Using Polymer Semiconductors and a 3-in-1 Plastic Electronics STEM

Article Cite This: J. Chem. Educ. XXXX, XXX, XXX-XXX

pubs.acs.org/jchemeduc

Using Polymer Semiconductors and a 3‑in‑1 Plastic Electronics STEM Education Kit To Engage Students in Hands-On Polymer Inquiry Activities Jessica L. Enlow,† Dawn M. Marin,‡ and Michael G. Walter*,‡ †

Science Department, Cox Mill High School, Concord, North Carolina 28027, United States Department of Chemistry, University of North Carolina at Charlotte, Charlotte, North Carolina 28223, United States

‡

S Supporting Information *

ABSTRACT: To improve polymer education for 9−12 and undergraduate students, a plastic electronics laboratory kit using polymer semiconductors has been developed. The three-module kit and curriculum use polymer semiconductors to provide hands-on inquiry activities with overlapping themes of electrical conductivity, light emission, and light-harvesting solar energy conversion. Many of these themes are critical to contemporary polymer molecular electronics research. The kit includes modules to synthesize and evaluate the electrical properties of conductive colloidal polyaniline (PAni), to construct a polymer lightemitting diode using poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), and to build a polymer solar cell using MEH-PPV and nanoparticulate TiO2. Designed initially for high school science classrooms, the activities developed also meet new ACS undergraduate education requirements for macromolecular, supramolecular, and nanoscale systems in the curriculum and can be used in undergraduate teaching laboratories. The modules and kit have also been implemented in professional development workshops for training 9−12 science educators to help integrate the activities into their classrooms. KEYWORDS: High School/Introductory Chemistry, Continuing Education, Polymer Chemistry, Public Understanding/Outreach, Hands-On Learning/Manipulatives, Inquiry-Based/Discovery Learning, Materials Science, Nanotechnology, Semiconductors, Physical Chemistry

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INTRODUCTION Polymer education has historically received little attention in undergraduate and 9−12 STEM education curricula. Recent American Chemical Society (ACS) guidelines from 2015 have recommended inclusion of macromolecular, supramolecular, and nanoscale (MSN) content for the undergraduate certified chemistry curricula.1 Increasing the awareness and practice of polymer education is also a key mission of IUPAC’s polymer education division, which has online resources and hosts symposia addressing international polymer education. 2 Although a significant portion of polymer educational curricula includes the basics of polymer synthesis, physical characteristics of polymers, and biological macromolecules,1,3 less attention has been devoted to the field of polymer-based molecular electronics and polymer semiconductors. Since the 2000 Nobel Prize in Chemistry was awarded to Heeger, MacDiarmid, and Shirakawa for their research on conducting polymers, progress in the utilization of conductive and semiconductive polymeric materials has led to innovative applications such as organic/ polymeric light-emitting diodes (OLEDs/PLEDs), printable plastic electronics, bioelectronics, antistatic coatings, and thinfilm materials for solar energy conversion.4,5 The commercial success and aggressive growth of OLED materials containing both conductive and semiconductive polymeric materials is clearly evident from their multiple uses in OLEDs for mobile phones and other digital displays.6 As polymer and material science ideas become a greater focus in undergraduate and advanced high school chemistry courses, instructors are seeking unique hands-on educational laborato© XXXX American Chemical Society and Division of Chemical Education, Inc.

ries that satisfy their content needs and include advanced science applications. Instructors are also anticipating a shift in educational focus to student-directed laboratories and inquiry activities. Lastly, many teachers and administrators at the high school level are looking for professional development that includes current content applications, new technologies, and innovative lessons with components to bring back to their classrooms. Therefore, a new polymeric semiconductor education kit has been developed. The three-module kit and curricula engage students and teachers on the applications of conjugated semiconductive polymers, their synthesis, and their physical properties. The education kit has been developed for students at the high school (secondary education) and undergraduate levels and contains fundamental concepts that exist in chemistry and physics laboratory activities. The hands-on activities are inquiry-based experiments in conductive polymer syntheses, OLEDs, and polymeric organic solar cells.7 The kit also provides opportunities to introduce and discuss concepts of nanotechnology. The three primary overlapping physical/ chemical phenomena that are presented in each module include electrical conduction (polyaniline sensor), light emission (OLED), and light-harvesting PV (polymer solar cell) (Figure Special Issue: Polymer Concepts across the Curriculum Received: August 9, 2017 Revised: October 17, 2017

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DOI: 10.1021/acs.jchemed.7b00332 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

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physics, and engineering concepts found in macromolecular chemical structure, photophysical properties of polymeric materials, and engineering of molecular electronic devices.

