Room Temperature as a Goldilocks Environment ... - ACS Publications

Article pubs.acs.org/JPCC

Room Temperature as a Goldilocks Environment for CH3NH3PbI3 Perovskite Solar Cells: The Importance of Temperature on Device Performance T. Jesper Jacobsson,*,† Wolfgang Tress,‡ Juan-Pablo Correa-Baena,† Tomas Edvinsson,§ and Anders Hagfeldt*,†,∥ Laboratory for Photomolecular Science, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, CH-1015-Lausanne, Switzerland ‡ Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, CH-1015-Lausanne, Switzerland § Department of Engineering Sciences, Solid State Physics, Uppsala University, Box 534, SE 751 21 Uppsala, Sweden ∥ Department of ChemistryÅngström Laboratory, Uppsala University, Box 538, 75121 Uppsala, Sweden †

S Supporting Information *

ABSTRACT: Terrestrial applications of solar cells during day−night cycling as well as operation in winter and summer involve substantial temperature variations, which influence the photophysics as well as the charge separation and transport properties in the various materials employed in a device. In this study, the optical absorption of methylammonium lead iodide (MAPbI3) and the device performance of MAPbI3 solar cells have been investigated in an extended temperature range between −190 and 80 °C. The optical properties were found to change by only a small amount in that temperature range. The device performance did, however, show more dramatic changes and decreased in a reversible manner for temperatures both higher and lower than room temperature. For temperatures up to 80 °C and down to −80 °C, the drop in performance was up to 25% compared to the room temperature value. Given thermal stability and reversible device performance, this is probably not a showstopper for terrestrial applications of perovskite solar cells but should be considered when evaluating the total energy yield under outdoor operations. At temperatures of −100 °C and below, which are relevant for outer atmosphere and space applications, the performance decreases rather dramatically and approaches zero at even lower temperature. Irreversible changes set in for temperatures above 50 °C. In addition, the hysteresis decreases at reduced temperatures. As the effects for the absorption properties are minor, the decrease in performance can be attributed to a temperature induced limitation in the transport and extraction of the photogenerated charge carriers which is seen as a strong increase of the series resistance at reduced temperature. The drop of the photovoltage for temperatures below −100 °C might be related to reduced charge carrier separation in the perovskite due to excitonic effects and a lower dielectric constant.

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achieved in the years after the first report.4−8 The field has expanded rapidly, and the top certified efficiency has now reached 22.1%.9 The high efficiencies, together with the prospect of cheap precursors and versatile synthesis methods,

INTRODUCTION The prospect of using hybrid organic−inorganic lead halide perovskites as photoabsorbers in solar cells has attracted considerable interest in the past few years. The first reports on these materials date as far back as 1978,1,2 but the first paper on lead halide perovskites for PV applications was published in 2009 by Kojima et al.3 After a slow start, a number of advances regarding material synthesis and device engineering have been © XXXX American Chemical Society

Received: March 19, 2016 Revised: May 10, 2016

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DOI: 10.1021/acs.jpcc.6b02858 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

In this paper, we report changes in both optical absorption and device performance of MAPbI3 in the temperature range from −190 to 80 °C. The effect of temperature on the optical properties was found to be rather small. A large reversible variation in the device performance was, however, observed, and cell efficiencies peaked close to room temperature.

make the perovskites real candidates for a competitive solar cell technology, either as standalone cells or as top cells in tandem configurations together with conventional solar cells.10−12 The perovskite that so far has attracted the most attention in the photovoltaic community is methylammonium lead iodide, (CH3NH3PbI3, or MAPbI3), which could be seen as the standard perovskite and as a model system for the range of perovskite compositions that are currently explored for solar cell applications. MAPbI3 is also the perovskite studied in this work. Most of the investigations on these perovskites, and especially measurements of solar cell device performance, have been conducted at room temperature. The operational window for a real solar cell normally deviates from room temperature and also experiences temperature cycling as well as temperature changes depending on the time of day, time of year, and geographical position. One concern is thermal stability of the perovskite itself, especially as stability overall has been an issue for the perovskite solar cells.13−17 Thermal stability is certainly a potential problem, and several reports demonstrate thermal decomposition at elevated temperature.14,17−19 That may, however, be a problem that could be solved, or at least severely reduced, by tweaking the perovskite composition by replacing some of the organic cations by other cations, such as cesium, which appears to lead to greater temperature stability.20,21 In this paper, the focus is not on thermal degradation, but instead on the reversible effects caused by temperature. Solar cells operating under outdoor conditions will experience a wide temperature range. During sunny summer days, the temperature of solar panels can reach over 80 °C, which is taken as the upper limit in established testing schemes, whereas the temperature of cold and sunny winter days in mid Sweden, which has a yearly solar irradiation comparable to parts of middle Europe, can reach below −20 °C. For space application, operational temperatures can vary broadly and reach substantially lower values.22 The organic−inorganic lead halide perovskites are fairly complex systems, and there are a plethora of parameters likely affecting the device performance that possibly are affected by temperature. Among those parameters are phase transformations, ion migration, and ion rotation. The MAPbI3 perovskite is known to have a phase transformation between tetragonal and cubic at around 54 °C,23−25 as well as a phase transformation from an orthorhombic to a tetragonal phase around −113 °C.26,27 Ion migration has been shown to occur to a fairly large extent in these perovskites28−31 and is assumed to have a direct influence on the hysteresis in the current− voltage curve commonly seen for these devices.30,32−35 At lower temperature, the activation barriers for ion movement will prevent some of the migration which likely is going to affect the hysteresis, and possibly also the device performance. The organic cations in the perovskites also have a dipole moment, which gives the possibility to form ferroelectric domains, and as the activation energy for ion rotation is rather small, they rotate freely at room temperature.36−39 This in turn gives rise to a large dielectric constant40,41 which screens the photogenerated charge carriers from each other and facilitates both in their separation and in their transport. At lower temperatures, this effect will likely be reduced, which will affect the device performance in a negative way. Furthermore, charge transfer and transport in the electron and hole transport layers is expected to change with temperature.

