Ag-Incorporated OrganicâInorganic Perovskite Films and Planar

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Ag-Incorporated Organic-Inorganic Perovskite Films and Planar Heterojunction Solar Cells Qi Chen, Lei Chen, Fengye Ye, Ting Zhao, Feng Tang, Adharsh Rajagopal, Zheng Jiang, Shenlong Jiang, Alex K. -Y. Jen, Yi Xie, Jinhua Cai, and Liwei Chen Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b00847 • Publication Date (Web): 24 Mar 2017 Downloaded from http://pubs.acs.org on March 25, 2017

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Ag-Incorporated Organic-Inorganic Perovskite Films and Planar Heterojunction Solar Cells Qi Chen†,‡, Lei Chen†, Fengye Ye†,#,§, Ting Zhao‡, Feng Tang†, Adharsh Rajagopal‡, Zheng Jiang┴, Shenlong Jiang§, Alex K.-Y. Jen‡, Yi Xie§, Jinhua Cai†* and Liwei Chen†,ǁ* † i-Lab, CAS Center for Excellence in Nanoscience, Suzhou Institute of Nano-Tech and NanoBionics, Chinese Academy of Sciences, Suzhou 215123, China ‡ Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195, USA # Department of Chemistry, University of Science and Technology of China, Hefei 230026, China § Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China ┴ Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese

Academy of Sciences, Shanghai 201204, China

ǁ CAS Key Laboratory of Nano-Bio Interface, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China

KEYWORDS: perovskite polarity, planar heterojunction perovskite solar cell, metal incorporation, carrier density, equivalent circuit model

ACS Paragon Plus Environment

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ABSTRACT Controlled doping for adjustable material polarity and charge carrier concentration is the basis of semiconductor materials and devices, and it is much more difficult to achieve in ionic semiconductors (e.g. ZnO and GaN) than in covalent semiconductors (e.g. Si and Ge), due to the high intrinsic defect density in ionic semiconductors. The organic-inorganic perovskite material, which is frenetically being researched for applications in solar cells and beyond, is also an ionic semiconductor. Here we present the Ag-incorporated organic-inorganic perovskite films and planar heterojunction solar cells. Partial substitution of Pb2+ by Ag+ leads to improved film morphology, crystallinity and carrier dynamics as well as shifted Fermi level and reduced electron concentration. Consequently, in planar heterojunction photovoltaic devices with inverted stacking structure, Ag incorporation results in an enhancement of the power conversion efficiency from 16.0% to 18.4% in MAPbI3 based devices, and from 11.2% to 15.4% in MAPbI3xClx

based devices. Our work implies that Ag incorporation is a feasible route to adjust carrier

concentrations in solution-processed perovskite materials in spite of high concentration of intrinsic defects.

Table of Contents Graphic


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Solar cells based on organic-inorganic perovskite active materials with a common chemical formula AMX3, in which A denotes Cs, CH3NH3 (MA), or HC(NH2)2 (FA), M denotes Pb or Sn, and X denotes halide Cl, Br, I, or their combinations such as I1-xClx, have enjoyed a stunning rise of power conversion efficiency ever since the first appearance of organic-inorganic perovskite in dye-sensitized solar cell in 2009.1-16 The prototypical organic-inorganic perovskite, MAPbI3, has a tetragonal structure and 1.4 ~ 1.6 eV band gap,1, 2 and all other organic-inorganic perovskites can be regarded as derivatives of MAPbI3 via elemental substitution on the sites of MA, Pb, or I. Ab initio calculations on the electronic structure of perovskite showed that the valence band and the conduction band are mainly contributed by the outer shell orbits of Pb and I, and hence the organic group MA does not take part in bonding.17, 18 Therefore, the electronic band structure and the corresponding materials properties such as light absorption coefficient, energy gap, and carrier diffusion length etc. can be significantly altered by element substitution on the Pb and/or I sites.19-21 Generally, element substitutions are categorized into isovalent and anisovalent substitutions according to the valence difference between the substituting and substituted ions. Up to date, large amounts of reports focused on isovalent substitution, e.g., Pb substituted by Sn, Sr, Cd, and Ca,21-24 or I substituted by Br, Cl.1, 19, 25, 26 Anisovalent substitution has been widely adopted in semiconductor industry to introduce donor or acceptor states within the band gap for controlled doping, thus determines the polarity of the material, adjusts the carrier concentration and tunes the position of Fermi level (EF).27 Recently, anisovalent substitution in organicinorganic perovskite has also attracted significant attention. Besides the case of self-doping, such as Sn2+ substituted by Sn4+,28 trivalent substitution such as In, Al, Sb, Bi etc.29-32 and monovalent substitution such as Cu, Na, Ag etc.33, 34 have been reported to significantly affect the materials properties as well as solar cell performance. In particular, Mojtaba Abdi-Jalebi et. al. reports Ag


