POROUS PEROVSKITE NANOCRYSTALS FOR PHOTOVOLTAIC APPLICATION, PhD student DIPARTIMENTO DI SCIENZE FISICHE E TECNOLOGIA DELLA MATERIA CNR-IOM - Istituto Officina dei Materiali Headquarters (Trieste, TASC) c/o Area Science Park - Basovizza Strada Statale 14 km 163,5-34149 Trieste Final report Thanks to the support of the Short Term Mobility (STM), I have performed my research activity in the group of Francesca Toma at the Joint Center for Artificial Photosynthesis (JCAP),in the Chemical Sciences Division at the Lawrence Berkeley National Laboratories (LBNL). My project has focused on the synthesis and characterization of i) a new deposition process called Low Pressure Vapor Assisted Solution Process (LP-VASP), previously developed at LBNL 20, and ii) of porous polycrystalline perovskites synthesized at our laboratories (TASC, Trieste). Introduction: Hybrid organic-inorganic lead halide perovskites (CH 3 NH 3 PbX 3, X = I, Br, Cl) are a new class of semiconductors that has emerged rapidly within the last few years. This material class shows excellent semiconductor properties, such as high absorption coefficient 1, tunable bandgap 2, long charge carrier diffusion length 3, high defect tolerance 4, and high photoluminescence quantum yield 5,6. The unique combination of these characteristics makes lead halide perovskites very attractive for application in optoelectronic devices, such as single junction 7,8 and multijunction photovoltaics 9,10, lasers 5,11, and LEDs 12. CH 3 NH 3 PbX 3 films can be fabricated by a variety of synthetic methods 13, which aim at improving the efficiency of this semiconducting material for energy applications 14. However, optimization of photovoltaic devices relies on the quality of the halide perovskite active layer, as well as its interfaces with charge selective contacts (i.e. electron and hole transport layers), which facilitate photocarrier collection in these devices. Specifically, continuous, pinhole-free active layers are necessary to minimize shunt resistance, thereby improving device performance. 1
Among the most widespread methods for fabricating halide perovskite thin films are solution-based and vacuum-based processes. The most common solution process uses equimolar ratios of lead halide and methylammonium halide dissolved in dimethylformamide (DMF), dimethylsulfoxide (DMSO), or γ- butyrolactone (GBL), or mixtures of these solvents. 2,15,16 Precursor molarity and solvent type, as well as annealing temperature, time and atmosphere, must be precisely controlled to obtain continuous and pinhole-free films. 15 For example, to improve surface coverage, a solvent-engineering technique was demonstrated to yield dense and extremely uniform films. 16 In this technique, a non-solvent (toluene) is dripped onto the perovskite layer during the spinning of the perovskite solution. 16 These approaches are usually well suited for mesoscopic heterojunctions, which employ mesoporous TiO 2 as an electron selective contact with increased contact area and reduced carrier transport length. However, planar heterojunctions, which use selective contacts based on thin (usually TiO 2 ) films, are more desirable because they provide a simple and scalable configuration that can be more easily adopted in solar cell technology. Therefore, the development of halide perovskite active layers that show high efficiency and stability under operation for planar heterojunctions may lead to technological advancements in this field. However, one of the main challenges to fabricate planar heterojunctions is still represented by the homogeneity of the active layer. A few attempts, based on vacuum processes, have been made to prepare uniform layers on thin TiO 2 films. For example, Snaith and collaborators have demonstrated a dual evaporation process, which yield highly homogenous perovskite layers with high power conversion efficiencies for photovoltaic applications. 17 While this work represents a significant advancement in the field, the use of high vacuum systems and the lack of tunability of the composition of the active layer limit the applicability of this method. i) Synthesis and characterization of low pressure vapor-assisted solution process (LP-VASP) Motivation: Interestingly, extremely high uniformity has been achieved with the vapor-assisted solution process (VASP) 18 and modified low pressure VASP (LP-VASP) 6,19. While the VASP, proposed by Yang and collaborators 18, requires higher temperatures and the use of a glove box, the LP-VASP is based on the annealing of a lead halide precursor layer in the presence of methyl ammonium halide vapor, at