Extending the Compositional Space of Mixed Lead Halide Perovskites

Article pubs.acs.org/JPCC

Cite This: J. Phys. Chem. C 2018, 122, 13548−13557

Extending the Compositional Space of Mixed Lead Halide Perovskites by Cs, Rb, K, and Na Doping T. Jesper Jacobsson,*,† Sebastian Svanström,‡ Virgil Andrei,§ Jasmine P. H. Rivett,∥ Nikolay Kornienko,§ Bertrand Philippe,‡ Ute B. Cappel,⊥ Håkan Rensmo,‡ Felix Deschler,∥ and Gerrit Boschloo† †

Department of Chemistry, Uppsala University, Box 538, 75121 Uppsala, Sweden Department of Physics and Astronomy, Uppsala University, Box 5516, 75120 Uppsala, Sweden § Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom ∥ Cavendish Laboratory, Department of Physics, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom ⊥ Department of Chemistry, KTH Royal Institute of Technology, Teknikringen 30, 10044 Stockholm, Sweden

J. Phys. Chem. C 2018.122:13548-13557. Downloaded from pubs.acs.org by UNIV OF ROCHESTER on 05/22/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: A trend in high performing lead halide perovskite solar cell devices has been increasing compositional complexity by successively introducing more elements, dopants, and additives into the structure; and some of the latest top efficiencies have been achieved with a quadruple cation mixed halide perovskite CsxFAyMAzRb1‑x‑y‑zPbBrqI3‑q. This paper continues this trend by exploring doping of mixed lead halide perovskites, FA0.83MA0.17PbBr0.51I2.49, with an extended set of alkali cations, i.e., Cs+, Rb+, K+, and Na+, as well as combinations of them. The doped perovskites were investigated with X-ray diffraction, energy-dispersive X-ray spectroscopy, scanning electron microscopy, hard X-ray photoelectron spectroscopy, UV−vis, steady state fluorescence, and ultrafast transient absorption spectroscopy. Solar cell devices were made as well. Cs+ can replace the organic cations in the perovskite structure, but Rb+, K+, and Na+ do not appear to do that. Despite this, samples doped with K and Na have substantially longer fluorescence lifetimes, which potentially could be beneficial for device performance.

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To increase the thermal stability23 and to shift the tetragonal to cubic phase transformation outside the operational range of terrestrial photovoltaics,24,25 the MA ions were exchanged with the larger and less volatile formamidinium ion (FA or CH(NH2)2+).26−29 FAPbI3 also has a lower band gap than MAPbI3 (∼1.45−1.52 eV vs ∼1.59 eV),30 which is closer to the single junction optimum,31 and cell efficiencies up to 20% have been reported.29 Unfortunately, the perovskite phase is not stable at room temperature where it decomposes into a yellow polymorph unsuitable for PV-applications.26,32,33 Mixing MA and FA within the same perovskite has turned out to be a way to avoid some of the problems found for both the single cation compositions.27,32,34 Iodide can be replaced with both chloride35 and bromide,36 either fully or gradually, for both the MA37−39 and FA perovskites.23 By gradually changing I to Br, the band gap can be tuned between 1.5 and 2.3 eV, which is highly advantageous for tandem applications. It is also possible to change the MA/ FA and Br/I-ratios simultaneously,29,40,41 and by accessing

INTRODUCTION The first report on lead halide perovskites for PV-applications was published in 2009.1 A few key advances in the following years2−6 triggered a rapid expansion of the field resulting in a stunningly fast improvement in device performance.7 Top solar cell efficiencies now reach beyond 22%,8 giving perovskites a reasonable chance to reach commercial competiveness, either alone or in tandem with conventional PV-technologies.9−11 There is an entire family of APbX3 perovskites of interest for PV-applications, where A and X are different cations and halogen anions (Figure 1a). Every perovskite has its own peculiarities, and to tackle problems of efficiency, stability, reproducibility and so on, there has been a drive to explore a wider set of compositions. The trend has been from the relatively simple to the progressively more complex, with multicomponent mixtures, dopants, and additives (Figure 1b). This work contributes with an additional step in this direction by exploring the impact of alkali cation doping, i.e., Cs, Rb, K, and Na, on mixed MA/FA−Pb-I/Br perovskites. The most investigated perovskite so far, as well as one of the simplest, is CH3NH3PbI3 (MAPbI3), with which much of the initial progress was accomplished. Cell efficiencies over 19% 12−14 have been accomplished but poor thermal stability,15−19 volatility of the MA ion, and a phase transformation at 54 °C,20−22 hampers its long-term potential. © 2018 American Chemical Society

