Insights into Planar CH3NH3PbI3 Perovskite Solar Cells Using

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Insights into Planar CHNHPbI Perovskite Solar Cells Using Impedance Spectroscopy Alexander Robert Pascoe, Noel W. Duffy, Andrew D Scully, Fuzhi Huang, and Yi-Bing Cheng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp509896u • Publication Date (Web): 13 Feb 2015 Downloaded from http://pubs.acs.org on February 15, 2015

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The Journal of Physical Chemistry

Insights into Planar CH3NH3PbI3 Perovskite Solar Cells Using Impedance Spectroscopy Alexander R. Pascoe,† Noel W. Duffy,‡ Andrew D. Scully, § Fuzhi Huang† and Yi-Bing Cheng†* † Department of Materials Engineering, Monash University, Victoria 3800, Australia ‡ CSIRO Energy Flagship, Clayton, Victoria 3169, Australia § CSIRO Manufacturing Flagship, Bayview Avenue, Clayton, Victoria 3169, Australia

1 ACS Paragon Plus Environment


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ABSTRACT Impedance spectroscopy (IS) is emerging as a valuable tool for the characterization of perovskite-based solar cells (PSCs). However, earlier reports of the IS response of mesoscopic PSCs have revealed notable discrepancies, with the interpretation of their spectra having been generalized to planar PSC devices despite fundamental differences in the device operation. The present study analyzes the impedance response of planar PSC devices through the characterization of cells employing a variety of constituent layers. Distinctive high-frequency and low-frequency features are observed in IS measurements and are attributed to the charge recombination across the perovskite/contact interfaces and the dielectric relaxation in the interfacial regions of the perovskite layer, respectively. Comparison of the characteristic IS time constants with time-resolved photoluminescence (TRPL) and open-circuit voltage decay (OCVD) measurements further substantiates the proposed impedance model. This work provides an empirical foundation for the interpretation of impedance spectra in planar PSCs, and develops the prospects of IS as a valuable diagnostic tool for future characterization of planar PSC devices. KEYWORDS: Perovskite, solar cells, impedance spectroscopy, photovoltaics.


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1. INTRODUCTION Hybrid organic-inorganic perovskite-based solar cells (PSCs) have displayed a rapid ascension in performance since first reported 5 years ago.1 Efficiencies of PSC devices are currently nearing the benchmarks of mc-Si based photovoltaics,2 which presents an important milestone in terms of the progression of the technology. In association with these high reported efficiencies, PSCs boast some enviable advantages in terms of their manufacturing requirements. The low-temperature, solution-based processes used to fabricate devices may ultimately lead to greatly reduced processing costs, thereby increasing the economic viability of the technology.3 Recently reported device architectures have also been able to dispense with costly metal contacts while still preserving relatively high conversion efficiencies.4 These benefits, coupled with the trending improvements in device performances, bode well for the future commercialization of PSC technologies. Impedance spectroscopic (IS) analyses of PSC devices have revealed some important information concerning device operation. Previous impedance studies have largely focused on a mesoscopic cell architecture,5-9 although there have also been preliminary studies into the impedance response of planar-structured devices.10, 11 Despite some clear similarities between the previously reported impedance spectra for both planar and mesoscopic devices, the interpretation of these results has presented some notable discrepancies. The observation of two or three distinct impedance features has been a common finding to most studies using a mesoporous TiO2 scaffold. The low/intermediate-frequency feature has been attributed to the accumulation of charge within the perovskite/TiO2 layer.8 This feature typically has been fitted using an RC component or a transmission-line model, as the revealing 45° transport feature is observed in some cell architectures but not others. Reports of an additional low-frequency



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feature have also been linked to charge accumulation at the ferroelectric domain walls of the pervoskite, and this feature has been distinguished by a characteristically slow time constant.12 The high-frequency feature has previously been attributed to the charge transport within the hole-conducting material.10 However, this feature is also observed in the absence of the holetransport layer, and has therefore been linked to a more general charge separation between the perovskite layer and adjacent materials.6 Judging from previous work, it is clear that the device architecture plays an integral role in the interpretation of the impedance response. For this reason it is important that a representative impedance model is established for planar PSC devices, rather than generalizing the formative work already reported for mesoscopic cells. While much progress has been made towards understanding the impedance response of perovskite-based devices, a more comprehensive understanding of PSC impedance spectra is necessary before the technique can be used as a diagnostic tool to compare and contrast devices. In the present study we develop a model for the interpretation of impedance spectra associated with planar perovskite-based photovoltaic devices. The relative electron affinities of the materials contacting the perovskite layer are found to be of key importance to the resulting impedance response. For this reason, the influence of contact materials was explored and the results are related to previous studies using mesoscopic devices. The kinetics of the impedance responses are also compared with photoluminescence (PL) and open-circuit voltage decays (OCVD) in order to ascertain the physical origins of the impedance spectra. The detailed analysis undertaken in this work provides a framework for the use of IS for the comparison of planar PSC devices.