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EXPERIMENTAL SECTION Device Preparation. Perovskite synthesis and device preparation were based on previously published procedures.42−44 A detailed account is given in the Supporting Information, and a condensed version is given below. FTO (NSG 10) coated glass sheets cleaned in freshly prepared piranha solution were used as substrates. On the cleaned FTO substrates, an electron selective contact of TiO2 was deposited by spray pyrolysis. On top of the compact TiO2, a layer of mesoporous TiO2 was deposited by spin-coating. Subsequently the substrates were sintered in air at 450 °C. The perovskite films were deposited by spin-coating in a nitrogen filled glovebox using a one-step method with the antisolvent approach.45−47 Chlorobenzene was used as the antisolvent, and anhydrous DMSO was used as solvent for the perovskite solution, which was 1.25 M with respect to PbI2 and 1.14 M with respect to CH3NH3I. The slight molar excess of PbI2 with respect to CH3NH3I has been shown to be beneficial for device performance.43,44 The spin coated films were annealed at 100 °C on a hot plate within the glovebox for 30−60 min. Prolonged times at those temperatures can lead to irreversible degradation, but 60 min is short enough for this not to happen, which is demonstrated by the fact that the record devices in excess of 21% efficiency employ this annealing scheme as well. Spiro-MeOTAD was used as a hole conductor and was deposited by spin coating from a 70 mM chlorobenzene solution with three different additives:48,49 4-tert-butylpyridine, Li-TFSI in acetonitrile, and Co[t-BuPyPz]3[TFSI]3 in acetonitrile. Gold was used as back contact and was deposited by vapor deposition Characterization. UV−vis absorption measurements were performed on an Ocean Optics spectrophotometer HR2000+ using the combined light of a deuterium and halogen lamp. In all measurements, a full spectrum from 190 to 1100 nm with 2048 evenly distributed points was collected. 100 consecutive spectra were averaged in order to obtain good statistics. For absorption measurements as a function of temperature, a THMs600 Linkam temperature control stage was used. The films were first heated to 80 °C, whereafter the temperature was decreased down to −190 °C with 10 °C per minute. Liquid nitrogen was used as a cooling medium, and dry nitrogen at room temperature was constantly blown over the windows of the temperature cell to decrease the condensation of water from the ambient atmosphere at lower temperatures. Room temperature measurements for evaluating accurate efficiencies of the cells were performed by a home-built system, which recently has been confirmed to give data in good agreement with data provided from independent certification agencies. The details of this system are found in the Supporting Information. For IV data measured as a function of temperature, a CH Instruments model 760C potentiostat was used together with a Linkam temperature control stage. An Oriel solar simulator xenon arc lamp equipped with a global AM1.5G air mass filter was used to simulate solar irradiance. The aperture in the Linkam cell where the light can enter is 0.0314 cm2. This B


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Figure 1. (a) XRD pattern of MAPbI3 spin coated onto a soda lime glass substrate. (b) Absorption and photoluminescence data for MAPbI3. The data is normalized and background corrected. (c) A top view SEM image of a MAPbI3 film. The scale bar is 300 nm.

Figure 2. (a) Absorption at selected temperatures. Data is background corrected. The full set of data is found in the Supporting Information. (b) A photo of one of the films on which absorption was measured. (c) Band gap deduced from absorption data as a function of temperature. (d). XRD data as a function of temperature illustrating the phase transformation from the tetragonal to the cubic phase at 54 °C for a drop-cast sample of MAPbI3. The data in panel d are adopted from one of our previous works.25

