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Fully Vacuum Processed Wide Bandgap Mixed Halide Perovskite Solar Cells Giulia Longo, Cristina Momblona, Maria Grazia I. La Placa, Lidon Gil-Escrig, Michele Sessolo, and Henk J Bolink ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01217 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 24, 2017

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ACS Energy Letters

Fully Vacuum Processed Wide Bandgap Mixed Halide Perovskite Solar Cells Giulia Longo, Cristina Momblona, Maria-Grazia La-Placa, Lidón Gil-Escrig, Michele Sessolo and Henk J. Bolink* Instituto de Ciencia Molecular, Universidad de Valencia, C/ Catedrático J. Beltrán 2, 46980 Paterna, Spain

Abstract. Methylammonium lead mixed-halide perovskites MAPb(BrxI1-x)3 are promising materials for the preparation of tandem devices. When exposed to light, MAPb(BrxI1-x)3 segregates in iodide- and bromiderich phases, limiting the achievable photovoltage and hence the attainable device efficiency. To date only solution-processed mixed-halide perovskites have been demonstrated. In this work we present fully vacuum deposited mixed halide perovskite thin films with bandgap of 1.72 eV and 1.87 eV, prepared by controlling the deposition rates of the different halide precursors. When used in thin-film devices, these materials lead to power conversion efficiencies of 15.9% and 10.5%, respectively, which are among the highest reported to date for these types of wide bandgap absorbers.

A proven successful route to increase the efficiency of photovoltaic devices is the fabrication of multijunction solar cells,1 the simplest being a tandem architecture obtained by placing a wide band-gap absorber on top of a narrow band-gap semiconductor (for example silicon). This can be done either using a separate but partially transparent top cell or by constructing a monolithic tandem device, referred to as four and two terminal tandem cells, respectively. In order to reach the highest efficiency in a two terminal tandem cell, the band-gap of the top cell semiconductor must be chosen ensuring current matching between the two sub cells. From previous calculations, an ideal top cell semiconductor in a tandem device with silicon should have a bandgap of about 1.7-1.8 eV.2 Perovskites, and in particular methylammonium 1 ACS Paragon Plus Environment

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lead mixed iodide-bromide perovskites, represents one of the best candidates for this application,3-6 due to their continuously tunable bandgap from 1.6 eV to 2.3 eV which scales with the bromide content.7 Whereas the high efficiency of methylammonium lead iodide (MAPbI3) perovskite in single-junction solar cells have been widely demonstrated,8 the performances of the wide bandgap mixed-halide compounds MAPb(BrxI1-x)3 have been limited by their relatively low photovoltage.9 In general, the open circuit voltage (Voc) of mixed halide perovskites increases when incorporating up to 20 % bromide (x = 0.2) with respect to the pure iodide compound, while it diminishes substantially at higher bromide concentrations.7 This phenomenon is due to a light-induced phase segregation into bromine-rich and iodide-rich domains.10 Due to the narrower bandgap of the iodide-rich domains, they would limit the Voc acting as efficient radiative recombination centers for the photogenerated charge carriers.11 It has been shown that the mixed MAPb(BrxI1-x)3 system exhibit a miscibility gap, being the compound with x = 0.2 the highest Br concentration leading to a stable phase.

12-13

The relatively high ionic mobility in

methylammonium lead halide perovskites, together with the different stable crystalline phase at room temperature for the pure iodide and bromide compounds (tetragonal and cubic, respectively) are the main responsible of this phase separation.7, 14-15 It has been demonstrated that the ion migration and the phase separation can be substantially diminished by changing the composition or the morphology of the films. The addition of Cs+ and formamidinium (FA+) cations to MAPb(BrxI1-x)3 helps to alleviate the associated lattice distortions, favoring the formation of uniform mixed-halide phases.16-19 Morphology also plays an important role in regulating the extent of the halide segregation, as a consequence of its direct influence on the ionic mobility. Uniform, highly crystalline perovskite films indeed show an enhanced photostability.20 By depositing large grain MAPb(BrxI1-x)3 films, high performance perovskite photovoltaic device with bandgap > 1.7 eV have been obtained. In particular, PCE as high as 16.6% and 14.9% where reported for compounds with bromide content of x = 0.17 and x = 0.27, respectively.21-22 In view of the above, the deposition method has a primary role in determining the perovskite photo-stability. This is especially true during wet deposition, where the different affinity between lead and iodide or bromide is a driving force for phase segregation already in solution.23-25 In addition, during solvent evaporation the precursor concentrations increase, yet not uniformly due to a different solubility of the PbI2 and PbBr2, leading to an uneven crystallization that may cause inhomogeneous films. Furthermore, one cannot exclude that small amounts of the high boiling point and polar solvents remain in the perovskite film even after annealing, increasing the mobility of ionic species through solvation. Vacuum deposition is an alternative technique for the preparation of perovskite thin films.26 In this method no solvents are employed ruling out their effect during the perovskite crystallization. Additionally, it permits a fine control over the thickness and the stoichiometry of the layer, leading to very uniformly distributed composition.27 Here, we report on the preparation of mixed halide MAPb(BrxI1-x)3 perovskites through 2 ACS Paragon Plus Environment

