Inkjet-Printed Triple Cation Perovskite Solar Cells - ACS Applied

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Inkjet-printed Triple Cation Perovskite Solar Cells Florian Mathies, Helge Eggers, Bryce Sydney Richards, Gerardo Hernandez-Sosa, Uli Lemmer, and Ulrich W. Paetzold ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00222 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 18, 2018

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ACS Applied Energy Materials

Inkjet-printed Triple Cation Perovskite Solar Cells Florian Mathies,1,2,* Helge Eggers,4 Bryce S. Richards,1,3 Gerardo Hernandez-Sosa,1,2 Uli Lemmer,1,3 and Ulrich W. Paetzold1,3 1

Light Technology Institute, Karlsruhe Institute of Technology, Engesserstr. 13, 76131 Karlsruhe, Germany 2

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InnovationLab GmbH, Speyererstr. 4, 69115 Heidelberg, Germany

Institute of Microstructure Technology, Karlsruhe Institute of Technology, Hermann-vonHelmholtz-Platz 1, 76344, Eggenstein-Leopoldshafen, Germany 4

Kirchhoff Institute for Physics, Heidelberg University, Im Neuenheimerfeld 225, 69120 Heidelberg, Germany

KEYWORDS: Inkjet printing, perovskite, solar cell, stability, triple cation, thickness

ACS Paragon Plus Environment

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ABSTRACT: Non-contact inkjet printing offers rapid and digital deposition combined with excellent control over the layer formation for printed perovskite solar cells. In this work, inkjet printing is used to deposit triple cation perovskite layers with 10 % cesium in a mixed formamidinium/methylammonium lead iodide/bromide composite for solar cells with high temperature and moisture stability. A reliable process control over a wide range of perovskite layer thickness from 175 to 780 nm and corresponding grain sizes is achieved by adjusting the drop spacing of the inkjet printer cartridge. A continuous power output at constant voltage, resulting in a power conversion efficiency of 12.9 % is demonstrated, representing a major improvement from previously reported inkjet-printed methylammonium lead triiodide perovskite solar. Moreover, this work highlights the extended resistance of triple cation perovskite solar cells against heat and moisture for our ambient inkjet printing approach. The presented results are a proof of concept for the processability of high efficient perovskite solar cells using digital inkjet printing for next generation photovoltaic applications.


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1. Introduction In recent years, the enormous potential of non-contact digital 7 printing techniques for low cost, high throughput and large area fabrication of a large variety of optoelectronic devices have been demonstrated. The high level of customization and compatibility with a broad range of substrates makes digital printing a promising process for a vast variety of applications such as integrated circuits, lab-on-a-chip systems, light emitting devices, and solar cells.1–6 Already to date, industrial inkjet printers, compatible with high throughput roll-to-roll or sheet-to-sheet processes, have been installed in organic light emitting device manufacturing sites.7 Regarding the printability of organometal halide perovskite solar cells (PSCs), most of the research has been focused on high throughput techniques like spray coating, blade coating, slot die and roll-to-roll processes.8–14 Power conversion efficiencies (PCEs) of up to 15.7 % have already been demonstrated for partially slot die coated devices.15 Digital inkjet printing has so far mainly been investigated for the purpose of fabricating PSCs of arbitrary shape and area.16 Beyond the freedom of design, inkjet printing of perovskites affords very good control over the crystallization behavior of organometal halide perovskite layers, which is of crucial importance for high performance solar cells. Recently, partially inkjet-printed PSCs with efficiencies of up to 16.5 % utilizing methylammonium lead iodide have been demonstrated, although this still lags well behind reported record PCE of 22.1 % for PSCs fabricated by spin coating.17–23 To date, research on printed PSCs has been mainly focused on maximizing the PCE. However, a more practical consideration, namely the long term stability of the devices, particularly of the perovskite absorber material, is just as important for photovoltaic (PV) applications. Especially methylammonium lead triiodide, the most prominent organometal halide perovskite absorber material for PV, strongly suffers from degradation upon contact with moisture, as well as thermal


