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Temperature effects in lead halide perovskites
C H A P T E R
8 Temperature effects in lead halide perovskites T. Jesper Jacobsson Department of Chemistry, Uppsala University, Uppsala, Sweden
8.1 Introduction Thermal motion is an omnipresent background to the web of reality, but also something that can have an extensive impact on the properties of materials. This is especially true for semiconductors where even the defining characteristics, conduction with limits, is highly temperature dependent. Semiconductor devices, like photovoltaic cells, are thus also affected by temperature. The difference between summer and winter, night and day, sunshine and shade, and the tropics and the north, means that the temperature for a solar cell can switch from below 220 C to above 80 C. For space applications, the operational temperature range can vary even more. For traditional solar cell materials, e.g. silicon, CIGS, GaAs, etc. the device performance gradually decrease when the temperature is increased. For lead halide perovskites, which are highly dynamic systems, the situation turns out to be more complex. The amount of thermal motion has a direct impact on the crystal structure of lead halide perovskites and the dynamics of its constituents, which has implications on, for example: defect formation, ionic movement, charge transport properties, dielectric behavior, recombination, and the interface chemistry. Temperature can be used as a variable parameter providing leverage for understanding physics of the system, e.g. activation energies for various processes. At low temperatures, new effects emerge, which give insights into the fundamental physics of the perovskites. Higher temperatures are on the other hand useful for investigating crystallization dynamics as well as degradation, which is a problem that must be supressed. For temperatures there between, where solar cells operate, plenty of physics is going on which impact the properties of the materials and the performance of devices. In this chapter, we look at how temperature play a role for the crystal structure of lead halide perovskites and the binding, motion, and importance of the organic ions. We discuss phase transformations, thermal
Characterization Techniques for Perovskite Solar Cell Materials DOI: https://doi.org/10.1016/B978-0-12-814727-6.00008-6
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expansion, optical properties, and stability. Finally, we go through what we known about how these can influence the performance of perovskite solar cell devices.
8.1.1 Crystal structure and phase transitions The most central motive out of which properties emerge is the crystal structure, i.e. the geometrical configuration in 3-dimentional space of the atoms in the compound. This configuration is very much constrained by the available thermal energy and the impact it has on the physical space occupied by the structure’s constituents, the excitation of the excitable, and the energetics of the bonding between neighboring atoms. In a discussion about the impact on temperature on the properties of perovskites, a recapture of the crystal structure is thus a natural starting point. The general perovskite structure has the composition ABC3, and the ideal structure has cubic symmetry and is composed of a backbone of corner sharing BC6-octahedra with cuboctahedra voids occupied by the A-cations (Fig. 8.1AC). Central for the structure is the relative size of the three ions. This can, at least partly, be captured by the concept of tolerance factor, t, as in Eq. 8.1 where rA, rB, and rC, are the ionic radius of the A, B, and C ions respectively [1]. Table 8.1 summarizes the impact the tolerance factors has on the structure.
FIGURE 8.1 (A) Ideal cubic unit cell of MAPbI3 illustrating the octahedral coordination around the lead ions. (B) Illustration of the extended structure and linking of the PbI6 octahedra. (C) The cuboctahedral coordination around the organic ion. (D) the MA ion. (E) Simulated energy landscape of the MA ion in the cubic phase in the form of a polar plot of the orientational energy surface [4]. (F and G) Illustration of the octahedra tilting pattern in the form of the PbI bond angel in the ideal tetragonal and cubic phase [3]. (H and I) Time distribution of simulated bond angels in the room temperature tetragonal phase (H) and the high temperature cubic phase (I) [3]. (E) Reproduced from J.S. Bechtel, R. Seshadri, A. Van der Ven, Energy landscape of molecular motion in cubic methylammonium lead iodide from first-principles, J. Phys. Chem. C 120 (23) (2016) 1240312410, with permission from the American Chemical Society; (FI) Adapted from C. Quarti, E. Mosconi, J.M. Ball, V. D’Innocenzo, C. Tao, S. Pathak, et al., Structural and optical properties of methylammonium lead iodide across the tetragonal to cubic phase transition: implications for perovskite solar cells, Energy Environ. Sci. 9 (1) (2016) 155163, permission from the Royal Society of Chemistry.
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TABLE 8.1 Tolerance factors for the perovskite structure. Tolerance factor, t
Structure
Comment
,0.7
0.70.9
Tetragonal/Ortorombic/Rhombohedral
A too small or B too large
0.91.0
Cubic
Ideal perovskite structure
.1.0
Various layered structures
A-cation too large
Reproduced from T.J. Jacobsson, et al., J. Phys. Chem. C 119 (46) (2015) 2567325683, with permission from the American Chemical Society.
rA 1 rC t 5 pffiffiffi 2ðrB 1 rC Þ
(8.1)
There is a large class of related perovskite compounds with promising PVcharacteristics. Out of those, methyl ammonium lead iodide, CH3NH3PbI3 (or MAPbI3), is so far the most extensively investigated, and can therefore be seen as a standard perovskite and a model compound. Much of the subsequent discussion therefore use MAPbI3 as a baseline case. Given the ionic radius of Pb21 5 0.132 nm, I2 5 0.206, and CH3NH31 5 0.18 nm [2], MAPbI3 should according to the tolerance factor form a tetragonal structure, which also is the case at room temperature. The room temperature structure of organicinorganic lead halide perovskites is by no mean static, but is a highly dynamic system with atoms having a large amplitude around their equilibrium positions [3]. Core reasons behind this dynamism are the non-spherical symmetry of the organic MA/FA ions (Fig. 8.1D) and their week binding to the inorganic backbone of PbI6-octahedra. The organic ions reside in cuboctahedral voids with a nonuniform energy landscape spanned by the halogen atoms (Fig. 8.1E) [4]. In this landscape, the MA ions interact with dipoledipole interactions, where the NH?I interactions are the strongest [46], and have a number of energetically favorable orientations. The weak interactions means that the energy barriers for turning the MA-ions away from their favorable orientations are small, thus enabling jumping between preferred orientations in the halide cages [5,7]. At room temperature, the time scale for flipping around the MA ions in MAPbI3 is in the range of femtosecond to picoseconds [3,79], where a distinction can be made between a faster wobbling motion around the crystal axis and a slower reorientation of the molecular dipole with respect to the iodide lattice [8]. Defects can complicate the picture even further [10]. Those rotating dipoles implies a mean for screening charges by aligning towards them. This ability is reflected in a high dielectric constant [11,12], and is contributing to the low exciton energy observed [1315], and for why the hybrid perovskites are not excitonic materials at room temperature [13,15]. The ability to reorient around a localized charge may also have an impact on charge transport and it has been suggested that it slow down recombination [16]. Exactly how important the reorientabilty of the organic cation is for device performance is still an open question, especially as purely inorganic perovskite solar cells have been made [17,18], even if with lower efficiencies. To which extent the organic dipoles collectively order themselves into ferroelectric domains, and how this affect the charge charier separation, transport, and recombination
