Temperature-dependent Local Electrical Properties of Organic

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Temperature-dependent Local Electrical Properties of OrganicInorganic Halide Perovskites: In-situ KPFM and c-AFM Investigation Jing-Yuan Ma, Jie Ding, Hui-Juan Yan, Dong Wang, and Jin-Song Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06418 • Publication Date (Web): 28 May 2019 Downloaded from http://pubs.acs.org on May 29, 2019

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ACS Applied Materials & Interfaces

Temperature-dependent Local Electrical Properties of Organic-Inorganic Halide Perovskites: In-situ KPFM and c-AFM Investigation Jing-Yuan Ma,†,‡ Jie Ding,†,‡ Hui-Juan Yan,† Dong Wang†,‡ and Jin-Song Hu*,†,‡ †

Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Key Laboratory of Molecular Nanostructure

and Nanotechnology, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡University

of Chinese Academy of Sciences, Beijing 100049, P. R. China

ACS Paragon Plus Environment

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ABSTRACT

Organic-inorganic halide perovskite materials are emerging as a new class of photoelectric materials for its low-cost, easy-preparation, and, especially, outstanding optoelectronic properties. Although the tremendous efforts have been made on the regulation and optimization of perovskite materials and their microscopic electrical properties for highefficiency solar cells, few reports focus on the evolution of electrical properties with temperature changes, especially at microscopic scale, which will directly affect the device performances at varying temperatures. Here we map the contact potential difference (CPD) and photocurrent distribution of MAPbI3 at different temperatures in situ by Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (c-AFM), emphasizing the different influence of variable temperature and phase transition on the photoelectric properties of grains and grain boundaries (GBs). It is discovered that both the Fermi level and photocurrent decrease as the sample is heated from 30 to 80 °C gradually due to the variation of effective carrier concentration and the degradation of carrier mobility implicated by lattice vibration scattering. The difference between Fermi level at GBs and that on grains ascends first and then descends, peaking at 50 °C, near which MAPbI3 transforms from tetragonal phase to cubic phase. This peak is speculated as a comprehensive consequence of the increasing difference of Fermi level of semiconductors with different doping concentrations and the converging properties of grains and GBs with temperature rising, because the lower ion activation energy of cubic phase at higher temperature facilitates greatly the ions’ movement between grains and GB. The variation trend of the difference of photocurrent is the same. These findings advance the knowledge on the temperature-induced variations of microscopic photoelectrical properties of organic-inorganic hybrid perovskite materials, which may guide the development of strategies for improving their thermal stability.

KEYWORDS: Solar cells, KPFM, c-AFM, PSCs, Grain boundaries


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Introduction Organic-inorganic halide perovskite solar cells (PSCs) have been in the spotlight since CH3NH3PbI3 (MAPbI3) acted as an efficient light absorber in solar cells in 2009. The record power conversion efficiencies (PCEs) of PSCs have soared rapidly from original 3.8% to current 24.2% in just a few years.1-3 As a direct band gap semiconductor with a band gap suitable for visible light absorbing, MAPbI3 is intrinsically an optimal alternative for photovoltaic application.4-6 Moreover, the fantastic set of electronic properties, such as ambipolar carrier transport properties, long charge carrier diffusion length, high charge carrier mobility and lifetime, and large absorption coefficient, makes perovskite a prospective candidate that can match or even surpass crystalline silicon solar cells.7-11 Another advantage for commercialization is the earth abundance of the elements in perovskite and the low-temperature solution-processing for thin film deposition and device fabrication, which diversifies the morphology and properties.12-14 As conspicuous as the remarkable PCEs of MAPbI3 solar cells are their inferior stabilities.15-17 Notoriously, MAPbI3 is exquisitely sensitive to the environment, including temperature, light, humidity and so forth.18-25 The operating temperature of photoelectric devices cannot be comfortable all the time, depending on the circumstances. The thermalization loss induced heating under illumination and extremely low thermal conductivity contribute to elevating the operating temperature (above 60 °C) for perovskite films under continuous illumistration.26-27 Therefore, the accurate knowledge of the temperature dependence of electric properties such as the Fermi levels, defect concentration as well as the charge-carrier transportation and recombination is undoubtedly a fatal problem for the practicability and commercialization of MAPbI3 solar cells. Atomic force microscopy (AFM) is a well-established scanning probe microscopy (SPM) technique with many additional modules, supplying the information about electrical, mechanical and magnetic properties as well as morphologies of the sample at the nanometer scale. Conventional methods for the characterization of photovoltaic materials, including X-ray diffraction (XRD), photoluminescence (PL), ultraviolet photoemission spectroscopy (UPS), external quantum efficiency (EQE), and I-V curve measurements and so on, usually show a limitation of spatial resolution.28-29 Most of them have been carried out to investigate the temperature-dependent properties of the perovskite.21-22,

