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Large Perovskite Grain Growth in Low Temperature SolutionProcessed Planar p-i-n Solar Cells by Sodium Addition Santanu Bag, and Michael F. Durstock ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11494 • Publication Date (Web): 10 Feb 2016 Downloaded from http://pubs.acs.org on February 17, 2016

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ACS Applied Materials & Interfaces

Large Perovskite Grain Growth in Low Temperature SolutionProcessed Planar p-i-n Solar Cells by Sodium Addition Santanu Bag,*,§,‡ and Michael F. Durstock*,§ §

Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433-7702, USA.

‡

National Research Council, Washington, District of Columbia 20001, USA.

KEYWORDS. Sodium additive, large grain, solution-processing, PEDOT surface, perovskite solar cell Supporting Information Placeholder ABSTRACT: Thin-film p-i-n type planar heterojunction perovskite solar cells have the advantage of full low temperature solution processability and can, therefore, be adopted in roll-to-roll production and flexible devices. One of the main challenges with these devices, however, is the ability to finely control the film morphology during the deposition and crystallization of the perovskite layer. Processes suitable for optimization of the perovskite layer film morphology with large grains are highly desirable for reduced recombination of charge carriers. Here, we show how uniform thin films with micron size perovskite grains can be made through the use of a controlled amount of sodium ions in the precursor solution. Large micron-size CH3NH3PbI3 perovskite grains are formed during low-temperature thin-film growth by adding sodium ions to the PbI2 precursor solution in a twostep interdiffusion process. By adjusting additive concentration, film morphologies were optimized and the fabricated p-i-n planar perovskite-PCBM solar cells showed improved power conversion efficiences (an average of 3-4% absolute efficiency enhancement) compared to the non-sodium based devices. Overall, the additive enhanced grain growth process helped to reach a high 14.2% solar cell device efficiency with low hysteresis. This method of grain growth is quite general and provides a facile way to fabricate large grained CH3NH3PbI3 on any arbitrary surface by an all solution processed route.

Organic-inorganic hybrid methylammonium lead trihalide (CH3NH3PbX3; X=Cl, Br, I) based perovskite materials have been the focus of immense research in recent years in the 1 quest for next-generation, efficient photovoltaic technology. This class of materials, pioneered by Dr. Mitzi and co2 workers, possess several unique features for photovoltaic applications, such as intense light absorption across the visible spectrum, long carrier diffusion lengths, tunable bandgap, excellent carrier transport, and insensitivity to defect 3 formation. Their low temperature solution processability, earth-abundant nature, and chemical tunability could be advantageous for low-cost roll to roll (R2R) coatings on 4 large-area flexible substrates. Rapid breakthroughs resulting in certified power conversion efficiencies (PCE) over 20% from these organolead halide perovskites in a short time has made them relevant to and competitive with commercialized 5 c-Si, thin film CIGS and CdTe photovoltaic technologies. To date, perovskite solar cells are based on two main device architectures; namely a mesostructured configuration and a thin-film planar heterojunction structure. In both cases, high 6,7 PCEs have been achieved for small area devices. The construction of complex mesostructured device architectures 6 require high-temperature sintering (>450°C) for the formation of electron-transporting metal-oxide layers, such as mesoporous or compact TiO2, which limits their applicability

on flexible roll-to-roll compatible plastic substrates. Thinfilm planar heterojunction (PHJ) structures, with no mesoporous TiO2, are advantageous for high-throughput manufacturing in terms of their simple device configuration and low 7,8 temperature processing. Several planar-heterojunction structures (p-i-n and n-i-p), which avoid the mesoporous scaffold and have different combinations of charge transporting interlayers, have been investigated by numerous research groups and the PCEs from these systems are on par with 7 those utilizing a mesostructured configuration. Planar heterojunction p-i-n structures consisting of poly(3,4ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) as a hole-transport layer (p-type) and phenyl-C61-butyric acid methyl ester (PCBM) as an electron-transport layer (ntype) are promising due to their low-temperature solution processability, excellent bendability, and tunable conductivi8-11 ty. A typical perovskite based p-i-n planar heterojunction solar cell device fabrication starts using glass as a substrate, indium-doped tin oxide (ITO) as a transparent conductive oxide front contact and PEDOT:PSS as a hole-transport layer. Then the perovskite active layer is deposited on top of PEDOT:PSS, followed by a thin layer of PCBM as an electron acceptor, and finally an aluminium (Al) metal layer as a cathode. At present, one of the main issues encountered in