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incorporation in mesoscopic structure perovskite devices, but the power conversion efficiency (PCE) of devices only exhibits a minor improvement from 14.01% to 14.18%.33 The role of Ag is proposed as an additive, similar to Cl, to improve surface coverage of the titania scaffold and reduce sub-band gap optical absorption, but the exact mechanism is unclear. Here we explore the effect of anisovalent substitution of Pb with Ag in organic-inorganic perovskite films and planar heterojunction photovoltaic devices. Ag is chosen as the substituting element because the ionic radius of Ag+ (129 pm) is similar to that of Pb2+ (133 pm),35 thus the substitution shall not severely disturb the crystal structure. Ag+ is proposed to substitute the Pb2+ site in perovskite lattice, and improves film morphology and crystallinity significantly. When codoping with otherwise n-type perovskites, Ag incorporation downshifts EF with respect to the conduction band edge (Ec), and reduces the electron concentration. These effects lead to improved carrier dynamics and balanced charge transport. Importantly, these effects of Ag incorporation are consistently observed in both MAPbI3 and MAPbI3-xClx materials. As a result, the power conversion efficiency (PCE) is enhanced from 16.0% to 18.4% in MAPbI3 based devices and from 11.2% to 15.4% in MAPbI3-xClx based devices.


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Figure 1. X-ray diffraction patterns of MAPbI3 films with different Ag-incorporation concentration from 0%-5.0%. The Ag incorporation concentration is defined as the molar ratio Ag/(Pb+Ag). The films show a tetragonal structure and maximum diffraction intensity at Ag concentration of 2.5%. Table 1. Lattice parameters and energy levels of Ag-incorporated MAPbI3 and MAPbI3-xClx

MAPbI3

MAPbI3-xClx

a

Ag (%) 0 0.5 1.0 2.5 5.0 0 2.5 5.0 7.5 10.0

aa (Å) 8.832 8.851 8.854 8.862 8.874 8.880 8.884 8.886 8.897 8.888

ba (Å) 8.832 8.851 8.854 8.862 8.874 8.880 8.884 8.886 8.897 8.888

ca (Å) 12.54 12.50 12.50 12.50 12.49 12.60 12.61 12.60 12.63 12.63

Ecb (eV) -3.79 -3.78 -3.77 -3.92 -4.03 -3.75 -3.91 -3.95 -3.93 -4.02

EFb (eV) -3.88 -3.97 -4.21 -4.68 -4.80 -3.98 -4.67 -4.72 -4.69 -4.79

Evb (eV) -5.38 -5.37 -5.36 -5.51 -5.63 -5.36 -5.51 -5.55 -5.52 -5.61

Crystal lattice parameters deduced from XRD, b energy level deduced by UPS and Tauc plot

The Ag incorporation in pure iodide (mixed halide) perovskite films was realized via partially replacing the PbI2 (PbCl2) in precursor solution of MAPbI3 (MAPbI3-xClx) with AgI (AgCl) (see the Experimental Section in Supporting Information for details). X-ray diffraction (XRD) patterns of Ag-incorporated MAPbI3 and MAPbI3-xClx films on ITO glass substrates are shown in Figure 1 and Figure S1, respectively; and the derived lattice constants are listed in Table 1 (the fitting method is detailed in Notes S1). Two strong diffraction peaks at 2θ = 14.12°, 28.44° and several weak diffraction peaks at 2θ = 19.94°, 23.46°, 24.48°, 30.20°, 31.88°, 35.20°, 40.70°, 43.16°, 50.56° are observed in the pure MAPbI3 sample. These peaks are assigned as the diffraction from (110), (220), (112), (211), (202), (104), (114), (312), (224), (314), and (404) planes of the perovskite film with a tetragonal crystal structure.3, 36 The diffraction peak positions are almost unchanged upon Ag incorporation, and there is no new diffraction peaks, indicating


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the phase purity and well-preserved crystal structure. Significant changes upon Ag incorporation are manifested in the increase in diffraction intensity and the reduction in full-width-of-halfmaximum (FWHM) of diffraction peaks, which indicate that the crystallinity is enhanced and the grain size is increased after Ag incorporation.

