent instead of the O-BPs prepared in Preparation Example 1.

Comparative Example 4

[0116] A perovskite solar cell was prepared in the same manner as in Example 1 except that 3 mg of TPPO (triphenylphosphine oxide) added to 1 mL of a SnCl.sub.2.Math.2H.sub.2O solution was used as a passivating agent instead of the O-BPs prepared in Preparation Example 1.

Test Examples

Test Example 1: Analysis of Characteristics of O-BPs

[0117] The synthesized O-BPs were used as a passivating agent to reduce oxygen vacancies in the SnO.sub.2-x layer. The O-BPs were selected as the passivating agent since P?O bonds were suitable candidates for Lewis acid-base passivation. The electronegativity difference between P and O atoms enables formation of a dative bond of the passivation with high binding energy. DPPO (diphenylphosphine oxide), TPPO (triphenylphosphine oxide) and PA (phosphoric acid) used in the comparative examples have been previously reported to improve the photoelectrical property of SnO.sub.2-x. However, compared with these P?O sources, it is expected that the O-BPs of the present disclosure can further improve the photoelectrical property of SnO.sub.2-x by offering multiple P?O bonds and exploiting the excellent electrical properties of the O-BPs themselves.

[0118] In this test example, the power conversion efficiency (PCE) of the perovskite solar cells (PSCs) prepared in Example 1 and Comparative Examples 1-3 wherein SnO.sub.2-x was passivated with various P?O sources was compared. The J-V curves of the PSCs of Example 1 and Comparative Examples 1-3 are shown in FIG. 1. It can be seen that the SnO.sub.2-x sample of Example 1 passivated with O-BPs showed the highest improvement, with the fill factor (FF) improved significantly by 80% or higher and the shunt resistance decreased. This result suggests that the O-BPs are a better source than conventional chemicals having P?O bonds.

[0119] The surface chemical state and size of the O-BPs of Example 1 were analyzed by high-resolution XPS and TEM, and the result is shown in FIG. 2 and FIG. 3. The peaks at about 130 eV are spin-orbit split peaks of the P 2p.sub.3/2 (129.49 eV) and P 2p.sub.1/2 (130.14 eV, ?E=0.81 eV compared with P 2p.sub.3/2) pair. The broad peak at approximately 133.14 eV is assigned to the oxidative state of the O-BPs, demonstrating that the oxygen atoms are bound to the phosphorus (P) atoms of the O-BPs. In addition, the average diameter of the O-BPs was confirmed to be 5 nm from the TEM image.

[0120] That is to say, it was confirmed that the O-BPs with P?O bonds were prepared successfully with a size suitable for application to a PSC, without formation of other unwanted chemical bonds.

Test Example 2: Analysis of Passivation of Oxygen Vacancies in SnO.SUB.2-x

[0121] The characteristics of the chemical phases of SnO.sub.2-x and O-BP-passivated SnO.sub.2-x (hereinafter, denoted as SnO.sub.2) were analyzed and the result is shown in FIGS. 4A to 4E. Quantitative phase analysis was conducted on SnO.sub.2-x and SnO.sub.2 by XPS (A). In addition, the characteristics of the O-BP passivation were analyzed by XANES (X-ray absorption near edge structure) (B) and EXAFS (extended X-ray absorption fine structure) spectroscopy (C). The XANES spectrum of SnO.sub.2-x shifted from the XANES spectrum of SnO.sub.2 to the spectrum of SnO, suggesting that SnO.sub.2-x consists of mixed SnO.sub.2 and SnO whereas SnO.sub.2 follows the spectrum of reference SnO.sub.2. Meanwhile, the Sn K-edge XAFS spectrum was extracted from the XANES spectrum using Fourier transform for analysis of chemical structure characteristics (C). Distinct SnO (1.61 ?) and SnSn (2.54 ? for SnO.sub.2-x) peaks appeared in both spectra. Unlike SnO.sub.2-x, the Sn K-edge XAFS spectrum of SnO.sub.2 shows a shoulder peak between 2.0 and 2.5 ?. This shoulder peak can be seen as the SnP peak resulting from O-BP passivation. These results indicate that the O-BPs are incorporated in the bulk or at the interface.