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2. EXPERIMENTAL Solar cells. The synthesis of the CH3NH3PbI3 solution used in this study has been reported previously.13 Glass substrates coated with indium-doped tin oxide (ITO) and fluorine-doped tin oxide (FTO) were cleaned using Hellmanex, distilled water and ethanol (96% purity). The dense TiO2 layer (ca. 30 nm) was formed by spray pyrolysis at 450 °C of titanium diisopropoxide bis(acetylacetonate) (Aldrich, 32525-2) in isopropanol with a volume ratio of 1:9. The perovskite solution (30 µL) was dropped onto an area of ~1cm2 and then spin coated at 6500 rpm. After 2 – 3 seconds a gas-gun was used to blow argon onto the surface of the perovskite film to rapidly remove the solvent composition of the perovskite solution, as has been previously reported.14 The perovskite films were then dried at 100 °C for ≈ 10 minutes. The hole-transport layer was formed by spin coating a solution of 41.6 mg of 2,20,7,70-tetrakis(N,N-bis(pmethoxphenyl)amino)-9,90-spirobifluorene (Spiro-OMeTAD) in 500 µL of chlorobenzene with the standard additives of 7.51 µL lithium bis(trifluoromethylsulphonyl)imide in acetonitrile (500 mg mL-1) and 16.88 µL 4-tert-butylpyridine. ITO (ca. 150 nm) and Au (ca. 65 nm) layers were sputter coated and evaporated respectively. Characterization. A solar simulator (Oriel) fitted with a filtered 1,000 W Xenon lamp was used to provide solar simulated irradiation (AM1.5, 1000 Wm-2). The light source was calibrated using a reference silicon photodiode (Peccell Technologies). Current-voltage measurements were performed using a Keithley 2400 source meter. Solar cells were masked using a non-reflective metal aperture of 0.16 cm2 which defined the active area. Impedance spectroscopy measurements were performed under illumination using a 435 nm LED powered by a PP210 potentiostat. Cells were measured using a 10 mV perturbation either with an applied potential or under open-circuit conditions. A Zahner Zennium electrochemical workstation ECW IM6 was used as a frequency



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response analyzer, and impedance measurements were performed in the 4 MHz to 50 mHz frequency range. Impedance data were analyzed using Zview equivalent circuit modeling software (Scribner). Photoluminescence (PL) measurements were performed using an Edinburgh Instruments Ltd. FLSP920 time-correlated single photon counting (TCSPC) spectrometer. Steady-state measurements used a Xenon lamp illumination source, while time-resolved measurements used a 465.8 nm pulsed diode laser excitation source with a ~ 100 ps pulse width and a laser irradiance of ~ 40 µW/cm2. The luminescence was collected using a grating monochromator. Open-circuit voltage decay (OCVD) measurements were performed using a 630 nm LED illumination source with a switch-off response time of ~ 20 µs. Voltage transients were measured using a NI-DAQmx USB-6212 data acquisition card. 3. RESULTS AND DISCUSSION One of the prerequisites for a high efficiency planar PSC device is the optimized crystal structure and surface coverage of the perovskite film.13, 15, 16 We have shown previously that an effective technique for preparing a high-performance CH3NH3PbI3 film is through the gasassisted fast crystallization method.14 This fabrication method is capable of producing uniform perovskite films having a thickness of approximately 300 nm through the rapid removal of solvents used in the deposition process. In order for impedance features to be meaningfully ascribed to a physical mechanism it is preferable that high-efficiency devices are used for characterization. One of the possible pitfalls associated with characterizing low-efficiency PSC devices is that faults in the material composition or in the device performance may be attributed erroneously to physical processes that are not representative of the typical device performance. To establish an accurate impedance model for planar PSC devices, cells using a series of selective contact materials adjacent to the CH3NH3PbI3 perovskite layer were prepared for IS


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measurements. Despite indications that perovskite doping densities vary widely between p-type to intrinsic to n-type depending on fabrication conditions,17 there is strong experimental evidence to support the p-i-n type behavior of a complete PSC device.18, 19 Therefore, it is appropriate to describe the operation of a PSC using a p-i-n model, regardless of whether the perovskite layer can be accurately defined as an intrinsic semiconductor. In this study, devices nomenclature was based on the constituent layers forming the solar cell. The complete PSC device, which employed both a p-type Spiro-OMeTAD layer and an n-type TiO2 layer contacting the absorber perovskite layer (i.e. Au/Spiro-OMeTAD/CH3NH3PbI3/TiO2/FTO/glass), is denoted as PIN. Devices that used a dense TiO2 layer without a Spiro-OMeTAD layer (i.e. Au/CH3NH3PbI3/TiO2/FTO/glass), and a Spiro-OMeTAD layer without the TiO2 layer (i.e. Au/Spiro-OMeTAD/CH3NH3PbI3/FTO/glass) are labelled IN and PI respectively, in reference to the materials contacting the perovskite layer. The current-voltage (J-V) performance of the PIN, PI and IN cell assemblies were measured under 1-sun illumination. Results for the ‘complete’ planar PSC device (PIN) used in the impedance measurements are shown in Figure 1. The blue squares represent the J-V measurements performed using a voltage sweep in the VOC to JSC direction, and the red circles represent a voltage sweep in the reverse direction. As is clearly evident from this figure, the measured performance of the device depends strongly on the direction of the voltage sweep. This hysteretic behavior of the device performance is well documented, and has been linked to the dielectric properties of the perovskite material.7, 20, 21 The steady-state efficiency, shown in the Figure 1 (inset), provides a more reliable assessment of the true device efficiency, while also revealing the slow transient response that is characteristic of PSC devices. The J-V curves for the PIN, PI and IN device architectures used in this impedance study are presented in Figure S1. As