known to have a large effect on device performance is the film morphology,50 and to evaluate that, a top view SEM image of a representative sample is given in Figure 1c. The film is observed to be rather uniform with a low number of microscopic pinholes. There is some surface roughness in the same scale as that of the underlying substrate. One of the most central functionalities of a solar cell material is the optical absorption, which is plotted against wavelength for temperatures stretching from 80 °C down to −190 °C in Figure 2a. The absorption was measured at every degree Celsius in that interval, and the full data set is presented in the Supporting Information. An experimental problem, encountered during measurements at low temperature, is condensation of ice on the window of the temperature cell which acts as an extra scattering layer. This problem was minimized by flushing dry nitrogen gas on the temperature stage window during measurements. At low temperatures (around −70 °C) partial ice formation complicated the measurements. This was compensated for by assuming that the ice contributes to a uniform and wavelength independent scattering, which is based on the fact that ice is white. Thus, a constant background was subtracted from each spectrum shifting the absorption minimum around 800 nm down to zero. The shift of the absorption onset was rather small when the temperature was changed. When the temperature decreases, the absorption onset gets sharper and shifts to lower energies in line with recently published absorption and photoluminescence results.27,51−53 A shift in the absorption onset is equivalent to a shift in the band gap, which is quantified in Figure 2c. The band gap was extracted by assuming parabolic bands close to the band edges and taking into account that MAPbI3 has a direct band gap. If the square of the absorption is plotted as a function

equipment was not as finely calibrated as the other setup and thus primarily serves for measurements of the relative device performance. Steady state photoluminescence was measured with a Fluorolog, Horiba Jobon Yvon, FL-1065, with a tungsten lamp as a luminous source and monochromators placed both before and after the sample. An excitation wavelength of 435 nm was used, and the emission spectrum was measured from 455 to 835 nm in steps of 1 nm. XRD measurements were measured using a Bruker diffractometer using a Bragg−Brentano geometry. Cu Kα radiation, with a wavelength of 1.54 Å, from a copper target was used as X-ray source. 2θ scans between 10° and 65° were collected using a step size of 0.008°. SEM imaging was carried out using a Zeiss Merlin scanning electron microscope. Photographs were taken using a Canon EOS 450 D with an EFS 60 macro lens.

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RESULTS AND DISCUSSION X-ray diffraction (XRD) was recorded for the deposited films to certify the perovskite structure and the crystallinity. An XRD pattern of one of the samples is shown in Figure 1a. The data verify the formation of MAPbI3, which except for small amounts of unreacted PbI2 is the only crystalline phase detected. The diffraction is dominated by two peaks. That indicates a strong degree of texture, which frequently has been observed for spin coated films of the tetragonal phase.25 By zooming in the XRD data, which is done in the Supporting Information, all the peaks expected for MAPbI3 can be identified. Absorption and photoluminescence data were also sampled, as illustrated in Figure 1b, and both are in good agreement with previous results for MAPbI3. A property that is C


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Figure 3. (a) An illustration of the solar cell architecture. (b) A cross section SEM image of a typical device. (c) A photo of one of the devices. The width of the substrate is 1.4 cm. (d) IV data under AM1.5G and at room temperature for the sample that later was evaluated as a function of temperature between room temperature and 80 °C.

of photon energy, the band gap can then be extracted if the linear region found for photon energies slightly above the band gap is extrapolated down to the baseline.54−56 This procedure is illustrated in the Supporting Information. While the temperature was changed from 80 °C down to −190 °C, the band gap shifted from 1.61 eV down to 1.58 eV, as seen in Figure 2c. That is an interesting observation as the band gap of most common semiconductors like Si, Ge, InP, InAs, GaAs, etc. generally increases when the temperature is decreased due to a reduced thermal expansion of the lattice and a modified electron phonon interaction.57 The reverse trend with a decreased band gap at lower temperatures has been observed for MAPbI3 before52,58 and can be explained by the antibonding nature of the valence states in MAPbI3.59 With a reduced interatomic distance at lower temperature, the energy of antibonding orbitals rises, which shifts the valence band up and in turn decreases the band gap.51 It has also been connected to the time dynamics of the rotation and tilt of the PbI6 octahedra that constitute the inorganic backbone of the perovskite structure.52 This dynamic shift, which makes the instantaneous structure deviate from the observed time averaged structure, is in turn closely connected to the rotation of the organic ions in the cuboctahedral voids they fill. No discontinuities or changes in the slope of the band gap versus temperature were observed around the phase transition temperatures: 54 °C for the tetragonal to cubic transition and −113 °C for the orthorhombic to tetragonal transition. Recent theoretical results have demonstrated that the cubic phase may be described more accurately as a macroscopic time average of a range of tetragonal configurations quickly interchanging with each other.52 This would comply with the smooth temperature trend seen in our data over the phase transition. The similarly regular trend in absorption data around the tetragonal to orthorhombic phase transition temperature at −113 °C provides an indication that a similar situation may occur also there. There is, however, also a recent paper which reports an increase of the band gap at that temperature when measured at MAPbI3 films on a quartz substrate.60 A possible reason behind this nonconformity might be a different film morphology including residual material or constraints due to adjacent layers such as the mesoscopic scaffold, which retard a phase transition. Some irregularities around −70 °C are apparent in the band gap trend in Figure 2c, but those can be attributed to measurement artifacts due to the condensation and ice formation mentioned above. A change of the band gap of 0.03 eV is from a practical point of view a small change, especially given that the temperature changes by almost 300 K. It can therefore be concluded that large changes in device