[0122] Furthermore, the accurate location of O-BP passivation was analyzed by STEM (scanning transmission electron microscopy) and EELS (electron energy loss spectroscopy). In the STEM image, eight consecutive points were selected for each sample and the corresponding EELS O-K edge profile was displayed. As shown in FIG. 4D, the EELS O-K edge profile of SnO.sub.2-x shows difference in the phase and oxidation state of Sn atoms from the SnO.sub.2-x adjacent to fluorine-doped tin oxide (FTO) to the SnO.sub.2-x adjacent to FAPbI.sub.3. That is to say, the SnO phase mixed with the SnO.sub.2 phase appears both in the bulk and at the interface, suggesting that the two sites are involved in charge loss along the SnO.sub.2-x layer. In contrast, from FIG. 4E, it can be seen that distinct SnO.sub.2 phase peaks are observed at all the eight consecutive points without impurity peaks in the EELS O-K edge profile of SnO.sub.2. Because the passivation effect occurs both in the bulk and at the interface, it can be seen that the O-BPs are located at both sites and hold greater potential as passivators than other surface passivators.

Test Example 3: Analysis of Charge Transport from FAPbI.SUB.3 .to SnO.SUB.2-x .or SnO.SUB.2

[0123] The result of analyzing the charge transport characteristics of SnO.sub.2-x and SnO.sub.2 is shown in FIGS. 5A to 5D.

[0124] Specifically, the analysis result using TRPL (time-resolved photoluminescence) measurement is shown in FIG. 5A. As the oxygen vacancies were reduced, a higher portion of electrons participated in fast charge transport process (?.sub.1) rather than in slow nonradiative (trap-mediated) recombination (?.sub.2) and radiative (band-to-band) recombination (?.sub.3). This result is in good agreement with the nonradiative quenching mechanism via oxygen vacancies of SnO.sub.2-x.

[0125] Confocal PL measurement was further carried out to analyze the uniformity of charge quenching within the film and the result is shown in FIG. 5B. Overall charge extraction from FAPbI.sub.3 to the SnO.sub.2 layer was improved and the uniformity was also improved. Average PL intensity was decreased by 20% after the vacancy passivation. The reduction of oxygen vacancies resulted in the decreased number of nonradiative recombination channels, thereby improving PL quenching and film uniformity.

[0126] The result of KPFM (Kelvin probe force microscopy) analysis is shown in FIG. 5C. After the vacancy passivation, the surface potential of SnO.sub.2-x was increased greatly. In addition, the energy band alignments of SnO.sub.2-x and SnO.sub.2 are schematically shown in FIG. 5D. It was confirmed that the increased surface potential can induce a smaller conduction band minimum (CBM) offset between FAPbI.sub.3 and SnO.sub.2 by upshifting the Fermi level toward the CBM of SnO.sub.2.

Test Example 4: Analysis of Phase Impurities at FAPbI.SUB.3./SnO.SUB.2-x .Interface

[0127] The incorporation of O-BPs into the SnO.sub.2-x layer adjusted the electronic properties of the passivated SnO.sub.2-x layer to be suitable for the interface with perovskite. Because the perovskite layer is deposited on the SnO.sub.2 layer, it is necessary to check whether the surface condition of SnO.sub.2 can affect the quality of the perovskite layer after deposition. Therefore, the XRD spectrum and morphology of the perovskite adjacent to SnO.sub.2-x and SnO.sub.2 were analyzed. The result is shown in FIGS. 6A to 6E. In this test example, FAPbI.sub.3 perovskite (Cs.sub.0.05(FAPbI.sub.3).sub.0.95(MAPbBr.sub.3).sub.0.05) using MAPbBr.sub.3 and Cs as phase stabilizers was used.

[0128] In order to investigate the crystal phase of the perovskite film adjacent to SnO.sub.2-x or SnO.sub.2, the perovskite film was peeled off using adhesive epoxy and analyzed by XRD. The result is shown in FIG. 6A. Prior to the XRD analysis, the morphology of the interface was observed by SEM to investigate damage during the peeling process. As shown in FIG. 6B, the perovskite film peeled off from the SnO.sub.2 layer showed no damage except the marks caused by the rough FTO. In contrast, the perovskite film peeled off from the SnO.sub.2-x layer showed additional damages such as irregular cracks and small particles, indicating the possibility of a heterogeneous phase. Unidirectional, regular cracks did not occur in either sample, indicating that no unintentional damage occurred during the peeling.