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apparent in this figure, the hysteretic behavior of the complete device is also reflected in the incomplete PI and IN structures, implying that the performance dependence on the voltage sweep direction is not associated specifically with the Spiro-OMeTAD/perovskite or perovskite/TiO2 interfaces. It has been shown by Snaith et al that the materials contacting the perovskite layer play a large role in the magnitude of this hysteretic behavior.20 Suggested likely origins of the PSC device hysteresis include the influence of trapping states within the perovskite band-gap, and the slow dielectric response of the perovskite film.12, 20 Both of these concepts are explored in the present study with reference to the impedance response of planar PSC devices.

Figure 1. Current-voltage performance of the complete PIN device used in the impedance studies measured under AM1.5 1000 Wm-2 irradiation. The blue squares represent the data obtained from sweeping in the VOC to JSC direction, and the red circles represent the data when sweeping in the reverse direction. Measurements were performed at a scan rate of 0.05 Vs-1. The photovoltaic performance parameters are documented in the table below the current-voltage curves. The inset shows the steady state efficiency when the device was held at the approximate maximum power potential of VMP ≈ 840 mV.

As a starting point, the impedance response of the simplest measurable PSC configuration was determined by fabricating a cell consisting of a perovskite film sandwiched between two transparent ITO contacts. The aim of characterizing this elementary device was to provide


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information concerning the physical processes occurring within the perovskite layer, as well as any impedance features arising due to the perovskite/ITO interface. The impedance response of this cell configuration as measured under illumination is presented in Figure 2(a). The graphs shown in Figure 2 display the imaginary and real components of the impedance responses at an applied potential of 0 to 100 mV for (a) ITO/perovskite/ITO, (b) ITO/perovskite/SpiroOMeTAD/ITO and (c) ITO/TiO2/perovskite/ITO cells. In all three cases, the magnitude of the impedance response general increases with an increase in the applied potential. From the Nyquist plot in Figure 2(a) a solitary high-frequency feature is observed for the ITO/perovskite/ITO symmetric device. The impedance spectra for this device displays no evidence of the transmission line response reported previously,10 and seems to resemble a single RC element. The absence of the transport resistance feature in Figure 2(a) is unsurprising given the high diffusion rates and mobilities reported for charges in organic-inorganic lead-halide perovskite materials.22-25 The impedance spectra measured for the device incorporating both Spiro-OMeTAD and perovskite layers, enclosed between two ITO contacts, is shown in Figure 2(b). In this instance the Nyquist plot reveals two distinct features; one occurring at high-frequency and the onset of a second feature at low-frequency. These high-frequency and low-frequency impedance features are again observed when measuring the ITO/TiO2/perovskite/ITO device, as shown in Figure 2(c). The high-frequency feature in planar PSC devices has been attributed to the charge transport within the Spiro-OMeTAD hole-conducting layer.10 However, as is shown in Figures 2(a) and 2(c), the high-frequency feature is evident even in the absence of the Spiro-OMeTAD layer. As has been reported for mesoporous PSC devices,6 our results imply that the highfrequency feature is due to charge separation at the interface between the perovskite layer and its



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adjacent contacts. The low-frequency feature evident in Figures 2(b) and 2(c) was not observed in the simplified ITO/perovskite/ITO device, and only occurred when a third material was included as a layer adjacent to the perovskite film. These high-frequency and low-frequency features are also present in the impedance spectra for the PIN, PI and IN devices shown in Figure S2. Therefore, the low-frequency feature is not unique to either the perovskite/Spiro-OMeTAD or perovskite/TiO2 interfaces as it is evident for a variety of different PSC assemblies.

Figure 2. Nyquist impedance spectra for a symmetric ITO/perovskite/ITO device (a), a ITO/perovskite/Spiro-OMeTAD/ITO device (b) and a ITO/TiO2/perovskite/ITO device (c). Impedance measurements were performed under illumination (270 Wm-2) with applied bias in the 4 MHz to 100 mHz frequency range.

