A Study of Inverted-Type Perovskite Solar Cells with Various

nanomaterials Article

A Study of Inverted-Type Perovskite Solar Cells with Various Composition Ratios of (FAPbI3)1−x(MAPbBr3)x Lung-Chien Chen *, Zong-Liang Tseng and Jun-Kai Huang Department of Electro-Optical Engineering, National Taipei University of Technology, No. 1, Sec. 3, Chung-Hsiao E. Rd., Taipei 10608, Taiwan; [email protected] (Z.-L.T.); [email protected] (J.-K.H.) * Correspondence: [email protected]; Tel.: +886-2-2772-2171 Academic Editor: Hao-chung Kuo Received: 5 September 2016; Accepted: 10 October 2016; Published: 13 October 2016

Abstract: This work presents mixed (FAPbI3 )1−x (MAPbBr3 )x perovskite films with various composition ratios, x (x = 0–1), which are formed using the spin coating method. The structural, optical, and electronic behaviors of the mixed (FAPbI3 )1−x (MAPbBr3 )x perovskite films are discussed. A device with structure glass/indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/mixed perovskite/C60 /BCP/Ag was fabricated. The mixed perovskite film was an active light-harvesting layer. PEDOT:PSS was a hole transporting layer between the ITO and perovskite. Both C60 and bathocuproine (BCP) were electron transporting layers. MAPbBr3 was added to FAPbI3 with a composition ratio of x = 0.2, stabilizing the perovskite phase, which exhibited a uniform and dense morphology. The optimal device exhibited band matching with C60 , resulting in a low series resistance (Rsh ) and a high fill factor (FF). Therefore, the device with composition (FAPbI3 )1−x (MAPbBr3 )x and x = 0.2 exhibited outstanding performance. Keywords: MAPbBr3 ; FAPbI3 ; solar cells; perovskite

1. Introduction Organometal halide perovskite solar cells have been intensively investigated owing to their high power conversion efficiency and fabrication in solution at low temperatures. Perovskite solar cells with an efficiency of over 20% have been fabricated [1–4]. The excellent performance of organometal halide perovskite solar cells has two causes: a small bandgap and a large exciton diffusion length. The low absorption bandgap (Eg of CH3 NH3 PbI3 (MAPbI3 ) ~1.5 eV) of organometal halide perovskite can harvest most wavelengths of incident sunlight. The long exciton diffusion length increases the thickness of active light-harvesting layers and ensures efficient carrier generation [5,6]. Moreover, the conventional structure of perovskite solar cells use TiO2 for electron transport, but the TiO2 needs to be processed using high temperatures (500–600 ◦ C), and many applications are limited. Inverted-type perovskite solar cells have been developed for low-temperature process and low hysteresis [7]. Interestingly, the compositional engineering of perovskite materials has been extensively utilized to adjust their bandgap and structural properties for use in efficient perovskite solar cells. CH3 NH3 PbI3−x Brx (x = 0.1–0.15), as an absorbing layer, has been reported to improve the open voltage of photovoltaic devices [8]. CH3 NH3 PbI3−x Clx has been used to increase the exciton diffusion length to improve device performance [9–12]. HC(NH2 )2 PbI3 (FAPbI3 ) [13–16] can reduce the optical bandgap (Eg ~1.48 eV), with an absorption edge of 840 nm, allowing photons to be absorbed over a broader solar spectrum. Accordingly, FAPbI3 absorbs more light than MAPbI3 . Another advantage of FAPbI3 is its thermal stability [17]. It can be processed at a higher temperature than can MAPbI3 . The typical annealing process temperature is approximately 130–170 ◦ C. In particular, Jeon et al. explained that (FAPbI3 )1−x (MAPbBr3 )x can provide a greater balance between electron transport and Nanomaterials 2016, 6, 183; doi:10.3390/nano6100183