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has gained a lot of attention [7,1922]. There is a balance between increased entropy by randomness and energetic gain by collective organization, and any collective ordering will decrease while the temperature increases. This balance will also depend on the perovskite composition. Given the current literature, much more investigations are required until we reach a comprehensive understanding and consensus of how it really works. The ionic rotation is highly temperature dependent. At higher temperatures, the probability to overcome the activation energy for flipping increases, which leads to faster rotation. It also decreases the relative energetic contribution from columbic interactions with stationary charges or external fields compared towards the thermal energy. This leads to a gradual decrease in the dielectric constant [11,12,23,24], and the probability of collective ordering. By changing the composition of the perovskite by gradually exchanging the halogen, or the organic ion, the energy landscape in the halogen cage can be tuned, and it has been observed that this can shift the behavior of the rotations, as well as the activation barriers [25]. If the temperature is low, the thermal energy will not be enough to overcome the rotational barriers and the organic ions freeze in position [26], most likely in an ordered fashion [23,27]. For MAPbI3, this results in a phase transformation to a low temperature orthorhombic phase around 2113 C [15,23,24,27], which has a substantially lower dielectric constant [11,12,23,24], a higher excition energy [23], lower mobility [28], and appears to be useless for solar cell applications [29]. At higher temperatures, the thermal energy can be substantially higher than the weak bonding energy between the organic ion and the halide cage, leading to essentially free organic cation rotation. One effect of this is that the time-average shape of the organic ions goes towards spherical symmetry, whereby they also effectively take up a larger volume. In terms of the tolerance factors in Eq. 8.1, this means a larger rA and consequently a larger t, which pushes the structure towards cubic symmetry. For MAPbI3, this leads to a phase transition between a tetragonal to a cubic high temperature phase, which in diffraction measurements is observed around 54 C [27,3032]. Accompanying this transition are changes in the tilt of the lead-halogen octahedrals (Fig. 8.1F and G) which can effects, for example, the band gap [33]. By changing the organic ion and the halogen, the temperature of this phase transition can be shifted, but the general trend towards higher symmetric phases at higher temperatures are general for perovskites, and observed also for completely inorganic perovskites. For MAPbBr3, the tetragonal to cubic phase transition is pushed down to 262 C [24], and for FAPbI3, it is observed at 273 C [32]. Mixed perovskites where part of the MA is replaced by FA and part of the I is replaced with Br can thus give a cubic phase at room temperature [34], and thereby shift the tetragonal to cubic phase transition out of the normal operational window of terrestrial solar cells. Even if the time average shape of the organic ions at higher temperatures are spherical, they still have a non-spherical molecular shape (Fig. 8.1D). If the inorganic lead halide framework dynamically can shift in line with the rotation of the organic ions, the local structure may deviate from the cubic time average seen in for example XRD and Raman measurements. Simulations have shown that the lead halide framework dynamically accommodate the motion on a time scale fast enough for this to be the case [3]. The high temperature cubic structure may thus better be described by a fast switching between different tetragonal local configurations, which average out to a cubic structure [3]. This is
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illustrated in Fig. 8.1FI with the simulated time distribution of lead-halide bond angels for both the tetragonal and the cubic phase. On the timescale of electronic transitions, the material may thus seldom experience a cubic environment, but rather a shifting distorted tetragonal one. This is perfectly in line with the observation that the phase transition between the tetragonal to the cubic phase of MAPbI3 at B55 C not give more than a small and gradual change in for example absorption and device characteristics [3,29].
8.1.2 Thermal expansion coefficients Phase transitions in perovskites can be investigated by, for example, X-ray diffraction measured as a function of temperature, which has been done both for MAPbI3 [30], and a few other compositions [27]. Polycrystalline MAPbI3 undergoes a phase transformation around 54 C, and single crystals around 55 C, between a tetragonal and a high temperature cubic phase, which is of special interest as it is within the operational temperature window of solar cells. The tetragonal and the cubic phase have similar, but still distinctly different, XRD-patterns as illustrated in Fig. 8.2A where a number of double peaks for the tetragonal phase merge into single peaks for the higher symmetry cubic phase. Experimental data demonstrating the change of the XRD-pattern over the phase transition is given in Fig. 8.2B [30]. While cycling the temperature between room
FIGURE 8.2 (A) Comparison between experimental XRD data and simulated data for both the tetragonal and the cubic phase. The peaks most clearly distinguishing the experimental room temperature structure of MAPbI3 as tetragonal are highlighted in gray. (B) 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. (C) A graphical illustration of the change in cell parameters with increased temperature for MAPbI3. (D) Primitive unit cell volume of MAPbI3 as a function of temperature. (A, C, and D) Adapted from T.J. Jacobsson, L.J. Schwan, M. Ottosson, A. Hagfeldt, T. Edvinsson, Determination of thermal expansion coefficients and locating the temperature-induced phase transition in methylammonium lead perovskites using X-ray diffraction, Inorg. Chem. 54 (22) (2015) 1067810685; (B) Reproduced from T.J. Jacobsson, W. Tress, J.P. Correa-Baena, T. Edvinsson, A. Hagfeldt, Room temperature as a goldilocks environment for CH3NH3PbI3 perovskite solar cells: the importance of temperature on device performance, J. Phys. Chem. C 120 (21) (2016) 1138211393, both published with permission from the American Chemical Society.
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temperature to 80 C, this transition is fully reversible, at least during a few cycles [30]. If the transition would be reversible also over the thousands of cycles perovskites in solar cells modules would experience under their lifetimes is still an open question. Something seen in experimental XRD-data (Fig. 8.2B) is a drift in the peak positions towards smaller angels with higher temperature, i.e. an expansion of the lattice [30]. With increased temperature, the tetragonal structure expands along the a-axis and contracts along the c-axis (Fig. 8.2C) and does so in a linear fashion [30]. After the phase transition at 54 C, the a-axis continues to expand, essentially without a discontinuity. This is in line with a gentle and gradual phase transition, which as discussed above is based on the rotational energy of the MA ions in the lead-halogen octahedron, and that the phase transition not requires atomic reorganizations in the form of breaking or formation of covalent or ionic bonds. Given the lattice parameters, the primitive cell volume can be extracted (Fig. 8.2D) which also is found to expand linearly with temperature. With lattice parameters and cell volume as a function of temperature, the thermal expansion coefficients are readily extracted, where the linear thermal expansion coefficient in the length dimension L, αL, is given by Eq. 8.2, and the volumetric expansion coefficient, αV, is given by Eq. 8.3 where V is volume. αL 5
1 @L L @T
(8.2)
αV 5
1 @V V @T
(8.3)
The thermal expansion coefficients are largely independent of temperature between 25 C and 80 C, and were determined to: αa_tet 5 1.32 1024 K21, αc_tet 5 21.06 1024 K21, and αa_cub 5 4.77 1025 K21 [30]. The volumetric thermal expansion coefficient was determined to; αv 5 1.57 1024 K21, and is independent of the perovskite phase. This is a rather high expansion coefficient. It is, for example, more than six times larger than for soda lime glass (αv 5 2.6 1025 K21), four times large than for steel (αvB3.5 1025 K21) and twice that of lead (αv 5 8.7 1025 K21). It is also considerably higher than for other thin film solar cell materials, like CIGS (αv 5 2.7 1025 K21) [35] and CdTe (αv 5 1.4 1025 K21) [36] which are reasonably close to the one of soda lime glass. Thermal expansion coefficients that are significantly different between the absorber layer and the substrate is potentially a problem as it results in mechanical stresses during temperature cycling. The temperature changes occurring under years of daynight cycling could thus pose a problem for the microscopic mechanical stability of perovskite solar cells. The severity of the problem is still an open question. To our knowledge, thermal expansion coefficients has not been extracted for other perovskite compositions than MAPbI3. Until further data is provided, it is reasonable to assume the expansion coefficients to be relatively high for all hybrid perovskites. The phase transition between the tetragonal and the cubic phase appears, at least from a crystallographic perspective, not to be a major problem. It is, however, probably best to avoid if possible, and by partially replacing the methyl ammonium ions with larger ions,
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e.g. formamidinium, the tolerance factors can be modified so that the phase transition is shifted to lower temperatures giving a preference for the cubic phase already at room temperature [34].