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this device fabrication process is the fine control of film morphology during the deposition and crystallization of the 12 perovskite layer. To avoid shunting in such planar structures, a homogeneous and pinhole-free perovskite layer is crucial. Besides surface coverage, the optimization of several other important material parameters, such as material crystallinity and grain structure, could lead to improved electronic properties of the perovskite films and thereby superior device performance. Since grain boundaries may act as recombination centers for photogenerated charge carriers and reduce device performance, it is anticipated that large grains, on the order of the film thickness, could facilitate charge transport by reducing the number of defects and trap 13,14 states.

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The diffusion enhanced grain growth, where precursor ions can penetrate through a longer distance than in a dry, allsolid state thermal annealing environment, is achieved by annealing the stacked film with DMF vapour for the same duration of time at the same temperature as done in the pure thermal annealing method (referred to as solvent anneal14 ing). Detailed film and device fabrication processes can be found in the electronic supplementary information (ESI). A sodium iodide (NaI) solution in DMF was used as a controlled source of sodium and was added in different amounts (from 1 mol% to 8 mol% in 2 mol% increments) to the PbI2 precursor solution.

In this paper, we demonstrate significant improvements of methylammonium lead tri-iodide (CH3NH3PbI3) perovskite grain size and planar heterojunction p-i-n solar cell device performance through the intentional introduction of a con+ trolled amount of sodium (Na ) ions into the precursor solution during film processing. While the effects of sodium and other alkali ions, such as potassium, have been extensively studied during the high temperature (>400°C) growth of polycrystalline CIGS, CZTS based thin-film solar cell devic15 es, it has never been demonstrated in the organic-inorganic hybrid methylammonium lead trihalide perovskite family. In recent reports, small molecule additives are shown to modulate the perovskite crystallization process and thereby thin16,17 film morphology. Irrespective of additives, micron size 11,18,19 perovskite grains are made by several other approaches. Herein, we show for the first time the fabrication of perovskite solar cells by incorporating a controlled amount of sodium ions into the film. The scope of this report is to demonstrate a methodology of increasing the grain size of perovskite thin films using a Na-based additive during film growth in order to increase the device performance. Additionally, we study the effect of processing environment on the additive-enhanced perovskite film morphology. Large, micron-size perovskite grains are formed when sodium is added and the films are annealed under a diffusion facilitated 14 environment (e.g. solvent annealing conditions). Here we use a sequential two-step solution based spin-coating tech14 nique as a representative deposition method to demonstrate the effect of sodium ion introduction on the properties of the perovskite film and the corresponding photovoltaic devices. The optimized device, incorporating 2 mol% of sodium into the perovskite film, showed a high efficiency of -2 14.2% under AM1.5G 100 mWcm conditions which is, on average, 3-4% higher in absolute efficiency than our control devices without sodium.

Figure 1. (a-e) Top down SEM images of the 300 nm perovskite films grown on CH3NH3PbI3 glass/ITO/PEDOT:PSS under different processing conditions: (a) Thermal annealing and (b) solvent annealing without Na additive, (c) thermal annealing and (d) solvent annealing with 2 mol% of NaI additive, and (e) solvent annealing with 2 mol% of NaBr additive. In all images, scale bars are 2 µm. (f) Grain size distributions of the solvent annealed films as measured from the SEM images in (b), (d) and (e).

The perovskite films were deposited on top of the PEDOT:PSS coated ITO/glass substrate via two-step deposition of two precursor solutions; a concentrated PbI2 solution in dimethylformamide (DMF) followed by a dilute CH3NH3I solution in 2-propanol. An intermediate annealing step (at 80°C for 10 minutes) before CH3NH3I deposition helps to partially evaporate the solvent and crystallize PbI2, while the final heat treatment (at 100°C for 80 minutes) of the deposited film drives the conversion of PbI2 to CH3NH3PbI3 by interdiffusion of the precursors, and facilitates crystallization and grain growth (referred to as pure thermal annealing).

