Doped Manipulation of Photoluminescence and Carrier Lifetime from

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Doped Manipulation of Photoluminescence and Carrier Lifetime from CH3NH3PbI3 Perovskites Thin Film Lixia Ren, Min Wang, Shuanhu Wang, Hong Yan, Zhan Zhang, Ming Li, Zhaoting Zhang, and Kexin Jin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01506 • Publication Date (Web): 05 Apr 2019 Downloaded from http://pubs.acs.org on April 5, 2019

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ACS Applied Materials & Interfaces

Doped Manipulation of Photoluminescence and Carrier Lifetime from CH3NH3PbI3 Perovskites Thin Film

Lixia Ren, Min Wang, Shuanhu Wang, Hong Yan, Zhan Zhang, Ming Li, Zhaoting Zhang and Kexin Jin∗ Shaanxi Key Laboratory of Condensed Matter Structures and Properties, School of Natural and Applied Science, Northwestern Polytechnical University, Xi’an 710072, China

Lixia Ren and Min Wang contributed equally to this work. 1

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ABSTRACT: The compositional doping techniques can delicately tune the band gap, carrier concentration and mobility of perovskite to optimize the photoelectric properties of materials. It is reported that the doped perovskites have been widely researched in the photovoltaic and photoelectronic field. Here, we show that the photoluminescence intensity and carrier lifetime of CH3NH3PbI3 films have been improved by three-orders of magnitude by incorporating abundant MnAc2∙4H2O in the perovskite precursor solution, which benefits from the morphological change and surface passivation induced by hydration water and surface manganese acetate. We also witness the increased photoluminescence quantum yield for film and the changed power conversion efficiency for perovskite solar cells. More importantly, the enhanced chemical stability of perovskite is displayed by immersing films into the water.

KEYWORDS: Perovskite; MnAc2 insertion; photoluminescence; carrier lifetime; surface passivation

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1. INTRODUCTION As one of the most promising light absorbers, methylammonium lead iodide perovskites (CH3NH3PbI3, denoted as MAPbI3) are receiving more and more research attention due to high absorption coefficient, long carrier lifetime and tunable band gap.1-6 It is reported that the power conversion efficiency (PCE) of perovskite solar cells rises exponentially up to 22.7%.7 Moreover, MAPbI3 has also been employed in other photophysical devices, such as light-emitting diodes,8 lasers,9 and photodetectors,10,

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because the band gap, carrier concentration and mobility of

materials can be delicately tuned by compositional doping techniques.12, 13 As MAPbI3 is one of the crystalline materials with the formula of ABX3, there are three substitution-doping positions in the lattice. Partial substitution of MA+ with formamidinium cation (FA+) can decrease the band gap and hence harvest further near-infrared photons.14 Inorganic Cs+ doping at the MA+ site can further enhance the chemical stability.15, 16 Substitution of Sn2+ at the Pb2+ site is beneficial to obtain higher charge mobility.17 Partially replacing I- with Br- or Cl- can improve the crystallization and morphology of perovskite films.18, 19 In addition, the usage of additive in perovskites has been widely investigated.7, 20-22 For examples, core/shell perovskites induced by in situ Ga addition apply to prepare highly efficient and stable solar cells.7 Sr additive can contribute to the charge extraction and enhance the fill factor in photovoltaic devices.21 Other elements, such as Co, Cu, Fe, Mg, Mn, Ni, and Sn, have been incorporated into perovskite films to improve the morphology of films and eliminate device hysteresis.22 Therefore, as one of the important photoelectric properties, the fluorescent emission of MAPbI3 modified by additive should be further revealed, which would provide a more comprehensive understanding of the carrier generation and recombination mechanisms for perovskites. 3



In this work, we investigate the influence of additive on the photoluminescence (PL) behavior and morphology of perovskite films by adding abundant manganese acetate tetra-hydrate (MnAc2∙4H2O) in MAPbI3 precursor solution. MAPbI3: (MnAc2∙4H2O)x (denoted as MAPbI3:x, x is the molar ratio of MnAc2/MAPbI3 or Mn/Pb, here, x=0, 0.05, 0.1, 0.5 and 0.95) films are deposited on glasses by solution method. The detailed method is described in the experimental section. It is observed that the PL intensity of perovskite films is enhanced to 1.06×106 counts and the carrier lifetime is extended up to 172.7 ns under a 468 nm excitation with increased x, as a result of the passivation effect of surface MnAc2. In addition, the changed PL quantum yield (PLQY), PCE and chemical stability for the counterpart of perovskites are demonstrated.