reduce pressure and relatively low temperature in a fume hood. These specific conditions enable access mixed perovskite compositions, and fabrication of pure MAPbI 3, MAPbI 3-x Cl x, MAPbI 3-x Br x, and MAPbBr 3 can be easily achieved. Specifically, CH 3 NH 3 PbI 3-x Br x films over the full composition space can be synthesized with high optoelectronic quality and reproducibility 6,19. In the LP-VASP, formation of CH 3 NH 3 PbX 3 films consists of a two-step procedure that comprises i) the spin-coating of the PbI 2 /PbBr 2 (or PbI 2, or PbI 2 /PbCl 2 ) precursor on glass substrate or fluorine-doped tin oxide (FTO) coated glass substrate with planar TiO 2, as electron transport layer, and ii) the low pressure vapor-assisted annealing in mixtures of CH 3 NH 3 I and CH 3 NH 3 Br that can be finely adjusted depending on the desired optical band gap (1.6 ev E g 2.3 ev). Under these conditions, the methylammonium halide molecules present in the vapor phase slowly diffuse into the lead halide thin film yielding continuous, pinhole-free halide perovskite films. This process yields a two-fold volume expansion from the starting lead halide precursor layer to the completed organic-inorganic lead halide perovskite. The standard thickness of the perovskite film is about 400 nm. It is possible to vary this thickness between 100-500 nm by changing the speed of the second spin coating step. The presented technique results in films of high optoelectronic quality, which translates to photovoltaic devices with power conversion efficiencies of up to 19% using a Au/spiro- OMeTAD /CH 3 NH 3 PbI 3-x Br x /compact TiO 2 /FTO/glass solar cell architecture. 19 2
Methodology and results: In detail, before the perovskite deposition several conducting and semiconducting materials need to be processed and deposited for the fabrication of a complete solar cell. On cleaned glass coated with fluorine-doped tin oxide (FTO),a compact TiO 2 layer is deposited by e- beam evaporation while the substrate is kept at the temperature of 350 C.E-beam deposited TiO 2 layers have native defects due to the presence of an oxygen-deficient environment during the electron beam evaporation. Therefore, the deep-level hole traps introduced in the TiO 2 layer contribute to a high power conversion efficiencies of and enhanced long-term stability. 19 Once the TiO 2 layer is deposited on the glass/fto substrate it is possible to start with the LP-VASP process which could be divided in two sequential steps: 1. Deposition of the lead halide (PbX 2 )precursor; 2. Conversion of the PbX 2 layer into perovskite with methylammonium halidech 3 NH 3 X(also called MAX). The lead halide precursor (PbI 2 or PbBr 2 or a mix of both) dissolved in N,N-dimethylformamide (DMF) is spin coated on the glass/fto/tio 2 substrateand dried on a hot plate for 15 at 110ᵒC. The setup (fig 1a) for the second step comprises a Schlenk line equipped with a vacuum pump and with Schlenk tubes. In these tubes, we place the MAX powder on the bottom, and the glass/fto/tio 2 /PbX 2 substrates slightly above the MAX precursor powder. The tubes (fig 1b) are immersed in an oil bath at 120 C and kept at a reduced pressure (0.185 torr) for 2 hours. The lead halide precursor thin film starts converting into the perovskite material due to the MAX incorporation: at this temperature and pressure the MAX powder sublimates and saturates the atmosphere inside the tube and slowly diffuses in the lead halide film. Figure 1 a) LP-VASP setup 3
Substrate Powder Figure 1 b) Tube immersed in oil bath at 120ᵒC; c) Tube with sample during the process. During this time, it is possible to observe the color change of the sample from yellow due to PbI 2 /PbBr 2 layer to dark brown for CH 3 NH 3 PbI 3 and again from yellow to orange forch 3 NH 3 PbBr 3 perovskite.after 2h the sample is carefully removed from the tube, rinsed in isopropyl alcohol (IPA) to wash away the non-reacted MAX form the surface and finally dried with the N 2 flow. By using different PbX 2 (PbI 2 or PbBr 2 ) precursors and MAX (CH 3 NH 3 I or CH 3 NH 3 Br) for the conversion, it is possible to tune the final composition of the perovskite from pure CH 3 NH 3 PbI 3 (through several CH 3 NH 3 PbI 3-X Br X mixed perovskites) to pure CH 3 NH 3 PbBr 3. Accessing different perovskite composition is key to tune the band gap of this material. After this step,the Spiro-OMeTAD hole transport material (HTM) and the gold back electrode are sequentially deposited through spin coating and thermal evaporation, respectively. Figure 2shows an example of a complete solar cell that I fabricated during my stay at LBNL. Figure 2: LP-VASP perovskite based solar cells with different photoactive layers: on leftch 3 NH 3 PbBr 3 while on the right CH 3 NH 3 PbI 3. Once the solar cell is complete, it is possible to characterize the device with JV curve measurements (Plot1). In this type of measurement, the current density (J) is measured as a function of the applied voltage (V). From this data, the relevant figures of merit of the solar cell (V OC, I SC and FF) can be extracted and used to calculate the power conversion efficiency (η). 4