1). These three concepts overlap in more advanced educational areas of excited-state charge transfer, chemical potential, and

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EXPERIMENTAL METHODS: POLYMER SEMICONDUCTOR EDUCATION KIT The three modules include (i) the synthesis and electrical conductivity of colloidal polyaniline (PAni), (ii) the construction and testing of a polymeric light-emitting diode, and (iii) a nanoparticulate TiO2 sensitized MEH-PPV solar cell. Each module has been designed along with teacher content background, a teacher lab preparation guide, and student procedures (the materials for module 1 (sections S1−S3), module 2 (sections S4−S6), and module 3 (sections S7−S9) are provided in the Supporting Information). Links are embedded in the supporting procedures to access additional safety resources online. The organic electronics and photochemistry themes of the kit and corresponding curriculum were specially chosen to correlate with current research at the UNC Charlotte Department of Chemistry. This serves to give context to the curriculum and creates the possibility for long-term partnerships and communication between local teachers and researchers. In the teacher instructional materials, instructors are provided with tips for implementing the activities. Tips address instructional time requirements, preparation guidelines, material management, and safety concerns. Prelab and postlab questions are provided throughout the student instructional materials to increase depth of thought and problem solving during the experience. Students can use the online mobile apps for ease of following the procedure step by step. The teacher guides provide answers to the pre/postlab questions as well as

Figure 1. Three overlapping optoelectronic concepts associated with each module in the polymer semiconductor kit.

charge transfer. The polymer semiconductor kit has been implemented using a collaborative professional development workshop model that bridges researchers and teachers. The three laboratory activities have been designed to provide an introduction to the rapidly advancing field of polymeric semiconductors. They build upon fundamental chemistry,

Figure 2. (a) Aniline hydrochloride monomer polymerization and polyaniline (PAni) product (conductive form). (b) Image of the resulting aqueous PAni dispersion from the polymerization. (c) PAni acid−base sensor device schematic. (d) Structures obtained by doping and dedoping using acid (vinegar) and base (ammonia). B



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MEH-PPV drop-cast from an organic solvent (chloroform or toluene) solution onto the surface of the electrode. Gallium− indium eutectic (Solution Materials, LLC) deposited onto another FTO is used as the top contact to the MEH-PPV film (Figure 3b and section S6). After the two electrodes are brought together, a voltage is applied to the device using a 9 V battery (Figure 3b), causing injection of charge carriers (a highenergy electron at the cathode and a hole at the anode) into the device layers. Upon recombination of the electrons and holes, an exciton forms in the polymer layer. Students see this exciton “relax” and release this excess energy through light emission, termed fluorescence or electrofluorescence (Figure 3c). Students engage with circuitry, photochemistry, and macromolecular chemistry in the analysis of the results (section S6). This module emphasizes concepts of energy and light−matter interactions and demonstrates how electrical energy can be transformed into light energy. Again, all laboratory instruction, detailed student procedures, instructor preparation, and content background are available in the Supporting Information (sections S4−S6).

other supporting information to help facilitate these activities in the classroom. Teachers can additionally participate in specialized workshops to get first-hand experience in completing modules 1, 2, and 3 prior to implementing them in their classroom. Module 1: Oxidative Polymerization of Aniline, Conductivity Testing, and Use of PAni as a Vapor Sensor

