Evaluation of Mulliken Electronegativity on CH3NH3PbI3 Hybrid

Activity Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

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Evaluation of Mulliken Electronegativity on CH3NH3PbI3 Hybrid Perovskite as a Thought-Provoking Activity Asiel N. Corpus-Mendoza,* Paola M. Moreno-Romero, and Hailin Hu Instituto de Energías Renovables, Universidad Nacional Autónoma de México, Temixco, Morelos 62580, Mexico

J. Chem. Educ. Downloaded from pubs.acs.org by UNIV OF NEW ENGLAND on 03/21/19. For personal use only.

S Supporting Information *

ABSTRACT: We implement an activity with graduate students to reinforce basic concepts of chemistry such as electron affinity, electronegativity, and ionization energy in order to improve their skills in designing and fabricating solar cells. At the same time, we use the Mulliken electronegativity method to evaluate energy levels of materials commonly applied in hybrid perovskite solar cells. Although this method does not always result in energy values comparable to those obtained experimentally or that are reported in literature, we consider that the limitations of this method can be an effective approach to generating thought in class and expanding the knowledge of students about the electronic properties, crystalline structure, and orbital states of some materials, such as CH3NH3PbI3. The analysis of a complementary activity and evaluation form shows that at least 75% of the students who worked on this activity improved their understanding of basic chemistry, the CH3NH3PbI3 molecule, and the design of solar cells. At the same time, they found this activity challenging enough to generate thought and make them consider new concepts. Also, the design of this activity allowed them to correctly evaluate the energy levels of CH3NH3PbI3 using the Mulliken method for the first time. This evaluation is possible only after considering the structural and electronic properties of CH3NH3PbI3. These results agree with experimental values reported in the literature. KEYWORDS: Graduate Education/Research, Demonstrations, Collaborative/Cooperative Learning, Semiconductors

T

a valuable tool for predicting energy levels of materials with potential PV applications without the need for relying completely on memorization. At the same time, the limitations of this procedure promote analytical thinking and discussion among students to correctly evaluate the energy levels of the perovskite molecule, which can only be achieved with a clear understanding of the geometrical structure and hybrid states of CH3NH3PbI3. This evaluation is described here and in the Supporting Information for the first time. Finally, we consider that this activity is suited for graduate students because PV researchers usually have a diversity of backgrounds where chemistry may not be the main focus. At the same time, students with a chemistry background can benefit from the PV concepts explained here by learning a valuable tool for estimating energy levels of novel materials and exploring the hybrid perovskite molecule.

he teaching of solar-energy conversion to electricity and fabrication of solar cells represents an important and exciting opportunity to prepare a young generation of students to face current global issues such as climate change, depletion of fossil fuels, and production of electricity from sustainable energy sources. In order to achieve a successful career in photovoltaics (PV), students and researchers need to have a good understanding of the periodic table and material properties as well as analytical skills to gain further experience on the fabrication and characterization of semiconductors. Often, students try to understand fundamental concepts and material properties such as electron affinity (EA), ionization energy (IE), and electronegativity (EN) through memorization. However, despite multiple research articles describing these properties and the trends they follow as a function of the atomic number,1−5 learning through memorization can create misconceptions and confusion of basic chemical concepts that lead to low performance in science.6,7 Therefore, some researchers have developed novel methodologies to understand the properties of elements and their periodicity through the use of didactic materials,8 3D-printing technology,9 and discussion of fictional elements.10 As a complement, some articles discuss anomalous properties of elements that do not follow expected trends.11,12 On the other hand, the teaching of solar cells often focuses on the concepts, fabrication, and characterization of solar cells13−16 without offering the chance to discuss or deeply analyze the impacts of the properties of materials in PV applications. Therefore, we have developed a homework activity in which students can evaluate energy levels of PV materials used in state-of-the-art perovskite (CH3NH3PbI3) solar cells following the Mulliken electronegativity empiric procedure.17 This activity offers the students © XXXX American Chemical Society and Division of Chemical Education, Inc.