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instability.24,25 Impressive results leading to both increased PCE and thermal stability have been presented, realized via mixing different cations following the Goldschmidt tolerance factor of ionic radii.26 In this regard, the most promising attempt based on a combination of methylammonium and formamidinium to form a mixed halide perovskite has been reported in literature.27,28 Furthermore, an increase of PCE and stability has been demonstrated for a triple cation perovskites, where a small amount of cesium (< 15 %) is found to improve the crystalline perovskite phase, hence solar cells exhibit higher thermal and humidity stability.29 Even multication PSCs with rubidium as a fourth cation have been investigated successfully.30 2. Results & Discussion In this work, we follow the triple cation approach to realize digital inkjet printing of stable and high PCE perovskite absorber materials for PSCs. For this purpose, we refined the inkjet printing process to fabricate smooth mixed cation perovskite layer on top of a conventional electron transport layer, namely compact titanium dioxide.17 We demonstrate control of the layer thickness, grain size and the surface roughness of the inkjet-printed triple cation perovskite absorber layers by variation of the drop spacing. Investigating the printed perovskite structure via cross-sectional scanning electron microscopy (SEM), we provide insights into the crystal growth of the perovskite layer and correlate it to the PCE of our prototype triple cation PSCs. Finally, we validate the improved stability of triple cation perovskites against temperature and moisture compared to PSCs based on a methylammonium lead triiodide absorber. In the following, we demonstrate the versatility of inkjet printing for high quality triple cation perovskite layers in PSCs. The employed triple cation perovskite layers exhibit 10 % cesium in a formamidinium/methylammonium lead iodide/bromide composition, which was first published by Saliba et al.29 We modified the original recipe in terms of concentration and solvent


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composition according to the needs of the inkjet printing process. See the Experimental section for more details. The triple cation perovskite layers were printed on structured fluorine doped tin oxide (FTO) substrates covered with a spin-coated compact layer of titanium dioxide (TiO2). Subsequent to the deposition of the perovskite absorber, a layer of 2,2',7,7'-Tetrakis-(N,N-di-4methoxyphenylamino)-9,9'-spirobifluorene (Spiro-MeOTAD) was spin-coated as hole transport layer (HTL), followed by an evaporated gold (Au) electrode. The printed perovskite layers and the full solar cell stack (both photograph and schematic) are depicted in Figure 1. All solution processes, as well as the measurement of the optoelectronic characteristics, were performed under controlled ambient conditions (23°C and 45-50 % humidity). Smooth perovskite absorber layers, with root mean square (RMS) values below 20 nm, were obtained by applying an additional vacuum drying step subsequent to the printing process. The importance of this step for inkjet-printed perovskite absorber layers was described in our previous work.17 In the supplementary material, we show an exemplary parameter optimization of the vacuum annealing time to form smooth perovskite layer on top of the compact TiO2 layer (see Figure S1). With regard to the inkjet printing of the perovskite layer, the drop spacing (ds) between next neighboring droplets allows controlling the amount of material deposition as well as steering the layer formation dynamics. In this work, the drop distance between next neighbor droplets was varied between 60 µm, 45 µm, 35 µm, and 25 µm. For the given concentration of the solution, this resulted in triple cation perovskite layer thicknesses of 175 nm, 380 nm, 520 nm, and 780 nm, respectively. In first approximation, considering the volume conservation, the thickness of the perovskite layer shall be inversely proportional to the square of the dropspacing. As illustrated in Figure S3 this trend is fulfilled for the printed layers. In Figure 2(a), cross-sectional SEM images of the printed perovskite layers in PSCs are depicted, highlighting the perovskite