For the devices studied in the present work, the only instance in which the low-frequency feature was not observed was when the two contacting layers adjacent to the perovskite film were the same material, and therefore maintained the same work function. The relationship


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between the different work functions/electron affinities of the materials contacting the perovskite film plays a pivotal role in the origins of this second feature, as will be discussed further below. It has been suggested that the two distinct impedance features may originate from a distribution of trapping states within the perovskite band-gap.26 Due to the low-temperature processes used to fabricate PSCs, it is likely that there exists a high concentration of sub-conduction band trapping states, and there is some experimental evidence to support this theory.22, 27 In the proposed trapping model, the two distinct impedance features are expected to arise due to the relative kinetics of trapping and release of electrons and holes within the perovskite layer. However, if the trapping of charges within the perovskite gives rise to the two impedance features observed for the ITO/perovskite/Spiro-OMeTAD/ITO and ITO/TiO2/perovskite/ITO cells, then these two features should also be present within the ITO/perovskite/ITO impedance spectrum. The solitary high-frequency feature observed in Figure 2(a) is inconsistent with the hypothesis that a distribution of trapping states produces the identified high-frequency and low-frequency features. To further investigate the characteristics of these two features, impedance spectroscopy was performed on the PIN, PI and IN cell types under varying illumination at open-circuit conditions. Under these measurement conditions, an increase in the illumination intensity causes a corresponding increase in the open-circuit voltage due to the rising concentration of chargecarriers and the associated separation of the electron and hole quasi-Fermi levels. Impedance spectroscopic measurements performed under open-circuit conditions and at different illumination levels essentially provide information concerning the performance of the device at different charge-carrier densities. The resulting impedance spectra obtained from these measurements were fitted using the equivalent circuit shown in Figure 3(a). This equivalent circuit incorporates a Debye dielectric relaxation component which Bisquert et al have identified



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as relating to the frequency dependence of the perovskite polarization.28 The elements used in this equivalent circuit included the perovskite recombination resistance Rrec and contact capacitance Ccon owing to the charge buildup at the interfaces between the perovskite film and its adjacent contacts, and the dielectric relaxation resistance Rdr and capacitance Cdr of the perovskite film. Typical impedance spectra are shown in Figure 3 for the (b) PIN, (c) PI and (d) IN devices.

Figure 3. (a) Equivalent circuit used to fit the impedance response. Typical Nyquist plots are shown for the (b) PIN, (c) PI and (d) IN devices measured under opencircuit conditions at high, low and medium illumination intensities of 273, 8 and 67 Wm-2 respectively. IS measurements are shown in the hollow markers, and equivalent circuit fits are represented by the dashed lines. The corresponding open-circuit potentials were 1010 mV, 361 mV and 497 mV respectively.


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The calculated resistance and capacitance values for the observed impedance spectra are shown in Figure 4. Parameters obtained from the impedance spectra fits for the PIN, PI and IN devices measured at open-circuit are presented in Figures 4(a) to 4(f), respectively. In each of the aforementioned PSC configurations, the red circles represent the Rrec and Ccon fits, and the blue triangles represent the Rdr and Cdr fits. For the complete PIN device (Figure 4(a) and 4(b)), it is apparent that the Rrec and Rdr resistive elements display an essentially exponential dependence on the open-circuit potential. Furthermore, both of these parameters are of similar magnitude, and display similar slopes with respect to the open-circuit potential. This log-linear trend of the Rrec and Rdr elements is also shared by the ‘incomplete’ PI and IN devices, although the magnitudes and slopes of the resistance values are not as closely matched as the PIN case, particularly for the IN structure. Whether the discrepancies between the magnitude and slope of the Rrec and Rdr elements for the different cell architectures are meaningful, or are within experimental uncertainty, is yet to be established. Likewise, further work is required to confidently form any conclusions about the significance of the differences between the relative magnitudes of the Rrec and Rdr components through the study of a broad variety of different hole and electrontransporting materials.



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Figure 4. Resistance and capacitance values for the contact (blue triangles) and dielectric response (red circles) impedance features for the PIN (a & b), PI (c & d) and IN (e & f) device architectures. Solid lines serve as a visual aid only. Impedance measurements were performed at open-circuit in the 4 MHz to 50 mHz frequency range. The equivalent circuit used, and a typical impedance spectra, are shown in Figure 3.

It has been shown in previous work29 that the charge density within the perovskite film is exponentially proportional to the open-circuit potential, therefore the Rrec and Rdr resistances show a direct dependence on the charge concentration. As is the case for mesoscopic PSC assemblies, the Rrec resistance represents the recombination resistance of carriers at the interfaces bordering the perovskite layer. The exponential dependence of Rrec on the open-circuit voltage is expected since charges are more likely to recombine across the contacting interfaces when there