8.1.3 Optical properties Optical absorption is the basis for charge carrier generation, and is thereby one of the most important aspects of a solar cell material. When the temperature decreases, the absorption onset for MAPbI3 has been observed to sharpen and shift to lower energies [3,15,3740]. This is illustrated in Fig. 8.3AE where the optical absorption for MAPbI3
FIGURE 8.3 (A) Absorption for MAPbI3 at selected temperatures. Data is background corrected. (B) A photo of the MAPbI3 film on which absorption was measured. (C) Band gap deduced from absorption data as a function of temperature. The small dip around 260 C is an artifact due to condensation on the measurement chamber. (D) and (E) Photos of the Lincam cell used for temperature dependent UVvis and IV measurements. Reproduced from T.J. Jacobsson, W. Tress, J.P. Correa-Baena, T. Edvinsson, A. Hagfeldt, Room temperature as a goldilocks environment for CH3NH3PbI3 perovskite solar cells: the importance of temperature on device performance, J. Phys. Chem. C 120 (21) (2016) 1138211393, published with permission from the American Chemical Society.
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was measured between 2190 C and 80 C [29]. The same behavior is reflected in steady state photoluminescence measurements [3739]. A shift in the absorption onset is equivalent to a shift in the band gap, which for MAPbI3 thus decreases with decreased temperature [3,41] (Fig. 8.3B), and while the temperature was changed from 80 C down to 2190 C, the band gap shifted from 1.61 eV down to 1.58 eV [29]. For most common semiconductors, e.g. Si, Ge, InP, InAs, GaAs, etc., the band gap does instead decrease with increased temperature, which can be explained by thermal expansion and changes in the electron-phonon interactions [42]. A possible explanation for the reversed behavior of MAPbI3 lies in the antibonding nature of its valence states [43]. At lower temperatures, the interatomic distances decreases which moves the energy of the antibonding states to higher energies due to increased orbital splitting, and thus decreases the band gap [37]. No changes or discontinuities were observed around the phase transition temperatures; 54 C for the tetragonal to cubic transition, and at 2113 C for the orthorhombic to tetrahedral transition. This behavior is in line with the phase transitions being gentle and gradual events. A change of the band gap of 0.03 eV is from a practical point of view a small change, especially given a temperature difference of almost 300 K. Significant temperature dependent changes in, for example, device performance can thus not be attributed to changes in the optical properties of the perovskite.
8.1.4 Degradation at higher temperature Perovskite solar cells have achieved impressive efficiencies over the last few years. Efficiencies that are high enough for devices to make a real technological impact if the long-term stability also can be controlled. Impressive improvements in stability have been reported over the last few years, i.e. from minutes to a few thousand hours under operational conditions, but further improvements are still needed. The pathways for perovskite degradation are plentiful and involves for example: moisture [44,45], light [46], oxygen [47,48], metal diffusion from contacts [49,50], etc. Increased temperature, which is the focus in this chapter, is also an important trigger for degradation. At elevated temperatures, MAPbI3 decompose into solid PbI2 and methyl ammonium and hydrogen iodine that can disappear in the gas phase (Eq. 8.4) [51]. Another suggested pathway involves decomposition of the methyl ammonium into ammonia and methyl iodine (Eq. 8.5) [52]. (8.4) CH3 NH3 PbI3 ðsÞ-PbI2 ðsÞ 1 CH3 NH2 ðgÞ 1 HI g (8.5) CH3 NH3 PbI3 ðsÞ-PbI2 ðsÞ 1 NH3 g 1 CH3 IðgÞ The exact temperature when MAPbI3 decomposes is non-trivial to pinpoint. Thermogravimetric analysis (TGA) indicates that MAPbI3 can be stable up to around 200 C [27,51,52], whereas considerably lower temperatures appears to be a problem in most environments. In part, this discrepancy is about timing as only a small part of the material needs to decompose before the device performance is compromised. It is also due
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to combined effects as increases in temperature can accelerate decomposition along other pathways requiring for example light or moisture [53]. A typical annealing temperature during crystallization of MAPbI3 is 100 C. That is probably not high enough to decompose the perovskite in itself, unless there are other decomposition pathways available effected by the elevated temperature. To make it even more complicated, a certain degradation can under some conditions be beneficial. If the outermost surface layer decomposes during the annealing, a small excess of lead iodine can form which is beneficial for device performance [5456], as long as the other decomposition products are vented out in the gas phase. In terms of thermal degradation of MAPbI3, the weak point appears to be MA ions, and attempts have thus been made to replace them. One approach is to go for all inorganic perovskites with Cs as the cation. Cs based all inorganic solar cells have been made [17,18], indicating that it is possible, and those perovskites are indeed more thermally stable. Efficiencies are so far, however, inferior, but it is an open question whatever or not they can be as good as the hybrid perovskites, or if the dipole moment of the organic ion is necessary for achieving the highest efficiencies. Another alternative is to replace MA with formamidinium, FA [57], or guanidinium [58]. The FA perovskites appears to be more thermally stable [59]. Unfortunately, the cubic (or pseudocubic) α-phase of FAPbI3 easily transform into a yellow polymorph at room temperature which has hexagonal symmetry (P63mc) and is unsuitable for PVapplications [32,57,60,61]. That transformation could be avoided by mixing in some MA or Cs. At the time of writing, an interesting composition is mixed FACs-perovskites [62,63]. FA stabilises the Cs-perovskite, and Cs stabilising the FA perovskite, giving a mixture that has a stable cubic phase at room temperature. Those compositions thereby avoid MA and gain in thermal stability. So far, those perovskites are, however, not the best performing ones. Some of the best composition are at the time of writing Cs and/or Cs-Rb doped FAMA perovskites [64,65] (e.g. Cs0.5FA0.79MA0.16PbBr0.51I2.49) which has demonstrated good stability under illumination at 80 C [64,65]. This indicates that a small amount of MA-ions can be beneficial in terms of crystallization and in stabilising the FA perovskite, and that they potentially not seriously compromise the thermal stability up to 80 C as long as they only are present in a small amount.
