Laboratory Experiment pubs.acs.org/jchemeduc
Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX
Fabrication and Characterization of Perovskite Solar Cells: An Integrated Laboratory Experience Vivien L. Cherrette,† Connor J. Hutcherson,† Jeremy L. Barnett,† and Monica C. So* California State University, Chico, California 95929-0210, United States S Supporting Information *
ABSTRACT: Perovskite solar cells have garnered exponential research interest due to their facile fabrication, solution processability, and low cost. However, there have been limited efforts to integrate this class of materials into the undergraduate laboratory curriculum. Therefore, we designed an integrated laboratory experiment in our upper-division integrated laboratory sequence to teach students about research procedures and tools used in physical, organic, inorganic, and materials chemistry. This laboratory sequence involves conversion of sunlight to electricity, which is one of the most challenging renewable energy issues we are facing as a society. In this work, upper-level undergraduates study four variables affecting the morphology and optical properties of perovskites: solvent treatment, percent of water added to a precursor mixture, cation substitution, and precursor temperature. To do so, students deposit uniform films of the material using spin-coating and annealing, and then probe the resulting film properties via scanning electron microscopy, X-ray diffraction, solid-state UV−vis spectroscopy, and current−voltage measurements. Students are able to execute the simple experimental setups and critically interpret, and compare, their results. Further, students are asked to question and understand structure−property relationships to arrive at a fuller understanding of the light-to-electricity conversion process. Importantly, this laboratory prepares students for cutting-edge inorganic and materials research topics. KEYWORDS: Upper-Division Undergraduate, Inorganic Chemistry, Hands-On Learning/Manipulatives, Collaborative/Cooperative Learning, Problem Solving/Decision Making, Materials Science, Physical Properties, Undergraduate Research, Laboratory Instruction
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INTRODUCTION Our society currently faces a high demand for energy; in fact, by 2050, our global power needs are expected to amount to 30 terawatts, which is double our current needs.1 One of the most abundant energy sources is from the sun. If sunlight is effectively captured, it potentially supplies 67 terawatts, satisfying the projected annual needs.1 To convert solar energy into electricity, solar cells are necessary. Perovskite solar cells have received exponential research interest in the past several years, due in part to their low cost of materials and interchangeable crystal structure and cell architecture. In fact, the expense of each perovskite solar cell is currently limited to $0.20 per cell, which is inexpensive compared to that of their silicon counterparts.2 There are few experimental protocols suitable for introducing this important solar energy conversion technology to undergraduates in a laboratory setting.2 This paper describes one such laboratory experiment that can be implemented in an upper-level undergraduate chemistry laboratory course. This chemistry laboratory experiment involves the fabrication and characterization of lead-iodide-based perovskite (APbI3, A = CH3NH3+, C2H(NH2)2+) solar cells. In general, solid-state materials chemistry was recently introduced into the chemistry laboratory curriculum.3,4 One of the core concepts of this experiment is structure−function relationships. Specifically, © XXXX American Chemical Society and Division of Chemical Education, Inc.
how does each part of the solar cell make a functional device? A device must include an anode (e.g., fluorine-doped tin oxide, FTO), light absorber (e.g., perovskite), electron-transport layer (e.g., titanium dioxide, TiO2), hole-transport layer (e.g., copper thiocyanide, CuSCN), and cathode (e.g., carbon). To operate the device, light absorbed by the perovskite layer excites an electron into the conduction band, leaving a hole (positive charge) in the valence band (Figure 1). While the holes inject into the hole-transporting layer (CuSCN), the electrons inject into the electron-transport layer (TiO2) and then flow through the circuit to fill the positive holes present in the cathode layer (carbon). To date, chemistry undergraduate laboratory experiments for solar energy conversion have implemented functionalized nanoparticles3 and copper-sulfide-based materials.4 One has focused on using perovskites for solar cells for general chemistry laboratory. None have focused on laboratory curriculum for upper-level chemistry courses.2 While the material combination in this experiment is mostly adapted from ref 2, it is novel for three pedagogical reasons. First, the students revisit the common perovskite crystal structure from Received: May 3, 2017 Revised: November 30, 2017
A
DOI: 10.1021/acs.jchemed.7b00299 J. Chem. Educ. XXXX, XXX, XXX−XXX
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group is responsible for film preparation, film characterization, and analysis for studying their variables. Film Fabrication
The first section of the experiment involves the fabrication of perovskite films through a seven-step process (Figure 2). The first step involves the cleaning of two pieces of 1.5 × 3 cm2 fluorine-doped tin oxide (FTO), glass coated with conductive material on one side (Figure 3). Figure 1. Solar cell operation depicting (1) light absorption, (2) charge separation, (3) charge collection, and (4) electrical power utilization.
general and inorganic chemistry; they experiment with the effects of cation substitution on the absorption characteristics of the perovskite thin films formed for the solar cell device. Second, the instructor reviews the effects of polarity of molecules on boiling points from organic chemistry; the students explore this relationship by studying how different solvents used to deposit the thin films affect their resulting morphology. Third, the students revisit the effects of reaction temperature from previous chemistry laboratories; in this materials chemistry experiment, the students visualize the color changes when the yellow precursors (due to PbI2) fully convert to the dark brown-black perovskites, as the reaction temperature increases. By exploring all these concepts and experimental parameters in a perovskite model, undergraduate students receive significant educational value. Given the need to use solar energy to meet future energy demands and the benefits of learning by doing, we believe this laboratory experiment is appropriate for upper-division undergraduate students.