2. EXPERIMENTAL SECTION 2.1.

Perovskite film Preparation. Glass substrates were cleaned with detergent solution,

deionized water, isopropanol and absolute ethyl alcohol with 15 min of sonication for each step and finally transferred to argon-filled compact plasma sputtering coater for 40 min. Then, the substrates were transferred to an N2-filled glovebox. Methylammonium iodide (CH3NH3I, MAI), lead acetate trihydrate (Pb(CH3CO2)2·3H2O or PbAc2·3H2O) and manganous acetate tetrahydrate (Mn(CH3CO2)2·4H2O or MnAc2·4H2O) were purchased from Xi'an Polymer Light Technology Corp. 0.6 mM of MAI were dissolved in 0.2 mL of anhydrous N, N-dimethylformamide (DMF) and then added to a mixture of 0.2 mM of PbAc2·3H2O and a variable amount of MnAc2·4H2O (0≤Mn/Pb≤0.95) depending on the desired concentration, thus obtaining precursor solution (MAPbI3 is 1M/L). The perovskite layers were prepared by spin-coating precursor solution at 6,000 rpm on glass substrates and the films were annealed at 100 ℃ for 5 min. 2.2.

DEVICE PREPARATION. Solar cells used here employed a device structure of

Glass/FTO/TiO2/Perovskite/Spiro-Ome TAD/Au. Fluoride-doped tin oxide glass sheets (FTO, 10 Ω/sq, NSG group, Japan) were patterned using a laser etcher (LMF-020F, Taiwan) and ultrasonically cleaned with commercial detergent (PK-LCG46, Parker Corporation) and deionized water for 30 min in sequence. FTO substrates were exposed to a UV-ozone environment for 5 min. A thin TiO2 compact layer was deposited on a cleaned FTO substrate by spin-coating using a solution composed of 0.3 M titanium diisopropoxide bis(acetylacetonate) (75 wt% in 2-propanol, Sigma-Aldrich) in 2-propanol (99.9% Sigma-Aldrich) at 4000 rpm for 30 s. The coated FTO substrate was then dried at 120 ℃ for 5 min, followed by calcination at 500 ℃ for 30 min. After cooling to ambient temperature, a TiO2 paste (30-TS, G24 power Ltd. UK) diluted in ethanol was spin coated on the compact TiO2-coated FTO substrate at 6000 rpm for 30 s to form a mesoporous TiO2 film. After coating, the bilayer film was dried at 120 ℃ for 5 min and then sintered at 500 ℃ for 30 min again. Prior to perovskite layer preparation, the bilayer TiO2 film was treated in a 40 mM TiCl4 solution at 70 ℃ for 30 min. After rinsing with deionized water and ethanol sequentially, the film was 4


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dried in air and re-sintered at 450 ℃ for 30 min. After the as-prepared FTO/bilayer TiO2/perovskite film cooled down from annealing, a layer of Spiro-OMe TAD was spin-coated on the top at 4000 rpm for 30 s. The

Spiro-OMe

TAD

solution

was

composed

of

75

mM

2,20,7,70-Tetrakis-(N,N-di-4-

methoxyphenylamino)-9,90-spirobifluorene (spiro-OMeTAD, >99%, Lumtec, Taiwan), 35 mM lithium bis(trifluoromethylsulphonyl)imide (Li-TFSI, 99.95%, Sigma-Aldrich), and 120 mM tert-butylpyridine (tBP, >96%, Sigma Aldrich) in chlorobenzene (99.8%, Sigma-Aldrich). Li-TFSI was predissolved in acetonitrile (99.5%, Merck) at a concentration of 340 mg/ml. Finally, a gold electrode was deposited on top of the Spiro-OMe TAD layer by thermal evaporation. The entire fabrication process was conducted in a controlled environment with a relative humidity level in the range of 30%～45% and temperature of approximately 25 ℃. 2.3.

MEASUREMENTS. X-ray diffraction (XRD) was performed using an X-ray diffractometer (XRD-

7000, Shimadzu) with Cu-Kα radiation source (λ=1.5406 Å) at a step size of 0.02°. The surface morphology of thin films was characterized by using a scanning electron microscope (SEM, JSM-6700F, JEOL). The ultraviolet-visible (UV-vis) absorption spectrum was measured using an ultraviolet-visible spectrophotometer (U-3010, Hitachi). The energy dispersive spectrometer (EDS, Oxford INCA) was used to obtain the content of the element. X-ray photoemission spectra (XPS) were taken using monochromated Al Kα radiation (1486.7 eV) at a resolution of 400 meV. PL spectra were measured by Transient Steady-state Fluorescence Spectrometer (Edinburgh FLS9). The current-voltage response of PVs was examined using a PECL15 solar simulator (Peccell, Japan) equipped with a Keithley 2400 digital source meter (USA) under standard 100 mW/cm2, AM 1.5 G illumination. The light intensity was calibrated through the 91150V model (Newport, UK).