8.1.5 Device performance From a solar cell perspective, the most important temperature dependent parameter is device performance. For MAPbI3, a few groups has investigated this both above [29,41,6668] and below [29,6668] room temperature. The details differ between the measurements but the general trends that here will be discussed are the same. Device performance as a function of temperature is in Fig. 8.4A 2 D given for MAPbI3 solar cells based on data from Jacobsson et al. [29], which are representative of the temperature dependent performance measurements so far reported. While increasing the temperature from room temperature up to 80 C, the highest efficiencies where measured at 3035 C. At higher temperatures, η, Voc, and Jsc, all gradually decreased (Fig. 8.4AC), whereas the FF peaked around 60 C (Fig. 8.4D). When the cells returned to room
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FIGURE 8.4 Device performance as a function of temperature for devices of standard configuration, i.e. FTO/ TiO2/mesoporous TiO2/MAPbI3/Spiro-MeOTAD/Au. Two different cells where used for the measurements; one for temperatures above room temperature, and another one for lower temperatures. (A) Efficiency. (B) Open circuit voltage. (C) Short circuit current. (D) Fill factor. Data has previously been published by T.J. Jacobsson, W. Tress, J. P. Correa-Baena, T. Edvinsson, A. Hagfeldt, Room temperature as a goldilocks environment for CH3NH3PbI3 perovskite solar cells: the importance of temperature on device performance, J. Phys. Chem. C 120 (21) (2016) 1138211393.
temperature, they recovered most of the loss in performance, showing a reasonable degree of reversibility. The decrease in measured efficiency with increased temperature was approximately 0.08% K21. A decrease in efficiency at elevated temperatures is expected based on the behavior of other solar cell materials [69]. The magnitude is, however, larger than observed for silicon, and large enough to be a concern with only 80% of the room temperature performance remaining at 60 C. This is problematic but not a showstopper for outdoor use of perovskite solar cell; especially given the rapid evolution of both device performance and stability, which makes it reasonable to assume further improvements. The open circuit voltage is expected to decrease at higher temperatures as an effect of increased entropy, which reduces the electrochemical energy of electrons and holes in the conduction and valence band, broadens the Fermi-Dirac distribution, and pushes the quasi-Fermi levels further away from the band edges. The data up to 50 C follows this
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trend (Fig. 8.4A) [29,41], indicating that the Voc change is governed by charge carrier recombination in the perovskite film within that temperature range. At higher temperatures, the Voc decrease accelerates, which was attributed to irreversible degradation [29]. It is assumed that the MA ions trigger a thermal instability [27,5153], which may be the cause for the observed degradation. Other compositions, especially Cs and Cs/Rb doped mixed perovskites (e.g. Cs0.5FA0.79MA0.16PbBr0.51I2.49) has showed higher thermal stability with up to 1000 hour of stability at 80 C [64,65], indicating that thermal stability probably can be accomplished within the operational window of terrestrial photovoltaics. A question of interest is if the perovskite phase, and in particular the phase transformation from tetragonal to cubic seen for MAPbI3 at 54 C, has an effect on the device performance. The downward trend in η, Vov and Jsc while the temperature is increased is uniform. The measurement point at 60 C, just after the phase transition, 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 that the best performing solar cells at the moment are based on formamidinium rich perovskites [57,7072], which have a cubic symmetry at room temperature [73]. 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. At lower temperatures, the changes in device performance are more dramatic. The Voc increases with decreasing temperature down to 280 C, which continues the trend seen for higher temperatures up to 50 C (Fig. 8.4B). That is in line with the expected behavior for inorganic semiconductor solar cells, and has been observed by different groups [29,66]. At even lower temperatures, the Voc decreases, and at temperatures below 2160 C, it goes to zero. The lower Voc is indicative of a reduction in charge carrier density, i.e. by either an increased recombination rate or by a reduced probability of charge carer generation by exciton dissociation [74]. The FF decreases unevenly down to 2120 C (Fig. 8.4D) where the IV-curve has gone from the characteristic shape of a solar cell response to a straight line. That is indicative of a highly resistive behavior, which for temperatures below 280 C could be attributed to a high series resistance [29]. The current was stable down to 260 C where after it continuously dropped down to almost zero at 2190 C (Fig. 8.4C), consistent with a high series resistance [29]. Also the hysteresis is strongly affected and vanishes completely at 280 C [29], which probably is an effect of restriction of the ionic motion often seen as responsible for the hysteresis [7580]. All those effects appears to be, given proper encapsulation, reversible with respect to temperature [29]. Several groups have reported similar device behavior at decreased temperatures [29,6668]. The best performance was also for the measurements at decreased temperatures found around room temperature [29,66,67], even if the drop in performance down to 280 C was reasonable low from an operational perspective (Fig. 8.4A). This behavior is in contrast to conventional PV-materials e.g. silicon, where a lower temperature is strictly better for the device performance, indicating that other physical mechanisms are in play in the perovskites. The behavior of the Voc and the increased series resistance were found to be unrelated pointing at more than one mechanism for decreasing performance at low temperatures
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[29]. One possible explanation for the increased resistance is a decrease of thermally activated charge hopping in the Spiro-MeOTAD hole conductor, which is in line with reported decrease in conductivity and hole mobility at lower temperature [81,82]. Other possible mechanisms relate to the perovskite phase. THz spectroscopy indicate that the mobility increases at lower temperature [39], in line with the simple Drude model [83], but the charge transport and recombination at grain boundaries might limit the device performance. Another possibility is the phase transformation from a tetragonal phase to a low temperature orthorhombic phase around 2113 C [15,23,24,27], which correlate reasonably well with the drop in Jsc and η as well as to a reported increase in the recombination constants [39]. A possible microscopic explanation is based on the movement and rotation of the organic dipolar cations, which at the lower temperatures is frozen in place when the thermal energy is not enough to flip them between their energetically preferred orientations in the lead halide cages [26], which possibly limits the current transport trough the perovskite. From a practical perspective. Elevated temperatures decrease cell performance, in line with expectations based on other solar cell technologies. The rate of decrease observed for MAPbI3 is 0.08% K21, which is high, but not catastrophically high. There is also a problem with thermal stability. It is, however, reasonable to assume that both aspects can be improved with compositional engineering of the perovskite and better interface control. At lower temperatures but within the boundaries for normal solar cell usage, performance decreases somewhat but not enough to be a problem. Room temperature thus seems to be something of a Goldilock’s zone for perovskite solar cells. In part because of peculiarities in the perovskite physics, and in part also because room temperature is the temperature where optimization has occurred. At even lower temperatures, perovskite solar cells does not appear to work at all. Why is still an open question, but it means that perovskites made with the present hole-selective layers not will be a good technology for space applications.