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thickness variations. Complementary atomic force microscopy images of the perovskite surface are given in Figure S2. All printing parameters result in closed wet films, due to the chosen lower drop distances compared to the single drop diameter of the perovskite ink of around 75 µm. It should be noted that the organic HTL (Spiro-MeOTAD) layer, above the perovskite layer is partially degraded by the gallium focused ion beam (FIB), which is used to prepare the solar cell cross-sections. Beyond the variation of thickness of samples I to IV [Figure 2(a)], the SEM images illustrate an increase of the perovskite grain size with decreasing drop spacing and increasing perovskite layer thickness, as illustrated in Figure 2(b). At this point, we suspect, that the reason for the grain size increase with decreasing drop spacing originates from a solvent annealing effect, described elsewhere.31,32 As the drop spacing is reduced, more solvent has to degas from the wet film in the vacuum chamber, which is supposed to increase the solvent concentration close to the perovskite layer. This is expected to lead to an increase of the grain sizes of printed perovskite layers with smaller drop spacings. For all thicknesses of the inkjetprinted layers, the lateral extension of the perovskite grains is smaller than the layer thickness. This result is in contrast to that achieved via spin-coated perovskite layers.29,32–34 As a consequence, for thick inkjet printed perovskite layers, charge carriers in our PSCs need to overcome one or two grain boundaries before they are extracted at the transport layer interfaces. Since enhanced non-radiative recombination is expected to occur at the interface between perovskite grains, such a crystal composition in the perovskite film is typically not preferred. The steep absorption edge at 780 nm indicates the high level of crystallinity of the inkjet-printed polycrystalline perovskite layers, as depicted in Figure 2(c). The in-plane grain boundaries do not affect the optical properties and are further investigated during electrical characterization of the inkjet-printed PSCs.


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Prototype triple cation PSCs were fabricated for different thick mixed cation perovskite layers by using the drop spacings discussed above. In order to demonstrate the reproducibility, for each perovskite layer thickness a statistical relevant quantity of samples were prepared. The current density – voltage (J-V) characteristics of representative devices and the statistical distribution of measured PCEs of around 20 cells per thickness are shown in Figure 3(a), (b), respectively. Further details are provided in Figure S4 in the supporting information. In general, an increase in short-circuit current density (JSC), open-circuit voltage (VOC) and fill factor (FF) with increasing perovskite absorber thickness is observed. The JSC increase up to 21.5 mA/cm² for the 520 nm thick perovskite layer is attributed to an increase in light absorption [Figure 2(c)]. Additionally, the VOC increases with the layer thickness to over 1.06 V, due to an improved surface coverage of the perovskite layer. This hypothesis is supported by an observed increase of shunt resistance (Rshunt) [Figure 3(d)]. The FF follows the same trend, increasing to a maximum of 67 % for the 520 nm thick devices, followed by a slight decrease for the thickest devices. Altogether, this leads to a maximum PCE of 15.3 % for the PSC with a triple cation perovskite layer of 520 nm thickness. To our knowledge this is, so far, the first reported value of inkjet-printed triple cation PSCs. From the exemplary J-V curves provided in Figure 3(a), we see a thickness dependent efficiency trend, suggesting an optimal layer thickness of 520 nm to obtain a good surface coverage and high efficient solar cells. Lower absorber thicknesses lead to lower light absorption and lower film coverage (see Figure 2). Layer thicknesses above 520 nm show no further increase of PCE, possibly due to a limitation of charge carrier collection, affected by the limited grain sizes. Compared to the spin-coated reference devices reported in literature, our best inkjet-printed PSCs with a thickness of 520 nm are already competitive in terms of JSC (21.5 mA/cm² vs.


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22.0 mA/cm²) and the VOC (1.06 V vs. 1.13 V). This suggests a comparable high photon absorption, charge carrier generation and surface coverage for the printed perovskite layers. In contrast, the FF (67 %) is still ~10 % (abs.) lower than that observed by Saliba et al. (77%).29 The difference between spin coated devices and inkjet-printed devices can be explained by the grain boundaries horizontally fragmenting the layer seen in Figure 2(a), which are limiting the charge carrier transport through the device. This explanation is supported by previous reports, which discuss the close correlation of the perovskite layer morphology and the PSCs device performance.35,36 In the work of Correa-Baena et al., it is explicitly demonstrated that for shuntfree layers the crystal size determines the charge carrier collection, which mainly effects the FF, while the recombination, which effects the VOC remains unmodified. Thus, we conclude that the lower FF of the inkjet-printed PSCs arises from the in-plane grain boundaries, depicted in Figure 2(a), resulting in reduced charge collection, which mainly affects the FF. Since our devices exhibit a large hysteresis effect with a hysteresis index of HI~0.7 in the J-V characteristic, it is generally advised to determine the power output of the solar cells under constant voltage around the maximum power point (mpp).37 As shown in Figure 3(c), the J-V scans of these so-called stabilized PCEs exhibit a similar trend, with a maximum PCE of 12.9 % for a 520 nm thick perovskite layer. Additional measurements of external quantum efficiency (EQE) spectra and forward J-V scans are depicted in Figure S5. In order to demonstrate the greatly-improved stability of inkjet-printed triple cation PSCs we compare the continuous power output measurements of these devices to printed methylammonium lead triiodide PSCs [refer to Figure 4(a) and (b)]. Details on the preparation of the methylammonium lead triiodide absorber layer can be found elsewhere.17 Inkjet-printed reference devices with a methylammonium lead triiodide perovskite absorber layer exhibit PCEs