Figure 2. Scanning electronic micrographs of Ag-incorporated MAPbI3 films with 0% (a), 0.5% (b), 1.0% (c), 2.5% (d) and 5.0% (e) Ag concentration. Scale bars are 1.0 µm, and the film thicknesses are ~ 300 nm. Figure 2 and Figure S2 show scanning electronic micrographs (SEM) of the Ag-incorporated MAPbI3 and MAPbI3-xCx films, respectively. The morphological changes in both MAPbI3 and MAPbI3-xClx films are apparent: the grain size clearly increases with the Ag concentration, which is consisted with reduction of FWHM of (110) XRD diffraction peak. Simultaneously, cracks and pinholes develop as Ag concentration increases, leading to reduced coverage of the perovskite film on the substrate. The high crystallinity and large grain size are beneficial for solar cells, but cracks and pinholes are detrimental since they may lead to low shunt resistance or even worse, shorted cells.37 The x-ray photoelectron spectra (XPS) in Figure S3 indicate that Ag has been incorporated into the perovskite materials. The intensities of Ag 3d5/2 and 3d3/2 lines monotonically increase with the Ag concentration. This confirms the presence of Ag in these films and that the Ag content


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increases with Ag fed ratio. Essentially, no significant changes are detected for binding energy positions of other spectrum lines.

Figure 3. (a) X-ray absorption spectroscopy and (b-c) X-ray absorption fine structures near Pb L-edge (b) and Ag K-edge (c) of the 5.0% Ag-incorporated MAPbI3-xClx sample. In panel (b-c), the black circles denote the experimental data, and the red lines are fitting curves. The symbols N and R denote the coordination number (number of iodide ions) and the bond length, respectively. Since XPS primarily probes the surface of the films, x-ray absorption spectroscopy (XAS) is employed to probe Ag in the bulk of the films. The Ag K-edge in XAS of Ag-incorporated MAPbI3-xClx films confirms presence of Ag in the bulk of the films (Figure 3a). Fitting of the extended x-ray absorption fine structure shows that the coordination number of Pb is ~ 5.9 (Figure 3b), consistent with theoretical coordination number of 6 (the fitting details and more parameters are included in Notes S2 and Table S1). The fitting of Ag K-edge yields a coordination number of ~ 4.0. Considering the coordination number of 6 for Pb and 12 for MA, it suggests that Ag ions primarily occupy Pb sites rather than MA sites in the perovskite film. Ag incorporation imparts significant changes in optical and electronic properties of the perovskite material. Figure 4a and Figure S4 show the absorption spectra of the Ag-incorporated MAPbI3 and MAPbI3-xClx films, respectively. The absorption spectrum of Ag-incorporated


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MAPbI3 exhibits improved absorption between 400 nm - 650 nm with increasing Ag concentration, while the optical gap Eg determined by the Tauc plot has little change (variation within 10 meV, Figure S5). The effect of Ag incorporation is similar in MAPbI3-xClx: the absorption spectrum of MAPbI3-xClx exhibits improved absorption for wavelength >500 nm with 0-5.0% Ag concentration and a new peak in the near-infrared range rises gradually as the Ag concentration further increases to 7.5% and 10.0%, while the difference in optical band gap is within 15 meV (Figure S5). These changes could result in a raised upper-limit of photocurrent density (Jph) if all absorbed photons are converted to photocurrent without any loss, which may lead to a higher short-circuit current density (Jsc) (Figure S6).