performance cannot be attributed to the optical properties of the perovskite. As mentioned above, MAPbI3 has one phase transformation between a tetragonal and a cubic phase at around 54 °C23−25 and one between a tetragonal and an orthorhombic phase at around −113 °C.26,27 In a previous study,25 we measured XRD from room temperature to up to 90 °C for MAPbI3. A subset of that data is reproduced in Figure 2d, which illustrates the phase transformation between tetragonal and cubic symmetry at 54 °C. We observe a gentle and fully reversible phase transformation which can be rationalized by the change in the time average of the tilt of the octahedral lead halogen in the structure.52 The thermal expansion coefficient for both the cubic and the tetragonal phases was found to be rather large, indicating that it is the change in lattice parameters that is the responsible for the observed changes in the band gap. In order to exploit the photovoltaic effect of the perovskite material, it must be assembled into a complete solar cell device. An illustration of the device architecture used in this study is given in Figure 3a. A cross section SEM of a typical device is given in Figure 3b, and a photo of one of the devices is given in Figure 3c. Making a good device is tricky and involves a fair bit of artisanship, and the final result is affected by a number of environmental parameters that at the moment are poorly understood and challenging to control. These factors are the reason for the cell-to-cell and batch-to-batch variation frequently observed by most research groups working in the field. There is also a clear dependence on the specific person constructing the cells, where the most skilled experimentalist with more refined artisanship obtains significantly better cells using the same starting materials. With that said, the solar cells we have been measuring on in this study were not made by an experimentalist at the peak of artisanship and were, therefore, not record devices, but they show an efficiency of almost 15% (Figure 3d). Nevertheless, these devices employ basically the same architecture and materials as record devices.42,43 Thus, despite changes in absolute values, general trends and the underlying physics are highly relevant for perovskite solar cells based on mesoscopic TiO2 and organic hole-transport layers. The key measurement was sampling of IV characteristics as a function of temperature. Those measurements were performed in a temperature controlled Linkam cell, which is pictured in the Supporting Information. The temperature of the cell was changed by a rate of 5 °C/min, and the samples were held at the measurement temperature for 5 min prior to the measurement to allow for thermal equilibration. This equilibration time appeared to be long enough to make the possible effect of scan speed negligible. During the thermal equilibration, the cell was kept in the dark. The IV curves were D


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Figure 4. Device performance as a function of temperature from room temperature up to 80 °C deduced from a scan with a rate of −0.1 V/s. (a) IV data. (b) Power-conversion efficiency. (c) Open circuit voltage. (d) Short circuit current. (e) Fill factor. (f) Hysteresis, H, as defined by eq 1. The yellow bar on the side of each plot represents data for the cell after it had cooled down to room temperature again, and it, therefore, shows the degree of reversibility in the measurement procedure.

Information where 50 IV scans were sampled in a direct sequence, with almost identical results for all 50 scans. Even if the scan speed affects the results for IV data measured at one temperature, we found the choice of scan rate and direction to have only a small effect on the temperature trend of the device performance. We have therefore used one single scan speed of 0.1 V/s in a backward scan for the following presentations of the results. We will come back to the role of the scan rate in the final discussion section. In Figure 4b−f, the device parameters extracted from the JV curves, i.e., the efficiency, η, the open circuit potential, Voc, the short circuit current density, Jsc, the fill factor, FF, and the hysteresis, H, are given as functions of temperature. The measurement started at 30 °C, and the temperature was gradually increased up to 80 °C. The yellow bar on the side of each plot represents data after the device had cooled down to room temperature. It is thus a measure of the degree of reversibility in the changes induced by a varied temperature. The highest efficiencies were reached at 30−35 °C. At higher temperatures, η, Voc, and Jsc gradually decreased. The fill factor did, however, increase somewhat at higher temperatures and peaked around 60 °C. A crucial question to answer when observing a decreased efficiency with a change in temperature is whether or not the observation is due to an irreversible degradation of the perovskite and the solar cell, or if it is truly a reversible temperature effect. The measurements performed on the cell at room temperature after the heating program reveals that a slight degradation occurred, mainly with respect to the Voc. That is not entirely surprising as thermal stability has been reported to be a problem,14,17−19 and given the, admittedly, rather harsh treatment of our unencapsulated cells. Diffusion of gold from the contact into the perovskite structure at high temperatures could also be a reason behind this degradation. The measurement performed after the heating program reveals that most of the decrease in performance observed at higher temperatures cannot be attributed to thermal degradation but instead is a reversible effect. The observed decrease in efficiency can be approximated by 0.08%/K. This is large