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ACS Energy Letters

vacuum deposition, using three-source co-sublimation of methylammonium iodide, lead iodide and lead bromide. The ratio of the halides was controlled via the sublimation rate of the corresponding precursors. These vacuum deposited films were integrated into solar cells by sandwiching them in between organic electron and hole transport layers that were also processed using thermal sublimation in vacuum. The solar cells present Voc and PCE similar or higher compared to previous reported methylammonium mixed-halide lead perovskite solar cells. In addition we observed that a low temperature annealing partially alleviates the halide segregation of the mixed phase, enhancing the performances of the devices. We prepared two different mixed-halide perovskites with increasing bromide/iodide ratios, MAPb(Br0.2I0.8)3 and MAPb(Br0.5I0.5)3, where the bromide content is x = 0.2 and x = 0.5, respectively (details in the Supporting Information). The compounds were obtained by simultaneous co-deposition of MAI, PbI2 and PbBr2 from separate temperature controlled crucibles, and by increasing the deposition rate of PbBr2 with respect to PbI2 while keeping the rate of MAI constant. The obtained absorption spectra are depicted in Figure 1a. All samples present steep absorption onsets blue-shifting as the bromide content increases, as expected for perovskites with lighter halides. The estimated bandgap values are 1.6 eV for MAPbI3, 1.72 eV for MAPb(Br0.2I0.8)3 and 1.87 eV for MAPb(Br0.5I0.5)3. After an annealing at 90 °C for 15 minutes, the absorption onset becomes steeper and more intense, while it does not change its position (Figure S1).

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ACS Energy Letters

MAPb(BrxI1-x)3

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2θ ( )

Figure 1. (a) Absorption spectra and (b) XRD patterns of vacuum deposited MAPb(BrxI1-x)3 perovskite films with increasing bromide content. The peaks marked with a star in (b) derive from the underlying ITO layer (Figure S4). The absorption spectra have been rescaled for ease of comparison.

To make sure that the intended bromide to iodide ratios were indeed obtained in the films, the samples were analyzed using x-ray photoelectron spectroscopy (XPS, Figure S2). From the quantitative analysis of the halide and lead signals we found the expected ratios for the low bandgap perovskite, MAPb(Br0.19I0.81)3, while for the wider bandgap mixed halide films the bromide content was found to be lower (x = 0.36). This is in contrast with the absorption spectrum (the estimate bandgap of 1.87 suggests a higher bromide inclusion), and it is likely due to the high air sensitivity observed for this particular compound. As XPS is a surface sensitive technique, the sample might have degraded due to air exposure prior to the analysis, and the calculated stoichiometry likely differs substantially from the bulk of the film. The mixed-halide perovskite films were further characterized by X-ray diffraction (XRD, Figure 1b). For these measurements, freshly deposited films on indium tin oxide (ITO) coated glass slides with a layer


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thickness of 450 nm were used. The diffractograms were collected both for pristine sample and for thermally treated films (15 minutes at 90 °C in inert atmosphere). As no substantial difference was observed upon annealing, only the data for the as-prepared samples are reported. The partial substitution of the iodide with the smaller halide causes a distortion of the tetragonal MAPbI3 lattice, which translates into a gradual shift to higher angles of the diffraction peaks, noticeable in Figure 1b. In particular, the characteristic peaks of the MAPbI3 film positioned at 14.2°, 28.5° and 32° shift to wider angles at higher bromide content (Figure S3). It is important to note that the diffractograms of the mixed-halide perovskites do not indicate the presence of multiple phases.10, 28 Continuous photoluminescence (PL) measurements is a valuable tool to monitor the presence of lightinduced halide segregation in real time. The photoluminescence spectra for the two mixed halide perovskite films were recorded as a function of exposure time under excitation with a continuous wave green laser (wavelength 515 nm, 300 mW/cm2), and the measurements were carried out in a nitrogenfilled glovebox on pristine and thermally annealed samples deposited on glass. The high power density used allows for accelerated testing of the halide segregation in the thin-film compound. The collected spectra are reported in Figure 2.