3. RESULTS AND DISCUSSION The perovskite films with different contents of MnAc2∙4H2O are characterized by measuring steadystate PL (SSPL) and time-resolved PL (TRPL) spectra. As shown in the inset of Figure 1a, the PL intensity distinctly is enhanced with the increased x, suggesting that the additive greatly promotes the radiative recombination of perovskite films. SSPL spectra are fitted by Gauss formula to obtain the corresponding intensity and position of PL peak in Figure 1a. The PL intensity increases logarithmically from 1.05×103 counts for the pure film to 1.06×106 counts for the film with x=0.95, expressing three-orders of magnitude improvement under a 468 nm laser excitation at the same excitation power. With the increased x, the position of PL peak exhibits mild blue-shift. Here, it is noticed that the lineshape of SSPL is visible asymmetric. So, we fit the PL results of MAPbI3:x films by

I ( E=) I ex ( E ) + I FC ( E ) ⊗ G ( Γ )

(1), 5



where I is the PL intensity, E is the photon energy, Iex is a Gaussian to describe the exciton recombination, IFC is the PL intensity of free carrier (FC) recombination and G (Γ) is a Gaussian of width Γ to account for the broadening from phonon interactions.23 The PL intensity originating from FC emission and bound exciton emission is shown in Figure 1b.23-25 It is clear that the bound exciton makes a significant contribution to PL emission. Then, FC recombination of samples is summarized in Figure 1c. It is observed that FC recombination becomes stronger with the increased x. Yet, FC recombination of the x=0.95 film is weaker than that of the x=0.5 film and still stronger than that of x=0.05 film. As shown in Figure S1, the enhanced PL intensity is also observed by using a 532 nm laser. Figure 1d shows the logarithm plot of PL intensity versus the excitation power of 532 nm laser. It is known that the integrated PL intensity (I) is a power-law function of the excitation power (L), I∝Lk, which is used to describe the one- or multiple-photon dominated process, with k photon number.26, 27 For a general single- (two-) photon excitation PL, k should be approximately equal to 1 (2). The k of all samples is listed in Table 1 at a series of excitation power (23.9 mW/cm2 ∽ 60.9 mW/cm2). The value of k is less than 2, changing from 0.96±0.06 to 1.37±0.06, which indicates that it is not relevant to the two-photon excitation process in this case.28 The increased k along with the increased x is ascribed to the increased contribution of FC recombination in PL emission process,26 which agrees with the fitted results of Figure 1b. TRPL spectra of the samples are shown in Figure 1e. As noted, the rapid TRPL decay for the pure film implies a short carrier lifetime, whereas the slow decay of MAPbI3:x (x=0.05∽0.95) thin films confirms that the additive can efficiently extend the carrier lifetime. As plotted in Figure 1f, data of radiative decay kinetics are fitted in the form of bi-exponential function and the decay times of τ1, τ2, τtotal are calculated (y = A1 exp(-t/τ1) + A2 exp(-t/τ2), τ1 and τ2 represent the fast and slow 6


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decay time, respectively. τ1 originates from the surface recombination process and can be ascribed to the defect trapping. τ2, generally attributed to recombination, derives from the recombination process taken place in the grains. τtotal is the carrier lifetime, here, τtotal=τ1 ×A1 + τ2×A2). 26, 29 It is observed that τtotal is dramatically extended from 0.89 ns for pure perovskite to 172.7 ns for the film with x =0.95 under 468 nm excitation. Under the low illumination, according to electron-hole radiative recombination and the effective PL lifetime equations,30 I ∝ Bn2 + BNn

(2)

t = 1/(A + Bn0)

(3),

where n is the photoexcited carrier density, B represents the electron-hole radiative recombination coefficient, N is the carrier density caused by unintentional doping in the sample, A is the carrier trapping rate and n0 is the photocarrier density just after the excitation. The enhanced PL intensity and the extended lifetime should be correlated to the increased radiative recombination and the decreased carrier trapping rate, respectively. Here, the substituted effect of Mn2+ can be ruled out in view of the extended lifetime, since it is revealed to generate a shorter lifetime for the heteroatom doping in MAPbI3.31 In order to further investigate the effects of additive in MAPbI3 films, the morphological evolution as a function of x is examined by SEM, because the morphology is crucial for improving the performance of optoelectronic, electronic, and photovoltaic devices.32 As shown in Figure 2, the additive has an obvious influence on the morphology of MAPbI3 films. Pure perovskite film is composed of densely packed crystals, with an average domain size of about 200-300 nm in Figure 2a. The morphology of films changes drastically and the grain size decreases with increasing x in Figure 2b and c. When x≤0.1, MnAc2∙4H2O additive is inclined to refine the grain size by 7