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the hole transport in cells, enabling highly efficient perovskite solar cells to be formed using a regular TiO2 mesoscopic structure (>20%) [4]. However, the origin of their favorable performances and their fabrication process are not yet fully understood. In this work, solution-processed (FAPbI3 )1−x (MAPbBr3 )x perovskites were prepared on poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)-coated indium tin oxide (ITO) substrates for use in inverted perovskite solar cells. The optical, structural, and surface properties of the (FAPbI3 )1−x (MAPbBr3 )x perovskite films were studied as functions of the composition ratio (x = 0–1). The dependence of cell performance and the properties of the perovskite films is discussed. 2. Materials and Methods In this work, a PEDOT:PSS (AI 4083) was spin-coated on a pre-cleaned ITO substrate at 5000 rpm for 30 s. Thereafter, the film was annealed at 120 ◦ C for 10 min. The perovskite layer was deposited as in our previous investigation [18]. HC(NH2 )2 I (FAI), PbI2 , CH3 NH3 Br (MABr), and PbBr2 were dissolved in 1 mL of cosolvent, which comprised dimethyl sulfoxide (DMSO) and γ-butyrolactone (GBL) (volume ratio = 1:1), to form perovskite precursor solutions. The mole ratios of FAPbI3 to MAPbBr3 in the mixed perovskite varied from 0 to 1. The concentrations of each precursor were 1.2 M. For example, 0.96 mmol of FAPbI3 and 0.24 mmol of MAPbBr3 (i.e., 165 mg of FAI, 27 mg of MABr, 443 mg of PbI2 , and 88 mg of PbBr2 ) were dissolved in the mixing solvent (1 mL) as a (FAPbI3 )0.8 (MAPbBr3 )0.2 precursor solution. The perovskite precursor solutions were then coated onto the PEDOT:PSS/ITO substrate in two consecutive spin-coating steps, at 1000 rpm and 5000 rpm for 10 s and 20 s, respectively, in a glove box that was filled with highly pure nitrogen (>99.999%). The wet spinning film was quenched by dropping 50 µL of anhydrous toluene at 17 s. After spin coating, the film was annealed at 100 ◦ C for 10 min. Subsequently, C60 , Bathocuproine (BCP), and a silver (Ag) electrode were deposited with thicknesses of 50, 5, and 100 nm, respectively, using a thermal evaporator. The sample was covered with a shadow mask to define an active area of 0.5 cm × 0.2 cm during C60 /BCP/Ag deposition. Figure 1a schematically depicts the complete structure. Material and Device Measurement The crystalline microstructures of the films were determined using a PAN analytical X’Pert Pro DY2840 X-ray diffractometer (PANalytical, Naerum, Denmark) with CuKα radiation (λ = 0.1541 nm). A field-emission scanning electron microscope (GeminiSEM, ZEISS, Oberkochen, Germany) was used to observe the surface morphology of the cells. Photoluminescence (PL) and absorption spectra were measured using a fluorescence spectrophotometer (Hitachi F-7000) and a UV/VIS/NIR spectrophotometer (Hitachi U-4100 spectrometers) (Hitachi High-Technologies Co., Tokyo, Japan), respectively. Current density–voltage (J-V) characteristics were measured using a Keithley 2420 programmable source meter (Keithley, Cleveland, OH, USA) under illumination by a 1000 W xenon lamp. The forward scan rate was 0.1 V/s. 3. Results and Discussion Mixed perovskite film can be flexibly modified by changing the concentration ratio of the precursors [4,19]. The lowest unoccupied molecular orbitals (LUMOs) of FAPbI3 , MAPbBr3 , and C60 are −4.0, −3.6, and −3.9 eV, respectively [20,21]. The bandgap of the mixed (FAPbI3 )1−x (MAPbBr3 )x film is between that of the FAPbI3 film and that of the MAPbBr3 film, from −4.0 to −3.6. To optimize the band matching with C60 , the composition ratio x of FAPbI3 to MAPbBr3 in the mixed (FAPbI3 )1−x (MAPbBr3 )x perovskite is approximately 0.2 because the LUMO of the (FAPbI3 )0.75 (MAPbBr3 )0.25 was determined by the interpolation to be −3.9, as shown in Figure 1b. Therefore, the device with the mixed (FAPbI3 )0.75 (MAPbBr3 )0.25 perovskite film should have the lowest series resistance (Rsh ).