Special Issue: Prashant V. Kamat Festschrift Received: December 19, 2017 Revised: January 24, 2018 Published: January 24, 2018 13548

DOI: 10.1021/acs.jpcc.7b12464 J. Phys. Chem. C 2018, 122, 13548−13557

Article

The Journal of Physical Chemistry C

Figure 1. (a) Idealized perovskite structure showing how the lead halide octahedra link together, and how they form cuboctahedral voids filled by organic cations and/or alkali substitutes. (b) Illustration of the trend toward a higher compositional complexity for the best performing perovskites.

would they be beneficial in the first place? Is it entropic stabilization, trap passivation, crystallization modifications, transport related, or something completely different? Would even smaller alkali cations have the same or different effects, and could an even further compositional complexity be beneficial? Those are some of the questions investigated in this paper. This was done by doping the baseline mixed perovskite (FA0.83MA0.17PbBr0.51I2.49) with CsI, RbI, KI, or NaI, as well as different combinations of them, and exploring how the doping affects the structure, composition, morphology, optical response, and device performance.

those additional degrees of freedom, more stable perovskites with higher efficiencies have been produced. At the time of writing, those mixed FAMAPbBrI perovskites could be seen as a base composition and the few last certified records in the NREL-chart42 are based on compositions in the vicinity of FA0.83MA0.17PbBr0.51I2.49. The detailed stoichiometry also plays a role, and the best devices contain a slight surplus of PbI2.12,43−45 Following the recent trend is an ongoing exploration of the compositional space beyond the mixed FAxMA1−xPbBryI3‑y perovskites, and in this paper, we focus on the effects of doping with alkali cations, i.e., Cs+, Rb+, K+, and Na+. Cs+ has the right size to fit into the lead halide perovskite structure, and inorganic Cs perovskites have been made.44,46−48 CsPbI3 has a band gap of 1.73 eV,23 but just like FAPbI3, it decomposes into a yellow orthorhombic polymorph at room temperature. CsPbBr3 is more stable49,50 but has a band gap of 2.32 eV,49 which is less suitable for PV applications. Partial Cs replacement is, however, promising, and FA-Cs perovskites overcome the stability problems encountered for both FAPbI3 and CsPbI3,51−53 potentially due to a more favorable mean cation size54 and increased mixing entropy.51 Triple cation perovskites with a bit of bromide, Cs0.05FA0.79MA0.16PbBr0.51I2.49, have been particularly promising with efficiencies over 21% and operational stabilities of a few hundred hours.53 How far can we continue this expansion of the compositional space, with its increased level of complexity, until the increased number of degrees of freedom no longer provide additional benefits? That is an open question. One limitation is the number of potential cations with the right size and charge. Too large a cation increases Goldsmith’s tolerance factor55,56 to above 1, whereupon layered 2D perovskites forms.57−61 FA is close to that limit. Too small a cation and other undesirable phases will form as well. For single atom ions, only Cs+ fits the size and charge criterion.62 Rb+ is, however, only slightly too small according to the simple theory. Based on this observation, attempts to synthesize Rb and Rb-doped perovskites were made.62 The Rb perovskite (RbPbBryI3‑y) does not form, in line with the simple reasoning based on tolerance factors, but mixed perovskites with a few percent Rb-doping have successfully been made.62 By combining a few percent Cs and Rb doping, quadruple cation mixed perovskites (CsxFAyMAzRb1‑x‑y‑zPbBrqI3‑q) with cell efficiencies as high as 21.6% have been demonstrated.62 Efficiencies, as well as both stability and reproducibility were improved by the Rb, and Cs doping,62 again demonstrating positive effects achievable with a higher compositional complexity. This development spurs additional questions. Does Rb replace cations in the perovskite structure, despite its small size, or does it result in other secondary beneficial effects; and why