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Characterization Techniques for Perovskite Solar Cell Materials
8 Temperature effects in lead halide perovskites T. Jesper Jacobsson Department of Chemistry, Uppsala University, Uppsala, Sweden
8.1 Introduction Thermal motion is an omnipresent background to the web of reality, but also something that can have an extensive impact on the properties of materials. This is especially true for semiconductors where even the defining characteristics, conduction with limits, is highly temperature dependent. Semiconductor devices, like photovoltaic cells, are thus also affected by temperature. The difference between summer and winter, night and day, sunshine and shade, and the tropics and the north, means that the temperature for a solar cell can switch from below 220 C to above 80 C. For space applications, the operational temperature range can vary even more. For traditional solar cell materials, e.g. silicon, CIGS, GaAs, etc. the device performance gradually decrease when the temperature is increased. For lead halide perovskites, which are highly dynamic systems, the situation turns out to be more complex. The amount of thermal motion has a direct impact on the crystal structure of lead halide perovskites and the dynamics of its constituents, which has implications on, for example: defect formation, ionic movement, charge transport properties, dielectric behavior, recombination, and the interface chemistry. Temperature can be used as a variable parameter providing leverage for understanding physics of the system, e.g. activation energies for various processes. At low temperatures, new effects emerge, which give insights into the fundamental physics of the perovskites. Higher temperatures are on the other hand useful for investigating crystallization dynamics as well as degradation, which is a problem that must be supressed. For temperatures there between, where solar cells operate, plenty of physics is going on which impact the properties of the materials and the performance of devices. In this chapter, we look at how temperature play a role for the crystal structure of lead halide perovskites and the binding, motion, and importance of the organic ions. We discuss phase transformations, thermal
Characterization Techniques for Perovskite Solar Cell Materials DOI: https://doi.org/10.1016/B978-0-12-814727-6.00008-6
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8. Temperature effects in lead halide perovskites
expansion, optical properties, and stability. Finally, we go through what we known about how these can influence the performance of perovskite solar cell devices.
8.1.1 Crystal structure and phase transitions The most central motive out of which properties emerge is the crystal structure, i.e. the geometrical configuration in 3-dimentional space of the atoms in the compound. This configuration is very much constrained by the available thermal energy and the impact it has on the physical space occupied by the structure’s constituents, the excitation of the excitable, and the energetics of the bonding between neighboring atoms. In a discussion about the impact on temperature on the properties of perovskites, a recapture of the crystal structure is thus a natural starting point. The general perovskite structure has the composition ABC3, and the ideal structure has cubic symmetry and is composed of a backbone of corner sharing BC6-octahedra with cuboctahedra voids occupied by the A-cations (Fig. 8.1AC). Central for the structure is the relative size of the three ions. This can, at least partly, be captured by the concept of tolerance factor, t, as in Eq. 8.1 where rA, rB, and rC, are the ionic radius of the A, B, and C ions respectively [1]. Table 8.1 summarizes the impact the tolerance factors has on the structure.
FIGURE 8.1 (A) Ideal cubic unit cell of MAPbI3 illustrating the octahedral coordination around the lead ions. (B) Illustration of the extended structure and linking of the PbI6 octahedra. (C) The cuboctahedral coordination around the organic ion. (D) the MA ion. (E) Simulated energy landscape of the MA ion in the cubic phase in the form of a polar plot of the orientational energy surface [4]. (F and G) Illustration of the octahedra tilting pattern in the form of the PbI bond angel in the ideal tetragonal and cubic phase [3]. (H and I) Time distribution of simulated bond angels in the room temperature tetragonal phase (H) and the high temperature cubic phase (I) [3]. (E) Reproduced from J.S. Bechtel, R. Seshadri, A. Van der Ven, Energy landscape of molecular motion in cubic methylammonium lead iodide from first-principles, J. Phys. Chem. C 120 (23) (2016) 1240312410, with permission from the American Chemical Society; (FI) Adapted from C. Quarti, E. Mosconi, J.M. Ball, V. D’Innocenzo, C. Tao, S. Pathak, et al., Structural and optical properties of methylammonium lead iodide across the tetragonal to cubic phase transition: implications for perovskite solar cells, Energy Environ. Sci. 9 (1) (2016) 155163, permission from the Royal Society of Chemistry.
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8.1 Introduction
TABLE 8.1 Tolerance factors for the perovskite structure. Tolerance factor, t
Structure
Comment
,0.7
0.70.9
Tetragonal/Ortorombic/Rhombohedral
A too small or B too large
0.91.0
Cubic
Ideal perovskite structure
.1.0
Various layered structures
A-cation too large
Reproduced from T.J. Jacobsson, et al., J. Phys. Chem. C 119 (46) (2015) 2567325683, with permission from the American Chemical Society.
rA 1 rC t 5 pffiffiffi 2ðrB 1 rC Þ
(8.1)
There is a large class of related perovskite compounds with promising PVcharacteristics. Out of those, methyl ammonium lead iodide, CH3NH3PbI3 (or MAPbI3), is so far the most extensively investigated, and can therefore be seen as a standard perovskite and a model compound. Much of the subsequent discussion therefore use MAPbI3 as a baseline case. Given the ionic radius of Pb21 5 0.132 nm, I2 5 0.206, and CH3NH31 5 0.18 nm [2], MAPbI3 should according to the tolerance factor form a tetragonal structure, which also is the case at room temperature. The room temperature structure of organicinorganic lead halide perovskites is by no mean static, but is a highly dynamic system with atoms having a large amplitude around their equilibrium positions [3]. Core reasons behind this dynamism are the non-spherical symmetry of the organic MA/FA ions (Fig. 8.1D) and their week binding to the inorganic backbone of PbI6-octahedra. The organic ions reside in cuboctahedral voids with a nonuniform energy landscape spanned by the halogen atoms (Fig. 8.1E) [4]. In this landscape, the MA ions interact with dipoledipole interactions, where the NH?I interactions are the strongest [46], and have a number of energetically favorable orientations. The weak interactions means that the energy barriers for turning the MA-ions away from their favorable orientations are small, thus enabling jumping between preferred orientations in the halide cages [5,7]. At room temperature, the time scale for flipping around the MA ions in MAPbI3 is in the range of femtosecond to picoseconds [3,79], where a distinction can be made between a faster wobbling motion around the crystal axis and a slower reorientation of the molecular dipole with respect to the iodide lattice [8]. Defects can complicate the picture even further [10]. Those rotating dipoles implies a mean for screening charges by aligning towards them. This ability is reflected in a high dielectric constant [11,12], and is contributing to the low exciton energy observed [1315], and for why the hybrid perovskites are not excitonic materials at room temperature [13,15]. The ability to reorient around a localized charge may also have an impact on charge transport and it has been suggested that it slow down recombination [16]. Exactly how important the reorientabilty of the organic cation is for device performance is still an open question, especially as purely inorganic perovskite solar cells have been made [17,18], even if with lower efficiencies. To which extent the organic dipoles collectively order themselves into ferroelectric domains, and how this affect the charge charier separation, transport, and recombination