Nanomaterials 2016, 6, 183 Nanomaterials 2016, 6, 183

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Figure Figure1.1.(a) (a)Complete Completestructure structureand and(b) (b)corresponding correspondingenergy energyband banddiagram diagramof of the the structure. structure. Figure 1. (a) Complete structure and (b) corresponding energy band diagram of the structure.

Figure Figure 22 displays displays aa high-resolution high-resolution image image of of the the cross-section cross-section of of the the obtained obtained perovskite perovskite solar solar Figure 2 displays a high-resolution image of the cross-section of the obtained perovskite solarnm), cell configuration, which clearly shows the presence of the layers ITO (200 nm), PEDOT:PSS (~50 cell configuration, which clearly shows the presence of the layers ITO (200 nm), PEDOT:PSS (~50 nm), cell configuration, which clearlynm), shows the presence of the layers ITO (200 nm), PEDOT:PSSperovskite (~50 nm), perovskite perovskite (~250 (~250 nm), nm), C C60 (~60 (~60 nm), and and BCP BCP (~10 (~10 nm). nm). The Thegrain grainsize size ofof the the perovskite isis perovskite (~250 nm),60C60 (~60 nm), and BCP (~10 nm). The grain size of the perovskite is approximately 200 as in Figure 2. Numerous voids (indicated by red arrows) between approximately 200 nm, nm,nm, aspresented presented inin Figure voids(indicated (indicated arrows) between approximately 200 as presented Figure2.2.Numerous Numerous voids by by redred arrows) between grain boundaries were observed. These are characteristic of mixed (FAPbI 3)1-x(MAPbBr3)x perovskite grain grain boundaries werewere observed. These areare characteristic x (MAPbBr 3 )x perovskite boundaries observed. These characteristicof ofmixed mixed (FAPbI (FAPbI33))1-x1− (MAPbBr 3)x perovskite films be to supersaturation and dynamic growth mechanism films and and may may be attributed attributed to the the supersaturation nucleation anddynamic dynamic growth mechanism [22]. films and may be attributed to the supersaturationnucleation nucleation and growth mechanism [22].[22].

Figure 2. Field emission scanning electron microscope (FESEM) cross-sectional image of device structure.

Figure 2. Field emission scanning electron microscope (FESEM) cross-sectional image of device structure. Figure 2.Figure Field emission microscope (FESEM) image3)of device structure. 3a showsscanning the X-rayelectron diffraction (XRD) patterns of cross-sectional (FAPbI3)1−x(MAPbBr x perovskite films

after thermal annealing at various temperatures. The spectrum of the MAPbBr3 film includes three

Figure 3a shows shows the diffraction patterns of (MAPbBr perovskite films main diffraction peaks at 14.04°, which(XRD) correspond to the δ-FAPbI 2, and α-FAPbI 3 phases, Figure 3a X-ray diffraction (XRD) patterns of(FAPbI (FAPbI 33))xxperovskite films 3 3)3)1,1−x −PbI x(MAPbBr respectively. As the value of x in the (FAPbI 3 ) 1−x (MAPbBr 3 ) x perovskite films increases, the position after thermal annealing at various temperatures. The spectrum of the MAPbBr film includes three after thermal annealing at various temperatures. The spectrum of the MAPbBr33 film includes three ◦ , which the α-FAPbI 3 phase shifts considerably to a high of diffraction, theα-FAPbI δ-FAPbI333 and main of diffraction peaks atpeak 14.04 correspond to degree the δ-FAPbI δ-FAPbI PbI22,and , and and α-FAPbI phases, main diffraction peaks at 14.04°, which correspond to the 33,, PbI phases, PbI 2 phase peaks disappear. The coexistence of the two FAPbI3 and PbI2 phases can be observed in respectively. As As the value of xx in (MAPbBr3)3x)xperovskite perovskitefilms filmsincreases, increases,the theposition position respectively. in the the(FAPbI (FAPbI33))11−x −x(MAPbBr ofthe theα-FAPbI α-FAPbI33phase phasepeak peakshifts shiftsconsiderably considerablyto toaahigh highdegree degreeof ofdiffraction, diffraction,and andthe theδ-FAPbI3 δ-FAPbI3and and of PbI2 phase peaks disappear. The coexistence of the two FAPbI3 and PbI2 phases can be observed in