Figure 1 shows scanning electron microscopy (SEM) images of the ≈300 nm thick perovskite films, with and without added sodium, deposited on glass/ITO/PEDOT:PSS by a twostep solution deposition process and annealed under differ-


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ent environments including both with and without solvent vapour. In order to get consistency of results, these thin perovskite films were prepared identically other than the annealing conditions and additive incorporation. Pinhole free perovskite films with complete surface coverage were obtained by fine tuning the process conditions. Consistent with previous reports, without an additive the average grain size of the solvent annealed perovskite films is slightly larger than 14 that of pure thermal annealed films, Figure 1a and b. However, when films are grown with a controlled amount of sodium in the precursor solution, the average grain size of the solvent annealed perovskite films increases substantially as seen in Figure 1c and d. Some grains of micrometer size range are clearly observed in those sodium added and solvent annealed films, Figure 1d. Similar results were found using both NaI and NaBr as an additive (Figure 1d and e), which suggests irrespective of the counter anions, sodium ions promote large grain growth under a diffusion facilitated, solvent annealing environment. Considering that a large excess of iodine is present in our preparation conditions in the form of CH3NH3I, the addition of a very small concentration of NaI is not expected to significantly change the total iodide concentration, which further suggests that the predominant effect of enhancing CH3NH3PbI3 grain growth is due to the Na ion. The grain size distributions of the solvent annealed perovskite films, both without and with sodium, are calculated from Figure 1b, and Figure 1d and 1e, respectively using ImageJ software and are plotted in Figure 1f.

sor solution was greater than 1 mol% and it becomes saturated at an amount of 5 mol%. At higher concentrations, a discontinuous perovskite film forms with many uncovered area of the substrate. The discontinuous nature starts on some areas of the substrate at 6 mol% as seen in Figure 2e and on the entire substrate at 8 mol%. While the average grain size of the film with 5 mol% sodium is the largest for our processing conditions (Figure S2), the film is comprised of rougher grains with irregular shapes. More ordered, large, flat, and polygonal grains with triple junction grain boundaries were obtained at 2 mol% of sodium (Figure 1d, e) and were further studied here.

The solvent annealed perovskite film without added sodium had a narrow grain size distribution with a sharp peak at 270 nm, while the peak became broader and shifted to 520585 nm after the addition of 2 mol% of sodium either as NaI or NaBr. Also, the largest grain size increased from 950 nm to 1.7 micrometers after sodium addition. It is worthwhile to mention here that in order to observe such a favourable effect of sodium additive on the perovskite grain growth, a diffusion facilitated support medium and annealing environments are highly desirable. A compact PbI2 layer in a twostep deposition process hinders diffusion of ions and impedes large grain formation even after sodium addition and solvent annealing. While the exact role of sodium in the perovskite film growth is unclear at this moment and subject to future investigations, it is believed that very low concentrations of sodium act as nucleation sites and promote large grain growth. It is also possible that the highly mobile small sodium ions act to facilitate grain boundary mobility during the solvent annealing process enabling the growth of larger grains (Figure S1). Since similar enhancements in grain size were previously observed by using a non-wetting polymeric 20 surface, it is likely that mobile grains are beneficial in improving the overall grain size.

Figure 2. Top down SEM images of the 300 nm perovskite films grown on glass/ITO/PEDOT:PSS under solvent annealing conditions at 100°C for 80 min with varied amount of Na additive (as NaI) in the PbI2 solution: (a) 1 mol%, (b) 3 mol%, (c) 5 mol%, (d, e) 6 mol%, but different area of the substrate, and (f) 8 mol%. In all images, scale bars are 2 µm.