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EXPERIMENTS Perovskite precursor solutions were prepared in a glovebox with a nitrogen atmosphere. Stock solutions of PbI2 and PbBr2 were prepared in advance, whereas the final precursor solutions were prepared just before perovskite deposition. Anhydrous DMF:DMSO in the proportion 4:1 was used as solvent for the perovskite solutions. Two master solutions and four doping solutions were prepared; (a) 1.25 M PbI2 and 1.14 M FAI, (b) 1.25 M PbBr2 and 1.14 M MABr, (c) 1.38 M CsI in DMSO, (d) 1.38 M RbI in DMSO, (e) 1.38 M KI in DMSO, and (f) 1.38 M NaI in DMSO. The final perovskite solutions were prepared by mixing the stock solutions according to Table 1. Table 1. Preparation of Final Precursor Solutions Were Done by Combining Solutions a−f in Volume Parts Per 100 According to the Tablea ID 1 2 3 4 5 6 7 8

doping 6% 6% 6% 6% 3% 3% 2%

Cs Rb K Na Cs, 3% Rb Cs, 3% K Cs, 2% Rb, 2% K

a

b

83 78 78 78 78 78 78 78

17 16 16 16 16 16 16 16

c

d

e

f

6 6 6 6 3 3 2

3 2

3 2

a

(a) 1.25 M PbI2, 1.14 M FAI (b) 1.25 M PbBr2, 1.14 M MABr, (c) 1.38 M CsI, (d) 1.38 M RbI, (e) 1.38 M KI, and (f) 1.38 M NaI.

The MA and FA salts were bought from Dyesol, the lead salts from TCI, solvents from Fisher, and the remaining chemicals from Sigma-Aldrich. All chemicals were used as received without further treatment. The perovskite deposition is described in detail in the Supporting Information (SI). In short, perovskites were spincoated in a N2 filled glovebox using a one-step antisolvent method with chlorobenzene as antisolvent. The deposited films were annealed at 100 °C in the glovebox for 30−60 min and then stored in dry air. Samples for TAS-measurements were spun from diluted solutions (diluted a factor of 2 and 4) to 13549


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The Journal of Physical Chemistry C ensure sufficient transparency. For X-ray diffraction (XRD), UV−vis, and fluorescence measurements, soda lime glass (SLG) substrates were used. As substrates for solar cell devices, FTO was spray coated with a TiO2 blocking layer and then covered with a spin-coated mesoporous TiO2 scaffold. Spin coated Spiro-MeOTAD doped with FK209, Li-TFSI, and TBP was used as hole-conductor. To finalize the devices, 80 nm Au was thermally evaporated, which gives the device stack: FTO \TiO2\TiO2 mesoporous\Perovskite\Spiro\Au (Figure 7f). UV−vis was measured on a Cary 50 Bio from Varian. XRD measurements were performed in ambient atmosphere with an Empyrean diffractometer from Panalytics. Steady state fluorescence was measured with a Spectrofluorometer FS5 from Edinburg Instruments with a Xe-light source, an excitation wavelength of 450 nm, a step size of 0.5 nm, a dwell time of 0.5 s, and an exit slit of 7 nm. The same equipment was used for time-dependent PL measurements but with an EPL 450 ps pulsed diode laser as a light source. IV-characteristics was measured with an Emstat3 from Palmsens using a Xe light source from Newport with an AM 1.5 filter and calibrated to 1 Sun with a Si-reference diode. Scanning electron microscopy (SEM) imaging was carried out using a Zeiss LEO 1550 scanning electron microscope. The energy-dispersive X-ray spectroscopy (EDX) measurements were done within the SEM using an 80 mm2 silicon drift detector and an excitation energy of 10 keV. Quantifications were performed using the Aztec software package from INCA energy. Hard X-ray photoelectron spectroscopy (HAXPES) was performed at the HIKE end station at the KMC-1 beamline at BESSY II.63 A photon energy of either 2100 or 4000 eV was selected using a double-crystal monochromator (OxfordDanfysik), and the emitted electrons were detected using a Gammadata Scienta R-4000 hemispherical analyzer. The binding energy was for all measurements calibrated against the Au 4f peak of a gold foil mounted on the manipulator in electrical contact with the sample. The perovskite core levels were fitted with Voigt functions for quantification. The relative concentrations of the perovskite were determined by dividing the area of the fitted Voigt function with the photoionization cross section64 and normalizing against the Pb 5d peak concentration at the same spot. The Pb 5d, I 4d, Br 3d, and C s4d core levels were measured at 3−5 spots for each excitation energy and sample in order to minimize the effect of surface variations. The O 1s, N 1s, and C 1s were measured on a single spot due to the significantly longer measurement time. Transient absorption measurements were carried out using the output of a Ti:sapphire amplifier system (Spectra-Physics Solstice) operating at 1 kHz and generating 90 fs pulses, which was split into pump and probe beam paths. Visible broadband probe beams were generated in home-built noncollinear optical parametric amplifiers (NOPAs), and visible narrowband (25 meV full-width at half-maximum) pump beams were provided by a TOPAS optical parametric amplifier (Light Conversion). The transmitted pulses were collected with an InGaAs dual-line array detector (Hamamatsu G11608−512), driven, and read out by a custom-built board from Stresing Entwicklungsbüro.