Students perform a simple reaction by mixing aqueous solutions of aniline hydrochloride (0.2 M) and ammonium persulfate (0.25 M) (Figure 2a−c and section S3; reagents can be obtained from Fisher Scientific). After approximately 5−10 min of vigorous mixing/shaking of the two solutions in a plastic vial, polymer aggregates begin forming, resulting in a visible dark-green precipitate of polyaniline colloidal nanofibers (Figure 2b) that continues to react, forming a very dark green solution. After the conductive polymer PAni forms, students “draw” conductive PAni wires on a piece of paper or paper towel using a pipet. After the PAni ink dries (approximately 5 min), the conductance is measured with a multimeter with contacts directly to the polymer or through predrawn (with a pencil) graphite pads (Figure 2c). After a brief discussion on the conductive properties of the various molecular forms of polyaniline, students test the chemical sensing ability of the fibers. Students can manipulate the various forms of polyaniline through acid−base protonation−deprotonation using store-bought vinegar and ammonia to test the properties of these different forms (section S3). Students experiment with the conductivity response of the fibers to acidic and basic vapors, leading them to postulate real world uses of this technology. This module emphasizes structure and function relationships, applications of conductive polymers and their pH sensitive molecular structure. All laboratory instruction, detailed student procedures, and instructor preparation, and content background are available (sections S1−S3).

Module 3: Polymer (MEH-PPV) Sensitized TiO2 Solar Cell

This module uses the same semiconducting polymer as in the OLED module, MEH-PPV, in order to exemplify the bimodal nature of the polymer. Here MEH-PPV is used to capture solar photons and transform the energy to electrical energy (Figure 4a and sections S7−S9). Initially, students learn about the materials used for the solar cell assembly (Figure 4b) and how organic semiconducting polymer solar cells can prove to be more cost-efficient than the inorganic solar cells that are commonly used today (sections S7−S9). The MEH-PPV solar cell is also constructed on FTO electrodes and follows an approach very similar to that used to make a dye-sensitized nanoparticulate TiO2 solar cell.7,8 In fact, the photoelectrode is also composed of a thin film of TiO2 nanoparticles applied to the FTO electrode from a TiO2 slurry/paste (4 g of Degussa P25 TiO2/10 mL of H2O/0.2 mL of Triton X-100 surfactant). Instead of using a dye to stain the TiO2 photoelectrode, this module uses MEH-PPV as a polymer sensitizer (sections S7− S9). A chloroform solution of MEH-PPV is drop-casted onto the TiO2 electrode and allowed to dry. Solar cell device construction involves clipping the MEH-PPV-sensitized TiO2 electrode to a counter electrode coated with graphite and filling the gap between the electrodes using an I−/I3− redox couple solution (0.05 M I2 and 0.5 M potassium iodide in ethylene glycol) (Figure 4a). After assembly, students expose the polymer-sensitized solar cell to light, test the photocurrent/ photovoltage using a multimeter, and calculate the power generated by the solar cell (Figure 4c). Typical photovoltages for devices made by students and instructors at workshops are in the range of 300−500 mV. Students can then experiment with different light filters to collect data on the most efficient wavelength of light for this selected polymer (section S9). All laboratory instruction, detailed student procedures, instructor preparation, and content background are available in the Supporting Information (sections S7−S9).

Module 2: Organic Light-Emitting Diode Using MEH-PPV

The OLED module involves the construction of a simple polymer LED device using the light-emissive semiconducting polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) (Sigma-Aldrich) (Figure 3a). After an initial exploration of the materials involved in OLED device construction, students construct the OLED device. The device is composed of a fluorine-doped tin oxide (FTO−TEC 15, Hartford Glass Co. Inc.) glass electrode with a thin film of

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HAZARDS

Module 1

Figure 3. (a) Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) polymer structure. (b) OLED device cross section with fluorine-doped tin oxide conductive glass electrodes (FTO), MEH-PPV, and indium−gallium eutectic (In−Ga) metal contact. (c) OLED emission by instructor workshop participant.

Aniline hydrochloride is considered toxic if swallowed, inhaled, or in contact with skin and is a suspected carcinogen. Ammonium persulfate is considered harmful if swallowed or in contact with skin. Therefore, protective eyewear, gloves, and C



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Figure 4. (a) Polymer semiconductor solar cell energy profile. (b) Anatomy of the solar cell device constructed using an MEH-PPV-sensitized nanoparticulate TiO2 photoelectrode, FTO electrodes, and I−/I3− redox electrolyte. (c) Image of the completed MEH-PPV/TiO2 solar cell device.