The effect that the concentration of Na ions has on thin film morphology is shown in the SEM images of Figure 2. Since similar effects were observed for NaI and NaBr in Figure 1, we limit further study to the addition of NaI. The ≈300 nm thick perovskite films were prepared with different amounts of added NaI but were annealed under the same solvent annealing conditions as shown above. The increase in average grain size of the perovskite film was significantly noticeable when the amount of added sodium in the precur-

To gain further insight into the formation and crystallization of perovskite phases using low concentrations of sodium, X-ray diffraction (XRD) data were collected on these thin-film samples. The XRD patterns in Figure 3 show that the perovskite films grown with added sodium (up to 2 21 mol%) form a tetragonal phase with no systematic shifting in the peak positions, and do not contain any PbI2 or



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CH3NH3I impurity peaks. Under identical processing conditions, the full width at half maximum (FWHM) value of the most intense (110) diffraction peak of the pristine perovskite film at 14.08° remains significantly unchanged after sodium addition. On the other hand, analysis of FWHM for the (022) reflection at 24.48° shows slightly narrower peaks with increasing amount of added Na, which is in fair agreement with the increased grain size. There does not appear to be any preferential effect on grain orientation at this point, but this is still under investigation. The normalized UV-Visible near infrared absorption spectra of the perovskite film shows no dramatic change of the band-edge absorption around 750800 nm with this low level of Na doping (Figure S3). Timeresolved photoluminescence (TRPL) measurements on these perovskite films further reveal no significant variation of the emission decay between control and 2 mol% Na added samples (Figure S4).

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for the best performing cells measured under light intensities 2 of 100 mW/cm . The overall PCE of the best cell improved from 10.2% to 14.2% when Na was used during perovskite film growth. Notably, this improvement results from significant enhancements in each of the device performance parameters including the fill factor (FF), open circuit voltage (Voc) and short-circuit current density (Jsc). Consistent with this, PV devices based on the Na added film show higher external quantum efficiency (EQE) in the entire visible (350750 nm) spectral region compared to control samples without Na, Figure 4c. We attribute these effects to a reduction in charge recombination in the perovskite layer due to the larger grain size and consequent smaller grain boundary area. These results were highly consistent for many different devices as is demonstrated by comparing the results of more than 25 different cells, as shown in Figure 4d. On average, a 3-4% increase in absolute PCE is observed when Na is used to induce larger grain sizes in perovskite films for photovoltaic cells. The appearance of hysteretic effect in perovskite based devices is a well-known issue and is attributable to a number of factors such as trapping/detrapping of charge carriers, 22-25 ferroelectricity, charge accumulation, and ion migration. The current-voltage (J-V) curves of our optimized samples do not show significant hysteresis presumably due to the absence of large density of charge traps at the perovskite grain boundaries which are passivated by the double fullerene layer. The J-V curves with variable scan rates ranging from 10 mV/s to 250 mV/s further reveal weak scan-rate dependence (Figure S6). This is consistent with the results reported pre9,14,20 viously on similar p-i-n type of device structures.

Figure 3. (a) Comparison of XRD patterns of the solvent annealed (SA) perovskite films on glass/ITO/PEDOT:PSS with and without Na additive. FWHM for the (b) (110) and (c) (022) peaks are compared among no Na, 1 mol% Na, and 2 mol% Na added perovskite samples (all solvent annealed). FWHM for the (110) reflection are 0.139, 0.136, and 0.140° for Na = 0, 1, and 2 mol%, respectively, and the corresponding values are 0.131, 0.132, and 0.113° for the (002) reflection (error in curve fitting: ±0.004°). The impact of sodium addition on solar cell device performance was explored by fabricating two sets of photovoltaic cells based on a sodium containing film (2 mol%) and a control sample (no Na), processed similarly under identical solvent annealing conditions with a glass/ITO/PEDOT:PSS/CH3NH3PbI3/PC71BM/C60/Al structure as shown in Figure 4a (device optimization results in Figure S5). Our control devices show an average PCE of 9.5% with the best cell exhibiting a PCE of 10.2%. This can be compared to optimized values of about 13% in the literature for similar 9,20 architectures prepared by two-step sequential process. Given that the PCE of these devices is highly dependent upon subtle changes in processing conditions, our baseline devices provide a reasonable basis for comparison. Most importantly, in our experiments the devices for which Na was used exhibit superior performance characteristics in all respects compared to the control samples as shown in Figure 4d and 4b,c