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is a higher population of free-carriers within the perovskite layer. This point is also supported by impedance measurements performed under illumination with an applied potential (Figure S3). For a cell under constant illumination, higher applied potentials (near open-circuit) correspond with higher charge concentrations, while lower potentials (near short-circuit) correspond with lower charge concentrations. IS results indicate that the Rrec value increases at the lower charge concentrations, resulting in increasingly favorable charge collection. The low-frequency capacitance Cdr values also displayed a direct dependence on the charge-carrier density, as shown by the log-linear correlation in Figure 4(b), 4(d) and 4(f). However, this is not the case for the capacitance of the adjacent contact layers, Ccon, as this parameter appears to be effectively independent of the VOC. The high-frequency feature, which determines the Ccon value, is linked to the charge separation at the interfaces between the perovskite layer and the neighboring interfacial materials. If the high-frequency feature reflected the chemical capacitance of the perovskite layer, Cµ, then it is expected that the magnitude of the high-frequency capacitance would increase at higher illumination intensities (higher VOC). The flat response of the Ccon parameter with VOC for all three assemblies implies that the buildup of charges within the perovskite film does not influence the capacitance at the perovskite contacts. From these results, it appears that the capacitance at the perovskite contacts is analogous to the Helmholtz response of an electrical double layer. The free-carrier concentrations within these contacting materials are also independent of the illumination intensity, and are principally determined by their relative doping densities. Considering that the high-frequency capacitance, Ccon, relates solely to the electrical double layer at the contact interfaces, it is consistent that this feature is also observed in the high-frequency arc of the symmetric ITO/perovskite/ITO device (Figure 2(a)).



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Due to the VOC dependence of the impedance response, the dielectric relaxation capacitance Cdr appears to reflect the relative charge concentration within the perovskite film. Previous reports on the impedance of mesoscopic devices have argued that the mid-frequency feature accounts for the charge accumulation within the perovskite film,8, 12 and the results from the present study indicate that the low-frequency feature may also provide information on the charge density. The larger Cdr values for the PIN and PI cells imply that the low-frequency capacitance is dominated by the charge separation at the p-type contact interface. Both PIN and PI devices establish a more effective charge separation at the perovskite/Spiro-OMeTAD interface than that achieved at the perovskite/gold interface of the IN device. For this reason, the Cdr values of the complete PIN device mirror those of the PI device. Real and imaginary components of the dielectric constants obtained from IS measurements are shown in Figure S4, and are in general agreement with previously reported values for PSC devices.30 The results from IS analysis show that the electrical response of PSC devices is largely influenced by the charge recombination across material interfaces. However, the transmissionline features previously reported indicate that recombination within the perovskite film is also measureable through IS.10 Time-resolved photoluminescence (TRPL) is a technique that can be used to analyze the recombination of charges within the neat perovskite film. Strong experimental evidence has been reported31-33 that suggests that excitons dissociate rapidly within the perovskite film at room temperature, and that the intense luminescence observed in PL measurements is most likely associated with the radiative recombination of free-carriers. Figure 5 shows the PL spectra and time-resolved PL decay curves for a pristine CH3NH3PbI3 perovskite film and a Spiro-OMeTAD/perovskite bi-layer film. The strong quenching effect of the contacting Spiro-OMeTAD layer is seen clearly in Figure 5(a). Similar effects have been noted


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for both planar and mesoscopic devices in previous work.23, 24 The PL decay curves (Figure 5(b)) illustrate the rapid charge transfer between the perovskite and the Spiro-OMeTAD layers, which occurs on a time scale that is significantly faster than the radiative recombination of charges within the perovskite material. Charge-carrier time constants of approximately 5 – 10 ns and 270 ns have been reported for pristine CH3NH3PbI3 and CH3NH3PbI3-xClx films, respectively.23, 24 The recombination time for charges within the pristine perovskite films studied in the present work was observed to be in the order of τ ≈ 250 ns, which was calculated by measuring the PL decay at an emission wavelength of 770 nm. It is important to note that differences are observed in the radiative recombination rate within the pristine perovskite film depending on the respective batch of films produced (Figure S5). However, the lower limit of the measured PL decay constants was ≈ 30 ns which still exceeds those previously reported for CH3NH3PbI3 films, suggesting that charge-carrier time constants in all cases were easily long enough to facilitate the extraction of charges at the perovskite contacts. Strong quenching by the contacting Spiro-OMeTAD layer was consistent for all devices tested, once again indicating the rapid transport of charges relative to the recombination rates.

Figure 5. Photoluminescence spectra (a) and time resolved decays (b). The blue lines represent the PL response of the pristine CH3NH3PbI3 film and the red lines represent the quenched perovskite/Spiro-OMeTAD films. The 1/e approximation for the decay time constant is shown by the dotted black line.



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The most important finding from the PL measurements is that the time constants for the charge recombination within the pristine perovskite film are in the order of 10 – 100 ns, and that the charge transfer into the adjacent Spiro-OMeTAD layer results in charge recombination time constants at least an order of magnitude smaller than this. By comparing our understanding of charge recombination rates and quenching efficiencies derived from PL measurements with the results from impedance measurements, it is possible to draw insights into the physical mechanisms represented by the impedance spectra. Figure 6 displays the time constants for the high-frequency and low-frequency features derived from impedance measurements under opencircuit conditions. Filled markers represent the high-frequency features and hollow markers represent the low-frequency features. Under the assumption that the dielectric capacitance is much larger than the perovskite contacts capacitance, Cdr >> Ccon, which holds in the case of our results, the high-frequency and low-frequency time constants for the equivalent circuit used in this study have been calculated as:28 τ hf = R||C con

(1)

τ lf = ( Rrec + Rdr ) Cdr

(2)

where the parallel resistance R|| is defined as:

R|| =

Rrec Rdr ( Rrec + Rdr )

(3)