sampled at seven different scan speeds with 1 min delay between each scan. The cell was kept under illumination for the entire measurement program. In Figure 4a, IV curves are given at a range of different temperatures starting from 30 °C and going up to 80 °C, measured on the same cell presented in Figure 3d. The full set of individual IV curves, including data for the forward scan, is found in the Supporting Information. The room temperature performance appears to be lower in Figure 4a than in Figure 3d. That could in part be a result of aging effects as there are a few weeks and half a continent between the fabrication of the cells and the temperature measurements. Changes in photocurrent might be due to a slight underestimation of the light intensity reaching the sample in the Linkam cell. These issues are not of severe relevance here as we focus on relative trends in the performance. It turns out that the scan speed chosen for the measurements has a significant effect on the measured values. That is neither uncommon nor unexpected for cells with hysteretic behavior which is something that frequently has been observed in perovskite cells.30,32−34 There are several hypotheses concerning the origin of the hysteresis, involving ion migration,28,35 interfacial charge transfer,61 capacitive effects,33 and ferroelectric effects.62 Hysteresis could, depending on the underlying mechanism, be a problem for device performance and stability. Undoubtedly, it results in problems to accurately evaluate device performance.63 The hysteresis can be quantified in a number of ways by defining a hysteresis index H.29,30 Here, according to eq 1, H is defined as the fraction of the difference between the “area”, swept by the current in the backward, Jb, and forward scan, Jf, and the area swept by the current of the backward scan. V

H=

V

∫0 oc Jb(V ) dV − ∫0 oc Jf (V ) dV V

∫0 oc Jb(V ) dV

(1)

Given a certain scan rate, the IV characteristics are stable over a large amount of scans, which is illustrated in the Supporting E


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Figure 5. Device performance as a function of temperature from room temperature down to −190 °C deduced from a scan with a rate of −0.1 V/s. (a) IV data. (b) Power conversion efficiency. (c) Open circuit voltage. (d) Short circuit current. (e) Fill factor. (f) Hysteresis, H, as defined by eq 1.

voltage increases with decreasing temperature down to −80 °C, which is in line with the behavior for inorganic semiconductor solar cells, and also with previously reported data for MAPbI3.65 At even lower temperatures, the Voc starts to decrease, and at temperatures below −160 °C, it completely crashes. The current is rather stable down to −60 °C and thereafter drops continuously down to essentially zero at −190 °C. The fill factor decreases in a jagged fashion down to −120 °C where the IV curve has gone from the characteristic shape of a solar cell response to a straight line. Also the hysteresis is strongly affected and vanishes completely at −80 °C, which is higher than where the solar cell performance starts to drop. The question of reversibility is more multifaceted than for the measurements at increased temperature. The room temperature measurements after the temperature program show a 30% degradation in the efficiency compared to the initial measurement, mostly due to a current loss. That indicates a severe degradation but it is still much better than the zero efficiency measured at −190 °C, which demonstrates a high degree of reversibility. In the original experimental design, the starting temperature was set at −190 °C, but as no current was measured at that temperature the cell was brought back toward room temperature under the assumption of contact problems. The cell recovered rapidly and almost completely when the temperature increased. The measurements at 0 °C and below are thus measured after the cell already had been once down to −190 °C. The irreversible degradation only occurred during the second, longer, temperature scan and can be attributed to the effect of moisture that condenses on both the Linkam cell and the solar cell at low temperatures, and moisture is known to be highly nonbeneficial for the performance of perovskite solar cells.73 It is thus reasonable to assume that, given proper encapsulation, low temperature will not be a problem for cell stability. The lower current and efficiency measured at 0 °C, compared to room temperature, might be an effect of too short time for thermal equilibration after the cell was heated from −190 °C. We have found three recent reports measuring IV data as a function of temperature below 0 °C.58,65,66 They are in line with our data in the sense that they report decreased efficiencies at lower temper-

enough to be an issue concerning an efficient operation of future perovskite modules, but it is not catastrophically detrimental. Having 80% of room temperature performance at 60 °C, and probably somewhat better if the problem of thermal degradation was solved, would be a nuisance but would not be a showstopper for technological outdoor use of perovskite solar cells. A decreased performance at elevated temperatures is also expected per default as it is the general behavior of convention solar cell technologies.64 We have found a few other reports that discuss IV data for MAPbI3 at elevated temperatures.58,65−67 Those works report a decrease in the device efficiency with elevated temperature as well, but the drop is generally larger than what is observed here, and any indications concerning the reversibility are not given. One of the questions addressed by this project is whether or not the phase transformation from tetragonal to cubic symmetry at 54 °C impacts the device performance. There is a fairly uniform downward trend in η, Vov and Jsc, with respect to increasing temperatures, but the measurement at 60 °C give somewhat higher values of all four solar cell parameters, η, Vov, Jsc, and FF, as well as a lower hysteresis. This indicates that the cubic phase may be somewhat advantageous, which is in line with the best performing solar cells at the moment being based on formamidinium rich perovskites18,19,43,68−71 which have a cubic symmetry at room temperature.72 The temperature trend is, however, stronger than this effect, and the increase in performance at 60 °C, just above the phase transition temperature, is merely a dent in the downward trend. This behavior is in line with the absorption data that shows the phase transformation to be an undramatic event with small consequences for device performance. Temperature dependent IV measurements were also performed in the other direction, starting from room temperature and going down to −190 °C. Those measurements were made on another sample, with somewhat lower performance (η = 11.5%), and are presented in Figure 5a−f. The room temperature JV curve and the entire set of JV curves are given in the Supporting Information. While decreasing the temperature, the variation is considerably larger than what was observed while increasing the temperature. The open circuit F