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of ~12 % measured in backward direction. However, these devices do not stabilize during 5 min of continuous power output measurements at constant voltage close to mpp. In contrast, the inkjet-printed triple cation PSC exhibit a stable power output of 12.9 %. Moreover, we prove the increased stability of the inkjet-printed triple cation PSC by measuring non-encapsulated devices at ambient conditions (23°C and 45 %). Figure 4(c) shows the normalized PCE values for the triple cation and pure methylammonium lead triiodide PSCs kept at 23°C and 80°C. The efficiency of methylammonium lead triiodide PSCs decreases to half of its initial efficiency after only 100 min if the devices are stored at 23°C. If stored at 80°C, the PCE drops to less than 15 % after 100 min. In contrast, the PCEs of the triple cation PSCs remain unaltered at 23°C, while a small decrease of 10 % at 80°C can be observed. These results validate the improved stability of cesium triple cation perovskites against moisture and heat, compared to reference methylammonium lead triiodide devices. Thus, the extremely promising combination of highly stable triple cation perovskite absorber layers and the versatile inkjet printing technique affords the possibility of digitally-printed and highly efficient PSCs to be adapted to future commercialization of perovskite PV applications. 3. Conclusion In summary, this work reports on the fabrication of high efficient inkjet-printed triple cation perovskite absorber layers for solar cell applications. For the first time, stable and high efficient inkjet-printed multi cation PSCs are demonstrated. By varying the neighbor drop distance during inkjet printing, the absorber layer thickness is varied from 175 to 780 nm as well as the crystallinity. Triple cation perovskite solar cells with a maximum efficiency of 15.3 % in backward scan direction and 12.9 % in continuous power output measurements are reported. To approach the state-of-the art PCEs, inkjet-printed devices have to solve the bottleneck of low


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FFs by reducing the in-plane grain boundaries, implying a better control over the perovskite crystallization. Furthermore, the printed triple cation PSCs outperform the printed reference devices based on methylammonium lead triiodide in terms of efficiency as well as stability against moisture and heat. Non-encapsulated triple cation PSCs withstand a temperature of 80°C for around 120 min with just a small PCE drop of around 10 %, whereas pure methylammonium lead triiodide devices are nearly totally degraded at this point. The results presented here reveal the ability of inkjet-printed perovskite solar cells, and a proof of concept towards digitallyprinted, low-cost and high-throughput solar cells. 4. Experimental Section Solar cell fabrication: The planar perovskite solar cells were prepared as following: Fluorine doped tin-oxide (FTO) coated glass from Dyenamo AB (12 Ω□, 1.1 mm) was patterned by etching with zinc powder and hydrochloric acid. Afterwards, a cleaning sequence in an ultrasonic bath with deionized water, acetone and isopropanol for 10 min each was performed, followed by a 5 min oxygen plasma treatment for better wettability. A 40 nm-thin compact TiO2 layer was spin-coated from a stock solution of titanium diisopropoxide bis(acetylacetonate) dissolved in ethanol (1:39 vol. %) by sequential spin coating at 3000 rpm for 30 s and annealing at ambient for 5 min at 100°C. These steps are repeated three times. The samples were sintered under standard atmospheric conditions in a furnace at 500°C for 1 h. A stock solution of riple cation perovskite solution (1.5 M) is prepared, by dissolving lead iodide (1.65 M, Alfa Aeser), formamidinium iodide (1.5 M, Lumtec Inc.), lead bromide (0.3 M, Dyesol) and methylammonium iodide (0.3 M, Dyesol) in dimethylformamide (DMF) and dimethylsulfoxide (DMSO) in a 4:1 ratio (both Sigma Aldrich) and a solution of cesium iodide (1.5 M, Dyesol) in DMSO. Both are stirred at least for 1 h at 65°C and then mixed together to the following