The high-frequency time constant τhf for all three measured assemblies displays an approximately exponential dependence on the open-circuit potential which, as already stated, signifies a direct dependence on the charge-carrier concentration. These τhf values are on the order of 0.01 – 1 ms which does not correlate well with PL-based recombination rates within the


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perovskite layer itself. It is, therefore, unlikely that the impedance spectra for a planar PSC device can provide information regarding the recombination of charges within the perovskite layer, particularly given the fast quenching by the neighboring materials shown by the TRPL decay. However, the physical interpretation of the high-frequency time constant τhf is complicated by the fact that the recombination resistance Rrec and the dielectric response resistance Rdr both contribute to the calculation of this time constant. The observation of a highfrequency and a low-frequency feature most notably represents two parallel processes occurring in the PSC devices; a fast and a slow component. For the IN device the low-frequency component of the impedance response, τlf, displays a similar slope with respected to the VOC as the high-frequency time constant. However, both the PIN and PI devices show relatively constant τlf values as a function of the VOC. The similarities in the data for the PIN and PI devices imply that the magnitude of τlf in a complete device is determined largely by the dielectric response of the perovskite material near the p-type contact. This conclusion is also supported by the larger low-frequency capacitance Cdr of the PIN and PI structures compared to the IN device. The nearly constant τlf response as a function of the VOC for the PIN and PI devices can be accounted for by the exponentially decreasing sum of the (Rrec + Rdr), coupled with the increasing slope of the Cdr element. The summed resistance elements and the parallel resistance are depicted in Figure S6. In contrast, the flat Ccon response for the three assemblies tested (Figures 4(b), 4(d) and 4(f)) means that the τhf values assume a slope equivalent to the R|| element.



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Figure 6. IS derived time constants for the high-frequency (filled markers) and lowfrequency (hollow markers) impedance features for the PIN (blue squares), PI (red circles) and IN (black triangles) cell structures. The calculation of the highfrequency and low-frequency time constants was performed in accordance with equations 1 and 2 respectively.

The fast and slow processes observed in the impedance measurements are equally represented in the open-circuit voltage decays (OCVD) in Figure 7. This technique involves the generation of charges under illumination at open-circuit, followed by the decay of the potential as the illumination is switched off and charges recombine. As shown in Figure 7(a), the voltage decays of the complete PIN device (blue), the PI device (red) and the IN device (black) display an initial fast component which then gives way to a slow decay at the lower potentials. If we assume a mono-exponential decay for the fast OCVD component then we are able to establish an approximate time constant for this process, as described in Figure S7. The time constant associated with the fast component of the decay is determined to be on the order of 0.1 – 10 ms for the three PSC devices, which is comparable with the magnitude of the time constant for the high-frequency feature obtained from impedance measurements. This provides further evidence that the high-frequency feature in planar PSC devices relates directly to the charge transfer at the interfaces between the perovskite and its bordering materials. It is important to note, that IS and


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OCVD measurements are performed at transient and steady-state conditions respectively. Time constants obtained under these two conditions may differ due to the influences of charge trapping, however our results indicate that there exists a clear relation between the time constants obtained from the two techniques. The results from the TRPL measurements indicate that the initial movement of charges from the perovskite to the Spiro-OMeTAD and TiO2 materials is significantly faster than recombination rates within the perovskite film alone. Considering the good correlation between OCVD and impedance time constants, it likely that the high-frequency time constant provides a good approximation of the free-carrier lifetime within the PSC device.

Figure 7. Open-circuit voltage decay (OCVD) curves for the PIN (blue), PI (red) and IN (black) devices. The fast components of the decays are shown in part (a) while the extremely slow decay for the PIN complete device is shown in part (b).

Impedance spectroscopy measurements and OCVDs both display a slow-response component of the device operation that has garnered a lot of recent attention in PSC research.12, 30, 34

Figure 7(b) shows that the slow component of the OCVD after a duration of 10 minutes is

still approximately 95% of its initial value for the PIN device. This is an unprecedentedly slow response for a thin film photovoltaic device. The characteristically slow response seen in PSCs has been linked to the dielectric nature of the perovskite material and has been postulated as a



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possible source for the hysteretic performance of PSC devices.12, 20 Under open-circuit conditions there is no spatial separation of charges within the bulk of the perovskite layer, and therefore any electric field experienced throughout the bulk of the film can be considered negligible compared to the field within the interfacial region. For these reasons, open-circuit conditions are not conducive to the alignment of the ferroelectric domains within the bulk of the perovskite layer. However, the slow response of the perovskite film is still observed in both IS and OCVD measurements conducted under open-circuit conditions. Judging by the depth of the fast decay, the slow component shown in Figure 7(a) appears to be far more prominent for the PIN and IN devices than for the PI device. This suggests that the TiO2 may play an important role in the origins of the slow photovoltage decay. It is likely that materials adjacent to the perovskite layer could have a significant bearing on the slow component observed in PSCs, as well as affecting the device performance. It has been shown that the relative electron affinities of the materials contacting the perovskite film result in a localized band bending at the perovskite/contacts interfaces,35 which is represented schematically in Figure 8. This band bending will cause an increased electron concentration at the perovskite/TiO2 interface and an increased hole concentration at the perovskite/Spiro-OMeTAD interface as charges move according to the changing electric potential. The charge concentration gradient at the interfacial regions of the perovskite layer results in a localized electric field not experienced throughout the bulk material. Due to the dielectric nature of the CH3NH3PbI3, this localized electric field may contribute to the polarization of the perovskite material giving rise to the low-frequency impedance feature and the slow response seen in transient measurements. Under this interpretation, the Cdr and Rdr equivalent circuit parameters represent the degree of electric polarization at the perovskite edges and the relaxation resistance of this polarization, as the polarization of the bulk perovskite is not