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PbI2 phase peaks disappear. The coexistence of the two FAPbI3 and PbI2 phases can be observed in the (FAPbI layers with FAPbI3 .3.The ofthe theMAPbBr MAPbBr includes 3 )1−x (MAPbBr 3 )x perovskite 3 film the (FAPbI 3)1−x(MAPbBr 3)x perovskite layers with FAPbI The spectrum spectrum of 3 film includes ◦ one diffraction whichcorresponds correspondstoto (100) phase. As presented in Figure one diffractionpeak peakat at 15.03 15.03°,, which thethe (100) phase. As presented in Figure 3b, the3b, thepeak peakposition position increases almost linearly with x, revealing that the crystalline FAPbI and MAPbBr increases almost linearly with x, revealing that the crystalline FAPbI3 and3MAPbBr3 are 3 arehomogeneous. homogeneous.

(a) (FAPbI3)

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Figure X-ray diffraction(XRD) (XRD)patterns patternsofof(FAPbI (FAPbI)3)1−x(MAPbBr 3)x perovskite films with various Figure 3. 3. (a)(a) X-ray diffraction 3 1−x (MAPbBr3 )x perovskite films with various compositions. (b) Relationship between degree of diffraction and composition of (FAPbI (FAPbI3))1−x(MAPbBr 3)x. compositions; (b) Relationship between degree of diffraction and composition of 3 1−x (MAPbBr3 )x .

Figure 4a presents the room-temperature PL spectra of (FAPbI3)1-x(MAPbBr3)x films with various Figure 4a presents thewere room-temperature PL spectra of (FAPbI 3 )1peak −x (MAPbBr 3 )x films with composition ratios that deposited on glass substrates. The PL shifts nonlinearly fromvarious 804 composition ratios that were deposited on glass substrates. The PL peak shifts nonlinearly from 804 to 533 nm as the composition ratio x is increased from 0 to 1, as displayed in Figure 4a. The bandgaps to 533ofnm as the 3composition is increased from to 1,1.5 as displayed in Figureand 4a. The bandgaps MAPbBr and FAPbIratio 3 are xapproximately 2.3 0and eV, respectively, correspond to of MAPbBr 2.3respectively. and 1.5 eV, respectively, and correspond to wavelengths 3 and FAPbI 3 are approximately wavelengths of around 540 and 820 nm, Therefore, bandgap values and PL results of around 540 bandgap and 820 nm, respectively. Therefore, bandgap values and PL results match.from The the bandgap match. The of the mixed (FAPbI 3)1−x(MAPbBr 3)x perovskite films is calculated PL of spectra, the mixed )1−Figure (MAPbBr ) perovskite films is calculated from the PL spectra, as shown as (FAPbI shown 3in 4b. The bandgap over the entire range of the (FAPbI 3 ) 1−x (MAPbBr 3)x x 3 x can be estimated PL spectra. Fitting3 )the spectra3 at room temperature in perovskite Figure 4b. films The bandgap over the from entirethe range of the (FAPbI (MAPbBr )x perovskite films can 1−xPL the following forFitting the bandgap, be yields estimated from theexpression PL spectra. the PLEg: spectra at room temperature yields the following expression for the bandgap, Eg: Eg(x) = 1.5 + 0.2x3 + 0.58x6. (1) 3 the traditional The expression is a sixth-order polynomial, rather than second-order polynomial for(1) Eg(x) = 1.5 + 0.2x + 0.58x6 . compound semiconductors, revealing that the bandgap of the mixed (FAPbI3)1−x(MAPbBr3)x