Figure 2. Fluorescence decay at the maximum steady state wavelength around 780 nm (Table 3) measured for different doping conditions after excitation with a 450 nm picosecond laser pulse. Perovskite samples were deposited on SLG. For corresponding measurements for perovskites deposited on FTO/TiO2 and exponential fits, see SI.

Table 2. Fit of a Two-Exponential Function (eq 1) to the Experimental Dataa ID 1 2 3 4 5 6 7 8

doping 6% 6% 6% 6% 3% 3% 2%


C1

τ1 [ns]

C2

τ2 [ns]

R2

0.76 0.47 0.30 0.64 0.42 0.55 0.56 0.79

774 1155 1135 4019 4756 1255 3952 4209

0.19 0.47 0.69 0.23 0.54 0.43 0.40 0.17

175 482 462 843 660 373 1509 1409

0.999 0.999 0.999 0.997 0.997 0.999 0.998 0.998

a Films deposited on SLG. For corresponding plots as well as fits of one and three exponentials, see SI.

substrates reproduce the trend. The decay was, however, faster for films deposited on FTO/TiO2 than on SLG (SI). To extract the fluorescence lifetimes, the data were fitted with one, two, and three exponentials (SI), where a two-exponential function (eq 1) gives a good fit in all cases (Table 2). The lifetimes for the K and Na doped perovskites were up to 4 μs. That is in the longer range of reported values in the literature,65−69 and is an indication of excellent crystal quality. A longer fluorescence lifetime means a higher probability for a diffusing charge carrier to be extracted at the charge selective contact before recombining in the perovskite. It is also an indication of good crystal quality with fewer defects. This indicates that something favorable is going on in the K- and Nadoped perovskites that potentially could be utilized for constructing higher performing devices. I(t ) = I0C1e−t / τ1 + I0C2e−t / τ2

(1)

X-ray diffractograms were measured on all compositions (SI). The undoped mixed perovskite, FA0.83MA0.17PbBr0.51I2.49 (Figure 3a), has a cubic structure and is the only crystalline phase besides PbI2 that is there by design.43 This is in line with previous studies.30 The diffractogram of the Cs-doped perovskite has the same appearance, whereas additional peaks are seen in the Rb-doped perovskite, potentially from the nonperovskite delta phase,62 or some other Rb-compounds. Both the K and Na-doped perovskites have a small yet unidentified peak around 27.3°, but are aside from that very similar to the standard mixed perovskite (Figure 3a). Cs in combination with other alkali ions stabilizes the perovskite

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RESULTS AND DISCUSSION The most interesting results concern the fluorescence decay, which for perovskites doped with KI or NaI was around 4 times slower than for the other samples (Figure 2, Table 2). The findings are robust in the sense that different samples synthesized at different occasions and deposited on different 13550


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Figure 3. (a) X-ray diffractograms for CsI, RbI, KI, and NaI doped perovskites compared to the undoped mixed perovskite. The background is subtracted and data is normalized toward the 001 reflex around 14°. For the complete set of figures, see SI. (b) Normalized data for the 001 reflex illustrating peak shifts and broadening. (c) EDX data on the same set of perovskites. Figures zoomed in on the regions around the peaks for the alkaline peaks, which more clearly show their presence, are found in the SI.