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expected under open-circuit conditions. The magnitude of the electric field at the interfacial regions of the perovskite layer is proportional to the degree of band bending at the respective interface, as well as the relative concentration of charges within the interfacial region and bulk of the perovskite material. If we consider a cell where the contact layers adjacent to the perovskite film possess the same electron affinity, as is in the case for the symmetric ITO/perovskite/ITO cell structure, then the localized band bending will not occur, and it is expected that the low-frequency impedance feature caused by the polarization of the perovskite film will not be observed. This situation is reflected in the impedance results shown in Figure 2(a), which produced a solitary highfrequency feature. However, upon introduction of a Spiro-OMeTAD or TiO2 layer between the perovskite and one of the ITO layers a low-frequency impedance feature is observed. This feature is due to the asymmetry in the electron affinities of the materials contacting the perovskite layer. The strong relationship between the low-frequency feature and the charge density within the perovskite film is evident from Figure 4. This dependence on the carrier concentration is another affirming link between the low-frequency impedance feature and the dielectric response of the perovskite.

Figure 8. Schematic representation of the difference in electron affinities for the two contacting materials giving rise to a localized electric field at the interfacial



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region of the perovskite layer. χspiro, χperov and χTiO2 represent the relative electron affinities of the Spiro-OMeTAD, CH3NH3PbI3 and TiO2 layers respectively. Conduction band electrons are shown as the red circles and valance band holes are shown as the blue circles.

The OCVD response of the PI device without the dense TiO2 layer (Figure 7) is markedly dissimilar to that of the PIN and IN cells. Additionally, the open-circuit voltage of the PI device displays a different dependence to the applied light intensity compared to the PIN and IN devices, as shown in Figure S8. From the OCVD results it appears that the n-type contact is an important factor for this slow dielectric response. Previous reports have proposed that the electric dipoles of the perovskite material are permanently aligned at the TiO2 interface, as shown through electroabsorption measurements and DFT calculations.36 A relatively low (< 1016 cm-3) n-type doping within the perovskite film has been shown experimentally,37 and it is thought that this dopant concentration is determined by the fabrication conditions.35 Due to the band bending, an excess of negative charges within the perovskite film builds up at the perovskite/TiO2 interface giving rise to the polarization of the perovskite layer at the interfacial regions of the material. It is worth noting that OCVD measurements generally assume ‘flat-band’ conditions, where there is no gradient in the potential across the profile of the photoactive layer. However, it is unlikely that the band-bending at the interfacial regions of the perovskite layer has any significant effect on the interpretation of OCVD transients. The dense TiO2 layer has proven to be an essential component for high-efficiency PSC devices, and is responsible for the effective charge separation of electrons and holes into the TiO2 film and the intrinsic perovskite layer, respectively.38 In the absence of this layer, electrons within the FTO film readily recombine across the FTO/perovskite interface thereby reducing the VOC of the device. It is possible that this accumulation of negative charges within the perovskite


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film results in a very long-lived dielectric polarization at the TiO2/perovskite interface. The alignment of the electric dipoles at this interface may sustain the high VOC values under low illumination conditions and give rise to the slow decay seen in OCVD measurements. As shown in Figure 8, the magnitude of the electric field at the interfacial regions of the perovskite layer is also determined by the degree of band bending due to the differences between the electron affinities of the perovskite contacts. From electron affinities reported previously in the literature, the difference between the n-type and p-type values for the PIN, PI and IN structures are 1.32, 0.62 and 1.1 eV respectively.39, 40 Therefore, it is expected that the band bending, and associated dielectric polarization in the PI architecture will be less significant than that of the PIN and IN structures. This difference may equally well account for the dissimilar OCVD and voltageillumination dependency of the PI device, as the slow dielectric response of the perovskite film appears to be comparatively weaker. Despite the anomalous behavior, impedance measurements of the PI cell structure showed a similar dielectric relaxation resistance Rdr to the complete PIN device. The low-frequency capacitance of this device is again similar to that of the PIN structure, and is significantly larger than the IN device. It is expected from our model that the weaker OCVD slow component of the PI cell would result in a significantly lower Cdr value compared to the PIN and IN devices. Open-circuit potentials under one-sun illumination (Figure S1) suggest that the perovskite/SpiroOMeTAD interface is a more effective junction than the perovskite/TiO2 at separating charges. Additionally, the dielectric capacitance Cdr in the complete device is dominated by the electric polarization at the perovskite/Spiro-OMeTAD interface. The strong dependence of the lowfrequency feature on the charge accumulation implies that the low-frequency impedance response is dominated by the dielectric relaxation of the perovskite/Spiro-OMeTAD interface.