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has gained a lot of attention [7,1922]. There is a balance between increased entropy by randomness and energetic gain by collective organization, and any collective ordering will decrease while the temperature increases. This balance will also depend on the perovskite composition. Given the current literature, much more investigations are required until we reach a comprehensive understanding and consensus of how it really works. The ionic rotation is highly temperature dependent. At higher temperatures, the probability to overcome the activation energy for flipping increases, which leads to faster rotation. It also decreases the relative energetic contribution from columbic interactions with stationary charges or external fields compared towards the thermal energy. This leads to a gradual decrease in the dielectric constant [11,12,23,24], and the probability of collective ordering. By changing the composition of the perovskite by gradually exchanging the halogen, or the organic ion, the energy landscape in the halogen cage can be tuned, and it has been observed that this can shift the behavior of the rotations, as well as the activation barriers [25]. If the temperature is low, the thermal energy will not be enough to overcome the rotational barriers and the organic ions freeze in position [26], most likely in an ordered fashion [23,27]. For MAPbI3, this results in a phase transformation to a low temperature orthorhombic phase around 2113 C [15,23,24,27], which has a substantially lower dielectric constant [11,12,23,24], a higher excition energy [23], lower mobility [28], and appears to be useless for solar cell applications [29]. At higher temperatures, the thermal energy can be substantially higher than the weak bonding energy between the organic ion and the halide cage, leading to essentially free organic cation rotation. One effect of this is that the time-average shape of the organic ions goes towards spherical symmetry, whereby they also effectively take up a larger volume. In terms of the tolerance factors in Eq. 8.1, this means a larger rA and consequently a larger t, which pushes the structure towards cubic symmetry. For MAPbI3, this leads to a phase transition between a tetragonal to a cubic high temperature phase, which in diffraction measurements is observed around 54 C [27,3032]. Accompanying this transition are changes in the tilt of the lead-halogen octahedrals (Fig. 8.1F and G) which can effects, for example, the band gap [33]. By changing the organic ion and the halogen, the temperature of this phase transition can be shifted, but the general trend towards higher symmetric phases at higher temperatures are general for perovskites, and observed also for completely inorganic perovskites. For MAPbBr3, the tetragonal to cubic phase transition is pushed down to 262 C [24], and for FAPbI3, it is observed at 273 C [32]. Mixed perovskites where part of the MA is replaced by FA and part of the I is replaced with Br can thus give a cubic phase at room temperature [34], and thereby shift the tetragonal to cubic phase transition out of the normal operational window of terrestrial solar cells. Even if the time average shape of the organic ions at higher temperatures are spherical, they still have a non-spherical molecular shape (Fig. 8.1D). If the inorganic lead halide framework dynamically can shift in line with the rotation of the organic ions, the local structure may deviate from the cubic time average seen in for example XRD and Raman measurements. Simulations have shown that the lead halide framework dynamically accommodate the motion on a time scale fast enough for this to be the case [3]. The high temperature cubic structure may thus better be described by a fast switching between different tetragonal local configurations, which average out to a cubic structure [3]. This is
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illustrated in Fig. 8.1FI with the simulated time distribution of lead-halide bond angels for both the tetragonal and the cubic phase. On the timescale of electronic transitions, the material may thus seldom experience a cubic environment, but rather a shifting distorted tetragonal one. This is perfectly in line with the observation that the phase transition between the tetragonal to the cubic phase of MAPbI3 at B55 C not give more than a small and gradual change in for example absorption and device characteristics [3,29].
8.1.2 Thermal expansion coefficients Phase transitions in perovskites can be investigated by, for example, X-ray diffraction measured as a function of temperature, which has been done both for MAPbI3 [30], and a few other compositions [27]. Polycrystalline MAPbI3 undergoes a phase transformation around 54 C, and single crystals around 55 C, between a tetragonal and a high temperature cubic phase, which is of special interest as it is within the operational temperature window of solar cells. The tetragonal and the cubic phase have similar, but still distinctly different, XRD-patterns as illustrated in Fig. 8.2A where a number of double peaks for the tetragonal phase merge into single peaks for the higher symmetry cubic phase. Experimental data demonstrating the change of the XRD-pattern over the phase transition is given in Fig. 8.2B [30]. While cycling the temperature between room
FIGURE 8.2 (A) Comparison between experimental XRD data and simulated data for both the tetragonal and the cubic phase. The peaks most clearly distinguishing the experimental room temperature structure of MAPbI3 as tetragonal are highlighted in gray. (B) 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. (C) A graphical illustration of the change in cell parameters with increased temperature for MAPbI3. (D) Primitive unit cell volume of MAPbI3 as a function of temperature. (A, C, and D) Adapted from T.J. Jacobsson, L.J. Schwan, M. Ottosson, A. Hagfeldt, T. Edvinsson, Determination of thermal expansion coefficients and locating the temperature-induced phase transition in methylammonium lead perovskites using X-ray diffraction, Inorg. Chem. 54 (22) (2015) 1067810685; (B) Reproduced from T.J. Jacobsson, W. Tress, J.P. Correa-Baena, T. Edvinsson, A. Hagfeldt, Room temperature as a goldilocks environment for CH3NH3PbI3 perovskite solar cells: the importance of temperature on device performance, J. Phys. Chem. C 120 (21) (2016) 1138211393, both published with permission from the American Chemical Society.
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temperature to 80 C, this transition is fully reversible, at least during a few cycles [30]. If the transition would be reversible also over the thousands of cycles perovskites in solar cells modules would experience under their lifetimes is still an open question. Something seen in experimental XRD-data (Fig. 8.2B) is a drift in the peak positions towards smaller angels with higher temperature, i.e. an expansion of the lattice [30]. With increased temperature, the tetragonal structure expands along the a-axis and contracts along the c-axis (Fig. 8.2C) and does so in a linear fashion [30]. After the phase transition at 54 C, the a-axis continues to expand, essentially without a discontinuity. This is in line with a gentle and gradual phase transition, which as discussed above is based on the rotational energy of the MA ions in the lead-halogen octahedron, and that the phase transition not requires atomic reorganizations in the form of breaking or formation of covalent or ionic bonds. Given the lattice parameters, the primitive cell volume can be extracted (Fig. 8.2D) which also is found to expand linearly with temperature. With lattice parameters and cell volume as a function of temperature, the thermal expansion coefficients are readily extracted, where the linear thermal expansion coefficient in the length dimension L, αL, is given by Eq. 8.2, and the volumetric expansion coefficient, αV, is given by Eq. 8.3 where V is volume. αL 5
1 @L L @T
(8.2)
αV 5
1 @V V @T
(8.3)
The thermal expansion coefficients are largely independent of temperature between 25 C and 80 C, and were determined to: αa_tet 5 1.32 1024 K21, αc_tet 5 21.06 1024 K21, and αa_cub 5 4.77 1025 K21 [30]. The volumetric thermal expansion coefficient was determined to; αv 5 1.57 1024 K21, and is independent of the perovskite phase. This is a rather high expansion coefficient. It is, for example, more than six times larger than for soda lime glass (αv 5 2.6 1025 K21), four times large than for steel (αvB3.5 1025 K21) and twice that of lead (αv 5 8.7 1025 K21). It is also considerably higher than for other thin film solar cell materials, like CIGS (αv 5 2.7 1025 K21) [35] and CdTe (αv 5 1.4 1025 K21) [36] which are reasonably close to the one of soda lime glass. Thermal expansion coefficients that are significantly different between the absorber layer and the substrate is potentially a problem as it results in mechanical stresses during temperature cycling. The temperature changes occurring under years of daynight cycling could thus pose a problem for the microscopic mechanical stability of perovskite solar cells. The severity of the problem is still an open question. To our knowledge, thermal expansion coefficients has not been extracted for other perovskite compositions than MAPbI3. Until further data is provided, it is reasonable to assume the expansion coefficients to be relatively high for all hybrid perovskites. The phase transition between the tetragonal and the cubic phase appears, at least from a crystallographic perspective, not to be a major problem. It is, however, probably best to avoid if possible, and by partially replacing the methyl ammonium ions with larger ions,
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e.g. formamidinium, the tolerance factors can be modified so that the phase transition is shifted to lower temperatures giving a preference for the cubic phase already at room temperature [34].