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BASIC CONCEPTS AND CALCULATIONS The valence band (EV) and the conduction band (EC) in inorganic semiconductors represent the highest value of energy at which an electron is found at temperature T = 0 K and the lowest energy with vacant states that an electron can occupy after absorbing energy, respectively. In organic semiconductors, the energy levels are identified for single molecules as the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), which represent the energy of the electron in its ground state and its energy in its excited state, respectively. Often, the concept of EV in Received: September 3, 2018 Revised: March 1, 2019

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DOI: 10.1021/acs.jchemed.8b00717 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Activity

Figure 1. Energy levels of materials commonly used to fabricate hybrid perovskite solar cells: FTO, TiO2, CH3NH3PbI3, Spiro-MeOTAD, and Au.

inorganic semiconductors is compared to the composition of multiple HOMO levels in organic semiconductors, and EC is compared to LUMO. In both cases, EV (HOMO) and EC (LUMO) are referenced to the vacuum level (EVAC). The energy difference between EV (HOMO) and EVAC is identified as IE and represents the minimum energy required to remove an electron from the system, whereas the energy difference between EC (LUMO) and EVAC is EA, which represents the amount of energy obtained when an electron is inserted from EVAC into EC (LUMO). Also, the energy difference between EV (HOMO) and E C (LUMO) defines the bandgap, E g (fundamental gap, Efund), which is the minimum energy required by an electron to be promoted from EV to EC. Finally, the Fermi level (EF) is the value of energy in which an energy state has a probability of occupation of 50% at any temperature, and the energy difference between EF and EVAC is known as EN, which represents the capacity of an atom to attract electrons. Finally, the work function (WF) for metals and degenerately doped semiconductors such as SnO2:F (FTO) represents the minimum energy required to remove an electron from a solid surface to a point in the vacuum. All these levels are graphically shown in Figure 1 for a typical structure of perovskite solar cells. The Mulliken electronegativity method17 proposes that the arithmetic mean of EA and the first IE of a neutral atom should be an indication of EN of the same atom in an absolute scale, as shown in eq 1. EN =

EA + IE 2

EA = EN −

IE = EN +

n+m

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(3)

Eg 2

(4)

DESCRIPTION OF ACTIVITY

This activity was implemented for the first time in the form of a class presentation to a group of 12 graduate students. Then, students had the chance to calculate the energy levels of compounds commonly found in solar cells following eqs 1−4. Unfortunately, it was difficult to estimate the level of understanding of the students just from the explanation and simple exercises in class. Therefore, we restructured this activity as a written homework in which a new group of students was guided through exercises with increasing difficulty to test their understanding of chemistry fundaments and to expand their knowledge about perovskite solar cells. Some of the numeric exercises in this activity were designed to evaluate energy values that do not match those obtained experimentally or those reported in the literature. However, we also included open questions in which the students could discuss discrepancies between their results and the values reported in the literature. This allowed the students to reflect on the knowledge acquired to identify the limitations of the Mulliken electronegativity procedure and to discover chemistry concepts previously unknown to them. Finally, an instructor guided the students through the correct solution of the activity in class to reinforce the content. It is also noteworthy to mention that the design of this activity allowed us to correctly evaluate the energy levels of CH3NH3PbI3 using the Mulliken electronegativity procedure in combination with experimental and theoretical results found in the literature. This result is reported here for the first time.

(1)

EN (X)n EN (Y)m

2

Overall, the Mulliken electronegativity method is a valuable tool for students and researchers to estimate energy levels of compounds from the EA and IE of its constituting elements.