[0129] After the morphological examination, XRD analysis was performed to track unfavorable phase transition in the perovskite layer depending on aging time. A SnO.sub.2 sample with an insufficient amount of O-BPs for passivation (expressed as SnO.sub.2 (10 ?L)) was used for comparison of the undesired phase transition related with the amount of oxygen vacancies in SnO.sub.2. From the J-V curves of the PSCs in which different amounts of O-BP solutions were added to the SnO.sub.2-x precursor, it was inferred that 20 ?L would be optimum and 10 ?L and 30 ?L would be insufficient or excessive for passivation, respectively.

[0130] For quantitative comparison, the relative intensity of the undesirable phase peak and the (001) peak was compared, and the result is shown in FIG. 6E. The perovskite film for XRD measurement was aged (at 25? C. and RH 35%) and then peeled off from the substrate. The FAPbI.sub.3 exfoliated from SnO.sub.2-x showed obvious increase in the intensity of ?-phase (I.sub.?-phase, 2?=11.54?) and PbI.sub.2 (I.sub.PbI2, 2?=12.60?) peaks. On day 9, the I.sub.?-phase/I.sub.?-phase (?10.sup.4) ratio was 50, which was about 5 times higher than those of the passivated SnO.sub.2 samples (SnO.sub.2 (10 ?L) and SnO.sub.2 (20 ?L)).

[0131] In particular, SnO.sub.2 (20 ?L) showed a very low I.sub.?-phase/I.sub.?-phase (?10.sup.4) ratio of about 10. In the case of PbI.sub.2, SnO.sub.2-x and SnO.sub.2 (10 ?L) showed gradual growth of the PbI.sub.2 phase and the I.sub.?-phase/I.sub.?-phase (?10.sup.4) ratio reached 23 and 18, respectively, on day 9. In contrast, SnO.sub.2 (20 ?L) almost maintained the initial ratio for 9 days with minimal increase from 10 to 12. This shows that the degree of unfavorable phase transition of the perovskite layer is proportional to the amount of oxygen vacancies in SnO.sub.2, indicating that the oxygen vacancies are the driving force of the unfavorable phase transition. Especially, the fastest increase of the I.sub.PbI2/I.sub.?-phase (?10.sup.4) ratio in SnO.sub.2-x shows that PbI.sub.2 is separated from FAPbI.sub.3 or formed from the incomplete transition to the perovskite phase.

[0132] Considering the hydrogen bonding between NH.sub.2 of FA (CH(NH.sub.2).sup.2+) and the I atom of the PbI.sub.6-bound FA cation in the perovskite lattice, the boundary may be unclear because the FA cation lacks the source of hydrogen bonding in the perovskite lattice at the FAPbI.sub.3/SnO.sub.2-x interface. Although the oxygen atom of SnO.sub.2-x may participate in hydrogen bonding, the possibility of oxygen vacancies disappears in that case. Therefore, it is more likely that transition occurs from the a phase to an unfavorable phase in FAPbI.sub.3/SnO.sub.2-x, which is supported by the higher ratio of the 5 phase and PbI.sub.2 in FAPbI.sub.3/SnO.sub.2-x. That is to say, it can be seen that the unfavorable phase is suppressed greatly when the oxygen vacancies are reduced in the adjacent SnO.sub.2-x.