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Figure 6. Analysis of temperature dependent JV data. Red and magenta: heated device. Green and cyan: cooled device. Circles and crosses are for backward scan at 0.1 V/s. (a) Voc(T), where lines show representatives of the Voc predicted according to eq 2. (b) Jsc(T), where the black dashed line shows the expected Jsc in the case of series resistance limit. (c) Arrhenius plot of the series resistance, Rs, showing two characteristic energies. The inset shows modeled JV curves according to the diode model assuming a photocurrent of 18 mA/cm2 and using a selection of the experimentally determined values of Rs displayed in the main figure. The values for Rs are 0, 3, 6, 11, 19, 25, 55, 150, 400, 1500, and 10000 ohm/cm2. (d) FF(T) for two different scan rates and for forward and backward scan. The single arrows visualize trends, and the double arrows indicate hysteresis.

atures.56,61,62 The observed decreases are, however, larger and start at higher temperatures than we see here,61,62 and they do not discuss the question of reversibility. The hysteresis, H, which is given in Figure 5f, decreases at lower temperature, which possibly is associated with decreased ionic migration.28−31,74,75 The hysteresis starts to decrease at a higher temperature than the photocurrent does. The best cell performance was also for this temperature set found at room temperature, but down to −80 °C the drop in efficiency is not extremely large. This is in contrast to the situation for silicon solar cells where lower temperatures are better. The observed decrease in efficiency is small enough for low temperature not to pose a major problem for terrestrial applications of perovskite solar cells. How repeated temperature cycling would affect the cells is a slightly different matter that will have to be evaluated within time. For space application, the drop at lower temperature may be a severe problem, but on the other hand, that has so far never really been the main target for perovskite cells.

and 6b respectively, including the reversibility check. Recall that data of two separate devices is combined here. For an ideal semiconductor solar cell at zero kelvins, the free, i.e., electrochemical, energy of one electron−hole pair equals the band gap energy, Eg. Therefore, eVoc = Eg with the elementary charge e. At higher temperatures, T, Voc will decrease as a consequence of the increased entropy, which reduces the electrochemical energy of electrons and holes in the conduction and the valence band. That means that the Fermi− Dirac distribution gets broader with higher T, which implies that, for a fixed charge carrier density, the quasi-Fermi levels will move further away from the band edges. In the simple case this can be formalized as in eq 2, where e is the elementary charge, kB is Boltzmann’s constant, n and p are the concentration of electrons and holes, and Nc and Nv are the density of states in the conduction and valence bands, respectively. ⎛NN ⎞ e·Voc(T ) = Eg − kBT ln⎜ c v ⎟ ⎝ np ⎠

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DISCUSSION OF TRENDS IN SOLAR CELL PERFORMANCE After having described the individual temperature measurements and their main results, we are now in a position where we can combine all the data in order to discuss trends for the solar cell parameters over the entire temperature range. The complete set of data for the Voc and the Jsc is given in Figures 6a

(2)

In Figure 6a, Voc is given as a function of temperature for the backward scans at a sweep rate of 0.1 V/s, where the data are from Figure 4c and 5c. As mentioned previously, changing the scan direction or the scan speed shifts absolute values by less than 50 mV, while preserving all trends. Red circles denote data of the sample measured at elevated temperatures, whereas green crosses represent data for the sample characterized while G


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for temperatures below −80 °C is due to a very high series resistance.