30-34

However, photovoltaic materials are commonly polycrystalline or heterogeneous, such as

polysilicon, CdTe, GaAs, copper indium gallium selenide (CIGS), the light-absorbing layer in the organic solar cell as well as organic-inorganic halide perovskite. The impact of grain boundaries (GBs) and grain orientations in


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polycrystalline films, components in heterogeneous phases as well as doping levels on the photoelectric properties cannot be neglected.35-39 Thus, AFM characterization, especially Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (c-AFM), spread out rapidly in the field of inorganic, organic and hybrid photoelectric materials in recent years, as providing more direct evidences for correlating the microscopic morphologies and electrical properties with the device performance.28, 40-43 In view of the lack of knowledge on temperature-dependent microscopic electrical properties of perovskites and their significance in device optimization, we investigated the local electrical properties of MAPbI3 films via in-situ KPFM and c-AFM while varying the operating temperature. With an emphasis on the temperature-dependent transformation in structural and electrical properties rather than the thermal degradation, the evolution of Fermi levels and photocurrents at the nanoscale as temperature changes is revealed. The results indicate that Fermi levels gradually shift down as temperature increases from 30 to 80 °C, and meanwhile, the photocurrents are diminishing. However, the superiority of AFM in spatial resolution capacitates us to explore MAPbI3 film at microscopic level, especially in grain interior and GBs. It is further discovered that the differences of surface potentials and photocurrents between grains and GBs both culminate at 50 °C roughly, which is ascribed to the phase transition from tetragonal phase to cubic phase.

Experimental Section Perovskite Film and Device Fabrication. Methylammonium iodide (CH3NH3I, MAI) was first synthesized by slowly dripping 10 mL HI (57 wt.% in H2O, Sigma-Aldrich) to 18 mL CH3NH2 (33 wt.% in absolute ethanol, Sigma-Aldrich) with O2 removed in the argon atmosphere at 0℃ and keep stirring 2 h more to ensure the complete reaction of HI. The reacted solution was then removed to rotational evaporation at 50 °C for 1 h to obtain white solid, which was subsequently dissolved in ethanol and recrystallized. By washing, filtering and vacuum drying, CH3NH3I was obtained for preparation. FTO (NSG, 15Ω/sq) was cleaned ultrasonically twice in deionized water, ethanol, acetone and isopropyl in sequence for 30 min before use, respectively. It was further treated in a UV-Ozone for 10 min to improve the surface wettability. A compact hole-blocking TiO2 (c-TiO2) in about 50 nm thick was deposited on FTO by spray pyrolysis of 20 mM titanium diisopropoxide bis(acetylacetonate) solution (75 wt.% in 2-propanol, Sigma-Aldrich) diluted in 1-butanol (≥99.8%, Sigma-Aldrich) at 450 °C, followed by heating to 500 °C for 1 h. The as-prepared compact TiO2