The value of EN for intrinsic compounds is evaluated as shown in eq 2, where n atoms of element X join m atoms of element Y. EN (X nYm) =

Eg

(2)

The evaluation of EN of semiconductor compounds is of significant importance because it corresponds to the intrinsic EF level of the semiconductor. Once a value of EN is evaluated, it is also possible to estimate EA and IE of the new compound if a value of Eg is known, as shown in eqs 3 and 4, respectively. B



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Activity

CONTENT OF THE ACTIVITY AND RESPONSES BY THE STUDENTS Table 1 and Supporting Information Table T1 require the students to compute the EA and IE values for elements Table 1. Energy Levels for Typical Elements Found in Perovskite Solar Cells Element

EA (eV)a

IE (eV)a

EN (eV)b

Sn O Ti C H N− Pb I Au

1.20 1.46 0.08 1.26 0.75 −0.07 0.36 3.06 2.31

7.34 13.62 6.83 11.26 13.60 14.53 7.42 10.45 9.23

4.27 7.54 3.46 6.26 7.18 7.23 3.89 6.75 5.77

Figure 2. Crystalline structure of CH3NH3PbI3.

molecule does not contribute to the energy levels of perovskite. At this point, students recognize this as a limitation of the Mulliken electronegativity procedure. They realize that certain arrangements of atoms can modify the electronic properties of a compound. Therefore, students are requested to evaluate and discuss EN, EA, and IE for perovskite once again in Table T3 and Question Q4 without considering the organic molecule. The next section reveals to the students that the EC (LUMO) in CH3NH3PbI3 is influenced mainly by Pb p-states, whereas the EV (HOMO) results from 70% I p-states and 25% Pb s-states, approximately.24 This helps the students to understand that the electronic properties of a compound can be influenced disproportionately by the hybrid states of its constituting elements. With this information, students can then figure out EA, IE, and EN for CH3NH3PbI3 as long as Eg is known (Tables 3 and T4). At this point, students should be

a

Values obtained from the literature.18−20 bMulliken electronegativity evaluated from eq 1.

commonly used in the fabrication of perovskite solar cells in order to calculate their EN values. Then, Question Q1 in the Supporting Information openly asks the students about any difficulties they had when completing the previous task. Interestingly, the students who reported difficulties such as not finding a particular EA or IE value in the reference provided or not being sure which values to use for the multivalent N atom performed better in the remaining exercises than those students who claimed not to have any issue. Noticing these difficulties demonstrates awareness of chemical concepts and a careful approach to completing the activity in most cases. Table 2 and Supporting Information Table T2 require the students to evaluate EN, EA, and IE values for compounds

Table 3. Energy Levels for the [PbI6]4− Cluster Element or Compound

EA (eV)

IE (eV)

EN (eV)

Pb (s-state) Pb (p-state) I (s-state) I (p-state) [PbI6]4−

    3.52b

    5.20c

13.44a 3.52a 17.06a 7.98a 4.36d

Table 2. Energy Levels for Typical Materials Used in Perovskite Solar Cells Compound

EN (eV)a

Eg (eV)b

EA (eV)c

IE (eV)d

SnO2 TiO2 CH3NH3PbI3

6.24 5.81 6.64

3.6 3.2 1.55

4.42 4.21 5.87

8.04 7.41 7.42

a EN values obtained from the literature.20 bEA evaluated from eq 2 considering that EC (LUMO) is the result of Pb p-states only.25 cIE evaluated from eq 2 considering that EV (HOMO) results from 70% I p-states (0.7 × 7.98 = 5.59) and 25% Pb s-states (0.25 × 13.44 = 3.36).25 dMulliken electronegativity evaluated from eq 1.

a

Mulliken electronegativity evaluated from eq 2. bEg obtained from the literature.21−23 cEA evaluated from eq 3. dIE evaluated from eq 4.