introducing more nucleation sites, changing from 200-300 nm to 50-100 nm. With further increasing x, the nanoscale grains are gradually accumulated into micro-sized clusters, particularly for the film with x=0.95 (Figure 2d and e). It is reported that the hydration water benefits to improve the morphology and electronic properties of perovskite films by reducing the surface roughness and the nonradiative pathways.33 Thus, the significant change of morphology is attributed to the appearance of hydration water induced by MnAc2∙4H2O, and then hydration water would volatilize in perovskite films after annealing.34 Moreover, we also examine the structural evolution as a function of x by using X-ray diffraction (XRD). The typical pattern of tetragonal MAPbI3 phase is noticed in all samples (Figure 2f) and no impure phases (especially, the phase of manganese acetate) are observed.35 The pattern of peaks is basically unaltered, which again rules out the substituted effect of Mn2+ in MAPbI3 lattice. To confirm the distribution of Mn2+ and Ac- in films, studies of X-ray photoelectron spectroscopy (XPS) at the surface of films are performed. Full XPS spectra, valence band spectra, and band alignment of films are shown in Figure 3. A large amount of O is found in the full XPS spectra of films with the increased x in Figure 3a. The valence-band maximum (VBM) is obtained from the edge of valence band spectra in Figure 3b. The optical gap calculated from the edge of UV-visible absorption spectra used the method of Tauc plot remains stable with the variation of x (Figure 3c). Here, it should be noted that the stable optical gap is different from the energy of the band-gap (the blue-shift in PL spectra), which is associated with the band filling.36 The surface MnAc2 passivates effectively trap states to decrease the band filling effect, resulting in a blue shift of PL peak.37 In Figure 3d, the conduction band minimum (CBM) is determined by adding the optical gap to VBM. Theoretically, if Mn atoms occupy Pb sites in perovskite lattice, the CBM and VBM would 8


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continuously shift with the increased x, as the conduction band is mainly derived from the unoccupied Pb p orbitals and the upper valence band is constituted mainly by halogen p orbitals interacted with Pb s orbitals in perovskites.38 For pure MAPbI3 film(x=0), VBM is located at -1.12 eV relative to Fermi level (EF). For films with x > 0, the VBM slightly increases about 80 meV in comparison with the pure film, which seems to be independent on the doping amount. Due to the stable optical gap, the trend of CBM agrees with VBM. These phenomena indicate the Pb site in the lattice might not be occupied by Mn atoms, which is consistent with the result of XRD in Figure 2f. Such a small energy difference (80 meV) further verifies that the electronic doping effect of Mn2+ can be neglected.21 As shown in Figure 4, areas of core level peaks for I 3d, N 1s and Pb 4f become smaller with increasing x, which is incompatible with the chemical stoichiometry of precursor solution (in our work, the molar concentration of I, N, Pb is 1 M/L for all films), whereas areas of the peaks of C 1s and Mn 2p become larger. Here, the emerging peak of C is noticed at 288.9 eV, which originates from C=O bonds.39 A large amount of O (the inset of Figure 3a) and the presence of C=O bonds, which are virtually absent in the pure perovskite, demonstrate the appearance of abundant MnAc2 at the surface of films. The elemental contents are calculated by XPS core level spectra and listed in Table S1. Notably, the Mn/Pb ratio at the surface of perovskite (x=0.5 and 0.95) exceeds the molar ratio used in the precursor solution. The content of elements is also summarized by using energy dispersive spectrometer (EDS) in Table S2. It is observed that the I/Pb and Mn/Pb ratios roughly follow the stoichiometry compound (Table 2), which clearly indicates that abundant MnAc2 is distributed at the surface of films. We assume that MAPbI3 is firstly formed along with the high miscibility of MnAc2 during the spin coating process, which is related to the different nucleation 9



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rates of the perovskite and MnAc2. Generally, the ubiquitous trap states in perovskites would give rise to nonradiative recombination to deteriorate the PL performance of perovskite.28,