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composition to Cs0.1(FA0.83MA0.17)0.9Pb(Br0.17I0.83)3. The solution was stable for more than a month. The solution was filtered with a PTFE filter (0.45 µm) prior to the filling the inkjet print head (DMC11610; 10 pl nozzle diameter 21 µm) of a Fujifilm Dimatix DMP 2831 inkjet printing system. The ink was held at 30°C while printing, and the perovskite layers were printed plate substrate of at 23°C. The drops were generated by a single bias pulse with a peak voltage of 22 V and a pulse width of 15 µs. The jetting frequency was set to 5 kHz. The wet film is transferred within 15 s to a vacuum chamber (~1 mbar) to outgas the excess solvent from the layer as depicted in the original publication.17 The films were than annealed at 100°C for 60 min under a nitrogen flow on a hotplate. The hole transport formulation was prepared in a nitrogen filled glovebox as follow: 80 mg 2,2,7,7-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9spirobifluorene (Spiro-MeOTAD) was added to 1 ml chlorobenzene, 28.5 µl 4-tert-butylpyridine (4-tBP) and 17.5 µl of a stock solution of 520 mg bis(trifluoromethane)sulfonimide lithium salt in acetonitrile. The hole transport layer was spin-coated at 4000 rpm for 30 s without any further annealing process. Finally, a 65 nm thick gold electrode was thermally evaporated on top. The cell area is defined by the overlap of FTO and Au electrodes to 9 mm². All wet film fabrication steps were performed under cleanroom conditions with a humidity of 45 % (± 5 %). Characterization: Layer thicknesses were measured using a Veeco Dektak 150 profilometer system. SEM images were obtained by a ZEISS, Cross Beam Auriga system. The surface roughness was calculated using white light interferometer images (Sensofar Plu neox). The absorbance was measured with an UV-Vis spectrometer and a white light source (Avantas Avalight-DH-S-BAL, Avaspec ULS3648). All J-V characteristics and PCE measurements were performed with a Keithley 2601B source meter. The light source was a filtered 1000 W Xenon calibrated to 100 mWcm-2 under AM 1.5 G solar light conditions with a certified silicon solar


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cell. The continuous power output measurements were performed, measuring first the J-V characteristic in backward direction to obtain the maximum power point (mpp) of the cell. The cell was then biased at the mpp before the solar light was turned on. The temperature stability was measured, using an external heat plate to control the temperature. The devices J-V characteristics were then measured after different times at the given temperature.

Figure 1. (a) 520 nm-thick inkjet-printed perovskite layer on FTO/TiO2-coated glass substrate (b) Photograph of inkjet-printed perovskite solar cells. The substrate contains eight cells with each 3 x 3 mm² active area. (c) Schematic diagram of the solar cell stack, denoting with the different layers: Glass/FTO/TiO2/triple cation perovskite (PVK)/Spiro-MeOTAD (HTL)/Au.


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Figure 2: (a) SEM cross-sectional images of inkjet-printed triple cation perovskite solar cells. The perovskite layer thickness varies with drop spacing (ds). I: 180 nm from 60 µm ds; II 380 nm from 45 µm ds; III: 520 nm from 35 µm ds; IV: 780 nm from 25 µm ds. The perovskite layer is highlighted in yellow, the TiO2 layer is highlighted in blue, while Spiro-MeOTAD is partially destroyed by the focused ion beam (FIB). Grain boundaries are enclosed by the black line. (b) Dependency of the calculated grain sizes as a function of perovskite layer thickness. (c) Measured absorbance as a function of the perovskite layer thickness.