found in perovskite solar cells. The obtained values are then discussed and compared with reported values of FTO and CH3NH3PbI3 in Questions Q2 and Q3, respectively. Here, students are expected to explain the lower EN values of FTO in comparison with those of SnO2 as a result of the shift of EF toward EC caused by n-type doping of F. Although most students recognize F doping as the cause of the discrepancy of values, they fail to provide clear answers explaining the effect of doping on EF. Similarly, Question Q3 requires the students to compare and explain discrepancies between the evaluated and reported values of perovskite. Here, most of the students manage to provide just one or a few possible reasons for the different values. Some answers include multiple valence states of an element such as N and the atomic structure of the compound. Figure 2 and its description are shown after Question Q3 in the Supporting Information in order to inform students about the atomic structure of CH3NH3PbI3 and how the organic

able to answer Question Q5 with multiple limitations of the Mulliken procedure, such as valence states, atomic structure, percentage contribution by element, hybrid states, and doping level. Finally, Questions Q6−Q8 are useful because they allow the students to reflect on the learning experience they had while completing this activity. Here, it is shown that 75% of the students had a positive experience that reinforced fundaments of chemistry and helped them learn about the design of solar cells and CH3NH3PbI3 and about a new method for approximating energy values for different elements and compounds. A summary of the quality of answers is shown in Figure 3.

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SUMMARY The activity described here reinforces basic concepts of chemistry such as electron affinity, ionization energy, and C



Activity

Figure 3. Quality of the answers given by students. Green: correct and detailed answer that demonstrates learning and a positive experience. Yellow: incomplete answer that demonstrates some understanding. Red: wrong or empty answer that does not demonstrate understanding.

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electronegativity in order to highlight the importance of these material properties in the design of solar cells. This activity also describes the use of the Mulliken electronegativity method to evaluate energy levels of common compounds found in PV applications while at the same time pointing out some of the limitations of this method. Particularly, this activity focuses on the evaluation of energy levels of the CH3NH3PbI3 molecule used in most hybrid perovskite solar cells. We are convinced that the limitations of the Mulliken electronegativity method on the CH3NH3PbI3 molecule result in an advantageous activity that generates thought in the students and can be used as an opportunity to learn more about the electronic properties, crystalline structure, and orbital states of the CH3NH3PbI3 molecule. This activity also demonstrates the correct evaluation of the energy levels of CH3NH3PbI3 using the Mulliken electronegativity procedure for the first time. These values agree with experimental results reported in the literature. As a future work, this activity can be integrated to a fabrication course to experimentally evaluate the energy levels of semiconductor materials in order to complement the conceptual, analytical, and technical skills of students.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00717. Activity sheet that includes an introduction to perovskite solar cells, definitions of energy levels, description of the Mulliken procedure, full tables with values, questions with expected answers, other descriptions, and references (PDF, DOCX)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Asiel N. Corpus-Mendoza: 0000-0003-1529-1923 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank the financial support granted by PAPIIT-UNAM (IN102619, Mexico) and CONACyT-Fronteras de la Ciencia 2016-2024 (Mexico). A.N.C.M. thanks DGAPA-UNAM for his postdoctoral fellowship and P.M.M.R. D



Activity

(24) Endres, J.; Egger, D. A.; Kulbak, M.; Kerner, R. A.; Zhao, L.; Silver, S. H.; Hodes, G.; Rand, B. P.; Cahen, D.; Kronik, L.; Kahn, A. Valence and conduction band densities of states of metal halide perovskites: A combined experimental-theoretical study. J. Phys. Chem. Lett. 2016, 7 (14), 2722−2729. (25) Brivio, F.; Butler, K. T.; Walsh, A.; Van Schilfgaarde, M. Relativistic quasiparticle self-consistent electronic structure of hybrid halide perovskite photovoltaic absorbers. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89 (15), 155204.

thanks CONACyT (Mexico) for her Ph.D. scholarship. We would also like to thank the students who participated in this activity.