40, 41

The

surface MnAc2 is able to effectively passivate surface defects to decrease trap rate,21, 42, 43 leading to the enhancement in PL intensity and the extension in carrier lifetime. In addition, it is obtained that the surface passivation effect of MnAc2 distinctly enhances the chemical stability of perovskites by comparing the colour of perovskite films (x=0 and 0.95) in water, as shown in Figure S2. In order to further exclude the substitution effect of Mn2+ ion in our samples, the precursor solution is prepared by using the mixed MAI and MnAc2 in DMF. However, MAMnI3 is not observed in the XRD result. (Figure S3) Based on the result of SSPL, PLQY, which is deduced by the PL intensity normalized with absorbance,44 also increases near three-orders of magnitude, as shown in Figure 5a. Furthermore, the quality of MAPbI3 film is confirmed by the performance of a testing cell (FTO/TiO2/MAPbI3/Spiro-OMe TAD/Au) in Figure 5b, in which the efficiency of 15.6 % is obtained and summarized in Table 3. However, PCE decreases from 15.6 % to 3.5 % with the increased x.

4.

CONCLUSIONS

We have investigated the influence of MnAc2∙4H2O on the PL performance and morphology of MAPbI3 films. It is found that the PL intensity is enhanced from 1.05×103 counts for pure perovskite film to 1.06×106 counts for the film with x =0.95 and the corresponding carrier lifetime is extended from 0.89 up to 172.7 ns, which benefits from the morphological improvement and surface passivation. Moreover, we obtain the tuned PLQY, PCE and chemical stability of perovskite by adding MnAc2∙4H2O into the MAPbI3 precursor solution. The results not only show the superiority 10


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by adding MnAc2∙4H2O in perovskite but also create an opportunity for future luminescence applications.

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ASSOCIATED CONTENT

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Supporting Information Steady-state PL spectra and the chemical stability of MAPbI3.



AUTHOR INFORMATION

Corresponding Author * E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. 

ACKNOWLEDGEMENTS

This work was supported by the National Natural Science Foundation of China (Nos. 51572222, 61601367 and 11604265) and the Fundamental Research Funds for Central Universities (Grant No. 3102016ZY028). LXR would like to thank Peng Zhai for helpful discussion and the measurement of PCE and Xin Li for PL measurement.

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REFERENCES

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Figures and captions

Figure 1. (a) The PL intensity and position as a function of x. The inset shows steady-state PL spectra of MAPbI3: x films under 468 nm laser. (Here, the strongest PL intensity is obtained by 468 nm excitation.) (b) The normalized PL spectra are fitted with bound-exciton emission plus free carrier (FC) emission. (c) FC emission of different samples. (d) The logarithm plot of PL intensity versus excitation density under a 532 nm excitation. The dots are experimental data and the full lines are linear fitting. (e) The time-resolved PL spectra of films under a 468 nm excitation with 2000 counts. The inset shows the amplifying spectrum of x=0 and 0.05. (f) The fitted parameters of fast (τ1), slow (τ2) and total (τtotal) components as a function of x, respectively.

Figure 2. SEM images of the surface. (a) for pure MAPbI3 thin films and (b)-(e) for films with x= 0.05, 0.1, 0.5, and 0.95. The scale bar is 200 nm. (f) XRD pattern for MAPbI3 thin films with increased x. 15



Figure 3. (a)-(c) Full XPS spectra, valence band spectra, and UV-visible absorption spectra of thin films. The inset of (b) shows the VBM as a function of x. (d) The band alignment of films, here, the conduction band minimum (CBM) is determined by adding the optical bandgap to the valence-band maximum (VBM).

Figure 4. XPS spectra of I 3d, N 1s, C 1s, Pb 4f, and Mn 2p regions for the same set of samples.

16


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Figure 5. (a) The normalized PL quantum yield as a function of x. (b) The current-voltage (J-V) characteristic of solar cells.

Table 1. The value of k for the films under 23.9 ~ 60.9 mW/cm2 excitation power. x

0

0.05

0.1

0.5

0.95

k

0.96±0.06

1.00±0.06

1.16±0.06

1.34±0.05

1.37±0.06

Table 2. The ratio of Mn/Pb and I/Pb from EDS. x

0

0.05

0.1

0.5

0.95

Mn/Pb

0

0.08

0.14

0.66

0.95

I/Pb

2.95

3.34

3.13

3.01

3.38

Table 3. Photovoltaic parameters of perovskite derived from different x. x

0

0.05

0.1

0.5

0.95

Jsc (mA·cm−2)

17.9

16.7

14.7

15.5

7.9

Voc (V)

1.07

1.10

1.08

1.02

0.99

FF

0.81

0.67

0.58

0.32

0.45

PCE (%)

15.6

12.3

9.2

5.1

3.5

17



311x238mm (96 x 96 DPI)


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Doped Manipulation of Photoluminescence and Carrier Lifetime from

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