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was then treated in a UV-Ozone for 10 min before the deposition of MAPbI3. 576.3 mg PbI2 (99%, Sigma-Aldrich) and 198.8 mg CH3NH3I were added into the mixture of 97.5 mg dimethyl sulfoxide (DMSO, ≥99.9%, Sigma-Aldrich) and 750 mg anhydrous N,N-dimethylformamide (DMF, ≥99.8%, Sigma-Aldrich), filtered before use. The precursor solution (60 μL) was dropped onto the c-TiO2 substrate and spincoated at a speed of 5000 rpm for 10 s. After that, the film was bathed into anhydrous chlorobenzene (≥99.8%, Sigma-Aldrich) for about 15 s, depending on its color change. Subsequently, the film was transferred onto a hotplate at 110 °C for 10 min to evaporate the solvents and crystallize the perovskite. 2,2’,7,7’-tetrakis[N,N-di(4methoxyphenyl)amino]-9,9’-spirobifluorene (spiro-OMeTAD) solution was formulated by 72.3 mg spiro-OMeTAD (LT-s922, Lumtec, Taiwan) in 1 mL chlorobenzene, 28.8 μL 4-tert-butylpyridine (96%, Sigma-Aldrich) and 17.5 μL Bis(trifluoromethane) sulfonamide lithium salt (Li-TFSI) solution (520 mg Li-TFSI dissolved in 1mL acetonitrile, 99.8%, Sigma-Aldrich), 60 μL solution was spin-coated on the perovskite film at a speed of 4000 rpm for 30 s when it cooled down to room temperature . Finally, 80 nm Au was thermally evaporated as top electrode to finish the device fabrication. Measurement and Characterization All the AFM characterizations were carried out using an AFM microscope (MFP-3D bio, Oxford Instruments Asylum Research Inc.) in air with a polyheater and an environmental controller to achieve the required temperature and deliver local photoelectric properties of the sample at variable temperatures in situ. Both KPFM and c-AFM were carried out with conductive probes (NSG03/Au, NT-MDT). In KPFM measurements, the sample was grounded. While c-AFM requires the connection of the sample with an Asylum Research ORCA cantilever holder. The continuous visible light (400~800 nm) illumination, derived from a Sun Full Spectrum Light Source (NBet, Beijing) optical fiber at 20 mW/cm2, was applied to the sample surface at an angle of ~45° so that the incident light wasn’t blocked by the probe. The light intensity reaching the sample surface was measured to be 14 mW/cm2. The characterizations were performed at every 10 °C as the temperature rises up from 30 to 80 °C and 2 min was reserved for thermal equilibrium and stabilization of atmosphere. Ultraviolet photoemission spectroscopy (UPS) was conducted on a Thermo Scientific ESCALab 250Xi with a He I (21.2 eV) source. The chamber pressure was below 6.0E-9 Torr. X-ray diffraction (XRD) analysis was performed in vacuum on a PANalytical Empyrean Powder X-ray diffractometer equipped with a Cu K1 radiation (  1.541 Å) operating at 40 kV and 40 mA with an HTK 1200N (Anton Paar) stage. The initial temperature was set at 25 °C. The diffractograms were collected at the following 17 temperatures: 30, 35, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60,


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65, 70, 75, 80 °C. 2 min was given for thermal equilibrium before each measurement. Masked devices were tested using a Keithley 2420 at AM1.5G and 100 W/cm2 illumination conditions, produced by a solar simulator (450W Model 91150, Newport). A hotplate was used to control the device temperature at 30, 40, 50, 60, 70, 80 °C. The photocurrent-voltage (J-V) curve was recorded when it remained stable.

Results and Discussion

Figure 1. (a) Schematic illustration of the in-situ AFM experiments. The sample structure is glass/FTO/cTiO2/MAPbI3 from bottom to top with a temperature-control equipment. (b) Typical AFM topographic image and (c) corresponding CPD distribution of the MAPbI3 perovskite film. (d) Typical AFM topographic images and (e) corresponding photocurrent distribution of the MAPbI3 perovskite film.


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Photovoltaic devices with a structure of FTO/compact-TiO2/mesoporous-TiO2/CH3NH3PbI3/spiro-OMeTAD/Au were firstly fabricated and characterized. A power conversion efficiency of 19.5% is achieved, demonstrating the good quality of the prepared MAPbI3 films (figure S1). The in-situ temperature-dependent KPFM and c-AFM measurements are illustrated in Figure 1a. The sample structure is glass/FTO/c-TiO2/MAPbI3 from bottom to top with a temperature-control equipment below. There is no hole-transport layer (HTL), conducive to the focus on the photoelectric properties of the perovskite thin film itself. The corresponding surface topography and contact potential difference (CPD) (Figure 1b, c) as well as the corresponding surface morphology and photocurrent (Figure 1d, e) can be obtained by KPFM and c-AFM, respectively. Generally, the 3.0 × 3.0 μm2 maps of morphology and corresponding CPD reveal that the surface properties of MAPbI3 film are quite uniform although the crystal grains is relatively small as the CPD of each grain is basically the same regardless of the morphology. As for c-AFM characterization, photocurrent through each grain is uniform, illustrating its good crystallinity. But the photocurrent distributions vary in some congeries consisting of several adjacent grains with similar photocurrent, showing inhomogeneity underside, including the internal of perovskite and c-TiO2. The dark patches dispersing on the surface of the sample suggest that the currents though these grains are zero under illumination. It could be ascribed to two possible reasons: (1) these grains are not MAPbI3, maybe corresponding PbI2 or other impurities derived from the reagents and during reaction process; (2) these grains do not penetrate the film to connect with electrode, indicating that the carriers transferred trough these grains longitudinally have to transport across the grain boundaries once more, compared to other carriers.41, 44 The uniform CPD of the sample shown above excludes the former reason. The difference in local electrical properties between grains and GBs is commonly seen in polycrystalline film. In this case, the CPD at GBs is lower (Figure 1b, c), while the photocurrent at GBs is larger (Figure 1d, e) than those on grains. A lower CPD value indicates a higher work function, thus a deeper Fermi level. Notably, the lower CPD at GBs means that Fermi level at GBs is deeper than that on the grains. This suggests the presence of a potential barrier for electron transport at the GBs due to the upwards band bending at equilibrium, which results from the difference of doping or defect concentration, more I- vacancies and Pb2+ interstitials at GBs while more Pb2+ vacancies and I- interstitials in grains.45 This is in accord with the higher photocurrent at GBs than that of adjacent grains, inferring that the defects at GBs are shallow and the carrier mobility at GBs is higher.46