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ACS Energy Letters

90 C - 30 min.

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Figure 2. Evolution of the photoluminescence spectra under continuous illumination for a) as deposited MAPb(Br0.2I0.8)3 and (b) MAPb(Br0.5I0.5)3, and for (c) MAPb(Br0.5I0.5)3 films after annealing. All the spectra were collected with 10 seconds delay under continuous excitation with a CW laser at 515 nm with an irradiance of 300 mW/cm2.

The MAPb(Br0.2I0.8)3 PL spectra (Figure 2a) present a single photoluminescence signal with a maximum at 728 nm, very close in energy (1.7 eV) to the bandgap determined from the absorption spectrum. During continuous excitation the photoluminescence signal decreases indicating a loss in quantum yield due to degradation of the film. More importantly, the position of the PL signal does not change with prolonged illumination and no additional signal emerges during the experiment. This is interesting as it indicates that no detectable phase segregation is taking place in for this particular I/Br ratio. Interestingly, when the film 5 ACS Paragon Plus Environment


is annealed the position and the shape of the PL signal do not change, but the intensity of the emission becomes stronger with time (Figure S5). On the other hand, perovskite films with higher bromide content as the MAPb(Br0.5I0.5)3 do show the appearance of a second low-energy component when exposed to the laser beam (Figure 2b). In particular, the evolution of the photoluminescence with exposure time is indicative of a phase segregation in two main phases: one with a wide bandgap of about 1.86 eV and another characterized by a narrower bandgap of 1.71 eV. We then performed an annealing of these perovskite films at 90 °C for 15 minutes in inert atmosphere. After the treatment, we noted that the segregation into I- and Br-rich phases is less clear, and the spectral shape is mainly defined by a component centered at about 670 nm and another one at 700 nm. This suggest that domains with bandgap energies closer to that of the initial mixed bromide-iodide film are formed, which in turn points at an increased stability of the films upon thermal treatment. The morphology of the mixed-halide perovskites plays a fundamental role in the stability under illumination, and it has been shown that the formation of large grains and uniform layers reduce the phase-segregation phenomena.21, 29-30 The vacuum deposition of perovskites permits to form high quality films, with smooth surfaces composed by closely packed grains, with a high control of the stoichiometry. The surface and cross sectional scanning electron microscopy (SEM, Figure 3) analysis of the MAPb(Br0.2I0.8)3 reveal that the morphology of the layer consists of grains with size ranging from 100 nm to 300 nm, forming a compact film without voids.


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Figure 3. a) Top-view and b) cross sectional SEM of MAPb(Br0.2I0.8)3.

The absence of any appreciable phase segregation in the MAPb(Br0.2I0.8)3 films might hence be favored by the high morphological quality of the vacuum deposited materials. In the case of MAPb(Br0.5I0.5)3, however, we could not record any meaningful SEM image due to the fast degradation of the material upon exposure to air. The mixed halide perovskite films were employed as light absorber in fully vacuum deposited n-i-p solar cells using a combination of doped and intrinsic organic charge transport layers (Figure 4a).31 Details on the device fabrication are reported in the Supporting Information. As a reference, we also prepared solar cells based on the pure MAPbI3 perovskite. The preparation and characterization of the devices were entirely conducted in inert atmosphere.


ACS Energy Letters

MAPb(BrxI1-x)3 C60 - 10 nm C60:PhIm - 40 nm

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Figure 4. a) Schematic illustration on the device architecture. b) EQE of the annealed devices. b) J-V curves of the perovskite solar cells. In these two graphs, the black curves represent MAPbI3, the red curves MAPb(Br0.2I0.8)3 and the green curves MAPb(Br0.5I0.5)3. Solid lines are used for forward scans while dashed lines for the reverse. Table 1. Photovoltaic parameters extracted from current-voltage measurements of the solar cells

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The lower bandgap cells exhibit a high external quantum efficiency (EQE, Figure 4a), about 80% on the whole visible spectrum, while the EQE is slightly reduced for the cells using MAPb(Br0.5I0.5)3. The increased bandgaps of the MAPb(Br0.2I0.8)3 and MAPb(Br0.5I0.5)3 perovskites is reflected in the blue-shift of the EQE onset for the devices employing them. The integrated short-circuit current density (Jsc) diminishes from 19.6 to 17.3 and 11.4 mA/cm2 for the devices using perovskites with bromide content x = 0, x = 0.2 and x = 0.5, respectively. The dark current density-voltage (J-V) curves (Figure S6) of the three types of solar cells show a low leakage current, indicating a low density of pinholes or defect in the device stack. When measured under illumination, the solar cells employing MAPbI3 do not show any relevant differences between the J-V curves measured in forward (from Jsc to Voc) or reverse (from Voc to Jsc) bias, while the wider bandgap perovskite devices do show a very small hysteresis. The high fill factors, between 77% and 82% for all cells, are an indication of the high quality of the diodes and in particular of an efficient current transport and collection at the electrodes. The open circuit voltage (Voc) 8 ACS Paragon Plus Environment