[0133] The STEM image analysis result for the interface of FAPbI.sub.3/SnO.sub.2-x or SnO.sub.2 is shown in FIGS. 7A to 7F. Specifically, the crystal lattice was analyzed further from the HAADF-STEM (high-angle annular dark-field STEM) image of FAPbI.sub.3/SnO.sub.2-x or SnO.sub.2. The cross-sectional STEM image of the entire solar cell (Au/2,2,7,7-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene (spiro-OMeTAD)/Cs.sub.0.05(FAPbI.sub.3).sub.0.95(MAPbBr.sub.3).sub.0.05/SnO.sub.2-x/FTO) is shown in FIG. 7A. The uniform and dense single-layer perovskite is adjacent to FAPbI.sub.3/SnO.sub.2-x. The result of analyzing d-spacings and Miller indices using HAADF from the magnified STEM image of the FAPbI.sub.3/SnO.sub.2-x interface is shown in FIG. 7B. In addition, the d-spacings of the crystal were measured for 4 selected interfacial sites of FAPbI.sub.3/SnO.sub.2-x, and the result is shown in FIG. 7C as well as crystal directions. The separate lattices exhibited different spacings and directions. The interplanar spacings of 0.33 nm and 0.37 nm matched well with the (002) and (111) crystal directions of ?-phase FAPbI.sub.3, respectively. The PbI.sub.2 and ?-phase FAPbI.sub.3 located near the FAPbI.sub.3/SnO.sub.2-x interface were in good agreement with the XRD result. The presence of PbI.sub.2 and ?-phase FAPbI.sub.3 even without the exfoliation process provides the reliability for the XRD result and also shows that SnO.sub.2-x forms an interface with PbI.sub.2 and mixed a/?-phase FAPbI.sub.3. In addition, FIG. 7D shows the cross-sectional STEM image of the entire solar cell in which SnO.sub.2-x has been replaced with SnO.sub.2. It can be seen that the morphology of the nanostructure is the same as that of its counterpart on a large scale. FIG. 7E shows the four interfacial sites of FAPbI.sub.3/SnO.sub.2 selected similarly to FAPbI.sub.3/SnO.sub.2-x. In FIG. 7F, the interplanar spacing of the distinct lattices is nearly 0.33 nm. This d-spacing is consistent with ?-phase FAPbI.sub.3. Unlike FAPbI.sub.3/SnO.sub.2-x, most of the FAPbI.sub.3 crystal adjacent to SnO.sub.2 grew into pure ?-phase FAPbI.sub.3 as can be seen from the XRD result.

Test Example 5: Analysis of Iodine Interstitials (I) Induced by Unfavorable Phase Transition

[0134] Density functional theory (DFT) calculation was performed to reveal the driving force of the unfavorable phase transition. Oxygen vacancies were introduced into the system to obtain appropriate conditions for DFT calculation. In SnO.sub.2-x, two types of oxygen exist at the FAPbI.sub.3/SnO.sub.2-x interface. One binds to the Pb atom and Sn atom of perovskite (V.sub.O1) and the other binds only to the Sn atom (V.sub.O2). When the oxygen vacancy formation energy was calculated for introduction of energetically more favorable oxygen vacancies into the system, it was found out that V.sub.O2 is energetically favorable by 0.41 eV than V.sub.O1.

[0135] FIGS. 8A to 8C show results of analyzing the unfavorable phase transition induced between iodine interstitials of tin oxide and the organic cation retention capacity. V.sub.O1 is relatively unsuitable because additional energy is required to break additional PbO bonds to form V.sub.O1. After the selection process, the longest PbI bond length at the interface was calculated (FIG. 8A). The PbI bond length may reflect the distortion of the PbI.sub.6 octahedron and the immobilization of I. According to the calculation, the PbI bond without V.sub.O2 interference was 3.452 ? but it was increased to 3.564 ? after the introduction of V.sub.O2. Oxygen vacancies affect the PbISn linkage by increasing positive charge near the Sn atoms and form a less stable chemical bond between Pb and I. The unstable PbI bond may increase the possibility of the distortion of the PbI.sub.6 octahedron by deviating from the ideal bond angle of PbIPb, which may lead to undesirable perovskite phase transition.

[0136] This distortion changes the iodine-mediated corner-sharing PbI.sub.6 octahedron to the edge-sharing PbI.sub.6 octahedron and further distortion changes the edge-sharing PbI.sub.6 octahedron to the face-haring PbI.sub.6 octahedron. The face-haring PbI.sub.6 octahedron demonstrates phase transition to ?-FAPbI.sub.3. Furthermore, because I is loosely bonded to Pb, it is assumed that the increased PbI bond length promotes the generation of iodine interstitials (I.sub.i). Based on this assumption, the energy of iodine Frenkel pair formation (generation of interstitial atoms at the vacancy sites) was compared with respect to the presence of oxygen vacancies (FIG. 8B). After the introduction of V.sub.O2 to the system, the formation energy was decreased from 2.37 eV to 1.24 eV, making it more likely that iodine interstitials (I.sub.i) would be generated. I.sub.i are known as initiators of ?-FAPbI.sub.3 formation, which lowers the activation energy of the phase transition from ?-FAPbI.sub.3 to ?-FAPbI.sub.3 and accelerates the decomposition process through propagation into the perovskite bulk.

[0137] Therefore, the PbI.sub.6 octahedron is distorted and the undesirable phase transition of ?-FAPbI.sub.3 occurs as shown in FIG. 8C.