Figure 3. Resistance measurements of fluorine-doped tin oxide silica glass (FTO) with a multimeter set on resistance mode (200 Ω mark). (a) The conductive fluorine-doped tin oxide layer will show a resistance reading, while (b) the nonconductive silica will not.
Second, the deposition of titanium dioxide (TiO2) is done by spin-coating onto the conductive side of one piece of FTO. The TiO2 film acts as the electron-transporting layer allowing for the injection of the negative charges, which are excited by light from the valence to the conduction band. Spin-coating involves deposition of precursor solution onto a substrate, which is then rotated at high speeds to produce uniform coatings. This is followed by annealing at 500 °C for 5 min (Figure 2, step 2); annealing involves heat treatment of films at high temperatures. The third step involves the deposition of the light-absorbing perovskite layer either by a one- or two-step deposition technique, detailed in the Supporting Information. Briefly, for students assigned to a one-step deposition process, they will also spin-coat a solution of 1 M lead(II) iodide (PbI2) and 1 M methylammonium iodide (MAI) in anhydrous dimethylformamide (DMF), and anneal at 100 °C for 20 min (Figure 2, step 4). For two-step deposition techniques, students will spin-coat a solution of PbI2 in anhydrous DMF (Figure 2, step 3). They will next dip-coat by fully submerging the PbI2 coated FTO in formamidinium iodide (FAI) in anhydrous 2-propanol, and anneal at 100 °C for 5 min (Figure 2, step 4). The fourth step involves deposition of the solution of copper thiocyanate (CuSCN) in diethyl sulfide. CuSCN is a holetransport layer which allows the resulting positive charges, created by removal of an electron from the valence band to the
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EXPERIMENTAL OVERVIEW The preparation and investigation of perovskites can be divided into two sections: (i) film fabrication and (ii) film characterization and analysis. The laboratory experiment can be carried out in four 3 h laboratory periods. Further, the experiment can be tailored to the level of rigor, variables of interest, and characterization instruments available in the lab setting. The following variables were tested: (a) solvent treatment,4−8 (b) cation substitution,9 (c) precursor solution temperature,7,9 and (d) percent of water added to a precursor mixture.10,11 To monitor changes in properties of their thin films, the students performed solid-state ultraviolet−visible (UV−vis) spectroscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and current−voltage measurements. Each student per
Figure 2. Schematic for solar cell fabrication. Note that A = MA+ (methylammonium) for one-step deposition, and A = FA+ (formamidinium) for two-step deposition. B
DOI: 10.1021/acs.jchemed.7b00299 J. Chem. Educ. XXXX, XXX, XXX−XXX
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significantly improves absorbance in the visible region (Figure 4a), compared to the control. Similarly, treatment with water
conduction band, to migrate to the respective anode. This is done by spin-coating and then annealing at 100 °C for 5 min (Figure 2, step 5). The fifth step involves the scattering of a thin layer of carbon particles to serve as the cathode (Figure 2, step 6), and finally protecting the multilayered structure with the second piece of FTO (Figure 2, step 7) (conductive side facing the CuSCN). The preparation of solutions and FTO teaches students about chemical versus physical adsorption of reagents, energetics of solid surfaces, and factors affecting solubility, while the sequential spin-coating and annealing of each layer onto a solid substrate informs students about the roles of each layer and factors affecting chemical solution deposition and film nucleation and growth. Film Characterization
The second section of the experiment requires students to characterize their perovskite films. Students will perform solidstate UV−vis absorption spectroscopy, SEM, and XRD on the perovskite film at step 4 (Figure 2), while students will measure current and voltage at step 7 with a multimeter. Note the solidstate UV−vis absorption spectroscopy can be performed using a solution UV−vis spectrometer modified with a solid-state sample holder. Further, if SEM or XRD is unavailable at the laboratory setting, SEM images and XRD raw data files are included in the Supporting Information. The UV−vis absorption measurement teaches students about how to derive the optical bandgap for a semiconducting material. While determining morphology of their films, students will think critically about secondary electron−sample surface interactions in SEM, and X-ray interactions with crystal lattice planes in XRD. Importantly, they will be able to concretely understand the solar cell operation. This is done by irradiating their solar cells with sunlight and measuring the resulting current and voltage using a multimeter, followed by calculating the efficiency of the device.