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REFERENCES

(1) Wheeler, J. C. Electron affinities of the alkaline earth metals and the sign convention for electron affinity. J. Chem. Educ. 1997, 74 (1), 123−127. (2) Myers, R. T. The periodicity of electron affinity. J. Chem. Educ. 1990, 67 (4), 307−308. (3) Sisler, H. H.; Vanderwerf, C. A. Ionization potentials in the teaching of elementary chemistry. J. Chem. Educ. 1945, 22 (8), 390− 401. (4) Haight, G. P. The use of tables of data in teaching: The students discover laws about ionization potentials. J. Chem. Educ. 1967, 44 (8), 468−470. (5) Lang, P. F.; Smith, B. C. Ionization energies of atoms and atomic ions. J. Chem. Educ. 2003, 80 (8), 938−946. (6) Osman, K.; Sukor, N. S. Conceptual understanding in secondary school chemistry: A discussion of the difficulties experienced by students. Am. J. Appl. Sci. 2013, 10 (5), 433−441. (7) Pfundt, H.; Duit, R. Students’ alternatives framework and science education, 3rd ed.; Institute for Science Education at the University of Keil: Kiel, Germany, 1991. (8) Hoffman, A.; Hennessy, M. The People Periodic Table: A Framework for Engaging Introductory Chemistry Students. J. Chem. Educ. 2018, 95 (2), 281−285. (9) LeSuer, R. J. Incorporating Tactile Learning into Periodic Trend Analysis Using Three-Dimensional Printing. J. Chem. Educ. 2019, 96 (2), 285−290. (10) Collins, S. N.; Appleby, L. Black Panther, Vibranium, and the Periodic Table. J. Chem. Educ. 2018, 95 (7), 1243−1244. (11) Chamizo, J. A. The “anomalous” electron affinity of lead: An example of isoelectronic behavior. J. Chem. Educ. 1984, 61 (10), 874. (12) Smith, D. W. The anomalous ionization potential of bismuth. J. Chem. Educ. 1975, 52, 576−577. (13) Cherrette, V. L.; Hutcherson, C. J.; Barnett, J. L.; So, M. C. Fabrication and characterization of perovskite solar cells: An integrated laboratory experience. J. Chem. Educ. 2018, 95 (4), 631− 635. (14) Chien, S.-I.; Su, C.; Chou, C.-C.; Li, W.-R. Visual Observation and Practical Application of Dye Sensitized Solar Cells in High School Energy Education. J. Chem. Educ. 2018, 95 (7), 1167−1172. (15) Shekhirev, M.; Goza, J.; Teeter, J. D.; Lipatov, A.; Sinitskii, A. Synthesis of cesium lead halide perovskite quantum dots. J. Chem. Educ. 2017, 94 (8), 1150−1156. (16) Fan, Q.; Munro, D.; Ng, L. M. Photoelectroconversion by semiconductors. A physical chemistry experiment. J. Chem. Educ. 1995, 72 (9), 842−845. (17) Mulliken, R. S. A new electroaffinity scale; together with data on valence states and on valence ionization potentials and electron affinities. J. Chem. Phys. 1934, 2, 782−793. (18) Andersen, T.; Haugen, H. K.; Hotop, H. Binding Energies in Atomic Negative Ions: III. J. Phys. Chem. Ref. Data 1999, 28 (6), 1511−1533. (19) Kittel, C. Introduction to solid state physics; John Wiley & Sons: New Delhi, India, 1996. (20) Bratsch, S. G. Revised Mulliken Electronegativities: I. Calculation and Conversion to Pauling Units. J. Chem. Educ. 1988, 65 (1), 34−41. (21) Grätzel, M. The Light and Shade of Perovskite Solar Cells. Nat. Mater. 2014, 13 (9), 838−842. (22) Tang, H.; Berger, H.; Schmid, P. E.; Lévy, F.; Burri, G. Photoluminiscence in TiO2 anatase single crystals. Solid State Commun. 1993, 87 (9), 847−850. (23) Ellmer, K. Resistivity of polycrystalline zinc oxide films: current status and physical limit. J. Phys. D: Appl. Phys. 2001, 34, 3097−3108. E


Evaluation of Mulliken Electronegativity on CH3NH3PbI3 Hybrid

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