Activity pubs.acs.org/jchemeduc
Gaining Hands-On Experience with Solid-State Photovoltaics through Constructing a Novel n‑Si/CuS Solar Cell Zexun Jin, Yecheng Li, and Jimmy C. Yu* Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China S Supporting Information *
ABSTRACT: A solid-state p−n junction photovoltaic cell can be fabricated in a common chemistry lab. It is prepared by coating a p-type copper sulfide film on an n-type silicon wafer via a simple chemical bath deposition method. The resulting p−n junction shows an obvious photovoltaic effect. This activity offers students an opportunity to gain hands-on experience with the technology of solid-state solar cells.
KEYWORDS: First-Year Undergraduate/General, Laboratory Instruction, Hands-On Learning/Manipulatives, Interdisciplinary/Multidisciplinary, Materials Science, Solid State Chemistry (CuS)25 thin film and an n-type Si (n-Si) wafer. As a typical ptype semiconductor, CuS has a high free hole-carrier density (∼1022 cm−3)26 and thus is an excellent hole transport material while forming a p−n junction with n-Si. Furthermore, the CuS film can be coated on the surface of the n-Si wafer using a simple chemical bath deposition method. The n-Si/CuS PV device exhibits a stable and remarkable photocurrent. To the best of our knowledge, this is the first teaching experiment involving the construction of a solid-state PV cell to appear in this Journal.
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he development of clean and renewable energy is a central issue in this century.1 Solar energy, an inexhaustible natural resource, is recognized as the most promising renewable energy source to meet the future energy demand and sustainable development of the planet.2 Photovoltaic (PV) cells, which convert solar energy to electricity, are becoming an increasingly popular form of alternative energy around the world.3−6 The fundamentals of PV cells and their pedagogical approach have been discussed in this Journal7−18 and elsewhere.19,20 However, laboratory experiments to illustrate clearly the working of p−n junction-based solid-state PV cells are still needed. Making usable PV cells for the general chemistry curriculum falls somewhere in the range of difficult to impractical to overly expensive. A dye-sensitized solar cell (DSSC) is a device that can be prepared in a common chemistry laboratory,21 but its principle is quite different from the mainstream solid-state PV cell driven by the built-in p−n junction. In addition, the assembly process of a DSSC involves multiple components, resulting in uncertain performance of the final device. It is not easy for an untrained novice to fabricate a durable DSSC. Perovskite photocells typically contain lead compounds. Their toxicity makes them impractical in the general chemistry curriculum. Silicon solar cells are currently the most successful commercial PV devices owing to their mature manufacturing industry.22−24 Unfortunately, it is impossible to make a pure silicon p−n junction in a teaching laboratory because of the high cost of the equipment involved. Herein we introduce a simple laboratory activity designed for an undergraduate chemistry or materials science program to construct a solid-state PV device composed of a copper sulfide © XXXX American Chemical Society and Division of Chemical Education, Inc.
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EXPERIMENTAL OVERVIEW
Preparation of CuS Layer on n-Si
CuS thin films were deposited on n-Si substrates using a chemical bath deposition method. Before deposition, a commercial n-type Si(100) wafer (20 mm × 15 mm) was cleaned sequentially in the ultrasonic bath of acetone, ethanol, and water, each for 15 min, followed by immersion in 30% H2O2 solution at 80 °C for 10 min to make the surface hydrophilic. For the deposition of the CuS thin film, 2 mL of 0.1 M copper acetate solution was placed in a glass container, to which 2 mL of 0.1 M Na2EDTA solution and 6 mL of 0.1 M thioacetamide solution were added successively. The solution was mixed thoroughly under stirring. Then the precleaned Si substrate was immersed vertically in the solution. The container Received: August 16, 2016 Revised: February 5, 2017
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DOI: 10.1021/acs.jchemed.6b00617 J. Chem. Educ. XXXX, XXX, XXX−XXX
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was placed in a thermostatic water bath maintained at 80 °C for 3 h. After deposition, the samples were rinsed with deionized water and dried at 80 °C.
should be handled carefully using appropriate personal protection equipment (gloves, goggles, laboratory coat, etc.). An explosion-proof refrigerator should be used for the storage of 30% H2O2 solution. Also, thioacetamide is classified as 2B (possible carcinogen for humans) by the International Agency for Research on Cancer (IARC)27 and has a bit of foul odor, so it should be handled in a fume hood.
Device Fabrication and Photoresponse Measurements
As shown in Figure 1, the as-prepared n-Si/CuS was covered with a piece of indium tin oxide (ITO) glass with the
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RESULTS AND DISCUSSION The deposition mechanism of CuS and characterization of the as-prepared n-Si/CuS are given in the Supporting Information. To evaluate the photovoltaic performance of this device, we tested its open-circuit voltage and short-circuit current change under chopped AM 1.5G illumination. The light was shined perpendicular to the face of the CuS thin film (ITO glass has high transmittance). The results show that the device could generate a stable photovoltage of 0.44 V (Figure 3a) and a photocurrent of 2.9 mA cm−2 (Figure 3b) under simulated sunlight. Furthermore, the photovoltage and photocurrent kept steady over many testing cycles (1.4% degradation over 50 cycles), indicating that this device is fairly stable when exposed to irradiation under ambient conditions. It is worth mentioning that such a device can still work after storage for two years. To show the requirement of a p−n junction in a PV cell, two additional devices (n-Si only and p-Si/CuS) were fabricated for comparison. As shown in Figure 4, both of them exhibited very poor photoresponse properties. This phenomenon indicates that the formation of p−n junction is crucial for a PV cell, i.e., that devices with no p-type semiconductor or combining two ptype semiconductors perform poorly, if at all. The above measurements involve a workstation and a standard light source to give quantitative results. In a more convenient demonstration, the students can take the PV device outdoors on a sunny day and use a multimeter to measure the voltage generated. As shown in Figure 2, a voltage of 197.7 mV emerges under sunlight, while the value becomes negligible in the dark.