Figure 5. Example screenshots of the mobile student interface app available for download with step-by-step laboratory procedures for the (a) PAni synthesis, (b) OLED, and (c) polymer-sensitized TiO2 solar cell activities.

to a fume hood is required for this activity because of the evaporation of small quantities of CHCl3 needed to form MEH-PPV thin films for the OLEDs and solar cells. The gallium−indium eutectic is not hazardous in its liquid form, but it should not be inhaled, so processes that generate dust should be avoided. When the OLED is tested with either a power supply or a battery pack, care must be taken not to short the power supply (batteries). A 9 V battery should be sufficient to produce light emission from the OLED. Handling titanium dioxide nanoparticle dry powder requires the use of a dust mask, eyeshields, and gloves because it is an inhalation hazard. The aqueous paste should be prepared prior to conducting the solar cell experiment. Additional information about chemical handling, safety, and disposal is included in sections S5 and S8 in the Supporting Information.

a well-ventilated area should be utilized when handling these solids. At the high school level, it may be appropriate for the instructor to prepare the dilute aniline hydrochloride and ammonium persulfate solutions prior to conducting the polymerization laboratory activity. Polyaniline from the resulting polymerization is not hazardous, but aqueous dispersions of the polymer could contain trace amounts of unreacted aniline hydrochloride and ammonium persulfate. Additional information about handling, safety, and disposal is included in section S2 in the Supporting Information. Modules 2 and 3

MEH-PPV is not considered a hazardous material. However, gloves and protective eyewear should be worn while handling MEH-PPV/CHCl3 solutions used in these experiments. Access D



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Figure 6. Images of a complete Polymer Semiconductor Education Kit for workshop participants and all labeled materials for the three modules contained within the kit.

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ELECTRONIC TOOLS FOR LABORATORY ACTIVITIES To address the long-term needs of the educational program, the authors have created a website to provide teachers with continual access and updates to curricular materials and information to access reusable laboratory materials.9 The website is also the hub for instructor discussions and suggested improvements. The kit and curricular materials are available online complete with ancillary instructional materials and optional technology enhancements. The curriculum also contains pictures and instructional videos about expected results when conducting the activities in addition to safety considerations for all of the materials used across the three modules. In an effort to further support teachers after the completion of the workshop, mobile apps that correspond with the kit module procedures can optionally be used during activity classroom implementation. Students can interact with the laboratory procedures (PAni, Figure 5a; OLED, Figure 5b; polymer-sensitized solar cell, Figure 5c) from their cell phones or other electronic devices that support Android apps.9 This provides teachers the option to allow students to preview the procedure, background content knowledge, and procedure how-to-videos before and during the activity. This option delivers assistance in implementing the laboratory activities where instructors are hesitant to try advanced procedures.

education platforms are needed in this area to support teachers who are integrating polymer concepts into existing science curricula. Contemporary innovations that impact students and teachers include polymeric organic electronics, bioelectronics, polymer sensors, and polymer semiconductor solar cells.5 The workshop was also developed so that instructors would receive a kit with enough materials to run the three modules for a class of 20−30 students once or twice a year for three years (Figure 6). The first day-long (6 h) workshops for instructors, conducted in Fall 2015 (12 science instructors) and Fall 2016 (eight science instructors), included a survey to evaluate the content knowledge for specific topics such as those introduced in the polymer semiconductor education kit. Results from the 20 educators (grades 9−12) show that teachers were least conf ident in teaching about monomers, polymers, and their properties; light−matter interactions; properties of insulators; semiconductors/conductors; and molecular orbital theory. Teachers were most excited to learn about monomer/polymer properties, insulator properties, semiconductors/conductors, batteries/circuits/devices, and renewable/alternative energy production. The workshop participants were required to fill out preregistration and postworkshop feedback surveys. In the preregistration survey, instructors were asked to state their motivation to attend this professional development workshop, which specified incorporation of advancements in the field of polymer semiconductor chemistry. Instructor reasons included:

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DISCUSSION: PROFESSIONAL DEVELOPMENT WORKSHOPS FOR SCIENCE EDUCATORS A professional-development-based workshop was also developed to train educators to use the three modules in their classrooms. Important characteristics of the workshop include: • Inclusion of local expert(s) to facilitate advanced content knowledge and developments in the field • Targets meaningful local and global issues in the content area • Hands-on activities with time allotted for inquiry-style experimentation • Dedicated time for educator collaboration to discuss classroom integration • Suggested curricular support for appropriate classroom implementation • Technology integration as a medium for distribution of the content and to aid in classroom implementation • Availability of outreach assistance in the local community Educators have expressed limited familiarity with polymer and material science concepts, revealing a knowledge deficiency in current advancements in science. Workshops and continuing

• Experience with techniques and procedures in polymer science to enhance advanced chemistry courses • Interest in using content in a problem-based learning project on solar energy • Showing students real-world applications of chemistry • Wanting personal experience in higher-level science • Modernizing student examples and applications of chemistry Teacher Feedback

Time was allotted for teacher reflection and feedback during and after the workshop.10 Teachers were asked to reflect on curriculum integration, student interest and engagement, and how this new knowledge and kit could be used to improve their content lessons. Workshop participants from the Fall 2015 and 2016 cohorts provided the following feedback on the workshop program, content, and kit activities. These teachers have gone on to use the laboratory activities each year in their classrooms. Some advantages or benefits of the workshop emerged from the 2015 and 2016 teacher participant feedback: E



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• Ease of procedure with use of visuals and procedures apps • Built-in “teaching moments” during procedures • WOW factor, fun learning for the students • Background, instructions, and questions included in activity • Practical application of content • Allows for creativity Workshop teacher participants from 2015 and 2016 characterized these drawbacks: • Specific content referenced may be too advanced • Concepts can be overwhelming for regular classes • Access to equipment for some procedures Three months following the Fall 2015 workshop additional teacher feedback was attained through a survey using an online questionnaire. Survey results indicate that teachers were enthusiastic about taking this knowledge and these activities back to their students (Table S10 in the Supporting Information). They also indicate that a majority of the instructors preferred to implement the PAni synthesis module first in their classrooms, although most had performed the OLED experiment.

the activities were received with great excitement and enthusiasm. Students were most excited about the opportunity to learn about how chemistry and physics were relevant and directly affecting their everyday lives. The students were surprised about the simplicity of the PAni polymerization reaction and were also interested to learn about how scientists were working to improve the efficiency and affordability of OLEDs and solar cell devices (section S11).

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CONCLUSION A comprehensive polymer semiconductor education kit and curriculum has been developed that focuses specifically on the advancements in organic electronics such as conductive polymers, OLED technology, and organic solar cells.5 The goals of the polymer semiconductor kit and teacher workshops are aimed to bring more opportunities to bridge the gap between 9−12 education and university-level research and education. Although initially developed for 9−12 educators and students, it is likely that the laboratory activities would also be appropriate at the undergraduate level. It is clear from the educator workshops, student participation in the project, and classroom outcomes that there is a great deal of interest in providing hands-on activities like these, which provide a new perspective for students learning basic chemical concepts and principles. Uniquely, these activities teach many aspects of contemporary polymeric materials in the area of molecular electronics that may or may not be available at the undergraduate level. Both instructors and students showed great enthusiasm for the experiments and were excited to learn more and/or experiment with new variations of the laboratory activities.