8.1.3 Optical properties Optical absorption is the basis for charge carrier generation, and is thereby one of the most important aspects of a solar cell material. When the temperature decreases, the absorption onset for MAPbI3 has been observed to sharpen and shift to lower energies [3,15,3740]. This is illustrated in Fig. 8.3AE where the optical absorption for MAPbI3
FIGURE 8.3 (A) Absorption for MAPbI3 at selected temperatures. Data is background corrected. (B) A photo of the MAPbI3 film on which absorption was measured. (C) Band gap deduced from absorption data as a function of temperature. The small dip around 260 C is an artifact due to condensation on the measurement chamber. (D) and (E) Photos of the Lincam cell used for temperature dependent UVvis and IV measurements. Reproduced from T.J. Jacobsson, W. Tress, J.P. Correa-Baena, T. Edvinsson, A. Hagfeldt, Room temperature as a goldilocks environment for CH3NH3PbI3 perovskite solar cells: the importance of temperature on device performance, J. Phys. Chem. C 120 (21) (2016) 1138211393, published with permission from the American Chemical Society.
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was measured between 2190 C and 80 C [29]. The same behavior is reflected in steady state photoluminescence measurements [3739]. A shift in the absorption onset is equivalent to a shift in the band gap, which for MAPbI3 thus decreases with decreased temperature [3,41] (Fig. 8.3B), and while the temperature was changed from 80 C down to 2190 C, the band gap shifted from 1.61 eV down to 1.58 eV [29]. For most common semiconductors, e.g. Si, Ge, InP, InAs, GaAs, etc., the band gap does instead decrease with increased temperature, which can be explained by thermal expansion and changes in the electron-phonon interactions [42]. A possible explanation for the reversed behavior of MAPbI3 lies in the antibonding nature of its valence states [43]. At lower temperatures, the interatomic distances decreases which moves the energy of the antibonding states to higher energies due to increased orbital splitting, and thus decreases the band gap [37]. No changes or discontinuities were observed around the phase transition temperatures; 54 C for the tetragonal to cubic transition, and at 2113 C for the orthorhombic to tetrahedral transition. This behavior is in line with the phase transitions being gentle and gradual events. A change of the band gap of 0.03 eV is from a practical point of view a small change, especially given a temperature difference of almost 300 K. Significant temperature dependent changes in, for example, device performance can thus not be attributed to changes in the optical properties of the perovskite.
8.1.4 Degradation at higher temperature Perovskite solar cells have achieved impressive efficiencies over the last few years. Efficiencies that are high enough for devices to make a real technological impact if the long-term stability also can be controlled. Impressive improvements in stability have been reported over the last few years, i.e. from minutes to a few thousand hours under operational conditions, but further improvements are still needed. The pathways for perovskite degradation are plentiful and involves for example: moisture [44,45], light [46], oxygen [47,48], metal diffusion from contacts [49,50], etc. Increased temperature, which is the focus in this chapter, is also an important trigger for degradation. At elevated temperatures, MAPbI3 decompose into solid PbI2 and methyl ammonium and hydrogen iodine that can disappear in the gas phase (Eq. 8.4) [51]. Another suggested pathway involves decomposition of the methyl ammonium into ammonia and methyl iodine (Eq. 8.5) [52]. (8.4) CH3 NH3 PbI3 ðsÞ-PbI2 ðsÞ 1 CH3 NH2 ðgÞ 1 HI g (8.5) CH3 NH3 PbI3 ðsÞ-PbI2 ðsÞ 1 NH3 g 1 CH3 IðgÞ The exact temperature when MAPbI3 decomposes is non-trivial to pinpoint. Thermogravimetric analysis (TGA) indicates that MAPbI3 can be stable up to around 200 C [27,51,52], whereas considerably lower temperatures appears to be a problem in most environments. In part, this discrepancy is about timing as only a small part of the material needs to decompose before the device performance is compromised. It is also due
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to combined effects as increases in temperature can accelerate decomposition along other pathways requiring for example light or moisture [53]. A typical annealing temperature during crystallization of MAPbI3 is 100 C. That is probably not high enough to decompose the perovskite in itself, unless there are other decomposition pathways available effected by the elevated temperature. To make it even more complicated, a certain degradation can under some conditions be beneficial. If the outermost surface layer decomposes during the annealing, a small excess of lead iodine can form which is beneficial for device performance [5456], as long as the other decomposition products are vented out in the gas phase. In terms of thermal degradation of MAPbI3, the weak point appears to be MA ions, and attempts have thus been made to replace them. One approach is to go for all inorganic perovskites with Cs as the cation. Cs based all inorganic solar cells have been made [17,18], indicating that it is possible, and those perovskites are indeed more thermally stable. Efficiencies are so far, however, inferior, but it is an open question whatever or not they can be as good as the hybrid perovskites, or if the dipole moment of the organic ion is necessary for achieving the highest efficiencies. Another alternative is to replace MA with formamidinium, FA [57], or guanidinium [58]. The FA perovskites appears to be more thermally stable [59]. Unfortunately, the cubic (or pseudocubic) α-phase of FAPbI3 easily transform into a yellow polymorph at room temperature which has hexagonal symmetry (P63mc) and is unsuitable for PVapplications [32,57,60,61]. That transformation could be avoided by mixing in some MA or Cs. At the time of writing, an interesting composition is mixed FACs-perovskites [62,63]. FA stabilises the Cs-perovskite, and Cs stabilising the FA perovskite, giving a mixture that has a stable cubic phase at room temperature. Those compositions thereby avoid MA and gain in thermal stability. So far, those perovskites are, however, not the best performing ones. Some of the best composition are at the time of writing Cs and/or Cs-Rb doped FAMA perovskites [64,65] (e.g. Cs0.5FA0.79MA0.16PbBr0.51I2.49) which has demonstrated good stability under illumination at 80 C [64,65]. This indicates that a small amount of MA-ions can be beneficial in terms of crystallization and in stabilising the FA perovskite, and that they potentially not seriously compromise the thermal stability up to 80 C as long as they only are present in a small amount.