Figure 4. EDX mapping of the doped elements. The elemental maps under each SEM images correspond to the dopant for that composition. For the full set of elemental maps for each composition, see the SI.

doping, the XRD data is not conclusive, but it points at an expanding lattice (SI) compared to the Cs-doped case. Rb is thus probably not fully incorporated into the perovskite structure, and if it does, it may be as an interstitial rather than as a replacement. This observation is in line with the few XRD data sets so far published on Rb-doped perovskites.62,70 Another possibility would be that Rb replaces lead, but we do not have support for that hypothesis. For the K and Na-doped samples, there is a weak trend, even though not conclusive, toward a contracting lattice (SI) indicating that some level of substitution potentially could occur, despite their small size. A more in-depth analysis and data sets are found in the SI. None of the dopants had any significant effect on the peak broadening (Figure 3b) EDX measurements were used to investigate the presence of the dopant atoms in the films (Figures 3c and 4, and the SI). Due to their low concentration, partially overlapping peaks (for Cs, Rb, and K), and low signal (for Na), the absolute quantification is not particularly reliable (SI). We observe signals from the alkali dopants, but Rb, K and Na appear to be less abundant than Cs, despite them all having the same concentration in their respective precursor solution. The spatial distribution of the doping atoms was analyzed using EDX mapping (Figure 4). The excitation volume of the electron beam limits the spatial resolution, but as far as we can tell, the alkali dopants are evenly distributed within the films. It would not be unreasonable to assume that the dopants would concentrate in the grain boundaries and in the secondary phases seen in the XRD measurements (Figure 3a and SI) for the Rb-doped case, especially if they do not substitute the

structure (SI). That is either due to Cs-induced stabilization or the fact that 6% Rb, K, and Na doping is enough to cause inclusions of a secondary phase whereas 3% is not. A recent report showed that 10% but not 5% Rb could cause a phase separation,70 which roughly is in line with our data. The XRD measurement does not capture a possible degradation in ambient air. The samples were loaded in a magazine in air and measured one after another with a 1 h delay for each sample. The secondary phases could thus be a result of storing the films in ambient humidity. A central question is if the dopants are incorporated into the dominant perovskite structure and substitute the organic cations. A smaller “A” cation should reasonably lead to a contraction of the lattice and a corresponding shift of the diffraction peaks toward higher diffraction angles; like when FA is replaced by the smaller MA ion, which shifts the (001) reflection at 14° by 0.2°.30 This is not observed (Figure 3b and SI). There is instead a small shift (0.07° for the 011 reflection) toward lower diffraction angles indicating an expanding lattice. This could be rationalized by a change in the I/Br ratio. The doped samples are spun from slightly more iodide rich solutions, as only iodide was used as counterions for the alkali metals (Table 1). Br tends to preferentially go into the perovskite structure,30 but a more iodide rich environment could lead to a more iodide rich perovskite with a larger lattice. That is the most likely rationalization for the Cs-doped samples, given that theory71 as well as a number of studies on ion exchange support complete Cs substitution.72,73 If the iodide-effect is accounted for by only comparing the doped samples, the difference is smaller. In the case of Rb13551


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Figure 5. (a) UV−vis of films deposited on SLG. (b) Steady state fluorescence. (c) Transient absorption map for the undoped perovskite. 400 nm excitation, 10 uW power, 450 um spot size. For the full set of TAS-maps; see the SI. (d) Transient absorption spectra between 1 and 10 ps.