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The expression Nanomaterials 2016, 6, 183is a sixth-order polynomial, rather than the traditional second-order polynomial 5 of 8 for compound semiconductors, revealing that the bandgap of the mixed (FAPbI3 )1−x (MAPbBr3 )x perovskite films films isisextremely extremely sensitive sensitive totothe thecomposition compositionofofthe themixed mixed(FAPbI (FAPbI 1−x(MAPbBr perovskite 3 )31)− x (MAPbBr33)x)x perovskitefilms filmswhen whenthe theconcentration concentration of of the the FAPbI FAPbI33 is perovskite is low. low. (FAPbI3)

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(b) Figure 4. (a) Photoluminescence (PL) spectra of mixed (FAPbI3)1−x(MAPbBr3)x perovskite films with Figure (a) Photoluminescence spectra(FAPbI of mixed (FAPbI3 )13− (MAPbBr3 )films, films with x perovskite various4.values of x. (b) Bandgap(PL) of mixed 3)1−x(MAPbBr )xxperovskite estimated from various values of x. (b) Bandgap of mixed (FAPbI ) (MAPbBr ) perovskite films, estimated from 3 1−x 3 x PL spectra. PL spectra.

Figure 5 plots the current density as a function of the voltage (J-V) of solar cells that are based on (FAPbI (MAPbBr 3)x films withasvarious composition ratios. 1 cells presents thebased power Figure 35)1−x plots the current density a function of the voltage (J-V)Table of solar that are on conversion efficiency (Eff), short-circuit current density (J sc), open-circuit voltage (V oc), andconversion fill factor (FAPbI ) (MAPbBr ) films with various composition ratios. Table 1 presents the power 3 1−x 3 x (FF) of the (FAPbI 3)1−x(MAPbBr 3)x solar cells. bandgap voltage of the (FAPbI 3)1−x(MAPbBr )x film is efficiency (Eff), short-circuit current density (Jsc ),The open-circuit (Voc ), and fill factor3(FF) of the reduced as the proportion of (MAPbBr 3 ) in the (FAPbI 3 ) 1−x (MAPbBr 3 ) x films increases. The power (FAPbI3 )1−x (MAPbBr3 )x solar cells. The bandgap of the (FAPbI3 )1−x (MAPbBr3 )x film is reduced as conversion efficiency increases with x in the (FAPbI3)1−x(MAPbBr3)x films because Jsc increases with the proportion of (MAPbBr 3 ) in the (FAPbI3 )1−x (MAPbBr3 )x films increases. The power conversion the strength of absorption and the amount of α-FAPbI3 formed. However, the power conversion efficiency increases with x in the (FAPbI 3 )1−x (MAPbBr3 )x films because Jsc increases with the strength efficiency decreases as more MAPbBr 3 is formed owing to a reduction in the photocurrent and series of absorption and the amount of α-FAPbI3 formed. However, the power conversion efficiency resistanceas(R sh). The optimal device, with a (FAPbI3)0.8(MAPbBr3)0.2 perovskite film, exhibited decreases more MAPbBr3 is formed owing to a reduction in the photocurrent and series resistance 2, Voc = 0.88 V, FF = 65.9%, and Eff = 12.0%. MAPbBr3 outstanding performance, = 20.6 mA/cm (Rsh ). The optimal device,where with aJsc(FAPbI 3 )0.8 (MAPbBr3 )0.2 perovskite film, exhibited outstanding was added to FAPbI x = 0.22 ,to stabilize phase a MAPbBr uniform and dense performance, where Jsc3 =where 20.6 mA/cm Voc = 0.88 V,the FF perovskite = 65.9%, and Eff = with 12.0%. 3 was added morphology [4]. xTherefore, the device the (FAPbI 3)0.8(MAPbBr3)0.2 perovskite film exhibited the to FAPbI3 where = 0.2 to stabilize thewith perovskite phase with a uniform and dense morphology [4]. lowest series resistance (R sh), the best FF, and therefore the best performance. Therefore, the device with the (FAPbI3 )0.8 (MAPbBr3 )0.2 perovskite film exhibited the lowest series The bandgap theFF, (FAPbI 3)1-x(MAPbBr3)x film is reduced as the amount of MAPbBr3 in the resistance (Rsh ), the of best and therefore the best performance. (FAPbI3)1−x(MAPbBr3)x film increases. Therefore, by increasing the composition ratio x, the (FAPbI3)1−x(MAPbBr3)x film increases the LUMO of the (FAPbI3)1-x(MAPbBr3)x film to match that of the C60 layer and increases the energy barrier to the transportation of electrons, resulting in a low FF.