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Figure 4. (a) Absorption spectra (left axis) and normalized steady-state photoluminescence spectra (right axis) of Ag-incorporated MAPbI3 films on glass substrate. (b) Time-resolved PL decay curves of Ag-incorporated MAPbI3 films on glass substrate. Steady state photoluminescence (PL) spectra of Ag incorporated MAPbI3 films (Figure 4a) show a stable emission peak at ~ 774 nm with > hole concentration nh. Ag incorporation causes the RH changes to more negative values, which is a direct evidence of reduction in electron concentration (the detailed data and interpretation are shown in Notes S3, Figure S7 and Table S4). However, the difficulty to carry out Hall effect measurement in MAPbI3 films due to their high sheet resistance (~ GΩ sq.-1) (Figure S8a) makes any further quantitative interpretation unreliable. This problem was also reported by Sang-Jin Moon et. al.34 To further explore the change in carrier concentration of Ag-incorporated MAPbI3, MottSchottky analysis of the capacitance versus voltage (C-V) curves was carried out (Figure 5c) based on Equation (1): 1 2 = (V − V ) 2 C ε 0ε qA2 N bi

(1)

where C is capacitance, A is device area, ε0 is the vacuum dielectric constant, ε is the relative dielectric constant of the material (~60.2 at low frequency limit for perovskite)38,

39

, q is

elecmentary charge, N is the upper limit of doping density, and Vbi is the built-in potential. Based on this relation, N is inversely proportional to the slope of Mott-Schottky plot (C-2 is plotted against the voltage V).40 For MAPbI3 with 0-1.0% Ag, N decreases as the Ag concentration


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increases from 0% (5.07 × 1016 cm-3) to 0.5% (3.28 × 1016 cm-3) and 1.0% (1.58 × 1016 cm-3), reflecting a reduction in electron concentration, that is consistent with the above interpretation of Hall effect measurements. At high Ag concentration (2.5%, 5.0% and beyond), measured C-V curves show large fluctuation (not shown here), making it difficult to quantify N. The reduction in electron concentration often implies improved electron mobility due to reduced scattering between carriers and/or from defect states. To confirm the improved carrier transport properties in Ag-incorporated MAPbI3 films, the space-charge-limited current (SCLC) method is employed to measure the carrier mobility.41 Figure S8b shows the J-V characteristics of electron only Ag-incorporated MAPbI3 devices with structure of ITO/TiOx/perovskite/[6,6]phenyl-C61-butyric acid methyl ester (PCBM)/Ag. The electron mobility µn is indeed improved by Ag incorporation, which is roughly estimated based on SCLC J=9εε0µV2/8d3 (d is film thickness) in trap-free Child’s regime (J∝V2) at high voltage regime as detailed in Supporting Information. Importantly, it is found that the hole mobility (µh) is also improved by Ag incorporation from hole only devices with structure of ITO/NiOx/perovskite/2,2′,7,7′tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene (Spiro-OMeTAD)/Ag (Figure S8c). This is rationalizable because the hole transport is suppressed in a heavily doped n-type material and the reduction in electron concentration benefits hole transport. The improved charge carrier mobility coupled with a reduction in conductivity (Figure S8a) of MAPbI3 by Ag-incorporation, indicates a reduction in electron concentration (by taking electron as the majority charge carrier according to the energy level diagram) and is consistent with Hall effect measurement and MottSchottky analysis. Dielectric force microscopy (DFM), a contactless imaging method for the characterization of charge carriers,42, 43 was employed to confirm the above discussed effect of Ag incorporation as


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detailed in Notes S4. The control film exhibits little variation in its dielectric response as the gate bias voltage Vg is changed from -2 V to +2 V (Figure S9), which suggests the material is either metallic or heavily doped such that the field effect generated by the gate voltage is not sufficient to tune the carrier concentration.43 In contrast, the Ag-incorporated MAPbI3-xClx film exhibits significant gate-dependent dielectric response with weak signal around Vg = 0 V, and much increased dielectric response as the absolute value of Vg increases at both positive and negative directions, indicating intrinsic-like bipolar property.43 Thus, the clear manifestation of polarity change by DFM measurements further validate the reduction of electron concentration in Agincorporated MAPbI3-xClx films identified above. Therefore, in both MAPbI3 and MAPbI3-xClx films, we definitively establish the reduction in electron concentration upon Ag incorporation via multiple characterization techniques, including Hall effect measurement, Mott-Schottky analysis, SCLC analysis, conductivity measurement and DFM scanning. The results are consistent with the energy band structure evolution determined from UPS measurements, that the EF moves far away from the EC towards the middle of the band gap. The effects of Ag incorporation on photovoltaic performances are examined in planar structure perovskite devices. J-V curves for MAPbI3 based devices with inverted stacking of ITO/NiOx/perovskite/PCBM/Ag under scan rate of 0.1 V/s and AM 1.5G illumination are presented in Figure 6a. More than 18 devices were fabricated for each Ag concentration, and the measurements were carried out in N2-filled glove box. Photovoltaic parameters including opencircuit voltage (Voc), Jsc, fill factor (FF) and PCE with standard deviation (S.D.) are summarized in Table 2. For the pure MAPbI3 control device, the highest PCE obtained is 16.0% with a Voc of 1.09 V, a Jsc of 19.3 mA cm-2, and a FF of 0.76, which lies at the baseline of state-of-the-art