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Figure 2. (a) AFM topographic image of the MAPbI3 perovskite films at 30 °C. Corresponding CPD distributions of the MAPbI3 perovskite films at (b) 30 °C, (c) 40 °C, (d) 50 °C, (e) 60 °C, (f) 70 °C, and (g) 80 °C. (h) Average CPD of the MAPbI3 perovskite film as a function of temperature from 30 to 80 °C.

Figure 2 shows the KPFM results over MAPbI3 surfaces. Figure 2a is the initial morphology of the sample, which does not change as the temperature varies from 30 to 80 °C (Figure S2). However, keeping the imaging field constant, the corresponding CPD of the sample decreases gradually as the temperature increases, as shown in Figure 2b-g. To obtain the reliable results and draw out the convincible conclusion, parallel experiments were carried out. Among them, in-situ KPFM characterization of the sample performed after it restored to 30 °C from 80 °C (Figure S3) exhibit little differences in topographies and CPD distributions before and after heating. This indicates that the sample didn’t decompose during the experiment. Figure 2h presents the variation of average CPD with temperature. An average CPD on the square scanning area is calculated to evaluate the temperature-dependent shift of Fermi level of MAPbI3, which is quite important for semiconductor materials. The average CPD of the sample at 30 °C in dark condition is 437 mV and inclines downward as the temperature increases, consistent with the temperature dependence of Fermi level of a n-type semiconductor. This indicates that the as-prepared perovskite is n-type, as proven before.45, 47


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To further illuminate the effect of temperature on the carrier transportation and recombination of perovskite, the c-AFM measurements were carried out to explore the photocurrent distribution over the films under illumination in the temperature range from 30 to 80 °C. Figure 3a-g shows the surface morphology and corresponding photocurrent mapping of the sample at varied temperatures. As temperature goes up, the average photocurrent diminishes. Similar to temperature-dependent KPFM measurements, parallel experiments are also implemented in c-AFM measurements. Quantitative data, the average photocurrent-temperature curve presented in Figure 3h, shows that the photocurrent decreases from 40.10 to 7.77 pA with the increase of temperature from 30 to 80 °C. It is acknowledged that the charge carriers increase under illumination due to the energy from the absorbed photons causing charge carriers (holes and electrons) separating, and transfer directionally under the influence of built-in electric field. Recombination occurs at the compound site in the progress of migration. When the temperature rises, the Fermi level shifts down as aforesaid, resulting in the weakening of built-in electric field. Meanwhile, carrier is almost constant, as the impurities have been completely ionized and the intrinsic excitation is not striking within the experimental temperature. While lattice vibration scattering, as the principal contradiction, increases the resistance during carrier migration, reducing carrier mobility. The comprehensive effect of these factors thus leads to the negative correlation between photocurrent of the sample and temperature, as indicated by the above experimental results.


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Figure 3. (a) AFM topographic image of the MAPbI3 perovskite film at 30 °C. Corresponding photocurrent distributions of the MAPbI3 perovskite films at (b) 30 °C, (c) 40 °C, (d) 50 °C, (e) 60 °C, (f) 70 °C and (g) 80 °C. (h) Average photocurrent of the MAPbI3 perovskite films as a function of temperature from 30 to 80 °C.