Plot 1: I-V curve of one of the solar cells with CH 3 NH 3 PbI 3 as perovksite photoactive layer. This device shows η= 9.02% that is a good value compared with the best efficiencies reached at the IOM- CNR laboratories (around 6%). The characterization reported represents my first attempt of synthesizing and characterizing a solar cell with this new technique. This measurement proves that LP-VASP is easy to manage also for non-expert users. Once the same setup will be reproduced at the IOM-CNR laboratories, I will acquire more experience with it and it will be possible to obtain high efficient devices. ii) Synthesis and characterization of porous polycrystalline perovskites Motivation: Through the LP-VASP, it has been demonstrated that thin film crystallographic texture (i.e. preferential grain orientation) can have a significant impact on photovoltaic performance. 6 By probing the photovoltaic properties at the nanoscale, it has been found that there exists facet-dependent defect concentrations that lead to significant photovoltage inhomogeneity within. 20 This research points to new ways to engineer interfaces and synthesis conditions of halide perovskites to obtain higher efficiency. Despite rapid progress in the field, a number of fundamental questions remain in order to understand the optoelectronic and transport properties of halide perovskite-based photovoltaic devices and to determine the role of local inhomogeneity on electronic properties correlated with specific crystallographic orientations. While in Trieste, I developed a synthetic methodology to obtain well-faceted perovskite cubes, which are particularly suitable for understanding how electronic properties correlate with crystallographic orientation through advance photoconductive atomic force microscopy characterization. The perovskite cubes I can produce show defined orientation and facets and allow to more precisely correlate local property variations with macroscopic characteristics. In addition, the achieved control over the structure and crystallographic orientation allow us to better understand roles of grain boundaries, local compositional disorder, and fundamental charge separation and transport mechanisms in this materials system. Methodology and results: Since the perovskite material is moisture sensitive it was necessary to prepare the samples at the hosting facility (LBNL). There I had to adapt my methodology to the infrastructure 5
and available equipment, as the perovskite crystal growth is heavily dependent on the conditions used for the process. I was eventually able to reproduce the synthesis and to achieve a better understanding of the process and on the parameters that determine the formation of the desired structure. For this experiment, I have first synthesized thech 3 NH 3 I and CH 3 NH 3 Br precursors. These molecules have been characterized with 1 H-NMR (Plot 2a and 2b)and 13 C-NMR techniques. Plot 2: a) 1 H-NMR of synthesized CH 3 NH 3 I and b) 1 H-NMR of synthesized CH 3 NH 3 Br. The porous polycrystalline perovskite film is obtained in N 2 filled glove bag by spin coating the PbI 2 (1.0 M in DMF) precursor and annealing it at 95ᵒC for 15 min. The sample is first dipped in isopropyl alcohol (IPA) for 10 to pre-wet the layer, then immersed in MABr solution (5 mg/ml in IPA) for 45, and finally rinsed in IPA to wash the MABr excess from the top of the film. After this first step a layer of well shaped cubic crystals is obtained (fig3a). The samples is then immersed in MAI solution (8 mg/ml in IPA) for 90, rinsed in IPA, and annealed at 95ᵒC for 15 min to yield the final porous material (fig 3b).In addition to understand and correlate electronic properties with crystal facets, this unique morphology could be exploited to increase the surface contact between the perovskite and the hole transporting material (HTM) such as Spiro-OMeTAD in order to enhance the holes extraction rate from the photoactive layer. Figure 3: a) SEM image of the polycrystalline perovskite after the first dipping in MABr/IPA solution; b) SEM image of the porous perovskite layer after the second dipping in MAI/IPA solution. Once I have obtained the layer, I have first characterized its morphology by SEM images. 6