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efficiency of planar structured perovskite solar cells.44 The device performance improves significantly with Ag incorporation concentration of 0.5% (the highest PCE = 18.4%, with a Voc of 1.10 V, a Jsc of 20.6 mA cm-2, and a FF of 0.81) and 1.0% (the highest PCE = 17.9%, with a Voc of 1.10 V, a Jsc of 20.3 mA cm-2, and a FF of 0.80). The hysteresis between forward and backward J-V scans of devices with 0-1.0% Ag concentrations were tested under different sweep rates of 0.01 V/s, 0.1 V/s and 0.1 V/s. The J-V curves (Figure S10 and Table S5) consistently show little hysteresis under these low Ag concentrations. Further increase in Ag concentration to 2.5% and 5.0% results in deterioration of device performance: decreased Voc, Jsc, and FF are observed and significant hysteresis between forward and backward J-V scans also starts to develop and becomes more serious at faster scan rates.


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Figure 6. (a) J-V curves of Ag-incorporated MAPbI3 solar cells with structure of ITO/NiOx/perovskite/PCBM/Ag under AM 1.5G illumination under scan rate of 0.1 V/s. The solid symbols are experimental data and solid lines are fitting curves. (b) The corresponding external quantum efficiency curves.

Table 2. Photovoltaic properties of Ag-incorporated MAPbI3 cells with sweep rate of 0.1V/s and fitting parameters based on equivalent circuit model Ag (%) 0 0.5 1.0 2.5 5.0

Voc (V) 1.09 1.10 1.10 1.05 1.00

Jsc (mAcm-2) 19.3 20.6 20.3 19.6 18.9

FF 0.76 0.81 0.80 0.74 0.69

PCE (%) 16.0(14.7±1.0) 18.4(17.2±1.1) 17.9(16.3±1.3) 15.2(13.6±1.3) 13.0(11.4±1.2)

Jph (mAcm-2) 19.4 20.9 20.5 19.7 19.2

J0 (nAcm-2) 2.3×10-3 3.8×10-7 1.1×10-7 4.4×102 1.3×105

n 1.85 1.36 1.31 2.32 3.29

Rs (Ω·cm2) 2.61 0.66 1.21 1.51 0.20

Rsh (Ω·cm2) 2294 1954 1692 1239 1055

The improved photovoltaic performance is also observed in Ag-incorporated MAPbI3-xClx devices

with

inverted

stacking

of

ITO/poly(3,4-ethylenedioxylenethiophene):

poly(styrenesulfonate) (PEDOT:PSS)/perovskite/PCBM/TiOx/Al. The cells with high Ag concentration > 5.0% are consistently shorted, so only device performance of Ag concentration from 0-5.0% are reported in Figure S11 and photovoltaic parameters of Jsc, Voc and FF are listed in Table S6. The highest PCE obtained for the control devices is 11.2% (Voc = 0.89 V, Jsc = 20.3 mA cm-2, and FF = 0.62). The highest PCE obtained for 2.5% Ag concentration is 15.4% (Voc = 1.07 V, Jsc = 21.5 mA cm-2, and FF = 0.67) and that for 5.0% Ag concentration is 13.6% (Voc = 0.97 V, Jsc = 22.9 mA cm-2, and FF = 0.61). These results are consistent with that of Agincorporated MAPbI3 devices that low concentration Ag incorporation improves device performance, but further increase in Ag concentration deteriorates it. Phenomenologically, multiple factors including improved crystallinity, morphology, light absorption, carrier dynamics as well as the changes in band structure and energy level alignment