Figure 4. (a) Temperature-dependent XRD patterns of the MAPbI3 perovskite film and (b) corresponding zoom-in XRD patterns in a 2θ range of 19-25°. Statistical analyses on the differences of (c) CPD and (d) photocurrent between grains and GBs of the MAPbI3 perovskite films as a function of temperature from 30 to 80 °C.

It is noted that the relationship between the average CPD and temperature shown in Figure 2h is non-linear: the average CPD decreases approximately by 23 mV every 10 °C as the temperature rises from 30 to 50 °C, varies little from 50 to 60 °C, and then the reduction rate returns or even exceed that before 50 °C. The reduction rate of average photocurrent also slows down at around 50 °C during heating (Figure 3h). Although subtly, the temperature-


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dependent downward trends of the average CPD and the average photocurrent both have a special point whose abscissa is between 50 to 60 °C, where the reduction rates change. To understand this phenomenon, the temperaturedependent XRD experiments of the MAPbI3 film were carried out. As shown in Figure 4a, b, the {211} reflection at 23.5° reduces gradually towards high temperature and disappears completely at 54 °C, inferring that the phase transition temperature from tetragonal phase to cubic phase is 54 °C. 30, 48-49 Thus, the phase transition at 54 °C does not lead to an abrupt change in Fermi levels or photocurrents, but the temperature-dependences of Fermi level and photocurrent are different in different phases. Furthermore, the differences of CPD and photocurrent between grains and GBs both peak at 50 °C were plotted as shown in Figure 4c and d according to the statistical analyses of parallel experiments. The statistical analysis of grain size and grain boundaries is performed by using the masks derived from surface topography as show in Figure S5. Since the prepared MAPbI3 film is n-type, the effective donor concentration at GBs is higher than that on grains. As the temperature rises, the Fermi level of grains is always higher than GBs’. Accordingly, the difference between the Fermi level at GBs and that on grains is equal to 𝑘0Tln

(

), which is calculated from the formulae of

ND,grains ― NA,grains ND,GBs ― NA,GBs

Fermi level of semiconductors, in accord with the data from 30 °C to 50 °C. The following diminishing difference of CPD at GBs and that on grains indicates their convergent properties, providing evidence for the claim that phase transition from tetragonal to cubic lowers the ion activation energy.49-51 The egregiously lattice-expanding phase transition contributes to a reduction of ion activation energy, which can be satisfied when temperature exceeds the phase transition temperature and the perovskite transforms into cubic phase. Therefore, ions can move easily between GBs and grains when the sample is heated to 60 °C, distributing the ion concentration or doping concentration more equably across the whole perovskite film. The difference in the photocurrent at GBs and that on grains is consistent with the difference in Fermi level, as the magnitude of CPD represents the magnitude of built-in electric filed. Moreover, ion migration due to lower activation energy after phase transition also leads to the convergence of carrier mobility at GBs and grains.

Conclusion In summary, we have deposited MAPbI3 perovskite film on the glass/FTO/c-TiO2 substrate and investigated the influence of temperature on the local electrical properties of MAPbI3 perovskite film in situ by KPFM and c-AFM. The results show that the Fermi level shifts down as temperature rises up from 30 to 80 °C, while the photocurrent


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also decreases. The negative correlation between the temperature and the Fermi level as well as the photocurrent indicates that photo-induced carriers in the n-type perovskite film move slower with the increase of temperature on account of the decrease of built-in electric field and enhancement of lattice vibration scattering. As the effective donor concentration on grains is larger than that at GBs, there is a slight difference in quantity between the variations of Fermi levels at GBs and on grains with the temperature change, peaking at 50 °C; and it’s the same for the photocurrent variations through GBs and grains. We deduce that the emergence of the special point relates to the phase transition from tetragonal phase to cubic phase as the lower ion activation energy in cubic phase leads to ion migration between GBs and grains and their convergent properties, as confirmed by in-situ temperature-dependent XRD experiments. Thus, the increasing difference of Fermi levels at lower temperature resulting from the disparity of effective donor concentrations at GBs and grains decreases gradually after phase transition.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Supplementary AFM images and photovoltaic measurements of the devices (PDF) AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21573249) and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12020100). ABBREVIATIONS KPFM, Kelvin probe force microscopy c-AFM, conductive atomic force microscopy CPD, contact potential difference GB, grain boundary


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Temperature-dependent Local Electrical Properties of Organic

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