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HAZARDS It is necessary to have students review the Material Safety Data Sheets for the chemicals involved in this experiment, and wear proper personal protective equipment. Since titanium isopropoxide is a skin irritant and oral inhalation toxin, it must be handled in the fume hood. Since concentrated hydrochloric acid and methyl sulfide both are flammable and are inhalation irritants, it should be handled in the fume hood. Lead iodide may cause harm to organs and tissues, if ingested. MAI and FAI are skin irritants. FAI is also extremely hygroscopic and must be purged with either nitrogen or argon after each opening. If there is skin contact with chemicals, wash thoroughly with soap and water for at least 15 min. If there is eye contact with chemicals, flush eyes with water for 15 min with eyewash.
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Figure 4. Solid-state UV−vis absorption spectra of the perovskite film upon (a) antisolvent treatment, (b) exposure to moisture, and (c) substitution of MAI with FAI and alteration of the FAI solution temperature. UV−vis spectra of FAPbI3 films are red-shifted from that of MAPbI3, due to differences in cation structure. The temperatures noted are those of the solvent used during the dipcoating process. Spectra were collected from 390 to 900 nm, but the data below 600 nm showed noisy data from light scattering and so were excluded from this data set.
RESULTS
slightly enhances absorbance of the perovskite thin films (Figure 4b). Another project variable involved addition of water. In the literature,10,11 water is regarded as detrimental to perovskite, since exposure to moisture increases degradation; students added 0−5% of water to the precursor solution in step 5 (Figure 2). The third project variable focused on using formamidinium (FA+) as the monovalent cation in place of MA+. Consequently, a red-shifting of the absorbance is observed in Figure 4c, which is attributed to the larger size and stability of FA+ compared to MA+.9 This suggests that a broader spectrum of visible light can potentially be converted from light to electricity in a solar cell. Lastly, as the temperature
Absorption Properties
Solid-state UV−vis spectroscopy provides the absorption characteristics of a film and is an important technique for students to learn to determine the optical properties of their samples. One of the project variables focuses on antisolvents, which are partially soluble with the precursor solution, and can improve film quality by reducing pinholes, acting as a dopant, and washing away excess methylammonium (MA+) and iodide (I−) ions.6,12,13 Students added the antisolvent while spincoating the precursor solution in step 5 (Figure 2). With high success rates, the students showed that antisolvent treatment C
DOI: 10.1021/acs.jchemed.7b00299 J. Chem. Educ. XXXX, XXX, XXX−XXX
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of the FAI solution increases, students observe an increase in absorbance in Figure 4c. Crystallinity
XRD yields a “fingerprint” of a crystalline material and teaches students an important identification tool. With reproducible success, students were able to confirm that the perovskite is indeed crystalline in all cells (Figure 5). Further, the addition of Figure 6. SEM images of perovskite thin films treated with (a) nothing, (b) antisolvent, (c) water, (d) 0 °C FAI precursor, and (e) 75 °C FAI precursor.
Electrical Properties
The open-circuit voltage (when current is zero) and shortcircuit current (when voltage is zero) values serve as important metrics in determining the efficiency of converting sunlight to electricity in a solar cell. Specifically, students learn how to measure open-circuit voltage and short-circuit current using a multimeter and how the fabrication protocol affects the solar cell operation. The solar cell devices described here give opencircuit voltage (Voc) from immeasurably low up to 0.95 V, compared to research-grade cells achieving up to 1.1 V. Similarly, up to 6.5 mA of short-circuit current (Jsc) can be achieved in our devices, compared to well over 10 mA in research-grade cells, assuming an active area of ca. 0.7 cm2 in both. The major loss in current occurs due to the thick films, limiting charge transport and increasing internal recombination of charge carriers. From the Voc and Jsc, the power conversion efficiencies (η) can be calculated by eq 1. η=
Figure 5. XRD of perovskite films treated with (a) antisolvents, (b) water, and (c) varying FAI solution temperatures. Peaks at 2θ = 13.6° show formed perovskite, while those at 2θ = 11.6° are PbI2, indicating incomplete perovskite formation.
VocJsc FF Pin active area
× 100%
(1)
Therefore, assuming a 50% fill factor (FF),2 100 mW/cm2 input power (Pin) from solar radiation, and an active surface area of 0.70 cm2, a measured voltage of 310 mV and a current of 2.8 mA result in an efficiency of 0.60%.
water does not degrade lead iodide (Figure 5b). The peak for perovskite at 13.5° < 2θ < 14.0° is observed, while there is an absence of PbI2 at 2θ≈12.5°. Under our procedures a freshly made MAPbI3 thin film has little to no PbI2 peak at 12.5°. As temperature of the FAI solution increases, unreacted PbI2 decreases,9 as seen in the decrease of the XRD peak at 12.5°. This suggests that more precursor conversion occurs at higher temperatures, as indicated by the disappearance of the PbI2 peak at 50−75 °C (Figure 5c).