thus be treated as an indicative fact among others, rather than as a conclusive statement. The steady state fluorescence (Figure 5b) behaved similar for all samples, with one distinct fluorescence peak shifted to slightly lower energies than the absorption edge (Table 3). While considering sample-to-sample variations, there is no clear trend with respect to fluorescence energy (Table 3 and SI). There was, however, a difference in the fluorescence intensity. The measurements are relative, but the observed intensities were higher for the K and Na doped perovskite films (Figure 5b and Table 3). That could indicate higher crystal core qualities and matches the longer fluorescence lifetimes (Figure 2). The fluorescence intensities were higher for films deposited on SLG than on FTO/TiO2 into which electrons can inject, but the trend among the samples was the same (SI). The time dynamics of the excitation process was explored with transient absorption spectroscopy (TAS). The hot carrier cooling was in the subpicosecond range, as seen from the TASmap of the undoped perovskite in Figure 5c. After the thermalization, a stable excited state was reached during the rest of the measurements time window, which agrees with the longlived steady state fluorescence (Figure 2). The TAS-maps for the doped perovskites are essentially identical (SI). A reasonable interpretation is that the alkali dopants do not significantly change either the crystal structure or the electronic structure, which both could affect the fast charge carrier dynamics in the perovskite. A slight red shift in the TAS measurements was, however, observed for the Na-doped sample (Figure 5d), which is surprising, as no corresponding shift was seen in the linear absorption spectra. A potential explanation is that Na-doping increases the tendency for a light induced phase separation and/or degradation. That couples to observed laser damage of one of the Na-doped samples during the lifetime measurements, to the lower device performance (Figure 7), and that Na-doped samples deposited on SLG after a few months of storage had decomposed, whereas the other samples had not. To gain further insights into the electronic structure and the elemental composition, HAXPES was measured at two excitation energies, 2100 and 4000 eV. The variations in stoichiometry of Pb, I, Br, and MA/FA between the samples were small (SI) and we were unable to correlate those to the alkali doping. For Cs, a Cs/Pb ratio of approximately 0.04 was observed, which is in good agreement with the amount of Cs added. We observed K and Na in very small amounts with a ratio estimated to 0.008 K/Pb and 0.01 Na/Pb, respectively, in their corresponding samples. However, these values are subject to great uncertainties due to overlapping peaks and due to being close to reliable detection limits. Furthermore, no signal of Rb were observed as far down in the films as can be probed with a 4000 eV measurement (around 20 nm82). Other recent HAXPES measurements on Rb-doped perovskites have shown

organic cations, but that is not supported by the EDX mapping data. The surface morphology was evaluated from the top view SEM images in Figure 4. A full set of SEM images at different magnification are found in the SI. Generally, there is a problem with pinholes for the baseline mixed perovskite (SI), which we attribute to technical problems with maintaining a good atmosphere in our glovebox. The Rb-doped films, and to a smaller extent the Cs-doped films (Figure 4), have a less even topography. That could, for the Rb-doped film, be a consequence of the secondary phase formed (Figure 3a). Both the K- and Na-doped films have smooth and even surface morphology similar to how the baseline case, FA0.83MA0.17PbBr0.51I2,49, normally looks like (SI). No difference in grain size can be seen from the SEM images, which is in line with the XRD-peak widths (Figure 3b and SI). The optical absorption is similar for all samples (Figure 5a), which is in line with the XRD data indicating that the perovskite phase is essentially the same. The optical band gaps were extracted from the absorption data using the parabolic band approximation74−76 (SI and Table 3). The organic cations Table 3. Band Gaps, Steady State Fluorescent Peak Positions, and Measured Intensity at the Peak Wavelengtha ID 1 2 3 4 5 6 7 8 a

doping 6% 6% 6% 6% 3% 3% 2%


Eg [eV]

λmax [nm]

Imax [a.u.] (1E6)

1.638 1.639 1.627 1.627 1.613 1.635 1.632 1.630

777 771 790 780 788 775 762 774

0.96 0.69 0.52 1.74 1.46 0.81 0.85 1.97

Extracted from the data in Figure 5a,b.

do not contribute to density of states close to the band edges,77,78 wherefore they have no direct impact on the band gap. They could, however, have an indirect effect by influencing the lattice parameters and the tilt between the lead halide octahedral.78−81 By exchanging the smaller MA ion (r = 217 pm56) with the larger FA ion (r = 253 pm56), the lattice expands and the band gap decreases by about 0.05 eV for iodide, bromide, as well as for mixed perovskites.30 Like the XRD data, the band gaps (Table 3) do not support a contraction of the perovskite lattice while doped with progressively smaller alkali cations. The trend is rather the opposite, pointing at interstitials rather than substitution. It should, however, be pointed out that the observed band gap shifts are of a size hard to distinguish from background noise and sample-to-sample variation. The observed trend should 13552


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Figure 6. (a) Valence band spectra of the different perovskites measured at an excitation energy of 4000 eV. (b) Fraction of metallic lead toward Pb(+II) as a function of time (expressed as the number of sweeps) as a proxy for stability under X-ray illumination.