cooling. Due to sample-to-sample variations, they show slightly different room temperature values of Voc. The experimental Voc data between −100 and 50 °C show good agreement with the trend predicted by eq 2 for the respective device as also observed by Leong et al.58 This is visualized by the dashed lines, which indicate how Voc(T) of solar cells with a band gap of 1.6 eV are expected to look in the case of unchanged recombination constraints. The good coincidence indicates that Voc is governed by recombination in the perovskite in that temperature range. At both higher and lower temperatures the experimental results do, however, deviate from what is expected based on eq 2. At higher temperatures, Voc decreased more strongly when the cell was heated further. This was due to degradation in the device. As the change in the band gap with temperature was small, as shown in Figure 2b, this implies an increase in the recombination rates. The degradation is proven by the Voc measured at room temperature after heating (marked by a red square in Figure 6a), which fits a novel theoretical line of Voc(T) with the point at 80 °C. Some irreversible degradation is thus observed for temperatures above 50 °C. The exact mechanistic cause of the degradation cannot be unambiguously concluded based on the given data, and this is a vital problem for future studies. As mentioned above, there is a phase transition occurring around 54 °C, which might be the reason behind the degradation, even though structural reversibility has been demonstrated.25 The optical characterization did not show any abrupt change either, although there might be secondary effects such as a modifications of the morphology of the perovskite inducing more recombination centers. A slight thermal decomposition, which is too small to be seen in the XRD and UV−vis measurements, could change the composition of the grain boundaries which potentially could be detrimental. Another possibility is crystallization of the hole transport layer which might cause more recombination losses. Gold migration into the perovskite structure at high temperatures may also play a role. At temperatures below −80 °C, Voc decreases instead of increasing which would be the expected response. This is indicative of a process reducing the charge carrier densities (np in eq 2), i.e., enhanced recombination or a reduced probability for charge carrier generation by exciton dissociation.76 By observing the temperature dependence of the photocurrent in Figure 6b, we find that this decrease in Voc coincides with a strong decrease of Jsc. Also the JV curves change at those temperatures and go from an S-shaped form toward almost straight lines, as seen in Figure 5a and in the Supporting Information. This is indicating a highly resistive behavior. To discriminate whether the Jsc is intrinsically limited at these low temperatures, or if it is just affected by a large (phenomenological) series resistance, Rs, we extract the differential resistance dV/dI measured in the forward direction at 1.1 V and use it as an approximation for Rs. The extracted Rs values are plotted in Figure 6c and are indeed seen to change over several orders of magnitude in the explored temperature range. By using those values of Rs together with the ideal diode equation (eq 3 with ideality factor n = 243,77 and photocurrent Iph = 18 mA/cm2), the JV curves can be predicted, as illustrated in the inset of Figure 6c. From those curves, an approximation of Jsc can be extracted, which is plotted as the black dashed line in Figure 6b. The simulated data based on the extracted Rs is consistent with the experimental data, which proves that observed decrease in Jsc

I = I0(e e(V − IRS)/ nkBT − 1) − Iph

(3)

Changes in Rs should, however, not affect the Voc. By comparing Rs and Voc(T), we clearly see that they are not correlated, which indicates that more than one mechanism is in play for decreasing the cell performance at lower temperatures. By looking more closely at the extracted Rs values in Figure 6c, they seem to be characterized by two processes with different activation energies Ea estimated from the slope of the conductance 1/R s in the shown Arrhenius plot (1/R s ∝ e−Ea / kBT ). The obtained values are 200 and 90 meV respectively, as illustrated in Figure 6c. The first one, valid at higher temperatures, could be explained by thermally activated hopping transport in the organic hole transport material spiroMeOTAD. The observed changes of Rs fit with recent literature, where the conductivity and hole mobility of spiroMeOTAD have been reported to decrease at lower temperature.78,79 Our measurements also indicate that spiroMeOTAD has a rather low conductivity below −50 °C. Also the S-shape in this temperature regime indicates that charge extraction is hindered due to nonoptimum contact properties. There is, however, other experimental data that indicates that the thin layers of heavily doped spiro-MeOTAD used here may not be a major obstacle for current transport at low temperatures.58 Thus, the mobility of charge carriers in the perovskite layer itself might decrease with temperature. This is, however, unexpected for a crystalline semiconductor where phonon scattering limits the mobility. In this case the mobility should increase with reduced T as verified by THz spectroscopy.60 Nevertheless, THz spectroscopy does not comprehensively monitor charge transport at grain boundaries and interfaces to the charge transport layers. Those processes are most likely activated by temperature and are found to limit the fill factor in perovskite solar cells with small grains. At the lowest temperatures, another process seems to dominate. It might be that the charge-selective pin structure cannot be maintained due to missing free charges, which might also explain why Voc decreases. Other explanations might again be related to the perovskite itself, where, for example, charge transport and recombination at grain boundaries might limit the device performance. One possibility could be the phase transformation from a tetragonal phase to a low temperature orthorhombic phase around −113 °C, which correlates reasonably well with the drop in Jsc and η at lower temperatures as well as to a reported increase in the recombination constants.60 Although the low Jsc can be rationalized with charge transport issues only, it is not excluded that charge separation (i.e., exciton splitting) is limited as well at low temperatures. By reducing np in eq 2, this process is compatible with a reduced Voc. Furthermore, lower photogenerated charge carrier densities would increase the series resistance due to the photoconductivity of MAPbI3.80 Although still under debate, the most recent lower values81 of reported exciton binding energies, which are in contrast to former data,27,82 support that photocurrent only should be reduced at low temperatures. A possible microscopic explanation is based on the movement and rotation of the organic dipolar cations. At room temperature, the energetic barrier for rotation of the organic ions located in the cuboctahedra voids in the perovskite H


