Article pubs.acs.org/JPCC
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, Melbourne, Victoria 3800, Australia CSIRO Energy Flagship, Clayton, Victoria 3169, Australia § CSIRO Manufacturing Flagship, Bayview Avenue, Clayton, Victoria 3169, Australia ‡
J. Phys. Chem. C 2015.119:4444-4453. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/08/18. For personal use only.
S Supporting Information *
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. 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 lowfrequency 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 hole-transport 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 toward understanding the impedance response of perovskite-based devices, a more comprehensive
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 multicrystalline (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 © 2015 American Chemical Society
Received: September 30, 2014 Revised: February 11, 2015 Published: February 13, 2015 4444
DOI: 10.1021/jp509896u J. Phys. Chem. C 2015, 119, 4444−4453
Article
The Journal of Physical Chemistry C
using a grating monochromator. 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.
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 opencircuit voltage decays (OCVDs) 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.
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 gas-assisted 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 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 SpiroOMeTAD layer (i.e., Au/CH3NH3PbI3/TiO2/FTO/glass), and a Spiro-OMeTAD layer without the TiO2 layer (i.e., Au/SpiroOMeTAD/CH3NH3PbI3/FTO/glass) are labeled 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 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 Supporting Information Figure S1. As 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
2. EXPERIMENTAL SECTION 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 ∼1 cm2 and then spin coated at 6500 rpm. After 2−3 s, 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 min. The hole-transport layer was formed by spin coating a solution of 41.6 mg of 2,20,7,70tetrakis(N,N-bis(p-methoxphenyl)amino)-9,90-spirobifluorene (Spiro-OMeTAD) in 500 μL of chlorobenzene with the standard additives of 7.51 μL lithium bis(trifluoromethylsulfonyl)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 1000 W xenon lamp was used to provide solar simulated irradiation (AM1.5, 1000 W m−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 nonreflective 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 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). 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 timeresolved measurements used a 465.8 nm pulsed diode laser excitation source with an ∼100 ps pulse width and a laser irradiance of ∼40 μW/cm2. The luminescence was collected 4445
DOI: 10.1021/jp509896u J. Phys. Chem. C 2015, 119, 4444−4453
Article
The Journal of Physical Chemistry C
Figure 1. Current−voltage performance of the complete PIN device used in the impedance studies measured under AM1.5 1000 W m−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 V s−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.
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. 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 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 2a. 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/Spiro-OMeTAD/ITO, and (c) ITO/TiO2/perovskite/ITO cells. In all three cases, the magnitude of the impedance response generally increases with an increase in the applied potential. From the Nyquist plot in Figure 2a, 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 2a 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 2b. In this instance the Nyquist plot reveals two distinct features; one
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 W m−2) with applied bias in the 4 MHz to 100 mHz frequency range.
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 2c. 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 Figure 2a,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 high-frequency feature is due to charge separation at the interface between the perovskite layer and its adjacent contacts. The low-frequency feature evident in Figure 2b,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 this feature is evident in a variety of different PSC assemblies. 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 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 4446
DOI: 10.1021/jp509896u J. Phys. Chem. C 2015, 119, 4444−4453
Article
The Journal of Physical Chemistry C Due to the low-temperature processes used to fabricate PSCs, it is likely that there exists a high concentration of subconduction 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 2a is inconsistent with the hypothesis that a distribution of trapping states produces the identified highfrequency 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 charge-carriers 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 3a. This equivalent circuit incorporates a Debye dielectric relaxation component that Bisquert et al. have identified 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. 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 Figure 4a−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 4a,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.
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 open-circuit conditions at high, low, and medium illumination intensities of 273, 8, and 67 W m−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.
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 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 4b,d,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 4447
DOI: 10.1021/jp509896u J. Phys. Chem. C 2015, 119, 4444−4453
Article
The Journal of Physical Chemistry C
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 transmission-line features previously reported indicate that recombination within the perovskite film is also measurable 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 freecarriers. Figure 5 shows the PL spectra and time-resolved PL
Figure 4. Resistance and capacitance values for the contact (blue triangles) and dielectric response (red circles) impedance features for the PIN (a and b), PI (c and d), and IN (e and 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.
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.
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 freecarrier 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 2a). 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 midfrequency feature accounts for the charge accumulation within the perovskite film,8,12 and the results from the present study indicate that the low-frequency
decay curves for a pristine CH3NH3PbI3 perovskite film and a Spiro-OMeTAD/perovskite bilayer film. The strong quenching effect of the contacting Spiro-OMeTAD layer is seen clearly in Figure 5a. Similar effects have been noted for both planar and mesoscopic devices in previous work.23,24 The PL decay curves (Figure 5b) 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 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 4448
DOI: 10.1021/jp509896u J. Phys. Chem. C 2015, 119, 4444−4453
Article
The Journal of Physical Chemistry C 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. The most important finding from the PL measurements is that the time constants for the charge recombination within the pristine perovskite film are on 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-
correlate well with PL-based recombination rates within the 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 respect 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 (Figure 4b,d,f) means that the τhf values assume a slope equivalent to the R∥ element. The fast and slow processes observed in the impedance measurements are equally represented in the OCVDs in Figure 7. This technique involves the generation of charges under
Figure 6. IS-derived time constants for the high-frequency (filled markers) and low-frequency (open markers) impedance features for the PIN (blue squares), PI (red circles), and IN (black triangles) cell structures. The calculation of the high-frequency and low-frequency time constants was performed in accordance with eqs 1 and 2, respectively.
frequency and low-frequency features derived from impedance measurements under open-circuit 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 as28
τhf = R Ccon
(1)
τlf = (R rec + R dr)Cdr
(2)
where the parallel resistance R∥ is defined as R recR dr R = (R rec + R dr)
Figure 7. OCVD curves for the PIN (blue), PI (red), and IN (black) devices. The fast components of the decays are shown in panel a, while the extremely slow decay for the PIN complete device is shown in panel b.
illumination at open-circuit, followed by the decay of the potential as the illumination is switched off and charges recombine. As shown in Figure 7a, 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 monoexponential 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 measure-
(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 4449
DOI: 10.1021/jp509896u J. Phys. Chem. C 2015, 119, 4444−4453
Article
The Journal of Physical Chemistry C ments. 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 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. 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 7b shows that the slow component of the OCVD after a duration of 10 min 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 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 7a 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 expected under opencircuit conditions. The magnitude of the electric field at the
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 region of the perovskite layer. χspiro, χperov, and χTiO2 represent the relative electron affinities of the SpiroOMeTAD, CH3NH3PbI3, and TiO2 layers, respectively. Conduction band electrons are shown as the red circles, and valence band holes are shown as the blue circles.
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 2a, which produced a solitary high-frequency feature. However, upon introduction of a Spiro-OMeTAD or TiO2 layer between the perovskite and one of the ITO layers, a lowfrequency 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. 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 (