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ACS Energy Letters

scales with the optical bandgap, increasing from 1095 mV to 1119 mV and to 1207 mV for increasing bromide content. This is interesting as all devices use the same organic charge transport layers, and it confirms that the photovoltage in perovskite cells is rather insensitive to the transport levels and mainly determined by the quasi-Fermi level splitting in the absorber.32 Overall, the vacuum deposited wide bandgap perovskite solar cells leads to a PCE that is among the highest reported for mixed halide methylammonium perovskite cells with similar compositions and bandgaps.7,

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The best cells

prepared with the MAPb(Br0.5I0.5)3 (x = 0.5) have a PCE exceeding 10%, and the narrower bandgap devices (x = 0.2) reach PCE as high as 15.9%. The maximum power point (mpp) tracking for this device, measured every 40 s under continuous 1 sun illumination, shows only a small decrease in the solar cell performance after 1 hour, to about 95% of the initial value (Figure S7). The data presented above were collected for devices annealed at 90 °C for 15 minutes. We also characterized the performances of asprepared solar cells, without thermal treatment. It is interesting to note that while the performances of MAPbI3 based devices do not vary upon annealing, they do change slightly for MAPb(Br0.2I0.8)3 and in particular for MAPb(Br0.5I0.5)3 (Figure S8). Before the thermal treatment, the EQE of MAPb(Br0.5I0.5)3 based cells is low, while the EQE of the narrower bandgap perovskite cell (x = 0.2) does not vary substantially. Importantly, the thermal treatment increases the Voc of both the mixed-halide perovskite compositions: for MAPb(Br0.2I0.8)3, the Voc increases from 1080 to 1119 mV while in the wider bandgap solar cell, the Voc gain is about 80 mV (reaching 1209 mV) with annealing. The small Voc gain measured with the low bromide content compound (x = 0.2) is likely related to the observed behavior of the photoluminescence, which is also enhanced after annealing (Figure S5), meaning that non-radiative recombination is somehow reduced. For the wider bandgap perovskite (x = 0.5), annealing partially alleviates the halide segregation as the low energy components of the PL spectra is blue-shifted compared to the as-deposited material (Figure 2b-c). As the photovoltage depends on the recombination within the compound with the smallest bandgap (in this case the iodide-rich phase responsible for the low energy PL signal), the increase in Voc observed for MAPb(Br0.5I0.5)3 is likely to be ascribed to a reduced phase segregation upon thermal treatment. In summary, we have prepared efficient vacuum deposited methylammonium lead mixed halide perovskites thin films. While at low bromide content no changes in the photoluminescence over time were observed, for high bromide concentrations halide segregation was observed, similarly to what reported for solution-processed perovskite films. Interestingly, we found a thermal treatment to be beneficial for mixed halide compounds, partially alleviating the phase segregation. In general, the perovskite properties are extremely dependent on the film morphology so that a comparison of their environmental and photostability with solution-processed compounds is rather difficult (even a comparison among solutionprocessed films deposited with different protocols would be problematic). Apart from the morphology 9 ACS Paragon Plus Environment


and the processing conditions (i.e. vacuum vs. solution), the use of more complex cation mixtures (Cs+/FA+) has proven to be a valuable route to stabilize mixed halide compounds, at least in solution processed perovskite films.18 The fully vacuum deposited solar cells prepared with the wide bandgap perovskites are characterized by a high power conversion efficiency and a large photovoltage. These results are extremely promising in view of the fabrication of tandem devices on textured substrates such as silicon, where conformal coating only accessible by vacuum deposition is required.

Acknowledgements We are grateful to Novaled GmbH, for the supply of the organic charge transport materials and dopants. We further acknowledge financial support from the European Union H2020 project INFORM (grant 675867), the Spanish Ministry of Economy and Competitiveness (MINECO) via the Unidad de Excelencia María de Maeztu MDM-2015-0538, MAT2014-55200-R and PCIN-2015-255 and the Generalitat Valenciana (Prometeo/2016/135). C. M. and M.S. thank the MINECO for their pre- and postdoctoral (JdC) contracts, MGL thanks the support of the Grisolia grant from the Generalitat Valenciana.

Supporting Information Available: Experimental methods, absorption spectra before and after annealing for perovskite thin-films, XPS and XRD analysis, PL spectra of annealed perovskite films, J-V curves in the dark, mpp tracking and J-V curves for not-annealed devices.


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