Student Evaluation and Results

Thirty-six students from an Advanced Placement (AP) high school chemistry class completed the polymer semiconductor education kit modules. Students worked in groups of two or three during the activities with a goal to effectively address procedural guidelines, communicate about findings, and engage collaboratively in the inquiry learning process. Students completed module 1 (polyaniline synthesis and properties testing) and module 2 (construction of a polymer LED) in one 90 min class period. Students were able to complete preparation of materials for PAni sensor testing (module 1, part 2; section S3) and polymer OLED simultaneously, using lab time efficiently. Students completed module 3 (polymer solar cell) in approximately 45 min. The classroom teacher prepared the solar cell TiO2 FTO glass slides the day before, as prescribed in the teacher laboratory manual (section S8). Student lab results were within the expected range: PAni synthesis occurred within 5 min of combining the reagent solutions, the PAni sensor baseline resistance was 65−70 kΩ, the doped PAni sensor resistance was 55−65 kΩ, and the dedoped PAni sensor resistance (PAni exposed to ammonia) was 75−80 kΩ. The polymer LED upon construction showed yellow-orange emission with each student group. These activities were completed as supplemental inquiry lab activities to extend the AP chemistry content to real-world applications. In particular, students compared typical physical properties of metallic, ionic, covalent, and network-covalent materials and then explored the nature of conductive polymers through polyaniline oxidative synthesis. Students compared the emission spectra of various forms of light and gases to that of the MEH-PPV polymer to conclude their discussion of light− matter interactions and the electromagnetic spectrum. Lastly, students used the polymer solar cell as an application of light− matter interactions for the purpose of energy production, specifically alternative energy resources such as solar energy. These modules could be used as replacement laboratory investigations for these topics at the time of instruction, as extension application activities, or as a means to demonstrate multiple concepts to an advanced chemistry class. In general,

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00332. Laboratory instruction, detailed student procedures, and instructor preparation with background/curriculum alignment manuals for the PAni module (module 1) (S1−S3) (PDF) Laboratory instruction, detailed student procedures, and instructor preparation with background/curriculum alignment manuals for the OLED−MEH PPV module (module 2) (S4−S6) (PDF) Laboratory instruction, detailed student procedures, and instructor preparation with background/curriculum alignment manuals for the MEH-PPV TiO2 solar cell module (module 3) (S7−S9) (PDF) Postworkshop high school teacher feedback (S10) (PDF) Student feedback from an AP high school chemistry class (S11) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Michael G. Walter: 0000-0002-9724-265X Notes

The authors declare no competing financial interest. F



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ACKNOWLEDGMENTS The outreach efforts were funded by the Department of Chemistry at the University of North Carolina at Charlotte, a grant from the Camille and Henry Dreyfus Foundation Special Grant Program in the Chemical Sciences, and the American Chemical Society (ACS) Science Coaches.

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REFERENCES

(1) 2015 ACS Guidelines for Undergraduate Professional Education in Chemistry. https://www.acs.org/content/dam/acsorg/about/ governance/committees/training/2015-acs-guidelines-for-bachelorsdegree-programs.pdf (accessed October 2017). (2) IUPAC Polymer Education Website. https://iupac.org/polymeredu/ (accessed October 2017). (3) Kosbar, L. L.; Wenzel, T. J. Inclusion of Synthetic Polymers within the Curriculum of the ACS Certified Undergraduate Degree. J. Chem. Educ. 2017, DOI: 10.1021/acs.jchemed.6b00922. (4) Hains, A. W.; Liang, Z.; Woodhouse, M. A.; Gregg, B. A. Molecular Semiconductors in Organic Photovoltaic Cells. Chem. Rev. 2010, 110 (11), 6689−6735. (5) Facchetti, A. π-Conjugated Polymers for Organic Electronics and Photovoltaic Cell Applications. Chem. Mater. 2011, 23 (3), 733−758. (6) Tremblay, J. F. OLED Growth Spurs Spending Spree. Chem. Eng. News 2017, 95 (7), 12. (7) Enlow, J. L.; Marin, D. M.; Walter, M. G. Developing a Polymer Semiconductor Education Kit and Curriculum for High School Science Classrooms. Macromol. Symp. 2015, 355 (1), 43−51. (8) Grätzel, M. Solar Energy Conversion by Dye-Sensitized Photovoltaic Cells. Inorg. Chem. 2005, 44 (20), 6841−6851. (9) Polymer Semiconductor Education Kit. http://polymereducation-kit.weebly.com/ (accessed October 2017). (10) Borko, H. Professional Development and Teacher Learning: Mapping the Terrain. Educ. Res. 2004, 33 (8), 3−15.

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Using Polymer Semiconductors and a 3-in-1 Plastic Electronics STEM

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