Figure 7. Device parameters for cells made with different dopants. (a) Efficiencies. (b) Short circuit current. (c) Open circuit voltage. (d) Fill factor. (e) Hysteresis defined as the ratio between the difference of the integrals of the current in the backward and forward scan and the integral of the current in the backward scan.88 (f) Device architecture.

being incorporated into the perovskite structure, form another phase that precipitates early on in the thermal annealing after the perovskite deposition, they may preferentially end up deep down in the film where they are invisible in HAXPES and shadowed in the EDX measurement. Such a phase would according to the XRD data be crystalline for Rb and possibly amorphous for K and Na. The longer lifetimes for the K and Na doped perovskites may thus not be attributed to cation substitution but to secondary effects, like for example trap passivation or modifications of the crystallization dynamics. A reduced defect density is likely involved, and it has recently been showed that K doping leads to a reduction in the low frequency capacitance, which could be linked to the defect density.85 That was also backed by theoretical arguments indicating that potassium ions are able to prevent the formation of Frenkel defects.85 Another possible secondary effect relates to the additional iodide counterions the alkali cations bring with them into the synthesis. An iodide rich synthesis, even though achieved by other means,8 appears to be the trick for the latest certified record efficiency of 22.1%.8 This hypothesis is speculative, and would require further investigations to be verified, but it fits with the data so far available.

Rb, but also there the Rb concentration was significantly depleted in the surface region,83 and lower than, or on the order of the magnitude of, our present detection limit (0.03 Rb/Pb at 4000 eV). Rb, K, and Na are thus likely concentrated deeper in the bulk of the films. No significant general shifts in the energy positions of the core levels, which could be related to a change in the Fermilevel, were observed between the samples. The valence band spectra gives information on the binding structure and is essentially identical for all compositions, except for the Csdoped perovskite, which has an additional feature (Figure 6a and SI) attributed to Cs 5p electrons.84 If the alkali cations are incorporated into the structure, they are expected to have an influence on, for example, the valence band spectra and the exact binding energies of nearby elements. We do not observe this, and the HAXPES measurements thus do not support that the smaller alkali cations would substitute the organic cations, at least to the extent where they can be detected reliably within a measurement time during which the perovskite remains unchanged by X-ray radiation. The observation of the low concentrations of the smaller alkali cations could potentially explain the low EDX signal for Rb, K, and Na (Figure 3c and SI). If Rb, K, and Na, rather than 13553


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the excited states are not significantly affected by the doping, mirroring that the structure is rather unaffected. The fluorescence lifetimes were, however, up to 4 times as long, in the range of a few microseconds, for perovskites doped with K and Na. The explanation behind the increase in fluorescence lifetime is still an open question, but potential mechanisms include, for example, trap state passivation or prevention of defect formation by the K and Na ions, or a modification in the crystallization dynamics leading to higher quality crystals and smoother surface morphology. It could also be the result of a secondary effect where Na and K do not participate in the perovskite crystallization, but, while not forming other harmful phases, they contribute with a higher iodide concentration, which could be beneficial for the crystal quality. Longer lifetimes may translate into a higher probability for charge extraction and is thereby potentially beneficial for device performance. Functional devices were made with the doped perovskites. None of the alkali dopants, with the exception of Na, had a significant negative impact. No significant positive impact, with the exception of Cs, was observed either, but two other resent studies86,87 indicate that the longer fluorescence lifetimes here observed with K-doping really can be a way toward increasing device performance.