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may all contribute to the improved device performance upon Ag incorporation. The improvement in Jsc with low concentration Ag incorporation is corroborated by higher external quantum efficiency (EQE) (Figure 6b), and is consistent with the increased solar absorption as manifested in the calculated upper-limit of Jph improvement (Figure S6) while the perovskite film thickness has little change (Table S7). The improvement in FF is attributed to improved morphology, crystallinity and carrier dynamics. At high Ag concentration, these factors such as the formation of pinholes, downshift of Ev and Ec, increase in non-radiative recombination may all be responsible for the decrease in Voc, Jsc and FF. The poor film morphology and hysteresis between forward and backward J-V scans at high Ag concentration are speculated to be related with the high noise and instability in C-V and SCLC measurements. This may arise from Ag induced halide vacancy (to satisfy charge neutralization) but the exact mechanism needs to be further investigated. The materials and device characterization provide an opportunity to analyse the unique role of anisovalent substitution in perovskite solar cells. The J-V curves of devices are analysed with the Equation (2) derived from an equivalent circuit model: J =J ph -J 0 {exp[

q(V +JRs ) V +JRs ]-1}nkT Rsh

(2)

where Jph, J0, n, Rs, Rsh, k and T are photocurrent density, reverse saturation current density, ideality factor, series resistance, shunt resistance, Boltzmann constant and temperature, respectively.45 The fitted curves are shown in Figure 6a and Figure S11, and the related parameters are listed in Table 2 and Table S6 for Ag-incorporated MAPbI3 and MAPbI3-xClx devices, respectively. While the Voc behaviour differs in MAPbI3 and MAPbI3-xClx based devices (the increase in Voc contributes significantly in the improvement of PCE in MAPbI3-xClx devices, but not in MAPbI3 based devices), a crucial observation is that optimized Ag incorporated


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devices consistently show lower J0 and n than the control devices in both MAPbI3 and MAPbI3xClx

based devices.

The reduction in J0 and n is directly related to partial substitution of Pb with Ag. It is derived in Notes S5 that J0∝ne1/2. Direct carrier concentration characterization such as Hall effect measurements, Mott-Schottky analysis, and conductivity-mobility measurements as well as indirect measurements such as UPS and DFM all indicate that the electron concentration ne in perovskite films is reduced upon low concentration of Ag incorporation, in an effect similar to ptype (co)doping to the otherwise n-type perovskite materials. Transient and steady state PL measurements also indicate that the gap state mediated carrier recombination, i.e. non-radiative recombination, is suppressed in materials with low concentration Ag incorporation. This shall lead to a decreased ideality factor n, since n is related with carrier recombination mode where n=1 indicates a band-to-band recombination dominated mode, while n=2 indicates a gap state assisted recombination dominated mode.27 J0 and n have significant influences on the J-V characteristics of devices. The slope of J-V curve near V=0 (short-circuit condition) is ~qJ0/nkT and the slope near J=0 (open-circuit condition) is ~qJph/nkT. Therefore, reduction in J0 and n leads to small slope near short-circuit point and steep slope near open-circuit point on the J-V curve, which result in higher FF. On the other hand, Voc is approximated as (nkT/q)ln(Jph/J0) assuming a large Rsh and thus shall increase with a lowered J0 and a higher n. In the case of Ag incorporation where J0 and n are reduced together, Voc may increase, decrease, or remain unchanged depending on the competitive effects of the two factors. This may explain the different behavior of Voc in MAPbI3 and MAPbI3-xClx based devices.


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In summary, incorporation of anisovalent substituent Ag in organic-inorganic perovskite films not only improves the morphology, crystallinity and carrier dynamics of the material but also reduces the electron concentration. The EF of the perovskite material moves towards middle of band gap upon Ag incorporation, suggesting a change of transport polarity from n-type to intrinsic-like behaviour. Consequently, photovoltaic performance is significantly improved in Ag incorporated perovskite solar cells. The PCE exhibits an enhancement from 16.0% to 18.4% in MAPbI3 devices and from 11.2% to 15.4% in MAPbI3-xClx devices. Our work has demonstrated the potential of anisovalent element substitution in tuning energy band structures of solution-processed organic-inorganic perovskite materials. It also reveals the importance of polarity control in the performance of planar heterojunction solar cells based on organic-inorganic perovskite materials. Remaining questions, such as the change in optical absorption, the energy band alignment depth profile and hysteresis behaviour in Ag-incorporated perovskite devices, are intriguing topics for future investigations.

