Beyond PCE: Looking at a Big Picture in Photovoltaic Research
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perovskite and organic materials as well as meticulous optimization from materials design, energetics, morphology, and interfacial structures. It is very exciting to see in five short years that perovskite solar cells have improved significantly in terms of materials stability with the use of nanocrystalline and 2D and 3D materials as well as different passivation methods.7 PPV devices have been shown to achieve a high Voc of 1.21 V without sacrificing photocurrent, with only a 0.41 V deficit, a bandgap of 1.62 eV, and a stabilized PCE approaching 21% at the maximum power point.5 These advancements have made new types of solar cells steps closer to commercialization. Meanwhile, OPV development also made fantastic advances in discoveries of new nonfullerene acceptor molecules, new donor polymers/small molecules, new donor/acceptor interfacial engineering to reduce loss in geminate and nongerminate charge recombination,8,9 as well as new approaches in characterization/optimization of active layer morphology for high device performance.10−12 While these devices and materials continue to make breakthroughs featured in headlines of scientific journals, a big picture may be useful to consider in terms of the PV impact on the entire energy landscape beyond the PCEs, such as life cycle impact, expense payback time, and elemental abundancy for sustainable commercialization of PPVs and OPVs. As a physical chemist, my own research on OPVs has been mainly focused on fundamental electronic processes in PV materials encompassed by three axes, structure (atomic positions r), energetics (E), and dynamics (t, time-dependent processes) in molecules and materials, as well as correlations of these three dimensions. The goal has been set to find the optimal point for materials and devices in this three-dimensional space where the optimal structure (i.e., local molecular structure and film morphology), energetics (i.e., sufficient driving force for exciton splitting with minimal loss in open-circuit voltage due to charge recombination), and dynamics (competitive charge separation and transport) will enable high performing PV devices. Thus, many of us have not thought about the other metrics beyond fundamental electronic processes and the device PCE. Therefore, it was enlightening to learn from the literature the big picture of the PV industry as well as its position in the entire energy landscape. For some of us who mainly work in laboratory settings, it is important to be knowledgeable about the energy required to produce solar cells and environmental impacts during the whole device lifetime, as well as the overall elemental abundance in the earth.13−15 Most of us are aware that OPV devices have relatively low efficiencies compared to silicon-based PV devices that dominate the PV market, but the payback time for OPVs can be 8−10 times faster than singlecrystal Si devices (Figure 1).13 Although perovskite solar cells
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ased on recent literature and the photovoltaic (PV) efficiency chart (https://www.nrel.gov/pv/assets/ images/efficiency-chart.png) published by the National Renewable Energy Laboratory (NREL) in the United States, the device efficiencies for two types of PVs, perovskite
Figure 1. Energy payback time for seven PV modules. P-1 represents the TiO2 perovskite module; P-2 represents the ZnO perovskite module. Reproduced from ref 13 with permission of the Royal Chemical Society.
photovoltaics (PPVs)1 and organic photovoltaics (OPVs),2 are approaching respectively >23%3 (tandem cell 27.3%)4 and 13−14%3,5,6 power conversion efficiencies (PCEs). These achievements represent tremendous research efforts on fundamental understanding of the electronic processes in
Figure 2. Abundance (atom fraction) of the chemical elements in Earth’s upper continental crust as a function of atomic number. (Gordon B. Haxel, Sara Boore, and Susan Mayfield from USGS; http://pubs.usgs.gov/fs/2002/fs087-02/). Notice the location of iodine (Z = 53). © 2018 American Chemical Society
Published: August 10, 2018 1967
DOI: 10.1021/acsenergylett.8b01166 ACS Energy Lett. 2018, 3, 1967−1968
Editorial
Cite This: ACS Energy Lett. 2018, 3, 1967−1968
ACS Energy Letters
Editorial
i−p Planar Perovskite Solar Cells by Processing with Glycol Ether Additives. ACS Energy Lett. 2017, 2, 1960−1968. (13) Gong, J.; Darling, S. B.; You, F. Perovskite Photovoltaics: LifeCycle Assessment of Energy and Environmental Impacts. Energy Environ. Sci. 2015, 8, 1953−1968. (14) Celik, I.; Philips, A. B.; Song, Z.; Yan, Y.; Ellingson, R. J.; Heben, M. J.; Apul, D. Energy Payback Time (EPBT) and Energy Return on Energy Invested (EROI) of Perovskite Tandem Photovoltaic Solar Cells. IEEE J. Photovoltaics 2018, 8, 305−309. (15) Yue, D.; Khatav, P.; You, P.; Darling, S. B. Deciphering the Uncertainties in Life Cycle Energy and Environmental Analysis of Organic Photovoltaics. Energy Environ. Sci. 2012, 5, 9163−9172.
have rather high efficiencies, their energy cost, environmental impact, as well as iodine abundance (Figure 2) are issues that cannot be completely ignored.14 Therefore, just like in fundamental electronic processes in PV materials and devices that need to balance structural (r), energetic (E), and dynamic (t) factors, the overall big picture in the PVs industry also needs to consider the balance of several metrics, as discussed above.
Lin X. Chen, Senior Editor
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Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, United States; Northwestern University, Evanston, Illinois 60208, United States
AUTHOR INFORMATION
ORCID
Lin X. Chen: 0000-0002-8450-6687 Notes
Views expressed in this editorial are those of the author and not necessarily the views of the ACS.
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REFERENCES
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DOI: 10.1021/acsenergylett.8b01166 ACS Energy Lett. 2018, 3, 1967−1968