8.1.5 Device performance From a solar cell perspective, the most important temperature dependent parameter is device performance. For MAPbI3, a few groups has investigated this both above [29,41,6668] and below [29,6668] room temperature. The details differ between the measurements but the general trends that here will be discussed are the same. Device performance as a function of temperature is in Fig. 8.4A 2 D given for MAPbI3 solar cells based on data from Jacobsson et al. [29], which are representative of the temperature dependent performance measurements so far reported. While increasing the temperature from room temperature up to 80 C, the highest efficiencies where measured at 3035 C. At higher temperatures, η, Voc, and Jsc, all gradually decreased (Fig. 8.4AC), whereas the FF peaked around 60 C (Fig. 8.4D). When the cells returned to room
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FIGURE 8.4 Device performance as a function of temperature for devices of standard configuration, i.e. FTO/ TiO2/mesoporous TiO2/MAPbI3/Spiro-MeOTAD/Au. Two different cells where used for the measurements; one for temperatures above room temperature, and another one for lower temperatures. (A) Efficiency. (B) Open circuit voltage. (C) Short circuit current. (D) Fill factor. Data has previously been published by T.J. Jacobsson, W. Tress, J. P. Correa-Baena, T. Edvinsson, A. Hagfeldt, Room temperature as a goldilocks environment for CH3NH3PbI3 perovskite solar cells: the importance of temperature on device performance, J. Phys. Chem. C 120 (21) (2016) 1138211393.
temperature, they recovered most of the loss in performance, showing a reasonable degree of reversibility. The decrease in measured efficiency with increased temperature was approximately 0.08% K21. A decrease in efficiency at elevated temperatures is expected based on the behavior of other solar cell materials [69]. The magnitude is, however, larger than observed for silicon, and large enough to be a concern with only 80% of the room temperature performance remaining at 60 C. This is problematic but not a showstopper for outdoor use of perovskite solar cell; especially given the rapid evolution of both device performance and stability, which makes it reasonable to assume further improvements. The open circuit voltage is expected to decrease at higher temperatures as an effect of increased entropy, which reduces the electrochemical energy of electrons and holes in the conduction and valence band, broadens the Fermi-Dirac distribution, and pushes the quasi-Fermi levels further away from the band edges. The data up to 50 C follows this
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trend (Fig. 8.4A) [29,41], indicating that the Voc change is governed by charge carrier recombination in the perovskite film within that temperature range. At higher temperatures, the Voc decrease accelerates, which was attributed to irreversible degradation [29]. It is assumed that the MA ions trigger a thermal instability [27,5153], which may be the cause for the observed degradation. Other compositions, especially Cs and Cs/Rb doped mixed perovskites (e.g. Cs0.5FA0.79MA0.16PbBr0.51I2.49) has showed higher thermal stability with up to 1000 hour of stability at 80 C [64,65], indicating that thermal stability probably can be accomplished within the operational window of terrestrial photovoltaics. A question of interest is if the perovskite phase, and in particular the phase transformation from tetragonal to cubic seen for MAPbI3 at 54 C, has an effect on the device performance. The downward trend in η, Vov and Jsc while the temperature is increased is uniform. The measurement point at 60 C, just after the phase transition, 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 that the best performing solar cells at the moment are based on formamidinium rich perovskites [57,7072], which have a cubic symmetry at room temperature [73]. 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. At lower temperatures, the changes in device performance are more dramatic. The Voc increases with decreasing temperature down to 280 C, which continues the trend seen for higher temperatures up to 50 C (Fig. 8.4B). That is in line with the expected behavior for inorganic semiconductor solar cells, and has been observed by different groups [29,66]. At even lower temperatures, the Voc decreases, and at temperatures below 2160 C, it goes to zero. The lower Voc is indicative of a reduction in charge carrier density, i.e. by either an increased recombination rate or by a reduced probability of charge carer generation by exciton dissociation [74]. The FF decreases unevenly down to 2120 C (Fig. 8.4D) where the IV-curve has gone from the characteristic shape of a solar cell response to a straight line. That is indicative of a highly resistive behavior, which for temperatures below 280 C could be attributed to a high series resistance [29]. The current was stable down to 260 C where after it continuously dropped down to almost zero at 2190 C (Fig. 8.4C), consistent with a high series resistance [29]. Also the hysteresis is strongly affected and vanishes completely at 280 C [29], which probably is an effect of restriction of the ionic motion often seen as responsible for the hysteresis [7580]. All those effects appears to be, given proper encapsulation, reversible with respect to temperature [29]. Several groups have reported similar device behavior at decreased temperatures [29,6668]. The best performance was also for the measurements at decreased temperatures found around room temperature [29,66,67], even if the drop in performance down to 280 C was reasonable low from an operational perspective (Fig. 8.4A). This behavior is in contrast to conventional PV-materials e.g. silicon, where a lower temperature is strictly better for the device performance, indicating that other physical mechanisms are in play in the perovskites. The behavior of the Voc and the increased series resistance were found to be unrelated pointing at more than one mechanism for decreasing performance at low temperatures
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[29]. One possible explanation for the increased resistance is a decrease of thermally activated charge hopping in the Spiro-MeOTAD hole conductor, which is in line with reported decrease in conductivity and hole mobility at lower temperature [81,82]. Other possible mechanisms relate to the perovskite phase. THz spectroscopy indicate that the mobility increases at lower temperature [39], in line with the simple Drude model [83], but the charge transport and recombination at grain boundaries might limit the device performance. Another possibility is the phase transformation from a tetragonal phase to a low temperature orthorhombic phase around 2113 C [15,23,24,27], which correlate reasonably well with the drop in Jsc and η as well as to a reported increase in the recombination constants [39]. A possible microscopic explanation is based on the movement and rotation of the organic dipolar cations, which at the lower temperatures is frozen in place when the thermal energy is not enough to flip them between their energetically preferred orientations in the lead halide cages [26], which possibly limits the current transport trough the perovskite. From a practical perspective. Elevated temperatures decrease cell performance, in line with expectations based on other solar cell technologies. The rate of decrease observed for MAPbI3 is 0.08% K21, which is high, but not catastrophically high. There is also a problem with thermal stability. It is, however, reasonable to assume that both aspects can be improved with compositional engineering of the perovskite and better interface control. At lower temperatures but within the boundaries for normal solar cell usage, performance decreases somewhat but not enough to be a problem. Room temperature thus seems to be something of a Goldilock’s zone for perovskite solar cells. In part because of peculiarities in the perovskite physics, and in part also because room temperature is the temperature where optimization has occurred. At even lower temperatures, perovskite solar cells does not appear to work at all. Why is still an open question, but it means that perovskites made with the present hole-selective layers not will be a good technology for space applications.
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