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Figure 3: (a) Solar cell parameters of representative inkjet-printed perovskite solar cells with triple cation absorber layer thicknesses ranging from 175 to 750 nm. The J-V characteristic was measured at 50 mV/s in backward direction (from 1.1 V to - 0.1 V). (b) Power conversion efficiency (PCE) statistics of around 20 triple cation perovskite solar cells fabricated for a given drop spacing/layer thickness. (c) PCE after five minutes of continuous power output at the maximum power point voltage. Further details are provided in Figure S3 in the supplementary information. (d) Rseries and Rshunt extracted from J-V- characteristics.


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Figure 4: (a) Continuous power output measurement of methylammonium lead triiodide PSCs. (b) Continuous power output measurement of a 520 nm thick triple cation PSC. (c) Normalized power conversion efficiency (PCE) values of triple cation and methylammonium lead triiodide perovskite solar cells after various times at 23°C and 80°C. Devices are not-encapsulated and measured at ambient conditions. TOC GRAPHICS


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ASSOCIATED CONTENT Further details on the surface topography optimization of the printed layers and the additional characteristics of the solar cells are given in the Supporting information. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources The authors would like to gratefully acknowledge financial support of the Initiating and Networking funding of the Helmholtz Association (HYIG of Dr. U.W. Paetzold; Recruitment Initiative of Prof. B.S. Richards; the Helmholtz Energy Materials Foundry (HEMF); and the Science and Technology of Nanostructures research program), as well as the Karlsruhe School of Optics and Photonics. The authors acknowledge financial support by the German Federal Ministry of Education and Research through grant 03SF0483 (PeroSol).


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors would like to gratefully acknowledge Tobias Rödlmeier and Stefan Schlisske for fruitful discussions concerning inkjet printing, and Noah Strobel for help with the EQE measurement setup. Thanks to the “KIT Perovskite Taskforce” group members for the great spirit. The manuscript was written through contributions of all authors.

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(a) 520 nm-thick inkjet-printed perovskite layer on FTO/TiO2-coated glass substrate (b) Photograph of inkjet-printed perovskite solar cells. The substrate contains eight cells with each 3 x 3 mm² active area. (c) Schematic diagram of the solar cell stack, denoting with the different layers: glass/FTO/TiO2/triple cation perovskite (PVK)/Spiro-MeOTAD (HTL)/Au. 98x176mm (300 x 300 DPI)


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(a) SEM cross-sectional images of inkjet-printed triple cation perovskite solar cells. The perovskite layer thickness varies with drop spacing (ds). I: 180 nm from 60 µm ds; II 380 nm from 45 µm ds; III: 520 nm from 35 µm ds; IV: 780 nm from 25 µm ds. The perovskite layer is highlighted in yellow, the TiO2 layer is highlighted in blue, while Spiro-MeOTAD is partially destroyed by the focused ion beam (FIB). Grain boundaries are enclosed by the black line. (b) Dependency of the calculated grain sizes as a function of perovskite layer thickness. (c) Measured absorbance as a function of the perovskite layer thickness. 85x43mm (300 x 300 DPI)



(a) Solar cell parameters of representative inkjet-printed perovskite solar cells with triple cation absorber layer thicknesses ranging from 175 to 750 nm. The J-V characteristic was measured at 50 mV/s in backward direction (from 1.1 V to - 0.1 V). (b) Power conversion efficiency (PCE) statistics of around 20 triple cation perovskite solar cells fabricated for a given drop spacing/layer thickness. (c) PCE after five minutes of continuous power output at the maximum power point voltage. Further details are provided in Figure S3 in the supplementary information. (d) Rseries and Rshunt extracted from J-V- characteristics. 149x321mm (300 x 300 DPI)


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(a) Continuous power output measurement of methylammonium lead triiodide PSCs. (b) Continuous power output measurement of a 520 nm thick triple cation PSC. (c) Normalized power conversion efficiency (PCE) values of triple cation and methylammonium lead triiodide perovskite solar cells after various times at 23°C and 80°C. Devices are not-encapsulated and measured at ambient conditions. 74x141mm (300 x 300 DPI)



TOC 50x33mm (300 x 300 DPI)


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Inkjet-Printed Triple Cation Perovskite Solar Cells - ACS Applied

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