ASSOCIATED CONTENT

Supporting Information. Experimental section regarding materials, device fabrication and characterization; XRD analysis, SEM, XPS, absorption spectra, PL analysis, UPS, energy level, Hall effect, DFM, XAS analysis, thickness of Ag-incorporated MAPbI3-xClx films; XPS, UPS, energy level, PL decay analysis, thickness of Ag-incorporated MAPbI3 films; J-V curves and EQE spectrums of Ag-incorporated MAPbI3-xClx devices; SCLC and hysteresis analysis of Agincorporated MAPbI3 devices; upper-limit of Jph, J0 of Ag-incorporated perovskite devices. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION

Corresponding Author *E-mail: (J.C.) [email protected], (Liwei Chen) [email protected]

Notes


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The authors declare no competing financial interest..

Author Contributions Liwei Chen, J.C. and Q.C. conceived the idea, designed the experiments and wrote the paper. For Ag-incorporated MAPbI3 materials and devices, T.Z. measured XRD and SEM; Q.C. fabricated devices and carried out all other related measurements and data analysis. For Ag-incorporated MAPbI3-xClx materials and devices, Z.J. measured XAS; Q.C., F.Y. measured XPS, PL lifetime and DFM; F.T. measured SEM and assisted in Hall effect measurement; Lei Chen and J.C. fabricated Ag-incorporated MAPbI3-xClx devices and carried out all other experiments and data analysis. A.R. and A.J. provided valuable discussions. Liwei Chen supervised the entire project. All of the authors commented on the manuscript. ACKNOWLEDGMENT This work was supported by the Ministry of Science and Technology of China (grant No. YFA0200700), the CAS Research Equipment Development Program (YZ201654) and National Natural Science Foundation of China (Nos: 21625304, 91233104, 61376063, 51473184, and 11504408). Partial support from Collaborative Innovation Center of Suzhou Nano Science and Technology (CICSNST) is also appreciated. Q.C. acknowledges Collaborative Academic Training Program for Post-doctoral Fellows support from CICSNST. L.C. acknowledges the support from Jiangsu Provincial Natural Science Foundation (Grant No. BK20130006). J.C. thanks Prof. Hua Qin for helpful discussions on Hall effect measurement. Dr. Jingyuan Ma at BL14W1 line of the Shanghai Synchrotron Radiation Facility is acknowledged for the XAS measurement. REFERENCES


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Figure 1. X-ray diffraction patterns of MAPbI3 films with different Ag-incorporation concentration from 0%5.0%. The Ag incorporation concentration is defined as the molar ratio Ag/(Pb+Ag). The films show a tetragonal structure and maximum diffraction intensity at Ag concentration of 2.5%. 76x63mm (300 x 300 DPI)


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Figure 2. Scanning electronic micrographs of Ag-incorporated MAPbI3 films with 0% (a), 0.5% (b), 1.0% (c), 2.5% (d) and 5.0% (e) Ag concentration. Scale bars are 1.0 µm, and the film thicknesses are ~ 300 nm. 160x31mm (300 x 300 DPI)


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Figure 3. (a) X-ray absorption spectroscopy and (b-c) X-ray absorption fine structures near Pb L-edge (b) and Ag K-edge (c) of the 5.0% Ag-incorporated MAPbI3-xClx sample. In panel (b-c), the black circles denote the experimental data, and the red lines are fitting curves. The symbols N and R denote the coordination number (number of iodide ions) and the bond length, respectively. 84x64mm (300 x 300 DPI)


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Figure 4. (a) Absorption spectra (left axis) and normalized steady-state photoluminescence spectra (right axis) of Ag-incorporated MAPbI3 films on glass substrate. (b) Time-resolved PL decay curves of Agincorporated MAPbI3 films on glass substrate. 84x113mm (300 x 300 DPI)


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Figure 5. (a) Ultraviolet photoelectron spectra (UPS) and (b) energy level diagram of Ag-incorporated MAPbI3 films. (c) Mott-Schottky analysis (violet solid lines) of the capacitance versus voltage (C-V) response of Ag-incorporated MAPbI3 devices with structure of ITO/NiOx/perovskite/PCBM/Ag. 177x48mm (300 x 300 DPI)


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Figure 6. (a) J-V curves of Ag-incorporated MAPbI3 solar cells with structure of ITO/NiOx/perovskite/PCBM/Ag under AM 1.5G illumination under scan rate of 0.1 V/s. The solid symbols are experimental data and solid lines are fitting curves. (b) The corresponding external quantum efficiency curves. Manuscript File 80x119mm (300 x 300 DPI)


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