To ensure structural integrity of the perovskites during the HAXPES measurements, time series were measured to establish the time window of stability under the X-ray illumination, in which subsequent measurements were kept. If the perovskite was converted to PbI2, this would be observed as a decrease of the I/Pb(+II) ratio as well as a significant reduction of the MA/ FA to Pb(+II) ratio. However, the conversion to PbI2 was negligible. The dominant change in the samples during X-ray illumination was instead a reduction of Pb(+II) to Pb(0), which is seen as an increase Pb(0)/Pb(+II) ratio followed by saturation (Figure 6b). An interesting observation in those time series measurements was that the doped perovskite was somewhat more stable than the other perovskites, and the Csdoped most so, with a slower increase of metallic lead (Figure 6b). Radiation hardness is not the same as operational solar cell stability, but it is still in line with the observation that doping can increase stability.53 Finally, full solar cell devices were assembled to explore the effect of the alkali dopants. Due to problems with the production line in the lab, the baseline cell performance was below expectations, with top cell performances in the range of 15−16% (Figure 7). That is low compared to the efficiencies in excess of 20% reported in the literature for the baseline compositions accomplished by some of the project’s device makers.30 Relative trends are, however, still of interest. Functional devices were made with all compositions mentioned in Table 1. The Na-doped samples had, despite the smooth surface morphology (Figure 4 and SI), significantly worse cell performance (Figure 7 and SI). Our main hypothesis for the poor performance of the Na-doped devices is that the Na-ions, as mentioned previously, appear to introduce an instability in the perovskite phase. For the Cs-, Rb-, and K-doped samples, the strongest conclusion we can draw is that none of those alkali ions are detrimental for device performance. The best devices were the Cs-doped ones, in line with previous reports,53 but that should also be seen in the light that more optimization work has been directed toward that composition. Based on the similarity in structure between the different doping conditions, it would not be unreasonable to assume that the longer fluorescence lifetimes for the K-doped perovskites (Figure 2) could be utilized for making better devices. While finishing the writing of this project two studies have recently been published on potassium doping where improved device performance with efficiencies in excess of 20% were demonstrated.86,87 Together with our data, this shows that potassium doping indeed can be a way towards better devices that should be explored in more depth.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b12464. A more detailed description of the experimental procedure; additional data, figures, and fits for the fluorescence lifetime measurements; individual plots of all XRD data, figures, and data for XRD peak heights and positions; additional EDX data; complete EDX elemental maps for all single doped compositions; top view SEM images for all single doped compositions at different magnifications; plots for band gap determination; additional data and figures for steady state fluorescence; graphical comparison between steady state fluorescence, optical absorption, and band gap for all compositions; transient absorption maps for all compositions; additional TAS data; additional data, figures, and quantifications from the HAXPES measurements; device data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; phone: +46 (0)705745116.

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CONCLUSIONS Mixed halide perovskites, FA0.83MA0.17PbBr0.51I2.49, doped with small amounts of Cs, Rb, K, and Na were synthesized. The same perovskite structure was maintained regardless of doping, but the smaller alkali ions, e.g., Rb, K, and Na, especially Rb, give a tendency for secondary phases to form as well, potentially formed early on in the thermal annealing when the perovskite crystallizes. Cs replaces the organic cation in the perovskite structure but the data do not support that also the smaller alkali ions would substitute the organic cations. That is in line with the classical Goldsmith’s tolerance factors stating that they are too small to fit the perovskite structure. Most of the optical properties, i.e., absorption, steady state fluorescence, and the ultrafast charge carrier dynamics and thermalization of

ORCID

T. Jesper Jacobsson: 0000-0002-4317-2879 Bertrand Philippe: 0000-0003-2412-8503 Ute B. Cappel: 0000-0002-9432-3112 Gerrit Boschloo: 0000-0002-8249-1469 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Swedish Energy Agency (Project No. P43294-1), the Swedish Foundation for Strategic Research (Project No. RMA15-0130), and the StandUP for Energy program are acknowledged for financial support. Part of the project has also 13554


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been funded by an EPSRC Impact Acceleration Account Follow on Fund, the Christian Doppler Research Association (Austrian Federal Ministry of Science, Research and Economy and the National Foundation for Research, Technology and Development), and the OMV Group. N.K. acknowledges the Royal Society Newton International Fellowship.

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Extending the Compositional Space of Mixed Lead Halide Perovskites

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