[0138] In general, excessive iodine interstitials (I.sub.i) are supplied from an external system. However, the spontaneous generation of I.sub.i at the interface is a serious problem. That is to say, the oxygen vacancies of SnO.sub.2-x increase the PbI bond length at the interface and form I.sub.i, resulting in unfavorable phase transition to ?-FAPbI.sub.3.

Test Example 6: Analysis of Organic Cation Retention Capacity of Tin Oxide

[0139] FA (formamidinium) cation is generally bonded to the PbI.sub.6 cage through hydrogen bonding between the I atom and H atom of FA. For the FAPbI.sub.3/SnO.sub.2-x interface, the 0 atoms of SnO.sub.2-x can provide hydrogen bonding sites in the absence of oxygen vacancies. To prove this, organic cation loss was traced at the FAPbI.sub.3/SnO.sub.2-x (or SnO.sub.2) interface by XPS. Precisely, the perovskite film ((FAPbI.sub.3).sub.0.95(MAPbBr.sub.3).sub.0.05)) deposited on SnO.sub.2-x or SnO.sub.2 was annealed at 85? C. (relative humidity 15%). It was because, among the important factors of perovskite degradation, heat stress accelerates the loss of organic cations mainly through evaporation and decomposition. After the heat treatment, the perovskite film was peeled off from the substrate and the exfoliated surface was characterized by XPS. In addition, the surface of the perovskite exposed to air before the peeling was analyzed for comparison of the degree of cation loss. The result is shown in FIG. 9.

[0140] In the N 1s spectrum of the perovskite film, two main peaks assigned to FA cation (=400 eV) and MA complex (=402 eV, byproduct of organic decomposition of MA cation) were observed. For all the prepared samples, the area ratio of the two peaks was 95/5, which proves that the perovskite film was prepared with the intended stoichiometry. After 48 hours of the heat treatment, the surface of the perovskite peeled off from SnO.sub.2-x showed slight decrease of FA cations, which was significant compared to other two samples. When thermal annealing was performed further for 72 hours, the loss of FA cations was accelerated from 80.71 mol % to 57.03 mol %. In contrast, the surface of the perovskite peeled off from SnO.sub.2 showed remarkable organic cation retention capacity. The molar ratio of the FA cations was decreased only by 6.07 mol % from 94.32 mol %. Surprisingly, the organic cation retention capacity of the perovskite film peeled off from SnO.sub.2 was much higher than that of the perovskite exposed to air before the peeling process. The absence of hydrogen bonding sources for the FA cations and the deleterious interactions with oxygen and moisture may have induced further loss of the FA cations on the surface of the perovskite exposed to air. That is to say, the XPS spectrum analysis result proves that the oxygen atoms at the interface are important in retaining the FA cations in the perovskite lattice.

Test Example 7: Analysis of Performance and Operational Stability of Perovskite Solar Cell (PSC)

[0141] In this test example, the performance of PSCs having SnO.sub.2-x or SnO.sub.2 layers was evaluated. The result is shown in FIGS. 10A and 10B. A FAPbI.sub.3-based PSC was fabricated with the composition Au/spiro-OMeTAD/Cs.sub.0.05(FAPbI.sub.3).sub.0.95(MAPbBr.sub.3).sub.0.05/SnO.sub.2-x or SnO.sub.2/FTO.

[0142] FIG. 10A shows the J-V curves of the maximum PCE for each tin oxide layer. As the oxygen vacancies were decreased in SnO.sub.2-x, i.e., in SnO.sub.2, the PCE was improved from 22.58% to 23.43% due to significant improvement in V.sub.oc and FF. The integrated J.sub.sc values were close to the J.sub.sc value of the J-V curves (integrated J.sub.sc: 24.59 mA/cm.sup.2 (SnO.sub.2-x) and 24.65 mA/cm.sup.2 (SnO.sub.2)). The average performance was improved with V.sub.oc from 1.13 V to 1.16 V, J.sub.sc from 24.37 mA/cm.sup.2 to 24.46 mA/cm.sup.2, FF from 79.48% to 80.11% and PCE from 21.89% to 22.73%. The improvement of V.sub.oc and FF was achieved by the decreased nonradiative recombination of oxygen vacancies and the suppression of 5-phase FAPbI.sub.3. The decrease of oxygen vacancies resulted in the decrease of the shunt resistance (R.sub.sh) of SnO.sub.2 and the increase of FF. Favorable energy band alignment may be induced by suppressing ?-phase FAPbI.sub.3 at the FAPbI.sub.3/SnO.sub.2 interface and V.sub.oc can be increased through this.