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The bandgap of the (FAPbI3 )1−x (MAPbBr3 )x film is reduced as the amount of MAPbBr3 in the (FAPbI3 )1−x (MAPbBr3 )x film increases. Therefore, by increasing the composition ratio x, the (FAPbI32016, )1−x (MAPbBr Nanomaterials 6, 183 of 8 3 )x film increases the LUMO of the (FAPbI3 )1−x (MAPbBr3 )x film to match6that of the C60 layer and increases the energy barrier to the transportation of electrons, resulting in a low FF. The value of Voc oc is is positively positively correlated correlated with with the the difference difference between between the highest occupied molecular orbital (HOMO) of the the (FAPbI (FAPbI33)11−x 3)3x)x film filmand andthe theLUMO LUMOofofthe theCC6060layer layer(Figure (Figure1b) 1b) [23]. [23]. −(MAPbBr x (MAPbBr Therefore, Voc oc is determined by the increase increase in inthe theHOMO HOMOlevel levelofofthe the(MAPbBr (MAPbBr 3 ) x (FAPbI 3 ) 1−x film. ) (FAPbI ) film. 3 x 3 1−x The bandgap (MAPbBr 3)x3 film increases with x, increasing Voc.VAdditionally, the bandgap of of the the (FAPbI (FAPbI33))1−x )x film increases with x, increasing oc . Additionally, 1− x (MAPbBr LUMO levellevel of the 3)x(FAPbI 3)1−x film lower that of C 60, so energy formed, the LUMO of(MAPbBr the (MAPbBr isthan lower than that ofan C60 , so anbarrier energyisbarrier is 3 )x (FAPbI 3 )1−is x film lowering FF. Additionally, the photo-generated current density declines as asthe formed, lowering FF. Additionally, the photo-generated current density declines theproportion proportion of MAPbBr33 in 3)x3 )films increases because thethe absorption range is is reduced. in the the (FAPbI (FAPbI33))1−x increases because absorption range reduced. x films 1−(MAPbBr x (MAPbBr 20 (FAPbI3) (FAPbI3)0.8(MAPbBr3)0.2

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Figure 5. J-V J-Vcurves curvesofof perovskite solar (Ag/BCP/C/(FAPbI 60/(FAPbI3)1−x(MAPbBr3)x/PEDOT:PSS/ITO) Figure 5. perovskite solar cell cell (Ag/BCP/C 60 3 )1−x (MAPbBr3 )x /PEDOT:PSS/ITO) obtained under standard 1 sun air mass (AM) 1.5 simulated solar irradiation. irradiation. obtained under standard 1 sun air mass (AM) 1.5 simulated solar Table 1. Parameters Parameters of of solar solar cells cells based basedon onperovskite perovskite(FAPbI (FAPbI)3)1−x(MAPbBr (MAPbBr3))x film with various Table 1. 3 1−x 3 x film with various composition composition ratios. ratios.