Classroom Testing and Teaching Objectives
The students tested the perovskite solar cell experiment for three semesters in the Integrated Chemistry Laboratory, an upper-division course at California State University, Chico (CSUC). In one laboratory period of 3 h, 3 groups of 4 students studied one project variable. During the first practice trial, each student per group rotated in executing all fabrication steps and received training on all characterization techniques on one sample. Note that one spin-coater was sufficient for a group of 12 students. Thereafter, to make multiple samples for their project, each student per group was responsible for executing one (or two) steps of the fabrication procedure and one characterization technique. To demonstrate different approaches in solid-state materials chemistry, the instructor discussed topics of general, physical, organic, inorganic, and materials chemistry including the following: • Global significance of solar energy conversion technology • Solution preparation and deposition • Chemistry and structure of perovskites • Spectroscopic characterization • Structural and morphological characterization • Device fabrication and solar cell operation At the end of the course, students were evaluated. They were assessed through group oral presentation and individually
Morphology
SEM images provide information about the morphology (e.g., size, shape, and density of the nanostructures)14 formed on a thin film of perovskite. This important electron imaging technique allows students to learn how interactions between electrons and surfaces of samples produce informative electron microscope images. Upon treatment with antisolvents, SEM images show an improvement in film morphology. Compared to the control (Figure 6a), the films treated with antisolvents (Figure 6b) and water (Figure 6c) showed significantly better coverage and layer uniformity. As temperature of the FAI solution increases from 0 °C (Figure 6d) to 75 °C, Figure 5e, unreacted PbI2 decreases, as observed by the decrease in needle-like crystals in Figure 6e. If SEM is inaccessible at a laboratory setting, raw data are provided in the Supporting Information. D
DOI: 10.1021/acs.jchemed.7b00299 J. Chem. Educ. XXXX, XXX, XXX−XXX
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written final laboratory reports. They also had to take a preand postlaboratory survey to self-rate confidence levels in executing both laboratory techniques (e.g., solution preparation, deposition, device fabrication, SEM) and laboratory tasks (e.g., using online search tools, writing reports in journal format). After performing the experiments, over 80% of the students over three semesters rated themselves as moderately to very confident in both laboratory techniques and tasks. Some common issues arise during the fabrication of the experiment but can be avoided with reminders from the instructor. Students often deposit reagents on the glass side of the FTO, resulting in a short-circuited device. Instead, students can verify the conductive side of the FTO before deposition of layers by reading resistance on a multimeter (Figure 1 of Supporting Information). To prepare the TiO2 solution, the instructor should also remind the students to first add anhydrous EtOH, followed by TTIP, and finally HCl. If students add the reagents in the incorrect order, the TiO2 solidifies, since the acid catalyzes the formation of TiO2. By providing these small reminders, the instructors can help students avoid problems during the laboratory experiment.
Author Contributions †
V.L.C., C.J.H., and J.L.B. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from California State University, Chico, and the Office of Research and Sponsored Programs. We would like to thank Nicole D. Mackie and the students of Integrated Laboratory (CHEM 381) at California State University, Chico, for taking part in the development and testing of this experiment. We would also like to acknowledge Duyen Cao at Northwestern University for consultation on this experiment.
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Summary
The authors adapted the fabrication and characterization of lead halide perovskite solar cells for use in an upper-level undergraduate integrated chemistry laboratory course. Perovskite thin films with absorption features, well-defined crystallinity, and unique morphology can be made with facile solution processing methods. Solar cell devices can also be made and tested for current, voltage, and efficiency. The diversity of chemical and processing changes that can be tested in the perovskite solar cell model is valuable for demonstration of structure−property relationships in materials chemistry. Furthermore, students revisit and discuss many concepts they learned in general, organic, inorganic, and physical chemistry. The fabrication process required students to think critically about physisorption, chemisorption, surface chemistry, and surface energetics, while the characterization techniques allowed students to relate the physical and chemical origins to the actual meanings of absorption, crystallinity, surface morphology, and electrical properties. Importantly, this laboratory experiment exposed students at the undergraduate level to how solid-state materials chemistry plays a central role in developing nanomaterials for energy conversion.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00299. Instructor notes, including experimental procedures that can be adapted to different laboratory settings, student handouts containing detailed instructions for the characterization of perovskite films, and raw SEM data (PDF, DOCX) Raw XRD data (XLSX)
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REFERENCES
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Monica C. So: 0000-0002-9044-4806 E
DOI: 10.1021/acs.jchemed.7b00299 J. Chem. Educ. XXXX, XXX, XXX−XXX