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Although the perovskite/TiO2 junction apparently regulates the OCVD transients, the stronger charge separation occurring at the perovskite/Spiro-OMeTAD means that the impedance spectrum will not reflect the slower dielectric relaxation of the TiO2/perovskite interface, but rather the slightly faster dielectric relaxation of the p-type interface. Whether the dominance of the p-type interface is unique to a Spiro-OMeTAD contact material is yet to be established, and further experimentation is required to analyze the impedance response of other p-type materials. It is also worth noting that the magnitude of the dielectric response in the perovskite layer differs significantly between illumination and dark conditions.30 The physical effects that give rise to this difference in the dielectric response could well account for the differences observed between IS and OCVD measurements. 4. CONCLUSION In this study, the impedance spectroscopic response of various planar PSC assemblies were measured and evaluated. The single impedance feature observed in the symmetric ITO/perovskite/ITO cell is attributed solely to the interface between the two materials, with no evidence from IS measurements to suggest a transmission-line response or a distribution of traps within the perovskite layer. This single impedance feature of the symmetric ITO/perovskite/ITO cell was unique with all other asymmetric configurations, displaying both high-frequency and low-frequency impedance response features. Using a theoretical foundation that accounts for the impedance response of the dielectric relaxation of the perovskite, the equivalent circuit parameters for a PIN, PI and IN cell were determined. The high-frequency feature associated with the charge separation at the perovskite contacts was observed to be largely independent of the charge-carrier density, whereas the low-frequency dielectric response showed a strong dependence on the carrier concentration. This flat high-frequency capacitance, as a function of


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the open-circuit potential, revealed that the high-frequency feature is not dominated by the chemical capacitance, Cµ, of the perovskite layer, but rather the double-layer capacitance immediately at the perovskite/contact interfaces. A comparison of PL decay constants and IS time constants revealed that it is unlikely that impedance measurements can provide any information on recombination rates within the perovskite film. However, OCVD data indicated that the high-frequency time constants correlate well with recombination rates across the perovskite/contact interfaces. The prominent slow component of the OCVD curves observed for the PIN and IN devices was not observed in the IS measurements due to the impedance response being dominated by the perovskite/SpiroOMeTAD interface. The extremely slow photovoltage decay found for the PIN and IN devices implies that the dense TiO2 layer plays a pivotal role in the characteristic delayed dielectric response of PSC devices. This work presents a solid foundation, not only for the analysis of planar PSC devices using IS, but also for the comparison of different planar assemblies and the comparison of IS measurements with complementary characterization techniques. The results of this study provide further evidence for the potential value of IS as a diagnostic tool for PSC technologies.

AUTHOR INFORMATION Corresponding Author Yi-Bing Cheng [email protected] Department of Materials Engineering, Monash University, Victoria 3800, Australia



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PH: (+61) (3) 9905 4930 Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We acknowledge financial support from the Australian Renewable Energy Agency (ARENA), the Australian Centre for Advanced Photovoltaics (ACAP) and the Australian Government Department of Education. ASSOCIATED CONTENT Supporting Information. Current-voltage performance of the PIN, PI and IN assemblies, as well as additional IS, PL and OCVD data is included in the supporting information. This information is available free of charge via the Internet at http://pubs.acs.org. REFERENCES (1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050-6051. (2) Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T. B.; Duan, H. S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Interface Engineering of Highly Efficient Perovskite Solar Cells. Science 2014, 345, 542-546. (3) Snaith, H. J. Perovskites: The Emergence of a New Era for Low-Cost, High-Efficiency Solar Cells. J. Phys. Chem. Lett. 2013, 4, 3623-3630. (4) Mei, A.; Li, X.; Liu, L.; Ku, Z.; Liu, T.; Rong, Y.; Xu, M.; Hu, M.; Chen, J.; Yang, Y. et al. A Hole-Conductor-Free, Fully Printable Mesoscopic Perovskite Solar Cell with High Stability. Science 2014, 345, 295-298. (5) Suarez, B.; Gonzalez-Pedro, V.; Ripolles, T. S.; Sanchez, R. S.; Otero, L.; Mora-Sero, I. Recombination Study of Combined Halides (Cl, Br, I) Perovskite Solar Cells. J. Phys. Chem. Lett. 2014, 5, 1628-1635. (6) Juarez-Perez, E. J.; Wuβler, M.; Fabregat-Santiago, F.; Lakus-Wollny, K.; Mankel, E.; Mayer, T.; Jaegermann, W.; Mora-Sero, I. Role of the Selective Contacts in the Performance of Lead Halide Perovskite Solar Cells. J. Phys. Chem. Lett. 2014, 5, 680-685.


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TOC Image


Insights into Planar CH3NH3PbI3 Perovskite Solar Cells Using

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