Figure 4. (a) The schematic of CH3NH3PbI3 photovoltaic device structure, (b) J-V curves (open circles: forward scan, closed circles: reverse scan) and (c) EQEs of the best performing solar cells built from solvent-annealed (100°C for 80 min) 300 nm perovskite films without and with 2 mol% Na additive, and (d) efficiency statistics of the photovoltaic devices in each category. In summary, we have used a simple additive enhanced, low temperature solution based crystallization process for CH3NH3PbI3 perovskite film growth. For the first time, it has been shown that the controlled addition of sodium ions in


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the perovskite precursors significantly improves the grain structure and the overall film morphology, which are important for achieving high efficiency polycrystalline thin film solar cells. The formation of pinhole-free, large-grained perovskite films on top of the PEDOT:PSS surface by incorporating very small amounts of a sodium additive during the crystallization process enables the fabrication of planar heterojunction solar cells with PCEs as high as 14.2%. We speculate that even greater efficiency enhancement can be achieved through further process optimization and integration of this approach to some of the state-of-the-art device fabrication protocols. The present approach is expected to provide an effective strategy to fabricate high quality perovskite films even by high-throughput solution-based deposition techniques like inkjet-printing, slot-die coating, or aerosol-jet 26,27 The applicability of this method is likely to printing etc. be extended to other perovskite device architectures comprising different interlayers, and material systems where the fabrication of large grains at low temperatures is an issue. Our results could further impact the fabrication of light28-30 weight perovskite based hybrid optoelectronic devices, such as field effect transistors, photodetectors and light emitting diodes on flexible polymer substrates.

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ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge via the Internet at http://pubs.acs.org. Experimental and measurement details, device fabrication, schematic of proposed grain growth mechanism, grain size distribution plot, spectroscopic data, device optimization results, and J-V curves.

AUTHOR INFORMATION Corresponding Author *[email protected]; [email protected]

ACKNOWLEDGMENT This work was supported by the Air Force Office of Scientific Research (AFSOR).

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(24) Tress, W.; Marinova, N.; Moehl, T.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Grätzel, M. Understanding the Rate-dependent J–V Hysteresis, Slow Time Component, and Aging in CH3NH3PbI3 Perovskite Solar Cells: the Role of a Compensated Electric Field. Energy Environ. Sci. 2015, 8, 995-1004. (25) Zhao, C.; Chen, B.; Qiao, X.; Luan, L.; Lu, K.; Hu, B. Revealing Underlying Processes Involved in Light Soaking Effects and Hysteresis Phenomena in Perovskite Solar Cells. Adv. Energy Mater. 2015, 5, 1500279-1-1500279-6. (26) Wei, Z.; Chen, H.; Yan, K.; Yang, S. Inkjet-printing and Instant Chemical Transformation of a CH3NH3PbI3/Nanocarbon Electrode and Interface for Planar Perovskite Solar Cells. Angew. Chem. Int. Ed. 2014, 53, 13239-13243. (27) Yang, Z.; Chueh, C. C.; Zuo, F.; Kim, J. H.; Liang, P. W.; Jen, A. K. Y. High-performance Fully Printable Perovskite Solar Cells via Bladecoating Technique under the Ambient Condition. Adv. Energy Mater. 2015, 5, 1500328-1-1500328-6. (28) Kagan, C. R.; Mitzi, D. B.; Dimitrakopoulos, C. D. Organicinorganic Hybrid Materials as Semiconducting Channels in Thin-film field-effect Transistors. Science 1999, 286, 945-947. (29) Dou, L.; Yang, Y.; You, J.; Hong, Z.; Chang, W. -H.; Li, G.; Yang, Y. Solution-processed Hybrid Perovskite Photodetectors with High Detectivity. Nat. Commun. 2014, 5, 5404-1-5404-6. (30) Tan, Z. -K.; Moghaddam, R. S.; Lai, M. L.; Docampo, P.; Higler, R.; Deschler, F.; Price, M.; Sadhanala, A.; Pazos, L. M.; Credgington, D.; Hanusch, F.; Bein, T.; Snaith, H. J.; Friend, R. H. Bright Lightemitting Diodes Based on Organometal Halide Perovskite. Nat. Nanotech. 2014, 9, 687-692.


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