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The Journal of Physical Chemistry C structure is low and easily surpassed, which allows them to flip around on a femtosecond time scale.36−39,74 As those carry a dipole moment, the rather unrestricted rotational freedom contributes to a high dielectric constant,40,41,83 which screens the photogenerated electrons and holes and facilitates their separation, and possibly also their transport through the perovskite structure. At lower temperature, the activation energy may start to be too high for free rotation of the organic ions given the thermal energy available. It is also known that the thermal expansion coefficient is rather high for the perovskite,25,66 which means that the space left over for the organic ions to flip around in will decrease at lower temperature, which may contribute to a higher activation energy for rotation. If the temperature gets low enough, the organic ions will freeze in their positions and may get a higher degree of collective order. There is experimental data indicating that this is the case in the orthorhombic phase,26,36 and that this leads to a lower dielectric constant,26 higher exciton binding energy, and less efficient charge carrier separation.36 Whether or not those truly are the most reasonable explanations requires more work of both experimental and theoretical nature. Figure 6b shows that Jsc decreases at higher temperature as well, which is not due to degradation as it is a reversible effect. This decrease cannot be explained by effects of contacts, but must instead be related to enhanced losses of charge carriers in the perovskite film. It is most likely also related to the changes in photocurrent observed due to hysteresis or accelerated reversible short-term aging.35 The hysteresis at a scan rate of 0.1 V/s was quantified with H above as seen in Figures 4f and 5f. In the following analysis, the FF itself is further analyzed with the aid of Figure 6d, which displays FF as a function of T for both the forward and the backward scan at two different voltage sweep rates. The behavior of the FF can be separated into three regimes. For temperatures below 200 K (regime I), the FF is low and decreases with lower T and saturates at approximately 25%. At this temperature regime the FF becomes independent of both scan rate and scan direction. This is compatible with the discussion above of an increased series resistance which finally makes the device behavior highly resistive (FF = 25%). For temperatures between 200 and 300 K (regime II), the FF depends on the scan rate. For a high scan rate, the values are rather scattered with respect to temperature but show a trend toward higher values at higher temperatures. This trend is rather independent of the scan direction for the high rates. For slower scan rates, the FF does, however, depend on the scan direction, which gives rise to a large hysteresis, H (cf. Figures 4f and 5f). For the forward scan, the FF even decreases at higher temperatures. This can be explained by the fact that the response time of the source of hysteresis matches the time required to sweep the voltage in this temperature range. As previously proposed, we assume that this source consists of mobile ions in the perovskite which can accumulate at the interfaces to the charge transport layers. Prior to starting the forward scan, the device was kept at Voc, which creates an ion distribution that is beneficial for charge extraction. When performing a slow scan, the ions do, however, respond at the low voltages and start to screen the electric field in the device which results in an S-shaped curve. As ions respond faster at higher temperatures due to thermal activation,74,84 this effect is more pronounced and the FF decreases when the temperature increases. By increasing the scan speed, this effect can be avoided, as ions do not respond sufficiently fast.

For temperatures above 300 K (regime III), the FF is less dependent on the scan rate as the ions react rather promptly during the scan. The extreme potentials previously applied do, however, still influence the ion distribution and consequently also the probability of charge extraction. The FF for the backward scan, which follows the forward scan and starts at a potential of 1.2 V, is thus high and comes along with a “bump” in the JV curve.

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SUMMARY AND CONCLUSIONS

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ASSOCIATED CONTENT

The optical properties and the device performance of perovskite solar cells, based on MAPbI3, have been investigated as a function of temperature from 80 °C down to −190 °C. The cells were found to perform best at room temperature, and the performance decreased at both higher and lower temperature. We did observe some degradation, but most of the observed effects were reversible in nature, and would be of relevance for technical use of perovskite solar cells. For temperatures up to 80 °C and down to −80 °C, the drop in performance was up to 25% referred to the room temperature value and is important to consider if the total energy yield for perovskite solar cells should be evaluated. Given thermal stability and the reversibility of the temperature performance, this is probably not a showstopper for terrestrial applications of perovskite solar cells. The observation that the best performance was obtained at ambient conditions indicates that room temperature would constitute somewhat of a Goldilocks environment for perovskite solar cells. This is in contrast to most other semiconductor technologies. It may, however, not be as strange an observation as it may seem at a first glance. Room temperature is after all the temperature where the performance of new materials is evaluated and optimized. At temperatures of −100 °C and below, the performance decreases rather dramatically and goes toward zero. The exact reason behind this observation is still unclear, but the effect is reversible. A high series resistance can explain a large part of the behavior, and we speculate that the separation and transport of the photogenerated charge carriers may be obstructed at very low temperatures due to constraints in the rotation and movement of the organic ions at those temperatures. The hysteresis is highest at room temperature and vanishes at low temperatures consistent with thermally activated ionic conductivity. We further find that the optical absorption and the band gap only change by a small amount in the temperature range investigated irrespective of passing temperatures of phase transitions. We can thus conclude that the temperature effects in the device performance cannot primarily be attributed to optical effects, but are instead related to the separation and transport of photogenerated charge carriers in the system.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b02858. Details of perovskite synthesis and device manufacturing, XRD data, IV data and curves, additional absorption figures, photos of the temperature cell, and solar cell parameters (PDF) I


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +46 (0)705745116. *E-mail: Anders.hagfeldt@epfl.ch. Tel: +41 (0)21 693 53 08. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS GRAPHENE project support by the European Commission Seventh Framework Program under Contract 604391 is gratefully acknowledged.

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