Intensity [arb units] Intensity [arb. units] Short Term Mobility 2016 Then, I have characterized these films by X-ray diffraction(xrd), after the first (Plot 3a) and the second dipping steps (Plot3b). In the XRD pattern in Plot3a, it is possible to notice that the peak at 2 Theta 12.68,assigned to PbI 2, is intense as well as the perovskite peak at 14.58, thereby showing that the perovskite formation is not still complete. In the second XRD pattern, while the perovskite peak increases, the PbI 2 peak decreases in intensity, there by indicating the conversion is almost complete. PbI 2 /MABr PbI 2 /MABr/MAI 800,00 600,00 400,00 200,00 0,00 1200,00 800,00 400,00 0,00 10,00 12,00 14,00 16,00 10,00 12,00 14,00 16,00 2 Theta [ᵒ] 2 Theta [ᵒ] Plot 3: a) XRD of the polycrystalline perovskite after the first dipping in MABr/IPA solution; b)xrd of the porous perovskite layer after the second dipping in MAI/IPA solution. In order to test the efficiency of this material, I have fabricated a solar cell with the porous polycrystalline perovskite.100 nm of TiO 2 were e-beam evaporated on cleaned glass-fto coated substrate. The perovskite layer was then deposited as described. To complete the device, Spiro- OMeTAD HTL was spin coated, then a gold back contact thermally evaporated. Unfortunately the so fabricated device was not working as expected, and more optimization is needed. From the J-V characterization, it was possible to observe that both series resistance (1.8E+7 Ω) and shunt resistance (8.1E+6Ω) are very high meaning that the device has shunt pathways due to the presence of pinholes. The total coverage of the perovskite layer is necessary to have high efficient solar cells. Plot 4: I-V curve porous polycrystalline perovksite based solar cell. However, these perovskite structure are now going to be characterized by PC-AFM, as a result of this collaboration. 7
Summary and outcome: To summarize it was possible to learn the LP-VASP process that I will reproduce at the IOM-CNR laboratory. This process could be exploited to reach high efficient solar cell and to combine our know-how on nano-fabrication with the more specific perovskite experience of the Toma s group. A secondary aspect that will be taken in consideration is the deposition of the TiO 2 layer through the e- beam evaporation. It seems to be highly reproducible and allows to reach high V OC. At the IOM-CNR it is possible to deposit the layer with this technique but an upgrade is needed to keep the sample at the desired temperature (350ᵒ). The porous polycrystalline perovskite even if promising needs a further optimization if it would be used for a solar cell. The next steps will be focused on the improving of crystal size control and coverage uniformity in order to obtain a suitable layer for the photovoltaic device. This project has allowed me to start a new collaboration between my supervisor Dr. Massimo Tormen(IOM-CNR) and Francesca Toma (JCAP-LBNL), in order to merge our knowledge and fabricate high efficient solar cells. References 1. De Wolf, S. et al. Organometallic Halide Perovskites: Sharp Optical Absorption Edge and Its Relation to Photovoltaic Performance. J. Phys. Chem. Lett.5, 1035 1039 (2014). 2. Noh, J. H., Im, S. H., Heo, J. H., Mandal, T. N. & Seok, S. I. Chemical Management for Colorful, Efficient, and Stable Inorganic Organic Hybrid Nanostructured Solar Cells. Nano Lett.13, 1764 1769 (2013). 3. Stranks, S. D. et al. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science342, 341 344 (2013). 4. Oga, H., Saeki, A., Ogomi, Y., Hayase, S. & Seki, S. Improved Understanding of the Electronic and Energetic Landscapes of Perovskite Solar Cells: High Local Charge Carrier Mobility, Reduced Recombination, and Extremely Shallow Traps. J. Am. Chem. Soc.136, 13818 13825 (2014). 5. Deschler, F. et al. High Photoluminescence Efficiency and Optically Pumped Lasing in Solution- Processed Mixed Halide Perovskite Semiconductors. J. Phys. Chem. Lett.5, 1421 1426 (2014). 6. Sutter-Fella, C. M. et al. High Photoluminescence Quantum Yield in Band Gap Tunable Bromide Containing Mixed Halide Perovskites. Nano Lett.16, 800 806 (2016). 7. Chen, W. et al. Efficient and stable large-area perovskite solar cells with inorganic charge extraction layers. Science350, 944 948 (2015). 8. Bi, D. et al. Efficient luminescent solar cells based on tailored mixed-cation perovskites. Sci. Adv.2, e1501170 (2016). 8
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