[0143] In addition, the hysteresis effect can be alleviated by the synergistic effect. To obtain reliable results, 30 devices were prepared for each condition. The statistical distribution of the PCEs is shown in FIG. 10B.

[0144] The same experiment was carried out for the chemical bath deposition-derived SnO.sub.2-x (CBD SnO.sub.2-x) prepared in Example 1 and other SnO.sub.2-x such as Alfa Aesar SnO.sub.2-x nanoparticles (SnO.sub.2-x NPs) and SnO.sub.2-x QDs. CBD SnO.sub.2-x and SnO.sub.2-x NPs require an aqueous precursor solution that can change the chemical state of O-BPs because the 0-BPs generally interact easily with H.sub.2O. The modified O-BPs may not act as an appropriate passivator for SnO.sub.2-x. Based on this concern, O-BPs were applied first to the precursor for CBD SnO.sub.2-x and SnO.sub.2-x NPs for bulk passivation. When the bulk passivation was applied, the PCE of the two SnO.sub.2-x-based PSCs was decreased. The FF was decreased significantly in both systems, indicating that the O-BPs acted as an insulator that increases the shunt resistance of the SnO.sub.2-x layer rather than as a passivator. The more severe decrease of FF in CBD SnO.sub.2-x suggests that the O-BPs had an additional negative effect on the growth of SnO.sub.2-x, which may be due to hydrochloric acid and the long process time of CBD.

[0145] Then, O-BPs were used as a surface passivator by post-processing with an O-BP dispersion. The post-treated SnO.sub.2-x may exhibit a passivation effect without exposing the O-BPs to an aqueous medium. Unlike the bulk passivation, the surface passivation increased V.sub.oc and FF without change in the chemical state of the O-BPs, thereby improving the PCE of the PSC. This indicates that the O-BPs can properly passivate various SnO.sub.2-x. In addition, the improvement of photovoltaic parameters was similar for the surface passivation to the bulk passivation of SnO.sub.2-x in a nonaqueous medium.

[0146] For further verification, SnO.sub.2-x QDs were synthesized through hydrolysis and dispersed in IPA, which were subjected to bulk passivation by adding O-BPs to the SnO.sub.2-x QD dispersion. After the bulk passivation, the average PCE of the PSC was increased from 19.25% to 20.18%. This result clearly shows that O-BPs can passivate various kinds of SnO.sub.2-x in the same manner.

[0147] Since the withdrawal of FA cations from the perovskite lattice is one of the causes of unfavorable phase transition, the thermal stability of the PSC was evaluated. The result is shown in FIG. 11. Heat can provide sufficient energy for the transport and volatilization of FA ions. The PCE of the PSC was tracked every 3 hours using J-V scan while exposing the PSC to 85? C. After 4 days of heating, the SnO.sub.2-x-based PSC of Example 1 maintained 70% of initial PCE, whereas the SnO.sub.2-based PSC of Comparative Example 1 maintained 90% of initial PCE.

[0148] Finally, operational stability was measured to confirm the stable power output of the cell that exhibited the best performance. The result is shown in FIG. 12. The photocurrent of the cell was monitored for nearly 70 hours in the ambient atmosphere with 0.9 V voltage biasing and light illumination (xenon lamp, 100 mW/cm.sup.2, no UV-blocking filter). The cell was encapsulated using a cover glass and epoxy before the measurement of long-term stability. Whereas the SnO.sub.2-x-based PSC of Comparative Example 1 lost 20% of initial photocurrent within 1 hour after light irradiation, the SnO.sub.2-based PSC of Example 1 maintained 90% or more of initial photocurrent for nearly 625 hours.

[0149] The J-V curve was obtained immediately after the long-term stability measurement for more accurate measurement of PCE deterioration. The result is shown in FIG. 13. The PCE decreased from 23.43% to 20.85%, with 11% of loss from initial PCE. A stable power output was exhibited under constant light irradiation.

[0150] While the exemplary embodiment of the present disclosure have been described, those having ordinary knowledge in the art will be able to change and modify the present disclosure variously through addition, change, deletion, etc. of elements without departing from the scope of the present disclosure defined in the appended claims.