(FAPbI3)1-x(MAPbBr3)x (FAPbI3 )1−x (MAPbBr3 )x (FAPbI3) (FAPbI3 ) 3)0.2 (FAPbI3)0.8(MAPbBr (FAPbI3 )0.8 (MAPbBr3 )0.2 (FAPbI3)0.6(MAPbBr3)0.4 (FAPbI3 )0.6 (MAPbBr3 )0.4 (FAPbI 3)0.4(MAPbBr3)0.6 (FAPbI 3 )0.4 (MAPbBr3 )0.6 (FAPbI 3)0.23(MAPbBr 3)0.8 (FAPbI )0.2 (MAPbBr 3 )0.8 (MAPbBr (MAPbBr 3)1 3 )1

Voc (V) Voc (V) 0.60 0.60 0.88 0.88 0.90 0.90 0.90 0.90 0.95 0.95 1.2 1.2

Jsc (mA/cm2) 17.3 17.3 20.6 20.6 17.63 17.63 11.01 11.01 8.86 8.86 7.23 7.23

Jsc (mA/cm2 )

FF (%) Eff (%) Rsh (Ω) 44.0 Eff (%) 5.68 Rsh (Ω) 20.3 44.0 5.68 12.0 20.3 4.6 65.9 65.9 12.0 52.9 9.41 4.6 8.5 52.9 9.41 8.5 51.4 51.4 5.51 5.51 19.8 19.8 49.4 49.4 4.18 4.18 21.2 21.2 47.7 4.19 4.19 30.6 30.6 47.7

FF (%)

4. Conclusions Conclusions In summary, this this work work presents presents mixed mixed(FAPbI (FAPbI3 )3)1-x 1-x(MAPbBr33))xx perovskite perovskite films films with various composition composition ratios ratios (x == 00 to to 1), formed using the spin coating method. The bandgap bandgap over over the entire range of the (FAPbI 3 ) 1−x (MAPbBr 3 ) x perovskite films can be estimated from the PL spectra. Fitting the of the (FAPbI3 )1−x (MAPbBr3 )x perovskite films can be estimated from the PL spectra. Fitting PL yields a sixth-order of the themixed mixed(FAPbI (FAPbI (MAPbBr33)xx the spectra PL spectra yields a sixth-orderpolynomial polynomialfor forthe the bandgap of 3 )13)−1−x x (MAPbBr perovskite films. films. This This result result reveals revealsthat thatthe thebandgap bandgapofofthe themixed mixed(FAPbI (FAPbI 3 ) 1−x (MAPbBr 3 ) x ) perovskite 3 1−x 3 x perovskite films is extremely extremely sensitive sensitive to tothe thecomposition compositionofofthe themixed mixed(FAPbI (FAPbI 3 ) 1−x (MAPbBr 3 ) x perovskite 3 )1 − x 3 x perovskite films when the concentration 3)x with x= concentration of of the theFAPbI FAPbI33isislow. low.The Theoptimal optimaldevice deviceuses uses(FAPbI (FAPbI3)31−x )1(MAPbBr −x (MAPbBr 3 )x with 0.2 exhibited outstanding performance, sc = 20.6 mA/cm mA/cm22,, x = and 0.2 and exhibited outstanding performance,where whereshort-circuit short-circuitcurrent current density density Jsc open-circuit voltage Voc 0.88V, V, fill fill factor factor FF FF == 65.9%, 65.9%, and and power power conversion conversion efficiency efficiency Eff == 12.0%, 12.0%, oc ==0.88 perhaps because the addition of MAPbBr3 to FAPbI3 where x = 0.2 stabilized the perovskite phase with a uniform and dense morphology. The optimum device exhibits band matching with C60, resulting in a low series resistance (Rsh) and high FF. Acknowledgments: Financial support of this paper was provided by the Ministry of Science and Technology of the Republic of China under Contract no. MOST 105-2221-E-027-055.


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perhaps because the addition of MAPbBr3 to FAPbI3 where x = 0.2 stabilized the perovskite phase with a uniform and dense morphology. The optimum device exhibits band matching with C60 , resulting in a low series resistance (Rsh ) and high FF. Acknowledgments: Financial support of this paper was provided by the Ministry of Science and Technology of the Republic of China under Contract no. MOST 105-2221-E-027-055. Author Contributions: Lung-Chien Chen wrote the paper, designed the experiments, and analyzed the data. Zong-Liang Tseng and Jun-Kai Huang prepared the samples and performed all the measurements. All authors read and approved the final manuscript. Conflicts of Interest: The authors declare no conflicts of interest.

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