Figure 1. Schematic diagram of device fabrication and photoresponse measurement.
conductive side in contact with the CuS film. The whole device was fastened using two binder clips or adhesive tapes. Afterward, the Si side (without CuS) and the ITO side were connected to the testing circuit via simple alligator clips. The voltage and current were measured on an electrochemical worksation or a SourceMeter. The light source was a 300 W xenon lamp with an AM 1.5G filter to simulate sunlight irradiation (100 mW/cm2). In a more convenient demonstration, the students can take the PV device outdoors under sunlight irradiation (if available) and test the photovoltage using a hand-held multimeter. Typically, we connected alligator clips to the Si side and the ITO side and then connected the other ends of the alligator clips to the probes of the multimeter test leads (Figure 2). The exposure of the device to sunlight is simply controlled by hand.
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CONCLUSIONS A new PV cell based on the heterojunction of n-Si and CuS can be fabricated via a convenient chemical bath deposition method. This activity may be appreciated by instructors of advanced chemistry or materials science courses. Under-
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HAZARDS The 30% H2O2 solution and thioacetamide are hazardous in case of skin contact (irritant) or eye contact (irritant). They
Figure 2. Measurement of the photovoltage from the n-Si/CuS PV device under sunlight (left) and dark conditions (right). B
DOI: 10.1021/acs.jchemed.6b00617 J. Chem. Educ. XXXX, XXX, XXX−XXX
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Figure 3. (a) V−t and (b) I−t characteristics of the n-Si/CuS device under chopped AM 1.5G illumination.
Vice-Chancellor’s One-Off Discretionary Fund of The Chinese University of Hong Kong (Project VCF2014016).
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Figure 4. I−t characteristics of other two devices: n-Si only and p-Si/ CuS. The device fabrication and photoresponse measurement processes are exactly the same as for n-Si/CuS.
graduate students will learn from this activity the principles and techniques of photovoltaics, semiconductor materials, and thinfilm coating. After seeing the potential and limitations of solar energy conversion, the students should be ready to discuss global energy issues.
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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.6b00617. Details of experimental section and characterization and notes for device testing (PDF, DOCX)
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jimmy C. Yu: 0000-0001-9886-3725 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work described in this paper was supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China, under Theme-Based Research Scheme through Project T23-407/13-N and a grant from the C
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(17) Moore, J. W. Energizing Students and Science. J. Chem. Educ. 2007, 84, 743. (18) Zoller, U. Science Education for Global Sustainability: What Is Necessary for Teaching, Learning, and Assessment Strategies? J. Chem. Educ. 2012, 89, 297−300. (19) Aurandt, J. L.; Butler, E. C. Sustainability Education: Approaches for Incorporating Sustainability into the Undergraduate Curriculum. J. Prof. Issues Eng. Educ. Pract. 2011, 137, 102−106. (20) Burmeister, M.; Rauch, F.; Eilks, I. Education for Sustainable Development (ESD) and chemistry education. Chem. Educ. Res. Pract. 2012, 13, 59−68. (21) Smith, Y. R.; Crone, E.; Subramanian, V. A Simple Photocell To Demonstrate Solar Energy Using Benign Household Ingredients. J. Chem. Educ. 2013, 90, 1358−1361. (22) Carlson, D. E.; Wronski, C. R. Amorphous silicon solar cell. Appl. Phys. Lett. 1976, 28, 671−673. (23) Tsakalakos, L.; Balch, J.; Fronheiser, J.; Korevaar, B. A.; Sulima, O.; Rand, J. Silicon nanowire solar cells. Appl. Phys. Lett. 2007, 91, 233117. (24) Yoo, J.; Yu, G.; Yi, J. Large-area multicrystalline silicon solar cell fabrication using reactive ion etching (RIE). Sol. Energy Mater. Sol. Cells 2011, 95, 2−6. (25) Covellite CuS, a stoichiometric member of the broad family of copper sulfides, has a unique metallic-like character that accounts for its elevated p-type conductivity. Although the exact bonding relationships and oxidation states of its constituent atoms are still debated, all proposed models of its electronic band structure suggest that CuS can intrinsically allow for a significant density of valenceband-delocalized holes without the need for intervening metal vacancies in the lattice. See: Xie, Y.; Carbone, L.; Nobile, C.; Grillo, V.; D’Agostino, S.; Della Sala, F.; Giannini, C.; Altamura, D.; Oelsner, C.; Kryschi, C.; Cozzoli, P. D. Metallic-like Stoichiometric Copper Sulfide Nanocrystals: Phase- and Shape-Selective Synthesis, NearInfrared Surface Plasmon Resonance Properties, and Their Modeling. ACS Nano 2013, 7, 7352−7369. (26) Nozaki, H.; Shibata, K.; Ohhashi, N. Metallic hole conduction in CuS. J. Solid State Chem. 1991, 91, 306−311. (27) Thioacetamide MSDS. http://www.sciencelab.com/msds. php?msdsId=9925237 (accessed January 2017).
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DOI: 10.1021/acs.jchemed.6b00617 J. Chem. Educ. XXXX, XXX, XXX−XXX
